Tuning Reactivity of Platinum(II) Complexes

associative mode of substitution mechanism, well known for d 8 square planar metal
complexes. 50,51–53
5.5 Conclusion
This work has demonstrated the connection between the structure and position of the N
atoms of the bridging diazine ligand and reactivity of the aqua Pt(II) complexes. The
substitution kinetics resulted in three k2 rate constants, the first and second rate
constants being due to stepwise substitution of the aqua ligands, while the third is due
to the release of the bridging ligand. The results of the observed second-order rate
constants for the stepwise substitution of the aqua ligand with thiourea nucleophiles in
monocyclic diazine bridged complexes increases in the order pzn > pmn > pdn. This
correlates with the mutual disposition of the nitrogen atoms and π-deficiency of the
diazine ligands. The higher π-acceptor action due to the added phenyl ring in the qzn
containing the pyrimidine moiety, accounts for the accelerated nucleophilic substitution
compared to the pht where the nitrogen atoms are in the ortho-position, despite both
complexes having an extended π-conjugation. The results are clearly supported by the
experimental pKa values and DFT computations especially for NBO charges and HOMO-
LUMO energy gap. While, 1H NMR measurement for the reactions of pzn–Cl and pdn–
OH2 with TU confirmed dissociation of the linker following the coordination of further
thiourea to the congested Pt(II) centre in the transition state. It can therefore, be
speculated that the biological importance of these complexes as potential anticancer Pt
drugs is likely to be limited by their instability.
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