Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a tert-Butylimido Ligand

A variety of molybdenum or tungsten complexes that contain a tert-butylimido ligand have been prepared. For example, the o-methoxybenzylidene complex W(N-t-Bu)(CH-o-MeOC6H4)(Cl)2(py) was prepared through addition of pyridinium chloride to W(N-t-Bu)2(CH2-o-MeOC6H4)2, while Mo(N-t-Bu)(CH-o-MeOC6H4)(ORF)2(tBuNH2) complexes (ORF = OC6F5 or OC(CF3)3) were prepared through addition of two equivalents of RFOH to Mo(N-t-Bu)2(CH2-o-MeOC6H4)2. An X-ray crystallographic study of Mo(N-t-Bu)(CH-o-MeOC6H4)[OC(CF3)3]2(t-BuNH2) showed that the methoxy oxygen is bound to the metal and that two protons on the tert-butylamine ligand are only a short distance away from one of the CF3 groups on one of the perfluoro-tert-butoxide ligands (H···F = 2.456(17) and 2.467(17) Å). Other synthesized tungsten tert-butylimido complexes include W(N-t-Bu)(CH-o-MeOC6H4)(pyr)2(2,2′-bipyridine) (pyr = pyrrolide), W(N-t-Bu)(CH-o-MeOC6H4)(pyr)(OHMT) (OHMT = O-2,6-(mesityl)2C6H3), W(N-t-Bu)(CH-t-Bu)(OHMT)(Cl)(py) (py = pyridine), W(N-t-Bu)(CH-t-Bu)(OHMT)(Cl), W(N-t-Bu)(CH-t-Bu)(pyr)(ODFT)(py), W(N-t-Bu)(CH-t-Bu)(OHMT)2, and W(N-t-Bu)(CH-t-Bu)(ODFT)2 (ODFT = O-2,6-(C6F5)2C6H3). Interestingly, W(N-t-Bu)(CH-t-Bu)(OHMT)2 does not react with ethylene or 2,3-dicarbomethoxynorbornadiene. Removal of pyridine from W(N-t-Bu)(CH-t-Bu)(BiphenCF3)(pyridine) (BiphenCF3 = 3,3′di-tert-butyl-5,5′-bistrifluoromethyl-6,6′-dimethyl-1,1′-biphenyl-2,2′-diolate) with B(C6F5)3 led to formation of a five-coordinate 14e neopentyl complex as a consequence of CH activation in one of the methyl groups in one tert-butyl group of the BiphenCF3 ligand, as was proven in an X-ray study. An attempted synthesis of W(N-t-Bu)(CH-t-Bu)(BiphenMe) (BiphenMe = 3,3′-di-tertbutyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) led to formation of a 1:1 mixture of W(N-t-Bu)(CH-t-Bu)(BiphenMe) and a neopentyl complex analogous to the one characterized through an X-ray study. The metallacyclobutane complexes W(N-t-Bu)(C3H6)(pyrrolide)(ODFT) and W(N-t-Bu)(C3H6)(ODFT)2 were prepared in reactions involving W(N-t-Bu)(CH-tBu)(pyr)2(bipy), ZnCl2(dioxane), and one or two equivalents of DFTOH, respectively, under 1 atm of ethylene.