Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato

DISCOVERY OF METHYLCHLOROSILYLENE (CH SiCl:) AS A KEY 3 INTERMEDIATE IN THE DIRECT SYNTHESIS OF DIMETHYLDICHLORO- SILANE, (CH3)2SiCl2

Kenrick M. Lewis, Momentive Performance Materials, 771 Old Sawmill River Rd., Tarrytown, NY 10591.

Preambulo sólido-gas catalizada por cobre. El Me2SiCl2 era conocido con anterioridad En uno de los números recientes de la desde antes de la Segunda Guerra revista Organometallics (2021, 40, 459- Mundial, sin embargo, su síntesis, se 462) en la que se incluye una sección basaba en una reacción de metátesis entre dedicada a personalidades pioneras en la tetracloruro de silicio (SiCl4) y un reactivo química organometálica y cuyas de Grignard, por ejemplo, MeMgBr. Se investigaciones han tenido gran influencia conocía que su hidrólisis permitía obtener en el campo industrial, se hace referencia el poli(dimetilsiloxano), por lo que al Dr. Kenrick M. Lewis, cuyos trabajos resultaba muy conveniente y atractiva su son reconocidos al impactar en aspectos síntesis debido a su estabilidad térmica y a fundamentales de la química de muchas aplicaciones que potencialmente organosilanos, entre ellos, la famosa se podrían desarrollar en aislantes para reacción directa de producción de motores eléctricos, radios de vuelo y diclorodimetilsilano (Me2SiCl2), muchas más, motivado a la vez por la compuesto químico esencial para la necesidad de materiales con aplicaciones industria moderna de los polímeros de militares. Sin embargo, la reacción no era silicio. En este número de la revista factible de ser escalada o trasladada a una Naturaleza y Tecnología, el Dr. Lewis producción industrial, lo que llevó a que el generosamente comparte los detalles que profesor Eugene Rochow en General condujeron al descubrimiento de una Electric, y durante la Segunda Guerra, especie química intermediaria clave en el descubriera que la reacción directa entre proceso de síntesis del referido compuesto silicio y cloruro de metilo en fase gas, y que en años recientes publicó, luego de catalizada por cobre, generaba qué durante un buen tiempo al ser parte de principalmente el compuesto deseado. un secreto industrial, la información estuvo reservada. 2CH3Cl(g) + Si(s) → (CH3)2SiCl2(l) (1) Con el propósito de obtener una mejor La referencia que se hace en el número de comprensión de la descripción que el Dr. Organometallics arriba citado, menciona Lewis presenta en este artículo de que el resultado de la reacción fue divulgación, conviene hacer referencia a la publicado por Rochow en un artículo de 3 reacción química involucrada, llamada páginas (The Direct Synthesis of Síntesis Directa o Proceso Directo, y que Organosilicon Compounds, J. Am. Chem. se clasifica como una adición oxidante Soc. 1945, 67, 963−965), que no

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Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato importando lo breve, se considera como Key words: methylchlorosilylene, Direct uno de los artículos más importantes en la Synthesis, Rochow-Müller, química orgánica de silicio por el impacto dimethyldichlorosilane que a lo largo de los años ha tenido en la industria química. Años después se conoció y se reconoció, que por los Resumen mismos años (1945) Richard Müller en Alemania, había trabajado de igual manera El propósito en esta memoria es dar a en la misma reacción, por lo que el proceso conocer, las etapas y eventos que dieron es llamado también la Síntesis Directa lugar al descubrimiento de la especie Rochow-Müller (Müller publicó su metilclorosilileno (sililenoid) como un experiencia en One hundred years of intermediario clave en la Síntesis Directa organosilicon chemistry. J. Chem. Educ. Rochow-Müller. De manera particular, 1965, 42, 41). El primer experimento deseo discutir la influencia de distintos exitoso de Rochow generó 5 mL de factores y situaciones específicas que dieron lugar al descubrimiento, y a la vez Me2SiCl2, cantidad obviamente difícil de considerar viable como un producto para pagar tributo a mis guías, a mis consejeros y a mis colaboradores. dar lugar a tantas aplicaciones. Sin embargo, luego de investigaciones y Palabras clave: metilclorosilileno, Síntesis desarrollos tecnológicos en los que se Directa Rochow-Múller, involucraron diseño de reactores de lecho diclorodimetilsilano fluidizado, de haber optimizado el empleo del catalizador de cobre, se sabe que en la When, in January 1979, I transferred from actualidad se producen algo así como 1.4 the Linde Research Department of Union millones de toneladas métricas anualmente Carbide to the Exploratory and Basic de esta molécula que ha sido pieza clave Group of Silicones R&D, one of my first en la industria química moderna de los assignments was to identify and develop polímeros de silicio. exploitable concepts and phenomena to improve the performance of the Rochow- Müller Direct Synthesis of methylchlorosilanes. Dr. Bernie Kanner, Abstract my Group Manager, had told me that there My intention, in this memoir, is to was much that was still unknown about the document the events leading to discovery reaction, despite almost forty years of of methylchlorosilylene (silylenoid) as a commercial practice at that point. I was key intermediate in the Rochow-Müller excited by the fact the Direct Synthesis is Direct Synthesis of a copper-catalyzed reaction. My dimethyldichlorosilane. In particular, I fascination with copper chemistry had want to discuss the influence of various started during my senior year at the factors and individuals and pay tribute to University of Alberta (Edmonton) with a mentors and contributors. lecture by Prof. Robert Jordan on the physiological role of copper enzymes and

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Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato copper proteins. It continued through 4. Promoters, particularly zinc, cadmium graduate school at the University of and antimony, enhance catalytic activity Massachusetts (Amherst), where I had and selectivity when used in small (< 1 studied the corrosion chemistry of the weight percent) amounts. copper intrauterine device (IUD) during 5. Neither methyl chloride adsorption nor my doctoral research. In the Linde copper diffusion is the rate-limiting step of Research Department, use of chelation the reaction. technology to recover copper and other metals from industrial wastes and 6. Methyl chloride is dissociatively hydrometallurgical solutions was one of chemisorbed on the catalytic surface. my projects. Now, in Silicones R&D, I was going to study the catalysis of the 7. The reaction follows Langmuir- Direct Synthesis with emphasis on the role Hinshelwood kinetics. of copper. 8. (CH3)2SiCl2 is formed from a surface My review of the published literature had intermediate with dimethylsilyl groups. revealed that the role of copper in the At the conclusion of his Plenary Lecture at catalysis had not been elucidated. the International Symposium on Prevailing ideas about the mechanism of Organosilicon Chemistry in 1969, Bazant the Rochow-Müller Direct Synthesis of acknowledged the difficulty of dimethyldichlorosilane had originated mechanistic research on a reaction as from the research of Voorhoeve and complex as the Rochow-Müller Direct coworkers, and van den Berg and Synthesis of methylchlorosilanes (see coworkers in The Netherlands, Bazant and Pure & Applied Chem., 19(1969) 473 – coworkers in Czechoslovakia and from 488). This is a copper-catalyzed, gas – Golubtsov, Lobusevich and coworkers in solid reaction, not the familiar reaction of the USSR. Voorhoeve’s monograph, gaseous reactants over a heterogeneous Organohalosilanes: Precursors to catalyst. Bazant’s remarks inspired the Silicones, was the primary authoritative surface-chemical investigations, which we source of published information. The initiated in 1980. Our objectives were: tenets of the prevailing hypothesis were: • To understand the significance of 1. Cu3Si (η phase) is the active catalyst bonding, thermochemical and leading to selective formation of morphological changes accompanying (CH3)2SiCl2. alloy formation in the Cu-Si binary and 2. The polarity of the metal – silicon bond Cu-Si-Zn ternary systems. in metal silicides determines or influences • To determine surface compositions in product selectivity. these systems in the temperature range 3. The Cu – Si bond energy (46 kcal/mole) (280° – 350°C) in which selectivity to is greater than the Si – Si bond energy (42 dimethyldichlorosilane is high. kcal/mole).

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Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato

• To characterize the valence states of Cu, retained. The Boulder team showed that Zn and Si in these systems in vacuo and silicon surface enrichment was predicted following methyl chloride chemisorption. by the empirical rule of Burton and Machlin. Their results were published in • To elucidate structure and composition of Applied Surface Science, 14(1982 – 83) the surface intermediates and active sites 359. We showed that the thermodynamics following chemisorption. of silicon enrichment was in agreement • To assess state-of-the-art mechanistic with the comprehensive model of proposals and make new ones consistent Wynblatt and Ku, but it was 1987 before with the surface analytical results. we were permitted to publish our results (Catalysis 1987, J. W. Ward, Editor, pp • To exploit findings that will improve the 415 – 434). performance of our commercial reactors. In the meantime, we had calculated the Although our research was begun Cu-Si bond energy to be 30.6 ± 2.4 independently, it soon became a joint kcal/mole, that is, less than the Si-Si bond effort with that of Prof. John Falconer’s energy of 53 kcal/mole. An independent group at the University of Colorado quantum mechanical calculation by Chou, (Boulder). John’s group used Auger et al. (J. Amer. Chem. Soc., 109(1987) Electron Spectroscopy (AES) and 1880)) found 30.7 kcal/mole as the Cu-Si Temperature Programmed Desorption/ bond energy. The reduced energy of Cu-Si Mass Spectrometry (TPD/MS) and we, at bonds compared to Si-Si bonds is Tarrytown, employed X-Ray consistent with copper catalysis and the Photoelectron Spectroscopy (XPS) and fact that it is the alloyed silicon, which is Mass Spectrometry. Early on, John and his reactive. The composition of the catalyst collaborators (Timothy Frank and Keith surface at reaction temperatures is 40 - 55 Kester) and Don McLeod and I both atom % Si and not the 25 atom % expected discovered the surface enrichment of from the formula, Cu3Si. In fact, it silicon which occurs on copper - silicon emerged that Voorhoeve and other alloys and solid solutions and the researchers had associated Cu3Si with high enhancement of this enrichment by zinc, a catalytic effectiveness because of the known promoter of selectivity to accident of its faster nucleation when dimethyldichlorosilane. Lead, a well- reacted copper-silicon masses had been known poison of the reaction, inhibited the cooled to room temperature for x-ray surface enrichment of silicon. High tin diffraction measurements. Calculation of concentrations also suppress silicon Auger Parameters and application of surface enrichment. Tin-induced zinc various electronegativity scales allowed surface enrichment provided evidence of Don McLeod and me to conclude that, interactive effects of zinc and tin on the while both copper and silicon are formally catalytic surface. Whereas both zinc and zerovalent in Cu3Si prior to exposure to tin were evaporated from the surface δ+ δ- –10 methyl chloride, the Cu - Si bond is above about 450 K at 10 torr, lead was polarized with a partial positive charge on

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Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato copper and a partial negative charge on Chem., 334(1964) 57 – 65), had suggested silicon. These results had challenged and that silylenes might be intermediates in the vanquished some of the key tenets of the Direct Synthesis, but no experimental prevailing hypothesis. verification of these suggestions had been undertaken under Direct Synthesis Chemisorption studies with methyl conditions. I wanted more proof before I chloride disclosed that there was a proclaimed so revolutionary an idea as an desorbable intermediate leaving the unknown (or unpublished) silylene as a catalytic surface above 160°C at 10-9 torr. key intermediate in a mechanism, which Based on the simultaneous decrease of had been so rife with controversy. At the silicon, carbon and chlorine intensities in 1966 International Symposium on the XPS spectra, I concluded that this Organosilicon Chemistry, Prof. Bazant intermediate contained these three responded to critical comments on his elements. Moreover, with a Si 2p binding proposed mechanism by asking the energy of 102.8 eV, this species had a questioners, “ were you there and did you silicon valence between those of elemental see what the active sites and activated silicon (Si 2p = 99.3 eV) and tetravalent complexes looked like?” I wanted to avoid silicon (Si 2p = 103.4 eV) and was most that type of contention. probably divalent. We did not yet have a mass spectrometer attached to our high So it was that in the summer of 1982 that I vacuum reaction chamber. So, we had no journeyed to Boulder to share results with data on the masses of the desorbates. We Prof. Falconer and review the status of his acquired one later. research. In our first conversation, he said that a desorbate of mass 78 was At that time, there were only two literature consistently observed following methyl references to the silylene, CH3SiCl, even chloride chemisorption on the silicon- though the early eighties was the era in enriched surfaces of copper - silicon which silylene chemistry was one of the catalysts. This species was more abundant hot organosilicon chemistry topics. Tom than could be accounted for by the mass Barton’s group (J. Amer. Chem. Soc., spectral fragmentation of 99(1977) 5199 - 5200), Morey Ring’s dimethyldichlorosilane and other group, (Inorg. Chem., 12(1973) 2968 - methylchlorosilanes. He could not explain 2972) and much later, G. Maier’s group its formation. You can well imagine my (Chem. Ber., 117(1984) 2369 – 2381; J. joy at having independent confirmation of Organomet. Chem., 366(1989) 25 - 38) the intermediacy of CH3SiCl and Heinecke, et al. (J. Anal. Appl. Pyrol., (methylchlorosilylene, mass 78) in the 28(1994) 93 - 105) demonstrated the Direct Synthesis as I wrote down, for intermediacy of CH3SiCl in gas phase John’s benefit, the silylene insertion trapping experiments. Others before me, reactions that made up my proposed for example Rochow, et al., (J. Amer. mechanism. We had just successfully Chem. Soc., 74(1952) 5545 – 5546), challenged another of the basic tenets of Hertwig (Z. Naturforsch., 6B(1951) 337 – Voorhoeve’s monograph on the Direct 338) and Schenk, et al., (Z. Anorg. Allg. 21

Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato

Synthesis. The Klebansky-Vikhtengol’ts- what additional confirmatory experiments Voorhoeve mechanism proposed a on the intermediacy of CH3SiCl in dimethylsilyl ((CH3)2Si) intermediate as (CH3)2SiCl2 formation could be the surface species leading to (CH3)2SiCl2. undertaken, how CH3SiCl might be Later, Timothy Frank showed that the trapped in laboratory reactors and how this intensity ratio of mass 78/mass 80 new knowledge might be exploited 35 37 (CH3Si Cl/CH3Si Cl) was 0.3 and commercially. Ethylene, acetylene and temperature independent as required by 1,3-butadiene were suggested as trapping the natural isotopic distribution of agents, but I was concerned about chlorine. extensive carbon formation in my fluidized bed reactor. My preferred As often happens in research, serendipity trapping agents were H2 and an Si-H was to intervene, not once but three times. monomer like (CH3)3SiH. A 6 – 15 fold First, Keith Kester, who had done his increase in the intensity of the mass 78 doctoral research on the Direct Synthesis peak had been observed in XPS/MS of dimethyldichlorogermane with Prof. experiments when CH3Cl – H2 mixtures Rochow at Harvard, was one of John’s had been used for chemisorption. It had colleagues and collaborators. He made us been reported in the literature that the aware of results, published only in his insertion of silylenes into H2 and into Si-H dissertation, that (CH3)2GeCl2 adsorbed on bonds proceeded with near zero activation germanium can decompose to CH3GeCl, energy. Furthermore, based on the findings (methylchloro-germylene) and CH3Cl of DeCooker, et al., (J. Organomet. (methyl chloride) under the influence of Chem., 99(1975) 371 – 377), I was already radio-frequency fields. This information studying the effects of H2 on product became significant when we sought to selectivity, especially in the absence of obtain reference XPS and AES/TPD zinc and other promoters. spectra for (CH3)2SiCl2 on copper – silicon alloys. The data were indistinguishable Our fluidized bed reactions confirmed that from those of methyl chloride on the selectivity to CH3SiHCl2 and (CH3)2SiHCl alloys. In addition, mass 78 and mass 50 is increased by the use of CH3Cl – H2 in (CH3Cl), both of which are weak in the the absence of promoters like zinc, mass spectrum of (CH3)2SiCl2, were quite antimony and tin. There was no evidence prominent. Thus, both the Boulder and of CH3SiH2Cl until one day in May 1984, Tarrytown groups demonstrated that when the methyl chloride supply from the CH3SiCl and CH3Cl are formed when cylinder was insufficient to fluidize the (CH3)2SiCl2 is chemisorbed on copper- bed. Thereafter, CH3SiCl trapping with H2 silicon surfaces. was done with the fluid bed reactor operated as a fixed bed. GC/MS, GC/FTIR John visited Tarrytown in November 1982 and 29Si NMR data and comparison of and we both presented seminars on the these results with those of an authentic discoveries and status of the research to the sample confirmed the presence of R&D staff. Later, there was a CH3SiH2Cl in the reaction product. Gas brainstorming session in Bernie’s office on 22

Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato circulation in the fluidized bed had • The active catalytic surface is a solid increased the probability of the solution containing 40 – 55 atom percent redistribution reactions, which consume Si along with Cu, Zn and Sn. CH3SiH2Cl. This was a good illustration • Zn is a structural promoter (affecting that batch or plug-flow reactors provide energy of activation) and Sn and P are better selectivity to early intermediates in textural promoters (affecting number of reaction pathways. However, the active sites) serendipitous decrease of the methyl chloride flow had allowed Rudy Cameron • M – Si bond polarities do not correlate and me to trap CH3SiCl in a large with product selectivity laboratory reactor as well as accomplish • Cu – Si bond energy (~ 31 kcal/mole) < the Direct Synthesis of CH3SiH2Cl. Si – Si bond energy (~ 53 kcal/mole) Jeff Larnerd and I performed fluidized bed • Surface precursor of (CH3)2SiCl2 is experiments with CH3Cl – H2 and periodic injections of methylchlorosilanes, CH3SiCl. Other silanes (except Si(CH3)4 and SiCl4) and the disilanes also originate particularly CH3SiCl3. The objective was from silylene intermediates to trap CH3SiCl as the disilane (CH3Cl2SiSiCl2CH3). The disilane was not • The divalent silicon intermediate detected. Instead, the rate of formation of mechanism is also applicable to other the methylchlorohydrosilanes increased 2 Direct Syntheses of silanes. - 5 fold. We had earlier expended a great deal of effort to find promoters for this Experimental evidence in support of these Direct Synthesis (see US 4,973,725). ideas has been published in Catalyzed Serendipity had intervened for a third time. Direct Reactions of Silicon (K. M. Lewis We pursued its lead and never got back to and D. G. Rethwisch, Editors), Elsevier, investigating the effect of injecting 1993. Nonetheless, very many questions methylchlorosilanes or Si-H monomers about the atomic and molecular events of into the fixed bed Direct Reaction with the reaction still remain unanswered. Some of these are: CH3Cl – H2. Based on our results and those of Michael Clarke and Ian Davidson, the • What is the structure of the active site Ono group and Gerhard Roewer’s group leading to selective formation of with 1,3-butadiene, I am confident that the (CH3)2SiCl2 ? silylene (silylenoid) intermediates will someday be trapped as disilanes. • Why is the reaction so much more sensitive to Sb, Sn and P than to Zn? The surface-chemical studies, silylene trapping experiments and other studies • Do silylenes (for example, RSiX, R = H, conducted by various groups from 1980 to CH3, X = Cl, Br, OCH3, N(CH3)2) the present permit an updated disproportionate (2RSiX ↔ R2Si + SiX2) understanding of the course of the Rochow under Direct Synthesis conditions, and – Müller Direct Synthesis: how significant is this reaction in product selectivity?

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Naturaleza y Tecnología Enero-Mayo 2021 ISSN 2007-672X Universidad de Guanajuato

• Why are the Direct Reactions with HCl, (Bernie’s Ph.D. adviser at Purdue ROH and (CH3)2NH, which are selective University and later a Nobel Laureate), for the trifunctional silanes, inhibited by began to realize the potential of promoters of (CH3)2SiCl2 formation? hydroboration he remarked that he felt like a guy who had fallen into a pit full of gold Throughout the telling of this story, I have nuggets. He was too busy filling his mentioned the names of my assistants, pockets to stop to query how they got collaborators and mentors. I want to there, or how he was going to climb out so acknowledge their contributions to the fully laden. Bernie was saying that successes we have had and to thank them organosilicon chemistry had provided a for their support, their assistance and their similar gold mine for us. I shall add that for inspiration. Others I want to recognize me copper also glitters. include the members of the analytical group and the Library staff at Tarrytown, About the author: Cynthia Vail, my secretary, and my wife Ken Lewis is currently a Corporate for putting up with my work habits. Bernie Research Fellow at Momentive, the Kanner stands out as an influential mentor. highest distinction for a scientist in the One day in 1983, following the company. While Ken’s interest in silicon methylchlorosilylene and silicon, zinc and and silicone chemistry has persisted since tin surface-enrichment discoveries, I this time, the names of his employers have reminded him of my question to him not. Over the years, Union Carbide’s during our initial meeting in January 1979. silicones business became OSi Specialties, The question was whether there were which was then sold to the Witco enough challenging problems in silicon Corporation but kept the name. Eventually chemistry yet to be solved to keep OSi Specialties became part of GE researchers interested and productive. At Advanced Materials, and when GE sold that time, silicon’s position in the periodic this business to Apollo Management, the table and the existence of inorganic company was named Momentive polymers called silicones were about the Performance Materials. limits of my knowledge of silicon chemistry. Bernie had replied that he and Ken ha sido profesor visitante del Cuerpo others like Don Bailey and Bela Prokai, Académico de Química y tecnología de had had very productive careers up to that Silicio del Departamento de Química de la point and that he was looking forward to DCNE y permanente apoyo en discusiones making many more contributions before sobre proyectos relacionados con los his retirement. He also said that there was temas de investigación de este cuerpo still much that was not known about the académico. Direct Synthesis of methylchlorosilanes, a copper-catalyzed reaction. Now, in 1983, in response to my reminder, Bernie said that when H. C. Brown,

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