Synthesis and structure of niobium and tantalum compounds containing 2,6-diphenyl-3,5-di-tert-butylphenoxide ligation

A series of mononuclear derivatives of niobium(V) and tantalum(V) containing the new ligand 2,6-diphenyl-3,5-di-tert-butyl-phenoxide (OC6HPh2-2,6-tBu2-3,5) have been isolated. The addition of the parent phenol (2 equiv. per Nb) to a benzene solution of [Nb2Cl10] leads to formation of the trichloride [Nb(OC6HPh2-2,6-tBu2-3,5)2Cl3] (1), while treatment of the tetra-chlorides [Cp*TaCl4] and [CpNbCl4] with [LiOC6HPh2-2,6-tBu2-3,5] leads to the mixed cyclopentadienyl, aryloxides [Cp*Ta(OC6HPh2-2,6-tBu2-3,5)Cl3] (2) and [CpNb(OC6HPh2-2,6-tBu2-3,5)Cl3] (3), respectively. Structural studies show a square pyramidal geometry for trichlorides 1–3 with a basal aryloxide in each case. An axial aryloxide, Cp* and Cp ligand are present for 1, 2 and 3, respectively. The addition of HOC6HPh2-2,6-tBu2-3,5 to the precursor [Ta(NMe2)5] results in formation of the mono-aryloxide [Ta(OC6HPh2-2,6-tBu2-3,5)(NMe2)4] (4), which is also square pyramidal with a basal aryloxide ligand. Activation of a solution of 1 in benzene with 3 equiv. of nBuLi, followed by treatment with H2 (1200 psi, 100°C) results in intramolecular hydrogenation of ortho-phenyl rings. Hydrolysis of the reaction mixture leads to isolation of 2,6-dicyclohexyl-3,5-di-tert-butylphenol (5), which has been structurally characterized.