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

Abstract

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···O hydrogen bonds of type R₂²(9) and R₂¹(7). Typically, the ionic tapes are further connected to form sheets or other assemblies by additional O–H···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···O hydrogen bonds of type R₂²(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