A Tris(Zn^II−1,4,7,10-tetraazacyclododecane) Complex as a New Receptor for Phosphate Dianions in Aqueous Solution

A new tris(ZnII−cyclen) (Zn3L3), which has three ZnII−macrocyclic tetraamine (cyclen) complexes connected through a 1,3,5-trimethylbenzene spacer, has been synthesized as a novel receptor for organic phosphate dianions in aqueous solution (cyclen = 1,4,7,10-tetraazacyclododecane and L3 = 1,3,5-tris(1,4,7,10-tetraazacyclododecan-1-ylmethyl)benzene). The design of Zn3L3 was based on X-ray crystal structure analysis of the 1:3 complex of 4-nitrophenyl phosphate (NPP2-) with ZnII−cyclen (ZnL1) complex. The potentiometric pH titration of Zn3L3·3H2O revealed the deprotonation constants of the three ZnII-bound H2O molecules to be 6.1 (pK1), 7.3 (pK2), and 8.6 (pK3) at 25 °C with I = 0.1 (NaNO3). These three stepwise deprotonations and 1H NMR spectra changes at various pD values in D2O suggest that strong intramolecular hydrogen bonds link each adjacent ZnII−OH2 (or ZnII−OH-) at neutral pH in aqueous solution. At higher pH (>9), the hydrogen bond network disrupts. The 31P NMR titration of Zn3L3·3H2O with phenyl phosphate dianion (PP2-) in D2O confirmed the formation of a 1:1 complex (Zn3L3−PP2-). By potentiometric pH titrations, the 1:1 complex affinity constants, log Kaff (Kaff = [phosphate complex]/[ZnII complex][phosphate] (M-1)), were determined to be 5.8 with NPP2-, 6.6 with PP2-, 7.0 with α-d-glucose-1-phosphate, and 7.9 with phenyl phosphonate in aqueous solution. The tris(ZnII−cyclen) complex is found to be a much better host toward phosphates than the parent ZnII−cyclen (ZnL1) (log Kaff = 3.3 for ZnL1−NPP2-) or a bis(ZnII−cyclen) linked with a m-xylene spacer (Zn2L2) (log Kaff = 4.0 for Zn2L2−NPP2-).