Reduction of Dinitrogen to Ammonia at a Well-Protected Reaction Site in a Molybdenum Triamidoamine Complex

We have synthesized a triamidoamine ligand ([(RNCH₂CH₂)₃N]^3-) in which R is 3,5-(2,4,6-i-Pr₃C₆H₂)₂C₆H₃ (HexaIsoPropylTerphenyl or HIPT). The reaction between MoCl₄(THF)₂ and H₃[HIPTN₃N] in THF followed by 3.1 equiv of LiN(SiMe₃)₂ led to formation of orange [HIPTN₃N]MoCl. Reduction of [HIPTN₃N]MoCl with magnesium in THF under dinitrogen led to formation of salts that contain the {[HIPTN₃N]Mo(N₂)}^- ion. The {[HIPTN₃N]Mo(N₂)}^- ion can be oxidized by zinc chloride to give [HIPTN₃N]Mo(N₂) or protonated to give [HIPTN₃N]Mo−NN−H. Other relevant compounds that have been prepared include {[HIPTN₃N]Mo−NNH₂}⁺, [HIPTN₃N]Mo⋮N, {[HIPTN₃N]MoNH}⁺, and {[HIPTN₃N]Mo(NH₃)}⁺. (The anion is usually {B(3,5-(CF₃)₂C₆H₃)₄}^- = {BAr‘₄}^-.) Reduction of [HIPTN₃N]Mo(N₂) with CoCp₂ in the presence of {2,6-lutidinium}BAr‘₄ in benzene leads to formation of ammonia and {[HIPTN₃N]Mo(NH₃)}⁺. Preliminary X-ray studies suggest that the HIPT substituent creates a deep, three-fold symmetric cavity that protects a variety of dinitrogen reduction products against bimolecular decomposition reactions, while at the same time the metal is left relatively open toward reactions near the equatorial amido ligands.