SCIENCE/TECHNOLOGY

Organometallic Chemistry Opens New Vistas In Medicine, Materials Science

• Use of metal compounds in immunoassays, cancer drugs, and oxide ceramics highlighted at symposium honoring F. Albert Cotton

Ron Dagani, C&EN Washington

Inorganic chemists are the cowboys of chemistry. They don't like to be confined to any corner of the peri­

odic table. They want the freedom to explore it all—from hydrogen to the transuranium elements. And they are at home in both the mineral and bio­logical realms. For them, adventure is discovering new reactions, new struc­tures, and new bonding modes, regard­less of which elements are involved.

This conception of inorganic chem­ists was borne out last month at a star-studded symposium celebrating the life and work of one of the premier chemists of our time. "Contemporary Inorganic Chemistry: A Symposium in Honor of F. Albert Cotton" was held, appropriately enough, at College Sta­tion, a city in the south central part of the sprawling Lone Star State. College Sta­tion is home to Texas A&M University, and Cotton is one of the luminaries on the faculty who has made that universi­ty synonymous with first-rate chemical research.

Cotton, 65, and his many students and collaborators over the years have ex­tended the boundaries of inorganic chemistry. The richness of the field was made evident at the symposium in a se­ries of one-hour lectures given by 23 of his colleagues. Some of them looked back, reflecting on the insights that have lit up dark corners of chemistry and even transformed the field. Others looked ahead, charting new directions.

Some of those new directions are in bioorganometallic chemistry—the use of transition-metal complexes in medicine

Jaouen: bioorganometallic explorations

and other biological areas. During the past few years, Gerard Jaouen, a profes­sor of chemistry at the Ecole Nationale Superieure de Chimie de Paris, and his coworkers have been exploring the po­tential of organometallics in such areas as immunoassays, antitumor drugs, ~and the study of active sites in proteins.

Immunoassays make use of specific antibodies to determine a hormone, drug, or other compound at nanogram or lower levels. In the simplest case, an antibody binds to the analyte or to a tagged version of the analyte in a com­petitive reaction. The tag, which typical­ly is a radioactive atom, allows the amount of tagged analyte-antibody complex to be determined. And from this, the concentration of the unlabeled analyte in the sample can be ascertained.

Some of the newer immunoassays use a nonradioactive tag such as an enzyme or a fluorescent dye. Jaouen's work aims to develop the use of a metal carbonyl complex as the tag. Such complexes have extremely intense infrared absorp­tion bands in the 2,200- to 1,850-cm"1 re­gion, where proteins do not absorb.

Thus, the organometallic-labeled mole­cule can be detected with great sensitivi­ty by Fourier-transform infrared spec­troscopy (FTIR).

Since each metal carbonyl marker ex­hibits its own characteristic absorption peaks, Jaouen says, it's conceivable that several different immunoassays could be performed at the same time. Such multi-analyte testing has been a "long-cher­ished" goal of the analytical chemist, he adds.

Jaouen has demonstrated the feasi­bility of such "multi-immunoassays" by attaching different metal carbonyl markers to three important antiepilep-tic drugs—carbamazepine, phenobarbi-tal, and phenytoin. In the case of car­bamazepine, for example, a side chain terminating in an alkyne function was added, and Co2(CO)6 was complexed to the C=C bond. In phenobarbital, the phenyl ring was derivatized with a group containing Mn(CO)3 complexed to a cyclopentadienyl ring. The tagged analytes are well recognized by the antibodies.

Jaouen and his coworkers have shown that a mixture of three such organome­tallic complexes, present in amounts from 10 to 100 picomoles, can be ana­lyzed simultaneously and quantitatively by FTIR. The margin of error in these determinations is less than 5%.

This multi-immunoassay might be of particular interest to hospitals because these three anticonvulsive drugs nor­mally are administered to patients at the same time. Assays for each drug are performed extensively, Jaouen ex­plains, since each drug's therapeutic ef­fect is related to its concentration in the patient's serum. Jaouen is not aware of any immunoassays that are routinely available and allow several drugs to be determined simultaneously.

The carbonyl metallo immunoassay (CMIA), as Jaouen calls his approach, also has been extended to the simulta­neous analysis of five organometallic tracers, "although in this case the super-

32 APRIL 10,1995 C&EN

Substituents on estradiol affect binding affinity to receptor

CF3SO3-

RBA = 0%

CIH2C C = C

RBA = 172%

Re(CO)3

RBA = 6% 11(3-Chloromethyl-17a-ethynylestradiol

tagged with rhenium complex

Estradiols tagged with organoruthenium cations

The relative binding affinity (RBA) indicates how strongly an estradiol derivative binds to the estrogen receptor compared to estradiol itself (RBA = 100%). The compound with RBA = 0% is not recognized by the receptor because of the positive­ly charged group at C-17. The compound with RBA = 172% has a very strong affinity for the receptor, possibly because the chlorine coordinates to a zinc atom at or near the binding site.

imposition of [IR] signals makes the analysis more complex/7 he says.

To make CMIA viable as a routine clinical method, it will have to be made into a solid-state immunoassay, in which one of the antibodies might be attached to a solid phase such as mi-crobeads. A second antibody labeled with a metal carbonyl would then be added to detect the analyte. Such a du­al-antibody system would allow the creation of a simpler to use immunoas­say in which the analyte to be deter­mined and the tracer would not have to compete for antibody binding sites. Jaouen's group is working toward this goal. Eventually, he expects that this type of assay will replace immunoas­says that depend on radioactivity.

Jaouen and his colleagues also use or-ganometallic complexes to study bind­ing sites of protein receptors, with an eye toward developing improved anticancer drugs. One protein class of particular in­terest to them is the estrogen receptor, which is involved in fertility, mammary gland development, and the regulation of many breast cancers. The first step in the stimulation of uterine growth, for ex­ample, is binding of the female sex hor­mone estradiol (an estrogen) to its recep­tor in the cytoplasm of uterine cells. The receptor dimerizes and then binds to a specific DNA sequence. This, it is thought, eventually leads to the modula­tion of gene transcription.

The detailed structure of the hor­mone-receptor complex is still unknown, as is the receptor's mechanism of action

at the molecular level. But Jaouen has been using organometallic chemistry and molecular modeling to find pieces to the structural and mechanistic puz­zle. Some of this information has been garnered by studying how well the re­ceptor binds to a variety of estradiol derivatives labeled with organometallic tags attached to different positions on the molecule.

Studies have shown that the hydroxyl groups at C-3 (on the A ring of estradi­ol) and C-17 (on the D ring) are neces-

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sary for the hormone to be recognized by the estrogen receptor. But when Jaouen attached a positively charged ruthenium metallocene to the hydroxyl-bearing C-17 of estradiol (at the 17a po­sition), the molecule was not recognized at all by the receptor. By contrast, the presence of a charged ruthenium com­plex at the opposite end of the steroid (on the A ring), did not significantly af­fect receptor recognition. This, Jaouen says, suggests that the binding partner in the natural receptor that comes near the C-17 position of estradiol also is posi­tively charged and thus would repulse a positively charged moiety approaching it. He thinks the binding site may contain protonated lysines and/or a Zn2+ ion.

One particularly interesting finding from Jaouen's lab concerns an estradiol that is substituted with a chloromethyl group at the l ip position and an ethy-nylcyclopentadienyl rhenium tricar-bonyl marker at the 17a position. This molecule shows a much stronger bind­ing affinity for the estrogen receptor than does estradiol itself. Jaouen sus­pects that this is a result of the presence of the chlorine atom, which may coor­dinate weakly to the zinc ion thought to be located near the binding site of the receptor. By labeling such an estra­diol derivative with a radioactive rhe­nium isotope, he suggests, it may be possible to make an anticancer agent that homes in on the receptor in the nucleus of the target cell and sticks to it long enough to irradiate the cancerous cell. Certain rhenium radioisotopes

APRIL 10,1995 C&EN 33

SCIENCE/TECHNOLOGY

could be used both to treat the tumor and to image it.

Another molecule that binds strong­ly to estrogen receptors is 4-hydroxyta-moxifen. This molecule is the active metabolite of tamoxifen, which is used in the adjuvant therapy of breast can­cer. Tamoxifen itself is known as an an-tiestrogen because its effect is the oppo­site of estradiol: It causes a down-regu­lation of estrogen receptors in breast tumors, preventing estradiol from stim­ulating tumor growth. 4-Hydroxyta-moxifen is considered to be a highly well-tolerated site-directed cytotoxic agent. But because patients who take ta­moxifen tend to develop resistance to it, scientists have been seeking alternative drugs.

Jaouen and coworkers are trying to create improved (more cytotoxic) ver­sions of tamoxifen using organometal-lic chemistry. For example, they pre­

pared an analog of 4-hydroxytamox-ifen in which a phenyl ring of the core triphenylethylene structure has been replaced with a ferrocenyl moiety. This is a logical derivative to try since ferro­cene has antitumor activity and is chemically stable in many media.

The ferrocenyl analog, dubbed ferro-cifen, was prepared in both its (Z) and (E) forms. Both isomers were found to bind to the estrogen receptor more strongly than tamoxifen (but less strong­ly than 4-hydroxytamoxifen). The ferro-cifen isomers also were tested for toxic­ity in a human cell line derived from a breast tumor. In these preliminary tests, both isomers were found to be some­what more cytotoxic than tamoxifen. Jaouen is encouraged by these results. He says the ferrocifens could well be prototypes for a new class of anticancer agents.

Concluding his lecture at the Cotton

Methyltrioxorhenium reacts with water to give 'polymer'

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/ I / I / I CH3 OH3 CH3

Simplified two-dimensional structural model of "polymethyltrioxorhenium"

Note: Circled rhenium atom in intermediate structure is missing a CH3 group (8% of the rhenium atoms in the final "polymeric" product have no CH3 group).

symposium, Jaouen noted that this work, which extends beyond the nor­mal realm of transition-metal complexes and into bioorganometallic chemistry, opens new vistas whose full potential can hardly be ascertained at present/'

Organometallic chemistry also has opened new dimensions in materials sci­ence. One recent example was described in College Station by chemistry profes­sor Wolfgang A. Herrmann of the Tech­nical University of Munich, in Garching, Germany. Herrmann and coworkers Richard W. Fischer and Wolfgang Scher-er discovered a process that they believe is unprecedented in organometallic chemistry: the transformation of an or­ganometallic oxide into a "polymeric" material in aqueous solution.

The starting oxide is methyltrioxo-rhenium(Vn)—CH3Re03, or MTO. This colorless compound, first reported in 1979, is the simplest organometallic ox­ide known, according to the research­ers. Herrmann and his coworkers have explored its chemistry extensively in the past decade. Their work shows that the oxide catalyzes a number of reac­tions, including olefin epoxidation, ole­fin metathesis, conversion of cyclic ke­tones to lactones (Baeyer-Villiger oxi­dation), oxidation of aromatics, and reductive olefination.

The big surprise came when the Mu­nich researchers dissolved MTO in wa­ter and observed, over the course of days, the formation of a gold-colored precipitate in 70% yield. The reaction goes faster if the mixture is heated. The resulting precipitate seemed to be poly­meric because it was found to be insolu­ble in all solvents, except those that react with it, like hydrogen peroxide. It also is totally nonvolatile—in striking contrast to MTO, which sublimes at room tem­perature.

The composition of the vacuum-dried precipitate, which Herrmann re­fers to simply as "poly-MTO," was found to be {H0.5[(CH3)0.92ReO3]}n. From this formula, the curious material appears to be basically CH3Re03 with 8% of the methyl groups missing and some additional hydrogen (half as much hydrogen as rhenium). The as-formed material is somewhat more complicated, though, because it con­tains water. And it isn't easy to isolate the compound without any water in the lattice, Herrmann points out.

Poly-MTO, which he calls the first known polymeric organometallic ox-

34 APRIL 10,1995 C&EN

Simplified two-dimensional structural model of "polymethyltrioxorhenium"

Herrmann (left) and Cotton posed together at the inorganic chemistry symposium held in Cotton's honor in College Station, Texas.

ide, has some striking physical proper­ties. It looks like graphite and feels smooth and slippery to the touch. It also is electrically conductive and weakly paramagnetic.

Poly-MTO behaves quite differently than MTO. MTO can withstand tem­peratures up to about 350 °C with no

decomposition. And when it does de­compose at 450 °C, it forms predomi­nantly Re02. By contrast, when poly-MTO is heated above 240 °C, it cleanly loses all its methyl groups (and water) to yield highly pure, crystalline Re03

and mostly methane. This, Herrmann points out, represents

the first organometallic route to pure, electrically conducting Re03 films that are free of any carbon or hydrogen con­tamination. These Re03 films adhere well to glass, plastics, and other surfaces. The polymer process thus seems to be an intriguing alternative to chemical va­por deposition techniques, which use organometallic compounds of well-de­fined composition and structure to lay down metallic or ceramic films for elec­tronic and other applications.

Another interesting aspect of the MTO-derived polymer is that it under­goes an unprecedented pressure-induced depolymerization on standing in air. Some 10 to 15 hours after a pellet of poly-MTO is made at a pressure of about 150 bar, little colorless crystals of CH3Re03

begin to form on the golden-yellow pel­let. After several weeks, the surface is covered with colorless crystals. When these are removed and pressure is ap­plied repeatedly to the remaining pellet, more CH3Re03 crystals form. In the end, what remains of the pellet is a violet ma­terial that is largely Re03.

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This process, Herrmann says, indi­cates that poly-MTO is a metastable ma­terial that uses various sources of energy to enter degradation pathways that fi­nally yield the thermodynamically stable products CH3Re03, Re63, and CH4.

The insolubility and nonvolatility of poly-MTO have made the elucidation of its structure very difficult. Neverthe­less, the Munich group has developed a structural model for the material, al­though the details are not yet clear. The polymer has a layered structure that contains methyl-deficient corner-shar­ing Re05(CH3) octahedra. In crystalline regions of poly-MTO, these octahedra form double layers that are separated by a single layer of intercalated water molecules. According to the model, the H20 molecules could form hydrogen bonds with the oxo groups on the rhe­nium atoms. However, the polymeric material also contains partially amor­phous regions where the rhenium ox­ide layers probably are not stacked in an orderly way and the water mole­cules are only loosely intercalated. Ad­jacent rhenium oxide layers in this ma­terial interact only weakly, leading to the flakiness, softness, and slipperiness characteristic of graphite.

The structural model for the polymer bears some similarity to the structure of Re03, a simple, three-dimensional lattice consisting of octahedral rhenium atoms and • • • Re-O-Re-O • • • chains. This sim­ilarity also helps explain how the lay­ered polymeric oxide is converted so cleanly to Re03 by heating: Only small structural changes—like elimination of the methyl groups—are necessary to convert one structure into the other, Herrmann explains. This is a new aspect of organometallic chemistry, he says, and it warrants further exploration.

He also believes the process that has led to poly-MTO offers a general new approach to oxide ceramics and to potentially interesting intercalation compounds.

The latest work of Herrmann and his collaborators on poly-MTO is described in three consecutive papers that were published a week after the symposium ended [/. Am. Chem. Soc.r 117,3223,3231, and 3244 (1995)].

Jaouen's and Herrmann's presenta­tions in College Station were not the only ones that dealt with opening new windows into the worlds of biomedi-cine or materials science. Kim R. Dun­bar, for instance, has her hands in both worlds. Dunbar, a chemistry professor at Michigan State University, East Lan­sing, has been collaborating with chem­istry professor George Christou of Indi­ana University, Bloomington, and their coworkers to develop dimetal com­pounds that have a metal-metal bond and show antitumor activity.

At the Cotton symposium, Dunbar discussed the synthesis of a new type of molecule—a metal-metal bonded com­plex in which the two transition-metal atoms are bridged both by carboxylate groups and purines (9-ethylguanine or 9-ethyladenine). The dirhodium, diru-thenium, and dirhenium complexes in this family exhibit antitumor activity. Dunbar's group plans to tailor these molecules to prepare clinically viable compounds with improved stability and water solubility.

The fact that DNA bases can be in­duced to bind to metals in this previous­ly unknown way raises the possibility that this mode of binding to DNA could be involved in the antitumor activity ob­served for other dinuclear metal carbox-ylates and related compounds.

Dunbar's group also is exploring ways to incorporate metal-metal bonds into extended arrays such as conduct­ing metallopolymers that could serve as a molecular wire. Possibilities being pursued include a chain or two-di­mensional sheet consisting of alternat­ing multiply bonded metal-metal units and conjugated organic molecules such as TCNQ (7,7,8,8-tetracyano-p-quinodi-methane). The incorporation of metal-metal bonded compounds into macro-molecular arrays "is a fascinating re­search area with enormous potential/' Dunbar says.

Metal-metal bonded systems also were the focus of several other lectures at the symposium. For example, chemis­try professor Richard D. Adams of the University of South Carolina, Columbia, wowed the audience with his elegant elucidation of the mechanism of a metal-cluster-catalyzed alkyne hydrogenation reaction. The metal carbonyl cluster at the heart of this work looks like a Pt3 tri­angle sandwiched between two Ru3 tri­angles. The unusually high activity of this so-called layer-segregated catalyst seems to be a result, in part, of a syner­gistic interaction between the platinum and ruthenium layers. Adams suspects that the platinum atoms activate the hy­drogen, and the ruthenium atoms acti­vate the alkyne.

For an event in honor of Cotton, it was not surprising that transition-metal chemistry dominated the proceedings. After all, that is the area of the periodic table that Cotton and most of his associ­ates have been mining. And it's also the area most familiar to the organizers of the symposium, Carlos A. Murillo, chemistry professor at the University of Costa Rica (and adjunct chemistry pro­fessor at Texas A&M), and John P. Fack-ler Jr., a chemistry professor at Texas A&M. Nevertheless, chemistry of the main group elements, lanthanides, and actinides also got their due. All in all, there was something for almost everyone.

Undoubtedly, the person who got the most out of the event was, appropriately enough, Cotton himself. Seated in front-row center for all the lectures, he listened appreciatively and beamed at his col­leagues. They, in turn, showered him with gifts of all kinds, most notably bot­tles of fine wine. And whether the speak­er explicitly said so or not, the underlying, heartfelt message of each talk seemed to be: 'Thanks, Al, for all you have done for chemistry." •

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" S n — CH2(CH3)2CC6H5

CH2(CH3)2CC6H5

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