Suit[4]ane – A Porphyrin Suits Up

Mechanically interlocked molecules (MIMs), such as rotaxanes, for example, are held together by mechanical bonds instead of chemical bonds. Their structures determine how the molecules can move in relation to one another, which can be used in the design of molecular machines. Suit[n]anes are MIMs with two components: one “torso” component with n protruding “limbs”, and a second component that encompasses the first one like a suit. This type of MIM is still challenging to synthesize and not well explored.

J. Fraser Stoddart, Northwestern University, Evanston, IL, USA, Tianjin University, China, and University of New South Wales, Sydney, Australia, and colleagues have synthesized a suit[4]ane (pictured), which is composed of a porphyrin with two phenyl groups and two bromine substituents (pictured in red) inside a tricyclic octacationic cyclophane cage (pictured in black). The suitane was synthesized via a “slippage” approach: First, the components are heated together to partially open the outer cage component and allow the formation of the product. Then the system is cooled to close the cage again and lock in the inner component. The team heated the porphyrin derivative and the cyclophane cage (in the form of its CF₃COO^– salt) to 100 °C in dimethylformamide (DMF) for 14 days. They obtained the desired product in a yield of 20 %.

The team characterized the suit[4]ane using ¹H NMR spectroscopy, UV–vis spectrocopy, and single-crystal X-ray crystallography. They confirmed the mechanically interlocked nature of the compound and found that the four “limbs” of the porphyrin unit protrude through the four entrances of the outer cage. NMR results showed that the porphyrin unit performs a rocking motion inside the cyclophane suit, at a rate of about 1,000 s^–1 at room temperature. According to the researchers, the work shows that the slippage approach is not limited to rotaxane synthesis, but can also be used to make other types of mechanically interlocked molecules.