A tetragonal polymorph of bis[hydrotris(pyrazol-1-yl)borato]iron(II)

The title compound, [Fe(C9H10BN6)2], is a polymorph of a compound reported previously [Oliver et al. (1980 ▶). Inorg. Chem. 19, 165–168]. In the previous report, the compound crystallized in the monoclinic space group P21/c (Z = 4), whereas the crystal symmetry of the compound reported here is tetragonal (P42/ncm, Z = 4). The molecular structure is comprised of two hydrotris(1-pyrazolyl)borate ligands (Tp−) and a central FeII ion, which is coordinated by six pyrazole N atoms from two two Tp− ligands, yielding a distorted bipyramidal FeN6 geometry. The complete molecule exhibits symmetry 2/m.


Related literature
For the crystal structure of the other polymorph measured at room temperature, see: Oliver et al. (1980). For iron(II) complexes with the Tp À derivative ligands, see: Janiak et al. (2000); Reger et al. (2005).

Comment
Recently, hydro-tris(1-pyrazolyl)-borate (Tp -)and its derivatives have been employed as tridentate ligands to assembly molecular functional materials such as cyanide-bridged magnetic complexes, spin cross-over compounds and optic materials.
In these cases, some mononuclear iron(II) complexes with two such tridendate ligands have been synthesized and crystal structures characterized (Janiak et al., 2000;Reger et al., 2005). The crystal structure of the title compound has been reported previously (Oliver et al., 1980) which was measured at room temperature and crystallized in monoclinic space group of P2 1 /c (Z = 4). Recently, we synthesized this compound and measured its crystal structure at temperature 123 K. The result indicated that the crystal structure of the compound is significantly from the previous report. Herein, we report the crystal structure of the title compound [Fe II (C 9 H 10 N 6 B) 2 ] (I).
The title compound in this paper crystallizes in a tetragonal space group P4 2 /ncm, suggesting there is a fourfold rotation symmetry axis in the unit cell. In the molecular structure of the title compound, there is a pseudo C 3 rotation axis. The geometry and labeling scheme for the crystal structure of the title complex are depicted in Figure 1. The molecular structure of title compound in this work comprises of two Tpligands and one central iron(II) ion. In the molecular structure, the cental metal iron(II) ion is coordinated by six pyrazole nitrogen atoms from the same two Tpligands, yielding a distorted bipyramidal FeN 6 geometry.

Experimental
The title complex was prepared as following: methanol solution (10 ml) of [Fe II (BF 4 ) 2 ].4H 2 O (30 mg, 0.1 mmol) was added slowly into a MeOH and aqueous solution (20 ml, water and methanol with v/v = 1/1) containing the ligand KTp (50.4 mg, 0.2 mmol). Then, the mixture was carefully filtered and the resulting solution was kept at room temperature for about two days, producing block brown crystals of (I) with yield 50%.

Refinement
The coordinates of the H atom bound to boron atom was found from difference Fourier maps and refined freely. H atoms bound to C atoms were placed using the HFIX commands in SHELXL-97, with C-H distances of 0.93 Å. All H atoms were allowed for as riding atoms with U iso (H) = 1.2U eq (C).