Issue Date: April 3, 2017 | Web Date: March 30, 2017
Tobin J. Marks: What is chemistry all about?
Let me begin by saying how honored I am to be selected as this year’s Priestley Medalist and to join the distinguished cohort of previous medalists. I also wish to salute this year’s ACS national award winners, an impressive group of chemists and accomplishments. Congratulations! In this address I would like to share a story with you, part biographical, part scientific, and part philosophical, relating something of my career, how that is intertwined with the science I have done and am currently doing, where it is going, and lastly, how it relates to the question of what chemistry is all about.
My grandparents on both sides emigrated from Czarist Russia and came to America for freedom and a better life. I was fortunate to grow up in the Washington, D.C., area (in Arlington, Va., and Bethesda, Md.) in a family having a deep respect for learning, hard work, and service. Like many of my generation, dynamic science teachers, highly motivated classmates, and Sputnik kindled my fascination with science. I owned a Gilbert Chemistry Set and tinkered with explosives and rockets, which I’m sure horrified my normally tolerant parents. I was fascinated with what things are made of and how that affects their properties.
On graduating from high school in Bethesda, I entered the University of Maryland, which always has had excellent science and engineering programs. I chose chemistry as a major partly because of my fascination with what things are made of and how to make them. I also took excellent physics and math courses, and like all chemistry majors then, German language courses.
There was great camaraderie among the “Terp” [University of Maryland]chemistry majors, and it seemed that the more noxious the odors emanating from a lab experiment, the more we relished it, possibly because students majoring in other subjects were repelled! We realized how much we were learning and how much we enjoyed laboratory work, and we were thrilled. I vividly remember one of my classmates pronouncing, “chemistry is my religion,” which captures some of that slightly wacky sentiment.
There was a generally good-natured rapport between professors and students in that era, and I learned that each professor has a different lecturing and teaching style, and that in itself was good for me. I vividly remember that a physics professor’s humorous request that the chemistry majors in the lecture hall erase the large blackboards was met with good-natured hisses and boos from the chemistry majors, after which we came to the front and dutifully erased the blackboards. Of course, we eventually got to know all of the Maryland chemistry faculty and felt unique as “chem majors.” With determination and diligence, most of us traversed the great divide between physical-chemistry-oriented freshman chemistry and phenomenology-, structure-, and mechanism-oriented organic chemistry. Chemistry majors had laboratory courses most weekday afternoons and sometimes on Saturdays, so we felt comfortable in the laboratory.
A transformative experience at Maryland was the National Science Foundation-funded summer undergraduate research program, which allowed us to work in a professor’s laboratory with a modest stipend. Most exhilarating for me was working with a young inorganic chemistry professor, Samuel Grim, synthesizing organophosphorus compounds and measuring their NMR parameters. I learned to work and think independently and to appreciate the power of “just-in-time learning,” meaning that when a puzzling result stares you in the face, the urgency of trying to understand it and planning what to do next greatly accelerates the learning process.
To me, the graduate students in the Grim group were stimulating, fun people, and although I didn’t quite understand who postdocs were, they were also impressive. From this experience I yearned for more research! Inorganic chemistry was taught by James Huheey, and his crystal-clear lectures kindled new interests, leading me to borrow Sam Grim’s copy of the landmark Cotton and Wilkinson inorganic textbook. I was entranced by the chemistry the vast periodic table offered, and attending graduate school was my ambition.
The honors chemistry program at Maryland required a written senior thesis and original research proposal, both to be defended before a faculty committee. The former task was straightforward because I had coauthored several publications by then, and the latter sent me exploring new literature. What captivated me was Rowland Pettit’s report that the elusive cyclobutadiene molecule could be stabilized by coordination to an iron carbonyl fragment. The ramifications of that chemistry formed the basis of my proposal, which emerged from the oral defense unscathed and stoked my interest in the organic-inorganic chemistry interface. Indeed, unusual interfaces between dissimilar substances continue to intrigue me.
Every time I climb the steps at the Massachusetts Ave. entrance to MIT, the adrenaline pumps as it did the first time when I entered graduate school as an NSF Predoctoral Fellow. The world-class MIT inorganic group consisted of Al Cotton, Alan Davison, Dick Holm, and Dietmar Seyferth, with George Whitesides closely associated. It had great intellectual power and breadth. I joined Al Cotton’s group because of his infectious enthusiasm and interesting projects on stereochemically nonrigid organometallic molecules.
MIT housed essentially all science and engineering in one giant complex of interconnected buildings, so in addition to chemistry coursework, I attended lectures and seminars in other departments and cross-registered at Harvard University for William Lipscomb’s famous chemical bonding course. MIT provided an extremely stimulating environment, filled with scientific leaders and a warm Cotton group camaraderie; Al gave me the freedom to sample this environment broadly, for which I will be forever grateful. After initial research speed bumps where difficultly synthesized new molecules turned out to be completely rigid, my persistence, bolstered by Al’s and George’s good advice, paid off. I enjoyed mentoring our research group’s junior members and vowed to create such an environment when I had my own research group. At Maryland I was an ACS student affiliate, at MIT I became a full ACS member, and I recently celebrated my 50th anniversary.
My quest for an academic position took me to U.S. regions I had never visited before, including Northwestern University, which, on the day I visited, was 0 °F with a 40 mile-per-hour wind. All science and engineering was housed in one large building, and I found the chemistry department to be research-intense, collegial, and well-equipped. The inorganic group and associated faculty, Fred Basolo (2001 Priestley Medalist; 1983 ACS president), Jim Ibers, Ralph Pearson, Duward Shriver, and Brian Hoffman, were world-class, and I happily joined as an assistant professor. I soon had my laboratory in place with enthusiastic graduate and undergraduate students, but what research to do? An empty canvas awaited my brush! Although I could not forget my organometallic roots, I was inspired by George Whitesides’ advice to strike boldly into new fields, to push the frontiers.
To my thinking, the f elements (lanthanides and actinides) offered untapped possibilities in that their properties differed radically from transition metals (for example, larger ionic radii, different bonding), and we set off to explore the chemistry of unknown metal-alkyl and metal-hydride bonds. My first NSF proposal was successful, although similar to what Bob Langer told us in his 2012 Priestley Address, one reviewer was critical to the point of irrationality. Undeterred, we forged ahead and soon could tell the world about remarkable new families of alkyls, aryls, acyls, tetrahydroborates, alkane activation products, and hydrides, as well as evidence for fleeting formyl, π-olefin, π-alkyne, π-arene, and H2 complexes (figure 1). One striking characteristic of these complexes was their extraordinarily high reactivity and catalytic activity versus that of their closest transition-metal analogs. We next extended this work to organolanthanides and again struck gold.
Invaluable collaborations to more deeply understand molecular and electronic structure with Roald Hoffmann of Cornell University; Andrew Streitwieser, Ken Raymond, and Dick Andersen of the University of California, Berkeley; Victor Day of the University of Nebraska; Ignazio Fragalá of the University of Catania; Herbert Schumann of the Technical University of Berlin; Geoffrey Cloke of Sussex University; and Jack Williams and Lester Morse of Argonne National Laboratory greatly enriched this field. We wondered what other unusual transformations could take place around large actinide centers, and, for example, found that in the presence of the uranyl ion (UO22+), condensation reactions that normally produce porphyrin-like metal phthalocyanine complexes—robust pigments used in blue jeans and auto paints (we’ll return to them later)—instead yield unusual expanded uranyl-encapsulated metallomacrocycles (figure 2). Analogous porphyrinic complexes now play a major role in photodynamic cancer therapy.
As we explored f-element catalytic chemistry, the enormous reaction rates intrigued us. If kinetics were not limiting, but only thermodynamics were, could we discover new catalytic transformations using bond energy information? Fortunately, stimulated by the work of Jack Halpern (and later John Bercaw, Bob Bergman, Carl Hoff, and others), we had already constructed sophisticated calorimetry equipment and were in a position to address these questions. Our bond enthalpy data provided new perspectives on the relative energetics of various metal-ligand bonds as a function of location in the periodic table (differences can be dramatic) but also clues to designing new catalytic transformations.
Combining exploratory chemistry with reaction mechanism analysis, we showed that hydroelementation processes involving heteroatom-H addition to carbon-carbon unsaturation are mediated by organolanthanides, and later other metal centers, to effect rapid hydroamination, hydrophosphination, hydroalkoxylation, hydrosilation, and hydrothiolation reactions of significant scope (figure 3). Moreover, many of these processes could be coupled to polymerizations to create heteroatom-functionalized polyolefins. In recent work, we coupled the microscopic reverse of homogeneous olefin hydroalkoxylation with tandem heterogeneous hydrogenation to hydrogenolyze C–O functionalities of the type found in biomass feedstocks.
During these times, I was also fascinated by the multifaceted role of metal ions “in biology and, with Jim Ibers, launched a synthetic, spectroscopic, and X-ray diffraction study to understand the molecular and electronic structures of the then-mysterious “blue” copper proteins. Using pyrazolylborate ligands to simulate the proposed multi-imidazole metal binding sites in the actual proteins, our students prepared very thermally labile facsimile complexes with most of the correct protein spectroscopic signatures—a significant advance at that time.
Later and of a more organometallic flavor, I was intrigued by reports that metallocene complexes of the type (C5H5)2MX2, where M = Ti, Zr, V, Mo, and X = halide, were potent antitumor agents in mice. Our question was whether the carcinostatic activity mechanism was similar to that established by Steve Lippard for cisplatin, involving key binding to specific DNA sequences. We launched detailed spectroscopic and X-ray crystallographic studies of (C5H5)2MX2(aqueous) interactions with oligonucleotides and whether (C5H5)2MX2(aqueous) inhibited the cleavage of small circular double-stranded DNAs by restriction endonucleases, in the manner of cisplatin. Our search for nonlabile DNA-metallocene coordination chemistry was inconclusive; however, we found that these metallocenes do inhibit certain important DNA processing enzymes, implicating a different anticancer mechanism.
When I arrived, Northwestern had a world-renowned heterogeneous catalysis group, founded when the distinguished Russian chemist, Vladimir Ipatieff fled the Soviet Union in 1931 to be jointly professor of chemistry at Northwestern and director of research at nearby Universal Oil Products. Ipatieff received many international recognitions (among them the 1939 ACS Willard Gibbs Medal) and, with faculty member Herman Pines, developed a catalytic process for high-octane aviation fuel, helping the U.K.’s Royal Air Force win the 1940 Battle of Britain. I am honored to now occupy Ipatieff’s chair.
In the late 1970s, I became intrigued with reports that chemisorption of simple and catalytically marginal early-transition-metal organometallic complexes on relatively mundane oxide surfaces such as dehydroxylated alumina (DA) yielded mysterious surface species that were highly active olefin hydrogenation and polymerization catalysts. In collaboration with Bob Burwell, who had the necessary catalytic equipment, we established that when adsorbed on DA, [(CH3)5C5]2M(CH3)2 (M = Th, U) and related Zr-methyl complexes hydrogenated simple olefins at rates comparable to supported platinum metals, and rapidly polymerized ethylene. Kinetic poisoning experiments established that only about 5% of these surface complexes were catalytically significant, not uncommon in heterogeneous catalysis. Furthermore, high-resolution cross-polarization/magic-angle spinning (CP-MAS) solid-state NMR argued that the active species were formed when Lewis acid sites on the DA surface abstracted a methide anion from the metal center, yielding for Zr, a formal 14-electron d0 cation. When I proposed this structure at an ACS meeting, it was met with skepticism. But what else could cause such enormous enhancements in catalytic activity?
At this time I was collaborating with Jim Stevens of Dow Chemical. The industrial and academic communities were ablaze with excitement when Walter Kaminsky of Hamburg University reported that ill-defined, partially hydrolyzed trimethylaluminum species (MAO) somehow activated organozirconium complexes to form potent and equally ill-defined homogenous olefin polymerization catalysts. What chemistry gave rise to such species? We quickly established that the CP-MAS NMR spectra of evaporated MAO-zirconacene solutions were virtually identical to the same zirconacenes on DA. For unambiguous characterization and well-defined, isolable catalysts, we explored Lewis acidic, hydrocarbon-soluble, and Teflon-coated perfluoroarylboranes as methide abstractors, starting with B(C6F5)3. (figure 4)
What followed was amazing: new families of isolable, highly active “single-site” polymerization catalysts, additional perfluoroaryl-borane/alane cocatalysts and weakly coordinating anions, numerous X-ray structures, and precision analysis of molecular dynamics, thermodynamics, and polymerization mechanisms. We showed how the electrostatic catalyst+···cocatalystˉ ion pairing can direct olefin insertion stereochemistry and polymer architecture. Partnering with Dow, our chemistry to date has enabled production of about 30 billion kg of stronger, more processable, more recyclable polyolefins, including those produced from sustainable sugarcane ethanol. Strong, lightweight, inexpensive, chemically inert polyolefins find use in products as diverse as automotive parts, food packaging, agriculture, clothing, solar cell coatings, and medical prosthetics.
In recent work, we have been creating binuclear complexes with mechanistically dissimilar metal catalytic sites and demonstrating enzyme-like cooperative interactions, scaling with M···M’ distance and affording new polymers. We are revisiting the original surface-bound catalysts and find that very strong Brønsted acidic surfaces create species with virtually 100% of the sites catalytically active for olefin polymerization, and even more intriguing, arene hydrogenation. The combination of EXAFS (with Jeff Miller of Argonne National Lab), two-dimensional solid state NMR (with Marek Pruski of Ames National Lab), DFT computation (with Alessandro Motta of Rome University), and detailed mechanistic analysis has illuminated the catalyst structures and explained why H2 delivery is selective for a single arene π-face. For mixtures of aromatics, these catalysts exhibit significant selectivity for carcinogenic benzene hydrogenation, modeling possible processes for detoxifying gasoline.
Soon after I arrived at Northwestern, I was intrigued by the work of Stanford University’s Jim Collman on metal complexes that crystallize in stacks like coins to provide a pathway for one-dimesional electrical conduction. Brian Hoffman, Jim Ibers, Mark Ratner, and I collaborated on synthetic, spectroscopic, structural, and theoretical studies of these systems and showed that metal or ligand “partial oxidation,” for example, by I2, creates empty bandlike states and, in cases such as metallophthalocyanines, “metallike” 1-D conductivity. However, for molecular systems, the challenge remained of ensuring that face-to-face stacking was preserved, ideally with controllable π-π distances. We addressed this issue with a “shish kebab” strategy in which phthalocyanine polymers were connected via central Si–O–Si, Ge–O–Ge, or Sn–O–Sn linkages (figure 5). We could then “dial in” the levels of oxidation while preserving the stacking, and analyze the results with a variety of incisive structural, spectroscopic, magnetic, electrical, and theoretical tools.
My fascination with charge transport phenomena in solids has continued, and today involves both “soft” (organic molecules and assemblies; polymers) and “hard” (oxides, chalcogenides) materials. The former studies focus on transistors, solar cells, and organic light-emitting diodes (OLEDs) using tunable, Earth-abundant organic materials. We seek to print these as “inks” in high-throughput production. The key materials in a transistor, the basic building block for all modern electronics, are the semiconductor, gate dielectric, and the source, drain, and gate electrodes. Our first target was relatively unexplored electron-transporting (n-type), versus established hole-transporting (p-type), organic semiconductors—necessary components of complementary circuits for high-speed, low-power operation. Collaborations with colleague Tony Facchetti, and later with Mike Wasielewski, yielded environmentally stable, high-mobility n-type semiconductors, and ultimately, devices with inkjet-printed complementary circuits. Theoretical studies with Mark Ratner convincingly defined the electronic structural and molecular architectural requirements for n-type semiconductors.
Gate dielectrics positioned between the transistor gate electrode and the semiconductor modulate the flow of current through the semiconducting channel and are critical to real world performance. Collaborating with Tony Facchetti and Mark Ratner, we invented self-assembled nanodielectrics (SANDs) that are compatible with organic electronics and deliver high dielectric constants, charge-trap-free interfaces, and nanoscale thicknesses. SANDs significantly enhance the performance of diverse organic and inorganic transistors, are radiation-hard, and are compatible with many substrate types; they stimulated fruitful device collaborations with Mark Hersam and Lincoln Lauhon of Northwestern; John Rogers, now of Northwestern; David Janes of Purdue University; and many companies. This also led to the cofounding of two Northwestern spinoffs with Tony Facchetti, Polyera and Flexterra, to transform these laboratory discoveries into printed products.
My interest in hard materials began with metal oxides. The late 1980s advent of cuprates with astounding superconducting properties caused me to ask whether such films could be grown using chemical processes. Therefore we designed, synthesized, and refined volatile metal-organic Y, Ba, Cu, Tl, Bi, and Ca precursors that enabled the controlled growth of excellent quality superconducting cuprate films by a scalable metal-organic chemical vapor deposition (MOCVD) process in collaboration with Northwestern engineers Bruce Wessels, Carl Kannewurf, and Bob Chang. These films and our ability to vary stoichiometry provided samples for studies aimed at understanding the intrinsic current-carrying capacity of these materials.
We next turned to other oxides, especially those combining optical transparency and electrical conductivity, which while seemingly contradictory, is realized in materials such as tin-doped indium oxide (ITO), used in flat-panel displays, solar cells, touch screens, and numerous other products as transparent electrodes. We reasoned that alternative growth techniques, doping, and oxide matrices might enhance conductivity at comparable transparencies, and allow growth on flexible plastics. Soon, MOCVD growth of cubic, optically transparent Cd1-xInxO films demonstrated record conductivity and benefited from theoretical collaboration with physicists Arthur Freeman of Northwestern and Julia Medvedeva, now of the University of Missouri). We also employed another low-temperature technique to grow In2O3 films on SAND, yielding transparent transistors with exceptional performance.
We next sought the solution-phase growth of electronically functional, ideally flexible semiconducting, dielectric, and conducting oxide films from solution at temperatures compatible with polymeric substrates. This was achieved via combustion synthesis, affording amorphous oxide films and flexible transparent transistors of electronic quality comparable to those grown by industrial sputtering processes (figure 6). Similar strategies afforded transparent flexible electrodes for rollable plastic solar cells, and unusual “antiambipolar” heterojunctions with carbon nanotubes and 2-D metal chalcogenides, in collaboration with Mark Hersam and Lincoln Lauhon. Such devices have potential in Wi-Fi and Bluetooth circuitry. Similar concepts were employed in our very successful design of polymer solar cells.
This concludes a brief summary of my research at Northwestern, crossing many disciplinary boundaries. These accomplishments would not have been possible without collaborators around the globe and the dedicated enthusiasm of my students and postdocs who contributed immensely to this enterprise. I dedicate this address to them.
As Jackie Barton noted in her 2015 Priestley address, many of life’s pathways are the products of accidents and opportunities. In that way I met and married my lifetime partner and fellow chemist, Indrani Mukharji. Coming from an academic family of two scientists, she understands the life we lead and amazingly puts up with me. Indrani’s father was Gilbert Stork’s first postdoc, moving with him from Harvard to Columbia University in the 1950s. We still keep in touch with Gilbert.
After spending a decade as a corporate scientist, Indrani has been a senior administrator at Northwestern for the past 22 years. Our daughter, Miriam Marks, wanted a different life. After studying public policy and economics at Stanford, she is now a student at New York University Law School. I will be forever indebted to them for their love, moral support, and endless patience.
And what of chemistry? Why is it unique? Steve Lippard argued in his 2014 Priestley address that the term “central science” might imply a mere service role. And I suspect that all scientists feel that their discipline is central. Lord Rutherford once said that “all science is either physics or stamp collecting.” So we might instead regard chemistry as the “all-pervasive science,” or the “interfacing science,” or the “indispensable science.” However, I feel that we should ask what we chemists do that is truly unique and that will be forever unique. I would argue that first and foremost, chemists make things, that with deep knowledge and skill we create “the stuff that dreams are made of.”
As you ponder these issues, you may feel the need for a libation. So I will close with a toast to our ACS award winners, to chemists in every walk of life, and to chemistry as an enduring, indispensable, and dynamic science! Good night and safe travels!
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