ARTICLE pubs.acs.org/Organometallics
Pyridylamido Hafnium and Zirconium Complexes: Synthesis, Dynamic Behavior, and Ethylene/1-Octene and Propylene Polymerization Reactions Kevin A. Frazier, Robert D. Froese, Yiyong He, Jerzy Klosin,* Curt N. Theriault, Paul C. Vosejpka, and Zhe Zhou Corporate R&D, The Dow Chemical Company, 1776 Building, Midland, Michigan 48674, United States
Khalil A. Abboud Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States
bS Supporting Information ABSTRACT: Hafnium and zirconium pyridylamido complexes were prepared by the reaction of deprotonated ligand with MCl4 followed by alkylation with MeMgBr. 1H NMR analysis of the isolated products revealed the presence of two complexes in each sample in about a 93:7 ratio. Both complexes were shown by 1D NOESY experiments to be in dynamic equilibrium with each other at ambient temperature. An analysis of the chemical shift differences between major and minor isomers together with low-temperature NOE experiments revealed that the difference between major and minor isomers is due to the rotation of the 2-iPr-phenyl group attached to the chiral center. Magnetization transfer experiments conducted at temperatures between 10 and 40 °C yielded ΔH‡ and ΔS‡ of 14.3(6) kcal/mol and 11(2) cal/ mol 3 K, respectively. Density functional theory calculations resulted in very similar activation parameters. Additionally, this fluxional process was calculated to occur in the cationic species with a rate comparable to that of propylene propagation kinetics. Both complexes have been studied as procatalysts for ethylene/1-octene copolymerization and propylene polymerization reactions. 13C NMR analysis of polypropylene obtained under high-temperature polymerization conditions allowed for the unequivocal determination of the identity of a second 2,1-regioerror.
’ INTRODUCTION The last two decades have seen extensive research devoted to the development of non-Cp-based molecular catalysts for olefin polymerization.13 We have been interested recently in iminoamido46 and imino-enamido7 type catalysts and uncovered an interesting diversity of polymerization characteristics. Closely related to the imino-amido catalyst family are catalysts containing the pyridylamido ligand framework, which were developed initially at Union Carbide8 (generation 1) and further elaborated via a joint collaboration between Symyx Technologies and The Dow Chemical Company9,10 (generation 2). The general structures of the pyridylamido catalysts and the chiral procatalyst 1 used in this work are depicted in Figure 1. This catalyst family has been the subject of much research due to unusual catalyst and polymer properties. Pyridylamido catalysts form high molecular weight, isotactic polypropylene, which is very unusual for the C1/Cs-symmetric catalysts.2d,11 This catalyst family was also identified as very effective for the preparation of olefin block copolymers due to their ability to undergo reversible and efficient chain-transfer reactions with some metal alkyl complexes (e.g., ZnEt2) during olefin polymerization.3,12 Pyridylamido catalysts were also r 2011 American Chemical Society
recently reported to exhibit living character.11 Interestingly, the formation of a multimodal polymer is often observed with catalysts of this type, indicating the lack of a single site. The activation mechanism of these catalysts is unusual and has been studied extensively via experimental and computational methods.13,14 It has been hypothesized that the active catalytic species is formed by insertion of an ethylene or R-olefin into the Hfaryl bond, leading to a modified catalyst, 1a (Figure 1). In ethylene/1-octene and propylene polymerization reactions, insertion of the olefin(s) into the Hfaryl bond can lead to multiple species, which results in the formation of multiple active polymerization sites generating a mixture of polymers. Most of the catalytic and mechanistic work reported thus far has focused on the study of chiral and achiral hafnium complexes. It was of interest to us to evaluate the zirconium8,15 analogue of 1 for both ethylene/1-octene and propylene polymerizations and compare its polymerization characteristics to that of 1. Depending on the ligand framework, zirconium and hafnium complexes can exhibit quite different polymerization behaviors; thus Received: February 21, 2011 Published: May 25, 2011 3318
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Organometallics synthesis and evaluation of 3 was warranted. In addition to the complications with catalyst activation mentioned above, it has been known that there is a persistent additional species in procatalyst 1, composition of which was unknown. This work will shed light on the identity of this additional species,
Figure 1. Pyridylamido procatalysts and their activation.
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ultimately adding an additional level of complexity to this intriguing catalyst family.
’ RESULTS AND DISCUSSION Previously, complex 1 was prepared by refluxing the corresponding ligand (2) with Hf(NMe2)4 followed by the reaction of the isolated triamide derivative, 2-Hf(NMe2)3, with AlMe3. We desired to prepare 1 and its Zr analogue by a more easily scalable
Figure 3. Thermal ellipsoid drawing of 3 at the 40% probability level. Hydrogen atoms have been removed for clarity.
Scheme 1. Synthesis of Procatalysts 1 and 3
Figure 2. 1H NMR spectrum of 1. Asterisks indicate some resonances of the minor component. 3319
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Organometallics method and developed an improved synthesis as outlined in Scheme 1. Ligand 216 was reacted with one equivalent of n-BuLi at ambient temperature in toluene followed by the reaction of the resulting anion with hafnium or zirconium tetrachloride at 110 °C for 1 h. The in situ formed trichloride intermediate was reacted with three equivalents of MeMgBr at ambient temperature to produce the desired complexes, 1 and 3. Both complexes were characterized by elemental analysis, 1D (Figure 2) and 2D NMR spectroscopy, and single-crystal X-ray analysis. X-ray structures of 1 and 3 are isostructural, and only the molecular structure of 3 is shown here (Figure 3).16 Interestingly, all regions of the 1H NMR spectra of both complexes 1 and 3 revealed the presence of a minor component at a level of about 7% (Figure 2). Since ethylene/1-octene copolymers with a broad polydispersity index (PDI) have been observed previously with 1, it was reasoned that the minor component might be catalytically active
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and directly responsible for polymer bimodality. Thus, it was important to us to determine the identity of this minor component. This minor component appeared to be structurally similar to 1, as all of its resonances exhibited very similar chemical shifts and multiplicity to those observed in 1. In an effort to identify these resonances, a comprehensive NMR analysis was carried out. The most important and unexpected piece of information that allowed for determining the structure of this minor component in the spectra of 1 came from 1D NOESY experiments. In addition to the expected positive NOEs, 1D NOESY spectra also revealed negative EXSY peaks between 1 and the minor component. This fact clearly indicates that the minor component is in chemical exchange with 1. Full analysis of the NOE and TOCSY data allowed for the full assignment of all resonances of the minor component, which from now on will be
Figure 4. Rotational isomers 1 and 10 .
Figure 5. 1H NMR and 1D NOESY spectra of 1 and 10 (in C6D6) at 7 °C (mixing time = 0.7 s). Asterisk designates residual toluene. 3320
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Figure 6. 1D NOE array spectra of 1 and 10 (in toluene-d8) at 30 °C.
Figure 7. Calculated structure of 1 (major isomer).
Figure 9. Calculated transition state (TS) for the interconversion of 1 and 10 .
Figure 8. Calculated structure of 10 (minor isomer).
referred to as either the minor isomer or 10 . The threedimensional structures of 1 and 10 were shown to differ in the orientation of the (2-iPr)Ph group relative to the remaining part of the ligand framework (Figure 4). This conclusion was reached from several independent pieces of data. The first clue as to the three-dimensional structure of the minor isomer came from the analysis of the chemical shift differences between both isomers. The chemical shift difference between most of the resonances of 1 and 10 was found to be small (within 0.05 ppm) except for the resonances associated with the (2-iPr)Ph group attached to the chiral center (C14) and its neighboring proton
H14.17 For example, the difference between the chemical shifts of H10/H100 , H11/H110 , H14/H140 , and H18/H180 are 0.68, 0.26, 0.44, and 0.56 ppm, respectively. If the (2-iPr)Ph fragment has two different orientations as depicted in Figure 4, then the NOE experiments might be able to provide unequivocal evidence for it. To minimize contributions from EXSY peaks, NOE NMR experiments were conducted at a lower temperature (7 °C). The 1H NMR and 1D NOESY spectra obtained at 7 °C are shown in Figure 5. Fortuitously, at 7 °C the H14 and H9 resonances are adequately separated (they overlap at 30 °C) such that they can be independently irradiated. Since the distance between protons H9/H14 in the major isomer is approximately the same as that of H90 /H140 in the minor isomer (the DFT-calculated distances are 2.67 Å for H9/H14 and 2.68 Å for H90 /H140 ), it was expected that irradiation of H14 and H140 would result in a similar intensity of NOEs for H9 and H90 , respectively (internal check). In the first NOE experiment, the H14 resonance was irradiated, resulting in strong NOEs of H9, H18, H20, and H25 (Figure 5, middle spectrum). It is important to note that strong NOEs are observed for two out of three methine resonances. There is also NOE enhancement for the H10 resonance, but its intensity is very small. On the other hand, irradiation of H140 resulted in very strong NOEs for H90 , H100 , H200 , and H250 (Figure 5, bottom spectrum). The appearance of 3321
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Table 1. Selected Bond Lengths and Angles for the Calculated and Experimentally Determined Molecular Structuresa bond/angle
1
1 (X-ray)
10
TS
HfN2
2.325
2.295(2)
2.331
2.328
HfN1 HfC27
2.094 2.220
2.081(2) 2.223(3)
2.099 2.218
2.122 2.220
HfC28
2.230
2.210(3)
2.226
2.217
HfC32
2.287
2.256(2)
2.288
2.294
N2C14
1.469
1.475(3)
1.469
1.491
N2C38
1.442
1.444(3)
1.444
1.443
C14C34
1.515
1.501(3)
1.514
1.522
C31C32
1.399
1.391(3)
1.398
1.395
C31C33 C27HfC28
1.480 107.8
1.489(3) 105.7(2)
1.480 106.2
1.478 111.1
N1HfC32
69.6
69.56(8)
69.4
69.6
N1HfN2
71.5
72.12(7)
71.5
72.3
C34C14N2
108.7
109.2(2)
109.1
105.2
C35C14N2
114.6
114.1(2)
117.2
112.7
a
Metric parameters are based on the geometries shown in Figure 79 for the calculated structures.
a strong NOE for H100 and the disappearance of the NOE for H180 is a strong indication that the difference between 1 and 10 is due to the different disposition of the (2-iPr)Ph fragment relative to the rest of the molecule. To evaluate the activation parameters for this fluxional process, a series of magnetization transfer experiments was conducted between 10 and 40 °C. An example set of arrayed 1D NOESY spectra is shown in Figure 6.18 The obtained rate constants and corresponding Eyring plot are shown in the Supporting Information. Data analysis results in ΔH‡ and ΔS‡ of 14.3(6) kcal/mol and 11(2) cal/mol 3 K, respectively. Integration of the resonances corresponding to the major and minor isomers in the 1H NMR spectrum gives an equilibrium constant (Keq) of 14 at 30 °C, which corresponds to a free energy (ΔG°) difference of 1.6 kcal/mol. For comparison, the rate constant for interconversion of 3 and 30 was measured at 30 °C using magnetization transfer experiments. The measured rate of 1.66(3) s1 is slightly faster than that of interconversion between 1 and 10 , which was determined to be 1.22(2) s1 at the same temperature. The calculated structures of 1, 10 , and the transition state (TS) for their interconversion are shown in Figure 79, respectively.19 Selected bond lengths and angles for the calculated structures are collected in Table 1. Except for the orientation of the (2-iPr)Ph fragment, both structures 1 and 10 are very similar. However, the transition state is distorted significantly compared to the two minima. First, the five-membered ring containing Hf N1C34C14N2 is relatively planar in 1 and 10 but is noticeably puckered in the TS. The HfN1C34C14 torsion angles for 1, 10 , and TS are 14.9°, 8.4°, and 38.7°, respectively. Second, the 2,6-diisopropylphenyl fragment is tilted away from the (2-iPr)Ph fragment in the TS. The C30 C38N2C14 torsion angles for 1, 10 , and TS are 105.7°, 107.7°, and 122.8°, respectively. Another notable aspect is that the (2-iPr)Ph fragment could rotate in either direction in the TS, but prefers to reside toward the pyridine rather than being directed toward the sterically encumbered 2,6-diisopropylphenyl group (see Figure 9). Thus the equilibrium process, 1 T 10 ,
Figure 10. Isomerization of 1/10 and the anticipated activation steps and possible isomerizations of various cationic species.
involves a windshield wiper motion (back and forth) rather than a full rotation. The enthalpy and entropy of activation as calculated using density functional theory are ΔH‡ = 16.9 kcal/mol and ΔS‡ = 9.9 cal/mol 3 K. These values agree well with the experimentally determined kinetic parameters of ΔH‡ = 14.3 kcal/mol and ΔS‡ = 11 cal/mol 3 K. Reaction thermodynamics for the isomerization process from 1 to 10 are computed to be ΔH° = 1.7 kcal/mol and ΔS° = 1.5 cal/mol 3 K, which at room temperature equates to an equilibrium mixture of 97:3. This value agrees very well with the experimentally determined equilibrium ratio (93.3:6.7). These same isomerization barriers were also determined for the cationic catalyst, formed by the removal of one methyl group, as well as the likely active catalyst,13 the olefin appended species (see 1a in Figure 1). The methyl cation is not necessarily the best representation of the active catalyst, as the open coordination site could be occupied by a solvent molecule, olefin, β-agostic interaction (in a longer polymer chain), or the counterion. However, our goal was to determine if there were gross differences for the rotation of the (2-iPr)Ph fragment in the dimethyl neutral catalyst (1) and the two cationic representations of active species. The isomerization processes for the neutral procatalysts 1 and 10 and their corresponding cationic species are depicted in Figure 10. The computed barrier for the cationic systems were determined to be even lower than that of the neutral species with ΔH‡ = 12.9 kcal/mol for activated 1 and ΔH‡ = 13.9 kcal/mol for ethylene-appended 1 (Figure 10). While the calculations for the active cationic catalysts are only estimates, these barriers are low and one would expect the corresponding rotation of the (2-iPr)Ph group to be as fast, if not faster, than the rate of polymerization. The relative rate at which the (2-iPr)Ph group rotates is important since the two possible procatalysts, 1 and 10 , are structurally different enough 3322
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Table 2. Ethylene/1-Octene Copolymerization Data for Complexes 1, 3, and CGCa Mw 103 (kDa)
yield (g)
catalyst activityb
1 (1.5)
19.9
13 267
3 (1.5)
8.4
8400
CGC (0.2)
46.7
233 500
59.1
150
1 (2.5)
17.5
7000
51.1
565
150
3 (2.0)
7.6
3800
71/114
301
150
CGC (0.4)
47.5
118 750
run #
poly. temp (°C)
1
120
2
120
3
120
4 5 6
catalyst (μmol)
Tm (°C) 53.8 68/106
55.4
Mw/Mn
octene content (mol %)
1420
3.1
867
3.3
8.5
2.2
15.6
8.2
12.7
50.2
28.6
136 1.8
12.1
8.4 14.6
a
Polymerization conditions: 533 mL of Isopar-E; 250 g of 1-octene; ethylene pressure = 460 psi; procatalyst:activator:MMAO = 1:1.2:10; activator: [HNMe(C18H37)2][B(C6F5)4]; hydrogen = 10 mmol; reaction time 15 min. b Activity: g of polymer/mmol cat. CGC = {(η5-C5Me4)(SiMe2-NtBu)}TiMe2. 1-Octene content determined by FTIR.
Table 3. Propylene Polymerization Data for Complexes 1 and 3 (high propylene concentration)a run
catalyst
yield
catalyst b
Tm
%
Mw 103
(°C) mmmm
(kDa)
23 800
153.9
94.2
198
10 900
146.6
92.6
#
(μmol)
(g)
activity
7
1 (1)
23.8
8
3 (1)
10.9
63.4
Mw/Mn 3.0 2.9
Polymerization conditions: temp = 90 °C; 667 mL of Isopar-E; 286 g of propylene; hydrogen = 8.3 mmol; procatalyst:activator:MMAO = 1:1.2:10; activator: [HNMe(C18H37)2][B(C6F5)4]; reaction time 10 min. b Activity: g of polymer/mmol cat. a
that they are likely to produce different types of polymers. The structural differences between the two isomers have already been discussed, but as an example of the contrasting geometries, one can compare the distance between the methine proton of the (2-iPr)Ph group and the hafnium atom. In 1, this distance is 5.570 Å, while in 10 this distance has been reduced to 3.050 Å, indicating a more sterically crowded environment. Since even achiral versions of pyridylamide catalysts {without (2-iPr)Ph)} produce multimodal polymers as a result of ligand modification,13 it is unclear what the role of the (2-iPr)Ph rotation is in 1 and 3 with regard to polymer properties. Ethylene/1-Octene Copolymerization Study. Ethylene/1octene copolymerization reactions was also conducted in a 2 L batch reactor at 120 °C containing 460 psi ethylene pressure and 250 g of 1-octene. In addition to complexes 1 and 3, (η5C5Me4)(SiMe2-N-tBu)TiMe2 (CGC) was also included in the study for comparison. Procatalysts were activated with 1.2 equivalents (relative to procatalyst) of [HNMe(C18H37)2][B(C6F5)4] activator. All polymerization reactions were carried out for 15 min and stopped by venting the ethylene pressure. At a polymerization temperature of 120 °C, the catalytic activity of 1 is about 60% higher than that of the zirconium analogue 3, but is 18 times lower than the catalytic activity of CGC. At 150 °C, the activity decreases but the relative activity trends between catalysts remain almost the same. The hafnium complex 1 is a good 1-octene incorporator, leading to polymers with 1-octene levels that are higher than those produced by 3 but somewhat lower than those produced by CGC (at both polymerization temperatures). The same 1-octene incorporation trends (hafnium leads to higher incorporation of 1-octene than the zirconium analogue) were observed also within the imino-amido and imino-enamido families of catalysts.4,7 Pyridylamido complex 1 is a better 1-octene incorporator than hafnium imino-amido and
Figure 11. 3D structures of four possible 2,1-insertion regioerrors in iPP.
imino-enamido catalysts.4,7 The most remarkable polymerization data included in Table 2 are the molecular weight of polymers produced by 1 and 3. At a polymerization temperature of 120 °C, procatalyst 1 gives an ultrahigh molecular weight ethylene/1-octene copolymer with a Mw of 1422 kDa, which is about 30 times higher than that produced by the CGC catalyst. The molecular weights of polymers produced by 1 at 120 °C are not only higher than those produced by CGC but also higher than those generated by hafnium imino-enamido complexes, which have Mw of about 1000 kDa under very similar polymerization conditions. The zirconium complex 3 results in polymers with high Mw that is about half of that produced by the Hf analogue. Procatalyst 1 is a very rare example of a catalyst that is capable of producing ethylene/ 1-octene copolymers at 120 °C with a Mw above 1000 kDa. Propylene Polymerization Study. Propylene polymerization reactions were conducted in a 1.8 L batch reactor under two different sets of conditions. The first set of experiments was conducted under high propylene concentration and 3323
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90 °C reactor temperature (Table 3). The data indicate that the zirconium complex 3 exhibits about 50% lower activity than 1. The Zr catalyst yields isotactic polypropylene (iPP) with significantly lower Mw and slightly lower tacticity as shown by a reduced melting point and 13 C NMR analysis. Previous studies2d,20 showed that pyridylamido catalyst 1 led to a 2,1-insertion regioerror (RE3, Figure 11), which is structurally different from the regioerrors observed in polymers prepared by standard C2-symmetric metallocene catalysts (RE1 and RE2). Initially, very low levels of the minor 2,1insertion regioerror in the polymers from 1 made it impossible to determine conclusively its nature. We decided to prepare several iPP samples under different reaction conditions using catalysts 1 and 3 in an effort to generate samples with significantly higher amounts of secondary minor 2,1-regioerrors, in order to allow us to establish its stereochemistry unambiguously. The second set of propylene polymerization reactions was performed at lower propylene pressure and two different reactor temperatures (60 and 120 °C), as shown in Table 4. Gratifyingly, 13 C NMR analysis of polymers produced at the higher temperature (120 °C) (runs 9 and 10) reveals significant formation of the
minor regioerror (Figure 12, label with # symbol). Chemical shift analysis of resonances of this minor regioerror suggests that they belong to either the RE1 or the RE4 microstructure. In order to assign this regioerror, a 2D HOESY experiment of iPP from run 9 was conducted (Figure 13). It was expected that the chemical shift difference of the two protons attached to carbons F and f (Figure 12) in RE1 and RE4, respectively, should be about 0.4 and 0.13 ppm, respectively. The chemical shift difference found in the sample from run 9 is 0.48 ppm (Figure 13), which indicates that this minor 2,1-regioerror belongs to the RE1
Table 4. Propylene Polymerization Data for Complexes 1 and 3 (low propylene concentration)a run temp
catalyst
yield
catalyst b
Tm
Mw 103
Mw/
#
(°C)
(μmol)
(g)
activity
(°C)
(kDa)
Mn
9
120
1 (0.5)
4.9
9800
144.7
88.7
2.7
10
120
3 (2.0)
2.8
1400
139.1
67.4
4.3
11
60
1 (0.5)
2.2
4400
156.7
702
2.2
12
60
3 (1.0)
7.2
7200
150.2
168
1.9
a
Polymerization conditions: 300 mL of Isopar-E; propylene = 80 g; hydrogen = 9.6 mmol; procatalyst:activator:MMAO = 1:1.2:10; activator: [HNMe(C18H37)2][B(C6F5)4]; reaction time 10 min. b Activity: g of polymer/mmol cat.
Figure 13. HOESY (1H13C) of iPP (resonances F and f) prepared at 120 °C (run 9). Mixing time is 100 ms.
Figure 12. 13C NMR of iPP prepared at 120 °C (run 9, top spectrum) and iPP prepared at 60 °C (run 13, bottom spectrum). Asterisks and number symbols designate RE3 and RE1 2,1-insertions, respectively. 3324
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Organometallics microstructure.20 The presence of RE1 and RE3 regioerrors in iPP produced by 1 and 3 indicates that these misinsertions are not triggered by the preceding stereoerror in the growing polymer chain and are formed by enchainment of 2,1-propylene coordinated with both enantiofaces to the metal center. Since procatalysts 1 and 3 can result in the formation of more than one active catalyst under polymerization conditions, it is not obvious if both regioerrors (RE3 and RE1) are formed by the same or different catalytic species.
’ CONCLUSIONS This work revealed that the second component present in the hafnium and zirconium pyridylamido procatalysts is a result of the presence of a minor diastereoisomer that is in dynamic equilibrium with the primary complex at ambient temperature. Analysis of chemical shift differences between the isomers together with lowtemperature NOE experiments and computational work showed that the difference between the major and minor isomers is due to the rotation of the 2-isopropylphenyl ring attached to the chiral center. Activation barriers obtained experimentally (ΔH‡ and ΔS‡ of 14.3(6) kcal/mol and 11(2) cal/mol 3 K, respectively, for the Hf complex) for this fluxional process compared favorably with computational data. Copolymerization studies of ethylene and 1-octene showed pyridylamide hafnium procatalyst 1 exhibits moderate activity at 120 °C, but it produces ultrahigh molecular weight (1420 kDa) copolymers at this temperature. Complex 1 is a very rare example of a procatalyst capable of producing such high molecular weight copolymers at temperatures above 100 °C. The zirconium analogue 3 gives lower activity and leads to polymers with a lower molecular weight and reduced 1-octene content compared to 1. Polypropylene produced by the pyridylamido zirconium 3 has slightly lower tacticity and lower molecular weight than the polymer prepared by the hafnium complex 1. Propylene polymerization at higher reactor temperature (120 °C) resulted in the formation of polypropylene with two distinct 2,1 insertion regioerrors. The minor regioerror was determined to be the same regioerror as the one observed in iPP produced by metallocene catalysts. ’ EXPERIMENTAL SECTION General Considerations. All syntheses and manipulations of airsensitive materials were carried out in a nitrogen-filled glovebox. Solvents were first saturated with nitrogen and then dried by passage through activated alumina and Q-5 catalyst prior to use. Deuterated NMR solvents (toluene-d8, C6D6) were dried over sodium/potassium alloy and filtered prior to use unless otherwise noted. NMR spectra were recorded on Varian Inova 300 (FT 300 MHz, 1H; 75 MHz, 13C) and VNMRS 500 (FT 500 MHz, 1H; 126 MHz, 13C) spectrometers. 1H NMR data are reported as follows: chemical shift, integration, multiplicity (br = broad, s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, hept = heptet, and m = multiplet) and assignment. Chemical shifts for 1H NMR data are reported in ppm downfield from tetramethylsilane (TMS, δ scale) using residual protons in the deuterated solvents (C6D6, 7.15 ppm, C6D5CD3, 2.09 ppm) as references. 13C NMR data were determined with 1H decoupling, and the chemical shifts are reported in ppm vs tetramethylsilane (C6D6, 128 ppm, toluene-d8, 20.4 ppm). 2,6-Bis(1-methylethyl)-N-[[6-(1-naphthalenyl)-2-pyridinyl]methylene]benzenamine and 2-isopropylphenyllithium were prepared according to published procedures.10c Sublimed grade HfCl4 was obtained from Strem, and the naphthylboronic acid used was obtained from Frontier Scientific. Elemental analyses were performed by Midwest Microlab, LLC. Preparation of N-[2,6-Bis(1-methylethyl)phenyl]-R-[2-(1-methylethyl)phenyl]-6-(1-naphthalenyl)-2-pyridinemethanamine (2). The 2,6-bis(1-
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methylethyl)-N-[[6-(1-naphthalenyl)-2-pyridinyl]methylene]benzenamine (2.20 g, 5.6 mmol) was magnetically stirred as a slurry in 6070 mL of dry ether under a nitrogen atmosphere. An ether solution of 2-isopropylphenyllithium (1.21 g, 9.67 mmol in 25 mL of dry ether) was added slowly using a syringe over a period of 45 min. After the addition was complete, a small sample was removed and quenched with 1 N NH4Cl, and the organic layer was analyzed by high-pressure liquid chromatography (HPLC) to check for complete consumption of starting imine. The remainder of the reaction mixture was quenched by the careful, slow addition of 1 N NH4Cl (10 mL). The mixture was diluted with more ether, and the organic layer was washed twice with brine, dried (anhydrous Na2SO4), filtered, and stripped of solvent under reduced pressure. The crude product, obtained as a thick red oil (2.92 g; 100% yield), was used without further purification. 1H NMR (500 MHz, toluene-d8): δ 8.14 (m, 1H), 7.68 (dd, 1H, J = 7.5, 1.5 Hz), 7.647.56 (m, 2H), 7.46 (dd, 1H, J = 7.1, 1.2 Hz), 7.297.20 (m, 3H), 7.197.09 (m, 4H), 7.087.00 (m, 5H), 5.66 (s, 1H, H14), 4.80 (s, 1H, NH), 3.29 (hept, 1H, J = 6.8 Hz, CH(CH3)2), 3.22 (hept, 2H, J = 6.8 Hz, CH(CH3)2), 1.081.03 (m, 6H, CH(CH3)2), 1.01 (d, 3H, J = 6.9 Hz, CH(CH3)2), 1.00 (d, 6H, J = 6.9 Hz, CH(CH3)2), 0.95 (d, 3H, J = 6.8 Hz, CH(CH3)2). 1H NMR (500 MHz, C6D6): δ 8.20 (m, 1H), 7.75 (m, 1H), 7.687.60 (m, 2H), 7.51 (dd, J = 7.1, 0.9 Hz, 1H), 7.307.23 (m, 3H), 7.217.05 (m, 8H), 7.02 (d, J = 7.6 Hz, 1H), 5.73 (s, 1H), 4.87 (s, 1H), 3.33 (hept, J = 6.8 Hz, 1H), 3.27 (hept, J = 6.7 Hz, 2H), 1.07 (d, J = 6.8 Hz, 6H), 1.02 (d, J = 6.8 Hz, 9H), 0.97 (d, J = 6.8 Hz, 3H). 13C{1H} NMR (126 MHz, C6D6): δ 163.01, 159.07, 146.71, 143.44, 143.41, 141.00, 138.86, 136.78, 134.55, 131.85, 129.24, 128.64, 128.52, 127.78, 127.68, 126.55, 126.53, 126.27, 126.00, 125.81, 125.33, 124.41, 123.96, 122.91, 120.19, 67.10, 28.85, 28.12, 24.36, 24.23, 23.81. 13C{1H} NMR (126 MHz, toluene-d8): δ 162.98 (quat), 159.06 (quat), 146.69 (quat), 143.47 (quat), 143.34 (quat), 141.04 (quat), 138.80 (quat), 136.73 (CH), 134.55 (quat), 131.83 (quat), 129.20 (CH), 128.62 (CH), 128.48 (CH), 127.70 (CH), 127.63 (CH), 126.54 (CH), 126.45 (CH), 126.22 (CH), 125.90 (CH), 125.73 (CH), 125.22 (CH), 124.37 (CH), 123.86 (CH), 122.84(CH), 120.09 (CH), 67.12 (CH), 28.85 (CH(CH3)2), 28.11 (CH(CH3)2), 24.32 (CH(CH3)2), 24.23 (CH(CH3)2), 24.20 (CH(CH3)2), 23.78 (CH(CH3)2). ES-HRMS (m/e): calcd for C37H41N2 (M þ H)þ 513.326, found 513.327. Preparation of [N-[2,6-Bis(1-methylethyl)phenyl]-R-[2-(1-methylethyl)phenyl]-6-(1-naphthalenyl-kC2)-2-pyridinemethanaminato(2-)-kN1, kN2]dimethylhafnium (1, 10 ). Inside a nitrogen-filled glovebox a glass jar was charged with N-[2,6-bis(1-methylethyl)phenyl]-R-[2-(1-methylethyl)phenyl]-6-(1-naphthalenyl)-2-pyridinemethanamine (10.21 g, 19.92 mmol) dissolved in 100 mL of toluene. To this solution was added nBuLi (8.4 mL of 2.5 M solution in hexanes, 21.0 mmol) by syringe. This solution was stirred for 1 h; then solid HfCl4 (6.41 g, 20.00 mmol) was added. The jar was capped with an air-cooled reflux condenser, and the mixture was heated to 100 °C for 2 h. After cooling, MeMgBr (23.2 mL of 3 M solution in diethyl ether, 69.7 mmol, 3.5 equivalents) was added by syringe, and the resulting mixture stirred overnight at ambient temperature. Solvent was removed from the reaction mixture using a vacuum system attached to the drybox. To the residue was added toluene and the mixture was filtered. The residue was washed with additional toluene (8 washes of ∼50 mL; until the eluent flow was colorless). Recovered eluent was stripped to dryness under vacuum, and hexane (30 mL) was added, then removed by vacuum. Hexane (50 mL) was again added, and the resulting slurry was stirred for 30 min. The suspension was filtered and the collected product was washed with cold hexane (50 mL) to give the desired product as a bright yellow powder (11.2 g, 15.46 mmol, 78%). 1: 1H NMR (500 MHz, C6D6, 30 °C): δ 8.57 (d, 1H, 3J = 7.5 Hz, H1), 8.25 (d, 1H, 3J = 8.5 Hz, H6), 7.81 (d, 1H, 3J = 7.5 Hz, H2), 7.71 (dd, 1H, 3J = 7.7 Hz, 3J = 1.7 Hz, H3), 7.51 (d, 1H, 3J = 8.0 Hz, H7), 7.34 (m, 1H, H10), 7.30 (m, H5), 7.27 (m, H4), 7.17 (dd, 1H, 3J = 7.5 Hz, 4J = 1.9, H17), 7.13 (t, 1H, 3J = 7.5 Hz, H16), 7.07 (dd, 1H, 3J = 7.5 Hz, 4J = 1.9, H15), 7.07 (m, 1H, H13), 7.00 (m, 2H, H11/H12), 6.84 (t, 1H, 3J = 7.5 Hz, H8), 6.57 (s, 1H, H14), 6.55 (d, 1H, 3J = 7.5 Hz, H9), 3.82 (sep., 1H, 3J = 6.8 3325
dx.doi.org/10.1021/om200167h |Organometallics 2011, 30, 3318–3329
Organometallics Hz, H19), 3.37 (sep., 1H, 3J = 7.0 Hz, H20), 2.89 (sep., 1H, 3J = 6.8 Hz, H18), 1.38 (d, 3H, 3J = 7.0 Hz, H25), 1.36 (d, 3H, 3J = 7.5 Hz, H24), 1.17 (d, 3H, 3J = 6.5 Hz, H21), 1.14 (d, 3H, 3J = 6.5 Hz, H26), 0.94 (s, 3H, H27), 0.689 (d, 3H, 3J = 6.5 Hz, H22), 0.685 (s, 3H, H28), 0.38 (d, 3H, 3 J = 6.5 Hz, H23). 10 : 1H NMR (C6D6, 30 °C): δ 8.55 (d, 1H, 3J = 7.5 Hz, H10 ), 8.30 (d, 1H, 3J = 8.5 Hz, H60 ), 7.77 (d, 1H, 3J = 8.0 Hz, H20 ), 7.69 (d, 1H, 3J = 8.0 Hz, H30 ), 7.55 (d, 1H, 3J = 8.0 Hz, H70 ), 7.30 (tm, 1H, 3J = 8.5 Hz, H50 ), 7.25 (tm, 1H, 3J = 7.0 Hz, H40 ), 7.18 (d, 1H, 3J = 7.5 Hz, H130 ), 7.07 (tm, 1H, 3J = 7.0 Hz, H120 ), 6.86 (t, 1H, 3J = 7.5 Hz, H80 ), 6.75 (t, 1H, 3J = 7.5 Hz, H110 ), 6.68 (d, 1H, 3J = 8.0 Hz, H100 ), 6.48 (d, 1H, 3J = 7.5 Hz, H90 ), 6.13 (s, 1H, H140 ), 3.92 (sep., 1H, 3 J = 7.0 Hz, H190 ), 3.45 (sep., 1H, 3J = 6.8 Hz, H180 ), 3.18 (sep., 1H, 3J = 6.8 Hz, H200 ), 1.41 (d, 3H, 3J = 7.0 Hz, H240 ), 1.40 (d, 3H, 3J = 7.0 Hz, H250 ), 1.27 (d, 3H, 3J = 6.5 Hz, H220 ), 1.18 (d, 3H, 3J = 6.5 Hz, H260 ), 0.88 (s, 3H, H270 ), 0.67 (s, 3H, H280 ), 0.48 (d, 3H, 3J = 6.5 Hz, H210 ), 0.29 (d, 3H, 3J = 7.0 Hz, H230 ). 1: 13C{1H} NMR (126 MHz, C6D6, 30 °C): δ 206.04 (C32), 170.53 (C34), 164.32 (C33), 147.35 (C37), 146.68 (36), 146.36 (C39), 145.55 (C38), 143.98 (C31), 140.81 (C35), 140.75 (C8, 1JCH = 163.6 Hz), 135.70 (C29), 134.11 (C1, 1JCH = 158.6 Hz), 130.72 (C30), 130.09 (C10), 129.92 (C3), 129.86 (C2), 127.95 (CH), 126.90 (C5), 126.79 (CH), 125.99 (C16), 125.46 (CH), 125.41 (C4), 125.14 (C17), 124.47 (CH), 124.17 (C6), 120.36 (C7, 1JCH = 166.2 Hz, 2JCH = 6.7 Hz),), 119.48 (C9, 1JCH = 166.4 Hz, 2JCH = 6.7 Hz), 76.75 (C14, 1JCH = 133.5 Hz), 66.89 (C28, 1JCH = 111.6 Hz), 62.85 (C27, 1JCH = 112.8 Hz), 28.70 (C20), 28.63 (18), 28.12 (C19), 27.44 (C24), 25.77 (C25), 25.44 (C26), 25.16 (C21), 23.73 (C23), 23.02 (C22). Anal. Calcd for C39H4HfN2: C, 65.12; H, 6.17; N, 3.89. Found: C, 65.56; H, 5.93, N, 3.73. Preparation of [N-[2,6-Bis(1-methylethyl)phenyl]-R-[2-(1-methylethyl)phenyl]-6-(1-naphthalenyl-kC2)-2-pyridinemethanaminato(2-)-kN1,kN2]dimethylzirconium (3, 30 ). A solution of N-[2,6-bis(1-methylethyl)phenyl]-R-[2-(1-methylethyl)phenyl]-6-(1-naphthalenyl)-2-pyridinemethanamine (2.25 g, 4.39 mmol) in hexane (40 mL) was cooled in a glovebox freezer (40 °C). nBuLi solution (1.93 mL of 2.5 M nBuLi in hexane, 4.83 mmol) was added by syringe, and the suspension was stirred for about 2 h after reaching ambient temperature. The reaction mixture was cooled again in the freezer, filtered, washed with cold hexane, and vacuum-dried overnight. The lithium amide product (2.38 g) as a light tan solid powder was carried on to the next step without further treatment or purification (quantitative recovery was assumed). This lithium salt (2.38 g crude from the previous reaction, 4.38 mmol) was stirred in toluene (∼50 mL) with ZrCl4 (1.02 g, 4.38 mmol). After mixing, the mixture became clear and darkened. The solution was heated to reflux for 2 h, then was allowed to cool to ambient temperature. The reaction solution was chilled slightly below ambient temperature in the glovebox freezer just prior to the addition of MeMgBr solution (5.06 mL of 3 M solution in ether, 15.18 mmol), which was added via syringe. This mixture was stirred at ambient temperature overnight. The solvents were removed completely under vacuum; then toluene (50 mL) was re-added and the mixture was filtered. The residual dark sludge was washed with toluene until the washes were almost colorless. The collected toluene solution was stripped to dryness under vacuum, and a crude product was obtained as a dark brown powder. Hexane (∼30 mL) was added to the crude product, and it was triturated briefly at ambient temperature before being cooled to 40 °C, filtered, and washed with additional cold hexane. The solid product recovered from the hexane filtering (1.36 g, 49% yield) was further purified by recrystallization (toluene/hexane) to provide X-ray quality crystals. 1H NMR (500 MHz, C6D6) δ 8.59 (d, 1H, J = 7.6 Hz, H1), 8.24 (d, 1H, J = 7.6 Hz, H6), 7.767.69 (m, 2H, H2/H3), 7.50 (d, 1H, J = 7.9 Hz, H7), 7.36 (m, 1H, H10), 7.327.25 (m, 2H, H4/H5), 7.197.10 (m, 2H, H16/H17), 7.097.05 (m, 2H, H13/H15), 7.046.97 (m, 2H, H11/H12), 6.83 (t, 1H, J = 7.8 Hz, H8), 6.54 (d, 1H, J = 7.7 Hz, H9), 6.44 (s, 1H, H14), 3.84 (hept, 1H, J = 6.9 Hz, H19), 3.40 (hept, 1H, J = 6.7 Hz, H20), 2.92 (hept, 1H, J = 6.7 Hz, H18), 1.38 (d, 3H, J = 6.8 Hz, H25), 1.36 (d, 3H, J = 6.9 Hz, H24), 1.19 (s, 3H), 1.18 (d, 3H, J = 7.3 Hz, H21), 1.13
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(d, 3H, J = 6.8 Hz, H26), 0.85 (s, 3H), 0.69 (d, 3H, J = 6.7 Hz, H22), 0.38 (d, 3H, J = 6.7 Hz, 3H). 13C{1H} NMR (126 MHz, C6D6): δ 192.35 (C32), 170.24 (C34), 164.34 (C33), 147.42 (quat), 146.47 (quat), 146.43 (quat), 144.71 (quat), 142.74 (quat), 140.78 (quat), 140.74 (C8), 135.76, 133.34 (C1), 130.25 (C10), 130.00, 129.89, 129.38, 127.93, 126.85, 126.82, 126.29, 125.45, 125.32, 125.23, 124.56, 124.18 (C6), 120.16 (C7), 119.65 (C9), 76.71 (C14), 53.61 (ZrCH3), 49.77 (ZrCH3), 28.86 (C20), 28.68 (C18), 28.17 (C19), 27.47 (C24), 25.83 (C25), 25.33 (C26), 25.04 (C21), 23.84 (C23), 23.15 (C22). Anal. Calcd for C39H4ZrN2: C, 74.12; H, 7.02; N, 4.43. Found: C, 74.22; H, 7.15, N, 4.42.
Polymerization Procedures and Polymer Characterizations. Ethylene/1-Octene Copolymerization. A 2 L Parr reactor was used in the polymerizations. All feeds were passed through columns of alumina and Q-5 catalyst (available from Engelhard Chemicals Inc.) prior to introduction into the reactor. Procatalyst and cocatalyst (activator) solutions were handled in the glovebox. A stirred 2 L reactor was charged with about 533 g of mixed alkanes solvent and 250 g of 1-octene comonomer. Hydrogen was added as a molecular weight control agent by differential pressure expansion from a 75 mL addition tank at 300 psi (2070 kPa). The reactor contents were heated to the polymerization temperature of 120 °C and saturated with ethylene at 460 psig (3.4 MPa). Catalysts and cocatalysts, as dilute solutions in toluene, were mixed and transferred to a catalyst addition tank and injected into the reactor. The polymerization conditions were maintained for 10 min with ethylene added on demand. Heat was continuously removed from the reaction vessel through an internal cooling coil. The resulting solution was removed from the reactor, quenched with isopropyl alcohol, and stabilized by addition of 10 mL of a toluene solution containing approximately 67 mg of a hindered phenol antioxidant (Irganox 1010 from Ciba Geigy Corporation) and 133 mg of a phosphorus stabilizer (Irgafos 168 from Ciba Geigy Corporation). Between polymerization runs, a wash cycle was conducted in which 850 g of mixed alkanes was added to the reactor and the reactor was heated to 150 °C. The reactor was then emptied of the heated solvent immediately before beginning a new polymerization run. Polymers were recovered by drying for about 12 h in a temperature-ramped vacuum oven with a final set point of 140 °C. Melting and crystallization temperatures of polymers were measured by differential scanning calorimetry (DSC 2910, TA Instruments, Inc.). Samples were first heated from room temperature to 180 at 10 °C/min. After being held at this temperature for 24 min, the samples were cooled to 40 at 10 °C/min, held for 24 min, and then heated to 160 °C. Weight average molecular weights (Mw) and polydispersity values (PDI) were determined by analysis on a Viscotek HT-350 gel permeation chromatographer (GPC) equipped with a low-angle/right-angle light-scattering detector, a 4-capillary inline viscometer, and a refractive index detector. The GPC utilized three Polymer Laboratories PLgel 10 μm MIXED-B columns (300 7.5 mm) at a flow rate of 1.0 mL/min in 1,2,4trichlorobenzene at either 145 or 160 °C. To determine octene incorporation, 140 μL of each polymer solution was deposited onto a silica wafer, heated at 140 °C until the trichlorobenzene had evaporated, and analyzed using a Nicolet Nexus 670 FTIR with 7.1 version software equipped with an AutoPro auto sampler. Propylene Polymerization. Propylene polymerization was conducted in a 1.8 L SS batch reactor. This reactor was manufactured by Buchi AG and sold by Mettler and is heated/cooled via the vessel jacket and reactor head. Syltherm 800 was the heat transfer fluid used and was controlled by a separate heating/cooling skid. Both the reactor and the heating/cooling system are controlled and monitored by a Camile TG process computer. The bottom of the reactor was fitted with a large orifice bottom dump valve, which empties the reactor contents into a 6 L SS dump pot. The dump pot was vented to a 30 gal blowndown tank, with both the pot and the tank N2 purged. All chemicals used for 3326
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Organometallics
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polymerization or catalyst makeup were run through purification columns, to remove any impurities that may effect polymerization. The propylene, toluene, and IsoparE were passed through two columns, the first containing A2 alumna, the second containing Q5 reactant. The N2 and H2 were passed through a single Q5 reactant column. The reactor was cooled to 50 °C for chemical additions. The Camile controls the addition of IsoparE, using a micromotion flowmeter to accurately add the desired amount. The addition of H2 was accurately achieved by pressuring up a 50 mL shot tank to 240 psi and slowly adding H2 until the desired decrease was reflected in the shot tank pressure. The propylene was then added through the micromotion flowmeter. After the chemicals were in the reactor, the reactor was heated to the polymerization temperature. The activator(s) and catalyst were handled in an inert glovebox, mixed together in a vial, drawn into a syringe, and pressure transferred into the catalyst shot tank. This was followed by three rinses of toluene, 5 mL each. Immediately after catalyst/activator addition, the run timer began. Usually within the first 2 min of successful catalyst runs, an exotherm was observed, as well as decreasing reactor pressure. These polymerizations were run for 10 min; then the agitator was stopped, the reactor pressured up to ∼500 psi with N2, and the bottom dump valve opened to empty reactor contents to the dump pot. The dump pot contents were poured into trays placed in a lab hood, where the solvent was evaporated off overnight. The trays containing the remaining polymer were then transferred to a vacuum oven, where they were heated to 145 °C under vacuum to remove any remaining solvent. After the trays cooled to ambient temperature, the polymers were weighed for yield/efficiencies and submitted for polymer testing. Weight average molecular weights (Mw) and polydispersity values were determined by analysis on a SYMYX high-throughput gel permeation chromatographer. The GPC utilized three Polymer Laboratories PLgel 10 μm MIXED-B columns (300 10 mm) at a flow rate of 2.5 mL/min in 1,2,4-trichlorobenzene at 160 °C. Melting and crystallization temperatures of polymers were measured by differential scanning calorimetry (DSC 2910, TA Instruments, Inc.). Samples were first heated from room temperature to 210 at 10 °C/min. After being held at this temperature for 4 min, the samples were cooled to 40 at 10 °C/min and were then heated to 215 at 10 °C/min after being held at 40 °C for 4 min. Magnetization Transfer Experiments.18 NMR measurements were performed on a Varian VNMRS 500 (FT 500 MHz) spectrometer using toluene-d8. The rate of this chemical exchange was measured by following transfer of magnetization (by measuring areas under the exchanging resonances) as a function of mixing time. A two-site exchange process is illustrated in eq 1. The rate laws for this two-site exchange are shown in eqs 2 and 3. Integration of these rate laws leads to expressions 4 and 5, where At and Bt are peak intensities (integrals) of resonances A and B at time t, Ao and Bo are peak intensities (integrals) of A and B at t = 0, k is the chemical exchange rate constant, t is the mixing time, and Ra and Rb are relaxation rates for A and B.21
Bt ¼ A 0 1 þ Bo 2
ka ðeLb t eLa t Þ La Lb
" 1þ
# Ra Rb þ ka kb Lb t Ra Rb þ ka kb La t þ 1 e e La Lb La Lb
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ðRa þ Rb þ ka þ kb Þ þ ½ðRa Rb þ ka kb Þ2 þ4ka kb 2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ðRa þ Rb þ ka þ kb Þ ½ðRa Rb þ ka kb Þ2 þ4ka kb Lb ¼ 2 La ¼
ð5Þ
DA ¼ kb ½B ka ½A Ra ½A Dt
ð2Þ
Magnetization transfer experiments were performed on a Varian VNMRS 500 (FT 500 MHz) spectrometer equipped with a pulse-field gradient probe using NOESY1D pulse sequences (double pulse field gradient spin echo NOE (DPFGSE-NOE) method22). Magnetization transfer data were collected at 10, 20, 30, and 40 °C. The resonance at 6.126.15 ppm was selectively excited in each of these experiments, and its return to equilibrium together with its exchange with the peak at 6.576.59 ppm was followed as a function of time (mixing time). Probe temperature was calibrated using ethylene glycol. Acquisition time was set to 2 s with a delay time of 1 s. Line broadening of 0.5 was used. The number of transients collected for each spectrum was set from 128 to 856. All the resonances were integrated and tabulated. Chemical exchange rate constants (k) were obtained by fitting time-dependent integrated values of both signals to eqs 4 and 5 using a nonlinear leastsquares routine. Five parameters (Ao, Bo, ka, kb, Ra, and Rb) were varied in order to minimize the sum of the squared deviations between the experimental and calculated data. Microsoft Excel solver was used to perform this least-squares analysis. Thermodynamic parameters (ΔH‡ and ΔS‡) were obtained by least-squares analysis of the nonlinear form of the Eyring equation.23 In this procedure, rate constants are calculated using the nonlinear form of the Eyring equation, and the ΔH‡ and ΔS‡ are adjusted until the sum of the error squared between the observed and calculated rate constants reaches a minimum. Microsoft Excel solver was used to perform this least-squares analysis. Errors were obtained by nonlinear error analysis using SolvStat macro.24 13 C NMR Analysis of Polypropylene Samples. The sample was prepared by adding approximately 2.7 g of stock solvent to a 0.211.2 g sample in a 10 mm NMR tube and then purging in an N2 box for 2 h. The stock solvent was made by dissolving 4 g of PDCB paradichlorobenzene in 39.2 g of ortho-dichlorobenzene with 0.025 M chromium acetylacetonate (relaxation agent). The sample was dissolved and homogenized by heating the tube and its contents at 140150 °C. The data were collected using a Bruker 400 MHz spectrometer equipped with a Bruker Dual DUL high-temperature CryoProbe.25 For 1D 13C NMR, the data were acquired using 320 transients per data file, a 7.3 s pulse repetition delay (6 s delay þ1.3 s acq. time), 90 degree flip angles, and a modified inverse gated decoupling26 with a sample temperature of 120 °C. All measurements were made on nonspinning samples in locked mode. Samples were homogenized immediately prior to insertion into the heated (125 °C) NMR changer and were allowed to thermally equilibrate in the probe for 7 min prior to data acquisition.
DB ¼ ka ½A kb ½B Rb ½B Dt
ð3Þ
’ COMPUTATIONAL DETAILS
Ra
ka
Rb
relaxation rs AhB sf relaxation kb
K eq ¼ ka =kb
ð1Þ
" 1 Ra Rb þ ka kb Lb t At ¼ Ao 1 þ e 2 La Lb Ra Rb þ ka kb La t kb ðeLb t eLa t Þ e þB0 þ 1þ La Lb La Lb ð4Þ
Calculations were carried out with the Gaussian0327 program. All optimizations and frequencies used the hybrid density functional theory (DFT) method, B3LYP.28 These calculations were performed with the LANL2TZ(f)29 basis set on hafnium, which includes triple-ζ functions on the valence and added f-functions, while the inner electrons are approximated by the LANL2DZ effective core potential. All other atoms utilize the 6-311G** basis set.30 Unless otherwise stated, energies are compiled in kcal/mol, and free energies are quoted at 298 K. 3327
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Organometallics Structure Determination of 1 and 3. Data for both structures were collected at 173 K on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo KR radiation (λ = 0.71073 Å). Cell parameters were refined using up to 8192 reflections in each case. A hemisphere of data (1381 frames) was collected using the ω-scan method (0.3° frame width) for each structure. The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was 2σ(I) were used to refine 387 parameters, and the resulting R1, wR2, and S (goodness of fit) were 2.31%, 5.33%, and 1.03, respectively. A correction for secondary extinction was not applied. For 3, in the final cycle of refinement, 21 193 observed reflections with I > 2σ(I) were used to refine 379 parameters, and the resulting R1, wR2, and S (goodness of fit) were 3.7%, 8.54%, and 0.924, respectively. A correction for secondary extinction was not applied due to the small crystal size. Refinement was done using F2. Structure Determination of 2. The crystal, mounted on a Mitegen Micromount, was automatically centered on a Bruker SMART X2S benchtop crystallographic system. Intensity measurements were performed using monochromated (doubly curved silicon crystal) Mo KR radiation (0.71073 Å) from a sealed microfocus tube. Generator settings were 50 kV, 30 mA. Data were acquired using three sets of omega scans at different phi settings. APEX2 software was used for preliminary determination of the unit cell. Determinations of integrated intensities and unit cell refinement were performed using SAINT. Data were corrected for absorption effects with SADABS using the multiscan technique. The structure was solved with XS, and subsequent structure refinements were performed with XL.
’ ASSOCIATED CONTENT
bS
Supporting Information. NMR spectra, X-ray data for 1, 2, and 3 including CIF file, table with total energies for all computed structures, mol file containing xyz data for all computed structures, and Excel spreadsheet containing magnetization transfer calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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