Tuning Reactivity of Platinum(II) Complexes

6.5 Conclusion
The present study has demonstrated that small modifications of the bridging moiety has
a strong influence on the properties and reactivity of diaqua Pt(II) complexes.
Spectrophotometric acid-base titrations of these complexes gave two pKa values. The 1 H
NMR measurement for the reaction of Pt1–Cl with TU confirmed the replacement of the
bridging pyridyl ligand following coordination of further thiourea to the already
congested Pt(II) centre in the transition state. The substitution process of the studied
systems remains associative mechanism. This is supported by the large negative
activation entropies.
The pKa1 values for deprotonation of the first water molecule increases with increasing
Pt---Pt distance in the order Pt1 < Pt3 < Pt2, showing there is direct correlation
between the Pt---Pt distance and the acidity of the coordinated aqua ligand. This has
been explained to be due to the charge additions and to the decreasing
communication/interactions between the two Pt atoms. The substitution kinetics
resulted into two k2 rate constants, simultaneous substitution of the coordinated H2O
molecules and the release of the linker ligand. The order of the reactivity is mostly
dependent on the structure of the complexes where steric hindrance plays a major role.
The second factor is the electronic effect where the charge addition has some influence.
Computation studies support the experimental trend of pKa values and substitution
reactivity of the diaqua Pt(II) complexes. It can be concluded that the dissociation of the
linker is a possible indication that if these compounds are used for treatment of cancer,
they are likely to be unstable.
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