Hydrogen-bonding chain and dimer motifs in pyridinium and morpholinium hydrogen oxalate salts

Three compounds consisting of pyridinium or morpholinium hydrogen oxalates each display different hydrogen oxalate hydrogen-bonding motifs, resulting in chains for 4-(dimethylamino)pyridinium hydrogen oxalate 0.22-hydrate, dimers for 4-tert-butylpyridinium hydrogen oxalate and chains for morpholinium hydrogen oxalate.


Chemical context
Oxalate is a common ligand in coordination chemistry, utilized for its ability to chelate and bridge metal ions to form complexes and coordination polymers (Decurtins, 1999). Its ability to facilitate strong magnetic interactions and stability under differing synthetic conditions makes it a ligand of choice for the rational design of magnetic materials (Pilkington & Decurtins, 2003). As the simplest dicarboxylic acid, it can also be found in differing states of deprotonation, providing a range of hydrogen-bonding motifs. Oxalate also has the unusual property of containing a C-C bond with a bond order of slightly less than one, resulting in the carboxylate moieties taking a perpendicular orientation in gas phase calculations (Herbert & Ortiz, 2000). While this structure is the most energetically favourable, the difference in energy between the 90 and 0 torsion angles is slight and is often overridden in hydrogen-bonded structures. Ammonium hydrogen oxalate salts are often useful precursors in the formation of transition metal complexes (Keene et al., 2003) and coordination polymers (Keene et al., 2004). Our research group has an interest in these precursors as part of our investigations into molecular magnets (Keene, et al. 2010), not only for their usefulness in this role, but for the complex hydrogen-bonded structures that often arise on crystallization. Previous work from our group has focused on the structure of discrete oxalate dianions and drawn correlations between torsion angles, bond lengths and the crystal packing (Keene et al., 2012).
Compound 2 crystallizes in the monoclinic space group P2 1 /c. The asymmetric unit of 2 ( Fig. 2) consists of two 4-tbutylpyridinium cations and two hydrogen oxalate anions.
Compound 3 crystallizes in the monoclinic space group P2 1 / c. The asymmetric unit of 3 (Fig. 3) consists of one morpholinium cation and one hydrogen oxalate anion. The hydrogen oxalate moiety is near to planar with torsion angles of À11.3 (2) and À12.0 (2) .
In compound 2, the hydrogen oxalate moieties form hydrogen-bonded pairs (Table 3) with a four-membered ring formed at the centre of the pair. The opposite sides of the oxalates form a bifurcated hydrogen bond to the 4-t-butylpyridinium groups, generating a supramolecular tecton. These are then built into the three-dimensional structure through C-HÁ Á ÁO interactions. The presence of the t-butyl groups suppressesstacking due to steric interference with no obvious C-HÁ Á Á interactions present.

Figure 2
Asymmetric unit of 2. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
Asymmetric unit of 3. Displacement ellipsoids are drawn at the 50% probability level.

Figure 1
Asymmetric unit of 1. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Selected geometric parameters (Å , ) for (1). In compound 3, the hydrogen oxalates form a chain along the a-axis direction. These chains form the core of the structure with hydrogen bonds (Table 4) coming from the morpholinium along with C-HÁ Á ÁO interactions that form the three-dimensional structure.

Database survey
Hydrogen-bonding motifs in hydrogen oxalate compounds often tend towards chain formation. Different chain types are formed depending on the conformation of the hydroxyl group, i.e. whether the O-H bond is cis or trans to the C-C bond. In compound 3, the hydrogen oxalate is the trans conformer and produces a chain along the a-axis direction and is comparable to compounds reported in the Cambridge Structural Database (CSD version 5.39, updated August 2018, Groom et al., 2016, such as ACOQER (Mora et al., 2017) and FOMBIU (Traut-Johnstone et al., 2014). The hydrogen oxalates in compound 2 are in the cis conformation and form a hydrogen-bonded pair, as seen in a small handful of structures: the combination of this pair-wise interaction with a birfurcated hydrogen bond to a pyridinium cation is also seen in EZECOC (Androš et al., 2011;Chen et al. 2012,), GULQOV (Thomas et al., 2015;Suresh et al., 2015), LOFMAW (Hu et al., 2014), YEPBAX (Said et al., 2006), YINVUO (Martin et al., 2013) and XEJRIQ (Edwards & Schafer, 2017). The chain type in 1 is not seen in any hydrogen oxalate compounds in the CSD.

Synthesis and crystallization
Compound 1 was synthesized by adding a solution of 4-dimethylaminopyridine (1.0 mmol, 122 mg) in water (10 ml) and oxalic acid dihydrate (126 mg, 1.0 mmol) in water (10 ml). The resultant solution was left to evaporate to a white powder and was then recrystallized from hot acetonitrile to give colourless crystals suitable for single-crystal X-ray diffraction.
The synthesis of compound 2 was achieved by addition of anhydrous oxalic acid (900 mg, 10 mmol) in distilled water (10 ml) to a non-miscible mixture of 4-t-butylpyridine (1.465ml, 10 mmol) and distilled water (10 ml) to give a homogenous solution. This was left to evaporate over five days and the white product recrystallized from hot methanol.
Compound 3 was synthesized by adding a solution of oxalic acid dihydrate (1271 mg, 10 mmol) in water (10 ml) to a solution of morpholine (862 ml, 871 mg, 10 mmol) in water (10 ml) and leaving the resultant solution to evaporate until crystals had formed.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. In all cases, the proton of the hydrogen oxalate was placed according to C-O bond lengths (O-H = 0.84 Å ). All other H atoms were positioned geometrically (N-H = 0.88, O-H = 0.97, C-H = 0.95-0.98 Å ) and refined as riding with U iso (H) = kU eq (parent atom) where k = 1.2 for all C-H and N-H groups and 1.5 for Cmethyl, Ohydroxy and Owater.

Figure 4
Hydrogen bonding in hydrogen oxalate groups: (a) chain formed in compound 1, (b) hydrogen-bonded dimer tecton in compound 2 and (c) chain formed in compound 3. [Please include the cell axes] O28 carboxylate in 1 were unsuccessful, leading to a poorquality refinement. Attempts to locate extra symmetry in compound 2 were unsuccessful, despite superficially appearing to have an inversion centre between the 4-tbpy moieties and between the hydrogen oxalate moieties.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.25550 (8) (10) 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.