Crystal structure and magnetic properties of bis[butyltris(1H-pyrazol-1-yl)borato]iron(II)

The title compound, bis[butyltris(1H-pyrazol-1-yl)borato]iron(II), belongs to the class of neutral scorpionate complexes. The crystal structure at low temperature and the magnetic susceptibility measurement in the high-temperature region reveal the low-spin character of the iron(II) ion.


Chemical context
Scorpionates, coordination metal complexes of poly(1pyrazolyl)borates, have been studied intensively since the pioneering work of Trofimenko (1999). Iron(II) derivatives are particularly interesting because of the spin-state crossover between 1 A 1 low-spin (LS) and 5 T 2g high-spin (HS) observed for several scorpionate ligands (Long et al., 2004;Halcrow, 2007). Complexes of this type are sensitive to the effects induced by substituents on the electronic structure of the ligand and/or steric crowding (Hamon et al., 2008). The prototypical [Fe(HB(pz) 3 ) 2 ], the LS compound at 295 K, undergoes a spin-state crossover to the HS state upon heating to ca 420 K (Long et al., 2004). Introducing methyl substituents to the pyrazole moieties decreases the ligand field and shifts the spin crossover down in temperature or completely stabilizes the high-spin state of the iron(II) ion (Long et al., 2004). In contrast, scorpionate ligands bearing an organic substituent instead of the hydrogen atom on the hub boron atom demonstrate stabilization of the low-spin state and shift of the spin transition to the higher temperature range (Hamon et al., 2008).

Structural commentary
The asymmetric unit of the title compound contains two half independent complex molecules. In each complex, the Fe II atom, that is located on an inversion center, is surrounded by two scorpionate ligands; each one providing three pyrazole moieties coordinated in a fac mode, thus a pseudo-octahedral [FeN 6 ] coordination polyhedron is formed (Fig. 1). The two complex molecules differ essentially in the conformation of the butyl groups. One of the methylene groups of the butyl substituents of the Fe1-based complex shows a gauche conformation, whilst the remaining two methylene groups are in the trans conformation. Oppositely, the three methylene groups of the butyl substituent of the Fe2-based molecule are close to a trans conformation (Fig. 1).
The average Fe-N bond length is 1.969 Å (Table 1), a typical value for the low-spin state of the iron(II) ion (Gü tlich & Goodwin, 2004). The average trigonal distortion parameters È = AE 1 24 (60i )/24, where i is the angle generated by superposition of two opposite faces of the octahedron (Chang et al. 1990,) and AE = AE 1 12 (| i À 90|), where i are the deviations from 90 of the cis-N-Fe-N angles in the coordination sphere (Drew et al. 1995), are 1.27 and 24.38 , respectively, which correspond to a relatively low distortion of the coordination polyhedron and are typical for the low-spin state of iron(II) (Guionneau et al., 2004). The averaged volume of the coordination polyhedron is equal to 10.155 Å 3 .

Supramolecular features
In the crystal, molecules are linked by C-HÁ Á Á interactions (Fig. 2

Figure 2
A view along the b axis of the crystal packing of the title compound, with the C-HÁ Á Á contacts (see Table 2 for details) represented by dashed lines. For clarity, only the H atoms involved are shown, as blue balls for the Fe1 complex molecule and red balls for the Fe2 complex molecule.
link the individual complex molecules to form layers parallel to the bc plane. The layers are linked by C-HÁ Á Á pyrazolepyrazole interactions (Table 2), leading to the formation of a supramolecular three-dimensional structure, as shown in Fig. 2.

Magnetic measurements
Variable-temperature magnetic susceptibility measurements were performed on single crystals (20 mg) of the title compound using a Quantum Design MPMS2 superconducting quantum interference device (SQUID) susceptometer operating at 1 T in the temperature range 10-400 K. Experimental susceptibilities were corrected for the diamagnetism of the holder (gelatine capsule) and of the constituent atoms by the application of Pascal's constants. The magnetic behaviour of the compound recorded at 2 K min À1 is shown in Fig. 3 in the form of M T versus T ( M is the molar magnetic susceptibility and T is the temperature). At 300 K, the M T value is close to zero, and on heating the value remains constant up to 400 K. This corroborates well with the observed short average Fe-N bond length at 120 K and identifies the low-spin state of the central iron(II) ion.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.39, update November 2017; Groom et al., 2016) for complexes containing the iron(II) ion based on a scorpionate ligand with a tri(1H-pyrazol-1-yl)borate fragment yielded 39 hits, with Fe-N bond lengths lying in the ranges 1.956-1.995 and 2.162-2.246 Å , respectively, for the low-and high-spin states of the iron(II) ion.

Synthesis and crystallization
The butyltris(1H-pyrazol-1-yl)borate ligand and the title compound were synthesized according to the reported procedures (Reger & Tarquini, 1982;Myers et al., 2008). The slow diffusion of hexane vapour into a chloroform solution of the title compound led to the separation of orange wellshaped crystals.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H-atoms were included in calculated positions and treated as riding atoms: C-H = 0.93-0.97 Å with U iso (H) = 1.2U eq (C).