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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
6
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
8
Concepts in Anion Host Design
• Preorganized• Bigger• Always positive• Best fit with iodide and bromide
9
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
10
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
11
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
15
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
16
Recognition of Carbonic Acids
Length based recognition of linear dicarboxyli c
acids by monocyclic hosts
24
Organometallic-Based Hosts
(a) free receptor(b) + Cl -
Br -
Charge-assisted, observable by cyclic voltammetry
25
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-
29
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
30
Neutral Hosts
Hosts with Lewis acid atoms,
such as organo boron, silicon, mercury and t in compounds
31
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