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Observation of Intramolecular CH⋅⋅⋅FC Contacts in Non‐Metallocene Polyolefin Catalysts: Model for Weak Attractive Interactions between Polymer Chain and Noninnocent Ligand

2003; Wiley; Volume: 42; Issue: 14 Linguagem: Inglês

10.1002/anie.200219832

1521-3773

S.C.F. Kui, Nianyong Zhu, Michael C. W. Chan,

Synthetic Organic Chemistry Methods

Attracting attention! The existence of attractive interactions between a functionalized ancillary ligand and a growing alkyl chain is a highly novel concept in olefin polymerization reactions (see scheme). Such interactions must be very weak so that the chain-propagation process is not perturbed, and "controversial" hydrogen bonds with strong dispersive character, such as CH⋅⋅⋅FC, are therefore ideal in this role (see scheme). The mechanisms of stereoselectivity for the polymerization of prochiral α-olefins by Group 4 metallocene catalysts are well understood and are principally derived from steric effects (i.e. repulsive nonbonded interactions).1 By comparison, the study of attractive interactions between the ancillary ligand and the polymer chain remains uncharted. In this context, multidentate ligands with mixed functionalities ("hemilabile" ligands) in which the labile moiety is employed to stabilize the metal center have been extensively utilized in late transition-metal catalysts2 and more recently in metallocene-type derivatives.3, 4 The application of attractive interactions between the ligand and growing alkyl chain to manipulate the reactivity of polyolefin catalysts becomes viable only if such interactions are weak and reversible, so that the intrinsic polymerization process is not disrupted. Significantly, the Fujita and Coates groups have recently described a novel class of fluorinated phenoxyimine titanium polyolefin catalysts. Fujita et al. reported that these catalysts can mediate living polyethylene formation at remarkably elevated temperatures5 as well as highly syndiotactic living polymerization of propylene,6a and elucidated that propylene insertions occur with predominant 2,1 regiochemistry,6b which is exceedingly rare for Group 4 catalysts. Coates and co-workers observed virtually exclusive 2,1 regiochemistry for these TiIV fluorine-rich catalysts in ethylene/propylene copolymerization and cyclopolymerization processes.7 Based on DFT calculations,5b Fujita and co-workers proposed that a fluorine atom adjacent to the imine nitrogen atom, which interacts with the polymer chain to curtail β-hydrogen-transfer pathways, is required for a living process. We now report the first direct observation of weak intramolecular CH⋅⋅⋅FC contacts in neutral and cationic Group 4 polyolefin procatalysts. The CH⋅⋅⋅FC interaction is amongst the weakest hydrogen bond to be proposed and apparent consensus regarding its validity has only just been reached.8 Herein, the potential importance of such weak attractive interactions between the ligand and polymer chain in olefin polymerization processes will be emphasized. We have targeted unsymmetric tridentate ligand sets that can support active olefin polymerization catalysts. In particular, we are evaluating the suitability of aromatic σ-carbanions, because the resultant metal–carbon bond should be more inert than aliphatic analogues and its relative covalency is expected to yield a highly electrophilic catalytic center. Metalation of 2-(2′-phenol)-6-arylpyridine substrates containing acidic protons ensues upon reaction with the basic metal precursors [M(CH2Ph)4] (M=Zr, Ti) and is accompanied by elimination of toluene to afford complexes 1–3 as orange to red crystalline solids (Scheme 1). The cyclometalation process readily proceeds at room temperature and appears highly regioselective; for complex 2, in which two cyclometalation sites are available, only the pictured compound (where R1 is less bulky) has been isolated or detected. We note that endeavors to synthesize Zr derivatives where R1=R2=tert-butyl groups yielded intractable mixtures, characterization of which (by 1H NMR spectroscopy) signified non-cyclometalation. Synthesis of 1–3. Characterization of complexes 1 and 3 by NMR spectroscopy was highly informative (also see Supporting Information). The 1H NMR spectrum (C6D6) of 1 is typical for η2-coordinated benzyl groups,9 except that one of the two expected doublets for the diastereotopic methylene hydrogen atoms, at δ=3.09 ppm, appears as an overlapping doublet of quartets (Figure 1). Furthermore, the 13C{1H} NMR spectrum displays a quartet signal at δ=70.50 ppm (J=5.9 Hz) for the methylene groups. The magnitudes of the coupling constants are clearly not consistent with formal six- or five-bond coupling. Subsequent 19F decoupling of the 1H NMR spectrum demonstrates coupling between one of the two CH2 protons and three equivalent 19F nuclei. Similarly, while the coupling in the 19F[1H] NMR spectrum is not clearly resolved, the 19F{1H} NMR spectrum undergoes significant narrowing for the downfield signal at δ=−58.09 ppm only (Figure 1). A 19F–1H 2D correlation experiment (see Supporting Information) also revealed a cross-peak between the upfield 1H multiplet and the downfield 19F signal only. These observations thus strongly implicate the existence of intramolecular "through-space" coupling through CH⋅⋅⋅FC hydrogen bonds.10 The effects of solvent polarity upon this coupling have been probed, and 1hJH,F values of 3.3 and 3.1 Hz in C6D6 and CD2Cl2, respectively, have been obtained, whereas in [D8]THF this coupling is unresolved yet still apparent (Figure 1). The decreases in the 1hJH,F value correspond to the increasing hydrogen-bonding capabilities of the solvents. The persistence of CH⋅⋅⋅FC coupling in [D8]THF is surprising and can be attributed to the "locked" conformation of the rigid molecular framework. 1H (400 MHz) and 19F (376 MHz, CF3 region) NMR spectra (C6D6, 300 K) of 1, demonstrating the effects of 19F and 1H decoupling (ppm axis displaced for clarity), respectively, and of solvent polarity upon the diastereotopic methylene hydrogen atoms (✶=C6D6, +=residual toluene). The proposed hydrogen-bonding interaction and the environment around the CF3 moieties in complex 1 were further investigated by 1H/19F NOE difference experiments (see Supporting Information). When the signal for the proximal CF3 group was selectively irradiated, strong enhancement was apparent for the multiplet exhibiting 19F coupling at δ=3.09 ppm only, and no response was perceived for the doublet at δ=3.26 ppm. Intriguingly, we also observed moderate enhancement for the ortho-benzyl protons. Irradiation of the distal CF3 substituent resulted in enrichment for the two adjacent aromatic proton signals only as anticipated. An illustration of the weak hydrogen bonding involving one of the methylene hydrogen atoms and the rapidly rotating trifluoromethyl unit is depicted in Figure 1. A virtually identical intramolecular CH⋅⋅⋅FC interaction in complex 3 is indicated by analogous NMR experiments, although the through-space coupling is slightly weaker (in C6D6: 1hJH,F≈2 Hz, 2hJC,F=5.3 Hz). We have also attempted to characterize the corresponding benzyl cationic species. The reaction of complex 1 with B(C6F5)3 in CD2Cl2 yielded a complicated mixture of products. However, in the presence of a few drops of [D8]THF, the transformation resulted in the clean generation of an η2-CH2Ph cation that is presumably stabilized by the OC4D8 ligand (Figure 2; see Supporting Information for 1H–1H COSY spectrum). Saliently, the broad upfield 1H doublet peak for one of the methylene protons and the downfield 19F signal for the proximal CF3 group are both partially sharpened upon decoupling of the 19F and 1H nuclei, respectively. Moreover, the 13C{1H} NMR spectrum reveals a rather broad but partially resolved quartet signal at δ=76.9 ppm (2hJC,F≈4 Hz) for the ZrCH2 group. These observations imply the presence of weak CH⋅⋅⋅FC contacts between the ancillary ligand and an alkyl fragment in a Group 4 metal cation. 1H (400 MHz) and 19F (376 MHz, CF3 region) NMR spectra (CD2Cl2 + [D8]THF, 300 K) for the η2-CH2Ph cation derived from the reaction of 1 with B(C6F5)3, demonstrating the effects of 19F and 1H decoupling (ppm axis displaced for clarity), respectively (✶=deuterated solvents). The molecular structures of complexes 1 (Figure 3) and 2 have been determined by X-ray crystallography.11 The zirconium center in both structures is chelated by a phenolate–pyridine–carbanion [O,N,C] ligand in a distorted trigonal-bipyramidal geometry, with axial O,C atoms and equatorial N and C(benzyl) groups. The most striking difference between 1 and 2 is the orientation of the benzyl moieties, which exist in the "anti,anti" and "syn,anti" configurations, respectively (see below). Nevertheless, like in their solution structures, the benzyl units can participate in η2 coordination to the electrophilic metal core,12 although this interaction appears fractionally weaker in 1. While there are no close Zr⋅⋅⋅F contacts4 (shortest Zr⋅⋅⋅F distance 2.940(2) Å) in the crystal lattice of 1, the structural parameters concerning the CH⋅⋅⋅FC fragment are consistent with a weak hydrogen bond8 (e.g. H⋅⋅⋅F 2.47 and 2.59 Å, CH⋅⋅⋅F 114°). The unusual anti,anti benzyl arrangement in 1 may be attributed to a combination of the stabilizing effect from the CH⋅⋅⋅FC interactions and the congested environment around the equatorial central cleft (see space-filling diagram in Supporting Information), although crystal packing effects cannot be disregarded. Interestingly, the N-Zr-C-Cipso dihedral angles for the anti benzyl ligands (which wrap around the pyridyl ring) are 8.4 and 26.5° (the corresponding angle in 2 is 2.8°), hence the CH2Ph units appear to incline towards the more crowded surroundings of the CF3 moiety (Figure 3 b). We note the position of the ortho-phenyl hydrogen atoms H34 and H41 relative to the CF3 group. a) Structure of 1 (30 % probability ellipsoids, only methylene hydrogen atoms are shown for clarity). Selected bond lengths [Å] and angles [°]: Zr1-C1 2.330(3), Zr1-N1 2.391(2), Zr1-O1 1.952(2), Zr1-C28 2.276(3), Zr1⋅⋅⋅C29 2.729(3), Zr1-C35 2.267(3), Zr1⋅⋅⋅C36 2.765(3); C13-O1-Zr1 147.3(2), C29-C28-Zr1 90.7(2), C36-C35-Zr1 92.8(2). b) View along the Zr1–N1 vector in 1, depicting the short CH⋅⋅⋅FC contacts (F1⋅⋅⋅H28b 2.471, F1⋅⋅⋅C28 2.996(3), F1⋅⋅⋅H35a 2.592, F1⋅⋅⋅C35 3.115(3) Å; F1⋅⋅⋅H28b-C28 113.8, F1⋅⋅⋅H35a-C35 114.0°) and the apparent "tilt" of the CH2Ph planes towards the CF3 moiety. The intramolecular CH⋅⋅⋅FC contacts depicted in this work, demonstrated in the solution state and supported by X-ray structural determinations, represent unique models of weak attractive interactions between a functionalized ligand and a polymer chain at an active catalytic center.13 Our results provide substantiating evidence for the proposition derived by Fujita et al. from DFT calculations regarding ligand–polymer interactions originating from (sp2)C–F moieties.5b In conclusion, this study highlights the potential of weak attractive interactions between the ligand and polymer chain as a new paradigm in olefin polymerization reactions. Fragile noncovalent contacts such as hydrogen bonds are ideal for this role, and their diverse nature paves the way for molecular recognition possibilities. These weak interactions may potentially be exploited for the stabilization of hitherto unstable intermediates and new polymer architectures. Synthetic procedure: The 2-(2′-phenol)-6-arylpyridine substrate in pentane/diethyl ether (5:1) was slowly added to a stirred solution of [M(CH2Ph)4] in pentane/diethyl ether (5:1) at −78 °C. The reaction mixture was allowed to warm up to room temperature and stirred for 12 h. The resultant solution was filtered, concentrated to about 10 mL and stored at −15 °C to afford a crystalline solid. Analytically pure products were obtained by recrystallization from pentane (see Supporting Information for detailed characterization data and labeling schemes). 1: red crystals; yield: 0.19 g, 62 %; elemental analysis calcd (%) for C41H39F6NOZr (766.98): C 64.21, H 5.13, N 1.83; found: C 63.99, H 5.57, N 1.89; selected NMR data (C6D6, 300 K): 1H NMR (400 MHz): δ=1.36 (s, 9 H; 5-tBu), 1.72 (s, 9 H; 3-tBu), 3.09 (dq, J=9.6 Hz, 1hJH,F=3.3 Hz, 2 H; ZrCH2), 3.26 (d, J=9.4 Hz, 2 H; ZrCH2), 6.18 (t, J=7.3 Hz, 2 H; p-Ph), 6.25 (t, J=7.7 Hz, 4 H; m-Ph), 6.54 (d, J=7.4 Hz, 4 H; o-Ph), 6.57 (d, J=7.9 Hz, 1 H; H10), 6.77 (t, J=8.0 Hz, 1 H; H9), 7.25 (d, J=8.0 Hz, 1 H; H8), 7.40 (d, J=2.3 Hz, 1 H; H6), 7.60 (s, 1 H; H13), 7.69 (d, J=2.4 Hz, 1 H; H4), 7.81 ppm (s, 1 H; H15); 19F NMR (376 MHz): δ=−58.09 (CF3 at R1), −62.56 ppm (CF3 at R2); 13C NMR (126 MHz): δ=70.50 ppm (q, 2hJC,F=5.9 Hz (JCH=133.3 Hz); ZrCH2). 2: orange-red crystals; yield: 0.23 g, 60 %; elemental analysis calcd (%) for C40H40F3NOZr (698.98): C 68.73, H 5.77, N 2.00; found: C 68.21, H 5.68, N 2.02; selected NMR data (C6D6, 300 K): 1H NMR (500 MHz): δ=1.36 (s, 9 H; 5-tBu), 1.58 (s, 9 H; 3-tBu), 2.27 (d, J=9.3 Hz, 2 H; ZrCH2), 2.49 (d, J=9.3 Hz, 2 H; ZrCH2), 6.64 (t, J=7.3 Hz, 2 H; p-Ph), 6.71 (d, J=7.2, 4 H; o-Ph), 6.80 (m, 5 H; m-Ph and H10), 6.86 (t, J=7.9 Hz, 1 H; H9), 7.20 (d, J=7.4 Hz, 1 H; H8), 7.39 (d, J=2.3 Hz, 1 H; H6), 7.41 (d, J=7.3 Hz, 1 H; H15), 7.63 (d, J=2.4 Hz, 1 H; H4), 7.70 (s, 1 H; H13), 7.79 ppm (d, J=7.3 Hz, 1 H; H16); 19F NMR (376 MHz): δ=−62.27 ppm; 13C NMR (126 MHz): δ=66.33 ppm (JC,H=135.0 Hz; ZrCH2). 3: dark red crystals; yield: 0.18 g, 57 %; elemental analysis calcd (%) for C41H39F6NOTi (723.66): C 68.05, H 5.43, N 1.93; found: C 68.08, H 5.58, N 2.09; selected NMR data (C6D6, 300 K): 1H NMR (500 MHz): δ=1.34 (s, 9 H; 5-tBu), 1.77 (s, 9 H; 3-tBu), 4.00 (dq, J=8.4 Hz, 1hJH,F=1.2 Hz, 2 H; TiCH2), 4.04 (d, J=8.3 Hz, 2 H; TiCH2), 6.25 (t, J=7.2 Hz, 2 H; p-Ph), 6.32 (t, J=7.1 Hz, 4 H; m-Ph), 6.42 (d, J=8.3 Hz, 1 H; H10), 6.44 (d, J=7.3 Hz, 4 H; o-Ph), 6.66 (t, J=8.0 Hz, 1 H; H9), 7.21 (d, J=8.0 Hz, 1 H; H8), 7.40 (d, J=2.1 Hz, 1 H; H6), 7.60 (s, 1 H; H13), 7.70 (d, J=2.3 Hz, 1 H; H4), 8.12 ppm (s, 1 H; H15); 19F (376 MHz): δ=−56.45 (CF3 at R1), −62.60 ppm (CF3 at R2); 13C (126 MHz): δ=96.19 ppm (q, 2hJC,F=5.3 Hz (JCH=138.9 Hz); TiCH2). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2003/z19832_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.