Article pubs.acs.org/JACS
Pentafluorosulfanyl Substituents in Polymerization Catalysis Philip Kenyon and Stefan Mecking* Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany S Supporting Information *
ABSTRACT: Highly electron-withdrawing pentafluorosulfanyl groups were probed as substituents in an organometallic catalyst. In Ni(II) salicylaldiminato complexes as an example case, these highly electron-withdrawing substituents allow for polymerization of ethylene to higher molecular weights with reduced branching due to significant reductions in β-hydrogen elimination. Combined with the excellent functional group tolerance of neutral Ni(II) complexes, this suppression of β-hydrogen elimination allows for the direct polymerization of ethylene in water to nanocrystal dispersions of disentangled, ultrahigh-molecular-weight linear polyethylene.
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INTRODUCTION The reactivity of molecules, including catalysts, is often controlled by substituents which are themselves inert. Within this principle, a tuning of the reactivity through electronic properties is ubiquitous. The number of practically viable electron-withdrawing substituents which are stable in various environments and can be introduced with reasonable effort is limited, however. The pentafluorosulfanyl (SF5 ) group is a relatively underutilized substituent considering its desirable properties, combining strong electron-withdrawing capability, high lipophilicity, and high thermal and chemical stability. Developments in the preparation of compounds containing SF5 groups and their establishment in areas such as medicinal chemistry and agrichemicals have been well documented in a number of recent reviews.1,2 The compatibility of SF5 groups with many synthetic transformations3−5 and the increasing number of commercially available precursors such as SF5-substituted arenes6 are now increasing the number of areas where the desirable properties of this group are utilized. It has found applications as a lipophilic alternative to the nitro (NO2) group in organocatalysis.7 It is also becoming increasingly common in materials science, namely in liquid crystals,8 polymers,9 triboluminescent materials,10 and photochromic switches.11 However, metal complexes containing the SF5 group are rarer. Although the complex [PtCl(SF5)(PPh3)2] with a metalbound SF5 group has been known since 1969,12 there are still few examples of metal complexes utilizing this group. Of the few reports of such complexes, several focus on either decomposition of the SF5 group to species such as alkylidene sulfur tetrafluorides13,14 or conversion of the potent greenhouse gas SF5CF3 into H2S.15 A notable recent example of metal complexes utilizing the desirable properties of the SF5 group are cationic iridium complexes with SF5-functionalized BIPY ligands. The SF5 substituent increased the blue shift in the phosphorescence when compared to ligands substituted with the more common trifluoromethyl (CF3) group.16 © 2017 American Chemical Society
To the best of our knowledge, there are no examples of organometallic complexes bearing ligands with SF5 substituents for catalysis, although their potential has been noted.17,18 Their lack is perhaps more surprising, as due to the high turnover numbers catalysts enable, the effort of incorporating the SF5 group is more easily offset by the potential gains such as improved catalyst performance and, in the case of polymerization catalysts, enhanced material properties. An example case arose from our studies of aqueous catalytic polymerizations. Late transition metal polymerization catalysts are unique in providing functional group tolerance. Another characteristic feature is their propensity for β-hydrogen elimination. This can result in chain transfer and branch formation.19 Neutral Ni(II) salicylaldiminato complexes display both of these properties.20−22 They are able to polymerize ethylene in aqueous media23−25 but at the same time are highly dependent on electron-withdrawing remote substituents (e.g., CF3, NO2) controlling β-hydrogen elimination to achieve high molecular weights and low branching.26−29 Through careful catalyst design, β-hydrogen elimination can be limited to the extent that aqueous dispersions of polyethylene nanocrystals can be generated.25 As electron-withdrawing substituents are crucial for achieving desirable polymer properties, this system is also potentially well suited to probe the utility of the role of SF5 substituents in polymerization catalysis.
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RESULTS AND DISCUSSION Synthesis of the desired SF5-substituted ligands was straightforward (Scheme 1). The boronic acid pincaol ester (3) used can be produced from the commercially available 1-bromo-3,5bis(pentafluorosulfanyl)benzene using [Pd(dppf)Cl2] (Scheme 2). Gas chromatography showed that conversion of the starting material to the product was >95% after 4.5 h, and the pure product could be isolated in a 76% yield. Due to this slightly Received: June 29, 2017 Published: September 18, 2017 13786
DOI: 10.1021/jacs.7b06745 J. Am. Chem. Soc. 2017, 139, 13786−13790
Article
Journal of the American Chemical Society
Scheme 1. Synthesis of SF5-Substituted Ligands and Complexation To Obtain Ni(II) Precatalysts, 1-SF5/Py and 2-SF5/Py
Scheme 2. Catalytic Transformation of SF5-Containing Brominated Compound to a Boronic Acid Ester
Figure 1. Structures of 1-SF5/TPPTS and CF3 analogues to the SF5substituted Ni(II) precatalysts.
higher yield, this route was preferred over the previously reported Grignard route (65% yield).7 From the boronic acid ester, the desired anilines (4, 6) were synthesized by Suzuki coupling either with 2,6-dibromo- or 2,4,6-tribromoaniline. Salicylaldimines (5, 7) were then synthesized by acid-catalyzed condensation of these anilines with the appropriate salicylaldehyde. Precatalysts (1-SF5/Py, 2SF5/Py) were obtained in near quantitative yields by reaction with (TMEDA)NiMe2 in the presence of pyridine. Interestingly, for both of these complexes, the resonances for pyridine in the 1H NMR spectrum are very broad. By comparison for the analogous compounds with CH3, tBu, and CF3 substituents, the pyridine resonances are well-defined multiplets.26,27 This broadening could be due to a more electron-deficient metal center binding the neutral pyridine ligand strongly and hindering its free rotation.30 In view of aqueous polymerizations, a water-soluble complex was also prepared. These are formed by introducing TPPTS (3,3′,3″-phosphanetriyltris(benzenesulfonic acid) trisodium salt) as a ligand to a labile dimethylformamide (DMF) complex, [(κ2-N^O)NiMe(DMF)] (cf. Supporting Information, SI).24 Exchange of DMF for TPPTS was incomplete, but by washing away the lipophilic intermediate catalyst, the watersoluble complex 1-SF5/TPPTS was obtained. The compound is mixed with free TPPTS and residual DMF; however, these do not hinder the polymerization. Analogous CF3 complexes were also prepared for direct comparison of polymerization behaviors (Figure 1). Polymerizations were carried out in toluene over a wide temperature range (30−70 °C), with the SF5-substituted complexes compared to analogous CF3 complexes to assess
the effect of the substituent on polymer properties and catalyst performance. As expected from the general reactivity pattern of polymerization catalysts, with an increased polymerization temperature, an increase in β-hydrogen elimination leads to an increase in branching (and a decrease in molecular weight). However, the SF5 substituent has a significant effect on polymer properties. NMR spectra of the polymers show that, over the entire temperature range studied, the SF5-substituted complexes produce polymers with far fewer methyl branches than the CF3 analogues (Figure 2). A similar trend is observed when comparing the molecular weights of the polymers produced. Molecular weights produced using precatalysts 1-SF5/Py and 2-SF5/Py are generally higher than those of the CF3 analogues. At industrially significant elevated temperatures when β-hydrogen elimination is much more significant, the SF5-substituted complexes produce polymers with molecular weights double (70 °C) or quadruple (50 °C) those produced by the CF3 complexes. At 30 °C where rates of β-hydrogen elimination are low in general, no such clear trend is observed (Table 1, entries 1, 4, 7, and 10). Note that poor activation at this temperature likely also plays a role, with only 0.4 of the nickel centers from precatalyst 2-SF5/Py producing a polymer chain compared to 2-CF3/Py, where each nickel center produces an average of 1.8 chains. This reduction in β-hydrogen elimination is also reflected in the melting temperature of the polymers produced. While the melting temperatures of the polymers produced at 30 °C are comparable, the polymers produced using SF5-substituted 13787
DOI: 10.1021/jacs.7b06745 J. Am. Chem. Soc. 2017, 139, 13786−13790
Article
Journal of the American Chemical Society
Scheme 3. Dissociation and Recombination of Neutral Pyridine Ligand Controlling the Concentration of the Active Species
aqueous media. As in aqueous polymerizations, different phases exist (polymer nanoparticles and continuous water phase), and the dissociated TPPTS ligand and the lipophilic active species may reside in different phases preferentially, and a change in productivity is not unexpected.22,24 Ethylene consumption in aqueous polymerization decreased significantly over time, and after an hour no further uptake of ethylene was observed. Remarkably complex 1-SF5/TPPTS produces a dispersion of polyethylene with unprecedented high molecular weights. These exceed 106 g mol−1 and are in the ultrahigh-molecularweight polyethylene (UHMWPE) regime. Adjusting the pH with CsOH·H2O,32 a gain of 106 g mol−1 above that of previously synthesized “ideal polyethylene nanocrystals” can be achieved (Table 2, entry 2).25 The polyethylene produced also displays characteristic melt properties of linear UHMWPE, i.e., an exaggerated first melting temperature of the nascent polymer ≥140 °C, while a melting temperature of ∼135 °C is obtained for all subsequent melting. In such polymerizations, the polyethylene is produced in the form of highly organized single crystals.25,33 The crystals produced by this catalyst are large (
