Towards Catalytic Ammonia Oxidation to Dinitrogen: A Synthetic Cycle by Using a Simple Manganese Complex

Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric [Ar₃ N][X] initiated a nitride coupling reaction to N₂ , a major step toward catalytic ammonia oxidation (salen=N,N'-bis(salicylidene)-ethylenediamine dianion; Ar=p-bromophenyl; X=[SbCl₆ ]^- or [B(C₆ F₅ )₄ ]^- ). N₂ production was confirmed by mass spectral analysis of the isotopomer, 1-¹⁵ N, and the gas quantified. The metal products of oxidation were the reduced Mn^III dimers, [(salen)MnCl]₂ (2) or [(salen)Mn(OEt₂ )]₂ [B(C₆ F₅ )₄ ]₂ (3) for X=[SbCl₆ ]^- or [B(C₆ F₅ )₄ ]^- , respectively. The mechanism of nitride coupling was probed to distinguish a nitridyl from a nucleophilic/electrophilic coupling sequence. During these studies, a rare mixed-valent Mn^V /Mn^III bridging nitride, [(salen)Mn^V (μ-N)Mn^III (salen)][B(C₆ F₅ )₄ ] (4), was isolated, and its oxidation-state assignment was confirmed by X-ray diffraction (XRD) studies, perpendicular and parallel-mode EPR and UV/Vis/NIR spectroscopies, as well as superconducting quantum interference device (SQUID) magnetometry. We found that 4 could subsequently be oxidized to 3. Furthermore, in view of generating a catalytic system, 2 can be re-oxidized to 1 in the presence of NH₃ and NaOCl closing a pseudo-catalytic "synthetic" cycle. Together, the reduction of 1→2 followed by oxidation of 2→1 yield a genuine synthetic cycle for NH₃ oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis.