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Effect of Coordination Environment in Grafted Single-Site Ti-SiO2 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 SiO2, and its effect on epoxidation catalysis by Ti(IV)-calixarene grafted on SiO2 is investigated. C2v 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 Cs 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-C1-Ti complexes can be more selective than mono-C1-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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Acknowledgments

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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Correspondence to Alexander Katz.

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Grosso-Giordano, N.A., Solovyov, A., Hwang, S. et al. Effect of Coordination Environment in Grafted Single-Site Ti-SiO2 Olefin Epoxidation Catalysis. Top Catal 59, 1110–1122 (2016). https://doi.org/10.1007/s11244-016-0630-y

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  • DOI: https://doi.org/10.1007/s11244-016-0630-y

Keywords

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