Monday, September 19, 2005
Reactive Intermediate Long-Suspected of Playing Key Role in Common Chemistry Ruled Out by Team of Scientists
Pittsburgh—Researchers at Carnegie Mellon University, The Ames Laboratory at Iowa State University and the University of Minnesota have succeeded for the first time in characterizing an elusive intermediate and ruling it out as a player in Fenton chemistry. The mechanistic complexity of the Fenton reaction and its importance in the biology of aging, catalytic chemistry, synthesis, and atmospheric and environmental processes have made it one of the most studied and debated reactions of all time. The new results, which definitively settle a long-standing debate, are described in an upcoming issue of prestigious journal Angewandte Chemie.
Introduced in 1894, the Fenton reaction is the oxidation of aqueous iron (II), or Fe (II), with hydrogen peroxide in an acidic aqueous solution. Scientists have advanced arguments both for hydroxyl radicals and for Fe (IV)-oxo complexes as the reactive species involved. (Tetra-valent iron, Fe (IV), is extremely rare).
“We’ve trapped a reactive intermediate and assigned it by spectroscopy and theory to be an Fe (IV)-oxo complex. Our colleagues at the Ames Lab have then shown that this intermediate could not participate in a Fenton reaction because it does chemistry different from that involved in Fenton chemistry,” said Eckard Münck, professor of chemistry at Carnegie Mellon and a co-author of the publication, which was given VIP (very important paper) status by the editors of the journal.
“Knowing the nature of the intermediate is crucial to understanding the role of Fenton chemistry in issues related to environmental and atmospheric chemistry, as well as human health and aging,” said Andreja Bakac, Ph.D., adjunct professor in the Department of Chemistry at Iowa State University and senior chemist at Ames Laboratory.
Fenton reaction kinetics is unfavorable for trapping the reactive intermediate(s). Therefore, the researchers used a detour and prepared a short-lived intermediate with a half-life of about 7 seconds by reacting Fe (II) with ozone. This intermediate forms in less than one millisecond, and it is so reactive with organic solvents, even at low temperature, that a reaction can not be quenched even by injecting it into isopentane cooled to 130 Kelvin (K).
In this latest study, the researchers successfully stopped the reaction within one tenth of a second. They mixed Fe (II) with ozone and shot the mixture against the inner wall of a brass pot that was kept at 77 K while rotating at 1000 rpm using a variable speed drill. In this way, samples were prepared for 4.2 K Mössbauer measurements at Carnegie Mellon. (Mössbauer spectroscopy is extremely well suited to identify the oxidation state of iron and provides unique insights into the electronic structure of a compound.) X-ray absorption studies of this intermediate were performed at the University of Minnesota (L. Que, Ph.D., and X. Shan, Ph.D.).
The Mössbauer studies, performed by Carnegie Mellon graduate student Sebastian Stoian, proved that the intermediate was an Fe (IV)-oxo complex. Working under the guidance of Carnegie Mellon research associate Emile Bominaar, Ph.D., Stoian used density functional theory to show that all the measured parameters indicated an Fe(IV) coordinated to an oxo group and five water molecules. This Fe (IV) assignment was confirmed by the Minnesota researchers.
With the nature of the reactive intermediate established, Bakac and Ames Lab assistant scientist Oleg Pestovsky, Ph.D., then showed that the Fe (IV)-oxo intermediate reacted with selected substrates to form a variety of products which differed from those obtained in Fenton chemistry. Under identical conditions, Fenton chemistry generates products known to be formed from reactions involving hydroxyl radicals.
The research was supported by grants from the Department of Energy and the National Institutes of Health.
By: Lauren Ward