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(II) 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−π/π–π/π–anion−π/π–lone
pair and lone pair−π/π–π/π–anion
involving various weak forces like lone pair−π, π–π,
and anion−π interactions. The strength of these π
contacts has been estimated using DFT calculations (M06/6-31+G*),
and the formation energy of the supramolecular 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.