Cooperative redox and spin activity from three redox congeners of sulfur-bridged iron nitrosyl and nickel dithiolene complexes

The synthesis of sulfur-bridged Fe–Ni heterobimetallics was inspired by Nature’s strategies to “trick” abundant first row transition metals into enabling 2-electron processes: redox-active ligands (including pendant iron–sulfur clusters) and proximal metals. Our design to have redox-active ligands on each metal, NO on iron and dithiolene on nickel, resulted in the observation of unexpectedly intricate physical properties. The metallodithiolate, (NO)Fe(N(2)S(2)), reacts with a labile ligand derivative of [Ni(II)(S(2)C(2)Ph(2))](0), Ni(DT), yielding the expected S-bridged neutral adduct, FeNi, containing a doublet {Fe(NO)}(7). Good reversibility of two redox events of FeNi led to isolation of reduced and oxidized congeners. Characterization by various spectroscopies and single-crystal X-ray diffraction concluded that reduction of the FeNi parent yielded [FeNi](−), a rare example of a high-spin {Fe(NO)}(8), described as linear Fe(II)(NO(–)). Mössbauer data is diagnostic for the redox change at the {Fe(NO)}(7/8) site. Oxidation of FeNi generated the 2[FeNi](+)⇌[Fe(2)Ni(2)](2+) equilibrium in solution; crystallization yields only the [Fe(2)Ni(2)](2+) dimer, isolated as PF(6)(−) and BArF(−) salts. The monomer is a spin-coupled diradical between {Fe(NO)}(7) and Ni(DT)(+), while dimerization couples the two Ni(DT)(+) via a Ni(2)S(2) rhomb. Magnetic susceptibility studies on the dimer found a singlet ground state with a thermally accessible triplet excited state responsible for the magnetism at 300 K (χ(M)T = 0.67 emu·K·mol(−1), µ(eff) = 2.31 µ(B)), and detectable by parallel-mode EPR spectroscopy at 20 to 50 K. A theoretical model built on an H(4) chain explains this unexpected low energy triplet state arising from a combination of anti- and ferromagnetic coupling of a four-radical molecular conglomerate.