Syntheses, characterisation, and catalytic role of (η5-C5Me5)Rh(iii) guanidinato complexes in transfer hydrogenation (TH) and TH–etherification

Abstract

A family of air stable half sandwich meal guanidinato complexes ([(η⁵-Cp*)MCl{κ²(N,N′)((ArN)₂C–N(H)Ar)}]) (M = Rh and Ir; Cp* = C₅Me₅; Ar = aryl) were synthesized in good yield and characterised by elemental analyses, IR, and NMR (¹H, ¹³C, and ¹⁹F) spectroscopy. The geometry of the metal and the conformations of the guanidinate ligands in the complexes were studied by single crystal X-ray diffraction. The solution behaviour of representative complexes was investigated by detailed NMR studies including variable temperature and variable concentration ¹H NMR measurements. The new complexes were screened as catalysts for transfer hydrogenation (TH) of acetophenone under basic and base free conditions and from these experiments, ([(η⁵-Cp*)RhCl{κ²(N,N′)((ArN)₂C–N(H)Ar)}]) (Ar = 3,5-(CF₃)₂C₆H₃; 3) was chosen as the preferred catalyst due to its slightly better catalytic activity than other complexes. The utility of 3 in TH of a variety of carbonyl compounds was explored under basic and base free conditions. Tandem catalysis involving TH of a carbonyl group and etherification of the resulting –CH₂OH group in reduction products of salicylaldehyde, 2-hydroxy-1-naphthaldehyde and 5-(hydroxymethyl)furfural was achieved in the presence of 3 under base free conditions. The role of the guanidinate ligands in the complexes for basic and base free TH of carbonyl compounds and TH–etherification tandem catalysis is discussed. Plausible mechanisms for TH and TH–etherification are outlined.