A. Keller

Bio: A. Keller is an academic researcher. The author has contributed to research in topics: Catalysis & Molybdenum. The author has an hindex of 1, co-authored 1 publications receiving 2 citations.

Preparation and characterization of active niobium, tantalum, and tungsten metathesis catalysts

Abstract: Complexes of the types M(CHCR/sub 3/)L/sub 2/X/sub 3/, M(CHCR/sub 3/)(OCR/sub 3/)/sub 2/LX, and WO(CHCR/sub 3/)L/sub 2/Cl/sub 2/, where M is Nb or Ta, R is methyl, L is a tertiary phosphine, and X is Cl or Br, showed good activities in metathesis of terminal olefins, including ethylene, propylene, styrene, 1-butene, and cis-2-pentene, at 25/sup 0/C in the presence of traces of AlCl/sub 3/.

Synthesis, X-ray structure and spectroscopic studies of Mo(O) 2 (3,5-di-tert-Busap)(EtOH) †

Abstract: Oxomolybdenum compounds are of great interest because of their function in several enzymes 1 and in many catalytic systems. 2 The presence of the cis-dioxomolybdenum(VI) fragment in the oxidative form of certain molybdoenzymes has stimulated search for new structures in which this moiety is co-ordinated to ligands containing nitrogen, oxygen and/or sulfur donors. Molybdenum(VI) Schiff-base complexes represent an important and interesting class of that kind of coordination compound. 3 The cis-dioxo moiety, cis-Mo(O) 2 ,i s the most commonly encountered structural form in Mo(VI) co-ordination chemistry. 4 cis-Mo(O) 2 complexes of dianionic tridentate Schiff base ligands can exist as monomers, Mo(O) 2 (L)(D) ‐ where: L = Schiff base, D = monodentate neutral ligand, which is dominant, 3 dimers, [Mo(O)(μ-O)(L)] 2 ‐ with asymmetric double oxygen bridge, 5 or oligomers with a postulated ······Mo=O·······Mo=O····· interaction. 6 This type of complex has a distinct advantage as models for oxo-transfer processes due to the presence of a labile (monomeric form) or easy accessible vacant co-ordination site (dimers and/or oligomers) for potential binding and/or activating of substrates. In this paper we present the syntheses of a Schiff-base complex, [Mo(O) 2 (3,5-di-tert-Busap)(EtOH)] (1), and its characterisation by spectroscopic and X-ray diffraction analysis. The complex is stable in the solid state (no changes were observed during 10 months storage) whereas the analogue [Mo(O) 2 (sap)(EtOH)] loses the co-ordinated ethanol slowly with simultaneous disappearance of its orange colour. The brown product formed from [Mo(O) 2 (sap)(EtOH)] is probably an oxo-bridged oligomer 6 or dimer 5 , revertible to its original mononuclear form when treated with ethanol. Thermogravimetric analysis shows that both complexes lose the co-ordinated ethanol in the ranges 413‐433 and 363‐373 K, respectively for [Mo(O) 2 (3,5-di-tert-Busap)(EtOH)] and [Mo(O) 2 (sap)(EtOH)]. The IR spectral data of the complex are typical for a co-ordinated anionic Schiff base and oxygen donor. The complex does not show a phenolic ν(O‐H) band and the δ(C‐O) band at 1258 and 1284 cm ‐1 has shifted to higher wavenumber compared to the free ligand (1170 and 1202 cm ‐1 ), both suggestive of coordination of the phenolic C‐O groups. The characteristic frequency ν(C=N) of the free ligand observed at 1616 cm ‐1 displays a shift to lower frequency at 1611 cm ‐1 , indicating coordination of the azomethine nitrogen to molybdenum. A broad band at 3269 cm ‐1 and medium one at 1047 cm ‐1 are characteristic of co-ordinated ethanol. An additional δ(Mo‐O) band at 575 cm ‐1 appears after complexation. The complex exhibits strong infrared absorptions at 933 and 916 cm ‐1 (Figure 1), which are characteristic of the cis-Mo(O) 2 group. 3 The complex free of ethanol (bands at 3269 and 1047 cm ‐1 disappear) as well as absorption within the 940‐900 cm ‐1 range, shows a very intensive absorptions at 862 and 836 cm ‐1 (Figure 1). The changes in the IR spectrum of both compounds are very similar to those observed for [Mo(O) 2 (sae)(H 2 O)] and [Mo(O)(μ-O)(sae)] 2 complexes. 5