Ferrocenyl Dithiophosphonate Stabilized Heterobimetallic (Co II /Fe II , Ni II /Fe II , and Cu I /Fe II ) Complexes: Synthesis, Characterization, Luminescence, and Electrochemical Behavior

Ferrocenyl dithiophosphonate metal-based molecular compounds are infrequent with untapped potential in electrocatalysis and luminescence. A stoichiometric reaction of late 3d metal(II) (Co, Ni, Cu) salts with ferrocenyl dithiophosphonate NH₄[S₂P(OnPr)Fc] in a 1:2 ratio produces mononuclear [M{S₂P(OnPr)Fc}₂] {M = CoII (1), NiII (2)} complexes and a tetranuclear [Cu₄{S₂P(OnPr)Fc}₄] (3) {Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅)} cluster. The distinct core of 3 compared to 1 and 2 indicates that NH₄[S₂P(OnPr)Fc] first serves as a sacrificial reductant and subsequently stabilizes the Cu(I) cluster. Heterometallic species (1-3) were characterized by NMR (¹H, ³¹P), FT-IR, TGA, UV-visible spectroscopy, ESI-MS, XPS, and single-crystal X-ray diffraction. Ligand [S₂P(OnPr)Fc]⁻ stabilized the redox-derived tetrahedral CuI core of 3 and the M(II)-centered square (MS₄) frame in 1 and 2 through (η³; μ¹, μ²) and (η; μ¹, μ¹) bonding patterns, respectively. The irreversible single wave voltammogram indicated nonuniform electrochemical behavior of 1-3. TD-DFT indicates MLCT and weak LLCT in isostructural 1-2 and 3m [CuII{S₂P(OnPr)Fc}₂], while 3 exhibits multielectron transitions with two types of MLCT. Third-order decay confirms singlet fluorescence, and multiple electronic transitions in 3 enhance its photoluminescence (27.5% QY) relative to 1-2, correlating the isostructural and different nuclearity features of late 3d metals with their physical and electrochemical properties.