Synthesis and structures of dinuclear RhIII and IrIII complexes supported by a tetraphosphine, meso- or rac-bis{[(diphenylphosphinomethyl)phenyl]phosphino]}methane

Abstract

Reactions of [Cp*MCl2]2 with meso- or rac-dpmppm in the presence of NH4PF6 afforded mixtures of stereoisomers formulated as [(Cp*MCl)2(meso- or rac-dpmppm)](PF6)2, from which the major isomers, rac-M2-[(Cp*MCl)2(meso-dpmppm)](PF6)2 (M = Rh (1a), Ir (1b)) and rac-M2-[(Cp*MCl)2(rac-dpmppm)](PF6)2 (M = Rh (2a), Ir (2b)), were isolated and characterized by IR, UV–vis, ESI mass, and 1H and 31P{1H} NMR spectroscopy and X-ray crystallography, where dpmppm is bis{[(diphenylphosphinomethyl)phenyl]phosphino}methane and Cp* is η5-pentamethylcyclopentadienyl. The isomeric tetraphosphine incorporates two {Cp*MCl} fragments with four-membered chelation through a pair of outer and inner phosphine units, and fixes them ca. 6.9 Å apart from each other. The configurations around the metal centers in 2 were determined to avoid repulsive interaction between the phenyl group on the inner P atom and the M-bound chloride anion, while those in 1 would be determined by minimizing not only the Ph/Cl repulsion but also repulsion between the two metal fragments. In the dichloromethane solutions, the major isomers 1 and 2 were not converted to any other minor isomers even at high temperature, but in dmf and dmso, only 1a was readily transformed into the minor isomer, meso-M2-[(Cp*RhCl)2(meso-dpmppm)](PF6)2. Cyclic voltammograms of 1a,b and 2a,b demonstrated two irreversible 2e− reduction waves corresponding to MIIIMIII → MIMIII → MIMI, in which the mixed-valence states with meso-dpmppm could be stabilized to a larger extent than with rac-dpmppm. Complex 1a heated in acetonitrile with AgOTf/NH4OTf afforded a Rh2Ag2 mixed-metal complex, [(Cp*Rh(CH3CN))2Ag2(OTf)2(meso-dpmppm)2](OTf)4 (5).