Tuning Reactivity of Platinum(II) Complexes

is the same as the electron-withdrawal ability of the bridge ligand and its ability to
stabilize the five-coordinate transition state which increases in the order: pyrazine
(pzn) > quinazoline (qzn) > pyrimidine (pmn) > pyridazine (pdn) > phthalazine (pht). 48
Therefore, the difference in reactivity can be ascribed to electronic and steric effects
operating around the Pt(II) centre.
To understand fully the influence of the diazine-bridge on the reactivity of the Pt(II)
metal centres, the complexes need to be grouped into two clusters. First, is the influence
of increasing the distance between the metal atoms, i.e. from being ortho to each other
(pdn) to para disposition (pzn) in the monocyclic diazines. Secondly, is by looking at
the influence of increasing the π-effect by comparing the monocyclic diazines to the
respective benzodiazines, i.e. pmn versus qzn and pdn against pht.
Using the k2(1 st ) values for TU as a reference one clearly sees that as the position of the
metal changes from ortho to each other to para, the reactivity increases by a factor of
15. One can infer from this that steric hindrance is playing a role in influencing the
reactivity of the complexes. This is because as the position changes from ortho to para
the dihedral angle (Pt–N1–N2–Pt), i.e. α in Table 5.1 increases reducing any possible
interference from each metal centre. This order of reactivity has been observed for
closely related substitution of chloride by guanosine-5’-monophosphate (GMP) studies
reported by Reedijk and co-workers. 32
There is also clear evidence that electronic effect is also playing a role in these reactions.
The DFT calculation of the NBO positive charges on the Pt-atoms, shows that the Pt(II)
centre in pzn is more electrophilic followed by that of pmn and then pdn, accounting
for the order of the reactivity. DFT calculations are supported further by the 195 Pt NMR
chemical shifts which shows the pyrazine-bridged complex (pzn) to have a lower field
(б = -2302.2 ppm) while that for the pyrimidine-bridged complex pmn appears at (б = -
2320.6 ppm) and for pyridazine bridged complex (pdn) at (б = -2244.8 ppm). This
trend has also been reported by other authors based on photo-physical data and
electrochemical properties. 29,49
30