Ligand Design by Ionomers. ESR of Mo^V in Perfluorinated Ionomer Supports

Abstract

Electron spin resonance (ESR) spectra of Mo 5+ in perfluorinated membranes (Nafion) neutralized by MoCl 5 suggest that the ionomer acts as an ion-selective medium and replaces the chlorine ligands of the molybdenum center by oxygen ligands from the sulfonic groups. This conclusion is based on an analysis of the ESR parameters that characterize the paramagnetic center and is supported by the detection of only one major paramagnetic molybdenum site, Mo(A) in the bulk ionomer, in the absence of solvents. Ligand design via the ionomer was thus achieved. We propose that the main molybdenum species is MoO(SO 3 -) 5 , where the unique oxygen ligand is from an sSdO group of the sulfonic moiety. Exposure of Mo/Nafion to water (up to 5 Torr) has no significant effect on the ESR parameters and on the intensity of the ESR signal. Exposure to 25 Torr of water leads to the reversible disappearance of the ESR signal. Brief exposure of Mo/Nafion to gaseous acetonitrile (as CD 3 CN) results in the appearance of two new Mo species, Mo(B) and Mo(C), which are thought to be formed via replacement of one equatorial or one axial sSdO ligand, respectively, based on an analysis of the effect on the corresponding g | values. Soaking of Mo/Nafion samples with acetonitrile results in the reversible disappearance of the ESR signals. All transformations of the ESR signals are explained by the replacement of one L-type ligand (donating two electrons), sSdO in the case of Nafion, by oxygen from water or by nitrogen from acetonitrile. The loss of the signal in the presence of adsorbates is explained by a replacement of one or more of the -S-O -ligands by ligands from the adsorbates. This process is a reversible reductive replacement of the ionic ligands and leads to the concomittant loss of the paramagnetism. The control of ligands around the paramagnetic center, the involvement of the molybdenum in redox processes, and the accessibility of the center to adsorbates are promising features for catalysis. Initial results for the oxidation of ethanol on Nafion and on Mo/Nafion supports are presented. Correlation plots (A | vs g | ) in paramagnetic molybdenum complexes, constructed from data in the literature and measured in this study, show trends that can be used for identification of the number and types of ligands in paramagnetic molybdenum centers.

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

“…The ESR line shape, Figure c, is complex, but the main line for nuclei with I = 0 is typical for Mo V complexes with g ∥ < g ⊥ . In analogy with previous results, , the g ∥ values of two new species, Mo(C) and Mo(D), are identified, and shown by arrows in Figure c. The intensity ratio of the indicated signals varies slightly with the conditions used for sample preparation, a clear proof that the signals are from different species and not from one species with rhombic symmetry.…”

“…ESR spectra of Mo V , a 5d 1 cation, consist of strong signals from nuclei with I = 0 ( 92 Mo, 94 Mo, 96 Mo, 98 Mo, and 100 Mo, total natural abundance 74.32), from which the g -tensor components can be deduced. The g values reflect the symmetry of the catalytic site, the type of ligand (through the spin−orbit coupling λ L of the ligand), and the changes that occur during reaction. − Additional structural information can be obtained from an analysis of the hyperfine splittings from Mo nuclei with I = 5/2 ( 95 Mo and 97 Mo, natural abundances 15.78% and 9.69%, respectively); these nuclei have similar nuclear magnetic moments, and separate signals are usually not detected in ESR spectra. Coupled with powerful simulation programs, , many complex ESR spectra consisting of contributions from several molybdenum centers have been interpreted.…”

“…ESR spectra, simulations, and Fourier transform infrared (FTIR) spectroscopy have been used to determine the ligands coordinated to the catalytic center for molybdenum catalysts on polymeric supports such as polyacetylene, polypyrrole, and polyaniline; , on poly(acrylic acid) (PAA) matrices; and on perfluorinated ionomers containing sulfonic groups (Nafion). , The decrease and even disappearance of the ESR signals are also important indications of the ligand exchange reactions that take place.…”

Two-Dimensional Spatial−Spectral ESR Imaging of Diffusion Based on Molybdenum(V)

“…Second, Cl -can be replaced by oxygen ligands from water adsorbed on the polymer and can cause the disappearance of the signal, as reported recently for Mo V on perfluorinated ionomer supports upon adsorption of water and acetonitrile. 9 In that study 9 we have suggested that water "unzips" the anions around paramagnetic Mo V to produce a diamagnetic species when more than one anionic ligand is replaced by oxygen from water. This conclusion was in agreement with the results of electron spin-echo (ESE) studies of Mo V on silica, which have provided evidence for only one hydroxyl group or one methanol molecule in the cation coordination sphere on exposure to water or methanol, respectively.…”

“…The g values measured in ESR spectra of this center are very sensitive to the ligands and can provide information on the symmetry of the catalytic site, the immediate environment of the catalytic center, and the changes that occur during reaction. − Additional structural information can be obtained from an analysis of the hyperfine interaction of magnetic nuclei 95 Mo and 97 Mo ( I = 5 / 2 , similar magnetic moments and natural abundances 15.78% and 9.69%, respectively). This approach was demonstrated recently for Mo V in poly(acrylic acid) (PAA) and perfluorinated ionomers as supports. , The assignments of the ligands was based on spectra simulations and correlation diagrams between g ∥ and g ⊥ 4,6 and between A ∥ and g ∥ …”