A Chiral Hexa-amino Cyclotriphosphazene for Enantioselective Recognition of Small Organic Compounds.

Chiral recognition and separation studies are of paramount importance in research in view of their applications in asymmetric catalysis and substrate recognition in biological processes. The efficiencies of these processes are governed by the subtle differences in noncovalent interactions between the host and guest molecules. Hexakis(organoamino)phosphazenes are versatile building blocks for host-guest chemistry for their ability to act as both the donor and the acceptor of H-bonds. Herein, we report an enantiomeric pair of cyclotriphosphazenes [P3N3(NHR*)6], [R* = (R)-(CH(CH3)Ph)] (1-R) and [(S)-(CH(CH3)Ph)] (1-S), with chiral α-methylbenzylamino substituents. The chiral recognition capabilities of its R-enantiomer were further probed for several chiral organic compounds containing a range of functional groups. Among these, a remarkable guest selectivity (ξ) value of 2506 was observed for binol (BOL) in favor of its R-enantiomer. DFT-optimized structures of the host-guest pairs suggest that the multiple noncovalent interactions between the molecules in the two unique types of binding sites at the phosphazenes play a crucial role in the observed high binding selectivities.

Ferroelectric Organic-Inorganic Hybrid Ammonium Halogenobismuthate(III) for Piezoelectric Energy Harvesting.

Halogenobismuthate(III) compounds are of recent interest because of their low toxicity and distinct electrical properties. The utility of these materials as ferroelectrics for piezoelectric energy harvesters is still in its early stages. Herein, we report a hybrid ammonium halogenobismuthate(III) [BPBrDMA]2·[BiBr5], crystallizing in a ferroelectrically active polar noncentrosymmetric Pna21 space group. Its noncentrosymmetric structure was confirmed by the detection of the second harmonic generation response. The ferroelectric P-E hysteresis loop measurements on the thin film sample of [BPBrDMA]2·[BiBr5] gave a saturation polarization (Ps) of 5.72 µC cm-2. The piezoresponse force microscopy analysis confirmed its ferroelectric and piezoelectric nature, showing characteristic domain structures and signature hysteresis and butterfly loops. The piezoelectric energy harvesting attributes of [BPBrDMA]2·[BiBr5] were further probed on its polylactic acid (PLA) composites. The 15 wt % [BPBrDMA]2·[BiBr5]-PLA polymer composite resulted in a high output voltage of 26.2 V and power density of 15.47 µW cm-2. The energy harvested from this device was further utilized for charging a 10 µF capacitor within 3 min.

Ligand-Directed Synthesis of a Self-Organized Chloro-Bridged Cubic Pd(II) Cage Showing Selective Encapsulation of Phenols.

The synthesis and guest recognition properties of a neutral Pd24-cubic cage, [{Pd3(NiPr)3PO}8(µ2-Cl)24] 1 are reported. The formation of the cubical assembly takes place by an exclusive one-pot ligand-assisted pathway directed by an oximido linker. The initial coordination of the oximido ligand pre-organizes the [Pd3(NiPr)3PO]3+ polyhedral building units into a tetrameric intermediate, which then transforms into an oximido-tethered tetrahedral assembly and to the cubical cage 1 in the presence of chloride ions. In the absence of the directing oximido linker, no cage formation was observed, and the Pd6-precursor was found to undergo self-condensation, giving rise to a new pentameric polyhedral cluster, [Pd5{(NiPr)3PO}2(OAc)2(OH)2] 2. The central cavity of the cube has been probed for guest encapsulation studies, which shows a high binding with phenolic guest molecules with association constants of the order of 104-105 M-1. The favorable formation of host-guest complexes was attributed to the strong hydrogen bonding interactions between the host and guest functional groups.

A Chiral B-N Adduct as a New Frontier in Ferroelectrics and Piezoelectric Energy Harvesting.

Within the burgeoning field of electronic materials, B-N Lewis acid-base pairs, distinguished by their partial charge distribution across boron and nitrogen centers, represent an underexplored class with significant potential. These materials exhibit inherent dipoles and are excellent candidates for ferroelectricity. However, the challenge lies in achieving the optimal combination of hard-soft acid-base pairs to yield B-N adducts with stable dipoles. Herein, we present an enantiomeric pair of B-N adducts [R/SC6H5CH(CH3)NH2BF3] (R/SMBA-BF3) crystallizing in the polar monoclinic P21 space group. The ferroelectric measurements on RMBA-BF3 gave a rectangular P-E hysteresis loop with a remnant polarization of 7.65 µC cm-2, a value that aligns with the polarization derived from the extensive density-functional theory computations. The PFM studies on the drop-casted film of RMBA-BF3 further corroborate the existence of ferroelectric domains, displaying characteristic amplitude-bias butterfly and phase-bias hysteresis loops. The piezoelectric nature of the RMBA-BF3 was confirmed by its direct piezoelectric coefficient (d33) value of 3.5 pC N-1 for its pellet. The piezoelectric energy harvesting applications on the sandwich devices fabricated from the as-made crystals of RMBA-BF3 gave an open circuit voltage (VPP) of 6.2 V. This work thus underscores the untapped potential of B-N adducts in the field of piezoelectric energy harvesting.

Design and Piezoelectric Energy Harvesting Properties of a Ferroelectric Cyclophosphazene Salt.

Cyclophosphazenes offer a robust and easily modifiable platform for a diverse range of functional systems that have found applications in a wide variety of areas. Herein, for the first time, it reports an organophosphazene-based supramolecular ferroelectric [(PhCH2 NH)6 P3 N3 Me]I, [PMe]I. The compound crystallizes in the polar space group Pc and its thin-film sample exhibits remnant polarization of 5 µC cm-2 . Vector piezoresponse force microscopy (PFM) measurements indicated the presence of multiaxial polarization. Subsequently, flexible composites of [PMe]I are fabricated for piezoelectric energy harvesting applications using thermoplastic polyurethane (TPU) as the matrix. The highest open-circuit voltages of 13.7 V and the maximum power density of 34.60 µW cm-2 are recorded for the poled 20 wt.% [PMe]I/TPU device. To understand the molecular origins of the high performance of [PMe]I-based mechanical energy harvesting devices, piezoelectric charge tensor values are obtained from DFT calculations of the single crystal structure. These indicate that the mechanical stress-induced distortions in the [PMe]I crystals are facilitated by the high flexibility of the layered supramolecular assembly.

Mapping the Assembly of Neutral Tetrahedral Cages Tethered by Oximido Linkers and Their Guest Encapsulation Studies.

A primary criterion for the design of polyhedral metal-organic cages is the requirement of geometrically matched pairs of metal ions and ligand moieties. However, understanding the pathway it takes to reach the final polyhedral structure can provide more insights into the self-assembly process and improved design strategies. In this regard, we report two neutral tetrahedral cages with the formulas {[Pd3(NiPr)3PO]4(L1)6} (1-TD) and {[Pd3(NiPr)3PO]4(L2)6} (2-TD) starting from the acetate-bridged cluster {[Pd3(NiPr)3PO]2(OAc)2(OH)}2·2(CH3)2SO (HEXA-Pd) and the respective oxamide precursors (L1H2: [C2(NH2)2O2]) and (L2H2: (C2(NHMe)2O2]). When subtle variations in the reaction conditions were made, two new tetrameric Pd12 assemblies, {[Pd3(NiPr)3PO]4(L1)2(OAc)4(OMe)4} (1-TM) and {[Pd3(NiPr)3PO]4(L2)2(OAc)4(OMe)4} (2-TM), were obtained from the same precursors. Detailed investigations using NMR, mass spectrometry, X-ray crystallography, and computational studies indicate that the macrocyclic complexes 1-TM and 2-TM are the reaction intermediates involved in the formation of the tetrahedral cages 1-TD and 2-TD, respectively. Moreover, the tetrahedral cages 1-TD and 2-TD exhibited intrinsic cavities of volume ∼85 Å3. Guest encapsulation studies revealed that the cage 1-TD can encapsulate a wide range of guest molecules such as CH2Cl2, CHCl3, CCl4, C6H6, and C6H5F. Interestingly, 1-TD was shown to exhibit a preferential binding of C6H5F and C6H6 over other halogenated guest molecules, as determined from NMR titrations and computational studies.

Encapsulation Studies on closo-Dicarbadodecaborane Isomers in Neutral Tetrahedral Palladium(II) Cages.

The encapsulation of icosahedral closo-dicarbadodecaborane (o-, m-, and p-carboranes, Cb) as guest molecules at the intrinsic cavities of the three isostructural tetrahedral cages [{Pd3(NiPr)3PO}4(Cl-AN)6] (1), [{Pd3(NiPr)3PO}4(Br-AN)6] (2), and [{Pd3(NiPr)3PO}4(H-AN)6] (3) was studied. The formation of definite host-guest assemblies was probed with mass spectrometry, IR, and NMR spectral analysis. 2D DOSY 1H NMR of the Cb⊂Cage systems showed similar diffusion coefficient (D) values for the host and guest species, signifying the encapsulation of these guests inside the cage assemblies. The hydrodynamic radius (RH) derived from the D values of the host and guest species further confirmed the encapsulation of the Cb isomers at the cage pockets. The single-molecule energy optimization of the host-guest assemblies indicated the preferential binding of o-Cb as a guest inside the cages (1-3). The stabilization of these Cb guests inside these cages was further attributed to various possible nonclassical C-H···X-type interactions.

Squarate-Tethered Enantiomeric Imido-Pd(II) Cages for Recognition and Separation of Chiral Organic Molecules.

Chiral coordination cages have emerged as an efficient platform for enantioselective processes via host-guest interactions. Here, we report an enantiomeric pair of tetrahedral cages of formula [(Pd3[PO(N(*CH(CH3)Ph)3])4(C4O4)6] supported by chiral tris(imido)phosphate trianions and squarate (C4O4)2- linkers. These cages exhibit unusual coordination isomerism for Pd(II)-linker bonds compared with the other Pd(II) cages of this family. Further, they were employed for the recognition and separation of small chiral molecules containing various functionalities. High enantioselectivities of 67 and 41 were found in the case of R-4-hydroxydihydrofuran-2(3H)-one and S-epichlorohydrin, recognized by the R-isomer of the cage. Chiral separation studies showed remarkable enantiomeric excess values of 93 and 85% for S-epichlorohydrin and R-4-benzyl-2-oxazolidinone, respectively, from their racemic mixtures. These studies showcase the potential of coordination cages for enantioselective applications.

A Ferroelectric Aminophosphonium Cyanoferrate with a Large Electrostrictive Coefficient as a Piezoelectric Nanogenerator.

Hybrid materials possessing piezo- and ferroelectric properties emerge as excellent alternatives to conventional piezoceramics due to their merits of facile synthesis, lightweight nature, ease of fabrication and mechanical flexibility. Inspired by the structural stability of aminophosphonium compounds, here we report the first A3 BX6 type cyanometallate [Ph2 (i PrNH)2 P]3 [Fe(CN)6 ] (1), which shows a ferroelectric saturation polarization (Ps ) of 3.71 µC cm-2 . Compound 1 exhibits a high electrostrictive coefficient (Q33 ) of 0.73 m4  C-2 , far exceeding those of piezoceramics (0.034-0.096 m4  C-2 ). Piezoresponse force microscopy (PFM) analysis demonstrates the polarization switching and domain structure of 1 further confirming its ferroelectric nature. Furthermore, thermoplastic polyurethane (TPU) polymer composite films of 1 were prepared and employed as piezoelectric nanogenerators. Notably, the 15 wt % 1-TPU device gave a maximum output voltage of 13.57 V and a power density of 6.03 µW cm-2 .

Efficient Piezoelectric Energy Harvesting from a Discrete Hybrid Bismuth Bromide Ferroelectric Templated by Phosphonium Cation.

Bismuth containing hybrid molecular ferroelectrics are receiving tremendous attention in recent years owing to their stable and non-toxic composition. However, these perovskite-like structures are primarily limited to ammonium cations. Herein, we report a new phosphonium based discrete perovskite-like hybrid ferroelectric with a formula [Me(Ph)3 P]3 [Bi2 Br9 ] (MTPBB) and its mechanical energy harvesting capability. The Polarization-Electric field (P-E) measurements resulted in a well-defined ferroelectric hysteresis loop with a remnant polarization value of 2.1 µC cm-2 . Piezoresponse force microscopy experiments enabled visualization of the ferroelectric domain structure and evaluation of the piezoelectric strain coefficient (d33 ) for an MTPBB single crystal and thin film sample. Furthermore, flexible devices incorporating MTPBB in polydimethylsiloxane (PDMS) matrix at various concentrations were fabricated and explored for their mechanical energy harvesting properties. The champion device with 20 wt % of MTPBB in PDMS rendered a maximum peak-to-peak open-circuit voltage of 22.9 V and a maximum power density of 7 µW cm-2 at an optimal load of 4 MΩ. Moreover, the potential of MTPBB-based devices in low power electronics was demonstrated by storing the harvested energy in various electrolytic capacitors.

Polyanionic Imido-P(V) Ligands: From Transition Metal Complexes to Coordination Driven Self-Assemblies.

The chemistry of the imido-anions of the main group elements has been studied for more than three decades. The imido (NR)- group is isoelectronic to the oxo (=O) group and can coordinate with metal ions through its lone pairs of electrons. The polyimido-P(V) anions are well explored as they resemble the phosphorus oxo moieties such as H3 PO4 , H2 PO4 - , HPO4 2- and PO4 3- species. These imido anions are typically generated using strong main group organometallic reagents such as n BuLi, Et2 Zn, Me3 Al and n Bu2 Mg, etc. As a result, their coordination chemistry has been restricted to reactions in anhydrous aprotic solvents for a few main group metal ions. This account presents our findings on using certain soft transition metal such Ag(I) and Pd (II) for isolating these imido-P(V) anions as their corresponding self-assembled clusters and cages. Using the various salts of Ag(I) ions in reaction with 2-pyridyl (2 Py) functionalized phosphonium salts and phosphoric triamides, we obtained the mono- and dianionic form of these imido ligands {[P(N2 Py)2 (NH2 Py)2 ]- , [P(N2 Py)2 (NH2 Py)]- , [PO(N2 Py)(NH2 Py)2 ]2- } and derived interesting examples of tri, penta, hepta and octanuclear Ag(I) clusters. Interestingly, by using the salts of Pd (II) ions, the elusive imido-phosphate trianions of the type [(RN)3 PO]3- (R=t Bu, c Hex, i Pr) were generated in a facile one pot reaction as their corresponding tri- and hexanuclear clusters of the type {Pd3 [(NR)3 PO](OAc)3 }n (n=1 or 2). These trianions acts as a cis-coordinated hexadentate ligand for a trinuclear Pd (II) cluster and serve as the polyhedral building units for constructing hitherto unknown family of neutral cages in tetrahedral {Pd3 [(Ni Pr)3 PO]4 (L)6 } and cubic {Pd3 [(Ni Pr)3 PO]8 (L)12 } structures in the presence of suitable linker ligands (L2- ). These cages show interesting host-guest chemistry and post-assembly reactions. Remarkably, by employing chiral tris(imido)phosphate trianions, enantiopure chiral cages of the type [(Pd3 X*)4 (L)6 ], ([X*]3- =RRR- or SSS-[PO(N(*CH(CH3 )Ph)3 ]3- ), were synthesized and used for the chiral-recognition and enantio-separation of small racemic guest molecules. Some of these chiral cages were also shown to exhibit polyradical framework structures. In future, these and other similar types of cages are envisioned as potential molecular vessels for performing the reactions in their confined environment. The enantiomeric cages can be probed for asymmetric catalysis and the separation of a range of small chiral molecules.

Enantiopure Polyradical Tetrahedral Pd12 L6 Cages.

The synthesis of cages with a polyradical framework remains a challenging task. Herein is reported an enantiomeric pair of quinoid-bridged polyradical tetrahedral palladium(II) cages that are stabilized by an unusual dianionic diradical form (dhbq..2- ). These cages have been characterized by electron paramagnetic resonance and UV-visible spectroscopy, squid magnetometry and mass spectrometry. Single-crystal-derived X-ray investigations of the iso-structural cages built on fluoranilate linkers confirm the tetrahedral structure of the obtained radical cages. Theoretical calculations showed that the diradical state of the dhbq anions is more stable than the usual monoradical state. A weak ferromagnetic exchange between adjacent radical centers was observed in DFT studies.

Construction of Entropically Favored Supramolecular Metal-Ligand Trimeric Assemblies Supported by Flexible Pyridylaminophosphorus(V) Scaffolds.

The self-assembly reactions of tetratopic metal acceptors with the flexible bidentate ligands are known to yield self-assembled molecular squares of the type [M4L8], triangles of composition [M3L6], or a mixture of these two. In this work, we demonstrate the preferential formation of a trimeric cage assembly of the formula [Pd3(L1)6·(BF4)6] (1a) over the tetrameric cage [Pd4(L1)8·(BF4)8] (1b) by employing a flexible dipodal phosphoramide ligand, [PhPO(NH(3-Py))2] (L1; 3-Py = 3-aminopyridine), in a reaction with [Pd(CH3CN)4·(BF4)2]. The entropically favored trimeric self-assembly of 1a is the predominant species in the solution [dimethyl sulfoxide (DMSO)-d6] at room temperature. In fact, at higher temperatures, 1a was found to be the only product, as observed from the disappearance of the peak due to 1b in the 31P NMR spectrum. However, in a 1:1 mixture of acetonitrile (MeCN)-d3 and DMSO-d6, the tetrameric species 1b is the preferred species, as revealed by the 31P NMR and electrospray ionization mass spectral analyses. The structure of the molecular trimer 1a has been established in the solid state by using single-crystal X-ray diffraction analysis. Interestingly, treatment of an another flexible ligand, [MePO(NH(3-Py))2] (L2), with the same Pd(II) acceptor resulted in exclusive formation of the trimeric cage [Pd3(L2)6·(BF4)6] (2).

Effective Enantioselective Recognition by Chiral Amino-Phosphonium Salts*.

The research on chiral recognition and selection is not only fundamental in deciding the mystery of homochirality, but also informative in terms of substrate recognition in biological processes and asymmetric catalysis. We report an enantiomeric pair of phosphonium salts having chiral (R and S) amino substituents that are utilized towards the enantioselective recognition of a variety of chiral compounds having functional groups, such as carboxylic acid, amine, and alcohol. These simple phosphonium salts are found to exhibit a high enantiomeric discrimination for 1-cyclohexylethylamine (CY). A remarkable guest selectivity (ξ) value of 5.3×108 is achieved for the enantiomer of R-CY over S-CY by using the R-isomer of the phosphonium salt. Such high binding selectivities and discrimination capabilities is attributed to multiple non-covalent interactions between the host and guest molecules as inferred from the DFT optimized structures of the host-guest pairs.

Anilate Tethered Neutral Tetrahedral Pd(II) Cages Exhibiting Selective Encapsulation of Xylenes and Mesitylene.

The design of porous materials for the recognition of multiple hydrocarbons is highly desirable for the energy-efficient separation and recognition of chemical feedstock. Herein, three new iso-structural porous discrete metal-organic cages of formula {[Pd3 (NiPr)3 PO]4 (R-AN)6 } (R-AN=anilate linkers) for the selective recognition of substituted aromatic hydrocarbons are reported. The tetrahedral cages 1, 2, and 3 containing anilate, chloranilate, and bromanilate linkers exhibited selective encapsulation of mesitylene, o-xylene, and p-xylene, respectively, over other analogous aromatic hydrocarbons. These selective encapsulations were driven by the variations in the portal diameters present at each of these cages and their interactions with the hydrocarbon guests. These observations are supported by mass spectrometry, NMR studies, and theoretical binding-energy calculations.

Piezoelectric Energy Harvesting from a Ferroelectric Hybrid Salt [Ph3 MeP]4 [Ni(NCS)6 ] Embedded in a Polymer Matrix.

Organic-inorganic hybrid ferroelectrics are an exciting class of molecular materials with promising applications in the area of energy and electronics. The synthesis, ferroelectric and piezoelectric energy harvesting behavior of a 3d metal ion-containing A4 BX6 type organic-inorganic hybrid salt [Ph3 MeP]4 [Ni(NCS)6 ] (1) is now presented. P-E hysteresis loop studies on 1 show a remnant ferroelectric polarization value of 18.71 µC cm-2 , at room temperature. Composite thermoplastic polyurethane (TPU) devices with 5, 10, 15 and 20 wt % compositions of 1 were prepared and employed for piezoelectric energy harvesting studies. A maximum output voltage of 19.29 V and a calculated power density value of 2.51 mW cm-3 were observed for the 15 wt % 1-TPU device. The capacitor charging experiments on the 15 wt % 1-TPU composite device shows an excellent energy storage performance with the highest stored energies and measured charges of 198.8 µJ and 600 µC, respectively.

Chiral Separation of Styrene Oxides Supported by Enantiomeric Tetrahedral Neutral Pd(II) Cages.

The separation of enantiomers is of considerable importance in the preparation of the compounds of biological interests, catalysis, and drug development. Here, we report a novel enantioseparation of styrene epoxides (SOs) resolved in the presence of a pair of enantio-enriched tetrahedral cages. Chiral neutral cages of formula [(Pd3X*)4(C6O4Cl2)6] ([X*]3- = RRR- or SSS-[PO(N(*CH(CH3)Ph)3]3-) are constructed from Pd3 building units supported by tris(imido)phosphate trianions and chloranilate linkers. These cages exhibit considerable enantioselective separation capabilities toward a series of styrene epoxides via a crystallization inclusion method. A highest enantiomeric excess (ee) value of up to 80% is achieved for the (R)-4-fluorostyrene oxide.

Hierarchical Frameworks of Metal-Organic Cages with Axial Ferroelectric Anisotropy.

Designing molecular crystals with switchable dipoles for ferroelectric applications is challenging and often serendipitous. Herein, we show a systematic approach toward hierarchical 1D, 2D and 3D frameworks that are assembled through successive linkage of metal-organic cages [Cu6 (H2 O)12 (TPTA)8 ]12+ with chloride ions. Their ferroelectric properties are due to the displacement of channel-bound nitrate counterions and solvated water molecules relative to the framework of cages. Ferroelectric measurements of crystals of discrete and 1D-framework assemblies showed axial ferroelectric anisotropy with high remnant polarisation. Both, the reversible formation of cage-connected networks and the observation of ferroelectric anisotropic behaviour are rare among metal-ligand cage assemblies.

A Flexible Composite Mechanical Energy Harvester from a Ferroelectric Organoamino Phosphonium Salt.

A new binary organic salt diphenyl diisopropylamino phosphonium hexaflurophosphate (DPDP⋅PF6 ) was shown to exhibit a good ferroelectric response and employed for mechanical energy harvesting application. The phosphonium salt crystallizes in the monoclinic noncentrosymmetric space group Cc and exhibits an H-bonded 1D chain structure due to N-H⋅⋅⋅F interactions. Ferroelectric measurements on the single crystals of DPDP⋅PF6 gave a well-saturated rectangular hysteresis loop with a remnant (Pr ) polarization value of 6 µC cm-2 . Further, composite devices based on polydimethylsiloxane (PDMS) films for various weight percentages (3, 5, 7, 10 and 20 wt %) of DPDP⋅PF6 were prepared and examined for power generation by using an impact test setup. A maximum output peak-to-peak voltage (VPP ) of 8.5 V and an output peak-to-peak current (IPP ) of 0.5 µA was obtained for the non-poled composite film with 10 wt % of DPDP⋅PF6 . These results show the efficacy of organic ferroelectric substances as potential micropower generators.

Mapping the Assembly of Metal-Organic Cages into Complex Coordination Networks.

Structural transformations of supramolecular assemblies play an important role in the synthesis of complex metal-organic materials. Nonetheless, often little is known of the assembly pathways that lead to the final product. This work describes the conversion of cubic metal-organic polyhedra to connected-cage networks of varying topologies. The neutral cubic cage assembly of formula {Pd3 [PO(NiPr)3 ]}8 (PZDC)12 has been synthesized from {Pd3 [(NiPr)3 PO](OAc)2 (OH)}2 ⋅2 (CH3 )2 SO and 2,5-pyrazenedicarboxilic acid (PZDC-2H). This 42-component self-assembly is the largest known among the neutral cages with PdII ions. The cage contains twenty-four vacant carboxylate O-sites at the PZDC ligands that are available for further coordination. Post-assembly reactions of the cubic cage with FeII and ZnII ions produced cage-connected networks of dia and qtz topologies, respectively. During these reactions, the discrete cubic cage transforms into a network of tetrahedral cages that are bridged by the 3D metal ions. The robustness of the [Pd3 {[PO(NiPr)3 }]3+ molecular building units made it possible to map the post-assembly reactions in detail, which revealed a variety of intermediate 1D and 2D cage networks. Such step-by-step mapping of the transformation of discrete cages to cage-connected frameworks is unprecedented in the chemistry of coordination driven assemblies.