A Tris(ZnII−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 (NPP 2 - ) 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 H 2 O molecules to be 6.1 (p K 1 ), 7.3 (p K 2 ), and 8.6 (p K 3 ) at 25 °C with I = 0.1 (NaNO 3 ). These three stepwise deprotonations and 1 H NMR spectra changes at various pD values in D 2 O suggest that strong intramolecular hydrogen bonds link each adjacent Zn II −OH 2 (or Zn II −OH - ) at neutral pH in aqueous solution. At higher pH (>9), the hydrogen bond network disrupts. The 31 P NMR titration of Zn 3 L 3 ·3H 2 O with phenyl phosphate dianion (PP 2 - ) in D 2 O confirmed the formation of a 1:1 complex (Zn 3 L 3 −PP 2 - ). 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 NPP 2 -, 6.6 with PP 2 -, 7.0 with α- 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 −NPP 2 - ) 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 −NPP 2 - ).