Synthesis and characterization of a new [4.0.4.0] porphyrin-like antiaromatic macrocycle

✍ Scribed by Martin R. Johnson; Carla Slebodnick; James A. Ibers

• Book ID: 101289053
• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 130 KB
• Volume: 01
• Category: Article
• ISSN: 1088-4246
• DOI: 10.1002/(sici)1099-1409(199701)1:1<87::aid-jpp4>3.0.co;2-0

✦ Synopsis

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 0 -T 1 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 0 -T 1 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 0 -T 1 band gap is substantially larger for the paratropic species than for the putative diatropic species.