Norman Sutin, Founding Editor of Comments on Inorganic Chemistry: A Remembrance and Tribute

Norman Sutin, a distinguished inorganic chemist who studied electron and charge-transfer reactions, died on January 31, 2022, at the age of 93. He was the founding editor of Comments on Inorganic Chemistry. Sutin pioneered the use of transition-metal complexes to study ground and excited-state reactions and to identify outer-sphere electron transfer reactions from other types of reactions. He was a master at using experiments to test theoretical descriptions of electron-transfer reactions, Figure 1. Sutin was born in Ceres, South Africa, on September 16, 1928. He was the third of three children and grew up in Paarl, an area northeast of Cape Town. In his late teens, he became interested in mysticism and meditation. During one meditation, he focused on how a candle burns. He believed this experience sparked his interest in how the exchange of different forms of energy can control chemical reactions. His initial work considered how energy released by radioactive decay controlled subsequent chemical reactions. Later, he studied how the thermal energy of the reactants controls rates of electron transfer between them, and then how absorption of light can provide the energy to drive a chemical reaction. Sutin received a bachelor’s degree with distinctions in Physics and Chemistry from the University of Cape Town (UCT) in 1948. He continued at UCT and received a master’s degree. He earned his doctorate in 1953 from Trinity College, Cambridge, in Alfred G. Maddock’s Radiochemistry group. He became friends with Dr. Garman Harbottle, a visiting scientist from Brookhaven National Laboratory (BNL), US. After a postdoc at Durham University, he moved to the United States and joined BNL as a Research Associate. He imagined Brookhaven as a bucolic area with green fields and

a rural lifestyle and was surprised to find that BNL was a discarded WW II Army base that had served as a prisoner of war camp. Despite his disappointment, he formed close friendships and spent the rest of his career at Brookhaven. He retired in 2002 but continued working for another 10 years.
At BNL Sutin became interested in inorganic reaction chemistry, both ligand exchange and electron transfer. He collaborated with the Chemistry Department Chairman, Richard W. Dodson, who had recently shown that the Fe(aq) 2+/3+ electron self-exchange reaction was much slower than many expected and could be easily measured by isotopic labeling. This work resulted in visits by Rudy A. Marcus to BNL for summers. Sutin and Marcus developed a deep friendship and a long-term collaboration. [4,5] Sutin quickly used Marcus' early theory of electron transfer reactions to interpret rates he was measuring for inorganic-metal complexes. Because the theory predicted a simple dependence of activation energy on driving force of the reaction, Sutin recognized that transition-metal coordination complexes were ideal candidates to test the theory. In 1960, Ford-Smith and Sutin used a series of iron complexes of substituted 2,2'-bipyridine and 1,10-phenanthroline ligands to experimentally determine the relationship between activation energy and driving force. [6] When plotted, their data were linear with a slope close to that predicted by theory ( Figure 2). The early theory ignored the contribution of inner-sphere reorganization energy. In another publication submitted that year, Sutin discussed the effect of inner-sphere reorganization energy on the rate constants. [7] Sutin also tested the prediction that rate constants for electron-transfer cross-reactions between two different complexes were equal to the square root of the product of the rate constants for the two self-exchange reactions and the equilibrium constant of the cross reaction. [8]  In a paper in the Annual Review of Nuclear Energy (1962), Sutin surveyed the field of electron-transfer reactions and put the experimental results in context with the developing theories of Marcus and Hush. [9] His description of the theory made it accessible to inorganic chemists. Using a simple formula, he calculated the inner-sphere contribution to the reorganization energy and showed that in many cases it was larger than the outer-sphere contribution. The review was pivotal in allowing inorganic chemists to employ the new theoretical models. Applications revealed that the models worked well for rates of outer-sphere reactions. Sutin followed these initial studies with a series of papers showing that careful choice of metal complexes allowed highlighting the effects of either the inner-sphere or outer-sphere reorganization energies. [10][11][12][13][14] He illustrated how dependence of rate constants on reorganization energies and driving force give insight into mechanisms for the reactions. He also developed methods to identify and characterize inner-sphere reactions. Sutin explored the implications of the classical Marcus model. [4,[15][16][17][18][19][20] and made particular use of the parabolic energy surfaces to understand both ground and excited state electron-transfer reactions, (Figure 3). [16,[21][22][23][24][25] From 1958 to 1962, Sutin worked parttime at Rockefeller University where he learned about metalloproteins. In 1961 he published pioneering papers on outer-sphere oxidation of ferro-hemoglobin and ferro-cytochrome-c by ferricyanide showing how electron-transfer reactions can play important roles in biological systems. [26,27] The results showed that biological molecules could be viewed as simple metal coordination complexes. Moreover, these studies were among the first to show that reactions between inorganic metal complexes and metals in biological systems can provide insights into how the biological systems work in vivo. [28][29][30][31][32] In particular, the conjugated porphyrin ring of cytochrome-c was shown to efficiently mediate electron transfer between the edge of the heme to the centrally located iron. Unfortunately, a few years later, the work with biological molecules ended when the Atomic Energy Commission, the funding agency for BNL, decided that biological systems were outside of its mission.
Sutin realized that the theories of electron-transfer reactions could be applied to reactions that involve a transfer between two different electronic surfaces such as the transitions between excited states or between different spin states. [33] In 1974, Sutin turned his attention to applying the techniques he had developed for thermal electron-transfer reactions to excited state reactions of transition metals. The idea had been established that excited states can be quenched by electron-transfer. For inorganic-metal complexes, however, there was no way to experimentally show that quenching occurred by electron-transfer, especially when the initially formed products could back react. Sutin showed that the dependence of quenching rates for the Ru(2,2'- bipyridine) 3 Ru(II) complex by Co(III) and Ru(III) complexes could be interpreted using the same type of formalism as for ground state electron-transfer reactions and that the Ru(bpy) 3 2+ excited state can undergo oxidative quenching. [34] The dependence of the logarithm of the rate constant on driving force for electron transfer allowed one to assign quenching mechanisms. The initial study was followed by papers probing excited state reactions of different transition-metal complexes. [35][36][37][38][39] BNL colleague Carol Creutz and Sutin showed that ruthenium bipyridine type complexes can undergo both oxidative and reductive quenching. [37] The formalism he developed for ground state electron transfer reactions worked well for long-lived excited states of many metal complexes. This made it possible to systematically correlate and predict excited state reactions.
The ability to distinguish quenching mechanisms made it possible to predict which inorganic complexes had excited states that would undergo facile electron-transfer reactions. Many inorganic metal complexes that had lifetimes longer than a few hundred picoseconds have been studied and the reaction mechanisms with various species assigned. With the increasing understanding of excited-state electron-transfer reactions, it became evident that they could be used to drive reactions "uphill" (i.e., reactions with ΔG 0 > 0 for reactants and products in their ground state). Thus, excited-state reactions can be used to convert the electronic energy of an excited state into chemical bonds. This led to the idea of using a reaction scheme initiated by absorption of sunlight to create a solar conversion device. [40] Sutin and his group developed schemes to understand how photoexcited molecules can be used to initiate a series of electron-transfers and other reactions to produce H 2 and/or O 2 from water. [22] This led to an understanding of the complexities in developing a homogeneous solution device for splitting water. [41] While the work on photoexcited molecules has not yet resulted in a deployable water splitting device, it has initiated a new field of organic synthesis. Understanding ways to use the electron-transfer properties of photoexcited transition-metals to catalyze organic reactions led to the field of organic photoredox chemistry. In this area, a transition-metal complex is photoexcited to initiate an electron-transfer to an organic species that leads to a series of reactions. This results in the catalytic activation of organic molecules to drive "uphill" synthetic reactions. This field has shown extensive growth in recent years with almost 500 citations in a 2022 review. [42] Along with studies of reactions of excited transition metals in homogeneous solution, Clark and Sutin (1977) were the first to report the heterogeneous sensitization of TiO 2 by a ruthenium(II) bipyridine type complex. [43] They showed that for some complexes the quantum efficiency for injection of an electron from an excited ruthenium complex into the TiO 2 conduction band was high. This was the beginning of TiO 2 sensitization schemes and an avalanche of studies on TiO 2 sensitization for splitting water that continues today.
Personally, Norman Sutin was very private and shy; however, once he got to know someone, he was extremely caring and warm. He loved to sit and discuss science and how to design experiments to test theoretical predictions. He insisted on doing careful experiments and writing precise scientific papers. He helped other researchers do better science and always offered his ideas with humility. In his work, he focused on the physics of electron transfers and one of his colleagues jested that he was a leading exponent of "parabolalogy" (i.e., use of parabolas to explain chemistry). A full list of the scientific publications of Norman Sutin is given in the Supplementary Material.
Norman's personality facilitated development of many close, long lasting professional and personal relationships. An autobiographical review he wrote in 2007 for J. Phys. Chem. B mentions over 60 coauthors and collaborators. [2] The collaborative culture between scientists at BNL, which extended to visitors, encouraged easy and open communication, and led to an integration of experiments with theory. Norman's close friendships with theoreticians Noel Hush and Rudy Marcus were significant. In 1985, Sutin and Marcus published a review in Biochimica et Biophysica Acta that has been cited over 7,500 times. [5] When Marcus won the Nobel Prize in 1992, he invited Norman to attend the ceremonies, including the Nobel dinner. Important extended collaborations within the Brookhaven community included those with Carol Creutz, Etsuko Fujita, Mai Chou, Marshall Newton, Harold Schwarz, and Bruce Brunschwig. Notably, none of the groups were large (Figures 4-7), thus illustrating the power of close collaboration.
Norman's personality shone beyond his professional life. He liked to entertain young children with card and magic tricks. He had a house in Bellport NY that overlooked the great South Bay and in summer often hosted parties on July 4 when Grucci Fireworks were set off on the town dock in front of his house. He loved to garden. He was the nephew of the painter Soutine and had prints of Soutine's work in his home. (It was suggested that there was a familial resemblance between them.) He also frequently hosted informal visitors to his house where one could sit on his screened porch and talk while watching the boats coming and going from the town dock. Doug Turner and Bruce Brunschwig got to know Norman when they were visitors to his laboratory during formative times in their lives. Doug came to BNL in 1970, as a 23-year-old graduate student in George Flynn's lab at Columbia University to start a thesis project. [44][45][46] Bruce, a junior faculty member at Hofstra University, arrived in 1974 to work summers. In both cases, Norman helped set the course of the rest of their scientific lives. For Doug, his thesis project and personal life had not been progressing. For Bruce, Norman rekindled his love of scientific research. Through extended personal    conversations, often at his home, Norman encouraged and inspired them. Doug remained in Norman's lab for 3 years to build a laser temperature jump apparatus conceived by Norman and Flynn and to study several nanoseconds time scale reactions. [33,[44][45][46][47] Bruce stayed and became a full-time BNL employee in Norman's group for 25 years.
Norman's enthusiasm, optimism, and scientific rigor continues to inspire us. He is deeply missed by all who knew him.

Disclosure statement
No potential conflict of interest was reported by the author(s).