This contribution addresses the role of water molecules in crystal engineering by studying the crystal structures and thermal stabilities of 11 new cocrystal hydrates, all of which were characterized by single crystal X-ray crystallography, powder X-ray diffraction (PXRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The cocrystal hydrates can be grouped into four categories based upon thermal stability: (1) water is lost at <100 °C; (2) water is lost between 100 and 120 °C; (3) water is lost at >120 °C; (4) dehydration occurs concurrently with the melt of the cocrystal. In order to address if there is any correlation between structure and stability, the following factors were considered: type of hydrate (tunnel hydrate or isolated hydrate); number of hydrogen bond donors and acceptors; hydrogen bond distances; packing efficiency. Category 1 hydrates exhibit water molecules in tunnels. However, no structure/stability correlations exist in any of the other categories of hydrate. To complement the cocrystal hydrates reported herein, a Cambridge Structural Database (CSD) analysis was conducted in order to address the supramolecular heterosynthons that water molecules exhibit with two of the most relevant functional groups in the context of active pharmaceutical ingredients, carboxylic acids, and alcohols. The CSD analysis suggests that, unlike cocrystals, there is great diversity in the supramolecular heterosynthons exhibited by water molecules when they form hydrogen bonds with carboxylic acids or alcohols. It can therefore be concluded that the promiscuity of water molecules in terms of their supramolecular synthons and their unpredictable thermal stability makes them a special challenge in the context of crystal engineering.
A second-generation aziridination catalyst supported by a borate-based dianionic macrocyclic tetracarbene ligand has been synthesized. The new macrocyclic tetracarbene iron(II) complex catalyzed the aziridination of alkyl azides and aliphatic alkenes showcasing the first fully aliphatic version of this C 2 + N 1 reaction. High isolated yields were obtained when no functional groups were present on the organic azides and alkenes, while modest yields were achieved when nonprotic functional groups were included. Even multiple functional groups can be added to the azide and alkene fragments to produce the most complex aziridines yet synthesized by this C 2 + N 1 catalytic reaction. The catalyst generated higher yields for aziridination with aryl azides and alkenes than the previously reported catalyst, [( Me,Et TC Ph )Fe(NCCH 3 ) 2 ](PF 6 ) 2 . The contrast is particularly apparent with functionalized aryl azides where the second-generation catalyst now provides practical yields for synthetic chemistry. Finally, catalytic intramolecular aziridination was investigated since many natural products with aziridines feature bicyclic tertiary aziridines. For fiveand six-membered rings, the bicyclic aziridines were formed catalytically, in contrast to previously studied catalyzed and uncatalyzed reactions.
18-Atom-ringed macrocyclic tetra-imidazolium ligands have been synthesized by a two-step procedure and are the smallest free tetra-imidazoliums to date. The structures of the tetra-imidazoliums were characterized by multinuclear NMR and high-resolution ESI/MS to distinguish them from the potential di-imidazolium species. These tetra-imidazolium ligands form monomeric tetra-carbene complexes of platinum through in situ deprotonation.
Macrocyclic
tetraimidazolium diborate ligand precursors with two different ring
sizes have been synthesized by ring-forming reactions between diimidazoles
and haloboranes. Deprotonation of the macrocyclic tetraimidazoliums
with n-butyllithium followed by the addition of divalent
metal salts of palladium or nickel leads to macrocyclic tetracarbene
complexes with an 18-atom macrocycle, but not the 16-atom variant.
These neutral palladium and nickel complexes are the first examples
of macrocyclic tetracarbene diborate complexes, and unlike their cationic
counterparts, they are highly soluble in nonpolar solvents. All macrocyclic
tetraimidazoliums and their corresponding metal complexes were characterized
by single-crystal X-ray diffraction and spectroscopic techniques.
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