Hydride elimination from an iridium(III) alkoxide complex:: a case in which a vacant cis coordination site is not required

Decomposition of trans-HIr(OCH3)(C6H5)(PMe3)(3) (2) formed by oxidative addition of methanol to Ir(C6H5)(PMe3)(3) (1) was studied in detail. Thermolysis of this complex yields trans-H2Ir(C6H5)(PMe3)(3) (2) and formaldehyde. Complex 2 is less stable than its two dihydrido isomers, showing that it is the kinetic product of this reaction. The elimination process follows first order kinetics and exhibits a kinetic isotope effect of k(H)/k(D) = 3.2 +/- 0.2, the observed activation parameters are Delta H-obs(double dagger) = 8.3 +/- 1.0 kcal mol(-1); Delta S-obs(double dagger) = -34 +/- 3.5 e.u. and Delta G(obs(298 K))(double dagger) = 18.4 +/- 2.0 kcal mol(-1). Catalysis by methanol was observed. The process does not involve a vacant coordination site cis to the coordinated methoxide, as shown by labeling experiments and by the lack of exchange with P(CD3)(3). Thus, in this case the beta-hydride elimination process noes not follow the usual pathway. A mechanism, in which following methoxide dissociation, C-H cleavage of free methanol takes place, is suggested. (C) 2000 Elsevier Science S.A. All rights reserved.