Complexes of tantalum with triaryloxides: Ligand and solvent effects on formation of hydride derivatives

A family of tantalum compounds supported by the triaryloxide [R–L]3− ligands are reported [H3(R–L) = 2,6-bis(4-methyl-6-R-salicyl)-4-tert-butylphenol, where R = Me or tBu]. The reaction of H3[Me–L] with TaCl5 in toluene gave [(Me–L)TaCl2]2 (1). The [tBu–L] analogue [(tBu–L)TaCl2]2 (2) was synthesized via treatment of TaCl5 with Li3[tBu–L]. A THF solution of LiBHEt3 was added to 1 in toluene to provide [(Me–L)TaCl(THF)]2 (3), while treatment of 2 with 2 equiv of LiBHEt3 or potassium in toluene followed by recrystallization from DME resulted in formation of [M(DME)3][{(tBu–L)TaCl}2(μ-Cl)] [M = Li (4a), K (4b)]. When the amount of MBHEt3 (M = Li, Na, K) was increased to 5 equiv, the analogous reactions in toluene afforded [{(bit-tBu–L)Ta}2(μ-H)3M] [M = Li(THF)2 (5a), Na(DME)2 (5b), K(DME)2 (5c)]. During the course of the reaction, the methylene CH activation of the ligand took place. Dissolution of 5a in DME produced [{(bit-tBu–L)Ta}2(μ-H)3Li(DME)2] (6), indicating that the coordinated THF molecules are labile. When the 2/LiBHEt3 reaction was carried out in THF, the ring opening of THF occurred to yield [(tBu–L)Ta(OBun)2]2 (7) along with a trace amount of [Li(THF)4][{(tBu–L)TaCl}2(μ-OBun)] (8). Treatment of 2 with potassium hydride in DME yielded [{(tBu–L)TaCl2K(DME)2}2(μ-OCH2CH2O)] (9), in which the ethane-1,2-diolate ligand arose from partial C–O bond rupture of DME. The X-ray crystal structures of 2, 3, 4, 5a, 6, 7, and 9 are described.