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

Interconversion of the molybdenum amido [((Ph)Tpy)(PPh2Me)(2)Mo(NHtBuAr)][BArF24] ((Ph)Tpy=4-Ph-2,2,6,2-terpyridine; tBuAr=4-tert-butyl-C6H4; ArF24=(C6H3-3,5-(CF3)(2))(4)) and imido [((Ph)Tpy)(PPh2Me)(2)Mo(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 [((Ph)Tpy)(PPh2Me)(2)Mo(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 52.3kcalmol(-1), in agreement with a DFT-computed value of 48kcalmol(-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.