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

Interconversion of the molybdenum amido [(Ph Tpy)(PPh2 Me)2 Mo(NHtBuAr)][BArF24 ] (Ph Tpy=4'-Ph-2,2',6',2"-terpyridine; tBuAr=4-tert-butyl-C6 H4 ; ArF24 =(C6 H3 -3,5-(CF3 )2 )4 ) and imido [(Ph Tpy)(PPh2 Me)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)(PPh2 Me)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 52.3 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.