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

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

Johansen, Christian M; Benazzi, Elisabetta; Peters, Jonas C

DOI: PMID:

Abstract

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• and [HEH2]•+. Transient absorbance data and EPR studies establish that the resulting [HEH2]•+ intermediate is rapidly deprotonated by Col to yield HEH•, in net furnishing HEH• and ColH• 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 π-bonds by HEH2, with a significant boost in rates and yield, and altered reactivity, observed on addition of Col-buffer. ColH• is posited as the most potent PCET donor generated (BDFEN−H of 28 kcal mol−1).

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