Electronically-coupled redox centers in trimetallic cobalt complexes

Abstract

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) (1³⁺), and benzene hexathiolate (BHT) (2³⁺), with 1,1,1,-tris(diphenylphosphinomethyl)ethane (triphos) employed as the capping ligand. Single crystal X-ray analyses of 1³⁺ and 2³⁺ display three five-coordinate cobalt centers bound to the triphos and dithiolene ligands in a distorted square pyramidal geometry. Cyclic voltammetry studies of 1³⁺ and 2³⁺ 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 (ΔE_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 2²⁺, 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 1³⁺, 2³⁺, 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 1³⁺, 2³⁺, and 3⁺.