Synthesis and crystal structure of 2,4,6,8-tetrakis(3,5-di-tert-butylphenoxy)pyrimido[5,4-d]pyrimidine: expansion of the Piedfort concept

The title host compound, C62H84N4O4, designed to self-assemble to form a new type of extended core Piedfort unit reminiscent of an eight-legged spider host, forms a number of crystalline inclusion compounds favouring oxygen-containing guest molecules. We have established the presence of this unit in the unsolvated molecular crystal at 100 K, which is monoclinic, space group P21/n, with Z = 8. The new Piedfort unit is chiral and its core structure closely approximates to D 2 symmetry, with both enantiomers present in the crystal. Rather than being superposed with a staggered arrangement of nitrogen atoms, the rings are rotated by an angle of approximately 45° with respect to each other, and the shortest contact between them is 3.181 (2) Å. The compound's significant inclusion properties may be taken to suggest the participation of an extended Piedfort unit in the microcrystalline adducts formed. The presence of such a dimeric host unit in the clathrates has, however, not yet been established because of the current lack of suitable single crystals for X-ray analysis.

Hydrogen-rich Inclusion Compounds at High-pressure

Molecular hydrogen (H2) has been proposed as an alternative fuel source for vehicles. Though H2has many benefits, such as clean combustion and the highest known energy density by mass, there are issues in how to store it in a safe and cost effective way. One solution is to store hydrogen in a chemical compound, and gas clathrates (crystalline inclusion compounds) have shown promising results. Pressure provides a powerful means to tune the properties of such compounds and its effects on potential hydrogen storage materials are widely explored. We have recently developed a hydrogen-compatible gas loader for the Paris-Edinburgh press, which enables the loading of high density hydrogen into a clamp with a sample volume suitable for neutron diffraction experiments using the Paris-Edinburgh press [1]. Neutron diffraction is the technique of choice for such materials since it can reveal the location and occupancy of the hydrogen sites. We will present recent data from high-pressure neutron diffraction experiments on hydrogen hydrates as well as other clathrate forming systems like urea and hydroquinone.