Investigations on the Synthesis of Chiral Ionic-Liquid-Supported Ligands and Corresponding Transition-Metal Catalysts: Strategy and Experimental Schemes

Ionic liquids have been utilized in numerous significant applications within the field of chemistry, particularly in organic chemistry, due to their unique physical and chemical properties. In the realm of asymmetric transition-metal-catalyzed transformations, chiral ionic-liquid-supported ligands and their corresponding transition-metal complexes have facilitated these processes in unconventional solvents, especially ionic liquids and water. These innovative reaction systems enable the recycling of transition-metal catalysts while producing optically active organic molecules with comparable or even higher levels of chemo-, regio-, and stereoselectivity compared to their parent catalysts. In this short review, we aim to provide an overview of the structures of chiral ionic-liquid-supported ligands and the synthetic pathways for these ligands and catalysts. Various synthetic methodologies are demonstrated based on the conceptual frameworks of diverse chiral ionic-liquid-supported ligands. We systematically present the structures and comprehensive synthetic pathways of the chiral ionic-liquid-supported ligands and the typical corresponding transition-metal complexes that have been readily applied to asymmetric processes, categorized by their parent ligand framework. Notably, the crucial experimental procedures are delineated in exhaustive detail, with the objective of enhancing comprehension of the pivotal aspects involved in constructing chiral ionic-liquid-tagged ligands and compounds for both scholars and readers. Considering the current limitations of such ligands and catalysts, we conclude with remarks on several potential research directions for future breakthroughs in the synthesis and application of these intriguing ligands.

Boron-Salphen Conjugate based Molecular Probe Exhibiting Fluorescence On-Off-On Response in Detection of Cu2+ and ATP through Displacement Approach

Synthesis and photophysical properties of a fluorescent probe HBD is described. Probe upon interaction with metal ions, anions and nucleoside pyrophosphates (NPPs) showed fluorescence quenching with Cu2+ due to chelation enhanced quenching effect (CHEQ). Moreover, interaction of ensemble HBD.Cu2+ with anions and NPPs showed fluorescence "turn-On" response with ATP selectively. "On-Off-On" responses observed with Cu2+ and ATP is attributed to an interplay between ESIPT and TICT processes. Cyclic voltammogram of probe exhibited quasi-reversible redox behaviour with three oxidation and two reduction potentials and the change in band gaps of probe suggested the interaction with Cu2+ and ATP. The 2 : 1 and 1 : 1 binding stoichiometry for an interaction between probe and Cu2+ (LOD, 62 nM) and ensemble, HBD.Cu2+ with ATP (LOD, 0.4 μM) respectively are realised by Job's plot and HRMS data. Cell imaging studies carried out to detect Cu2+ and ATP in HeLa cells. Also, the output emission observed with Cu2+ and ATP is utilized to construct an implication (IMP) logic gate. Test paper strips showed naked-eye visible color responses to detect Cu2+ and ATP. In real water samples probe successfully detected copper (0.03 μM) between 5-6.5 ppb level (ICP-MS method).

Synthetic and Mechanistic Studies into the Reductive Functionalization of Nitro Compounds Catalyzed by an Iron(salen) Complex

We report on the use of a simple, bench-stable [Fe(salen)2]-μ-oxo precatalyst in the reduction of nitro compounds. The reaction proceeds at room temperature across a range of substrates, including nitro aromatics and aliphatics. By changing the reducing agent from pinacol borane (HBpin) to phenyl silane (H3SiPh), we can chemoselectively reduce nitro compounds while retaining carbonyl functionality. Our mechanistic studies, which include kinetics, electron paramagnetic resonance (EPR), mass spectrometry, and quantum chemistry, indicate the presence of a nitroso intermediate and the generation of an on-cycle iron hydride as a key catalytic intermediate. Based on this mechanistic insight, we were able to extend the chemistry to hydroamination and identified a simple substrate feature (alkene lowest unoccupied molecular orbital (LUMO) energy) that could be used to predict which alkenes would result in productive catalysis.

Synthesis, Crystal Structure, Fluorescence and Theoretical Calculations of Three Zn(II)/Cd(II) Complexes with Bis-dentate N,N-Quinoline Schiff Base

Three d10 metal complexes, ZnL(OAc)2 (1), CdL(OAc)2 (2) and [CdL2(NO3)2]·CH3CN (3) were synthesized using the ligand (E)-N-(3-methoxy-4-methylphenyl)-1-(quinolin-2-yl)methanimine (L) and characterized by FT-IR spectra, NMR spectra, and CHN elemental analysis. Single-crystal X-ray diffraction analysis revealed that complexes 1 and 2 are isostructural, with the central metal adopting a hexacoordinate octahedral geometry, while complex 3 adopts a triangular dodecahedron geometry. Thermal gravimetric analysis showed that these complexes exhibit good thermal stability. Solid-state fluorescence spectroscopy measurements demonstrated that complexes 1-3 exhibit bright yellow-green fluorescence (λem = 564 nm for 1; 524 nm for 2; 542 nm for 3), suggesting their potential as photoluminescent materials. Furthermore, DFT calculations, including frontier molecular orbitals, energy levels, and surface electrostatic potential, provided insights into the structural and electronic spectral properties of complexes 1-3.

Zn-Salphen Acrylic Films Powered by Aggregation-Induced Enhanced Emission for Sensing Applications

Zn(II) complexes possess attractive characteristics for supramolecular chemistry, catalysis, and optoelectronic applications, while Zn-Salphen counterparts are also suitable as chemical sensors, although limited by solution-based to date. In this study, we report the synthesis of new polymers from methyl methacrylate, n-butyl acrylate, and a non-symmetrical Zn-Salphen complex. We show that this low-fluorescent complex exhibits aggregation-induced emission enhancement (AIEE) properties and that, the incorporation of AIEE complexes into a polymeric matrix make it possible to achieve fluorescent films with enhanced fluorescence suitable for sensing applications. As a proof of concept, these films could detect acetic acid, showing a decrease of up to 73% in the original fluorescence. Host/guest studies showed a subtle disruption of the emission in aggregates upon treatment with anion guests. These results indicate that an interaction between the guest and Zn-Salphen complex may occur, stabilizing or destabilizing the complex and causing a concomitant increase or decrease in emission.

Altering the Localization of an Unpaired Spin in a Formal Ni(V) Species

The participation of both ligand and the metal center in the redox events has been recognized as one of the ways to attain the formal high valent complexes for the late 3d metals, such as Ni and Cu. Such an approach has been employed successfully to stabilize a Ni(III) bisphenoxyl diradical species in which there exist an equilibrium between the ligand and the Ni localized resultant spin. The present work, however, broadens the scope of the previously reported three oxidized equivalent species by conveying the approaches that tend to affect the reported equilibrium in CH3 CN at 233 K. Various spectroscopic characterization revealed that employing exogenous N-donor ligands like 1-methyl imidazole and pyridine favors the formation of the Ni centered localized spin though axial binding. In contrast, due to its steric hinderance, quinoline favors an exclusive ligand localized radical species. DFT studies shed light on the novel intermediates' complex electronic structure. Further, the three oxidized equivalent species with the Ni centered spin was examined for its hydrogen atom abstraction ability stressing their key role in alike reactions.

Flexible and sewable electrode based on Ni-Co@PANI-salphen composite-coated on textiles for wearable supercapacitor

Flexible electrodes with high deformability and energy density are critical for electronic textiles. The key factor for achieving high-performance supercapacitors with superior power and energy density is the evaluation of materials that exhibit exceptional capacitive performance. Herein, we have prepared Ni-Co nanoparticles at the surface of polyaniline-salphen (Ni-Co@PS). Then, followed by casting Ni-Co@PS on a conductive carbon cloth (CC) as a substrate through a facile in-situ polymerization strategy. The morphologies of Ni-Co@PS composite were characterized by different methods such as FE-SEM, XPS, XRD, BET, and electrochemical methods. This nanocomposite showed high tolerability and a large surface area with excellent behavior as a new nanomaterial for supercapacitor application. Thus, the optimum composite designed with a metal ratio (nickel-cobalt 3:1 w/w) satisfactorily possesses a specific capacitance of up to 549.994 C g-1 (1447.2 F g-1) under 0.5 A g-1 and long-term cyclic stability featuring capacity retention of 95.9% after 5000 cycles at a current density of 9.0 A g-1. The Ni-Co@PS-CC, is a material with great potential as an electrode in asymmetric wearable supercapacitor (AWSC) apparatus, demonstrating a remarkable specific capacity of 70.01, and accompanied by an energy density of 23.46 Wh k g-1 at a power density of 800 W k g-1.

Diversifying the functions of heme proteins with non-porphyrin cofactors

Heme proteins perform diverse biochemical functions using a single iron porphyrin cofactor. This versatility makes them attractive platforms for the development of new functional proteins. While directed evolution and metal substitution have expanded the properties, reactivity, and applications of heme proteins, the incorporation of porphyrin analogs remains an underexplored approach. This review discusses the replacement of heme with non-porphyrin cofactors, such as porphycene, corrole, tetradehydrocorrin, phthalocyanine, and salophen, and the attendant properties of these conjugates. While structurally similar, each ligand exhibits distinct optical and redox properties, as well as unique chemical reactivity. These hybrids serve as model systems to elucidate the effects of the protein environment on the electronic structure, redox potentials, optical properties, or other features of the porphyrin analog. Protein encapsulation can confer distinct chemical reactivity or selectivity of artificial metalloenzymes that cannot be achieved with the small molecule catalyst alone. Additionally, these conjugates can interfere with heme acquisition and uptake in pathogenic bacteria, providing an inroad to innovative antibiotic strategies. Together, these examples illustrate the diverse functionality that can be achieved by cofactor substitution. The further expansion of this approach will access unexplored chemical space, enabling the development of superior catalysts and the creation of heme proteins with emergent properties.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Group 13 salphen compounds (In, Ga and Al): a comparison of their structural features and activities as catalysts for cyclic carbonate synthesis

Many complexes based on group 13 elements have been successfully applied as catalysts for the synthesis of cyclic carbonates from epoxides and CO₂ and to date these have provided some of the most active catalysts developed. It is notable that most reports have focused on the use of aluminium-based compounds likely because of the well-established Lewis acidity of this element and its cost. In comparison, relatively little attention has been paid to the development of catalysts based on the heavier group 13 elements, despite their known Lewis acidic properties. This study describes the synthesis of aluminium, gallium and indium compounds supported by a readily prepared salphen ligand and explores both their comparative structures and also their potential as catalysts for the synthesis of cyclic carbonates. In addition, the halide ligand which forms a key part of the compound has been systematically varied and the effect of this change on the structure and catalytic activity is also discussed. It is demonstrated that the indium compounds are actually, and unexpectedly, the most active for cyclic carbonate synthesis, despite their lower Lewis acidity when compared to their aluminium congeners. The experimental observations from this work are fully supported by a Density Functional Theory (DFT) study, which provides important insights into the reasons as to why the indium catalyst with bromide, [InBr(salphen)], is most active.

Lewis Acidic Zinc(II) Complexes of Tetradentate Ligands as Building Blocks for Responsive Assembled Supramolecular Structures

This review presents representative examples illustrating how the Lewis acidic character of the Zn(II) metal center in Zn(salen)-type complexes, as well as in complexes of other tetradentate ligands, and the nature of the medium govern their supramolecular aggregation, leading to the formation of a variety of supramolecular structures, either in solution or in the solid state. Stabilization of these Lewis acidic complexes is almost always reached through an axial coordination of a Lewis base, leading to a penta-coordinated square-pyramidal geometry around the metal center. The coverage is not exhaustive, mainly focused on their crystallographic structures, but also on their aggregation and sensing properties in solution, and on their self-assembled and responsive nanostructures, summarizing their salient aspects. The axial ligands can easily be displaced, either in solution or in the solid state, with suitable Lewis bases, thus being responsive supramolecular structures useful for sensing. This contribution represents the first attempt to relate some common features of the chemistry of different families of Zn(II) complexes of tetradentate ligands to their intrinsic Lewis acidic character.

Ultrasensitive NO Sensor Based on a Nickel Single-Atom Electrocatalyst for Preliminary Screening of COVID-19

A new coronavirus, SARS-CoV-2, has caused the coronavirus disease-2019 (COVID-19) epidemic. A rapid and economical method for preliminary screening of COVID-19 may help to control the COVID-19 pandemic. Here, we report a nickel single-atom electrocatalyst that can be printed on a paper-printing sensor for preliminary screening of COVID-19 suspects by efficient detection of fractional exhaled nitric oxide (FeNO). The FeNO value is confirmed to be related to COVID-19 in our exploratory clinical study, and a machine learning model that can accurately classify healthy subjects and COVID-19 patients is established based on FeNO and other features. The nickel single-atom electrocatalyst consists of a single nickel atom with N₂O₂ coordination embedded in porous acetylene black (named Ni-N₂O₂/AB). A paper-printed sensor was fabricated with the material and showed ultrasensitive response to NO in the range of 0.3-180 ppb. This ultrasensitive sensor could be applied to preliminary screening of COVID-19 in everyday life.

Variation of the Emission Efficiency and Wavelength from Fluorescent Zinc Salen Complexes upon Systematic Structural Modifications

Understanding the photophysical properties of metal salen complexes is not straightforward because the emission efficiency is altered irregularly upon structural modifications. The present study prepared zinc salen complexes with systematic structural variations to pinpoint critical factors to determine the emission efficiency. One of the important experimental observations is the regiochemistry of a phenolate substituent affecting emission efficiency from a salicylidene fluorophore, which is nicely assigned as arising from the photoexcited electronic structure of metal salen complexes. Another significant finding is the thermal fluctuation of a salen ligand arising from the mismatched ligand-metal interaction, which has a significant impact on fluorescence lifetime. The present study sheds light on hidden factors that alter photophysical properties of a metal salen complex, which provide valuable insights into designing new photoactive salen ligands.

Saccharide-Functionalized Poly(Zn-salphen)-alt-(m- and p-phenyleneethynylene)s as Dynamic Helical Metallopolymers

The study of metallopolymers with controllable helical sense remains in its infancy. We report arabinose-functionalized (Zn-salphen)-based conjugated polymers that display mirror-image circular dichroism spectra for L- and D-sugar sidechains respectively, signifying ordered (helical) coiling of the polymer backbone with opposite screw-sense preferences. The observation of different spectroscopic behavior and Cotton effects for a variety of solvents (in a reversible manner) and temperatures, ascribed to changes in the extent of intrachain (Zn⋅⋅⋅O(salphen) and π-stacking) interactions between Zn-salphen moieties, thus indicate the flexible, responsive and dynamic nature of the folded helical conformation in these systems. An application study signifying that activity can be governed by the structure and helical sense of the polymer is described.

2D Salphen-based heteropore covalent organic frameworks for highly efficient electrocatalytic water oxidation

The construction of heteroporous covalent organic frameworks (COFs) is still a challenge. Herein, a series of 2D COFs with hexagonal and quadrilateral pores were constructed via in situ salphen or metal salphen formation. Metallized salphen-based COFs can be used as electrocatalysts towards water oxidation with an overpotential of 266 mV at 10 mA cm^-2.

Theoretical Studies on Bimetallic Salen Complexes Catalyzed Epoxide Hydration: Effects of Metal Centers, Substrates, and Ligands

To provide guiding information for developing efficient and stable catalysts for epoxide hydration, we investigated the mechanism of propylene oxide (PO) to 1,2-propylene glycol (PG) using density functional theory (DFT) calculations. The mechanism was identified to follow the cooperative bimetallic mechanism in which a metal-salen complex activated H₂O attacks the middle carbon atom of a metal-salen complex activated PO from the oxygen side of three-membered ring. Analyses reveal that the distortion energy correlates linearly with the barrier, and the hydrogen bonding between H₂O and PO increases from reaction precursors to transition states. A nice linear relationship exists between the ratio of square root of ionic potential to the square of the distance from the metal ion spherical surface to the oxygen atom center of PO. It is demonstrated that the substrates with larger polarizability tend to have lower hydration barriers and the influence of ligands is less than that of metal centers and substrates. Modifying metal ions is the first choice for developing metal-salen catalysts, and metal ions with more formal charges and larger radius are expected to exhibit high activity. These findings shed lights on the mechanism and provide guiding information for developing efficient metal-salen catalysts for epoxide hydration.

Morphology Engineering of Fullerene[C70 ] Microcrystals: From Perfect Cubes, Defective Hoppers to Novel Cruciform-Pillars

Controlled crystallization of fullerene molecules into ordered molecular assemblies is important for their applications. However, the morphology engineering of fullerene[C₇₀ ] assemblies is challenging, and complicated architectures have rarely been reported due to the low molecular symmetry of C₇₀ molecules, which makes their crystallization difficult to control and the low production yield as well. Herein, with the assistance of solvent intercalation, a general reprecipitation approach is reported to prepare morphologically controllable C₇₀ microcrystals with mesitylene as a good solvent and n-propanol as a poor solvent in one solvent system without replacing specific solvents. A series of C₇₀ microcrystals with high uniformity from perfect cubes and defective hoppers to novel cruciform-pillars are obtained by intentionally tuning C₇₀ concentration and the volume ratio of mesitylene to n-propanol. Among them, novel cruciform-pillar-shaped microcrystals are obtained for the first time by further decreasing the amount of mesitylene in the solvent-intercalated microcrystals. Notably, the C₇₀ concentration is a key parameter for the selective growth of C₇₀ hopper, rather than the volume ratio of mesitylene to n-propanol. Interestingly, the hopper-shaped microcrystals exhibit excellent photoluminescence properties relative to those of cubes and cruciform-pillars owing to the enhanced light absorption, proving their potential applications in optoelectronic devices. This study offers new insights into the morphology-controlled synthesis of other micro/nanostructured organic microcrystals and the fine tuning of photoluminescence properties of organic crystals.

Lewis acidic zinc(II) salen-type Schiff-base complexes: sensing properties and responsive nanostructures

In this frontier article some peculiar characteristics of Zn(salen)-type Schiff-base complexes are reviewed. The paper is mainly focused on the most recent and relevant achievements on responsive supramolecular nanostructures and sensing properties, both of them related to the Lewis acidic character of the ZnII centre in these molecular species, providing an interpretation of these features. The sensing properties of Zn(salen)-type complexes mainly originate from optical spectroscopic changes associated with the formation of the adducts upon addition of a Lewis base (analyte), either by deaggregation of dimeric species or displacement of the solvent coordinated to the metal centre. In both cases the direct sensing is related either to the Lewis acidic character of the complex as well as to the Lewis basicity of the analyte. The formation of responsive nanostructures with fluorescent, and/or vapochromic, mechanochromic, and thermochromic characteristics is driven by non-mutual intermolecular ZnO interactions, further stabilized by π-π stacking interactions and/or interdigitation of the alkyl side groups. The Lewis acidic character is not a prerogative of Zn(salen)-type complexes of tetradentate Schiff-bases. Many other classes of ZnII complexes can possess this property. A correct interpretation of their chemistry is certainly useful for further development of these classical coordination compounds as new molecular materials.

Triptycene-supported bimetallic salen porous organic polymers for high efficiency CO2 fixation to cyclic carbonates

Triptycene units in bimetallic salen POPs are envisaged to support the alignment of bimetallic salen macrocycles in side walls of channels for exposing more metal active sites resulting in the high efficiency coupling reaction of epoxides with CO₂.

Manganese(ii) Schiff-base-mediated reversible deactivation controlled radical polymerization of vinyl acetate

The manganese Schiff-base complexes were applied in the OMRP of vinyl acetate.

Schiff-base [4]helicene Zn(II) complexes as chiral emitters

The controlled preparation of chiral emissive transition metal complexes is fundamental in the field of circularly polarized luminescence (CPL) active molecular materials. For this purpose, enantiopure Zn(ii) complexes 1 and 2 based on a tetradentate salen ligand surrounded by [4]helicene moieties, together with their racemic counterpart 3, have been herein synthesized. Chirality is primarily brought about by chiral 1,2-cyclohexane-diamines. Alternatively, achiral complex 4 based on ortho-phenylene-diamine has been prepared as well. Single crystal X-ray diffraction analyses have been performed on helicenic intermediates 8 and 9 and complexes 1 and 4. Complexes 1 and 4 display the typical tetradentate O,N,N,O coordination around Zn(ii) characteristic of salen ligands, and bear two [4]helicene moieties. The zinc complexes are luminescent in the visible range around 560 nm at room temperature in aerated solutions with the QY reaching ca. 15% for a luminescence lifetime of 5.5 ns. The optical activities of these complexes have been assessed by CD and CPL, and compared to DFT calculations.

Pt(II) and Au(III) complexes containing Schiff-base ligands: A promising source for antitumor treatment

The effective application of cisplatin in the clinic as an antitumor treatment has stimulated widespread interest in inorganic metal drugs. In particular, complexes containing the transition metals platinum and gold have attracted considerable attention due to their antitumor effects. The Pt(II) and Au(III) Schiff-base complexes are potential antitumor agents because of their remarkable biological activities and good stability, lipophilicity, and electroluminescent properties. These complexes act via various antitumor mechanisms that are unlike those of the classic platinum drugs, providing a feasible solution for improving the serious side effects caused by metal chemotherapy. In this review, promising antitumor agents based on Pt(II) and Au(III) complexes containing Schiff-base ligands, and their biological targets, including G-quadruplex DNA and thioredoxin reductase, are comprehensively summarized.

Acrylic Polymers Containing a Nickel Salphen Complex: An Approach to Supramolecular and Macromolecular Systems

The synthesis, characterization and crystallographic analysis is reported of a new Nickel Salphen complex and its radical copolymerization with n-butyl acrylate and methyl methacrylate to produce novel host macromolecules with tunable association against guest anions. Spectrophotometric titrations of the complex and of the polymers revealed that a supramolecular regulation of guest-binding accessibility was enabled by the number of Ni-Salphen units per chain. The latter content in turn, determined the chain size and molecular weight uniformity upon polymerization, and likely increased the strength in interchain/intrachain non-covalent interactions over the nickel center and the acrylic domains. The study also showed that incorporation of the monomer into the acrylic polymer backbone opened the possibility for the nickel binding site to gain access to host:guest stoichiometric discrimination, switching from 1 : 1 (major) and 1 : 2 (minor) both coexisting for the host when in the free form, to mostly 1 : 2 when in the polymerized version.

Anion-Dominated Copper Salicyaldimine Complexes-Structures, Coordination Mode of Nitrate and Decolorization Properties toward Acid Orange 7 Dye

A salicyaldimine ligand, 3-tert-butyl-4-hydroxy-5-(((pyridin-2-ylmethyl)imino)methyl)benzoic acid (H₂L_salpyca) and two Cu(II)−salicylaldimine complexes, [Cu(HL_salpyca)Cl] (1) and [Cu(HL_salpyca)(NO₃)]_n (2), have been synthesized. Complex 1 has a discrete mononuclear structure, in which the Cu(II) center is in a distorted square-planar geometry made up of one HL_salpyca⁻ monoanion in an NNO tris-chelating mode and one Cl⁻ anion. Complex 2 adopts a neutral one-dimensional zigzag chain structure propagating along the crystallographic [010] direction, where the Cu(II) center suits a distorted square pyramidal geometry with a τ value of 0.134, consisted of one HL_salpyca⁻ monoanion as an NNO tris-chelator and two NO₃⁻ anions. When the Cu∙∙∙O semi coordination is taken into consideration, the nitrato ligand bridges two Cu(II) centers in an unsymmetrical bridging-tridentate with a μ, κ⁴O,O′:O′,O″ coordination. Clearly, anion herein plays a critical role in dominating the formation of discrete and polymeric structures of copper salicyaldimine complexes. Noteworthy, complex 2 is insoluble but highly stable in H₂O and various organic solvents (CH₃OH, CH₃CN, acetone, CH₂Cl₂ and THF). Moreover, complex 2 shows good photocatalytic degradation activity and recyclability to accelerate the decolorization rate and enhance the decolorization performance of acid orange 7 (AO7) dye by hydrogen peroxide (H₂O₂) under daylight.

Organic polymorphs based on an AEE-active tetraphenylethene salicylaldehyde Schiff-base derivative: the effect of molecular conformation on luminescence properties

An aggregation-enhanced emission (AEE)-active tetraphenylethene salicylaldehyde Schiff-base derivative, TPE-Nap, was prepared using a facile synthesis. The AEE property of TPE-Nap was studied by luminescence and absorption spectra, and was attributed to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N isomerization restriction and the excited-state intramolecular proton transfer (ESIPT) process. Polymorphs TPE-Nap-Y and TPE-Nap-O were prepared from TPE-Nap, and their emission color and intensity were compared. TPE-Nap-Y is a yellow block crystal with a very weak yellow emission, with its main peak at 565 nm, while TPE-Nap-O is an orange plate crystal that gave a stronger orange emission, with its main peak at 583 nm. Single crystal diffraction data were used to demonstrate the structure–property relationship. The most unique feature was that the torsion angle of TPE-Nap-Y between the benzene ring of the TPE unit and the Nap unit was 54.08°, while that of TPE-Nap-O was 14.19°. Interestingly, the TPE unit assumed propeller-like nonplanar conformations that likely led to different intermolecular interactions, such as C–H⋯O interactions (2.529 Å and 2.617 Å) in TPE-Nap-O and C–H⋯π interactions (3.224 Å and 3.791 Å) in TPE-Nap-Y. These were influenced by the torsion angle, although the molecules in both crystals were arranged in a similar end-to-end slip-stacking mode. These results inferred that the molecular conformation was evidently affected by luminescent properties. Crystals possessing a slightly twisted molecular conformation exhibited stronger emission than those possessing a heavily twisted molecular conformation. These investigations will expand the research on the relationship between the molecular conformation and the emission properties of organic solids, and might provide a new development strategy for organic polymorphs.

Organic polymorphs displaying different emission colors and intensities were obtained from aggregation-enhanced emission (AEE)-active tetraphenylethene derivatives, and their luminescent properties were affected mainly by molecular conformation.

Cathecol and Naphtol Groups in Salphen-Type Schiff Bases for the Preparation of Polynuclear Complexes

In this paper, we show a strategy to modify salphen-type Schiff base ligands with naphtol (SYML1) and pyrocathecol (2,3-dihydroxyphenyl) groups (SYML2), or a combination of both (ASYML). Each of these ligands can be used to obtain polynuclear metal complexes following two different strategies. One relies on using metals that are either too large for the N₂O₂ cavity or not fond of coordination number 4 and the other one relies on forcing the polynuclear species by adding functional groups to the hydroxybenzaldehayde in order to have extra coordination sites in the ligand. We report and characterize the mononuclear complexes SYML1-Cu and SYML1-Ce, along with the dinuclear complex SYML1-Fe and the tetranuclear species SYML2-Mn. The asymmetric ligand ASYML routinely hydrolyzes into the symmetric ligands in the reaction mixtures. SYML1-Fe displays a nearly linear Fe-O-Fe bridge with very strong antiferromagnetic coupling between the Fe(III) ions.

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118-119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N2O2-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed.

Unprecedented reductive cyclisation of salophen ligands to tetrahydroquinoxalines during metal complex formation

The synthesis of novel tetrahydroquinoxalines by a metal induced one-electron reductive cyclisation of salophen ligands was found to occur when a salophen ligand was treated with chromium(ii) chloride or decamethylcobaltocene.

A ferrocene functionalized Schiff base containing Cu(ii) complex: synthesis, characterization and parts-per-million level catalysis for azide alkyne cycloaddition

Atom economy is one of the major factors in developing catalysis chemistry. Using the minimum amount of catalyst to obtain the maximum product yield is of the utmost priority in catalysis, which drives us to use parts-per-million (ppm) levels of catalyst loadings in syntheses. In this context, a new ferrocene functionalized Schiff base and its copper(ii) complex have been synthesized and characterized. This Cu(ii) complex is employed as a catalyst for popular 'click chemistry', where 1,2,3-triazoles are the end product. As low as 5 ppm catalyst loading is enough to produce gram scale product, and highest turnover number (TON) and turnover frequency (TOF) values of 140 000 and 70 000 h^-1 are achieved, respectively. Furthermore, this highly efficient protocol has been successfully applied to the preparation of diverse functionalized materials with pharmaceutical, labelling and supramolecular properties.

Dinuclear zinc(ii) salen-type Schiff-base complexes as molecular tweezers

In this contribution, the synthesis and the unusual aggregation/deaggregation properties in solution of two dinuclear ZnII Schiff-base complexes of tetradentate Schiff-base units, having non-conjugated spacers between each molecular unit, are reported in comparison to the mononuclear model complex. Through detailed 1H NMR, DOSY NMR, optical absorption, fluorescence emission, and multivariate analysis of optical absorption data, emerge some interesting findings. In solution of non-coordinating solvents, these Lewis acidic species are characterized as monomers stabilized by formation of intramolecular aggregates, having distinct spectroscopic properties in comparison to intermolecular aggregates derived from the mononuclear model analogue. Instead, in coordinating solvents they exhibit a typical behaviour, with formation of stable adducts showing a strong fluorescence. Deaggregation studies using pyridine as reference Lewis base allowed establishing a larger thermodynamic stability of these intramolecular aggregates, in comparison to intermolecular aggregates, even larger than that of aggregates of conjugated multinuclear complexes. The combined analysis of spectroscopic data upon deaggregation with ditopic Lewis bases unambiguously demonstrated the formation of stable 1 : 1 adducts, with higher binding constants in comparison to those related to monotopic species. Therefore, the present Lewis acidic, dinuclear complexes behave as molecular tweezers of ditopic guests having a strong Lewis basicity.

Surface-Confined Macrocyclization via Dynamic Covalent Chemistry

Surface-confined synthesis is a promising approach to build complex molecular nanostructures including macrocycles. However, despite the recent advances in on-surface macrocyclization under ultrahigh vacuum, selective synthesis of monodisperse and multicomponent macrocycles remains a challenge. Here, we report on an on-surface formation of [6 + 6] Schiff-base macrocycles via dynamic covalent chemistry. The macrocycles form two-dimensional crystalline domains on the micrometer scale, enabled by dynamic conversion of open-chain oligomers into well-defined ∼3.0 nm hexagonal macrocycles. We further show that by tailoring the length of the alkyl substituents, it is possible to control which of three possible products-oligomers, macrocycles, or polymers-will form at the surface. In situ scanning tunneling microscopy imaging combined with density functional theory calculations and molecular dynamics simulations unravel the synergistic effect of surface confinement and solvent in leading to preferential on-surface macrocyclization.