Neutral and cationic di(tert-butyl) cyclopentadienyl titanium, zirconium and hafnium complexes. Dynamic NMR study of the ligand-free cations [M(1,3-tBu2-η5-C5H3)(η5-f5H5)(CH3)]+(M=Zr, Hf)

Abstract

Group 4 metal complexes containing the di(tert-butyl)cyclopentadienyl ligand (l,3-tBu2-r/5-CsH3) 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 - tBu2CsH3)]-(THF)I_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-tBu2C5H3XCH3)3 3. The silyl derivative 3 is an excellent precursor for monocyclopentadienyl trichlorotitanium and zirconium compounds M(1,3 -t Bu 2-r/5-C 5 H 3)C13 [M = Ti (4), Zr (5)]. Addition of a stoichiometric amount of water in the presence of NEt 3 to a toluene solution of 4 affords the oxo trimer compound [Ti(1,3-tBu2-~75 - CsH3)CI( p,-O)] 3 6. The reaction of 4 with 2 equiv, of LiMe affords the chloro dimethyl derivative Ti(1,3-tBu2-'r/5-CsH3)CI(CH3)2 7. The mixed dicyclopentadienyl compounds M(1,3-tBu2-r/5-CsH3XCsHs)CI2 [M = Ti (8); Zr (9)] were prepared by reaction of complexes 4 and 5 respectively with TI(CsHs). 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-@-C5H3XCsHs)CIR [M = Ti, R = Me (10); M = Zr, R = Me (11), Bz (12)] or the dialkyl complexes M(1,3-tBu2-@-CsH3)(CsHs)Rz [M = Ti, R = Me (13); M = Zr, R = Me (14), Bz (15), Nf (16)]. When 8 reacts with 2 equiv. of MgBz2(THF) 2 or LiCH2CMe2Ph the metallacyclic complexes Ti(1-tBu-3-CMe2CH~-r/5-C~Ha)(CsHs)R [R = Bz (17); Nf (18)] were isolated as red oils at room temperature, with the elimination of toluene or ten-butyl benzene respectively. The previously reported cationic mono 1,3-di(tert-butyl)cyclopentadienyl dibenzyl zirconium species [Zr(1,3 -t Bu 2-'05-C 5 H 3 XCH 2 Ph) 2 ] + (19) can be stabilized by reaction with tBuNC or PMe 3, in CD2C12 at -78°C, and the formation of the new cationic species [Zr(1,3-tBu2-r/5- CsHa)(L)(CH2Ph)2] + [L=tBuNC (20); PMe 3 (21)] was identified by NMR spectroscopy. The reaction of B(CrFs) 3 with the monocyclopentadienyl trimethyl derivatives M(1,3-tBu2-r/5-CsH3XCH3)3 [M = Ti (22), Zr (23)], in the presence of PMe 3, gives the cationic species [M(I,3-tBu2-@-C~H3)(PMe3)2(CH3)2] + [M = Ti (24); Zr (25)], obtained as orange-yellow solids, stable at room temperature. The reaction of B(C6Fs) 3 with the metallocene dimethyl derivatives M(1,3-tBu2-r/5-CsHa)(@-CsHs)(CH3)z [M = Zr (14); Hf (26)], in a 1:1 molar ratio and in hydrocarbon solvents gives the cationic derivatives [M(1,3-tBu2-@-CsH3)(@ - CsHsXCH3)]+[(CH3)B(CrFs)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(C6Fs) 3 are carried out in a 2:1 molar ratio at room temperature, the complexes {[M(1,3-tBu2-@-CsH3X@-CsH5)Me]2(/.L-Me)}[MeB(C6Fs)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(C6Fs) 3 bonds, leading to the formation of the free [M(I,3-tBu2-r/5-CsH3)(r/5-CsHs)(CH3)] + 14-electron species, verified by 1H DNMR spectroscopy. When compound (27) was heated at 50°C the metallacyclic cation [Zr(1-tBu-3-CMezCH2-@-C5H3)(@-CsHs)] + (31) was formed. The alkyl derivatives synthesized and reported herein, activated with MAO, B(C6Fs) 3 or [Ph3C][B(C6Fs)4], polymerize ethylene with very low activity. The molecular structure of [Ti(1,3-tBu2-r/5- C5H3)C1(/x-O)] 3 6 has been determined by X-ray diffraction methods.