Coordination sphere flexibility at copper: chemistry of a unipositive copper(II) macrocycle, [Cu(cyclops)]+

Carbazole appended trans-dicationic pyridinium porphyrin finds supremacy in DNA binding/photocleavage over a non-carbazolyl analogue

A carbazolyl appended trans-pyridyl porphyrin (1) was synthesized and its dicationic form 2 was obtained by methylation of the pyridyl group. Copper and zinc complexes of porphyrin 2 (Cu(II), 3; Zn(II), 4) were isolated and characterized by various modern spectroscopic techniques. The DNA binding properties of 2, 3, and 4 have been explored against calf thymus-DNA (CT-DNA). DNA binding was quantized using the intrinsic binding constant (K_b) that was calculated by UV-visible absorption spectroscopy, and the value K_b = 1.6 × 10⁶ M^-1 for compound 2 reveals a better interaction of 2 towards CT-DNA than those of 3 (3.1 × 10⁵ M^-1) and 4 (3.4 × 10⁵ M^-1), which follows the order 2 > 4 > 3. The fluorescence quenching efficiency and ethidium bromide quenching assay also indicated a good binding affinity of all the compounds towards CT-DNA. Furthermore, the spectroscopic data suggest that the possible mode of interaction is intercalation. The docking studies were in accordance with the experimental results. Notably, DNA cleavage studies reveal that 2 shows better damage than 3 and 4 which is in accordance with the binding affinity order 2 > 4 > 3. The observed quantum yield (2: 0.65, 3: 0.33, and 4: 0.97) and no change in DNA cleavage in the presence of NaN₃ reveal the involvement of singlet oxygen. The singlet excited state lifetimes were in the range of 6.3-1.2 ns. Furthermore, these porphyrins can be investigated as interesting photosensitizers in photodynamic therapy and photochemotherapy.

Mixed-ligand copper(ii) complexes of guanidine derivatives containing ciprofloxacin: synthesis, characterization, DFT calculations, DNA interactions and biological activities

Two copper(ii) complexes of guanidine derivatives and ciprofloxacin are good candidates for the next generation of anticancer and/or antibacterial agents.

Copper Coordination Chemistry of Sulfur Pendant Cyclen Derivatives: An Attempt to Hinder the Reductive-Induced Demetalation in 64/67Cu Radiopharmaceuticals

The Cu²⁺ complexes formed by a series of cyclen derivatives bearing sulfur pendant arms, 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO3S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-10-acetamido-1,4,7,10-tetraazacyclododecane (DO3SAm), and 1,7-bis[2-(methylsulfanyl)ethyl]-4,10-diacetic acid-1,4,7,10-tetraazacyclododecane (DO2A2S), were studied in aqueous solution at 25 °C from thermodynamic and structural points of view to evaluate their potential as chelators for copper radioisotopes. UV-vis spectrophotometric out-of-cell titrations under strongly acidic conditions, direct in-cell UV-vis titrations, potentiometric measurements at pH >4, and spectrophotometric Ag⁺-Cu²⁺ competition experiments were performed to evaluate the stoichiometry and stability constants of the Cu²⁺ complexes. A highly stable 1:1 metal-to-ligand complex (CuL) was found in solution at all pH values for all chelators, and for DO2A2S, protonated species were also detected under acidic conditions. The structures of the Cu²⁺ complexes in aqueous solution were investigated by UV-vis and electron paramagnetic resonance (EPR), and the results were supported by relativistic density functional theory (DFT) calculations. Isomers were detected that differed from their coordination modes. Crystals of [Cu(DO4S)(NO₃)]·NO₃ and [Cu(DO2A2S)] suitable for X-ray diffraction were obtained. Cyclic voltammetry (CV) experiments highlighted the remarkable stability of the copper complexes with reference to dissociation upon reduction from Cu²⁺ to Cu⁺ on the CV time scale. The Cu⁺ complexes were generated in situ by electrolysis and examined by NMR spectroscopy. DFT calculations gave further structural insights. These results demonstrate that the investigated sulfur-containing chelators are promising candidates for application in copper-based radiopharmaceuticals. In this connection, the high stability of both Cu²⁺ and Cu⁺ complexes can represent a key parameter for avoiding in vivo demetalation after bioinduced reduction to Cu⁺, often observed for other well-known chelators that can stabilize only Cu²⁺.

Copper(ii) complexes of 2-methyl-8-hydroxyquinoline and tri/diimine co-ligand: DFT calculations, DNA and BSA binding, DNA cleavage, cytotoxicity and induction of apoptosis

Four-coordinate copper(ii) complex cleaves supercoiled ϕX174 RF DNA without reductant while five-coordinate complex cleaves with reductant. The cytotoxicity against cancer cells are higher than cisplatin, less-toxic to normal cells and induce apoptosis.

Electrochemical Conversion of CO2 to CO by a Competent FeI Intermediate Bearing a Schiff Base Ligand

Iron complexes with a N₂ O₂ -type N,N'-bis(salicylaldehyde)-1,2-phenylenediamine salophen ligand catalyze the electrochemical reduction of CO₂ to CO in acetonitrile with phenol as the proton donor, giving rise to 90-99 % selectivity, faradaic efficiency up to 58 %, and turnover frequency up to 10³  s^-1 at an overpotential of 0.65 V. This novel class of molecular catalyst for CO₂ reduction operate through a mononuclear Fe^I intermediate, with phenol being involved in the process with first-order kinetics. The molecular nature of the catalyst and the low cost, easy synthesis and functionalization of the salophen ligand paves the way for catalyst engineering and optimization. Competitive electrodeposition of the coordination complex at the electrode surface results in the formation of iron-based nanoparticles, which are active towards heterogeneous electrocatalytic processes mainly leading to proton reduction to hydrogen (faradaic efficiency up to 80 %) but also to the direct reduction of CO₂ to methane with a faradaic efficiency of 1-2 %.

Metal-mediated diradical tuning for DNA replication arrest via template strand scission

A series of M(PyED)·X (X = 2Cl-, SO42-) pyridine-metalloenediyne complexes [M = Cu(II), Fe(II), or Zn(II)] and their independently synthesized, cyclized analogs have been prepared to investigate their potential as radical-generating DNA-damaging agents. All complexes possess a 1:1 metal-to-ligand stoichiometry as determined by electronic absorption spectroscopy and X-ray diffraction. Solution structural analysis reveals a pπ Cl [Formula: see text] Cu(II) LMCT (22,026 cm-1) for Cu(PyED)·2Cl, indicating three nitrogens and a chloride in the psuedo-equatorial plane with the remaining pyridine nitrogen and solvent in axial positions. EPR spectra of the Cu(II) complexes exhibit an axially elongated octahedron. This spectroscopic evidence, together with density functional theory computed geometries, suggest six-coordinate structures for Cu(II) and Fe(II) complexes and a five-coordinate environment for Zn(II) analogs. Bergman cyclization via thermal activation of these constructs yields benzannulated product indicative of diradical generation in all complexes within 3 h at 37 °C. A significant metal dependence on the rate of the reaction is observed [Cu(II) > Fe(II) > Zn(II)], which is mirrored in in vitro DNA-damaging outcomes. Whereas in situ chelation of PyED leads to considerable degradation in the presence of all metals within 1 h under hyperthermia conditions, Cu(II) activation produces >50% compromised DNA within 5 min. Additionally, Cu(II) chelated PyED outcompetes DNA polymerase I to successfully inhibit template strand extension. Exposure of HeLa cells to Cu(PyBD)·SO4 (IC50 = 10 μM) results in a G2/M arrest compared with untreated samples, indicating significant DNA damage. These results demonstrate metal-controlled radical generation for degradation of biopolymers under physiologically relevant temperatures on short timescales.

CuAAC-based assembly and characterization of a ruthenium-copper dyad containing a diimine-dioxime ligand framework

The design of molecular dyads combining a light-harvesting unit with an electroactive centre is highly demanded in the field of artificial photosynthesis. The versatile Copper-catalyzed Azide-Alkyne Cycloaddition (CuAAC) procedure was employed to assemble a ruthenium tris-diimine unit to an unprecedented azide-substituted copper diimine-dioxime moiety. The resulting Ru^IICu^II dyad 4 was characterized by electrochemistry, ¹H NMR, EPR, UV-visible absorption, steady-state fluorescence and transient absorption spectroscopies. Photoinduced electron transfer from the ruthenium to the copper centre upon light-activation in the presence of a sacrificial electron donor was established thanks to EPR-monitored photolysis experiments, opening interesting perspectives for photocatalytic applications.

The role of ligand covalency in the selective activation of metalloenediynes for Bergman cyclization

One of the key concerns with the development of radical-generating reactive therapeutics is the ability to control the activation event within a biological environment. To that end, a series of quinoline-metal-loenediynes of the form M(QuiED)·2Cl (M = Cu(II), Fe(II), Mg(II), or Zn(II)) and their independently synthesized cyclized analogs have been prepared in an effort to elucidate Bergman cyclization (BC) reactivity differences in solution. HRMS(ESI) establishes a solution stoichiometry of 1:1 metal to ligand with coordination of one chloride counter ion to the metal center. EPR spectroscopy of Cu(QuiED)·2Cl and Cu (QuiBD)·2Cl denotes an axially-elongated tetragonal octahedron (g║ > g⊥ > 2.0023) with a dx2-y2 ground state, while the electronic absorption spectrum reveals a pπ Cl→Cu(II) LMCT feature at 19,000 cm -1, indicating a solution structure with three nitrogens and a chloride in the equatorial plane with the remaining quinoline nitrogen and solvent in the axial positions. Investigations into the BC activity reveal formation of the cyclized product from the Cu(II) and Fe(II) complexes after 12 h at 45 °C in solution, while no product is observed for the Mg(II) or Zn(II) complexes under identical conditions. The basis of this reactivity difference has been found to be a steric effect leading to metal-ligand bond elongation and thus, a retardation of solution reactivity. These results demonstrate how careful consideration of ligand and complex structure may allow for a degree of control and selective activation of these reactive agents.

Two macrocyclic pentaaza compounds containing pyridine evaluated as novel chelating agents in copper(II) and nickel(II) overload

Two pentaaza macrocycles containing pyridine in the backbone, namely 3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene ([15]pyN(5)), and 3,6,10,13,19-pentaazabicyclo[13.3.1]nonadeca-1(19),15,17-triene ([16]pyN(5)), were synthesized in good yields. The acid-base behaviour of these compounds was studied by potentiometry at 298.2K in aqueous solution and ionic strength 0.10 M in KNO(3). The protonation sequence of [15]pyN(5) was investigated by (1)H NMR titration that also allowed the determination of protonation constants in D(2)O. Binding studies of the two ligands with Ca(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Pb(2+) metal ions were performed under the same experimental conditions. The results showed that all the complexes formed with the 15-membered ligand, particularly those of Cu(2+) and especially Ni(2+), are thermodynamically more stable than with the larger macrocycle. Cyclic voltammetric data showed that the copper(II) complexes of the two macrocycles exhibited analogous behaviour, with a single quasi-reversible one-electron transfer reduction process assigned to the Cu(II)/Cu(I) couple. The UV-visible-near IR spectroscopic and magnetic moment data of the nickel(II) complexes in solution indicated a tetragonal distorted coordination geometry for the metal centre. X-band EPR spectra of the copper(II) complexes are consistent with distorted square pyramidal geometries. The crystal structure of [Cu([15]pyN(5))](2+) determined by X-ray diffraction showed the copper(II) centre coordinated to all five macrocyclic nitrogen donors in a distorted square pyramidal environment.

Synthesis and spectroscopic characterization of copper(II) tetraazaiminooxime macrocyclic complexes--a tetragonal distortion analysis

Herein we describe the synthesis and spectroscopic (infrared and UV-vis) analysis of [Cu(II)(dohpn)(L)](n+) (dohpn=imineoximic tetraazamacrocyclic ligand 2,3,9,10-tetramethyl-1,4,8,11-tetraazaundecane-1,3,8,10-tetraen-11-ol-1-olate) and L=SCN(-), I(-), Cl(-) (n=0) and 4-aminopyridine (ampy), 4,4'-bipyridine (bipy), imidazole (im), 2-aminopyrazine (ampz) and water (n=1+). The following order of the Jahn-Teller stabilization energy (cm(-1)) was observed: I(-)(6452)<Cl(-)(6477)<NCS(-)(6494)<ampy(6562)<im(6614)<bipy(6685)<ampz(6739)<H(2)O(7813) and the energy of the crystal field transition was correlated with the tetragonal distortion in the structures.

Dioxadiaza- and trioxadiaza-macrocycles containing one dibenzofuran unit selective for cadmium

The binding properties of dioxadiaza- ([17](DBF)N2O2) and trioxadiaza- ([22](DBF)N2O3), macrocyclic ligands containing a rigid dibenzofuran group (DBF), to metal cations and structural studies of their metal complexes have been carried out. The protonation constants of these two ligands and the stability constants of their complexes with Ca2+, Ba2+, and Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Cd2+, were determined at 298.2 K in methanol-water (1:1, v/v), and at ionic strength 0.10 mol dm-3 in KNO3. The values of the protonation constants of both ligands are similar, indicating that no cavity size effect is observed. Only mononuclear complexes of these ligands with the divalent metal ions studied were found, and their stability constants are lower than expected, especially for the complexes of the macrocycle with smaller cavity size. However, the Cd2+ complex with [17](DBF)N2O2 exhibits the highest value of stability constant for the whole series of metal ions studied, indicating that this ligand reveals a remarkable selectivity for cadmium(II) in the presence of all the metal ions studied, except copper(II), indicating that this ligand reveals a remarkable selectivity for cadmium(II) in the presence of the mentioned metal ions. The crystal structures of H2[17](DBF)N2O3(2+) (diprotonated form of the ligand) and of its cadmium complex were determined by X-ray diffraction. The Cd2+ ion fits exactly inside the macrocyclic cavity exhibiting coordination number eight by coordination to all the donor atoms of the ligand, and additionally to two oxygen atoms from one nitrate anion and one oxygen atom from a water molecule. The nickel(II) and copper(II) complexes with the two ligands were further studied by UV-vis-NIR and the copper(II) complexes also by EPR spectroscopic techniques in solution indicating square-pyramidal structures and suggesting that only one nitrogen and oxygen donors of the ligands are bound to the metal. However an additional weak interaction of the second nitrogen cannot be ruled out.

Synthesis and biodistribution of a Lipophilic 64Cu-labeled monocationic Copper(II) complex

The lipophilic, monocationic copper(II) complex of the diiminedioxime ligand 2,10-di-n-butyl-3,9-dimethyl-1,4,8,11-tetraazaundeca-1,3,8,10-tetraen-1,11-dione dioxime was synthesized and labeled with 64Cu. The biological properties of the 64Cu-labeled complex were measured in vivo and in vitro. In vivo, the complex shows uptake by the heart similar to that of 99mTc-tetrofosmin. In vitro, its uptake by multidrug resistant and non-resistant MES-SA tumor cells parallels that of 99mTc-MIBI, a well-characterized marker of multidrug resistance. These results suggest that this class of copper complexes may form the basis for the development of a 64Cu PET radiopharmaceutical for the functional imaging of multidrug resistance and/or myocardial perfusion.

Synthesis and biodistribution of 64Cu-labeled monocationic diiminedioxime copper(II) complexes

Monocationic copper(II) complexes of three derivatives of the diiminedioxime ligand 2,10-dioximino-3,9-dimethyl-4,8-diazaundeca-3,8-diene were labeled with 64Cu in high radiochemical yield, and the biodistribution of the complexes was measured in mice. The concentration of the complexes in the heart was low, but the partition coefficients of the complexes are less than optimal for a myocardial perfusion agent. All three complexes are resistant to reduction by glutathione in vitro. This suggests that more lipophilic derivatives of these compounds merit further investigation as possible myocardial perfusion agents.