Large easy-axis magnetic anisotropy in a series of trigonal prismatic mononuclear cobalt(II) complexes with zero-field hidden single-molecule magnet behaviour: the important role of the distortion of the coordination sphere and intermolecular interactions in the slow relaxation

Abstract

The complexes [Co(L))X center dot S (X = CoCl42-, S = CH3CN (1); X = ZnCl42- 2, S = CH3OH (2)), [Co(L))X-2 center dot S (X = ClO4-, S = 2CH(3)OH (3) and X = BF4- (4)) and [Co(L(NCS)(2)] (5), where L = the N-6-tripodal ligand tris(pyridylhydrazonyl)phosphorylsulfide, were prepared and studied by X-ray crystallography, ac and dc magnetic data, FIRMS and HFEPR spectra, and theoretical calculations. On passing from 1 to 4, the change of the counteranion decreases slightly the distortion of the CoN6 coordination polyhedron from trigonal prismatic to octahedral, with a parallel increase of the easy-axis magnetic anisotropy. Compound 1 does not show slow magnetic relaxation, even in the presence of a dc magnetic field, due to fast QTM triggered by dipolar interactions. Although the complexes 2-4 show a weak frequency and temperature dependence of the ac susceptibility below 10 K at zero field, they exhibit slow relaxation and single-molecule magnet (SMM) behaviour under the corresponding optimal field. The relaxation of the magnetization takes place mainly through a Raman relaxation process above 4 K, whereas below this temperature QTM and/or direct processes dominate. The relaxation time increases with the parallel increase of the uniaxial anisotropy on passing from 1. to 4. The width of the hysteresis for the trigonal prismatic complexes at 0.4 K decreases in the order 3 > 2 > 4 > 1, which is due to combined effects of QTM relaxation and axial anisotropy. Magnetic dilution of complexes 3 and 4 with Zn-II triggers the slow relaxation of the magnetization at zero-field, so that these complexes can be considered as "hidden mononuclear SMMs". Compound 5, with a compressed octahedral geometry, exhibits easy-plane magnetic anisotropy (D = +34.7 cm(-1)), and it is a field-induced mononuclear SMM with magnetization relaxation faster than compounds 2-4 and a smaller hysteresis loop.