The deleterious effect of pyrollic-nitrogen on the substitution reactivity of tridentate N^C^N platinum(II) complexes. A kinetic and mechanistic study

Abstract

The rate of substitution of the chloride ligand in 1,3-bis(pyridyl)benzene platinum(II) chloride (Pt1), 1,3-bis(N-pyrazolyl)benzene platinum(II) chloride (Pt2), 1,3-bis(8-quinolyl)benzene platinum(II) chloride (Pt3), and 1,3-bis(7-azaindolyl)benzene platinum(II) chloride (Pt4) by three neutral nucleophiles with different steric demands, thiourea (Tu), N,N’-dimethylthiourea (Dmtu), and N,N,N,’N-tetramethylthiourea (Tmtu), was studied under pseudo-first-order conditions in methanol. The investigation was performed as a function of concentration and temperature using stopped-flow and UV–visible spectrophotometric techniques. The observed pseudo-first-order rate constants for substitution obeyed the rate law: . The reactivity of the rigid planar five-membered chelate complexes (Pt1 and Pt2) is influenced by the push and pull effect of the σ/π-character of the cis moieties, while in the six-membered complexes (Pt3 and Pt4) it is dependent on their structural topology and electronic features. The study showed that the presence of the pyrrolic-N π-donor within the chelate, irrespective of the chelate size, rigidity/flexibility, and symmetry, retards ligand substitution reactions. The second-order kinetics and large negative entropies support an associative mode of activation. DFT calculations were performed to support the interpretation and discussion of the experimental data.