Lalit Rajput

Robust hydrogen bonding synthon in one-dimensional and two-dimensional coordination polymers of pyridine-appended reverse amides and amides

Two dimensional iso-structurality and control was achieved in coordination polymers by carefully selecting the ligands that contain two types of bis-amido pyridines which have different connectivities. These networks were found to include a variety of large aromatic guest molecules.

Assembling coordination networks of bis-amido pyridines via hydrogen bonds: isostructurality and large hydrophobic cavities for guest inclusion

The reactions of bis(3-pyridylcarboxamido)alkanes (amides, 1) and bis(3-pyridyl)alkanediamides (reverse amides, 2) with Cu(SCN)2 in the presence of various guests afforded 22 crystalline complexes containing 1D and 2D coordination networks with or without guest inclusion. In these complexes, the ratio of metal to ligand is either 1:1 or 1:2, despite carrying out all the reactions by taking metal and ligand in 1:2 ratio. The recognition of coordination networks via amide-to-amide hydrogen bonds in the form of β-sheet was observed in the complexes containing metal to ligand ratio (1:2). Various guest molecules such as DMF, nitrobenzene, benzonitrile, 1,4-dihalobenzenes, phenanthrene, biphenyl, anthracene, 9-anthraldehyde and pyrene were found to include in these networks depending on the lengths of the spacers in 1 or 2. The ligands 1 or 2 with shorter spacers such as –(CH2)2– and –(CH2)4– have shown no consistency or preference to any particular network geometry. However the ligands with longer spacers such as –(CH2)6–, –(CH2)8– or phenyl have exhibited consistency in network geometries. The iso-structurality between the coordination polymers of amides and reverse amides was observed for ligands with longer spacers such as –(CH2)6– and –(C6H4)–. The spacer –(CH2)n– found to exhibit different conformations for accommodating the guests of different sizes and shapes. The guest occupied volume in these crystal lattices varies from 27% to 60%.

Crystal Engineering Studies on Ionic Crystals of Pyridine and Carboxylic Acid Derivatives Containing Amide Functional Groups

Seven crystal structures of pyromellitic acid or trimesic acid salts of molecules that contain pyridine and amide functionalities were determined and their structures were analyzed in detail in terms of various intermolecular interactions. The presence of multiple functionalities (acid, pyridine, amide, and hydroxy groups) in these structures resulted in diversified supramolecular architectures. Amide-to-amide hydrogen bonds are not observed in any of these structures because of interference by the anions, water molecules, or pyridinium cations. The symmetry of the components was found to be important in determining the resultant supramolecular synthon and, therefore, the overall architecture. The pyromellitate anions exhibited four types of geometries which, differ in valencies and intramolecular hydrogen bonding, and these anions also exhibit self stacks when they have planar geometries.

Exploring the salt–co-crystal continuum with ssNMR using natural abundance samples

There is a significant recent interest in differentiating multi-component solid forms such as salt, cocrystal, and their continuum, owing to the direct relationship of property to clinical, regulatory and legal requirements for an active pharmaceutical ingredient (API). In this context, detection of the H-atom position in a hydrogen bond X–H•••A–Y is a matter of fundamental and practical importance.[1] In the present study, solid forms of simple cocrystals/salts were investigated by high field (700 MHz) solid-state NMR (ssNMR) technique using the samples with naturally abundant 15N nuclei. Several model compounds in a series of prototypical salt/cocrystal/continuum systems exhibiting the {PyN•••H–O–}/{PyN+–H•••O-} type of hydrogen bonds were selected and prepared. The crystal structures were determined at low temperature and room temperature using X-ray diffraction. Accurate H-atom positions were determined by measuring the 15N–1H distances through 15N-1H dipolar interactions using 2D inversely proton-detected cross-polarization with variable contact-time (invCP-VC) 1H→15N→1H experiments at ultra-fast (νR ≥ 60–70 kHz) magic angle spinning (MAS) frequency.[2] The experiment is sensitive enough to determine the proton position even in a continuum where an ambiguity of terminology for the solid form often arises[3] and can be performed on minimum amounts of microcrystalline or even amorphous solids with natural abundance 15N samples. The crystal structures of the relevant solids have also been determined at a high level of accuracy and the results of the X-ray and NMR experiments are compared. This work has implications in the pharmaceutical industry where the salt/cocrystal/continuum condition of the APIs is seriously considered.