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

Ofer Blum, David Milstein

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Abstract

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-H2IrCl-(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 (PEt3 > PMe3), 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-(OCH3)Cl(PMe3)3are ΔHobs = 24.1 ± 1.8 kcal mol-1, ΔSobs = 0.6 ± 5.9 eu, and ΔGobs (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(OCD3)Cl(PMe3)3 at 22 °C is Kh/Kd= 2.45± 0.10, and the secondary kinetic isotope effect for the decomposition of DIr(OCH3)Cl(PMe3)3at 22 °C is 1.10 ± 0.06. Both DIr(OCH3)Cl(PMe3)3and HIr(OCD3)Cl(PMe3)3 produce only the two mer-cis isomers of HDIrCl(PMe3)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.

Original languageEnglish
Pages (from-to)4582-4594
Number of pages13
JournalJournal of the American Chemical Society
Volume117
Issue number16
DOIs
Publication statusPublished - Apr 1995

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ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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