Hydrogen bonded binary molecular adducts derived from exobidentate N-donor ligand with dicarboxylic acids: AcidÁÁÁimidazole hydrogen-bonding interactions in neutral and ionic heterosynthons Amal Cherian Kathalikkattil, Subin Damodaran, Kamal Kumar Bisht, Eringathodi Suresh ⇑ Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G.B. Marg, Bhavnagar 364 002, Gujarat, India article info Article history: Received 2 October 2010 Accepted 10 November 2010 Available online 16 November 2010 Keywords: Cocrystal Organic salt Supramolecular assembly Hydrogen bond Molecular adduct pK a values abstract Four new binary molecular compounds between a flexible exobidentate N-heterocycle and a series of dicarboxylic acids have been synthesized. The N-donor 1,4-bis(imidazol-1-ylmethyl)benzene (bix) was reacted with flexible and rigid dicarboxylic acids viz., cyclohexane-1,4-dicarboxylic acid (H 2 chdc), naph- thalene-1,4-dicarboxylic acid (H 2 npdc) and 1 H-pyrazole-3,5-dicarboxylic acid (H 2 pzdc), generating four binary molecular complexes. X-ray crystallographic investigation of the molecular adducts revealed the primary intermolecular interactions carboxylic acidÁÁÁamine (via O–HÁÁÁN) as well as carboxyl- ateÁÁÁprotonated amine (via N–H + ÁÁÁO À ) within the binary compounds, generating layered and two- dimensional sheet type H-bonded networks involving secondary weak interactions (C–HÁÁÁO) including the solvent of crystallization. Depending on the differences in pK a values of the selected base/acid (DpK a ), diverse H-bonded supramolecular assemblies could be premeditated. This study demonstrates the H- bonding interactions between imidazole/imidazolium cation and carboxylic acid/carboxylate anion in providing sufficient driving force for the directed assembly of binary molecular complexes. In the two- component solid form of hetero synthons involving bix and dicarboxylic acid, only H 2 chdc exist as cocrys- tal with bix, while all the other three compounds crystallized exclusively as salt, in agreement with the DpK a values predicted for the formation of salts/cocrystals from the base and acid used in the synthesis of supramolecular solids. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Crystal engineering offers a rational approach to the design of materials with new compositions, properties and crystal struc- tures. Much as an organic chemist employs the covalent bond in the design of target molecules, non-covalent bonds can be exploited in the design of supramolecular assemblies [1,2]. Such non-covalent interactions include hydrogen bonding, van der Waals, p–p stacking, and electrostatic interactions. These kind of molecular interactions could be utilized in designing cocrystals, solvates, organic salts, polymorphs and pseudopolymorphs, etc. [3–7], out of which cocrystals are of widespread interest. Cocrystal [8–10], also referred to as molecular complex, is a homogeneous phase of two or more different components in a stoi- chiometric composition, and often relies on hydrogen bonded assemblies between neutral molecules. Strong and directional hydrogen bonds have been used in rational strategies to design binary/ternary cocrystals in crystal engineering, materials science, host–guest inclusion compounds, and pharmaceutical solids [11]. Understanding of highly specific and mutually complementary non-covalent interactions between molecules is the basic aspect in molecular recognition, cocrystallization and supramolecular synthetic chemistry [12]. By the judicious choice of the supramo- lecular synthons with specific and directional non-covalent connectivity, multidimensional solid state assembly can be gener- ated by the effective close packing of the discrete building blocks [13–15]. Among all the non-bonded interactions, hydrogen bond- ing has proved to be the most useful and reliable because of its strength and directional properties [16]. Many molecular solids with novel properties have been prepared using hydrogen bonding as the main steering force [17–19]. Molecular cocrystals have been known for a long time, still there is wide scope for further applica- tions and structural studies in this field. In recent years, many pre- meditated synthesis of binary and ternary cocrystals [20–24] have been reported. Variety of cocrystals with multidimensional topol- ogy, based on hydrogen bonded networks can be achieved when a N-heterocycle moiety is allowed to interact with a suitable carboxylic acid [14,25]. The ability of carboxylic acids to readily aggregate through the dimer homosynthon have been observed traditionally in structural chemistry. 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.11.022 ⇑ Corresponding author. Tel.: +91 278 2567760x662/667 (office), +91 278 2565006 (residence), moblie: +91 9426910756; fax: +91 278 2567562. E-mail addresses: esuresh@csmcri.org, sureshe123@rediffmail.com (E. Suresh). Journal of Molecular Structure 985 (2011) 361–370 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc