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.