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

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- firstorder 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: k(obs) = k (reverse) + k(2) [ Nu]. The reactivity of the rigid planar five- membered chelate complexes ( Pt1 and Pt2) is influenced by the push and pull effect of the sigma/ Pi- 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 Pi- 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. GRAPHICS