A series of tetraazalene radical-bridged M₂ (M = Cr^III, Mn^II, Fe^II, Co^II) 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 (^NMePhLH₂) was metalated to give the series of dinuclear complexes [(TPyA)₂M₂(^NMePhL^2-)]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 = Mn^II and Fe^II, respectively. One-electron reduction of the complexes affords the reduced analogues [(TPyA)₂M₂(^NMePhL^3-•)]⁺. Following a slightly different synthetic procedure, the related complex [(TPyA)₂CrIII2(^NMePhL^3-•)]³⁺ was obtained. X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy indicate the presence of radical ^NMePhL^3-• 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)₂Fe₂(^NMePhL^3-•)]⁺ 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.