Robert G. Surbella

The exploration of supramolecular interactions stemming from the [UO2(NCS)4(H2O)]2− tecton and substituted pyridinium cations

The synthesis and crystal structures of eight compounds containing the anionic uranyl isothiocyanate tecton [UO2(NCS)4(H2O)]2−, charge balanced by substituted pyridinium cations (4XpyH: X = Cl, Br, I, SCN, CH3, NH2, N(CH3)2) are reported. Hydrogen bonding interactions between coordinated water molecules and the non-coordinated sulfur atoms (O–H2⋯S) of the adjoining tectons facilitate the formation of infinite ‘pseudo’ chains of inorganic tectons. The substituted pyridinium cations were paired with these fixed uranyl isothiocyanate tectons due to their ability to sustain multiple intermolecular interactions with the sulfur atoms of the uranyl tectons, thus promoting assembly. The variation of the substituent group bound to the organic cations allowed for the resulting non-covalent interactions to be systematically assessed in order to determine their relative effects upon packing. Additionally, respective acceptor–donor pairings as well as the relative strengths of the halogen, pseudohalogen [C–X⋯A (X = Cl, Br, I, SCN and A = O, S)] and hydrogen bonding [R–H⋯A (R = N, C and A = O, S, π)] interactions were investigated. The acceptor–donor relationships emulated hard–soft pairings and consequently increased in strength as a function of this trend.

Transuranic Hybrid Materials: Crystallographic and Computational Metrics of Supramolecular Assembly

Assembly of a family of 12 supramolecular compounds containing [AnO2Cl4]2- (An = U, Np, Pu), via hydrogen and halogen bonds donated by substituted 4-X-pyridinium cations (X = H, Cl, Br, I), is reported. These materials were prepared from a room-temperature synthesis wherein crystallization of unhydrolyzed and valence-pure [An(VI)O2Cl4]2- (An = U, Np, Pu) tectons is the norm. We present a hierarchy of assembly criteria based on crystallographic observations and subsequently quantify the strengths of the non-covalent interactions using Kohn-Sham density functional calculations. We provide, for the first time, a detailed description of the electrostatic potentials of the actinyl tetrahalide dianions and reconcile crystallographically observed structural motifs and non-covalent interaction acceptor-donor pairings. Our findings indicate that the average electrostatic potential across the halogen ligands (the acceptors) changes by only ∼2 kJ mol-1 across the AnO22+ series, indicating that the magnitude of the potential is independent of the metal center. The role of the cation is therefore critical in directing structural motifs and dictating the resulting hydrogen and halogen bond strengths, the former being stronger due to the positive charge centralized on the pyridyl nitrogen, N-H+. Subsequent analyses using the quantum theory of atoms in molecules and natural bond orbital approaches support this conclusion and highlight the structure-directing role of the cations. Whereas one can infer that Columbic attraction is the driver for assembly, the contribution of the non-covalent interaction is to direct the molecular-level arrangement (or disposition) of the tectons.

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.