Gellert Mezei

Influencing the size and anion selectivity of dimeric Ln 3+[15-Metallacrown-5] compartments through systematic variation of the host side chains and central metal

Dimeric Ln(3+)[15-metallacrown-5] compartments selectively recognize carboxylates through guest binding to host metal ions and intermolecular interactions with the phenyl side chains. A systematic study is presented on how the size, selectivity, and number of encapsulated guests in the dimeric containers is influenced by the Ln(3+)[15-metallacrown(Cu(II))-5] ligand side chain and central metal. Compartments of varying heights were assembled from metallacrowns with S-phenylglycine hydroxamic acid (pgHA), S-phenylalanine hydroxamic acid (pheHA), and S-homophenylalanine hydroxamic acid (hpheHA) ligands. Guests that were examined include the fully deprotonated forms of terephthalic acid, isonicotinic acid, and bithiophene dicarboxylic acid (btDC). X-ray crystallography reveals that the side-chain length constrains the maximum and minimum length guest that can be encapsulated in the compartment. Compartments with heights ranging from 9.7 to 15.2 Å are formed with different phenyl side chains that complex 4.3-9.2 Å long guests. Up to five guests are accommodated in Ln(3+)[15-metallacrown(Cu(II))-5] compartments depending on steric effects from the host side chains. The nine-coordinate La(3+) central metal promotes the encapsulation of multiple guests, while the eight-coordinate Gd(3+) typically binds only one dicarboxylate. Electrospray ionization mass spectrometry reveals that the dimerization phenomenon occurs beyond the solid state, suggesting that these containers can be utilized in solid-state and solution applications.

Tuning of the [Cu3(μ-O)]4+/5+ Redox Couple: Spectroscopic Evidence of Charge Delocalization in the Mixed-Valent [Cu3(μ-O)]5+ Species

Trinuclear Cu (II)-complexes of formula [Cu (II) 3(mu 3-E)(mu-4-R-pz) 3X 3] (+/- n ), E = O and OH; R = H, Cl, Br, CH(O) and NO 2; X = Cl, NCS, CH 3COO, and py, have been synthesized and characterized and the effect of substitution of terminal ligands, as well as 4-R-groups, in the one-electron oxidation process has been investigated by cyclic voltammetry. In situ UV-vis-NIR spectroelectrochemical characterization of the mixed valence Cu 3 (7+)-complex [Cu 3(mu 3-O)(mu-pz) 3Cl 3] (-) revealed an intervalence charge transfer band at 9550 cm (-1) (epsilon = 2600 cm (-1) M (-1)), whose analysis identifies this species as a delocalized, Robin-Day class-III system, with an electronic coupling factor, H ab, of 4775 cm (-1).

Single-color pseudorotaxane-based temperature sensing

Colored pseudorotaxane solutions can be used to assess temperature changes over large temperature windows. The color intensity of our novel pseudorotaxane systems decreases gradually from −50 to +50 °C with no shift in absorption maximum, making these and similar pseudorotaxanes attractive candidates for single-wavelength colorimetric temperature sensors.

Survival of the Fittest Nanojar: Stepwise Breakdown of Polydisperse Cu27-Cu31 Nanojar Mixtures into Monodisperse Cu27(CO3) and Cu31(SO4) Nanojars

Nanojars are emerging as a class of anion sequestration agents of unparalleled efficiency. Dinegative oxoanions (e.g., carbonate, sulfate) template the formation of a series of homologous nanojars [Cu(OH)(pyrazolato)]n (n=27-31). Pyridine selectively transforms less stable, larger CO3(2-) nanojars (n=30, 31) into more stable, smaller ones (n=27, 29), but leaves all SO4(2-) nanojars (n=27-29, 31) intact. Ammonia, in turn, transforms all less stable nanojars into the most stable one and allows the isolation of pure [CO3(2-)⊂{Cu(OH)(pz)}27] and [SO4(2-)⊂{Cu(OH)(pz)}31]. A comprehensive picture of the solution and solid-state intricacies of nanojars was revealed by a combination of variable temperature NMR spectroscopy, tandem mass spectrometry, and X-ray crystallography.

Three-component 1D and 2D metal phosphonates: structural variability, topological analysis and catalytic hydrocarboxylation of alkanes

Herein, we report the use of diphosphonate building blocks and chelating auxiliary N,N-ligands to generate novel polymeric architectures. Specifically, we report new 1D and 2D coordination polymers incorporating three components: transition metal ions (Co2+, Cu2+, Mn2+ or Zn2+), diphosphonate ligands (methane-diphosphonate, MDPA, or 1,2-ethanediphosphonate, EDPA) and N,N-heterocyclic chelators (1,10-phenanthroline, phen, or 2,2′-bipyridine, bpy). Six compounds were isolated under mild synthesis (ambient temperature) conditions: [Cu2(phen)2(EDPA)2(H2O)4]∞ (1), [Co(phen)(EDPA)(H2O)2]∞ (1a), {[Cu(phen)(MDPA)]·H2O]}∞ (2), [Mn(bpy)(EDPA)(H2O)2]∞ (3), [Zn(bpy)(EDPA)]∞ (4), and, lastly, a discrete Ni2+ molecular derivative [Ni(phen)(H2O)4](EDPA) (5). Synthetic details, crystal structures, and intermolecular interactions (π–π stacking and hydrogen bonding) in 1–5 are discussed. Topological analyses and classification of the underlying metal–organic networks in 1–4 were performed, revealing the uninodal 1D chains with the 2C1 topology in 1–3 and the binodal 2D layers with the 3,4L13 topology in 4. In 1–3 and 5, multiple hydrogen bonds lead to the extension of the structures to give 3D H-bonded nets with the seh-4,6-C2/c topology in 1 and 3, 2D H-bonded layers with the 3,5L52 topology in 2, and a 3D H-bonded net with the 6,6T1 topology in 5. The catalytic activity of compounds 1 and 1a was evaluated in a single-step hydrocarboxylation of cyclic and linear C5–C8 alkanes to furnish the carboxylic acids with one more carbon atom. These reactions proceed in the presence of CO, K2S2O8, and H2O at 60 °C in MeCN/H2O medium. Compound 1 showed higher activity than 1a and was studied in detail. Substrate scope was investigated, revealing that cyclohexane and n-pentane are the most reactive among the cyclic and linear C5–C8 alkanes, and resulting in the total yields of carboxylic acids (based on substrate) of up to 43 and 36%, respectively. In the case of cycloalkane substrates, only one cycloalkanecarboxylic acid is produced, whereas a series of isomeric monocarboxylic acids is generated when using linear alkanes; an increased regioselectivity at the C(2) position of the hydrocarbon chain was also observed.

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

Nanojars are large (2 nm wide) anion-incarcerating coordination complexes of the composition [anion⊂{Cu(μ-OH)(μ-pz)}n] (n = 27-36), formed by the self-assembly of simple Cu(2+), HO(-), and pyrazolate (pz(-) = C3H3N2(-)) ions in the presence of certain anions with large hydration energy (e.g., CO3(2-), SO4(2-), PO4(3-), HPO4(2-)). Nanojars display spectacular chemical properties, such as unparalleled anion binding strength and, as shown herein, extraordinary resistance to extreme alkalinities (10 M NaOH). To shed light on the mechanism of the self-assembly process leading to these distinctive constructs, we employed an array of complementary techniques including mass spectrometry, pH titration, UV-vis and NMR spectroscopies, chemical synthesis, and single-crystal X-ray diffraction. In the reaction of Cu(NO3)2, pyrazole, NaOH, and Na2CO3 in tetrahydrofuran (THF), the first major intermediate is a trinuclear copper pyrazolate complex, [Cu3(μ3-OH)(μ-pz)3(NO3)2(H2O)], which was separately isolated and characterized. As the THF-insoluble NaOH slowly reacts, the nitrate ions are gradually precipitated out as NaNO3 and replaced by hydroxide ions. The resulting species, [Cu3(μ3-OH)(μ-pz)3(OH)x(NO3)3-x](-) (x = 1-3), have unstable terminal Cu-OH groups and react with each other to yield OH-bridged units, such as [Cu3(μ3-OH)(μ-pz)3(NO3)2]2(μ-OH) and then [{Cu3(μ3-OH)(μ-pz)3(μ-OH)2}x(NaNO3)y(Na2CO3)z] oligomers. The Cu3(OH)3(pz)3 repeating units of these oligomers have the same composition as the [Cu(OH)(pz)]n (n = 3x) nanojars and rearrange to the final products, Na2[CO3⊂{Cu(μ-OH)(μ-pz)}n] (n = 27, 29, 31), while eliminating the last amounts of NaNO3. pH titration, UV-vis monitoring, and chemical synthesis also confirm the formation of the trinuclear intermediate, followed by its clean transformation to nanojars. While displaying an unusual stability to high pH, nanojars are sensitive to acids stronger than water, a property exploitable for the recovery of the incarcerated anion. On lowering the pH, nanojars first break down to trinuclear complexes and finally to copper ions and pyrazole. This process is fully reversible, and nanojars are reassembled as pH is increased.

Temperature-, molar ratio- and counterion-effects on the crystal growth of bipyridinium-bis(alkylcarboxylic acid)–crown ether pseudorotaxanes

Five new dicarboxylic acid-functionalized pseudorotaxanes, comprised of a bis(1,5-naphtho)-38-crown-10 ring-component and a N,N′-di(n-carboxyalkyl)-4,4′-bipyridinium dihexafluorophosphate (n = 4 or 5, alkyl = butyl or pentyl) rod-component, have been synthesized and characterized by UV-Vis and 1H-NMR spectroscopic, ESI-MS and/or X-ray crystallographic methods. Depending on the temperature of crystallization and the molar ratio between the N,N′-di(4-carboxybutyl)-4,4′-bipyridinium rod and the crown ether, three different polymorphs were obtained. In the case of the N,N′-di(5-carboxypentyl)-4,4′-bipyridinium rod, two different pseudorotaxane-structures were obtained at different temperatures and in the presence of one or two different counterions. The latter one is an unprecedented example of a pseudorotaxane in which the bipyridinium rod is not sandwiched in-between the crown ether ring’s two naphthalene units, as expected, but forms intermolecular π–π interactions with neighboring pseudorotaxanes.

Green protection of pyrazole, thermal isomerization and deprotection of tetrahydropyranylpyrazoles, and high-yield, one-pot synthesis of 3(5)-alkylpyrazoles

We report a new synthetic approach that opens up the possibility of large scale, one-pot pyrazole derivatization by a wide variety of functionalities, including alkyl, halogen, hydroxyl, amino, azido, carbonyl, and organo-element (e.g., B, Si, P) groups. The approach is illustrated by the highly efficient synthesis of fourteen 3(5)-alkylpyrazoles, including the novel isopentyl- and n-hexadecyl derivatives, as well as 1,6-bis(pyrazol-3(5)-yl)hexane. The value of the new approach lies in the discovery of a green (solvent- and catalyst-free, quantitative) protection of pyrazole, followed by a high-yield lithiation/alkylation/deprotection sequence in the same pot. For the first time, the corresponding N-tetrahydropyran-2-yl (THP) intermediates have been isolated and characterized. Thermal isomerization of the 5-alkyl-1-(THP) to the 3-alkyl-1-(THP) isomer is shown to be an advantageous, green alternative to the acid-catalyzed, sequential protecting-group switching methodology in pyrazole chemistry. The X-ray crystal structures of 1,6-bis(pyrazol-3(5)-yl)hexane and 5-n-hexadecyl-1-(tetrahydropyran-2-yl)pyrazole reveal supramolecular architectures that shine light on the remarkable affinity for water and unexpected insolubility in organic solvents of alkylene-bridged bis(pyrazoles). 3(5)-Alkylpyrazoles are obtained in high yield from pyrazole by a one-pot procedure.

Deprotonation of Methyl-Substituted, Five-Membered Aromatic Molecules: A Surprising Case of Mixed Conjugation, Rehybridization, and Induction Contributions

Methyl-substituted, six-membered aromatic molecules are deprotonated to benzylic carbanions, which are stabilized by π conjugation. In contrast, deprotonation of 3(5)-methylpyrazole (NH protected) occurs at an endocylic CH group. Computational analyses showed that the reduction of π conjugation in substituted five-membered rings plays a major role, while the reduced bond angles, in addition to the strengthened induction of Csp(2) versus Csp(3), further favor the deprotonation of endocyclic carbon sites rather than that of the methyl group.

Mapping the supramolecular chemistry of pyrazole-4-sulfonate: layered inorganic–organic networks with Zn2+, Cd2+, Ag+, Na+ and NH4+, and their use in copper anticorrosion protective films

Five compounds based on the versatile pyrazole-4-sulfonate anion (4-SO3-pzH = L−) were synthesized by the reaction of ligand HL with ZnO, CdCO3, Ag2O, NaOH and NH3, respectively. Crystals of Zn(4-SO3-pzH)2(H2O)2, Cd(4-SO3-pzH)2(H2O)2, Ag(4-SO3-pzH), Na(4-SO3-pzH)(H2O) and NH4(4-SO3-pzH) were obtained from aqueous solutions upon evaporation, and were characterized by single-crystal X-ray diffraction, IR and NMR spectroscopy, thermogravimetric analysis and copper corrosion inhibition experiments. We found that the non-isomorphous, 3-dimensional inorganic–organic layered solid state structure of these compounds is determined by an intricate interplay between the size, charge and coordination preference of the cation, and an extended lattice of hydrogen bonds and aromatic interactions. Ligand L− incorporates a host of different binding capabilities (metal coordination through the pyrazole N-atom and/or the sulfonate O-atom, hydrogen-bonding both as donor and acceptor, π–π and C–H⋯π interactions). Thin films formed by these complexes on copper metal surfaces were studied by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. ZnL2, CdL2, AgL and NH4L, in addition to the free ligand HL, were tested as corrosion inhibitors on copper metal surfaces at three different pH values (2, 3 and 4), and the corrosion rates were quantified. Significant corrosion protection was observed with all compounds at pH 4 and 3.