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Dr. Sarah Chen
Stanford · NLP & interpretability
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Hannah Sayre
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Northeastern University · Chemical and Biomolecular Engineering
Hannah Sayre is an Assistant Professor in the Department of Chemical Engineering at Northeastern University, with a joint appointment in Chemistry and Chemical Biology. She joined the university in August 2021. Her research focuses on designing photocatalysts to improve their efficiency and reactivity, as well as elucidating light-activated chemical reaction mechanisms using time-resolved spectroscopy. Her group, the Sayre Research Group, designs, synthesizes, and investigates light-activated catalysts (photocatalysts) with the goal of light-powered chemical manufacturing. They utilize spectroscopy and electrochemistry to investigate photocatalysis mechanisms and apply mechanistic insights to enhance photocatalyst performance.
Dual photoredox catalysis is a powerful tool to achieve challenging bond transformations. Ir(III)/Ni(II) mediated cross-electrophile coupling is mechanistically underexplored, particularly the role of nucleated Ni(0). To exploit excited state interactions between Ir(III)* and Ni(II), we synthesized and isolated an Ir(III)/Ni(II) supramolecular photocatalyst in which the Ir(III) and Ni(II) are tethered by a bridging ligand (BL). The supramolecular Ir-Ni (Ir = [Ir(dF(CF 3 )ppy) 2 (BL)] + ; dF(CF 3 )ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; BL = 1,2-Bis(4'-methyl-2,2'-bipyridin-4-yl)ethane; Ni = NiBr 2 ) and the monometallic analog Ir-BL have similar photophysical and electrochemical properties to [Ir(dF(CF 3 )ppy) 2 (dtbbpy)] + (dtbbpy = 4,4'-di-tert-butyl-2,2'bipyridine). In dual photoredox coupling with 4-bromobenzotrifluoride and bromocyclohexane, Ni(0) nucleation is observed after 24 hours. A model cross-electrophile coupling reaction with supramolecular Ir-Ni does not nucleate Ni(0) and has comparable cross-coupled product yields to dual photoredox reactions. Fresh reagent addition after 24 h resulted in continued photocatalysis in all reactions, revealing nucleated Ni(0) noninnocence.
Nickel photocatalysis has recently become vital to organic synthesis, but how the Ni(II)X2L pre-catalyst (X = Cl, Br; L = bidentate ligand) becomes activated to Ni(I)XL has remained puzzling and is typically addressed on a case-by-case basis. Here, we reveal a general mechanism where light induces photolysis of the Ni(II)-X bond, either via direct excitation or triplet energy transfer. Photolysis produces Ni(I)XL and a halogen radical, X•. Subsequent hydrogen atom abstraction, often from the solvent, produces a C(sp3) radical, R•, that recombines with Ni(I) to form organonickel(II) complexes, Ni(II)XRL. Rather than acting as a loss pathway, Ni(II)XRL behaves as a light-activated reservoir of Ni(I) via photolysis of the Ni(II)-C bond. These results explain the role of the solvent in protecting the catalyst from off-cycle dimerization, demonstrate that two photons are often required to drive the reaction, and show how tuning the ligand can control the concentration of active Ni(I) species. Nickel(II) dihalide precatalysts with bidentate nitrogen ligands are widely used in cross-coupling reactions, notably in combination with photosensitizers, forming catalytic systems that currently drive major conceptual and synthetic thrusts within organic chemistry. Here the authors show a general mechanism by which these precatalysts are converted to the reduced, catalytically active species, using a range of characterization and spectroscopic techniques.
Conjugates between molecules and quantum dots (QDs) have been explored for a range of potential applications from photocatalysis and photovoltaics to quantum information science technologies. A particularly ubiquitous material in many of these applications are ZnO QDs since they can accept and transport electrons and can also act as hosts for unique spin states. Conjugates between molecular light absorbers and ZnO QDs have been explored for decades as components in dye-sensitized solar cells. Recently, these materials have also attracted interest for their ability to produce spin-polarized states upon photoexcitation. The current paper employs a series of light absorbing perylene molecules with different ZnO QD sizes to explore key features of these QD-molecule conjugates: (1) chemical interactions, (2) charge dynamics, and (3) spin polarization. The chemical interactions between the molecules and QDs are determined with binding equilibria and reveal dramatic impact of ligand size. The charge transfer dynamics from photoexcited perylenes to ZnO QDs were found to depend exponentially on the linker length. Finally, time-resolved electron paramagnetic resonance experiments reveal that these conjugates generate spin-polarized states in the form of radical pairs and triplets. These spin states hold promise as potential qubits and also offer an avenue to efficiently sensitize molecular triplets.
Photocatalysis harvests energy from light to power high-energy reactions and has potential in sustainable manufacturing. Enhanced efficiency is essential for large-scale applications. Mechanistic insight into photocatalyzed C(sp 3 )−C(sp 2 ) cross-coupling offers a roadmap to increase quantum yields. The presumed mechanism for C(sp 3 )−C(sp 2 ) cross-coupling proceeds by nucleophilic reductive quenching of an Ir(III) photosensitizer, which subsequently reduces Ni(II/0). Stern-Volmer analysis and transient absorption spectroscopy experiments reveal dominant energy transfer mechanisms in both cross-electrophile coupling and C(sp 3 )−C(sp 2 ) coupling with an alkyl borate nucleophile. Although reductive quenching was observed with some nucleophiles, pulse radiolysis experiments reveal a rate constant for electron transfer from the reduced Ir(III) •− photosensitizer to Ni(II) that is an order of magnitude less than the energy transfer rate constant. Energy transfer is a competitive productive pathway and drives cross-coupling reactions in which electron transfer is not possible.
Dual IrIII/LnNiII metallaphotoredox catalyzed C(sp3)–C(sp2) cross-coupling reactions are widely assumed to proceed by photoinduced single electron transfer steps due to the highly oxidizing IrIII* excited state (IrIII = [Ir(dF(CF3)ppy)2(dtbbpy)]+[PF6]−; dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; Ln = dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine). Using time-resolved absorption and emission spectroscopy, we reveal that energy transfer between IrIII* and various LnNiII precatalysts and intermediates with kq ≥ 108 M–1 s–1 also drives catalysis. Specifically, the excited states of LnNiII dihalide precatalysts/organometallic intermediates accessible by energy transfer appear to drive bond homolysis, halogen radical elimination, and reductive elimination reactions that facilitate formation of cross-coupled products. Energy transfer dynamics consequently circumvent the need for photoinduced electron transfer, thereby extending substrate scopes to coupling partners that cannot be oxidized by IrIII*. Within a cross-electrophile coupling model reaction between 4-bromobenzotrifluoride and bromocyclohexane, energy transfer activates the LnNiII precatalyst at early reaction times before nucleophilic reductants are present. In the absence of IrIII, direct excitation of LnNiII(Br)2 also activates the precatalyst to form a LnNiII(Br)(Aryl) intermediate. To compare energy transfer and electron transfer kinetics, we determined rate constants for reductive quenching by Br– (kSET = 4.1 × 108 M–1 s–1) and for the subsequent electron transfer from reduced IrIII•– to LnNiII(Br)2 (kSET = 4.1 × 107 M–1 s–1) using Stern-Volmer analysis and pulse radiolysis, respectively. Energy transfer rate constants are competitive with the electron transfer rate constants and energy transfer is a parallel pathway within metallaphotoredox catalysis. Exploiting the energy transfer mechanism, we demonstrate highly selective cross-electrophile coupling between 4-chlorobenzotrifluoride and bromocyclohexane to form exclusively cross-coupled product. With alkyl-trifluoroborate nucleophiles that do not reductively quench IrIII* emission, transmetalation with LnNiII(Br/Cl)(Aryl) followed by energy transfer also drives excited state reductive elimination to form C(sp3)–C(sp2) cross-coupled product. Similarly, energy transfer rather than NiII oxidation drives C(sp2)–OR reductive elimination, despite the strongly oxidizing ability of IrIII*. In total, these reactions demonstrate energy transfer processes from IrIII* to LnNiII in metallaphotoredox catalysis that can unlock alternative reactive pathways.
We show that Ni-mediated carbon-carbon cross coupling can be initiated by trans- formation of Ni(II)X2(dtbbpy) to Ni(I)X(dtbbpy) by light. Photolysis of the Ni(II)- X bond (X= Cl, Br) either via direct excitation or triplet energy transfer produces Ni(I)X(dtbbpy) and a halogen radical, X•. Hydrogen atom abstraction, often from the solvent, subsequently produces a C(sp3) radical, R•, that recombines with Ni(I) to form novel organonickel(II) complexes, Ni(II)XR(dtbbpy). These solvent-derived organonickel(II) species can be subsequently photolyzed back to Ni(I), thereby serving as a reservoir state that protects the system from Ni dimer formation and deactivation. A combination of x-ray absorption (XAS), nuclear-magnetic resonance (NMR), elec- tronic absorption (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies confirm the identity of the final photo-products, whilst the R• addition to Ni(I) was in- dependently observed via pulse radiolysis and found to form the same NiXR(dtbbpy) complex on the μs timescale. Finally, subsequent irradiation of the NiXR(dtbbpy) reservoir state converts it to known NiX(Caryl)(dtbbpy) compounds in the presence of an aryl bromide. These results explain how Ni(II) pre-catalysts convert to Ni(I) and ultimately drive oxidative addition of aryl halides in many pivotal methodologies for C(sp2)-C(sp3) bond formation, for example in cross electrophile couplings and C-H activations.
Journal of the American Chemical Society · 2024-09-20 · 20 citations
article
Photoredox catalysis is a powerful tool to access challenging and diverse syntheses. Absorption of visible light forms the excited state catalyst (*PC) but photons may be wasted if one of several unproductive pathways occur. Facile dissociation of the charge-separated encounter complex [PC•–:D•+], also known as (solvent) cage escape, is required for productive chemistry and directly governs availability of the critical PC•– intermediate. Competitive charge recombination, either inside or outside the solvent cage, may limit the overall efficiency of a photochemical reaction or internal quantum yield (defined as the moles of product formed per mole of photons absorbed by PC). Measuring the cage escape efficiency (ϕCE) typically requires time-resolved spectroscopy; however, we demonstrate how to estimate ϕCE using steady-state techniques that measure the efficiency of PC•– formation (ϕPC). Our results show that choice of electron donor critically impacts ϕPC, which directly correlates to improved synthetic and internal quantum yields. Furthermore, we demonstrate how modest structural differences between photocatalysts may afford a sizable effect on reactivity due to changes in ϕPC, and by extension ϕCE. Optimizing experimental conditions for cage escape provides photochemical reactions with improved atom economy and energy input, paving the way for sustainable design of photocatalytic systems.
In this work we seek to understand the pre-catalytic initiation steps in a classic metallaphotoredox catalysis paring Ir(III) ((Ir[dF(CF$_{3}$)ppy]$_{2}$(dtbbpy))PF$_{6}$) and Ni(II) ((4,4'-dtbbpy)NiCl$_{2}$) in dimethoxyethane. We use a combination of transient X-ray and optical absorption spectroscopies to track both nuclear and electronic excited-state dynamics, revealing two steps. First, photoexcitation produces the expected intramolecular oxidation of the iridium center, Ir(III), to Ir(IV)(dtbbpy)$^{\bullet -}$ correlated to the Ir metal-to-ligand charge transfer state. Second, interaction with Ir(IV)(dtbbpy)$^{\bullet -}$ drives the tetrahedral Ni(II) starting material to an unexpected octahedral Ni(II) species. We conclude by proposing the identity of the octahedral Ni(II) and suggest both an electron and an energy transfer mechanism for producing it that are equally consistent with our observations.
