Experimental observation of supramolecular carbonyl-pi/pi-pi/pi-carbonyl and carbonyl-pi/pi-pi/pi-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) [ntaH(3) = nitrilotriacetic acid; C6H8N2H = protonated 2-amino-4-picoline; maIH(2) = 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 R-2(2)(8) motifs are observed between the protonated heteroaromatic N-rings and the metal-carboxylate complexes. Moreover, the aromatic molecules are engaged in carbonyl center dot center dot center dot pi and anion center dot center dot center dot pi interactions with the noncoordinated carbonyl moieties of the metal complexes and with a nitrate ion respectively, giving rise to remarkable carbonyl center dot center dot center dot pi/pi center dot center dot center dot pi/carbonyl center dot center dot center dot pi and carbonyl center dot center dot center dot pi/pi center dot center dot center dot pi/pi center dot center dot center dot anion interactions having sandwich type topologies. In I and 3, the carbonyl center dot center dot center dot pi 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 center dot center dot center dot pi/pi center dot center dot center dot pi/lone pair center dot center dot center dot pi supramolecular self-assembly observed in compound 2 and confirm the favorable formation of the lone pair center dot center dot center dot pi/pi center dot center dot center dot pi/pi center dot center dot center dot 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)center dot 5H(2)O)(n) (4), {[Cu(mal)(2)](picH)(2)center dot 2H(2)O}(n) (5), (picH)(2)[M(mal)(2)(H2O)(2)]center dot 4H(2)O (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.)center dot center dot center dot pi and anion center dot center dot center dot pi interactions are examined and related.