Editor's Page pubs.acs.org/Organometallics
Gregory L. Hillhouse: His Life, His Art, His Science, and the Rise of “Double Nickel”
P
Greg (Figures 1 and 2) began exploring fundamental chemistry of energy-rich nitrogen compounds such as organic azides and diazoalkanes.1−3 In parallel, he pursued the synthesis of metal hydride complexes that underwent insertion reactions with unsaturated molecules.2,4 At this early stage of his career, the seeds of Greg’s passion for metal−ligand multiple bonding were sown.2,5 After completing his Ph.D., Greg headed to the California Institute of Technology to work with Professor John Bercaw (Figure 1). There he developed the reaction chemistry of Zr and Hf decamethylmetallocene complexes with small molecules such as water and ammonia.6 This classic work showed that N− H and O−H activation could occur by σ-bond metathesis or oxidative addition. These fundamentally important reactions are relevant to water oxidation and conversion of ammonia to more value-added products, which is why this chemistry is still cited today. During this time, Greg’s passion for high-energy molecules such as azides continued, as he embarked on new chemistry with group 4 transition metal complexes.7 The early 1980s were an exhilarating time in the Bercaw group, which was a wellspring of aspiring academicians. During this period group members who later pursued academic careers included Don Berry (University of Pennsylvania), Michael Fryzuk (University of British Columbia), Vernon Gibson (Imperial College and now Chief Scientific Adviser, Ministry of Defense in the U.K.), James Mayer (Yale University), Gerard Parkin (Columbia University), Dean Roddick (University of Wyoming), Mark Thompson (University of Southern California), T. Don Tilley (UC Berkeley), Antonio Togni (ETH Zurich), and Peter Wolczanski (Cornell University). From the start of his independent career at the University of Chicago in 1983, Greg’s research program was framed by the interactions of reactive species (carbon suboxide, OCC CO) with transition metal complexes,8−11 the activation of kinetically inert molecules (N2O) by d0 organometallic complexes,12−14 and the stabilization of reactive species (diazenes, HNNR) at transition metal centers.15−22 The nitrous oxide chemistry was unique in that d0 systems could not be oxidized, precluding the formation of metal−oxo complexes, and N2O activation was achieved by utilizing coordinative unsaturation and relief of ring strain of coordinated organic ligands. Remarkably, some of the products were arrested O atom transfer intermediates where the N2O framework remained intact after insertion into metal−carbon bonds.14,23,24 These compounds subsequently reacted to eject N2, affording the formal O atom inserted products. The Hillhouse group showed in later work that O atom insertion could be extended to d8 Ni complexes,25−29 thus demonstrating generality beyond the early transition metals. Greg’s novel
rofessor Gregory Lee Hillhouse was a purist who explored at a fundamental level structure, bonding, and reactivity of organic and inorganic molecules. Whether at the canvas or in the laboratory, he was a creative artist who lived life to its fullest, and enriched the lives of those around him. As an undergraduate at the University of South Carolina with Professor Edward Mercer, Greg became interested in the chemistry of transition metals. In his graduate and postdoctoral studies, Greg focused on the roles of transition metals in stabilizing reactive molecules and activating inert ones, which would become hallmarks of his independent career. In his early days as a graduate student in the laboratories of Professor Barry Haymore (Figure 1) at the Indiana UniversityBloomington,
Figure 1. Mentors and their protégé at the 2013 Spring ACS National meeting in New Orleans. Pictured from left to right are Dr. Barry Haymore, Prof. Greg Hillhouse, and Prof. John Bercaw. (Photographed by Prof. Daniel Mindiola.)
Figure 2. 1976 snapshot at Indiana UniversityBloomington (Reprinted with permission from Prof. Surrey Walton. Photographed by the late Prof. Greg Hillhouse.) © 2015 American Chemical Society
Special Issue: Gregory Hillhouse Issue Published: October 12, 2015 4633
DOI: 10.1021/acs.organomet.5b00527 Organometallics 2015, 34, 4633−4636
Organometallics
Editor's Page
approach still holds great promise as a route to using N2O, a greenhouse gas, as a reagent for O atom transfer. Hillhouse’s diazene chemistry is germane to both oxidations of hydrazines and reduction of N2, and his group demonstrated the unique reactivity of coordinated NHNH.20 Importantly, the oxidation of coordinated hydrazine was extended to coordinated hydroxylamine, affording coordinated nitroxyl (NHO).30 Additional routes to this family of compounds were developed,31,32 providing insight into this fundamentally important molecule, whose chemical relevance ranges from biochemical signaling to astrochemistry. In the early 1990s (Figure 3), Greg began exploring oxidation reactions as a route to promote bond-forming
nases, are instrumental in catalyzing over 75% of metabolic reactions in our body. Greg’s passion for nitrenes was fueled by the fact that late transition metals were considered too electron rich to stabilize such a group. Early transition metals were known to form stable nitrene complexes, but these often proved to be unreactive because of the very strong M−N multiple bond. Low coordination numbers are required for chemistry to be unlocked with the early transition metals. Greg combined two and two, and his epiphany was to support a late transition metal nitrene by constraining the geometry to three-coordinate and by using strongly donating ligands to stabilize the metal center. Greg drew inspiration for this approach from a textbook published by one of his late colleagues at the University of Chicago, Professor Jeremy Burdett. In his coauthored textbook, Orbital Interactions in Chemistry,36 it is stated that bent, twocoordinate fragments of electronic configuration identical with that of nickel(II) are isolobal to a nitrene, carbene or oxene fragment (Figure 4)! Taking advantage of this insight, Greg
Figure 3. Greg Hillhouse ca. 1990 visiting the Stonewall Inn in New York City. (Photographed by Prof. Milton Smith III.)
Figure 4. Orbital matching of a d8-C2v symmetric fragment with an isolobal nitrene (or a carbene or oxene) group.
processes via reductive elimination. Oxidatively induced reductive elimination is an important reaction to forming C− X bonds (where X is a heteroatom such as N, O, or S), and the Hillhouse group played an integral part in understanding how oxidants participate in this process.33−35 In the late 1990s Greg began exploring the chemistry of nitrenes (NR, where R is an organic group), a type of reactive radical species integral to the formation of molecules such as aziridines that are important in pharmaceuticals.34 Nitrenes, carbenes, and oxenes are all reactive species essential in oxidation chemistry. Since the 1880s chemists have been exploring their potential use in bond insertion chemistry (known as the Buchner−Curtius−Schlotterbeck reaction); more recently organic chemists have applied this methodology of group transfer for the construction of biologically important compounds. Physical chemists have been also fascinated by the properties of these fragments, given their diradical character. Biology has found a way to harness the reactivity of molecules such as oxenes by using highly sophisticated architectures such as cytochromes. These molecular machines, the monoxyge-
devoted his efforts to prepare stable forms of late transition metal nitrenes. In 2001, Greg constructed such a molecule, “double nickel”,37 and soon thereafter, the carbene38 and phosphinidene (the heavier congener of a nitrene)39 derivatives were prepared. Greg later demonstrated that these systems could indeed perform transfer reactions to form important organic molecules such as azirdines, cyclopropanes, and phosphiranes.40 Depending on the nature of the reagent, Greg discovered compounds that could deliver the nitrene group catalytically.41 It was only a question of time before even lower coordinate nitrenes of nickel were assembled. By using a sterically “bodacious” N-heterocyclic carbene coligand, Greg was able to take advantage of the intrinsically reactive Ni−N multiple bond and thus explore remarkable transformations such as the amination of carbon−hydrogen bonds.42 For Greg, the next target on the horizon was the synthesis of stable oxene complexes. These oxene species, if prepared, could teach us about electrophilic single atom oxygen sources and provide insights into processes such as water oxidation, currently so important in the context of energy and a future hydrogen 4634
DOI: 10.1021/acs.organomet.5b00527 Organometallics 2015, 34, 4633−4636
Organometallics
Editor's Page
Figure 5. 2013 recipient Gregory L. Hillhouse (center) being presented his award by sponsor representative James C. Stevens (right) and ACS President Marinda Li Wu (left). (Reproduced with permission from the American Chemical Society. Copyright 2013 American Chemical Society. Photographed by Cutts Photography.)
Figure 6. Greg’s “Man Sitting” (left). (Reprinted with permission from Prof. Surrey Walton.). Greg’s 1973 watercolor (right). (Reprinted with permission from Prof. Surrey Walton.)
economy. Unfortunately, his life was cut short in early 2014 due to liver and pancreatic cancer. Greg’s legacy in chemistry is not limited to experimental results that will provide a wealth of information for decades to come, but by the number of scientists he fostered over the three decades he taught at the University of Chicago. In 2013, Greg’s contributions to the chemical community were recognized at the 245th ACS National Meeting and Exposition in New Orleans, where he was presented the ACS Award in Organometallic Chemistry (Figure 5). He helped those who worked under his supervision find their groove in chemistry and in life and instilled in all of them the desire to get the most out of life. Greg took pride in his science, but he was perhaps even prouder of the accomplishments of those who had been fortunate to spend time in his laboratories, once they embarked on their independent careers. His support extended beyond his
own group, as it was a common sight to see Greg engaging budding chemists in conversation at conferences. For those who worked under his supervision, he always found the best in each of them and knew how to nurture it and then let go to allow them to become the independent thinkers they are now. To you Greg, scientist, artist (Figure 6), mentor, and beloved colleague and friend, we all dedicate this special issue in Organometallics.
Daniel J. Mindiola*,† Milton R. Smith, III*,‡ John E. Bercaw*,§ †
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia Pennsylvania 19104, United States 4635
DOI: 10.1021/acs.organomet.5b00527 Organometallics 2015, 34, 4633−4636
Organometallics
Editor's Page
‡
■
(27) Matsunaga, P. T.; Mavropoulos, J. C.; Hillhouse, G. L. Polyhedron 1995, 14, 175−185. (28) Koo, K.; Hillhouse, G. L. Organometallics 1998, 17, 2924−2925. (29) Harrold, N. D.; Waterman, R.; Hillhouse, G. L.; Cundari, T. R. J. Am. Chem. Soc. 2009, 131, 12872−12873. (30) Southern, J. S.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc. 1997, 119, 12406−12407. (31) Melenkivitz, R.; Hillhouse, G. L. Chem. Commun. 2002, 660− 661. (32) Melenkivitz, R.; Southern, J. S.; Hillhouse, G. L.; Concolino, T. E.; Liable-Sands, L. M.; Rheingold, A. L. J. Am. Chem. Soc. 2002, 124, 12068−12069. (33) Koo, K.; Hillhouse, G. L. Organometallics 1995, 14, 4421−4423. (34) Han, R. Y.; Hillhouse, G. L. J. Am. Chem. Soc. 1997, 119, 8135− 8136. (35) Lin, B. L.; Clough, C. R.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 2890−2891. (36) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interactions in Chemistry; Wiley: New York, 1985. (37) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623−4624. (38) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 9976−9977. (39) Melenkivitz, R.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 3846−3847. (40) Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2003, 125, 13350−13351. (41) Laskowski, C. A.; Hillhouse, G. L. Organometallics 2009, 28, 6114−6120. (42) Laskowski, C. A.; Miller, A. J. M.; Hillhouse, G. L.; Cundari, T. R. J. Am. Chem. Soc. 2011, 133, 771−773.
Department of Chemistry, Michigan State University, 578 South Shaw Lane, East Lansing, Michigan 48824, United States § Division of Chemistry and Chemical Engineering, California Institute of Technology, 200 East California Boulevard, Pasadena, California 91125, United States
AUTHOR INFORMATION
Corresponding Authors
*E-mail for D.J.M.:
[email protected]. *E-mail for M.R.S.:
[email protected]. *E-mail for J.E.B.:
[email protected]. Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
■
REFERENCES
(1) Hillhouse, G. L.; Haymore, B. L. J. Organomet. Chem. 1978, 162, C23−C26. (2) Hillhouse, G. L.; Haymore, B. L.; Herrmann, W. A. Inorg. Chem. 1979, 18, 2423−2426. (3) Hillhouse, G. L.; Haymore, B. L. J. Am. Chem. Soc. 1982, 104, 1537−1548. (4) Hillhouse, G. L.; Haymore, B. L. Inorg. Chem. 1987, 26, 1876− 1885. (5) Herrmann, W. A.; Hubbard, J. L.; Bernal, I.; Korp, J. D.; Haymore, B. L.; Hillhouse, G. L. Inorg. Chem. 1984, 23, 2978−2983. (6) Hillhouse, G. L.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5472− 5478. (7) Hillhouse, G. L.; Bercaw, J. E. Organometallics 1982, 1, 1025− 1029. (8) Hillhouse, G. L. J. Am. Chem. Soc. 1985, 107, 7772−7773. (9) List, A. K.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc. 1988, 110, 4855−4856. (10) List, A. K.; Hillhouse, G. L.; Rheingold, A. L. Organometallics 1989, 8, 2010−2016. (11) List, A. K.; Smith, M. R., III; Hillhouse, G. L. Organometallics 1991, 10, 361−362. (12) Vaughan, G. A.; Rupert, P. B.; Hillhouse, G. L. J. Am. Chem. Soc. 1987, 109, 5538−5539. (13) Vaughan, G. A.; Hillhouse, G. L.; Lum, R. T.; Buchwald, S. L.; Rheingold, A. L. J. Am. Chem. Soc. 1988, 110, 7215−7217. (14) Vaughan, G. A.; Sofield, C. D.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 5491−5493. (15) Smith, M. R., III; Hillhouse, G. L. J. Am. Chem. Soc. 1988, 110, 4066−4068. (16) Smith, M. R., III; Hillhouse, G. L. J. Am. Chem. Soc. 1989, 111, 3764−3765. (17) Smith, M. R., III; Keys, R. L.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 8312−8314. (18) Smith, M. R., III; Cheng, T. Y.; Hillhouse, G. L. Inorg. Chem. 1992, 31, 1535−1538. (19) Smith, M. R., III; Cheng, T. Y.; Hillhouse, G. L. J. Am. Chem. Soc. 1993, 115, 8638−8642. (20) Cheng, T. Y.; Peters, J. C.; Hillhouse, G. L. J. Am. Chem. Soc. 1994, 116, 204−207. (21) Cheng, T. Y.; Ponce, A.; Rheingold, A. L.; Hillhouse, G. L. Angew. Chem., Int. Ed. Engl. 1994, 33, 657−659. (22) Peters, J. C.; Hillhouse, G. L. Polyhedron 1994, 13, 1741−1746. (23) Vaughan, G. A.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc. 1990, 112, 7994−8001. (24) Mindiola, D. J.; Watson, L. A.; Meyer, K.; Hillhouse, G. L. Organometallics 2014, 33, 2760−2769. (25) Matsunaga, P. T.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 2075−2077. (26) Koo, K. M.; Hillhouse, G. L.; Rheingold, A. L. Organometallics 1995, 14, 456−460. 4636
DOI: 10.1021/acs.organomet.5b00527 Organometallics 2015, 34, 4633−4636
