Bonding of Molybdenum(V) to Poly(ethylene-co-methacrylic acid) Ionomers from X- and W-Band ESR and IR Spectroscopies

Abstract

The bonding of MoV dispersed in poly(ethylene-co-methacrylic acid) (EMAA) ionomers was deduced from ESR measurements at X (9.5 GHz) and W (95 GHz) frequencies, spectral simulations, and FTIR spectra and compared with previous results for MoV in polyacrylic acid (PAA) and in perfluorinated ionomers (“Nafion”) that contain sulfonic acid groups. Molybdenum dispersed in EMAA ionomers was obtained by grafting the ionomer with gaseous MoCl5 in the absence of air, to prevent oxidation to MoVI. This method produced both physically and chemically bonded MoV species, and the ratio of the two species depended on the temperature of MoCl5 during grafting. The type of ligands was deduced from the g-values, and the hyperfine splittings of magnetic Mo nuclei (I = 5/2). The major species detected at the lower grafting temperature was MoV bonded to five chlorine ligands and to an oxygen ligand from a carboxylic group of the ionomer. Heating in vacuum to 400−500 K (“activation”) led to the gradual replacement of the chlorine ligands by oxygen ligands and to the removal of some molybdenum. The nature of the oxygen ligands was deduced from FTIR spectra and depends on the methacrylic acid (MAA) content in the ionomer: in Mo/EMAA samples activated at ≈400 K, molybdenum is bonded to the substrate by bidentate carboxylic groups in the ionomer containing 1.3 mol % MAA and by monodentate carboxylic groups for an MAA content of 11.2 mol %. Exposure of Mo/EMAA samples to ethanol vapor resulted in an increase in the total ESR signal intensity and in the solubilization of ESR-silent MoV species; at 300 K the ESR spectra are isotropic, in contrast to the anisotropic signals observed at the same temperature in the absence of solvents. For water as adsorbate, two MoV species differing in line widths were detected and assigned to MoV species located in the water pools of the ionomer and in the interior of the nonpolar domains, respectively. This result is in support of the model proposed for EMAA ionomers swollen by water, which was based on ESR spectra of amphiphilic spin probes. The bonding of MoV to EMAA ionomers is similar to that described previously for Mo/PAA and different from that in Mo/Nafion, indicating that the determining factor is the type of acid in the ionomers. Small differences between the Mo/EMAA and Mo/PAA systems were assigned to the different distribution of carboxylic groups in the polymer.

Structural characterization of MoOCl3(THF)2, the pre-reagent for Kauffmann olefination reactions

“…A g value of 1.94 at 347.46 mT was determined at ambient temperature, which is exactly the value reported for the molecule dissolved in CH 2 Cl 2 [5]. Typical g values for molecular Mo(V) compounds are around 1.95 [10]. This indicates one unpaired electron, which fits our structure model containing a terminal oxygen atom O1.…”

Interactions of Chromium Ions with Starch Granules in an Aqueous Environment

“…The reaction delivered not only the "bare", nonhydrated, dynamic Cr(III) ions but also the new carbonyl and/or carboxyl functional groups, which are known for their formation of complexes with transition-metal ions. 40,20 3.6. Electron Imaging and X-ray Microanalysis.…”

“…Our observations allowed for the conclusion that the Cr(VI) → Cr(III) reduction was an important step in the hexavalent chromium elimination from aqueous solutions. The reaction delivered not only the “bare”, nonhydrated, dynamic Cr(III) ions but also the new carbonyl and/or carboxyl functional groups, which are known for their formation of complexes with transition-metal ions. , …”

Memorial Viewpoint for Shulamith Schlick

“…She made a great contribution to understanding the radical degradation processes of polymer membranes, especially perfluorinated ones, used in fuel cells. Her total scientific output (1960–2018) consists of about 240 publications, including original research articles, reviews as well as book chapters, in addition to being the editor of three highly valued monographs; some of these are noted here. − Most of these works were realized jointly with foreign partners. She enthusiastically combined travel and research, establishing long lasting international collaborations.…”