Ligand-Driven Electrochemical Tuning of Co₆Se₈ Chevrel Clusters

Molecular "Chevrel-type" clusters of the formula Co₆Se₈L₆ (L = neutral ligand) are a well-studied class of clusters due to their utility as molecular analogues to the Chevrel extended solid phase and their application as subunits in hierarchical materials. However, their solution and optical properties remain relatively underexplored. Aiming to develop the fundamental relationships between the molecular and electronic structures of these clusters and their electrochemical and photophysical properties, this work reports the preparation of a series of Co₆Se₈(P(C₆H₄R)₃)₆-type clusters with R = Cl (1), F (2), H (3), CH₃ (4), and OCH₃ (5) via a stepwise synthetic approach. Solution and solid-state experimental characterization and density functional theory calculations reveal that the Co₆Se₈ cores of 1-5 maintain consistent electronic and structural properties despite the variation of the triarylphosphine ligand para-substituent Hammett parameters (σ_p). However, cyclic voltammetry measurements indicate that the electron transfer energetics of 1-5 are strongly influenced by ligand substitution, with the E_1/2 of a given redox event spanning ∼0.5 V depending on the triarylphosphine ligand's σ_p. These findings support the characterization of Co₆Se₈ clusters as atomically precise nanoclusters with both the structural robustness and the electrochemical tunability needed to act as components in larger charge transfer assemblies.