Synthesis and Characterization of a New [4.0.4.0] Porphyrin-like Antiaromatic Macrocycle

Abstract

<jats:p> The antiaromatic macrocycle 2 has been prepared by the reaction of the bipyrrole dialdehyde 1 with hydrazine. In the crystal structure of 2· DMSO molecule 2 is nearly planar, the mean deviation of the 28 atoms of the core from the best least-squares plane being 0.227 Å. AM1 geometric optimization of the core of 2 leads to a fully planar macrocycle. Attempts to oxidize 2 to 3 were largely unsuccessful, yielding only limited evidence for the formation of 3. The antiaromatic precursor 2 turns out to be much more stable than the putative aromatic product 3. Molecular orbital calculations at the AM1/PM3 level indicate the S <jats:sub>0</jats:sub>- T <jats:sub>1</jats:sub> gap for 3 is only about 0.2 eV and hence is readily accessible via intersystem crossing from any visible transition. This small band gap of 3 contrasts starkly with the much larger S <jats:sub>0</jats:sub>- T <jats:sub>1</jats:sub> band gap of 1.86 eV calculated for 2 itself. Paratropicity in antiaromatic systems is usually attributed to a low-lying paramagnetic state, with band gaps ranging up to 2.1 eV. For the 2/3 system the S <jats:sub>0</jats:sub>- T <jats:sub>1</jats:sub> band gap is substantially larger for the paratropic species than for the putative diatropic species. </jats:p>