Spin‐Crossover and Field‐Induced Single‐Molecule Magnet Behaviour in Metal(II)‐Dipyrazolylpyridine Complexes

The reaction between the 2,6‐di(pyrazol‐1‐yl)‐4‐(bromomethyl)pyridine (L) ligand and divalent transition metal salts led to the formation of three mononuclear complexes of formula [M(L)2](BF4)2·xMeNO2 [M = FeII (1·xMeNO2), x = 0 and 4, M = CoII, x = 2 (2·2MeNO2) and]. Static magnetic measurements highlighted thermal spin crossover at high temperature (320–340 K) for the two FeII solvatomorphs nevertheless both the solvation degree and crystal packing had drastic influence on the shape and width of the hysteresis loop. Thus the 1 displayed abrupt and straight (6 K) hysteresis loop while for 1·4MeNO2 the thermal spin transition could be described as a six‐step gradual transition with the widest magnetic bistability range (35 K) for this kind of complexes. Irradiation at 530 nm of 1·4MeNO2 provoked an efficient LIESST effect at low temperature (TLIESST = 55 K). Finally dynamic magnetic measurements of 2·2MeNO2 shown a field‐induced mononuclear Single‐Molecule Magnet behavior with Δ = 14.6(4) cm–1 and τ0 = 6.28(8) × 10–7 s.