Anion Induced Formation of Supramolecular Associations Involving Lone pair-π and Anion-π Interactions in Co(II) Malonate Complexes: Experimental Observations, Hirshfeld Surface Analyses and DFT Studies

Three Co(II)-malonate complexes, namely, (C5H7N2)(4)[Co(C3H2O4)(2)(H2O)(2)](NO3)(2) (1), (C5H7N2)(4)[Co(C3H2O4)(2)(H2O)(2)](ClO4)(2) (2), and (C5H7N2)(4)[Co(C3H2O4)(2)-(H2O)(2)](PF6)(2) (3) [C5H7N2 = protonated 2-aminopyridine, C3H4O4 = malonic acid, NO3- = nitrate, ClO4- = perchlorate, PF6- = hexafluorophosphate], have been synthesized from purely aqueous media, and their crystal structures have been determined by single crystal X-ray diffraction. A thorough analysis of Hirshfeld surfaces and fingerprint plots facilitates a comparison of intermolecular interactions in 1-3, which are crucial in building supramolecular architectures. When these complexes are structurally compared with their previously reported analogous Ni(II) or Mg(H) compounds, a very interesting feature regarding the role of counteranions has emerged. This phenomenon can be best described as anion-induced formation of extended supramolecular networks of the type lone pair-pi/pi-pi/-anion-pi/pi-lone pair and lone pair-pi/pi-pi/pi-anion involving various weak forces like lone pair-pi, pi-pi, and anion-pi interactions. The strength of these Ir contacts has been estimated using DFT calculations (M06/6-3I+G*), and the formation energy of the supramolec-ular networks has been also evaluated. The influence of the anion (NO3-, ClO4-, and PF6-) on the total interaction energy of the assembly is also studied.