Jack Halpern (1925–2018) (2018)

Jack Halpern, Louis Block Professor of Chemistry Emeritus at the University of Chicago, died on January 31, 2018 at the age of 93. Halpern was a preeminent scholar, editor, mentor, and consultant. He innovated strategies for rigorous mechanistic and thermodynamic evaluation of fundamental inorganic transformations that laid the groundwork for organometallic and bioinorganic chemistry. Homogeneous catalysis by transition-metal complexes, a field now interwoven throughout modern chemistry, is deeply rooted in Halpern's work. He developed the first molecular catalyst for the hydrogenation of olefins; ultimately this led to his elucidation of the key principles underlying the field of asymmetric catalysis. More so than any other chemist, Halpern helped to build the intellectual framework of modern catalytic science. Halpern was born in Poland and moved to Montreal at the age of four. After completing BS and PhD (with Carl Winkler) studies at McGill, Halpern took an NRC Overseas Postdoctoral Fellowship with A. G. Evans at the University of Manchester. Although trained in physical-organic methods, Halpern's first academic position was in the Department of Metallurgy and Mining at the University of British Columbia. Accordingly, he turned his attention to the chemistry of metal complexes. Beginning in 1954, Halpern published a series of papers that demonstrated heterolytic (bimolecular) and homolytic (termolecular) pathways for dihydrogen activation by aquated transition-metal ions. This work set the stage for the first catalytic hydrogenation of alkenes by homogeneous metal complexes, as reported by Halpern in 1961. And, thus, modern homogeneous catalysis science was born. In 1962, Halpern moved to the University of Chicago and the scope of his studies broadened to include many of the elementary steps—oxidative addition, reductive elimination, insertion, ligand substitution, and radical processes—that form the basis of modern organometallic chemistry. His establishment of analogies between metal and organic reactivities (later generalized as isolobal analogies by Roald Hoffmann), and determinations of metal–carbon bond thermodynamics reflected his drive to establish the fundamental principles of transition-metal reactivity. Few scientists exhibit Halpern's uncanny ability to identify the heart of a mechanistic problem and resolve it with elegant experimentation and clear logic. Halpern's definitive kinetic studies of both the overall catalytic reaction and many of the constituent elementary steps for alkene hydrogenation catalysts revealed insights, sometimes referred to as “Halpern's Rules”, that continue to guide modern studies of catalysts: kinetic studies are essential because catalysis is wholly a kinetic phenomenon, many of the true intermediates along the cycle exist only fleetingly, and those species with sufficient stability for direct observation commonly lie off-cycle (to quote Halpern “If you can isolate it, it is probably not the catalyst”). Such themes figured prominently in Halpern's seminal investigation of asymmetric hydrogenation systems. Here, mechanistic studies revealed that the more stable diastereomeric adduct of the alkene substrate with the catalyst was relatively inert toward hydrogenation; the majority of catalytic flux funneled through the spectroscopically invisible minor diastereomer. With implications well beyond asymmetric hydrogenation, this work undercut the then widely accepted “lock-and-key” paradigm of catalysis. Halpern's studies also strongly impacted the then nascent field of bioinorganic chemistry by establishing a deeper understanding of the reactivities of vitamin B12 (a result of studies of organocobalt complexes) and hydrogenase (resulting from studies of catalytic reduction of one-electron oxidants by dihydrogen). Halpern found many ways to contribute to the chemistry community: through acting as a journal editor for many years, his service as Vice President of the National Academy of Sciences, and his participation on many advisory boards. He was also a highly valued consultant, and was the recipient of numerous lectureships, fellowships, honorary doctorates, and awards. Halpern's quick mind, skill at debate, and love of scientific arguments led to many lively exchanges at seminars and conferences. Seated in the front row, Jack's hand would shoot up at a lecture's end and he would announce “I have a question and a comment”. Halpern would distill a talk to its essence, place the work in context, and identify any weak arguments. This was hugely appreciated by those who knew him well—perhaps less so by those unfamiliar with his style—but it was always a teaching moment that challenged and enriched all who were present.