Twists, Turns, and Spins: Copper-Guided Electronic Communication in Sulfur-Bridged Trimetallic Complexes

Sulfur-bridged multimetallic complexes provide a platform for probing long-range electronic communication relevant to metalloproteins and molecular electronic materials. Trimetallic complexes of the type (MN₂S₂)₂Cu (M = [FeNO]²⁺ or [VO]²⁺), in which these two redox-active S = 1/2 spin probes are linked by a diamagnetic Cu-(I) center in a tetrahedral S₄ coordination environment, are developed. Structural analysis shows that the Cu-(I) bridge enforces a twisted arrangement of the two MN₂S₂ units, in contrast to square-planar NiS₄-bridged analogues, [QuirozM., , Chem. Sci.2023, 14, 9167-9174]. leading to distinct metal-ligand orbital alignment and sulfur-mediated bonding pathways. Despite a large interspin separation of approximately 6 Å between the spin probes, magnetic susceptibility measurements reveal appreciable ferromagnetic exchange coupling between these paramagnetic centers, although weaker than the coupling observed for the d⁸ (M' = Ni-(II), Pd-(II), and Pt-(II))-bridged stair-step complexes. These differences are attributed to the contrasting geometries of the CuS₄ and M'S₄ cores and their impact on superexchange through the bridging sulfurs. In addition to the solid-state X-ray crystallography, this new class of trimetallics was characterized by multiple methods including electrochemistry, vibrational spectroscopy, magnetic susceptibility, EPR, Mössbauer, and DFT computations. Redox and vibrational studies further probe electronic communication between the FeNO units: ν-(NO) stretching frequency indicates that one-electron reduction is largely localized on a single FeNO center, whereas two-electron reduction leads to electron delocalization across both units. Together, these results demonstrate that copper-imposed geometric distortion modulates sulfur-mediated spin coupling and electron delocalization, providing design principles for multimetallic, spin-active architectures.