Experimental Observation of Supramolecular Carbonyl-π/π-π/π-carbonyl and Carbonyl-π/π-π/π-anion Assemblies Supported by Theoretical Studies

Mononuclear nickel(II) and copper(II) complexes, namely, (C6H9N2)2[Ni(ntaH)2] (1), (C5H7N2)2[Cu(mal)2(H2O)2] (2), and (C5H7N2)4[Ni(mal)2(H2O)2](NO3)2 (3) [ntaH3 = nitrilotriacetic acid; C6H8N2H = protonated 2-amino-4-picoline; malH2 = malonic acid; C5H7N2 = protonated 2-aminopyridine] have been synthesized in water and their crystal structures have been determined by single crystal X-ray diffraction. In all the complexes, robust bimolecular cyclic hydrogen bonding R22(8) motifs are observed between the protonated heteroaromatic N-rings and the metal-carboxylate complexes. Moreover, the aromatic molecules are engaged in carbonyl···π and anion···π interactions with the noncoordinated carbonyl moieties of the metal complexes and with a nitrate ion respectively, giving rise to remarkable carbonyl···π/π···π/carbonyl···π and carbonyl···π/π···π/π···anion interactions having sandwich type topologies. In 1 and 3, the carbonyl···π interactions are one of the weak forces responsible for the stabilization of the final assemblies, whereas in 2, it strongly contributes to the stabilization of the one-dimensional tape generated from monomeric [Cu(mal)2(H2O)2]2− units. Density functional theory studies reveal a high stability of the unique lone pair···π/π···π/lone pair···π supramolecular self-assembly observed in compound 2 and confirm the favorable formation of the lone pair···π/π···π/π···anion array present in compound 3. Compounds 1−3 are compared in terms of synthetic aspects and supramolecular interactions with analogous complexes, that is, {[Cu(mal)2](picH)2·5H2O}n (4), {[Cu(mal)2](picH)2·2H2O}n (5), (picH)2[M(mal)2(H2O)2]·4H2O (M = Ni/Co/Mn) (6−8), recently obtained with picoline (picH = protonated picoline). The self-assembly pathways involved in the recognition of the heterocyclic amines by the metal-carboxylate complexes and the occurrence of lone pair (l.p.)···π and anion ···π interactions are examined and related.