Hydrogen Bond Triggers the Self-Assembly of Dihydrogen Arsenates into Supramolecular Anion···Anion Adducts

Eight H-bonded complexes of arsenic acid with nitrogen bases (diethylamine, 4-methoxypyridine, pyridine-2,6-diamine, 2,4,6-trimethylpyridine, N1,N1,N2,N2-tetraethylethane-1,2-diamine, 1,3,5-triazine-2,4,6-triamine, pyridin-4-ol and 4-methoxyaniline) were studied in the solid state by single crystal X-ray diffraction technique and DFT calculations. In all cases quite short (≤ 2.65 Å) OHO bonds were found in the self-assembled supramolecular infinite chains or ribbons of dihydrogen arsenates, constituting a repertoire of five different H-bonding patterns (motifs). The electron localization function maps revealed the spots of the nucleophilic sites on oxy-gen atoms that determine the preferable directions for H-bonding of H2AsO4– anions observed in the crystal packing. Analysis of the electrostatic potential maps for isolated species has demonstrated that upon H-bonding between H2AsO4– anions and proto-nated nitrogen bases, NH+···–OAsO(OH)2, the redistribution of electron density within the anion provides otherwise virtually non-existent electrophilic sites on hydrogen atoms, which balances the Coulomb repulsion and allows for the anion···anion pairing within the crystal. The topological analysis of calculated electron density after relaxation of the hydrogen atoms’ positions was used to classify the OHO bonds as moderately strong ones (with an interaction energy up to 65 kJ/mol) and revealed a high degree of ionicity of molecular moieties within zwitterions (with an absolute charge up to 0.87 e). For the strongest OHO and NHO bonds the noticeable covalent character was shown by using the crystal orbital Hamiltonian population analysis.