π–π Stacking Interaction in an Oxidized CuII–Salen Complex with a Side-Chain Indole Ring: An Approach to the Function of the Tryptophan in the Active Site of Galactose Oxidase

In order to gain new insights into the effect of the π–π stacking interaction of the indole ring with the Cu^II–phenoxyl radical as seen in the active form of galactose oxidase, we have prepared a Cu^II complex of a methoxy-substituted salen-type ligand, containing a pendent indole ring on the dinitrogen chelate backbone, and characterized its one-electron-oxidized forms. The X-ray crystal structures of the oxidized Cu^II complex exhibited the π–π stacking interaction of the indole ring mainly with one of the two phenolate moieties. The phenolate moiety in close contact with the indole moiety showed the characteristic phenoxyl radical structural features, indicating that the indole ring favors the π–π stacking interaction with the phenoxyl radical. The UV/Vis/NIR spectra of the oxidized Cu^II complex with the pendent indole ring was significantly different from those of the complex without the side-chain indole ring, and the absorption and CD spectra exhibited a solvent dependence, which is in line with the phenoxyl radical–indole stacking interaction in solution. The other physicochemical results and theoretical calculations strongly support that the indole ring, as an electron donor, stabilizes the phenoxyl radical by the π–π stacking interaction.     

Formation of the CuII–Phenoxyl Radical by Reaction of O2 with a CuII–Phenolate Complex via the CuI–Phenoxyl Radical

Reaction of Cu(ClO₄)₂ ⋅ 6 H₂O with a tripodal 2N2O ligand, H₂Me₂NL, having a p-(dimethylamino)phenol moiety, in CH₂Cl₂/MeOH (1:1 v/v) under basic conditions under an inert gas atmosphere gave [Cu(Me₂NL)(H₂O)] ( 1 ). The same reaction carried out under aerobic conditions gave [Cu(Me₂NL)(MeOH)]ClO₄ ( 2 ), which could be obtained also from the isolated complex 1 by reaction with O₂ in CH₂Cl₂/MeOH. The X-ray crystal structures of 1 and 2 revealed similar square-pyramidal structures, but 2 showed the (dimethylamino)phenoxyl radical features. Complex 1 exhibits characteristic Cu^II EPR signals of the d ground state in CH₂Cl₂/MeOH at 77 K, whereas 2 is EPR-silent. The EPR and X-ray absorption fine structure (XAFS) results suggest that 2 is assigned to the Cu^II–(dimethylamino)phenoxyl radical. However, complex 1 showed different features in the absence of MeOH. The EPR spectrum of the CH₂Cl₂ solution of 1 exhibits distortion from the d ground state and a temperature-dependent equilibrium between the Cu^II–(dimethylamino)phenolate and the Cu^I–(dimethylamino)phenoxyl radical. From these results, Cu^II–phenoxyl radical complex 2 is concluded to be formed by the reaction of 1 with O₂ via the Cu^I–phenoxyl radical species.     

Characterization and Reactivity of a Tetrahedral Copper(II) Alkylperoxido Complex

A tetrahedral Cu^II alkylperoxido complex [Cu^II(TMG₃tach)(OOCm)]⁺ ( 1^OOCm ) (TMG₃tach={2,2′,2′′-[(1s,3s,5s)-cyclohexane-1,3,5-triyl]tris-(1,1,3,3-tetramethyl guanidine)}, OOCm=cumyl peroxide) is prepared and characterized by UV/Vis, cold-spray ionization mass spectroscopy (CSI-MS), resonance Raman, and EPR spectroscopic methods. Product analysis of the self-decomposition reaction of 1^OOCm in acetonitrile (MeCN) indicates that the reaction involves O−O bond homolytic cleavage of the peroxide moiety with concomitant C−H bond activation of the solvent molecule. When an external substrate such as 1,4-cyclohexadiene (CHD) is added, the O−O bond homolysis leads to C−H activation of the substrate. Furthermore, the reaction of 1^OOCm with 2,6-di-tert-butylphenol derivatives produces the corresponding phenoxyl radical species (ArO^.) together with a Cu^I complex through a concerted proton-electron transfer (CPET) mechanism. Details of the reaction mechanisms are explored by DFT calculations.     

Kinetic and ESR studies of the Cu<Superscript>II</Superscript>-halides mediated oxidation of promazine by dioxygen in acidic aqueous solutions

The Cu^II-mediated oxidation of promazine by dioxygen to form promazine 5-oxide was studied in the presence of a large excess of dioxygen, Cu^II-halides (Cl⁻, Br⁻) and H⁺ ions using u.v.–vis and ESR spectroscopies. The first step is a fast reaction between promazine and Cu^II-halides leading to the production of a stable promazine radical with much higher yield in bromide than chloride media. ESR results provide clear evidence for the formation of this radical. In the second step the cation radical is oxidized by dioxygen to a dication hydrolyzing to promazine 5-oxide. The promazine-superoxide complex, concentration of which is determined by steady-state approximation, is postulated as a significant intermediate resulting from the reduction of dioxygen by the cation radical. The final product, promazine 5-oxide, is formed via a spontaneous and a Cu^II-assisted reaction path way. Cu^II controls the reaction rate through: (i) oxidation of promazine to the promazine radical, (ii) acting as a scavenger of superoxide, and (iii) slow oxidation of the promazine radical in the parallel reaction. The rate is independent of [H⁺], linearly dependent on [O₂] and only slightly dependent on [Cu^II] within the excess concentration range of the Cu^II complexes used. Mechanistic consequences of all these results are discussed.     

A Stable Organic π‐Radical of a Zinc(II)–Copper(I)–Zinc(II) Complex of Decaphyrin

A Zn^II‐Cu^I‐Zn^II heterotrimetal complex of decaphyrin was synthesized by stepwise metalations: metalation of a [46]decaphyrin with Zn^II ions to produce a 46π decaphyrin bis(Zn^II) complex and its subsequent metalation with Cu^II ion. In the second metalation step, it has been shown that Cu^II ion is reduced to a Cu^I ion in the complex and a dianionic bis(Zn^II) containing [46]decaphyrin ligand is oxidized to the corresponding monoanionic [45]decaphyrin ligand, indicating a non‐innocent nature of the decaphyrin ligand. Despite the radical nature, the heterotrimetal complex is fairly stable under ambient conditions and exhibits almost no intermolecular magnetic interaction, owing to extensive delocalization of an unpaired electron in the large π‐conjugated circuit of decaphyrin moiety.     

Bis{[μ‐N,N′‐bis­(salicyl­idene)‐1,4‐butane­di­amine‐N,N′,O,O′‐copper(II)]‐μ‐chloro‐chloromercury(II)}

A new tetranuclear Cu^II–Hg^II–Hg^II–Cu^II complex, [Cu₂Hg₂Cl₄(C₁₈H₁₈N₂O₂)₂], has been prepared by means of a copper complex found in the literature. The molecular structure of this complex was determined by X‐ray diffraction and the Cu–Hg–Hg–Cu chain was seen to be non‐linear. The change in magnetic susceptibility with temperature was recorded for this complex and observed to abide by the Curie–Weiss law. The coordination around the Hg^II ions is square pyramidal. The Cu?Hg bridging distance is 3.5269 (7) Å.     

Synthesis and properties of a triarylamine derivative with a coordination site and its copper (II) complex

A ligand, 4-(bis(2-picolyl)aminomethyl)-4′,4″-dimethyltriphenylamine ((2-py)2TPA) and its copper complex were designed and prepared in order to examine intramolecular interactions of organic cation radical–metal ion. CV measurements of the copper complex showed reversible Cu^I/Cu^II and TPA/TPA^+· redox couples. The spin–spin interaction in [Cu((2-py)2TPA)Cl]²⁺ generated upon one electron oxidation of the copper complex was examined by ESR measurements.     

Oxidation of Cu(II) aminopolycarboxylates by carbonate radical: A flash photolysis study

Reactions of carbonate radical (Co₃ ⁻) generated by photolysis or by radiolysis of a carbonate solution, with Cu(II) complexes of aminopolycarboxylic acids viz., Cu(II)ethylenediamine tetraacetate [Cu^IIEDTA]²⁻ and Cu(II)-iminodiacetate [Cu^IIIDA] were studied at pH 10. 5 and ionic strength 0.2 mol·dm⁻³. Time-resolved spectroscopy and kinetics for the transients were studied using flash photolysis and stable products arising from the ligand degradation of the complex were ascertained by steady-state radiolysis experiments. From the kinetic data it is observed that CO₃ ⁻, radical reacts initially with Cu^II-complex to form a transient intermediate having maximum absorption at 335 nm and 430 nm. From the subsequent reactions of this intermediate it was assigned to be Cu^III. species. This Cu(III) species undergoes intermolecular electron transfer with the Cu^II-complex to give a radical intermediate which again slowly reacts with Cu^II-complex to give a long lived species containing Cu−C bond. This long lived species, however, slowly decomposed to give glyoxalic reaction between Cu^III-complex and a suitable donor, the one electron reduction potential for [Cu^IIIEDTA]¹⁻/[Cu^IIEDTA]²⁻ and [Cu^IIIIDA]⁺¹/Cu^IIIDA was determined.     

Mechanistic Insight into Peptidyl-Cysteine Oxidation by the Copper-Dependent Formylglycine-Generating Enzyme

The copper-dependent formylglycine-generating enzyme (FGE) catalyzes the oxygen-dependent oxidation of specific peptidyl-cysteine residues to formylglycine. Our QM/MM calculations provide a very likely mechanism for this transformation. The reaction starts with dioxygen binding to the tris-thiolate Cu^I center to form a triplet Cu^II-superoxide complex. The rate-determining hydrogen atom abstraction involves a triplet-singlet crossing to form a Cu^II−OOH species that couples with the substrate radical, leading to a Cu^I-alkylperoxo intermediate. This is accompanied by proton transfer from the hydroperoxide to the S atom of the substrate via a nearby water molecule. The subsequent O−O bond cleavage is coupled with the C−S bond breaking that generates the formylglycine and a Cu^II-oxyl complex. Moreover, our results suggest that the aldehyde oxygen of the final product originates from O₂, which will be useful for future experimental work.     

Intermolecular interaction and magnetic coupling mechanism of a mixed-valence CuIICuI tetranuclear complex with iodide bridge

A tetranuclear Cu^ICu^II mixed oxidation state complex, [Cu^II ₂(μ-I)₂Cu^I ₂(μ-I)₂(phenP)₂I₂] (phenPE: 2-(1H-pyrazol-1-yl)-1,10-phenanthroline), has been prepared and its crystal structure is determined by X-ray crystallography. In the complex, Cu^II is a distorted square pyramid and Cu^I is a distorted trigonal planar coordination environment; Cu^II and Cu^I are bridged by iodide. It is rare to form a Cu^II-iodide bond and for Cu^II and Cu^I to be bridged by iodide. In the crystal, there is a slipped π–π stacking between adjacent Cu^II complexes, which resulted in the formation of the 1-D chain along the c axis. The fitting for the variable-temperature magnetic susceptibility data gave magnetic coupling constant 2J?=??1.16?cm^?1 and it may be ascribed to the intermolecular π–π magnetic coupling pathway.     

Synthesis and properties of a redox active ligand with bispicorylamino groups and its dinuclear complex

A ligand, 4-methoxy-4′,4″-bis[N,N-bis(2-pyridylmethyl) aminomethyl]triphenyl-amine, and its palladium and copper dinuclear complexes were designed and prepared in order to examine intramolecular interactions between an organic cation radical and the metal ion. Novel NMR techniques, COSY and NOESY, were applied to the palladium complex to examine its conformation in solution. The palladium complex was found to prefer a folded conformation at ambient temperature, indicating the occurrence of intramolecular stacking interaction. CV measurements of the copper complex showed reversible Cu^I/Cu^II and TPA/TPA⁺ redox couples. The spin–spin interaction in the radical pendant copper complex generated upon one electron oxidation of the copper complex was examined by cw-ESR measurements.     

A Boron Dipyrromethene (BODIPY)‐Based CuII–Bipyridine Complex for Highly Selective NO Detection

A BODIPY‐containing Cu^II–bipyridine complex for the simple selective fluorogenic detection of NO in air and in live cells is reported. The detection mechanism is based on NO‐promoted Cu^II to Cu^I reduction, followed by demetallation of the complex, which results in the clearly enhanced emission of the boron dipyrromethene (BODIPY) unit.     

Redox switchable ligands suitable for transition metal ions: Protonation,complexation and electrochemical properties of a ferrocene-modified tetraamine diketone and its saturated analogue

Abstract

The new ferrocene-containing water-soluble ligands 1 and 2 were synthesized and their protonation and complexation properties toward Ni^II and Cu^II studied as a function of pH, by means of potentiometric titration experiments. Electrochemical measurements were performed in aqueous solution on pure 1 and 2 and in the presence of Ni^II and Cu^II cations, in the pH range 2–12, allowing us to determine the redox potential values relative to the ferrocene oxidation in the free ligands and in their Ni^II and Cu^II complexes. 1 and 2 behave as redox switchable ligands, the former enhancing, the latter decreasing its binding ability upon oxidation of the appended ferrocene function. Besides, the Cu^II complex of ligand 1 and the Ni^II complex of ligand 2 behave as two-centre two-electron redox systems, the complexed metal cation being subject to further oxidation to M^III.     

Synthesis,Structures, and Properties of Crystalline Salts with Radical Anions of Metal‐Containing and Metal‐Free Phthalocyanines

Radical anion salts of metal‐containing and metal‐free phthalocyanines [MPc(3?)]^.?, where M=Cu^II, Ni^II, H₂, Sn^II, Pb^II, Ti^IVO, and V^IVO ( 1 – 10 ) with tetraalkylammonium cations have been obtained as single crystals by phthalocyanine reduction with sodium fluorenone ketyl. Their formation is accompanied by the Pc ligand reduction and affects the molecular structure of metal phthalocyanine radical anions as well as their optical and magnetic properties. Radical anions are characterized by the alternation of short and long C?N_imine bonds in the Pc ligand owing to the disruption of its aromaticity. Salts 1 – 10 show new bands at 833–1041 nm in the NIR range, whereas the Q‐ and Soret bands are blue‐shifted by 0.13–0.25 eV (38‐92 nm) and 0.04–0.07 eV (4–13 nm), respectively. Radical anions with Ni^II, Sn^II, Pb^II, and Ti^IVO have S=1/2 spin state, whereas [Cu^IIPc(3?)]^.? and [V^IVOPc(3?)]^.? containing paramagnetic Cu^II and V^IVO have two S=1/2 spins per radical anion. Central metal atoms strongly affect EPR spectra of phthalocyanine radical anions. Instead of narrow EPR signals characteristic of metal‐free phthalocyanine radical anions [H₂Pc(3?)]^.? (linewidth of 0.08–0.24 mT), broad EPR signals are manifested (linewidth of 2–70 mT) with g‐factors and linewidths that are strongly temperature‐dependent. Salt 11 containing the [Na^IPc(2?)]^? anions as well as previously studied [Fe^IPc(2?)]^? and [Co^IPc(2?)]^? anions that are formed without reduction of the Pc ligand do not show changes in molecular structure or optical and magnetic properties characteristic of [MPc(3?)]^.? in 1 – 10 .     

Synthesis,Characterization, DNA‐binding Properties,and ­Antioxidant Activity of a Copper(II) Complex with 1, 3‐Bis­(1‐allaylbenzimidazol‐2‐yl)‐2‐oxopropane

A new complex of copper(II) picrate (pic) with 1, 3‐bis(1‐allaylbenzimidazol‐2‐yl)‐2‐oxopropane (aobb), with the composition [Cu(aobb)₂](pic)₂, was synthesized and characterized. The crystal structure of the copper(II) complex revealed that the coordination environment around the central copper(II) atom is a distorted octahedral arrangement. Electronic absorption spectroscopy, ethidium bromide displacement experiments and viscosity measurements indicate that the ligand and the Cu^II complex can strongly bind to calf thymus DNA, presumably by an intercalation mechanism. Furthermore, the antioxidant activity of the Cu^II complex was determined by superoxide and hydroxyl radical scavenging method in vitro, which indicate that the Cu^II complex has the activity to suppress OH ^· and O₂ ^{· –}.     

A bis‐copper(II)–[d‐βVal3,7]ascidiacyclamide complex enveloping two square pyramids and sharing an apex atom from a carbonate anion

The four azole rings place structural restrictions on ascidiacyclamide (ASC). As a result, the structure of ASC exists in an equilibrium between two major forms (i.e. folded and square). [d ‐βVal^3,7]Ascidiacyclamide (βASC) was synthesized by replacing two d ‐Val‐Thz (Val is valine and Thz is thiazole) blocks with d ‐β‐Valine (D‐βVal‐Thz). This modification expands the peptide ring; the original 24‐membered macrocycle of ASC becomes a 26‐membered ring. Circular dichroism (CD) spectra showed that, in solution, the structural equilibrium is maintained with βASC, but the folded form is dominant. A copper complex was prepared, namely [[d ‐βVal^3,7]ascidiacyclamide(2?)]aqua‐μ‐carbonato‐dicopper(II) monohydrate, [Cu₂(C₃₈H₅₄N₈O₆S₂)(CO₃)(H₂O)]·H₂O, to determine the effect of the change in ring size on the coordinated structure. The obtained bis‐Cu^II–βASC complex contains two water molecules and a carbonate anion. Two Cu^II ions are chelated by three N‐donor atoms of two Thz–Ile–Oxz (Ile is isoleucine and Oxz is oxazoline) units. An O atom of the carbonate anion bridges two Cu^II ions, forming two square pyramids. These features are similar to the previously reported structure of the Cu^II–ASC complex, but the two pyramids are enveloped inside the peptide and share one apex. In the Cu^II–ASC complex, the apex of each square pyramid is an O atom of a water molecule, and the two pyramids are oriented toward the outside of the peptide. The incorporated β‐amino acids of βASC make the space inside the peptide large enough to envelop the two square pyramids. The observed structural changes in the bis‐Cu^II–βASC complex arising from ring expansion are particularly interesting in the context of the previously reported structure of the Cu^II–ASC complex.     

Superoxide Dismutase Activity of the Naturally Occurring Human Serum Albumin–Copper Complex without Hydroxyl Radical Formation

The superoxide radical anion (O₂^.?) is biologically toxic and contributes to the pathogenesis of various diseases. Here we describe the superoxide dismutase (SOD) activity of human serum albumin (HSA) complexed with a single Cu^II ion at the N‐terminal end (HSA–Cu complex). The structure of this naturally occurring copper‐coordinated blood serum protein has been characterized by several physicochemical measurements. The O₂^.? dismutation ability of the HSA–Cu (1:1) complex is almost the same as that of the well‐known SOD mimics, such as Mn^III‐tetrakis(N‐methylpyridinium)porphyrin. Interestingly, the HSA–Cu complex does not induce a subsequent Fenton reaction to produce the hydroxyl radical (OH^.), which is one of the most harmful reactive oxygen species.     

The assembly,synthesis and properties of a trinuclear cyano-bridged CuII–MoIV–CuII complex

A new trinuclear cyano-bridged Cu^II–Mo^IV–Cu^II compound has been prepared, characterized spectroscopically (UV–Vis and IR) and its structure determined by X-ray crystallography. The title complex 1 exhibits an antiferromagnetic exchange interaction between copper(II) ions mediated by [Mo(CN)₈]^4? diamagnetic units.     

Macrocyclic effect of auxiliary ligand on the gas‐phase dissociation of ternary copper(II)–GGX complexes

Previous studies into the dissociation of [Cu^II(dien)peptide] ^{. 2+} ions (dien = diethylenetriamine) have shown that NH‐containing auxiliary ligands do not favor the formation of [peptide] ^{. +} species; instead, they promote proton‐transfer reactions, especially for peptides containing basic amino residues. Formation of radical cationic tripeptides of the form GGX ^{. +} [GGX = glycylglycyl(residue X)] becomes feasible upon substituting the open‐chain tridentate ligand dien with its analogous cyclic ligand, 1,4,7‐triazacyclononane (9‐aneN₃); i.e., from [Cu^II(9‐aneN₃)GGX] ^{. 2+} ions. Similar enhancements occur when using 1,4,7,10‐tetraoxacyclododecane (12‐crown‐4) in place of its open‐chain analog, 2,5,8,11‐tetraoxadecane (triglyme). We have demonstrated that a sterically encumbered auxiliary macrocyclic ligand within [Cu^II(L)GGX] ^{. 2+} complex ions [where L = 9‐aneN₃ or 12‐crown‐4] facilitates the formation of radical cationic peptides through gas‐phase fragmentation. We verified our experimental observations by examining the reactivities of a series of 19 tripeptides of the type GGX that differ only in the identity of their C‐terminal residue. The energy of the electron‐transfer reaction correlates well with the bond‐dissociation energy of the peptide–Cu(II) interaction; the presence of a constrained macrocyclic ligand weakens metal–peptide chelation through steric repulsion between the ligand and the peptide, and this situation may lead to more favorable radical cationic peptide formation. Copyright © 2006 John Wiley & Sons, Ltd.