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KIE in Metal Hydrides Polyhedron Vol. 8, No. 4, pp. 383405, 1989 0277-5387189 $3.Oi+.W Printed in Great Britain 0 1989 Pergamoo Pres.3 plc POLYHEDRON REPORT NUMBER 26 KINETIC DEUTERIUM ISOTOPE EFFECTS IN TRANSITION METAL HYDRIDE CLUSTERS EDWARD ROSENBERG Department of Chemistry, California State University, Northridge, CA 91330, U.S.A. CONTENTS 1. INTRODUCTION . 383 2. THE SMALL PRIMARY ISOTOPE EFFECT (kH/kD = 1.4-2.0) ........ 386 2.1 Direct exchange of two p-hydrides ................... 386 2.2 Axial-radial exchange of carbonyl groups ................ 388 2.3 Face migrations of hydrocarbon ligands ................. 390 2.4 Chemical reactions involving hydride migrations .............. 393 3. THE LARGE PRIMARY ISOTOPE EFFECT (kH/kD = 7-50) ......... 398 3.1 Ligand to metal hydrogen transfers in protonations of clusters ......... 398 3.2 Metal to ligand hydrogen transfers ................... 40 1 4. CONCLUSIONS AND FUTURE WORK . 403 ACKNOWLEDGEMENTS . _ 404 REFERENCES . 404 APPENDIX . 405 1. INTRODUCTION Primary deuterium kinetic isotope effects have been used by organic chemists for many years to elucidate the rate-determining steps and the nature of the transition states in a wide range of organic reactions. However, the mechanistic interpretation of the magnitude of observed kH/kD ratios has always been somewhat problematic. The “maximum” primary isotope effect can be estimated from the observed differences in zero point energies (i.e. differences in the X-D versus X-H stretching frequencies) using the Bigeleisen equation [eq. (l)] (1) for a reaction which is thought to have a linear symmetrical transition state and in which substitution of deuterium for hydrogen has no effect on the energy of the transition state. These constraints are rarely realized. Non-linear transition states are thought to attenuate the size of the isotope effect. Reactions with late transition states (i.e. the rate of bond breaking exceeds the rate of bond making for the hydrogen transfer) would be expected to lower the energy of the transition state and also attenuate the size of the isotope effect. Recently it has been proposed that reactions with linear transition states should show temperature dependent kH/kD ratios while those with non-linear transition states should show temperature independent kH/kD ratios.’ 383 384 E. ROSENBERG In addition, there is a separate class of isotope effects in which the observed kH/kD ratio is larger than that predicted from eq. (1). ‘9’ These large kH/kD values are thought to be associated with ‘barrier tunnelling’. Reactions in which the ground state geometry of the reactant resembles that of the product very closely would be expected to have a very narrow barrier. If the barrier width approaches the wavelength of the hydrogen atom, proton or hydride involved in the reaction, the particle can tunnel through the barrier rather than go over it. Substitution of deuterium for hydrogen would effectively turn off the tunnelling component of the reaction leading to unexpectedly high kH/kD ratios. The observation of a high kH/kD ratio is not, however, a sufficient criterion for ascribing barrier tunnelling to a reaction. The observed difference in activation energy on going from the protium to the deuterium system must be greater than that calculated from differences in zero point energy (i.e. AE,g >>AE&. ’ In order to meet this criterion, details of the differences in zero point energy must be available and it must be possible to obtain kH/kD values over a range of temperatures. These requirements are not often met by many kinetic studies and thus there are few well documented examples of barrier tunnelling for different reaction types. The best documented cases of barrier tunnelling exist for reactions involving proton transfer. l-3 In the organometallic chemistry of mononuclear and dinuclear species, kinetic deuterium isotope effects have been used extensively to elucidate the mechanisms of H-H and C-H activation processes and in processes involving M-H bond cleavage. No attempt will be made here to be comprehensive, but rather to present examples which illustrate the range of systems studied with regard to reaction type, metal centre and the magnitude of observed isotope effects. Perhaps the most straightforward example of a kinetic deuterium isotope effect in transition metal chemistry comes from the work of Norton et al. who studied the proton self-exchange between cyclopentadienyl metallates [eq. (2)].4 (q-C,H,)M(CO),H+(?-C,H,)M’(CO); L-- (q-C,H,)M(CO); +(rl-C,H,)M’(CO),H (2) M=M’=Cr,Mo,W. The observed kH/kD ratios of 3.7 for eq. (2) are in excellent agreement with the value of 3.39 calculated from eq. (1).4 These results clearly suggest a linear symmetrical transition state taking place directly between metal atoms and demonstrate that there is nothing intrinsically anomalous about the magnitude of kinetic deuterium isotope effects involving the M-H bond. The largest kinetic deuterium isotope effects involving transition metal centres appear to be associated with C-H activation processes at electrophilic metal centres. For example, Bercaw and co-workers have noted a kH/kD = 9.7 at 34°C for the formation of an azametallocyclopropane [eq. (3)l. 5 Cp*Ta(N(CH&)(CH&- Cp*Ta{N(CH3)CH&CH3),+CH4 (3) Cp* = r&(CH&. Given that the kH/kD calculated from eq. (1) for cleavage of an aliphatic C-H bond is 7 at room temperature, this value implies a significant tunnelling component for this intramolecular hydrogen transfer. Similarly kH/kD values of 4.8 (114°C) and 5.2 (180°C) have been reported for the elim- ination of toluene from a zirconium centre [eqs (4) and (5)16 Zr(OAr’)2(CHzC6H5)2-~r(OAr’)(OC6H3Bu’C(CH3)2C H2)(CH2C6HS)+CH3C6H5 (4) A Ta(OAr’)2(CH3)3 - Ta(OAr’)(OC6H3Bu’C(CH3)&H~)(CH3)2+CH4 (5) A (OAr’) = 2,6-di-t-butylphenoxide. The reverse reaction (i.e. cleavage of a metallocycle by a hydrocarbon at an electrophilic metal centre) also shows a relatively large isotope effect. Thus cleavage of a metallocyclobutane at a thorium(IV) centre by methane [eq. (6)] gives kH/kD = 6 k 2.’ FH’\ CH4 PH3 (Cp’),Th,CH2,W%h x (Cp')zTh (6) \ CHIC(CHB)I Isotope effects in transition metal hydride clusters 385 Activation of the aromatic C-H bond in benzene by an electrophilic metal centre gives a kH/kD = 5.5 [eq. (7)]* (Cp*)~LuCH~+C&r-+ (Cp*),LuC,jH, +CH+ (7) This isotope effect is operative for the bimolecular term of a two-term rate law (i.e. rate = [k, +k&H,)][complex]). The first-order term shows no isotope effect with C6Db and is thought to arise from the reversible formation of a ring metallated intermediate.* Much smaller isotope effects are observed for C-H activation at low valent late transition metal centres. Thus the photochemically generated intermediate (Cp*)IrP(CH3),, shows a kH/kD = 1.38 for the activation of cyclohexane [eq. (S)].’ (Cp*)IrP(CH&Hz-$+ (Cp*)IrP(CHs)3 C6H1Zb (Cp*)Ir(PCH,),(C,H, i)H. (8) Activation of the C-H bond in b’enzene at an electron rich osmium centre gave a kH/kD = 2.2 [es. PW” L CHs C6H6 i/L L7ts /“,A2 ,-CH, - -C,Hs + C(CH3h ‘,/is (9) L CHs L L = P(CH& The large differences between kH/kD ratios at electrophilic metal centres and at electron rich metal centres delineated here, can be rationalized on the basis of early (for electrophilic metal centres) versus late (for electron rich metal centres) transition states for cleavage of the C-H bond. On the other hand, all the examples presented here for electrophilic metal centres involve direct transfer of hydrogen between carbon atoms, a reaction which could involve a large tunnelling component. The electron rich metal centres generally proceed via hydride intermediates which necessarily involve significant changes in geometry at the metal centre and might be expected to have little or no tunnelling component. These ideas need to be investigated further by carrying out variable tem- perature studies on the kH/kD ratios. Reductive eliminations of C-H bonds from late electron rich transition metal centres also give relatively small kH/kD values. Thus, reductive elimination from a dihydride-alkene rhodium complex gives a kH/kD = 1.15 for the formation of a rhodium alkyl in the rate-determining step for olefin reductions [eq. (lo)]. ’ ’ ii” C6H1,, i/H i D c”7p-L -L_ (10) ;Ty ’ +S -0 - 3-’ L L S L = PRB S = Solvent Also, the binuclear reductive elimination mechanism elucidated by Norton et al. for osmium(I1) hydrido alkyls shows a kH/kD = 1.5 [eq. (1 l)].” ~OS(CO)~(H)CH~--+ CH4+H(CO),0sOs(CO)4(CH3). (11) A It can be concluded from these examples that both C-H oxidative addition to and reductive elimination from mono- and binuclear electron rich metal centres will involve relatively small deuterium kinetic isotope effects (i.e. kH/kD = l-2). The oxidative addition of H2 to electron rich metal centres also shows relatively small isotope effects (kH/kD = l-2). Oxidative addition of H,(DJ to the photochemically generated intermediate Rh(Cl)(P(C,H,),}, shows a kH/kD = 1.4 [eq. (12)]. l3 386 E. ROSENBERG RhCl(CO)~P(C6H3~~z~RhCl{P(C,HS)s~2+H* - H8hC@‘KciH,),) 2. (12) This value is in excellent agreement with the calculated value of 1.48 based on a theoretical model of the transition metaldihydrogen bond involving donation from the H2 cr-bond to empty orbitals on the metal (i.e. side-on coordination) and back donation from filled metal orbitals to the Hz rr*- orbital.
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