Crystal structure of [{FeCl3}2(μ-PCHP)2] [PCHP = 1,3-bis(2-diphenylphosphanylethyl)-3H-imidazol-1-ium] with an unknown solvent

In the crystal structure of the title compound, binuclear centrosymmetric molecules are present. The FeIII cation exhibits a trigonal–bipyramidal environment, being coordinated by three Cl ligands in the equatorial plane and two P atoms of symmetry-related PCHP ligands at the axial sites. The complex molecules are linked into layers by intermolecular C—H⋯Cl hydrogen bonding.


Chemical context
The conversion of dinitrogen into ammonia is an interesting reaction in the area of bioinorganic chemistry. In nature, the enzyme nitrogenase comprising the iron molybdenum cofactor (an MoFe 7 S 9 C-cluster), catalyses the derivatization of dinitrogen (Burgess & Lowe, 1996;Spatzal et al., 2011;Lancaster et al., 2011). Based on spectroscopic, biochemical and theoretical investigations, one of the iron atoms of the MoFe cofactor is considered to be the binding site of the dinitrogen molecule (Hoffman et al., 2009(Hoffman et al., , 2014. For this reason, the synthesis of model systems based on iron complexes serving as N 2 ! NH 3 catalysts has gained in importance over the past few years (Stucke et al., 2018). In particular, iron(II) dinitrogen complexes containing a PCP pincer ligand with a central N-heterocyclic carbene (Lee et al., 2004) are of significant interest because they are able to bind and activate dinitrogen. As a result of the strong -donor property of the central carbene unit, electron density is transferred to the central metal atom and to the N 2 ligand (Gradert et al., 2015). In this way, the dinitrogen molecule coordinating to the iron(II) cation should be activated sufficiently in order to get protonated, which is the first step in the N 2 ! NH 3 conversion (Yandulov & Schrock, 2003;Del Castillo et al., 2016) ISSN 2056-9890 In this context we are interested in the synthesis of iron dinitrogen complexes containing PCP pincer ligands. In the course of this project we serendipitously obtained crystals of the title compound by the reaction of the PC H P pincer ligand and the dinuclear iron(II) precursor [{FeCl(tmeda)} 2 (-Cl) 2 ]. To prove the identity of this compound, a single crystal X-ray structure determination was performed, which revealed that the central carbene C atom is protonated and that a dimeric iron(II) trichlorido complex has formed. Comparison of the experimental X-ray powder diffraction pattern with the calculated pattern on the basis of single-crystal data shows that the obtained product contained the title compound as the major phase but is contaminated with small amounts of other unknown crystalline phase(s) (Supplementary Fig. S1).

Structural commentary
The asymmetric unit of the title compound consists of one Fe III cation, three chlorido ligands and one PC H P ligand. The binuclear molecule is completed by inversion symmetry. The Fe III cation has a distorted trigonal-bipyramidal environment, being coordinated by two phosphorus atoms of two symmetryrelated PC H P ligands that occupy the axial positions and by three chlorido ligands that are located in the trigonal plane of the bipyramid (Figs. 1 and 2). The Fe-Cl bond lengths range from 2.3193 (5) to 2.3499 (4) Å and are much shorter than the Fe-P bond lengths of 2.6014 (5) and 2.6329 (5) Å (Table 1). In the binuclear molecule, the two iron(II) cations are linked by pairs of PC H P ligands (Figs. 1,2). The protonation of the central carbene moiety and hence the +2 oxidation state of iron of was proven by localization of the H atom attached to C1 and free refinement of its position. We also looked for trichlorido iron complexes with a trigonal-bipyramidal configuration in which the central iron atom has an oxidation state of +3. In comparison with the title compound, the Fe-Cl bond lengths in these complexes are significantly shorter (2.21 to 2.27 Å ; Walker & Poli, 1989;Feng et al., 2017), thus confirming the oxidation state +2 of the iron cation in [{FeCl 3 } 2 (-PC H P) 2 ].
Finally it is noted that within the dimer, a pair of intramolecular C-HÁ Á ÁCl hydrogen bonds between the aromatic    Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 1.   H atom attached to C1 and one of the chlorido ligands is observed (Fig. 2, Table 2). There is an additional intramolecular contact between the H atom attached to C16 and Cl1, but at a much longer HÁ Á ÁCl distance (Table 2).

Supramolecular features
In the crystal structure, the dimers are linked by centrosymmetric pairs of C-HÁ Á ÁCl hydrogen bonds between the H atom attached to C2 and the Cl3 atom of a neighbouring complex into layers parallel to (101) (Fig. 3, Table 2). Within these layers there are a number of additional C-HÁ Á ÁCl contacts, but either at much longer HÁ Á ÁCl distances or with angles deviating strongly from linearity (Table 2). These layers are stacked along [100] with no pronounced intermolecular interactions between them (Fig. 4, Table 2). By this arrangement, large cavities are formed in which disordered solvent molecules of unknown identity are present (see Refinement).

Database survey
To the best of our knowledge, no other iron complexes with the PC H P ligand have been reported in the literature. However, a few iron complexes where iron is coordinated by three chlorido and two phosphine ligands in a trigonalbipyramidal environment are known (Walker & Poli, 1989;Feng et al., 2017). Furthermore, other metal complexes of silver, palladium, rhodium and molybdenum with the metal coordinated by the deprotonated PC H P ligand have been reported and are well investigated (Lee et al., 2004;Zeng et al., 2005;Gradert et al., 2013). The difference between these complexes and the title complex [{FeCl 3 } 2 (-PC H P) 2 ] is the coordination of the carbene unit to the central metal cation, leading to the formation of mononuclear complexes. Nevertheless, a dinuclear gold complex with two bridging PC H P ligands was obtained by Bestgen et al. (2015). Here, the PC H P pincer ligands exhibit the same coordination mode as in the title complex [{FeCl 3 } 2 (-PC H P) 2 ], i.e. the pincer ligand binds to the central metal merely with the two phosphine donor groups. Polynuclear silver complexes with the PC H P ligand have also been reported, but in contrast to the aforementioned gold complex the central carbene unit does coordinate to the silver atom (Chiu et al., 2005).

Synthesis and crystallization
Synthetic procedures were performed according to Xiang et al. (2011). To 230 mg (435 mmol) of 1,3-bis(2-diphenylphosphanylethyl)-3H-imidazol-1-ium chloride (PC H PÁCl), which was prepared according to literature procedures (Lee et al., 2004), and 54.0 mg (482 mmol) of KO t Bu was added toluene (20 ml). The mixture was stirred at room temperature for 2 h. Afterwards, the suspension was filtered and added to 100 mg (207 mmol) of [{FeCl(tmeda)} 2 (-Cl) 2 ] in 5 ml of toluene. The iron complex had been prepared according to a literature protocol (Davies et al., 1997). After the reaction mixture had been stirred at room temperature overnight, the solution was concentrated under vacuum to 15 ml. The precipitate was filtered off, washed with toluene and dried under vacuum. The product was obtained as a light-brown solid (145 mg). Colourless crystals suitable for single-crystal X-ray diffraction were grown by diffusion of diethyl ether into a methanol solution of the product. Presumably, the protonation of the central carbene unit results from the crystallization process in protic methanol.
In the first stage of structure refinement, the hydrogen atom bound to the carbene C1 atom was clearly discernible in a difference map and was refined with varying coordinates and varying isotropic displacement parameters to prove that the carbene C atom is definitely protonated. Some very weak residual electron density peaks were present after the final refinement, indicating disordered solvent molecules. Since the disorder could not be resolved by various split models and the nature and number of solvent molecules (diethyl ether, methanol) could not be determined, all electron density associated with the solvent molecule(s) was removed using the SQUEEZE procedure in PLATON (Spek, 2015). SHELXL2014 (Sheldrick, 2015); molecular graphics: XP (Sheldrick, 2008) and DIAMOND (Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis{µ-1,3-bis[2-(diphenylphosphanyl)ethyl]-1H-imidazole-κ 2 P:P′}bis[trichloridoiron(III)]
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.