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(N₂S₂), reacts with a labile ligand derivative of [Ni^II(S₂C₂Ph₂)]⁰, Ni_DT, yielding the expected S-bridged neutral adduct, <b>FeNi</b>, containing a doublet {Fe(NO)}⁷. Good reversibility of two redox events of <b>FeNi</b> led to isolation of reduced and oxidized congeners. Characterization by various spectroscopies and single-crystal X-ray diffraction concluded that reduction of the <b>FeNi</b> parent yielded <b>[FeNi]</b>^-, 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 <b>FeNi</b> generated the 2<b>[FeNi]</b>⁺⇌<b>[Fe₂Ni₂]</b>²⁺ equilibrium in solution; crystallization yields only the <b>[Fe₂Ni₂]</b>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 Ni_DT⁺ 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 (χ_MT = 0.67 emu·K·mol^-1, <i>µ</i>_eff = 2.31 <i>µ</i>_B), and detectable by parallel-mode EPR spectroscopy at 20 to 50 K. A theoretical model built on an H₄ chain explains this unexpected low energy triplet state arising from a combination of anti- and ferromagnetic coupling of a four-radical molecular conglomerate.