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

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 NaBPh4 and a phosphane produced the corresponding cationic phosphane derivatives [M(1)L][BPh4] [L = PPh3 (Rh, Ir), PPh2Me (Ir)]. The analogous reaction of [RhCl(1)] with CNXy (Xy = 2,6-Me2-C6H3) and NaBPh4 yielded the monosubstituted complex [Rh(1)(CNXy)][BPh4], whereas the reaction between [IrCl(1)] and isonitriles led to the disubstituted complexes [Ir(1)(L)2][BPh4] (L = CNBn, CNCy). Ethylene compound [Rh(1)(C2H4)][BPh4] was obtained from the reaction of [RhCl(1)] with NaBPh4 under ethylene, whereas [Ir(1)(C2H4)][BPh4] was synthesized by a treatment of [{IrCl(COE)2}2] with ethylene followed by addition of 1 and NaBPh4. 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)(PPh3)][BPh4], [Rh(1)(MeCN)][BPh4] and [Ir(1)(PPh2Me)][BPh4] displays a square-planar structure with ligand 1 coordinated in a pincer fashion for these complexes. In addition, the ethylene derivative [Rh(1)(C2H4)][BPh4] shows a near in-plane conformation of the ethylene ligand, with a short C–C distance (1.319 A). 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.