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 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.