SCIENCE & TECHNOLOGY FIXATED Cummins and his group at MIT are devising new approaches to activate N2, P4, and other small molecules to create synthetic reaction cycles. From left: McKellar, Piro, and Cummins.
FIXING PHOSPHORUS Niobium complex allows MIT chemists to build compounds from elemental phosphorus STEPHEN K. RITTER, C&EN WASHINGTON
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HEMISTS HAVE LONG BEEN ENAM-
ored with nitrogen fixation, the process nature employs to help plants capture atmospheric nitrogen and convert it to ammonia for use as a nutrient. Besides the famous Haber-Bosch industrial process to convert N 2 to NH 3 , hundreds of transition-metal complexes involving N 2 have been prepared over the years in an effort to mimic nitrogen fixation. The exact enzymemediated mechanism that nature uses to fix nitrogen is still a bit of a mystery, however, so deciphering the chemistry continues to captivate researchers. Be that as it may, N 2 isn't the only small molecule amenable to "fixation" that has piqued the curiosity of chemists. Massachusetts Institute of Technology inorganic chemistry professor Christopher C. (Kit) Cummins and his research group have spent the past decade pursuing metal complexes that also can fix elemental phosphorus (P4) and other small molecules. A breakthrough for the Cummins group came a fewyears ago in the form of a niobium complex having a terminal niobium-phosphorus triple bond. This niobium complex now serves as a workhorse to transfer phosphorus atoms to organic molecules or to ligands bound to transition
metals under mild reaction conditions. Unlike N 2 , elemental phosphorus isn't readily available in a pure form in nature, Cummins points out. White phosphorus, or P4, one of the element's allotropes, is derived by reduction of apatite, a phosphate mineral. White phosphorus is used to make phosphoric acid, phosphorus trichloride, and other commercial reagents vital to research and industrial syntheses. "Making monophosphorus compounds directly from P4, rather than first making commodity reagents, would be more efficient, safer, and greener chemistry," Cummins says. This approach could someday be standard for production of polymers, agricultural chemicals, and pharmaceuticals, he notes. One potential application area for the triple-bonded complexes is reactivity akin to alkyne metathesis, a reaction that hasn't yet been developed for phosphorus triple bonds, Cummins observes. Alkyne metathesis is the triple-bond version of Nobel Prize-winning olefin metathesis, which has had a tremendous impact on organic synthesis and polymer chemistry. Cummins expects the metal-phosphide chemistry will also lead to novel phosphine catalysts or ligands. "Cummins' group has been the pioneer in synthesis of terminal phosphide ligands
in transition-metal complexes," notes chemistry professor Gregory L. Hillhouse of the University of Chicago. "This work is not only highly creative but of fundamental importance in that it explores and develops the most basic of chemical transformations— atom-transfer reactions—and the synthetic precursor of these transformations is simply the element itself." Cummins' niobium-phosphorus chemistry began as an offshoot of his research on molybdenum trisamide complexes. In 1995, his group reported the synthesis from P4 of one of the first complexes containing a metal-phosphorus triple bond, the molybdenum trisamide complex P=Mo(NRR) 3 , where R is tert-butyl and R' is 3,5-dimethylphenyl. Cummins' group proceeded to do extensive work on molybdenum and other metal trisamide complexes, which has resulted in reactions with N 2 , P4, and CO to form terminal anionic Mo=N, Mo=P, and Mo=C complexes. A number of researchers are exploring the potential of these complexes for transfer of the terminal ligands to organic molecules. For example, the carbide complex is a potential precursor for alkyne metathesis catalysts (Angew. Chem. Int. Ed. 2006,45, 862). But the Mo=P complex has proven to be relatively unreactive for phosphorus atom transfer, Cummins says. AFTER GLEANING some clues from the literature about the reactivity of niobium complexes, Cummins' group decided to try to migrate the chemistry from molybdenum, in group 6 of the periodic table, to niobium, in group 5. The researchers settled on a trisamide complex dubbed niobaziridine hydride. In this complex, a hydrogen atom from the methylene carbon of the neopentyl group [-CH2C(CH3)3] on one of the amide ligands migrates to the niobium atom, and the neopentyl chain doubles back on the metal center to form a three-membered Nb-C-Nring. Niobaziridine hydride readily reacts with P4 to form a dimer in which two niobium atoms are coupled together by a diphosphorus bridge, forming an Nb 2 P 2 "butterfly" core structure, Cummins explains. Subsequent reduction of the dimer by Na/Hg amalgam
"Making monophosphorus compounds directly from P4 would be more efficient, safer, and greener chemistry/' 26
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in tetrahydrofuran cleaves the P2 bridge, isotopically labeled terminal nitride ligand, Heavier group 14 aromatic rings haven't forming the desired niobium phosphide salt, Nb= 5N, reacts with acyl chlorides to form received much attention, even though they NalP^NbCNRRO^, where R is neopentyl nitriles, RC^ 5 N, which are useful precur- are analogs of important carbon species, and R' is 3,5-dimethylphenyl. sors for heterocyclic compounds contain- Cummins says. For example, the EP2 tri"The neat thing about this chemistry is ing the isotope marker (J. Am. Chem. Soc. angles are isolobal with the aromatic cyclopropenium ion (C3H3+). But there's growing how we were able to movefromthe analo- 2006,128,940). interest in noncarbon aromatic ring systems, gous molybdenum phosphide complex in group 6 to niobium in group 5 and main- ANOTHER VERSATILE reaction with the Cummins notes, and his group is working tain the same electron count," Cummins niobium phosphide creates ligands directly to find a way to liberate the EP2 triangles notes. "That leaves a negative charge on the on a metal center by simply tacking on an ad- as potential reagents for further chemical niobium complex, which introduces nucleo- ditional molecularfragmentIn one example, reactions. philicity and allows us to use the complex as reaction with chlorodiorganophosphanes "Cummins' research emphasizes the a platform to create new phosphorus-con- (R2PQ) leads to addition of the phosphane importance of doing exploratory inorganic taining molecules." to the phosphide, creating a newphosphorus synthesis," comments chemistry professor In a series of papers during the past few ligand, P=PR2, where R is phenyl or tert-bu- Philip P. Power of the University of Caliyears, Cummins andgraduate studentJoshua tyl. This ligand is bound side-on to the nio- fornia, Davis. New compound classes, such S. Figueroa, who is now a postdoc in chemis- bium atom by 7C-complexation, similar to an as the niobium-phosphorus complexes, "open up exciting possibilities for activatry professor Gerard Parkin's group at Co- olefin or alkyne binding to a metal. lumbia University, have presented 'Vignettes" Diphosphorus ligands have been de- tion of small molecules and development on the chemistry of the niobium phosphide veloped for late transition metals, such as of reagents for metathesis or atom-transfer complex (Palton Trans. 2006,2161). platinum, Cummins points out. But until reactions," he says. One reaction is a new route to make phos- now, they have been made only by transfer "Chemistry of metal-ligand multiple bonds is extremely important, as exemplified phaalkynes, RC=P, by treating the phos- of the ligand to the metal. phide complex with various by the 2005 Nobel Prize in Chemistry," adds T. Don Tilacyl chlorides, such as pivaloyl chloride, (CH^CCOCl. The ley, a chemistry professor at the n carbonyl carbon and oxygen University of California, Berke*-RC = P + atoms of the acyl chloride iniley. "Many aspects of this field -Nb tially add to the Nb^P bond, have yet to be mapped out, and / forming afour-memberedring. further development of metalThese intermediates are stable Niobium oxo ligand multiple-bond chemistry complex and can be isolated, but upon should be expected to produce heating the ring undergoes useful new reactions." fragmentation to form a nioTilley notes that his group bium oxo complex and spits has used the niobium-phosI =P P out a phosphaalkyne. Thus, phorus chemistry to explore i the triple-bonded phosphorus cycloaddition reactions of -Nb — is traded, as in a metathesis rephosphaalkynes and other / action, to form a C=P bond. triply bonded substrates with metal-silicon triple bonds. Phosphaalkynes are reactive And the complexes containing analogs of nitriles, and the tertthe main-group EP2 ligands butyl compound derived from Niobium phosphide i GeCl , SnCl , or Pb(0S0 CF L "represent great examples of pivaloyl chloride is aparticularly the potential of this system," useful reagent for 1,2-addition L= he adds. and cycloaddition reactions to make aminophosphines and "Cummins and his group are rNbL, alkenes, Cummins says. Phosmaking valuable contributions E = Ge,Sn,Pb phaalkynes typically are made to this topic through their beauvia organosilicon chemistry untiful and brilliant manipulation der harsh conditions, he adds, ofthe chemistry ofthe niobium PHOSPHORUS ZOO Niobium phosphide complex, but the niobium phosphide complexes," Tilley concludes. containing a Nb=P bond, is a new workhorse to prepare complex "offers a milder route "I expect much more to come a variety of phosphorus-containing species, including for in situ generation of phosfrom their studies." phosphaalkynes (RC=P), phosphorus ligands (P=PR2), and phaalkynes that might not be main-group aromatic rings (EP2). Indeed, work on niobiumavailable by other methods." phosphorus chemistry in the In yet another example of atom transfer, Cummins lab continues apace. "It's certainly A bonusforthe phosphaalkyne synthesis is that the niobium oxo by-product is easily Cummins and Figueroa investigated reac- true that we have developed a fixation with recycled back to niobaziridine hydride. This tions of the niobium phosphide with diva- phosphorus chemistry," Cummins quips. type of "synthetic cycle" could be used to lent group 14 salts, such as GeCl2, SnCl2, or Ongoing work by graduate student Nichogenerate many different types of nitrogen Pb(OS02CF3)2. These reactions form novel las A. Piro and undergraduate Jessica T. and phosphorus compounds, he notes. For quasiaromatic EP2 triangles (E stands for McKellar is leading to a few surprises that example, Cummins' group has shown that Ge, Sn, or Pb), which are suspended between are waiting to be unveiled in an important upcoming publication, he says. • the niobium trisamide complex having an a pair of niobium trisamide complexes. 2
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