Dual Catalysis of the Selective Polymerization of Biosourced Myrcene

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Dual Catalysis of the Selective Polymerization of Biosourced Myrcene and Methyl Methacrylate Promoted by Salicylaldiminato Cobalt(II) Complexes with a Pendant Donor Xiaoyu Jia,‡,§ Wenxin Li,† Junyi Zhao,† Feiyan Yi,† Yunjie Luo,† and Dirong Gong*,† †

Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, 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 Organometallics Downloaded from pubs.acs.org by TULANE UNIV on 12/17/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Cobalt complexes (Co1−Co6) bis-chelated by phenoxy (4,6-di-tert-butyl-2-phenol, naphthalen-2-ol)-imine ligands containing pendant hard oxygen (Co1 and Co4), soft sulfur (Co2 and Co5), or phosphine donors (Co3 and Co6) are reported for dual catalysis of selective polymerization for biosourced myrcene and methyl methacrylate (MMA). X-ray analysis reveals that the complexes features a distorted octahedral coordination of the cobalt center binding with phenolate oxygen, imine nitrogen, and pendant donor (P for Co3 and S for Co5). Activated by MMAO or AliBu3, all precursors are able to not only convert myrcene to cis-1,4-polymyrence but also polymerize MMA resulting in poly(methyl methacrylate) (PMMA) with moderate syndiotacticity. The introduction of pendant donor concurrently improves activity and stability particularly at 60−80 °C for polymerizations of both monomers. The enhanced activity and selectivity observed in myrcene cis-1,4 polymerization promoted by the sulfur and oxygen containing catalysts may relate to the weak Co− X (X = O, S) interaction, facilitating myrcene coordination. However, the donor effect on MMA polymerization activity is reversed, with phosphine donated catalyst demonstrating remarkably enhanced activity. This study represents the first example of cobalt catalyzed polymerization of both polar and nonpolar functions via possible mechanisms differentiated by pendant donor and also may provide valuable insights into their essential role in mediating the polymerization process.

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INTRODUCTION The polymerization of isoprene and butadiene has been one of the most important industrial processes to access to high performance rubber; however, the field is encountering the increased likelihood of gradual exhaustion of petroleum resources and environment pollution.1 Motivated by the surge of sustainability in this field, exploitation of renewable polymers derived mostly or entirely from biosources that can resemble and replace the fossil based elastomer has attracted considerable attention from industry and academic sectors.1c β-Myrcene (7-methyl-3-methylene-1,6-octadiene), resembling the chemical structure of many petro-based olefins, can be found within the essential oil of various plants and can be produced on a commercial scale by the pyrolysis of β-pinene.2 Radical (co)polymerization of myrcene has provided an entry to a novel type of biosourced elastomer.3 In the meanwhile, metal catalyzed highly specific polymerization of myrcene has been also reported for regular polymyrcene based materials; however, examples have been limited to the Nd(BH4)3(THF)3/BEM complex for cis-1,4-polymyrcene,4 Lu© XXXX American Chemical Society

(CH2Si(CH3)3)2PNP and LuCH2Si(CH3)3SNS for 3,4-polymyrcene (PNP, iminophosphonamide; SNS, β-diimidosulfonate),5 Lu(CH2Ph)2PNP for trans-1,4-polymyrcene,6 and titanium dithiolate (TiOSSO) complexes for polymyrcene with trans-1,4, 3,4, and cis-1,4 mixed structures.7 In polymerization of conjugated dienes, cobalt-based systems have received the earliest recognition in both academic and commercial production of polybutadiene due to easy preparation,8 high stability, and versatile selectivities, 9 particularly, the unique 1,2-stereoselectivities in polymerization of some substituted butadienes.10 Nevertheless, they have been rarely relevant for syntheses of cis-1,4-polyisoprene and -polymyrcene,11,12 probably due to steric factors disfavoring monomer coordination and insertion to a smaller radius Co+. In the meanwhile, production of functionalized olefinic polymers with polar groups by direct synthesis has been one of the most challenging research goals hardly achieved by Received: September 30, 2018

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DOI: 10.1021/acs.organomet.8b00708 Organometallics XXXX, XXX, XXX−XXX

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Organometallics conventional industrial chemical process.13 In this aspect, cobalt catalyst has been also attractive due to low oxophilic and higher tolerance to moisture and oxygen, avoiding masking polar functionalities. Although many cobalt complexes have been developed for mediating radical (co)polymerization of polar monomers such as MMA,14 vinyl acetate,15 acrylonitrile,16 and ethylene,17 coordinative polymerization of polar monomers by cobalt metal is still a challenging endeavor, because simple coordination of the functional group to the metal may hamper monomer activation. Till now, reported catalysts have been limited to N,N-di(2-picolyl)amine Co(II),18 bis(salicylaldiminate)-Co(II)19 complexes, and Co(II)Pyridyl bis(imine)/MAO20 for low efficiency MMA polymerization. The strong binding of ester functionality to the metal center via η4-chelation retarding the chain-growth reaction is responsible for the low activities, and so far there have been no general ways to improve the catalytic activity and selectivity, as well as the stability. Generally, the development of coordination catalysts has met limited monomer versatilities, for example, catalysts efficient for nonpolar monomers are usually not active for polar monomers, and vice versa. The discoveries of a handful of competent polymerization catalysts capable of performing both polar and nonpolar functions at our disposal seemingly ease the dilemma, but the mechanism has not yet been elucidated. On the other hand, it becomes more and more difficult, as well as costly, to synthesize new catalysts that are applicable for limited monomers and as a result increasingly difficult to discover new polymerization strategies or new materials. Therefore, studies into alternative catalyst design strategies are important for addressing both functions. Considering different polymerization mechanisms that could be operating for different type of monomers, a single catalyst that is able to polymerize different monomers, namely, “dual catalysis”, via a switchable mechanism is highly attractive. And, their potential dual catalysis performance for both functions is also desirable but still challenging. In the late 1990s, the exploration of catalysts incidentally led three groups to independently discover that certain salicylaldimine (phenoxy-imine) compounds could serve as excellent ligands for olefin insertion polymerization catalysts with transition metals.21 These discoveries have invoked renewed interest in this conventional and yet versatile ligand motif and spurred intense research into its associated chemistry around the globe. These ligands benefit from ease of synthesis and amenability to structural and electronic modifications, which have resulted in extremely rich and diverse ligand libraries. Moreover, the phenoxy-imine ligands can act on different metals over a wide range of the periodic table, resulting in superb olefin,22 diene,23 acrylate,24 cyclic ester, and alcohol25 (co)polymerization catalysts, and a very similar ligand design (position, size, and nature of substituents) has been often used in order to maximize catalytic performance. The phenoxyimine ligands can be further classified into bridged tetradentate [O,N,N,O] ligands, including classical salen and salphen,21 and tridentate [O,N,X] or [O,N,L] phenoxy-imine ligands that have pendant donors (X or L) bearing a Lewis base group in addition to an inert fragment.21,26 The tridentate ligands have been attracting substantial interests in olefin polymerization catalysis, where judiciously designing the pendant donor is able to delicately tune the electronic and steric environment of the metal center via proximate and flexible metal-donor coordination. Despite the wide research work on the syntheses and

structural and catalytic properties of the ligand-supported transition metal complexes,27 very little is known about the structures of their cobalt counterparts and few have been reported for polymerizations of myrcene or polar monomers.8c,d,28 In order to gain insight into the polymerizations of both functions by cobalt catalysts, the current work focuses on syntheses, structure analyses, and dual polymerization catalysis by a series of cobalt complexes with tridentate salicylaldiminate paired with oxygen, sulfur, and phosphine donors.

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RESULTS AND DISCUSSION Synthesis and Characterization of Ligands and Complexes. Phenoxy imine-thio (S), ester (O), and phosphine (P) ligands including different combinations of phenoxy and donor substituents have been designed. Their phenolate complexes, Co1−Co6, depicted in Scheme 1, have been prepared by treating cobalt acetate with 2.0 equiv of the corresponding ligands in refluxed ethanol under a nitrogen atmosphere. Scheme 1. Structures of Cobalt Complexes Studied in the Work

1

H NMR spectra were recorded at room temperature, and those of Co1 and Co3 are displayed in Figure 1 and Figure 2, respectively. Cobalt(II) complexes Co1, Co2, Co4, and Co5 give rise to paramagnetic NMR spectra with relatively sharp resonances in the range of −7.76 to 57.12 ppm probably due to the rapid electron relaxation of the high spin d7 (t2g5eg2 configuration) systems.29 As shown in Figure 1, the cobalt(II) complex Co1 displays resonances arising from eight of the nine nonequivalent H atoms, but one signal is not detected, presumably due to its close vicinity to the paramagnetic metal center and a resulting substantial dipolar relaxation.30 However, 2-D NMR for full assignment for each peak is discouraging due to its low intensity even if the acquisition time is prolonged. The isostructural Co2 displays a similar NMR behavior to that of Co1. As anticipated, the -OMe complex Co5 shows 1H NMR spectrum similar to that of the -SMe complex Co4. The magnetic measurements calculated by Evans method for Co1, Co2, Co4, and Co5 lead to effective magnetic moments between 3.99 and 4.39 μB, which are in the expected range for Co(II) with octahedral geometry and d7 high spin (t2g5eg2) configuration.30 The phosphine donated complexes Co3 and Co6, however, are fall in the range of B


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Organometallics

Figure 1. 1H NMR spectrum of complex Co1.

Figure 2. 1H NMR spectrum of complex Co3.

8.38−0.76 ppm in 1H NMR spectra with all detectable protons. As in Figure 2, two tert-butyl proton sets down-shift to 0.75 and 1.13 ppm, respectively, and protons in the aromatic region also slightly shift with respect to those of free ligand. Inferred from the effective magnetic moments valued at 1.99 μB and 2.14 μB, respectively, the Co(II) for Co3 and Co6 probably adopts d7 low spin (t2g6eg1) configuration.30 The low spin state for both Co3 and Co6 supports that phosphine has more electron donating capability than oxygen and sulfur atoms, though steric effect may also play a minor role. The mass spectra display an intact molecule of [M]+ as the highest m/z ion peak. The cyclic voltammetry was also applied for analyzing the electron property which is an important parameter for evaluating catalysis properties. All the complexes exhibit two successive anodic irreversible waves, Iox and IIox, at potential ranges of −0.75 to −0.54 V and 0.0 to 0.3 V versus SCE, respectively (see example in Figure 3 for Co1). The former wave can be tentatively assigned to the Co(II)

Figure 3. Cyclic voltammetry of complex Co1 (analysis conditions are included in the Experimental Section).

C


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Organometallics conversion to Co(III) and the latter to ligand based oxidation, which are close to those found for salicylaldimine cobalt(II) compounds.29,30 Intense, broad, and irreversible cathodic waves (Ered p ) were also detected in the ranges of −0.15 to −0.13 V and −0.75 to −0.55 V versus SCE, which are believed to involve the Co(II) to Co(I) and Co(0) conversions. The cyclic voltammetry, however, does not allow us to distinguish the complexes supported by different donors. In addition, crystals of complexes Co3 and Co5 suitable for structure analyses were developed from dichloromethane solutions. The complex Co6, however, is cocrystallized with 2-hydroxynaphthalene-1-carbaldehyde decomposed from the complex, and its crystal parameters are appended in a footnote.a Crystallographic data for Co3 and Co5 are provided in the Supporting Information. Selected bond lengths and angles for Co3 and Co5 are provided in Figures 4 and 5, respectively.

Figure 5. Drawing of complex Co5. Hydrogen atoms are omitted for clarity. Selected bond length (Å) and angles (deg): Co1−O1 1.907(3), Co1−O1 1.907(3), Co1−N1 1.922(4), Co1−N1 1.922(4), Co1−S1 2.2186(13), Co1−S2 2.2186(13), O1−Co1−O1 90.00(18), O1−Co1−N1 86.23(12), O1−Co1−N1 94.86(12), O1− Co1−N1 94.86(12), O1−Co1−N1 86.23(12), N1−Co1−N1 178.46(16), O1−Co1−S1 175.77(9), O1−Co1−S1 91.92(10), N1− Co1−S1 89.85(9), N1−Co1−S1 89.03(9), O1−Co1−S1 91.92(10), O1−Co1−S1 175.77(9), N1−Co1−S1 89.02(9), N1−Co1−S1 89.85(9), S1−Co1−S1 86.42(7).

2.2053(14) for Co3) are slightly longer than the Co−S (Co1− S1 2.2186(13) Å, Co5), probably due to the more electronic donating capability of P donor,23 and this also means Co−S is more labile. The bond angles sum of C18−S1−Co1 115.6(4)°, C18−S1−C1 101.6(4)°, and C1−S1−Co1 96.69(14)° is 313.89°, suggesting that the S atom in Co5 is sp3-hybridized.23 These results show that the side arm could readily modulate the spatial and the electronic properties of the metal center and thus possibly tune the catalytic performance, which will be discussed in the following sections. Polymerization of Myrcene. Polymerizations of myrcene were conducted with variable cocatalysts. MMAO shows the best cocatalyst activity (entry 1, Table 1), whereas the use of aluminumalkyls proves detrimental to the catalyst, partially (AliBu3, entry 2, Table 1) or completely (AlEt3, entry 3, Table 1) inhibiting the polymerization activity. Thus, the effects of

Figure 4. Drawing of complex Co3. Hydrogen atoms and H2O are omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1−O2 1.915(4), Co1−O1 1.914(3), Co1−N1 1.923(3), Co1−N2 1.934(4), Co1−P2 2.2053(14), Co1−P1 2.2075(14), O2−Co1−O1 85.88(14), O2−Co1−N1 84.85(14), O1−Co1−N1 96.20(15), O2− Co1−N2 95.91(15), O1−Co1−N2 84.68(15), N1−Co1−N2 178.88(16), O2−Co1−P2 172.70(13), O1−Co1−P2 88.00(13), N1−Co1−P2 91.71(15), N2−Co1−P2 87.59(15), O2−Co1−P1 88.17(13), O1−Co1−P1 172.68(11), N1−Co1−P1 87.46(12), N2−Co1−P1 91.74(12), P2−Co1−P1 98.07(5).

Table 1. Myrcene Polymerization by Cobalt Catalystsa entry 1 2b 3c 4 5 6 7 8 9 10 11 12 13

As shown in Figures 4 and 5, X-ray crystallographic analyses of complexes reveal that two ligands are situated perpendicularly to minimize the steric congestion and coordinate the cobalt atom with phenolate oxygen, imine nitrogen, and pendant donor (P for Co3 and S for Co5), similar to the titanium analogues.31 Both complexes feature a distorted octahedral coordination of the cobalt center in which the five atoms, that is, Co, O1, O2, and two pendant atoms, are nearly coplanar, with the sum of four equatorial angles of nearly 360°. The N atoms occupy at the two axial points via transconfiguration and form an axial angle N1−Co−N2 of nearly 180° (the angle of N1−Co−N2 is in the range of 178.42(18)°−178.99(19)°). Alternatively, two oxygen anions and the two pendant atoms are both cis-positioned. Complex Co3 differs in bond distances Co−O1 vs Co−O2, Co−N1 vs Co−N2, and Co−P1 vs Co−P2; however, complex Co5 is CS symmetrical, and bond distances for each pair are identical. The Co−P bond distances (Co1−P1 2.2075(14), Co1−P2

T complex monomer (°C) Co1 Co1 Co1 Co1 Co1 Co1 Co1 Co1 Co2 Co3 Co4 Co5 Co6

MY MY MY MY MY MY IP BD MY MY MY MY MY

30 30 30 0 60 90 60 60 60 60 60 60 60

conv (%)

Mnd × 10−4

PDId

3,4/cis-1,4e

83.2 24.2 5.4 78.5 89.2 80.4 85.9 86.5 94.3 55.1 90.1 92.1 43.1

20.8 7.9 f 22.9 20.4 17.2 15.7 15.6 21.5 25.3 27.2 28.2 31.2

2.5 3.1 f 2.1 2.6 3.2 1.9 2.5 2.1 2.3 2.2 2.0 1.8

4.7/95.3 8.3/91.7 f 3.3/96.7 5.6/94.4 9.7/90.3 23.5/76.5 2.8/97.2 8.3/91.7 24.7/75.3 7.9/92.1 9.2/90.8 25.1/74.9

a

Polymerization conditions: isoprene and myrcene, 10.0 mmol; activator, MMAO, [Al]/[Co] = 300 except as noted; solvent, toluene; total volume, 10 mL; reaction time, 2 h. Tg for all polymyrcene was in the range of −61.3 to −70.3 °C. bAl(iBu)3. cAlEt3. dDetermined by GPC. eCalculated by 1H NMR. fNot determined. D


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Organometallics

Figure 6. 1H NMR of polymyrcene obtained from entry 5, Table 1.

resulting in polymyrcene with a much higher Mn of (2.72− 3.12) × 105 g/mol and a lower PDI of 1.8−2.2 (entries 11−13, Table 1). The donor induces the same trend in stereocontrol, -OMe (Co4, cis-1,4 = 92.1%) > -SMe (Co5, cis-1,4 = 90.8%) > − PPh2 (Co6, cis-1,4 = 74.9%). The reason for this reduced activity in phosphine ligated complexes is possibly too strong binding and too great a steric hindrance that to a certain degree causes substantially reduced monomer access. The coordinative flexibility in O and S donors could facilitate metal−donor dissociation thereby increasing monomer−metal binding and simultaneously increasing the selectivity. Probably, in these cases only the P atom remains coordinated to the cobalt atom, as suggested by production of a polymer with a higher 1,2 content.8f,g Polymerization of MMA. In order to explore the catalytic potential of Co1−Co6 for MMA polymerization and on PMMA properties, the reaction parameters such as Al type, [Al]/[Co] ratio, reaction temperature, and MMA feed were systematically modified using Co1 as precatalyst, and the other complexes were compared at the optimized conditions. As the data collected in Table 2, when they were associated with an exogenous organic aluminum compound, all cobalt complexes are active in the MMA polymerization. Polymerizations by either metal compounds or MAO alone show no activity, demonstrating that binary components are necessary to initiate the polymerization of MMA. MAO (entry 1, Table 2), MMAO (entry 2, Table 2), and AlEt3 (entry 3, Table 2) are all efficient in activating Co1 for polymerization with a lower degree of monomer conversion and syndiotactic selectivity (rr) than those initiated by AliBu3 (entry 4, Table 2). The molecular weights of the resultant PMMAs are also lower than that achieved with AliBu3. This may reflect the occurrence of transfer to the MAO, MMAO, and AlEt3 side reactions under such conditions, as supported by the fairly broad dispersity values. Considering the feeding amount and the cost of activator, herein, AliBu3 was selected for further study. Reaction changes in the [Al]/[Co] ratio and temperature displays a profound effect on the catalytic activities and characteristic properties of the resultant PMMA. When the

cocatalyst loading, reaction temperature, and catalyst were investigated under activation with MMAO. Varying the temperature shows that the best activity is reached at 60 °C (entry 5, Table 1), which provides a polymer yield of 89.2%. The stereospecificity of Co1 shows little sensitivity to changes in temperature (entries 1 and 4−6, Table 1), which implies that conformational dynamics or changes responsible for the different polymer microstructures are negligible despite the temperature variation (Figure 6, 1H NMR of polymer entry 5, Table 1,). It should be noted that acceptable activity and selectivity are still maintained at 90 °C (entry 6, Table 1), suggesting improved stability of the active species possibly via saturation of the metal coordinative vacancy by the O-donor. These enhanced performances are consistent with those of titanium and vanadium catalysts25b,26a and are remarkable compared to the Schiff-base cobalt analogues without pendant donors.8c,d The molecular weight distribution of the polymer becomes broader but still unimodal in the GPC, in agreement with the accelerated chain transfer rate diagram. The catalyst is also efficient for isoprene (entry 7, Table 1) and butadiene (entry 8, Table 1) polymerizations, providing essential cis-1,4 polymers known for cobalt catalysts. To establish the correlation of structural variations in the precatalysts with their catalytic performances in myrcene polymerization, all of the remaining cobalt complexes (Co2− Co6) were also studied under these conditions. Indeed, the nature of donor on the phenoxy-imine ligand affects the ability of the catalyst to control monomer enchainment in the polymerization of myrcene. The complexes Co1 and Co2 (entry 9, Table 1) with pendant groups containing oxygen and sulfur donors, respectively, show much higher activity than that of Co3 (entry 10, Table 1) under comparable conditions. With respect to the selectivity, it seems to follow grossly the type of donor, -OMe (Co1, cis-1,4 = 94.7%) > -SMe (Co2, cis-1,4 = 91.7%) > -PPh2 (Co3, cis-1,4 = 75.3%). Compared with Co1, Co2 and Co3 give polymers with higher molecular weight and narrower molecular weight distribution, probably resulting from synergic effects of steric hindrance and the electronic properties of the pendant group. We observed comparable specificity and activity of catalysts of the other series but the E


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Organometallics Table 2. MMA Polymerization Results by Cobalt Catalystsa entries

complexes

[Al]/ [Co]

T (°C)

conv (%)

Mn × 10−4

PDI

mm/mr/rr

1b 2c 3d 4 5 6 7 8 9 10 11 12 13 14 15 16

Co1 Co1 Co1 Co1 Co1 Co1 Co1 Co1 Co1 Co1 Co1 Co2 Co3 Co4 Co5 Co6

300 300 60 60 20 40 80 60 60 60 60 60 60 60 60 60

60 60 60 60 60 60 60 0 30 80 100 60 60 60 60 60

43.6 40.6 52.6 59.9 35.1 48.0 56.4 32.4 51.5 47.8 42.6 53.3 75.6 40.3 43.3 72.6

4.44 6.57 8.57 11.57 13.95 10.8 7.45 7.76 8.02 5.84 5.21 8.34 14.8 7.83 8.94 14.1

2.72 2.87 2.38 1.42 1.81 1.60 1.91 1.41 1.50 1.93 2.23 1.67 2.05 1.62 1.99 2.67

8/35/57 9/38/53 4/35/61 5/29/66 8/31/61 2/30/68 2/28/70 4/21/75 8/25/67 16/29/55 16/31/53 6/30/64 2/13/85 5/29/66 7/30/63 6/21/73

those of salicylaldiminato cobalt complexes without additional donor.18a The reaction proceeded with higher syndiotactic selectivity (75%) when it was carried out at a lower temperature (entry 8, Table 2), indicating that active species geometry is more stable. Notably, as the temperature increased from 0 to 100 °C or as the molar ratio of [Al]/[Co] of AliBu3 increased from 20 to 100, the molecular weight of PMMA almost monotonously drops and displays a wider molecular weight distribution. 18 These findings agree with the literature20,21 and can be ascribed to either the higher molar ratio of [Al]/[Co] or higher temperature increasing the rate of chain transfer in comparison to chain propagation. However, all of the polymer samples at different temperatures or molar ratios of [Al]/[Co] display unimodal and narrow molecular weight distributions, suggesting that single site active species are involved throughout the polymerizations. To rule out a radical mechanism, the reactivity of the propagating polymer end was examined by highly active monomer acrylonitrile (AN); however, the reaction of Co1 with an AN/MMA mixture produces PMMA with no AN incorporation. Additionally, Co1 is unable to homopolymerize AN. Complex Co2, with a pendant -SMe group, gives increased activity (53.3% in yield, entry 12, Table 2); nevertheless, complex Co3, bearing -PPh2 group, produces substantially increased polymer under the same polymerization conditions (75.6% in yield, entry 13, Table 2). The stereoselectivity also benefits from the -PPh2 group. The catalytic activity and selectivity changed in the same order Co4 < Co5 < Co6, but their performances are slightly lower than those of the corresponding 2,4-di-tert-butylsacylialdehyde counterparts Co1, Co2, and Co3, respectively. These may be related to the use of donor of stronger binding capability to saturate the coordination sphere of the metal center, which can retard the ester group. As a result, formation of thermally stable deactivated η4-α,β-unsaturated ester intermediate is avoided, which has been revealed in the literature.21,27 This dramatic switch of catalytic performance on two different monomers is not fully understood but is potentially related to changes in monomer−metal coordination mode that are affected by the character of donor. The results probably support that myrcene is more likely coordinated to metal by η4-mode; thus more

a

Polymerization conditions: MMA, 10.06 mmol; activator, Al(i-Bu)3 except as noted; solvent, toluene; total volume, 10 mL; reaction time, 4 h. bMAO. cMMAO. dAlEt3.

[Al]/[Co] ratio was increased from 20 to 80 and the temperature was kept constant at 60 °C (entries 4−7, Table 2), an [Al]/[Co] ratio of 60 was found to be the best; a lower or higher ratio of cocatalyst shows reduced activities. The syndiotactic selectivity is in the range of 61−70%, and a slightly beneficial effect from the higher cocatalyst feeding is seen (typical 1H NMR of entry 6, Table 2 shown in Figure 7). When the reaction temperature was increased from 0 to 100 °C with the [Al]/[Co] ratio fixed at 60 and the reaction running time at 4 h, the activity gradually increased to a maximum level of 59.9% (entry 4, Table 2). Above 60 °C, the catalytic activity was maintained at 47.8% at 80 °C (entry 10, Table 2), but the selectivity slightly dropped at elevated temperature. Still, a high activity of 42.6% (entry 11, Table 2) is maintained at 100 °C; the lack of catalyst decomposition or change highlights the high thermal stability probably ascribed to the pendant arms, which are again more pronounced than

Figure 7. 1H NMR of PMMA obtained from entry 6, Table 2. F


Article

Organometallics

3423 (broad, −OH), 2948 (Ph−H), 1624 (-CN-), 1245 (−C− O−). 2-((2′-(Methylthio)phenylimino)methyl)-4,6-di-tert-butylphenol (L2). Yield: 61.3%. 1H NMR (400 MHz, CDCl3, ppm) δ: 8.71 (s, 1H, −CHN), 7.45 (s, 1H, Ar−H), 7.22−7.00 (m, 5H, Ar−H), 2.52 (s, 3H, S−CH3), 1.50 (s, 9H, −CH3), 1.35 (s, 9H, −CH3). 13C NMR (100 MHz, CDCl3, ppm) δ: 164.26 (-CN-), 158.85 (HO-Ar), 153.04 (Ar−C), 140.49 (Ar−C), 138.12 (Ar−C), 127.82 (Ar−C), 126.76 (Ar−C), 121.21 (Ar−C), 120.29 (Ar−C), 118.84 (Ar−C), 111.97 (Ar−C), 56.11 (S−CH3), 35.52 (CMe3), 34.19 (CMe3), 31.82 (-C−CH3), 29.71 (-C−CH3). IR (KBr, cm−1) ν: 3420 (broad, −OH), 2956 (Ph-H), 1625 (-CN-), 1244 (−C−O−). 2-((2′-(Diphenylphosphino)phenylimino)methyl)-4,6-di-tert-butylphenol (L3). Yield: 76.1%. 1H NMR (400 MHz, CDCl3, ppm) δ: 8.42(s, 1H, −CHN), 7.29−6.89 (m, 16H, Ar−H), 1.42 (s, 9H, −CH3), 1.30 (s, 9H, −CH3). 13C NMR (100 MHz, CDCl3, ppm) δ: 163.73 (-CN-), 158.32 (HO-Ar), 152.11 (Ar−C), 140.36 (Ar−C), 137.33 (Ar−C), 136.14 (Ar−C), 134.42 (Ar−C), 133.10 (Ar−C), 130.20 (Ar−C), 128.61 (Ar−C), 125.44 (Ar−C), 118.31 (Ar−C), 117.65 (Ar−C), 35.25 (-CMe3), 34.33 (-CMe3), 31.69 (-C−CH3), 29.31 (-C−CH3). 31P NMR (162 MHz, CDCl3, ppm): 13.71. IR (KBr)ν/cm−1: 3424 (broad, −OH), 3056(Ph−H), 1623(-CN-), 1450(-CC−), 1250(−C−O−). 1-((2′-Methoxylphenylimino)methyl)naphthalen-2-ol (L4). Yield: 42.6%. 1H NMR (400 MHz, CDCl3, ppm) δ: 9.19 (s, 1H, −CHN), 7.99 (d, 1H, Ar−H), 7.70 (d, 1H, Ar−H), 7.61 (d, 1H, Ar−H), 7.40−7.22 (m, 2H, Ar−H), 7.13−7.01 (m, 2H, Ar−H), 6.94−6.90 (m, 2H, Ar−H), 3.99 (s, 3H, O−CH3). 13C NMR (100 MHz, CDCl3, ppm) δ: 175.33(-CN-), 153.11 (Ar−C), 149.93 (Ar−C), 139.44 (Ar−C), 136.19 (Ar−C), 133.42 (Ar−C), 130.44 (Ar−C), 129.25 (Ar−C), 127.09 (Ar−C), 126.28 (Ar−C), 125.36 (Ar−C), 122.45 (Ar−C), 119.21 (Ar−C), 118.44 (Ar−C), 112.34 (Ar−C), 108.98 (Ar−C), 57.10 (O−CH3). IR (KBr, cm−1) ν: 3433(broad, −OH), 1621 (-CN-), 1445 (-CC−), 1229 (−C−O−). 1-((2′-(Methylthio)phenyl)imino)methyl)naphthalen-2-ol (L5). Yield: 41.2%. 1H NMR (400 MHz, CDCl3, ppm) δ: 9.21 (s, 1H, −CHN-), 7.98 (d, 1H, Ar−H), 7.72 (d, 1H, Ar−H), 7.63 (d, 1H, Ar−H), 7.47−7.27 (m, 2H, Ar−H), 7.23−7.07 (m, 2H, Ar−H), 6.97−6.93 (m, 2H, Ar−H), 4.02 (s, 3H, S−CH3). 13C NMR (100 MHz, CDCl3, ppm) δ: 178.13(-CN-), 151.06 (Ar−C), 149.21 (Ar−C), 138.52 (Ar−C), 134.29 (Ar−C), 131.12 (Ar−C), 129.67 (Ar−C), 128.21 (Ar−C), 126.89 (Ar−C), 125.18 (Ar−C), 123.86 (Ar−C), 121.35 (Ar−C), 118.18 (Ar−C), 117.12 (Ar−C), 111.97 (Ar−C), 108.28 (Ar−C), 56.26 (S−CH3). IR (KBr, cm−1) ν: 3420(broad, −OH), 1625 (-CN-), 1486 (-CC−), 1247 (−C− O−). 1-((2′-(Diphenylphosphino)phenylimino)methyl)naphthalen-2ol (L6). Yield: 47.1%. 1H NMR (400 MHz, CDCl3, ppm) δ: 9.11 (s, 1H, -CN-), 7.99−7.41 (m, 2H, Ar−H), 7.34−7.01 (m, 18H, Ar− H). 13C NMR (100 MHz, CDCl3, ppm) δ: 177.54 (-CN-), 154.11 (Ar−C), 149.87 (Ar−C), 139.12 (Ar−C), 135.11 (Ar−C), 131.17 (Ar−C), 129.66 (Ar−C), 128.87 (Ar−C), 128.24 (Ar−C), 127.78 (Ar−C), 127.11 (Ar−C), 126.24 (Ar−C), 125.78 (Ar−C), 110.15 (Ar−C). IR (KBr, cm−1) ν: 3420 (broad,−OH), 3045 (Ph-H), 1625 (-CN-), 1440 (-CC−), 1250 (−C−O−). Complexes. To an anhydrous ethanol solution (10 mL) of ligand L1 (0.21 mmol) in a 50 mL round-bottomed flask under nitrogen atmosphere, was slowly added an anhydrous ethanol solution (20 mL) of cobalt acetate (0.1 mmol). The formed solution was refluxed overnight. A precipitate then formed, which was filtered, washed with anhydrous n-hexane (30 mL × 3), and dried under vacuum to furnish a red solid. Complexes Co2, Co4, and Co5 were prepared with a similar procedure. No precipitate formed when phosphine donated complexes Co3 and Co6 were being prepared; therefore, ethanol was removed under vacuum, and a residue resulted, which was washed with anhydrous n-hexane (30 mL × 3) and dried under vacuum to furnish a dark red solid. Cobalt Bis(1-((E)-(2′-(methoxy)phenylimino)methyl)-2,4-di-tertbutylphenolate) (Co1). Yield: 78.5%, red solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 57.58 (s, 1H, -CNH-), 38.66 (s, 1H, -Ar),

labile O−Co and S−Co binding favors monomer access, while η2-mode is possibly favorable for MMA coordination where the stronger Co−P interaction is more beneficial. This elucidation is also good agreement with the distinctive spin configurations of the two types of complexes as revealed by the Evans method.

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CONCLUSIONS In summary, this paper presents using a series of tridentate phenoxy-imine ligands with a pendant donor (O, S, and P) for dual catalysis for polymerization of myrcene and methyl methacrylate. The catalytic system has enhanced thermal tolerance in polymerization of two monomers, and cis-1,4polymyrcene and syndiotactic poly(methyl mathacrylate) result with acceptable yields in the range of 0−100 °C. The nature of donor possibly plays a distinctive role in affecting the method of monomer insertion, and the stronger binding capability of the P donor catalyst has pronounced activity toward MMA, whereas more labile O and S type catalysts are much more active in myrcene polymerization. This work may be important for potential dual catalysis processes for both polar and nonpolar monomers.

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EXPERIMENTAL SECTION

Materials and Methods. All reactions sensitive to air and moisture were performed under a dry and air free argon atmosphere by means of standard Schlenk technique or under an argon atmosphere in a glovebox. Myrcene and methyl methacrylate was distilled from CaH2 and stored under nitrogen atmosphere. Toluene and THF were refluxed over sodium and vacuum transferred to flask under N2 atmosphere prior to use. Tri-isobutyl aluminum, triethyl aluminum, MAO (methylaluminoxane), MMAO (tributylaluminoxane modified MAO), and anhydrous cobalt acetate were purchased from Energy Chemical. 3,5-Di-tert-butyl-2-hydroxybenzaldehyde, 2hydroxynaphthalene-1-carbaldehyde, 2-(diphenylphosphino)aniline, 2-methoxyaniline, and 2-(methylthio)aniline were available from Alfa Aesar without further purification. Ligands (L1−L6) were synthesized via salicylaldehyde and aliphatic amine condensation methods. NMR analyses (1H, 400 MHz; 13C, 100 MHz; and 31P, 162 MHz) for ligands and polymers were recorded on a Bruker AV400 spectrometer. Elemental analyses for C, H, N, and Co were carried on Flash 2000-Thermo Scientific CHNO Analyzer and Inductively Coupled Plasma Mass Spectrometry (icap Q). Mass spectra of cobalt complexes were analyzed on LC-MS Xeov GS-2 QTOF in methanol solution. The molecular weights (Mn) and molecular weight distributions (Mw/Mn) of poly(methyl methacrylate) and polymyrcene were measured at 30 °C by gel permeation chromatography (GPC) equipped with a Waters 515 HPLC pump, four columns (HMW7 THF, HMW6E THF×2, HMW2 THF), and a Waters 2414 refractive index detector. THF was used as eluent at a flow rate of 1.0 mL/min. The values of Mn and Mw/Mn were determined using the polystyrene calibration. Ligand and Complex Synthesis and Characterization. Ligands. Condensation with salicylaldehydes and aliphatic amines furnished the desired tridentate phenoxy-imine compound in high yields. This reaction was fast and proceeded without any problem at refluxing temperature.26a 2-(-(2′-Methoxyphenylimino)methyl)-4,6-di-tert-butylphenol (L1). Yield: 57.8%. 1H NMR (400 MHz, CDCl3, ppm) δ: 8.69 (s, 1H, −CHN−), 7.43 (s, 1H, Ar−H), 7.20−6.99 (m, 5H, Ar−H), 3.91 (s, 3H, O−CH3), 1.48 (s, 9H, −CH3), 1.31 (s, 9H, −CH3). 13C NMR (100 MHz, CDCl3, ppm) δ: 163.27 (−CN−), 158.03 (HO−Ar), 152.19 (Ar−C), 139.60 (Ar−C), 137.36 (Ar−C), 136.31 (Ar−C), 127.17 (Ar−C), 125.82 (Ar−C), 120.43 (Ar−C), 119.24 (Ar−C), 118.04 (Ar−C), 111.15 (Ar−C), 55.12(O−CH3), 34.45 (CMe3), 33.40 (CMe3), 31.00 (-C−CH3), 28.61 (C−CH3). IR (KBr, cm−1) ν: G


Article

Organometallics 18.45 (s, 1H, -Ar), 10.82 (s, 1H, -Ar), 8.14 (br, 3H, -SCH3), 7.41 (s, 9H, −C(CH3)3), 3.47 (s, 9H, −C(CH3)3), 1.38 (s, 1H, -Ar), −7.83 (s, 1H, -Ar). IR (KBr, cm−1): 2927, 2881, 1627 (νC=N), 1434, 1190, 990. Anal. Calcd for Co1, C44H56CoN2O4: C, 71.82; H, 7.67; N, 3.81; Co, 8.01%. Found: C, 71.79; H, 7.79; N, 3.88; Co, 8.01%. UV− visible: 325 nm; μeff = 3.99 μB. EI-MS: required 735.3572, found 735.73504 [M]+. Cobalt Bis(1-((E)-(2′-(methylthio)phenylimino)methyl)-2,4-ditert-butylphenolate) (Co2). Yield: 78.5%, red solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 57.57 (s, 1H, -CNH-), 38.71 (s, 1H, -Ar), 18.49 (s, 1H, -Ar), 10.81 (s, 1H, -Ar), 8.12 (br, 3H, -SCH3), 7.42 (s, 9H, −C(CH3)3), 3.55 (s, 9H, −C(CH3)3), 1.34 (s, 1H, -Ar), −7.74 (s, 1H, -Ar). IR (KBr, cm−1): 2932, 2880, 1621 (νC=N), 1434, 1090; Anal. Calcd for Co2, C44H56CoN2S2O2: C, 68.81; H, 7.35; N, 3.65; Co, 7.67%. Found: C, 68.79; H, 7.49; N, 3.58; Co, 7.81%. UV− visible: 323 nm; μeff = 4.04 μB. EI-MS: required 767.3115, found 767.3578 [M]+. Cobalt Bis(1-((E)-(2′-(diphenylphosphino)phenylimino)methyl)2,4-di-tert-butylphenolate) (Co3). Yield: 78.5%, dark red solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 8.38 (s, 1H, -CNH-), 8.18 (s, 1H, -Ar), 7.83 (s, 1H, -Ar), 7.49 (d, 2H, -Ar), 7.11 (s, 1H, -Ar), 7.01 (b, 3H, -Ar), 6.92 (b, 3H, -Ar), 6.77 (b, 2H, -Ar), 6.67 (b, 2H, -Ar), 6.61 (s, 1H, -Ar), 1.14 (s, 18H, −C(CH3)3), 0.75 (s, 18H, −C(CH3)3). IR (KBr, cm−1): 2935, 2879, 1626 (νC=N), 1431, 1090, 690 (νP‑Ph). Anal. Calcd for Co3, C66H70CoN2O2P2: C, 75.92; H, 6.76; N, 2.68; Co, 5.64%. Found: C, 75.77; H, 6.79; N, 2.59; Co, 5.80%. UV−visible: 335 nm; μeff = 1.99 μB. EI-MS: required 1039.3932, found 1039.3973 [M]+. Cobalt Bis(1-((E)-(2′-(methoxy)phenylimino)methyl)naphthalen2-olate) (Co4). Yield: 78.5%, red solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 53.08 (s, 1H, -CNH-), 23.19 (s, 1H, -Ar), 19.31 (s, 1H, -Ar), 18.31 (m, 3H, -Ar), 7.36 (b, 1H, -Ar), 5.78 (b, 3H, -Ar), 3.76 (s, 1H, -Ar), −5.48 (s, 3H, -SMe). IR (KBr, cm−1): 1621 (νC=N), 1437, 1117, 700. Anal. Calcd for Co4, C36H28CoN2O4: C, 70.70; H, 4.61; N, 4.58; Co, 9.64%. Found: C, 70.77; H, 4.49; N, 4.49; Co, 9.50%. UV−visible: 321 nm; μeff = 4.31 μB. EI-MS: required 611.1381, found 611.17892 [M]+. Cobalt Bis(1-((E)-(2′-(methylthio)phenylimino)methyl)naphthalen-2-olate) (Co5). Yield: 78.5%, red solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 53.06 (s, 1H, -CNH-), 23.10 (s, 1H, -Ar), 19.41 (s, 1H, -Ar), 18.09 (m, 3H, -Ar), 7.32 (b, 1H, -Ar), 5.73 (b, 3H, -Ar), 3.73 (s, 1H, -Ar), −5.50 (s, 3H, -SMe). IR (KBr, cm−1): 1622 (ν C=N ), 1433, 1077, 690 (ν P‑Ph ). Anal. Calcd for Co5, C36H28CoN2S2O2: C, 67.17; H, 4.38; N, 4.35; Co, 9.16%. Found: C, 66.81; H, 4.47; N, 4.27; Co, 9.20%. UV−visible: 323 nm; μeff = 4.39 μB. EI-MS: required 643.0924, found 643.0757 [M]+. Cobalt Bis(1-((E)-(2′-(diphenylphosphino)phenylimino)methyl)naphthalen-2-olate) (Co6). Yield: 68.5%, dark solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 8.36 (s, 1H, -CNH-), 8.18 (s, 1H, -Ar), 7.83 (s, 1H, -Ar), 7.37−7.11 (m, 8H, -Ar), 7.07−6.79 (m, 10H, -Ar). IR (KBr, cm−1): 1621 (νC=N), 1437, 1093, 692 (νP‑Ph). Anal. Calcd for Co6, C58H42CoN2O2P2: C, 75.73; H, 4.60; N, 3.05; Co, 6.41%. Found: C, 75.89; H, 4.69; N, 2.99; Co, 6.37%. UV−visible: 332 nm; μeff = 2.14 μB. EI-MS: required 920.2104, found 920.9356 [M]+. Magnetic Susceptibility Measurements. Magnetic measurements were performed by solution 1H NMR using the Evans’ method on a Bruker Avance 400 spectrometer operating at 400.13 MHz at room temperature. The measurements were performed in a standard 5 mm NMR tube containing the cobalt complex sample dissolved in DMSO-d6 (4.1 wt % TMS), against a coaxial reference tube filled with the same solvents. Cyclic Voltammetry Studies. The CV measurements were conducted on a CHI1100A electrochemical workstation (Chenhua Instrument Company, Shanghai, China) with scan rates of 0.1 V/s. The electrolytic cell used is a conventional three-electrode system, which contains a glass carbon electrode (GCE, 3 mm diameter, Gaoss Union, Wuhan, China) as working electrode, a Ag/AgCl (3 M KCl) electrode as reference electrode, and a platinum coil as counter electrode. The electrode was immersed in dimethyl formamide

(DMF) and 0.5 M C16H36BrN as electrolyte, and potential was scanned from −1.7 to +0.7 V with a scan rate of 100 mV/s. X-ray Structure Analysis. Crystal data were collected on a Bruker SMART APEX diffractometer with a CCD area detector with a graphite monochromated Mo K radiation (λ = 1.71073 Å). The SMART program package was used to calculate the crystal class and unit cell, and the reflection data file was generated from the raw frame data by SAINT and SADABS. The space group of crystal was determined by XPREP implemented in APEX2. The structures of complexes were solved by using software SHELXTL-97 and refined by using full-matrix least-squares method on F2. All the hydrogen atoms were added at the calculated positions after hybridization. Procedure for Myrcene and MMA Polymerization. Polymerization of MMA and myrcene were handled in a glovebox. A 50 mL sealed flask was sequentially charged with toluene (10.0 mL), monomer (2 mmol), and the cobalt complex (10 μmol). The polymerization was initiated by the addition of aluminum cocatalyst at a set temperature. After a given time, the polymer was poured into a 0.5 wt % acidified methanol solution with 1.0% 2,6-dibutyl-4-methyl phenol. The precipitated polymers were suction filtered, washed with methanol (30 mL × 3), and dried under vacuum to a constant weight.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00708. Crystallographic data for Co3 and Co5 (PDF) Accession Codes

CCDC 1849236−1849237 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Feiyan Yi: 0000-0003-0733-9712 Yunjie Luo: 0000-0002-2480-1385 Dirong Gong: 0000-0002-9791-9261 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of Zhejiang (Grant LY17E30002), the Natural Science Foundation of China (Grants 21507126 and 21304050), and the Natural Science Foundation of Ningbo (Grants 2016A610045, and 2017A610055). This work is also sponsored by K. C. Wong Magna Fund in Ningbo University.

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ADDITIONAL NOTE Complex Co6·2C 11 H 8 O 2 (deposited CCDC number 1849238). Formula: C80H58CoN2O6P2. Molecular weight: 1264.15. Crystal system: triclinic. Space group: P1̅. a = 13.940(3) (Å). b = 14.879(3) (Å). c = 16.575(3) (Å). α = 85.062(2) (deg). β = 74.111(2)°. γ = 70.772(2)°. V = 3122.1(10)Å3. Z = 2. Dcalcd = 1.345 Mg/m3. Absorp coeff = 0.386 mm−1. F(000) = 1314. Crystal size, 0.14 × 0.13 × 0.11 mm3. θ range: 1.6 to 25°. No. of reflns collected: 9513. No. of a

H


Article

Organometallics

(7) Naddeo, M.; Buonerba, A.; Luciano, E.; Grassi, A.; Proto, A.; Capacchione, C. Stereoselective Polymerization of Biosourced Terpenes β-Myrcene and β-Ocimene and their Copolymerization with Styrene Promoted by Titanium Catalysts. Polymer 2017, 131, 151−159. (8) (a) Cariou, R.; Chirinos, J.; Gibson, V. C.; Jacobsen, G.; Tomov, A.; Elsegood, K. M. R. J. 1,3-Butadiene Polymerization by Bis(benzimidazolyl)amine Metal complexes: Remarkable Microstructural Control and a Protocol for In-Reactor Blending of trans1,4-, cis-1,4-, and cis-1,4-co-1,2-Vinylpolybutadiene. Macromolecules 2009, 42, 1443−1444. (b) Endo, K.; Kitagawa, T.; Nakatani, K. Effect of an Alkyl Substituted in Salen Ligands on 1,4-Cis Selectivity and Molecular Weight Control in the Polymerization of 1,3-Butadiene with (Salen)Co(II)Complexes in Combination with Methylaluminoxane. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4088−4094. (c) Chandran, D.; Kwak, C.; Ha, C.; Kim, I. Polymerization of 1,3Butadiene by Bis(salicylaldiminate)cobalt(II) Catalysts Combined with Organoaluminium Cocatalysts. Catal. Today 2008, 131, 505− 512. (d) Jie, S.; Ai, P.; Li, B.-G. Highly Active and Stereospecific Polymerization of 1,3-Butadiene Catalyzed by Dinuclear Cobalt(II) Complexes Bearing 3-Aryliminomethyl-2-hydroxybenzaldehydes. Dalton Trans 2011, 40, 10975−10982. (e) Cai, Z.; Shinzawa, M.; Nakayama, Y.; Shiono, T. Synthesis of Regioblock Polybutadiene with CoCl2-Based Catalyst via Reversible Coordination of Lewis Base. Macromolecules 2009, 42, 7642−7643. (f) Gong, D.; Jia, W.; Chen, T.; Huang, K.-W. Polymerization of 1,3-Butadiene Catalyzed by Pincer Cobalt(II) Complexes Derived from 2-(1-arylimino)-6-(pyrazol-1yl)Pyridine Ligands. Appl. Catal., A 2013, 464−465, 35−42. (g) Gong, D.; Zhang, X.; Huang, K.-W. Regio- and Stereo-selective Polymerization of 1,3-Butadiene Catalyzed by Phosphorus-nitrogen PN3-pincer Cobalt(II) Complexes. Dalton Trans 2016, 45, 19399− 19407. (h) Liu, W.; Pan, W.; Wang, P.; Li, W.; Mu, J.; Weng, G.; Jia, X.; Gong, D.; Huang, K.-W. Synthesis of Mixed-ligand Cobalt Complexes and Their Applications in High cis-1,4-Selective Butadiene Polymerization. Inorg. Chim. Acta 2015, 436, 132−138. (9) (a) Ricci, G.; Forni, A.; Boglia, A.; Motta, T. Synthesis, Structure, and Butadiene Polymerization Behavior of Alkylphosphine Cobalt(II) Complexes. J. Mol. Catal. A: Chem. 2005, 226, 235−241. (b) Ricci, G.; Forni, A.; Boglia, A.; Motta, T.; Zannoni, G.; Canetti, M.; Bertini, F. Synthesis and X-Ray Structure of CoCl2(PiPrPh2)2. A New Highly Active and Stereospecific Catalyst for 1,2 Polymerization of Conjugated Dienes When Used Associated with MAO. Macromolecules 2005, 38, 1064−1070. (c) Ricci, G.; Forni, A.; Boglia, A.; Sommazzi, A.; Masi, F. Synthesis, Structure and Butadiene Polymerization Behavior of CoCl2(PRxPh3‑x)2 (R = Methyl, Ethyl, Propyl, Allyl, Isopropyl, Cyclohexyl; x = 1,2). Influence of the Phosphorous Ligand on Polymerization Stereoselectivity. J. Organomet. Chem. 2005, 690, 1845−1854. (d) Ricci, G.; Sommazzi, A.; Masi, F.; Ricci, M.; Boglia, A.; Leone, G. Well Defined Transition Metal Complexes with Phosphorus and Nitrogen Ligands for 1,3-Dienes Polymerization. Coord. Chem. Rev. 2010, 254, 661−676. (e) Nath, D. C. D.; Shiono, T.; Ikeda, T. cis-Specific Living Polymerization of 1,3-Butadiene with CoCl2 and Methylaluminoxane. Macromol. Chem. Phys. 2002, 203, 756−760. (f) Takeuchi, M.; Shiono, T.; Soga, K. Polymerization of 1,3-Butadiene with MgCl2-Supported Cobalt Catalysts Activated by Ordinary Alkylaluminums. Polym. Int. 1995, 36, 41−46. (10) (a) Boccia, A. C.; Leone, G.; Boglia, A.; Ricci, G. Novel Stereoregular cis-1,4 and trans-1,2 Poly(diene)s: Synthesis, Characterization, and Mechanistic Considerations. Polymer 2013, 54, 3492− 3503. (b) Ricci, G.; Leone, G.; Boglia, A.; Bertini, F.; Boccia, A. C.; Zetta, L. Synthesis and Characterization of Isotactic 1,2-Poly(E-3methyl-1,3-pentadiene). Some Remarks about the Influence of Monomer Structure on Polymerization Stereoselectivity. Macromolecules 2009, 42, 3048−3056. (11) (a) Barbotin, F.; Spitz, R.; Boisson, C. Heterogeneous ZieglerNatta Catalyst Based on Neodymium for the Stereospecific Polymerization of Butadiene. Macromol. Rapid Commun. 2001, 22, 1411−1414. (b) Fischbach, A.; Meermann, C.; Eickerling, G.; Scherer, W.; Anwander, R. Discrete Lanthanide Aryl(alk)oxide

indep reflns: 10953 (Rint= 0.0291). Selected bond length (Å) and angles (deg): Co1−O2 1.9138(13), Co1−O1 1.9273(13), Co1−N1 1.9271(16), Co1−N2 1.9420(15), Co1−P1 2.2024(6), Co1−P2 2.2129(6), O2−Co1−O1 86.73(5), O2−Co1−N1 84.56(6), O1−Co1−N1 94.57(6), O2−Co1− N2 94.58(6), O1−Co1−N2 85.12(6), N1−Co1−N2 179.10(6), O2−Co1−P1 89.33(4), O1−Co1−P1 175.75(4), N1−Co1−P1 86.62(5), N2−Co1−P1 93.63(5), O2−Co1−P2 172.60(4), O1−Co1−P2 86.31(4), N1−Co1−P2 93.49(5), N2−Co1−P2 87.34(5), P1−Co1−P2 97.69(2).

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