Polymerization Catalysts Bearing a Fluorinated Cyclophane Ligand

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Organometallics 2009, 28, 4452–4463 DOI: 10.1021/om900302r

Nickel(II) and Palladium(II) Polymerization Catalysts Bearing a Fluorinated Cyclophane Ligand: Stabilization of the Reactive Intermediate1 Chris S. Popeney,† Arnold L. Rheingold,‡ and Zhibin Guan*,† †

Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697, and ‡ Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0332 Received April 21, 2009

The synthesis and characterization of Ni(II) and Pd(II) R-diimine olefin polymerization catalysts bearing a fluorinated cyclophane-based ligand were performed. Fluorine was placed in such a manner as to interact with the metal center from the axial direction. The catalysts were active in the polymerization of ethylene, showing substantial differences in both catalytic behavior and polymer size and structure as compared to their nonfluorinated analogues. Both catalysts afforded polymer of comparatively low branching density and high molecular weight. The Ni(II) catalysts, from precursor [Ni(acetylacetonato)(F-Cyc)]+ salts (F-Cyc=fluorinated cyclophane), exhibited enhanced thermal stability by remaining active after 70 min with little loss in polymerization activity at 105 °C. The Pd(II) catalysts from salts of [Pd(F-Cyc)Me(NCR)]+ (NCR=nitrile) afforded polymer of molecular weights far higher than the nonfluorinated analogue. Additionally, polymerization activity was directly related to ethylene feed pressure for the Pd(II) system, and NMR analysis could not detect the presence of bound olefin, indicating that the polymerization proceeded via different kinetics involving an olefin-free 14 e- complex as the catalyst resting state. Furthermore, NMR 1H-19F coupling data provide clear evidence that the fluorine atoms were indeed interacting with the metal axial site. The unusual properties of these new complexes are thus attributed to stabilization of the highly reactive 14 e- intermediate by donation of the fluorine lone pair to the metal center.

Introduction Ligand structure has a dramatic effect on the reactivity of organometallic complexes. This has been especially evident in the recent development of late transition metal olefin polymerization catalysts. A major breakthrough occurred over a decade ago when Ni(II) and Pd(II) catalysts were prepared bearing bulky R-diimine ligands,2-4 which are crucial for blockage of the axial coordination sites on the metal toward incoming monomer and reduction of chain transfer. Although the Ni catalysts are highly active and the Pd catalysts are attractive because of the unique polymer architectures they afford and their high functional group tolerance, both systems suffer from relatively low thermal stability.5,6 In response to the limitations of existing late transition metal polymerization *Corresponding author. E-mail: [email protected]. (1) This paper has been adapted in part from the following thesis : Popeney, C. S. Ph.D. Dissertation, University of California, Irvine, 2008. (2) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414–6415. (3) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267–268. (4) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169–1203. (5) Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320–2334. (6) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686–6700. pubs.acs.org/Organometallics

Published on Web 07/06/2009

catalysts, our laboratories created new Ni(II) and Pd(II) catalysts bearing a cyclophane-based R-diimine ligand 1 (Chart 1).7,8 The thermal stability of these catalysts, evident by their relatively high activity, ability to produce high molecular weight polyethylene at elevated temperatures, and living polymerization for R-olefins at temperatures up to 75 °C,9 is attributed to the influence of the unique ligand environment of 1. Ligand moieties remain roughly fixed above the metal center, retarding chain transfer and limiting deactivation processes. In addition to their enhanced stability, these catalysts afforded polymers with significantly higher branching densities when compared to catalysts bearing acyclic ligands, presumably by facilitating β-hydride elimination/reinsertion with respect to chain propagation.9,10 During our investigation of the Pd(II) catalysts formed from 1, it was observed that one pair of ligand aromatic protons in a position above the metal underwent a significant upfield chemical shift of more than 1 ppm in the 1H NMR when halide was abstracted from the neutral methyl chloro complex (Nˆ N)PdMeCl to afford a cationic complex of type (7) Camacho, D. H.; Salo, E. V.; Ziller, J. W.; Guan, Z. Angew. Chem., Int. Ed. 2004, 43, 1821–1825. (8) Camacho, D. H.; Salo, E. V.; Guan, Z. Org. Lett. 2004, 6, 865– 868. (9) Camacho, D. H.; Guan, Z. Macromolecules 2005, 38, 2544–2546. (10) Popeney, C. S.; Levins, C. M.; Guan, Z. Manuscript in preparation. r 2009 American Chemical Society

Article Chart 1. Cyclophane-Based r-Diimine Ligands 1 and 2 and Corresponding Ni(II) and Pd(II) Complexes

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data provide evidence of direct palladium-fluorine interactions that may contribute to the unusual catalytic properties for these complexes.

Results

[(Nˆ N)PdMe(L)]+(NˆNˆ=cyclophane, L=neutral ligand).11 As elucidated by NOE and 2D NMR, these protons were found to lie nearest to the exiting chloro and incoming neutral ligands, suggesting an H-agostic interaction existed with the more electrophilic metal center of the cationic complexes.10 Prompted by this possibility, we proposed to introduce Lewis basic groups in this position on the cyclophane ligand for direct interaction with the metal center through the axial coordination site. We reasoned that this hemilabile interaction may stabilize the most reactive 14 e- intermediate in the catalytic cycle and hence suppress side reactions associated with this species such as catalyst deactivation (Scheme 1). Binding of an olefin monomer should afford the less electrophilic 16 e- complex, release the fluorine interaction, and allow monomer insertion to proceed. Fluorine was chosen as the Lewis basic group because of its small size and weakly donating nature. Interactions between fluorine and late transition metals are uncommon;12,13 however a reactivity enhancement was recently reported in a fluorinated Ru metathesis catalyst believed to involve a metal-fluorine interaction.14 Fluorinated titanium phenoxy-imine olefin polymerization catalysts were shown to afford living polymerizations with a reduction in chain transfer processes due to direct interaction of β-hydrogen with fluorine and stabilization toward elimination.15 Although fluorine substituents have been introduced to a few late metal polymerization complexes,5,16-18 no experimental evidence regarding direct fluorine-metal interactions has been reported. Herein, we describe the synthesis of Ni(II) and Pd(II) catalysts bearing a fluorinated cyclophane (F-Cyc) ligand 2 and polymerizations with ethylene (Chart 1). The fluorine atoms are appropriately positioned to serve as axial ligand donors to stabilize electrophilic polymerization intermediates, particularly the important 14 ealkyl agostic complex. The new complexes exhibit unusual polymerization properties. In addition, NMR spectroscopic (11) Salo, E. V. Masters thesis, Chapter 2, University of California, Irvine, 2005. (12) Mezzetti, A.; Becker, C. Helv. Chim. Acta 2002, 85, 2686–2703. (13) Grushin, V. V. Chem.;Eur. J. 2002, 8, 1007–1014. (14) Ritter, T.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 11768–11769. (15) Mitani, M.; Mohri, J.; Yasunori, Y.; Saito, J.; Ishii, S.; Tsuru, K.; Matsui, S.; Furuyama, R.; Nakano, T.; Tanaka, H.; Kojoh, S.; Matsugi, T.; Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2002, 124, 3327–3336. (16) Chen, Y.; Qian, C.; Sun, J. Organometallics 2003, 22, 1231–1236. (17) Ionkin, A. S.; Marshall, W. J.; Adelman, D. J.; Fones, B. B.; Fish, B. F.; Schiffhauer, M. F. Organometallics 2008, 27, 1147–1156. (18) Helldorfer, M.; Militus, W.; Alt, H. G. J. Mol. Catal. A: Chem. 2003, 197, 1–13.

Synthesis and Characterization of F-Cyc Ni(II) and Pd(II) Catalysts. Preparation of the fluorinated cyclophane ligand began with the Pd-catalyzed Negishi coupling of acetal 9 to 2,6dibromo-4-methylnitrobenzene (10) to afford 11. Following deprotection to give crude dialdehyde 12, vinyl groups were installed by Wittig reaction to afford 13. The nitro group was then subsequently reduced by tin(II) chloride to provide the divinyl meta-terphenyl aniline 14 (Scheme 2). Further construction of the complete ligand 2 was performed in a manner similar to the unsubstituted ligand: diimine condensation followed by ring-closing metathesis and hydrogenation (Scheme 3).7,8 Initial attempts at complexation of the ligand to Ni and Pd appeared to be problematic. Unlike the unsubstituted cyclophane ligand 1,7 the F-Cyc ligand 2 was unable to complex to any neutral precursors to afford, for example, the commonly employed nickel dihalo or palladium methyl chloride complexes, possibly because of electronic effects or steric repulsion. Instead, facile preparation of the Ni(II) complex [(F-Cyc)Ni(acac)]B(C6F5)4 (4, acac = acetylacetonato)19 and Pd(II) nitrile complexes [(F-Cyc)PdMe(NCMe)]BAr4 (7, Ar=3,5-bis-(trifluoromethyl)-phenyl)6 and [(F-Cyc)PdMe(NCAr)]BAr4 (8) was carried out (Scheme 4), presumably via more reactive cationic intermediates. To allow for direct comparison, the nonfluorinated analogues of Ni(II) acetylacetonato and Pd(II) methyl acetonitrile complexes, 3 and 6, were also prepared (Chart 1). Single crystals of Ni(II) complex 4 were subjected to X-ray diffraction analysis.20 As expected, complex 4 exhibits C2v symmetry in the X-ray crystal structure (Figure 1). The fluorine atoms do not closely approach the nickel atom since the axial phenyl rings they are attached to lie roughly perpendicular to the central aniline phenyl rings. In contrast, the two nonequivalent cis ligands in the Cs-symmetric Pd(II) complexes impart noticeable deflection of the phenyl-phenyl dihedral angle. Although no crystals of Pd complexes of the F-Cyc ligand were grown of sufficient quality for X-ray analysis, previous diffraction studies on the nonfluorinated cyclophane Pd(II) methyl chloride complex Pd(Cyc)MeCl (15) exhibited such phenyl ring rotation.7 The rings rotate away from the methyl ligand on the left side of complex 15 but turn inward toward the chloro ligand on the right side, (19) Moody, L. S.; Mackenzie, P. B.; Killian, C. M.; Lavoie, G. G.; Ponasik, J. A., Jr.; Barrett, A. G.; Smith, T. W.; Pearson, J. C. WO 00/ 50470, 2002. (20) Crystals of 4 were grown over several days by slow vapor diffusion of n-pentane into a dichloromethane solution of the complex (15 mg/mL). Crystallographic data for 4: C83H39BF28N2NiO2 3 CH2Cl2: red plate, triclinic,Mr=1782.61, P1, a=12.010(3) A˚, b=14.126(4)A˚, c= 22.928(6) A˚, R=103.802(4)°, β=101.778(4)°, γ=100.135(4)°, V=3593.1 (16) A˚3, Z=2, T=100(2) K, λ=0.71073 A˚. The asymmetric unit consists of one anion, one molecule of the recrystallization solvent, CH2Cl2, and two half-cations residing on inversion centers; of necessity, while the outer macrocyclic ring possesses the crystallographically required symmetry, the inner Ni coordination sphere does not and is consequentially fully disordered, but easily modeled based on the planar geometry about Ni. R1=0.0695, wR2=0.1567 for 12 613 independent reflections. CCDC702264 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44)1223-336033; or [email protected]).

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Scheme 1. Proposed Hemilabile Interaction between Fluorine and the Metal in the 14 e- Intermediate and Displacement by Incoming Ethylene

Scheme 2. Synthesis of Aniline 14

allowing two hydrogen atoms to make a relatively close approach of 2.7 A˚ to the metal center.11 Since all active Ni (II) and Pd(II) polymerization intermediates, such as the alkyl olefin and the alkyl agostic complexes, exhibit Cs symmetry, there should be two fluorine atoms in the corresponding F-Cyc derivatives near the metal at any given time. Furthermore, strong 1H-19F coupling seen by NMR in the Pd(II) acetonitrile complex 7 indicated close approach of the fluorine atoms to the coordination sphere. The resonances of protons on groups that lie in the coordination plane were noticeably split by fluorine, in addition to the expected doublets seen for protons ortho to fluorine on the axial phenyl rings. Most prominently, signals of the methyl ligand exhibited a triplet pattern split by two nearby fluorine atoms (FA) with JHF = 4.5 Hz, while protons of the acenaphthyl diimine backbone were split in triplet multiplicity

with similar magnitude by other nearby fluorine atoms (FB and FC) (Figure 2). Analogous triplet splitting was also seen in 13C NMR. Coupling of similar magnitude has been observed in other rigid complexes containing fluorine and has been typically attributed to nonbonding, close-contact interactions.21-24 In complex 7, these interactions were confirmed by selective 19F decoupling of the four fluorine resonances, each corresponding to a pair of F atoms lying (21) Chambers, R. D.; Sutcliffe, L. H.; Tiddy, D. J. T. Trans. Faraday Soc. 1970, 66, 1025–1038. (22) Belt, S. T.; Helliwell, M.; Jones, W. D.; Partridge, M. G.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 1429–1440. (23) Fornies, J.; Fortuno, C.; Gomez, M. A.; Menjon, B. Organometallics 1993, 12, 4368–4375. (24) Noveski, D.; Braun, T.; Neumann, B.; Stammler, A.; Stammler, H. Dalton Trans. 2004, 4106–4119.

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Scheme 3. Synthesis of Fluorinated Cyclophane Ligand 2

Scheme 4. Preparation of Ni(II) acac Complexes 3 and 4 and Pd(II) Nitrile Complexes 7 and 8

above and below the metal coordination plane as expected from the Cs symmetry of the complex. A summary of the 1 H-19F interactions is shown in Figure 2, while the results of 19 F decoupling experiments are shown below in Figure 3. In this manner, it was possible to determine the location of the fluorine pairs themselves, assuming the observed coupling arises chiefly from close contact. However, we also observed

weak coupling of the FD fluorines to the acetonitrile methyl group in 13C NMR and by line broadening of its 1H NMR signal. The coupling of the acetonitrile methyl group with fluorine is surprising, given that this group should be too far away from fluorine for coupling by close contact. Instead, we believe that a through-bond contribution, mediated by an F-Pd interaction, is responsible for the observed coupling.

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Figure 1. X-ray crystal structure of cation 4, [(F-Cyc)Ni(acac)]+ (borate anion and hydrogen atoms omitted for clarity). Selected interatomic distances (A˚): Ni1-N1A 1.734(3), Ni1-N1 1.770(3), Ni1-O26 1.861(3), Ni1-O24 1.897(3), N1-C1 1.434(4), N1AC23A 1.498(8), N1-C22A 1.535(8). Selected bond angles (deg): N1A-Ni1-N1 95.17(13), N1A-Ni1-O26 89.86(15), N1-Ni1O26 174.3216, N1A-Ni1-O24 176.47(14), N1-Ni1-O24 88.21 (14), O26-Ni1-O24 86.71(14), C1-N1-C22A 108.9(4), C1-N1Ni1 140.1(3), C22A-N1-Ni1 111.0(3), C34-O26-Ni1 132.8(4). Selected dihedral angles (deg): N1-C22A-C23A-N1A -0.8(6), C23A-N1A-C1A-C2A 84.4(6), C6-C1-N1-C22A -85.9(6).

Figure 2. Diagram of H-F interactions in complex 7. Arrows indicate 1H and 19F pairs in close proximity by observed J couplings of 19F with 1H and 13C. Atoms are labeled as assigned in the characterization provided in the Experimental Section.

Polymerizations with Ni(II) and Pd(II) F-Cyc Catalysts. Treatment of Ni(II) complex 4 with trialkylaluminum reagents led to the formation of catalytically active species for the polymerization of ethylene. The highest yields of polymer were obtained with triethylaluminum (TEA) and triisobutylaluminum (TIBA), while less polymer was produced with trimethylaluminum (TMA). Both modified methylaluminoxane (MMAO) and dimethylaluminum chloride were ineffective. The activity, expressed as turnover number (TON), of complex 4/TIBA was low at temperatures below 70 °C. Its activity was increased significantly at elevated temperature and peaked at 105 °C (Table 1, entries 4-9), with 1 μmol of 4 yielding 0.35 g of polyethylene after 10 min upon treatment with 1500 equiv of TIBA and 200 psi ethylene pressure (entry 4). By comparison, under otherwise similar conditions, the catalyst 3/TIBA was roughly 10 times more active than 4/TIBA, but lost activity rapidly at temperature above 70 °C (entries 1 and 2). Despite the

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high purity of complex 4 employed, the polymer obtained from 4/TIBA exhibited a bimodal molecular weight distribution, consisting of a high molecular weight major fraction (Mn ≈ 105 g/ mol) and a minor proportion of ultrahigh molecular weight polymer (Mn>106 g/mol). The main fraction of lower molecular weight became increasingly dominant at higher polymerization temperatures, according to GPC analysis (Supporting Information, Figure S1). We attribute this behavior to complicated but temperature-dependent activation kinetics, leading to two active species. Like other Ni(II) catalysts,5,25 changing the Al/Ni ratio had no observed effect on the polymer molecular weight distribution, suggesting that chain transfer to aluminum is negligible. The polymer molecular weight distribution also underwent little significant change with time (Supporting Information, Figure S2), indicating that the relative concentrations of both active species remain constant. To account for changes in ethylene solubility at different temperatures, TON was also adjusted for ethylene concentration, as estimated from solubilities in toluene (see Experimental Section and Supporting Information). This resulted in, overall, a relatively minor correction due to a reduction of solubility of about 30% from 80 to 120 °C. Although polymer productivity of 4 is lower than the nonfluorinated analogue 3, the catalyst exhibits increased thermal stability. As seen in Figure 4, after 70 min of polymerization at 105 °C, little decrease in activity can be noted. In contrast, roughly 40% of catalyst 3, itself a dramatic improvement over the standard acyclic catalyst, had deactivated after 30 min at 80 °C.7 The microstructure of the polymer produced by 4 was also considerably different than that of 3. Most notably, the polymer was much more linear, as indicated by a pronounced decrease in branching density (B, branches per 1000 carbon atoms). Values of B lie within 30-40 at polymerization temperatures between 80 and 120 °C for polymers obtained from 4, as compared to the heavily branched polymers obtained from 3 (B>100). The presence of fluorine has a significant inhibiting effect on the rate of chain walking, likely by reducing the rate of β-hydride elimination. The fluorinated Pd(II) cyclophane catalyst exhibited dramatically different behavior compared to the nonfluorinated analogue. The addition of ethylene at ambient temperature and pressure to a solution of Pd(II) F-Cyc acetonitrile adduct 7 led to the formation of negligible polymer. However, at an ethylene pressure of 88 psi, the catalyst was quite active (Table 2, entry 4). The catalytic activity of 7 was considerably higher than for the nonfluorinated acetonitrile catalyst 6 (entries 2 and 3) and comparable to that of the commonly employed nonfluorinated ester chelate catalyst 5 (entry 1).7,26 As competitive binding of the nitrile ligand has been observed to retard polymerization in the nonfluorinated cyclophane Pd(II) complexes (compare catalysts 5 and 6, entries 1, 2, and 3),27,28 the analogous fluorinated cyclophane Pd(II) complex bearing the weakly donating nitrile 3,5-bis(trifluoromethyl)benzonitrile (8) was prepared.29 (25) Meinhard, D.; Wegner, M.; Kipiani, G.; Hearley, A.; Reuter, P.; Fischer, S.; Marti, O.; Rieger, B. J. Am. Chem. Soc. 2007, 129, 9182–9191. (26) Popeney, C. S.; Camacho, D. H.; Guan, Z. J. Am. Chem. Soc. 2007, 129, 10062–10063. (27) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888–899. (28) Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085–3100. (29) Acetonitrile adduct 7 was synthesized along with an inseparable unidentified minor product, 70 . This was additional motivation for the synthesis of arylnitrile 8, which could be isolated in high purity. Despite the impurity, GPC traces of polymer produced by 7 were monomodal, indicating it was not active toward polymerization.

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Figure 3. Representative NMR spectra for complex 7: (a) 19F NMR and (b) 1H NMR without 19F decoupling. The results of selective 19 F decoupling experiments are shown for resonances (c) FA, (d) FB, (e) FC, and (f) FD. While not shown here, decoupling of FD led to a narrowing of the line width at half-height of the acetonitrile methyl resonance from 1.6 to 1.3 Hz. All resonances are labeled as assigned in the Experimental Section. For clarity, only the decoupling resonances are labeled in (c)-(f).

Interestingly, the polymerization behavior of 8 (entries 5-9) was nearly identical to that of 7 (entry 4), suggesting any inhibition of polymerization by nitrile was insignificant for the fluorinated system. Once ethylene solubility was taken into account, it appeared that the initial activity of 8 was rather constant from 35 °C up to 80 °C, although significant deactivation took place with time. As with the Ni(II) catalyst 4, the fluorinated Pd(II) catalysts 7 and 8 displayed distinctly different polymerization behavior when compared to its nonfluorinated analogues. First, while the polymers from cyclophane catalysts 5 and 6 were of low molecular weight (Table 2, entries 1-3),30 catalysts 7 and 8 afforded polyethylenes with over 20-fold increase in molecular weight (Table 2, entries 4-9). Since the turnover frequency of the fluorinated catalyst was comparable to the nonfluorinated catalyst, chain transfer processes must have been suppressed in the former to account for the large molecular weight increase. Furthermore, as with the Ni system, the branching (30) The low molecular weight of polymer produced by the cyclophane Pd(II) catalysts is inconsistent with evidence that suggests a reduction in associative chain transfer rates (see refs 6, 8). Instead, an additional chain transfer mechanism, possibly dissociative, may operate preferentially . Popeney, C. S.; Levins, C. M.; Guan, Z. Manuscript in preparation.

densities of the polymers from the F-Cyc Pd(II) catalysts were reduced 2-fold. The higher linearity led to a corresponding increase in polymer crystallinity, as evident by the observation of strong melting transitions seen in differential scanning calorimetry.

Discussion Introduction of fluorine onto the cyclophane ligand imparts significant effects on the polymerization catalytic properties for its late transition metal complexes. The fluorinated Ni(II) and Pd(II) cyclophane catalysts polymerize ethylene to give polymers exhibiting much less branching density than their nonfluorinated counterparts. This, in turn, imparts crystallinity to the polymers and significantly influences their macroscopic properties (Tables 1 and 2). Since polymer branching is introduced by the chain-walking mechanism, the presence of fluorine apparently inhibits this process. On the basis of mechanistic studies by Brookhart and co-workers,6,31 chain walking results from repetitive hydride elimination and reinsertion reactions of highly reactive and electron-deficient alkyl agostic intermediates such (31) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 11539–11555.

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Table 1. Polymerization Results for Ni(II) Complexes 3 and 4a entry

cat.

Trxn (°C)

time (min)

load (μmol)b

yield (mg)

TON ( 10-3)c

TON/[C2H4] ( 10-3 M-1)d

Mn (kg/mol)e

Mw/Mne

Bf

Tm (°C)g

3 80 10 1.0 4520 160 140 267 1.9 104.0 N/Ah 3 115 10 1.0 920 33 38 131 1.5 116.7 N/A 1.4, 1.3i 33.3 86, 114 4 80 10 1.0 147 5.2 4.5 1664, 166i 4 105 10 1.0 345 12 13 159 2.3 33.2 81, 122 4 105 10 0.50 174 12 13 N/A 4 105 20 0.50 223 16 17 197 2.9 N/A 4 105 30 0.50 318 23 24 185 3.0 35.6 N/A 4 105 45 0.50 476 34 36 N/A 4 105 70 0.50 639 46 48 190 2.7 N/A 4 120 10 1.0 170 6.3 7.2 115 4.1 40.2 80, 126 a Polymerization conditions: 1500 equiv of TIBA, 200 psi of ethylene. Solvent = 1,2-dichlorobenzene for runs >80 °C, 4:1 toluene/1,2dichlorobenzene for other runs. b Catalyst load. c Turnover number (moles of ethylene per mole of catalyst). d Turnover number adjusted for ethylene monomer concentration (see Supporting Information). e Determined by size exclusion chromatography in 1,2,4-trichlorobenzene at 140 °C with polyethylene standard. f Branching density per 1000 carbons by 1H NMR. g Melting temperature of crystalline domains determined by differential scanning calorimetry. h Amorphous. i Individual distributions could be resolved.

1 2 3 4 5 6 7 8 9 10

Table 2. Polymerization Results for Pd(II) Catalystsa entry

cat.

Trxn (°C)

time (h)

catalyst load (mol-6)

yield (g)

TOF (h-1)b

TOF/ [C2H4] (h-1 M-1)c

Mn (kg/mol)d

Mw/Mn

Be

Tm (°C) f

5 35 18 10 2.28 450 670 12.9 1.7 106 N/Ag 6 35 18 10 0.50 100 150 8.31 1.2 106 N/Ag 6 60 6 7.7 0.12 93 180 3.97 1.2 114 N/Ag 7 60 18 10 1.79 350 680 166 1.3 55.5 64.4 8 35 6 4.0 0.60 890 1300 264 1.3 51.3 66.5 8 35 18 4.0 0.72 360 530 269 1.6 53.2 66.4 8 60 6 3.0 0.28 560 1100 161 1.4 56.4 63.8 8 60 18 5.0 0.76 300 580 134 1.5 60.8 64.7 8 80 6 4.0 0.30 450 1100 79.5 1.4 58.3 62.9 a Polymerization conditions: in 100 mL of toluene, 88 psi of ethylene pressure. b Turnover frequency = moles of ethylene per mole of catalyst per hour. c Turnover frequency adjusted for ethylene monomer concentration (see Supporting Information). d Number-averaged molecular weight determined by size exclusion chromatography coupled to multi-angle light scattering. e Branching density per 1000 carbon atoms as determined by 1H NMR. f Melting temperature as determined by differential scanning calorimetry. g Amorphous. 1 2 3 4 5 6 7 8 9

Figure 4. Polymerization activity of 4/TIBA expressed as turnover number (TON, mol ethylene polymerized/mol catalyst) at 105 °C.

as II (Figure 5). Our results from NMR analysis have proven that the fluorine atoms are close enough to the metal to interact with groups in the metal coordination plane through spin-spin coupling and by through-bond coupling facilitated by donation to Pd. Therefore, the fluorine atoms seem appropriately positioned to allow the stabilization of electrophilic species by electron donation to the metal (Figure 5). On the basis of the molecular model of the ligand geometry, this interaction should occur through the axial coordination site on the metal when in a 14 e- state. Unlike the H-agostic interaction, the interaction with fluorine should occur by donation from the lone pair on fluorine and not from the C-F bond, giving rise to an intermediate resembling I. This interaction with fluorine is expected to be stronger than

Figure 5. Fluorinated cyclophane complexes: axial F-donation in intermediate I and alkyl agostic intermediate II (M = Ni or Pd).

the H-agosic interaction presumed to occur in the nonfluorinated cyclophane catalysts. Molecular weights of polymers prepared by the fluorinated Pd(II) complex are much higher than polymers prepared by the analogous nonfluorinated Pd(II) complexes. Since the turnover frequency of the fluorinated catalyst is only slightly higher than the nonfluorinated catalyst (Table 2), chain transfer processes must be suppressed in the former to account for the large MW increase. This suppression can also be attributed to the stabilization of the highly active catalytic intermediates through interaction with fluorine. The electronic donation of fluorine from the axial direction should stabilize the 14 e- alkyl intermediate I, suppressing β-hydride elimination. The suppression of β-hydride elimination would reduce the rate of chain transfer,

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Scheme 5. The Presence of the Stabilized Resting State II and the Relative Destabilization of Reactive Alkyl Agostic Complex I Lead to Reduced Rates of Chain Walking, Chain Transfer, and Decomposition with the F-Cyc Series Ni(II) and Pd(II) Catalysts

which is presumed to operate by associative monomer displacement from the formed olefin hydride intermediate.6,31 This proposal is consistent with the observed reduction in branching density because β-hydride elimination is critical for chain walking. Alternatively, as proposed by Ziegler,32 chain transfer could occur by concerted hydride transfer from the polymer chain to bound ethylene in the 16 e- alkyl olefin complex (III). As mentioned previously, in the F-Cyc system this 16 e- complex is a short-lived intermediate that is not detectable by NMR. Therefore, the occurrence of chain transfer by this mechanism would also be suppressed in the F-Cyc case because of the limited availablility of III. Another dramatic effect of cyclophane ligand fluorination is the significant enhancement of thermal stability for the Ni(II) complex. Although knowledge of the deactivation processes of Ni(II) catalysts is limited, the alkyl agostic complex II, being the most reactive intermediate in the system, is likely involved. Therefore, stabilization of this intermediate by electronic donation from fluorine could account for the high thermal stability of the Ni(II) catalyst. A further interesting observation is that while the polymerization activity of Pd(II)-R-diimine catalysts are typically independent of monomer concentration,2,3,6,31 the productivity of the fluorinated Pd(II) catalyst (7) was strongly dependent on the ethylene pressure. In in situ NMR polymerization experiments, polyethylene production was observed although bound ethylene signals were not. These observations suggest that the ethylene trapping step, rather than monomer insertion,6 is rate limiting and that a 14 especies lacking bound olefin is the catalytic resting state. On the basis of all these observations, we propose that axial fluorine donation to the metal center helps to stabilize the 14 e- alkyl intermediates and the olefin-free F-ligated complex I is the catalytic resting state (Scheme 5). The exact structure of complex I could involve the interaction of either one fluorine to form a four-coordinate complex with significant tetrahedral distortion33 or two fluorines above and below the metal coordination plane in a distorted trigonal-bipyramidal complex.34 The stabilization of the 14 e- reactive catalytic intermediates suppresses a number of potential pathways as outlined in Scheme 5: chain transfer (molecular weight), chain walking (branching architecture), (32) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 6177–6186. (33) Dawson, J. W.; Gray, H. B.; Hix, J. E.Jr.; Preer, J. R.; Venanzi, L. M. J. Am. Chem. Soc. 1972, 94, 2979–2987. (34) Antsishkina, A. S.; Porai-Koshits, M. A.; Nivorozhkin, A. L.; Vasilchenko, I. S.; Nivorozhkin, L. E.; Garnovsky, A. D. Inorg. Chim. Acta 1991, 180, 151–152.

and catalyst deactivation (thermal stability). The fluorine-stabilized 14 e- alkyl intermediates are less prone to β-hydride elimination and, hence, less chain walking and less chain transfer. Because the most reactive 14 e- alkyl agostic complexes are also key intermediates along the paths of other processes such as catalyst deactivation, its stabilization may account for the enhanced thermal stability of the Ni(II) catalyst.

Conclusions A significant effect on reactivity and polymer properties was observed in Ni(II) and Pd(II) olefin polymerization catalysts bearing a fluorinated cyclophane-based ligand. Elevated thermal stability was observed for the Ni(II) complex, and significant molecular weight increase was obtained from the Pd(II) complex. In both metal systems, a large reduction in chain-branching density was noted, which suggests a significant reduction in chain-walking isomerization. As chain walking was proposed to proceed through repetitive β-hydride elimination followed by nonregioselective reinsertion of the hydride, the reduced extent of chain walking for the fluorinated complexes most likely operates through restriction of the β-hydride elimination process. Evidence for interaction between fluorine and Ni(II) and Pd(II) was gathered by NMR spectroscopy. The strong coupling observed in the NMR between the fluorine atoms and protons and carbons of coordinating ligands provided evidence for a direct interaction between fluorine and the metal center. Lastly, the lack of observation of bound olefin species in the fluorinated Pd(II) system and the large effect of olefin concentration on polymerization activity implied increased stability of the 14 e- polymerization intermediates. To the best of our knowledge, this represents the first example of direct fluorine interactions with late metal centers in polymerization catalysts and their positive effects on olefin polymerization.

Experimental Section General Considerations. All catalyst handling was carried out in a Vacuum Atmospheres glovebox filled with nitrogen. All moisture- and air-sensitive reactions were carried out in flamedried glassware using magnetic stirring under a positive pressure of nitrogen. Removal of organic solvents was accomplished by rotary evaporation and is referred to as concentrated in vacuo. Flash column chromatography was performed using forced flow on 230-400 mesh silica gel purchased from Dynamic Adsorbents, Inc. NMR spectra were recorded on Bruker DRX400, DRX500, and AVANCE600 FT-NMR instruments. Proton and carbon NMR spectra were recorded in ppm and were

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referenced to the indicated solvents. Fluorine NMR spectra were recorded in ppm and were referenced to CFCl3. Data were reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet), integration, and coupling constant(s) in hertz (Hz). Multiplets (m) were reported over the range (ppm) at which they appear at the indicated field strength. Elemental analysis was performed by Atlantic Microlab, Norcross, GA. High-resolution mass spectrometry (HR-MS) was recorded on a Micromass LCT or a Micromass Autospec. Infrared spectrometry was performed by the thin-film method with a Prospect PRS-102 spectrometer from Midac Corp. High-pressure polymerizations were conducted in a mechanically stirred water-cooled 600 mL Parr autoclave. Materials. Toluene, tetrahydrofuran (THF), diethyl ether, and dichloromethane were purified by passing through solvent purification columns and are referred to herein as dry.35 Pentane was dried by distillation from the solution with sodium benzophenone ketyl. 1,2-Dichlorobenzene was purified by passage through activated basic alumina under a nitrogen atmosphere. Unless otherwise stated, all other solvents and reagents were purchased from commercial suppliers and used as received. Grubbs second-generation catalyst was generously donated by Materia Inc. All catalysts were stored in a glovebox under a nitrogen atmosphere. The syntheses and characterization of the nonfluorinated cyclophane ligand 1 and Pd(II) complex 5 can be found in the literature.7,8 2-Dicyclohexylphosphino-2 0 ,6 0 -dimethoxybiphenyl (S-Phos) 36 and sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAr 4)37 were prepared by known literature procedures. Spectral data for the BAr4 anion can be found elsewhere and will not be repeated in the spectroscopic data below.27 2-(3,5-Difluorophenyl)-1,3-dioxolane (9). 1,3-Propanediol (48.33 mL, 50.89 g, 669 mmol) was added to a solution of 3,5-difluorobenzaldehyde (47.53 g, 334 mmol) and 500 mg of p-toluenesulfonic acid monohydrate in 100 mL of toluene. The flask was equipped with a Dean-Stark trap and heated to reflux with stirring for 4 h, cooled, and poured into 100 mL of water and 100 mL of saturated NaHCO3 solution. The organic phase was separated, and the aqueous phase was extracted with diethyl ether (2 100 mL). The organic phases were combined, washed with water, and concentrated in vacuo. Vacuum distillation afforded 9 as a colorless oil (63.64 g, 95%): 1H NMR (400 MHz, CDCl3) δ 1.42 (d, 1H, J=13.6 Hz), 2.17 (m, 1H), 3.94 (t, 2H, J=12.1 Hz), 4.20 4.24 (m, 2H), 5.42 (s, 1H), 6.75 (tt, 1H, JHF=8.8 Hz, J=2.3 Hz), 7.01 (pseudo d, 2H, JHF=8.0 Hz); 13C NMR (125 MHz, CDCl3) δ 25.8, 67.5, 99.88 (d, 4JCF=1.3 Hz), 104.1 (t, 2JCF=23.9 Hz), 109.4 (dd, 2JCF=20.1 Hz, 4JCF=6.3 Hz), 142.7 (t, 3JCF=10.1 Hz), 163.1 (dd, 1JCF=249.0 Hz, 3JCF=12.6 Hz). 1,3-Dibromo-5-methyl-2-nitrobenzene (10).38 2,6-Dibromo-4methylaniline (22.64 g, 85.5 mmol) and 3-chloroperoxybenzoic acid (84.26 g, 70 wt %, 342 mmol) were dissolved in 600 mL of CH2Cl2 and heated to reflux for 6 h. The mixture was cooled in an ice bath and filtered. Then the filtrate was washed with 1.0 N KOH solution (4 200 mL). The organic phase was concentrated in vacuo to afford a tan solid. The residue was redissolved in 350 mL of glacial acetic acid, and to this was added 120 mL of 30% hydrogen peroxide solution and 22 mL of concentrated nitric acid. The mixture was heated to reflux for 2 h, then cooled and poured into 2.0 L of ice water. The suspension was left to stand overnight, then filtered, and the solid was washed with water, then dried under suction. Recrystallization in 99:1 hexanes/methanol afforded 10 as a pale yellow solid (20.32 g, 81%): 1H NMR (400 MHz, CDCl3) δ 2.38 (s, 3H), 7.42 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 21.1, (35) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520. (36) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc. 2005, 127, 4685–4696. (37) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579– 3581. (38) Shen, D.; Diele, S.; Pelzl, G.; Wirth, I.; Tschierske, C. J. Mater. Chem. 1999, 9, 661–672.

Popeney et al. 113.6, 133.6, 143.3, 149.8. Anal. Calcd for C7H5NO2Br2: C, 28.51; H, 1.71; N, 4.75. Found: C, 28.59; H, 1.65; N, 4.66. F-Terphenyl Nitro Diacetal (11). To a solution of 9 (8.38 mL, 9.987 g, 49.89 mmol) in 100 mL of dry THF cooled to -78 °C was dropwise added 18.86 mL of a 2.86 M solution of butyllithium in hexanes (53.93 mmol). After 1 h, anhydrous zinc chloride (7.354 g, 53.93 mmol) was added with rapid stirring, and the mixture was slowly warmed to room temperature over 1 h. The mixture was transferred via cannula to a solution of 10 (6.628 g, 22.47 mmol), S-Phos (185 mg, 0.449 mmol), and Pd2(dba)3 (206 mg, 0.225 mmol) in dry THF (40 mL), and the mixture was heated to reflux overnight. The reaction was cooled, and 200 mL of water and 50 mL of saturated NH4Cl solution were added. The organic layer was separated and the aqueous layer extracted twice with diethyl ether (100 mL). The organic phases were recombined, washed with brine, and dried over MgSO4. Purification by flash chromatography in 4:1 hexanes/ethyl acetate afforded 11 as a tan solid (6.84 g, 57%): 1 H NMR (500 MHz, CDCl3) δ 1.46 (m, 2H), 2.21 (m, 2H), 2.49 (s, 3H), 3.99 (td, 4H, J=12.1, 1.2 Hz), 4.27 (dd, 4H, J=11.5, 4.8 Hz), 5.48 (s, 2H), 7.13 (d, 4H, JHF =7.8 Hz), 7.26 (s, 2H); 13C NMR (125 MHz, CDCl3) δ 21.3, 25.6, 67.4, 99.4, 109.5 (dd, 2 JCF=21.3 Hz, 4JCF=6.0 Hz), 114.2 (t, 2JCF=20.3 Hz), 124.1, 133.5, 141.76, 142.08 (t, 3JCF=9.1 Hz), 148.2, 159.6 (dd, 1JCF= 249.2 Hz, 3JCF = 6.9 Hz); 19F NMR (376 MHz, CDCl3) δ 112.14 (d, JHF = 7.8 Hz). Anal. Calcd for C27H23NO6F4: C, 60.79; H, 4.35; N, 2.63. Found: C, 60.11; H, 4.41; N, 2.49. F-Terphenyl Nitro Dialdehyde (12). A solution of diacetal 11 (5.00 g, 9.37 mmol) in 90 mL of glacial acetic acid and 15 mL of water was heated to reflux for 1 h, then cooled and concentrated in vacuo to 20 mL. The mixture was neutralized with saturated NaHCO3 solution, and water (50 mL) was added and extracted with diethyl ether (3100 mL). The organic layers were combined, washed once with water and once with brine, and dried over MgSO4. The solution was concentrated in vacuo to 10 mL and stored at -30 °C for 2 h. The mixture was decanted, and the remaining crystals were dried in vacuo to afford the dialdehyde 12 as pale brown crystals (2.88 g, 74%): 1H NMR (500 MHz, CDCl3) δ 2.55 (s, 3H), 7.39 (s, 2H), 7.53 (d, 4H, JHF =6.5 Hz), 9.98 (s, 2H). 13C NMR (125 MHz, CDCl3) δ 21.4, 112.4 (dd, 2J CF=20.1 Hz, 4JCF =5.0 Hz), 120.2 (t, 2JCF =20.4 Hz), 123.8, 133.8, 141.76, 138.3 (t, 3JCF =7.4 Hz), 142.9, 160.1 (dd, 1JCF = 252.9 Hz, 3JCF =6.0 Hz), 189.0; 19F NMR (376 MHz, CDCl3) δ -109.60 (d, JHF=6.2 Hz); HR-MS calcd for [C21H11NO4F4 H]- 416.0546, found 416.0544. F-Terphenyl Nitro Diolefin (13). In a separate flask, a suspension of 8.37 g of methyltriphenylphosphonium bromide (23.4 mmol) and potassium tert-butoxide (2.98 g, 8.15 mmol) in 1500 mL of THF was stirred for 1 h at room temperature, then cooled to -78 °C. A solution of dialdehyde 12 (3.26 g, 7.81 mmol) in 100 mL of THF was added dropwise over 15 min, and the resulting mixture was stirred at - 78 °C for 4 h, then slowly allowed to warm to 0 °C over 1.5 h. The reaction was quenched with 150 mL of water and 150 mL of saturated NH4Cl solution, and the organic layer was separated and concentrated to 200 mL. The aqueous layer was extracted twice with diethyl ether (50 mL), and the organic layers were combined, washed with brine, and dried over MgSO4. The solvent was removed in vacuo and the product purified by flash chromatography in 9:1 hexanes/ethyl acetate to afford 13 as a light brown solid (2.46 g, 76%): 1H NMR (500 MHz, CDCl3) δ 2.51 (s, 3H), 5.42 (d, 2H, J=10.8 Hz), 5.81 (d, 2H, J=17.5 Hz), 6.66 (dd, 2H, J=17.5, 10.8 Hz), 7.02 (d, 4H, JHF =8.3 Hz), 7.33 (s, 2H); 13C NMR (125 MHz, CDCl3) δ 21.5, 109.4 (dd, 2JCF=21.4 Hz, 4JCF = 6.3 Hz), 113.3 (t, 2JCF = 21.4 Hz), 117.4, 124.3, 133.7, 134.8, 140.8 (t, 3JCF =10.1 Hz), 141.9, 148.4, 160.1 (dd, 1 JCF =247.8 Hz, 3JCF =7.5 Hz); 19F NMR (376 MHz, CDCl3) δ -112.93 (d, JHF=8.2 Hz). Anal. Calcd for C23H15NO2F4: C, 66.83; H, 3.66; N, 3.39. Found: C, 67.06; H, 3.80; N, 3.27.

Article F-Terphenyl Aniline Diolefin (14). To a mixture of diolefin 13 (2.09 g, 5.06 mmol) and tin(II) chloride dihydrate (6.85 g, 30.4 mmol) in 200 mL of ethanol was added 15 mL of concentrated hydrochloric acid. The mixture was heated to reflux and stirred for 8 h. The organic solvent was removed in vacuo, 200 mL of water was added, and the mixture was made strongly basic by addition of solid potassium hydroxide. The mixture was extracted with CH2Cl2, and the organic layer was separated, washed once with water, and concentrated in vacuo. The residue was recrystallized in 5:1 ethanol/CH2Cl2 to afford 14 as a light tan solid identifiable by pale blue fluorescence at 365 nm (1.715 g, 79%): 1H NMR (500 MHz, CDCl3) δ 2.34 (s, 3H), 3.47 (br s, 2H), 5.42 (d, 2H, J=10.8 Hz), 5.83 (d, 2H, J=17.5 Hz), 6.69 (dd, 2H, J=17.5, 10.8 Hz), 7.03 (s, 2H), 7.07 (d, 4H, JHF =8.0 Hz); 13C NMR (125 MHz, CDCl3) δ 20.4, 109.5 (dd, 2 JCF=20.1 Hz, 4JCF=5.0 Hz), 114.6 (t, 2JCF =21.7 Hz), 115.4, 116.7, 127.3, 132.6, 134.8, 139.9 (t, 3JCF =9.7 Hz), 140.7, 160.7 (dd, 1JCF = 247.8 Hz, 3JCF = 8.8 Hz); 19F NMR (376 MHz, CDCl3) δ -111.36 (d, JHF = 7.9 Hz). Anal. Calcd for C23H17NF4: C, 72.06; H, 4.47; N, 3.65. Found: C, 71.86; H, 4.50; N, 3.62. Open F-Diimine (15). Aniline 14 (1.538 g, 4.01 mmol), acenaphthenequinone (329 mg, 1.81 mmol), and 80 mg of pTSA were reacted in 70 mL of benzene for two days, then purified by flash chromatography in 6:1 hexanes/ethyl acetate to afford 15 as an orange solid (782 mg, 47%): 1H NMR (500 MHz, CDCl3) δ 2.43 (s, 3H), 5.25 (d, 4H, J=10.8 Hz), 5.59 (d, 4H, J=17.5 Hz), 6.43 - 6.50 (m, 8H), 6.64 (d, 4H, JHF = 9.7 Hz), 6.95 (d, 2H, J=7.1 Hz), 7.16 (s, 4H), 7.28 (t, 2H, J=8.2 Hz), 7.68 (d, 2H, J= 8.2 Hz); 13C NMR (125 MHz, CDCl3) δ 20.8, 108.2 (d, 2JCF = 26.4 Hz), 109.2 (d, 2JCF = 23.9 Hz), 115.7 (t, 2JCF = 20.1 Hz), 116.0, 118.9, 127.0, 127.99, 128.36, 130.2, 132.6, 133.5, 135.0, 138.6 (t, 3JCF =10.1 Hz), 139.9, 147.1, 160.3 (dd, 1JCF =245.3, 3 JCF = 8.3 Hz), 160.7, 161.6; 19F NMR (376 MHz, CDCl3) δ -112.88 (dd, J=8.2, 5.6 Hz), -108.18 (dd, J=9.1, 5.6 Hz). Anal. Calcd for C58H36N2F8: C, 76.31; H, 3.97; N, 3.07. Found: C, 76.48; H, 4.11; N, 3.02. F-Cyclophane, Unhydrogenated (16). Diimine 15 (257 mg, 0.282 mmol) and Grubbs catalyst (45 mg) were reacted in 280 mL of degassed CH2Cl2 for 3 h, concentrated in vacuo, then purified by flash chromatography in CH2Cl2 to afford 16 as a yellow solid (157 mg, 65%): 1H NMR (500 MHz, CDCl3) δ 2.47 (s, 6H), 6.14 (d, 4H, JHF =9.7 Hz), 6.54 (d, 4H, JHF =9.2 Hz), 6.83 (s, 4H), 6.85 (d, 2H, J=7.3 Hz), 7.25 (s, 4H), 7.34 (t, 2H, J= 7.3 Hz), 7.80 (d, 2H, J=8.3 Hz); 13C NMR (125 MHz, CDCl3) δ 20.8, 111.2 (dd, 2JCF=23.1 Hz, 4JCF=3.2 Hz), 112.8 (dd, 2JCF= 23.1 Hz, 4JCF =3.2 Hz), 114.2 (t, 2JCF =21.4 Hz), 118.7, 123.8, 127.0, 128.4, 129.2, 130.3, 132.69, 132.85, 133.15, 139.1 (t, 3JCF= 10.1 Hz), 140.2, 148.3, 159.26 (dd, 1JCF = 251.5 Hz, 3JCF = 8.2 Hz), 159.94 (dd, 1JCF =247.8 Hz, 3JCF=7.9 Hz), 162.8; 19F NMR (376 MHz, CDCl3) δ -112.32 (dd, J = 9.4, 6.2 Hz), -108.90 (dd, J=8.7, 6.2 Hz); HR-MS calcd for [C54H28N2F8+ H]+ 857.2203, found 857.2202. F-Cyclophane, Hydrogenated (F-Cyc, 2). Palladium on activated carbon (20 mg, 10 wt % Pd) was added to a N2-degassed solution of diimine 16 (133 mg, 0.155 mmol) in 10 mL of methanol, 10 mL of CH2Cl2, and 0.5 mL of triethylamine. The mixture was purged with hydrogen for 5 min, stirred under a hydrogen atmosphere for 1 h at room temperature, filtered through Celite, and concentrated in vacuo. The residue was washed with 10 mL of methanol and dried overnight under high vacuum to afford 2 as a yellow solid (126 mg, 94%): 1H NMR (500 MHz, CDCl3) δ 2.48 (s, 6H), 2.86 - 3.07 (m, 8H), 6.16 (d, 4H, J=9.5 Hz), 6.57 (d, 2H, J=7.2 Hz), 6.61 (d, 4H, J=9.1 Hz), 7.19 (s, 4H), 7.33 (t, 2H, J=7.7 Hz), 7.81 (d, 2H, J=8.3 Hz); 13C NMR (125 MHz, CDCl3) δ 21.0, 33.6, 111.0 (dd, 2JCF=22.2 Hz, 4 JCF=3.2 Hz), 112.5 (dd, 2JCF=22.2 Hz, 4JCF=3.2 Hz), 113.2 (t, 2 JCF = 21.7 Hz), 119.0, 125.1, 126.9, 128.30, 128.65, 130.5, 132.71, 133.13, 140.63, 141.28 (t, 3JCF =8.8 Hz), 149.4, 159.67 (dd, 1JCF=250.6 Hz, 3JCF=9.3 Hz), 160.26 (dd, 1JCF=246.5 Hz,


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JCF =7.4 Hz), 162.5; 19F NMR (376 MHz, CDCl3) δ -113.12 (t, J=7.0 Hz), -109.54 (dd, J=9,1, 6.3 Hz); HR-MS calcd for [C54H32N2F8 + Na]+ 883.2335, found 883.2359. [(Cyc)Ni(acac)]B(C6F5)4 (3). 19 A scintillation vial was charged with diimine 1 (54 mg, 0.075 mmol), trityl tetrakis(pentafluorophenyl)borate (70 mg, 0.075 mmol), and Ni(acac)2 (19 mg, 0.075 mmol). To this was added 2 mL of dry CH2Cl2 and 8 mL of dry diethyl ether, and the reaction was stirred for 1 h at room temperature. Pentane (10 mL) was added, the mixture was decanted, and the solid was washed with 10 mL of diethyl ether and 10 mL of pentane. The solid was dried for two days in vacuo to afford 3 as a dark red solid (72 mg, 93%): 1H NMR (500 MHz, CD2Cl2) δ 1.79 (s, 6H), 2.38 (s, 6 H), 3.03 - 3.22 (m, 8H), 5.56 (s, 1H), 6.44 (d, 4H, J=7.8 Hz), 6.60 (d, 4H, J= 7.8 Hz), 6.85 (d, 2H, J=7.2 Hz), 7.11 (s, 4H), 7.24 (d, 4H, J= 7.8 Hz), 7.30 (d, 4H, J=7.8 Hz), 7.67 (t, 2H, J=8.2 Hz), 8.22 (d, 2H, J=8.0 Hz); 13C NMR (125 MHz, CD2Cl2) δ 21.5, 25.1, 35.2, 102.7, 125.4, 126.84, 127.58, 128.84, 129.68, 130.29, 130.81, 131.83, 132.20, 133.63, 135.85, 135.97, 137.0, 139.3, 140.8, 173.7, 187.8. Anal. Calcd for C83H47N2O2BF20Ni: C, 64.16; H, 3.05; N, 1.80. Found: C, 64.38; H, 3.08; N, 1.74. [(F-Cyc)Ni(acac)]B(C6F5)4 (4). 19 A scintillation vial was charged with diimine 2 (43 mg, 0.050 mmol), trityl tetrakis (pentafluorophenyl)borate (46 mg, 0.050 mmol), and Ni(II) acetylacetonate (13 mg, 0.050 mmol). To this was added 10 mL of dry CH2Cl2, and the reaction was stirred for 1 h at room temperature. Pentane (10 mL) was added, and the mixture was decanted. The solid was suspended in 10 mL of diethyl ether precipitated with 10 mL of pentane, decanted, and dried for two days in vacuo to afford 4 as a dark red solid (45 mg, 58%): 1 H NMR (500 MHz, CDCl3) δ 1.65 (s, 6H), 2.44 (s, 6H), 3.13 3.26 (m, 8H), 5.38 (s, 1H), 6.38 (d, 4H, JHF=9.9 Hz), 6.85 (d, 4H, JHF=9.8 Hz), 7.09 - 7.13 (m, 2H), 7.21 (s, 4H), 7.57 (t, 2H, J= 8.0 Hz), 8.18 (d, 2H, J=8.3 Hz); 13C NMR (125 MHz, CDCl3) δ 21.2, 23.7, 33.0, 102.0, 111.38 (t, 2JCF =21.7 Hz), 112.37 (dd, 2 JCF=22.9 Hz, 4JCF=3.2 Hz), 113.24 (dd, 2JCF=22.2 Hz, 4JCF= 3.2 Hz), 122.8, 125.0, 128.2, 129.4 (t, 3JCF = 7.9 Hz), 130.8, 133.54, 133.85, 138.57, 139.12, 143.7 (t, 3JCF =9.7 Hz), 159.64 (dd, 1JCF=249.7 Hz, 3JCF=7.9 Hz), 159.75 (dd, 1JCF=248.3 Hz, 3 JCF =7.9 Hz), 174.0, 187.2; 19F NMR (376 MHz, CDCl3) δ 109.45, -108.03. Anal. Calcd for C83H39N2O2BF28Ni: C, 58.72; H, 2.32; N, 1.65. Found: C, 58.74; H, 2.58; N, 1.66. [(Cyc)PdMe(NCMe)]BAr4, Ar = 3,5-(CF3)2C6H3 (6). An oven-dried flask was charged with 26 mg of (Cyc)PdMeCl7 (0.030 mmol) and 27 mg of NaBAr4 (0.030 mmol). To this was added 5 mL of CH2Cl2 and 1.0 mL of acetonitrile, and the mixture was stirred overnight at room temperature. The mixture was filtered through Celite and concentrated in vacuo to 1 mL. Pentanes (15 mL) were added, the mixture was left to stand for 30 min, and the supernatant was decanted. The solid residue was dried for 24 h in vacuo to afford 6 as a rust-colored solid (46 mg, 88%). Representative 2D NMR spectra and NOE measurements used during assignment are given in the Supporting Information. 1H NMR (500 MHz, CD2Cl2) δ 0.72 (s, 3H, PdMe), 2.20 (s, 3H, NCMe), 2.45 (s, 3H, Ar-Me), 2.46 (s, 3H, ArMe), 2.81 - 3.22 (m, 8H, N), 6.23 (m, 4H, L/M), 6.50 (d, 1H, J= 7.2 Hz, K), 6.59 (d, 2H, J=7.8 Hz, J), 6.75 (d, 2H, J=7.9 Hz, I), 6.89 (d, 2H, J=7.9 Hz, H), 6.97 (d, 2H, J=7.2 Hz, G), 7.16 (d, 1H, J=8.0 Hz, E), 7.17 (s, 2H, F), 7.19 (s, 2H, F), 7.21 (d, 2H, J=7.8 Hz, D), 7.26 (d, 2H, J=7.9 Hz, A), 7.57-7.62 (m, 2H, C), 8.12 (d, 1H, J=8.3 Hz, B0 ), 8.15 (d, 1H, J=8.3 Hz, B); 13C NMR (125 MHz, CD2Cl2) δ 3.9 (NCMe), 7.0 (Pd-Me), 21.27 (Ar-Me), 21.33 (Ar-Me), 34.95 (af), 35.01 (af), 121.79 (NCMe), 125.70 (ae), 126.52 (ab), 126.90 (ad), 127.80 (aa), 127.91 (v/ac), 128.66 (y), 128.96 (n), 129.37 (z), 129.56 (x), 129.76 (t), 129.83 (u), 130.01 (w), 130.49 (s), 131.46 (o), 131.88 (m), 131.94 (q), 132.30 (p), 133.09 (r), 135.03 (j/j’), 135.26 (l), 135.42 (k), 138.05 (h), 138.68 (i), 138.81 (d), 140.37 (f/g), 140.41 (f/g), 140.75 (e), 145.25 (c), 169.7 (b), 176.3 (a). Anal. Calcd for 3

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C89H58N3PdBF24: C, 61.34; H, 3.35; N, 2.41. Found: C, 61.64; H, 3.32; N, 2.39. [(F-Cyc)PdMe(NCMe)]BAr4, Ar = 3,5-(CF3)2C6H3 (7). An oven-dried flask was charged with 47 mg of Pd(COD)MeCl (0.18 mmol), 153 mg of diimine 2 (0.18 mmol), and 158 mg of NaBAr4 (0.18 mmol). To this was added 10 mL of dry CH2Cl2 and 1.0 mL of acetonitrile. The mixture was stirred for 10 h at room temperature, then filtered through Celite. The solution was concentrated to 2 mL, pentanes were added, and the mixture was left to stand for 1 h. The supernatant was decanted, and the solid residue was dried in vacuo for 24 h to afford 225 mg (75% yield by weight) of a mixture of 7 (75%) and side product 70 (25%). Anal. Calcd for C89H50N3PdBF32: C, 56.66; H, 2.67; N, 2.23. Found: C, 57.17; H, 2.71; N, 2.09. The following spectra are separately given for cations of 7 and 70 . Assignments for the cation 7 were corroborated by HMQC, HMBC, and 1H-decoupled 19F methods. The characterization of side product 70 was incomplete because of the small quantities present. [(F-Cyc)PdMe(NCMe)]BAr4, Ar=3,5-(CF3)2C6H3 (7): Characterization of 7 was aided by 1H NMR with selective 19F decoupling. Standard 1H pulses were applied over a continuous weak 19F decoupling signal targeting a single fluorine resonance. Assignments of carbons were aided by the 2D methods HMQC and HMBC. 1H NMR (500 MHz, CD2Cl2) δ 0.80 (t, 3H, JHF= 4.5 Hz, Pd-Me), 2.00 (s, 3H, NCMe), 2.48 (s, 3H, Ar-Me), 2.50 (s, 3H, Ar-Me), 3.02 - 3.24 (m, 8H, HJ), 6.25 (d, 2H, JHF = 10.1 Hz, HI), 6.37 (d, 2H, JHF=9.7 Hz, HH), 6.74 (d, 2H, JHF= 9.8 Hz, HG), 6.79 (d, 2H, JHF = 9.7 Hz, HF), 6.88 (dt, 1H, JHH =7.3 Hz, JHF =4.6 Hz, HE), 7.00 (dt, 1H, JHH =7.3 Hz, JHF=4.2 Hz, HD), 7.29 (s, 2H, HC’), 7.33 (s, 2H, HC), 7.46 - 7.54 (m, 2H, partially obscured by BAF-, HB), 8.11 (d, 1H, J = 8.3 Hz, HA), 8.13 (d, 1H, J=8.3 Hz, HA0 ); 13C NMR (125 MHz, CD2Cl2) δ 3.0 (t, JCF=2.3 Hz, NCMe), 8.0 (t, JCF=10.6 Hz, PdMe), 21.14 (Ar-Me), 21.26 (Ar-Me), 32.91 (af), 33.01 (af), 111.22 (t, 2JCF=20.8 Hz, ae), 111.73 (t, 2JCF=21.3 Hz, ad), 112.66 (dd, 2 JCF =22.2 Hz, 4JCF =3.4 Hz, ac), 112.86 (dd, 2JCF =21.8 Hz, 4 JCF=3.2 Hz, ab), 112.95 (dd, 2JCF=22.2 Hz, 4JCF=3.2 Hz, aa), 113.14 (dd, 2JCF =22.2 Hz, 4JCF =2.3 Hz, z), 121.43 (NCMe), 122.33 (y), 123.96 (t, JCF=2.5 Hz, x), 124.22 (w), 125.03 (t, JCF= 4.4 Hz, v), 128.09 (t/u), 128.20 (t/u), 128.58 (t, JCF =5.1 Hz, s), 129.76 (t, JCF = 6.6 Hz, r), 130.98 (q), 132.64 (p), 133.21 (o), 134.11 (n), 134.77 (m), 138.33 (k/l), 139.38 (k/l), 142.00 (j), 142.86 (i), 143.87 (t, 3JCF =9.4 Hz, h/h0 ), 143.92 (t, 3JCF =9.4 Hz, h/h0 ), 146.37 (g), 159.53 (dd, 1JCF=247.8 Hz, 3JCF=8.3 Hz, f), 159.61 (dd, 2JCF = 245.6 Hz, 4JCF = 7.9 Hz, e), 160.29 (dd, 2 JCF=247.8 Hz, 4JCF=6.0 Hz, d), 160.33 (dd, 2JCF=248.3 Hz, 4 JCF = 6.9 Hz, c), 171.2 (b), 177.3 (a); 19F NMR (376 MHz, CD2Cl2) δ -110.34 (dd, 2F, 3JFH=9.8 Hz, 4JFF=4.9 Hz, FD), 110.07 (ddd, 2F, 3JFH=9.7 Hz, 4JFF=4.9 Hz, JFH=4.2 Hz, FC), -107.70 (ddd, 2F, 3JFH=10.1 Hz, JFH=4.6 Hz, 4JFF=3.9 Hz, FB), -107.50 (ddd, 2F, 3JFH = 9.7 Hz, JFH = 4.5 Hz, 4 JFF =3.9 Hz, FA); IR (thin film) 2175.3 cm-1 [ν(CtN)]. Side product (70 ): 1H NMR (500 MHz, CD2Cl2) δ 2.50 (s, 3H), 2.85 - 3.05 (m, 8H, partially obscured by 7), 6.26 (d, 4H, JHF= 9.2 Hz), 6.61 (d, 4H, JHF=10.3 Hz), 6.64 (d, 2H, J=7.3 Hz), 7.32 (s, 4H), 7.40 (t, 2H, J = 7.8 Hz), 7.94 (d, 2H, J= 8.1 Hz); 19F NMR (376 MHz, CD2Cl2) δ -112.09 (br), -117.79 (br).

[(F-Cyc)PdMe(NCAr)]BAr4, Ar = 3,5-(CF3)2C6H3 (8). An oven-dried flask was charged with 16 mg of Pd(COD)MeCl (0.058 mmol), 50 mg of diimine 2 (0.058 mmol), and 52 mg of NaBAr4 (0.058 mmol). To this was added 10 mL of dry CH2Cl2

Popeney et al. and 0.10 mL of 3,5-bis(trifluoromethyl)benzonitrile (140 mg, 0.59 mmol). The mixture was stirred for 10 h at room temperature, then filtered through Celite. The solution was concentrated to 1 mL, pentanes were added, and the mixture was left to stand for 1 h. The supernatant was decanted and the solid residue was dried in vacuo for 24 h to afford 8 as an orange solid (107 mg, 88%): 1H NMR (500 MHz, CD2Cl2) δ 0.97 (t, 3H, JHF=4.3 Hz), 2.47 (s, 3H), 2.51 (s, 3H), 3.02 - 3.20 (m, 8H), 6.26 (d, 2H, 3JHF= 9.8 Hz), 6.39 (d, 2H, 3JHF=9.8 Hz), 6.73 (d, 2H, 3JHF=10.0 Hz), 6.80 (d, 1H, 3JHF =9.9 Hz), 6.88-6.94 (m, 1H), 7.03-7.09 (m, 1H), 7.34 (s, 2H), 7.36 (s, 2H), 7.48-7.56 (m, 2H, obscured by BAF-), 8.14 (pseudo t, 2H, J=8.3 Hz), 8.20 (s, 1H); 13C NMR (125 MHz, CD2Cl2) δ 9.5 (t, JCF =9.7 Hz), 20.93, 21.30, 32.88, 32.97, 111.34 (t, 2JCF = 20.8 Hz), 111.51 (t, 2JCF = 21.3 Hz), 112.89 (dd, 2JCF =22.6 Hz, 4JCF =2.9 Hz), 113.16 (dd, 2JCF = 21.8 Hz, 4JCF = 2.8 Hz), 113.23 (dd, 2JCF = 22.7 Hz, 4JCF = 2.9 Hz), 113.28 (dd, 2JCF = 22.2 Hz, 4JCF = 2.9 Hz), 119.22, 119.72, 122.50 (quartet, 1JCF = 273.3 Hz), 122.59, 123.67 (t, JCF =2.3 Hz), 124.22, 124.86 (t, JCF =4.2 Hz), 128.20, 128.26, 128.81 (t, JCF=5.6 Hz), 128.97 (m, obscured by BAF-), 130.00 (t, JCF=6.9 Hz), 133.06 (septet, 3JCF=3.8 Hz), 133.44 (quartet, 3 JCF =3.7 Hz), 133.52, 134.17, 134.85, 138.90, 139.64, 141.78, 142.82, 143.98 (t, 3JCF = 9.7 Hz), 144.18 (t, 3JCF = 9.3 Hz), 146.69, 159.46 (dd, 1JCF=246.9 Hz, 3JCF=7.9 Hz), 159.53 (dd, 1 JCF =245.5 Hz, 3JCF =7.9 Hz), 160.32 (dd, 1JCF =249.2 Hz, 3 JCF = 7.8 Hz), 160.35 (dd, 2JCF = 247.8 Hz, 4JCF = 7.4 Hz), 171.55, 177.8; 19F NMR (376 MHz, CD2Cl2) δ -110.54 (m, 2F), -109.63 (m, 2F), -107.71 (m, 4F), -63.89 (s, 6F). Anal. Calcd for C96H50N3PdBF38: C, 55.31; H, 2.42; N, 2.02. Found: C, 55.61; H, 2.41; N, 2.15. Estimation of Ethylene Concentration. The solubilities of ethylene in this work was estimated from those in toluene reported by Lee et al.,39 which provided values at 50, 100, and 150 °C at a range of pressures. Solubilities at the desired pressure (200 psi/13.79 bar or 88 psi/6.07 bar) at these temperatures were first obtained by a linear fit of the literature data and then adjusted to other temperatures by quadratic fit. The concentrations were converted from grams of ethylene per gram of solvent to moles of ethylene per liter of solvent, considering 200 mL of toluene was used as solvent and assuming the same volume during the course of the reaction. The calculated values are shown in the Supporting Information, Table S1, and the quadratic fits are shown in the Supporting Information, Figure S3. General Procedure for Ni(II)-Catalyzed Polymerizations. A 600 mL autoclave was heated under vacuum to 120 °C for 2 h, then twice purged with ethylene and adjusted to the desired polymerization temperature by external oil bath. A flask was charged with a desired amount of catalyst 3 or 4 under a nitrogen atmosphere, and to this was added 10 mL of 1,2-dichlorobenzene. In a separate flask was added 80 mL of dry toluene or 1,2dichlorobenzene, the latter solvent if the polymerization was conducted at a temperature higher than 80 °C, and a suitable amount of aluminum coactivator solution in toluene ([Al]/[Ni]= 1500). An additional 10 mL of 1,2-dichlorobenzene was set aside. The solution of Al activator was then transferred into the reactor by vacuum under nitrogen stream. Ethylene was introduced to a pressure of 200 psi, and the mixture was rapidly stirred for 10 min and then vented. The catalyst solution was transferred into the reactor by vacuum under nitrogen stream and flushed with the final 10 mL of 1,2-dichlorobenzene. The reactor was filled with ethylene to 200 psi, and the polymerization was allowed to proceed for the desired time with rapid stirring. The reactor was vented, a mixture of 70 mL of acetone, 30 mL of methanol, and 1 mL of concentrated hydrochloric acid was transferred into the reactor by vacuum, the autoclave was opened, and the polymer was collected, decanted, and dried (39) Lee, L.-s.; Ou, H.-j.; Hsu, H.-l. Fluid Phase Equilib. 2005, 231, 221–230.

Article in vacuo at 80 °C overnight. The reactor was twice cleaned with boiling toluene before subsequent polymerizations. Variable-Temperature Screen. The general procedure for Ni (II)-catalyzed polymerizations was followed using 1.0 μmol of catalyst 4 and triisobutylaluminum (TIBA) as the coactivator (1.5 mL of a 10% w/w solution in toluene, 1.0 M). Polymerizations were run for 10 min at the desired temperature. Thermal Stability Study at 105 °C. The general procedure for Ni(II)-catalyzed polymerizations was followed using 0.5 μmol of catalyst 4 and TIBA as the coactivator (1.5 mL of a 10% w/w solution in toluene, 1.0 M). Polymerizations were started at 110 °C, held there for 1 min, then allowed to cool to 105 °C, and left at that temperature for the duration of the polymerization. General Procedure for Pd(II)-Catalyzed Polymerizations. A 600 mL autoclave was heated under vacuum to 120 °C for 2 h, then twice purged with ethylene and adjusted to the desired polymerization temperature by external oil bath. A flask was charged with the desired amount of catalyst 5, 6, 7, or 8 under a nitrogen atmosphere, and to this was added 20 mL of dry toluene. The catalyst solution was then transferred into the reactor by vacuum under nitrogen stream followed by toluene and comonomer, if any, to bring the total volume up to 100 mL. The reactor was then filled with ethylene to 88 psi, and the polymerization was allowed to proceed for the desired time at the desired temperature. The reactor was then vented, the mixture was concentrated in vacuo, and the polymer was dried for 24 h under vacuum at 70 °C. SEC Characterization of Polymers. All polymers were characterized by size-exclusion chromatography (SEC). All polymers produced by the Pd(II) catalysts were soluble in THF and were eluted through a 30 cm size-exclusion column (Polymer Laboratories PLgel Mixed C, 5 μm particle size) to separate polymer samples. Measurements were made on highly dilute fractions eluting from a SEC consisting of a HP Aglient 1100 solvent delivery system/auto injector with an online solvent degasser, a temperature-controlled column compartment, and an Aglient 1100 differential refractometer. The mobile phase was THF, and the flow rate was 1.0 mL/min. A Dawn DSP 18angle multiangle light scattering (MALS) detector (Wyatt Technology, Santa Barbara, CA) was coupled to the SEC to (40) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999, 283, 2059–2062. (41) Cotts, P. M.; Guan, Z.; McCord, E.; McLain, S. Macromolecules 2000, 33, 6945–6952.


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measure both the MW and sizes for each fraction of the polymer eluted from the SEC column.40,41 Both the column and the differential refractometer were held at 35 °C. A 30 μL sample of a 2 mg/mL solution was injected into the column. ASTRA 4.7 from Wyatt Technology was used to acquire data from the 18 scattering angles (detectors) and the differential refractometer. The Mn and Mw data were obtained by following classical light scattering treatments. All polymers from Ni(II) catalysts were insufficiently soluble in THF and were analyzed by SEC in 1,2,4-trichlorobenzene at 140 °C by the Polymer Characterization Facility at the Cornell Center for Materials Research, Cornell University, Ithaca, NY. The Mn and Mw data were obtained by comparison to polyethylene standards. As mentioned above, polymers from the fluorinated Pd(II) catalyst produced at 60 and 80 °C had their molecular weights accurately determined by MALS. The calculated molecular weights of these polymers by GPC using the polyethylene standards were closer to the actual MALSdetermined value than the values calculated using the more commonly used polystyrene standards, which were roughly a factor of 2 larger. NMR Analysis of Polymers. Branching densities B were calculated by the ratio of methyl group integration to total hydrocarbon integration using 1H NMR. Samples were dissolved in hot 1,1,2,2-tetrachloroethane-d2, and spectra were acquired at 140 °C.

Acknowledgment. We thank the National Science Foundation (Chem-0456719, Chem-0723497, and DMR-0703988) for funding. We thank Materia, Inc. for the generous donation of Grubbs catalyst. C.P. acknowledges the Allergan Foundation, the UCI Department of Chemistry, and Joan Rowland for financial support. We also thank Dr. Phil Dennison for valuable NMR assistance, Chris Levins and Alex Nguyen for aid in ligand synthesis, Yen Kong and Zach Tolstykia (UCLA) for DSC assistance, and Dr. Anthony Condo and Jeffrey Rose of Cornell University for GPC assistance. Supporting Information Available: Selected GPC traces of polyethylenes and estimations of ethylene concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

Polymerization Catalysts Bearing a Fluorinated Cyclophane Ligand

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