[Fe(μ-btzmp)2(btzmp)2](ClO4)2:: a doubly-bridged 1D spin-transition bistetrazole-based polymer showing thermal hysteresis behaviour

The reaction of btzmp (1,2-bis(tetrazol-1-yl)-2-methylpropane) with Fe(ClO4)(2) generates a 1D polymeric species, [Fe(mu-btzmp)(2)(btzmp)(2)](ClO4)(2), showing a steep spin transition (T-1/2 up arrow = 136 K and T-1/2 down arrow = 133 K) with a 3 K thermal hysteresis. The crystal structure at 100 and 200 K reveals that, in contrast to other bistetrazole based spin-transition systems such as [Fe(endi)(3)](BF4)(2) and [Fe(btzp)(3)](ClO4)(2), the present compound has only two ligands bridging the metallic centres, while the other two coordination positions are occupied by two mono-coordinated (non-bridging) btzmp ligands. This peculiarity confers an unprecedented crystal packing in the series of 1D bistetrazole based polymers. The change in spin state is accompanied by an order/disorder transition of the ClO4- counterion. A careful examination of the structural changes occurring upon the spin transition indicates that this order/disorder is most likely affected by the modification of the [tetrazole-centroid]-N-D-Fe angle (which is typical of bistetrazole spin-transition materials). Apart from X-ray analysis, also magnetic susceptibility, Mossbauer and UV-vis spectroscopies have been used to characterise the HS and the LS states of [Fe(mu-btzmp)(2)(btzmp)(2)](ClO4)(2).