Half-Sandwich Zirconium and Hafnium Amidoborane Complexes: Precursors of Hydride Derivatives

Half-sandwich zirconium(IV) and hafnium(IV) complexes with amidoborane and hydride ligands have been isolated in the stoichiometric reactions of mono(pentamethylcyclopentadienyl)metal alkyl and amido derivatives with the amine-boranes NHR₂BH₃ (R₂ = H₂, Me₂, HtBu). Treatment of the tris(trimethylsilylmethyl) complexes [M(η⁵-C₅Me₅)(CH₂SiMe₃)₃] with NH₃BH₃ (3 equiv) gives the seven-coordinate species [M(η⁵-C₅Me₅)(NH₂BH₃)₃] (M = Zr (1), Hf (2)) with three κ²N,H-NH₂BH₃ ligands. The tris(neophyl) [M(η⁵-C₅Me₅)(CH₂CMe₂Ph)₃] or tris(dimethylamido) [M(η⁵-C₅Me₅)(NMe₂)₃] derivatives react with NHMe₂BH₃ (≥3 equiv) to afford bis(dimethylamidoborane) hydride complexes [M(η⁵-C₅Me₅)H(NMe₂BH₃)₂] (M = Zr (3), Hf (4)) via thermally unstable [M(η⁵-C₅Me₅)(NMe₂BH₃)₃] species. The reaction of [M(η⁵-C₅Me₅)(NMe₂)₃] and NH₂tBuBH₃ (≥4 equiv) affords analogous mixed amidoborane hydride derivatives [M(η⁵-C₅Me₅)H(NHtBuBH₃)(NMe₂BH₃)] (M = Zr (5), Hf (6)) with κ²N,H-NHtBuBH₃ and κ³N,H,H-NMe₂BH₃ ligands. The addition of NHR₂BH₃ (≥1 equiv) on the mono(dimethylamido) complexes [M(η⁵-C₅Me₅)Cl₂(NMe₂)] in hexane leads to the precipitation of the ionic compounds [(NHR₂)₂BH₂][{M(η⁵-C₅Me₅)Cl₂}₂(μ-H)₃] (R₂ = Me₂, M = Zr (7), Hf (8); R₂ = HtBu, M = Zr (9), Hf (10)). Molecular hydride species [Cl₂(η⁵-C₅Me₅)M(μ-Cl)(μ-H)₂M(η⁵-C₅Me₅)Cl(NH₂tBu)] (M = Zr (11), Hf (12)) could be isolated from mixtures of complexes [M(η⁵-C₅Me₅)Cl₂(NMe₂)] and lower ratios of NH₂tBuBH₃. The zirconium complex 11 decomposes in solution to give the mononuclear tert-butylamido derivative [Zr(η⁵-C₅Me₅)Cl₂(NHtBu)] (13) along with other byproducts.