Supramolecular Chirogenesis in a Sterically Hindered Porphyrin: A Critical Theoretical Analysis

The determination of molecular stereochemistry and absolute configuration is an important part of modern chemistry, pharmacology, and biology. Electronic circular dichroism (ECD) spectroscopy is a widely used tool for chirality assignment, especially with porphyrin macrocycles employed as reporter chromophores. However, the mechanisms of induced ECD in porphyrin complexes are yet to be comprehensively rationalized. In this work, the ECD spectra of a sterically hindered hexa-cationic porphyrin with two camphorsulfonic acids in dichloromethane and chloroform were experimentally measured and computationally analyzed. The influence of geometric factors such as the position of chiral guest molecules, distortion of the porphyrin macrocycle, and orientation of aromatic and non-aromatic peripheral substituents on the ECD spectra was theoretically studied. Various potential pitfalls, such as a lack of significant conformations and accidental agreement of experimental and simulated spectra, are considered and discussed.

Circularly Polarized Microscopy of Thin Films of Chiral Organic Dyes

We introduce an optical microscopy technique, circularly polarized microscopy or CPM, able to afford spatially resolved electronic circular dichroism (ECD) of thin films of chiral organic semiconductors through a commercial microscope equipped with a camera and inexpensive optics. Provided the dichroic ratio is sufficiently large, the spatial resolution is on the order of the μm and is only limited by the magnification optics integrated in the microscope. We apply CPM to thin films of small chiral π-conjugated molecules, which gave rise to ordered aggregates in the thin layer. Primarily, conventional ECD can reveal and characterize chiral supramolecular structures and possible interferences between anisotropic properties of solid samples; however, it cannot generally account for the spatial distribution of such properties. CPM offers a characterization of supramolecular chirality and of commingling polarization anisotropies of the material, describing their local distribution. To validate CPM, we demonstrated that it can be adopted to quantify the local ECD of samples characterized by intense signals, virtually on any standard optical microscope.

New Dinuclear Macrocyclic Copper(II) Complexes as Potentially Fluorescent and Magnetic Materials

Two dinuclear copper(II) complexes with macrocyclic Schiff bases K1 and K2 were prepared by the template reaction of (R)-(+)-1,1'-binaphthalene-2,2'-diamine and 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde K1, or 4-tert-butyl-2,6-diformylphenol K2 with copper(II) chloride dihydrate. The compounds were characterized by spectroscopic methods. X-ray crystal structure determination and DFT calculations confirmed their geometry in solution and in the solid phase. Moreover, intermolecular interactions in the crystal structure of K2 were analyzed using 3D Hirshfeld surfaces and the related 2D fingerprint plots. The magnetic study revealed very strong antiferromagnetic Cu^II-Cu^II exchange interactions, which were supported by magneto-structural correlation and DFT calculations conducted within a broken symmetry (BS) framework. Complexes K1 and K2 exhibited luminescent properties that may be of great importance in the search for new OLEDs. Both K1 and K2 complexes showed emissions in the range of 392-424 nm in solutions at various polarities. Thin materials of the studied compounds were deposited on Si(111) by the spin-coating method or by thermal vapor deposition and studied by scanning electron microscopy (SEM/EDS), atomic force microscopy (AFM), and fluorescence spectroscopy. The thermally deposited K1 and K2 materials showed high fluorescence intensity in the range of 318-531 nm for K1/Si and 326-472 nm for the K2/Si material, indicating that they could be used in optical devices.

Characterization of Diastereomeric Equilibria of Pseudotetrahedral Bis[(R or S)-N-1-(Ar)Ethylsalicylaldiminato-κ2 N,O]zinc(II) with Λ/Δ-Chirality-At-Metal Induction

A family of bis[(R or S)‐N‐1‐(Ar)ethylsalicylaldiminato‐κ²N,O]‐Δ/Λ‐zinc(II) {Ar=C₆H₅ (ZnRL¹ or ZnSL¹), p‐CH₃OC₆H₄ (ZnRL² or ZnSL²) and p‐ClC₆H₄ (ZnRL³ or ZnSL³)} compounds was synthesized and investigated by multiple methods. They feature Λ/Δ‐chirality‐at‐metal induction along the pseudo‐C₂ axis of the molecules. The chirality induction is quantitative in the solid state, explored by X‐ray crystallography and powder X‐ray diffraction (PXRD), where R or S‐ligated complexes diastereoselectively yield Λ or Δ‐configuration at the metal. On the other hand, Λ and Δ‐diastereomers co‐exist in solution. The Λ⇆Δ equilibrium is solvent‐ and temperature‐dependent. Electronic circular dichroism (ECD) spectra confirm the existence of a diastereomeric excess of Λ‐ZnRL¹⁻³ or Δ‐ZnSL¹⁻³ in solution. DSC analysis reveals thermally induced irreversible phase transformation from a crystalline solid to an isotropic liquid phase. ECD spectra were reproduced by DFT geometry optimizations and time‐dependent DFT (TD‐DFT) calculations, providing ultimate proof of the dominant chirality atmetal in solution.

Sign control of circularly polarized luminescence of chiral Schiff-base Zn(II) complexes through coordination geometry changes

The handedness of circularly polarized luminescence (CPL) of Zn(II) complexes with a chiral Schiff-base ligand can be controlled by coordination geometry changes in solution and in the solid state. The relationships between molecular structure and chiroptical properties were discussed with the results of X-ray diffraction analysis and DFT calculations.

Investigation of the influence of chirality and halogen atoms on the anticancer activity of enantiopure palladium(ii) complexes derived from chiral amino-alcohol Schiff bases and 2-picolylamine

In order to evaluate the impact of chirality and halogen-substitution pattern on biological activity, four mixed-ligand enantiomeric pairs of Pd(ii) complexes were synthesized and characterized.

Pseudotetrahedral Zn(II)-(R or S)-dihalogen-salicylaldiminato complexes with Λ- or Δ-chirality induction at-metal

Reactions of enantiopure (S or R)-N-1-(phenyl)ethyl-2,4-X1,X2-salicylaldimine (S-H or R-H; X1, X2 = dihalogen) with Zn(II)-nitrate give bis[(S or R)-N-1-(phenyl)ethyl-2,4-X1,X2-salicylaldiminato-κ2N,O]-zinc(II), (Δ-ZnS or Λ-ZnR) with Δ/Λ-chirality induction at-metal in the C2-symmetric molecules....

Enantioselective Gel Phase Synthesis of Metal-Organic Materials

We report the asymmetric synthesis of homochiral metal-organic materials (MOMs) in chiral gels from achiral components. The enantioselectivity of MOMs depends on the chirality of the gel, whereas the synthesis performed in solution phase and achiral gels resulted in conglomerates.

Synthesis and Characterization of Bis[(R or S)-N-1-(X-C6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]-Λ/Δ-cobalt(II) (X = H, p-CH3O, p-Br) with Symmetry- and Distance-Dependent Vibrational Circular Dichroism Enhancement and Sign Inversion

The enantiopure Schiff bases (R or S)-N-1-(X-C₆H₄)ethyl-2-hydroxy-1-naphthaldimine {X = H [(R or S)-HL1], p-CH₃O [(R or S)-HL2], and p-Br [(R- or S)-HL3]} react with cobalt(II) acetate to give bis[(R or S)-N-1-(X-C₆H₄)ethyl-2-oxo-1-naphthaldiminato-κ²N,O]-Λ/Δ-cobalt(II) {X = H [Λ/Δ-Co-(R or S)-L1], p-CH₃O [Λ/Δ-Co-(R or S)-L2], and p-Br [Λ/Δ-Co-(R or S)-L3]} (1–3), respectively. Induced Λ and Δ chirality originates at the metal center of the C₂-symmetric molecule in pseudotetrahedral geometry. Differential scanning calorimetry analyses explored the thermal stability of the complexes, which undergo reversible phase transformation from crystalline solid to isotropic liquid phase for 1 and 3 but irreversible phase transformation for 2. Like other cobalt(II) complexes, compounds 1–3 exhibit a continuous ensemble of absorption and circular dichroism bands, which span from the UV to IR region and can be collected into a superspectrum. Infrared vibrational circular dichroism (IR-VCD) spectra witness the coupling between Co²⁺-centered low-lying electronic states and ligand-centered vibrations. The coupling produces enhanced and almost monosignate VCD spectra, with both effects being mode-dependent in terms of the A or B symmetry (in the C₂ point group) and distance from the Co²⁺ core.

Chiroptical superspectra of naphthaldiminatocobalt(II) complexes 1−3 exhibit a continuous ensemble of absorption and circular dichroism bands from the UV to IR region, including peculiar and almost monosignate vibrational circular dichroism (VCD) spectra dominated by the coupling between Co²⁺-centered low-lying electronic states and ligand-centered vibrations, which depend on the normal-mode symmetry and distance from the Co²⁺ core

Enantiomeric copper based anticancer agents promoting sequence-selective cleavage of G-quadruplex telomeric DNA and non-random cleavage of plasmid DNA

Copper-based binuclear enantiomeric complexes 1S and 1R were synthesized as anticancer chemotherapeutic agents to target G-quadruplex rich region of DNA and thoroughly characterized by various spectroscopic and single X-ray crystal diffraction studies. The structure elucidation of Schiff base ligand LS and complexes 1S & 1R, was carried out by single crystal X-ray studies which showed that ligand crystallized in the monoclinic P21/n space group while complexes 1S and 1R crystallized in triclinic space groups P1[combining macron] and P1, respectively with two copper units connected to each other via an alkoxide bridge to exhibit square planar geometry which is in good agreement with other spectroscopic studies {IR, ESI-MS, EPR and magnetic moment values}. In vitro binding studies of complexes 1S and 1R were carried out with G-quadruplex DNA and CT-DNA which showed higher binding affinity and selectivity toward quadruplex DNA over the duplex DNA. To validate the potential of complexes to act as therapeutic drug candidates, the cleavage studies of complexes 1S and 1R were carried out with G-quadruplex telomeric DNA by PAGE Gel assay which showed sequence selective cleavage of 22G4via oxidative cleavage pathway. The major cleavage sites identified were G15, T6, G8, G9, G14 for complex 1S whereas for 1R G15, G20, G21, G14 cleavage sites were observed. Furthermore, these complexes were capable of cleaving pUC19 plasmid DNA in double-stranded non-random fashion which is considered to be more potent than single-strand cleavage as a source of lethal DNA lesions. Cellular studies of 1S and 1R were performed on a panel of human cancer cell lines; Huh7, MCF7, BxPC3 and AsPC1, which displayed significant cytotoxicity and differential responses toward different cancer phenotypes.

Heteroleptic enantiopure Pd(ii)-complexes derived from halogen-substituted Schiff bases and 2-picolylamine: synthesis, experimental and computational characterization and investigation of the influence of chirality and halogen atoms on the anticancer activity

To investigate the effect of chirality and different halogen substituents on the anticancer activity, seven enantiomeric pairs of palladium complexes were synthesized and characterized.

Schiff base ligands derived from 1,2-bis(2′-nitro-/amino-phenoxy)-3-R-benzene and 2-hydroxy-1-naphthaldehyde and their Cu/Zn(ii) complexes: synthesis, characterization, X-ray structures and computational studies

Racemic crystals of Cu/Zn(ii)-tetradentate Schiff base ligands with Λ/Δ-chirality induction at-metal center.

Pseudotetrahedral copper(II)-complexes with enantiopure (R or S)-2-(((aryl)ethylimino)ethyl)phenolate Schiff base ligands

Condensation of 2-hydroxy-benzophenone (HL') with (R or S)-(Ar)ethylamine yields the enantiopure Schiff bases (S or R)-2-((E)-1-(1-(Ar)ethylimino)ethyl)phenol {Ar = C6H5 (S- or R-HL1), p-CH3OC6H4 (S- or R-HL2)}. These Schiff bases react with copper(ii) acetate under reflux to give green microcrystals of bis[(R or S)-2-((E)-1-(1-(Ar)ethylimino)ethyl)phenolato-κ2N,O]-Λ/Δ-copper(ii), {Ar = C6H5 (Λ/Δ-Cu-R- or S-L1), p-CH3OC6H4 (Λ/Δ-Cu-R- or S-L2)} with induction of Λ/Δ-chirality at-metal. The presence of Schiff base ligands in the paramagnetic green microcrystals is confirmed by decomplexation reaction with NaCN via reduction of Cu(ii) to Cu(i) in DMSO-d6 solution. Crystallization attempts of the green microcrystalline Schiff-base Cu complexes provide deep-green block-shaped crystals of an about equal admixture of bis[2-oxo-benzophenonato-κ2O,O']-copper(ii), (CuL'2) and bis[2-(imino(phenyl)methyl)phenolato-κ2N,O]copper(ii), (CuL''2) via in situ hydrolysis of the coordinated Schiff base ligands back to 2-hydroxy-benzophenone (HL') and to 2-(imino(phenyl)methyl)phenol (HL''), which in-turn bind with the copper(ii) ion. Powder X-ray diffraction (PXRD) patterns of R-HL1 and Cu-R-L1 allowed their structure determinations using the program Expo-2014 followed by Rietveld refinement. The Cu structures refined to four-coordinated Λ/Δ-copper(ii)-complexes by the two phenolate-oxygen and two imine-nitrogen atoms from two Schiff base ligands in a pseudotetrahedral geometry. DFT optimized structures (at gas-phase) reveal the Δ-Cu-S-L1 or Λ-Cu-R-L1 diastereomer as slightly more stable than the corresponding Λ-Cu-S-L1 or Δ-Cu-R-L1 by ca. 7.60 kcal mol-1, resulting from diastereoselectively induced Λ vs. Δ-chirality at-metal. Electronic circular dichroism (ECD) spectra display mirror-image relationships and comparisons of experimental and simulated ECD spectra by TDDFT suggest an excess of the Δ-Cu-S-L1 or Λ-Cu-R-L1 diastereomer in solution. The cyclic voltammograms demonstrate two one electron charge transfer processes for Cu2+/Cu+ and Cu+/Cu0 couples in acetonitrile, respectively.

Solid-state to solution helicity inversion of pseudotetrahedral chiral copper(ii) complexes with 2,4-dihalo-salicylaldiminate ligands

Steric constraints by the halogen substituents show an efficient control of stereochemical behaviours of the complexes.

Benchmarking computational methods and influence of guest conformation on chirogenesis in zinc porphyrin complexes

Different computational methods and influence of the guest conformation and solvent effect to analyze chirogenesis in zinc porphyrins by several chiral compounds have been investigated.

Broad-Range Spectral Analysis for Chiral Metal Coordination Compounds: (Chiro)optical Superspectrum of Cobalt(II) Complexes

Chiroptical broad-range spectral analysis extending from UV to mid-IR was employed to study a family of Co(II) N-(1-(aryl)ethyl)salicylaldiminato Schiff base complexes with pseudotetrahedral geometry associated with chirality-at-metal of the Δ/Λ type. While common chiral organic compounds have well-separated absorption and circular dichroism spectra (CD) in the UV/vis and IR regions, chiral Co(II) complexes feature an almost unique continuum of absorption and CD bands, which cover in sequence the UV, visible, near-IR (NIR), and IR regions of the electromagnetic spectrum. They can be collected in a single (chiro)optical superspectrum ranging from the UV (230 nm, 5.4 eV) to the mid-IR (1000 cm^-1, 0.12 eV), which offers a fingerprint of the structure and stereochemistry of the metal complexes. Each region of the superspectrum contributes to one piece of information: the NIR-CD region, in combination with TDDFT calculations, allows a reliable assignment of the metal-centered chirality; the UV-CD region facilitates the analysis of the Δ/Λ diastereomeric equilibrium in solution; and the IR-VCD region contains a combination of low-lying metal-centered electronic states (LLES) and ligand-centered vibrations and displays characteristically enhanced and monosignate VCD bands. Circular dichroism in the NIR and IR regions is crucial to reveal the presence of d-d transitions of the Co(II) core which, due to the electric-dipole forbidden character, would be otherwise overlooked in the corresponding absorption spectra.

Enantiopure Schiff bases of amino acid phenylhydrazides: impact of the hydrazide function on their structures and properties

New chiral Schiff bases have been synthesized and characterized with an important contribution from quantum chemical calculations at the DFT level of theory. They have been evaluated for their physical and biological properties.

Spontaneous resolution of a Δ/Λ-chiral-at-metal pseudo-tetrahedral Schiff-base zinc complex to a racemic conglomerate with C–H⋯O organized 41- and 43-helices

Weak supramolecular interactions can be decisive!

Synthesis, characterization, DNA/BSA interactions and anticancer activity of achiral and chiral copper complexes

Six novel copper(ii) complexes of [CuCl]ClO4 (), [Cu(acac)]PF6 (), [CuCl]2(PF6)2 (), [CuCl]2(PF6)2 (), [Cu(acac)]PF6 () and [Cu(acac)]PF6 (), ( = 1-naphthyl-N,N-[bis(2-pyridyl)methyl]amine, = R/S-1-naphthyl-N,N-[bis(2-pyridyl)methyl]ethanamine, acac = diacetone) were synthesized to serve as artificial nucleases. All complexes were structurally characterized using X-ray crystallography. The crystal structures showed the presence of distorted square-planar CuLCl (, and ) and distorted tetragonal-pyramidal CuL(acac) (, and ) geometry. The interaction of these complexes with calf thymus DNA (CT-DNA) was researched by means of several spectroscopy methods, which indicated that the complexes were bound to CT-DNA by an intercalation binding mode. DNA cleavage experiments revealed that the complexes exhibited remarkable DNA cleavage activities in the presence of H2O2, and single oxygen ((1)O2) or hydroxyl radicals may serve as the major cleavage active species. In particular, the in vitro cytotoxicity of the complexes on four human cancer cell lines (HeLa, MCF-7, Bel-7404 and HepG-2) demonstrated that the six compounds had broad-spectrum anti-cancer activity with low IC50 values. The stronger cytotoxicity and DNA cleavage activity of the chiral enantiomers compared with chiral analogues verified the influence of chirality on the antitumor activity of complexes. Meanwhile, the protein binding ability was revealed by quenching of tryptophan emission with the addition of complexes using BSA as a model protein. The results indicated that the quenching mechanism of BSA by the complexes was a static process.

Chirality at metal and helical ligand folding in optical isomers of chiral bis(naphthaldiminato)nickel(ii) complexes

Ni-Schiff base complexes combine ligand chirality, chirality at metal, and helical ligand folding in a concerted way.