Spin crossover in a supramolecular FeII-FeIII system

The structure of [FeII(H3L)](ClO4)(2).3H(2)O, where H3L is a <LF>tripodal hexadentate ligand derived from the 1:3 condensation of tris(2-aminoethyl)amine and 4-formylimidazole, has been determined by X-ray crystallography at 113 and 293 K. A spin transition was inferred from the Fe-N bond distances. The temperature dependence of the magnetic susceptibility revealed that the complex undergoes a gradual spin transition in the temperature range of 150-270 K. A mixed-valence <LF>complex, [Fe-II(H3L)][Fe-III(L)]-(BF4)(2).1.5H(2)O, was <LF>prepared by the controlled deprotonation of the protonated species, [Fe-II(H3L)](BF4)(2).1.5H(2)O, under aerobic conditions, and the X-ray structure was determined at 293 K. Two species, [Fe-II(H3L)](2+) and [Fe-III(L)], are linked by imidazole-imidazolate (NH...H) hydrogen bonds to form a puckered sheet structure. Magnetic susceptibility measurements and Mossbauer spectra provided an evidence for spin-crossover at both the Fe-II and Fe-III sites. There are three accessible electronic states: (LS Fe-II-LS Fe-III), (HS Fe-II-LS Fe-III), and (HS Fe-II-HS Fe-III) that occur in passing from lower to higher temperatures.