Solvate-Dependent Spin Crossover and Exchange in Cobalt(II) Oxazolidine Nitroxide Chelates

Two oxazolidine nitroxide complexes of cobalt(II), [CoII(L•)2]- (B(C6F5)4)2·CH2Cl2 (1) and [CoII(L•)2](B(C6F5)4)2·2Et2O (2), where, L• is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [CoII(L•)2]- (NO3)2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures 1 displays low-spin Co(II)-to-radical exchange with differences in fitted J values in comparison to DFT (broken symmetry) calculated J values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co(dz2) and the trans-oriented O atoms of the NO radical moieties in L•. Spin crossover in 1 is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex 2 remains in the high-spin Co(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co−radical and radical−radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in 1 and 2 have yielded satisfying details on the mechanism of metal−radical and radical−radical exchange, the radical spins being in π*NO orbitals.