Synthesis of inexpensive chiral half-sandwich nickel N-heterocyclic carbene complexes: X-ray diffraction study of the D-menthyl-functionalized complex [Ni(iPr2Ph-NHC-CH2OMent)ClCp]

• Chiral Cp-Ni(NHC) complexes bearing menthoxymethyl arms are readily accessible from inexpensive D-menthol.
• One complex has been structurally characterized.
• The complexes are effective catalysts for the hydrosilylation of ketones.
• No chiral induction was observed.

The imidazolium salts, 1-(2,4,6-trimethylphenyl)-3-[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride (Mes-NHC-CH2OMent·HCl) (1a) and 1-(2,6-diisopropylphenyl)-3-[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride (iPr2Ph-NHC-CH2OMent·HCl) (1b), are readily accessible from inexpensive (1R,2S,5R)-(−)-menthol. They react with nickelocene to give two new chiral nickel-N-heterocyclic carbene (NHC) complexes, [Ni(Mes-NHC-CH2OMent)ClCp] (2a) and [Ni(iPr2Ph-NHC-CH2OMent)ClCp] (2b), in good yields. The new complexes were fully characterized by standard spectroscopic techniques and by a single crystal X-ray diffraction study on complex 2b. Complex 2b crystallizes in the chiral spacegroup P1, with two independent molecules in the unit cell, which are slightly different from each other. Preliminary studies show that these complexes are effective catalysts for the hydrosilylation of ketones. However no chiral induction was observed.

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