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)-2-oxoethyl]-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® ME 100, Na(2x)Mg(3.0-x)Si4O10(F(y)OH(1-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 ((23)Na, (19)F, and (29)Si) of the Somasif structure. The intercalation process can be conveniently monitored using (23)Na MAS-NMR. Guest-guest interactions have been probed by magnetic susceptibility measurements as well as EPR and (1)H MAS NMR experiments. The (1)H 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.
In all but the simplest crystal structures, the identification of all relevant interactions between magnetic sites as well as the setup of magnetic model spaces, which are necessary for modeling macroscopic magnetism, are tedious and error-prone tasks. Here, we present a procedure to generate magnetic susceptibility versus temperature curves using only a crystal structure as input. The procedure, which is based on the first-principles bottom-up approach [Deumal et al., J. Phys. Chem. A, 2002, 106, 1299], is designed in a way to require as little user interference as possible. We employ quantum chemical calculations to parametrize a Heisenberg Hamiltonian, which is set up and diagonalized for different magnetic model spaces to ensure convergence of the model. We apply the procedure to several 6-oxo-verdazyl radical structures, including newly synthesized compounds, and compare the results to data we obtained from magnetic susceptibility measurements as well as published data to further benchmark our procedure. Furthermore, the different impact of certain dominating coupling constants is systematically analyzed.
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