Effect of Coordination Environment in Grafted Single-Site Ti-$ SiO_{2} $ Olefin Epoxidation Catalysis

Abstract The effect of calixarene ligand symmetry, as dictated by lower-rim substitution pattern, on the coordination to a Ti(IV) cation is assessed in solution and when grafted on $ SiO_{2} $, and its effect on epoxidation catalysis by Ti(IV)-calixarene grafted on $ SiO_{2} $ is investigated. $ C_{...
Ausführliche Beschreibung

Abstract The effect of calixarene ligand symmetry, as dictated by lower-rim substitution pattern, on the coordination to a Ti(IV) cation is assessed in solution and when grafted on $ SiO_{2} $, and its effect on epoxidation catalysis by Ti(IV)-calixarene grafted on $ SiO_{2} $ is investigated. $ C_{2v} $ symmetric Ti-tert-butylcalix[4]arene complexes that are 1,3-alkyl disubstituted at the lower rim (di-R-Ti) are compared to previously reported grafted $ C_{s} $ symmetric complexes, which are singly substituted at the lower rim (mono-R-Ti). 13C MAS NMR spectra of complexes isotopically enriched at the lower-rim alkyl position indicate that di-R-Ti predominantly grafts onto silica as the conformation found in solution, exhibiting a deshielded alkyl resonance compared to the grafted mono-R-Ti complexes, which is consistent with stronger alkyl ether→Ti dative interactions that are hypothesized to result in higher electron density at the Ti center. Moreover, 13C MAS NMR spectroscopy detects an additional contribution from an “endo” conformer for grafted di-R-Ti sites, which is not observed in solution. Based on prior molecular modeling studies and on 13C MAS NMR spectroscopy chemical shifts, this “endo” conformer is proposed to have similar Ti–(alkyl ether) distances at the lower-rim and electron density at the Ti center relative to grafted mono-R-Ti complexes. Differences between grafted mono-R-Ti and di-R-Ti sites can be observed by ligand-to-metal charge transfer edge-energies, calculated from diffuse-reflectance UV–visible spectroscopy at 2.24 ± 0.02 and 2.16 ± 0.02 eV, respectively. However, rates of tert-butyl hydroperoxide consumption in the epoxidation of 1-octene are found to be largely unchanged when compared to those of the grafted mono-R-Ti complexes, with average rate constants of ~1.5 $ M^{−2} $ $ s^{−1} $ and initial TOF of ~4 $ ks^{−1} $ at 323 K. This suggests that an “endo” conformation of grafted di-R-Ti may prevail during catalysis. Despite this, grafted di-$ C_{1} $-Ti complexes can be more selective than mono-$ C_{1} $-Ti complexes (45 vs. 34 % at a 50 % conversion at 338 and 353 K), illustrating the importance of the Ti coordination environment on epoxidation catalysis. Ausführliche Beschreibung

Autor*in:	Grosso-Giordano, Nicolás A. [verfasserIn] Solovyov, Andrew [verfasserIn] Hwang, Sonjong [verfasserIn] Katz, Alexander [verfasserIn]

Format:	E-Artikel
Sprache:	Englisch

Erschienen:	2016

Schlagwörter:	Terminal olefin epoxidation Titanium calixarene Organic hydroperoxide Tert-butyl hydroperoxide Heterogeneous catalysis Coordination environment Ligand-to-metal charge-transfer Grafted organometallic complex conformation

Übergeordnetes Werk:	Enthalten in: Topics in catalysis - Bussum : Baltzer, 1994, 59(2016), 13-14 vom: 10. Juni, Seite 1110-1122
Übergeordnetes Werk:	volume:59 ; year:2016 ; number:13-14 ; day:10 ; month:06 ; pages:1110-1122

Links:	Volltext

DOI / URN:	10.1007/s11244-016-0630-y

Katalog-ID:	SPR018118496