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

5 years ago

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

Gabriel Ménard, Guang Wu, Raúl Hernández Sánchez, Madeline Peterson, Megan Keener, Victoria F. Oswald

Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric [Ar3N][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(C6F5)4]−). N2 production was confirmed by mass spectral analysis of the isotopomer, 1-15N, and the gas quantified. The metal products of oxidation were the reduced MnIII dimers, [(salen)MnCl]2 (2) or [(salen)Mn(OEt2)]2[B(C6F5)4]2 (3) for X=[SbCl6]− or [B(C6F5)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 MnV/MnIII bridging nitride, [(salen)MnV(μ-N)MnIII(salen)][B(C6F5)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 12 followed by oxidation of 21 yield a genuine synthetic cycle for NH3 oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis. Store that H2! Oxidation of a simple (salen)Mn≡N resulted in the 6e− nitride coupling reaction, central to NH3 oxidation to N2. A complete synthetic cycle was established with implications for catalytic NH3 oxidation for H2-storage applications. An unusual mixed-valent MnV/MnIII species was also isolated and characterized (see scheme).

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201703153