Steric Effects on the Binding of Phosphate and Polyphosphate Anions by Zinc(II) and Copper(II) Dinuclear Complexes of m-Xylyl-bis-cyclen

Abstract

The triethylbenzene-bis-cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) compound (tbmce) was designed with an imposed structural rigidity at the m-xylyl spacer to be compared to a less restrained and known parent compound (bmce). The framework of both compounds differs only in the substituents of the m-xylyl spacer. The study was centered in the differences observed in the acid-base reactions of both compounds, their copper(II) and zinc(II) complexation behaviors, as well as in the uptake of phosphate and polyphosphate anions (HPPi, ATP, ADP, AMP, PhPO, and HPO). On the one hand, the acid-base reactions showed lower values for the third and fourth protonation constants of tbmce than for bmce, suggesting that the ethyl groups of the spacer in tbmce force the two cyclen units to more conformational restricted positions. On the other hand, the stability constant values for copper(II) and zinc(II) complexes revealed that bmce is a better chelator than tbmce pointing out to additional conformational restraints imposed by the triethylbenzene spacer. The binding studies of phosphates by the dinuclear copper(II) and zinc(II) complexes showed much smaller effective association constants for the dicopper complexes. Single-crystal X-ray and computational (density functional theory) studies suggest that anion binding promotes the formation of tetranuclear entities in which anions are bridging the metal centers. Our studies also revealed the dinuclear zinc(II) complex of bmce as a promising receptor for phosphate anions, with the largest effective association constant of 5.94 log units being observed for the formation of [Znbmce(HPPi)]. Accordingly, a colorimetric study via an indicator displacement assay to detect phosphates in aqueous solution found that the [Znbmce] complex acts as the best receptor for pyrophosphate displaying a detection limit of 2.5 nM by changes visible to naked eye.

Exciplex Formation and Aggregation Induced Emission in Di‐(N‐benzyl)cyclen and Its Complexes – Selective Fluorescence with Lead(II), and as the Cadmium(II) Complex, with the Chloride Ion

“…A possible structure for the dimeric Zn II DB‐cyclen complex that would allow for exciplex formation, based on the hydroxy‐bridged MB‐cyclen complex, but with a bridging NO 3 – group, generated by DFT calculation, is seen in Figure . One notes a recently reported structure of a Cu II complex of a cyclen derivative that is bridged by a nitrate group as suggested in Figure . The benzyl groups are quite well separated, but can be brought into a parallel π‐stacking type of arrangement as would be required for exciplex formation by simply rocking the two Zn(DB‐cyclen) moieties with moderate bending of the Zn–O bonds to the bridging nitrate, or simply rotating them around the Zn–O bonds which can bring the aromatic rings close together.…”

Pentacyclic Nano‐Trefoil

“…reported a cyclic dipicolinate and its Pb 2+ and Ba 2+ complexes [11] . They also reported dinuclear Cu 2+ and Zn 2+ complexes of cyclen ligands and their association with phosphate anions [12] . Hancock et al.…”

“…Cyclen is a 12‐membered cyclic polyamine that forms stable complexes with several metal ions [11–22] . Tripier et al.…”

“…[5,6] Cyclen is a 12-membered cyclic polyamine that forms stable complexes with several metal ions. [11][12][13][14][15][16][17][18][19][20][21][22] Tripier et al reported a cyclic dipicolinate and its Pb 2+ and Ba 2+ complexes. [11] They also reported dinuclear Cu 2+ and Zn 2+ complexes of cyclen ligands and their association with phosphate anions.…”

“…[11] They also reported dinuclear Cu 2+ and Zn 2+ complexes of cyclen ligands and their association with phosphate anions. [12] Hancock et al prepared Pb 2+ and Cd 2+ complexes of di-(N-benzyl)cyclen and studied their selective fluorescence properties. [13] We have previously reported Ag + complexes with cyclen with two, three, or four aromatic side arms that allow the complex to function like an insectivorous plant (e.g., a Venus flytrap) towards Ag + .…”