During last decades we have been using organometallic cyclo-Pn complexes (n = 4, 5) as building blocks for the rational design of giant supramolecules, up to 4.6 nm in size [1-4]. The supramolecules consisting of hundreds of atoms frequently demonstrate weak scattering power due to the severe crystallographic disorder. In many cases, the use the high-flux synchrotron sources becomes the only remedy.
The samples of organometallic supramolecules must be permanently protected from oxygen to avoid the oxidation of cyclo-Pn fragments. Any traces of water in the organic solvents also should be avoided. Therefore, the proper sample handling requires a vacuum-argon line (Schlenk line) that protects the sample from air during the extraction of the portion of crystals for the diffraction study. Within the long-term project II-20180597 with DESY (Hamburg, Germany) we have installed such line in the sample preparation lab of P24 beamline.
To obtain quality data at dmin > 1 Å for giant supramolecules we developed optimal strategies to perform the single-crystal diffraction experiments at both P11 and P24 beamlines. For P11 beamline, the most critical is to choose radiation energy as a compromise between dmin and quantum efficiency (QE) of a PILATUS 6M detector. Higher energies improves the resolution by the cost of significantly lower QE.
The optics at the P24 beamline allows using hard X-ray radiation with E up to 44 keV. It helps in reducing the absorption and radiation damage in Ag and Ta-containing crystals. Helium open-flow cryostat provides temperature down to 10 K.
Optimization of the experimental strategy allowed us to obtain high-quality diffraction data even from weakly scattering crystals (Fig. 1) and to investigate such subtle structural effects as superstructural ordering.
This work was supported by the German Research Foundation (DFG) within the project Sche 384/44-1.
Fig. 1a) Giant cationic supramolecule [(Cp''Fe(η5-P5)12(CuNCCH3)8]8+ with an external diameter of 2.5 nm . Hydrogen atoms are omitted for clarity; b) the diffraction pattern from the single crystal at P11 beamline (E = 18 keV, 0.1° scan)
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