Theoretical and spectroscopic study of nickel(II) porphyrin derivatives

A set of substituted (sulfonate, amino) nickel porphyrin derivatives such as phthalocyanine and phenylporphyrin was studied by spectroscopic (UV-vis, FTIR, XPS) and quantum-chemical methods. The Q and Soret bands were identified in the UV-vis spectra of aquo solutions of the tetrasulfo-substituted complexes and in DMF and ACN solutions of the amino-substituted phenylporphyrin and phthalocyanine Ni(II) complexes, respectively. In all the complexes the frontier molecular orbitals predict that the oxidation and reduction sites are localized on the ligand rather than in the metal atom. A natural bonding orbital (NBO) analysis of all the complexes showed that a two-center bond NBO between the pyrrolic nitrogens (N-pyrr) and the nickel atom does not exist, the N-pyrr center dot center dot center dot Ni interaction occurring instead by a delocalization from one lone pair of each N-pyrr toward one lone pair of the nickel atom, as estimated by second-order perturbation theory. The calculated values of electronic transitions between the frontier molecular orbitals are in good agreeement with the UV-vis data. At the theoretical level, we found that while the ligand effect is more important in the Q-band (similar to 16 kcal/mol), the substituent effect is more significant in the Soret band (similar to 9 kcal/mol). A good agreement was also found between the experimental and calculated infrared spectra, which allowed the assignment of many experimental bands. The XPS results indicate that the Ni(II) present in the phenylporphyrin structure is not affected by a change of the substituent (sulfonate or amino).