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 H3HIPTN3N] 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]MoN, {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.