Phosphate diester hydrolysis by mono- and dinuclear lanthanum complexes with an unusual third-order dependence

A series of mono- and dinuclear lanthanum complexes of 15,31-dimethyl-3,11,19,27,33,35-hexaazapentacyclo-[27.3.1.1(5,9)1.(13,17).1(21,25)]hexatriaconta-5,7,9(33),13,15,17(34),21,23,25(35),29,31,1(36)-dodecaene-34,36-diol (24RBPyBC, L) have been defined in solution. Their ability to hydrolyze bis(4-nitrophenyl) phosphate, a phosphate diester, was studied. The various metal-coordinated hydroxide nucleophiles that form in solution attack the substrate in the hydrolysis reaction. The dihydroxo dilanthanum complex, L-2La-2(OH), is the most effective catalyst. Its rate constant is 75 times larger than the rate constant for the monohydroxo dilanthanum complex, L-2La-OH. The mononuclear complexes are not as successful as the dinuclear complexes because they have fewer metal ions per complex to act as Lewis acids. They also cannot generate hydroxide nucleophiles at low pH values like the dinuclear complexes can. The reaction has an unusual third-order dependence on the catalyst concentration which is valid for the dinuclear complexes as well as the mononuclear complexes. This implies a mechanism where a metal-coordinated hydroxide nucleophile attacks the phosphorus of the substrate on the side opposite the negatively charged oxygens.