R. S. Brown

Potentiometric titration of metal ions in methanol

The potentiometric titrations of Zn2+, Cu2+ and 12 Ln3+ metal ions were obtained in ethanol to determine the titration constants (defined as the at which the [-OEt]/[Mx+]t ratios are 0.5, 1.5, and 2.5) and in two cases (La3+ and Zn2+) a complete speciation diagram. Several simple monobasic acids and aminium ions were also titrated to test the validity of experimental titration measurements and to establish new constants in this medium that will be useful for the preparation of buffers and standard solutions. The dependence of the titration constants on the concentration and type of metal ion and specific counterion effects is discussed. In selected cases, the titration profiles were analyzed using a commercially available fitting program to obtain information about the species present in solution, including La3+ for which a dimer model is proposed. The fitting provides the microscopic values for deprotonation of one to four metal-bound ethanol molecules. Kinetics for the La3+-catalyzed ethanolysis of paraoxon as a function of are presented and analyzed in terms of La3+ speciation as determined by the analysis of potentiometric titration curves. The stability constants for the formation of Zn2+ and Cu2+ complexes with 1,5,9-triazacyclododecane as determined by potentiometric titration are presented.

An immobilized ortho-palladated dimethylbenzylamine complex as an efficient catalyst for the methanolysis of phosphorothionate pesticides.

The methanolysis of a series of P=S phosphorothionate pesticides (fenitrothion, coumaphos, diazinon, and dichlofenthion) catalyzed by an ortho-palladated complex covalently attached to two different solid supports, macroporous polystyrene and amorphous silica gel, was studied. Both the polystyrene and the silica-based catalysts showed excellent activity in methanol near neutral pH (neutral s(s)pH = 8.38) at ambient temperature. These heterogeneous catalysts can be readily recovered and reused without significant loss of activity. Fifty milligrams of the silica-supported catalyst SiPd1 offered an acceleration of up to 8.6 x 10(9)-fold for the methanolysis of fenitrothion (2) over the methoxide-promoted background reaction at s(s)pH = 8.8. For the same reaction, 50 mg of polystyrene-supported complex PSPd2 provided a 3.7 x 10(9)-fold acceleration at s(s)pH = 8.8. When accounting for the amount of palladium in the solid, the slight superiority of silica over polystyrene as a solid support is believed to be a result of several possible factors including a higher concentration of active sites accessible to the reaction solvent and a more hydrophilic surface environment that allows better interaction of the methanol solvent with the attached palladacycle. Unlike the behavior in homogeneous solution, the rate of methanolysis of the substrates catalyzed by the solid catalysts was relatively insensitive to the nature of the substrate, probably indicating that a mass transport process is rate limiting. The solid-supported materials effectively decompose malathion at roughly stoichiometric ratios, but they are strongly inhibited by the thiol product resulting from the cleavage of the P=S(SR) linkage.