A DFT and 59Co Solid-State NMR Study of the Chemical Shielding Property and Electronic Interaction in the Metalloporphyrin System

Abstract

The hybrid density functional theoretical method has been successfully applied to the computation of the 59Co NMR chemical shifts and shift tensor components in a hexacoordinated Co(III) porphyrin system. The calculated results of [Co(TDCPP)(MeIm)2]BF4 at the B3LYP/6-311G** level using experimental geometry give excellent agreement with the observed values obtained by the solid-state NMR method. Furthermore, the electronic properties of a series of [Co(TPxP)(RIm)2]+ (X and R = substituents) systems have been demonstrated to give good correlation with the chemical shielding properties of the central metal as demonstrated by the theoretical calculation. It is proposed that electron-releasing substituents on the porphyrinate ligand transfer electron density to the metal not only via the porphyrin e(π) orbitals but also with the participation of porphyrin nitrogens σ orbitals. Moreover, a number of unresolved issues in the experimental literature concerning the origin of the behavior of the cobalt line widths, axial ligand substitution, and hydrogen bonding have been addressed. An interpretation taking into consideration the influence of the axial ligand orientation on the shielding property of the central metal within the framework of the libration model is proposed. This study demonstrated that the hybrid density functional theoretical method, with its much higher efficiency than that of post-Hartree−Fock methods such as MPn models, will provide a way for understanding the electronic and molecular structures of reasonably large molecules.