EXAFS AND X-RAY-POWDER DIFFRACTION STUDIES OF THE SPIN TRANSITION MOLECULAR MATERIALS [FE(HTRZ)A(TRZ)](BF4) AND [FE(HTRZ)(3)](BF4)(2)CENTER-DOT-H2O (HTRZ=1,2,4-4H-TRIAZOLE, TRZ=1,2,4-TRIAZOLATO)

The goal of this paper is to obtain structural information on the comp ounds [Fe(Htrz)(2)(trz)](BF4) (modifications 1a and 1b) and [Fe(Htrz)( 3)](BF4)(2) . H2O (2) with Htrz = 1,2,4-4H-triazole and trz = 1,2,4-tr iazolato. These compounds, in particular 1a, exhibit cooperative spin transitions between low-spin (LS) and high-spin (HS) states around roo m temperature, with large thermal hysteresis and well-pronounced therm ochromic effect. Both EXAFS spectroscopy at the iron K-edge and X-ray powder diffraction have been used. The EXAFS study has also been conce rned with two model compounds, the crystal structures of which were kn own, namely, [Fes(p-MeOptrz)(8)(H2O)(4)](BF4)(6) (3) and [Fes(p-MeOptr z)(6)(H2O)(6)] (tos)(6) . 4H(2)O (4), with p-MeOptrz = 4-p-methoxyphen yl-1,2,4-triazole and tos = tosylate. When the LS --> HS transition oc curs, as expected the length of the Fe-N bonds is increased by about 0 .18 Angstrom, and the FeN6 core is more distorted. The spectra of all the compounds in the LS state show a very characteristic peak around 7 Angstrom, which cannot be interpreted in the frame of the single scat tering approach of the standard EXAFS formula. This peak has been assi gned to a three-atom path involving aligned iron atoms. These EXAFS da ta perfectly agree with a linear chain structure for both 1 and 2, wit h Fe(Htrz)(6) chromophores linked to each other through the 1,2 nitrog en positions of three triazole ligands. The stoichiometry of 1 require s a proton-hole disordered over each Fe(Htrz)(2)(trz)Fe bridging netwo rk. The X-ray data have provided two important pieces of information: first, the spin transition in both 1a and 1b most likely occurs withou t change of space group; second, the two modifications 1a and 1b do no t crystallize in the same space group, the lattice symmetry of la bein g lower. The relation between these structural data and the mechanism of the spin transition in these compounds is briefly discussed.