Titanium and Zirconium Permethylpentalene Complexes, Pn*MCpRX

Aug 1, 2016 - Well-defined homogeneous group 4 complexes are of interest as alternatives to Ziegler–Natta or Phillips-type polymerization systems, o...
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Titanium and Zirconium Permethylpentalene Complexes, Pn*MCpRX, as Ethylene Polymerization Catalysts Duncan A. X. Fraser, Zoë R. Turner, Jean-Charles Buffet, and Dermot O’Hare* Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K. S Supporting Information *

ABSTRACT: A family of group 4 permethylpentalene complexes, Pn*MCpRX (M = Ti, Zr; CpR = Cp, CpMe, t n Cp Bu, Cp Bu, CpMe3, Ind; X = Cl, Me), has been synthesized and fully characterized by multinuclear NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction studies. These complexes were immobilized on an insoluble polymethylaluminoxane (sMAO), MAO-modified silica (ssMAO), and a MAO-modified layered double hydroxide (LDH-MAO). The effect of substitution around the Cp ligand was examined in relation to their performance (activity, Mw, PDI, polymer morphology) for ethylene polymerization measured both in solution and in slurry phase. Maximum solution-phase activities of 3585 kg/mol·h·bar were recorded at modest [Zr]:[Al] ratios of 1:250. These were compared to the activities recorded using the equivalent solid-supported complexes, and it was observed that sMAO was a superior support material with average increases in activity of 5.3 and 2.3 times relative to ssMAO and LDH-MAO, respectively. Most striking was the observation that slurry-phase ethylene polymerization activities using equivalent precatalysts supported on sMAO showed enhanced performance compared to the solution phase up to a maximum of 4486 kg/mol·h·bar.

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INTRODUCTION The pentalene molecule (Pn, C8H6), a lower homologue of naphthalene and heptalene, is a highly unstable 8π antiaromatic.1 Stabilization of the pentalene moiety can be achieved with large substituents at the ring periphery,2 which has led to the investigation of pentalene derivatives as highperformance organic conducting materials.3 Alternatively, twoelectron reduction of the neutral precursor generates a 10πelectron, aromatic dianion.4 In 2007, Ashley et al. published a solution-phase synthesis of the permethylpentalene dianion (Li2Pn*, Li2[C8Me6]),5 which has provided an accessible route into the organometallic chemistry of permethylpentalene.6 As a ligand, pentalene is capable of multiple types of coordination.7 The η8-binding mode,8 where the ligand bends around the bridgehead bond, disrupting aromaticity, is of particular interest since it can be viewed as two ring-fused cyclopentadienyl (Cp) fragments; the pentalene framework contributes two fewer electrons, and overall it may be classified as an eight-electron L3X2 ligand.9 Jonas et al. have previously demonstrated the reaction of Li2Pn with CpTiCl2 to produce PnTiCp, which could be oxidized with dichloroethane to the diamagnetic 18-electron complex PnTiCpCl (Chart 1, A). The zirconium congener, PnZrCpCl, could more simply be synthesized by combination of Cp2ZrCl2 and Li2Pn (Chart 1, B).8b O’Hare and co-workers subsequently reported the synthesis of the permethylpentalene analogues, Pn*MCpCl (M = Ti, Zr, Chart 1, C, D), prepared from the corresponding © XXXX American Chemical Society

Chart 1. Summary of Previous Related Work Carried out on Group 4 η8-Pentalene Complexes

dimeric metal dichlorides, E and F, respectively.6e Complex D was found to be an excellent precatalyst for the polymerization of ethylene despite the electronic saturation and the crowded environment at the metal center; traditional metallocene precatalysts are of the form Cp2MCl2, and the active species Received: May 24, 2016

A

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was therefore concluded that the small, first-row group 4 metal was simply too crowded to accommodate these sterically demanding ligands. Synthesis of the analogous zirconium complexes was achieved by reaction of [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li·thfX (F)

of polymerization is the cationic [Cp2MR]+ (M = group 4 metal, R = alkyl group).10 Well-defined homogeneous group 4 complexes are of interest as alternatives to Ziegler−Natta or Phillips-type polymerization systems, owing to the improved control over polymer properties.11 Alternatively, the use of surface organometallic chemistry12 to immobilize these well-defined complexes on solid supports targets the preparation of single-site heterogeneous catalysts and is known to limit reactor fouling while retaining desirable polymer properties.12d,13 The O’Hare group has been exploring this methodology to prepare ethylene polymerization catalysts and to study both structure−activity relationships and the nature of the active species on the surface.14 Herein we report the synthesis of a series of complexes of general formula Pn*MCpRX (M = Ti, Zr; X = Cl, Me; CpR = t n Cp, CpMe, Cp Bu, Cp Bu, CpMe3, Ind) in order to investigate the effects of substitution around the Cp ligand on the observed activities for the polymerization of ethylene. We develop the surface organometallic chemistry of pentalene complexes, reporting the first η8-pentalene compounds immobilized on inorganic supports, and the capacity of these catalysts to polymerize ethylene.

t

n

with LiCpR (CpR = Cp, CpMe, Cp Bu, Cp Bu, CpMe3, Ind) at −78 °C in diethyl ether over 1 h to afford Pn*ZrCpRCl (D, 2.1− 2.5) in moderate to good yields (54−80%) (Scheme 2). In this case, the increased ionic radius of zirconium compared to titanium allows more sterically demanding cyclopentadienyl derivatives to be appended to the metal.15 t

Scheme 2. Synthesis of Pn*ZrCpRCl (CpR = Cp, CpMe, Cp Bu, n Cp Bu, CpMe3, Ind)

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RESULTS AND DISCUSSION Synthesis of Pn*MCpRCl (M = Ti, Zr). [TiPn*Cl(μ-Cl)]2 t (E) was reacted with LiCpR (CpR = Cp, CpMe, Cp Bu, Ind) in toluene at −78 °C over 1 h to produce a family of complexes Pn*TiCpRCl (C, 1.1−1.3), which are isolated as orange to dark red solids in good yields (61−70%) (Scheme 1). The synthesis of complexes with larger CpR ligands was also attempted (R = Cp*, CpMe3). t

Scheme 1. Synthesis of Pn*TiCpRCl (CpR = Cp, CpMe, Cp Bu, Ind) Methylation of the metal center can be achieved using a stoichiometric amount of MeLi at −78 °C in toluene to afford zirconium complex 2.6 (Scheme 3). A cold workup is required; Scheme 3. Synthesis of Pn*ZrCpMeMe

at temperatures above 0 °C decomposition is observed in the presence of the bulk reaction mixture, presumably due to irreversible formation of an “ate”-complex, as has previously been observed with these motifs in the presence of alkyl lithium reagents.6j Once isolated, 2.6 shows minimal decomposition of the solid or toluene solution for 1 month. Solid-State Structural Analysis. The solid-state structures were obtained for complexes 1.1−1.3 and 2.1−2.6; representative structures are shown in Figure 1, with a comparison of some key structural features provided in Table 1 (full details provided in the Supporting Information). In the solid state, all complexes are C1 symmetric with pseudotetrahedral geometry

However, as the steric demands of the ligand increased, reduction of the Ti(IV) dimer E was observed to compete with the desired salt metathesis, producing an intractable mixture of Ti(IV) and Ti(III) species. This can be readily demonstrated with the attempted synthesis of Pn*TiCp*Cl; at room temperature in toluene minimal reaction was observed to occur with LiCp*, while heating to 60 °C for 2−16 h produced a complex 1H NMR spectrum indicating a mixture of diamagnetic and paramagnetic components that could not be assigned. Varying the solvent failed to permit the synthesis of the target compound, as did sonication, stannylation of the carbocycle, and addition of crown-ether to solubilize LiCp*. It B

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t

n

Figure 1. Thermal displacement ellipsoid plots (50% probability) of (a) Pn*TiCpMeCl (1.1); (b) Pn*ZrCp BuCl (2.2); (c) Pn*ZrCp BuCl (2.3); (d) Pn*ZrCpMe3Cl (2.4); (e) Pn*ZrIndCl (2.5); and (f) Pn*ZrCpMeMe (2.6). H atoms are omitted for clarity. Only a single enantiomer is depicted.

Table 1. Comparison of Selected Bond Lengths (Å), Angles (deg), and Selected 1H NMR Spectroscopic Chemical Shifts (ppm)a H NMR δ

1

label C 1.1 1.2d 1.3d D 2.1 2.2 2.3d 2.4 2.5 2.6

complex b

Pn*TiCpCl PnTiCpClc Pn*TiCpMeCl t Pn*TiCp BuCl Pn*TiIndCl Pn*ZrCpClb PnZrCpClc Pn*ZrCpMeCl t Pn*ZrCp BuCl n Pn*ZrCp BuCl Pn*ZrCpMe3Cl Pn*ZrIndCl Pn*ZrCpMeMe

d(M−Cl)

Pn* fold angle

d(M−Pn*cent)

2.5068(8) 2.512(1) 2.4769(6) 2.4608(6) 2.4668(13) 2.5484(8) 2.540(1) 2.5309(4) 2.5319(5) 2.5285(14) 2.5340(5) 2.5317(4) 2.348(2)

30.98(12) 33.57 31.29(10) 31.9(2) 31.7(2) 30.33(14) 32.64 30.56(7) 30.00(9) 30.7(2) 30.56(9) 30.42(7) 30.10(11)

1.9924(11) 1.969 1.9912(9) 1.991(2) 1.981(2) 2.1094(13) 2.095 2.1062(6) 2.1117(8) 2.1028(18) 2.0984(8) 2.0880(7) 2.1089(9)

d(M−CpRcent) 1.9967(12) 1.969 1.9942(10) 1.993(2) 1.993(2) 2.1099(11) 2.095 2.1065(7) 2.1181(8) 2.1058(23) 2.1259(8) 2.1172(7) 2.1203(10)

2.0631(14) 2.057 2.0704(10) 2.056(2) 2.119(2) 2.2189(17) 2.215 2.2219(7) 2.2338(8) 2.2192(23) 2.2310(8) 2.2515(7) 2.2262(11)

NWT

WT

1.51

1.64

2.19

1.51 1.58 1.35 1.64

1.68 1.69 1.83 1.75

2.20 2.16 2.00 2.10

1.66 1.71 1.70 1.64 1.49 1.63

1.81 1.83 1.84 1.90 1.91 1.99

2.12 2.09 2.11 2.14 1.94 2.00

a

Relevant literature comparisons are provided in italics. bRef 6e. cRef 8b. dSolid-state parameters are given as averages of the two independent molecules in the asymmetric unit.

terminal Ti(IV)−Cl bonds; the longest example can be found in TiCp2(4-MeO-TEMPO)Cl (2.5346(12) Å).16 Although the bond length does not always correlate exactly with bond strength when sterics are limiting, this does suggest a rather activated Ti−Cl bond. The indenyl ligand in 1.3 has slip-fold characteristics consistent with distorted η5-coordination,17 implying steric congestion around the metal center (see Supporting Information). By comparison, the Zr−Cl bond lengths (2.5285(14)−2.5340(5) Å) are somewhat shorter than expected based on consideration of the ionic radii of the metal centers (four-coordinate Zr(IV), 0.59 Å; Ti(IV), 0.42 Å).15 Zr−

about the metal center. The Pn* ligand is bound asymmetrically, canting away from chloride (or methyl) ligand. The effects of this tilt are amplified by the Pn*−Me groups, which lead to a strong steric preference for substituents appended to the Cp ligand to point away from the downward cant of the Pn*, while avoiding eclipsing interactions with the chloride (or methyl) group. The Ti−Cl bond lengths fall in the range 2.4608(6)− 2.4769(6) Å and are slightly shorter than both the parent pentalene analogue PnTiCpCl (2.512(1) Å) and Pn*TiCpCl (2.5068(8) Å). They are all, however, long compared to typical C

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case, coupling was observed between 13CO and the proximal nonwingtip substituents (Si and H) of the Pn† ring (JC−H = 3.2 Hz; JC−Si = 1.5 Hz) and was attributed to through-space scalar coupling.22 Solution-Phase Ethylene Polymerization Studies. The new complexes were tested as ethylene polymerization catalysts in solution, using MAO as a cocatalyst in a modest ratio ([M]: [MAO] = 1:250). The polymerization activities when using titanium precatalysts C, E, and 1.1−1.3 were moderate on the Gibson scale,23 and no significant difference is observed across the series (Figure 2). The highest activity was recorded at 35

Cl bond lengths for structurally related compounds generally fall in the range 2.3−2.5 Å.18 The slip-fold characteristics of zirconium indenyl complex 2.5 indicate a slightly less distorted η5-coordination relative to the titanium analogue 1.3, which reflects the radially expanded and less crowded coordination sphere of the metal center (see Supporting Information). The Pn* fold-angle, defined as the angle by which the ligand deviates from planarity,7 is marginally smaller for the zirconium complexes than the titanium analogues, correlating inversely with the ionic radius of the metal center, as has previously been observed.6e However, in all cases the fold angle of the Pn* ligand is less than the Pn analogues, which is likely a result of the improved donor characteristics of Pn*; inductive donation of electron density from the six methyl groups around the pentalene framework allows the ligand to satisfy electron deficiency at the metal center with less extreme distortion of the wingtip positions. Density functional theory calculations were performed at the BP8619 and B3LYP19a,20 level of theory for complexes 1.1−1.3 and 2.1−2.6 (Tables S4−S12, Figures S61−S64). Geometry optimizations were generally found to reproduce well the metrical data established experimentally, although reproduction of the R−CpRcentroid−M1−Cl1 torsion angles did appear to be more sensitive to the functional in use. The calculations indicated that, for all structures, the HOMO primarily consisted of M−Pn* π-bonding interactions, while the LUMO was centered on metal d-orbitals. NMR Spectroscopic Characterization. The 1H NMR spectra of all complexes under study demonstrate three sharp singlets of intensity 6:6:6 corresponding to the Pn*−Me groups, which is expected for these pseudotetrahedral complexes of the general formula Pn*MXY; two singlets correspond to the nonwingtip (NWT: C1, C3, C5, C7; see Supporting Information) methyl groups, and a further singlet is observed for the wingtip (WT: C2, C6; see Supporting Information) methyl substituents. This is consistent with Cs molecular symmetry in solution due to rotation of the CpR ligand; a complex of C1 symmetry would result in six discrete resonances in a 3:3:3:3:3:3 ratio for the Pn*−Me groups. There are no clear relationships between the observed 1H NMR spectral resonances and either the donor capacity of the CpR substituent or structural parameters in the solid state. Complexes bearing indenyl ligands (1.3 and 2.5) show large differences in the chemical shifts of the Pn*−Me resonances relative to compounds with alkyl-Cp substituents. This is presumably due to nascent aromatic character at the benzannulated position producing ring currents that influence the chemical shifts of the Pn* protons. On breaking the 5-fold symmetry of the Cp ligand fragment by functionalizing one or more positions, the 1H NMR spectrum shows the expected splitting resulting from reduced symmetry of a rotating ring. For example, in complexes with a CpR ligand (R = Me, tBu, n Bu), the AA′XX′ spin system leads to the observation of two triplets (for example, in 1.1 and 2.1, 3JH−H = 2.7 and 2.6 Hz, respectively).21 Interestingly, the 13C−1H HMBC spectrum of methyl complex 2.6 indicates a correlation between Zr−CH3 (Figure 1f; C21) and one Pn*−CH3 environment (Figure 1f; C11 and C12); given that an interaction is observed and that the proton interacts with only one Pn*−CH3 environment, it can be concluded that ring whizzing of the Pn* is not operative on the NMR spectroscopic time scale. Similar features have previously been noted by Kilpatrick et al. in (μ:η5,η5Pn†)2Ti2(μ:η2−13CO) (Pn† = [C8H4(1,4-SiiPr3)2]2−) In this

Figure 2. Solution-phase ethylene polymerization activities of (left to right) [TiPn*Cl(μ-Cl)]2 (E) and Pn*TiCpRCl (CpR = Cp, CpMe, t

Cp Bu, Ind) (C, 1.1−1.3). [Ti]:[MAO] = 1:250; 2 bar of ethylene; 0.5 mg catalyst loading; 50 mL of toluene; 60 °C; 30 min.

kgPE/(mol·h·bar) for complex 1.2. The moderate activity of E is particularly noteworthy in this context given that it is more electron deficient and sterically available than the other complexes. By comparison the zirconium complexes are very active on the Gibson scale, with most complexes 2 orders of magnitude more active than their titanium analogues (Figure 3). The best precatalyst is the indenyl complex 2.5, which shows an increase in activity of 57% relative to the parent compound E (3585 and 2281 kgPE/(mol·h·bar), respectively). Direct comparison of

Figure 3. Ethylene polymerization activities in solution (gray) and supported on polymethylaluminoxane (orange) of Pn*ZrCpRCl (CpR t

n

= Cp, Cp Me , Cp Bu , Cp Bu , Cp Me3 , Ind) (D, 2.1−2.5) and Pn*ZrCpMeMe (2.6). Solution conditions: [Zr]:[MAO] = 1:250; 2 bar of ethylene; 0.5 mg catalyst loading; 50 mL of toluene; 60 °C; 5 min. Supported conditions: [Zr]:[sMAO] = 1:200; 150 mg of TiBA cocatalyst; 2 bar of ethylene; 10 mg catalyst loading; 50 mL of toluene; 60 °C; 30 min. D

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The activities of 2.1ssMAO, 2.1LDH‑MAO, and 2.1sMAO were measured over the temperature range 40−80 °C to investigate the role of the support (Figure 4). The silica-supported catalyst

activities with previous reports should be treated with caution due to the significant effect reaction conditions (e.g., cocatalyst ratio, dilution, stirring efficiency) can have on the observed activities. For example, the zironocene rac-(SBI*)ZrCl2 (SBI* = dimethylsilylbis(hexamethylindenyl)) displays an ethylene polymerization activity of 22 622 kgPE/(mol·h·bar) under similar conditions to this study,14d whereas rac-(EBI*)ZrCl2 (EBI* = ethylenebis(hexamethylindenyl)) is a highly active precatalyst (61 800 kgPE/(mol·h·bar)), but the polymerization conditions involve 10 bar of ethylene pressure and 1.8 L of isobutene solvent.24 The ethylene polymerization activity of the unsubstituted parent pentalene complex PnZrIndCl has previously been described in the patent literature as 5423 kgPE/(mol·h·bar), albeit at much higher cocatalyst loadings of [Zr]:[MAO] = 1:1000 and on a larger scale.25 The indenyl ligand is often observed to increase the rate of reaction in transformations that involve associative rate-determining steps due to the ease with which the indenyl ligand can transiently adopt an η3-configuration, allowing for the coordination of additional ligands such as ethylene.26 However, in this case it would appear that this is not the major factor that dictates the increased activity. 2.1 and 2.3 are similar in activity to 2.5, but instead contain n-alkyl-substituted CpR ligands (CpMe and

Figure 4. Comparison of ethylene polymerization activities of (top to bottom) 2.1sMAO, 2.1LDH‑MAO, and 2.1ssMAO. Slurry conditions: [Zr]: [Al] = 1:200; 150 mg of TiBA cocatalyst; 2 bar of ethylene; 10 mg catalyst loading; 50 mL of toluene; 30 min.

n

Cp Bu, respectively). Electron-donating substituents are known to inhibit ring slippage, so a rate decrease relative to D would be expected.27 It is more likely that these substituents aid in the stabilization of charged intermediates in the polymerization as a result of their improved donor capacity. When n-alkyl-CpR (2.1, 2.3, 2.4) is substituted for tert-alkyl-CpR (2.2), a 66% decrease in activity is observed relative to D (789 and 2281 kgPE/(mol·h· bar), respectively), with the steric requirements of the tert-butyl group now appearing to dominate the rate of polymerization over any favorable electronic advantages. Interestingly when Cl− is substituted for Me− (2.6), no statistically significant change in activity is observed, implying facile activation of the precatalyst 2.1 under polymerization conditions. Synthesis and Characterization of Solid-Supported Zirconium Complexes and Slurry Ethylene Polymerization Studies. Given the high activities of the zirconium precatalysts in solution, Pn*ZrCpMeCl (2.1) was immobilized on MAO-modified silica (ssMAO), on MAO-modified aqueous miscible organic solvent treated layered double hydroxide (LDH-MAO), and on polymethylaluminoxane (sMAO) to afford 2.1ssMAO, 2.1LDH‑MAO, and 2.1sMAO, respectively for further study as slurry polymerization precatalysts. To achieve immobilization, 2.1 and the appropriate support material were swirled in toluene at 60 °C for 1 h to furnish the supported complexes as pale yellow pyrophoric powders in good isolated yields (64−75%). MAO-modified silica (ssMAO) has been well documented as an effective support in surface organometallic chemistry28 and is commonly employed to support olefin polymerization precatalysts.29 By comparison, LDH-MAO, where LDH has the general formula [Mz+1−xM′y+x(OH)2]a+(An−)a/n·bH2O· c(AMO-solvent), have only recently been demonstrated as effective supports for the immobilization of single-site olefin polymerization and oligomerization catalysts.14d,e,30 sMAO is a polymeric, porous aluminoxane that is a solid, insoluble form of MAO suitable for supporting complexes. The structure, like MAO, is poorly defined, and efforts within our group are currently ongoing to better understand this material.

demonstrated the lowest activities (504−943 kgPE/(mol·h· bar)), while 2.1LDH‑MAO produced activities generally twice as large (1116−1612 kgPE/(mol·h·bar)) with both showing a general increase in activity with increasing temperature. 2.1LDH‑MAO is one of the most active known zirconocene LDH-MAO precatalysts, only marginally slower than n (Cp Bu)2ZrCl2LDH‑MAO (2141 kgPE/(mol·h·bar)) and over 18 times faster than the (hydro)permethylpentalene complex [Pn*(H)ZrCl3]2LDH‑MAO (89 kgPE/(mol·h·bar)).14e The polymethylaluminoxane-based precatalyst 2.1sMAO demonstrated up to a 3-fold increase in activity relative to the same complex supported on LDH-MAO, with a maximum activity observed at 4209 kgPE/(mol·h·bar) at 60 °C. The observation of such high polymerization activity of 2.1sMAO relative to other inorganic-supported systems and, more unusually, the solution-phase system (Figure 3) led us to prepare polymethylaluminoxane-supported complexes DSMAO and 2.2SMAO−2.6SMAO for further study; the complexes were also isolated as pale yellow pyrophoric powders in excellent yields (71−80%). Characterization of the polymethylaluminoxane-supported complexes was achieved by solid-state 13C CPMAS and 27Al DPMAS NMR spectroscopy (Figure 5 and Supporting Information). The low loading of the complexes on the surfaces precluded meaningful characterization by IR spectroscopy. The 13C CPMAS spectrum of sMAO exhibits a major, broad resonance at −13 ppm corresponding to the sMAO−Me environments, while benzoate resonances can be observed at 119−130 ppm for the aromatic environments and 171 ppm for α-C(O)O (benzoic acid is used as a mild oxygen source in the synthesis of sMAO). Additionally, 2.1SMAO−2.6SMAO all present a resonance at around 6 ppm corresponding to Pn*−Me environments, which are no longer resolved as discrete resonances due to line broadening in the solid state. Further resonances are observed around 106−111 ppm due to Cp ringcarbon environments. These resonances are best resolved in complexes with minimally substituted CpR ligands containing multiple CCp‑ring−H bonds due to magnetization transfer from E

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be observed in which the supported complexes with no substitution or n-alkyl substitution of one position of the Cp ring were found to be the most effective ethylene polymerization catalysts. This can be compared with cases where there is substitution of multiple positions of the Cp ring (2.4sMAO and 2.5sMAO). Here, a significant decrease in rate is observed, with both catalysts approximately 66% slower than 2.6sMAO (1981 and 1971 kgPE/(mol·h·bar) compared to 4486 kgPE/(mol·h· bar)). However, the most dramatic effect is found for tert-alkyl substitution of the Cp ring, echoing the trend observed for the solution-phase precatalysts, with activities of 294 kgPE/(mol·h· bar) recorded for 2.2sMAO, a 94% decrease compared to 2.6sMAO. An interesting comparison can be made of these data with the solution-phase polymerizations. For complexes bearing CpR ligands with no substitution or n-alkyl substitution at only a single position (DsMAO, 2.1sMAO, 2.3sMAO, and 2.6sMAO), an increase in activity is observed. The largest increase is 59% from 2281 kgPE/(mol·h·bar) (D) to 3624 kgPE/(mol·h·bar) (DsMAO). This is surprising given the general trend that supported polymerization catalysts usually demonstrate lower activities compared to their homogeneous counterparts.31 The solid surface can be assumed to occupy a large volume and inhibit approach of monomer units to the metal center. When supported complexes bearing larger CpR ligands are employed (2.2sMAO, 2.4sMAO, and 2.5sMAO), the steric requirements of the support have a noticeable impact, and the activities are found to decrease relative to the homogeneous analogues as expected. For example, there is a 63% decrease from 2.2 (789 kgPE/(mol· h·bar)) to 2.2sMAO (294 kgPE/(mol·h·bar)). This more conventional trend is also clearly observed in the comparison of 2.1LDH‑MAO and 2.1ssMAO with the solution-phase polymerization using 2.1. The polyethylene produced from selected catalysts was characterized by gel-permeation chromatography, and the experimental molecular weights and polydispersities (PDI) were found to vary significantly across the series (Figure 6).

Figure 5. (Top) Solution 13C{1H} NMR spectrum (400 MHz, C6D6) of 2.3. (Bottom) Solid-state 13C CPMAS NMR spectrum (10 kHz, contact time 3000 μs) of 2.3sMAO. Highlighted in red: nBu resonances; highlighted in green: Pn*−Me resonances.

the protons in these cross-polarization experiments, such as DsMAO, 2.1sMAO, 2.3sMAO, and 2.6sMAO. Examples where this is not the case are 2.5sMAO, where the indenyl resonances are split over a large frequency range, and 2.4sMAO, where fewer CCp‑ring−H bonds are present in the Cp ligand and do not produce readily observable signals in this region. Nonetheless complexation can still be confirmed by the presence of the characteristic Pn*−Me resonances at 6 ppm. In the case of 2.2sMAO and 2.3sMAO, resonances can be observed for the CpR tert-butyl and n-butyl groups at 33 ppm and 15−28 ppm, respectively, with the latter showing three well-resolved resonances for the carbon environments along the n-butyl chain (Figure 5). The solid-state 27Al DPMAS NMR spectra for the supported species are complicated with four major resonances around 340, 200, −80, and −245 ppm, which correspond to the different aluminum environments within the polymethylaluminoxane. We have previously examined the surface structure of (EBI)ZrCl2LDH‑MAO (EBI = rac-ethylenebis(indenyl)) using Zr K-edge EXAFS; the zirconium is in a tetrahedral environment, bound to the indenyl ligand, with a single Zr−Me bond and a close contact to a surface oxygen.14c Studies on the immobilization of W(NDipp)Cl4(thf) (Dipp = 2,6-iPr-C6H3) on sMAO indicated that three methylations occur in situ, while a W−Cl−Al interaction stabilizes the resulting species.30 Although a resonance accounting for a Zr−Me environment cannot be observed in the 13C CPMAS NMR spectra of DSMAO or 2.1SMAO−2.6SMAO, it is likely that methylation has occurred. The slurry-phase ethylene polymerization activities of these supported complexes (DSMAO and 2.1SMAO−2.6SMAO) were measured (Figure 3). The highest activities were observed for 2.1sMAO, 2.3sMAO, and 2.6sMAO (4206, 4096, and 4486 kgPE/ (mol·h·bar), respectively). This can be compared with the unsubstituted analogue DsMAO, which demonstrated somewhat lower activities of 3624 kgPE/(mol·h·bar). A general trend can

Figure 6. Polymer molecular weight, M w, for solution and polymethylaluminoxane-supported ethylene polymerization at 60 °C t

with Pn*ZrCpRCl (CpR = Cp, CpMe, Cp Bu, CpMe3, Ind) (D, 2.1, 2.2, 2.4, 2.5). PDIs are given in parentheses.

Although 2.1 and 2.5 displayed the highest activities of any solution-phase system, the polymers produced were inhomogeneous in 1,2,3-C6Cl3H3 on heating at 140 °C for 4 h followed by 160 °C for 2 h. Supporting these catalysts on sMAO allows for the production of a homogeneous polymer at the expense of a lower Mw. In general, the supported catalysts produce polymers with larger PDIs, except in the case of 2.1, which may F

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A maximum activity for DsMAO was observed at 60 °C; however increasing temperature decreased the observed Mw by 35% from a maximum of 450 000 kg/mol at 50 °C to 295 000 kg/mol at 70 °C (Figure 9), a trend that has previously been observed.32

be an artifact of the inhomogeneous polymer formed by the solution-phase catalyst. The highest Mw observed was for 2.1 employed as a solution-phase precatalyst, which produced a polymer Mw of 527 000 kg/mol, while the highest Mw for a supported catalyst was 2.4sMAO (505 000 kg/mol), although a rather large PDI (3.8) is also observed for the latter. The polymers produced using 2.2 and 2.2sMAO were found to exhibit a bimodal molecular weight distribution (Figure 7); 2.2 exhibits a major peak at 40 600 kg/mol and a minor peak at 136 200 kg/mol, while 2.2sMAO has maxima at 61 300 and 579 100 kg/mol.

Figure 9. Temperature dependence of ethylene polymerization activity and polymer molecular weight, Mw, with DsMAO. [Zr]:[sMAO] = 1:200; TiBA cocatalyst; 2 bar of ethylene; 10 mg catalyst loading; 50 mL of toluene; 30 min. PDIs are given in parentheses.

The activity and molecular weight of the polymer produced by DsMAO were observed to decrease with increasing time of polymerization (Figure 10), consistent with literature reports.33 Figure 7. Molecular weight distribution of polymer produced by precatalysts 2.2 (red) and 2.2sMAO (black), determined by GPC.

The polymer morphology was also highly dependent on whether the catalyst was employed in solution phase or supported on polymethylaluminoxane. Scanning electron microscopy (SEM) highlights the significant polymer aggregation that results from homogeneous conditions; Figure 8 shows

Figure 10. Time dependence of slurry-phase ethylene polymerization activity and polymer molecular weight, Mw, with DsMAO. [Zr]:[sMAO] = 1:200; TiBA cocatalyst; 2 bar of ethylene; 10 mg catalyst loading; 50 mL of toluene; 60 °C. PDIs are given in parentheses.

Provided is a table summarizing the polymerization activities and GPC data presented in this study (Table 2).

■

CONCLUSION Nine new permethylpentalene group 4 complexes have been synthesized and fully characterized, and their ability to polymerize ethylene in solution phase has been evaluated. All nine complexes have also been successfully immobilized on a polymethylaluminoxane support, which produced a number of solid catalysts that demonstrated excellent activities, in some instances greater than the solution-phase analogues. Maximum solution-phase ethylene polymerization activities of 3585 kgPE/ (mol·h·bar) were recorded at modest [Zr]:[Al] ratios of 1:250. Maximum slurry-phase ethylene polymerization activities of 4486 kgPE/(mol·h·bar) were measured. These were compared to the activities recorded using conventional MAO-modified silica and MAO-modified AMO-LDH as supports, which demonstrated sMAO as the superior support material, with average increases in activity of 5.3 and 2.3 times relative to

Figure 8. SEM of polymer produced: (a) 2.1 at 100× magnification; (b) 2.1sMAO at 100× magnification; (c) 2.1 at 250× magnification; (d) 2.1sMAO at 250× magnification.

SEM images of the polymer produced by 2.1 (a) and 2.1sMAO (b), respectively, both at 100× magnification. Clearly, aggregation is significantly decreased by immobilization of the catalyst on sMAO, producing a polymer with a typical particle size of 13−23 μm, compared to >1 mm, as seen with the product of homogeneous conditions. G

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Organometallics Table 2. Summary of Polymerization Activities at 60 °C and, Where Relevant, GPC Data for the Complexes in This Study precatalyst

activity (kgPE/(mol·h·bar))

A B 1.1 1.2 1.3 E DsMAO 2.1 2.1sMAO 2.1ssMAO 2.1LDH‑MAO 2.2 2.2sMAO 2.3 2.3sMAO 2.4 2.4sMAO 2.5 2.5sMAO 2.6 2.6sMAO

24 30 25 35 34 2281 3624 3353 4209 458 1501 789 294 3300 4096 2773 1981 3585 1971 3307 4486

Mw (kg/mol)

PDI

470 000 325 000 527 000 250 000

2.4 2.4 2.7 2.4

145 000 310 000

4.1 7.0

195 000 505 000 392 000 290 000

2.4 3.8 2.4 3.4

Crystals were mounted on MiTeGen MicroMounts using perfluoropolyether oil and rapidly transferred to a goniometer head on a diffractometer fitted with an Oxford Cryosystems Cryostream open-flow nitrogen cooling device.36 Data collections were carried out at 150 K either using an Oxford Diffraction Supernova diffractometer using mirror-monochromated Cu Kα radiation (λ = 1.541 78 Å) with data processed using CrysalisPro37 or using an Enraf-Nonius Kappa CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å), where raw frame data were reduced using the DENZO-SMN package38 and corrected for absorption using SORTAV.39 The structures were solved using direct methods (SIR92)40 or a charge-flipping algorithm (SUPERFLIP)41 and refined by full-matrix least-squares procedures using the Win-GX software suite.42 Pn*TiCpMeCl (1.1). LiCpMe (29 mg, 0.337 mmol) was slurried in toluene (10 mL) and transferred onto [Pn*TiCl(μ-Cl)]2 (E) (100 mg, 0.164 mmol) in toluene (10 mL) at −78 °C. The reaction was warmed to room temperature and stirred for 1 h. The solution was filtered, and the solids were extracted with toluene (3 × 5 mL) before the volatiles were removed in vacuo. The solid was washed with −78 °C hexane (2 × 2 mL) before being dried under dynamic vacuum for 3 h, affording 1.1 as a red powder in 67% yield (77 mg, 0.221 mmol). Single crystals suitable for an X-ray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C20H25ClTi: C, 68.88 (66.79); H, 7.23 (7.20). 1H NMR (400 MHz C6D6) δ (ppm): 1.51 (s, 6H, 2,6-Me-Pn*); 1.68 (s, 6H, 3,5-Me-Pn*); 2.11 (s, 3H, Me-Cp); 2.20 (s, 6H, 1,7-Me-Pn*); 4.65 (t, 2H, 3JH−H = 2.7 Hz, 2,5-H-Cp); 5.47 (t, 2H, 3JH−H = 2.7 Hz, 3,4-H-Cp). 13C{1H} NMR (100 MHz, C6D6) δ (ppm): 10.9 (2,6-Me-Pn*); 13.7 (3,5-MePn*); 14.0 (1,7-Me-Pn*); 15.2 (Me-Cp); 102.9 (2,5-CH-Cp); 113.5 (3,5-Pn*); 116.7 (3,4-CH-Cp); 117.5 (4-Pn*); 120.8 (1,7-Pn*); 122.0 (2,6-Pn*); 124.2 (1-Cp); 128.6 (8-Pn*).

ssMAO and LDH-MAO, respectively. The polymers produced by these systems were characterized by SEM and GPC, which demonstrates the reduction in aggregation observed when polymethylaluminoxane-supported catalysts are employed. The mechanistic rationale for the very high activities of these electronically saturated catalysts is currently under investigation; the traditional polymerization mechanism for 16-electron Cp2MCl2 complexes that proceeds via the 14-electron alkyl cation [Cp2MR]+ does not appear to be accessible for the Pn*MCpRX system. The relevance of alkylated and cationic complexes to the active species will be reported in due course.

■

t

Pn*TiCp BuCl (1.2). To a solution of [Pn*TiCl(μ-Cl)]2 (E) (250 t

mg, 0.409 mmol) in toluene (10 mL) at −78 °C was added LiCp Bu (105 mg, 0.819 mmol) slurried in toluene (10 mL) at −78 °C. The reaction mixture was allowed to warm to room temperature and stirred for 1 h. The toluene was removed under vacuum, and the resulting solid extracted into benzene (3 × 3 mL) and lyophilized. This solid was then washed with −78 °C pentane (2 × 3 mL) before being dried under vacuum for 3 h to afford 1.2 as a brown powder (195 mg, 0.499 mmol, 61% yield). Analytical samples were prepared by recrystallizing the product from pentane at −78 °C. Single crystals suitable for an Xray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C23H31ClTi: C, 70.69 (69.79); H, 8.00 (7.92). 1H NMR (400 MHz, C6D6) δ (ppm): 1.36 (s, 9H, tBuCp); 1.58 (s, 6H, 2,6-Me-Pn*); 1.69 (s, 6H, 3,5-Me-Pn*); 2.16 (s, 6H, 1,7-Me-Pn*); 4.62 (t, 2H, 3JH−H = 2.7 Hz, 2,5-H-Cp); 5.72 (t, 2H, 3 JH−H = 2.7 Hz, 3,4-H-Cp). 13C{1H} NMR (100 MHz C6D6) δ (ppm): 11.5 (2,6-Me-Pn*}; 13.5 (3,5-Me-Pn*); 13.9 (1,7-Me-Pn*); 32.1 (CMe3-Cp); 33.5 (CMe3-Cp); 101.8 (2,5-CH-Cp}; 114.0 (3,4-CHCp); 114.1 (3,5-Pn*); 117.9 (4-Pn*); 120.7 (1,7-Pn*); 123.0 (2,6Pn*); 129.2 (8-Pn*); 141.7 (1-Cp). Pn*TiIndCl (1.3). To a solution of [Pn*TiCl(μ-Cl)]2 (E) (250 mg, 0.410 mmol) in toluene (10 mL) at −78 °C was added a slurry of LiInd (100 mg, 0.820 mmol) in toluene (10 mL). The reaction was warmed to room temperature and stirred for 1 h. The solution was filtered, and the solids were extracted with toluene (3 × 5 mL). The volatiles were removed in vacuo, and the solid was washed with −78 °C hexane (2 × 2 mL) before being dried under dynamic vacuum for 4 h to afford 1.3 as a crimson red powder (153 mg, 0.398 mmol, 67% yield). Single crystals suitable for an X-ray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C23H25ClTi: C, 71.80 (71.88); H, 6.55 (6.47). 1H NMR (400 MHz, C6D6) δ (ppm): 1.35 (s, 6H, 2,6-Me-Pn*); 1.83 (s, 6H, 3,5-Me-Pn*); 1.99 (s, 6H, 1,7-Me-Pn*); 5.29 (d, 2H, 3JH−H = 3.4 Hz, 2,9-H-Ind); 5.45 (t, 1H, 3JH−H = 3.4 Hz, 1-H-Ind); 6.90 (m, 2H, 5,6-HInd); 7.45 (m, 2H, 4,7-H-Ind). 13C{1H} NMR (100 MHz, C6D6) δ (ppm): 10.1 (2,6-Me-Pn*); 13.6 (3,5-Me-Pn*); 13.8 (1,7-Me-Pn*); 96.9 (2,9-Ind); 114.2 (3,5-Pn*); 116.1 (1-Ind); 119.7 (4-Pn*); 121.4

EXPERIMENTAL SECTION

All reactions were performed under an inert N2 atmosphere using standard Schlenk line techniques and, where required, an MBraun UNIlab glovebox. Pentane, hexane, benzene, and toluene were dried and degassed using an MBraun SPS-800 solvent purification system. They were stored over a potassium mirror and degassed under partial vacuum before use. thf was distilled from Na/benzophenone and stored over 4 Å molecular sieves. Et2O was distilled from Na/K and stored over a potassium mirror. Benzene-d6 and toluene-d8 were purchased from Goss Scientific and were dried under partial vacuum over Na/K. They were degassed with three freeze−pump−thaw degas cycles and stored under N2. Solution samples for NMR spectroscopy were made up in the glovebox, using 5 mm Young’s tap NMR tubes. Spectra were recorded either on a Bruker Avance III HD Nanobay 400 MHz NMR or on a Bruker Avance III 500 MHz NMR and were referenced to the residual protio-solvent peak. Solid-state NMR spectra were recorded on a Bruker Avance III HD in 4 mm OD rotors using contact times for CPMAS spectra of 3000 μs. MeLi (1.6 M in Et2O) was purchased from Sigma-Aldrich; LiCp t

n

was purchased from Alfa Aesar; LiCpMe, LiCp Bu, and LiCp Bu were purchased from MCat GMBH, and these products were used as supplied. The following compounds were synthesized according to the literature procedure: polymethylaluminoxane,34 LiCpMe3,35 [Pn*TiCl(μ-Cl)] 2 (E), 6d [ZrPn*(μ-Cl) 3/2 ] 2 (μ-Cl) 2 Li·thf X Et 2 O Y (F), 6d Pn*TiCpCl (C),6e and Pn*ZrCpCl (D).6e H

DOI: 10.1021/acs.organomet.6b00417 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

mixture was stirred at this temperature for 1 h. The reaction mixture was then slowly warmed to room temperature before being stirred for 1 h. The solvent was removed in vacuo, and the solids were extracted into benzene (3 × 2 mL) and lyophilized overnight. The solid was washed with −78 °C pentane (2 × 3 mL) and dried under vacuum overnight, giving 2.4 as a light tan powder in 53% yield (137 mg, 0.326 mmol). Single crystals suitable for an X-ray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C22H29ClZr: C, 62.89 (61.69); H, 6.96 (7.15). 1H NMR (400 MHz, C6D6) δ (ppm): 1.64 (s, 6H, 2,6-Me-Pn*); 1.76 (s, 6H, 1,3-Me-Cp); 1.90 (s, 6H, 3,5-Me-Pn*); 2.03 (s, 3H, 2-Me-Cp); 2.14 (s, 6H, 1,7-Me-Pn*); 4.82 (s, 2H, 4,5-H-Cp). 13C{1H} NMR (100 MHz, C6D6) δ (ppm): 10.5 (2,6-Me-Pn*); 12.1 (2-Me-Cp); 12.4 (1,3-MeCp); 12.8 (1,7-Me-Pn*); 13.4 (3,5-Me-Pn*); 104.1 (3,5-Pn*); 105.1 (4,5-Cp); 112.0 (1,7-Pn*); 118.9 (1,3-Cp); 119.3 (4-Pn*); 125.2 (2,6Pn*); 126.8 (8-Pn*); 127.3 (2-Cp). Pn*ZrIndCl (2.5). To [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li·thf(1.02) (F) (300 mg, 0.369 mmol) in Et2O (20 mL) at −78 °C was transferred a slurry of LiInd (90.1 mg, 0.738 mmol) in Et2O (15 mL) at −78 °C, and the contents were stirred for 1 h. The reaction mixture was allowed to warm to room temperature, then stirred for 1 h, before the solvent was removed under vacuum. The solid was extracted into benzene (3 × 2 mL) and lyophilized overnight. The powder was recrystallized from pentane at −80 °C, giving 2.5 as a yellow powder in 75% yield (239 mg, 0.558 mmol). Single crystals suitable for an Xray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C23H25ClZr: C, 64.53 (65.19); H, 5.89 (5.96). 1H NMR (400 MHz, C6D6) δ (ppm): 1.49 (s, 6H, 2,6Me-Pn*); 1.91 (s, 6H, 3,5-Me-Pn*); 1.94 (s, 6H, 1,7-Me-Pn*); 5.51 (d, 2H, 3JH−H = 3.4 Hz, 2,9-H-Ind); 5.78 (t, 1H, 3JH−H = 3.4 Hz, 1-H-Ind); 6.93 (m, 2H, 5,6-H-Ind); 7.51 (m, 2H, 4,7-H-Ind). 13C{1H} NMR (100 MHz, C6D6) δ (ppm): 10.4 (2,6-Me-Pn*); 12.5 (1,7-Me-Pn*); 13.1 (3,5-Me-Pn*); 95.1 (2,9-Ind); 105.7 (3,5-Pn*); 112.4 (1,7-Pn*); 119.1 (1-Ind); 119.8 (4-Pn*); 123.4 (4,7-Ind); 124.0 (5,6-Ind); 126.3 (2,6-Pn*); 126.5 (3,8-Ind); 128.6 (8-Pn*). Pn*ZrCpMeMe (2.6). A solution of Pn*ZrCpMeCl (200 mg, 0.510 mmol) in toluene (15 mL) at −78 °C was added to a solution of MeLi (1.6 M in Et2O, 319 μL, 0.510 mmol) in toluene at −78 °C. The reaction mixture was stirred for 1 h before being exposed to dynamic vacuum while still in the cold bath. The solution was removed from the cold bath so that the removal of the solvent kept the temperature below 0 °C. The solid was extracted with hexane (4 × 5 mL), and the combined extracts were concentrated to 15 mL and cooled to −80 °C, yielding a yellow microcrystalline solid, after standing overnight. The supernatant was removed, and the solid dried in vacuo for 4 h to yield 2.6 in 50% yield (95 mg, 0.256 mmol). Single crystals suitable for an X-ray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C21H28Zr: C, 67.86 (67.73); H, 7.59 (7.71). 1H NMR (400 MHz, C6D6) δ (ppm): −0.74 (s, 3H, Zr-Me); 1.63 (s, 6H, 2,6-Me-Pn*); 1.85 (s, 3H, Me-Cp, 3H, s); 1.99 (s, 6H, 3,5Me-Pn*); 2.00 (s, 6H, 1,7-Me-Pn*); 5.15 (t, 2H, 3JH−H = 2.6 Hz, 2,5H-Cp); 5.37 (t, 2H, 3JH−H = 2.6 Hz, 3,4-H-Cp). 13C{1H} NMR (100 MHz, C6D6) δ (ppm): 10.6 (Zr-CH3); 10.9 (2,6-Me-Pn*); 12.4 (3,5Me-Pn*); 13.4 (1,7-Me-Pn*); 14.0 (Me-Cp); 102.2 (1,7-Pn*); 105.5 (3,4-Cp); 106.4 (3,5-Pn*); 111.4 (2,5-Cp); 116.9 (8-Pn*); 119.8 (1Cp); 123.1 (2,6-Pn*); 123.4 (4-Pn*). General Procedure for Supporting Catalysts on Solid Support. The polymethylaluminoxane, MAO-modified silica, or MAO-modified LDH was combined with the precatalyst and stirred together dry for 5 min. The stirring was halted, and toluene (10 mL) was added to the mixture and heated to 60 °C for 1 h. The contents were manually swirled every 5 min and after 1 h were allowed to settle, leaving a colored solid and a colorless solution. The supernatant was removed via cannula, and the solid dried under vacuum for 4 h. ZrPn*CpCl-sMAO (EsMAO). 13C CPMAS (10 kHz) δ (ppm): −13.3 (sMAO-Me); 6.3 (Pn*−Me); 106.2 (Cp-ring; Pn*-ring); 123.0 (sMAO-benzoate); 126.4 (sMAO-benzoate); 172.0 (sMAO-benzoate α-C(O)O). 27Al DPMAS (15 kHz) δ (ppm): −387.4; −250.7; −83.1; 75.7; 211.2; 347.0.

(1,7-Pn*); 123.2 (2,6-Pn*) 124.0 (4,7-Ind); 124.6 (5,6-Ind); 126.7 (3,8-Ind); 129.9 (8-Pn*) Pn*ZrCpMeCl (2.1). To [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li.Et2O(1.21) (F) (300 mg, 0.362 mmol) in Et2O (20 mL) at −78 °C was added a slurry of LiCpMe (62.3 mg, 0.724 mmol) in Et2O (15 mL) at −78 °C. The reaction mixture was allowed to warm to room temperature over the course of 1 h and then stirred for 1 h. The volatiles were removed under vacuum, and the resulting solid was extracted into benzene (3 × 2 mL) and lyophilized. The solid was washed with −78 °C pentane (2 × 3 mL) and dried under vacuum for 4 h to afford 2.1 in 54% yield (152 mg, 0.388 mmol). Single crystals suitable for an X-ray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C20H25ClZr: C, 61.27 (61.30); H, 6.43 (6.38). 1H NMR (400 MHz, C6D6) δ (ppm): 1.66 (s, 6H, 2,6-Me-Pn*); 1.81 (s, 6H, 3,5-Me-Pn*); 2.12 (s, 9H, 1,7-Me-Pn*, Cp-Me; overlapping resonances); 5.13 (t, 2H, 3JH−H = 2.6 Hz, 2,5-H-Cp); 5.59 (t, 2H, 3 JH−H = 2.6 Hz, 3,4-H-Cp). 13C{1H} NMR (100 MHz, C6D6) δ (ppm): 11.1 (2,6-Me-Pn*); 12.7 (1,7-Me-Pn*); 13.4 (3,5-Me-Pn*); 14.8 (Cp-Me); 105.2 (3,5-Pn*); 106.4 (2,5-Cp); 112.3 (1,7-Pn*); 114.0 (3,4-Cp); 119.5 (4-Pn*); 124.6 (1-Cp); 125.5 (2,6-Pn*); 128.6 (8-Pn*). t

Pn*ZrCp BuCl (2.2). To [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li.Et2O(1.21) (F) (300 mg, 0.362 mmol) in Et2O (20 mL) at −78 °C was added a slurry t

of LiCp Bu in Et2O (15 mL) at −78 °C. The reaction mixture was warmed to room temperature over the course of 1 h and then stirred for 1 h. The volatiles were removed under vacuum, and the solids extracted into benzene (3 × 2 mL) and lyophilized. The solid was washed with −78 °C pentane (2 × 3 mL) and dried under vacuum for 4 h to afford 2.2 in 80% yield (253 mg, 0.583 mmol). Analytical samples were prepared by recrystallizing the product from pentane at −78 °C. Single crystals suitable for an X-ray diffraction study were grown from slow evaporation of a benzene solution. Anal. Calcd (found) for C23H31ClZr: C, 63.63 (63.55); H, 7.20 (7.33). 1H NMR (400 MHz, C6D6) δ (ppm): 1.33 (s, 9H, tBu-Cp); 1.71 (s, 6H, 2,6-MePn*); 1.83 (s, 6H, 3,5-Me-Pn*); 2.09 (s, 6H, 1,7-Me-Pn*); 5.04 (t, 2H, 3 JH−H = 2.8 Hz, 2,5-H-Cp); 5.96 (t, 2H, 3JH−H = 2.8 Hz, 3,4-H-Cp). 13 C{1H} NMR (100 MHz, C6D6) δ (ppm): 11.6 (2,6-Me-Pn*); 12.6 (1,7-Me-Pn*); 13.3 (3,5-Me-Pn*); 32.2 (CMe3-Cp); 104.2 (2,5-Cp); 105.0 (3,5-Pn*); 112.3 (1,7-Pn*); 113.9 (3,4-Cp); 119.4 (4-Pn*); 126.1 (2,6-Pn*); 128.6 (8-Pn*); 138.8 (1-Cp). n

n

Pn*ZrCp BuCl (2.3). LiCp Bu (79 mg, 0.617 mmol) was ground with an agate pestle and mortar and added to an ampule containing [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li·thf(0.988) (F) (250 mg, 0.308 mmol). Et2O (20 mL) was cooled to −78 °C and transferred onto the solids and stirred vigorously for 1 h. The ampule was removed from the cold bath and sonicated for 1 h. The reaction mixture was then stirred for a further hour at room temperature before the solvent was removed under vacuum to afford an orange oil that crystallizes slowly on standing. Following extraction into benzene (3 × 2 mL) and lyophilization, 2.3 was afforded as a brown solid in 67% yield (179 mg, 0.412 mmol). Single crystals suitable for an X-ray diffraction study were grown from a saturated (Me3Si)2O solution at −35 °C. Anal. Calcd (found) for C23H31ClZr: C, 63.63 (63.71); H, 7.20 (7.21). 1H NMR (400 MHz, C6D6) δ (ppm): 0.87 (t, 3H, 3JH−H = 7.2 Hz, CH3(CH2)3-Cp); 1.30 (m, 2H, CH3CH2(CH2)2-Cp); 1.42 (m, 2H, CH3CH2CH2CH2-Cp); 1.70 (s, 6H, 2,6-Me-Pn*); 1.84 (s, 6H, 3,5-MePn*); 2.11 (s, 6H, 1,7-Me-Pn*); 2.6 (m, 2H, CH3CH2CH2CH2-Cp); 5.15 (t, 2H, 3JH−H = 2.7 Hz, 3,4-H-Cp); 5.68 (t, 2H, 3JH−H = 2.7 Hz, 2,5-H-Cp). 13C{1H} NMR (100 MHz, C6D6) δ (ppm): 11.3 (2,6-MePn*); 12.6 (1,7-Me-Pn*); 13.3 (3,5-Me-Pn*); 14.3 (CH 3 CH 2 CH 2 CH 2 -Cp); 23.0 (CH 3 CH 2 CH 2 CH 2 -Cp); 29.3 (CH3CH2CH2CH2-Cp); 33.8 (CH3CH2CH2CH2-Cp); 105.2 (3,5Pn*); 106.0 (3,4-Cp); 112.2 (1,7-Pn*); 113.4 (2,5-Cp); 119.5 (4Pn*); 125.6 (2,6-Pn*); 128.6 (8-Pn*); 130.0 (1-Cp). Pn*ZrCpMe3Cl (2.4). [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li·thf(1.02) (F) (250 mg, 0.308 mmol) was dissolved in Et2O (20 mL) and cooled to −78 °C. A slurry of LiCpMe3 (70.2 mg, 0.615 mmol) in −78 °C Et2O (15 mL) was transferred to this solution via cannula, and the reaction I

DOI: 10.1021/acs.organomet.6b00417 Organometallics XXXX, XXX, XXX−XXX

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Organometallics ZrPn*CpMeCl-sMAO (2.1sMAO). 13C CPMAS (10 kHz) δ (ppm): −13.3 (sMAO-Me); 6.7 (Pn*-Me); 15.1 (Cp-Me); 106.7 (Cp-ring; Pn*-ring); 122.9 (sMAO-benzoate); 126.0 (sMAO-benzoate); 130.8 (sMAO-benzoate); 172.4 (sMAO-benzoate α-C(O)O). 27Al HAHNECHO (15 kHz) δ (ppm): −242.4; −86.4; 97.6; 179.2; 330.3.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank Dr. Nicholas H. Rees for solidstate NMR spectroscopy, Dr. Alexander Kilpatrick for the synthesis of polymethylaluminoxane support, Norner AS, Norway, for GPC measurements, Mr. Phakpoom Angpanitcharoen for SEM, Chemical Crystallography (University of Oxford) for use of the diffractometers, and SCG Chemicals Co. Ltd, Thailand, for funding (D.A.X.F., Z.R.T., and J.-C.B.). Z.R.T. also thanks Trinity College Oxford for a Junior Research Fellowship.

t

ZrPn*Cp BuCl-sMAO (2.2sMAO). 13C CPMAS (10 kHz) δ (ppm): −13.3 (sMAO-Me); 6.7 (Pn*-Me); 26.9 (Cp-C-Me3); 111.2 (Cp-ring; Pn*-ring); 123.0 (sMAO-benzoate); 126.0 (sMAO-benzoate); 130.7 (sMAO-benzoate); 171.9 (sMAO-benzoate α-C(O)O). 27Al DPMAS (15 kHz) δ (ppm): −246.8; −80.5; 74.2; 204.4; 351.0. n

ZrPn*Cp BuCl-sMAO (2.3sMAO). 13C CPMAS (10 kHz) δ (ppm): −13.3 (sMAO-Me); 6.5 (Pn*-Me); 15.9 (Cp-nButyl); 23.0 (CpnButyl); 28.4 (Cp-nButyl); 108.6 (Cp-ring; Pn*-ring); 123.1 (sMAObenzoate); 126.2 (sMAO-benzoate); 131.2 (sMAO-benzoate); 172.5 (sMAO-benzoate α-C(O)O). 27Al HAHNECHO (15 kHz) δ (ppm): −241.1; −84.2; 72.4; 204.3; 343.2. ZrPn*CpMe3Cl-sMAO (2.4sMAO). 13C CPMAS (10 kHz) δ (ppm): −13.4 (sMAO-Me); 6.2 (Pn*-Me); 107.2 (Cp-ring; Pn*-ring); 123.0 (sMAO-benzoate); 126.0 (sMAO-benzoate); 130.7 (sMAO-benzoate); 172.0 (sMAO-benzoate α-C(O)O). 27Al DPMAS (15 kHz) δ (ppm): −385.1; −247.7; −78.1; 32.7; 203.7; 349.0. ZrPn*IndCl-sMAO (2.5sMAO). 13C CPMAS (10 kHz) δ (ppm): −13.3 (sMAO-Me); 6.3 (Pn*-Me); 122.9 (sMAO-benzoate); 130.8 (sMAO-benzoate); 172.8 (sMAO-benzoate α-C(O)O). 27Al DPMAS (15 kHz) δ (ppm): −382.5; 247.1; −75.2; 33.2; 209.4; 353.0. ZrPn*CpMeMe-sMAO (2.6sMAO). 13C CPMAS (10 kHz) δ (ppm): −13.3 (sMAO-Me); 6.2 (Pn*-Me); 106.4 (Cp-ring; Pn*-ring); 122.8 (sMAO-benzoate); 126.0 (sMAO-benzoate); 130.0 (sMAO-benzoate); 171.7 (sMAO-benzoate α-C(O)O). 27Al HAHNECHO (15 kHz) δ (ppm): −240.0; − 81.1; 83.0; 208.6; 345.9. General Procedure for Solution-Phase Polymerization of Ethylene. The precatalyst (2 mg) was dissolved in toluene (2 mL). An ampule was charged with MAO (250 equiv) and toluene (50 mL), before 500 μL of the catalyst solution was transferred to the ampule. The reaction mixture was placed in an oil bath at the required temperature and allowed to equilibrate for 5 min while the headspace was degassed. The flask was opened to ethylene (2 bar) and stirred at 1200 rpm for the duration of the experiment. The polymer was then filtered, washed with pentane (2 × 20 mL), and dried at 5 mbar overnight. General Procedure for Slurry-Phase Polymerization of Ethylene. An ampule was charged with TiBA (150 mg, 0.756 mmol), toluene (50 mL), and the support complex (10 mg). The contents were placed in an oil bath at the required temperature and allowed to equilibrate for 5 min while the headspace was degassed. The flask was opened to ethylene (2 bar) and stirred at 1200 rpm for the duration of the experiment. The polymer was then filtered, washed with pentane (2 × 20 mL), and dried at 5 mbar overnight.

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00417. Definitions of structural parameters for pentalene complexes, NMR spectra for all compounds, crystallographic details, DFT calculation details. (PDF) Crystallographic data (CIF)

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DOI: 10.1021/acs.organomet.6b00417 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.6b00417 Organometallics XXXX, XXX, XXX−XXX