HYDRATION OF THE CARBONYL GROUPS IN 1,10-PHENANTHROLINE-5,6-DIONE INDUCED BY BINDING PROTONS OR METAL-CATIONS TO THE PYRIDINE NITROGEN SITES

In acidic solutions, the chemical or slow electrochemical reduction of the organic ligand-oxidant 1,10-phenanthroline-5,6-dione consumes two electrons and two protons. However, when the electrochemical reductio n is examined on shorter time scales using cyclic or rotating disk vol tammetry, fewer than two electrons are consumed in the reduction. The magnitude of the electron deficit depends upon pH and the deficit is e liminated at pH values where the pyridine nitrogen sites on the molecu le are not protonated. Complexation of metal cations, e.g., Os(II) or Ni(II), to the 1,10-phenanthroline portion of the molecule also produc es an electron deficit in electrochemical reductions of the dione. A p roton- or metal ion-induced addition of H2O to the quinone carbonyl gr oups is shown to be responsible for the observed behavior. Rate and eq uilibrium constants for the hydration-dehydration equilibrium were eva luated by fitting experimental and calculated parameters in a digital simulation. The possible consequences of the results to studies in whi ch the dione is used to oxidize amines and other organic reductants ar e pointed out.