Proximally bridged calix[4]arene compounds (DESC)H2 (3), (DMSHC)H2 (4), (DMSMC)H2 (5), and (DPSC)H2 (6), in which one R2Si group (R=alkyl or aryl) bridges adjacent oxygens, were synthesized via reaction between dialkyl- or diaryldichlorosilane and the corresponding calix[4]arene. Treatment of p-tert-butylcalix[4]arene with Ph2SiCl2 at room temperature or (o-MeC6H4)2SiCl2 at 80 °C gave (ClPh2SiCl)2Calix-H2 (7) and (o-Tol2SiCl)2Calix-H2 (8), respectively. Titanium dichloride complexes 9–12 (L2TiCl2, where L2=DESC, DMSHC, DMSMC, or DPSC) were prepared in high yield from reaction of 3–6 with TiCl4. The molecular structures of 7 and 12 were established by single-crystal X-ray diffraction studies. Reduction of 9, 11, and 12 with activated magnesium (Mg*) in the presence of an excess of Me3SiCCH produced titananorbornadiene complexes L2Ti{η6-1,2,4-C6H3(SiMe3)3} (13–15, L2=DESC, DMSMC, or DPSC), which were characterized in solution. Catalytic cyclotrimerization of both terminal and internal alkynes was achieved using catalyst systems derived from L2TiCl2 complexes 9–12 and Mg*. For unsymmetrically substituted internal alkynes, preference for 1,2,4-substitution decreased as the size difference of the substituent groups decreased. The cyclotrimerization of PhCCMe was more facile when the calixarene-derived bis(aryloxide) ligand was DPSC versus DMSMC, suggesting that the DPSC ligand may provide a less crowded titanium center and exert greater kinetic control over the course of the cyclotrimerization.Titananorbornadiene complex formation and catalytic alkyne cyclotrimerization occur when titanium dichloride complexes containing calix[4]arene-derived bis(aryloxide) ligands are reduced by magnesium in the presence of excess alkyne. Our results indicate that steric properties of the R2Si bridging group of calixarene-derived bis(aryloxide) ligands affect reactivity of the titanium complexes.

Titanium(IV)mono(salicylaldiminato) complexes [L1Ti(NMe2)3] (1) and [L1TiCl3] (2) have been synthesized by treatment of Ti(NMe2)4 or TiCl4 with one equivalent of [4,6-Bu2t-2-(CHNBut)C6H3OH] (L1H) or [4,6-Bu2t-2-(CHNBut)C6H3OSiMe3] (L1SiMe3), respectively. The compounds are monomeric in solution and in the solid-state. Reactions of TiCl4 with one equivalent of [4,6-Bu2t-2-(CHNCH2Ph)C6H3OH] (L2H) and [4,6-Bu2t-2-{CHN(2-C6H4OH)}C6H3OH] (L3H2) produced [L2TiCl2(μ-Cl)]2 (3) and [L3TiCl2]2 (4), respectively. [L3TiCl2(THF)] (5) was also produced in quantitative yield when 4 was stirred in THF for 16 h. The reaction of TiCl4 with L3H2 (Two equivalents) in toluene gave [(L3)2Ti] (6). The molecular structures of 2–4 and 6 were established by single-crystal X-ray diffraction studies; and 4 displayed a rare face to face π–π stacking interaction in its structure. Compounds 1 and 2 showed modest ethylene polymerization activities at 25 °C with 900 molar equivalents of methylalumoxane (MAO) as co-catalyst.Monomeric titanium(IV)mono(salicylaldiminato) complexes [L1Ti(NMe2)3] (1) and [L1TiCl3] (2) (L1=4,6-Bu2t-2-(CHNBut)C6H3O−) were synthesized and characterized by spectroscopic methods and X-ray crystallography. Reactions of TiCl4 with one equivalent of [4,6-Bu2t-2-(CHNCH2Ph)C6H3OH] (L2H) and [4,6-Bu2t-2-{CHN(2-C6H4OH)}C6H3OH] (L3H2) produced dimeric [L2TiCl2(μ-Cl)]2 (3) and [L3TiCl2]2 (4), respectively. The structure of 4 contained a rare face to face π–π stacking interaction. With methylalumoxane as co-catalyst, 1 and 2 showed modest activities in ethylene polymerization.

We investigated the activity, epoxide selectivity, H2O2 efficiency, and recyclability of heterogeneous alkene epoxidation catalysts prepared by encapsulation of tripodal Ti silsesquioxane complexes in polydimethylsiloxane (PDMS) membrane. We found that [Ti(NMe2){(c-C6H11)7Si7O12}]/PDMS (3) and [Ti(NMe2){(i-C4H9)7Si7O12}]/PDMS (4) displayed high activity, epoxide selectivity (⩾97%), and H2O2 efficiency (⩾97%) in cyclohexene- and 1-octene epoxidation with aqueous H2O2. Moreover, these catalysts were highly recyclable. The high H2O2 efficiency can be attributed to the uniformly non-polar environment provided about the Ti silsesquioxane complex in 3 and 4 by the PDMS membrane, which presumably results in low local water concentrations and higher [alkene]:[H2O2] ratios at the Ti center than in the bulk reaction medium; both of these effects favor the epoxide selectivity and minimal leaching of titanium. Our study of the effect of solvent on catalyst activity revealed that acetonitrile is to be preferred as solvent, since methanol inhibits the epoxidation at long reaction times.Polydimethylsiloxane (PDMS)-immobilized Ti silsesquioxane catalysts [Ti(NMe2){(c-C6H11)7Si7O12}]/PDMS (3) and [Ti(NMe2){(i-C4H9)7Si7O12}]/PDMS (4), displayed high activity and recyclability, as well as excellent epoxide selectivity (⩾97%), and H2O2 efficiency (⩾97%) in cyclohexene- and 1-octene epoxidation with aqueous H2O2.Download high-res image (75KB)Download full-size image