PH-Dependent Color-Change Behavior of Bis(o-phenylenediamine)platinum(II) Complex and pH-Dependent Redox of Bis(o-semibenzoquinonediimine)platinum(II) Complex

Abstract

Abstract pH-Dependent color changes of an innocent diamineplatinum(II) complex, [Pt(H2L)2]Cl2 ([1]Cl2), where H2L is o-phenylenediamine, and the pH-dependent redox of non-innocent diimineplatinum(II) complexes, [Pt(L)2] (2) and [{Pt(L)2}2]Cl2 ([22]Cl2), where L is o-semibenzoquinonediimine monoanion or neutral o-benzoquinonediimine, have been investigated by vis–NIR spectroscopy and cyclic voltammetry in solutions of various pH. The crystals of [1]Cl2·2H2O and [22]Cl2·6H2O have also been characterized by X-ray crystallography. [1]Cl2 in an aqueous solution changes from colorless either to purple or to yellow-green depending on the pH of the solution. The purple solution gave a neutral complex 2 and the yellow-green solution yielded [22]Cl2·6H2O. In the latter complex, the monocation, [2]+, dimerizes forming a weak Pt–Pt bond [3.0109(4) Å] with no bridging ligands. The spectroscopy of [1]Cl2 in the buffer solutions shows that the complex 2 is formed in the range of pH 4.6–5.8, while the complex [2]+ is generated in the range of pH 2.1–4.5. These results indicate that the one-electron redox process of [2]+/2 depends on the pH of the solution. By adjusting the pH of the solution by adding acid or base, it is possible to reversibly control this redox process. This behavior is a kind of pH-dependent chromism because of their color being different.