Computational Study of Copper(II) Complexation and Hydrolysis in Aqueous Solutions Using Mixed Cluster/Continuum Models
- 5 August 2009
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry A
- Vol. 113 (34), 9559-9567
- https://doi.org/10.1021/jp904816d
Abstract
We use density functional theory (B3LYP) and the COSMO continuum solvent model to characterize the structure and stability of the hydrated Cu(II) complexes [Cu(MeNH2)(H2O)n−1]2+ and [Cu(OH)x(H2O)n−x]2−x (x = 1−3) as a function of metal coordination number (4−6) and cluster size (n = 4−8, 18). The small clusters with n = 4−8 are found to be the most stable in the nearly square-planar four-coordinate configuration, except for [Cu(OH)3(H2O)]−, which is three-coordinate. In the presence of the two full hydration shells (n = 18), however, the five-coordinate square-pyramidal geometry is the most favorable for Cu(MeNH2)2+ (5, 6) and Cu(OH)+ (5, 4, 6), and the four-coordinate geometry is the most stable for Cu(OH)2 (4, 5) and Cu(OH)3− (4). (Other possible coordination numbers for these complexes in the aqueous phase are shown in parentheses.) A small energetic difference between these structures (0.23−2.65 kcal/mol) suggests that complexes with different coordination numbers may coexist in solution. Using two full hydration shells around the Cu2+ ion (18 ligands) gives Gibbs free energies of aqueous reactions that are in excellent agreement with experiment. The mean unsigned error is 0.7 kcal/mol for the three consecutive hydrolysis steps of Cu2+ and the complexation of Cu2+ with methylamine. Conversely, calculations for the complexes with only one coordination shell (four equatorial ligands) lead to a mean unsigned error that is >6.0 kcal/mol. Thus, the explicit treatment of the first and the second shells is critical for the accurate prediction of structural and thermodynamic properties of Cu(II) species in aqueous solution.Keywords
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