Fifty Years of ZieglerâNatta Polymerization: From Serendipity to

Review pubs.acs.org/Organometallics

Fifty Years of Ziegler−Natta Polymerization: From Serendipity to Science. A Personal Account† John J. Eisch*

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Department of Chemistry, The State University of New York at Binghamton, Binghamton, New York 13902-6000, United States

PROLOGUE At the end of some 55 years in university teaching and research, I find myself considered as an organic researcher well-known in the discipline of organometallic chemistry. With their air- and moisture-sensitive carbon−metal bonds, organometallics are clearly the antithesis of the naturally occurring organic compounds and their simpler derivatives, which are composed of carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus, and for over a century have been the focus of mainstream organic chemistry. Prominence in organometallics was not at all my goal when I began the study of chemistry at Marquette University. Although I was already committed to organic chemistry from work in my home lab and high school chemistry, my attraction narrowed rapidly to the field of natural products and thereafter to carbohydrates.1 As a result I carried out research on the sugars found as hydrolysis products of okra pods and prepared a honors senior thesis2 under the direction of Professor Clifford Haymaker3 (Figure 1) of the Chemistry Department and Dr. Kenneth Brown, a young biochemist in the Marquette School of Medicine.4 On the strength of this thesis and my overall academic record, I was awarded a competitive, four-year scholarship for doctoral study in paper chemistry at the Institute of Paper Chemistry, then in Appleton, Wisconsin. I was assured that such study could lead to a position as a research chemist in a paper mill. I thus undertook the first semester of study at Lawrence College, affiliated with the IPC, with great enthusiasm. As the semester wore on, however, I enjoyed the organic chemistry greatly5 but found the parallel classes in engineering and physics rather routine and unexciting. Discussion with my classmates gradually made clear to me that upon graduation I would most likely end up managing a paper mill. That outcome was not what I wanted, and so again with the guidance of Professor Haymaker, I resigned my scholarship after one semester and obtained a research fellowship at Iowa State University with Professor Henry Gilman. In this way began my journey from conventional natural products chemistry to “unnatural” organometallic compounds.

of not only natural products but also the large-scale production of organic compounds of broad pharmaceutical importance. To the inorganic or physical chemist, on the other hand, the structural and physical properties of organometallics challenged the boundaries of existing valence theory and promised deeper insights into the complex nature of chemical bonding. Although these two separate viewpoints, the value in fundamental organic synthesis versus the complex structure of organometallic compounds, enjoyed broad importance in academic chemistry and the production of specialty organics, neither of these efforts had generated any great interest in industrial chemical processes prior to 1950. This compartmentalized academic pursuit of organometallics into utility in synthesis or into relevance to chemical structure was to be swept away with the completely unexpected discoveries in the pentad of 1950 to 1954, embracing as structural surprises the syntheses of ferrocene and other cyclopentadienyl and arene−transition-metal complexes and the discovery of facile, stereoselective olefin oligomerizations or polymerizations by transition-metal salt combinations with

I. INTRODUCTION A. Organometallic Chemistry Prior to 1950. The status of organometallic chemistry, right after World War II, was that of an esoteric, marginal academic discipline bordering on both organic and inorganic chemistry and concerned with the structure and properties of ionic or covalently bonded organic derivatives containing a direct carbon−metal bond. Already a century-old discipline since Frankland’s pioneering research on zinc alkyls in 1849,6 organometallics were regarded from two distinct points of view. The organic chemist valued the C−C bond-making ability of Grignard and organolithium reagents as presenting unique and selective methods for the complex and stereoselective synthesis © XXXX American Chemical Society

Figure 1. Clifford R. Haymaker (1907−1981). Born sightless in Milwaukee, Wisconsin, he made his way with sighted tutors through grade school, high school, and Marquette University with the highest honors and finally obtained the Ph.D. degree from Marquette University in 1938 with the dissertation “The Chemistry of Atomic Nuclei”. He offered lectures at Marquette on all levels of organic chemistry with the aid of student scribes from 1938 to 1968. He retired with the rank of professor and the warm regard of his many grateful students. Biography: Eisch, J. J.; Haworth, D. T. J. Chem. Educ. 2003, 80, 275. Received: April 26, 2012

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dx.doi.org/10.1021/om300349x | Organometallics XXXX, XXX, XXX−XXX

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main-group alkyls as an unexpected boon to synthetic organic chemists. This list of some pathfinders in this period, chiefly Reppe, Roelen, Pauson, E. O. Fischer, Wilkinson, Ziegler, and Natta, conjure up the heroes in these revolutionary developments.7a,b B. The Gilman−Ziegler Connection during My Doctoral Studies at Iowa State University. When I began doctoral studies at Iowa State University in January of 1953, I was following the trusted counsel of my major advisor at Marquette University, Professor Clifford Haymaker, who had sent a number of Marquette seniors to study with Professor Henry Gilman.8 Gilman was already world famous both for his voluminous publications on main-group organometallics and heterocyclic compounds and as a legendary taskmaster in his training of graduate students (Figure 2). I was initially more interested in heterocycles than, to me then, the peripheral field of main-group organometallics, and so Professor Gilman and I agreed upon a doctoral topic combining both themes. In early 1956, accordingly I presented a doctoral dissertation entitled, “Comparison of Phenanthridine with Other Aza-Aromatic Heterocycles”,9 which concerned the use of Hückel molecular orbital theory to rationalize the site and relative reactivity displayed by pyridine and bi- and tricyclic pyridines toward electrophilic, nucleophilic, and radical reagents. Naturally, among the nucleophiles that I evaluated were both Grignard and organolithium compounds. The further relevant connections that completed my total immersion in organometallics as my career field were of two very different types: one a scientific relationship and the other a cultural

interest. As will be brought out later, I had developed a strong interest in the German language and literature since my youth and from my family heritage. However, the more important and immediate connection was the long-standing relationship between Henry Gilman and Karl Ziegler. Gilman and Ziegler had been

Figure 2. Henry Gilman (1893−1986). After he received a Ph.D. degree at Harvard, he spent his entire academic career at Iowa State, where his graduate students and he published over 1000 papers. About half of these publications were made after 1947, as retinal detachments and eventually glaucoma rendered him sightless. He was, arguably, internationally the most towering figure in organometallic chemistry. Biography and appreciation: Eisch, J. J. J. Organomet. Chem. 1988, 338, 281. Eisch, J. J. Organometallics 2002, 21, 5439. B


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unique study of reaction mechanisms, a field that had seized my interest while at Iowa State, under the influence of two younger professors, George Hammond and Charles DePuy.16 Furthermore, while in Gilman’s group, I was extensively exposed to Gilman’s empirical studies on the rankings of organometallic reactivity.17 Although these rankings were then without any clear molecular explanation, they served to stimulate curiosity as to mechanism. In such rankings of the reactivity of RM types toward carbonyl substrates, the RLi reagents showed a greater reactivity than RMgBr types and the R3Al type displayed a much reduced reactivity compared with either. Yet Ziegler now found that R3Al reagents showed a much greater reactivity than either RLi and RMgBr reagents toward olefins both in additions and in transition-metal-catalyzed polymerizations. (For further discussion of the diminishing effect of coordinating solvents, such as ethers and tertiary amines, on the reactivity of aluminum alkyls, cf. section III.A.) As mentioned above, a second factor enhancing my interest in such an appointment was my German heritage. My grandfather on my father’s side came from Bavaria through a long line of glass workers reaching back to Bohemia before the 1600s. My grandfather, Josef Eisch, immigrated to the United States in 1880 with his (then) family of six. Three further children were born in the United States, of which my father, Frank Joseph, was the youngest.18 Although a glassblower by trade, my grandfather had to abandon that work, as mechanical bottle-making machines took over his function in the X-ray tube and even glass bottle industries. My father eventually moved to a German community in Milwaukee,19 where with his brother William he established an automobile, battery, and tire-repair shop. Remaining in Bavaria were my grandfather’s brother, whose offspring lived in Frauenau, Bavaria, and founded a glass works, the Glashütte Eisch, in 1949 after working for centuries for local gentry. After World War II my much older first cousin in Milwaukee, coincidentally named Joseph Ziegler (!) reestablished contact with the Bavarian Eisches. Many of my friends were also of German background. In fact, my best friend in high school, Robert Heinen, came from a German-speaking family, and through him I developed an early interest in spoken German and its literature. Finally, I studied German for 2 years at Marquette University and to my personal satisfaction won first prize in the Gottfried Keller competition in German literature translation. I attribute my success in mastering spoken and written German to my four years of studying Classical Latin at St. John Cathedral. With its five declensions of nouns and four conjugations of verbs, Latin was an excellent preparation for the intricacies of German. Thus, in my meeting with Dr. Gillette and his inquiry about my knowledge of German, he became most interested in my suitability for the Ziegler Fellowship upon learning about my keen interest in German and promptly offered me a 2 year postdoctoral appointment to start in April of 1956. After consulting with my wife, Joan, I readily accepted the offer. However, it was the courage of Joan that really was decisive: at the time she had just borne our first child, Margaret, who was then 11 months old and was expecting our second in June 1956. She had graduated in 1952, also from Marquette, with the Bachelor of Science degree in nursing and thus was better prepared than most American women for the surprises and apparent vagaries of German medical practice.20,21

Figure 3. Karl Ziegler (1898−1973). After he received a Ph.D. degree from Marburg at age 21 (!), he held professorships at Frankfurt (Main), Heidelberg, and Halle (1925−1943). Already world famous in 1936 for his ingenious research on the synthesis of stable free radicals and large carbon rings, he was guest professor at the University of Chicago. In 1943 he became director of the Kaiser-Wilhelm-Institut für Kohlenforschung (after 1949, Max-Planck-Institut), where his research on olefin polymerization earned him, jointly with Natta, the Nobel Prize in Chemistry in 1963. He retired from the directorship in 1969, and his chief research associate, Guenther Wilke, was his successor. He passed away in 1973, upon returning from viewing an eclipse of the sun, astronomy being his keen avocation. Biography and appreciation: Wilke, G. Liebigs Ann. Chem. 1975, 805−833. Eisch, J. J. J. Chem. Educ. 1983, 60, 1009−1014.

friendly rivals in the field of organolithium compounds since their independent discoveries of organolithium reagents beginning in the early 1930s10 extending into the 1940s.11 Although their contacts were interrupted during the war, communication recommenced after 1945. During my graduate studies Gilman provided me with reprints of Ziegler’s renewed and interesting research with ethylene and organolithium reagents in the postwar period12 and most impressive reports of his completely original studies with α-olefins and aluminum alkyls.13 C. Prospect of a Postdoctoral Appointment in Germany. As my research drew to a close, Dr. Roger Gillette,14 a European scientist with Union Carbide, invited me to Chicago for an interview on a postdoctoral associateship that Carbide wished to establish in the research laboratory headed by Karl Ziegler at the Max-Planck-Institut für Kohlenforschung in Mulheim, in the Ruhr region of Germany (Figure 3).15 I frankly was doubtful if I wanted more exposure to organometallics and whether I wanted to work in industry, a likely outcome of such an appointment. However, two factors favored my acceptance of any offer made. The mechanisms involved in Ziegler olefin polymerization were completely unknown then and thus seemed to offered me a great scientific challenge for a

II. RESEARCH IN THE STRANGE REALM OF RECUPERATING EUROPE A. Passage to Germany. We departed from Milwaukee in April 1956 after I received my doctorate degree that March. After a short stopover in New York City to visit administrators of Union C


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either aluminum alkyls or combinations of aluminum alkyls with transition-metal salts, the question naturally arose: how would the alkyls of gallium and of indium (metals congeneric with aluminum in group 13) behave toward unsaturated hydrocarbons? In addition, Ziegler noted how relatively expensive gallium metal was, in comparison to aluminum (in 1956, about 10 times more costly). He reasoned that a knowledge of the difference in physical properties among these three types of alkyls might offer a way to separate small amounts of the expensive gallium from the much cheaper aluminum. His co-workers told me later that his interest in isolating gallium from bulk aluminum was typical of the economic motivation underlying all of Ziegler’s fundamental studies. In parting, Ziegler presented me with a 20 gram lump of gallium (which softens in the hand) and then admonished me to be careful, both because of the metal’s expense and because of the pyrophoric organometallics that I would be working with. Although I began my initial work in Wilke’s personal lab in the main building constructed in 1912 (Figure 5), a long, one-floor building known as die Baracke (hut, in German) to house Wilke’s group was complete soon after my arrival (Figure 6).

Carbide and the German Consulate for the required visas, we undertook, with baby Margy in our laps, the long, tedious, propellerdriven flight to Brussels, where Union Carbide had just established the European Research Associates Laboratory in an old chateau in the Brussels suburb of Uclle. This was to be a fundamental corporate research facility meant to attract talented European scientists who could not be lured away from their European roots. After several days of getting acquainted with ERA scientists with my adequate English and German but minimal French, my family and I took the evening train to Cologne, where we were met by my principal mentor while at the Max-Planck-Institut für Kohlenforschung, Dr. Guenther Wilke, one of Karl Ziegler’s chief co-workers (Figure 4). Switching to conversational German proved to be difficult initially but I had resolved against retrogressing into English. (No German was to learn English from me!) During the first 2 weeks in Mülheim, a devastated industrial city in the Ruhr near Essen and only barely restored, we resided in the only hotel left standing from the war, as Dr. Wilke guided us in finding a furnished apartment, one above a small candy factory, and a serviceable auto, an Opel.22 B. Ziegler’s Proposal of My Research Project on Gallium and Indium Alkyls. I began research under Wilke’s direct guidance on a problem presented to me in my first discussion with Professor Ziegler. With all of the novel and unusual chemical reactions the Ziegler group had discovered involving olefins and

Figure 5. Façade of the Max-Planck-Institut für Kohlenforschung; founded in 1912 as a Kaiser-Wilhelm Institut. The length of the original building is about three times the width of this façade (cf. Figure 10). A lecture hall was added behind the main building in 1929.

To the left in Figure 6 and shown in a different viewing was the new Versuchsanlage (pilot plant), where large-scale olefin−aluminum alkyl reactions were conducted (Figure 7) Although externally of wood framework, internally die Baracke was all ceramic, concrete, and steel to minimize the fire hazard (Figure 8). As my first demonstration of the dangers I would encounter, one of the group leaders, Hans Breil, equipped with a face shield and leather gauntlets, loaded a 10 mL pipet having a plunger with triethylaluminum under argon and squirted the contents down the lab’s central aisle. The vivid flame-thrower effect of the burning alkyl made an indelible impression on me, and I needed no further reminder to be aware, careful, and frightened. The average German graduate student then might have been less acquainted with modern electronic theory than his American counterpart would have been in planning actual laboratory experiments, but the German’s training in research execution was far superior to any skill that I and my American contemporaries would have acquired during doctoral study. Because of both the moisture and oxygen sensitivity of main-group organometallics, as well as the poor quality of research chemicals and solvents then available in Germany, chemists at the MPI had to undertake lengthy purification procedures to secure reagents and solvents

Figure 4. Guenther Wilke. Born in Heidelberg in 1925, at whose christening Karl Ziegler served as godfather. After World War II he obtained the Ph.D. at Heidelberg and joined the MPI as research associate. His research focused on the influence of transition-metal catalysts in novel Ziegler organoaluminum-promoted oligomerization and polymerization of olefins and diolefins and became a dominant figure in transition-metal-catalyzed hydrocarbon transformations. He was the principal mentor of John Eisch’s research at the MPI during the 1956−1957 residence. He became the director of the MPI beginning in 1969 and continued in this office until his own retirement in 1993. D


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suitable for organometallic research. Furthermore, the preferred inert atmosphere for such reactions, the denser-than-air argon, had to be thoroughly deoxygenated and freed of any proton sources, ranging from water through alcohols and extending to terminal acetylenes. Finally, the root cause of many of Ziegler’s astonishing discoveries stemmed from the catalytic action of transition-metal salts, such as nickel, zirconium, and titanium, on main group alkyls, so that after the astonishing and unexpected effects of such salts on aluminum alkyls were revealed, excluding any trace of such salts from the aluminum used or from the metal autoclave reactors became a crucial factor. My fellow co-workers in Wilke’s group and Guenther Wilke himself freely offered indispensable advice to this neophyte in undertaking this hazardous, demanding, but most exciting research. Figure 6. The façade of the MPI-Baracke, the temporary laboratory for the Wilke research group during 1956 to 1981, is in the foreground. The length of this one-story building was four times the width of the façade. At the same time, the pilot plant (Versuchsanlage) on the left was also constructed. The new 10 story research center in the left background (Laborhochhaus) was opened in 1967.

III. MÜ LHEIM AS THE WORLDWIDE MECCA FOR THE CHEMICAL INDUSTRY A. Advances in Main-Group Organometallics after 1950. After 1945 research on the prominent main-group-metal alkyls resumed along the same lines, where the applications of Grignard and organolithium reagents to organic synthesis were vigorously explored, principally in the research groups of Henry Gilman and Morris Kharasch in the United States and by Georg Wittig and Karl Ziegler in Germany. Most of the synthesis procedures involved diethyl ether as the preferred solvent, although the introduction of tetrahydrofuran (THF) as an adjuvant medium by Henri Normant in 19547b was immediately accepted as a great improvement as to reaction rate or yield. As a result, when Gilman carried out studies of the comparative reactivity of phenyllithium, phenyl Grignard reagents, and triphenylaluminum toward ketones, such as benzophenone, the reagents showed a reactivity decrease in the order PhLi > PhMgBr > Ph3Al. It has been subsequently shown by our group23 that aluminum reagents in ether solution form a Lewis complex (3), which reacts at a much slower rate than the unsolvated monomer (1) or dimer (2) (eq 1). Because of this retarding effect of Lewis bases on the reactivity of R3Al, their reactivity toward olefins or acetylenes remained undetected until Ziegler’s work in the early

Figure 7. The pilot plant ((Versuchsanlage) where large-scale reactions of ethylene with aluminum alkyls were conducted under pressure. Occasionally when such a reaction went out of control, a spectacular fiery explosion would result.

2R ′2 O

(R3Al) ⇌ 2R3Al ⎯⎯⎯⎯⎯→ 2R3Al·OR′2 2

1

3

(1)

1950s. (An anonymous reviewer has poetically referred to solvated aluminum alkyls 3 with their muted reactivity as “sleeping beauties”.) B. Ziegler’s Research with Unsolvated Main-Group Alkyls.24 Actually in his pioneering work with Colonius, Ziegler had first synthesized alkyllithium (5) from alkyl halides (4) and lithium metal in alkanes or benzene, from which the resulting lithium halide (6) cleanly precipitates (eq 2). Furthermore, the RLi reagent proved to be much more stable in hydrocarbons than in ethers. Gilman, Zoellner, and Selby soon found that the preparation and subsequent reactions of lithium organyls (R = alkyl, aryl)

proceeded rapidly and more efficiently in ether in the range of 25 °C or lower (eq 3). Thus, subsequent research was conducted in diethyl ether or tetrahydrofuran, solvents that coordinate with aluminum or organyls and slow down addition to either carbonyl or carbon−carbon unsaturation.

Figure 8. The interior of the Baracke, where all lab benches, shelves, and apparatus supports were made of cement, brick, ceramic, glass, and/or metal, not combustible in case of fire. John Eisch’s research bay was the third bench toward the rear on the right side: in an emergency, comfortably close to an exit. E


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α-olefins (eq 6), the latter formed from displacement from 10 by excess ethylene (11 → 10).

When Ziegler recommenced his research with lithium alkyls in the late 1940s, employing as his model systems ethyllithium and n-butyllithium,12 it was natural that he would employ these reagents in a hydrocarbon solvent, as prepared according to eq 2. The choice of a hydrocarbon solvent for these reactions of formaldehyde or ethylene proved to be fortuitous, since ethyl ether would have inhibited the RLi from addition of ethylene to the carbon− lithium bond of RLi (vide supra, eq 1) and instead the ether would have been cleaved (eq 4). Not only did the alkyllithium added still add slowly to formaldehyde but also under ethylene pressure added 1−15 ethylene units to the carbon−lithium bonds to give oligomeric α-olefins after the elimination of LiH (Scheme 1). Although the

LiH

LiAlH4 (9)

no reaction ←⎯⎯⎯⎯⎯⎯ 4H 2CCH 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ LiAl(CH 2CH3)4 Δ, pres.

(5)

Since the hydroalumination of ethylene by LiAlH4 (9) (eq 4) and the subsequent stepwise carbalumination of 10 proved much more rapid than the process for the corresponding uncomplexed LiH, Ziegler and co-workers removed the LiH from 10 completely and began the unexpectedly fertile study of the behavior of ethylene toward the labile AlH3 and the more stable R3Al. Although some previous studies of the reactions of aluminum aryls with unsaturated compounds had been made by Gilman during the 1930s, the results indicated that R3Al adds to the CC group far less readily than R2Mg or RLi, and then only efficiently when the CC bond is conjugated with a carbonyl group. However, Ziegler and co-workers found that Et2AlH and Et3Al add readily to ethylene at 100 °C and further ethylenes were inserted to give higher statistical mixtures of R3Al (11) (eq 7).

Scheme 1

oligomerization was slow and complicated by resin formation via 8, Ziegler reasoned that such a repetitive insertion of ethylene units into an appropriate carbon−metal bond could lead to a linear polymerization of the hydrocarbon chain.

Reinforcing this surmise of possible formation of a linear polymeric alkyllithium were related observations made in the attempted purification of ethyllithium by sublimation under high vacuum. In the immediate postwar period the necessary vacuum equipment simply was not available. Hence, upon heating under the attainable reduced pressure ethyllithium underwent complete decomposition into lithium hydride and a homologous series of even-numbered α-olefins ranging from ethylene up to 1-dodecene in sharply diminishing proportions. In a neat, serendipitous explanation Ziegler concluded that such products were explicable by the reactions in Scheme 1, proposed for the multiple insertion of ethylene units into the starting ethyllithium. C. Ziegler’s Pioneering New Departure (Auf bruch, in German) in Organometallic Chemistry. With the hope of bypassing the preparation of ethyllithium, the addition of ethylene under autoclave pressures to powdered lithium hydride (mp 680 °C) was studied (eq 5) but was found to be thermodynamically impractical. In the search for a more covalent metal hydride having a smaller negative lattice energy, Ziegler’s group examined the unstable LiAlH4 (9), recently reported by Schlesinger’s group at the University of Chicago. Indeed, four units of ethylene added to the Al−H bonds, a process later termed hydroalumination, to form lithium tetraethylaluminate (10). At higher temperatures and pressures (under argon) each of the Al−C bonds of 10 underwent multiple insertions of ethylene units to form lithium tetraalkylaluminates (11), whose alkyl groups consisted of even-numbered carbons statistically distributed between C4 and about C30. Hydrolysis of 11 yielded the corresponding mixtures of even-numbered alkanes or

In retrospect, we now understand why the high reactivity of organoaluminum compounds toward olefins went unnoticed by previous workers. If such reactions are conducted in ether, as was ordinarily done, stable ethereates, R3Al·OEt2, are formed, which are unreactive toward olef ins or acetylenes.23 Fortunately, Ziegler had employed the ether-free R3Al in his work. The lengthening of the C−C chains of Et3Al to mixtures of higher trialkylalumium mixtures of 11 (eq 7), termed the “growth” (in German, “Aufbau”) reaction, was immediately recognized as a great achievement in principle and in industrial practice. Such mixtures of 11 could be oxidized by dry air and then hydrolyzed to produce higher even-numbered alcohols. Thus, a mixture of Et3Al and ethylene as shown in Scheme 2 now permitted the oligomerization of ethylene into a Poisson distribution of higher fatty alcohols, which proved to be eminently suitable for preparing biodegradable detergents. Ziegler’s research at this stage had not achieved a true linear polymerization of ethylene to polyethylene or, via terminal oxidation and hydrolysis, a 1-polyalkenol of high molecular weight, but the oligomeric 1-alkanols (n = 1−30) rapidly found their place in industry in soaps and detergents and as potential sources of fatty alcohols, acids, and esters. Furthermore, Ziegler had established a new and multifold synthesis of σ-bonded organometallics, namely by hydrometalation, in which covalent metal hydrides add to olefins, acetylenes, and dienes, as shown in eq 7 with Et2AlH. Through the efforts of other researchers such hydroalumination was extrapolated to other metal hydrides, such as those of boron (Köster, Brown), magnesium (Bogdanovich), gallium (Eisch), silicon (Speier), and tin (Neumann, van der Kerk).7b F


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Scheme 2

D. Ziegler’s Discovery of the Diverse Effects of Transition Metals on Organoaluminum Chemistry. In the early 1950s “Ziegler chemistry” constituted a body of new reactions for the preparation and transformations of aluminum alkyls, whose molecular basis was a rational extension of the known chemistry of main-group alkyls. However, his truly revolutionary discoveries emerged from unexpected observations made during the reactions of ethylene and other α-olefins with triethylaluminum. Whether such astonishing findings should be attributed to pure chance or serendipity, on the one hand, or to a rational and consequential research plan, on the other, is problematic. Serendipity connotes the making of fortunate but unexpected discoveries through chance by an observer who is alert to their manifestation. The other extreme, observations made as a direct result of a consequential research plan, implies that the researcher have a premonition of what kind of reaction is being observed, on the basis of previous experiments. In these transition-metal effects, which variously caused olefin oligomerization, unprecedented polymerization, metal hydride elimination, and hydrometalation, it is safe to say that even an experienced researcher such as Karl Ziegler could not have had a strong premonition whether one or more of these reactions would be selectively catalyzed by a transition-metal salt. As a consequence, Ziegler and co-workers would seem to have enjoyed serendipity in these discoveries, even if they were alert to the possibilities of unusual products arising from transition metals in the reactions depicted in eq 7. In this sense, their serendipity was aided by the dictum of Louis Pasteur: “Chance favors the prepared mind.” Serendipity certainly played a central role in the next revolutionary advance, but such chance undoubtedly found Karl Ziegler with a prepared mind. As mentioned above, in 1950 Ziegler had observed the capability of unsolvated ethyllithium to bring about the stepwise oligomerization of ethylene (Scheme 1) and shortly thereafter the superior ability of AlH3 or Et3Al to achieve similar carbon chain growth (eqs 7 and 8). Certainly Ziegler was alert to the possibility of carbon polymer chain growth. In repeating a “growth” reaction (eq 8) in 1953, an anomalous result was obtained: instead of ethylene being converted into a mixture of higher aluminum alkyls (11), its dimer, 1-butene (13), was almost the only product.

Figure 9. Karl Ziegler observing the low-pressure polymerization of ethylene into snow-white polyethylene in a 5 L preserves jar borrowed from Mrs. Ziegler’s kitchen.

enormously accelerated the “growth” reaction (eq 7). Simply passing ethylene, at atmospheric pressure, into a catalytic amount of TiCl4 and Et2AlCl dissolved in a higher alkane led to the prompt deposition of polyethylene (14) (Figure 9). The ease with which such a “Ziegler” catalyst polymerized ethylene was recognized as astounding; an earlier process developed by Imperial Chemicals Industries required temperatures up to 200 °C and pressures up to 2000 atm, together with various radical promoters. The properties of the Ziegler polyethylene were no less remarkable: a melting point of 137 °C and an average molecular weight of 3 000 000 (in comparison with various waxy polyethylenes of about 100 000 molecular weight, which were obtained under much more severe conditions) (eq 9).

The implications of Ziegler’s discovery of this new process were numerous. Within Ziegler’s own institute the following projects were inspired by these findings:25 (1) a search for processes to prepare aluminum alkyls in an economical and convenient way, (2) an investigation of the utility of aluminum alkyls for preparing other organometallics in a thermal or electrochemical way, (3) research on the use of organometallics as a means of purifying or plating metals, (4) development of industrial processes for preparing higher alcohols or α-olefins from ethylene, and (5) extrapolation of Ziegler’s ethylene oligomerization and polymerization findings to other olefins, dienes, and acetylenes. These prospects offered such an abundance of research opportunities that industrial laboratories throughout the world soon launched similar studies. Of much broader impact, both within the MPI and in the worldwide industrial and academic chemistry community, was the revelation of the revolutionary importance of transitionmetal catalysis in organic synthesis. Previous work by Morris Kharasch had reported the useful carbon−carbon coupling induced by late-transition-metal salts of iron, cobalt, and nickel on Grignard reagents, and again Kharasch as well as Birch had shown that small amounts of copper salts included with stoichiometric RMgX could change the course of reaction with α,β-unsaturated ketones from 1,2-addition to principally 1,4addition (eq 10).26

In the manner depicted in eq 8, intermediate 12 seemed to have eliminated R2AlH more rapidly than it reacted with further ethylene. The promoter for this unexpectedly facile elimination was eventually found to be traces of nickel salts, which arose accidentally during the cleaning of the metal autoclave. Since a nickel salt could have such a dramatic influence on the course of an ethylene−aluminum alkyl reaction, curiosity was aroused as to the influence of other transition-metal salts. Then by an “Edisonian” search Ziegler, Holzkamp, and Breil found that salts of chromium, zirconium, and especially titanium did not promote the R2AlH elimination (as did nickel) but, instead, G


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research facilities; however, we in the laboratories told them nothing about what research was in progress. Mum was the word until the particular firm signed a secrecy agreement or a nonexclusive patent licensing agreement. Within the MPI the rumor among the staff was that in this way Karl Ziegler would become at once the most famous and the richest scientist in Germany (Figure 10).

E. Emergence of Unanticipated Transition-Metal π-Complexes. However, what really thrust transition metals into the chemical limelight were the astonishing syntheses of novel transition-metal organometallics achieved contemporaneously with Ziegler’s findings during the period of 1951− 1955. Adding to the excitement of Ziegler’s publication of his findings in 1955 were contemporaneous announcements of the discovery and the unusual structures of ferrocene or bis(η5cyclopentadienyl)iron (15) (Kealy and Pauson; Woodward) and the nature of Hein’s compounds as charged bis(η6-arene) chromium complexes (16), (Ernst Otto Fischer; Zeiss).7b These astonishing structural prototypes of what came to be known as transition-metal π-complexes or “sandwich compounds” inspired the research groups of Geoffrey Wilkinson in Great Britain and of Ernst Otto Fischer in Germany to undertake the synthesis of many analogues of 15 and 16, some of which were to be the basis of homogeneous metallocene olefin catalyst systems, such as the titanium or zirconium derivatives of the open metallocene 17 and the ansa-bridged metallocene 18, the latter of which could be obtained as enantiomeric mixtures, resolvable into chiral isomers.7b The availability of such hydrocarbon-soluble metallocenes was to have a great influence on Ziegler−Natta mechanistic studies in the 1970s.

Figure 10. Final developed layout of the physical plant of the MaxPlanck Institut für Kohlenforschung at the end of Karl Ziegler’s directorate in 1969 (aerial photo). In the foreground stands the 10 story Research Center (Laborhochhaus), to the upper right lies the main laboratory of the original MPI, and to the left behind the Research Center is the future Max-Planck Institut für Strahlenchemie. The low building, at the upper center behind the Research Center, is die Laborbaracke of Guenther Wilke’s research group, where John Eisch worked during his 1956−1957 residence.

F. Commercial Development of the Mülheim Atmospheric Polyethylene Process and Persistent Legal Controversy. Not only did Karl Ziegler and the MPI stand to profit greatly from the patents issuing in 1955 onward on this revolutionary polymerization process but also those industrial firms holding nonexclusive licenses on such technology could envision the huge value in the large-scale production of polyethylene by this technology. This aspect of what is now known as Ziegler−Natta polymerization and catalysts has generated legal controversy lasting up to the 1980s. However, the origin of the various disputes arose before I arrived at the MPI from a secrecy agreement between the MPI and the Italian chemical company Montecatini. This agreement gave their consultant, Giulio Natta, Director of the Institute of Industrial Chemistry at the Polytechnic in Milan, access to details of any Ziegler process dealing with the reactions of olefins with organometallic catalysts. In 1953 Natta took such information on the Ziegler polymerization process back to Milan and began to modify the TiCl4 used by Ziegler and eventually found that TiCl3, a solid, with aluminum alkyls could convert propylene into largely a stereoregular isomer of polypropylene, namely isotactic polypropylene, as determined by X-ray analysis. Naturally Natta and Montecatini began to obtain patents on their own, in direct competition with Ziegler’s patents at the MPI. With now two startling discoveries, the low-pressure polymerization of ethylene, propylene, and other α-olefins and the stereoregular variants of isotactic, syndiotactic, and atactic of α-olefin and 1,3-alkadiene polymers of differing properties, industrial laboratories worldwide took up the chase for new catalysts. In addition to Montecatini, some of the first competitors in this search were DuPont, Standard Oil of Indiana, Phillips Petroleum, and Hercules Powder.27 Even Karl Ziegler agreed that Giulio Natta’s independent discovery of stereoregular polymerization was an

The outcome of this heightened interest in organometallics was 2-fold: (1) the launching of a new branch of organic coordination chemistry having mainly hydrocarbon or carbonyl ligands bonded to a transition metal and (2) a thorough reexamination of all accessible organometallics as to their stoichiometric or catalytic behavior toward organic compounds. As an immediate consequence, all aspects of “Ziegler chemistry”, embracing novel organoaluminum and transitionmetal catalytic reactions, had by 1956 converted Ziegler’s Max Planck Institute in Mülheim into a chemical Mecca for all chemical and petrochemical companies throughout the world. When I arrived in Mülheim in April 1956, I found that there was a large group of German chemists and engineers occupying every available square meter of the original institute of 1912 and that a new “temporary” outdoor pilot plant (Versuchsanlage) and a new one-story laboratory (die Baracke) for Guenther Wilke’s group were under construction (Figures 6 and 7). Almost every week groups of visiting industrial scientists and engineers, from the United States, Europe, or Japan, were given a conducted tour through our laboratories, where they could learn much about the H


Organometallics

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important scientif ic f inding that Ziegler’s team would not have conceived (during my stay Wilke told me of Ziegler’s open admission of this contribution). When the Nobel Prize was jointly awarded in 1963 to Ziegler and Natta for the scientif ic discovery of Ziegler−Natta polymerization, there was apparent harmony between the MPI and Natta’s group. However, the Nobel Prize did not end but only intensified the legal dispute over the inventorship of what Ziegler’s associates considered “the Ziegler catalyst”. This and many other legal contentions continued on between 1954 and 1975 concerning various heterogeneous olefin polymerization catalysts. Fortunately, for those interested in the myriads of patents, publications, and litigations exemplifying this persistent legal battle, an excellent, well-considered review of such empirical studies has been published by an impartial industrial scientist, John Boor, Jr.27 Long after Ziegler and Natta died, in 1973 and 1979, respectively, the legal struggle carried on between the MPI, headed by Heinz Martin and other co-workers of Karl Ziegler, and various industrial firms over the legal inventorship of the catalyst for this olefin polymerization process not only for ethylene but especially for propylene. The MPI won final victory over its opponents and their respective patent licensees by Civil Action No. 3952 in the case of the Studiengesellschaft Kohle (a legal arm of the MPI) against Dart Industries in the U.S. District Court for the District of Delaware on October 5, 1982.28 This most complex and labyrinthine series of lawsuits leading up to this definitive decision has been described in great legal detail by Martin in 2002 and is possibly best appreciated by experienced patent lawyers.29 Guenther Wilke mentioned to me in 1956 how distracting from genuine scientific research was the experimentation Ziegler and co-workers had to carry out in legal defense of his patents. Heinz Martin documents in this monograph how extensive, detailed, and well documented such lab work had to be in these many litigations. Such copious defensive experimentation to strengthen a patent application or to undermine an opponent’s patent was another reason that industrial chemistry seemed to me to be a melancholy career.

Figure 11. Guenther Wilke’s Baracke group. Seated, from left: Peter Borner, postdoctorate, Schering Corporation; Hans Breil, chief associate with Ziegler in discovering the Mülheim Polyethylene Process; Herbert Müller, first doctoral student with Wilke, who explored the reactions of aluminum alkyls with acetylenes. Standing: Guenther Wilke (third person from left), doctoral students, and laboratory technicians (Laboranten).

IV. MY RESEARCH ON GALLIUM AND INDIUM ALKYLS A. Preconceptions and Expectations. The 1 year residency at the MPI was my first, and last, experience of working at an industry-oriented research laboratory, whose investigations were aimed at important proprietary chemical processes. Even though Karl Ziegler had always attacked the most fundamental problems of organic chemistry, be it C−C bond formation, its strength or its mode of rupture, free radicals, cationic or halonium ions, carbanions, carbocyclic rings, polymerization, or natural products, he kept a steady eye focused on the organic’s commercial value and utility throughout all his research. Many of his discoveries early and late in his career found their way into important patents. Especially at the MPI his extensive cadre of co-workers, staff members, and postdoctorates (Assistenten) such as myself and doctoral students (through the University of Aachen) worked studiously on data for strengthening patent applications. As a necessary consequence, much proprietary research was not shared with a temporary outsider like me. Nevertheless, I was kept informed of enough of such secret work to help my own endeavors. Especially helpful were my direct advisor, Guenther Wilke, as well as research associates such as Roland Köster and Hans Breil, all of whom were members of our Kegelklub (nine-pin bowling club), and postdoctorates Wilhelm Neumann and Peter Borner (Figure 11).

The last two were also especially congenial in after-hours activities, such as Karneval parties and excursions. After my meeting with Professor Ziegler on the theme of my research, gallium and indium alkyls, I learned from others at the MPI that absolutely no prior work with such alkyls had been performed at the MPI and that the mention of such alkyls in previous Ziegler patent applications was simply of a “prophetic” nature. When I began my gallium research there was obvious concern whether or not gallium alkyls would surpass aluminum alkyls in their cocatalytic effect in ethylene polymerization. This concern was evident when I first synthesized pure triethylgallium (Figure 12). With Wilke’s concurrence the MPI catalyst group obtained a sample of our triethylgallium, while I was away on brief vacation, and tested it in the standard ethylene polymerization procedure used for triethylaluminum. At the time I was upset by this action, but I came to realize that Ziegler had a proprietary right to this information. However, I was never informed of the results of their comparative runs. Had the polymerization group waited for my testing results, I could have told them that Et3Ga showed activity comparable to I


Organometallics

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factual organic chemistry, embracing principally name reactions, together with their scope and limitations, generally surpassed what I had acquired in my graduate studies at Iowa State. In contrast, however, I was surprised at their lack of knowledge of modern structural and reaction mechanistic theory, which I would summarize as Erich Hückel−C.K. Ingold−Paul Bartlett−Saul Winstein physical organic chemistry. When one considers, however, that the first up-todate advanced organic chemistry monograph, published in German, the second edition of Eugen Müller’s Neuere Anschauungen der Organischen Chemie, containing explanations of molecular orbital theory and detailed homolytic and heterolytic reaction mechanisms, appeared as late as 1957, it is no surprise that German doctoral training had not yet caught up with such advances in the United States. B. Differing Approaches to Reaction Research: Empirical Gathering of the Facts versus the Testing of Molecular Mechanistic Models. If the question of bond breaking in a proposed reaction mechanism would arise, they would favor homolysis to free radicals (path a) rather their heterolysis by polarity (path b) or single-electron-transfer (SET) rupture (path c) (Scheme 3). In fact, in Scheme 3

the 1930s German chemists such as Karl Ziegler himself, Georg Wittig, and Eugen Müller had corroborated Moses Gomberg’s theory of the trivalent (i.e., free radical) character of the triphenylmethyl group by synthesizing even more stable (persistent, in modern terms) free radicals, typified by pentaphenylcyclopentadienyl and 1,1,3,3-tetraphenylallyl. Despite the research of the Ingold and Winstein groups on the solvolysis of organic halides or pseudohalides, the older generation of chemists such as Karl Ziegler, Roger Adams, and Henry Gilman did not readily accept the intermediacy of ephemeral ions in organic solvents. However, with organic halide or organoalkali reagents they would recognize the polarity of the carbon−element bond and the possibility of the organic compound leading to a cryptocarbonium or cryptocarbanion fragments, never a free ion, at the moment of reaction (Kryptoionenreaktionen in Meerwein’s conception). In any case, Ziegler and Gilman considered that such ionic mechanisms were fraught with too many assumptions to be useful. In seminar presentations Gilman would express his reservations about reaction mechanisms in his usual courtly and thoughtful manner, while in lectures at MPI Ziegler would bluntly state his disdain for any detailed theoretical discussion of a novel and little explored reaction. I can now see the merits in their critical approach: they wanted to find out what happened in a novel reaction and judged that the known facts were still too inadequate to determine how it happened on a molecular level. As a young scientist, I was disappointed that almost no serious effort was given at the MPI to the rich network of reaction mechanisms likely involved in Ziegler’s novel organoaluminum- and transition-metal-catalyzed olefin and acetylene oligomerizations and polymerizations. In my time at the MPI I therefore resolved to begin to gather the facts required for a fruitful study of their reaction mechanisms.

Figure 12. John Eisch at the laboratory bench in the Baracke. Setting up a glass reaction apparatus, at a point where the mandatory eye protection was not yet required.

that of Et3Al in such TiCl4/Et3M polymerization procedures30 and, as a bonus in information, Et3In had about one-tenth the activity. The cost of utilizing Et3Ga in place of Et3Al in ethylene polymerization thus made it prohibitive in commercial ethylene polymerization, but its comparable activity provided a clue, not immediately discerned by me, to the likely reaction mechanism. From my background with my awareness of Gilman’s empirical reactivity trends, I was from the start pessimistic about my research project. Gilman and Jones had found that, in additions to various carbonyl substrates, Ph3Al was decidedly more reactive than Ph3Ga or Ph3In. In further comparisons Ph3Ga was sometimes slightly more reactive than Ph3In (with benzoyl chloride) and sometimes less reactive (with benzaldehyde). In any event, in such approximate reactivity rankings the order was clearly Al ≫ Ga ≅ In > Tl.31,32 Naturally, therefore, I expected that gallium and indium alkyls would be slower in additions to α-olefin than Et3Al, and their greater cost would make gallium and indium alkyls poor competitors. Furthermore, in covalent radius, electronegativity, and C−M bond strength, I saw little promise in gallium or indium alkyls being superior substitutes in Ziegler aluminum chemistry. In any discussions I had with the German doctoral or postdoctoral staff, I noticed that their knowledge of J


Organometallics

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R3Al > R3Ga ≫ R3In, with R3In further showing sensitivity to laboratory fluorescent lighting, which led to indium metal deposition. In the basic reactions of hydrolysis, oxidation, and Lewis acidity these gallium and indium derivatives displayed activity distinctly lessened from that of their aluminum analogues. With either Et3Ga or Me3In the action of dry air led cleanly to the cleavage of one C−M bond (Scheme 4), instead of all three with R3Al. Solvolysis with

One nagging doubt about my ultimate ability to be successful in such research arose from the necessity of performing kinetic studies on the heterogeneous reaction mixtures formed in Ziegler’s transition-metal-catalyzed olefin polymerizations. Such studies were beyond my competence and would, in my opinion then, have yielded only empirical, ad hoc data capable of evaluating the polymer productivity of a specific polymerization reactor of a given volume and configuration. Avoidance of heterogeniety and attainment of kinetics on the molecular level were not to be solved until the availability of practical soluble metallocene catalyst systems after 1975. C. Preparation and Properties of Gallium and Indium Alkyls. Since I was given complete freedom in my line of investigation, I decided to make a careful comparison of the known Ziegler organoaluminum chemistry with that of the analogous organogallium and organoindium alkyls. By noting the trend of chemical properties in the congeneric triad of R3Al, R3Ga, and R3In, I hoped to gain insight into the role of such alkyls into their reactions with olefins alone and thereafter with olefins combined with titanium(IV) chloride. Since the most interesting organoaluminum derivatives were of the types Et3Al, iBu3Al, iBu2AlH, R2AlCl, and R2Al−OR′, I set out by attempting to prepare the gallium and indium analogues by known conventional methods. The strong tendency of almost all organoaluminum compounds to coordinate with ethers or other donor solvents dictated that any preparation of the gallium or indium alkyls or salts be conducted in hydrocarbons. Although ether complexation with gallium or indium alkyls was weaker, obtaining the unsolvated R3Ga and R3In from ether solution presented difficulties. Because metallic gallium and indium were convenient starting materials, the first step was the treatment of the metal in an evacuated flask with dry Cl2. With gallium a immediate combustion of the metal globule into flame led to the deposition of GaCl3 on the cooler wall (eq 10). This organic chemist had never before witnessed such burning without oxygen! 2Ga + 3Cl 2 ⎯⎯⎯⎯→ Ga 2Cl 6 −ΔH

Scheme 4

ethanol likewise produced the monoethoxyl alkyls. Finally, the metal trichlorides rank in Lewis acidity in the order AlCl3 ≫ GaCl3 > InCl3. A striking confirmation of this trend is my observation that even Et2AlCl behaves as a stronger Lewis acid than GaCl3, as evidenced by its 3:1 complex with 1 equiv of GaCl3 (eq 12). The distillable, fairly stable complex 19 is formed by the exothermic admixture of its components at 25 °C. Although neither the molecular mass nor the detailed structure was then known, I suggested that each Cl ligand on Ga behaved as a Lewis base for each Et2AlCl unit (eq 14). GaCl3 + 3Et 2AlCl ⎯→ ⎯ Ga[AlEt 2Cl 2]3 −Δ

(10)

150 − 160 °C

iBu3Ga + 3H 2CCHR ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ga(CH 2 − CH 2R)3 −3iC4 H8

(15)

Likewise, the so-called Auf bau or growth reaction of triethylgallium with ethylene was found to proceed productively under pressures double that needed for such a reaction with Et3Al and at temperatures 50−75 °C higher (eq 16).

(in theory)

(11)

(2) Even at the 3:1 ratio of Et3Al and GaCl3, the isolated yield of Et3Ga approached but did not exceed 50%. In discussions with Roland Köster he suggested that the strongly Lewis acidic Et2AlCl coordinated with each of the polarized Ga−Cl bonds to form a complex (eq 12), tying up half of the GaCl3. 3Et3AlCl + GaCl3 → Ga[AlEt 2Cl 2]3

(CH3CH 2)3 Ga + 3nH 2CCH 2 170 °C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [CH3CH 2(CH 2−CH 2)n ]3 Ga 100 − 125 atm

(16)

Thus, for the inorganic and organic Ziegler chemistry I explored in the gallium and indium counterparts, I concluded that such reactions were too slow, too impractical, or too expensive to represent a feasible alternative to achieving reactions such as those found with aluminum alkyls. Such a devaluation in practical importance also holds for two interesting results discussed in the next section: namely, the reaction of gallium alkyls with 1-alkynes and the obvious practical reason for this study, the possible utility of gallium alkyls as a component with a transition metal salt in olefin polymerization. However, there I wish to point out the valuable theoretical implications of our gallium findings to the molecular mechanisms of Ziegler chemistry.

(12)

In order to set free the GaCl3 and bind the Et2AlCl, the reaction in eq 11 was repeated in the presence of 3 equiv of powdered and dry KCl. In so doing, the yield of Et3Ga rose from 42% without KCl to 84% with KCl. The liberation of the complexed GaCl3 is conceived as in eq 13. Ga[AlEt 2Al 2]3 + 3KCl → GaCl3 + 3K[AlEt 2Cl 2]

(14)

The activity trends discussed in the foregoing section are very similar to those of the reactions of gallium alkyls and alkylgallium hydrides with olefins. (A parallel study of the possible reactions of indium alkyls was not undertaken because such alkyls undergo total decomposition to indium at the requisite temperatures for olefin interactions.) The temperature required for the olefin displacement reaction of a 1-alkene with iBu3Ga (eq 15) lies some 50 °C higher than that for the analogous reaction with iBu3Al.

Instead of using a Grignard reagent (containing ether) or organolithium reagents (in the 1950s not easily prepared in hydrocarbons (eq 2), I availed myself of triethylaluminum and triisobutylaluminum, both readily accessible in the MPI. Two complications arose from using R3Al, however. (1) Only one alkyl group of R3Al was exchangeable in the transmetalation (eq 11): 3Et3Al + GaCl3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Et3Ga + 3Et 2AlCl

19

bp 98 − 102 °C (2.5 mmHg)

(13)

By appropriate modifications Et3In, Et2Ga, iBu3Ga, and Et2GaH were accessible in fair to excellent yields. The thermal stability of this triad of alkyls decreased in the order K


Organometallics

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D. Molecular View of the Properties of Group 13 Alkyls.33,34 In a discussion of organometallic derivatives of group 13 it is useful to exclude those of boron and of thallium: the former because of the low polarity of the carbon−boron bond and its smaller covalent radius and therewith the magnified steric effect of ligands and the latter because of the highly screened nuclear charge on thallium and the much weaker carbon− thallium bond. The atomic and molecular bonding data of the remaining triad are given in Table 1. To be borne in mind is the aforementioned empirical Lewis acidic ranking of MCl3 as AlCl3 ≫ GaCl3 > InCl3. Of the parameters in Table 1 the only

Scheme 6

Table 1. Atomic and Molecular Bonding Parameters of Aluminum, Gallium, and Indium property

Al

Ga

In

electronegativity (Allred−Rochow) covalent radius, Å atomic radius, Å ionic radius (+3), Å ionization energy (3p), eV mean M−C bond enthalpy, kcal/mol

1.47 1.25 1.43 0.51 6.0 65.5

1.82 1.26 1.22 0.62 6.0 59

1.49 1.44 1.43 0.81 5.8 38

Scheme 7

one that correlates qualitatively with such relative Lewis acidities is the ionic radius. A possible electronic explanation of this correlation might be that the effective nuclear charge at ionic radius of 0.51 Å for Al might be the greatest for Al and determine its greatest attraction for electron pairs of Lewis bases. I therefore proposed that the greater activity of R3Al over R3Ga or R3In principally stems from its Lewis acidity acting on π-basic olefin and acetylenes to form a π-complexlike transition state (20, Scheme 5), which in turn leads into the syn adduct 21. A similar explanation would account for facile hydrometalation of alkynes by iBu3Al or iBuAlH (Scheme 6). Even though alkynes are weaker π-bases than alkenes, the transition state for 1-alkenes in carboalumination and hydroalumination has higher energy steric repulsing effects in ultimate cis addition (24, Scheme 6), in comparison with 22. In such π-complexes as transition states there is an explanation for the observed difference of reactions between acetylene and Et3Al or Et3Ga at 25 °C (Scheme 7). The carbalumination product 23 can readily be accommodated into the π-complex mechanistic depicted in Schemes 5 and 6 (20 and 22), but the proton−gallium exchange leading to 26 clearly involves a transition state having the gallium center close to the terminal acetylenic carbon 25 and resembling a σ-complex. With the known weaker Lewis acidity of Et3Ga, its π-complex transition state should be of higher energy than that of aluminum (27). Consequently, it may find a lower energy transition state bonding to a single carbon: i.e. a σ-complex. It is noteworthy that the covalent radii of Al and Ga centers are almost identical, so that no front strain steric effect is a tenable explanation for the reactions depicted in Scheme 7.

E. Implications for the Mechanism of Ziegler Olefin Polymerization. A key observation mentioned at the beginning of my discussion (vide supra) was a comparative study of the yields of polyethylene obtained from titanium(IV) chloride and triethylaluminum according to a published procedure by Ziegler and Martin for a lecture demonstration. Strictly according to this procedure but on a 11% scale, I substituted for the Et3Al, in consecutive reactions, first the equivalent amount of pure Et3Ga and then that of Et3In. The yields of polyethylene that a 11% scale of Et3Al (of variable purity) should have produced was 28−50 g; the yield of polymer isolated with Et3Ga as the cocatalyst was 47 g and that from Et3In only 5 g, with traces of In metal. As was to be expected from our foregoing results, the indium alkyl was essentially worthless as a substitute for Et3Al. However, what was a complete surprise was the high activity of Et3Ga, which in this polymerization procedure was very comparable with that of Et3Al. One interesting deduction permissible from this comparative study is that ethylene insertions in polymerization are not occurring into carbon−gallium bonds. If such a bond were to be involved in the rate-determining step, that polymerization would be slower and less productive. The logical alternative, which frankly I did not dwell on then, would be that carbon−titanium bonds must be the sites of ethylene insertions. Early in the mechanistic studies of Ziegler−Natta polymerization, whether the carbon−aluminum bond or the carbon−titanium bond was the active site was still an open question.

Scheme 5

L


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organic chemistry. Thus, once finally established at Catholic University in 1963, my flourishing research in organoaluminum and organoboron chemistry made up for this hiatus in research reports by leading to over 60 publications in my 9 years there. Epilogue. As a final note, I had avoided undertaking mechanistic studies of any Ziegler polymerization catalyst system because of the difficulty, uncertainties, and pitfalls of performing reliable kinetic studies in heterogeneous systems. After the accessibility in 1975 and thereafter of soluble metallocene procatalysts and main-group alkyl cocatalysts, such as MAO and perfluorinated boron aryls, the necessary NMR IR and UV spectral measurements could be made on the catalyst systems, both by themselves and with a monomer before polymer began to precipitate. With such favorable experimental advances, I joined many other academic and industrial researchers in mapping out the molecular mechanisms of the organoaluminum chemistry itself and the related ethylene polymerization and the stereoselective αolefin polymerizations.35 Now some 50 years after Ziegler and Natta’s pioneering and empirical research, it is generally agreed that the key mechanistic insight into such processes on a molecular level is the growth of the hydrocarbon chain by the electrophilic carbometalating attack on the inserting olefin by an alkylated high-valent transition-metal cation (28 as the proposed transition state, where Cp′ ≠ Cp′′ and M+ is a chiral center; stereoselective α-olefin insertion may generate isotactic polymer chains).

V. AFTERMATH With the completion of my 12 month work with gallium and indium, I was offered an additional year in residence at the MPI under the Union Carbide Fellowship. Upon reflection I thought that my wife and now our two small girls were longing for a return home where the English language and a social life could be enjoyed again. Moreover, I had declined a generous offer of a lucrative industrial research position at Union Carbide and accepted instead an assistant professorship at St. Louis University, applied for and offered at less than half the Carbide salary. It took some persuasion to convince my wife that my attraction for academia meant more to me than money. As a compromise, before I left Europe for St. Louis, Union Carbide offered me a 4 month appointment at their fundamental research center in Brussels, the European Research Associates Center mentioned earlier. Top academically inclined European scientists devoted their time to pondering industrially important themes, such as Ziegler olefin polymerizations and Fischer−Tropsch and Roelen hydrocarbonylations. Situated in an old chateau and within the most cosmopolitan metropolis in Europe, the ERA was a most enjoyable environment for science, culture, and a pleasant social life. For my first academic position, St. Louis University offered several appealing aspects: (1) like Marquette, it was a medium-sized Jesuit-administered institution striving to improve its graduate programs in the sciences, (2) Professor George Schaeffer, a doctoral graduate of Herman Schlesinger’s metal hydride group at the University of Chicago, had been appointed chair to spearhead such efforts in chemistry, and (3) promptly Schaeffer set up thriving research on boron hydrides sponsored by the U.S. Air Force and he judged that my work with Ziegler would nicely complement his research efforts. Indeed, in my first year there I was able to start research on organolithium and aluminum hydride chemistry. Tragically, by the fall of 1958, however, George Schaeffer was diagnosed with serious respiratory disease and he passed away on August 17, 1959, in his early 40s. His promising research program collapsed, and I was forced to seek and obtain my next position at the University of Michigan for the fall of 1959. Before I had left Mü lheim in April of 1957, I deposited, with Professor Ziegler, two full manuscripts for intended publication under his name and with my coauthorship. Upon my periodic inquiries from abroad I was first informed that publication would have to await completion of suitable patent matters. Finally, after about 4 years, when I was at Michigan, I was informed that Professor Ziegler had decided not to publish the results under his name and I was now free to do so independently. Thereafter, I submitted the two manuscripts to the Journal of the American Chemical Society, in which they appeared in 1962.33,34 By that time, I had ascertained that any fruitful mechanistic studies with group 13 organometallics were best carried out with those of aluminum and boron rather than with gallium and indium. This unexpected 5 year hiatus in my scientific publication sequence was to cause me professional difficulties at Michigan, where it was taken as a sign of particularly unproductive postdoctoral research. Also, the organic faculty there was unappreciative of my organometallic research. As one organic colleague jocosely put it, I was introducing “mighty peculiar elements” into

The status of the multiple mechanistic insights gained from such chiral and achiral metallocene-catalyzed olefin polymerizations has been superbly and cogently reviewed by leaders in this research.36 The wide-ranging studies involved in metallocene synthesis, characterization of catalyst systems, and selectivity in controlling oligomerization versus polymerization processes have revolutionized the roles of organic and inorganic chemistry in modern chemistry and have had an enduring impact on the textile, petrochemical, materials, and even specialty-chemicals industries.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. † The term “Ziegler-Natta polymerization” in this article is intended to designate the scientific discoveries involved in this catalytic process as well as in the structure of the polymers formed from ethylene, α-olefins, 1,3-alkadienes, or acetylenes. For such findings the Nobel Prize in Chemistry was jointly awarded to Karl Ziegler and Giulio Natta in 1963. However, this term does not M


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imply anything definitive about the inventorship or patent rights of such polymerization catalysts (cf. section III.F).

postdoctoral associates. The results have been reported in over 380 scientific publications, in some 275 invited lectures worldwide, in the monograph The Chemistry of Organometallic Compounds (Macmillan, 1967), and in the edited series Organometallic Syntheses (J. J. Eisch and R. B. King, editors). He has been an industrial consultant on organometallic chemistry and expert witness in several patent litigations on Ziegler−Natta polymerization catalysis.

Biography

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ACKNOWLEDGMENTS I thank Dr. Guenther Wilke, my direct mentor during my postdoctoral fellowship at the Max-Planck-Institut für Kohlenforschung in 1956−1957 under the auspices of the former Union Carbide Corporation. He offered valuable experimental assistance during the planning and execution of my research. Moreover, as director of the MPI he extended the hospitality of the Institut and scientific advice during my periodic residence over some 35 years. The photographs appearing as Figures 3, 4, 8, 9, and 10 are from the archives of the MPI and appear here with the written permission of Professor Dr. Matthias W. Haenel of the MPI. All other photographs in this article are from my private collection. Furthermore, I thank Jeanne E. LaBonté for her expert preparation of several printed versions of this intricate paper. Finally, I gratefully dedicate this article to the memory of the professors and mentors who aided greatly in the launching of my career in organometallic chemistry: Clifford Haymaker, Henry Gilman, and Karl Ziegler. In addition, I had the pleasure of the friendship and scientific counsel of Wilhelm Neumann (1926−1993), starting at the MPI after completing his habilitation thesis on organotin chemistry at Giessen and continuing at Dortmund as Ordinarius in Organic Chemistry. Brief biography: Eisch, J. J. Eur. J. Inorg. Chem. 1999, 153−162.

John Joseph Eisch was born in Milwaukee, Wisconsin, in 1930 and received his secondary education at St. John Cathedral High School with concentration on the sciences, mathematics, and Classical Latin. After graduation as class salutatorian he attended Marquette University, where he was awarded the B.S. degree in chemistry, summa cum laude, in 1952. In 1953 he and his former grade school classmate, Joan Scheuerell, were married and over the years have raised a family of five, Margaret (dec. 1976), Karla, Joseph, Paula, and Amelia. After initial studies under scholarship at the Institute of Paper Chemistry, he thereafter earned the Ph.D. degree from Iowa State University with Henry Gilman in March 1956 with the dissertation, “Comparison of Phenanthridine with Other Azaaromatic of main-group organometallics (Al, Ga, In) with unsaturated hydrocarbons at the Max-Planck-Institut fü r Kohlenforschung, Mü lheim, Germany, and then research fellow at Union Carbide’s European Research Associates Center in Brussels, Belgium. After junior appointments at St. Louis University (1957−1959) and the University of Michigan (1959−1963) and research on such novel Ziegler organometallic chemistry, he was appointed Associate Professor in 1963 at the Catholic University of America and thereafter became Ordinary Professor of Chemistry and Department Head. Finally, in 1972 he was chosen as Chair of Chemistry in the State University of New York at Binghamton. In 1983 he was promoted to the SUNY-wide rank of Distinguished Professor of Chemistry. Over the years he has held visiting appointments at Kyoto University, Max-Planck-Institut fü r Kohlenforschung, the University of Munich and the Technical University of Munich, the Polytechnic University of Warsaw, the California Institute of Technology, the University of California at San Diego, and Cornell University. He was named Fellow of the Japan Society for the Promotion of Science in 1979 and has received the first Gilman Research Award from Iowa State University in 1995 and Alexander von Humboldt Senior Scientist Awards in 1993 and in 2005. The scholarly work of Professor Eisch has been devoted to the structures and reaction mechanisms of organometallic compounds, as well as the chemistry of fully π-bonded nitrogen and metallocyclic rings. Such research has involved the fruitful collaboration of over 140 masters, doctoral, and

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REFERENCES

(1) My home basement lab had its start with a small Gilbert chemistry kit, which was a Christmas gift after my 10th birthday. Although the scope of the experiments grew widely by the time of my graduation from Marquette, my father insisted that I stay away from explosives and flammables such as ether. My experiments ranged from qualitative identification of inorganics and organics through food dyes and the pyrolysis products of wood, soft coal and various plastics. At the time I dismantled my lab in 1952, I was surprised by the impressive array of chemicals and lab supplies I was able to donate to my former high school, St. John Cathedral in Milwaukee. (2) Eisch, J. J. “Qualitative Identification of the Components of the Okra Pod Polysaccharide Fraction by Paper Chromatography”, Bachelor of Science Thesis, Marquette University, 1952, 98 pp, jointly directed by Professors C. R. Haymaker and K. Brown. (3) Professor Clifford R. Haymaker, from whom I gained all my education in organic chemistry while at Marquette, was an extraordinary teacher and human being. Although he was born sightless, he pursued his education through high school by tutors, earning top grades, and then finally obtaining first the B.S. degree in chemistry and then the Ph.D. in theoretical nuclear chemistry at Marquette. He taught introductory and advanced organic chemistry for 30 years at Marquette. A younger colleague and this author have published a professional appreciation of this remarkable educator and mentor: Eisch, J. J.; Haworth, D. T. The Professional Career of Clifford R. Haymaker: A Life of Chemistry Imagined and Bequeathed. J. Chem. Educ. 2003, 80, 278. (4) Dr. Kenneth Brown, my introductory organic chemistry laboratory instructor over two semesters at Marquette, put his experience as an exofficer in the Marines during World War II to excellent use in his teaching style. N


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in the treatise: Organic Chemistry; Gilman, H., Ed.; Wiley: New York, 1943; Vol. 1, pp 520−524. (18) As a most compatible complement to my father, my mother, Gladys née Riordan, was second-generation Irish-American, whose forebears from County Cork, Ireland, were farmers and school teachers. (19) Distinct enclaves of first Germans and then Irish, Italian, and Polish were formed throughout Milwaukee. Our district of Bay View was predominantly German before World War II, and our Catholic Church, St. Augustine, originally had German legends on its walls and stainedglass windows. (20) Although our obstetrician had a private clinic, we learned when Joan had our Karla Maria delivered deep at night on June 13, 1956, that it was actually a midwife (Hebamme) who would perform the delivery, as required under German law. The obstetrician could only intervene if some serious problem were to arise. Joan was surprised indeed by this unforeseen procedure. (21) After a 9 day stay at the clinic the mother and baby returned to our apartment. We had had the baby christened Karla Maria, because I was fond of the music of Carl Maria von Weber and thought the name very melodious. When the Zieglers heard about the birth, Professor Ziegler’s wife, Maria, appeared at our door with a book of German fairy tales. Then she thanked us for honoring them with the naming of our daughter. We smiled rather sheepishly but did not have the heart to reveal the primary reason for her name. (22) The somber façades of all buildings left standing in Mülheim were heavily pockmarked from the bombing and yet necessarily were now provided with new windows and casements. Fortunately, the MPI was situated on a hill some distance from the city center and escaped destruction. Our apartment was close to heavy industry so that air pollution covered all autos with particulate matter. Every morning I had to wipe off all the windows of my parked Opel. Ironically, the same pollution created most beautiful sunsets. (23) Eisch, J. J.; Hordis, C. K. J. Am. Chem. Soc. 1971, 93, 4496− 4502. (24) Karl Ziegler’s pioneering research on the reactions of olefins with lithium alkyls and with aluminum alkyls, with or without transition-metal salts, which culminated in the discovery of the Mülheim low-pressure polymerization of polyethylene has been presented in the following reviews: (a) Wilke, G. Liebigs Ann. Chem . 1975, 805− 833 (in German). (b) Eisch, J. J. Karl Ziegler: Master Advocate for the Unity of Pure and Applied Research. J. Chem. Educ. 1983, 60, 1009− 1014. (25) Lehmkuhl, H.; Ziegler, K.; Gellert, H. G. In Methoden der Organischen Chemie; Müller, E., Ed.; Georg Thieme: Stuttgart, Germany, 1970; Vol. 13/4, pp 9−314. (26) Kharasch, M. S.; Reinmuth, O. Grignard Reactions of Nonmetallic Substances; Prentice-Hall: New York, 1954; pp 219−221. (27) Boor, J. J., Jr. Ziegler-Natta Catalysts and Polymerization; Academic Press: New York, 1979. This book was completed by the staff of the Shell Development Company after the tragic death of Dr. Boor in an automobile accident on December 31, 1974. (28) Civil Action No. 3952 of Studiengesellschaft Kohle vs Dart Industries, U.S. District Court of the District of Delaware, October 5, 1982. This decision invalidated Dart’s patents on polypropylene and those of others linked to Natta’s patents from Montecatini. Thereby, the MPI could legally claim the right to call such olefin polymerization catalysts “Ziegler catalysts”. (29) Martin, H. Polymere und Patente (in German); Wiley-VCH: Weinheim, Germany, 2002; pp 310 ff. (30) Ziegler, K.; Martin, H. Makromol. Chem. 1956, 18/19, 186−194. (31) Gilman, H.; Jones, R. G. J. Am. Chem. Soc. 1940, 62, 980−982. (32) Gilman, H.; Jones, R. G. J. Am. Chem. Soc. 1940, 62, 2353−2357. (33) Eisch, J. J. J. Am. Chem. Soc. 1962, 84, 3605−3610. (34) Eisch, J. J. J. Am. Chem. Soc. 1962, 84, 3830−3836.

(5) Professor Stephen Darling of Lawrence College and Dr. John Green of the IPC offered superb lectures on the structures and properties of organic compounds. The world expert on lignin chemistry, Dr. Friedrich E. Brauns, maintained a vigorous research program on the chemistry of wood at the IPC, and I certainly would have undertaken doctoral research under his guidance had I stayed. (6) Frankland, E. Liebigs Ann. Chem. 1849, 71, 171; 1853, 85, 347; 1855, 95, 33. (7) (a) Historical survey up to 1967: Eisch, J. J. The Chemistry of Organometallic Compounds; Macmillan; New York, 1957. Historical survey up to 2005: Elschenbroich, C. Organometallics, 3rd ed.; WileyVCH: Weinheim, Germany, 2006. (b) Providing specific literature citations for individual aspects of the history and background of organometallics, such as π-complexes, hydrometalation, etc., would add unnecessary detail to this review. Instead, the reader is referred to Elschenbroich’s excellent foregoing monograph for such leading references. (8) For scientific evaluations of the career of Henry Gilman: Eisch, J. J. Henry Gilman (1893−1986): An Appreciation. J. Organomet. Chem. 1988, 338, 281−287. Eisch, J. J. Henry Gilman: American Pioneer in the Rise of Organometallic Chemistry. Organometallics 2002, 21, 5439− 5463. (9) (a) Eisch, J. J. “Comparison of Phenanthridine with Other AzaAromatic Heterocycles”, Doctoral Dissertation, Iowa State University, March 17, 1956, 225 pp. (b) Eisch, J. J.; Gilman, H. The doctoral dissertation was published under the same title as a review in: Chem. Rev. 1957, 57, 525−581. (10) (a) Ziegler, K.; Colonius, H. Liebigs Ann. 1930, 479, 135. (b) Gilman, H.; Zoellner, E. A.; Selby, W. M. J. Am. Chem. Soc. 1932, 54, 1957;(c) 1933, 55, 1252. (11) Henry Gilman’s more vigorous pursuit of organolithium chemistry is readily followed because he composed detailed reviews. (a) Review of lithium−hydrogen exchanges: Gilman, H.; Morton, J. W., Jr. Organic Reactions; Adams, R., Ed.; Wiley: New York, 1954; Vol. 8, p 258. (b) Review of lithium−halogen exchanges: Jones, R. G.; Gilman, H. Organic Reactions; Adams, R., Ed.; Wiley: New York, 1951; Vol. 6, p 339. (12) Ziegler, K. Brennst. Chem. 1949, 30, 181 and reference therein. (13) Ziegler, K.; Gellert, H. G.; Martin, H.; Nagel, K.; Schneider, J. Liebigs Ann. Chem. 1954, 589, 91 and references therein. (14) Gillette had been an infrared spectroscopist at the University of Michigan, who became involved with evaluating German scientific research carried on during World War II and subsequently became director of the European Research Associates Laboratory established by Union Carbide in Brussels after the war. He himself had a strong attachment to German, especially with a Viennese accent. (15) The Max-Planck-Institut für Kohlenforschung was founded in 1912 as the Kaiser-Wilhelm Institut für Kohlenforschung, in order to foster the technical development of the closely connected RhineWestphalia coal and steel industry. The eminent organic chemist and Nobel Prize recipient Emil Fischer attended the inauguration of the institute. The first director was Franz Fischer, who developed with Hans Tropsch the world-famous process for producing hydrocarbons from carbon monoxide and hydrogen. The name change of KWI to MPI finally occurred in 1949. (16) Hammond had received the doctorate with the legendary Paul Bartlett at Harvard and then established a vigorous program in modern physical organic chemistry at Iowa State, dealing with free radical mechanisms, photochemistry, and aromatic substitution and rearrangements. His courses and seminars were open to all and were a great stimulus to graduate students. Also strengthening the graduate program was Charles DePuy, who studied with William von Eggers Doering at Yale and brought to Iowa State an excellent course on Hückel molecular orbital theory, which was to prove essential for my doctoral research.9 (17) The most reliable formulations of Gilman’s empirical relative reactivities of organometallic compounds are given by him in Chapter 5 O


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(35) Related mechanistic studies by the Eisch group on organoboron-, organoaluminum-, and titanocene-derived Ziegler olefin polymerization catalysts has been reviewed, as of 1995, in: Eisch, J. J. Forty Years of Umpolung in Organometallic Chemistry. J. Organomet. Chem. 1995, 500, 101−115. (36) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143−1170.

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