[Cu(H2O)n]2+ (n=1-6) complexes in solution phase: a DFT hierarchical study

For the theoretical modeling of energy profiles of Cu2+ reactions with biological ligands, it is indispensable to know the structure of its solvation sphere. Not withstanding the experimental and theoretical studies on this topic, nature of Cu-water complexes is still subject of intense debate. In order to gain insight into the structural features and relative stabilities of the [Cu(H2O)(n)](2+) (n = 1-6) species, several of their coordination modes considering water molecules in the first and second hydration shell of Cu2+, have been considered. Electronic structure calculations were performed with the G09 quantum chemistry suite of programs. Geometries are optimized by means of DFT functionals (allocate in different rungs of the Jacob's ladder) and the ab initio MP2 perturbation theory applying the def-SVP valence split basis set; formation of complexes is studied in gas phase and solution by application of CPCM and SMD protocols. Stabilization energies for each stoichiometry are calculated at the MP2/aug-cc-pVTZ level of theory. [Cu(H2O)(2)](2+), [Cu(H2O)(3)](2+) and [Cu(H2O)(4)](2+) complexes show lineal, planar trigonal and square planar structures, respectively; gas phase and CPCM results indicate that [Cu(H2O)(5)](2+) is isoenergetic with [Cu(H2O)(4)](2+)(H2O), but SMD solvent effects favor formation of intramolecular hydrogen bonds. For sixfold complexes, [Cu(H2O)(4)](2+)(H2O)(2) is consistently found to be the most stable compared with [Cu(H2O)(6)](2+) and [Cu(H2O)(5)](2+)(H2O). For n = 5 and 6, hydrogen bond formation in the second hydration shell competes with coordination in axial positions; results indicate that this intramolecular hydrogen bond stabilizes the complex more than axial coordination of water. In gas phase as well as solution, BHLYP outcomes are always consistent with MP2 results, evidencing the importance of exact exchange in the functional. SMD solvent effects show that water ligands are more loosely bond to the central ion that might explain a variety of complexes coexisting in solution, contrasting with gas phase where releasing a water molecule entails a higher energetic cost. Results indicate a clear tendency to improve the geometry, the symmetry and the relative stabilization energy of the Cu-water complex, toward the MP2 value, when either advancing on the Jacob's ladder or the percentage of exact exchange in the functional increases.