Acceleration of the methanolysis of phosphate diesters promoted by La(OTf)3 — The analysis of non-integer pH/rate profiles resulting from changes in metal ion speciation

In a previous publication (A.A. Neverov and R.S. Brown. Inorg. Chem. 40, 3588 (2001)) we reported very effective catalysis of the methanolysis of some phosphate diesters (methyl p-nitrophenyl phosphate (1), bis(p-nitrophenyl) phosphate (2), and diphenyl phosphate (3)) promoted by La3+, and noted a general observation that the plots of logkcat vs. [Formula: see text]pH had non-integer gradients. In this report the origins of that behaviour are studied and analyzed through determination of the speciation of La3+(–OCH3)n, (La3+)2(–OCH3)m, (La3+)2:phosphate:(–OCH3)y forms in solution as a function of [Formula: see text]pH. Potentiometric titrations of solutions of La(OTf)3 in methanol at low (<1 × 10–4 mol/L) and high (>1 × 10–3 mol/L) concentration were analyzed through fits of the data to various models to provide speciation diagrams of the various La3+ forms in the absence of phosphate. Titrations of La3+ in the presence of diphenyl phosphate were also analyzed to provide speciation diagrams for phosphate bound forms. The kinetic data for the La3+ catalyzed methanolysis of 1 were analyzed through fitting the kinetic data at low and high [La3+] as a function of [Formula: see text]pH to a linear combination of the individual kinetic contributions of each species. Overall the data are best analyzed in the low [La3+] domain as resulting from methoxide attack on a transient complex of phosphate bound to La3+(–OCH3)0,1. In the high [La3+] domain the data fit two kinetically equivalent processes involving either a spontaneous decomposition of (La3+)2:1–:(–OCH3)2,3,4,5 or external methoxide attack on (La3+)2:1–:(–OCH3)1,2,3,4. Key words: lanthanides, phosphate diester, methanolysis, kinetics, speciation, metal ion catalysis of methanolysis, DNA model methanolysis.