Metal‐Ligand Cooperative Synthesis of Benzonitrile by Electrochemical Reduction and Photolytic Splitting of Dinitrogen

Abstract

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N À Nb ond energy. Photochemical strategies that drive N 2 fixation are scarcely developed. Here,t he synthesis of ad inuclear N 2 -bridged complex is presented upon reduction of arhenium(III) pincer platform. Photochemical splitting into terminal nitride complexes is triggered by visible light. Clean nitrogen transfer with benzoyl chloride to free benzamide and benzonitrile is enabled by cooperative 2H + /2 e À transfer of the pincer ligand. Athreestep cycle is demonstrated for N 2 to nitrile fixation that relies on electrochemical reduction, photochemical N 2 -splitting and thermal nitrogen transfer. Figure 3. Left:CVof7 in presence of 0-15 equiv.b enzoic acid.Right: CV of 7 with 10 equiv 2,6-dichlorophenol under N 2 before CPE (orange), after 8hCPE at À1.65 V( pink) and after subsequent 5h CPE at À1.85 V(blue).Scheme 3. Optimized, three-steps ynthetic cycle. Angewandte ChemieCommunications 833

Dinitrogen Reduction to Ammonium at Rhenium Utilizing Light and Proton-Coupled Electron Transfer

“…Sharp, paramagnetically shifted 1 H and 31 P NMR signals suggested an even-electron species with TIP, similar to other d 5 -d 5 µ-N2 complexes. 30,43 In an attempt to trigger N≡N bond scission, complex 3 t,t was heated at 130 °C in toluene-d8 ( Figure 3A). There was no indication of nitride formation, but the minor species present in samples of (3 c,c ), on the basis of NMR experiments including 1 H, 31 P{ 1 H}, 1 H-1 H COSY, and 1 H-31 P HMBC.…”

“…[13][14][15][16][23][24][25][26][27][28] More recently, Schneider reported the first examples of thermal and photochemical N2 splitting reactions at rhenium to form nitride complexes B and C (Figure 1), respectively. 29,30 Figure 1. Selected N2-derived terminal nitrides A, 16 B, 29 C, 30 and D. Sites of ligand protonation highlighted in red.…”

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

“…In a recent spectacular case of PCMET involving a lightdriven reaction step, dinitrogen splitting was achieved using a rhenium complex with a pincer ligand that can act as a 2 e À /2 H + reservoir via interconversion between imine and amine forms. 87 Benzamide and benzonitrile can be cleanly formed from N 2 in overall 61% yield in a three-step cycle. The cycle starts with electrochemical N 2 activation leading to the dinuclear N 2 -bridged complex, continues with photochemical splitting of the latter into nitride complexes, and ends with thermal nitrogen transfer from the nitride complexes to the benzoyl chloride substrate.…”

Dinitrogen Reduction to Ammonium at Rhenium Utilizing Light and Proton-Coupled Electron Transfer

“…confirming that the observed nitride ligand originates from the bridging N2. 30,32 Illumination of the 3 t,c :3 c,c mixture using 455 nm LED resulted in slower conversion to 4 c (47% yield in 1.5 days, F = 0.004 at 1 h),…”

“…29,31 For rhenium complex B (Figure 1), for example, protonation occurs at the central amide donor, resulting in a cationic rhenium nitride product. 29 Although elegant syntheses of nitriles from N2-derived rhenium nitride complexes has been achieved, 30,32 the complete transformation from N2 to NH3 at rhenium has not been reported to date.…”