NMR kinetic study of ligand exchange on a (µ-oxo)bis(µ-acetato)diruthenium(III) complex. A special kinetic case of introduction of a more labile ligand

Abstract

We have prepared the new [Ru 2 (μ-O)(μ-O 2 CCH 3 ) 2 (4-pic) 6 ](PF 6 ) 2 complex and followed the kinetics of exchange of the 4-picoline ligands, trans to the oxo bridge, with dimethyl sulfoxide (DMSO-d 6 ) by 1 H NMR at 30, 40, 50, and 60 °C. The 1 H NMR spectra of this complex in DMSO gradually change in the acetyl methyl region as solvent substitution of the 4-picoline trans to the oxo bridge takes place. This substitution is in accordance with the lability of the ligands trans to the Ru III -O-Ru III bridge in this kind of complex. The kinetic data fit in a series of double reversible reactions, pseudo first order in the forward direction and second order in the reverse direction, with ΔH ≠ = 115, 105, 116, and 106 kJ mol −1 and ΔS ≠ = 35.7, 17.2, 38.8, and 16.9 JK −1 mol −1 associated with k 1 , k −1 , k 2 , and k −2 rate constants, respectively.[Formula: see text]A, B and C stand for the nonexchanged, monoexchanged, and diexchanged complexes, respectively. The large ΔH ≠ and positive ΔS ≠ values agree with the dissociative nature of the mechanism in both forward and reverse reactions. The differences in 1 H and I3 C NMR chemical shifts of the three complexes are explained in terms of the effect of the ligand exchange on the N—Ru bond lengths and the geometry of the dimetal core. It is evident from our data that the exchange on one metallic site modifies the reactivity at the other site in this kind of dinuclear complex. Keywords: diruthenium complex, variable temperature NMR, ligand exchange kinetics.