Autobiography

Scientific Career Autobiography

From the time before I was old enough to enter school I have been curious about the nature of matter at a scale smaller than the eye can see. By the time I was eight years old my tools for looking deeper had progressed from a magnifying glass to a microscope to a chemistry bench. My interest in tools for exploring matter at the molecular level led naturally to my later development of photoelectron spectroscopy for probing molecular electronic structure.

As an undergraduate student at Indiana University I was drawn to physical inorganic and organometallic chemistry by the multitude of fascinating structures and properties of matter composed of elements throughout the periodic table, with a focus on the electronic structures that determine these properties. My explorations of electronic structure and bonding initially centered on theory and computations and later expanded to experimentally probing electronic structure via photoelectron spectroscopy. I was fortunate to be in the right place at the right time. First, as a graduate student at the University of Wisconsin I was fortunate to have Professor Richard F. Fenske and a research advisor and I shared an office with Michael B. Hall while he was in the midst of programming an approximate method for molecular orbital calculations that later came to be known as the Fenske-Hall method (named by Larry Dahl). Second, while I was at Wisconsin Professor Fenske and the Department obtained one of the first instruments for what was called ESCA at the time (Electron Spectroscopy for Chemical Analysis, now XPS). I developed the instrument for the study of molecules in the gas phase rather than as solids and with UV sources rather than X-ray sources (UPS), and was able to pioneer the acquisition of  detailed quantitative information on the valence electronic structures of organometallic molecules.

With photoelectron spectroscopy I showed how to experimentally ‘observe’ the metal d electron configurations, orbital overlap interactions, and electronic symmetry at the metal centers. I obtained energy measures of fundamental electronic bonding and backbonding interactions of metal carbonyls, thiocarbonyls, dinitrogen, ammonia, cyclopentadienyls, and related ligands. The research also emphasized the value of collaborative interactions in projects with Larry Dahl, Tom Whitesides, Bob Angelici, Dieter Sellmann, Steve Nelson, and Chuck Casey, to name a few. Such collaborations have been a continuing characteristic of my career, and have led to many exciting discoveries at the forefronts of chemistry with many of the most distinguished scientists of this era.

Postdoctoral Research. Following my Ph.D. research I sought to broaden my experience in other methods and in particular to gain experience in preparing new molecules with properties to advance our understanding of inorganic and organometallic electronic structure and behavior. While working with Professor Theodore L. Brown at the University of Illinois, Champaign-Urbana, I was the first to freeze out by NMR the Berry pseudorotation process in five-coordinate d⁸ metal carbonyl complexes, and showed that steric factors were dominant in determining the rate of fluxionality. In a related study I was able to show that the cis labilization of carbonyl substitution, which was counter to expectations based on electron richness at the metal center, followed from stabilization of the transition states and intermediates by π donor ligands. I also explained the stability of different structures of dicobalt octacarbonyl observed in the IR. Interestingly, these studies of dimetal carbonyl structure, fluxionality, and reaction mechanisms with donor ligands all relate to my recent investigations of the mechanisms of electrocatalytic production of hydrogen with mimics of the active sites of [FeFe]-hydrogenase enzymes.

The University of Arizona. Up to this time publications of photoelectron spectra were generally accompanied by electronic structure computations to assign and interpret the ionizations. I felt that for the technique to be truly valuable it needed to provide chemically useful information independent of computations. I took two approaches toward this goal. The first approach was to develop the instrumentation and the photoelectron experiment so that (a) we could obtain higher resolution and precision in the measure of the ionization energies, (b) we had stable photon sources of different energy to take advantage of the variable ionization cross-sections for assigning the ionizations, (c) we had advanced data analysis procedures based on the fundamental physics of the photo-ionization process to extract chemical information, and (d) we could obtain data on large and reactive molecules. For many of the systems we have investigated there is no other instrument capable of making the measurements.

The second approach to developing photoelectron spectroscopy was to build the relationships of the ionization energies themselves to thermodynamic cycles of bond energies, protonation energies, and other reaction processes and physical properties.In one sense, all chemical behavior may be viewed as the movement of electrons. An obvious example is oxidation and reduction processes, but so too is the selective making and breaking of bonds in catalysis, the transport of electrons in molecular wires, and the interactions of molecules with light. Most recently I have bridged the detailed gas-phase energy measures of photoelectron spectroscopy to the energies in the Marcus theory of electron transfer, to the electronic energies in solid-state assemblies such as light-emitting diodes, and to the redox free energies in solution related to the photoelectrocatalysis of sustainable solar fuels.

Selected Advances

• Obtained quantitative energy measures of metal-ligand electronic structure interactions and bonding fundamental to organometallic chemistry and catalysis. Studies included a wide range of metals with hydrides, halides, carbonyls, phosphines, nitrosyls, alkenes, alkynes, alkynyls, methylenes, alkylidenes, alkylidynes, agostic C-H interactions, Si-H interactions, cyclopentadienyls vs. pentadienyls, cyclopropenyl, and more.

• First to resolve vibrational fine structure in the ionizations of metal-metal, metal-carbonyl, metal-hydride, and other metal-ligand bonds. This structure quantifies the influence of the orbital electron on the metal-ligand bond distance and force constant.

• First to measure the photoelectron ionizations of C₆₀ and confirm experimentally the high electronic symmetry of the molecule.  This was recognized by The Scientist as one of the “Hot Papers”. It was the fourth most cited paper in all of the sciences in the year in which it was published, which underscores the value of the technique. This was followed by our work on the high resolution gas-phase photoelectron spectroscopy of C₆₀ and C₇₀ and by STM studies showing charge delocalization into the low-lying virtual levels of C₆₀, in which C₆₀ on gold was behaving simply as a large organometallic molecule with backbonding from the metal to the ligand π* orbitals.

• Discovered the non-Aufbau orbital filling in certain metalloporphyrins and metallophthalocyanine complexes, in which a single unpaired electron localized in a d orbital on the metal is much more strongly bound than several electron pairs in orbitals on the ligands. Despite an enormous amount of prior research on metalloporphyrins, these energy relationships had not been recognized previously.

• Experimentally demonstrated principles (with J. H. Enemark) of the thiolate buffering effect and the enedithiolate fold angle modulation effect on the redox chemistry of Mo-S and Fe-S metalloenzymes with the aid of organometallic model complexes.

• Developed a method to experimentally measure small inner-sphere electron reorganization energies (with H. B. Gray and others) from analysis of the shape of the first ionization band in high-resolution gas-phase photoelectron spectroscopy, and applied the method to electron transfer processes of both metalloporphyrins and organic electronic devices.

• Made numerous contributions to the understanding of metal-metal and metal-ligand multiple bonding (many with F. A. Cotton and M. H. Chisholm) including measure of the lowest ionization energy ever recorded for a laboratory-prepared molecule, which is substantially lower than that of any atom in the periodic table. This discovery was selected by C&E News as one of the highlights in chemistry in 2002, and the discovery received substantial coverage by the press.

• Explored the synthesis, characterization, kinetics and mechanisms of some of the most effective electrocatalysts for production of hydrogen from weak acids, including water.

• Developed the relationship, both experimentally and theoretically, between gas-phase ionization energies, condensed phase ionization energies, and solution oxidation potentials, and showed that in many important cases, ranging from organometallic metal-phosphine complexes to organic electronic devices, that the properties built into molecules by chemical substituents can be reversed in the condensed and solution phases.

• First to find fluxionality in an iron-sulfur cluster core and showed how this fluxionality is critical to understanding the reduction of protons to hydrogen by organometallic mimics of the active sites of hydrogenases. 

Teaching and Mentoring. Classroom teaching has spanned honors freshman chemistry, senior level inorganic chemistry and inorganic preparations, and graduate level advanced inorganic chemistry, physical inorganic chemistry, organometallic chemistry, and computational chemistry. I have always enjoyed teaching, and the class evaluations have been uniformly high. In 1994 I was awarded the Faculty of Science Distinguished Teaching Award at the University of Arizona, and in 2008 I was awarded the University Graduate and Professional Education Teaching and Mentoring Award. In addition to mentoring over 60 graduate and postdoctoral research associates, I have provided research experience opportunities to undergraduate students, with over 100 undergraduates having worked in my laboratories for periods from several weeks to several semesters. In the summer I provide research experiences to high school teachers and students, most recently to high school teachers in schools with high Native American student populations. The participants have covered a wide range of diversity in underrepresented groups in science.

Professional Service. I have contributed in all areas in the Department, including Department Head from 1994-2002. In the American Chemical Society, I have served in many capacities at the local and national level, including Chair of the Organometallic subdivision of the Inorganic Division, and as an associate editor of the ACS journal Organometallics.