Ligand effect on the structures and redox properties of the incomplete cubane-type Mo-Fe-S clusters

Iron-sulfur clusters play a pivotal role in biochemical processes and have attracted considerable attention in biomimetic chemistry. Within synthetic iron-sulfur cluster chemistry, studying ligand effects is significant for both understanding structure and function of natural iron-sulfur clusters and regulating structure and properties of synthetic analogs to mimic natural systems. This study systematically investigates how thiolate/thiophenolate ligands at in-core and/or out-core sites govern structural and electrochemical behavior of incomplete cubane clusters with a [MoFe₂S₄] core. A series of eight well-defined Mo–Fe–S clusters sharing the same [MoFe₂S₄] core were synthesized via the previously developed site-differentiated ligand substitution strategy; these are either mono-substituted at the in-core bridging site or tri-substituted at both in-core bridging and out-core terminal sites using thiolate/thiophenolate ligands with tunable electronic and steric properties (MeS⁻, EtS⁻, PhS⁻, pTolS⁻). Comprehensive characterization by single-crystal X-ray diffraction reveals that types and/or numbers of in-core bridging/out-core terminal ligands exert only limited influence on structural metrics of the [MoFe₂S₄] core. However, electrochemical studies using cyclic voltammetry indicate that ligand effects significantly govern potentials of reversible redox events.