1804 https://doi.org/10.1107/S2056989018015827 Acta Cryst. (2018). E74, 1804–1807 research communications Received 5 November 2018 Accepted 7 November 2018 Edited by P. McArdle, National University of Ireland, Ireland Keywords: oxalate; hydrogen bonding; ammo- nium cations; crystal structure. CCDC references: 1877733; 1877732; 1877731 Supporting information: this article has supporting information at journals.iucr.org/e Hydrogen-bonding chain and dimer motifs in pyridinium and morpholinium hydrogen oxalate salts David Z. T. Mulrooney, Eimear C. Madden, Rhona F. Lonergan, Valentyna D. Slyusarchuk, Helge Mu ¨ller-Bunz and Tony D. Keene* School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland. *Correspondence e-mail: tony.keene@ucd.ie We present here three compounds consisting of pyridinium or morpholinium hydrogen oxalates, each displaying different hydrogen-bonding motifs, resulting in chains for 4-(dimethylamino)pyridinium hydrogen oxalate 0.22-hydrate, C 7 H 11 N 2 + C 2 HO 4  0.22H 2 O(1), dimers for 4-tert-butylpyridinium hydrogen oxalate, C 9 H 14 N + C 2 HO 4  (2), and chains for morpholinium hydrogen oxalate, C 4 H 10 NO + C 2 HO 4  (3). 1. 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). 2. Structural commentary Compound 1 crystallizes in the triclinic space group P 1. The asymmetric unit of 1 (Fig. 1) consists of two 4-dimethyl- aminopyridinium cations, two hydrogen oxalate anions and a partial-occupancy water molecule [44.3 (4)% occupancy]. The ISSN 2056-9890