Crystal structure of 3-aminopyridinium 1′-carboxyferrocene-1-carboxylate

The title structure consists of 3-aminopyridinium cations and 1′-carboxyferrocene-1-carboxylate monoanions held together by N—H⋯O and O—H⋯O hydrogen bonds.


Chemical context
The idea behind this research was to use ferrocenedicarboxylic acid as a dianionic building block in supramolecular polymer and conventional polymer design (Amer et al., 2013;Sun et al., 2016;Zheng et al., 2016).
We tried to apply the trio of available aminopyridines, namely 2-, 3-and 4-aminopyridine, as basic counterparts to ferrocenedicarboxylic diacid. One of the ideas was to check the possibility of obtaining gels with a supramolecular arrangement of the constituents in alcoholic media. All those reactions were carried out in a 1:2 ratio of acid-amine in order to exploit both carboxylic acid groups of the diacid. The experiments revealed, however, that while in cases of 2-and 4-aminopyridine, only amorphous powders could be obtained, the reaction of 3-aminopyridine led to a crystalline salt, 3-aminopyridinium 1 0 -carboxyferrocene-1-carboxylate, (1), but with a 1:1 composition.

Structural commentary
The crystal structure of (1) consists of one 3-aminopyridinium cation and one 1 0 -carboxyferrocene-1-carboxylate monoanion (Fig. 1). In the cation, the pyridine N atom is protonated. The ferrocenyl moieties adopt the characteristic sandwich structure, with typical Fe-C distances in the range 2.0270 (15)- ISSN 2056-9890 2.0568 (17) Å (Table 1). The Fe II atom is slightly ($0.01 Å ) shifted towards the substituted C11 and C21 atoms. The C16-O bond lengths within the carboxylate anion are almost equal [1.2604 (19) and 1.2636 (19) Å ], whereas, in contrast, they differ greatly within the carboxylic acid group, with C26 O22 = 1.2128 (19) Å and C26-O21 = 1.326 (2) Å , the latter involving the OH group. The planes of the cyclopentadienyl (Cp) rings are almost parallel to the planes of the corresponding carboxy/carboxylate groups, with O-C-C-C torsion angles less than 13 . The conformation of 1,1 0 -disubstituted ferrocenes is described by the torsion angle C subst -Cp cent -Cp cent -C subst , where C subst stands for a ferrocene C atom with an additional bonding partner and Cp cent for the centre of gravity of the C atoms of the ring; this angle is hereafter referred to as '. In (1), the anion possesses an eclipsed conformation with ' = 66.0 (ideal value 72 ) (Fig. 2).

Supramolecular features
In the title crystal, adjacent cationic and anionic units are combined into a layered arrangement parallel to (100) by charge-supported NHÁ Á Á À O 2 C hydrogen bonds of mediumstrong-to-weak nature and of CO 2 HÁ Á Á À O 2 C hydrogen bonds of strong nature (   Figure 1 The structures of the molecular components in (1). Displacement ellipsoids are shown at the 50% probability level. Hydrogen bonding is shown as dashed lines.

Figure 3
The formation of hydrogen-bonded layers parallel to (100) in the crystal. Hydrogen bonds are drawn as dashed lines.

Synthesis and crystallization
5.1. Preparation of ferrocene-1,1 0 0 0 -dicarboxylic acid (Gao et al., 2009) An 8% NaOCl aqueous solution (100 ml) was added dropwise to 1,1 0 -diacetylferrocene (5.37 g, 20 mmol) under stirring at a temperature of 317-320 K. The solution was stirred at this temperature for 2 h. Three more 25 ml portions of NaOCl solution were added every 2 h. The reaction mixture was filtered and acidified to a pH of 1.1 with 10% hydrochloric acid and cooled to 277 K overnight. The yellow precipitate which formed was filtered off and recrystallized from ethanol to give an orange microcrystalline powder (yield 2.18 g, 40%).

Preparation of 3-aminopyridinium 1
Ferrocene-1,1 0 -dicarboxylic acid (50 mg, 0.18 mmol) was dissolved in methanol and mixed with a methanolic solution of 3-aminopyridine (33.8 mg, 0.36 mmol). The reaction mixture was filtered and subjected to slow evaporation at room temperature to give orange crystals of the title salt.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were located from a difference Fourier synthesis and refined isotropically without constraints or restraints. Computer programs: APEX2 and SAINT (Bruker, 2008) and SHELXTL (Sheldrick, 2008).

3-Aminopyridinium 1′-carboxyferrocene-1-carboxylate
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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.