Anion receptors

Question 1

Roles of anions

Answer 1

Medicinal e.g. deregulation of Cl- channels in CF, maintenance of sulphate anion conc during dialysis
Catalysis
Pollution e.g. eutrophication of rivers from the run-off of phosphate-containing fertilisers
Carcinogenesis e.g. metabolites of nuclear fuel reprocessing TcO4^2- (also an environmental concern)

Question 2

Practical applications of anion binding

Answer 2

Crystal engineering/design (templation)
Separations/extractions e.g. HPLC separation of oligonucleotides on anion-receptor-modified silica
Binding/sensing e.g. for F- (from sarin?)

Question 3

Problems with anions

Answer 3

Negatively charged (electrons have been added) so are larger than cations within the same periodic row
Most anions also consist of multiple atoms (not generally seen with cations) which magnifies this effect - also means anions come in many different shapes
Anions can form acids (e.g. oxo acids) - but the forms of the anion is pH dependent i.e. low pH = protonated = no longer an anion

Question 4

Why is design of receptors for binding anions particularly challenging?

Answer 4

Due to their size/geometry/pH dependence
Generally a small optimum pH window for anion binding
Too low = protonation, neutral
Too high = increase deprotonation of anion, becomes strong base

Question 5

Hofmeister series

Answer 5

Classification of ions by the degree of their aqueous solvation
i.e. from hydrophobic organic anions to hydrophilic anions e.g. F-, SO4^2-, HPO4^2-

Question 6

Designing an effective anion receptor

Answer 6

Use electron pair donor-acceptor interactions i.e. where donor = anion
Acceptor must be of correct location/orientation
e.g. H-bond (classic electron pair donor/acceptor interaction)

Question 7

Examples of classic recognition elements for H bonding

Answer 7

Amides
Thioamides
Sulfonamides
Ureas
Thioureas
Guanidiniums
Pyrroles

Question 8

Cyclic thioureas

Answer 8

Selective binding of nitrate and bromide

Question 9

How can the binding of bicyclic and polycyclic amines to anions be enhanced?

Answer 9

Through protonation of N (so becomes positively charged)

Question 10

Calixpyrroles

Answer 10

Bind strongly to F-
Free = 1,3-alternate conformation
Adopts ‘cone’ conformation when F- binds to maximise no. of H-bond interactions
(can also bind Cl-)