Kinetics and Mechanism of Substitution Reactions of cis‐[PtMe2(dmso)2] with Pyridine

Abstract

AbstractThe substitution reactions of cis‐[PtMe,(dmso),] with pyridine (py) to produce cis‐[PtMe2py2] in toluene proceeds in two steps. In the absence of added dimethylsulfoxide (dmso), these steps can not be separated due to the rate constants being very similar. In the presence of added dmso, the rate of the first step, the formation of the monopyridine complex is retarded, which is indicative of a dissociative mechanism. A parallel associative reaction path with pyridine could also be observed. This pathway is independent of the concentration of added dmso. Above a 40‐fold excess of dmso, the dissociative pathway is suppressed and only the associative reaction occurs. A plot of Kobs vs the pyridine concentration for this pathway is linear at low [py], but shows a saturation at high [py]. This suggests that the reaction occurs via the formation of a precursor‐complex, for which the formation constant was found to be 0.32 · 0.03 M−1. The volume of activation at a high pyridine concentration is −11.4 · 0.8 cm3 mol−1, which indicates that the ligand interchange process is of the associative type. The second step, the formation of the bispyridine complex, can clearly be separated from the first reaction step. This step occurs via a dissociative mechanism, as demonstrated by the decrease in kobs with increasing pyridine concentration. The dissociation of dmso was characterized by a rate constant of (8.1 · 0.9).10−45−1 at 25°C, ·H = 116 ·9 kJ mol−1 and AS. = 86 · 29 JK−1 mol−1. At higher pyridine concentrations evidence for a parallel associative reaction was found, for which the rate constant is (1.3 · 0.2).10−3 M−1S−1 25°C. The results are discussed in reference to available literature data.