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 (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 (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 4d z 2 and S 3p z 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.

Investigating Metal–Metal Bond Polarization in a Heteroleptic Tris-Ylide Diiron System

“…These absorption bands are similar to the features observed by Lu for a series of complexes bearing the ligand in C (Figure ). Coll and Dunbar also observed features in the same range for a series of thiolate bridged dirhodium complexes . They characterized those transitions as intervalence charge transfer (IVCT) bands; however, their observed extinction coefficients are nearly an order of magnitude larger than those for 3 and [4-OTf] 0 .…”

“…Coll and Dunbar also observed features in the same range for a series of thiolate bridged dirhodium complexes. 26 They characterized those transitions as intervalence charge transfer (IVCT) bands; however, their observed extinction coefficients are nearly an order of magnitude larger than those for 3 and [4-OTf] 0 . TD-DFT computational analyses were not able to accurately reproduce the experimental results, likely due to the significant ground state static correlation in the Fe−Fe interaction (see below), limiting our ability to model the origin of these features (Figure 4).…”