The objectives of this project are:
- To produce guidelines for terms to use in the topological description of metal-organic frameworks, coordination polymers and other crystalline materials that can be described as network compounds.
- To produce guidelines for the use of topological descriptions in the mentioned areas.
- To ensure permanent and stable access to the database containing topological nets and net descriptors.
- To determine the procedure for depositing new network topologies.
- To elaborate recommendations for including the information about network descriptors and topological properties into crystallographic databases.
This project resolves outstanding issues identified in project 2009-012-2-200: “Coordination polymers and metal organic frameworks: terminology and nomenclature guidelines” as reported in Pure Appl. Chem., Vol. 85, No. 8, pp. 1715-1724, 2013 (https://dx.doi.org/10.1351/PAC-REC-12-11-20)
Network solids comprise classical and technological important materials such as diamond and quartz as well as many allotropes of the important group 14 elements. The new materials known as metal-organic frameworks (MOFs) and coordination polymers are also in the majority of cases network solids (more specifically coordination networks). In addition many crystalline materials, be they pure organic molecules, inorganic or coordination compounds, can be well described as forming networks.
In current literature scientist’s description of these network compounds range from simply stating “compound X forms a 3D-network” to the use of a set of well defined terms and topology descriptors found in the Reticular Chemistry Structural Resource* database and the TOPOS# TTD collection. The preceding IUPAC project (PAC 2013, see above) strongly suggested the use of these descriptors, but could not make definitive recommendations as issues with terms, procedures for determining the topology, and the long-term availability of the database were identified.
This project aims at resolving these issues, specifically:
- Topology descriptors
- Terms and the corresponding numerical parameters (topological indices) used in the identification of these topologies using computer programs. (coordination sequence, vertex, point symbol, etc.)
- Terms used in the detailed discussion about network topologies, how they are determined and related. (rings, strong rings, dual nets etc.)
- Terms used for description of topological properties of network solids (dimensionality, periodicity, entanglement, coordination figure, etc.)
The two first points are aimed directly at the needs of the synthetic chemist or crystallographer whereas the third and fourth issues are specialist questions, but nevertheless the foundation on which everyday usage is built.
B: Procedures for determining a network topology
Currently two major methods are in use for the assignment of nodes and linkers in a network compound. The first is strictly based on the connectivity of the individual molecular units used in the synthesis. The second is a more flexible approach where both the mapping of the network on the actual structure using Structural Building Units (SBUs) and a possible expansion of the SBUs to a series of isoreticular compounds are considered. At the very least the two methods need to be clearly distinguished and have names so that possible confusion can be eliminated. At the least, clear guidance will be given and a way of pointing out the relationships between these topologies will be recommended. What we hope to do, however, is to suggest strict algorithms for these methods to avoid subjectivity in the description of network solids.
C: Securing and aligning the Reticular Chemistry Structural Resource database and the TOPOS TTD collection
The RCSR database has been compiled over the last 15 years by Michael O’Keeffe and co-workers. It is an invaluable source for researchers in the field and is currently hosted by the Australian National University Supercomputer Centre in Canberra. The TOPOS TTD collection is supported by a research group in Samara State University, Russia, since 2008. The relation between the two databases needs to be clarified. Moreover, they need permanent homes (RCSR) and the support of an international organization (could be the IUCr). Another way of securing the data is to publish the current contents of the RCSR as an e-book. Both questions need detailed scrutiny. These databases could hopefully be approved by IUPAC and IUCr as world-wide collections of crystal network topologies.
D. Determining the procedure for depositing new network topologies
An important point is to establish a procedure for depositing new network topologies. When a new network solid is obtained the authors should have a clear instruction how to assign a descriptor to the network, and how to deposit the network topological indices to the world-wide collections.
E. Recommendations for including the information about network descriptors and topological properties into crystallographic databases
At present, the information on occurrence of network topologies in crystalline solids is stored in the RCSR and the TOPOS TTD collection. It should be recommended to include descriptors and topological properties of networks into crystallographic databases (CSD, ICSD, Pearson’s Crystal Data). The format and method of deposition of such information should be determined.
* Average monthly access to RCSR is around 1050 visits per month, from 87 countries in total (44 of those have visits in double figures).
# Current number of TOPOS users registered: 2746 from 70 countries
April 2015 update: A workshop including lectures by members of the task group, will take place in Samara, Russia, 21-23 May 2015. The workshop is organized by the Samara Center for Theoretical Materials Science
October 2015 update:
– Report from the project meeting and workshop was held in Samara: https://english.sctms.ru/novosti_centra/20150619_01/
– Poster presentation EuroMOF Berlin (Potsdam, 11-14 Oct 2015) Metal-Organic Frameworks (https://www.dechema.de/euromof2015.html): doi.org/10.13140/RG.2.1.1875.0169
January 2017 update:
A further update will be part of the TGC upcoming oral presentation at the ACS meeting in San Francisco in April 2017.
May 2018 update:
See Cryst. Growth Des., 2018, 18 (6), pp 3411–3418 (http://doi.org/10.1021/acs.cgd.8b00126) on the Relevance for Terminology Guidelines and Crystallographic Databases.
This communication briefly reviews why network topology is an important tool (for understanding, comparing, communicating, designing, and solving crystal structures from powder diffraction data) and then discusses the terms of this IUPAC project dealing with various aspects of network topology. One is the ambiguity in node assignment, and this question is addressed in more detail. First, we define the most important approaches: the “all node” deconstruction considering all branch points of the linkers, the “single node” deconstruction considering only components mixed, and the ToposPro “standard representation” also considering linkers as one node but, if present, takes each metal atom as a separate node. These methods are applied to a number of metal–organic framework structures (MOFs, although this is just one example of materials this method is applicable on), and it is concluded that the “all node” method potentially yields more information on the structure in question but cannot be recommended as the only way of reporting the network topology. In addition, several terms needing definitions are discussed.
Page last updated 3 January 2019