Removal of Metal−Metal Bonding in a Dimetallic Paddlewheel Complex: Molecular and Electronic Structure of Bis(phenyl) Dirhodium(III) Carboxamidate Compounds

International audienceThe unique structural features and chemical stabilities of bis(phenyl) tetracarboxamidatodirhodium(III) are reported, and their electronic structures are mapped through XPS, electrochemical, and computational methods. Comparison with the structures of dirhodium(II,II) and dirhodium(II,III) oxidative precursors portrays the diphenyl dirhodium(III) compounds as two square-pyramidal rhodium units that have undergone conrotatory motion in order to optimize metal−ligand bonding. Axial phenyl ligands are severely distorted from their expected Rh−Rh−C linear array. XPS data for this series of dirhodium compounds are consistent with the absence of a rhodium−rhodium bond for the diphenyl dirhodium(III) compounds, and electrochemical measurement shows a single reversible Rh26+/Rh27+ redox couple. Notably, they exhibit high thermal stability, and Brønsted acid removal of carboxamidate ligands precedes the formation of benzene. The ability of a phenyl group to impart unusual stability to rhodium(III) compounds is explained by theoretical analysis of the electronic structure of bis-σ-(phenyl)-tetrakis-μ-(carboxamido)dirhodium(III), and comparison is made with previously reported dinitrosyl dirhodium(III) complexes. Formally, two phenyl radicals in combination with a rhodium−rhodium bond are transformed into two phenyl−rhodium bonds. The severe distortion of phenyl rings from linearity is suggested to result from a long-range interaction between the electron-deficient rhodium and distal oxygen atoms