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Coordination polymer

A coordination compound (M-L bonds) with repeating coordination entities in one-, two- and three-dimensions


1D coordination polymer



Example of 1D coordination polymer

Two coordinate metal


2D coordination polymer

Trigonal planar metal centre


3D coordination polymer

Tetrahedral metal centre


What are two- and three-dimensional coordination polymers sometimes referred to as?

Coordination networks



When two or more networks are not directly connected, but cannot be separated without breaking bonds
Often occurs in network structures as a way of avoiding void space - the networks exist in each other's pores



A collection of nodes with a clearly defined connectivity
A polymeric collection of interlinked nodes
Each link connects 2 nodes and each node is linked to 3 or more other nodes


Reasons for the common use of polycarboxylates as ligands in forming coordination networks

Carboxylates are anionic, so often form neutral networks with metal cations - therefore no need to accommodate counter-ions in the pores (already charge-balanced)
Carboxylates can bridge between metal centres, giving aggregates rather than single metal centres as the nodes in the framework (therefore less interpenetration)
Carboxylate-based networks are often relatively stable to heating - heating is required to remove solvent from the pores of the 'as-synthesised' materials
Metal-carboxylate bonds re strong enough to form robust porous materials, but not so strong that their formation ins't reversible - important for forming crystalline materials



Metal-organic framework
A coordination network with organic ligands containing potential voids (i.e. potentially porous)



Secondary building unit
Metal-carboxylate aggregates


How can 3D MOFs be formed?

By carrying out the reaction in the presence of a neutral bridging ligand such as dabco (bridges 2D sheets into a 3D network)


Isoreticular series

Compounds with the same gross/net structure
Can have different pore sizes through changing the length of the linking group between the carboxylates (longer linker = larger pore)


How can the structure of a coordination network be controlled?

Changing the relative orientation of the carboxylate groups


Non-carboxylate MOFs

e.g. anionic polydentate N-donor linkers to produce neutral frameworks e.g. imidazolate



Zeolitic imidazolate frameworks
2nd most important MOFs after polycarboxylates
Imidazolates contain a similar angle between the N lone pairs to the Si-O-Si angle in zeolites (140 degrees)
As a result, many imidazolate networks have similar open structures to zeolites but with larger pores


MOF synthesis

Heating metal salt with a protonated linker (e.g. polycarboxylic acid, imidazole) in a weakly basic solvent e.g. DMF, H2O


Purpose of weakly basic solvent in MOF synthesis

Gives low concentration of the anionic linker in solution, ensuring slow crystallisation
This helps with the growth of single crystals (often required for characterisation of new materials)


Alternative methods of MOF synthesis

1. Microwave synthesis - faster reactions but crystals tend to be very small (so less useful when making a new MOF - crystals too small for single crystal X-ray crystallography)
2. Mechanochemistry - grinding reagents together in the absence of solvent, but also gives small particle size
3. Electrochemical synthesis - using an electrode as the source of the metal. Used in large scale synthesis/commercial MOF production


Uniform net

All pairs of links give the same sized circuits


Platonic uniform net

All nodes have the same connectivity


Vertex symbol

(2D net)
a = number of nodes in the smallest loop
b = number of routes


Can 3D nets also be described using vertex symbols?

A unique 3 letter code is often also used to describe a particular net e.g. dia for diamond structure


What is the removal of solvent molecules from the pores of MOFs called?



MOF stability

As the charge on the metal centre increases, the M-L bonds tend to become stronger, making the MOFs more stable to decomposition by e.g. hydrolysis
e.g. many Zn(II) MOFs decompose in moist air, whereas Cr(III) and Zr(IV) MOFs are stable in boiling water, and can even by boiled in concentrated acids

M(II) < M(III) < M(IV)


Why is it easier to grow single crystals for Zn(II) MOFS than it is for MOFs with more highly charged metal centres?

Bond formation in Zn(II) MOFs is more reversible


Other methods of MOF activation (other than heating to remove solvent)

1. Solvent exchange with a more volatile solvent followed by evacuation
2. Using supercritical carbon dioxide
3. Freeze drying


Applications of MOFs

Methane adsorption e.g. to store natural gas in fuel tanks
Hydrogen adsorption - to safely store H2 for use with fuel cells
CO2 adsorption e.g. to selectively remove CO2 from waste gas streams. Amine groups in ligands increases interaction of MOF with CO2 through H bonds
Heterogeneous catalysts e.g Lewis acid catalysts, chiral epoxidation catalyst, polymerisation catalyst
Separation of gases e.g. selective adsorption of CO2 from mixtures of CO2 and CH4, separation of p-xylene from m-xylene, separation of cis- and trans-alkenes
Drug delivery


Separation factor of HKUST-1 for cis-2-octene over trans-2-octene



How many classes of porous materials are there?

1st gen, 2nd gen & 3rd gen materials


How have porous materials been divided into classes?

Based on their behaviour on guest removal


1st generation materials

Collapse on guest removal


2nd generation materials

Have rigid and robust frameworks so become porous on guest removal


3rd generation materials

Transformable, change structure on guest removal
Also known as "soft porous crystals"


Define 'breathing'

The ability of a MOF to adjust its structure in the presence or absence of a guest within the pores


What are the 6 different types of flexibility in MOFs?

1. 1D chains
2. 2D stacked layers
3. 2D interdigitated layers
4. 3D pillared layers
5. 3D expanding and shrinking grids
6. 3D interpenetrated grids


Example of class III flexibility

CID class - has a 2D structure with interdigitated layers
This class is typified by [Zn(1,3-bdc)(4,4'-bipy)] (CID-1)


Example of class V flexibility

MIL-53 series of MOFs, of general formula [M(OH)(bdc)] where M = Al, Ga, Cr, Fe


MOFs that are capable of breathing...

...often show unusual adsorption behaviour e.g. multiple steps in the isotherms and/or hysteresis
Can be useful for separation


Crystalline molecular flasks

Hollow molecular structures that are able to encapsulate guest species within their cavities and influence their reactivity in a controlled fashion


Example of a crystalline molecular flask

Can include a wide variety of guests within its pores, many of which are crystallographically ordered
Allows for structural characterisation of such guests, which cannot form single crystals on their own/be prepared in sufficient quantities to crystallise
Only micro- or even nanograms of the guest are required
e.g. co-crystal of [(ZnI2)3(tpt)2] and santonin (anti-parastic worm drug) enabled absolute structure of santonin to be determined


Post-synthetic modification

The process by which a functional group (or "tag") in a MOF is converted into another
A solid-state reaction, occurs after the initial MOF synthesis ("post-synthetic")
Most useful when the modified MOF cannot be prepared directly


Examples of post-synthetic modifications

e.g. conversion of an amine group into an amide
Would work well with acetic anhydride (small enough to access the pores)
Use of a larger reagent may mean that the reaction does not go to completion
e.g. attaching a metal centre to a MOF


Uses of post-synthetic modifications

Preparation of non-interpenetrated versions of MOFs that are normally interpenetrated
Carried out via a deprotection interaction with convert a large group into a smaller one


Post-synthetic exchange

Many MOFs have labile M-L bonds so can undergo either metal or ligand exchange reactions
e.g. [Zn(Me-im)2] ---> [Zn(Him)2], i.e. [Zn(Me-im)2] undergoes exchange with imidazole (Him)
This occurs without a change of phase
Enables the preparation of a more porous material than can be synthesised directly, as [Zn(Him)2] is normally formed in a dense phase


Surface modifications

Limited to crystal surfaces either by sterics or incompatibility with the pores
Reactions can also be targeted on terminal rather than bridging ligands - a fluorescent reagent can be used to illustrate this


Mixed-component MOFs

MOFs that have different linkers or metals with the same structural role
Many mixed-linker/mixed-metal MOFs are solid solutions in which the proportions of the ligands/metals can be adjusted/controlled
Mixed-linker MOFs can be prepared by using a mixture of linkers in the synthesis
As many as 8 different linkers have been incorporated into the same MOF crystal


Analysis of mixed-component MOFs

To show that each crystal contains the different linkers as opposed to the product being a physical mixture of 2 or more single-linker MOFs
NMR or MS analysis of individual digested single crystals
PXRD studies showing that cell parameters vary with Vegard's law
Thermogravimetric analysis


Optimising properties of MOFs

e.g. for selective adsorption of CO2 from a CO2/CH4 mixture [Zn(1,3-bdc-X)(4,4'-bipy)], where X = NO2 or OMe
NO2 compound = selective for CO2 but very low uptake
OMe compound = higher uptake but less selective for CO2
Mixed-linker MOFs could combine the selectivity for CO2 of one MOF with the faster rate of uptake of another MOF


Macroporous materials

Pores larger than 50 nm (500 A)


Mesoporous materials

Pores between 2 nm (20 A) and 50 nm (500 A)


Microporous materials

Pores smaller than 2 nm (20 A)


Nanoporous materials

Pores smaller than 50 nm
i.e. microporous and mesoporous


Why do MOFs form as crystalline materials with regular structures?

Because the bond formation within them is reversible, so the bond can break and reform if it doesn't form correctly at first


Amorphous materials

Have no long range order


Example of a covalent bond that forms reversibly

Dehydration of a boronic acid to form a boroxine
Imine formation


Formula of a boronic acid



Boroxine formula

6-membered, heterocyclic compound composed of alternating oxygen and singly-hydrogenated boron atoms



Covalent organic frameworks
Crystalline, polymeric materials from reactions employing diboronic acids (or diboronic acids + diols)
Have similar high surface areas to MOFs


Trigonal nodes

Lead to hexagonal pores


Square planar nodes

Lead to square pores


The use of non-planar building blocks can lead to...

...3D structures


COF-1, COF-5

2D sheet structures



3D structure
Built from a non-planar building block



Shows the highest uptake capacity of any porous material known for ammonia
Uptake is reversible - ammonia can be released by heating at 200 oC
High uptake is as a result of Lewis acid-Lewis base reactions between ammonia and the boron centres



5-fold interpenetrated diamondoid structure


How are triazine-based COFs formed?

From the self-condensation of nitriles (e.g. 1,4-dicyanobenzene) when heated to 400-700 oC in the presence of ZnCl2
The network formed from 1,4-dicyanobenzene is crystalline but increasing the size of the linker leads to amorphous materials



Hypercrosslinked polymers
When irreversible reactions are used to prepare porous materials
Reactions need to be very high yielding so that all available groups react e.g. Friedel Crafts reactions


Amorphous porous materials

Have all the benefits and potential uses of crystalline porous materials
But characterisation can be problematic


Problems with amorphous porous materials

No long range order so X-ray diffraction does not provide much structural information
Can be difficult to predict/model properties without long range order
Amorphous materials can contain a broader range of pore sizes than crystalline materials which can affect their adsorption properties
Properties of amorphous materials can very from batch to batch (but using high-yielding reactions can minimise this)



Conjugated microporous polymers
A subclass of HCPs
Contain multiple C-C bonds and/or aromatic rings in an extended conjugated network
Formed from Sonogashira reaction



Porous aromatic frameworks
Have the highest known surface areas for microporous organic polymers
Produced from the homocoupling of tetrahedral monomers using the Yamamoto reaction



Polymers of intrinsic microporosity
Polymers that are porous because of inefficient packing of the individual polymer strands
Have kinked chains due to the presence of spirocentres (this is what causes them to pack inefficiently)


Why is there currently considerable interest in PIMs?

They are processable
Most porous materials exist as insoluble powders whereas PIMs are soluble so can be dissolved and the solvent slowly evaporated to form free-standing flexible films


Post-translational modification of PIM-1

PIM-1 contains a dicyanobenzene
Can react with NaN3 in the presence of ZnCl2 to form a tetrazole-functionalised PIM that shows exceptional CO2 uptake and separation from other gases (due to N-H groups)


Condensation reaction for preparation of PIMs

o-diphenol + o-difluorobenzene
Produces benzene-O-benzene linkage


Condensation routes to nanoporous materials

Generally reversible


Metal-catalysed routes to nanoporous materials

Generally irreversible
Generally high-yielding


Advantages of MOFs c.f. organic framework materials

MOFs tend to be more crystalline (more regular/ordered structures)
Wide variation of structure possible through variation of SBU and linker
3D structures easily accessible (from 3D metal centres)


Advantages of organic framework materials c.f. MOFs

Structures are often more robust (C-C bonds are generally stronger than coordination bonds)
Use of lighter atoms means structures are often lower density
Processability is possible


What are the 2 ways in which molecular solids can be porous?

Extrinsic and intrinsic porosity


Extrinsic porosity

Porosity that arises from the inefficient packing of molecules in the solid state


Intrinsic porosity

Porosity that arises from pores within molecules


How does extrinsic porosity arise?

Due to molecular shape or because of strong intermolecular interactions
Or a combination of both



The spontaneous generation of a complex structure from simple components
Involves a loss of entropy but a gain of enthalpy through the interactions


How can molecular boxes be prepared?

Through using square planar metal centres e.g. Pd(II) and Pt(II)
e.g. [M(en)(ONO2)2] + 4,4'-bipy ---> [{M(en)}4(4,4'-bipy)4]8+
Metal ion and ligand both coordinatively saturated


Extension of a molecular box into three dimensions gives...

...a molecular cube
Requires a precursor with 3 labile ligands all at 90 degrees to each other


Pt c.f. Pd

Pt has a higher stability and lower lability


Effect of reducing angle between pyridyl groups in ligand from 180 degrees

Can increase cage size



Naturally occurring cyclic oligomers of D-glucose
Produced from the action of bacteria on starch
Bucket shape


Cyclodextrin cavity

Can bind non-polar molecules with high association constants