Rubin Huang’s research while affiliated with Eindhoven University of Technology and other places

Transmetallation of 4,4'-bis{(2,6-bis[(dimethylamino)methyl]phenylgold)diphenyl-phosphino}biphenyl (3) with MCl(4) (M = Ti, NbCl, V) in benzene gave the corresponding transition metal pincer complexes (4) and insoluble 4,4'-bis[P-(chloro gold(I))diphenylphosphino]biphenyl (2), which can be quantitatively recovered and recycled. Interestingly, 3 did not react with TiCl(3). However, reaction of 2,6-bis[(dimethylamino)methyl]phenyllithium (1) with TiCl(3) resulted in formation of the novel diaryltitanium(IV) compound 5 (16% yield), comprising one N,C,N-mer bound NCN-pincer ligand and a second NCN-pincer ligand that is rearranged from a 1,2,6-isomer to a 1,2,4 one. The latter NCN-ligand is dianionic and is bidentate bonded; one of the CH(2)NMe(2) substituents (para to C'(ipso)) is non-coordinated, while the second CH(2)NMe(2) group, after C-H activation of one of the Me groups, is η(2)-C,N-bonded to the titanium centre trans to C(ipso) of the mer-NCN ligand. The new NCN-pincer metal complexes 2,6-bis[(dimethylamino)methyl]phenylTiCl(3) (4a) and 2,6-bis[(dimethylamino)methyl]-phenylVCl(2) (4d) gave, after immobilization on MgCl(2)-based supports, very high activity in ethene polymerisation.

Reaction of (S,S)-2,6-bis(4′-isopropyl-2′-oxazolinyl)phenyllithium (i-Pr-Phebox-Li) (2a) with 4,4′-bis[P-(chlorogold(I))diphenylphosphino]biphenyl [(dppbp)(AuCl)2] (5) afforded the new, bimetallic gold complex 4,4′-bis[P-(η1-C-i-Pr-Phebox-gold)diphenylphosphino]biphenyl [(P-Au(η1-C-i-Pr-Phebox))2(dppbp)] (6). Transmetalation of 6 with 2 equiv of Cl3MX (MX = TiOi-Pr, VCl, CrPy, ZrCl, HfCl, NbO) afforded the corresponding monopincer compounds [MCl2X(i-Pr-Phebox)] (M = Ti, V, Cr) (7) and [MCl2X(i-Pr-Phebox)]2 (M = Zr, Hf, Nb) (8) in high yields and the gold starting material 5, which could be quantitatively recovered and reused. The structures in the solid state of the digold compound 6, the monopincer compounds R-PheboxAu(PPh3) (R = i-Pr, t-Bu), and a number of Phebox-ETM complexes (7a, 7b, 7c, 8a, 8b, and 8c) were obtained. In each of these structures the i-Pr-Phebox monoanion is mer-N,C,N-tridentate bonded. The monopincer Phebox-Zr and -Hf compounds are dimeric because of two bridging chlorides, while the corresponding Nb compound has bridging oxygen atoms. Reaction of 6 with iron(III) chloride resulted in the formation of a N4(FeCl2)2 complex, characterized by X-ray crystal structure determination, comprising a bridging, tetradentate N4 ligand formed by C–C coupling of two Phebox anions to a biphenyl species. The new Phebox complexes 7 and 8 have been tested as olefin polymerization precatalysts. Rapid catalyst deactivation was observed in ethene polymerization under homogeneous conditions, whereas stable activity was obtained after immobilization on MgCl2-based supports. From preliminary investigations on the reaction of 8a with either methylmagnesium chloride or methyllithium we assume that the low reactivity in homogeneous polymerization is due to the alkylation of the Phebox ligand.

The ability of a MgCl 2 support to activate a transition metal catalyst has been found to depend both on the crystallographic structure of the support and on the nature of the catalyst. A high degree of crystallographic disorder can be very effective for the immobilization and activation of titanium and vanadium complexes, but is not necessarily effective for zirconocene activation. A highly disordered support prepared by the reaction of MgBu 2 with HCl gave high activity with TiCl 4 but low activity with ( n ‐PrCp) 2 ZrCl 2 . High polymerization activities with the zirconocene were only obtained with supports of type MgCl 2 /AlR n (OEt) 3− n prepared from the reaction of AlR 3 with MgCl 2 · 1.1EtOH. These supports are characterized by additional peaks in the X‐ray diffraction pattern, indicating the presence of a crystalline structure which is absent in the other supports and contains highly Lewis acidic sites able to generate the active metallocenium species. magnified image

The effect of cyclopentadienyl ring substitution on the immobilization and activation of zirconocenes on supports of type MgCl2/AlRn(OEt)3-n has been investigated. Comparison of the activity of immobilized zirconocenes (RCp)2ZrCl2 in ethylene polymerization, using simple aluminum alkyls such as AlEt3 or AliBu3 as a cocatalyst, revealed a remarkable effect of the substituent R in the cyclopentadienyl ring. Low activity (<300 kg/mol bar h) was obtained with R = H or Et, but longer alkyl substituents, notably n-Pr or n-Bu, gave more than an order of magnitude increase in activity. Zirconocenes with branched substituents (i-Pr, t-Bu) were less easily immobilized and gave relatively low activity, while the activities of zirconocenes of composition Cp(RCp)ZrCl2 (R = n-Pr, n-Bu, n-pentyl) were intermediate between those of Cp2ZrCl2 and (RCp)2ZrCl2. Experiments carried out under homogeneous conditions with aluminoxane cocatalysts indicated that the effect of cyclopentadienyl ring substitution can be related to differences in the ease of formation of the active species. An inverse relationship between activity and the catalyst loading on the support was found. At a low loading of 1 μmol (n-PrCp)2ZrCl2/g support, an activity greater than 40 000 kg/mol bar h was obtained in ethylene polymerization at 70 °C, with AliBu3 as the cocatalyst. The results demonstrate that with certain MgCl2-immobilized zirconocenes it is possible to achieve very high polymerization activity without the use of methylaluminoxane or a borate activator, but the strong effect of catalyst loading on activity implies the presence of a relatively low number of active species.

Immobilization of combinations of early- and late-transition metal catalysts on MgCl2 supports has led to significant increases in catalyst activity when a nickel diimine is incorporated into a Fe-, Cr- or Ti-based catalyst system, and to the formation of intimately mixed, bimodal blends of high- and low-molecular weight polyethylene via coimmobilization of Cr and Fe catalysts.

Investigation of the melt rheological properties of polyethylenes prepared with chromium, titanium, vanadium, and zirconium catalysts immobilized on MgCl2 supports has confirmed narrow (Schulz−Flory) molecular weight distribution, indicative of single-center catalysis, for Cr-, V-, and Zr-based systems, but not for Ti. In the case of polymers prepared with MgCl2-immobilized Ti complexes and having narrow MWD (Mw/Mn 2−3) according to GPC, deviation from a Schulz−Flory distribution was evident from a decrease in storage modulus with decreasing angular frequencies, whereas polyethylenes prepared with analogous vanadium complexes exhibited a constant (plateau) modulus over a wide frequency range. The presence of the constant plateau modulus in high molecular weight polyethylenes is characteristic for narrow MWD, so that melt rheology provides a valuable tool to prove, or disprove, the presence of a Schulz−Flory distribution in cases where GPC does not provide a definitive answer.

The effects of hydrogen in ethylene polymerization and oligomerization with different bis(imino)pyridyl iron(II) complexes immobilized on supports of type MgCl2/AlEtn(OEt)3–n have been investigated. Hydrogen has a significant activating effect on polymerization catalysts containing relatively bulky bis(imino)pyridyl ligands, but this is not the case in ethylene oligomerization with a catalyst containing relatively little steric bulk in the ligand. It was found that the presence of hydrogen in the latter system led to decreased activity and an overall increase rather than a decrease in product molecular weight, indicating deactivation of active species producing low molecular weight polymer and oligomer. Decreased formation of vinyl-terminated oligomers in the presence of hydrogen can therefore contribute to the activating effect of hydrogen in ethylene polymerization with immobilized iron catalysts, if it is assumed that hydrogen activation is related to chain transfer after a 2,1-insertion of a vinyl-terminated oligomer into the growing polymer chain. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4054–4061, 2007

Significant increases in the productivity of iron-, chromium-, and titanium-based MgCl2-supported catalysts for ethylene polymerization have been obtained by incorporation of a limited amount of a nickel diimine catalyst giving branched polyethylene. Formation of the latter during the early stages of polymerization reduces the monomer diffusion limitation inherent in ethylene homopolymerization, thereby increasing the productivity of the main catalyst component. The final products are essentially linear, high-density polyethylenes containing very small amounts of branched polymer. As well being effective for catalysts prepared by coimmobilization of the Ni and Fe, Cr, or Ti component on the MgCl2 support, it is also shown that the productivity of preformed Ziegler−Natta catalysts can be improved significantly by impregnation with the nickel diimine component. In the latter case, more rapid increases in rate throughout the polymerization indicated easier fragmentation of the support in the presence of the nickel complex.

Ethylene polymerizations carried out with various bis(imino)pyridyl iron, chromium and vanadium complexes immobilized on a MgCl2/AlRn(OEt)3−n support gave relatively broad polyethylene molecular weight distributions in the case of iron, but high molecular weight and a very narrow molecular weight distribution with vanadium, indicative of a single active species. The narrow MWD was confirmed by melt rheometry. Similar results were obtained after reaction of the bis(imino)pyridyl complex LVCl3 (6) with MeLi or AlEt3, where alkylation of the pyridine ring gives a complex L′VCl2 (7). In the case of chromium, a bimodal distribution was obtained, with evidence of incomplete catalyst immobilization. The polyethylene molecular weights obtained with the iron complexes were strongly dependent on the substituents in the bis(imino)pyridyl ligand, and were somewhat higher than have been obtained in homogeneous polymerization. In contrast, the molecular weights obtained with the bis(imino)pyridyl chromium and vanadium complexes were much higher that those previously obtained under homogeneous conditions. In all cases, the activities of the immobilized catalysts were higher than those found in homogeneous polymerization.Graphical abstractIn contrast to the relatively broad molecular weight distribution of polyethylene prepared with various bis(imino)pyridyl iron complexes immobilized on a MgCl2/AlRn(OEt)3−n support, immobilization of a bis(imino)pyridyl vanadium(III) complex on the same support gave very narrow molecular weight distribution, indicative of a single active species in the case of vanadium but not with iron. The vanadium-based system also gave significantly higher molecular weight.Shear frequency dependence of storage modulus (G′) at constant strain of (♦ and ▴) polyethylene () prepared using an immobilized V catalyst, compared to (■) a reference polyethylene () prepared using an immobilized iron catalyst.