Aromaticity in tautomers of triazoleporphyrazine

Abstract

The effect of tautomerism on the aromaticity was studied for the free base triazoleporphyrazine at the Density Functional Theory level, using double-zeta double polarized basis sets. The porphyrazine (1), the three tautomers (2a, 2b and 2c) as well as the Ni complex (3) of triazoleporphyrazine were examined. Two independent aromaticity criteria, the geometry-based index HOMA and the magnetic properties-based index NICS, were used to estimate the aromaticity of the complete and selected fragments of the above compounds. Results show that structure 2a, which has the H atom in position 1 of the triazole ring, has low aromaticity, although it contains structural fragments with large conjugation. The aromatic character of both tautomers 2b and 2c is similar to that of both the porphyrin and the porphyrazine. The central ring contour was found to be the most aromatic part of the triazoleporphyrazine tautomers. Occupancy of the central position of the macrocycle cavity of the triazoleporphyrazine by a metal atom increases the aromaticity of the macrocycle as a whole, but reduces the conjugation of the central cross sub-unit. In conclusion, both criteria can be used confidently for evaluating the aromaticity for the studied systems.

Structural analogs of tetrapyrrole macrocycles and their biological properties

“…64 Calculations with full geometry optimization demonstrated that the configurations correspond ing to minima on the potential energy surface are planar.…”

“…64, as well as Table 2 and Scheme 11) using the HOMA (Harmonic Oscillator Model of Aroma ticity) 65 and NICS (Nucleus Independent Chemical Shifts) criteria. 66, 67 As can be seen from Table 2, the overall aromaticity increases in the series 40A < 40B < 40C < 41, which corresponds to the tendency to equalization of the bond lengths in these molecules.…”

Aromaticity from the Viewpoint of Molecular Geometry: Application to Planar Systems

“…Similar results were obtained for planar triazoleporphyrazines. 272 The aromaticity increases in the sequence a < b < c (see Figure 10) with increasing stability of the tautomer. The values of HOMA for the whole molecule and NICS in the center of the macrocycle correlate fairly well and are similar to those obtained for free base porphyrin for the most stable tautomer (0.666 and −16.5 ppm, respectively).…”

Vibrational analysis of Ni(II) complex with 4,12-ditolyl-16,24-diphenyl-3-thiaporphyrin (SDTDPPNi(II)Cl) and its isotopic labeled (61Ni(II), −d6, and −d10) derivatives

“…Increase of the EN value confirms this suggestion. Due to the presence of the symmetry plane, During recent years, many papers concerning the HOMA indexes of the porphyrin core with protons (-H) in the meso-positions were published [33,34], while studies on the meso-substituted porphyrins are few [35]. For example, Cyrański and coworkers optimized the structure of the TPPMg(II) complex at B3LYP/6-31+G* DFT level and received HOMA total = 0.671; HOMA pyrrole = 0.556; HOMA internal envelope = 0.906 [35].…”