Crystal engineering with coordination compounds of NiII, CoII, and CrIII bearing dipicolinic acid driven by the nature of the noncovalent interactions

Five new proton transfer compounds, (Hapy)[Cr(pydc)(2)]center dot 5H(2)O center dot[pydcH(2)] (1), (Hacr)[Cr(pydc)(2)]center dot 2H(2)O (2), (H9a-acr)[Cr(pydc)(2)]center dot 4.27H(2)O (3), [Co(pydc)(pydcH(2))]center dot 3H(2)O (4) and [Ni(pydcH)(2)]center dot H3O center dot 2H(2)O (5) (where pydcH(2) = pyridine-2,6-dicarboxylic acid; apy = 2-aminopyridine; acr = acridine and 9a-acr = 9-aminoacridine) have been synthesized and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction techniques. Crystallographic analysis revealed that compounds 1-5 have distorted octahedral geometries with pydc(2-) and pydcH(-) ions or pydcH(2) coordinated as tridentate ligands to each metal ion through one oxygen atom of each carboxylate group and the nitrogen atom of the pyridine ring. Compounds 1-3 consist of cationic complexes where the dipicolinic acid is coordinated to the metal ion in the dianionic form, whereas 4 and 5 are neutral complexes. In 4, the acid is coordinated to the metal ion in the neutral form, which is quite rare. In the crystal structures of the different complexes, extensive O-H center dot center dot center dot O, N-H center dot center dot center dot O and C-H center dot center dot center dot O hydrogen bonds, as well as other noncovalent interactions involving aromatic rings (C-H center dot center dot center dot pi, C-O center dot center dot center dot pi and pi center dot center dot center dot pi stacking), were described and analysed by means of density functional theory (DFT) calculations since they play an important role in the construction of three-dimensional supramolecular frameworks.