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.