Catalytic N−H Bond Activation and Breaking by a Well-Defined Co^II ₁O₄ Site of a Heterogeneous Catalyst

Catalytic N−H bond activation and breaking by well-defined molecular complexes or their heterogeneous analogues is considered to be a challenge in chemical science. Metal(0) nanoparticles catalytically decompose NH₃; they are, however, ill defined and contain a range of contiguous metal sites with varying coordination numbers and catalytic properties. So far, no well-defined/molecular Mⁿ⁺-containing materials have been demonstrated to break strong N−H bonds catalytically, especially in NH₃, the molecule with the strongest N−H bonds. Recently, noncatalytic activation of NH₃ with the liberation of molecular H₂ on an organometallic molybdenum complex was demonstrated. Herein, we show the catalytic activation and breaking of N−H bonds on a singly dispersed, well-defined, and highly thermally resistant (even under reducing environments) Co^II ₁O₄ site of a heterogeneous catalyst for organic (ethylamine) and inorganic (NH₃, with the formation of N₂ and H₂) molecules. The single-site material serves as a viable precursor to ultrasmall (2.7 nm and less) silica-supported cobalt nanoparticles; thus, we directly compare the activity of isolated cationic cobalt sites with small cobalt nanoparticles. Density functional theory (DFT) calculations suggest a unique mechanism involving breaking of the N−H bonds in NH₃ and N−N coupling steps taking place on a Co₁O₄ site with the formation of N₂H₄, which then decomposes to H₂ and N₂H₂; N₂H₂ subsequently decomposes to H₂ and N₂. In contrast, Co₁N₄ sites are not catalytically active, which implies that the ligand environment around a single atom of a heterogeneous catalyst largely controls reactivity. This may open a new chapter for the design of well-defined heterogeneous materials for N−H bond-activation reactions.