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 NaBPh 4 and a phosphane produced the corresponding cationic phosphane derivatives [M( 1 ) L ][BPh 4 ] [ L = PPh 3 (Rh, Ir), PPh 2 Me (Ir)]. The analogous reaction of [RhCl( 1 )] with CNXy (Xy = 2,6‐Me 2 ‐C 6 H 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 2 H 4 )][BPh 4 ] was obtained from the reaction of [RhCl( 1 )] with NaBPh 4 under ethylene, whereas [Ir( 1 )(C 2 H 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 2 Me)][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 2 H 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.