Topological crystal chemistry: Polycatenation weaves a 3D web

news & views

TOPOLOGICAL CRYSTAL CHEMISTRY

Polycatenation weaves a 3D web

Mechanical linking of small cage structures leads to a type of metalorganic framework with an architecture topologically distinct from those constructed so far.

Davide M. Proserpio

a

b

c

Figure 1 | Interpenetrated and polycatenated arrays. a, The Hopf link (left) is the basic unit of inextricable entanglement. Multiple Hopf links (right) result in an [n]-catenane and an increase in dimensionality. b, Polycatenation of 0D cages results in a 3D octahedral array of cages. In the nal structure, two of these octahedral arrays (shown in blue and red) are interpenetrated. c, Cd(imidazolate)2 is an example of the type of MOF more commonly observed in which there is only interpenetration, and precisely twofold dia. This MOF is formed between cadmium ions acting as nodes and the organic imidazolate ligands acting as bidentate linkers.

20 Macmillan Publishers Limited. All rights reserved

10

Metalorganic frameworks (MOFs) and porous coordination polymers are promising materials for a variety

of applications, including sorption, selection, catalysis, sensing or microelectronics, and the eorts of many research groups worldwide have produced numerous extended systemsof ever growing structural complexity1. A mathematical (topological) description ofthe structure of these complex systems is necessary to achieve a correct classication and thus help to clarify the fundamental relationship between structure and properties. Indeed, new topological congurationsare continuously discovered, including mechanically linked arrays derived from the simplest two-ring link, the Hopf link (Fig. 1). Hopf links are well known to the chemistry community as they appear in catenanes compounds that consist of molecular rings held together by mechanical bonds2.

A recent review3 suggested the extensionof catenation to create innite objects inone, two or even three dimensions by the formation of several Hopf links. At the time, however, no real examples had actually been observed. It is only recently that a one-dimensional (1D) [n]-catenane (Fig. 1a) has been observed in the realm of coordination polymers4, and no examples extending into two or three dimensions have appeared until now. Writing in this issue of Nature Chemistry, Can-Zhong Lu and co-workers5 describe the exciting discovery of a coordination material in which the catenation of adamantane-like molecular cages considered...