A cyanostar (pentacyanopentabenzo 5nnulene) is a shape-persistent macrocycle that binds anions. Cyanostar STM

Synthesis

The cyanostar structure is synthesized in a one-pot process among five

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s of a

benzaldehyde Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. It is a colorless liquid with a characteristic almond-like odor. ...

bearing a ''

meta Meta (from the Greek μετά, '' meta'', meaning "after" or "beyond") is a prefix meaning "more comprehensive" or "transcending". In modern nomenclature, ''meta''- can also serve as a prefix meaning self-referential, as a field of study or ende ...

''- cyanomethyl substituent. A series of

Knoevenagel condensation In organic chemistry, the Knoevenagel condensation () reaction is a type of chemical reaction named after German chemist Emil Knoevenagel. It is a modification of the aldol condensation. A Knoevenagel condensation is a nucleophilic addition o ...

reactions catalyzed by various bases stitches them together to make the C5-symmetric structure.

Anion binding

Cyanostar binds anions through

hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...

from the C–H hydrogen bonds, as the hydrogen has a slight positive charge. It is the first binder to make use of cyanostilbene's electropositive CH groups. The hydrogen bonds create an electropositive region in the center of the macrocycle, creating a

binding pocket In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...

. Cyanostar strongly binds anions that usually can only be bound weakly. The increased binding arises from the formation of a 2:1 complex, with two cyanostars sandwiching the anion on each side. An extended version of this structural pattern is a 4:3 alternating stack of cyanostar molecules complexing a hydrogen-bonded chain of

dihydrogen phosphate Dihydrogen phosphate or dihydrogenphosphate ion is an inorganic ion with the formula 2PO4sup>−. Phosphates occur widely in natural systems. These sodium phosphates are artificially used in food processing and packaging as emulsifying agents, ...

units.

Rotaxanes

Two cyanostars can be threaded onto a phosphate diester structure, forming a

rotaxane In chemistry, a rotaxane () is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle (see graphical representation). The two components of a rotaxane are kinetically t ...

. Because they have a high affinity for the central phosphate group only when it is in its anionic form, there is a substantial and reversible structural change in response to acid–base changes in solution.

References

{{reflist Macrocycles