Bridge-Splitting Reactions of Platinum(II) Complexes with Parametalated Pyridine Spacer Groups: A Kinetic and Mechanistic Study

Abstract

Substitution reactions of three dinuclear Pt(II) complexes connected by a pyridine-bridging ligand of variable length, namely [ cis-{PtOH2(NH3)2}2–μ–L]4+, where L = 4,4′-bis(pyridine)sulfide (Pt1), 4,4′-bis(pyridine)disulfide (Pt2), and 1,2-bis(4-pyridyl)ethane (Pt3) with S-donor nucleophiles (thiourea, 1,3-dimethyl-2-thiourea, and 1,1,3,3-tetramethyl-2-thiourea) and anionic nucleophiles (SCN−, I−, and Br−) were investigated. The substitutions were followed under pseudo–first-order conditions as a function of the nucleophile concentration and temperature, using stopped-flow and UV–visible spectrophotometric methods. The observed pKa values were, respectively, Pt1 (pKa1: 4.86; pKa2: 5.53), Pt2 (pKa1: 5.19; pKa2: 6.42), and Pt3 (pKa1: 5.04; pKa2: 5.45). The second-order rate constants for the lability of aqua ligands in the first step decreased in the order Pt2 > Pt3 > Pt1, whereas for the second step it is Pt1 > Pt2 > Pt3. The obtained results indicate that introduction of a spacer atom(s) on the structure of the bridging ligand influences the substitution reactivity as well as acidity of the investigated dinuclear Pt(II) complexes. Also nonplanarity of the bridging ligand of Pt1 complex significantly slows down the rate of substitution due to steric hindrance, whereas release of the strain enhances the dissociation of the bridging ligand. The release of the bridging ligand in the second step was confirmed by the 1H NMR of Pt1-Cl with thiourea in DMF-d7. The temperature dependence of the second–order rate constants and the negative values of entropies of activation (ΔS#) support an associative mode of the substitution mechanism.