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Tertiary phosphine complexes of zirconium(IV) and hafnium(IV) Carter, Alan 1985

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TERTIARY PHOSPHINE COMPLEXES OF ZIRCONIUM(IV) AND HAFNIUM(IV) by ALAN CARTER B.Sc.(Hons), Trent Polytechnic, 1982 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH ^ OLtJRlftA-January 1985 © Alan Carter, 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of QUvt'Av^ «rtJ/ The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 requirements for an advanced degree at the University / R l "4 i i ABSTRACT The tetra-halides of zirconium and hafnium were reacted with one equivalent of the p o t e n t i a l l y tridentate hybrid ligand, N(SiMe 2CH 2PR 2) 2~, (R = Me, i - P r , t-Bu) to generate the corresponding mono-ligand complexes MCl 3{N(SiMe 2CH 2PR 2) 2), (M - Zr, Hf). Based on the re s u l t s obtained from the solution spectroscopic data and the single c r y s t a l X-ray d i f f r a c t i o n analyses of HfCI 3{N(SiMe 2CH 2PMe 2) 2> and Z r C l 3 { N ( S i M e 2 C H 2 P ( i - P r ) 2 ) 2 ) the stereochemistries of a l l the MCI 3{N(SiMe 2CH 2PR 2) 2) complexes were found to be meridional i n so l u t i o n , but both f a c i a l and meridional geometries were displayed i n the s o l i d state dependent on the ligand. The mono-ligand derivatives served as useful s t a r t i n g materials for the generation of zirconium- and hafnium-carbon bonds. Thus the addition of three equivalents of MeMgCl to one equivalent of MCl 3{N(SiMe 2CH 2PR 2) 2} generated the trimethyl complexes M(CH 3) 3{N(SiMe 2CH 2PR 2) 2}. When two equivalents of MeMgCl was added to the mono-ligand complexes, an inseparable mixture of the monomethyl and dimethyl derivatives was obtained. The stereochemistry of Hf(CH 3) 3{N(SiMe 2CH 2PMe 2) 2} i s f a c i a l i n the s o l i d state but displays unusual f l u x i o n a l behaviour i n s o l u t i o n . This behaviour i s observed for a l l the trimethyl derivatives as a consequence of the d i s s o c i a t i v e nature of the phosphine donors. Several possible rearrangement pathways for these compounds are discussed i n an attempt to interpret t h i s behaviour i n s o l u t i o n . i i i ACKNOWLEDGEMENTS F i r s t l y , I would l i k e to thank, my supervisor, Dr. Michael Fryzuk, for his guidance throughout my two years here. Many thanks also to Dr. Pat MacNeil for her help during the course of th i s project. Thanks also to Dr. Axel Westerhaus for his supervision during the early part of th i s work. F i n a l l y I would l i k e to thank my proof readers, Dr. Ian Butler, John Haynes and Dr. Pat MacNeil for the time and e f f o r t they put i n and the i r l i b e r a l use of pen and p e n c i l , and also T i l l y Schreinders for her perseverence i n typing this manuscript. i v To my Mother and Father V TABLE OF CONTENTS Page Abstract i i Acknowledgements i v Table of Contents v l L i s t of Figures v l l L i s t of Tables v i i i L i s t of Abbreviations i x CHAPTER 1: Introduction 1 1.1 Homoleptic Alkyls of Titanium(IV), Zirconium(IV), and Hafnium(IV) 7 1.2 Heteroleptic Alkyls of Titanium(IV), Zirconium(IV) , and Hafnium(IV) 9 1.2.1 Cyclopentadienyl Complexes of Titanium(IV), Zirconium(IV), and Hafnium(IV) 9 1.2.2 Phosphine Complexes of Titanium(IV), Zirconium(IV), and Hafnium(IV) 12 1.2.3 Amide Complexes of Titanium(IV), Zirconium(IV), and Hafnium(IV) 15 1.3 Design and Synthesis of a Hybrid Multidentate Ligand 18 v i Table of Contents (contd) Page CHAPTER 2. Results and Discussion 23 2.1 Preparation of LiN(SiMe 2CH 2PR 2) 2 23 2.2 Preparation and Stereochemistry of MCl 3{N(SiMe 2CH 2PR 2) 2} 24 2.3 Preparation and Stereochemistry of M(CH 3) 3{N(SiMe 2CH 2PR 2) 2} 31 CHAPTER 3. Experimental 45 CHAPTER 4. Summary i . . 50 Appendix 52 Bibliography 69 v i i LIST OF FIGURES Page F i g . 1 Representation of d-d o r b i t a l overlap i n a metal-phosphorus bond 12 Fi g . 2 Representation of a phosphine cone angle 9 13 F i g . 3a,3b, Bonding p o s s i b i l i t i e s a v ailable to t r a n s i t i o n metal 3c amides 15 F i g . 4 Representation of p-d o r b i t a l overlap i n a metal-nitrogen bond 16 F i g . 5a,5b Coordination modes ava i l a b l e to { N ( S i M e 2 C H 2 P R 2 ) * * 2 0 F i g . 6 80 MHz *H NMR spectrum of mer-HfCI,{N(SiMe ?CH^Me 0) ,} 26 F i g . 7 Proposed isomerisation of fac-HfCl 3{N(SiMe 2CH 2PMe 2) 2) to mer-HfCl 3{N(SiMe 2CH 2PMe 2) 2} 27 F i g . 8 Structure and numbering scheme of HfCl 3{N(SiMe 2CH 2PMe 2) 2} (8a) 28 F i g . 9 Structure and numbering scheme of ZrCl 3{N(SiMe 2CH 2P(i-Pr) 2) 2} (9b) 28 F i g . 10 80MHz XH NMR spectrum of mer-HfCI^{N(SiMe 2CH 2P(t-Bu) 2) 2} 30 F i g . 11 3 1P{ 1H} NMR spectrum following the reaction between HfCl 3{N(SiMe 2CH 2P(i-Pr) 2) 2} and two equivalents of MeMgCl 33 Fi g . 12 80MHz *H NMR spectrum of Hf (CH 3) 3{N(SiMe ^ H ^ t - B u ) 2) 2> 36 Fi g . 13 Variable-temperature *H and 3 1P{ 1H} NMR spectra of Hf(CH 3) 3{N(SiMe 2CH 2P(t-Bu) 2) 2} 37 F i g . 14 Structure and numbering scheme for fac-Hf(CH 3) 3{N(SiMe 2CH 2P(t-Bu) 2) 2} 44 v i i i LIST OF TABLES Page Table 1 Bond lengths i n Hf(CH 3) 3(N(SiMe 2CH 2PMe 2) 2> 67 Table 2 Bond angles i n Hf(CH 3) 3{N(SiMe 2CH 2PMe 2) 2) 68 i x ABBREVIATIONS Me methyl i-Pr = iso-propyl t-Bu t e r t i a r y - b u t y l Ph = phenyl sec = secondary fac f a c i a l mer = meridional e = electron TBP = t r i g o n a l bipyramid BPR = Berry pseudorotation A = Angstroms mL = m i l l i l i t r e s °C = centigrade s = s i n g l e t d 2 doublet bd = broad doublet t = t r i p l e t m = multiplet m • p • = melting point I.R. inf r a - r e d 1 CHAPTER 1 Introduction. There has been a growing development i n the chemistry of the early group t r a n s i t i o n metals, with emphasis on the formation of metal-carbon and metal-hydrogen bonds. The synthetic p o t e n t i a l of such metal complexes was recognised with the advent of Ziegler-Natta c a t a l y s i s . The Ziegler-Natta process i s thought to involve the intramolecular migration of an a l k y l group to an adjacent coordinated o l e f i n , although the actual mechanism i s s t i l l controversial. l a» b The o r i g i n a l Ziegler-Natta process u t i l i s e d T i C l ^ and E t 3 A l in the polymerisation of ethylene (Scheme 1). M R M-CH2CH2R ^ e | j ' m CH2CH2 polymer M—H Termination Scheme 1 For the metals on the l e f t of the t r a n s i t i o n metal series k >kv and high a b 2 molecular weight polymers are produced. Ethylene polymerisation can also be carried out homogeneously employing M( TI3-C 3H 5 ) ^ ( M - Zr, Hf : (r) 3-C 3H 5) = a l l y l ) as the c a t a l y s t , 2 although the r e a c t i v i t y of these systems i s an order of magnitude less than that of the Ziegler-Natta type c a t a l y s t s . The a l k y l s of the group IVb metals have subsequently proven to be important reagents i n organic and organometallic chemistry. Their synthetic u t i l i t y i n organic chemistry was demonstrated with the discovery of the hydrozirconation r e a c t i o n , 3 which involves the reaction of (TI 5-C 5H 5) 2 Z r ( H ) C l with o l e f i n s (Scheme 2). CsHi8 17 Scheme 2 The organometallic complex 2 can be converted to a v a r i e t y of organic products upon reaction with a suitable e l e c t r o p h i l e . ** The early development of o-hydrocarbyl chemistry throughout the t r a n s i t i o n metal series was hampered by the misconception that metal a l k y l s were i n t r i n s i c a l l y unstable. Investigations which sought to ascertain the nature of t r a n s i t i o n metal a l k y l decomposition pathways gave, as a 3 consequence, invaluable information regarding their successful preparation and i s o l a t i o n . There are several decomposition pathways av a i l a b l e to t r a n s i t i o n metal a l k y l s . They are most commonly referred to as or-, fj- and y-elimination, reductive elimination, and homolysis (eqns. 1, 2, 3, 4, and 5 r e s p e c t i v e l y ) . a-Elimination H M-CHjjCHgR r1 ' ^ M=CHCH2R ( l ) p-Elimination H CHR M-CH2CH2R ^ M 1| ( 2 ) CH2 y-Elimination MCCH^CH.R ^ M ( ) C H 2 CHR Reductive Elimination M N " 2 • XZ <*> 4 Homolysis M — R ^ ^ M* + R* (5) Intramolecular p-hydrogen abstraction involves the migration of a p-hydrogen from a hydrocarbyl group to the metal centre. The unsaturated species formed during t h i s process may be eliminated or remain i n the coordination sphere. The p-elimination pathway i s r e v e r s i b l e ; i n fact both metal-carbon bond making and bond breaking by t h i s route are important phenomena i n organic synthesis (e.g. hydroformylation, h y d r o s i l a t i o n , and hydroboration). E n e r g e t i c a l l y this process i s the most f a c i l e decomposition pathway. The p r o b a b i l i t y of p-elimination increases when a) the hydrocarbyl group has a p-hydrogen; b) the metal i s coordinatively unsaturated; c) the metal centre i s s t e r i c a l l y unhindered. These e f f e c t s are exemplified in the following s t a b i l i t y sequence of a series of titanium homoleptic a l k y l s , i l l u s t r a t e d below (Scheme 3). T i ( E t ) 4 « T i ( M e ) l t<Ti(CH 2CMe 3) 4<Ti(CH 2SiMe 3) 4 Scheme 3 * Homoleptic refers to compounds containing only one type of ligand with the general formula MRX (M = t r a n s i t i o n metal, R » hydrocarbyl group) i n contrast to heteroleptic which i s a term used to describe complexes containing several d i f f e r e n t types of ligands. 5 The complex, TiCEt ) ^ , which i s extremely susceptible to ^-elimination, has not been i s o l a t e d , while i n contrast, Ti(CH 2SiMe 3)^ which contains the s t e r i c a l l y bulky -CH 2SiMe 3 ligand, has no B-hydrogens and i s subsequently stable to temperatures approaching 80°C. Py r o l y s i s studies have also shown the homoleptic a l k y l s MR^  (M = Zr, Hf, R = Me 3SiCH 2-, Me3CCH2-, Me3SnCH2-) decompose v i a reductive elimination rather than ^ - a b s t r a c t i o n . 5 The process of crelimination proceeds with the migration of a proton from the orcarbon of an a l k y l group to the metal centre to generate an alkylidene-hydride intermediate or product. The f i r s t documented example was reported i n 1974 6 (eqn. 6). Ta(CH2CMe 3) 3 C 1 2 + 2LiCH 2CMe 3 — + [Intermediate] —->• Ta(CH 2CMe 3) 5 —•*• (CH2CMe 3) 3Ta=CHCMe 3 + CMe^ + 2LiCl (6) B C The neopentyl ligands on B are s t e r i c a l l y too demanding, therefore, subsequently a proton on one of the neopentyl groups i s abstracted by a second neopentyl ligand with the ensuing elimination of neopentane. An alternate mechanism has since been proposed which suggests LiCH 2CMe 3 metallates one of the protons on Ta(CH 2CMe 3)^Cl to generate {Ta(CH 2CMe 3 ) 3 ~ (CHCMe 3)Cl}~Li + which eliminates L i C l to give product C. Recent studies indicate that ct-elimination i s f a c i l i t a t e d by the removal of electron density from the a l k y l C-H bond to the metal c e n t r e . 7 6 The Y ~ e H - m i n a t i o n pathway i s the least documented of the afore-mentioned decomposition pathways. I t i s a process which has a high a c t i v a t i o n energy, a t t r i b u t a b l e to the s t e r i c requirements of the reaction. Reductive elimination provides a route for the cleavage of metal-carbon, and metal-hydrogen bonds. It i s confined to those metals having stable oxidation states d i f f e r i n g by two u n i t s . I t i s an infrequent process within the early group t r a n s i t i o n metal chemistry, becoming more prominent for the l a t e r t r a n s i t i o n metals. Reductive elimination usually involves c i s - o r i e n t a t e d groups of either an alkyl/hydride or an a l k y l / a l k y l combination 8 (eqn. 7). P d ( P E t 3 ) 2 + Me 2 (7) Homolytic cleavage for the group IVb t r a n s i t i o n metal a l k y l s i s an uncommon decomposition pathway. The infrequent occurrence of this process was i n i t i a l l y a t t ributed to the r e l a t i v e l y large t r a n s i t i o n metal-carbon bond strength. However t h i s idea was d i s p e l l e d with the tabulation of metal-carbon bond force constants for TiMeCl 3 and TiMe^ which were s i m i l a r to those found for t h e i r main group counterparts ( c . f . MeSnCl,). In f a c t , E t a P 3 ^ ^ P e r Et^P Me 7 horaolysis has now been Implicated i n the decomposition of a few organo-titanium and organozirconium compounds9 (eqns. 8, 9). (C 6H 5CH 2) 3Zr-CH 2Ph — - • (C 6H 5CH 2) 3Zr * + *CH2Ph (8) (C 6H 5CH 2) 3Zr-CH 2Ph — ( C 6 H 5 C H 2 ) 3ZrCH 2 + *Ph (9) A r e l a t i o n s h i p between the various modes of decomposition and the s t a b i l i t y of a metal a l k y l complex cannot be given; however, the following s t a b i l i t y sequence i s generally the most accurate and widely a c c e p t e d 1 0 (Scheme 4). PhCH 2 > Me 3SiCH 2 > Me 3CCH 2 > Ph > M e » Et > Sec-alkyl Scheme 4 1.1 Homoleptic Alkyls of Titanium(IV) , Zirconium(IV), and Hafnium(IV). Homoleptic a l k y l s of the group IVb metals are generally prepared by reacting an a l k y l lithium or Grignard reagent with a t r a n s i t i o n metal h a l i d e . The coordination number of the metal with respect to the number of a l k y l substituents, can be influenced by regulating the size of the a l k y l group (Scheme 5). 8 4LiR 3LiR -[MCIR3 ] M = Hf,Zr R=fTH(SiMe3^] [ M R 4 ] ^ MCI 4 M = Hf;Zr R =CH2CMe3 CH 2SiMe3 Scheme 5 The bulky b i s ( t r i m e t h y l s i l y l m e t h y l ) ligand precludes four-coordination about the metal centre, f a i l i n g to displace a l l the chloride ligands from Zr and H f 1 1 . A l l the group IVb homoleptic a l k y l s are a i r s e n s i t i v e ; for example, zirconium tetrabenzyl takes up two moles of to generate, among other products, benzyl alcohol and benzaldehyde a f t e r h y d r o l y s i s . The group IVb metal a l k y l s usually generate RH upon hydrolysis; t h e i r s e n s i t i v i t y to moisture i s of the general order T i < Zr <Hf. 1 2 From calculated heats of formation and metal-carbon bond energies,* 3 the thermal s t a b i l i t y of the a l k y l s follows the same trend, with the corresponding metal-carbon bond energies decreasing i n the sequence Q^PhX^H^e >CH^CMe^. The thermal decomposition of the t e t r a - a l k y l s [MR^] often produces alkanes and a reduced metal species, however, even subtle changes i n the nature of the hydrocarbyl group can influence the decomposition products (Scheme 6). 9 H RR + RH «• [ZrRj R=CH2CMe3 CH2SiMe3 R =CH2SnMe RR + RH +CgH6+C2H6 R = CH2Ph Scheme 6 1.2 Heteroleptlc a l k y l s of Tltanlum(IV), Zlrconlum(IV) and Hafnlum(IV). Three categories w i l l be reviewed: 1) derivatives containing c y c l o -pentadienyl groups, 2) complexes u t i l i s i n g phosphine ligands, 3) systems containing nitrogen donors. 1.2.1 Cyclopentadienyl complexes of Titanium(IV), Zirconium(IV), and  Hafnium(IV). There are few examples of mono-cyclopentadienyl complexes of Zr and Hf. The e a r l i e s t reported derivatives have the general formula {ZrCl R_ (n 5-CcHr)} (n • 1,2) (R - neopentyl). 7 These complexes are n J—n 3 3 thermally unstable; however, i n the presence of PMe3, i s o l a b l e compounds are formed. The bis-cyclopentadienyl d e r i v a t i v e s : ( TI 5-C 5H 5) jMR2 ( R * M e ) (M • T i , Zr, H f ) , l , are far more numerous. 10 .R M r " The bis-cyclopentadienyl complexes are commonly prepared from the appropri-ate metallocene d i c h l o r i d e and a suitable a l k y l lithium or Grignard reagent (eqn. 10). ( n 5-C 5H 5) 2MC12 + 2RLi —•*• ( n5-C 5H 5) jMRj + 2L i C l (10) The products are usually monomeric, colourless, c r y s t a l l i n e s o l i d s which may be p u r i f i e d by sublimation. The metal-carbon bonds are extremely reactive towards p r o t i c media, forming metal-oxygenated derivatives and eliminating an alkane (eqn. 11). ( n5-C 5H 5) 2ZrMe 2 + ROH — + ( n5-C5H5) 2Zr(0R)Me + Me (11) In contrast to their homoleptic counterparts they are r e l a t i v e l y unreactive towards 0 2 and C0 2» The* --hetaroleptic complexes also exhibit a greater degree of thermal s t a b i l i t y . For example, ZrMe 4 decomposes at temperatures above -70°C, while Cp 2ZrMe 2 sublimes between 100-110°C at 10"*• mm Hg. The 11 s t a b i l i t y of these systems i s in part attributed to the cyclopentadienyl ligand, which i s not only s t e r i c a l l y demanding but s u b s t i t u t i o n a l ^ i n e r t . The i n s e r t i o n of CO into the metal-carbon bonds of zirconocene and hafnocene d i a l k y l d e rivatives has recently undergone exhaustive i n v e s t i g a -t i o n (eqn. 12). ^  1 5 > 1 6 ( n 5 ^ C 5H 5)2MR 2 + CO — + ( n 5-C 5H 5) ^ (COIOR (12) M *= Zr, Hf R = Me, CH2Ph The reaction with CO generates n — a c y l s , 2. The acyl group may be considered to be a uninegative four-electron donor ligand which r e s u l t s i n an 18 electron complex. I s o l a t i o n has so far only been possible for the zirconium d e r i v a t i v e s . 2 12 1.2.2 Phosphine Complexes of Tltanlum(IV), Zirconium(IV), and  Hafnium(IV). Phosphine complexes of the early group t r a n s i t i o n metals are rare. Many of the known phosphine complexes display substantial l a b i l i t y when the phosphine donor i s coordinated to metals which have no d electron density, such as Zr(IV) or Hf(IV). One possible explanation for t h i s behaviour i s found by reference to the two basic i n t e r a c t i o n s believed to be involved i n a phosphorus-metal bond, namely, a donation from the lone pair of electrons on the phosphorus atom into an empty metal d o r b i t a l and % backdonation from f u l l or pa r t l y f u l l d - o r b i t a l s on the metal to low energy vacant d - o r b i t a l s on phosphorus ( F i g . 1). dir-dir bacxdonation M e t a l —c d o rb i ta l Phosphorus d o rb i ta l Figure 1. Representation of d-d o r b i t a l overlap i n a metal-phosphorus bond. The extent of du-du i n t e r a c t i o n i s dependent on two fact o r s : a) the degree of d - o r b i t a l overlap, and b) the electron density at the metal centre available for backdonation, which i s presumably small i n a formally 13 d° metal complex. However, a large du-d-jt i n t e r a c t i o n does not nec e s s a r i l y generate a non-labile phosphine complex. Investigations on the s t a b i l i t y of a series of Ni(0) complexes containing a v a r i e t y of phosphine donors concluded that i t i s the size of the phosphine donors rather than th e i r e l e c t r o n i c character that p r i n c i p a l l y determines the s t a b i l i t y of the c omplex. 1 8' 1 9 A large number of phosphine ligands have been ranked according to cone angle ( F i g . 2), (the angle subtended by a cone i n a Ni-PR, fragment). Figure 2. Representation of a phosphine cone angle, 9. Early t r a n s i t i o n metal phosphine chemistry i s s t i l l i n i t s infancy, for example, there are only a few reported phosphine derivatives of zirconium i n which the metal i s i n a +4 oxidation state. One p a r t i c u l a r complex has just recently been d e s c r i b e d . 2 0 The reaction involved the 14 carbonylation of Cp 2Zr(Cl)CH 2PPh 2 to generate product B v i a an interaolecular proton transfer pathway (Scheme 7). ,PPh CH 2PPh 2 2 C p ZrCICH 2PPh 2- CO CH 3PPh 2 Scheme 7 The l a b i l i t y of the phosphine donor was also displayed during the unsuccessful attempts to induce intramolecular proton-abstraction i n a 7 series of neopentyl phosphine complexes of^Zr(IV) (eqn. 13). (13) Zr(CH 2CMe 3) 2Cl2(}pHe3) 2 + 2PMe 3 ~^ ZrCl 2&He 3)( 2PMe 3) ( C H 2 C M e 3 ) 2 + 1PMe 3 Only one si g n a l was present i n the 3 1P NMR when PMe3 was added to ZrCl 2(CH 2CMe 3) 2(PMe 3) 2, which suggested the phosphine ligands were 38 exchanging r a p i d l y on the NMR time scale (eqn. 13). 15 1.2.3 Amide Complexes of T i t a n i u m ( I V ) , Z i r c o n i u m ( I V ) , and H a f n i u m ( I V ) . A m e t a l amide i s a compound which c o n t a i n s one or more NR"^  u n i t s c o o r d i n a t e d to a m e t a l (R = a l k y l , a r y l or s i l y l ) 2 1 . I n c o n t r a s t t o the r e l a t i v e l y s m a l l number of phosphine complexes i s o l a t e d f o r the e a r l y group t r a n s i t i o n m e t a l s , a wide v a r i e t y of amide d e r i v a t i v e s have been c h a r a c t e r i s e d . There are s e v e r a l bonding p o s s i b i l i t i e s a v a i l a b l e t o t r a n s i t i o n m e t a l amides. These are i l l u s t r a t e d below ( F i g . 3a, 3b, 3 c ) . Kl M R-R 3a M 3b N—- M F i g u r e 3a, 3b, 3c. Bonding p o s s i b i l i t i e s a v a i l a b l e to t r a n s i t i o n m e t a l amides A m e t a l amide can adopt a p y r a m i d a l c o n f i g u r a t i o n w i t h an approximate s p 3 mode of h y b r i d i s a t i o n of the n i t r o g e n atom ( 3 a ) , or s e r v e as a b r i d g i n g u n i t between two m e t a l c e n t r e s ( 3 b ) . The t h i r d p o s s i b i l i t y , 3c, w h i c h i n v o l v e s a p^-du i n t e r a c t i o n i n a d d i t i o n to a a component was put f o r w a r d to e x p l a i n the r e l a t i v e l y s h o r t M-N bond d i s t a n c e s and the p l a n a r R 2NM u n i t s found i n s e v e r a l of these complexes ( F i g . 4 ) . 16 Figure 4. Representation of p-d o r b i t a l overlap i n a metal-nitrogen bond The pu-dn component of the bonding involves the i n t e r a c t i o n of the lone pair of electrons on the nitrogen atom with low energy vacant d - o r b i t a l s on the metal. Zr(IV) and Hf(IV), which have vacant d - o r b i t a l s , can accept pit electron density from a nitrogen donor and therefore form stable amide de r i v a t i v e s , in contrast to the l a t e r group t r a n s i t i o n metals which have pa r t l y f u l l or f u l l d - o r b i t a l s and as a consequence behave as poor rc-electron acceptors. Both homoleptic and h e t e r o l e p t i c t r a n s i t i o n metal amides are known. There are two common procedures employed i n t h e i r preparation: transmetal-l a t i o n and transamination. The former involves the addition of a lithium 17 amide to the appropriate metal halide (eqn. 14). This i s the method generally used to prepare homoleptic amides. 2 2 4Li(NMe 2) + MCl^ * M(NMe2) ^  + 4L i C l (14) M - T i , Zr, Hf. The preparation of he t e r o l e p t i c amides can be accomplished by trans-amination (eqn. 15), a process which involves the exchange of one amide for another at the metal centre. The more v o l a t i l e amine i s usually displaced. Zr(NMe 2) 4 + 2HN(i-Pr) 2 — * Zr(NMe 2) 2(N( i-Pr) 2.) 2 + 2HNMe 2 (15) The use of bulky amido ligands allows the i s o l a t i o n of complexes having low coordination numbers, for example, T i {N( (SiMe 3) 2 ) ^ 3 compared to Ti(NMe 2) 1 +. The homoleptic amides M(NR 2) l 4 (M - T i , Zr, Hf, R - a l k y l ) are thermally stable; however, they are re a d i l y hydrolysed and react with protic solvents to generate the metal a l k o x i d e 2 3 (eqn. 16). Zr(NR 2)^ + 4R0H —— » Z r ( O R ) l 4 + 4R2NH (16) The addition of lithium or sodium (hexamethyldisilyl)amide to zirconium or hafnium t e t r a c h l o r i d e generates the hydrocarbon-soluble compounds 18 24 25. M C 1 4 - n { N ( S i M e 3 > 2 > n ( n " X' 2> ( e c l n - 1 7>' ' nLi{N(SiMe 3) 2} + MCl^ • MCl 4_ n {N(SiMe 3) 2>n+ nLi C l (17) The tris-amido d e r i v a t i v e s , MCI{N(SiMe 3) 2) 3, are a i r stable, i n contrast to the b i s - d e r i v a t i v e s , MCl 2(N(SiMe 3) 2> 2, which are s l i g h t l y a i r and moisture s e n s i t i v e . The addition of methyl lithium to these complexes gives the corresponding metal-alkyl amides (eqn. 18). MCl 2{N(SiMe 3) 2} 2 + 2MeLi — * M(Me) 2{N(SiMe 3) 2) 2 + 2LiCl (18) The d i - a l k y l metal amides are unreactive towards 0 2 and moisture. They are also k i n e t i c a l l y i n e r t , a property which i s at t r i b u t a b l e to the large silylamide ligands which tend to preclude r e a c t i v i t y at the metal centre. 1.3 Design and Synthetic U t i l i t y of a Hybrid Multidentate Ligand. The concept of ligand design has been a prominent feature in the development of t r a n s i t i o n metal ch e m i s t r y . 1 7 By c o n t r o l l i n g the s t e r i c and elec t r o n i c environment about the metal centre i t i s possible to influence the s t a b i l i t y and r e a c t i v i t y of the complex. The d i f f i c u l t y i n i s o l a t i n g 19 and successfully characterising phosphine complexes of the group IVb metals has been overcome by developing a ligand system which contains both phosphine donors and an amide donor i n a chelating array, 3. :PR. 3 The ligand, 3, may formally be regarded as a uninegative 6e donor, or a uninegative Ae donor similar i n nature to the a l l y l group C 3H 5~, depending upon the tendency of the ligand to coordinate i n a bidentate or tridentate manner (Figs. 5a, 5b) r e s p e c t i v e l y . 20 Me Me Me ,Me Ml"* PR Me Me Me Me Si" V" R2P +M< PR2 5a 5b Figures 5a and 5b. Coordination modes ava i l a b l e to [NCSiMejCHjPI^) 2]~* I n i t i a l investigations which u t i l i s e d t h i s ligand i n conjunction with metals from the l a t e r groups of the t r a n s i t i o n metal s e r i e s : Ni, Pd, Pt 26 27 gave a range of reactive t r a n s i t i o n metal chloro d e r i v a t i v e s , 4. ' PPh2 Mezsi N- -M- CI Me-) Si 21 Coordination of the ligand to zirconium and hafnium was also successful, generating b i s - l i g a n d derivatives of the formula MCI2{N(SiMe 2CH 2PR2)2 ^  2» 5 > 28» 29 5 These derivatives displayed l i m i t e d r e a c t i v i t y , a r e s u l t attributed to s t e r i c crowding at the metal centre. Addition of excess H f C l 4 to 5^  produced a hafnium t e t r a c h l o r i d e adduct of empirical formula Hf 2Cl 7{N(SIMe 2CH 2PMe 2) 2}, 6. This d e r i v a t i v e was found to be p a r t i c u l a r l y useful i n generating hydride d e r i v a t i v e s . However, 6 was r e l a t i v e l y insoluble and i t s chemical nature was poorly characterised. 22 6 This thesis examines the formation and stereochemistry of the mono-ligand d e r i v a t i v e s , MCI 3{N(SiMe 2CH 2PR 2) 2} a n d explores t h e i r chemical r e a c t i v i t y with p a r t i c u l a r emphasis on the formation of metal-carbon bonds• 23 CHAPTER 2 2.1 Synthesis of LlN(SlMe ?CH ?PR 2) ?. The addition of NH(SiMe 2CH 2C1) 2 to three equivalents of L i P R 2 i n THF generates the corresponding lithium-amido-phosphine derivatives LiN(SiMe 2CH 2PR 2) 2 (eqn. 19). Me, N H 3LIPR, CI CI Me N Li R,P Me +HPR2 + 2 L i C l (19) PR, R=Me la* R=i-Pr 7b* R=t-Bu 7c The products are col o u r l e s s , c r y s t a l l i n e , a i r and moisture s e n s i t i v e s o l i d s , which are soluble i n a va r i e t y of organic solvents. They are pu r i f i e d by f r a c t i o n a l c r y s t a l l i s a t i o n from hexanes at -30°C. In addition, 7c occludes a small amount of THF from the reaction mixture which The compounds 7a and 7b were o r i g i n a l l y prepared by Dr. Axel Westerhaus 24 cannot be removed by r e c r y s t a l l l s a t i o n from d i f f e r e n t solvents, and It Is therefore used as a THF solvate. 2.2 Preparation and stereochemistry of MCI ?{N(SiMe ; C H 2 ^ * The addition of LiN(SiMe 2CH 2PR 2) 2 (R - Me, i - P r , t-Bu) to the appropriate metal t e t r a c h l o r i d e MCl^ (M - Zr, Hf) generates the mono-ligand derivatives MCI 3{N(SiMe 2CH 2PR 2) 2) (eqns. 20, 21). 5 days LiN(SiMe 2CH 2PMe 2) 2 + MCl^ * MCI 3{N(SiMe 2CH jPMe 2) 2) + L i C l (20) M - Hf, 8a* M - Zr, 9a* 1 day LiN(SiMe 2CH 2PR 2) 2 + MCl^ MCI 3{N(SiMe 2CH 2PR 2) 2) + L i C l (21) M R Hf i - P r 8b* Hf t-Bu 8c Zr i-Pr 9b* Zr t-Bu 9c * The compounds $a, 8J>, 9ja and 9J> were o r i g i n a l l y prepared by Dr. A. Uesterhaus. The X-ray c r y s t a l structures of 8a and 9b were obtained from samples also prepared by Dr. Westerhaus. ^ 7 25 I n i t i a l l y , o i l y products and poor yields of 8a were obtained; however, monitoring this reaction by 3 1P{ 1H} NMR established that the b i s - l i g a n d derivative 5, was formed f i r s t which then underwent a conproportionation reaction with excess H f C l ^ to generate 6 (eqn. 22). (22) 5 6 In an attempt to optimise the y i e l d s of 8a (R = Me), the procedure was refined to include longer reaction times and higher reactant solution d i l u t i o n . The reaction of zirconium or hafnium tet r a c h l o r i d e with the bulkier phosphine s a l t s 7b (R = i - P r ) , and 7c (R = t-Bu) (eqn. 21), proceeds d i r e c t l y to give the mono-ligand derivatives i n good y i e l d s . The products c r y s t a l l i s e from hexane/toluene at -30°C. They are c o l o u r l e s s , a i r and moisture s e n s i t i v e s o l i d s , which are only s l i g h t l y soluble i n hexanes or d i e t h y l ether but are r e a d i l y soluble in aromatic solvents. 26 Stereochemistry of the mono ligand d e r i v a t i v e s . The proton NMR of 8a (R = Me) displays one resonance for the s i l y l methyl protons (SiCH 3), and two " f i l l e d - i n " doublets for the methylene (PCH 2) and methyl (PCH 3) protons ( F i g . 6). The X-ray c r y s t a l l o g r a p h i c analysis of 8a (R = Me) s u r p r i s i n g l y showed that the ligand i s bound i n a f a c i a l o r i e n t a t i o n . This i s inconsistent with the proton NMR data which i n f e r s a meridional mode of l i g a t i o n for the amido-phosphine ligand, ascribable to the equivalent s i l y l m e t h y l (SiCH 3), methylene (PCH 2), and methyl (PCH 3) protons. If a s t a t i c , f a c i a l l y orientated species e x i s t s i n solu t i o n then two d i s s i m i l a r environments for In a meridional geometry, the methyl protons can be described as belonging to an A 2 A 2 XX' spin system which reduces to an A i |X 2 spin system when JyyJ i s large. This generates a v i r t u a l t r i p l e t for the A protons. ^0 If however ^xx'^AX'' a ^ i H - e £ * - i - n ' doublet i s observed rather than the v i r t u a l t r i p l e t one might have expected to see from trans-disposed phosphines. PMe SiMe ppm Figure 6. 80MHz, XH NMR spectrum of mer-HfCI 3{N(SiMe 2CH 2PMe 2) 2> 27 the aforementioned groups would be generated. To account for t h i s inconsistency an isomerisation of the f a c i a l l y orientated ligand i s thought * to take place in solution to generate the meridionally bound analog ( F i g . 7). Figure 7. Proposed isomerisation of fac-HfCI,{N(SiMe 2CH 2PMe 2) 2 } to mer-HfCl 3{N(SiMe 2CH 2PMe 2) 2} i n s o l u t i o n . Fluxional processes involving fac-mer interconversions have been dismissed on the basis of 3 1 P {-^H} variable-temperature NMR experiments on 8a, which displays only one signal down to -90°C. 28 CIS) C(3) Figure 8. Structure and numbering scheme of HfCI 3{N(SiMe 2CH 2PMe 2) 2) (8a). Figure 9. Structure and numbering scheme of Z r C l 3 { N ( S i M e 2 C H 2 P ( i - P r ) 2 ) 2 ) (9b). 29 Increasing the size of the a l k y l group on the phosphorus donor induces dramatic stereochemical changes i n the molecule. The c r y s t a l l o -graphic analysis of 9b (R = i-Pr) shows the ligand to be bound in a meridional o r i e n t a t i o n ( F i g . 9), with i r r e g u l a r octahedral geometry. (This i s i n contrast to 8a i n which the ligand i s bound f a c i a l l y , see F i g . 8). One possible reason for th i s geometrical modification would be the minimisation of ligand-ligand repulsions between the bulkier iso-propyl phosphine groups. To interpret the stereochemistry of 9b (R • i-Pr) from the proton NMR i s d i f f i c u l t owing to the m u l t i p l i c i t y of the iso-propyl s i g n a l s . In solution 9b (R = i-Pr) appears to r e t a i n i t s meridional geometry; only one type of si l y l m e t h y l signal i s observed down to -90°C. A much simpler spectral pattern i s observed with the bulkier t e r t i a r y - b u t y l analog 8c (R = t-Bu) ( F i g . 10), which i s sim i l a r to that described for 8a (R = Me); a si n g l e t for the s i l y l m e t h y l protons (SiCH 3) together with " f i l l e d - i n " doublets for both the t e r t i a r y - b u t y l (PC(CH 3) 3) and methylene (PCH 2) groups are present i n the proton NMR which i s once again consistent with a meridional geometry. In fact i t i s believed a l l the mono-ligand derivatives MC1 3{N(SiMe 2CH 2PR 2) 2>; 8 and 9 (R = Me, i - P r , t-Bu, M - Zr, Hf) exist i n a meridional conformation i n solu t i o n . 30 Figure 10. 80MHz *H NMR spectrum of mer-HfCI 3{N(SiMe 2CH 2P(t-Bu) 2) 2> 31 2.3 The preparation and stereochemistry of M(CH 3) 3{N(SiMe 2CH 2PR 2) 2) The addition of three equivalents of MeMgCl i n THF to an ether sol u t i o n of the mono-ligand complexes MCI 3{N(SiMe 2CH 2PR 2) 2> 8_ and 9 (R = Me, i - P r , t-Bu, M = Zr, Hf) generates the corresponding trimethyl derivatives M(CH 3) 3{N(SiMe 2CH 2PR 2) 2) (eqn. 23). MCl 3{N(SiMe 2CH 2PR 2) 2} + 3MeMgCl —-* M(CH3) 3{N(SiMe 2CH 2PR 2) 2) + 3 MgCl 2 M R Hf Me 10a Hf i-Pr 10b Hf t-Bu 10c Zr Me U a * Zr i-Pr U b Zr t-Bu 11c These compounds are col o u r l e s s , moisture s e n s i t i v e , c r y s t a l l i n e s o l i d s , which are unstable at room temperature and ra p i d l y decompose i n a i r . The thermal s t a b i l i t y of these complexes decreases i n the order t-Bu > i-Pr > Me and Hf > Zr. Although the t e r t i a r y - b u t y l phosphine donors are s l i g h t l y more basic than the isopropyl or methyl analogs, the e l e c t r o n i c influences which contribute to the s t a b i l i t y of the t r i - a l k y l derivatives i s presumably minimal when compared to the s t e r i c e f f e c t s . 11a was only observed i n the proton NMR and was not i s o l a t e d . 32 The analogous reaction to the trimethyl preparation, using 2 moles of MeMgCl to generate the dimethyl derivatives M(CH3) 2C1 {N(SiMe 2CH jPRj) 2> (eqn. 24) was unsuccessful. The addition of two equivalents of MeMgCl to the mono-ligand d e r i v a t i v e s 8a (R - Me) and 8b (R • i-Pr) generated a mixture of both the mono- and d i - a l k y l products which could not be separated on the basis of the i r physical or chemical properties. HfCl 3{N(SiMe 2CH 2PR 2) 2} + 2MeMgCl -»• Hf(Me ) xCl Y{N(SiMe 2CH 2PR 2) 2) } x-0, y=3 (24) x=l, y=2 x=2, y=l The addition of two equivalents of MeMgCl to 8c (R - t-Bu) resulted i n the i s o l a t i o n of the trimethyl derivative 10c (R - t-Bu) and s t a r t i n g m a t e r i a l . Monitoring the reaction of 8b (R • i-Pr) with 2 equivalents of MeMgCl by 3 1P{ 1H} NMR indicated that the trimethyl d e r i v a t i v e i s i n i t i a l l y formed within 3 minutes ( F i g . 11). Two add i t i o n a l s i n g l e t resonances then begin to appear concurrent with the disappearence of the trimethyl resonance. The two new peaks correspond to the mono- and d i - a l k y l d e r i vatives Hf(CH 3)C1 2{N(SiMe 2CH 2P(i-Pr) 2) 2) and Hf(CH 3) 2C1{N(SiMeg-33 CH 2P(t-Pr) 2) 2 ) . The chemical s h i f t s of these derivatives l i e between those of the trimethyl complex and s t a r t i n g m a t e r i a l . L = N(SiMe2CH2PR2) LHfMeCU t = 3 min LHfMe,CI W f M e 3 30 2 0 10 P Pm t = 30 min 30 To" "7b~ ~o P Pm t = 1 h To To" Figure 11. ppm JO 3 1 P NMR spectrum following the reaction between HfCl 3{N(SiMe 2CH2P(i-Pr) 2) 2> and two equivalents of MeMgCl. 34 Presumably the formation of these products occurs v i a a conproportionation reaction between the trimethyl species 10b (R • i-Pr) and s t a r t i n g material 8b (R - i-Pr) (eqn. 25, scheme 8). L - W l .+ LMCH. L1M CI ML CH •L1MCH3+ LMCI L = N(SiMe2CH2PR ) (25) L H f C l 3 + 2MeMgCl -»•-| L HfMe 3 + -j L H f C l 3 + 2 M g C l 2 conproportionation L HfMe CI x y (x - 1,2,3) Scheme 8 35 For 8c (R - t-Bu), s t e r i c repulsion between the t e r t i a r y - b u t y l groups may prevent the bridging conproportionation step, which precludes methyl-halide exchange. This would account for the i s o l a t i o n of s t a r t i n g material i n the reaction of 8c (R - t-Bu) with 2 equivalents of MeMgCl. An equilibrium i s eventually established between the monomethyl and dimethyl derivatives and st a r t i n g material (as demonstrated by the fact that a f t e r monitoring the reaction over a period of 24 h i n the 3 1P{ 1H} NMR, the concentrations, of a l l species i n solution remain constant). To further investigate the proposal of methyl-halide exchange, the reaction of 8b (R • i-Pr) and 10b (R = i-Pr) was monitored by 3 1P{ 1H} NMR. Within minutes the si n g l e t corresponding to 10b (R = i-Pr) disappeared and two new resonances belonging to the monomethyl and dimethyl analogs appear, which indicated methyl-halide exchange was taking place. The proton NMR spectra for a l l the trimethyl derivatives show one type of si l y l m e t h y l (SiCH 3), phosphine (PR 2)» and methylene (PCH 2) proton environment ( F i g . 12). One anomaly, however, i s the presence of only one t r i p l e t resonance for the methyl groups attached to the metal centre. 36 *CCH, SiCH, H tCH 3 -CH, 1 p pm Figure 12. 80MHz *H NMR spectrum of Hf(CH 3) 3{N(SiMe 2CH,P(t-Bu),),} 2 / 2 J If one assumes a r i g i d octahedral geometry around the metal there should be two signals generated by the methyl protons. One signal for the methyl group trans to the amide and one for the two methyl groups c i s to the amide donor. The variable-temperature 3 1P{ :H} NMR of 10c (R = t-Bu) provides a possible answer to this dilemma ( F i g . 13). 10c (R = t-Bu) displays one signal at room temperature (as one might predict for two phosphorus n u c l e i i n equivalent environments). This signal broadens at -70°C and f i n a l l y , on cooling to -90°C, two resonances appear. One possible i n t e r p r e t a t i o n of the variable-temperature NMR data requires a process whereby one of the phosphine donors dis s o c i a t e s from the metal to generate a f i v e coordinate intermediate. This i s then followed by recoordination of the phosphine donor. It would appear that the coordi-nated and uncoordinated phosphorus n u c l e i are only distinguishable i n 37 e 13. Variable-temperature *H and 3 1P{ 1H) NMR of Hf (CH 3) 3{N(SiMe 2CH 2P(t-Bu) 2) 2>. 38 the 3 1P{ 1H} NMR spectrum at low temperature when the process of as s o c i a t i o n / d i s s o c i a t i o n i s slowed down or stopped. In other words, the exchange rate of the phosphorus n u c l e i between the two environments at room temperature i s fast i n comparison to the frequency difference between the two si g n a l s , and as a consequence, the two signals merge to form one resonance peak. The process i s said to be fast on the NMR time s c a l e . 3 8 To pursue the idea of phosphine d i s s o c i a t i o n , AlMe, was added to a solut i o n of 10c (R = t-Bu) i n an attempt to prevent recoordination of the phosphine donor by generating the corresponding phosphorus/AlMe 3 adduct, 12. 12 39 M o n i t o r i n g t h i s r e a c t i o n i n the 3 1 P { 1 H } NMR spectrum, t h e r e i s an u p f i e l d s h i f t of a p p r o x i m a t e l y 4ppm i n the peak, p o s i t i o n of 10c (R=t-Bu) a f t e r the a d d i t i o n of e x c e s s A l M e 3 . T h i s was an e n c o u r a g i n g r e s u l t , i n l i g h t of the f a c t t h a t t h e r e was no r e a c t i o n between A l M e 3 and 8a (R = Me) o r 8c (R = t - B u ) ; the t r i c h l o r o d e r i v a t i v e s , which i s i n a c c o r d a n c e w i t h the c o n t e n t i o n t h a t the phosphine donors i n t h e s e complexes do not d i s s o c i a t e i n s o l u t i o n . I t i s , however, e x t r e m e l y u n l i k e l y t h a t the new s p e c i e s formed i s ' 12J . N o r m a l l y adduct f o r m a t i o n i n v o l v i n g phosphorus and a l u m i n i u m , i s accompanied by a d o w n f i e l d s h i f t i n the phosphorus peak 39 p o s i t i o n . F u r t h e r m o r e , one cannot r u l e out the p o s s i b i l i t y of c o m p e t i t i o n o c c u r r i n g between the two Lewis a c i d c e n t r e s of hafnium and aluminium f o r t h e phosphine donors r a t h e r than s i m p l e adduct f o r m a t i o n . To e x p l a i n the o b s e r v a t i o n of o n l y one t r i p l e t resonance i n the *H NMR spectrum f o r the m e t h y l groups a t t a c h e d to the m e t a l c e n t r e u t i l i s i n g the i d e a of phosphine d i s s o c i a t i o n , one must examine the p o s s i b l e rearrangement pathways a v a i l a b l e to p e n t a c o o r d i n a t e complexes 31*32 ( a p e n t a c o o r d i n a t e s p e c i e s would be g e n e r a t e d i n the t r i m e t h y l complexes I f one of the phosphine arms d i s s o c i a t e s ) . One p o s s i b l e mechanism r e q u i r e s the n i t r o g e n donor and t h r e e m e t h y l groups to be a r r a n g e d t e t r a h e d r a l l y , ( a r e q u i r e m e n t which i s not 40 u n r e a l i s t i c based on the c r y s t a l l o g r a p h i c data obtained for 10a (R » Me)) with the phosphorus donors coordinated to the metal through two d i s s i m i l a r faces of the tetrahedron. If one considers the nitrogen atom to play the role of pivot, each phosphorus donor i s dissociated i n turn and rotated through 120°, whereupon recoordination to the metal takes place through a d i f f e r e n t face of the tetrahedron. After a complete 360° r o t a t i o n of the chelating phosphine a l l the methyl groups w i l l have occupied equivalent positions (Scheme 9). Me Scheme 9 The Berry pseudorotation mechanism 3 3 involves the pairwise exchange of a p i c a l and equatorial ligands i n pentacoordinate TBP complexes. In the scheme shown below (Scheme 10), E' i s a pivot and does not p a r t i c i p a t e i n any p o s i t i o n a l exchange. 41 Scheme 10 This mechanism i s demonstrated below i s an attempt to account for the behaviour of the methyl groups i n the M(CH3) 3{N(SiMe 2CH 2PR 2) 2) c o m P l e x e s (Scheme 11). (For c l a r i t y , only the coordinated arms of the chelating amido-phosphine ligand are shown). Scheme 11 On moving from structure A to structure B (Scheme 11) one proceeds through two consecutive BPR's each structure generated by permutation of the four ligands preceding i t with CI and C3 acting as pivots. In comparing structures A and B i t has been shown 1(CH 3)=2(CH 3), 3(CH 3)=1(CH 3), 2(CH 3)=3(CH 3). 42 A further possible mechanism to account for isoraerisations involving TBP complexes i s known as t u r n s t i l e r o t a t i o n 3 2 , (TR). The mechanism corresponds to an i n t e r n a l rotation of one a p i c a l and one equatorial ligand rotating as a pair versus the oppositely rotating t r i o of three remaining ligands (Scheme 12). 4 5 Scheme 12 Once again by manipulation of the M(CH3) 3{N(SiMe 2CH 2PR 2) 2) derivatives to accommodate two successive TR's one generates equivalent methyl groups (Scheme 13). 43 Scheme 13 Comparing structures C and D, Once again i t has been shown 3(CH 3)«1(CH 3) , 2(CH 3)-1(CH 3), 2(CH 3)-3(CH 3). There Is at the present time no evidence to support a l l or any of the preceding mechanisms. They are shown only to i l l u s t r a t e the possible rearrangement pathways av a i l a b l e to the trimethyl complexes. The s o l i d state structure of 10a ( F i g . 14) shows the ligand i s bound in a f a c i a l mode as previously encountered for the t r i c h l o r o d e r i v a t i v e 8a (R - Me). There are large deviations from an octahedral geometry. The C(5)-Hf-N bond angle i s 136° 1"; the nitrogen atom l y i n g approximately 44° out of a trans o r i e n t a t i o n . The nitrogen atom and three carbon atoms of the methyl groups appear to be arranged in the form of a pseudo-tetrahedron. Although the C(4)-Hf-N angle of 113° and C(3)-Hf-N 44 angle of 102° show s l i g h t deviations from the expected tetrahedral angle of 109°, the C(5)-Hf-N of 136° i s much too large for a regular tetrahedron. Figure 14. Structure and numbering scheme for fac-Hf(CH 3) 3{N(SiMe 2CH 2PMe 2) 2}. The Hf-P bond lengths, ranging from 2.761 to 2.806 A,are 0.055 to 0.070' larger than the corresponding bond lengths found for the t r i c h l o r o d e r i v a t i v e s . 3 7 This may t e n t a t i v e l y be attributed to an increase i n electron density at the metal centre when the more electronegative c h l o r i d e atoms are replaced by the less electronegative methyl groups. The far IR region on s o l i d samples (Csl disk) of the trimethyl derivatives did not provide any d e f i n i t i v e information on the i r B o l i d - s t a t e structures. A l l band assignments were made by comparison to previous l i t e r a t u r e values given for metal-nitrogen and metal-halogen l.R. stretching f r e q u e n c i e s 2 ^ 2 5 . 45 CHAPTER 3 EXPERIMENTAL General Information. A l l manipulations (unless otherwise stated) were performed under prepurified nitrogen i n a vacuum atmospheres HE-553-2 glovebox equipped with a M0-40-2H p u r i f i c a t i o n system or i n standard Schlenk-type glassware. The compounds Z r C l ^ (Aldrich) and H f C ^ (Alfa) were sublimed p r i o r to use. The secondary phosphines, HPMe 2» H P ( i - P r ) 2 and HP(t-Bu) 2 were prepared according to l i t e r a t u r e p r o c e d u r e s . 3 4 ' 3 5 ' 3 6 The lith i u m amide derivatives 7a and 7b and the mono-ligand complexes 8a, 8b, 9a, 9b were o r i g i n a l l y prepared by Dr. A. Westerhaus. 3 7 Carbon, hydrogen, nitrogen, and halogen analyses were performed by Mr. P. Borda of t h i s Department. Solvents were dried, d i s t i l l e d and degassed by standard procedures. C r y s t a l structure analyses were performed by Dr. S.J. Rettig of this department. *H NMR spectra were run on one of the following instruments depending upon the complexity of the p a r t i c u l a r spectrum: Bruker WP-80, Varian XL-100, or Bruker WH-400. 3 1P{ 1H} NMR spectra were obtained at 32.442 MHz on the WP-80; a l l 3 1 P chemical s h i f t s were referenced to external P(OMe) 3 set at 141.00 ppm r e l a t i v e to 85% HjPO^. Infrared spectra were run on a Nicolet 5D-X F.T. instrument. Deuterated benzene (CgDg) and deuterated toluene (C 7Dg) were obtained from A l d r i c h Chemical Company, dried over activated 4A molecular sieves and vacuum transferred prior to use. 46 Preparation of LiN{SiMe 2CH 2 P ( t - B u ) 2 ) To a cold (-78°C) toluene s l u r r y (=30 mL) of L i P ( t - B u ) 2 (7.0 g, 46 mmol), prepared by s t i r r i n g n-BuLi and HP(t-Bu) 2 for 6 days, was added THF (100 mL) and the r e s u l t i n g yellow solution allowed to warm slowly to -15°C; to this solution was added neat HN(SiMe 2CH 2C1) 2 (3.53 g 15.3 mmol) dropwise. After allowing the solu t i o n to warm to room temperature, the v o l a t i l e s were removed under vacuum; the residue was extracted with hexanes (3 x 100 mL), f i l t e r e d through a medium-porosity f r i t and reduced i n volume u n t i l c r y s t a l l i s a t i o n began. Cooling to -30°C afforded a 60% y i e l d of sticky white c r y s t a l s . XH NMR (C 6D 6, ppm): PC(CH 3) 3, 1.10 (br d. 3 J p = 12Hz); PCH 2Si, 0.56 (d, 2 J p = 8.2 Hz); SiCH 3, 0.34 ( d , . \ j p - 1Hz). 3 1P{ 1H} (C 6D 6, ppm): +18.2(s). So far we have been unable to obtain a n a l y t i c a l l y pure samples of t h i s m a t e r i a l . Preparation of HfCI 3{N(SiMe 2CH 2P(t-Bu) 2) 2) A sol u t i o n of LiN(SiMe 2CH 2P(t-Bu) 2) 2 (4.0 g, 5.7 mmol) i n toluene (30 mL) was added dropwise to a s l u r r y of Hf C l ^ (2.18 g, 5.79 mmol) i n toluene (150 mL). The reaction mixture was s t i r r e d at room temperature for 1 day, f i l t e r e d through C e l i t e , and the toluene removed under vacuum. The residue was r e c r y s t a l l i s e d from minimum toluene/hexanes and cooling to -30°C gave white needles: y i e l d 5.24 g (81%). XH NMR (C 6D f i, ppm): PC(CH 3) 3, 1.30 ("filled-in'doublet, | 3 J p + 5 J p | = 13.2 Hz); PCH 2Si, 1.07 ( f i l l e d i n doublet, | 2 J p + \ j p | - 10.5 Hz); SiCH 3, 0.47 ( s ) . 3 1P{ 1H} 47 (C 6D 6, ppm); +38.5 ( s ) . Anal. Calcd. for C 2 2 H 5 2 C 1 3 H f ^ 2 S i 2 : C» 3 6 * 0 2 ! H » 7.14; N, 1.91; CI, 14.50. Found: C, 35.86; H, 7.19; N, 1.85; CI, 14.30. m.p. 249-251°C. IR ( C s l , cm - 1): Hf-N, 398; Hf-Cl, 296. Preparation of ZrCl 3{N(SiMe ^ ^ ( t ^ B u ) 2) 2) The i d e n t i c a l conditions described above for the hafnium derivative were used to prepare the zirconium complex. Y i e l d : 85%. *H NMR (C 6D &, ppm): PC(CH 3) 3, 1.26 ( f i l l e d i n doublet | 3 J p + 5 J p | = 13.5 Hz); PCH 2Si, 1.08 C f i H e d - i r f doublet | 2 J p + \ j p | - 10.1 Hz); SiCH 3, 0.50 ( s ) . 3 1P{ 1H} (C 6D 6, ppm) +33.85 ( s ) . Anal. Calcd. for C 2 2 H 5 2 C l 3 N P 2 S i 2 Z r : C, 40.88; H, 8.11; N, 2.17; CI, 16.45. Found: C, 41.16; H, 8.30; N, 2.13; CI, 16.21. m.p. 234-236°C. IR ( C s l , cm - 1): Zr-N, 399; Z r - C l , 325. 304. Preparation of Hf(CH 3) 3{N(SiMe 2CH 2P(t-Bu) 2) 2) To a s t i r r e d s o l u t i o n of HfCl 3{N(SiMe 2CH 2P(t-Bu) 2) 2) (1.5 g, 2.1 mmol) in E t 2 0 , MeMgCl (5.58 mL, 1.1M i n THF, 6.1 mmol) was added dropwise. The r e s u l t i n g mixture was s t i r r e d for three hours whereupon the E t 2 0 was removed under vacuum. The s o l i d was extracted with hexanes (15 mL), f i l t e r e d through a f i n e - p o r o s i t y f r i t , and allowed to r e c r y s t a l i s e from minimum hexanes at -30°C for one day. Y i e l d 65% (0.9 g). *H NMR (C gD 6, ppm): PC(CH 3), 1.12 (d, 3 J p - 11.2Hz); PCH 2, 0.78 (d, 2 J p - 6.1Hz); SiCH 3, 0.48 ( s ) ; HfCH 3, 0.87 ( t , 3 J p = 1.2Hz). 3 1P{ 1H} (CgD 6): +20.88 ( s ) . Anal. 48 Calcd. for C 2 5 H 6 1 H f P 2 S i 2 N : C, 44.66; H, 9.14; N, 2.08. Found C, 44.46; H, 9.14; N, 1.88, IR ( C s l , cm"*1); Hf-N 502,481. Preparation of Hf(CH 3) 3{N(SiMe 2CH 2P(i-Pr) 2) 2) The analogous conditions described above for the t e r t i a r y - b u t y l d e r i v a t i v e were used. Y i e l d 70%. *H NMR (C 6D 6, ppm): SiCH 3, 0.41 ( s ) ; PCH 2, 0.89 (d, 2 J p - 6.3Hz); HfCH 3, 0.74 ( t , 3 J p = 3.5Hz); PCH(CH 3), 1.13 (m); PCH(CH3) 1.94 (m). 3 1P{ 1H} (C 6D 6, ppm): +2.4 ( s ) . Anal. Calcd. f o r C 2 1 H 5 3 H f N P 2 S i 2 : C, 40.92; H, 8.67; N, 2.27. Found C, 41.03; H, 8.65; N, 2.39. IR ( C s l , cm - 1): Hf-N 488, 457. Preparation of Hf (CH 3) 3{N(SiMe 2CH2PMe 2) 2) . The i d e n t i c a l conditions described above for the t e r t i a r y butyl d e r i v a t i v e were used. Y i e l d 73%. XH NMR (C 5D 6, ppm): PCH 3, 0.96 (d, 2 J p = 4.5Hz); PCH 2 0.67 (d, 2 J p = 8Hz); SiCH 3, 0.20 ( s ) ; HfCH 3, 0.47 ( t , 3 J p = 6.7Hz). 3 1P{ 1H} (C 6D 6, ppm): -17.3 ( s ) . Anal. Calcd. for C 1 3 H 3 ? H f N P 2 S i 2 : C, 30.98; H, 7.39 N, 2.77. Found: C, 30.78; H, 7.32; N, 2.80. IR ( C s l , cm - 1): Hf-N 450,445. Preparation of Zr(CH 3) 3{N(SiMe 2CH 2P(t-Bu) 2) 2). MeMgCl, (4.2 mL, 1.1M in THF, 4.6 mmol), was added to a s t i r r e d solution of ZrCl 3{N(SiMe 2CH 2P(t-Bu) 2) 2} (1.0 g. 1.5 mmol) i n Et 20 (25 mL). The r e s u l t i n g s o l u t i o n was l e f t to s t i r for f i v e minutes a f t e r which 49 time the E t 2 0 was removed under vacuum and the remaining s o l i d extracted with hexanes (20 mL) and f i l t e r e d through a fine f r i t . Cooling to -30°C for one day generated spiky c r y s t a l s . Y i e l d 70%. *H NMR (C gD 6, ppm): SiCH 3, 0.45 ( s ) , PCH 2, 0.73 (d, 2 J p - 6 Hz); PC(CH 3), 1.03 (d. 3 J p - 11 Hz); ZrCH 3, 1.07 ( t , 1 Hz). 3 1P{ 1H} (C 7D 8, ppm): +20.35 ( s ) . Anal. Calcd. for C 2 5 H 6 1 N P 2 S i 2 Z r : C, 51.22; H, 10.51; N, 2.39. Found: C. 51.34; H, 10.45; N, 2.49. IR ( C s l , cm - 1). Zr-N 505, 476. Preparation of Zr(CH 3) 3{N(SiMe 2CH 2P(i-Pr) 2) 2>. The i d e n t i c a l conditions described above for the analogous zirconium t e r t i a r y butyl d e r i v a t i v e were used. Y i e l d 67%. *H NMR (CgDg, ppm): SiCH 3, 0.37 ( s ) ; PCH 2, 0.84 (d, 2 J p = 13.8 Hz); ZrCH 3, 0.9 ( t , 3 J p -3.8 Hz); PCH(CH 3), 1.17 (m); PCH(CH 3), 2.04 (m). 3 1P{ 1H} (C 6D f i, ppm): +8.7 ( s ) . Anal. Calcd. for C 2 1 H 5 3 N P 2 S i 2 Z r : C, 47.68; H, 10.10; N, 2.65. Found: C, 47.63; H, 10.13; N, 2.80. IR ( C s l , cm - 1): Zr-N 432, 419. Preparation of Zr(CH 3) 3{N(SiMe 2CH 2PMe 2) 2} The i d e n t i c a l conditions described for the analogous zirconium t e r t i a r y - b u t y l d e r i v a t i v e were used. However, at the present time we have not been able to i s o l a t e an a n a l y t i c a l l y pure product. 50 CHAPTER 4 Summary A series of mono-amido' phosphine derivatives of zirconium and hafnium of the formula MCI 3{N(SiMe 2CH 2PR 2) 2} were prepared. The single c r y s t a l X-ray d i f f r a c t i o n analysis of HfCl 3{N(SiMe 2CH 2PMe 2) 2) indicates that the molecule i s f a c i a l , which, i n accordance with the NMR spectroscopic data, isomerises to a meridional geometry i n s o l u t i o n . In contrast, i n both the s o l i d state and sol u t i o n , ZrCl 3{N(SiMe 2CH 2P(i-Pr) 2) 2> i s meridional, a geometry which i s adopted by a l l the MCI 3{N(SiMe 2CH 2PR 2) 2> derivatives i n s o l u t i o n . Presumably t h i s stereochemistry i s adopted to minimize the non-bonding repulsions between the PR 2 groups on the ligand and to reduce the s t r a i n generated upon the planar Si-N-Si u n i t , which i s less when the phosphine ligands are trans disposed. These mono-ligand de r i v a t i v e s were used as s t a r t i n g materials for the formation of the trimethyl complexes, M(CH3) 3{N(SiMe-^CH^R^ 2 (M = Zr. Hf, R » Me, i - P r , t-Bu) i n addition to the analogous monomethyl and dimethyl complexes which were i s o l a t e d as a mixture. The generation of the monomethyl and dimethyl products a r i s e v i a a conproportion reaction between the corresponding t r i - a l k y l complex and s t a r t i n g material, which f a c i l i t a t e s a l k y l - h a l i d e exchange between the two metal centres i n these d e r i v a t i v e s . The NMR data indicates the trimethyl complexes are f l u x i o n a l i n s o l u t i o n . This behaviour i s believed to occur as a consequence of the 51 d i s s o c i a t i v e nature of the phosphine donors of the chelating l i g a n d . Several possible mechanisms which u t i l i s e the idea of phosphine d i s s o c i a t i o n were considered i n an attempt to explain the equivalence of the methyl groups attached to the metal centre i n the ^ NMR. The f i r s t mechanism requires the trimethyl complexes to move towards a f a c i a l o r i e n t a t i o n which would not only increase the s t r a i n on the planar Si-N-Si unit but also lead to an increase i n the non-bonding repulsions between the phosphine groups. For these reasons I would tend to favour the Berry pseudorotation or t u r n s t i l e mechanisms, because i n both mechanisms the chelating ligand does not have to a t t a i n a f a c i a l conformation. 52 APPENDIX PCCH, Si 1 P pm 80 MHz *H NMR of LiN(SiMe 2CH 2P(t-Bu) 2) 2 54 a ESC "IB 6E'. -tC Z2+ 'B* BSC BE IBB 'OE IZT'CZ r S t ' t : F.T.-I.R. of Hf(CH 3) 3{N(SiMe 2CH 2P(i-Pr) 2) 2} 55 POM. SiCH, HfCH' -CH2 J I I I I 5 I 0 ppm 80 MHz *H NMR of Hf(CH 3) 3(N(SiMe 2CH 2PMe 2) 2) 56 F.T.-I.R. of Hf(CH 3) 3{N(SiMe 2CH 2PMe 2) 2} 57 PCCH, HfCH3 SiCH, 1 ppm 80 MHz *H NMR of Hf(CH 3) 3{N(SiMe 2CH 2(t-Bu) 2) 2) 58 * I OBX'M ^ B t E 6 BE O £ Y BOBO *E C H I 't "O 1 3 N y n I MSNVMJ.X F.T.-I.R. of Hf(CH 3) 3{N(SiMfi 2CH 2P(t-Bu) 2) 2} 60 ZSE *CB TOB-BS OE» *»C 000*06 6 » S '6* B63*l» I r e 'BE F.T.-I.R. of 2 r(CH 3) 3{N(SiMe 2CH 2(t-Bu) 2) 2) 61 -PCCH 3 SiCH, 1# p p m 0 80 MHz *H NMR of ZrCl 3{N(SiMe 2CH 2P( t - B u ) 2 ) 2 ) F.T.-I.R. of ZrCl 3{N(SiMeJCHJPCt-Bu) 2) 2} 63 PCCH3 SiCH3 1 ppm *H NMR of HfCl 3{N(SiMe 2CH 2(t-Bu) 2) 2} 64 o • 0 O B i M 2 ZBS 81 BBC *BT BB! "IT C U B 'B Z*LC'% 1££.S'Z 13NVJ.1 I H S N V U I X F.T.-I.R. of HfCl 3{N(SiMe 2CH 2(t-Bu) 2) 2> o o N IS 1 CO if ro O r H-I •0 H v ' to PCHCH, PCHCH, TV 2 ppm ZrCH, CH-SiCH, 0 66 F.T.-I.R. of Z r ( O T 3 ) 3 { N ( S i t e 2 C H 2 P ( i - P r ) 2 ) 2 } 67 Table 1 Bond lengths (A) with estimated standard deviations in parentheses of Hf(CH 3) 3{N(SiMe 2CH 2PMe 2) 2} Bond Length(A) Bond Length( A) Hf - P ( l ) 2.761(2) P(2)-C(8) 1.836(8) Hf -P(2) 2.806(2) P(2)-C(9) 1.827(8) Hf -N 2.156(4) S i ( l ) - N 1.735(5) Hf -C(3) 2.272(7) S i ( l ) - C ( l ) 1.884(6) Hf -C(4) 2.246(7) Si(l)-C(10) 1.851(8) Hf -C(5) 2.243(8) S i ( l ) - C ( l l ) 1.869(8) P ( l ) - C ( l ) 1.803(7) Si(2)-N 1.739(5) P(D-C(6) 1.821(8) Si(2)-C(2) 1.892(7) P(D-C(7) 1.835(8) Si(2)-C(12) 1.869(7) P(2)-C(2) 1.811(7) Si(2)-C(l3) 1.870(7) 68 Table 2 Bond angles (deg) with estimated standard deviations in parentheses of Hf(CH 3) 3{N(SiMe 2CH 2PMe 2) 2) Bonds Angle(deg) Bonds Angle(deg) P ( l ) -Hf -P(2) 115.32(6) Hf -P(2)-C(8) 115.8(3) P(l) -Hf -N 79.60(13) Hf -P(2)-C(9) 127.3(2) P( l ) -Hf -C(3) 162.6(2) C(2)-P(2)-C(8) 103.3(3) P( l ) -Hf -C(4) 80.0(2) C(2)-P(2)-C(9) 105.4(3) P( l ) -Hf -C(5) 78.3(2) C(8)-P(2)-C(9) 102.3(4) P(2) -Hf -N 73.62(11) N - S i ( l ) - C ( l ) 110.0(3) P(2) -Hf -C(3) 80.8(2) N -Si(l)-C(10) 114.3(3) P(2) -Hf -C(4) 162.4(2) N - S i ( l ) - C ( l l ) 112.2(4) P(2) -Hf -C(5) 83.2(2) C ( l ) - S i ( l ) - C ( 1 0 ) 105.8(4) N -Hf -C(3) 113.0(3) C ( l ) - S i ( l ) - C ( l l ) 105.6(3) N -Hf -C(4) 102.4(2) C ( 1 0 ) - S i ( l ) - C ( l l ) 108.4(5) N -Hf -C(5) 136.8(3) N -Si(2)-C(2) 108.3(3) C(3) -Hf -C(4) 85.3(3) N -Si(2)-C(12) 112.0(3) C(3) -Hf -C(5) 98.1(3) N -Si(2)-C(13) 114.5(3) C(A) -Hf -C(5) 109.6(3) C(2)-Si(2)-C(12) 107.7(4) Hf -P(D - C ( l ) 100.3(2) C(2)-Si(2)-C(13) 106.0(3) Hf -P(D -C(6) 117.2(3) C(12)-Si(2.)-C(13) 107.9(4) Hf -P(D -C(7) 123.0(3) Hf -N - S i ( l ) 122.9(2) C(l ) -PCD -C(6) 104.4(4) Hf -N -Si(2) 118.2(3) C(l ) -P(D -C(7) 107.4(4) S i ( l ) - N -Si(2) 117.3(3) C(6) -P(D -C(7) 102.7(5) P ( l ) - C ( l ) - S i ( l ) 109.6(3) Hf -P(2) -C(2) 99.8(2) P(2)-C(2)-Si(2) 108.3(3) 69 Bibliography l a . 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