SCIENCE/TECHNOLOGY
Capping Carbon Chains With Metals Gives 'Molecular Wires' And More • Metal complexes having chains of up to 20 carbon atoms provide entry into new materials, electron transport studies
Pacifichem '95 Honolulu Ron Dagani, C&EN Washington arbon is a wondrous element. Combined with hydrogen, oxygen, nitrogen, and a few other elements, it forms the basis for all life on Earth. In its "naked" or uncombined forms, carbon is no less fascinating—ask anyone familiar with the structure and properties of diamond, graphite, fullerenes, and carbon nanotubes. This list would not be complete without the linear, one-dimensional form of carbon. This substance exists as a chain of sp-hybridized carbon atoms, which are joined either by alternating triple and single bonds or by an uninterrupted series of double bonds. This linear form of carbon has attracted growing interest in recent years. And at last month's Pacifichem '95 in Honolulu, the latest insights into its synthesis and structure were featured at a symposium on metal complexes of carbon. Over the years, chemists at a number
C
of laboratories have found ways to prepare and stabilize such carbon chains by capping them with various organic end groups. But because these syntheses produce mixtures of oligomers having different chain lengths, defining the properties of individual oligomers can be very difficult. By far the longest acetylenic carbon chains have been produced in the laboratory of chemistry professor Richard J. Lagow at the University of Texas, Austin. He and his coworkers have used both conventional organic synthesis and laser-based syntheses to prepare chains of hundreds of carbon atoms capped with a trifluoromethyl or trifluorosilyl group at either end. Lagow refers to these compounds as "extraordinary new classes of fluorocarbons" and is exploring their many potential applications. Other research teams have turned to transition-metal complexes as the endcapping groups. The synthetic methods they use are designed to yield samples in which all the carbon-chain moieties are the same length. But the interest in these metal-capped carbon chains goes far beyond the desire for homogeneous samples. For example, some of these metal-capped chains exhibit potentially useful materials properties. Conjugated carbon rods are expected to have a high degree of mechanical strength and stability, which is important for making strong, tough materials. Their expected electrical conductivity could lead to their use as "molecular wires" in electronic devices. This same property would make them of interest for studying how charge is transported
Gladysz's C 20 complex
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JANUARY 22, 1996 C&EN
from one metal atom to another through a long molecule. And they also could potentially be useful as nonlinear optical materials and liquid crystals. In addition, metal-capped carbon chains could serve as models for surface carbides that are generated on heterogeneous catalysts used in processing basic chemical feedstocks. And aside from these practical considerations, chemists also are motivated to study these elemental chains for their "purely aesthetic appeal." John A. Gladysz, a chemistry professor at the University of Utah, Salt Lake City, and one of the symposium's organizers, has been synthesizing and studying these metal-carbon complexes for years, gradually building up to longer and longer chains. In a recent paper [/. Am. Chem. Soc, 117, 11922 (1995)], he and his coworkers detailed the synthesis and properties of complexes containing, for instance, Re-C=C-C^C-Re and ReC^C-C^C-Pd-C^C-C^C-Re linkages. Using cyclic voltammetry, they were able to remove an electron from each complex to create a radical cation and study its behavior. The odd electron is rapidly delocalized between the two rhenium atoms in the first complex, they found, but the palladium atom in the second complex apparently is a barrier to efficient charge transfer. In Hawaii, Gladysz described how his group synthesized complexes in which a C12, C16, or C20 chain is strung between two rhenium atoms. These are by far the longest sp carbon chains tethered between two metals, he pointed out. The Utah researchers began with the
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'wire' has hybrid electronic structure
atoms. In a paper pub- quency of impinging laser light—a lished last summer property of great importance for non[/. Am. Chem. Soc., 117, linear optical devices. 7129 (1995)], Lapinte Lapinte also discussed some very rea n d c o w o r k e r s d e - cent work on the synthesis of a C 8 comscribed the synthesis plex—{Fe*} (C =C) 4 {Fe* (—by oxidative and spectroscopic prop- coupling of two {Fe*}(C=C-C^CH) molerties of {Fe*}(C=C- ecules. Although this diiron complex is C=Q{Fe*}, in which the not the first C 8 complex to be reported iron atoms also are (Gladysz's group can take the bow for coordinated to other that), Lapinte was able to chemically oxgroups. idize it to yield the longest chain mixedLike Gladysz's com- valence [Fe(II)-Fe(III)] radical cation plexes, Lapinte's buta- known to date. Infrared spectroscopy of ie thermally stable mixed-valence complex [Fe11—C8— diynyl complex so far the isolated complex reveals that the !in]+[PF6]~ has an odd electron that is fully delocalized behas been found to be electronic structure of its C 8 bridge is reen the two iron centers. The electronic structure of the C8 stable in three oxida- intermediate between a polyacetylenic idge is intermediate between a polyacetylenic chain (top) tion states—the neutral chain and a cumulenic chain (all double id a cumulenic chain (bottom), according to Lapinte and compound, in which bonds). Furthermore, the redox states of ançoise Coat of the University of Rennes I. The bidentate both irons are Fe(II); the the two metals are indistinguishable by jand on iron is l,2-bis(diphenylphosphino)ethane. monocation, in which all of the standard spectroscopic criteone of the irons has ria. "This means that the electron transbutadiynyl complex {Re*}(C=C-C=CH), been oxidized to Fe(III); and the dication, fer rate or 'communication' between the where {Re*} represents [ T I 5 - C 5 ( C H 3 ) 5 ] - in which both irons are Fe(III). The termini is very high, and the carbon Re(NO)[P(C 6 H 5 ) 3 ]. This was converted monocation is the first M-C 4 -M' com- chain can be viewed as a 'conducting into the copper-substituted alkyne plex (in which M and M' are different wire' with little or no resistance," Glad{Re*}(C=C-C=CCu). The carbon chain oxidation states of the metal) to be struc- ysz said. could then be extended by coupling this turally characterized. Lapinte told So far, both Lapinte and Gladysz have copper reagent with bromine-substitut- C&EN that his group has seen a fourth accessed no more than three or four oxied alkynes—a known procedure but one oxidation state corresponding to Fe(III)- dation states in these complexes. But that apparently had not been applied to C4-Fe(IV). The researchers are trying to graduate student Paul J. Low, working molecules containing an additional met- isolate this species before reporting on it. in chemistry professor Michael I. Bruce's In Hawaii, Lapinte revealed that his lab at the University of Adelaide, South al such as rhenium. Thus, two molecules of {Re*}(C=C-C=CCu), for example, diiron complexes exhibit an unexpect- Australia, has observed five. The system were coupled to one molecule of BrC^C- ed second-order nonlinear optical ef- Low and Bruce are studying, in collaboC^CBr to give {Re*}(C=C)6{Re*}. Similar fect. Specifically, they double the fre- ration with two colleagues in Moscow, but longer multistep routes furnished the C 16 and C 20 analogs in moderate yields. Comparing the electrochemical beButadiyne units, complexée rings form carbon arrays havior of these complexes with their shorter chain analogs revealed some striking trends, Gladysz said. For example, as the chain length increases, losing the first electron becomes thermodynamically less favorable, but the favorability of losing the second electron doesn't change as much. Values of the first and second oxidation potentials approach each other and, with the C 20 complex, they merge to give a single— presumably two-electron—oxidation. At C20/ Gladysz said, the rheniums are far enough apart that they behave independently in a redox sense—that is, the two atoms get oxidized simultaneously rather than sequentially. Claude Lapinte, director of research at the Laboratory of the Chemistry of TranBunz and coworkers at the Max Planck Institute for Polymer Research in sition Metal Complexes & Organic SynMainz, Germany, have synthesized linear structures incorporating butadiyne thesis at the University of Rennes I in units and sandwich or half-sandwich metal complexes. France, also has been studying carbon chains, but his are tipped with iron JANUARY 22, 1996 C&EN
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JANUARY 22,1996 C&EN
SCIENCE/TECHNOLOGY is {Ru*}(C=C-C=C){Ru*}, where each ruthenium is coordinated to one cyclopentadienyl and two triphenylphosphine ligands. During cyclic voltammetry, this complex loses a total of four electrons sequentially and semireversibly, giving the mono-, di-, tri-, and tetracation. As the oxidation proceeds, the C4 chain progressively changes from the neutral diacetylide through a dou bly charged dicarbene, [{Ru*)=C=C= C=C={Ru*}]2+, to the tetracationic dicarbyne,
[{RU*}^-C^EC-C={RU*}]4+.
Spectral data indicate that electron delocalization occurs over both metals. More complicated metal-carbon mole cules, some linear and some not, were presented by Uwe H. F. Bunz, an organ ic chemist at the Max Planck Institute for Polymer Research in Mainz, Germany. Bunz and coworkers have used metal sandwich or half-sandwich complexes, along with ethyne and butadiyne units, as building blocks to construct a variety of carbon arrays. For example, Bunz has made rigid linear polymers that consist of butadiyne units alternating with 1,3substituted cyclobutadienes that are sta bilized by complexation to a cyclopentadienylcobalt moiety. Using a related chemical system, Bunz's group pro duced ''bent wires" that zigzag because butadiyne units connect metal-support ed cyclopentadienyl rings at two adja cent ring carbons. The Mainz researchers also have created "molecular stars" by attaching rigid butadiyne units to all the ring carbons of metal-complexed cy clobutadienes or cyclopentadienes. The aim in all these efforts has been to synthesize simple segments that might one day be connected to form more complicated carbon networks. The "bent wires" and cyclopentadienyl-based "5 stars" are elements of an "exploded fullerene," a hypothetical, spherical car bon network in which butadiyne "struts" link together cyclopentadienyl "connec tors." And the butadiyne-cydobutadiene polymer and cyclobutadiene-based "4 stars" are segments of a hypothetical twodimensional network that looks like a grid. Synthesizing these grid segments is fairly straightforward, but connecting them into large sheets of the grid will be "extremely difficult," Bunz conceded. Nevertheless, if these types of carbon networks are ever realized, they will tingle the imaginations of chemists just as surely as fullerenes, linear carbon, and other unusual carbon structures have in the past decade. Π
