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)]
A number of alkyl and alkylidene complexes of niobium and primarily tantalum containing 2,6-dialkylphenoxide ligation, OC6H3R2-2,6 (R = Me, Pri, or But) 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(OC6H3But2-2,6)2Cl3] with Li(CH2SiMe3) or Mg(CH2Ph)2 leads to the mixed alkyl–alkylidene complexes [Ta(OC6H3But2-2,6)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].