Very Large Cooperative Effects in Heterobimetallic Titanium-Chromium Catalysts for Ethylene Polymerization/Copolymerization

Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization

The preparation of heterodinuclear complexes, especially those comprising early-late transition metals coordinated by a simple or symmetrical ancillary ligand, represents a fundamental challenge and an opportunity to prepare catalysts benefitting from synergic properties. Here, two new mixed titanium(iv)-zinc(ii) complexes, [LTi(OⁱPr)₂ZnEt] and [LTi(OⁱPr)₂ZnPh], both coordinated by a diphenolate tetra(amine) macrocyclic ligand (L), are prepared. The synthesis benefits from the discovery that reaction of the ligand with a single equivalent of titanium tetrakis(iso-propoxide) allows the efficient formation of a mono-Ti(iv) complex, [LTi(OⁱPr)₂]. All new complexes are characterized by a combination of single crystal X-ray diffraction, multinuclear NMR spectroscopy and mass spectrometry techniques. The two heterobimetallic complexes, [LTi(OⁱPr)₂ZnEt] and [LTi(OⁱPr)₂ZnPh], feature trianionic coordination by the macrocyclic ligand and bridging alkoxide groups coordinate to both the different metal centres. The heterodinuclear catalysts are compared to the mono-titanium analogue, [LTi(OⁱPr)₂], in various polymerization reactions. In the alternating copolymerizations of carbon dioxide and cyclohexene oxide, the mono-titanium complex is totally inactive whilst the heterodinuclear complexes show moderate activity (TOF = 3 h^-1); it should be noted the activity is measured using just 1 bar pressure of carbon dioxide. In the ring opening polymerization of lactide and ε-caprolactone, the mono-Ti(iv) complex is totally inactive whilst the heterodinuclear complexes show moderate-high activities, qualified by comparison to other known titanium polymerization catalysts (l-lactide, k_obs = 11 × 10^-4 s^-1 at 70 °C, 1 M in [lactide]) and ε-caprolactone (k_obs = 5 × 10^-4 s^-1 at 70 °C, 0.9 M in [ε-caprolactone]).

Group 4 permethylindenyl constrained geometry complexes for ethylene polymerisation catalysis

The synthesis and characterisation of six permethylindenyl constrained geometry complexes is reported. Slurry phase ethylene polymerisation studies demonstrate that activities increase with increasing electron donating character of the amido fragment.

Synthesis of poly(α-olefins) containing rare short-chain branches by dinuclear Ni-based catalysts

Chain walking mechanism with regard to dinuclear Ni-based catalyst stereoisomers in the polymerization of α-olefins: the effects of bridge, backbone and ortho-substituents.

Polyolefins, a Success Story

This paper reports the principal discoveries which have played a major role in the polyolefin field and have positioned polyolefins as the most produced plastics. The early development of polyolefins covering the production of LDPE (Low density polyethylene) at ICI (Imperial Chemical Industries) and the discovery of Phillips or Ziegler-Natta catalysts are highlighted in the first section. In the second part, the impact of the implementation of molecular catalysts on the research in polyolefins is discussed together with the most recent advances leading to high-performance tailor-made resins.

Synthesis and structural analysis of aryloxo-modified trinuclear half-titanocenes, and their use as catalyst precursors for ethylene polymerisation

The synthesis and structural analysis of aryloxo-modified trinuclear half-titanocenes have been explored: the Cp* analogues showed higher activities for ethylene polymerisation than the related mononuclear analogues.

Functional Short-Bite Ligands: Synthesis, Coordination Chemistry, and Applications of N-Functionalized Bis(diaryl/dialkylphosphino)amine-type Ligands

The aim of this review is to highlight how the diversity generated by N-substitution in the well-known short-bite ligand bis(diphenylphosphino)amine (DPPA) allows a fine-tuning of the ligand properties and offers a considerable scope for tailoring the properties and applications of their corresponding metal complexes. The various N-substituents include nitrogen-, oxygen-, phosphorus-, sulfur-, halogen-, and silicon-based functionalities and directly N-bound metals. Multiple DPPA-type ligands linked through an organic spacer and N-functionalized DRPA-type ligands, in which the PPh2 substituents are replaced by PR2 (R = alkyl, benzyl) groups, are also discussed. Owing to the considerable diversity of N-functionalized DPPA-type ligands available, the applications of their mono- and polynuclear metal complexes are very diverse and range from homogeneous catalysis with well-defined or in situ generated (pre)catalysts to heterogeneous catalysis and materials science. In particular, sustained interest for DPPA-type ligands has been motivated, at least in part, by their ability to promote selective ethylene tri- or tetramerization in combination with chromium. Ligands and metal complexes where the N-substituent is a pure hydrocarbon group (as opposed to N-functionalization) are outside the scope of this review. However, when possible, a comparison between the catalytic performances of N-functionalized systems with those of their N-substituted analogs will be provided.

Phosphinosilylenes as a novel ligand system for heterobimetallic complexes

The first heterobimetallic complexes comprising interconnected silylene and phosphine donors are reported. In a stepwise fashion, first the silylene coordinates to iron and subsequently the phosphine coordinates to tungsten. Another heterobimetallic complex can be obtained by the insertion of platinum into the P–H bond.

Ligand-Unsymmetrical Phenoxyiminato Dinickel Catalyst for High Molecular Weight Long-Chain Branched Polyethylenes

An unsymmetrical bimetallic catalyst, (CF₃/SO₂)FI²-Ni₂, having a CF₃-functionalized phenoxyiminato Ni(II) center, which produces linear high-M_w polyolefin (CF₃/Ni) joined to an adjacent SO₂-functionalized phenoxyiminato Ni(II) center, which produces highly branched low-M_w polyolefin (SO₂/Ni), was synthesized and fully characterized. In ethylene homopolymerizations, (CF₃/SO₂)FI²-Ni₂ affords monomodal (Đ = 1.7), highly, long-chain branched (69 branches/1000 C) polyethylenes with M_w = 24 kg/mol. In contrast, bimetallic (CF₃)FI²-Ni₂ and (SO₂)FI²-Ni₂ produce high-M_w (25 kg/mol) exclusively methyl-branched (40 branches/1000 C) and low-M_w (4.5 kg/mol) highly branched (105 branches/1000 C) polyethylenes, respectively, while tandem monometallic (CF₃)FI-Ni + (SO₂)FI-Ni catalyst mixtures yield a bimodal polyolefin mixture (Đ = 6.4).

Preparation of octahydro- and tetrahydro-[1,10]phenanthroline zirconium and hafnium complexes for olefin polymerization

Post-metallocenes were constructed for olefin polymerization using 1,2,3,4,7,8,9,10-octahydro[1,10]phenanthroline and 1,2,3,4-tetrahydro[1,10]phenanthroline derivatives. A series of zirconium complexes - LZrCl2(NHMe2)2 [L = 2,9-H2-C12H12N2 (4), 2,9-Me2-C12H12N2 (5), 2,9-nBu2-C12H12N2 (6), and 2,9-iPr2-C12H12N2 (7)] - and hafnium complexes - LHfCl2(NHMe2)2 [L = 2,9-H2-C12H12N2 (8), 2,9-Me2-C12H12N2 (9), 2,9-nBu2-C12H12N2 (10), and 2,9-iPr2-C12H12N2 (11)] - were synthesized via the reaction of octahydro[1,10]phenanthrolines (2,9-R2-C12H12(NH)2) with (Me2N)2MCl2 (DME). The reaction of 2,9-R2-C12H12(NH)2 with (PhCH2)2ZrCl2 in the presence of a small amount of THF afforded a series of THF adduct analogs, i.e., LZrCl2(THF)2 [L = 2,9-H2-C12H12N2 (12), 2,9-Me2-C12H12N2 (13), 2,9-nBu2-C12H12N2 (14), and 2,9-iPr2-C12H12N2 (15)]. The treatment of 12 and 13 with excess Me3Al resulted in the formation of unexpected complexes, i.e., (η(4)-LAlMe2)ZrCl2(Me) [L = 2,9-H2-C12H12N2 (16) and 2,9-Me2-C12H12N2 (17)], in which the Me2Al unit forms a five-membered ring through binding with the two nitrogen donors and the MeCl2Zr unit slips to an η(4)-binding mode containing the N-C-C-N fragment. The treatment of tetrahydro[1,10]phenanthrolines [2,9-R2-C12NH9(NH)] with M(CH2Ph)4 afforded tribenzyl zirconium complexes LZr(CH2Ph)3 - [L = 2,9-Me2-C12NH9N (18) and 2,9-nBu2-C12NH9N (19)] - and hafnium complexes - LHf(CH2Ph)3 [L = 2,9-Me2-C12NH9N (20), 2,9-nBu2-C12NH9N (21), and 2,9-iPr2-C12NH9N (22)]. The structures of 4, 5, 12, 17, and 22 were elucidated by X-ray crystallography. The newly prepared complexes were screened for ethylene/1-octene copolymerization activity: 12 and 16 were potent catalysts (activities of 74 × 10(6) g mol-Zr h(-1) at ∼120 °C under 30 bar ethylene) for the production of wax-like low-molecular weight polyethylene (Mn: ∼5000), which is widely used in industry.

Tetraphenolate niobium and tantalum complexes for the ring opening polymerization of ε-caprolactone

Reaction of the pro-ligands α,α,α′,α′-tetra(3,5-di-tert-butyl-2-hydroxyphenyl-p-(or -m-)xylene-para-(or -meta-)tetraphenol with [MCl₅] (M = Nb, Ta) or [Nb(O)Cl₃(NCMe)₂] afforded complexes capable of the ROP of e-caprolactone at high temperature.

Chromium-catalyzed transformations with Grignard reagents – new opportunities for cross-coupling reactions

Recent advances in chromium-catalyzed cross-couplings of C–X and C–H bonds with Grignard reagents are highlighted.

Titanium complexes supported by imidazo[1,5-a]pyridine-containing pyrrolyl ligand as catalysts for hydroamination and polymerization reactions, and as an antitumor reagent

The syntheses, structures, catalytic properties and antitumor activities of three titanium complexes supported by an imidazo[1,5-a]pyridine-containing pyrrolyl ligand are reported.

Multinuclear group 4 catalysis: olefin polymerization pathways modified by strong metal-metal cooperative effects

Polyolefins are produced today catalytically on a vast scale, and the manufactured polymers find use in everything from artificial limbs and food/medical packaging to automotive and electrical components and lubricants. Although polyolefin monomers are typically cheap (e.g., ethylene, propylene, α-olefins), the resulting polymer properties can be dramatically tuned by the particular polymerization catalyst employed, and reflect a rich interplay of macromolecular chemistry, materials science, and physics. For example, linear low-density polyethylene (LLDPE), produced by copolymerization of ethylene with linear α-olefin comonomers such as 1-butene, 1-hexene, or 1-octene, has small but significant levels of short alkyl branches (C2, C4, C6) along the polyethylene backbone, and is an important technology material due to outstanding rheological and mechanical properties. In 2013, the total world polyolefin production was approximately 211 million metric tons, of which about 11% was LLDPE. Historically, polyolefins were produced using ill-defined but highly active heterogeneous catalysts composed of supported groups 4 or 6 species (usually halides) activated by aluminum alkyls. In 1963, Karl Ziegler and Giulio Natta received the Nobel Prize for these discoveries. Beginning in the late 1980s, a new generation of group 4 molecule-based homogeneous olefin polymerization catalysts emerged from discoveries by Walter Kaminsky, a team led by James Stevens at The Dow Chemical Company, this Laboratory at Northwestern University, and a host of talented groups in Germany, Italy, Japan, the United Kingdom, and the United States. These new "single-site" catalysts and their activating cocatalysts were far better defined and more rationally tunable in terms of structure, mechanism, thermodynamics, and catalyst activity and selectivity than ever before possible. An explosion of research advances led to new catalysts, cocatalysts, deeper mechanistic understanding of both the homogeneous and heterogeneous systems, macromolecules with dramatically altered properties, and large-scale industrial processes. It is noteworthy that many metalloenzymes employ multiple active centers operating in close synergistic proximity to achieve high activity and selectivity. Such enzymes were the inspiration for the research discussed in this Account, focused on the properties of multimetallic olefin polymerization catalysts. Here we discuss how modifications in organic ligand architecture, metal···metal proximity, and cocatalyst can dramatically modify polyolefin molecular weight, branch structure, and selectively for olefinic comonomer enchainment. We first discuss bimetallic catalysts with identical group 4 metal centers and then heterobimetallic systems with either group 4 or groups 4 + 6 catalytic centers. We compare and contrast the polymerization properties of the bimetallic catalysts with their monometallic analogues, highlighting marked cooperative enchainment effects and unusual polymeric products possible via the proximate catalytic centers. Such multinuclear olefin polymerization catalysts exhibit the following distinctive features: (1) unprecedented levels of polyolefin branching; (2) enhanced enchainment selectivity for linear and encumbered α-olefin comonomers; (3) enhanced polyolefin tacticity and molecular weight; (4) unusual 1,2-insertion regiochemistry for styrenic monomers; (5) modified chain transfer kinetics, such as M-polymer β-hydride transfer to the metal or incoming monomer; (6) LLDPE synthesis with a single binuclear catalyst and ethylene.