N–H Bond Formation in a Manganese(V) Nitride Yields Ammonia by Light-Driven Proton-Coupled Electron Transfer

Abstract

A method for the reduction of a manganese nitride to ammonia is reported, where light-driven proton-coupled electron transfer enables the formation of weak NH bonds. Photoreduction of (saltBu)­MnVN to ammonia and a Mn­(II) complex has been accomplished using 9,10-dihydroacridine and a combination of an appropriately matched photoredox catalyst and weak Brønsted acid. Acid-reductant pairs with effective bond dissociation free energies between 35 and 46 kcal/mol exhibited high efficiencies. This light-driven method may provide a blueprint for new approaches to catalytic homogeneous ammonia synthesis under ambient conditions.

Facile conversion of ammonia to a nitride in a rhenium system that cleaves dinitrogen

“…Some of the weakest N–H bonds were calculated for Mo–NH intermediates in the Chatt 78 (37 kcal mol −1 ) and Schrock 76 (42 kcal mol −1 ) systems, which can undergo complete N 2 reduction to ammonia. 79 Consistent with earlier systems ( Table 2 ), amide intermediates consistently exhibit stronger N–H bonds than their corresponding imide intermediates. However, the difference between these two bond energies is particularly large in the Re systems, especially 3 where ΔBDFE N–H = 36 kcal mol −1 .…”

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

“…90c,d Recently, the scope of PCET photocatalysis has been extended to the depolymerization of lignin 91 and activation of metal-nitrides for ammonia synthesis. 92 In a work reported back-to-back with Rovis' related contribution, 93 Knowles described remote C−H alkylation of amides, which is mediated by the generation of a highly reactive amidyl radical by means of PCET under the cooperative action of an Ir photocatalyst and a phosphate base(Figure 23). 94 Recently, Alexanian and Knowles also reported an intermolecular C−H alkylation reaction via multisite-PCET.…”

“…Thus, this strategy allows for the generation of reactive intermediates by breaking homolytically very strong chemical bonds. For instance, the Knowles group described a diverse suite of photocatalytic transformations including inter- and intramolecular hydroamination, hydroetherification reactions, and oxidative C–O and C–C activation reactions. , Recently, the scope of PCET photocatalysis has been extended to the depolymerization of lignin and activation of metal-nitrides for ammonia synthesis . In a work reported back-to-back with Rovis’ related contribution, Knowles described remote C–H alkylation of amides, which is mediated by the generation of a highly reactive amidyl radical by means of PCET under the cooperative action of an Ir photocatalyst and a phosphate base­(Figure ).…”

Highly Selective Hydroboration of Carbonyls by a Manganese Catalyst: Insight into the Reaction Mechanism

“…Recently, the environmental and sustainability concerns have prompted the development of catalysts derived from inexpensive, earth-abundant, and low-toxicity first row transition metals, − such as Ti, , Fe, − and Cu, , because of their abundance and biocompatibility. As the third most abundant metal in terms of global reserve, manganese catalysts have been employed in a wide range of reactions, such as hydrogenation, − hydrosilylation, , nitrogen transfer, , and oxidation processes; , their excellent efficiency in hydroboration of carbonyl compounds has only been established in recent contributions by Zhang, Gade, and others. − Taking this into consideration along with concerns about sustainability, manganese is of particular interest, since it is one of the most abundant transition metals in the earth’s crust and is biocompatible. − …”