Multisite PCET with photocharged carbon nitride in dark

Abstract

A combination of photochemistry and proton coupled electron transfer (PCET) is a primary strategy employed by biochemical systems and synthetic chemistry to enable uphill reactions under mild conditions. Degenerate nanometer‐sized n‐type semiconductor nanoparticles (SCNPs) with the Fermi level above the bottom of the conduction band are strongly reducing and act more like metals than semiconductors. Application of the degenerate SCNPs is limited to few examples. Herein, we load microporous potassium poly(heptazine imide) (K‐PHI) nanoparticles with electrons (e‒) and charge balancing protons (H+) in an illumination phase using sacrificial agents. e‒/H+ in the K‐PHI nanoparticles are weakly bound and therefore could be used in a range of PCET reactions in dark, such as generation of aryl radicals from aryl halides, ketyl radicals from ketones, and 6e‒/6H+ reduction of nitrobenzene to aniline. The integration of several features that until now were intrinsic for plants and natural photosynthesis into a transition metal free nanomaterial composed of abundant elements (C, N, and K) offers a powerful tool for synthetic organic chemistry.

Photocharging of Semiconductor Materials: Database, Quantitative Data Analysis, and Application in Organic Synthesis

“…Due to plenty of electrons available in photocharged semiconductors at nearly the same potential, at least one example—reductive dimerization of enones—unambiguously point that reactivity of semiconductor particles under continuous light illumination (classical photocatalysis) [ 151 ] and semiconductors photocharged with e − /H + (ref. [36]) is different. Studying photocharged SCP(e − /H + ) as PCET reagents may establish a whole new field of preparative organic chemistry.…”

“…[ 151 ] Addition of a ketyl radical to the starting enone 1a followed by HAT/PCET and elimination of H 2 O yields substituted hexadienone 2a . [ 36 ] Kurpil and Savateev found that under continuous illumination using triethanolamine as electron donor, selectivity is shifted to cyclopentanol 3a as the major product. [ 151 ] PCET mechanism for ketyl radical formation was concluded taking into account CB potential of K‐PHI (−0.75 V vs SCE), [ 152 ] which is not sufficiently negative to reduce enone 1a to its radical anion via electron transfer ( E 1/2 = −1.4 V vs SCE).…”

“…These results and results obtained by Lotsch and co‐workers who found that the amount of H 2 that H‐PHI(e − / H + ) and NCN‐CNx(e − / H + ) can release upon addition of Pt nanoparticles in dark decreases after storage of the materials for several hours at room temperature, [ 34,38 ] suggest that electrons might adopt thermodynamically more stable, less reactive, trap states in the bulk of the materials. Indeed, reactions that involve transfer of 1e − /1H + to α,β‐unsaturated ketones (Figure 25b), [ 151,154 ] and 2e − /2H + transfer reactions (Figure 26b) proceed faster when conducted at 80 °C compared to room temperature, [ 36 ] which point at the kinetic control of e − /H + extraction from the micropores of K‐PHI.…”

“…Reduction of nitroarenes to aromatic amines has been accomplished by TiO 2 (e − /H + ), [ 132 ] V‐doped V‐TiO 2 (e − /H + ) [ 66 ] and K‐PHI(e − /H + ) [ 36 ] ( Figure 27 ). Unlike to reduction of nitrocompounds employing semiconductors under continuous illumination, which typically gives a range of products, such as diazo‐, azoxy‐, nitroso‐compounds, and N ‐hydroxylamines, [ 157 ] photocharged SCP(e − /H + ) yields selectively amines likely due to plenty of electrons and protons readily available for multielectron transfer.…”

“…[29][30][31][32][33] However, there is emerging interest in ionic photochargeable graphitic carbon nitride (gCN) materials. [13,[34][35][36] Ideal structures of these materials are depicted in Figure 3. All these materials are composed of heptazine units, in which both C and N are sp 2 -hybridized.…”