However, in spite of the many attempts made, 4 the crucial cationic d 0 metal-alkene complex has only been observed and studied by X-ray diffraction 5 in the zirconium complex [ZrCp 2 (OCMe 2 CH 2 CH 2 CHNCH 2 )] + , in which the alkenic double bond shows a rather weak interaction with the metal center. The isolation of dimethyl titanium 6a and zirconium 6b complexes containing the 1-(but-3-enyl)-2,3,4,5-tetramethylcyclopentadienyl ligand has been reported, for which extensive low temperature NMR studies were unable to detect the alkenecoordinated metal complex. An yttrium pentenyl chelate has also been studied, 7 where the exchange between the diastereotopic C 5 H 4 Me ligands is proposed to occur via rapid and reversible alkene dissociation followed by rate limiting inversion of the pyramidal d 0 yttrium center. A related compound reported by Erker 8 was isolated by reaction of the butadiene zirconium complex [ZrCp 2 (C 4 H 6 )] with B(C 6 F 5 ) 3 , resulting in the formation of a betaine system in which an anionic allyl system is coordinated to the dicyclopentadienyl zirconium cation.In order to favour alkene coordination we decided to synthesize zirconium complexes containing the more electronwithdrawing di(methyl)(allyl)silyl(cyclopentadienyl) ligand to create a more acidic metal center with a slightly longer and more fluxional pendant chain, owing to the bulkier silicon atom.As shown in Scheme 1 the dichloro complex [Zr(h 5 -C 5 H 5 ){h 5 -C 5 H 4 SiMe 2 (CH 2 CHNCH 2 )}Cl 2 ] 1 was prepared † by reaction of the lithium salt of di(methyl)(allyl)silylcyclopentadiene 9 with stoichiometric amounts of ZrCpCl 3 ·DME. Treatment of 1 with MgBn 2 ·2THF yielded the dibenzyl complex 2.NMR spectra recorded after mixing 1 equiv. of either Stabilization of this 14-electron cationic zirconium species is usually achieved by benzallylic coordination of the benzyl ligand, evidenced by the loss of the C 2v symmetry of the phenyl ring and a substantial difference between the 1 H chemical shifts for the two methylenic protons (Dd = 2.80 ppm) observed 1c in the solid by X-ray analysis and in solution by NMR spectroscopy at low temperatures. However, this behaviour is not observed in the 1 H NMR spectra of 3 + at 203 K which shows C 2v symmetry for the phenyl ring, an AB spin system (d av 3.13, Dd = 0.27 ppm) for the two diastereotopic methylenic protons, two SiMe signals at d 0.31 and 0.59, and four resonances at d 5. 74, 6.13, 6.35, 7.10 due to the CpSi ring protons, consistent with an asymmetric species. The 1 H NMR spectra of 3 + did not show a typical vinylic signal between d 4.9 and 5.6, but instead five new multiplets were observed at d 1.81, 2.05, 2.13, 2.78, 7.30. Table 1 summarizes the chemical shifts and proton-proton coupling constants for the allyl chain protons of complexes 2 and 3 + .All of the spin-spin coupling constants for H 3 , H 4 and H 5 in complexes 2 and 3 + are very similar indicating that the sp 2 character of the alkenic carbon atoms is not lost. However the H 3 signal is shifted [Dd = d(3 + ) 2 d(2)]...
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The reaction of the chlorosilyl-substituted cyclopentadienyltitanium compound [Ti(η 5 -C 5 H 4 -SiMe 2 Cl)Cl 3 ] with N,N′-dimethylethylenediamine in the presence of 2 equiv of NEt 3 gave a mixture of two complexes, [Ti{η 5 -C 5 H 4 SiMe 2 NMe(CH 2 ) 2 -η-NMe}Cl 2 ] (1) and [Ti{η 5 -C 5 H 4 SiMe 2η-N(CH 2 ) 2 -η-NHMe}Cl 2 ] (2), in a molar ratio of 3:1. However, a similar reaction with N-methylethylenediamine afforded complex 2 regiospecifically. The dialkyl derivatives [Ti- 3), CH 2 Ph (4)) were prepared by reacting the dichloride complex 1 with 2 equiv of MgClMe or 1 equiv of Mg(CH 2 Ph) 2 •2THF, respectively. The analogous reaction with LiNMe 2 afforded the diamido derivative [Ti{η 5 -C 5 H 4 SiMe 2 NMe(CH 2 ) 2 -η-NMe}(NMe 2 ) 2 ] (5) in high yield. Complex 1 reacts with dry CO 2 via insertion of a molecule of CO 2 into each of the Ti-N and Si-N bonds to yield the dicarbamate compound [Ti{(η 5 -C 5 H 4 SiMe 2 OC(O)NMe(CH 2 ) 2 NMe(η 2 -CO 2 )}Cl 2 ] (6), in which the carbamate groups are bound in η 2 -and η 1 -fashion. Thermal decomposition of complex 6 afforded the known oxo derivatives [Ti{µ-(η 5 -C 5 H 4 SiMe 2 -η-O)}Cl 2 ] 2 and [(TiCl 2 ) 2 (µ-O)-[µ-{(η 5 -C 5 H 4 SiMe 2 ) 2 -(µ-O)}] with elimination of CO 2 and 1,3-dimethyl-2-imidazolidinone. The conformational interconversion of 1 and 6 and the reversible coordination of the terminal "NHMe" group to the metal center in 2 were studied in solution by DNMR spectroscopy. The crystal structure of 1 was determined by X-ray diffraction methods.
Reaction of TaCp" C! 4 with MOR (M -Li, Na) in different molar ratios gives halo alkoxides TaCp" CI,(OR)4_,, (n -3: R -tBu 1; $iPh 3 2; 2,6-Me2C6H 3 3; n --2: R -~Bu 4; SiPh 3 5; 2,6-Me2C6H3 6; a -!: R -SiMe 3 7) in good yields. The alkylidene complex TaCp'(CH2SiMe,02(CHSiMe ~) is hydrolyzed in the presence of traces of water to give the oxo dialkyl derivative [TaCp'(CH2SiM¢3)~O],, 8, and reacts with I equivalent of 2,6-MezC6H~NC affording a new 'q2-iminoacyi compound TaCp" (CH 2$iMe3XCHSiMe3){~ 2"C(CH 2SiMe3 ) = N(2,6"MeaC6 H 3)} 9. Reactions of TaCp "(CH 2SiMe3)a(CHSiMe3 ) with I equivalent of C6HsOH and 4-MeC6H3(OH) 2 result in the formation of the alkyl phenoxo TaCp'(CH2SiMe3)3(OC6Hs) 10 and 4-methyl pyrocatecholate TaCp'(CH2SiMe~)2(O2C6H3Me) 11, whereas the related re,",,:don with 2,6-Me2C6H 3OH leads to the cyclic alkyi-aikoxo compound TaCp (CH2SiMej)2(~2-O(2-CH2-6-MeC6H3)) 12. All the complexes were characterized by IR and NMR (tH and 13C) spectroscopy. The crystal and molecular structure of 12 has been determine~i. C~ystals of 12 are triclinic, space group P'[ with Z -2 in a unit cell of dimensions a-9.
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