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Hafnium catalysts for olefin polymerization are often very sensitive to the nature of cocatalysts, especially if they contain "free" aluminium trialkyls. Herein, cocatalyst effects in Hf-catalysed propene polymerization are examined for four Hf catalysts belonging to the family of CS-symmetric (Hf-CS-Met) and C2-symmetric (Hf-C2-Met) metallocenes, as well as of octahedral (Hf-OOOO) and pentacoordinated (Hf-PyAm) "post-metallocenes". The performance of the recently developed {[iBu2(PhNMe2)Al]2(μ-H)}+[B(C6F5)4]− (AlHAl) cocatalyst is compared with that of established systems like methylalumoxane, phenol-modified methylalumoxane and trityl borate/tri-iso-butylaluminium. The worst catalytic performance is observed with MAO. Conversely, the best cocatalyst varies depending on the Hf catalyst used and the performance indicator of interest, highlighting the complexity and importance of selecting the right precatalyst/cocatalyst combination. AlHAl proved to be a suitable system for all catalysts tested and, in some cases, it provides the best performance in terms of productivity (e.g. with hafnocenes). Furthermore, it generally leads to high molecular weight polymers, also with catalysts enabling easy chain transfer to Al like Hf-PyAm. This suggests that AlHAl has a low tendency to form heterodinuclear adducts with the cationic active species, therefore preventing the formation of dormant sites and/or termination events by chain transfer to Al. [ABSTRACT FROM AUTHOR]

The dinuclear aluminum salt {[iBu2(DMA)Al]2(μ-H)}+[B(C6F5)4]− (AlHAl; DMA = N,N-dimethylaniline) is the prototype of a new class of molecular cocatalysts for catalytic olefin polymerization, its modular nature offering easy avenues for tailoring the activator to specific needs. We report here, as proof of concept, a first variant (s-AlHAl) bearing p-hexadecyl-N,N-dimethylaniline (DMAC16) units, which enhances solubility in aliphatic hydrocarbons. The novel s-AlHAl was used successfully as an activator/scavenger in ethylene/1-hexene copolymerization in a high-temperature solution process. [ABSTRACT FROM AUTHOR]

"Structural" AlMe2(bht) was found to be an integral building block of the "TMA‐free" olefin polymerization cocatalyst MAO/BHT (TMA = trimethylaluminum, MAO = methylaluminoxane, BHT = 2,6‐di‐tert‐butyl‐4‐methylphenol). NMR studies show that treatment of MAO/BHT with the neutral N‐donor pyridine (py) leads to selective generation of neutral AlMe2(bht)(py). The reaction of MAO/BHT with 2,2'‐bipyridine (bipy) generates cationic [AlMe(bht)(bipy)]+ fragments (bht = BHT phenolate), analogously to what happens when unmodified MAO is treated with bipy, causing [AlMe2(bipy)]+ formation. This suggests that the activation of an olefin polymerization precatalyst LnMX2 (X = Me or Cl) can occur via an indirect pathway involving [AlMeR]+ (R = Me or bht) transient species (rather than direct Cl/Me abstraction by the Al‐cages) not only when unmodified MAO is exploited but also in the case of "TMA‐free" BHT‐modified MAO. The high chemoselectivity of py for neutral Al adducts however presents a distinct difference to unmodified MAO. These observations indicate that a) "structural" TMA is converted into "structural" AlMe2(bht) upon reaction of MAO with BHT and that b) MAO/BHT is harder to ionize than unmodified MAO. A refined formula and cluster size estimation [(AlOMe)0.87(AlMe2bht)0.13]n (n = 57–84) is proposed for MAO/BHT based on this new experimental evidence, accounting for the presence of "structural" AlMe2(bht) units. [ABSTRACT FROM AUTHOR]

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 ( Mw>2 MDa and Mw/ Mn = 1.3-1.6) has been produced if pre-catalysts were activated with 1000 equivalents of DMAO (based on Al) [activity >1000 kgPE (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 Al iBu2H (60 equiv). In the case of propene polymerisation, activity has been observed only if pre-catalysts were treated with an excess of Al iBu2H 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⋅⋅⋅η3-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 Al iBu2H is crucial to split the homodimers. [ABSTRACT FROM AUTHOR]

The reactions of metallaaziridinium ion pairs [Cp2M(η2-CH2NMePh)][B(C6F5)4] [M=Zr ( 1), Hf ( 2)] with 1-hexene and 2-methyl-1-heptene were investigated by means of NMR spectroscopy and DFT calculations. We found that a single insertion of the olefin into the MC bond led to the formation of stable five-membered heterocycles ( 3- 6). The reactions were 1,2-regioselective and afforded two diastereoisomers whose ratio was mainly controlled by the nature of the olefin, whilst the nature of the metal only played a marginal role. In particular, the reaction with 1-hexene was found to be not only more favored than that with 2-methyl-1-heptene from a thermodynamic point of view but it was also more diastereoselective. In all cases, the observed diastereoisomeric ratio was dictated by thermodynamics because the products interconverted into each other, as indicated by EXSY NMR spectroscopy and by DFT calculations. [ABSTRACT FROM AUTHOR]

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N-,N,C-} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Brønsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N-,N,C-}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf(N-,N,C-}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4} selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N-,N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/η²-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N-,N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N-,N,C-}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N-,C-,N,C-}NR3][B(C6F5)4] (12) through activation of a CH bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N-,N}NMe2C6H4][B(C6F5)4] (7) through orthometalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N-,C-,N,C-}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf(N-,N,C-}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Brønsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems. [ABSTRACT FROM AUTHOR]

The article examines the influence of mixed-ion clusters in metallocene catalysts on ligand exchange dynamics and catalyst activity. According to the authors, the ion pair symmetrization rate constants in ion pairs [(L)Zr-R+⋯B(C6F5)4-] in the absence of olefin were followed by variable-temperature ¹H nuclear magnetic resonance spectroscopy and calculated with the help of simulated spectra.

Counteranion effects on propylene polymerization rates and stereoselectivities are compared using Cs-symmetric Me2C(Cp)(Flu)ZrMe2 (1; Cp = C5H4,η5-cyclopentadienyl; Flu = C13H8, η5-fluorenyl) and C1-symmetric Me2Si(OHF)(CpR*)ZrMe2 (2; OHF = C13H16, η5-octahydrofluorenyl; CpR* = η5-3-(-)- menthylcyclopentadienyl) precatalysts activated with the mononuclear and polynuclear perfluoroarylborate, -aluminate, and -gallate cocatalysts/activators B(C6F5)3 (3), B(o-C6F5C6F4)3- (4), Al(C6F5)3 (5), Ph3C+B(C6F5)4- (6) Ph3C+FAl(o-C6F5C6F4)3- (7), Ga(C6F5)3- (8), and recently reported mono- and polymetallic trityl perfluoroarylhalometalates Ph3C+FB(C6F5)3- (9), Ph3C+FB(o-C6F5C6F4)3- (10), (Ph3C+)xFx[Al(C6F5)3]yx- (X = 1, y = 1, 11; x = 1, y = 2, 12; x = 2, y= 3, 13), Ph3C+(C6F5)3AlFAl(o-C6F5C6F4)3- (14), Ph3C+XAl(C6F5)3- (X = Cl, 15; X = Br, 16), and Ph3C+F[Ga(C6F5)3]2- (17). Temperature, propylene concentration, and solvent polarity dependence are surveyed in polymerizations catalyzed by 1 activated with cocatalysts 3–16 and with a 1:2 ratio of Ph3CCI and 5, and with a 1:2 ratio of Ph3CBr and 5, and by 2 activated with 3, 6, 7, 12, and 14. Remarkable stereocontrol with high activities is observed for 1 + 12 and 1 + 14. Polypropylene samples produced using C1-symmetric precatalyst 2 are subjected to microstructural analyses using stochastic models describing the relative contributions of enantiofacial misinsertion and backskip processes. A powerful technique is introduced for calculating interparametric correlation matrices for these nonlinear stochastic models. The collected results significantly extend what is known about ion-pairing effects in the case of Cs-symmetric precatalyst 1 and allow these findings to be applied to the case of C1-symmetric precatalyst 2 as an agent of isospecific propylene polymerization. [ABSTRACT FROM AUTHOR]

Complexes [Pd(η1, η2-5-OMe–C8H12)(N,O)]BF4 (N,O=2,6-(i-Pr)2(C6H3)N&z.dbnd6;C(Ph)–C(Ph)&z.dbnd6;O, 1; 2,6-(i-Pr)2(C6H3)N&z.dbnd6;C(Me)–C(Ph)&z.dbnd6;O, 2; 2-benzoylpyridine, 3) were synthesized by the reactions of [Pd(η1,η2-5-OMe–C8H12)Cl]2 with the suitable N,O-ligand. They were tested as catalysts for olefin or alkyne polymerizations. During such reactions 1–3 quantitatively transformed into their η1,η2-1-OMe–C8H12 isomers (1a–3a). The same isomerization occurred in methylene chloride, even in the absence of olefins or alkynes, with a much slower rate. All complexes were fully characterized in solution by multinuclear and multidimensional low temperature NMR spectroscopy. The solid state structures of complexes 1 and 1a were investigated by X-ray single crystal studies. 19F, 1H-HOESY NMR experiments carried out in methylene chloride-d2 at 217 K indicated that the anion prefers to locate on the side of N,O-ligand shifted toward the O-arm in 1–1a and 2–2a while it approaches the N-arm in 3 and 3a compounds. [Copyright &y& Elsevier]

Complexes [M(η1,η2-C8H12OMe)((2,6-(R)2&z.sbnd;C6H3)N&z.dbnd6;C(R′)&z.sbnd;C(R′)&z.dbnd6;N((2,6-(R)2&z.sbnd;C6H3))]PF6 (where M=Pd, R=H and R′2=Me2 (1), M=Pd, R=Me and R′2=Me2 (2), M=Pd, R=Et and R′2=Me2 (3), M=Pd, R=iPr and R′2=Me2 (4), M=Pd, R=iPr and R′2=An (5), M=Pt, R=iPr and R′2=An (6)) were synthesized by the reaction of [M(η1,η2-C8H12OMe)Cl]2 with the appropriate α-diimine ligand in the presence of NH4PF6. Their ion pair structure in solution was investigated by detecting dipolar interactions between protons belonging to the cation and fluorine nuclei of the anion (interionic contacts) in the 19F, 1H-HOESY NMR spectra. In complexes 1–4, the anion in solution is located close to the peripheral protons of the α-diimine ligand and it interacts with the R′ protons and with the R protons that point toward the R′ groups. The steric protection of apical position exerted by the R substituents is clearly illustrated by the absence of interionic contacts between any protons of the cycloctenylmethoxy-moiety and the anion for R≥Me in 1–4. In complexes 5 and 6 the interactions between the anion and the peripheral N,N protons also predominate but other anion–cation orientations are significantly present and, consequently, the interionic structure is less specific. [Copyright &y& Elsevier]