Tuning Reactivity of Platinum(II) Complexes

Comparing the reactivity of qzn against pmn and pht versus pdn provides an
understanding of the role of adding π-moiety onto the phenyl part of the chelate. The
results show that in case of qzn, the π-backbonding into the aromatic ring is dominant.
The net result is that the Pt-metal of qzn is more electropositive and as such is more
reactive than pmn. In case of pht, the opposite is true, the N=N bond in both pyridazine
(pdn) and phthalazine (pht) is a strong electron-withdrawer than the π-backbonding
due to the introduced aromatic ring, which makes it more of an electron donor than
electron-withdrawer. Because of this the metal centre for pht is more electronegative or
less electrophilic than the rest of the complexes; as such it is the least reactive. The
optimised structures of the LUMO clearly show the difference between the two
complexes with respect to electron density around the metal atom.
On the whole, the rate of substitution of the second aqua ligand from the starting
complex is slower than in the first substitution step (Table 5.3). The order of
substitution with TU decreases by a factor of 140: 247: 71: 19: 9 for pzn: qzn: pmn:
pdn: pht, respectively. The obtained rate constants show the same order of nucleophile
reactivity as was observed in the first step. Again, a combination of steric and electronic
properties control reactivity at the Pt(II) centre. Finally, a slower third step was
observed for all the metal complexes that involves the release of the linker and
formation of PtS4 units, due to the influence of the trans-effect of the S-donor ligands.
Also the electron density on the metal centre is increased when the number of
coordinated S-donor atoms per Pt(II) centre is increased further. This weakens the
bond between the metal and the leaving group, but simultaneously obstructs the
formation of a bond between the Pt(II) centre and the entering group in the transition
state, thus slowing down the reaction.
On the basis of the results summarised in Table 5.3, it can be concluded that the
reactivity order of the nucleophiles is the same for all the cases and follows the trend TU
> DMTU >TMTU, which is in relation to their order of nucleophilicity and steric
properties of the respective nucleophile. 50 The reported activation entropies
(ΔS ≠ (1 st /2 nd/3 rd ) for all systems studied are significantly negative, which is in line with an
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