Two-step spin transition in an iron(II) coordination network based on flexible bitopic ligand 1-(tetrazol-1-yl)-3-(1,2,3-triazol-1-yl)propane.
Flexible bitopic ligand 1-(tetrazol-1-yl)-3-(1,2,3-triazol-1-yl)propane (ptrtz) reacts with Fe(ClO4)2·6H2O to form two-dimensional (2D) coordination network [Fe(ptrtz)3](ClO4)2·MeCN (1) in which two chemically different spin crossover (SCO) FeII centers are observed. Although two 1,2,3-triazole and four tetrazole rings are in the first coordination sphere of the Fe1 ions, Fe2 ions are coordinated to four 1,2,3-triazole and two tetrazole rings. Symmetry-related FeII ions are bridged by two disordered ptrtz molecules resulting in the formation of chains that are extended in the same direction. Alternately arranged chains containing Fe1 or Fe2 ions are cross-linked by single ptrtz molecules to form 2D layers. At room temperature, the complex remains in a high-spin (HS) form (χMT = 3.46 cm3 K mol–1). Spin crossover in 1 occurs in two steps separated by an inflection point at 140 K (χMT = 2.68 cm3 K mol–1, γHS ≈ 0.76). The first step is gradual and between 220 and 140 K about a quarter of all the iron(II) ions change their spin state (T1↓ = T1↑ = 165 K). Single-crystal X-ray diffraction studies revealed that upon lowering the temperature from 220 to 140 K, spin crossover occurs for only half of the iron(II) ions occupying Fe2 sites. It is accompanied by an alteration of the occupancy factor of ptrtz molecules bridging the Fe2 ions. Iron(II) ions occupying Fe1 sites remain in the HS form, which inhibits compression of the polymeric layer in the Fe1···Fe1 and Fe2···Fe2 linking direction. Upon further lowering the temperature, iron(II) ions occupying Fe1 sites and the rest of the iron(II) ions occupying Fe2 sites change their spin state. The second step of the spin transition is very abrupt and is accompanied by a hysteresis loop (T2↓= 90 K, T2↑ = 96 K). The change in the spin state by iron(II) ions occupying Fe1 and Fe2 sites involves, contrary to first step, a significant shortening of the Fe1···Fe1 and Fe2···Fe2 separations.
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