Diversities of Coordination Geometry Around the Cu²⁺ Center in Bis(maleonitriledithiolato)metalate Complex Anions: Geometry Controlled by Varying the Alkyl Chain Length of Imidazolium Cations

Six new ion-pair metal-bis­(dithiolene) complexes with the formulas [C₉H₁₄N₄]­[Cu­(mnt)₂] <b>(1a</b>), [C₁₀H₁₆N₄]­[Cu­(mnt)₂] (<b>1b</b>), [C₁₁H₁₈N₄]­[Cu­(mnt)₂] (<b>1c</b>), [C₁₂H₂₀N₄]­[Cu­(mnt)₂] (<b>1d</b>), [C₁₃H₂₂N₄]­[Cu­(mnt)₂] (<b>1e</b>), and [C₁₄H₂₄N₄]­[Cu­(mnt)₂] (<b>1f</b>) have been synthesized starting from Cu­(II) salt, Na₂mnt (disodium maleonitriledithiolate), and bromide salts of alkyl-bis­(imidazolium) cations [C₈H₁₂(CH₂)_nN₄Br₂] (<i>n</i> = 1–6, <b>a</b>–<b>f</b>). In this series of ion-pair compounds <b>1a</b>–<b>1f</b>, a common [Cu­(mnt)₂]^2– complex anion is associated with alkyl imidazolium cations of varied alkyl chain lengths. We have described a systematic study of deviation from square planar geometries (in terms of distortion) around the metal ion in customary square planar metal-dithiolene complexes. The distortion in the geometry around the metal ion can be explained on the basis of center of symmetry along C–H···Cu supramolecular interaction and unbalanced supramolecular interactions, such as S···H, N···H, and M···S type weak contacts. Dianionic copper­(II) complexes <b>1a</b>–<b>1f</b> show an electronic absorption in the near-infrared (NIR) region, which has been attributed to the charge transfer transition from the highest occupied molecular orbital level of copper dithiolate anion [Cu­(mnt)₂]^2– to the lowest unoccupied molecular orbital level of alkyl imidazolium cation [C₈H₁₂(CH₂)_<i>n</i>N₄]²⁺. All these compounds are unambiguously characterized by single crystal X-ray crystallography and further characterized by IR, ¹H NMR, electron spin resonance, LC/MS spectroscopic techniques, and electrochemical studies

Diverse Supramolecular Architectures Having Well-Defined Void Spaces Formed from a Pseudorotaxane Cation: Influential Role of Metal Dithiolate Coordination Complex Anions

This paper describes the influence of a group of classical inorganic coordination complex anions on assembling a particular pseudorotaxane cation (the crown ether, dibenzo-24-crown-8 threaded by an axle, 1,2-bis­(4,4′-bipyridinium) ethane) resulting in a series of supramolecular ion pair compounds, namely, [pseudorotaxane]­[Cu­(mnt)₂] (<b>1</b>), [pseudorotaxane]­[Ni­(mnt)₂] (<b>2</b>), [pseudorotaxane]­[Pd­(mnt)₂] (<b>3</b>), and [pseudorotaxane]­[Zn­(dmit)₂] (<b>5</b>) of varying dimensions in terms of their topology; dithiolene = mnt^2–(1,2-dicyanoethylenedithiolate) and dmit^2–(1,3-dithiole-2-thione-4,5-dithiolate). The shapes of supramolecular framework void spaces of diverse dimensions, that are observed in the crystal structures of compounds <b>1</b>–<b>3</b>, are influenced by the geometry of particular coordination complex anions, used in the relevant synthesis, and the concerned coordination complex gets encapsulated in the void spaces of respective supramolecular pseudorotaxane frameworks. The platinum compound [pseudorotaxane]­[Pt­(mnt)₂] (<b>4</b>) is found to be an exception in forming well-defined void spaces. The crystal structure of compound [pseudorotaxane]­[Zn­(dmit)₂] (<b>5</b>) reveals an interesting aggregation of supramolecular ladders, in which each compartment of the ladders accommodates the complex anion Zn­(dmit)₂]^2–. The shape of this coordination complex anion seems to be responsible for such ladderlike arrangement in the relevant crystals. Compounds <b>1</b> through <b>5</b> have been characterized by routine analysis, such as IR, ¹H NMR, UV–Vis–NIR, and electron paramagnetic resonance spectroscopic techniques including elemental analysis, and unambiguously by single crystal X-ray crystallography. The stabilization of such cationic supramolecular pseudorotaxane architectures having well-defined grid-type void spaces is achieved through hydrogen bonding interactions that include C–H···S, C–H···N, and C–H···O, and π–π stacking interactions. The exchange of the complex anion in one of these ion pair compounds (compound <b>1</b>) with Br^– anions (in a solid-to-solid transformation through solid–liquid interface reaction) results in the formation compound [pseudorotaxane]­Br₂ whose X-ray powder pattern is different than that of <b>1</b> indicating a new phase formation in the crystals of [pseudorotaxane]­Br₂

Perceptive Approach in Assessing Rigidity versus Flexibility in the Construction of Diverse Metal–Organic Coordination Networks: Synthesis, Structure, and Magnetism

By associating rigidity and flexibility within the organic building blocks, we have synthesized four new metal–organic coordination polymers, formulated as {Co­(ADA)­(bpbix)}_<i>n</i>·<i>n</i>H₂O (<b>1</b>), {Co­(ADC)­(bpbix)}_<i>n</i> (<b>2</b>), {Co­(ADA)­(bpim)}_<i>n</i>·<i>n</i>H₂O (<b>3</b>), and {Co₂(ADC)₂(bpim)}_<i>n</i> (<b>4</b>), by using the adamantane based flexible dicarboxylate ligand H₂ADA and rigid dicarboxylate ligand H₂ADC along with flexible bis-imidazole linker, bpbix and rigid bis-imidazole linker, bpim as coligands (where H₂ADA = 1,3-adamantanediacetic acid; H₂ADC = 1,3-adamantanedicarboxylic acid; bpbix = 4,4′-bis­((1<i>H</i>-imidazol-1-yl)­methyl)­biphenyl; bpim = 4,4′-di­(1<i>H</i>-imidazol-1-yl)­biphenyl). Compounds <b>1</b>–<b>4</b> have been characterized by routine elemental analysis, IR spectroscopy, thermogravimetric (TG) analysis and unambiguously by single crystal X-ray diffraction analysis. In the crystal structures of these compounds <b>1</b>–<b>4</b>, diverse architectures, have been observed, formation of which is facilitated by the conformation rigidity and flexibility of the ligands. The role of the interchanging between flexibility and rigidity of both the adamantine- and bis-imidazole-based ligands in assessing the diversity in the resulting architectures has been discussed. In addition, temperature-dependent magnetic studies for the compounds <b>1</b>, <b>3</b>, and <b>4</b> have been described

Hydrothermal Synthesis and Structural Characterization of Metal Organophosphonate Oxide Materials: Role of Metal-Oxo Clusters in the Self Assembly of Metal Phosphonate Architectures

Two new metal organophosphonate oxide materials with formulas [Cu^II₄Cu^I₂(L)₂(2,2′-bpy)₆(HPW₁₂O₄₀)]_<i>n</i>·4<i>n</i>H₂O (<b>1</b>) and [Cu­(2,2′-bpy)­VO₂(OH)­(H₂L)]_<i>n</i> (<b>2</b>) have been synthesized starting from the Cu­(II) salts, 2,2′-bipyridine (2,2′-bpy), <i>p</i>-xylylenediphosphonic acid (H₄L), and sodium tungstate (for <b>1</b>)/ammonium metavanadate (for <b>2</b>). Both the compounds <b>1</b> and <b>2</b> are characterized by routine elemental analyses, IR spectroscopy, thermogravimetric (TG) analysis, and unambiguously characterized by single crystal X-ray crystallography. The crystal structure of compound <b>1</b> consists of 2D copper phosphonate layers connected by the Keggin heteropolyanion to form a three-dimensional (3D) framework. The copper phosphonate layers in compound <b>1</b> are fabricated by the rare copper hexanuclear clusters in which the four terminal Cu­(II) centers form two eight-membered Cu-dimer (Cu₂P₂O₄) rings (top and the bottom) that are connected to each other by the two central Cu­(I) atoms of four-membered Cu₂O₂ rings. These hexanuclear assemblies are connected to each other along the plane through the p-xylyl linkers to form a two-dimensional (2D) layer. Compound <b>1</b> is a unique example in terms of the existence of a hexanuclear copper phosphonate cluster in the 3D coordination matrix. Compound <b>2</b> has a 2D structure, in which the one-dimensional [Cu­(2,2′-bpy)­(H₂L)]_<i>n</i> chains are connected by the VO₂OH subunits to from a 2D layer. The formation of VO₂OH in compound <b>2</b> ceases the formation of eight-membered Cu-dimer rings. The self-assembly of the polyoxometalates plays an important role in the formation of the metal organophosphonate phases

Hydrothermal Synthesis and Structural Characterization of Metal Organophosphonate Oxide Materials: Role of Metal-Oxo Clusters in the Self Assembly of Metal Phosphonate Architectures

Two new metal organophosphonate oxide materials with formulas [Cu^II₄Cu^I₂(L)₂(2,2′-bpy)₆(HPW₁₂O₄₀)]_<i>n</i>·4<i>n</i>H₂O (<b>1</b>) and [Cu­(2,2′-bpy)­VO₂(OH)­(H₂L)]_<i>n</i> (<b>2</b>) have been synthesized starting from the Cu­(II) salts, 2,2′-bipyridine (2,2′-bpy), <i>p</i>-xylylenediphosphonic acid (H₄L), and sodium tungstate (for <b>1</b>)/ammonium metavanadate (for <b>2</b>). Both the compounds <b>1</b> and <b>2</b> are characterized by routine elemental analyses, IR spectroscopy, thermogravimetric (TG) analysis, and unambiguously characterized by single crystal X-ray crystallography. The crystal structure of compound <b>1</b> consists of 2D copper phosphonate layers connected by the Keggin heteropolyanion to form a three-dimensional (3D) framework. The copper phosphonate layers in compound <b>1</b> are fabricated by the rare copper hexanuclear clusters in which the four terminal Cu­(II) centers form two eight-membered Cu-dimer (Cu₂P₂O₄) rings (top and the bottom) that are connected to each other by the two central Cu­(I) atoms of four-membered Cu₂O₂ rings. These hexanuclear assemblies are connected to each other along the plane through the p-xylyl linkers to form a two-dimensional (2D) layer. Compound <b>1</b> is a unique example in terms of the existence of a hexanuclear copper phosphonate cluster in the 3D coordination matrix. Compound <b>2</b> has a 2D structure, in which the one-dimensional [Cu­(2,2′-bpy)­(H₂L)]_<i>n</i> chains are connected by the VO₂OH subunits to from a 2D layer. The formation of VO₂OH in compound <b>2</b> ceases the formation of eight-membered Cu-dimer rings. The self-assembly of the polyoxometalates plays an important role in the formation of the metal organophosphonate phases

Isolation of Blackberry-Shaped Nanoparticles of a Giant {Mo₇₂Fe₃₀} Cluster and Their Transformation to a Crystalline Nanoferric Molybdate

When an aqueous solution of sodium molybdate is added to an aqueous solution of ferric chloride, acidified with acetic acid, a giant {Mo₇₂Fe₃₀} cluster is instantaneously formed as the amorphous substance Na₂­[Mo₇₂Fe₃₀O₂₅₂­(CH₃COO)₄­(OH)₁₆­(H₂O)₁₀₈]·180 H₂O (<b>1</b>). Compound <b>1</b> consists of aggregated nanovesicles of {Mo₇₂Fe₃₀} clusters, as confirmed by field-emission scanning electron microscopy and transmission electron microscopy images of <b>1</b>. An aqueous suspension of <b>1</b> upon moderate heating results in the formation of crystalline nanoferric molybdate, which gives insight into understanding the formation of a yellow coating mineral, ferrimolybdite, frequently found on the ores of molybdenum

Design of Flexible Metal–Organic Framework-Based Superprotonic Conductors and Their Fabrication with a Polymer into Proton Exchange Membranes

In recent times, the deployment of metal–organic frameworks (MOFs) to develop efficient proton conductors has gained immense popularity in the arena of sustainable energy research due to the ease of structural and functional tunability in MOFs. In this work, we have focused on developing “flexible MOF”-based proton conductors with Fe-MIL-53-NH2 and Fe-MIL-88B-NH2 MOFs using postsynthetic modification (PSM) as the tool. Taking advantage of the porous nature of these frameworks, we have carried out PSM on the primary amine groups present on the MOFs and converted them to −NH(CH2CH2CH2SO3H) groups. The PSM increased the number of labile protons in the channels of the modified MOFs as well as the extent of H-bonded networks inside the framework. The modified Fe-MIL-53-NH2 and Fe-MIL-88B-NH2 MOFs, named hereafter as 53-S and 88B-S, respectively, showed proton conductivity of 1.298 × 10–2 and 1.687 × 10–2 S cm–1 at ∼80 °C and 98% relative humidity (RH), respectively. This reflects ∼10-fold and ∼5-fold increases in their proton conductivity than their respective parent MOFs. Since MOFs as such are difficult to make directly into flexible membranes, and these are essential for practical applications as proton conductors, we have incorporated 53-S and 88B-S as fillers into a robust imidazole-based polymer matrix, namely, OPBI [poly(4,4′-diphenylether-5,5′-bibenzimidazole)]. The resulting polymer–MOF mixed matrix membranes (MMMs) after doping with phosphoric acid (PA) performed as flexible proton exchange membranes (PEMs) above 100 °C under anhydrous conditions and were found to be much more efficient and stable than the pristine OPBI membrane (devoid of any filler loading). By optimizing the amount of filler loading in the membrane, we obtained the highest proton conductivity of 0.304 S cm–1 at 160 °C under anhydrous conditions

Polyoxometalate-Supported Bis(2,2′-bipyridine)mono(aqua)nickel(II) Coordination Complex: an Efficient Electrocatalyst for Water Oxidation

A polyoxometalate (POM)-supported nickel­(II) coordination complex, [Ni^II(2,2′-bpy)₃]₃[{Ni^II(2,2′-bpy)₂(H₂O)}­{HCo^IIW^VI₁₂O₄₀}]₂·3H₂O (<b>1</b>; 2,2′-bpy = 2,2′-bipyridine), has been synthesized and structurally characterized. We could obtain a relatively better resolved structure from dried crystals of <b>1</b>, Ni^II(2,2′-bpy)₃]₃[{Ni^II(2,2′-bpy)₂(H₂O)}­{HCo^IIW^VI₁₂O₄₀}]₂·H₂O (<b>D1</b>). Because the title compound has been characterized with a {Ni^II(2,2′-bpy)₂(H₂O)}²⁺ fragment coordinated to the surface of the Keggin anion ([H­(Co^IIW₁₂O₄₀]^5–) via a terminal oxo group of tungsten and the [Ni^II(2,2′-bpy)₃]²⁺ coordination complex cation sitting as the lattice component in the concerned crystals, the electronic spectroscopy of compound <b>1</b> has been described by comparing its electronic spectral features with those of [Ni^II(2,2′-bpy)₂(H₂O)­Cl]­Cl, [Ni^II(2,2′-bpy)₃]­Cl₂, and K₆[Co^IIW₁₂O₄₀]·6H₂O. Most importantly, compound <b>1</b> can function as a heterogeneous and robust electrochemical water oxidation catalyst (WOC). To gain insights into the water oxidation (WO) protocol and to interpret the nature of the active catalyst, diverse electrochemical experiments have been conducted. The mode of action of the WOC during the electrochemical process is accounted for by confirmation that there was no formation/participation of metal oxide during various controlled experiments. It is found that the title compound acts as a true catalyst that has Ni^II (coordinated to POM surface) acting as the active catalytic center. It is also found to follow a proton-coupled electron-transfer pathway (two electrons and one proton) for WO catalysis with a high turnover frequency of 18.49 (mol of O₂)­(mol of Ni^II)⁻¹ s^–1

Fate of a Giant {Mo₇₂Fe₃₀}‑Type Polyoxometalate Cluster in an Aqueous Solution at Higher Temperature: Understanding Related Keplerate Chemistry, from Molecule to Material

When the giant icosahedral {Mo₇₂Fe₃₀} cluster containing compound [Mo₇₂Fe₃₀O₂₅₂­(CH₃COO)₁₂­{Mo₂O₇­(H₂O)}₂­{H₂Mo₂O₈­(H₂O)}­(H₂O)₉₁]·150H₂O (<b>1</b>) is refluxed in water for 36 h, it results in the formation of nanoiron molybdate, Fe₂(MoO₄)₃, in the form of a yellow precipitate; this simple approach not only generates nanoferric molybdate at a moderate temperature but also helps to understand the stability of {Mo₇₂Fe₃₀} in terms of the linker–pentagon complementary relationship

Mechanistic Aspects for the Formation of Copper Dimer Bridged by Phosphonic Acid and Extending Its Dimensionality by Organic and Inorganic Linkers: Synthesis, Structural Characterization, Magnetic Properties, and Theoretical Studies

Six new copper metal complexes with formulas [Cu­(H₂O)­(2,2′-bpy)­(H₂L)]₂·H₄L·4H₂O (<b>1</b>), [{Cu­(H₂O)­(2,2′-bpy)­(H₃L)}₂(H₂L)]·2H₂O (<b>2</b>), [Cu­(H₂O)­(1,10-phen)­(H₂L)]₂·6H₂O (<b>3</b>), [Cu­(2,2′-bpy)­(H₂L)]_<i>n</i>·<i>n</i>H₂O (<b>4</b>), [Cu­(1,10-phen)­(H₂L)]_<i>n</i>·3<i>n</i>H₂O (<b>5</b>), and [{Cu­(2,2′-bpy)­(MoO₃)}₂(L)]_<i>n</i>·2<i>n</i>H₂O (<b>6</b>) have been synthesized starting from <i>p</i>-xylylenediphosphonic acid (H₄L) and 2,2′-bipyridine (2,2′-bpy) or 1,10-phenanthroline (1,10-phen) as secondary linkers and characterized by single crystal X-ray diffraction analysis, IR spectroscopy, and thermogravimetric (TG) analysis. All the complexes were synthesized by hydrothermal methods. A dinuclear motif (Cu-dimer) bridged by phosphonic acid represents a new class of simple building unit (SBU) in the construction of coordination architectures in metal phosphonate chemistry. The initial pH of the reaction mixture induced by the secondary linker plays an important role in the formation of the molecular phosphonates <b>1</b>, <b>2</b>, and <b>3</b>. Temperature dependent hydrothermal synthesis of the compounds <b>1</b>, <b>2</b>, and <b>3</b> reveals the mechanism of the self-assembly of the compounds based on the solubility of the phosphonic acid H₄L. Two-dimensional coordination polymers <b>4</b>, <b>5</b>, and <b>6</b>, which are formed by increasing the pH of the reaction mixture, comprise Cu-dimers as nodes, organic (H₂L) and inorganic (Mo₄O₁₂) ligands as linkers. The void space-areas, created by the (4,4) connected nets in compounds <b>4</b> and <b>5</b>, are occupied by lattice water molecules. Thus compounds <b>4</b> and <b>5</b> have the potential to accommodate guest species/molecules. Variable temperature magnetic studies of the compounds <b>3</b>, <b>4</b>, <b>5</b>, and <b>6</b> reveal the antiferromagnetic interactions between the two Cu­(II) ions in the eight-membered ring, observed in their crystal structures. A density functional theory (DFT) calculation correlates the conformation of the Cu-dimer ring with the magnitude of the exchange parameter based on the torsion angle of the conformation