Profile of Malcolm H. Chisholm

A Fiery Start

Born in Bombay, India, in 1945, Chisholm spent only the first 6 months of his life there before moving back to his family's home of Inverness, Scotland. Soon after, at the age of 3, Chisholm moved to southern England. He remembers being captivated by science even at the young age of 4. “My first interests in science were meteorological, trying to predict storms, rain, and when it would snow,” he says. “Of course, that was frustrating because it doesn't snow much in the south of England.”

By the time he was 9, Chisholm's interests had moved on to astronomy. With his telescope, he looked at the moons of Jupiter and, in 1956, spied the Aaron Rowland comet. “About then, I got a chemistry set,” he recalls. He found fodder for his experiments at the local pharmacy. “In those days, one could buy a lot of chemicals from a chemist's shop.” He recounts that sodium chlorate, sold as a pesticide, along with sugar, made good homemade rockets. Chisholm's experiments took place unfettered by his parents until a particular incident drew the local fire brigade. “I had a bit of a fire in the garden shed,” he says. “That was the end of my independent experiments.” His parents suppressed his would-be pyromania, but Chisholm still found enjoyment in chemistry and studied it in school.

Chisholm entered London University (London, England) in 1963 where early on he had to declare a major. He briefly considered meteorology, but then, he says, “I discovered you had to have a degree in math.” Chisholm settled on a chemistry major because, he says, “it was one of the subjects that came most easily to me.” But he still did not envision himself becoming a chemist. It was not until his third year, during independent research on iron enneacarbonyl in the laboratory of Alan Massey, that Chisholm decided to pursue a career in chemistry. Chisholm enjoyed getting back to doing what he calls “original, inquiry-driven experiments.” In its solid form, iron enneacarbonyl, composed of two ferric carbonyl groups linked with three CO ligands, is insoluble in almost all known solvents. Chisholm found, however, that it showed a beautiful mass spectrum. His work on iron-carbonyl garnered him his first publication, a brief communication in Nature (2).

Complex Coordination

Chisholm received his B.Sc. in 1966 and stayed at London University for the doctoral program in inorganic chemistry. He wanted to continue the work he had begun as an undergraduate but explains that he was recruited to join a new laboratory. The change proved beneficial. Chisholm calls his advisor, Donald C. Bradley, “a very influential man in my life. He was a good role model, both as a mentor and as a person.” For his doctoral work, Chisholm says, “We made some unusually low coordinate metal complexes” (3). Coordination compounds, alternately known as metal complexes, are made up of a metal complexed, or coordinated, to a ligand. The coordination number refers to the number of ligands attached to the metal atom. Low coordination compounds have large ligands attached to small metals, like chromium. Chisholm wanted to understand the nature of the bonds that occur when a ligand π-donates a pair of electrons into the empty valence shell of a metal. He used group 4, 5, and 6 transition metals complexed with organic π-donor ligands paired with bulky groups, amides and alkoxides, to help him formulate his theory of electric structure. Chisholm explains that, at the time, the Ligand Field Theory could explain the color and magnetic properties of coordination compounds, but he had a hunch that the bonding properties were much more organic and covalent in nature, which indeed they turned out to be. “I developed some of the early bonding theories involved in π-donor complexes. For example, employing a simple molecular orbital approach, I could explain why a four-coordinate d² ion could be diamagnetic as was seen for Mo(NMe₂)₄,” he says (4).

Synthesizing compounds and inspecting their molecular properties did not proceed at the same rate as it does today, however. In the 1960s, Chisholm recalls that the analytical tools were limited and slow. X-ray crystallography techniques, important for visualizing the structure of the molecule, a process that currently takes 1 day, were far from routine. Chisholm recalls that analysis for a single crystal took several months. NMR spectroscopy, for identifying individual atoms, was restricted to very low fields, ≈60 MHz. Accoutrements like dry boxes and controlled atmospheres were not common. “It was slower because of that,” Chisholm explains, adding, “We had to do most of the glassblowing ourselves.”

Out of England

In 1969, Chisholm was ready to start his postdoctoral work. “I decided to get away from those miserable English winters.” He explains that if it were going to be cold, he would at least like to be where it snowed. Bradley recommended that Chisholm study with Howard Clark at the University of Western Ontario (London, ON, Canada). Over the next 3 years, Chisholm and Clark published 18 papers in the relatively new field of cationic organoplatinum chemistry. Chisholm explains that the field is now referred to as “organometallic chemistry,” the study of the bonding of metals to carbon or organic ligands. Compared with his earlier research, Chisholm says, “the work was much easier synthetically.” He learned NMR spectroscopic techniques and formulated ideas about the reactivity of the organoplatinum compounds. Organoplatinum cations catalyze insertion reactions. The cationic metal complex can start a polymerization process. Chisholm recognized that the polarization of the olefin, also known as an alkene, is important for the reaction that takes place during Ziegler–Natta olefinic polymerization. He considers his evolving theory of metal-induced carbonium ion reactivity to be the important part of his postdoctoral work (5).

When Chisholm began to look for his first faculty position in 1972, he recalls, “There were very few job openings in England and Canada.” Although he had not considered options in the United States, he received a call from Kurt Mislow at Princeton University (Princeton, NJ) and soon found himself an assistant professor in the chemistry department. Chisholm intended to continue his work on organoplatinum cations, but he had trouble finding funding. As he explains, “The grant referees thought it would be a better idea for me to work on something different.” Chisholm turned to his work on metal–metal (MM) coordination compounds and began what would become a long collaboration with F. A. Cotton at Texas A&M University (College Station, TX). Cotton pursued the single crystal structure determination while Chisholm focused on developing an explanation of the reactivity of the MM compounds of molybdenum and tungsten. Chisholm imagined how they might behave if they were organic. He explains that organic chemistry has double and triple bonds with particular reactivity patterns. For instance, carbon–carbon (C–C) double bonds are known to undergo addition and elimination reactions leading to stepwise changes in C–C bond order. Chisholm expected that it should be the same for MM compounds, to some extent. In addition, it was soon found that carbon–carbon, carbon–nitrogen, and carbon–oxygen multiple bonds would add to MM multiple bonds in unusual ways and sometimes lead to metathesis reactions (6) The work was fruitful, and Chisholm enjoyed having his own laboratory with one change from the independent experiments in the backyard laboratory of his youth. “Now that I had my own lab, I didn't like the idea of explosions or fires.”

Finding Parallels

In 1978, Chisholm moved to Indiana University (Bloomington, IN) where he continued working on MM bonds. Throughout the 1980s, “I really focused on developing the reactivity of MM bonds,” he says. With MM bonds, he again looked for analogies from organic chemistry. He explains that there are both similarities and differences. For instance, similar to C–C double bonds, MM bonds can change bond order 3–2–1 or vice versa. Chisholm finds the differences interesting, too, and notes that the famous Woodward–Hoffmann rules of organic chemistry are relaxed by the presence of the metal d orbitals.

For his work, Chisholm used either molybdenum or tungsten compounds. Both elements are in the same period on the periodic table, have the same number of valence electrons, six, and differ only in electronic configuration. Chisholm explains that tungsten is reducing and thus more electrically tunable. Some of the difficulty in working with these compounds was because of their solubility. “The compounds were very air sensitive,” he explains. The metal compounds are also soluble in hydrocarbons, necessitating careful work when isolating single crystals. The compounds are useful, however, because they are diamagnetic, which enables applications of NMR spectroscopy to the elucidation of their reactions. For example, by employing ¹³C¹⁶O/¹³C¹⁸O, one can monitor the reactions and intermediates leading to the formation of carbide-oxo products by NMR and IR spectroscopy.

Going Green

In the late 1990s, Chisholm found himself interested in materials research and moved to Ohio State University (OSU; Columbus, OH). OSU had a materials science department as well as an engineering college, two elements that Chisholm thought necessary for fruitful collaborations. He began working on polymeric materials that incorporated MM bonds. The materials, some of which can be put into thin films, show interesting properties like electroluminescence and light-emitting diodes. Chisholm explains that the synthesis of the materials is challenging because it requires control of solubility as well as the ability to make discrete compounds instead of mixtures. With each new material, he attempts to elucidate the mixed valence state, asks what the photophysical properties are, and looks at whether the new polymers are good or poor conductors. Chisholm's initial work in this area has focused on incorporating MM quadruple bonds of molybdenum and tungsten into oligothiophenes by the use of the carboxylate linker. The M₂δ and CO₂π orbitals interact to allow conjugation through the metal centers (7). Chisholm's work involves extensive collaborations, and in particular he acknowledges the support of Art Epstein, Terry Gustafson, Bruce Bursten, Claudia Turro, Pat Woodward, and Christopher Hadad.

Chisholm says that now, in 2007, half of his laboratory's work is devoted to “green chemistry.” In keeping with the principles of green chemistry, which seek to reduce hazardous substances, Chisholm's lab is currently attempting to “make organic polymers from sustainable sources.” One project is attempting to synthesize polyesters with lactide as the fundamental unit. Lactide is made from corn and readily available. Chisholm thinks the projects are interesting in terms of catalysis design because his lab must make metal complexes that will carry out the reaction. The goal is to design a catalyst that will polymerize the lactide, but the catalyst must be immortal and stereoselective. Steroselectivity is important in making copolymers, other molecules inserted within the lactides. Chisholm says that another important research endeavor is the use of carbon dioxide in the formation of polycarbonates. He also has worked on molecules that decompose cleanly to solid-state materials. For instance, he developed a way to make tungsten carbide at 300°C versus 1,000°C (8). As Chisholm explains, “We try to make known materials by new routes.”

Liquid Crystal Limbo

For his Inaugural Article (1), Chisholm synthesizes and characterizes molybdenum and tungsten MM multiple bonds in a specific, ordered manner to discern their optical and electronic properties. He uses two strategies, each based on a different method for ordering the bonds. One method is to make and study liquid crystal phases. Liquid crystals are caught in limbo between a liquid and a crystalline state and thus represent a distinct state of matter. This limbo has subphases determined by the order induced by the type of molecules and their responsiveness to outside influences such as magnetic fields or temperature. Chisholm examines how the MM axes are aligned in magnetic fields, characterizes the properties with regard to temperature, and looks for third-order responses (9, 10). He says it was disappointing not to find any large third-order responses because it means that the MM bonds cannot be used in laser applications. Another way to order the bonds is to insert the MM into a conjugated organic polymer and see how it behaves. Chisholm finds this strategy interesting because it enables him to characterize simple, dinuclear units. “By studying individual model compounds, we can estimate what we will see in the polymer,” he says.

The synthetic techniques involved in studying MM bonds also intrigue Chisholm. His laboratory recently developed a technique to synthesize extended chains, enabling him to study electron delocalization (11). Chisholm explains that electron delocalization is necessary for charge separation, and, in turn, charge separation is an indicator of useful properties like absorbing visible light and photo harvesting for photocells. When studying charge delocalization, Chisholm says he considers a series of questions, such as: “How far does the electron move from the metal center? How long does it stay separated?” For the compound Mo₂(O₂C-9-anthracene)₄ and related Mo₂-oligothiophenes, Chisholm has found separation of up to 100 μs, which he explains is a relatively long time (12). “The next step is to look for applications of these organic metallopolymers,” he says. “We want to put them in thin films and see if we can make molecular switches out of them.” For something to be an effective molecular switch, it must exist in two or more different states, each having different properties, such as color and conductivity, and these properties must be reactive to an external stimulus, either physical or chemical. Therefore, Chisholm is looking for metallopolymers that can be sensitive to their environment.

In terms of Chisholm's own environment, science runs in the family. His wife, Cynthia, has helped him as an editorial assistant. He has three sons, one of whom, Calum, has his own start-up company working on fuel cells. Chisholm, who has garnered numerous teaching and mentoring awards, including the 1988 American Chemical Society Nobel Laureate Signature Award that he shares with his doctoral student David L. Clark, is also proud of his academic legacy: “I'm most happy for the successes of my students who have gone on in the field.”