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

• Published in: Inorganic chemistry 2018-12, Vol.57 (24), p.15566-15574
• Main Authors: Wickramasinghe, Lasantha A, Schrock, Richard R, Tsay, Charlene, Müller, Peter
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.8b02903

[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–Namido–Cipso 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[CAC­(OMe)3(H)3]­Mo­(N) complex (9) was prepared in which the three methoxides pointing into the cavity in 3 have been replaced by protons.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).