Stability and Lability of Dicopper Double-Stranded Helicates in Solution

Stability constants have been measured for a series of ligands based on a 2,2′‐(pyridine‐2,6‐diyl)bis[1H‐benzimidazole] unit which forms dinuclear double‐stranded helical complexes with copper(I). Variation of different structural parameters confirms the importance of the coordinate bond, the stacking interactions, and the weakly bridging pyridine units observed by X‐ray crystallography. The stabilities of the complexes depend strongly on the solvent, and in MeCN, which is a good solvent for copper(I), the complexes are less stable and assemble in a stepwise manner. The interconversion of the enantiomers may be followed by 1H‐NMR and takes place on a millisecond time scale around room temperature. The trends in lability are similar to those found for the stability of the complexes

Control of Redox Potential by Deprotonation of Coordinated 1<i>H</i>-Imidazole in Complexes of 2-(1<i>H</i>-Imidazol-2-yl)pyridine

Complexes [ML₃]²⁺ of the bidentate ligand 2‐(1H‐imidazol‐2‐yl)pyridine were prepared with iron(II), cobalt(II), and ruthenium(II). The electronic spectra suggest the ligand to be a weaker σ‐donor and π‐acceptor than the closely related 2,2′‐bipyridine. The complexes are readily deprotonated by addition of base, and the effect of the deprotonation is to lower the MIII/MII redox potential by roughly 900 mV. This is roughly 75% of the drop observed for related complexes of 2,6‐di‐1H‐imidazol‐2‐ylpyridine, and suggests the effect to be largely coulombic in origin

Conjoint analysis of kinetic and equilibrium data for mechanistic elucidation in polynuclear complexation reactions, exemplified by metal(II) helicate complex formation

Combined kinetic and equilibrium studies of the complexation of the ligand N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide with copper(ii) and nickel(ii), examined conjointly employing advanced chemometric methods, provides elucidation of speciation and formation pathways in solution for complexes such as M₂L₂ and M₂L₃ helicates, respectively

EuIII complexes of octadentate 1-hydroxy-2-pyridinones: Stability and improved photophysical performance

The luminescence properties of lanthanoid ions can be dramatically enhanced by coupling them to antenna ligands that absorb light in the UV/visible and then efficiently transfer the energy to the lanthanoid center. The synthesis and the complexation of Ln(III) cations (Ln=Eu; Gd) for a ligand based on four 1-hydroxy-2-pyridinone (1,2-HOPO) chelators appended to a ligand backbone derived by linking two L-lysine units (3LI-bis-LYS) is described. This octadentate Eu(III) complex ([Eu(3LI-bis-LYS-1,2-HOPO)](−)) has been evaluated in terms of its thermodynamic stability, UV/visible absorption and luminescence properties. For this complex the conditional stability constant (pM) is 19.9, which is an order of magnitude higher than diethylenetriaminepentacetic acid (DTPA) at pH= 7.4. This Eu(III) complex also shows an almost two-fold increase in its luminescence quantum yield in aqueous solution (pH= 7.4) when compared to other octadentate ligands. Hence, despite a slight decrease of the molar absorption coefficient, a much higher brightness is obtained for [Eu(3LI-bis-LYS-1,2-HOPO)](−). This overall improvement was achieved by saturating the coordination sphere of the Eu(III) cation, yielding an increased metal centered efficiency by excluding solvent water molecules from the metal’s inner sphere