Axial-ligand substitution reactions of a head-to-head pivalamidato-bridged Pt(III) binuclear complex bearing equatorial bromide ligands: A mechanistic study

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Axial ligand-substitution reactions of head-to-head (HH) a binuclear pivalamidato-bridged platinum(III) complex bearing equatorial bromide ligands, [(H2O)(NH3)2Pt(μ-pivalamidato)2Pt(Br)2(OH2)]2+ (1), with chloride and bromide ions were thermodynamically and kinetically investigated in acidic aqueous solutions. Reactions of 1 with p-styrenesulfonate and 4-penten-1-ol were also kinetically investigated in order to clarify the reaction mechanisms involving olefins. In contrast to the reactions of the HH tetraammine pivalamidato-bridged platinum(III) binuclear complex, [(H2O)(NH3)2Pt(μ-pivalamidato)2Pt(NH3)2(OH2)]4+ (2′), all substitution processes on 1 involve single-step reactions. The effect of the equatorial halide ligands on the axial ligand substitution reactions of 1 is discussed in relation to 2′; the rate-determining first substitution of H2O by X− at the Pt(Br2O2) site in 1 is followed by a fast second substitution of H2O by X− at Pt(N4). In contrast, axial ligand substitution on 2′ by X− proceeds in two consecutive steps. Reactions of 1 with p-styrenesulfonate or 4-penten-1-ol proceed in one step to form only mono-π complexes that are in rapid equilibrium with σ complexes, whereas reactions of 2′ with these olefins proceed in three consecutive steps via di-π complexes. These mechanistic differences are interpreted in terms of enhanced charge localization in the mono-π complexes of 1 ([PtII(NH3)2(μ-pivalamidato)2PtIV(Br)2(p-styrenesulfonate or 4-penten-1-ol)]2+).