Mass Spectrometric Mechanistic Investigation of Ligand Modification

Article pubs.acs.org/Organometallics

Mass Spectrometric Mechanistic Investigation of Ligand Modification in Hafnocene-Catalyzed Olefin Polymerization Anthony P. Gies,*,† Roger L. Kuhlman,‡ Cristiano Zuccaccia,§ Alceo Macchioni,§ and Richard J. Keaton† †

Core R&D, The Dow Chemical Company, 2301 North Brazosport Boulevard, B-1820, Freeport, Texas 77541, United States Univation Technologies, LLC, Freeport, Texas 77541, United States § Department of Chemistry, Biology and Biotechnology, University of Perugia and CIRCC, Via Elce di sotto, 8, I-06123 Perugia, Italy ‡

S Supporting Information *

ABSTRACT: We recently reported evidence that a cyclometalated intermediate can facilitate the polymerization of 1-hexene to append polymer chains to the termini of propyl groups of the Me2Hf(Cpn‑Propyl)2 catalyst precursor. Herein we provide further mechanistic details on the activation of Me2Hf(Cpn‑Propyl)2 by B(C6F5)3 and the polymerization of 1-hexene mainly by applying a battery of mass spectrometry-based techniques. First, a combination of MALDI and CID fragmentation is used to characterize the high molecular mass region (up to 6 kDa) of the isolated poly(1-hexene) material with attached metallocene. The CID fragmentation patterns are explained by relatively low-energy ligand losses and higher energy hydrocarbon chain degradation via C−C bond cleavage and 1,3-hydrogen shift reactions. Further mechanistic insights are gained by investigating 1-hexene polymerization reaction employing a properly 13C-labeled catalyst activated by B(C6F5)3. Mass spectrometry analyses, along with supporting NMR experiments, indicate that polymer chain growth from the propylcyclopentadienyl ligand proceeds via a series of 2,1-insertion ring expansions of the hafnium metallacycle. In contrast, free poly(1-hexene) chains are generated by conventional 1,2-insertions. In addition, six boron-containing species were identified from negative ion mode ESI-QqTOF: [B(C6F5)3]−•, [H−B(C6F5)3]−, [CH3−B(C6F5)3]−, [HO−B(C6F5)3]−, [C6H13− B(C6F5)3]−, and [B(C6F5)4]−.

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INTRODUCTION Polyethylene/1-alkene copolymers are prepared on an industrial scale as the largest volume synthetic polymer.1,2 Polymer properties such as durability, elasticity, and processability are controlled by microstructural details such as molecular weight, molecular weight distribution, and comonomer content of the polymers. In turn, the polymer microstructure is controlled by the catalyst along with the polymerization process.1−5 Due to the increasing use of group IV molecular catalysts, such as metallocenes, to produce tailored polyolefin materials for a variety of applications, there has been considerable focus on identifying the key factors that affect their reactivity and the mechanism of polymerization reactions catalyzed by them.1,2,6−8 A distinguishing feature of molecular catalysts, as compared to conventional Ziegler−Natta or chromium catalysts, is that the polymerization behavior can be better controlled by a proper selection of the ligand environment around the active metal center.1−3 In principle, a pure metallocene catalyst should produce polyethylene with a single Schulz−Flory molecular weight distribution and random incorporation of comonomer.1,2 However, even simple metallocene9,10 and postmetallocene11−19 precursors often generate multimodal polyethylene compositions. Many explanations have been offered for such behavior, often involving interactions with activators and/or supports.20−24 Surprisingly, © XXXX American Chemical Society

little research has been reported regarding activation and polymerization chemistry that may affect the nature of the catalyst’s active site(s), especially during the polymerization reaction itself. We have been exploring such chemistry for the Me2Hf(Cpn‑Propyl)2 catalyst system, which is a simple metallocene but can generate multimodal polymers.10 We recently reported the surprising observation that activation of this metallocene by activators such as B(C6F5)3 in the presence of 1-hexene promotes a series of reactions to append poly(1hexene) to the terminus of metallocene propyl substituents. In this way, new metallocene species containing HfCp(CH2)3‑Poly(1‑hexene) moieties are generated (Scheme 1) that might themselves be active polymerization catalysts. Herein, we report the results of mass spectrometric studies to elucidate peculiar aspects of Me2Hf(Cpn‑Propyl)2 activation by B(C6F5)3 and subsequent 1-hexene polymerization. While these analytical methods are seldom employed for such highly reactive species, we find that their careful application to these systems can be quite informative. First, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used in combination with collisioninduced dissociation (CID) to characterize the various Received: April 16, 2017

A

DOI: 10.1021/acs.organomet.7b00293 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 1. Idealized Mechanism for the Metallocene (Ai) Activation, (Aii) Initiation, (Aiii) Migratory Insertion, and (Aiv) Polymerization of 1-Hexene (via Repeated 1,2-Insertions) and Proposed n-Propyl Metalation and “Self-Modification” Reactions of the Hf-Based Metallocene Catalyst (Bi−Biv)

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observed high mass, [Hf-Cp(CH2)3‑Poly(1‑hexene)]-containing series in the isolated polymer.25−30 Fragmentation mechanisms are proposed to explain the series of [(dithranol) 2 -HfCp(CH2)3‑Poly(1‑hexene)]+ cations and [H-Cp(CH2)3‑Poly(1‑hexene)]+• radical cations observed by CID.29,30 These fragmentation mechanisms require a certain connectivity of enchained 1hexene units, conveying additional mechanistic implications. Building from these detailed characterizations, mass spectrometry methods are applied to an active 1-hexene polymerization solution using a metallocene with 13C-labeled Cpn‑Propyl ligands. This study is particularly informative in three ways: (i) The 13C labels induce “fingerprint” mass shifts to indicate the number of

EXPERIMENTAL SECTION

Materials. The metallocenes Me2Hf(Cpn‑Propyl)2 and Cl2Hf(Cpn‑Propyl)2 were purchased from Boulder Scientific. The 13C-labeled version was synthesized starting from 13CH313CH213CH2Br (purchased from Sigma-Aldrich) to incorporate the labels as described below. 1Hexene was purged with nitrogen, dried over activated molecular sieves, and stored in a nitrogen-filled glovebox. Tris(pentafluorophenyl)borane was obtained from AGC Wakasa. Hafnium tetrachloride was purchased from Strem and methylmagnesium bromide and dicyclopentadiene were purchased from Sigma-Aldrich. All synthetic reactions were performed in a nitrogen-filled glovebox. Conversions and purity were confirmed by 1H and/or 13C analyses using a Bruker Ascend 400 NMR instrument, using benzene-d6, toluene-d8, or chlorobenzene-d5 as solvents. Referencing of chemical shift (ppm) is relative to residual 1H and 13C solvent signals. Synthesis of (n-PrCp)(η5,κ1-C5H4CH2CH2CH2-)Hf(n-Bu). The metallocene dichloride Cl2Hf(Cpn‑Propyl)2 (2.00 g, 4.31 mmol) was dissolved in ether (30 mL) and cooled to −33 °C, and n-BuLi (5.4 mL, 1.6 M hexanes, 8.64 mmol) was added dropwise. The reaction mixture was stirred overnight at ambient temperature. The light brown liquid was filtered to remove fine white solids, and volatiles were removed from the filtrate in vacuo. This intermediate product (nBu)2Hf(Cpn‑Propyl)2 (2.121g, 4.18 mmol) was previously reported.10 To induce the cyclometalation reaction, the dibutyl metallocene was dissolved in toluene (20 mL), and the solution was held at 75 °C overnight, then at 90 °C for an additional 2 h. The toluene was removed in vacuo to yield the product as an oily brown liquid (1.596 g,

Cpn‑(*CH2)3‑H ligands present in the observed peak masses (e.g., 3 Da shift for one Cpn‑(*CH2)3‑H ligand and a 6 Da shift for two Cpn‑(*CH2)3‑H ligands). (ii) The “live” polymerization reaction is examined without polymer isolation by methanol washing. (iii) A combination of MALDI, electrospray, atmospheric pressure photoionization (APPI), and tandem mass spectrometry (MS/ MS) are used to examine different chemical species present within the reaction solution. These results are combined with supporting NMR experiments to create a consistent mechanistic description of this polymerization system. B


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Organometallics

C5H4CH2CH2CH2-)Hf(n-Bu)] (10 mg, 0.0223 mmol) and [CPh3][B(C6F5)4] (20.5 mg, 0.0222 mmol) were introduced into two separate vials, dissolved in 0.3 mL of C6D5Cl, and cooled down to −30 °C. After 1 h, the solutions were mixed. The final solution was slowly warmed up to room temperature and transferred into a J. Young NMR tube for NMR analysis. Relative 1H NMR signal integration with respect to HCPh3 indicates that the title compound is formed as the main species in 45 ± 5% yield. 1H NMR (C6D5Cl, 243 K) (Figure S7): 5.92 (1H, pseudo q, 3JHH = 2.8 Hz), 5.76 (1H, pseudo q, 3JHH = 2.8 Hz), 5.66 (1H, pseudo q, 3JHH = 2.8 Hz), 5.58 (1H, pseudo q, 3JHH = 2.8 Hz), 5.52 (1H, pseudo q, 3JHH = 2.8 Hz), 5.46 (1H, pseudo q, 3JHH = 2.8 Hz), 5.43 (1H, pseudo q, 3JHH = 2.8 Hz), 4.93 (1H, pseudo q, 3 JHH = 2.8 Hz), 2.56 (2H, m), 2.49 (1H, m), 2.17 (1H, m), 2.13 (2H, dd, 3JHH = 6.8 Hz, 3JHH = 8.2 Hz), 1.25 (2H, m), 1.14 (8H, br), 1.10 (1H, m), 0.88 (t, 3JHH = 6.8 Hz), 0.48 (1H, m). 13C{1H} NMR (C6D5Cl, 243 K) (Figure S8): 154.86 (s), 133.61 (s), 115.79 (s), 114.51 (s), 112.63 (s), 111.77 (s), 110.93 (s), 110.53 (s), 109.73 (s), 109.27 (s), 74.61 (s), 42.00 (s), 32.07 (s), 32.03 (s), 29.82 (s), 29.56 (s), 29.43 (s), 29.39 (s), 23.01 (s), 14.33 (s). 19F NMR (C6D5Cl, 243 K): −132.06 (8F, brd), −161.54 (4F, 3JFF = 20.7 Hz), −165.55 (8F, pseudo t). MALDI-TOF/TOF CID Measurements. All samples were analyzed using a Bruker UltrafleXtreme MALDI-TOF/TOF MS (Bruker Daltronics Inc., Billerica, MA) equipped with a 355 nm Nd:YAG laser. Spectra were obtained in the positive ion reflection mode with a mass resolution greater than 20 000 full-width at half-maximum height (fwhm); isotopic resolution was observed throughout the entire mass range detected. The laser intensity was set approximately 10% greater than threshold. Instrument voltages were optimized for each spectrum to achieve the best signal-to-noise ratio. External mass calibration was performed using protein standards (Peptide Mix II) from a Peptide Mass Standard Kit (Bruker Daltronics) and a seven-point calibration method using Bradykinin (clip 1−7) (m = 757.40 Da), Angiotensin II (m = 1046.54 Da), Angiotensin I (m = 1296.68 Da), Substance P (m = 1347.74 Da), ACTH (clip 1−17) (m = 2093.09 Da), ACTH (clip 18− 39) (m = 2465.20 Da), and Somatostatin 28 (m = 3147.47 Da) to yield monoisotopic mass accuracy better than Δm = ±0.05 Da. The instrument was calibrated before each measurement to ensure constant experimental conditions. For CID fragmentation experiments, argon was used as a collision gas at a pressure of 1.5 × 10−6 Torr, and the collision energy amounts to 20 keV.31,32 All spectra were acquired in the reflection mode with a mass resolution greater than 20 000 full-width at half-maximum height (fwhm); isotopic resolution was observed throughout the entire mass range detected. MALDI spectra were run in either a 2,5dihydroxybenzoic acid (DHB; Aldrich) or dithranol (Aldrich) matrix, either doped with sodium trifluoroacetate (NaTFA; Aldrich) or neat. Polymer samples were prepared using the dried-droplet method. Dithranol (20 mg/mL in toluene), sodium trifluoroacetate (when used) (15 mg/mL in THF), and poly(1-hexene) (in toluene) were mixed using the following ratios: 50 μL of dithranol/10 μL of poly(1hexene) solution/1.5 μL of NaTFA. After vortexing the mixture for 30 s, 1 μL of the mixture was pipetted on the MALDI sample plate and allowed to air-dry at room temperature. MS and MS/MS data were processed using Polymerix 3.0 software supplied by Sierra Analytics (Modesto, CA). APPI-QqTOF MS and CID Fragmentation Measurements. All samples were analyzed using a Waters Synapt G2 High-Resolution Mass Spectrometer, equipped with an atmospheric pressure photoionization (APPI) source (Waters Corp., Milford, MA). Mass spectra were obtained in the positive ion mode with the capillary (4000 V), cone (15 V), APPI nebulizer temperature (250 °C), source temperature (110 °C), desolvation chamber (220 °C), and TOF mass analyzer potentials optimized to achieve the best signal-to-noise ratio. A curtain of nitrogen drying gas was utilized to assist in the APPI process. All spectra were acquired in the reflectron mode (resolution “V” mode) of the TOF mass spectrometer at mass resolutions greater than 20 000 fwhm; isotopic resolution was observed throughout the entire mass range detected. External mass calibration was performed using sodium formate and a 15-point calibration method to yield

3.56 mmol, 82.4%), which was stored in a glovebox freezer when not in use. 1H NMR (C7D8) (Figure S1): 6.32 (1H, pseudo q, 3JHH = 2.7 Hz), 5.67 (1H, pseudo q, 3JHH = 2.7 Hz), 5.56 (1H, pseudo q, 3JHH = 2.8 Hz), 5.51 (1H, pseudo q, 3JHH = 2.7 Hz), 5.43 (2H, pseudo t, 3JHH = 2.7 Hz), 5.04 (1H, pseudo q, 3JHH = 2.8 Hz), 4.94 (1H, pseudo q, 3 JHH = 2.8 Hz), 2.68 (2H, m), 2.53 (1H, m), 2.25 (3H, m), 1.56 (1H, m), 1.48 (1H, m), 1.35 (3H, m), 1.03 (3H, t, 3JHH = 7.1 Hz), 0.88 (3H, t, 3JHH = 7.2 Hz), 0.40 (1H, m), 0.25 (1H, m), −0.045 (1H, m), −0.145 (1H, m). 13C{1H} NMR (C7D8) (Figure S2): 140.92 (s), 126.05 (s), 114.18 (s), 110.46 (s), 108.63 (s), 107.89 (s), 107.60 (s), 106.43 (s), 105.60 (s), 102.75 (s), 60.64 (s), 56.94 (s), 39.60 (s), 33.95 (s), 31.85 (s), 30.71 (s), 30.34 (s), 25.05 (s), 13.88 (s), 13.69 (s). Synthesis of Bis(13C3-n-propylcyclopentadienyl)hafnium(IV) Dichloride, Cl2Hf(Cpn‑Pr*)2. Freshly cracked cyclopentadiene (1.00 g, 15.1 mmol) was combined with calcium oxide (1.34 g, 24 mmol), sodium hydroxide (0.96 g, 24 mmol), and diethylene glycol dimethoxyethane (20 mL), and triply 13C-labeled bromopropane (1.51 g, 12.0 mmol) was added to the stirred homogeneous mixture. After stirring for 3 h, the mixture was filtered into a tared jar and the filtrate exposed to light vacuum (4−5 Torr) for 4 h to remove unreacted cyclopentadiene. Sodium (0.20 g. 8.8 mmol) was added, and the mixture was heated to 85 °C overnight while stirring with a glasscoated stirbar. A small amount of residual sodium metal was removed by gravity filtering through a polyethylene frit. The filtrate was cooled to −40 °C, and hafnium tetrachloride (1.28 g, 4.0 mmol) was added. After stirring overnight, the mixture was filtered and solvents removed from the filtrate in vacuo. The white solid obtained was recrystallized from hot hexane to yield the metallocene dichloride product (Yield = 0.884 g, 1.88 mmol, 31% overall yield from bromopropane as limiting reagent). 1H NMR (C6D6) (Figure S3): 5.79 (4H, td, 3JHH = 3 Hz, 3 JCH = 1 Hz), 5.63 (4H, t, 3JHH = 3 Hz), 2.61 (4H, dtdd, 1JCH = 128 Hz, 3JHH = 7 Hz, 2JCH = 4 Hz, 3JCH = 4 Hz), 1.43 (4H, dtqdd, 1JCH = 127 Hz, 3JHH = 7 Hz, 3JHH = 7 Hz, 2JCH = 4 Hz, 2JCH = 4 Hz), 0.77 (6H, dtdd, 1JCH = 125 Hz, 3JHH = 7 Hz, 2JCH = 6 Hz, 3JCH = 4 Hz). 13 C{1H} NMR (C6D6) (Figure S3): 115.47 (s), 110.71 (s), 110.68 (s), 32.45 (d, 1JCC = 34 Hz), 24.39 (t, 1JCC = 34 Hz), 14.02 (d, 1JCC = 34 Hz). Synthesis of Bis(13C3-n-propylcyclopentadienyl)hafnium(IV) Dimethyl, Me2Hf(Cpn‑Pr*)2. The metallocene dichloride (0.200 g, 0.426 mmol) was dissolved in ether (10 mL), MeMgBr (0.28 mL, 3.0 M in ether, 0.85 mmol) was added at room temperature, and the mixture stirred overnight. Hexane (10 mL) was added and the mixture filtered to remove white solids. Volatiles were removed from the filtrate in vacuo to yield the product as a pale yellow, clear oil. Yield = 0.167 g (91.3%). 1H NMR (C6D6) (Figure S4): 5.61 (4H, td, 3JHH = 2.7 Hz, 3JCH = 1.5 Hz), 5.46 (4H, td, 3JHH = 2.7 Hz, 3JCH = 0.7 Hz), 2.35 (4H, dtdd, 1JCH = 126.5 Hz, 3JHH = 7.5 Hz, 2JCH = 4.5 Hz, 3JCH = 4.5 Hz), 1.49 (4H, dtqdd, 1JCH = 127 Hz, 3JHH = 7.5 Hz, 3JHH = 7.5 Hz, 2 JCH = 4 Hz, 2JCH = 4 Hz), 0.88 (4H, dtdd, 1JCH = 127 Hz, 3JHH = 7.5 Hz, 2JCH = 5.5 Hz, 3JCH = 4.5 Hz), −0.30 (6H, s). 13C{1H} NMR (C6D6) (Figure S5): 126.68 (ddd, 1JCC = 45.6 Hz, 2JCC = 4.4 Hz, 3JCC = 1.0 Hz), 110.22 (dd, 2JCC = 3.0 Hz, 3JCC = 1.5 Hz), 107.00 (d, 3JCC = 3.1 Hz), 36.00 (t, 3JCC = 0.6 Hz), 32.03 (dd, 1JCC = 33.6 Hz, 2JCC = 0.7 Hz), 24.88 (dd, 1JCC = 34.6 Hz, 1JCC = 33.6 Hz), 13.90 (dd, 1JCC = 34.6 Hz, 2JCC = 0.7 Hz). Metallocene Catalyzed Synthesis of Poly(1-hexene). 1Hexene was polymerized by combining (labeled) metallocene (0.0151g, 35 μmol) and 1-hexene (0.87 mL, 7.0 mmol) in a vial and cooling to −35 °C in a glovebox freezer then adding B(C6F5)3 (20.4 mg, 39 μmol) dissolved in toluene or toluene-d8 (0.76 g) and stirred as the mixture warmed to room temperature (Scheme 1). An aliquot (50 μL) was diluted in additional toluene (1 mL) for mass spectrometric experiments. Alternatively, the resulting poly(1-hexene) solution was quenched with dilute HCl, dried in vacuo and washed thoroughly with methanol, then again dried in vacuo. These isolated samples were shown by NMR to contain metallocene residues (Figure S6, vide inf ra). Generation of [(n-heptCp)(η5,κ1-C5H4CH2CH2CH2-)Hf][B(C6F5)4]. The cyclometalated metallocene precursor [(n-PrCp)(η5,κ1C


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Organometallics

Table 1. Structural Assignments for the Peaks of Interest Observed in the Positive Ion MALDI-TOF Spectra of Activated Metallocene and Poly(1-hexene) Samplesa

a

Using dithranol as the MALDI matrix.

monoisotopic masses exhibiting a mass accuracy better than Δm = ±0.01 Da. The instrument was calibrated before each measurement to ensure constant experimental conditions. Sample solutions were initially prepared in toluene (35 μg/mL) and were introduced into the APPI interface by direct infusion using a Harvard Apparatus PHD Ultra syringe pump at a flow rate of 50 μL/min. For CID fragmentation experiments, argon was used as a collision gas. Mass spectral data were processed using Polymerix 3.0 software by Sierra Analytics (Modesto, CA).

ESI-QqTOF MS and CID Fragmentation Measurements. All samples were analyzed using a Waters Synapt G2 High Resolution Mass Spectrometer (Waters Corp., Milford, MA). Mass spectra were obtained in both the positive and negative ion modes, with the capillary (4000 V), cone (30 V), source temperature (110 °C), desolvation chamber (250 °C), and TOF mass analyzer potentials optimized to achieve the best signal-to-noise ratio. A curtain of nitrogen drying gas was utilized to assist in the ESI process. All spectra were acquired in the reflectron mode (resolution “V” mode) of the D


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Organometallics

Table 2. Structural Assignments for the Peaks of Interest Observed in the Positive Ion MALDI-TOF Spectra of Poly(1-hexene) Samplesa

a

Using dithranol as the MALDI matrix.

TOF mass spectrometer at mass resolutions greater than 20 000 fwhm; isotopic resolution was observed throughout the entire mass range detected. External mass calibration was performed using sodium formate and a 15-point calibration method. Internal mass calibration was subsequently performed using the peptide leu-enkephalin (TyrGly-Gly-Phe-Leu) to yield monoisotopic masses exhibiting a mass accuracy better than Δm = ±0.001 Da. The instrument was calibrated

before each measurement to ensure constant experimental conditions. Sample solutions were initially prepared in toluene (35 μg/mL), with 50 μL of methanol to assist in the ionization, and introduced into the ESI interface by direct infusion using a Harvard Apparatus PHD Ultra syringe pump at a flow rate of 10 μL/min. For CID fragmentation experiments, argon was used as a collision gas Mass spectral data were E


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Organometallics

Figure 1. MALDI-TOF mass spectrum of the methanol-washed and isolated poly(1-hexene). The MALDI matrix was dithranol. The mass ranges are listed in the tables for ions having different “n” values. Calculated masses are listed in Tables 1 and 2, while observed masses are reported in the figures−their deviations being primarily linked to the MALDI ionization/calibration process.

processed using Polymerix 3.0 software by Sierra Analytics (Modesto, CA). Nomenclature. All figures show structures and peaks labeled according to the following key: (i) Precursor ion peaks are labeled in the x-y format (x = table number, y = structure number) and the table and structure number are followed by the ion which provides the charge added to the oligomers during the MALDI process (e.g., M−, M+, or H+). The activated catalyst is designated with “A,” and “ACy” for the metallacycle intermediate. Structural modifications to poly(1hexene) are designated with “II” (for a vinylidene end group) and “Hy” (for reaction of the metallacycle with a H2O molecule). (ii) Dithranol MALDI matrix/metallocene catalyst precursor ions are labeled with a “D”, and their structures are listed in Table 1. (iii) Fragment ion peaks are labeled with the letter “F” and are numbered in accordance to their respective fragmentation scheme. (iv) The number of repeat units (n) corresponds to the calculated mass numbers found in Table 2. For example, the Cp(CH2)3‑Poly(1‑hexene) precursor ion peak labeled “2-2 II+” corresponds to structure 2-2 in Table 2 that is intrinsically cationic (M+), possesses a vinylidene end group, and some defined number of 1-hexene repeat units (n). The structures are shown in Tables 1 and 2.

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RESULTS AND DISCUSSION The activation and olefin polymerization features of Me2Hf(Cpn‑Propyl)2 were studied by mass spectrometry and NMR using B(C6F5)3 as a molecular well-defined activator and 1hexene, as a liquid olefin, in order to obtain more reliable mechanistic information. Polymerizations of 1-hexene yielded both conventional polyhexene, having “free” all-hydrocarbon polymer chains, and polymer chains attached to a hafnocene fragment. The two types of polymers have not been fractionated or separated. Nonetheless, relatively detailed analyses are possible because the hafnocene centers allow for selective in situ analyses. As a matter of fact, the isolated poly(1hexene) samples show 1H NMR resonances characteristic of metallocene complexes, even after the reaction has been F


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Organometallics quenched with HCl and the product thoroughly washed with methanol (Figure S6).10 These observations were surprising because the conventionally expected Cl2Hf(Cpn‑Propyl)2 is quite soluble in methanol and should have been removed in the washing process. Additional diffusion NMR experiments showed molecular volumes associated with the metallocene resonances that are more consistent with relatively high molecular weight polymers than metallocene molecules.10 To further elaborate these observations, we have analyzed these polymeric samples by a collection of mass spectrometry-based techniques. Traditionally, saturated polyolefins have not been amenable to mass spectral analysis. This is primarily due to their lack of polar, unsaturated, or aromatic groups, which enable the use of conventional ionization techniques, commonly performed through association with sodium or silver cations. A unique feature of the present studies is that the poly(1-hexene) has been inadvertently “labeled” with the ionizable cyclopentadienyl-Hf groups (e.g., [H−Hf(Cpn‑Propyl)Cp(CH2)3‑Poly(1‑hexene)]+), which can be further enhanced through exchange reactions with the dithranol MALDI matrix to form species such as ([(dithranol)2HfCp(CH2)3‑Poly(1‑hexene)]+), vide inf ra. MALDI-TOF MS of the Isolated Poly(1-hexene). Although the low-mass region shows expected dithranol adducts (Figures S9−S11), MALDI analysis of the methanolwashed poly(1-hexene) indeed reveals an 84 Da repeat series in the mass range of 750−6000 Da (bottom spectrum in Figure 1), consistent with a 1-hexene repeat unit. The intrinsically charged [(dithranol)2HfCp(CH2)3‑Poly(1‑hexene)]+ cation series (Structure 2-2 II) is a very good fit for the observed molecular masses, consistent with polymer appended to the metallocene. Furthermore, expansion of the 2501 Da mass peak and its collective isotopes reveals the presence of a hafnium atom in the ionized polymer (cf., the observed and predicted isotopic patterns at the top of Figure 1). MALDI-TOF/TOF CID Fragmentation of the [(Dithranol)2HfCp(CH2)3‑Poly(1‑hexene)]+ Cation (Structure 2-2 II). Having confirmed from MALDI-TOF analysis that the polymer and hafnocene moieties are connected, we next elaborated the atom connectivity within a representative single molecular species selected from the polymeric series. The intrinsically charged [(dithranol)2HfCp(CH2)3‑Poly(1‑hexene)]+ cation (Structure 2-2 II) with mass of 1492 (n = 9) was selected, and CID ionization was used to generate a series of ion fragments. The fragmentation pattern is quite complex, implying multiple degradation pathways. Five different types of mass loss have been identified, and the associated peaks are labeled F1−F5 in Figure 2. The first three ion fragments can be assigned as simple ligand losses from the parent ion. As depicted in Scheme 2, two low energy fragmentations are proposed: (i) loss of Cp(CH2)3‑Poly(1‑hexene) to form (dithranol)2Hf (F1) and (ii) loss of dithranol to generate (dithranol)HfCp(CH2)3‑Poly(1‑hexene) (F2). The latter fragment ion can lose a second dithranol to yield a trace of HfCp(CH2)3‑Poly(1‑hexene) (F3). Additional higher kinetic energy degradation pathways require cleavage of the poly(1-hexene) C−C bonds along the polymer backbone to generate the F4 and F5 series of molecular fragments. As expected, molecular fragmentation is not a simple matter of depolymerizing 1-hexene units from the polymer chain. In fact, neither of these series retains an integral number of 1-hexene repeat units: F4 comprises (dithranol)-

Figure 2. MALDI-TOF/TOF CID fragmentation spectrum of the 1491.9719 Da precursor ion (Structure 2-2 II), from the washed and isolated poly(1-hexene). The MALDI matrix was dithranol.

HfCp(CH2)3‑Poly(1‑hexene) and three additional methylene units, while F5 is made up of (O)HfCp(CH2)3‑Poly(1‑hexene) and three additional methylene units. A mechanistic explanation for the generation of these species is proposed in Scheme 3. Importantly, this mechanism requires a base molecular structure derived from a series of 2,1-insertions of 1-hexene to form the polymeric portion of the molecule. We are unable to derive a feasible mechanism that starts from a structure derived from 1,2-insertions (Scheme S1). Thus, this analysis lends support to the assignment of 2,1-regiochemistry for polymerization initiated at the propyl chain end. The first step in the polymer chain degradation mechanism can be either C−C cleavage along the polymer backbone (Scheme 3Ai), or cleavage of a butyl side chain (Scheme 3Bi). Backbone cleavage generates an all-hydrocarbon primary radical (not observed) and a hafnium-containing secondary radical. The latter undergoes one or more β-scission reactions (Scheme 3Aii) to produce the F4 fragment series. Alternatively, butyl side-chain cleavage forms a midchain secondary radical (Scheme 3Bi) that may subsequently follow any of three pathways. First, it may undergo β-scission toward the metal center (Scheme 3BiiL) to generate a fragment ion that is isobaric with F4. Second, it may undergo β-scission away from the metal center (Scheme 3BiiL) to generate the previously described product of pathway 3Ai, which also ultimately leads to the F4 series. Third, it is likely that the midchain radical may undergo a 1,5-H transfer reaction (Scheme 3Ci) to produce a more stable tertiary radical. This tertiary radical may undergo βscission toward the metal (Scheme 3CiiL) to once again funnel into the generation of F4. Alternatively, it may undergo βscission cleavage away from the hafnium center (Scheme 3CiiR) to generate a (dithranol)HfCp(CH2)3‑Poly(1‑hexene) fragment ion with a methylene end group and a secondary radical poly(1hexene) fragment. However, this fragment is not observed, presumably due to the relatively low statistical probability of formation. The low level fragment series (F5) observed at 763.2, 847.3, and 931.5 Da can be analogously derived from C− C cleavage and β-scission reactions starting from the 1057.8564 Da fragment ion (F3). NMR Study on the Regiochemistry of Metallocycle Insertion Reactions. Evidence for the proposed 2,1-insertion reaction of the cyclometalated cationic complex was provided G


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Organometallics Scheme 2. Proposed Degradation Pathway for Structure 2-2 II (1491.9719 Da)

by reacting (n-PrCp)(η5,κ1-C5H4CH2CH2CH2-)Hf(n-Bu) with [CPh3][B(C6F5)4]. This generates 1 equiv of 1-butene directly at the catalyst center via β-hydride abstraction from a Hf−nBu group,13 thus avoiding the possibility that multiple olefin insertion events occur due to the fact that the first insertion is slower than subsequent ones. The reaction was carried out in C6D5Cl by mixing the reagents at 243 K and allowing the mixture to slowly warm to room temperature. The 1H and 13C NMR spectra, recorded at 243 K, indicate the formation of one main product (yield ca. 50% with respect to the starting metallocene, Supporting Information). The presence of a single CH3 moiety (δH = 0.88 ppm and δC = 14.33 ppm) and nine different methylene units was confirmed via 13C{1H} Jmodulated NMR techniques. Analysis of homo- and heteronuclear scalar 2D NMR correlation experiments provided granularity of the latter into two subsets. One comprises three resonances (δC = 74.61, 42.00, and 29.82 ppm), assigned to a Cp-propyl-metallacycle, in agreement with previous results.10 The other subset consists of six methylene units that are connected to a Cp-ring on one side and to a methyl group on the other side. On the basis of such evidence, the main product is formulated as the ion pair III (Scheme 4), containing nheptyl-substituted and n-propyl-metalated Cp rings. The linear heptyl substituent is likely formed via 2,1-insertion of 1-butene into the Hf−C bond of putative intermediate I to form II,

followed by C−H activation and cyclometalation of the other ring to produce III (Scheme 4). Attempts to detect I and II by low-temperature NMR spectroscopy were unsuccessful. Aliphatic CH moieties could not be detected, ruling out the occurrence of 1,2-insertion which would have resulted in the formation of an ion pair containing a 4-methyl-heptyl substituted Cp ring (Supporting Information). The combination of the results coming from NMR and mass spectrometry provides a clear picture of the polymer chain growth into the propyl substituent by 2,1-insertions. With these observations in hand, we approached the more challenging issue of directly probing active polymerization reactions. To facilitate this, 13C labeled metallocene catalyst was synthesized by employing a cyclopentadiene with a triply labeled propyl group, made from 13CH313CH213CH2Br (see the Experimental Section). 13 C NMR and Mass Spectroscopic Studies of 1-Hexene Polymerization by the Activated 13C-Labeled Metallocene Catalyst. Catalyst activation and 1-hexene polymerization were initially examined by NMR experiments. The labeled propyl groups facilitate the identification of metallocene-derived species by 13C NMR, even in the presence of excess 1-hexene. As previously reported, the initial B(C6F5)3 activation product is the [(Cpn‑Pr)2HfMe(μ-Me) B(C6F5)3] inner-sphere ion pair (bottom, Figure 3), formed by reaction of H


Article

Organometallics Scheme 3. Additional β-Scission Reactions of the Poly(1-hexene) Backbone for Structure 2-2 II (1491.9719 Da)

Scheme 4. Proposed Pathway for the Formation of IIIa

a

The observation of the n-heptyl-substituted product indicates a 2,1-insertion mechanism of the cyclometalated cation complex, followed by cyclometallation of the n-propyl moiety of the other ring.

Similar to the NMR experiments, a series of mass spectrometry techniques were used to study first the 13Clabeled catalyst activation and then the 1-hexene polymerization reaction. The purposes of these experiments include: (1) assessment of the most effective ionization techniques for examining the active 1-hexene polymerization process, (2) characterization of the various molecular species and series of species generated during catalyst activation and 1-hexene polymerization, and (3) detailed structural elaborations to provide insight into the associated mechanistic processes. Positive Ion Mode ESI-QqTOF MS Analysis of the 13CLabeled Activated Catalyst. Electrospray analysis of the 13C-

the metallocene with one equivalent of B(C6F5)3. When 1hexene (ca. 250 equiv) is added, the expected cyclometalated intermediate (Structure 1-1ACy) is observed at an early stage of the polymerization (Figure 3, middle), along with several resonances associated with poly(1-hexene). As the reaction proceeds and additional 1-hexene is added to total 400 equiv, the cyclometalated species is consumed (Figure 3, top), presumably indicating at least one 1-hexene insertion into each of the originally formed cyclometalated intermediates. Of course, cyclometalated species with larger ring sizes may still be present, but the 13C signal intensities are too low to be observed at natural abundance. I


Article

Organometallics

Figure 3. Direct observation of the polymerization.

13

C-labeled cyclometalated intermediate, using

13

C NMR to follow catalyst activation and 1-hexene

ion mode generates a fairly simple spectrum (Figure S16) with only two peaks of interest: a smaller peak for the B(C6F5)3 activator radical anion [B(C6F5)3]−• (511.9902 Da) and a more prominent one for [CH3−B(C6F5)3]− (527.0121 Da). These measurements complement the positive-mode ESI observations for the hafnocene species, again showing the bridging methyl group can retain bonding to either hafnium or boron. Under negative ionizing conditions, there seems to be preference for retaining the boron−methyl bonding. In fact, the spectrum generated by negative ion mode MALDI (Figure S15) shows only [CH3−B(C6F5)3]−. Polymerization of 1-Hexene Using a 13C-Labeled Metallocene Catalyst. Negative Ion Mode ESI-QqTOF MS Analysis of the 13C-Labeled Cp(CH2)3‑Poly(1‑hexene) Reaction Solution: Identification of the Borate Anions. Figure 4 shows the negative ion mode electrospray spectrum of the “live” polymerization reaction after 3 h reaction time at −35 °C. The negative ion mode was utilized to examine the borate anion. There are six species of interest in this spectrum: the B(C6F5)3 activator radical anion [B(C6F5)3]−• (511.9902 Da), [H−B(C6F5)3]− (512.9956 Da), [CH3−B(C6F5)3]− (527.0121 Da), [HO−B(C6F5)3]− (528.9895 Da), [C6H13−B(C6F5)3]− (597.0876 Da), and [B(C6F5)4]− (678.9762 Da). Each of these products can be reasonably formed by conventional metallocene activation chemistry33,34 and associated side reactions. For example, during catalyst activation, B(C6F5)3 abstracts a methyl group from the Hf complex to form [CH3−B(C6F5)3]− anion (Scheme 1Ai).33,34 β-Hydride

labeled activated catalyst simply shows two species, with molar masses of 431 Da (major) and 417 Da (minor). These are assigned the structures [(CH3)2-Hf-(Cpn‑Propyl)2][H]+ (Structure 1-1ACH3 H+) and [(CH3)(H)-Hf-(Cpn‑Propyl)2][H]+ (Structure 1-1AH H+), respectively. (Figure S12). During this electrospray process, the bridged methyl appears to preferentially retain its bond to the Hf atom. Explanations of these observations invoking adventitious unreacted [(CH3)2-Hf(Cpn‑Propyl)2] are ruled out because attempts to ionize this neutral metallocene starting material proved fruitless; these structures could only be observed after the addition of B(C6F5)3. MALDI-TOF MS Analysis of the 13C-Labeled Activated Catalyst. MALDI analysis of the 13C-labeled activated catalyst solution identified the expected activated catalyst (Structure 11AH). Interestingly, unlike ESI, MALDI generates the cyclometalated intermediate (Structure 1-1ACy) as well (Figure S13). The partial conversion to 1-1ACy may be induced by the ionization process in this method. As expected, the observed mass shifts are consistent with the addition of the 13 C-labeled Cpn‑Propyl ligands. These species were observed at trace level peak intensities due to the excessive dithranol-Hf adducts that were formed (Figure S14). In the negative ion mode, only the anion [CH3−B(C6F5)3]− (527.0121 Da) is observed (Figure S15). Negative Ion Mode ESI-QqTOF MS Analysis of the 13CLabeled Activated Catalyst. Negative ion mode electrospray was utilized to examine the boron-containing species. Negative J


Article

Organometallics

Hf(Cpn‑Propyl)Cp(CH2)3‑Poly(1‑hexene)]+ (Structure 2-1 II). The lowest intensity peak series is assigned as [(dithranol)2Hf(OH)Cp(CH2)3‑Poly(1‑hexene)]+ (Structure 2-2 Hy). The expected 3 Da mass shift for Structure 2-2 II and 6 Da mass shift for Structure 2-1 II series correspond to the number of Cp rings in the structures. Inspection of the expanded spectrum (2160−2195 Da, Figure 6) reveals that the distinct hafnium isotope pattern has

Figure 4. Negative ion mode electrospray mass spectrum of the 13Clabeled metallocene catalyzed polymerization of 1-hexene (t = 3 h), covering the mass range of 500−700 Da.

elimination is a common chain termination mechanism that transfers a hydrogen atom to the Hf metal center,34 which is then available to be extracted by an equivalent of B(C6F5)3 to form [H−B(C6F5)3]− anion.34 Similarly, hexyl transfer from the hafnium center to B(C6F5)3 can form the [hexyl-B(C6F5)3]− anion.33 Reaction of B(C6F5)3 with adventitious moisture can readily form the [HO−B(C6F5)3]− anion.34,35 The trace of tetra-aryl borate anion observed at 678.9762 Da is not expected as an impurity in B(C6F5)3 but may be generated via multiple C6F5/alkyl exchanges between boron and hafnium centers.36 M A L D I- T O F M S A n al y s i s o f t h e 1 3 C - L a b e l e d Cp(CH2)3‑Poly(1‑hexene) Reaction Solution. There are some significant differences in the MALDI analysis of an active polymerization solution compared to that of the isolated polymer samples. By not terminating the poly(1-hexene) reaction, there is a considerable boost in the observed peak intensities of the polymer-containing species. This increase may be due to the more facile reaction of dithranol with active hafnium species compared to isolated polymer-modified hafnocene dichloride. Furthermore, two additional molecular series are observed (cf., Figures 1 and 5). The predominant polymer series remains [(dithranol)2HfCp(CH2)3‑Poly(1‑hexene)]+ (Structure 2-2 II). The next most intense series is [dithranol-

Figure 6. Expanded positive ion mode MALDI-TOF mass spectrum of the 13C-labeled poly(1-hexene) solution, covering the mass range of 2160−2195 Da. The MALDI matrix was dithranol.

been perturbed. This apparent discrepancy arises from the presence of both unsaturated and saturated poly(1-hexene) end groups within this sample, creating overlapping isotope patterns. The isolated polymer sample had proceeded to completion, allowing all polymer chains to generate unsaturated end groups by β-hydride elimination, generating Structure 2-2 II. However, the “live” reaction solution is expected to contain Hf-polymer metallacycle chains. When the dithranol MALDI matrix contacts the “live” polymerization solution, the dithranol quenches the metallacycle to form saturated end groups on the HfCp(CH2)3‑Poly(1‑hexene) polymer chains (Structure 2-2). Through a similar process, adventitious moisture reacts with the hafnium metal center and ring-opens the metallacycle, to form saturated end groups on the Hf(OH)Cp(CH2)3‑Poly(1‑hexene) polymer chains (Structure 2-2 Hy). When the isotope patterns for Structures 22 II and 2-2 are combined, they collectively form isotope distributions similar to those observed in Figure 6. Hypothesis for the Multisite Behavior of the Metallocene Catalyst. In addition to the expected conventional 1hexene chain growth at the hafnium metal center, primarily through repeated 1,2-insertions (Scheme 1Aiv), the Hf-based metallocene catalyst can undergo “self-modification” reactions that affect the ligand environment surrounding the Hf metal center. For example, the metallocene can undergo n-propyl C− H bond activation to generate a metallacycle (Scheme 1Bi).37,38 Next, a 1-hexene molecule can enter the active site and incorporate into the Cpn‑propyl−Hf bond, through a 2,1-insertion (Scheme 1Bii). Chain growth can occur by ring expansion through repeated 1-hexene insertions, until chain termination. Termination can occur through at least two mechanisms: (1) βhydride elimination opens the metallacycle and generates a Cp(CH2)3‑poly(1‑hexene) ligand with a terminal double bond (Scheme 1Biii), or, occasionally, (2) H-transfer from the adjacent Cpn‑propyl ligand to the polymer metallacycle ring

Figure 5. Positive ion mode MALDI-TOF mass spectrum of the 13Clabeled poly(1-hexene) reaction solution, covering the mass range of 2050−2550 Da. The MALDI matrix was dithranol. K


Article

Organometallics generates a ring-opened Cp(CH2)3‑poly(1‑hexene) with a saturated chain end and a new metallacycle (Scheme 1Biv). On the basis of the observed data, we hypothesize that the metallocene introduces polymer chains into its structure via cyclometalated intermediates: (1) The metallocene structure is modified, but not deactivated; (2) multiple cyclometalation/ polymerization sequences generate additional catalyst structure modifications. These “self-modification” reactions may contribute to the multicomponent polymer compositions that are sometimes observed for this catalyst.

Alceo Macchioni: 0000-0001-7866-8332 Richard J. Keaton: 0000-0002-1989-0205 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed under the auspices of Univation Technologies, LLC. Additional gratitude is extended to Prof. David M. Hercules and Prof. Ned A. Porter, at Vanderbilt University, for their helpful discussions on materials characterization and degradation mechanisms.

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CONCLUSIONS By combining all pieces of information coming from a collection of mass spectrometry-based techniques and NMR, precious mechanistic details have been obtained for the activation of Me2Hf(Cpn‑Propyl)2 by B(C6F5)3, and subsequent polymerization of 1-hexene: (1) A cyclometalated catalystintermediate is observed near the beginning of the reaction, which disappears with 1-hexene consumption. (2) A combination of MALDI and CID fragmentation identify the presence of poly(1-hexene) attached to the metallocene (HfCp(CH2)3‑Poly(1‑Hexene)). (3) Data suggest that ring expansion of the Hf-CH2CH2CH2Cp metallacycle occurs through repeated 2,1-insertions of the 1-hexene monomer. Additional experiments, using a metallocene with 13C-labeled Cpn‑Propyl ligands, allow for the “fingerprint” identification of the Cpn‑Propyl moiety within the structure of the [(dithranol)2-HfCp(CH2)3‑Poly(1‑hexene)]+ and [H-Cp(CH2)3‑Poly(1‑hexene)]+• species series. The observations described in this report augment our previous discussion of this unusual chain growth mechanism as a means of tuning ligand structures to generate a multisite catalyst in situ from a single molecular precatalyst. The vast majority of chain growth events from this Hf center still must proceed via the conventional polymerization mechanism to generate metal-free polymer chains, either before or after the growing metallacycle undergoes a chain termination event. In addition to being a curious example of an unexpected molecular geography for chain growth, the polymer-appended metallocene provides a useful ionizable modification of the polymer. This Hf-containing group enables the selective investigation of the structures of greatest interest, those containing Hf, even in the presence of considerable simple polymer “contaminants.” With this availability of relatively mild ionization conditions for analyzing polyolefins, even the degradation of poly(1-hexene) chains could be meaningfully investigated by ionization methods such as MALDI-TOF and ESI.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00293. Additional mass spectra and NMR data are presented for the poly(1-hexene) materials (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (979) 238-1778. E-mail: [email protected]. ORCID

Anthony P. Gies: 0000-0002-3558-593X Cristiano Zuccaccia: 0000-0002-9835-2818 L


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Mass Spectrometric Mechanistic Investigation of Ligand Modification

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