Preparation of a novel clay/metal complex hybrid film and its catalytic oxidation to chiral 1,1′-binaphthol

An ITO electrode modified with a hybrid film of chiral metal complex (Λ-[Os(phen)3]2+) and a clay (montmorillonite) has been prepared for the purpose of chiral sensing. As a first step, a floating monolayer of amphiphilic Os(II) complex, [Os(phen)2(dC18bpy)](ClO4)2 (phen=1,10-phenanthroline, dC18bpy=4,4′-dioctadecyl-2,2′-bipyridyl), was formed on an aqueous dispersion of sodium montmorillonite. The monolayer acted as an organic part for the hybridization of clay particles in an aqueous phase. The hybrid film of clay and amphiphilic metal complex was transferred onto an indium tin oxide (ITO) substrate by the vertical dipping method. The next step was to immerse the electrode in chloroform, during which the amphiphilic Os(II) complex was removed from the clay surface. Thereafter the electrode was immersed in an aqueous solution of 0.5 mM Λ-[Os(phen)3](ClO4)2 and rinsed with water. Cyclic voltammetric measurements were performed at each step of the above procedures. When the observed curves were simulated on the basis of a double-layered modified electrode, the electron transfer rate constant (k1) for Λ-[Os(phen)3]2+/Λ-[Os(phen)3]3+ was determined to be 0.25 s−1. This OsII/OsIII redox couple was found to mediate the electrochemical oxidation of chiral 1,1′-2-binaphthol in a stereoselective way: i.e., the S-isomer was oxidized at a 1.4 times higher rate than the R-isomer.