Novel Allyl Cobalt Phosphine Complexes: Synthesis, Characterization and Behavior in the Polymerization of Allene and 1,3-Dienes

Abstract

Abstract:Novel allyl cobalt complexes, i.e., (η 3 -C 4 H 7 )(η 4 -C 4 H 6 )Co(PCyPh 2 ) (1), (η 3 -C 4 H 7 )(η 4 -C 4 H 6 )Co(PMePh 2 ) (2) and (η 3 -C 5 H 9 )(η 4 -C 5 H 8 )Co(PMePh 2 ) (3), were synthesized by reacting CoCl 2 (PRPh 2 ) 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 ( 1 H 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.

Dienes Polymerization: Where We Are and What Lies Ahead

“…In recent years, new catalytic systems have been obtained by combining MAO with CoCl 2 complexes with phosphine ligands − (Figure and Table ). The peculiarity of these new systems consists in the fact that they allow to obtain polybutadiene with a controlled microstructure ( cis -1,4; 1,2; mixed cis -1,4/1,2 structure) simply by varying the type of ligand coordinated to the cobalt atom; it is well-known that phosphines may exhibit different steric and electronic properties depending on the substituents on the phosphorus atom. − Polybutadiene with a high cis content was obtained in the case of bulky aliphatic phosphines (e.g., P t Bu 3 and P i Pr 3 ), while polybutadiene with a cis -1,4/1,2 mixed structure was obtained with aliphatic phosphines with a smaller steric hindrance (e.g., PEt 3 and P n Pr 3 ) …”

“…The use of catalysts based on complexes with aromatic phosphines instead led to the formation of essentially 1,2-polybutadiene, showing an increasing of the syndiotacticity degree with increasing the phosphine steric hindrance. − …”

Some Novel Cobalt Diphenylphosphine Complexes: Synthesis, Characterization, and Behavior in the Polymerization of 1,3-Butadiene

“…In the polymerization of 1,3-butadiene with cobalt phosphine complex-based catalysts, the structure of the active site, as reported in our previous papers [ 1 , 2 , 3 , 5 , 13 ], is that shown in Figure 4 A, with only one phosphine ligand on the cobalt atom, the incoming monomer cis -η 4 coordinated, and the growing chain bonded to the cobalt atom through a η 3 −allyl bond. Most likely, a sort of equilibrium between cobalt and aluminum (MAO) was established ( Figure 4 ), so that the phosphine ligand, initially on the cobalt atom ( Figure 4 A), may migrate onto the aluminum atom ( Figure 4 B), causing a drastic change in the selectivity of the catalytic center from specific 1,2 to specific cis -1,4.…”

Catalysts for the Controlled Polymerization of Conjugated Dienes

“…Particular behavior was observed vs. polymerization, related in particular to the internal or external substitution of the substrate diene monomer, also depending on the molecular structure (exo-exo or exo-endo orientation of the allyl group and steric bulk of additional phosphine ligands) of the pre-catalyst. Hypotheses were given to account for these observations [16].…”