Catalytic effect of sulfoxides on the fluxional motion of 2,9-dimethyl-1,10-phenanthroline in a platinum(II) complex: Evaluation of steric and electronic contributions

Addition of external weak nucleophiles to a chloroform solution of the cationic complex ion [Pt(Me)(dmphen)(PPh3)](+) (1) accelerates the fluxional motion of the symmetric chelating ligand 2,9-dimethyl-1,10-phenanthroline (dmphen) between nonequivalent exchanging sites, The rates of the dynamic process can be measured by line-shape analysis of the H-1 NMR spectra. Concentration-dependent measurements were carried out with the ligands SOMe2, SO(CH2)(4), SO(n-Bu)(2), SO(sec-Bu)(2), SO(i-Pr)(2), SOEt(Ph), SOPr(Ph), SO(Bz)(2), SO(p-MeC6H4)(2), SOPh2, SO(p-ClC6H4)(2), SOMe(p-MeOC6H4), SOMe(p-MeC6H4), SOMe(Ph), SOMe(p-BrC6H4), and SOMe(P-ClC6H4), The rate constants k(obsd), when plotted against the concentration of the added ligands SOR(R'), give a family of straight lines with a common intercept, indicating that the two-term rate law k(obsd) = k(1) + k(2)[SOR(R')] is obeyed, The same rate law applies to the displacement of SOMe2 from [Pt(Me)(phen)(SOMe2)](+) (2) (phen = 1,10-phenanthroline) by sulfoxides (SOMe2, SO(i-Pr)(2), SOMe(p-MeOC6H4), SO(p-MeC6H4)(2), SOPh2, SO(p-ClC6H4)(2), and SO(sec-Bu)(2)). The fluxional rates in 1 are 6-7 orders of magnitude higher than the substitution rates in 2. The values of the rate constants for the two processes were resolved quantitatively into steric and electronic contributions by use of quantitative analysis of ligand effects (QALE), Inhibitory steric effects are linearly operative for the entire set of ligands, the rates of the reactions are enhanced with increasing electron donor capacity of the sulfoxides, and there is a small but significant Ear effect that enhances the reactivity of the aryl sulfoxides, The strict similarity of the patterns of the two processes and of their dependence upon the stereoelectronic properties of the ligands, combined with the intrinsic lability of the platinum-nitrogen bonds, would suggest the operation of stereospecific consecutive ring-opening and ring-closure steps for the fluxional motion of dmphen in 1. However, the available evidence does not allow alternative mechanisms involving intramolecular rearrangements of the five-coordinate intermediate to be ruled out.