Precise Investigation of the Axial Ligand Substitution Mechanism on a Hydrogenphosphato-Bridged Lantern-Type Platinum(III) Binuclear Complex in Acidic Aqueous Solution

Abstract

Detailed equilibrium and kinetic studies on axial water ligand substitution reactions of the "lantern-type" platinum(III) binuclear complex, [Pt(2)(mu-HPO(4))(4)(H(2)O)(2)](2)(-), with halide and pseudo-halide ions (X(-) = Cl(-), Br(-), and SCN(-)) were carried out in acidic aqueous solution at 25 degrees C with I = 1.0 M. The diaqua Pt(III) dimer complex is in acid dissociation equilibrium in aqueous solution with -log K(h1) = 2.69 +/- 0.04. The consecutive formation constants of the aquahalo complex () and the dihalo complex () were determined spectrophotometrically to be log = 2.36 +/- 0.01 and log = 1.47 +/- 0.01 for the reaction with Cl(-) and log = 2.90 +/- 0.04 and log = 2.28 +/- 0.01 for the reaction with Br(-), respectively. In the kinetic measurements carried out under the pseudo-first-order conditions with a large excess concentration of halide ion compared to that of Pt(III) dimer (C(X)()- >> C(Pt)), all of the reactions proceeded via a one-step first-order reaction, which is a contrast to the consecutive two-step reaction for the amidato-bridged platinum(III) binuclear complexes. The conditional first-order rate constant (k(obs)) depended on C(X)()- as well as the acidity of the solution. From kinetic analyses, the rate-limiting step was determined to be the first substitution process that forms the monohalo species, which is in rapid equilibrium with the dihalo complex. The reaction with 4-penten-1-ol was also kinetically investigated to examine the reactivity of the lantern complex with olefin compounds.

Kinetics and mechanisms of the axial ligand substitution reactions of platinum(III) binuclear complexes with halide ions

“…Most of the complexes of group (1) and (2) are considerably stable in solid and/or in solution, and have been widely studied. However, no detailed analysis in the reactivity and the reaction mechanism has been reported for platinum(III) dimers except one before our recent studies [27][28][29][30][31][32].…”

“…Table 2 Stepwise formation constants of the monohalo and dihalo complexes for the reaction of platinum(III) binuclear complexes with halide ions determined spectrophotometrically at 25 1C [29]. e The unconditional value was re-calculated [32]. f Ref.…”

“…Furthermore, the kinetic analyses in both reports are insufficient to clarify the detailed reaction mechanism. Thus, re-investigation was carried out on the axial water substitution reaction of the HPO 4 -bridged lantern-type complex with halide and pseudo-halide ions (X À ¼ Cl À , Br À , SCN À ) in acidic aqueous solution (Scheme 7) [32], and the detailed mechanisms were determined based on the mechanistic information obtained from the amidato-bridged platinum(III) binuclear complexes [27][28][29][30][31].…”

“…The formation constants of the reaction with thiocyanate ion, however, could not be determined because the complex(es) decomposed [32].…”

“…In this paper, we review our recent kinetic and mechanistic studies on the axial water ligand substitution reactions of the two types of platinum(III) binuclear complexes, the platinumblues derived bis(amidato)-bridged complexes, head-to-head (HH) and head-to-tail (HT) [Pt 2 (NH 3 ) 4 (m-amidato) 2 (OH 2 ) 2 ] 4+ (amidato ¼ a-pyridonato, a-pyrrolidonato, or pivalamidato) [27][28][29], and the tetrakis(hydrogenphosphato)-bridged lantern-type complex, [Pt 2 (m-HPO 4 ) 4 (OH 2 ) 2 ] 2À [32], with halide ions in acidic aqueous solution. These mechanistic studies provide the first detailed information concerning the reactivity of Pt(III) binuclear complexes.…”