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]

Abstract

Two new complexes belonging to the family [Fe^II(<b>L</b>)₂(NCX)₂] have been synthesized and structurally characterized. Complexation of the ligand <b>L</b>^<b>O</b> = 2,5-di­(2-pyridyl)-1,3,4-oxadiazole results in the formation of two derivatives of [Fe^II(<b>L</b>^<b>O</b>)₂(NCS)₂] (<b>1</b>) 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 <b>L</b>^<b>Se</b> = 2,5-di­(2-pyridyl)-1,3,4-selenadiazole, compound [Fe^II(<b>L</b>^<b>Se</b>)₂(NCS)₂]·1.4DCM·0.6MeOH (<b>2</b>·1.4DCM·0.6MeOH) was structurally characterized by single crystal X-ray diffraction. Bulk material of [Fe^II(<b>L</b>^<b>Se</b>)₂(NCS)₂]·MeOH (<b>2</b>·MeOH) exhibits a thermally induced SCO with small hysteresis [<i>T</i>_1/2(↓) = 91 K, <i>T</i>_1/2(↑) = 96 K]. LIESST and reflectivity studies have been performed on the SCO complexes [Fe^II(<b>L</b>^<b>S</b>)₂(NCS)₂], [Fe^II(<b>L</b>^<b>S</b>)₂(NCSe)₂], [Fe^II(<b>L</b>^<b>S</b>)₂(NCBH₃)₂]·H₂O [<b>L</b>^<b>S</b> = 2,5-di­(2-pyridyl)-1,3,4-thiadiazole], and <b>2</b>·MeOH. All complexes belong to the <i>T</i>₀ = 90 K line [<i>T</i>(LIESST) = <i>T</i>₀ – 0.3<i>T</i>_1/2]. [Fe^II(<b>L</b>^<b>S</b>)₂(NCS)₂], that exhibits a two-stepped thermal SCO process, has been found to also exhibit two well-separated <i>T</i>(LIESST) temperatures [<i>T</i>(LIESST, 1) = 44 K; <i>T</i>(LIESST, 2) = 53 K]