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RESEARCH PRODUCT

Cooperative Spin‐Crossover Behaviour in Polymeric 1D Fe II Coordination Compounds: [{Fe(tba) 3 }X 2 ]· n H 2 O

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A new family of 1D cooperative spin-crossover polymers with general formula [{Fe(tba)3}X2]·nH2O [tba = N-(4H-1,2,4-triazol-4-yl)benzamide; X = CF3SO3–, n = 2 (1), n = 0 (4); BF4–,n = 3 (2), n = 0 (5); 4-CH3C6H4SO3–, n = 3 (3), n = 0 (6)] has been synthesised and characterised using a series of spectroscopic methods, X-ray powder diffraction, magnetic susceptibility measurements and differential scanning calorimetry. The copper analogue of 1, [{Cu(tba)3}(CF3SO3)2]·3H2O (7), has also been synthesised and its crystal structure solved at 293 K. Compound 7 crystallises in the P space group. The bidentate N-(4H-1,2,4-triazol-4-yl)benzamide ligand bridges the copper ions through the 1,2-nitrogen positions leading to a one-dimensional infinite polymer. The EXAFS data collected at 80 K for 1 agree perfectly with a linear chain structure, as observed for 7, with [Fe(tba)3] chromophores linked to each other through the adjacent nitrogen atoms of three N-(4H-1,2,4-triazole-4-yl)benzamide ligands. IR spectroscopic studies confirm the retention of the C2v symmetry for the triazole rings in 2 and 3, thereby suggesting a similar chainlike arrangement in these compounds. Compound 1 is low spin (LS) at room temperature while 2 and 3 are high spin (HS). The latter two complexes undergo cooperative thermal spin transition at Tc = 250 and Tc = 258 K in 2 and Tc = 226 and Tc = 238 K in 3. The spin transition in 2 and 3 is accompanied by a change of colour from white in the HS state to violet in the LS state. The thermodynamic parameters associated with the spin transition estimated from DSC measurements are ΔH = 17.8 (2) and 14.6 kJ mol–1 (3), and ΔS = 67.2 (2) and 63.5 J K–1 mol–1 (3). In contrast, the dehydrated compounds 4, 5 and 6 show very different spin-transition behaviour. Complex 4 exhibits an abrupt spin transition centred at room temperature and is accompanied by a relatively large hysteresis loop with a width of 25 K (ΔH = 19.7 kJ mol–1 and ΔS = 65.6 J K–1 mol–1). Like 2 and 3, 4 shows a dramatic colour change upon the spin transition. Complex 5 shows a continuous and incomplete spin transition, and 6 is HS over the whole temperature range investigated (10–400 K). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)