Tuning Reactivity of Platinum(II) Complexes

Inspection of the data in Table 4.2 shows that the pKa1 values of the deprotonation of the
first coordinated aqua molecule increase in the order pzn (pKa 3.29) < 2,6pzn (pKa 3.48)
< 2,3pzn (pKa 3.98) ≈ 2,5pzn (pKa 3.96). Also noted is that the second deprotonation of
the aqua molecule from the aqua/hydroxo species to form the hydroxo/hydroxo species
occurs at higher pH values than the first.
Since pKa values have been associated with the electrophilicity of the metal centre, 33-36 it
can be concluded that introduction of the methyl groups to the pyrazine moiety
decreases the electrophilicity of the metal centre through σ-electron donation via the
pyrazine nitrogen bound to the Pt(II) centre. The position of the methyl groups also
influences the electron density on the pyrazine ligand. In cases of 2,3pzn and 2,5pzn,
the charges are symmetrically distributed resulting in pKa values that are approximately
equal, since the methyl groups are symmetrically distributed at the ortho position of the
pyrazine linker. However, in 2,6pzn, the N6 being more negative than the N3 results in
Pt1 centre being slightly more positively charged compared to Pt2. This is seen in the
ΔpKa values (Table 4.2) for which 2,6pzn has the biggest difference compared to the
other complexes, signifying the difference in terms of electrophilicity between the two
platinum centres. This is supported by the 195 Pt NMR which shows chemical shift signals
at δ ( 195Pt) = -2320.36 for Pt1 and -2461.82 ppm for Pt2. These differences exist for the
other complexes, but the gap is relatively constant across the other complexes. In
general, the DFT calculated NBO charges in Table 4.1 are in agreement with the
observed experimental data.
When the current pKa results are compared to that of trans-analogues (shown in Table
4.2), the pKa values due to the first deprotonation of the coordinated aqua ligands are
similar in magnitude within the limits of experimental errors. The second pKa values for
the cis-complexes are less acidic, a factor that can be linked to their geometric
differences. First, introduction of the methyl groups on the pyrazine moiety in the trans-
complexes results in elongation of the Pt-OH2 bond due to the trans-effect, 32 and as a
consequent enhances the lability and acidity of coordinated aqua ligands of these
complexes. On the other hand, the influence of the cis ligand is mainly to bring about
change of electron density at the Pt(II) centre. This causes a decrease in the
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