Electron Transfer Reactions - ACS Publications - American Chemical

Henry Taube, 1983 Nobel Laureate

Downloaded by 80.82.77.83 on May 26, 2018 | https://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.pr001

H E N R Y T A U B E WAS B O R N O N N O V E M B E R 30,1915, in

Neudorf, Saskatchewan, Canada. H e received his education at the University of Saskatchewan (B.S., 1935; M.S., 1937) and at the University of California at Berkeley (Ph.D., 1940). He became a naturalized U.S. citizen in 1941. He served as an instructor at Berkeley from 1940 to 1941; as an instructor and assistant pro -fessor at Cornell University from 1941 to 1946; as assistant, associate, and full professor at the University of Chicago from 1946 to 1961; and as professor of chemistry at Stanford University from 1961 to the pre -sent. He served as department chairman at Chicago from 1955 to 1959 and as chairman at Stanford from 1972 to 1974. Taube's major awards include the Nobel Prize in chemistry in 1983 and the Priestley Medal, the highest award of the American Chemical Society, in 1985. Taube is a physical inorganic chemist whose contributions have been in the discovery of new chemical reactions and the study of their mechanistic pathways. Taube's professional career began at Cornell in the years from 1941 to 1946. During that time, his research continued in the same areas as during his graduate student investigations—that is, redox reactions of oxygen- and halo -gen-containingoxidizing agents. (Interestingly, during this period and up until 1950, Taube had very few publications concerned with transition metal chem -istry. However, that would soon change.) In studies of aqueous solutions of chlorate, Taube applied oxygen-atom-labeling studies, using oxygen-18 and radioactive chlorine to study the mechanisms of the redox reactions of oxochlo -rine species and related molecules in aqueous solutions. In 1955 Taube received the American Chemical Society Award for Nuclear Applications i n Chemistry in recognition of his perceptive applications of isotopic labeling techniques. (Excerpted from Nobel Laureates in Chemistry 1901-1992, pp. 660-666, © 1993 American Chemi-calSociety and the Chemical Heritage Foundation. With permission.) xi

Isied; Electron Transfer Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1997.


In the area of the inorganic chemistry of metal ions, the contributions of Alfred Werner, leading to an understanding of the composition and structure of coordination complexes, are well known. Although a great deal was known about the reactivity of coordination complexes by 1950, it remained for Henry Taube to place reactivity into a unified concept based on the electronic nature of the metal and on the influence of the ligands around the metal center. In many ways, Taube is the leading figure in modern inorganic chemistry, because he laid the groundwork for many conceptual advancements and probed various aspects of chemical reactivity with new techniques. The wide-ranging contributions of Henry Taube to the advancement of chemistry are apparent from just a casual survey of the areas in which he has published. In addition to major contributions in classical substitution and redox chemistry of transition metal complexes of Cr, Co, Ru, and Os, Taube's publications include papers on (1) main group nonmetal ions and molecules, particularly oxygen compounds and chlorine oxide compounds; (2) organic acids and amino acids; (3) rare earth compounds; (4) main group metal com plexes; (5) sulfur ligands i n coordination complexes; (6) organometallic com pounds of transition metals. The impressive aspect of these studies is the way in which Taube universally applies the fundamental concepts and techniques that he so masterfully developed and refined. The greatness of a scientist is not measured only by the number (more than 600) and significance of his publications. A n important parameter i n the measure of a scientist is the overall impact of his "school" on the scientific community. Taube associates (students, postdocs, visiting faculty, and coau thors) make an impressive list of scientists who have made tremendous contri butions i n their own right. O f over 250 associates, nearly half have served as faculty on university campuses throughout the United States and the world. The Taube school is represented at places such as Cornell, Iowa State, George town, the State University of New York at Stony Brook, Texas Tech, Georgia Tech, Michigan State, Rutgers, Boston, North Carolina, Indiana, MIT, Rice, and Pittsburgh, to name a few. Over a dozen Taube associates are prominent scientists i n foreign countries. A similar number hold positions at national research laboratories such as Los Alamos, Brookhaven, and Argonne. A d d to this the number of associates in major industrial laboratories and the influence of Henry Taube on the scientific community is indeed significant. Henry Taube's work was celebrated and honored in a symposium at a national meeting of the American Chemical Society (in Las Vegas, Nevada, 1982) and in Volume 30 of Progress in Inorganic Chemistry, entitled "An Appre ciation of Henry Taube". The impact of Henry Taube is reflected in the promi nence of the science and the scientists who contributed to these celebrations. O n a personal level, Henry Taube has been characterized as a modest, unpretentious scientist who generously credits others for inspiration and accomplishments. H e is greatly respected by his students and colleagues for his guidance and inspiration to approach chemical problems from a critical but xii



rational viewpoint. The success of the Taube school is due not so much to their training in chemistry as to the infectious enthusiasm and creative approach to chemistry that Taube has passed on to his colleagues. Taube associates transmit a sense of high esteem and appreciation for Taube in all references to their pro fessional and personal associations with him. A reflection of Taube's generosity and respect for others may be best rep resented by his choice of topics for his Priestley Medal address. On that occa sion, he honored William C. Bray, his Ph.D. mentor, four decades after Bray's death. The warmth, inspiration, and generosity of Bray was obviously a strong influence on Taube, who is surely gratified to have earned similar praise from his students. Taube is an emeritus professor at Stanford and he continues to investigate and inspire the development of new ideas and new areas of inorganic chem istry that are, to him, an extension of his original ideas on chemical reactivity and mechanisms. H e continues to publish on organometallic osmium chem istry, osmium dihydrogen complexes, and a variety of other new reactions with the same enthusiasm that many of his associates have witnessed throughout his career. His immediate family consists of his wife, Mary; two daughters, Linda and Marianna, and two sons, Heinrich and Karl. H e enjoys listening to old opera records from his vast collection and is said to have a passion for garden ing and sour mash whiskey. The scientific contributions of Henry Taube have inspired revolutionary advancements i n inorganic reaction mechanisms. His direct achievements have laid much of the groundwork, and his associates have built on that to define much of what we know as inorganic reaction chemistry. A simple state ment from the nomination papers for the Nobel Prize may be the most appro priate way to end. "Henry Taube founded the modern study of inorganic reac tion mechanisms." JERRY WALSH

University of North Carolina at Greensboro

Bibliography "An Appreciation of Henry Taube"; Lippard, S. J., Ed.; Prog. Inorg. Chem. 1983,30. Gray, H . B.; Collman, J. P. "The 1983 Nobel Prize in Chemistry Science 1983, 222, 986-987. Taube, H . "Electron Transfer between Metal Complexes: Retrospective"; Science 1984, 226, 1028-1036. Taube, H . Electron Transfer Reactions of Complex Ions in Solution; Academic Press: New York, 1970. Taube, H . "Rates and Mechanisms of Substitution in Inorganic Complexes in Solution"; Chem. Rev. 1952,50, 69-126. ,,

;

xiii





The Taube Revolution, 1952-1954 ONE DAY, 40 YEARS A G O , I was sitting in the science library, browsing through recent issues of the Journal of the American Chemical Society, search ing in vain for something that was both new and interesting in inorganic coor dination chemistry. For the nth time, I was tempted to abandon my Ph.D. the sis and turn to more promising fields, such as organic chemistry or biochemistry. I sat there, deploring the sad story of classical coordination chemistry: it was created by Alfred Werner in a " B i g Bang" in 1893 (I) and flourished for a quarter of a century. Although Werner was not the only con tributor to the field he created, he was by far the most important and influen tial one. The decline of classical coordination chemistry started when Werner's i l l ness forced him to abandon his research around 1915. His premature death in 1919 marked the end of the Classical Age of coordination chemistry and the beginning of its Middle Ages, or rather its Dark Age. Very little important work was accomplished in the three decades that followed. This was in remarkable contrast to organic chemistry and biochemistry, which flourished during the same period: synthetic polymers, kinetics and mechanisms of organic substitu tion reactions, and the discovery of biochemical cycles, vitamins, and antibi otics were some of the highlights of this period. Meanwhile, inorganic coordination chemistry was in a state of deep hiber nation. Even Pauling's valence bond theory, which furnished new insight and understanding of the nature of chemical bonds of coordination compounds and of their physical properties, failed to awaken it. Engulfed by these depressing thoughts, my eyes focused on a paper enti tled "Evidence for a Bridged Activated Complex for Electron Transfer Reac tions" (1954) (2). I was not expecting anything unusual, because I had missed the earlier communication to the Editor (1953) (3) and the review on "Rates and Mechanism of Substitution Reactions in Inorganic Complexes in Solution" (1952) (4). As soon as I started reading, my pulse accelerated with excitement, and my eyes raced from one line to the other, trying to swallow it all i n one gulp. When I reached the end, literally out of breath, I rushed to the shelves of

xv



Chemical Reviews to find the 1952 review {4) by Henry Taube. As I finished reading it, late that evening, I knew that the Dark Age of inorganic chemistry had come to an end, and its Renaissance had just begun. The shift of interest from the traditional topics of coordination chemistry toward the mechanism of its reactions in aqueous solutions was one of the main features of this Renaissance. The first results were, however, disappoint ing. Attempts to repeat the success story of Ingold's "English Heresy" of the 1930s by applying it to the far more complicated situation existing in aqueous solutions of inorganic reagents had only very limited success (5). The first step forward was made when Taube, in his definitive review (4), created order in this primordial field by establishing the dichotomy between inert and labile complexes. The confusion between thermodynamic and kinetic stability of coordination compounds had plagued coordination chemistry for decades. No real advance in the field of inorganic reaction mechanism could be achieved until this confusion had been cleared. Although ligand substitu tion reactions was the main subject of this review, its greatest impact was else where—on the study of the mechanism of redox reactions. It would seem that to investigate the mechanism of substitution reactions of aqua ions was compli cated enough—in the early 1950s almost hopelessly complicated. To try and elucidate the mechanism of electron transfer processes between these i l l defined, labile, and elusive species, before they themselves were fully charac terized, would seem to be almost foolhardy. Fortunately, Taube's intuition and vision drove him in this direction. His choice of the prototypical reaction Co(NH ) Cl 3

5

2 +

+ Cr> + 5 H -> C o +

+

2 +

+ 5NH

4

+

+ CrCl

2 +

for gaining insight into the mechanism of a redox reaction between two metal ions was a stroke of genius. It was a carefully planned experiment devised to drive nature into a corner that left it no choice but to confirm the suspicion that electron transfer can take place through a ligand bonded directly to both the reducing and the oxidizing metal ion. Often, in the history of science, important theories or experiments were proposed simultaneously and independently by two scientists because they were based on new findings that had just been made. In the case of Taube, the opposite happened. The stage was set for his revolutionary work long before it was carried out. The experimental bases, including the crucial rate and equi librium data, were known for at least 20 years and most of them for more than 30 years. The rate and equilibrium data of the reaction between the chromic aqua ion and the chloride ion were known since the work of Niels Bjerrum, at the beginning of the century (6). The distinction between inert and labile com plexes could have been made at any time after that and probably was, subcon sciously, in the minds of many coordination chemists. Some even used the term "robust" (instead of "inert") and were obviously quite familiar with this xvi


distinction, but no one realized its importance. Some of the techniques used by Taube, such as the use of O and of radioactive chloride, were not available in the 1920s or 1930s, but he used them only to reconfirm the results of product analysis of the reaction between the cobaltic and chromous ions. The latter were clear and unequivocal, even without this reconfirmation. In other words, a coordination chemist of the intellectual stature of Henry Taube could have done this work a whole generation earlier. There was none. l s

MICHAEL ARDON

Hebrew University of Jerusalem


References 1. 2. 3. 4. 5. 6.

Werner, A. Z. Anorg. Chem. 1893, 3, 279. Taube, H.; Myers,H.J.Am. Chem. Soc. 1954, 76, 2103. Taube, H.; Myers, H.; Rich, R. L. J. Am. Chem. Soc. 1953, 75, 4118. Taube, H. Chem. Rev. 1952, 50, 69. Ingold, C. K., et al. J. Chem. Soc. 1953, 2674, 2696. Bjerrum, N. Z. Phys. Chem. 1907, 59, 339, 581.

xvii




Preface


H E N R Y T A U B E ' S I D E A S H A V E H A D A P R O F O U N D I M P A C T on the development

of inorganic chemistry i n the second half of the 20th century. H e began his research studying the chemistry and photochemistry of nonmetallic oxidants such as ozone, hydrogen peroxide, and halogens and their reactions with a variety of inorganic and organic species. Throughout these studies, his early insights into mechanistic questions were very apparent. In 1952 he published a classic series of papers and a review summarizing his ideas on the rates and mechanisms of substitution and electron transfer reactions of complex ions i n solution. Taube's definitions of inert and labile metal ions in terms of valence bond theory are of fundamental and historical importance in relating electronic structure theory to the reactivity of metal ions i n solution. This relationship constitutes one of the foundations of mechanistic inorganic chemistry and pro -videdinorganic chemists with the concepts and methodologies needed to pur -sueand advance the field to its present state. His elegant experiments in the use of radioisotopes and N M R methods to assign hydration numbers to water molecules around a metal ion in solution and to determine the rate of water exchange constitute the experimental basis for studying the reactions of these species in solution. In his research on charge transfer complexes, Taube was able to describe metal-ligand bonds in terms of simple molecular orbital language. As a result of this work the new field of mixed-valence compounds developed into a new area for spectroscopic and kinetic investigations. Ligands with σ-donor, π-donor, and π-acceptor properties are now used to dissect and rationalize the affinity and dynamics of metal-ligand interactions. His recent experiments in the area of organometallic osmium ammines have resulted in reuniting the divergent fields of classical coordination chemistry and organometallic chemistry. This research showed that simple ammine complexes of osmium exhibit many of the reactivity characteristics of organometallic complexes with phosphine, arene, and cyclopentadienyl-type ligands. From his experiments in this area, powerful predictions on the reactivity of organic fragments in the coordination sphere of osmium ammines can be made and put to use in organic synthesis. Taube's ideas on intramolecular electron transfer i n extended binuclear complexes set the stage for the more recent electron transfer studies i n pepxix



tides, proteins, and other more complex biomolecules. Overall, Taube's inte grated approach of experimentation on the kinetics, equilibria, and reactivity of complex ions in solution is a continuing theme throughout his studies and constitutes a foundation for collecting and understanding new facts about inor ganic systems. In March of 1995, the year of his 80th birthday, Taube's students, friends, collaborators, and colleagues from around the world assembled at the Stanford University Chemistry Department for a two-day symposium to honor Taube and to remember the contributions he has made to inorganic chemistry and to their research. Only a limited number of topics influenced by Taube's research could be included in the two-day symposium. However, the participants at the symposium found the experience very rewarding and enjoyable and had a chance to review some of the diverse areas of chemistry that have been affected by Taube's ideas and insights. The symposium included lectures in the area of electron transfer and mixed-valence chemistry, mechanistic inorganic chemistry and photochemistry, organometallic chemistry and catalysis, and electron transfer and the use of transition metal ions in biological systems. This book presents many of the symposium lectures, emphasizing the common themes of Taube's insights that have led to many advances in these areas. Many readers will find these underlying Taube themes applicable to their research interests. The book begins with a chapter by Taube, a historical perspective of his work on the study of electron transfer reactions and the effects of π-backbonding on the reactivity of metal complexes. In the first sec tion of the book (Chapters 2-7), the theory, mechanisms, and applications of Taube's work in inorganic synthesis organic oxidation, polymerization catalysis, zeolite matrices, and catalytic peroxide reactions are discussed. In the second section of the book (Chapters 8-18), the various aspects of electron transfer mechanisms of cobalt macrocycles, barriers to atom transfer reactions, nuclear factors in main-group electron transfer reactions, redox reac tions of mixed-valence ions, electron derealization in disulfide ligands, reac tion of cysteines with aquo iron(III) species, design of oscillating reactions, and hydrolysis of coordinated nitriles are discussed. The area of inorganic photo chemistry is represented by three chapters (11-13) on the calculation of the rate of nonradiative decay of metal-to-ligand charge transfer ( M L C T ) from spectra, ligand-induced relaxation of excited states in chromium(III) com plexes, and time-resolved studies of migratory insertions in manganese carbonyl compounds. In the third section of the book (Chapters 19-24), Taube's influence on the areas of long-range electron transfer reactions and metal ions in biology is pre sented. Chapters on electron transfer and the entatic state and electron trans fer across protein and peptide networks demonstrate how Taube's ideas have been extended to macromolecular systems. Two chapters discuss the use of ruthenium complexes as D N A probes and potential drugs, showing how the wealth of information on the mechanism and reactions of ruthenium complexes XX


can lead to rational drug design. The last two chapters discuss the role of inor ganic chemistry in cellular mechanisms of host resistance to disease and appli cations to diagnostic imaging reagents in nuclear medicine. In this book the authors highlight the influence of Taube insights on their research. Whether in the area of inorganic chemistry, organometallic chem istry, or biology, the underlying themes of kinetics and mechanisms, and bind ing and affinity, as they affect reactivity provide a foundation for continuing research and discovery. These principles will continue to be the engine that generates new knowledge and opens new areas of research and applications for inorganic reactions.


S T E P H A N S. I S I E D

Department of Chemistry Rutgers, The State University of New Jersey P.O. Box 939 Piscataway, NJ 08550

xxi


Acknowledgments


W Ε G R A T E F U L L Y A C K N O W L E D G E the following organizations: Air Products, B A S E BioMetailics, Catalytica, Mallinckrodt, Miles, and the Stanford Univer -sityDepartments of Chemistry and Chemical Engineering, for their participa -tion and financial contributions to the Henry Taube Symposium and to this book.

xxii


Electron Transfer Reactions - ACS Publications - American Chemical

Recommend Documents