Molecular Recognition of Activated and Unactivated Phosphate Diesters

The molecular recognition of two simple phosphate diesters by a terpyridine−copper (TCu) complex was studied
by X-ray crystallography and potentiometry. Molecular recognition of the substrate is the first step in a hydrolysis
reaction. The two phosphate diesters bis(p-nitrophenyl) phosphate (BNP) and diphenyl phosphate (DPP) coordinate
axially to an approximate square pyramidal copper complex where a chloride and three terpyridine nitrogens
form the corners of the base. The coordination geometries are almost identical. The molecular recognition constants
for the formation of TCu−BNP and TCu−DPP were measured potentiometrically and have log values of 1.2 and
2.5, respectively. By a significant margin, the TCu complex favors DPP over BNP in solution. A pH versus rate
constant profile shows that TCu−OH can hydrolyze BNP but not DPP. Activated and unactivated phosphate
diesters are typically hydrolyzed by similar mechanisms. In this study the difference in reactivity lies outside the
process of molecular recognition. Activating (electron-withdrawing) effects of the p-nitro groups on the nature of
the leaving groups must play an important role in the susceptibility of phosphate diesters to hydrolysis.