Thermodynamics of N-H bond formation in bis(phosphine) molybdenum(ii) diazenides and the influence of the trans ligand.

A series of bis(phosphine) molybdenum(ii) diazenides [(dppe)2Mo(NNCy)(I)], [(dppe)2(CH3CN)Mo(NNCy)][BArF24] and [(dppe)2)(3,5-(CF3)2C6H3CN)Mo(NNCy)][BArF24] (dppe = 1,2-bis(diphenylphosphino)ethane; Cy = cyclohexyl; ArF24 = (3,5-(CF3)2C6H3)4) were synthesized and structurally characterized. Treatment of the diazenido complexes with a stoichiometric amount of [H(OEt2)2][BArF24] afforded the corresponding molybdenum(iv) hydrazido species [(dppe)2Mo(NNHCy)(I)][BArF24], [(dppe)2(CH3CN)Mo(NNHCy)][BArF24]2 and [(dppe)2(3,5-(CF3)2C6H3CN)Mo(NNHCy)][BArF24]2, enabling the study of N-H bond dissociation free energies (BDFEs) in the classical Chatt-type bis(phosphine) diazenide platform as a function of ligand (L) trans to the nitrogenous fragment. Deprotonation and electrochemical experiments established that the trans nitrile 3,5-(CF3)2C6H3CN afforded the least reducing molybdenum(iv) hydrazido complex in the series ( = -1.32 V vs. Fc/Fc+) with the most acidic N-H bond (pKa < 2.6, THF), whereas the ligands CH3CN ( = -1.60 V, pKa < 5.5) and I- ( = -2.03 V, pKa = 9.3) gave more reducing complexes with less acidic N-H bonds. Computational (DFT) studies confirm weak N-H bond strengths of 32.8 (L = I-), 35.4 (L = CH3CN) and 36.2 kcal mol-1 (L = 3,5-(CF3)2C6H3CN) in the hydrazido series.