Supramolecular assemblies involving anion-pi and lone pair-pi interactions: experimental observation and theoretical analysis

Two nickel(II) malonate coordination compounds, namely (C5H7N2)(4)[Ni(C3H2O4)(2)(H2O)(2)](ClO4)(2) (1) and (C5H7N2)(4)[Ni(C3H2O4)(2)(H2O)(2)](PF6)(2) (2) [ where C5H7N2 = protonated 2-aminopyridine, C3H4O4 = malonic acid, ClO4- = perchlorate, PF6- = hexafluoridophosphate], have been synthesized from purely aqueous media, and their single-crystal structures have been determined by X-ray diffraction. As anticipated, various weak forces, i.e. lone pair-pi, pi-pi and anion-pi interactions, play a key role in stabilizing the self- assembly process observed for both compounds. Intricate combinations of lone pair-pi, pi-pi and anion-pi interactions of the type lone pair-pi/pi-pi/pi-anion-pi/pi-lone pair and lone pair-pi/pi-pi/pi-anion are observed that are fully described, including by computational methods. The theoretical calculations allow estimating the strength of these p contacts.