Mechanistic elucidation of linker and ancillary ligand substitution reactions in Pt(II) dinuclear complexes

The rate of substitution of aqua ligands by three nucleophiles, thiourea (TU), N,N-dimethylthiourea (DMTU) and N,N,N,N-tetramethylthiourea (TMTU), for the complexes [cis-{PtOH2(NH3)(2)}(2)-mu-pyrazine]-(ClO4)(2) (pzn), [cis-{PtOH2(NH3)(2)}(2)-mu-2,3-dimethylpyrazine](ClO4)(2) (2,3pzn), [cis-{PtOH2(NH3)(2)}(2)-mu-2,5-pyrazine](ClO4)(2) (2,5pzn) and [cis-{PtOH2(NH3)(2)}(2)-mu-2,6-dimethylpyrazine](ClO4)(2) (2,6pzn) was investigated under pseudo first-order conditions as a function of concentration and temperature by stopped-flow and UV-Visible spectrophotometry. The reaction proceeded in three consecutive steps; each step follows first order kinetics with respect to each complex and nucleophile. The pseudo first-order rate constants, k(obs(1/2/3)), for sequential substitution of the aqua ligands and subsequent displacement of the linker obeyed the rate law: k(obs(1/2/3)) = k((1/2/3))[nucleophile]. The steric hindrance properties of the pyrazine-bridging ligand control the overall reaction pattern. The order of reactivity of the complexes is 2,3pzn approximate to 2,5pzn < 2,6pzn < pzn. The difference in reactivity attributed to the steric crowding at the Pt(II) centre imposed by the methyl groups reduces the lability of the aqua complexes. The order of reactivity of the nucleophiles decreases with the increase in steric demand TU > DMTU > TMTU. H-1 and Pt-195 NMR spectroscopic results confirmed the observed dissociation of the bridging ligand from the metal centre of the cis-dinuclear complexes and its derivatives in the third step. The dissociation process is accelerated by the introduction of the steric effect on the linker in conjunction with the increased ligand field strength imparted by additional thiourea ligands at each metal centre. The large negative entropy of activation Delta S-not equal values in all cases support an associative substitution mechanism.