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

By M. Quesada, H. Kooijman, P. Gamez, J. Sánchez Costa, P.J. van Koningsbruggen, P. Weinberger, M. Reissner, A.L. Spek, J.G. Haasnoot and J. Reedijk


The reaction of btzmp (1,2-bis(tetrazol-1-yl)-2-methylpropane) with Fe(ClO4)2 generates a 1D polymeric species, [Fe(µ-btzmp)2(btzmp)2](ClO4)2, showing a steep spin transition (T1/2 = 136 K and T1/2 = 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]–ND–Fe angle (which is typical of bistetrazole spin-transition materials). Apart from X-ray analysis, also magnetic susceptibility, Mössbauer and UV-vis spectroscopies have been used to characterise the HS and the LS states of [Fe(µ-btzmp)2(btzmp)2](ClO4)2

Year: 2007
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