Ab Initio Study of the Exchange Coupling in Oxalato-Bridged Cu(II) Dinuclear Complexes

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 104; no. 44; pp. 9983 - 9989
• Main Authors: Cabrero, J, Ben Amor, N, de Graaf, C, Illas, F, Caballol, R
• Format: Journal Article
• Language: English
• Published: American Chemical Society 09.11.2000
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp001908e

The structural dependence of the coupling constant in a series of [L3Cu(μ-C2O4)CuL3]2+complexes is analyzed by means of ab initio difference-dedicated configuration interaction (DDCI2) calculations on the model (μ-oxalato)bis[triamminecopper(II)] cation, [(NH3)6Cu2(μ-C2O4)]2+, in which the nitrogen-coordinated ligands have been substituted by NH3. Two types of geometrical structures have been considered:  three different C 2 h geometries and four crystallographic centrosymmetric geometries taken from [(Et5dien)2Cu2(μ-C2O4)](BPh4)2 and [(Et5dien)2Cu2(μ-C2O4)](PF6)2 (Et5dien = 1,1,4,7,7-pentaethyldiethylenetriamine), [(tmen,2-MeIm)2Cu2(μ-C2O4)](PF6)2 (tmen = N,N,N‘,N‘-tetramethylethylenediamine and 2-MeIm = 2-methylimidazole), and [(dien)2Cu2(μ-C2O4)](ClO4)2 (dien = diethylenetriamine). The results show that the antiferromagnetic coupling is strongly underestimated when pure DDCI2 calculations are performed, but when the CI space includes the relaxation of the oxalato-copper charge transfer, quantitative agreement with the experimental results is reached with an error smaller than 5 cm-1. The role of the external ligands in the model is also discussed by means of broken symmetry DFT calculations. At this level of theory, a very different influence of the ligands is predicted by different exchange-correlation functionals. Therefore, the use of DFT to investigate this effect should be considered with caution.