Stepwise Complexation Research Articles

Three-Dimensional Fe(II)-based Metallo-Supramolecular Polymers with Electrochromic Properties of Quick Switching, Large Contrast, and High Coloration Efficiency

A series of Fe(II)-based metallo-supramolecular polymers with three-dimensional (3-D) structures were synthesized by the stepwise complexation of an Fe(II) salt with different ratios of a linear bis(terpyridine) ligand and a branched tris(terpyridine) ligand. Atomic force microscopy images of the polymer films showed a drastic change in the surface morphology upon varying the amount of the branched ligand. The surface of a designed 3-D construction film showed a highly porous structure (pore size: approximately 30-50 nm in diameter), probably due to the formation of a hyperbranched polymer structure. All the 3-D polymers had a blue color based on the metal-to-ligand charge-transfer (MLCT) absorption and exhibited excellent electrochromic properties. The most highly porous 3-D-structured film showed the best electrochromic performance; as compared with a 1-D linear polymer, the switching times were improved 38.7% for the coloring (0.31 → 0.19 s) and 37.9% for the bleaching (0.58 → 0.36 s). The transmittance change (ΔT) increased 21.8% (41.6 → 50.7%). Also, the coloration efficiency (η) was enhanced 45.3% (263.8 → 383.4 cm(2) C(-1)). The redox in the 3-D film was diffusion-controlled, as supported by the linear relationship between the current and square root of the scan rate. It is considered that the porous structure of the 3-D polymer films contributed to smooth ionic transfer during the redox and to the improved electrochromic properties.

Hysteretic vapour response of a heterodinuclear platinum(ii)–copper(ii) complex derived from the dimer-of-dimer motif and the guest-absorbing site

A heterodinuclear complex, syn-[PtCu(μ-pyt)2(bpy)2](2+) (syn-[PtCu]; Hpyt = pyridine-2-thiol, bpy = 2,2'-bipyridine), was synthesized by stepwise complexation. Three different crystal structures of the hexafluorophosphate salts with solvent molecules as guests at the axial sites of the Cu(ii) ions, i.e. syn-[PtCu-G] (G = acetonitrile, acetone, and methanol), were determined by X-ray analyses. In the solid-state, these complexes adopt dimer-of-dimer motifs characteristic of the syn isomers of dinuclear complexes with two pyt bridging ligands arranged in a head-to-head configuration. These dinuclear complexes have a short PtCu distance (2.75-2.81 Å) and slightly different intermolecular PtPt distances (3.43-3.51 Å), which affect the colour of the solid complexes. All the syn-[PtCu-G] systems absorbed/desorbed vapour molecules; however, they exhibit different chromic changes because of the unique structural hysteresis of the desorbed form, i.e. syn-[PtCu], according to powder X-ray diffraction measurements. In addition, guest exchange occurred in the syn-[PtM-G] complexes upon exposure to the vapour of different solvents.

Formation of a Pt12 Cluster by Single‐Atom Control That Leads to Enhanced Reactivity: Hydrogenation of Unreactive Olefins

A platinum subnanocluster catalyst composed of 12 atoms was synthesized using a phenylazomethine dendrimer, which can assemble twelve PtCl4 units by stepwise complexation, followed by reduction to Pt(0). Unreactive olefins that were not activated by conventional 2 nm Pt nanoparticles were successfully hydrogenated by the subnanocluster. EWG = electron-withdrawing group.

Formation of a Pt12 Cluster by Single‐Atom Control That Leads to Enhanced Reactivity: Hydrogenation of Unreactive Olefins

A platinum subnanocluster catalyst composed of 12 atoms was synthesized using a phenylazomethine dendrimer, which can assemble twelve PtCl4 units by stepwise complexation, followed by reduction to Pt(0). Unreactive olefins that were not activated by conventional 2 nm Pt nanoparticles were successfully hydrogenated by the subnanocluster. EWG = electron-withdrawing group.

Dendrimer‐Encapsulated Bimetallic Pt‐Ni Nanoparticles as Highly Efficient Catalysts for Hydrogen Generation from Chemical Hydrogen Storage Materials

AbstractBimetallic Pt‐Ni dendrimer‐encapsulated nanoparticles (DENs) with different Pt/Ni ratios have been successfully synthesized through the co‐complexation of Pt2+ and Ni2+ cations to the internal tertiary amine of fourth‐generation hydroxyl‐terminated poly(amidoamine) dendrimers (G4‐OH) followed by coreduction by NaBH4 in aqueous solution. The catalytic activities for the decomposition of hydrous hydrazine and the hydrolysis of ammonia borane over the Pt‐Ni DENs have been studied in aqueous solution, and the Pt‐Ni DENs show high catalytic performances for these hydrogen‐generation reactions. TEM measurements reveal that the size‐controlled Pt‐Ni NPs, which have an average particle size of 1.1±0.3 nm, are encapsulated in the G4‐OH dendrimer.

Vapour and mechanically induced chromic behaviour of platinum complexes with a dimer-of-dimer motif and the effects of heterometal ions

Heterodinuclear complexes, syn-[MPt(μ-pyt)2(bpy)2](n+) (syn-[MPt], M = Pd(2+), Au(3+), Hpyt = pyridine-2-thiol, bpy = 2,2'-bipyridine), were synthesized as a selective geometrical isomer by stepwise complexation. X-ray analyses of the hexafluorophosphate salts of these complexes proved their dinuclear structures with short M···Pt distances (2.9084(4) Å for syn-[PdPt] and 2.9071(4) Å for syn-[AuPt]), similar to the homodinuclear complex (2.9292(2) Å for syn-[PtPt]). In the syn-[PdPt] crystal, two dinuclear motifs are arranged closely in a head-to-head manner with a short Pt···Pt distance (3.3757(3) Å), forming a dimer-of-dimer structure as in the case of syn-[PtPt], whereas the corresponding crystal of syn-[AuPt] has a discrete arrangement of the dinuclear motifs. By the isomerisation of syn-[PdPt], anti-[PdPt] with equivalent environments of the Pd(2+) and Pt(2+) ions was also obtained successfully. Syn-[PdPt](PF6)2 exhibits vapochromic behaviour based on the absorption/desorption of CH3CN vapour, similar to that observed for syn-[PtPt](PF6)2. The reversible structural transformations induced by the uptake and release of CH3CN molecules were investigated by powder and single-crystal X-ray diffraction studies. These revealed that the vapochromic behaviour was based on the interconversion between two phases, the dimer-of-dimer structure with a short Pt···Pt distance and a π-π stacked arrangement with no Pt···Pt intermolecular interaction. The introduction of the heterometal ions enabled control of the colour region: orange ↔ red for syn-[PdPt] vs. light red ↔ dark red for syn-[PtPt], reflecting the weaker metal-metal interaction between Pd(2+) and Pt(2+) ions in the dinuclear motif. In addition, these complexes were found to exhibit mechanochromic behaviour based on a crystal-to-amorphous transformation upon grinding, and the reconstruction of the crystal structures by vapour sorption.

An alternately introduced heterometallo-supramolecular polymer: synthesis and solid-state emission switching by electrochemical redox

By utilizing different coordination properties of a lanthanide ion and a transition metal ion, a heterometallo-supramolecular polymer with Eu(III) and Fe(II) ions introduced alternately (polyEuFe) was precisely prepared via stepwise complexation of the metal ions to a new unsymmetrical ligand with both a dicarboxylate-substituted terpyridine and an unsubstituted terpyridine. The polymer showed unique, reversible "on-off" switching of the Eu(III) luminescence by electrochemical redox of the Fe ions in the solid-state.

Spectroscopic Studies on the Thermodynamics of the Complexation of Trivalent Curium with Propionate in the Temperature Range from 20 to 90 °C

The thermodynamics of the stepwise complexation reaction of Cm(III) with propionate was studied by time resolved laser fluorescence spectroscopy (TRLFS) and UV/Vis absorption spectroscopy as a function of the ligand concentration, the ionic strength and temperature (20–90 °C). The molar fractions of the 1:1 and 1:2 complexes were quantified by peak deconvolution of the emission spectra at each temperature, yielding the log10 \( K_{n}^{\prime } \) values. Using the specific ion interaction theory (SIT), the thermodynamic stability constants log10 \( K_{n}^{0} (T) \) were determined. The log10 \( K_{n}^{0} (T) \) values show a distinct increase by 0.15 (n = 1) and 1.0 (n = 2) orders of magnitude in the studied temperature range, respectively. The temperature dependency of the log10 \( K_{n}^{0} (T) \) values is well described by the integrated van’t Hoff equation, assuming a constant enthalpy of reaction and \( \Updelta_{\text{r}} C^\circ_{{p,{\text{m}}}} = 0, \) yielding the thermodynamic standard state \( \left( {\Updelta_{\text{r}} H^\circ_{\text{m}} ,\Updelta_{\text{r}} S^\circ_{\text{m}} ,\Updelta_{\text{r}} G^\circ_{\text{m}} } \right) \) values for the formation of the \( {\text{Cm(Prop)}}_{n}^{3 - n} \), n = (1, 2) species.

A Green Copper‐Based Metallo‐Supramolecular Polymer: Synthesis, Structure, and Electrochromic Properties

Stepwise complexation: A green copper-based metallo-supramolecular polymer was successfully synthesized through complexation of CuII ions with new bis(1,10-phenanthroline) ligands. The obtained polymer showed an excellent electrochromic color change from green to colorless upon electrochemical reduction of CuII to CuI. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Synthesis of a Dendrimer Reactor for Clusters with a Magic Number

Abstract A new type of phenylazomethine dendrimer (DPA) that has one more coordination site compared to the tetraphenylmethane core DPA was synthesized for the “magic number 13” metal cluster template that has cubic or hexagonal densely packed structures. The stepwise complexation property was revealed, and this property is expected to be the “magic number 13” metal cluster template.

Bromide Complexation by the EuIII Lanthanide Cation in Dry and Humid Ionic Liquids: A Molecular Dynamics PMF Study

We report a molecular dynamics study on the EuBr(n)(3-n) complexes (n=0 to 6) formed upon complexation of Br(-) by Eu(3+) in the [BMI][PF(6)], [BMI][Tf(2)N] and [MeBu(3)N][Tf(2)N] ionic liquids (ILs), to compare the effect of the IL anion (PF(6)(-) versus Tf(2)N(-)), the IL cation (BMI(+) versus MeBu(3)N(+)) and the "IL humidity" on their solvation and stability. In "dry" solutions all complexes remain stable and the first coordination shell of Eu(3+) is purely anionic (Br(-) and IL anions), surrounded by IL cations (BMI(+) or MeBu(3)N(+) ions). Long range "onion type" solvation features (up to 20 Å from Eu(3+)), with alternating cation-rich and anion-rich solvent shells, are observed around the different complexes. The comparison of gas phase-optimized structures of EuBr(n)(3-n) complexes (that are unstable for n=5 and 6) with those observed in solution points to the importance of solvation forces on the nature of the complex, with a higher stabilization by imidazolium- than by ammonium-based dry ILs. Adding water to the IL has different effects, depending on the IL. In the highly hygroscopic [BMI][PF(6)] IL, Br(-) ligands are displaced by water, to finally form Eu(H(2)O)(9)(3+). In the less "humid" [BMI][Tf(2)N], the EuBr(n)(3-n) complexes do not dissociate and coordinate at most 1-2 H(2)O molecules. We also calculated the free-energy profiles (Potential of Mean Force calculations) for the stepwise complexation of Br(-), and found significant solvent effects. EuBr(6)(3-) is predicted to form in both [BMI][PF(6)] and [BMI][Tf(2)N], but not in [MeBu(3)N][Tf(2)N], mainly due to weaker interactions with the cationic solvation shell. First steps are found to be more exergonic in the PF(6)(-)- than in the Tf(2)N(-)-based IL. Molecular dynamics (MD) comparisons between ILs and classical solvents (acetonitrile and water) are also reported, affording good agreement with the experimental observations of Br(-) complexation by trivalent lanthanides in these classical solvents.

Controlling the size of electrostatically self-assembled nanoparticles with cyclodextrin as external trigger

This study shows how electrostatic self-assembly of polyelectrolytes and organic counterions can be combined with cyclodextrin host–guest chemistry into a novel route to size-controllable supramolecular nanoparticles in aqueous solution. Nanoparticles are formed from fourth generation polyamidoamine dendrimers (PAMAM G4) as positively charged macroions and an excess of the oppositely charged anionic diazo dye Acid Yellow 38 (Ay38), yielding negatively charged supramolecular particles with hydrodynamic radii of 30–60 nm. Stepwise complexation of the dye by α-cyclodextrin leads to a decreasing charge density and thereby caused stepwise size increase up to particles with a hydrodynamic radius of 240 nm. Dynamic light scattering and ζ-potential measurements were used to elucidate assembly size and stabilization and isothermal titration calorimetry to determine association stoichiometry, enthalpies and equilibrium constants.

7Li NMR chemical shift titration and theoretical DFT calculation studies: solvent and anion effects on second‐order complexation of 12‐crown‐4 and 1‐aza‐12‐crown‐4 with Lithium cation in several aprotic solvents

(7)Li NMR titration was used to determine stepwise complexation constants for the second-order complexation of lithium cation with 12-crown-4 in acetonitrile, propylene carbonate and a binary mixture of propylene carbonate and dimethyl carbonate. The anions used were perchlorate, hexaflurophosphate and trifluromethanesulfonate. A second ligand 1-aza-12-crown-4 was similarly investigated. The exchange between the free and complexed cation in these reactions is fast on an NMR timescale resulting in a single lithium peak which is a concentration-weighted average of the free and bound species. Solvent effects show that the 1:1 complex is much more stable in acetonitrile than in propylene carbonate or in the propylene carbonate dimethyl carbonate mixture. Anion effects for a given solvent were small. Optimized geometries of the free ligands and the 1:1 and 1:2 (sandwich) metal-ligand complexes were predicted by hybrid-density functional theory using the Gaussian 03 software package. Results were compared to literature values for 1:1 stability constants found by microcalorimetry for several of these systems and are found to be in good agreement. Although microcalorimetry only considered the formation of 1:1 complexes, (7)Li NMR shows evidence that both 1:1 and 1:2 complexes should be considered.

Formation of (FeCl3)@phenylazomethine dendrimer (DPA): Fine control of the release and encapsulation of Fe ions in dendrimers

Abstract Phenylazomethine dendrimers (DPAs) act as a strong coordination site for metal ion assembly. DPA G4 is regarded as a molecular capsule having 30 metal-assembling sites with a 2.7 nm diameter. We have reported the radial stepwise complexation with Sn2+ ions in the dendrimers, which means the location and number of metal ions can be controlled. Therefore, DPA G4 should realize a ferritin-like redox nanocapsule with precise control of the number of Fe ions. On the other hand, the Fe ion is a typical paramagnetic molecule. For creating an advanced memory with a high density, ferritin is one of the candidates for use as a magnetic quantum dot. Many attempts to use biomaterials, for example, ferritins and chapero-nins, as metal storage capsules have been demonstrated. Some research groups fabricate a device by assembling ferritins on a plate using their rigid and uniform structure. The attempts to use dendrimers have also been demonstrated. We now describe the successful attempt to control the "encapsulation and release" of iron ions in a dendrimer in order to mimic a ferritin through the redox reaction. Furthermore, the assembling structures of (FeCl3) n @DPA on a plate were first observed by scanning tunneling microscopy (STM) as a dendrimer complex, which shows that a highly oriented film is formed on a plate only by solvent casting.

Size-specific catalytic activity of platinum clusters enhances oxygen reduction reactions

Colloidal platinum nanoparticles with diameters of 2-5 nm on carbon supports are currently regarded as the best catalysts for the oxygen reduction reaction. However, the particle size is limited by the conventional preparation methods that are used to synthesize small platinum particles; the inherent activity of ultrasmall nanoparticles has not yet been revealed. We present a practical synthesis for ultrafine subnanometre platinum clusters using a spherical macromolecular template with no disorder in molecular weight or structure. The template, a phenylazomethine dendrimer, offers control of the number of metal complexes in an assembly through stepwise complexation, allowing the complexes to accumulate in discrete nano-cages. Subsequent reduction of Pt(IV) chloride to Pt(0) results in the formation of platinum clusters composed of a defined number of atoms. As a result of exceptionally small particle size, the clusters exhibit very high catalytic activity for the four-electron reduction of oxygen molecules.

Heterometal Assembly in Dendritic Polyphenylazomethines

Abstract Fine-controlled metal–organic hybridization is strongly desired for advanced nano-materials. Dendrimers are highly branched organic macromolecules with successive layers or “generations” of branch units surrounding a central core. Metal–organic hybrid versions have also been produced by trapping metal ions or metal clusters within the voids of dendrimers. Here, we report a technique to form a dendritic organic structure, which includes various metals, with fine control over the number and location of the metal salts through a radial stepwise complexation. FeCl3, GaCl3, VCl3, and SnCl2 were decorated on a phenylazomethine dendrimer with 4th generations at the first, second, third, and forth layers, respectively. These stepwise complexation and heterometal assembling were also supported by TEM observation, 57Fe Mössbauer spectroscopy, XPS measurement, and NMR spectroscopy.

Investigation of a Molecular Morphology Effect on Polyphenylazomethine Dendrimers; Physical Properties and Metal‐Assembling Processes

A series of novel dendritic polyphenylazomethines (DPA) with asymmetric morphologies was synthesized. Their physical properties, such as encapsulating effect, molecular dynamics, and metal assembly, are strongly dependent on the entire conformation of the molecules. The most important property is layer-by-layer metal assembly in the dendrimer structure from the core to the outside. Bis- and tris-substituted DPAs of the fourth generation also act as frameworks for stepwise assembly of a metal component (SnCl2), like the fully substituted symmetric DPA. However, extensive investigation of metal assembly in specific DPAs revealed that they do not follow the stepwise process. The molecular density calculated from the experimental hydrodynamic volume indicated that bis- and tris-substituted DPAs with asymmetric morphology still retain a free space similar to that of fully substituted symmetric DPA. The monosubstituted DPA, however, displayed a slightly higher density (smaller space) than the other DPAs. The experimental results suggest a bent conformation of the dendrimer in which the core moiety is folded into the dendron structure. In addition, the molecular dynamics were probed by means of the 1H NMR signals of the porphyrin core. It was demonstrated that the conformation is not fixed at room temperature in solvated DPAs, especially in monosubstituted DPA. A similar observation was for the smaller DPAs (third generations) with asymmetric morphologies. These dendrimers do not follow the stepwise complexation process. The structures of bis- and tris-substituted dendrimers which accurately follow the stepwise process are fixed. These observations provide a new insight into the finely controlled metal-assembly chemistry of dendritic macromolecules, and a rigid and fixed conformation is one of important factors for their unique properties.

Modification and Electrochemical Properties of Poly(phenylazomethine) Derivatives

AbstractSummary: Oligo(phenylazomethine)s (OPAs) and aniline‐capped OPAs (OPA's) are used as model compounds of polyphenylazomethine (PPA), and their fundamental properties and their modification methods are investigated. Cyclic voltammograms of bis(diphenylmethyleneamino)benzene (OPA2′) showed irreversible redox response in the presence of trifluoroacetic acid. A selective synthesis of oligophenanthridine was achieved through the photocyclization of OPA2′ in concentrated sulfuric acid. Stepwise complexation behavior in dendritic poly(phenylazomethine)s (DPAs) was supported by the shell‐selective reduction of the imines. Using the shell‐selective reduction method and the terminal‐modification method of the DPAs, the core and terminals of DPAs were functionalized by ferrocene units, respectively.

Solubility of β-FeF3·3H2O in Mixtures of Nitric and Hydrofluoric Acid

The solubility of β-FeF3·3H2O has been measured in solutions with different concentrations of nitric acid (0 to 7 m) and hydrofluoric acid (1 to 6 m) at the temperatures of (30, 40, and 50) °C. The total iron concentration at equilibrium was measured with inductively coupled plasma (ICP) spectroscopy. The solubility was evaluated in terms of the stepwise complexation of iron by fluoride ions at different ionic strength and temperature. The solubility of β-FeF3·3H2O increases with decreasing concentration of HF and HNO3 and increasing temperature.

Probing Stepwise Complexation in Phenylazomethine Dendrimers by a Metallo-Porphyrin Core

A series of dendritic phenylazomethines (DPA), which have a meso-substituted zinc porphyrin core (DPAGX-ZnP, X = 1-4), were synthesized. Structural studies of these dendrimers were carried out using Tri-SEC (triple detection after size exclusion chromatography), intrinsic viscosity analysis, TEM (tunneling electron microscopy), and molecular modeling calculations by AM1. As a result, a sphere-like structure within a single-nanometer scale (R(h) = 22 A for DPAG4-ZnP) was observed. In addition, encapsulating effects by the DPA shell in the larger dendrimers were confirmed as fundamental properties, based on the UV-vis abosorption spectra, cyclic voltammograms, and 1H NMR spin-lattice relaxation times (T1). The DPAGX-ZnP acts as a multi-metal ion reservoir for SnCl2 and FeCl3. The generation-4 dendrimer (DPAG4-ZnP) can take up to 60 molar amounts of metal complexes around the porphyrin core. A quantitative study of the metal assembling reaction by UV-vis titration revealed stepwise layer-by-layer complexations from the inner imines nearest to the core to the surface. The redox behavior and fluorescence of the zinc porphyrin in these metal-assembled dendrimers also support the stepwise complexation of the metal ion. These analyses suggest that the finely assembled metal complexes in a dendrimer architecture strongly affect the electronic status of the porphyrin core. Results from transient absorption measurements strongly indicate a very fast electron transfer on a subpicosecond time scale between the core and assembled metal complexes.