COMPARATIVE INVESTIGATION OF THE SPIN-CROSSOVER COMPOUNDS FE(BTZ)2(NCS)2 AND FE(PHEN)2(NCS)2 (WHERE BTZ = 2,2'-BI-4,5-DIHYDROTHIAZINE AND PHEN = 1,10-PHENANTHROLINE) - MAGNETIC-PROPERTIES AND THERMAL DILATATION BEHAVIOR AND CRYSTAL-STRUCTURE OF FE(BTZ)2(NCS)2 AT 293 AND 130-K

The crystal structure of Fe(btz)2(NCS)2 (1) was determined by X-ray diffraction at almost-equal-to 293 and almost-equal-to 130 K in order to detect the structural changes associated with the spin transition. The space group is Pbcn with Z = 4 at both temperatures. Lattice constants are as follows: a = 13.2880 (I 1) angstrom, b = 10.86 10 (I 1) angstrom, and c = 16.9199 (19) angstrom at almost-equal-to 293 K and a = 13.0551 (17) angstrom, b = 10.6504 (17) angstrom, and c = 16.6717 (39) angstrom at 130 K. The data were refined to R = 0.059 (0.062) at almost-equal-to 293 K (almost-equal-to 130 K) for 1102 (1095) observed independent reflections ((Fo)2 > 2.5sigma(Fo)2). These results compare with those previously reported for Fe(phen)2(NCS)2(2). However, the temperature dependence of (chiM)T (chiM = molar magnetic susceptibility and T = temperature) is different for the two compounds: 2 shows a very abrupt singlet <-- --> quintet spin conversion and 1 a gradual one. At the molecular scale, the structural modifications associated with the spin change are also found to be similar in 1 and 2; they mainly consist in a large reorganization of the iron(II) environment: when the temperature is lowered from 293 to 130 K, the Fe-N(L) (L = btz, phen) and Fe-N(CS) distances decrease by 0.20 (mean value) and 0.1 0-0. 1 1 angstrom, respectively, and a noticeablc variation of the N-Fe-N angles, leading to a more regular shape of the [Fe-N6] octahedron, is observed. Moreover, in both compounds, the molecular packing may be described as sheets of molecules parallel to the a-b plane. However, the framework of the intermolecular interactions is different in 1 and 2, and the reinforcement of these interactions upon the high-spin --> low-spin conversions is more pronounced between consecutive sheets in 1 and between molecules of the same sheet in 2, leading to a higher bidimensional character for the latter compound. This difference in structural anisotropy, which might be at the origin of the difference in cooperativity of the two spin transitions, is reflected on the thermal variation of the lattice parameters measured for both complexes in the 130-293 K temperature range: in 1, all the parameters evolve continuously; in 2 the evolution of the a and b parameters clearly shows a discontinuity in the close vicinity of the spin transition, whereas the c parameter varies continuously. The values found for the total variation of the unit cell volume are of the same order of magnitude for the two compounds: DELTAV = 124 angstrom3 for 1 and 119 angstrom3 for 2. The same holds for the values of the component DELTAV(SC) (which corresponds to the spin-crossover alone), estimated at 89 angstrom3 for 1 and 72 angstrom3 for 2 in the case of a complete spin conversion.