Biological Anion Receptors

Transcription

1 Binding of Anions 1

2 Binding of Anions 2

3 Biological Anion Receptors 3

4 Concepts in Anion Host Design FACTORS WITCH AFFECT ANION COMPLEXATION Prevailing interactions which take place in anion binding: hydrogen bonding ion-dipole and ion-ion interactions van der Waals interactions 1. Size match between anion and host cavity 2. Complementarity (topological and shape selectivity) 3. Anion and host charge and anion polarisability 4. Solvent (polarity, hydrogen bonding and coordination ability), anion and host free energies of solvation 5. Anion basicity and host acidity 6. Other kinetic, enthalpic and entropic contributions to the anion-host interactions 4

5 Concepts in Anion Host Design 1. Size match between anion and host cavity Anions are relatively large and therefore require receptors of considerably greater size than cations Cation Diameter (Å) Anion Diameter (Å) Li Na Na K F Cs Cl Br - I Ca ClO Zn CO 2-3, NO Al SO PO NH La H 2 PO 4 - PdCl

6 Concepts in Anion Host Design hexacyclen Hexacyclen cavity is partially filled with NH protons it is too small to include anions Its 4H + form binds halide and nitrate anions via hydrogen bonds with perching geometry 6

7 Concepts in Anion Host Design From Cation Hosts to Anion Hosts a Simple Change in ph 7

8 Concepts in Anion Host Design Preorganized Bigger Always positive Best fit with iodide and bromide 8

9 Concepts in Anion Host Design 2. Complementarity (topological and shape selectivity) Even simple inorganic anions occur in a range of shapes and geometries Geometry spherical linear trigonal planar tetragonal planar tetrahedral octahedral Anions F -, Cl -, Br -, I - SCN -, N - 3 CO 2-3, NO - 3 PtCl 2-4 BF - 4, ClO - 4, SO 2-4, PO 3-4 PF - 6, Fe(CN) 3-6 9

10 Concepts in Anion Host Design 2. Complementarity (topological and shape selectivity) Complexes of 6H + forms of nitrogen macrobicycles with anions: F - : tetrahedral binding; log K = 4.19 Cl - : octahedral coordination; log K = 3.0 N - 3 : log K = 4.3 I - : log K = 2.15 Fluoride Chloride Azide 10

11 Concepts in Anion Host Design 2. Complementarity (topological and shape selectivity) a b Complexes of 6H + forms of nitrogen macrobicycles with fluoride anions: (a) tetrahedral binding (b) octahedral binding logk (in water): (a) 4.2 (b)

12 Concepts in Anion Host Design 12

13 Concepts in Anion Host Design

14 Concepts in Anion Host Design 14

15 Concepts in Anion Host Design Anion basicity and host acidity Many anions exist in a relatively narrow ph window (anion basicity/acidity). Some receptors (especially polyammonium salts) are present in desired oligoprotonated form at relatively low ph values. n = 4 Complex [H 4 LFe(CN) 6 ] [H 5 LFe(CN) 6 ] + [H 6 LFe(CN) 6 ] 2+ [H 7 LFe(CN) 6 ] 3+ [H 8 LFe(CN) 6 ] 4+ logk Distribution diagram for the system H + / [Fe(CN) 6 ] 4- / 27<N corand-6> and the Fe(II)/Fe(III) redox potential 15 versus ph

16 Recognition of Carbonic Acids Length based recognition of linear dicarboxylic acids by monocyclic hosts 16

17 Porphyrine Hosts methanol 17

18 Cyclophane Hosts Diphenylmethane used for: Curvature Rigidity Binding ability 18

19 Cyclophane Hosts 4.18 still able of induced fit (180 pm) 19

20 Guanidinium-Based Hosts 20

21 Guanidinium-Based Hosts 21

22 Guanidinium-Based Hosts Binding of amino acids O O H 2 N CH C OH + H 2 N CH C OH CH 2 CH 2 HN 22

23 Guanidinium-Based Hosts 23

24 Organometallic-Based Hosts Charge-assisted, observable by cyclic voltammetry (a) free receptor Br - (b) + Cl - 24

25 Organometallic-Based Hosts Cl - Selective for carboxylates (parallel orientation of N-H groups) K (AcO - ) = M -1 K (Cl - ) = 70 M -1 measured in DMSO 25

26 Neutral Hosts especially for aprotic solvents (lipophobic effect) 26

27 Neutral Hosts 27

28 Neutral Hosts In aprotic solvents 28

29 Neutral Hosts H O N H + F- + Calix[4]pyrrole Perching mode 50-fold selectivity for F - over Cl - K (F - ) = 1.7 x 10 4 M -1 in DCM 29

30 Neutral Hosts Hosts with Lewis acid atoms, such as organo boron, silicon, mercury and tin compounds 30

31 Lewis-Acid Chelation Methoxide chelation F B F F B (4.47) + F Me O Me F (Ph 3 C) + O + B - F B - F F Hydroxide chelation 31

32 Lewis-Acid Chelation Fluoride chelation Bridge Si-F: 190 and 207 pm Si-F: pm Chloride chelation 2 : 1 complex 32

33 Anticrowns Anticrowns: oxygen atoms of crown ethers are replaced with Lewis acid atoms 33

34 Anticrowns Corands: anticrowns and cryptand analogues Cl - bridging mode hopping of chloride between Sn atoms n = 6: F - n = 8: Cl - logk = 4 34