Group 13 compounds incorporating Salen ligands

Metal Complexes in the Synthesis of Biodegradable Polymers: Achievements and Prospects

This review describes recent advances in the synthesis of homopolymers of lactide and related cyclic esters via ring-opening polymerization (ROP) in the presence of metal complexes based on group 1, 2, 4, 12, 13 and 14 metals. Particular attention is paid to the influence of the initiator structure on the properties of the obtaining homo- and copolymers. Also, a separate chapter is devoted to the study of metal complexes in the synthesis of copolymers of lactide and lactones. This review highlights the efforts made over the last ten years or so, and shows how main-group metals have received increasing attention in the field of the polymerization of lactide and related cyclic esters.

Highly emissive 4-carbazole-appended salen-indium complex: the effect of strong donor-acceptor interaction

Herein, we report our findings on 4-carbazole (CBZ)-appended salen-based indium complexes, CBZIn1 and CBZIn2, which feature diimine bridges exhibiting different electron-accepting properties. Notably, CBZIn2 exhibited a significantly higher photoluminescence quantum efficiency (PLQY, ΦPL) in toluene than CBZIn1, with a value over 15 times greater (ΦPL = 57.7% for CBZIn2; ΦPL = 3.7% for CBZIn1). In particular, in the rigid state of THF at 77 K, CBZIn2 exhibited a near-unity PLQY of 98.2%. Even in the PMMA film, CBZIn2 maintained a high level of PLQY (ΦPL = 70.2%). These results can be attributed to the highly efficient radiative decay process based on intramolecular charge-transfer (ICT) transition between the moderately twisted CBZ, characterized by its conformational rigidity and the 1,2-dicyanoethylene-bridged salen, which exhibits a strong electron-accepting ability. Furthermore, these findings are supported by theoretical calculations.

The curious cases of tetrahydrosalen-type ligands interacting with Ni(II): structures and ligand-based oxidation reactions

This work centers around the nickel complexes derived from two tetrahydrosalen-type proligands: N,N'-bis(2-hydroxybenzyl)-o-phenylenediamine (H2salophan) and N,N'-bis(2-hydroxy-3-methylbenzyl)-o-phenylenediamine (H2salophan_Me). The reaction of H2salophan with Ni(OAc)2·4H2O generates a dinuclear complex Ni2(Hsalophan)2(OAc)2 or Na[Ni2(salophan)2(OAc)] when NaOH is added to assist ligand deprotonation. The reaction of H2salophan_Me with Ni(OAc)2·4H2O, however, yields a mononuclear complex Ni(Hsalophan_Me)2. Unlike the corresponding salen-type nickel complexes, these tetrahydrosalen-type complexes are paramagentic and air sensitive (in solution). Oxidation by O2 or peroxides results in dehydrogenation of the ligand backbone to form the salen-type complexes.

Investigating of the anticancer activity of salen/salophen metal complexes based on graphene quantum dots: Induction of apoptosis as part of biological activity

This research work is the first report on the synthesis and stabilization of [Fe-Salophen] and [Fe-Salen] complexes by two methods of surface modification and anchoring of synthesized Schiff base ligand on the surface of graphene quantum dots (GQDs). The GQDs contain oxygenated functional groups that can act as non-radiative electron-hole recombination centers. Therefore removing these oxygen functional groups may improve quantum yield by reducing or deactivating the surface. In this work, GQDs with the amine functional group were synthesized with a quantum yield of 37.48%. The physicochemical properties of GQDs were investigated by Ultraviolet-visible (UV-Vis) and fluorescence spectroscopies, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Powder X-ray diffraction (PXRD), Transmission electron microscope (TEM). The synthesis of GQDs-[Fe-Salen] and GQDs-[Fe-Salophen] was evaluated by FT-IR, Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and Energy dispersive X-Ray analysis (EDX) analyses. Then, using MTT- assay, annexin V-FITC/PI, DAPI staining and cellular uptake assays, the biochemical activity of these complexes on the MCF7 cell line was investigated. The results shows that GQDs-[Fe-Salen] and GQDs-[Fe-Salophen] affect the survival of MCF7 cancer cells and, by nuclear fragmentation cause 35.77% and 19.41% of early apoptosis in cells, respectively. Also was found cellular uptake of GQDs-[Fe-Salen] is higher than that of GQDs-[Fe-Salophen].

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 (M_n up to 15,000 g mol^-1), relatively high polydispersities (Ð~1.8) and high number-average functionalities (F_n up to 85%).

Highly red-emissive salen–indium complexes: impact of 4-amino-substitution on the photophysical properties

4-NR2-appended salen–indium complexes were prepared via a one-pot synthetic pathway. The complexes exhibited narrow-bandwidth red emissions with high photoluminescence quantum yields that are the highest among the reported salen-based luminophores.

Synthesis, DFT Calculations, Antiproliferative, Bactericidal Activity and Molecular Docking of Novel Mixed-Ligand Salen/8-Hydroxyquinoline Metal Complexes

Despite the common use of salens and hydroxyquinolines as therapeutic and bioactive agents, their metal complexes are still under development. Here, we report the synthesis of novel mixed-ligand metal complexes (MSQ) comprising salen (S), derived from (2,2′-{1,2-ethanediylbis[nitrilo(E) methylylidene]}diphenol, and 8-hydroxyquinoline (Q) with Co(II), Ni(II), Cd(II), Al(III), and La(III). The structures and properties of these MSQ metal complexes were investigated using molar conductivity, melting point, FTIR, ¹H NMR, ¹³C NMR, UV–VIS, mass spectra, and thermal analysis. Quantum calculation, analytical, and experimental measurements seem to suggest the proposed structure of the compounds and its uncommon monobasic tridentate binding mode of salen via phenolic oxygen, azomethine group, and the NH group. The general molecular formula of MSQ metal complexes is [M(S)(Q)(H₂O)] for M (II) = Co, Ni, and Cd or [M(S)(Q)(Cl)] and [M(S)(Q)(H₂O)]Cl for M(III) = La and Al, respectively. Importantly, all prepared metal complexes were evaluated for their antimicrobial and anticancer activities. The metal complexes exhibited high cytotoxic potency against human breast cancer (MDA-MB231) and liver cancer (Hep-G2) cell lines. Among all MSQ metal complexes, CoSQ and LaSQ produced IC₅₀ values (1.49 and 1.95 µM, respectively) that were comparable to that of cisplatin (1.55 µM) against Hep-G2 cells, whereas CdSQ and LaSQ had best potency against MDA-MB231 with IC₅₀ values of 1.95 and 1.43 µM, respectively. Furthermore, the metal complexes exhibited significant antimicrobial activities against a wide spectrum of both Gram-positive and -negative bacterial and fungal strains. The antibacterial and antifungal efficacies for the MSQ metal complexes, the free S and Q ligands, and the standard drugs gentamycin and ketoconazole decreased in the order AlSQ > LaSQ > CdSQ > gentamycin > NiSQ > CoSQ > Q > S for antibacterial activity, and for antifungal activity followed the trend of LaSQ > AlSQ > CdSQ > ketoconazole > NiSQ > CoSQ > Q > S. Molecular docking studies were performed to investigate the binding of the synthesized compounds with breast cancer oxidoreductase (PDB ID: 3HB5). According to the data obtained, the most probable coordination geometry is octahedral for all the metal complexes. The molecular and electronic structures of the metal complexes were optimized theoretically, and their quantum chemical parameters were calculated. PXRD results for the Cd(II) and La(III) metal complexes indicated that they were crystalline in nature.

Organometallic complexes of neodymium: an overview of synthetic methodologies based on coordinating elements

Organometallic complexes of neodymium have unique coordinating ability to form both micro and macromolecules as well as metal-based polymers. These complexes have been reported in different fields and play a tremendous role in luminescence, catalytic, biological and magnetic applications. So, the current study will comprise all possible routes for the synthesis of organometallic complexes of neodymium. Neodymium complexes have been synthesized of single, double, triple and tetra linkages with H, C, N, O as well as S, B, and X. The detailed synthetic routes have been classified into four categories but in brief, neodymium forms complexes by reacting metal chloride, nitrate or oxide (hydrated or dehydrated) as precursor along with appropriate ligand. Most applied solvents for neodymium complexes were Toluene and THF. These complexes required a range of temperature based on the nature of complexes as well as linkages. The authors have surveyed the research work published through 2011–2020 and provide a comprehensive overview to understand the synthetic routes of organometallic complexes of neodymium.

Cationic aluminum, gallium, and indium complexes in catalysis

The introduction of cationic charge allows cationic group 13 complexes to be excellent Lewis acid catalysts. Cationic aluminum, gallium, and indium complexes in catalysis are comprehensively reviewed based on the reaction type.

Salen-type aluminum and zinc complexes as two-faced Janus compounds: contribution to molecular sensing and polymerization catalysis

The aim of the present review is to highlight the most recent achievements in different fields of application of salen-based zinc and aluminum complexes. More specifically this article focuses on the use of aluminum and zinc salen-type complexes as optical probes for biologically relevant molecules, as catalysts for the ring opening polymerization (ROP) of cyclic esters and co-polymerization of epoxides and anhydrides (ROCOP) and in the chemical fixation of carbon dioxide (CO2). The intention is to provide an overview of the most recent results from our group within the framework of the state-of-art-results in the literature.

Aluminum complexes with Schiff base bridged bis(indolyl) ligands: synthesis, structure, and catalytic activity for polymerization of rac-lactide

A series of Schiff base bridged bis(indolyl) ligands were developed for aluminum chemistry. The reactions of AlEt3 or AlMe3 with the Schiff base bridged bis(indolyl) proligands R1(-N[double bond, length as m-dash]CHC8H5NH)2 (R1 = -CH2CH2- (H2L1); -CH2CH2CH2- (H2L2); -CH2CMe2CH2- (H2L3); rac-Cy (H2L4); and R,R-Cy (H2L5)) were studied leading to the synthesis of a series of aluminum alkyl complexes L1AlEt (1)-L5AlEt (5) and L3AlMe (3b) in good yields, while the reaction of H2L3 with Al(OiPr)3 gave the aluminum alkoxide complex L3AlOiPr (3a). These aluminum complexes were characterized by spectroscopic methods and elemental analyses. The solid state structures of the aluminum complexes 1-5 and 3a were confirmed by the X-ray diffraction study. X-ray analyses revealed that the aluminum centre in these complexes is five-coordinated. The coordination geometry is between square pyramidal and trigonal bipyramidal. In the presence of 1 equiv. of isopropanol, the aluminum alkyl complexes exhibited notable activity towards the ring-opening polymerization of rac-lactide at 70 °C in toluene, with good control over molecular weights and dispersities. The substituents and the length of the bridging part between the two Schiff base nitrogen atoms have an influence on either the tacticity of isolated polymers or the rate of polymerization. The kinetics of complex L3AlOiPr (3a) in C6D6 was also investigated, and the experimental results revealed that the rate of polymerization was first-order with respect to rac-lactide.

Al(Salen) Metal Complexes in Stereoselective Catalysis

Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N'-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published.

Water-Soluble Sulfonate Schiff-Base Ligands as Fluorescent Detectors for Metal Ions in Drinking Water and Biological Systems

The ability to detect and selectively identify trace amounts of metal ions is of major importance for drinking water identification and biological studies. Herein, we report a series of water-soluble Schiff-base ligands capable of being fluorescent and colorimetric sensors for metal ions. Upon coordination of the metal ion to the ligand, quenching of fluorescence is observed, typically in a 1:1 ratio. The selectivity of metal ions Cu²⁺, Ni²⁺, Cr₃₊, and Co₂₊ is exhibited via fluorescence quenching accompanied by colorimetric changes, whereas that of Ag⁺ and Co²⁺ is observed through colorimetric changes alone. Additionally, pH sensing studies were performed for the potential use of these ligands in biological applications.

Homogeneous photochemical water oxidation with metal salophen complexes in neutral media

The development of water oxidation catalysts based on Earth-abundant metals that can function at neutral pH remains a basic chemical challenge.

Strong Fluorescent Lanthanide Salen Complexes: Photophysical Properties, Excited-State Dynamics, and Bioimaging

The synthesis, excited-state dynamics, and biological application of luminescent lanthanide salen complexes (Ln = Lu, Gd, Eu, Yb, salen = N, N'-bis(salicylidene)ethylenediamine-based ligands) with sandwich structures are described. Among them, Lu(III) complexes show unusually strong ligand-centered fluorescence with quantum yields up to 62%, although the metal center is close to a chromophore ligand. The excited-state dynamic studies including ultrafast spectroscopy for Ln-salen complexes revealed that their excited states are solely dependent on the salen ligands and the ISC rates are slow (10⁸-10⁹ s^-1). Importantly, time-dependent density functional theory calculations attribute the low energy transfer efficiency to the weak spin-orbital coupling (SOC) between the singlet and triplet excited states. More importantly, Lu-salen has been applied as a molecular platform to construct fluorescence probes with organelle specificity in living cell imaging, which demonstrates the advantages of the sandwich structures as being capable of preventing intramolecular metal-ligand interactions and behaviors different from those of the previously reported Zn-salens. Most importantly, the preliminary study for in vivo imaging using a mouse model demonstrated the potential application of Ln coordination complexes in therapeutic and diagnostic bioimaging beyond living cells or in vitro.

Unexpected Structure of a Helical N4 -Schiff-Base Zn(II) Complex and Its Demetallation: Experimental and Theoretical Studies

A new Zn-N₄ -Schiff base L=((±)-trans-N,N'-Bis(2-aminobenzylidene)-1,2-diaminocyclohexane) complex was synthesized and fully characterized, showing an unexpected self-assembled double-stranded helicate structure. The X-ray crystal analysis of the Zn₂ L₂ complex ((C₄₀ H₄₄ N₈ Zn₂ ,CH₂ Cl₂ , a=14.2375(3) Å, b=16.7976(4) Å, c=16.1613(4) Å, monoclinic, P2₁ /n, Z=4) shows a centrosymmetrical structure in which zinc atoms are in distorted tetrahedral environments, revealing an M- (R, R) left-handed helicity in its asymmetric unit. However, it was observed that this dinuclear complex is thermodynamically unstable in the presence of small water amounts and undergoes demetallation into free N4-Schiff base ligand and ZnO nanoparticles. This hydrolysis process was thoroughly identified and monitored through detailed ¹ H NMR, DOSY NMR analysis. The reaction mechanism of this demetallation event was elucidated by using the DFT method, involving an activation energy smaller than 13 kcal/mol. Besides, a theoretical mechanism of the demetallation process is given for the first time.

Spectrochemical Properties and Solvatochromism of Tetradentate Schiff Base Complex with Nickel: Calculations and Experiments

The nickel(II) complex with a tetradentate Schiff base ligand obtained by condensation of 1,3‐propanediamine with salicylaldehyde (H₂salpn) was studied in a variety of solvents at room temperature. The product, that is, the N′,N′′‐propylenebis(salicylaldiminato)nickel(II) ([Ni(salpn)]) complex, is brown in color in the solid state. The properties of the ligand and complex were characterized by elemental analysis, solubility in common solvents, molar conductivities, and ultraviolet (UV) and visible (Vis) spectroscopy. The [Ni(salpn)] complex is easily soluble in common solvents such as chloroform, methanol, ethanol, dimethyl formamide, dimethyl sulfoxide, acetonitrile, dioxane, acetone, 2‐propanol, and toluene—a necessary condition for observing solvatochromism. The molar conductivity values, equal to 0.0 S mol^‐1 cm² in these solvents, point to a typical non‐electrolyte behavior for this complex. Spectroscopic measurements were used to confirm the square‐planar geometry of the species in solution and to determine the coordination properties of the donor atoms and their bonding abilities (CFM/AOM parameters), as well as trichromaticity coordinate calculations. The results obtained show that the interactions of the metal with the donors depend on the polarity of the solvent.

Phenoxyaluminum(salophen) Scaffolds: Synthesis, Electrochemical Properties, and Self-Assembly at Surfaces of Multifunctional Systems

Salophens and Salens are Schiff bases generated through the condensation of two equivalents of salicylaldehyde with either 1,2-phenylenediamines or aliphatic diamines, respectively. Both ligands have been extensively exploited as key building blocks in coordination chemistry and catalysis. In particular, their metal complexes have been widely used for various catalytical transformations with high yield and selectivity. Through the modification of the phenol unit it is possible to tune the steric hindrance and electronic properties of Salophen and Salen. The introduction of long aliphatic chains in salicylaldehydes can be used to promote their self-assembly into ordered supramolecular structures on solid surfaces. Herein, we report a novel method towards the facile synthesis of robust and air-stable [Al(Salophen)] derivatives capable of undergoing spontaneous self-assembly at the graphite/solution interface forming highly-ordered nanopatterns. The new synthetic approach relies on the use of [MeAl^III (Salophen)] as a building unit to introduce, via a simple acid/base reaction with functionalized acidic phenol derivatives, selected frameworks integrating multiple functions for efficient surface decoration. STM imaging at the solid/liquid interface made it possible to monitor the formation of ordered supramolecular structures. In addition, the redox properties of the Salophen derivatives functionalized with ferrocene units in solution and on surface were unraveled by cyclic voltammetry. The use of a five-coordinate aluminum alkyl Salophen precursor enables the tailoring of new Salophen molecules capable of undergoing controlled self-assembly on HOPG, and thereby it can be exploited to introduce multiple functionalities with subnanometer precision at surfaces, ultimately forming ordered functional patterns.

A salen-Al/carbazole dyad-based guest-host assembly: enhancement of luminescence efficiency via intramolecular energy transfer

A novel class of salen-Al/carbazole dyads (D1 and D2) was synthesized and fully identified. The emission spectra of the dyads presented intriguing dual-emission patterns via an intramolecular energy transfer (IET) state in solution. Furthermore, the IET feature of the dyads was clearly observed in the rigid state. Interestingly, the emission efficiency of the dyads was enhanced by the significant IET process from the carbazole group to the salen-Al moiety. Particularly, D1 exhibited a nearly three-fold enhanced luminescence efficiency compared to the corresponding mononuclear aluminum complexes (A1). Such an emission process of these guest-host systems was further supported by theoretical calculation.

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

Tracking vesicle transport from the Golgi apparatus to lysosomes based on an Al³⁺–phospholipid coordination strategy.

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.

Highly active aluminium catalysts for room temperature ring-opening polymerisation of rac-lactide

New aluminium catalysts for room temperature rac-lactide ROP.

Synthesis and characterization of thallium–salen derivatives for use as underground fluid flow tracers

[Tl₂(salo-Bu^t)] (1, shown) and [Tl₂(saloPh-Bu^t)] (2) were synthesized and characterized for use as monitors for deep subterranean fluid flows.

Indium Catalysts for Ring Opening Polymerization: Exploring the Importance of Catalyst Aggregation

Inexorably, the environmental persistence and damage caused by polyolefins have become major drawbacks to their continued long-term use. Global shifts in thinking from fossil-fuel to renewable biobased resources have urged researchers to focus their attention on substituting fossil-fuel based polymers with renewable and biodegradable alternatives on an industrial scale. The recent development of biodegradable polyesters from ring opening polymerization (ROP) of bioderived cyclic ester monomers has emerged as a promising new avenue toward this goal. Ever increasing numbers of metal-based initiators have been reported in the literature for the controlled ROP of cyclic esters, in particular for the polymerization of lactide to produce poly(lactic acid) (PLA). PLA has several material weaknesses, which hinder its use as a replacement for commodity plastics. Despite many advances in developing highly active and controlled catalysts for lactide polymerization, no single catalyst system has emerged to replace industrially used catalysts and provide access to PLA materials with improved properties. We reported the first example of indium(III) for the ring opening polymerization of lactide. Since then, indium(III) has emerged as a useful Lewis acid in initiators for the controlled polymerization of lactide and other cyclic esters. In particular, we have developed a large family of chiral dinuclear indium complexes bearing tridentate diaminophenolate ligands and tetradentate salen and salan ligands. Complexes within our tridentate ligand family are highly active initiators for the moderately isoselective living and immortal polymerization of rac-lactide, as well as other cyclic esters. We have shown that subtle steric effects influence aggregation in these systems, with polymerization typically proceeding through a dinuclear propagating species. In addition, profound effects on polymerization activities have been observed for central tertiary versus secondary amine donors in these and other related systems. In contrast, our well-controlled and highly active chiral indium salen systems are more isoselective than the tridentate analogues and polymerize lactide via a mononuclear propagating species. Again, we have noticed that subtle steric and electronic changes to the ligand can influence both polymerization activity and stereoselectivity via aggregation phenomena. Recently, we have reported a promising new chiral indium catalyst supported by a tetradentate salan ligand. This catalyst is remarkably water and air stable and can be activated by linear and branched alcohols to provide controlled access to multiblock copolymers in air. This catalyst represents an important step forward toward generating new, commercially relevant catalysts for ROP of cyclic esters to produce novel biodegradable polymers, and highlights the unique value of indium-based catalysts in the field.