Crystal structure and magnetic properties of (tris{4-[1-(2-methoxyethyl)imidazol-2-yl]-3-azabut-3-enyl}amine)iron(II) bis(hexafluoridophosphate)

The title compound, [Fe(C27H41N10O3)](PF6)2, is an example of an iron(II) spin-crossover compound. In this compound, C⋯F and CH⋯F/O contacts, present between the cations and anions, extend the structure into a three-dimensional supramolecular network.

In the complex cation of the title compound, [Fe(C 27 H 41 N 10 O 3 )](PF 6 ) 2 , the tripodal tris{4-[1-(2-methoxyethyl)imidazol-2-yl]-3-azabut-3-enyl}amine ligand is coordinated to an Fe II ion through the nitrogen atoms of three imidazole and three imino groups. The Fe atom exhibits a distorted octahedral geometry. In the crystal, l and d antipodes are arranged in layers in the bc plane. Weak CÁ Á ÁF and C-HÁ Á ÁF/O contacts exist between the ligands of the complex cation and the PF 6 À anions, generating a three-dimensional network. At 120 K, the Fe II ion is in a low-spin state, with an average Fe-N bond distance of 1.970 (2) Å . On heating, the Fe II ion converts to the high-spin state, as demonstrated by magnetic susceptibility measurements.

Chemical context
One of the most investigated groups of switchable molecular materials are the pseudo-octahedral Fe II spin-crossover (SCO) complexes, which can change between high-spin (HS, t 2g 4 e g 2 ) and low-spin (LS, t 2g 6 e g 0 ) electronic states on application of physicochemical stimuli. The LS-to-HS conversion involves an electron transfer between the e g and t 2g orbitals and is strongly coupled to structural changes in the coordination sphere of the Fe II ions, affecting the Fe-ligand bond lengths and angles (Gü tlich & Goodwin, 2004). The spin-state change is reversible and can be controlled, for example by the action of temperature, pressure or light. It is accompanied by a change in a number of physical properties, including magnetic susceptibility, colour, dielectric constant and NLO properties (Kö nig, 1991;Nakamoto et al., 2005;Bonhommeau et al., 2006Bonhommeau et al., , 2012. Tripod-based iron(II) complexes represent one of the well-studied classes of SCO complexes owing to the suitable ligand-field strength and readily achievable functionalization of their complex ligands (Hardie et al., 2004;Seredyuk et al., 2007;Klug et al., 2012;Hagiwara et al., 2014), particularly with aliphatic chains (Seredyuk et al., 2008a(Seredyuk et al., ,b, 2013(Seredyuk et al., , 2014. In this work, we report the synthesis, structure and magnetic properties of a new Fe II complex based on the tripodal ligand tris{4-[1-(2-methoxyethyl)imidazol-2-yl]-3-azabut-3-enyl}amine, which can be crystallized in the presence of hexafluoridophosphate anions (Fig. 1).

Structural commentary
The unit cell of the title compound contains two pairs of crystallographically identical complex cations of l and d chirality and eight PF 6 À counter-ions (on two crystallographically distinct sites) to balance the charge. In the complex cation, the Fe II ion is wrapped by three 1-(2-methoxyethyl)-imidazol-2-ylimino moieties, defining a pseudooctahedral [FeN 6 ] coordination environment (Fig. 1). The average Fe-N bond length is 1.970 Å and is typical for the low-spin state of the Fe II ion (Gü tlich & Goodwin, 2004) ( Table 1). The average trigonal distortion parameters, È = AE i 24 (60 À i )/24 [where i is the angle generated by superposition of two opposite faces of an octahedron (Chang et al., 1990)] and AE = AE i 12 (| i À 90|) [where i is the deviation from 90 of the cis-N-Fe-N angles in the coordination sphere (Drew et al., 1995)] are 57.72 and 5.23 , respectively. These values are comparable to those reported previously for a similar low-spin compound with n-butyl substituents (Seredyuk et al., 2013). The capping tertiary nitrogen atom, N4, is situated at a distance of 3.375 (2) Å from the Fe atom and does not participate in coordination to the metal ion. Each of the methylene groups of the 2-methoxyethyl substituents directly attached to the imidazole moieties shows a gauche conformation, whilst the remaining methylene groups are in a trans conformation.

Supramolecular features
Supramolecular interactions occur between the complex cations and PF 6 À anions, with van der Waals contacts, CÁ Á ÁF, lying in the range 2.934 (2)-3.137 (2) Å , linking the ions into two-dimensional layers running parallel to [011] (Fig. 2). These contacts are observed mostly for the carbon atoms belonging to the imidazole moieties of the ligand (Table 2). In addition, there are numerous C-HÁ Á ÁF and C-HÁ Á ÁO contacts between the complex cations and anions, extending the crystal structure into a three-dimensional supramolecular network.

Magnetic properties
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 2-300 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 1 K min À1 between 150 and 300 K, Table 1 Selected bond lengths (Å ).
3.074 (4) x, 3 2 À y, À 1 2 + z Figure 1 Molecular structure of the complex cation and anions of the title compound showing the atom labelling. Short CÁ Á ÁF contacts less than the sum of the van der Waals radii are shown as dashed lines. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) x, is shown in Fig. 3 in the form of M T vs T ( M is the molar magnetic susceptibility and T is the temperature). At 300 K, the M T value is close to 1.8 cm 3 K mol À1 , displaying at this temperature an incomplete transition of the Fe II ion to the paramagnetic high-spin state (S = 2). On cooling, a gradual decrease of M T value down to 0.07 cm 3 K mol À1 is observed corresponding to an almost compete transformation to the diamagnetic low-spin state (S = 0). 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 Fe II ion wrapped by a tripodal ligand with a tris{imidazol-2-yl-3-azabut-3-enyl}amine fragment yielded 29 hits, for which the Fe-N bond lengths lie in the ranges 1.926-2.016 and 2.151-2.286 Å for the low-spin and high-spin spin states of the Fe II ion, respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H = 0.95-0.99 Å and U iso (H) = 1.2-1.5U eq (parent atom). The highest peak is located 1.21 Å from atom C24 and the deepest hole is located 0.65 Å from atom P2.

(Tris{4-[1-(2-methoxyethyl)imidazol-2-yl]-3-azabut-3-enyl}amine)iron(II) bis(hexafluoridophosphate)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.