SECONDARY BETA-DEUTERIUM ISOTOPE EFFECTS ON THE RATES AND EQUILIBRIA OF ORGANOMETALLIC OXIDATIVE ADDITION REDUCTIVE ELIMINATION-REACTIONS

We report a study of beta-deuterium isotope effects on the rate and equilibrium of an organometallic transformation: the oxidative addition of silane R3SiH (R = Et) and the corresponding deuteride R3SiD to the iridium center in the heterodinuclear complex Cp2Ta(mu-CH2)2Ir(CO)2 (1) and to its tetradeuterated analogue Cp2Ta(mu-CD2)2Ir(CO)2 (1-d4). The Si-H(D) bond contributes only a small normal isotope effect to the forward rate constant k1 (k1H/k1D = 1.13 +/- 0.06). An inverse effect (k1H/k1D = 0.875 +/- 0.022) is caused by isotopic tetrasubstitution at the bridging methylenes. For the reductive elimination of Et3SiX from Cp2Ta(mu-CX2)2Ir(X)(SiEt3)(CO)2 (X = H, D), the Si-H(D) bond again contributes a small normal isotope effect to the rate constant k-1 (k-1H/k-1D = 1.45 +/- 0.08). However, a normal effect (k-1H/k-1D = 1.25 +/- 0.03) is caused by the secondary isotopic substitution. The combined primary and secondary equilibrium isotope effect on the silane oxidative addition/reductive elimination equilibrium constant K(eq)H/K(eq)D = [k1/k-1]H/[k1/k-1]D is 0.53 +/- 0.04 at 0-degrees-C. A kinetic isotope effect analogous to that observed with 1 is measured for the oxidative addition of methyl iodide to (PPh3)2Ir(CO)(CX3) (X = H, D) (k1H/k1D = 0.922 +/- 0.030 in THF at 0-degrees-C). This suggests that the inverse beta-deuterium isotope effect on oxidative addition is a general phenomenon. Kinetic isotope effects of similar direction and magnitude were also observed for oxidative addition of CH3I (k1H/k1D = 0.752 +/- 0.018 in THF at 0-degrees-C) and Ph3SiH (k1H/k1D = 0.898 +/- 0.077 in THF at 10-degrees-C) to the tantalum/iridium complex 1, indicating that steric effects are not the source of the measured difference in rate behavior. The tendency of deuterium to act as an inductive electron donor relative to hydrogen is suggested to account for its ability to enhance the rate of oxidative addition reactions that convert iridium(I) to iridium(III).