Slow magnetic relaxation in hexacoordinated cobalt(ii) field-induced single-ion magnets

To gain a better insight into the factors affecting the enhancement of the energy barrier in single molecule (single ion) magnets, three hexacoordinate cobalt(ii) complexes based on the tridentate ligand 2,6-bis(pyrazol-1-yl)pyridine (pypz) and pseudohalide ions have been synthesized and investigated. It was found that dicyanoamido and azido ligands act as bridges to form a one dimensional network based on a single mu(1,5)-dca bridge [Co(pypz)(dca)(H2O]center dot dca (1) and dimer [Co-2(pypz)(2)(mu(1,1)-N-3)(2)(N-3)(2)]center dot 2CH(3)OH (2), while tcm(-)counterbalanced the charge of [Co(pypz)(2)](2+)in [Co(pypz)(2)](tcm)(2)(3), where dca(-)= dicyanamide ion; tcm(-)= tricyanomethanide ion, respectively. The DC magnetic data show a sizable magnetic anisotropy, which was confirmed by high-field/high frequency EPR measurements. Two of them are SIMs (1and3) and the other one is a SMM (2). All complexes exhibit field induced slow magnetic relaxation with two (1and2) or three relaxation channels (3) and an exceptionally long relaxation time for the low-frequency channel upon application of an external fieldB(DC)= 0.4, 0.3, and 0.4 T atT= 1.9 K;tau(LF) = 1.9, 2.1 and 0.9 s, respectively. Additionally, the high spin-reversal barriers,U= 103 K for1and 95 K for2, are among the largest for field-induced SIMs for cobalt(ii) reported in the literature.