p-Quinquephenyl diamine, C6H5–p-C6H4–p-C6H2(2,3-NH2)–p-C6H4–C6H5, and its corresponding diimine–Ru(II) complex: Crystal structure and electrochemical and optical properties

The molecular structure of an o-phenylenediamine unit-containing oligophenylene (1), Ph–Ph′–Ph′(2,3-NH2)–Ph′–Ph (Ph = phenyl; Ph′ = p-phenylene; Ph′(2,3-NH2) = 2,3-diamino-p-phenylene), was determined by X-ray crystallography. 1 has a twisted structure, and forms an intermolecular C–H⋯π interaction network. The –NH2 group of 1 was air-oxidized to an imine, NH, group in the presence of [RuCl2(bpy)2] (bpy = 2,2′-bipyridyl) and gave a ruthenium(II)–benzoquinone diimine complex [Ru(2)(bpy)2](PF6)2 (2: Ph–Ph′–Ph′(2,3-imine)–Ph′–Ph). The molecular structure of [Ru(2)(bpy)2](PF6)2 was confirmed by X-ray crystallography. [Ru(2)(bpy)2](PF6)2 underwent two-step electrochemical reduction with E1/2 = −0.889 V and −1.531 V versus Fc+/Fc. The E1/2’s were located at higher potentials by 91 mV and 117 mV, respectively, than those of reported [Ru(bqdi)(bpy)2](PF6)2 (bqdi = benzoquinone diimine). Electrochemical oxidation of [Ru(2)(bpy)2](PF6)2 occurred at a lower potential by 180 mV than that of [Ru(bqdi)(bpy)2](PF6)2. Occurrence of the easier reduction and oxidation of [Ru(2)(bpy)2](PF6)2 than those of [Ru(bqdi)(bpy)2](PF6)2 is ascribed to the presence of a large π-conjugation system in 2.