Comparisons of MN2S2vs. bipyridine as redox-active ligands to manganese and rhenium in (L-L)M'(CO)3Cl complexes

Abstract

The bipyridine ligand is renowned as a photo- and redox-active ligand in catalysis; the latter has been particularly explored in the complex Re(bipy)(CO)₃Cl for CO₂ reduction. We ask whether a bidentate, redox-active MN₂S₂ metallodithiolate ligand in heterobimetallic complexes of Mn and Re might similarly serve as a receptor and conduit of electrons. In order to assess the electrochemical features of such designed bimetallics, a series of complexes featuring redox active MN₂S₂ metallodithiolates, with M = Ni²⁺, {Fe(NO)}²⁺, and {Co(NO)}²⁺, bound to M'(CO)₃X, where M' = Mn and Re, were synthesized and characterized using IR and EPR spectroscopies, X-ray diffraction, cyclic voltammetry, and density functional theory (DFT) computations. Butterfly type structures resulted from binding of the convergent lone pairs of the cis-sulfur atoms to the M'(CO)₃X unit. Bond distances and angles are similar across the M' metal series regardless of the ligand attached. Electrochemical characterizations of [MN₂S₂·Re(CO)₃Cl] showed the redox potential of the Re is significantly altered by the identity of the metal in the N₂S₂ pocket. DFT calculations proved useful to identify the roles played by the MN₂S₂ ligands, upon reduction of the bimetallics, in altering the lability of the Re-Cl bond and the ensuing effect on the reduction of Re^I to Re⁰.