Dynamic lanthanide exchange between quadruple-stranded cages: the effect of ionic radius differences on kinetics and thermodynamics

Advances in the coordination chemistry of multinuclear compounds have been exploited to drive the self-assembly of many new discrete metallo-supramolecular motifs. Due to the nature of the metal-ligand interactions, many of these systems have a dynamic character with reversible association and dissociation able to generate complex mixtures. Unveil such dynamic behaviours, it is a priority to fully understand, control and design their functional properties. Among metallo-supramolecular systems, lanthanide (Ln) based architectures attracts much attention due to their remarkable optical and magnetic properties. However, design and control of the final supramolecule is very challenging due to the inner nature of the 4f orbitals and consequent small ligand-field effects. There is, however, a steady variation of the effective ionic radius (EIR) across the series, the so called “lanthanide contraction”. Although the radii difference (ΔEIR) is quite small (ca. 0.20 Å between La3+ and Lu3+ and ca. 0.02 Å between two consecutive lanthanides), it can have important chemical consequences on the nature and features of supramolecular complexes.

Recently, a group headed by Marzio Rancan of ICMATE-CNR (Italy) and collaborators from the University of Padova (Italy) and Dortmund University (Germany) have demonstrated that ΔEIR strongly affects the kinetics of Ln ions exchange between preassembled quadruple-stranded [Ln2L4]2 cages (Figure 1).

Figure 1. (a) Self-assembly of seven [Ln2L4]2− cages (Ln = La, Nd, Eu, Tb, Er, Tm and Lu). (b) Dynamic Ln3+ ion exchange equilibrium between two pre-assembled cages and (c) exponential trend of the kinetic constants depending on the Ln ΔEIR.

The process has been qualitatively and quantitatively characterized by time-dependent electrospray ionization mass spectrometry (ESI-MS). Mixing a series of two homonuclear [LnA2L4]2− and [LnB2L4]2− with increasing Ln3+ ΔEIR always leads to the formation of a statistical mixture of homo- and heteronuclear helicates due to the Ln exchange. All the studied systems have an equilibrium constant close to K = 4. The Ln3+ ΔEIR, hence, does not affect the thermodynamics of the process that is mainly governed by statistical factors and entropy-driven. On the other hand, they demonstrate that the rate of the dynamic ion exchange is Ln radius-dependent (Figure 1b). The kinetic constants of the forward and backward reactions revealed an exponential trend depending on the Ln3+ ΔEIR of the two homonuclear pre-assembled cages (Figure 1c): from the minimum to the maximum value of ΔEIR, the kinetic constants increase by three orders of magnitude. This fundamental study hints new tools and guidelines to study dynamic processes in metallo-supramolecular ensembles, and for the precise preparation and control of lanthanide-based mixed coordination-driven systems.

Corresponding author:

Marzio Rancan is a Research Fellow at ICMATE-CNR (Italy). He received his PhD in Molecular Sciences at the University of Padova in 2009. He did post-doctoral studies at CNR, University of Padova and spent one year in the Molecular Magnetism Group at The University of Manchester (UK).  His current research is focused on the synthesis and characterization of coordination-driven molecular and supramolecular architectures with functional properties. He is the author of about 60 articles.

WEBSITE: http://wwwdisc.chimica.unipd.it/FMNLab/index.html

ORCID: https://orcid.org/0000-0001-9967-5283

RESEARCHGATE: https://www.researchgate.net/profile/Marzio-Rancan

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