Synthesis of mixed-ligand cobalt complexes and their applications in high cis-1,4-selective butadiene polymerization
Graphical abstract
Mixed-ligands ligated cobalt complexes were prepared. The complexes are high active in butadiene polymerization. The obtained polymer has cis-1,4 unit up to 95.5–97.8%.
Introduction
One of our main interests lies in the field of homogeneous catalysis of butadiene to cis-1,4-polybutadiene, as it has been one of the most regarded materials suitable for many practical applications such as tire, tube and shoes. The development of catalysts plays an important role for efficient control in chain architectures such as stereochemistry, functionality and chain length in polybutadiene synthesis [1], [2], [3], [4], [5], [6]. Recently, tri-dentated pincer ligands have emerged as attractive auxiliary ligands in iron and cobalt based catalysts promoted for olefin and diene polymerizations [7], [8]. We have currently been exploring well-defined cobalt catalysts for (co-) controllable (co-) polymerization for conjugated dienes to afford desirable product. The extensive and diverse coordination chemistry of the cobalt complexes have provided diversified catalytic properties as the product microstructures, such as cis-1,4, trans-1,4, 1,2 enchainment and their combinations crucially depends on catalyst formulation, controlling of the product’s properties by catalyst design and chosen has thus been feasible. Publications to date have described various ligands supported metal complexes, in particular, asymmetry PN2 and PN3 type ligands, which developed by us and Milstein, respectively, [9], [10], [11], [12], [13] have attracted much attention as the dissociation of labile metal-phosphine bond (metal-P bond is weak with respective to metal-N bond) tends to generation of open site available for small molecule coordination and activation, while the intact two metal-N bonds could effectively chelated, stabilize the metal center, as well as induce regio- and stereoselectivity. Relevant ligands in this context are specified in scheme 1. Besides the well known PN2 ligand A which forms a labile five-membered-ring chelate with ruthenium, iron and iridium, e.g. {2-diethylamine-6-(N,N-di-tert-butylphosphine)methylpyridine, [14]}, there are also established ligands recently explored in our lab which give rise to various frameworks and the corresponding coordination chemistry e.g. B {2-diethylamine-6-(N,N-di-tert-butylphosphine)aminepyridine, [9]}, C {2-pyrazol-6-(N,N-di-tertbutylphosphine)aminepyridine, [10]}, D {6-(N,N-di-tert-butylphosphine)amine-2-(3′,3′dimethyloxaline)pyridine, [11]}, E {6-(N,N-di-tert-butylphosphine)amine-2,2-bipyridine, [12]}, F {2-O-di-tert-butylphosphine)-6-aryliminepyridine, [15]}. However, due to the air sensitivity of phosphine (III) moiety of PN3 ligand in presence of oxidant, transformation to it’s oxide PO(V) analogues is usually resulted, as such, a twist six-membered-rings chelate could be produced and the formed metal-O bond is typical weaker than the corresponding metal-P, which ultimately could lead to a more open active center for monomer activation and insertion in process of polymerization. For investigating structural variation before and after oxidation and their effect on the catalytic behavior, we design, synthesize (N-di-tert-butylphosphino)-6-(2′-methyl-2′H-benzoimidazole)-2-aminepyridine ligand, complex it with anhydrous CoCl2, and oxidize the corresponding cobalt complex in DMF in presence of H2O2, interestingly, recrystallization of product results in a cocrystal comprised of CoCl2PN3, the oxidized CoCl2OPN3 (with a coordinated DMF) and a free DMF molecule. The resultant complexes have been examined for selective polymerization of butadiene, and comparison of the catalytic performance with respect to the activity and selectivity to that of the intact complex has been also discussed.
Section snippets
General procedure
All manipulation of air or moisture sensitive compounds was carried out under nitrogen or argon atmosphere. Ammonia, phenylenediamine, 6-bromopyridine-2-carboxylic acid, diethyl aluminum chloride, methyl iodide, polyphosphoric acid (PPA), n-butyllithium and di-tert-butylphosphinechloride were commercially available and used as received. All solvents were purified by the standard procedures. Polymerization grade 1,3-butadiene was purified by passing columns packed with KOH and molecular sieves
Synthesis and characterization of ligand and complexes
The PN3 ligand was prepared via reaction of 6-bromopyridine-2-carboxylic acid and phenylene diamine in presence of PPA in microwave conditions, followed by the N-methylation of NH in benzoimidazole moiety and N-substitution with di-tert-butylphosphine chloride in NH2. The obtained ligand was well characterized by NMR (see in the Supporting information), and the results are well in consistent with expected formula. The corresponding cobalt complex was synthesized by the conventional refluxing
Conclusion
We have prepared N-di-tert-butylphosphino-6-(2′-methylbenzoimidazole)-2-aminepyridine ligand supported CoCl2PN3 complex. Reaction with H2O2 in DMF provides an incomplete cocrystal I comprised of intact CoCl2PN3, oxidized CoCl2OPN3 and free DMF. In combination with AlEt2Cl, the complexes are found to be high active in butadiene polymerization, producing polybutadiene with high cis-1,4 enchainment. The cis-1,4 specified selectivity and monomodal of molecular weight distribution imply that a
Acknowledgment
This work is supported by the Natural Science Foundation of China under Grant [21304050]; Natural Science Foundation of Ningbo [2014A610109] and [2014A610115]; Natural Science Foundation of Fujian Province [2015H0045] and sponsored by K.C. Wong Magna Fund in Ningbo University.
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