Kumar Biradha

Crystal engineering: from weak hydrogen bonds to co-ordination bonds

This article highlights the importance of crystal engineering in designing functional materials. Various rational design strategies will be discussed for controlling the molecular architectures of the materials using C–H⋯O, C–H⋯π, O–H⋯O, N–H⋯O and O–H⋯N hydrogen bonds and co-ordination bonds. The results described here show the role of guest molecules in templating the isomeric network structures with one set of molecular components. In particular, the materials that are designed based on strong hydrogen bonds and coordination bonds exhibited zeolite like and clay like properties.

Molecular paneling via coordination

This article deals with a coordination approach to three-dimensional assemblies via ‘molecular paneling’. Families of planar exo-multidentate organic ligands (molecular panels) are found to assemble into large three-dimensional assemblies through metal-coordination. In particular, cis-protected square planar metals, (en)Pd2+ or (en)Pt2+ (en = ethylenediamine), are shown to be very useful to panel the molecules. Metal-assembled cages, bowls, tubes, capsules, and polyhedra are efficiently constructed by this approach.

Post-synthetic modification of isomorphic coordination layers: exchange dynamics of metal ions in a single crystal to single crystal fashion

A diverse set of guest molecules template the consistent formation of fifteen iso-structural layered structures containing a pyridine based tripodal ligand, divalent metal ions and anions. These materials have shown a remarkable ability to interconvert from Zn(II) to Cu(II), Cd(II) to Cu(II), Cu(II) to Cd(II) complexes in a SCSC (single crystal-to-single crystal) fashion.

A molecular sphere of octahedral symmetry

Complexation of tridentate ligand 1 with Pd(NO3)2 leads to the quantitative self-assembly of M6L8 molecular sphere 2.

Crystal engineering with acid and pyridine heteromeric synthon: neutral and ionic co-crystals

The reaction of trimesic acid (H3TMA) with 2,6-bis(4-pyridylmethylene)cyclohexanone (1) in MeOH and phenol resulted in a co-crystal H3TMA·1 (1 : 1). The crystal structure of this co-crystal was found to contain a triply interpenetrated (6,3) hydrogen bonded network. The repeat of this reaction without PhOH resulted in crystals of an ionic compound which also contains both the components in 1 : 1 ratio. The crystal structure of this ionic solid exhibits a doubly interpenetrated three-dimensional hydrogen bonded network.

Molecular Paneling by Coordination: An M15L6 Hexahedral Molecular Capsule having Clefts for Reversible Guest Inclusion

Only one positional isomer is obtained from the assembly of 21 small components. A triangular molecular panel with five coordination sites is assembled upon complexation with [PdII (en)] (en=ethylenediamine) into a unique M15 L6 hexahedral coordination capsule (see X-ray structure). The capsule can encapsulate/exchange organic guests reversibly through the clefts at the nonbinding sites of the capsule.

Halogen⋯halogen interactions in assembling β-sheets into 2D layers in the bis-(4-halo-phenylamido)alkanes and their co-crystals via inter-halogen interactions

Halogen substituted bis(amido)alkanes were synthesized and their crystal structures were studied in terms of N–H⋯O hydrogen bonds and halogen⋯halogen interactions. The N–H⋯O hydrogen bonded β-sheets of bis-amides were shown to be assembled in to 2D layers viahalogen⋯halogen interactions. Out of 17 structures studied here 13 structures were found to form β-sheets. The iodo- and bromo derivatives consistently formed β-sheets, while chloro- and fluoro derivatives resulted in β-sheets or 2D layers. In the case of fluoro compounds, C–H⋯F hydrogen bonds are preferred compared to F⋯F contacts. Four co-crystals of inter halogen (IBr or BrCl) compounds were prepared and their crystal structures determined. These results suggest that the crystal structure of co-crystals will be isomorphous with that of parents provided that the crystal structures of the parents are also isomorphous to each other. In case of non-isomorphous parents, the resultant crystal structure of a co-crystal may resemble one of the parents or may have a new structure. The X-ray powder diffraction studies also confirm these results.

Stepwise dimerization of double [2 + 2] reaction in the co-crystals of 1,5-bis(4-pyridyl)-1,4-pentadiene-3-one and phloroglucinol: a single-crystal to single-crystal transformation

A double [2 + 2] reaction was shown to occur in a crystal to crystal manner in an O–H⋯N hydrogen-bonded co-crystal and was found to occur in a stepwise manner, i.e. one site reacts at a time.

Multifunctional white-light-emitting metal-organic gels with a sensing ability of nitrobenzene.

In this study, three novel luminescent nanofibrous metal-organic gels (MOGs) have been synthesized by the reaction of 1,3,5-tris(3-pyridylmethoxyl)benzene (L) with chloride salts of Cd(II), Hg(II), and Cu(II). The metal-ligand coordination, intermolecular π-π stacking and several other weak interactions found to play an important role in the formation of nanofibrous materials. The gel materials are characterized by rheology, diffuse reflectance spectra and various microscopic techniques such as TEM, FESEM, and AFM. The gels MOG-1 and MOG-2 were found to exhibit significant white photoluminescence, whereas the MOG-3 exhbits green emission upon excitation at 325 nm. Furthermore, the MOG-1 has shown its application as a chemosensor for the remarkable detection of nitroaromatics such as nitrobenzene (NB), 2,4-dinitrophenol (DNP). The significant quenching response for NB and DNP is attributed to the strong charge-transfer interactions between the electron-deficient aromatic ring of NB and the electron rich aromatic group of L in MOG-1. The crystal structure of Cd(II) complex of L reveals the formation one-dimensional network which contains strong π-π interactions within and between the networks and these strong π-π interactions generate the free charge carrier in all these nanofibrous gels.

Solid state double [2 + 2] photochemical reactions in the co-crystal forms of 1,5-bis(4-pyridyl)-1,4-pentadiene-3-one: establishing mechanism using single crystal X-ray, UV and 1H NMR

Double [2 + 2] photochemical reaction of 1,5-bis(4-pyridyl)-1,4-pentadiene-3-one (1P) was observed with 100% yield in four co-crystal forms of 1P with the hydrogen bonding template molecule phloroglucinol (PG) or 5-methoxy resorcinol (MR). The crystal structure of compound 1P does not have the required geometry to produce the double [2 + 2] reaction product, but it gives a single [2 + 2] reaction product with 15% yield upon time restricted irradiation. All the reactions in co-crystals of 1P resulted in the stereospecific exo–exotetrapyridyl tricyclo[6.2.0.03,6]-decane. 1H NMR and the UV-vis spectroscopic studies clearly established a stepwise mechanism for this reaction through the formation of monocyclobutane intermediate. One co-crystal form of 1P with PG and another co-crystal form of 1P with resorcinol were found to be unreactive due to different reasons. Furthermore the mechanochemical grinding products of 1P with RN or MR have exhibited similar structural and reactivity correlations with those crystallized from solution.