La3+-Catalyzed Methanolysis of Phosphate Diesters. Remarkable Rate Acceleration of the Methanolysis of Diphenyl Phosphate, Methyl p-Nitrophenyl Phosphate, and Bis(p-nitrophenyl) Phosphate

Abstract

The interaction of La(3+) with diphenyl phosphate, methyl p-nitrophenyl phosphate, and bis(p-nitrophenyl) phosphate (4, 5, and 3) and the ensuing catalysis of methanolysis has been studied in methanol under completely homogeneous conditions of known pH. (31)P NMR shows that phosphates 3 and 4 are each associated with La(3+) as a rapidly interconverting mixture of complexes having 2:1, 1:1 (or 2:2), and 1:2 stoichiometry. At [La(3+)] > 2 x 10(-4) M, the kinetically dominant species in the pH regions of interest are phosphate-bound La(3+) dimers having 2-5 associated methoxides. Potentiometric titration reveals that 4 mM La(3+) alone in MeOH exhibits two apparent pK(a)s at 7.86 and 10.44 consuming 1 and 1.5 CH(3)O(-) per metal, the suggested structures being La(3+) dimers with first two and then five associated methoxides. In the presence of 0.5 equiv of 4 the potentiometric titration reveals strong complexation of 4 to La(3+) and a profile with apparent pK(a)s at 7.8 and 11.1 consuming 1 and 1.5 CH(3)O(-) per metal corresponding to phosphate-bridged La(3+) dimers with first two and then five associated methoxides. Methanolysis of 3-5 is strongly accelerated by La(3+). At pH 8.9 or 11.1, respectively, as little as 5 x 10(-4) M La(OTf)(3) accelerates the methanolysis of 3 or 4 by 10(10)-fold relative to the background reaction. Detailed kinetic studies of the methanolysis of 5 at varying [La(3+)] and pH indicate that both La(3+) monomers and dimers with associated methoxides are reactive species. The rate constants for La(3+) monomer and dimer catalysis of the methanolysis of 5 are dependent on pH, the slopes of the log plots being 0.35 and 0.5, respectively. A scheme is proposed to account for the non-first-order dependence on [CH(3)O(-)] wherein the La(3+)(CH(3)O(-))(x) + 5 <--> La(3+)(CH(3)O(-))(x):5 and (La(3+))(2)(CH(3)O(-))(y) + 5 <--> 5:(La(3+))(2)(CH(3)O(-))(y) equilibria are driven to the left with increasing numbers of associated methoxides and that attack on these complexes, whether by external or metal-associated methoxide, is slower due to the reduced net positive charge on the complex.