Cooperative spin-crossover behaviour in polymeric 1D FeII coordination compounds:: [{Fe(tba)3}X2]•nH2O

A new family of ID cooperative spin-crossover polymers with general formula [IFe(tba)(3))X-2].nH(2)O [tba = N-(4H-1,2,4triazol-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)].3H(2)0 (7), has also been synthesised and its crystal structure solved at 293 K. Compound 7 crystallises in the Pi 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 C-2, 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 down arrow = 250 and Tc up arrow = 258 K in 2 and T-c down arrow = 226 and Tc up arrow = 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 Delta H = 17.8 (2) and 14.6 kJmol(-1) (3), and Delta S = 67.2 (2) and 63.5 JK(-1)mol(-1) (3). In contrast, the dehydrated compounds 4, 5 and 6 show very different spintransition 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 (Delta H = 19.7 kJ mol(-1) and Delta S = 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). (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)