[Fe-II(TRIM)(2)]F-2, the first example of spin conversion monitored by molecular vibrations

The new [Fe-II(TRIM)(2)]F-2 spin-crossover complex (TRIM = 4-(4-imidazolylmethyl)-2-(2-imidazolylmethyl)imidazole) has been synthesized, crystallizing in the monoclinic system, space group P2/n, with Z = 2, a = 9.798(2) Angstrom, b = 8.433(2) Angstrom, c = 14.597(3) Angstrom, and beta = 90.46(1)degrees. The structure was solved by direct methods and refined to conventional agreement indices R = 0.032 and R(w) = 0.034 with 1378 unique reflections for which I > 3 sigma(I). The molecular structure consists of [Fe(TRIM)(2)](2+) complex cations hydrogen-bonded to six fluoride anions. The crystal packing results from this highly symmetrical and dense 3D network of hydrogen bonds. The coordination geometry of the iron(II) center can be described as a weakly distorted octahedron, including six nitrogen atoms originating from the two TRIM ligands coordinated to Fe(II) through their imine nitrogen atoms. Investigation of [Fe-II(TRIM)(2)]F-2 by magnetic susceptibility measurements and Mossbauer spectroscopy as a function of temperature indicates a 5% thermal variation of the spin fraction between 50 and 150 K, at variance with all previous litterature data. The spin conversion is gradual with 6% LS fraction below 50 K and less than 1% above 150 K. A theoretical approach based on the Ising-like model, completed with harmonic oscillators associated with the 15 vibration modes of the FeN6 coordination octahedron, successfully fits the data with an energy gap of similar to 40 K between the lowest LS and HS electrovibrational states, an average vibration frequency omega(LS) of 232 K in the LS state, and an average omega(LS)/omega(HS) ratio of 1.3. Taking these results into account, the computed molar entropy change Delta S associated with a complete conversion between the HS and LS states of Fe-II(TRIM)(2)F-2 (similar to 40 J . K-1. mol(-1)) is in fair agreement with the expected value.