A kinetic investigation into the rate of chloride substitution from chloro terpyridine platinum(II) and analogous complexes by a series of azole nucleophiles

The substitution kinetics of the complexes [Pt(terpy)Cl]Cl center dot 2H(2)O (PtL1), [Pt((t)Bu(3)terpy)Cl]ClO4 (PtL2), [Pt{4'-(2'''-CH3-Ph)terpy}Cl]BF4 (PtL3), [Pt{4'-(2'''-CF3-Ph)terpy}Cl]CF3SO3 (PtL4), [Pt{4'-(2'''-CF3-Ph)-6-Ph-bipy}Cl] (PtL5) and [Pt{4'-(2'''-CH3-Ph)-6-2''-pyrazinyl-2,2'-bipy}Cl]CF3SO3 (PtL6) with the nucleophiles imidazole (Im), 1-methylimidazole (MIm), 1,2-dimethylimidazole (DIm), pyrazole (Pyz) and 1,2,4-triazole (Trz) were investigated in a methanolic solution of constant ionic strength. Substitution of the chloride ligand from the metal complexes by the nucleophiles was investigated as a function of nucleophile concentration and temperature under pseudo first-order conditions using UV/Visible and stopped-flow spectrophotometric techniques. The reactions follow the rate law. The results indicate that changing the nature or distance of influence of the substituents on the terpy moiety affects the pi-back-donation ability of the chelate. This in turn controls the electrophilicity of the metal centre and hence its reactivity. Electron-donating groups decrease the reactivity of the metal centre, while electron-withdrawing groups increase the reactivity. Placing a strong sigma-donor cis to the leaving group greatly decreases the reactivity of the complex, while the addition of a good pi-acceptor group significantly enhances the reactivity. The results indicate that the metal is activated differently by changing the surrounding atoms even though they are part of a conjugated system. It is also evident that substituents in the cis position activate the metal centre differently to those in the trans position. The kinetic results are supported by DFT calculations, which show that the metal centre is less electrophilic when a strong sigma-donor is cis to the leaving group and more electrophilic when a good pi-acceptor group is part of the ring moiety. The temperature dependence studies support an associative mode of activation. An X-ray crystal structure of Pyz bound to PtL3 was obtained and confirmed the results of the DFT calculations as to the preferred N-atom as a binding site.