research papers 682 https://doi.org/10.1107/S2053229617011172 Acta Cryst. (2017). C73, 682–687 Received 6 May 2017 Accepted 28 July 2017 Edited by M. Kubicki, Adam Mickiewicz University, Poland Keywords: bifurcated hydrogen bonding; fluorous weak interaction; crystal structure; halogen bonding; pyridinium salt; supra- molecular interactions; synthon. CCDC references: 1565383; 1565382 Supporting information: this article has supporting information at journals.iucr.org/c Weak hydrogen and halogen bonding in 4-[(2,2-difluoroethoxy)methyl]pyridinium iodide and 4-[(3-chloro-2,2,3,3-tetrafluoropropoxy)- methyl]pyridinium iodide Norman Lu, a * Rong-Jyun Wei, a Hsing-Fang Chiang, a,b Joseph S. Thrasher, b Yuh-Sheng Wen c and Ling-Kang Liu a,c a Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan, b Department of Chemistry, Clemson University, Advanced Materials Research Laboratory, 91 Technology Drive, Anderson, SC, USA, and c Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan. *Correspondence e-mail: normanlu@mail.ntut.edu.tw To enable a comparison between a C—HX hydrogen bond and a halogen bond, the structures of two fluorous-substituted pyridinium iodide salts have been determined. 4-[(2,2-Difluoroethoxy)methyl]pyridinium iodide, C 8 H 10 F 2 - NO + I , (1), has a –CH 2 OCH 2 CF 2 H substituent at the para position of the pyridinium ring and 4-[(3-chloro-2,2,3,3-tetrafluoropropoxy)methyl]pyridinium iodide, C 9 H 9 ClF 4 NO + I , (2), has a –CH 2 OCH 2 CF 2 CF 2 Cl substituent at the para position of the pyridinium ring. In salt (1), the iodide anion is involved in one N—HI and three C—HI hydrogen bonds, which, together with C—HF hydrogen bonds, link the cations and anions into a three-dimensional network. For salt (2), the iodide anion is involved in one N—HI hydrogen bond, two C—HI hydrogen bonds and one C—ClI halogen bond; additional C— HF and C—FF interactions link the cations and anions into a three- dimensional arrangement. 1. Introduction The role of hydrogen bonding has long been recognized as controlling structures in a wide variety of materials (Jeffrey & Saenger, 1991; Jeffrey, 2003; Steiner, 2002; Nagy, 2014). In general, hydrogen bonding spans from the covalent limit (e.g. HF 2 anion, 40 kcal mol 1 ; 1 kcal mol 1 = 4.184 kJ mol 1 ) to the electrostatic limit (e.g. NH 4 + Cl , 15 kcal mol 1 ). The even weaker C—HF hydrogen bond is on the van der Waals limit (0.25 kcal mol 1 ) (Desiraju, 2002; Desiraju & Steiner, 1999). Alternatively, studies on halogen-bonding interactions with other electronegative species have shown that halogen bonding, where the halogen atom acts as a Lewis base acceptor, is driven by a region of positive electrostatic potential (i.e. the -hole) which arises from the anisotropic distribution of electron density within the halogen atom (Jagarlapudi et al., 1986; Price et al. , 1994; Metrangolo et al. , 2008; Politzer et al., 2010). Halogen bonds have found increasing practical applications, such as in the discovery and development of active pharmaceutical ingredients (APIs) (Wilcken et al., 2012; Baldrighi et al., 2013). A halide ion may serve as a hydrogen-bond acceptor or as a halogen-bond acceptor. For example, pyridinium halides in the solid state were known to have the halide ion exhibiting four hydrogen bonds, arranged in a distorted tetrahedral geometry (Mootz & Hocken, 1989), forming alternating cation–anion chains that are crosslinked using halide ions as ISSN 2053-2296 # 2017 International Union of Crystallography