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

Abstract

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 (pK_a < 2.6, THF), whereas the ligands CH₃CN ( = −1.60 V, pK_a < 5.5) and I⁻ ( = −2.03 V, pK_a = 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.