Efficient Catalysis of Transfer Hydrogenation of Ketones and Oxidation of Alcohols with Newly Designed Half-Sandwich Rhodium(III) and Iridium(III) Complexes of Half-Pincer Chalcogenated Pyridines

Abstract

The in situ generated ArE¯ (E = S or Se) reacts with (2-chloromethyl)pyridine in N2 atmosphere, resulting in half-pincer (N, S/Se) ligands (2-arylchalcogenomethyl)pyridine (L1–L3; aryl = Ph/2-pyridyl for S, Ph for Se). Half-sandwich complexes [(η5-Cp*)Rh(L)Cl][PF6] (1, 2), [(η5-Cp*)Rh(L2)CH3CN][PF6]2 (3), and [(η5-Cp*)Ir(L)Cl][PF6] (4–6), where L = L1–L3, have been synthesized by reacting L with [(η5-Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μ-Cl)]2 respectively. The ligands and complexes have been characterized by IR, HR-MS, and 1H, 13C{1H}, and 77Se{1H} NMR spectra. The single-crystal structures of all the complexes (1–6) have been determined with X-ray crystallography [Rh–S, Rh–Se, Ir–S, Ir–Se bond distances: 2.383(2)/2.353(2), 2.4867(9), 2.343(2)/2.341(3), 2.453(1) Å, respectively]. Each metal has pseudo-octahedral half-sandwich “piano-stool” geometry. The PF6– ions in crystals are packed between half-sandwich complex molecules, resulting in various H···F secondary interactions. Complexes 1–6 have been explored for catalysis of oxidation of secondary alcohols with N-methylmorpholine-N-oxide and transfer hydrogenation reaction of ketones with 2-propanol and found very efficient, as revealed by TON values, which are up to 9.6 × 103 and 9.8 × 103 respectively for the two catalytic reactions. The formation of the M–H bond has been noticed in the intermediates of both the catalytic reactions. DFT calculations indicate a reactivity order for the complexes of Rh > Ir, which is also found for the two catalytic reactions experimentally. The calculated (DFT) and observed bond distances and angles of 1–6 are reasonably close.