Sadhika Khullar

Hierarchical importance of coordination and hydrogen bonds in the formation of homochiral 2D coordination polymers and 2D supramolecular assemblies

In exploring the chemistry of reduced Schiff base derivatives of amino acids with Cu(II) ions, a series of homochiral two-dimensional (2D) coordination polymers (CPs) with a unique loop-like structure comprised of five Cu(II) centers, {[Cu2(Hsersal)2(H2O)]·2.5H2O}n, (1), {[Cu2(Hser-5OMe-sal)2(H2O)]·DMF}n (2), [Cu2(Hser-5NO2-sal)2(H2O)]n (3), {[Cu2(Hser-5Cl-sal)2(H2O)]·2H2O}n (4), {[Cu2(Hser-3Cl-sal)2(H2O)]·3H2O}n (5) and {[Cu2(Hser-o-Van)2(H2O)]·3H2O}n (6) [where H3sersal = N-(2-hydroxybenzyl)-serine, H3ser-5OMe-sal = N-(2-hydroxy-5-methoxybenzyl)-serine, H3ser-5NO2-sal = N-(2-hydroxy-5-nitrobenzyl)-serine, H3ser-5-Cl-sal = N-(2-hydroxy-5-chlorobenzyl)-serine, H3ser-3-Cl-sal = N-(2-hydroxy-3-chlorobenzyl)-serine, H3ser-o-van = N-(2-hydroxy-3-methoxybenzyl)-serine], have been isolated in good yields from the reaction of a methanolic solution of CuSO4·5H2O and potassium salt of the respective ligands (in a 1 : 1 ratio) either at room temperature or under reflux. In these CPs, the two Cu(II) centers have different coordination environments with one coordinated to a water molecule. Using a bifunctional linker, such as 4,4′-bipyridine, four of these 2D CPs are converted in methanol under reflux to the corresponding 2D supramolecular coordination complexes (SCCs) constructed through very strong hydrogen bonding interactions, [Cu2(4,4′-bpy)(Hsersal)2]·2H2O (7), [Cu2(4,4′-bpy)(Hser-5-OMe-sal)2]·6H2O (8), [Cu2(4,4′-bpy)(Hser-5-NO2-sal)2]·H2O (9) and [Cu2(4,4′-bpy)(Hser-5-Cl-sal)2]·4H2O·DMF (10). This chemical conversion of a CP to an SCC is unknown in the literature and indicates the hierarchical importance of coordination and hydrogen bonds in their formation. The complexes are structurally characterized by elemental analysis, UV-Vis spectroscopy, circular dichroism, IR and Raman spectroscopy, ESI mass spectrometry, single crystal and powder X-ray diffraction, polarimetry and thermogravimetric analysis. A magneto-structural correlation for the change from 1 to 7 is established through variable temperature magnetic susceptibility measurements (2–390 K) indicating strong antiferromagnetic coupling (2J = −278 cm−1) in 1 and no interaction in 7 between the Cu(II) centers. As an example, water adsorption studies of 1 and 7 were carried out to demonstrate the porous nature of the SCCs compared to the CPs.

Structural diversity of the encapsulated water clusters in the 3D supramolecular assemblies: a cyclic quasi-planar hexamer of water constructed through strong hydrogen bonding interactions

In the three-dimensional (3D) supramolecular assemblies having the general formula [Mn2(dicarboxylate)2(tpa)2]·xH2O (where tpa = N,N,N-tris(2-pyridylmethyl)-amine; dicarboxylate = acetylene dicarboxylate (adc) and x = 6 (1); fumarate and x = 8 (2) and succinate and x = 6 (3)), the dimanganese subunits differing in the dicarboxylate with a variation in the aliphatic chain structure (triple bond to double bond to single bond, respectively) provide the structural diversity of the encapsulated water clusters. These neutral supramolecular assemblies are held together by strong hydrogen bonding and π–π interactions, where the two dimensional (2D) water clusters are formed through strong hydrogen bonding (O–H⋯O). In each case, these water clusters connect the dimanganese subunits through hydrogen bonding with the uncoordinated carboxylate oxygen atoms of the dicarboxylates that bridge the Mn(II) centers of the subunit. The strength of the hydrogen bonding observed in these clusters (the range of O⋯O distances is 2.724 A to 2.845 A) is very similar to that found in water and ice. These are synthesized in high yields from a one-pot self-assembly reaction using Mn(OAc)2·4H2O, tpa and the corresponding acid in methanol at ambient conditions. Each of these is characterized by elemental analysis, single crystal and powder X-ray diffraction, IR and Raman spectroscopy and thermogravimetric analysis (TGA). Based on the X-ray crystal structures and TGA, the unusual stability of 2 compared to 1 and 3 is the result of different motifs including a cyclic quasi-planar hexamer formed through hydrogen bonding interactions. In 1 and 2, the Mn(II) centers are hexa-coordinated in a distorted octahedral geometry (N4O2), bonded to four nitrogen atoms of the ligand and two oxygen atoms from two monodentate carboxylate groups, while in 3 each Mn(II) center is heptacoordinated in a pentagonal bipyramidal geometry (N4O3), where in addition to the tpa ligand each succinate binds in chelated as well as monodentate mode, from opposite ends.

Tuning the formation of dicarboxylate linker-assisted supramolecular 1D chains and squares of Ni(II) using coordination and hydrogen bonds

Based on the variation in the multitopic dicarboxylates differing in the aliphatic chain structure (triple bond to double bond to single bond, respectively) in the formation of diverse coordination architectures for the same metal center and the ancillary ligand bpta, three new metal organic coordination networks (MOCNs) [Ni(bpta)(adc)(H2O)2]·2H2O (2), [Ni4(bpta)4(fumarate)4(H2O)4]·4H2O (3) and [Ni(bpta)(succinate)(H2O)2]·3H2O (4) are reported here (where bpta = N,N′-bis(2-pyridylmethyl)-tert-butylamine and adc = acetylene dicarboxylate). These are synthesized in good yields from a one-pot self-assembly reaction using Ni(OAc)2, bpta and the corresponding acid in methanol at ambient conditions. In order to shed light into their formation, the intermediate compound [Ni(bpta)(OAc)2(H2O)]·H2O (1) from the two-component mixture (Ni(OAc)2 and bpta), for all MOCNs has also been isolated and crystallographically characterized. Through strong hydrogen bonding between coordinated water, uncoordinated oxygen atoms of the two acetate groups and the lattice water molecule, 1 has a 1D chain structure. In 2 and 4, only one end of the dicarboxylate is coordinated to Ni(II) due to the fact that its other end (a carboxylate group not a carboxylic acid group which is commonly found for numerous polycarboxylic acids under hydrothermal conditions) is strongly hydrogen bonded to two coordinated water molecules of the adjacent Ni(II) center of the monomeric unit generating a 1D chain which is further hydrogen bonded through the lattice water molecules to form the respective 2D supramolecular assemblies. On the other hand, bis(monodentate) syn–syn and syn–anti bridging modes of fumarates between two Ni(II) centers result in the formation of a square in 3. These squares in 3 are further associated via hydrogen bonding through the formation of a six membered hexagonal motif R64(6) between two lattice water molecules, uncoordinated oxygen atoms of the fumarate and two coordinated water molecules on Ni(II) centers. The strengths of hydrogen bonding observed in the networks of 2–4 (the range of O–O distances is 2.638 A to 2.949 A) are very similar to those found in water and ice. All these are characterized by elemental analysis, single crystal X-ray diffraction, IR and Raman spectroscopy. Their thermal behavior in the solid state analyzed by thermogravimetric analysis (TGA) shows unusual stability of 3 up to 300 °C as compared to 2 and 4.

New conformational polymorph of hydrochlorothiazide with improved solubility

Abstract Context: To characterize a new conformation of hydrochlorothiazide (HCT) with better solubility and establishing its relationship with previously reported form I, obtained during attempted crystallization experiments. Objective: The aim of present investigation is to unveil a new conformational polymorph (form IA) having a higher solubility compared to commercially available form I. Materials and methods: New form (IA) was obtained from slow evaporation as well as by solvent–antisolvent method and was then characterized by DSC, FTIR, PXRD and SCXRD. Equilibrium solubility profile shows that it is more soluble than form I. Results: Appearance of phase transition endotherm at 215.87 °C in DSC spectra indicated the existence of new polymorph which was further confirmed by FTIR and PXRD. Single crystal study showed significant difference in various bond angles and torsion angles of the two forms. The solubility exhibited by form IA was (938 µg/mL) compared to form I (791 µg/mL) in water. Discussion: Complete structural analysis and molecular arrangements in the unit cell along with the DSC and FTIR data confirm the existence of new conformer of HCT. Conclusion: This study reveals the existence of a new conformational polymorph of HCT molecule having higher solubility could prove to be promising in pre-formulation.

Imidazolyl-substituted silatranes derived from triethanolamine and tris(isopropanol)amine: syntheses and structural characterization

The syntheses of seven new silatranes, N-[3-(3,7,10-trimethylsilatranyl)propyl]imidazole (7), N-(3-silatranylpropyl)-4-methylimidazole (8), N-[3-(3,7,10-trimethylsilatranyl)propyl]-4-methylimidazole (9), N-(3-silatranylpropyl)-2-methylimidazole (10), N-[3-(3,7,10-trimethylsilatranyl)propyl]-2-methylimidazole (11), N-(3-silatranylpropyl)-2-imidazoline (12), and N-[3-(3,7,10-trimethylsilatranyl)propyl]-2-imidazoline (13), are reported. These silatranes were prepared by transesterification of triethoxysilanes, N-(3-triethoxysilylpropyl)imidazole (3), N-(3-triethoxysilylpropyl)-4-methylimidazole (4), N-(3-triethoxysilylpropyl)-2-methylimidazole (5) and N-(3-triethoxysilylpropyl)-2-imidazoline (6) with triethanolamine (1), and tris(isopropanol)amine (2). The structures of five silatranes (7, 8, 9, 10, and 12) were confirmed by X-ray crystallography. The compounds were investigated by elemental analysis, IR, 1H, 13C NMR spectroscopy, thermogravimetric analysis, and mass spectrometry.

A green synthesis of thieno[2,3-c]xanthen-6-ones through the tandem photochemical sigmatropic shift and cyclization

A green and clean route for the synthesis of 5-hydroxy-4-methyl-6H-thieno[2,3-c]xanthen-6-ones (2) has been developed by the photoirradiation of 3-(prop-2-ynyloxy)-2-(thiophen-3-yl)-4H-chromen-4-ones (1). This photoreaction of the 3-propynyloxychromenones being observed for the first time is very interesting and appears to be the result of a tandem sigmatropic shift and cyclization. The structure of 2 has been determined by the spectroscopic (Fourier transform infrared [FTIR], nuclear magnetic resonance [NMR], and mass) and single crystal X-ray crystallographic studies.

Selective mercury ion recognition using a methyl red (MR) based silatrane sensor

A new methyl red (MR) based silatrane was synthesized in high yield by the transesterification reaction of methyl red-amidopropyltrimethoxysilane 1 (MR-APTMS) and trisisopropanolamine. Single crystal X-ray diffraction analysis was used to deduce the structure of silatrane 2 which was also supported by NMR (1H, 13C) spectroscopy, mass spectrometry, FTIR, UV-Vis and DFT studies. Silatrane 2 when surveyed for cation recognition ability with a library of metal ions in CH3CN/H2O (9 : 1, v/v) was found to act as an excellent UV-Vis probe for the selective recognition of Hg2+in vitro. Also, quantum mechanical calculations of the interaction complex [MR-APS-Hg]2+3 using DFT at the B3LYP level in conjunction with the LanL2DZ basis set determined the geometric and the stability parameters of the surveyed interaction.