*2.6. Single-Crystal X-ray Diffraction (SCXRD)*

Measured crystals were prepared under inert conditions immersed in perfluoropolyether as protecting oil for manipulation. Suitable crystals were mounted on MiTeGen Micromounts™ (MiTeGen, Ithaca, NY, USA), and these samples were used for data collection. Data for FUR–ETZ and FUR–PRX were collected with a Bruker D8 Venture diffractometer (Bruker-AXS, Karlsruhe, Germany) with graphite monochromated MoKα (FUR–ETZ, λ = 0.71073 Å, at 298(2) K) or CuKα radiation (FUR–PRX, λ = 1.54178 Å, at 298(2) K). The data were processed with APEX3 suite [22]. The structures were solved by Intrinsic Phasing using the ShelXT program [23], which revealed the position of all non-hydrogen atoms. These atoms were refined on F2 by a full-matrix least-squares procedure using

anisotropic displacement parameter [24]. All hydrogen atoms were located in difference Fourier maps and included as fixed contributions riding on attached atoms with isotropic thermal displacement parameters 1.2- or 1.5-times those of the respective atom. The OLEX2 software was used as a graphical interface [25]. Intermolecular interactions were calculated using PLATON [26]. Molecular graphics were generated using Mercury [27]. The crystallographic data for the reported structures were deposited with the Cambridge Crystallographic Data Center as supplementary publication No. CCDC 2114160 and 2114161. Additional crystal data are shown in Table 1. Copies of the data can be obtained free of charge at http://www.ccdc.cam.ac.uk/products/csd/request (accessed on 30 October 2021).


**Table 1.** Crystallographic data and structure refinement details of FUR cocrystals.
