Aqua substitution from mononuclear Pt(II) complexes with 2-(pyrazol-1-ylmethyl)quinoline non-leaving ligands

Abstract

The rates of aqua substitution from [Pt{2-(pyrazol-1-ylmethyl)quinoline}(H2O)2](ClO4)2, [Pt(H2Qn)], [Pt{2-(3,5-dimethylpyrazol-1-ylmethyl)quinoline}(H2O)2](ClO4)2, [Pt(dCH3Qn)], [Pt{2-[(3,5-bis(trifluoromethyl)pyrazol-1-ylmethyl]quinoline}(H2O)2](ClO4)2, [Pt(dCF3Qn)], and [Pt{2-[(3,5-bis(trifluoromethyl)pyrazol-1-ylmethyl]pyridine}(H2O)2](ClO4)2, [Pt(dCF3Py)], with three sulfur donor nucleophiles were studied. The reactions were followed under pseudo-first-order conditions as a function of nucleophile concentration and temperature using a stopped-flow analyzer and UV/visible spectrophotometry. The substitution reactions proceeded sequentially. The second-order rate constants for substituting the aqua ligands in the first substitution step increased in the order Pt(dCH3Qn) < Pt(dCF3Qn) < Pt(H2Qn) < Pt(dCF3Py), while that of the second substitution step was Pt(dCH3Qn) < Pt(dCF3Qn) < Pt(dCF3Py) < Pt(H2Qn). The reactivity trends confirm that the quinoline substructure in the (pyrazolylmethyl)quinoline ligands acts as an apparent donor of electron density toward the metal center rather than being a π-acceptor. Measured pKa values from spectrophotometric acid–base titrations were Pt(H2Qn) (pKa1 = 4.56; pKa2 = 6.32), Pt(dCH3Qn) (pKa1 = 4.88; pKa2 = 6.31), Pt(dCF3Qn) (pKa1 = 4.07; pKa2 = 6.35), and Pt(dCF3Py) (pKa1 = 4.76; pKa2 = 6.27). The activation parameters from the temperature dependence of the second-order rate constants support an associative mechanism of substitution.