Revised molecular structure and vibrational spectra of tetraaqua(orotato)nickel(II) monohydrate: band assignment based on density functional calculations

Abstract

The crystal and molecular structure and vibrational spectra of [Ni(orotato)(H2O)4]·H2O have been reinvestigated. Single crystal X-ray diffraction studies have revealed new structural features. It is shown that two uracilate rings from the orotate ligands are linked through the pair of N3–H3⋯O2 (−x+1, −y, −z+1) hydrogen bonds which lead to the formation of centrosymmetric dimer of the title complex in the crystal unit cell. The nickel atom is bound to the deprotonated N1 pyrimidine atom and to the carboxylate oxygen atom of the orotate ligand, and to four water molecules. The lattice water molecule is involved in hydrogen bond to the coordinated water molecule, and not to the N3H group, as previously reported. The Raman and infrared spectra (4000–50cm−1) of the title complex are presented, for the first time. The theoretical frequencies, infrared intensities and Raman scattering activities have been calculated by the second-order Möller–Plesset and density functional (B3LYP) methods with the LanL2DZ and D95V∗∗/LanL2DZ basis sets, respectively. The B3LYP-predicted infrared and Raman spectra show a very good agreement with experiment. Detailed band assignment has been made on the basis of the calculated potential energy distribution (PED). The results provide new information on the strength of nickel–ligand bonding in this complex. The Ni–N1(pyrimidine) stretching vibration is assigned to the strong Raman band at 256cm−1, and the Ni–O(carboxylate) stretching vibration is assigned at 357cm−1. It is demonstrated that B3LYP method is extremely helpful in the reliable assignment of vibrational spectra of the Ni(II) complex with orotic acid.