Subtlety of the Spin-Crossover Phenomenon Observed with Dipyridylamino-Substituted Triazine Ligands

Reactions of the new, closely related ligands 4,6-dichloro-N,N-di(pyridine-2-yl)-1,3,5-triazine-amine (Cldpat) and 6-chloro-N'-phenyl-N, N-di(pyridin-2-yl)-1,3,5-triazine-2,4-di-amine (Cladpat) with iron(II) thiocyanate produced coordination compounds with drastically distinct magnetic properties. The compound trans-[Fe(Cldpat)(2)(NCS)(2)](H2O) (1) is a high-spin complex from room temperature down to 5 K whereas the analogous compound trans-[Fe(Cladpat)(2)(NCS)(2)] (2) exhibits spin-crossover (SCO) properties with T-1/2 = 178 K. Compounds 1 and 2 (both in its low-spin and high-spin states) have been structurally characterized by X-ray diffraction studies, which revealed identical metal coordination spheres. The SCO properties of 2 have been thoroughly investigated by temperature-dependent magnetic susceptibility measurements and differential scanning calorimetry (DSC), and a LIESST process with rapid relaxation of the trapped HS species has been observed. The equivalent coordination compound with selenocyanate anions, namely [Fe(Cladpat)(2)(NCSe)(2)] (3) also displays SCO properties, although more gradual and with a lower T-1/2 value of 166 K.