Synthesis and structure of a novel supramolecular Zn3Co8 assembly containing a linear Zn3 core

Reaction of Zn(OAc)(2).2H2O (OAc = acetate) with LH2 in THF followed by reaction with Co2(CO)8 yields the novel supramolecular assembly Zn3L2(OAc)2[Co2(CO)6](4).0.5CH2Cl2, which was characterized by X-ray diffraction (LH2 = (CH2)3(N=CH-2-OH-5-C identical to CSiMe3C6H3)2 or N,N'-propylenebis(2-hydroxy-5-trimethylsilylethynl-1-aldimine).     

Structural chemistry of a green fluorescent protein Zn biosensor

We designed a green fluorescent protein mutant (BFPms1) that preferentially binds Zn(II) (enhancing fluorescence intensity) and Cu(II) (quenching fluorescence) directly to a chromophore ligand that resembles a dipyrrole unit of a porphyrin. Crystallographic structure determination of apo, Zn(II)-bound, and Cu(II)-bound BFPms1 to better than 1.5 A resolution allowed us to refine metal centers without geometric restraints, to calculate experimental standard uncertainty errors for bond lengths and angles, and to model thermal displacement parameters anisotropically. The BFPms1 Zn(II) site (KD = 50 muM) displays distorted trigonal bipyrimidal geometry, with Zn(II) binding to Glu222, to a water molecule, and tridentate to the chromophore ligand. In contrast, the BFPms1 Cu(II) site (KD = 24 muM) exhibits square planar geometry similar to metalated porphyrins, with Cu(II) binding to the chromophore chelate and Glu222. The apo structure reveals a large electropositive region near the designed metal insertion channel, suggesting a basis for the measured metal cation binding kinetics. The preorganized tridentate ligand is accommodated in both coordination geometries by a 0.4 A difference between the Zn and Cu positions and by distinct rearrangements of Glu222. The highly accurate metal ligand bond lengths reveal different protonation states for the same oxygen bound to Zn vs Cu, with implications for the observed metal ion specificity. Crystallographic anisotropic thermal factor analysis validates metal ion rigidification of the chromophore in enhancement of fluorescence intensity upon Zn(II) binding. Thus, our high-resolution structures reveal how structure-based design has effectively linked selective metal binding to changes in fluorescent properties. Furthermore, this protein Zn(II) biosensor provides a prototype suitable for further optimization by directed evolution to generate metalloprotein variants with desirable physical or biochemical properties.     

Direct catalytic asymmetric Michael reaction of hydroxyketones: asymmetric Zn catalysis with a Et2Zn/linked-BINOL complex

Full details of our direct Michael addition of unmodified ketones using new asymmetric zinc catalysis are described. Et(2)Zn/(S,S)-linked-BINOL complexes were successfully applied to direct 1,4-addition reactions of hydroxyketones. The first generation Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was effective for 1,4-addition of 2-hydroxy-2'-methoxyacetophenone (3). Using 1 mol % of (S,S)-linked-BINOL 1 and 2 mol % of Et(2)Zn, we found that a 1,4-addition reaction of beta-unsubstituted enone proceeded smoothly at 4 degrees C to afford products in high yield (up to 90%) and enantiomeric excess (up to 95%). In the case of beta-substituted enones, however, the first generation Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was not at all effective. The second generation Et(2)Zn/(S,S)-linked-BINOL 1 = 4/1 with MS 3A system was developed and was effective for various beta-substituted enones to afford products in good dr, yield (up to 99%), and high enantiomeric excess (up to 99% ee). With the Et(2)Zn/1 = 4/1 systems, catalyst loading for beta-unsubstituted enone was reduced to as little as 0.01 mol % (substrate/chiral ligand = 10 000). The new system was also effective for 1,4-addition reactions of 2-hydroxy-2'-methoxypropiophenone (9) to afford chiral tert-alcohol in high enantiomeric excess (up to 96% ee). Mechanistic investigations as well as transformations of the Michael adducts into synthetically versatile intermediates are also described.     

Synthesis and near infrared luminescence of a tetrametallic Zn2Yb2 architecture from a trinuclear Zn3L2 Schiff base complex

Near infrared luminescence is observed in tetrametallic [Zn2Yb2L2(mu-OH)2Cl4].2MeCN which is obtained from the Zn3 Schiff-base complex [Zn3L2(NO3)2].MeOH, (H2L =N,N'-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine).     

Saturation of hydrogen sorption in Zn benzenedicarboxylate and Zn naphthalenedicarboxylate

We report excess hydrogen saturation values from high-pressure isotherms of metal organic framework structures taken at 77 K. Zn benzendicarboxylate (IRMOF-1) and Zn naphthalendicarboxylate (IRMOF-8) linker structures show identical saturation values of 137 hydrogen molecules on a per unit cell basis, despite the higher sorption potential of IRMOF-8 of 6.1 kJ/mol over that of IRMOF-1 of 4.1 kJ/mol. Charge transfer between linker and vertex, as well as surface area, appear to dominate the sorption behavior, over that of linker length in these two systems.     

Direct catalytic asymmetric aldol reaction of hydroxyketones: asymmetric Zn catalysis with a Et(2)Zn/linked-BINOL complex

Full details of our newly developed catalyses with asymmetric zinc complexes as mimics of class II zinc-containing aldolase are described. A Et(2)Zn/(S,S)-linked-BINOL complex was developed and successfully applied to direct catalytic asymmetric aldol reactions of hydroxyketones. A Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was initially developed, which efficiently promoted the direct aldol reaction of 2-hydroxy-2'-methoxyacetophenone (7d). Using 1 mol % of (S,S)-linked-BINOL 1 and 2 mol % of Et(2)Zn, we obtained 1,2-dihydroxyketones syn-selectively in high yield (up to 95%), good diastereomeric ratio (up to 97/3), and excellent enantiomeric excess (up to 99%). Mechanistic investigation of Et(2)Zn/(S,S)-linked-BINOL 1, including X-ray analysis, NMR analysis, cold spray ionization mass spectrometry (CSI-MS) analysis, and kinetic studies, provided new insight into the active oligomeric Zn/(S,S)-linked-BINOL 1/ketone 7d active species. On the basis of mechanistic investigations, a modified second generation Et(2)Zn/(S,S)-linked-BINOL 1 = 4/1 with molecular sieves 3A (MS 3A) system was developed as a much more effective catalyst system for the direct aldol reaction. As little as 0.1 mol % of (S,S)-linked-BINOL 1 and 0.4 mol % of Et(2)Zn promoted the direct aldol reaction smoothly, using only 1.1 equiv of 7d as a donor (substrate/ligand = 1000). This is the most efficient, in terms of catalyst loading, asymmetric catalyst for the direct catalytic asymmetric aldol reaction. Moreover, the Et(2)Zn/(S,S)-linked-BINOL 1 = 4/1 system was effective in the direct catalytic asymmetric aldol reaction of 2-hydroxy-2'-methoxypropiophenone (12), which afforded a chiral tetrasubstituted carbon center (tert-alcohol) in good yield (up to 97%) and ee (up to 97%), albeit in modest syn-selectivity. Newly developed (S,S)-sulfur-linked-BINOL 2 was also effective in the direct aldol reaction of 12. The Et(2)Zn/(S,S)-sulfur-linked-BINOL 2 = 4/1 system gave aldol adducts anti-selectively in good ee (up to 93%). Transformations of the aldol adducts into synthetically versatile intermediates were also described.     

Zn6[MeN(CH2CO2)(CH2PO3)](6)(Zn)]4- anion: the first example of the oxo-bridged Zn6 octahedron with a centered Zn(II) cation

Hydrothermal reactions of N-(phosphonomethyl)-N-methylglycine, MeN(CH(2)CO(2)H)(CH(2)PO(3)H(2)) (H(3)L), with zinc(II) acetate resulted in the formation of a novel zinc carboxylate-phosphonate, [Zn(6)L(6)(Zn)][Zn(H(2)O)(6)](2) x 22H(2)O (1). The structure of 1 contains a heptanuclear zinc phosphonate cluster anion, [Zn(6)L(6)(Zn)](4-), in which seven zinc(II) cations form an unusual Zn(6)(Zn) centered octahedron with six of its Zn(3) triangle faces each further capped by a phosphonate group. The Zn(II) cations of the Zn(6) octahedron are five-coordinated whereas the centered Zn(II) cation is octahedrally coordinated. Packing of these cluster anions creates micropores occupied by the hydrated zinc(II) cations as well as lattice water molecules. The structural skeleton of 1 is retained after the removal of the lattice water molecules.     

Assembly and disassembly of a Zn10 high-nuclearity circular helicate

A nano-scale decanuclear Zn(II) circular helicate is synthesized without the aid of counteranions during the assembly process, and can be totally disassembled into its reactants by specific anions.     

Role of the Zn1 and Zn2 sites in metallo-beta-lactamase L1

In an effort to probe the role of the Zn(II) sites in metallo-beta-lactamase L1, mononuclear metal ion containing and heterobimetallic analogues of the enzyme were generated and characterized using kinetic and spectroscopic studies. Mononuclear Zn(II)-containing L1, which binds Zn(II) in the consensus Zn1 site, was shown to be slightly active; however, this enzyme did not stabilize a nitrocefin-derived reaction intermediate that had been previously detected. Mononuclear Co(II)- and Fe(III)-containing L1 were essentially inactive, and NMR and EPR studies suggest that these metal ions bind to the consensus Zn2 site in L1. Heterobimetallic analogues (ZnCo and ZnFe) analogues of L1 were generated, and stopped-flow kinetic studies revealed that these enzymes rapidly hydrolyze nitrocefin and that there are large amounts of the reaction intermediate formed during the reaction. The heterobimetallic analogues were reacted with nitrocefin, and the reactions were rapidly freeze quenched. EPR studies on these samples demonstrate that Co(II) is 5-coordinate in the resting state, proceeds through a 4-coordinate species during the reaction, and is 5-coordinate in the enzyme-product complex. These studies demonstrate that the metal ion in the Zn1 site is essential for catalysis in L1 and that the metal ion in the Zn2 site is crucial for stabilization of the nitrocefin-derived reaction intermediate.     

Effect of Zn...Zn separation on the hydrolytic activity of model dizinc phosphodiesterases

From the study of highly preorganized model systems, experimental support has been obtained for a possible functional role of the Zn-(H)O...HO(H)-Zn motif in oligozinc hydrolases. The mechanistic relevance of such an array, which may be described as a hydrated form of a pseudo-terminal Zn-bound hydroxide, has recently been supported by DFT calculations on various metallohydrolase active sites. In the present targeted approach, the Zn...Zn distance in two related dizinc complexes has been controlled through the use of multifunctional pyrazolate-based ligand scaffolds, giving either a tightly bridged Zn-O(H)-Zn or a more loosely bridged Zn-(H)O...HO(H)-Zn species in the solid state. Zn-bound water has been found to exhibit comparable acidity irrespective of whether the resulting hydroxide is supported by strong hydrogen-bonding in the O(2)H(3) moiety or is in a bridging position between two zinc ions, indicating that water does not necessarily have to adopt a bridging position in order for its pK(a) to be sufficiently lowered so as to provide a Zn-bound hydroxide at physiological pH. Comparative reactivity studies on the cleavage of bis(4-nitrophenyl)phosphate (BNPP) mediated by the two dizinc complexes have revealed that the system with the larger Zn...Zn separation is hydrolytically more potent, both in the hydrolysis and the transesterification of BNPP. The extent of active site inhibition by the reaction products has also been found to be governed by the Zn...Zn distance, since phosphate diester coordination in a bridging mode within the clamp of two zinc ions is only favored for Zn...Zn distances well above 4 A. Different binding affinities are rationalized in terms of the structural characteristics of the product-inhibited complexes for the two different ligand scaffolds, with dimethyl phosphate found as a bridging ligand within the bimetallic pocket.     

Crystal structure of a new transition metal-organic Zn(II) complex

A novel Zn(II) coordination polymer [Zn₂(phen)₂]L₄·3H₂O(1) is synthesized by the reaction of Zn(NO₃)₂, Phen(1,10-phenanthroline), and L(2-mercaptonicotinic acid) at room temperature and structurally characterized by X-ray single crystal diffraction along with IR spectra and elemental analysis. Title complex 1 belongs to the triclinic system with the space group (P-1), a = 10.9373(11) Å, b = 11.6201(12) Å, c = 13.1371(14) Å; α = 116.100(1)°, β = 97.717(2)°, γ = 108.652(2)°, V = 1344.4(2) ų; Z = 2, ρ_calc = 1.596 g·cm^?3, F(000) = 664, R ₁ = 0.0708 and wR ₂ = 0.1823 independent reflections for 18523 observed ones (I > 2σ(I)), and the zinc atom is rendered five-coordinated in a distorted tetragonal pyramid coordination geometry by two nitrogen atoms from the phen molecule, two oxygen atoms from two L molecules, and an oxygen atom from the H₂O molecule. Complex 1 forms a 1D chain by O-H…O hydrogen bonds from free-water, while the 2D layer structure is formed by C-H…O hydrogen bonds through the L ligand of adjacent chains. These compounds further result in a 3D network structure by the intermolecular π…π stacking interaction of the neighbouring layers.     

Electrochemical Reduction of Monochlorinated Hydrocarbons at a Hydrophobic Zn/PTFE Composite‐Electroplated Zn Electrode in a Water‐Acetonitrile Mixture

Composite‐electroplating of zinc particles with polytetrafluoroethylene at a zinc substrate resulted in a working electrode which could be used at high negative potentials (to ?2.5 V vs. Ag/AgCl) without significant hydrogen evolution. The electrodes were characterized by SEM and CV and were used to detect organochloride compounds in a water‐acetonitrile mixture, by potential step methods.     

Stereoselective recognition of vicinal diamines with a Zn(II) complex

Zn(II) complex of L (N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane) binds chiral vicinal diamines (1,2-diphenylethylenediamine (dpen) and 1,2-diaminocyclohexane (dach)) stereoselectively. Crystallographic studies reveal that the ternary complex has the C2 symmetric cis-alpha topology. 1H NMR shows that the R,R form of the tetradentate zinc complex binds rapidly and reversibly to the R,R form of the diamine over the S,S form with a stereoselectivity of about 5:1. Although the diamine exchange rate is rapid it is slower than the NMR time scale, and distinct signals for the diastereomeric complexes are observed when racemic mixtures of the host and guest molecules are mixed. Origin of stereoselectivity is discussed in terms of steric effects.     

Metal-directed self-assembly of a Zn(II)-salpyr complex into a supramolecular vase structure

A new type of supramolecular building block, Zn(II)-salpyr [salpyr = N,N'-3-pyridylenebis(salicylideneimine)], is described that contains both a pyridyl donor and a Lewis acidic Zn(II) acceptor site in the salen framework. As a consequence, this building block self-organizes into a stable tetrameric vase structure via cooperative intermolecular Zn-N(pyr) interactions.     

Creation of a nonsymmetric dimethanolpyridine ligand: a rare Zn(salphen) template effect

An interesting reversible, templated synthesis of unsymmetrical pyridine-ligated Zn complexes has been observed in solution and solid-state phases when Zn(salphen) complexes are reacted with methanol-substituted pyridine substrates. The X-ray molecular structure of an unusual Zn(3) supramolecular assembly is reported, in which the central Zn atom is embedded in a distorted octahedral geometry and each of the single anionic O atoms of the pyridine groups coordinates to a Zn(salphen) unit.     

Sodium zinc hydroxide sulfite with a novel Zn3OH geometry

The synthesis and crystal structure of a new sodium zinc hydroxide sulfite, Na[(Zn(OH)]3(SO3)2, with novel structural features and bonding geometry are reported. In Na[(Zn(OH)]3(SO3)2, Zn2+ and O2- alternate to form an interesting corrugated hexagonal sheet with rings consisting of (ZnO)3. Another rare occurrence is the bonding geometry of one-third of the O sites within the sheet. These O sites form a planar Zn3O unit, unlike other known compounds, in which Zn3O is pyramidal.     

Kinetics of Zn/Zn(II) reactions in acidified solutions of potassium chloride

The title subject has been studied using stationary, single-pulse, and chronopotentiometric polarization measurements on the Zn/Zn(II) electrode and equilibrium measurements on the same and the Ag/AgCl electrode in 0.5–4 M chloride solutions at 25°C. The Zn/Zn(II) electrode reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate. The ion-transfer step Zn/Zn(I) is too fast to exhibit its kinetics. The electron-transfer step Zn(I)/Zn(II) mostly occurs by the couple ZnCl₂(H₂O)_y/ZnCl₂(H₂O)_y, but species with one or no chloride ligand take over as the main electroactive ones at chloride (or salt) concentrations below 1 M. The value of y is not clearly revealed by the data. Some sluggishness in complex equilibration and some, double-layer effects are observed. A convenient scale for single-ion activities is described, used, and recommended.     

Synthesis and characterization of a pair of temperature and cosolvent-dependent Zn(II)-organic frameworks containing a novel discrete single-walled Zn(II)-organic coordination polymer nanotube

A pair of temperature and cosolvent-controlled assemblies of one dimensional (1-D) and two dimensional (2-D) Zn(II)-organic frameworks (ZOFs] based on 5-iodoisophthalic acid (H₂IIP) and an auxiliary flexible ligand, 1,4-bis(triazol-1-ylmethyl)benzene (bbtz) with different structures, has been rationally designed and successfully synthesized. Results show that when the reaction was carried out under ambient condition, the novel discrete single-walled Zn(II)-organic coordination polymer nanotube {[Zn(IIP)(bbtz)(H₂O)]·H₂O}_n (SWCPNT-1), which shows a fascinating 3-D supramolecular interdigitated columnar microporous architecture supported by face to face π?π stacking interactions and hydrogen bonds, was generated, whereas under solvothermal condition at 120?°C, an interesting 3-D-polycatenated array of layers, [Zn(IIP)(bbtz)] (2), which further extends into a threefold-interpenetrated 3-D supramolecular mesoporous framework with 1-D channels (ca. 3.46?×?1.54?nm²) through C–I?O halogen bonds would be obtained. Interestingly, the reversible in situ rapid rehydration from static air is significantly observed in the discrete SWCPNT-1, revealing its potential application as water absorbent and sensing material. The dehydrated SWCPNT-1 shows selective gas adsorption of CO₂ over N₂. Luminescent studies show that SWCPNT-1, dehydrated SWCPNT-1, and 2 exhibit blue fluorescence in the solid state. The water molecules in SWCPNT-1 affect its fluorescent property.     

Autocatalytic demetalation of a Zn(salphen) complex provoked by unprotected N-heterocycles

The interaction of Zn(salphen) complex with biologically important structures such as (benz)imidazoles and purine has been studied and revealed in the case of unprotected purine and (benz)imidazole derivatives a demetalated product, whereas for structurally related 1-methyl-(benz)imidazole the formation of 1 : 1 coordination complexes was evidenced by NMR, MS and X-ray crystallography.     

Electrochemical impedance spectroscopy study of a hydrogen electrode reaction at a Zn electrode in a molten LiCl-KCl-LiH system

The hydrogen electrode reaction involving hydride ion, H-, at a Zn electrode is investigated in a molten LiCl-KCl-LiH system at 673 K. The charge-transfer resistances were measured by electrochemical impedance spectroscopy in the overpotential region of 0.10 < or = eta < or = 0.35 V and over the H- concentrations of 1.5 x 10(-4) < or = C(H)- < or = 1.2 x 10(-3) mol cm(-3). The logarithm plot of the charge-transfer resistance against the overpotential at C(H)- = 3.0 x 10(-4) mol cm(-3) gives the symmetry factor, beta, of 0.50 and the exchange current density, j0, of 5.8 x 10(-3)A cm(-2), respectively. Analysis of the dependence of j0 on H- concentration independently gives a beta of 0.55. The reasonable beta values indicate that the H- <==> H(ad)(M) + e- step is rate-determining.