Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts for Carbon Dioxide and Cyclohexene Oxide Ring Opening Copolymerizations

Evolution of Copolymers of Epoxides and CO2: Catalysts, Monomers, Architectures, and Applications

The copolymerization of CO2 and epoxides presents a transformative approach to converting greenhouse gases into aliphatic polycarbonates (CO2-PCs), thereby reducing the polymer industry's dependence on fossil resources. Over the past 50 years, a wide array of metallic catalysts, both heterogeneous and homogeneous, have been developed to achieve precise control over polymer selectivity, sequence, regio-, and stereoselectivity. This review details the evolution of metal-based catalysts, with a particular focus on the emergence of organoborane catalysts, and explores how these catalysts effectively address kinetic and thermodynamic challenges in CO2/epoxides copoly2merization. Advances in the synthesis of CO2-PCs with varied sequence and chain architectures through diverse polymerization protocols are examined, alongside the applications of functional CO2-PCs produced by incorporating different epoxides. The review also underscores the contributions of computational techniques to our understanding of copolymerization mechanisms and highlights recent advances in the closed-loop chemical recycling of CO2-sourced polycarbonates. Finally, the industrialization efforts of CO2-PCs are discussed, offering readers a comprehensive understanding of the evolution and future potential of epoxide copolymerization with CO2.

Synthesis and Characterization of Heterodinuclear Indium(III)/Sodium(I) Complexes Containing Benzotriazole-Derived Phenolate Ligands: Effective Catalysts for Ring-Opening Copolymerization of Carbon Dioxide with Epoxides

This study reported for the first time the facile synthesis of a series of novel structurally well-characterized heterodinuclear indium(III)/sodium(I) dihalide complexes containing benzotriazole-based bis(amino-phenolate) derivatives. All heterobimetallic In(III)/Na(I) complexes were found to be active single-component catalysts for the copolymerization of carbon dioxide (CO2) with cyclohexene oxide (CHO). Noteworthily, In/Na chloro complex 1 has been shown to give high copolymerization selectivity possessing >99% carbonate repeated units for CO2-derived poly(cyclohexene carbonate) production and displayed a turnover number of >1400 under the optimized conditions. Apart from the CO2/CHO copolymerization, the same complex was capable of mediating the CO2-copolymerization of 4-vinyl-1,2-cyclohexene oxide or cyclopentene oxide to deliver the related CO2-based polycarbonates. To the best of our knowledge, complex 1 in this work appears to be the first example of In/Na halide complex-promoted CO2/epoxide copolymerization that enabled the generation of aliphatic polycarbonates with good productivity and high product selectivity.

Air-Stable Cobalt(III) and Chromium(III) Complexes as Single-Component Catalysts for the Activation of Carbon Dioxide and Epoxides

Cobalt(III) and chromium(III) salophen chloride complexes were synthesized and tested for the cycloaddition of carbon dioxide (CO2) with epoxides to obtain cyclic carbonates. The cat1, cat2, cat4, and cat5 complexes presented high catalytic activity without cocatalysts and are solvent-free at 100 °C, 8 bar, and 9 h. At these conditions, the terminal epoxides (1a-1k) were successfully converted into the corresponding cyclic carbonates with a maximum conversion of ∼99%. Moreover, cat5 was highlighted due to its capability of opening internal epoxides such as limonene oxide (1l) with a 36% conversion to limonene carbonate (2l), and from cyclohexene oxide (1m), cyclic trans-cyclohexene carbonate (2m) and poly(cyclohexene carbonate) were obtained with 15% and 85% selectivity, respectively. A study of the coupling reaction mechanism was proposed with the aid of electrospray ionization mass spectrometry (ESI-MS) analysis, confirming the single-component behavior of the complexes through their ionization due to epoxide coordination. In addition, crystallographic analysis of cat1 single crystals grown in a saturated solution of pyridine helped to demonstrate that the substitution of chloride ion by pyridine ligands to form an octahedral coordination occurs (Py-cat1), supporting the proposed mechanism. Also, a recyclability study was performed for cat5, and a total turnover number of 952 was obtained with only minor losses in catalytic activity after five cycles.

Trivalent Cations Slow Electron Transfer to Macrocyclic Heterobimetallic Complexes

Incorporation of secondary redox-inactive cations into heterobimetallic complexes is an attractive strategy for modulation of metal-centered redox chemistry, but quantification of the consequences of incorporating strongly Lewis acidic trivalent cations has received little attention. Here, a family of seven heterobimetallic complexes that pair a redox-active nickel center with La3+, Y3+, Lu3+, Sr2+, Ca2+, K+, and Na+ (in the form of their triflate salts) have been prepared on a heteroditopic ligand platform to understand how chemical behavior varies across the comprehensive series. Structural data from X-ray diffraction analysis demonstrate that the positions adopted by the secondary cations in the crown-ether-like site of the ligand relative to nickel are dependent primarily on the secondary cations' ionic radii and that the triflate counteranions are bound to the cations in all cases. Electrochemical data, in concert with electron paramagnetic resonance studies, show that nickel(II)/nickel(I) redox is modulated by the secondary metals; the heterogeneous electron-transfer rate is diminished for the derivatives incorporating trivalent metals, an effect that is dependent on steric crowding about the nickel metal center and that was quantified here with a topographical free-volume analysis. As related analyses carried out here on previously reported systems bear out similar relationships, we conclude that the placement and identity of both the secondary metal cations and their associated counteranions can afford unique changes in the (electro)chemical behavior of heterobimetallic species.

Bimetallic nickel complexes containing imidazole-based phenolate ligands as efficient catalysts for the copolymerization of carbon dioxide with epoxides

The utilization of hexadentate imidazole-derived diamine-bisphenolate ligands to construct structurally well-defined bimetallic nickel catalysts that enable the mediation of the copolymerization of carbon dioxide with alicyclic epoxides was reported for the first time. A series of dinickel carboxylate/nitrophenolate complexes were facilely prepared through a one-pot procedure and their structures were fully determined by single crystal X-ray structural analysis. Dinickel complexes 1-10 were used as single-component catalysts, and were evaluated for the copolymerization of CO2 and cyclohexene oxide (CHO), for which acetato-incorporated complex 1 was proved to exhibit the best activity. Not only has the controllability of binickel catalyst 1 for CO2/CHO copolymerization been demonstrated, but also an "immortal" character for the same polymerization has been realized. Furthermore, detailed kinetic studies of polymerization catalysis of this type were undertaken, and the kinetics results revealed a first-order dependence on both Ni complex 1 and CHO concentrations. This is a successful example of the introduction of the easily accessible nitrogen-heterocycle group, the imidazole moiety, into phenolate ligands for the development of high-performance homogeneous catalysts towards the bimetallic complex-catalyzed copolymerization of CO2 and epoxides.

Correlating Metal Redox Potentials to Co(III)K(I) Catalyst Performances in Carbon Dioxide and Propene Oxide Ring Opening Copolymerization

Carbon dioxide copolymerization is a front-runner CO2 utilization strategy but its viability depends on improving the catalysis. So far, catalyst structure-performance correlations have not been straightforward, limiting the ability to predict how to improve both catalytic activity and selectivity. Here, a simple measure of a catalyst ground-state parameter, metal reduction potential, directly correlates with both polymerization activity and selectivity. It is applied to compare performances of 6 new heterodinuclear Co(III)K(I) catalysts for propene oxide (PO)/CO2 ring opening copolymerization (ROCOP) producing poly(propene carbonate) (PPC). The best catalyst shows an excellent turnover frequency of 389 h-1 and high PPC selectivity of >99 % (50 °C, 20 bar, 0.025 mol% catalyst). As demonstration of its utility, neither DFT calculations nor ligand Hammett parameter analyses are viable predictors. It is proposed that the cobalt redox potential informs upon the active site electron density with a more electron rich cobalt centre showing better performances. The method may be widely applicable and is recommended to guide future catalyst discovery for other (co)polymerizations and carbon dioxide utilizations.

Cationic tetranuclear macrocyclic CaCo3 complexes as highly active catalysts for alternating copolymerization of propylene oxide and carbon dioxide

We found that a cationic hetero tetranuclear complex including a calcium and three cobalts exhibited high catalytic activity toward alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO2). The tertiary anilinium salt [PhNMe2H][B(C6F5)4] was the best additive to generate the cationic species while maintaining polymer selectivity and carbonate linkage, even under 1.0 MPa CO2. Density functional theory calculations clarified that the reaction pathway mediated by the cationic complex is more favorable than that mediated by the neutral complex by 1.0 kcal mol-1. We further found that the flexible ligand exchange between Ca and Co ions is important for the alternating copolymerization to proceed smoothly.

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.

Vanadyl as a Spectroscopic Probe of Tunable Ligand Donor Strength in Bimetallic Complexes

Incorporation of secondary metal ions into heterobimetallic complexes has emerged as an attractive strategy for rational tuning of compounds' properties and reactivity, but direct solution-phase spectroscopic interrogation of tuning effects has received less attention than it deserves. Here, we report the assembly and study of a series of heterobimetallic complexes containing the vanadyl ion, [VO]²⁺, paired with monovalent cations (Cs⁺, Rb⁺, K⁺, Na⁺, and Li⁺) and a divalent cation (Ca²⁺). These complexes, which can be isolated in pure form or generated in situ from a common monometallic vanadyl-containing precursor, enable experimental spectroscopic and electrochemical quantification of the influence of the incorporated cations on the properties of the vanadyl moiety. The data reveal systematic shifts in the V-O stretching frequency, isotropic hyperfine coupling constant for the vanadium center, and V(V)/V(IV) reduction potential in the complexes. These shifts can be interpreted as charge density effects parametrized through the Lewis acidities of the cations, suggesting broad potential for the vanadyl ion to serve as a spectroscopic probe in multimetallic species.

Experimental and Theoretical Studies of the Optical Properties of the Schiff Bases and Their Materials Obtained from o-Phenylenediamine

Two macrocyclic Schiff bases derived from o-phenylenediamine and 2-hydroxy-5-methylisophthalaldehyde L1 or 2-hydroxy-5-tert-butyl-1,3-benzenedicarboxaldehyde L2, respectively, were obtained and characterized by X-ray crystallography and spectroscopy (UV-vis, fluorescence and IR). X-ray crystal structure determination and DFT calculations for compounds confirmed their geometry in solution and in the solid phase. Moreover, intermolecular interactions in the crystal structure of L1 and L2 were analyzed using 3D Hirshfeld surfaces and the related 2D fingerprint plots. The 3D Hirschfeld analyses show that the most numerous interactions were found between hydrogen atoms. A considerable number of such interactions are justified by the presence of bulk tert-butyl groups in L2. The luminescence of L1 and L2 in various solvents and in the solid state was studied. In general, the quantum efficiency between 0.14 and 0.70 was noted. The increase in the quantum efficiency with the solvent polarity in the case of L1 was observed (λex = 350 nm). For L2, this trend is similar, except for the chloroform. In the solid state, emission was registered at 552 nm and 561 nm (λex = 350 nm) for L1 and L2, respectively. Thin layers of the studied compounds were deposited on Si(111) by the spin coating method or by thermal vapor deposition and studied by scanning electron microscopy (SEM/EDS), atomic force microscopy (AFM), spectroscopic ellipsometry and fluorescence spectroscopy. The ellipsometric analysis of thin materials obtained by thermal vapor deposition showed that the band-gap energy was 3.45 ± 0.02 eV (359 ± 2 nm) and 3.29 ± 0.02 eV (377 ± 2 nm) for L1/Si and L2/Si samples, respectively. Furthermore, the materials of the L1/Si and L2/Si exhibited broad emission. This feature can allow for using these compounds in LED diodes.

Dinuclear Nickel Complexes Using Hexadentate Benzothiazole-Based Diamine-Bisphenolate Ligands: Highly Active Catalysts for Copolymerization of Carbon Dioxide with Epoxides

We reported for the first time the utilization of hexadentate benzothiazole-based diamine-bisphenolate ligands to synthesize structurally well-characterized dinickel dicarboxylate complexes and studied their catalysis for copolymerization of carbon dioxide with epoxides. Dinickel carboxylate complexes having a 1,3-diamine-bridged backbone were demonstrated to be high-performance catalysts for alternating copolymerization of CO₂ and cyclohexene oxide (CHO) with high product selectivity. Particularly, acetate-supported nickel complex 2 enabled us to promote such CO₂-copolymerization of this kind with a maximum turnover frequency of up to 2600 h^-1 and gave good molecular weight controllability under high-pressure conditions. It is worth noting that bimetallic Ni catalyst 2 was also capable of mediating the catalytic CO₂-polymerization of alicyclic epoxides at atmospheric pressure. Kinetic investigations of CO₂/CHO copolymerization by 2 allowed us to determine the rate equation of -d[CHO]/dt = k_p[2]¹[CHO]¹, and such catalysis exhibited a first-order dependence on both dinickel complex and CHO concentrations.

Synergic Heterodinuclear Catalysts for the Ring-Opening Copolymerization (ROCOP) of Epoxides, Carbon Dioxide, and Anhydrides

The development of sustainable plastic materials is an essential target of chemistry in the 21st century. Key objectives toward this goal include utilizing sustainable monomers and the development of polymers that can be chemically recycled/degraded. Polycarbonates synthesized from the ring-opening copolymerization (ROCOP) of epoxides and CO₂, and polyesters synthesized from the ROCOP of epoxides and anhydrides, meet these criteria. Despite this, designing efficient catalysts for these processes remains challenging. Typical issues include the requirement for high catalyst loading; low catalytic activities in comparison with other commercialized polymerizations; and the requirement of costly, toxic cocatalysts. The development of efficient catalysts for both types of ROCOP is highly desirable. This Account details our work on the development of catalysts for these two related polymerizations and, in particular, focuses on dinuclear complexes, which are typically applied without any cocatalyst. We have developed mechanistic hypotheses in tandem with our catalysts, and throughout the Account, we describe the kinetic, computational, and structure–activity studies that underpin the performance of these catalysts. Our initial research on homodinuclear M(II)M(II) complexes for cyclohexene oxide (CHO)/CO₂ ROCOP provided data to support a chain shuttling catalytic mechanism, which implied different roles for the two metals in the catalysis. This mechanistic hypothesis inspired the development of mixed-metal, heterodinuclear catalysts. The first of this class of catalysts was a heterodinuclear Zn(II)Mg(II) complex, which showed higher rates than either of the homodinuclear [Zn(II)Zn(II) and Mg(II)Mg(II)] analogues for CHO/CO₂ ROCOP. Expanding on this finding, we subsequently developed a Co(II)Mg(II) complex that showed field leading rates for CHO/CO₂ ROCOP and allowed for unique insight into the role of the two metals in this complex, where it was established that the Mg(II) center reduced transition state entropy and the Co(II) center reduced transition state enthalpy. Following these discoveries, we subsequently developed a range of heterodinuclear M(III)M(I) catalysts that were capable of catalyzing a broad range of copolymerizations, including the ring-opening copolymerization of CHO/CO₂, propylene oxide (PO)/CO₂, and CHO/phthalic anhydride (PA). Catalysts featuring Co(III)K(I) and Al(III)K(I) were found to be exceptionally effective for PO/CO₂ and CHO/PA ROCOP, respectively. Such M(III)M(I) complexes operate through a dinuclear metalate mechanism, where the M(III) binds and activates monomers while the M(I) species binds the polymer change in close proximity to allow for insertion into the activated monomer. Our research illustrates how careful catalyst design can yield highly efficient systems and how the development of mechanistic understanding aids this process. Avenues of future research are also discussed, including the applicability of these heterodinuclear catalysts in the synthesis of sustainable materials.

Syntheses of Heterometallic Neodymium-Zinc Complexes and Their Performance in the Copolymerization of CO2 and Cyclohexene Oxide

A series of Nd-Zn heterometallic complexes bearing o-phenylenediamine-bridged tris(phenolato) ligands (L) were synthesized and characterized. By tuning the backbones of ancillary tris(phenolato) ligands and initiating benzyloxy groups, a Nd-Zn heterometallic complex 12 (^ClLNdZnOBn^CF3) was found to be highly active for the copolymerization of CO₂ and cyclohexene oxide (CHO) to produce perfect alternating poly(cyclohexene carbonate) with a high turnover frequency up to 5640 h^-1 under the polymerization of 90 °C and 20 bar CO₂ pressure. The kinetics study showed that CO₂/CHO copolymerization catalyzed by 12 was the first order dependence of 12 and CHO concentration and the zero-order dependence of CO₂ pressure. The reaction of 12 with CO₂ generated a carbonate-coordinated [NdZnNd] trinuclear complex 13, which was believed to be the key intermediate to initiate CO₂/CHO copolymerization. On the basis of some experiments, a plausible synergistic polymerization mechanism was proposed.

Mg(ii) heterodinuclear catalysts delivering carbon dioxide derived multi-block polymers

Carbon dioxide derived polymers are emerging as useful materials for applications spanning packaging, construction, house-hold goods and automotive components. To accelerate and broaden their uptake requires both more active and selective catalysts and greater structural diversity for the carbon dioxide derived polymers. Here, highly active catalysts show controllable selectivity for the enchainment of mixtures of epoxide, anhydride, carbon dioxide and lactone. Firstly, metal dependent selectivity differences are uncovered using a series of dinuclear catalysts, Mg(ii)Mg(ii), Zn(ii)Zn(ii), Mg(ii)Zn(ii), and Mg(ii)Co(ii), each exposed to mixtures of bio-derived tricyclic anhydride, cyclohexene oxide and carbon dioxide (1 bar). Depending upon the metal combinations, different block structures are possible with Zn(ii)Zn(ii) yielding poly(ester-b-carbonate); Mg(ii)Mg(ii) or Mg(ii)Co(ii) catalysts delivering poly(carbonate-b-ester); and Mg(ii)Zn(ii) furnishing a random copolymer. These results indicate that carbon dioxide insertion reactions follow the order Co(ii) > Mg(ii) > Zn(ii). Using the most active and selective catalyst, Mg(ii)Co(ii), and exploiting reversible on/off switches between carbon dioxide/nitrogen at 1 bar delivers precision triblock (ABA), pentablock (BABAB) and heptablock (ABABABA) polymers (where A = poly(cyclohexylene oxide-alt-tricyclic anhydride), PE; B = poly(cyclohexene carbonate), PCHC). The Mg(ii)Co(ii) catalyst also selectively polymerizes a mixture of anhydride, carbon dioxide, cyclohexene oxide and ε-caprolactone to deliver a CBABC pentablock copolymer (A = PE, B = PCHC C = poly(caprolactone), PCL). The catalysts combine high activity and selectivity to deliver new polymers featuring regularly placed carbon dioxide and biomass derived linkages.