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 ( iPr PDI)Mo(═NTol) 2 (Tol = 4-methylphenyl) was synthesized by the addition of 2 equiv of 4-methylphenylazide to the corresponding molybdenum benzene derivative, ( iPr PDI)Mo(η 6 -C 6 H 6 ) [ iPr PDI = 2,6-(2,6-iPr 2 C 6 H 3 N═CMe) 2 C 5 H 3 N]. Protonation of ( iPr PDI)Mo(═NTol) 2 with 2,6-lutinidum triflate yielded a cationic molybdenum amido complex, [( iPr PDI)Mo(NHTol)(═NTol)][OTf], which was further transformed into the neutral molybdenum amido ( iPr PDI)Mo(NHTol)(═NTol) by reduction with zinc powder. A series of spectroscopic, synthetic, and p K 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, ( iPr PDI)Mo(NHTol)(═NTol). Full-molecule density functional theory studies provided a computed value of 61 kcal/mol. By contrast, reduction of ( iPr PDI)Mo(═NTol) 2 with KC 8 afforded the corresponding anionic molybdenum complex K[( iPr PDI)Mo(═NTol) 2 ], which has a potassium cation intercalated with the pyridine and tolyl groups. Protonation of K[( iPr PDI)Mo(═NTol) 2 ] with the weak amidinium acid [TBD(H)][BArF 24 ] (TBD = triazabicyclodecene; BArF 24 = B[3,5-(CF 3 ) 2 C 6 H 3 ] 4 ) also produced the neutral molybdenum amido complex ( iPr PDI)Mo(NHTol)(═NTol). Measurement of the p K a and oxidation potential of K[( iPr PDI)Mo(═NTol) 2 ] provided a range of 69-73 kcal/mol for the N-H BDFE of ( iPr PDI)Mo(NHTol)(═NTol), in good agreement with the protonation-reduction route and completing the square scheme. The similar p K a and redox potentials obtained from each pathway demonstrate that both sequences are energetically feasible for proton-coupled 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.