Tuning Reactivity of Platinum(II) Complexes

the number of sulphur bound molecules per Pt(II) atom in the products, thereby
weakening the amine N–Pt bond, which becomes susceptible to further substitution
reactions, leading to the simultaneous displacement of the linker and formation of
[Pt(TU)4] 2+. Based on the NMR studies one can conclude that the third step observed in
all these dinuclear complexes is due to degradation of the dinuclear Pt(II) complex to
release the bridging ligand and form [Pt(TU)4] +2. The results show that the rate
constants of the third step are not dependent on the nature of the pyrazine linker, but
are sensitive to the steric nature of the nucleophile.
The value of the rate constant of the third step from the 1 H NMR spectral data was
calculated from the observed first-order rate constant, kobs, obtained from Figure 4.10
and converted to second-order rate constant (k2 = kobs/ [TU]) based on equation 1 and
found to be (k3,3 rd = 2.44 x 10 -3 M -1 s -1), which is in close agreement with the value (k3 =
2.86 x10 -3 M -1 s -1 ) obtained from the concentration dependence study at 25 °C for pzn,
as the thiourea in each case attacks a thiourea substituted Pt(II) centre.
When the TU results of the cis-complexes pzn, 2,3pzn and 2,5pzn are compared with
those of the recently published trans-complexes 32 , the second-order rate constants (k1)
of the cis-complexes for the first step are slightly bigger. This indicates that cis-analogue
undergoes substitution reactions slightly faster than its trans-counterpart. This small
difference is possibly due to a very weak hydrogen bond between NH---OH2 in case of
pzn or CH---OH for the other complexes, which stabilizes the five-coordinate trigonal
bipyramidal transition state as illustrated in Scheme S4.1 and Figure S4.1. This
phenomenon was reported by Klein and co-workers, 53 while investigating
intramolecular C-H bond activation by the Pt(II) centre and its ability to form hydrogen
bonding amongst the dimethylpyrazine ligands; 54 and has also been reported by Munro
and his group in crystallographic studies. 55
The results as tabulated in Table 4.4 show that the activation entropies (ΔS # 1/2/3) are
large and negative whereas the activation enthalpies (ΔH # 1/2/3) are small and positive. It
is also noted that steric effects decelerate the reactivity between the cis-complexes
2,3pzn, 2,5pzn and 2,6pzn and the S-donor nucleophiles, which helps confirm that the
33