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

Ofer Blum, David Milstein

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65 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)479-484
Number of pages6
JournalJournal of Organometallic Chemistry
Volume593-594
Publication statusPublished - Jan 15 2000

Fingerprint

Iridium
alkoxides
iridium
Hydrides
hydrides
Methanol
elimination
methyl alcohol
Kinetics
kinetics
Thermolysis
formaldehyde
Catalysis
Formaldehyde
Isotopes
Isomers
isotope effect
Labeling
marking
catalysis

Keywords

  • Alkoxide
  • Elimination
  • Hydride
  • Iridium
  • Mechanism
  • O-H activation

ASJC Scopus subject areas

  • Biochemistry
  • Inorganic Chemistry
  • Organic Chemistry
  • Physical and Theoretical Chemistry
  • Materials Chemistry

Cite this

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title = "Hydride elimination from an iridium(III) alkoxide complex: A case in which a vacant cis coordination site is not required",
abstract = "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.",
keywords = "Alkoxide, Elimination, Hydride, Iridium, Mechanism, O-H activation",
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T1 - Hydride elimination from an iridium(III) alkoxide complex

T2 - A case in which a vacant cis coordination site is not required

AU - Blum, Ofer

AU - Milstein, David

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Y1 - 2000/1/15

N2 - 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.

AB - 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.

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