Thermal Spin Crossover and LIESST Effect Observed in Complexes [Fe(L^Ch)₂(NCX)₂] [L^Ch = 2,5-Di(2-Pyridyl)-1,3,4-Chalcadiazole; Ch = O, S, Se; X = S, Se, BH₃]

Two new complexes belonging to the family [FeII(L)2(NCX)2] have been synthesized and structurally characterized. Complexation of the ligand LO = 2,5-di­(2-pyridyl)-1,3,4-oxadiazole results in the formation of two derivatives of [FeII(LO)2(NCS)2] (1) with the common s-trans and the rarely observed s-cis conformation. No thermal spin crossover (SCO) was observed for the amorphous bulk material of the mixture. Using the new ligand LSe = 2,5-di­(2-pyridyl)-1,3,4-selenadiazole, compound [FeII(LSe)2(NCS)2]·1.4DCM·0.6MeOH (2·1.4DCM·0.6MeOH) was structurally characterized by single crystal X-ray diffraction. Bulk material of [FeII(LSe)2(NCS)2]·MeOH (2·MeOH) exhibits a thermally induced SCO with small hysteresis [T1/2(↓) = 91 K, T1/2(↑) = 96 K]. LIESST and reflectivity studies have been performed on the SCO complexes [FeII(LS)2(NCS)2], [FeII(LS)2(NCSe)2], [FeII(LS)2(NCBH3)2]·H2O [LS = 2,5-di­(2-pyridyl)-1,3,4-thiadiazole], and 2·MeOH. All complexes belong to the T0 = 90 K line [T(LIESST) = T0 – 0.3T1/2]. [FeII(LS)2(NCS)2], that exhibits a two-stepped thermal SCO process, has been found to also exhibit two well-separated T(LIESST) temperatures [T(LIESST, 1) = 44 K; T(LIESST, 2) = 53 K].