Tropylium and Porphyrinoid Character in Carbaporphyrinoid Systems. Relative Stability and Aromatic Characteristics of Azuliporphyrin and Tropiporphyrin Tautomers, Protonated Species, and Related Structures

Abstract

DFT studies have been carried out on 46 azuliporphyrin (AzP) and tropiporphyrin (TrP) tautomers, cations, and dications. The structures were minimized using DFT-B3LYP/6-311++G(d,p), and the relative stabilities of the tautomers for each series were computed with M06-2X and B3LYP-D3 functionals. Nucleus independent chemical shifts, both NICS(0) and NICS(1) zz , were calculated for both the center of the macrocycles and the individual pyrrolic and carbocyclic subunits. In addition, anisotropy of induced current density (AICD) plots were generated for all of the species under investigation. The results provided insights into the favored conjugation pathways and aromatic properties of these important carbaporphyrinoid systems. The π delocalization pathways for AzP were complex, and contrary to previous speculations, there was no evidence for tropylium characteristics in the favored free base structure. The global diatropicity of AzP was found to increase with increasing solvent polarity, in agreement with experimental observations. TrP was shown to be planar and to possess a large global diamagnetic ring current. However, NICS calculations and AICD plots indicated that the cycloheptatrienyl components of the two most favored TrP tautomers have paratropic characteristics. These results are not fully consistent with the proton NMR spectra for TrPs. A dihydroTrP and a Diels−Alder adduct were investigated, and these showed exceptionally large shielding effects with NICS(1) zz values of −36.53 and −36.92 ppm, respectively, in agreement with experimental results. A silver(III) complex of tropiporphyrin was also examined, and this showed no paratropic character for the seven-membered ring, although in this case, the system was found to be severely twisted. Overall, the results provide important information about the electronic properties of AzP and TrP tautomers, as well as for related protonated species, and complement the extensive experimental studies that have been conducted on these systems.

Synthesis and Characterization of N-Methylporphyrins, Heteroporphyrins, Carbaporphyrins, and Related Systems

“…It was anticipated that this factor could also play a role in altering the coordination chemistry of other carbaporphyrinoid systems such as tropiporphyrin and oxybenziporphyrin. Tropiporphyrin 7a (Figure ) is a porphyrin analogue in which a pyrrole subunit has been replaced with a cycloheptatriene ring. , The system also has global aromatic properties, although computational studies indicate that the seven-membered ring is paratropic. , Cycloheptatriene dialdehyde 34 was reacted with tripyrrane 12b in the presence of TFA under relatively dilute conditions followed by oxidation with an aqueous ferric chloride solution to give 25-methyltropiporphyrin 35 in 32% yield (Scheme ). The product proved to be somewhat unstable and prone to protonation, and the proton NMR spectrum of the free base form could only be obtained when the NMR solution was treated with potassium carbonate.…”

“…Addition of a trace amount of TFA to a solution of 35 in CDCl 3 gave a monoprotonated species and the proton NMR spectrum (Figure 16) showed an increased diamagnetic ring current (Δδ = 15.20 ppm for 35H + compared to Δδ = 14.27 ppm in the free base form). DFT studies indicate that the sevenmembered rings in tropiporphyrin monocations are also paratropic, 51 but again the proton NMR spectra for 7H + or 35H + do not really support this suggestion. It is clear that further studies will be required to clarify these interpretations.…”

“…48,49 The system also has global aromatic properties, although computational studies indicate that the seven-membered ring is paratropic. 50,51 Cycloheptatriene dialdehyde 34 was reacted with tripyrrane 12b in the presence of TFA under relatively dilute conditions followed by oxidation with an aqueous ferric chloride solution to give 25-methyltropiporphyrin 35 in 32% yield (Scheme 8). The product proved to be somewhat unstable and prone to protonation, and the proton NMR spectrum of the free base form could only be obtained when the NMR solution was treated with potassium carbonate.…”

Aromaticity of Hückel and Möbius Topologies Involved in Conformation Conversion of Macrocyclic [32]Octaphyrin(1.0.1.0.1.0.1.0): Refined Evidence from Multiple Visual Criteria

“…AICD is another widely accepted visual function based on magnetic property to investigate the molecular aromaticity, which can describe the density and direction of the induced ring current in a molecule under an external magnetic field. ,− …”

Azulichlorins and Benzocarbachlorins Derived Therefrom

“…42 Attempts to rationalize the conjugation pathways in terms of resonance led to the proposal of similar somewhat unlikely canonical forms, and it was concluded that azuliporphyrins cannot be satisfactorily assessed in this fashion. 51 The calculated bond lengths for the favored free-base and monocations AzC-c-22H and AzC-c-22,24H + showed very little variation for the individual bonds in the seven-membered ring apart from the position for ring fusion. In fact, this bond (C2−C3) was 0.078 Å or 0.067 Å longer, respectively, for these two species.…”