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Effect of Coordination Environment in Grafted Single-Site Ti-SiO_2 Olefin Epoxidation Catalysis

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.


Item Type:Article
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http://dx.doi.org/10.1007/s11244-016-0630-yDOIArticle
http://link.springer.com/article/10.1007%2Fs11244-016-0630-yPublisherArticle
http://rdcu.be/rDatPublisherFree ReadCube access
ORCID:
AuthorORCID
Grosso-Giordano, Nicolás A.0000-0002-0912-366X
Hwang, Sonjong0000-0002-3210-466X
Katz, Alexander0000-0003-3487-7049
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.
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG02-05ER15696
NSFIIP-1542974
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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