Selectivity of tungsten mediated dinitrogen splitting vs. proton reduction

Abstract

Mo complexes are currently the most active catalysts for nitrogen fixation under ambient conditions. In comparison, tungsten platforms are scarcely examined. For active catalysts, the control of N₂vs. proton reduction selectivities remains a difficult task. We here present N₂ splitting using a tungsten pincer platform, which has been proposed as the key reaction for catalytic nitrogen fixation. Starting from [WCl₃(PNP)] (PNP = N(CH₂CH₂PtBu₂)₂), the activation of N₂ enabled the isolation of the dinitrogen bridged redox series [(N₂){WCl(PNP)}₂]^0/+/2+. Protonation of the neutral complex results either in the formation of a nitride [W(N)Cl(HPNP)]⁺ or H₂ evolution and oxidation of the W₂N₂ core, respectively, depending on the acid and reaction conditions. Examination of the nitrogen splitting vs. proton reduction selectivity emphasizes the role of hydrogen bonding of the conjugate base with the protonated intermediates and provides guidelines for nitrogen fixation.