Catalytic hydrolysis of phosphate diesters by lanthanide(III) cryptate (2.2.1) complexes

Abstract

Lanthanide(III) Cryptate (2.2.1) chlorides (Ln(2.2.1)Cl-3; Ln = La (1a), Ce(1b), and Eu(1c); (2.2.1) = 4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane) are effective for the catalytic hydrolysis of bis(4-nitrophenyl) phosphate. Kinetic studies reveal that the europium(III) complex (1c) catalyzes the hydrolysis to produce 6 equiv of 4-nitrophenol with a significant rate (k(1) = 1.5 x 10(-4) s(-1) at 0.40 mM) at pH 8.5 and 50 degrees C. The catalytic activity of the complexes is increased with decreasing the ionic size, i.e, La < Ce < Eu. While the use of hydrogen peroxide further increase the activity of 1b (k(1) = 1.6 x 10(-3) s(-1) at 0.40 mM), the presence of molecular oxygen does not affect the activity at all. Crystal of 1a . CH3OH([La(2.2.1)(Cl)(2)](Cl)(CH3OH)) belongs to the space group Pnma with a = 17.072(3) Angstrom, b = 19.037(3) Angstrom, c = 14.725(2) Angstrom, V = 4786(1) Angstrom(3), Z = 8, D-x = 1.691 g cm(-3), mu = 21.7 cm(-1). The encryptated metal ion is nine-coordinated, and all the heteroatoms of the cryptate (2.2.1) ligand coordinate the metal center to form a bowl-shaped structure. Two coordinating chloride anions are located on the open face with a cis geometry. The existence of coordinated water to the europium(III) complex 1c in the aqueous solution was confirmed by time-resolved Eu(III) luminescence spectroscopy. From the decay constants in H2O and D2O. the numbers of coordinated water molecules (q) are found to be 3.02 at pH of 5.0. The above kinetic and spectroscopic observation are supportive of mechanisms in which the metal complexes act as a center for binding and activation as well as a source of nucleophilic metal hydroxides.