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A series of two-dimensional (2D) spin crossover complexes, FeIIH3L(Me)FeIIL(Me)X (X-=ClO4-, BF4-, PF6-, AsF6-, SbF6-) 1-5, have been synthesized, where H3L(Me) denotes an hexadentate N6 tripodlike ligand containing three imidazole groups, tris2-(((2-methylimidazol-4-yl)methylidene)amino)ethylamine. Compounds 1-5 exhibit a two-step (HS-FeIIH3L(Me)(2+) + HS-FeIIL(Me)-) (HS-FeIIH3L(Me)(2+) + LS-FeIIL(Me)-) (LS-FeIIH3L(Me)(2+) + LS-FeIIL(Me)-) spin-transition. The crystal structure of FeIIH3L(Me)FeIIL(Me)PF6 (3) was determined at 295, 200, and 100 K. The structure consists of homochiral extended 2D puckered sheets, in which the complementary FeIIH3L(Me)(2+) and FeIIL(Me)- capped tripodlike components, linked together by imidazole-imidazolate hydrogen bonds, are alternately arrayed in an up-and-down mode. The Fe-N bond distances and angles revealed that the FeII sites of both constituting units are in the high-spin (HS) state at 295 K; at 200 K, the FeII sites of FeIIH3L(Me)(2+) and FeIIL(Me)- are in the HS and low-spin (LS) states, respectively. The FeII sites of both constituting units are in the LS state at 100 K. The size of the counteranion affects significantly the intra- and interlayer interactions leading to modifications of the spin crossover behavior. The onset of the second spin-transition of the ClO4- (1) and BF4- (2) salts adjoins the first spin-transition, while a mixed (HS-FeIIH3L(Me)(2+) + LS-FeIIL(Me)-) spin-state spans a temperature range as wide as 70 K for salts 3-5 with larger counteranions, PF6-, AsF6-, and SbF6-, respectively. Compounds 1 and 2 showed remarkable LIESST (light induced excited spin state trapping) and reverse-LIESST effects, whereas 3-5 showed no remarkable LIESST effect. The interlayer interaction due to the size of the counteranion is an important factor governing the spin crossover behavior and LIESST effect.
Yamada et al. (Sat,) studied this question.