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Pressure is known to trigger unusual chemical reactivity in molecular solids. In particular, small molecules containing unsaturated bonds are subject to oligo- or polymerization, effectively synthesizing new compounds. These are tipically energetic materials which can be amorphous, as in the case of carbon monoxide,[1] or crystalline, as for carbon dioxide phase V.[2] In more complex molecular systems, where unsaturated bonds can be only one of the present moieties, stereo-controlled reactivity can be exploited to synthesize topo-tactic structures. We performed a synchrotron single crystal experiment on oxalic acid dihydrate up to 54.7 GPa, using He as pressure transmitting medium to ensure hydrostatic behavior. This is, to the best of our knowledge, the highest pressure ever achieved in a single crystal study on an organic molecule. It had been reported that the species undergoes a proton transfer at mild pressures,[3] and further compression confirms the major role played by hydrogen bonds. After the proton transfer, the species undergoes two phase transitions, caused mainly by a rearrangement of hydrogen bonding patterns, that does not demage the singly crystal nature of the sample. At ~40 GPa an initial bending of the flat oxalic molecule is observed, sign of an enhanced nucleophilic interaction between one oxygen and the carbon of a neighbor molecule. At the highest pressure achieved, a further phase transition is observed. Although the crystallinity is decreased, the new unit cell shows a drastic shrinking in one specific direction. Periodic DFT calculations reveal this metric is compatible with an ordered polymerization of the oxalic acid created by a nucleophilic addition: a monodimensional covalent organic framework is the resulting material (figure). This observation, unique up to now in its kind, is of high relevance for crystal engineering and highlights the potential of high pressure to stimulate new chemistry.
abstracts
Accurate electron density mapping is quite a common practice for crystals cooled at low temperature and accurately measured. This is not true for species under external perturbation, due to complicated experimental conditions. Studying molecular crystals in excited states is a challenge, Coppens(2009), and a purely experimental electron density mapping is not possible at present. So far, the same limitation affected molecular crystals at high pressure, with only few attempts to use theoretical multipoles to fit experimental data, Fabbiani (2011). Here we report on the first unconstrained multipolar model, refined for syn-l,6;8,13 biscarbonyl[14]annulene (BCA) at P=7.7 GPa. BCA was the subject of a low temperature data collection by Destro (1995). The molecule (close to C2v symmetry) has a fair aromaticity, but it progressively localizes double and single bonds as a function of pressure. At 7.7 GPa the geometrical distortion is quite evident and mirrored by the electron density. The experiment, carried out at Diamond Light Source, was possible combining: a) high energy (40 Kev) to overcome the resolution problems caused by diamond anvil cells and reduce absorption and extinction; b) microfocused beam (30 micron) to minimize spurious X-ray diffusion; c) two crystals in the DAC, to increase data coverage; d) sufficient pressure to quench atomic motion. The final agreement is obviously worse than what typically obtained at ambient pressure. However, the model is satisfactory because: a) the deformation density is sensible and in agreement with the calculated one; b) the distribution of residuals is normal and no significant error is evident. The study proves that aromatic molecules are more reactive when squeezed, in keeping with the recent theoretical study by Hoffmann et al. on benzene. The Figure shows the static deformation density of BCA in 3D, obtained from a multipolar model refined against the experimental structure factors.
