Molybdenum Complexes that Contain a Calix[6]azacryptand Ligand as Catalysts for Reduction of N₂ to Ammonia

[CAC(OMe)₆]Mo(N) (3, where [CAC]^3- is a calix[6]azacryptand ligand derived from a [6]calixarene) has been prepared in a reaction between Li₃[CAC(OMe)₆] and ( t-BuO)₃Mo(N). An X-ray structural study showed 3 to have a structure similar to that of [HIPTN₃N]Mo(N) (where [HIPTN₃N]^3- is [(3,5-(2,4,6-triisopropylphenyl)₂C₆H₃NCH₂CH₂)₃N]^3-). The relatively rigid [CAC(OMe)₆]^3- ligand in 3 forms a bowl-shaped cavity defined by a 24-atom macrocyclic ring. The Mo-N_amido-C_ipso angles are ∼8° smaller in 3 than they are in [HIPTN₃N]Mo(N). Methoxides on the three linking units point into the cavity above the nitride in 3, whereas the three methoxides on phenyl rings attached to the amido nitrogen atoms point away from the cavity. An analogous [CAC(OMe)₃(H)₃]Mo(N) complex (9) was prepared in which the three methoxides pointing into the cavity in 3 have been replaced by protons. Its structure differs little from that of 3. The nitride could be protonated in 3 to give {[CAC(OMe)₆]Mo(NH)}⁺, which could be reduced (reversibly) to [CAC(OMe)₆]Mo(NH). Catalytic reduction of molecular nitrogen under a variety of conditions with either Ph₂NH₂OTf or HBArf (BArf^- = {B[3,5(CF₃)₂C₅H₃]₄}^-) as the acid and a Co metallocene or KC₈ as the reducing agent between -78 and 22 °C in diethyl ether shows that 1.20-1.34 equivalents of ammonia are formed starting with either [CAC(OMe)₆]Mo(N) (50% ¹⁵N) or [CAC(OMe)₃(H)₃]Mo(N) (50% ¹⁵N).