Nishiuchi Tomohiko, Kuwatani Yoshiyuki, Nishinaga Tohru, Iyoda Masahiko
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan.
Chemistry. 2009 Jul 13;15(28):6838-47. doi: 10.1002/chem.200900623.
Novel dynamic molecular tweezers (DMTs) 3 a, 3 b, 4 a, 4 b, and 5 b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units, were designed and synthesized. The cyclooctatetraene (COT) rings of these DMTs readily invert in solution, and the molecular structure shows rigid syn and anti forms in an equilibrium mixture in solution. The syn and anti conformers can be observed by NMR. The isomerization barriers of 3 a, 3 b, 4 a, 4 b, and 5 b are in the range of 16.5-21.3 kcal mol(-1), depending on steric repulsion between substituents of the COT rings and protons of the central benzene ring. These DMTs form complexes with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 1,2,4,5-tetracyano-benzene (TCNB) in solution and in the solid state. The binding abilities of these DMTs increase with electron-donating substituents on COT, which increase the electron densities of the cavity of the syn form, as supported by theoretical calculations. In addition, elongation of the terminal alkoxy chains of the DMTs was found to cause the enhancement of van der Waals contact with guest molecules. Therefore, 5 b, which has CH(2)OMe groups on the COT rings and longer ethoxy groups on the terminal benzene rings, showed the highest electron density of the cavity and hence the highest binding ability with the electron-deficient guest molecules. Interestingly, solutions of 3 b, 4 b, and 5 b show thermochromism in the presence of DDQ. A solution of 3 b or 4 b with DDQ in CHCl(3) is green due to charge-transfer interaction at room temperature and the color changes from green to yellow upon heating to 60 degrees C and from green to blue upon cooling to -40 degrees C, whereas the high complexation ability of 5 b with DDQ only shows a change in the shade of blue.
设计并合成了由两个桶状二苯并环辛四烯（DBCOT）单元组成的新型动态分子镊子（DMTs）3a、3b、4a、4b和5b。这些DMTs的环辛四烯（COT）环在溶液中容易翻转，并且分子结构在溶液中的平衡混合物中呈现刚性的顺式和反式形式。顺式和反式构象体可以通过核磁共振（NMR）观察到。3a、3b、4a、4b和5b的异构化能垒在16.5 - 21.3千卡·摩尔⁻¹范围内，这取决于COT环的取代基与中心苯环质子之间的空间排斥作用。这些DMTs在溶液和固态下都能与2,3 - 二氯 - 5,6 - 二氰基 - 1,4 - 苯醌（DDQ）和1,2,4,5 - 四氰基苯（TCNB）形成配合物。理论计算表明，这些DMTs的结合能力随着COT上供电子取代基的增加而增强，供电子取代基增加了顺式形式空腔的电子密度。此外，发现DMTs末端烷氧基链的延长会导致与客体分子的范德华接触增强。因此，在COT环上具有CH₂OMe基团且在末端苯环上具有较长乙氧基的5b显示出最高的空腔电子密度，因此与缺电子客体分子具有最高的结合能力。有趣的是，3b、4b和5b的溶液在DDQ存在下表现出热致变色现象。在CHCl₃中，3b或4b与DDQ的溶液在室温下由于电荷转移相互作用呈绿色，加热至60℃时颜色从绿色变为黄色，冷却至 - 40℃时从绿色变为蓝色，而5b与DDQ的高络合能力仅表现为蓝色深浅的变化。
