Hydrolysis and DFT structural studies of dinuclear Zn(II) and Cu(II) macrocyclic complexes of m-12N(3)O-dimer and the effect of pH on their promoted HPNP hydrolysis rates

Abstract

The synthesis of the ligand, m-12N(3)O-dimer (1,3-bis(1-oxa-4,7,10-triazacyclododecan-7-yl)methyl)benzene, L), and the stability and hydrolysis constants of its dinuclear Zn(II) and Cu(II) complexes are reported, in addition to the effect of pH on HPNP (2-hydroxypropyl-4-nitrophenylphosphate) hydrolysis reaction rates promoted by these complexes. Various structural possibilities of the [Zn2L] and [Cu2L] hydrolytic species derived from solution equilibrium modeling are predicted from density functional theory (DFT) studies to correlate with the promoted HPNP hydrolysis reaction rates and to establish the structure-function-reactivity relationship. Upon deprotonation [Zn2L(OH)](3+) tends to form a structure with a closed-form conformation where it is not possible for para-isomers. At pH >8, the formation of the closed-form [Zn2L(OH)(2)](2+) and [Zn2L(mu-OH)(OH)(2)](+) species led to faster promoted HPNP hydrolysis rates than the [Zn2L(OH)](3+) species. On the other hand, the observed rates of the Cu2L-promoted HPNP hydrolysis reaction were much slower than those of the [Zn2L]-promoted ones due to formation of the inactive, di-mu-OH- bridged closed-form [Cu2L(mu-OH)(2)](2+) structure at high pH. The effects of solvent molecules and the use of higher DFT computation levels, i.e., M06 and M06-2X, in conjunction with cc-pVDZ and cc-pVTZ basis sets on the DFT-predicted structures for both [Cu(12N(4))(H2O)](2+) and [Zn(12N(3)O)(H2O)(2)](2+) complexes were also evaluated and compared with those using the B3LYP/6-31G* method. [GRAPHICS]