Self-Assembly Mechanism of a Bimetallic Europium Triple-Stranded Helicate

Abstract

We report the self-assembly process of a supramolecular edifice based on the coordination of europium(III) by a ditopic strand L bearing tridentate bis(benzimidazolyl)pyridine subunits. Varying the metal/ligand ratio and using a fruitful combination of electrospray mass spectrometry and absorption spectrophotometry, we characterized three major complexes (EuL(2), Eu(2)L(2), and Eu(2)L(3)) in acetonitrile. Kinetic investigations showed an alternative "braiding" and "keystone" mechanism leading to Eu(2)L(3). The formation mechanism of the dinuclear triple-stranded helicate, which is mainly governed by electrostatic interactions, goes via the "side-by-side" Eu(2)L(2) intermediate. Our thermodynamic and kinetic data allow the prediction of the apparent "magic" self-assembly of Eu(2)L(3) which is fast and efficient only under a strict set of conditions.

Lanthanide Homobimetallic Triple‐Stranded Helicates: Insight into the Self‐Assembly Mechanism

“…This is in contrast to the formation mechanism of Eu 2 (L A ) 3 for which we propose the formation of a labile intermediate Eu 2 (L A ) 2 . [30] As far as we can tell, the increased inertness of the key intermediate is the main reason why the rate constant of the last step in the self-assembly process of Eu 2 L 3 is 10 5 times larger than Eu 2 (L A ) 3 . Moreover, the addition of the successive ligand strands leads to a strong diminishing of the electrostatic interactions between the two coordination sites.…”

“…This kinetic behavior points to the Eu 2 L 2 intermediate probably adopting a structure different from the ''side-by-side'' arrangement observed for Eu 2 (L B ) 2 in the solid state. [22] Indeed the formation of a 2:2 helical structure at this stage will decrease the steric hindrance and, moreover, the presence of negatively charged terminal carboxylate groups obviously contributes to prevent the formation a labile ''side-by-side'' [30,57] species. This is in contrast to the formation mechanism of Eu 2 (L A ) 3 for which we propose the formation of a labile intermediate Eu 2 (L A ) 2 .…”

“…[27Ϫ30] As part of our ongoing kinetic investigations of supramolecular edifices with polytopic ligands, dealing with either copper() trimetallic double-stranded [29] or lanthanide() bimetallic triplestranded [30] helicates in acetonitrile, we present here a detailed physicochemical study of the Eu 2 L 3 helicate in aqueous solution. The work is focused on getting a better understanding of the key steps of the self-assembly process.…”

“…The formation mechanism of EuL is at odds with the one determined for the EuL A analogue, for which conformational changes lead to a slower binding rate constant and to an increased inertness. [30] The stability constant K EuL ϭ (1.8Ϯ0.9) ϫ10 ) [70] gives a good indication of a partial binding to Eu III by one of the tridentate subunits in L. The dissociation rate constant k Ϫ1 ϭ k 1 /K EuL can then be estimated ). [66,67] Step 1 can therefore be considered as being a fast pre-equilibrium in the formation process of the target Eu 2 L 3 .…”

The First Self‐Assembled Trimetallic Lanthanide Helicates Driven by Positive Cooperativity

“…Since b 13 is not accessible, the cooperativity process refers to the fixation of the third lanthanide ion with respect to the average binding of the metal ions in [Ln 2 (L7) 3 ] 6 , and we conclude that electrostatic repulsion associated with the complexation of the third Ln III is more than compensated by the preorganization of the receptor brought about by the first two metal ions. Related Scatchard plots for the bimetallic helicates [Ln 2 (L1) 3 ] 6 (Ln La, Eu, Lu) display convex curves and negative cooperativity; [29] this highlights the crucial role played by 1) the ligand design (i.e., the replacement of terminal benzimidazole groups in L1 or L6 [20] with carboxamide groups in L7) and 2) the overall nuclearity of the assembly processes. (Figure 4a, detailed assignments and discussion are given in the section dedicated to the solution structure of [Ln 3 (L7) 3 ] 9 ).…”