Abstract
In this study, the reactions of the bimetallic compound Na[W2(µ-Cl)3Cl4(THF)2]·(THF)3 (1, (W 3 W)6+, a'2e'4) with norbornene (NBE) and some of its derivatives (5-X-2-NBE; X = COOH (NBE–COOH), OH (NBE–OH), CN (NBE–CN), COOMe (NBE–COOMe), CH=CH2 (VNBE); norbornadiene (NBD)) are described. Complex 1 contains a tungsten–tungsten triple bond, bearing three halide bridges and two labile THF ligands, in a cisoidal relationship along the metal–metal axis. The complex was found to be a highly efficient room temperature homogeneous and heterogeneous unicomponent initiator for the catalytic ring opening metathesis polymerization (ROMP) of most substrates. NBE provides polynorbornene (PNBE) of high molecular weight (Mw) in high yields, soluble in organic solvents. The reaction proceeds with high cis-stereoselectivity (80%–86% cis), independently of the reaction conditions. Strongly coordinating pendant groups (–COOH, –OH, –CN) deactivate 1, whereas substrates bearing softer ones (–COOMe, –CH=CH2) are quantitatively polymerized. NBD gives quantitatively insoluble PNBD. The polymers have been characterized by 1H, 13C NMR and Size Exclusion Chromatography (SEC). Monitoring the reactions in situ by 1H NMR (1/NBD or NBE) provides direct evidence of the metathetical nature of the polymerization with the observation of the active tungsten alkylidene propagating polymeric chains. Mechanistic aspects of the reactions are discussed.
Highlights
The olefin metathesis reaction induces the mild cleaving and redistribution of carbon–carbon double bonds, and ever since its discovery, it has been highly utilized in polymer science and organic synthesis [1,2,3,4,5,6]
In view of: (a) the continuing interest on classical-type catalysts in industrial processes for highly active substrates, which may not justify the use of well-defined initiators [1,2,3,4,5,6,7,22], and (b) the importance of metal oxidation state in the search for new metathesis catalysts [12], as well as general comparison reasons (W(VI), W26+ halides), we have extended our investigations into the reactions of 1 with NBE and some selected derivatives (5-X-2-NBE; X = COOH (NBE–COOH), OH
It must be noted that the isostructural and isoelectronic perhalocomplex n ( Bu4N)3[W2(μ-Cl)3Cl6] (2) [26] under the same conditions was found inactive towards the ring opening metathesis polymerization (ROMP) of NBE, whereas the monosubstituted W27+ derivative[W2(μ-Cl)3Cl5(THF)] (3) [27] provided very small yield (~10%)
Summary
The olefin metathesis reaction induces the mild cleaving and redistribution of carbon–carbon double bonds, and ever since its discovery, it has been highly utilized in polymer science and organic synthesis (basic chemicals, pharmaceuticals) [1,2,3,4,5,6]. An interesting aspect of this initiator is its marked predilection to produce highly cis-polyacetylenes, which is unusual within the mononuclear tungsten halide analogs In this respect, and in view of: (a) the continuing interest on classical-type catalysts in industrial processes for highly active substrates, which may not justify the use of well-defined initiators [1,2,3,4,5,6,7,22], and (b) the importance of metal oxidation state in the search for new metathesis catalysts [12], as well as general comparison reasons (W(VI), W26+ halides), we have extended our investigations into the reactions of 1 with NBE and some selected derivatives (5-X-2-NBE; X = COOH (NBE–COOH), OH (NBE–OH), CN (NBE–CN), COOMe (NBE–COOMe), CH=CH2 (VNBE); norbornadiene (NBD)). Characterization of the polymers formed and mechanistic aspects of these reactions are presented
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