Tuning Reactivity of Platinum(II) Complexes

The decrease in reactivity of 2,6pzn for the first substitution is due to the positive
inductive effect of the 2,6-dimethylpyrazine ring that leads to accumulation of electron
density at metal centre. This makes the Pt1(II) centre less electrophilic, resulting in
lower reactivity towards the nucleophiles. This is well supported by the large HOMO-
LUMO energy gap coupled with reduced positive NBO charges (Table 4.1) for 2,6pzn.
Smaller values of k1 were observed for the first substitution step of the aqua ligand in
the complexes 2,3pzn and 2,5pzn by a factor of 10 than pzn and also smaller than that
of 2,6pzn. Previous studies have shown that methyl groups and other electron-donating
groups collectively lead to a reduction in the rate of ligand substitution, by reducing the
positive charge density on the platinum atom, thereby lowering the electrophilicity of
the metal centre. 3,31,32,36,44,45 As a result, the incoming nucleophile is repelled by the
increased amount of electron density around the metal centre, leading to a less stable
five-coordinate transition state and decreased rate of substitution.
The difference between the 2,6pzn and the 2,3pzn and 2,5pzn indicates that the
distribution of the methyl groups around the pyrazine linker plays a role in controlling
the reactivity. This is because the position of the methyl group influences the electronic
as well as the steric effects around the Pt(II) centres. The reactivity of 2,3pzn and
2,5pzn are similar because of having symmetrically equal effects in terms of electronic
and steric hindrance.
The reactions for the second step are significantly slower, by a factor of 10, than for the
first in all cases. This is probably because of the electronic communication between the
two platinum atoms and also steric hindrance from the first substitution at the Pt(II)
centre. The DFT calculations show a change in NBO charges if TU is substituted to one
platinum centre as can be seen in Table 5, suggesting that electronic information is
somehow conveyed through the bridging ligand. The order of reactivity for the second
step is pzn > 2,3pzn ≈ 2,5pzn > 2,6pzn. In case of 2,6pzn there is additional steric
effect which blocks entry of the incoming nucleophile on both sides of the Pt2 centre
(Figure 4.14), 3, 39,46-52 accounting for the slowest rate of reactivity.
30