C& EN TA L KS W ITH
ANTHONY J. ARDUENGO III Inventor of stable N-HETEROCYCLIC CARBENES reflects on the impact of his discovery 20 years later STEPHEN K. RITTER, C&EN WASHINGTON
from elements all across the perimolecules that break the rules. The odic table. They also have branched University of Alabama chemistry out from catalysis and into elecprofessor has spent his career cretronics and other applications. ating compounds with abnormal Arduengo’s interest in tinkering valencyandbondingarrangements with molecules comes honestly, he leading to unusual molecular says. He has always had an interest electronic properties. Although in working with tools and doing many chemists might not recogmechanical work. When he was 16, nize Arduengo by name, they will for example, he teamed up with his recognize one of his milestone rulefather to build a car from miscelbreaking achievements: the discovlaneous parts. His fascination with ery 20 years ago of the first isolable cars continues. Now 60, he owns Arduengo N-heterocyclic carbene, or NHC. a yellow 2008 Lotus Elise. “Aside Arduengo was a researcher at from really liking the car, I feel a DuPont when he made the cyclic diamine (an imidazole) bearing kinship to it because it has a DuPont automotive waterborne coatadamantyl substituents. In carbenes, the central carbon has a valening,” Arduengo says. “It’s nice to be able to look at something and cy of two instead of the usual four, Arduengo explains. A lone pair of see reflected in it the chemistry you’ve worked on.” electrons fills one of the carbon’s orbitals, leaving one orbital empty. Chemists once viewed carbenes as highly reactive, transient speALTHOUGH HE ENJOYED working at DuPont, Arduengo sensed cies that were important intermediates in some reactions, Arduenin the late 1990s that the firm’s research culture was shifting togo says. Although predictions were made that the electron imbalward project-driven applied research, where success or failure ance in carbenes could be stabilized with structural modifications, depended more on whether something worked and less on fundano one had been able to crystallize one and put it in a reagent bottle. mental discoveries like uncovering stable carbenes. He decided to He credits the research culture at DuPont for making it possible. change gears, and in 1999 he took a faculty position at Alabama. He “We worked on applied projects, but we also were free to look also has a joint research appointment at the Technical University for new ideas and concepts. This discovery-driven approach liberof Braunschweig, in Germany. ates you from the philosophical confines of what is conventionally The transition to academia enabled Arduengo to continue disknown,” he says. “Within that culture, if you keep your ear to the covery-driven applied research on carbenes, he says. Some of his rail and listen for research problems that come along, you can allatest work includes developing modified NHC ligands and using ways be thinking about how you might provide a quick solution.” NHCs for alkyne metathesis reactions. But his penchant for tinkerArduengo had been working on developing cross-linking cataing with tools has led him in new directions. lysts used in paints and other polymer-based systems. Meanwhile, One of his targets is developing NHC-based dyes to capture light DuPont was trying to develop an environmentally friendly waterin solar cells. He’s reluctant to share details pending patent protecborne coating with low volatile organic compound emissions, he tion, but his group’s chromophores absorb light from the infrared says. He recognized that the catalytic imidazole thiones he had into the ultraviolet and convert light energy into electrical energy been working on provided a solution. without the semiconductor layer that most solar cells require. It struck Arduengo that the carbene intermediates that captured Arduengo says his foray into stable carbenes has been overwhelmsulfur to make the thiones had an unusual tolerance to moisture in ingly gratifying to him as a scientist and educator. “I have had the the air and to a range of different types of substituent groups on the opportunity to meet and interact with some of the world’s luminary nitrogen atoms. “When you notice something like that in the lab, it scientific figures and help many hardworking young scientists in my tends to gnaw at you as to why,” Arduengo notes. “It finally dawned group,” he says. “As a trumpet player, I can tell you that one’s musical on me that these particular carbenes were really stable.” Although performance improves with the quality of the musicians with whom Arduengo’s DuPont managers thought he was chasyou associate and perform. This intellectual concert ing shadows trying to isolate a stable carbene, he enhancement holds true among chemists.” gave it a shot anyway. He nailed it on his first try. He’s also thrilled that many chemists have picked “It’s nice to be When news about Arduengo’s discovery spread, able to look at up on and found success with NHCs. “It’s invigoratchemists quickly grouped NHCs with cyclopentadiing to know you’ve made a contribution that will enes and phosphines as go-to ligands for transition- something and leave a mark,” Arduengo says. “As a scientist, that metal catalysts. They also found that NHCs function see reflected in is something we all hope for I think, to push the sciit the chemistry as metal-free organocatalysts. The carbenes now ence forward and give others tools to work with so come in many varieties, and analogs have been made you’ve worked on.” they can go even further.” ◾ COURTESY O F AN T HO N Y AR DU EN GO
ANTHONY J. ARDUENGO III likes
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