Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe 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 Fe complexes [(MeTPyA)Fe(L)] (MeTPyA = tris(6-methyl-2-pyridylmethyl)amine; = 2: LH = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, LH = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na[L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; = 4: L = 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 Fe centers in the oxidized species, with exchange constants of = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO, SMe) cm. 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 = -57(10), -60(7), -58(6), and -65(8) cm for R = OMe, Cl, NO, and SMe, respectively. The minimal effects of substituents in the 3- and 6-positions of L 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 = 50(1), 41(1), 38(1), and 33(1) cm for R = OMe, Cl, NO, and SMe, 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.