Arbin Rajbanshi

Mapping out the synthetic landscape for re-crystallization, co-crystallization and salt formation

In order to examine the balance between co-crystallization and proton transfer in a set of acid–base reactions, molecular electrostatic potential (MEP) surface calculations for substituted pyridines were correlated with their ability to communicate with a series of carboxylic acids via intermolecular interactions in the solid state. The calculated (AM1) charges on the hydrogen-bond acceptor in these N-heterocyclic compounds provide overall excellent guidelines for predicting when a salt or a co-crystal will form. The charges can also be related to the supramolecular yield of the reactions between seven derivatives of 2-aminopyridine and fifteen aromatic/aliphatic carboxylic acids. The outcome of all reactions was screened using IR spectroscopy, and twelve crystal structures were used to verify the spectroscopic assignments and to examine the exact nature of the primary intermolecular interactions.

N, N′-Dicyclohexyl-N″-Isotridecylguanidine as Suppressor for the Next Generation Caustic Side Solvent Extraction (NG-CSSX) Process

The purity, concentration, and source of the N,N′-dicyclohexyl-N″-isotridecylguanidine (DCiTG) suppressor (guanidine) used in the NG-CSSX process were found to influence solvent performance. As the starting isotridecanol used in the preparation of DCiTG is comprised of a mixture of branched-chain aliphatic alcohols, varying in composition with manufacturer, the resulting DCiTG itself is a mixture. Thus, it is necessary to address how the solvent performance will be affected by the different preparations of the DCiTG solvent component. In this study, four preparations of DCiTG from three sources were analyzed and evaluated for purity and performance, both in the absence and presence of a deliberately added anionic surfactant impurity.

Exploring the structural landscape of 2-aminopyrazines via co-crystallizations

A correlation between the electrostatic charge on the hydrogen-bond acceptor sites of 2-aminopyrazine derivatives and the ability of the compound to form intermolecular hydrogen bonds with carboxylic acids in the solid state has been established. The charge on the hydrogen-bond acceptor can be modulated which leads to a predictable lowering of the supramolecular yield of the reaction. The outcome of all reactions was screened using IR spectroscopy, and twelve new crystal structures are reported to verify the spectroscopic assignments, and to examine the exact nature of the primary intermolecular interactions. The binding preference of carboxylic acids towards the two possible binding sites of 2-aminopyrazines has also been examined, and the main driving force for the assembly of the heteromer between bases and carboxylic acids is the two-point O–H⋯N/O⋯H–N synthon. However, seven out of twelve times carboxylic acids also bind via a single-point O–H⋯N synthons. This ‘synthon crossover’ is unavoidable due to highly competitive binding sites present in the N-heterocyclic bases chosen.

Structure and selectivity trends in crystalline urea-functionalised anion-binding capsules

A tripodal trisurea receptor (L1) persistently self-assembles with various divalent oxoanion salts M n X (M = Na, K, Mg, Ca, Cd; X = SO4 2 − , SO3 2 − , SeO4 2 − , CrO4 2 − ) into isomorphous series of crystalline frameworks in three different compositions: MX(L1)2(H2O)6 (M = Mg, Ca, Cd) (1), Na2X(L1)2(H2O)4 (2) and K2X(L1)2(H2O)2 (3). Single-crystal X-ray structural analysis revealed that all three series of structures adopt a NaCl-type topology, consisting of alternating anionic X(L1)2 2 − capsules and M(H2O)6 2+, Na2(H2O)4 2+ or K2(H2O)2 2+ hydrated cations. The capsules provide a complementary environment to tetrahedral oxoanions via 12 hydrogen bonds from six urea groups lining the cavities of the capsules. The persistent formation of the capsules facilitated the investigation of structural trends and structure–selectivity relationships across series 1–3. First, it was found that the size of the capsules is relatively unresponsive to the change in the encapsulated anion, resulting in good shape and size recognition in the separation of anions by competitive crystallisations. Second, it was found that the size of the capsules varies linearly with the size of the external cation, which provides a way for tuning the anion encapsulation selectivity. However, no straightforward dependence was found between the size of the capsules and the relative selectivity for different-sized tetrahedral oxoanions in competitive crystallisations.

Sulfate Separation from Hanford Waste Simulants by Selective Crystallization of Urea-Functionalized Capsules

Crystallization of urea-functionalized capsules self-assembled from a tripodal anion receptor (L1) was evaluated as a means to selectively separate sulfate from aqueous alkaline solutions simulating Hanford waste compositions. The crystallizing solids consist of anionic capsules, and or hydrated cations, alternating in three-dimensional frameworks with NaCl-type topology. While both frameworks encapsulate sulfate selectively upon crystallization through the formation of complementary hydrogen bonds from the urea groups, the separation efficacy depends strongly on the nature of the cation, the pH, and the nature and concentration of competing anions in the solution. Crystallization of the Mg-based capsules provides an efficient sulfate separation from mildly alkaline solutions (pH < 9.5), with more basic conditions leading instead to Mg(OH)2 and L1 precipitation. On the other hand, crystallization of the Na-based capsules proved efficient from highly alkaline solutions (pH = 14) with compositions similar to those found in the Hanford wastes.