非手性单齿配体对反式双二胺合钴（Ⅲ）配合物圆二色谱的影响

摘要

过渡金属配合物手征结构单元与光学活性的研究，是手性研究的重要课题之一。然而，一直以来人们主要针对八面体中心手性、螯合环构象效应以及邻位效应对配合物圆二色（ECD ）光谱的贡献进行了研究，而忽略了非手性单齿配体对 ECD 谱的影响。使用含时密度泛函理论（TDDFT）在B3LYP／6‐311＋＋ G（2d ，p）水平上，计算了一系列含单齿配体X的反式双二胺合钴（Ⅲ）配合物的激发能、振子强度和旋转强度等激发态性质。结果表明，三种含非轴向对称单齿配体（X＝ N3－，SCN －，NO2－）的化合物中，N3－和SCN －的手性取向（δ／λ）支配着配合物的ECD谱；而硝基配合物中，虽然螯合环的构象手性对 ECD谱做出了主要贡献，但单齿配体手性取向的贡献也是非常重要的，表现在δ‐异构体得到强负弱正的 ECD曲线，而λ‐异构体则给出强正弱负的吸收谱带，且前者的计算ECD与实验谱吻合得很好。因此，反式双二胺合钴配合物中，单齿配体的手性取向效应不仅存在，而且这种贡献在 trans‐[Co（pn）2（NO2）2]＋配合物中是可观察的。此外，通过对配合物最稳定构型的计算ECD谱的分析表明，所有反式配合物在长波区的 ECD带形是单齿配体依赖性的，与它们的光谱化学序列有关，强场配体的第一吸收带的符号为负，而弱场配体则为正。%To assess the influence of achiral unidentate ligands on electronic circular dichroism spectra ,the excitation energies ,oscillator and rotational strengths for a series of trans‐bis‐ethyl‐enediamine and propylenediamine cobalt (Ⅲ) complexes containing the unidentate ligands X (X=Cl- ,CN - ,NH3 ,NO2- ,N3- ,etc) have been calculated at the TDDFT level using the B3LYP functional and 6‐311+ + G(2d ,p) basis set .It was found that ,for the chelates with nonaxially symmetry unidentate ligands (X= N3- ,SCN - ,NO2- ) ,the chiral orientations (δ/λ) of the N3-and SCN - dominate the ECD spectra .However ,the contribution from the last one ligand is rela‐tively smaller than that of the first two .Such phenomenon has been successfully observed in the experimental ECD spectra .The δ‐orientation of NO2- exhibits one strong negative and another weak positive band shapes in the ECD curves ,and such distributions is consistent with the exper‐imental observation of trans‐[Co(pn)2 (NO2 )2 ]+ .For all of the chelates ,the ECD patterns in the long‐wavelength region are dependent on the spectrochemical series of the unidentate ligands X in the sequence from weak to strong :SCN - ,Cl- ,N3- ,H2O ,NCS - ,NH3 ,en ,NO2- ,CN - .The first absorption peak of the ECD spectra is negative in sign for the chelates with stronger unidentate ligands ,while it is positive in sign for the complexes with weaker unidentate ligands .