BridgeForward Essay

Jonathan Dabbs

At the University of Virginia, a black-and-white photo of Dr. Henry Taube hangs in Dr. W. Dean Harman’s lab. Harman repeatedly emphasizes that this photo is both a tribute to his late advisor and an acknowledgement that the ongoing research conducted in the Harman lab rests upon the shoulders of the work carried out by Taube.

The primary research interest of the Harman Lab is accessing novel chemical space and synthesizing topologically-complex small-molecules that are rich in stereocenters. This is done by the dihapto (η²) coordination of various aromatic compounds stoichiometrically to a π-basic transition metal complex.¹ Once η² bound, the aromatic molecule is then activated for highly regio- and stereoselective tandem electrophilic/nucleophilic additions across the remaining uncoordinated olefins. Libraries of these highly functionalized small molecules are then assembled after oxidatively liberating them from the metal.

The keystone of this research is the η² bond, which consists of a σ-donor and π-acceptor qualities of aromatic ligands. The π* orbital of the aromatic ligand accepts π-electron density backbonded from the π-symmetric t_2g orbitals of the d⁶ W(0) metal center thus forming a metallacyclopropane-like structure between the metal and the coordinated olefin.

The first reported instance of an aromatic molecule η²-bound to a metal complex was reported by both Harman and Taube.² At the time, Taube’s research focus was on electron transfer (ET) between transition metals, work he was famously awarded the Nobel Prize for in 1983.³ After investigating ET between pentaamine-Co³⁺ complexes bridged to Cr²⁺ and proposing concepts such as “Inner-Sphere” and “Outer-Sphere” mechanisms,^4,5  Taube began examining ET with pentaamine-Ru³⁺ and pentaamine-Os³⁺ complexes.^6–8  These complexes, which were chosen due to their π-symmetric acceptor orbitals, displayed acute backbonding. The pentaamine-Os complex backbonds so effectively that it was serendipitously discovered to dearomatize benzene and other aromatic molecules through an η² bond.

Taube shifted his research focus to investigating ET between inorganic complexes after preparing to teach a course by reading about transition metal cations at the University of Chicago. Before this, he examined oxidation-reduction reactions with a strong focus on reactive oxygen species (ozone, peroxides, oxygen radicals, etc.).^9–11 This body of work stemmed from his doctoral work at the University of California at Berkeley with his advisor, Dr. William C. Bray.

The two Canadian chemists had an excellent rapport. After winning the 1985 ACS Priestly Medal, Taube dedicated his speech to his late advisor in which he hailed Bray as the “herald of inorganic reaction kinetics.”¹² In 1940, Taube and Bray published a noteworthy paper examining the effects of halogen ions on the rates of ozone and peroxide reactions, which likely jumpstarted Taube’s initial research interests. ¹³

While conducting impactful research on main group reaction kinetics and catalysis,^15–17 Bray also transformed chemical education with his textbook A Course in General Chemistry and a strong belief in the importance of an active-learning based laboratory course. He recognized that research and teaching are both critical not only students but also professors in advancing chemical understanding. Taube, who stated this was Bray’s greatest contribution to the field, himself greatly benefitted from teaching a course that led to his Nobel-winning research. Keeping with this idea, Harman continues to teach general chemistry at Virginia where has won numerous faculty teaching awards.

The careers of Harman, Taube, and Bray demonstrate the power of an advanced understanding of electron movement. Despite having various research interests and applications ranging from developing druglike small-molecule libraries or inventing a catalyst for oxidizing carbon monoxide during World War I (as Bray did), the common thread is a firm grasp of the behavior of redox reactions in main group and transition metals.

References

(1) Liebov, B. K.; Harman, W. D. Group 6 Dihapto-Coordinate Dearomatization Agents for Organic Synthesis. Chemical Reviews. American Chemical Society November 22, 2017, pp 13721–13755. https://doi.org/10.1021/acs.chemrev.7b00480.

(2) Harman, W. D.; Taube, H. Reactivity of Pentaannnineosmium(II) with Benzene. J. Am. Chem. Soc. 1987, 109 (6), 1883–1885. https://doi.org/10.1021/ja00240a061.

(3) Henry Taube – Nobel Lecture https://www.nobelprize.org/prizes/chemistry/1983/taube/lecture/ (accessed Nov 29, 2020).

(4) Taube, H.; Myers, H. Evidence for a Bridged Activated Complex for Electron Transfer Reactions. J. Am. Chem. Soc. 1954, 76 (8), 2103–2111. https://doi.org/10.1021/ja01637a020.

(5) Taube, H.; Myers, H.; Rich, R. L. Observations on the Mechanism of Electron Transfer in Solution. Journal of the American Chemical Society. American Chemical Society August 1, 1953, pp 4118–4119. https://doi.org/10.1021/ja01112a546.

(6) Endicott, J. F.; Taube, H. Studies on Oxidation-Reduction Reactions of Ruthenium Ammines. Inorg. Chem. 1965, 4 (4), 437–445. https://doi.org/10.1021/ic50026a001.

(7) Stritar, J. A.; Taube, H. Electron-Transfer Reactions of Ruthenium(III) Pentaammines with Chromium(II), Vanadium(II), and Europium(II). Inorg. Chem. 1969, 8 (11), 2281–2292. https://doi.org/10.1021/ic50081a013.

(8) Magnuson, R. H.; Taube, H. Synthesis and Properties of Osmium(II) and Osmium(III) Ammine Complexes of Aromatic Nitrogen Heterocyclics. J. Am. Chem. Soc. 1975, 97 (18), 5129–5136. https://doi.org/10.1021/ja00851a018.

(9) Espenson, J. H.; Taube, H. Tracer Experiments with Ozone as Oxidizing Agent in Aqueous Solution. Inorg. Chem. 1965, 4 (5), 704–709. https://doi.org/10.1021/ic50027a023.

(10) Halperin, J.; Taube, H. The Transfer of Oxygen Atoms in Oxidation-Reduction Reactions. IV. The Reaction of Hydrogen Peroxide with Sulfite and Thiosulfate, and of Oxygen, Manganese Dioxide and of Permanganate with Sulfite. J. Am. Chem. Soc. 1952, 74 (2), 380–382. https://doi.org/10.1021/ja01122a027.

(11) Forchheimer, O. L.; Taube, H. Evidence for the Exchange of Hydroxyl Radical with Water. Journal of the American Chemical Society. UTC July 1, 1952, pp 3705–3706. https://doi.org/10.1021/ja01134a518.

(12) Taube, H. William C. Bray—Teacher and Herald of Inorganic Reaction Kinetics. Chem. Eng. News 1985, 63 (18), 40–45. https://doi.org/10.1021/cen-v063n018.p040.

(13) Taube, H.; Bray, W. C. Chain Reactions in Aqueous Solutions Containing Ozone, Hydrogen Peroxide and Acid. J. Am. Chem. Soc. 1940, 62 (12), 3357–3373. https://doi.org/10.1021/ja01869a027.

(14) Cervellati, R.; Greco, E. Periodic Reactions: The Early Works of William C. Bray and Alfred J. Lotka. J. Chem. Educ. 2017, 94 (2), 195–201. https://doi.org/10.1021/acs.jchemed.6b00342.

(15) Bray, W. C. A Periodic Reaction in Homogeneous Solution and Its Relation to Catalysis. J. Am. Chem. Soc. 1921, 43 (6), 1262–1267. https://doi.org/10.1021/ja01439a007.

(16) Hershey, A. V; Bray, W. C.; 58, V.; Brav, W. C. Kinetic and Equilibrium Measurements of the Reaction 2Fe+++ + 2I- = 2Fe++ + I2 . J. Am. Chem. Soc 1936, 58, 1760–1772.

(17) Bray, W. C.; Livingston, R. S. The Catalytic Decomposition of Hydrogen Peroxide in a Bromine-Bromide Solution, and a Study of the Steady State. J. Am. Chem. Soc 1923, 45