Reversible Photoswitching of the Coordination Numbers of Silicon in Organosilicon Compounds Bearing a 2-(Phenylazo)phenyl Group

Several organosilicon compounds bearing a 2-(phenylazo)phenyl group were synthesized from
the corresponding chlorosilanes and 2-lithioazobenzene prepared by halogen−lithium transmetalation of
2-iodoazobenzene. Their structures were determined by 1H, 13C, 19F, and 29Si NMR spectra, UV−vis spectra,
and X-ray crystallographic analyses. In the UV−vis spectra, silyl groups caused red shifts of both the n−π*
and π−π* transitions of the azo group compared with the transitions of the unsubstituted azobenzene. The
E-isomers of the fluorosilanes showed an intramolecular interaction between a nitrogen atom of the azo
group and the silicon atom, leading their intermediate structures between a distorted trigonal bipyramidal
structure and a tetrahedral structure around the silicon atoms, which were revealed by the X-ray
crystallographic analyses and the NMR spectra. On the other hand, silanes without fluorine atoms showed
tetrahedral structures in the absence of such an interaction. The photoirradiation of the E-isomers of the
fluorosilanes afforded reversibly the corresponding Z-isomers in good yields. The silicon atoms of the
Z-isomers were found to be tetracoordinate in the absence of Si−N interactions by the 29Si NMR spectra.
The coordination numbers of the silicon atom of the fluorosilanes were reversibly switched between four
and five by photoirradiation. These properties were compared to those of a tetrafluoro[2-(phenylazo)phenyl]silicate.

本研究以相应的氯硅烷与由2-碘偶氮苯经卤素-锂金属转移反应制得的2-锂代偶氮苯为原料，合成了多款带有2-(苯基偶氮)苯基基团的有机硅化合物。通过¹H、¹³C、¹⁹F及²⁹Si核磁共振（NMR）光谱、紫外-可见（UV−vis）光谱以及X射线晶体衍射分析，确定了这些化合物的分子结构。相较于未取代偶氮苯的电子跃迁，硅烷基团会使偶氮基团的n−π*与π−π*跃迁均发生红移。X射线晶体衍射与核磁共振光谱分析显示，含氟硅烷的E型异构体存在偶氮基团氮原子与硅原子之间的分子内相互作用，使得硅原子周围的配位结构介于扭曲三角双锥结构与四面体结构之间。而不含氟原子的硅烷则无此类相互作用，硅原子周围呈现标准四面体配位结构。对含氟硅烷的E型异构体进行光照射，可高效可逆地得到对应的Z型异构体。通过²⁹Si核磁共振光谱分析可知，Z型异构体的硅原子不存在Si−N相互作用，为四配位结构。经光照射后，含氟硅烷的硅原子配位数可在4与5之间可逆切换。本研究还将上述性质与四氟[2-(苯基偶氮)苯基]硅酸盐的相关性质进行了对比。