Photoinduced transfer hydrogenation of nitrogen to ammonia using a Mo-catalyst and a Hantzsch ester donor is demonstrated with and without an Ir-photoredox co-catalyst

Whereas photoredox catalysis using molecular systems enjoys considerable utility in small molecule transformations and reactions relevant to organic synthesis, to date there are no related examples of photodriven catalytic nitrogen fixation. We wondered whether a photoinduced transfer hydrogenation strategy might provide a viable pathway toward such a reaction. Hantzsch esters (and related organic structures) offer an opportunity for catalysis design in this context as they can behave as photoreductants, though to our knowledge they had yet to be shown to be compatible with such a redox intensive process (6 e–/6 H+). In the present study we demonstrate that fully reduced Hantzsch esters (abbreviated as HEH2) successively deliver stored H2-equivalents to N2, producing NH3 catalytically, in the presence of a molecular precatalyst (Mo) under blue-light irradiation but otherwise ambient conditions. While not required for the observed photocatalysis, the addition of a photoredox catalyst (Ir) to the reaction mixture enhances both the rate and turnover number of the net transformation. Encouraging with respect to future studies toward recycling the donor, electrochemically or via hydrogenation, other N-heterocycle H2-donors are also compatible with catalysis in the presence of the photoredox catalyst. The reduction of N2 to NH3 by HEH2 or H2 are thermodynamically very similar (ΔΔGf(NH3) = 1.8 kcal mol–1 in acetonitrile). However, whereas the combination of H2 with N2 to produce NH3 is accomplished via high temperature and pressure over a metal catalyst, the needed overpotential to drive the reduction of N2 by HEH2 can instead be derived from light. This study hence illustrates a promising photoredox catalysis approach toward deep reduction of robust small molecule substrates via photoinduced transfer hydrogenation, with the complete reduction of the triple bond of N2 providing a vivid example.