The synthesis and structure of Group 5 metal alkyl and alkylidene complexes containing 2,6-dialkylphenoxide ligands: X-ray crystal structures of [Ta(OC6H3Me2-2,6)2(CH2Ph)3], [Ta(OC6H3Me2-2,6)4Me], and [Ta(OC6H3But2-2,6)2(CHSiMe3)(CH2SiMe3)]

Abstract

A number of alkyl and alkylidene complexes of niobium and primarily tantalum containing 2,6-dialkylphenoxide ligation, OC₆H₃R₂-2,6 (R = Me, Prⁱ, or Bu^t) have been synthesized and characterized. For the sterically less demanding 2,6-dimethyl- and 2,6-di-isopropyl-phenoxides a series of compounds [Ta(OC₆H₃R₂-2,6)₂(CH₂R′)₃](R′= H, SiMe₃, or Ph) can be readily obtained by alkylation of the corresponding mixed chloro-aryloxide. The remaining two aryloxides are more resistant to further substitution. Spectroscopic evidence supports the σ-bonded compounds adopting a trigonal-bipyramidal structure with trans-axial aryloxide ligands. This structure was confirmed by a single-crystal X-ray diffraction study on [Ta(OC₆H₃Me₂-2,6)₂(CH₂Ph)₃]. The di- and mono-alkyls [Ta( OC₆H₃Me₂-2,6)₃( CH₂Ph)₂] and [Ta(OC₆H₃Me₂-2,6)₄Me] have also been isolated and the latter shown to adopt a square-pyramidal structure in the solid state with an axial methyl group. In contrast the reaction of [Ta(OC₆H₃Bu^t₂-2,6)₂Cl₃] with Li(CH₂SiMe₃) or Mg(CH₂Ph)₂ leads to the mixed alkyl–alkylidene complexes [Ta(OC₆H₃Bu^t₂-2,6)₂(CHR′)(CH₂R′)](R′= SiMe₃ or Ph). The presence of the alkylidene functional group was confirmed both spectroscopically and by a single-crystal X-ray diffraction study on the trimethylsilyl derivative. A related complex, [Ta(OC₆H₃Prⁱ₂-2,6)₂(CHSiMe₃)(CH₂SiMe₃)], was obtained by photolysis of [Ta(OC₆H₃Prⁱ₂-2,6)₂(CH₂SiMe₃)₃].