Cubane-Type Mo₃FeS₄^4+,5+ Complexes Containing Outer Diphosphane Ligands: Ligand Substitution Reactions, Spectroscopic Studies, and Electronic Structure

A general protocol to access Mo₃FeS₄⁴⁺ clusters selectively modified at the Fe coordination site is presented starting from the all-chlorine Mo₃(FeCl)­S₄(dmpe)₃Cl₃ (<b>1</b>) [dmpe = 1,2-bis­(dimethylphosphane-ethane)] cluster and tetrabutylammonium salts (<i>n</i>-Bu₄NX) (X = CN^–, N₃^–, and PhS^–). Clusters Mo₃(FeX)­S₄(dmpe)₃Cl₃ [X = CN^– (<b>2</b>), N₃^– (<b>3</b>), and PhS^– (<b>4</b>)] are prepared in high yield, and comparison of geometric and redox features upon modification of the coordination environment at the Fe site at parity of ligands at the Mo sites is also presented. The existence of the cubane-type Mo₃FeS₄^4+,5+ redox couple is demonstrated by cyclic voltammetry and for compound <b>1</b> by cluster synthesis and X-ray structure determinations. Ground states for the <b>1</b>/<b>1</b>⁺ redox couple are evaluated on the basis of magnetic susceptibility measurements, electron paramagnetic resonance, and ⁵⁷Fe Mössbauer spectroscopy aimed at providing an input of experimental data for electronic structure determination based on density functional theory calculations