Rare stoichiometry of carboxyl–carboxylate benzbetaine complexes: in vitro versus in silico

Abstract

The first betaine–mineral acid complex of 3 : 2 stoichiometry, as well as a 1 : 1 complex common for this class of compounds, has been obtained. Crystalline [di-(3-trimethylammonium-benzoic acid)–(3-trimethylammonium-benzoate)]diiodide (1) is the only betaine–mineral acid 3 : 2 complex obtained so far. Two concomitant polymorphs of this 3 : 2 complex, 1-I and 1-II, have been isolated and characterized by X-ray diffraction and FTIR spectroscopy. Polymorphs 1-I, of monoclinic space group P2₁/c, and 1-II, of orthorhombic space group Pccn, have very similar lattices and aggregates of one zwitterionic molecule and two cations O–H⋯O bonded into a 3-membered catemeric carboxyl–carboxylate–carboxyl interval, but polymorph 1-II dissolves better in methanol than 1-I. Alternative hypothetical aggregates of 3-trimethylammonium-benzoate hydroiodide 1 : 1, 2 : 1 and 3 : 2 complexes optimized at the B3LYP/6-31G(d,p) level of theory suggest that the betaine-to-acid 3 : 2 stoichiometry is favoured at the stage of small cluster aggregation and not due to the complex crystal packing preferences.