Synthesis of Molybdenum(VI) Monoimido Alkyl and Alkylidene Complexes

In this paper we report the synthesis of several new Mo(NR)Cl 4 (THF) species (R = C 6 F 5, 3,5-(CF 3 ) 2 C 6 H 3, 1-adamantyl, CPh 3, and 2,6-i-Pr 2 C 6 H 3 ) via the treatment of MoCl 4 (THF) 2 with azides and their reactions with neopentyl reagents. Addition of Mo(NR)Cl 4 (THF) complexes in toluene to a cold solution of NpMgCl in ether gave Mo(NR)Np 3 Cl species (R = C 6 F 5, 3,5-(CF 3 ) 2 C 6 H 3, Ad, Ph 3 C, and 2,6-i-Pr 2 C 6 H 3 (Ar); Np = CH 2 -t-Bu) in poor (35%) to modest (51%) yields. Heating Mo(NAr)Np 3 Cl in C 6 D 6 to 50 °C results in α-hydrogen abstraction to give neopentane and a molecule whose NMR spectra are consistent with it being Mo(NAr)(CH-t-Bu)(CH 2 -t-Bu)Cl; it decomposed bimolecularly upon attempted isolation. The other Mo(NR)Np 3 Cl species were found to be more stable than Mo(NAr)Np 3 Cl, but when they did decompose at elevated temperatures, no neopentylidene complex could be observed. Addition of neopentyllithium to Mo(NR)Np 3 Cl species (R = Ar, CPh 3, or Ad) yielded Mo(NR)(CH-t-Bu)Np 2 species, the adamantylimido version of which is unstable toward bimolecular decomposition. Addition of neopentyllithium to Mo(NR)Np 3 Cl complexes in which R = pentafluorophenyl or 3,5-trifluoromethylphenyl led to intractable mixtures. Addition of 1 equiv of 2,6-diisopropylphenol, 2,6-dimethylphenol, or 3,5-(2,4,6-i-Pr 3 C 6 H 2 ) 2 C 6 H 3 OH (HIPTOH) to Mo(NCPh 3 )(CH-t-Bu)Np 2 led to formation of Mo(NCPh 3 )(CH-t-Bu)Np(OR) species, while treatment of Mo(NCPh 3 )(CH-t-Bu) 2 (CH 2 -t-Bu) with C 6 F 5 OH gave Mo(NCPh 3 )Np 3 (OC 6 F 5 ). The three monophenoxide neopentylidene complexes showed poor to moderate metathesis activity for ring-closing a small selection of substrates. X-ray studies were completed for Mo[N-3,5-(CF 3 ) 2 C 6 H 3 ]Cl 4 (THF), Mo[N-3,5-(CF 3 ) 2 C 6 H 3 ]Np 3 Cl, Mo(NCPh 3 )Np 3 Cl, and Mo(NCPh 3 )(CH-t-Bu)(CH 2 -t-Bu)(OHIPT).