Publication: Detailed mechanistic study on ligand substitution reactions in dinuclear platinum(II) complexes: effect of alkanediamine linker
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2014-03-14
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Transition Metal Chemistry
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Abstract
Substitution of the coordinated water ligands from the cis-[{Pt(NH3)2H2O}2-μ-NH2(CH2) n NH2]4+ (n = 2–4, 6, 8, 10) complexes: EnPt, PropPt, ButPt, HexPt, OctPt and DecPt with S-donor nucleophiles thiourea, N,N-dimethyl-2-thiourea and N,N,N,N-tetramethyl-2-thiourea 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, followed by the displacement of the ammine ligands in the trans-position due to the strong trans-effect of the coordinated thiourea nucleophiles, 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, k 2,1st and k 2,2nd, indicates that the rate constants of the first step are 1–2 orders larger than those of the second step in all cases. The large negative ΔS ≠ values support an associative mode of substitution mechanism for both reaction steps. 1H and 195Pt NMR spectroscopy established that the α,ω-alkanediamine linkers remained coordinated to the metal centres, possibly due to their cis geometry to the incoming thiourea nucleophiles.