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 kH/kD=3.2±0.2, the observed activation parameters are ΔH‡obs=8.3±1.0 kcal mol−1; ΔS‡obs=−34±3.5 e.u. and ΔG‡obs (298 K)=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 β-hydride elimination process does not follow the usual pathway. A mechanism, in which following methoxide dissociation, CH cleavage of free methanol takes place, is suggested.