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

Abstract

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⋯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⋯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⋯S bond in the tetramer is stronger (−7.50 kcal mol⁻¹ per metal) than the NH⋯O bond in the dimer (−4.01 kcal mol⁻¹ per metal), indicating a preference for the formation of tetrameric units. The NH⋯N hydrogen bonds calculated in the Cu(II) crystal are stronger (−9.15 kcal mol⁻¹ per metal) than the NH⋯S ones in Ni(II) and Zn(II).