⁵⁷Fe Mössbauer spectroscopy study of a 2D spin transition coordination polymer built from a tris-1R-tetrazole ligand

[Fe{N(entz)₃}₂](ClO₄)₂ (N(entz)₃ = tris(2-(1H-tetrazol-1-yl)ethyl)-amine) is a 2D Fe^II coordination polymer built from a tris-1-R tetrazole building block. This thermochromic Fe^II complex which was investigated by variable temperature ⁵⁷Fe Mossbauer spectroscopy (78–300 K) displays on cooling a complete, abrupt and hysteretic spin transition at $T_c^{\uparrow }=170\left(1\right)$ K and $T_c^{\downarrow }=149\left(1\right)$ K. The reversibility of the spin transition over several thermal cycles was deduced from SQUID magnetometry. Differential scanning calorimetry reveals a reversible first order phase transition on cooling below room temperature, with entropy data consistent with the highly cooperative character of the spin transition.