Antiferromagnetic Exchange and Metal-Metal Bonding in Roussin's Black Sulfur and Selenium Salts.

Atom-efficient syntheses of the tetraethylammonium Roussin black sulfur and selenium salts ((Et4N)[Fe4E3(NO)7], E = S, Se) as well as their 15N-labeled counterparts are described herein. Broken-symmetry DFT calculations were conducted on both complexes to model an antiferromagnetic interaction between the apical {FeNO}7 unit, Sap = 3/2, and the three basal {Fe(NO)2}9 units, Sbas = 1/2. The calculated J values are -1813 and -1467 cm-1 for the sulfur and selenium compounds, respectively. The mechanism for antiferromagnetic exchange in both compounds was deduced to be direct exchange on the basis of the partially overlapping magnetic orbitals with orbital density only residing on the Fe-centers. The obtained Mossbauer parameters are most consistent with the calculated MS = 0 broken-symmetry state for both complexes. The values for J have been determined with variable-temperature 15N NMR experiments. Values of -1660 and -1430 cm-1 for the sulfur and selenium compounds, respectively, were obtained by fits to the variable-temperature NMR data, further validating the broken-symmetry MS = 0 model of the electronic structure.