A kinetic study of aqua ligand substitution in dinuclear Pt(II) complexes containing four non-coplanar pyridine ligands

Abstract

Substitution reactions of the aqua ligands from azine-bridged dinuclear platinum(II) complexes of the type [{cis-Pt(py)2(OH2)} 2(μ-pzn)](ClO4)4 [pzn = pyrazine (Pt-PZN), 2,3-dimethylpyrazine (Pt-2,3PZN), 2,5-dimethylpyrazine (Pt-2,5PZN) or 2,6-dimethylpyrazine (Pt-2,6PZN)] by thiourea nucleophiles were investigated under pseudo first-order conditions as a function of concentration and temperature using the stopped-flow technique. The experimental results are discussed in reference to structures obtained by DFT calculations. The results are in good agreement with the pKa values of the complexes as well as DFT calculations. Compared to [{cis/trans-Pt(NH3)2(OH2)} 2(μ-pzn)](ClO4)4, the complexes in this series react faster by a factor of 10 or 23 respectively due to the presence of pyridine rings, which forces the geometry to allow π-back bonding to take place such that the electrons from the metal centres are accepted to the empty π*-orbitals of the pyridine subunits. The reactivity of the nucleophile is sterically dependent, with N,N,N′,N′-tetramethylthiourea reacting three times slower than thiourea. In all complexes and for both substitution steps, the mode of activation remains associative in nature. The presence of pyridine rings in [{cis-Pt(py)2(OH2)} 2(μ-pzn)](ClO4)4 where py = pyridine, pzn = pyrazine (Pt-PZN), 2,3-dimethylpyrazine (Pt-2,3PZN), 2,5-dimethylpyrazine (Pt-2,5PZN) and 2,6-dimethylpyrazine (Pt-2,6PZN) allows the substitution reactions by thiourea nucleophiles to proceed faster by a factor of 10 than [{cis-Pt(NH3)2(OH2)} 2(μ-pzn)](ClO4)4 through π-back bonding.