Interlayer interaction of two-dimensional layered spin crossover complexes [FeIIH3 LMe] [FeIILMe]X (X- = CIO4-, BF4-, PF6-, AsF6-, and SbF6-; H3LMe = tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine)

A series of two-dimensional (2D) spin crossover complexes, [(FeH3LMe)-H-II][(FeLMe)-L-II]X (X- = ClO4-, BF4-, PF6-, AsF6-, SbF6-} 1-5, have been synthesized, where H3LMe denotes an hexadentate N-6 tripodlike ligand containing three imidazole groups, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. Compounds 1-5 exhibit a two-step (HS-[(FeH3LMe)-H-II](2+) + HS-[(FeLMe)-L-II](-)) <----> (HS-[(FeH3LMe)-H-II](2+) + LS-[(FeLMe)-L-II](-)) <----> (LS-[(FeH3LMe)-H-II](2+) + LS-[(FeLMe)-L-II](-)) spin-transition. The crystal structure of [(FeH3LMe)-H-II][(FeLMe)-L-II]PF6 (3) was determined at 295, 200, and 100 K. The structure consists of homochiral extended 2D puckered sheets, in which the complementary [(FeH3LMe)-H-II](2+) and [(FeLMe)-L-II](-) 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 Fell sites of both constituting units are in the high-spin (HS) state at 295 K; at 200 K, the Fell sites of [(FeH3LMe)-H-II](2+) and [FeIILMe](-) are in the HS and low-spin (LS) states, respectively. The Fell 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-[(FeH3LMe)-H-II](2+) + LS-[(FeLMe)-L-II](-)) 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.