Grafted Metallocalixarenes as Single-Site Surface Organometallic Catalysts

Abstract

Metallocalixarenes were grafted onto silica using a surface organometallic approach and shown to be active and selective catalysts for epoxidation of alkenes using organic hydroperoxides. Calixarene-Ti(IV) precursors were anchored at surface densities from 0.1 to near-monolayer coverages (0.025-0.25 calixarene nm(-2)). Several spectroscopic methods independently detected calixarene-Ti(IV) connectivity before and after epoxidation catalysis. Kinetic analyses of cyclohexene epoxidation confirmed that the active sites were anchored on the silica surface and were significantly more active than their homogeneous analogues. The steric bulk and multidentate binding of the calixarenes led to structural stability and to single-site behavior during epoxidation catalysis. Rate constants were independent of surface density for cyclohexene epoxidation with tert-butyl hydroperoxide (11.1 +/- 0.3 M(-2) s(-1)) or cumene hydroperoxide (25 +/- 2 M(-2) s(-1)). The materials and methods reported here allow the assembly of robust surface organometallic structures in which the active sites behave as isolated species, even near saturation monolayer coverages. In turn, this makes possible the rational design and synthesis of a class of heterogeneous oxide catalysts with atomic-scale precision at the active site.

Catalytic consequences of open and closed grafted Al(III)-calix[4]arene complexes for hydride and oxo transfer reactions

“…When comparing 6-SiO 2 (450) as catalyst at either dilute or saturation surface coverage of grafted Al(III)-calix [4]arene sites, there is no coverage dependence on the turnover frequency (SI Appendix). These data are consistent with the single-site nature of 6-SiO 2 (450) as catalyst, as observed for other grafted metallocalixarene-on-silica catalysts (36). In addition, the slightly lower rate for 6-SiO 2 (450) relative to 6-SiO 2 (800) suggests that a high degree of silica dehydroxylation may promote monodentate attachment of open precursors to the silica surface, because bidentate grafting is expected to synthesize MPV-inactive sites within a closed configuration.…”

“…A conceptually related approach has been used to site isolate grafted metal cations on silica surfaces within non-octahedral coordination geometries, using Lewis acidic metallocalixarene complexes as precursors for anchoring (36,37). Using the proven ability of tert-butyl-calix [4]arene ligands to site-isolate and template specific environments surrounding grafted metal cation catalysts on inorganic-oxide surfaces (36, 37), we synthesize both open and closed variants of a grafted metal catalyst active site (as represented by SI Appendix, Fig.…”

Site Isolation and Epoxidation Reactivity of a Templated Ferrous Bis(phenanthroline) Site in Porous Silica

“…[13] Two methods were used for the immobilization of 1 onto SBA-15 silica: metal templating and random ligand grafting. 4 ]PF 6 reestablishes the 2:1 ligand/metal ratio as determined by ICP analysis, thereby indicating fully reversible metal binding at the templated sites. The random ligand grafting method involves covalent attachment of 1 on SBA-15 in the absence of a metal template to give the grafted material G. [15] At a loading of about 0.11 mmol g À1 of 1 on either T or G, the average surface area per ligand is 500 2 , which allows (on average) effective siteisolation of each ligand.…”

“…[1][2][3] While the reactivity of many immobilized olefin oxidation catalysts is less than their homogeneous counterparts, some species show enhanced efficiency and/or greater enantioselectivity. [4] The manganese catalysts reported by Jacobs and co-workers benefit remarkably from the site isolation afforded by immobilization: while the homogeneous analogues are prone to forming Mn dimers that decompose H 2 O 2 , site isolation through immobilization yields more oxidant-efficient and substrate-efficient epoxidation catalysts. [2,5] An additional advantage of such site isolation is the potential to create metal-coordination environments with labile exogenous ligands not readily accessible in a homogeneous medium.…”

Study on an Aldol Reaction Catalyzed by Ti(IV)/Calix[n]arene Complexes

“…[11] Thus, some interesting biomimetic systems have been proposed, [12] which mainly possess hydrolytic activities, [13] whereas only a very few examples deal with the catalysis of reactions relevant to organic synthesis. [14] These include the enantioselective allylation of aldehydes catalyzed by a zirconium-BINOL catalyst activated by ptert-butylcalix [4]arene, [15] the reductive coupling of alkynes and ketones, [16] the epoxidation of allylic alcohols [17] and alkenes, [18] the Mukaiyama-aldol reaction of aromatic aldehydes with silyl enol ethers, [19] and the cyclotrimerization of alkynes.…”