Synthesis, Structural Elucidation, and Catalytic Properties in Olefin Epoxidation of the Polymeric Hybrid Material [Mo3O9(2-[3(5)-Pyrazolyl]pyridine)](n)

Abstract

The reaction of [MoO2Cl2(pzpy)] (1) (pzpy = 2-[3(5)-pyrazolyl]pyridine) with water in an open reflux system (16 h), in a microwave synthesis system (120 degrees C, 2 h), or in a Teflon-lined stainless steel digestion bomb (100 degrees C, 19 h) gave the molybdenum oxide/pyrazolylpyridine polymeric hybrid material [Mo3O9(pzpy)](n) (2) as a microcrystalline powder in yields of 72-79%. Compound 2 can also be obtained by the hydrothermal reaction of MoO3, pzpy, and H2O at 160 degrees C for 3 d. Secondary products isolated from the reaction solutions included the salt (pzpyH)(2)(MoCl4) (3) (pzpyH = 2-[3(5)-pyrazolyl]pyridinium), containing a very rare example of the tetrahedral MoCl42- anion, and the tetranuclear compound [Mo4O12(pzpy)(4)] (4). Reaction of 2 with excess tert-butylhydroperoxide (TBHP) led to the isolation of the oxodiperoxo complex [MoO(O-2)(2)(pzpy)(4)] (5). Single-crystal X-ray structures of 3 and 5 are described. Fourier transform (FT)-IR and FT Raman spectra for 1, 4, and 5 were assigned based on density functional theory calculations. The structure of 2 was determined from synchrotron powder X-ray diffraction data in combination with other physicochemical information. In 2, a hybrid organic inorganic one-dimensional (1D) polymer, (1)(infinity)[Mo3O9(pzpy)], is formed by the connection of two very distinct components: a double ladder-type inorganic core reminiscent of the crystal structure of MoO3 and 1D chains of corner-sharing distorted {MoO4N2} octahedra. Compound 2 exhibits moderate activity and high selectivity when used as a (pre)catalyst for the epoxidation of cis-cyclooctene with TBHP. Under the reaction conditions used, 2 is poorly soluble and is gradually converted into 5, which is at least partly responsible for the catalytic reaction.