Metal-Organic Cages

Question 1

What is a metal-organic cage?

Answer 1

They are 3D structures with a well-defined void or cavity

Question 2

What two types of ligands do w need to make a metal-organic cage?

Answer 2

• Designing a ligands for each edge of the 3D shape we want to make - Edge-Capping ligands
• Designing a ligand for each face of the 3D shape we want to make - Face-Capping ligands

Question 3

What is the name of this platonic sold
How many faces, edges and vertices does it have?

Answer 3

Tetrahedron
Faces: 4
Edges: 6
Vertices: 4

Question 4

What is the name of this platonic sold
How many faces, edges and vertices does it have?

Answer 4

Cube
Faces: 6
Edges: 12
Vertices: 8

Question 5

What is the name of this platonic sold
How many faces, edges and vertices does it have?

Answer 5

Ocetahedron
Faces: 8
Edges: 12
Vertices: 6

Question 6

What is the name of this platonic sold
How many faces, edges and vertices does it have?

Answer 6

Dodecahedron
Faces: 12
Edges: 30
Vertices: 20

Question 7

What is the name of this platonic sold
How many faces, edges and vertices does it have?

Answer 7

Icosahedron
Faces: 20
Edges: 30
Vertices: 12

Question 8

What is a Face-Capped Cage?

Answer 8

Can be made with tetrahedra and cubes
Three faces meet at a vertex

Question 9

What types of ligands do we need for face-capped cages for tetrahedrons and cubes?

Answer 9

• In a tetrahedron, three triangular faces meet at each vertex, therefore our ligand would need to be a tritopic ligand which is triangular
• In a cube, three square faces meet as each vertex, therefore our ligand would need to be tetratopic ligand which is square in shape

Question 10

What type of metal ion would we use to coordinate all the ligands when forming these face-capped structures?

Answer 10

• As three faces meet at each vertex, we can use an octahedral metal ion and have that coordinated to 3 bidentate chelating groups
• One from each face capping ligand that is meeting at a given vertex
• These ligands then wrap around the metal ion in the same way we saw in a triple helicate
• The use of 3 bidentate groups ensures the exterior coordination positions around the metal ion are filled

Question 11

The following is a ligand that can be used to face-cap a tetrahedron
It is a…

Answer 11

Tritopic bidentate ligand

Question 12

The ligands contains functional imine (C=N) which are made in reversible reactions from anilines (or amine) and aldehyde
Why are they special

Answer 12

This ligand is special in that the ligand itself is only formed during the self assembly process
e.g. forming the imine makes a bidentate chelating group for a metal, so this coordinates to a metal ion, which helps to move the equilibrium to the right
This happens 3 times to make this ligand

Question 13

This tetrahedral cage can self-assemble if 4eq of the trisaniline, 12eq of the aldeyde, and 4eq of the metal ion are mixed
What is the formula of the overall cage formed?

Answer 13

[M₄L₄]⁸⁺

Question 14

To construct a face-capped cube which also has 3 faces meeting at each vertex
Which type of ligand would we use?

Answer 14

The ligand would now be a tetratopic bidentate ligand
(This ligand contains 4 imines and so is also made in situ from the condensation of anilines and aldehydes)

Question 15

Describe the structure of the middle of this ligand

Answer 15

The middle of this ligand contains a nickel(II) porphyrin (just a spectator)
The nickel(II) has a d⁸ electron configuration and is in a square planar envionment
The Ni(II) is held tightly in the porphyrin and is not involved in the self-assembly - just used to make this square unit

Question 16

Where does the self-assembly occur to allow this ligand to form?

Answer 16

The self-assembly takes place around the Iron(II) at the corners of the cube as this d⁶ metal ion prefers octahedral coordination

Question 17

How many tetra-aniline, aledyde and metal ion are required to produce this ligand?

Answer 17

This cubic cage can self-assemble if 6eq of tetra-aniline, 24eq of aledyde and 8eq of metal ion are mixed

Question 18

What is the formular of this overall cubic cage

Answer 18

[M₈L₆]¹⁶⁺

Question 19

What is the cavity volume of the tetrahedral cage vs the cubic cage?

Answer 19

• The tetrahedral cage has a cavity volume of 45 ų
• The cubic cage is much larger and has a cavity of 1300 ų
• Cubic cage can bind things which are much larger

Question 20

The cavity of the cubic cage is hydrophobic and shows a strong affinity for binding …

Answer 20

Fullerences (C₆₀ and C₇₀)
Perfers C₇₀ over C₆₀ due to better size match - hence it can be used to seperate different fullerenes

Question 21

How are Octehedral cage formed?

Answer 21

Constructing an octahedral cage requires a different strategy, as now 4 face-capping ligands meet at each vertex
The structure is bases the metal-organic cage on an octahedron, but only every alternative face is capped with a ligand
The resulting cage only has four face-capping ligands and there are only two of these which meet at each vertex

Question 22

Describe the structure of the ligand used to form an ocetehedral cage

Answer 22

The ligand is a tritopic monodentate ligand as the faces are triangular
Two sites are taken up with the bonding from tritopic ligands and the other two are blocked by the bidentate chelating group
(the resulting cage has four of the faces without a panel and therefore much more open than other face-panelled cages)

Question 23

What is the formula of the overall structure

Answer 23

[M₆L₄]¹²⁺

Question 24

The four open faces of the octehedral cage means that is is good at binding guests
One such guest can be…

Answer 24

Adamantane carboxylate
4 of these are bound - one in each open face
The carboxylate is orientated out into solution and the hydrocarbon part buried in the cage

Question 25

The number of guest that bind in the cage depends upon the size of the guests: If a small guest is used...

Answer 25

1 cage : 4 guests

Question 26

The number of guest that bind in the cage depends upon the size of the guests: If a medium guest is used...

Answer 26

1 cage : 2 guest

Question 27

The number of guest that bind in the cage depends upon the size of the guests: If a large guest is used...

Answer 27

1 cage : 1 guest

Question 28

How may the conformation of a long or large guest molecule adapt from what it is in solution when it bound to a cage

Answer 28

* Long molecule that normally have quite an extended conformation have to curl to to bind * Very large molecules cannot fit entirely in the cage. For example, the end of one molecule can poke out one of the open faces

Question 29

Linear diterpenoids are one such molecule that must adopt a curled up, "U-shape" conformation to bind The terminal prenyl group shown in orange pokes out of the open face which allows the cage to behave as a...

Answer 29

Non-covalent protecting group

Question 30

Stilbene molecules undergo isomerisation between trans and cis forms when irradiated by light Normally equilibrium lies on the side of trans, with a trans:cis ratio of 6:1 due to sterics How can we just form the cis form?

Answer 30

The cis isomer is encapsulated in the cage, where the trans isomer is to long and rigid Two cis stilbenes are encapsulated per cage where it is stable to irradiation into the trans form

Question 31

One way to make edge-capped cages is using square-planar metal ions (such as Pd(II) and Pt(II) with bent ligands) If we change the metal source to a suitable metal salt without a bidentate capping group then cages with...

Answer 31

two metals and four ligands are formed The have a lantern shape

Question 32

How can we construct a latent shaped edge-capped cage?

Answer 32

Using palladium(II) nitrate This is because nitrate ions are weakly coordinating, they are easily displaced by monodentate ligands Only one nitrate is bound in the centre of the cage

Question 33

We would refer to the cage as...

Answer 33

[M₂L₄ ɔ (NO₃)]³⁺

Question 34

Why is the nature of the ligand important to the formation of the edge-capped cages?

Answer 34

The bent nature of the ligand is crucial It is the bend in the ligand that enables a closed, discrete cage to be formed This is entropically favourable over polymer formation

Question 35

How do the ligands in these edge-capped cages bend?

Answer 35

* These lantern cages have very bent ligands * They are bent in such a way that the two metal-igand bonds made by the same ligand are parallel and so there is an angle of 180°

Question 36

The larger the angle in the latern cages...

Answer 36

The greater number of vertices there are in the cage that forms This is because there is less curvature between adjacent vertices These cages always have the same metal:ligand ratio of 1:2

Question 37

What is an Archimedean solid?

Answer 37

These Archimedean solids have different polygons as their faces, but all their vertices are the same and all the edges are the same length One ligand lies along each edge

Question 38

As the angle between the metal-ligand coordination vectors gets larger, the...

Answer 38

the resultant cages have more metal centres, get larger, and get closer to being spherical

Question 39

Edge-capped cages can be made from ligands that are not bent If we do this for a tetraheddra with 3 edges meeting at each vertex as well as three face...

Answer 39

This means that they can be constructed from octahedral metal ions which are coordinated by 3 bidentate ligands, one from each edge

Question 40

The following ligands is a ditopic bidentate ligand which is flexible meaning it has two conformers What does each conformer form?

Answer 40

Ligand A - Tetrahedral conformation Ligand B - Helicate conformation This is due to the 2 bidentate groups being aligned

Question 41

The following ligand is a ditopic bidentate ligand which is flexible meaning it has two conformers Why can this ligand not form a helicate

Answer 41

Conformation 1 is suitable for a tetrahedron Conformation 2 is unsuitable for a helicate as the two bidentate groups offset from the ligand backbone (Hence we would use this type of ligand to form a tetrahedron as a helicate would usually be favoured)

Question 42

What is the charge on the ligands This results in the cage being...

Answer 42

* The 4 phenol groups become deprotonated in the self-assembly process make the ligand have an overall charge of -4 * Due to the high negative charge on the hard ligand, this overall cage is now negatively charged * This cage can bind to a range of cationic species, a useful contrast to many other cages