Naphthyridine Cyclopentadienyl Chromium ... - ACS Publications

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Naphthyridine Cyclopentadienyl Chromium Complexes as SingleSite Catalysts for the Formation of Ultrahigh Molecular Weight Polyethylene David Sieb,† Robert W. Baker,‡ Hubert Wadepohl,† and Markus Enders*,† †

Anorganisch-Chemisches Institut der Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany ‡ School of Chemistry, University of Sydney, NSW 2006, Australia S Supporting Information *

ABSTRACT: Cyclopentadienyl ligands functionalized with a 1,5-naphthyridine donor have been used for the formation of chromium(III) half-sandwich complexes. In addition to standard analytical characterization, the complexes have been investigated by paramagnetic 1H NMR spectroscopy combined with DFT calculations. After activation, highly active single-site catalysts are obtained that lead to the formation of ultrahigh molecular weight polyethylene (UHMW-PE). Evaluation of polymer formation after different polymerization times at low temperature shows that the number of active centers is low at the beginning, but increases with polymerization time.

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INTRODUCTION

chromium complexes with such ligands and their polymerization behavior.

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Chromium-based catalysts are capable of polymerizing ethylene with excellent activities. A large proportion of the worldwide production of high-density polyethylene (HDPE) is realized by the heterogeneous, silica-supported Phillips catalyst,1 which is based on chromium salts without the need of a cocatalyst. Homogeneous chromium catalysts have also been developed, and some of them show similarities, but also significant differences, compared to the well-established heterogeneous systems.2 Very promising single-site chromium catalysts are half-sandwich Cr(III) compounds that are coordinated by an additional nitrogen donor.3 Jolly et al. have demonstrated that chromium complexes with chelating N-donor Cp ligands show good properties for the polymerization of ethylene.4 We have incorporated the C2 linker of such ligands into a rigid aromatic moiety and developed aniline-5 and quinoline-substituted Cp ligands.6 Chromium complexes of such ligands show improved catalytic properties.7 The molecular weight of the resulting polyethylenes lies in the range between 2 × 105 and 106 g mol−1 when the polymerization reactions are carried out in homogeneous phase, whereas a higher molecular weight is obtained upon addition of modifiers8 or after the complexes have been heterogenized on solid supports.9,7c Recently, we reported a naphthyridine Cp ligand system where one of the two N atoms can bind to the Cp-bound metal in a chelating manner, whereas the second N atom is situated in distal position and is therefore not able to interact with the same metal center.10 Here we describe our results on the synthesis of © 2012 American Chemical Society

RESULTS AND DISCUSSION Ligand and Complex Synthesis. The synthesis of the new protio ligands 2 and 3 follows the published procedure for 1.10 Coupling of 8-methylsulfinyl-1,5-naphthyridine with tertbutylcyclopentadienyllithium or (2-methyl-2-adamantyl)cyclopentadienyllithium, respectively, leads to the formation of the 1,3-disubstituted cyclopentadiene derivatives in good yields (Scheme 1). After deprotonation of the protio ligands 1− 3 with potassium hydride or n-butyllithium, CrCl3(THF)3 was added and the green, air-stable chromium(III) complexes 4, 5, and 6 were obtained. The complexes have been characterized by elemental analysis, paramagnetic NMR, and X-ray structure determination. Single crystals have been obtained from dichloromethane solutions. Molecular structures and selected bond lengths and angles are presented in Figure 1. Due to the unsymmetrical 1,3-disubstitution pattern of the Cp rings, the complexes 5 and 6 are chiral and both enantiomers are present in the single crystals. The coordination geometry can be considered as a distorted three-legged “pianostool” arrangement. The distortion is due to the constrained geometry of the naphthyridine unit, which leads to a bending of the three-leg unit (i.e., Cr−Cl(1), Cr−Cl(2), and Cr−N(1) Received: June 26, 2012 Published: October 25, 2012 7368

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Scheme 1. Synthesis of the Protio Ligands 1−3 and the Complexes 4−6a

a

AdMe = 2-methyl-2-adamantyl.

This results in somewhat increased Cr−N bond lengths when going from complex 4 to the bulkier ligand in 6 (Cr−N = 2.086(2), 2.090(2), and 2.097(2) Å, respectively). pNMR Analysis.12,13 The complexes 4−6 are paramagnetic due to three unpaired electrons located in the valence d-orbitals of the Cr atoms. The interaction of the 1H nuclei with the unpaired electrons leads to strongly broadened and shifted NMR signals. The observed chemical shift is the sum of the socalled hyperfine shift (δhf) and the orbital shift (δorb). A comprehensive theoretical analysis of the hyperfine shift leads to many isotropic chemical shift contributions for systems with S > 2.14 In a nonaxial S = 3/2 complex like the Cr complexes described in this paper eight nonzero hyperfine shift contributions and the orbital shift have to be considered.15 However, many pNMR spectra can be interpreted by considering only three contributions: the orbital shift, which is also relevant for diamagnetic compounds, the Fermi-contact shift (δcon), and the pseudocontact shift (δpc), which arises from dipolar electron−nucleus interactions. We have shown that for similar 8-quinolylcyclopentadienyl chromium complexes the experimental chemical shift can be approximated by the sum of the orbital shift and the Fermi-contact shift alone.16 Theory predicts a linear correlation between the contact shift (δcon) and the DFT-calculated spin-density at the corresponding atom nucleus (ραβ) by δcon = 2.0051 × 105ραβ

(1)

This prefactor results from the product of several physical constants, the absolute temperature and the total spin S of the compound, and it is valid for S = 3/2 and T = 293 K. Fitting of a bigger number of experimental NMR shift values against the calculated spin densities leads to the following equation, which deviates slightly from the theoretical equation by a somewhat smaller prefactor and a nonzero intercept:16

Figure 1. Solid-state molecular structures of 4, 5, and 6. Selected bond lenghts (Å) and angles (deg): 4: N(1)−Cr 2.086(2), Cr−Cl(1) 2.303(1), Cr−Cl(2) 2.293(1), N(1)−Cr−Cl(1) 94.19(5), Cl(2)−Cr− N(1) 96.03(5), Cl(1)−Cr−Cl(2) 100.52(3). 5: N(1)−Cr 2.090(2), Cr−Cl(1) 2.289(1), Cr−Cl(2) 2.290(1), N(1)−Cr−Cl(1) 96.75(5), Cl(2)−Cr−N(1) 92.74(4), Cl(1)−Cr−Cl(2) 98.11(3). 6: N(1)−Cr 2.097(2), Cr−Cl(1) 2.3039(9), Cr−Cl(2) 2.294(1), N(1)−Cr−Cl(1) 89.40(5), Cl(2)−Cr−N(1) 97.88(4), Cl(1)−Cr−Cl(2) 99.44(4). Naph.−Cp: 4: 89.35(5)°. 5: 86.97(5)°. 6: 74.26(6)°.

δexp ≅ δorb + δcon = δorb + 1.8484 × 105ραβ + 6.1 ppm (2) 1

bonds) away from the line defined by the Cp−Cr bonding vector. The extent of this distortion can be estimated by the angle between the centroid of the Cp ring, the chromium, and the nitrogen atom. In two comparable complexes with nonchelating donors, this angle is about 120°,11 whereas in complexes 4−6 these range from 111° to 113°. The corresponding quinoline complex [C5(CH3)4C9H6N]CrCl2 exhibits a value of 112.7°. Complex 4, with the symmetrically substituted ligand, shows an almost orthogonal arrangement between the naphthyridine unit and the Cp ring, and thus the NMR spectrum is in accordance with a Cs symmetric molecule (see below). In complex 5, the angle between the best planes of the naphthyridine unit and the Cp ring is 87.0°, also close to orthogonality, whereas this angle deviates more in 6 (74.3°).

Figure 2 shows the H NMR spectra of 4−6. The two resonances of the CH3 groups in 4 give additional signals in comparison with the spectrum of 5 or 6. The signals with smaller intensities (e.g., around +50 and −50 ppm) are present in all three spectra and stem from H atoms of the naphthyridine units. The assignment to the individual positions is done with the help of DFT calculations (see below). Additional information comes from the line widths of the individual 1H NMR signals. Signal broadening is a consequence of the relaxation behavior of the corresponding nuclei. In paramagnetic NMR spectra with dominating Fermi-contact shift contribution, the signal widths are often proportional to the square number of the contact shift. However, if the resonating nucleus is in spatial proximity to the unpaired electrons, 7369

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are given in Table 1. The good linear correlation for the complexes 4−6 is presented in Figure 3.

Figure 2. 1H NMR spectra of 4−6 (4 and 5 in C6D6, 6 in CD2Cl2) at 199.92 MHz. The inset shows a 10-fold increased plot of the sprectrum of 6. The broad resonance at ca. −80 ppm stems from H2 (see Figure 1 for numbering, H2 is connected to C2, etc.).

Figure 3. Correlation of calculated and experimental hyperfine shifts of complexes 4−6.

We have also evaluated the solvent dependence of the 1H NMR signal shifts. This is important, as the DFT calculation gives the data for the gas phase but the experimental data are obtained in solution. As can be seen from Figure 4, only signals

relaxation due to dipolar interaction, which is proportional to r−6 (r = distance between Cr and H), becomes important. The H2 atom of the naphthyridine17 has a distance to the Cr center of 3.13 Å, and the line width of the corresponding 1H signal at room temperature is ∼4300 Hz (width at half-height, signals around −80 ppm in Figure 2; see expansion plot). The solidstate molecular structure of the methyladamantyl derivative 6 shows a conformer where the CH3 group points to the Cr center and the bulky adamantyl substituent is on the opposite side (see Figure 1). Consequently the hydrogen atoms of the CH3 group are on average 3.52 Å apart from the unpaired electrons at the Cr center. The signal due to this group is located at +58.2 ppm, and with a half-width of ∼2400 Hz it is also considerably broader compared to the other signals. The ratio of signal widths of this CH3 group and the H2 atom is 0.56 and reflects well the r−6 dependence of dipolar relaxation ([r(Cr−CH3):r(Cr−H2)]−6 = 1.125−6 = 0.49). This leads to the conclusion that the intermediate orientation of the methyladamantyl substituent in complex 6 in solution is analogous to the conformer determined in the solid state. The assignment of the 1H NMR signals from the aromatic CH groups was done by calculation of the Fermi-contact coupling constants by DFT methods. It has been demonstrated that the B3LYP functional gives good results, whereas the use of functionals with more Hartree−Fock exchange like BHandHLYP, which is popular for the calculation of EPR parameters, gives poorer results for Fermi-contact couplings.15 The calculated together with the experimental 1H NMR values

Figure 4. 1H NMR spectra of 4 in different solvents. Dipole moments in units of 10−30 Cm are given in parentheses.

of the CH3 substituents at the Cp rings show considerable solvent shift dependence. Both resonances for the CH3 groups in complex 4 shift to higher field when going to solvents with larger dipole moments. The best correlation with the calculated NMR shifts is obtained for a solvent with no dipole moment (benzene). Including solvent effects in the calculation using the SCRT method (self-consistent reaction field) as implemented in the Gaussian 09 program package18 did not fully reproduce

Table 1. Experimental and Calculated Contact Shifts for Complexes 4−6 Using Eq 2a 4 5 6

δ δ δ δ δ δ

(exp) (calc) (exp) (calc) (exp) (calc)

H2

H3

H4

H6

H7

Cp-CH3

Cp-CH3

−80.0 −71.9 −84.8 −71.9 −78.8 −81.2

50.7 53.8 47.9 51.9 45.1 48.2

−55.0 −42.3 −50.0 −40.5 −45.5 −44.2

9.4 11.3 0−11b 9.4 8.7 11.3

5.3 5.7 0−11b 5.7 4.1 5.7

26.1 17.3

−40.0 −34.7

a

The program package Gaussian0918 was used. All structures were fully optimized using unrestricted DFT with the B3LYP functional and 6-311G* basis set. The calculated Fermi-contact coupling (expressed in atomic units) was used for the calculation of the chemical shift according to eq 2 (δorb ≈ 2 ppm for aliphatic C−H and 7 ppm for aromatic C−H). The numbering follows Figure 1 with H2 connected to C2, etc. bSignals covered in the diamagnetic range. 7370

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Table 2. Polymerization of Ethylene at Atmospheric Pressure and Room Temperature entry

precat.a

nkat [μmol]

t [min]

polymer [g]

activity [g·mmol−1·h−1· bar−1]

Mw [106 g·mol−1]

PDI [Mw/Mn]

Tm [°C]

deg of cryst. [%]

1 2 3 4 5 6 7 8 9f 10g 11g 12g 13g 14g

Cp2ZrCl2 CpQCrCl2b 4 4c 5 5d 5e 6 5 4 4 4 4 4

10.30 2.8 1.88 1.77 6.85 6.99 6.99 2.37 3.04 7.79 10.75 9.94 12.10 7.25

12 30 20 10 20 20 20 20 2 10 15 20 25 30

4.78 4.97 3.24 12.81 3.53 3.04 0.97 2.98 0.05 0.041 0.043 0.138 0.180 0.260

2332 3550 5173 8673 1546 1332 424 3775 493 32 16 42 36 72

0.60 0.52 1.24 0.48 3.54

2.4 2.8 2.9 2.5 4.4

132.3 132.1 135.3 136.9 133.7

57 55 47 56 37

1.63 1.16 0.33 0.39 0.45 0.57 0.73

3.1 3.6 4.5 3.9 3.7 3.1 2.9

133.3

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a Cocatalyst = pMAO (Al:Cr = 1000:1). bCpQCrCl2 = dichloro-η5-[2,3,4,5-tetramethyl-1-(8-quinolyl)cyclopentadienyl]chromium(III). 7a cAutoclave: 40 °C, 5 bar ethylene. dStart of polymerization after 45 min of preactivation. eStart of polymerization after 48 h of preactivation. f0 °C. g−30 °C.

the experimental solvent effects, so that we did not use SCRT calculations for the correlation of experimental paramagnetic shifts with calculated spin-densities. Ethylene Polymerization. The complexes were examined as catalysts for ethylene polymerization in the presence of pMAO19 as a cocatalyst. We used pMAO instead of conventional MAO, as it is more stable in terms of aging, leading to a higher reproducibility of our polymerization results. The lower content of trimethylaluminum in pMAO leads to somewhat higher PE molecular weights.8 The activation was performed with 1000 equivalents of Al. The polymerization results are summarized in Table 2. For comparison of catalyst activities, zirconocene dichloride was used as a reference under the same conditions (entry 1). The complexes 4, 5, and 6 show an activity for ethylene polymerization between 1550 and 5170 g·mmol−1·h−1·bar−1 as a function of substitution on the cyclopentadienyl ligand. At 5 bar ethylene pressure, higher pressure normalized activities are observed and lie around 8670 g·mmol−1·h−1·bar−1 for complex 4. The molecular weights are in the UHMW range from 1.24 to 3.54 × 106 g·mol−1. A comparison with the analogous quinoline chromium complex (entry 2) demonstrates the increase in molecular weight on the installation of the second N atom in the ligand. The active species is stable at room temperature for several hours. After 48 h of preactivation, a growing UV band of an inactive decomposition product can be observed spectroscopically, and in accordance with this the activity decreases in the corresponding experiments (entries 6, 7). Figure 5 shows a specific polymerization course of the ethylene absorption of the naphthyridine chromium complex 5. The catalytic activity reaches a maximum after approximately 6 min and decreases after 10 min to a relatively constant ethylene consumption over a period between 10 and 20 min. If all catalytically active centers react uniformly, the ethylene consumption would be the highest at the beginning of the reaction. The constant increase of activity within the first minutes can be explained by an increase in the number of active centers within that time. Theoretical investigations of the insertion barrier of ethylene into the chromium−alkyl bond showed that the second and subsequent insertion steps run 1000 times faster than the first insertion. Therefore, at the beginning of the reaction only a few Cr complexes polymerize, resulting in a delayed onset of full polymerization activity.20 In

Figure 5. Ethylene absorption of a toluene solution of 8.7 μmol of 5 and methylaluminoxane (MAO). Total amount of PE produced: 4.1 g.

order to support this interpretation of the observed polymerization activity profile, we tried to analyze polymer samples at the start of the polymerization experiment. At the beginning of a chain growth polymerization, the chain lengths should grow linearly with time in a quasi living polymerization manner. In order to study this initial phase of the polymerization reaction by analyzing individual samples, the insertion must be slow enough. We conducted the reaction at 0 °C, but at this temperature a molecular weight of over 1 × 106 g/mol was determined already after 2 min of polymerization time (Table 2, entry 9). At −30 °C, the insertion rate of ethylene is sufficiently slow, so that individual samples after different polymerization times could be analyzed. Figure 6 shows approximately a linear relationship between the molecular weight and polymerization time over a 30 min period at −30 °C. The yield of the polymer and the activity increase with proceeding polymerization. The number of active sites can be estimated by dividing the yield of the resulting polymer by the corresponding average molecular weight Mn. After 10 min 7% of the chromium centers are active, and this number rises to 14% after 30 min. After this time there is no longer a linear dependence on molecular weight, so that the number of active centers can no longer be calculated. However, at the end of a typical polymerization experiment the number of produced polymer chains is higher than the number of chromium atoms. 7371

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Olexis-type columns from Polymer Laboratories (300 × 8 mm). Melting points and crystallinity were measured on a Mettler Toledo DSC821e. The peak maximum of the second melting transition was taken as the melting point of the polymer. Polymerization Conditions. (a) Polymerization at atmospheric pressure: A 250 mL round-bottom Schlenk flask was charged with 100 mL of dry toluene (water content below 10 ppm as verified by Karl Fischer titration). The solution was magnetically stirred with a 25 × 5 mm Teflon-coated stirrer bar at 1000 rpm. The flask was placed in a water bath at 22 °C (internal temperature measurement in a few typical polymerization runs showed that temperature increase during polymerization is around 3 °C). One third of the pMAO solution was added to the 250 mL flask, and ethylene gas was led over the stirred solution. The rest of the pMAO was added to a second flask containing the precatalyst in 2 mL of toluene. The catalyst activation time was usually 5 min. After that time, the activated catalyst solution was transferred to the polymerization flask by a syringe. The polymerization was stopped by adding 10 mL of a solution of methanol/HClconc (4:1). The polymer was collected by filtration, subsequently washed with methanol/HCl and three times with acetone, and dried at 80 °C for 15−20 h. (b) Measuring the ethylene consumption: In specific polymerization runs, the ethylene consumption was determined by subtracting the inlet and outlet flow, which have been measured by conventional gas-flow meters or by digital mass-flow controllers. (c) Polymerization at 5 bar: A Büchi glass autoclave, equipped with mechanical stirrer, internal cooling tube, external tempering mantle, and internal thermometer, was used. The polymerization was started by injection of the preactivated catalyst solution into the ethylene-saturated toluene solution at 40 °C, and the autoclave was then quickly pressurized to 5 bar. Polymerization was stopped by releasing the ethylene pressure and injecting the methanol/ HCl mixture as mentioned above. X-ray Crystal Structure Determinations. Crystal data and details of the structure determinations are listed in Table S1 (see Supporting Information). Full shells of intensity data were collected at low temperature (100 K) with a Bruker AXS Smart 1000 CCD diffractometer (Mo Kα radiation, sealed tube, graphite monochromator, λ = 0.71073 Å). Data were corrected for air and detector absorption, Lorentz, and polarization effects;21 absorption by the crystal was treated with a semiempirical multiscan method.22 The structures were solved by the heavy atom method combined with structure expansion by direct methods applied to difference structure factors23 (complexes 5·CH2Cl2 and 6) or by the charge flip procedure24 and refined by full-matrix least-squares methods based on F2 against all unique reflections.25 All non-hydrogen atoms were given anisotropic displacement parameters. Hydrogen atoms were input at calculated positions and refined with a riding model. Synthesis of 2. A solution of tert-butylcyclopentadienyllithium was prepared by dissolving tert-butylcyclopentadiene (2.0 g, 16.35 mmol) in 50 mL of THF at 0 °C and adding n-BuLi (10.75 mL of 1.6 M, 17.31 mmol) dropwise. The solution was warmed to rt and stirred for 30 min. After cooling to −78 °C, the sulfoxide (1.54 g, 8.02 mmol) dissolved in 45 mL of THF was added dropwise. After 2 h at −78 °C, the reaction was quenched by addition of 10% aqueous NH4Cl (100 mL). The mixture was diluted with CH2Cl2, and the organic layer separated. The CH2Cl2 layer was washed a further two times with water and dried with MgSO4, and the solvent was evaporated. Flash chromatography (Alox (neutral)) eluting with 25% EtOAc in CH2Cl2 gave two isomers of the product (ratio ∼1:1) as a red-brown oil. Yield: 1.57 g (6.27 mmol = 78%). 1H NMR (CDCl3, 400 MHz): 2a: δ 1.24 (s, 9H, CH3), 3.68 (dd, 2H, 4JH12,H14 = 1.5 Hz, 4JH12,H15 = 1.5 Hz, H12), 6.36 (dt, 1H, 3JH14,H15 = 1.5 Hz, 4JH14,H12 = 1.5 Hz, H14), 7.53 (d, 1H, 3JH7,H6 = 4.8 Hz, H7), 7.56 (dd, 1H, 3JH3,H2 = 8.3 Hz, 3JH3,H4 = 4.2 Hz, H3), 7.92 (dt, 1H, 3JH15,H14 = 1.5 Hz, 4JH15,H12 = 1.5 Hz, H15), 8.33 (dd, 1H, 3JH4,H3 = 8.6 Hz, 4JH4,H2 = 1.1 Hz, H4), 8.78 (d, 1H, 3 JH6,H7 = 4.7 Hz, H6), 8.94 (dd, 1H, 3JH2,H3 = 4.0 Hz, 4JH2,H4 = 1.8 Hz, H2); 2b: δ 1.24 (s, 9H, CH3), 3.69 (dd, 2H, 3JH15,H14 = 1.5 Hz, 4 JH15,H12 = 1.5 Hz, H15), 6.21 (td, 1H, 3JH14,H15 = 1.5 Hz, 4JH14,H12 = 1.5 Hz, H14), 7.56 (dd, 1H, 3JH3,H2 = 8.3 Hz, 3JH3,H4 = 4.2 Hz, H3), 7.58 (d, 1H, 3JH7,H6 = 4.8 Hz, H7), 8.10 (dt, 1H, 4JH12,H14 = 1.5 Hz, 4JH12,H15

Figure 6. Increase of molecular weight of PE (red) and quantity of polymer (black) at −30 °C with precatalyst 4.

For example in entry 3 in Table 2, 7.58 μmol of polymer chains was produced with 1.88 μmol of Cr catalyst, and consequently one chromium center produced at least four polymer chains in this run (taking as a basis 100% of active chromium centers). At this stage of our investigations we do not know if much more than 14% of the employed Cr complex molecules leads to active catalysts.

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CONCLUSION Naphthyridine chromium(III) complexes have been synthesized and characterized. The complexes can be judged as longterm stable, highly active ethylene polymerization catalysts with impressive turnover frequencies. The modification by an additional nitrogen atom compared to the corresponding quinoline chromium(III) complexes caused a shift of the molecular weight distribution to the ultrahigh molecular weight range. We were able to show that the number of catalytically active centers is low at the beginning but rises in the course of a polymerization reaction. This behavior leads to an activity profile with a smooth rise of activity within the first minutes. We interpret these results by a model with initial Cr-methyl complexes with low activity as dormant species and highly active catalysts after the first insertion has taken place.

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EXPERIMENTAL SECTION

Materials and General Considerations. Unless noted otherwise, all manipulations were carried out under an inert argon or nitrogen atmosphere using standard Schlenk techniques. All glassware was heated and dried under vacuum before use. Toluene, tetrahydrofuran (THF), dichloromethane, and n-hexane were dried using a solvent purifier system based on molecular sieves supplied by VAC and were degassed prior to use. Commercially available starting materials were used as purchased. Ethylene gas in a quality 3.0 was passed through columns packed with molecular sieves and P2O5 (Sicapent), respectively. NMR spectra were recorded on a Bruker DRX 200, Bruker Avance II 400, or Bruker Avance III 600 spectrometer. 1H and 13C NMR chemical shifts were referenced to the solvent signal. NMR assignments were confirmed by H,H-COSY, HSQC, and HMBC experiments, and the numbering of the assignments is based on the numbers shown in Figure 1. Mass spectra were recorded on a Finnigan MAT8230 and a Jeol JMS-700 spectrometer. Elemental analyses were performed on a CHN-O-vario EL (Elementar) by the Mikroanalytisches Labor, Organisch-Chemisches Institut, University of Heidelberg. High-temperature GPC analysis was carried out at the Deutsches Kunststoff Institut at Darmstadt using a Polymer Laboratories PL 220 GPC at 150 °C equipped with a refractive index detector. The samples were dissolved in 1,2,4-trichlorobenzene (1 mg/mL), and a flow rate of 1 mL/min was applied. The stationary phase consisted of three 7372

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(42) [M − 2×Cl]+. Anal. Calcd for C17H17Cl2CrN2 (372.23): C, 54.85; H, 4.60; N, 7.53. Found: C, 55.02; H, 4.94; N 7.31. UV/vis (THF), λmax (ε): 283 (9260), 305 (9770), 361 (5420). Synthesis of 6. 1-(1,5-Naphthyridin-8-yl)-3-(2-methyl-2adamantyl)cyclopentadiene (3; 0.33 g, 0.95 mmol) was dissolved in 20 mL of THF, and KH (0.04 g, 0.95 mmol) was added. After stirring overnight the reaction mixture was slowly added to a solution of CrCl3(THF)3 (0.36 g, 0.95 mmol) in 20 mL of THF. After stirring for 3 days, the solvent was evaporated under vacuum, and the residue was extracted several times with toluene and CH2Cl2. The combined solutions were evaporated, and the resulting green powder was washed with n-hexane. Yield: 0.05 g (0.11 mmol, 11%). 1H NMR (CDCl3, 200 MHz): δ −78.8 (hw ∼4300 Hz, H2), −45.5 (H4), −11.9, −7.6, −3.8, 1.0−2.5 (Had), 3.2, 4.1 (H7), 8.7 (H6), 15.1, 45.1 (H3), 58.2 (hw ∼2400 Hz, CH3). MS(EI) m/z (%): 463 (9) [M]+, 427 (7) [M − Cl]+, 391 (45) [M − 2×Cl]+. C24H25Cl2CrN2 (464.37): No elemental analysis values within the ACS guidelines could be obtained for this compound (calcd C, 62.07; H, 5.43; N, 6.03; found C, 60.23; H, 5.74; N, 5.52).

= 1.5 Hz, H12), 8.33 (dd, 1H, 3JH4,H3 = 8.6 Hz, 4JH4,H2 = 1.1 Hz, H4), 8.80 (d, 1H, 3JH6,H7 = 4.7 Hz, H6), 8.96 (dd, 1H, 3JH2,H3 = 4.0 Hz, 4 JH2,H4 = 1.8 Hz, H2). 13C{1H} NMR (CDCl3, 100 MHz): 2a: δ 30.7 (CH3), 33.6 (Cq), 42.2 (CH2), 120.1 (C7), 123.8 (C3), 125.7 (C14), 137.4 (C4), 138.8 (Cq), 140.0 (C15), 142.0, 142.1, 144.3 (3×Cq), 149.6 (C2), 150.6 (C6), 163.5 (Cq); 2b: δ 29.8 (CH3), 32.2 (Cq), 42.9 (CH2), 120.5 (C7), 123.8 (C3), 125.9 (C14), 137.3 (C4), 138.8 (Cq), 139.9 (C12), 141.8, 142.0, 144.2 (3×Cq), 149.8 (C2), 150.8 (C6), 158.3 (Cq). MS(EI) m/z (%): 250 (47) [M]+, 235 (100) [M − CH3]+, 220 (26) [M − 2×CH3]+, 193 (10) [M − tBu]+. Synthesis of 3. A solution of (2-methyl-2-adamantyl)cyclopentadienyllithium was prepared by dissolving (2-methyl-2adamantyl)cyclopentadiene (0.72 g, 3.35 mmol) in 25 mL of Et2O at 0 °C and adding n-BuLi (2.1 mL of 1.6 M, 3.38 mmol) dropwise. After stirring overnight the white suspension was dissolved in 15 mL of THF and cooled to −78 °C. The sulfoxide (0.32 g, 1.67 mmol) dissolved in 45 mL of THF was added dropwise. After 2 h at −78 °C, the reaction was quenched by addition of 10% aqueous NH4Cl (100 mL). The mixture was diluted with CH2Cl2, and the organic layer separated. The CH2Cl2 layer was washed a further two times with water and dried with MgSO4 and the solvent was evaporated. Flash chromatography (Alox (neutral)) eluting with 25% EtOAc in CH2Cl2 gave the product as a reddish-brown solid. Tautomer ratio 3a:3b ≈ 3:2. Yield: 0.5 g (1.46 mmol, 83%). 1H NMR (CDCl3, 400 MHz): 3a: δ 1.27 (s, 3H, CH3), 1.57−2.21 (m, 14H, Had), 3.67 (dd, 2H, 4JH12,H14 = 1.2 Hz, 4JH12,H15 = 1.2 Hz, H12), 6.40 (dt, 1H, 3JH14,H15 = 1.7 Hz, 4 JH14,H12 = 1.7 Hz, H14), 7.57 (d, 1H, 3JH7,H6 = 4.8 Hz, H7), 7.60 (dd, 1H, 3JH3,H2 = 8.4 Hz, 3JH3,H4 = 4.1 Hz, H3), 8.08 (dt, 1H, 3JH15,H14 = 2.4 Hz, 4JH15,H12 = 2.4 Hz, H15), 8.37 (dd, 1H, 3JH4,H3 = 7.8 Hz, 4JH4,H2 = 1.8 Hz, H4), 8.80 (d, 1H, 3JH6,H7 = 4.8 Hz, H6), 8.98 (dd, 1H, 3JH2,H3 = 3.8 Hz, 4JH2,H4 = 1.8 Hz, H2); 3b: δ 1.28 (s, 3H, CH3), 1.57−2.21 (m, 14H, Had), 3.78 (dd, 2H, 3JH15,H14 = 1.6 Hz, 4JH15,H12 = 1.6 Hz, H15), 6.27 (td, 1H, 3JH14,H15 = 1.7 Hz, 4JH14,H12 = 1.7 Hz, H14), 7.60 (dd, 1H, 3 JH3,H2 = 8.4 Hz, 3JH3,H4 = 4.1 Hz, H3), 7.63 (d, 1H, 3JH7,H6 = 4.8 Hz, H7), 8.14 (dt, 1H, 4JH12,H14 = 1.6 Hz, 4JH12,H15 = 1.6 Hz, H12), 8.37 (dd, 1H, 3JH4,H3 = 7.8 Hz, 4JH4,H2 = 1.8 Hz, H4), 8.83 (d, 1H, 3JH6,H7 = 4.8 Hz, H6), 8.98 (dd, 1H, 3JH2,H3 = 3.8 Hz, 4JH2,H4 = 1.8 Hz, H2). 13 C{1H} NMR (CDCl3, 100 MHz): 3a: δ 30.7 (CH3), 33.6 (Cq), 42.2 (CH2), 120.1 (C7), 123.8 (C3), 125.7 (C14), 137.4 (C4), 138.8 (Cq), 140.0 (C15), 142.0, 142.1, 144.3 (3×Cq), 149.6 (C2), 150.6 (C6), 163.5 (Cq); 3b: δ 29.8 (CH3), 32.2 (Cq), 42.9 (CH2), 120.5 (C7), 123.8 (C3), 125.9 (C14), 137.3 (C4), 138.8 (Cq), 139.9 (C12), 141.8, 142.0, 144.2 (3×Cq), 149.8 (C2), 150.8 (C6), 158.3 (Cq). MS(EI) m/z (%): 342 (44) [M]+, 327 (22) [M − CH3]+. Synthesis of 4. 1-(1,5-Naphthyridin-8-yl)-2,3,4,5-tetramethylcyclopentadiene (1; 0.33 g, 1.33 mmol) was dissolved in 35 mL of THF, and KH (0.06 g, 1.33 mmol) was added. After stirring overnight the reaction mixture was slowly added to a solution of CrCl3(THF)3 (0.49 g, 1.33 mmol) in 20 mL of THF. After stirring for 3 days, the solvent was evaporated under vacuum, and the residue was extracted several times with toluene and CH2Cl2. The combined solutions were evaporated, and the resulting green powder was washed with n-hexane. Yield: 0.13 g (0.35 mmol, 26%). 1H NMR (CDCl3, 200 MHz): δ −80.0 (H2), −55.0 (H4), −40.0 (CH3‑b), 5.3 (H7), 9.4 (H6), 26.1 (CH3‑a), 50.7 (H3). UV/vis (THF), λmax (ε): 376 (178), 467 (66). MS(EI) m/z (%): 371 (54) [M]+, 335 (100) [M − Cl]+, 299 (27) [M − 2×Cl]+. Anal. Calcd for C17H17Cl2CrN2 (372.23): C, 54.85; H, 4.60; N, 7.53. Found: C, 54.91; H, 4.87; N, 7.39. Synthesis of 5. 1-(1,5-Naphthyridin-8-yl)-3-(tert-butyl)cyclopentadiene (2; 0.85 g, 3.41 mmol) was dissolved in 50 mL of THF, and KH (0.14 g, 3.41 mmol) was added. After stirring overnight the reaction mixture was slowly added to a solution of CrCl3(THF)3 (1.29 g, 3.4 mmol) in 20 mL of THF. After stirring for 3 days, the solvent was evaporated under vacuum, and the residue was extracted several times with toluene and CH2Cl2. The combined solutions were evaporated, and the resulting green powder was washed with n-hexane. Yield: 0.43 g (1.16 mmol, 34%). 1H NMR (CDCl3, 200 MHz): δ −84.8 (H2), −50.0 (H4), −3.11 (H6, H7), 47.9 (H3), 220−230 (H8, H9, H10). MS(EI) m/z (%): 371 (50) [M]+, 336 (22) [M − Cl]+, 249

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ASSOCIATED CONTENT

* Supporting Information S

Table of details of crystal structures of 4−6 and CIF file giving crystallographic data for compounds 4, 5·CH2Cl2, and 6. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +49-6221-546247. Fax: +49-6221-541616247. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (SFB 623). We thank Dr. Robert Brüll (Deutsches Kunststoff Institut at Darmstadt/Germany) for the GPC analytical data.

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REFERENCES

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