Two-step spin crossover behaviour in the chiral one-dimensional coordination polymer [Fe(HAT)(NCS)(2)](infinity)

Solvated and unsolvated forms of the complex [Fe(HAT)(NCS)(2)](infinity)center dot(nMeOH) (1) (n = 1.5, 0; HAT = 1,4,5,8,9,12-hexaazatriphenylene) were prepared. The structure of 1 center dot(1.5MeOH), measured at 120 K, showed that this system crystallizes in the homochiral P4(3) tetragonal space group. The solid is constituted of stacks of one-dimensional coordination polymers running along c-axis. All the Fe-II centres have the same Lambda or Delta conformation and are in the LS state at 120 K. In the range of temperatures 10-300 K the magnetic properties of 1 center dot(1.5MeOH) shows the occurrence of reversible spin crossover behaviour. However, above ca. 310 K complete desolvation of 1 center dot(1.5MeOH) to give 1 was observed from crystal structure analysis, magnetic behaviour and thermal analysis. Compound 1 displays a two-step spin crossover behaviour characterised by a plateau 60 K wide. Simulation of the two-step behaviour in the frame of the regular solutions theory afforded, respectively, the critical temperatures (T-ci), the interaction parameters (Gamma(i)), and the enthalpy (Delta H-i) and entropy (Delta S-i) variations for steps i = 1 and 2: T-c1(T-c2) = 172 (358) K, Gamma(1)(Gamma(2)) = 1.6 (3.0) kJ mol(-1), Delta H-1(Delta H-2) = 5.7 (18.3) kJ mol(-1) and Delta S-1(Delta S-2) = 33.4 (51.0) J K mol(-1).