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
✍ Scribed by Andreas Friedrich; Udo Radius
- Publisher
- John Wiley and Sons
- Year
- 2004
- Tongue
- English
- Weight
- 367 KB
- Volume
- 2004
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Abstract
New mononuclear titanium complexes [TiCl(Rcalix)] [R = Me (1), Bz (2), SiMe~3~ (3)] supported by p‐tert‐butylcalix[4]arene mono(organyl)ethers (Rcalix) were prepared in good yield from H~2~R~2~calix and [TiCl~4~(THF)~2~]. The crystallographically characterized complex [TiCl(Mecalix)] (1) reacts readily with NaCp and LiNR~2~ to afford the complexes [TiCp(Mecalix)] (4) and [Ti(NR~2~)(Mecalix)] [R = Me (5), Et (6), __i__Pr (7), Ph (8)], respectively. Reactions of [Ti(NR~2~)(Mecalix)] (5) with alcohols, phenols and thiols proceed cleanly with amide exchange, as exemplified by the synthesis of [Ti(OR)(Mecalix)] [R = Me (10), __i__Pr (11), __t__Bu (12), 4‐__t__BuC~6~H~4~ (13), 2,6‐__i__Pr~2~C~6~H~3~ (14)], and [Ti(S__t__Bu)(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(OC~6~H~3~‐2,6‐__i__Pr~2~)(Mecalix)] (14) and [Ti(S__t__Bu)(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 Å and 2.438 Å 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 Å and 2.80 Å. 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(OC~6~H~4~‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)