On the mechanism of photodriven hydrogenations of N2 and other substrates by Hantzsch ester: Buffer is key to reactive H- atom donors

• 成果类型: Article
• 署名作者: Johansen, Christian M.; Benazzi, Elisabetta; Peters, Jonas C.
• 署名单位: California Institute of Technology
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-8950
• DOI: 10.1073/pnas.2502484122
• 发表日期: 2025-07-01
• 关键词: coupled electron-transfer; hydride-transfer; model compounds; excited-state; nadh; activation; radicals; photoexcitation; reduction; iridium

摘要

The Hantzsch ester (HEH2) has found considerable utility as a photoreductant in synthesis, with photodriven transfer hydrogenation reactions typically limited to activated substrates. We recently established that the addition of an organic buffer of collidinium triflate [(ColH)OTf] and collidine (Col) allows photodriven transfer hydrogenation from HEH2 to N2 forming NH3 (nitrogen reduction; N2R) in the presence of a Mo catalyst. Given the requirements for Mo-catalyzed thermally driven N2R, this result suggested the generation of a significant driving force for proton-coupled electron transfer (PCET) when irradiating HEH2 in the presence of Col-buffer. In this study, we probe how Col-buffer enables efficient photodriven proton-coupled reductions with HEH2. Wavelength-dependent NH3 yields are consistent with HEH2 photoexcitation, and the combination of HEH2 with Col-buffer is privileged. Data are presented, suggesting that HEH2 is statically quenched via ET to [ColH]OTf through an H-bonded association complex to release ColH center dot and [HEH2]center dot+. Transient absorbance data and EPR studies establish that the resulting [HEH2]center dot+ intermediate is rapidly deprotonated by Col to yield HEH center dot, in net furnishing HEH center dot and ColH center dot as potent H-atom donors. Broader utility of this reagent combination is demonstrated in the photoreduction of a range of C=O and N=O pi-bonds by HEH2, with a significant boost in rates and yield, and altered reactivity, observed on addition of Col-buffer. ColH center dot is posited as the most potent PCET donor generated (BDFEN-H of 28 kcal mol-1). Significance Solar-driven ammonia synthesis is an interesting approach to consider for fertilizer production and zero-carbon fuels. However, photocatalytic nitrogen fixation by well-defined synthetic catalysts was first demonstrated onlyvery recently. Our group was able to show that a reduced Hantzsch ester (HEH2) can behave as 2e-/2H+ photoreductant when partnered with a suitable catalyst (Mo) under blue light irradiation. HEH2 facilitates delivery of successive H2-equivalents for the 6e-/6H+ catalytic reduction of N2 to NH3. This photodriven transfer hydrogenation raises fascinating mechanistic questions that are addressed herein, where it is shown that the presence of a specific buffer enables generation of reactive radical H-atom donors that drive N2 reduction and affords interesting reductive reactivity patterns with other substrates.