Herein we report efficient catalytic hydrosilylations of nitroarenes to form the corresponding aromatic amines using a well-defined manganese(II)−NNO pincer complex with a low catalyst loading (1 mol %) under solvent-free conditions. This base-metal-catalyzed hydrosilylation is an easy and sustainable alternative to classical hydrogenation. A large variety of nitroarenes bearing various functionalities were selectively transformed into the corresponding aromatic amines in good yields. The potential utility of the present catalytic protocol was demonstrated by the preparation of commercial drug molecules.

The one-pot reaction between ethylenediamine, paraformaldehyde, and Ph 2 PH or t Bu 2 PH gave a new diphosphine compound 1,3bis((diphenylphosphaneyl)methyl)imidazolidine L1 or 1,3-bis((di-tertbutylphosphaneyl)methyl)imidazolidine L2, respectively, in excellent yields. Metalation with [NiCl 2 (DME)] in the presence of KPF 6 afforded pincer carbene nickel chloride complexes [( Ph PCP)NiCl]PF 6 , 1, and [( tBu PCP)NiCl]-PF 6 , 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 [( Ph PCP)Ni(SPh) 2 ], 3, and [( Ph PCP)NiSPh]PF 6 , 4, and [( tBu PCP)NiSPh]PF 6 , 5, depending upon the reaction stoichiometry. Interestingly, the reaction of 2 with an excess of NaBH 4 gave the new hydride complex [( tBu PCP)NiH]PF 6 , 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 PhSiH 3 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 19 Si NMR method with H 2 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.

In recent years, catalysis with base metal manganese has received a significant amount of interest. Catalysis with manganese complexes having N-heterocyclic carbenes (NHCs) is relatively underdeveloped in comparison to the extensively investigated manganese catalysts possessing pincer ligands (particularly phosphine-based ligands). Herein, we describe the synthesis of two imidazolium salts decorated with picolyl arms (L 1 and L 2 ) as NHC precursors. Facile coordination of L 1 and L 2 with MnBr(CO) 5 in the presence of a base resulted in the formation manganese(I)−NHC complexes (1 and 2) as an air-stable solid in good isolated yield. Single-crystal X-ray analysis revealed the structure of the cationic complexes [Mn(CO) 3 (NHC)][PF 6 ] with tridentate N,C,N binding of the NHC ligand in a facile fashion. Along with a few known manganese(I) complexes, these Mn(I)−NHC complexes 1 and 2 were tested for the hydrosilylation of terminal alkynes. Complex 1 was proved to be an effective catalyst for the hydrosilylation of terminal alkynes with good selectivity toward the less thermodynamically stable β-(Z)-vinylsilanes. This method provided good regioselectivity (anti-Markovnikov addition) and stereoselectivity (β-(Z)-product). Experimental evidence suggested that the present hydrosilylation pathway involved an organometallic mechanism with manganese(I)−silyl species as a possible reactive intermediate.