Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers

The use of multimetallic complexes is a rapidly advancing route to enhance catalyst performance in the ring-opening polymerization of cyclic esters and ethers. Multimetallic catalysts often outperform their monometallic analogues in terms of reactivity and/or polymerization control, and these improvements are typically attributed to "multimetallic cooperativity". Yet the origins of multimetallic cooperativity often remain unclear. This review explores the key factors underpinning multimetallic cooperativity, including metal-metal distances, the flexibility, electronics and conformation of the ligand framework, and the coordination environment of the metal centers. Emerging trends are discussed to provide insights into why cooperativity occurs and how to harness cooperativity for the development of highly efficient multimetallic catalysts.

Bimetallic Manganese Catalysts: A Route to Controlled and Switchable Polymerization of Lactones

The sustainable solution to the environmental problem of polymeric materials calls for efficient and well-controlled ring-opening polymerization catalytic systems. Inspired by the highly reactive and stereospecific bimetallic catalysts, three kinds of bimetallic Salen-Mn catalysts supported by biaryl linking moieties are synthesized and applied to polymerization catalysis of lactide (LA) and ϵ-caprolactone (ϵ-CL) in this work. The polymerization is initiated in situ by the ring-opening of epoxide compounds, in which the ionic cocatalyst could accelerate the reaction process. The Mn-Mn coordination effect contributes to the higher activity and iso-selectivity towards LA compared to the mononuclear Salen-Mn catalyst. The reactivity and stereoselectivity are determined by the conformation of catalysts, specifically the Mn-Mn separation and dihedral angle. Finally, the CO2 -controlled switchable polymerizations are carried out with LA and ϵ-CL. The reversibility of the on-off switching operation is influenced by the combination between CO2 molecules and active species. The success in binuclear Salen-Mn catalysts not only expands the range of bimetallic catalyst analogues but also claims the promising potential of Mn-based catalysts in practical and theoretical research.

Understanding catalytic synergy in dinuclear polymerization catalysts for sustainable polymers

Understanding the chemistry underpinning intermetallic synergy and the discovery of generally applicable structure-performances relationships are major challenges in catalysis. Additionally, high-performance catalysts using earth-abundant, non-toxic and inexpensive elements must be prioritised. Here, a series of heterodinuclear catalysts of the form Co(III)M(I/II), where M(I/II) = Na(I), K(I), Ca(II), Sr(II), Ba(II) are evaluated for three different polymerizations, by assessment of rate constants, turn over frequencies, polymer selectivity and control. This allows for comparisons of performances both within and between catalysts containing Group I and II metals for CO2/propene oxide ring-opening copolymerization (ROCOP), propene oxide/phthalic anhydride ROCOP and lactide ring-opening polymerization (ROP). The data reveal new structure-performance correlations that apply across all the different polymerizations: catalysts featuring s-block metals of lower Lewis acidity show higher rates and selectivity. The epoxide/heterocumulene ROCOPs both show exponential activity increases (vs. Lewis acidity, measured by the pKa of [M(OH2)m]n+), whilst the lactide ROP activity and CO2/epoxide selectivity show linear increases. Such clear structure-activity/selectivity correlations are very unusual, yet are fully rationalised by the polymerization mechanisms and the chemistry of the catalytic intermediates. The general applicability across three different polymerizations is significant for future exploitation of catalytic synergy and provides a framework to improve other catalysts.

Diruthenium Catalyst for Hydrogen Production from Aqueous Formic Acid

Diruthenium complexes [{(η⁶-arene)RuCl}₂(μ-κ²:κ²-benztetraimd)]²⁺ containing the bridging bis-imidazole methane-based ligand {1,4-bis(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)benzene} (benztetraimd) are synthesized for catalytic formic acid dehydrogenation in water at 90 °C. Catalyst [{(η⁶-p-cymene)RuCl}₂(μ-κ²:κ²-benztetraimd)]²⁺ [1-Cl₂] exhibited a remarkably high turnover frequency (1993 h^-1 per Ru atom) and long-term stability over 60 days for formic acid dehydrogenation, while the analogous (η⁶-benzene)diruthenium and mononuclear catalysts displayed low activity with poor long-term stability. Notably, catalyst [1-Cl₂] also displayed an appreciably high turnover number of 93 200 for the bulk-scale reaction. In addition, the in-depth mass and nuclear magnetic resonance investigations under the catalytic and control experimental conditions revealed the active involvement of several crucial catalytic intermediate species, such as Ru-aqua species [{(η⁶-p-cymene)Ru(H₂O)}₂(μ-L)]²⁺ [1-(OH₂)₂], Ru-formato species [{(η⁶-p-cymene)Ru(HCOO)}₂(μ-L)] [1-(HCOO)₂], and Ru-hydrido species [{(η⁶-p-cymene)Ru(H)}₂(μ-L)] [1-(H)₂], in the catalytic formic acid dehydrogenation reaction.

Cooperative Initiation in a Dinuclear Indium Complex for CO2 Epoxide Copolymerization

Dinuclear indium complexes have been synthesized and characterized. These include neutral and cationic indium complexes supported by a Schiff base ligand bearing a binaphthol linker. The new compounds were investigated for alternating copolymerization of CO₂ and cyclohexene oxide. In particular, the neutral indium chloride complex (±)-[(O_NapN_iN)InCl₂]₂ (4) showed high conversion of cyclohexene oxide and selectivity for poly(cyclohexene carbonate) formation without cocatalysts at 80 °C under various CO₂ pressures (2-30 bar). Importantly, the reactivity of the dinuclear indium chloride complex 4 is drastically different from that of the mononuclear indium chloride complex (±)-(NN_iO_tBu)InCl₂ (5), suggesting a cooperative initiation mechanism involving the two indium centers in 4.

Salen-type aluminum and zinc complexes as two-faced Janus compounds: contribution to molecular sensing and polymerization catalysis

The aim of the present review is to highlight the most recent achievements in different fields of application of salen-based zinc and aluminum complexes. More specifically this article focuses on the use of aluminum and zinc salen-type complexes as optical probes for biologically relevant molecules, as catalysts for the ring opening polymerization (ROP) of cyclic esters and co-polymerization of epoxides and anhydrides (ROCOP) and in the chemical fixation of carbon dioxide (CO2). The intention is to provide an overview of the most recent results from our group within the framework of the state-of-art-results in the literature.

Highly Isoselective Ring-Opening Polymerization of rac-Lactide Using Chiral Binuclear Aluminum Catalyst

The microstructure can significantly affect the physical and mechanical properties of polymers. Five chiral binuclear aluminum methyl complexes (rac1 to rac5) are synthesized by using chiral binaphthalene diamine ligand and characterized by both ¹ H and ¹³ C NMR spectroscopy. The crystal structures of rac3 and rac5 are also identified by single crystal X-ray diffraction. In the presence of ⁱ PrOH initiator, these aluminum complexes catalyze the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in a controlled manner and produce polymers with high to excellent isoselectivity (P_m up to 0.93). Even with a catalyst loading as low as 0.1%, polylactide with a P_m of 0.83 can still be obtained. Kinetic studies reveal the first-order dependence on monomer concentration whereas kinetic resolution polymerization provides the evidence to support that these binuclear aluminum catalysts catalyzed ROP of rac-LA adopts enantiomorphic site control mechanism. Furthermore, this strategy can also be applied to the ROP of both ε-caprolactone and δ-valeroactone.

Synthesis of Ultrahigh Molecular Weight PLAs Using a Phenoxy-Imine Al(III) Complex

l- and d-lactide polymerization kinetics using phenoxy-imine ligands of the type Me2Al[O-2-tert-Bu-6-(C6F5N=CH)C6H3] in the presence of n-butanol and benzyl alcohol by ring-opening polymerization into polylactide are investigated. Effects of initiator concentration, catalyst concentration, polymerization temperature, and time on the molecular weight of poly-l-lactide are also investigated. Purification and drying of l-lactide are found to significantly influence the polymerization kinetics and the final molecular weight achieved. Ultrahigh molecular weight poly(l-lactic acid) PLLA (M w = 1.4 × 106 g/mol with Đ = 1.8) and ultrahigh molecular weight poly(d-lactic acid) PDLA (M w = 1.3 × 106 g/mol with Đ = 2.0) are obtained when polymerization is performed with a molar ratio of monomer to catalyst (LA/Al) of 8000 for 72 h at 120 °C in the presence of benzyl alcohol with conversions of 96 and 91%, respectively. We report for the first time the synthesis of ultrahigh molecular weight poly-l- and d-lactide using the Me2Al[O-2-tert-Bu-6-(C6F5N=CH)C6H3] catalyst. The identified catalyst is found to be suitable for the synthesis of a broad range of molecular weights.

Dinucleating Amino-Phenolate Platform for Zinc Catalysts: Impact on Lactide Polymerization

We report imine- and amine-based dinucleating ligands bearing a bisphenol backbone and explore their coordination chemistry with zinc to form zinc alkyl, alkoxide, acetate, and amide complexes. Full characterization of the complexes shows that this ligand framework can support dinuclear and trinuclear complexes. We explore the reactivity of the zinc alkyl and alkoxide complexes as catalysts for the ring opening polymerization of lactide and compared this reactivity to analogous mononuclear complexes. We show that 1) The amine-based complexes are more reactive than the imine-based analogues; 2) The trinuclear zinc alkyl species show unusual control and reproducibility for lactide polymerization; and 3) The extent of bimetallic cooperation is hampered by the ability of the ligand framework to form trinuclear clusters.

Dinuclear aluminum complexes bearing methylene-bridged phenoxy-imine ligands and their application in the ring-opening polymerization of rac-lactide

The first report on the use of dinuclear aluminum complexes supported by methylene-bridged phenoxy-imine ligands for the ring-opening polymerization of rac-lactide.

Dinuclear Aluminum Halide Complexes Based on Bis(β-diketiminate) Ligands: Synthesis, Structures, and Electrochemical Characterization

Twenty-three dinuclear aluminum halide complexes based on bis(β-diketiminate) ligands with various flexible and rigid bridging groups have been synthesized and fully characterized by NMR and IR spectroscopy, cyclic voltammetry, and elemental analysis. In addition, nine complexes were structurally characterized by means of single-crystal X-ray diffraction. Attempts to reduce the dinuclear species using C₈K remained unsuccessful, but the reduction potentials investigated by cyclic voltammetry indicate the principle feasibility.

Synthesis of Lewis Acidic, Aromatic Aminotroponiminate Zinc Complexes for the Ring-Opening Polymerization of Cyclic Esters

Three novel aminotroponiminate (ATI) zinc complexes I-III (I = [(Ph₂)ATI]Zn-N(SiMe₃)₂, II = [(C₆H₃-2,6-C₂H₅/Ph)ATI]Zn-N(SiMe₃)₂, and III = [(C₆H₃-2,6-CH(CH₃)₂/Ph)ATI]Zn-N(SiMe₃)₂) were synthesized and tested in the ring-opening polymerization of the lactones β- rac-butyrolactone (BBL) and rac-lactide (LA). The ligands, with two of them literature unknown, were readily obtained via a three-step synthesis from tropolone. Forming a five-membered metallacycle with zinc, the complexes were further structurally examined via single-crystal X-ray analysis and compared with that of the established, 6-ringed β-diiminate (BDI) complex IV ([CH(CMeNPh)₂]Zn-N(SiMe₃)₂). The influence of the varying metallacycle ring size on the polymerization was evaluated. In situ IR measurements indicate a higher catalytic activity of the novel ATI complexes I-III for BBL compared with the BDI system IV. The activity and degree of control were further improved by an in situ generated alkoxy initiating group generated after the addition of 2-propanol. An enhanced initiator efficiency allowed the synthesis of polymers with controlled molecular weights and narrow polydispersities. Furthermore, II and III exhibited a high activity in the ring-opening polymerization of rac-LA. Hereby, reaction time and initiator efficiency could also be optimized at a higher temperature or by the addition of 2-propanol.

Aggregated initiators: defining their role in the ROP of rac-lactide

Reported examples of aggregated initiators for the ring-opening polymerisation (ROP) of lactide often lack detailed investigations as to the nature of the active species, making it difficult to reconcile ligand design with performance. Here, we offer additional stability to the polynuclear titanium complexes, TiL(OiPr) (L = 9-14), through a bridging carboxylate anchored to the supporting amine bis(phenolate) ligands. An in-depth study of solution-state behaviour determined the process of assembly was driven by interactions between the carboxylate and a vacant site on a neighbouring titanium centre. Furthermore, we establish that mononuclear units form dynamic mixtures of polynuclear aggregates, with a clear relationship between nuclearity of the aggregates and the steric bulk on the ligand. Smaller aggregates displayed increased activity towards the ROP of rac-lactide. Furthermore, addition of a chiral centre, on the ligand framework, was investigated as a route to influence the selectivity of the polymerisation via easily-accessible initiators.

Copolymerization of cyclic esters, epoxides and anhydrides: evidence of the dual role of the monomers in the reaction mixture

A block copolyester derived from ROCOP of CHO/SA followed by ROP of ε-CL was obtained from the mixture of monomers. The lactone comonomer acts as an endogen cocatalyst during the SA/CHO ROCOP step and as a monomer during ROP.

Dinuclear zirconium complex bearing a 1,5-bridged-calix[8]arene ligand as an effective catalyst for the synthesis of macrolactones

“Macrocyclization of Cycles with a Macrocycle.” The intramolecular cooperation of metal centres in a novel dinuclear complex has been postulated as responsible for the observed high rate of formation of macrocyclic polyesters.

Low valent Al(ii)–Al(ii) catalysts as highly active ε-caprolactone polymerization catalysts: indication of metal cooperativity through DFT studies

A dinuclear Al(ii)–Al(ii) species acts as a highly effective ROP catalyst of ε-caprolactone at room temperature, with a DFT computed ROP mechanism indicative of metal cooperativity.

Improvement in aluminum complexes bearing a Schiff base in ring-opening polymerization of ε-caprolactone: the synergy of the N,S-Schiff base in a five-membered ring aluminum system

A series of five-membered ring aluminum complexes bearing thiol-Schiff base ligands were synthesized, and their application in the ring-opening polymerization of ε-caprolactone (CL) was investigated.

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

The impact of the metal size and Lewis acidity on the polymerization activity of group 13 metal complexes was studied, and it was shown that, within the same ligand family, indium complexes are far more reactive and selective than their gallium analogues. To this end, gallium and aluminum complexes supported by a tridentate diaminophenolate ligand, as well as gallium complexes supported by N,N'-ethylenebis(salicylimine)(salen) ligands, were synthesized and compared to their indium analogues. Using the tridentate ligand set, it was possible to isolate the gallium chloride complexes 3 and (±)-4 and the aluminum analogues 5 and (±)-6. The alkoxygallium complex (±)-2, supported by a salen ligand, was also prepared and characterized and, along with the three-component system GaCl₃/BnOH/NEt₃, was tested for the ring-opening polymerization of lactide and ε-caprolactone. The polymerization rates and selectivities of both systems were significantly lower than those for the indium analogues. The reaction of (±)-2 with 1 equiv of lactide forms the first insertion product, which is stable in solution and can be characterized at room temperature. In order to understand the differences of the reactivity within the group 13 metal complexes, a Lewis acidity study using triethylphosphine oxide (the Gutmann-Beckett method) was undertaken for a series of aluminum, gallium, and indium halide complexes; this study shows that indium halide complexes are less Lewis acidic than their aluminum and gallium analogues. Density functional theory calculations show that the Mulliken charges for the indium complexes are higher than those for the gallium analogues. These data suggest that the impact of ligands on the reactivity is more significant than that of the metal Lewis acidity.

Investigation of the dinuclear effect of aluminum complexes in the ring-opening polymerization of ε-caprolactone

Al complexes bearing hydrazine-bridging Schiff base ligands showed the best catalytic activity, approximately 3- to 11-fold higher than that of dinuclear Al complexes bearing Salen ligands and mononuclear Al complexes bearing Schiff base ligands.

Zinc bimetallics supported by a xanthene-bridged dinucleating ligand: synthesis, characterization, and lactide polymerization studies

Di-zinc complexes of a new dinucleating xanthene-based bis(iminophenolate) ligand have been synthesized, and their coordination chemistry and lactide polymerization activity have been investigated.

Synthesis of aluminum complexes supported by 2-(1,10-phenanthrolin-2-yl)phenolate ligands and their catalysis in the ring-opening polymerization of cyclic esters

Phenanthroline-phenolate based N,N,O-chelate aluminum complexes were demonstrated to catalyze the ROP of ε-caprolactone, rac-lactide, and rac-β-butyrolactone, as well as their block copolymerization.

Aluminum methyl, alkoxide and α-alkoxy ester complexes supported by 6,6'-dimethylbiphenyl-bridged salen ligands: synthesis, characterization and catalysis for rac-lactide polymerization

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6'-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe3 with various amounts of the proligands (6,6'-[(6,6'-dimethyl-[1,1'-biphenyl]-2,2'-diyl)bis(nitrylomethilidyne)]-bis(2-R(1)-4-R(2)-phenol): , R(1) = R(2) = Me; , R(1) = (t)Bu, R(2) = Me; , R(1) = R(2) = cumyl; , R(1) = Br, R(2) = (t)Bu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [AlMe (), Al2Me4 ()]. Aluminum alkoxide complexes AlO(i)Pr (), AlOBn (), and α-alkoxy ester complexes Al(OCMe2CO2Me) (), Al[(S)-OCHMeCO2Me] () were prepared via in situ alcoholysis of the parent aluminum methyl complex with the corresponding alcohols. The molecular structures of mononuclear complexes , dinuclear complex , alkoxide complexes and α-alkoxy ester complexes were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69-70 ppm and 25-41 ppm were observed in the (27)Al NMR spectra of complexes and , indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex is also a rare example of an O-lactate model complex that mimics the first insertion of l-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes in the presence of (i)PrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03-1.05) and 65-73% isotacticities. Particularly, complex showed an "immortal" behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.

Dizinc Lactide Polymerization Catalysts: Hyperactivity by Control of Ligand Conformation and Metallic Cooperativity

Understanding how to moderate and improve catalytic activity is critical to improving degradable polymer production. Here, di- and monozinc catalysts, coordinated by bis(imino)diphenylamido ligands, show remarkable activities and allow determination of the factors controlling performance. In most cases, the dizinc catalysts significantly out-perform the monozinc analogs. Further, for the best dizinc catalyst, the ligand conformation controls activity: the catalyst with "folded" ligand conformation shows turnover frequency (TOF) values up to 60 000 h(-1) (0.1 mol % loading, 298 K, [LA]=1 m), whilst that with a "planar" conformation is much slower, under similar conditions (TOF=30 h(-1) ). Dizinc catalysts also perform very well under immortal conditions, showing improved control, and are able to tolerate loadings as low as 0.002 mol % whilst conserving high activity (TOF=12 500 h(-1) ).

Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

The synthesis of the first alkoxide-bridged indium complex supported by a chiral dinucleating ligand platform (1), along with its zinc analogue (2), is reported. Both complexes are synthesized in a one-pot reaction starting from a chiral dinucleating bis(diamino)phenolate ligand platform, sodium ethoxide, and respective metal salts. The dinucleating indium analogue (7) based on an achiral ligand backbone is also reported. Indium complexes bearing either the chiral or achiral ligand catalyze the ring-opening polymerization of racemic lactide (rac-LA) to afford highly heterotactic poly(lactic acid) (PLA; Pr > 0.85). The indium complex bearing an achiral ligand affords essentially atactic PLA from meso-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed.

Bimetallic Schiff base complexes for stereoselective polymerisation of racemic-lactide and copolymerisation of racemic-lactide with ε-caprolactone

Bimetallic Schiff-base complexes with different bridging parts and silyl-substituents were applied for stereoselective ROP ofrac-LA, and copolymerisation ofrac-LA/ε-CL.