ChemComm Zwitterionic Aluminabenzene-Alkylzirconium Complex Having Half-Zirconocene Structure: Synthesis and Application for an Additive-Free Ethylene Polymerization Journal: ChemComm Manuscript ID CC-COM-03-2018-002186 Article Type: Communication Page 1 of 5 Please doChemComm not adjust margins Journal Name COMMUNICATION Zwitterionic Aluminabenzene-Alkylzirconium Complex Having Half-Zirconocene Structure: Synthesis and Application for an Received 00th January 20xx, Additive-Free Ethylene Polymerization Accepted 00th January 20xx a ,b ,b DOI: 10.1039/x0xx00000x Taichi Nakamura, Katsunori Suzuki* and Makoto Yamashita* www.rsc.org/ The aluminabenzene-alkylzirconium complex having half- resulting zwitterionic complexes catalyses ethylene zirconocene structure was synthesized. X-ray crystallographic polymerization as a highly active singlecomponent catalyst.5b analysis of this complex revealed a zwitterionic structure However, the reported group 4 metal catalyst systems with a consisting of cationic alkylzirconium chloride and four-coordinated pendent aluminum-based Lewis acidic moiety were limited,6 aluminate. In the presence of catalytic amount of this complex, while the aluminum additives have been commonly used in ethylene polymerization could proceed without any additives to Kaminsky catalyst. Nomura reported utilization of titanium t form ultra-high molecular weight polyethylene. complex bearing Me2Al(O Bu)(OR) moiety as a single- component catalyst for ethylene polymerization (Scheme In homogeneous catalytic polymerization of olefins, an 1B).6a,b In their plausible mechanism, the dissociation of admixture of group 4 transition-metal complex, such as Me2Al(OtBu)(OR) moiety as an anion generates zwitterionic metallocene or halfmetallocene complexes, and Lewis acidic complex involving cationic titanium centre. This catalyst additive, have been known to construct an effective catalyst system can form high molecular weight PE (> 1,000,000) due 1 system, represented as Kaminsky catalyst. The Lewis acidic to the suppression of chain transfer by taking advantages of the additive, such as methylalumoxane (MAO), plays a crucial role absence of external Lewis acid. These reports indicate that for the generation of catalytically active species. Generally, a incorporation of Lewis acidic aluminum on an ancillary ligand large excess of the additive is required to form the active will be unique strategy for the active catalyst for high molecular 1 catalyst effectively. Using an excess of additive causes side weight polyethylene (PE). In this context, common catalysts, reactions, such as catalyst degradation by contaminated AlMe3 such as metallocene/halfmetallocene type complex, bearing and chain transfer that decreases the molecular weight of the aluminum moiety should be mentioned. However, in our 2 resulting polymer. Thus, the catalytic polymerization system knowledge, no example of metallocene or halfmetallocene- with stoichiometric amount of Lewis acidic additive or with an type olefin polymerization catalyst bearing a Lewis acidic sp2 isolation of catalytically active cationic complex has been aluminum moiety incorporated in the ancillary Cp type ligand 3 developed. As an alternative strategy, Reetz and Bochmann have been reported.7,8 reported group 4 metal complexes with pendent Lewis acid Recently, we reported synthesis and characterization of functionality, which could capture an anionic ligand from metal anionic aluminabenzene, which has aromatic 1 and ambiphilic centre to generate catalytically active cationic metal centre 1’ resonance contributions (Scheme 2).9 Due to the resonance (Scheme 1A).4 After these findings, several examples of such complexes possessing a boron-based Lewis acidic moiety have A BR'2 BR'2 been reported to be applied as an active catalyst for ethylene X X LnM LnM R polymerization.5 For example, Piers reported catalytic activity R of zwitterionic complex synthesized by the reaction of "tuck-in" M = group 4 metal zirconocene with HB(C6F5)2 or B(C6F5)3. In this case, the B t t Bu Me Bu Me Al O O OAl O O Ti O O Ti O N N Scheme 1 (A) Formation of active catalyst by intramolecular ligand migration proposed by Reetz and Bochmann. (B) Reported single-component olefin polymerization catalyst based on intra-molecular combination of aluminum and titanium This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please doChemComm not adjust margins Page 2 of 5 COMMUNICATION Journal Name Previous work 0 1 on Cl and the d Zr centre. The H NMR spectrum of 3 in C6D6 exhibited broadened signals probably due to a change in Cl Zr CpZrCl3 hapticity of the Zrattached Bn group or exchanging Bn groups Si Al Si Si Al Si Cl Si between Al and Zr atoms. The complex 3 was unstable in Mes Mes Al solution at room temperature even under argon atmosphere 1 1’ Mes Si 2 (45% of 3 was converted to unidentified product after 24 h in Aromatic Ambiphilic i C6D6). Singlecrystal Xray diffraction analysis of 3 revealed a Si = Si Pr3 Mes = 2,4,6-trimethylphenyl zwitterionic halfmetallocene structure involving This work dibenzylaluminate and cationic zirconium moieties (Fig. 1). Si Synthesis The Al1–Zr1 distance [2.9686(9) Å] is longer than the sum of Bn Al Structural analysis the covalent radii (2.70 Å), indicating no tangible interaction Bn 13 Si Application f or ethylene polymerization between Al and Zr atoms. The aluminabenzene ligand Zr without any additives 5 Cl Bn coordinated to zirconium in η fashion at pentadienyl moiety, 3 which had almost same C–C bond lengths [1.407(4)–1.423(4) Scheme 2 Complexation of aluminabenzene with zirconium Å]. The Zr–C1 and Zr1-C5 distances [2.204(3), 2.292(3) Å] were shorter compared to those of the previously reported contribution of 1’, the aluminum atom act as Lewis acid to aluminabenzene zirconium chloride.10a This result would accept the coordination of Lewis base. In addition, anionic attribute to the strong interaction between cationic zirconium aluminabenzene can coordinate to early and late transition and most negatively charged ortho-carbon atoms due to the 5 metals in η -fashion with retaining the Lewis acidity of the substitution of electropositive aluminum and silicon atoms. The 10 aluminum atom in aluminabenzene ligand. In fact, aluminum atom adopts tetrahedral geometry with two benzyl aluminabenzene-chlorozirconium complex 2 exhibited ligands. The Al–C1 and Al-C5 distances [2.188(3), 2.112(3) Å] intramolecular interaction between the aluminum atom and were longer compared to those of 1 [1.922(2)1.930(2) Å] due chloride ligand on zirconium. If anionic aluminabenzene would to the disappearance of unsaturated character of Al–C bonds coordinate to alkylzirconium complex, one could expect that upon coordination. The Zr1–C38 and Zr1–C39 distances aluminum atom on the aluminabenzene ligand would abstract [2.293(3), 2.693(2) Å] indicated η2-coordination mode of an alkyl ligand from zirconium to generate cationic zirconium benzyl ligand to Zr because they were comparable to those of centre, which would be capable for the olefin polymerization. η2Bn ligand in the previously reported cationic zirconium Herein, we report the synthesis of the alkylzirconium complex + - 14 complex [ZrBn3] [BnB(C6F5)3] . having aluminabenzene ligand 3. The Xray crystallographic To obtain further insight into the electronic structure of 3, analysis of 3 revealed a zwitterionic structure bearing anionic we performed DFT calculations at the M06L/[LanL2DZ for Zr, aluminate and cationic alkylzirconium moieties. Catalytic 631G(d) for Al, Si, Cl, C, H] level of theory.15 The optimized activity of 3 toward ethylene polymerization in the absence of structure of 3 is comparable to the experimentally observed additive was also investigated. structure determined by X-ray crystallography. The HOMO and Although we initially attempted synthesis of LUMO of the optimized 3 are shown in Fig. 2. The HOMO aluminabenzene-alkylzirconium complex by an alkylation of localizes around the AlBn2 moiety, while the LUMO was the previously reported aluminabenzenezirconium chloride, located around zirconium atom. The localization of HOMO and 10 such as 2, the alkylation did not proceed probably due to the LUMO would be reasonable to the zwitterionic character of steric hindrance around zirconium centre. In contrast, red- crystalline aluminabenzenealkylzirconium complex 3 could be synthesized by the reaction of aluminacyclohexadiene 411 with tetrabenzylzirconium in 80% yield (Scheme 3). Although no intermediate was observed during this reaction, the mechanism for the formation of complex 3 would be explained by the complexation of aluminabenzene ligand with zirconium followed by the ligand redistribution between zirconium and aluminum atoms.12 The formation of ZrCl might be preferable due to the presence of the pπdπ interaction between a lone pair Si ZrBn4 Bn Al Bn Si Si Al - toluene Si Zr Cl NEt3 - NEt3 80% Cl Bn 4 3 i Si = Si Pr3 Scheme 3 Synthesis of aluminabenzene-alkylzirconium complex 3 Fig. 1 Crystal structure of 3 (50% thermal ellipsoid probability). Hydrogen atoms are omitted for clarity. 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Page 3 of 5 Please doChemComm not adjust margins Journal Name COMMUNICATION Higher pressure of ethylene resulted in higher yield and activity of 6.8 kg(PE)/mol(Zr)·h·MPa (entry 3). All the obtained PEs were not soluble in solvent for GPC analysis, probably due to their high molecular weight. Therefore, we tried the ethylene polymerization in shorter time to carry out the GPC analysis (entries 4 and 5). The Mw of the resulting PEs in entry 4 and 5 were estimated to be about 3.4×106 and 10.2×106 by high- temperature GPC analysis, although the edge on the higher molecular weight of GPC chromatogram exceeded the exclusion limit of the column (4.0×106). Instead of GPC analysis, the intrinsic viscosity measurement provided viscosity average molecular weight (Mv > 7,000,000; [η] > 35) for entries 13. Thus, all the obtained PEs are classified as ultrahigh Fig.