Studies of the alkylation of niobium and tantalum halides containing 2,6-diarylphenoxide ancillary ligation

Abstract

The mixed halo(aryloxide) compounds [Nb{OC₆H(C₆H₄Me-4)₂-2,6-Ph₂-3,5}₂Cl₃], [Ta(OC₆HPh₄-2,3,5,6)₂Br₃] and [Nb(OC₆HPh₄-2,3,5,6)₂I₃] have been isolated and structurally characterized. Although the chloride and bromide compounds are square pyramidal with an axial aryloxide, the tris(iodide) compound was determined to adopt a trigonal bipyramidal geometry in the solid state with trans, axial aryloxides. The reaction of the tantalum tris(chlorides) [Ta(OC₆H₃Ph₂-2,6)₂Cl₃] or [Ta(OC₆HPh₄-2,3,5,6)₂Cl₃] with RMgCl (R = CH₂Ph or CH₂SiMe₃) forms the corresponding mono-, bis- and tris-(alkyls), all shown to adopt a trigonal bipyramidal geometry in the solid state with trans, axial aryloxides. In contrast the mono(cycloalkyl) compounds [Ta(OC₆H₃Ph₂-2,6)₂(C_nH_2n+1)Cl] (n = 5, 6) were found to adopt square pyramidal structures in the solid state with the alkyl group axial (metal pseudo-equatorial) and mutually cis aryloxides. The alkylation of the corresponding niobium substrate [Nb(OC₆HPh₄-2,3,5,6)₂Cl₃] with Me₃SiCH₂MgCl produced a complex mixture of products. Careful analysis including the use of ²⁹Si NMR identified the d⁰-alkyl products [Nb(OC₆HPh₄-2,3,5,6)₂Cl₂(CH₂SiMe₃)], [Nb(OC₆HPh₄-2,3,5,6)₂Cl(CH₂SiMe₃)₂] and [Nb(OC₆HPh₄-2,3,5,6)₂(CH₂SiMe₃)₃], the alkylidene compounds [Nb(OC₆HPh₄-2,3,5,6)₂Cl(=CHSiMe₃)] and [Nb(OC₆HPh₄-2,3,5,6)₂(CH₂SiMe₃)(=CHSiMe₃)] and the d¹-alkyl [Nb(OC₆HPh₄-2,3,5,6)₂(CH₂SiMe₃)₂]. Treating a benzene solution of [Nb(OC₆HPh₄-2,3,5,6)₂(CH₂SiMe₃)₂]/[Nb(OC₆HPh₄-2,3,5,6)₂(CH₂SiMe₃)(=CHSiMe₃)] and PMe₃ at 85 °C under 1200 psi of H₂ produced deep-green crystals of the d¹-di(hydride) [Nb(OC₆HCy₂-2,6-Ph₂-3,5)₂(H)₂(PMe₃)₂].