Narrow Results

The structure and properties of the N${}_{(\mathrm{\text{ax}})}$–Mn–O bonds of high valent manganese-oxo of the second and third generation porphyrins in the presence of imidazole have been studied by means of density functional (DFT) method with 6-31G* basis set in the gas phase as well as water solution. The geometric structures, frontier molecular orbitals, thermodynamic parameters, aromaticity indices and physical properties such as chemical potential and chemical hardness of [(TPP)(ImH)MnO]${}^{+}$ and its derivatives were calculated. The obtained results showed that [(TPP)(ImH)MnO]${}^{+}$ bearing halogen atoms at the $\beta$-pyrrole positions had a saddle conformation with low Mn–O strength. The electron density ($\rho )$ and Laplacian (${\nabla }_{\rho }^2)$ properties at critical points of the N${}_{(\mathrm{\text{ax}})}$–Mn–O bonds, estimated by AIM calculations, indicate that Mn–O bonds in third generation porphyrins have lower $\rho$ and Mn–N${}_{(\mathrm{\text{ax}})}$ distances have higher $\rho$ than the second generation ones. The calculations of aromaticity indices for chelated rings at the porphyrin center show that HOMA and NICS in third generation porphyrins are generally lower than that of second ones which is in agreement with their saddle conformation. These results are supported by natural bond orbital (NBO) analysis.

The stereo electronic effects as well as hydrogen bonding effects of imidazole, pyridine and 2,6-dimethylpyridine as N-donor axial ligands on the C–H oxidation activity of high-valent manganese(V)–oxo meso-tetraphenylporphyrin (TPP) and meso-tetrakis(pentaflourophenyl)porphyrin (TPFPP), are investigated by DFT calculations. The electronic and steric properties of axial donors and porphyrin ligands affected on the activation energy of cyclohexane hydroxylation as well as the Mn–O bond strength of the oxo species in transition state. Imidazole with the strong $\pi$-donating ability and the least steric hindrance showed greater co-catalytic activity than those of pyridine and in particularly hindered 2,6-Me₂ pyridine in the presence of simple [(TPP)MnO]${}^{+}$. Nevertheless, the C–H bond activation by hindered and electron-deficient perfluorinated catalyst [(TPFPP)MnO]${}^{+}$ is in the order of pyridine >2, 6-Me₂ pyridine >imidazole. AIM analysis showed hydrogen bonding (HB) between the C–H $\sigma$ bonds of pyridine (C${}_{\mathrm{\text{sp}}2}$-H of ring) and 2,6-Me₂Py (C${}_{\mathrm{\text{sp}}3}$-H of methyl groups) with ortho-C–F bond of phenyl rings of TPFPP in Mn$=$O species (C–H…..F–C hydrogen bond) which might be responsible for this unusual behavior. These results are supported by natural bond orbital (NBO) analysis.