In Search of Organic Zeolites – Does Modern Information Retrieval Inevitably Become a ‘Sieving-the-Desert' Exercise?

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday Information retrieval for planning and executing research projects and for publishing results is considered a routine task that is usually neither mentioned explicitly in a scientific publication nor described in any detail. In the information searches for the preceding publication (−Building an Organic Zeolite from a Macrocyclic TADDOL Derivative or How to Teach an Old Dog New Tricks×), we were confronted with so many problems during retrieval of the desired information about related work that we decided to deviate from this tradition. We had to use the Cambridge Structural Database, the Chemical Abstracts structure and literature databases, and the Beilstein database to the full extent of their contents, indexing, and search facilities to retrieve the necessary information about −organic zeolites×. In the process, we found important limitations and deficiencies in any one of these databases, and we had to conceive search procedures that we considered rather unusual even after more than 20 years of experience in searching chemistry databases. The results and, particularly, the problems encountered underline the necessity for enhanced integration of individual compound and property databases and improved standardization as a prerequisite for this.

Nitrogen and Hydrogen Adsorption by an Organic Microporous Crystal

“…These organic clathrates are usually unstable toward the removal of the included species, although some allow the exchange of one type of guest for another and a few survive the evacuation of included solvent 6. Such crystals are sometimes termed “organic zeolites”,7, 8 although their structures and properties have little in common with those of the zeolites 9. In particular, rapid and reversible gas adsorption is a difficult challenge for microporous organic crystals 10.…”

Nitrogen and Hydrogen Adsorption by an Organic Microporous Crystal

“…[6] Such crystals are sometimes termed "organic zeolites", [7,8] although their structures and properties have little in common with those of the zeolites. [9] In particular, rapid and reversible gas adsorption is a difficult challenge for microporous organic crystals.…”

“…Hence, we set out to discover unrecognized microporous crystals by searching through the many crystal structures (more than 4.5 10 5 ) held within the Cambridge Structural Database (CSD). Such an approach has been likened previously to "sieving the desert", [8] however, we narrowed the search criteria to identify candidates that might possess enhanced microporosity over existing examples of microporous crystals. The initial structure selection was based upon the following criteria: 1) a calculated density of less than 0.9 g cm À3 (the lowest density of any established microporous organic crystal is 0.96 g cm À3 for p-tert-butylcalix [4]dihydroquinone on removal of included water), [16,17] 2) the crystal should be composed predominantly of rigid aromatic molecules to aid stability, and 3) the apparent porosity should exist as small micropores (diameter < 10 ) to be suitable for strong gas adsorption through multiwall interactions.…”

Building an Organic Zeolite from a Macrocyclic TADDOL Derivative or How to Teach an Old Dog New Tricks

“…Since H-bonds constitute the strongest intermolecular interactions, it is not surprising that H-bridged frameworks often constitute the basic structural motif in other examples of purely organic, nanoporous structures [31] [32]. An exhaustive literature search has shown that, in any case, the total number of organic isomorphous one-and two-component crystal structures is very small [25]. Only one of these resembles the structure of A in that the inclusion compound of octakis(m-tolylthio)naphthalene and its −empty× form are lacking stabilization by strong intermolecular forces, the frameworks being held together only by hydrophobic interactions [33].…”

“…± The crystal structure of A includes for some structural reasons [...] vacant spaces in the solid state that are suited to accommodate guest molecules [34]. To the best of our knowledge [25], this system constitutes the only case of a solid, purely organic compound held together by hydrophobic interactions and where the guests can be removed reversibly while the crystal lattice remains intact. This behavior demonstrates that the vacant spaces are in the form of open channels providing ingress and egress [34].…”