NMR Spectroscopy and X‐Ray Characterisation of Cationic N‐Heteroaryl‐Pyridylamido Zr^IV Complexes: A Further Level of Complexity for the Elusive Active Species of Pyridylamido Olefin Polymerisation Catalysts

Abstract

New [(N(-),N,N(-))ZrR2] dialkyl complexes (N(-),N,N(-)=pyrrolyl-pyridyl-amido or indolyl-pyridyl-amido; R=Me or CH2Ph) have been synthesised and tested as pre-catalysts for ethene and propene polymerisation in combination with different activators, such as B(C6F5)3, [Ph3C][B(C6F5)4], [HNMe2Ph][B(C6F5)4] or solid AlMe3-depleted methylaluminoxane (DMAO). Polyethylene (M(w)>2 MDa and M(w)/M(n)= 1.3-1.6) has been produced if pre-catalysts were activated with 1000 equivalents of DMAO (based on Al) [activity >1000 kg(PE)(mol([Zr]) h mol atm)(-1)] or by using a higher pre-catalyst concentration and a mixture of [HNPhMe2][B(C6F5)4] (1 equiv) and AliBu2H (60 equiv). In the case of propene polymerisation, activity has been observed only if pre-catalysts were treated with an excess of AliBu2H prior to addition of DMAO, which led to highly isotactic polypropylene ([mmmm]>95%). Neutral pre-catalysts and ion pairs derived from their activation have been characterised in solution by using advanced 1D and 2D NMR spectroscopy experiments. The detection and rationalisation of intercationic NOEs clearly showed the formation of dimeric species in which some pyrrolyl or indolyl π-electron density of one unit is engaged in stabilising the metal centre of the other unit, which relegates the counterions in the second coordination sphere. The solid-state structure of the dimeric indolyl-pyridyl-amidomethylzirconium derivative, determined by X-ray diffraction studies, points toward a weak Zr···η(3)-indolyl interaction. It can be hypothesised that the formation of dimeric cationic species hampers monomer coordination (especially of less reactive α-olefins) and that addition of AliBu2H is crucial to split the homodimers.

Benzoimidazole‐Pyridylamido Zirconium and Hafnium Alkyl Complexes as Homogeneous Catalysts for Tandem Carbon Dioxide Hydrosilylation to Methane

“…However, a viscous precipitate formed on the tube walls upon standing within a few minutes, which limits the complete spectroscopic characterization of the activated form once again. The moderate solubility of the activated species suggests that outer‐sphere ion pairs (OSIPs) are formed. The 19 F{ 1 H} NMR spectrum of the cationic benzyl‐hafnium species 7 b* with [PhCH 2 B(C 6 F 5 ) 3 ] − as a counteranion (Figure S44) confirms this hypothesis as inferred by the relatively small [Δ δ =( para F)−( meta F)] value calculated for the complex (Δ δ m F, p F =2.90 ppm) .…”

NMR and EPR trapping of the active species in the ethylene polymerization and trimerization catalyst systems

“…This tendency supports stable interest to the detailed mechanistic studies of metallocene and post-metallocene catalyzed polymerization [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, quite often the available information on the reaction mechanism remains insufficient.…”

Olefin Polymerization Reactivity of Group 4 Post‐Metallocene Catalysts Bearing a Four‐Membered C(sp3)‐Donor Chelate Ring

“…DFT calculations with the borate indicate that the o-CF 3 group as well as the anion would reduce the accessibility of the active site, and comparatively high insertion barriers are obtained. However, with the MAO cocatalyst, intraligand C(methine)-H···FÀ C interactions [3,5,[41][42][43][44] with the o-CF 3 unit (as observed by DFT [for 3 + without anion: H···F = 2.40 Å, CÀ H···F = 115°; H···F = 2.49 Å, CÀ H···F = 109°] and in [ 1 H, 19 F]-HMQC spectrum of 3), could conceivably occur distal to the active site, so that incoming monomers are relatively unhindered and activity is enhanced.…”

“…All complexes have been characterized by 1 H, 13 19 F]-HMBC for 2, 3 and 5). Unlike [O,N,C(σ-aryl)] derivatives that exhibit C 2v symmetry due to a virtual mirror plane through the meridional ligand, the [O,N,CH-(Ar)] complexes adopt C 1 symmetry because of the presence of the Ar unit at the racemic C(methine) atom, as revealed by the appearance of four resonances in the 1 H (and two in the 13 C) NMR spectrum for the CH 2 (benzyl) groups ( Figure S1).…”

“…Various NMR experiments have been performed to probe the possibility of CÀ H···FÀ C interactions within complexes 2, 3 and 5. While studies on 2 and 5 indicated the absence of CÀ H···FÀ C interactions, the [ 1 H, 19 F]-HMQC spectrum for 3 ( Figure S4) reveal a strong crosspeak between the o-CF 3 group and the methine (H 12 ) proton, and a noticeably weaker crosspeak with one of the four methylene protons, while the selective [ 1 H, 19 F]-HMBC (J) spectrum ( Figure S5) demonstrates scalar coupling for these interactions. [6] Considering the highly sensitive nature of these NMR pulse sequences, and the absence of discernible 1h J H,F or 2h J C,F coupling in the 1 H (the H 12 resonance is slightly broad but 19 F-coupling is unresolved) and 13 C NMR spectra, we conclude that (1) the intraligand C-(methine)À H···FÀ C interaction is weak but could persist during polymerization in the absence of competing interactions, and (2) the apparent C(methylene)-H···FÀ C interaction is likely to be too weak (particularly for a benzyl group with greater flexibility within the cationic species) to impact appreciably with regards to WALPI-type catalysis (see below).…”