Host-guest networks hydrogen-bonded

Structural reasons must exist for explaining such a behaviour. They concern two types of situations (i) the intrinsic flexibility of the framework itself, induced by the existence of weak points within the skeleton, which allow the deformation of the network under the action of the stimulus (this was explained for MIL-88 and MIL-53 [118]) and (ii) the host-guest interactions (hydrogen bonds, VDW forces, 71 — 71 interactions)... 

This host network, termed the helical tubuland structure type 7,8), is unique. The walls of the canals are lined only by aliphatic hydrocarbon, and the hydrogen bonded spines are insulated from the guest canals. Powder diffraction and IR measurements indicate that when 1 is crystallised from acetonitrile the same host crystal structure occurs, but devoid of guest. This has the unusually low calculated density, 1.02 g cm 3. 

The melting point of the empty host crystals is 189-191 °C. Therefore it was apparent that this is an independently stable host structure containing large yet chemically inert guest canals, in which guest species cannot interfere with the strong hydrogen bonds which maintain the host network. 

The network structures to be discussed will all involved hydrogen bonding as the supramolecular synthon. It should be noted however that other interactions such as coordinate bonds and host-guest interactions may also organise host molecules into network structures. Coordination polymers constructed from molecular hosts may involve functionalised calixarenes [8-11], cyclotriveratrylene [12], or cucurbituril [13]. Calixarenes have also been used to build up network structures via host-guest interactions [14,15]. It is also notable that volatile species may be trapped within the solid state lattice of calix[4] arene with a structure entirely composed of van der Waals interactions [16]. 

---