Nucleophilic substitution of chloride from the [Pt(chlorido)(NNN)/(NCN)]+, (NNN) = bis(2-pyridyl/2-quinolyl)pyridine, (NCN) = bis(2-pyridyl/2-quinolyl)benzene) complexes by azoles

Abstract The rates of chloride substitution from [Pt(chlorido)(1,3-bis(2-pyridyl)benzene)]Cl, (PtL2); [Pt(chlorido)(2,6-bis(2-quinolinyl)pyridine)]Cl, (PtL3) and [Pt(chlorido)(1,3-bis(2-quinolinyl)benzene)]Cl (PtL4) by azoles viz., imidazole (Im), 1-methylimidazole (MIm), 1,2-dimethylimidazole (DIm), pyrazole (Pyz) and 1,2,4-triazole (Trz) were measured under pseudo-first-order conditions as a function of concentration and temperature using spectroscopic techniques. The rate data fitted to the equation, kobs. = k2[azole]. Taken together with the literature data of [Pt(chlorido)(2,2′:6,2″-terpyridine)]Cl·2H2O, (PtL1), the order of reactivity was PtL2 > PtL4 > PtL1 > PtL3, and showed a dependent on the strength of the π-back bonding from the metal centre to the tridentate spectator ligand. The cis-quinolinyl moieties of PtL3 and PtL4 accumulated electron density onto the cis Pt-N bonds through a net σ/π-induction donation towards the Pt(II) centre which impeded the attack of the incoming azole nucleophiles. The rate of chloride substitution was thus lowered relative to that of their respective analogues with cis-pyridyl moieties. Moreover, by changing the trans-donor ring from pyridyl (PtL1 and PtL3) to a phenyl (PtL2 and PtL4) in the spectator ligand framework, the trans-labilizing effect on the leaving chloride ligand was accentuated by factors in the range 102-103. The substituting power of the nucleophiles was influenced kinetically by the basicity (Im > Pyz > Trz) and steric effects of the entering nucleophile (MIm > Im > DIm) for all complexes except for PtL3, where MIm and DIm had similar reactivities. The trends in magnitudes of the rate constants correlated well with key DFT-calculated metrics of the geometry-optimized structures of the complexes. The reactions form substitution products through a common reaction pathway which is characterized by a decrease in the entropy of activation, and at a rate that is dependent on the concentration as well as the steric bulk of the azoles. These observations are characteristic of an associative mode of activation for substitution processes.