Research Articles Photocontrol of the Coordination Numbers of the Main Group Elements: Properties of Azobenzenes Bearing a Main-Group-Element Substituent and Their Application | TCI

Each main group element in main group compounds usually takes its own specific coordination number, oxidation number and bonding state. An unusual valence can be achieved by appropriate molecular design and synthetic methods, and such an unusual valence results in characteristic, unique structures and properties. The reactivity of the main group compounds markedly depends on the coordination number. For instance, silicon, which belongs to group 14 and is located just under carbon in the periodic table, generally shows the tetracoordination state l ike an sp carbon. Sil icon can also take a high coordination state, such as pentaor hexa-coordination states, when the appropriate ligands are used under suitable condit ions. In highly coordinated sil icon compounds, the electrophilicity of the central silicon increases compared to that of the tetracoordinated silicon compounds, and the nucleophilicity of the substituents on the silicon also increases (Figure 1). Thus, some synthetic reactions have been developed by using these properties of highly coordinated sil icon compounds as their intermediates or transition states, as seen in the Tamao oxidation reaction. In other examples, organophosphorus compounds, whose phosphorus center takes a trivalent and tricoordination state, have widely been used as ligands for transition metals. When the phosphorus takes pentavalent and tetracoordination or pentavalent and pentacoordination states, the usefulness of these phosphorus compounds is dramatically reduced. Considering elemental properties and their relationship with the coordination numbers, control of the oxidation state and the coordination number of the main group elements by using some external stimuli is quite interesting. This will lead to new methods for control of organic reactions, based on the control of physical properties and reactivity of main group compounds. For example, if the coordination state could be easily switched so that an element could take an activated coordination state at a certain period but an inactivated state at another, this would be useful for the regulation of copolymerization reactions. Moreover, if this could be used as a substrate, it could possibly provide a new method for protection or deprotection of the ligands. However, main group compounds taking unusual coordination states are unstable in general, and some stabilization modification is required for actual use. For example, steric protection methods are often used for stabilization of low-coordinated main group compounds, by introducing a bulky substituent onto the main group element in order to prevent o l igomer izat ion or polymerization. In addition, elimination reactions are considered to proceed easily in the highly coordinated compounds because the highly coordinated compounds often behave as intermediates in organic reactions. Therefore, it is required that an electron-withdrawing group should be introduced to reduce the electron density of the central element and to stabilize the polarized bonds, and that multidentate l igands should be used to take advantage of the favorable entropy contribution. There are limitations in molecular design for the stabilization of