Structure and stability of 2-aza-21-carbaporphyrin tautomers prearranged for coordination

2-Aza-21-carba-5,10,15,20-tetraarylporphyrin (CTPPH2, inverted porphyrin) and its methylated derivatives, acting as ligands, can be structurally related to the following hypothetical tautomers: 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (2-NH-CTPPH); 2-aza-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (21-H-21-CH3CTPPH); 2-aza-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (2-NH-21-CH3CTPPH); 2-aza-2,21-dimethyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (2-NCH3-21-CH3CTPPH). Their structures and electronic energies have been investigated applying the density functional theory (DFT) for idealized 21-carbaporphyrin, created by a replacement of phenyl and methyl groups with hydrogen, yielding CPH2, 21-H-CPH, 2-NH-CPH, and 2-NH-21-H-CP. The B3LYP/6-31G optimized bond lengths and angles of 21-carbaporphyrin skeletons are in satisfactory agreement with X-ray crystallographic values when compared in structurally related pairs, i.e., CPH2 and CTPPH2, 21-H-CPH and (21-CH3TPP)Ni-II, and 2-NH-CPH and (2-NCH3-21-CH3CTPP)(NiI)-I-II. The calculated total electronic energies, using the B3LYP/6-31-G**//B3LYP/6-31G approach, demonstrate that relative stability of the postulated tautomers decreases in the order CPH2 > 21-H-CPH > 2-NH-CPH much greater than 2-NH-CPH2 > 2-NH-21-H-CP. The difference between the electronic energy of the regular porphyrin PH2 and the fundamental tautomer 21-carbaporphyrin CPH2 is calculated to be 20.41 kcal/mol. The DFT calculation reproduced properly the distribution of mobile protons for the tautomer of the lowest energy for each stable 21-carbaporphyrin and provided some insight into the analysis of energy required for the suggested prearrangement process imposed by insertion of a metal ion. The spectacular flexibility of the ligand structure including a reversible trigonal-pyramidal geometry change at the C(21) center has been also discussed.