Copper Complexes In Aqueous Solution Research Articles

Assessment of the isodesmic method in the calculation of standard reduction potential of copper complexes.

Molecular phenomena involving electron transfer and reduction/oxidation processes are of the utmost importance in chemistry. However, accurate computational calculations of standard reduction potentials (SRPs) for transition metal complexes are still challenging. For this reason, some computational strategies have been proposed in order to overcome the main limitations in SRP calculations for copper complexes. However, these strategies are limited to particular coordination spheres and do not represent a general methodology. In this work, we present standard reduction potential calculations for copper complexes in aqueous solution covering a wide range of coordination spheres. These calculations were performed using the M06-2X density functional, and by employing the direct and isodesmic approaches. Result analysis reveals that values obtained with the use of the isodesmic method are in better agreement with experimental values than those obtained from the direct method (mean unsigned error 0.39 V with the direct and 0.08 V with the isodesmic method). This approach provides values with errors comparable to the experimental uncertainty due to the proper cancellation of computational errors. These results strongly suggest the isodesmic approach as an adequate methodology for the calculation of SRPs for copper complexes with diverse coordination spheres. Graphical Abstract Comparison between direct and isodesmic methods in the calculation of standard reduction potentials for copper complexes using DFT methods.

Copper Oxidation/Reduction in Water and Protein: Studies with DFTB3/MM and VALBOND Molecular Dynamics Simulations.

We apply two recently developed computational methods, DFTB3 and VALBOND, to study copper oxidation/reduction processes in solution and protein. The properties of interest include the coordination structure of copper in different oxidation states in water or in a protein (plastocyanin) active site, the reduction potential of the copper ion in different environments, and the environmental response to copper oxidation. The DFTB3/MM and VALBOND simulation results are compared to DFT/MM simulations and experimental results whenever possible. For a copper ion in aqueous solution, DFTB3/MM results are generally close to B3LYP/MM with a medium basis, including both solvation structure and reduction potential for Cu(II); for Cu(I), however, DFTB3/MM finds a two-water coordination, similar to previous Born-Oppenheimer molecular dynamics simulations using BLYP and HSE, whereas B3LYP/MM leads to a tetrahedron coordination. For a tetraammonia copper complex in aqueous solution, VALBOND and DFTB3/MM are consistent in terms of both structural and dynamical properties of solvent near copper for both oxidation states. For copper reduction in plastocyanin, DFTB3/MM simulations capture the key properties of the active site, and the computed reduction potential and reorganization energy are in fair agreement with experiment, especially when the periodic boundary condition is used. Overall, the study supports the value of VALBOND and DFTB3(/MM) for the analysis of fundamental copper redox chemistry in water and protein, and the results also help highlight areas where further improvements in these methods are desirable.

Anion binding to a tetracopper resorcinarene-based complex

UV–vis and electron paramagnetic resonance (EPR) spectroscopic studies have been carried out on the multinuclear copper(II) complex Cu4BpaRes. The copper atoms are in a tetragonal distorted geometry with nitrogens from bispicolylamine (Bpa) coordinating in the equatorial plane and water molecules or anions completing four coordination sites. The interaction of anions with this polynuclear copper complex in aqueous solution supports the formation of different complex species, which depend on the type and the concentration of the anions. In the presence of excess anions, frozen solution EPR parameters show the formation of species in which the in-plane coordination is characterised by the presence of three nitrogen atoms coming from the ligand and a donor atom from the specific anion. For the bidentate anion ligands and especially for malonate, UV–vis titrations indicate the formation of a 1:4 (Cu4BpaRes:anion) species. EPR experiments support the formation of such a species and indicate that the four copper centres are equivalent and reach penta-coordination via the coordination of both oxygens from the bidentate ligand.

Hydrogen Peroxide and Dioxygen Activation by Dinuclear Copper Complexes in Aqueous Solution: Hydroxyl Radical Production Initiated by Internal Electron Transfer

Dinuclear Cu(II) complexes, CuII2Nn (n = 4 or 5), were recently found to specifically cleave DNA in the presence of a reducing thiol and O2 or in the presence of H2O2 alone. However, CuII2N3 and a closely related mononuclear Cu(II) complex exhibited no selective reaction under either condition. Spectroscopic studies indicate an intermediate is generated from CuII2Nn (n = 4 or 5) and mononuclear Cu(II) solutions in the presence of H2O2 or from CuI2Nn (n = 4 or 5) in the presence of O2. This intermediate decays to generate OH radicals and ligand degradation products at room temperature. The lack of reactivity of the intermediate with a series of added electron donors suggests the intermediate discharges through a rate-limiting intramolecular electron transfer from the ligand to the metal peroxo center to produce an OH radical and a ligand-based radical. These results imply that DNA cleavage does not result from direct reaction with a metal-peroxo intermediate but instead arises from reaction with either OH radicals or ligand-based radicals.

Copper coordination in blue proteins.

The spectroscopic and electrochemical properties of blue copper proteins are strikingly different from those of inorganic copper complexes in aqueous solution. Over three decades ago this unusual behavior was ascribed to constrained coordination in the folded protein; consistent with this view, crystal structure determinations of blue proteins have demonstrated that the ligand positions are essentially unchanged on reduction as well as in the apoprotein. Blue copper reduction potentials are tuned to match the particular function of a given protein by exclusion of water from the metal site and strict control of the positions of axial ligands in the folded structure. Extensive experimental work has established that the reorganization energy of a prototypal protein, Pseudomonas aeruginosa azurin, is approximately 0.7 eV, a value that is much lower than those of inorganic copper complexes in aqueous solution. The lowered reorganization energy in the protein, which is attributable to constrained coordination, is critically important for function, since the driving forces for electron transfer often are low (approximately 0.1 eV) between blue copper centers and distant (>10 A) donors and acceptors.

A spectrophotometric determination of the second dissociation constant of 5-nitrosalicyclic acid and the stability of its 1:1 copper complex in aqueous solution at 25°C

The second dissociation constant of 5-nitrosalicyclic acid has been determined spectrophotometrically in different buffer solutions at an ionic strength of 0·1 M. The stability constant of copper 5-nitrosalicylate (1 : 1 complex) at the same ionic strength has also been determined.

Protonation of 1,2-Diamino-2-methyl-propane and Its Tendency to Form Copper Complexes in Aqueous Solution.

Protonation of 1,2-Diamino-2-methyl-propane and Its Tendency to Form Copper Complexes in Aqueous Solution.