Cluster Chemistry Broadens Scope Of OrganometaUic Research

SCIENCE

Cluster Chemistry Broadens Scope Of OrganometaUic Research Studies on heteronuclear clusters result in theoretical, commercial breakthroughs in catalysis, ceramics, electronic materials Joseph Haggin, C&EN Chicago

An exponential growth of interest in cluster chemistry has considerably broadened the already broad venue of organometallics. Anticipated breakthroughs that would be a boon for catalysis are behind much of that interest. But cluster chemistry also has opened up some new vistas in materials science in which the chemist should play a central role—for example, in ceramics and metalloceramics. And encompassing it all is the continually expanding field of organometallic chemistry, which seems to be constantly producing discoveries of theoretical and commercial importance. These directions in cluster chemistry research were strongly underlined last month at an International Workshop on the Chemistry of Heteronuclear Clusters and Multimetallic Catalysts, which convened at Kônigstein/Taunus near Frankfurt, West Germany. Known as Kônigstein-Konferenz II, it was sponsored jointly by the U.S. National Science Foundation and the Deutsche Forschungsgemeinschaft. Cochairmen for the conference were chemistry professor Wolfgang A. Herrmann of Technische Universitât Munchen and chemistry professor Richard D. Adams of the University of South Carolina. The first conference of this kind took place two years ago, and a third is now being planned for 1989. The discovery that certain metal

alloys display catalytic activity, selectivity, and stability superior to the performance of individual components of the alloys was an important development in heterogeneous catalysis two decades ago. The more recent discovery that heteronuclear clusters and bimetallic mixtures also display enhanced properties appears to be of equal importance for homogeneous catalytic reactions. There seems to be general agreement that these phenomena are manifestations of a greater unity in organometallic chemistry, and the current era is one of very competitive research worldwide in search of that unity. Discoveries of potential benefit to catalysis are not the only ones

expected to accrue from research in heteronuclear clusters, however. Although the Kônigstein conference spent most of the week dealing with catalytic matters, it did touch briefly on research related to the possibility of producing some advanced electronic and severe-service materials. Of basic interest in this regard are phenomena of aggregation and structural variation in the transition from single atoms and molecules to macroscopic bulk materials. Heteronuclear clusters are usually made by agglomeration of metal atoms from complexes via redox coupling, pyrolysis, and other means. Alloy catalysts usually take the form of metal clusters highly dispersed

Metal cluster contains quadruply bridging carbonyl ligands

This mixed metal cluster Is the first molecule ever made that contains quadruply bridging carbonyl ligands, and It contains two. The Importance of bridging ligands rests, In part, on the stability they Impart to clusters. They also affect reactivity and catalytic selectivity In many cases October 5, 1987 C&EN

31

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Conference recommendations outline cluster chemistry research directions Taken together, the numerous presentations made at the Konigstein conference leave little doubt about the general importance of cluster chemistry or the breadth of interest in it. Overall, there is a pervasive feeling of imminent breakthrough. Although this is typical of much, if not most, research, in the case of cluster chemistry additional substance was provided at Konigstein by the attendance and active participation of industrial researchers. There is a spectrum of interest in cluster chemistry stretching from the very basic to frie very practical. In a series of summary recommendations made following the conference, participants outlined many of the needs and opportunities for future research. For example, in the area of supported bimetallic catalysts made from clusters, a major need is the synthesis of new precursor clusters with industrially important metal combinations. There is a corresponding need for improved techniques for structural characterization, particularly with extend-

on inert supports. These clusters are generally larger than molecular clusters, but the preparative methods have similarities. An important consideration for catalysis is establishing the structural identity of catalytic sites. The atomic arrangements in clusters and the possibility of precisely characterizing their structures offer potential means of unambiguously establishing the identity of catalytic sites, particularly if the clusters are highly controllable catalyst precursors. A case in point is the application of nuclear magnetic resonance spectroscopy, which is often used to study molecular complexes, to studies of the structure and chemistry of surfaces and adsorbed species. Electronic effects that change the reactivity of bimetallic catalysts and alloys can be attributed to changes in electronic structure caused by the influence of neighboring atoms. The neighboring atoms may be nonmetallic, as is usually the case in homogeneous media. On catalytic surfaces, the neighbors are other metal atoms, and heteronuclear complexes 32

October 5, 1987 C&EN

ed x-ray absorption fine structure (EXAFS) analysis. A basic understanding of the role of cationic promoters still eludes researchers, and there is a growing need for in-depth investigations of some of the more unusual supports, such as molecular sieves, carbon, and metal sulfides. The entire subject of metal/support interactions is only beginning to be understood in any detail. Mixed-metal clusters may serve as precursors to these bimetallic catalysts and supported catalysts. There is a pressing need for fundamental investigations of metal-metal bonding. Also in the area of catalysis, clusters may produce new transformations of small molecules and a route to new compounds. A fundamental understanding of site selectivity is still needed, and there is a corresponding need for the development of conceptual models for catalytic cycles to represent the steps in homogeneous clustercatalyzed processes. In the area of heteronuclear com-

may provide a way to examine electronic effects on surfaces. Geometric effects depend on the nature of the active site itself. Of importance in this context are the nuclearity, or size, of the ensemble making up the site and the nature of the immediate molecular environment. The factors that influence geometric effects may be operable equally in molecular clusters and alloy surfaces. Ion promoters used with catalysts might affect both geometric and electronic factors. Such effects have been observed in some studies with molecular clusters. The processes of bond making and breaking are crucial steps in catalytic reactions of small molecules. In the past, classic experiments have provided much information about bond rearrangements, but the details remained obscure. The advent of cluster chemistry and the studies of alloys have begun to yield more information on the intermediates that usually have a crucial but transient existence in transformations. One of the most valuable contributions of alloys and heteronuclear

plexes, it is desirable to prepare new homo- and heteronuclear clusters to serve as models for the interaction of small molecules on surfaces. These may aid the surface scientist in identifying surface species. Synthesis of very-high-nuclearity clusters with structural features resembling those of bulk metals will be important in developing new materials with unusual electronic, magnetic, and optical properties. Also in noncatalytic areas, there is a need to build alloy powders from large metal clusters, to gain basic information on nucleation and particle growth, and to prepare ultrafine powders for ceramic applications. There is also a need for ways to prepare new alloy phases via low-temperature deposition and the laser "writing" of alloy combinations. These and many more recommendations will eventually appear in the conference proceedings. They offer ample future opportunities for researchers and indicate potential benefits that could accrue from such research.

clusters may be the insight that they provide about the nature of intermediate species. Cluster chemistry is not inevitably esoteric. There have been some very practical, commercial results. At the Konigstein conference, one example came from Boy Cornils of Hoechst, who was recently named to manage the technical matters associated with the merger between Hoechst and Celanese. The example had to do with some recent changes in the venerable oxo process, which converts olefins to aldehydes by the addition of carbon monoxide and hydrogen to olefinic double bonds. This hydroformylation reaction was classically catalyzed by hydrido-transition metal carbonyls, for example, hydridocobalttetracarbonyl. This catalyst had been used in one plant to produce 150,000 metric tons per year of n-butyraldehyde from 134,000 tons per year of propylene. In recounting the development of an improved process, Cornils noted that the catalytic properties of the carbonyl could be varied by ei-

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Science ther changing the central metal or by substitution of CO ligands of the carbonyl. However, in cooperation with Rhône-Poulenc, Hoechst succeeded in developing a catalyst with water-soluble ligands by using transalkylated, metal-substituted triphenyl phosphates. The catalyst remains in the aqueous phase at all times, thereby providing heterogeneous catalysis in what was formerly a homogeneous system. Restriction of the catalyst to the aqueous phase also provides some economic advantages owing to process simplifications. Separation of the organic oxo-products from the catalyst aqueous phase can be done simply by decanting, without addition of energy. The heat of reaction is thus used to raise steam. In addition, there is no problem with lowboiling-point products. There is no leaching of the rhodium-bearing catalyst, which appears to have a long operational life. According to Cornils, the new process has proved to be highly reliable. It is operable using a small fraction of the space formerly required for the same output. Perhaps most important is the reduction in propylene requirements from 134,000 to 90,000 tons per year, indicating a sharp increase in aldehyde selectivity. Another practical application of cluster chemistry comes from M. David Curtis, chemistry professor at the University of Michigan. He has been developing sulfur-tolerant catalysts from sulfido-bimetallic clusters for use with synthesis gas. In particular, he has supported isomers of Cp2Mo2Fe2S2(CO)s (where Cp = cyclopentadienyl) on alumina, silica, titania, and magnesia. The surface species on these supports have been identified with in-situ Môssbauer spectroscopy, Fourier transform infrared spectroscopy, molybdenum and iron extended x-ray absorption fine structure (EXAFS) analysis, and temperature-controlled decomposition. The cluster-derived catalysts on alumina and silica supports produce carbon dioxide and methane as the principal hydrocarbon from syngas feed. Titania supports yield higher proportions of Q and C3 hydrocarbons. The MoFeS/MgO catalyst ex-

hibits transient behavior during which 90 mole % of the product is C2H4 and C2H6. Of particular interest is that these catalysts are all but unaffected by 13 ppm of H2S in the feed stream. The activity of MoFeS/ T1O2 is enhanced by the presence ofH 2 S. A new chapter in Ziegler-Natta catalysis may be in prospect. Walter Kaminsky, a chemistry professor at

the University of Hamburg, has found that extremely high ethylene polymerization activities are obtained with a homogeneous catalyst of a zirconocene compound and methylaluminoxane. Regulation of the molecular weight in this homogeneous system is obtained by changing the reaction temperature, varying the concentration of the zirconium compound (Cp2ZrCl2), or

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Science adding a small amount of comonomer, such as 1-butene or 1-hexene. With increasing zirconium concen tration, the molecular weight de creases almost linearly. At 10 °C, the polyethylene formed has a mo lecular weight of 1.5 million. At 50 °C, this drops to 180,000. At tem peratures above 100 °C, the prod ucts are α-olefins with an even num ber of carbon atoms. According to Kaminsky, unlike with heterogene ous catalysts, only a trace of hydro gen is needed to lower the molecu lar weight over a wide range. Recently Kaminsky and his co workers have used ring-substituted zirconocene dichlorides as transi tion metal components because of their activity in ethylene polymer ization. If [Cp(CH 3 ) 5 ]2ZrCl 2 a n d oligomeric methylaluminoxane are combined, a highly active ZieglerNatta catalyst is formed. This cata lyst has provided the greatest mo lecular w e i g h t of p o l y e t h y l e n e achieved in the experiments.

H i g h l y isotactic p o l y p r o p y l e n e may be prepared with chiral racemicethylene bis-(tetrahydroindenyl)zirconium dichloride and methylalu minoxane as a cocatalyst. Following the resolution of this racemic mix ture, Kaminsky obtained optically active polypropylene and polybutylene for the first time. Control of the reaction temperature allowed control of t h e molecular w e i g h t from 300,000 d o w n to 1500. Some particularly elegant chem istry was described at Konigstein by chemistry professor Masanobu Hidai of the University of Tokyo. In previous work, Hidai had dis covered that in the homologation of methanol by cobalt/ruthenium mixed-metal catalysts, ruthenium was not only involved in the hy drogénation of acetaldehyde to ethanol but to some degree in enhanced aldehyde formation. It had been expected that these bimetallic systems might also show high catalytic activity for the hydroformylation

of olefins, since both processes involve alkyl and acyl complexes as common intermediates. The expectations were realized w h e n it was found that bimetallic catalysts consisting of Co 2 (CO) 8 and Ru 3 (CO)i 2 had a higher activity than either did alone. The initial rate of hydroformylation of cyclohexene with the mixed catalyst was 19 times faster than with the cobalt catalyst alone. In discussing the chemistry of cobalt/ruthenium mixed clusters, Hidai noted that the reaction of R u C l 3 - 3 H 2 0 with Na[Co(CO) 4 ] results in formation of mixed cluster, Na[RuCo 3 (CO)i 2 ], in high yield. This is readily converted via photolysis to HRuCo 3 (CO)i 2 . The substitution of cobalt ligands takes place preferentially at the ruthenium atom, but the substitution of phosphines takes place exclusively at the cobalt atoms. In dealing with clusters, particularly mixed-metal clusters, the problems of synthesis are rather complex, often because of the syner-

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Science gism that appears. Some syntheses aimed at preparation of mixed-metal crystallites in thin films were described at the conference by Herbert D. Kaesz, chemistry professor at the University of California, Los Angeles. Kaesz and his coworkers have been concentrating on rhodium/ ruthenium and iridium/gallium systems. Rhodium/ruthenium mixedmetal catalysts on supports, or homogeneous solutions of these two metals, display some potential as catalysts for converting syngas to ethylene glycol. One of the problems, however, is stability of the complex. Kaesz and his coworkers are now attempting to synthesize these complexes with bridging C or CH groups between the metals. Current work focuses on the combination of suitable rhodium carbyne complexes with hydrido-triruthenium complexes. Istvan T. Horvath of the department of industrial and engineering chemistry of the Swiss Federal In-

stitute of Technology has been concentrating on the influence of a substrate on the activity of clusters. The implication is that there is a substratedependent heterosite reactivity of cobalt/rhodium clusters and that this may be the origin of the often observed cobalt-rhodium synergism in catalytic hydroformylation. Reactions of cobalt/rhodium mixedmetal clusters with P(C2H5)3, tetrahydrofuran, CH3CN, CNBu+, and Cl~ occur on the rhodium site. Reactions with alkynes occur at the cobalt site. Several reversible tetranuclear-dinuclear transformations were observed, with preferential retainment of the cobalt-rhodium bond. The reactions of mixed-metal clusters and alkynes may produce either substitution or degradation of the cluster to lower nuclearity complexes. One question remaining is at which site the substitution takes place. Another is how the metal is distributed in the fragments. The

fragmentation is, generally, not reversible. However, Horvath and his associates have discovered a reversible reaction involving diphenylacetylene and/or carbon monoxide. He believes this is the first example of such a transformation. The mechanism is not known. The University of South Carolina's Richard Adams and his coworkers have been investigating the activation of trinuclear metal cluster complexes by metal atom substitution. In addition to the usual variation of ligands to control the catalytic properties of complexes, there is sometimes the additional option of varying the identities of the metal atoms themselves. In particular, Adams has investigated the reactivity of the cluster RuMo2(CO)7(C5H 5 ) 2 (M3-S), which is formed from the substitution of two Ru-carbonyl units in RU 3 (CO) 9 (M3-CO)(M 3 -S) by two cyclopentadienyl-Mo carbonyl units with HC2Ph (where Ph = phenyl). The trinuclear center has been

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shown to be active toward HC2PI1 oligomerization. What has been shown by this work is that an active alkyne cluster complex can be prepared from an inactive one. Adams believes that, in general, it may be possible to tune the reactivity of polynuclear metal complexes by a process of selective metal atom substitution, a concept that should be valuable in the future systematic design of new multimetallic catalysts. Cochairman Herrmann and his associates have directed their interest toward some new organorhenium compounds that exhibit the metal in some high and intermediate oxidation states. The compound of greatest interest at the present time is trioxo(i75-pentamethylcyclopentadienyl)rhenium(VII). This compound was stable—surprisingly, since it had been believed that the metal in such a highly oxidized state would "consume" the organic ligands. A number of closely related

organorhenium compounds also have been shown to have equally interesting chemistry. The chemistry of the compound mentioned is governed by reduction processes that lead to derivatives with pentavalent rhenium. Alkyl groups may be introduced by using such appropriate sources as trialkylaluminum. In some cases, both mono- and dialkyl products have been obtained. Plans for future work include the possible applications of organometallic oxides in catalysis-related studies. Some interesting research on large clusters was described at the Kônigstein conference by Georg Schmid of the University of Essen, West Germany. He dealt with two-shell clusters of the type M55L12QX (where M = metal and L = ligand). These clusters are synthesized by reduction of appropriate complexes by B2H6 in solution. The triphenylphosphine ligands in Au55(PPh3)12 can be completely exchanged by

Ph 2 (Na + 03-S-C 6 H4)P, leading to water-soluble clusters. Schmid also reported that the first five-shell cluster consisting of 561 palladium atoms can be prepared using Pd(Ac)2, phenanthroline as a ligand, and hydrogen as the reducing agent. The cluster can be stabilized when oxidized by air in solution. The formula Pd56i(Ph)36Oi9o-200 is idealized, but Schmid says that x-ray studies show that the cluster is "very close to the magic number of 561." The M55 clusters can be degraded in CH2CI2 solution using platinum electrodes and 10 to 20 volts dc. The outer shell of 42 metal atoms and ligands is peeled off and the naked M13 clusters are revealed. These assemble into different modifications, [(Mi 3 ) 13 ] n , using the M i 3 clusters as basic building blocks. These come into contact via the triangle faces of the cuboctahedra and form a close-packed structure of MJ3 units. It is often said that selectivity is

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Science the name of the game in hydrocarbon catalysis, particularly in Q catalysis. Among the more active researchers in the field of CO hydrogénation is Masaru Ichikawa of the Research Institute for Catalysis at Hokkaido University. He noted at the conference that in catalytic CO hydrogénation with multimetallic catalysts, electropositive ions such as manganese, titanium, and zirconium increase yields; in contrast, iron, molybdenum, and zinc improve selectivity toward such oxygenates as alcohols. To understand the phenomena of promotion by metal ions, Ichikawa has used molecular iron-containing rhodium, platinum, and palladium carbonyl cluster compounds to plant bimetal ensembles on silica, titania, and magnesia and inside zeolite supercages. These compounds are very active for migratory CO insertion and conversion to alcohols. The results suggest that Fe 3+ in the bimetals acts not only as an anchor to fix rhodium, platinum, and palladium clusters, but also renders RhFe 3+ and like pairs highly active in olefin hydroformylation and alcohol formation in the carbon monoxide/ hydrogen reaction. The problems of finding a systematic procedure for developing polynuclear systems useful in catalytic or stoichiometric transformations is often a seemingly never ending filling in of blank spaces. An attempt at systematic development has recently been started by chemistry professor Arthur J. Carty of the University of Waterloo, Ont. Among the fundamental problems facing organometallic chemistry is that of finding means to stabilize polynuclear complexes against fragmentation. This is the reason for the use of main-group metal ligands in many cases. It also remains to be seen if site selectivity can be improved by the use of mixed-metal systems. Carty suggests that an alternative to mixed-metal systems may be the use of multifunctional ligands and potentially bridging groups with different donor atoms. In his laboratory, Carty has begun the systematic investigation of polynuclear complexes incorporating phosphidoxo ligands. This work is still in the early stages. D 44


Workshop airs research ethics and monitoring of scientific misconduct Pamela S. Zurer, C&EN Washington

"The difficulty of these issues is offset by their interest," said Patricia K. Woolf, Princeton University sociologist, at the start of a recent workshop on scientific fraud and misconduct. After three days of thought-provoking, sometimes emotionally draining sessions in which a diverse group of individuals struggled with the complex problem, her comment seemed right on the mark. About 35 scientists, lawyers, government regulators, and university officials were brought together last month for a weekend retreat in Hedgesville, W.Va., by the National Conference of Lawyers & Scientists (NCLS)—a joint project of the American Bar Association and the American Association for the Advancement of Science. The workshop, according to Albert H. Teich, head of AAAS's office of public sector programs and organizer of the conference, was designed to enable people with widely different backgrounds to share their knowledge and experience. Each participant had a unique perspective on scientific misconduct to offer to the group. Some, like Woolf, are undertaking scholarly research on the issue. Others have been charged by their institutions to develop procedures for handling instances of alleged misconduct. Two were attorneys for government funding agencies responsible for safeguarding public funds. Several scientists had had personal experience with cases of scientific fraud. The conference, the first of three planned by NCLS and funded by a grant from the Sloan Foundation, had two major objectives. One was to assess the principles for conducting high-quality research. The second goal, discussion of which ended up dominating the weekend, was to describe procedures for dealing with scientific misconduct so that the rights of all parties are protected. Woolf and Warren Schmaus, professor of philosophy at Illinois Institute of Technology, kicked off the

discussion by presenting commissioned papers that attempted to assess the scope and consequences of scientific misconduct. From their presentations and the ensuing discussion, it became clear that not only is there a lack of hard data about how often fraud occurs in research, but there is no generally accepted definition of scientific misconduct. Everyone agreed that falsification or misrepresentation of data is clearly wrong and cannot be tolerated. However, the group was less of one mind with respect to whether action could or ought to be taken to curb such practices as multiple publication of the same results, honorary authorship (when a researcher who had little or no direct hand in a project is named as a coauthor), or failure to acknowledge the source of an idea. "We live in an era of cacophonous misrepresentation," said Woolf, citing as an example the recent revelations of lying by certain political candidates. "How do we decide which lies matter?" Lewis M. Branscomb, director of Harvard University's science, technology, and public policy program, expressed his opinion that the system should intervene only in the most flagrant cases of misconduct, for fear that policies to suppress fraud may result in suppression of thought. "Science is extraordinarily inefficient," he said. "It has to be inefficient to be creative. People have to be able to fail. The line between deliberate and self deception is thin." Eleanor G. Shore, associate dean for faculty affairs at Harvard medical school, thinks it is fruitless and not even necessary to try to come up with an exact definition of misconduct. "It comes down to scientific conduct that is not acceptable," she said. "There are certain standards that one's peers or colleagues expect." To complicate matters still further, sociologist Daryl E. Chubin, a senior analyst with Congress' Office of Technology Assessment,

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