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

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 (BDFE_N−H of 28 kcal mol⁻¹).