Configurational preferences and alkylidene-centered reactivity of tantalum alkoxide complexes containing an aryldiamine spectator Ligand

Article abstract of DOI:10.1021/om00020a044

Transmetalation of Ta(=CH-t-Bu){C₆H₃(CH₂NMe₂) ₂-2,6}Cl₂ with LiO-t-Bu or KOMe affords monoalkoxide complexes Ta(=CH-t-Bu){C₆H₃(CH₂NMe₂) ₂-2,6}Cl(OR) (R = t-Bu, Me) in moderate yield. The tert-butoxide complex Ta(=CH-t-Bu)){C₆H₃(CH₂NMe₂) ₂-2,6}Cl(O-t-Bu) reacts with (trimethylsilyl)ethene to afford a (trimethylsilyl)methylidene analogue via a quantitative alkene metathesis reaction. Transmetalation of Ta(=CH-t-Bu)Cl(O-t-Bu)₂(PMe₃) with 1/2 equiv of [Li{C₆H₃(CH₂NMe₂) ₂-2,6}]₂ gives the dialkoxide complex Ta(=CH-t-Bu){C₆H₃(CH₂NMe₂) ₂-2,6}(O-t-Bu)₂, 3d, in moderate yield. Complex 3d belongs to space group P1 with a = 9.3519(5) ?, b = 9.4443(3) ?, c = 18.1110(9) ?, α = 90.432(3)°, β = 100.241(4)°, γ = 116.593(3)°, and Z = 2. The pentacoordinate tantalum center in 3d has a distorted trigonal bipyramidal geometry with one of the NMe₂ units of the C,N-bidentate bonded aryldiamine (Ta-N(2) = 2.415(3) ?) and one of the alkoxide ligands in axial positions, while the second o-CH₂NMe₂ substituent is free (Ta?N(1) = 3.426(3) ?) but still trans-oriented to the alkylidene carbon atom. Complex 3d reacts with (trimethylsilyl)ethene to afford its (trimethylsilyl)methylidene analogue. A dialkoxide complex, Ta(=CH-t-Bu)(C₆H₄CH₂NMe ₂2)(O-t-Bu)₂, 3b, that is related to 3d but which contains only one o-amino substituent, can be prepared in a one-pot procedure by a sequence of transmetalation reactions starting from Ta(=CH-t-Bu)Cl₃(THF)₂ (THF = tetrahydrofuran), Zn(C₆H₄CH₂NMe₂-2)₂, and LiO-t-Bu. All alkoxide complexes are fluxional in solution at room temperature and undergo dissociation/ association processes involving the coordinative Ta-N bonds of the arylamine ligands.