A calix[4]arene monoalkyl ether as a model of a tris(phenolate) ligand with a hemilabile anisole moiety: Syntheses, molecular structures and bonding of calix[4]arene ether supported titanium complexes and their catalytic activity in epoxidation reactions

New mononuclear titanium complexes [TiCl(Rcalix)] [R = Me (1), Bz (2), SiMe3 (3)] supported by p-tert-butylcalix[4]arene mono(organyl)ethers (Rcalix) were prepared in good yield from H(2)R(2)calix and [TiCl4(THF)(2)]. The crystallographically characterized complex [TiCl(Mecalix)] (1) reacts readily with NaCp and LiNR2 to afford the complexes [TiCp(Mecalix)] (4) and [Ti(NR2)(Mecalix)] [R = Me (5), Et (6), iPr (7), Ph (8)], respectively. Reactions of [Ti(NR2)(Mecalix)] (5) with alcohols, phenols and thiols proceed cleanly with amide exchange, as exemplified by the synthesis of [Ti(OR)(Mecalix)] [R = Me (10), iPr (11), tBu (12), 4-tBuC(6)H(4) (13), 2,6-iPr(2)C(6)H(3) (14)], and [Ti(StBu)(Mecalix)] (15). Depending on the steric demand of the ligand coordinated to the [Ti(Mecalix)] complex fragment these compounds are monomeric or dimeric in the solid state, as demonstrated by the molecular structures of monomeric [Ti(OC6H3-2,6-iPr(2))(Mecalix)] (14) and [Ti(StBu)(Mecalix)] (15), or dimeric [{Ti(OMe)(Mecalix)}(2)] (10). In all complexes the calix[4]arene ligand adopts an elliptically distorted cone conformation in which the distance of the titanium atom from the anisole oxygen atom varies between 2.342 Angstrom and 2.438 Angstrom for the structurally characterized complexes, with the exception of the Cp complex 4. MP2 calculations on model complexes demonstrate that the titanium-anisole oxygen bond is weak, with a particularly shallow potential energy surface in the region between 2.30 Angstrom and 2.80 Angstrom The Mecalix ligand system therefore might be described as a tris(phenolate) ligand with a hemilabile anisole group in the titanium compounds reported. Despite the labile anisole ether titanium bond, isomerization of the calix[4]arene ligand to a paco coordination mode, as described earlier for the Me(2)calix complexes [Ti(OC6H4-4-R)(2)(Me(2)calix)], has not been observed in these complexes. Preliminary results on the catalytic epoxidation of cyclooctene with TBHP using calix[4]arene-stabilized titanium(iv) complexes are also briefly presented. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.