Grosso-Giordano, Nicolás A. and Solovyov, Andrew and Hwang, Sonjong and Katz, Alexander (2016) Effect of Coordination Environment in Grafted Single-Site Ti-SiO_2 Olefin Epoxidation Catalysis. Topics in Catalysis, 59 (13-14). pp. 1110-1122. ISSN 1022-5528. doi:10.1007/s11244-016-0630-y. https://resolver.caltech.edu/CaltechAUTHORS:20160915-101225482
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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). ^(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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Additional Information: | © 2016 Springer Science+Business Media New York. First Online: 10 June 2016. The authors are grateful to Prof. Dr. Udo Radius for providing structural data from previous molecular modeling studies [24] of Ti-calixarene complexes, and Dr. Kathleen Durkin for assistance with data analysis. Funding from the U.S. Department of Energy Office of Basic Energy Sciences (DE-FG02-05ER15696) and the National Science Foundation (IIP 1542974) is gratefully acknowledged. | ||||||||||||
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Subject Keywords: | Terminal olefin epoxidation; Titanium calixarene; Organic hydroperoxide; Tert-butyl hydroperoxide; Heterogeneous catalysis; Coordination environment; Ligand-to-metal charge-transfer; Grafted organometallic complex conformation | ||||||||||||
Issue or Number: | 13-14 | ||||||||||||
DOI: | 10.1007/s11244-016-0630-y | ||||||||||||
Record Number: | CaltechAUTHORS:20160915-101225482 | ||||||||||||
Persistent URL: | https://resolver.caltech.edu/CaltechAUTHORS:20160915-101225482 | ||||||||||||
Official Citation: | Grosso-Giordano, N.A., Solovyov, A., Hwang, S. et al. Top Catal (2016) 59: 1110. doi:10.1007/s11244-016-0630-y | ||||||||||||
Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. | ||||||||||||
ID Code: | 70369 | ||||||||||||
Collection: | CaltechAUTHORS | ||||||||||||
Deposited By: | Tony Diaz | ||||||||||||
Deposited On: | 29 Sep 2016 23:44 | ||||||||||||
Last Modified: | 11 Nov 2021 04:28 |
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