Rapid Intermolecular C-H Activation of Aromatic Substrates at a Cationic, Electrophilic Molybdenum (VI) Nitrido Complex: Mechanistic Nuance in C-H Amination

When the cationic, octahedral terminal nitrido complex of Mo(VI) supported by a diborate pentadentate ligand is generated in the absence of trapping nucleophiles, it rapidly activates the C(sp2)-H bonds of simple arenes to form the cationic imido complexes [[ArMo(V)]=NR]+ (R = H; PhX, X = H, o,m,p-F). Detailed Density Functional Theory computational work and extensive experimental studies support a mechanism with homolytic aromatic substitution character, wherein electrophilic attack of the arene system initiates N-C bond formation, but use of electrons from the Mo≡N triple bond completes the bond formation through an open-shell singlet transition state that formally reduces the metal center to Mo(V) and imbues the arene ring with significant radical character. This transition state is enthalpically 15 kcal mol-1 lower in energy than the classical “Wheland-type” transition state common to electrophilic aromatic substitution paths proposed in related systems. Subsequent intramolecular transfer of the arene hydrogen to the imido nitrogen atom leads to the cationic Mo(IV) anilido complex [[ArMoIV]-NHPh]+, which rapidily reacts with available [[ArMoVI]≡N]+ by a proton-coupled electron transfer step to generate the observed 1 : 1 mixture of imido products [[ArMo(V)]=NR]+ (R = H; PhX, X = H, o,m,p-F). This second phase of the mechanism is also supported computationally and by an experimental crossover experiment and is driven by significant coordination-induced bond weakening (CIBW) in the N-H bond of the [[ArMoIV]-NHPh]+ and the lack of CIBW in the imido N-H product. The novel mechanism of C-H addition to the electrophilic terminal nitrido opens new low energy pathways for the development of systems capable of catalytic nitrogen atom transfer to C-H bonds.