Molecular Networks Created by Charge-Assisted Hydrogen Bonding in Phosphonate, Phosphate, and Sulfonate Salts of Bis(amidines)

Two bis(amidines), 2,2'-bi-2-imidazoline (BI) and fluoflavin (FF), were treated with phosphonic, phosphoric, and sulfonic acids in an effort to produce crystalline salts composed of ions linked by networks of charge-assisted hydrogen bonds. As intended, mixing bis(amidine) BI with 1,4-benzenediphosphonic acid and 1,3,5-benzenetriphosphonic acid yielded crystals of phosphonate salts of dication H2BI+2. Structural analyses showed that such salts tend to incorporate tapes composed of alternating dications and anions linked by multiple charge-assisted N-H center dot center dot center dot O hydrogen bonds of type R-2(2)(9) and R-2(1)(7). Typically, the ionic tapes are further connected to form sheets or other assemblies by additional O-H center dot center dot center dot O hydrogen bonds involving phosphonate anions. An analogous reaction of the more weakly basic bis(amidine) FF with 1,4-benzenedisulfonic acid yielded only a sulfonate salt of monocation HFF+; however, diprotonation could be achieved by phosphoric acid to produce a crystalline salt built from stacks of H2FF+2 dications linked to phosphate anions by charge-assisted N-H center dot center dot center dot O hydrogen bonds of type R-2(2)(8). Together, these results demonstrate that acids with multiple PO(OH)(2) and SO2OH groups can react with bis(amidines) to produce salts with structural features resulting predictably from the geometry of the ions and their ability to engage in multiple charge-assisted hydrogen bonds according to standard patterns.