Adjacent Metal Centers Research Articles

Luminescent Metal Complexes within Polyelectrolyte Layers: Tuning Electron and Energy Transfer

The electrochemical and photophysical properties of a luminescent metal center, [Os(bpy)(3)](2+), are significantly modified by encapsulation within a conducting polymer composite film. Cyclic voltammetry reveals that the encapsulation in an inherently conducting polymer, polyaniline (Pani) or polypyrrole (PPy), can dramatically influence the charge-transfer rates between the metal centers. The increased electron transport, most likely mediated through the conducting polymer backbone, significantly enhances the electrochemiluminescence (ECL) efficiency. The increased communication between adjacent metal centers can also result in other interesting properties, such as photoinduced electron-transfer processes. In situ electron spin resonance (ESR) spectroscopy has been used to probe the photo-oxidation of an osmium metal center encapsulated in a PPy composite film. The irradiation of PPy in the presence of the osmium metal center resulted in the photo-oxidation of the Os(2+) to Os(3+) state and the consequent reduction of the PPy polyelectrolyte. The degree of communication between luminescent metal centers allows the composite properties to be tuned for various applications including ECL sensor devices and light-switching and light-harvesting systems.

Syntheses, crystal structures and fluorescent properties of two 4,4′-dipyridylsulfide based zinc(II) and cadmium(II) helical complexes

Two allomeric helical coordination polymers, {[Zn(4,4′-dps)2(HSSA)(H2O)2] · 3(H2O)} n (1) and {[Cd(4,4′-dps)2(HSSA)(H2O)2] · 3(H2O)} n (2) (4,4′-dps = 4,4′-dipyridylsulfide, H3SSA = 5-sulfosalicylic acid), have been synthesized by similar experimental methods and characterized by elemental analysis, single-crystal X-ray diffraction, and fluorescence spectra. Both crystallize in monoclinic system, space group P21/n, with a = 11.7947(13) Å, b = 13.7475(15) Å, c = 20.183(2) Å, and Z = 4 for 1 and a = 11.940(7) Å, b = 14.068(8) Å, c = 20.323(12) Å, and Z = 4 for 2. In both complexes, the metal (ZnII for 1 and CdII for 2) are six-coordinate with a N3O3 donor set in a distorted octahedron. Half of the 4,4′-dps are μ 2-bridging, linking adjacent metal centers and forming P21/n dimensional helical structures along the b-axis. Fluorescence measurements show that 1 and 2 have medium fluorescent emission at 402 and 405 nm, respectively.

Enantioselective Recognition and Separation by a Homochiral Porous Lamellar Solid Based on Unsymmetrical Schiff Base Metal Complexes

Our Schiff has come in: A homochiral porous lamellar solid based on chiral unsymmetrical Schiff base metal complexes was assembled by using coordination and hydrogen bonds (see picture). The host features a flexible helical framework and chiral amphiphilic channel surfaces capable of multiple interactions with guest species.A homochiral lamellar supramolecular host system has been developed by treating 3-[(E)-{[(1R,2R)-2-aminocyclohexyl]imino}methyl]-4-hydroxybenzoic acid (H(2)L) with copper nitrate and characterized by a variety of techniques including microanalysis, IR spectroscopy, thermogravimetric analysis (TGA), circular dichroism (CD) spectroscopy, and powder and single-crystal X-ray diffraction. Tridentate Schiff base L ligands link adjacent metal centers to form 1D coordination polymeric chains using tridentate N(2)O donors and carboxylate groups, while interchain metal-oxygen and hydrogen-bonding interactions further link 1D polymeric chains to a porous lamellar solid. The host features a flexible helical framework for guest inclusions and chiral amphiphilic channel surfaces lined with amine hydrogen atoms and aliphatic groups capable of multiple interactions with guest species. It allows for the development of an efficient approach for highly enantioselective separation of racemic secondary alcohols by inclusion crystallization followed by distillation (>99.5 % ee). In addition, the host including different alcohols exhibits guest-responsive ferroelectric behavior.

Catena-Poly[[[di-μ2-hydroxido-bis[(di-2-pyridylamine)nickel(II)]]-μ-fumarato] dihydrate

The NiII ion in the one-dimensional title complex, {[Ni2(C4H2O4)(OH)2(C10H9N3)2]·2H2O}n, has a distorted square-pyramidal coordination environment formed by three O atoms from two bridging hydroxide groups and one carboxyl­ate group of the fumarate ligand and two pyridine N atoms from a di-2-pyridylamine (dpa) ligand. Two hydroxide groups link adjacent metal centers, forming a centrosymmetric four-membered [Ni2(OH)2] ring. In the crystal structure, the H atoms of the bridging hydroxide groups form inter­molecular hydrogen bonds to both water mol­ecules. These are further linked to the uncoordinated O atoms of the carboxyl­ate groups and the NH group of a dpa ligand to generate a three-dimensional network from the chains of the coordination polymer.

Synthesis, Structure and Fluorescence Properties of a Trinuclear Zn(II) Complex with N,N,O-donor Schiff Base Ligands and Bridging Acetates

A new centrosymmetric trinuclear Zn(II) complex [Zn3L2(CH3COO)4] (1) has been synthesized by the reaction of Zn(CH3COO)2 · 2H2O with a tridentate N,N,O-donor Schiff base ligand HL [C5H4NC(CH3)=NC6H4(OH)] and systematically characterized by elemental analysis, FT-IR, UV/Vis and thermal methods. Single crystal X-ray structure analysis reveals that three Zn(II) centers are in two different coordination environments. Two terminal Zn(II) centers adopt a distorted squarepyramidal geometry leaving the central Zn(II) in a distorted octahedral environment. Two adjacent metal centers are connected through single μ2-phenolato as well as double μ-acetato-O,O´ syn-syn bridges. Fluorescence properties of the complex as compared to the ligand indicate that the former can serve as a potential photoactive material.

Poly[diaqua(μ-oxalato)(μ-2-oxidopyridinium-3-carboxylato)praseodymium(III)

In the title complex, [Pr(C6H4NO3)(C2O4)(H2O)2]n, each PrIII ion is coordinated by eight O atoms from two 2-oxynicotinate ligands, two oxalate ligands and two water mol­ecules, displaying a distorted bicapped square-anti­prismatic geometry. The carboxyl­ate groups link adjacent praseodymium metal centres, forming layers parallel to the bc plane. The crystal packing is stabilized by inter­molecular O—H⋯O and N—H⋯O hydrogen bonds.

A new polymorph of poly[bis(μ2-perchlorato-κ2O:O′)(2,2′:6′,2′′-terpyridine-κ3N,N′,N′′)lead(II)] with a greatly extended chain repeat distance

In the title compound, [Pb(ClO(4))(2)(C(15)H(11)N(3))](n), two molecules occupy general positions while the third lies on a crystallographic twofold axis, giving a total of two and a half molecules per asymmetric unit. Each metal centre is coordinated equatorially by three 2,2':6',2''-terpyridine (terpy) N-donor atoms and axially by two perchlorate O-donor atoms. The distorted pentagonal bipyramidal geometry is completed by two equatorial O-donor atoms from two perchlorate anions which bridge to two different adjacent metal centres. The coordination about each metal centre is very similar to that seen at the unique Pb(II) centre in the previously published polymorph [Engelhardt, Harrowfield, Miyamae, Patrick, Skelton, Soudi & White (1996). Aust. J. Chem. 49, 1135-1146], but the new polymorph differs from it by the insertion on each side of an existing [bis(perchlorato)(terpy)lead(II)] molecule of two additional such units. Pairs of asymmetrically bridging perchlorate anions link irregularly spaced Pb(II) centres into undulating chains parallel to [201] which exhibit a repeat distance of 26.280 (4) A. The significance of this new polymorph lies in the fact that, while it is chemically identical to the known polymorph, it is structurally distinct from it.

Solution and solid-state photoluminescence of bis{succinatobis[2,6-bis(2-benzimidazolyl)] lead(II)}: A comment on the lone-pair stereochemistry and its effect on supramolecular interaction

A new Pb(II) coordination compound, namely [Pb(suc)(H 2bbp)] 2 ( 1) has been synthesized by treating succinic acid (H 2suc), and 2,6-bis(2-benzimidazolyl) pyridine (H 2bbp) with Pb(II) nitrate under hydrothermal conditions. Dicarboxylate as a bridging ligand is used to link adjacent metal centers forming dinuclear metal complex. The arrangement of the H 2bbp ligand and carboxyl groups suggest a gap in the coordination around the Pb(II) ion, occupied possibly by a stereoactive lone-pair of electrons on Pb(II) where the coordination around the lead atoms is hemidirected. At room temperature, in the solid-state, the compound displays blue luminescence at 438 nm, which is about 50 nm red-shifted compared to the emission in DMF solution, resulting from long Pb···O interactions, a closer inspection of the pack reveals π–π stacking interactions that are crucial to lowering the HOMO–LUMO energy gaps.

2-Benzothiazolylthioacetic acid and 4,4′-bipyridine as ligands in designing low-dimensional coordination polymers: Synthesis, architectures and magnetic properties

The combined use of 4,4′-bipyridine (4,4′-bipy) and 2-benzothiazolylthioacetic acid (HBTTAA) as ligands with Mn(II), Cd(II), Co(II) and Cu(II) ions afforded six polymeric complexes, namely {[Mn 3(BTTAA) 4(4,4′-bipy) 4](ClO 4) 2 · 2H 2O} n ( 1), [Mn(BTTAA) 2(4,4′-bipy) 2] n ( 2), [Cd(BTTAA) 2(4,4′-bipy) 2] n ( 3), [Cd(BTTAA)(4,4′-bipy)(NO 3)(H 2O)] n ( 4), [Co(BTTAA) 2(4,4′-bipy)(H 2O) 2] n ( 5) and [Cu(BTTAA) 2(4,4′-bipy)] n ( 6). All these complexes have been characterized by a combination of analytical, spectroscopic and crystallographic methods. Complex 1 is a novel 2D network formed by two different 4 4 grid networks, whereas isomorphous complexes 2 and 3 exhibit a 2Dl coordination architecture formed by the same 4 4 grid network. In 4– 6, extended 1D chains are formed, with the 4,4′-bipy molecules acting as rigid rod-like links between adjacent metal centers. The carboxylato groups of BTTAA in these complexes exhibit four different coordination modes, namely monodentate, chelating, bridging and bridging-chelating modes. The magnetic properties of 1, 2, 5 and 6 were investigated in the temperature range 2.0–300.0 K. Variable temperature magnetic susceptibility measurements show weak antiferromagnetic interactions in these complexes.

Synthesis, Structural, Thermal and Magnetic Characterization of a Pyrophosphato‐Bridged Cobalt(II) Complex

AbstractThe reaction in water of CoII sulfate heptahydrate with 1,10‐phenanthroline (phen) and sodium pyrophosphate (Na4P2O7) in a 2:4:1 stoichiometric ratio resulted in the crystallization of a neutral dinuclear CoII complex, {[Co(phen)2]2(μ‐P2O7)}·6MeOH (1), as revealed by a single‐crystal X‐ray diffraction study. The bridging pyrophosphato ligand between the two [Co(phen)2]2+ units in a bis(bidentate) coordination mode places the adjacent metal centers at 4.857 Å distance, and its conformation gives rise to intramolecular π–π stacking interaction between adjacent phen ligands. Indeed, intermolecular π–π stacking interactions between phen ligands from adjacent dinuclear complexes create a supramolecular 2D network in 1. Magnetic susceptibility measurements on a polycrystalline sample of 1 in the temperature range 1.9–295 K are typical of an overall antiferromagnetic coupling with a maximum of the magnetic susceptibility at 3.0 K. The analysis of the magnetic data in the whole temperature range allows the determination of the value of the intramolecular magnetic coupling (J = –1.23 cm–1). The ability of the pyrophosphato ligand to mediate magnetic interactions between different first‐row transition‐metal ions when adopting the bis(bidentate) bridging mode is analyzed and discussed in the light of the small number of magneto‐structural reports on this type of compound, bearing in mind the number of unpaired electrons and type of magnetic orbitals on each metal center. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

New microporous copper(II) coordination polymers based upon bifunctional 1,2,4-triazole/tetrazolate bridges

A series of Cu(II) coordination polymers, [Cu(μ2-L1)3]PO3F (1), [Cu2(μ2-L1)4(μ4-L1)](NO3)4·2H2O (2), [Cu(μ2-L1)2(μ2-X)]X·12H2O (X = Cl, 3; Br, 4), [Cu4(μ2-OH)2(μ4-L1)3(μ2-L1)2(H2O)2](CF3SO3)6·10H2O (5), [Cu3(μ2-OH)2(μ4-L1)(H2O)2(μ3-SO4)2] (6), [Cu(μ4-L1)(μ2-SO4)]·6H2O (7), [Cu2(μ4-L2)3]Cl·12H2O (8) involving new bifunctional p-phenylene bridged bi(1,2,4-triazole) and mixed 1,2,4-triazole-tetrazolate based ligands (L1= 1,4-phenylene-4,4′-bi(1,2,4-triazole), HL2= 5-(4-[1,2,4]triazol-4-yl-phenyl)-1H-tetrazole) has been prepared under hydrothermal conditions and their structures have been established by means of X-ray diffraction. In crystal structures 1–8, the organic ligands, utilizing two neighboring nitrogen atoms (N1, N2 in triazole (trz) and N2, N3 in tetrazolate), behave either in μ2 or in μ2 + μ4 manner binding the adjacent metal centers or Cu3(μ2-OH)2 clusters into 1D columns (1, 3, 4), 2D zigzag layers (2) and 3D frameworks (5–8). The mutual coplanarity between uncoordinated trz moiety and p-phenylene spacer as well as its radial disposition around the “propeller-like” [Cu2(η2-trz)3] subunits is a crucial factor, which specifically mediates multiple π–π interactions through intercalation of the neighboring Cu(η2-trz)n (n = 2,3) axles, and as a consequence, affording one-dimensional channels with trigonal (1, 2) or rhombic geometry (3, 4). Large rectangular channels have been realized in neutral [Cu(μ4-L1)(μ2-SO4)]n (7) and cationic [Cu2(μ4-L2)3]nn+ (8) frameworks, in which the remaining void space is filled by water molecules and counter anions (Cl−).

A pyrazole-based orthogonal ferromagnetically coupled [2 × 2] homoleptic square Cu 4 grid: Magnetostructural correlations

The synthesis and structure of a pyrazole-based orthogonal ferromagnetically coupled tetracopper(II) 2 × 2 homoleptic grid complex [Cu 4(PzOAPyz) 4(ClO 4) 2](ClO 4) 2 · 6H 2O ( 1), formed by the reaction between the ditopic ligand PzOAPyz and Cu(ClO 4) 2 · 6H 2O, are described. The ligand contains terminal pyrazole and pyrazine residues bound to a central flexible diazine subunit (N–N) as well as one potentially bridging alkoxo group. The two adjacent metal centers are linked by an alkoxo oxygen forming essentially a square Cu 4(μ-O 4) cluster. In the Cu 4(μ-O 4) core, out of the four copper centers, two copper centers are penta-coordinated and the remaining two are hexa-coordinated. In each case of hexa-coordination, the sixth position is occupied by one of the oxygen atoms of a coordinated perchlorate ion. Complex 1 has been characterized structurally and magnetically. Although the large Cu–O–Cu bridge angles (137–138°) and short Cu–Cu distances (3.964–3.970 Å) are suitable for the transmission of the expected antiferromagnetic coupling, the square-based Cu 4(μ-O 4) cluster exhibits an intramolecular ferromagnetic exchange ( J = 7.47 cm −1) between the metal centers with an S = 2 magnetic ground state associated with the quasi orthogonal arrangement of the magnetic orbitals ( d x 2 - y 2 ). The exchange pathway parameters have been evaluated from density functional calculations.

Synthesis and Structural and Magnetic Characterization of {[(phen)2Ni]2(μ-P2O7)}·27H2O and {[(phen)2Mn]2(μ-P2O7)}·13H2O: Rare Examples of Coordination Complexes with the Pyrophosphate Ligand

The reaction in water of M(II) [M = Ni or Mn] with 1,10-phenanthroline (phen) and sodium pyrophosphate (Na4P2O7) in a 2:4:1 stoichiometry resulted in the crystallization of dinuclear complexes featuring the heretofore rare bridging pyrophosphate. Single-crystal X-ray diffraction studies revealed the complexes to be {[(phen)2Ni]2(micro-P2O7)} . 27H2O (1) and {[(phen)2Mn]2(micro-P2O7)} . 13H2O (2) where the asymmetric M(phen)2 units are bridged by bis-bidentate pyrophosphate, each metal ion exhibiting a distorted octahedral geometry. The bridging pyrophosphate places adjacent metal centers at 5.031 A in 1 and 4.700 A in 2, and its conformation also gives rise to an intramolecular pi-pi interaction between two adjacent phen ligands. Intermolecular pi-pi interactions between phen ligands from adjacent dinuclear complexes create an ornate 3D network in 1, whereas a 2D sheet results in 2. The hydrophilic nature of the pyrophosphate ligand leads to heavy hydration with the potential solvent-accessible area for 1 and 2 accounting for 45.7% and 26.4% of their unit cell volumes, respectively. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 1 and 2 revealed net weak intramolecular antiferromagnetic coupling between metal centers in both compounds with J = -3.77 cm(-1) in 1 and J = -0.88 cm(-1) in 2, the Hamiltonian being defined as H = -JSA.SB. The ability of the bis-bidentate pyrophosphate to mediate magnetic interactions between divalent first row transition metal ions is discussed bearing in mind the number and nature of the interacting magnetic orbitals.

Effects of Metal Cation Coordination on Fluorescence Properties of a Diethylenetriamine Bearing Two End Pyrene Fragments

Fluorescence properties of a diethylenetriamine bearing two end pyrene fragments (L) have been studied in water, where effects of adding metal cations (Zn2+, Cd2+, Cu2+, Hg2+, Ag+) on the emission properties of L have been studied. Without metal cations, L shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission intensity (I(M)) is strong at acidic pH but decreases with a pH increase because of an electron transfer (ET) from the unprotonated nitrogen atoms to the excited pyrene fragment. The excimer emission is due to the static excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments. The excimer emission intensity (I(E)) is weak at acidic pH but increases with a pH increase because of the GSD stability increase associated with the deprotonation of the polyamine chain. Addition of metal cations leads to I(M) decrease, where chelation-driven I(M) enhancement does not occur even with diamagnetic Zn2+ and Cd2+ at any pH. This is because a pyrene-metal cation pi-complex, formed via a donation of pi-electron of the pyrene fragment to the adjacent metal center, suppresses the monomer photoexcitation. I(E) also decreases upon addition of metal cations because the pyrene-metal cation pi-complex weakens pi-stacking interaction of the end pyrene fragments, leading to GSD stability decrease. The emission properties of L-Zn2+ complexes were studied by means of time-resolved fluorescence decay measurements, and the effects of adding a less-polar organic solvent were also studied to clarify the detailed emission properties.

Carbon–Fluorine Bond Activation in Fluoroolefins: Clear Documentation of Cooperative CF Bond Activation by Adjacent Metal Centers

Carbon–Fluorine Bond Activation in Fluoroolefins: Clear Documentation of Cooperative CF Bond Activation by Adjacent Metal Centers

Carbon–Fluorine Bond Activation in Fluoroolefins: Clear Documentation of Cooperative CF Bond Activation by Adjacent Metal Centers

Carbon–Fluorine Bond Activation in Fluoroolefins: Clear Documentation of Cooperative CF Bond Activation by Adjacent Metal Centers

Copper(II) and lead(II) complexes of 4,4′-bipyridine-N,N′-dioxide

Cu(II) and Pb(II) complexes of 4,4′-bipyridine-N,N′-dioxide have been prepared using a solvent-layering system. [Cu2Cl4(bpdo)3(H2O)2] . 2(CH3)2SO (1) crystallises in P-1, a=8.731(2), b=8.943(2), c=14.408(3) A, α=102.85(3), β=97.49(3), γ=109.77(3)°. The Cu(II) complex is a z-shaped discrete molecule with a DMSO molecule hydrogen bonded to the host through coordinated water molecule. Crystallisation of PbCl2, PbBr2 and PbI2 with bpdo afforded isostructural 2D coordination polymers. [PbCl2(bpdo)]n is monoclinic, C2/c with a=16.3274(7), b=4.0708(1), c=18.6146(8) A, β=93.73(1)°; [PbBr2(bpdo)]n is monoclinic, C2/c with a=16.403(3), b=4.2412(8), c=18.846(4) A, β=92.59(3)° and [PbI2(bpdo)]n is monoclinic, C2/c with a=16.438(3), b=4.538(1), c=18.973(4) A, β=91.04(3)°. The adjacent metal centres of these polymers are bridged by coordinated Cl−, Br− or I− anions as well as by bpdo ligands. These polymers possess no conventional hydrogen bonds.

Catena-Poly[[(2,2′-bipyridine-κ2 N,N′)manganese(II)]-μ-3,5-dinitro-2-oxidobenzoato-κ4 O,O′:O′,O 2

In the title complex, [Mn(C7H2N2O7)(C10H8N2)] n , the MnII atom has a distorted octahedral coordination geometry, defined by two N atoms from the 2,2′-bipyridine ligand and four O atoms from two 3,5-dinitrosalicylate dianions. One carboxylate O atom acts as a bridge between adjacent metal centres to form a one-dimensional chain. The Mn...Mn distance is 4.315 (6) Å. The crystal packing is stabilized by intermolecular C—H...O hydrogen interactions.

Linear copper ‘chain’ complexes with bulky tritopic hydrazone ligands—structural and magnetic studies

Tritopic 2,6-picolyl-bis-hydrazone ligands with bulky terminal groups derived from phenyl-pyridyl ketone do not form the expected [3 x 3] grids on reaction with copper(II), but instead form Cu8 'pinwheels', and in the present case linear trinuclear, pentanuclear and chain structures also. Direct bridging between copper ions occurs through micro2-N-N diazine groups, and longer O-C-N hydrazone connections, leading to moderately strong antiferromagnetic exchange between adjacent metal centres. Structural and magnetic properties are discussed in the context of specific orthogonal and non-orthogonal bridges, which can be distinguished and quantified.

Changes in magnetic properties from solid state to solution in a trinuclear linear copper(ii) complex

A linear trinuclear copper(II) complex containing phenoxido- and alkoxido-bridges between the metal centers has been isolated and structurally characterized. The complex cation consists of a linear array of three copper ions, assembled by means of two doubly deprotonated ligands. The octahedral coordination sphere of the two peripheral copper(II) ions is completed by weakly bound methanol molecules, and the square-planar central metal ion is located on an exact, crystallographic inversion center. Temperature-dependent magnetic susceptibility studies reveal the presence of antiferromagnetic exchange coupling between the copper(II) ions in the trinuclear unit along with small intermolecular antiferromagnetic interactions in the low temperature range. The results were fitted in two different ways, (i) taking into account solely the exchange interaction between the adjacent metal centers or, (ii) regarding exchange interactions between both adjacent and non-adjacent copper(II) ions. Solid-state temperature-dependent X-band EPR studies in the range 4.2–250 K indicate a doublet ground spin state |½, 1〉. In solution, the ground spin state of the complex is found to be a quartet (S = ), suggesting a modification of the exchange coupling interactions between the copper(II) ions. The simulation of the 4.2 K solution spectrum gives rise to the best parameters D > 0.8 cm−1, g⊥ = 2.04 and g∥ = 2.21.