Cooperative Spin Transition in the Two-Dimensional Coordination Polymer [Fe(4,4′-bipyridine)2(NCX)2)•4CHCl3 (X = S, Se)

Two new isostructural two-dimensional (2D) coordination polymers exhibiting spin crossover (SCO) behavior of formulation [Fe(4,4'-bipy)(2)(NCX)(2)]center dot 4CHCl(3) (4,4'-bipy = 4,4'-bipyriciine; X = S [1 center dot 4CHCl(3)], Se [2 center dot 4CHCl(3)]) have been synthesized and characterized, and both undergo cooperative spin transitions (ST). For 1 center dot 4CHCl(3) the ST takes place in two steps with critical temperatures of Tc1(down) = 143.1 K, T-c2(down) = 91.2 K, T-c1(up) = 150.7 K, and T-c2(uP) = 112.2 K. 2 center dot 4CHCl(3) displays half ST characterized by T-c(down) = 161.7 K and T-c(up) = 168.3 K. The average enthalpy and entropy variations and cooperativity parameters associated with the ST have been estimated to be Delta H-1(av) = 5.18 kJ mol(-1), Delta S-1(av) = 35 J K-1 mol(-1) and Gamma(1) = 2.8 kJ mol(-1) and Delta H-2(aw) = 3.55 kJ mol(-1) Delta S-2(av) = 35 J K-1 mol(-1) and Gamma(2) = 2.6 kJ mol(-1) for 1 center dot 4CHCl(3), and Delta H-av = 6.25 kJ mol(-1), Delta S-av = 38.1 J K-1 mol(-1) and Gamma = 3.2 kJ mol(-1) for 2 center dot 4CHCl(3). At T > [T-c1 (1 center dot 4CHCl(3)); T-c (2 center dot 4CHCl(3))], both compounds are in the space group P2/c while at T < [T-c1 (1 center dot 4CHCl(3)); T-c (2 center dot 4CHCl(3))] they change to the C2/c space group and display an ordered checkerboard-like arrangement of iron(II) sites where the high- and low-spin states coexist at 50%.