PCP Pincer Carbene Nickel(II) Chloride, Hydride, and Thiolate Complexes: Hydrosilylation of Aldehyde, Ketone, and Nitroarene by the Thiolate Complex

Abstract

The one-pot reaction between ethylenediamine, paraformaldehyde, and Ph2PH or tBu2PH gave a new diphosphine compound 1,3-bis((diphenylphosphaneyl)methyl)imidazolidine L1 or 1,3-bis((di-tert-butylphosphaneyl)methyl)imidazolidine L2, respectively, in excellent yields. Metalation with [NiCl2(DME)] in the presence of KPF6 afforded pincer carbene nickel chloride complexes [(PhPCP)NiCl]PF6, 1, and [(tBuPCP)NiCl]PF6, 2, by the simultaneous double C–H bond activations of the methylene protons. The reaction of 1 and 2 with PhSNa gave both neutral five- and ionic four-coordinate thiolate complexes [(PhPCP)Ni(SPh)2], 3, and [(PhPCP)NiSPh]PF6, 4, and [(tBuPCP)NiSPh]PF6, 5, depending upon the reaction stoichiometry. Interestingly, the reaction of 2 with an excess of NaBH4 gave the new hydride complex [(tBuPCP)NiH]PF6, 6. Their structures were confirmed by the X-ray diffraction method. Of these, only the thiolate complexes 3 and 4 are efficient catalysts for the hydrosilylation of aldehydes, ketones, and nitroarenes to give primary, secondary alcohols, and aromatic amines using PhSiH3 after hydrolysis. Aldehydes containing different substituents and conjugated double bonds, aliphatic, and heterocyclic aldehydes are converted to alcohols in excellent isolated yields with 0.5 mol % complex 3 in 2 h under neat conditions at room temperature. The hydrosilylation of ketones requires heating conditions in toluene to give products in moderate yields. Eight nitroarenes were converted to their aromatic amines in very good yields including chemoselective reductions. The poor performance of the nickel hydride suggests that the bound thiolate group in 3 or 4 plays a key role in mediating these reactions possibly via metal–ligand cooperation. Preliminary mechanistic studies indicated the formation of a nickel silyl complex detected by the 19Si NMR method with H2 evolution among others. For the nitroarene reduction, the detection of intermediates followed by the catalytic reduction of N-phenylhydroxylamine to give aniline indicated the direct method mechanism.