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 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.

Dynamic Reorganization and Confinement of TiIV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes

“…To understand the molecularstructural effects controlling these differences in reactivity, we first consider whether changes to inner-sphere ligands of Ti IV active centers exist across cTi/X catalysts. UV−visible calix(O)−Ti LMCT edge-energies, known to be sensitive to the number of covalent connections between calix [4]arene oxygens and grafted Ti IV cations, 93 are shown in Table 1. Their uniform value of 2.28 ± 0.02 eV for all cTi/X catalysts is indicative of identical connectivity across all Ti IV surface complexes, consisting of a tridentate connection to phenolic structural units of the calix [4]arene, and single-point connectivity to the support, 15 as depicted in Figure 1.…”

Characterization of a Molecule Partially Confined at the Pore Mouth of a Zeotype

“…[36] Such aconformational change can be expected, given the well-known conformational flexibility of the calix- [4]arene scaffold. [34,42,46,47] Experimental observations by XAS in Figure 1and Table 1determined that Ti-O distances in cTi/ 2 are lengthened (indicated by alarger value of R)and exhibit stronger distortion (as indicated by alarger value of s 2 )from an ideal tetrahedron than in all other cTi/X materials (R = 1.86 , s 2 = 5 10 À3 2 in cTi/2 versus R = 1.82 , s 2 = 3 10 À3 2 ). Consistent with this,e lectronic structure calculations show lengthened and less symmetric Ti À Obond lengths in cTi-cone/I (hd Ti-O i = 1.83 AE 0.04 )r elative to those modeled without partial confinement in pore mouths,incTi-cone (hd Ti-O i = 1.82 AE 0.01 )( section 2.4, Figure 3).…”

“…UV-visible spectra of all materials (Figure S5) exhibit broad absorption features at 290-500 nm, arising from calix- [4]arene(O)-Ti IV ligand-to-metal charge transfer (LMCT), [27] consistent with calix [4]arene coordination to Ti IV after grafting. Themeasured edge energies associated with these LMCT bands are known to be sensitive to the number of covalent connections between calix [4]arene oxygens and grafted Ti IV cations, [34] and their values are shown in Table 1. Theobserved uniform value of 2.29 AE 0.03 eV for all cTi/X materials is indicative of identical connectivity within grafted cTi centers across all materials.This result confirms uniform chelation of Ti IV centers by calix [4]arene macrocycles,inamanner that is irrespective of the support.…”

“…To further investigate the conformation of cTi,w e isotopically enriched the lower-rim methoxy position of cTi with 13 C( red in Scheme 1) and characterized all samples via 13 C{ 1 H} CPMAS NMR spectroscopy.O ur approach involves using d( 13 C) of this substituent as as ensitive reporter of the dative interaction distance between the methoxy oxygen and Ti IV center (red dashed line in Scheme 1), [27] because we expect this distance to be affected by changes in the calix- [4]arene conformation. [34,42,43] Figure 2s hows the 13 C{ 1 H} CPMAS NMR spectra, with resonance chemical shifts summarized in Table 1. All cTi/X samples except cTi/2 exhibit alower-rim methoxy-substituent resonance at 64.2 AE 0.2 ppm, in reasonable agreement with values observed for analogous complexes in solution.…”

Use of Titanium Complexes Bearing Diphenolate or Calix[n]arene Ligands in α-Olefin Polymerization and the ROP of Cyclic Esters

“…Even lower conversion (11%) was observed under air when employing the same reaction conditions. Furthermore, complex 66 was supported on silica by following a slightly modified reported procedure [41], and tested in the ROP of all monomers previously investigated (Table 22). The catalyst efficiently promoted the polymerization of ε-CL, and complete conversion was observed after 24 h at 130 • C, regardless of the use of benzyl alcohol (Table 22, entries 1 and 2).…”