Optical chemosensors for Cu(II) ion based on BODIPY derivatives: an experimental and theoretical study

Two BODIPY derivatives for Cu²⁺ ion chemosensors containing 4-[2-(diethylamino)-2-oxoethoxy]phenyl (BDP1) and 3,4-bis[2-(diethylamino)-2-oxoethoxy]phenyl (BDP2) were synthesized by coupling appropriate N,N-diethyl-2-(4-formylphenoxy)acetamide and 2,4-dimethylpyrrole moieties in the presence of trifluoroacetic acid and anhydrous dichloromethane at room temperature. The binding abilities between these chemosensors and 50 equivalents of Na⁺, K⁺, Ag⁺, Ca²⁺, Fe²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ ions were studied using UV-vis and fluorescence spectrophotometry. The results show that, compared to other ions, both the UV-vis absorption and fluorescence emission intensity of BDP2 decreased dramatically when Cu²⁺ ion was added. To explain this behavior, ab initio quantum chemical calculations were performed using correlated second-order Møller-Plesset perturbation theory (MP2/LanL2DZ). The calculated orbital energies indicated that the decrease in UV-vis absorption intensity and the quenching of fluorescene emission were due to the single-electron reduction of Cu²⁺ to Cu⁺ ion.

Synthesis, characterization, crystal structures and magnetic properties of dissymmetrical double schiff base heterotrinuclear complexes: [CuL(H2O)CoCuL] · H2O · CH3OH and [(CuL)2Ni] · 2H2O

A dissymmetrical double Schiff base Cu(II) mononuclear complex: CuHL (1) (where H₃L is N-3-carboxylsalicylidene-N^′-salicylaldehyde-1,2-diaminoethane) and two trinuclear complexes: [CuL(H₂O)CoCuL] · H₂O · CH₃OH (2) and [(CuL)₂Ni] · 2H₂O (3) have been synthesized and characterized by means of elemental analyses, IR and electronic spectra. The crystal structures of two heterotrinucler complexes were determined by X-ray analysis. Each dissymmetrical cell unit of the complex 2 contains two heterotrinucler neutral molecules. In each neutral molecule, the central Co²⁺ ion is located at the site of O₆ with a distorted octahedral geometry and one terminal Cu²⁺ ion at the four-coordination site of N₂O₂, but the other one at the square-pyramidal environment of N₂O₃. Each dissymmetrical unit of the complex 3 contains a heterotrinucler neutral molecule, whose structure is similar to that of 2 except two terminal Cu²⁺ ions both at the inner site of N₂O₂. The magnetic properties of two heterotrinucler complexes have been determined in the temperature range of 5-300 K, which indicate that the interaction between the central Co²⁺ ion or Ni²⁺ ion and the outer Cu²⁺ ions is antiferromagnetic. The exchange integrals are equal to −26.2 cm⁻¹ for 2 and −50.6 cm⁻¹ for 3.     

Effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+ and Zn2+) and water coordination on the structure and properties of l-histidine and zwitterionic l-histidine

Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of l-histidine is examined. The effect of metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) and water on structures of His·M(H₂O)m, m = 0.1 complexes have been determined theoretically employing density functional theories using extended basis sets. Of the five stable complexes investigated the relative stability of the gas-phase complexes computed with DFT methods (with one exception of K⁺ systems) suggest metallic complexes of the neutral l-histidine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of l-histidine in the presence of the metal cations Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to l-histidine is exhibited by the Cu²⁺ cation. The computed Gibbs energies ΔG are negative, span a rather broad energy interval (from ?130 to ?1,300 kJ/mol), and upon hydration are appreciably lowered.     

A theoretical study on the coordination behavior of some phosphoryl,carbonyl and sulfoxide derivatives in lanthanide complexation

The selectivity of phosphoryl P(O)R₃, sulfoxide S(O)R₂, and carbonyl C(O)R₂ (R?=?NH₂, CH₃, OH, and F) derivatives with lanthanide cations (La³⁺, Eu³⁺, Lu³⁺) was studied by density functional theory calculations. Theoretical approaches were also used to investigate energy and the nature of metal–ligand interaction in the model complexes. Atoms in molecules and natural bond orbital (NBO) analyses were accomplished to understand the electronic structure of ligands, L, and the related complexes, L–Ln³⁺. NBO analysis demonstrated that the negative charge on phosphoryl, carbonyl, and sulfoxide oxygen (O_P, O_C, and O_S) has maximum and minimum values when the connected –R groups are –NH₂ and –F. The metal–ligand distance declines as, –F?>?–OH?>?–CH₃?>?–NH₂. Charge density at the bond critical point and on the lanthanide cation in the L–Ln³⁺ complexes varies in the order –F?<?–OH?<?–CH₃?<?–NH₂, due to greater ligand to metal charge transfer, which is well explained by energy decomposition analysis. It was also illustrated that E⁽²⁾ values of Lp(N)?→?σ*(Y–N) vary in the order P=O ? S=O ? C=O and the related values of Lp(N)?→?σ*(Y=O) change as C=O ? S=O ? P=O in (NH₂)_nYO ligands (Y?=?P, C, and S). Trends in the L–Ln³⁺ CP–corrected bond energies are in good accordance with the optimized O_Y?Ln distances. It seems that, comparing the three types of ligands studied, NH₂–substituted are the better coordination ligands.

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Graphical Abstract Density functional theory (B3LYP) calculations were used to compare structural, electronic and energy aspects of lanthanide (La, Eu, Lu) complexes of phosphine derivatives with those of carbonyls and sulfoxides in which the R– groups connected to the P=O, C=O and S=O are –NH₂, –CH₃, –OH and –F.

     

DFT and docking studies of rhodostreptomycins A and B and their interactions with solvated/nonsolvated Mg2+ and Ca2+ ions

The interactions of L-aminoglucosidic stereoisomers such as rhodostreptomycins A (Rho A) and B (Rho B) with cations (Mg²⁺, Ca²⁺, and H⁺) were studied by a quantum mechanical method that utilized DFT with B3LYP/6-311G**. Docking studies were also carried out in order to explore the surface recognition properties of L-aminoglucoside with respect to Mg²⁺ and Ca²⁺ ions under solvated and nonsolvated conditions. Although both of the stereoisomers possess similar physicochemical/antibiotic properties against Helicobacter pylori, the thermochemical values for these complexes showed that its high affinity for Mg²⁺ cations caused the hydration of Rho B. According to the results of the calculations, for Rho A–Ca²⁺(H₂O)₆, ΔH = ?72.21 kcal?mol^?1; for Rho B–Ca²⁺(H₂O)₆, ΔH = ?72.53 kcal?mol^?1; for Rho A–Mg²⁺(H₂O)₆, ΔH = ?72.99  kcal?mol^?1 and for Rho B–Mg²⁺(H₂O)₆, ΔH = ?95.00  kcal?mol^?1, confirming that Rho B binds most strongly with hydrated Mg²⁺, considering the energy associated with this binding process. This result suggests that Rho B forms a more stable complex than its isomer does with magnesium ion. Docking results show that both of these rhodostreptomycin molecules bind to solvated Ca²⁺ or Mg²⁺ through hydrogen bonding. Finally, Rho B is more stable than Rho A when protonation occurs.

Fluorescent sensors based on BODIPY derivatives for aluminium ion recognition: an experimental and theoretical study

Two BODIPY derivative sensors for metal ion recognition containing 10-(4-hydroxyphenyl) (L1) and 10-(3,4-dihydroxyphenyl) (L2) were synthesized in a one-pot reaction of benzaldehyde derivative and 2,4-dimethylpyrrole in the presence of trifluoroacetic acid as catalyst. The binding abilities between these sensors and 50 equivalents of Na⁺, K⁺, Ag⁺, Ca²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Al³⁺ and Cr³⁺ ions were studied using UV–vis and fluorescent spectroscopic methods. Of all the metal ions tested, Al³⁺ ion showed the greatest decrease in intensity in the spectra of the sensors, and therefore Al³⁺ ion forms the strongest complex. The binding abilities of BODIPY receptors with Na⁺, Ag⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Al³⁺ ions were also investigated using density functional theory (DFT) calculations at B3LYP/LanL2DZ theoretical level. The calculated results point to the same conclusion. DFT calculations also provided the HOMO–LUMO energy levels, which can explain the spectrum change upon complexation.

Preparation, structures and properties of dinuclear and trinuclear copper(II) complexes bridged by one oximato and one hydroxo ligands

The dinuclear and trinuclear copper(II) complexes [Cu₂(L)(OH)(ClO₄)(phen)(H₂O)]ClO₄ · [Cu₂(L)(OH)(ClO₄)₂(phen)(CH₃OH)] (1) and [Cu₃(L)₂(OH)₂(H₂O)₂](NO₃)₂ (2) (HL=2-[2-(α-pyridyl)ethyl]imino-3-butanone oxime and phen=1,10-phenanthroline) were prepared and their crystal structures have been determined by X-ray crystallography. Complex 1 is composed of [Cu₂(L)(OH)(ClO₄)(phen)(H₂O)]ClO₄ (1a) and [Cu₂(L)(OH)(ClO₄)₂(phen)(CH₃OH)] (1b). In 1a and 1b, one oximato of L and one hydroxo group bridge two copper(II) ions. The linear trinuclear cation [Cu₃(L)₂(OH)₂(H₂O)₂]²⁺ in 2 is centrosymmetric, and one oximato and one hydroxo group bridge the central and terminal copper(II) ions. The strong antiferromagnetic interactions within the dinuclear and trinuclear complexes 1 and 2 have been observed (2J=∼−900 cm⁻¹ for 1 and 2, respectively, H=−2JS₁·S₂).     

N-acetyl-L-cysteine in the presence of Cu<Superscript>2+</Superscript> induces oxidative stress and death of granule neurons in dissociated cultures of rat cerebellum

Addition into the culture medium of the antioxidant N-acetylcysteine (NAC, 1 mM) in the presence of Cu²⁺ (0.0005-0.001 mM) induced intensive death of cultured rat cerebellar granule neurons, which was significantly decreased by the zinc ion chelator TPEN (N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine). However, the combined action of NAC and Zn²⁺ did not induce destruction of the neurons. Measurement of the relative intracellular concentration of Zn²⁺ with the fluorescent probe FluoZin-3 AM or of free radical production using a CellROX Green showed that incubation of the culture for 4 h with Cu²⁺ and NAC induced an intensive increase in the fluorescence of CellROX Green but not of FluoZin-3. Probably, the protective effect of TPEN in this case could be mediated by its ability to chelate Cu²⁺. Incubation of cultures in a balanced salt solution in the presence of 0.01 mM Cu²⁺ caused neuronal death already after 1 h if the NAC concentration in the solution was within 0.005–0.05 mM. NAC at higher concentrations (0.1–1 mM) together with 0.01mM Cu²⁺ did not cause the death of neurons. These data imply that the antioxidant NAC can be potentially harmful to neurons even in the presence of nanomolar concentrations of variable valence metals.     

Stability and isomerization of complexes formed by metal ions and cytosine isomers in aqueous phase

We present a systematic study of the stability of the formation of complexes produced by four metal ions (M^+/2+) and 14 cytosine isomers (C_n). This work predicts theoretically that predominant product complexes are associated with higher-energy C₄M^+/2+ and C₅M^+/2+ rather than the most stable C₁M^+/2+. The prediction resolves successfully several experimental facts puzzling two research groups. Meanwhile, in-depth studies further reveal that direct isomerization of C₁?C₄ is almost impossible, and also that the isomerization induced by either metalation or hydration, or by a combination of the two unfavorable. It is the single water molecule locating between the H1(?N1) and O2 of the cytosine that plays the dual roles of being a bridge and an activator that consequently improves the isomerization greatly. Moreover, the cooperation of divalent metal ion and such a monohydration actually leads to an energy-free C₁←C₄ isomerization in the gas phase. Henceforth, we are able to propose schemes inhibiting the free C₁←C₄ isomerization, based purely on extended hydration at the divalent metal ion.

A resin buffered method for controlling metal speciation in nutrient solutions for plant toxicity tests

Aims

Root elongation tests are sensitive bioassays for testing metal toxicity in nutrient solutions. The metal speciation and, hence, metal exposure conditions are little controlled in the traditional set-up. A resin buffered solution system was developed to overcome this issue.

Methods

Barley (Hordeum vulgare L.) root elongation was tested in aerated 140 mL solution batch systems supplied with 3.3 g Dowex resin for two plants. Copper toxicity was measured in presence or absence of the resin (+R/?R) and in presence or absence of a metal complexing ligand (+NTA; nitrilotriacetic acid/?NTA). In addition, the toxicity in the traditional set without resin and with daily solution replacement was included as a reference.

Results

Metal desorption from the resin is fast in these systems (k?=?0.82 h^?1). Total dissolved Cu roughly halved during 4 days in ?R/?NTA systems due to uptake, while it increased by 30 % in the +R/?NTA, probably due to complexation reactions by root-derived molecules. The toxicity (50 % reduction in root length, EC50) of the initial free Cu²⁺ was equal in all resin or chelate buffered systems and in the solutions with daily replacement, whereas this threshold was significantly larger in the ?R/?NTA due to Cu²⁺ uptake and complexation reactions.

Conclusion

The resin method is a convenient system for high throughput screening of metal toxicity and avoids uncertainties in metal speciation inherent to chelator buffered systems. Details are given how to prepare the resin to obtain a target metal ion activity.     

Comparison of the structural characteristics of Cu2+-bound and unbound α-syn12 peptide obtained in simulations using different force fields

The effects of Cu²⁺ binding and the utilization of different force fields when modeling the structural characteristics of α-syn12 peptide were investigated. To this end, we performed extensive temperature replica exchange molecular dynamics (T-REMD) simulations on Cu²⁺-bound and unbound α-syn12 peptide using the GROMOS 43A1, OPLS-AA, and AMBER03 force fields. Each replica was run for 300 ns. The structural characteristics of α-syn12 peptide were studied based on backbone dihedral angle distributions, free-energy surfaces obtained with different reaction coordinates, favored conformations, the formation of different Turn structures, and the solvent exposure of the hydrophobic residues. The findings show that AMBER03 prefers to sample helical structures for the unbound α-syn12 peptide and does not sample any β-hairpin structure for the Cu²⁺-bound α-syn12 peptide. In contrast, the central structure of the major conformational clusters for the Cu²⁺-bound and unbound α-syn12 peptide according to simulations performed using the GROMOS 43A1 and OPLS-AA force fields is a β-hairpin with Turn_9-6. Cu²⁺ can also promote the formation of the β-hairpin and increase the solvent exposure of hydrophobic residues, which promotes the aggregation of α-syn12 peptide. This study can help us to understand the mechanisms through which Cu²⁺ participates in the fibrillation of α-syn12 peptide at the atomic level, which in turn represents a step towards elucidating the nosogenesis of Parkinson’s disease.

Competition between halogen, dihalogen and hydrogen bonds in bromo- and iodomethanol dimers

O-H…X and O-H…O H-bonds as well as C-X…X dihalogen and C-X…O halogen bonds have been investigated in halomethanol dimers (bromomethanol dimer, iodomethanol dimer, difluorobromomethanol…bromomethanol complex and difluoroiodomethanol…iodomethanol complex). Structures of all complexes were optimized at the counterpoise-corrected MP2/cc-pVTZ level and single-point energies were calculated at the CCSD(T)/aug-cc-pVTZ level. Energy decomposition for the bromomethanol dimer complex was performed using the DFT-SAPT method based on the aug-cc-pVTZ basis set. OH…O and OH…X H-bonds are systematically the strongest in all complexes investigated, with the former being the strongest bond. Halogen and dihalogen bonds, being of comparable strength, are weaker than both H-bonds but are still significant. The strongest bonds were found in the difluoroiodomethanol…iodomethanol complex, where the O-H…O H-bond exceeds 7 kcal mol^-1, and the halogen and dihalogen bonds exceed 2.5 and 2.3 kcal mol^-1, respectively. Electrostatic energy is dominant for H-bonded structures, in halogen bonded structures electrostatic and dispersion energies are comparable, and, finally, for dihalogen structures the dispersion energy is clearly dominant.

Di- and polynuclear silver(I) saccharinate complexes of tertiary diphosphane ligands: synthesis, structures, in vitro DNA binding, and antibacterial and anticancer properties

A series of new silver(I) saccharinate (sac) complexes, [Ag₂(sac)₂(μ-dppm)H₂O]·H₂O (1), {[Ag₂(μ-sac)₂(μ-dppe)]·3H₂O·CH₂Cl₂} _n (2), [Ag₂(μ-sac)₂(μ-dppp)] _n (3), and [Ag(sac)(μ-dppb)] _n (4) [dppm is 1,1-bis(diphenylphosphino)methane, dppe is 1,2-bis(diphenylphosphino)ethane, dppp is 1,3-bis(diphenylphosphino)propane, and dppb is 1,4-bis(diphenylphosphino)butane], have been synthesized and characterized by C, H, N elemental analysis, IR spectroscopy, ¹H NMR, ¹³C NMR, and ³¹P NMR spectroscopy, electrospray ionization mass spectrometry, and thermogravimetry–differential thermal analysis. Single-crystal X-ray studies show that the diphosphanes act as bridging ligands to yield a dinuclear complex (1) and one-dimensional coordination polymers (2 and 4), whereas the sac ligand adopts a μ₂-N/O bridging mode in 2, and is N-coordinated in 1 and 4. The interaction of the silver(I) complexes with fish sperm DNA was investigated using UV–vis spectroscopy, fluorescence spectroscopy, and agarose gel electrophoresis. The binding studies indicate that the silver(I) complexes can interact with fish sperm DNA through intercalation, and complexes 1 and 3 have the highest binding affinity. The gel electrophoresis assay further confirms the binding of the complexes with the pBR322 plasmid DNA. The minimum inhibitory concentrations of the complexes indicate that complex 1 exhibits very high antibacterial activity against standard bacterial strains of Escherichia coli, Salmonella typhimurium, and Staphylococcus aureus, being much higher than those of AgNO₃, silver sulfadiazine, ciprofloxacin, and gentamicin. Moreover, complexes 1–3 exhibit very high cytotoxic activity against A549 and MCF-7 cancer cell lines, compared with AgNO₃ and cisplatin. The bacterial and cell growth inhibitions of the silver(I) complexes are closely related to their DNA binding affinities.     

DFT studies of the conversion of four mesylate esters during reaction with ammonia

The energetics of the Menshutkin-like reaction between four mesylate derivatives and ammonia have been computed using B3LYP functional with the 6-31+G** basis set. Additionally, MPW1K/6-31+G** level calculations were carried out to estimate activation barrier heights in the gas phase. Solvent effect corrections were computed using PCM/B3LYP/6-31+G** level. The conversion of the reactant complexes into ion pairs is accompanied by a strong energy decrease in the gas phase and in all solvents. The ion pairs are stabilized with two strong hydrogen bonds in the gas phase. The bifurcation at C2 causes a significant activation barrier increase. Also, bifurcation at C5 leads to noticeable barrier height differentiation. Both B3LYP/6-31+G** and MPW1K/6-31+G** activation barriers suggest the reaction 2 (2a?+?NH₃) to be the fastest in the gas phase. The reaction 4 is the slowest one in all environments.

Modeling the effect of H-bonding interactions and molecular packing on the molecular structure of [Ag(ethylnicotinate)2]NO3 complex

The gas phase molecular structure of a single isolated molecule of [Ag(Etnic)₂NO₃];1 where Etnic = Ethylnicotinate was calculated using B3LYP method. The H-bonding interaction between 1 with one (complex 2) and two (complex 3) water molecules together with the dimeric formula [Ag(Etnic)₂NO₃]₂;4 and the tetrameric formula [Ag(Etnic)₂NO₃]₄;5 were calculated using the same level of theory to model the effect of intermolecular interactions and molecular packing on the molecular structure of the titled complex. The H-bond dissociation energies of complexes 2 and 3 were calculated to be in the range of 12.220–14.253 and 30.106–31.055 kcal?mol^?1, respectively, indicating the formation of relatively strong H-bonds between 1 and water molecules. The calculations predict bidentate nitrate ligand in the case of 1 and 2, leading to distorted tetrahedral geometry around the silver ion with longer Ag–O distances in case of 2 compared to 1, while 3 has a unidentate nitrate ligand leading to a distorted trigonal planar geometry. The packing of two [Ag(Etnic)₂NO₃] complex units; 4 does not affect the molecular geometry around Ag(I) ion compared to 1. In the case of 5, the two asymmetric units of the formula [Ag(Etnic)₂NO₃] differ in the bonding mode of the nitrate group, where the geometry around the silver ion is distorted tetrahedral in one unit and trigonal planar in the other. The calculations predicted almost no change in the charge densities at the different atomic sites except at the sites involved in the C–H?O interactions as well as at the coordinated nitrogen of the pyridine ring.

Mono- and bis-N-functionalised cyclen with benzimidazolylmethyl pendant arms: Sensitive and selective fluorescent probes for zinc and copper ions

The Zn²⁺ and Cu²⁺ complexes of L¹ and L² ligands (L¹: 1-(benzimidazol-2-ylmethyl)-1,4,7,10-tetraazacyclododecane, L²: 1,7-bis(benzimidazol-2-ylmethyl)-1,4,7,10-tetraazacyclododecane) were synthesised and characterised by means of NMR, EPR, and UV-Vis spectroscopies, X-ray determination and molecular modelisation (HF-DFT(B3LYP)/LANL2DZ). These studies showed that the 1:1 complexes were formed in which the benzimidazole arm(s) are coordinated to the metal ion. On addition of successive amounts of Zn²⁺ in CH₃CN, the fluorescence emission of L¹ increased linearly by a factor of 50 and the one of L² by a factor of 5 while on addition of successive amounts of Cu²⁺ in CH₃CN, the fluorescence emission of L² decreased linearly to 80% of its initial value.     

Synthesis of a series of first-row tris-imidazolylphosphine sandwich complexes and their potential for formation of polymetallic species

A series of tris-[2-(1-methylimidazolyl)phosphine] sandwich complexes with Co²⁺ (1), Ni²⁺ (2), Cu²⁺ (3) and Zn²⁺ (4) have been synthesized and characterized with the intent of using these complexes as templates for building polymetallic species. X-ray analysis reveals that each metal is six-coordinate with octahedral geometry. Compound 3 exhibits a tetragonal distortion resulting from a Jahn-Teller distortion of a Cu²⁺ (d⁹) complex. Electrochemistry of 1-4 displayed peaks assigned to both ligand-based oxidation at 1.45-1.56 V and a perchlorate-based reduction at −2.36 to −2.78 V versus FcH⁺/FcH. Metal-based reduction was also observed for 2, with a reversible Ni^2+/3+ oxidation at 0.89 V, consistent with the relatively poor donor ability of the imidazolyl nitrogen bases; and 3, with a irreversible Cu^2+/1+ reduction at −1.29 V versus FcH⁺/FcH, consistent with the decomposition of the complex upon reduction. X-ray structures indicate large steric crowding around the phosphorus atom resulting from the orientation of the methyl substituent on the imidazoles. This crowding may prohibit the formation of polymetallic species through phosphorus-metal bonding.     

Conversion of 2-(aminomethyl) substituted pyridine and quinoline to their dicarbonyldiimides using copper(II) acetate

In air, hydrated ethanolic (95%) solution of 2-(aminomethyl) substituted pyridine and quinoline, on stirring with half equivalent of Cu(OAc)₂·H₂O, respectively afforded [Cu(bpca)(OAc)(H₂O)]·H₂O (1) and [Cu(bqca)(OAc)(H₂O)] (2) {bpca = bis(2-pyridylcarbonyl)diimide ion and bqca = bis(2-quinolylcarbonyl)diimide ion} in good yields. These reactions involve oxidation of the methylene group and formation of the bond between nitrogen and carbon in N-C(O) through coupling. The complex [Cu(pqca)(OAc)(H₂O)]₃[Cu₂(OAc)₄(EtOH)₂]_1.5 (3) {pqca = (2-pyridylcarbonyl)(2-quinolylcarbonyl)diimide ion} was synthesized by stirring an ethanolic solution of the Schiff base [(2-pyridyl)-N-((2-quinolyl)methylene)methanamine] (L1) and with one equivalent of Cu(OAc)₂·H₂O. A plausible mechanism for the conversion has been proposed. The free ligands were isolated as crystalline solids from compounds 1-3, by extrusion of Cu²⁺ ion using EDTA²⁻. The molecular structures of 1-3 and bqcaH were established by X-ray crystallography and compounds having quinolyl group have π-stacking interactions.     

Unprecedented quadruple Me2biim-bridged di- and tetranuclear complexes: Syntheses, structures and magnetic properties

Quadruple Me₂biim-bridged di- and tetranuclear complexes[Cu₂(H₂O)₂(Me₂biim)₄](SO₄)₂ (1) and [Cr₄(μ-Cl)(μ-Me₂biim)₈(H₂O)_1.67(OH)_0.33](ClO₄)_6.67 (2) have been prepared and characterized. The repulsion between the methyl groups forces the two imidazole rings out of coplanarity, responsible for the much weaker magnetic exchange coupling between the Cu²⁺ ions in 1.     

Investigation of the encapsulation of metal cations (Cu<Superscript>2+</Superscript>, Zn<Superscript>2+</Superscript>, Ca<Superscript>2+</Superscript> and Ba<Superscript>2+</Superscript>) by the dipeptide Phe–Phe using natural bond orbital theory and molecular dynamics simulation

Complexes of the dipeptide phenylalanine–phenylalanine (Phe–Phe) with divalent metal cations (Cu²⁺, Zn²⁺, Ca²⁺ and Ba²⁺) were studied at the B3LYP and MP2 levels of theory with the basis sets 6-311++G(d,p) and 6-31 + G(d) in the gas phase. The relative energies of these complexes indicated that cation–π bidentate/tridentate conformations are more favourable than other conformations with uncoordinated rings. These findings were confirmed by the calculated values of thermodynamic parameters such as the Gibbs free energy. Natural bond orbital (NBO) analysis was carried out to explore the metal–ligand coordination in Phe–Phe–Cu²⁺/Zn²⁺ complexes. Possible orbital transitions, types of orbitals and their occupancies were determined for a range of Phe–Phe–Cu²⁺/Zn²⁺ complexes. The charge transfer involved in various orbital transitions was explored by considering the second-order perturbation energy. NBO analysis revealed that the change transfer is stronger when the metal cation uses both the 4s + 4p subshells rather than just its 4p subshell. We also performed molecular dynamics (MD) simulations to check the stability and consistency of the most favourable binding motifs of Cu²⁺, Zn²⁺, Ca²⁺ and Ba²⁺ with Phe–Phe over time. The structures of the Phe–Phe–Cu²⁺/Zn²⁺/Ca²⁺/Ba²⁺ complexes obtained using MD simulation were found to be in good agreement with those obtained in the DFT-based calculations.

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Graphical Abstract Conformational search on encapsulation of divalent metal cations (Ca²⁺, Zn²⁺, Ca²⁺, Ba²⁺) by the Phe-Phe dipeptide