Mark Kalaj

Harnessing uranyl oxo atoms via halogen bonding interactions in molecular uranyl materials featuring 2,5-diiodobenzoic acid and N-donor capping ligands

The syntheses and crystal structures of five new compounds containing the UO22+ cation, 2,5-diiodobenzoic acid, and a chelating N-donor (2,2′-bipyridine (bipy) (1), 1,10-phenanthroline (phen) (2 and 3), 2,2′:6′,2′′-terpyridine (terpy) (4), or 4′-chloro-2,2′:6′,2′′-terpyridine (Cl-terpy) (5)) are described and the spectroscopic properties (both vibrational and luminescent) and stretching and interaction force constants of complexes 2, 4, and 5 are reported. Single crystal X-ray diffraction analysis of these materials shows that variation of the chelating N-donor with the same benzoic acid featuring multiple, polarizable halogens at the periphery allows for the systematic accessing of uranyl oxo atoms for non-covalent assembly, which is notable as these atoms are generally terminal. Spectroscopic characterization of complexes 2, 4, and 5 indicate that oxo atom participation in halogen bonding interactions may complement the effects of the electron donating ability of the capping ligand on corresponding uranyl luminescence and vibrational spectra, each contributing to the observed bathochromic shifts.

Utilizing bifurcated halogen-bonding interactions with the uranyl oxo group in the assembly of a UO2–3-bromo-5-iodo­benzoic acid coordination polymer

The synthesis and crystal structure of a new uranyl coordination polymer featuring 3-bromo-5-iodobenzoic acid is described and the luminescent and vibrational properties of the material have been explored. Compound (1), [UO2(C7H3BrIO2)2]n, features dimeric uranyl units chelated and then linked by 3-bromo-5-iodobenzoic acid ligands to form a one-dimensional coordination polymer that is subsequently assembled via bifurcated halogen-bonding interactions with uranyl oxo atoms to form a supramolecular three-dimensional network. The asymmetric, bifurcated halogen-bonding interaction in (1) is notable as it represents the first observation of this synthon in a uranyl hybrid material. Raman and IR spectroscopy showed that halogen-bonding interactions with the uranyl oxo atoms result in small shifts in υ1 and υ3 frequencies, whereas luminescence spectra collected at an excitation wavelength of 420 nm reveal partially resolved uranyl emission.

Restricted Speciation and Supramolecular Assembly in the 5f Block

Hybrid materials bearing elements from the 5f block display a rich diversity of coordination geometries, connectivities, and assembly motifs. Exemplary in this regard have been uranyl coordination polymers, which feature a wide range of secondary building units resulting from hydrolysis and oligomerization of the [UO2 ]2+ cation. An alternative approach to novel materials, however, suppresses hydrolysis and relies on non-covalent interactions (e.g. hydrogen or halogen bonding) to direct assembly of a more limited suite of species or building units. This may be achieved through the use of high-anion media to promote singular actinyl anions that are assembled with organic cations, or by way of functionalized chelating ligands that produce complexes suited for assembly through peripheral donor/acceptor sites. Presented in this Concept article is therefore an overview of our efforts in this arena. We highlight examples of each approach, share our thoughts regarding delineation of assembly criteria, and discuss the opportunities for exploring structure-property relationships in these systems.

Probing hydrogen and halogen-oxo interactions in uranyl coordination polymers: a combined crystallographic and computational study

The syntheses and crystal structures of four compounds containing the UO22+ cation and either benzoic acid (1), m-chlorobenzoic acid (2), m-bromobenzoic acid (3), or m-iodobenzoic acid (4) are described and the vibrational spectroscopic properties for compounds 3 and 4 are reported. Single crystal X-ray diffraction analysis of these materials shows that uranyl oxo atoms are engaged in non-covalent assembly via either hydrogen (1 and 2) or halogen bonding (3 and 4) interactions. The halogen bonding in compounds 3 and 4 is notable as the crystallographic metric percentage of the sum of the van der Waals radii indicates these interactions are of similar strength. Characteristics of the halogen-oxo interactions of 3 and 4 were probed via Raman and infrared spectroscopy, which revealed significant differences in stretching frequency values for the two compounds. Additionally, compounds 3 and 4 were characterized via quantum chemical calculations and density-based quantum theory of atoms in molecules (QTAIM) analysis, which indicated that the I-oxo interaction in 4 is likely the stronger of the two interactions, with differences between the two interactions resulting from both inductive effects and halogen polarizability.

Engaging the terminal: highlighting routes for promoting non-covalent interactions with uranyl oxo atoms

Harnessing the nominally terminal oxo atoms of the linear uranyl (UO2) cation represents both a significant challenge and opportunity within the field of f-element hybrid materials. We have developed multiple approaches for promoting oxo atom participation in halogen-oxo and cation-cation interactions via synthetic strategies based on the judicious selection of halogen atoms and selected transition metal (TM) cations. These synthesis efforts have yielded a diverse suite of hybrid materials including uranyl molecular complexes, coordination polymers, and heterometallic hybrid materials, which have all been characterized via single crystal X-ray diffraction, Raman, Infrared (IR), and luminescence spectroscopy. Raman and IR spectroscopy results are used to generate stretching force and interaction force constants, which indicate that both halogen-oxo and cation-cation interactions weaken the U=O bond. Presented will be an overview of the routes that yield oxo participation in non-covalent interactions along with the relevant spectroscopy data that highlight our ongoing efforts to enhance structure-property delineations in this area at the frontier of uranyl crystal chemistry.