The Solution Chemistry of Cu2+–tren Complexes Revisited: Exploring the Role of Species That Are Not Trigonal Bipyramidal

Potentiometric and spectrophotometric titrations indicate that aqueous solutions that contain equimolar amounts of Cu2+ and tren contain the HCuL3+, CuL2+ and CuL(OH)+ species and that their relative concentrations depend on the pH of the solution. The stability constants and the UV/Vis and EPR spectra of the three species have been determined. The position of the absorption maximum clearly corresponds to a trigonal bipyramidal (tbp) geometry for CuL2+, whereas for HCuL3+ and CuL(OH)+ there are also bands that could correspond to square pyramidal (sp) complexes, but the EPR spectra indicate that only HCuL3+ can be considered to be sp. When any of these species is mixed with an excess of acid, an intermediate is formed within the mixing time of the stopped-flow technique. This intermediate undergoes complete decomposition in a second slower step. Interestingly, the spectrum of this intermediate is typical of sp geometry. Kinetic studies on complex formation in general indicate that complexation occurs in a single step, although under certain conditions an additional step has been observed that probably corresponds to the conversion of CuL2+ to HCuL3+, and the spectral changes indicate that the process involves structural reorganization from tbp to sp geometry. DFT and TDDFT calculations have been carried out for the three stable species, as well as for species in a higher protonation state. The results indicate that CuL2+ exists as a species with tetradentate tren and tbp geometry, although a wide range of distortions between the ideal tbp geometry and a geometry closer to sp is possible with a very modest energy cost. The energy change associated with hydrolysis of one of the Cu– N bonds to give a species with tridentate tren was found to be slightly higher than that previously found for a related ligand, which contains a substituent at one of the terminal amino groups. For CuL(OH)+, the calculations suggest that an equilibrium exists between species with essentially the same energy but different geometries, each one of the species is closer to one of the ideal tbp and sp limits. For HCuL3+, the relevance of the sp geometry was confirmed by the calculations

Hydrothermal replacement of biogenic and abiogenic aragonite by Mg-carbonates – Relation between textural control on effective element fluxes and resulting carbonate phase

Dolomitization, i.e., the secondary replacement of calcite or aragonite (CaCO3) by dolomite (CaMg[CO3]2), is one of the most volumetrically important carbonate diagenetic processes. It occurs under near surface and shallow burial conditions and can significantly modify rock properties through changes in porosity and permeability. Dolomitization fronts are directly coupled to fluid pathways, which may be related to the initial porosity/permeability of the precursor limestone, an existing fault network or secondary porosity/permeability created through the replacement reaction. In this study, the textural control on the replacement of biogenic and abiogenic aragonite by Mg-carbonates, that are typical precursor phases in the dolomitization process, was experimentally studied under hydrothermal conditions. Aragonite samples with different textural and microstructural properties exhibiting a compact (inorganic aragonite single crystal), an intermediate (bivalve shell of Arctica islandica) and open porous structure (skeleton of coral Porites sp.) were reacted with a solution of 0.9 M MgCl2 and 0.015 M SrCl2 at 200 °C. The replacement of aragonite by a Ca-bearing magnesite and a Mg-Ca carbonate of non-stoichiometric dolomitic composition takes place via a dissolution-precipitation process and leads to the formation of a porous reaction front that progressively replaces the aragonite precursor. The reaction leads to the development of porosity within the reaction front and distinctive microstructures such as gaps and cavities at the reaction interface. The newly formed reaction rim consists of chemically distinct phases separated by sharp boundaries. It was found that the number of phases and their chemical variation decreases with increasing initial porosity and reactive surface area. This observation is explained by variations in effective element fluxes that result in differential chemical gradients in the fluid within the pore space of the reaction rim. Observed reaction rates are highest for the replacement of the initially highly porous coral and lowest for the compact structure of a single aragonite crystal. Therefore, the reaction progress equally depends on effective element fluxes between the fluid at the reaction interface and the bulk solution surrounding the test material as well as the reactive surface area. This study demonstrates that the textural and microstructural properties of the parent material have a significant influence on the chemical composition of the product phase. Moreover, our data highlight the importance of effective fluid-mediated element exchange between the fluid at the reaction interface and the bulk solution controlled by the local microstructure