Quantum chemical modeling of ligand substitution in cationic nitrosyl iron complexes

The substitution of the glutathione ligand for penicillamine and cysteamine ligands in the cationic nitrosyl iron complexes with the penicillamine thiyl ligand, [Fe₂(SC₅H₁₁NO₂)₂(NO)₄]SO₄·2H₂O, and the cysteamine thiyl ligand, [Fe₂(S(CH)₂NH₃)₂(NO)₄]SO₄·2H₂O, was studied by quantum chemical methods. Quantum chemical calculations were performed with the full geometry optimization of the starting and final complexes by the DFT method using the local BP86 and OPBE functionals. The structures of the intermediate and final complexes were predicted. The S-C bond in the complexes with the penicillamine ligand is longer than in other complexes of this type. The calculated energy of the detachment of the ligand from the complex with penicillamine is lower than that for the complex with the cysteamine ligand. The ligand substitution in the complex with the penicillamine ligand does not require considerable energy and can easily proceed in aqueous solution, whereas this reaction in the complex with the cysteamine ligand is thermodynamically unfavorable.