Synthesis and Structural Characterization of Pincer Pyridine Diphosphite Complexes of Rhodium and Iridium

<jats:title>Abstract</jats:title><jats:p>The synthesis of a novel pyridine diphosphite ligand <jats:bold>1</jats:bold> has been described. From this ligand, rhodium– and iridium–chlorido complexes of formula [MCl(<jats:bold>1</jats:bold>)] (M = Rh, Ir) have been prepared. Chloride abstraction by treatment with NaBPh<jats:sub>4</jats:sub> and a phosphane produced the corresponding cationic phosphane derivatives [M(<jats:bold>1</jats:bold>)<jats:bold>L</jats:bold>][BPh<jats:sub>4</jats:sub>] [<jats:bold>L</jats:bold> = PPh<jats:sub>3</jats:sub> (Rh, Ir), PPh<jats:sub>2</jats:sub>Me (Ir)]. The analogous reaction of [RhCl(<jats:bold>1</jats:bold>)] with CNXy (Xy = 2,6‐Me<jats:sub>2</jats:sub>‐C<jats:sub>6</jats:sub>H<jats:sub>3</jats:sub>) and NaBPh<jats:sub>4</jats:sub> yielded the monosubstituted complex [Rh(<jats:bold>1</jats:bold>)(CNXy)][BPh<jats:sub>4</jats:sub>], whereas the reaction between [IrCl(<jats:bold>1</jats:bold>)] and isonitriles led to the disubstituted complexes [Ir(<jats:bold>1</jats:bold>)(<jats:bold>L</jats:bold>)<jats:sub>2</jats:sub>][BPh<jats:sub>4</jats:sub>] (<jats:bold>L</jats:bold> = CNBn, CNCy). Ethylene compound [Rh(<jats:bold>1</jats:bold>)(C<jats:sub>2</jats:sub>H<jats:sub>4</jats:sub>)][BPh<jats:sub>4</jats:sub>] was obtained from the reaction of [RhCl(<jats:bold>1</jats:bold>)] with NaBPh<jats:sub>4</jats:sub> under ethylene, whereas [Ir(<jats:bold>1</jats:bold>)(C<jats:sub>2</jats:sub>H<jats:sub>4</jats:sub>)][BPh<jats:sub>4</jats:sub>] was synthesized by a treatment of [{IrCl(COE)<jats:sub>2</jats:sub>}<jats:sub>2</jats:sub>] with ethylene followed by addition of <jats:bold>1</jats:bold> and NaBPh<jats:sub>4</jats:sub>. An IR analysis of the isocyanide complexes indicates a very poor π‐donor ability of the [M(<jats:bold>1</jats:bold>)]<jats:sup>+</jats:sup> fragment, therefore the isocyanide metal bond is mostly due to σ donation from the isocyanide. Characterization by X‐ray crystallography of [Rh(<jats:bold>1</jats:bold>)(PPh<jats:sub>3</jats:sub>)][BPh<jats:sub>4</jats:sub>], [Rh(<jats:bold>1</jats:bold>)(MeCN)][BPh<jats:sub>4</jats:sub>] and [Ir(<jats:bold>1</jats:bold>)(PPh<jats:sub>2</jats:sub>Me)][BPh<jats:sub>4</jats:sub>] displays a square‐planar structure with ligand <jats:bold>1</jats:bold> coordinated in a pincer fashion for these complexes. In addition, the ethylene derivative [Rh(<jats:bold>1</jats:bold>)(C<jats:sub>2</jats:sub>H<jats:sub>4</jats:sub>)][BPh<jats:sub>4</jats:sub>] 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 <jats:italic>meso</jats:italic> 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(<jats:bold>1</jats:bold>)] is an active catalyst in the reduction of 2‐methylquinoline and 2‐methylquinoxaline.</jats:p>