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W. Donald Cotter

Research in the History of Chemistry

My research focuses on the life and career of Alexander Smith (1865 - 1922), an early member of the University of Chicago faculty, and Chairman of Chemistry at Columbia University from 1911 - 1919. Smith is little remembered today, probably because his research output, while respectable, was modest. However, it is not too much to say that he wrote the establishing work in what would become the American General Chemistry textbook genre. His influence over contemporary practice remains both deep and broad.

Another reason for Smith's relative obscurity is likely the paucity of primary sources concerning him. I have uncovered an uncataloged collection of his papers in the Columbia University Archives, and much of my current activity is centered on exploiting this new resource. Smith's life will tell us about more than textbook evolution and teaching practices. He was a quietly influential figure in the political struggles within the American Chemical Society, becoming embroiled in and professionally threatened by the intersection of science and wartime patriotism during World War I. He greatly influenced the early work of the College Board in establishing chemistry examinations with a national clientele. And he underwent profound intellectual transformations over the course of his career that reveal much about the dynamics of competition between various chemical schools of the day.

I have spoken about Smith's life and intellectual career before the Chemical Heritage Foundation, the IHPST, and the Society for the Social Study of Science.

Research in Organometallic Chemistry

• Mechanisms of transmetalation between heavy main group and transition metals

  My research in organometallic chemistry at Mount Holyoke explored the mechanisms of transmetalation, specifically the processes by which organic groups are transferred from the main-group metal tin to the transition metal palladium. This is the crucial step in the powerful Stille coupling reaction. More generally, transmetalation is one of the least well-understood basic mechanistic events in organometallic chemistry.

  Lauren Barbour, '96, began studying the transfer of organic groups between tin and nickel in AY '94-95. During the summer of 1995, she was joined by Kristin McNamara, '97. Kristin prepared and studied the chemistry of a palladium triflate complex first described by Venanzi and Rimml, while Lauren prepared the previously unknown nickel analog. Together, they found that these triflate complexes catalyze the hydrolysis of certain organotin compounds. In her honors thesis, Lauren described complex formation between vinyltributyltin and the cationic palladium species, including an estimate of the thermodynamic characteristics of that process.

  Kristin's continuing work in the summer of 1996 found that this equilibrium process is strongly perturbed by the presence of very low concentrations of water. Her work has begun to reveal the multiple mechanistic roles played by water in this very complex system.

  Rachel Hechter, '96, in her honors thesis, described the first stoichiometric transmetalation reaction we have successfully observed--that between the palladium triflate and 2-tributylstannylfuran. The organopalladium product, shown left, has been crystallographically characterized. Recent observations in our laboratory have shown that formation of this furyl complex is reversible. Our description of this equilibrium, completed by WDC during a 1997-98 sabbatical, appeared as a Communication to the Editor in the Journal of the American Chemical Society (W. D. Cotter, L. Barbour, K. McNamara, R. Hechter, and R. J. Lachicotte, "Mechanisms and Thermodynamics of the Reversible Tin-to-Palladium Transmetalation of the Furyl Group," J. Am. Chem. Soc., 1998, 120, 11016-11017), and was presented at the 1998 Gordon Conference on Organometallic Chemistry and at the August, 1998, ACS Conference in Boston. Study of the furyl transfer reaction has provided two unprecedented pieces of information about a carbon transmetalation reaction: spectroscopic characterization of a reaction intermediate (a dihapto furan adduct, itself a highly unusual species) and measurement of thermodynamic parameters for the two-step process. Masters student Laurie Lentz-Marino will attempt to isolate the dihapto intermediate in the presence of very weakly or non-nucleophilic counterions. She will also begin our attempt to exploit this electrophilic palladium system for the catalytic modification of furans. Kirsten Norman, '99, performed parallel studies with more electron-rich complexes containing dialkylphosphino moieties.

  Emily Gussenhoven, '99, initiated a study of fluoride-assisted transfer reactions. This strategy was intended to provide synthetic access to transmetalations we have not been able to cleanly observe. In fact, Emily's work raised some interesting new questions about the mechanistic role of nucleophiles in the transmetalation process. The palladium triflate complex and vinyltributyltin undergo no net reaction with one another in the absence of fluoride. In the presence of excess LiF, which dissolves completely in acetone under the reaction conditions, no organometallic product is observed; rather, the chelating ligand dissociates from the metal center, synthetically modified. We suspect that the ligand is fluorinated under these conditions, perhaps by reductive elimination from a fluoropalladium intermediate. When fluoride is present on a polymer surface, however, an organometallic product containing the vinyl moiety precipitates from acetone. Emily's preliminary results are strongly suggestive of competing kinetic and thermodynamic pathways.

  Other projects in this area that have been pursued in our labs include those of Audi Okullo, '99, who has explored the use of pentacoordinate compounds of tin as transmetalation precursors, and Christine Algozo, '97, who attempted a study of the binding equilibria of non-functionalized olefins.

• Structure and reactivity of electronically modified metallocenes

  A sabbatical collaboration with Professor Guillermo Bazan (then at the University of Rochester, now at UC-Santa Barbara), and Rochester graduate students Caroline Sperry and Rip Lee, provided an opportunity to study in detail some of the structural and mechanistic implications of the use of the borollide and boratabenzene ligands in place of cyclopentadienyl. Caroline's synthetic and crystallographic work has provided a remarkable database of structurally related borollide-tantalum complexes; an attempt to mine this database for information about electronic structure in these highly ambiguous compounds is described in "Tantalum Borollide Trichloride: A Versatile Entry to Tantalum Borollide Complexes", C. K. Sperry, W. D. Cotter, R. Lee, R. Lachicotte, and G. C. Bazan, J. Am. Chem. Soc., 1998, 7791-7806. We also addressed the structural preferences of a very different set of cyclopentadienyl analogs, trimethylene- and tribenzylidenemethanes (G. Rodriguez, J. P. Graham, W. D. Cotter, C. K. Sperry, G. C. Bazan, and B. E. Bursten, "Binding Preferences of the Tribenzylidenemethane Ligand in High-Oxidation State Tantalum Complexes," J. Am. Chem. Soc., 1998, 120, 12512).

  The bulk of my collaboration in Rochester was devoted to an in-depth examination of hydrogenation of borollide-tantalum alkyl complexes, which yielded an unusually detailed description of the approach of dihydrogen to a low-valent organometallic fragment, its activation, and the structural characteristics and subsequent rearrangement of the product. This work was also published in JACS (W. Donald Cotter, Caroline K. Sperry, Guillermo C. Bazan, "Hydrogenation Mechanisms in (Boratacycle)tantalum Analogs of Dimethylzirconocene," J. Am. Chem. Soc., 1999, 121, 1513).

  Finally, while at Rochester I returned to the study of polymerization catalysis via dynamic NMR studies of the electronic influence of boratabenzene substituents on ionization equilibria in analogs of homogeneous, zirconocene-based Ziegler-Natta catalysts (Guillermo C. Bazan, Rip A. Lee, W. Donald Cotter, Zachary J. A. Komon, and Rene J. Lachicotte, "Electron Donating Properties of Boratabenzene Ligands," J. Am. Chem. Soc. 2000, 122, 1371 - 1380).

• Redox-active nickel complexes as models for enzymatic active sites

  Tammy Louie, '96, began pursuing the challenging synthesis of sterically hindered dithiolene ligands, for eventual use in model studies of carbon monoxide dehydrogenase activity, in AY '94-95. Tammy was also an English major interested in information science, and was involved in early work with our science librarian, Sandy Ward, to develop a campus-wide system for on-line information retrieval in the chemical sciences.

Financial Support

• Dreyfus Foundation Start-up Grant for Undergraduate Institutions
• Research Corporation Cottrell Grant
• The Petroleum Research Fund
• Dreyfus Foundation Special Grant Program in the Chemical Sciences
• Council on Undergraduate Research/Academic-Industrial Undergraduate Research Partnership (LB)
• Mount Holyoke College

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