Crystal structure of (ferrocenylmethyl)dimethylammonium hydrogen oxalate

In the title salt, [Fe(C5H5)(C8H13N)](HC2O4), the anions are linked via strong O—H⋯O hydrogen bonds into linear [100] chains. The cations connect to the anion through bifurcated N—H⋯(O,O′) hydrogen bonds.


Chemical context
Our group has been working on the interactions between alkylammonium ions with oxalic acid, and we have recently reported the crystal structure of (H 3 C) 2 NH + ÁHC 2 O 4 À Á-0.5H 2 C 2 O 4 (Diallo et al., 2015). Numerous other reports have described crystal structures containing acidic or neutral oxalate molecules interacting with a protonated amine, see for example: Vaidhyanathan et al. (2002); Braga et al. (2013); Said et al. (2006); Hathwar et al. (2010); Matulková et al. (2008); Olenik et al. (2003); Anda et al. (2004). Braga et al. have reported several structures of columnar metallocenium sandwich compounds interacting with hydrogen oxalate (Braga et al., 2002). However, none of these structures features the hydrogen oxalate anion alone. It is crystallized either with neutral oxalic acid and/or a water molecule. The crystal structure of the title salt, [Fe(C 5 H 5 )(C 8 H 13 N)] + Á[HC 2 O 4 ] À , (I), features only the hydrogen oxalate anion. This compound was obtained when studying the interaction of (ferrocenylmethyl)dimethylamine and oxalic acid in aqueous solution.

Structural commentary
The asymmetric unit of (I) contains one hydrogen oxalate anion and one (ferrocenylmethyl)dimethylammonium cation (Fig. 1). As previously observed in structures featuring this ISSN 2056-9890 cation (Wang, 2010;Guo, 2006;Guo et al., 2006a,b), the two Cp rings exhibit a nearly eclipsed conformation. They are planar and almost parallel, as demonstrated by the dihedral angle of 0.96 (5) between their least-square planes. The Fe-C distances range from 2.0394 (10) to 2.0578 (12) Å . The Fe binding with the Cp rings is somewhat asymmetric as suggested by both the FeÁ Á ÁCp plane distances [1.6601 (6) and 1.6514 (6) Å for the unsubstituted and the substituted ligand, respectively], and the Cp1-Fe-Cp2 dihedral angle of 170.96 (3) . This behaviour was previously described as a consequence of an electron-withdrawal effect of the methyldimethylamine group that results in the less electron-rich substituted ring being slightly closer to the metal (Winter & Wolmershä user, 1998). The oxalate anion is essentially planar and the dihedral angle between carboxylate and the carboxyl groups is only 4.6 (3) . The C-OH bond is at 1.3052 (13) significantly longer than the other three C-O bonds with an mean of 1.24 (2) Å .

Supramolecular features
The hydrogen oxalate anions are held together via a strong intermolecular O4-H4AÁ Á ÁO2 hydrogen bond, resulting in the formation of linear chains running parallel to [100] (Fig. 2). Within a chain, successive hydrogen oxalate anions are rotated by 30.89 (11) . The cation is linked to the anionic chain through a bifurcated N1-H1Á Á Á(O1,O4) hydrogen bond (Table 1). In addition to Coulomb forces and hydrogen bonds, a weak C-HÁ Á Á interaction involving the centroid Cg2 of the Cp ligand (C6-C10; Table 1) is present and consolidates the three-dimensional supramolecular network.

Database survey
A search in the Cambridge Structural database (Version 5.36 with three updates, Groom & Allen, 2014) returned only eight entries for seven independent crystal structures containing the (ferrocenylmethyl)dimethylammonium cation. These include simple salts with Cl À (Winter & Wolmershä user, 1998) and its hydrated form (Guo et al., 2006a), Br À (Wang, 2010), NO 3 À (Guo et al., 2006b), sulfate pentahydrate (Guo, 2006), tetrachloridozincate monohydrate (Gibbons & Trotter, 1971) and a benzene solvate with dodecaborane (Yongmao et al., 1983). The investigation of hydrogen-bonded hydrogen oxalate chains returned 119 unique structures of which 32 are characterized by a bifurcated hydrogen bond with an ammonium counter-cation. The molecular components in the structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as small spheres of arbitrary radii. Fe-Cp interactions and hydrogen bonds are shown as dashed lines.

Figure 2
Partial packing diagram in the structure of the title compound viewed along [001]. The chains running along [100] as defined by the hydrogenbonded hydrogen oxalate anions and the (ferrocenylmethyl)dimethylammonium cations linked by a bifurcated hydrogen bond are shown. Hydrogen bonds are shown as black lines. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
Crystals of the title compound were obtained by slow evaporation of an aqueous solution in which (ferrocenylmethyl)dimethylamine was mixed with oxalic acid in a 1:2 ratio.

Special details
Experimental. X-ray crystallographic data for I were collected from a single-crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker Venture diffractometer equipped with a Photon 100 CMOS Detector, a Helios MX optics and a Kappa goniometer. The crystal-to-detector distance was 4.0 cm, and the data collection was carried out in 1024 x 1024 pixel mode. 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) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.