Highly conjugated systems with pedal motion in uranyl crotonate compounds with 1,2-bis(4-pyridyl)ethylene as a neutral ligand or a counter cation

Abstract

Crystal structures of [UO2(crt)2(bpe)2] (I) and (H2bpe)2(OH)2[UO2(crt)3]2[UO2(crt)2(H2O)2] (II), where crt = crotonate ion C3H5COO− and bpe = trans-1,2-bis(4-pyridyl)ethylene C12H10N2, were solved via single crystal X-ray diffraction analysis and characterized via FTIR spectroscopy. Even though I and II are composed of similar species, their FTIR spectra feature certain differences that can be used for rapid identification of the two phases. The bpe species in I and II act either as monodentate ligands coordinated to uranyl ions through nitrogen atoms, or as protonated counter-cations. Both compounds represent π-systems conjugated in all three dimensions due to the prevalence of aromatic pyridine rings, double CC bonds and carboxylate anions with lots of π-stacking, CHπ and CHN interactions. The XRD data shows temperature variable disorder patterns of one bpe specie in II implying its pedal motion at 120 K. Analysis of all available to date uranyl compounds with bpe molecules/cations in the CSD by means of Voronoi–Dirichlet tessellation is provided. It reveals that the arrangement of bpe is topologically equivalent in most of the studied compounds and represents the thinnest covering of 3D space with 14 neighbors as opposed to classical densest sphere packing with 12 neighbors. This is consistent with the earlier proposed mechanism of crystal growth which implies such a packing of bulky species that allows to minimize the overlap of their outer electron shells.