Carbon-Halogen Bond Activation by Selenium-Based Chalcogen Bonding.
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ABSTRACT:
SUBMITTER: Wonner P
PROVIDER: S-EPMC5638094 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:The synthesis and anion binding properties of novel halogen-bonding (XB) bis-iodotriazole-pyridinium-containing acyclic and [2]catenane anion host systems are described. The XB acyclic receptor displays selectivity for acetate over halides with enhanced anion recognition properties compared to the analogous hydrogen-bonding (HB) acyclic receptor. A reversal in halide selectivity is observed in the XB [2]catenane, in comparison to the acyclic XB receptor, due to the interlocked host's unique three-dimensional binding cavity, and no binding is observed for oxoanions. Notable halide anion association constant values determined for the [2]catenane in competitive organic-aqueous solvent mixtures demonstrate considerable enhancement of anion recognition as compared to the HB catenane analogue. X-ray crystallographic analysis of a series of halide catenane complexes reveal strong XB interactions in the solid state. These interactions were studied using Cl and Br K-edge X-ray Absorption Spectroscopy (XAS) indicating intense pre-edge features characteristic of charge transfer from the halide to its bonding partner (?(AX?X(-))(*) ? X1s), and providing a direct measure of the degree of covalency in the halogen bond(s). The data reveal that the degree of covalency is similar to that which is observed in transition metal coordinate covalent bonds. These results are supported by DFT results, which correlate well with the experimental data.
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Project description:1,3-Bis(benzimidazoliumyl)benzene-based chalcogen-bonding catalysts were previously successfully applied in different benchmark reactions. In one of those examples, i.e. the activation of quinolines, sulfur- and selenium-based chalcogen-bonding catalysts showed comparable properties, which is unexpected, as the selenium-containing catalysts should show superior catalytic properties due to the increased polarizability of selenium compared to sulfur. Herein, we present four crystal structures of the respective 1,3-bis(benzimidazoliumyl)benzene-based chalcogen-bonding catalyst containing sulfur (3S) and selenium (3Se, three forms) as Lewis acidic centres. The sulfur-containing catalyst shows weaker chalcogen bonding compared to its selenium analogue, as well as anion-π interactions. The selenium-based analogues, on the other hand, show stronger chalcogen-bonding motifs compared to the sulfur equivalent, depending on the crystallization conditions, but in every case, the intermolecular interactions are comparable in strength. Other interactions, such as hydrogen bonding and anion-π, were also observed, but in the latter case, the interaction distances are longer compared to those of the sulfur-based equivalent. The solid-state structures could not further explain the high catalytic activity of the sulfur-containing catalysts. Therefore, a comparison of their σ-hole depths from density functional theory (DFT) gas-phase calculations was performed, which are again in line with the previously found properties in the solid-state structures.
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