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

Abstract

[eng] 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) [C5 H7N2 = 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.