Rhodium Dimers with 2,2-Dimethyl-1,3-diisocyano and Bis(diphenylphosphino)methane Bridging Ligands

Abstract

Four rhodium dimers have been synthesized with a bridging diisocyanide ligand, dmb (2,2-dimethyl-1,3-diisocyanopropane): [Rh2(dmb)4](BPh4)2, [Rh2(dmb)4Cl2]Cl2, [Rh2(dmb)4I2](PF6)2, and [Rh2(dmb)2(dppm)2](BPh4)2 (dppm = bis(diphenylphosphino)methane). The complexes have been characterized by elemental analysis and mass spectrometry, as well as UV-visible, IR, and 1H NMR spectroscopies. X-ray crystal structures of the rhodium(I) complexes, [Rh2(dmb)4](BPh4)2 . 1.5CH3CN (3.2330(4), 3.2265(4) A) and [Rh2(dmb)2(dppm)2](BPh4)2.0.5CH3OH . 0.2H2O (3.0371(5) A), confirm the existence of short Rh...Rh interactions. The metal-metal separation for the rhodium(II) adduct, [Rh(2)(dmb)4Cl2]Cl2.6CHCl3 (2.8465(6) A), is consistent with a formal Rh-Rh bond. For the two luminescent rhodium(I) dimers and six previously investigated diisocyano-bridged dimers with and without dppm ligands, the intense spin-allowed dsigma-->psigma absorption band maximum shifts to longer wavelengths with decreasing Rh...Rh separation, and there is an approximate correlation between band energy and the inverse of the metal-metal separation cubed. Both [Rh2(dmb)4]2+ and [Rh2(dmb)4(dppm)2]2+ undergo oxidative addition in the presence of iodine. In the conversion of [Rh2(dmb)4]2+ to [Rh2(dmb)4I2]2+, the observed intermediate is tentatively assigned to a tetramer composed of two rhodium dimers. In the case of [Rh2(dmb)2(dppm)2]2+, no intermediate was detected.