Tuning Reactivity of Platinum(II) Complexes

CHAPTER 4
Substitution of Aqua Ligands in Pyrazine-Bridged Pt(II) Dinuclear
Complexes: Influence of the Ancillary Substituents.
4.0 Abstract
The rate of substitution of coordinated water molecules by three nucleophiles of
different steric demand, viz. TU, DMTU and TMTU for the complexes: [cis-
{PtOH2(NH3)2}2-μ-pzn](ClO4)2 (pzn), [cis-{PtOH2(NH3)2}2-μ-2,3pzn](ClO4)2 (2,3pzn),
[cis-{PtOH2(NH3)2}2-μ-2,5pzn](ClO4)2 (2,5pzn) and [cis-{PtOH2(NH3)2}2-μ-2,6pzn](ClO4)2
(2,6pzn) was investigated under pseudo first-order conditions as a function of
concentration and temperature by stopped-flow and UV-Visible spectrophotometry .
The reaction proceeded in three consecutive steps; each step follows first order kinetics
with respect to each complex and nucleophile. The pseudo first-order rate constants,
kobs1/2/3 for sequential substitution of the aqua ligands and subsequent displacement of
the linker obeyed the rate law: kobs1/2/3 = k1/2/3[nucleophile].
The order reactivity of the complexes is 2,3pzn ≈ 2,5pzn < 2,6pzn < pzn. The difference
in reactivity is due to electronic and steric effects at the Pt(II) centre. An increase in
steric crowding at the Pt(II) centre imposed by the methyl groups decelerates the
lability of the aqua ligands in the dinuclear complexes by increasing the negative charge
and obstructs the entry of the incoming nucleophile from above or below the reaction
metal centre. The 1H and 195Pt NMR spectroscopic results confirmed the observed
dissociation of the bridging ligand from the metal centre of the cis-dinuclear complexes,
caused by thiourea and its derivatives in the third step.
The order of reactivity of the nucleophiles is based on the order of steric hindrance,
which is TU > DMTU > TMTU. The low enthalpy of activation ∆H ≠ and large negative
entropy of activation ∆S ≠ values support an associative mode of activation for both the
aqua ligand substitution reactions and the displacement of the linker.
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