A series of tetraazalene radical-bridged M2 (M = CrIII, MnII, FeII, CoII) 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 N, N', N '', N '''-tetra(2-methylphenyl)-2,5-diamino-1,4-diiminobenzoquinone ((LH2)-L-NMePh) was metalated to give the series of dinuclear complexes [(TPyA)(2)M-2(NMePhL2-)](2+) (TPyA = tris(2-pyridylmethyl) amine, M = Mn-II, Fe-II, Co-II). 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 J = -1.64(1) and -2.16(2) cm(-1) for M = MnII and FeII, respectively. One-electron reduction of the complexes affords the reduced analogues [(TPyA)(2)M-2(NMePhL3-.)](+). Following a slightly different synthetic procedure, the related complex [(TPyA)(2)Cr-2(III) (NMePhL3-.)](3+) was obtained. X-ray diffraction, cyclic voltammetry, and Mossbauer spectroscopy indicate the presence of radical NMePhL3-. 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 J = -626(7), -157(7), -307(9), and -396(16) cm(-1) for M = Cr-III, Mn-II, Fe-II, and Co-II, 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)(2)Fe-2(NMePhL3-.)]+ behaves as a single-molecule magnet with a relaxation barrier of U-eff = 52(1) cm(-1). 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.