Add time:08/28/2019         Source:sciencedirect.com

A new functional macrocyclic ligand, 2,4-dinitrophenylcyclen (= 1-(2,4-dinitrophenyl)-1,4,7,10-tetraazacyclododecane), has been synthesized and isolated as its trihydrochloric acid salt (L·3HCl). The protonation constants (log Kn) for three secondary nitrogens of L were determined by potentiometric pH titration to be 10.10, 7.33 and <2 with I = 0.10 (NaNO3) at 25°C. The 2,4-dinitrophenylaniline chromophore was proven to be a good reporter signaling proton- and metal-binding events in the macrocyclic cavity. The UV absorption band (λmax 370 nm, ɛ 8200) of the 2,4-dinitrophenylaniline moiety at pH ≥ 9 becomes quenched as pH is lowered (to pH 3.1, where the major species is L·2H+), due to the strong protonation effect extended to the aniline moiety within the macrocyclic cavity. This is in sharp contrast to the pH-independent UV absorption (λmax 390 nm, ɛ 14 000) of a reference compound, N,N-diethyl-2,4-dinitroaniline. The UV absorption band of L is shifted to lower wavelengths with Zn2+ (λmax 320 nm), Cd2+ (λmax 316 nm) and Pb2+ (λmax 317 nm), while it almost disappears with Cu2+ and Ni2+. The 1:1 Zn2+ and Cu2+ complexes with L were isolated and characterized. The Zn2+ complex recognizes 1-methylthymine anion (MT−) in aqueous solution at physiological pH to yield a stable ternary complex ZnL-MT−. The X-ray crystal structure of ZnL-MT− showed that Zn2+ is four-coordinate with three secondary nitrogens of L and the deprotonated imide anion that is cofacial to the 2,4-dinitrophenyl ring.

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