Inhibition by Water during Heterogeneous Brønsted Acid Catalysis by Three-Dimensional Crystalline Organic Salts

Abstract

A new self-assembled and self-healing class of metal free, recyclable, heterogeneous Brønsted acid catalysts has been developed by the protonation of aniline derivatives (tetrakis(4aminophenyl)methane, leuco-crystal violet, benzidine, and p-phenylenediamine) with aromatic sulfonic acids (tetrakis(phenyl-4-sulfonic acid)methane, and 2,6-naphthalenedisulfonic acid). As a result, five three-dimensional crystalline organic salts (F-1a, F-1b, F-1c, F-2, and F-3) were obtained, linked by hydrogen bonds and additionally stabilized by the opposite charges of the components. Frameworks F-2 and F-3 were prepared for the first time and characterized by elemental analysis, X-ray structural analysis (for F-2), thermogravimetry, SEM, and FTIR spectroscopy. The catalytic activities of crystalline organic salts F-1−3 have been explored in industrially important epoxide ring-opening and acetal formation reactions. The presence of encapsulated water inside frameworks F-1a and F-2 had an inhibitory effect on the performance of the catalysts. Xray diffraction analysis of hydrated and dehydrated samples of F-1a and F-2 indicated that water of crystallization served as a crosslinking agent, diminishing the substrate induced "breathing" affinities of the frameworks.

Palladium Nanoparticles Entrapped In a Hydrogen Bonded Crystalline Organic Salt Matrix as a Selective Heterogeneous Reduction Catalyst

“…The strongest Pd(111) peak is observed at ∼40.1° (which is in accord with record 46–1043 of the ICDD PDF‐2 database). The low‐angle region of the diffraction patterns (5° to 45°) reveal narrow diffraction peaks of F2 in full agreement with the experimental pattern reported previously [23] . The experimental patterns of A‐2 ( Pd@F2 ) and the regenerated catalyst A‐4 obtained from a diphenylacetylene reduction reaction are essentially identical to each other, which indicates that they possess identical or very similar components and crystallographic characteristics of their constituents.…”

“…The materials are stable up to 200 °C as previously reported for F-2. [23] The components of HOF A-2 could be readily separated by adding DMSO to a sample of Pd@F2. The organic components completely dissolved in DMSO, retaining the ratio of the two components (Figure S18) whereas palladium nanoparticles precipitated as a black powder.…”

“…Interestingly, saturation of A-2 with water or drying the sample before use did not change its catalytic performance as would have been predicted if the framework took any catalytic part in the reaction; analogous to the epoxide ring-opening reaction with nucleophiles promoted by F-2 alone as a Bronsted acid. [23] The selectivity of the reduction of cinnamaldehyde (17) is another example of possible framework induced modification of A-2 reactivity (Scheme 6). Table 3 summarises the ratio of the hydrocinnamaldehyde (18) and 3-phenylpropanol (19) obtained under various reduction conditions.…”

“…[20,21] In our earlier work, a family of crystalline, heterogeneous, supramolecular acid catalysts based on ionic interactions of the components, supplemented by hydrogen bonds was prepared by mixing basic and acidic components in water followed by separation of the formed precipitate. [22,23] For example, an efficient catalyst for epoxide ring opening was obtained by mixing a solution of the tetrahydrochloride salt of tetrakis(4aminophenyl)(TAPM) in THF with an aqueous solution of the tetrasodium salt of tetraphenylmethane-tetrasulfonate (TPSM). The resulting precipitate was filtered to give a crystalline salt F-2, the structure of which was established by analytical methods including X-ray crystallography.…”

“…The resulting precipitate was filtered to give a crystalline salt F-2, the structure of which was established by analytical methods including X-ray crystallography. [23] A related crystalline organic salt was shown by White et al to be an efficient media for the heterogenation and stabilization of enzymes by encapsulation. [24] Therefore, we anticipated that metal nanoparticles could also be stabilized by hydrogen bonded organic salts.…”