Supramolecular Assembly of Mg(II) Complexes Directed by Associative Lone Pair−π/π−π/π−Anion−π/π−Lone Pair Interactions

Two Mg(II) malonate complexes with protonated 2-aminopyridine and protonated 2-amino-4-picoline as counterions, namely, (C5H7N2)4[Mg(C3H2O4)2(H2O)2](ClO4)2 (1) and (C6H8N2H)2[Mg(C3H2O4)2(H2O)2]·4H2O (2) [C5H7N2 = protonated 2-aminopyridine, C3H4O4 = malonic acid, C6H8N2H = protonated 2-amino-4-picoline], have been synthesized from purely aqueous media, and their crystal structures have been determined by single-crystal X-ray diffraction. The role of lone pair···π interactions in stabilizing the self-assembly process appears to be of great importance in both complexes. Additional weak forces like anion···π and noncovalent O···O interactions are also found to be operating in 1. A rare combination of lone pair···π and anion···π interactions in 1, of the type lone pair···π/π···π/π···anion···π/π···lone pair, is observed, and this unusual supramolecular network is fully described here. An attempt to prepare an analogous complex with 2-amino-4-picoline resulted in 2, which is isomorphous with our recently reported transition-metal complexes of the type (C6H8N2H)2[M(C3H2O4)2(H2O)2]·4H2O (M = Ni/Co/Mn). A high-level DFT-D study (RI-B97-D/TZVP) has been used to characterize the different noncovalent interactions present in the solid state. We have also analyzed some crystal fragments to examine energetically some important assemblies that drive the crystal packing. Finally, we have studied the influence of magnesium on some hydrogen-bonding interactions.