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New Riches in Carbaporphyrin Chemistry: Silver and Gold Organometallic Complexes of Benzocarbaporphyrins†
journal contribution
posted on 2004-08-23, 00:00 authored by Timothy D. Lash, Denise A. Colby, Lisa F. SzczepuraThe NH−N−NH−N core of the porphyrin system represents one of the best studied and most versatile platforms
for coordination chemistry. However, the replacement of one or more of the interior nitrogens with carbon atoms
would be expected to diminish the ability of these systems to form metallo derivatives considerably. Despite this
expectation, carbaporphyrinoid systems have been shown to form stable organometallic derivatives. Although
azuliporphyrins and benziporphyrins act as dianionic ligands, benzocarbaporphyrins are trianionic ligands. Treatment
of five different meso unsubstituted benzocarbaporphyrins and two different meso tetraarylbenzocarbaporphyrins
with excess silver(I) acetate afforded 65−97% yields of the corresponding silver(III) organometallic derivatives. The
insertion of silver metal was confirmed by mass spectrometry and X-ray crystallography. The UV−vis spectra showed
a strong Soret band at wavelengths between 437 and 451 nm, together with a series of Q-type bands at longer
wavelengths. The new metallo carbaporphyrins demonstrate the presence of a strong diatropic ring current in their
proton NMR spectra, and carbon-13 NMR spectroscopy indicates that the derivatives retain a plane of symmetry.
The reaction of meso tetraaryl carbaporphyrins with gold(III) acetate afforded the related gold(III) complexes, and
these also showed strongly porphyrin-like aromatic characteristics. The UV−vis spectra for the gold complexes
again showed a strong Soret band between 437−439 nm, but a secondary band near 400 nm is somewhat intensified
for the gold species compared to the spectra for the related silver(III) meso tetrasubstituted carbaporphyrins. The
ring currents observed for the gold(III) complexes by proton NMR spectroscopy were comparable to those of the
silver(III) derivatives, implying that both series have similar macrocyclic conformations. Cyclic voltammetry was
performed on two different carbaporphyrins, their silver(III) derivatives, and a gold(III) complex. The silver complexes
display a reversible cathodic wave that is assigned to the Ag(III/II) couple. However, the gold porphyrinoid gave a
value for the reductive wave that could be due to a gold(III/II) couple or a ligand-based process.