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.