Synthesis, stereochemistry, bonding and fluxionality of 2-(inden-3-yl)phenols and their cyclopentadienyl titanium derivatives
The phenolic reagents 2-(inden-3-yl)-4,6-di-tert-butylphenol (1) and its 2-methyl (2), 1,2-dimethyl (3), 2,4,7-trimethyl (4) and 1,2,4,7-tetramethyl (5) derivatives have been obtained. The solid-state structures of 2, 3 and 4 have been determined by X-ray diffraction and torsion angles (between the vinyl group and phenoxy ring) of 57, 61 and 79° measured. Both the (aR) and (aS) forms of the ligands are present in the solid state. Solution 1H NMR spectra show that rotation about the indenyl–phenoxy bond is facile for 1 but introduction of the 2-methyl substituent leads to restricted rotation on the NMR timescale. In the crystals of 3 analyzed, only the (aS,S) and (aR,R) forms were present, with the 1-methyl group pointing away from the OH group. In the 1H and 13C NMR spectra of 3 and 5 there are two, equal intensity sets of signals. Hence, both diastereoisomeric forms are present in equal concentrations in solution and two sharp OH singlets are observed for 3 even at 130 °C in p-xylene-d10. Reaction of the phenols 1–4 with [CpTiCl3] in the presence of pyridine has previously generated the corresponding mono-aryloxides 7–10 with no evidence of the de-protonation of the indenyl ring in these reactions. In all four compounds both the (aR) and (aS) forms are present within the unit cells. Variable temperature NMR studies of 7 allow the barrier to inden-3-yl rotation (enantiomer interconversion) to be estimated at 13.9(5) kcal mol−1 (at 20 °C). In solution only one major set of 1H and 13C NMR resonances were observed for 1,2-dimethyl derivative 9. Hence, it appears that replacement of the phenolic proton by the much bulkier [CpTiCl2] unit destabilizes the (aS,R) and (aR,S) forms in solution. Attempted de-protonation of the inden-3-yl ring in 7 by treatment with n-BuLi or MeLi did not lead to the formation of chelate rings. Instead formation of a Ti(III) dinuclear compound 11 and dimethyl derivative 12 occurred. The barrier to indenyl rotation in 12 can be estimated to be 13.4(5) kcal mol−1 at −5 °C, slightly lower than that measured for the dichloride 7. Reaction of [CpTiCl3] with the di-lithio (doubly deprotonated ligand) did lead to the formation of a chelated indenyl-phenoxide derivative 13. In the solid state only the (pS,R) and (pR,S) forms were observed (indenyl oriented towards the Cp group, and in solution only one set of 1H and 13C NMR signals were observed indicating the presence of this single diastereoisomer. The bonding of the indenyl group and Cp ligands in 13 have been compared.