Crystal structures of trans-dichloridotetrakis[1-(2,6-diisopropylphenyl)-1H-imidazole-κN 3]iron(II), trans-dibromidotetrakis[1-(2,6-diisopropylphenyl)-1H-imidazole-κN 3]iron(II) and trans-dibromidotetrakis[1-(2,6-diisopropylphenyl)-1H-imidazole-κN 3]iron(II) diethyl ether disolvate1

The title compounds are iron(II) dihalide complexes of the bulky arylimidazole ligand 1-(2,6-diisopropylphenyl)-1H-imidazole. The FeCl2 and FeBr2 complexes are isotypic, while the third compound, also an FeBr2 complex, crystallizes as a diethyl ether disolvate.

The title compounds, [FeCl 2 (C 15 H 20 N 2 ) 4 ], (I), [FeBr 2 (C 15 H 20 N 2 ) 4 ], (II), and [FeBr 2 (C 15 H 20 N 2 ) 4 ]Á2C 4 H 10 O, (IIb), respectively, all have triclinic symmetry, with (I) and (II) being isotypic. The Fe II atoms in each of the structures are located on an inversion center. They have octahedral FeX 2 N 4 (X = Cl and Br, respectively) coordination spheres with the Fe II atom coordinated by two halide ions in a trans arrangement and by the tertiary N atom of four arylimidazole ligands [1-(2,6-diisopropylphenyl)-1H-imidazole] in the equatorial plane. In the two independent ligands, the benzene and imidazole rings are almost normal to one another, with dihedral angles of 88.19 (15) and 79.26 (14) in (I), 87.0 (3) and 79.2 (3) in (II), and 84.71 (11) and 80.58 (13) in (IIb). The imidazole rings of the two independent ligand molecules are inclined to one another by 70.04 (15), 69.3 (3) and 61.55 (12) in (I), (II) and (IIb), respectively, while the benzene rings are inclined to one another by 82.83 (13), 83.0 (2) and 88.16 (12) , respectively. The various dihedral angles involving (IIb) differ slightly from those in (I) and (II), probably due to the close proximity of the diethyl ether solvent molecule. There are a number of C-HÁ Á Áhalide hydrogen bonds in each molecule involving the CH groups of the imidazole units. In the structures of compounds (I) and (II), molecules are linked via pairs of C-HÁ Á Áhalogen hydrogen bonds, forming chains along the a axis that enclose R 2 2 (12) ring motifs. The chains are linked by C-HÁ Á Á interactions, forming sheets parallel to (001). In the structure of compound (IIb), molecules are linked via pairs of C-HÁ Á Áhalogen hydrogen bonds, forming chains along the b axis, and the diethyl ether solvent molecules are attached to the chains via C-HÁ Á ÁO hydrogen bonds. The chains are linked by C-HÁ Á Á interactions, forming sheets parallel to (001). In (I) and (II), the methyl groups of an isopropyl group are disordered over two positions [occupancy ratio = 0.727 (13):0.273 (13) and 0.5:0.5, respectively]. In (IIb), one of the ethyl groups of the diethyl ether solvent molecule is disordered over two positions (occupancy ratio = 0.5:0.5).

Chemical context
The use of organometallic complexes as catalysts is an important development in the field of material chemistry. However, despite this, only a very few of them contain iron(II), except the tridentate diimine pyridine complex Britovsek et al., 1998) used in olefin polymerization. Unfortunately, this model ISSN 1600-5368 suffers from its lack of tolerance towards the minor changes carried out in its envelope, resulting in a drastic reduction of its catalytic activity. Neutral and cationic complexes of iron(II) chloride and bromide with nitrogen bases are well known for imidazole, pyridine and pyrazoles (Schrö der et al., 2009;Christie et al., 1993). For this reason, we set out to prepare new iron complexes containing more electron-donating and bulky ligands. Only a few analogous bulky arylimidazoles have been reported so far (Reisner et al., 2007).
We focused our attention on the use of bis-N-heterocyclic carbene Fe II complexes in hydrogenation and polymerization of olefins (Mafua, 2006). During the preparation of these complexes, several other complexes of Fe II and Fe III were isolated, among them the title compounds, (I), (II) and (IIb). Compound (I) was isolated by deprotonation of bisimidazoliummethylene tetrachloridoferrate(III) (L1 in Fig. 7) with NaH in THF at reflux. When the same reaction was conducted at room temperature, only the starting material was recovered after recrystallization. Compounds (II) and (IIb) were isolated when bisimidazoliummethylene tetrabromidoferrate(III) (L2 in Fig. 7) was reacted with NaH in THF at reflux. The main result in the structure of these compounds is the loss of the bridging methylene group of the starting bisimidazolium cation. Thus two independent N-1-arylimidazolyl groups are formed for each starting bisimidazolium cation. Additionally, this result demonstrates a possible fragility of methylene-bisimidazole ligands when used in harsh reaction conditions. The question of the reduction of Fe III to Fe II remains to be elucidated.

Structural commentary
The structures of (I) and (II) are isotypic whereas (IIb) differs due to the presence of solvent diethyl ether molecules. The whole molecule of each compound, (I), (II) and (IIb), is generated by inversion symmetry (Figs. 1, 2 and  A view of the molecular structure of complex (II), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level; disordered parts are not shown. H atoms have been omitted for clarity.

Figure 1
A view of the molecular structure of complex (I), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level; disordered parts are not shown. H atoms have been omitted for clarity.

Figure 5
A view along the c axis of the crystal packing of compound (II). Hydrogen bonds and C-HÁ Á Á interactions are shown as dashed lines (see Table 2 for details; H atoms not involved in these interactions have been omitted for clarity). 61.55 (12) in (I), (II) and (IIb), respectively, while the benzene rings (C4-C9 and C19-C24) are inclined to one another by 82.83 (13), 83.0 (2) and 88.16 (12) , respectively. The various dihedral angles involving (IIb) differ slightly from those in (I) and (II) due to steric hindrance owing to the close proximity of the diethyl ether solvent molecule of crystallization.

Supramolecular features
In the crystal structures of all three compounds, (I), (II) and (IIb), molecules are linked via pairs of C-HÁ Á Áhalogen hydrogen bonds, forming chains along the a axis [for (I) and (II)] and the b axis, respectively, for (IIb) that enclose R 2 2 (12) ring motifs (Figs. 4, 5 and 6, respectively, and Tables 1, 2 and 3, respectively). They are linked by C-HÁ Á Á interactions, forming sheets parallel to (001). In the crystal structure of compound (IIb), the diethyl ether solvent molecules are attached to the chains via C-HÁ Á ÁO hydrogen bonds, and within the chains there are a series of C-HÁ Á Á interactions present ( Fig. 6 and Table 3).

Synthesis and crystallization
The synthesis of the precursors bisimidazolium methylene tetrachlorido-and tetrabromidoferrate(III) (L1 and L2, respectively, in Fig. 7) have been reported elsewhere (Mafua, 2006). Compound (I) was prepared as follows: to a solution of (L1) [0.34 g, 0.5 mmol] in 20 ml of THF was added 0.09 g (2.3 mmol) of NaH 60% and 0.01 g (0.1 mmol) of t BuOK, and the reaction mixture was heated at 340 K for 8 h. The solution was then filtered and the solvent evaporated under vacuum yielding an orange solid. Yellow crystals were obtained by slow diffusion of diethyl ether into a THF solution of the isolated orange solid. UV-vis (THF, 200-800 nm): 364, 290. Compounds (II) and (IIb) were prepared in a similar manner. To a solution of (L2) [0.29 g, 0.5 mmol] in 20 ml of THF was added 0.09 g (2.3 mmol) of NaH 60% and 0.01 g (0.1 mmol) of t BuOK at 273 K, and the reaction mixture was heated at reflux for 8 h. The solution was then filtered and the solvent evaporated under vacuum yielding a yellow-brown solid. Yellow crystals were obtained by slow diffusion of diethyl ether into a THF solution of the isolated yellow-brownish solid. UV-vis (THF, 200-800 nm): 292. Two types of crystals were obtained: yellow plates for (II) and yellow blocks for (IIb).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. In all three compounds, the H atoms were included in calculated positions and treated as riding atoms: C-H = 0.95, 1.00 and 0.98 Å for CH(aromatic), CH and CH 3 H atoms, respectively, with U iso (H) = 1.5U eq (Cmethyl) and = 1.2U eq (C) for other H atoms. In (I) and (II), the methyl groups of an isopropyl group are disordered over two positions [occupancy ratio = 0.727 (13):0.273 (13) in (I) and fixed at 0.5:0.5 for (II)]. In (IIb), one of the ethyl groups of the diethyl ether solvent molecule is disordered over two positions (occupancy ratio fixed at 0.5:0.5).  A view along the c axis of the crystal packing of compound (IIb). Hydrogen bonds are shown as dashed lines (see Table 3 for details; H atoms not involved in these interactions have been omitted for clarity).

(I) trans-Dichloridotetrakis[1-(2,6-diisopropylphenyl)-1H-imidazole-κN 3 ]iron(II)
Crystal data Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (