Electronically-coupled redox centers in trimetallic cobalt complexes

Synthesis and isolation of molecular building blocks of metal-organic frameworks (MOFs) can provide unique opportunities for characterization that would otherwise be inaccessible due to the heterogeneous nature of MOFs. Herein, we report a series of trinuclear cobalt complexes incorporating dithiolene ligands, triphenylene-2,3,6,7,10,11-hexathiolate (THT) (1(3+)), and benzene hexathiolate (BHT) (2(3+)), with 1,1,1,-tris(diphenylphosphinomethyl)ethane (triphos) employed as the capping ligand. Single crystal X-ray analyses of 1(3+) and 2(3+) display three five-coordinate cobalt centers bound to the triphos and dithiolene ligands in a distorted square pyramidal geometry. Cyclic voltammetry studies of 1(3+) and 2(3+) reveal three redox features associated with the formation of mixed valence states due to the sequential reduction of the redox-active metal centers (Co-III/II). Using this electrochemical data, the comproportionality values were determined for 1 and 2 (log K-c = 1.4 and 1.5 for 1, and 4.7 and 5.8 for 2), suggesting strong resonance-stabilized coupling of the metal centers, with stronger electronic coupling observed for complex 2 compared to that for complex 1. Cyclic voltammetry studies were also performed in solvents of varying polarity, whereupon the difference in the standard potentials (Delta E-1/2) for 1 and 2 was found to shift as a function of the polarity of the solvent, indicating a negative correlation between the dielectric constant of the electrochemical medium and the stability of the mixed valence species. Spectroelectrochemical studies of in situ generated multi-valent (MV) states of complexes 1 and 2 display characteristic NIR intervalence charge transfer (IVCT) bands, and analysis of the IVCT transitions for complex 2 suggests a weakly coupled class II multi-valent species and relatively large electronic coupling factors (1700 cm(-1) for the first multi-valent state of 2(2+), and 1400 and 4000 cm(-1) for the second multi-valent state of 2(+)). Density functional theory (DFT) calculations indicate a significant deviation in relative energies of the frontier orbitals of complexes 1(3+), 2(3+), and 3(+) that contrasts those calculated for the analogous trinuclear cobalt dithiolene complexes employing pentamethylcyclopentadienyl (Cp*) as the capping ligand (Co3Cp*3THT and Co3Cp*3BHT, respectively), and may be a result of the cationic nature of complexes 1(3+), 2(3+), and 3(+).