Role of Imidazole and Chelate Ring Size in Copper Oxidation Catalysts: An Experimental and Theoretical Study.

In this work, the structural, solution, electrochemical, and catalytic properties of the complexes with ligands derived from imidazole and pyridines were studied. A comparative study of five bioinspired copper catalysts with or without coordinated imidazole and with different chelate ring sizes is presented. Catalytic efficiency on the oxidation of 3,5-di-tert-butylcatechol (DTBC) and ortho-aminophenol (OAP) in a MeOH/H2O medium was assessed by means of the Michaelis-Menten model. Catalysts comprising imidazole-containing ligands and/or a six-membered chelate ring proved to be more efficient in both oxidation reactions. Determination of stability constants and electrochemical parameters of the copper complexes supported the explanation of the catalytic behavior. A catalytic cycle similar for both reactions has been proposed. The results of density functional theory (DFT) free energy calculations for all five complexes and both catalytic reactions agree with the experimental results.

Comment on "Mixed azido/phenoxido bridged trinuclear Cu(ii) complexes of Mannich bases: synthesis, structures, magnetic properties and catalytic oxidase activities", Dalton Trans., 2018, 47, 9385-9399 and "Tri- and hexa-nuclear NiII-MnII complexes of a N2O2 donor unsymmetrical ligand: synthesis, structures, magnetic properties and catalytic oxidase activities", Dalton Trans., 2018, 47, 13957-13971.

This letter comments on two examples of numerical results of kinetic parameters which are inconsistent with the experimental data given in the same articles. Since this seems to be a trend in inorganic chemistry articles dealing with the catalytic oxidation of 3,5 di tert- butylcatechol and ortho-aminophenol, we call the attention of editors, reviewers and readers about grossly overestimated catalytic parameters in the literature.

Coumarin Derivative Directly Coordinated to Lanthanides Acts as an Excellent Antenna for UV-Vis and Near-IR Emission.

A chelating coumarin-derived ligand sensitizes all emitting lanthanide ions in the solid state and gives high absolute quantum yields for ethanol solutions of complexes of Sm, Eu, Tb, and Dy, above 20% for the last two. Crystal structures of these four complexes are [Ln(Cum)3(H2O)(X)]·X where X = MeOH or EtOH.

Dinuclear copper complexes with imidazole derivative ligands: a theoretical study related to catechol oxidase activity.

Catechol oxidase is a very important and interesting metalloprotein. In spite of the efforts to understand the reaction mechanism of this protein, there are important questions that remain unanswered concerning the catalytic mechanism of this enzyme. In this article, dinuclear copper compounds are used as biomimetic models of catechol oxidase to study plausible reaction paths. These dinuclear copper(II) complexes have distant metal centers (of 7.5 Å approximately) and superior catalytic activity to that of many dicopper complexes with shorter Cu-Cu distances. One mononuclear copper(II) complex is also analyzed in this investigation in order to see the influence of the two metal centers in the catalytic activity. Density functional theory calculations were performed to obtain optimized structures, vertical ionization energies, vertical electron affinities, the electrodonating power (ω(-)), the electroaccepting power (ω(+)) and the energy difference of several reaction paths. The K(M) experimental results that were previously reported compare well with the electroaccepting power (ω(+)) of the copper compounds that are included in this article, indicating that this index is useful for the interpretation of the electron transfer capacity and therefore the catalytic activity. The catechol moiety coordinates to only one Cu ion, but two metal atoms are needed in order to have a good electron acceptor capacity of the biomimetic models.

Mechanistic insight on the catecholase activity of dinuclear copper complexes with distant metal centers.

The catecholase activity of two dinuclear Cu(II) complexes with distant metal centers is discussed together with solid state and solution studies. The crystal structure for one of them, [Cu(2)(diep)(H(2)O)(4)](ClO(4))(4)·2H(2)O, is described, showing the two copper ions are 7.457 Å apart and in a square pyramidal coordination. Both complexes display a weak antiferromagnetic coupling in the solid state that is manifest in the dimer EPR spectra obtained in frozen solution. The pH-potentiometric speciation performed in 1:1 MeOH-H(2)O allowed the assignment of hydrolyzed copper species as those catalytically active in the oxidation of 3,5-di-tert-butylcatechol (DTBC). The kinetic measurements led us to propose behavior consistent with Michaelis-Menten plus a linear dependence of the initial rate on [DTBC]. This can be associated with the presence of more than one catalytically active species, which is consistent with the evidence of several differently hydrolyzed species shown in the predominance diagrams. Product characterization studies led to establishing the formation of hydrogen peroxide during the catalytic cycle, while semiquinone and superoxide radicals were detected by EPR spectroscopy, supporting one-electron transference at each of the copper centers.

Coexistence of intramolecular ligand-mediated and through hydrogen-bond magnetic interactions in a chain of dicopper(II) units.

The reaction of 2,8-dimethyl-5,11-bis(pyridin-2-ylmethyl)-1,4,5,6,7,10,11,12-octahydroimidazo[4,5-h]imidazo[4,5-c][1,6]-diazecine (dimp) with copper(II) nitrate in water produces the compound [Cu(2)(dimp)(H(2)O)(2)(NO(3))(2)](NO(3))(2). The single-crystal X-ray structure shows the formation of hydrogen-bonded chains in the lattice that are formed by dicopper(II) units doubly connected by nitrate/water bridges. Within the one-dimensional chains, the Cu ions are separated by either intramolecular or intermolecular distances of 7.309(2) Å or 6.255(2) Å, respectively. The magnetic susceptibility data revealing weak antiferromagnetic exchange interactions between the copper(II) ions were interpreted by considering two possible models, namely, an isolated dinuclear and a 1-D chain picture. The latter leads to an alternation J(1) = -11.6 and J(2) = -3.0 cm(-1) along the chain. In order to clarify the relative strengths of the exchange couplings through hydrogen bonds and via the bridging dimp ligand, solution EPR studies and quantum chemical calculations were carried out. EPR studies unambiguously conclude on the existence of an exchange interaction J(a) mediated by the dinucleating dimp ligand, while the through-H coupling J(b) is physically absent in solution. On the basis of dinuclear units extracted from the X-ray data, J(a) was estimated around -5.0 cm(-1) from DFT-based calculations (M06 functional), whereas J(b) is negligible. In contrast, wave function configuration interaction calculations (DDCI) support a description where both inter- and intramolecular pathways coexist with a preeminent role of H bonds with J(a) = -2.8 and J(b) = -10.4 cm(-1). Not only are these values very consistent with the extracted set of parameters (J(1), J(2) = -11.6, -3.0 cm(-1)) but the possibility to generate leading exchange coupling through weak bonds is evidenced by means of wave function-based calculations.

Solvent and pH effects on the redox behavior and catecholase activity of a dicopper complex with distant metal centers.

A dicopper complex is described for which significant catecholase activity was found, particularly for a compound in which the two metal ions are more than 7A apart. Variations on the catecholase activity of this complex were explored in a range of pH values from 5.5 to 9.0 in two solvent mixtures, MeCN/H(2)O and MeOH/H(2)O. The catalytic performance of the complex was found to be substantially better in the second, where the maximum activity was achieved at a pH value one unit lower than in the first. Electrochemical studies of the complex in the absence and presence of dioxygen revealed a very different behavior in each of the two solvent mixtures, which may account for the correspondingly distinct catalytic activity.

Copper(ii) complexes of a polydentate imidazole-based ligand. pH effect on magnetic coupling and catecholase activity.

A potentially dodecadentate N8O4-donor ligand obtained from 2,2'-biimidazole and l-valine and its tetranuclear Cu(ii) complexes in different degrees of protonation were characterized by chemical and spectroscopic methods. The extensive solution studies performed reveal that the rise in pH media leads successively to the formation of imidazolato (pKa(1) and pKa(2) and hydroxido (pKa(3) and pKa(4)) bridges. A frozen solution EPR study shows a decrease in the signal intensity until an EPR silent spectrum is observed, upon increasing the basicity of the solution. The catalytic performance of the oxidation of 3,5-di-tert-butylcatechol to its corresponding quinone was studied using UV-Vis-NIR absorption spectroscopic methods in CH3CN-H2O and in CH3OH-H2O at pH = 7.5, 8.0 and 8.5. A marked increase in activity, consistent with the formation of the hydroxide bridged species, is observed at pH = 8.5 in both solvent mixtures, but the activity is significantly higher in CH3OH-H2O.

A dicopper complex with distant metal centers. Structure, magnetic properties, electrochemistry and catecholase activity.

The crystal structure and magnetic properties of a dinuclear copper(II) complex of the ligand [2,8-dimethyl-5,11-di-(dimethylethyleneamine) 1,4,5,6,7,10,11,12-octahydroimidazo [4,5-h] imidazo [4,5-c] [1,6]diazecine] dimeim have been investigated. Also, its catecholase activity has been explored in different solvent mixtures: MeCN/H2O and OH/H2O, each at several pH values. In CH3OH/H2O, where the activity was superior, the optimal pH value for the catalytic activity was found to be lower than in CH3CN/H2O. The study of the complex's electrochemical behavior (cyclic voltammetry) which was also investigated in these various media, revealed that although an increase in pH in both solvent mixtures results in an increase both in Me oxidizing power (E(1/2)) and reversibility (ipa/ipc) the change of solvent system seems to be a more influencing factor. The superior catalytic activity found in MeOH/H2O pH=8.0, is associated with a significantly more reversible behavior displayed in this medium. Potentiometric determination of the overall formation constant and three successive pKas for the complex, suggest the formation of stable hydroxo complexes which could be the catalytically active species.

[µ-2,8-Dimethyl-1,4,5,6,7,10,11,12-octa-hydro-diimidazo[4,5-h;4',5'-c][1,6]diaze-cine-5,11-diacetato]bis-[diaqua-nitrato-copper(II)] trihydrate.

The title compound, [Cu(2)(C(16)H(20)N(6)O(4))(NO(3))(2)(H(2)O)(4)]·3H(2)O, crystallizes with two dinuclear Cu(II) complex mol-ecules, each lying on an inversion center, and six solvent water mol-ecules per unit cell. The central 1,6-diazecine ring adopts the common chair conformation invariably found in the family of complexes bearing such ligands. The Cu(II) atoms have an octa-hedral geometry, with a very strong tetra-gonal distortion due to the Jahn-Teller effect. Axial sites are occupied by a nitrate ion and a water mol-ecule. The Cu⋯Cu separations [7.3580 (9) and 7.3341 (9) Å] are compatible with a potential catecholase activity. Neighboring mol-ecules in the crystal structure are connected via O-H⋯O hydrogen bonds formed by water mol-ecules and carboxyl-ate O atoms. N-H⋯O hydrogen bonds are also present.

Diaqua-(nitrato-κO,O')bis-(l-phenyl-alaninato-κO,O')lead(II) nitrate.

In the title complex, [Pb(C(9)H(11)NO(2))(2)(NO(3))(H(2)O)(2)]NO(3), the cation is a monomeric species including zwitterionic amino-acids. In both zwitterions, rotation of the NH(3) (+) groups about their C-N bonds is blocked by inter-molecular N-H⋯O hydrogen bonds. Assuming a limit for Pb-O bond lengths of 3 Å, the Pb(II) ion is coordinated by eight O atoms. Each phenyl-alaninate ligand coordinates asymmetrically, with one short and one long Pb-O bond. Coordinated water mol-ecules are also found at significantly different distances, while the bidentate nitrate ion coordinates symmetrically. The resulting [Pb(II)O(8)] core is hemi-directed, with a void placed almost trans to a carboxyl-ate group. However, the 6s(2) lone pair of the metal center can not be considered as stereochemically active, as a non-coordinating O atom of a nitrate belonging to a symmetry-related cation is placed in the empty hemisphere, with a short Pb⋯O separation of 3.035 (10) Å.