Probing the factors that stabilize mononuclear rhodium(II) bis(phosphine), eta(6)-arene complexes with piano-stool geometries

The synthesis, characterization, and electrochemistry of a series of Rh(I) bis(phosphine), eta(6)-arene, piano-stool complexes are reported. The study reported herein elucidates several of the important factors which lead to the stabilization of Rh(II) in this coordination environment. From the electrochemical data for a series of complexes of the type [Rh(eta(2)-dppe)(eta(6)-C(6)H(6-n)X(n))]BF4 (X = CH3, n = 0-6) (1-7) it was shown that the addition of methyl groups to the arene ligand kinetically stabilize the Rh(II) center and thermodynamically stabilize the Rh(II) species by 16 mV per added methyl group. Furthermore, complexes which contain chelation to the arene ligand, such as [Rh(eta(6):eta(1)-Ph(CH2)(3)PPh(2))(eta(1)-Ph(CH2)(3)PPh(2))]BF4 (12), kinetically stabilize the Rh(II) form, presumably from ligand substitution based decomposition reactions. The electrochemical studies of five isostructural and isoelectronic complexes, [Rh(eta(2)-dppe)(eta(6)-C6H5CH3)]BF4 (2), [Rh(eta(1)-n-BUPPh(2))(2)(eta(6)-C6H5CH3)]BF4 (8), [Rh(eta(2)-dppp)(eta(6)-C6H5-CH3)]BF4 (9), [Rh(eta(2)-dppb)(eta(6)-C6H5CH3)]BF4 (10), and 12 show that those complexes which contain bidentate, bis(phosphine) chelation with an ethyl or butyl bridge, 2 and 10, thermodynamically destabilize the Rh(II) form relative to those complexes which contain a less restricted bis(phosphine) chelate or no bis(phosphine) chelation. Using single-crystal X-ray data and extended Huckel calculations, these counterintuitive electrochemical trends were explained in terms of not only the properties of the Rh(I) complex but also, in terms of the structural changes which are likely to occur upon oxidation of the metal center from Rh(I) to Rh(II).