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Synthesis, Structural Elucidation, and Application of a Pyrazolylpyridine–Molybdenum Oxide Composite as a Heterogeneous Catalyst for Olefin Epoxidation

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posted on 06.08.2012, 00:00 by Sónia Figueiredo, Ana C. Gomes, Patrícia Neves, Tatiana R. Amarante, Filipe A. Almeida Paz, Rosário Soares, André D. Lopes, Anabela A. Valente, Martyn Pillinger, Isabel S. Gonçalves
The reaction of [MoO2Cl2(pypzEA)] (1) (pypzEA = ethyl­[3-(pyridin-2-yl)-1H-pyrazol-1-yl]­acetate) with water in a Teflon-lined stainless steel autoclave (100 °C) or in an open reflux system leads to the isolation of the molybdenum oxide/pyrazolylpyridine composite material [Mo2O6(HpypzA)] (2; HpypzA = [3-(pyridinium-2-yl)-1H-pyrazol-1-yl]­acetate). The solid state structure of 2 was solved through single crystal and powder X-ray diffraction analyses in conjunction with information derived from FT-IR and 13C CP MAS NMR spectroscopies and elemental analyses. In the asymmetric unit of 2, two crystallographically distinct Mo6+ centers are bridged by a syn,syn-carboxylate group of HpypzA. The periodic repetition of these units along the a axis of the unit cell leads to the formation of a one-dimensional composite polymer, 1[Mo2O6(HpypzA)]. The outstretched pyrazolylpyridine groups of adjacent polymers interdigitate to form a zipper-like motif, generating strong onset π–π contacts between adjacent rings of coordinated HpypzA molecules. The composite oxide 2 is a stable heterogeneous catalyst for liquid-phase olefin epoxidation.