2-Propanol vs Glycerol as Hydrogen Source in Catalytic Activation of Transfer Hydrogenation with (η6-Benzene)ruthenium(II) Complexes of Unsymmetrical Bidentate Chalcogen Ligands

Abstract

1H-Benzoimidazole on subjection to a sequence of reactions with benzyl bromide, PhECH2Cl (E = S, Se), and elemental S or Se results in 1-benzyl-3-phenylchalcogenylmethyl-1,3-dihydrobenzoimidazole-2-chalcogenones (L1–L4), which are unsymmetrical bidentate chalcogen ligands having a unique combination of chalcogenoether and chalcogenone donor sites. Half sandwich complexes, [(η6-C6H6)Ru(L)Cl][PF6] (1–4), have been synthesized by reactions of [(η6-C6H6)RuCl(μ-Cl)]2 with the appropriate L at room temperature followed by treatment with NH4PF6. L1–L4 and their complexes 1–4 have been authenticated with HR-MS and 1H, 13C{1H}, and 77Se{1H} NMR spectra. The single-crystal structures of 1–4 have been determined by X-ray crystallography. Each L acts as an unsymmetric (E,E) or (E,E′) bidentate ligand. The Ru atom in 1–4 has pseudo-octahedral half-sandwich “piano-stool” geometry. The Ru–S and Ru–Se bond distances (Å) respectively are 2.358(3)/2.3563(18) and 2.4606(11)/2.4737(10) (thio- and selenoether), and 2.4534(17)/2.435(3) and 2.5434(9)/2.5431(10) (thione and selone). Catalytic activation with complexes 1–4 has been explored for the transfer hydrogenation (TH) of aldehydes and ketones using various sources of hydrogen. 2-Propanol and glycerol have been compared and found most suitable among the sources screened. The catalytic efficiency of other sources explored, viz. formic, citric, and ascorbic acid, is dependent on the pH of reaction medium and is not promising. A comparative study of 2-propanol and glycerol as hydrogen sources for catalytic activation of TH with 1–4 has revealed that with glycerol (for comparable conversion in the same time) more amount of catalyst is needed in comparison to that of 2-propanol. The catalytic process is more efficient with 3 (where Ru is bonded with selone), followed by 1 ≈ 4, and 2 showing the least activity among all four complexes. The transfer hydrogenation involves an intermediate containing a Ru–H bond and follows a conventional alkoxide intermediate based mechanism. The results of DFT calculations appear to be generally consistent with experimental catalytic efficiencies and bond lengths/angles.