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

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 (E_1/2 = 0.014, 0.52 V vs Ag/AgCl). The large degree of separation between these redox states (ΔE = 0.51 V, K_C = 4.17 × 10⁸) allows for the chemical isolation of three distinct complexes 1, 2, and 3, in which the oxidation states of each Rh center are Rh₂^I,I, Rh₂^I,II, and Rh₂^II,II, respectively, and whose solid-state structures were elucidated by single crystal X-ray diffraction studies. Complex 2 is an unprecedented type of mixed valence dirhodium species whose electron paramagnetic resonance spectrum revealed a delocalization of the unpaired electron through the thiolate-bridging ligand. Intervalence charge transfer occurs between the Rh centers, as evidenced by a broad absorption in the near-infrared region (λ_max = 1187 nm). The structure of 3 is quite rare in that it lacks the typical Rh^II-Rh^II σ bond, but significant orbital overlap between the Rh 4d² and S 3p orbitals results in a strong antiferromagnetic coupling (computed J = -1516.9 cm^-1). Complex 3 also absorbs low-energy light (λ_max = 779 nm). Spectroscopic and magnetic measurements are supported by density functional theory methods, which further elucidate the nature of the ground state energies, frontier orbital characters, excited state transitions, and presence of weak Rh-Rh natural bond orbital interactions.