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1 3 CATION BINDING HOSTS, PART II SELECTIVITY OF CATION COMPLEXATION TEMPLATE EFFECT HIGH-DILUTION SYNTHESIS PREORGANISATION AND COMPLEMENTARITY SOFT LIGAND VS. HARD LIGAND COMPLEXATION OF ORGANIC CATIONS THE CALIXARENES CARBON DONOR AND Π -ACID LIGANDS THE SIDEROPHORES SELECTIVITY OF CATION COMPLEXATION EFFECTING FACTORS: Size match between cation and host cavity Electrostatic charge Solvent (polarity, hydrogen bonding and coordinating ability) Degree of host preorganisation Enthalpic and entropic contributions to the cation-host interaction Cation and host free energies of solvation Nature of the counter-anion and its interactions with solvent and the cation Cation binding kinetics Chelate ring size STUDYING SELECTIVITY Selectivity of the cation binding can be studied simply by eluting ions through an ion exchange column, in which a polymer containing the studied host (e.g. 18C6 as in the picture) acts as stabile-phase The eluant is fractioned when it comes out of the column The order of eluation is inversely proportional to the binding constant (ion that complex best comes last) In the case of crown ethers, the selectivity towards potassium can be detected 2 SELECTIVITY OF CROWN ETHERS, LARIAT ETHERS AND CRYPTANDS FOR CATIONS Crown ethers are highly flexible -> quite similar affinity for several metal ions All crown ethers are selective for all cations in some extent [18]crown-6 and its derivatives are selective for K+ For crown ethers, the most important factors in selectivity of binding are the size match (host cavity and guest), number of donor atoms and the solvation of cation and ligand as well as the chelate ring size Also the charge of the cation has its effect: o monocation has much less affinity compared to same-sized dication (e.g. K+ vs. Ba2+) -> this feature can be used in separation of metals Large crown ethers can change their shape when binding the guest Lariat ethers can use their side-arms in binding guests -> threedimensionality Cryptands are also three-dimensional Because of their more rigid and restricted cavities, cryptands express selectivity in cation binding In Cryptands the size of the cavity as well as preorganization controls the metal complexation In cryptands, the donor atoms are arranged threedimensionally in a suitable fashion in order to bind a guest which effect their binding properties TEMPLATE EFFECT Metal-cation can be used to template the syntheses of macrocyclic compounds (e.g. crown ethers) In the syntheses of 18C6, KOH can be used both as a base and as a template -> K+-cation organizes the componets in preorganised cycle array -> coordination of oxygens to K+-cation brings the reactive Cl- and OHgroups near each other -> cyclisation occurs rather than polymerisation When triethylamine (also base) is used instead of KOH, polymerisation takes place (this is because there is no cation, which could preorganise the components for cyclisation) K2CO3 is the most widely used base in the syntheses of 18- membered crown ethers because of their selectivity towards K+ Template effect assisted cyclisation can be used e.g. in the syntheses of benzo[18]crown-6. 3 Template effect discussed here is kinetic template effect, but there also exists another kind of template effect, thermodynamic template effect In kinetic template effect the preorganised metal complex actually forms In thermodynamic template effect the effect is based on the ability of the metal cation to select the complementary ligand from the equilibrium mixture of products -> this way te equilibrium of the reaction can be driven towards the desired product HIGH-DILUTION SYNTHESIS High-dilution method is used for the preparation of macrocycles Small quantities of reactants in large volume of solvent Specific apparatus Dropwise addition of reactants at a very slow rate Formation of cyclic product is more likely and faster than formation of polymer in dilute solutions The rate of reaction can yet be improved by using more reactive groups (e.g. acid chloride instead of acid etc.) when possible A Schematic picture of high dilution apparatus designed by Professor Vögtle. References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.123-133, Wiley, (2000). 4 PREORGANISATION AND COMPLEMENTARITY THERMODYNAMIC EFFECTS The closer the host is to the conformation in which the complexation occurs, the easier the complexation can take place Spherands are very rigid, their structure in complex is very similar to the structure of free ligand (only minor changes can take place during the complexation) Spherand structure is thus very highly preorganised This kind of preorganised structure can be very selective Molecules naturally tend to optimize their structures so that inside the molecule remains no empty space This is achieved by different conformation depending on the situation KINETIC AND DYNAMIC EFFECTS Host preorganisation and degree of flexibility affects also the rates of complexation and decomplexation General phenomenon is that the more rigidly preorganized a host is for cation binding, the slower the kinetics of the process is (cation has to desolvate) More flexible hosts can change easily from solvation to host complexation (without the need to pass through unstable intermediates) Macrocyclic molecules naturally tend to adopt their conformations so that no empty space remains inside the molecule When the structure is macrocyclic and very rigid so that it can not change its conformation to eliminate empty space -> Other molecule goes inside the ring of the first one to fill the empty space References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.133-141, Wiley, (2000). SOFT LIGAND VS. HARD LIGANDS WHAT IS A SOFT OR A HARD LIGAND? Metal ions have been devided into two classes based on the stability of their halogen complexes (Class “a” and “b”) Further this is generalized as a principle of Hard and Soft Acids and Bases (HSAB) Base=Lewis base (electron donor, usually ligand) Acid=Lewis acid (electron acceptor, usually metal ion) Hard acids prefer coordination to hard bases, and soft acids prefer coordination to soft bases 5 HARD ACIDS: High positive charge Low polarisability Small size (H+), Al3+) HARD BASES: High electronegativity Difficult to oxidise Low polarisability (F–) SOFT ACIDS: Low positive charge High polarisability Larger size (Ag+) SOFT BASES: Low electronegativity Easily oxidized High polarisability High negative charge (H–) EXAMPLE OF THE PROPERTIES OF METAL CATIONS: ALKALI METAL CATIONS Hard, nonpolarisable spheres that maintain certain shape Do not have fixed coordination geometries (although usually they are octahedral) Interact very strongly with water Interact strongly with similar negative charges Ionic radius of alkali metal cations Based on the ligand, one can draw a conclusion about cations with which it can interact HETEROCROWNS Pure crown ethers are the hardest ligands If one or more of the oxygens are replaced with another donor-atom the ligand is a heterocrown 6 The replacing donor can be softer donor than oxygen (such as N or S), and the replacement changes properties of the ligand o E.g. Ag+ complex of 18C6 requires nitrate to bond with Ag+, but when oxygens are replaced with sulphur, Ag+ coordinates in the middle of the atom with every S-atom HETEROCRYPTANDS The name is a little misleading, because in cryptands there always is at least four kinds of atoms in the structure Hetero in this term means that some other than a bridge atom is replaced e.g. by nitrogen MIXED CRYPTATES Two different binding sites of which one can complex hard and another one soft cations In the picture the cryptand has two different binding sites: one on the crown for alkali metal cation and one for CO between redox-active transition metal and alkalimetal cation A mixed cryptate After J.W. Steed and J.L. Atwood in Supramolecular Chemistry, Wiley, (2000), Chichester SCHIFF’S BASES Very important The key Schiff´s base condensation reaction involves simply the reaction of an amine with aldehyde to eliminate (condence) water and give an imine The product may be reduced to give amine or secondary amine-based macrocycle 7 An original Schiff´s base macrocycle First artificial metal macrocycles were synthesized using thermodynamic template effect Templating metal ions: by changing the template one can modify the desired molecule Afterwards the metal can be removed to give pure Schiff´s base Extremely versatile and high yielding Wide range of metallomacrocyclic and macrobicyclic as well as podandanalogues have been prepared using Shiff´s base approach References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.141-152, Wiley, (2000). COMPLEXATION OF ORGANIC CATIONS Organic cations are more complicated They can be chiral (chiral recognition), which makes them interesting 95% are ammonium cations (positively charged salts of nitrogen) BINDING OF AMMONIUM CATIONS BY CORANDS Ammonium cation complex via H-bonding Ammonium cations are usually aliphatic, but they can also be aromatic (e.g. ) pyridinium-cation Positively charged ammonium ions can help H-bonding Steric effects are important BINDING OF AMMONIUM CATIONS BY CORANDS - CONTINUED An example of binding organic cations:complexation of N-heteroatomic cations by crown ethers 8 Complexation is mainly by H-bonding, but π···π/cation···π and CH···π interactions are also involved DB18C6 is a rather rigid shallow bowl-like host with two perpendicular interactions sites Large crowns (DB24C8 ->) can act as molecular tweezers towards planar cations Read more DITOPIC RECEPTORS Receptor with two binding sites Polytopic receptors = hosts possessing three or more binding sites Affinity towards bifunctional guest is greater Better chances for selective molecular recognition Possibly macro-chelate effect (multipoint binding at more than one site is greater than the sum of individual interactions) An example of a ditopic receptor After Steed and Atwood, Supramolecular Chemistry, Wiley, (2000), Chichester CHIRAL RECOGNITION In molecular recognition the chemical features do not affect -> attractive forces between the molecules Molecular recognition can be selective, e.g. CMR=chiral molecular recognition Very important in biochemical systems (enzymes and substrates) 9 Synthetic systems can be used as: o models and mimics for those in nature o abiotic chiral catalysis o in synthesis or separation of chiral pharmaceuticals In 1978, the first chiral corand by Peacock et al. twisted conformation, a chiral barrier The basis of chiral recognition has its bases on unfavourable steric interactions between the bulky substituents and the protruding methyl substituents of the host Two-phase liquid-liquid extraction experiment -> selective extraction of Denantiomers of amino acid and ester guest (Cram et al.) First chiral chromatographic column Nowadays extensively used, although expensive, technique for assessing excesses in organic reactions and separating small quantities of enantiomers AMPHIPHILIC RECEPTORS Combines two or more forms of guest recognition -> synergic enhancement of the binding References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.152-167, Wiley, (2000). THE CALIXARENES Today calixarenes is a well studied “receptor family”, because they can quite easily be made to serve as receptors for “all of the guests” (metal-, and ammonium cations, inorganic and organic anions as well as neutral molecules) Named after their shape which resembles the shape of a Greek vase called a calix crater t-butyl-calix[4]arene 10 COMPLEXATION OF CATIONS BY CALIXARENES The complexation properties of calixarenes has been widely studied during the last two decades The complexation properties of calixarenes has been widely studied during the last two decades Calixarenes have too different kind of binding sites formed by upper rim substituents and by lower rim substituents Lower rim binding site is suitable for cation-binding It resembles the binding site of spherands: phenoxylic oxygen atoms (as part of the hydroxyl group or as part of alkyl ether derivatives) are suitable in binding eg. Na+-cation Tetramethyl ether of calix[4]arene forms a complex in which a Na+cation has bound to lower rim ether-substituents and a toluene-molecule sits in the hydrophobic pocket -> acts as a simultaneous receptor for both cationic and neutral guest After Steed and Atwood, Supramolecular Chemistry, (2000) , Wiley, Chichester References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.169-182, Wiley, (2000). CARBON DONOR AND π -ACID LIGANDS soft metal ions are not restricted to complexation by heteroatom donors synergic backbonding noncovalent cation···π interactions MIXED C-HETEROATOM HOSTS an example of a mixed c-heteroatom host R = substituent (e.g. 2-NO2,4-CN etc.) 11 In Ag+ complexes of these kind of lariat ethers there occurs cation···π interactions between Ag+ and aromatic ring An electron-withdrawing substituent (such as -NO2) in the aryl-ring decreases the stability of the Ag+-complex HYDROCARBON HOSTS Ag+-complex of [2.2.2]-paracyclophane (move your mouse curson on the picture to see the spacefill-view of the molecule) a π-ligand (π-prismand) metal complexation via doublebond References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.183-187, Wiley, (2000). THE SIDEROPHORES Natural system which complex Fe3+-ion (cf. ionophore= metal cation binder) Essential microorganism grown promoters Essentially three-armed podands, binding through deprotonated hydroxyl groups Ligands have overall charge of 6– It is known that o-dihydroxybenzene (catechol) is a common feature to effective iron chelators There are number of synthetic podand and cryptand analogues An example of a synthetic siderophore, MECAM References: J.W. Steed, J.L. Atwood, Supramolecular chemistry, p.187-191, Wiley, (2000). 12 Following are some questions on the topics you just read about. Please, take some time to think back what you´ve just learned about cation binding hosts and try to answer these questions. Notice that there may be more than one correct answer for the questions. What aspects should be concidered when designing hosts that would selectively bind some particular cation? What aspects are the most important ones of these? What kind of "extra aids" (besides normal reagents and conditions...) you could use when prepairing macrocyclic hosts for cations and on what bases? What makes chiral recognition so important in the nature and how it could be utilized by synthetic systems?
