Version 2 2020-03-20, 18:40Version 2 2020-03-20, 18:40
Version 1 2020-03-20, 16:34Version 1 2020-03-20, 16:34
journal contribution
posted on 2020-03-20, 18:40authored byAgnes
E. Thorarinsdottir, Ragnar Bjornsson, T. David Harris
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.