Monopicolinate Cyclen and Cyclam Derivatives for Stable Copper(II) Complexation

Abstract

The syntheses of a new 1,4,7,10-tetraazacyclododecane (cyclen) derivative bearing a picolinate pendant arm (HL1), and its 1,4,8,11-tetraazacyclotetradecane (cyclam) analogue HL2, were achieved by using two different selective-protection methods involving the preparation of cyclen-bisaminal or phosphoryl cyclam derivatives. The acid-base properties of both compounds were investigated as well as their coordination chemistry, especially with Cu(2+), in aqueous solution and in solid state. The copper(II) complexes were synthesized, and the single crystal X-ray diffraction structures of compounds of formula [Cu(HL)](ClO(4))(2)·H(2)O (L = L1 or L2), [CuL1](ClO(4)) and [CuL2]Cl·2H(2)O, were determined. These studies revealed that protonation of the complexes occurs on the carboxylate group of the picolinate moiety. Stability constants of the complexes were determined at 25.0 °C and ionic strength 0.10 M in KNO(3) using potentiometric titrations. Both ligands form complexes with Cu(2+) that are thermodynamically very stable. Additionally, both HL1 and HL2 exhibit an important selectivity for Cu(2+) over Zn(2+). The kinetic inertness in acidic medium of both complexes of Cu(2+) was evaluated by spectrophotometry revealing that [CuL2](+) is much more inert than [CuL1](+). The determined half-life values also demonstrate the very high kinetic inertness of [CuL2](+) when compared to a list of copper(II) complexes of other macrocyclic ligands. The coordination geometry of the copper center in the complexes was established in aqueous solution from UV-visible and electron paramagnetic resonance (EPR) spectroscopy, showing that the solution structures of both complexes are in excellent agreement with those of crystallographic data. Cyclic voltammetry experiments point to a good stability of the complexes with respect to metal ion dissociation upon reduction of the metal ion to Cu(+) at about neutral pH. Our results revealed that the cyclam-based ligand HL2 is a very attractive receptor for copper(II), presenting a fast complexation process, a high kinetic inertness, and important thermodynamic and electrochemical stability.

Electrocatalytic water oxidation by CuII complexes with branched peptides

“…Parallel X-Band EPR, UV/ VIS and CD spectroscopic changes with increasing pH suggested a {NH 2 ,2N À ,NH 2 } equatorial ligand set for 1. 15 Based on analogous examples, 13,16 a further deprotonation step resulting in Cu II H À3 L (1-H + ) in the case of 1 has been associated with proton loss from an axially coordinated water molecule. In 2, Cu II is bound by a {NH 2 ,2N À ,N Im } equatorial set, involving the C-terminal histidine residue.…”

Current advances in ligand design for inorganic positron emission tomography tracers 68Ga, 64Cu, 89Zr and 44Sc

“…The TRAP and NOPO ligands (Fig. 125 In comparison, Cu-cyclam has a halflife of 2.7 days in 5 M HCl despite having no pendant arms. 120 TRAP(CHX) 3 (t 1/2 = 10 h -pH 3.5, 25°C) 65 was shown to be more kinetically stable than TRAP-Pr (t 1/2 = 2 h -pH 3.5, 25°C).…”

Influence of Introduced Substituents on the Anion-selectivity of [14]Tetraazaannulene Complexes

“…7 Although the unsubstituted cyclam works as a highly selective and efficient carrier for the transport of the Pb 2+ ion, 8,9 the introduction of various substituents on the nitrogen atoms of the cyclam can significantly vary the ion-selectivity. For example, the introduction of the tosyl (p-toluenesulfonyl) 10 and 2-pyridylmethyl 11,12 groups as substituents into the cyclam led to the formation of complexes with high Ni 2+ and Cu 2+ ionselectivities, respectively. Furthermore, fluoroionophores for the Hg 2+ and Zn 2+ ions were synthesized by the introduction of 1-pyrenyl acetamide,…”