Dipak Samanta

Self‐Assembled Pd6 Open Cage with Triimidazole Walls and the Use of Its Confined Nanospace for Catalytic Knoevenagel‐ and Diels–Alder Reactions in Aqueous Medium

The two-component self-assembly of a 90° Pd(II) acceptor and a triimidazole donor led to the formation of a water-soluble semi-cylindrical cage with a hydrophobic cavity, which was separately crystallized with hydrophilic- and hydrophobic guests. The parent cage was found to catalyze the Knoevenagel condensation reaction of a series of aromatic mono-aldehydes with active methylene compounds, such as Meldrums acid or 1,3-dimethylbarbituric acid. The confined hydrophobic nanospace within this cage was also used in the catalytic Diels-Alder reactions of 9-hydroxymethylanthracene with N-phenylmaleimide or N-cyclohexylmaleimide.

A Pd24 Pregnant Molecular Nanoball: Self-Templated Stellation by Precise Mapping of Coordination Sites

We found that Pd(II) ion (M) and the smallest 120° bidentate donor pyrimidine (L(a)) self-assemble into a mononuclear M(L(a))4 complex (1a) instead of the expected smallest M12(L(a))24 molecular ball (1), presumably due to the weak coordination nature of the pyrimidine. To construct such a pyrimidine bridged nanoball, we employed a new donor tris(4-(pyrimidin-5-yl)phenyl)amine (L); which upon selective complexation with Pd(II) ions resulted in the formation of a pregnant M24L24 molecular nanoball (2) consisting of a pyrimidine-bridged Pd12 baby-ball supported by a Pd12 larger mother-ball. The formation of the baby-ball was not successful without the support of the mother-ball. Thus, we created an example of a self-assembly where the inner baby-ball resembling to the predicted M12(L(a))24 ball (1) was incarcerated by the giant outer mother-ball by means of geometrical constraints. Facile conversion of the pregnant ball 2 to a smaller M12(L(b))24 ball 3 with dipyridyl donor was achieved in a single step.

Sunlight-induced covalent marriage of two triply interlocked Pd6 cages and their facile thermal separation.

A template-free triply interlocked Pd6 cage (2) was synthesized by two-component self-assembly of cis-blocked 90° acceptor cis-(tmen)Pd(NO3)2 (M) and 1,3,5-tris((E)-2-(pyridin-3-yl)vinyl)benzene (L). Assembly 2 was characterized by (1)H NMR and ESI-MS, and the structure was confirmed by X-ray crystallography, which revealed a parallel conformation of the olefin double bonds belonging to the adjacent cages in the solid state at a distance of 3.656 Å, thereby indicating the feasibility of [2+2] photochemical reaction. Two adjacent interlocked cages were covalently married together by intermolecular [2+2] cycloaddition in a single crystal-to-single crystal fashion upon exposure to sunlight/UV irradiation. Most surprisingly, the covalently married pair was easily separated thermally in aqueous medium under mild reaction conditions.

Component Selection in the Self‐Assembly of Palladium(II) Nanocages and Cage‐to‐Cage Transformations

Dynamic supramolecular systems involving a tetratopic palladium(II) acceptor and three different pyridine- and imidazole-based donors have been used for self-selection by a synergistic effect of morphological information and coordination ability of ligands through specific coordination interactions. Three different cages were first synthesized by two-component self-assembly of individual donor and acceptor. When all four components were allowed to interact in a reaction mixture, only one out of three cages was isolated. The preferential binding affinity towards a particular partner was also established by transforming a non-preferred cage into a preferred cage by interaction with the appropriate ligand. Computational studies further supported the fact that coordination interaction of imidazole moiety to Pd(II) is enthalpically more preferred compared to pyridine, which drives the selection process. Analysis of crystal packing of both complexes indicated the presence of strong hydrogen bonds between nitrate and water molecules and also H-bonded 3D networks of water. Both complexes exhibit promising proton conductivity (10(-5) to ca. 10(-3)  S cm(-1) ) at ambient temperature under a relative humidity of circa 98 % with low activation energy.

Pillar height dependent formation of unprecedented Pd8 molecular swing and Pd6 molecular boat via multicomponent self-assembly

Three-component self-assembly of a cis-blocked 90° Pd(II) acceptor with a mixture of a tetraimidazole and a linear dipyridyl donor self-discriminated into unusual Pd(8) molecular swing (1) and Pd(6) molecular boat (2), which are characterized by single-crystal X-ray diffraction analysis; their ability to bind C(60) in solution is established by fluorescence titration.

Structural Diversity in Multinuclear PdII Assemblies that Show Low‐Humidity Proton Conduction

Systematic investigation on synergetic effects of geometry, length, denticity, and asymmetry of donors was performed through the formation of a series of uncommon Pd(II) aggregates by employing the donor in a multicomponent self-assembly of a cis-blocked 90° Pd(II) acceptor and a tetratopic donor. Some of these assemblies represent the first examples of these types of structures, and their formation is not anticipated by only taking the geometry of the donor and the acceptor building units into account. Analysis of the crystal packing of the X-ray structure revealed several H bonds between the counteranions (NO3 (-) ) and water molecules (OH⋅⋅⋅ON). Moreover, H-bonded 3D-networks of water are present in the molecular pockets, which show water-adsorption properties with some variation in water affinity. Interestingly, these complexes exhibit proton conductivity (1.87×10(-5) -6.52×10(-4)  Scm(-1) ) at 296 K and low relative humidity (ca. 46 %) with activation energies of 0.29-0.46 eV. Moreover, the conductivities further increase with the enhancement of humidity. The ability of these assemblies to exhibit proton-conducting properties under low-humidity conditions makes these materials highly appealing as electrolytes in batteries and in fuel-cell applications.

Covalent Postassembly Modification and Water Adsorption of Pd3 Self-Assembled Trinuclear Barrels

Three new ditopic imidazole ligands (2-4) were synthesized in high yields and characterized by various spectroscopic techniques. These ligands resulted in the formation of [3 + 6] self-assembled trinuclear barrels (5-7) in quantitative yields by stoichiometric combination of individual ligands and Pd(NO3)2 in DMSO. All the three assemblies (5-7) were characterized by (1)H NMR and ESI-MS analysis, and subsequently, structures of the complexes 5 and 6 were confirmed by single-crystal X-ray diffraction studies. Structure analysis reveals the presence of NO3(-) counteranions in the intermolecular channels/pockets, which could potentially act as H-bonding sites between adsorbed water molecules within the channels. In fact, both the assemblies (5 and 6) showed water uptake (136.58, and 123.78 cm(3) g(-1), respectively) at ambient temperature under maximum allowable humidity. In addition, free aldehyde group present in the bridging ligand in complex 7 provides reactive site for postassembly modification. Herein, Knoevenagel condensation with Meldrums acid was utilized under mild conditions by targeting aldehyde group appended in prefabricated complex 7 and transformed into a different complex (8) with altered functional group. Such postassembly functionalization enables incorporation of a new functional group without disrupting the integrity of the trifacial structure.

Stepwise Construction of Self-Assembled Heterometallic Cages Showing High Proton Conductivity

Two neutral tripodal metalloligands (CoL and FeL where L=C18 H21 N10 ) containing a clathrochelate core were synthesized and characterized in one-step. Reactions of these ligands with three different metal acceptors cis-(tmen)Pd(NO3 )2 (tmen = tetramethylethylenediamine), Zn(NO3 )2 and Mn(ClO4 )2 separately yielded a series of heterometallic coordination cages (1 a-3 a and 1 b-3 b) in high yields. Depending on the nature of coordination geometry of the acceptors, the resulting assemblies have trigonal- bipyramidal (1 a/1 b), open-cubic (2 a/2 b), and closed-cubic structures (3 a/3 b). The structures of the complexes 1 a, 2 a, 2 b, 3 a, and 3 b were confirmed by single-crystal X-ray diffraction studies. Analysis of crystal packing of the complexes 3 a and 3 b revealed the presence of several coordinated and lattice water molecules in the intermolecular channels. Both these complexes (3 a and 3 b) showed very high water adsorption under humid conditions. In addition, 3 a and 3 b exhibited promising proton conductivity of 3.31×10-3 and 1.05×10-4  S cm-1 at 70 °C under 98 % relative humidity (RH) respectively, with activation energy of 1.00-0.78 eV.