Elsevier

Inorganica Chimica Acta

Volume 436, 1 September 2015, Pages 132-138
Inorganica Chimica Acta

Synthesis of mixed-ligand cobalt complexes and their applications in high cis-1,4-selective butadiene polymerization

https://doi.org/10.1016/j.ica.2015.07.033Get rights and content

Highlights

  • The novel PN3 and the CoCl2PN3 are synthesized and characterized.

  • The CoCl2PN3 is half-oxidized to CoCl2Odouble bondPN3 in presence of H2O2.

  • X-ray finds CoCl2PN3 and CoCl2Odouble bondPN3 are co-crystallized in one unit.

  • The complexes are active in cis-1,4 selective butadiene polymerization.

Abstract

Incomplete oxidation of (N-di-tert-butylphosphino)-6-(2-methyl-2′H-benzoimidazole)-2-aminepyridine dichlorocobalt (PN3CoCl2) in DMF results in a unique co-crystal I formed with three parts including DMF, unit A and unit B complex with Co1 and Co2, respectively (PN3 ligand in unit A: (N-di-tert-butylphosphino)-6-(2′-methyl-2′H-benzoimidazole)-2-aminepyridine, and Odouble bondPN3 ligand in unit B: (N-di-tert-butylphosphinoxide)-6-(2′-methyl-2′H-benzoimidazole)-2-aminepyridine), with 1:1:1 molar ratio. Co1 and Co2 complexes both display a five-coordinated distorted-square-pyramidal geometry around the metal center. The Co1 center is coordinated with PN3 ligand via two N atoms from pyridine, benzoimidazole moiety as well as one P atom, and the Co2 center is coordinated with the oxidized ligand Odouble bondPN3 via two N atoms from pyridine, benzoimidazole moiety as well as one O atom from DMF molecule, while the oxidized phosphine moiety (Odouble bondP) being excluded from the coordination sphere. Activated with AlEt2Cl, the co-crystallized complexes I are able to actively convert butadiene to polybutadiene, affording cis-1,4 polybutadiene with cis-1,4 unit up to 95.5–97.8% and number average molecular weight of cal. 105 g/mol. The high cis-1,4 selectivity and monomodal GPC curve of resultant polymer imply that the identical active species generated from two distinctive cobalt centers.

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%.

  1. Download : Download full-size image

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 Pdouble bondO(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 CoCl2Odouble bondPN3 (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 CoCl2Odouble bondPN3 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.

References (24)

  • D. Gong et al.

    J. Mol. Catal. A: Chem.

    (2014)
  • D. Gong et al.

    J. Organomet. Chem.

    (2012)
  • D. Gong et al.

    J. Organomet. Chem.

    (2011)
  • D. Gong et al.

    Appl. Catal., A

    (2013)
  • H. Liu et al.

    J. Mol. Catal. A: Chem.

    (2014)
  • Y. Nakayama et al.

    Polym. Int.

    (2011)
  • H.T. Ban et al.

    J. Polym. Sci., Part A: Polym. Chem.

    (2004)
  • G. Ricci et al.

    Macromolecules

    (2001)
  • G. Ricci et al.

    J. Organomet. Chem.

    (2004)
  • R. Cariou, J.J. Chirinos, V.C. Gibson, G. Jacobsen, A.K. Tomov, G.J.P. Britovsek, A.J.P. 39, 2010,...
  • R. Cariou et al.

    Macromolecule

    (2009)
  • G.J.P. Britovsek et al.

    J. Am. Chem. Soc.

    (1999)
  • Cited by (25)

    • Polymerization of butadiene, isoprene and 1-substituted dienes using cobalt catalysts

      2022, Inorganica Chimica Acta
      Citation Excerpt :

      Cis-1,4 polydiene has been prepared at industrially scale mainly using catalysts Nd(vers)3/AliBu3/AlEt2Cl [4], TiI4/AliBu3 [5], Co(2-ethylhexanoate)3/AlEt2Cl [6] and Ni(naphthenate)2/AlEt3/BF3OEt2 [7] catalyzed polymerization. Cobalt catalysts supported by bidentate N,O-[8], N,N-[9], O,O- [10] and tridentate N,N,O-[11,12], N,N,N-[13-15], N,O,S-[16,17] and N,O,P- [18,19] have been also reported for cis-1,4 selectivity in the literatures, and control of molecular weight and sequence can be achieved in some cases [16,17,20]. 1,2 Polybutadiene, particularly with syndiotactic regularity can be synthesized by iron catalysts carrying bipyridine ligand [3], or with phosphite and phosphate an additive [21-25], cobalt catalysts using trialkyl(aryl) phosphine ligand [26-29] and chromium [30] catalysts supported with biphosphine ligand.

    • Thermally robust α-diimine nickel complexes for 1,3-Butadiene polymerization enhanced by intra-ligand H…F interaction

      2022, Molecular Catalysis
      Citation Excerpt :

      As one of the most important synthetic rubber, cis-1,4- polybutadiene (PB) nowadays finds wide applications in our daily life, such as tire treads, side walls, thermoplastic modifications, golf ball cores, etc. [1–16] Nickel-based catalytic systems, i.e. Ni(OCOR)2-AlR3-BF3·OEt2, serve as a pivotal role in producing the currently commercially available PB commodity, and annually, ca. 1.5 million metric tons of Ni-PB is produced around the world, which is much higher than other metal based coordination polymerization catalytic systems, such as Ti, Co, Nd, etc.

    View all citing articles on Scopus
    View full text