Directly observed β-H elimination of unsaturated PCP-based rhodium(III)-alkyl complexes

Milko van der Boom, Catherine L. Higgitt, David Milstein

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Abstract

The unsaturated PCP-type complexes Rh(L){2,6-(CH2PtBu2)2C 6H3}X (L = Et, nPr; X = Cl, I) complexes convert upon heating to the corresponding Rh(III)-hydride complexes Rh(H)-{2,6-(CH2PtBu2)2C 6H3}X (X = Cl, I) and ethylene or propylene, products indicative of a β-H elimination process. The iPr analogue is observed upon reaction of Rh(η1-N2){2,6-(CH2Pt-Bu 2)2C6H3} with iPrI at -10°C and decomposes readily at room temperature to give Rh-(H){2,6-(CH2PtBu2)2C 6H3}I and propylene. Analogous alkyl complexes - lacking β-hydrogens - are stable under the applied reaction conditions. The mechanism of this process has been studied by NMR, using 13C and deuterium labeling of the alkyl ligand (L = Et-d5, 13CH2-CH3). 13C labeling shows that the β-H elimination is irreversible. A deuterium isotope effect of kEt/kEt-d5 = 1.4 and a rate order of Et <nPr ≪ iPr were observed. The overall process follows first-order kinetics in the Rh(III)-alkyl complexes. The activation parameters for the thermolysis of Rh(Et)(2,6-(CH2PtBu2)2C 6H3)I in toluene were determined: ΔH‡ = 21.2 kcal/mol, ΔS‡ = -21.1 eu, and ΔG‡298K = 27.5 kcal/mol.

Original languageEnglish
Pages (from-to)2413-2419
Number of pages7
JournalOrganometallics
Volume18
Issue number13
Publication statusPublished - Jun 21 1999

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ASJC Scopus subject areas

  • Inorganic Chemistry
  • Organic Chemistry

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