The conformation effect of the diamine bridge on the stability of dinuclear platinum(II) complexes and their hydrolysis

• Supermolecule approach has been used for the hydrolysis reactions.
• Five transition states were proposed differing by conformation of the bridge group.
• The degree of folding contributes to the overall reactivity.
• Calculated energy barrier is in agreement with experiment.

In this paper, the hydrolysis process of a bisplatinum complex containing the flexible chain 1,6-hexanediamine between the two metal centers was investigated through the use of density functional theory (DFT) with the analysis of the role of the spacing group arrangement on the values of free energy activation barrier. All structures were fully optimized in aqueous solution using implicit model for solvent at DFT level. The energy profiles for the hydrolysis reaction were determined by using the supermolecule approach. Five transition states were proposed differing by the conformation of the bridge group, and the activation free energy calculated as a weighted average within the selected forms. The Gibbs population for reactant was used as a statistical weight leading to the predicted value of 23.1 kcal mol−1, in good accordance with experiment, 23.8 kcal mol−1. Our results suggests that for 1,6-hexanediamine bridge ligand, the extend forms with average torsional angle over the carbon chain larger than 130° have the greatest contribution to the hydrolysis kinetics. The results presented here point out that the hydrolysis mechanism might follow different paths for each conformation and each of these contributes to the observed energy barrier.

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