| Research Abstract |
Spin-crossover(SC)between the low-spin(LS)and high-spin(HS)states is the representative example of molecular bistability and can be utilized for information processing. The interaction between SC sites or the cooperativity governs the physical properties of SC such as the abruptness, multi-steps, and hysteresis. The rational molecular design is the important subject for SC materials. We report several families of SC compounds such as [Fe^<II>H_3L<Me>] Cl-X(X- = PF_6-, AsF_6-, SbF_6-, CF_3SO_3-). The complexes involved in one family were not only crystallized in the same monoclinic crystal system with the similar cell dimensions, but also assume the same space group P2_1/n at the HS state, P2_1 at the HS + LS state, and P2_1/n at the LS state. The general crystal structure consists of a 2D extended network structure constructed by NH... Cl- hydrogen bonds between the Cl- and the imidazole NH groups of three neighboring cations [Fe^<II>H_3L<Me>]^<2+>, while the anion X exists just as an isolated counter anion and occupies the space among the 2D sheets. Nevertheless the only counter anion is different in these complexes, a variety of SC behaviors, 1: a one-step SC of 2HS 〓 HS + LS, 2 : a two-step SC of 2HS HS + LS 〓2LS, 3 : a gradual one-step SC of HS 〓 LS, and 4 : a steep one-step SC of HS 〓 LS with hysteresis, has been observed. The molecular volumes of the counter anions are : V = 53.4 A^3 for BF_4-, V = 54.4 A^3 for C1O_4-, V = 73.0 A^3 for PF_6-, V = 78.5 A^3 for AsF_6-, V 88.7 A^3 for SbF_6-, V = 86.9 A^3 for CF_3S0_3-, and V = 94.4 A^3 for I_3-. These estimated relative sizes of the counter anions correlate fairly well with the SC behaviors. The present SC complexes supply the most useful model-compounds to experimentally and theoretically investigate the exact microscopic origin of the multi-step spin transition
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