Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe₂ Complexes

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged FeII2 complexes [(Me3TPyA)2Fe2(RL)]n+ (Me3TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: OMeLH2 = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, ClLH2 = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na2[NO2L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; n = 4: SMe2L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between FeII centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO2, SMe2) cm-1. In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm-1 for R = OMe, Cl, NO2, and SMe2, respectively. The minimal effects of substituents in the 3- and 6-positions of RLx-• on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of Ueff = 50(1), 41(1), 38(1), and 33(1) cm-1 for R = OMe, Cl, NO2, and SMe2, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.