Abstract
While inherent complexation properties and propensity for self-organization of cyclodextrins (CDs) render them potentially promising scaffolds of magnetic materials, this research area is still at an embryonic stage. We report on the synthesis and structure characterization of a new sandwich-type complex, [(α-CD)2Co3Li6(H2O)9] (α-1), which represents a smaller analogue of the previously characterized [(γ-CD)2Co4Li8(H2O)12] (γ-1) cluster. A comprehensive structural analysis of α-1 and a careful reinvestigation of γ-1 reveal how the symmetry of CD ligands determines the molecular composition and supramolecular arrangements of Co/Li sandwich-type complexes. Furthermore, the first comparative studies of the magnetic properties in this type of system point to subtle differences in the magnetic behavior of both compounds. The sandwich-type complexes α-1 and γ-1 exhibit field-induced slow magnetic relaxation, defining a new family of magnetic materials with a pillared grid-like supramolecular structure composed of weakly interacting CoII centers forming an SMM.
Highlights
Occurring cyclodextrins (CDs) are readily available macrocyclic entities with an inherent hydrophobic internal cavity and hydrophilic external surface that display a combination of interesting molecular recognition and complexation properties.[1]
The first structurally well-characterized CD−metal complex, [(β-CD)2Cu4Li7(H2O)7], was reported by Klüfers in 1993.2 because of the challenges associated with the isolation of well-defined systems and/or reliable characterization methods, the interactions of native cyclodextrins with metal ions have remained a largely undeveloped research area, and the scarcity of structural data and mechanistic insights are some of the key obstacles in the rational design of new CDbased functional systems.1c comprehensive analysis of reported CD−metal complexes indicates that there are several distinguishable patterns of CD−metal interactions, which may be harnessed to control the chemical environment and spatial arrangement of metal centers.1c In particular, native CDs possess a tendency for stabilization of sandwich-type metal complexes composed of macrocyclic systems of metal ions enclosed between two CD ligands.1c
The reported studies nicely substantiate that native cyclodextrins are favorable compounds in formulating small molecule clusters and exiting supramolecular structures as well as in developing single-molecule magnet systems
Summary
Occurring cyclodextrins (CDs) are readily available macrocyclic entities with an inherent hydrophobic internal cavity and hydrophilic external surface that display a combination of interesting molecular recognition and complexation properties.[1]. The molecular structure of γ-1 represents a larger sandwichtype analogue of α-1 and comprises the metallamacrocycle composed of four {Co,Li,Li} units in a square geometry.[5] The coordination environment of the metal centers is similar in both complexes (Table 1), and the analogous separation of CoII centers by pairs of Li+ ions provides comparable intramolecular Co···Co distances of about 10.3 and 10.7 Å, respectively. Raman process as additional contribution cannot be unambiguously determined from the data, which was noted elsewhere for similar compounds.[5,21] Because a satisfactory and sound correlation between the energy states of the paramagnetic centers and the parameters deduced from magnetic ac measurements data could still not be derived, partially due to the models including (virtual) phonon processes, we present both results that have almost the exact same fit quality. This could be an indirect effect since only the individual spin contributions may be affected
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