Synthesis and characterization of inorganic-organic hybrid materials based on the intercalation of stable organic radicals into a fluoromica clay

Hybrid materials, in which stable organic radical cations are intercalated into layered inorganic host materials, can be successfully synthesized via an ion exchange reaction in a layered fluoromica clay, to yield recyclable heterogeneous catalysts for oxidation of various alcohols. We have conducted systematic synthetic and structural studies on the intercalation of the radical cations 4-(diethylmethylammonium)-2,2,6,6-tetramethylpiperidin-1-oxyl (DEMTEP), 1-[2-(4-amino-2,2,6,6-tetramethyl-1-piperidinyloxyl)-2oxoethyl]-1'-methyl-4,4'-bipyridinium (VIOTEP), and 2-(3-N-methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide (m-MPYNN) into a synthetic fluoromica clay named Somasif (R) ME 100, Na2xMg3.0-xSi4O10(FyOH1-y)(2) (x = 0.33, y = 0.98). The guest-host interactions in these intercalation compounds have been characterized by X-ray powder diffraction and solid-state NMR of the constituent nuclei (Na-23, F-19, and Si-29) of the Somasif structure. The intercalation process can be conveniently monitored using Na-23 MAS-NMR. Guest-guest interactions have been probed by magnetic susceptibility measurements as well as EPR and H-1 MAS NMR experiments. The H-1 MAS-NMR line widths and chemical shifts probe modifications in the electron spin density distributions and/or intermolecular interactions between the electron spins of the guest species. Despite these indications of weakly interacting spins, magnetic susceptibility measurements are consistent with the near-absence of cooperative magnetism. The VIOTEP and DEMTEP inclusion compounds demonstrate catalytic activity for the oxidation of benzyl alcohol, using NaOCl as a co-oxidant. Although the radical ion species is partially leached out under these conditions (ionic exchange with Na+ in solution) the catalytic activity remains for up to 40 subsequent cycles. Fully leached materials can be regenerated by catalyst re-loading and this process can be conveniently monitored by X-band EPR spectroscopy.