Size-Exclusion Mechanism Driving Host-Guest Interactions between Octahedral Rhenium Clusters and Cyclodextrins

Area of Science:

• Supramolecular Chemistry
• Coordination Chemistry
• Materials Science

Background:

• Cyclodextrins (CDs) are cyclic oligosaccharides known for their ability to form host-guest complexes with various molecules in aqueous solutions.
• Rhenium clusters, specifically octahedral hexametallic clusters like [Re₆Q₈(CN)₆]^4- (Q = S, Se, Te), are water-soluble and possess unique structural and electronic properties.

Purpose of the Study:

• To systematically investigate the supramolecular interactions between native cyclodextrins (α- and β-CD) and water-soluble rhenium clusters ([Re₆Q₈(CN)₆]^4-).
• To elucidate the factors governing the formation and stability of inclusion complexes, including host-guest size matching and solvent effects.
• To explore the chirality induction from cyclodextrins to rhenium clusters based on host-guest interaction strength.

Main Methods:

• X-ray crystallography to determine the solid-state structures and supramolecular arrangements.
• Isothermal titration calorimetry (ITC) to quantify binding thermodynamics.
• Multinuclear Nuclear Magnetic Resonance (NMR) spectroscopy to study solution-state interactions.
• Cyclic voltammetry (CV) to assess electrochemical properties.
• Electrospray ionization mass spectrometry (ESI-MS) for molecular identification and complex analysis.
• Circular dichroism (CD) spectroscopy to investigate chirality transfer.

Main Results:

• Six new crystal structures were obtained, showcasing diverse supramolecular arrangements between CDs and rhenium clusters.
• β-CDs encapsulated all tested rhenium clusters ([Re₆Q₈(CN)₆]^4-, Q = S, Se, Te) regardless of cluster size, forming inclusion complexes with varying degrees of inclusion (none, partial, or deep).
• α-CD selectively formed inclusion complexes only with the smaller [Re₆S₈(CN)₆]^4- cluster.
• Solution studies corroborated solid-state findings, highlighting the critical roles of size-matching and solvent effects in complex stability.
• Circular dichroism studies demonstrated that chirality induction from CDs to rhenium clusters is strongly dependent on the host-guest interaction strength.

Conclusions:

• The study reveals distinct supramolecular assembly behaviors between cyclodextrins and rhenium clusters, governed by host-guest size complementarity and solvent effects.
• The formation and stability of these inclusion complexes are tunable, offering possibilities for designing novel supramolecular architectures.
• The observed chirality transfer provides insights into host-induced stereochemical control in coordination chemistry.