Two isostructural complexes of Ni(ii) and Zn(ii) with violurate and pyridine: a detailed structural, theoretical, magnetic, and NMR investigation

Abstract

Two new metal(II) complexes [M(dmv)₂(py)₂]·3H₂O {M = Ni (1), Zn (2)} were self-assembled from a metal(II) chloride salt, N,N′-dimethylvioluric acid (Hdmv), and pyridine (py). They were isolated as stable crystalline solids and characterized, including by single crystal X-ray diffraction. Structures 1 and 2 are composed of discrete [M(dmv)₂(py)₂] blocks in which the six-coordinate metal(II) atoms show a slightly distorted {MN₄O₂} octahedral geometry. The adjacent [M(dmv)₂(py)₂] units are hydrogen bonded with an involvement of disordered water clusters into 1D H-bonded chains. DFT calculations were used to analyse the unconventional anti-parallel CO⋯CO interaction observed between the monomeric units in 1 and 2. Complex 1 also refers to an elongated tetragonal bipyramid for which a positive zero-field splitting is confirmed by magnetic studies. The complex matches the magnetostructural D-correlation for hexacoordinate Ni(II) complexes with moderate structural and magnetic anisotropy with D_str = +7.45 pm, and D_mag = +5.11 cm⁻¹. Solution state NMR spectroscopy reveals a slow isomerization process related to the bidentate-N,O and bidentate-O,O coordination of dimethylviolurate in 2, with the barrier ranging from 76.4 to 77.9 kJ mol⁻¹. The ¹H NMR spectra confirm the presence of three water molecules which exhibit significant chemical shift dependence on temperature indicating strong binding to the complex in solution phase.