Highly Holey: MOFs with Record-Breaking Surface Areas

​Researchers at Northwestern University in the US have recently published a series of metal-organic crystal structures with record-breaking microporous properties. The structures (known as NU-109E and NU-110E) are part of a set of compounds known as metal-organic frameworks, or MOFs. Synthesis and study of these MOF compounds is a highly popular area of research at the moment due to their potential for use as gas storage materials, molecular sieves, catalytic reaction vessels and even drug delivery systems.

The packed unit cell of MOF structure NU-110E (CSD refcode SEMNIJ) shown using capped sticks (left) and with the framework replaced by the contact surface (right) as calculated by Mercury

These two newly published structures are now available through WebCSD, under the CSD refcodes SEMNIJ (see image above) and SEMNEF, alongside a growing number of structurally characterised MOFs. Although the Cambridge Structural Database (CSD) is often regarded as being focussed just on small organic molecules, it does in fact contain both organics and metal-organics. Over half of the structures in the CSD (~54%) actually now contain at least one transition metal atom.

We are often asked at the CCDC how many metal-organic framework structures there are in the database, but this quite a tricky question and depends to a large extent on the scientific definition of a MOF. The least controversial statistics on this are based on a simple search consisting of a transition metal bonded to an oxygen or nitrogen via a polymeric bond. This ensures the metal-organic nature of the structure and the repeating framework, but does not specifically require a 3D coordination-bonded network (in line with provisional recommendations on MOF terminology from IUPAC). This search returns 38,062 structures based on version 5.34 of the CSD with updates up to and including November 2012. That number equates to roughly 6% of the CSD, a proportion that has been steadily rising over the last few decades (see graph below). For more up-to-date statistics on MOFs, please re-run the search using your own version of the CSD System.  

Graph of the number of MOFs published per year since 1970 (left) and the change in the percentage of MOFs in the overall CSD since 1970 (right) 

It is clear that the metal-organic framework research field is still expanding quickly and we are looking forward to seeing many more MOFs in the CSD in the future as well as finding out more about the industrial applications of these exciting materials.