Rotationally Active Ligands: Dialing-Up the Co-conformations of a [2]Rotaxane for Metal Ion Binding

A novel [2]rotaxane was constructed that has a bidentate N,N'-chelate as part of a rigid, H-shaped axle and a 24-membered crown ether macrocycle containing six ether O-atoms and an olefinic group as the wheel. This unique topology produces a ligand with the ability to dial-up different donor sets for complexation to metal ions by simply rotating the wheel about the axle. The solution and solid-state structures of the free ligand and complexes with Li+ and Cu+ show how the ligand adopts different rotational co-conformations for each. The Li+ ion uses the N,N'-chelate and O-donors while the Cu+ center is coordinated to both O-donors and the olefinic group. This concept of rotationally active ligands should be possible with a wide variety of donor sets and could find broad application in areas of coordination chemistry, such as catalysis and metal sequestration.