Evaluation of excited state bond weakening for ammonia synthesis from a manganese nitride: stepwise proton coupled electron transfer is preferred over hydrogen atom transfer

Abstract

Concepts for the thermodynamically challenging synthesis of weak N–H bonds by photoinduced proton coupled electron transfer are explored. By harvesting visible light as driving force, ammonia synthesis was achieved and mechanistically elucidated.

Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia

“…Previous studies have established that addition of L-type ligands to ammonia synthesis reactions with MnN increases the yield of free NH 3 , likely by outcompeting ammonia coordination to the manganese. 25,26 Repeating the hydrogenation of MnN with 10 mol% of 2 at higher concentration and addition of 2 equiv of TMEDA (N,N,N′,N′-tetramethylethylenediamine) increased the yield of NH 3 to 40% (entry 4). Again, 1 produced no NH 3 under these conditions (entry 5).…”

“…Recently, our laboratory reported the application of photodriven proton-coupled electron transfer for the synthesis of ammonia from the manganese nitride ( tBu Salen)MnN (MnN) using a combination of 9,10-dihydroacridine, a ruthenium photocatalyst, and a proton source (Scheme 1c). 25,26 The computed, successive N−H BDFEs of 60, 84, and 85 kcal mol −1 make this specific metal nitride attractive for ammonia formation. Because of the requirement for 9,10dihydroacridine as the stoichiometric hydrogen atom source and light as the energy source, this reaction is not ideal for…”

“…For homogeneous ammonia synthesis catalysts to operate near thermodynamic potential, dihydrogen (H 2 ) is the optimal stoichiometric reductant. − The challenge lies in the synthesis of weak N–H bonds, as many intermediates in the catalytic cycle range between 30 and 50 kcal mol –1 , below the driving force for spontaneous H 2 formation . Proton-coupled electron transfer (PCET) has emerged as a promising strategy for the synthesis of these bonds, and examples relevant to ammonia synthesis from metal nitrides include SmI 2 · x H 2 O, , 9,10-dihydro­acridine, , and TEMPO-H . Our group demonstrated this concept using (η 5 -C 5 Me 5 )­Rh­(ppy)H ( 1 ) (ppy = 2-phenyl­pyridine) − as the hydrogen-atom transfer catalyst for the synthesis of ammonia from a titanocene amide complex with H 2 as the terminal reductant (Scheme a). , A key feature of the hydrogen transfer catalyst is a M–H bond dissociation free energy (BDFE) higher than 48.6 kcal mol –1 (Δ G f °(H • )) to promote H 2 cleavage but also as close to this value as possible to maximize the driving force for N–H bond formation.…”

Equilibrium Geometries, Adiabatic Excitation Energies and Intrinsic C=C/C–H Bond Strengths of Ethylene in Lowest Singlet Excited States Described by TDDFT

“…However, one needs to be aware that such a simplified picture overlooks the different types of electronic excitations and the heterogeneity in the molecular charge distribution Therefore, to understand the chemical bond strength and its change caused by electronic excitation is non-trivial. A bond weakening effect in excited states has been discovered in cyclopropyl ketones [16], double-bond species [17], metal-ligand bonds [18] and ligands within a manganese-nitride complex [19]. Han and co-workers pioneered the investigation of the so-called excited state hydrogen bond between a chromophore and protic solvents.…”

Cationic Effects on the Effective Hydrogen Atom Bond Dissociation Free Energy of High Valent Manganese Imido Complexes

“…[53][54][55][56][57][58][59][60][61][62][63][64][65][66] Lau and coworkers previously reported salen manganese nitrido complexes that were activated for stoichiometric aziridination of alkenes following protonation with a Brønsted acid or addition of electrophiles. [67][68][69] Chirik and coworkers demonstrated proton-coupled electron transfer (PCET) at salen manganese nitrido complexes to form ammonia through either photodriven 70,71 or thermal 72 pathways. Although the bond dissociation free energies (BDFEs) at transition metal imidos are central in catalytic nitrogen cycles [73][74][75][76][77][78] and C-H activation, [79][80][81][82][83][84] few BDFE values have been measured compared to isoelectronic metal oxido analogues.…”