Reduction and Protonation of Mo(IV) Imido Complexes with depe Coligands: Generation and Reactivity of a S = 1/2 Mo(III) Alkylnitrene Intermediate

Abstract

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.

Molybdenum dinitrogen complexes supported by a silicon-centred tripod ligand and dppm or dmpm: tuning the activation of N2

“…8 We have performed extensive spectroscopic and theoretical investigations on the elementary steps of this cycle. [9][10][11][12][13][14][15][16][17] Importantly, there exist some side reactions which prevent the conversion of N 2 from being conducted in a catalytic manner. For instance, the intermediate Mo(I) complex coordinated by the conjugate base X of the employed acid undergoes a disproportionation reaction, which implies a 50% loss of catalyst at each cycle.…”

(Dinitrogen)molybdenum Complexes Supported by Asymmetric Silicon‐Centered Tripod Ligands: Steric and Electronic Influences on the Coordination of Mono‐ and Diphosphine Coligands

“…So far, a large number of N 2 complexes have been synthesized, only a few of which, however, allow the reduction of N 2 to NH 3 in a cyclic or even catalytic fashion. [11][12][13][14][15][16][17][18] More recently we focused our attention on the design of new ligand environments. On the basis of the latter systems, the first complete mechanistic scenario of N 2 reduction has been established by Chatt and co-workers.…”

Molybdenum Bis(Dinitrogen) Complexes Supported by NP3 Ligands containing Propylene Linkages: Symmetry Breaking and Geometrical Isomerism

“…The first mechanistic scheme in this regard has been the Chatt cycle, although based on Chatt‐type complexes a truly catalytic formation of NH 3 has not been achieved to date. We have studied the elementary reactions of the Chatt cycle in a number of experimental and theoretical investigations 11–14. One of the main problems of these systems is the exchange of the ligand in trans position to the coordinated dinitrogen.…”