Hydrolysis and DFT structural studies of dinuclear Zn(II) and Cu(II) macrocyclic complexes of m -12N3O-dimer and the effect of pH on their promoted HPNP hydrolysis rates

Article abstract of DOI:10.1080/00958972.2016.1157177

The synthesis of the ligand, m-12N₃O-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(μ-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-μ-OH^- bridged closed-form [Cu₂L(μ-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(12N₄)(H₂O)]²⁺ and [Zn(12N₃O)(H₂O)₂]²⁺ complexes were also evaluated and compared with those using the B3LYP/6-31G? method.