PHOSPHONATE ESTER HYDROLYSIS CATALYZED BY 2 LANTHANUM IONS - INTRAMOLECULAR NUCLEOPHILIC-ATTACK OF COORDINATED HYDROXIDE AND LEWIS-ACID ACTIVATION

(8-Hydroxy-2-quinolyl)methyl (8-hydroxy-2-quinolyl)methyl phosphonate (I) has been synthesized as a model compound and investigated in terms of catalysis of hydrolysis by two metal ions in concert. The divalent metal ions, Zn2+, Ni2+, Co2+ Cu2+ as well as Al3+, form only 1:1 complexes in which the metal ion is sequestered by coordination with the two 8-hydroxyquinoline moieties of I. In these complexes the >PO2- linker points away from the metal ion, and the latter does not act as a catalyst for hydrolysis. The spectrophotometric titration of I with La3+ indicates that the complexes (I)La and (I)La-2 exist in solution with association constant (log K-as1, 6.54; log K-as2, 4.81) greater than that (log K-as, < 4.0) for La3+ complexation of 8-hydroxy-2-methylquinoline. The large values of K-as with I suggest that La3+ interacts with both the 8-hydroxyquinoline ligands and the negative charge of the phosphonate linkage (>PO2-). La3+ is a very effective catalyst for hydrolysis of I. Studies of the rates of hydrolysis of I as a function of [La3+] establish that the hydrolysis proceeds via two reaction paths:.(i) spontaneous hydrolysis of a complex with (I)La-2 stoichiometry and (ii) La3+ promoted hydrolysis of the (I)La-2 complex. On the other hand, removal of one of two 8-hydroxyquinoline ligands of I to provide (8-hydroxy-2-quinolyl)methyl methylphosphonate (II) leads to the formation of the 1:1 complex (II)La, which is hydrolytically inert but subject to catalysis by free La3+. Thus, the remarkable spontaneous hydrolysis of (I)La-2 is due to double metal ion cooperativity in catalysis. The pH-rate profile supports a mechanism for the spontaneous hydrolysis of (I)La-2 which involves the intramolecular attack of HO- ligated to one of the two La3+ in the complex (I)La-2(OH2)(n-1)(OH). The pK(a) value of the water bound to La3+ in (I)La-2(OH2)(n) and rate constant for the hydrolysis of (I)La-2(OH2)(n-1)(OH) are determined as 7.19 and 1.36 x 10(-3) s(-1), respectively. The [La3+] and [HO-] dependence of La3+ promoted hydrolysis of (I)La-2(X)(n) and (II)La(X)(n) {X = H2O or HO-} is in accord with the transition state compositions of {(I)La-2(X)n}{La(OH)(2+)} and {(II)La(X)(n)}{La(OH)(2+)}, respectively. The bimolecular rate constant for the hydrolysis of (I)La-2(X)(n) and (II)La(X)(n) by La(OH)(2+) were calculated as 2.62 x 10(-1) and 7.61 x 10(-2) M(-1) s(-1), respectively. The similarity of these second-order rate constants suggests that one 8-hydroxyquinoline {probably in leaving group} ligated La3+ is effectively associated with the leaving group oxyanion and negative charge of the >PO2- to activate I and II by Lewis-acid catalysis. Based on an estimation of the rate constant for hydroxide-catalyzed hydrolysis of I with P-O bond cleavage, a rate enhancement of similar to 10(13) fold has been observed on the spontaneous hydrolysis of (I)La-2(OH2)(n-1)(OH). This is attributed to a combination of Lewis-acid activation {La3+ ligated to negative >PO2- and leaving oxygen} and intramolecular nucleophilic attack of hydroxide bound to the second La3+.