Article pubs.acs.org/Organometallics
Highly Fluorinated (σ-Aryl)-Chelating Titanium(IV) Post-Metallocene: Characterization and Scalar [C−H···F−C] Coupling Cham-Chuen Liu,† Loi-Chi So,† Jerry C. Y. Lo,† Michael C. W. Chan,*,† Hiromu Kaneyoshi,‡ and Haruyuki Makio§ †
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China R&D Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan § Mitsui Chemicals Singapore R&D Centre Pte. Ltd., 50 Science Park Road, #06−08 The Kendall Singapore Science Park II, Singapore 117406 ‡
S Supporting Information *
ABSTRACT: The observation of weak intramolecular C−H···F−C interactions in group 4 (σ-aryl)-chelating complexes using NMR spectroscopic and neutron diffraction studies was recently reported. In this work, a new titanium(IV) catalyst precursor supported by a tridentate pyridine-2-phenolate-6-(σ-aryl) ligand, featuring a metal center surrounded by multiple CF3 substituents, has been synthesized. The nature of intramolecular interactions in the bis(benzyl) complex in solution was probed using multinuclear NMR spectroscopic experiments (including [1H,19F]-HMQC and -HMBC), which reveal scalar coupling across C−H···F−C interactions between methylene hydrogens and the proximal CF3 group on the σ-aryl (but not the phenolate) moiety. High activities are observed for ethylene polymerization at different temperatures, which exceed those for the t Bu-phenolate congener.
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by NMR spectroscopic observation of 19F-coupling for the αcarbon atom of the methyl cation, has been invoked to rationalize the suppression of chain termination.9 Furthermore, with regard to late transition metal catalyst systems, two recent reports are of particular relevance. The improved performance of Ni(II) and Pd(II) α-diimine catalysts incorporated into a highly fluorinated cyclophane framework was attributed to stabilization of the reactive intermediate by M···F−C interactions (C−H···F−C interactions are feasible and detected by 1H−19F coupling for a Pd+−Me complex),10 while the observation of suppressed β-H elimination for CF3-substituted Ni(II) phenoxyimine catalysts (relative to the Me-substituted congener) was ascribed to stabilization afforded by the presence of Cβ−Hβ···F−C interactions, based on the appearance of 1 H···19F dipolar interactions in [1H,19F]-HOESY NMR experiments.11 We have described the observation of intramolecular C− H···F−C interactions in F-functionalized pyridine-2-phenolate6-(σ-aryl) [O,N,C] group 4 complexes using NMR spectroscopic and neutron diffraction studies,12 which constitute synthetic models of weak attractive ligand−polymer interactions with potential applicability to polymerization processes.7,11,13 In the wider context of the growing and diverse
INTRODUCTION The generation of highly electrophilic active sites remains an intrinsic and critical objective in the quest for new postmetallocene olefin polymerization catalysts.1 In general, enhanced electrophilicity at the metal center should promote olefin binding and activation for insertion, leading to increased catalytic activity. In this regard, the incorporation of fluorinated moieties as electron-withdrawing substituents to reduce the basicity of the ancillary ligand(s) is an attractive design strategy.2,3 Moreover, the presence of repulsive fluorinecontaining fragments would be expected to decrease undesirable intermolecular interactions, such as ion-pairing4 between cationic metal alkyl species and perfluorinated cocatalysts, during the polymerization reaction. On the other hand, the role of organofluorine groups as attractive moieties in catalysis has become more widespread, especially in the form of noncovalent M···F−C coordination.5 The remarkable living olefin polymerization capabilities6 of fluorine-rich bis(phenoxyimine) group 4 catalysts have been attributed, based on DFT calculations, to the presence of C− H···F−C interactions in the active species between a βhydrogen atom of the polymer chain and an ortho-F substituent in the arylimine moiety,7 which is proposed to curtail termination (β-H elimination/transfer) processes. In contrast, for the related living o-fluorinated bis(enolatoimine) titanium catalysts,8 the existence of Ti···F−C interactions, as evidenced © 2012 American Chemical Society
Received: March 21, 2012 Published: July 25, 2012 5274
dx.doi.org/10.1021/om3002334 | Organometallics 2012, 31, 5274−5281
Organometallics
Article
Scheme 1. Synthesis of Ti Complex 1, Shown with tBu Analogue 2
importance of fluorinated substituents, and the debate14 over the existence and significance of C−H···F−C interactions,15,16 the development of spectroscopic techniques to probe the environment and connectivity around fluorine atoms is warranted. Very recently, we published the first investigation to probe noncovalent 1H−19F coupling using [1H,19F]-HMBC NMR experiments, which confirmed the existence of significant scalar (J) coupling across C−H···F−C interactions in (σ-aryl)2-phenolate-6-pyridyl [O,C,N-RF]-Ti (RF = CF3, F) complexes, but indicated that 1H−19F coupling occurs predominantly through M···F−C interactions in the [O,C,N-CF3]-Zr and -Hf congeners.17 Aromatic σ-carbanions are predominantly σ-donors and can impart greater electrophilicity at metal centers compared with π-donors.18 The aim of this work is to modify the previously reported [O,N,C]-Ti complex bearing a bis(trifluoromethyl)(σ-aryl) moiety12 by replacing the tert-butyl phenolate substituent with an additional CF3 group. The possible effects of such an F-dominated catalytic environment, including enhanced electrophilicity and separation of cation and fluorinated cocatalyst, and the consequences of these upon catalytic and polymer properties will be examined. A variety of multinuclear NMR experiments have been employed to probe the environments around the multiple CF3 units and their participation in intramolecular interactions. In particular, selective [1H,19F]-HMBC NMR experiments have been performed to evaluate the scalar coupling for individual CF3 groups.
acid, the additions of the chromic and acetic acid reagents were carefully performed at −50 °C to minimize its removal. Subsequent deprotection was achieved during flash chromatography using mildly acidic silica gel to afford the desired H2L(CF3)3 ligand in relatively low yields (ca. 20%). The molecular structure of H2L(CF3)3 (Supporting Information) revealed an intramolecular O−H···N hydrogen bond as well as multiple close intermolecular C−H···F−C interactions (shortest 2.57 Å; C−H···F 138°) that are similar to those previously detected in organofluorine molecules.15 Cyclometalation of the H2L(CF3)3 ligand with Ti(CH2Ph)4 was performed in a diethyl ether/n-pentane mixture at −78 °C and proceeded upon warming to 20 °C. After stirring for 12 h, the reaction mixture was filtered, concentrated, and cooled at −78 °C to afford the titanium complex 1 as a dark red crystalline solid in an isolated yield of 30% (the relatively low yield is partly ascribed to its high solubility in nonpolar solvents). Preparation of the Zr and Hf analogues has been attempted several times using different solvents and mixtures thereof (diethyl ether, THF, n-pentane, toluene); the nonpolar solvents were used because etherate solvents were subsequently suspected to promote reduction processes. Nevertheless, the solids obtained from these reactions failed to give discernible 1 H NMR signals, indicating reduced products. The attempted reaction of Zr(CH2Ph)2Cl2(thf)2 with H2L(CF3)3 in various solvents yielded intractable mixtures. Radical mechanisms and migratory insertion and reduction of imine have been described for related tetradentate Schiff-base zirconium bis(benzyl) complexes.22 Multinuclear NMR Characterization. Complex 1 has been characterized in C6D6 by 1H, 19F, and 13C NMR spectroscopy and a range of 2-D NMR experiments. According to conventionally accepted criteria,23 the diminished 2JH,H (8.0 Hz) and large 1JC,H (137.4 Hz) values, plus the high-field orthoPh 1H NMR shift (6.42 ppm) for the benzyl groups, are strongly indicative of η2-coordination (involving ipso-Ph···Ti), thus reflecting the high electrophilicity of the Ti center in 1. Nevertheless, the corresponding parameters for 2 (2JH,H = 8.3 Hz, 1JC,H = 138.0 Hz, 1H NMR (o-Ph): 6.44 ppm) are comparable and do not exhibit the effects of different phenolate
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RESULTS AND DISCUSSION Synthesis. The fluorinated 2-phenol-6-arylpyridine ligand was synthesized by modification of a literature procedure19 involving the sequential coupling of two substituted acetophenones (Scheme 1).20 Ortho-directed lithiation21 of protected 2trifluoromethylphenol, followed by ethanal addition then oxidation, gave the required acetophenone (I-2). The prop-1ene intermediate (I-3) was treated with a mixture of potassium tert-butoxide and I-2, followed by ammonium acetate in acetic acid, to give the protected [O,N,C] ligand. It should be noted that since the ethoxymethyl protecting group is sensitive to 5275
dx.doi.org/10.1021/om3002334 | Organometallics 2012, 31, 5274−5281
Organometallics
Article
Figure 1. Methylene region in (a) 1H and (b) 13C NMR spectra and (c) [1H,19F]-HMQC spectrum of 1 (C6D6,* 298 K, + = residual toluene).
substituents (CF3 versus two tBu). The previously reported molecular structures of [O,N,C] complexes, bearing a σ-aryl substituent adjacent to the metal, show bis(benzyl) moieties that point toward the pyridine ring in an “anti, anti” configuration12,24 (rather than the “syn, anti” arrangement observed for sterically undemanding [O,N,C] ligands12b and typical in related post-metallocenes25), and careful scrutiny of the benzyl 1H NMR resonances for such derivatives reveals a noticeably high-field para-Ph shift (
