CONFIGURATIONAL PREFERENCES AND ALKYLIDENE-CENTERED REACTIVITY OF TANTALUM ALKOXIDE COMPLEXES CONTAINING AN ARYLDIAMINE SPECTATOR LIGAND

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}CI(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{C6Hs(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 P1BAR with a = 9.3519(5) angstrom, b = 9.4443(3) angstrom, c = 18.1110(9) angstrom, alpha = 90.432(3)-degrees, beta = 100.241(4)-degrees, gamma = 116.593(3)-degrees, and Z = 2. The pentacoordinate tantalum center in Sd 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) angstrom) and one of the alkoxide ligands in axial positions, while the second o-CH2NMe3 substituent is free (Ta...N(l) = 3.426(3) angstrom) 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)(C6H4CH2NMe2-2)(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.