Tuning Reactivity of Platinum(II) Complexes

eaction proceeds via bimolecular path with no hydrolysis step. All these are in support
of associative mode of substitution mechanism known for d 8 square-planar
complexes. 47,48,56
In terms of steric effects, substitution of coordinated aqua ligands shows a clear
dependence on the steric hindrance of the incoming nucleophiles. Thus, the order of
reactivity exhibited by the S-donor nucleophiles at the Pt(II) centres increased in the
order: TU > DMTU > TMTU and is line with the steric retardation in the case of TMTU.
4.6 Conclusion
From the results of this study, we have shown that all the dimethylpyrazine substituted
Pt(II) complexes: 2,3pzn, 2,5pzn and 2,6pzn reacted more slowly with thiourea ligands
compared to unsubstituted pzn. The order of reactivity of the complexes is 2,5pzn ≈
2,3pzn < 2,6pzn < pzn. Substitution in the pyrazine backbone influences the rate of the
substitution at the Pt(II) centre by controlling mainly the steric effects. A highly
symmetrical bridging ligand like the unsubstituted pyrazine ring with electronegative N-
atoms, will pull electron-density away from the positively charged metal centre, making
the Pt(II) centre more electrophilic and susceptible to attack by the S-donor
nucleophiles. On the other hand, the bridging ligands possessing the methyl moiety,
which is an electron-donating moiety, reduces the positive charge at the Pt(II) centre.
This makes the platinum centre less electrophilic, and hence, less susceptible to
nucleophilic attack resulting into a slowed down substitution reactions. In addition, the
bulky methyl substituent on the pyrazine linker introduces steric crowding on the
reaction centre retarding the reactivity of 2,3pzn, 2,5pzn and 2,6pzn. The electronic
communication between the two metal centres results in two distinct pKa values and
substitution reactions.
The 1 H and 195 Pt NMR results collectively support the experimentally observed
dissociation of the linker by thiourea nucleophiles as the third step in all the cases. The
study confirmed the release of free pyrazine (singlet, at δ 1 H = 8.64 ppm) and [Pt(TU)4] 2+
(δ 195Pt = -4146.6 ppm), for the reaction of pzn with excess TU. Given the high
nucleophilicity of the sulphur bearing thiourea nucleophiles at the Pt(II) centres coupled
34