Double-layer effects on simple electrode reactions III. The reduction of Eu3+ and Cr3+ in the presence of thiocyanate anions. Evidence for ligand bridging

The rates of reduction of Eu³⁺ and Cr³⁺ have been measured in mixed perchlorate + thiocyanate electrolytes at constant ionic strength, using low concentrations of thiocyanate to minimize its association with the cationic reactants. The effect of adsorbed thiocyanate anions on the reduction kinetics of Cr³⁺ resembled those produced by iodide and bromide on both Cr³⁺ and Eu³⁺. However, thiocyanate exhibited an unusual catalytic effect on the reduction of Eu³⁺ which was identified as arising from a reaction pathway involving thiocyanate-bridging between the mercury surface and the Eu³⁺. The diagnostic criteria used to support the proposed mechanism included analysis of the rate—potential behavior and of the effects of competitively adsorbed iodide ions upon the reduction rates.     

Double-layer effects on simple electrode reactions II. The effects of specifically adsorbed anions and ion-pairing on the reduction of Eu3+ and Cr3+

The kinetics of the reduction of Eu³⁺ and Cr³⁺ at mercury electrodes have been studied in various mixed perchlorate electrolytes containing iodide, bromide, and p-toluene-sulfonate as adsorbing anions. Specific adsorption data were obtained by means of the constant ionic strength approach due to Hurwitz and Parsons. The rate enhancements observed in the presence of the first two (monatomic) anions were in good agreement with the predictions of the simple Frumkin model only when the experimental, rather than calculated diffuse-layer capacitances were employed in the data analysis. It was demonstrated that the effects of ion-pairing and ligand-bridging of the reactants with the adsorbing anions were negligible under the experimental conditions chosen. From experiments with these systems and also with weakly adsorbing chloride electrolytes it was concluded that ion-pairing did not enhance the electrochemical reactivity of either Eu³⁺ or Cr³⁺. The value of the analyses described in separating various contributions to the catalytic effects of adsorbing anions is emphasized.     

Enhancement of the Catalytic Activity of a Macrocyclic Cobalt(II) Complex for the Electroreduction of O(2) by Adsorption on Graphite

In solution, the [(tim)Co](2+) complex (tim = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) reacts only slowly with O(2), but upon adsorption on graphite electrodes, it becomes an active catalyst for the reduction of O(2) to H(2)O(2). The electroreduction of O(2) proceeds in a single voltammetric step at close to the diffusion-controlled rate at a relatively positive potential (0.25 V vs SCE). The remarkable enhancement in catalytic activity is attributed to a higher affinity for O(2) of the adsorbed complex as a result of its interactions with functional groups on the surface of roughened or oxidized graphite. A possible mechanism for the catalytic reduction of O(2) is proposed. It differs from the one employed by the analogous [(hmc)Co](2+) complex (hmc = C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) which operates at less positive potentials and exhibits two separated voltammetric steps in the reduction of O(2), via [(hmc)CoOOH](2+), to H(2)O(2).     

Antiferromagnetic three-dimensional order induced by carboxylate bridges in a two-dimensional network of [Cu3(dcp)2(H2O)4] trimers

A new Cu(II) complex, [Cu(3)(dcp)(2)(H(2)O)(4)](n), with the ligand 3,5-pyrazoledicarboxylic acid monohydrate (H(3)dcp) has been prepared by hydrothermal synthesis, and it crystallizes in the monoclinic space group P2(1)/c with a = 11.633(2) A, b = 9.6005(14) A, c = 6.9230(17) A, beta = 106.01(2) degrees, and Z = 2. In the solid state structure of [Cu(3)(dcp)(2)(H(2)O)(4)](n), trinuclear [Cu(3)(dcp)(2)(H(2)O)(4)] repeating units in which two dcp(3-) ligands chelate the three Cu(II) ions with the central Cu(II) ion, Cu(1) (on an inversion center), link to form infinite 2D sheets via syn-anti equatorial-equatorial carboxylate bridges between Cu(2) atoms in adjacent trimers. These layers are further linked by syn-anti axial-equatorial carboxylate bridging between Cu(1) atoms in adjacent sheets resulting in the formation of a crystallographic 3D network. A detailed analysis of the magnetic properties of [Cu(3)(dcp)(2)(H(2)O)(4)](n) reveals that the dcp(3-) ligand acts to link Cu(II) centers in three different ways with coupling constants orders of magnitude apart in value. In the high temperature region above 50 K, the dominant interaction is strongly antiferromagnetic (J/k(B) = -32 K) within the trimer units mediated by the pyrazolate bridges. Below 20 K, the trimer motif can be modeled as an S = 1/2 unit. These units are coupled to their neighbors by a ferromagnetic interaction mediated by the syn-anti equatorial-equatorial carboxylate bridge. This interaction has been estimated at J(2D)/k(B) = +2.8 K on the basis of a 2D square lattice Heisenberg model. Finally, below 3.2 K a weak antiferromagnetic coupling (J(3D)/k(B) = -0.1 K) which is mediated by the syn-anti axial-equatorial carboxylate bridges between the 2D layers becomes relevant to describe the magnetic (T, H) phase diagram of this material.