Porphyrin-like macrocyclic complexes: a spectroscopic and theoretical study of dibenzo [b,i] [1,4,8,11] tetraaza [14] annulenenickel (II)

The electronic structure of dibenzo [b,i][1,4,8,11]tetraaza [14]annulenenickel(II), Nidbtaa, is studied by density functional LCAO approach and is presented together with a detailed description of the bonding. A quantitative analysis of the energetic contributions of the σ and π components to the metal-macrocycle total bond strength points out that the σ contribution is by far dominant. The π-back donation appears to have little energetic contribution. Ultraviolet photoelectron and optical spectra are reported for Nidbtaa and, for comparison purpose, for the free ligand, H2dbtaa. Satisfactory assignments of the spectra are provided by ΔSCF density functional calculations. Similarities and differences with the spectroscopical behaviour and metal porphyrins, MP, are highlighted. It is stressed that a not negligible π metal-macrocycle interaction is responsible for the red shift of the Soret and Q bands in Nidbtaa, not observed in porphyrins. The gross population analysis of the excited states shows that, as in porphyrins, these bands are due to transitions which consists of a charge rearrangement occurring only within the macrocycle π system.