Decomposition of nitrous oxide in hydrated cobalt(i) clusters: a theoretical insight into the mechanistic roles of ligand-binding modes

Abstract

Hydrated cobalt(I) cluster ions, [Co(H₂O)_n]⁺, can decompose the inert nitrous oxide molecule, N₂O. Density functional theory suggests that N₂O can anchor to Co⁺ of [Co(N₂O)(H₂O)_n]⁺ through either O end-on (η¹-OL) or N end-on (η¹-NL) coordinate mode. The latter is thermodynamically more favorable resulting from a subtle π backdonation from Co⁺ to N₂O. N₂O decomposition involves two major processes: (1) redox reaction and (2) N–O bond dissociation. The initial activation of N₂O through an electron transfer from Co⁺ to N₂O yields anionic N₂O⁻, which binds to the metal center of [Co²⁺(N₂O⁻)(H₂O)_n] also through either O end-on (η¹-O) or N end-on (η¹-N) mode and is stabilized by water molecules through hydrogen bonding. From η¹-O, subsequent N–O bond dissociation to liberate N₂, producing [CoO(H₂O)_n]⁺, is straightforward via a mechanism that is commonplace for typical metal-catalyzed N₂O decompositions. Unexpectedly, the N–O bond dissociation directly from η¹-N is also possible and eliminates both N₂ and OH, explaining the formation of [CoOH(H₂O)_n]⁺ as observed in a previous experimental study. Interestingly, formation of [CoO(H₂O)_n]⁺ is kinetically controlled by the initial redox process between Co⁺ and the O-bound N₂O, the activation barriers of which in large water clusters (n ≥ 14) are higher than that of the unexpected N–O bond dissociation from the N-bound structure forming [CoOH(H₂O)_n]⁺. This theoretical discovery implies that in the present of water molecules, the metal-catalyzed N₂O decomposition starting from an O-bound metal complex is not mandatory.