Synthesis, structure and reactivity of high-valent early transition metal complexes containing pi coordinated carbocycles and hard pi donor ligands.

Abstract

Ta(DIPP)Cl₄ (1) has been prepared from the reaction of TaCl₅ with 1 equiv of Me₃SiDIPP and is shown to be dimeric. While Ta(DIPP)Cl₄ reacts with Et₂O to form trans-Ta(DIPP)Cl₄(OEt₂) (2), TaCl₅ and Me₃SiDMP react in the presence of Et₂O to provide trans-Ta(DMP)Cl₄(OEt₂) (3) directly. TaCl₅ reacts with 2 equiv of Me₃SiDMP to form the bis phenoxide complex Ta(DMP)₂Cl₃(OEt₂) (6). All of these compounds cyclize 3-hexyne upon their two electron reduction, to form (η⁶-C₆Et₆)Ta(OR)ₓCl₃₋ₓ (OR = DIPP, DMP, x = 1; OR = DMP, x = 2), but do not readily undergo the analogous reaction with 2-butyne. However, (η⁶-C₆Me₆)Ta(DIPP)Cl₂ (10) can be prepared in essentially quantitative yield from the reaction of (η⁶-C₆Me₆)Ta(DIPP)₂Cl (9) with Ta(DIPP)₂Cl₃(OEt₂). The arene ligand in (η⁶-C₆Me₆)Ta(DIPP)Cl₂ (10) is characterized by a folded structure and considerable localization of the π electron density in a 1,4-diene fashion. The thermolysis of (η⁶-C₆Me₆)Ta(DIPP)₂Cl 9 produces free C₆M₆ along with the "tucked in" complex (η¹-C₆Me₅CH₂)Ta(DIPP)₂Cl₂ (11). Compound 11 can be prepared in higher yields from 9 by its thermal decomposition in the presence of Me₃SiCl. Alkylation of tantalum(III) arene complexes affords stable dialkyl (η⁶-C₆Me₆)Ta(DIPP)R₂ and monoalkyl halide species (η⁶-C₆Me₆)Ta(DIPP)RX. The alkyl hydride complexes (η⁶-C₆Me₆)Ta(DIPP)R(H) are also prepared from (η⁶-C₆Me₆)Ta(DIPP)RX and LiBEt₃H. The arene ring in (η⁶-C₆Me₆)Ta(DIPP)Et₂ (18) exhibits a structure consistent with a diene-diyl distortion. The first evidence for the formation of a d¹ arene species is presented in cyclic voltammetry experiments on these compounds. The addition of 4 equiv of LiNHAr to a solution of ZrCl₄(THF)₂ in THF/PY yields Zr(=NAr)(NHAr)₂(PY)₂ (26). Zr(=NAr)(NHAr)(PY)₂ reacts with 1 or 2 equiv of TMSCl in THF/PY to provide Zr(=NAr)(NHAr)Cl(PY)₂ (27) and Zr(=NAr)Cl₂(PY)₃ (28) respectively. MCl₄(THF)₂ (M = Zr or Hf) react with 4 equiv LiNHAr followed by the addition of 2 equiv TMSCl to provide M(=NAr)Cl₂(THF)₂ (24, M = Zr; 25 M = Hf). Both 24 and 25 react with K₂ (C₈H₈) to provide ((η⁸-C₈H₈)M(=NAr))₂ (31, M = Zr; 32, M = Hf). Zr(=NAr)Cl₂(THF)₂ reacts with 1 equiv Li(C₅H₄Me) to provide ((η⁵-C₅H₄Me)Zr(=NAr)Cl)₂ (30). An imprecise structure of this dimeric compound has been determined which exhibits bridging imido functionalities. (η⁵-C₅Me₅)ZrCl₃(THF) reacts with 3 equiv LiNHAr in THF/PY to provide ((η⁵-C₅Me₅)Zr(=NAr)(NHAr)(PY))₂ (33). Compound 33 reacts with 1 equiv TMSCl in THF/PY to provide monomeric (η⁵-C₅Me₅)Zr(=NAr)Cl(PY)₂ (35).