Histidine-containing mixed amino acid and dipeptide copper(II) complexes. Cyclic voltammetric studies on the reduction in aqueous media

Summary Cyclic voltammetric studies on the reduction of the title complexes were carried out in aqueous media at HMDE. The histidine-containing mixed amino acid copper(II) complexes exhibit a relatively sharp reduction peak which is identified with an adsorption-controlled reduction process. The histidine-containing copper(II) dipeptide complexes undergo a one-electron reduction generating an intermediate copper(I) species which subsequently decomposes to Cu⁰. The mechanism of these processes is discussed. The cyclic voltammograms of these complexes are not complicated by adsorption processes.     

Cyclic voltammetric studies of copper(II) amino acid complexes: electrochemical evidence for the generation of copper(I) complex as an intermediate

The reduction of the title complexes is studied by cyclic voltammetry in aqueous media. It proceeds through a one-electron process generating the corresponding copper(I) amino acid complexes. The reduced copper(I) species undergo chemical reactions generating Cu(O) at the mercury electrode. The unreacted fraction of the copper(I) species is re-oxidised to the copper(II) complexes. The Cu(O) generated undergo a two-electron oxidation to Cu²⁺ at less cathodic potentials which get reduced to Cu(O) subsequently. pH-dependence of these complexes is also investigated.     

Mixed ligand Cu(II) amino acid complexes containing 2,2′-bipyridyl. Cyclic voltammetric studies of the reduction in aqueous media

The reduction of the title ternary complexes was studied by cyclic voltammetry in aqueous media. Reversible one-electron reduction was involved generating an intermediate Cu(I) species which subsequently dissociated to Cu⁰ at the mercury electrode. The intermediate ternary Cu(I) amino acid complexes were found to be less unstable than the corresponding Cu(I) binary complexes. The pH-dependence of these ternary complexes was also studied.     

Unusual electrochemical reduction of copper(II) to copper(I) in polyoxotungstates

The electrochemical behaviour of the copper-substituted Keggin-type and sandwich-type polyoxotungstate anions of the compounds α-[(C₄H₉)₄N]₄H[PW₁₁Cu^IIO₃₉] and α-B-[(C₄H₉)₄N]₇H₃[Cu^II₄(H₂O)₂(PW₉O₃₄)₂] was studied by cyclic voltammetry in acetonitrile. In both cases two copper 1-electron reduction waves were detected in the cathodic scan. The first one was due to the reduction of one Cu^II to Cu^I in the polyoxoanion and the second one to the consecutive reduction of the preformed Cu^I to Cu⁰, with the consequent deposition/adsorption of the ejected metal atom at the glassy carbon electrode surface. In the anodic scan, Cu⁰ was re-oxidised with regeneration of the initial copper(II) complexes, via a Cu^I intermediate. The observed two-step reduction of copper(II) to copper(0) and the formation of intermediate species containing copper(I) is here reported for the first time for copper substituted polyoxotungstates. The co-ordination of the acetonitrile molecules to the copper ions must play a role in the formation of the copper(I) species, which are not detected in aqueous solution.     

Template-directed synthesis of macrocyclic copper(II) complexes of diazacyclam, 1,3,6,10,12,15-hexaazatricyclo[13.3.1.16,10]eicosane

Template reaction of copper(II) nitrate with N-(2-aminoethyl)-1,3-diaminopropane and formaldehyde yields a macrocyclic copper(II) complex of 1,3,6,10,12,15-hexaazatricyclo[13.3.1.1^6,10]eicosane (L), [CuL(NO₃)₂] (1). Replacement of nitrate with perchlorate gives [CuL(ClO₄)₂] (2). These complexes have been characterized by FT-IR and Raman spectroscopies, electronic absorption, cyclic voltammetry, and X-ray crystallography. The crystal structure of 1 shows that copper has distorted octahedral geometry with two secondary and two tertiary amines of the macrocycle and two oxygen atoms from nitrate coordinating the axial positions. The copper in 2 has the same geometry with axial positions occupied by one oxygen atom of perchlorate. Copper lies on the plane of four coordinated nitrogen atoms and there is no rms deviation from this plane. Cyclic voltammetry of 1 and 2 gives two one-electron waves corresponding to Cu^II/Cu^III (?0.75,??0.72) and Cu^II/Cu^I (0.48, 0.24) processes. For understanding of geometry parameters in diazacyclam-based copper(II) complexes, a survey on complexes from CSD structures is presented. In this study the macrocycle hole size was estimated by ionic radii of metal ions located inside of it.     

Synthesis and characterization of copper(II) hexaazamacrotetracyclic complexes containing organic ligands

Two mononuclear copper(II) complexes [Cu(L)(NO₂)](ClO₄) (1) and [Cu(L)(MO₄)]₂· 5H₂O (2) (L = 1,3,10, 12,16,19-hexaazatetracyclo[17,3,1,1^12.16,0^4.9]tetracosane) have been synthesized and their structures determined. Both compounds show a distorted square-pyramidal geometry with the two secondary and two tertiary amines of the macrocycle and one ligand coordinated at the axial position. Cyclic voltammetry of the complexes gives two one-electron waves corresponding to Cu^II/Cu^III and Cu^II/Cu^I processes. The electronic spectra and electrochemical behavior of the complexes are significantly affected by the nature of the organic ligands.     

Preparation, characterization and crystal structures of copper(II) hexaazamacrotetracyclic complexes with organic ligands

The reaction of [Cu(L)](ClO₄)₂ · H₂O (L=1,3,10,12,16,19-hexaazatetracyclo[17,3,1,1^12.16,0^4.9]tetracosane) with NaN₃ and Na₂tp yields mononuclear and dinuclear copper(II) complexes, [Cu(L)(N₃)](ClO₄) (1) and [Cu(L)(μ-tp)](ClO₄) · 2H₂O (2). These complexes have been characterized by X-ray crystallography, electronic absorption, cyclic voltammetry and magnetic susceptibility. The crystal structure of (1) shows that the copper(II) ion has a distorted square-pyramidal geometry with the two secondary and two tertiary amines of the macrocycle and one nitrogen atom from the azide group coordinating the axial position. The copper(II) ions in (2) are bridged by the terephthalate anion to form a dinuclear complex, in which each copper(II) ion reveals a distorted square-pyramid with four nitrogen atoms of the macrocycle and the oxygen atom of bridging tp ligand. Cyclic voltammetry of the complexes gives two one-electron waves corresponding to Cu^II/Cu^III and Cu^II/Cu^I processes. The magnetic susceptibility measurement for (2) exhibits a weak antiferromagnetic interaction between copper(II) centers with a 2J value of −2.21 cm⁻¹ (H = −2JΣS₁ · S₂). The electronic spectra and electrochemical behavior of the complexes are significantly affected by the nature of the organic ligands.     

Synthesis and characterization of mononuclear copper(II) and dinuclear cadmium(II) complexes with di-(2-picolyl)sulfide

The reaction of CuCl₂ · 2H₂O and CdCl₂ with di-(2-picolyl)sulfide (dps) leads to the formation of mononuclear copper(II) and binuclear cadmium(II) complexes, [Cu(dps)Cl₂] · H₂O (1) and [(dps)(Cl)Cd^II(μ-Cl)₂Cd^II(Cl)(dps)] (2). The copper atom in (1) is coordinated to one sulfur and two nitrogen atoms from the dps ligand and two chlorides in a distorted square-pyramidal environment. Complex (2) has two distorted octahedra sharing the basal edge that contain the bridging chloro ligands, each of which resides at a center of inversion. Cyclic voltammetric data show that (1) undergoes two reversible one-electron waves corresponding to Cu^II/Cu^III and Cu^II/Cu^I processes. However, cyclic voltammetry of (2) gives two irreversible reduced waves.     

Synthesis,properties, and crystal structures of mononuclear nickel(II) and copper(II) complexes with 2-oximino-3-thiosemicarbazone-2,3-butanedione

The tridentate ligand 2-Oximino-3-thiosemicarbazone-2,3-butanedione (Hotsb) reacts with MCl₂ (M = Ni²⁺ or Cu²⁺) to give rise to the mononuclear complexes [Ni(Hotsb)₂]Cl₂ · H₂O (1) and [Cu(Hotsb)Cl₂] · H₂O (2). These complexes have been characterized by X-ray crystallography, spectroscopy, and cyclic voltammetry. The nickel(II) ion in (1) is in a six-coordinate octahedral environment being bonded to the two protonated tridentate ligands which occupy mer positions. The copper(II) ion in (2) is in a five-coordinate square-pyramidal geometry, in which the basal plane is made up the two nitrogens, sulfur, and chloride atom, while the other chloride atom is coordinated at the axial position. The cyclic voltammogram of the complexes displays two one-electron waves corresponding to M^II/M^III and M^II/M^I processes. The electronic as well as infrared spectral properties of the title complexes are reported and discussed.     

Amination and amidation of aryl iodides catalyzed by copper(I)-phenanthroline complexes

Four kinds of copper(I)-phenanthroline complexes ([Cu^I(phen)₂]Cl, [Cu^I(phen)Cl]₂, [Cu^I(phen)₂]BF₄, and Cu^I(phen)PPh₃Cl) were prepared and used as catalysts for amination and amidation of aryl iodide to investigate the influence on the yields of products due to differences of the structures. These complexes were found to work as catalysts on these reactions and showed that the differences of structures of copper(I) complexes significantly influenced the yield of aryl-nitrogen bond forming processes.     

Nickel(II) and copper(II) complexes of unsymmetrical tetradentate reduced Schiff base ligands

Two new reduced Schiff base ligands, [HL¹ = 4-{2-[(pyridin-2-ylmethyl)-amino]-ethylimino}-pentan-2-one and HL² = 4-[2-(1-pyridin-2-yl-ethylamino)-ethylimino]-pentan-2-one] have been prepared by reduction of the corresponding tetradentate unsymmetrical Schiff bases derived from 1:1: 1 condensation of 1,2-ethanediamine, acetylacetone and pyridine-2-carboxaldehyde/2-acetyl pyridine. Four complexes, [Ni(L¹)]ClO₄ (1), [Cu(L¹)]ClO₄ (2), [Ni(L²)]ClO₄ (3), and [Cu(L²)]ClO₄ (4) with these two reduced Schiff base ligands have been synthesized and structurally characterized by X-ray crystallography. The mono-negative ligands L¹ and L² are chelated in all four complexes through the four donor atoms to form square planar nickel(II) and copper(II) complexes. Structures of 3 and 4 reveal that enantiomeric pairs are crystallized together with opposite chirality in the nitrogen and carbon atoms. The two Cu^II complexes (2 and 4) exhibit both irreversible reductive (Cu^II/Cu^I; E_pc, −1.00 and −1.04 V) and oxidative (Cu^II/Cu^III; E_pa, +1.22 and +1.17 V, respectively) responses in cyclic voltammetry. The electrochemically generated Cu^I species for both the complexes are unstable and undergo disproportionation.     

Ligand influence on the electrochemical behavior of some copper(II) thiosemicarbazone complexes

The redox properties of the title mono- and binuclear copper(II) chelates have been investigated by cyclic voltammetry in DMF at a working platinum electrode. The cathodic reduction and anodic oxidation of the investigated chelates produced the corresponding electrochemical Cu^I and Cu^IIIspecies stable only in the voltammetric time scale, The effects of substituents on E_1/2, redox properties and stability towards oxidation of the complexes were related to the electron-withdrawing or releasing ability of the substituents on the C=N¹[H, CH₃ or C₆H₅] and/or N⁴H [H, C₂H₅, C₆H₅ or pClC₆H₄] groups, The electron attracting substituents stabilize the Cu(II) complexes while electron-donating groups favor oxidation to Cu(III). Changes in the E_1/2 for the complexes due to remote substituent effects could be related to changes in basicity of N⁴H.Thus, variation in N⁴1-J has more influence on E_1/2 than changes in C=N¹. The correlation between E_1/2 of the complexes and pK_a of the ligands has been attributed to the spherical potential generated by the electron density of the donor atoms at the antibonding d orbitals.     

Carboxylate-bridged copper(II) complexes: synthesis,crystal structures,and superoxide dismutase activity

Two copper(II) complexes, [Cu₂(μ-benzoato)(L¹)₂]NO₃·2H₂O (1) and [Cu₂(μ-succinato)(L²)₂(H₂O)]ClO₄ (2), have been synthesized, where L¹ = N′-[(E)-phenyl(pyridin-2-yl)methylidene]benzoylhydrazone and L² = N′-[(E)-pyridin-2-ylmethylidene]benzoylhydrazone. These complexes were characterized including by single-crystal X-ray diffraction studies. The copper is five-coordinate in 1 while in 2 one copper is five-coordinate and the other is six-coordinate. Electrochemical behavior of these complexes was measured by cyclic voltammetry. The conproportionation equilibrium constants (K_con) for both complexes have been estimated. The superoxide dismutase (SOD) activities of 1 and 2 were measured by nitro blue tetrazolium assay. Complex 1 has better SOD activity than 2.     

Copper(II) complexes of entwined 2,9-di(o-substituted phenyl)-1,10-phenanthroline type ligands and intramolecular quenching in copper(I) complexes

Cu¹¹ complexes of 1,10-phenanthroline, disubstituted at the 2 and 9 positions or monosubstituted at the 2 position by phenyl moieties possessingortho substituents, were prepared and investigated by spectroscopic and electrochemical methods. The electronic spectral d-d band position varies from 14 500 to 13 200 cm⁻¹. E.s.r. g_‖ values are between 2.256 to 2.283 and A_‖ between 164 to 117×10⁻⁴ cm⁻¹. Thebis[2,9-di(o-substituted phenyl)-1,10-phenanthroline]Cu¹¹ complexes undergo reversible one-electron electrochemical reduction (Cu¹¹/Cu¹) in the +0.536 to +0.825 V potential range versus s.c.e., whereas thebis[2-mono(o-substituted phenyl)-1,10-phenanthroline]Cu¹¹ complexes undergo reduction in the +0.360 to +0.405 V range; the redox couple is found to be quasireversible. Emission studies on copper(I) complexes show that onlybis[2,9-di(o-tolyl)-1,10-phen]Copper(I) complex exhibits emission properties. Emission behaviour of other structurally similar compounds is explored. TMC 2555     

Trinuclear copper(II) complexes of some aza macrocycles

The syntheses of copper(II) complexes with neutral macrocyclic ligands 1,4,7,10,12,- 15,17,20,23,26,27,30-dodecaazadispiro[10·4·10·4]triacontane (DDST), 2,5,7,10,13,15,18,21,-23,26,29,32-dodecaazatricyclo[20·10·0·0^6,17]dotriacontane (DOCD) and 2,5,7,10,13,16,18,-21,23,26,29,32-dodecaaza-1,6,17,22-tetrachlorotricyclo[20·10·0·0^6,17]dotriacontane (DTTD) derived from triethylenetetramine, 1,2-diaminoethane and chlorocarbons (carbon tetrachloride, 1,l,2,2-tetrachloroethane and hexachloroethane, respectively) have been studied. Complexes [Cu₃(DDST)]Cl₆, [Cu₃(DOCD)]Cl₆ and [Cu₃(DTTD)]Cl₆?·?H₂O and the copper ion-free ligand hydrochlorides DDST?·?12HCl and DOCD?·?12HCl are supported by elemental analyses, conductivity measurements and spectroscopic studies. Potentiometric equilibrium studies on DDST and DOCD hydrochlorides and their copper complexes also support the structures.     

Synthesis,spectroscopic and voltammetric studies of mixed-ligand copper(II) complexes of amino acids

Two Cu^II complexes of the type [Cu(L¹)(L²)] (where L¹ = tryptophanate or phenylalaninate; L² = cysteine thiolate) have been prepared and characterised, and their spectrophotometric and voltammetric behaviour has been investigated. The results obtained by means of FT-IR, e.s.r., u.v.–vis. spectroscopy and by voltammetry revealed the existence of two different [Cu(L¹)(L²)] complexes. A significance decrease in the g _|| value and, concomitantly, an increase in the d–d transition energy was observed when a mixed-ligand complex is present. The observed anisotropic g-values indicate the presence of Cu^II in a tetragonally distorted octahedral geometry. Formation of a mixed-ligand copper complex can be considered as a type of synergism in the presence of cysteine. The redox state Cu^II or Cu^I of copper in the Cu(L²) complex depends on the analysing conditions, i.e., cysteine forms a Cu^II complex under aerobic conditions and a Cu^I complex in anaerobic media. Tryptophan or phenylalanine is bound to Cu^II ions in the Cu(L¹) complexes.     

Copper(I) complexes with fluorinated hydrotris(pyrazolyl)borate: Influence of electronic effects on their structure,physicochemical properties,and reactivity

Copper(I) coordination complexes of the anionic fluorinated ligand, hydrotris(3-trifluoromethyl-5-methyl-1-pyrazolyl)borate (L0f⁻), i.e. the copper(I) carbonyl complex, [Cu^I(L0f)(CO)] (1), the copper(I) triphenylphosphine complex, [Cu^I(L0f)(PPh₃)] (2), the copper(I) acetonitrile complex, [Cu^I(L0f)(NCMe)] (3), and the corresponding copper(I) triphenylphosphine complex with hydrotris(3,5-diisopropyl-1-pyrazolyl)-borate anion (L1⁻), i.e. [Cu^I(L1)(PPh₃)] (4), were synthesized in order to investigate the influence of the electron-withdrawing groups on the pyrazolyl rings. The structures of complexes 1, 2, and 4 were determined by X-ray crystallography. While X-ray crystallography did not show definitive trends in terms of copper(I) atom geometry, the clear influence of the electronic structure of the pyrazolyl rings is observed by spectroscopic techniques, namely, IR and multinuclear NMR spectroscopy. Finally, the relative stability of the copper(I) complexes is discussed.     

Synthesis,characterization, spectrophotometric and electrochemistry studies,and DNA cleavage of copper(II) complexes of a new azacrown bis-macrocycle and its mono-cyclic analogue

A new azacrown bis-macrocycle (5) and its mono–cyclic analogue (7) were synthesized and characterized by FT-IR, ¹H NMR, ¹³C NMR, DEPT ¹³C NMR, MS, and elemental analysis. The reaction with copper(II) nitrate yielded the corresponding complexes, formulated as Cu(5)(NO₃)₂·3H₂O (8), and Cu(7)(NO₃)₂·2.5H₂O (9). Also the stoichiometries of the complexes were determined in alcoholic solution and the results show that for both complexes the ratio of ligand to metal was 1:1 in methanol. The redox behavior of both complexes has been studied by cyclic voltammetry in DMF. Cyclic voltamogram of 8 shows quasi-reversible Cu^II/Cu^I redox couple whereas 9 shows a reversible Cu^II/Cu^I redox couple. Mono- and bis-macrocycle copper(II) complexes (8 and 9 respectively) cleaved plasmid pGS2 DNA by using an oxidative mechanism with 3- mercaptopropionic acid (MPA) as the reductant under aerobic conditions. The bis-macrocycle copper(II) complex 8 showed higher cleavage efficiency than their mono-macrocycle analogue 9 at the same Cu²⁺ concentration.     

Identification of Cu(II)/Cu(III) couples in binuclear copper(II) complexes

A new series of binuclear copper(II) complexes were synthesised and studied by magnetic, spectral, ESR and cyclic voltammetry methods. The μ_eff values per copper atom correspond to the values observed for mononuclear copper(II) complexes. ESR spectral data in solution indicate weak interactions resulting from the electron delocalisation through the ligand system. Two nearly reversible red-ox couples are identified at $\text{+}\text{?}$0.50 V and $\text{+}\text{?}$0.75 V vs SCE. They correspond to Cu(II)αCu(III) red-ox processes, successively occurring at the two copper sites in the binuclear complexes.     

Mixed-ligand complex formation of (2,2′,2″-terpyridine)copper(I) and (2,2′2″-terpyridine)copper(II) with some sulfur-containing amino acids

Summary Mixed-ligand complex formation of copper(I) with 2,2,2-terpyridine (terpy) and various biologically significant sulfurcontaining amino acids, L [L = cysteine, penicillamine,N-acetylcysteine andN-acetylpenicillamine] was investigated. WithN-acetylcysteine andN-acetylpenicillamine, copper(I) complexes [Cu(terpy)L]HCl have been isolated and characterised. The amino acid in each case bonds to the copper(I) through the sulfur atom only. These complexes are readily oxidised to the corresponding copper(II) complexes which contain a Cu^II-S(mercaptide) bond as demonstrated by the (S)Cu^II LMCT at 420 nm. The e.p.r. and electronic spectroscopic results are consistent with a distorted octahedral structure. Reactions of copper(I) with terpy and cysteine/ penicillamine in weakly acidic medium produce complexes containing two different copper(II) centres. From a strongly acidic medium, a binary copper(I) terpyridine complex is obtained instead and is shown from i.r. spectral absorptions to contain a protonated pyridyl ring.