Coordination-driven self-assembly of thiocyanate complexes of Co(II), Ni(II) and Cu(II) with picolinamide: a structural and DFT study

The new heteroleptic complexes, [M(NCS)(2)(pia)(2)] M Co(II) (1), Ni(II) (2), and [Cu(SCN)(2)(pia)(2)] (3), pia = pyridine-2-carboxamide, were synthesized and characterized. Their single crystal X-ray diffraction structures showed octahedral units, with the two thiocyanate ligands occupying cis-positions and binding through the nitrogen atom in Co(II) and Ni(II). In the Cu(II) complex, they were trans and S-bound. The crystal structures display a 2-d structure with NH center dot center dot center dot S hydrogen bonds for the Co(II) and Ni(II), forming tetrameric units, and are isomorphous with the Zn(II) complex, [Zn(NCS) 2(pia) 2], and a 2-d structure with NH center dot center dot center dot N for the Cu(II). DFT calculations were performed on the new complexes and the analogous polymorphs of [Zn(NCS)(2)(pia)(2)], including a second one containing dimeric motifs. The calculated vibrational modes of the thiocyanate ligands corroborate the experimental ones and reflect the coordination mode of the ligand. A comparison between the two Zn(II) polymorphs showed that the NH center dot center dot center dot S bond in the tetramer is stronger (-7.50 kcal mol(-1) per metal) than the NH center dot center dot center dot O bond in the dimer (-4.01 kcal mol(-1) per metal), indicating a preference for the formation of tetrameric units. The NH center dot center dot center dot N hydrogen bonds calculated in the Cu(II) crystal are stronger (-9.15 kcal mol(-1) per metal) than the NH center dot center dot center dot S ones in Ni(II) and Zn(II).