NEWS OF THE WEEK ENZYME
MIMIC
A CLEARER PATH TO N2 REDUCTION Single-site Mo complex provides improved model for nitrogen fixation 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 NH 3 . None of the systems studied previously uses a relatively mild reducing agent or reveals many details of the reduction steps, he adds. AMMONIA OR BUST Schrock and YanMolybdenum complex forms series of intermediates in catalytic dulov used a molycycle to make 2NH3 from N2 •N9 Mo(N2] bdenum complex Mo(NH3) -NH3 R ι _ with bulky hexaisoel " • ' propylterphenyl [Mo(NH 3 )] + Mo — N = NH groups to prepare |H and isolate several H*î Ν R + Ν —ΝΗ ] nitrogen reduction M o N H 2 [Mo= HIPT ie- A intermediates at R \ e-î • room temperature HIPTwl''>lo—N' [Mo-NH 2 ] + Mo == Ν — Ν Η 2 and pressure under jH+ a variety of reaction H*Î ^ N ^ + conditions. As a reMo=NH [ M o = Ν — Ν Η3] R = isopropyl I e" suit of 1 5 N labeling, HIPT = hexaisopropylterphenyl e-î N + NH NMRspectroscopy, Mo = N + [Mo = NH] + < and X-ray studies of six of these inter duction of nitrogen mediates, they discovered that to ammonia. Using the ligand creates a pocket a bulky molybdearound the single molybdenum num aryltriamicenter that protects N 2 and its doamine complex, reduced products. Schrock and postIn additional experiments, the doctoral researcher M I T chemists found that sever Dmitry V. Yanal of the intermediates could be dulov have finally made in high yield using other indone it [Science, termediates as a starting point 301,76(2003)}. when a constant flux of electrons REDUCTIONISTS Schrock (left) and hydrogen ions was supplied Nitrogen-fixing and Yandulov nail down by cobaltocene and 2,6-lutidinienzymes with one intermediate steps in nitrogen um tetraarylborate, respectively or more transitionreduction. Reduction of the intermediates metal centers (Mo, all the way to N H 3 didn't occur, Fe, V) are known but Yandulov and Schrock rec for producing two equivalents of ognized that a slightly stronger N H 3 from N 2 under ambient reducing agent might be able to conditions. Hundreds of transicomplete the reaction cycle. tion-metal N 2 complexes have
F
OR TWO DECADES, MASSA-
chusetts Institute of Tech nology chemistry professor Richard R. Schrock and mem bers of his research lab have been part of a stalwart group of scien tists seeking to unravel one of the essential chemical conversions in nature—the stepwise catalytic re-
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Using decamethylchromocene with the Mo complexes in hep tane, they indeed were able to pro duce seven to eight equivalents of N H 3 from four of the isolated in termediates. Efficiencies of the reactions were 63 to 66%—sec ond only to the 75% observed with the Fe-Mo nitrogenase en zyme, 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 consid erably weaker than that required in the past for any reduction of dinitrogen," Schrock says. In an accompanying commen tary 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 wa ter and is not as stable as nitrogenases, it "may finally allow us to draw realistic and empirically based chemistry parallels with dinitrogenase reductions." Vana dium nitrogenases probably un dergo similar reactions, he adds, but more work will be needed to answer the question of how irononly nitrogenases reduce N 2 . The work presents "a beauti ful culmination" of experimental design, comments Jonas C. Pe ters, an assistant chemistry pro fessor at California Institute of Technology whose group is in vestigating iron-based systems as models for N 2 reduction. "This is without question the most mechanistically well-defined sys tem that reduces N 2 to N H 3 un der modest temperature and pressure in a catalytic fashion," Peters says. Schrock plans to explore the chemistry of the related vanadi um or iron complexes next, in ad dition to more elaborate molyb denum complexes. A practical large-scale process is still not around t h e corner, he tells C&EN. "It is what we can learn about reducing difficult sub strates catalytically that is im portant."—STEVE RITTER HTTP://WWW.CEN-ONLINE.ORG
