On the importance of non covalent interactions in the structure of coordination Cu(ii) and Co(ii) complexes of pyrazine- and pyridine-dicarboxylic acid derivatives: experimental and theoretical views

Abstract

Three Cu and one Co complexes of three pyridine-2,6-dicarboxylic acid derivatives were synthesized by proton transfer reactions. They are formulated as {Cu[(ampym)(hypydc)(H₂O)]}·H₂O (1), {[Co(pyzdc)(H₂O)2]·H₂O}n (2), [Cu(dipic)(μ-dipic)Cu(II)(H₂O)5]·2H₂O (3) and [Cu3(dipic)4(en)2]·enH₂·4H₂O (4) where ampym = 2-amino-4-methylpyrimidine, hypydc-4-hydroxypyridine-2,6-dicarboxylic acid, pyzdc = pyrazine-2,3-dicarboxylic acid, dipic = pyridine-2,6-dicarboxylic acid, en = ethylenediamine. The complexes have been characterized by single crystal X-ray diffraction. Complex 1 is five-coordinated with a distorted square pyramidal geometry around Cu(II) where hypydc acts as a tridentate ligand, ampym as a monodentate ligand and one water molecule coordinated in the axial position. Complex 2 is a linear polymer containing a six-coordinated Co(II) metal center, which is surrounded by N and O atoms from two pyzdc ligands and two coordinated water molecules. Complex 3 is a binuclear compound containing two six-coordinated Cu(II) ions, one coordinated to two dipic ligands while the other one is coordinated by five water molecules and one oxygen atom belonging to the bridging carboxylate group. Complex 4 is a trinuclear complex where two identical ions are hexa-coordinated to four carboxylate groups belonging to two dipic ligands and central ion is coordinated to two ethylenediamine ligands and two oxygen atoms of the dipic ligands. The non-covalent interactions that play important roles in the stabilization of the crystal structures have been analysed for several compounds by means of Density Functional Theory (DFT) calculations and characterized using the Bader's theory of “atoms in molecules” (AIM). The evaluation of the binding energies associated to each noncovalent interaction is useful for rationalizing their influence in the crystal packing. The formation of an unprecedented lp–π-hole interaction in 1 is remarkable.