Comparative density functional theory study of the binding of ligands to Cu+ and Cu2+: Influence of the coordination and oxidation state

BP86, B3LYP and MP2 methods, generally used to study large systems containing transition metals, were compared for their ability to accuratly evaluate bond dissociation energies of copper complexes. Various Cu-L]+ and Cu-L]2+ complexes in which L are small ligands and the higher coordinated complexes, Cu(NH3)(4)]+ and Cu(NH3)4]2+ were studied. For monoligated complexes, the BDEs calculated by the three methods differed by 2 to 60 kcal/mol, the larger differences being obtained for Cu-L]2+ complexes. The BDEs calculated using the B3LYP functional were in general close to the experimental values whereas the BDEs calculated using the BP86 functional were too high and the BDEs calculated using the MP2 were too low. If we rank the whole ligands according to their increased bond strength, the resulting orders obtained with the three methods are different for the Cu-L]+ complexes, the B3LYP giving the same order as the experimental one. This result indicates that the BDEs of Cu-L]+ complexes are better modeled using the B3LYP than using the BP86 and MP2 methods. For Cu-L]2+, B3LYP also gave the most reliable results whereas BP86 gave too large BDEs and MP2 gave too small BDEs. However, symmetries of ground states can be different using DFT and post-Hartree-Fock methods. For Cu-N2O]2+ the use of the B1LYP provides a better symmetry of the complex than the B3LYP, as has been recently shown in the literature for Cu-H2O]2+. MP2 led to an incorrect bent structure for Cu-N2]2+ in contrast to a linear structure obtained with the other methods, including CCSD(T). However, due to the lack of experimental data for Cu-L]2+ complexes and to contrasted results for the methods, it is not possible to conclude definitely. For the high coordinated complexes Cu(NH3)4]+ and Cu(NH3)4]2+, the PBE calculation method was used in addition to the BP86, B3LYP and MP2. The BDE values were very close to each other when there is no change of the oxidation state during the reaction. On the basis of these calculations, the choice of the method was less crucial for high coordinated complexes Cu(NH3)4]+ and Cu(NH3)4]2+ so long as the oxidation state remained the same during the reaction. In contrast, when Cu(NH3)4]2+ is reduced in Cu(NH3)3]+ and NH3, the BDE calculated using the four methods were markedly different.