Unearthing the mechanism of prebiotic nitrile bond reduction in hydrogen cyanide through a curious association of two molecular radical anions

HCN is clearly associated with the prebiotic chemical evolution of life. It has been known for decades that the radiolysis of HCN solutions produces sugars, amino acids and nucleobases. Remarkably, recent experimental studies have shown that the photolytic reduction of aqueous HCN by a photoredox reagent [Cu(CN)3](2-) specifically yields sugars, which are the essential building blocks of RNA. Although a mechanistic understanding of such reductions with solvated electrons is poor, the general consensus is that they involve neutral free radicals. We show herein through the use of electronic structure studies and molecular simulations that the reduction of the nitrile bond of HCN is initiated through the formation of a molecular dipole-bound anion from the photoredox reagent. Our theoretical studies show how HCN binds to the photoexcited reagent and then extracts an electron from the reagent and is ultimately detached as a dipole-bound anion. The dipole-bound anionic form of [HCN](-) can easily convert into a solvated valence-bound form of [HCN](-). After the formation of solvated [HCN](-), an extraordinary chemical event ensues through a counter-intuitive coupling of two valence-bound anions to form a solvated molecular dianionic intermediate, [HCN]2(2-). Finally, a proton-coupled electron transfer occurs within the dianionic entity to complete the reduction. This mechanistic scenario is applicable to the reduction of other prebiotic nitrile species and avoids neutral radical-based pathways, thereby preventing the proliferation of reactive species and preserving chemical selectivity. Furthermore, we show how such similar nitrile reduction pathways operate to yield the sugar precursors.