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Reduction and protonation of Mo(IV) imido complexes with depe coligands: generation and reactivity of a S = 1/2 Mo(III) alkylnitrene intermediate.

Reduction and protonation of Mo(IV) imido complexes with diphosphine coligands constitutes the second part of the Chatt cycle for biomimetic reduction of N2 to ammonia. In order to obtain insights into the corresponding elementary reactions we synthesized the Mo(IV) ethylimido complex [Mo(CH3CN)(NEt)(depe)2](OTf)2 (2-MeCN) from the Mo(IV)-NNH2 precursor [Mo(NNH2)(OTf)(depe)2](OTf) (1). As shown by UV-vis and NMR spectroscopy, exchange of the acetonitrile ligand with one of the counterions in THF results in formation of the so far unknown complex [Mo(OTf)(NEt)(depe)2](OTf) (2-OTf). 2-MeCN and 2-OTf are studied by spectroscopy and X-ray crystallography in conjunction with DFT calculations. Furthermore, both complexes are investigated by cyclic voltammetry and spectroelectrochemistry. The complex 2-OTf undergoes a two-electron reduction in THF associated with loss of the trans ligand triflate. In contrast, 2-MeCN in acetonitrile is reduced to an unprecedented Mo(III) alkylnitrene complex [Mo(NEt)(CH3CN)(depe)2]OTf (5) which abstracts a proton from the parent Mo(IV) compound 2-MeCN, forming the Mo(III) ethylamido complex 5H and a Mo(II) azavinylidene complex 6. Compound 5 is also protonated to the Mo(III) ethylamido complex 5H in the presence of externally added acid and further reduced to the Mo(II) ethylamido complex 7. The results of this study provide further support to a central reaction paradigm of the Schrock and Chatt cycles: double reductions (and double protonations) lead to high-energy intermediates, and therefore, every single reduction has to be followed by a single protonation (and vice versa). Only in this way the biomimetic conversion of dinitrogen to ammonia proceeds on a minimum-energy pathway.