Preparation, Structural Characterization, Assessment of Potential Antiaromaticity and Metalation of 21-Oxyazuliporphyrins

Oxidation of tetraarylazuliporphyrins with silver­(I) acetate in refluxing chloroform–acetonitrile afforded good yields of 21-oxyazuliporphyrins. Although hydroxyazuliporphyrin tautomers can be considered for this system, spectroscopic results and density functional theory calculations indicate that the keto form is favored, and this was confirmed by single-crystal X-ray diffraction. Oxyazuliporphyrins formally possess a 24π electron delocalization pathway, but the proton NMR spectra are consistent with macrocycles that have diatropic ring currents. Nucleus independent chemical shift and anisotropy of induced current density calculations also confirmed the diatropic nature of these macrocycles, although these results indicated that the seven-membered ring is antiaromatic. However, while the NMR spectra showed the azulene protons at atypically high field values, the results are consistent with a nonaromatic cycloheptatrienyl unit. Protonation gave dicationic products that exhibited enhanced diatropic character. Oxyazuliporphyrins readily form metalated derivatives with Ni­(II), Pd­(II), and Pt­(II), and these complexes exhibited significant diatropic character even though the macrocycle is highly distorted. X-ray diffraction characterization of palladium­(II) and platinum­(II) complexes demonstrated that these derivatives are structurally virtually identical to a previously reported copper­(II) oxyazuliporphyrin.