Issue 40, 2016

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 (Image ID:c6dt01932c-t1.gif = −1.32 V vs. Fc/Fc+) with the most acidic N–H bond (pKa < 2.6, THF), whereas the ligands CH3CN (Image ID:c6dt01932c-t2.gif = −1.60 V, pKa < 5.5) and I (Image ID:c6dt01932c-t3.gif = −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.

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

Supplementary files

Article information

Article type
Paper
Submitted
15 mei 2016
Accepted
28 jun 2016
First published
29 jun 2016

Dalton Trans., 2016,45, 15922-15930

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

M. J. Bezdek and P. J. Chirik, Dalton Trans., 2016, 45, 15922 DOI: 10.1039/C6DT01932C

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