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 (E1/2 = 0.014, 0.52 V vs Ag/AgCl). The large degree of separation
between these redox states (ΔE = 0.51 V, KC = 4.17 × 108) allows for the
chemical isolation of three distinct complexes 1, 2, and 3, in which the oxidation states of each
Rh center are Rh2I,I, Rh2I,II, and Rh2II,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 (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 a strong antiferromagnetic coupling (computed J = −1516.9 cm–1). Complex 3 also absorbs low-energy light (λmax = 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.