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 [Fe(II)(L)2(NCX)2] have been synthesized and structurally characterized. Complexation of the ligand L(O) = 2,5-di(2-pyridyl)-1,3,4-oxadiazole results in the formation of two derivatives of [Fe(II)(L(O))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 L(Se) = 2,5-di(2-pyridyl)-1,3,4-selenadiazole, compound [Fe(II)(L(Se))2(NCS)2]·1.4DCM·0.6MeOH (2·1.4DCM·0.6MeOH) was structurally characterized by single crystal X-ray diffraction. Bulk material of [Fe(II)(L(Se))2(NCS)2]·MeOH (2·MeOH) exhibits a thermally induced SCO with small hysteresis [T(1/2)(↓) = 91 K, T(1/2)(↑) = 96 K]. LIESST and reflectivity studies have been performed on the SCO complexes [Fe(II)(L(S))2(NCS)2], [Fe(II)(L(S))2(NCSe)2], [Fe(II)(L(S))2(NCBH3)2]·H2O [L(S) = 2,5-di(2-pyridyl)-1,3,4-thiadiazole], and 2·MeOH. All complexes belong to the T0 = 90 K line [T(LIESST) = T0 - 0.3T(1/2)]. [Fe(II)(L(S))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].