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(CnH2n+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].