DFT investigation onto axial ligand effects on the TPP ligand and its manganese complexes [Mn(TPP)(O)(X)] (X=F-, Cl-, Br-)

A theoretical study on the structure and nature of the Mn-X bond in tetraphenylporphyrin and its manganoporphyrines containing halogen as an axial ligand [Mn(TPP)X] {X=F, Cl and Br} was carried through density functional B3LYP/6-311G and M06/def2TZVP methods. Comparison of length and angle of bonds in the optimized geometry of [Mn(TPP)Cl] to its X-ray single crystal structure revealed that M06/def2TZVP basis set resulted in more realistic outcomes. Furthermore, the Mn–X bond was analyzed using AIM and NBO to obtain its nature and order in the presence of different halogen axial ligands. The results showed that the Mn-X bond has a σ nature with no π-back donation from Mn to X ligand and the bond order decreased from F to Br. In addition, the effect of axial ligands on the global and local chemical reactivity descriptors of manganoporphyrins has been discussed.