Catalyst activation by loss of cyclopentadienyl ligands in hydrogen transfer catalysis with Cp*IrIII complexes

J. Campos, U Hintermair, T.P. Brewster, M.K. Takase, R.H. Crabtree

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Abstract

The activity of the two related complexes [Cp*Ir(IMe) 2X]BF4 (X = Cl (1), H (2)) in transfer hydrogenation from isopropyl alcohol to acetophenone was investigated. The results suggest that the commonly accepted monohydride mechanism for transfer hydrogenation mediated by cyclopentadienyl iridium species does not apply to chloride 1. We have found evidence that, although the two monodentate NHC ligands are retained in the coordination sphere, the Cp* ligand is completely released under mild conditions in a precatalytic activation step. Synthesis of modified versions of the initial precatalyst 1 with different cyclopentadienyl and NHC ligands demonstrated that increasing the steric pressure around the iridium center facilitates precatalyst activation and thus enhances the catalytic performance. Study of five new iridium(III) complexes bearing mono- or diphosphines helped us monitor Cp* ligand loss under mild conditions. An unusual P-C bond cleavage was also noted in a 1,2-bis(dimethylphosphino)methane (dmpm) ligand. On the basis of these findings, a novel catalyst activation mechanism is proposed for [(η5-C5R5)Ir] transfer hydrogenation based on the lability of the cyclopentadienyl ligand.
Original languageEnglish
Pages (from-to)973-985
Number of pages13
JournalACS Catalysis
Volume4
Issue number3
Early online date5 Feb 2014
DOIs
Publication statusPublished - 7 Mar 2014

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Catalysis
Hydrogen
Chemical activation
Ligands
Iridium
Catalysts
Hydrogenation
Bearings (structural)
2-Propanol
Methane
Chlorides
Alcohols

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Catalyst activation by loss of cyclopentadienyl ligands in hydrogen transfer catalysis with Cp*IrIII complexes. / Campos, J.; Hintermair, U; Brewster, T.P.; Takase, M.K.; Crabtree, R.H.

In: ACS Catalysis, Vol. 4, No. 3, 07.03.2014, p. 973-985.

Research output: Contribution to journalArticle

Campos, J. ; Hintermair, U ; Brewster, T.P. ; Takase, M.K. ; Crabtree, R.H. / Catalyst activation by loss of cyclopentadienyl ligands in hydrogen transfer catalysis with Cp*IrIII complexes. In: ACS Catalysis. 2014 ; Vol. 4, No. 3. pp. 973-985.
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AB - The activity of the two related complexes [Cp*Ir(IMe) 2X]BF4 (X = Cl (1), H (2)) in transfer hydrogenation from isopropyl alcohol to acetophenone was investigated. The results suggest that the commonly accepted monohydride mechanism for transfer hydrogenation mediated by cyclopentadienyl iridium species does not apply to chloride 1. We have found evidence that, although the two monodentate NHC ligands are retained in the coordination sphere, the Cp* ligand is completely released under mild conditions in a precatalytic activation step. Synthesis of modified versions of the initial precatalyst 1 with different cyclopentadienyl and NHC ligands demonstrated that increasing the steric pressure around the iridium center facilitates precatalyst activation and thus enhances the catalytic performance. Study of five new iridium(III) complexes bearing mono- or diphosphines helped us monitor Cp* ligand loss under mild conditions. An unusual P-C bond cleavage was also noted in a 1,2-bis(dimethylphosphino)methane (dmpm) ligand. On the basis of these findings, a novel catalyst activation mechanism is proposed for [(η5-C5R5)Ir] transfer hydrogenation based on the lability of the cyclopentadienyl ligand.

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