Thiosemiquinoid Radical-Bridged Cr ^III ₂ Complexes with Strong Magnetic Exchange Coupling

Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA) ₂ Cr ₂ ( ^R L ^4- )] ²⁺ ( ^O LH ₄ = 1,2,4,5-tetrahydroxybenzene, ^OS LH ₄ = 1,2-dithio-4,5-dihydroxybenzene, ^S LH ₄ = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between Cr ^III centers in these complexes, with exchange constants of J = -2.83(3) ( ^O L ^4- ), -2.28(5) ( ^OS L ^4- ), and -1.80(2) ( ^S L ^4- ) cm ^-1 . Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA) ₂ Cr ₂ ( ^R L ^3-• )] ³⁺ . Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal-semiquinoid radical coupling, with exchange constants of J = -352(10) ( ^O L ^3-• ), - 401(8) ( ^OS L ^3-• ), and -487(8) ( ^S L ^3-• ) cm ^-1 . These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.