Tuning Reactivity of Platinum(II) Complexes

Chapter 7
The Influence of α,ω-Diamine Linker on Ligand Substitution of
Dinuclear Pt(II) Complexes: A Thermodynamic and Kinetic Study
7.0 Abstract
Substitution of the coordinated water molecules from the [{cis–Pt(NH3)2H2O}2–μ–
NH2(CH2)nNH2] +4 ( n = 2, 3, 4, 6, 8, 10) complexes: EnPt, PropPt, ButPt, HexPt, OctPt
and DecPt with S-donor nucleophiles of different steric demand, thiourea (TU), N,N-
dimethyl-2-thiourea (DMTU) and N,N,N,N-tetramethylthiourea (TMTU) was studied
under pseudo first-order conditions as a function of concentration and temperature,
using stopped-flow and UV–Vis Spectrophotometric techniques. The substitution
reaction proceeded in two steps: simultaneous substitution of the aqua ligands and
subsequently, release of the ammine ligand in the trans-position by the strong trans–
effect of the coordinated thiourea, with each of the steps being sensitive to steric and σ-
electronic properties of the alkanediamine linker. A comparison of the second-order rate
constants, k2,1 st and k2,2 nd, shows that the rate constants of the first step are 1-2 orders of
magnitude larger than those of the second step in all cases.
The pKa of the coordinated water molecules in the complexes were determined by
spectrophotometric acid-base titrations. The obtained pKa values clearly demonstrated
their dependency on the σ-donor capacity of the bridging ligand which increases with
increasing aliphatic chain length. This effect reflects a less electrophilic and less acidic
Pt(II) centre as the aliphatic chain is increased further. This relationship shows also that
the calculated second-order rate constants decreases as the alkanediamine chain length
is increased from EnPt to DecPt. The experimental data is supported by the Density
Functional theory (DFT) calculated data, where reduction in the NBO charges on the Pt
atom and increased HOMO-LUMO energy gap in the ground state of the Pt(II) complexes
clearly demonstrates an increase in the σ-inductive effect of the alkanediamine bridging
ligand, thereby leading to a less reactive metal centre. The large negative values of
activation entropy, ΔS ≠, confirmed an associative mode of substitution mechanism for
both steps. Proton ( 1 H) and 195 Pt NMR spectroscopy established that α,ω-alkanediamine
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