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Syntheses, vibrational and solution studies of niobium(V) and tantalum(V) fluoride fluorosulfates Zhang, Dingliang 1991

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SYNTHESES, VIBRATIONAL AND SOLUTION STUDIES OF NIOBIUM(V) AND TANTALUM(V) FLUORIDE FLUOROSULFATES by DINGLIANG ZHANG B.Sc, Hangzhou U n i v e r s i t y , Hangzhou, Zhe j i a n g , China, 1982 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF MASTER OF SCIENCE IN THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard The U n i v e r s i t y of B r i t i s h Columbia © D. ZHANG, August 1991 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada Date Qrtr.SC , l_9 9 1 DE-6 (2/88) ABSTRACT A one-step metal o x i d a t i o n / l i g a n d r e d i s t r i b u t i o n r e a c t i o n was developed t o s y n t h e s i z e niobium(V) and tantalum(V) f l u o r i d e f l u o r o s u l f a t e s a c c o r d i n g t o the equation: (5-n)M + nMF 5 + excess S 2 0 6 F 2 — > 5 M F n ( S 0 3 F ) 5 _ n F i v e compounds, N b 2 F 9 ( S 0 3 F ) , NbF 4(S0 3F), N b F 3 ( S 0 3 F ) 2 , T a F 4 ( S 0 3 F ) , and T a F 3 ( S 0 3 F ) 2 , were i s o l a t e d as c o l o r l e s s v i s c o u s l i q u i d s i n pure form. Nb 2F 9(S0 3F) appears t o be u n s t a b l e , and d i s p r o p o r t i o n a t e s i n t o s o l i d [ N b F 5 ] 4 and l i q u i d NbF 4(S0 3F) on lon g s t a n d i n g . T a 2 F 9 ( S 0 3 F ) cannot be s y n t h e s i z e d i n t h i s manner. The attempted i s o l a t i o n of compounds with h i g h e r f l u o r o s u l f a t e content, M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n<3), was not s u c c e s s f u l . Raman and i n f r a r e d s p e c t r a of the i s o l a t e d compounds, M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n>3), are i n t e r p r e t e d i n terms of a s s o c i a t e d s t r u c t u r e s w i t h approximately o c t a h e d r a l c o o r d i n a t i o n of niobium or tantalum by f l u o r i d e s and f l u o r o s u l f a t e groups. B i d e n t a t e f l u o r o s u l f a t e groups, presumably b r i d g i n g , are found i n a l l t hese d e r i v a t i v e s . In N b F 3 ( S 0 3 F ) 2 and T a F 3 ( S 0 3 F ) 2 both monodentate and b i d e n t a t e f l u o r o s u l f a t e groups are found. B r i d g i n g f l u o r i n e s are suggested f o r l i q u i d Nb 2F 9(S0 3F) and, t o a l e s s e r extent, NbF 4(S0 3F) and TaF 4(S0 3F) as w e l l . T e n t a t i v e assignments of the v i b r a t i o n a l bands are d i s c u s s e d . The behavior of the s t a b l e compounds, M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n=2, 3), i n HS0 3F was i n v e s t i g a t e d by conductometric s t u d i e s . i i i A t low Lewis a c i d c o n c e n t r a t i o n s , the r e s u l t s show t h a t T a F 3 ( S 0 3 F ) 2 i s a weak Lewis a c i d i n HS0 3F and t h a t N b F 3 ( S 0 3 F ) 2 , NbF 4(S0 3F) and TaF 4(S0 3F) behave as very weak e l e c t r o l y t e s and poor S0 3F~ a c c e p t o r s . C o n d u c t i v i t i e s of the s e s o l u t i o n s i n HS0 3F f o l l o w the o r d e r : T a F 3 ( S 0 3 F ) 2 » N b F 3 ( S 0 3 F ) 2 > T a F 4 ( S 0 3 F ) > NbF 4(S0 3F) . The g e n e r a l t r e n d p r e v i o u s l y r e p o r t e d f o r HS0 3F/SbF 5/S0 3 and HS0 3F/AsF 5/S0 3 systems i s v a l i d f o r HS0 3F/NbF n(S0 3F) 5_ n and HS0 3F/TaF n(S0 3F) 5_ n systems, i . e . , the more f l u o r i n e s r e p l a c e d by f l u o r o s u l f a t e groups, the s t r o n g e r the r e s u l t i n g s u p e r a c i d . At h i g h Lewis a c i d c o n c e n t r a t i o n s , v i s c o u s and h i g h l y c o n d u c t i n g l i q u i d s are p r e s e n t . For the c o n d u c t i v i t i e s of the neat l i q u i d s , the f o l l o w i n g order was observed: N b F 3 ( S 0 3 F ) 2 > T a F 3 ( S 0 3 F ) 2 > NbF 4(S0 3F) > T a F 4 ( S 0 3 F ) . i v TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES v i i i LIST OF SYMBOLS AND ABBREVIATIONS i x ACKNOWLEDGEMENT X 1. GENERAL INTRODUCTION 1 1.1 Acid-base Concepts 1 1.2 A c i d i t y S c a l e s f o r P r o t o n i c A c i d s and T h e i r S o l u t i o n s 3 1.3 S u p e r a c i d Concepts 4 1.4 Bronsted A c i d s Used i n Conjugate Bronsted/Lewis Superacids 8 1.5 Lewis A c i d s Used i n Conjugate Bronsted/Lewis Superacids 10 1.6 C h a r a c t e r i z a t i o n of F l u o r o - and F l u o r o s u l f a t o -D e r i v a t i v e s by V i b r a t i o n a l Spectroscopy 18 1.7 O b j e c t i v e s of T h i s Research 25 2. EXPERIMENTAL 2 8 2.1 General Comments 28 2.2 Chemicals Used i n T h i s Study 29 2.3 Apparatus and Equipment 31 2.4 I n s t r u m e n t a t i o n and Methods 3 6 V 3. SYNTHESIS AND CHARACTERIZATION OF FLUORIDE FLUOROSULFATES OF NIOBIUM(V) AND TANTALUM(V) 40 3.1 I n t r o d u c t i o n 40 3.2 Experimental ...42 3.3 R e s u l t s and D i s c u s s i o n 45 4. CONDUCTOMETRIC STUDIES OF Nb(V) AND Ta(V) FLUORIDE FLUOROSULFATES IN FLUOROSULFURIC ACID 81 4.1 I n t r o d u c t i o n 81 4.2 Experimental 83 4.3 R e s u l t s and D i s c u s s i o n 85 5. CONCLUSIONS 101 5.1 C o n c l u s i o n s 101 5.2 P r o p o s a l s f o r F u r t h e r S t u d i e s 103 REFERENCES. 105 APPENDIX 116 v i LIST OF TABLES 1-1 P h y s i c a l P r o p e r t i e s of Some Bronsted Superacids 7 1-2 P h y s i c a l P r o p e r t i e s of Some B i n a r y P e n t a f l u o r i d e s 12 1- 3 Hammett A c i d i t y F u n c t i o n s of Some Conjugate Bronsted/Lewis Superacids I n v o l v i n g HF and HS0 3F.... 12 2- 1 Source and P u r i t y of Chemicals 29 3- 1 Experimental D e t a i l s of Syntheses o f M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta) 44 3-2 Elemental A n a l y s i s Data f o r M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta) 51 3-3 Raman Bands f o r N b 2 F 9 ( S 0 3 F ) , Molten NbF 5, NbF 4 ( S 0 3 F ) , S b 2 F 9 ( S 0 3 F ) and SbF 5 59 3-4 Assignment of V i b r a t i o n a l Bands of Nb 2F 9(S0 3F) 60 3-5 Assignments of V i b r a t i o n a l Bands of NbF 4(S0 3F) and N b F 3 ( S 0 3 F ) 2 66 3-6 Assignments of V i b r a t i o n a l Bands of TaF 4 ( S 0 3 F ) and T a F 3 ( S 0 3 F ) 2 70 3-7 C o r r e l a t i o n of Raman Bands of F l u o r o s u l f a t e Group f o r M F 3 ( S 0 3 F ) 2 (M=Nb, Ta, As and Sb) 72 3-8 Comparison of V i b r a t i o n a l Spectra of S o l i d and L i q u i d TaF 4(S0 3F) 74 v i i 4-1 C o n d u c t i v i t y Data f o r L i q u i d Niobium(V) and Tantalum(V) F l u o r i d e F l u o r o s u l f a t e 86 4-2 C o n d u c t i v i t y Data f o r S o l u t i o n s of NbF 4(S0 3F) or N b F 3 ( S 0 3 F ) 2 i n HS0 3F (25.00°C) 89 4-3 C o n d u c t i v i t y Data f o r S o l u t i o n s of Ta F 4 ( S 0 3 F ) or T a F 3 ( S 0 3 F ) 2 i n HS0 3F (25.00°C) 90 4-4 Conductometric T i t r a t i o n of NbF 4(S0 3F) or N b F 3 ( S 0 3 F ) 2 w i t h KS0 3F i n HS0 3F 96 4-5 Conductometric T i t r a t i o n of Ta F 4 ( S 0 3 F ) or T a F 3 ( S 0 3 F ) 2 w i t h KS0 3F i n HS0 3F 96 v i i i LIST OF FIGURES 1- 1 Frequency Range of V i b r a t i o n a l Fundamentals f o r the F l u o r o s u l f a t e Group 22 2- 1 T y p i c a l Pyrex R e a c t i o n V e s s e l s Used i n T h i s Study 3 2 2-2 A K e l - F T ubular Reactor 33 2-3 Vacuum-Adapted F i l t r a t i o n and D i s t i l l a t i o n Apparatus ... 34 2-4 E l e c t r i c a l C o n d u c t i v i t y C e l l 38 2- 5 A d d i t i o n Apparatus Used During C o n d u c t i v i t y Measurements 39 3- 1 I n f r a r e d and Raman Spec t r a of Nb 2F 9(S0 3F) 57 3-2 I n f r a r e d and Raman Spec t r a of NbF 4(S0 3F) 64 3-3 I n f r a r e d and Raman Spec t r a of N b F 3 ( S 0 3 F ) 2 65 3-4 I n f r a r e d and Raman Spec t r a of TaF 4(S0 3F) 68 3-5 I n f r a r e d and Raman Spec t r a of T a F 3 ( S 0 3 F ) 2 69 3-6 1 9 F NMR Spec t r a of Nb(V) F l u o r i d e F l u o r o s u l f a t e s 77 3- 7 1 9 F NMR Sp e c t r a of Ta(V) F l u o r i d e F l u o r o s u l f a t e s 78 4- 1 Temperature-dependent c o n d u c t i v i t i e s of L i q u i d Nb(V) and Ta(V) F l u o r i d e F l u o r o s u l f a t e s 87 4-2 Concentration-dependent C o n d u c t i v i t i e s of S o l u t i o n s of Nb(V) and Ta(V) F l u o r i d e F l u o r o s u l f a t e s i n HS0 3F 91 4-3 T i t r a t i o n Curves of Nb(V) and Ta(V) F l u o r i d e F l u o r o s u l f a t e s with KS0 3F i n HS0 3F 97 4-4 Concentration-dependent C o n d u c t i v i t i e s of S o l u t i o n s of M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta) i n HS0 3F 99 i x LIST OF SYMBOLS AND ABBREVIATIONS // t he a n a l y z i n g h a l f wave p l a t e s e t i s parallel t o the plane of p o l a r i z a t i o n of the i n c i d e n t r a d i a t i o n i n Raman s p e c t r a _L t he a n a l y z i n g h a l f wave p l a t e s e t i s perpendicular t o the plane of p o l a r i z a t i o n of the i n c i d e n t r a d i a t i o n i n Raman s p e c t r a * background peak i n v i b r a t i o n a l s p e c t r a v v i b r a t i o n a l s t r e t c h i n g mode, or v i b r a t i o n a l frequency v sym symmetric v i b r a t i o n a l s t r e t c h i n g mode v a s asymmetric v i b r a t i o n a l s t r e t c h i n g mode v i b r a t i o n a l r o c k i n g mode S v i b r a t i o n a l deformation mode T v i b r a t i o n a l t o r s i o n mode m m o l a l i t y (mole/kg) k c o n d u c t i v i t y X ACKNOWLEDGEMENT I would l i k e t o express my thanks and g r a t i t u d e t o my r e s e a r c h s u p e r v i s o r , P r o f e s s o r F . Aubke, f o r h i s guidance and encouragement throughout t h i s graduate work. Shah Roshan Cader, Germaine Hwang, Fred M i s t r y and Jun X i a are a l s o thanked f o r t h e i r h e l p and u s e f u l d i s c u s s i o n s d u r i n g my t e n u r e . My g r a t i t u d e i s a l s o extended t o the e x c e p t i o n a l s e r v i c e s t a f f of t h i s department: Mr. P. Borda f o r the elemental a n a l y s e s , Ms. L. Darge f o r r e c o r d i n g NMR s p e c t r a , and the G l a s s b l o w i n g , E l e c t r o n i c and Mechanical E n g i n e e r i n g S e r v i c e s f o r c o n s t r u c t i n g and m a i n t a i n i n g the equipment used f o r t h i s r e s e a r c h . F i n a l l y , I would l i k e t o thank my w i f e , Weihua Chen, whose encouragement and support d u r i n g these years has been i n v a l u a b l e . 1 CHAPTER 1 GENERAL INTRODUCTION In t h i s chapter, a number of concepts and terms a p p l i c a b l e t o t h i s work w i l l be d i s c u s s e d . T h i s d i s c u s s i o n i s f o l l o w e d by a l i t e r a t u r e review and the f o r m u l a t i o n of the p r e s e n t r e s e a r c h p r o j e c t . 1.1 Acid-base Concepts S e v e r a l g e n e r a l reviews have been p u b l i s h e d on aci d - b a s e c o n c e p t s . l a _ l c The f i r s t c l e a r d e f i n i t i o n of " a c i d " and "base" was g i v e n by A r r h e n i u s . 2 He d e f i n e d a c i d s as hydrogen-containing substances and bases as h y d r o x y l - c o n t a i n i n g substances t h a t d i s s o c i a t e i n water t o y i e l d hydrogen and hydroxide i o n s , r e s p e c t i v e l y : HCl(aq) > H +(aq) + Cl"(aq) [ 1 - l a ] acid NaOH(aq) > Na +(aq) + OH"(aq) [1-lb] base In 1923, Bronsted extended t h i s concept t o other p r o t o n i c s o l v e n t s by d e f i n i n g a c i d s as proton donors and bases as proton a c c e p t o r s . 3 T h i s d e f i n i t i o n can be used f o r any p r o t o n i c s o l v e n t . For example, the a u t o p r o t o l y s i s of H 2S0 4 i n v o l v e s two conjugate a c i d and base p a i r s : H 2S0 4 + H 2S0 4 ===== H 3 S 0 4 + + HS0 4" [1-2] acid 1 base 1 acid 2 base 2 In a d d i t i o n , the s o l v e n t system concept, proposed o r i g i n a l l y 2 by Cady and E l s e y , i s a very u s e f u l model f o r both p r o t o n i c and n o n - p r o t o n i c i o n i z i n g s o l v e n t s . 4 The s e l f - i o n i z a t i o n of a s o l v e n t g i v e s r i s e t o a c a t i o n i c s p e c i e s (acidium ion) and an a n i o n i c s p e c i e s (base i o n ) : 2 B rF 3 5==^ B r F 2 + + B r F 4 " [1-3] acidium base ion ion Substances t h a t i n c r e a s e the acidium i o n c o n c e n t r a t i o n are termed a c i d s and substances t h a t i n c r e a s e the base i o n c o n c e n t r a t i o n are termed bases. N o n e l e c t r o l y t e s do not a f f e c t the c o n c e n t r a t i o n of e i t h e r the acidium i o n or the base i o n . There are two c r i t e r i a f o r the use of the s o l v e n t system concept: ( i ) the p o s t u l a t e d acidium and base ions must be r e a l i s t i c chemical s p e c i e s , and ( i i ) t h e r e must be p h y s i c a l evidence ( e l e c t r i c a l conductance) and chemical evidence (chemical r e a c t i o n s i n the s o l v e n t i n v o l v i n g the p a r t i c u l a r s p e c i e s ) f o r the proposed s e l f - i o n i z a t i o n of the s o l v e n t . For the s e l f -i o n i z a t i o n r e a c t i o n [1-3], the compounds c o n t a i n i n g the acidium and base i o n s , B r F 2 [ A s F 6 ] and K [ B r F 4 ] , are w e l l known. T h e i r i n t e r a c t i o n i n l i q u i d B r F 3 i s an example of a t y p i c a l n e u t r a l i z a t i o n r e a c t i o n : B r F 2 [ A s F 6 ] + K [ B r F 4 ] — > K[AsF 6] + 2BrF 3 [l-3b] Meanwhile, Lewis proposed an a l t e r n a t i v e s e t of a cid-base d e f i n i t i o n s t h a t c h a r a c t e r i z e d acid-base r e a c t i o n s i n terms of the r e d i s t r i b u t i o n of the r e a c t a n t s 1 v a l e n c e e l e c t r o n s . 5 He 3 d e f i n e d a c i d s as substances t h a t can accept e l e c t r o n p a i r s , and bases as substances t h a t can donate unshared e l e c t r o n p a i r s . Thus, f o r example, BF 3 i s a Lewis a c i d and NH 3 i s a Lewis base. NH 3 donates an e l e c t r o n p a i r t o BF 3 t o form an a c i d - b a s e adduct f e a t u r i n g a d a t i v e or c o o r d i n a t e bond: BF 3 + :NH3 — > F 3B NH 3 [1-4] acid base adduct 1.2 A c i d i t y Scales for Protonic Acids and Their Solutions The a c i d s d e f i n e d above cover a wide range of a c i d s t r e n g t h s . In t h i s s e c t i o n , o n l y a c i d i t y s c a l e s s u i t a b l e f o r p r o t o n i c s o l v e n t s and t h e i r s o l u t i o n s are d i s c u s s e d . The a c i d i t y of a d i l u t e aqueous s o l u t i o n may be c o n v e n i e n t l y expressed by the pH s c a l e d e f i n e d as: pH = - l o g [ H 3 0 + ] [1-5] where [H 30 +] i s the c o n c e n t r a t i o n of the s o l v a t e d p r o t o n . The e x p e r i m e n t a l d e t e r m i n a t i o n of pH v a l u e s i n v o l v e s the measurement of the p o t e n t i a l of a hydrogen e l e c t r o d e or a g l a s s e l e c t r o d e i n e q u i l i b r i u m w i t h the s o l u t i o n s t o be t e s t e d . The pH range i s l i m i t e d t o 14 pH u n i t s by the a u t o p r o t o l y s i s c o n s t a n t of water, K a P = [ H 3 0 +][°H~] = l . O x l O - 1 4 a t 25°C. P r a c t i c a l l y , the u s e f u l n e s s of the pH s c a l e i n aqueous s o l u t i o n s i s l i m i t e d t o an approximate range of 1 < pH < 13. Beyond t h a t range, the pH s c a l e no longer has any r e a l s i g n i f i c a n c e . The a c i d i t y of s t r o n g p r o t o n i c a c i d s or t h e i r s o l u t i o n s i n 4 s t r o n g p r o t o n i c a c i d s can be expressed by the Hammett A c i d i t y F u n c t i o n , 6 H 0, d e f i n e d as: H 0 = P K B H + " l o g ( [ B H + ] / [ B ] ) [1-6] where K B H+ = the e q u i l i b r i u m c o n s t a n t of the p r o t o n a t i o n r e a c t i o n of a s u i t a b l e o r g a n i c weak base B, which a c t s as an i n d i c a t o r , [BH +] = the c o n c e n t r a t i o n of the p r o t o n a t e d i n d i c a t o r , [B] = the c o n c e n t r a t i o n of the unprotonated i n d i c a t o r . The i o n i z a t i o n r a t i o of the i n d i c a t o r , [BH +]/[B], may be measured by U V - v i s i b l e spectroscopy or by NMR spectroscopy. Using a s e t of s u i t a b l e o r g a n i c bases wi t h v a r i o u s pK B H+, the a c i d i t y of a v e r y s t r o n g a c i d system can thus be determined. 1.3 Superacid Concepts The term " s u p e r a c i d s " f i r s t appeared i n chemical l i t e r a t u r e i n 1927 and i t r e f e r r e d t o those s t r o n g p r o t o n i c a c i d s t h a t were capable of p r o t o n a t i n g weak bases such as c a r b o n y l compounds. 7 In the 1960's, G i l l e s p i e d e f i n e d s u p e r a c i d s as a c i d s t h a t are s t r o n g e r than 100% s u l f u r i c a c i d (-H 0>12). 8 T h i s d e f i n i t i o n , a lthough somewhat a r b i t r a r y , has been w i d e l y accepted. Furthermore, the concept has a l s o been extended t o Lewis a c i d s . Olah e t a l . d e f i n e d Lewis s u p e r a c i d s as those s t r o n g e r than A1C1 3. 9 Superacids have been used t o generate and s t a b i l i z e unusual 5 c a t i o n s . These generated c a t i o n s may be i s o l a t e d o r s t u d i e d in situ u s i n g s p e c t r o s c o p i c t e c h n i q u e s . The a p p l i c a t i o n o f su p e r a c i d s i n o r g a n i c chemistry i n v o l v e s t he s t a b i l i z a t i o n o f c a r b o c a t i o n s , i n c l u d i n g c l a s s i c a l carbenium i o n s and " n o n c l a s s i c a l " t e t r a - o r pen t a - c o o r d i n a t e d carbonium i o n s , 1 0 which may not be d i r e c t l y observed i n l e s s a c i d i c media and the c a t a l y s i s o f o r g a n i c r e a c t i o n s v i a t r a n s i e n t c a r b o c a t i o n i n t e r m e d i a t e s . 9 Superacids are a l s o used t o generate unusual i n o r g a n i c c a t i o n s such as polyatomic c a t i o n s of the n o n - m e t a l l i c elements o f Group 15, 16 and 17, which i n t u r n may be used i n v a r i o u s s u b s t i t u t i o n r e a c t i o n s as i n t e r m e d i a t e s . 1 1 , 1 2 When one c o n s i d e r s the p r i n c i p a l ways of i n c r e a s i n g the a c i d i t y of a Bronsted a c i d , the s o l v e n t system concept i s a ver y u s e f u l model. For example, HF undergoes a u t o p r o t o l y s i s a c c o r d i n g t o : 2HF ===== H 2 F + + F~ [1-7] One way t o i n c r e a s e the a c i d i t y i n v o l v e s p r o t o n a t i o n by another s t r o n g Bronsted a c i d , such as HS03F, t h a t can donate a prot o n t o HF t o i n c r e a s e the c o n c e n t r a t i o n of the acidium i o n : HF + HS0 3F ===== H 2F + + SO3F" [ l - 7 a ] O b v i o u s l y t h i s r o u t e i s ve r y l i m i t e d , s i n c e the a c i d s t r e n g t h s of HF and HSO3F are comparable and both rank h i g h among the s t r o n g e s t Bronsted a c i d s . An a l t e r n a t e way t o i n c r e a s e the a c i d i t y of the Bronsted 6 a c i d HF i s the a d d i t i o n of a Lewis a c i d , l i k e BF 3, t o remove the counter anion F~ from the a u t o p r o t o l y s i s e q u i l i b r i u m [1-7] by c o n v e r t i n g F~ i n t o a l e s s b a s i c anion, BF 4~, where the n e g a t i v e charge i s d e l o c a l i z e d over f i v e atoms. The e q u i l i b r i u m i s now s h i f t e d t o a h i g h e r H 2 F + i o n c o n c e n t r a t i o n because of the f o r m a t i o n of BF 4~: F" + BF 3 — > BF 4" [l-7b] The combination of [1-7] and [l-7b] g i v e s r i s e t o the o v e r a l l r e a c t i o n : 2HF + BF 3 ;==^  H 2 F + + BF 4" [1-8] The r e s u l t i n g system r e p r e s e n t s a conjugate Bronsted/Lewis s u p e r a c i d . In summary, as suggested by Olah e t a l . , 9 s u p e r a c i d s can hence be d i v i d e d i n t o two g e n e r a l c l a s s e s : (a) P r o t o n i c s u p e r a c i d s which i n c l u d e the Bronsted a c i d s w i t h h i g h e r a c i d i t i e s than 100% H 2S0 4 (Table 1-1) and conjugate Bronsted/Lewis s u p e r a c i d s . They are u s u a l l y l i q u i d s a t room temperature. (b) Lewis s u p e r a c i d s which have s t r o n g e r e l e c t r o n - p a i r donating a b i l i t y than A1C1 3. They may be s o l i d s (NbF 5 and T a F 5 ) , l i q u i d s (SbF 5) or gases (AsF 5) a t room temperature. The s o - c a l l e d " s o l i d s u p e r a c i d s " may be i n c l u d e d i n the above c l a s s i f i c a t i o n s i n c e they behave as e i t h e r Bronsted or Lewis a c i d s . 9 They may be s o l i d a c i d s i n which the a c i d i t y i s a 7 Table 1-1 P h y s i c a l P r o p e r t i e s of Some Bronsted Superacids Properties H C 1 0 4 HF a H S 0 3 F H S O 3 C I HSO3CF3 m e l t i n g p o i n t ( ° C ) -112 -89.9 -89 -81 -34 b o i l i n g p o i n t (•C) 110 (explosive) 19.5 162.7 151-152 162 d e n s i t y (g/cm 3) 1.767 (20°C) 1.002 (0°C) 1.726 (25°C) 1.753 (25°C) 1.698 (25°C) v i s c o s i t y (cp) - 0.256 (0 ° O 1.56 (25°C) 3.0 (15°C) 2.87 (25°C) d i e l e c t r i c c o n s t a n t - 84 (0°C) 120 (25°C) 60±10 (25°C) c o n d u c t i v i t y (ohm - 1" cm - 1) - «10~6 (0°C) l . l x l O - 4 (20°C) 2 - 3 X 1 0 " 4 (20°C) 2 x l 0 - 4 (20°C) -H 0 «13.0 15.1 15.1 14.1 13.8 a Data for HF are from ref.15 A l l others are from ref.9 except -HQ value, which i s from r e f . 21. p r o p e r t y of the s o l i d s due t o t h e i r chemical s t r u c t u r e s They can a l s o be supported or i n t e r c a l a t e d s u p e r a c i d s which can be obt a i n e d by d e p o s i t i o n on or i n t e r c a l a t i o n i n t o an otherwise i n e r t o r l o w - a c i d i t y support. Although s o l i d s u p e r a c i d s r e p r e s e n t a s p e c i a l group of compounds w i t h a wide p o t e n t i a l i n a p p l i e d chemistry, they a re not i n v e s t i g a t e d i n t h i s study. T h e r e f o r e , the " s o l i d s u p e r a c i d s " w i l l not be i n v o l v e d i n any f u r t h e r d i s c u s s i o n . Of t h e p r o t o n i c s u p e r a c i d s , the conjugate Bronsted/Lewis s u p e r a c i d appears t o have a g r e a t e r p o t e n t i a l i n the development of s u p e r a c i d s w i t h h i g h a c i d i t y , e.g. HF/SbF 5, HS0 3F/SbF 5 /S0 3 and HSO3F/AU(S03F)3 systems. The a c i d i t y of such a s u p e r a c i d 8 system depends on the a c i d i t y of the Bronsted a c i d as w e l l as t h a t of the Lewis a c i d . Some commonly used Bronsted and Lewis a c i d s i n t h i s system are d i s c u s s e d i n the f o l l o w i n g s e c t i o n s . 1.4 Bronsted Acids Used i n Conjugate Bronsted/Lewis Superacids As one component of the conjugate Bronsted/Lewis s u p e r a c i d system, the Bronsted a c i d used has t o be a s t r o n g a c i d (-H0>10) f o r the a c i d i t y of the conjugate system t o extend beyond the l i m i t s e t by G i l l e s p i e ' s d e f i n i t i o n . Some commonly used s t r o n g Bronsted a c i d s w i t h -H0>10 w i l l be d i s c u s s e d here. S u l f u r i c a c i d , H 2S0 4, i s a c l a s s i c a l s t r o n g i n o r g a n i c a c i d . 1 4 I t i s used as the standard t o e v a l u a t e s u p e r a c i d s . I t s o x i d i z i n g power, h i g h v i s c o s i t y (24.54 cp a t 25°C), r e l a t i v e l y low -H 0 v a l u e and other p h y s i c a l p r o p e r t i e s (m.p. 10.4°C, b.p. 290-317°C) l i m i t i t s a p p l i c a t i o n s i n s u p e r a c i d systems. Only a few Lewis a c i d s have been r e p o r t e d t o form conjugate Bronsted/Lewis s u p e r a c i d s w i t h H 2S0 4. One example i s S 0 3 , 1 6 which g i v e s d i s u l f u r i c a c i d H 2S 20 7 i n H 2S0 4: S0 3 + H 2S0 4 — > H 2S 20 7 [ l ~ 9 a ] H 2S 20 7 + H 2S0 4 =^*= H 3 S 0 4 + + HS 20 7~ [l-9b] Another conjugate Bronsted/Lewis s u p e r a c i d i n v o l v i n g H 2S0 4 i s the H 2S0 4/B(OH) 3/S0 3 system, where hydrogen t e t r a k i s ( h y d r o g e n s u l f a t o ) b o r a t e , H [ B ( H S 0 4 ) 4 ] , forms a c c o r d i n g t o : B(0H) 3 + 3S0 3 + H 2S0 4 —*> H[B(HS0 4) 4] [l-10a] H[B(HS0 4) 4] + H 2S0 4 ^ H 3 S 0 4 + + [ B ( H S 0 4 ) 4 ] " [l-10b] 9 Hydrogen f l u o r i d e , HF, i s a very important s o l v e n t i n i n o r g a n i c c h e m i s t r y . 1 5 I t a l s o possesses the a b i l i t y t o d i s s o l v e many c l a s s e s of o r g a n i c compounds. I t s r e a c t i v i t y towards g l a s s and q u a r t z r e s t r i c t e d the i n v e s t i g a t i o n of t h i s s o l v e n t u n t i l t he development of s t a b l e f l u o r i n e - c o n t a i n i n g polymers t h a t are i n e r t t o HF. S i n c e then the chemistry i n HF has been s t u d i e d e x t e n s i v e l y and many conjugate Bronsted/Lewis s u p e r a c i d s i n v o l v i n g HF have been d e v e l o p e d . 1 7 - 2 4 However, the complete removal of t r a c e s of H 20 or o t h e r i m p u r i t i e s , which behave as bases i n HF, i s v e r y d i f f i c u l t . Hence the h i g h e s t a c i d i t y of 100% HF i s e s t i m a t e d t o have a -H 0 v a l u e of 15.1±0.1 from the S-shaped curve of -H 0 v a l u e s f o r the KF/HF/SbF 5 s y s t e m , 2 1 w h i l e the measured -H 0 has r a r e l y exceeded a v a l u e of 12. F l u o r o s u l f u r i c a c i d , HS0 3F, i s s y n t h e s i z e d from S0 3 and HF. HS0 3F may a l s o be regarded as a m o n o f l u o r o - s u b s t i t u t e d d e r i v a t i v e of H 2S0 4. R e l a t i v e t o HF and H 2S0 4, i t has the f o l l o w i n g a d v a n t a g e s : 8 , 2 5 (a) With a d i r e c t l y measurable -H 0 v a l u e of 15.1, f l u o r o s u l f u r i c a c i d i s one of the s t r o n g e s t simple monobasic a c i d s known. The a c i d can be e a s i l y p u r i f i e d by d i s t i l l a t i o n i n N 2 a t atmospheric p r e s s u r e . (b) I t s l i q u i d range, from -89°C t o 163°C, p r o v i d e s a r e a c t i o n medium over a very wide and convenient temperature range and a l l o w s the study of chemical r e a c t i o n s by NMR s p e c t r o s c o p y a t low temperatures. (c) Although f l u o r o s u l f u r i c a c i d a t t a c k s rubber, cork and 10 wood, i t s i n e r t n e s s towards g l a s s i n anhydrous c o n d i t i o n s a l l o w s h a n d l i n g i n c o n v e n t i o n a l g l a s s or quartz apparatus. The s e l f - d i s s o c i a t i o n i n t o S0 3 and HF may p r e s e n t problems at e l e v a t e d temperatures but i s n e g l i g i b l e a t room temperature. (d) Compared t o H 2S0 4, HS0 3F has a much lower v i s c o s i t y ( c l o s e t o the v i s c o s i t y of water), so t h a t m a n i p u l a t i o n s such as f i l t r a t i o n s o r d e c a n t a t i o n s are e a s i e r . (e) The o x i d i z i n g agent b i s ( f l u o r o s u l f u r y l ) p e r o x i d e , S 20gF 2, i s m i s c i b l e w i t h HS0 3F. The weak 0-0 bond, l i n k i n g two S0 3F-r a d i c a l s , i s e a s i l y c l e a v e d and S 20gF 2 a c t s as a t w o - e l e c t r o n o x i d a n t a c c o r d i n g t o : S 2 0 6 F 2 ===== 2S0 3F« [ 1 - l l a ] 2S0 3F. + 2e — > 2S0 3F" [ 1 - l l b ] g i v i n g r i s e t o a r i c h s y n t h e t i c chemistry. For a l l these reasons, HS0 3F, b e s i d e s HF, has been the most w i d e l y used Bronsted a c i d i n s u p e r a c i d s y s t e m s . 2 3 - 2 4 , 2 6 - 3 4 Hence, HS0 3F w i l l be used i n t h i s work as w e l l . 1.5 Lewis Acids Used i n Conjugate Bronsted/Lewis Superacids S i n c e the counter anions of s t r o n g Bronsted a c i d s are g e n e r a l l y h i g h l y e l e c t r o n e g a t i v e , of low n u c l e o p h i l i c i t y , weakly b a s i c and p o o r l y c o o r d i n a t i n g , the conjugated Lewis a c i d s should have a c e n t r a l atom i n a h i g h o x i d a t i o n s t a t e w i t h p o t e n t i a l l y u n s a t u r a t e d c o o r d i n a t i o n , surrounded by h i g h l y e l e c t r o n e g a t i v e l i g a n d s (F~, C l ~ or S0 3F~ e t c . ) . The Lewis a c i d s may have 11 o l i g o m e r i c s t r u c t u r e s . However, i d e a l l y these oligomers should break up i n s o l u t i o n . The c e n t r a l atom should not have r e a d i l y a v a i l a b l e lower o x i d a t i o n s t a t e s t o p r e c l u d e redox r e a c t i o n s as s i d e r e a c t i o n s . The most i n t e r e s t i n g and important groups of such Lewis a c i d s are b i n a r y f l u o r i d e s and b i n a r y f l u o r o s u l f a t e s of elements i n Groups 4, 5, 13, 14, and 15. The p r e v i o u s s t u d i e s on t h e a p p l i c a t i o n of these Lewis a c i d s i n the conjugate Bronsted/Lewis s u p e r a c i d s are b r i e f l y reviewed below. 1.5.1 Binary Fluorides as Lewis Acids i n Conjugate Bronsted/Lewis Superacids Most b i n a r y t r i - and t e t r a - f l u o r i d e s are e x t e n s i v e l y F - b r i d g e d and show low s o l u b i l i t i e s i n anhydrous H F 1 5 and H S 0 3F. 2 5 , 2 6 Of the s o l u b l e compounds, B r F 3 , 2 6 A s F 3 and S b F 3 4 0 are known t o be weak bases i n HS03F. AuF 3 i s b e l i e v e d t o be a weak a c i d i n HS0 3F. 2 6 BF 3 i s a weak a c i d i n anhydrous HF, 2 0 but no a c i d i c b ehavior has been observed i n HSO3F. 2 6 T i F 4 shows l i m i t e d s o l u b i l i t y i n HS03F and i t s i o n i z a t i o n b ehavior cannot be p r e d i c t e d w i t h c e r t a i n t y a c c o r d i n g t o conductometric s t u d i e s . 2 8 P e n t a f l u o r i d e s of Group 5 and Group 15 elements show e x c e p t i o n a l Lewis a c i d i t y . Table 1-2 l i s t s some of these p e n t a f l u o r i d e s t o g e t h e r w i t h t h e i r p h y s i c a l p r o p e r t i e s . T h e i r use as Lewis a c i d s i n conjugate Bronsted/Lewis s u p e r a c i d s has been e x t e n s i v e l y i n v e s t i g a t e d . 1 8 - 2 4 , 2 8 - 3 4 Table 1-3 shows some of these s u p e r a c i d systems t o g e t h e r w i t h t h e i r Hammett A c i d i t y F u n c t i o n v a l u e s . Although the -H 0 v a l u e s o b t a i n e d from d i f f e r e n t 12 Table 1-2 P h y s i c a l P r o p e r t i e s of Some B i n a r y P e n t a f l u o r i d e s Property PF 5 a AsF 5 a SbF 5 a NbF5 TaF 5 m e l t i n g p o i n t (°C) -93.8 -79.8 8.3 72-73 97 b o i l i n g p o i n t (°C) -84.6 -53.2 141 236 229 d e n s i t y (g/cm 3) 2.33 (b.p.) 3 .145 (15°C) 2.7 (15°C) 3.9 (15°C) s t r u c t u r e monomer t r i g o n a l bipyramid (gas) monomer t r i g o n a l bipyramid (gas) oligomer c i s - F -b r i d g e d ( l i q u i d ) tetramer c i s - F -b r i d g e d ( s o l i d ) tetramer c i s - F -b r i d g e d ( s o l i d ) indicates data from ref.42a. Other data are from ref.42 Table 1-3 Hammett A c i d i t y F u n c t i o n s of Some Conjugate Bronsted/Lewis Superacids I n v o l v i n g HF and HS0 3F Bronsted/Lewis Levis Acid Concentration -H0 Method Reference HF/NbF 5 0.36 M (sat.) -13.5 a 18 HF/TaF 5 2 M (sat.) 13.5 b 32 HF/SbF 5 2 M 15. 3 b 32 HF/PF 5 0.4 mole% 11. 98 b 21 HF/NbF 5 0.4 mole% 16.98 b 21 HF/TaF 5 0.4 mole% 18.60 b 21 HF/AsF 5 0.4 mole% 19. 31 b 21 HF/SbF 5 0.4 mole% 20. 64 b 21 HS0 3F/TaF 5 2 M 16.7 b 32 HS0 3F/SbF 5 2 M >18 b 32 HS0 3F/SbF 5 90 mole% «26.5 c 34 a estimated from conductivity data. b ionization ratio of the indicator measured by UV-visible spectroscopy, c ionization ratio of the indicator measured by NMR spectroscopy. 13 methods and authors i n d i c a t e i n c o n s i s t e n c y , i t can be concluded t h a t t he r e l a t i v e Lewis a c i d s t r e n g t h of the p e n t a f l u o r i d e s i n HF or HSO3F i n c r e a s e s i n the f o l l o w i n g o r d e r : PF 5 < NbF 5 < TaF 5 < A s F 5 < SbF 5 i n H F 2 0 ' 2 1 PF 5 ~ NbF 5 < TaF 5 < A s F 5 < B i F 5 < SbF 5 i n HSO3F 9 ' 2 8 ' 3 2 Antimony p e n t a f l u o r i d e i s a c o l o r l e s s , v e r y v i s c o u s l i q u i d a t room temperature. I t has a pol y m e r i c c h a i n s t r u c t u r e w i t h cis-fluorine-bridges i n the l i q u i d phase, but i t i s r e p o r t e d t o c r y s t a l i z e as a tetramer s i m i l a r t o [ N b F 5 ] 4 . 4 2 b The antimony(V) i s surrounded by s i x f l u o r i n e atoms i n a d i s t o r t e d o c t a h e d r a l arrangement. I t s s o l u t i o n s i n H F 1 8 - 2 1 ' 2 3 ' 2 4 and HS03F 23,24,27,29-34 are p r o b a b l y the most thoroughly i n v e s t i g a t e d conjugate Bronsted/Lewis s u p e r a c i d s . The l a t t e r i s a l s o termed "Magic A c i d " . 9 The -H 0 v a l u e of the HS0 3F/SbF 5 system has been measured as a f u n c t i o n of SbF 5 content. The i n c r e a s e i n -H 0 v a l u e s i s ver y sharp a t low SbF 5 c o n c e n t r a t i o n c o r r e s p o n d i n g t o the i n i t i a l i o n i z a t i o n : 2HS0 3F + SbF 5 ===== H 2 S 0 3F + + S b F 5 ( S 0 3 F ) ~ [1-12] As the SbF 5 c o n c e n t r a t i o n i n c r e a s e s , the -H 0 v a l u e o f the system i n c r e a s e s up t o 26.5, which i s the h i g h e s t v - H 0 v a l u e p r a c t i c a l l y measured t o date, f o r a s o l u t i o n of 90 mole% S b F 5 . 3 4 The a c t u a l composition of t h e system i s very complex and depends on the SbF 5 c o n c e n t r a t i o n . A v a r i e t y of o l i g o m e r i c antimony(V) f l u o r i d e f l u o r o s u l f a t e anions b r i d g e d v i a S0 3F~ or F~ are r e p o r t e d t o form by two p r o c e s s e s : o l i g o m e r i z a t i o n and l i g a n d r e d i s t r i b u t i o n 14 r e a c t i o n s . T y p i c a l r e a c t i o n s observed by 1 9 F NMR a r e : .29,30 SbF 5(S0 3F) + ( S b F 5 ) m ^ (SbF 5) ( m + 1 ) S 0 3 F -2 S b F 5 ( S 0 3 F ) " S b F 4 ( S 0 3 F ) 2 " + SbF 6" [1-13] [1-14] SbF 5 + SbF 6" ?==i S b 2 F 1 1 [1-15] SbF 5 can a l s o a c t as an o x i d i z i n g reagent and undergo r e d u c t i o n t o SbF 3. T h i s p r e s e n t s a l i m i t a t i o n t o i t s use i n the conjugate Brosnted/Lewis s u p e r a c i d s . A r s e n i c p e n t a f l u o r i d e i s a c o l o r l e s s gas a t room temperature, condensing t o a y e l l o w l i q u i d a t -53°C. Vapor d e n s i t y measurements i n d i c a t e a low degree of a s s o c i a t i o n i n the condensed phase. As a Lewis a c i d , A s F 5 i s weaker than SbF 5. A s F 5 i s used i n the Bronsted/Lewis s u p e r a c i d s i n v o l v i n g e i t h e r H F 1 9 ' 2 1 ' 2 2 or H S 0 3 F 2 8 ' 3 1 . However, i t s h i g h t o x i c i t y and l i m i t e d s o l u b i l i t y i n HS0 3F as w e l l as i t s o x i d i z i n g and f l u o r i n a t i n g a b i l i t y s e v e r e l y r e s t r i c t more d e t a i l e d i n v e s t i g a t i o n s and p o s s i b l e a p p l i c a t i o n s of the HF/AsF 5 and HS0 3F/AsF 5 systems. Both niobium and tantalum p e n t a f l u o r i d e s have s i m i l a r p h y s i c a l and chemical p r o p e r t i e s and a common t e t r a m e r i c s t r u c t u r e i n the s o l i d s t a t e . They are s t r o n g Lewis a c i d s , capable of complexing w i t h a v a r i e t y of donors such as e t h e r s , s u l f i d e s and amines. 4 2 Both are a l s o used i n s u p e r a c i d systems i n c o n j u n c t i o n w i t h s t r o n g p r o t o n i c a c i d s such as anhydrous HF, HS0 3F and HS0 3CF 3. 9 However, the s o l u b i l i t i e s of T aF 5 and NbF 5 i n s t r o n g Bronsted a c i d s are l i m i t e d compared t o t h a t of SbF 5, r e s t r i c t i n g t h e i r use t o some extent. TaF 5 appears t o be 15 s l i g h t l y more s o l u b l e i n HS0 3F than i n HF although no exact data have been r e p o r t e d . 9 The low s o l u b i l i t i e s of both NbF 5 and TaF 5 are p r o b a b l y due t o t h e i r s t a b l e t e t r a m e r i c s t r u c t u r e i n the s o l i d s t a t e . Furthermore, i n a conductometric study, NbF 5 was found t o be n e a r l y a n o n e l e c t r o l y t e i n HS0 3F. 2 8 N e v e r t h e l e s s , t h e i r h i g h redox p o t e n t i a l s and l i m i t e d v o l a t i l i t i e s g i v e us a wider c h o i c e i n the a p p l i c a t i o n of s u p e r a c i d systems. The use of other t r a n s i t i o n metal p e n t a f l u o r i d e s as Lewis a c i d s i s l i m i t e d s i n c e they are u s u a l l y s t r o n g f l u o r i n a t i n g r e agents and a t t a c k c o n v e n t i o n a l g l a s s a p p a r a t u s . 3 5 ' 3 6 , 3 8 In a d d i t i o n , some of them undergo d i s p r o p o r t i o n a t i o n r e a c t i o n s . Hexa- or h e p t a - f l u o r i d e s can a c t as Lewis a c i d s o n l y i f they expand t h e i r c o o r d i n a t i o n spheres from s i x t o seven or e i g h t . However, t h e i r h i g h l y o x i d i z i n g and f l u o r i n a t i n g a b i l i t i e s p r e c l u d e t h e i r use as Lewis a c i d s i n s u p e r a c i d systems. 1.5.2 Binary Fluorosulfates as Lewis Acids i n Conjugate Bronsted/Lewis Superacids The f l u o r o s u l f a t e r a d i c a l S0 3F" and the anion S0 3F~ have been termed a "pseudohalogen" and "pseudohalide" r e p e c t i v e l y because t h e i r c hemistry resembles t h a t of the halogens. With r e g a r d t o e l e c t r o n e g a t i v i t y and c o o r d i n a t i n g a b i l i t y , the anion S0 3F~ resembles the f l u o r i d e i o n more than the c h l o r i d e i o n . The e l e c t r o n e g a t i v i t y of the -S0 3F group has been estimated t o be 3.83 on the P a u l i n g s c a l e (F, 3.98; CI, 3.16) from 1 1 9 S n Mossbauer s t u d i e s of K 2[SnX 6] wi t h X=F, CI, S 0 3 F . 4 3 The T a f t 16 i n d u c t i v e c o n s t a n t a*, a measurement of the e l e c t r o n i n d u c t i v e e f f e c t , i s found t o be 3.68 f o r the S0 3F group from Mossbauer s t u d i e s of X 2SnF 2 and X 2 S n ( S 0 3 F ) 2 w i t h X = CH 3, F, CI, Br, which have v a l u e s of 0, 3.08, 2.94, 2.80 r e s p e c t i v e l y . 4 4 T h i s r e s u l t i n d i c a t e s t h a t the S0 3F group has a g r e a t e r a b i l i t y t o withdraw e l e c t r o n i c charge v i a both a- and rr- bonds than f l u o r i n e does. The charge can be d e l o c a l i z e d over the e n t i r e S0 3F group. Rather s i m i l a r l i g a n d f i e l d s p l i t t i n g parameters (Dq) of the f l u o r o s u l f a t e a nion and the f l u o r i d e i o n i n d i c a t e t h a t they are both weak f i e l d l i g a n d s . 4 5 The f l u o r o s u l f a t e anion, l i k e F~, i s capable of p o l y d e n t a t e c o o r d i n a t i o n . The c o o r d i n a t i o n modes of S0 3F~ may be s t u d i e d c o n v e n i e n t l y by v i b r a t i o n a l s p e c t r o s c o p y because of i t s m o l e c u l a r nature (see s e c t i o n 1.6.2). On account of the resemblance of f l u o r o s u l f a t e s t o f l u o r i d e s , b i n a r y f l u o r o s u l f a t e s s hould form another group of good Lewis a c i d s , p o t e n t i a l l y a p p l i c a b l e i n the Bronsted/Lewis s u p e r a c i d systems. T h e i r c h e m i s t r y i n HS0 3F i s expected t o be l e s s c o m p l i c a t e d than t h a t of b i n a r y f l u o r i d e s because l i g a n d exchange between s o l u t e and s o l v e n t w i l l not cause a change i n composition. The a p p l i c a t i o n of the b i n a r y f l u o r o s u l f a t e s as Lewis a c i d s i n the conjugate Bronsted/Lewis s u p e r a c i d s i n v o l v i n g HS0 3F i s l i m i t e d s i n c e most of the known b i n a r y f l u o r o s u l f a t e s are not s u f f i c i e n t l y s o l u b l e i n HS0 3F, and some b i n a r y f l u o r o s u l f a t e s w i t h the metals i n h i g h o x i d a t i o n s t a t e s are t h e r m a l l y u n s t a b l e . 5 2 - 8 2 However, t h e r e are a few e x c e p t i o n s . Gold t r i s ( f l u o r o s u l f a t e ) , A u ( S 0 3 F ) 3 , which i s d i m e r i c i n the s o l i d 17 s t a t e , 7 U d i s s o l v e s s l o w l y i n HS0 3F a t room temperature. Conductometric s t u d i e s show t h a t A u ( S 0 3F) 3 i s a moderately s t r o n g monobasic a c i d i n HS0 3F. 7 9 The l i n e a r dependence o f c o n d u c t i v i t y on c o n c e n t r a t i o n and the Raman s p e c t r a of the s o l u t i o n s i n d i c a t e the absence o f e x t e n s i v e s o l u t e a s s o c i a t i o n w i t h i n c r e a s i n g c o n c e n t r a t i o n . Platinum t e t r a k i s ( f l u o r o s u l f a t e ) , P t ( S 0 3 F ) 4 , i s found t o be a ver y good f l u o r o s u l f a t e anion a c c e p t o r , forming a d i b a s i c a c i d , H 2 [ P t ( S 0 3 F ) 6 ] , i n a s o l u t i o n o f HS0 3F. 7 4 In s p i t e of the h i g h a c i d i t y and thermal s t a b i l i t y o f these s u p e r a c i d systems, the hi g h c o s t of g o l d and platinum l i m i t s t h e i r a p p l i c a t i o n i n s y n t h e t i c chemistry. I r ( S 0 3 F ) 4 i s s o l u b l e i n HS0 3F and appears t o behave as an a c i d but no d e t a i l e d s o l u t i o n s t u d i e s a re r e p o r t e d , probably due t o the extreme d i f f i c u l t y encountered i n the p r e p a r a t i o n of the compound. 7 3 R e c e n t l y , two no v e l s u p e r a c i d systems, H S 0 3F/Ta(S0 3F) 5 (soiv) and HS0 3F/Nb (S0 3F) 5 (solv), are obt a i n e d by the o x i d a t i o n o f the cor r e s p o n d i n g metals w i t h S 2 0 6 F 2 i n HS0 3F but attempts t o i s o l a t e the pure compounds, Nb ( S 0 3F) 5 and T a ( S 0 3 F ) 5 , are u n s u c c e s s f u l . 6 6 Compared t o H S 0 3F / S b F 5 ("Magic Acid") and H S 0 3F/SbF 5 / 3 S 0 3 (the s t r o n g e s t s u p e r a c i d known so f a r ) , the order o f Lewis a c i d i t y of these b i n a r y f l u o r o s u l f a t e s i n HS0 3F i s estimated as f o l l o w s : SbF 5 * 3 S 0 3 > P t ( S 0 3 F ) 4 > A u ( S 0 3 F ) 3 > T a ( S 0 3 F ) 5 (solv) > SbF 5 > Nb (S0 3F) 5 (solv) The e s t i m a t i o n i s based on the comparison of the s l o p e s o f c o n d u c t i v i t y v s . Lewis a c i d c o n c e n t r a t i o n p l o t s and the t i t r a t i o n c urves w i t h KS0 3F as the standard base. 18 1.6 Characterization of Fluoro- and Fluorosulfato-Derivatives by V i b r a t i o n a l Spectroscopy S t r u c t u r a l i n f o r m a t i o n on i n o r g a n i c f l u o r i n e - c o n t a i n i n g compounds can be a c q u i r e d by v a r i o u s p h y s i c a l t e c h n i q u e s . In the s o l i d s t a t e , X-ray powder d i f f r a c t i o n or, i f p o s s i b l e , s i n g l e c r y s t a l X-ray d i f f r a c t i o n are the most d e f i n i t i v e t e c h n i q u e s f o r a c c u r a t e s t r u c t u r a l d e t e r m i n a t i o n . However, the h i g h r e a c t i v i t y of f l u o r i n e - c o n t a i n i n g compounds has f r e q u e n t l y prevented the p r e p a r a t i o n of s i n g l e c r y s t a l s and has l i m i t e d c r y s t a l l o g r a p h i c s t u d i e s . Mossbauer sp e c t r o s c o p y i s a p p l i c a b l e t o a l i m i t e d number of compounds c o n t a i n i n g n u c l e i such as 5 7 F e and 1 1 9 S n . NMR s p e c t r o s c o p y has been used e x t e n s i v e l y f o r l i q u i d s and s o l u t i o n s , but i t i s now p o s s i b l e t o use t h i s technique t o study s o l i d s . V i b r a t i o n a l s p e c t r o s c o p y ( i n f r a r e d and Raman) has been the most w i d e l y used technique t o g a i n i n i t i a l s t r u c t u r a l i n f o r m a t i o n s i n c e the spectrometers are r e a d i l y a v a i l a b l e and the sampling procedures are uncomplicated. I t can be c o n v e n i e n t l y used f o r the study of s o l i d s , l i q u i d s , s o l u t i o n s or gases. Raman s p e c t r a are o f t e n s i m p l e r than i n f r a r e d s p e c t r a s i n c e u s u a l l y o n l y the fundamental v i b r a t i o n s are observed. Overtones or combination modes are u s u a l l y too weak t o be observed u n l e s s Fermi resonance o c c u r s . In a d d i t i o n , u n l i k e i n f r a r e d spectroscopy, Raman spe c t r o s c o p y does not r e q u i r e the use of s p e c i f i c window m a t e r i a l s , which may r e a c t w i t h the compounds s t u d i e d . In some i n s t a n c e s , however, i n t e n s c o l o r s or l i m i t e d thermal s t a b i l i t y of compounds or the occurrence of f l u o r e s c e n c e prevent the 19 r e c o r d i n g o f Raman s p e c t r a , even though the advent o f l a s e r s as e x c i t a t i o n sources has widened the range of compounds which can be s t u d i e d by t h i s technique. 1.6.1 V i b r a t i o n a l Spectra of Binary Fluorides The v i b r a t i o n a l s p e c t r o s c o p y of b i n a r y f l u o r i d e s has been e x t e n s i v e l y r e v i e w e d . 3 8 In common w i t h o r g a n i c f u n c t i o n a l groups, the r e g i o n s o f the spectrum of an i n o r g a n i c b i n a r y f l u o r i d e may be as s i g n e d t o s p e c i f i c v i b r a t i o n s o f the f l u o r i n e atoms, termed "group frequency". The band p o s i t i o n s may be b r o a d l y d e f i n e d f o r a n e u t r a l b i n a r y f l u o r i d e as the " t e r m i n a l " s t r e t c h i n g r e g i o n (600-800 cm - 1) , the " b r i d g i n g " s t r e t c h i n g r e g i o n (400-600 cm - 1) and the deformation mode r e g i o n (100-400 cm - 1) . The a n a l y s i s of the v i b r a t i o n a l bands on the b a s i s of group frequency i s t h e r e f o r e r e l a t i v e l y simple f o r b i n a r y f l u o r i d e s , p r o v i d e d the above mentioned band p o s i t i o n s are not obscured by oth e r i n t e r n a l bands or l a t t i c e modes. S l i g h t c o m p l i c a t i o n s may a r i s e w i t h l i g h t e r c e n t r a l atoms and i o n i c s p e c i e s . As a g e n e r a l r u l e , bands of a n i o n i c f l u o r i d e d e r i v a t i v e s a re observed a t lower f r e q u e n c i e s w h i l e those of c a t i o n i c f l u o r i d e d e r i v a t i v e s s h i f t t o h i g h e r f r e q u e n c i e s , r e l a t i v e t o comparable n e u t r a l f l u o r i d e s . When o n l y Raman or i n f r a r e d data are a v a i l a b l e , c a u t i o n must be e x e r c i s e d . An i n t e r p r e t a t i o n based s o l e l y on Raman or i n f r a r e d s p e c t r a can l e a d t o erroneous c o n c l u s i o n s , i n p a r t i c u l a r where molecules w i t h h i g h symmetry or w i t h i n t e r - m o l e c u l a r 20 a s s o c i a t i o n are encountered. Such e r r o r s have o c c u r r e d f o r weak f l u o r i n e - b r i d g e d polymers, e.g. SbF 5, VF 5, molten NbF 5 and TaF 5, where bands i n the b r i d g i n g r e g i o n appear t o be m i s s i n g from t h e i r Raman s p e c t r a . T h i s would suggest an assignment based on t h e presence of a monomer wit h D 3 h symmetry. However, a c o n s i d e r a t i o n of the p h y s i c a l p r o p e r t i e s , i n f r a r e d s p e c t r a and 1 9 F NMR s p e c t r a of the above p e n t a f l u o r i d e s would have made the s u g g e s t i o n of monomeric s p e c i e s untenable. 1.6.2 V i b r a t i o n a l Spectra of Fluorosulfato- Derivatives The f l u o r o s u l f a t e group may be assumed t o e x h i b i t a t l e a s t f i v e d i f f e r e n t bonding or c o o r d i n a t i o n modes: ( i ) i o n i c ; ( i i ) c o v a l e n t monodentate; ( i i i ) c o v a l e n t b i d e n t a t e ; ( i v ) c o v a l e n t t r i d e n t a t e ; and (v) c o v a l e n t t e t r a d e n t a t e . Bonding t o o t h e r atoms occ u r s through the more b a s i c oxygens except i n the case of the t e t r a d e n t a t e mode, where f l u o r i n e w i l l be i n v o l v e d as w e l l . Each of t h e s e f i v e bonding or c o o r d i n a t i o n modes has i t s own d i s t i n c t f e a t u r e s i n the v i b r a t i o n a l s p e c t r a , mainly i n the s t r e t c h i n g r e g i o n (700-1500 cm - 1) . D i f f e r e n c e s a r i s e from two g e n e r a l reasons: l o c a l symmetry and bond s t r e n g t h s . F l u o r o s u l f a t e groups w i t h i o n i c , t r i d e n t a t e , and t e t r a d e n t a t e c o o r d i n a t i o n modes have C 3 v l o c a l symmetry. For the remaining bonding or c o o r d i n a t i o n modes, C s l o c a l symmetry i s expected. T h i s d i f f e r e n c e i s r e f l e c t e d i n the number of fundamentals: s i x f o r C 3 V and n i n e f o r C s or lower p o i n t groups, w i t h a l l bands i n f r a r e d and Raman a c t i v e . W i t h i n the two "symmetry groups", as 21 i l l u s t r a t e d i n F i g u r e 1-1, d i f f e r e n c e s i n band p o s i t i o n s due t o v a r y i n g bond s t r e n g t h s a l l o w a reasonable d i f f e r e n t i a t i o n by " d i a g n o s t i c bands", which are shown as hatched ar e a s . T h i s f i g u r e i s based on the p r e v i o u s l y r e p o r t e d v i b r a t i o n a l s p e c t r a of f l u o r o s u l f a t o - d e r i v a t i v e s . P u r e l y i o n i c , c o v a l e n t t r i d e n t a t e and t e t r a d e n t a t e bonding and c o o r d i n a t i o n modes of f l u o r o s u l f a t e groups have common C 3 v symmetry, which g i v e s r i s e t o s i x fundamentals (3A2+3E). The S-0 s t r e t c h i n g bands have a s i m i l a r s p e c t r a l p a t t e r n f o r a l l these modes. The major d i f f e r e n c e between the i o n i c bonding mode and t r i d e n t a t e c o o r d i n a t i o n mode i s the p o s i t i o n of the v(S-F) band: below 800 cm - 1 f o r the former and above 800 cm - 1 f o r the l a t t e r . A t r i d e n t a t e f l u o r o s u l f a t e group i s u s u a l l y found among the p o l y m e r i c metal b i s ( f l u o r o s u l f a t e ) s , l i k e Z n ( S 0 3 F ) 2 / 7 5 and a p u r e l y i o n i c f l u o r o s u l f a t e group may be found i n a l k a l i metal f l u o r o s u l f a t e s , such as KS0 3F whose s t r u c t u r e has been determined by a s i n g l e c r y s t a l X-ray d i f f r a c t i o n a n a l y s i s . 8 6 A t e t r a d e n t a t e c o o r d i n a t i o n mode i s p o s t u l a t e d o n l y f o r T i 3 C l 1 0 (S0 3F) 2 . 9 0 Because the S-F bond i s g r e a t l y weakened as a r e s u l t of the c o o r d i n a t i o n of the f l u o r i n e atom t o t i t a n i u m , the v(S-F) appears a t a much lower frequency (660 cm - 1) than any o t h e r bonding or c o o r d i n a t i o n modes. P e r t u r b a t i o n s are o f t e n observed i n the i o n i c systems which are caused by ( i ) n o n - s p h e r i c a l c a t i o n s such as NO+ i n NOS0 3F, 8 7 ( i i ) s t r o n g l y p o l a r i z i n g c a t i o n s such as L i + i n L i S 0 3 F , 8 8 or Symmetry Bonding or Coordination Mode Frequency Range (cm _ 1 ) Symmetry Stretching Band Deformation Band >• r-O C_J> COVALENT TRIDENTATE u„ (S-O) ^ ( S - O ) „ ( S - F ) I I I I r / / / / j «a. (S03) ( 1 Proc* • 0 «.ym (SOj) PURELY IONIC "o. (s-o) ^ ( s - o ) „(S-F) «<» (so 3) « ^ ( s o 3 ) P r e c k u u • tn IONIC PERTURBED »«. (so 2) X , , , , A . . f(S-O) f(S-F) VZZZA ^ ( s o j , , CZZ3 cw(S0 2) r—, ^ (SOzF) , _ p r ( s - r t 9 " ' T(S0 2 F) Troek(SOa) COVALENT MONODENTATE "o, (S02) ff*m(S02) [y/^y///A I ' ^ f(S-F) 'rock (S02; • a • <WS0 2) 7 „ , ( S - F ) CD T(S0 2F) COVALENT BIDENTATE (SOj) 1 1 T7777\ ^(SOz) «5b.nd(S02) r.ogCS-F) T(S0 2F) • • • CZD C D r^ockCsOj) r t w W ( s o 2 F ) ~i 1 1 1— 1 r 1400 1200 1000 800 600 400 Figure 1-1 Frequency Range of V i b r a t i o n a l Fundamentals f o r the F l u o r o s u l f a t e Group 23 ( i i i ) s i t e symmetry e f f e c t s where the S0 3F~ i o n i s i n a c r y s t a l l o g r a p h i c s i t e o f lower symmetry than C 3 v . A l l these cause s p l i t t i n g of each doubly degenerate E mode i n t o a p a i r of A' and A" modes, g i v i n g r i s e t o a t o t a l of nine bands. T h i s bonding mode can be d i s t i n g u i s h e d from o t h e r modes by i t s " d i a g n o s t i c bands" i n the s t r e t c h i n g r e g i o n , i . e . a c l o s e l y spaced p a i r of v a s ( S 0 2 ) and v s y m ( S 0 2 ) bands w i t h i n the 1230-1350 cm - 1 range and a v(S-F) band under 800 cm - 1. For a c o v a l e n t l y bonded f l u o r o s u l f a t e group as i n C1S0 3F, 9 1 or a monodentate c o o r d i n a t i n g f l u o r o s u l f a t e group as i n [ A u ( S 0 3 F ) 4 ] ~ , 7 9 i t s C s l o c a l symmetry suggests nine fundamentals (eA'+SA"). Covalent bonding or c o o r d i n a t i o n of the f l u o r o s u l f a t e group through an oxygen atom t o another atom, X, causes the v(S-O) of the S-O-X moiety t o s h i f t t o lower f r e q u e n c i e s , e.g. from 1084 cm"1 i n i o n i c KS0 3F t o about 1000 cm - 1 i n C s 2 [ M ( S 0 3 F ) 6 ] (M=Pd,Pt,Sn or G e ) , 9 2 even down t o 856 cm"1 i n the case of a s t r o n g l y c o v a l e n t l y bonded f l u o r o s u l f a t e group as i n C1S0 3F, 9 1 whereas the average v a l u e of v a s and v s y m f o r the -S0 2- moiety of the unbonded oxygens s h i f t s t o h i g h e r v a l u e s . Furthermore, the e l e c t r o n withdrawing e f f e c t of the atom X bonded t o oxygen causes the S-F s t r e t c h i n g frequency t o i n c r e a s e t o 810-860 cm"1 f o r monodentate c o o r d i n a t i n g f l u o r o s u l f a t e groups from under 800 cm"1 f o r the i o n i c mode. I f the f l u o r o s u l f a t e group c o o r d i n a t e s i n a s y m m e t r i c a l l y b i d e n t a t e mode as i n ( C H 3 ) 2 S n ( S 0 3 F ) 2 a c c o r d i n g t o a s i n g l e c r y s t a l X-ray d i f f r a c t i o n s t u d y , 9 3 the number of fundamentals 24 remains n i n e as expected f o r i t s C s p o i n t group. T h i s conformation can be d i s t i n g u i s h e d from the monodentate bonding mode by the d i f f e r e n c e i n band p o s i t i o n s , p a r t i c u l a r l y i n the S0 3 s t r e t c h i n g r e g i o n . As expected, v(S-O) of the unbonded oxygen occ u r s a t h i g h e r frequency («1400 cm - 1) . The v a s and v s y m f o r the -S0 2- moiety of the c o o r d i n a t i n g oxygens appear a t c l o s e p r o x i m i t y i n the lower frequency range (1150-1200 cm - 1) . In a number of compounds such as A u ( S 0 3 F ) 3 , 7 9 P t ( S 0 3 F ) 4 , 7 4 or S n ( S 0 3 F ) 4 , 5 7 a both monodentate and b i d e n t a t e modes are suggested by t h e i r complex s p e c t r a . Of these compounds, o n l y the molecular s t r u c t u r e of A u ( S 0 3 F ) 3 has r e c e n t l y been determined by a s i n g l e c r y s t a l X-ray d i f f r a c t i o n s t u d y . 7 8 A complexity i s observed i n t e r n a r y f l u o r o s u l f a t e s , as i n P d ( I I ) [ S n ( I V ) ( S 0 3 F ) 6 ] , where a n i s o b i d e n t a t e groups r e f l e c t the d i f f e r e n c e s i n a c c e p t o r a b i l i t i e s o f a d i v a l e n t v ersus a t e t r a v a l e n t metal c e n t e r . The above d i s c u s s i o n has focused o n l y on s t r e t c h i n g modes because they are spread over a wider s p e c t r a l range, and hence are more d i a g n o s t i c than deformation modes. However, the f o l l o w i n g f e a t u r e i s worth n o t i n g : o n l y f o r b i d e n t a t e c o o r d i n a t i o n , the bending mode may occur w630 cm - 1 as seen i n F i g . 1-1. T h i s a n a l y s i s i s based on the l o c a l symmetry of the f l u o r o s u l f a t e group. L o c a l symmetry i s d e f i n e d as a s i t u a t i o n where a t t e n t i o n i s focused s o l e l y on the f l u o r o s u l f a t e group and e f f e c t s of o t h e r groups a t t a c h e d t o the c e n t r a l atom, which may reduce the o v e r a l l symmetry, are i g n o r e d . C a u t i o n i s r e q u i r e d 25 when s p e c t r a are taken on s o l i d samples. A number of f a c t o r s , termed s o l i d s t a t e e f f e c t s , may cause band s p l i t t i n g . Among these f a c t o r s , s i t e symmetry and f a c t o r group s p l i t t i n g are the most prominent. Another c o m p l i c a t i o n a r i s e s i f the u n i t c e l l c o n t a i n s more than one type of f l u o r o s u l f a t e i o n w i t h s l i g h t l y d i f f e r e n t l o c a l symmetries due t o the o r i e n t a t i o n s of c a t i o n s and anions. In t h i s s i t u a t i o n , a l l or most of the fundamentals w i l l be d u p l i c a t e d , r e g a r d l e s s of symmetry, as i n S n ( S 0 3 F ) 2 . 7 5 In the a n a l y s i s of the f l u o r o s u l f a t e group f r e q u e n c i e s i t has been assumed t h a t p o l y d e n t a t e f l u o r o s u l f a t e groups are b r i d g i n g r a t h e r than c h e l a t i n g . Indeed a l l m o l e c u l a r s t r u c t u r e s r e p o r t e d so f a r j u s t i f y t h i s assumption, as do the g e n e r a l p h y s i c a l p r o p e r t i e s of the r e s u l t i n g compounds, which are c o n s i s t e n t w i t h an a s s o c i a t e d o l i g o m e r i c s t r u c t u r e l i k e A u ( S 0 3 F ) 3 , 7 8 or a p o l y m e r i c s t r u c t u r e such as ( C H 3 ) 2 S n ( S 0 3 F ) 2 . 9 3 1.7 Objectives of This Research P r e v i o u s s t u d i e s on the HS0 3F\SbF 5\S0 3 system have shown t h a t the a c i d i t y of a s o l u t i o n of SbF 5 i n HS0 3F can be enhanced by the a d d i t i o n of S 0 3 . 2 7 The a c i d i t y of the s o l u t i o n i n c r e a s e s w i t h the amount of S0 3 added, up t o a maximum of 3 moles of S0 3 per mole of SbF 5. The i n c r e a s e i n a c i d i t y i s presumably due t o t h e f o r m a t i o n of complex Sb(V) f l u o r i d e f l u o r o s u l f a t e s a c i d s of the type H[SbF n(S0 3F) 6_ n] (n=2, 3, 4), s u g g e s t i n g i n s e r t i o n of S0 3 i n t o Sb-F bonds. The f a c t t h a t f u r t h e r a d d i t i o n of S0 3 does not i n c r e a s e the a c i d i t y of the s o l u t i o n i n d i c a t e s the e x i s t e n c e of 26 an e q u i l i b r i u m between i n s e r t e d and f r e e S0 3 i n the system. The same phenomenon has been observed i n the analogous a r s e n i c system t o a l e s s e r e x t e n t . 2 7 T h i s p o s t u l a t i o n i s supported by NMR s t u d i e s of the s o l u t i o n s i n H S0 3F 2 7 and the syntheses of t h e i r a n solvo a c i d s S b F 4 ( S 0 3 F ) , 8 3 , 8 4 S b F 3 ( S 0 3 F ) 2 , 8 4 S b 2 F 9 ( S 0 3 F ) , 8 4 and. A s F 3 ( S 0 3 F ) 2 • I t i m p l i e s t h a t t e r n a r y f l u o r i d e f l u o r o s u l f a t e s can be another group of p o t e n t i a l l y u s e f u l Lewis a c i d s i n the conjugate Bronsted/Lewis a c i d systems. I t i s a l s o i n t e r e s t i n g t o note t h a t i n the s e r i e s of compounds w i t h g e n e r a l formula SbF n(S0 3F) 5 _ n (n>2), the Lewis a c i d s t r e n g t h s i n f l u o r o s u l f u r i c a c i d i n c r e a s e as n decreases, i . e . the h i g h e r the f l u o r o s u l f a t e content, the s t r o n g e r the Lewis a c i d . As a l r e a d y i n d i c a t e d , the mixture of HS0 3F/SbF 5/3S0 3 i s the s t r o n g e s t conjugate Bronsted/Lewis s u p e r a c i d known t o d a t e , 2 7 although i t s ansolvo a c i d S b F 2 ( S 0 3 F ) 3 i s s t i l l t o be s y n t h e s i z e d . One of the l i m i t a t i o n s i n the a p p l i c a t i o n s of the HS0 3F/SbF n(S0 3F) 5 _ n system i s the h i g h redox p o t e n t i a l of the Sb(V)/Sb(III) couple, which may l e a d t o u n d e s i r a b l e redox r e a c t i o n s . T h i s s i t u a t i o n does not e x i s t f o r NbF 5 and TaF 5 compounds on account of the i n a c c e s s i b i l i t y o f s t a b l e low valence s t a t e s . However, the use of NbF 5 and TaF 5 i n the conjugate Bronsted/Lewis systems i s l i m i t e d t o some extent by t h e i r low s o l u b i l i t y . I t has been r e p o r t e d t h a t N b ( S 0 3 F ) 5 and T a ( S 0 3 F ) 5 are extremely s o l u b l e i n HS0 3F so t h a t the i s o l a t i o n of the pure Lewis a c i d s by d i s t i l l i n g o f f HS0 3F i s not f e a s i b l e . 6 6 HS0 3F/Nb(S0 3F) 5 ( s o i v ) i s r e p o r t e d t o be a s u p e r a c i d system 27 s t r o n g e r than HS0 3F/NbF 5. T h i s seems t o be i n c o n t r a s t t o the r e p o r t , where the a d d i t i o n of S0 3 t o s o l u t i o n s of NbF 5 i n HS0 3F shows no e f f e c t on the c o n d u c t i v i t i e s of the s o l u t i o n s . 2 7 T h e r e f o r e , i t i s of i n t e r e s t t o i n v e s t i g a t e whether t h i s r e l a t i o n s h i p between the a c i d i t y and the f l u o r o s u l f a t e content of the Lewis a c i d found f o r the HS0 3F/SbF n (S0 3F) 5 _ n system i s a l s o a p p l i c a b l e t o the HS0 3F/NbF n (S0 3F) 5 _ n and HS0 3F/TaF n (S0 3F) 5 _ n systems. In t h i s work, an attempt w i l l be made t o develop a g e n e r a l s y n t h e t i c r o u t e t o the p r e p a r a t i o n of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s , i n order t o study t h e i r b ehavior i n HS0 3F by the conductometric method. The products w i l l be c h a r a c t e r i z e d p r i m a r i l y by v i b r a t i o n a l s p ectroscopy. 28 Chapter 2 EXPERIMENTAL T h i s chapter d e a l s w i t h the sources and p r e p a r a t i o n s of s t a r t i n g m a t e r i a l s , apparatus and g e n e r a l experimental techniques used i n t h i s study. D e t a i l s of s p e c i f i c syntheses and product a n a l y s i s w i l l be d e s c r i b e d i n the a p p r o p r i a t e c h a p t e r s . 2.1 General Comments S i n c e most of the compounds used i n t h i s study were moisture s e n s i t i v e , extreme care had t o be taken t o a v o i d c o n t a c t with a i r . In a d d i t i o n , some s t a r t i n g m a t e r i a l s and products were c o r r o s i v e and t o x i c . Most m a t e r i a l s were handled i n s i d e a dry box and on g l a s s or metal vacuum l i n e s . V o l a t i l e l i q u i d s were t r a n s f e r r e d in vacuo u s i n g a T-shaped b r i d g e c o n s i s t i n g of a Pyrex tube w i t h a BIO ground g l a s s socket a t e i t h e r t e r m i n a l . The b r i d g e was a t t a c h e d t o the vacuum l i n e v i a a Kontes T e f l o n stem stopcock, and c o u l d be heated d u r i n g the t r a n s f e r . Less v o l a t i l e l i q u i d s and s o l i d s were handled i n s i d e the dry box. Ground g l a s s c o n n e c t i o n s were l u b r i c a t e d w i t h Halocarbon grease ( S e r i e s 25-10M) or s e a l e d w i t h Halocarbon wax ( S e r i e s 12-00), which e x h i b i t low r e a c t i v i t y towards h a l o g e n - c o n t a i n i n g compounds. Both were ob t a i n e d from the Halocarbon Products C o r p o r a t i o n (New J e r s e y , USA). O c c a s i o n a l l y , mixtures of grease and wax were used t o achieve a d e s i r e d c o n s i s t e n c y . In the conductometric s t u d i e s , T e f l o n j o i n t s l e e v e s (Nalge Company, New York, USA) were used t o s e a l the ground g l a s s j o i n t s . 29 2.2 Chemicals Used i n This Study Some chemicals were used without f u r t h e r p u r i f i c a t i o n (Table 2-1). Other chemicals were p u r i f i e d o r s y n t h e s i z e d a c c o r d i n g t o the methods d e s c r i b e d below. B i s ( f l u o r o s u l f u r y l ) p eroxide, S 2 0 6 F 2 , was prepared by the d i r e c t r e a c t i o n between F 2 and S0 3 c a t a l y z e d by A g F 2 . 9 4 The s y n t h e s i s was c a r r i e d out a t 150-180°C i n a flow r e a c t o r made of Monel t u b i n g , u s i n g N 2 as c a r r i e r gas f o r S0 3. The crude product was condensed as a l i q u i d i n Pyrex t r a p s c o o l e d t o -78°C w i t h dry i c e . Condensation of the p o t e n t i a l l y hazardous byproduct, FS0 3F, was l a r g e l y avoided a t t h i s temperature. Trace amounts of FS0 3F Table 2-1 Source and P u r i t y of Chemicals Chemical Source Purity Nb, -60mesh Morton T h i o k o l ( A l f a ) 99.9% Ta, -60mesh Johnson Matthey ( A l f a ) 99.9% NbF 5 Ozark Mahoning (Pennwalt) 3 99% T a F 5 Ozark Mahoning (Pennwalt) a 99% KCl Matheson Coleman & B e l l >99% C s C l Matheson Coleman & B e l l >99% CsF Johnson Matthey ( A l f a ) 99% HF Matheson of Canada reagent grade H 2 P t C l 6 A l d r i c h 8 wt% aq. s o l n . P 2°5 BDH 98% a now known as ATO-CHEM NORTH AMERICA 30 dissolved i n the crude product were removed by intermittently warming the product to room temperature and cooling down to -78°C, before pumping on the Pyrex traps. Any excess S03 was extracted with concentrated H 2S0 4 i n a separatory funnel. The product obtained i n t h i s manner frequently contained a small amount of d i s u l f u r y l d i f l u o r i d e , S2O5F2, which has no e f f e c t on the synthetic reactions, except those i n which a stoichiometric amount of S 20gF 2 i s required. The p u r i f i e d product was stored i n one-part Pyrex storage vessels (500-1000 mL) equipped with Kontes Teflon stem stopcocks. The puri t y was tested by in f r a r e d and, more r e l i a b l y , by 1 9 F NMR spectroscopy. Technical grade f l u o r o s u l f u r i c acid, HS03F, (Orange County Chemicals, C a l i f o r n i a , USA) was p u r i f i e d by d o u b l e - d i s t i l l a t i o n at atmospheric pressure under a counter flow of dry N 2, as described by Barr et a l . . 4 0 The f l u o r o s u l f a t e s , CsS0 3F and KS03F, were prepared by the reaction of CsCl and KCl with an excess of HS03F r e s p e c t i v e l y . 4 0 The products were i s o l a t e d a f t e r removal of a l l v o l a t i l e s in vacuo while heating up to «80°C to avoid formation of H-bridged solvates l i k e Cs[H(S0 3F) 2] or K[H(S0 3F) 2]. Cesium hexafluoroniobate(V), Cs[NbF 6], was synthesized i n HF medium by the reaction of CsF with NbF 5. 1 0 4 P u r i f i e d propylene carbonate, 0=COCH2CH2CH26, was obtained from Moli Energy Inc., Burnaby, B.C.. 31 2.3 Apparatus and Equipment 2.3.1 Reaction Vessels One-part reactors (Fig. 2-la) were used when the reaction product could be i s o l a t e d by removal of a l l the v o l a t i l e s in vacuo. The reactors were made from 25, 50 or 100 mL round bottom f l a s k s , or from either 2 mm or 3 mm thick wall glass tubing depending on the reaction conditions. The product was removed from the reactor by either pouring through the valve or cutting the reactor stem o f f inside the dry box. Two-part reactors (Fig. 2-lb) were used when subsequent i s o l a t i o n procedures were necessary and high pressures were not anticipated. The reactor consisted of a 25, 50 or 100 mL round bottom f l a s k or normal wall glass tubing, with a B19 ground glass cone f i t t e d with a "drip l i p " to trap possible grease-contaminated l i q u i d s and prevent them from mixing with the product. The corresponding adaptor top had a Kontes Teflon stem stopcock between a B19 socket and a B10 cone. After completion of the reaction, the top adaptor could be substituted by appropriate equipment such as a f i l t r a t i o n or a d i s t i l l a t i o n apparatus. A Kel-F tube reactor (Fig. 2-2) was used when l i q u i d HF was the reaction medium. The Kel-F tube (2 cm o.d. and 1.2 cm i . d . , Argonne National Laboratory, I l l i n o i s , USA) was held by a Monel adaptor top (made by Mechanical Engineering Services, Chemistry Department, UBC) which was f i t t e d to a metal vacuum l i n e using a Whitey valve (type 1KS4-316). 32 Kontes T e f l o n B 1 ° ground a. One-part r e a c t o r s b. Two-part r e a c t o r s Figure 2-1 T y p i c a l Pyrex R e a c t i o n V e s s e l s Used i n T h i s Study 33 Whitey Valve Monel Alloy Top Kel-F Tube Brass Nut Copper Ferrule ^ 2cm • |3cmJ 6 o CM 1 s u in 1 F i g u r e 2-2 A K e l - F Tubular Reactor 2.3.2 F i l t r a t i o n and D i s t i l l a t i o n Apparatus A vacuum f i l t e r ( F i g . 2-3a) was used f o r the f i l t r a t i o n of mois t u r e s e n s i t i v e compounds in vacuo. The d e s i g n of the f i l t e r was adapted from t h a t d i s c u s s e d by S h r i v e r 9 5 . Two Kontes T e f l o n stem stopcocks were used t o c o n t r o l the p r e s s u r e on e i t h e r s i d e of t he g l a s s f r i t . 34 A two-part dynamic vacuum d i s t i l l a t i o n apparatus ( F i g . 2-3b) was designed f o r the d i s t i l l a t i o n of products which had a low v o l a t i l i t y a t room temperature and were t h e r m a l l y u n s t a b l e under h e a t i n g . A dynamic vacuum was employed and the vapor of the p r o d u c t s was condensed i n s i d e a t u b u l a r r e a c t o r c o o l e d w i t h e i t h e r l i q u i d n i t r o g e n (-196°C) or dry i c e (-78°C). a. F i l t r a t i o n Apparatus b. D i s t i l l a t i o n Apparatus Figure 2-3 Vacuum-Adapted F i l t r a t i o n and D i s t i l l a t i o n Apparatus 35 2.3.3 Vacuum Lines A Pyrex g l a s s vacuum l i n e was employed f o r g e n e r a l purposes. The g l a s s vacuum l i n e had a 60cm-long m a n i f o l d w i t h f i v e BIO soc k e t s , equipped w i t h Kontes T e f l o n stem stopcocks. A s a f e t y t r a p c o o l e d w i t h l i q u i d n i t r o g e n was l o c a t e d between the m a n i f o l d and a r o t a r y o i l vacuum pump t o p r o t e c t the pump from v o l a t i l e , c o r r o s i v e m a t e r i a l s . A metal l i n e was used f o r r e a c t i o n s i n l i q u i d HF medium. The m a n i f o l d was c o n s t r u c t e d of 6 mm o.d. Monel t u b i n g equipped w i t h Whitey v a l v e s (type 1KS4-316), and was connected t o a Pyrex s a f e t y t r a p v i a a T e f l o n adaptor. The Pyrex s a f e t y t r a p , c o o l e d w i t h l i q u i d N 2 , was s e t be f o r e a r o t a r y o i l vacuum pump t o p r o t e c t the pump from HF and other v o l a t i l e c o r r o s i v e m a t e r i a l s . 2.3.4 Dry Box For the h a n d l i n g and sto r a g e of h y g r o s c o p i c s o l i d s and l i q u i d s w i t h low v o l a t i l i t y , a "DRI-LAB" Model DL-001-S-G dry box (Vacuum Atmosphere C o r p o r a t i o n , C a l i f o r n i a , USA) was used, f i l l e d w i t h dry n i t r o g e n . The removal of moisture i n s i d e t he dry box was accomplished by c i r c u l a t i n g the n i t r o g e n over m o l e c u l a r s i e v e s l o c a t e d w i t h i n the "DRI-TRAIN" Model HE-493 (Vacuum Atmosphere C o r p o r a t i o n , C a l i f o r n i a , USA). The molecular s i e v e s were p e r i o d i c a l l y regenerated by h e a t i n g i n a stream of 10% H 2 mixed w i t h N 2 . 36 2.4 Instrumentation and Methods 2.4.1 Elemental Analyses The s u l f u r content of some samples was determined by Mr. P e t e r Borda of t h i s Department. A l l o t h e r elemental analyses were performed by the A n a l y t i s c h e L a b o r a t o r i e n , Gummersbach, Germany. 2.4.2 I n f r a r e d Spectroscopy I n f r a r e d s p e c t r a were rec o r d e d on a Perkin-Elmer Model 598 g r a t i n g spectrometer, o p e r a t i n g i n the range of 250-4000 cm - 1. AgBr windows, w i t h an approximate t r a n s m i s s i o n range down t o about 400 cm - 1, were used. S i n c e samples were extremely h y g r o s c o p i c and r e a c t i v e towards most m u l l i n g agents such as N u j o l or HCB, they were packed as t h i n f i l m s between two AgBr windows i n s i d e the dry box and b l a c k e l e c t r i c a l tape was wrapped around the edges of the window p l a t e s t o p r o t e c t the samples from m o i s t u r e o u t s i d e the dry box. The i n f r a r e d s p e c t r a were rec o r d e d immediately a f t e r t a k i n g the sample out of the dry box. 2.4.3 Raman Spectroscopy Raman s p e c t r a were rec o r d e d on a Spex Ramalog 5 spectrophotometer equipped w i t h a S p e c t r a - P h y s i c s Model 164 argon i o n l a s e r . The green l i n e a t 514.5 nm was used f o r e x c i t a t i o n . L i q u i d samples were loaded i n t o NMR tubes or 6 mm o.d. g l a s s tubes and s o l i d samples were packed i n t o m e l t i n g p o i n t c a p i l l a r y 37 tubes i n s i d e the dry box. The tubes were f l a m e - s e a l e d under vacuum as soon as they were taken out of the dry box. Holders f o r sample tubes were made by the Mechanical E n g i n e e r i n g S e r v i c e s of t h i s department. In some cases, Raman s p e c t r a of the samples i n g l a s s v e s s e l s were a l s o recorded f o r the purpose of m o n i t o r i n g r e a c t i o n s i n p r o c e s s or p u r i f i c a t i o n procedures. 2.4.4 NMR Spectroscopy NMR s p e c t r a were recorded on a V a r i a n XL-300 m u l t i n u c l e a r spectrometer. The l i q u i d samples were loaded i n t o 5 mm o.d. NMR tubes i n s i d e the dry box and f l a m e - s e a l e d under vacuum. A s o l u t i o n of CFC1 3 i n d 6-acetone was used as a r e f e r e n c e f o r 1 9 F NMR and a s a t u r a t e d s o l u t i o n of Cs[NbF 6] i n propylene carbonate f o r 9 3Nb NMR. 2.4 .5 E l e c t r i c a l C o n d u c t i v i t y Measurements A d e t a i l e d d e s c r i p t i o n of the g e n e r a l methods and apparatus i n v o l v e d has been g i v e n p r e v i o u s l y . 4 0 The c o n d u c t i v i t y c e l l s used are shown i n F i g . 2-4. The c o a t i n g of p l a t i n u m b l a c k on the e l e c t r o d e s was renewed a f t e r every two runs. Before each measurement, the c e l l c o nstant was c a l i b r a t e d u s i n g a 0.01000 M KCl aqueous s o l u t i o n as d e s c r i b e d p r e v i o u s l y . 9 6 The c o n d u c t i v i t y was measured wi t h a Wayne-Kerr U n i v e r s a l B r i d g e Model B221A conductometer. The temperature was kept c o n s t a n t a t a g i v e n v a l u e w i t h i n 0.01°C u s i n g a l a r g e o i l bath and was c o n t r o l l e d by a Model ST Sargent Thermonitor. A s p e c i a l a d d i t i o n b u r e t 38 (Fig. 2-5a) and a s o l i d addition container (Fig. 2-5b) were used during the measurements. So l i d was added v i a an adapter (Fig. 2-5c) to minimize the leakage of a i r into the conductivity c e l l . B-10 Ground platinum electrodes Figure 2 -4 E l e c t r i c a l Conductivity C e l l 39 B-19 Ground G l a s s Cone T e f l o n S t o p c o c k B-4 Ground G l a s s J o i n t Jb. S o l i d a d d i t i o n c o n t a i n e r T e f l o n R o t a r y V a l v e i. L i q u i d a d d i t i o n buret c * adaptor Figure 2-5 A d d i t i o n Apparatus Used During C o n d u c t i v i t y Measurements 40 Chapter 3 SYNTHESIS AND CHARACTERIZATION OF FLUORIDE FLUOROSULFATES OF NIOBIUM(V) AND TANTALUM(V) 3.1 Introduction The chemical p r o p e r t i e s of niobium and tantalum, and t h e i r compounds are v e r y s i m i l a r , but they d i f f e r i n many important a s p e c t s from those of vanadium and i t s c o r r e s p o n d i n g compounds. For example, niobium and tantalum compounds wi t h the metal i n a h i g h o x i d a t i o n s t a t e are more s t a b l e towards r e d u c t i o n and thermal decomposition than those of vanadium. For the compounds w i t h metal i n a lower o x i d a t i o n s t a t e , the s i t u a t i o n i s r e v e r s e d . In t h e i r h i g h e s t o x i d a t i o n s t a t e (+5), niobium and tantalum have v e r y s i m i l a r chemical p r o p e r t i e s t o those of the Group 15 elements a r s e n i c and antimony, which may be regarded as m e t a l l o i d s . There are v i r t u a l l y no c a t i o n i c d e r i v a t i v e s known, but numerous a n i o n i c complexes have been r e p o r t e d . 4 2 In h a l i d e s and o x y - h a l i d e s , bonding i s l a r g e l y c o v a l e n t and the compounds are r e a d i l y h y d r o l y z e d . The i n t r i n s i c a c c e p t o r a b i l i t y of Nb(V) and Ta(V) i s apparent from the a s s o c i a t e d o l i g o m e r i c s t r u c t u r e s of t h e i r c h l o r i d e s and f l u o r i d e s . NbF 5 and TaF 5 have a common c i s - b r i d g e d t e t r a m e r i c s t r u c t u r e and d i s p l a y very s i m i l a r chemical behavior. Both r e a c t as Lewis a c i d s towards Lewis bases t o form adducts, and both are used i n s u p e r a c i d systems i n c o n j u n c t i o n w i t h s t r o n g p r o t o n i c a c i d s such as anhydrous HF, HS0 3F and H S 0 3CF 3. 9 41 However, t h e i r l i m i t e d s o l u b i l i t i e s i n these p r o t o n i c a c i d s r e s t r i c t s t h e i r a p p l i c a t i o n s t o some extent. In s p i t e o f t h e i r low s o l u b i l i t i e s , T a F 5 and, t o a l e s s e r extent, NbF 5 have r e c e i v e d much a t t e n t i o n i n s u p e r a c i d chemistry and a number of a c i d - c a t a l y z e d r e a c t i o n s are r e p o r t e d f o r b o t h . 9 ' 9 a The most d i s t i n c t i v e f e a t u r e of NbF 5 and TaF 5 i s the l a c k of o x i d i z i n g a b i l i t y on account of the i n a c c e s s i b i l i t y of s t a b l e low valence s t a t e s . In c o n t r a s t , A s F 5 and SbF 5 are good o x i d i z i n g agents, and may l e a d t o u n d e s i r a b l e redox r e a c t i o n s , s i n c e the o x i d a t i o n s t a t e (+3) i s v e r y common f o r a r s e n i c and antimony. NbF 5 has been found t o be a very weak e l e c t r o l y t e i n HSO3F. 2 8 S i m i l a r s t u d i e s on the HS0 3F/TaF 5 system have not been p u b l i s h e d so f a r . Meanwhile, two n o v e l s u p e r a c i d s , HS0 3F/Ta (SO3F) 5 (solv) and HS0 3F/Nb(S0 3F) 5(solv) systems, have been r e p o r t e d . 6 6 Attempts t o i s o l a t e the pure Lewis a c i d s , Nb ( S 0 3F) 5 and Ta ( S 0 3F) 5, from the s o l u t i o n by d i s t i l l a t i n g t he s o l v e n t o f f r e s u l t i n decomposition, probably via S0 3 e l i m i n a t i o n . I t i s noteworthy t h a t niobium p e n t a k i s ( f l u o r o s u l f a t e ) and tantalum p e n t a k i s ( f l u o r o s u l f a t e ) are extremely s o l u b l e i n HS0 3F and cannot be separated, w h i l e the corres p o n d i n g p e n t a f l u o r i d e s show l i m i t e d s o l u b i l i t y i n HS0 3F. One Nb(V) f l u o r i d e f l u o r o s u l f a t e , N b F 2 ( S 0 3 F ) 3 , i s i s o l a t e d as a p o s s i b l e decomposition product from the Nb(S0 3F) 5(solv)/HS0 3F s y s t e m . 9 7 TaF 4(S0 3F) i s s y n t h e s i z e d by a l i g a n d r e d i s t r i b u t i o n r e a c t i o n o f TaF 5 and s o l v a t e d T a ( S 0 3 F ) 5 i n HS0 3F. Both compounds are r e p o r t e d t o be white solids g i v i n g r i s e t o complicated 42 v i b r a t i o n a l s p e c t r a . 9 7 These f i n d i n g s d i f f e r from a p r e v i o u s r e p o r t , i n which the r e a c t i o n of NbF 5 or TaF 5 w i t h excess S0 3 r e s u l t s i n viscous colorless liquids of the compositions TaF 5.2.6S0 3 and NbF 5 2.1S0 3. 9 8 In o r d e r t o i n v e s t i g a t e the behavior of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s i n the s t r o n g p r o t o n i c a c i d HS0 3F, i t i s necessary t o develop a g e n e r a l l y a p p l i c a b l e , s y s t e m a t i c s y n t h e t i c r o u t e t o o b t a i n a s e r i e s of compounds of the type M F n ( S 0 3 F ) 5 _ n (M=Nb, T a ) . In t h i s chapter, the attempted syntheses o f Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s by a one-step metal o x i d a t i o n / l i g a n d r e d i s t r i b u t i o n r e a c t i o n w i l l be d i s c u s s e d . The v i b r a t i o n a l s p e c t r a of the compounds prepared w i l l a l s o be i n t e r p r e t e d . To a v o i d the problems encountered i n product i s o l a t i o n , our attempts aim a t s y n t h e t i c r e a c t i o n s which do not i n v o l v e HS0 3F. 3.2 Experimenta1 A l l syntheses were c a r r i e d out a t room temperature a c c o r d i n g t o the f o l l o w i n g g e n e r a l equation: (S-nJM + nMF 5 + excess S 2 0 6 F 2 — » 5MF n(S0 3F) 5 _ n [3-1] (M=Nb, Ta; n=0, 1, 2, 3, 4, 4.5) In a t y p i c a l r e a c t i o n , metal powder and the c o r r e s p o n d i n g metal p e n t a f l u o r i d e were added i n s t o i c h i o m e t r i c q u a n t i t i e s . An excess of S 20gF 2 was added by vacuum t r a n s f e r t o a c t both as a f l u o r o s u l f o n a t i n g reagent and as a r e a c t i o n medium. As the 43 mixture warmed up t o room temperature the r e a c t i o n proceeded v i g o r o u s l y and e x o t h e r m i c a l l y . The r e a c t o r needed t o be c o o l e d i n an i c e - w a t e r bath t o a v o i d p r e s s u r e b u i l d u p and the p o s s i b l e decomposition of the products a t h i g h e r temperatures. A f t e r t h i s i n i t i a l phase, the r e a c t i o n proceeded smoothly a t room temperature and the mixture was s t i r r e d u n t i l a l l the metal powder was consumed. The r e s u l t i n g white murky mixture was f i l t e r e d in vacuo t o remove s m a l l amounts of a white p r e c i p i t a t e which i s observed as a byproduct i n a l l r e a c t i o n s other than the p r e p a r a t i o n of N b 2 F 9 ( S 0 3 F ) . In a l l i n s t a n c e s , the amounts of the p r e c i p i t a t e were so s m a l l t h a t no i s o l a t i o n and i d e n t i f i c a t i o n was p o s s i b l e . Excess S 20gF 2 was d i s t i l l e d o f f , f i r s t i n a s t a t i c vacuum and then i n a dynamic vacuum. The removal of S 20gF 2 was monitored both by weight and by the absence of the 800 cm - 1 0-0 s t r e t c h i n g band of S 205F 2 i n the Raman spectrum of the p r o d u c t . 8 4 A f t e r removal of the excess S 205F 2, p a l e - y e l l o w v i s c o u s l i q u i d s were o b t a i n e d . The c o l o r was a t t r i b u t e d t o i m p u r i t i e s which were unavoidable when two-part r e a c t o r s were used. Two-p a r t r e a c t o r s had t o be used s i n c e f i l t r a t i o n was necessary t o remove the s m a l l amounts of the white p r e c i p i t a t e . C o l o r l e s s v i s c o u s l i q u i d s were o b t a i n e d by the d i s t i l l a t i o n of the crude product i n a dynamic vacuum a t room temperature. Experimental d e t a i l s of the r e a c t i o n s are l i s t e d i n Ta b l e 3-1. 44 Table 3 -1 Experimental D e t a i l s of Syntheses of M F N ( S 0 3 F ) 5 _ n (M=Nb, Ta) M g (mmol) MFS g (mmol) S 2 0 6 P 2 g (mmol) Reaction Time a Product Expected Product Obtained & Comments 0.0931 (1.00) 1.6889 (9.003) 3.009 (15.19) 18 hours N b 2 F 9 ( S 0 3 F ) N b 2 F 9 ( S 0 3 F ) b 0.1414 (1.522) 1.1328 (6.038) 1.9303 (9.744) 2.5 days N b F 4 ( S 0 3 F ) N b F 4 ( S 0 3 F ) c 0.5377 (5.787) 1.605 (8.555) 6.3671 (32.14) 3 days N b F 3 ( S 0 3 F ) 2 N b F 3 ( S 0 3 F ) 2 c 0.6290 (6.770) 0.8477 (4.519) 9.8808 (49.88) 4 days N b F 2 ( S 0 3 F ) 3 c, d 0.7277 (7.832) 0.3638 (1.939) 3.8791 (19.58) 3 days N b F ( S 0 3 F ) 4 d 0.3398 (3.657) 0 8.6129 (43.48) 3 days N b ( S 0 3 F ) 5 e 0.1377 (0.7391) 1.8164 (6.591) 3.6895 (18.63) 18 hours T a 2 F 9 ( S 0 3 F ) f 0.2043 (1.129) 1.2527 (4.545) 2.5441 (12.84) 2 days T a F 4 ( S 0 3 F ) T a F 4 ( S 0 3 F ) c 0.8277 (4.575) 1.8894 (6.856) 8.069 (40.73) 2 days T a F 3 ( S 0 3 F ) 2 T a F 3 ( S 0 3 F ) 2 c 1.0422 (5.761) 1.0607 (3.849) 7.0125 (35.40) 2 days T a F 2 ( S 0 3 F ) 3 c, d 0.5358 (2.962) 0.2045 (0.7420) 5.4889 (19.58) 2.5 days T a F ( S 0 3 F ) 4 c, d 0.5534 (3.058) 0 4.5809 (23.12) 3 days T a ( S 0 3 F ) 5 e t i m e b e t w e e n t h e i n i t i a l p h a s e a n d t h e b e g i n n i n g o f t h e w o r k u p . c o l o r l e s s l i q u i d b u t b e c o m e s a m i x t u r e o f N b F £ a n d a l i q u i d o n l o n g c d s t a n d i n g . c o l o r l e s s v i s c o u s l i q u i d . f l a k y S 0 3 o b s e r v e d d u r i n g d i s t i l l a t i o n . e v e r y v i s c o u s p a l e - y e l l o w l i q u i d s a s c r u d e p r o d u c t s ; n o d i s t i l l a t i o n w a s s u c c e s s f u l . * m i x t u r e o f T a F 5 a n d a l i q u i d 45 3.3 Results and Discussion 3.3.1 Syntheses of Nb(V) and Ta(V) Fluoride Fluorosulfates A number of g e n e r a l r o u t e s t o the p r e p a r a t i o n of t e r n a r y f l u o r i d e f l u o r o s u l f a t e s are d i s c u s s e d here i n o rder t o determine the most s u i t a b l e and g e n e r a l l y a p p l i c a b l e s y n t h e t i c r o u t e . The f o l l o w i n g methods have been used p r e v i o u s l y : (a) P a r t i a l i n s e r t i o n of S 0 3 i n t o b i n a r y metal f l u o r i d e s : MF n + mSC-3 — » M F ( n _ m ) ( S 0 3 F ) m [3-2] T h i s was the f i r s t r o u t e a p p l i e d t o the p r e p a r a t i o n of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s . 9 7 NbF 5 and TaF 5 were found t o r e a c t w i t h S0 3, g i v i n g r i s e t o c o l o r l e s s v i s c o u s l i q u i d s formulated as N b F 3 ( S 0 3 F ) 2 and T a F 3 ( S 0 3 F ) 2 , but having the a c t u a l compositions of NbF 5i2.1S0 3 and TaF 5»2.6S0 3, r e s p e c t i v e l y . The d i f f e r e n c e between the proposed formulae and a c t u a l compositions was a t t r i b u t e d t o the incomplete removal of unreacted S0 3. T h i s r o u t e was not used i n our attempts f o r the f o l l o w i n g reasons: ( i ) the c o n t r o l of the s t o i c h i o m e t r y of F vs. S 0 3 F i s very d i f f i c u l t , ( i i ) t h e r e i s a p p a r e n t l y some d i f f i c u l t y i n removing the unreacted S0 3 from the product and i t i s not c l e a r from the r e p o r t whether S0 3 i s c ompletely i n s e r t e d or p a r t i a l l y d i s s o l v e d i n the p r o d u c t s , ( i i i ) based on the r e p o r t e d IR s p e c t r a which have o n l y two bands a t 1057 and 767 cm - 1, the e x i s t e n c e of f l u o r o s u l f a t e groups i n the product cannot be s a f e l y deduced, 46 and ( i v ) r e c e n t work i n our group has shown t h a t a very complex r e a c t i o n takes p l a c e i n the analogous MoF 6/S0 3 system, y i e l d i n g a wide range of p r o d u c t s . 6 4 (b) O x i d a t i o n of lower v a l e n t b i n a r y f l u o r i d e s by S 206F 2 or FS0 3F a c c o r d i n g t o : 2MF n + mS 20 6F 2 — » 2 M F n ( S 0 3 F ) m [3-3a] MF n + mFS0 3F — » M F ( n + m ) ( S 0 3 F ) m [3-3b] A number of As(V) and Sb(V) f l u o r i d e f l u o r o s u l f a t e s have been prepared i n t h i s manner. 8 4' 8 5 T h i s r o u t e i s not a p p l i c a b l e t o the s y n t h e s i s of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s s i n c e the p r e p a r a t i o n of the s t a r t i n g m a t e r i a l s , NbF 4 and TaF 4, i s very d i f f i c u l t and o n l y l i m i t e d i n f o r m a t i o n on t h e i r lower v a l e n t f l u o r i d e s i s a v a i l a b l e . 4 2 Furthermore, r e c e n t r e s u l t s from our group have shown t h a t FS0 3F i s an extremely dangerous chemical and o f t e n l e a d s t o u n p r e d i c t a b l e and e x p l o s i v e r e a c t i o n s . 6 4 (c) S u b s t i t u t i o n of c h l o r i d e by f l u o r o s u l f a t e u s i n g S 20gF 2: 2 M F n C l m + mS 20 6F 2 — » 2 M F n ( S 0 3 F ) m + m.Cl2 [3-4] T h i s r o u t e r e q u i r e s the use of excess S 205F 2 which i s used both as a r e a c t a n t and as a r e a c t i o n medium. I t i s l i k e l y t h a t S 205F 2 r e a c t s f u r t h e r w i t h C l 2 t o produce C1S0 3F and C 1 0 2 S 0 3 F . 4 3 The l a t t e r i s very d i f f i c u l t t o separate from the product. Furthermore, the p r e c u r s o r s , Nb(V) or Ta(V) c h l o r i d e f l u o r i d e s , are d i f f i c u l t t o o b t a i n and have o f t e n complex c o m p o s i t i o n s . 4 2 I t should a l s o be r e c a l l e d t h a t the 47 r e a c t i o n s of MC1 5 (M=Nb, Ta) wit h S 20gF 2 r e s u l t i n the fo r m a t i o n of M O ( S 0 3 F ) 3 a c c o r d i n g t o K l e i n k o p f and S h r e e v e . 9 9 (d) Decomposition of b i n a r y f l u o r o s u l f a t e s a c c o r d i n g t o : M ( S 0 3 F ) n — > M F m ( S 0 3 F ) n _ m + mS03 [3-5] T h i s i s a c t u a l l y an a c c i d e n t a l r a t h e r than a w e l l - d e s i g n e d s y n t h e t i c r o u t e . For example, attempts t o prepare G e ( S 0 3 F ) 4 by the o x i d a t i o n of germanium w i t h S 20gF 2 i n HS0 3F r e s u l t e d i n the p r e c i p i t a t i o n of G e F 2 ( S 0 3 F ) 2 i n h i g h y i e l d . 5 7 b I t should a l s o be mentioned t h a t a l t e r n a t i v e d i s s o c i a t i o n modes of b i n a r y f l u o r o s u l f a t e s are known. Among them, the fo r m a t i o n of o x y - f l u o r o s u l f a t e s o f t e n occurs a c c o r d i n g t o : M ( S 0 3 F ) n —-> M O ( S 0 3 F ) n _ 2 + S 2 0 5 F 2 [3-5a] (e) L i g a n d r e d i s t r i b u t i o n of b i n a r y f l u o r i d e and f l u o r o s u l f a t e : nMF x + mM(S0 3F) x — » x M F n ( S 0 3 F ) m [3-6] The l i g a n d r e d i s t r i b u t i o n r e a c t i o n i s the most p r o m i s i n g r o u t e t o Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s . The method i s s t r a i g h t f o r w a r d and the composition of the compounds can be e a s i l y c o n t r o l l e d by the use of s t o i c h i o m e t r i c amounts of the b i n a r y f l u o r i d e s and corres p o n d i n g b i n a r y f l u o r o s u l f a t e s , p r o v i d e d both of them are a v a i l a b l e . There i s a l s o a precedent f o r t h i s r o u t e . TaF 4(S0 3F) i s r e p o r t e d t o be o b t a i n e d as a white s o l i d i n about 52% y i e l d v i a a l i g a n d 48 r e d i s t r i b u t i o n r e a c t i o n between TaF 5 and s o l v a t e d T a ( S 0 3 F ) 5 i n HSO3F. 9 7 There are a number of requirements t h a t must be met i n the s y n t h e s i s of w e l l - d e f i n e d products by l i g a n d r e d i s t r i b u t i o n : ( i ) t h e p r e c u r s o r s must be a v a i l a b l e i n h i g h p u r i t y , ( i i ) t h e r e a c t i o n s hould occur i n a homogeneous medium, such as melt or s o l u t i o n . In the l a t t e r case, the s o l v e n t chosen should be r e a d i l y s e p a r a b l e from the product, and ( i i i ) i f monomeric compounds are expected t o form, the r e d i s t r i b u t i o n r e a c t i o n must be n o n - s t a t i s t i c a l , l e a d i n g t o a w e l l - d e f i n e d product. S i n c e N b ( S 0 3 F ) 5 and T a ( S 0 3 F ) 5 cannot be i s o l a t e d from the r e s u l t i n g s o l u t i o n a f t e r the o x i d a t i o n of the c o r r e s p o n d i n g metals by S2C>6F2 i n HS0 3F, the o x i d a t i o n of metals and the l i g a n d r e d i s t r i b u t i o n are c o n v e n i e n t l y combined i n t o a one-step r e a c t i o n w i t h the g e n e r a l equation: (5-n)M + nMF 5 + excess S 2 0 6 F 2 — > 5MF n(S0 3F) 5 _ n [3-1] (M=Nb, Ta; n=0, 1, 2, 3, 4, 4.5) There are a few cases i n which the o x i d a t i o n of t r a n s i t i o n metals w i t h S 20gF 2, i n the absence of HS0 3F, goes t o completion a t e l e v a t e d temperatures (60-130°C) and long r e a c t i o n times: M + excess S 2 0 6 F 2 — > M ( S 0 3 F ) n [3-7] Ag(S0 3F) 2 7 6 , H g ( S 0 3 F ) 2 and O s ( S 0 3 F ) 3 7 1 have a l l been prepared a c c o r d i n g t o r e a c t i o n [3-7]. An e a r l y r e p o r t by K l e i n k o p f and Shreeve s t a t e s t h a t niobium and tantalum r e a c t s l o w l y w i t h 49 S 2 ° 6 F 2 a n < * l e a d t o i n c o m p l e t e l y o x i d i z e d m a t e r i a l s due t o a c o a t i n g on the s u r f a c e of metals. I t i s hence s u r p r i s i n g t o us t h a t the o x i d a t i o n of niobium and tantalum w i t h S2O5F2 alone goes t o completion a t room temperature, a l b e i t slower than i n t h e presence of HS0 3F. Surf a c e c o a t i n g of unreacted metal does not appear t o be a problem s i n c e the l i q u i d p roducts are s o l u b l e i n S 2 ° 6 F 2 a n c * r e a c t i o n times are r e l a t i v e l y s h o r t . Another f o r t u i t o u s f i n d i n g i s the o b s e r v a t i o n t h a t NbF 5 and T a F 5 are r e a d i l y s o l u b l e i n S 20gF2, whereas they are s p a r i n g l y s o l u b l e i n HSO3F. T h e r e f o r e excess S 20gF 2, i n s t e a d of HS0 3F, i s used as the medium f o r r e a c t i o n [3-1]. An obvious advantage i s t h a t excess S 2 ° 6 F 2 I s e a s y t o remove in vacuo. These two f i n d i n g s made a t the onset of t h i s r e s e a r c h form the b a s i s f o r the s u c c e s s f u l use of r e a c t i o n [3-1]. The f a c t t h a t both niobium and tantalum can be o x i d i z e d completely a t room temperature by S 20gF 2 i n the absence of HS0 3F, and t h a t t e t r a m e r i c NbF 5 and T a F 5 are s o l u b l e i n S2C>6F2, p r o v i d e a sound b a s i s f o r two s y n t h e t i c o b j e c t i v e s : ( i ) a renewed attempt a t the p r e p a r a t i o n of the b i n a r y f l u o r o s u l f a t e s N b ( S 0 3 F ) 5 and T a ( S 0 3 F ) 5 w i t h i s o l a t i o n h o p e f u l l y s i m p l i f i e d because HS0 3F i s not i n v o l v e d , and ( i i ) the s y s t e m a t i c s y n t h e s i s of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s . 3.3.1.1 Attempted Syntheses of Nb 2F 9(S0 3F) and Ta 2F 9(S0 3F) The p o s s i b l e e x i s t e n c e of Nb 2F 9(S0 3F) and T a 2 F 9 ( S 0 3 F ) i s suggested by the p r e v i o u s l y r e p o r t e d i s o l a t i o n of S b 2 F 9 ( S 0 3 F ) 8 4 50 and the e x i s t e n c e of o l i g o m e r i c Nb(V) and Ta(V) f l u o r i d e c h l o r i d e s of the type [ M 4 F n C l 2 o - n ] • 4 2 T n e syntheses of Nb 2F 9(S0 3F) and T a 2 F 9 ( S 0 3 F ) are attempted a c c o r d i n g t o e q u a t i o n [3-1] f o r n=4.5. In case of niobium, a v i s c o u s c o l o r l e s s l i q u i d i s o b t a i n e d . The elemental a n a l y s e s f o r the l i q u i d are i n agreement w i t h the expected v a l u e s f o r Nb 2F 9(S0 3F) (Table 3-2). However, Nb 2F 9(S0 3F) appears t o be an u n s t a b l e compound, because n e e d l e - l i k e c r y s t a l s form i n l i q u i d Nb 2F 9(S0 3F) upon s t a n d i n g f o r s e v e r a l weeks. The c r y s t a l s are i d e n t i f i e d as t e t r a m e r i c [ N b F 5 ] 4 by Raman s p e c t r o s c o p y . 1 0 0 The l i q u i d r e s i d u e g i v e s a Raman spectrum s i m i l a r t o t h a t of N b F 4 ( S 0 3 F ) . T h i s suggests t h a t Nb 2F 9(S0 3F) undergoes d i s p r o p o r t i o n a t i o n a c c o r d i n g t o : Nb 2F 9 (S0 3F) (1) — > NbF 5(s) + NbF 4(S0 3F) (1) [3-8] R e a c t i o n [3-8] cannot be r e v e r s e d by h e a t i n g the r e s u l t i n g mixture up t o 50°C. In the case of tantalum, the attempted r e a c t i o n g i v e s a mixture of a white s o l i d and a c o l o r l e s s l i q u i d . The s o l i d i s i d e n t i f i e d as the s t a r t i n g m a t e r i a l , TaF 5. T h i s behaviour appears t o be i n c o n t r a s t w i t h t h a t of the antimony analogue, which can be s y n t h e s i z e d by a l i g a n d r e d i s t r i b u t i o n r e a c t i o n , i n an o p p o s i t e manner t o r e a c t i o n [3-8]: SbF 4(S0 3F)(1) + SbF 5(l) — > S b 2F 9(S0 3F)(1) [3-9] 51 T a b l e 3-2 Elemental A n a l y s i s Data f o r M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta) Compound M (%) S (%) F (%) Nb 2F 9 ( S 0 3F) found 40.55 7.03 7.54a, 7.49a 41.68 calculated 40.81 7.04 41.72 NbF 4 (S0 3F) found 34.40 12.11 11.93a,11.82a 35.27 calculated 34.71 11.98 35.49 NbF 3 ( S 0 3F) 2 found 27.00 18.15 27.58 26.90 18.33 27.48 calculated 26.72 18.44 27.32 »NbF 2(S0 3F) 3» found 26.40 18.70 26.94 calculated 21.72 22.48 22.21 " N b F ( S 0 3 F ) 4 " b found 21.55 22.20 21.99 calculated 18.30 25.25 18.71 TaF 4 ( S 0 3 F ) found 50.55 9.15 26.69 calculated 50.87 9.01 26.71 T a F 3 ( S 0 3 F ) 2 found 41.35 14.46 22.06 calculated 41.52 14.71 21.80 " T a F 2 ( S 0 3 F ) 3 " found 41 .20 14.47 21 .91 calculated 35.08 18.65 18.42 "TaF ( S 0 3F) 4" found calculated 34.80 30.36 18.42 21.52 18.37 15.94 "Ta(S0 3F) 5 , , b found 41.20 14.82 21 .54 calculated 26.77 23.71 14.05 a data from Mr. P. Borda in the Chemistry Department, UBC. A l l others are from Analytische Laboratorien, Gummersbach, Germany b crude product after removing a l l the excess S 2OgF 2 and before d i s t i l l a t i o n . The raw product of "Nb(S0 3F) 5" is an extremely viscous liquid and no elemental analysis has been attempted 52 S b 2 F 9 ( S 0 3 F ) i s b e l i e v e d t o be a s t a b l e s p e c i e s w i t h a poly m e r i c s t r u c t u r e i n the l i q u i d phase and a monomeric s t r u c t u r e i n the gaseous phase, a c c o r d i n g t o vapor d e n s i t y measurements. 8 4 The c o n t r a s t i n g behavior r e g a r d i n g the fo r m a t i o n of compounds of the type M 2F 9(S0 3F) may be a t t r i b u t e d t o the s t r u c t u r a l d i f f e r e n c e between SbF 5 and MF 5 (M=Nb, Ta) . At room temperature, NbF 5 and TaF 5 are s o l i d s w i t h t e t r a m e r i c s t r u c t u r e s w h i l e SbF 5 i s a l i q u i d w i t h an o l i g o m e r i c c h a i n s t r u c t u r e . Conversely, i t may be argued t h a t a s i m i l a r d i s p r o p o r t i o n a t i o n of S b 2 F 9 ( S 0 3 F ) on long s t a n d i n g c o u l d c o n c e i v a b l y remain undetected because a l l t h r e e compounds i n t he system ( S b 2 F 9 ( S 0 3 F ) , SbF 4(S0 3F) and SbF 5) are c o l o r l e s s v i s c o u s l i q u i d s , and may be m i s c i b l e w i t h one another. 3.3.1.2 syntheses of MF 4(S0 3F) and MF 3(S0 3F) 2 (M=Nb, Ta) M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n=4, 3) were s u c c e s s f u l l y s y n t h e s i z e d a c c o r d i n g t o r e a c t i o n [3-1]. A l l compounds are c o l o r l e s s v i s c o u s l i q u i d s . The elemental a n a l y s e s agree w i t h the expected v a l u e s (Table 3-2). In no i n s t a n c e i s the form a t i o n of s o l i d NbF 5 or TaF 5 observed, even over a long time. The p h y s i c a l appearance of the f o u r compounds i s not s u r p r i s i n g i n view of the f i r s t r e p o r t e d p r e p a r a t i o n of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s by C l a r k and Emeleus, 9 8 although the incomplete removal of excess S0 3 was claimed t o account f o r the d i f f e r e n c e between the proposed formulae and a c t u a l compositions. In a d d i t i o n , a l l known As(V) and Sb(V) f l u o r i d e f l u o r o s u l f a t e s a re c o l o r l e s s l i q u i d s . 8 4 ' 8 5 T h e i r c l o s e 53 resemblance t o these Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s extends not o n l y t o the p h y s i c a l appearance, but a l s o t o t h e i r v i b r a t i o n a l s p e c t r a , as d i s c u s s e d l a t e r i n t h i s c h a p ter. Some ambiguity a r i s e s when two other r e c e n t l y r e p o r t e d Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s , N b F 2 ( S 0 3 F ) 3 and T a F 4 ( S 0 3 F ) , are i n v o l v e d . 9 7 Both of them are r e p o r t e d t o be o b t a i n e d from HS0 3F s o l u t i o n as white s o l i d s w i t h h i g h m e l t i n g p o i n t s . The d i f f e r e n c e i n composition and the claimed incomplete removal of S0 3 had been thought t o be a p l a u s i b l e e x p l a n a t i o n f o r the apparent d i f f e r e n c e between the solid and the p r e v i o u s l y r e p o r t e d liquid Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s . 9 7 An a d d i t i o n a l d i s t u r b i n g f e a t u r e i s t h a t s o l i d T a F 4 ( S 0 3 F ) , prepared by a s t o i c h i o m e t r i c r e a c t i o n between TaF 5 and s o l v a t e d T a ( S 0 3 F ) 5 i n HS0 3F, i s i s o l a t e d i n o n l y 52% y i e l d a f t e r removal of a l l v o l a t i l e s . 9 7 Since both white s o l i d s , T aF 4(S0 3F) and N b F 2 ( S 0 3 F ) 3 , are o b t a i n e d from HS0 3F s o l u t i o n w h i l e the l i q u i d m a t e r i a l s are o b t a i n e d from r e a c t i o n s e i t h e r i n S0 3 or i n S 20gF 2, i t was suspected t h a t Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s may have d i f f e r e n t c o o r d i n a t i o n environments around the metal c e n t e r and p o s s i b l y show a d i f f e r e n t degree of o l i g o m e r i z a t i o n , and hence d i f f e r e n t p h y s i c a l appearance, depending on d i f f e r e n t r e a c t i o n c o n d i t i o n s . However, the t r a n s f o r m a t i o n of liquid T a F 4 ( S 0 3 F ) i n t o solid TaF 4(S0 3F) i n HS0 3F i s not completely s u c c e s s f u l . In an attempt t o t r a n s f o r m l i q u i d TaF 4(S0 3F) t o s o l i d T a F 4 ( S 0 3 F ) , a s o l u t i o n was made up from 1.818 g l i q u i d T a F 4(S0 3F) and 3.217 g 54 HS0 3F. A f t e r removing a l l v o l a t i l e s i n vacuo, o n l y a s m a l l amount of s o l i d (0.06g, «3%) remained i n the r e a c t o r , and gave a s i m i l a r i n f r a r e d spectrum t o the one p r e v i o u s l y r e p o r t e d f o r s o l i d T a F 4 ( S 0 3 F ) , 9 7 Furthermore, i t was found d u r i n g the conductometric s t u d i e s (next chapter) t h a t these l i q u i d compounds were m i s c i b l e w i t h HS0 3F i n any p r o p o r t i o n . The o b s e r v a t i o n t h a t compounds of the type M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n=3, 4), may be d i s t i l l e d in vacuo without decomposition has an a d d i t i o n a l i m p l i c a t i o n . The unexpectedly low y i e l d of s o l i d T a F 4( S 0 3F ) r e p o r t e d 9 7 i s most l i k e l y due t o some product having been removed in vacuo t o g e t h e r w i t h HS0 3F. 3.3.1.3 Attempted Syntheses o f Nb(V) and Ta(V) F l u o r i d e F l u o r o s u l f a t e s w i t h Higher S0 3F Content A l l attempted syntheses of MF n(S0 3F) 5 _ n , (M=Nb, Ta; n<3), were c a r r i e d out i n the same manner. Although S 20gF 2 i s very v o l a t i l e a t room temperature, the i s o l a t i o n of S 2 0 6 F 2 - f r e e products i s s t i l l not easy because of the h i g h v i s c o s i t y and the thermal i n s t a b i l i t y of the products. The v i s c o s i t y of the products seems t o i n c r e a s e with i n c r e a s i n g S0 3F content. E l e v a t i n g the temperature or extending the pumping time may r e s u l t i n decomposition, and p u r i f i c a t i o n by d i s t i l l a t i o n i s no lon g e r p o s s i b l e . During the attempted d i s t i l l a t i o n s , a f i n e f i l m i s observed a t low temperature, which appears t o be S0 3 formed i n a p a r t i a l decomposition, i n a d d i t i o n t o a c o l o r l e s s and g l a s s y s o l i d as observed i n the d i s t i l l a t i o n of the low SO^F content 55 compounds. T h i s o b s e r v a t i o n and the elemental a n a l y s i s (Table 3-2) i n d i c a t e t h a t M F n ( S 0 3 F ) 5 _ n w i t h a h i g h S0 3F content may not be t h e r m a l l y s t a b l e . The lower s u l f u r c o ntents than the expected v a l u e s can be e x p l a i n e d by the decomposition of the p r o d u c t s v i a l o s i n g S0 3 stepwise a c c o r d i n g t o : M F n ( S 0 3 F ) 5 _ n —-> M F ( n + 1 ) ( S 0 3 F ) 4 _ n + S0 3 [3-10] M=Nb, Ta; n<3 3.3.1 .4 Attempted S y n t h e s i s of C s [ T a F 4 ( S 0 3 F ) 2 ] The s y n t h e s i s of t e r n a r y cesium t e t r a f l u o r o -b i s ( f l u o r o s u l f a t o ) - t a n t a l a t e ( V ) , C s [ T a F 4 ( S 0 3 F ) 2 ] , was attempted a c c o r d i n g t o : CsS0 3F + TaF 4(S0 3F) — > C s [ T a F 4 ( S 0 3 F ) 2 ] [3-11] HS0 3F was used as s o l v e n t s i n c e CsS0 3F i s not s o l u b l e i n S 2 0 6 F 2 . No p r e c i p i t a t e appeared i n the r e s u l t i n g mixture. Removal of HS0 3F from the mixture by pumping at 50°C in vacuo r e s u l t e d i n the l o s s of v o l a t i l e T a F 4(S0 3F) as w e l l . E v e n t u a l l y a wax-like m a t e r i a l was o b t a i n e d . I t s weight was s i g n i f i c a n t l y lower than expected ( o b t a i n e d «1.87 g, expected 2.1658 g ) . Hence i t was suspected t o be a mixture of CsS0 3F and the r e s t of T a F 4 ( S 0 3 F ) . I t seems t h a t r e a c t i o n [3-11] does not go t o completion. T h i s r e s u l t i s c o n s i s t e n t w i t h the poor S0 3F~ a c c e p t o r a b i l i t y of T a F 4 ( S 0 3F ) (see next c h a p t e r ) . 56 3.3.2 V i b r a t i o n a l Spectra of Nb(V) and Ta(V) F l u o r i d e F l u o r o s u l f a t e s V i b r a t i o n a l s p e c t r a are r e p o r t e d here f o r the Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s , f o r which p u r i f i c a t i o n by d i s t i l l a t i o n i s p o s s i b l e and t h e i r compositions have been e s t a b l i s h e d by elemental a n a l y s i s . T h e i r Raman and i n f r a r e d s p e c t r a a re i l l u s t r a t e d i n F i g . 3-1 t o F i g . 3-5. The Raman s p e c t r a o f the v i s c o u s l i q u i d s show r a t h e r broad bands. As a consequence, c l o s e l y spaced bands are o f t e n p o o r l y r e s o l v e d and t h e i r f r e q u e n c i e s can be determined o n l y w i t h some u n c e r t a i n t y . In a d d i t i o n , unambiguous p o l a r i z a t i o n measurements are not p o s s i b l e f o r weak bands. I n f r a r e d s p e c t r a , o b t a i n e d from t h i n f i l m s between two AgBr p l a t e s a t room temperature, may be i n f l u e n c e d by the samples a t t a c k i n g the windows. As a r e s u l t , f o r m a t i o n o f " i o n i c " AgS0 3F i s p o s s i b l e , and some weak bands observed i n the s p e c t r a are a t t r i b u t a b l e t o the S0 3F~ i o n . Th e r e f o r e , the assignments are p r i m a r i l y based on Raman s p e c t r a t o a v o i d p o s s i b l e m i s i n t e r p r e t a t i o n . 3.3.2.1 V i b r a t i o n a l Spectra of Nb 2F 9(S0 3F) S i n c e Nb 2F 9(S0 3F) i s not i n d e f i n i t e l y s t a b l e and w i l l d i s p r o p o r t i o n a t e on long s t a n d i n g i n t o s o l i d [ N b F 5 ] 4 and, presumably, l i q u i d N bF 4(S0 3F), i t may be suspected t h a t a s o l u t i o n of s o l i d NbF 5 and l i q u i d NbF 4(S0 3F) would form i n i t i a l l y , from which s o l i d ( N b F 5 ) 4 e v e n t u a l l y c r y s t a l l i z e s . T h e r e f o r e , a comparison of the v i b r a t i o n a l s p e c t r a of 57 a 1600 1400 1200 1000 800 600 400 200 V (cm"1) 1600 1400 1200 1000 8 0 0 6 0 0 4 0 0 2 0 0 AV (cm*1) F i g u r e 3-1 I n f r a r e d (a) and Raman (Jb) S p e c t r a of Nb 2F 9(S0 3F) 58 Nb 2F 9(S03F), NbF 4(S0 3F) and molten NbF 5 A U J- i s necessary t o d i s p e l the ambiguity. T h i s comparison was made p r i m a r i l y u s i n g the Raman s p e c t r a . An a d d i t i o n a l comparison w i t h S b 2 F 9 ( S 0 3 F ) and SbF 5 i s made u s i n g t h e i r r e p o r t e d Raman s p e c t r a (Table 3 - 3 ) . 8 4 , 1 0 Although the same c o o r d i n a t i o n mode of the f l u o r o s u l f a t e group e x i s t s i n both Nb(V) f l u o r i d e f l u o r o s u l f a t e s , NbF 4(S0 3F) seems t o show a more complex s p e c t r a l p a t t e r n i n the S-0 s t r e t c h i n g r e g i o n . In a d d i t i o n t o the s h i f t o f the S-0 s t r e t c h i n g band from 1147 cm - 1 f o r Nb 2F 9(S0 3F) t o 1180 cm - 1 f o r N b F 4 ( S 0 3 F ) , two weak shou l d e r s appear near the S-0 s t r e t c h i n g band a t 1108 c m - 1 i n the Raman spectrum of N b F 4 ( S 0 3 F ) . N e i t h e r shoulder i s d e t e c t a b l e i n N b 2 F 9 ( S 0 3 F ) . NbF 5, Nb 2F 9(S0 3F) and NbF 4(S0 3F) show s i m i l a r s p e c t r a l p a t t e r n s i n t h e Nb-F s t r e t c h i n g r e g i o n . However, the band a t 767 cm - 1 f o r NbF 5 s h i f t s t o 758 cm - 1 f o r Nb 2F 9(S0 3F) and f u r t h e r down t o 748 cm - 1 f o r N b F 4 ( S 0 3 F ) . Another d i f f e r e n c e i s observed i n the IR s p e c t r a where the r e l a t i v e i n t e n s i t y of an IR band a t «510 cm - 1 i s reduced from Nb 2F 9(S0 3F) t o NbF 4(S0 3F) and i s absent i n the IR spectrum of N b F 3 ( S 0 3 F ) 2 . A l l these d i f f e r e n c e s i n the s p e c t r a suggest t h a t Nb 2F 9(S0 3F) i s a genuine compound and not a mixture of NbF 5 and N b F 4 ( S 0 3 F ) . Nb 2F 9(S0 3F) has a v e r y s i m i l a r Raman spectrum t o t h a t of the p r e v i o u s l y c h a r a c t e r i z e d S b 2 F 9 ( S 0 3 F ) . 8 4 The v i b r a t i o n a l f r e q u e n c i e s f o r N b 2 F 9 ( S 0 3 F ) , a l o n g w i t h t h e i r p o s s i b l e assignments, are l i s t e d i n T a b l e 3-4. In i d e n t i f y i n g the c o o r d i n a t i o n modes of the f l u o r o s u l f a t e group, the most i n f o r m a t i v e r e g i o n i n the v i b r a t i o n a l s p e c t r a i s the 59 T a b l e 3 - 3 Raman Bands f o r Itt^FgCSC^F), Molten NbF 5, NbF 4(S0 3F), S b 2 F 9 ( S 0 3 F ) and SbF 5 Nb 2F 9 (S0 3F) NbF 5(molten) ref.100 NbF 4 (S0 3F) S b 2 F 9 ( S 0 3 F ) ref.84 SbF 5 ref.100 A v ( c m - 1 ) A v ( c m - 1 ) Av ( c m - 1 ) A v ( c m - 1 ) A v ( c m - 1 ) 1408 wm 1408 wm 1430 m (p) 1147 wm,b 1180 1128 w, vb w, sh 1103 m (p) 1108 1092 ms (p) w, sh 1068 m (p) 878 ms (p) 878 ra (p) 898 s (P) 758 vs (p) 767 VS (p) 748 vs (p) 710 vs (p) 718 s (P) 710 w, sh 726 w 718 w, sh 700 s, sh 678 s (P) 683 s (p) 678 s (P) 661 vs (p) 670 s (P) 630 vw 628 vw 606 w 600 vw 600 vw 590 vw 568 w 565 472 w w 556 w (dp) 425 w 428 w 434 m (p) 315 vw 271 s 302 w 349 w 254 s (dp) 253 s (dp) 252 ra (dp) 245 s 2 68 mw 231 mw 200 sh(dp) 226 ms(dp) 200 wm(dp) 201 ms 189 mw 130 vw 136 w 130 w 129 w, sh 140 vw 116 VW a b b r e v i a t i o n s : ( a l s o a p p l i c a b l e t o a l l the f o i l i n g v i b r a t i o n a l data t a b l e s ) w=weak, m=medium, s=strong, b=broad v=very, sh=shoulder, dp=depolarized, p=polarized 60 T a b l e 3-4 Assignment of V i b r a t i o n a l Bands f o r Itt^FgtSG^F) I n f r a r e d Raman Approximate v(cm-1) Av(cm-1) Assignment 1400 vs 1408 wm v(S-O) 1170 vs, vb 1147 wm,b v a s(S0 2) 1092 s 1103 m (P) vsym( S 02) 970 w, sh 873 s 878 ms (P) v(S-F) 760 w, sh 758 vs (P) 705 vs, b 710 w, sh >• v(Nb-F) 680 w, sh 678 s (P) 625 vw, sh 630 vw 600 w 600 vw > <S(S03F) 570 m 568 w 508 s 425 w v(Nb-F-Nb) S(S03) 254 200 130 s sh vw (dp) (dp) > T(S03F) + T(NbFnOm) S0 3F s t r e t c h i n g range (1500-700 cm"1 > • A c c o r d i n g t o c h a r a c t e r i s t i c v i b r a t i o n a l f r e q u e n c i e s f o r d i f f e r e n t c o o r d i n a t i o n modes of f l u o r o s u l f a t e group ( F i g . 1-1), the bands a t 1408, 1147 and 1103 cm - 1 are a s s i g n e d t o S0 3 s t r e t c h e s , and the band a t 878 cm - 1 t o the S-F s t r e t c h i n g v i b r a t i o n . T h e i r occurrence i n t h i s r e g i o n c l e a r l y i n d i c a t e s the presence of a b i d e n t a t e , presumably 61 b r i d g i n g , f l u o r o s u l f a t e group i n Nb 2F9(S0 3F). A weak band a t «630 cm - 1 i n both Raman and IR a l s o supports t h i s c o n c l u s i o n , as i s d i s c u s s e d i n the g e n e r a l i n t r o d u c t i o n . The bands a t t r i b u t e d t o t e r m i n a l Nb-F s t r e t c h i n g v i b r a t i o n s s h o u l d appear i n a lower frequency range (800-650 cm - 1) . A p r e v i o u s l y r e p o r t e d study on the v i b r a t i o n a l s p e c t r a of N b F 5 1 0 0 i s a good guide i n the assignment of bands due t o Nb-F s t r e t c h i n g v i b r a t i o n s . A symmetry c o n s i d e r a t i o n and a normal-coordinate a n a l y s i s f o r c r y s t a l l i n e t e t r a m e r i c [ N b F 5 ] 4 p r e d i c t s 23 Raman and 14 IR fundamentals, however, onl y 12 Raman and 12 IR bands are observed f o r N b F 5 . 1 0 0 For s o l i d NbF 5, Raman bands a t 766, 752, 716, 668, 656 cm - 1 are a t t r i b u t e d t o t e r m i n a l v(Nb-F) whereas on l y two p o l a r i z e d Raman bands a t 763 and 680 cm - 1 a r e c l e a r l y observed i n t h i s r e g i o n f o r l i q u i d N b F 5 . 1 0 0 These bands may be i n t e r p r e t e d as the two A± modes expected f o r l o c a l C 2 v symmetry of c i s - F - b r i d g e d t e r m i n a l NbF 4 moiety. S i m i l a r l y , two s t r o n g l y p o l a r i z e d Raman bands a t 758 and 678 cm - 1, and a v e r y weak Raman band a t about 700 cm - 1 t h a t i s a l s o p r e s e n t i n the IR spectrum, are observed f o r Nb 2F 9(S0 3F) i n t h i s r e g i o n . T h i s i m p l i e s t h a t t h e c i s - b r i d g e d t e r m i n a l NbF 4 moiety i n NbF 5 i s most l i k e l y r e t a i n e d i n N b 2 F 9 ( S 0 3 F ) . I t i s hence assumed t h a t i n Nb 2F 9(S0 3F) some of b r i d g i n g f l u o r i n e s are r e p l a c e d by f l u o r o s u l f a t e groups w i t h both the b r i d g i n g l i g a n d s i n c i s c o n f i g u r a t i o n . T h i s assumption i s supported by the f a c t t h a t t e r m i n a l monodentate f l u o r o s u l f a t e groups are not observed. In a d d i t i o n , as seen p r e v i o u s l y i n t e r n a r y f l u o r i d e f l u o r o s u l f a t e s , b r i d g i n g by the 62 f l u o r o s u l f a t e group takes precedence over b r i d g i n g by f l u o r i n e . The deformation bands of the f l u o r o s u l f a t e group and the s t r e t c h i n g bands of the f l u o r i n e - b r i d g e d Nb-F-Nb group are expected t o be found i n the lower r e g i o n of the spectrum (650-400 cm - 1) . P r e v i o u s l y r e p o r t e d Raman s p e c t r a on s o l i d o r l i q u i d samples of NbF 5 do not show any d e t e c t a b l e bands i n t h e range 300-650 cm - 1. However, t h i s enables us t o a s s i g n the observed Raman bands a t 630, 600, 568 and 425 cm - 1 t o the deformation bands of the f l u o r o s u l f a t e group. While the normal-coordinate a n a l y s i s f o r t e t r a m e r i c [ N b F 5 ] 4 p r e d i c t s f o u r bands a t 109, 488, 106 and 478 cm - 1 due t o the Nb-F-Nb b r i d g e , o n l y two bands a t 514 and 479 cm-1 are observed i n the IR spectrum of s o l i d NbF 5. They are, however, not found i n the Raman s p e c t r a of e i t h e r s o l i d or l i q u i d N b F 5 . 1 0 0 No IR spectrum of l i q u i d NbF 5 has been r e p o r t e d , p r o b a b l y due t o the h i g h r e a c t i v i t y of the compound. N e v e r t h e l e s s , we t e n t a t i v e l y a s s i g n an IR band a t 508 cm - 1 t o the s t r e t c h i n g v i b r a t i o n of the Nb-F-Nb b r i d g e s . The decrease i n the r e l a t i v e band i n t e n s i t y from Nb 2F 9(S0 3F) t o NbF 4(S0 3F) and the absence of the band i n the IR spectrum of N b F 3 ( S 0 3 F ) 2 support t h i s assignment. Hence i t appears t h a t the b r i d g i n g l i g a n d s i n Nb 2F 9(S0 3F) are both f l u o r i d e and f l u o r o s u l f a t e . The t o r s i o n bands of the S0 3F group and the s k e l e t a l v i b r a t i o n s of the NbF mO n e n t i t y should appear i n the r e g i o n below 400 cm - 1. However, the AgBr windows used f o r the IR spectrum are not t r a n s p a r e n t i n t h i s r e g i o n , and o n l y t h r e e broad Raman bands are observed a t 254, 200, 130 c m - 1 f o r N b 2 F g ( S 0 3 F ) . Hence a 63 d e t a i l e d unambiguous assignment of these bands i s not p o s s i b l e . 3.3.2.2 V i b r a t i o n a l Spectra of NbF 4(S0 3F) and NbF 3(S0 3F) 2 Although t h e r e are some d i f f e r e n c e s , the v i b r a t i o n a l s p e c t r a of NbF 4(S0 3F) are comparable t o those of N b 2 F 9 ( S 0 3 F ) . In the IR spectrum of NbF 4 ( S 0 3F), two bands are seen a t 1190 and 1130 cm - 1, whereas o n l y one S-0 s t r e t c h i n g band i s observed a t 1175 cm - 1 f o r N b 2 F 9 ( S 0 3 F ) . These two bands are a l s o d e t e c t a b l e i n the Raman spectrum of N b F 4 ( S 0 3 F ) . V i b r a t i o n a l c o u p l i n g o f the two c i s -b r i d g i n g f l u o r o s u l f a t e groups c o o r d i n a t e d t o niobium i s seen as a probable cause f o r the occurrence of two bands i n t h i s r e g i o n . N e v e r t h e l e s s , the f l u o r o s u l f a t e group remains i n the b r i d g i n g b i d e n t a t e c o o r d i n a t i o n mode i n NbF 4 ( S 0 3F). The v(Nb-F) bands d i s p l a y a s i m i l a r p a t t e r n i n terms of both band p o s i t i o n and band shape. However, when compared t o the S0 3F s t r e t c h i n g bands, the r e l a t i v e i n t e n s i t y of v(Nb-F) bands i s l e s s i n NbF 4 ( S 0 3F) than i n N b 2 F 9 ( S 0 3 F ) . A s t r o n g IR band a t a510 cm - 1 observed f o r Nb 2F 9 ( S 0 3F) and as s i g n e d t o v(Nb-F-Nb) i s weak i n the IR spectrum of N b F 4 ( S 0 3 F ) . A p l a u s i b l e i n t e r p r e t a t i o n i s t h a t most of the b r i d g i n g f l u o r i d e s are r e p l a c e d by b r i d g i n g b i d e n t a t e f l u o r o s u l f a t e s . The s i x Raman bands i n the S-0 s t r e t c h i n g r e g i o n and the two bands i n the S-F s t r e t c h i n g r e g i o n of the v i b r a t i o n a l s p e c t r a of N b F 3 ( S 0 3 F ) 2 i n d i c a t e two d i f f e r e n t c o o r d i n a t i o n modes f o r the f l u o r o s u l f a t e group. The Raman bands a t 1408, 1114, 1098 and 878 cm - 1 are a t t r i b u t e d t o the b i d e n t a t e b r i d g i n g f l u o r o s u l f a t e 64 F i g u r e 3-2 I n f r a r e d (a) and Raman (Jb) S p e c t r a of N b F 4 ( S 0 3 F ) 65 66 Tabl e 3 - 5 Assignments of V i b r a t i o n a l Bands of NbF 4(S0 3F) and N b F 3 ( S 0 3 F ) 2 NbF 4 (S0 3F) NbF 3 (S0 3F) 2 Approximate I n f r a r e d Raman I n f r a r e d Raman Assignment v ( c m - 1 ) Av(cm ^) v ( c m _ 1 ) A v ( c m - 1 ) 1430 w, sh 1448 w M , v a s ( S 0 2 ) 1400 vs 1408 wm 1400 vs 1408 m (P) B,v(S-O) 1232 m, sh 1235 s (P) M , V g y m ( S 0 2 ) 1190 vs 1180 w, vb 1186 s 1150 vw, b B , v a s ( S 0 2 ) 1130 vs ,b 1128 w, sh 1130 s 1108 ms (p) 1114 s (P) B , v s y m ( S 0 2 ) 1090 s 1092 w, sh 1085 s 1090 m, sh 955 vw 955 s 958 m, b M, v(S-O) 870 s 878 m (p) 878 s 878 ms (P) B , v(S-F) 838 vw, sh 840 m (P) M,v(S-F) 740 m, sh 748 vs (p) 740 m, sh 738 vs (P) 700 vs ,b 718 w, sh 702 vs 710 w, sh > v(Nb-F) 675 w, sh 678 s (P) 670 vw, sh 680 ms (P) 630 vw , sh 628 vw 630 w 638 ms (P) 600 w 600 vw 600 w 602 vw > 5(S0 3F) 570 m 565 w 565 s 560 w 505 w v(Nb-F-Nb) 450 w 428 w 445 m 420 275 w w, sh 6*(S03F) 252 200 m (dp) wm (dp) 252 200 s (dp) wm (dp) 1 r(NbF mO n) > + J T(S0 3F) 130 w 130 vw J 67 group, by analogy t o N b 2 F 9 ( S 0 3 F ) . The ot h e r f o u r bands a t 1438, 1240, 1080 and 840 cm - 1 are a t t r i b u t e d t o a monodentate t e r m i n a l f l u o r o s u l f a t e group. I t i s noted t h a t the appearance of a t e r m i n a l f l u o r o s u l f a t e group changes the t e r m i n a l NbF 4 moiety i n NbF 4(S0 3F) i n t o a t e r m i n a l c i s - b r i d g e d NbF 3-moiety i n N b F 3 ( S 0 3 F ) 2 , r e s u l t i n g i n an approximately o c t a h e d r a l c o o r d i n a t i o n f o r niobium. The t h r e e Nb-F s t r e t c h i n g bands may i n d i c a t e e i t h e r a mer arrangement f o r the t e r m i n a l NbF 3-moiety or a mixture of mer and fac isomers w i t h some band o v e r l a p s . In the lower frequency range, the r e l a t i v e i n t e n s i t i e s of the deforma t i o n bands are s t r o n g e r f o r N b F 3 ( S 0 3 F ) 2 than f o r NbF 4(S0 3F) . I t i s very d i f f i c u l t t o d i s t i n g u i s h the S0 3F deformation bands c o r r e s p o n d i n g t o the two c o o r d i n a t i o n modes s i n c e an o v e r l a p of these deformation bands i s apparent. 3.3.2.3 V i b r a t i o n a l S p e c t r a of TaF 4(S0 3F) and T a F 3 ( S 0 3 F ) 2 The v i b r a t i o n a l s p e c t r a of TaF 4(S0 3F) and T a F 3 ( S 0 3 F ) 2 are q u i t e s i m i l a r t o those of t h e i r niobium analogs, i n terms of both band p o s i t i o n s and r e l a t i v e i n t e n s i t i e s . Frequency s h i f t s are expected f o r some bands. For example, v(Ta-F) bands are found a t s l i g h t l y lower f r e q u e n c i e s than the c o r r e s p o n d i n g v(Nb-F) bands. Such s h i f t s have been observed f o r the analogous p e n t a f l u o r i d e s and h e x a f l u o r o m e t a l l a t e s of niobium and t a n t a l u m . 3 7 A d e t a i l e d d i s c u s s i o n of the assignments of the v i b r a t i o n a l bands i s deemed unnecessary a t t h i s p o i n t , s i n c e the arguments i n v o l v e d are s i m i l a r t o those presented i n the p r e c e d i n g s e c t i o n s . 68 _ | 1600 1400 I2CX5 icXX) 855 660 4 0 0 2CXD V ( c m - 1 ) b 1600 (400 1200 1000 800 600 400 200 Av (cm"1) F i g u r e 3-4 I n f r a r e d (a) and Raman (b) S p e c t r a of TaF 4 ( S 0 3F) 69 1600 1400 1200 1000 800 600 400 200 •o (cm*1) 1600 1400 1200 1000 800 600 400 200 A*o (cm*1) F i g u r e 3-5 I n f r a r e d (a) and Raman (Jb) S p e c t r a of T a F 3 ( S 0 3 F ) 2 70 T a b l e 3-6 Assignments of V i b r a t i o n a l Bands of T a F 4 ( S 0 3 F ) and T a F 3 ( S 0 3 F ) 2 T a F 4 ( S 0 3 F ) T a F 3 ( S 0 3 F ) 2 Approximate I n f r a r e d Raman I n f r a r e d Raman Assignment v ( c m - 1 ) AV( cm" 1 ) V ( c m Av( c m - 1 ) 1440 sh 1448 w M,v a s(S0 2) 1410 vs 1410 wm 1410 1325 vs,b w, sh 1415 m (p) B,v(S-O) 1235 s 1235 s (p) M ,v s y m(S0 2) 1200 s 1180 vw, vb 1170 m 1190 vw B, v a g ( S 0 2 ) 1130 s 1130 vw, sh 1116 s (p) 1100 m 1115 s (p) B , v s y m ( S 0 2 ) 1098 s 1098 w, sh 1040 m 1090 sh 980 vw 980 s 980 w, b M,V(S-O) 880 vs 885 s (P) 880 m, sh 886 s (p) B,v(S-F) 840 s 840 ms (p) M,V(S-F) 735 m,sh 742 vs (p) 730 m, sh 740 vs (p) 700 m,sh 710 w, sh 710 w, sh » v(Ta-F) 662 s 690 mw (p) 680 s 685 w, sh 635 w 632 w 630 w 640 s (p) 1 610 w 608 vw 600 vw " <S (SO3F) 575 m 568 w 565 s 560 m (dp) 520 w v(Ta-F-Ta) 455 W 432 w 440 m 430 280 wm m, sh (P) 6*(S03F) 1 250 s (dp)' 240 s (dp) I T(NbF mO n) 180 vw, sh 180 w T(SO3F) 130 vw 130 vw 71 3.3.2.4 Comparison of F l u o r i d e F l u o r o s u l f a t e s of Group 5 and Group 15 Elements The s i m i l a r i t i e s between Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s and the corres p o n d i n g As(V) and Sb(V) compounds are r a t h e r s t r i k i n g , as seen from the c o r r e l a t i o n o f v i b r a t i o n a l f r e q u e n c i e s f o r NbF 3(S0 3F)2, T a F 3 ( S 0 3 F ) 2 , A s F 3 ( S 0 3 F ) 2 8 5 and SbF 3(S0 3F) 2 8 4 (Table 3-7). The s i m i l a r i t y of t h e i r p h y s i c a l appearance and t h e i r v i b r a t i o n a l s p e c t r a i s not unexpected i f one c o n s i d e r s the p o s i t i o n s of these elements i n the p e r i o d i c t a b l e . I t can be concluded a t t h i s stage t h a t the l i q u i d niobium(V) and tantalum(V) f l u o r i d e f l u o r o s u l f a t e s s y n t h e s i z e d i n t h i s study have p o l y m e r i c s t r u c t u r e s w i t h an approximately o c t a h e d r a l c o o r d i n a t i o n around the metal c e n t e r s , which are a s s o c i a t e d over b i d e n t a t e b r i d g i n g S0 3F groups, and b r i d g i n g f l u o r i n e as w e l l i n the case of compounds wi t h lower S0 3F content, as suggested p r e v i o u s l y f o r the cor r e s p o n d i n g a r s e n i c 8 5 and antimony compounds . 3.2.2.5 Comparison of L i q u i d and S o l i d M F n(S0 3F) 5_ n (M=Nb, Ta) Two bands were observed a t 767 and 1057 cm - 1 i n a p o o r l y r e s o l v e d IR spectrum of MF3(S03F)2 (M=Nb, Ta) and were a t t r i b u t e d t o v(S-F) and v(S-O) bands r e s p e c t i v e l y . 9 8 We b e l i e v e t h a t an assignment of the former as v(M-F) (M=Nb, Ta), and the l a t t e r as v(S-O) i s more a p p r o p r i a t e . However, n e i t h e r of our compounds N b F 3 ( S 0 3 F ) 2 nor T a F 3 ( S 0 3 F ) 2 e x h i b i t bands a t these f r e q u e n c i e s . A 72 Ta b l e 3-7 C o r r e l a t i o n of Raman Bands of F l u o r o s u l f a t e Group f o r M F 3 ( S 0 3 F ) 2 (M=Nb, Ta, As and Sb) N b F 3 ( S 0 3 F ) 2 T a F 3 ( S 0 3 F ) 2 A S F 3 ( S 0 3 F ) 2 ref.85 S b F 3 ( S 0 3 F ) 2 ref.84 Approximate Assignment 1448 w 1448 w 1460 W, sh 1461 W M, v a s ( S 0 2 ) 1408m (p) 1415 m (p) 1426 m (P) 1424 m (P) B, v(S-O) 1395 m 1235 s (p) 1235 s (P) 1237 s (P) 1242 s (P) M, Vsym(S02) 1150 vw, b 1190 vw B, V a s ( S 0 2 ) 1114 s (p) 1115 s (P) 1069 ItV (P) 1078 m (P) B, v Sym ( S 0 2 ) 1090 w,sh 1090 w, sh 958 m,b 980 w, b 1030 w 1030 w M, V(S-O) 878 ms (p) 886 s (P) 891 ms (P) 885 s (P) B, V(S-F) 840 m (p) 840 ms (p) 863 m 861 s, sh M, V(S-F) 738 vs (p) 740 vs (p) 750 s (P) 700 s (P) 710 w,sh 710 w, sh > V(M-F) 680 ms (p) 685 w, sh 670 vs (P) 650 vs (P) J 638 ms (p) 640 s (P) 610 vw 635 s, sh 602 vw 600 vw 590 wm (P) 585 W (P) 560 w 560 m (dp) 550 m (dp) 556 w (dp) > <S(S03F) 420 w 430 wm 455 410 377 ms w w (P) 442 s 413 w (P) 275 w, sh 280 m,sh(p) 293 m, sh 299 m, sh > 252 s (dp) 240 s (dp) 268 245 m, b 266 m, 251 s sh > r(MFmOn) + 200 wm (dp) 180 w r(S0 3F) 130 vw 130 vw 125 73 comparison t o our s p e c t r a based on o n l y two r e p o r t e d bands i s not u s e f u l . A study i n v o l v i n g the s y n t h e s i s and v i b r a t i o n a l s p e c t r a of s o l i d T a F 4 ( S 0 3 F ) and N b F 2 ( S 0 3 F ) 3 has been r e p o r t e d . 9 7 However, i n t h i s work, N b F 2 ( S 0 3 F ) 3 c o u l d not be o b t a i n e d i n s u f f i c i e n t l y pure form t o enable a d e t a i l e d comparison of t h e v i b r a t i o n a l s p e c t r a . I t may however be mentioned t h a t the r e p o r t e d v i b r a t i o n a l s p e c t r a f o r s o l i d N b F 2 ( S 0 3 F ) 3 shows some s i m i l a r i t y t o those of U F 2 ( S 0 3 F ) 3 , 1 0 2 which i n t u r n suggests t h a t niobium, j u s t l i k e uranium, i s capable of expanding i t s c o o r d i n a t i o n number beyond s i x , r e s u l t i n g i n a m a t e r i a l t h a t i s h i g h l y p o l y m e r i c i n nature. The l a c k of s o l u b i l i t y of s o l i d N b F 2 ( S 0 3 F ) 3 i n HS0 3F i s c o n s i s t e n t w i t h t h i s c o n c l u s i o n . In our s y n t h e s i s , which does not i n v o l v e HS0 3F, a l l products are c o l o r l e s s v i s c o u s l i q u i d s , except f o r the presence of a very s m a l l amount of an u n i d e n t i f i e d white s o l i d s eparated from the main product by f i l t r a t i o n . T h e r e f o r e , the p o s s i b i l i t y t h a t d i f f e r e n t s t r u c t u r a l forms might be p r e s e n t f o r the same composition cannot be r u l e d out, based on the a b i l i t y of niobium and tantalum t o e x h i b i t h i g h e r c o o r d i n a t i o n numbers. T h i s , coupled w i t h a p o s s i b l e h i g h e r d e n t i c i t y of the f l u o r o s u l f a t e group, can r e s u l t i n p o l y m e r i c s o l i d s . C o n s i d e r a b l e d i f f e r e n c e s are apparent i n the v i b r a t i o n a l s p e c t r a of s o l i d and l i q u i d T aF 4(S0 3F) (Table 3-8). T h i s comparison s u f f e r s from the r e p o r t e d o b s e r v a t i o n t h a t both IR and Raman s p e c t r a f o r the s o l i d are not o p t i m a l . Some bands which 74 Tabl e 3-8 Comparison of V i b r a t i o n a l S p e c t r a of S o l i d and L i q u i d T a F 4 ( S 0 3 F ) S o l i d T a F 4 ( S 0 3 F ) ref.97 L i q u i d T a F 4 ( S 0 3 F ) Approximate Assignment I n f r a r e d Raman I n f r a r e d Raman V ( c m - 1 ) A V ( c m - 1 ) V ( c m - 1 ) A\V(cm _ 1 ) 1403 m 1410 V S 1410 wm 1180 s 1191 w 1200 s 1180 vw, vb • 1130 vw, sh 1112 s,b 1123 s 1130 s 1116 s (P) > V(S0 3) 1105w,sh 1098 s 1098 w, sh 1075 s,sh 1083 m 980 vw 895 m, sh 880 vs 885 s (P) v(S-F) 879 m, sh J 748 vs 735 m, sh 742 vs (P) 733 m 725 m 700 m, sh 710 w, sh 708 m 716 m 684 m 691 m )>. v(Ta-F) 671 m 663 m, sh 660 vw, sh 662 s 690 mw (P) > 644 m 648 w 635 w 632 w -611 w 620 vw, b 610 575 520 w m w 608 568 vw w U <S(S03F) v(Ta-F-Ta) 486 w 488 w 464 vw, sh 470 308 283 268 w w m, sh m 455 w 432 w 6-(S03F) 241 w 250 s (dp) T(NbF mO n) 228 w > + 220 w, sh 180 vw, sh T(S0 3F) 200 w 130 vw 130 vw 75 are weak f o r l i q u i d T a F 4 ( S 0 3 F ) , l i k e the S0 3F deformation modes a t 500-600 cm - 1 and a S-0 s t r e t c h i n g band a t «1400 cm - 1, are not observed i n the Raman spectrum of the s o l i d . A l s o , f o r s o l i d T a F 4 ( S 0 3 F ) , v(S-F) i s found o n l y i n the IR spectrum. With these problems o n l y a v e r y c u r s o r y comparison i s p o s s i b l e . Three d i f f e r e n c e s emerge: (a) For s o l i d T a F 4 ( S 0 3 F ) , the S0 3F s t r e t c h i n g r e g i o n i s more complex and i s i n t e r p r e t e d as due t o the presence of both b i d e n t a t e and t r i d e n t a t e S0 3F groups w i t h two d i f f e r e n t v(S-F) observed i n the IR spectrum. 9 7 (b) The Ta-F s t r e t c h i n g r e g i o n (650-750 cm - 1) i s v e r y complex f o r s o l i d T a F 4 ( S 0 3 F ) . The w e l l r e s o l v e d Raman spectrum has s i x bands and the IR spectrum has f i v e bands i n t h i s r e g i o n , w h i l e l i q u i d T aF 4(S0 3F) has o n l y t h r e e bands i n both IR and Raman s p e c t r a . T h i s suggests e i t h e r e x c l u s i v e v i b r a t i o n a l c o u p l i n g i n the s o l i d or two s l i g h t l y d i f f e r e n t t e r m i n a l T a F 4 - m o i e t i e s i n a p o l y m e r i c s t r u c t u r e . (c) Weak bands a t 464 cm - 1 and 488 cm - 1 are o n l y observed f o r the s o l i d . The o r i g i n of these bands i s u n c l e a r and the i n t e r p r e t a t i o n of these bands as s h i f t e d S0 3F deformation bands or as b r i d g i n g Ta-F-Ta v i b r a t i o n s i s p l a u s i b l e . There are some s i m i l a r i t i e s between the two compounds and t h i s i s not unexpected s i n c e a b i d e n t a t e b r i d g i n g S0 3F group i s common t o both. However, these s i m i l a r i t i e s should not be over 76 emphasized because the compounds d i f f e r i n t h e i r p h y s i c a l p r o p e r t i e s . 3.2.3 NMR Spectra of Nb(V) and Ta(V) F l u o r i d e f l u o r o s u l f a t e s 3.2.3.1 1 9 F NMR Spectra of Nb(V) and Ta(V) F l u o r i d e f l u o r o s u l f a t e s The room temperature 19F-NMR s p e c t r a of N b 2 F 9 ( S 0 3 F ) , NbF 4(S0 3F) and N b F 3 ( S 0 3 F ) 2 o b t a i n e d u s i n g CFC1 3 as an e x t e r n a l r e f e r e n c e are shown i n F i g . 3-6. Only one s i g n a l a t t r i b u t e d t o the f l u o r i n e on s u l f u r of the f l u o r o s u l f a t e groups i s observed i n the 39 ppm range, although two c o o r d i n a t i o n modes are suggested f o r N b F 3 ( S 0 3 F ) 2 based on the v i b r a t i o n a l s p e c t r a . T h i s suggests the presence of a chemical exchange process between monodentate t e r m i n a l and b i d e n t a t e b r i d g i n g f l u o r o s u l f a t e groups. The broad bands a t «210-250 ppm are a t t r i b u t e d t o the f l u o r i n e s connected d i r e c t l y t o the niobium atoms. The broadness of the s i g n a l i s p r o b a b l y due t o c o u p l i n g w i t h the quadrupolar 9 3Nb n u c l e i ( s p i n 9/2, Q=-0.22xl0~ 2 8 cm 2). D e t a i l e d s t r u c t u r a l i n f o r m a t i o n i s u n o b t a i n a b l e from the 1 9 F NMR s p e c t r a of these compounds s i n c e f i n e s t r u c t u r e cannot be observed on a l l the s i g n a l s . The 1 9 F NMR s p e c t r a of TaF 4(S0 3F) and T a F 3 ( S 0 3 F ) 2 are i l l u s t r a t e d i n F i g . 3-7. A s i g n a l a t 38 ppm and two broad bands a t 145 and 178 ppm are observed f o r T a F 4 ( S 0 3 F ) . For T a F 3 ( S 0 3 F ) 2 , one s i g n a l a t 39 ppm and t h r e e broad bands a t 191, 183 and 148 ppm. The s i g n a l s a t 145 and 178 ppm f o r T a F 4 ( S 0 3 F ) and 148, 183, 193 ppm f o r T a F 3 ( S 0 3 F ) 2 are a t t r i b u t e d t o the f l u o r i n e s N b 2 F 9 ( S 0 3 F ) ro ro CO - f — T r — i , , 1 , 1 . 1 1 1 1 1 1 1 1 1 1 1 1—1 250 200 150 100 50 -50 PPM N b F 4 ( S 0 3 F ) oo ro ro cu i ' ' 1 i 250 200 150 100 •—r~ 50 0 -50 PPM N b F 3 ( S 0 3 F ) ; CD ro • • • i ' • • ' i ' ' ' • ' • • ' • i 1 ' ' ' i • • ' • i ' ' • • i 1 250 200 1^0 100 50 0 -50 PPM F i g u r e 3 - 6 1 9 F NMR Sp e c t r a of Nb(V) F l u o r i d e F l u o r o s u l f a t e s 78 TaF4(S03F) LO oo I r 180 160 140 A . CO ro | l l l l | l l l i | l i i i | i i i i | l l l i ) i i i i | i i l i | i l i r | i i i i | l i i l | i i i i | l l l l | i l i l | i l l l | i i i i | i i l i | i l i i | i i i i | l l l l | i i i i | l i l i | i i i i | i i i H ' i i i | i n n i i n i 240 200 160 120 80 40 0 PPM TaF 3(S0 3F) 2 180 • ""| " i " 1 1 i' 240 200 160 _A A_ 140 CD ro T|'ll'[ I I I | I I I 1 | • • ' ' | ' ' | " I1"' 120 80 40 'i '"i 0 PPM iiii i i | iniii i i 160 F i g u r e 3-7 1 9 F NMR Spe c t r a of Ta(V) F l u o r i d e F l u o r o s u l f a t e s 79 connected d i r e c t l y t o tantalum, and the s i g n a l s a t 38 and 39 ppm t o the f l u o r i n e s of the S 0 3F groups. The s p e c t r a l p a t t e r n s f o r f l u o r i n e s d i r e c t l y connected t o the metal c e n t e r d i s p l a y more bands than those of Nb(V) f l u o r i d e f l u o r o s u l f a t e s , but d e t a i l e d s t r u c t u r a l c o n c l u s i o n s cannot be s a f e l y d e r i v e d from these s p e c t r a s i n c e no s p i n - s p i n c o u p l i n g can be observed f o r e i t h e r T a F 4 ( S 0 3F) or T a F 3 ( S 0 3 F ) 2 . 3.2.3.2 Attempted 9 3Nb NMR 9 3Nb ( s p i n 9/2) i s 100% abundant and has a n a t u r a l abundance r e c e p t i v i t y of 2736 ( 1 3C=1), ra n k i n g t h i r d a f t e r 1H and 1 9 F among a l l the elements i n r e c e p t i v i t y . Most 9 3Nb NMR has been done on niobium h a l i d e or pseudo-halide d e r i v a t i v e s . 1 0 4 R e c e n t l y The 9 3Nb chemical s h i f t s f o r Nb ( S 0 3F) 5 ( s o l v ), a 1 : 1 mixture of NbF 5 and Nb (S0 3F) 5 ( s o l v ), and NbF 5 i n H S 0 3F were r e p o r t e d t o be - 8 0 , 20 and 60 ppm r e s p e c t i v e l y , r e l a t i v e t o L i N b F 6 , w i t h r a t h e r broad bands w i t h width at h a l f h e i g h t of 1 6 , 0 0 0 t o 2 7 , 0 0 0 H z . 1 0 5 A c c o r d i n g l y , we expected t h a t the 9 3Nb chemical s h i f t s f o r Nb(V) f l u o r i d e f l u o r o s u l f a t e s would not be f a r beyond these v a l u e s . However, no s i g n a l was d e t e c t e d i n the 9 3Nb NMR f o r l i q u i d NbF 4 ( S 0 3F) and N b F 3 ( S 0 3 F ) 2 a t room temperature i n t h e range from - 4 0 0 t o 600 ppm r e l a t i v e t o Cs[NbF 6] i n propylene carbonate. Under the same i n s t r u m e n t a l c o n d i t i o n s , the r e f e r e n c e g i v e s r i s e t o a s e p t e t p a t t e r n c l e a r l y r e f l e c t i n g the c o u p l i n g of 9 3Nb t o s i x e q u i v a l e n t o c t a h e d r a l l y c o o r d i n a t e d 1 9 F ( s p i n 1/2). I t c o u l d be argued t h a t 9 3Nb has a low symmetry environment i n h i g h l y 80 a s s o c i a t e d l i q u i d NbF 4(S0 3F) and N b F 3 ( S 0 3 F ) 2 , i n c o n t r a s t t o t h a t of HS0 3F/Nb(S0 3F) 5(solv) and HS0 3F/NbF 5 systems, where the i n t e r a c t i o n s of the compounds wi t h the s o l v e n t s a re apparent and the f o r m a t i o n of h i g h l y symmetrical s p e c i e s , l i k e [ N b ( S 0 3 F ) 6 ] ~ or NbF 6", i s p o s s i b l e . The f a i l u r e t o observe any s i g n a l i n the r e g i o n i s pro b a b l y due t o the l a r g e quadrupole moment of 9 3Nb, the lower symmetry around 9 3Nb or the a n i s o t r o p y r e s u l t i n g from the v i s c o s i t y o f the compounds. 1 8 1 T a i s 100% abundant and has a d e t e c t i o n r e c e p t i v i t y of 204 r e l a t i v e t o 1 3 C . I t s l a r g e quadrupole moment of 3 x l 0 ~ 2 4 cm2, l a r g e r than t h a t of 9 3Nb, has r e s u l t e d i n s e v e r a l u n s u c c e s s f u l attempts t o o b t a i n i t s NMR s p e c t r a . 1 0 4 The o n l y s u c c e s s f u l 1 8 1 T a NMR spectrum was ob t a i n e d f o r the h i g h l y symmetrical [ T a F 6 ] ~ i o n , i n which a s i g n a l w i t h width a t h a l f h e i g h t of 3 6,000 Hz i s o b s e r v e d . 1 0 4 No 1 8 1 T a NMR was attempted f o r TaF 4(S0 3F) or T a F 3 ( S 0 3 F ) 2 because i t was expected t h a t the s i g n a l w i l l not be observed due t o the r a t h e r low symmetry about tantalum. 81 CHAPTER 4 CONDUCTOMETRIC STUDIES OP Nb(V) AND Ta(V) FLUORIDE FLUOROSULFATES IN FLUOROSULFURIC ACID 4.1 Introduction A v a r i e t y of methods have been a p p l i e d t o study conjugate Bronsted/Lewis s u p e r a c i d s . C o n d u c t i v i t y measurements, Hammett A c i d i t y F u n c t i o n (H 0) d e t e r m i n a t i o n s , e l e c t r o c h e m i c a l techniques, and s e l e c t i v i t y parameter measurements have been used t o rank the r e l a t i v e a c i d s t r e n g t h s of Lewis a c i d s i n Bronsted a c i d s . The Hammett A c i d i t y F u n c t i o n (H 0) p r o v i d e s an a c c e p t a b l e q u a n t i t a t i v e i n d i c a t o r of a c i d i t y i n s u p e r a c i d systems. I t measures the a b i l i t y of an a c i d , or s u p e r a c i d , t o pr o t o n a t e a very weakly b a s i c i n d i c a t o r . I t s r e l i a b i l i t y r e s t s on the proper s e l e c t i o n of a s e r i e s of Hammett i n d i c a t o r s . The measurements of the Hammett A c i d i t y F u n c t i o n can be extended from d i l u t e t o ve r y c o n c e n t r a t e d s o l u t i o n s and have been wi d e l y a p p l i e d i n s u p e r a c i d s t u d i e s . 1 6 ' 2 0 " 2 4 ' 3 1 ' 3 3 ' 3 4 E l e c t r i c a l c o n d u c t i v i t y measurement has been a p p l i e d t o a number of conjugate Bronsted/Lewis s u p e r a c i d s . 1 8 ' 1 9 ' 2 2 ' 2 7 ' 6 5 ' 7 4 ' 7 9 I t i s based on the f a c t t h a t the acidium i o n and the base i o n are the most mobile i o n s i n these p a r t i c u l a r a c i d systems w i t h the conductance i n v o l v i n g a pro t o n t r a n s f e r mechanism, r e s u l t i n g i n a v i r t u a l , r a t h e r than r e a l , i o n movement f o r the acidium and base i o n s . At low Lewis a c i d c o n c e n t r a t i o n s , the e l e c t r i c a l c o n d u c t i v i t y i n s u p e r a c i d systems becomes an approximate measure 82 of the acidium i o n c o n c e n t r a t i o n . I t i s customary t o use a c o n d u c t i v i t y v s . Lewis a c i d c o n c e n t r a t i o n p l o t t o i l l u s t r a t e the d i f f e r e n c e i n acidium i o n c o n c e n t r a t i o n f o r v a r i o u s Lewis a c i d s . The s l o p e of the p l o t i s used as an i n i t i a l measure of the r e l a t i v e s t r e n g t h of the s u p e r a c i d s . C o n d u c t i v i t y measurements are taken a t low Lewis a c i d c o n c e n t r a t i o n s , u s u a l l y below 1 0 _ 1 m o l • k g - 1 , s i n c e the Lewis a c i d i s p r i m a r i l y i n v o l v e d i n the g e n e r a t i o n o f the acidium i o n . At h i g h e r Lewis a c i d c o n c e n t r a t i o n , i n t e r m o l e c u l a r a s s o c i a t i o n of the Lewis a c i d competes wi t h the i n t e r a c t i o n between the Lewis a c i d and the Bronsted a c i d . As a r e s u l t , the p l o t of c o n d u c t i v i t y v s . Lewis a c i d c o n c e n t r a t i o n e x h i b i t s c u r v a t u r e , and i s no lon g e r a t r u e r e f l e c t i o n of the a c i d i t y a t h i g h Lewis a c i d c o n c e n t r a t i o n . Conductometric t i t r a t i o n s e s t a b l i s h whether a s o l u t e i s an a c i d by v e r i f y i n g the f a c t t h a t the c o n d u c t i v i t i e s of s o l u t i o n s of Lewis a c i d s i n Bronsted a c i d s are indeed l a r g e l y due t o the acidium i o n c o n c e n t r a t i o n . The t i t r a t i o n s a l l o w us t o e v a l u a t e the r e l a t i v e a c i d s t r e n g t h s of Lewis a c i d s by comparing the shapes of the curves. The two f e a t u r e s worthy of note are the p o i n t of minimum c o n d u c t i v i t y , and the e q u i v a l e n c e p o i n t , the p o i n t beyond which the c o n d u c t i v i t y r e f l e c t s the base i o n c o n c e n t r a t i o n . For monomeric s u p e r a c i d s , the e q u i v a l e n c e p o i n t w i l l be a t a 1:1 b a s e : a c i d mole r a t i o . For a s t r o n g a c i d l i k e Au ( S 0 3F) 3 i n HS0 3F, the minimum c o n d u c t i v i t y and the e q u i v a l e n c e p o i n t c o i n c i d e , r e s u l t i n g i n sharp t i t r a t i o n c u r v e s . 7 9 For 83 weaker a c i d s , the minimum c o n d u c t i v i t y w i l l occur a t lower b a s e : a c i d mole r a t i o s than t h a t a t the e q u i v a l e n c e p o i n t , and r a t h e r broad t i t r a t i o n curves w i l l be o b t a i n e d . The two prime methods f o r d e t e r m i n a t i o n of a c i d i t y , H 0 measurement and conductometric s t u d i e s , probe two d i f f e r e n t but p o s s i b l y r e l a t e d f e a t u r e s : the o v e r a l l p r o t o n a t i o n a b i l i t y , and the acidium i o n c o n c e n t r a t i o n . While conductometric s t u d i e s are u s u a l l y c a r r i e d out i n the low Lewis a c i d c o n c e n t r a t i o n range, Hammett A c i d i t y F u n c t i o n measurements extend t o a h i g h e r Lewis a c i d c o n c e n t r a t i o n range, where most p r a c t i c a l a p p l i c a t i o n s are mani f e s t e d . The p o t e n t i a l uses of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s as Lewis a c i d s i n conjugate Bronsted/Lewis s u p e r a c i d s are suggested by the s i m i l a r i t y of t h e i r p h y s i c a l appearance and t h e i r v i b r a t i o n a l s p e c t r a t o the As(V) and Sb(V) f l u o r i d e f l u o r o s u l f a t e s . In t h i s chapter, a p r e l i m i n a r y i n v e s t i g a t i o n of t h e i r p o t e n t i a l a p p l i c a t i o n s i n s u p e r a c i d systems w i l l be d i s c u s s e d . The work i s r e s t r i c t e d t o compounds of the type M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n=3, 4), s i n c e these compounds are t h e r m a l l y s t a b l e and can be o b t a i n e d i n pure form. 4 .2 Experimental 4 . 2 . 1 Conductivity Measurements of Neat Compounds The measurement of the e l e c t r i c a l c o n d u c t i v i t i e s of pure compounds was c a r r i e d out i n a s m a l l c o n d u c t i v i t y c e l l w i t h a 84 volume of ca. 3.5 mL. The c e l l c o nstant was c a l i b r a t e d as 3.544 cm - 1 w i t h r e f e r e n c e t o a 0.0100M KCl aqueous s o l u t i o n (k=l.4078X10-3 ohm - 1'cm - 1 a t 2 5 . 0 0 ° C ) . 9 6 L i q u i d samples were t r a n s f e r r e d i n t o the c o n d u c t i v i t y c e l l i n a dry box. The temperatures were kept c o n s t a n t t o ±0.1°C by a l a r g e o i l bath. Conductance data were recorded a f t e r m a i n t a i n i n g a c o n s t a n t temperature f o r 10 min. i n order f o r the samples t o e q u i l i b r a t e . 4 . 2 . 2 Conductivity Measurements of Solutions of Nb(V) and Ta(V) Fluoride Fluorosulfates i n HS0 3F C o n d u c t i v i t y measurements on s o l u t i o n s of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s i n HS0 3F were c a r r i e d out i n a l a r g e c o n d u c t i v i t y c e l l w i t h a c e l l c o n s t a n t of 5.67710.15 cm - 1. The c e l l c o n s t a n t was c a l i b r a t e d a f t e r every two s e r i e s of measurements. For the measurements i n the low Lewis a c i d c o n c e n t r a t i o n range, f r e s h l y d i s t i l l e d HS0 3F was added d i r e c t l y i n t o the c o n d u c t i v i t y c e l l from the d i s t i l l a t i o n apparatus. The s o l u t i o n s were prepared and t r a n s f e r r e d i n t o an a d d i t i o n b u r e t i n s i d e a dry box. For the measurements i n the h i g h Lewis a c i d c o n c e n t r a t i o n range, f r e s h l y d i s t i l l e d HS0 3F was t r a n s f e r r e d i n t o an a d d i t i o n buret, and the prepared s o l u t i o n s were t r a n s f e r r e d i n t o the c o n d u c t i v i t y c e l l i n s i d e the dry box. The a d d i t i o n b u r e t was then f i x e d onto the c o n d u c t i v i t y c e l l b e f o r e being taken out of the dry box. During the measurements, upon the a d d i t i o n of each a l i q u o t , the c e l l was shaken t o mix the s o l u t i o n s thoroughly, and the c o n c e n t r a t i o n s were then 85 c a l c u l a t e d . A l l conductance data were reco r d e d a t 25.00°C. 4.2.3 Conductometric T i t r a t i o n i n HS03F Solution Conductometric t i t r a t i o n of s o l u t i o n s of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s i n HS0 3F, a g a i n s t KS0 3F, were c a r r i e d out i n the l a r g e c o n d u c t i v i t y c e l l . The c e l l c o n s t a n t was c a l i b r a t e d as d e s c r i b e d e a r l i e r . S o l i d KS0 3F was loaded i n t o a pre-weighed s o l i d a d d i t i o n c o n t a i n e r ( F i g . 2-5b) and the prepared s o l u t i o n was t r a n s f e r r e d i n t o the c o n d u c t i v i t y c e l l i n s i d e the dry box. The t i t r a t i o n s were performed w i t h the c o n d u c t i v i t y c e l l h e l d a t 25.00±0.01°C. S o l i d KS0 3F was added through an adaptor w i t h a T e f l o n stopcock t o minimize the leakage of a i r i n t o the c o n d u c t i v i t y c e l l , and the c o n d u c t i v i t y data were r e c o r d e d a f t e r every a d d i t i o n . The amount of added KS0 3F was determined by weighing by d i f f e r e n c e , and the mole r a t i o of KS0 3F/MF m(S0 3F) 5 _ n was c a l c u l a t e d a c c o r d i n g l y . 4.3 Results and Discussion 4.3.1 Conductivity of Neat Compounds Compounds of the type M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n=3, 4) are v i s c o u s l i q u i d s , and t h e i r v i b r a t i o n a l s p e c t r a have been i n t e r p r e t e d i n terms of a s s o c i a t e d s t r u c t u r e s p r i m a r i l y b r i d g e d by f l u o r o s u l f a t e groups. The s y n t h e t i c method used r e q u i r e s F vs. S0 3F exchange t o take p l a c e . T h i s exchange may i n v o l v e v a r i o u s a s s o c i a t i o n - d i s s o c i a t i o n e q u i l i b r i a , i n c l u d i n g i o n i c 86 d i s s o c i a t i o n . T h e r e f o r e , i t becomes i n t e r e s t i n g t o o b t a i n some i n f o r m a t i o n about the extent of i o n i c s e l f - d i s s o c i a t i o n of the neat compounds by measuring t h e i r e l e c t r i c a l c o n d u c t i v i t i e s a t v a r i o u s temperatures. The measured c o n d u c t i v i t i e s of Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s a t d i f f e r e n t temperatures are l i s t e d i n T a b l e 4-1, and t h e i r temperature dependence i s shown i n F i g . 4-1 t o g e t h e r w i t h p r e v i o u s l y r e p o r t e d c o n d u c t i v i t i e s of A s F 3 ( S 0 3 F ) 2 • 8 5 The c o n d u c t i v i t i e s of these Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s i n c r e a s e w i t h i n c r e a s i n g temperature, which i s c o n s i s t e n t w i t h c o n d u c t i o n by i o n s . The c o n d u c t i v i t y a t 25.00°C f o r these compounds i s c l o s e t o the v a l u e f o r VF 5 (&=2.4xl0~ 4 ohm - 1•cm - 1 a t 25.00°C), where a s e l f - i o n i z a t i o n mechanism i s p o s t u l a t e d . 1 0 6 I n t e r m o l e c u l a r a s s o c i a t i o n and i o n i c d i s s o c i a t i o n i n the l i q u i d phase are apparent i n the M F n ( S 0 3 F ) 5 _ n compounds. The extent of T a b l e 4-1 C o n d u c t i v i t y Data f o r L i q u i d Niobium(V) and Tantalum(V) F l u o r i d e F l u o r o s u l f a t e s Temperature c o n d u c t i v i t y k (10 4 ohm 1«cm~ 1) (°C) NbF 4(S0 3F) N b F 3 ( S 0 3 F ) 2 TaF 4(S0 3F) T a F 3 ( S 0 3 F ) 2 20.0 0.504 1.48 1.07 25.0 0.580 1.76 0.110 1.28 30.0 0.669 2.11 1.53 35.0 0.772 2.373 1.78 40.0 0.861 2.539 2.05 87 i o n i z a t i o n i s s m a l l , i n comparison with h i g h l y c o n d ucting l i q u i d s l i k e C10 2S0 3F (7.51xl0~ 2 o h m " 1 * ™ - 1 a t 25.00°C) and SeO(S0 3F) 2 (1.592X10" 3 ohm" 1«cm" 1 a t 2 5 . 0 0 ° C ) . 1 0 7 However, more e x t e n s i v e i o n i z a t i o n appears t o occur f o r the Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s than f o r t h e i r c o r r e s p o n d i n g parent compounds NbF 5 (/c=1.63xl0~5 ohm" 1-cm - 1 a t 80.0°C) and TaF 5 (fc=l.56xl0 - 5 ohm'^-'cm-1 a t 9 5 . 1 ° C ) . 1 0 6 The o v e r a l l o r d e r of c o n d u c t i v i t y seems t o be: N b F 3 ( S 0 3 F ) 2 > T a F 3 ( S 0 3 F ) 2 > NbF 4(S0 3F) > TaF 4(S0 3F) > NbF 5 > TaF 5, where data a t d i f f e r e n t temperatures a re used f o r the l a t t e r two. 3.00 - i 6 O I s 0 2.00 ->i 4J • H > • H •U U 3 •O C O o 1.00 -0.00 / A s F 3(S0 3 F ) 2 NbF 3(S0 3F) 2 r T a F 3 ( S 0 3 F ) 2 NbF 4(S0 3F) 0— 10 I 20 ^ TaF 4(S0 3F) - l — — ~T~ 30 40 50 Temperature (»c) F i g u r e 4-1 Temperature-dependent c o n d u c t i v i t i e s of L i q u i d Nb(V) and Ta(V) F l u o r i d e F l u o r o s u l f a t e s (Data f o r A s F 3 ( S 0 3 F ) 2 are from ref.85) 88 4.3.2 Conductometric Studies on Solutions of Nb(V) or Ta(V) Fluoride Fluorosulfates i n HS03F F l u o r o s u l f u r i c a c i d undergoes s e l f - i o n i z a t i o n via a proton t r a n s f e r mechanism, and the h i g h e s t e l e c t r i c a - 1 c o n d u c t i v i t y measured f o r pure HS0 3F a t 25°C i s 1.085xl0 - 4 ohm - 1 • cm - 1. 2 5 L i q u i d Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s have e l e c t r i c a l c o n d u c t i v i t i e s on the order of 1 0 - 4 ohm" 1•cm - 1, comparable t o t h a t of pure HS0 3F. I t becomes i n t e r e s t i n g t o study the e f f e c t s of the a d d i t i o n of HS0 3F t o these f l u o r i d e f l u o r o s u l f a t e s on t h e i r e l e c t r i c a l c o n d u c t i v i t i e s . The m i s c i b i l i t y of a l l f o u r Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s w i t h HS0 3F over the e n t i r e c o n c e n t r a t i o n range permits an i n t e r e s t i n g experiment — the study of e l e c t r i c a l c o n d u c t i v i t i e s over the e n t i r e c o n c e n t r a t i o n range. Such s t u d i e s are not always p o s s i b l e ; i n systems such as HS0 3F/Au(S0 3F) 3, H S 0 3 F / P t ( S 0 3 F ) 4 , HS0 3F/NbF 5 and HS0 3F/TaF 5, the s o l u b i l i t y of the s o l i d Lewis a c i d s i n HS0 3F i s l i m i t e d . The c o n d u c t i v i t i e s of the s o l u t i o n s of NbF 4(S0 3F) or N b F 3 ( S 0 3 F ) 2 i n HS0 3F are l i s t e d i n Table 4-2, and the cor r e s p o n d i n g data f o r s o l u t i o n s of TaF 4 ( S 0 3F ) or T a F 3 ( S 0 3 F ) 2 i n HS0 3F are l i s t e d i n Table 4-3. P l o t s of c o n d u c t i v i t y vs. Lewis a c i d c o n c e n t r a t i o n are shown i n F i g . 4-2. The p l o t s f o r a l l f o u r s p e c i e s have a s i m i l a r shape. An i n i t i a l i n c r e a s e a t low Lewis a c i d c o n c e n t r a t i o n l e v e l s o f f g r a d u a l l y t o a maximum i n c o n d u c t i v i t y a t 15- 20 mole% Lewis 89 Tabl e 4-2 C o n d u c t i v i t y Data f o r S o l u t i o n s of NbF 4(S0 3F) or N b F 3 ( S 0 3 F ) 2 i n HS0 3F (25.00°C) HS0 3F/NbF 4(S0 3F) HS0 3F/NbF 3(S0 3F) 2 Lewis A c i d c o n c e n t r a t i o n (mole%) c o n d u c t i v i t y k (10" 4 ohm - 1 •cm"1) Lewis A c i d c o n c e n t r a t i o n (mole%) c o n d u c t i v i t y k (10~ 4 ohm - 1-cm - 1) 0 3 .218 0 4. 070 0.43 7.416 0.27 10.13 0.89 9.655 1. 05 14.28 1.41 11.54 1. 62 16.23 2.41 14.13 2 . 02 17.58 2.89 15.34 2:52 19.28 3.40 15.84 3.57 20.86 4.03 16.83 4 . 34 23 . 56 4.82 18.31 5.93 28 . 01 6. 06 20. 52 8.09 33 .96 7 . 57 22 .92 11.23 39 . 97 9.62 25.52 14 .41 42 . 57 11.54 27.34 20.73 43 . 27 15.70 29 . 62 28.11 39.84 20.97 32 . 05 43.31 27.26 28.26 31. 69 100 1.76 58.41 15.95 100 0.580 a c i d and then f a l l s o f f t o e v e n t u a l l y reach the c o n d u c t i v i t i e s of the neat Lewis a c i d s . The c o n d u c t i v i t y of the mixture of the M F n ( S 0 3 F ) 5 _ n s p e c i e s and HS0 3F i s hi g h e r than a l i n e a r combination of the c o n d u c t i v i t i e s of the i n d i v i d u a l compounds a t a l l c o n c e n t r a t i o n s . T h i s s y n e r g i s t i c e f f e c t on the c o n d u c t i v i t y of the mixture i m p l i e s a s t r o n g i n t e r a c t i o n between the Lewis a c i d 9 0 T a b l e 4-3 C o n d u c t i v i t y Data f o r S o l u t i o n s of TaF 4(S0 3F) or T a F 3 ( S 0 3 F ) 2 i n HS0 3F (25.00°C) HS0 3F/TaF 4(S0 3F) H S 0 3 F / T a F 3 ( S 0 3 F ) 2 Lewis A c i d c o n d u c t i v i t y Lewis A c i d c o n d u c t i v i t y c o n c e n t r a t i o n k c o n c e n t r a t i o n k (mole%) ( 1 0 - 4 ohm - 1-cm - 1) (mole%) ( 1 0 - 4 ohm - 1-cm - 1) 0 3.58 0 2.948 0.03 3.69 0.22 11.79 0.14 5.036 0.47 19.98 0.26 6.989 0.71 26.73 0.41 8.697 0.93 31.17 0.57 10.06 1.53 42.06 0.93 11.95 2.25 52.34 1.35 13.17 3.09 62.07 1.87 14.23 4.50 74.5 2.34 15.04 7.33 93.07 2.95 15.91 8.68 102.9 3.72 17.22 11.13 108.0 4. 68 19.68 15.38 109.1 5.57 21.68 20.48 103.1 8.88 27.54 27.48 88.43 10.89 30.61 40.68 56.87 16.35 35.74 61.50 20. 76 20.90 37.99 100 1.28 24.32 37.18 26.27 36.83 29.72 35.53 31.16 34.64 31.99 34.51 33.99 33.74 35.26 32.92 37.73 31.49 44.21 27.08 45.69 25.96 47.90 24.09 53.51 19.70 63.90 12.14 68.42 9.210 74.49 5.727 78.8 3.958 82.8 2.610 92.4 0.7049 100 0.110 91 120.0 - i 100.0 -i 80.0 6 U i H I * e xi o o 60.0 4 J •P U ^ 40.0 c o o 20.0 0.0 r N 40.0 30.0 a u a o o ^ 20.0 3 10 .0 - ) ? . ' ' / T3 C o u l o v c o n c e n t r a t i o n r e g i o n 0.0 s i \ 0.00 C.02 0.04 0.06 0.08 0.10 L e w i s A c i d c o n c e n t r a t i o n (mole*) Legend 0 NbF 4 ( S 0 3F) • N b F 3 ( S 0 3 F ) 2 + T a F 4 ( S 0 3 F ) x T a F 3 ( S 0 3 F ) 2 " i — i — | i — i — i — i — | — i — i — i — i — | — i — i — i — i — | — i — r 0.00 0.20 0.40 0.60 0.80 Lewis A c i d c o n c e n t r a t i o n (mole%) F i g u r e 4-2 Concentration-dependent C o n d u c t i v i t i e s of The S o l u t i o n s of Nb(V) or Ta(V) F l u o r i d e F l u o r o s u l f a t e s i n HS0 3F. 1.00 92 and the Bronsted a c i d i n such a way as t o cause i n c r e a s e d c o n d u c t i v i t y . In the h i g h c o n c e n t r a t i o n r e g i o n the c o n d u c t i v i t i e s f o l l o w the o r d e r : T aF 4(S0 3F) < NbF 4(S0 3F) < T a F 3 ( S 0 3 F ) 2 < N b F 3 ( S 0 3 F ) 2 . However, c r o s s - o v e r s occur and f o r the m a j o r i t y of the c o n d u c t i v i t y v s . Lewis a c i d c o n c e n t r a t i o n p l o t s the order i s : NbF 4 (S0 3F) < TaF 4 ( S 0 3F) < NbF 3 ( S 0 3F) 2 « T a F 3 ( S 0 3 F ) 2 . While the f i r s t t h r e e compounds i n t h i s s e r i e s g i v e data o f the same magnitude, T a F 3 ( S 0 3 F ) 2 appears t o be a c o n s i d e r a b l y s t r o n g e r e l e c t r o l y t e , as i s d e p i c t e d i n F i g 4-2. Taking i n t o account the l i t e r a t u r e data on N b F 5 2 8 and S b F 5 , 2 7 the o v e r a l l o r d e r of c o n d u c t i v i t i e s i n the d i l u t e domain i s : NbF 5 < NbF 4 (S0 3F) < TaF 4(S0 3F) < NbF 3 ( S 0 3F) 2 « T a F 3 ( S 0 3 F ) 2 < SbF 5 which may be viewed as an approximate order of t h e i r Lewis a c i d i t i e s . As d i s c u s s e d e a r l i e r , the acidium i o n , H 2S0 3F +, and base i o n , S0 3F~, are the main c o n t r i b u t o r s t o the e l e c t r i c a l c o n d u c t i v i t y o f pure HS0 3F. In the low M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n=4, 3) c o n c e n t r a t i o n range, where MF n (S0 3F) 5 _ n i s viewed as the s o l u t e and HS0 3F as the s o l v e n t , M F n ( S 0 3 F ) 5 _ n accepts S0 3F~ ions from HS0 3F t o y i e l d the H 2S0 3F + i o n , as g i v e n i n the f o l l o w i n g g e n e r a l e q u a t i o n : [ M F n ( S 0 3 F ) 5 _ n ] m + 2HS0 3F ===== H 2 S 0 3 F + + [ ( M F n ( S 0 3 F ) 5 _ n ) m S 0 3 F ] - [4-1] 93 where m i n d i c a t e s the degree of i n t e r m o l e c u l a r a s s o c i a t i o n of the s o l u t e , which i n c r e a s e s w i t h s o l u t e c o n c e n t r a t i o n . At v e r y low Lewis a c i d c o n c e n t r a t i o n s , m i s c l o s e t o u n i t y . At h i g h e r c o n c e n t r a t i o n s , s o l u t e a s s o c i a t i o n occurs and m i n c r e a s e s , r e s u l t i n g i n a l e v e l l i n g o f f of the c o n d u c t i v i t y vs. Lewis a c i d c o n c e n t r a t i o n p l o t s . A l t e r n a t i v e l y , i t can be argued t h a t o l i g m e r i c Lewis a c i d s l i k e [MF n (S0 3F) 5 _ n ] m are not completely broken down i n t o monomers a t h i g h e r c o n c e n t r a t i o n s . Evidence f o r the r e t e n t i o n of s o l u t e a s s o c i a t i o n i s o b t a i n e d from the o b s e r v a t i o n t h a t the b i d e n t a t e S0 3F s t r e t c h i n g bands i n the Raman s p e c t r a o b t a i n e d on the HS0 3F/TaF 4(S0 3F) system are v e r y s i m i l a r t o those seen i n the Raman s p e c t r a of the pure Lewis a c i d s . In d i l u t e Lewis a c i d s o l u t i o n s i n HS03F, the p r i n c i p a l c o n t r i b u t i o n t o e l e c t r i c a l c o n d u c t i v i t y a r i s e s from H 2S0 3F + v i a the p r o t o n t r a n s f e r mechanism. A d i f f e r e n t , but n e v e r t h e l e s s e f f e c t i v e conductance mechanism must p r e v a i l i n pure MF n(S0 3F) 5 _ n . I t can be suggested t h a t i o n s form v i a s e l f - i o n i z a t i o n of the o l i g m e r i c Lewis a c i d s : 2 [ M F n ( S 0 3 F ) 5 _ n ] m ^ [ ( M F n ) m ( S 0 3 F ) m(5-n)+l ] + [ ( M F n ) m ( S 0 3 F ) m(5 - n ) -U T h i s p r o p o s a l i s based on the s o l v e n t system concept. Both S0 3F~ and F~ t r a n s f e r are f e a s i b l e i n M F n ( S 0 3 F ) 5 _ n compounds. The former i s more l i k e l y on account of the p r e f e r e n t i a l b r i d g i n g f u n c t i o n of S0 3F~ (Chapter 3), which f a c i l i t a t e s i o n t r a n s f e r j u s t as hydrogen b r i d g i n g f a c i l i t a t e s p roton t r a n s f e r i n s t r o n g 94 p r o t o n i c a c i d s . I t should be noted t h a t F - b r i d g i n g and F t r a n s f e r occurs i n F ~ / S 0 3 F ~ exchange r e a c t i o n s : M ( S 0 3 F ) 5 + 4MF5 — > 5 MF 4(S0 3F) [4-3] Although i t i s reasonably c l e a r how enhanced c o n d u c t i v i t y a t low Lewis a c i d c o n c e n t r a t i o n s a r i s e s , i t i s not so obvious i n the h i g h Lewis a c i d c o n c e n t r a t i o n range, where HS0 3F may be viewed as a s o l u t e and M F n ( S 0 3 F ) 5 _ n as a s o l v e n t . I t i s a l s o u n c l e a r a t which Lewis a c i d c o n c e n t r a t i o n proton m i g r a t i o n becomes the dominant mechanism. I t must be assumed t h a t the i n i t i a l a d d i t i o n of H S 0 3 F t o M F n ( S 0 3 F ) 5 _ n must e i t h e r promote S 0 3 F ~ t r a n s f e r or generate a d d i t i o n a l i o n s v i a p r o t o n a t i o n and complexation: [ M F n ( S 0 3 F ) 5 _ n ] m + HS0 3F ===== H [ M F n ( S 0 3 F ) 5 _ n ] i + + [ M F n ( S 0 3 F ) 5 _ n ] j S 0 3 F - [4-4] where i and j i n d i c a t e d i f f e r e n t degrees of a s s o c i a t i o n and i+j=m. From the o b s e r v a t i o n of the h i g h l y conducting, and presumably h i g h l y i o n i z i n g , l i q u i d s i t may be i m p l i e d t h a t ions formed a t h i g h Lewis a c i d c o n c e n t r a t i o n s are i n v o l v e d i n s u p e r a c i d - c a t a l y z e d r e a c t i o n s as much as those formed a t low Lewis a c i d c o n c e n t r a t i o n s . The L e w i s - a c i d - c o n c e n t r a t e d s u p e r a c i d systems thus p r o v i d e a b e t t e r model f o r a c t u a l s u p e r a c i d c h e m i s t r y than do the very d i l u t e s o l u t i o n s . I t has been assumed so f a r t h a t f o r the d i l u t e Lewis a c i d s o l u t i o n s i n HS0 3F, e l e c t r i c c o n d u c t i v i t i e s are l a r g e due t o the 95 H 2 S 0 3 F + i o n , which shows the h i g h e s t m o b i l i t y i n HS0 3F. 2 5 Experimental p r o o f f o r t h i s i s e v i d e n t from conductometric t i t r a t i o n s . In such experiments, the s o l u t i o n s of N b F 4 ( S 0 3 F ) , N b F 3 ( S 0 3 F ) 2 , T a F 4 ( S 0 3 F ) and T a F 3 ( S 0 3 F ) 2 i n H S 0 3 F are t i t r a t e d a g a i n s t the standard base, K S 0 3 F . The c o n d u c t i v i t i e s a t the c o r r e s p o n d i n g K S 0 3 F / M F N ( S 0 3 F ) 5 _ n mole r a t i o s (M=Nb, Ta; n=4,3) are l i s t e d i n T a b l e s 4-4 and 4-5. The p l o t s of measured s p e c i f i c conductance vs. K S 0 3 F / M F N ( S 0 3 F ) 5 _ n mole r a t i o are shown i n F i g . 4-3. The r e s u l t s of the conductometric t i t r a t i o n s are c o n s i s t e n t w i t h the r e s u l t s of the c o n d u c t i v i t y measurements f o r d i l u t e s o l u t i o n s i n H S 0 3 F . Of the f o u r s p e c i e s s t u d i e d , o n l y T a F 3 ( S 0 3 F ) 2 shows a minimum c o n d u c t i v i t y i n the t i t r a t i o n curve a t a K S 0 3 F / T a F 3 ( S 0 3 F ) 2 mole r a t i o of «0.25, i n d i c a t i n g t h a t T a F 3 ( S 0 3 F ) 2 behaves as a weak a c i d i n HS0 3F. No minima i n c o n d u c t i v i t y are observed i n the t i t r a t i o n curves of the other t h r e e s p e c i e s , which have low a c i d s t r e n g t h s ; t h e i r t i t r a t i o n curves do not a l l o w a c l e a r r a n k i n g . I t i s known t h a t the a d d i t i o n of S0 3 t o a s o l u t i o n of MF 5 (M=Sb, As) i n HS0 3F has a marked e f f e c t on the a c i d i t y of the s o l u t i o n , presumably due t o the formation of M F N ( S 0 3 F ) 5 _ n (M=Sb, A s ) . The more f l u o r i n e s t h a t are r e p l a c e d by f l u o r o s u l f a t e groups i n the Lewis a c i d , the s t r o n g e r i s the r e s u l t i n g s u p e r a c i d . 96 Table 4-4 Conductometric T i t r a t i o n of NbF 4(S0 3F) or N b F 3 ( S 0 3 F ) 2 w i t h KS0 3F i n HS0 3F NbF 4(S0 3F) (0.2961 m) NbF 3(S0 3F) 2 (0.2584 m) Mole r a t i o c o n d u c t i v i t y Mole r a t i o c o n d u c t i v i t y ( KSO3F \ k f KSO^F \ k \NbF 4(S0 3F)/ ( 1 0 - 4 ohm"1-cm-1) \NbF 3(S0 3F) 2/ (10~ 4 ohm - 1-cm - 1) 0 15.34 0 19.28 0.071 20.94 0.038 21.08 0.168 29.36 0.150 26.46 0.3001 40.93 0.4209 41.27 0.4479 53.19 0.4970 46.01 0.6225 66.62 0.6052 52.10 0.8676 90.38 0.6977 58.16 1.059 113.3 0.7845 63.24 1.172 131.6 0.8870 69.08 1.306 156.2 1.006 77.44 1.466 194.4 1.153 90.76 1.321 110.5 1.440 131.0 Table 4-5 Conductometric T i t r a t i o n of TaF 4(S0 3F) or T a F 3 ( S 0 3 F ) 2 w i t h KS0 3F i n HS0 3F TaF 4(S0 3F) (0.3084 m) TaF 3(S0 3F) 2 (0. 3400 m) Mole r a t i o / KSO3F \ c o n d u c t i v i t y k ( 1 0 - 4 ohm - 1 • cm - 1) Mole r a t i o / KSO3F \ c o n d u c t i v i t y k (10~ 4 ohm - 1 • cm - 1) VTaF4(S03F); \jraF 3(S0 3F) 2/ 0 16.89 0 63.67 0.0235 18.44 0.0437 60.24 0.0580 20.77 0.1100 56.10 0.1209 25.04 0.1788 53.91 0.2176 31.52 0.2638 53.32 0.3126 38.07 0.3485 54.29 0.4264 45.69 0.4514 56.65 0.5104 51.31 0.6036 62.68 0.6128 58.27 0.7305 68.09 0.7081 64.79 0.9457 78.04 0.8155 71.97 0.9791 79.87 0.8379 74.87 1.011 82.02 0.9144 81.62 1.100 87.71 1.018 93.61 1.171 94.40 1.116 105.7 1.271 114.68 1.271 127.3 1.366 142.7 1.368 146.9 1.481 179.4 200.0 - l 180.0 H 160.0 H 0 140.0 O •g 120.0 o w 100.0 O 8 0 0 3 X J c o CJ 60.0 H 40.0 H 20.0 0.0 | i i i I | i i i I | i i i I | I I I I | I I I I | I I I I | I I I I | I i i I | 0.00 0 .20 0.40 0 .80 0 .80 1.00 1.20 1.40 1.60 Mole Ratio of K/Nb 180.0 - i 160.0 140.0 B u 120.0 H I o 100.0 80 .0 a TJ O 60.0 o 40.0 20 .0 A 0.0 / // I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 0.00 0 .20 0 .40 0 .60 0 .80 1.00 1.20 1.40 1.60 Mole Ratio of K/Ta F i g u r e 4 - 3 T i t r a t i o n Curves of Nb(V) and Ta(V) Fluoride- Fluorosulfates with KSO3F i n HSO3F vo 98 This trend i s also found i n niobium(V) and tantalum(V) f l u o r i d e f l u o r o s u l f a t e s of the type MF n(S0 3F) 5_ n (Fig. 4-4). In the series of the NbF n(S0 3F) 5_ n compounds, which have r e l a t i v e l y low acid strengths, NbF 3(S0 3F) 2 seems to be a stronger acid than NbF 4(S0 3F) and both are stronger than NbF5, which i s claimed to have n e g l i g i b l e a c i d i c p r o p e r t i e s . 2 8 A l l three are weaker than Nb(S03F) 5 ( s o i v ) . 6 6 However, the addition of S0 3 to a soluti o n of NbF5 i n HS03F has no e f f e c t on the a c i d i t y of the s o l u t i o n . 2 7 I t might be argued that the i n s e r t i o n of S0 3 into NbF5 does not occur to any s i g n i f i c a n t extent i n HS03F although the reaction of NbF5 with an excess of S0 3 i s reported to y i e l d l i q u i d niobium(V) f l u o r i d e f l u o r o s u l f a t e s . 1 0 0 As f a r as the TaF n(S0 3F) 5_ n series i s concerned, TaF 3(S0 3F) 2 i s a much stronger acid than TaF 4(S0 3F) but weaker than T a ( S 0 3 F ) 5 6 6 . In summary, the conductometric studies indicate TaF 3(S0 3F) 2 i s a weak Lewis acid i n HS03F. TaF 4(S0 3F), NbF 3(S0 3F) 2 and NbF 4(S0 3F) behave as weak e l e c t r o l y t e s i n d i l u t e solutions of HS03F, and t h e i r acid strengths are extremely weak. The conductivity measurements show that a l l the Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s studied are highly associated i n HS03F, which i s also evident from the Raman spectra of the solution of TaF 4(S0 3F) and HS03F. The intermolecular association of the Lewis acids reduces t h e i r a b i l i t y to accept the S0 3F~ ion. A general tendency i s applicable to MF n(S0 3F) 5_ n/HS0 3F (M=As, Sb, Nb or Ta) superacid systems: the higher the fl u o r o s u l f a t e content i n the Lewis acid (the smaller the n i n the general formula), the 25.00 - i 20.00 fl O a a i5.oo o o lo.oo 3 TJ C o o 5.00 i g 5^ 0.00 0.00 -1—I—I—I—I—I—r 0.05 - | — | — I — I — I — I — | — I — I — I — I — | — i — I — I — I — | 0.10 0.15 0 20 0.25 Lewis Acid Concentration (m) 55.00 50.00 -45.00 40.00 I B U V 35.00 o 7 30.00 o 25.00 \> '-*-» o T> 20.00 c o CJ 15.00 10.00 -5.00 0.00 ft. 4 • r I: 0.00 -I—I—I—I—I—I—I—I—I—I—I—I I I I r 0.05 0.10 0.15 Lewis Acid Concentration (m) l ' ' ' ' l 0.20 0.25 F i g u r e 4-4 Concentration-dependent C o n d u c t i v i t i e s o f s o l u t i o n s of M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta) i n HS0 3F (Data f o r NbF 5 are from r e f . 2 8 and data f o r Nb(S0 3 F ) 5 and T a ( S 0 3 F ) 5 are from r e f . 66) vo 100 s t r o n g e r the r e s u l t i n g s u p e r a c i d . However, the thermal s t a b i l i t y of such Lewis a c i d s l i m i t s the p o s s i b l e p r e p a r a t i o n of v e r y s t r o n g s u p e r a c i d s i n t h i s way, as has been d i s c u s s e d i n the p r e c e d i n g chapter. 101 Chapter 5 CONCLUSIONS 5.1 C o n c l u s i o n s In t h i s work, the d i r e c t s y n t h e s i s o f niobium and tantalum f l u o r i d e f l u o r o s u l f a t e s of the g e n e r a l type M F n ( S 0 3 F ) 5 _ n (n=0, 1, 2, 3, 4, 4.5) was attempted. A one-step metal o x i d a t i o n / l i g a n d r e d i s t r i b u t i o n r e a c t i o n was developed a c c o r d i n g t o the g e n e r a l e q u a t i o n : (5-n)M + nMF 5 + excess S 2 0 6 F 2 > 5MF n(S0 3F) 5 _ n [5-1] F i v e niobium(V) and tantalum(V) f l u o r i d e f l u o r o s u l f a t e s — N b 2 F 9 ( S 0 3 F ) , NbF 4(S0 3F), N b F 3 ( S 0 3 F ) 2 , TaF 4(S0 3F) and T a F 3 ( S 0 3 F ) 2 — were s y n t h e s i z e d i n t h i s manner. They were found t o be s u f f i c i e n t l y s t a b l e t o a l l o w p u r i f i c a t i o n by d i s t i l l a t i o n in vacuo. The i s o l a t i o n of h i g h e r f l u o r o s u l f a t e c o n t a i n i n g compounds M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n<3) was not p o s s i b l e on account of t h e i r h i g h v i s c o s i t y and l a c k of v o l a t i l i t y a t room temperature, and t h e i r thermal i n s t a b i l i t y a t e l e v a t e d temperature. The p r i n c i p a l decomposition pathway i n v o l v e d S0 3 e l i m i n a t i o n . A l l Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s o b t a i n e d are c o l o r l e s s , v i s c o u s , m o i s t u r e - s e n s i t i v e l i q u i d s . A l l are a s s o c i a t e d i n the l i q u i d s t a t e , p r e f e r e n t i a l l y w i t h the f l u o r o s u l f a t e group i n v o l v e d i n b r i d g i n g . Nb 2F 9(S0 3F) appears t o be u n s t a b l e , d i s p r o p o r t i o n a t i n g t o g i v e s o l i d [ N b F 5 ] 4 and l i q u i d 102 NbF 4(S0 3F) on long s t a n d i n g . The s y n t h e t i c approach taken i s e f f e c t i v e f o r two reasons: ( i ) metal o x i d a t i o n by S 20gF 2 occurs a t room temperature over s e v e r a l days, even i n the absence of HSO3F, and ( i i ) NbF 5 and T a F 5 are s o l u b l e i n S 20gF 2. Raman and i n f r a r e d s p e c t r a of the i s o l a t e d compounds, M F n ( S 0 3 F ) 5 _ n (M=Nb, Ta; n>3) are i n t e r p r e t e d i n terms of a s s o c i a t e d s t r u c t u r e s w i t h approximately o c t a h e d r a l c o o r d i n a t i o n of the metal by f l u o r i n e s and f l u o r o s u l f a t e groups. B i d e n t a t e b r i d g i n g f l u o r o s u l f a t e groups are found i n a l l these f l u o r o s u l f a t e d e r i v a t i v e s . Monodentate f l u o r o s u l f a t e groups are found o n l y i n N b F 3 ( S 0 3 F ) 2 and T a F 3 ( S 0 3 F ) 2 . B r i d g i n g f l u o r i n e s are suggested f o r l i q u i d N b 2 F 9 ( S 0 3 F ) , and MF 4(S0 3F) (M=Nb, Ta) as w e l l , although t o a l e s s e r e xtent. T e n t a t i v e assignments of the v i b r a t i o n a l bands are based on the p r e v i o u s l y r e p o r t e d v i b r a t i o n a l s p e c t r a of the parent compounds NbF 5 and TaF 5, t h e i r As(V) and Sb(V) analogues and v a r i o u s other f l u o r o s u l f a t e s . The s t r u c t u r a l s i m i l a r i t y t o the analogous As(V) and Sb(V) compounds i s i n t e r p r e t e d i n terms of a common approximately o c t a h e d r a l c o o r d i n a t i o n f o r Nb, Ta, As and Sb. The Nb(V) and Ta(V) f l u o r i d e f l o u r o s u l f a t e s i s o l a t e d are m i s c i b l e w i t h HS0 3F i n any p r o p o r t i o n , e n a b l i n g s o l u t i o n s t u d i e s over the e n t i r e c o n c e n t r a t i o n range. At the low c o n c e n t r a t i o n range, the conductometric study shows t h a t T a F 3 ( S 0 3 F ) 2 i s a weak Lewis a c i d i n HS0 3F. N b F 3 ( S 0 3 F ) 2 , NbF 4(S0 3F) and TaF 4(S0 3F) are found t o behave as very weak e l e c t r o l y t e s and poor S0 3F~ a c c e p t o r s . The presence of h i g h l y a s s o c i a t e d s t r u c t u r e s i n 103 s o l u t i o n s i s supported by the presence of a b i d e n t a t e f l u o r o s u l f a t e group observed i n the Raman s p e c t r a of the TaF 4(S0 3F)/HSO3F system. C o n d u c t i v i t i e s of these s o l u t i o n s i n HSO3F f o l l o w the o r d e r : T a F 3 ( S 0 3 F ) 2 > N b F 3 ( S 0 3 F ) 2 > T a F 4 ( S 0 3 F ) > N b F 4 ( S 0 3 F ) . A t h i g h Lewis a c i d c o n c e n t r a t i o n s , h i g h l y conducting l i q u i d s are p r e s e n t . For neat l i q u i d s , the observed order of c o n d u c t i v i t i e s i s N b F 3 ( S 0 3 F ) 2 > T a F 3 ( S 0 3 F ) 2 > N b F 4 ( S 0 3 F ) > T a F 4 ( S 0 3 F ) , w i t h NbF 5 and T a F 5 r e p o r t e d l y l e s s c o n ducting. The g e n e r a l t r e n d r e p o r t e d f o r S b F n ( S 0 3 F ) 5 _ n and A s F N ( S 0 3 F ) 5 _ n h o l d s f o r N b F n ( S 0 3 F ) 5 _ n and T a F n ( S 0 3 F ) 5 _ n as w e l l : the h i g h e r the f l u o r o s u l f a t e content i n the Lewis a c i d MF n(SO3F) 5 _ n , the s t r o n g e r i s the a c i d i t y of the r e s u l t i n g conjugated Bronsted/Lewis s u p e r a c i d s HS0 3F/MF n(S0 3F) 5 _ n . 5 . 2 P r o p o s a l s f o r F u r t h e r S t u d i e s As s t a t e d e a r l i e r , the liquid Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s o b t a i n e d from S 2 0 6 F 2 medium i n t h i s work show some d i f f e r e n c e s t o the solid Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s o b t a i n e d from HSO3F medium i n terms of both p h y s i c a l appearance and v i b r a t i o n a l s p e c t r a . Attempts t o t r a n s f o r m the l i q u i d T a F 4 ( S 0 3F ) i n t o s o l i d T aF 4(S0 3F) i n HS0 3F medium have been u n s u c c e s s f u l . I t would be u s e f u l t o e s t a b l i s h s u i t a b l e e x p e r i m e n t a l c o n d i t i o n s f o r such t r a n s f o r m a t i o n s . Syntheses of s a l t s of the t e r n a r y Nb(V) and Ta(V) f l u o r i d e f l u o r o s u l f a t e s should be attempted a c c o r d i n g t o : 104 M IS0 3F + M F n ( S 0 3 F ) 5 _ n > M 1 [MF n (S0 3F) ( 6 _ n ) ] ( M I = a l k a l i metal, C 10 2 +, R 4N +; M=Nb, Ta; n<6) T h i s r o u t e i s suggested based on the presumption t h a t ( i ) the use of bulky c o u n t e r c a t i o n s may promote the p r e c i p i t a t i o n of the s a l t s , and ( i i ) the use of other s o l v e n t s as r e a c t i o n media may circumvent the problem of l i g a n d exchange t h a t may occur between M F n ( S 0 3 F ) 5 _ n and HS0 3F. The s o l v e n t chosen sh o u l d a l s o be easy t o remove from the r e s u l t i n g mixture. Only a p r e l i m i n a r y i n v e s t i g a t i o n of the MF n(S0 3F) 5_ n/HS0 3F s u p e r a c i d systems based on c o n d u c t i v i t y measurements has been completed. D e t a i l e d s t u d i e s , e s p e c i a l l y i n the h i g h Lewis a c i d c o n c e n t r a t i o n range, would be u s e f u l f o r a b e t t e r understanding. I t would be advantageous t o expand these s t u d i e s t o s o l u t i o n s i n anhydrous HF, and t o determine the Hammett A c i d i t y F u n c t i o n s (H 0) of these systems, p o s s i b l y l e a d i n g t o new and i n t e r e s t i n g s u p e r a c i d systems. 105 REFERENCES l a . Huheey, J . E., "In o r g a n i c C h e m i s t r y — P r i n c i p l e s o f S t r u c t u r e and R e a c t i v i t y " , Harper and Row, New York, 2nd ed., 1978, p. 258. l b . F i n s t o n , H. L.; Rychtman, A. 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Heyak, E.; Czaloun, A.; Krismer, B., Monatsh. Chem., 1956, 87, 741. 57a. Yeats, P. A.; Poh, B. L.; Ford, B. F. E.; Sams, J . R.; Aubke, F., J. Chem. Soc. (A), 1970, 2188. 57b. M a l l e l a , S. P.; Lee, K. C.; Aubke, F., Inorg. Chem., 1984, 23, 653. I l l 58. C a r t e r , H. A.; Mi l n e , C. A.; Aubke F., J. Inorg. Nucl. Chem., 1975, 37, 282. 59. M u e t t e r t i e s , E. L. ; Coffman, D. D. , J. Am. Chem. Soc, 1958, 80, 5914. 60. P a u l , R. C.; Singh, S.; Kumar, R. C.; Sharma, R. D.; Verma, R.D., J. Inorg. Nucl. Chem., 1979, 17A, 273. 61. G i l l e s p i e , R. J . ; M i l n e , J . B., Inorg. Chem., 1966, 5, 1326. 62. G i l l e s p i e , R. J . ; Morton, M. J . , Inorg. Chem., 1972, 11, 586. 63. Singh, S.; Bedi, M.; Verma R. D., J. Fluorine. Chem., 1982, 20, 107. 64. M i s t r y , F.; Aubke, F., unpublished work. 65. P a u l , R. C.; Kumar, R. C.; Verma, K. D., J. Fluorine Chem., 1978, 11, 203. 66. C i c h a , W. V.; Aubke, F. , J. Am. Chem. Soc, 1988, 111, 4328. 67. Brown, S. 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Giinther, H., "NMR S p e c t r o s c o p y — A n I n t r o d u c t i o n " , John Wiley & Sons, New York, 1985, Chap. 10, p. 342. 104. Granger, P. i n "NMR of Newly A c c e s s i b l e N u c l e i " , Ed. L a s z l o P., Academic Press, New York, 1983, V o l . 2, p. 386, and r e f e r e n c e s t h e r e i n . 105. C i c h a , W. V., Ph.D T h e s i s , The U n i v e r s i t y of B r i t i s h Columbia, 1989. C l a r k , H. C. ; Emeleus, H. J . , J. Chem. Soc, 1957, 2119, C a r t e r , H. L., Ph.D. T h e s i s , The U n i v e r s i t y of B r i t i s h Columbia, 1971. 116 APPENDIX Structures of Some Fluorosulfato Derivatives There are s e v e r a l p o s s i b l e bonding or c o o r d i n a t i o n modes a v a i l a b l e t o the f l u o r o s u l f a t e group: (a) i o n i c ; (b) monodentate; (c) b i d e n t a t e , e i t h e r i s o - or a n i s o - c o o r d i n a t e d ; (d) t r i d e n t a t e and (e) t e t r a d e n t a t e as i l l u s t r a t e d below. (a) i o n i c (b) monodentate (c) b i d e n t a t e (d) t r i d e n t a t e (e) t e t r a d e n t a t e 117 The modes ( a ) , (b) and (c) can be found i n the s o l i d compounds of which the s t r u c t u r e s have been determined by the s i n g l e c r y s t a l X-ray d i f f r a c t i o n s t u d i e s . A b i d e n t a t e b r i d g i n g f l u o r o s u l f a t e group can be found i n ( C H 3 ) 2 S n ( S 0 3 F ) 2 . Both monodentate and b i d e n t a t e c o o r d i n a t i o n modes of the f l u o r o s u l f a t e group are found i n A u ( S 0 3 F ) 3 . Mode (d) has been proposed f o r some t r a n s i t i o n metal b i s ( f l u o r o s u l f a t e s ) l i k e P d ( S 0 3 F ) 2 and mode (e) has been assumed on l y f o r T i 3 C l 1 0 (S0 3F) 2 . The S t r u c t u r e of (CH 3) 2 S n ( S 0 3 F ) 2 where the F l u o r o s u l f a t e Group Fu n c t i o n s as a B i d e n t a t e B r i d g i n g Ligand. (Ref.93) 118 The S t r u c t u r e of A u ( S 0 3 F ) 3 where both Monodentate and B i d e n t a t e C o o r d i n a t i o n Modes of the F l u o r o s u l f a t e Group are Found, (ref.78) • © Metal S O © 0 F The Proposed S t r u c t u r e of P d ( S 0 3 F ) 2 where the F l u o r o s u l f a t e Group F u n c t i o n s as a T r i d e n t a t e B r i d g i n g Ligand. (ref.72) 

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