A novel complex of orotic acid (vitamin B 13) with nickel, [Ni(HOr)(NH 3) 2(H 2O) 2]: Crystal structure, vibrational spectra and density functional study

Abstract

The crystal and molecular structures of a novel orotate complex, [Ni(HOr)(NH 3) 2(H 2O) 2], have been determined by the single crystal X-ray diffraction and vibrational spectroscopic methods. The blue complex crystallizes in the monoclinic system (space group P2 1/ n). The nickel atom is chelated by the orotate ligand through the deprotonated carboxylate oxygen atom and the deprotonated nitrogen atom (N1) from the uracilate ring. One ammonia nitrogen atom and one water oxygen atom are lying in the molecular plane. The other water and ammonia molecules complete the distorted octahedral environment around nickel. The Raman and infrared spectra of the title complex are reported. The molecular geometry, vibrational frequencies, infrared intensities and Raman scattering activities have been calculated by two unrestricted density functional theory methods, B3LYP, and the newly developed M05-2X functional. It is demonstrated that M05-2X performs better than B3LYP in prediction of the nickel–ligand atom distances and the bond lengths in the uracil ring. However, the M05-2X method significantly overestimates the strength of intramolecular (interligand) hydrogen bond, in the complex. On the other hand, the frequencies of the nickel–ligand vibrations calculated by M05-2X are in good agreement with experiment. Detailed vibrational assignments have been made on the basis of the calculated potential energy distribution, PED. The very strong Raman band at 1222 cm −1 is diagnostic for the N1-deprotonation of the uracil ring and formation of the nickel–nitrogen bond. The Ni–O (carboxylate) stretching vibration is observed at 289 cm −1 in the FT-IR spectrum, while the Ni–N1 (ring) stretching vibration contributes predominantly to the bands at 269 cm −1 (Raman) and 189 cm −1 (IR). These assignments can be very helpful in the interpretation of vibrational spectra of other Ni(II) complexes with vitamin B 13.