Intramolecular coordination of an alkene to a mixed dicyclopentadienyl benzyl zirconium cation studied by NMR spectroscopy

Abstract

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)]...

Alkyl–η2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl–η5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations

“…This is consistent with the three 13 C signals observed at  ~134-136 (internal CH),  ~113-114 (terminal CH 2 ) and  ~25-26 (SiCH 2 ). The chemical shifts and coupling constants of these signals are only very slightly affected by the remaining substituents being almost the same for group 4 metal(IV) 3,4 and for group 5 metal(III) complexes 13 and 14.…”

“…The dichloro niobium(IV) and tantalum(IV) 1-3 and the dialkyl group 4 metal complexes [M( 5 -C 5 R 5 ){ 5 -C 5 H 4 SiMe 2 (CH 2 -CHCH 2 )}R′ 2 ] used as precursors for the preparation of the olefin coordinated compounds are symmetric molecules for which both right and left equatorial positions are equivalent. Therefore, treatment with either, a reducing agent (Na/Hg for 4), an alkylating agent (5-12) or B(C 6 F 5 ) 3 (cationic group 4 metal compounds) may eliminate any of the two symmetric chloro or alkyl ligands. After coordination of the terminal alkene moiety all the resulting compounds are characterized by two chiral principles, namely the enantioface of the coordinated vinyl moiety and the metal atom.…”

“…Exchange between two enantiomeric structures was observed and studied by DNMR spectroscopy 3 between 193 and 253 K.…”

“…We reported 3,4 previously the synthesis of group 4 metal dialkyls containing an allyldimethylsilyl-cyclopentadienyl ligand, which after treatment with a Lewis acid B(C 6 F 5 ) 3 , coordinate the olefinic system of the allyl moiety to give alkyl cations with a chelated  5 -Cp- 2 -alkene ligand. Exchange between two enantiomeric structures was observed and studied by DNMR spectroscopy 3 between 193 and 253 K.…”

Unprecedented Alkene Complex of Zinc(II): Structures and Bonding of Divinylzinc Complexes

“…To increase the binding of olefins to d 0 metal complexes, initial efforts focused on the use of ligands with pendent vinyl groups to exploit the chelation effect. This approach has proven successful with main group, 42-44 transition, [14][15][16][17][18][19][24][25][26] and lanthanide 13 metal complexes lacking d-electrons available for back bonding. In some cases, convincing NMR spectroscopic evidence has been reported, while in others, compounds were sufficiently crystalline for X-ray structure determination.…”