A series of tetraazalene radical-bridged M (M = Cr, Mn, Fe, Co) complexes with strong magnetic exchange coupling.

The ability of tetraazalene radical bridging ligands to mediate exceptionally strong magnetic exchange coupling across a range of transition metal complexes is demonstrated. The redox-active bridging ligand ,','','''-tetra(2-methylphenyl)-2,5-diamino-1,4-diiminobenzoquinone (LH) was metalated to give the series of dinuclear complexes [(TPyA)M(L)] (TPyA = tris(2-pyridylmethyl)amine, M = Mn, Fe, Co). Variable-temperature dc magnetic susceptibility data for these complexes reveal the presence of weak superexchange interactions between metal centers, and fits to the data provide coupling constants of = -1.64(1) and -2.16(2) cm for M = Mn and Fe, respectively. One-electron reduction of the complexes affords the reduced analogues [(TPyA)M(L˙)]. Following a slightly different synthetic procedure, the related complex [(TPyA)CrIII2(L˙)] was obtained. X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy indicate the presence of radical L˙ bridging ligands in these complexes. Variable-temperature dc magnetic susceptibility data of the radical-bridged species reveal the presence of strong magnetic interactions between metal centers and ligand radicals, with simulations to data providing exchange constants of = -626(7), -157(7), -307(9), and -396(16) cm for M = Cr, Mn, Fe, and Co, respectively. Moreover, the strength of magnetic exchange in the radical-bridged complexes increases linearly with decreasing M-L bond distance in the oxidized analogues. Finally, ac magnetic susceptibility measurements reveal that [(TPyA)Fe(L˙)] behaves as a single-molecule magnet with a relaxation barrier of = 52(1) cm. These results highlight the ability of redox-active tetraazalene bridging ligands to enable dramatic enhancement of magnetic exchange coupling upon redox chemistry and provide a rare opportunity to examine metal-radical coupling trends across a transmetallic series of complexes.