Electrochemical Oxidative Addition Involving Dirhodium(I) Complexes Containing Transoid Bridging Bis(diphenyIphosphino)methane Ligands. Crystal Structure of a Dirhodium(II) Complex with an Unusual Folding of the Diphosphine Ligand

The two-electron electrochemical oxidation of complexes such as [Rh2(μ-dppm)2(r-BuNC)2(μ-A)]PF6 (dppm = bis(diphenylphosphino)methane; A = pyrazolate derivative) is facilitated by the presence of neutral Lewis bases such as pyridine derivatives or anionic ligands such as Cl−, Br−, I−, SCN−, NO3−, and CH3COO−. Several of the oxidative-addition products have been isolated and characterized. The NO3− and CH3CO− derivatives were also obtained by the reaction of the pyrazolato-bridged dirhodium(I) complexes with HNO3 and CH3COOH, respectively. The paramagnetic Rh23+ species generated by a one-electron oxidation of the parent dirhodium(I) complexes undergo disproportionation in the presence of the anionic ligands (except NO3−). The addition of NO3− to the Rh23+ species results in oxidation to form a dinitrato Rh24+ complex. The mechanism of the disproportionation reaction of the Rh23+ species in the presence of halide ions is discussed. One mechanism is believed to involve electron exchange between two Rh23+ species with simultaneous addition of two anions to the resulting Rh24+ species. The operative mechanism depends on the nature of the Rh23+ species and the nature of the anion. If the potential at which the anion ion is oxidized is more negative that the potential at which the Rh23+ species is reduced, the mechanism involves the oxidation of the anion followed by an oxidative-addition reaction. This second mechanism was found to be operative for the reaction of some of the dicarbonyl Rh23+ species with Br− and I−. The occurrence of different mechanisms for the disproportionation reactions is supported by the 31P{1H} NMR data. When the electrochemical oxidation of the dirhodium(I) or the disproportionation of the dirhodium(I,II) complexes that contain the pyrazolate anion (pz) is conducted in CH2Cl2 containing CH3COO− ions, the complex abstracts a Cl from the solvent to form [Rh2(μ-dppm)2(t-BuNC)2(μ-pz)CI2]PF6, which was completely characterized by a single-crystal X-ray analysis. Crystals of the complex are monoclinic, space group P21/c, with a = 12.355 (2) Å, b = 20.759 (9) Å, c = 24.689 (5) Å,β = 96.82 (2)°, V = 6287 (5) Å3, and Z = 4. The methylene moieties of the transoid bridging dppm ligands are folded away from the bridging pyrazolate ligand, and thus they have an orientation which contrasts with that previously found and believed to be favored for dppm-bridged A-frame complexes. © 1990, American Chemical Society. All rights reserved.

Duke Scholars

Author Andrew T. McPhail Chemistry