Determination of the N-H Bond Dissociation Free Energy in a Pyridine(diimine)molybdenum Complex Prepared by Proton-Coupled Electron Transfer

The pyridine(diimine)molybdenum bis(imido) complex ((PDI)-P-iPr)Mo(=NTol)(2) (Tol = 4-methylphenyl) was synthesized by the addition of 2 equiv of 4-methylphenylazide to the corresponding molybdenum benzene derivative, ((PDI)-P-iPr)Mo(eta(6)-C6H6) [(TDI)-T-iPr = 2,6-(2,6-iPr(2)C(6)H(3)N=CMe)(2)C5H3N]. Protonation of ((PDI)-P-iPr)Mo(=NTol)(2) with 2,6-lutinidum triflate yielded a cationic molybdenum amido complex, [((PDI)-P-iPr)Mo(NHTol)(=NTol)][OTf], which was further transformed into the neutral molybdenum amido ((PDI)-P-iPr)Mo(NHTol)(=NTol) by reduction with zinc powder. A series of spectroscopic, synthetic, and pK(a) determination studies along with electrochemical measurements by the protonation-reduction pathway were used to establish an N-H bond dissociation free energy (BDFE) between 65 and 69 kcal/mol for the molybdenum imido-amido compound, ((PDI)-P-iPr)Mo(NHTol)(=NTol). Full-molecule density functional theory studies provided a computed value of 61 kcal/mol. By contrast, reduction of ((PDI)-P-iPr)Mo(=NTol)(2) with KC8 afforded the corresponding anionic molybdenum complex K[rTDI)Mo(=NTol)(2)], which has a potassium cation intercalated with the pyridine and tolyl groups. Protonation of K[(PDI)-P-iPr)Mo(=NTol)(2)] with the weak amidinium acid [TBD(H)][BArF24] (TBD = triazabicyclodecene; BArF24 = B[3,5-(CF3)(2)C6H3](4)) also produced the neutral molybdenum amido complex ((PDI)-P-iPr)Mo(NHTol)(=NTol). Measurement of the pK(a) and oxidation potential of K[((PDI)-P-iPr)Mo(=NTol)(2)] provided a range of 69-73 kcal/mol for the N-H BDFE of ((PDI)-P-iPr)Mo(NHTol)(=NTol), in good agreement with the protonation-reduction route and completing the square scheme. The similar pK(a) and redox potentials obtained from each pathway demonstrate that both sequences are energetically feasible for protoncoupled electron-transfer (PCET) events. This study on the determination of N-H BDFE of the molybdenum amido complex renders fundamental insight into the N-2 reduction cycle by PCET.