Single Crystal Electron and Powder Diffraction of the Anhydrous and Dihydrate Crystalline Forms of a 24-Atom Triazine Macrocycle

• DOI: 10.1063/4.0000321
• Publication Date: 2025-04-10T18:24:05Z
• Authors: Joseph Reibenspies; Nattamai Bhuvanesh; Mark Del Campo; Casey Patterson-G...; Eric Simanek

The molecular structures of an anhydrous and dihydrate forms of a 24-atom triazine macrocycle containing proline and methine hydrazine residues, were determined by MicroED (Single Crystal Electron Diffraction, ScED, 3DED). The whole powder pattern profile decomposition (WPPD) Pawley refinement of the X-ray Powder Pattern (RT, XRPD) confirms the two crystalline forms of the macrocycle in the sample and indicates one or more additional unidentified crystalline states. The macrocycle is protonated at the triazines (pKa ∼6) by trifluoroacetic acid (TFA, pKa 0.25) to yield a dication. In the anhydrous form, the TFA anions form strong hydrogen bonds to the methine hydrazine groups (NHN=CH) while in the dihydrate the TFA anions form hydrogen bonds to water molecules. The macrocycle folds into a “taco shell” shape for both structures. The unit cell from MicroED for the anhydrous macrocycle (RT, SG P21212) is: a = 22.936(228)Å, b = 7.2766(222)Å, c = 12.4713(404) Å, V = 2081(96) Å3 and from the WPPD Pawley refinement (RT,XRPD): a = 22.754(18) Å, b = 7.475(6) Å, c = 12.562(13)Å, V = 2139(3) Å3. The dihydrate macrocycle MicroED cell (100K, SG C2) is: a = 15.074(406) Å, b = 12.206(156) Å, c = 23.8816(150) Å, β = 97.817(492)°,V = 4273(132)Å3 and from the WPPD Pawley refinement (RT) a = 15.093(3)Å, b = 12.241(3)Å, c = 23.900(6)Å, β = 98.092(14)°, V = 4379.2(2) Å3. The XRPD analysis showed that the refined unit cell parameters were consistent with the MicroED results for both forms. This study highlights the combined use of XRPD and MicroED to gain a comprehensive understanding of crystalline samples.