Mechanistic Studies on Fast Ligand Substitution Reactions of Pt(II) in Different Ionic Liquids: Role of Solvent Polarity and Ion-Pair Formation

Abstract

The effect of several imidazolium-based ionic liquids on the mechanism of a classical ligand substitution reaction of [Pt(terpyridine)Cl] (+) with thiourea was investigated. A detailed kinetic study as a function of the nucleophile concentration and temperature was undertaken under pseudo-first-order conditions using stopped-flow techniques. Polarity measurements were performed for the employed ionic liquids on the basis of solvatochromic effects, and they show similarities with conventional polar solvents. Density-functional theory calculations (RB3LYP/LANL2DZp) were employed to predict the ion-pair stabilization energy between the ionic components of the ionic liquids and/or between the anions of the ionic liquids and the cationic Pt (II) complex. These data illustrate how the anions of the ionic liquids can affect the investigated substitution reaction. In general, the substitution mechanism in ionic liquids was found to have an associative character similar to that in conventional solvents. The observed deviations reflect the influence of the ionic liquid on the interaction between the anionic component of the liquid and the positively charged complex.

Mechanistic Studies on Fast Ligand‐Substitution Reactions of a Very Labile PdII Complex in Several Ionic Liquids

“…A similar effects was especially noted for this IL in our earlier studies for the corresponding Pt II complex. [8] In comparison to the reaction in are once again in line with an associative or associative interchange mechanism as expected.…”

“…[6,7] Earlier work from our group in this field involved ligandsubstitution reactions of inert and labile Pt II complexes with thiourea (TU) or thiocyanate (SCN -) as entering nucleophiles. [8][9][10] The results indicate that the nature of the underlying substitution mechanism, in this case associative (A) or associative-interchange (I a ), remains the same on -1 s -1 at 25°C, respectively, which can be accounted for in terms of the protective interaction of the anionic components of the ionic liquids with the electrophilic metal centre.…”

“…The negative activation entropy ∆S # of -69 Ϯ 3 J K -1 mol -1 supports the assignment of an associative (A) or associative interchange (I a ) mechanism, characteristic for substitution reactions of square-planar complexes. [8][9][10]15] In the case of SCN -as entering nucleophile, the concentration dependence of k obs exhibits a significant intercept (see Figure 4), indicating the participation of a back or parallel reaction as shown in Scheme 3, for which k obs = k 2 …”

“…To be able to compare the results on Pd II with our earlier work on the analogous Pt II complex, [8,10] some reactions had to be studied in water since only a few selected reactions in water/methanol mixtures have been studied before. [11] When complex I is dissolved in a water/CF 3 SO 3 H mixture, it undergoes spontaneous aquation to form the corresponding aqua complex.…”

“…A summary of the physical properties of the ILs used has already been given in our earlier report. [8] Results and Discussion…”