Advanced Coordination Chemistry Flashcards
Describe the solubility of crown ethers.
> Crown ethers show unusual solubility: they are all soluble in all common solvents (both lipophilic and hydrophilic).
> This occurs due to the flexibility of the chain which allows the lone pairs (polar bits) to be either internalised or exposed to solvent.
How are corand complexes formed?
> Crown ethers can coordinated with high affinity to alkali ions forming corand complexes.
How do crown ether ligands affect the solubility of alkali metal ions?
> The ligand lipophilises alkali metal ions e.g. in the presence of [18] crown-6, inorganic salts (KCl, KOH and KMnO4) become soluble in organic solvents such as toluene.
Explain the selectivity of crown ethers.
> Selectivity was originally thought to be due to hole size of the macrocycle i.e. each crown specifically binds to a certain alkali metal cation. THIS IS WRONG
> There is some correlation between affinities and cavity diameters but the simplest explanations is that as the number of donor groups increase (as the size of the crown ether increases), there is an increase in enthalpy due to more coordination interactions.
> ALSO: the free energy of solvation (cost before the benefit of coordination) for K+ is lower than that of Na+, Li+ and Ca2+ i.e. it is easier to remove solvated water from K+ before crown coordination. (smaller charge density = weaker solvation) .
What are podands?
> Podands are acyclic polyether analogues of crowns.
Describe the binding affinity behaviour of podands.
> Although they have multiple donor groups, podands bind to metals ions with much lower affinities than the crowns.
> This is due to the MACROCYCLIC EFFECT (different to the chelate effect which is entropy driven).
What is the chelate effect?
> Polydentate chelate ligands form more stable complexes (have a higher affinity for metal ions) compared to their monodentate analogues.
> Macrocyclic complexes are stabilised by chelate effects.
What is preorganisation?
> In addition to the chelate effect, macrocycles undergo less conformational change on binding to a metal ion than a podand (the entropic cost of binding is already paid) - this phenomenon is preorganisation.
What 3 factors contribute to the MACROCYCLIC EFFECT?
(i) A macrocycle undergoes less conformation change on binding to a metal ion than a podand - PREORGANISATION.
(ii) Donor lone pairs are held together in the macrcocycle framework so the enthalpic cost of unfavourable electrostatic interactions had been paid.
(ii) Macrocycles are less solvated than analogous podands as they have less solvent accessible surfaces.
What are cryptands?
> Cryptands are 3D bicyclic crown analogues. They are more preorganised than crowns due to the 3D element.
> Complexes of cryptands are known as cryptates.
How can binding preferences of cryptands be adjusted?
> Cryptands show a “cavity diameter effect” and binding preference can be adjusted by changing 1 arm of the cryptands i.e. the rigid structure of the macrocycle cavity can be fine-tuned.
How can cryptand rigidity dictate selectivity?
> Rigidity means that cryptands cannot flex around ions that are too small for their cavity and cannot expand to accommodate ions larger than the cavity.
> Therefore, each cryptand selectively binds specific metal ions (can be used in metal detection).
How do cryptand binding affinities compare to that of crowns?
> Cryptand binding affinities for metal ions are HIGHER than those for corresponding crowns by several orders of magnitude.
> Although cryptands contain more donor groups than crowns, their increased affinities have a larger preorganisation component.
What are spherands?
> Spherands= most rigid macrocycles and more preorganised.
> They contain binding pockets where convergent donor groups are held more rigidly in the correct geometry for metal binding (claw type structure that surrounds the metal ion).
> There is NO conformation change on binding to metal ion, therefore, it binds 10^12 more effectively than the equivalent acyclic podand (NO entropic cost of binding).
What is the effect of changing donor atoms?
> Macrocycles based on hard O donors bind to hard metal ions e.g. alkali earth metals.
> Incorporation of softer donor atoms (e.g. S) has a radical effect on the metal ion binding selectivities (LogKb).
How do crowns and cryptands bind to non-metallic cations?
> Crowns and cryptands can complex cations such a [NH4}+ via hydrogen bonding.
> Crowns interact via O——H-N interactions to give what is known as 2perching2 complex, with the [NH4]+ sitting above the crown and making usually 3 H-bond contacts.
> Cryptands can show higher selectivity through preorganisation → the large ball shaped cryptand selectively bind to [NH4]+ over K+ (x500) form 4 tetrahedrally arranged N—H-N interactions.
What 2 methods are used to synthesise macrocyclic ligands?
(i) Template Synthesis
(ii) High Dilution Techniques
Note: The methods are not mutually exclusive (sometimes syntheses require both methods).
Outline the synthesis of crown ethers.
> Most common method: 2 presynthesised fragments are broight together in the final macrocyclisation step. (OTs is a good LG for this in THF using Base (KOH)).
Why are these reactions so high yielding?
> There are no oligomeric/polymeric material produced.
> If KOH is replaced with NBu4OH, a mixture of polymeric ether products result → NO CROWN. This is a example of the TEMPLATE EFFECT.
What is the TEMPLATE EFFECT?
> Template effect: A template organises an assembly of atoms in a specific geometry in order to achieve a particular linking of atoms.
> Template process: where a metal ion (or centre) with definite stereochemistry and electronic properties serves as mould for forming a product that would normally be difficult or impossible to obtain.
> In this specific case, the intramolecular macrocyclisation step has to compete against the unwanted intermolecular polymerisation step - the metal template facilitates the intramoleular reaction.
What is the mechanism of the template effect?
> The mechanism of the template effect can be either kinetic or thermodynamic (or a mixture of the 2).
> Kinetic effect: when the metal ion provides a site for the reactants to assemble themselves in the right arrangement to produce the target product structure.
> Thermodynamic effect: coordination to a metal ion shifts a reaction equilibrium towards a desired product (stabilises intermediate to create a product not usually accessible).
Outline the synthesis of cryptands.
> Cryptand synthesis usually has several steps where A≠B≠C. Therefore unsymmetrical cryptands are easily accessed (high control).
(i) similar to crown ether - template synthesis
(ii) Classical high dilution (tiny amounts of reagent in ~1L solvent - always very low concentration).
amine + acid chloride → (High dilution) → amide → (LiAlH4) → amine
> 2nd order intermolecular reaction is favoured by high concentration (low dilution) → gives polymeric products X
> 1st order intramolecular reaction is favoured by low concentration (high dilution) → gives desired cryptand :)
Why does high dilution work?
rcycl/rpol α 1/[A-B]
> inverse proportionality means that rcycl (rate of cyclisation) increases as [A-B] (reactant concentration) decreases.
What role does [18] C-6 play in oxidations using KMnO4?
> Addition of catalytic amounts of [18]C-6 (~3-5 crystals) allows oxidations using KMnO4 to be completed at room temperature in quantitative yields.
Reactions: Alkene → Carboxylic acids, Alcohol → Ketone (using KMnO4 in Toluene → “purple toluene”)
> [18]C-6 binds to KMnO4 and forces it to dissolve in the toluene where is performs the reaction.
> [18]C-6 then precipitates out and can be reused (reason why only a small amount is needed).
What role do crowns and cryptands play in activating inorganic bases?
> Crowns and cryptands are phase transfer catalysts → they are able to transport species from one phase into another e.g. from aqueous solvent into non-aqueous solvent ( can therefore facilitate reaction of hydrophilic inorganic salts with lipophillic organic compounds).
> e.g. In organic solutions, CO32- also behaves as a very strong base (usually a very mild base) i.e. it is activated in non-aqueous solvent. In water, CO32- is a weak base due to solvation sphere. It is taken across into non aqueous solvent where it is a much stronger base (bare carbonate).
Why does a solvation sphere inhibit the action of inorganic ions?
> When ions are solvated by water molecules, these water molecules need to be removed so that a reaction can occur (part of the reaction activation energy).
