Half-Sandwich Rhodium/Iridium(III) Complexes Designed with Cp* and 1,2-Bis(phenylchalcogenomethyl)benzene as Catalysts for Transfer Hydrogenation in Glycerol Om Prakash, Kamal Nayan Sharma, Hemant Joshi, Pancham L. Gupta, and Ajai K. Singh* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India * S Supporting Information ABSTRACT: The reactions of 1,2-bis(phenylthiomethyl)benzene(L1) and 1,2- bis(phenylselenomethyl)benzene(L2) with [(η 5 -Cp*)MCl(μ-Cl)] 2 (M = Rh or Ir) at room temperature, followed by treatment with NH 4 PF 6 have resulted in air and moisture insensitive half-sandwich complexes of composition [(η 5 -Cp*)M(L)Cl]- [PF 6 ] (Rh, 1-2; Ir, 3-4; L = L1 or L2). Their HR-MS, 1 H, 13 C{ 1 H}, and 77 Se{ 1 H} NMR spectra were found to be characteristic. The single crystal structures of 1-4 have been established by X-ray crystallography. The complexes 1-4 have been found efficient for catalytic transfer hydrogenation (TH) of aldehydes and ketones in glycerol, which acts as a solvent and hydrogen source. Complexes 1-2 are the first examples of Rh species explored for TH in glycerol. The catalysis appears to be homogeneous. The complexes of the (Se, Se) ligand are marginally efficient than the corresponding complexes of the (S, S) ligand. The reactivity of Rh complexes in comparison to those of Ir also appears to be somewhat more. The results of DFT calculations appear to be generally consistent with experimental catalytic efficiencies and bond lengths/angles. ■ INTRODUCTION The combination of an efficient catalyst and a nontoxic solvent like glycerol is attractive. Further, edible and biodegradable glycerol 1 is attractive due to its low cost, ready availability, 2 and renewability, 3 being the main byproduct in oleochemical production. Several processes of the conversion of biomass to chemicals and fuels occur via glycerol. 4 Nonhazardous glycerol is also a good solvent as it dissolves inorganic salts, acids, bases, enzymes, transition-metal complexes, and organic compounds (poorly miscible in water). Thus, in glycerol a variety of transformations is feasible. Hydrophobic solvents such as ethers and hydrocarbons being immiscible with glycerol may be used to remove products from it by simple extraction. The high boiling point (290 °C) makes it a suitable solvent for reactions to be carried out at a high temperature (not possible with the 2- propanol known for its use in transfer hydrogenation (TH)). Distillation is a feasible technique for the separation of the products of reactions carried out in glycerol, which being a nonflammable solvent does not require special handling or storage. 5 TH is a convenient and versatile method for the reduction of carbonyl groups of ketones and aldehydes. 7 2-Propanol has been widely used as a source of hydrogen in TH. It eliminates inflammable hydrogen gas and the need for pressure vessels. 8 Use of glycerol as a hydrogen source is successful for TH reactions 6 but has received less attention. In TH reactions, glycerol is dehydrogenated to several products including dihydroxyacetone. 9 However, low yield of dihydroxyacetone is not a big concern as glycerol is very cheap, and low recovery of this main byproduct is not going to cut the cost very significantly. TH of organic carbonyl compounds requires a catalyst. Several ruthenium species including half-sandwich ones have been reported to catalyze TH reactions in glycerol, 9 However, rhodium/iridium complexes are much less explored for TH reactions, particularly in glycerol. 10 In view of our current research interest in the designing of transition metal catalysts for reactions such as C-C coupling, oxidation of alcohol, and TH 11 using organochalcogen ligands, it was thought worthwhile to synthesize complexes of 1,2- bis(phenylchalcogenomethyl)benzene (L1-L2, Chart 1) with η 5 -Cp*Rh(III)/Ir(III) and explore them as TH catalysts in Received: February 10, 2014 Chart 1 Article pubs.acs.org/Organometallics © XXXX American Chemical Society A dx.doi.org/10.1021/om500149n | Organometallics XXXX, XXX, XXX-XXX