Dialing in single-site reactivity of a supported calixarene-protected tetrairidium cluster catalyst.

A closed Ir4 carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulky tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl) ligands at the basal plane, was characterized with variable-temperature 13C NMR spectroscopy, which show the absence of scrambling of the CO ligands at temperatures up to 313 K. This demonstration of distinct sites for the CO ligands was found to extend to the reactivity and catalytic properties, as shown by selective decarbonylation in a reaction with trimethylamine N-oxide (TMAO) as an oxidant, which, reacting in the presence of ethylene, leads to the selective bonding of an ethyl ligand at the apical Ir site. These clusters were supported intact on porous silica and found to catalyze ethylene hydrogenation, and a comparison of the kinetics of the single-hydrogenation reaction and steady-state hydrogenation catalysis demonstrates a unique single-site catalyst-with each site having the same catalytic activity. Reaction orders in the catalytic ethylene hydrogenation reaction of approximately 1/2 and 0 for H2 and C2H4, respectively, nearly match those for conventional noble-metal catalysts. In contrast to oxidative decarbonylation, thermal desorption of CO from silica-supported cluster 1 occurred exclusively at the basal plane, giving rise to sites that do not react with ethylene and are catalytically inactive for ethylene hydrogenation. The evidence of distinctive sites on the cluster catalyst leads to a model that links to hydrogen-transfer catalysis on metals-involving some surface sites that bond to both hydrocarbon and hydrogen and are catalytically engaged (so-called "*" sites) and others, at the basal plane, which bond hydrogen and CO but not hydrocarbon and are reservoir sites (so-called "S" sites)

Recognition and Binding of Aliphatic Dicarboxylic Acids C4 – C10 by Diiminocalix[4]arene

Host-Guest complexation of 5,17-bis-(N-tolyliminomethyl)-25,27-dipropoxycalix[4]arenewithaliphatic dicarboxylicacidsC4 – C10 hasbeenstudiedin water-organic solution by the RP HPLC and molecular modeling methods. The stability constants (log KA =2.56– 3.05) of the supramolecular complexes are depended on structure, pKa and log Pvalues of the acids. The complexation is determined by the hydrogen bonds of the COOH group of the dicarboxylic acids with nitrogen atoms at the upper rim or oxygen atoms at the lower rim of the calixarene

Stabilizing Single Sites on Solid Supports: Robust Grafted Ti(IV)-Calixarene Olefin Epoxidation Catalysts via Surface Polymerization and Cross-Linking

This manuscript develops a surface polymerization and cross-linking approach for the stabilization of single-site catalysts on solid surfaces, which is demonstrated here for grafted Ti(IV)-calixarene Lewis acids on silica. Our approach relies on cationic polymerization that is initiated by an adsorbed B(C_6F_5)_3 and uses styrene as the monomer and diisopropenylbenzene as the cross-linking agent. The mildness of this polymerization method is demonstrated by its lack of blocking micropores and only slight consumption of mesopore internal surface area on the basis of N2 physisorption data at 77 K, both of which are in contrast to previously reported surface-polymerization approaches. Catalysis of samples before and after polymerization and cross-linking was investigated with a probe reaction consisting of the epoxidation of 1-octene with tert-butyl hydroperoxide as oxidant, which is known to be catalyzed by Lewis-acid sites, and a comparison of catalyst hydrolytic stability was performed. Added water in the latter was used as a a trigger to induce site aggregation, as a stress test to determine the effectiveness of site protection by our polymerization approach. Consistent with the N2 physisorption data, catalysis data demonstrate that surface polymerization does not block small-molecule reactant and product access to Lewis-acid sites on the surface, since the conversion remains essentially unchanged before and after surface polymerization and cross-linking. DR UV–vis, TGA, and catalysis data reveal that the grafted Ti(IV)-calixarene sites on silica maintain their catalytic activity even after being treated with corrosive protic stress-test solution. In sharp contrast, grafted sites without the polymer layer leach nearly all of their calixarene and Ti contents during similar stress testing, resulting in the near complete loss of catalytic activity. We hypothesize that the surface polymer acts as a nanoreactor gatekeeper, which prevents the large Ti(IV)-calixarene site from leaching and keeps surface complexes as single sites grafted on the silica surface, by blocking access for the migration of sites from the surface to bulk solution

Stabilizing Single Sites on Solid Supports: Robust Grafted Ti(IV)-Calixarene Olefin Epoxidation Catalysts via Surface Polymerization and Cross-Linking

This manuscript develops a surface polymerization and cross-linking approach for the stabilization of single-site catalysts on solid surfaces, which is demonstrated here for grafted Ti(IV)-calixarene Lewis acids on silica. Our approach relies on cationic polymerization that is initiated by an adsorbed B(C_6F_5)_3 and uses styrene as the monomer and diisopropenylbenzene as the cross-linking agent. The mildness of this polymerization method is demonstrated by its lack of blocking micropores and only slight consumption of mesopore internal surface area on the basis of N2 physisorption data at 77 K, both of which are in contrast to previously reported surface-polymerization approaches. Catalysis of samples before and after polymerization and cross-linking was investigated with a probe reaction consisting of the epoxidation of 1-octene with tert-butyl hydroperoxide as oxidant, which is known to be catalyzed by Lewis-acid sites, and a comparison of catalyst hydrolytic stability was performed. Added water in the latter was used as a a trigger to induce site aggregation, as a stress test to determine the effectiveness of site protection by our polymerization approach. Consistent with the N2 physisorption data, catalysis data demonstrate that surface polymerization does not block small-molecule reactant and product access to Lewis-acid sites on the surface, since the conversion remains essentially unchanged before and after surface polymerization and cross-linking. DR UV–vis, TGA, and catalysis data reveal that the grafted Ti(IV)-calixarene sites on silica maintain their catalytic activity even after being treated with corrosive protic stress-test solution. In sharp contrast, grafted sites without the polymer layer leach nearly all of their calixarene and Ti contents during similar stress testing, resulting in the near complete loss of catalytic activity. We hypothesize that the surface polymer acts as a nanoreactor gatekeeper, which prevents the large Ti(IV)-calixarene site from leaching and keeps surface complexes as single sites grafted on the silica surface, by blocking access for the migration of sites from the surface to bulk solution

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

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

Al^III–Calix[4]arene Catalysts for Asymmetric Meerwein–Ponndorf–Verley Reduction

Chiral Al^III-calixarene complexes were investigated as catalysts for the asymmetric Meerwein–Ponndorf–Verley (MPV) reduction reaction when using chiral and achiral secondary alcohols as reductants. The most enantioselective catalyst consisted of a new axially chiral vaulted-hemispherical calix[4]­arene phosphite ligand, which attained an enantioselective excess of 99%. This ligand consists of two lower-rim hydroxyl groups, with the remaining two lower-rim oxygens directly connected to the phosphorus of the phosphite, which is derived from a chiral diol. The results emphasize the importance of the rigid calix[4]­arene lower-rim substituents and point to a possible role of a lower-rim chiral pocket and Lewis-basic phosphorus lone pairs in enhancing asymmetric hydride transfer

Nanoporous gold assemblies of calixarene-phosphine-capped colloids.

The synthesis of high surface-area colloidal assemblies of calixarene-phosphine-capped nanoporous gold is reported under reductive electrochemical conditions. These materials uniquely exhibit a remarkably thin wall thickness down to 10 nm, while possessing pore sizes on the order of up to hundreds of nanometers, which can be controlled via choice of organic ligand

Interfacial Interactions of Cations with Amphiphilic Dihydroxyphosphonyl-calix-[4]-arene Mesosystems

International audienc

Silica-Supported Phosphonic Acids as Thermally and Oxidatively Stable Organic Acid Sites

Organic–inorganic materials consisting of organophosphonic-acid-supported-on-silica materials <b>C3</b>/<b>SiO₂</b> and <b>C4/SiO₂</b> are described, where <b>C3</b> is propane-1,2,3-triphosphonic acid and <b>C4</b> is butane-1,2,3,4-tetraphosphonic acid. Solid-state structures of both of these phosphonic acids are analyzed using single-crystal X-ray diffraction, and these data reveal extensive intermolecular hydrogen bonding and no intramolecular hydrogen bonds. Thermogravimetric analysis/mass spectroscopy (TGA/MS) data show a lack of combustion for these materials in air at temperatures below 400 °C, and only release of water corresponding to reversible organophosphonic acid condensation below 150 °C. A comparative series of silica-supported materials were synthesized, consisting of organophosphonic acid <b>CX8</b>, which represents a calixarene macrocycle that is decorated with a high density of organophosphonic-acid substituents on both the lower and upper rim, as well as polyvinylphosphoric acid (<b>PVPA</b>). Material <b>CX8</b>/<b>SiO₂</b> possesses a significantly lower thermal stability and lower combustion temperature of 300 °C in air, whereas <b>PVPA</b> demonstrates comparable thermal stability as observed with <b>C3</b> and <b>C4</b>. TGA coupled with base-probe titration was used to determine the Brønsted acid site density of all silica-supported phosphonic acids at various coverages and temperatures. Material <b>C4/SiO</b>_<b>2</b><b>-37%</b> (corresponding to 37% (by mass) loading and half-monolayer coverage on silica) exhibited the highest Brønsted acid-site density of all materials, corresponding to 0.84 mmol/g at 150 °C, and 0.62 mmol/g at 300 °C. All supported phosphonic acids treated with pyridine at room temperature were strong enough acids to protonate pyridine at room temperature as exhibited by a distinct pyridinium cation band in the infrared spectrum; however, in contrast to much stronger acid sites in silica-supported phosphoric acid materials, almost all adsorbed pyridine was lost by 150 °C. Use of a stronger base for acid-site titration consisting of diisopropylamine (DIPA) demonstrates acid sites in all materials up to 300 °C, at which temperature the acid site was too weak to adsorb DIPA. Thus, these oxidatively stable materials are deemed to be useful in applications requiring weak Brønsted acid sites, while exhibiting high-temperature oxidative stability up to 400 °C