Synthesis and structural characterization of pincer pyridine diphosphite complexes of rhodium and iridium

Abstract

The synthesis of a novel pyridine diphosphite ligand 1 has been described. From this ligand, rhodium- and iridium-chlorido complexes of formula [MCl(1)] (M = Rh, Ir) have been prepared. Chloride abstraction by treatment with NaBPh 4 and a phosphane produced the corresponding cationic phosphane derivatives [M(1)L][BPh 4] [L = PPh 3 (Rh, Ir), PPh 2Me (Ir)]. The analogous reaction of [RhCl(1)] with CNXy (Xy = 2,6-Me 2-C 6H 3) and NaBPh 4 yielded the monosubstituted complex [Rh(1)(CNXy)][BPh 4], whereas the reaction between [IrCl(1)] and isonitriles led to the disubstituted complexes [Ir(1)(L) 2][BPh 4] (L = CNBn, CNCy). Ethylene compound [Rh(1)(C 2H 4)][BPh 4] was obtained from the reaction of [RhCl(1)] with NaBPh 4 under ethylene, whereas [Ir(1)(C 2H 4)][BPh 4] was synthesized by a treatment of [{IrCl(COE) 2} 2] with ethylene followed by addition of 1 and NaBPh 4. An IR analysis of the isocyanide complexes indicates a very poor π-donor ability of the [M(1)] + fragment, therefore the isocyanide metal bond is mostly due to σ donation from the isocyanide. Characterization by X-ray crystallography of [Rh(1)(PPh 3)][BPh 4], [Rh(1)(MeCN)][BPh 4] and [Ir(1)(PPh 2Me)][BPh 4] displays a square-planar structure with ligand 1 coordinated in a pincer fashion for these complexes. In addition, the ethylene derivative [Rh(1)(C 2H 4)][BPh 4] shows a near in-plane conformation of the ethylene ligand, with a short C-C distance (1.319 Å). Moreover, in all the structures, the diphosphite ligand exhibits a meso conformation irrespective of the size of the neutral ancillary ligand. An examination of the behaviour of some of these complexes in catalytic hydrogenation has shown that [IrCl(1)] is an active catalyst in the reduction of 2-methylquinoline and 2-methylquinoxaline. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.