Oxo 2-Adamantylidene Complexes of Mo(VI) and W(VI)

Molybdenum and tungsten oxo 2-adamantylidene (Adene) complexes that contain two nonafluoro-tert-butoxide (ORF9) ligands have been prepared through addition of 2-methylene- or 2-ethylideneadamantane to neophylidene or neopentylidene complexes. The isolated oxo complexes include W(O)(Adene)(ORF9)(2)(PPh2Me) (1(w)), W(O)(Adene)(ORF9)(2) (2(w)), W(O)Adene)(2,5-dimethylpyrrolide)(2) (3(w)), W(O)(Adene)(2,5-dimethylpyrrolide)-(2,6-dimesitylphenoxide) (4(w)), Mo(O)(Adene)(ORF9)(2) (PPhMe2) (1(Mo)), and Mo(O)(Adene)(ORF9)(2) (2(Mo)). Compound 2(w) is a dimer that contains unsymmetrically bridging oxo ligands; it dissociates readily and reversibly into monomers, especially in the presence of a donor such as THF. In contrast, 2(Mo) is a monomer. Both 2(w) and 2(Mo) are remarkably stable thermally. The pale-blue complexes Mo(Adene)(ORF9)(2)Cl-2 (5(Mo)) and W(Adene)(ORF9)(2)Cl-2 (5(w)) are formed upon addition of PCl5 to 2(Mo) and 2(w), respectively. The oxo complexes are reactive olefin metathesis initiators, while 5(Mo) and 5(w) are relatively poor initiators. We ascribe the thermal stability of 2(Mo) and 2 w to resistance of the 2-adamantylidene ligands to couple bimolecularly, to the absence of an alpha-hydrogen in the alkylidene, or both.