Aluminum Salen Complexes Research Articles

Aluminum Salen Complexes Modified with Unsaturated Alcohol: Synthesis, Characterization, and Their Activity towards Ring-Opening Polymerization of ε-Caprolactone and D,L-Lactide.

A highly efficient one-step approach to the macromonomer synthesis using modified aluminum complexes as catalysts of ring-opening polymerization (ROP) of ε-caprolactone and D,L-lactide was developed. The syntheses, structures, and catalytic activities of a wide range of aluminum salen complexes, 3a-c, functionalized with unsaturated alcohol (HO(CH2)4OCH=CH2) are reported. X-Ray diffraction studies revealed a tetragonal pyramidal structure for 3c. Among the complexes 3a-c, the highest activity in bulk ROP of ε-caprolactone and D,L-lactide was displayed by 3b, affording polyesters with controlled molecular weights at low monomer to initiator ratios (Mn up to 15,000 g mol-1), relatively high polydispersities (Ð~1.8) and high number-average functionalities (Fn up to 85%).

Cyclic Carbonate Synthesis from Epoxides and CO2 Catalyzed by Aluminum-Salen Complexes Bearing a nido-C2B9 Carborane Ligand.

The active and well-designed Schiff base ligands are considered "privileged ligands". The so-called salen ligands, i.e., the tetradentate [O, N, N, O] bis-Schiff base ligands, have also found broad applications in many homogeneous catalytic reactions. Modification of the salen ligands has concentrated on altering the substituents in the phenolate rings and variations in the diamine backbones. Herein, o-carborane-supported salen ligands (2) were designed and prepared. A series of aluminum-salen complexes (3·(sol)2), which were supported by the nido-C2B9 carborane anions, were synthesized. These Al(III) complexes showed high activities (TOF up to 1500 h-1) in catalyzing the cycloaddition of epoxides and CO2 at atmospheric pressure and near room temperature. Complexes 3·(sol)2 are one of the rare examples of Al-based catalysts capable of promoting cycloaddition at 1 bar pressure of CO2. Density functional theory (DFT) studies combined with the catalytic results reveal that the catalytic cycles occur on two axial sites of the Al(III) center.

Synthesis of Semi‐Aromatic Di‐Block Polyesters by Terpolymerization of Macrolactones, Epoxides, and Anhydrides

AbstractIn this contribution, the catalytic behavior of a phenoxy‐imine aluminum catalyst and of a bimetallic salen aluminum complex in ring‐opening polymerization (ROP) of macrolactones such as ω‐pentadecalactone (PDL), ω‐6‐hexadecenlactone (HDL) and ethylene brassylate (EB), and in the ring‐opening copolymerization (ROCOP) of cyclohexene oxide (CHO) and phthalic anhydride (PA) is described. A significant difference in terms of activity emerged between the two catalysts in the ROP of the macrolactones, while similar behaviors were observed in the ROCOP process. The synthesis of diblock polyesters, by combination of two distinct processes, was performed in a one‐pot procedure. The semi‐aromatic polyester block was formed first, followed by the polyethylene‐like portion produced by ROP of macrolactones.

Bifunctional Catalysis Prevents Inhibition in Reversible-Deactivation Ring-Opening Copolymerizations of Epoxides and Cyclic Anhydrides.

Reversible-deactivation chain transfer is a viable strategy to increase the catalytic efficiency of ring-opening polymerizations, such as the alternating copolymerization of epoxides and cyclic anhydrides. In conjunction with the catalyst, protic chain transfer agents (CTAs) initiate polymerization and facilitate rapid proton transfer between active and dormant chains. Functional-group-tolerant Lewis acid catalysts are therefore required to successfully apply protic CTAs in reversible-deactivation ring-opening copolymerizations (RD-ROCOP), yet the predominant binary Lewis acid catalyst/nucleophilic cocatalyst systems suffer lower polymerization rates when used with protic CTAs. New mechanistic insight into the inhibition pathways reveals that the alcohol chain ends compete with epoxide binding to the Lewis acid and hydrogen-bond with anionic chain ends to impede epoxide ring opening. We report that a bifunctional aminocyclopropenium aluminum salen complex maintains excellent activity in the presence of protic functionality, exhibiting resilience against these inhibition pathways, even at high CTA concentrations. We apply reversible-deactivation chain transfer in the bifunctional ROCOP system to demonstrate precise molecular-weight control, CTA functional group scope, and accessible polymer architectures.

Cover Feature: Aluminum(III) Salen Complexes as Active Photoredox Catalysts (Eur. J. Org. Chem. 10/2020)

Cover Feature: Aluminum(III) Salen Complexes as Active Photoredox Catalysts (Eur. J. Org. Chem. 10/2020)

Aluminum(III) Salen Complexes as Active Photoredox Catalysts

Metallosalen are privileged complexes that have found important applications in catalysis. In addition, their luminescent properties have also been studied and used for sensing and biological applications. Salen metal complexes can be efficient photosensitizers, but they can also participate to electron transfer processes. Indeed, we have found that commercially available [Al(Salen)Cl] is an efficient photoredox catalyst for the synergistic stereoselective reaction of alkyl aldehydes with different bromo ketones and malonates to give the corresponding enantioenriched α‐alkylated derivatives. The reaction was performed in the presence of a MacMillan catalyst. [Al(Salen)Cl] is able to replace ruthenium complexes, showing that also aluminum complexes can be used in promoting photoredox catalytic reactions.

Ring‐opening polymerization of (Macro)lactones by highly active mononuclear salen–aluminum complexes bearing cyclic β‐ketoiminato ligand

ABSTRACTIn this contribution, we explored the catalytic ring‐opening polymerization (ROP) of (macro)lactones using salen–aluminum complexes bearing cyclic β‐ketoiminato ligand. The effects of bridge moiety and ring size in the benzocyclane skeleton on the catalytic activity of these complexes were thoroughly investigated. Complex 5 with 2,2‐dimethylpropylene bridge and five‐membered cyclane ring can efficiently catalyze the ROP of ω‐pentadecalactone (ω‐PDL), showing higher catalytic activity (turnover frequency [TOF] up to 309.2 h−1) than the typical Al‐salen analogs bearing salicylaldiminato ligand (TOF = 227.2 h−1). Thus, polyethylene‐like polyester with high‐molecular weight (up to 164.5 kg/mol) could be easily prepared under optimal conditions. In addition, complex 5 can also catalyze the ROP of lactide (LA) and ε‐caprolactone (ε‐CL) with extremely high activity (TOF is high up to 147.6 h−1 and 4752 h−1, respectively). Here, we demonstrated a rare mono‐nuclear salen‐Al complex that can prompt the ROP of (macro)lactones with unprecedentedly high efficiency. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 973–981

Breaking the Paradox between Catalytic Activity and Stereoselectivity: rac-Lactide Polymerization by Trinuclear Salen–Al Complexes

Polylactide (PLA), the popular bio-based plastics, has attracted more interest as a benign alternative for oil-based plastics. Lowering the manufacture cost of PLA and improving the performance of PLA products are the essential challenges in PLA study. PLA stereocomplex is a promising choice to improve the thermal and mechanical properties. Efficient preparation of the PLA stereocomplex by the direct ring-opening polymerization (ROP) of inexpensive racemic lactide (rac-LA) requires the ROP catalysts with high activity and stereoselectivity. For this purpose, the novel trinuclear Salen–aluminum complexes were prepared in this work and exerted extraordinary improvement of catalytic activity and stereoselectivity at the same time. These complexes showed the excellent isoselectivity (typically Pm = 0.98), high activity (typically kp = 15.4 L mol–1 min–1), and low catalyst loading amount (0.01 mol %). The Tm of PLA was 219.9 °C, which is the highest value among the reported PLA derived directly from rac-LA unt...

A luminescent aluminium salen complex allows for monitoring dynamic vesicle trafficking from the Golgi apparatus to lysosomes in living cells.

The Golgi apparatus is well-known as the center of vesicle trafficking whose malfunction might cause the breakdown of overall cellular architecture and ultimately cell death. The development of fluorescent probes to not only precisely stain the Golgi apparatus but also monitor dynamic vesicle trafficking is of great significance. While fluorescent proteins and fluorescent lipid analogs have been reported, they are sometime limited by either overexpression and toxicity or lack of high selectivity, respectively. We herein report a novel approach based on metal-induced coordination between the phosphate anions of phospholipids and the metal center of a luminescent Alsalen complex AlL, which can in situ track membrane vesicle trafficking from the Golgi apparatus to the lysosomes in living cells. This work opens a new avenue for designing luminescent metal probes based on the Lewis acidity of metal ions and allows the use of metal ions with different charge states, polarities, and reactivities within a similar structural scaffold to expand coordination chemistry for biological studies.

Functionalized Polyesters with Tunable Degradability Prepared by Controlled Ring-Opening (Co)polymerization of Lactones

Various well-defined diblock copolymers with poly(α-methylene−β-butyrolactone) (PMβBL) segment were prepared by salen–aluminum complexes or aminoalkoxybis(phenolate)yttrium amido complex mediated living ring-opening polymerization of racemic α-methylene-β-butyrolactone (rac-MβBL) and different lactones via one-pot, two-step method. These diblock copolymers all consist of semicrystalline and amorphous segments, possessing both a melting point and a glass transition temperature. With the use of simple salenAlMe with bulky (tert-butyldimethylsilyl) groups on the phenolate ortho position as catalyst in conjunction with equivalent benzyl alcohol, MβBL and β-butyrolactone (BL) have nearly identical reactivity in their copolymerization, affording the copolymers P(MβBL-ran-BL) with random distributions of the two monomers. Notably, these copolymers exhibited tunable degradability in the presence of Lewis base, dependent on the MβBL unit content in the random copolymers. Moreover, the vinylidene groups in P(MβBL-r...

Correction to Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes.

Correction to Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes.

Crystalline Polyesters from CO2 and 2-Butyne via α-Methylene-β-butyrolactone Intermediate

The selective transformation of carbon dioxide into useful chemicals has attracted much attention in recent decades due to the economic and environmental benefits arising from the utilization of renewable source. Nevertheless, the reactions on incorporating CO2 into polymeric materials are very limited. The copolymerization of CO2 and alkynes to synthesize degradable polyesters is a thermodynamically unfavorable process, since the propagation step involving CO2 is a major obstacle. Herein, we report a strategy to conquer the thermodynamic and kinetic barriers for the copolymerization of CO2 and 2-butyne via α-methylene-β-butyrolactone (MβBL) intermediate. Subsequent ring-opening polymerization of the lactone intermediate mediated by achiral Salen aluminum complexes afforded syndiotactic-enriched polyesters with controllable molecular weight and narrow polydispersity. Notably, the resultant syndiotactic-enriched poly(MβBL) is a typical semicrystalline material. The present method provides access to a new c...

ChemInform Abstract: Lewis Acid—Base Bifunctional Aluminum—Salen Catalysts: Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides.

AbstractThe Lewis acid—base bifunctional monometallic aluminum—salen complexes (I) and (II) are prepared, characterized and shown to be efficient and recyclable homogeneous catalysts for the organic solvent‐free synthesis of cyclic carbonates from epoxides and CO2.

Copolymerization of l-lactide and ε-caprolactone catalyzed by mono-and dinuclear salen aluminum complexes bearing bulky 6,6′-dimethylbipheyl-bridge: random and tapered copolymer

Mono- and dinuclear salen aluminum complexes bearing a racemic 6,6′-dimethyl-biphenyl bridge could efficiently catalyze the copolymerization of l-LA and ε-CL in the melt, producing practically random and tapered copolymers, respectively.

Bimetallic salen aluminum complexes: cooperation between reactive centers in the ring-opening polymerization of lactides and epoxides.

Three dinuclear aluminum alkyl complexes of the general formula LAl2Me4, where L are salen ligands with an alkyl backbone of different lengths between the nitrogen atoms (1,3-propylene (1), 1,5-pentylene (2) and 1,12-dodecaylene (3)), have been prepared through alkane elimination reactions between each ligand and two equivalents of AlMe3. The related hemi-salen aluminum complex 4 was prepared by an analogous reaction between a phenoxy-imine ligand and a single equivalent of AlMe3. The activities of these aluminum complexes in the ring-opening polymerization (ROP) of rac-lactide and of several epoxides have been investigated and compared. The dinuclear complex 1, bearing the salen ligand with the shortest alkyl bridge, was the most active in the ROP of LA producing isotactic enriched PLA. Otherwise, the other complexes (2 and 3), in which the metal centers are remote, produced atactic PLA with inferior activity. Analogous differences in terms of activity emerged in the ROP of epoxides. The comparison of the catalytic behavior of the dinuclear complexes as well as their mononuclear counterparts suggests the cooperation between the two aluminum metal centers of the dinuclear species in which these are close enough.

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.

Non-symmetrical aluminium salen complexes: Synthesis and their reactivity with cyclic ester

Non-symmetrical aluminium salen complexes that contained different substituents were designed and synthesized. All the ligands and their complexes were characterized by 1H, 13C NMR and elemental analysis. These complexes can be used as catalysts to produce polylactide and poly-ε-caprolactone. All polymerizations were living and molar mass distributions were narrow. The Mn(obsd) of the isolated polymers were in good agreement with Mn(calcd). The polymerization rate of electrophilic substituted complex was higher than the non-electrophilic substituted analogies. The bulky substituents with more steric hindrance of the complexes had relatively lower activity. Kinetic studies showed that the polymerizations were both first-ordered with respect to lactide and ε-caprolactone monomers.

Effect of the phenyl ring substituent on stereoselectivity in the ring-opening polymerization of the rac-lactide initiated by salen aluminum complexes

A number of unreported salen aluminum complexes bearing Schiff base ligands starting from (R,R)-1,2-diammoniumcyclohexane mono-(+)-tartrate salt were synthesized. These complexes were characterized by 1H, 13C NMR, and elemental analysis. These complexes were employed as initiators for the ring-opening polymerization (ROP) of L-lactide and rac-lactide. Complex 3 (R = Br) showed the highest activity for the ROP of L-lactide among these complexes, and complex 2 (R = iPr) possessed the highest stereoselectivity for the ROP of rac-lactide among these aluminum isopropoxides. The kinetics studies of the polymerization employed complex 2 as initiator indicated that the polymerization rate was first-ordered in lactide and initiator.

ChemInform Abstract: Asymmetric Hydrocyanation of Nitroolefins Catalyzed by an Aluminum(III) Salen Complex.

Chiral AlIII salen complexes were synthesized and used as catalysts for the asymmetric hydrocyanation of nitroolefins using 4-phenylpyridine N-oxide as an additive and trimethylsilyl cyanide (TMSCN) as a source of cyanide. An excellent yield of β-nitronitrile (87 %) and enantioselectivity (90 %) were achieved if (2-nitrovinyl)cyclohexane was used as a substrate at −15 °C in 16 h. To understand the interaction of the AlIII salen complex and additive, NMR and IR spectroscopic studies revealed that 4-phenylpyridine N-oxide acts both as an axial ligand and helps to activate the cyanide source TMSCN, which thereby increases the reactivity. A catalytic cycle was proposed based on the spectroscopic studies.

Alternating Copolymerization of Propylene Oxide with Biorenewable Terpene‐Based Cyclic Anhydrides: A Sustainable Route to Aliphatic Polyesters with High Glass Transition Temperatures

The alternating copolymerization of propylene oxide with terpene-based cyclic anhydrides catalyzed by chromium, cobalt, and aluminum salen complexes is reported. The use of the Diels-Alder adduct of α-terpinene and maleic anhydride as the cyclic anhydride comonomer results in amorphous polyesters that exhibit glass transition temperatures (Tg ) of up to 109 °C. The polymerization conditions and choice of catalyst have a dramatic impact on the molecular weight distribution, the relative stereochemistry of the diester units along the polymer chain, and ultimately the Tg of the resulting polymer. The aluminum salen complex exhibits exceptional selectivity for copolymerization without transesterification or epimerization side reactions. The resulting polyesters are highly alternating and have high molecular weights and narrow polydispersities.