1 INTRODUCTION AND CONCEPTS

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1 1 1 INTRODUCTION AND CONCEPTS WHAT IS SUPRAMOLECULAR CHEMISTRY? DEVELOPMENT OF SUPRAMOLECULAR CHEMISTRY HOST-GUEST CHEMISTRY SOME BASIC CONCEPTS WEAK INTERACTIONS DESIGNING SUPRAMOLECULAR HOSTS WHAT IS SUPRAMOLECULAR CHEMISTRY? "Supramolecular chemistry is the chemistry of the intermolecular bond, covering the structures and functions of the entities formed by the association of two or more chemical species" J.-M. Lehn "In contrast to molecular chemistry, which is predominantly based upon the covalent bonding of atoms, supramolecular chemistry is based upon intermolecular interactions, i.e. on the association of two or more building blocks, which are held together by intermolecular bonds" F. Vögtle "Supramolecular chemistry is defined as chemistry "beyond the molecule", as chemistry of tailor-shaped intermolecular interaction. In 'supramolecules' information is stored in the form of structural peculiarities. Moreover, not only the combined action of molecules is called supramolecular, but also the combined action of characteristic parts of one and the same molecule." F. Vögtle supra (Latin) = above, beyond... One of the most popular and fastest growing areas of experimental chemistry Interdisciplinary field One field is host-guest chemistry Host selectively binds a particular guest in order to produce a host/guest complex, a supramolecule. SUPRAMOLECULAR COMPOUND: A molecular assembly where two or more compounds interact with each other via various weak intermolecular interactions such as H-bonding, cation π-, π π-, CH π-interactions, hydrophobic effects etc., showing inclusion, selectivity or other functionality.

2 2 SUPRAMOLECULAR COORDINATION COMPOUND: A Supramolecular compound where the selectivity or other functionality is achieved via metal ion coordination. SUPRAMOLECULAR ASSEMBLY: A molecular assembly where two or more compounds interact with each other via various intermolecular interactions such as metal coordination, H-bonding, cation π -, π π -, CH π -interactions etc., resulting in large entity "a supermolecule".

3 3 Some of the first supramolecules (at that time, these were not called supramolecules) a) Curtis 1961: First Schiff s base macrocycle from asetone and ethylene diamine b) Busch 1964: Schiff s base macrocycle c) Jäger 1964: Schiff s base macrocycle d) Pedersen 1967: Crown ethers References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.4-5, Wiley, (2000). HOST-GUEST CHEMISTRY A molecule (host) can bind another molecule (guest) to produce a hostguest complex Interactions between host and guest are noncovalent Guest may be o a monoatomic cation o o a simple inorganic anion a more sophisticated molecule such as a hormone, pheromone or neurotransmitter Host possesses convergent binding site

4 4 Guest possesses divergent binding site References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.3-4, Wiley, (2000). CLASSIFICATION OF SUPRAMOLECULAR HOST-GUEST COMPOUNDS Division into two major classes according to type of the host-guest aggregate: a) Cavitand = host with intramolecular cavities host-guest aggregate is cavitate b) Clathrand = host with extramolecular cavities, host-guest aggregate is clathrate After F. Vögtle, Angew. Chem., Int. Ed. Engl., 1985, 24, 728 Further these can be divided by the forces between the host and the guest: When the forces are electrostatic the host-guest system is a complex When the forces are nondirectional, less specific such as hydrophobic, van der Waals or crystal close-packing effects, then the names used are cavitate and clathrate Division relating to the stability of a host-guest complex in solution: o Clathrate-hosts (such as gas hydrates and urea clathrates) are stable only in the solid state o Cation- and anion-binding hosts (such as crown ethers, cryptands and spherands) and hosts for neutral molecules are stable both in solid state and in solution, sometimes also in the gas-phase o There also exists purely liquid-phase phenomena: liquid crystals and liquid clathrates References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p. 6-7, Wiley, (2000).

5 5 SOME BASIC CONCEPTS RECEPTORS, COORDINATING AND THE LOCK-AND-KEY PRINCIPLE Selective binding must involve attraction or mutual affinity between host and guest -> theory of coordination chemistry by Alfred Werner in 1893 Enzyme (host) has a binding site to witch its substrate (guest) fits (like a glove in the hand) The guest has a geometric size or shape complementary to the receptor (host) -> lock and key principle by Emil Fischer in 1894 Paul Ehrlich in 1906: molecules do not act if they do not bind -> the concept of a biological receptor CHELATE- AND MACROCYCLIC EFFECTS The interaction of the whole system is synergically greater than the sum of the parts Such extra stabilization has its basis in the chelate and macrocyclic effects Metal complexes of bidental ligands are significantly more stable than closely related materials that contain unidentate ligands Stabilization via chelate effect in solution is due to both thermodynamic and kinetic effects The stabilization offered by the chelate effect is highly dependent on the size of the chelate ring The chelate effect can be used to stabilize the system a) Chelate effect (Blue pentagon represents a donor) When the host-guest system is even more stabile than would be expected from the chelate effect alone, there usually are also macrocyclic effects present The host of these species is usually macrocyclic The macrocyclic effect relates not only to the chelation of the guest by multiple binding sites but also to the organization of those sites in space Macrocyclic effect was first introduced by Cabbines and Margerum in 1970 Macrocyclic effect has both enthalpic and entropic contributions

6 6 b) Chelate and macrocyclic effects Bicyclic hosts such as cryptands are found to be even more stable than monocyclic corands because of the macrobicyclic effect c) Chelate and macrobicyclic effects (a), b) and c) after J.W. Steed and J.L. Atwood in Supramolecular Chemistry, (2000)) References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.9-13, Wiley, (2000). PREORGANIZATION AND COMPLEMENTARITY In order to bind, a host must have binding sites that are of the correct electronic character to complement those of the guest Those binding sites must also sterically match to the conformation of the binding site of the guest The host that fulfils these criteria is said to be complementary If the host molecule does not undergo any significant changes in conformation when binding the guest it is said to be preorganised

7 7 A good example of a preorganized host is spherand which is well preorganized in order to bind alkali metal cations References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.13-14, Wiley, (2000). WEAK INTERACTIONS (SUPRAMOLECULAR INTERACTIONS) Supramolecular chemistry is the chemistry of weak interactions, i.e. noncovalent intermolecular forces Categories: o (Ion-ion interactions) (Strongest) o Ion-dipole interactions o Dipole-dipole interactions o Hydrogen bonding o Interactions involving p-systems o Van der Waals forces o Close packing o Hydrophobic effects (Weakest) Classification is not unambiguous ION-ION INTERACTIONS KJ/MOL Comparable in strength to covalent bonding -> is it really a weak interaction? Examples

8 8 ION-DIPOLE INTERACTIONS KJ/MOL Ion interacts with a polar part of a molecule Typical examples alkali metal-crown ether complexes Examples of ion-dipole interactions in supramolecules. N.B.! Notations for complexes. Coordinative (dative) bonds between nonpolarisable metal cations and hard bases: Lewis base (ligand) and Lewis acid (metal) are attached by means of lone-pair electrons Coordinative bond has a significant covalent component Strong, which causes the structures to be stabile, but yet kinetically labile allowing reorganisation Well-defined geometries (coordination number of metals) -> useful in crystal engineering and synthesis o coordination numbers vary from 2 to 8 (usually 2 or 6) Formation of coordinative bonds requires some level of complementarity (conformational and stereoelectronic, size) Examples [Ru(BiPy) 3 ] 2+ and grid-type of supramolecular assemblies DIPOLE-DIPOLE INTERACTIONS 5-50 KJ/MOL Typical with organic carbonyl compounds

9 9 HYDROGEN BONDING KJ/MOL VERY IMPORTANT! May be considered as particular kind of dipole-dipole interaction Directional, electrostatic, certain geometry Donor is partially positive hydrogen: partial charge arises from the large difference in electronegativity of hydrogen and the atom to which it is attached (typically O, N, S, F, (C)) -> highly polarised covalent bond Acceptor is (partially) negative atom with unshared valence electrons or polarisable π-electrons Formation of hydrogen bonds causes a singnificant decrease in energy -> in solid state as many H-bond as possible are formed (keep in mind geometric restrictions and the closest packing) The solvent is the defining factor when forming hydrogen bonds Very polar solvent causes hydrogen bonds to collaps Hydrogen bonding is strongest in nonpolar/low polarity solvents Hydrogen bonding often orders the overall shape of a biological molecule In DNA the hydrogen bond system is sheltered by the double helix-structure (In DNA also π π -interactions have effect in the maintaining of the structure) Hydrogen bonding has also an important role in tautomerism

10 10 Subcategories: strong, moderate and weak hydrogen bonding Strong H-bonds are formed when there is electron density deficiency in the donor or an excess of electron density in the acceptor or when configuration or conformation of the molecule forces the donor and acceptor closer than normal H-bonding distance Moderate bonds (most common type) form typically between neutral group Weak H-bonds are formed when hydrogen is bonded to only slightly more electronegative atom (C, Si) or when acceptor has π-electrons instead of lonepairs or it is otherwise poor acceptor (Br, Se). Weak H-bonds have more geometrical freedom Donor properties of C-H depend on the hybridisation of C, the electronegativity of the atom C is attached to, steric environment of hydrogen and on the basicity of the acceptor Cooperativity: although a single interaction (e.g. H-bonding) is quite weak, several simultaneously acting interactions increase the stability significantly σ-cooperativity: hydrogen bonding to continuous chains or cycles π-cooperativity (resonance-assisted H-bonding): hydrogen bonding between molecules with conjugated multiple π-bonds Hydrogen-bonding of non-linear molecules produces non-linear H-bonded polymers and may also result in H-bonded macrocycle Hydrogen bonding of planar molecules with more than two H-bonding possibilities produces H-bonded sheets Hydrogen bonding of molecules with three-dimensionally organized H-bonding sites can result in 3D H-bonded network/lattice

11 11 WEAK INTERACTIONS INVOLVING π-systems Three subcategories: D-H π, π π and cation π Note: D-H π could also be classified as weak H-bonding and the properties of weak H-bonding apply for this interaction (e.g. directionality) π π interaction (π-stacking; 0-50 kj/mol) is non-directional, electrostatic, attractive force, which occurs when attraction between π-electrons and π- framework overcome the unfavorable π-π repulsions Most typical geometries edge-to-face (herringbone pattern) and offset faceto-face -> direct face-to-face is repulsive Polarisation of π-systems by heteroatoms may lead to direct face-to-face geometry Significant both in nature (e.g. DNA) and in artificial systems, but difficult to predict and control (weakness, weak directionality) Cation π interactions (5-80 kj/mol) occur between metallic or organic cations and aromatic or double/triple bonded regions of the molecule Electrostatic force, but relates also to the polarisability of the aromatics (ioninduced dipole, donor-acceptor, charge transfer or dispersion force)

12 12 An example of cation π interactions VAN DER WAALS FORCES (< 5 KJ/MOL) Collective name for non-directional dispersion (London) forces weakly bonding at long distance and exchange-repulsion forces strongly non-bonding at short distances Attractive dispersion forces are caused by fluctuating multipoles (polarisation of electron cloud) Proportional to the size of the molecule and inversely proportional to the sixth power of distance Repulsive forces balance the dispersion forces and define molecular shape and geometry -> important in crystal packing Usually term VDW-forces refers to C C, C H and H H interactions CLOSE PACKING Solid state phenomena -> Nature abhors vacuum Maximisation of favourable isotropic VDW interactions o move you mouse cursor on the picture in order to see the close packed structure

13 13 HYDROPHOBIC EFFECTS Relate to the exclusion of large or weakly solvated (hydrophobic) species from polar media Can be devided to enthalpic and entropic components Enthalpic effect: stabilisation of polar solvent excluded from the cavity upon guest binding Entropic effect: combination of host and guest results in less disruption to the solvent structure -> entropic gain Typical examples: Binding of organic guests by cyclodextrins or cyclophanes in water After J.W. Steed and J.L. Atwood in Supramolecular Chemistry, (2000). References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.19-30, Wiley, (2000).

14 14 DESIGNING SUPRAMOLECULAR HOSTS Selectivity towards a particular guest can be achieved by using chelate and macrocyclic effects, complementarity and host preorganisation Definition and careful consideration of the target -> properties of the host system Organisation Selection of the type and the number of binding sites The nature of the organic framework of the host itself SOME LINKS: Www-pages of the Supramolecular Chemistry - textbook Structures of different kinds of supramolecular compounds A list of links to supramolecular resources Following are some questions on the topics you just read about. Please, take some time to think back what you ve just learned about the concepts of supramolecular chemistry and try to answer these questions. Notice that there may be more than one correct answer for the questions. How would you define the concept "supramolecular chemisty"? In your opinion, why is supramolecular chemistry so widely spread and interdisciplinary field of science? Why are supramolecular or weak interactions so important? Supramolecular interactions are present in countless amount of "events" of biological life. Can you find out some of them?