Crystal structure of tetrakis(tetrahydrofuran-κO)bis(trifluoromethanesulfonato-κO)iron(II)

The title high-spin iron(II) complex is six-coordinated with two trifluoromethanesulfonato and four tetrahydrofuran ligands. It is isostructural with the corresponding Co, Ni and Zn complexes known from the literature.

The title compound, [Fe(CF 3 SO 3 ) 2 (C 4 H 8 O) 4 ], is octahedral with two trifluoromethanesulfonate ligands in trans positions and four tetrahydrofurane molecules in the equatorial plane. By the conformation of the ligands the complex is chiral in the crystal packing. The compound crystallizes in the Sohncke space group P2 1 2 1 2 1 and is enantiomerically pure. The packing of the molecules is determined by weak C-HÁ Á ÁO hydrogen bonds. The crystal studied was refined as a two-component inversion twin.

Chemical context
The trifluoromethanesulfonato anion is usually weakly coordinating to metals, and the salts thereof are consequently important starting compounds for the exchange with other ligands. In an attempt of such a synthesis on iron(II) we obtained the starting material back with tetrahydrofuran (THF) molecules from the solvent completing the sixfold coordination environment. The overall composition of the title compound (I) is then [Fe(CF 3 SO 3 ) 2 (C 4 H 8 O) 4 ].

Structural commentary
A molecular plot of (I) is shown in Fig. 1 with selected bond lengths and bond angles given in Table 1. The present Fe compound is isostructural to the corresponding Co, Ni and Zn compounds known from the literature (Amel'chenkova et al., 2006). An isostructural Cu compound is mentioned in the same publication but no further details are given. An overlay of the isostructural compounds is presented in Fig. 2. The comparison of metal-oxygen distances in Table 2 follows the trend of effective ionic radii (Shannon, 1976) with 0.92 Å for ISSN 2056-9890 octahedral Fe 2+ (high-spin), 0.885 Å for Co 2+ (high-spin), 0.83 Å for Ni 2+ and 0.88 Å for Zn 2+ . From this comparison we can conclude that the Fe ion in (I) has a high-spin electronic configuration. It should also be noted that there are no significant differences in metal-oxygen distances between the partially negative triflate and the neutral THF.
In the octahedral compound (I), the triflate ligands are in trans positions and the equatorial plane is formed by O atoms of THF. The Fe atom is approximately in the equatorial plane at a distance of 0.0079 (3) Å from the least-squares plane of the THF oxygen atoms. The FeO 6 octahedron is nearly undistorted with a quadratic elongation of 1.001 and an angle variance of 2.79 2 (Robinson et al., 1971). To the best of our knowledge, the crystal structure of compound (I) is the first of a trans triflate Fe complex with an FeO 6 chromophore. Similar complexes with N atoms in the equatorial plane are known from the literature. In the acetonitrile complex [Fe(CF 3 SO 3 ) 2 -(CH 3 CN) 4 ], the core octahedron is similarly undistorted (Hagen, 2000), while the pyridine complex [Fe(CF 3 SO 3 ) 2 -(C 5 H 5 N) 4 ] is slightly tetragonally compressed (Haynes et al., 1986).
As expected, all four coordinated THF molecules are puckered. The rings at O7 and O8 are best described as having an envelope conformation, the rings at O9 and O10 as being in a twist conformation. The O atoms are coordinated to the metal in a trigonal geometry with angle sums of 358.7 (2)-360.0 (2) .
The two triflate ligands adopt a staggered conformation with O-S-C-F torsion angles between 56.6 (2) and 64.11 (19) . The S-O distances to the coordinating oxygen atoms are significantly longer than to the non-coordinating oxygen atoms (Table 1)

Figure 2
Overlay plot of the isostructural Co, Ni, and Zn complexes (Amel'chenkova et al., 2006) with respect to the Fe complex (I). The coordinates of the Ni and Zn complexes have been inverted for this comparison. Hydrogen atoms are omitted for clarity. The quaternion fit algorithm (Mackay, 1984) as implemented in PLATON (Spek, 2009) was used for the preparation of the plot. Color scheme: Fe complex (blue),Co complex (green), Ni complex (red), and Zn complex (black).

Figure 1
A view of the molecular structure of (I), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. For clarity, H atoms have been omitted.  (9) in compound (I) are well within this range. The octahedral symmetry of the inner-sphere coordination environment (see above) is reduced to approximate C 2 symmetry by the arrangement of the triflate anion ( Fig. 3). If the THF molecules are considered as well, the overall symmetry reduces to C 1 . Despite the achiral ligands, the metal complex is thus chiral in the crystal.

Supramolecular features
The crystal structure of (I) has a packing index (Kitajgorodskij, 1973) of only 68.7%, which is at the lower end of the 65-75% range expected for organic solids (Dunitz, 1995). Indeed, the packing is determined by only weak C-HÁ Á ÁO interactions with the THF atoms as donors and the noncoordinated triflate oxygen atoms as acceptors (Table 3). Every molecule of (I) is the donor and acceptor of three intermolecular C-HÁ Á ÁO hydrogen bonds and has thus a coordination number of six. This results in a three-dimensional network.

Synthesis and crystallization
The title compound was obtained from an experiment aimed at synthesizing an iron coordination compound based on an oxazine ligand. In a glovebox under a dinitrogen atmosphere, 4a,8a-dimethyloctahydro-[1,4]oxazino[3,2-b][1,4]oxazine (159 mg, 0.923 mmol) and Fe(OTf) 2 Á2MeCN (400 mg, 0.917 mmol) were placed in separate vials. The ligand was dissolved in THF (about 12 mL) and added to the vial containing Fe(OTf) 2 Á2MeCN under gentle stirring. The color of the solution turned from black to dark red and stirring was maintained overnight at room temperature. The resulting compound was precipitated twice by dropwise addition of a concentrated THF solution into hexane. The slightly pinkcolored supernatants were removed by decantation. The precipitated solids were washed with hexanes and dried under vacuum. The decanted solutions were stored in a freezer at 238 K and over a month light-pink crystals slowly grew.
A second crystallization starting from the isolated precipitate in an 1:1 THF:hexane solution grew similar crystals over several months at 238 K. 1 H-NMR in d 3 -MeCN showed no paramagnetic peaks but small diamagnetic peaks of THF (3.64, 1.79 ppm) and hexane (1.28, 0.89 ppm). 19 F-NMR showed a single sharp peak at À79.36 ppm.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms were placed in calculated positions (C-H = 0.99 Å ) and refined as riding with U iso (H) = 1.2U eq (C).
The reflection profiles in Eval15 (Schreurs et al., 2010) were based on a split-mosaic model. Two fragments were rotated by 0.56 with respect to each other. An example for a reflection profile is shown in Fig. 4.

Figure 4
Height plot of the pixel intensities of reflection hkl = (5,15,12). The central frame (scan width 0.3 ) is shown. Observed intensities (left) and model intensities (right). A split-mosaic model was assumed for the prediction of the profile.

Figure 3
The of 99, resulting in a slope of 0.885 and an intercept of À0.037. The student-t probability plot is linear with a correlation coefficient of 1.000. All of these different methods give a consistent result for the present crystal. The measurement of a second crystal results in x = 0.015 (11) from an inversion twin refinement, but very low standard uncertainties in the values of z = 0.015 (2) and y = 0.0012 (1) leave reasons for doubt concerning its enantiopurity, although the Bijvoet difference related probabilities P2/P3 (true) are 1.000 and the probability P3 (false) is 0.000 in both crystals, suggesting that both crystals are enantiopure.

Tetrakis(tetrahydrofuran-κO)bis(trifluoromethanesulfonato-κO)iron(II)
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. 0.0245 (9) 0.0269 (9) 0.0363 (10) 0.0057 (7) 0.0080 (9) 0.0020 (9) Geometric parameters (Å, º)