Cyclic Tetrathiophenes Planarized by Silicon and Sulfur Bridges Bearing Antiaromatic Cyclooctatetraene Core: Syntheses, Structures, and Properties

• Published in: Journal of the American Chemical Society 2010-01, Vol.132 (3), p.1066-1074
• Main Authors: Ohmae, Takeshi, Nishinaga, Tohru, Wu, Mo, Iyoda, Masahiko
• Format: Article
• Language: English
• Subjects: Crystallography, X-Ray; Cyclooctanes - chemistry; Models, Molecular; Molecular Structure; Silicon - chemistry; Stereoisomerism; Sulfur - chemistry; Thiophenes - chemical synthesis; Thiophenes - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja908161r

Cyclic tetrathiophenes 1, 2, and 3 planarized by dimethylsilyl, sulfur, and sulfone bridges bearing an antiaromatic cycloocatatetraene (COT) core were designed and synthesized to investigate the relationship among the bent angle, paratropicity, and HOMO−LUMO gap of the COT ring. The bent angles of the central COT rings of 1−3 were theoretically estimated and experimentally determined, and it was found that the planarity of the COT ring was finely adjusted in the order of 2 > 3 > 1 by using the small differences in the bond lengths between the bridging units and thiophene rings. From the comparisons of NICS values and calculated HOMO−LUMO gaps of cyclooctatetraene at various bent angles as well as the optimized structures of cyclic tetrathiophenes 1−3, similar enhancement of the paratropicity and narrowing of the HOMO−LUMO gap with decreasing bent angle of the COT rings were shown in both cyclooctatetraene and cyclic tetrathiophenes 1−3. Such predictions were experimentally proved for the first time by means of 1H NMR and UV−vis measurements of 1−3. In comparison of the 1H NMR chemical shifts of 1−3 with those of the corresponding precursors, upfield shifts due to a paratropic ring current in the COT ring were observed and the degree of shift increased with increasing planarity of the COT ring. Furthermore the colors of the solutions of 1 (λmax = 483 nm), 2b (λmax = 618 nm), and 3b (λmax = 575 nm) were orange, purple, and red in CH2Cl2, respectively, indicating that the HOMO−LUMO gaps of 1−3 become increasingly narrow with increasing planarity of the COT ring. Reflecting these electronic properties, CV measurements demonstrated the amphoteric redox properties of 1 and 2b, and the radical cation 1 •+ , radical anion 1 •− , and dianion 1 2− were chemically generated and successfully characterized by means of UV−vis, ESR, and NMR spectroscopies.