Tetrazol-2-yl as a Donor Group for Incorporation of a Spin-Crossover Function Based on Fe(II) Ions into a Coordination Network

The coordination polymer {[Fe(pbtz)3](ClO4)2·2EtOH}∞ (1) has been prepared in a reaction between Fe(ClO4)2·6H2O and 1,3-di(tetrazol-2-yl)propane (pbtz). The formation of the second product {[Fe(pbtz)3](ClO4)2}∞ (2) was
also noticed. Both complexes crystallize in the R3 space group. The single-crystal X-ray diffraction study of 1 (295,
90 and 230 K) revealed that the 2-substituted tetrazole rings (2tz) coordinate monodentately to the metal ions,
forming Fe(2tz)6 cores. There are two crystallographically independent iron(II) ions in 1. At 295 K the Fe−N4 bond
lengths are equal to 2.173(5) and 2.196(5) Å for Fe1 and 2.176(5) and 2.190(4) Å for Fe2. The pbtz ligand molecules
act as N4,N4‘ connectors, bridging central atoms in the three directions, which leads to the formation of the 3D
network. The crystal lattice of 1 is solvated by ethanol molecules. At 295 K the solvent and ligand molecules are
disordered. The results of temperature-dependent magnetic susceptibility measurements (5−300 K), and the single-crystal X-ray diffraction studies (90 K) have exhibited that 1 undergoes the thermally induced spin transition HS↔LS
(SCO). The χMT(T) dependence shows in the range 200−75 K gradual SCO. Below 75 K the transition is finished
and ∼20% of the HS fraction is present in the sample. The HS→LS transition is accompanied by a shortening of
the Fe−N bonds of 0.15 Å. At 90 K the ligand molecules are ordered. The presence of 2 in the reaction product
was disclosed accidentally, and only the X-ray diffraction studies (250, 90 K) were performed. Also in 2 iron(II) ions
serve as topological nodes of the 3D network. Despite the same network topology, 2 crystallizes without ethanol
molecules solvating the crystal lattice. The pbtz molecules bridge the neighboring iron(II) ions, coordinating through
N4,N4‘ atoms of the 2-substituted tetrazole rings forming the Fe(2tz)6 cores. At 250 K the Fe−N bond lengths are
equal to 2.208(5) and 2.218(5) Å. In contrast to 1, the cooling of the crystal of 2 from 250 to 90 K does not involve
the shortening of the Fe−N bond lengths. At this temperature, the Fe−N distances remain characteristic for the HS
form of the complex and are equal to 2.203(3) and 2.208(3) Å.