Mechanism of the Alternating Copolymerization of Epoxides and CO₂ Using β-Diiminate Zinc Catalysts:  Evidence for a Bimetallic Epoxide Enchainment

Eberhardt R., Allmendinger M., Rieger B.

Highly efficient formation of poly(propylene carbonate) can be achieved in the coupling of CO 2 and propylene oxide assisted by 4-(N,N-dimethylamino)pyridine (DMAP) and catalyzed with salen chromium(III) chloride by using DMAP/Cr ratios of less than 2. Under these conditions a possible backbiting mechanism is suppressed, leading to only minor amounts of cyclic carbonate as a side product.

Stevens E.S.

Eberhardt R., Allmendinger M., Luinstra G.A., Rieger B.

The quantitative synthesis of a series of new zinc(II) sulfinate complexes (4a−e) by insertion of SO2 into zinc−ethyl bonds of β-diiminate complexes ((ArNC(CH3)CH(CH3)CNAr)Zn(O(SO)Et); Ar = 2,6-diisopropylphenyl (4a), 2,6-diethylphenyl (4b), 2-ethyl-6-isopropylphenyl (4c), 2,6-diphenylphenyl (4d), 2,6-bis(4-tert-butylphenyl)phenyl (4e)) is described. X-ray structure analysis reveals a dinuclear, μ-ethylsulfinate-bridged structure for 4a in the solid state, which in solution exists in an equilibrium with the mononuclear species. The easily accessible complexes 4a−c are highly active catalysts for the alternating copolymerization of CO2 and cyclohexene oxide, leading to products with narrow molecular weight distributions.

Rieth L.R., Moore D.R., Lobkovsky E.B., Coates G.W.

Polymerization of beta-butyrolactone (BBL) and beta-valerolactone (BVL) using the zinc alkoxide initiator (BDI-1)ZnO(i)()Pr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] proceeds very rapidly under mild conditions to produce poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyvalerate) (PHV), respectively. For the monomer-to-initiator ratio 200:1, PHB number-average molecular weights (M(n)) are proportional to conversion throughout the reaction and polydispersity indices (PDIs) are narrow, consistent with a living polymerization. Higher monomer-to-initiator ratios can be used to achieve high molecular weight PHB (M(n) > 100 000). End-group analysis verifies that the polymerization of BBL follows a coordination-insertion mechanism, where complexes of the form (BDI-1)ZnOCH(Me)CH(2)CO(2)R are the active species. Variable temperature NMR experiments show that (BDI-1)ZnO(i)()Pr is monomeric in benzene-d(6) solution. In contrast, (BDI-2)ZnO(i)()Pr [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] is a poor initiator at room temperature because it prefers to form a bis-mu-isopropoxide dimer in solution. According to kinetic studies, propagation by (BDI-1)ZnO(i)()Pr is first order in both monomer as well as (BDI-1)ZnO(i)()Pr concentration. These results lead us to propose a monometallic active species. Several results suggest that elimination side reactions are slowly catalyzed by zinc alkoxides, leading to degradation of the polymer.

Allen S.D., Moore D.R., Lobkovsky E.B., Coates G.W.

Despite recent advances regarding catalysts for CO2/epoxide copolymerization, the development of high-activity catalysts for alternating polymerization of CO2 and commodity epoxides, such as propylene oxide, remains a challenge. A new class of unsymmetrically substituted beta-diiminate zinc complexes is reported that exhibits unprecedented activity for CO2/propylene oxide copolymerization. The polymers formed are of high molecular weight (Mn approximately 35 kg/mol) and have narrow polydispersities (PDI approximately 1.1), consistent with a living polymerization.

Gross R.A., Kalra B.

Biodegradable polymers are designed to degrade upon disposal by the action of living organisms. Extraordinary progress has been made in the development of practical processes and products from polymers such as starch, cellulose, and lactic acid. The need to create alternative biodegradable water-soluble polymers for down-the-drain products such as detergents and cosmetics has taken on increasing importance. Consumers have, however, thus far attached little or no added value to the property of biodegradability, forcing industry to compete head-to-head on a cost-performance basis with existing familiar products. In addition, no suitable infrastructure for the disposal of biodegradable materials exists as yet.

Moore D.R., Cheng M., Lobkovsky E.B., Coates G.W.

Darensbourg D.J., Wildeson J.R., Yarbrough J.C.

Zinc complexes derived from benzoic acids containing electron-withdrawing substituents have been synthesized from ZnII(bis-trimethylsilyl amide)2 and the corresponding carboxylic acid (2,6-X2C6H3COOH, where X = F, Cl, or OMe) in THF and structurally characterized via X-ray crystallography. The 2,6-difluorobenzoate complex crystallizes from THF or CH3CN as a seven membered zinc aggregate, where the metal atoms are interconnected by a combination of 10 μ-benzoates and μ4-oxo ligands, that is, [(2,6-difluorobenzoate)10O2Zn7](solvent)2, solvent = THF (1) and CH3CN (1a). On the other hand, the 2,6-dichlorobenzoate zinc derivative crystallizes from THF as a dimer, [(2,6-dichlorobenzoate)4Zn2](THF)3 (2), where the two zinc centers are bridged by three benzoate ligand. One of the zinc centers possesses a tetrahedral ligand environment where the fourth ligand is a unidentate benzoate, and the other zinc center has an octahedral arrangement of ligands which is accomplished by the additional binding of three THF molecules. Upon dissolution of complex 1 or 2 in the strongly binding pyridine solvent, disruption of these zinc carboxylates occurs with concomitant formation of mononuclear zinc bis-benzoates with three pyridine ligands in the metal coordination sphere. Complexes 1 and 2 were found to be effective catalysts for the copolymerization of cyclohexene oxide and carbon dioxide to afford polycarbonates devoid of polyether linkages, that is, completely alternating copolymers. Although these catalysts or catalyst precursors in the presence of CO2/propylene oxide afforded mostly propylene carbonate, they did serve as efficient catalysts for the terpolymerization of carbon dioxide/cyclohexene oxide/propylene oxide. The reactivities of these zinc carboxylates were very similar to those previously reported analogous complexes which have not been structurally characterized. Hence, it is suggested here that all of these zinc carboxylates provide similar catalytic sites for CO2/epoxide coupling processes.

Okada M.

Commercially available synthetic biodegradable polymers had been limited to aliphatic polyesters until polyesters containing aromatic moieties recently appeared on the market. Currently, several different types of biodegradable polymers have been developed and are being evaluated for practical uses. The present article reviews recent advances in chemical syntheses of biodegradable polymers from the standpoint of molecular design. Thus, synthetic biodegradable polymers are herein classified into three groups: (1) polyesters, (2) polymers containing both ester and other heteroatom-containing linkages in the main chains, and (3) polymers with heteroatom-containing linkages other than ester linkages in the main chains. Progress in the synthesis of the representative polymers in each category is described with emphasis on controlled synthesis, and their biodegradability is discussed in relation to the molecular structure.

Paddock R.L., Nguyen S.T.

Arakawa H., Aresta M., Armor J.N., Barteau M.A., Beckman E.J., Bell A.T., Bercaw J.E., Creutz C., Dinjus E., Dixon D.A., Domen K., DuBois D.L., Eckert J., Fujita E., Gibson D.H., et. al.

The goal of the "Opportunities for Catalysis Research in Carbon Management" workshop was to review within the context of greenhouse gas/carbon issues the current state of knowledge, barriers to further scientific and technological progress, and basic scientific research needs in the areas of H2 generation and utilization, light hydrocarbon activation and utilization, carbon dioxide activation, utilization, and sequestration, emerging techniques and research directions in relevant catalysis research, and in catalysis for more efficient transportation engines. Several overarching themes emerge from this review. First and foremost, there is a pressing need to better understand in detail the catalytic mechanisms involved in almost every process area mentioned above. This includes the structures, energetics, lifetimes, and reactivities of the species thought to be important in the key catalytic cycles. As much of this type of information as is possible to acquire would also greatly aid in better understanding perplexing, incomplete/inefficient catalytic cycles and in inventing new, efficient ones. The most productive way to attack such problems must include long-term, in-depth fundamental studies of both commercial and model processes, by conventional research techniques and, importantly, by applying various promising new physicochemical and computational approaches which would allow incisive, in situ elucidation of reaction pathways. There is also a consensus that more exploratory experiments, especially high-risk, unconventional catalytic and model studies, should be undertaken. Such an effort will likely require specialized equipment, instrumentation, and computational facilities. The most expeditious and cost-effective means to carry out this research would be by close coupling of academic, industrial, and national laboratory catalysis efforts worldwide. Completely new research approaches should be vigorously explored, ranging from novel compositions, fabrication techniques, reactors, and reaction conditions for heterogeneous catalysts, to novel ligands and ligation geometries (e.g., biomimetic), reaction media, and activation methods for homogeneous ones. The interplay between these two areas involving various hybrid and single-site supported catalyst systems should also be productive. Finally, new combinatorial and semicombinatorial means to rapidly create and screen catalyst systems are now available. As a complement to the approaches noted above, these techniques promise to greatly accelerate catalyst discovery, evaluation, and understanding. They should be incorporated in the vigorous international research effort needed in this field.

Chamberlain B.M., Cheng M., Moore D.R., Ovitt T.M., Lobkovsky E.B., Coates G.W.

A series of zinc(II) and magnesium(II) alkoxides based upon a beta-diiminate ligand framework has been prepared. [(BDI-1)ZnO(i)Pr](2) [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] exhibited the highest activity and stereoselectivity of the zinc complexes studied for the polymerization of rac- and meso-lactide to poly(lactic acid) (PLA). [(BDI-1)ZnO(i)()Pr](2) polymerized (S,S)-lactide to isotactic PLA without epimerization of the monomer, rac-lactide to heterotactic PLA (P(r) = 0.94 at 0 degrees C), and meso-lactide to syndiotactic PLA (P(r) = 0.76 at 0 degrees C). The polymerizations are living, as evidenced by the narrow polydispersities of the isolated polymers in addition to the linear nature of number average molecular weight versus conversion plots and monomer-to-catalyst ratios. The substituents on the beta-diiminate ligand exert a significant influence upon the course of the polymerizations, affecting both the degree of stereoselectivity and the rate of polymerization. Kinetic studies with [(BDI-1)ZnO(i)Pr](2) indicate that the polymerizations are first order with respect to monomer (rac-lactide) and 1.56 order in catalyst. Polymerization experiments with [(BDI-1)MgO(i)Pr](2) revealed that this complex is extremely fast for the polymerization of rac-lactide, polymerizing 500 equiv in 96% yield in less than 5 min at 20 degrees C.

Darensbourg D.J., Wildeson J.R., Yarbrough J.C., Reibenspies J.H.

Dimeric phenoxide derivatives of zinc and cadmium have been synthesized from the reaction of the corresponding metal bistrimethylsilylamide and two equivalents of 2,6-F2C6H3OH in tetrahydrofuran. The zinc analogue, [Zn(O-2,6-F2C6H3)2·THF]2 (1), has been characterized in the solid state via X-ray crystallography, where the zinc centers are shown to possess distorted tetrahedral geometry containing two bridging phenoxides and a terminal phenoxide and THF ligand. The distance between the metal centers (Zn···Zn) was found to be 3.059 A, and the THF ligands lie on opposite sides of the plane formed by the two zinc atoms and two bridging phenoxide ligands' oxygen atoms. There are several Zn···F nonbonding distances involving the bridging phenoxide ligands that are less than the van der Waals internuclear distance. In addition, both the zinc and cadmium dimeric derivatives have been prepared such that the labile THF ligands are replaced by the sterically encumbering basic phosphine, PCy3. The solid-state structu...

Konno T., Chikamoto Y., Okamoto K., Yamaguchi T., Ito T., Hirotsu M.

Octahedral fac(S)-[CoIII(aet)3] molecules (aet=NH2CH2CH2S−) react with square-planar PdII ions to form a sulfur-bridged CoPdII trinuclear complex [Pd{Co(aet)3}2]2+, accompanied by the geometrical isomerization from fac(S) to mer(S). This complex was found to act as a sulfur-donating trinuclear-metalloligand toward linear AuI and AgI ions to afford CoPdM metallacycles with 16 chiral centers, [M{PdII[CoIII(aet)3]2}2]6+ (M=Ag, Au (1)), which were successfully resolved into the (Δ)4(R)12 and (Λ)4(S)12 enantiomers.

Gerngross T.U., Slater S.C.

Zeng C., Ren J., Wang R., Qiu Z., Wang L., Zhang S., Ji P., Wang C., Wang H.

Rusconi Y., D'Alterio M.C., De Rosa C., Coates G.W., Talarico G.

The factors contributing to the enantioselective ring opening copolymerization of CO2 and meso-epoxide have been unveiled by DFT calculations combined with activation strain model and noncovalent interaction analysis.

Liu Z., Liu G., Ko B.

Mashima K., Tsurugi H., Nagae H.

Multiple metal cluster complexes supported by alkoxide or carboxylate ligands as well as oxo, imido or nitride, and hydride serve as unique and intrinsic catalysts with peculiar reactivity and selectivity based on the concerted catalytic performance of two or more proximately arranged homo or hetero metals. This chapter highlights the catalytic reactions of these clusters for Lewis acid–base reactions, oxidation reactions, radical reactions, etc.

Suresh L., Lalrempuia R., Fjermestad T., Törnroos K.W., Bour J., Frache G., Nova A., Le Roux E.

Yang G., Xie R., Zhang Y., Xu C., Wu G.

Naik P.K., Gu Z., Comito R.J.

We report the first binucleating aniline ligand (1), and compare its dizinc complexes to analogous phenolate complexes both structurally and in ring-opening polymerizaion catalysis.

Xu C., Lu C., Zhao S., Yang G., Li W., Wang J., Wu G.

Fiorentini F., Eisenhardt K.H., Deacy A.C., Williams C.K.

Yang A., Wu X., Guo K., Wen Y., Duan Z., Liu B.

Nie Y., Mei Y., Zhu Y., Mu Y., Wu Y., Sun L., Xi Z., Liu Z.

Fan P., Liu S., Zhang R., Zhuo C., Gao F., Pang X., Chen X., Wang X.

Jia Y., Li B., Sun Y., Hu C., Li X., Liu S., Wang X., Pang X., Chen X.

Adams F.

AbstractPolymers with well‐defined structures, synthesized through metal‐catalyzed processes, and having end groups exhibiting different polarity and reactivity than the backbone, are gaining considerable attention in both scientific and industrial communities. These polymers show potential applications as fundamental building blocks and additives in the creation of innovative functional materials. Investigations are directed toward identifying the most optimal and uncomplicated synthetic approach by employing a combination of living coordination polymerization mediated by rare‐earth metal complexes and C–H bond activation reaction by σ‐bond metathesis. This combination directly yields catalysts with diverse functional groups from a single precursor, enabling the production of terminal‐functionalized polymers without the need for sequential reactions, such as termination reactions. The utilization of this innovative methodology allows for precise control over end‐group functionalities, providing a versatile approach to tailor the properties and applications of the resulting polymers. This perspective discusses the principles, challenges, and potential advancements associated with this synthetic strategy, highlighting its significance in advancing the interface of metalorganic chemistry, polymer chemistry, and materials science.

Chiarcos R., Sparnacci K., Antonioli D., Ivaldi C., Gianotti V., Po R., Biagini P., Losio S., Laus M.

Three poly(cyclohexene carbonates) with molecular weights ranging from 4.9 to 9.4 kg/mol were synthesized from cyclohexene oxide and CO2 using macrocyclic phenolate dimetallic catalysts and purified by conventional purification procedure. A decrease in thermal stability of approximately 100 °C was observed in comparison to poly(cyclohexene carbonates) with similar molecular weights synthesized using salen metal catalysts. This decrease derives from the presence of traces of dimetallic catalyst which is able to promote the depolymerisation of poly(cyclohexene carbonate) to CO2 and cyclohexene oxide in contrast to the usual backbiting mechanism that leads to cyclic carbonate. The onset of the degradation can be precisely tuned by changing the amount of residual dimetallic catalyst or including species with functional groups that can reduce the availability of the catalytic centers. Therefore, the possibility of controlling the thermal stability of poly(cyclohexene carbonates) by varying the concentration of the catalyst and the surrounding chemical environment paves the way for the use of these polymers as components in self-sacrificial materials of interest for advanced applications.