Structural Assessment and Catalytic Consequences of the Oxygen Coordination Environment in Grafted Ti−Calixarenes

Stabilisation of molecular TiO4 species on the pore surface of mesoporous silica for photocatalytic H2 evolution

Although molecular tetrahedral Ti-oxo species exhibit unique electronic and photochemical properties due to their discrete energy levels, which are different from those of anatase and rutile, such Ti-oxo species are generally unstable and readily transformed to amorphous/crystalline TiO2 (bulk phases, nanoparticles and clusters) via hydrolysis and condensation. Here, molecular Ti-oxo species were immobilised within mesoporous silica SBA-15 by grafting titanium(IV) oxyacetylacetonate using the surface silanol groups of SBA-15 as a scaffold, followed by chemical etching with dilute hydrochloric acid to form molecular TiO4 species. These Ti species mainly exist as isolated tetrahedrally coordinated structures, as was confirmed by diffuse reflectance UV-vis and Raman spectroscopy. The SBA-15-immobilised molecular TiO4 exhibited higher photocatalytic activity for H2 evolution from an aqueous methanol solution than conventional Ti-incorporated mesoporous silica (Ti-MCM-41) and reference TiO2 (P25).

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

We investigate the interaction between a molecule and a pore mouth-a critical step in adsorption processes-by characterizing the conformation of a macrocyclic calix[4]arene-Ti^IV complex, which is grafted on the external surface of a zeotype (*-SVY). X-ray absorption and ¹³ C{¹ H} CPMAS NMR spectroscopies independently detect a unique conformation of this complex when it is grafted at crystallographically equivalent locations that lie at the interface of 7 Å hemispherical microporous cavities and the external surface. Electronic structure calculations support the presence of this unique conformation, and suggest that it is brought about by a specific orientation of the macrocycle that maximizes non-covalent interactions between calix[4]arene upper-rim tert-butyl substituents and the microporous-cavity walls. Our comparative study provides a rare "snapshot" of a molecule partially confined at a pore mouth, an essential intermediate for adsorption into micropores, and demonstrates how surrounding environment controls this confinement in a sensitive fashion.

Use of titanocalix[4]arenes in the ring opening polymerization of cyclic esters

Titanocalix[4]arenes are shown to be active for the ROP of cyclic esters under N₂ or air with the mono-methoxy complex [Ti(NCMe)Cl(p-tert-butylcalix[4]arene(O)₃(OMe))] performing best.

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

The effect of dynamic reorganization and confinement of isolated Ti^IV catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene-Ti^IV centers or their calcined counterparts. Their location is synthetically controlled to be either unconfined at terminal T-atom positions (denoted as type-(i)) or within confining 12-MR pockets (denoted as type-(ii); diameter ∼7 Å, volume ∼185 ų) composed of hemispherical cavities on the external surface of zeotypes with *-SVY topology. Electronic structure calculations (density functional theory) indicate that active sites consist of cooperative assemblies of Ti^IV centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM Ti^IV, 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M^-2 s^-1; whereas within confining type-(ii) 12-MR pockets, there is a ∼5-fold enhancement to 48 ± 8 M^-2 s^-1. When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M^-2 s^-1), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are Δ H^‡_app = 43 ± 1 kJ mol^-1 irrespective of active site location, confining environments exhibit diminished entropic barriers (Δ S^‡_app = -68 J mol^-1 K^-1 for unconfined type-(i) vs -56 J mol^-1 K^-1 for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ∼ 225 ų). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow external-surface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.

Synthesis, crystal structure, and cytotoxicity studies of titanacalix[4 and 8]arene complexes

Reaction of 5,11,17,23-tetra-tert-butyl-dihydroxy-26,28-bis(2-pentoxy)calix[4]arene (L(OH)2(Opentyl)2) with [TiCl4] afforded the dichlorotitanoacalix[4]arene complex [TiCl2L(O)2(Opentyl)2] (1) in good yield. Hydrolysis of 1 led to the isolation of the complex {[TiL(O)3(Opentyl)]2(μ-OH)(μ-Cl)} (2). Reaction of 5,11,17,23,29,35,41,47-p-tert-butyl-49,50,51,52,53,54,55,56-octapropoxycalix[8]arene (L'(Opropyl)8) with [TiCl4] in refluxing toluene afforded, following work-up, a 35 : 65 mixture (3) of the complex [Ti(NCMe)Cl]2[TiCl(μ-O)]2L' and the silicone grease derived complex [Ti(NCMe)Cl]2[Ti(μ-O)]2[OSi(CH3)2OSi(CH3)2O]L' in which the grease replaces two chloride ligands. The molecular structures of 1·2MeCN, 2·7¼MeCN, and 3·10MeCN have been determined. The complexes were studied using in vitro cell assays and were found to have CC50 values in the range 111-186 μM, i.e. they have low toxicity.

Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-Ti^IV single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as supports. A chelating calix[4]arene macrocyclic ligand helps enforce a constant Ti^IV inner-sphere, as characterized by UV-visible and X-ray absorption spectroscopies, thus enabling the rigorous comparison of outer-sphere environments across different siliceous supports. These outer-sphere environments are characterized by solid-state ¹H NMR spectroscopy to comprise proximally organized silanols confined within 12 membered-ring cups in crystalline UCB-4, and are responsible for up to 5-fold enhancements in rates of epoxidation by Ti^IV centers.

Epoxidation of 1-octene under harsh tail-end conditions in a flow reactor I: a comparative study of crystalline vs. amorphous catalysts

The crystalline Ti-UCB-4 epoxidation catalyst is more active, selective, and stable than the amorphous Ti–SiO₂.

Economic Hydrophobicity Triggering of CO2 Photoreduction for Selective CH4 Generation on Noble-Metal-Free TiO2-SiO2

On the basis of the fact that the competitive adsorption between CO2 and H2O on the catalyst plays an important role in the CO2 photoreduction process, here we develop an economic NH4F-induced hydrophobic modification strategy to enhance the CO2 competitive adsorption on the mesoporous TiO2-SiO2 composite surface via a simple solvothermal method. After the hydrophobic modification, the CO2 photoreduction for the selective generation of CH4 over the noble-metal-free TiO2-SiO2 composite can be greatly enhanced (2.42 vs 0.10 μmol/g in 4h). The enhanced CO2 photoreduction efficiency is assigned to the rational hydrophobic modification on TiO2-SiO2 surface by replacing Si-OH to hydrophobic Si-F bonds, which will improve the CO2 competitive adsorption and trigger the eight-electron CO2 photoreduction on the reaction kinetics.

Straightforward synthesis and catalytic applications of rigid N,O-type calixarene ligands

Here, we report a simple one-step access to new rigid N,O-calixarene ligands which is based on copper-catalized amination at the lower rim. We also present their main group and late transition metal complexes which show superior catalytic activity, in several organic transformations, compared with known metal calixarene complexes.

Zinc calixarene complexes for the ring opening polymerization of cyclic esters

A number of zinc calixarene complexes have been structurally characterized and screened for the ROP of ε-caprolactone and rac-lactide.

Fifty years of oxacalix[3]arenes: A review

Hexahomotrioxacalix[3]arenes, commonly called oxacalix[3]arenes, were first reported in 1962. Since then, their chemistry has been expanded to include numerous derivatives and complexes. This review describes the syntheses of the parent compounds, their derivatives, and their complexation behaviour towards cations. Extraction data are presented, as are crystal structures of the macrocycles and their complexes with guest species. Applications in fields as diverse as ion selective electrode modifiers, fluorescence sensors, fullerene separations and biomimetic chemistry are described.

Enhancement of epoxidation efficiencies for Ta-SBA15 catalysts. The influence of modification with -EMe3 (E = Si, Ge, Sn) groups

Site-isolated Ta(V) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. After thermal treatment under oxygen, the resulting Si-OH and Ta-OH sites of TaSBA15-O(2)were modified with a series of trimethyl group 14 species, Me(3)E-, by treatment with Me(3)E-NMe(2) (E = Si, Ge, Sn) reagents. The resulting surface-modified catalysts (Me(3)E)(cap)TaSBA15 exhibit a significantly increased rate of cyclohexene epoxidation with H(2)O(2) as an oxidant, and provided a decreased amount of allylic oxidation products with respect to the unmodified material, TaSBA15-O(2). The rate of nonproductive H(2)O(2) decomposition, as monitored via (1)H NMR spectroscopy, significantly decreased after the surface modification. The structure of the TaSBA15 catalysts and potential Ta(V) epoxidation intermediates (formed upon treatment of Ta(V) materials with H(2)O(2)) were probed using UV-visible absorbance and diffuse-reflectance UV-visible spectroscopy. A Ta(V)(η(2)-O(2)) intermediate species is proposed for the TaSBA15-O(2), (Me(3)Si)(cap)TaSBA15, and (Me(3)Ge)(cap)TaSBA15 catalysts, while intermediate species for the (Me(3)Sn)(cap)TaSBA15 catalysts could not be characterized.

Optimizing acid-base bifunctional mesoporous catalysts for the henry reaction: effects of the surface density and site isolation of functional groups

We report on the effects of the surface density and the spacing between grafted organoamines (and residual ungrafted silanols) of amine-functionalized mesoporous materials on their (cooperative) catalytic activity in the Henry reaction. The spacing between the bifunctional groups (amines and silanols), their site isolation, and their surface density were controlled by one-step or two-step grafting of a series of organosilanes containing linear alkylamine, alkyldiamine, alkyltriamine, and meta- and para-substituted aromatic amines onto mesoporous silica in ethanol and/or toluene. The grafting in ethanol produced site-isolated, flexible alkylamines, alkyldiamines, and alkyltriamines of different tether lengths and rigid meta- and para-substituted aromatic amines and high surface area materials, whereas the grafting in toluene resulted in closely spaced organoamines and materials with lower surface areas. The spacing between the organoamine groups was probed by complexing cupric ions with the amines and by measuring the electronic spectra of the complexes. The materials' catalytic activities were dependent not only on the degree of site isolation of the amine groups and the surface areas of the materials, but also on the relative spacing between the functional groups and their surface density. Samples grafted with monoamine groups in ethanol and samples grafted with diamine or triamine groups in toluene for 5 h gave approximately 100% conversion in 16 min of the Henry reaction between p-hydroxybenzaldehyde and nitromethane. However, the corresponding monoamine-grafted sample in toluene and diamine- and triamine-grafted samples in ethanol gave approximately 100% conversion after 1 h. On the basis of turnover number (TON) and TON per surface area, the samples containing optimum concentrations of approximately 0.8 - 1.5 mmol of grafted organoamines/g, which we dubbed as the critical density of organic grafted groups, gave the highest catalytic efficiencies. These samples have the most favorable amine-silanol cooperative catalytic activity. Furthermore, samples functionalized with rigid meta-substituted aminophenyl groups in ethanol showed higher catalytic efficiency than the corresponding sample containing the amine groups at the para-position, possibly due to the close proximity of the bifunctional groups in the former. The capping of ungrafted silanols with noncatalytic organosilanes in toluene resulted in reduction of catalytic activities, confirming the involvement of silanols.