Abstract
Novel allyl cobalt complexes, i.e., (η3-C4H7)(η4-C4H6)Co(PCyPh2) (1), (η3-C4H7)(η4-C4H6)Co(PMePh2) (2) and (η3-C5H9)(η4-C5H8)Co(PMePh2) (3), were synthesized by reacting CoCl2(PRPh2)2 (R = methyl, cyclohexyl) with 1,3-butadiene or isoprene in presence of metallic zinc. The complexes were fully characterized by Nuclear Magnetic Resonance (NMR) spectroscopy (1H and 2D experiments); in case of 1, single crystals, suitable for X-ray analysis, were obtained and the molecular structure was determined. The allyl cobalt phosphine complexes alone gave highly crystalline 1,2 polymers from 1,2-propadiene, but they did not polymerize 1,3-dienes. Nevertheless, in the presence of a stoichiometric amount of methylaluminoxane (MAO), they were able to polymerize 1,3-butadiene and substituted 1,3-butadienes such as isoprene, (E)-1,3-pentadiene, (E)-1,3-hexadiene, and (E)-3-methyl-1,3-pentadiene. Specifically, 1/MAO gave predominantly syndiotactic 1,2 polymers from 1,3-butadiene and terminally substituted 1,3-butadienes (e.g., 1,3-pentadiene and 1,3-hexadiene), but it was practically not active in the polymerization of internally substituted 1,3-butadienes (e.g., isoprene and 3-methyl-1,3-pentadiene); 2/MAO and 3/MAO exhibited instead an opposite behavior, giving predominantly isotactic 1,2 polymers from 3-methyl-1,3-pentadiene, and showing very low activity in the polymerization of 1,3-butadiene, 1,3-pentadiene and 1,3-hexadiene. The results obtained are interesting from the mechanistic point of view, and some hypotheses to explain this particular behavior were formulated.
Highlights
Transition metal catalysts commonly used for the polymerization of 1,3-dienes can be grouped into the following classes: (i) catalysts derived from aluminum compounds and transition metal compounds (Ziegler–Natta catalysts); (ii) catalysts derived from precursors not containing preformed metal-carbon bonds; (iii) catalysts based on allyl derivatives of transition metals [1,2,3,4,5]
The allyl cobalt phosphine 1–3 complexes were prepared by reacting CoCl2(PRPh2)2
No metal-carbon bond for the monomer insertion is available; following route b no more ligand remains coordinated to the cobalt atom, meaning that we should not observe any influence of the ligand on the polymerization stereoselectivity, while, as reported above, 1,2 poly(1,3-butadiene)s having different tacticity, depending on the type of ligand coordinated to the cobalt atom, are obtained
Summary
Transition metal catalysts commonly used for the polymerization of 1,3-dienes can be grouped into the following classes: (i) catalysts derived from aluminum compounds (i.e., aluminum alkyls, aluminum hydrides and aluminoxanes) and transition metal compounds (Ziegler–Natta catalysts);(ii) catalysts derived from precursors not containing preformed metal-carbon bonds; (iii) catalysts based on allyl derivatives of transition metals [1,2,3,4,5]. Extensive work has been carried out with catalysts based on transition metal allyl derivatives [1]; these systems, which include simple allyl derivatives of transition metals and more. Catalysts 2017, 7, 381; doi:10.3390/catal7120381 www.mdpi.com/journal/catalysts acceptorscatalysts or Lewisderived acids, are of particular interest mainly because they are electron good models of the active complex from the reaction of the allyl complexes with donors, electron site structure, and provide useful information to elucidate the catalysis of polymerization. The firstand examples of provide stereospecific polymerization inducedthe bycatalysis simple of allyl derivatives of site structure, useful information to elucidate polymerization. The first examples of stereospecific polymerization induced by simple allylwas derivatives with (allyl)3Cr andwere a predominantly polymer with (allyl). A crystalline of transition metals first reported incis‐1,4
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