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

Abstract

The coordination polymer {[Fe(pbtz)3](ClO4)2 . 2EtOH}infinity (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}infinity (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) A for Fe1 and 2.176(5) and 2.190(4) A 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 chiMT(T) dependence shows in the range 200-75 K gradual SCO. Below 75 K the transition is finished and approximately 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 A. 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) A. 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) A.