Since Pedersen first synthesized a series of crown ether compounds in 1967, chemists from various countries have done a lot of work on the synthesis, properties, and applications of crown ethers. These types of ligands are widely used in the basic research of coordination chemistry for alkali metals, alkaline earth metals, and lanthanide element metals.There are currently hundreds of synthesized crown ethers, and the following are the most common crown ethers:
1. Coordination properties of crown ethers
Crown ethers have a hydrophobic outer skeleton and a hydrophilic inner cavity that can bond with metal ions. Crown ether compounds have a definite human ring structure and can form relatively stable complexes with many metal ions.
2. The structure of crown ether complexes
In recent years, extensive research has been conducted on the structure of crown ether complexes, which can be divided into five categories based on the positional relationship between ligands and cations in the complexes:
The first type is the pores where cations are exactly suitable for ligands, such as the complexes of 18-crown-6 and KSCN [K (18-crown-6) (SCN)]K＋with weak interaction forces with SCN¯.
The second type is the pore where the cation is slightly larger than the ligand and slightly above the pore. The complexes Rb＋ and Cs¯ ions of 18-crown-6 with CsSCN and RbSCN leave the plane 1.2Åand 1.44Å, respectively. At the same time, the ammonia atoms of two SCN¯ also weakly bind with metal ions, bridging the two complexes together.
When the cation ratio of the ligand is small, the ligand can simultaneously encapsulate two metal ions, forming a third type of structure. For example, the complex of dibenzo-24-crown-8 with K＋ions: out of the 8 atoms in the ligand, every 4 atoms coordinate with 1K＋.
The fourth type of complex is a part of the coordinating atoms of the ligand that does not coordinate with metal ions. What is the complex Pd2＋of PdCl2 with 1,10-disulfide-18-crown-6 Coordinate with 2 sulfur atoms and 2C1¯atoms of the ligand, while the oxygen atom in the ligand does not participate in coordination.
The fifth type is sandwich structure, such as the structure of [K＋-(benzo-15-crown-5)2]in the complex of benzo-15-crown-5 and K＋.
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