Central Zn Ion Research Articles

Photocatalytic CO2 Reductions Catalyzed by meso-(1,10-Phenanthrolin-2-yl)-Porphyrins Having a Rhenium(I) Tricarbonyl Complex.

We have prepared Zn and free‐base porphyrins appended with a fac‐Re(phen)(CO)3Br (where phen is 1,10‐phenanthroline) at the meso position of the porphyrin, and performed photocatalytic CO2 reduction using porphyrin–Re dyads in the presence of either triethylamine (TEA) or 1,3‐dimethyl‐2‐phenyl‐2,3‐dihydro‐1H‐benzo[d]imidazole (BIH) as an electron donor. The Zn porphyrin dyad showed a high turnover number for CO production compared with the free‐base porphyrin dyad, suggesting that the central Zn ion of porphyrin plays an important role in suppressing electron accumulation on the porphyrin part and achieving high durability of the photocatalytic CO2 reduction using both TEA and BIH. The effect of acids on the CO2 reduction was investigated using the Zn porphyrin–Re dyad and BIH. Acetic acid, a relatively strong Brønsted acid, rapidly causes the porphyrin's color to fade upon irradiation and dramatically decreases CO production, whereas proper weak Brønsted acids such as 2,2,2‐trifluoroethanol and phenol enhance the CO2 reduction.

Ellagic acid: A potent glyoxalase-I inhibitor with a unique scaffold.

The glyoxalase system, particularly glyoxalase-I (GLO-I), has been approved as a potential target for cancer treatment. In this study, a set of structurally diverse polyphenolic natural compounds were investigated as potential GLO-I inhibitors. Ellagic acid was found, computationally and experimentally, to be the most potent GLO-I inhibitor among the tested compounds which showed an IC50 of 0.71 mmol L-1. Its binding to the GLO-I active site seemed to be mainly driven by ionic interaction via its ionized hydroxyl groups with the central Zn ion and Lys156, along with other numerous hydrogen bonding and hydrophobic interactions. Due to its unique and rigid skeleton, it can be utilized to search for other novel and potent GLO-I inhibitors via computational approaches such as pharmacophore modeling and similarity search methods. Moreover, an inspection of the docked poses of the tested compounds showed that chlorogenic acid and dihydrocaffeic acid could be considered as lead compounds worthy of further optimization.

A Metal-organic Framework with Paddle-wheel Zn2(CO2)4 Secondary Building Units and Cubane-1,4-dicarboxylic Acid Linkers

A new crystal structure of catena-(bis((µ4-cubane-1,4-dicarboxylato)-(N-methyl-2-pyrrolidone)-zinc(II)) N-methyl-2-pyrrolidone solvate) (1) was prepared by solvothermal method. The crystal structure of the compound was analyzed by single-crystal X-ray diffraction. It has a P1 space group, with lattice parameters a = 10.7190(4) Å, b = 10.8245(5) Å, c = 10.8403(8) Å, α = 80.291(5)°, β = 70.0015(5)°, γ = 77.531(4)°, V = 1147.97(12) Å3. The secondary building units of 1 consist of 2 central Zn ions, coordinated by 4 carboxylate groups in a bis-monodentate way, forming a square planar configuration of Zn2(CO2)4, known as paddle-wheel units. The paddle-wheels are connected by cubane-1,4-dicarboxylic acid linkers at the edges, resulting in a two-dimensional coordination polymer with a square lattice (sql) underlying network topology. The axial sites of the zinc atoms are occupied by N-methyl-2-pyrrolidone molecules. In this new crystal structure the two-dimensional polymer planes are interstacked by weak dispersion bonds. The axial N-methyl-2-pyrrolidone solvent molecules determine the distances of planar polymer planes. The thermal properties of this new structure were studied by thermogravimetry/mass spectrometry in inert atmosphere. It was found, that the organic linkers in the framework structure do not decompose below 200 °C. The stochiometry of the activated compound is Zn2[C8H6(COO)2]2(C5H9NO)2, as determined by thermogravimetry in oxidative atmosphere.

Synthesis, characterization and activity of imidazolate-bridged and Schiff-base dinuclear complexes as models of Cu,Zn-SOD. A comparative study

Two imidazolate-bridged diCuII and CuIIZnII complexes, [CuZn(dien)2(μ-Im)](ClO4)3·MeOH (1) and [Cu2(dien)2(μ-Im)](ClO4)3 (2) (Im = imidazole, dien=diethylenetriamine), and two complexes formed with Schiff base ligands, [CuZn(salpn)Cl2] (3) and [Cu2(salbutO)ClO4] (4) (H2salpn=1,3-bis(salicylidenamino)propane, H3salbutO=1,4-bis(salicylidenamino)butan-2-ol) have been prepared and characterized. The reaction of [Cu(dien)(ImH)](ClO4)2 with [Zn(dien)(H2O)](ClO4)2 at pH≥11 yields complex 1; at lower pH, the Cu3Zn tetranuclear complex [{(dien)Cu(μ-Im)}3Zn(OH2)(ClO4)2](ClO4)3 (1a) forms as the main reaction product. X-ray diffraction of 1a reveals that the complex contains a metal centered windmill-shaped cation having three blades with a central Zn ion and three peripheral capping Cu(dien) moieties bound to the central Zn ion through three imidazolate bridges. The four complexes are able to disproportionate O2− in aqueous medium at pH7.8, with relative rates 4>1>2≫3. [Cu2(salbutO)]+ (4) is the most easily reducible of the four complexes and exhibits the highest activity among the SOD models reported so far; a fact related to the ligand flexibility to accommodate the copper ion in both CuI and CuII oxidation states and the lability of the fourth coordination position of copper facilitating stereochemical rearrangements.

Self-organization principles in the formation of multiporphyrin complexes and "semiconductor quantum dot-porphyrin" nanoassemblies

In this paper, we review several aspects of molecular recognition (based on non-covalent binding interactions) occurring between meso-pyridyl substituted tetrapyrrole extra-ligands and chemical dimers of tetrapyrrolic macrocycles containing central Zn ions and spacers of various nature and flexibility. Experimental results obtained by us earlier are analyzed using a novel approach (based on steady-state absorption/fluorescence measurements) for the evaluation of complexation constants KC for the formation of porphyrin triads. It was found that KC values [ KC ~ (0.5 – 70) × 106 M-1] show noticeable dependence on the structural parameters of the interacting subunits as well as on the solvent nature. The same self-assembly approach has been used to attach meso-pyridyl substituted porphyrins to the surface of semiconductor CdSe / ZnS quantum dots (QD). It was comparatively found that in contrast to self-assembled porphyrin triads, the formation of "QD-porphyrin" nanoassemblies takes place in competition with surface stabilizing tri-n-octyl phosphine oxide (TOPO) ligand molecules and attached porphyrin molecules. It manifests in a temporal dynamics of QD photoluminescence caused by ligand exchange, TOPO layer reorganization, QD surface reconstruction, solvent properties. It was shown that the sensitivity of QD surface morphology to attached organic ligands (e.g. porphyrins) provides an opportunity to control the dynamics and pathways of the exciton relaxation in "QD-dye" nanoassemblies by changing the structure and electronic properties of these ligands.

Control formation of rigid linear and flexible zig-zig complexes based on Zn(II) and hydroxyquinoline carboxylate ligand system

Abstract Two novel zinc complexes, [Zn(H-hqc)2(bpy)(H2O)2] (1) and [Zn(H-hqc)2(bpe)]∙H2O (2), where H2-hqc = 2-hydroxyquinoline-4-carboxylic acid, bpy = 4,4′-bipyridine and bpe = 1,2-bi(4-pyridyl)ethylene, have been prepared under hydrothermal condition. Complex 1 displays a 1D rigid linear chain structure while complex 2 displays 1D flexible zig-zag chain structure. Coordination number of the central Zn ion in complex 1 is six and in complex 2 is four. The structural differences of these complexes suggest the effect of size of the bridging ligands on the construction of metal organic frameworks with the same metal ion under solvothermal conditions.

Photoinduced electron transfer in supramolecular donor–acceptor dyads of Zn corrphycene

Porphyrins have been used by various researchers as important building blocks of photofunctional molecules, while the number of studies on the excitation properties of the structural isomers of porphyrins is small when compared to those of porphyrins. In the present study, photoinduced electron transfer (ET) processes of supramolecular donor-acceptor dyads of 2,3,6,7,11,12,17,18-octaethylcorrphycenatozinc (ZnCn), one of the porphyrin isomers, and some imide compounds, which can coordinate to the central Zn ion as an electron acceptor, were investigated. Formation of the supramolecular donor-acceptor dyads was confirmed by steady state absorption change. Charge separation and charge recombination processes upon photoexcitation of ZnCn of the supramolecular dyads were successfully observed by subpicosecond laser flash photolysis. The estimated ET rates (kET) were compared with those of other porphyrin isomers. Differences in the driving force dependence of kET values of porphyrin isomers were attributed to the electronic coupling and internal reorganization energy. Electronic and structural factors, which brought about the difference in kET values, were reasonably explained on the basis of the theoretical calculation.

Effect of Oligothiophene π-Bridge Length on the Photovoltaic Properties of D–A Copolymers Based on Carbazole and Quinoxalinoporphyrin

A series of low-bandgap donor–acceptor (D–A) copolymers, P(C-T-QP), P(C-BT-QP), P(C-TT-QP), and P(C-TT-QP-Zn), using 2,7-carbazole (C) as an electron-rich unit and quinoxalino[2,3-b′]porphyrins (QP) or quinoxalino[2,3-b′]porphyrinatozinc(QP-Zn) as an electron-deficient unit with different length of oligothiophene π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridge between the C donor unit and the QP acceptor unit is thiophene (T) in P(C-T-QP), bithiophene (BT) in P(C-BT-QP), and terthiophene (TT) in P(C-TT-QP) or P(C-TT-QP-Zn). These copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap ranging from 1.66 to 1.73 eV. The influence of the π-bridge and the central Zn ion on the electronic and photovoltaic properties was investigated and discussed in detail. It was found that the π-bridge played an important role in tuning the effective conjugation length and therefore significantly affected the molecular architecture and opt...

Electron Transfer in the Supramolecular Donor−Acceptor Dyad of Zinc Porphycene

The electron transfer processes of Zn octaethylporphycene (ZnPcn), a structural isomer of Zn octaethylporphyrin, have been investigated mainly using transient absorption spectroscopy. To form a supramolecular donor-acceptor dyad, imide compounds bearing a pyridine group at the N position of the imides have been used as an acceptor. The N atom of the pyridine ring can coordinate to the central Zn ion of ZnPcn. Formation of a supramolecular donor-acceptor dyad, that is, pentacoordinated ZnPcn, was confirmed by steady-state absorption spectroscopy using toluene as a solvent. Charge separation upon excitation of ZnPcn was indicated by efficient fluorescence quenching, especially when pyromellitic diimide was used as the acceptor. Electron transfer processes were confirmed by subpicosecond transient absorption spectroscopy, in which generation of a radical anion of the acceptor and a radical cation of ZnPcn, which was identified by means of gamma-ray radiolysis, was confirmed. It became clear that the charge separation rate was smaller than that of the corresponding supramolecular dyads of Zn tetraphenylporphyrin and Zn octaethylporphyrin despite a similar driving force. This observation indicates a larger internal reorganization energy and a smaller coupling element of the ZnPcn dyad.

Synthesis, luminescence, and structural characterization of Zn and Cd coordination polymers of flexible bis(imidazolyl) derivatives

Coordination polymers [Zn(imc)(L 1)] · H 2O, ( 1, imc = iminodiacetate, L 1 = bis( N-imidazolyl)methane) [Zn(hba) 2(L 2)] 2 · EtOH · 3H 2O, ( 2, hba = p-hydroxybenzoate, L 2 = bis( N-benzimidazolyl)methane), [Cd(mal)(H 2O)(L 3)]( 3, L 3 = 1,4-bis( N-imidazolyl)butane, mal = maleate) have been prepared and structurally characterized. Complex 1 consists of hexa-coordinated central Zn ions and exhibits 2D network structure. The Zn atoms in 2 have tetrahedral coordination geometry, and are linked by bis(imidazolyl) ligands into 1D chain structure. The cadmium ions in 3 are hepta-coordinated with pentagonal bipyramidal geometry. Complex 3 displays 2D grid structure. The TGA showed that the coordination polymers are stable up to 200 °C. All the three complexes are emissive at room temperature in their solid state.

Interfacial Behavior of Tetrapyridylporphyrin Monolayer Arrays

The behavior of monolayer films of free base 5,10,15,20-tetrapyridylporphinato (TPyP) and 5,10,15,20-tetrapyridylporphinato zinc(II) (ZnTPyP) on pure water, 0.1 M CdCl2, and 0.1 M CuCl2 subphases was investigated by surface pressure-area isotherms, specular X-ray reflectometry, and polarized total reflection X-ray absorption spectroscopy (PTRXAS). Surface pressure-area isotherms showed significant differences in the area per molecule on pure water compared to that on salt subphases, with a marked increase in the area observed on the salt solutions. This behavior was noted for both forms of the porphyrin and both salts investigated. Modeling of specular X-ray reflectometry data indicated that thinner and more electron dense layers on salt subphases best fit the observed profiles. These data suggest that the porphyrin macrocycle is oriented parallel to the interface on salt subphases and takes on a tilted conformation on pure water. In the case of ZnTPyP, PTRXAS was used to determine the orientation of the porphyrin moiety relative to the surface and to probe the coordination of the central Zn ion. In agreement with the pressure-area isotherms and reflectometry, the PTRXAS data indicate a change in orientation on the salt subphases.

Ivestigation of energy‐ and charge‐transfer processes using self‐organized porphyrin aggregates

AbstractSelf‐assembled triads and pentads are formed from Zn‐porphyrin or Zn‐chlorin dimers and pyridyl‐substituted free base of porphyrins, chlorins or tetrahydroporphyrins. A two‐point co‐ordination of the central Zn ions in the dimer with the nitrogen atoms of the pyridyl substituents of the free bases is essential for the complex formation. The complexation process has an equilibrium constant of the order of 107 M−1 at room temperature. An activation energy of the thermal dissociation of the complexes of 0.8 eV is found, which does not differ very much for different kinds of aggregates. Competing energy and charge transfer processes taking place in the aggregates have been identified by temperature dependent steady state fluorescence measurements.

Formation and optical properties of self-organized pentameric porphyrin arrays

Principles of formation, electronic absorption and fluorescence spectra are reported for self-organized pentameric arrays of tetrapyrrolic macrocycles. In these arrays two molecules of Zn-porphyrin dimers, Zn(II 1,4-bis[5-(10,15,20-tri- p-hexylphenylporphyrinyl)]-benzene ((ZnHTPP) 2) are bound via one molecule of a tetrapyridyl-substituted free base of porphyrin or tetrahydroporphyrin. The process of self-assembly is based on the twofold coordination of the central Zn ions in the dimer with the nitrogen atoms of the pyridyl rings in the free base which is strong enough to make the complexes stable at room temperature. The formation of the complexes can be followed by changes in the absorption bands of (ZnHTPP) 2 characteristic of an axial extra-ligation of Zn-porphyrins with pyridine or pyridyl-substituted compounds. The spectral behavior of the free bases in the pentads is determined by a non-planar distortion of their macrocycle caused by the two-point binding with the dimers. The fluorescence intensity of the Zn-porphyrin dimer decreases essentially upon complexation with the tetrapyridyl-substituted free bases. This quenching effect is assigned to a singlet-singlet energy transfer from the complexed Zn-porphyrin dimers to the free base subunit in the pentad.

The crystal structure of zinc 8-hydroxyquinolinate dihydrate

Abstract : The crystal structure of Zn 8-hydroxyquinolinate dihydrate was determined by single crystal methods. THe unit cell is monoclinic with a = 11.28 A, b= 5.42 A, c =13.16 A. and beta = 106 deg 18 ft. The space group is P2 sub 1/ a -(c(superscript5)) sub 2h; there are 2 molecules per unit cell. Atomic positions were determined by Fourier projections and refined by a 3-dimensional Fourier electron-density synthesis and by the method of least squares. Structural factors were obtained from visually estimated intensities on Weissenberg photographs taken with Cu K alpha radiation. The molecule, with the exception of the Hsub2O groups is essentially planar. When the Hsub2O molecules are included, A distorted octahedral arrangement of bonds results around the central Zn ion. The Zn-O and Zn-N bond lengths are 2.05 A and 2.06 A, respectively; the Zn-OHsub2 bond length is 2.27 A.