Fluoride Ion Sensing and Caging by a Preformed Molecular D4R Zinc Phosphate Heterocubane

Double-4-ring (D4R) zinc phosphate [Zn­(dipp)­(DMSO)]₄ (<b>1</b>, dipp = 2,6-di-<i>iso</i>-propylphenylphosphate, DMSO = dimethyl sulfoxide), on treatment with a free fluoride ion source, exhibited ability to sense and capture fluoride ions from a variety of sources, as evidenced by extensive solution ³¹P and ¹⁹F NMR spectral titration studies. The fluoride ion-encapsulated cage [^<i>n</i>Bu₄N]­[F@{Zn­(dipp)­(DMSO)}₄] (<b>2</b>) was isolated in good yield from an equimolar reaction between <b>1</b> and ^<i>n</i>Bu₄NF in methanol and characterized by analytical and spectroscopic methods. When 1-methyl-4,4′-bipyridin-1-ium fluoride (MeQ-F) was used as the fluoride ion source a zwitterionic cage [F@{Zn₄(dipp)₄(MeQ)­(DMSO)₃}] (<b>3</b>) was isolated. Crystal structure determination for <b>3</b> confirmed not only fluoride incorporation inside the D4R cage but also a weak interaction of the central fluoride ion with all four zinc centers of the cubane, resulting in a trigonal bipyramidal geometry around the zinc centers. To establish the selectivity, cubane <b>1</b> was treated with 2 equiv of MeQ-X (X = various anions) under similar conditions to isolate [F@{Zn₄(dipp)₄(MeQ)₂(MeOH)₂}]­[X] (X = I <b>4</b>; BF₄ <b>5</b>; PF₆ <b>6</b>) in good yields. The crystal structure determination of <b>4</b> and <b>5</b> showed that the iodide and tetrafluoroborate anions are found outside the cage while fluoride ion has entered the cavity. The final fluoride encapsulated D4R cage is anionic in <b>2</b>, neutral in <b>3</b>, and cationic in <b>4</b>–<b>6</b>, showing the versatility of the cubane framework to stabilize fluoride ions in all three forms. NMR titrations showed that <b>1</b> can sense even 1 ppm level of fluoride ions and sequester them from fluoridated water and toothpaste extract

Containment of Polynitroaromatic Compounds in a Hydrogen Bonded Triarylbenzene Host

Co-crystallization of energetic materials has emerged as an important technique to modify their critical properties such as stability, sensitivity, etc. Using 1,3,5-tris­(4′-aminophenyl)­benzene (TAPB) as a novel co-crystal former, we have prepared co-crystals of 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (TNP), and <i>m</i>-dinitrobenzene (<i>m</i>DNB). Molecular structures of the co-crystals have been determined from single crystal X-ray diffraction data. The diffraction data analysis reveals that strong intermolecular π–π interaction directs the intercalation of polynitroaromatic explosives (PNACs) between the layers of TAPB molecules, which leads to the formation of vertically overlapped -A-B-A-B- types of π-stacks. Both TNT and TNP form π-interactions with the center of TAPB with 1:1 molar ratios, while <i>m</i>DNB forms a complex in a 1:3 stoichiometry through stacking between peripheral rings. The crystal lattices are further stabilized through interstack hydrogen bonds (N–H···N and N–H···O) between amino groups of TAPB and nitro groups of PNACs. NMR and Fourier transform infrared spectra further provide the information about the presence of various interactions in the crystal systems. Owing to the π electron-rich nature and ease of synthesis, triphenylbenzene systems are promising host candidates for co-crystallization of PNAC analytes

Lanthanide Organophosphate Spiro Polymers: Synthesis, Structure, and Magnetocaloric Effect in the Gadolinium Polymer

Spirocyclic lanthanide organophosphate polymers, {[Ln­(dipp)­(dippH)­(CH₃OH)­(H₂O)₂]­(CH₃OH)₂}_<i>n</i> [Ln = La (<b>1</b>), Ce (<b>2</b>), Pr (<b>3</b>), Nd (<b>4</b>), Sm (<b>5</b>), Eu (<b>6</b>), Gd (<b>7</b>), Tb (<b>8</b>), Dy (<b>9</b>), Ho (<b>10</b>), Er (<b>11</b>)], have been prepared from the reaction of Ln­(NO₃)₃·<i>x</i>H₂O with sterically hindered 2,6-diisopropylphenyl phosphate (dippH₂) using aqueous NaOH as the base. The one-dimensional chainlike lanthanide (III) organophosphate coordination polymers have been characterized with the aid of analytical and spectroscopic methods. The single crystal structure determination of polymers (<b>2</b>–<b>5</b> and <b>7</b>–<b>11</b>) reveals that in these compounds the hydrophobic organic groups of the phosphate provide a protective coating for the inorganic lanthanide phosphate polymeric chain. The encapsulation of inorganic lanthanide phosphate core, which has very low solubility product, within the organic groups assists in the facile crystallization of the polymers. The di- and monoanionic organophosphate ligands dipp^2– and dippH^– display [2.111] and [2.110] binding modes, respectively, in <b>2</b>–<b>5</b> and <b>7</b>. However, they exhibit only [2.110] binding mode in the case of <b>8</b>–<b>11</b>. This results in the formation of two different types of polymers. While the lighter rare-earth metal ions in <b>2</b>–<b>5</b> and <b>7</b> display eight coordinate biaugmented trigonal prismatic geometry, the heavier rare-earth metal ions in <b>9</b>–<b>11</b> exhibit a seven coordinate capped trigonal prismatic environment. The Tb­(III) ion in <b>8</b> displays distorted pentagonal bipyramidal geometry. Magnetic studies reveal the presence of weak antiferromagnetic interactions between the Ln­(III) ions through the organophosphate ligand. The isotropic Gd­(III) polymer <b>7</b> exhibits a maximum entropy change of 17.83 J kg^–1 K^–1 for a field change of 7.0 T at 2.5 K, which is significant considering the high molecular weight of the organophosphate ligand. These polymers represent the first family of any structurally characterized rare-earth organophosphate polymers derived from monoesters of phosphoric acid

Dimensionality Alteration and Intra- versus Inter-SBU Void Encapsulation in Zinc Phosphate Frameworks

4,4′-Bipyridine-<i>N</i>-oxide (BIPYMO, <b>1</b>), a less commonly employed coordination polymer linker, has been used as a ditopic spacer to bridge double-four-ring (D4R) zinc phosphate clusters to form novel framework coordination polymers. Zinc phosphate framework compounds [Zn₄(X-dipp)₄(BIPYMO)₂]_<i>n</i>·2MeOH [X = H (<b>2</b>), Cl (<b>3</b>), Br (<b>4</b>), I (<b>5</b>); dipp = 2,6-diisopropylphenyl phosphate] have been obtained by treating a methanol solution of zinc acetate with X-dippH₂ and BIPYMO (in a 1:1:1 molar ratio) at ambient conditions. Framework phosphates <b>2</b>–<b>5</b> can also be obtained by treating the preformed D4R cubanes [Zn­(X-dipp)­(DMSO)]₄ with required quantities of BIPYMO in methanol. Single-crystal X-ray diffraction studies reveal that these framework solids are two-dimensional (2D) networks as opposed to the diamondoid networks obtained when the parent unoxidized 4,4′-bipyridine is used as the linker (<i>Inorg. Chem.</i> <b>2014</b>, <i>53</i>, 8959). The two types of voids (viz., smaller intra-D4R and larger inter-D4R) present in these framework solids can be utilized for different types of encapsulation processes. For example, the in situ generated 2D framework <b>2</b> encapsulates fluoride ions accompanied by a change in the dimensionality of the framework to yield {[(<i>n</i>C₄H₉)₄N]­[F@(Zn₄(dipp)₄(BIPYMO)₂)]}_<i>n</i> (<b>6</b>). The three-dimensional framework <b>6</b> represents the first structurally characterized example of a fluoride-ion-encapsulated polymeric coordination compound or a metal–organic framework. The possibility of utilizing inter-D4R voids as hosts for small organic molecules has been explored by treating in situ generated <b>2</b> with a series of organic molecules of appropriate size. Framework <b>2</b> has been found to be a selective host for benzil and not for other structurally similar molecules such as benzoquinone, benzidine, anthracene, naphthalene, α-pyridoin, etc. The benzil-occluded isolated framework [benzil@{Zn₄(dipp)₄(BIPYMO)₂}]_<i>n</i> (<b>7</b>) has been isolated as single crystals, and its crystal structure determination revealed the binding of benzil molecules to the framework through strong π–π interactions

Is Single-4-Ring the Most Basic but Elusive Secondary Building Unit That Transforms to Larger Structures in Zinc Phosphate Chemistry?

Haloaryl phosphates (X-dippH₂, X = Cl, Br, I) react with zinc acetate in the presence of collidine or 2-aminopyridine (2-apy) to yield zinc phosphate clusters [Zn­(X-dipp)­(collidine)]₄ (X = Cl (<b>1</b>), Br (<b>2</b>), I (<b>3</b>)) and [Zn­(X-dipp)­(2-apy)]₄·2MeOH (X = Cl (<b>4</b>), Br (<b>5</b>), I (<b>6</b>)), respectively. Single-crystal X-ray diffraction studies reveal that collidine and 2-apy capped zinc phosphates <b>1</b>–<b>6</b> exist as discrete tetrameric zinc phosphate molecules, exhibiting a cubane-shaped D4R core. In contrast, when the same reaction has been carried out in the presence of 4-cyanopyridine (4-CNpy), polymeric zinc phosphates {[Zn₄(X-dipp)₄(4-CNpy)₂(MeOH)₂]·2H₂O}_<i>n</i> (X = Cl (<b>7</b>), Br (<b>8</b>), I (<b>9</b>)) have been isolated. Compounds <b>7</b>–<b>9</b> are square-wave-shaped, one-dimensional polymers composed of fused S4R repeating units. The common structural motif found both in D4R cubanes <b>1</b>–<b>6</b> and polymers <b>7</b>–<b>9</b> is the S4R building block, which presumably undergoes further fusion because of the coordinative unsaturation at zinc and the simultaneous presence of free PO. The closed shell cubanes <b>1</b>–<b>6</b> are obviously formed by a <i>face-to-face</i> dimerization involving two S4R units in which the two PO groups are in cis-configuration. On the other hand, the one-dimensional (1-D) square-wave polymers <b>7</b>–<b>9</b> are formed from a face-to-face association of S4R building units in which the two PO groups are in a trans-configuration. In order to stabilize these elusive S4R zinc phosphates, the reaction between Cl-dippH₂ and zinc acetate was carried out in the presence of excess imidazole as an ancillary ligand (1:1:4), although only an imidazole decorated cubane cluster [Zn­(Cl-dipp)­(imz)]₄.2MeOH (<b>10</b>) was isolated. The chelating <i>N</i>,<i>N</i>′-donor 1,10-phenanthroline ligand was used to eventually isolate cyclic S4R phosphate [Zn­(μ₂-Cl-dipp)­(1,10-phen)­(OH₂)]₂·MeOH·H₂O (<b>11</b>). The change of Zn²⁺ source to zinc nitrate and the phosphate source to 2,6-dimethylphenyl phosphate (dmppH₂) led to the isolation of another polymeric phosphate [Zn­(dmpp)­(MeOH)]_<i>n</i> (<b>12</b>), with a zigzag backbone, formed through an <i>edge-to-edge</i> to polymerization of S4R building units with PO groups in trans-configuration. The isolation of four different structural types of zinc phosphates <b>A</b>–<b>D</b> in the present study can be rationalized in terms of fusion of S4R rings in a variety of ways to either produce discrete clusters or 1-D polymers

Thermolabile Organotitanium Monoalkyl Phosphates: Synthesis, Structures, and Utility as Epoxidation Catalysts and Single-Source Precursors for TiP₂O₇

The reaction of [Cp*TiCl₃] (Cp* = C₅Me₅) with monoalkyl phosphates (RO)­PO₃H₂ (R = Me, Et, and ^<i>i</i>Pr) in tetrahydrofuran (THF) at 25 °C leads to the formation of binuclear complexes [Cp*₂Ti₂(μ-O₂P­(OH)­OR)₂(μ-O₂P­(O)­OR)₂] [R = Me (<b>1</b>), Et (<b>2</b>), and ^<i>i</i>Pr (<b>3</b>)]. On the other hand, the reaction of (^<i>t</i>BuO)₂PO₂K with [Cp*TiCl₃] in acetonitrile or THF results in isolation of either the dinuclear [Cp*₂Ti₂(μ-O₂P­(OH)­O^<i>t</i>Bu)₂(μ-O₂P­(O)­O^<i>t</i>Bu)₂] (<b>4</b>) or the trinuclear titanophosphate [Cp*₃Ti₃(μ-O₃PO^<i>t</i>Bu)₂(μ-O)₂(μ-O₂P­(O^<i>t</i>Bu)₂)] (<b>5</b>), respectively. The formation of compounds <b>4</b> and <b>5</b> is facilitated by partial hydrolysis of the <i>tert</i>-butoxy groups of (^<i>t</i>BuO)₂PO₂K. New titanophosphates <b>1</b>–<b>5</b> have been characterized by spectroscopic and analytical methods, and the molecular structures have further been confirmed by single-crystal X-ray diffraction studies. Thermal decomposition studies of <b>1</b>–<b>5</b> reveal the initial loss of thermally labile alkyl substituents of the organophosphate ligands, followed by the loss of C₅Me₅ groups to form an organic-free amorphous titanophosphate in the temperature range 300–500 °C. This material transforms to highly crystalline titanium pyrophosphate TiP₂O₇ at 800 °C. Compounds <b>1</b>–<b>5</b> and the TiP₂O₇ materials obtained at 300, 500, and 800 °C through the thermal decomposition of <b>3</b> have been employed as efficient homogeneous catalysts for the alkene epoxidation reaction. Using hydrogen peroxide as the oxidant in an acetonitrile medium, these catalysts exhibit >90% alkene conversion with >90% epoxide selectivity in 4 h at temperatures below 100 °C

Bulky Isopropyl Group Loaded Tetraaryl Pyrene Based Azo-Linked Covalent Organic Polymer for Nitroaromatics Sensing and CO₂ Adsorption

An azo-linked covalent organic polymer, Py-azo-COP, was synthesized by employing a highly blue-fluorescent pyrene derivative that is multiply substituted with bulky isopropyl groups. Py-azo-COP was investigated for its sensing and gas adsorption properties. Py-azo-COP shows selective sensing toward the electron-deficient polynitroaromatic compound picric acid among the many other competing analogs that were investigated. Apart from its chemosensing ability, Py-azo-COP (surface area 700 m² g^–1) exhibits moderate selectivity toward adsorption of CO₂ and stores up to 8.5 wt % of CO₂ at 1 bar and 18.2 wt % at 15.5 bar at 273 K, although this is limited due to the electron-rich −NN– linkages being flanked by isopropyl groups. Furthermore, the presence of a large number of isopropyl groups imparts hydrophobicity to Py-azo-COP, as confirmed by the increased adsorption of toluene compared to that of water in the pores of the COP

Octanuclear Zinc Phosphates with Hitherto Unknown Cluster Architectures: Ancillary Ligand and Solvent Assisted Structural Transformations Thereof

Structural variations in zinc phosphate cluster chemistry have been achieved through a careful selection of phosphate ligand, ancillary ligand, and solvent medium. The use of 4-haloaryl phosphates (X-dippH₂) as phosphate source in conjunction with 2-hydroxypyridine (hpy) ancillary ligand in acetonitrile solvent resulted in the isolation of the first examples of octameric zinc phosphates [Zn₈(X-dipp)₈(hpy)₄­(CH₃CN)₂(H₂O)₂]·4H₂O (X = Cl <b>2</b>, Br <b>3</b>) and not the expected tetranuclear D4R cubane clusters. Use of 2,3-dihydroxypyridine (dhpy) as ancillary ligand, under otherwise similar reaction conditions with the same set of phosphate ligands and solvent, resulted in isolation of another type of octanuclear zinc phosphate clusters {[(Zn₈(X-dipp)₄(X-dippH)₄­(dhpyH)₄­(dhpyH₂)₂(H₂O)₂]·2solvent} (X = Cl, solvent = MeCN <b>4</b>; Br, solvent = H₂O <b>5</b>), as the only isolated products. X-ray crystal diffraction studies reveal that <b>2</b> and <b>3</b> are octanuclear clusters that are essentially formed by edge fusion of two D4R zinc phosphates. Although <b>4</b> and <b>5</b> are also octanuclear clusters, they exhibit a completely different cluster architecture and have been presumably formed by the ability of 2,3-dihydroxypyridine to bridge zinc centers in addition to the X-dipp ligands. Dissolution of both types of octanuclear clusters in DMSO followed by crystallization yields D4R cubanes [Zn­(X-dipp)­(DMSO)]₄ (X = Cl <b>6</b>, Br <b>7</b>), in which the ancillary ligands such as hpy, H₂O, and CH₃CN originally present on the zinc centers of <b>2</b>–<b>5</b> have been replaced by DMSO. DFT calculations carried out to understand the preference of Zn₈ versus Zn₄ clusters in different solvent media reveal that use of CH₃CN as solvent favors the formation of fused cubanes of the type <b>2</b> and <b>3</b>, whereas use of DMSO as the solvent medium promotes the formation of D4R structures of the type <b>6</b> and <b>7</b>. The calculations also reveal that the vacant exocluster coordination sites on the zinc centers at the bridgehead positions prefer coordination by water to hpy or CH₃CN. Interestingly, the initially inaccessible D4R cubanes [Zn­(X-dipp)­(hpy)]₄·2MeCN (X = Cl <b>8</b>, Br <b>9</b>) could be isolated as the sole products from the corresponding DMSO-decorated cubanes <b>6</b> and <b>7</b> by combining them with hpy in CH₃CN

Octanuclear Zinc Phosphates with Hitherto Unknown Cluster Architectures: Ancillary Ligand and Solvent Assisted Structural Transformations Thereof

Structural variations in zinc phosphate cluster chemistry have been achieved through a careful selection of phosphate ligand, ancillary ligand, and solvent medium. The use of 4-haloaryl phosphates (X-dippH₂) as phosphate source in conjunction with 2-hydroxypyridine (hpy) ancillary ligand in acetonitrile solvent resulted in the isolation of the first examples of octameric zinc phosphates [Zn₈(X-dipp)₈(hpy)₄­(CH₃CN)₂(H₂O)₂]·4H₂O (X = Cl <b>2</b>, Br <b>3</b>) and not the expected tetranuclear D4R cubane clusters. Use of 2,3-dihydroxypyridine (dhpy) as ancillary ligand, under otherwise similar reaction conditions with the same set of phosphate ligands and solvent, resulted in isolation of another type of octanuclear zinc phosphate clusters {[(Zn₈(X-dipp)₄(X-dippH)₄­(dhpyH)₄­(dhpyH₂)₂(H₂O)₂]·2solvent} (X = Cl, solvent = MeCN <b>4</b>; Br, solvent = H₂O <b>5</b>), as the only isolated products. X-ray crystal diffraction studies reveal that <b>2</b> and <b>3</b> are octanuclear clusters that are essentially formed by edge fusion of two D4R zinc phosphates. Although <b>4</b> and <b>5</b> are also octanuclear clusters, they exhibit a completely different cluster architecture and have been presumably formed by the ability of 2,3-dihydroxypyridine to bridge zinc centers in addition to the X-dipp ligands. Dissolution of both types of octanuclear clusters in DMSO followed by crystallization yields D4R cubanes [Zn­(X-dipp)­(DMSO)]₄ (X = Cl <b>6</b>, Br <b>7</b>), in which the ancillary ligands such as hpy, H₂O, and CH₃CN originally present on the zinc centers of <b>2</b>–<b>5</b> have been replaced by DMSO. DFT calculations carried out to understand the preference of Zn₈ versus Zn₄ clusters in different solvent media reveal that use of CH₃CN as solvent favors the formation of fused cubanes of the type <b>2</b> and <b>3</b>, whereas use of DMSO as the solvent medium promotes the formation of D4R structures of the type <b>6</b> and <b>7</b>. The calculations also reveal that the vacant exocluster coordination sites on the zinc centers at the bridgehead positions prefer coordination by water to hpy or CH₃CN. Interestingly, the initially inaccessible D4R cubanes [Zn­(X-dipp)­(hpy)]₄·2MeCN (X = Cl <b>8</b>, Br <b>9</b>) could be isolated as the sole products from the corresponding DMSO-decorated cubanes <b>6</b> and <b>7</b> by combining them with hpy in CH₃CN

Ab Initio Chemical Synthesis of Designer Metal Phosphate Frameworks at Ambient Conditions

Stepwise hierarchical and rational synthesis of porous zinc phosphate frameworks by predictable and directed assembly of easily isolable tetrameric zinc phosphate [Zn­(dipp)­(solv)]₄ (dippH₂ = diisopropylphenyldihydrogen phosphate; solv = CH₃OH or dimethyl sulfoxide) with D4R (double-4-ring) topology has been achieved. The preformed and highly robust D4R secondary building unit can be coordinatively interconnected through a varied choice of bipyridine-based ditopic spacers L1–L7 to isolate eight functional zinc phosphate frameworks, [Zn₄(dipp)₄(L1)_1.5(DMSO)]·4H₂O (<b>2</b>), [Zn₄(dipp)₄(L2)_1.5(CH₃OH)] (<b>3</b>), [Zn₄(dipp)₄(L1)₂] (<b>4</b>), [Zn₄(dipp)₄(L3)₂] (<b>5</b>), [Zn₄(dipp)₄(L4)₂] (<b>6</b>), [Zn₄(dipp)₄(L5)₂] (<b>7</b>), [Zn₄(dipp)₄(L6)₂] (<b>8</b>), and [Zn₄(dipp)₄(L7)₂] (<b>9</b>), in good yield. The preparative procedures are simple and do not require high pressure or temperature. Surface area measurements of these framework solids show that the guest accessibility of the frameworks can be tuned by suitable modification of bipyridine spacers