Epoxide/CO2 Cycloaddition Reaction Catalyzed by Indium Chloride Complexes Supported by Constrained Inden Schiff-Base Ligands.

Cyclic carbonates have received significant interests for uses as reagents, solvents, and monomers. The coupling reaction of epoxides with carbon dioxide (CO2 ) to produce cyclic carbonate is an attractive route which can significantly reduce greenhouse gas emissions and environmental hazards. Herein, a series of five indium chloride complexes supported by inden Schiff-base ligands were reported along with four X-ray crystal structures. The constrained five-membered rings were added to the ligands to enhance the coordination of epoxides to the In metal. From the catalyst screening, In inden complex having tert-butyl substituents and propylene backbone in combination with tetrabutylammonium bromide (TBAB) exhibited the highest catalytic activity (TON up to 1017) for propylene oxide/CO2 coupling reaction with >99 % selectivity for cyclic carbonate under solvent-free conditions. In addition, the catalyst was shown to be active at atmospheric pressure of CO2 at room temperature. The catalyst system can be applied to various internal and terminal epoxide substrates to exclusively produce the corresponding cyclic carbonates.

Ring-Opening Co- and Terpolymerization of Epoxides, Cyclic Anhydrides, and l-Lactide Using Constrained Aluminum Inden Complexes.

Seven constrained aluminum inden complexes having different substituents and diamine backbones were developed for the ring-opening copolymerization (ROCOP) of epoxides and bulky cyclic anhydrides giving alternating polyesters with Tg ranging from 49 to 226 °C. Among several catalyst/cocatalyst screenings, the aluminum inden complex having a rigid phenylene backbone coupled with 4-dimethylaminopyridine showed the best performance giving linear polyesters. In the case of cyclohexene oxide (CHO) and succinic anhydride (SA), the linear poly(CHO-alt-SA) could be transformed to cyclic polymer when the polymerization was left under prolonged reaction time to induce intramolecular transesterification. The kinetic studies of the ROCOP revealed a zeroth-order dependence on cyclic anhydride and a first-order dependence on epoxide and the catalyst. The catalysts can be extended efficiently to the one-pot CHO/PA/l-lactide terpolymerization giving uncommon tapered copolymers of poly(CHO-alt-PA) and PLA via switchable polymerization.

Enhancement of Ethylene and Ethylene/1-Hexene (Co)polymerization Activities by Titanium(IV) and Zirconium(IV) Complexes Bearing Constrained Hydroxyindanone-Imine Ligands.

A new series of catalysts for ethylene and ethylene/1-hexene (co)polymerizations bearing constrained hydroxyindanone-imine ligands was developed for titanium(IV) and zirconium(IV) metals with variations of steric and electronic contributions on the ligands. X-ray crystal structures revealed significantly higher open space for the constrained titanium and zirconium complexes, compared to the conventional FI counterparts. Upon activation with MAO, significantly higher ethylene polymerization activities (up to 379.4 kg-PE/mmol-M h for Zr) and notably almost doubled 1-hexene content in the ethylene/1-hexene copolymerizations were observed as a result of the constrained five-membered ring backbone.

Antioxidant and Anticancer Potential of Bioactive Compounds from Rhinacanthus nasutus Cell Suspension Culture.

The potential benefits of natural plant extracts have received attention in recent years, encouraging the development of natural products that effectively treat various diseases. This is the first report on establishing callus and cell suspension cultures of Rhinacanthus nasutus (L.) Kurz. A yellow friable callus was successfully induced from in vitro leaf explants on Murashige and Skoog medium supplemented with 1 mg/L 2,4-dichlorophenoxyacetic acid and 1 mg/L 1-naphthalene acetic acid. A selected friable callus line was used to establish the cell suspension culture with the same medium. The antioxidant assays showed that the leaf- and ethanolic-suspension-cultured cell (SCC) extracts exhibited high antioxidant potential. In addition, the in vitro cytotoxicity revealed by the MTT assay demonstrated potent antiproliferative effects against the oral cancer cell lines ORL-48 and ORL-136 in a dose-dependent manner. Several groups of compounds, including terpenoids, phenolics, flavonoids, quinones, and stilbenes, were identified by UHPLC-QToF-MS, with the same compounds detected in leaf and SCC extracts, including austroinulin, lucidenic acid, esculetin, embelin, and quercetin 3-(2″-p-hydroxybenzoyl-4″-p-coumarylrhamnoside). The present study suggests the value of further investigations for phytochemical production using R. nasutus cell suspension culture.

Complexity of imine and amine Schiff-base tin(II) complexes: drastic differences of amino and pyridyl side arms.

A series of homoleptic and heteroleptic imine and amine Schiff-base tin(II) complexes containing dimethylamino and pyridyl side arms were synthesized and structurally characterized. While the tin complexes containing the dimethylamino side arm could be prepared in good yield, unusual cyclodimerization and ligand dimerization were observed for the first time for Schiff-base tin complexes having a pyridyl side arm due to the added stabilization offered by the pyridyl ring. The amine Schiff-base tin(II) complexes were active for the ring-opening copolymerization of succinic anhydride (SA) and cyclohexene oxide (CHO) at 110 °C giving highly alternating poly(SA-alt-CHO).

Highly Active Homoleptic Zinc and Magnesium Complexes Supported by Constrained Reduced Schiff Base Ligands for the Ring-Opening Polymerization of Lactide.

New homoleptic zinc and magnesium complexes containing constrained reduced Schiff base ligands based on substituted 7-hydroxy-1-indanone were successfully synthesized and used as a catalyst for the polymerization of lactide. The ligands contain a side arm having different basicity because dimethylamino, pyridyl, and furfuryl groups are shown to greatly affect the polymerization rates. The homoleptic zinc complex containing constrained reduced Schiff base ligands and a dimethylamino side arm was highly active, giving a 92% conversion of l-lactide in 3 min using [LA]:[Zn]:[BnOH] = 500:1:2 at room temperature. The polymerization is pseudo-first-order dependent on the LA concentration. Well-controlled and living behavior of the zinc complex was observed and demonstrated in the preparation of stereodiblock and triblock copolymers of l-, d-, and rac-lactide in a one-pot sequential synthesis with a predictable block length, block sequence, and narrow dispersity rapidly in 10 min. Stereocomplex formation was observed for PLA made sequentially from 100 l-LA, 100 rac-LA, and 100 d-LA having a high Tm of up to 220 °C.

Bifunctional zinc and magnesium Schiff-base complexes containing quaternary ammonium side-arms for epoxide/CO2 coupling reactions.

Novel bifunctional zinc and magnesium Schiff-base complexes containing quaternary ammonium halide side-arms were developed. Zinc complex 1Et-I (0.02 mol%) having an iodide anion has shown the highest TOF for the propylene oxide/CO2 coupling reaction of up to 459 h-1. This TOF value was maintained even when the catalyst loading was reduced to 0.005 mol%.

Cycloaddition of carbon dioxide to epoxides by highly active constrained aluminum chloride complexes.

The transformation of carbon dioxide (CO2) and epoxides to cyclic carbonates has gained much interest due to its low cost, abundance, low toxicity, and renewability. Therefore, novel constrained aluminum chloride complexes were developed based on bis(salicylimine) ligands for epoxides/CO2 coupling reactions. The five-membered rings attached to the aromatic rings were designed to enlarge the coordination pocket around the aluminum center as demonstrated by single-crystal X-ray crystallography. Addition of propylene oxide (PO) to a mixture of an aluminum chloride complex and tetrabutylammonium bromide (TBAB) rapidly gave (ligand)Al-OCH(Me)CH2Cl and (ligand)Al-OCH(Me)CH2Br in similar quantities. The anion exchange between (ligand)Al-Cl and TBAB was found to be faster than the ring-opening of PO. From a series of catalyst screening and optimization, the combination of catalyst 2g having no substituent on the aromatic rings and TBAB displayed very high activity (TOF up to 10 800 h-1) for the PO/CO2 coupling reaction. This catalyst system was extended to eleven more examples of epoxides. Moreover, excellent selectivity for cyclic carbonate production was observed for both terminal and internal epoxides.

Highly Active Chromium Complexes Supported by Constrained Schiff-Base Ligands for Cycloaddition of Carbon Dioxide to Epoxides.

Novel constrained Schiff-base ligands (inden) were developed based on the well-known salen ligands. Chromium complexes supported by the constrained inden ligands were successfully synthesized and used as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The catalyst having tert-butyl (tBu) groups as substituents in combination with tetrabutylammonium bromide (TBAB) as a cocatalyst exhibited very high catalytic activity with a turnover frequency of up to 14800 h-1 for the conversion of CO2 and propylene oxide into propylene carbonate exclusively at 100 °C and 300 psi of CO2 under solvent-free conditions. The catalyst was found to be highly active for various epoxide substrates to produce terminal cyclic carbonates in 100% selectivity.

Comparative bindings of lactones, lactide, and cyclic carbonates: experimental insights into the coordination step of polymerization.

Comparative bindings of several renowned monomers were investigated experimentally using B(C6F5)3 as a Lewis acid model for the coordination step in ring-opening polymerization. A complete series of the X-ray crystal structures of the B(C6F5)3 adducts with the monomers was reported. The X-ray structural studies and spectroscopic data revealed a coordination strength in the order lactones > tetrahydrofuran > cyclic carbonates > lactide.

Ring-Opening Copolymerizaton of Cyclohexene Oxide and Succinic Anhydride by Zinc and Magnesium Schiff-Base Complexes Containing Alkoxy Side Arms.

The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is a promising method for the synthesis of new polyesters with various polymer properties. Among previously reported metal catalysts for ROCOP, the Schiff-base complexes have gained significant attention because of their ease of synthesis and modification. In this work, zinc and magnesium complexes containing Schiff-base ligands with different alkoxy side arms [-(CH2)2O- and -(CH2)3O-] were synthesized and shown to have a cubane metal core by X-ray crystal structures. All complexes were studied in the ROCOP of cyclohexene oxide (CHO) and succinic anhydride (SA) in toluene at 110 °C. The zinc complex having a shorter side arm is the most active catalyst for copolymerization, giving poly(CHO-alt-SA) with narrow dispersity and negligible ether linkage. On the other hand, magnesium complexes were not active because of the formation of stable carboxylate species. The detailed analysis of polyester obtained from zinc complexes unexpectedly revealed three different types of polymer structures occurring at different polymerization times. Cyclic polymer was generated at the beginning by intramolecular transesterification of the alkoxy side arm, giving a low-molecular-weight polyester. At higher conversion, cyclization diminished, giving just a linear polyester but with minor competitive formation of higher-molecular-weight polyester having cyclohexanediol as an end group. On the basis of a thorough understanding of the polymerization mechanism, the desired cyclic poly(CHO-alt-SA) was successfully synthesized using a low monomer/catalyst ratio.

Synthesis and characterization of guanidinate tin(ii) complexes for ring-opening polymerization of cyclic esters.

Novel homoleptic and heteroleptic (guanidinate)tin(ii) complexes were successfully synthesized and structurally characterized. The first heteroleptic (guanidinato)tin(ii) alkoxide complex was synthesized but found to be unstable leading to the corresponding bis(guanidinate)tin(ii) complex. The catalytic activities of bis(guanidinate)tin(ii) complexes having different substituents at the nitrogen atoms (isopropyl (1), cyclohexyl (2), and p-tolyl (3)) were investigated in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and lactide (LA). The lone-pair electrons of the tin(ii) atom were proposed to act as an initiator similar to N-heterocyclic carbenes. Among the synthesized catalysts, complex 1 having less steric hindrance efficiently catalyzed both homo- and copolymerizations of ε-CL and LA giving high molecular weight cyclic polyesters. Transesterification was found to be the major contributor to the cyclization to cyclic polyesters.

Influence of catalyst nuclearity on copper-catalyzed aerobic alcohol oxidation.

Reactions of CuX with the bis(triazolyl) ligand Hbtm [bis(1-benzyl-1H-1,2,3-triazol-4-yl)phenylmethanol] in CH2Cl2 afforded trinuclear copper(ii) complexes with a core structure (µ-X)Cu3(µ-κ3-N,O,N-btm)3(L)2+ [X = Cl, L = CH3OH (1); X = Br, L = H2O (2)], while a similar reaction of [Cu(CH3CN)4](PF6) with the mono(triazolyl) ligand HPhtm [(1-benzyl-1H-1,2,3-triazol-4-yl)diphenylmethanol] resulted in the mononuclear complex [Cu(κ2-N,O-Phtm)(κ2-N,O-HPhtm)(κ1-N-HPhtm)][PF6] (3). The structural characterization of these complexes was made by single-crystal X-ray crystallography in combination with elemental and ESI mass analyses. Catalytic studies toward aerobic oxidation of benzyl alcohol to benzaldehyde revealed that the trinuclear 1 and 2 exhibited higher activities than the mononuclear 3 in both CH3CN and EtOH/H2O solvent systems.

Highly efficient metal(iii) porphyrin and salen complexes for the polymerization of rac-lactide under ambient conditions.

Ligated metal(iii) complexes, Al(iii), In(iii), Cr(iii), and Co(iii), containing tetraphenylporphyrin (TPP) and cyclohexylsalen (Cy-salen) ligands were investigated for the polymerization of rac-lactide (rac-LA). A combination of metal complexes, co-catalyst, and co-initiator was shown to be highly efficient for the ring-opening polymerization of rac-LA at room temperature. Influences of metal centers, ligands, and co-catalysts were investigated. Higher Lewis acidity of metal centers and ligands enhanced catalytic activity. Ionic co-catalysts showed increased polymerization rates. Polymerization of rac-LA catalyzed by tetraphenylporphyrin aluminum chloride ((TPP)AlCl) and bis(triphenylphosphine)iminium chloride (PPN+Cl-) in cyclohexene oxide (CHO) as a solvent produced isotactic-enriched polylactide. The order of reactivities based on the metals supported by the TPP ligand is Cr(iii) > Al(iii) > In(iii) > Co(iii) while the reactivities of the catalysts supported by Cy-salen ligands are in the order In(iii) > Cr(iii) > Al(iii) > Co(iii).

Polymerization of ε-Caprolactone Using Bis(phenoxy)-amine Aluminum Complex: Deactivation by Lactide.

Polymerizations of biodegradable lactide and lactones have been the subjects of intense research during the past decade. They can be polymerized/copolymerized effectively by several catalyst systems. With bis(phenolate)-amine aluminum complex, we have shown for the first time that lactide monomer can deactivate the aluminum complex during the ongoing polymerization of ε-caprolactone to a complete stop. After hours of dormant state, the aluminum complex can be reactivated again by heating at 100 °C without the addition of any external chemicals still giving polymer with narrow dispersity. Studies using NMR, in situ FTIR, and single-crystal X-ray crystallography indicated that the coordination of the carbonyl group in lactyl unit was responsible for the unusual behavior of lactide. In addition, the unusual methyl-migration from methyl lactate ligand to the amine side chain of the aluminum complex was observed through intermolecular nucleophilic-attack mechanism.

Hierarchical FAU/ZIF-8 Hybrid Materials as Highly Efficient Acid-Base Catalysts for Aldol Condensation.

The composite of hierarchical faujasite nanosheets and zeolitic imidazolate framework-8 (Hie-FAU-ZIF-8) has been successfully prepared via a stepwise deposition of ZIF-8 on modified zeolite surfaces. Compared to the direct deposition of metal organic frameworks (MOFs) on zeolite surfaces, ZIF-8 nanospheres were selectively attached to the external surfaces of the MOF ligand-grafted FAU crystals because of the enhancing interaction between the zeolite and MOF in the composite. In addition, the degree of surface functionalization can be greatly enhanced because of the presence of hierarchical structures. This behavior leads to an increase in the deposited MOF content, improving the hydrophobic properties of the zeolite surfaces. Interestingly, the designed hierarchical composite exhibits outstanding catalytic properties as an acid-base catalyst for the aldol condensation of 5-hydroxymethylfurfural with acetone. Compared to the isolated FAU and ZIF-8, a high yield of the product, 4-[5-(hydroxymethyl)furan-2-yl]but-3-en-2-one (67%), can be observed in the composite because of the synergistic effect between the Na+-stabilized zeolite framework and the imidazolate linkers bearing basic nitrogen functions. This opens up interesting perspectives for the development of new organic and inorganic hybrid materials as heterogeneous acid-base catalysts.

Synthesis of cyclic polyesters: effects of alkoxy side chains in salicylaldiminato tin(II) complexes.

A new class of salicylaldiminato tin(II) catalysts having different alkoxy side chains has been developed. The ligands were modified to have different lengths and flexibilities such as ­(CH2)2­ (2a), ­(CH2)3­ (2b), ­(ortho-C6H4)CH2­ (2c) and ­(CH2)2­O­(CH2)2­ (2d). Complexes 2a, b were characterized crystallographically revealing a more constrained environment around the metal in complex 2a. These catalysts are active for the solvent-free polymerization of L-lactide and ε-caprolactone. Complex 2a having a shorter side chain was shown to better promote intramolecular transesterification affording cyclic polylactides and cyclic poly(ε-caprolactone). Complexes 2b and 2d having longer side chains produced cyclic poly(ε-caprolactone) as a major product but failed to give cyclic polylactides.

Ethylene polymerisation and ethylene/norbornene copolymerisation by using aryloxo-modified vanadium(V) complexes containing 2,6-difluoro-, dichloro-phenylimido complexes.

Aryloxo-modified vanadium(V) dichloride complexes containing a dichloro- or difluoro-phenylimido ligand of the type, V(N-2,6-X2C6H3)Cl2(O-2,6-R2C6H3) [X = Cl (1), F (2); R = Me (a), F (b)], showed remarkable catalytic activity in ethylene polymerisation and the copolymerisation with norbornene (NBE) in the presence of Et2AlCl; the activities were higher than that of V(N-2,6-Me2C6H3)Cl2(O-2,6-Me2C6H3) reported previously. Complex 1a is a suitable catalyst precursor in terms of not only the activity, but also synthesis of high molecular weight copolymers with both unimodal molecular weight distributions and uniform compositions.

Syntheses of bis(pyrrolylaldiminato)aluminum complexes for the polymerisation of lactide.

Seven bis(pyrrolylaldiminato)aluminum methyl complexes were synthesized from the reactions of AlMe3 and two equiv. of the corresponding pyrrolylaldimine ligands. The ligands were modified to have different steric hindrances (C6H5 (1), 2,6-Me2C6H3 (2), 2,4,6-Me3C6H2 (3), 2,6-Et2C6H3 (4) and 2,6-(i)Pr2C6H3 (5)) and electronic contributions (4-CF3C6H4 (6) and 4-OMeC6H4 (7)). Crystal structures of complexes 3-7 were determined and shown to have distorted trigonal bipyramidal geometry (4, 6, 7) and intermediates between trigonal bipyramidal and square pyramidal geometries (3 and 5). The rotation around the N-C(aryl) bond was fast for ligands having small ortho substituents and became slower as the size of the substituents increased. Polymerisations of l-lactide using complexes 1-7 and benzyl alcohol as an initiator were carried out giving the rate dependence on steric hindrance (5 < 4 < 3 < 2 < 1) and electronic contribution (6 < 7 < 1). Larger substituents and electron withdrawing groups were found to suppress the polymerisation rates. Despite having C2 symmetry in the crystal structures of compounds 3-7, only slight enhancement for isotactic enchainment was found in the polymerisation of rac-lactide.

Synthesis of cyclic polylactide catalysed by bis(salicylaldiminato)tin(II) complexes.

Eight bis(salicylaldiminato)tin(II) complexes have been synthesized from the reaction of Sn[N(SiMe(3))(2)](2) and 2 equiv of the corresponding ligands at room temperature. The ligands, synthesized from salicylaldehyde and amines, were designed to have different electronic and steric properties using different amines to synthesize the tin(II) complexes as aniline (2a), 2,6-dimethylaniline (2b), 2,6-diisopropylaniline (2c), 4-methoxyaniline (2d), 4-trifluoromethylaniline (2e), methylamine (2g), and tert-butylamine (2h). Ligand variation at the salicyl group synthesized from 4-bromosalicylaldehyde and 2,6-diisopropylaniline was used to form complex 2f. Complex 2c was characterized crystallographically. All catalysts were active for the neat polymerization of L-lactide at 115 °C. At a lactide : Sn molar ratio of 10 : 1, cyclic polylactide (PLA) was obtained as demonstrated by (1)H NMR and mass spectrometry. Addition of 1 equiv of benzyl alcohol in the polymerization produced linear PLA. At a higher lactide : Sn molar ratio of 200 : 1, high molecular weight PLAs with M(n) up to 132,200 Daltons were obtained. Results from GPC coupled with light scattering detector and viscometer suggested that they are cyclic PLA. The order of reactivity based on conversion was determined to be 2c < 2b < 2a in accordance with lower steric hindrance. For electronic contribution, the order of 2e < 2a < 2d was observed in agreement with the increasing electron donation of the ligands. Complex 2g having the smallest substituents was found to be the most active catalyst.