Communication pubs.acs.org/Organometallics
ZnCl2 Capture Promotes Ethylene Polymerization by a Salicylaldiminato Ni Complex Bearing a Pendent 2,2′-Bipyridine Group Abigail J. Smith, Eric D. Kalkman, Zachary W. Gilbert, and Ian A. Tonks* Department of Chemistry, University of MinnesotaTwin Cities, 207 Pleasant Street SE, Minneapolis Minnesota 55455, United States S Supporting Information *
ABSTRACT: The effect of ZnCl2 additives on a series of (salicylaldiminato)Ni ethylene polymerization catalysts is reported. While ZnCl2 acts solely as a pyridine scavenger for simple imine catalyst frameworks such as the biphenylimine 4, in the case of complexes containing a 2,2′-bipyridine pendent group such as 5, ZnCl2 can coordinate to generate a bimetallic Ni/Zn active species that produces a polymer with significantly higher Mn value. 5 is not catalytically active in the absence of ZnCl2, and control experiments indicate that Zn coordination of the bpy pocket to generate a heterobimetallic Ni/Zn complex is critical for productive catalysis to occur. A heterobimetallic Ni/ZnCl2 precatalyst 7 has also been synthesized and structurally characterized and shows activity similar to that of the in situ bimetallic generated from 5 + ZnCl2.
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As a result, we are interested in designing new bimetallic ligand scaffolds in which main-group metals can be placed in close proximity to active Ni polymerization catalysts, ultimately to serve as a model for chain transfer. By placing main-group metals in the secondary coordination sphere of Ni, it should be easier for the metal centers to adopt bridging intermediate structures that are necessary for productive chain transfer to occur. Furthermore, proximal main-group metals may affect polymerization behavior through either electronic contributions or their large steric profile, similar to the case for recent multimetallic24 group 10 systems applied in polymerization catalysis25−27 and other catalytic reactions.28−32 Herein, we report on the synthesis and polymerization behavior of several mono- and bimetallic Ni catalysts based on a new salicylaldiminato ligand33,34 bearing a pendent 2,2′-bipyridine group. In these systems, ZnCl2 coordination by the pendent 2,2′-bipyridine ligand plays an important role in promoting effective polymerization. The 2,2′-bipyridine-appended salicylaldimine ligands 2 and 3 and the biphenyl-functionalized control 1 were prepared via condensation of the corresponding salicylaldehyde with the
ver the past several decades, advances in single-site organometallic catalysts for the production of polyolefins have resulted in powerful methods to tune the molecular weight, dispersity, and microstructure of polymers.1−3 Chain transfer and chain shuttling polymerizations, wherein a growing polymer chain is transferred onto a second metal (or catalyst), have recently established a new model for precision control over olefin polymerizations.4−6 For example, chain shuttling polymerizations have been used to synthesize block copolymers,7,8 modulate polymer molecular weight,9 and catalyze Aufbau-like chain growth on main-group metals such as Zn.10 While chain transfer processes in olefin polymerization have been observed across the transition-metal series, the most well established and practical systems have been reported using early-transition-metal catalysts in combination with maingroup-metal alkyls.5,9−17 In many of these early-transitionmetal systems, chain transfer is fast, efficient, and reversible. In contrast, there are very few reports of effective chain transfer with late-transition-metal polymerization catalysts, particularly the group 10 metals Ni and Pd.4,9,18−22 Of the few examples involving Ni, chain transfer rates are significantly slower than chain propagation and are highly sensitive to steric and electronic perturbations.23 © XXXX American Chemical Society
Received: June 15, 2016
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DOI: 10.1021/acs.organomet.6b00485 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics appropriate 6-amino-biaryl (eq 1, An = anthracenyl). The monometallic Ni tolyl species 4−6 were synthesized via salt
metathesis of deprotonated 1−3 with (tmeda)NiCl(o-tolyl) (eq 2).35a,b Ni pyridine adducts were selected as precatalysts because they typically do not require Lewis acid activators to initiate polymerization, while the tolyl functional groups were selected due to their stability relative to Ni methyl analogues. In fact, the monometallic complexes 4−6 are stable as solids in air for numerous weeks, although in solution they are susceptible to decomposition and should be handled under inert conditions. The 1H NMR spectra of 1−3 show the characteristic −OH and −NCH singlets (∼14−15.5 and 8.5−10 ppm, respectively), and in the spectra of 4−6 the −OH signal disappears while the −NCH signal shifts upfield, as expected for the metalated complexes. A similar synthesis was performed to construct a Grubbs-like control complex that contained an otolyl group on Ni (6-tBu-salicyl(2,6-diisopropylphenyl)iminato)Ni(o-tolyl)(py) (8) (Supporting Information). The bimetallic Ni/Zn complex 7 was synthesized via slow diffusion of a THF solution of ZnCl2 into a cold toluene solution of 5 (eq 3). This synthetic technique was utilized
Figure 1. Thermal ellipsoid drawing of 7. Hydrogens and solvents of crystallization are removed for clarity. Relevant bond distances (Å): Ni1−C18 1.899(2); Ni1−N1 1.887(2); Ni1−N4 1.898(2); Ni1−O1 1.900(1); Ni1−Cl1 4.0060(7); Zn1−N2 2.066(2); Zn1−N3 2.060(2); Zn1−Cl1 2.1966(7); Zn1−Cl2 2.2274(6).
fact, ZnCl2 appears to be a practical and effective Lewis acid cocatalyst even for the standard Grubbs-type salicylaldimine ethylene polymerization catalyst 8 (entries 21 and 22).36 In contrast, complex 5 exhibits no gas uptake in the absence of an activator/cocatalyst, likely because the electron-withdrawing bipyridine increases the barrier for pyridine decoordination (Table 1, entry 9) in comparison to the biphenyl framework of 4. However, upon addition of ZnCl2, polymerization turns on and precatalyst 5 yields low activities for the formation of higher Mn polyethylene, albeit with very broad dispersity (entries 10−12, Chart 1). The molecular weight dispersity decreased significantly upon addition of a large excess of ZnCl2 (entry 12, Chart 2). Polymers produced by the tested catalysts are fairly linear with varying amounts of methyl branching (entries 10, 18, 21, and 22), and 1H NMR showed that for catalysts 5 and 7 less than 20% of the polymer produced contained terminal vinyl end groups (Supporting Information). As is the case for 4, other Lewis acids such as CuCl2 or AlCl3 yield negligible polymerization behavior (entries 15 and 16). The more electron rich, less sterically encumbered complex 6 yielded no polymerization activity, even with ZnCl2 additives (entry 17). Much like the case with 4, the polymerization activity of 5 scales with the concentration of ZnCl2 (Chart 1). In this case, ZnCl2 can sponge excess pyridine and coordinate to the bpy pocket of the catalyst, generating a heterobimetallic active species analogous to 7 (Figure 2). In agreement, using the pregenerated heterobimetallic 7 as a precatalyst yields polymerization behavior similar to that of the in situ generated heterobimetallic from 5 + ZnCl2 (Table 1 entries 18−20) in terms of activity, molecular weight, and melting point of the resulting polymer. This heterobimetallic active species has significantly more apical steric protection than 4 or 5 (as evidenced by the structure of 7) and as a result is capable of generating higher molecular weight polymer as β-H transfer/ termination processes are slowed (Figure 2).37 Control reactions of 5 with bpy and (bpy)ZnCl2 additives (entries 13 and 14) do not yield polymer, indicative that the polymerization results above with ZnCl2 stem from intramolecular effects of ZnCl2 capture by the catalyst. In fact, the addition of bpy decreases activity, suggesting that a possible pathway to catalyst 5 inactivity is intermolecular binding of the free iminobpy moiety to Ni.
because direct mixing of excess ZnCl2 with 5 and 6 resulted in decomposition due to pyridine scavenging by the Lewis acid. The 1H NMR of 7 shows signals shifted slightly upfield from those of the parent complex 5 (Figures S13 and S15 in the Supporting Information). Due to the broad features in the 1H NMR of 7, we speculate that the solution-state structure is slowly fluxional in comparison to the NMR time scale, perhaps via hindered rotation of the bpy arm. The crystal structure of 7 is presented in Figure 1. Most notably, the bpy moiety is situated perpendicular to the Ni square plane, resulting in blockage of the apical binding site of Ni by a chlorine from the bpy-bound ZnCl2 (Ni1−Cl1 4.006(1) Å). The results of ethylene polymerization reactions catalyzed by 4−7 are summarized in Table 1. Reactions catalyzed by the control complex 4 yielded moderate activity (as measured by gas uptake) for the formation of short chain oligomers in a Shulz−Flory type distribution (entry 1). This result is expected, given that the biphenylimine provides no steric protection of the apical Ni binding sites; thus, termination through β-H elimination or transfer should be rapid.36 Upon addition of ZnCl2 to 4 in order to remove pyridine (entries 2−4), activities for oligomerization increased significantly. Other Lewis acids such as CuCl2 and AlCl3 were less effective (entries 7 and 8). In B
DOI: 10.1021/acs.organomet.6b00485 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Table 1. Ethylene Polymerization Experiments Carried Out with 4−8a entry
catalyst
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 6 7 7 7 8d 8d
additive (amt (equiv)) ZnCl2 (1) ZnCl2 (2) ZnCl2 (10) (bpy)ZnCl2 (1) bpy (1) CuCl2 (1) AlCl3 (1) ZnCl2 (1) ZnCl2 (2) ZnCl2 (50) (bpy)ZnCl2 (1) bpy (1) CuCl2 (1) AlCl3 (1) ZnCl2 (1) ZnCl2 (1) ZnCl2 (50) ZnCl2 (1)
time (h)
gas uptake (mmol)
activity (g mol−1 h−1)b
Mn
Đ
Tm (°C)
branches/1000C
12 9.2 5.5 3.0 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 9.8 6.3
3.74 28.4 30.9 25.9 5.17 1.97 2.25 6.36 0.100 0.574 0.632 11.0 0.149 0.055 0.042 0.133 0.018 0.277 1.00 3.10 30.6 30.9
3500 34800 63100 96800 4800 1800 2100 5900 93 537 591 10300 139 51 39 124 17 259 935 2860 35000 54800
c c c c c c c c n.d. 18300 21600 37100 n.d. n.d. n.d. n.d. n.d. 10900 21900 31600 56700 45100
c c c c c c c c n.d. 11.0 6.61 3.80 n.d. n.d. n.d. n.d. n.d. 6.38 8.07 6.75 3.13 3.26
c c c c c c c c n.d. 133 133 133 n.d. n.d. n.d. n.d. n.d. 133 132 133 116 112
c c c c c c c c n.d. 4
n.d. n.d. n.d. n.d. n.d. 7
11 20
a Conditions: [catalyst] = 0.83 mM in 3 mL of PhCH3, 30 °C, 400 psi of C2H4. bDetermined from ethylene uptake. cPredominantly generates C4− C12 oligomers (GC). See Figure S46 in the Supporting Information. d8 = (6-tBu-salicyl(2,6-diisopropylphenyl)iminato)Ni(o-tolyl)(py).
Chart 1. ZnCl2 Effects on the Ethylene Polymerization Activities of 5 and 7
Figure 2. Effects of ZnCl2 activation and capture on the ethylene polymerization behavior of 5.
Chart 2. ZnCl2 Effects on the Molecular Weight and Dispersity of Polyethylene Generated from 5 and 7 The large molecular weight distributions observed in polymerizations carried out with 5 and 7 are a result of catalyst multispeciation via ZnCl2 loss (Figure 2). Complex 7 is unstable in CDCl3 or CD2Cl2 solution over the course of several hours, resulting in ZnCl2 loss and ultimately catalyst agglomeration. During polymerization, ZnCl2 loss yields an active species with very little apical steric protection, resulting in significantly more termination events and lower molecular weights. Consistent with this, the GPC traces of ethylene polymerizations by 5 and 7 in the absence of significant excesses of ZnCl2 all contain low-molecular-weight “tails.” Addition of large excesses of ZnCl2 (50 equiv) disfavors ZnCl2 loss to some degree and molecular weight dispersities
