A tris(Zn-II-1,4,7,10-tetraazacyclododecane) complex as a new receptor for phosphate dianions in aqueous solution

A new tris(Zn-II-cyclen) (Zn(3)L(3)), which has three Zn-II-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 L(3) = 1,3,5-tris(1,4,7,10-tetraazacyclododecan-1-ylmethyl)benzene), The design of Zn(3)L(3) was based on X-ray crystal structure analysis of the 1:3 complex of 4-nitrophenyl phosphate (NPP2-) with Zn-II-cyclen (ZnL(1)) complex. The potentiometric pH titration of Zn(3)L(3) . 3H(2)O revealed the deprotonation constants of the three Zn-II-bound H2O molecules to be 6.1 (pK(1)), 7.3 (pK(2)), and 8.6 (pK(3)) at 25 degrees C with I = 0.1 (NaNO3). These three stepwise deprotonations and H-1 NMR spectra changes at various pD values in D2O suggest that strong intramolecular hydrogen bonds link each adjacent Zn-II-OH2 (or Zn-II-OH-) at neutral pH in aqueous solution. At higher pH (>9), the hydrogen bond network disrupts. The P-31 NMR titration of Zn(3)L(3) . 3H(2)O With phenyl phosphate dianion (PP2-) in D2O confirmed the formation of a 1:1 complex(Zn(3)L(3)-PP2-). By potentiometric pH titrations, the 1:1 complex affinity constants, log K-aff (K-aff = [phosphate complex]/[Zn-II complex][phosphate] (M(-1))), were determined to be 5.8 with NPP2-, 6.6 with PP2-, 7.0 with alpha-D-glucose-1-phosphate, and 7.9 with phenyl phosphonate in aqueous solution. The tris(Zn-II-cyclen) complex is found to be a much better host toward phosphates than the parent Zn-II-cyclen (ZnL(1)) (log K-aff = 3.3 for ZnL(1)-NPP2-) or a bis(Zn-II-cyclen) linked with a m-xylene spacer (Zn(2)L(2)) (log K-aff = 4.0 for Zn(2)L(2)-NPP2-).