SCIENCE/TECHNOLOGY
Bimetallic Cluster Complex Found To Exhibit Catalytic Activity • Work tvith platinumruthenium cluster complexes could establish new directions for development of clusters for catalysis A n unusual example of highly se/ % lective catalysis by a bimetallic JL J L cluster complex has been discovered. It may show the way to further development of clusters for this purpose. For a long time, chemists have been looking for a cluster complex that would catalyze a reaction at a rate that would be of commercial importance. Richard D. Adams and a research group at the University of South Carolina, Columbia, believe they have found such a complex [/. Am. Chem. Soc, 114,10657 (1992)]. The researchers suggest that, although all of the implications of the discovery aren't yet known, there is little doubt that
it will be a significant advance in the development of cluster complexes as catalysts. It may also aid in providing a better understanding of the function of mixed-metal catalysts and metal-alloy catalysts. Investigators, Adams notes, have long speculated that heteronuclear complexes might exhibit synergistic effects that would provide superior catalytic activity. Until the present discovery, however, there has been little evidence of such effects in metal complexes. A few years ago, the group at South Carolina had observed the tendency shown by certain high-nuclearity mixedmetal clusters for their metals to spontaneously segregate into layers. The chemical implications of this effect were not clear at the time, but it now seems that one implication is increased catalytic activity of the cluster. In work supported by the National Science Foundation, Adams' group has prepared a high-nuclearity platinumruthenium cluster exhibiting the tendency of the platinum and ruthenium to segregate at dif-
ferent places in the cluster. One member of this cluster family exhibits a coordination preference for triruthenium sites in the monoalkyne derivative. This derivative exhibits a high activity for the catalytic hydrogenation of diphenylacetylene (C6H5C2C6H5). The Pt3Ru6(CO)21(|i-H)3(ji3-H) cluster complex was prepared by reacting Ru4Pt2(CO)18 with hydrogen in heptane solution. The structure of the complex includes three triangular layers of nine metal atoms arranged in the form of a face-centered bioctahedron. The center layer contains three platinum atoms. The other two, one on either side, contain only ruthenium atoms. Three hydride ligands bridge each edge of the Ru(4)-Ru(5)-Ru(6) triangle while a fourth hydride is a triple-bonding ligand across the Ru(l)-Ru(2)-Ru(3) triangle. In addition, the molecule contains 21 carbonyl ligands. The cluster complex is converted to Pt3Ru6(CO)20(ji3-PhC2Ph)([i3-H)([i-H) when it is reacted with diphenylacetylene in refluxing hexane solution. Like the original clus-
Platinum D Ruthenium • Hydrogen • Oxygen • Carbon
The complex Pt3Ru6(CO)21(\i-H)3(\i3-H), shown at left, exhibits the natural tendency of metals in a bimetallic cluster to segregate spontaneously. Its derivative, Pt3Ru6(CO)20(\i3-PhC2Ph)(yL3-H)([!L-H), on right, exhibits 100% selectivity for the hydrogenation of diphenylacetylene Z-stilbene 32
JANUARY 18,1993 C&EN
ter complex, the derivative has a layered structure, but also has a triply bridging diphenylacetylene ligand coordinated to the Ru(l)-Ru(2)-Ru(3) triangle. First experiments indicate that the derivative is an unusually active catalyst for the selective hydrogenation of diphenylacetylene to Z-stilbene
at 1 atm and 50 °C. In these experiments, the diphenylacetylene was converted to Z-stilbene with 100% selectivity for the isomer and corresponding to a turnover frequency of 31 units per enzyme site per hour. The catalytic activity was significantly higher than the activities for either platinum or ruthenium clusters. Adams and his research group are still speculating about why there is a preference by the alkyne for the triruthenium catalytic site. Although the reason is not clear, it may be related to the idea that metal selectivity for catalysis is sometimes enhanced when the metal in question functions "cooperatively" with neighboring metals. Similarly, the mechanism for this particular reaction has yet to be established. The suspicion, however, is that the platinum in the central triangle layer plays a key role because of its ability to activate hydrogen. The catalytic activity of this cluster complex is only a fraction of that of the best mononuclear hydrogenation catalyst. However, the catalytic activity of the cluster is also at least an order of magnitude better than those for all the previously studied cluster catalysts. The derivative was recovered in essentially quantitative yield during the first few hours of the experiments. Although the activity of the derivative may be less than that of mononuclear catalysts, many previous efforts with clusters failed for other reasons, such as the extremely fragile nature of clusters, difficulty in preparation, and inability to recycle the catalysts. All of these difficulties appear to have been overcome in the case of the platinumruthenium complexes produced in the University of South Carolina work. These clusters demonstrate the frequently cited virtue of specificity which is highly desirable in catalysis. Until now, clusters have often been relegated to roles as precursors for more robust catalysts, usually supported on active
substrates. The South Carolina result is an encouraging beginning for the possible development of clusters with practical applications in catalytic chemistry. During the past decade, at least three national committees have looked into the future of chemistry and chemical engineering in the U.S. All have concluded that one of the major, if not the
most important, areas of research and development is catalysis. Also noted was that a major effort in this area might well be directed toward an examination of fundamental catalysis through the study of new entities such as clusters and complexes. The South Carolina research illustrates that idea. Joseph Haggin
Solid-phase synthesis of benzodiazepines A general and expedient method for the synthesis of 1,4-benzodiazepine derivatives on a solid support has been developed by chemists at the University of California, Berkeley [/. Am. Chem. Soc., 114,10997 (1992)]. The work represents the first critical step in the develop ment of a method for the simultaneous synthesis of a variety of derivatives of this important class of therapeutic agents. The research was carried out by Berkeley assistant chemistry professor Jonathan A. Ellman and his graduate student Barry A. Bunin. The two chemists applied a technique that has been ex-
tremely useful in synthesizing biological polymers such as proteins and nucleic acids to the synthesis of nonpolymeric organic molecules. This is one of the first examples of a methodology for synthesizing a general class of nonpolymeric organic molecules on a solid support, Ellman says. The work opens the door to developing a combinatorial synthesis of the 1,4-benzodiazepines—that is, the simultaneous synthesis of an array of different, identifiable, spatially separate 1,4-benzodiazepine derivatives. Once synthesized, such a "library" of benzodiazepines could be screened for bio-
Synthetic strategy adds amino acid to 2-aminobenzophenone NHFMOC
NHFMOC
FMOC-protected amino acid
Support
Support 2-AmJnobenzophenone derivative linked to solid support
Alkylating agent
Support
Support R = C H 3 , CH(CH 3 ) 2 , CH 2 COOH, (CH 2 ) 4 NH 2 , C H 2 - / ^ \ , or C H j - T ' V o H
1,4-Benzodiazepine derivative linked to solid support
R' = H, CH 3 , CH 2 CH 3 , C H 2 C H = C H 2 , or C H FMOC = fluorenylmethoxycarbonyl
2
-T^
Note: 2-Aminobenzophenone derivative can also be linked to the solid support through a carbonyl group at the 4-position of the phenyl ring carrying the amine group.
JANUARY 18,1993 C&EN
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