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.

Adaptive coordination assemblies based on a flexible tetraazacyclododecane ligand for promoting carbon dioxide fixation

Coordination hosts based on flexible ligands have received increasing attention due to their inherent adaptive cavities that often show induced-fit guest binding and catalysis like enzymes. Herein, we report the controlled self-assembly of a series of homo/heterometallic coordination hosts (Me₄enPd)_2n(ML)_n [n = 2/3; M = Zn(ii)/Co(ii)/Ni(ii)/Cu(ii)/Pd(ii)/Ag(i); Me₄en: N,N,N′,N′-tetramethylethylenediamine] with different shapes (tube/cage) from a flexible tetraazacyclododecane-based pyridinyl ligand (L) and cis-blocking Me₄enPd(ii) units. While the Ag(i)-metalated ligand (AgL) gave rise to the formation of a (Me₄enPd)₄(ML)₂-type cage, all other M(ii) ions led to isostructural (Me₄enPd)₆(ML)₃-type tubular complexes. Structural transformations between cages and tubes could be realized through transmetalation of the ligand. The buffering effect on the ML panels endows the coordination tubes with remarkable acid–base resistance, which makes the (Me₄enPd)₆(ZnL)₃ host an effective catalyst for the CO₂ to CO₃²⁻ conversion. Control experiments suggested that the integration of multiple active Zn(ii) sites on the tubular host and the perfect geometry match between CO₃²⁻ and the cavity synergistically promoted such a conversion. Our results provide an important strategy for the design of adaptive coordination hosts to achieve efficient carbon fixation.

A series of coordination hosts were prepared and their applications in CO₂ fixation were studied.

Mechanistic Dichotomy in Solvent Dependent Access to E vs Z-allylic Amines via Decarboxylative Vinylation of Amino Acids

The solvent plays an important role in the photophysical properties of donor–acceptor based photocatalysts. The solvent-dependent access to E vs. Z-allylic amines was achieved via decarboxylative vinylation of amino acids with vinyl sulfones. Detailed experimental studies have been conducted to understand the role of the solvent in the reactivity and stereoselectivity of the vinylation reactions.

A solvent-dependent access to E vs. Z-allylic amines was achieved via decarboxylative vinylation of amino acids. Detailed experimental studies have been conducted to understand the role of the solvent in the reactivity and stereoselectivity of the vinylation reactions.

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.

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.

Manganese(II), lead(II) and cadmium(II) coordination complexes containing a tetrazole-based acylamide ligand: synthesis and the influence of the metal ions on the structures

Three new manganese(II), lead(II) and cadmium(II) coordination complexes have been prepared by reaction of N-(1H-tetrazol-5-yl)cinnamamide (HNTCA) with divalent metal salts (MnCl2, PbCl2 and CdCl2) in a mixed-solvent system, affording mononuclear to trinuclear structures namely, bis(methanol-κO)bis[5-(3-phenylprop-2-enamido)-1H-1,2,3,4-tetrazol-1-ido-κ2 N 1,O]manganese(II), [Mn(C10H8N5O)2(CH3OH)2], (1), bis[μ-5-(3-phenylprop-2-enamido)-1H-1,2,3,4-tetrazol-1-ido]-κ3 N 1,O:N 2;κ3 N 2:N 1,O-bis{aqua[5-(3-phenylprop-2-enamido)-1H-1,2,3,4-tetrazol-1-ido-κ2 N 1,O]lead(II)}, [Pb2(C10H8N5O)4(H2O)2], (2), and hexakis[μ2-5-(3-phenylprop-2-enamido)-1H-1,2,3,4-tetrazol-1-ido-κ3 N 1,O:N 2]tricadmium(II), [Cd3(C10H8N5O)6], (3). The structures of these three compounds reveal that the nature of the metal ions and the side groups of the organic building blocks have a significant effect on the structures of the coordination compounds formed. Intermolecular hydrogen bonds link the molecules into two-dimensional [complex (1)] and three-dimensional hydrogen-bonded networks. Complexes (2) and (3) show significant fluorescence, while complex (1) displays no fluorescence.

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.