Multinuclear Olefin Polymerization Catalysts

Catalytic performance of bis(imino)pyridine Fe/Co complexes toward ethylene polymerization by 2D-/3D-QSPR modeling

The two-dimensional and three-dimensional quantitative structure-property relationship (2D- and 3D-QSPR) approaches are applied to investigate the catalytic performance for a total data set of 55 bis(imino)pryridine iron and cobalt complexes, including the catalytic activity, molecular weight, and melting temperature of the product. The obtained models for the catalytic performance of interest exhibit good results by both 2D- and 3D-QSPR modeling, meanwhile higher predictive and validation powers observed in the 3D type. The modeling results indicate that the bulky substituents on ortho-position of the singular side phenyl ring and positive charge on para-position of the phenyl ring within the ligand are favorable to catalytic activity, while unfavorable to the molecular weight of product. Based on the obtained QSPR models, four new complexes are designed and predicted with good catalytic activity and very high molecular weight, which are in good agreement with our recent experimental report. © 2019 Wiley Periodicals, Inc.

Polymerization of phenylacetylenes by binuclear rhodium catalysts with different para-binucleating phenoxyiminato linkages

A binuclear rhodium catalyst 3b with a 2,5-phenyloxydiiminato linkage and an nbd ligand exhibits cooperative effects in terms of enhanced catalytic activity in the polymerization of phenylacetylene and its functional derivatives.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

Here we will summarize some of the recent advances in α-diimine ligand developments, as well as some new and challenging monomers that this class of catalysts can address through these ligand improvements.

Salen complexes of zirconium and hafnium: synthesis, structural characterization and polymerization studies

Salen complexes of zirconium and hafnium were synthesized and used as effective catalysts for the polymerization of lactide and ε-CL and homopolymerization, copolymerization and coupling of epoxides with CO₂.

Rare-earth/zinc heterometallic complexes containing both alkoxy-amino-bis(phenolato) and chiral salen ligands: synthesis and catalytic application for copolymerization of CO2 with cyclohexene oxide

Ten rare-earth/zinc heterometallic complexes were synthesized and their catalytic application for copolymerization of CO₂ with cyclohexene oxide was explored.

Preparation and in situ chain-end-functionalization of branched ethylene oligomers by monosubstituted α-diimine nickel catalysts

Chain-end-functionalization of (highly) branched ethylene oligomers was achieved in situ with the most/least bulky α-diimine nickel catalysts for the first time.

1,5-Naphthyl-linked bis(imino)pyridines as binucleating scaffolds for dicobalt ethylene oligo-/polymerization catalysts: exploring temperature and steric effects

The title dinuclear Co complexes showed high activity for ethylene oligo-/polymerization with the run temperature having an unexpected effect on the PE to oligomer ratio.

Synthesis of bimetallic complexes bridged by 2,6-bis(benzimidazol-2-yl) pyridine derivatives and their catalytic properties in transfer hydrogenation

A series of binuclear rhodium(i) and iridium(i) complexes with 2,6-bis(benzimidazol-2-yl) pyridine (bzimpy) derivatives were synthesized and characterized by elemental analysis and spectroscopic methods. The molecular and crystal structures of complex 3d were determined by the single crystal X-ray diffraction technique. Their monometallic analogues were prepared to compare the catalytic properties of the bimetallic complexes. To determine the catalyst properties that result in a cooperative, bimetallic enhancement of the reaction rate, the systematic variation of the intermetallic distance and the ligand donor properties of the bimetallic complexes were explored based on the transfer hydrogenation reactions of ketones.

Copolymerization of epoxides with cyclic anhydrides catalyzed by dinuclear cobalt complexes

The alternating copolymerization of epoxides with cyclic anhydrides (CAs) is a highly diverse synthetic method for polyesters as the polymers’ architectures and properties can be easily controlled depending on the combination of two monomers. Thus, a variety of catalyst designs has been reported to prepare the desired copolymers efficiently. We herein report dinuclear cobalt–salen complexes with a benzene ring as a linker and their activities in copolymerization reactions. The dinuclear cobalt complexes showed a higher catalytic activity for the copolymerization of propylene oxide with phthalic anhydride than the corresponding mononuclear cobalt–salen complex and achieved one of the highest turnover frequencies ever reported. A variety of epoxides and CAs were also found to be copolymerized successfully by the dinuclear cobalt complex with a high catalytic activity.

Aluminates with Fluorinated Schiff Bases: Influence of the Alkali Metal⁻Fluorine Interactions in Structure Stabilization

New heterometallic aluminium-alkali metal compounds have been prepared using Schiff bases with electron withdrawing substituents as ligands. The synthesis of these new species was achieved via the reaction of AlMe₃ with the freshly prepared alkali-metallated ligand. The derivatives formed were characterized by NMR in solution and by single crystal X-ray diffraction in the solid state. Aluminate derivatives with lithium and sodium were prepared and a clear influence of the alkali metal in the final outcome is observed. The presence of a Na···F interaction in the solid state has a stabilization effect and the species [NaAlMe₃L]₂ can de isolated for the first time, which was not possible when using Schiff bases without electron withdrawing substituents as ligands.

Synthesis, APPI Mass-Spectrometric Characterization, and Polymerization Studies of Group 4 Dinuclear Bis(ansa-metallocene) Complexes

New ligand platforms of the type p- or m-Ph{-CR(3,6-tBu2Flu)(Cp)}2 (para-, R = Me (2a), H (2b); meta-, R = Me (2c)) were synthesized via nucleophilic addition of the 3,6-tBu2-fluorenyl-anion onto the parent phenylene-bridged difulvenes (1a–c). The corresponding discrete homodinuclear zirconium and hafnium bis(dichloro ansa-metallocene) complexes, Ph[{-CR(3,6-tBu2Flu)(Cp)}MCl2]2 (p-, R = Me (3a-Zr2, 3a-Hf2), R = H (3b-Zr2); m-, R = Me (3c-Zr2), were prepared by salt metathesis reactions. An attempt to generate in situ a heterodinuclear complex 3a-Zr-Hf was also undertaken. For the first time, Atmospheric Pressure PhotoIonization (APPI) mass-spectrometric data were obtained for all dinuclear compounds and found to be in excellent agreement with the simulated ones. Preliminary studies on the catalytic performances of these dinuclear complexes, upon activation with MAO, in ethylene homopolymerization and ethylene/1-hexene copolymerization revealed a few differences as compared to those of the monometallic analogues. In particular, slightly lower molecular weights and a greater formation of short methyl and ethyl branches were obtained with the dinuclear systems.

Mechanochemical routes for the synthesis of acetyl- and bis-(imino)pyridine ligands and organometallics

Organometallic precatalysts play a pivotal role in organic synthesis. However, their preparation often relies on multiple time, energy, and solvent intensive steps, including the synthesis of supporting organic ligand structures, and finally installation on the desired metal centres. We report the sustainable mechanochemical synthesis of acetyl- and bis-(imino)pyridine pincer complexes, a ubiquitous ligand class for organometallic precatalysts. The approach is extended to the one-pot synthesis of acetyl(imino)pyridine-CoCl₂, where the ligand is formed in situ.

Highly Active Dinuclear Titanium(IV) Complexes for the Catalytic Formation of a Carbon-Heteroatom Bond

A series of mononuclear titanium(IV) complexes with the general composition κ³-[R{NHPh₂P(X)}₂Ti(NMe₂)₂] [R = C₆H₄, X = Se (3b); R = trans-C₆H₁₀, X = S (4a), Se (4b)] and [{κ²-N(PPh₂Se)₂}₂Ti(NMe₂)₂] (6b) and two dinuclear titanium(IV) complexes, [C₆H₄{(NPh₂PS)(N)}Ti(NMe₂)]₂ (3c) and [{κ²-N(PPh₂Se)}Ti(NMe₂)₂]₂ (6c), are reported. Dinuclear titanium(IV) complex 6c acts as an efficient catalyst for the chemoselective addition of an E-H bond (E = N, O, S, P, C) to heterocumulenes under mild conditions. The catalytic addition of aliphatic and aromatic amines, alcohol, thiol, phosphine oxide, and acetylene to the carbodiimides afforded the corresponding hydroelemented products in high yield at mild conditions with a broader substrate scope. The catalytic efficiency of the dinuclear complex depends on the cooperative effect of the Ti^IV ions, the systematic variation of the intermetallic distance, and the ligand's steric properties of the complex, which enhances the reaction rate. Most interestingly, this is the first example of catalytic insertion of various E-H bonds into the carbodiimides using a single-site catalyst because only the titanium-mediated insertion of E-H into a C═N unsaturated bond is reported to date. The amine and alcohol insertion reaction with the carbodiimides showed first-order kinetics with respect to the titanium(IV) catalyst as well as substrates. A most plausible mechanism for hydroelementation reaction is also proposed, based on the spectroscopic data of the controlled reaction, a time-course study, and the Hammett plot.

Bottom-Up Assembly of a Highly Efficient Metal-Organic Framework for Cooperative Catalysis

In this study, we demonstrate a bottom-up assembly of a monomeric copper complex and a two-dimensional (2-D) heterometallic metal-organic framework (MOF) from a carboxylate-functionalized tridentate Schiff base ligand and metal ions. The obtained 2-D MOF features a unique bimetallic copper center which is different from its monometallic precursor and acts as an efficient heterogeneous catalyst for the Friedel-Crafts reaction and Henry reaction. The MOF catalyst shows a remarkably superior activity compared to its homogeneous counterparts in a wide range of substrates. It is presumably ascribed to the dual activation of the substrates by the active bimetallic copper center confined in the MOF network, which is supported by the significant changes in catalytic activity at low catalyst/substrates ratios when using the 2-D MOF and its precursor as catalysts, respectively. Moreover, the MOF catalyst also shows an excellent stability and recyclability. Our work, therefore, provides a stepwise strategy to design a heterogeneous cooperative catalyst, by taking advantage of the modulated structure of MOF and tunable functionality of the tridentate Schiff base, with high performance in a variety of organic synthesis.

Synthesis of di- and trinuclear iridium polyhydride complexes surrounded by light-absorbing ligands

New di- and trinuclear iridium (Ir) penta- and hexahydride complexes containing light-absorbing diphosphine ligands have been developed. The trinuclear Ir complex possessed a higher absorption coefficient (standardized per Ir) than that of the parent mononuclear Ir complex, indicating the assembly effect. The trinuclear complex showed high reactivity toward catalytic hydrogenation of diphenylacetylene under mild conditions, and the reaction was considerably accelerated under photoirradiation.

Nuclearity effects in supported, single-site Fe(ii) hydrogenation pre-catalysts

Dimeric and monomeric supported single-site Fe(ii) pre-catalysts on SiO2 have been prepared via organometallic grafting and characterized with advanced spectroscopic techniques. Manipulation of the surface hydroxyl concentration on the support influences monomer/dimer formation. While both pre-catalysts are highly active in liquid-phase hydrogenation, the dimeric pre-catalyst is ∼3× faster than the monomer. Preliminary XAS experiments on the H2-activated samples suggest the active species are isolated Fe(ii) sites.

Iron-based trinuclear metal-organic nanostructures on a surface with local charge accumulation

Coordination chemistry relies on harnessing active metal sites within organic matrices. Polynuclear complexes-where organic ligands bind to several metal atoms-are relevant due to their electronic/magnetic properties and potential for functional reactivity pathways. However, their synthesis remains challenging; few geometries and configurations have been achieved. Here, we synthesise-via supramolecular chemistry on a noble metal surface-one-dimensional metal-organic nanostructures composed of terpyridine (tpy)-based molecules coordinated with well-defined polynuclear iron clusters. Combining low-temperature scanning probe microscopy and density functional theory, we demonstrate that the coordination motif consists of coplanar tpy's linked via a quasi-linear tri-iron node in a mixed (positive-)valence metal-metal bond configuration. This unusual linkage is stabilised by local accumulation of electrons between cations, ligand and surface. The latter, enabled by bottom-up on-surface synthesis, yields an electronic structure that hints at a chemically active polynuclear metal centre, paving the way for nanomaterials with novel catalytic/magnetic functionalities.

N,N-chelated nickel catalysts for highly branched polyolefin elastomers: a survey

The physical properties and end applications of polyolefin materials are defined by their chain architectures and topologies. These properties can, in part, be controlled by a judicious choice of the steric and electronic properties of the catalyst and, in particular, the ligand framework. One major achievement in this field is the discovery of thermoplastic polyolefin elastomers that combine the processing and recyclable characteristics of thermoplastics with the flexibility and ductility of elastomers. These polymers are highly sought after as alternative materials to thermoset elastomers. In this perspective, works in the literature related to the development of nickel catalysts as well as their implementations for the synthesis of polyolefin elastomers are summarized in detail. Throughout the perspective, attention has been focused on developing the relationship between catalyst structure and performance, on strategies for the synthesis of polyolefin elastomer using nickel catalysts, on properties of the resultant polyolefin, such as degree of branching and crystallinity, as well as on their effects on mechanical properties. The future perspective regarding the most recent developments in single-step production of polyethylene elastomers will also be presented.

NMR chemical shift analysis decodes olefin oligo- and polymerization activity of d0 group 4 metal complexes

d⁰ metal-alkyl complexes (M = Ti, Zr, and Hf) show specific activity and selectivity in olefin polymerization and oligomerization depending on their ligand set and charge. Here, we show by a combined experimental and computational study that the ¹³C NMR chemical shift tensors of the α-carbon of metal alkyls that undergo olefin insertion signal the presence of partial alkylidene character in the metal-carbon bond, which facilitates this reaction. The alkylidene character is traced back to the π-donating interaction of a filled orbital on the alkyl group with an empty low-lying metal d-orbital of appropriate symmetry. This molecular orbital picture establishes a connection between olefin insertion into a metal-alkyl bond and olefin metathesis and a close link between the Cossee-Arlmann and Green-Rooney polymerization mechanisms. The ¹³C NMR chemical shifts, the α-H agostic interaction, and the low activation barrier of ethylene insertion are, therefore, the results of the same orbital interactions, thus establishing chemical shift tensors as a descriptor for olefin insertion.

Strong Preference of the Redox-Neutral Mechanism over the Redox Mechanism for the TiIV Catalysis Involved in the Carboamination of Alkyne with Alkene and Diazene

Titanium catalysis generally prefers redox-neutral mechanisms. Yet it has been reported that titanium could promote bond formations in a way similar to reductive elimination. Accordingly, redox catalytic cycles involving Ti^IV /Ti^II cycling have been considered. By studying, as an example, the carboamination of alkynes with alkenes and azobenzene catalyzed by the [Ti^IV ]=NPh imido complex, we performed DFT computations to gain an understanding of how the "abnormal" catalysis takes place, thereby allowing us to clarify whether the catalysis really follows Ti^IV /Ti^II redox mechanisms. The reaction first forms an azatitanacyclohexene by alkyne addition to the [Ti^IV ]=NPh bond, followed by alkene insertion. The azatitanacyclohexene can either undergo C^α -C^γ coupling, to afford bicyclo[3.1.0]imine, or β-H elimination, to yield a [Ti^IV ]-H hydride, which then undergoes C^α =C^β or C^γ =C^δ insertion to give an α,β- or β,γ-unsaturated imine, respectively. Both the geometric and electronic structures indicate that the catalytic cycles proceed through redox-neutral mechanisms. The alternative redox mechanisms (e.g., by N-H or C-H reductive elimination) are substantially less favorable. We concluded that electronically, the Ti^IV catalysis intrinsically favors the redox-neutral mechanism, because a redox pathway would involve Ti^II structures either in the triplet ground state or in the high-lying open-shell singlet state, but the involvement of triplet Ti^II structures is spin-forbidden and that of singlet Ti^II structures is energetically disadvantageous.

Unsymmetrical difunctionalization of cyclooctadiene under continuous flow conditions: expanding the scope of ring opening metathesis polymerization

Functionalized cyclooctenes (FCOEs) are important monomers in ring-opening metathesis polymerization (ROMP). Herein, a new library of disubstituted FCOEs bearing adjacent heteroatoms were synthesized and applied in ROMP. To address the issues associated with the handling of the reactive thienyl chloride intermediate, a two-step continuous flow method has been developed to prepare 5-thio-6-chlorocyclooctene compounds from abundant cyclooctadiene starting materials. These newly synthesized FCOE monomers were subsequently polymerized through ROMP, giving rise to a range of functionalized polymers with high molecular weights. Furthermore, we demonstrated that the thermal properties of these polymers could be fine-tuned by changing the functional groups in the FCOE monomers. We expect that this functionalization-polymerization strategy will enable the preparation of a range of polymeric materials with complex structures.

Advancing polyethylene properties by incorporating NO2 moiety in 1,2-bis(arylimino)acenaphthylnickel precatalysts: synthesis, characterization and ethylene polymerization

A new family of nickel halides (bromides Ni1-Ni5 and chlorides Ni6-Ni10) ligated by 1-(2,6-dibenzhydryl-4-nitrophenylimino)-2-(arylimino)acenaphthylene (Aryl = 2,6-Me₂C₆H₃L1, 2,6-Et₂C₆H₃L2, 2,6-ⁱPr₂C₆H₃L3, 2,4,6-Me₃C₆H₂L4, and 2,6-Et₂-4-MeC₆H₂L5) have been prepared and well characterized, and the scope of their catalytic properties toward the polymerization of ethylene has been investigated. Upon activation with either Et₂AlCl or EASC, the nickel bromide complexes displayed better activities than their nickel chloride counterparts and produced higher-molecular-weight polyethylene in the range of 10⁶ g mol^-1 with a very narrow range of molecular weight distributions. In comparison with reference precatalysts with non-nitro substituents (CH₃, F or Cl), the title complexes experienced a modest negative effect on catalytic activity upon replacement with a NO₂ moiety (activity up to 4.61 × 10⁶ g PE (mol Ni)^-1 h^-1 at 20 °C). Conversely, the NO₂ moiety exerted a positive effect to increase the molecular weight of the resulting polyethylene, and Ni4/Et₂AlCl gave polyethylene with a maximum molecular weight of as high as 32.8 × 10⁵ g mol^-1, which is not only the highest among the title complexes but also higher than any literature values reported with 1,2-diiminoacenaphthylnickel halides.

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]).

Synthesis, characterization and olefin polymerization behaviors of phenylene-bridged bis-β-carbonylenamine binuclear titanium complexes

Binuclear and multinuclear complexes have attracted much attention due to their unique catalytic performances for olefin polymerization compared with their mononuclear counterparts. In this work, a series of phenyl-bridged bis-β-carbonylenamine [O⁻NS^R] (R = alkyl or phenyl) tridentate ligands and their binuclear titanium complexes (Ti²L₁–Ti²L₅) were synthesized and characterized by ¹H NMR, ¹³C NMR, FTIR and elemental analysis. The molecular structure of ligand L₂ (R = ⁿPr) and its corresponding Ti complex Ti²L₂ were further investigated by single-crystal X-ray diffraction, which showed that each titanium coordinated with six atoms to form a distorted octahedral configuration along with the conversion of the ligand from β-carbonylenamine to β-imino enol form. Under the activation of MMAO, these complexes catalyzed ethylene polymerization and ethylene/α-olefin copolymerization with extremely high activity (over 10⁶ g mol (Ti)⁻¹ h⁻¹ atm⁻¹) to produce high molecular weight polyethylene. At the same time, wider polydispersity as compared with the mononuclear counterpart TiL₆ was observed, indicating that two active catalytic centers may be present, consistent with the asymmetrical crystal structure of the binuclear titanium complex. Furthermore, these complexes possessed better thermal stability than their mononuclear analogues. Compared with the complexes bearing alkylthio sidearms, the complex Ti²L₅ bearing a phenylthio sidearm exhibited higher catalytic activity towards ethylene polymerization and produced polyethylene with much higher molecular weight, but with an appreciably lower 1-hexene incorporation ratio. Nevertheless, these bis-β-carbonylenamine-derived binuclear titanium complexes showed much higher ethylene/1-hexene copolymerization activity and 1-hexene incorporation ratios as compared with the methylene-bridged bis-salicylaldiminato binuclear titanium complexes, and the molecular weight and 1-hexene incorporation ratio could be flexibly tuned by the initial feed of α-olefin commoners and catalyst structures.

Phenyl-bridged bis-β-carbonylenamine binuclear titanium complexes were synthesized, characterized and used to catalyze ethylene (co)polymerization with extremely high activity.