Tuning Reactivity of Platinum(II) Complexes

introduction of a greater number of (CH2)n from n = 2 up to 10. A further factor is the
decrease in-space electrostatic charge additions at the metal centre due to the increase
in the linker chain. 13,29,30 This leads to a decrease in electrophilicity at the Pt(II) centre
thereby making the metal centre less reactive.
It has been demonstrated that the 1,1/c,c geometry results in slower substitution
reactions because of the steric hindrance of the bridge around the Pt(II) centre. 14 The
steric factor is mainly due to the flexibility of the (CH2)n units of the alkanediamine
ligand. 26 In this study the overall steric effect was found to have been non-existent or
had no significant influence on the first substitution step of the aqua ligands. It can
therefore be concluded that electronic effect is the main factor that controls the first
substitution step.
A general look at the results in Table 7.3 shows that the rate constants, k2,2 nd , for the
second substitution step are about 1-2 orders of magnitude smaller than those obtained
for the simultaneous substitution of the aqua ligands in the first step in all the six
complexes. The second substitution reaction is slower than the first step due to a
number of reasons. Firstly, a facilitated attack (due to the strong trans-effect of the first
coordinated TU) by a second thiourea molecule on the trans-positioned ammine
nitrogen is slowed down as a result of steric constraints around the Pt(II) centre from
the first substitution. Secondly, is the increase in electron donation to the Pt metal due to
the coordinated TU from the first substitution reactions. The third reason is the fact that
the leaving group in the first substitution is different to that in the second substitution.
The Pt–OH2 bond is stronger than that of Pt–NH3. 54 Finally, the trans-effect of the two
substitution process is also different, as such influencing the overall reactivity
In this study, the proton ( 1 H) and 195 Pt NMR data have demonstrated that for 1,1/c,c
dinuclear Pt(II) complexes, a second step which involved release of ammine ligand
following the coordination of a further thiourea to the Pt(II) centre is observed. In
contrast, the first data reported in literature on di-platinum metal complexes for the
trans analogues (1,1/t,t) 26 gave only a single reaction step, which is most likely a
combination of the two steps. The results of the trans analogues (1,1/t.t) also contradicts
NMR studies by Farrell et al. 31(a) for the reaction of 1,1/t,t (n =6) with N-
27