From Ordinary to Extraordinary: Insights into the Formation Mechanism and pH-Dependent Assembly/Disassembly of Nanojars

Conversion of Metal Pyrazolate/(Hydr)oxide Clusters into Nanojars: Solution vs Solid-State Structure and Magnetism

Nanojars are a class of anion binding and extraction agents composed of a series of [Cu(μ-OH)(μ-pz)]n (pz = pyrazolate; n = 26-36) supramolecular metal-organic complexes. In contrast to other anion binding agents amenable to liquid-liquid extraction, nanojars only form by self-assembly around the target anion, and guest-free nanojar hosts cannot be isolated. An extraordinary binding strength toward highly hydrophilic anions such as carbonate and sulfate was demonstrated by the inability of Ba2+ ions to precipitate the corresponding insoluble barium salts from nanojars. Herein, we provide an additional proof for the superior robustness of the nanojar framework based on competition experiments with other transition metal pyrazolate/(hydr)oxide complexes. In addition to the mass spectrometric characterization, we present variable-temperature nuclear magnetic resonance studies with an emphasis on the influence of the paramagnetic Cu2+ centers on 1H hyperfine shifts, along with X-ray crystallographic analysis of two polymorphs of (MePh3P)2[CO3⊂{Cu(OH)(pz)}27], including the highest (cubic) symmetry nanojar crystal lattice obtained to date as well as magnetism studies for the first time. Furthermore, we provide evidence for the first molybdate-incarcerating nanojars, [MoO4⊂{Cu(μ-OH)(μ-pz)}n]2- (n = 28, 31-33), formed by rearrangement from [MoVI8O12(μ-O)9(μ-pz)6(pzH)6·3pzH] in the presence of Cu2+ ions.

An octa-nuclear nickel(II) pyrazolate cluster with a cubic Ni8 core and its methyl- and n-octyl-functionalized derivatives

The mol-ecular and crystal structure of a discrete [Ni8(μ4-OH)6(μ-4-Rpz)12]2- (R = H; pz = pyrazolate anion, C3H3N2 -) cluster with an unprecedented, perfectly cubic arrangement of its eight Ni centers is reported, along with its lower-symmetry alkyl-functionalized (R = methyl and n-oct-yl) derivatives. Crystals of the latter two were obtained with two identical counter-ions (Bu4N+), whereas the crystal of the complex with the parent pyrazole ligand has one Me4N+ and one Bu4N+ counter-ion. The methyl derivative incorporates 1,2-di-chloro-ethane solvent mol-ecules in its crystal structure, whereas the other two are solvent-free. The compounds are tetra-butyl-aza-nium tetra-methyl-aza-nium hexa-μ4-hydroxido-dodeca-μ2-pyrazolato-hexa-hedro-octa-nickel, (C16H36N)(C4H12N)[Ni8(C3H3N2)12(OH)6] or (Bu4N)(Me4N)[Ni8(μ4-OH)6(μ-pz)12] (1), bis-(tetra-butyl-aza-nium) hexa-μ4-hydroxido-dodeca-μ2-(4-methyl-pyrazolato)-hexa-hedro-octa-nickel 1,2-di-chloro-ethane 7.196-solvate, (C16H36N)2[Ni8(C4H5N2)12(OH)6]·7.196C2H4Cl2 or (Bu4N)2[Ni8(μ4-OH)6(μ-4-Mepz)12]·7.196(ClCH2CH2Cl) (2), and bis-(tetra-butyl-aza-nium) hexa-μ4-hydroxido-dodeca-μ2-(4-octylpyrazolato)-hexa-hedro-octa-nickel, (C16H36N)2[Ni8(C11H19N2)12(OH)6] or (Bu4N)2[Ni8(μ4-OH)6(μ-4-nOctpz)12] (3). All counter-ions are disordered (with the exception of one Bu4N+ in 3). Some of the octyl chains of 3 (the crystal is twinned by non-merohedry) are also disordered. Various structural features are discussed and contrasted with those of other known [Ni8(μ4-OH)6(μ-4-Rpz)12]2- complexes, including extended three-dimensional metal-organic frameworks. In all three structures, the Ni8 units are lined up in columns.

Chromate Incarceration by Nanojars and Its Removal from Water by Liquid-Liquid Extraction

The unprecedented liquid-liquid extraction of the dinegative chromate ion (CrO₄^2-) from neutral aqueous solutions into aliphatic hydrocarbon solvents using nanojars as extraction agents is demonstrated. Transferring chromate from water into an organic solvent is extremely challenging due to its large hydration energy (ΔG_h° = -950 kJ/mol) and strong oxidizing ability. Owing to their highly hydrophilic anion binding pockets lined by a multitude of hydrogen bond donor OH groups, neutral nanojars of the formula [cis-Cu^II(μ-OH)(μ-4-Rpz)]_n (n = 27-33; pz = pyrazolate anion; R = H or n-octyl) strongly bind the CrO₄^2- ion and efficiently transfer it from water into n-heptane or C₁₁ - C₁₃ isoalkanes (when R = n-octyl). The extracted chromate can easily be recovered from the organic layer by stripping with an aqueous acid solution. Electrospray ionization mass spectrometric, UV-vis and paramagnetic ¹H NMR spectroscopic, X-ray crystallographic, and thermal stability studies in solution and chemical stability studies toward NH₃, methanol, and Ba²⁺ ions are employed to explore the binding of the CrO₄^2- ion by nanojars. Titration of carbonate nanojars [CO₃ ⊂ {Cu(OH)(pz)}_n]^2- with H₂CrO₄ leads to anion exchange and the formation of chromate nanojars [CrO₄ ⊂ {Cu(OH)(pz)}_n]^2-. Details of chromate binding by H-bonding based on single-crystal structures of (Bu₄N)₂[CrO₄ ⊂ {Cu(OH)(pz)}₂₈], four pseudopolymorphs of (Bu₄N)₂[CrO₄ ⊂ {Cu(OH)(pz)}₃₁], and also the methoxy-substituted derivative (Bu₄N)₂[CrO₄ ⊂ {Cu₃₁(OH)₃₀(OCH₃)(pz)₃₁}] are presented.

Integration of Trinuclear Triangle Copper(II) Secondary Building Units in Octacyanidometallates(IV)-Based Frameworks

The design and synthesis of high-dimensional materials based on secondary building blocks (SBUs) play a pivotal role in the further development of functional molecular materials. Herein, the self-assembly of Cu(II) ions, pyrazole (Hpz), and octacyanidometallate(IV) anions in the presence of water produced two new isostructural three-dimensional systems {[Cu₃(μ₃-OH)(μ-pz)₃(H₂O)₃]₂[M(CN)₈]}·nH₂O (M = W, 1, and Mo, 2). 1 and 2 consist of trinuclear triangle copper(II) (TTC) SBUs and octacyanidometallates(IV). At room temperature, both assemblies display strong antiferromagnetic interactions within the TTC entities with an average Cu^II···Cu^II isotropic magnetic coupling constant of about -145 cm^-1. Moreover, a detailed analysis of magnetic data revealed the presence of spin frustration with antisymmetric magnetic exchange-coupling constants of around +32 and +46 cm^-1 for 1 and 2, respectively. Finally, quantum chemical calculations explained their magnetic and optical properties.

Supramolecular Incarceration and Extraction of Tetrafluoroberyllate from Water by Nanojars

The previously unexplored noncovalent binding of the highly toxic tetrafluoroberyllate anion (BeF₄^2-) and its extraction from water into organic solvents are presented. Nanojars resemble anion-binding proteins in that they also possess an inner anion binding pocket lined by a multitude of H-bond donors (OH groups), which wrap around the incarcerated anion and completely isolate it from the surrounding medium. The BeF₄-binding propensity of [BeF₄⊂{Cu^II(OH)(pz)}_n]^2- (pz = pyrazolate; n = 27-32) nanojars of different sizes is investigated using an array of techniques including mass spectrometry, paramagnetic ¹H, ⁹Be, and ¹⁹F NMR spectroscopy, and X-ray crystallography, along with thermal stability studies in solution and chemical stability studies toward acidity and Ba²⁺ ions. The latter is found to be unable to precipitate the insoluble BaBeF₄ from nanojar solutions, indicating a very strong binding of the BeF₄^2- anion by nanojars. ⁹Be and ¹⁹F NMR spectroscopy allows for the unprecedented direct probing of the incarcerated anion in a nanojar and, along with ¹H NMR studies, reveals the fluxional structure of nanojars and their inner anion-binding pockets. Single-crystal X-ray diffraction provides the crystal and molecular structures of (Bu₄N)₂[BeF₄⊂{Cu(OH)(pz)}₃₂], which contains a novel Cu_x-ring combination (x = 9 + 14 + 9), (Bu₄N)₂[BeF₄⊂{Cu(OH)(pz)}₈₊₁₄₊₉], and (Bu₄N)₂[BeF₄⊂{Cu(OH)(pz)}_6+12+10] and offers detailed structural parameters related to the supramolecular binding of BeF₄^2- in these nanojars. The extraction of BeF₄^2- from water into organic solvents, including the highly hydrophobic solvent n-heptane, demonstrates that nanojars are efficient binding and extracting agents not only for oxoanions but also for fluoroanions.

Pyrene-Functionalized Fluorescent Nanojars: Synthesis, Mass Spectrometric, and Photophysical Studies

Nanojars are a class of supramolecular coordination complexes based on pyrazolate, Cu2+, and OH- ions that self-assemble around highly hydrophilic anions and serve as efficient anion binding and extraction agents. In this work, the synthesis, characterization, and photophysical properties of pyrene-functionalized fluorescent nanojars are presented. Three pyrene derivatives, 4-(pyren-1-yl)pyrazole (HL1), 4-(5-(pyren-1-yl)pent-4-yn-1-yl)pyrazole (HL2), and 4-(3-(pyrazol-4-yl)propyl)-1-(pyren-1-yl)-1,2,3-triazole (HL3), and the corresponding nanojars were synthesized and characterized using nuclear magnetic resonance spectroscopy and mass spectrometry. Electronic absorption, steady-state, and time-resolved fluorescence measurements were carried out to understand the interaction between the pyrene fluorophore and copper nanojars. Optical absorption measurements have shown minor ground state interaction between the fluorophore and nanojars. The fluorescence of pyrene is significantly quenched when attached to nanojars, suggesting strong contribution from the paramagnetic Cu2+ ions. Significant static quenching is observed in the case of L1, when pyrene is directly bound to the nanojar, whereas in the case of L2 and L3, when pyrene is attached to the nanojars using flexible tethers, both static and dynamic quenching are observed.

Fascinating interlocked triacontanuclear giant nanocages

We report herein three air, thermal and solvent stable interlocked triacontanuclear giant nanocages, generated using a node and spacer concept. Interestingly, the crystal structures of the cages are not only nano-dimensional but also exist in the nano-dimension range, which was corroborated with microscopic images. The combination of microscopic and crystallographic data, in effect, led us to a unique advantageous situation of generating nanomaterials with hard-to-come-by structural information at the molecular level.

Selective binding of anions by rigidified nanojars: sulfate vs. carbonate

Selective binding and transport of highly hydrophilic anions is ubiquitous in nature, as anion binding proteins can differentiate between similar anions with over a million-fold efficiency. While comparable selectivity has occasionally been achieved for certain anions using small, artificial receptors, the selective binding of certain anions, such as sulfate in the presence of carbonate, remains a very challenging task. Nanojars of the formula [anion⊂{Cu(OH)(pz)}_n]^2- (pz = pyrazolate; n = 27-33) are totally selective for either CO₃^2- or SO₄^2- over anions such as NO₃^-, ClO₄^-, BF₄^-, Cl^-, Br^- and I^-, but cannot differentiate between the two. We hypothesized that rigidification of the nanojar outer shell by tethering pairs of pyrazole moieties together will restrict the possible orientations of the OH hydrogen-bond donor groups in the anion-binding cavity of nanojars, similarly to anion-binding proteins, and will lead to selectivity. Indeed, by using either homoleptic or heteroleptic nanojars of the general formula [anion⊂Cu_n(OH)_n(L2-L6)_y(pz)_n-2y]^2- (n = 26-31) based on a series of homologous ligands HpzCH₂(CH₂)_xCH₂pzH (x = 0-4; H₂L2-H₂L6), selectivity for carbonate (with L2 and with L4-L6/pz mixtures) or for sulfate (with L3) has been achieved. The synthesis of new ligands H₂L3, H₂L4 and H₂L5, X-ray crystal structures of H₂L4 and the tetrahydropyranyl-protected derivatives (THP)₂L4 and (THP)₂L5, synthesis and characterization by electrospray-ionization mass spectrometry (ESI-MS) of carbonate- and sulfate-nanojars derived from ligands H₂L2-H₂L6, as well as detailed selectivity studies for CO₃^2-vs. SO₄^2- using these novel nanojars are presented.

Capped Nanojars: Synthesis, Solution and Solid-State Characterization, and Atmospheric CO2 Sequestration by Selective Binding of Carbonate

Nanojars are a class of supramolecular anion-incarcerating coordination complexes that self-assemble from Cu²⁺ ions, pyrazole, and a strong base in the presence of highly hydrophilic anions. In this work, we show that if the strong base (e.g., NaOH or Bu₄NOH) is replaced by a weak base such as a trialkylamine, capped nanojars of the formula [{Cu₃(μ₃-OH)(μ-pz)₃L₃}CO₃⊂{Cu(μ-OH)(μ-pz)}_n] (pz = pyrazolate anion; L = neutral donor molecule; n = 27-31) are obtained instead of the conventional nanojars. Yet, to obtain capped nanojars, the conjugate acid side product originating from the weak base must be separated by transferring it to water either by precipitation of the water-insoluble capped nanojars or by liquid-liquid extraction. Full characterization using electrospray ionization mass spectrometry, UV-vis and variable-temperature ¹H NMR spectroscopy in solution, and single-crystal X-ray diffraction, elemental analysis, and solubility studies in the solid state reveals similarities as well as drastic differences between capped nanojars and nanojars lacking the [Cu₃(μ₃-OH)(μ-pz)₃L₃]²⁺ cap. Acid-base reactivity studies demonstrate that capped nanojars are intermediates in the pH-controlled assembly-disassembly of nanojars. During the self-assembly of capped nanojars, CO₂ is selectively sequestered from air in the presence of other atmospheric gases and converted to carbonate, the binding of which is selective in the presence of NO₃^-, ClO₄^-, BF₄^-, Cl^-, and Br^- ions.

Investigation on the interconversion from DMF-solvated to unsolvated copper(ii) pyrazolate coordination polymers

Enlarging the family of CPs with the {Cu(μ-pz)₂} building unit: synthesis, thermal behaviour, crystal structure and spectroscopic properties of [Cu(μ-4-Xpz)₂(μ-DMF)]_n (4-Xpz = 4-Xpyrazolate, X = H, Cl, Br, I) and the non-solvated counterparts.

Molecular enneanuclear CuII phosphates containing planar hexanuclear and trinuclear sub-units: syntheses, structures, and magnetism

Highly symmetric enneanuclear copper(ii) phosphates [Cu9(Pz)6(μ-OH)3(μ3-OH)(ArOPO3)4(DMF)3] (PzH = pyrazole, Ar = 2,6-(CHPh2)2-4-R-C6H2; R = Me, 2MeAr; Et, 2EtAr; iPr, 2iPrAr; and Ar = 2,6-iPr2C6H3, 2Dip) comprising nine copper(ii) centers and pyrazole, hydroxide and DMF as ancillary ligands were synthesized by a reaction involving the arylphosphate monoester, 1, copper(i)chloride, pyrazole, and triethylamine in a 4 : 9 : 6 : 14 ratio. All four complexes were characterized by single crystal structural analysis. The complexes contain two distinct structural motifs within the multinuclear copper scaffold: a hexanuclear unit and a trinuclear unit. In the latter, the three Cu(ii) centres are bridged by a μ3-OH. Each pair of Cu(ii) centers in the trinuclear unit are bridged by a pyrazole ligand. The hexanuclear unit is made up of three dinuclear Cu(ii) motifs where the two Cu(ii) centres are bridged by an -OH and a pyrazole ligand. The three dinuclear units are connected to each other by phosphate ligands. The latter also aid the fusion of the trinuclear and the hexanuclear motifs. Magnetic studies reveal a strong antiferromagnetic exchange between the Cu(ii) centres of the dinuclear units in the hexanuclear part and a strong spin frustration in the trinuclear part leading to a degenerate ground state.

Amplification of impurity upon complex formation: how a 2% ligand impurity lowers the corresponding complex purity to 50%

Statistical incorporation of small amounts of ligand impurities has devastating consequences on the purity of metal–organic complexes derived from the respective ligands.

Six novel coordination polymers based on the 5-(1H-tetrazol-5-yl)isophthalic acid ligand: structures, luminescence, and magnetic properties

Six coordination polymers containing clusters or chains have been synthesized by a combination of one tetrazolate-isophthalate ligand and N-donor ligands. Their structures, magnetic properties and luminescence properties were discussed in detail.

Interactions between H-bonded [CuII3(μ3-OH)] triangles; a combined magnetic susceptibility and EPR study

X-band EPR spectroscopy and magnetic susceptibility studies elucidate the magnetic exchange scheme within a triangular CuII3(μ₃-OH) complex and the intermolecular dipolar interactions between two H-bonded CuII3(μ₃-OH) units.

Accessing the inaccessible: discrete multinuclear coordination complexes and selective anion binding attainable only by tethering ligands together

The use of tethers between ligands allows for the preparation of novel multimetallic complexes, not obtainable without tethers, and leads to unexpected properties, such as selective binding of the carbonate vs. sulfate anion by nanojars.

Halogen-bonded network of trinuclear copper(II) 4-iodo-pyrazolate complexes formed by mutual breakdown of chloro-form and nanojars

Crystals of bis-(tetra-butyl-ammonium) di-μ3-chlorido--tris-(μ2-4-iodo-pyrazolato-κ2N:N')tris-[chlorido-cuprate(II)] 1,4-dioxane hemisolvate, (C16H36N)2[Cu3(C3H2IN2)3Cl5]·0.5C4H8O or (Bu4N)2[CuII3(μ3-Cl)2(μ-4-I-pz)3Cl3]·0.5C4H8O, were obtained by evaporating a solution of (Bu4N)2[{CuII(μ-OH)(μ-4-I-pz)} n CO3] (n = 27-31) nanojars in chloro-form/1,4-dioxane. The decomposition of chloro-form in the presence of oxygen and moisture provides HCl, which leads to the breakdown of nanojars to the title trinuclear copper(II) pyrazolate complex, and possibly CuII ions and free 4-iodo-pyrazole. CuII ions, in turn, act as catalyst for the accelerated decomposition of chloro-form, ultimately leading to the complete breakdown of nanojars. The crystal structure presented here provides the first structural description of a trinuclear copper(II) pyrazolate complex with iodine-substituted pyrazoles. In contrast to related trinuclear complexes based on differently substituted 4-R-pyrazoles (R = H, Cl, Br, Me), the [Cu3(μ-4-I-pz)3Cl3] core in the title complex is nearly planar. This difference is likely a result of the presence of the iodine substituent, which provides a unique, novel feature in copper pyrazolate chemistry. Thus, the iodine atoms form halogen bonds with the terminal chlorido ligands of the surrounding complexes [mean length of I⋯Cl contacts = 3.48 (1) Å], leading to an extended two-dimensional, halogen-bonded network along (-110). The cavities within this framework are filled by centrosymmetric 1,4-dioxane solvent mol-ecules, which create further bridges via C-H⋯Cl hydrogen bonds with terminal chlorido ligands of the trinuclear complex not involved in halogen bonding.

Sulfate-Incarcerating Nanojars: Solution and Solid-State Studies, Sulfate Extraction from Water, and Anion Exchange with Carbonate

A series of 9 homologous sulfate-incarcerating nanojars [SO₄⊂{Cu(OH)(pz)}_n]^2- (Cu_n; n = 27-33; pz = pyrazolate), based on combinations of three [Cu(OH)(pz)]_x rings (x = 6-14, except 11)-namely, 6 + 12 + 9 (Cu₂₇), 6 + 12 + 10 (Cu₂₈), 8 + 13 + 8 (Cu₂₉), 7 + 13 + 9 (Cu₂₉), 8 + 14 + 8 (Cu₃₀), 7 + 14 + 9 (Cu₃₀), 8 + 14 + 9 (Cu₃₁), 8 + 14 + 10 (Cu₃₂), and 9 + 14 + 10 (Cu₃₃)-has been obtained and characterized by electrospray-ionization mass spectrometry (ESI-MS), variable-temperature ¹H NMR spectroscopy, and thermogravimetry. The X-ray crystal structure of Cu₂₉ (8 + 13 + 8) is described. Cu₃₂ and Cu₃₃, which are the largest nanojars in this series, are observed for the first time. Despite extensive overlap at a given temperature, monitoring the temperature-dependent variation of paramagnetically shifted pyrazole and OH proton signals in 60 different ¹H NMR spectra over a temperature range of 25-150 °C and a chemical shift range from 41 ppm to -59 ppm permits the assignment of individual protons in six different sulfate nanojars in a mixture. As opposed to ESI-MS, which only provides the size of nanojars, ¹H NMR offers additional information about their detailed composition. Thus, nanojars such as Cu₂₉ (8 + 13 + 8) and Cu₂₉ (7 + 13 + 9) can easily be differentiated in solution. High-temperature solution studies unveil a significant difference in the thermal stability of nanojars of different sizes obtained under kinetic control at ambient temperature, and aid in predicting the structure of the Cu₃₃ nanojar, as well as in explaining the absence of the Cu₁₁ ring from the Cu₆-Cu₁₄ series. Anion exchange studies using sulfate and carbonate reveal that, although each anion is thermodynamically preferred by a nanojar of a certain size, the exchange of an already incarcerated anion is hampered by a substantial kinetic barrier. The remarkably strong binding of anions by nanojars allows for the extraction of highly hydrophilic anions, such as sulfate and carbonate, from water into organic solvents, despite their very large hydration energies.