Mechanism of a Directly Observed β-Hydride Elimination Process of Iridium Alkoxo Complexes

摘要

The octahedral alkoxo complexes mer-cis-HIr(OR)Cl(PR'_3)_3 (R = Me, Et, i-Pr; R' = Me, Et; H trans to Cl) decompose at room temperature in an alcohol/benzene solution, forming the dihydrido products mer-cis- H_2IrCl-(PR'_3)_3 and the corresponding aldehyde or ketone. The reaction rate is of first order in the iridium complex and of 1.33 order in the alcohol, which serves as a catalyst. The rate depends on the nature of the phosphine (PEt_3 > PMe_3), on the alkyl substituent of the alkoxide (Me > Et > > i-Pr), and on the medium (benzene > N-methylpyrrolidone) but is not effected by excess phosphine. The activation parameters obtained for the decomposition of mer-cis-HIr-(OCH_3)Cl(PMe_3)_3 are ΔH_(obs) = 24.1 ± 1.8 kcal mol~(-1), ΔS_(obs) = 0.6 ± 5.9 eu, and ΔG_(obs) (298 K) = 23.9 ± 3.6 kcal mol~(-1). The kinetic isotope effect (combined primary and secondary effects) for the decomposition of mer-cis-DIr(OCD_3)Cl(PMe_3)_3 at 22℃ is k_H/k_D = 2.45 ± 0.10, and the secondary kinetic isotope effect for the decomposition of DIr(OCH_3)Cl(PMe_3)_3 at 22℃ is 1.10 ± 0.06. Both DIr(OCH_3)Cl(PMe_3)_3 and HIr(OCD_3)Cl-(PMe_3)_3 produce only the two mer-cis isomers of HDIrCl(PMe_3)_3, but in different ratios. The following steps are involved in the β-hydride elimination process: (a) pre-equilibrium generation of a free coordination site by chloride dissociation, which is induced by hydrogen bonding of a methanol molecule to the chloride; (b) irreversible rate-determining β-C-H cleavage through the sterically favored transition state; (c) facile, irreversible dissociation of the aldehyde; (d) ligand rearrangement; and (e) irreversible reassociation of the chloride. Selective deuterium labeling enables the elucidation of a competing minor mechanism through the electronically favored transition state, operative for the trimethylphosphine complex only.