1

Binding of Anions

2

Binding of Anions

3

Biological Anion Receptors

4

Concepts in Anion Host Design

FACTORS WITCH AFFECT ANION COMPLEXATION

1. Size match between anion and host cavity

2. Complementarity (topological and shape select ivity)

3. Anion and host charge and anion polarisabil ity

4. Solvent (polarity, hydrogen bonding and coord ination ability), anion

and host free energies of solvation

5. Anion basicity and host acidity

6. Other kinetic, enthalpic and entropic contri butions to the anion-host

interactions

Prevailing interactions which take place in a nion binding:

• hydrogen bonding

• ion-dipole and ion-ion interactions

• van der Waals interactions

5

Concepts in Anion Host Design

1. Size match between anion and host cavity

3.58CO32-, NO3

-1.50Zn2+

4.60SO42-1.06Al 3+

4.76PO43-2.1La3+

4.00H2PO4-2.96NH4

+

4.4Na-2.04Na+

2.66F-2.76K+

3.62Cl-3.40Cs+

3.90Br -

4.32I-

PdCl 62-

ClO4-

Anion

2.00

1.38

Diameter (Å)

6.38

5.00Ca2+

Li +

Diameter (Å)Cation

Anions are relatively large and therefore req uire receptors of considerably

greater size than cations

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Concepts in Anion Host Design

Hexacyclen cavity is partially filled with NH protons – it is

too small to include anions

Its 4H+ form binds halide and nitrate anions via hyd rogen

bonds with perching geometry

hexacyclen

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Concepts in Anion Host Design

From Cation Hosts to Anion Hosts – a Simple Change i n pH

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Concepts in Anion Host Design

• Preorganized• Bigger• Always positive• Best fit with iodide and bromide

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Concepts in Anion Host Design

2. Complementarity (topological and shape sel ectivity)

Even simple inorganic anions occur in a rang e of shapes and geometries

SCN-, N3-linear

CO32-, NO3

-trigonal planar

PtCl42-tetragonal planar

BF4-, ClO4

-, SO42-, PO4

3-tetrahedral

PF6-, Fe(CN)6

3-octahedral

spherical

Geometry

F-, Cl-, Br -, I-

Anions

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Concepts in Anion Host Design

Complexes of 6H + forms of nitrogen

macrobicycles with anions:

F- : tetrahedral binding; log K = 4.19

Cl- : octahedral coordination; log K = 3.0

N3- : log K = 4.3

I- : log K = 2.15

2. Complementarity (topological and shape sel ectivity)

Fluoride Chloride Azide

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Concepts in Anion Host Design

Complexes of 6H + forms of nitrogen

macrobicycles with fluoride anions:

(a) tetrahedral binding

(b) octahedral binding

log K (in water):

(a) 4.2

(b) 11.2

2. Complementarity (topological and shape sele ctivity)

a b

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Concepts in Anion Host Design

13

Concepts in Anion Host Design

4.12

14

Concepts in Anion Host Design

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Concepts in Anion Host Design

Anion basicity and host acidityMany anions exist in a relatively narrow pH window (anion basicity/acidity).

Some receptors (especially polyammonium salts) a re present in desired

oligoprotonated form at relatively low pH value s.

Distribution diagram for the system

H+ / [Fe(CN)6]4- / 27<N929corand-6>

and the Fe(II)/Fe(III) redox potential

versus pH

n = 4

6.2[H6LFe(CN)6]2+

4.8[H5LFe(CN)6]+

9.0[H8LFe(CN)6]4+

7.9[H7LFe(CN)6]3+

3.7[H4LFe(CN)6]

log KComplex

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Recognition of Carbonic Acids

Length based recognition of linear dicarboxyli c

acids by monocyclic hosts

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Porphyrine Hosts

methanol

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Cyclophane Hosts

Diphenylmethane used for:• Curvature• Rigidity• Binding ability

19

Cyclophane Hosts

4.18 still able of induced fit (180 pm)

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Guanidinium-Based Hosts

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Guanidinium-Based Hosts

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Guanidinium-Based Hosts

Binding of amino acids

H2N CH C

CH2

OH

O

H2N CH C

CH2

OH

O

HN

+

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Guanidinium-Based Hosts

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Organometallic-Based Hosts

(a) free receptor(b) + Cl -

Br -

Charge-assisted, observable by cyclic voltammetry

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Organometallic-Based Hosts

Selective for carboxylates

(parallel orientation of N-H groups)

K (AcO -) = 41500 M-1 K (Cl-) = 70 M-1

measured in DMSO

Cl-

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Neutral Hosts

especially for aprotic solvents (lipophobic effect)

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Neutral Hosts

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Neutral Hosts

In aprotic solvents

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Neutral Hosts

Perching mode

50-fold selectivity for F - over Cl -

K (F-) = 1.7 x 104 M-1 in DCM

HNO

+H+ F-

Calix[4]pyrrole

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Neutral Hosts

Hosts with Lewis acid atoms,

such as organo boron, silicon, mercury and t in compounds

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Lewis-Acid Chelation

Methoxide chelation

Hydroxide chelation

B

F

F

B

F

F

(4.47)

+OMe

(Ph3C)+ B-O+

B-

FF

FMe

F

32

Lewis-Acid Chelation

Fluoride chelation

Chloride chelation 2 : 1 complex

Bridge Si-F:190 and 207 pm

Si-F:160-165 pm

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Anticrowns

Anticrowns:

oxygen atoms of crown ethers are replaced with Lewis acid atoms

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Anticrowns

Corands: anticrowns and cryptand analogues

bridging mode hopping of chloride between Sn atoms

Cl-

n = 6: F - n = 8: Cl -

log K = 4