Closing the loop in the synthesis of heteroscorpionate-based aluminium helicates: catalytic studies for cyclic carbonate synthesis

Novel polynuclear helical aluminium complexes supported by bulky heteroscorpionate ligands have been developed and characterised. The use of bulkier ligands has allowed the isolation of unprecedented intermediates for the preparation of helical aluminium complexes. The catalytic activity of these aluminium complexes for cyclic carbonates formation has also been investigated under mild reaction conditions. The combination of complex 16 and Bu₄NBr catalysed the synthesis of a broad range of monosubstituted cyclic carbonates from their corresponding epoxides and CO₂ at 25 °C and one bar of CO₂ pressure. This catalyst system also showed good catalytic activity for the preparation of disubstituted cyclic carbonates from internal epoxides and CO₂.

Ring-Opening Polymerization of ε-Caprolactone and Styrene Oxide-CO2 Coupling Reactions Catalyzed by Chelated Dehydroacetic Acid-Imine Aluminum Complexes

A series of chelated dehydroacetic acid-imine-based ligands L1H~L4H was synthesized by reacting dehydroacetic acid with 2-t-butylaniline, (S)-1-phenyl-ethylamine, 4-methoxylbenzylamine, and 2-(aminoethyl)pyridine, respectively, in moderate yields. Ligands L1H~L4H reacted with AlMe3 in toluene to afford corresponding compounds AlMe2L1 (1), AlMe2L2 (2), AlMe2L3 (3), and AlMe2L4 (4). All the ligands and aluminum compounds were characterized by IR spectra, 1H and 13C NMR spectroscopy. Additionally, the ligands L1H~L4H and corresponding aluminum derivatives 1, 3, and 4 were characterized by single-crystal X-ray diffractometry. The catalytic activities using these aluminum compounds as catalysts for the ε-caprolactone ring-opening polymerization (ROP) and styrene oxide-CO2 coupling reactions were studied. The results show that increases in the reaction temperature and selective solvent intensify the conversions of ε-caprolactone to polycaprolactone. Regarding the coupling reactions of styrene oxide and CO2, the conversion rate is over 90% for a period of 12 h at 90 °C. This strategy dispenses the origination of cyclic styrene carbonates, which is an appealing concern because of the transformation of CO2 into an inexpensive, renewable and easy excess carbon feedstock.

Syntheses, Characterization, and Application of Tridentate Phenoxyimino-Phenoxy Aluminum Complexes for the Coupling of Terminal Epoxide with CO2: From Binary System to Single Component Catalyst

A series of binuclear aluminum complexes 1–3 supported by tridentate phenoxyimino-phenoxy ligands was synthesized and used as catalysts for the coupling reaction of terminal epoxide with carbon dioxide. The aluminum complex 1, which is catalytically inactive toward the coupling of epoxide with CO2 by itself, shows moderate activity in the presence of excess nucleophiles or organic bases at high temperature. In sharp contrast to complex 1, bifunctional complexes 2 and 3, which incorporate tertiary amine groups as the built-in nucleophile, are able to efficiently transform terminal epoxide with CO2 to corresponding cyclic carbonates as a sole product by themselves at 100 °C. The number of amine groups on the ligand skeleton and the reaction temperature exert a great influence on the catalytic activity. The bifunctional complexes 2 and 3 are also active at low carbon dioxide pressure such as 2 atm or atmospheric CO2 pressure. Kinetic studies of the coupling reactions of chloropropylene oxide/CO2 and styrene oxide/CO2 using bifunctional catalysts under atmospheric pressure of CO2 demonstrate that the coupling reaction has a first-order dependence on the concentration of the epoxide.

Catalytic Conversion of Carbon Dioxide Using Binuclear Double-Stranded Helicates: Cyclic Carbonate from Epoxides and Diol

The construction of sophisticated molecular architectures from chemical subunits requires careful selection of the spacers, precise synthetic strategies, and substantial efforts. Here, we report a series of binuclear double-stranded helicates synthesized from different combinations of pyridyl hydrazone-based multidentate ligands (H₂ 1, H₂ 2, H₂ 3) by increasing the methylene spacer and transition metals (Co, Ni, and Zn). The ligands H₂ 1 (N'1,N'3-bis((E)-pyridin-2-ylmethylene)malonohydrazide), H₂ 2 (N'1,N'4-bis((E)-pyridin-2-ylmethylene)succinohydrazide), and H₂ 3 (N'1,N'5-bis((E)-pyridin-2-ylmethylene)glutarohydrazide) and their respective complexes with Co, Ni, and Zn were obtained. Single-crystal X-ray diffraction studies of these binuclear metallohelicates confirm the double-stranded helical structure of the complexes derived from H₂ 2. The set of helicates Co-1, Co-2, and Co-3; Ni-1, Ni-2, and Ni-3; and Zn-1, Zn-2, and Zn-3 were investigated for its catalytic activity in the cyclic carbonate formation reaction. Intriguingly, among the synthesized catalyst, Co-1 was found to be better in terms of conversions with the calculated TOF (turnover frequency) of 128/h. The catalytic performance was significantly improved by adding 0.2 mmol of tetrabutylammonium bromide by achieving 76% conversion in 30 min, with the observed TOF of 15,934 h^-1/molecule and 7967 h^-1/Co center. The results obtained herein show that the double-stranded helicates are effective catalysts for converting both terminal and non-terminal epoxides into their corresponding cyclic carbonates. The striking feature of this catalytic protocol lies in demonstrating the catalytic activity for the conversion of diol to cyclic carbonate, and the detailed kinetic experiments tempted us to propose a tentative reaction mechanism for this conversion.

Self-assembly of tetranuclear 3d–4f helicates as highly efficient catalysts for CO2 cycloaddition reactions under mild conditions

A series of 4-nuclear lanthanide clusters supported by organic ligands Zn3LnL4 (Ln = Dy(1), Gd(2), Er(3)) were synthesized. These helicates could be used to convert CO₂ into cyclic carbonates with TOF up to 38 000 h⁻¹, without being influenced by moisture or air.

Synthesis of helical aluminium catalysts for cyclic carbonate formation

Helical aluminium complexes have been prepared and used as catalysts for cyclic carbonate synthesis.

Zinc 2-N-methyl N-confused porphyrin: an efficient catalyst for the conversion of CO2 into cyclic carbonates

A zinc 2-N-methyl N-confused porphyrin (Zn(NCP)Cl) catalyst was developed for the solvent-free synthesis of cyclic carbonates from epoxides and CO₂.

Alternating Copolymerization of Epoxides and Anhydrides Catalyzed by Aluminum Complexes

The optimization of an organoaluminum catalytic system for the copolymerization of epoxides and anhydrides is presented. For this purpose, the influence of different variables in the process, such as catalysts, cocatalyst, solvent, or substrates, has been analyzed. Kinetic studies, a proposal for the catalytic mechanism, and full characterization of the copolymers obtained are also discussed. Finally, a new copolymer, poly(limonene succinate), obtained by the optimized catalytic system is reported.

Anion-induced 3d–4f luminescent coordination clusters: structural characteristics and chemical fixation of CO2 under mild conditions

Anion-induced clusters with luminescence exhibit excellent catalysis to transform CO₂ into cyclic carbonates under mild conditions.

Recent Developments in the Synthesis of Cyclic Carbonates from Epoxides and CO2

The use of CO₂ as a C1 building block will be of essential importance in the future. In this context the synthesis of cyclic carbonates from epoxides and CO₂ gained great attention recently. These products are valuable compounds in a variety of chemical fields. The development of new catalysts and catalytic systems for this atom-economic, scalable, and industrially relevant reaction is a highly active research field. Over the past 17 years great advances have been made in this area of research. This chapter covers the survey of the important known classes of homogeneous catalysts for the addition of CO₂ to epoxides. Besides pioneering work, recent developments and procedures that allow this transformation under mild reaction conditions (reaction temperatures of ≤100 °C and/or CO₂ pressures of 0.1 MPa) are especially emphasized.

One-Component Aluminum(heteroscorpionate) Catalysts for the Formation of Cyclic Carbonates from Epoxides and Carbon Dioxide

New neutral and zwitterionic chiral NNO-donor scorpionate ligands 1 and 2 were designed to obtain new mononuclear and dinuclear NNO-heteroscorpionate aluminum complexes. Reaction of 1 with [AlR₃ ] (R=Me, Et) in a 1:1 or 1:2 molar ratio afforded the neutral mononuclear alkyl complexes [AlR₂ (κ² -bpzappe)] {R=Me (3), Et (4); bpzappeH=2,2-bis(3,5-dimethylpyrazol-1-yl)-1-[4-(dimethylamino)phenyl]-1-phenylethanol} and bimetallic complexes [{AlR₂ (κ² -bpzappe)}(μ-O){AlR₃ }] [R=Me (5), Et (6)]. By reaction of complexes 3-6 with PhCH₂ Br, mononuclear and dinuclear cationic aluminum complexes [AlR₂ (κ² -bbpzappe)]Br {R=Me (7), Et (8); bbpzappeH=N-benzyl-4-[2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-hydroxy-1-phenylethyl]-N,N-dimethylbenzenaminium bromide} and [{AlR₂ (κ² -bbpzappe)}(μ-O){AlR₃ }]Br [R=Me (9), Et (10)] were synthesized. Both neutral aluminum complexes in the presence of Bu₄ NBr and cationic aluminum complexes were investigated as catalysts for cyclic carbonate formation from epoxides and carbon dioxide. Amongst them, complex 10 was found to be an efficient one-component catalyst for the synthesis of cyclic carbonates from both monosubstituted and internal epoxides and was shown to have broad substrate scope.

A Bimetallic Aluminium(Salphen) Complex for the Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide

A bimetallic aluminium(salphen) complex is reported as a sustainable, efficient and inexpensive catalyst for the synthesis of cyclic carbonates from epoxides and carbon dioxide. In the presence of this complex and tetrabutylammonium bromide, terminal and internal epoxides reacted at 50 °C and 10 bar carbon dioxide pressure to afford their corresponding cyclic carbonates in yields of 50-94 % and 30-71 % for terminal and internal cyclic carbonates, respectively. Mechanistic studies using deuterated epoxides and an analogous monometallic aluminium(salphen) chloride complex support a mechanism for catalysis by the bimetallic complex, which involves intramolecular cooperative catalysis between the two aluminium centres.

Aluminum complexes derived from a hexadentate salen-type Schiff base: synthesis, structure, and catalysis for cyclic carbonate synthesis

The unexpected aluminum complex was synthesized and found to be active in the cycloaddition reaction of terminal epoxides and CO₂ at atmospheric pressure.

Synthesis of Cyclic Carbonates Catalysed by Chromium and Aluminium Salphen Complexes

Chromium and aluminium salphen complexes have been found to display remarkable catalytic activity in the synthesis of cyclic carbonates from a range of epoxides and carbon dioxide. The Al(salphen) complex is more reactive towards terminal epoxides at ambient temperature and pressure, whereas the Cr(salphen) complex exhibits higher catalytic activity towards more challenging internal epoxides at elevated temperature and pressure.

Aluminium salabza complexes for fixation of CO2 to organic carbonates

A highly stable and easy to synthesize aluminium complex bearing a flexible N₂O₂-donor salabza ligand (N,N′-bis(salicylene)-2-aminobenzylamine) in combination with tetrabutylammonium bromide forms an active binary catalytic system for the cycloaddition of CO₂ to epoxides (TOFs 120–3434 h⁻¹) under mild conditions (10 bar, 80 °C) and low catalyst loadings (0.05–0.2 mol%).

Ring-opening copolymerisation of cyclohexene oxide and carbon dioxide catalysed by scorpionate zinc complexes

New bimetallic heteroscorpionate zinc complexes have been developed and used as efficient catalysts for the synthesis of poly(cyclohexene carbonate).

Catalytic behaviour in the ring-opening polymerisation of organoaluminiums supported by bulky heteroscorpionate ligands

A series of alkyl organoaluminium complexes based on bulky heteroscorpionate ligands were designed as catalysts for the ring-opening polymerisation of cyclic esters. Thus, the treatment of AlX3 (X = Me, Et) with bulky acetamide or thioacetamide heteroscorpionate ligands nbptamH (1) [nbptamH = N-naphthyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide], fbpamH (2) [fbpamH = N-fluorenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide], ptbptamH (3) [ptbptamH = N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide], ntbptamH (4) [ntbptamH = N-naphthyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide], ptbpamH (5) [ptbpamH = N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)acetamide] and (S)-mtbpamH (6) [(S)-mtbpamH = (S)-(−)-N-α-methylbenzyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)acetamide] for 1 hour at 0 °C afforded the dialkyl aluminium complexes [AlX2{κ(2)-nbptam}] (X = Me 7, Et 8), [AlX2{κ(2)-fbpam}] (X = Me 9, Et 10), [AlX2{κ(2)-ptbptam}] (X = Me 11, Et 12), [AlX2{κ(2)-ntbptam}] (X = Me 13, Et 14), [AlX2{κ2(-)ptbpam}] (X = Me 15, Et 16) and [AlX2{κ(2)-(S)-mtbpam}] (X = Me 17, Et 18). The structures of the complexes were determined by spectroscopic methods and the X-ray crystal structure of 14 was also established. The alkyl-containing aluminium complexes 7–18 can act as efficient single-component initiators for the ring-opening polymerisation of ε-caprolactone and rac-lactide. The polymerisations are living, as evidenced by the narrow polydispersities of the isolated polymers and the linear nature of the number average molecular weight versus conversion plot. Finally, a comparative study of ring-opening polymerisation for new bulky heteroscorpionate aluminium initiators and the less congested aluminium analogues is reported.

Synthesis of new heteroscorpionate iridium(i) and iridium(iii) complexes

The reactivity of different heteroscorpionate ligands based on bis(pyrazol-1-yl)methane, with different iridium-(i) and -(iii) precursors is reported.

Comparing kinetic profiles between bifunctional and binary type of Zn(salen)-based catalysts for organic carbonate formation

Zn(salen) complexes have been employed as active catalysts for the formation of cyclic carbonates from epoxides and CO₂. A series of kinetic experiments was carried out to obtain information about the mechanism for this process catalyzed by these complexes and in particular about the order-dependence in catalyst. A comparative analysis was done between the binary catalyst system Zn(salphen)/NBu₄I and a bifunctional system Zn(salpyr)·MeI with a built-in nucleophile. The latter system demonstrates an apparent second-order dependence on the bifunctional catalyst concentration and thus follows a different, bimetallic mechanism as opposed to the binary catalyst that is connected with a first-order dependence on the catalyst concentration and a monometallic mechanism.

Catalytic production of cyclic carbonates mediated by lanthanide phenolates under mild conditions

Readily available lanthanide complexes stabilized by a bridged poly(phenolate) ligand have been used for the first time as efficient catalysts for the insertion of CO2 into epoxides to generate cyclic carbonates with high activity, high selectivity, and a wide substrate scope under mild conditions.

Nucleophile-directed selectivity towards linear carbonates in the niobium pentaethoxide-catalysed cycloaddition of CO2 and propylene oxide

The reaction between CO₂ and propylene oxide in the presence of Nb(EtO)₅ and 4-dimethylaminopyridine leads to the formation of an acyclic carbonate through an unprecedented pathway.

Synthesis of cyclic carbonates catalysed by aluminium heteroscorpionate complexes

A trimetallic aluminium complex catalyses the synthesis of cyclic carbonates from epoxides and CO₂ at room temperature and pressure.