The tetracarbonyl complexes cis-[Mo(CO)(4){2-13(5)-pyrazolyl]pyridine}] (1) and cis-[Mo(CO)(4){ethyl[3-(2-pyridyl)-1-pyrazolyllacetate}] (2) were obtained in excellent yields by microwave-assisted heating of a mixture of Mo(CO)(6) and the organic ligand in toluene at 110 degrees C for 30 min. Complexes 1 and 2 are air stable, storable catalyst precursors for the epoxidation of olefins by tert-butyl hydroperoxide (TBHP). In situ oxidative decarbonylation of 1 gives the tetranuclear compound [Mo4O12{2-[3(5)-pyrazolyl]pyridine}(4)] (3). Elemental analysis and infrared data indicate that the oxidation of 2 gives an octameric species with the molecular formula [Mo8O24{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}(4)] (4). The molybdenum oxide/pyrazolylpyridine compounds 3 and 4, which can be readily prepared in very good yields from the parent carbonyls upon reaction with TBHP in n-decane/CH2Cl2 at room temperature, are active and stable heterogeneous catalysts for the epoxidation of cis-cyclooctene (Cy) by TBHP, giving 1,2-epoxycyclooctane as the only product. The highest activity is found for 4: ca. 90% epoxide yield after 6h reaction at 55 degrees C. Further experiments confirmed that 2 is an effective catalyst precursor for (i) the epoxidation of Cy by aqueous TBHP, giving the corresponding epoxide as the only product, (ii) the selective epoxidation of R-(+)-limonene by TBHP (in n-decane), and (iii) the selective oxidation of methyl phenyl sulfide to the corresponding sulfoxide by aqueous TBHP or H2O2 at 35 degrees C. (c) 2011 Elsevier B.V. All rights reserved.