Structure and Physical Properties of [μ-Tris(1,4-bis(tetrazol-1-yl)butane-N4,N4‘)iron(II)] Bis(hexafluorophosphate), a New Fe(II) Spin-Crossover Compound with a Three-Dimensional Threefold Interlocked Crystal Lattice

[μ-Tris(1,4-bis(tetrazol-1-yl)butane-N4,N4‘)iron(II)] bis(hexafluorophosphate), [Fe(btzb)₃](PF₆)₂, crystallizes in a three-dimensional 3-fold interlocked structure featuring a sharp two-step spin-crossover behavior. The spin conversion takes place between 164 and 182 K showing a discontinuity at about T_1/2 = 174 K and a hysteresis of about 4 K between T_1/2 and the low-spin state. The spin transition has been independently followed by magnetic susceptibility measurements, ⁵⁷Fe-Mössbauer spectroscopy, and variable temperature far and midrange FTIR spectroscopy. The title compound crystallizes in the trigonal space group P3̄ (No. 147) with a unit cell content of one formula unit plus a small amount of disordered solvent. The lattice parameters were determined by X-ray diffraction at several temperatures between 100 and 300 K. Complete crystal structures were resolved for 9 of these temperatures between 100 (only low spin, LS) and 300 K (only high spin, HS), Z = 1 [Fe(btzb)₃](PF₆)₂:  300 K (HS), a = 11.258(6) Å, c = 8.948(6) Å, V = 982.2(10) ų; 100 K (LS), a = 10.989(3) Å, c = 8.702(2) Å, V = 910.1(4) ų. The molecular structure consists of octahedral coordinated iron(II) centers bridged by six N4,N4‘ coordinating bis(tetrazole) ligands to form three 3-dimensional networks. Each of these three networks is symmetry related and interpenetrates each other within a unit cell to form the interlocked structure. The Fe−N bond lengths change between 1.993(1) Å at 100 K in the LS state and 2.193(2) Å at 300 K in the HS state. The nearest Fe separation is along the c-axis and identical with the lattice parameter c.