Our next webinar will take place via the internet on Tuesday October 5th at 11 AM EDT/ 4PM GMT. Sign up on our mailing list to receive the Zoom link!
We hope to see/hear from you all at one of our sessions or as one of the next speakers. If you are an early career scientist and would like to present your research, don't hesitate to submit an abstract today! For now, please learn more about our current speakers and their research below. We also thank the generous support from Cell Reports Physical Science, Merck, and the Royal Society of Chemistry.
Our featured speakers this week are Colleen Chernowsky (graduate student, University of Wisconsin-Madison, USA), and Dr. Sébastien Lapointe (postdoctoral researcher, Ruhr-Universitat Bochum, Germany). The seminar will be guest-moderated by Prof. Anne McNeil from the University of Michigan.
LEARN MORE ABOUT THE SPEAKERS AND THEIR TALKS BELOW
Biography: Colleen Chernowsky is a fourth-year graduate student in the Wickens group at the University of Wisconsin-Madison. She is originally from Michigan and graduated from Ohio Wesleyan University in 2018 with B.A.s in Chemistry and Geology. As an undergraduate, she participated in an REU internship with Dr. Jonah Jurss at the University of Mississippi investigating the synthesis of catalyst structures for efficient CO2 reduction. Her current graduate work revolves around harnessing multiple energy sources to amplify the reductive power of photoredox catalysis and unlock challenging substrates for radical coupling.
Title of Talk: Electrochemical Activation of Diverse Conventional Photoredox Catalysts Induces New Potent Photoreductant Activity
Abstract: Reductive activation of organic molecules through single electron transfer (SET) is an elementary step at the heart of a myriad of useful transformations. In recent years, photoredox catalysis has emerged as a mild and chemoselective method to induce redox events. Unfortunately, while 400 nm light possesses sufficient energy for a driving force of 3.1 eV, this energy is diminished by 25–50% through vibrational relaxation, and intersystem crossing. As a consequence, many abundant but thermodynamically stable molecules remain inert to photoredox activation. To address this, the development of new strategies to deliver extreme reduction potentials (significantly more negative than –2 V vs SCE) with the chemoselectivity profile of photoredox catalysis is an emerging area of interest. We leverage electrochemistry to examine the photocatalytic activity of a range of structurally diverse persistent radical anions and find that many are effective electrophotocatalysts. These studies uncover a new electron-primed photoredox catalyst capable of promoting the reductive cleavage of strong C(sp2)–N and C(sp2)–O bonds even when reduction potentials hundreds of mV more negative than Li0 are required. We illustrate several examples of the synthetic utility of these deeply reducing but otherwise safe and mild catalytic conditions. Finally, we employ electrochemical current measurements to perform a reaction progress kinetic analysis that reveals that the improved activity of this new catalyst is a consequence of an enhanced stability profile.
DR SEBASTIEN LAPOINTE (on Twitter @SLapointeChem)
Biography: Sébastien did his undergrad studies at the Université de Montréal under Pr. Davit Zargarian on cationic Pincer complexes of nickel, where his love for both nickel and pincer complexes began. He then pursued his love further by doing a PhD in Japan (Okinawa institute of Science and Technology) under Pr. Julia R. Khusnutdinova also on nickel and other metal pincer complexes. He is now working with Pr. Viktoria H. Gessner on Phosphorous ylides complexes at the Ruhr-Universitat Bochum in Germany! His talk gives an overview of the work he did in his PhD.
Title of Talk: Nickel Complexes of New Electron-Rich, Sterically-Hindered PNP Pincer Ligands
Abstract: The PNP pincer ligands (PNP = 2,6-bis[(dialkylphosphino)methyl]pyridine) have been extensively studied in catalysis and in small molecule activation due to their ability to stabilize reactive species, or to induce new modes of reactivity that lead to new catalytic reactions with their associated metal complexes. They often feature Metal-Ligand Cooperation (MLC) via participation of the CH2 arm in protonation/deprotonation.1 While sometimes such reactivity can be instrumental in catalysis, in other cases it can lead to undesired irreversible ligand modification or decomposition.
In that light, we have recently synthesized some very bulky, electron rich pincer PNP ligand in which ligand reactivity is blocked by substitution with the alkyl groups. We investigated the effect of this modification on nickel complexes and found that when metal-ligand cooperative modes are disabled, one-electron reactivity is observed, which has not been previously reported for the parent unmethylated PNP pincer complexes. Indeed, the presence of methyl groups on the pincer arm blocks MLC and allows for the stabilization of paramagnetic nickel(I) species. The increased bulkiness from the methyl groups also helps to create new C-C bonds through dimerization at the para position of the pyridine ring when nickel methyl complexes are reacted with a strong reducing agent. Moreover, no reactivity with small molecules were observed with nickel(II) hydride or methyl species due to the increased bulkiness of the ligand, except when iPr groups are present on the phosphine. In that case, reacting a nickel(II) hydride species with O2 forms a highly reactive superoxide adduct that has been confirmed by EPR spectroscopy. Finally, we explore the lability of the nickel-methyl bond through UV irradiation and observed the formation of methyl radical species and a radical nickel center through EPR spectroscopy. Through careful selection of reducing conditions and ligand modification, it should be possible to obtain anionic PNP pincer ligands that can undergo different chemistry and open up new reactivity that the parent PNP ligand might not be able to achieve.