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Solution reactions of HX molecules (X = SH, Cl, Br) with dinuclear palladium(I) complexes containing… Barnabas, Freddy A. 1989

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e Solution Reactions of H X Molecules (X = SH, CI, Br) With Dinuclear Palladium(I) Complexes Containing Bis(diphenylphosphino)methane by Freddy A. Barnabas B. Sc. (Chemistry) Madras University, India, 1975 M . Sc. (Chemistry) Madras University, India, 1977 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S C H E M I S T R Y We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March 1989 © Freddy A. Barnabas, 1989 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 of CJ\Cm l$>b^j The University of British Columbia Vancouver, Canada DE-6 (2/88) Abstract This thesis describes kinetic and spectroscopic studies on the reaction be-tween Pd 2X 2(dpm) 2, 1 (X = CI, a; Br, b; I, c; dpm = bis(diphenylphosph-ino)methane) and HX molecules (X = SH, CI, Br) in solution. The reaction of 1 with hydrogen sulfide leads to the formation of the corresponding /i-sulfide, 2, and H 2 gas. Kinetic studies carried out in CH2CI2 solutions over the temper-ature range 0 - 35°C reveal a first order dependence on both palladium dimer and hydrogen sulfide concentrations. The value of the bimolecular rate con-stant, k2, is 1.71 x IO - 3 M _ 1 s - 1 at 25°C, and the activation parameters for the reaction are AH* = 55 ± 5 kJ mole"1 and AS* = -115 ± 10 J K" 1 mole"1. Low temperature J H and 3 1P{ 1H} nmr spectroscopic investigations comple-ment the kinetic studies, and show that the reaction proceeds via the forma-tion of a hydride intermediate. The observations indicate that the reaction proceeds through oxidative-addition of H2S across the metal-metal bond and the data are discussed in terms of the following reactions: Pd2X2(dpm)2 + H 2S v _ Pd2X2(dpm)2(H2S) 1 Pd2X2(dpm)2(H2S) v =^ Pd2X2(dpm)2(H)(SH) Pd2X2(dpm)2(H)(SH) • Pd2X2(dpm)2(M-S) + H 2 2 There is no direct evidence for the first reaction listed, however. The analogous reactions between 1 and HX (X = CI, Br) were studied with UV-visible, *H and 3 1P{ 1H} nmr spectroscopy. The reaction between ii HCl and l a in CH 2C1 2 results in the 'direct' formation of Pd(II) monomer, PdCl2(dpm), with no intermediates being seen. The corresponding reaction between the bromide, l b and HBr, however, is seen to take place in several steps. Reaction with one mole of HBr results in the formation of a hydride intermediate; this then reacts with a second mole of HBr to form H 2 gas and a tetrabromo complex, Pd2Br4(dpm)2, which subsequently fragments slowly to the Pd(II) monomer, PdBr2(dpm). All the intermediate species involved in the bromide/HBr reaction were detected by UV-visible and low temperature J H and 3 1P{JH} nmr spectroscopy. An effective way of abstracting the bound //-sulfide ligand in order to regenerate the palladium dimer, 1, has been demonstrated using reactions with dpm and with diphenyl; the abstraction by dpm results in formation of dpmS, the monosulfide of dpm, as the only sulfur-containing product. A new and as yet incompletely characterized system that catalytically desulfurizes H2S was generated by having excess dpm present in the reaction between lb and H2S. This catalytic system tranfers sulfur from H2S to the bisphosphine dpm to form dpmS along with some uncharacterized palladium compounds. in Table of Contents Abstract Table of Contents List of Figures List of Tables List of Abbreviations Acknowledgement 1 I N T R O D U C T I O N 1.1 General introduction 1.2 Transition metal/H2S chemistry 1.3 The chemistry of transition metal dpm dinuclear species . . . 1.4 Aim of work 1.5 References 2 E X P E R I M E N T A L 2.1 MATERIALS 2.1 .1 Solvents 2 . 1 . 2 Gases 2 . 1 . 3 Phosphines and other materials 2 . 1 . 4 Palladium Compounds 2 .1 .4 .1 Pd(PhCN) 2Cl 2  2 . 1 . 4 . 2 Pd,(dba) s.CHCl s  iv 2.1.4.3 Pd 2 Cl 2 (dpm) 2 20 2.1.4.4 Pd 2 Br 2 (dpm) 2 21 2.1.4.5 Pd 2 I 2 (dpm) 2 .CH 2 Cl 2 21 2.1.4.6 Pd 2Cl 2(dpm) 2t>S) 22 2.1.4.7 Pd 2 Br 2 (dpm) 2(/i -S) 23 2.1.4.8 Pd2I2(dpm)2(/x-S) 23 2.1.4.9 PdCl 2(dpm) 24 2.1.4.10 PdBr2(dpm) 24 2.2 Instrumentation 25 2.3 Procedure for a typical kinetic run 26 2.4 Low temperature nmr experiments with Pd 2 X 2 (dpm) 2 and H 2 S. 28 2.4.1 Reaction of Pd 2 Br 2 (dpm) 2 with H 2 S 28 2.4.2 Reaction of Pd 2I 2(dpm) 2 with H 2 S -28 2.5 Interaction of Pd 2 Br 2 (dpm) 2 with HX 29 2.5.1 Room temperature reaction 29 2.5.2 Low temperature reaction 29 2.6 Sulfur abstraction reactions 29 2.6.1 Sulfur abstraction by dpm 29 2.6.2 Desulfurization of H 2 S 30 2.7 Gas solubility measurements 30 2.8 References 31 3 THE INTERACTION OF H 2S WITH Pd(I) DIMERS 32 3.1 T h e paHad ium( I ) d p m d imer , P d 2 X 2 ( d p m ) 2 32 3.2 R e a c t i o n w i t h D 2 S 33 3.3 Kinetics and rate measurements 34 v 3.4 Spectroscopic detection of intermediates 45 3.4.1 Reaction between Pd 2Br2(dpm) 2 and H 2 S 45 3.4.2 Reaction between Pd 2I 2(dpm) 2 and H 2S 49 3.5 Discussion of kinetic and spectroscopic data 52 3.6 References . . 61 4 T H E I N T E R A C T I O N O F H X (X = CI, Br ) W I T H Pd(I) D I M E R S 64 4.1 Introduction 64 4.2 Stoichiometry of the reaction and product identification . . . . 67 4.2.1 Reaction of Pd 2 Cl 2 (dpm) 2 with HCl at room temperature 67 4.2.2 Reaction of Pd 2 Br 2 (dpm) 2 with HBr at room temperature 70 4.2.3 Low temperature spectroscopic studies of the reaction between l b and HBr 75 4.3 Discussion 78 4.4 References 83 5 S U L F U R T R A N S F E R R E A C T I O N S 84 5.1 Introduction 84 5.2 Sulfur abstraction reactions 87 5.2.1 Sulfur abstraction by phosphines 87 5.2.2 Sulfur abstraction by organics 88 5.3 The desulfurization of H 2S 90 5.4 References 94 6 G E N E R A L C O N C L U S I O N S 96 6.1 Conclusions 96 vi Recommendations for future work vii L i s t o f F i g u r e s 2.1 Anaerobic spectral cell used for UV-visible spectroscopy 27 3.1 Visible absorption spectral changes of a CH2CI2 solution of Pd2Br2(dpm)2 upon addition of H2S at 25°C 35 3.2 Rate plot for the reaction between Pd2Br2(dpm)2 (1.01 x 10 - 3 M) and H2S (0.53 M) in CH 2C1 2, at 25°C 36 3.3 A rate plot analysed for first order Pd\ dependence in CH 2C1 2 at 25°C. ([Pd!2] = 1.01 x 10"3 M and [H2S] = 0.53 M) 37 3.4 Solubility of H2S in CH 2C1 2 at various pressures 40 3.5 Dependence of reaction rate on [Pd\] in CH 2C1 2 at 25°C 42 3.6 Dependence of reaction rate on [H2S] at 1.01 x 10~3 M Pd 2 in CH 2C1 2 at 25°C 43 3.7 Temperature dependence of the rate constant for the reaction between lb (at 1.01 x 10"3 M) and H2S in CH 2C1 2 44 3.8 The J H nmr spectra in CD 2C1 2 solution for (A) -CH 2 - region of l b ; (B) -CH 2 - region of l b + H2S at -70°C and (C) hydride region of l b + H2S at -70°C 46 3.9 The1H nmr spectra in CD 2C1 2 solution of (A) -CH 2 - region of l b + E 2 S warmed up to room temperature and (B) -CH 2 -region of 2b 47 3.10 The 3 1 P { 1 H } nmr spectra in CD 2C1 2 of l b and H2S 48 viii 3.11 T h e ! H n m r spect ra i n C D 2 C 1 2 so lu t ion for ( A ) - C H 2 - region of l c ; ( B ) - C H 2 - region of l c + H 2 S kept at ~ - 7 4 ° C for 8 h a n d ( C ) - C H 2 - region of l c + H 2 S kept at ~ - 7 4 ° C for 8 h then war-m e d u p to - 4 0 ° C 50 3.12 T h e 1E n m r spect ra in C D 2 C 1 2 so lu t ion of ( A ) l c + H 2 S kept at — 7 4 ° C for 8 h (hydr ide a n d - S H region) and ( B ) 2 c 51 3.13 T h e 3 1 P { 1 H } n m r spectra i n C D C 1 3 of ( A ) l c at 20°C; ( B ) l c + H 2 S kept at — 7 4 ° C for 8 h and ( C ) l c + H 2 S kept at — 7 4 ° C for 8 h and then warmed up to - 4 0 ° C 53 3.14 T h e ^ P ^ H } n m r spect ra i n C D C 1 3 of ( A ) l c + H 2 S kept at ~ - 7 4 ° C for 8 h and then wa rmed to r o o m tempera tu re and ( B ) au then t i c sample of 2 c at 20°C 54 4.1 T h e 3 1 P { J H } n m r spect ra of the react ion p roduc ts of l b and anhydrous H C I 65 4.2 T h e mechan is t i c pa thway of the react ion between l b and B r 2 . 66 4.3 T h e : H n m r spect ra ( - C H 2 - region) i n C D C 1 3 so lu t ion at 20°C of ( A ) l a ; ( B ) l a + H C l ( g ) (~3:1) a n d ( C ) l a + H C l ( g ) (1:2). 68 4.4 T h e 3 1 P { 1 H } n m r spect ra i n C D C 1 3 so lu t ion at 20°C of ( A ) l a ; ( B ) i a + H C l ( g ) (~3:1) a n d ( C ) l a + H C l ( g ) (1:2) 69 4.5 T h e gas chromatograms of (A) the gaseous p roduc ts evo lved i n the react ion of l a + 2 H C l ( g ) i n DMA; ( B ) the gaseous p roduc ts evo lved i n the react ion of l b + 2 H B r ( g ) i n C H 2 C 1 2 a n d ( C ) au then t i c d ihydrogen gas 71 i x 4.6 Visible absorption spectrum of l b upon addition of anhydrous HBr at 25°C, as a function of time 72 4.7 The *H nmr spectra (-CH 2- region) in CDC1 3 solution at 20°C of (A) l b ; (B) l_b 4 HBr(g) after 15 min; (C) l b 4 HBr(g) after 6 h and (D) authentic sample of PdBr 2(dpm) 73 4.8 The 3 1P{ 1H} nmr spectra in CDC1 3 solution at 20°C of (A) l b ; (B) l b 4 HBr(g) after 15 min; (C) l b + HBr(g) after 6 h and (D) authentic sample of PdBr 2(dpm) 74 4.9 The ] H nmr spectra (-CH 2- region) in CDC1 3 solution at -40°C of (A) l b 4 HBr(g) (2:1); (B) l b 4 HBr(g) (1:2) after ~15 min and (C) l b 4 HBr(g) (1:2) after -30 min 76 4.10 The J H nmr spectra in CDC1 3 solution at -40°C of (A) l b 4 HBr(g)(l:l), the high field region after ~ 2 min; (B) l b 4 HBr(g) (1:2) after 60 min, - C H 2 - region and (C) l b 4 HBr(g) (1:2) after 6 h,-CH 2 -region 77 4.11 The ^ P-^H} nmr spectra in CDC1 3 solution at -40°C of (A) l b 4 HBr(g) (1:2) after ~15 min; (B) l b -f HBr(g) (1:5) after ~20 min and (C) l b 4 HBr(g) (1:10) after ~25 min 79 5.1 The schematic representation of the regeneration of 1 from 2 by the oxidation of the /i-sulfide to S 0 2 86 5.2 The mechanistic, pathway for the abstraction of sulfur by dpm from 2 89 5.3 The 3 1P{ 1H} nmr spectra of the reaction product of lc and H 2 S in the presence of 20-fold excess dpm 91 x 5.4 The 3 1 P { X H } nmr spectra of the reaction product of lh and H 2S in the presence of 20-fold excess dpm 92 xi L i s t o f T a b l e s III-2 Solubility of H 2 S in CH 2 Cl2 at various pressures at 25°C. . . . 39 III-2 Dependence of reaction rate on [Pd\] in CH 2 C1 2 at 25°C. . . . 39 III-3 Dependence of reaction rate on [H2Sj at 1.01 x 10"3 M Pd 2 in CH 2 C1 2 at 25°C 41 III-4 Temperature dependence of the rate constant for the reaction between lb and H 2 S in CH 2 C1 2 41 x i i L i s t o f A b b r e v i a t i o n s The following list of abbreviations, most of which are commonly adopted in chemical literature, will be employed in this thesis. br broad Cp cyclopentadienyl, C 5 H 5 d doublet dba dibenzylideneacetone, C 6 H 5 CH:CHC(0)CH:CHC 6 H 5 dd doublet of doublets DMA N,N'-dimethylacetamide, CH 3 CON(CH 3 ) 2 dpm bis(diphenylphosphino)methane, ( C 6 H 5 ) 2 P C H 2 P ( C 6 H 5 ) 2 dpmS bis(diphenylphosphino)methane monosulfide, (thio(diphenyl)phosphino(diphenyl)phosphinomethane), (C 6H 5) 2PCH 2P(S)(C 6H 5) 2 dpmS2 bis(diphenylphosphino)methane disulfide, (bis(thio(diphenyl)phosphino)methane), (C 6H 5) 2P(S)CH 2P(S)(C 6H 5) 2 dpe l,2-bis(diphenylphosphino)ethane, ( C 6 H 6 ) 2 P C H 2 C H 2 P ( C 6 H 5 ) 2 i.r. infra-red J couphng constant, Hz m multiplet 3 1P{ 1H} proton broad-band decoupled phosphorus nmr PPh 3 triphenylphosphine, (C 6 H 5 ) 3 P PPh3S triphenylphosphine sulfide, (C6H5)3P(S) PPh2Me methyldiphenylphosphine, (C 6H 5) 2PMe Pd* Pd2X2(dpm)2 x i i i q quintet s singlet or second t triplet UV ultra-violet X halide ligand, CI, Br, or I 8 chemical shift, ppm e molar extinction coefficient or dielectric constant Some of the compounds used during the course of this work have been referred to by numerical symbols in this thesis for the sake of brevity and simplicity. Following is the list of complexes corresponding to each such symbol: 1 P d 2X 2 ( d p m ) 2 la P d 2 C l 2 ( d p m ) 2 lb P d 2 B r 2 ( d p m ) 2 l c P d 2 I 2 ( d p m ) 2 2 Pd 2X 2(/i-S)(dpm) 2 2a Pd 2 Cl 2 ( /x-S)(dpm) 2 2b Pd 2 Br 2 ( /*-S)(dpm) 2 2c Pd 2 I 2(/i-S)(dpm) 2 3b P d 2 B r 2 ( d P m ) 2 ( H ) ( S H ) 3c Pd 2 I 2 (dpm) 2 (H)(SH) 4b Pd 2 Br 2 (dpm) 2 (H) (Br) 5b Pd 2Br4(dpm) 2 6a PdCl 2 (dpm) 6b PdBr 2 (dpm) Pd 2 Br 2 (dpm) s P d 2 B r 2 ( M - S ) ( d p m ) 5 xiv A c k n o w l e d g e m e n t s I am most grateful to Professor B. R. James for his guidance and encour-agement throughout the course of this work. I would also like to express my gratitude to the members of Professor James'group for making valuable suggestions regarding this work. I am also indebted to Dr. Dominique Sallin and Mr. Ken MacFarlane for their support, friendship and suggestions. Special thanks must go to Dr. K. Venkateswaran, who helped me with plots and text processing. The assistance and cooperation of various departmental services are grate-fully acknowledged. xv Chapter 1 I N T R O D U C T I O N 1.1 G e n e r a l i n t r o d u c t i o n Recent studies 1 carried out in this laboratory to separate CO from gas mix-tures using Pd(I) dimers of the type P d 2 X 2 ( d p m ) 2 (X = CI, Br, I; dpm = P h 2 P C H 2 P P h 2 ) led to the discovery of reaction 1.1, while testing the reactiv-ity of the palladium complex toward H 2S, an impurity often present in such gas mixtures. Pd2X2{dpm)2 + H2S —• Pd2X2(dpm)2(fi - S) + H2 (1.1) [X = Cl,Br,I] For X = CI and Br the reaction is quantitative and is the first of its kind in which hydrogen gas is generated from H 2S by a transition metal complex in solution. Gaseous H 2S, a noxious pollutant, is produced as a result of both natural and man-made processes.2 Two main mechanisms account for the generation of the gas in the natural sources: bacterial reduction of sulfate, sulfur and organic sulfur compounds in plant and animal matter, and geochemical formation as in volcanic activity and thermal hot springs. These natural sources contribute an important part of atmospheric emission of H 2S (3 - 30 /ig/m 3) that is part of the natural global sulfur cycle. 1 Though significant quantities of H2S are generated in many industrial pro-cesses, most of it is consumed for the production of S and SO2. Important production sources include petroleum refining operations, natural gas plants, petrochemical plants, coke oven plants and kraft pulp mills; H2S is an occa-sional pollutant, and the main pollution sources are gas-well blow outs and pulp mills. 2 Though H2S is as toxic as cyanide to most organisms, nature has evolved a few microbes, that depend on it for their survival. 3 For example, a class of bacteria known as 'sulfate reducers' produce H 2S by the reduction of sulfates in anoxic conditions. The generated H 2S is utilized in turn by other types of bacteria, and the subsequent oxidation results in elemental sulfur and sulfate. It is interesting to note that most of the world's native sulfur deposits are said to be formed as a result of bacterial action during the warm and sunny Permian and Jurasic periods (~ 2 x 108 years ago). As a result of a geochemical reaction that takes place at a newly formed ocean floor, H2S is generated. 4 When sulfate bearing sea water percolates and comes in contact with hot crustal rocks, iron present in the rock reduces the sulfate to H2S gas. Thus the hot water, rising to the ocean floor through the so-called 'hydrothermal vents' or 'hot springs', is heavily charged writh hydrogen sulfide. In the waters surrounding these vents, nature has evolved a ecosystem that solely depends on a bacterium that derives energy by oxidizing H2S to sulfate. This unique, diversified, ecosystem apart from the bacterium includes giant worms, clams and crabs that tolerate the high concentration of H2S and perhaps are the only higher organisms unaffected by H 2 S . The fossil fuels, petroleum, shale, oil bearing sands, coal and natural gas have varying amounts of sulfur depending on their origin. For example, in petroleum the sulfur is mainly present as mercaptans (RSH), sulfides (RSR), disulfides (RSSR) and sulfur heterocyclics.5 The presence of sulfur organics in fossil fuels has the following implications: (i) the refining of petroleum involves the catalytic cracking and catalytic reforming processes that utilize transition metal catalysts. If sulfur or-ganics are not removed prior to these processes, then the catalysts are rendered ineffective by poisoning. (ii) Combustion of sulfurous fossil fuels produces sulfur oxides (mainly S 0 2 ) that are not only corrosive to the machinery but if vented into the atmo-sphere cause many environmental problems. It is interesting to note that a substantially larger amount of SCs than it is utilized industrially is dis-charged into the atmosphere.6 Acid rain, smog and other environmental hazards result because of this discharge. Thus sulfur removal is essential and is removed from petroleum by a process called hydrodesulfurization(HDS), prior to the cracking and reforming. In HDS processes, organic sulfur is removed as H 2 S by reacting with H 2 gas in the presence of sulfided Mo-Co or W - N i oxide catalysts as in reactions 1.2 to 1.5.5 RSH + H2—>RH + H2S (1.2) RSR + 2H2—>2RH + H2S (1.3) RSSR + 3H2—>2RH + 2H2S (1.4) C4H4S(thiophene) + 2H2 —• CH2 = CH-CH = CH2 + H2S (1.5) The resulting H 2 S is either oxidized to elemental sulfur (as in Claus, Stretford, Takahax, Giammarco-Vetrocoke-sulfur and Konox processes7) or converted to 3 CaS0 4 . In the currently available technologies, the industrially expensive H 2 is lost and thus sulfur removal becomes an expensive process especially for petroleum crude with high sulfur content.However, the organic sulfur removal could become an industrially less expensive operation with little or no con-sumption of H 2, provided H 2 could be generated from H 2S by a catalytic process, perhaps one based on reacion 1.1. Such a process would be valuable, if not now, at least in the future, once currently usable petroleum reserves of low sulfur content become exhausted. Thus the study of the interaction of H 2S with transition metal complexes, as in reaction 1.1, is of fundamental importance because of the biological, environmental and industrial importance of the gas. Further, the interesting reactivity pattern of H 2S, as exhibited in reaction 1.1, could lead to the devel-opment of an efficient catalytic system, perhaps a homogeneous one (the first of its kind) to desulfurize H 2S. 1.2 T r a n s i t i o n m e t a l / H 2 S c h e m i s t r y Transition metals and sulfur have an 'affinity' for one another and readily form metal sulfides. This strong tendency to form metal sulfides can be readily seen in the Pearson's acid-base scale, 8 where d"- transition metal ions (with higher n values) and sulfide ions are both classified as 'soft' groups. The interaction of H 2S with transition metal complexes typically gives insoluble sulfides often polymeric in nature, but more interesting reactivity patterns are known. The first reported case of quantitative H 2 generation from H 2S by a transition metal complex was unearthed in this laboratory as noted in sec 1.1.la This involved the insertion of S between the Pd-Pd metal-metal bond of Pd 2X 2(dpm) 2, X 4 = CI, Br and I, to give a //-sulfide complex of the type Pd2X2(dpm)2(/x-S), Pd2X2{dpm)2 + H2S — • Pd2X2{dpm)2{fi -S) + H2 (1.6) [X = Cl,Br,I] Apart from this reaction, there is only one other system that generates H2 quantitatively from H 2S. 9 2CP'2Zr{CO)2 + 2H2S—> [Cp'2Zr(fi - S)]2 + 2H2 + ACO (1.7) [Cp1 = T J 5 - C5H& or r,5 - C 5Me 5] The interaction of H 2 S with the ruthenium complexes R u H 2 ( P P h 3 ) 4 , 1 0 , 1 1 and Ru(CO)2(PPh3)3,12 results in H 2 production. The H 2 generated from the reaction of RuH2(PPh3)4 and H2S is a consequence of the hydride content of the complex, and the net reaction, ignoring a labelling of the hydrogen atom, is the more usual oxidative addition of H2S via cleavage of the S-H bon d . 1 1 - 1 3 RuH2{PPh3)4 + H2S —» RuH(SH){PPh3)3 + H2 + PPh3 (1.8) The ruthenium complexes, R u ( C O ) 2 ( P P h 3 ) 3 1 2 and [ R u ( N H 3 ) 5 ( H 2 S ) ] 2 + , 1 4 reduce H2S to H2 and 2 S H " . Ru(CO)2{PPh3)3 + H2S RuH{SH)(CO)(PPh3)2 Ru(SH)2(CO)2{PPh3)2 -f H2 (1.9) 2\Ru{NH3\{H2S))2+ -^2[Ru(SH){NH3)Bf+ + H2 (1.10) The reaction was suggested tentatively because the precursor complex was not. obtained in a pure state. 1 4 The only other well characterised isolated H2S com-plex appears to be W(CO) 6(H 2S), 1 6 although X H nmr spectroscopic evidence 5 has been presented for the species Pt(PPh3)2(H2S) en route to formation of more stable (hydrido)(mercapto) complex. 1 6 , 1 7 Equation 6, analogous to Eqs 1 and 2, has been invoked for a solid state reaction to account for the filling of vacant anionic sites by sulfur in WS 2 lattices. 1 8 2W3+ + H2S A n i ^ t i t e 2W4+ + S2- + H2 (1.11) 1.3 T h e c h e m i s t r y of t r a n s i t i o n m e t a l d p m di n u c l e a r species The bisphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexi-bility in the distance between the two metal ions involved. Though dinuclear metal complexes of dpm are known wTith one, 1 9 two 2 0 or three 2 1 bridging fx-dpm ligands, two trans-/z-dpm ligands are most common and these include homobimetallic Rh, 2 2 l r , 2 3 Pd, 2 4 P t 2 5 and M n 2 6 and heterobimetallic Pd/Pt, 2 7 Pd/Fe 2 8 and Pd/Mn 2 9 dinuclear complexes. Typically, these dinuclear com-plexes exhibit any one of three main structural types. Firstly, a structure without either a metal-metal bond or other bridg-ing groups between the two metal centers and the two metal ions are linked by two bridging dpm ligands. This structure is most common with Rh(I) dimers and is more elusive with other metals; R h 2 ( C O ) 2 C l 2 ( d p m ) 2 3 0 and Rh 2(MeNC ) 4(dpm) 2 3 1 are good examples of complexes having this structure, usually described as a face-to-face structure. Though face-to-face dinuclear complexes are known in the Pt(II) and Pd(II) systems, P t 2 ( C = C R ) 4 ( d p m ) 2 (R = Me, C F 3 , Ph, 4-tolyl) 3 2 and P d 2 C l 2 ( C H 3 ) 2 ( d p m ) 2 , 3 3 those with other non-bridging, terminal ligands (i.e. X in M 2 X 4 ( d p m ) 2 ) rearrange rapidly to monomelic forms. 3 4 6 Secondly, a structure with no other bridging groups but with interaction between the metal centres is commonly adopted by both homo- and hetero-bimetallic complexes of dpm.The bond between the two metal centres is usually single for the majority of metals, but in certain Mo and Re systems 3 5 multiple bonds are known. The singly metal-metal bonded derivatives, MM'X2(dpm)2 (M, M' = Pd, Pt; X = CI) display high reactivity either by displacement of terminal chloride ligands by anionic or neutral ligands or by insertion of Sn C l 2 into the metal-chloride bond. 2 4 0' 2 5 0 , 3 6 The displacement of the terminal chloride is effected by various ligands including Br~, I - , N C O - , NCS~, N j , N O 3 , this resulting in the alteration of the metal-metal bond strength. The change of bond strength is attributed to the trans-influence exerted by the terminal ligand. As a result, the reactivity of the metal-metal bond is altered and this is evident in most of the reactions in which the metal-metal bond is broken. However, in the reaction between CO and Pd 2X2(dpm) 2 species (X = CI, Br, I, NCO), the rate was governed by the Pd-CO bond strength rather than Pd-Pd bond energy. 3 7 A number of unique chemical features have been observed as a result of the proximity of the two metal ions in the dpm complexes. Many of these complexes are able to co-ordinate (sometimes re-versibly), in a bridging manner, atoms or small molecules such as H, 2 0 , 2 4 , 2 5 0' 3 8 S la ,25a ,39 g e 4 0 g Q ^ B a . S M l SQ42 C 0 20,24,25a,37,43 RQ~, 4 4 RS", 2 6 0 CN", 2 6° CS 2 ) 2 2 a , 2 5 a C N R 24.25«,36,45 36,46 Q^R-, 47 N j R + ) 20,24b,36 N 2CRR' *>>*">** and CH2. 2 0 , 2 4 f c , 2 5 o , 3 3 ,' w This type of co-ordination results in the third structural type that includes both A-frame structures with one bridging group, and dou-ble A-frame structures with two bridging groups. Both single and double atom bridges are known; CH 2, CO, S 0 2 and P h N j connect the metals centres via single atoms while CS2 and R C = C R utilise two atoms. A metal-metal bond 7 may be present in complexes with either one and two bridging groups. In dinuclear Pd complexes of the type Pd 2X2(dpm) 2, a structure with a single metal-metal bond exists. However, in the analogous A-frame complexes with a single bridging group, the Pd-Pd bond is absent. The terminal, non-bridging l i g a n d s 2 4 a (i.e. X and X' in Pd 2XX'(dpm) 2) can be simple inorganic anions such as CI", Br", I - , NCS~, N 3 , N C O - or predominately non-ionic organic groups hke CeFs, 3 6 CeCls, 3 6 or C6H5S.24° Complexes are also known with two different terminal ligands. In these mixed terminal ligand dimers, C 6C1 5 or CeFs is usually one of the ligands and the other terminal ligand is usually a simple inorganic anion mentioned above. Both types of P d 2 dimers with the same or mixed terminal ligands exhibit reactivity toward the insertion of small molecules. The reactivity is, however, altered considerably by changing the terminal ligands. For example, the CO insertion shows such a reactivity trend by changing ligands (CI, Br, I, NCO), but the resulting p-CO complexes are isolable. 3 7 With CeFs or CeCls as one or both terminal ligands, CO inserts but the fi-CO complex is non-isolable 3 6 and is observed only as a transient intermediate. The insertion of S0 2 , on the other hand, results in the formation of stable / / -S0 2 complexes 3 6 with X = CeF 5 or CeCl 5; spontaneous loss of S0 2 occurs above 0°C when X = CI, Br or I . 4 1 a Dinuclear complexes with related bisphosphine ligands such as ( C H 3 ) 2 P C H 2 P ( C H 3 ) 2 (dmpm) 4 9 and P h 2 P C H ( C H 3 ) P P h 2 (dpmMe) 4 2 gener-ally exhibit reactivity toward the insertion of small molecules. In the case of Pd 2X 2(dpmMe)(dpm) species (X = CI, Br, I, NCO), CO insertion, and S in-sertion from H 2S, take place, but with P d 2 X 2 ( d p m M e ) 2 (X = CI, Br, I, NCO) neither the insertion of CO or S occurs because of steric factors imposed by the additional methyl group. 8 1.4 A i m o f w o r k The main objective of this thesis was to investigate in greater detail the kinetic and mechanistic aspects of reaction 1.1, the first of its kind to show full recovery of H2 from H2S. This study was aimed at answering some of the pertinent questions connected with reaction 1.1, such as the possible role of the acidic -CH2- protons of dpm, the nature of attacking species, the nature of reaction intermediates, the reasons for the non-reactivity of other sulfur compounds (RSH and R S S R ) 4 6 and the effect of solvent polarity (i.e. dielectric constant). Further aims of the project were to study the reactivity of the Pd 2X 2(dpm)2 dimers with analogous HX molecules (HCl, HBr and HI), and to find an ef-fective way to remove the sulfur from the fi-S complex in order to regenerate the Pd dimer. 9 1.5 References 1. (a) C . L . Lee, G . Besenyei, B . R . James, D . A . Nelson and M . A . Lilga, J. Chem. Soc, Chem. Commun., 1175 (1985). (b) S. E . Lyke, M . A . Lilga, D . A . Nelson, B . R. James and C . L . Lee, Ind. Eng. Chem. Prod. Res. Dev., 25, 517 (1986). 2. (a) Hydrogen sulphide - Environmental technical information for problem spills, Environmental Protection Service, Ottawa, O N , Canada, (1984), p.26. (b) Hydrogen sulfide in atmospheric environment: Scientific criteria for assessing its effects on environmental quality, N R C C No. 18467, Na-tional Research Council of Canada, Ottawa, O N , Canada, (1981). 3. (a) J . R. Postgate, The sulphate-reducing bacteria, 2nd edition, Cam-bridge University Press, Cambridge, (1984), p.61. (b) J . R. Postgate, New Scientist (July 1988), p.58. 4. (a) J . Edmond, Scientific American, (April 1983), p.78. (b) J . Edmond, Oceanus, (Summer 1982), p.22. 5. S. C . Schuman and H . Shalit, Cat. Rev., 4, 245 (1970). 6. TJ. H . F . Sander, H . Fischer, U . Rothe and R. Kola , Sulphur, sulphur dioxide and sulphuric acid, Verlag Chemie International Inc., Deerfield Beach, F l . , U S A , (1984), p.241. 7. U . H . F . Sander, H . Fischer, U . Rothe and R. Kola , Sulphur, sulphur dioxide and sulphuric acid, Verlag Chemie International Inc., Deerfield Beach, F l . , U S A , (1984), p.41. 10 8. R. G. Pearson, J. Am. Chem. Soc, 85, 3533 (1963). 9. F. Bottomley, D. F. Drummond, G . 0. Egharevba a n d P. S. White, organometallics, 5, 1620 (1986). 10. K . Osakada, T. Yamamoto, A. Yamamoto, A. Takenaka and Y. Sasada, Inorg. Chim. Acta, 105, L9 (1985). 11. K . Osakada, T. Yamamoto, A. Yamamoto, Inorg. Chim. Acta, 90, L5 (1984). 12. C. L. Lee, J. Chisholm, B. R. James, D. A. Nelson and M. A. 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Rev., 12, 99 (1983). 21. (a) E. W . Stern and P. K. Maples, J. Catol., 27, 120 (1972). (b) K. A. Azam, R. J. Puddephatt, M. P. Brown and A. Yavari, J. Organomet. Chem., 234, C31 (1982). 22. See, for example: (a) M. Cowie and S. K. Dwight, J. Organomet. Chem., 214, 233 (1981). (b) C. Woodcock and R. Eisenberg, Organometallics, 1, 886 (1982). (c) J. T. Mague, Inorg. Chem., 22, 45 (1983). (d) C. Woodcock and R. Eisenberg, Organometallics, 4, 4 (1985). (e) J. A. Ladd, M. M. Olmstead and A. L. Balch, Inorg. Chem., 23, 2318 (1984). 23. (a) B. R. Sutherland and M. Cowie, Organometallics, 3, 1869 (1984). (b) J. T. Mague, C. L. Klein, R. J. Majeste and E. D. Stevens, Organo-metallics, 3, 1860 (1984). 24. (a) A. L. Balch, in Catalytic Aspects of Metal Pkosphine Complexes, Edited by E. C. Alyea and D. W. Meek, Advances in Chemistry Series 196, Am. Chem. Soc, Washington, D.C. (1982) p.243. 12 I (b) A. L. Balch, in Homogeneous Catalysis with metal Phosphine Com-plexes, Edited by L. Pignolet, Plenum, New York (1983) p.167. 25. See, for example, (a) M. P. Brown, J. R. Fisher, S. J. Franklin, R. J. Puddephatt and M. A. Thomson, in Catalytic Aspects of Metal Phosphine Complexes, Edited by E. C. Alyea and D. W. Meek, Advances in Chemistry Series 196, Am. Chem. Soc, Washington, D. C. (1982) p.231. (b) A. T. Hutton, B. Shebanzadeh and B. L. Shaw, J. Chem. Soc, Chem. Commun., 549 (1984). (c) P. G. Pringle and B. L. Shaw, J. Chem. Soc, Dalton Trans., 849 (1984). (d) M. C. Grossel, J. R. Batson, R. P. Moulding and K. R. Seddon, J. Organomet. Chem., 304, 391 (1986). 26. See, for example, (a) H. C. Aspinall, A. J. Deeming and S. Donovan-Mtunzi, J. Chem. Soc, Dalton Trans., 2669 (1983). (b) G. Ferguson, W. J. Laws, M. Parvez and R. J. Puddephatt, Organo-metallics, 2, 276 (1983). (c) H. C. Aspinall and A. J. Deeming, J. Chem. Soc, Dalton Trans., 743 (1985). 27. P. G. Pringle and B. L. Shaw, J. Chem. Soc, Chem. Commun., 81 (1982). 28. G. B. Jacobson and B. L. Shaw, J. Chem. Soc, Dalton Trans., 2005 (1987). 13 29. (a) P. Braunstein, J. M. Jud and J. Fischer, J. Chem. Soc, Chem. Commun., 5 (1983). (b) B. F. Hoskins, R. J. Stein and T. W. Turney, Inorg. Chim. Acta, 77, L69 (1983). (c) P. Braunstein, C. de Meric de Bellefon and M. Ries, Organometallics, 7, 332 (1988). 30. A. L. Balch, J. W. Labade and G. Delker, Inorg. Chem., 18, 1224 (1979). 31. (a) A. L. Balch, J. Am. Chem. Soc, 98, 8049 (1976). (b) J. T. Mague and S. H. DeVries, Inorg. Chem., 19, 3743 (1980). 32. R. J. Puddephatt and M. A. Thomson, J. Organomet. Chem., 2 3 8 , 231 (1982). 33. S. J. Young, B. Kellenberger, J. H. Reibenspies, S. E. Himmel, M. Man-ning, O. P. Anderson and J. K. Stille, J. Am. Chem. Soc, 1 1 0 , 5744 (1988). 34. C. H. Lindsay and A. L. Balch, Inorg. Chem., 2 0 , 2267 (1981). 35. See, for example, (a) S. A. B e s t , T . J. S m i t h a n d R. A. W a l t o n , Inorg. Chem., 17, 99 (1978). (b) E . H. A b b o t , K. S. B o s e , F. A. C o t t o n , W . T . Hall a n d J. C . Seku-t owsk i , Inorg. Chem., 17, 3240 (1978). (c) J. R . E b n e r , D . R . T a y l o r a n d R. A. W a l t o n , Inorg. Chem., 15, 833 (1976). 14 (d) M. Y. Darensbourg, R. L. Mehdawi, T. J. Delord, F. R. Fronczek and S. F. Watkins, J. Am. Chem. Soc, 106, 2583 (1984). 36. P. Espinet, J. Fornies, C. Fortune-, G. Hidalgo, F. Martines, M. Tomas and A. J. Welch, J. Organomet Chem., 317, 105 (1986). 37. C. L. Lee, B. R. James, D. A. Nelson and R. T. Hallen, Organometallics, 3, 1360 (1984). 38. (a) J. R. Fisher, A. J. Mills, S. Sumner, M. P. Brown, M. A. Thomson, R. J. Puddephatt, A. A. Frew, L. Manojlovic-Muir and K. W. Muir, Organometallics, 1, 1421 (1982). (b) R. H. Hill and R. J. Puddephatt, J. Am. Chem. Soc, 105, 5797 (1983) . (c) B. R. Sutherland and M. Cowie, Inorg. Chem., 23, 1290 (1984). (d) D. W. Prest, M. J. Mays and P. R. Raithby, J. Chem. Soc, Dalton Trans., 2021 (1982). 39. A. L. Bakh, L. S. Benner and M. M. Olmstead, Inorg. Chem., 18, 2926 (1979). 40. G . Besenyei, C . L. Lee and B. R. James, J. Chem. Soc, Chem. Com-mun., 1750 (1986). 41. (a) L. S. Benner, M. M. Olmstead, H. Hope and A. L. Balch, J. Organomet. Chem., 153, C31 (1978). (b) M. Cowie, R. S. Dickson and B. W. Hames, Organometallics, 3, 1879 (1984) . 15 42. G . Besenye i , C . L . Lee , J . G u l i n s k i , S. J . R e t t i g , B . R. J a m e s , D . A . Ne lson and M . A . L i l g a , Inorg. Chem., 26, 3622 (1987). 43. P . G . P r i ng l e and B . L . S h a w , J. Chem. Soc, Dalton Trans., 889 (1983). 44. S. P . De ran i yaga la and K . R. G r u n d y , Inorg. Chem., 24, 50 (1985). 45. (a) K . R. G r u n d y and K . N . R o b e r t s o n , Organometallics, 2, 1736 (1983). (b) D . L . De lae t , D . R. Powe l l and C . P . K u b i a k , Organometallics, 4, 954 (1985). 46. (a) J . T . M a g u e , Inorg. Chem., 22, 45 (1983). (b) C . L . Lee , C . T . H u n t and A . L. B a l c h , Inorg. Chem., 20, 2498 (1981). 47. (a) A . B l a g g , A . T . H u t t o n , P. G . P r i n g l e a n d B . L . S h a w , J. Chem. Soc, Dalton Trans., 1815 (1984). (b) M . C o w i e and S. J . L o e b , Organometallics, 4, 852 (1985). 48. I. R . Mckeer and M . C o w i e , Inorg. Chim. Acta, 6 5 , L107 (1982). 49. M . L . K u l l b e r g and C . P . K u b i a k , Inorg. Chem., 25, 26 (1986). 16 C h a p t e r 2 E X P E R I M E N T A L 2.1 M A T E R I A L S 2.1.1 Solvents Spectral or analytical grade solvents were obtained from MCB, BDH, Aldrich, Eastman, Fisher or Mallinckrodt Chemical Co.. Benzene, hexanes and toluene were refluxed with and distilled from sodium/benzophenone under an atmo-sphere of N 2. N,N'-Dimethylacetamide (DMA) was stirred over C a H 2 for 24 h prior to fractional distillation under vacuum, and subsequently stored in the dark. Methanol, ethanol, dichloromethane and acetone were distilled after re-fluxing with the appropriate drying agents (Mg/I 2 for methanol and ethanol, P 2 0 5 for dichloromethane, and CaSCu for acetone). Acetonitrile was stored over molecular sieves (Fisher : Type 5A, Grade 522, 8-12 mesh) prior to use. Anhydrous diethyl ether and pentanes were used without further purification. A l l solvents were deoxygenated prior to use. 2.1.2 Gases Purified argon (H.P.), nitrogen (U.S.P.) and hydrogen (U.S.P.) were obtained from Union Carbide Canada Ltd.. Hydrogen was passed through an Engelhard Deoxo catalytic hydrogen purifier to remove traces of oxygen. Hydrogen sulfide (CP.), anhydrous hydrogen chloride and anhydrous hydrogen bromide were 17 obtained from Matheson Gas Co.. All gases, except hydrogen, were used without further purification. D2S was prepared by the action of DC1/D20 (10 mL) on CaS (2 g). The resulting gas was first bubbled through D 20 to remove any sulfur oxide impu-rities and HCl, and then dried over CaCl2 and P 2 0 5 prior to use. 2.1.3 Phosphines and other materials Reagent grade triphenylphosphine, bis(diphenylphosphino)methane (dpm) and l,3-(diphenylphosphino)propane (dpp) were supplied by Strem Chemicals Inc., while methyldiphenylphosphine was kindly made available by K.Bhangu of this department. All phosphines except dpp were used as such, without further pu-rification, while dpp was crystallized from hot ethanol/hexane mixture under argon. The disulfide of dpm (dpmS2) was prepared by refluxing a mixture of dpm (0.2 gm, 0.55 mmol) and elemental sulfur (S8) (36 mg, 0.14 mmol) in deoxygenated toluene (25 mL) under argon atmosphere for 2 h. The white product which precipitated after the addition of ethanol (50 mL) was then filtered, washed with ethanol (2 x 10 mL) and dried. An attempt to prepare dpm monosulfide (dpmS) employing the above procedure, using one half the amount of elemental sulfur resulted in a mixture containing dpm (~20%), dpmS (~60%) and dpmS2 (~20%). The purity of all phosphines was checked by 31P{1ii"} nmr prior to use. Benzonitrile (Aldrich), biphenyl (BDH), CaS (Alfa Products) and DC1/D20 (35% (w/w) DC1) were used as such, without further purification. The ligand dibenzylideneacetone (dba) was prepared 1 by the condensation of acetone (1.45 g, 25 mmol) with benzaldehyde (5.3 g, 50 mmol) in a solution of sodium hydroxide in aqueous ethanol (90 mL); the yellow product was filtered, washed 18 with water (2 x 20 mL) and dried in vacuo. The crude product was recrystal-lized from hot ethyl acetate. The adduct DMA.HCI2 was prepared by bubbling anhydrous HCl(g) into DMA(30 mL) to produce a copious white precipitate. The mixture was filtered under Ar, washed well with diethyl ether and vacuum dried. Recrystallization from acetone/diethyl ether afforded colourless, extremely hygroscopic crystals of DMA.HCI. 2.1.4 Palladium Compounds The palladium, on loan by Johnson Matthey Ltd., was obtained as PdCl2. All synthetic reactions, unless specified otherwise, were carried out under an atmosphere of argon, employing Schlenk techniques. 2.1.4.1 Pd(PhCN) 2Cl 2 Trans-dichlorobis(benzonit rile) palladium ( I I ) . 3 , 4 Palladium(II) chloride, PdCl2 (2.0 g, 11.3 mmol) was suspended in ben-zonitrile (50 mL) and the mixture warmed to 100°C. After 20 minutes the greater part of palladium(II) chloride has dissolved to give a red solution. This red solution was filtered while still warm and the filtrate was poured into hexanes (300 mL). The light yellow product was filtered, washed with hexanes and vacuum dried. Yield - 3.9 g(90%) calculated for C14H10N2Cl2Pd; C : 43.84, H : 2.63. Found C : 44.01, H : 2.69%. 19 2.1.4.2 P d 2 ( d b a ) 3 . C H C l 3 T r i s ( d i b e n z y l i d e n e a c e t o n e ) d i p a l l a d i u m ( 0 ) - c h l o r o f o r m solvate. 5 Palladium chloride (1.05 g, 5.9 mmol) was added to hot (~50°C) methanol (150 mL), containing dibenzylideneacetone (dba) (4.6 g, 19.6 mmol) and sodium acetate (3.9 g, 47.5 mmol). The mixture was stirred at 40°C to give a reddish-purple precipitate, cooled to complete the precipitation, filtered, and the solid then washed successively with water and acetone and dried in vacuo. The precipitate (3.39 g) was dissolved in hot chloroform (120 mL), and filtered to give a deep violet solution. The deep purple needles, which precipitated after the slow addition of diethyl ether (170 mL), were filtered, washed with diethyl ether and dried in vacuo. Yield-2.5 g(85%) calculated for CS2H43Cl303Pd2> C : 60.34, H : 4.19. Found C : 60.12, H : 4.10%. 2.1.4.3 P d 2 C l 2 ( d p m ) 2 Dichlorobis-/x-[bis(diphenylphosphino)methane]dipalladium(I), 6 l a The title compound was prepared by a procedure slightly modified from that reported. P d ( P h C N ) 2 C l 2 (0.41 g, 1.01 mmol), Pd 2 ( d b a ) 3 . C H C l 3 (0.55 g, 0.53 mmol) and P h 2 P C H 2 P P h 2 (0.82 g, 2.1 mmol) were refluxed i n oxygen-free dichloromethane (50 mL), under a nitrogen atmosphere for 30 min. After being cooled, the resulting red solution was filtered to remove any insoluble materials, and filtrate reduced in volume to ~10 mL. The yellow-orange product, which precipitated after the addition of diethyl ether (25 mL), was then filtered, washed with acetone (2 x 10 mL) to remove any palladium(II) monomer, and dried in vacuo. 20 Yield - 1.0 g (90%) calculated for C50H44P4Cl2Pd2; C : 57.05, H : 4.21. Found C : 57.45, H : 4.10%. *H nmr 6fgCh : 4.17 ppm (-CH2-, q, iP_H = 4 Hz). 3 1P{ 1f/'} nmr in CD2CI2 showed a single peak at -3.46 ppm. The spectroscopic data for this complex 7 and the others described in the fol-lowing sections : Pd 2Br 2(dpm )2 , 8 Pd 2I 2(dpm )2, 8 Pd2Cl2(dpm)2(/i-S),9 P d 2 B r 2 -(dpm) 2(/i-S), 9 Pd 2l2(dpm) 2(/x-S), 9 PdCl 2(dpm), 1 0- 1 1 and PdBr 2(dpm), 1 0 agree with those reported in the literature. 2.1.4.4 Pd 2 Br 2 (dpm) 2 Dibromobis-/x-[bis(diphenylphosphino)methane]dipalladium(I), 8 lb To a solution of Pd2Cl2(dpm)2 (0.23 g, 0.22 mmol) in dichloromethane (10 mL), a solution of NaBr (0.20 g, 2 mmol) in aqueous methanol (10 mL) was added. The resulting solution was filtered, and concentrated under vacuum until orange crystals were formed. Aqueous methanol was added to complete the precipitation and the product was filtered. Recrystallization from dichloro-methane/aqueous methanol followed by vacuum drying yielded an orange-red crystalline product. Yield - 2.35 g (95%) calculated for C50H44P4Br2Pd2; C : 52.62, H : 3.89. Found C : 52.20, H : 3.80%. lH nmr 8f%£h : 4.26 ppm (-CH2-, q, iP-B = 4 Hz). nmr in CDC1 3 showed a single peak at -5.5 ppm. 2.1.4.5 Pd2l 2(dpm)2.CH 2Cl2 Duodobis-/i-[bis(diphenylphosphino)methane]dipalladium(I)dichloro-methane solvate,8 lc 21 The complex was prepared using a procedure similar to that described for Pd 2Br 2(dpm) 2. To a solution of P d 2 C l 2 ( d p m ) 2 (0.23 g, 0.22 mmol) in dichl-oromethane (10 mL), a solution of N a l (0.15 g, 1 mmol) in aqueous methanol (10 mL) was added. The resulting solution was filtered and concentrated until brownish-violet crystals were formed. Aqueous methanol was added to complete the precipitation; the solid was filtered, washed with acetone (2 x 10 mL) and dried in vacuo. Yield -0.27 g (92%) calculated for C^H^ChhPd^ C : 46.39, H : 3.51, I : 19.20. Found C : 46.70, H : 3.60, I : 19.11%. *H nmr $ * £ g J s : 4.23 ppm (-CH2-, q, J P _ H = 4 Hz). zlP{lH} nmr in C D C I 3 showed a single peak at -11.3 ppm. 2.1.4.6 P d 2 C l 2 ( d p m ) 2 ( / i - S ) Dichlorobis-/x-[bis(diphenylphosphino)methane]-/x-sulfidodipalladium-(n),9 2a P d 2 C l 2 ( d p m ) 2 (0.50 g, 0.48 mmol) was dissolved in dichloromethane (50 mL) and H 2S gas was bubbled through the solution for 20 min at 20°C; the colour changed from orange-red to brown with accompanying precipitation of brown solid that was completed by gradual addition of diethyl ether (50 mL). The product was filtered, washed successively with acetone (2 x 10 mL), diethyl ether (10 mL) and dried in vacuo. Yield - 0.50 g (97%) calculated for C^H^CliSPdz; C : 55.36, H : 4.09. Found C : 55.60, H : 4.27%. 1H nmr S££h : -CH2-, 2.79 ppm ( dq, J H _ H = 13 Ez, J P _ H = 4 Hz), 4.70 ppm (m, 2E-H = 13 Hz, J P _ H = 6 Hz). nmr in CD 2C1 2 showed a singlet at 5.50 ppm. 22 2.1.4.7 Pd 2Br 2(dpm) 2(^-S) Dibromobis-/i-[bis(diphenylphosphino)methane]-/i-sulfidodipalladium(II), 2b H 2S (50 mL, 1 atm at 25°C) was injected into a Schlenk tube stoppered with a rubber septum, containing a solution of P d 2 B r 2 ( d p m ) 2 (0.50 g, 0.47 mmol), in oxygen-free dichloromethane (50 mL) and allowed to react for 3h; the colour changed from orange-red to brown with accompanying precipitation of brown product. The precipitation was completed by the addition of diethyl ether (50 mL); the solid was filtered, washed successively with acetone (2 x 10 mL), diethyl ether (10 mL) and dried in vacuo. Yield - 0.51 g (98%) calculated for CwH^PABr2SPd2\ C : 51.13, H : 3.78. Found C : 51.41, H : 3.96%. 'H nmr 5*£fCTj : -CH 2 - , 2.85 ppm (dq, 3H-H = 13 Hz, 3P.H = 4 Hz), 4.80 ppm (m, 3H-E = 13 Hz, 3P-H = 6 Hz). 3 1 P { J H } nmr in CD 2C1 2 showed a singlet at 5.80 ppm. 2.1.4.8 Pd 2I 2(dpm) 2(//-S) Diiodobis-/x-[bis(diphenylphosphino)rnethane]-/x-sulfidodipalladium(n), 9 2c To a solution of Pd 2Cl 2(dpm) 2(/i-S) (0.25 g, 0.23 mmol), in oxygen-free di-chloromethane (25 mL) a solution of Nal (0.30 g, 2 mmol) in aqueous methanol (10 mL) was added. The volume of the solution was reduced to ~10 mL by rotovap to yield brown crystals that were collected, dissolved in dichlorometh-ane (20 mL) and reprecipitated using methanol (50 mL). The precipitate was filtered, washed with methanol (2 x 10 mL) and dried in vacuo. 23 Yield - 0.32 g (90%) calculated for C50H44P4hSPd2; C : 47.34, H : 3.50. Found C : 47.20, H : 3.46%. 1H nmr Sg$£la : -CH 2 - , 3.07 ppm (dq, JH.H = 14 Hz, J P _ f f = 3 Hz), 4.94 ppm (m, 3 E - H = 14 Hz, Jp-j? = 6 Hz). 31 P^H} nmr in CDC13 showed a singlet at 5.77 ppm. 2.1.4.9 P d C l 2 ( d p m ) D i c h l o r o [ b i s ( d i p h e n y l p h o s p h i n o ) m e t h a n e ] p a l l a d i u m ( I I ) , 1 0 1 3 6a To a dichloromethane (10 mL) solution of dpm (0.30 g, 0.78 mmol), a solution of Pd(PhCN) 2Cl 2 (0.30 g, 0.77 mmol) in dichloromethane (10 mL) was added with stirring. The stirring continued for 2h and diethyl ether (50 mL) was added to the yellow solution, which afforded a pale yellow solid. The solid was filtered, and after reprecipitation twice from dichloromethane/diethyl ether, washed with diethyl ether, and dried in vacuo. Yield - 0.35 g (80%) calculated for C2sH22P2Cl2SPd; C : 53.46, H : 3.95. Found C : 53.14, H : 3.90%. *H nmr $ £ g , 3 : 4.28 ppm (-CH2-, t, J P _ H = 10.8 Hz). ^P^H} nmr in CDC13 showed a singlet at -54 ppm. 2.1.4.10 PdBr 2(dpm) Dibromo[bis(diphenylphosphino)methane]palladium(II), 1 0 6b T o a dichloromethane solution (10 m L ) of PdCl2(dpm) (0.25 g, 0.45 mmol), a solution o f NaBr (0.20 g, 2 mmol) i n aqueous methanol (10 m L ) was added. T h e resulting solution was filtered, and t he volume o f the solution reduced 24 to ~5 mL in a rotovap. Aqueous methanol (25 mL) was added to com-plete the precipitation of the yellow solid. Recrystallization from dichloromet-hane/methanol, followed by vacuum drying, yielded a yellow product. Yield - 0.23 g (79%) calculated for C25H22P2Br2SPd; C : 46.15, H : 3.41. Found C : 46.00, H : 3.63%. lH nmr f$£la : 4.32 ppm (-CH 2 - , t, 3P_H = 10.5 Hz). 3 1 P { 2 H } nmr in C D C I 3 showed a singlet at -55.8 ppm. 2.2 Instrumentation Infrared spectra were recorded on a Nicolet DX FT-IR spectrometer, as Nujol mulls between Csl plates or as KBr pellets. UV-visible spectra were recorded on a Perkin-Elmer 552A spectrophotometer with thermostated cell compart-ments using anaerobic spectral cells of path length 1.0 or 0.1 cm (Fig 2.1). 1H nmr spectra were recorded on Bruker WP-80, Varian XL-300 or Bruker WH-400 spectrometers, with tetramethylsilane (TMS) at 8 0.0 ppm as stan-dard. 3 1 P{lH} nmr spectra were recorded on a Varian XL-300 (121.4 MHz) or a Bruker WP-80 (32.3 MHz). The reference for the zlP{lH) nmr spectra was the signal for triphenylphosphine at -6 ppm (relative to 85% /J3PO4). 1 1 Chemical shifts are positive in the direction of decreasing field and are reported relative to 85% H3PO4. All spectrometers were operating in the Fourier trans-form mode and were equipped with variable temperature control. All samples were sealed under argon, unless otherwise specified. Gas chromatographic analyses were performed on a temperature program-mable Hewlett Packard 5890A instrument equipped with a thermal conduc-tivity detector (TCD). A packed molecular sieve column was used with helium 2 5 as a carrier gas. Mass spectra were recorded on a mass spectrometer (AE1MS9) by the mass spectrometry services of this department. Elemental analyses were carried out by P . Borda of this department. 2.3 Procedure for a typical kinetic run The kinetics of reaction between the Pd\ complex and H2S were monitored spectrophotometrically, under anaerobic conditions by using cells shown in Fig 2.1. In a typical experiment where P J J 2 S w a s 1 atm, the study was carried out as follows: a weighed amount of complex was placed in the quartz side of the cell which was then evacuated and then filled with H2S gas. A slow stream of H2S gas was then allowed to flow while a saturated solution of H2S (25°C, 1 atm) in dichloromethane (10 mL) was pipetted into the side-arm flask. The solid and the solvent then were mixed and shaken until a homogeneous solution was obtained. The cell was placed in a thermostated cell compartment and the change in optical density was monitored at a fixed wavelength. The cell was agitated periodically to maintain the "physical equilibrium" of the solution (i.e. to avoid any diffusion problems). For kinetic runs requiring Pjy2s values different from 1 atm a slightly mod-ified procedure was adopted. A weighed amount of solid complex was placed in the cell whilst an appropriate volume of solvent was pipetted into the flask. The solvent was degassed by employing a freeze and thaw static vacuum tech-nique. The solid and the solvent were then mixed and shaken until a homo-geneous solution was obtained. The cell was placed in the thermostated cell compartment to allow the solution temperature to equilibrate; when this was 26 100 m m 100 m m Sidearnv Flask 60 m m 360 m m •-Quartz Cell 12 m m F i g u r e 2 .1 : A n a e r o b i c B p e c t r a l cel l used for U V - v i s i b l e spectroscopy. 27 achieved, an appropriate amount of H2S gas or a solution of H2S in the solvent was injected into the cell with the aid of a gas-tight syringe, the experiment then carried out as described above. 2.4 Low temperature nmr experiments with Pd 2X 2(dpm)2 and H 2 S . 2.4.1 Reaction of Pd 2Br 2(dpm)2 with H 2 S A C D 2 C 1 2 (0.6 mL) solution of Pd 2 Br 2 (dpm) 2 , lb , (5 mg) in a 5 mm nmr tube, stoppered with rubber septum, was degassed employing a freeze and thaw static vacuum technique, and : H and 3lP{lH} nmr spectra were run at - 7 8 ° C . The sample was then removed from the probe, and H2S (5 mL, l atm, at 25 CC) was injected into the cold tube with a gas-tight syringe. The H2S gas condensed inside the tube (b.p. - 6 5 ° C ) and the liquid interface (CD2CI2 solution of lb and liquid H2S) turned green. The nmr tube was shaken, replaced into the -78°C probe, and *H and 3lP{lH} spectra were run first at -78°C and then at higher temperatures. 2.4.2 Reaction of Pd 2 I 2 (dpm)2 with H 2 S A CD2CI2 (2.5 mL) solution of lc (20 mg) in a Schlenk tube (10 mL volume) was degassed three times employing the freeze and thaw static vacuum tech-nique. A large excess of H 2 S (~50 mL, 1 atm, 25°C) was then administered into the Schlenk tube, which was then cooled to - 7 4 ° C in a low-temperature slush bath (solid C0 2/ethanol), and the system allowed to react for several hours (8 - 20 h). The initially brownish-red solution of lc , which turned green over this period, was transferred (at - 7 4 ° C ) to an nmr tube with the aid of a cannula, and 1 H and 3lP{}H} nmr spectra were then run at different 28 temperatures, from -74 to 20°C. 2.5 Interaction of Pd 2 Br 2 (dpm )2 with H X 2.5.1 Room temperature reaction A solution of l b (3 mg) in oxygen-free CDC13 (5 mL) or C 6 D 6 , in a small Schlenk tube (5 mL), was degassed three times by the freeze and thaw static vacuum technique. On addition of anhydrous hydrogen bromide gas (50 fiL, 1 atm, 25°C) from a gas-tight syringe, the orange-red solution of l b turned green. UV-visible and J H and 3 1 P{lH} nmr spectra were run immediately after the addition of HBr gas, and after several hours when the solution had become pale yellow. 2.5.2 Low temperature reaction The reaction between the CDCI3 solution of l b and HBr was carried out at low temperature (-40°C) in a nmr tube, stoppered with a rubber septum. The solution was first degassed several times by employing the freeze and thaw technique, then cooled in a low temperature bath (-40°C), and then anhydrous HBr gas was injected into the nmr tube in small increments; after each addition, the *H and 3 1P{ 1if} nmr spectra were run at -40°C. 2.6 Sulfur abstraction reactions 2.6.1 Sulfur abstraction by dpm In a typical experiment, an equimolar mixture of 2b or 2c and dpm (5 x 10 - 2 mmol) in dichloromethane solution (10 mL) were stirred under Ar for 4-10 29 h at room temperature. The solvent was then pumped off and the solid left behind was characterized by *H and 31P{lH} nmr spectra(see section 5.2.1). 2.6.2 Desulfurizat ion of H 2S The reaction between the complex 2 and H 2 S was carried out in the presence of dpm. The molar ratios of P d 2 to dpm were varied from 1:1 to 1:200 in several experiments, always keeping the H 2 S concentration in large excess. A solution of 2b or 2c (3 mg) and a large excess of dpm (100 mg) in dichloromethane solution (25 mL) were stirred with excess H 2 S (50 mL, 1 atm, 25°C) at room temperature for 4-6 h. The resulting solid was isolated by pumping off the solvent and characterized by J H and ^P^H} nmr spec.tra(see section 5.3). 2.7 Gas solubil i ty measurements The solubility of H 2 S in dichloromethane at specific pressures at 25°C was determined using ] H nmr spectroscopy. In a typical experiment, a known volume of CD 2 C1 2 solution containing either benzene (3 //L) or acetonitrile (5 /J.L.) was taken in an nmr tube sealed with a rubber septum. The CD 2 C1 2 solution was degassed employing the freeze and thaw static vacuum technique. An appropriate volume of H 2 S gas was injected into the nmr tube, which was then shaken well to attain physical equilibrium; the 1 H nmr spectrum was run at 25°C after allowing the solution temperature to equilibrate. From the integral peak intensities, using the benzene or acetonitrile peaks as standard, the H 2 S solubility was readily measured. H 2 S solubility was measured directly at various temperatures for the solution initially saturated with H 2 S at room temperature and at 1 atm. 30 2.8 References 1. C. R. Conard and M. A. Dolliver, Org. Syn., Coll. Vol. 2, John Wiley, Toronto, p.167. 2. I. Thorburn, Ph. D. Dissertation, University of British Columbia, Van-couver, B. C, (1985). 3. M. S. Kharasch, R. C. Seyler and F. R. Mayo, J. Am. Chem. Soc., 63, 2088 (1941). 4. J. R. Doyle, P. E. Slade and H. B. Jonassen, Inorg. Synth., 6, 216 (1960). 5. T. Ukai, H. Kawazura, Y. Ishii, J. Bonnet and J. A. Ibers, J. Organomet. Chem., 6 5 , 253 (1974). 6. A. L. Balch and L. S. Benner, Inorg. Synth., 21, 47 (1982). 7. A. L. Balch, L. S. Benner and M. M. Olmstead, Inorg. Chem., 18, 2996 (1979). 8. L. S. Benner and A. L. Balch, J. Am. Chem. Soc, 100, 6099 (1978). 9. G. Besenyei, C. L. Lee, J. Gulinski, S. J. Rettig, B. R. James, D. A. Nelson and M. A. Lilga, Inorg. Chem., 2 6 , 3622 (1987). 10. J. M. Jenkins and J. G. Verkade, Inorg. Synth., 11, 108 (1968). 11. W. L. Steffen and G. J. Palenik, Inorg. Chem., 15, 2432 (1976). 12. G. M. Kosolapoff and L. Maier, Organic Phosphorus Compounds, 2nd ed., Wiley Interscience, Toronto, Vol. 1, 1972, p.130. 31 Chapter 3 T H E I N T E R A C T I O N O F H 2 S W I T H Pd(I) D L M E R S 3.1 The palladium (I) dpm dimer, Pd 2 X 2 (dpm) 2 . The palladium(I) diphenylphosphinomethane dimers, Pd 2 X 2 (dpm) 2 , [X = CI (la), Br (lb), I (lc); dpm = P h 2 P C H 2 P P h 2 ] , first reported by Colton et al.,1 contain an unusually reactive metal-metal bond. The crystal structure of Pd 2 Br 2 (dpm) 2 , Jjb, reveals that the two palladium atoms are connected by a metal-metal bond, of length 2 . 6 6 9 A.2 The coordinating geometries about each metal are approximately square planar, but the two coordination planes(Pd, P, P, Br) are twisted about the Br-Pd-Pd-BR axis which results in a dihedral angle of 3 9 ° between the planes. In solutions 1 undergoes addition of a number of small molecules, including S,3'4 R N C , 2 , 5 , 6 S0 2 , 3 ' 7 C S 2 , 8 C O , 2 , 5 , 6 , 9 and activated acetylenes.10 Addition of these molecules involves their insertion into the Pd-Pd bond, which breaks, the Pd, Pd separation increasing by ~0.5 A. The insertion of sulfur into the Pd-Pd bond results in the formation of so-called A-frame complexes, Pd 2X 2(dpm) 2(/j-S), 2. Elemental Bulfur and organic episulfides effect the conversion of 1 to 2. Recent studies4 in this laboratory have unearthed reaction ( 3 . 1 ) , which occurs quantitatively in solution at am-bient conditions for X = CI or Br. Pd2X2{dpm)2 + H2S —> Pd2X2(dpm)2{fi - S) + H2 ( 3 . 1 ) 3 2 This reaction is of considerable interest because it generates dihydrogen quan-titatively from the environmentally hazardous and common industrial by-product H 2S. So it was decided to investigate the kinetic and mechanistic aspects of this reaction in detail. The results of these studies are discussed in the following sections. 3.2 R e a c t i o n w i t h D 2 S A solution of l a (25 mg) in oxygen-free C H 2C1 2 in a 10 mL Schlenk tube under vacuum was allowed to react with D 2S (4 mL, 1 atm, 25°C). The red-orange solution of l a turned brown with accompanying precipitation of 2a. The solution was stirred for 30 min to complete the reaction. The possible gaseous products evolved (H 2, HD, D 2) were identified as follows: the contents of the Schlenk flask were cooled at liquid-N 2 temperature (to freeze CH 2C1 2 and D 2S), and the mass spectrum of a sample of the gaseous phase was run at low mass range (0-10). The spectrum revealed the presence of only D2. The brown precipitate of 2a was isolated after the addition of ether (5 mL) to complete the precipitation. The isolated solid was dissolved in CD 2C1 2 and the X H nmr spectrum was run. The integral intensities of the -CH 2- proton peaks (4 H) and those of the phenyl proton peaks (40 H) were compared and found to be in a 1:10 ratio, reconfirming that there was no deuterium exchange between D 2S and the -CH 2- protons of l a in CH 2C1 2 solution during the reaction conditions. Thus the H 2 formed via the reaction (3.1) comes exclusively from the H 2S reactant. 33 3.3 K i n e t i c s a n d r a t e m e a s u r e m e n t s A dichloromethane solution of l b is yellow-orange with the following electronic absorptions: 5 A nm 428 10,600 364 17,500 301 23,100 while the dichloromethane solution of Pd 2Br 2(dpm) 2(/z-S), 3 2b, is pale yellow with visible absorption bands at 473 nm (1200 M~lcm~l) and 348 nm (15200 M - 1 c m _ 1 ) . When a dichloromethane solution of l b reacts with hydrogen sulfide, the visible absorption bands at 428 and 364 nm decrease in intensity while the 473 and 348 nm bands of 2b grow in intensity, as shown in Fig 3.1. Anaerobic spectral cells with path lengths of either 1.0 or 0.1 cm were used to monitor the reaction between complex l b and H2S by observing the disappearance of the 428 nm band in a UV-visible spectrometer with a well thermostatted cell compartment. A l l rates were measured under pseudo-first order conditions in that the concentration of H2S was at least 100 times greater than that of complex lb. The reactions were monitored for"up to 2^ - 3 half-lives. A typical rate plot and a plot of the data, which analysed for first order in P d j dependence, are shown in Figs 3.2 and 3.3. The rates were measured at palladium concentrations ranging from 8.41 x 10" B to 2.23 x 10~ 3 M, at different H2S concentrations (7.1 x 10~ 2 to 5.3 x 1 0 _ 1 M) and at different temperatures (0-35°C). Only one parameter at a time was varied. The dependences of the rate on palladium concentration and hydrogen sul-fide concentration were studied at 25° C, i n dichloromethane. The solubility 34 b 350 400 450 500 550 600 nm Figure 3.1: Visible absorption spectral changes of a dichloromethane solution of Pd2Br2(dpm)2 upon addition of H2S at 25°C. 35 1 | 1 r .9 -Time, s F i g u r e 3.2: R a t e p lot for the react ion between P d 2 B r 2 ( c l p m ) 2 (1.01 x 10 3 M ) a n d H 2 S (0.53 M ) i n C H 2 C 1 2 ) at 25°C . 36 500 1000 1500 2000 2500 Time, s F i g u r e 3.3: A rate p lot ana lysed for f irst order Pd\ dependence i n C H 2 C 1 2 at 25°C . ( [Pd*] = 1.01 x l O " 3 M a n d [H 2 S] = 0.53 M ) . 37 of H 2S in CH 2C1 2 at 25°C at various H 2S pressures was determined using the procedure described in section 2.4, and found to obey Henry's law at least up to about 1 atm of H 2S pressure (Table III-1, Fig 3.4). Fur-ther, the solubility of H 2S was calculated to be 1.14 M a t m - 1 or 1.5 x 1 0 - 3 M t o r r - 1 . The kinetic data for the l b and H 2S reaction are summarized in Tables III-2 and III-3, and Figs. 3.5 and 3.6. The reaction is first order in both [Pd 2] and in H 2S ; k 0f, s is the pseudo-first order rate constant deter-mined in the presence of excess [H 2S], and equals k 2[H 2S], where k 2 is the true bimolecular, second-order rate constant for the reaction. The temperature dependence of the rates over the range 0-35°C was investigated at a single palladium concentration (Table III-4). The plot of ln(k 2/T) vs. 1/T yields a reasonably good straight line (Fig. 3.7). Values of AH* = 55 ± 5 kJ mole - 1 and AS* = -115 ± 10 J K - 1 mole - 1 were calculated from the slope and in-tercept, respectively. Attempts were made to investigate the effect of the added bromide and phosphine on the reaction rate.With these ligands, however, reacton 1.1 took a completely different course; reaction of lb_ with H 2S in the presence of a five-fold excess of tetraethylammonium bromide ( N E t 4 B r - ) resulted in the formation of 2b in only ~ 7 0 % , and several other products were evidenced by 3 1 P { J H } nmr and UV-visible spectroscopy, especially an unidentified species with 8p = 22.2 ppm. A five-fold excess of dpm rendered a catalytic system for the removal of H 2 from H 2S, and resulted in the formation of dpm monosulfide along with other palladium compounds (See Ch.5). 38 Table III-1: Solubility of H 2S in CH2CI2 at various pressures at 25°C. P# 2s, atm [ H 2 S W / 2 , M 0.16 0.20 0.22 0.25 0.33 0.38 0.43 0.50 0.46 0.55 0.50 0.56 0.54 0.62 0.58 0.67 0.59 0.69 0.67 0.74 0.78 0.91 Table III-2: Dependence of reaction rate on [Pd!2] at 0.53 M [H 2S] in CH 2C1 2 at 25°C. [Pd 2 ] / 10 - 4 , M W 1 0 - 4 , s- 1 0.84 8.91 1.81 9.05 3.37 8.91 4.42 8.90 5.13 8.92 7.45 8.62 8.09 9.07 10.0 8.84 22.3 8.73 39 F igu re 3.4: So lub i l i t y of H 2 S i n CH2CI2 at var ious pressures. 40 Tab le III-3: Dependence of react ion rate on [H 2 S] at 1.01 x 1 0 - 3 M P d 2 i n C H 2 C 1 2 at 25°C. [ H 2 S ] / 1 0 - a , M Wio-4, s~> 0.70 1.18 1.3 2.24 1.4 2.80 1.8 3.01 2.1 3.85 2.5 4.01 2.7 4.47 2.8 4.90 3.2 5.25 5.3 8.90 Tab le III-4: Tempera tu re dependence of the rate constant for the react ion between l b (at 1.01 x 1 0 - 3 M ) and H 2 S in C H 2 C 1 2 . T e m p . K [H 2 S] * M k2/10"4, M " 1 * - 1 T-710-3, K " 1 Mkj / T y i O " 1 273 0.50 1.59 3.66 -1.44 279 0.51 3.61 3.58 -1.36 288 0.52 6.53 3.47 -1.30 298 0.53 17.1 3.36 -1.21 303 0.54 20.1 3.30 -1.19 308 0.55 25.5 3.25 -1.17 * : so lub i l i t y measured d i rec t ly b y JH n m r exper iments . 41 11 i r - i 1 1 1 i i r 10 I o \ V) O 8 h 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 - 4 ; P d 2 ] / 1 0 " , M Figure 3.5: Dependence of reaction rate on [Pd ]^ at 0.53 M [H2S] in CH 2 C1 2 at 25°C. 42 0 1 2 3 4 5 6 [ H 2 S ] / 1 0 , M F i g u r e 3.6: Dependence of react ion rate on [H 2 S] at 1.01 x 10 3 M P d 2 i n C H 2 C 1 2 at 25°C . 43 -1 T 1 1 1 1 1 I I I ~" -1.1 •1.2 -1.3 h •1.4 -1.5 J I L I I L 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 1/T/10" 3 , K"1 F i g u r e 3.7: T h e tempera ture dependence of the rate constant for the react ion between l b (at 1.01 x 1 0 " 3 M ) a n d H 2 S i n C H 2 C 1 2 . 44 3.4 S p e c t r o s c o p i c d e t e c t i o n o f i n t e r m e d i a t e s . 3.4.1 R e a c t i o n b e t w e e n P d 2 B r 2 ( d p m ) 2 a n d H 2 S . The low temperature (—78°C) a H nmr spectrum of l b in CD 2 C1 2 shows the characteristic -CH2- proton signal of dpm (8 = 4.26 ppm, q, J P _ H = 4 Hz) along with that of phenyl proton signals (8 between 7 and 8 ppm), which are less informative. The nmr spectra of the l b solution after the addition of H2S showed, in addition to the H2S signal (0.9 ppm), two additional sets of peaks (in the -CH2- region 8 = 3.4 ppm, 8 = 3.6 ppm, both broad, 4H, and in the Pd-H region (8 = —8.45 ppm, t, Jp-jj = 18 Hz, 1H) attributed to the formation of a new species in a small amount (~10%), and presumed to be a metal hydride (Figs. 3.8 A - C). Spectra recorded at higher temperatures showed a decrease in the intensity of both sets of peaks up to —65°C, with the gradual formation of Pd2Br2(dpm)2(//-S), 2b, as evidenced by the appearance of peaks in the -CH 2- region (8 = 2.85 ppm, J H - H = 13 Hz, J p _ H = 4 Hz, 8 = 4.8 ppm, J H - H = 13 Hz, Jp_# = 6 Hz) (Figs. 3.9 A & B). At temperatures above —65°C, only signals due to l_b and 2b were observed. Comparison of the 31P{1//"} nmr spectrum of l b , and that of l b with H2S, at —78°C in CD2C12 showed the formation of the intermediate hydride. The spectrum of Jjb with H2S had at least four new unresolved broad peaks (8 ~ 5, 8, 19 and 23 ppm) in addition to the single sharp peak of l b at —5.5 ppm (Fig. 3.9). Similar to the *H nmr spectra, the spectrum run at higher temperatures showed a decrease in intensity of the unresolved broad peaks, with the appearance of a new peak (5.8 ppm), that of Pd2Br2(dpm)2(//-S). At —65°C and above, the nmr spectra showed the presence of only l b and 2b. 45 F i g u r e 3.8: T h e J H n m r spec t ra i n CD2CI2 so lu t ion for ( A ) - C H 2 - reg ion of lb; ( B ) - C H 2 - region o f lb + H 2 S at - 7 0 ° C and ( C ) hyd r i de region of lb + H 2 S at - 7 0 ° C . 46 (A) 4.26 ppm F i g u r e 3.9: T h e J H n m r spec t ra i n C D 2 C 1 2 so lu t ion of ( A ) - C H 2 - reg ion of lb + H 2 S w a r m e d u p to r o o m tempera tu re and ( B ) - C H 2 - region of 2b. 47 -5.5 ppm 19 ppm 8 ppm 23 ppm 5 ppm -78°C 5.80 p p m -70°C - 6 0 ° C - 5 0 ° C 20° C F i g u r e 3.10: T h e S 1 P { 1 H } n m r spect ra i n C D 2 C 1 2 o f lb a n d H 2 S . 48 These observations, to be discussed later (Section 3.5) are explained by: (i) oxidative addition H 2S to P d 2Br 2(dpm) 2, l b , to form a Pd(H)(SH) inter-mediate, observable at temperatures below —65°C, and (ii) the formation of Pd2Br2(dpm)2(/i-S), 2b, from the hydride intermediate at temperatures above ~ -78°C. Pd2Br2{dpm)2 + H2S —• Pd2Br2{dpm)2(H){SH) l b 3b Pd2Br2{dpm)2{H)(SH)+ —> Pd2Br2(dpm)2(pL - S) + H2 (3.2) 3b 2b 3.4.2 R e a c t i o n between P d 2 I 2 ( d p m ) 2 and H 2S. Compared to the : H nmr spectrum of the complex Pd 2I 2(dpm) 2, l c , in CD 2C1 2, recorded at ~ —78°C, the spectrum of the mixture of l c and H 2S in CD 2C1 2 showed additional peaks in the -CH 2- region (8 = 5.02 ppm, br singlet, 4H) and in the Pd-H region (8 = —6.05 ppm, br singlet, 1 H) in addition to the H 2S peak (8 = 0.93 ppm, br ), again indicating the formation of an intermediate hydride (Fig. 3.11 A - C). The spectra recorded at higher temperatures (> —70°C) showed a general decrease in the intensity of both the 5.02 and —6.05 ppm peaks along with the appearance of a new set of peaks (8 3.07 and 4.97 ppm), which are due to the -CH 2- protons of Pd 2I 2(dpm) 2(/j-S), 2c (Fig. 3.12 A & B). The room temperature spectrum showed the presence of only the Pdl. dimer, l c , and the /i-sulfide, 2c. The 31P{lH} spectrum of the hydride intermediate, which was recorded at — 74°C in C D 2C1 2 solutions and more conveniently in neat H 2S liquid contain-ing a small amount of CDCI3, showed two broad peaks (—2.7, —6.7 ppm) in 49 4.23 p p m 5 0 (A) -6.05 ppm F i g u r e 3.12: T h e 1 H n m r spect ra i n CD2CI2 so lu t ion of ( A ) l c -f H 2 S kept at — 7 4 6 C for 8 h (hydr ide a n d - S H region) a n d ( B ) 2jc. 51 addition to a broadened singlet of lc (-11.3 ppm) (Fig. 3.13 A & B). However, as the temperature was increased, the 31P{lH} spectrum of the intermediate underwent two changes. Firstly, increasing amounts of the intermediate de-composed to form the //-sulfide, 2c, as evidenced from the decrease in the in-tensity of the intermediate's peaks, along with the appearance of the //-sulfide peak at 5.7 ppm. Secondly, the two peaks of the intermediate coalesced to form a single peak at —4.0 ppm near the vicinity of — 4 0 ° C (Fig. 3.13 C ) . As in case of the ] H nmr spectra the room temperature spectrum showed the presence of only lc and 2c (Fig 3.14 A & B). These observations are explained in terms of the same general processes shown above in equation (3.2) for the corresponding bromide system (see sec-tion 3.4.1). 3.5 Discussion of kinetic and spectroscopic data The first-order dependence of rate on both [Pd\] and [H 2S] can be explained by either of the following mechanisms: (a) Pd2X2{dpm)2 + H2S Pd2X2(dpm)2(H){SH) (3.3) 1 3 Pd2X2{dpm)2(H)(SH)+ Pd2X2(dpm)2(p - S) + H2 (3.4) 3 2 where fci, k-i and ki are the rate constants of the individual steps as written. Application of the steady state treatment to the intermediate, Pd 2X 2(dpm)2(H)(SH), 52 (A) -11.3 ppm (B) -2.7 ppm 11.3 p p m (C) 5.7 p p m - 4 p p m -11.3 p p m F i g u r e 3.13: T h e " P ^ H } n m r spec t ra i n C D C 1 3 o f ( A ) lc at 20 °C ; ( B ) lc + H 2 S kept at — 7 4 ° C for 8 h a n d ( C ) l c + H 2 S kept at — 7 4 ° C for 8 h a n d then w a r m e d u p to - 4 0 ° C . (S im i la r d a t a are ob ta ined i n C D 2 C 1 2 . ) 53 (A) -11.3 p p m o.< p p m ( B ) 5.7 p p m F i g u r e 3.14: T h e 3 1 P { 1 H } n m r spec t ra i n CDC13 o f ( A ) l c + H 2 S kept at ~ - 7 4 ° C for 8 h and then w a r m e d to r o o m tempera tu re a n d ( B ) au then t i c samp le of 2 c at 2 0 ° C . (S im i la r d a t a are ob ta ined i n CD2CI2) 54 3 gives the fo l low ing rate law. -d\Pd\] d[Pd2(fx-S)} kMPdDlHtS] Rate T h a t i s , where dt dt -+- k2 Rate = k'[Pd\][H2S] k^k2 k' = fc-i - f k2 If k2 » fc_1} the rate s impl i f ies to fc^Pd^fH^S], m e a n i n g that the reac-t ion 3.3 is rate de te rmin ing . If ^> k2, the rate expression becomes fcifc2/fc_i[Pd2][H2S] or Kfc 2[Pd2][H2S] where K is now7 the equ i l i b r i um constant (ki/k-i) for react ion 3.3, and react ion 3.4 is rate de te rmin ing . T h e more complete express ion for a r a p i d p re -equ i l i b r i um (eq 3.3) fo l lowed by a rate de te rmin ing k2 step is k2K[Pd\}T[H2S}  R a t e = 1 + K[H2S) where the T subscr ip t imphes to ta l Td\. T h e s t r ic t ly first order dependence on H 2 S requires tha t up to [H 2 S] = 5.3 x I O - 1 M ( F i g 3.4), at 25°C, K [ H 2 S ] < 1, and that 3_b is no t detectable at the condi t ions where the H 2 S dependence was measured. (b) A n ex t ra step w i t h a c c o m p a n y i n g k i n e t i c / e q u i l i b r i u m parameters is i n -vo lved i f the first step i n the react ion sequence is fo rma t ion of an H 2 S adduc t : Pd2X2{dpm)2 + H2S ^=h0=^ Pd2X2(dpm)2(H2S) (3.5) Pd2X2(djrm)2{H2S) ^ = f c , = * Pd2X2(dpm)2{H){SH) (3.6) 55 Pd2X2{dpm)2{H){SH)+ -X Pd2X2(dpm)2{u - S) + H2 (3.7) Consistency with the observed kinetic dependences requires one of the fol-lowing: (i) the ka step is slow and rate determining; rate = fca[Pd2][H2S]. (ii) fcj, governs the rate determining step, following a rapid K„ pre-equilibrium (ka/k_a) with Pd 2(H 2S) being undetectable; rate = fcfeKa[Pd2][H2S]. (iii) the k2 step is rate determining, the two pre-equilibria (ka/k_a and kb/k^b) now again generating undetectable amounts of P d 2(H 2S) and Pd 2X 2(dpm) 2(H)(SH); rate = fc2KaKfc[Pd2][H2S]. The simple kinetics measured do not distinguish between the mechanisms of reactions 3.3 and 3.4, and that outlined in 3.5 to 3.7. Nevertheless, further insight into the process, along with information about a reaction intermediate, is gained from the low temperature nmr experiments. The oxidative-addition of H 2S across the metal-metal bond of 1 would presumably lead to a hydride intermediate of the type 3, with terminal hydride and terminal SH. 3 ( X = CI, a ; Br, b ; I , c) 56 Such oxidative-addition type reactions to the P d 2 X 2 ( d p m)2 complexes, in which the metal-metal bond acts as a nucleophile, are well established.11 In the H 2 S/Pd 2 Br 2 (dpm) 2 reactions, a hydride intermediate thought to be (3b) is formed at -78°C quite rapidly in a liquid H 2 S interface region and this species rapidly decomposes ( t i < 30 s) to form 2_b at temperatures above ~-65°C. Thus at 0-35°C, the rate determining step will not be decomposition of 3b, (i.e. the k2 steps of the mechanisms discussed). The rate determining step could be the formation of the hydride 3b (or the H 2 S adduct), and this would be consistent with the non-detection of 3b at ambient conditions. It then seems surprising, however, that 3b is detected at low temperatures. Its formation could presumably result because of one or both of the following factors: (i) the H 2 S addition at low temperatures (below the H 2 S liquefaction tem-perature) produces a very high localized concentration of H 2 S. (ii) optimum temperature conditions prevail so that the oxidative-addition takes place (perhaps because of high H 2 S concentration), while decom-position of the hydride is slowed down sufficiently to build up the con-centration of 3b. It was initially thought that the presence of liquid H 2 S might increase the dielectric constant of the medium, and rates of oxidative-addition of polar molecules are typically enhanced in more polar media. 1 3 However, dielectric constant data for C H 2 C 1 2 and liquid H 2 S at -65°C give e values of 15 and 9, respectively, and thus the changes in dielectric constant are unlikely to be important. Two qualitative observations support the rationale presented above in (i) and (ii). When liquid H 2 S comes in contact with solid lb at ~-78°C a green solution is produced, presumably because of the formation of 3b; based 57 on nmr data, the green solution subsequently yields 2b on warming. Secondly, warming an nmr tube containing successively frozen layers of a solution of l b and H 2S (liquid N 2 temperature) generates a green colour in the upper part of the liquid but only in a very narrow temperature range (between ~ - 7 0 and -60°C). The hydride a H nmr signal (8 = -8.9 ppm, triplet, J P - H = 18 Hz) is characteristic of a terminal Pd-H hydride cis to a phosphine ligand. 1 4 The non-observation of the 6(Pd-SH) J H nmr signal (typically at 8 between 1.5 and -1.5 ppm) 1 5 could be attributed to two possible factors: (i) the burial of the S-H resonance under the large, broad H 2S signal (~0.9 ppm), because the intermediate 3b is generated only under high H 2S concentration. (ii) the S-H proton, which will certainly be a c i d i c 1 6 may undergo rapid ex-change with H 2S protons thus producing a broad resonance not detected under the experimental conditions. Additional structural information could not be obtained from the 3 1 P { 1 H } nmr spectrum as it was of poor quality (Fig 3.9) because of the low concen-tration of 3b (~10%). However, the solutions of 3b should generate a AA' BB' type 3 1 P { ] f f } nmr which typically is seen as a pattern centred at four major resonances, 1 7 and the observed unresolved peaks could result from such a spectrum. The hydride intermediate for the analogous Pd 2I 2(dpm) 2/H 2S system is slow i n forming (~8 h at ~-78°C) but could be obtained at a much higher concentration (>50%); the hydride 3c is much more stable than 3b but rapidly decomposes at temperatures above -30°C. This further indicates that the oxidative-addition is the rate determining step. 58 It shou ld be noted that fo rmat ion of a poss ib le H 2 S - a d d u c t (i.e. the K a equ i l i b r i um i n react ion 3.5) is expected to be favoured at lower tempera tu res , because so lu t ion react ions of th is type must be ent rop ica l ly un favourab le a n d therefore exo thermic i f they are to occur at a l l . T h u s K a wou ld be favoured at lower temperatures. However , the J H n m r d a t a show the presence of an in te rmed ia te hyd r ide and not H 2 S complex fo rma t ion ; J p _ H f ° r th ree-bond coup l i ng w i t h i n an P d - S H 2 w o u l d be m u c h less t h a n the observed 18 H z w h i c h is t yp i ca l for 2Jp-jj values w i t h hydr ide cis to p h o s p h o r u s . 1 4 T h e ac t i va t ion parameters ( A H 1 = 55 K J , A S * = -115 J K - 1 ) are qui te typ-ica l of those observed for ox ida t i ve add i t i on react ions at one meta l c e n t r e , 1 8 - 2 0 bu t there are not any usefu l , comparab le da ta for d inuc lear systems. Fo r the present P d 2 —> P d ° , fo rmat ion of a species such as 3_b necessitates b reak ing the P d - P d b o n d but the e lect ronic p romo t i ona l energy 1 8 - 2 0 is now on ly for a s ingle e lect ron at P d centre a n d w i l l be less t h a n tha t requi red for 2e -ox ida t i ve add i t i on at one ' comparab le ' me ta l centre; the other factors con t r i bu t i ng to A H 1 are b reak ing of an S - H b o n d and fo rmat ion of the P d - H and P d - S H bonds . O v e r a l l , the ac t i va t ion parameter for ox ida t i ve add i t i on process for the same mo lecu le at one or two me ta l centres m a y t u rn out to be s imi la r . It is not c lear f rom the present s tudy i f A H * and A S * refer to k j (eqn. 3.3), k a or k{ ,K a (eqn. 3.5 & 3.6). C o r r e s p o n d i n g ambigu i t ies arise i n ox ida t i ve a d d i -t i on of H 2 t o g ive a d ihydrogen species, now tha t » 7 2 - H 2 complexes as poss ib le in termedia tes have been r e c o g n i z e d . 2 1 O f note , the react ion of lb w i t h C O to give P d 2 B r 2 ( d p m ) 2 ( / j - C O ) i n d ime thy lace tamide so lu t ion , w i t h A H * = 15 K J , A S * = - 1 2 1 J K _ 1 , 9 is some 10 t imes faster t h a n H 2 S react ion i n C H 2 C 1 2 at comparab le temperatures , a n d the difference is ref lected ent i re ly i n the A H * va lues; p resumab ly , the h igher va lue for the H 2 S system results f r om energy 59 required to break an S-H bond. It would be particularly useful to have activation parameters for the other halide systems; unfortunately the iodide system is photosensitive, but at 25°C the H 2S reaction is about 10 times slower than for the bromide. 2 2 The reac-tivity trend of 1 toward H 2S (X = CI > Br > I) is opposite to that normally encountered for oxidative addition of gas molecules to a single metal centre, where more basic auxiliary ligands promote r e a c t i o n s . 1 8 - 2 0 Within 1, the P d -Pd bond strength is expected to increase in the order I < Br < CI, 5 the reverse of the trans effect of the halides, 2 3 and thus the reactivity trend is not domi-nated by differences in the metal-metal bond strength. Reactivity of 1 toward CO in terms of equilibrium constants is also CI > Br > I, and this is governed by the off-rates that decrease in the reverse order (I > Br > CI), the on-rates being essentially the same;9 it has been suggested that the strength of metal-carbonyl bond in the product (I < Br < CI) governs the reactivity trend. Such could be the case in the H 2S systems, if the k _ i step of the mechanism outlined in eqns. 3.3 and 3.4, or the k_ a step of the mechanism given in eqn. 3.5, case (ii), become increasingly important (i.e. these rate constants increase in the order kci < k f i r < kj). 60 3.6 References 1. R . Colton, H. Farthing and M. J. McConnick, Aust. J. Chem., 26, 2607 (1973). 2. R . J. Holloway, B. R . Penhold, R. Colton and M. J. McConnick, J. Chem. Soc, Chem. Commun., 485 (1976). 3. A . L. Balch, L. S. Benner and M. M. Olmstead, Inorg. Chem., 18, 2996 (1979). 4. C. L. Lee, G . Besenyei, B. R. James, D . A . Nelson and M. A . Lilga, J . Chem. Soc, Chem. Commun., 1175 (1985). 5. L. S. Benner and A . L. Balch, J. Am. Chem. Soc, 100, 6099 (1978). 6. M. M. Olmstead, H. Hope, L. S. Benner and A . L. Balch, J. Am. Chem. Soc, 9 9 , 5502 (1977). 7. L. S. Benner, M. M. Olmstead, H. Hope and A . L. Balch, J. Organomet. Chem., 153, C31 (1978). 8. M. L. KuUberg and C. P. Kubiak, Inorg. Chem., 25, 26 (1986). 9. C. L. Lee, B. R. James, D. A . Nelson and R. T. Halien, Organometallics, 3, 1360 (1984). 10. A . L. Balch, C. L. Lee, C. H. Lindsay and M. M. Olmstead, J. Organomet. Chem., 177, C22 (1979). 11. R. J. Puddephatt, Chem. Soc Rev., 12, 99 (1983). 61 12. Handbook of Chemistry and Physics, 6 8 t h E d i t i o n , C R C , B o c a R a t o n , F l a . (1988), p .E50 . 13. R. G . Pearson and C . T . K r e s g e , Inorg. Chem., 20, 1878 (1981). 14. S. J . Y o u n g , B . Ke l lenberger , J . H . Re ibensp ies , S. E . H i m m e l , M . M a n -n i n g , 0. P . A n d e r s o n and J . K . St i l le , J. Am. Chem. Soc, 110, 5744 (1988). 15. See, for example : (a) G . Besenye i , C . L . Lee , J . G u l i n s k i , S. J . R e t t i g , B . R . J a m e s , D . A . Ne lson a n d M . A . L i l g a , Inorg. Chem., 2 6 , 3622 (1987). (b) M . R. D u B o i s , M . C . Vanderveer , D . L . D u B o i s , R . C . Ha t t iw inge r and W. K . M i l l e r , J. Am. Chem. Soc, 102, 7456 (1980). (c) K . O s a k a d a , Y . Y a m a m o t o , A . Y a m a m o t o , Inorg. Chim. Acta, 90, L 5 (1984). (d) R . Y u g o , G . L a M o n i c a , S . C e n i n i , A . Segre a n d F . C o n t i , J. Chem. Soc A, 522 (1971). (e) I. M . B l a c k l a w s , E . A . V . E b s w o r t h , D . W . H . R a n k i n a n d H . E . R o b e r t s o n , / . Chem. Soc, Dalton Trans., 753 (1978). 16. M . C . J e n n i n g , N . C . P a y n e a n d R. J. P u d d e p h a t t , J. Chem. Soc, Chem. Comrnun., 1809 (1986). 17. NMR, Vol 5, E d s . P . D i e h l , E . F l u c k a n d R. K o s f e l d , Spr inger V e r l a g , N e w Y o r k , 1971, p.110. 18. L . V a s k a , Acc. Chem. Res., 21, 120 (1968). 19. J . P . C o l l m a n a n d W . R . R o p er, Adv. Organomet. Chem. 7, 54 (1968). 62 20. J . Halpern, Pure. Appl. Chem., 20, 59 (1969). 21. G. J . Kubas, Acc. Chem. Res., 21, 120 (1988). 22. (a) D. Sallin Private Communication. (b) A. F. Barnabas, D. Sallin and B. R. James, Can. J. Chem. (submit-ted). 23. T. G. Appleton, H. C. Clark and L. E. Manzer, Coord. Chem. Rev., 10, 333 (1973). 63 C h a p t e r 4 T H E I N T E R A C T I O N O F H X ( X = C I , B r ) W I T H P d ( I ) D L M E R S 4.1 I n t r o d u c t i o n T h e in terac t ion between anhydrous H C I gas and P d ( I ) d imer , P d 2 B r 2 ( d p m ) 2 l b , was discovered acc identa l l j ' , wh i l e test ing for the effect of H X impur i t ies present i n D 2 S gas (sect ion 3.2.) on its react ion w r i th l b . T h e ye l lowish-orange ch lo ro form or d ich lo romethane solut ions of l b tu rned dark green on exposure to anhydrous H C I gas, en route to the fo rmat ion of the pa le yel low Pd( I I ) monomer i c species P d ( d p m ) X 2 ( X = ha l ide) , at amb ien t cond i t ions . A p p r o x i m a t e l y 2 mo le equivalents of anhydrous H C I per mo le equivalent of P d 2 comp lex were requi red for the complete convers ion of Pd\ d imer to a Pd( I I ) monomer , as ev idenced by n m r spectroscopy. T h e green in te rmed ia te , in i t i a l l y f o rmed , subsequent ly broke down to the pa le yel low Pd ( I I ) complexes of the t ype P d ( d p m ) X 2 , i n a slow react ion (t i ~ 30 m i n ) . T h e final Pd ( I I ) p roduc ts compr ised a m ix tu re of P d ( d p m ) C l 2 , P d ( d p m ) B r 2 a n d P d ( d p m ) C l B r ( F i g 4.1). T h e ox ida t i ve add i t i on of X 2 ( X = C I , B r , I) to P d 2 X 2 ( d p m ) 2 h a d been s tud ied b y B a l c h et a l . 1 T h e react ion proceeded th rough the ox ida t i ve add i t i on of X 2 across the me ta l -me ta l b o n d , resu l t ing i n a green te t raha lo in te rmed ia te , wh i ch was detected at low temperatures before the subsequent b reak -down to 64 Pd(dpm)(Cl)(Br) (-54.5, -55, -57.2 k -57.7 ppm) Pd(dpm)Cl 2 (-55.2 ppm) Pd(dpm)Br 2 (-56.9 ppm) Figure 4.1: The 3 1 P { J H } nmr spectra in CDCI3 of the reaction products of l b and anhydrous HCl; the A B pattern J w = 60 Hz is assigned to Pd(dpm)(Cl)(Br). The 6 values for Pd(dpm)Cl 2 and Pd(dpm)Br 2 are shifted slightly to those given in sections 2.1.4.9 and 2.1.4.10 because of the presence of HCl. Pd(Il) monomers (eq 4.1, Fig 4.2). Pd2X2(dpm)2 + A'2 — • 2PdX2{dpm) (4.1) As the H X reaction with the P d ( I ) dimers seemed to take a similar route, it was decided to investigate the reaction in more detail to unravel the mech-anistic path way of the H X additions. 65 F i g u r e 4.2: T h e mechan is t i c pa thway of the react ion between l b and B r 2 . ( A d a p t e d f rom ref. 1). 66 4.2 Stoichiometry of the reaction and product identification 4.2.1 Reaction of Pd2C l 2(dpm) 2 with HCl at room temperature The Pcl(I) complex la reacted with 2 mole equivalents of HCl, or with the adduct DMA.HCl 2 in D M A , resulting in formation of Pd(dpm)Cl2 and H 2 . Pd2Cl2(dprn)2 + 2HCI —• 2PdCl2(dpm) + H2 (4.2) la 6a The reaction was monitored in D M A as well as in CH2C12 by UV-visible, and J H and 3 1 P{*H} nmr spectroscopy. The nmr spectra are presented in Figs. 4.3 and 4.4 along with that of an authentic sample of Pd(dpm)Cl2, which was prepared from Pd(PhCN)2Cl2 (section 2.1.4.9.). Both J H and 3lP{lH} nmr data for la in CDC13 indicated the disap-pearance of the l a signals (JH, -CH 2- protons, 8 = 4.17 ppm, q, Jp_# = 4 Hz; 31P{1i/}, singlet, —3.46 ppm), and the appearance of the signals of Pd(dpm)Cl2, (JH, 8 = 4.28 ppm, t, J P _ H = 10 Hz; ^P^H}, -55.2 ppm) on gradual additions of HCl gas. When the Pd2 dimer to HCl ratio was 1:2 mole equivalents, the la signals were completely lost. The rapid interaction (ti < 30 s) of anhydrous HCl(g) with la in CH2C12 or CHC13 solutions at room temperature resulted in the appearance (UV-visible spectra) of a new band at 320 nm at the expense of the 416 and 347 nm bands of la. A similar reaction was observed in D M A solutions between l a and DMA.HCl but at a much slower rate (ti ~ 45 min). v 2 The gaseous product, dihydrogen, was detected and identified by gas chro-matography by comparison with authentic H 2 gas. A packed molecular sieve 67 (A) 4.17 ppm (B) 4.17 ppm (C) 4.28 ppm Figure 4.3: The J H nmr spectra (-CH2- region) in CDC13 solution at 20°C of (A) la; (B) l a + HCl(g) (~3:l) and (C) l a + HCl(g) (1:2). 68 (A) -3.46 ppm I* (B) -3.46 ppm -55.2 ppm (C) -55.2 ppm F i g u r e 4.4: T h e 3 1 P { ' H } n m r spec t ra i n C D C 1 3 so lu t ion at 20°C of ( A ) l a ; ( B ) l a + H C l ( g ) ( -3:1) a n d ( C ) l a + H C l ( g ) (1:2). 69 column was used in a Hewlett Packard 5890A instrument equipped with a ther-mal conductivity detector (TCD); a retention time of 1.05 min was measured for the gaseous product and authentic H 2 (Fig 4.5). An attempt to detect reaction intermediates at low temperature (—78°C), akin to C l 2 addition to l a 1 (cf. Fig 4.2), was not successful. 4.2.2 React ion of P d 2 B r 2 ( d p m ) 2 w i th H B r at room temperature As summarised in section 4.1 and shown in section 4.2.1, the chloroform and dichloromethane solutions of l a or l b required ~2 mole equivalents of H X per mole of palladium complex for reaction. This is readily rationalized in terms of (i) oxidative-addition of a mole of, for example, HBr to l b leading to a palladium hydride product, 4b: Pd2Br2(dpm)2 + 2HBr • Pd2Br2(dpm)2(H){Br) (4.3) l b 4b (ii) the reaction of 4b with a second mole of HBr leading to a tetrabromopal-ladium(II) product, 5_b, and hydrogen (H 2 ): Pd2Br2(dpm)2{H)(Br) + HBr > Pd2Br4(dpm)2 + H2 (4.4) 4b 5b and (iii) the break-down of 5b to the monomeric Pd(II) species: Pd2Br4(dpm)2 — • 2PdBr2(dpm) (4.5) 5b 6b Evidence for the pathways of reactions (4.3) - (4.5) was obtained from the 70 Figure 4.5: T h e gas chromatograms of ( A ) the gaseous products evolved in the react ion of la 4- 2 HCl(g) in D M A ; (B) the gaseous products evolved in the react ion of lb + 2 HBr(g) in CH 2 C 1 2 and (C) authentic dihydrogen gas. P d 2 B r 2 ( d p m ) 2 / H B r system. The reaction of lb with HBr in CDC13 was mon-itored by *H nmr, 3 1 P { 1 f f } nmr and UV-visible spectroscopy (Fig 4.6). Wavelength(A) nm Figure 4.6: Visible absorption spectrum of dichloromethane solution of Pd 2 Br2(dpm) 2 upon addition of anhydrous HBr at 25°C, as a function of time; the P d 2 B r 2 ( d p m ) 2 ( A m a a ; = 364, 428 nm) 'instantly' gives P d 2 B r 4 ( d p m ) 2 ( A m 0 i E = 360, 600 nm) that slowly converts to PdBr 2(dpm)(A m o i e = 338, 380 nm). nmr spectra at 20° C are presented in figures 4.7 and 4.8, along with the spec-trum of an authentic sample of PdBr2(dpm), 6b, which was prepared from PdCl2(dpm) (see section 2.1.4.10). In the * H nmr spectrum, two sets of peaks, a singlet (6 — 4.63 ppm) and a triplet (6 = 4.31 ppm, Jp_jy = 10 H z ) appeared in the - C H 2 - proton region, immediately on addition of anhydrous hydrogen bromide gas to l b in solution; the signal due to the - C H 2 - proton of the dpm of lb (6 = 4.26 ppm, q, Jp_jj = 4 H z ) had disappeared. The singlet at 4.63 72 (A) 4.26 ppm 1 (C) 4. J ! 31 ppm i ! ( B ) 4.31 p p m 4.63 p p m (D) 4 J 31 p p m I Figure 4 .7: The *H nmr spectra (-CH2- region) in CDC1S solution at 20°C of (A) lb; (B) l b + HBr(g) after 15 min.; (C) l b + HBr(g) after 6 h and (D) authentic sample of PdBr2(dpm). 73 (A) (C) -o.o ppm -56.9 ppm (B) (D) - 56 .9 p p m - 5 6 . 9 p p m 5.2 p p m Figure 4 .8: The 8 1 P^H} nmr spectra in C D C 1 3 solution at 20°C of ( A ) l b ; (B) l b 4 HBr(g) after 15 min.; ( C ) l b + HBr(g) after 6 h and (D) authentic sample of PdBr2(dpm). 74 ppm then lost its intensity slowly ( t i ~ 30 min), while the triplet grew in intensity as the reaction proceeded. A singlet appeared at 6 5.2 ppm in the 31P{lH} spectrum nmr along with a low intensity singlet at —56.9 ppm. The high field singlet (6 = —56.9 ppm) grew in intensity as the reaction proceeded. The J H and 31P{*H} nmr spectra (Figs. 4.7 C, D, 4.8 C &: D) show that the identity of the final palladium product is PdBr 2(dpm). The J H (6 = 4.63 ppm) and 3 1P{ 1//} nmr data along with absence of high field resonance in the hydride region show that the intermediate is 5b rather than 4b. Both the *H and 3 1P{ 177} nmr data are in good agreement with those of 5_b observed by Balch et al., 1 on reacting l b with B r 2 in C D C I 3 solution at -40°C {lE: 6 = 4.6 ppm and 3 1 P { 1 i ? } : 6 = 4.90 ppm). The gaseous product dihydrogen formed during the reaction was detected as described in section 4.2.1 for the l a/HCl reaction (Fig 4.5). 4.2.3 Low temperature spectroscopic studies of the reaction be-tween lb and HBr. A low temperature titration between a CDCI3 solution of lb and anhydrous HBr gas was carried out at — 40°C, and the changes monitored with *H and 3 1 P { J i / } nmr spectroscopy, (section 2.5.2.). Addition of up to 1 mole equiv-alent of anhydrous HBr to lb in solution resulted in the formation of a new species, a green hydride intermediate with *H nmr signals at 6= 4.60 ppm (br, s, 4 H) and at 6= -8.9 ppm (br, s, 1 H) (Figs 4.9 A & 4.10 A). Further addition of HBr gas resulted in the depletion of the hydride intermediate, as evidenced by the decrease in the intensity of hydride intermediate's *H nmr signals, with concomitant formation of the tetrabromo intermediate, P d 2Br 4(dpm) 2. 75 (A) 4.26 ppm 4.60 ppm Hydride lb (B) 4.63 ppm 4.60 ppm Hydride P d 2 B r 4 ( d p m ) 2 4.31 ppm <-PdBr 2(dpm) (C) Figure 4.9: The *H nmr spectra (-CH 2- region) in CDC13 solution at -40°C of (A) l b + HBr(g) (2:1); (B) l b + HBr(g) (1:2) after ~15 min and (C) l b + HBr(g) (1:2) after ~30 min. 76 (A) l 8 - 9 P P m L m e W \ (B) 4 -4.63 p p m 1 P d 2 B r 4 ( d p m ) 2 —> ^ J 31 p p m l ^ - P d B r 2 ( d p m ) ( C ) 4 J .31 p p m I Figure 4.10: The *H nmr spectra in CDC13 solution at -40°C of (A) l b + HBr(g)(l:l), the high field region after ~ 2 min; (B) l b + HBr(g) (1:2) after 60 min, - C H 2 - region and (C) l b + HBr(g) (1:2) after 6 h, - C H 2 - region. 77 The tetrabromo intermediate slowly broke down at low temperature to Pd(II) monomer or more rapidly if the solution was warmed up to room tem-perature. The 31P{XH} nmr spectrum of a solution of l b containing 2 mole equiva-lents of HBr (analogous to Fig 4.9 B) had signals at 8p = -55.6 ppm (sharp, s) and at Bp — 6.2 ppm (br, s) (Fig 4.11 A) . The high field signal corresponds to that of PdBr 2(dpm) ( see section 2.1.4.10). The low field signal is not eas-ily assigned but could arise from the time averaging of various signals ( for example, Pd 2Br 4(dpm)2 and Pd2Br2(dpm)2(H)(Br) species.) Further, in the presence of excess HBr, the 8P = 6.2 ppm peak shifted to the low field side and is seen as a multiplet, while the corresponding shift in the high field signal was negligible (Fig 4.11 B & C ) . 4.3 Discussion The H B r / i b reaction proceeds stepwise and the changes taking place at the low temperatures are depicted as follows: fast Pd2Br2(dpm)2 + HBr ^ Pd2Br2(dpm)2(H)(Br) (4.6) l b . yellow orange 4b, green Pd2Br2(dpm)2(H)(Br) + HBr ^ Pd2Br4(dpm)2 + H2 (4.7) 4b. green 5b, green Pd2Br4(dpm)2 slow 2PdBr2(dpm) (4.8) 5b, green 6b, pale yellow 78 -55.6 ppm <-PdBr2(dp>m) (B) : .8 ppm -55.7 ppm (C) 8 ppm -55.8 ppm Figure 4.11: The 3 1P{ JH} nmr spectra in CDC13 solution at -40°C of (A) l b + HBr(g) (1:2) after ~15 min; (B) l b + HBr(g) (1:5) after ~20 min and (C) l b + HBr(g) (1:10) after ~25 min. l b + HBr(g) (2:1) after ~15 min, the 31P{*H} nmr spectra in CDC13 shows peaks at -55.6 ppm (sharp, s) and 5.4 ppm (br, s). 79 The first mole of HBr oxidatively adds across the Pd, Pd bond, and leads to the formation a transient palladium hydride intermediate observable at low temperatures (-40°C) by nmr spectroscopy. Such a hydride intermediate could have any one or more of the following structures: (i) A face-to-face bridged structure as observed in the Rh(I) dimers Rli2(dpm)2-( C 0 ) 2 C 1 2 ) 2 and Rh 2(dam)2(CO) 2Cl2, 3 and in the Pd(II) dimers Pd 2(dpm) 2-Cl 2(CH- 3) 2, 4 Pd 2Cl 4[Bu 2P ( C H 2) 7PBu 2 ] 2 , 5 , 6 and Pd 2Cl4[Bu 2P (CH2) 1 0PBu 2 ]2 . 5 ' 6 (ii) A /i-hydrido /j-bromo bridged structure commonly observed in doubly bridged dimers 7. ( i i i ) A d imer hav i ng a A - f r a m e s t ruc ture , w i t h a hyd r i do b r idge , as f ound 80 in many Pt and Pd dimers with dpm as the bridging ligand. [Pt 2(dpm) 2-C1 2(H)] + ,8 [Pt 2(dpm) 2Me 2(H)] + ,9 and [Pd 2(dpm) 2Me 2(H)] + 4 are examples of cationic complexes exhibiting A-frame structures with bridging hydride. + Y " The observed J H (broad -CH 2- signal and hydride signal) and 3 3 P { 1 H } (broad singlet) nmr spectra of 4b can be accounted for by any one of the above structures or by two or more of the structures in equilibrium with each other. The reaction of the second mole of HBr with the hydride intermediate leads to the generation of H 2 gas along with formation of a tetrabromide intermedi-ate. The presence of a sharp singlet in both the *H and 3 1 P { J H } nmr spectra indicates a more symmetric structure persumably with equivalent phosphorus atoms and methylene protons. A face-to-face bridged structure, a dibromo bridged structure, or a /i-bromo bridged A - frame structure with rapidly ex-changing methylene protons, would give the observed 1 H and 3 1 P { J H } nmr spectra. The subsequent break down of the tetrabromo intermediate, 5b, to Pd(II) monomer, 6b, is a much slower process ( t i ~ 30 min at 20°C) than that of the corresponding iodide species, P d 2 I 4 ( d p m ) 2 ( U = 6 min at 15°C).1 The breakdown of 5b, if it exibits the face-to-face dimer structure, would pro-ceed simply through the scission of Pd-P bonds. Of interest, the complexes of 81 the type P d 2 C L ; [ B u 2 P ( C H 2 ) n P B u 2 ] 2 (n = 7, 10), 5 , 6 in which the Pd centers are well seperated by the long methylene chains of the bridging bisphosphine Lig-ands, are found to be indefinitely stable in solution; obviously, in these cases, the formation of a halide bridge between the metal centers is not possible and this indicates that the formation of a halogen bridge between the metal atoms is a prerequisite for the scission of a Pd-P bond. Thus the breakdown of 5_b is likely to take place through the structures (ii) and /or (iii). If the reaction between l a and HCI proceeds through the above mech-anism, then at low temperatures the reaction would generate the transient intermediates Pd 2Cl 2(dpm) 2(H)(Cl) and Pd 2 C l 4 ( d p m ) 2 , analogous to the l b system. The tetrachloro intermediate has been observed in the oxidative ad-dition of C l 2 to l a at low temperatures (-40°C) in CH 2C1 2 solutions. 1 Failure to detect any intermediate in the present work even at -78°C indicates that the HCI/la reaction proceeds either through a mechanistic pathway in which Pd 2CLi(dpm) 2 is not formed or, more likely, the tetrachloro species is formed but its subsequent breakdown to the monomer is fast (perhaps catalysed by HCI) and no longer rate determining as in the reaction between l a and C l 2 -82 4.4 References 1. C. H. Lindsay and A. L. Balch, Inorg. Chem., 20, 2267 (1981). 2. M. Cowie and S. K. Dwight, Inorg. Chem., 19, 2500 (1980). 3. J. T . Mague, Inorg. Chem., 8, 1975 (1969). 4. S. J. Young, B. Kellenberger, J. H. Reibenspies, S. E. Himmel, M. Man-ning, O. P. Anderson and J. K. Stille, J. Am. Chem. Soc, 110, 5744 (1988). 5. N. A. Al-Salem, H. D. Empsall, R. Markham, B. L. Shaw and B. Weeks, J. Chem. Soc, Dalton Trans., 1972 (1979). 6. B. L. Shaw, in Catalytic aspects of metal phosphine complexes, Edited by C. Alyea and D. W. Meek, Advances in Chemistry Series 196, Am. Chem. Soc, Washington, D. C. (1982), Ch.6. 7. See, for example: C. Bianchini, C. Meali, A. Meli and M. Sabat, Inorg. Chem., 25, 4617 (1986). 8. M. P. Brown, R. J. Puddephatt, M. Rashidi and K. R. J. Seddon, Inorg. Chim. Acta, 23, L27 (1977). 9. M. P. Brown, J. R. Fisher, S. J. Franklin, R. J. Puddephatt and M. A. Thomson, in Catalytic aspects of metal phosphine complexes, Edited by C. Alyea and W. Meek, Advances in Chemistry Series 196, Am. Chem. Soc, Washington, D. C. (1982), Ch.13. 83 C h a p t e r 5 S U L F U R T R A N S F E R R E A C T I O N S 5.1 I n t r o d u c t i o n The key factor in transforming the stoichiometric reaction 3.1 into some useful type of catalytic process lies in the successful regeneration of 1, by the re-moval of the bound //2-sulnde ligand. In principle, numerous strategies could be applied (scheme 5.1) for the sulfur removal, because // 2-sulfur exhibits both nucleophilic and electrophilic nature in transition metal complexes. 1 - 5 P d 2 + H 2 S - P d 2S + H 2 (3.1) I 2 P d 2 + S 1 P d 2 + S 0 2 1 P d 2 + E S / N S 1 P d 2 = P d 2 X 2 ( d p m ) 2 ) I , X = Cl(a), Br(b), 1(c); P d 2S = Pd 2X 2(dpm ) 2 ( / i-S), 2a - 2c; P d 2 S 0 2 = Pd 2X 2(dpm) 2(//-S0 2); [O] = oxidizing agent such as m-Cl-C6H4C(0)02H; N = nucleophilic S acceptor and E = electrophilic S acceptor. Scheme 5.1 84 Successful regeneration of 1. could lead to a catalytic process that would desul-furize H2S and generate dihydrogen gas. The interaction between S 0 2 and 1 leads to the formation of the known A-frame complex Pd 2 X2(dpm)2(/i-S02), which loses SO2 reversibly 6 and thus has a much weaker Pd-S bond than that of 2; oxidation of 2 thus presented a plausible pathway to regenerate 1, and this was accomplished using peracids, 7 but the S 0 2 co-product in the net catalytic reaction (equation 5.1) makes the process relatively unattractive. H2S + 2RC(0)02H—>H2 + S02 + 2RC02H (5.1) Photolysis of 2 has been considered 8 to lead to the possible regeneration of 1 along with elemental sulfur, which would precipitate out in solvents like dibutylphthalate of low sulfur solubility. Finally fi2-S could perhaps be ab-stracted by transferring it to a sulfur-accepting moiety. Both electrophilic ( H + , R + ) and nudeophilic (PR 3, P A r 3 , CN") reagents offer possibilities. Of the above methods outlined for the sulfur removal, the oxidation with peracids is the only successful route utilized so far for the regeneration of 1.. The oxidation of the /^-sulfide proceeded through the formation of an fi-SO isolable intermediate, which was subsequently oxidized to the fi-S02 complex, which i n turn lost SO2 spontaneously at room temperature to yield 1 (Fig 5.1). Some attempts to dislodge the /i 2-sulfide by 'non-oxidative' routes (i.e. to sulfur oxides) are discussed in the following sections. 85 Pd^dpm). CH,CI, Pd2Xj(dpm)20x-S) +H, so, 30X H,0„ 25'C CH,Cli/MeOH PdjX^dpm^Oi-SO,) PdjXj(dpm)jOt-SO) m-CI-C,H4C(0)0,H, -60*C Figure 5.1: The schematic representation of the regeneration of 1 from 2. by the oxidation of the -^sulfide to S02-86 5.2 Sulfur abstraction reactions 5.2.1 Sulfur abstraction by phosphines Co-ordinated sulfur atoms are effectively removed as SPPh3, by reacting with PPh 3. 9 , 1 0. Attempts to regenerate l a by reacting 2_a_ with excess PPh3 were unsuccessful; a slow reaction in CH2C12 at 40°C destroyed much of the 2a but l a was never recovered in more than 20% yield.7 In the present work both 2b and 2_c showed inertness toward /i2-sulfide abstraction by PPh3, and indeed could be recovered back unchanged in good yields (>90%). In an attempt to abstract the /x2-sulfide ligand, the bisphosphines dpm and dpe and the monophosphine PPh2Me were also tried. The reaction between 2b or 2c and dpm in CH2C12 at 2 0 ° C generated Jjb in ~ 7 5 % yield or lc quantitatively after ~ 6 h stirring. Interestingly, in each case (eq 5.2), dpm monosulfide (nmr : J H, 8 — 3.32 ppm, dd, Jp_# = 1 Hz, Jp (S)-H = 12-8 Hz ; and 31P{]#} 8 = - 2 8 ppm, d, ; 40 ppm, d, J P _ P = 76 Hz) was the only phosphorus-sulfur compound formed and the nmr data are in excellent agreement with literature data.11 P<z2JY2(<zprn)2(^  — S) -f dpm • Pcf2X2(^pm)2 -f dpmS (5.2) 2 1 The reactions between 2b or 2c with dpe or PPh2Me were, like the PPh3 reactions, unsuccessful for the regeneration of l b or l c , respectively. In the dpe case, l b or l c was generated in ~50% yield along with numerous(from J H & 31 P{VH}), as yet unidentified, Pd complexes. The PPh2Me reaction did not regenerate any ljb or l c , but much of the respective 2b or 2c was destroyed, presumably by the break down of the Pd2 and ^ -sulfide dimers. 87 T h e i nab i l i t y of P P h 3 to mob i l i ze sul fur f rom 2 b / 2 c ind ica tes that ei ther 2 b / 2 c is t h e r m o d y n a m i c a l l y stable w i t h respect to the fo rmat ion of l b / l c and S P P h 3 , or else the react ion is t h e r m o d y n a m i c a l l y favourable bu t k ine t i ca l l y dif-ficult. T h e faci le fo rmat ion of d p m monosu l f ide i n react ion (5.2) ind ica tes that the P P h 3 react ion does not take p lace because of k ine t ic reasons. T h e specif ic t ransfer of su l fur to one phosphorus a t o m of the d p m ( d p m d isu l f ide fo rma t ion was faci le i n a react ion between d p m S a n d e lementa l sulfur(S8), and in a d p m react ion w i t h su l fur (1:|), a long w i t h some d p m S ) suggests tha t the react ion may invo lve a dang l i ng (monodenta te) d p m in te rmed ia te , w i t h perhaps the b i nd ing of d p m at one end be ing necessary d u r i n g the sul fur t ransfer. Fu r the r , such a dang l i ng d p m l igand wou ld severely weaken the P d - S b o n d because the / / -su l f ide wou ld now be t rans to two l igands ( B r and the P a t o m of the added d p m ) w i t h different t r ans - i n f l uences . 1 2 T h e react ion m a y proceed th rough the fo l lowing p laus ib le m e c h a n i s m , as shown i n F i g u r e 5.2; the b rom ine at the P d meta l be ing a t tacked cou ld a l ternat ive ly d issoc ia te , and t h e n finally reas-socdate, as b rom ide . Fur ther , p re l im ina ry k ine t i c i n v e s t i g a t i o n s 1 3 carr ied out subsequent to th is work , ind ica te that the react ion between 2 c a n d d p m is wel l behaved , as ev idenced f r om U V - v i s i b l e spect roscopy and shows a first order dependence on bo th [Pd 2 ] a n d [dpm] i n accordance w i t h the proposed m e c h a n i s m i f t he first step is r a te -de te rm in ing . 5.2.2 Sulfur abstraction by organics T h e successful abs t rac t ion of / ^ - s u l f i d e by d p m s ignal led searches for o ther ways of abs t rac t ing the b o u n d su l fur f r om 2, because the abs t rac t ion of fi2-sul f ide b y d p m is un l i ke ly to b e un ique . Benzo th iophene and d ibenzo th iophene der ivat ives are qu i te inert t oward hydrodesu l fu r i za t ion (i.e. remova l of sul fur 88 B r — P d P d — B r . + P P F i g u r e 5.2: T h e proposed mechan is t i c p a t h w a y for the abs t rac t ion of su l fur by b i s (d ipheny lphosph ino )me thane from 2 . 89 as H2S), indicating a stable carbon-sulfur framework. Thus, sulfur abstraction from 2 was considered likely to occur if it led to the formation of benzo-thiophene or dibenzothiophene derivatives,and so biphenyl and styrene were judged to be likely molecules for abstraction of the bound /z-sulfide from 2. The reaction between 2_c and styrene was unsuccessful, however, for the sulfur abstraction. With diphenyl a small amount (~ 15%) of l c was regener-ated from 2c at room temperature in 2 h. A much higher conversion (~45%) was effected in refiuxing toluene in 2 h. From the 3 1 P{lH} spectrum, the reaction appeared to be clean, showing the presence of only l_c and 2_c. The organic sulfur product formed remains to be identified. 5.3 The desulfurization of H 2S As discussed in section 5.1, the coupling of reaction 3.1 and reaction 5.2 should yield a process which catalytically desulfurizes H2S and result in the generation of H 2 and dpmS, eq 5.3. H2S + dpm • H2 -+• dpmS (5.3) Such proved to be the case. In a blank reaction containing no Pd complexes, dpm was recovered quantitatively, unchanged after reacting with H2S(excess) in CH2C12 at 25°C even after 2 days. However, in the presence of a small amount of 1 (b or c) (Pd2 : dpm = 1 : 200, CH2C12, 25°C, 8h) dpm was cat-alytically converted to dpmS, indicating the generation of an efficient, catalytic system for desulfurization of H2S (Figs. 5.3 and 5.4). Of interest, on closer examination, the desulfurization of H2S perhaps proceeds through a route dif-ferent from the one predicted by the coupling of the reactions 3.1 and 5.2. Catalysis via these reactions should generate (finally or during the catalysis) 90 "1 (A) dpmS l. .. 11 . . 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 40 3d 20 10 0 -]0 -20 -30 -36 ppm (B) II i I I I I I | I I I I | I I I I | I I I I | I I I I J I I I I | I I I I J I I I I | 3 5 3 0 2 5 2 0 1 5 1 0 5 0 - 5 P P ™ Figure 5.3: The nP{*H} nmr spectra in CDC1 3 solution at 20°C of (A) the reaction product of l c and H 2 S in the presence of 20 fold excess dpm after 2 h; (B) the expanded region of (A) between -5 and 36 ppm. 91 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 30 25 20 15 10 5 0 -5 P P m Figure 5.4: The 3 1 P{lH) nmr spectra in CDC13 solution at 20°C of (A) the reaction product of lfc and H 2 S in the presence of 20-fold excess dpm after 2 h, the dpm signal had completely disappeared after 8 h. (B) the expanded region of (A) 92 either l ( b or c) in the system if dpm was in excess, or 2 ( b or c) if H2S was in excess. However, in desulfurization reactions catalyzed by 1 species neither 1 nor 2 was detected. Instead, a few unidentified palladium products, presumably dpmS substituted derivatives of 1 and/or 2, were detected in the 31P{17J'} nmr spectra (Figs. 5.3B and 5.4B). . 93 5.4 References 1. C. J. Casewit, R. C. Haltiwagner, J. Noordik and M. Rakowski DuBois, Organometallics, 4, 119 (1985). 2. (a) R. G. W. Gingerich and R. J. Angelici, J. Am. Chem. Soc, 101, 5804 (1979). (b)R. J. Angelici, Acc. Chem. Res., 21, 387 (1988). 3. C. M. Bolinger, T. B. Rauchfuss and S. Wilson, J. Am. Chem. Soc, 104, 7313 (1982). 4. M. G. B. Drew, P. C. H. Mitchell and C. F. Pygall, J. Chem. Soc, Dalton Trans., 1213 (1979). 5. C. P. Kubiak and R. Eisenberg, Inorg. Chem., 19, 2726 (1980). 6. A. L. Balch, L. S. Benner and M. M. Olmstead, Inorg. Chem., 18, 2996 (1979). 7. G. Besenyei, C. L. Lee, J. Gulinski, S. J. Rettig, B. R. James, D. A. Nelson and M. A. Lilga, Inorg. Chem., 26, 3622 (1985). 8. B. R. James, Personal Communication. 9. K. K. Pandey, Spectrvchim. Acta, Part A, 39, 925 (1983). 10. (a) G. Schmid and G. Ritter, Angew. Chem. Int. Ed. Engl, 14, 645 (1983). (b) G. Schmid and G.Ritter, Chem. Ber., 103, 3008 (1975). 11. S. 0. Grim and E. D. Walton, Inorg. Chem., 19, 1982 (1980). 94 12. T. S. A . Hor and A. L. C. Tan, Inorg. Chim. Acta , 142, 173 (1988). 13. D. Sallin, Personal Communucation. 95 Chapter 6 G E N E R A L C O N C L U S I O N S 6.1 Conclusions The analysis of kinetic data for the reaction between Pd 2 Br 2 (dpm) 2 , l b and H 2 S is consistent with, and can be accounted for, by either'of mechanisms (a) (Eqs. 3.3 and 3.4) and (b) (Eqs. 3.5 - 3.7). The first order dependence on both P d 2 complex and H 2 S does not distinguish between the two mechanisms. The detection of a transient hydride intermediate at low temperatures shows that the reaction proceeds through the oxidative addition of H 2 S across the metal-metal bond. Further, the rapid decomposition of the hydride intermediate at ambient conditions indicates that the formation of the hydride (or the H 2 S adduct in Eq 3.5) is the rate determining step. Obviously, more extensive kinetic studies on related systems, especially for complexes of the same metal, are necessary before any conclusion regarding the generality of any mechanism can be reached for H 2 S reactivity toward dimers. Low temperature timr experiments show that the reaction between HBr and lb proceeds stepwise. The oxidative addition of the first mole of HBr results in the formation of a hydride intermediate; this then reacts with the second mole of HBr to form dihydrogen and a tetrabromo complex, Pd 2 Br 4 (dpm) 2 , which subsequently fragments slowly to the Pd(II) monomer, PdBr 2(dpm). The bound ^-sulfide ligand was effectively abstracted from the A-frame 96 complex, Pd2X2(dpm)2(/i-S), by dpm to generate the Pd(I) dimer, Pd2X2(dpm)2, along with dpm monosulfide as the only sulfur-containing co-product. It was also demonstrated that the sulfur could be transferred to an organic moiety such as biphenyl. Apart from these systems with stoichiometric sulfur trans-fer, an effective catalytic system was unearthed. Catalytic desulfurization of H2S was effected in the presence of Pd2X2(dpm)2 and excess dpm along with the formation of dpmS as the only sulfur-containing product. 6.2 Recommendations for future work Further studies should be pursued to distinguish between the proposed mech-anisms (a) and (b). This could be accomplished by utilizing Pd(I) dpm com-plexes with different auxiliary ligands (i.e. varying X in Pd2X2(dpm)2) as well as other dinuclear complexes of transition metals. So far, there are no kinetic models available for the reactions between H2S and dinuclear metal complexes that generate dihydrogen gas. There is little doubt that future investigations in these areas would lead to the improvement of the present system as well as to new systems for the generation of dihydrogen. Understanding the kinetic and the mechanistic aspects of the the reaction that leads to the desulfuriza-tion of H2S by Pd2X2(dpm)2 could lead to the utilization of an economically more viable sulfur acceptor moiety instead of the bisphosphine dpm. Also the synthetic potential of this reaction to transfer sulfur to specific sites should be explored in greater detail. 97 

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