Neutral and cationic di( tert-butyl) cyclopentadienyl titanium, zirconium and hafnium complexes dynamic NMR study of the ligand-free cations [M( 1,3-tBu2-μ5-C5H3)(μ5-C5H5)(CH3)]+ (M = Zr, Hf)

Article abstract of DOI:10.1016/S0022-328X(97)00034-X

Group 4 metal complexes containing the di(tert-butyl)cyclopentadienyl ligand (1,3-^tBu₂-η⁵-C₅H₃) have been synthesized. The reaction of a mixture of 1,3- and 1,4-di(tert-butyl)cyclopentadiene isomers with KH in THF at -78°C gives the salt K⁺[(1,3-Bu₂C₅H₃)]^-(THF)_1-3 2 as a white solid. Treatment of 2 with chlorotrimethylsilane in a 1:1 molar ratio gives the air-stable trimethylsilylcyclopentadienyl derivative Si(1,3-^tBu₂C₅H₃)(CH₃)₃ 3. The silyl derivative 3 is an excellent precursor for monocyclopentadienyl trichlorotitanium and zirconium compounds M(1,3-^tBu₂-η⁵-C₅H₃)Cl₃ [M = Ti (4), Zr (5)]. Addition of a stoichiometric amount of water in the presence of NEt₃ to a toluene solution of 4 affords the oxo trimer compound [Ti(1,3-^tBu₂-η⁵-C₅H₃)Cl(μ-O)]₃ 6. The reaction of 4 with 2equiv. of LiMe affords the chloro dimethyl derivative Ti(1,3-^tBu₂-η⁵-C₅H₃)Cl(CH₃)₂ 7. The mixed dicyclopentadienyl compounds M(1,3-^tBu₂-η⁵-C₅H₃)(C₅H₅)Cl₂ [M = Ti (8); Zr (9)] were prepared by reaction of complexes 4 and 5 respectively with T1(C₅H₅). Treatment of complexes (8) and (9) with the appropriate alkylating reagent and molar ratio, in hexane at - 78°C, gives the chloro alkyl derivatives M(1,3-^tBu₂-η⁵-C₅H₃)(C₅H₅)CiR [M = Ti, R = Me (10); M = Zr, R = Me (11), Bz (12)] or the dialkyl complexes M(1,3-^tBu₂-η⁵-C₅H₃)(C₅H₅)R₂ [M = Ti, R = Me (13); M = Zr, R = Me (14), Bz (15), Nf (16)]. When 8 reacts with 2equiv. of MgBz₂(THF)₂ or LiCH₂CMe₂Ph the metallacyclic complexes Ti(1-^tBu-3-CMe₂CH₂-η⁵-C₅H₃)(C₅H₅)R [R = Bz (17); Nf (18)] were isolated as red oils at room temperature, with the elimination of toluene or tert-butyl benzene respectively. The previously reported cationic mono 1,3-di(tert-butyl)cyclopentadienyl dibenzyl zirconium species [Zr(1,3-^tBu₂-η⁵-C₅H₃)(CH₂Ph)₂]⁺ (19) can be stabilized by reaction with ^tBuNC or PMe₃, in CD₂Cl₂ at - 78°C, and the formation of the new cationic species [Zr(1,3-^tBu₂-η⁵-C₅H₃)(L)(CH₂Ph)₂]⁺ [L=^tBuNC (20); PMe₃ (21)] was identified by NMR spectroscopy. The reaction of B(C₆F₅)₃ with the monocyclopentadienyl trimethyl derivatives M(1,3-^tBu₂-η⁵-C₅H₃)(CH₃)₃ [M = Ti (22), Zr (23)], in the presence of PMe₃, gives the cationic species [M(1,3-^tBu₂-η⁵-C₅H₃)(PMe₃)₂(CH₃)₂]⁺ [M = Ti (24); Zr (25)], obtained as orange-yellow solids, stable at room temperature. The reaction of B(C₆F₅)₃ with the metallocene dimethyl derivatives M(1,3-^tBu₂-η⁵-C₅H₃)(η⁵-C₅H₅)(CH₃)₂ [M = Zr (14); Hf (26)], in a 1:1 molar ratio and in hydrocarbon solvents gives the cationic derivatives [M(1,3-^tBu₂-η⁵-C₅H₃)(η⁵-C₅H₅)(CH₃)]⁺[(CH₃)B(C₆F₅)₃]^- [M = Zr (27); Hf (28)] as yellow oils which can be stored for weeks under an inert atmosphere. When the same reactions of (14) and (26) with B(C₆F₅)₃ are carried out in a 2:1 molar ratio at room temperature, the complexes {[M(1,3-^tBu₂-η ⁵-C₅H₃)(η⁵-C₅H₅)Me]₂(μ,-Me)}[MeB(C₆F₅)₃] [M = Zr (29), Hf (30)] can be obtained as a mixture of syn- and anti-isomers as shown by NMR spectroscopic observations. The formation of (29) and (30) implies the stabilization of the 14-electron cationic intermediate by interaction with one methyl group of the neutral complexes (14) and (26). Complexes (27) and (28) undergo heterolytic dissociation of the Metal-MeB(C₆F₅)₃ bonds, leading to the formation of the free [M(1,3-^tBu₂-η⁵-C₅H₃)(η⁵-C₅H₅)(CH₃)]⁺ 14-electron species, verified by 1H DNMR spectroscopy. When compound (27) was heated at 50°C the metallacyclic cation [Zr(1-^tBu-3-CMe₂CH₂-η⁵-C₅H₃)(η⁵-C₅H₅)]⁺ (31) was formed. The alkyl derivatives synthesized and reported herein, activated with MAO, B(C₆F₅)₃ or [Ph₃C][B(C₆F₅)₄], polymerize ethylene with very low activity. The molecular structure of [Ti(1,3-^tBu₂-η⁵-C₅H₃)Cl( μ-O)]₃ 6 has been determined by X-ray diffraction methods.