Metal-Organic Cages Flashcards
What is a metal-organic cage?
They are 3D structures with a well-defined void or cavity
What two types of ligands do w need to make a metal-organic cage?
- 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
What is the name of this platonic sold
How many faces, edges and vertices does it have?
Tetrahedron
Faces: 4
Edges: 6
Vertices: 4
What is the name of this platonic sold
How many faces, edges and vertices does it have?
Cube
Faces: 6
Edges: 12
Vertices: 8
What is the name of this platonic sold
How many faces, edges and vertices does it have?
Ocetahedron
Faces: 8
Edges: 12
Vertices: 6
What is the name of this platonic sold
How many faces, edges and vertices does it have?
Dodecahedron
Faces: 12
Edges: 30
Vertices: 20
What is the name of this platonic sold
How many faces, edges and vertices does it have?
Icosahedron
Faces: 20
Edges: 30
Vertices: 12
What is a Face-Capped Cage?
Can be made with tetrahedra and cubes
Three faces meet at a vertex
What types of ligands do we need for face-capped cages for tetrahedrons and cubes?
- 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
What type of metal ion would we use to coordinate all the ligands when forming these face-capped structures?
- 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
The following is a ligand that can be used to face-cap a tetrahedron
It is a…
Tritopic bidentate ligand
The ligands contains functional imine (C=N) which are made in reversible reactions from anilines (or amine) and aldehyde
Why are they special
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
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?
[M₄L₄]⁸⁺
To construct a face-capped cube which also has 3 faces meeting at each vertex
Which type of ligand would we use?
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)
Describe the structure of the middle of this ligand
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
Where does the self-assembly occur to allow this ligand to form?
The self-assembly takes place around the Iron(II) at the corners of the cube as this d⁶ metal ion prefers octahedral coordination
How many tetra-aniline, aledyde and metal ion are required to produce this ligand?
This cubic cage can self-assemble if 6eq of tetra-aniline, 24eq of aledyde and 8eq of metal ion are mixed
What is the formular of this overall cubic cage
[M₈L₆]¹⁶⁺
How are Octehedral cage formed?
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
Describe the structure of the ligand used to form an ocetehedral cage
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)
What is the formula of the overall structure
[M₆L₄]¹²⁺
The four open faces of the octehedral cage means that is is good at binding guests
One such guest can be…
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
How may the conformation of a long or large guest molecule adapt from what it is in solution when it bound to a cage
- 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
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…
Non-covalent protecting group
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?
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
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…
two metals and four ligands are formed
The have a lantern shape
How can we construct a latent shaped edge-capped cage?
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
We would refer to the cage as…
[M₂L₄ ɔ (NO₃)]³⁺
What is an Archimedean solid?
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
As the angle between the metal-ligand coordination vectors gets larger, the…
the resultant cages have more metal centres, get larger, and get closer to being spherical
The following ligands is a ditopic bidentate ligand which is flexible meaning it has two conformers
What does each conformer form?
Ligand A - Tetrahedral conformation
Ligand B - Helicate conformation
This is due to the 2 bidentate groups being aligned
The following ligand is a ditopic bidentate ligand which is flexible meaning it has two conformers
Why can this ligand not form a helicate
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)
What is the charge on the ligands
This results in the cage being…
- 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
