Omar Yaghi is convinced that chemists should embrace complexity. ‘Right now, materials in general have one structure, one property, one operation,’ he says. ‘[Yet] it would be great if we could design materials… where several things are happening at once.’
This is surprising for a chemist who helped establish the field of metal–organic frameworks (MOFs), a contribution recognised by last year’s Nobel prize in chemistry, which Yaghi shared with Susumu Kitagawa and Richard Robson. Yet in recent years, Yaghi has moved away from creating conventional MOFs built from a single type of linker and metal node. Instead, he and other chemists have found ways to incorporate multiple organic ligands and different metal building blocks within a single structure.
It would be great if we could design materials… where several things are happening at once
This new class of materials – known as multivariate- or MTV-MOFs – drastically expands the scope and capability of porous frameworks. ‘[Chemists] look at [MTV-MOFs] and they say “Oh my God, this is just in one structure? You have too many things happening”,’ says Yaghi. ‘A lot of people shy away from it [but] I think that [it’s] a big opportunity to transform our chemistry.’
MTV-MOFs allow chemists to create materials that have pores of various sizes and chemistries, which often leads to structures with multiple functions. This could be, for example, a material that captures carbon dioxide in one part of a framework and then uses the bound molecules in a catalytic reaction in another region.
Yaghi notes that these materials take inspiration from nature, which thrives on combining multiple building blocks together for a specific function. Collections of different amino acids decorate an enzyme’s active site, for example, producing a finely tuned catalyst. Similarly, having a mixture of metals and ligands within an MTV-MOF could help control the function and specificity of these materials. Combining a range of building blocks also drastically increases ‘the possibilities of different compounds being made with different combinations,’ Yaghi explains.
Neil Champness at the University of Birmingham, UK, shares Yaghi’s enthusiasm for these materials. ‘There were already enormous numbers of MOFs, but [these multivariate materials] basically take the cap off… [it’s] essentially an infinite number.’ While for many chemists this may seem daunting, Champness thinks, like Yaghi, that chemists should embrace complexity. ‘You can make 1000 different MOFs until you’ve found the one which is right, or you [can] make a multivariate system that does the reaction anyway.’
Making MTV-MOFs
In 2010, Yaghi’s team at the University of California, Berkeley in the US produced one of the first MTV-MOFs by reacting zinc oxide clusters with functionalised terephthalic acid linkers. This led to a series of materials that had up to eight different organic ligands in one structure.1 ‘Typically in inorganic chemistry… when you have multiple building blocks coming together, you make multiple compounds,’ Yaghi explains. He describes realising that these framework materials don’t behave in the same way as a ‘big discovery’. This prompted Yaghi’s team to propose a new class of materials, which is when they coined the term MTV-MOFs to describe them.
Fifteen years later, his team has now created several MTV-MOFs, some of which have up to 36 different ligands.2 Yaghi’s group noted at the time that this particular material had a ‘record number of linkers and functionalities’ in a single framework. This may seem like an exercise in seeing just how complex these materials can become. But the team think that MTV-MOFs with this number of linkers could help chemists quickly identify which ligands or combinations thereof work best for a specific application. This drastically cuts the number of homogeneous MOFs that chemists would need to make, saving time and resources.
Yaghi’s team was able to synthesise such MTV-MOFs using similar methods to typical framework materials. This involves heating multiple linkers with metal building blocks in solution for several days, before repeatedly washing the crystals with solvent. Adjusting the ratio of the linkers and metals then allows chemists to create different MTV-MOFs.
However, this method doesn’t work for all MTV-MOF structures. For example, certain frameworks are challenging to make from scratch as introducing multiple ligands can affect their solubility or cause linkers to compete for interactions with metal nodes.
Chemists have since found alternative ways to synthesise these materials. Seth Cohen at the University of San Diego in the US says that as ‘these [framework] materials are just basically acting like big coordination compounds’, chemists can exploit this property to make MTV-MOFs. This involves swapping out organic linkers within a pre-made framework – a technique known as postsynthetic ligand exchange. Typically, this is limited to replacing one linker with an organic molecule of a similar length and connectivity, as this helps maintain the MOF’s overall structure. Cohen adds that this process usually occurs from the outside-in, which can create layers that have different linkers. Leaving the reaction for longer would eventually replace all ligands, creating a different but homogenous MOF, rather than an MTV-MOF.
His team showed that this technique could work back in 2012 by swapping out the brominated ligands in a zirconium-based MOF for an amino-functionalised linker.3 Reacting the homogeneous amino- and brominated-MOF structures together also led to a MOF containing both linkers, as the ligands of either MOF partially exchanged.
Rather than swapping out ligands, chemists are also able to chemically modify the linkers themselves. Cohen’s team, for example, was able to react amino-functionalised ligands with anhydrides of various chain lengths, with conversion rates ranging from 11 to 99%.4 ‘Basically, any time we had a post-synthetic modification reaction on a ligand that didn’t go to completion, by definition, we had created a multivariate MOF,’ says Cohen.
These modifications allowed the group to alter the hydrophobicity of a zinc-based MOF, as well as its internal surface area. Synthesising these materials in this way ensured ‘reliable access to a family of closely related MOFs,’ the group noted, which made it easier to compare material properties. Ligand modification also works for various other functional groups, including azide click reactions5 to incorporate fluorescent tags or Schiff base reactions with amines.6
What are multivariate metal-organic frameworks?
A quick introduction to why complexity is the next challenge for MOF materials
What is a metal-organic-framework (MOF?)
MOFs are 3D network materials made of metal ions or clusters connected by organic linkers. They are often highly porous, with voids that can capture molecules selectively, making them useful in a wide variety of applications such as separations, sensing, gas storage and catalysis.
What is a multivariate MOF (MTV-MOF)?
Conventional MOFs typically contain a single type of metal node and linker, but MTV-MOFs incorporate multiple linkers and nodes. The result is a material that combines an ordered framework with chemical heterogeneity.
How do you make MTV-MOFs?
The simplest way is to mix the metals and linkers in a solvent, then heat, just like a normal MOF. Researchers often select linkers with similar shapes but different functional groups, thereby introducing extra functionality without changing the framework structure.
You can also make a MOF first and then modify it in various ways:
- Ligand exchange can be used to replace linkers with new ones
- Metal exchange can swap out some of the metal ions with different metals
- Chemical modification can transform functional groups on some of the linkers
Why are chemists excited by these materials?
MTV-MOFs offer a way to introduce complexity into highly ordered materials, mimicking the kind of controlled complexity found in biological systems.
An MTV-MOF could, for example, capture a molecule in one part of the framework and transform it chemically in another, allowing several functions to operate within a single material.
Is that as simple as it sounds?
No, it’s much harder. The feature that makes MTV-MOFs attractive also makes them difficult to study: complexity.
Like conventional crystalline MOFs, MTV-MOFs retain an ordered framework, but there is chemical diversity within it, blurring the line between order and disorder.
The lack of uniform composition means they are also much harder to characterise – the local composition varies across the material in ways that can be hard to predict.
Give me some examples
MTV-MOF-74 is based on a framework in which divalent metal ions are linked by 2,5-dioxidoterephthalate. It can contain up to 10 different metal ions and chemists can tune its overall composition to alter its properties. For example, introducing Ni2+ or Co2+ into Mg-MOF-74 can improve its stability in water.
MOF-303/MIL-160 combines two linkers, 3,5-pyrazoledicarboxylic acid from MOF-303 and 2,5-furandicarboxylic acid from MIL-160, with aluminium-based nodes. MOF-303 provides high carbon dioxide uptake, while MIL-160 offers stronger carbon dioxide selectivity. By combining both features, the MTVMOF achieves improved carbon dioxide separation from methane compared with either parent material.
All pictures © SciComm Studio/Anna Tanczos
The simplicity of the idea drags you in … and you get sucked into this whirlpool of possibilities
Aside from capturing biological-like activity in synthetic materials, Yaghi’s team has also created MTV-MOFs to harvest water from the atmosphere, even in desert regions with low humidity.11 The team has developed a series of aluminium-based MTV-MOFs by varying the ratio of two organic ligands during synthesis: a dicarboxylate thiophene and a dicarboxylate pyrazole. Increasing the proportion of pyrazole linkers makes it more energetically favourable for water molecules to adsorb at lower humidities. Equally, MOFs that have a higher proportion of thiophene can desorb water at lower temperatures, such as at night. Balancing the ratio of these linkers has led to an MTV-MOF with optimised properties, which the team has scaled up to produce several kilograms in one batch with yields reaching up to 90%. Yaghi’s team has recently filed a patent for this material.
For Yaghi, this water-harvesting material represents a shift from chemists simply making, characterising and studying a material, to pushing compounds ‘all the way to commercialisation’. ‘That would be what a chemistry lab would 1789376891 be doing – not just making the molecule and the material but also integrating that material into a device.’
Expanding the scope of these multivariate frameworks is going to require collaboration between chemists, as well as material scientists, biologists, mathematicians and physicists. ‘I think the beauty of MOFs is that almost anybody with a chemistry degree can come up with an idea for a new [structure],’ says Champness. ‘The simplicity of the idea drags you in, and then there’s endless complexity, and you kind of get sucked into this whirlpool of possibilities.’ For Champness, ‘multivariate [MOFs] are just kind of an extension of that experience’.
