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(N2S2), reacts with a labile ligand derivative of [NiII(S2C2Ph2)]0, Ni-DT, yielding the expected S-bridged neutral adduct, FeNi, containing a doublet fFe(NO)g(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 fFe(NO)g(8), described as linear Fe-II(NO-). Mossbauer data is diagnostic for the redox change at the {Fe(NO)}(7/8) site. Oxidation of FeNi generated the 2[FeNi](+) -> [Fe2Ni2](2+) equilibrium in solution; crystallization yields only the [Fe2Ni2](2+) dimer, isolated as PF6- and BArF- salts. The monomer is a spin-coupled diradical between {Fe(NO)}(7) and Ni-DT(+), while dimerization couples the two NiDT+ via a Ni2S2 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 (chi(T)(M) = 0.67 emu.K.mol(-1), mu(eff) = 2.31 mu(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.

Automated summary

Inspired by strategies found in nature, sulfur-bridged Fe-Ni heterobimetallics have been synthesized and exhibit intricate physical properties. Experimental and theoretical investigations have revealed the structures, redox reactions, and magnetic properties of these compounds.