Interconversion of Molybdenum Imido and Amido Complexes by Proton‐Coupled Electron Transfer

Abstract

Interconversion of the molybdenum amido [(PhTpy)(PPh2Me)2Mo(NHtBuAr)][BArF24] (PhTpy=4′‐Ph‐2,2′,6′,2“‐terpyridine; tBuAr=4‐tert‐butyl‐C6H4; ArF24=(C6H3‐3,5‐(CF3)2)4) and imido [(PhTpy)(PPh2Me)2Mo(NtBuAr)][BArF24] complexes has been accomplished by proton‐coupled electron transfer. The 2,4,6‐tri‐tert‐butylphenoxyl radical was used as an oxidant and the non‐classical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] as the reductant. The N−H bond dissociation free energy (BDFE) of the amido N−H bond formed and cleaved in the sequence was experimentally bracketed between 45.8 and 52.3 kcal mol−1, in agreement with a DFT‐computed value of 48 kcal mol−1. The N−H BDFE in combination with electrochemical data eliminate proton transfer as the first step in the N−H bond‐forming sequence and favor initial electron transfer or concerted pathways.

Proton-Coupled Electron Transfer Guidelines, Fair and Square

“…Meanwhile, new reactions proposed by synthetic chemists may give feedback to fundamental PCET research, raising questions like the distinctions between hydrogen atom transfer (HAT) and CEPT ( section 2 ) and whether proton-coupled inner-sphere electron transfer can be described as PCET. 84 − 86 In many studies, arguments based on models for proton tunneling in non-adiabatic CEPT ( section 2 ) are frequently mixed with those based on classical models, discussing, e.g., polarization and/or asymmetry of the transition state based on Hammett plots or direct free-energy correlations ( section 3.5 ). 64 , 67 , 68 An interesting development in PCET would be to harmonize the theoretical description behind the above phenomena.…”

Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe(iii) amido intermediate

“…13 The oxidation of NH 3 into N 2 is an inherently challenging process due to the difficulty in breaking all three of its strong N-H bonds (the rst N-H bond dissociation energy, BDE, of NH 3 is 107.6 kcal mol À1 ), 14 as well as the multiple electron and proton transfers involved in the formation of N 2 (Scheme 1). 15 The energy associated with N-H bond breaking can be lowered by coordination of NH 3 to a metal center, and subsequent strategies such as N-H oxidative addition, 16 heterolytic cleavage through metal-ligand cooperativity, [17][18][19][20][21] or hydrogen atom abstraction (HAA) 8,9,[22][23][24][25][26][27] have been successful at mediating N-H bond cleavage of NH 3 . In addition to N-H bond cleavage, N-N bond formation is a key process.…”

Diversion of Catalytic C–N Bond Formation to Catalytic Oxidation of NH3 through Modification of the Hydrogen Atom Abstractor

“…The first N–H bond dissociation free energy (BDFE) of ammonia is 99.4 kcal/mol, but the energy is decreased by coordination to a metal. Several approaches in transition metal systems have overcome the high bond strength to cleave N–H bonds, including N–H oxidative addition, metal–ligand cooperativity, , 1,2 addition across a metal–metal bond, and H atom abstraction (HAA). − The formation of a N–N bond is a requisite step in the oxidation of NH 3 to N 2 , so understanding the factors that control that reaction are crucial in the design of metal catalysts for oxidation of NH 3 . − To the best of our knowledge, only four molecular systems have been reported to catalytically oxidize NH 3 to N 2 (Figure , top); − all have turnover numbers (TONs) less than 20.…”