Subhadip Neogi

Heteroleptic metallosupramolecular racks, rectangles, and trigonal prisms: stoichiometry-controlled reversible interconversion.

A simple approach toward preparation of heteroleptic two-dimensional (2D) rectangles and three-dimensional (3D) triangular prisms is described utilizing the HETPYP (HETeroleptic PYridyl and Phenanthroline metal complexes) concept. By mixing metal-loaded linear bisphenanthrolines of varying lengths with diverse (multi)pyridine (py) ligands in a proper ratio, six different self-assembled architectures arise cleanly and spontaneously in the absence of any template. They are characterized by (1)H and DOSY NMR, ESI-FT-ICR mass spectrometry as well as by Job plots and UV-vis titrations. Density functional theory (DFT) computations provide information about each structure. A stoichiometry-controlled supramolecule-to-supramolecule interconversion based on the relative amounts of metal bisphenanthroline and bipyridine forces the rectangular assembly to reorganize to a rack architecture and back to the rectangle, as clearly supported by variable temperature and DOSY NMR as well as dynamic light scattering data. The highly dynamic nature of the assemblies represents a promising starting point for constitutional dynamic materials.

Structural variation of transition metal coordination polymers based on bent carboxylate and flexible spacer ligand: polymorphism, gas adsorption and SC-SC transmetallation

Reaction of the bent dicarboxylate ligand H2OBA (H2OBA = 4,4′-oxybisbenzoic acid) and the flexible linker 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L1), under diverse reaction conditions, forms two polymorphic Co(II) coordination polymers (CPs): {[Co(OBA)(L1)]·DMF}n (1), as a three dimensional (3D) framework with a pcu alpha-Po primitive cubic topology, and {[Co(OBA)(L1)]·DMF}n, (2), as a two dimensional (2D) structure with a 6-c uninodal net topology. Gas adsorption measurements of the desolvated Co(II) CPs show negligible uptake of all gases in 1, while 2 exhibits moderate uptake of CO2, with good selectivity over N2 and CH4. With Zn(II), reaction of H2OBA and L1 produces a different 2D CP, {[Zn0.5(OBA)0.5(L1)0.5]}n (3). Finally, three isostructural Cd(II) CPs, {[Cd(OBA)(L1)]·DMF}n (4), {[Cd(OBA)(L1)]·DEF}n (5), and {[Cd(OBA)(L1)]·DMA}n (6) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, DMA = N,N-dimethylacetamide), that differ only in the lattice solvent molecules and show 2D structural arrangements are prepared. Interestingly, CP 4 undergoes single-crystal to single-crystal (SC-SC) transmetallation reaction at room temperature, yielding isostructural {[Cu(OBA)(L1)]·DMF}n (7) that cannot be synthesized independently. Moreover, the luminescence properties of compounds 1, 2, 3, and 4 have been studied in the solid state at room temperature. All the complexes are characterized by elemental analysis, IR, TGA, PXRD and single crystal X-ray diffraction.

Versatile Tailoring of Paddle-Wheel Zn(II) Metal-Organic Frameworks through Single-Crystal-to-Single-Crystal Transformations.

A new tetracarboxylate ligand having short and long arms formed 2D layer Zn(II) coordination polymer 1 with paddle-wheel secondary building units under solvothermal conditions. The framework undergoes solvent-specific single crystal-to-single crystal (SC-SC) transmetalation to produce 1Cu . With a sterically encumbered dipyridyl linker, the same ligand forms non-interpenetrated, 3D, pillared-layer Zn(II) metal-organic framework (MOF) 2, which takes part in SC-SC linker-exchange reactions to produce three daughter frameworks. The parent MOF 2 shows preferential incorporation of the longest linker in competitive linker-exchange experiments. All the 3D MOFs undergo complete SC-SC transmetalation with Cu(II) , whereby metal exchange in different solvents and monitoring of X-ray structures revealed that bulky solvated metal ions lead to ordering of the shortest linker in the framework, which confirms that the solvated metal ions enter through the pores along the linker axis.

Substitution at the metal center of coordination polymers in single-crystal-to-single-crystal (SC-SC) transformation

With the explosive growth in the synthesis of coordination polymers, several systems are now available which exhibit transformation without losing single crystallinity. Among different types of single-crystal-to-single-crystal (SC-SC) transformations, those that involve the first coordination sphere of the metal centers are only a few in number. Nevertheless, they constitute an important case of SC-SC transformation because they are likely to activate small molecules inside the pores, can lead to separation of geometrical isomers, and so on. In this highlight, we have focused our attention on several examples of SC-SC transformations at the metal center. As the crystallinity is maintained in these cases, great insights into the entire process can be obtained that are potentially important in the development of new and technologically useful nano-scale devices and sensors. Besides, they can help in designing new catalytic systems.

Guest dependent reversible single-crystal to single-crystal structural transformation in a flexible Gd(III)-coordination polymer

The ligand 2,6,2′,6′-tetranitro-biphenyl-4,4′-dicarboxylic acid (H2L) reacts solvothermally with [Gd(NO3)3]·6H2O to produce a flexible and porous metal–organic framework, {[Gd2(L)3(DMF)4]·(4DMF)·(3H2O)}n (1) (DMF = N,N′-dimethylformamide). An X-ray crystallographic study reveals that compound 1 contains a 3D framework structure with two different 1D channels (A and B) that are occupied by solvent DMF and water molecules. Crystals of 1 when kept in a dichloromethane solution of 4-chlorobenzaldedhyde (4-ClPhCHO) afford the daughter product {[Gd2(L)3(DMF)4]·(4-ClPhCHO)·(4DMF)}n (1a), via single-crystal to single-crystal (SC–SC) transformation, where lattice water molecules of channel B are replaced by guest aldehyde molecules. Likewise, exposure of 4-fluorobenzaldehyde (4-FPhCHO) and 4-methylbenzaldehyde (4-MePhCHO) vapors to fresh crystals of 1 afforded two isostructural daughter frameworks, {[Gd(L)1.5(DMF)(H2O)3]·(4-FPhCHO)·(DMF)·(3H2O)}n (1b) and {[Gd(L)1.5(DMF)(H2O)3]·(4-MePhCHO)·(2DMF)·(H2O)}n (1c), respectively. Here, the guest aldehyde molecules occupy both the channels of the framework. Interestingly, the latter transformations exhibit a drastic rearrangement of the framework channels followed by several ‘carboxylate-shift’ processes, and concomitant movement of the water molecules from the cavity to the metal center. Importantly, all the host–guest complexes revert back to the as-synthesized crystal when kept in fresh DMF, rendering the mother framework a flexible and dynamic container for the aromatic aldehydes. All these transformations transpire through an SC–SC fashion under ambient conditions, pointing to the high flexibility of the framework and “guest-responsive fitting” of the channels. All the compounds are characterized by X-ray crystallography, thermogravimetry, elemental analysis, powder X-ray diffraction measurements and infrared spectroscopy.

Porous Lanthanide Coordination Polymers Built With a Podand and its Decomposition Product Oxalate: Identification of Discrete Water Clusters of Different Nuclearity

A tripodal ligand, tris‐(4‐carboxy‐2‐phenoxy‐ethyl‐)amine [ptaH3], bearing one carboxylate group at each terminal reacts hydrothermally with Dy(III), Er(III) and Ho(III) nitrates to afford porous co‐ordination polymers, {[Dy(ptaH)(1/2ox) · H2O]5H2O} n , (1), {[Er(ptaH)(1/2ox) · H2O]5H2O} n , (2) and {[Ho2(ptaH)(ox)2 · H2O]4H2O} n (3) [ox=oxalate]. The oxalate group is formed as the decomposition product of ptaH3 under hydrothermal condition. The structure of 1 and 2 consists of an array of interlinked metallocycles with oval shaped cavity. Compound 3 however, forms a 2D sheet structure built with Ho(III) and oxalate and the podand units bind to this sheet to extend the porous structure in the third dimension. Supramolecularly assembled water clusters with different shapes and sizes occupy the voids in the MOFs. In 1 and 2, discrete (H2O)12 clusters as open‐cube octamers buttressed on two sides by dimers have been identified. In 3, both (H2O)4 and (H2O)6 clusters could be found. All the three compounds are characterized by X‐ray crystallography.