Two ways spin crossover in iron(II) coordination polymer associated with conformational changes of bridging ligand

1,4-di(1-ethyl-1,2,3-triazol-5-yl)butane (bbtre) was prepared by lithiation of 1-ethyl-1,2,3-triazole followed then by alkylation with 1,4-dibromobutane. Bbtre forms with Fe(II) three-dimensional network [Fe(bbtre)3](ClO4)2·2CH3CN exhibiting thermally induced spin crossover (SCO). Change of temperature or change of spin state results in various types of structural transformations. It leads to formation of different structures which are stable in strictly defined temperature ranges. As a result, there are three spin crossover transitions arranged in two different paths. Thusly, cooling below 280 K involves HT(HS)→LT(HS) (HT – high temperature structure; LT – low temperature structure) phase transition (PT) which is associated with conformational changes of bbtre molecules and with deformation of the polymeric skeleton. In the LT phase there occurs incomplete and reversible LT(HS)↔LT(HS/LS) spin crossover. In contrast, rapid cooling (not previously thermally treated sample) allows to avoid HT(HS)→LT(HS) phase transition and there occurs complete HT(HS)→HT1(LS) SCO. It means that PT plays a role of switch which allows to choose one of two ways in which SCO will proceed. After rapid cooling, further heating to 150 K and subsequent cooling causes reversible HT1(HS)↔HT1(LS) spin crossover (T1/2↓= 130 K, T1/2↑= 131 K). However raising the temperature to 170-200 K leads to formation of modulated structure HT2(HS) exhibiting next reversible HT2(HS)↔HT2(LS) SCO (T1/2↓= 121 K, T1/2↑ =123 K). Finally heating above 200 K involves HT2(HS)→LT(HS) PT and resulted LT(HS) structure exhibits incomplete LT(HS)↔LT(HS/LS) spin crossover.