Chapter 6
Coordination Chemistry and Applications of Vanadium Aikoxides in Catalysis Esther C. E. Rosenthal, Huiling Cui, and Juliane Koch Department of Chemistry, Technische Universität Berlin, 10623 Berlin, Germany
The synthesis of oxovanadium aikoxides with 2-aryloxy- and 2-alkoxyethanol ligands and their use for the polymerization and oxidation of unfunctionalized olefins is described. The catalytic epoxidation of the complexes with tert-butyl hydroperoxide is discussed. Catalytic polymerizations in combination with different cocatalysts and methyl trichloro acetate as promoter generate different reaction products than without promoter.
Introduction The present paper contains new results on the synthesis and application of oxovanadium aikoxides with ether alcohol ligands. For a better understanding of the recent results a summary of earlier work by the authors (1-3) is included, completed by an extended discussion. The ligands studied (Figure 1) were chosen because of their proposed ability to stabilize the obtained vanadium compounds in their high oxidation states by forming chelating structures with the additional oxygen donor when complexed to vanadium. With this approach we were able to synthesize compound classes, which were previously inaccessible and to prove their existence by single crystal x-ray diffraction (2-4). The presence of remaining chloride ligands in the mixed alkoxo chloro complexes obtained makes the metal atom functionalizable e.g. to introduce alkyl groups.
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© 2007 American Chemical Society
71 Chloro ligands either in the catalysts or in the cocatalysts are also essential for olefin polymerization (5). Heterometallic alkyl aikoxides derived from the vanadium alkoxide precatalyst and the alkyl aluminum cocatalysts have been postulated to act as the active catalysts in Ziegler-Natta polymerization with such systems (6). With regard to an approximation to the so far unidentified active species in vanadium-based homogeneous Ziegler-Natta polymerization, alkylation reactions as well as reactions of the vanadium complexes with Lewis bases have been studied. Our catalytic investigations on this class of compounds revealed activity for the oligomerization of styrene, 1-decene and 1-hexene and for the polymerization of styrene and ethene as well as for the epoxidation of unfunctionalized olefins. In the following part, the syntheses of the oxovanadium complexes as well as the catalytic reactivities of the vanadium complexes will be discussed in detail. For a more complete description of the systems, including discussion of spectral and structural data, and for experimental details, the reader is referred to references 1-3.
Chlorooxovanadium Aikoxides Although mixed oxovanadium(V) alkoxide chlorides are known since 1913 (7), knowledge of the structural features of the systems in the solid state was limited prior to our work as the chloro aikoxides usually are moisture and light sensitive liquids, which have only a limited lifetime even when stored under nitrogen (8-14). Therefore no such compound derived from a mono alcohol has been structurally characterized without additional donor coordination and the corresponding rise of the coordination number to five or six (15-19). This prompted us to choose Lewis base free reaction media for the following reactions in order to avoid the coordination of external solvent molecules to the vanadium center. Indeed tetrahedral coordination at the V ( V ) center could be generated (Figure 1) by equimolar reaction of V O C l with 2-phenoxyethanol in hexane giving the moderately air-sensitive, orange [VOCl (OCH CH OPh)] 1, which is the first simply four-coordinated dichlorooxovanadium(V) alkoxide to be structurally characterized (7). Electronic saturation of the Lewis acidic V ( V ) center in this neither sterically nor coordinatively saturated complex is reached by rather short bond lengths and through a second, non-bonding coordination sphere formed by 3 neighboring molecules and the phenyl ether, whose electronegative atoms lie above the faces of the coordination tetrahedron. The analogous reaction with 2-alkoxyethanols yields the octahedrally coordinated chloride bridged dimers 2-7 in Figure 1 (2, 3). Use of the corresponding lithium alcoholates is a synthetic alternative in all cases. With exception of the benzyl derivative 5 the solid state structures of all aikoxides 3
2
2
2
72 have been proven by single crystal x-ray diffraction and are the first of chloride bridged oxovanadium(V) aikoxides. Whereas in the monomeric 1 electronic saturation of the Lewis acidic V(V) center results from the second coordination sphere complexes 2-7 dimerize and draw additional electron density from intramolecular donor coordination: the two bridging chloride ligands build a shared edge of 2 V 0 C 1 octahedrons, which incorporate a five-membered chelate ring. The unequal coordination behaviour results from the different a donor bond strengths of the ether functions. No bonding interaction is observed with the weakest a donor, the phenyl ether. Consistently, the shortest vanadium ether oxygen bond within the series of dimeric complexes 2-7 is found for the strongest a donor, the cyclic ether. 3
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