Synthesis, characterization and theoretical studies of tris-cyclometalated rhodium(III) complexes: applications to asymmetric transfer hydrogenation of ketones

This paper presents the synthesis of {Rh[PyCH=C(O)Rn]3} complexes, where R1 = 4-cyanophenyl (L1), R2 = 3-nitrophenyl (L2), and R3 = 4-fluorophenyl (L3). These complexes were derived from the reactions of potassium hexachlororhodate(III) with related pyridinium ylides: PyCH=C(O)C6H4-p-CN (Y1), PyCH=C(O)C6H4-m-NO2 (Y2), and PyCH=C(O)C6H4-p-F (Y3). The reactions were carried out in methanol-water mixtures under mild conditions. The synthesized complexes were characterized using FT-IR, ESI-MS, multinuclear NMR (1H and 13C), and elemental analysis techniques (CHN). Furthermore, these Rh(III) compounds demonstrated excellent catalytic activity, high turnover number (TON), and turnover frequency (TOF) in the hydrogenation of various ketones, resulting in a series of alcohol products with outstanding yields and efficiencies. To investigate the structures of L1, L2, and L3, density function theory (DFT) calculations were performed at the BP86/def2-SVP level of theory. The study employed natural bond orbital analysis (NBO) and energy-decomposition analysis (EDA) to examine the strength and nature of the donor-acceptor bonds between the pyridinium ylide (Y) and Rh fragments in the complexes. Additionally, variations in natural orbitals for chemical valence (NOCV) were investigated.