Yb. Lei et Fc. Anson, HYDRATION OF THE CARBONYL GROUPS IN 1,10-PHENANTHROLINE-5,6-DIONE INDUCED BY BINDING PROTONS OR METAL-CATIONS TO THE PYRIDINE NITROGEN SITES, Journal of the American Chemical Society, 117(39), 1995, pp. 9849-9854
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.