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(NS), reacts with a labile ligand derivative of [Ni(SCPh)], Ni, yielding the expected S-bridged neutral adduct, , containing a doublet {Fe(NO)}. Good reversibility of two redox events of led to isolation of reduced and oxidized congeners. Characterization by various spectroscopies and single-crystal X-ray diffraction concluded that reduction of the parent yielded , a rare example of a high-spin {Fe(NO)}, described as linear Fe(NO). Mössbauer data is diagnostic for the redox change at the {Fe(NO)} site. Oxidation of generated the 2⇌ equilibrium in solution; crystallization yields only the dimer, isolated as PF and BArF salts. The monomer is a spin-coupled diradical between {Fe(NO)} and Ni, while dimerization couples the two Ni via a NiS 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 (χT = 0.67 emu·K·mol, = 2.31 ), and detectable by parallel-mode EPR spectroscopy at 20 to 50 K. A theoretical model built on an H chain explains this unexpected low energy triplet state arising from a combination of anti- and ferromagnetic coupling of a four-radical molecular conglomerate.