Steric influences in cyclopentadienyl-monophenoxide complexes of titanium(iv) arising from ortho-substitution of the phenoxide ligand

Reaction of [CpTiCl3] with 1 equiv. of Me3SiOC6H4CMe3-4 in benzene or thermalisation with an excess of neat Me3SiOC6H4CMe3-4 gives good yields of crystalline [CpTiCl2(OC6H4CMe3-4)] (1) for which an X-ray crystal structure determination shows a distorted tetrahedral geometry. Reaction of [CpTiCl3] and Me3SiOC6H3CMe3-2-Me-4 via the benzene solvent method or the direct thermalisation gives rise to crystalline [CpTiCl2(OC6H3CMe3-2-Me-4)] (2) for which an X-ray crystal structure determination shows a distorted tetrahedral geometry little changed from complex 1 but with the phenoxide ligand ring rotated so that two of the tert-butyl group methyls straddle one chloro ligand. Refluxing [CpTiCl3] and Me3SiOC6H4Ph-4 in toluene gave crystalline [CpTiCl2(OC6H4Ph-2)] (3) which X-ray crystallography shows has a distorted tetrahedral geometry little changed from 1 or 2. The phenoxide ligand ring is rotated less in 1 than in 2 or 3 with the 2-phenyl substituent sited between the chloro and Cp ligands. The complexes differ only in the interplanar angle between the Cp and phenoxide ligand phenyl rings [6.4(1)° for 1, 29.4(1)° for 2 and 22.2(1)° for 3]. DFT calculations carried out on 2 are in good agreement with the X-ray crystal structure. A relaxed PES scan shows that the tert-butyl group in the complex cannot freely rotate past the adjacent chloro ligand and is essentially locked in close proximity to it. There is a shallow minimum (2 kJ/mol within a range of 60 degrees) for the Ti–O–C–C dihedral angle indicating there is sufficient torsional motion within the molecule to accommodate the tert-butyl group in the observed position.

Graphical abstract[CpTiCl2(OC6H4CMe3-4)] (1), [CpTiCl2(OC6H3CMe3-2-Me-4)] (2) and [CpTiCl2(OC6H4Ph-2)] (3) are formed by reaction of the appropriate TMS phenol with [CpTiCl3]. X-ray crystal structures show similar tetrahedral distortions for the three complexes but with different rotations of the phenoxide ring depending on the substituent at the ortho-position. DFT calculations on 2 shows that the tert-butyl group can not rotate past the adjacent chloro ligand and there is sufficient flexibility in the Ti–O–C–C torsional angle to accommodate it.Figure optionsDownload full-size imageDownload as PowerPoint slide