Chiral Induction in a Self-Assembled Pd4 Coordination Cage with Chiral Guests.

The dynamic interplay of coordination bonds within metal-organic cages offers a unique avenue for structural evolution in response to external stimuli, presenting a promising strategy for the construction of chiral assemblies. This adaptability is crucial for the selective synthesis of homochiral assemblies and advancement of asymmetric catalysis. In this study, we report the self-assembly of an achiral square-planar Pd(II) acceptor with a C2-symmetric tetrapyridyl donor resulted in the formation of a racemic mixture of the chiral octahedral cage Pd4L2. The existence of this racemic mixture was confirmed using circular dichroism spectroscopy as well as single crystal X-ray diffraction analysis. We encoded chiral information into the asymmetric cavity of the cage by encapsulating chiral aromatic guests through efficient π-π stacking and hydrophobic interactions in aqueous media. The inclusion of a chiral guest induces a preference for one enantiomeric conformation of the cage over the other, effectively shifting the equilibrium towards a single, enantiopure host-guest complex. While the concept of chiral guest recognition by a chiral host is well-established, this work constitutes a remarkable example of guest-mediated chirality transfer leading to the formation of a single enantiopure coordination complex from achiral building blocks.

Improving Fatigue Resistance and Autonomous Switching of pH Responsive Hydrazones by Pulses of a Chemical Fuel.

The chemically triggered reversible switching of pH-responsive hydrazones involves rotary motion-induced configurational changes, serving as a prototype for constructing an array of molecular machines. Typically, the configurational isomerization of such switches into two distinct forms (E/Z) occurs through the alteration of the pH the medium, achieved by successive additions of acid and base stimuli. However, this process results in intermittent operation due to the concomitant accumulation of salt after each cycle, limiting switching performance to only a few cycles (5-6). In this context, we introduce a novel strategy for the autonomous E/Z isomerization of hydrazones in acetonitrile using pulses of trichloroacetic acid as a chemical fuel. The use of this transient acid enabled reversible switching of hydrazones even after 50 cycles without causing significant fatigue. To test the broad viability of the fuel, a series of ortho/para-substituted hydrazones were synthesized and their switching performance was investigated. The analysis of kinetic data showed a strong dependency of switching operations including the lifetime of transient state, on the electronic properties of substituents. Finally, a distinct color change from yellow to orange due to reversible switching of the para-methoxy substituted hydrazone was employed for the creation of rewritable messages on commercially available paper.

Reversible photoswitching of encapsulated azobenzenes in water.

Efficient molecular switching in confined spaces is critical for the successful development of artificial molecular machines. However, molecular switching events often entail large structural changes and therefore require conformational freedom, which is typically limited under confinement conditions. Here, we investigated the behavior of azobenzene-the key building block of light-controlled molecular machines-in a confined environment that is flexible and can adapt its shape to that of the bound guest. To this end, we encapsulated several structurally diverse azobenzenes within the cavity of a flexible, water-soluble coordination cage, and investigated their light-responsive behavior. Using UV/Vis absorption spectroscopy and a combination of NMR methods, we showed that each of the encapsulated azobenzenes exhibited distinct switching properties. An azobenzene forming a 1:1 host-guest inclusion complex could be efficiently photoisomerized in a reversible fashion. In contrast, successful switching in inclusion complexes incorporating two azobenzene guests was dependent on the availability of free cages in the system, and it involved reversible trafficking of azobenzene between the cages. In the absence of extra cages, photoswitching was either suppressed or it involved expulsion of azobenzene from the cage and consequently its precipitation from the solution. This finding was utilized to develop an information storage medium in which messages could be written and erased in a reversible fashion using light.

Publisher Correction: Reversible chromism of spiropyran in the cavity of a flexible coordination cage.

The original version of this Article contained an error in Fig. 4c, in which the right-most chemical structure included an 'N+' rather than an 'N'. This has been corrected in both the PDF and HTML versions of the Article.

Reversible chromism of spiropyran in the cavity of a flexible coordination cage.

Confining molecules to volumes only slightly larger than the molecules themselves can profoundly alter their properties. Molecular switches-entities that can be toggled between two or more forms upon exposure to an external stimulus-often require conformational freedom to isomerize. Therefore, placing these switches in confined spaces can render them non-operational. To preserve the switchability of these species under confinement, we work with a water-soluble coordination cage that is flexible enough to adapt its shape to the conformation of the encapsulated guest. We show that owing to its flexibility, the cage is not only capable of accommodating-and solubilizing in water-several light-responsive spiropyran-based molecular switches, but, more importantly, it also provides an environment suitable for the efficient, reversible photoisomerization of the bound guests. Our findings pave the way towards studying various molecular switching processes in confined environments.

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.

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 Meldrum's 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.

Light-controlled self-assembly of non-photoresponsive nanoparticles.

The ability to guide the assembly of nanosized objects reversibly with external stimuli, in particular light, is of fundamental importance, and it contributes to the development of applications as diverse as nanofabrication and controlled drug delivery. However, all the systems described to date are based on nanoparticles (NPs) that are inherently photoresponsive, which makes their preparation cumbersome and can markedly hamper their performance. Here we describe a conceptually new methodology to assemble NPs reversibly using light that does not require the particles to be functionalized with light-responsive ligands. Our strategy is based on the use of a photoswitchable medium that responds to light in such a way that it modulates the interparticle interactions. NP assembly proceeds quantitatively and without apparent fatigue, both in solution and in gels. Exposing the gels to light in a spatially controlled manner allowed us to draw images that spontaneously disappeared after a specific period of time.

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.

Structural diversity in multinuclear Pd(II) 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.

Self-assembled multicomponent Pd6 aggregates showing low-humidity proton conduction.

Two Pd6 molecular aggregates (1 and 2), self-sorted via a template-free three-component self-assembly process, represent new examples of discrete architectures exhibiting very high proton conductivity [0.78 × 10(-3) S cm(-1) (1) and 0.22 × 10(-3) S cm(-1) (2)] at 300 K at low relative humidity (∼46%) with low activation energy comparable to that of currently used Nafion in fuel cells.

PtII6 nanoscopic cages with an organometallic backbone as sensors for picric acid.

An organometallic building block 1,3,5-tris(4-trans-Pt(PEt3)2I(ethynyl)phenyl)benzene (1) incorporating Pt-ethynyl functionality has been synthesized and characterized. [2 + 3] self-assembly of its nitrate analogue 1,3,5-tris(4-trans-Pt(PEt3)2(ONO2)(ethynyl)phenyl)benzene (2) with "clip" type bidentate donors (L1-L3) separately afforded three trigonal prismatic architectures (3a-3c), respectively. All these prisms were characterized and their shapes/sizes are predicted through geometry optimization employing molecular mechanics universal force field (MMUFF) simulation. The extended π-conjugation including the presence of Pt-ethynyl functionality makes them electron rich as well as luminescent in nature. Macrocycles 3b and 3c exhibit fluorescence quenching in solution upon addition of picric acid [PA], which is a common constituent of many explosives. Interestingly, the non-responsive nature of fluorescent intensity towards other electron-deficient nitro-aromatic explosives (NAEs) makes them promising selective sensors for PA with a detection limit predicted to be ppb level. Furthermore, solid-state quenching of fluorescent intensity of the thin film of 3b upon exposure to saturated vapor of picric acid has drawn special attention for infield applications.

Multicomponent self-sorting of a Pd7 molecular boat and its use in catalytic Knoevenagel condensation.

Unique three-component self-assembly of a cis-blocked 90° Pd(II) acceptor with a mixture of tri- and tetra-imidazole donors led to the self-sorting of a Pd7 molecular boat with an internal nanocavity, which catalyses the Knoevenagel condensation of a series of aromatic aldehydes with 1,3-dimethylbarbituric acid and Meldrum's acid in aqueous media.

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 Meldrum's 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.

Self-assembly of Ru(4) and Ru(8) assemblies by coordination using organometallic Ru(II)(2) precursors: Synthesis, characterization and properties.

Coordination-driven self-assembly of binuclear half-sandwich p-cymene ruthenium(II) complexes [Ru(2)(µ-η(4)-C(2)O(4))(MeOH)(2)(η(6)-p-cymene)(2)](O(3)SCF(3))(2) (1a) or [Ru(2)(µ-η(4)-N,N'-diphenyloxamidato)(MeOH)(2)(η(6)-p-cymene)(2)](O(3)SCF(3))(2) (1b) separately with an imidazole-based tetratopic donor L in methanol affords two tetranuclear metallamacrocycles 2a and 2b, respectively. Conversely, the similar combination of L with 2,5-dihydroxy-1,4-benzoquinonato (dhbq) bridged binuclear complex [Ru(2)(µ-η(4)-C(6)H(2)O(4))(MeOH)(2)(η(6)-p-cymene)(2)](O(3)SCF(3))(2) (1c) in 1:2 molar ratio resulted in an octanuclear macrocyclic cage 2c. All the self-assembled macrocycles 2a-2c were isolated as their triflate salts in high yields and were characterized fully by multinuclear ((1)H, (13)C and (19)F) NMR, infrared (IR) and electrospray ionization mass spectrometry (ESIMS). In addition, the molecular structure of macrocycle 2a was established unequivocally by single-crystal X-ray diffraction analysis and adopts a tetranuclear rectangular geometry with the dimensions of 5.53 Å × 12.39 Å. Furthermore, the photo- and electrochemical properties of these newly synthesized assemblies have been studied by using UV-vis absorption and cyclic voltammetry analysis.

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.

Coordination-driven self-assembly of 2D-metallamacrocycles using a shape-selective Pt(II)2-organometallic 90° acceptor: design, synthesis and sensing study.

Synthesis of a series of two-dimensional metallamacrocycles via coordination-driven self-assembly of a shape-selective Pt(II)(2)-molecular building unit incorporating carbazole-ethynyl functionality is described. An equimolar (1 : 1) combination of a Pt(II)(2)-organometallic 90° acceptor, 1, with rigid linear ditopic donors (L(a) and L(b)) afforded [4 + 4] self-assembled octanuclear molecular squares, 2 and 3, in quantitative yields, respectively [L(a) = 4,4'-bipyridine; L(b) = trans-1,2-bis(4-pyridyl)ethylene]. Conversely, a similar treatment of 1 with an amide-based unsymmetrical flexible ditopic donor, L(c), resulted in the formation of a [2 + 2] self-sorted molecular rhomboid (4a) as a single product [L(c) = N-(4-pyridyl)isonicotinamide]. Despite the possibility of several linkage isomeric macrocycles (rhomboid, triangle and square) due to the different connectivity of L(c), the formation of a single and symmetrical molecular rhomboid (4a) as the only product is an interesting observation. All the self-assembled macrocycles (2, 3 and 4a) were fully characterized by multinuclear NMR ((1)H and (31)P) and ESI-MS analysis. Further structural insights about the size and shape of the macrocycles were obtained through energy minimization using density functional theory (DFT) calculations. Decoration of the starting carbazole building unit with Pt-ethynyl functionality enriches the assemblies to be more π-electron rich and luminescent in nature. Macrocycles 2 and 3 could sense the presence of electron deficient nitroaromatics in solution by quenching of the initial intensity upon gradual addition of picric acid (PA). They exhibited the largest quenching response with high selectivity for nitroaromatics compared to several other electron deficient aromatics tested.