A new 3-D polymeric spin transition compound: [tris(1,4-bis(tetrazol-1-yl)butane-N1,N1′)iron(II)] bis(perchlorate)

Abstract

A series of novel polymeric compounds of formula [M(btzb)₃][ClO₄]₂ (M^II = Fe, Ni or Cu) with btzb = 1,4-bis(tetrazol-1-yl)butane have been prepared and their physical properties investigated. The btzb ligand has been prepared and its crystal structure determined, together with a tentative crystal structure of the 3-D compound [Fe(btzb)₃][ClO₄]₂. The model of the latter shows two symmetry-related, interpenetrating Fe–btzb networks in which the iron(II) ions approach each other as close as 8.3 and 9.1 Å. This supramolecular catenane undergoes a sharp thermal spin transition around 160 K with hysteresis (20 K) along with a pronounced thermochromic effect. The spin crossover behaviour has been followed by magnetic, DSC, optical spectroscopy and ⁵⁷Fe Mössbauer spectroscopy measurements. Irradiation with green light at low temperature leads to population of the metastable high-spin state for the thermally active iron(II) ions. The nature of the spin crossover behaviour has been discussed in detail.