Half sandwich complexes of chalcogenated pyridine based bi-(N, S/Se) and terdentate (N, S/Se, N) ligands with (η6-benzene)ruthenium(ii): synthesis, structure and catalysis of transfer hydrogenation of ketones and oxidation of alcohols

Abstract

The half sandwich complexes [(η⁶-C₆H₆)Ru(L)Cl][PF₆] (1–5) have been synthesized by the reactions of (2-arylchalcogenomethyl)pyridine [L = L1–L3] and bis(2-pyridylmethyl)chalcogenide [L = L4–L5] (chalcogen = S, Se; Ar = Ph/2-pyridyl for S, Ph for Se) with [(η⁶-C₆H₆)RuCl₂]₂, at room temperature followed by treatment with NH₄PF₆. Their HR-MS, ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectra have been found characteristic. The single crystal structures of 1–5 have been established by X-ray crystallography. The Ru has pseudo-octahedral half sandwich “piano-stool” geometry. The complexes 1–5 have been found efficient for catalytic oxidation of alcohols with N-methylmorpholine-N-oxide (NMO) and transfer hydrogenation of ketones with 2-propanol (at moderate temperature 80 °C) as TON values are up to 9.9 × 10³ and 9.8 × 10³ respectively for the two catalytic reactions. On comparing the required catalyst loading for good conversions and reaction time for the present complexes with those reported in literature for other transfer hydrogenation/oxidation catalysts, it becomes apparent that 1–5 have good promise. The complexes of Se ligands have been found more efficient than their sulphur analogues. The complexes of bidentate ligands are more efficient than those of terdentate, due to difficult bond cleavage in the case of latter. These orders of efficiency are supported by DFT calculations. The calculated bond lengths/angles by DFT are generally consistent with the experimental ones.