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‐diamine (Cladpat) with iron(II) thiocyanate produced coordination compounds with drastically distinct magnetic properties. The compound trans ‐[Fe(Cldpat) 2 (NCS) 2 ](H 2 O) ( 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.