Macrocyclic And Supramolecular Chem Flashcards
(80 cards)
1
Q
Define the term macrocycle
A
- a cyclic molecule with three or more potential donor atoms in a ring of at least 9 atoms
- capable of binding a metal ion within the central cavity
2
Q
What are the two routes employed in macrocycle synthesis?
A
- high dilution methods - reactants kept apart this minimises polymer formation but results in slow reactant rates
- template synthesis - metal ion pre-organises the precursor to a macrocyclic configuration
3
Q
What are there types of macrocyclic ligands
A
- Crown ether - O- donor
- Cryptand (macrobicycle) - O-donor
- Cyclam (polyamine)- N-donor
4
Q
What are crown ethers
A
Macrocycles that consist of 9- to 30- members rings with 3 to 10 mainly O-donors
5
Q
What are cryptands
A
- they are 3D crown ethers
- macrocyclic analogues of crown ethers where the metal is bound in a 3D cavity
- typically contain bridgehead N-atoms
6
Q
Describe a typical cryptand synthesis
A
- synthesis achieved by reacting a strap across a pre-formed macrobicycle
- performed under high dilution conditions
- does not require a metal ion to template reaction
7
Q
Give two examples of macrocycles with N-donor atoms
A
- cyclam
- porphyrin
8
Q
What is the key method of assembly of N-donor macrocycles
A
Schiff base condensation
9
Q
Describe the synthetic strategy of Schiff base condensation
A
- reaction between an aldehyde/ ketone and an amine to give an imine
- key method used in assembling N-donor macrocycles
- imines are sensitive to hydrolysis and are stabilised by reduction to amines
10
Q
What is the main disadvantage of template synthesis
A
- metal must be removed without destroying the macrocyclic frame work
- for stable complexes especially for transition metal ions long reaction times are needed
11
Q
What are the 3 main methods of metal removal from cyclic complexes
A
- Use a strongly coordinating competitive ligand e.g. CN- to sequester the metal ion from the macrocycle
- Change the metal oxidation state to produce a kinetically more labile species e.g. Co(III) to Co(II)
- Treat with a strong acid e.g. Protonated ion of N-donor groups
12
Q
Define the term donor atom
A
- organic molecules that act as ligands and have one or more atoms that bind directly to metal ion/s
- examples - O, N, S, Se, P
13
Q
What is a multi-dentate ligand
A
A ligand with more than one donor atom
14
Q
Define the term homoleptic donor set
A
- ligands with the same donor atom type
15
Q
Define the term heteroleptic donor set
A
- ligands with different donor atom types
16
Q
What are the 3 main classes of multidentate,if and topologies
A
- Linear
- Cyclic
- Branched
17
Q
Describe the chelate effect
A
- complexes of chelating ligands are always more thermodynamically stable than complexes of equivalent mondentate ligands
18
Q
What is the macrocyclic effect
A
- the thermodynamic stability constant of a complex with a macrocyclic ligand is usually higher than that for a complex of an equivalent multidentate but non-cyclic ligand
19
Q
Describe the entropic rationalisation of the macrocyclic effect
A
- overall ΔS is positive due to the increase in the number of species in solution
- loss of ligand flexibility on coordination gives an unfavourable negative contribution to ΔS
- for linear multidentate ligands metal coordination reduces conformational flexibility - entropically unfavourable
- macrocycles have less conformation flexibility giving a smaller loss of entropy on complexation
20
Q
Describe the enthalpic rationalisation of the macrocyclic effect
A
- overall ΔH is negative due to formation of new M-L bonds but additional smaller ΔH terms are superimposed
- solvation effects give an unfavourable positive contribution to ΔH
- linear multidentate ligands are easily solvated which are displaced on coordination - positive ΔH
- in macrocycles donor groups are less available for solvation hence smaller positive ΔH cost on metal coordination
21
Q
Describe the kinetic aspects of the macrocyclic effect.
A
- rate of metal removal (e.g. Demetallation, dissociation, decomplexation) from a macrocyclic ligand is slow even with kinetically labile metal ions
- only need to break one bond for removal of ligand from metal with mondentate ligands- very fast reactions
- for linear or branched ligands can break one bond at a time and unzip from the metal
- macrocycles have little or no conformational flexibility so break several M-L bonds at once to dissociate - large activation energy
- kinetics of complex formation with macrocyclic ligands can be slow
22
Q
What are the three main factors that contribute to thermodynamic and kinetic stability of metal-ligand complexes
A
- Ligand configuration and design
- Metal
- Solvent and counter anion
23
Q
In terms of factors that contribute to the thermodynamic and kinetic stability of metal-ligand complexes, what factors contribute to ligand configuration and design
A
- Ligand flexibly
- Chelate ring size
- Number and identity of ligand donor atoms (Hard/Soft base)
24
Q
In terms of factors that contribute to the thermodynamic and kinetic stability of metal-ligand complexes, what factors contribute to the metal of the complex
A
- Ionic charge and radius (hard/soft acid)
2. Irving Williams series (transition metals)
25
In terms of factors that contribute to the thermodynamic and kinetic stability of metal-ligand complexes, what factors contribute to the solvent and counter anion
1. Solvent polarity and extent of solvation of metal, ligand and anion
26
Describe the effect of 5 membered chelate rings
- the most thermodynamically favoured
| - allows the non-metal atoms to have approximately tetrahedral geometry i.e. 109o bond angles
27
Describe the stability of 6 membered chelate rings
- after 5-membered rings they are the next most thermodynamically favoured
- slightly larger angles at non-metal atoms
- preferred for smaller metal ions e.g. Li2+
28
In hard-soft acid-base theory what is a soft base
- ligands are bases
- bonding tends towards covalent
- soft bases generally have less electronegative donor groups
E.g. Most sulphur donor groups, phosphorus
29
In hard-soft acid-base theory what is a hard base
- ligands with principally electrostatic bonding
- hard bases generally have highly electronegative donor groups
- examples : F-, Cl-, most oxygen donor groups
30
In hard-soft acid-base theory what is a hard acid
- metals are acids
- hard acids are highly electropositive and or highly charged (>+2) and /or small
- examples include s block ions, f block ions and early d block ions
31
In hard-soft acid-base theory what is a soft acid
- weakly electropositive, low charge and large
| - examples include the late 4d-, and 5d- ions
32
What are the rules of HSAB theory
1. Hard acids bind hard bases
2. Soft acids bind soft bases
3. Hard acids do not bind soft bases
4. Soft acids do not bind hard bases
5. Borderline bases bind everything
6. Borderline acids bind everything
33
What are the 3 main factors for molecular binding in cation recognition in complex formation
1. Requires strong binding and selectivity e.g. Bind K+ but not Na+ or Cs+
2. Design ligand (host/ receptor) to provide specifics
3. Use factors such as cavity size , identity of ligand donor atoms
34
What are some examples of the importance of cation binding in the real world
- biological process
- Transport of metals across cell membranes
- pharmaceuticals
- mining technology
35
What are the 3 main principles in ligand design attributed to metal ion selectivity
1. Macrocycle cavity size and configuration
2. Identity of donor atoms - principles based on HASB theory
3. Directionality of host/guest interactions
36
What is the key non-covalent interaction for cation binding that holds together host and guest
Electrostatic ion-dipole
37
For macrocycle cavity size what is the hole size fit concept
The highest binding constant for best match between the cavity diameter and cation diameter
38
What are the 4 main limitations associated with the hole size fit concept
1. Crown ethers are flexible ligands they can modify their exact conformation on binding to a metal cation - seen in the range of sizes for cavity diameters
2. Highest stability constants correspond with best fit between cavity size and cation diameter
3. Binding of other cations is possible but with reduced stability
4. Most accurate measure of hole size where ligand conformational strain is minimised
39
For structures of complexes with mis-matched ligand cavity and metal cation sizes what is the effect of the cation being too large
1. Cation sits above the ligand cavity
| 2. Or a 2:1 (ligand:metal) sandwich complex forms
40
For structures of complexes with mis-matched ligand cavity and metal cation sizes what is the effect of cation being too small
1. A large ligand conformational change
| 2. Or not all the ligand donor sites connect to the metal
41
What are cryptates
- metal complexes of cryptand ligands
- the ligands provide a 3D cavity
- group 1 metal binds through 6 oxygen atoms
42
What are 3 main advantageous features in the selectivity of metal binding by cryptands
1. Better selectivity than crown ethers- stability constants peak where cation size matches the cavity size
2. They are more rigid, there is less conformational stability
3. Pre-organisation of binding sites
43
What are the ligand features of spherands
1. Pre-organised receptor sites. Six O-donor atoms are arranged in a very rigid octahedral cavity
2. Very high binding constants to Li+ and Na+. No unfavourable energy terms from reorganisation of ligand on binding
3. K+ not bound as the cavity in a spherand is too small
44
How is selective through choice of ligand donor atoms usually achieved
- replacing hard donor (oxygen p) with softer donor (nitrogen) giving increased preference for softer metal (Ag+)
- other factors important for transition metal ions such as Irving Willams series - maximum stability for Cu(II) impedes selectivity by ligand design
45
Describe the factors associated with selectivity by directionality of bonding
- ion-dipole interactions which are key to M-L bonding are not strongly directional
- H-bonding is strongly directional - most effective with a linear arrangement between H-bond donor and H-bond acceptor
46
Give some of the importance associated with anion binding
- biological processes
- important for biological molecules such as ATP and DNA which are polyp ions
- environmental issues
47
What are the main ligand design principles in anion binding
- negative charge - anion receptors exploit electrostatic ion-ion or ion-dipole interactions
- size and geometry - many anions consist of several atoms e.g. NO3-, TcO4- with a specific shape
- binding constants affected by charge density and extent of solvation
48
In terms of ligand design principles what are the effects of anion size and geometry
1. Larger binding sites required
| 2. Shape specific complementary binding site. Potential for enhanced specificity
49
Describe the effects of electrostatic interactions of anion recognition
- use array of positively charge groups
- hold positive centres in a rigid system to constrain repulsion
- receptor design
50
In poly-aza macrocycles such as hexacyclen what are the effects of high and low pH
- high pH - NH groups acts as good donors to cations
- low pH - protonation of NH groups give positively charged ammonium sites. These provide combined and H-bonding interactions with anions
51
What are the limitations associated with Hexacyclen as an anion receptor
- N-heteroatom analogue of 18-crown-6 but it's binding cavity is much smaller - too small for large anions to bind
- difficult to protonate all N, very low pH required - but at low pH anions may be protonated and lose their negative charge
52
What is a solution to the issues associated with hexacyclen as an anion receptor
- larger aza-crowns with bigger separation between N atoms
- this allows full protonation of N-atoms within a pH range where common anions remain deprotonated
- leads to the cavity size being larger enough to fit anions
53
What are the main features of a macrocyclic host when considering shape and size selective binding of α,ω- dicarobxylates by poly-aza macrocycles
1. Elongated CH2 changes between N+ centres reduce repulsions allowing protonation of N within pH range where dicarboxylate is an anion
2. (CH2)n spacers linking two sides of macrocycle allow tuning of cavity size to length of dicarboxylate chain
3. Host-guest binding via combination of electrostatic and H-bonding interactions
54
What are the non-covalent interactions available for ligand design with neutral guest binding
1. H-bonding strongly directional to give specificity
2. π-π stacking interactions between aromatic groups
3. Hydrophobic effects - exclusion of non-polar groups or molecules from aqueous solution
55
Describe the principles of complementarity in H-bonding
1. host/receptor has multiple H-bonding groups that are complementary to the guest - matching of H-bond donors and acceptors
2. H-bonding groups are pre-organised and rigidly held within the receptor molecule so that directionality in interaction with guest is retained
- Example of complementary array DAD-ADA
56
Describe the two main configurations of π-π stacking
1. face-to-face
- parallel ring systems separated by around 3.5 Å
- aligned so that the centre of one ring lies over the corner of the other
- maximises π-electron cloud/ σ-framework interaction
2. edge-to-face
- H atom from one ring interacts in a perpendicular orientation with the centre of π-cloud of the second ring
57
Describe π-π stacking interactions
- attraction between negatively charged π-electron cloud of an aromatic system with positively charge σ- framework of a neighbouring aromatic molecule
- two main configurations - face to face and edge to face
58
Describe the molecular hinge seen in the π-π stacking of a receptor naphthalene group with a thymine base
- receptor naphthalene unit is hinged on a flexible chain to allow it to move into position above the heterocyclic ring - face to face interaction
- π-π stacking interaction acts in combination with DAD H-bonding array
59
What are the main principles associated with hydrophobic effects
- bases on the exclusion of non-polar groups or molecules from aqueous solution
- mainly an enthalpic effect- process has negative ΔH
- water molecules prefer interacting with other water molecules or polar groups
- it is unfavourable to trap water in a non-polar hydrophobic cavity
- replacing water by a non-polar guest allows displaced water to interact with the bulk solvent outside the cavity
60
What is an example of hydrophobic interactions in neutral guest binding
Biding of aromatics by cyclophanes
61
Define the term self-assembly in supramolecular chemistry
Spontaneous and reversible association of two or more components to form a larger non-covalently bound aggregate
62
Describe thermodynamic control in metal directed self-assembly
- self-assembled aggregate represents thermodynamically most stable product
- a variety of products may form irreversibly
- mistakes during assembly are corrected via reversible processes
63
What are the two building blocks of metal direct self assembly of aggregates
1. Metal
| 2. Ligands
64
In metal directed self assembly of aggregates what are properties of the metal building blocks
- use transition metal ions with defined geometric requirements e.g. Square planar, tetrahedral, octahedral
- this provides code information for self assembly
65
In metal directed self assembly of aggregates what are properties of the ligand building blocks
- use rigid poly-pyridine systems - N donor atoms are suitable for coordination to transition metals
- the number and configuration of N-donor atoms adds further control to self assembly
66
Define the term topological connection
- the interlocking of two or more species without need for a chemical bond between the components
- a mechanical rather than chemical bond connection
- examples
- catenanes - interlocked macrocycles
- rotaxane - threaded macrocycle
67
What is a catenane
- interlocked macrocyclic rings
68
What is a catanate
The metal complex of a catanane
69
Define the term topological isomers
- compounds with the same covalent connectivity but are topologically unique
- conversion cannot occur without breaking bonds
70
What are the two synthetic strategies of catenanes
1. Metal directed pre-assembly + clip
| 2. Thread and clip
71
What are the main principles of metal directed catenane synthesis
1. Form helix by metal coordination
2. Link ends
- the product formed depends on which ends are joined
- preferred geometry of metal coordination gives some control
72
In the thread and clip catenane synthesis how is the threading process promoted
1. Statistical approach gives very low yields
| 2. Use non-covalent interaction (π-π stacking) to encourage threading of linear component through ring
73
What are the key features of the Paraquat/ BPP[34]C10 interaction
- π-π stacking between electron rich crown and electron poor Paraquat aromatic rings
- H-bonding between crown O and acidic aromatic H or Paraquat
74
Describe the principles of the synthesis of higher order catenanes
- requires larger crown ether ring e.g. TPP[68]C20 (dimer of BPP[34]C10) - reduces steric constraints
- needs more forcing conditions - high pressure, long reaction
75
What are the 3 main synthetic strategy of rotaxanes
1. Threading + stopper
2. Clipping
3. Slipping
76
What aids the assembly of rotaxanes
- non-covalent interactions between wheel and axle or by metal-direction
77
Describe the assembly process of threading in rotaxanes synthesis
1. self assemble linear components to give pseudo rotaxanes
| 2. stopper ends of linear axle with bulky end groups to prevent wheel slipping off axle
78
Describe the assembly process of clipping in rotaxanes synthesis
- pre- stoppered linear component reacts with self assembling component which undergoes macrocyclic ring closure around the axle
79
Describe the assembly process of slipping in rotaxanes synthesis
- relatively small stoppers only
- slip macrocycle over stopper at high temperatures
- at lower temperature interactions between wheel and macrocycle prevent reverse process
80
Describe an example of a threading synthesis
1. Threading of axle through macrocycle facilitated by Paraquat/ aromatic ether type interactions as for catanane so
2. Connection of stopper group is assisted by good leaving group
