| STUDY |
For two decades, Massachusetts Institute of Technology chemistry professor Richard R. Schrock and members of his research lab have been part of a stalwart group of scientists seeking to unravel one of the essential chemical conversions in nature--the stepwise catalytic reduction of nitrogen to ammonia. Using a bulky molybdenum aryltriamidoamine complex, Schrock and postdoctoral researcher Dmitry V. Yandulov have finally done it [Science, 301, 76 (2003)]. REDUCTIONISTS Schrock (left) and Yandulov nail down intermediate steps in nitrogen reduction. MIT PHOTO Nitrogen-fixing enzymes with one or more transition-metal centers (Mo, Fe, V) are known for producing two equivalents of NH3 from N2 under ambient conditions. Hundreds of transition-metal N2 complexes have been prepared over the years in an effort to mimic this natural process, Schrock notes, yet scientists know of only one other example of "an authentic catalytic reduction" to NH3. None of the systems studied previously uses a relatively mild reducing agent or reveals many details of the reduction steps, he adds. Schrock and Yandulov used a molybdenum complex with bulky hexaisopropylterphenyl groups to prepare and isolate several nitrogen reduction intermediates at room temperature and pressure under a variety of reaction conditions. As a result of 15N labeling, NMR spectroscopy, and X-ray studies of six of these intermediates, they discovered that the ligand creates a pocket around the single molybdenum center that protects N2 and its reduced products. In additional experiments, the MIT chemists found that several of the intermediates could be made in high yield using other intermediates as a starting point when a constant flux of electrons and hydrogen ions was supplied by cobaltocene and 2,6-lutidinium tetraarylborate, respectively. Reduction of the intermediates all the way to NH3 didn't occur, but Yandulov and Schrock recognized that a slightly stronger reducing agent might be able to complete the reaction cycle. Using decamethylchromocene with the Mo complexes in heptane, they indeed were able to produce seven to eight equivalents of NH3 from four of the isolated intermediates. Efficiencies of the reactions were 63 to 66%--second only to the 75% observed with the Fe–Mo nitrogenase enzyme, the researchers say. "We believe this is the first time that catalytic reduction of dinitrogen has been achieved in an aprotic environment and with a reducing agent that is considerably weaker than that required in the past for any reduction of dinitrogen," Schrock says. |
| COMMENTS |
In an accompanying commentary in Science, emeritus chemistry professor G. Jeffery Leigh of the University of Sussex, in Brighton, England, notes that even though the system doesn't operate in water and is not as stable as nitrogenases, it "may finally allow us to draw realistic and empirically based chemistry parallels with dinitrogenase reductions." Vanadium nitrogenases probably undergo similar reactions, he adds, but more work will be needed to answer the question of how iron-only nitrogenases reduce N2. The work presents "a beautiful culmination" of experimental design, comments Jonas C. Peters, an assistant chemistry professor at California Institute of Technology whose group is investigating iron-based systems as models for N2 reduction. "This is without question the most mechanistically well-defined system that reduces N2 to NH3 under modest temperature and pressure in a catalytic fashion," Peters says |
| UPDATE | 07.03 |
| AUTHOR | Massachusetts Institute of Technology chemistry professor Richard R. Schrock and members of his research lab |
| LITERATURE REF. |
[Science, 301, 76 (2003)]. |
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