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

Abstract

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.