Hydrogenation of cationic dicyclopentadienyl zirconium (IV) alkyl complexes. Characterization of cationic zirconium(IV) hydrides

Article abstract of DOI:10.1021/om00148a023

The cationic Zr(IV) alkyl complex [Cp₂Zr(CH₃)(THF)][BPh₄] (1) undergoes hydrogenation to the insoluble hydride complex [Cp₂Zr(H)(THF)][BPh₄] (6) under mild conditions (23°C, 1 atm of H₂). This reaction is faster in CH₂Cl₂ (t_1/2 = 5 h) than in THF (t_1/2 = 21 h) and in the latter solvent is ca. 5 times faster than hydrogenation of Cp₂Zr(CH₃)₂ (2) to [Cp₂Zr(CH₃)(μ-H)]₂ (8). In CH₃CN, 1 forms the 18-electron complex [Cp₂Zr(CH₃)(CH₃CN)₂][BPh ₄] (3) which does not undergo significant reaction with H₂. Reaction of 1 with PMe₃ yields [Cp₂Zr(CH₃)(PMe₃)₂][BPh₄] (10) which undergoes rapid PMe₃ exchange at 23°C in THF and CH₂Cl₂ and very rapid (t_1/2 < 2 min) hydrogenation to the nonlabile hydride complex [Cp₂Zr(H)-(PMe₃)₂][BPh₄] (13). Complex 13 crystallizes in the monoclinic space group P2₁/c with a = 11.249 (4) A?, b = 19.082 (6) A?, c = 17.391 (5) A?, β = 99.57 (3)°, Z = 4, and R_wF = 6.53%. 13 exhibits a normal bent metallocene structure with the hydride ligand in the central position in the plane between the two Cp^- ligands. PMe₂Ph coordinates weakly to 1; in the presence of 17 equiv of PMe₂Ph, 1 undergoes rapid (t_1/2 = ca. 8 min) hydrogenation to the nonlabile hydride complex [Cp₂Zr(H)(PMe₂Ph)₂][BPh₄] (15) which by NMR is isostructural with 13. Neither PMePh₂ nor PPh₃ react with 1 to form detectable phosphine complexes. The presence of 17 equiv of PMePh₂ produces a minor acceleration of the hydrogenation of 1 (t_1/2 = 5 h, THF) and results in the formation of [Cp₂Zr(H)(PMePh₂)₂][BPh₄] (16) which by NMR is isostructural with 13 and 15. PPh₃ does not accelerate the hydrogenation of 1 and does not produce a phosphine hydride product. The 18-electron complex [Cp₂Zr(CH₃)(dmpe)][BPh₄] (11) does not react with H₂ even at elevated temperatures. As for neutral Cp₂Zr(R)(X) complexes, the hydrogenation reactivity of Cp₂Zr(CH₃)⁺ complexes depends strongly upon the availability of a low-energy Zr LUMO for interaction with H₂. The acceleration of the hydrogenation of 1 by PMe₃ and PMe₂Ph is ascribed to the removal of Zr-O π-bonding upon substitution of THF by phosphine.