Three Reversible Redox States of Thiolate-Bridged Dirhodium Complexes without Metal–Metal Bonds

Abstract

An unusual dinuclear rhodium complex with the anionic 2-mercapto-6-methylpyridinate (mmp) bridging ligand is reported which is capable of undergoing significant variations in its structural and coordination environments as a result of two reversible redox events at accessible potentials (E1/2 = 0.014, 0.52 V vs. Ag/AgCl). The large degree of separation between these redox states (ΔE = 0.51 V, KC = 4.17x108) allows for the chemical isolation of three distinct complexes 1, 2, and 3, in which the oxidation states of each Rh center are described as Rh2I,I, Rh2I,II, and Rh2II,II, respectively, and whose structures were elucidated by single crystal X-ray diffraction studies. Complex 2 is an unprecedented type of mixed valence dirhodium species whose EPR spectrum revealed a delocalization of the unpaired electron through the thiolate bridging ligand. Intervalence charge transfer (IVCT) occurs between the Rh centers as evidenced by a broad absorption in the near-infrared region (λmax¬=1187 nm). The structure of 3 is quite rare in that it lacks the typical RhII-RhII σ bond but significant orbital overlap between the Rh 4dz2 and S 3pz orbitals results in strong antiferromagnetic coupling (computed J = -1516.9 cm-1). Complex 3 also absorbs low energy light (λmax¬ = 779 nm) and spectroscopic and magnetic measurements are supported by DFT methods which further elucidate the nature of the ground state energies, frontier orbital characters, excitated state transitions, and the presence of weak Rh-Rh NBO interactions.