Effect of Coordination Environment in Grafted Single-Site Ti-SiO2 Olefin Epoxidation Catalysis
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 -...
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Published in | Topics in catalysis Vol. 59; no. 13-14; pp. 1110 - 1122 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
New York
Springer US
01.08.2016
|
Subjects | |
Online Access | Get full text |
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Summary: | 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).
13
C 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,
13
C 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
13
C 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. |
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ISSN: | 1022-5528 1572-9028 |
DOI: | 10.1007/s11244-016-0630-y |