Structure of Metal-Organic Frameworks (MOFs) Flashcards
- If a square planar metal ion was combined with a linear ditopic monodentate linker like 4,4’-bipy, the pieces could assemble into a 2D square lattice
- However, this often does not work, as there are several factors that disfavour this
- Why?
- Entropy
- Flexibility
- Charge balance
why does entropy mean an extended polymeric species doesn’t form?
- Entropy favours multiple different discrete species forming, (-)ve entropy change
- Therefore, for the overall assembly to be thermodynamically favourable, strong bonds
- Pd(II)-pyridin bond is only reasonably strong due to low ionic contribution as ligand isn’t charged
Why does flexibility mean an extended polymeric species doen’t form?
- 4-4’-bipy ligand is fairly rigid and planar
- However, there is nots of flexibility in the M(II)-pyridine bonds
- This can lead to triangles and squares with discrete boxes
- An extended porous structure such as this would rapidly become irregular if these is flexibility
- (need to reduce flexibility)
Why does charge balance mean an extended polymeric species doesn’t form?
- 4,4’-bipy ligand is neutral and Pd(II) is cationic
- For every metal centre there is a charge of +2 that needed balancing
- These can be balanced by non-coordinating anions that sit around the discrete structure
- However, in an extended structure they cannot just all be around the edge
- This can be overcome by putting the anions between the layers
What is the issue with charge balance in the analogous 3D structure as a primitive cubic lattice, where there is an octehdral metal ion such as Fe(II) at each lattice point
There would now be no space between the layers and counteranions would have to be put in the voids in the middle
This is an issue however, because the voids within these structures allowing guest to bind are what make them useful to science, and having anions in them would remove this and porosity would be lost
Define Coordination polymer
A coordination compound continuously extending in one, two or three dimensions through coordination bonds
Define Coordination network
A coordination compound extending, through coordination bonds, in one dimension, but with cross-links between two or more individual chains, loops or spiro-links, or a coordination compound extending through coordination bonds in two or three dimersions
Define Metal Organic Framework (MOFs)
is a coordination polymer (or alternatively coordination network) with an open framework containing potential voids
What is the most widely used ligand for making MOFs is
Carboxylates
(different to discrete structures which was pyridines)
What is the charge on carboxylates and what is the benefit of this?
- Carboxylates carry a charge of -1
- THis helps them form strong bonds to metal centres as there is an ionic contribution to the interaction as well as a covalent contribution
What are the two ways a ditopic carboxulate ligand can bind?
- It is possible for the 2 donor atoms tto coordinate to the same metal in a bidentate fashion
- OR for each carboxylate to coordinate to two nearby metal ions. Both metal ions sit at the same site
Why is it rarer for a carboxylate to coordinate in a bidentate fashion?
- This makes a 4-membered chelate ring which is strained
- With bidentate ligands at a single metal ion, we would rapidly run out of available coordination sites
- for example, a square lattice where each ditopic linker was bidentate would require 8 sites on the metal ion
What is the benefit of the structure of the dicarboxylate for the MOF formation?
- Whilst strictly being monodentate to any individual metal ion, the carboxylate is bidentate to the node or cluster of metal ions at the corner of the framework
- This chelating nature makes the structure much more rigid and is crucial for MOF formation
- The carboxylate coordinates to 2 nearby metal ions. Both metal ions sit at the same node of the structure
The simplest example of the dicarboxylate ligand is in a paddlewheel
Describe the structure of it?
- The centre of the paddlewheel has two metal ions
- Four carboxylate ligands span out from the centre of the paddlewheel at angles of 90 degrees to each other
- The two metal ions are close together and there is a metal-metal bond between them
- In this structure here the metal ions are Cu(II)
What is the overall charge for each node
- 2 x Cu²⁺
- 4 x RCO₂⁻
- Overall charge for each node is zero
Is the copper in the paddlewheel para- or diamagnetic
- Cu(II) has a d⁹ electron configuration meaning the material is paramagnetic
- This is observed at elevated temperatures
- At low temperatures however, the unpaired electron spind undergo antiferromagnetic exchange coupling making the overall node diamagnetic
We have seen that Cu(II) often likes to be 5-coordinate or Jahn Teller distorted octehedral
How is a Jahn Teller distorted octehedral formed?
- Each copper makes one bond to another copper
- 4 bonds to carboxylates
- making 5 in total
- There is always a sixth ligand that coordinates on the top and bottom of the paddlewheel
- It is possible to remove the solvent under certain conditions
- However, even with the solvent removed, the top and bottom of the paddlewheel still need a sixth ligand to coordinate to the metal
- How this this overcome?
- This is now provided by one of the oxygen atoms on a carboxylate of the next layer
- The nearest copper metal centres of the adjacent layers are shown in gold
- The resultant structure is much more porous as no additional coordated solvent molecules are needed
The addition of second ligand to coordinate to the sixth position of the padlewheels can hold the layers further apart
This ligand should be neutral (4,4’-bipy) given that we do not want to build up charge in the framwork
What overall structure is formed
- There are two interpenetrating versions of the same framework (one in red and other blue)
- This is analogous to the catenanes that can form in equilirbrium with molecular boxes
- The interpenetrating structure maximises other through space interactions such as π-π stacking + van der Waals interactions
When thinking about designing a connecting node to span 3D as opposed to the 2D of the paddlewheel
This can be achieved with a structed based on four Zn²⁺ ions
Wht is the geometry of this?
- An ocetahedral node would have six carboxylate ligands meeting, with 90° between each of the carboxylate ligands coming off
On its own, at the node this would give 4x Zn²⁺ and 6x RCO₂⁻, leaving an overall charge of +2
How is this overcome?
- The 4 zinc(II) ions are arranged in a tetrahedron
- In the centre of the tetrahedron there is an oxgyen atom (formally O²⁻)
- This oxygen is four-coordinate and surrounded by a tetrahedron of four zinc(II) ions
- The zincs are to far apart now however so no Zinc-zinc bonding
- Overall charge is zero
The geometry arond each zinc in this compound is?
- Each of the four zinc(II) ions is in a tetrahedral coordination environment provides by four oxygen atoms
- Around any one zinc, three of the oxygens come from carboxyate ligands and the fourth comes from the central oxgyen
- These four ZnO₄ tetrahedra share their central vertex
What is the overall structure formed when these unit polymerise?
- The overall structure of the solid is based on a primitive cubic lattice
- There are channels with a square cross section running in all three directions (x, y, z)
As the ligand get longer, what happens to the properties of the MOF?
As the ligand gets longer, the potential cavity in the MOF gets larger and the structure more porous
What can we disfavour the formation of interpenetrating structures?
Substituent groups can also be added to the dicarboxylate ligand to disfavour interpenetrating structures
(These also, however, reduce the amount of free space in the structure)
MOF-235: consists of a secondary building unit consructed from 3 Fe(III) ions. These three metal ions form a…
A central oxgyen sits in the middle of the triangle (O²⁻). A carboxylate ligand bridges each edge on the top of the triangle and each edge on th bottom of the triangle. This gives the SBU a ….. shape
- Triangle linked by metal-metal bonds
- Trigonal prismatic shape
MOF-235: Each Fe(III) ion is in an overall octahedral environment, consisting of four bonds to carboxylate oxygens, one bond to the central oxgyen and one bond to a coordinated DMF solvent
What is the charge on the node?
- 3 x Fe³⁺
- 6 x RCO₂⁻
- 1 x O²⁻
- = overall +1 charge
- (Hence this structure must have one anion in the void for every node of the structure. In this case the anion is [FeCl₄]⁻
MOF HKUST-1: what is the difference of the ligands on this MOF? And how does this affect the structure?
- The ligands linking these SBUs are now trotopic as it is a benzentricarboxylate
- This means the paddlewheels are linked together in different ways
- In particular, the paddlewheels no longer form layers and are not all orientated in the same direction
What is the structure of the overall polymer formed from this unit?
- Octahedral units within this structure with alternating faces capped by the tricarboxylate ligand (four out of eight faces capped)
- Now at each vertex of the octahedron are two metals of the copper paddlewheel SBU rather than just one
