Anionic Organic Guests Incorporated in Zeolites: Adsorption and Reactivity of a Meisenheimer Complex in Faujasites

Abstract

Zeolites are suitable microporous hosts for positively charged organic species, but it is believed that they cannot adsorb organic anions. Pure Meisenheimer complex, derived from reduction of 2,4-dinitroaniline with NaBH4, was adsorbed inside faujasite cavities. Evidence for the internal incorporation of this negatively charged reaction intermediate comes from 1) XPS elemental analysis as a function of the depth of penetration into the particle, 2) the remarkable blue shift in lambda(max) of the Meisenheimer complex adsorbed on zeolite (ca. 470 nm) as compared to that in acetonitrile (580 nm) and 3) from the lack of reactivity with size-excluded hydride-acceptor reagents. Evidence is provided in support of an adsorption mechanism in which a neutral ion pair (alkali metal ion + Meisenheimer anion) is the actual species being adsorbed. In fact it appears that there is remarkable increase in the association constant for the ion-pair complex within the zeolite cavities as compared to DMF solution. Although this mechanism of adsorption as an ion-pair complex has precedents in the adsorption of some inorganic salts, what is novel is the notable increase in the stability and persistence of the Meisenheimer anion (a anionic reaction intermediate) as a result of zeolite inclusion. Adsorbed Meisenheimer complex exhibits much lower reactivity towards electron acceptors, oxygen, and water. Cyclic voltammetry of zeolite-modified electrodes reveals for the Meisenheimer complex adsorbed on LiY a reversible redox peak that is not observed in solution and has been interpreted as arising from site isolation and stabilisation of the electrochemically generated species.

Electrochemical Monitoring of Compartmentalization Effects in the Stability of Meisenheimer Anions Supported in Hydrotalcite and X and Y Zeolites

“…9,10,19 The adsorption of Meisenheimer complex within the solids was carried out by stirring under inert atmosphere a solution of the Meisenheimer complex in MeCN in the presence of the thermally dehydrated solid, as reported elsewhere. 8 Purity of the Meisenheimer complex in solution was checked by 1 H-NMR and 13 C-NMR prior to incorporation. The presence of MS − in the solids was monitored by diffuse reflectance UV-vis spectrometry and XPS analysis.…”

“…The characteristic band at 578 nm recorded for MS-in MeCN solution, responsible for this intense purple coloration, is blue-shifted above 100 nm upon zeolite attachment. 8 Commercially available Paraloid B72, an ethyl acrylate ͑70%͒-methyl acrylate ͑30%͒ copolymer ͑P͓EMA/MA͔͒ was selected for polymer-film electrode preparation because of its mechanical stability and ability to form porous films able to adhere the zeolite microparticles to the electrode surface.Polymer-film sample-modified electrodes were prepared, as previously described, 18,20 by transferring a few microliters ͑typically 50 L͒ of a dispersion of the solid ͑10 mg͒ in acetone ͑5 mL͒ to the surface of a freshly polished glassy carbon electrode ͑GCE, geometrical area 0.071 cm 2 ͒ and allowing the coating to dry in air. After the electrode surface was air dried, one drop of a solution of the acrylic resin ͑1%͒ in acetone was added and the modified electrode was air dried.…”

“…Contrary to that expectance, we have previously reported the adsorption of negatively charged Meisenheimer complex on faujasites, the adsorption mechanism involving a neutral ion pair complex. 8 Meisenheimer complexes are the negatively charged anion intermediates arising from the hydride addition to 2,4-dinitroaniline. 9,10 The evidence for the internal location of the Meisenheimer complex comes from a combination of spectrophotometric data ͓uv-visible ͑UV/Vis͒ and X-ray photoelectron spectroscopy ͑XPS͔͒ and studies on its reactivity with p-benzoquinone and malachite green.…”

“…9,10 The evidence for the internal location of the Meisenheimer complex comes from a combination of spectrophotometric data ͓uv-visible ͑UV/Vis͒ and X-ray photoelectron spectroscopy ͑XPS͔͒ and studies on its reactivity with p-benzoquinone and malachite green. [8][9][10] It is presented here electrochemical evidence for the stabilization of Meisenheimer anions, MS − in Na + -and Li + -exchanged zeolites X and Y ͑MS@NaX, MS@NaY, MS@LiX, MS@LiY, respectively͒. The electrochemical response of such materials is compared with that of MS − ions supported on H + -Beta zeolite ͑MS@HBeta͒ and a layered hydrotalcite material ͑MS@HT͒ and MS − in solution phase.…”

Second Harmonic Generation of C60Incorporated in Alkali Metal Ion Zeolites and Mesoporous MCM-41 Silica

“…[1][2][3][4][5][6][7][8] The geometry and topology of the zeolite micropores can be used to organize spatially the organic guest, while at the same time provides site isolation and stabilization of the guests by impeding the attack of atmospheric reagents. [9][10][11][12] Second harmonic generation (SHG) is one of the properties in which the molecular response of the guest can be strongly influenced by incorporation inside the zeolite framework. [13][14][15][16] Among the best studied examples, one paradigmatic case is p-nitroaniline, which is a molecule with a large dipole moment that could serve in principle for nonlinear optics.…”