Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Polymerization of Isoprene Promoted by Aminophosphine(ory)Fused Bipyridine Cobalt Complexes: Precise Control of Molecular Weight and cis-1,4-alt-3,4 Sequence Junyi Zhao,† Huafeng Chen,† Wenxin Li,† Xiaoyu Jia,§,∥ Xuequan Zhang,‡ and Dirong Gong*,† †
Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, P. R. China Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China § Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, No. 1799, Jimei Road, Xiamen, Fujian 361021, P. R. China ∥ Ningbo Urban Environment Observation and Research Station-NUEORS, Chinese Academy of Sciences, Ningbo, Zhejiang 315830, P. R. China ‡
S Supporting Information *
ABSTRACT: Ligands N-(dialkyl or arylphosphino)-(2,2′-bipyridin)-6-amine (L1, aryl = Ph; L2, alkyl = tBu; L3, alkyl = adamantyl (Ad)) as well as the corresponding oxidized N-(2,2′-bipyridin-6-yl)-P,P-dialkyl or aryl phosphinic amide (L4, aryl = Ph; L5, alkyl = tBu; L6, alkyl = Ad) congeners were designed and coordinated to cobalt dichloride. The structures of formed complexes were characterized by IR and elemental analyses, as well as characterizations of the X-ray diffractions for complexes Co4 and Co6, which revealed the cobalt center is expectedly pentacoordinated in a distorted trigonal bipyramidal configuration with a prolonged CoO(P) bond. In combination with MMAO, complex Co2 was highly active in cis-1,4-alt-3,4 enchained polymerization. The hemilabile nature of OP is possible for the alternating η4-cis-1,4 and η2-3,4 coordination, and insertion at the metal−carbon bond ensued. In combination with AlEt2Cl, each of complexes Co4, Co5, and Co6 was capable of converting isoprene to polyisoprene in a control mode with observed polymerization rate constants (kobs = 0.1531 L mol−1 min−1 (Co4), 0.1382 L mol−1 min−1 (Co5), and 0.0902 L mol−1 min−1 (Co6)). The activation energy of the polymerization by Co4 falls in the range of 27−31 kJ/mol by determining kobs values at 0, 30, and 50 °C. The 13C NMR analyses of the obtained polyisoprene revealed that complexes Co4, Co5, and Co6 have a cis-1,4 selectivity of 86.6−93.4% with a 3,4 selectivity of 6.6−13.4%. This catalyst system can also be applied to block copolymerization of isoprene and myrcene in a living cis-1,4 fashion; therefore, a new biosourced monomer-based elastomer has been achieved.
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cis-1,4-alt-3,46 polymers from a sole isoprene monomer, whose properties can be varied by tuning the composition and sequence distributions. Since the introduction of the Nobel Prize winning Ziegler− Natta catalyst, the field for conjugated diene specific polymerization has enjoyed tremendous successes in both academia and industry.7 Nevertheless, some major limitations exist for the
INTRODUCTION As the demand for high performance synthetic rubbers has increased, the development of high quality elastomers by polymerization of conjugated diene has grown in importance. In addition, the limited supply of natural rubber has promoted the need for synthetic rubber. Isoprene is an attractive, convenient monomer and can be incorporated in 1,2,1 cis1,4,2 trans-1,4,3 and 3,4 incorporation4 fashions in the process of polymerization. Meanwhile, the versatility of isoprene insertion allows for the synthesis of cis-1,4-b-trans-1,45 and © XXXX American Chemical Society
Received: January 31, 2018
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DOI: 10.1021/acs.inorgchem.8b00270 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. Synthetic Route to Ligands L1−L7 and Cobalt Complexes Co1−Co7
the architecture and their further functionalizations, has been achieved.5,14 Compared to the lanthanide and early transition metals, cobalt complexes are attractive due to their amenability in being synthesized and isolated, high moisture and air stability, high activity, and versatile selectivity for conjugated diene polymerization, which therefore makes them applicable for large scale production of poly(conjugated dienes). For example, Co(acac)2/AlEt3/CS215 and CoBr2(PPh3)2/AlEt3/H2O16 have been patented for industrial production of syndiotactic 1,2 polybutadiene, and CoCl2·Py/AlEtCl28h has been patented for cis-1,4 polybutadiene. Representative well-defined complexes are those bearing phosphine ligands, which are highly active and stereospecific in the polymerization of butadiene and alkylsubstituted butadienes,17 among which isoprene is able to be polymerized via cis-1,4-alt-3,4.6 Bidentate-N,N18 and tridentateN,N,N19 chelated cobalt complexes were also reported, but either the high activity or unique cis-1,4 specificity was lost. For all reported cobalt catalysts, even for the first transition metals, the capability of controlled polymerization of isoprene with mainly cis-1,4 enchainment has not yet been attained. In ongoing efforts toward poly(conjugated diene) materials, our laboratory has explored stereoselective iron20 and cobalt21 catalysts for conjugated diene polymerizations. In the seminal work, we developed types of (X)PN322,23 compounds and investigated the effect of the heteroatom (X = S and O donor) of a hemilabile ligand on the cobalt-catalyzed isoprene polymerization.23 We here report a new catalyst system based on dichlorocobalt bearing an ancillary amino-phosphory(ine)fused 2,2′-bipyridine ligand. With MMAO as the co-catalyst, this catalyst system actively polymerizes isoprene with a cis-1,4alt-3,4 sequence; meanwhile, activated by AlEt2Cl, the catalyst system provides cis-1,4 selectivity and excellent “livingness” (Mw/Mn = 1.05−1.35) at 0, 30, and 50 °C and is also efficient for sequential copolymerization of isoprene and myrcene.
class of catalysts, such as multisite behavior, poor copolymerization capability, and reduced sequence and molecular weight control in most cases.8 In the search for soluble versions of the Ziegler−Natta catalysts, well-defined homogeneous catalysts have played a central role in illuminating the mechanistic points of the initiation, propagation, and termination steps of the coordination polymerization. Metallocene catalysts (with cyclopentadiene derivatives Cp as the ligand), as one type of homogeneous catalyst, possess a welldefined active site that permits control over polymer stereochemistry, unit sequence, and polymer molecular weight and distribution, resulting in the formation of a wide range of differentiated poly(conjugated diene) materials with new or enhanced qualities.9 The first utility of Cp as a metal nonspectator ligand for conjugated diene polymerization was traced back to CpTiCl3 for living and cis-1,4 polymerization of butadiene and its biphenolate derivatives for copolymerization of isoprene with styrene.10 Subsequently, the Cp derivatives were extended to lanthanide metals for 3,4 isotactic polymerization of isoprene and alternating copolymerization with styrene and ethylene.11 Recently, a pyridine-fused fluorene ligand supported yttrium complex (Flu−CH 2 −Py)Y(CH2SiMe3)2 has been used for temporal and stereocontrolled 3,4 polymerization of isoprene by alternating additions of pyridine and AliBu3; with this strategy, the switchable copolymerization of isoprene and styrene with dual 3,4 and syndiotactic selectivity is also effective.12 In the past two decades, interest in nonmetallocene catalysts (another type of homogeneous catalyst) has grown. A large number of new high activity and selective catalysts based on a wide array of late and early transition metals, therefore, have been discovered. Middle and later transition metal catalysts, such as (dmpe)2CrCl2-MAO (dmpe: 1,2-bis(dimethylphosphino)ethane)4c, and ferric complexes chelated by bidentate,4d,e tridentate,4o,p and terpyridine ligands4f were reported for 3,4 regio-polymerization of isoprene. The trans-1,4 copolymerization of isoprene and ethylene was realized by using titanium dichloride complexes bearing OSSO ligands.10b Perfect specific cis-1,42l,k and trans-1,42p polymerizations of isoprene have been obtained with various non-Cp rare earth metal catalysts. In parallel with the development of new catalysts, the methodology of polymerizations, such as catalyzed chain growth or coordinative chain transfer polymerization13 and chain shuttling polymerization by using a combination of metal catalysts with various co-catalysts and/or chain transfer agents, has been developed as well. Therefore, control of both the growth and microstructure of polyisoprene chains, as well as
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RESULTS AND DISCUSSION Synthesis and Characterization of Ligands and Complexes. The ligands 2,2′-bipyridine-6-N-(di-tert-butylphosphine) (L1), 2,2′-bipyridine-6-N-(diphenylphosphine) (L2), and 2,2′-bipyridine-6-N-(diadamantylphosphine) (L3) were synthesized via deprotonation of 2,2′-bipyridine-6-amine, followed by the substitution reaction with dialkyl(aryl)phosphine chloride (Scheme 1), with moderate to high yields. The corresponding oxidized ligands L4−L6 were prepared from L1−L3 in the presence of 1.0 equiv of H2O2 in satisfactory yields. All ligands were confirmed by 1H, 13C, and 31 P NMR (see Figures 1S−18S in the Supporting Information). B
DOI: 10.1021/acs.inorgchem.8b00270 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry The cobalt complexes (Co1−Co6) were prepared via refluxing the corresponding ligand and cobalt dichloride in THF in an inert atmosphere. After the solvent was removed, the raw products were dissolved in a methanol/Et2O mixture at −30 °C to afford crystals in moderate yields. Co7 (Scheme 1) was prepared from Co1 via de-aromatization by 1.0 equiv of KOtBu in anhydrous THF. Complexes Co1−Co6 were ionized in the form of (M−HCl−Cl)+, and Co7 was (M−Cl)+ in the ESI-MS condition. The elemental analyses of the complexes were quite consistent with the calculated values. IR spectra (Figure 18S) show the imine CN absorption red shift to the region of 1618−1636 cm−1 in comparison with that of the pro-ligand, verifying the coordination indeed occurs. The solid-state structures of complexes Co4 and Co6 were further determined by single-crystal X-ray diffraction analyses. The crystal parameters are listed in Table 1S in the Supporting Information. Co4 crystallizes with two independent molecules (Co1 in Figure 1) per asymmetric unit, and due to the fact that
Figure 2. Drawing of complex Co6, with hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1−Cl1 2.3143(18), Co1−Cl2 2.3512(16), Co1−O1 1.978(3), Co1−N2 2.204(4), Co1−N3 2.076(5); Cl1−Co1−Cl2 105.31(6), O1−Co1− Cl1 113.25(12), O1−Co1−Cl2 89.61(10), O1−Co1−N2 85.42(13), O1−Co1−N3 141.03(16), N2−Co1−Cl1 94.00(11), N2−Co1−Cl2 160.46(12), N3−Co1−Cl1 101.87(13), N3−Co1−Cl2 97.18(13), N3−Co1−N2 75.50(16).
complex Co2 displayed an acceptable activity toward isoprene. IR of the resulting polyisoprene showed that the 3,4 (890 cm−1) and 1,4 (836 cm−1) incorporations are predominate (Figure 20S). 1H NMR analyses revealed that the integrals of peaks at 5.15 and 4.77−4.69 ppm are close to 1/2, indicating that the equibinary 1,4 and 3,4 are enchained (Figure 21S). A thorough inspection of the polymer structure by 13C NMR spectroscopy revealed more interesting structural details, and the cis-1,4-cis-1,4 and 3,4−3,4 sequences are both invisible (Figure 20S). Careful correlation of the chemical shifts with each type of carbon verified that the cis-1,4 and 3,4 units are in an alternating sequence in the current sample, which was observed in the literature.6 The glass transition temperature falls in the range from −17.6 to −18.5 °C. Varying the amount of MMAO (runs 2−4) with the Co2 catalyst did not significantly change the microstructures, namely, the mixed type with a predominant amount of cis-1,4 and 3,4 having similar levels of the cis-1,4-alt-3,4 sequence. Replacing MMAO with AlEt2Cl to activate Co2 generated more active species; however, the alternating sequence was not preserved, and more cis-1,4-cis-1,4 homosequences were produced (run 5). Nevertheless, this catalyst is highly active and cis-1,4 specific for 1,3butadiene polymerization (run 6). The polymers produced by other Co1 and Co3 catalysts in the presence of MMAO, however, have less 3,4 incorporation (33.7−37.9%, runs 7 and 8), indicating that the steric effect is important for the catalytic selectivity. MMAO was unable to activate either Co4 (run 9) or Co7 (run 10) for the isoprene and 1,3-butadiene polymerization as AlR3 and AlR2Cl were preferred for iron and cobalt complexes with a PO moiety containing ligand or donor.4a,d GPC (gel permeation chromatography) analyses revealed that these polymers have molecular weights of 64−157 kg/mol and unimodal distributions (PDI: 1.29−2.38), consistent with a homogeneous and single-site catalytic process. Living Polymerization of Isoprene. Activated by AlEt2Cl, complexes Co4, Co5, and Co6 have each been proven to be efficient precursors for polymerization (Table 2). The polymer yields were over 80% within 40 min above 30 °C. The molecular weights of the polymers were in the range of 120− 216 kg/mol; importantly, the PDIs fell in a narrow range of 1.08−1.35, indicative of their possible controlled natures.
Figure 1. Drawing of complex Co4, with hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1−Cl1 2.3497(9), Co1−Cl2 2.3047(9), Co1−O1 2.015(2), Co1−N2 2.223(3), Co1−N3 2.106(3); Cl2−Co1−Cl1 100.98(3), O1−Co1− Cl1 107.87(7), O1−Co1−Cl2 95.79(7), O1−Co1−N2 85.50(9), O1− Co1−N3 132.76(9), N2−Co1−Cl1 88.32(7), N2−Co1−Cl2 169.67(7), N3−Co1−Cl1 113.91(7), N3−Co1−Cl2 96.97(8), N3− Co1−N2 74.92(10).
there are only a few differences in the coordination geometry of the central metals in the two independent molecules, reference can be made to the molecule identified by Co1. The N,N,Opincer tridendate ligand coordinates to the central metal ion in an expected N,N,O tridentate mode to form a meridional conformation. The P and O1 atoms lie out of the bipyridine− Co plane by 0.132 Å, and the six member ring consisting of Co1, N1, C1, N2, P, and O1 is, therefore, distorted; this could be the reason for the Co−O1 hemilabile characteristic. The Co1−O1 bond distance is 2.015(2) Å, which is a typical coordinative bond but with a prolonged distance compared to most (P)O−M. The angle of P1−O1−Co1 is 129.41(13)°, indicating O1 adopts an sp2 hybridization. Complex Co6 (Figure 2) has a geometry identical to that of Co4, with a reduced Co1−O1 distance of 1.978(3) Å and a contracted P1− O1−Co1 angle of 127.7(3)°, possibly due to the steric congestion. cis-1,4-alt-3,4 Polymerization of Isoprene. Isoprene homopolymerizations by MMAO activated Co1−Co7 were investigated in toluene, and the results are summarized in Table 1. When activated with 300 equiv of MMAO (run 1, Table 1), C
DOI: 10.1021/acs.inorgchem.8b00270 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. cis-1,4-alt-3,4 Polymerization of Isoprene with Cobalt Complexes/MMAOa microstructure run
cat
[Al]/[Co]
yield (wt %)
cis-1,4
3,4
Mn (10 kg/mol)
Mw/Mn
Tg (°C)
1 2 3 4 5b 6c 7 8 9d 10
Co2 Co2 Co2 Co2 Co2 Co2 Co1 Co3 Co4 Co7
300 500 800 1200 300 200 300 300 300 300
74.9 82.7 86.3 80.1 100 100 84.5 80.3
