A series of tetraazalene radical-bridged M(2) (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 ((NMePh)LH(2)) was metalated to give the series of dinuclear complexes [(TPyA)(2)M(2)((NMePh)L(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)(2)M(2)((NMePh)L(3–)˙)](+). Following a slightly different synthetic procedure, the related complex [(TPyA)(2)CrIII2((NMePh)L(3–)˙)](3+) was obtained. X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy indicate the presence of radical (NMePh)L(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)(2)Fe(2)((NMePh)L(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.