Bridging Linker Research Articles

Amide Coupling Reaction for the Synthesis of Bispyridine-based Ligands and Their Complexation to Platinum as Dinuclear Anticancer Agents

Amide coupling reactions can be used to synthesize bispyridine-based ligands for use as bridging linkers in multinuclear platinum anticancer drugs. Isonicotinic acid, or its derivatives, are coupled to variable length diaminoalkane chains under an inert atmosphere in anhydrous DMF or DMSO with the use of a weak base, triethylamine, and a coupling agent, 1-propylphosphonic anhydride. The products precipitate from solution upon formation or can be precipitated by the addition of water. If desired, the ligands can be further purified by recrystallization from hot water. Dinuclear platinum complex synthesis using the bispyridine ligands is done in hot water using transplatin. The most informative of the chemical characterization techniques to determine the structure and gross purity of both the bispyridine ligands and the final platinum complexes is 1H NMR with particular analysis of the aromatic region of the spectra (7-9 ppm). The platinum complexes have potential application as anticancer agents and the synthesis method can be modified to produce trinuclear and other multinuclear complexes with different hydrogen bonding functionality in the bridging ligand.

Amide coupling reaction for the synthesis of bispyridine-based ligands and their complexation to platinum as dinuclear anticancer agents.

Amide coupling reactions can be used to synthesize bispyridine-based ligands for use as bridging linkers in multinuclear platinum anticancer drugs. Isonicotinic acid, or its derivatives, are coupled to variable length diaminoalkane chains under an inert atmosphere in anhydrous DMF or DMSO with the use of a weak base, triethylamine, and a coupling agent, 1-propylphosphonic anhydride. The products precipitate from solution upon formation or can be precipitated by the addition of water. If desired, the ligands can be further purified by recrystallization from hot water. Dinuclear platinum complex synthesis using the bispyridine ligands is done in hot water using transplatin. The most informative of the chemical characterization techniques to determine the structure and gross purity of both the bispyridine ligands and the final platinum complexes is (1)H NMR with particular analysis of the aromatic region of the spectra (7-9 ppm). The platinum complexes have potential application as anticancer agents and the synthesis method can be modified to produce trinuclear and other multinuclear complexes with different hydrogen bonding functionality in the bridging ligand.

Coordination site induced controllable assembly of metal–organic units in polyoxometalate-based hybrids

Two hybrid materials based on Keggin-type polyoxometalate, [Ag4(mtrz)4(H2O)2(SiW12O40)] (1), [Ag4(mtez)5(SiW12O40)]·H2O (2), (mtrz=1-methyl-1,2,4-triazole, mtez=1-methyl-1,2,3,4-tetrazole), have been hydrothermally synthesized and structurally characterized. Through the use of the azole ligands mtrz and mtez, compounds 1 and 2 with totally different metal–organic units are obtained. In 1, the mtrz ligands with N atoms in meta-positions induced the formation of chain-like metal–organic units, while in 2, the mtez ligands with adjacent N atoms induced the assembly of tetranuclear units. The SiW12 anions act as bridging linkers to evolve the structural dimensionality of the two title compounds into 2D network and 3D framework, respectively.

Syntheses, structures, and magnetic properties of six coordination polymers based on 4,5-di(4′-carboxylphenyl)phthalic acid and different bis(imidazole) bridging linkers

Six multi-dimensional metal–organic polymers based on 4,5-di(4′-carboxylphenyl)phthalic acid and bis(imidazole) bridging linkers were synthesized and characterized.

Syntheses, structures, and magnetic properties of five coordination polymers constructed from biphenyl-3,4′,5-tricarboxylic acid and (bis)imidazole linkers

Five coordination polymers (CPs) based on biphenyl-3,4′,5-tricarboxylic acid and bis(imdizole) bridging linkers were synthesized under hydrothermal conditions.

Investigations of the Binding of [Pt2(DTBPA)Cl2](II) and [Pt2(TPXA)Cl2](II) to DNA via Various Cross-Linking Modes

We have constructed models for a series of platinum-DNA adducts that represent the binding of two agents, [Pt2(DTBPA)Cl2](II) and [Pt2(TPXA)Cl2](II), to DNA via inter- and intra-strand cross-linking, and carried out molecular dynamics simulations and DNA conformational dynamics calculations. The effects of trans- and cis-configurations of the centers of these di-nuclear platinum agents, and of different bridging linkers, have been investigated on the conformational distortions of platinum-DNA adducts formed via inter- and intra-strand cross-links. The results demonstrate that the DNA conformational distortions for the various platinum-DNA adducts with differing cross-linking modes are greatly influenced by the difference between the platinum-platinum distance for the platinum agent and the platinum-bound N7–N7 distance for the DNA molecule, and by the flexibility of the bridging linkers in the platinum agent. However, the effects of trans/cis-configurations of the platinum-centers on the DNA conformational distortions in the platinum-DNA adducts depend on the inter- and intra-strand cross-linking modes. In addition, we discuss the relevance of DNA base motions, including opening, shift and roll, to the changes in the parameters of the DNA major and minor grooves caused by binding of the platinum agent.

Enhanced Photocatalytic Activity of Au/TiO2 Nanocomposite Prepared Using Bifunctional Bridging Linker

Using a simple bifunctional bridging linker, nanosized gold and titanium dioxide composites are prepared containing different Au loadings. Linker is synthesized to contain both catechol and thiol moieties to enable binding to the TiO2 and Au surface respectively. Au/TiO2 nanocomposites are prepared using simple synthetic route that allows the control over the amount of Au nanoparticles, a property which plays a significant role in the catalytic activity of hybrid materials. Photocatalytic activity of materials prepared using different TiO2 precursors is investigated using reactive oxygen species sensitive assay based on activation of horseradish peroxidase (HRP) enzyme. Significant increase in catalytic activity is observed for all Au/TiO2 nanocomposites with Au/TiO2 prepared by use of the bridging linker being up to 5.5 times more active than bare commercial TiO2 nanoparticles. In addition to 365 nm light excitation, less energetic 470 nm light, which is more suitable for the use with biological systems, is used to induce photocatalytic activity. Finally, prepared photocatalytic materials are successfully used to exert temporal control over enzymatic activity, a feature which is important for the study of both enzymatic activity and design of novel bio‐sensing platforms.

Multimetallic Complexes Featuring a Bridging N-heterocyclic Phosphido/Phosphenium Ligand: Synthesis, Structure, and Theoretical Investigation

By incorporating an N-heterocyclic phosphenium/phosphide (NHP) ligand into a chelating pincer ligand framework (PPP(+)/PPP(-)), we have elucidated several different and unprecedented binding modes of NHP ligands in homobimetallic, heterobimetallic, and trimetallic metal complexes. One-electron reduction of the previously reported (PPP)(-)/M(II) complexes (PPP)M-Cl (M = Pd (1), Pt (2)) results in clean formation of the symmetric homobimetallic M(I)/M(I) complexes [(μ-PPP)Pd]2 (5) and [(μ-PPP)Pt]2 (6). The tridentate NHP ligand has also been utilized as a bridging linker in the M/Co heterobimetallic compounds (OC)3Co(u-PPP)M(CO) (M = Pd (7), Pt (8)), synthesized via salt elimination from mixtures of 1 and 2 and Na[Co(CO)4]. Furthermore, an NHP-bridged trimetallic complex (PPP)2Pd3Cl2 (9) can be synthesized in a manner similar to precursor 1 (Pd(PPh3)4 + (PPP)Cl) via careful adjustment of reaction stoichiometry. Examination of the interatomic distances and angles in complexes 5-9, in tandem with density functional theory calculations have been used to evaluate and characterize the bonding interactions in these complexes.

Small Molecule- and Amino Acid-Induced Aggregation of Gold Nanoparticles

To understand which organic molecules are capable of binding to gold nanoparticles and/or inducing nanoparticle aggregation, we investigate the interaction of gold nanoparticles with small molecules and amino acids at variable pH. Dynamic Light Scattering (DLS) and ultraviolet-visible (UV-vis) spectra were measured on mixtures of colloidal gold with small molecules to track the progression of the aggregation of gold nanoparticles. We introduce the 522 to 435 nm UV-vis absorbance ratio as a sensitive method for the detection of colloidal gold aggregation, whereby we delineate the ability of thiol, amine, and carboxylic acid functional groups to bind to the surfaces of gold nanoparticles and investigate how combinations of these functional groups affect colloidal stability. We present models for mechanisms of aggregation of colloidal gold, including surface charge reduction and bridging linkers. For all molecules whose addition leads to the aggregation of gold nanoparticles, the aggregation kinetics were accelerated at acidic pH values. Colloidal gold is maintained only in the presence of anionic carboxyl groups, which are neutralized by protonation at lower pH. The overall reduced charge on the stabilizing carboxyl groups accounts for the accelerated aggregation at lower pH values.

Lanthanide coordination polymers with hexa-carboxylate ligands derived from cyclotriphosphazene as bridging linkers: synthesis, thermal and luminescent properties

Solvothermal reactions of two ligands with different geometries, derived from cyclotriphosphazene, hexakis(4-carboxylato-phenoxy)cyclotriphosphazene (H6L1) and hexakis(3-carboxylato-phenoxy)cyclotriphosphazene (H6L2) with Eu(NO3)3·6H2O in H2O/DMF under similar synthesis conditions produced three new compounds, namely, {[Eu2(L1)(H2O)4]·(H2O)4·(DMA)2}n (1), {[Eu2(L1)(H2O)8]·(H2O)2·DMA}n (2), and {[Eu2(L2)(H2O)3(DMF)]·(H2O)2·(DMA)}n (3). Compounds 1 and 2 display a 2D layer crystal structure with a distinct topological network incorporating the extended hexa-carboxylate ligand L1; in contrast, 3 has a 1D crystal structure with the highly distorted hexa-carboxylate ligand L2. In these three compounds, the ligands L1 and L2 are fully deprotonated, whose six extended carboxyl arms connect six different/same metallic nodes to generate metal–organic frameworks. The luminescence properties of three compounds have been studied at room temperature in relative detail.

A hybrid nanocomposite precursor route to synthesize dispersion-enhanced Ni catalysts for the selective hydrogenation of o-chloronitrobenzene

Highly-dispersed Ni nanoparticles over carbon nanotubes (CNTs) were successfully prepared from a hybrid nanocomposite of Ni–Al layered double hydroxide (NiAl-LDH) and poly acrylic acid (PAA) functionalized CNTs. The physicochemical properties of materials were investigated by powder X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (TPD), transmission electron microscopy (TEM), low-temperature nitrogen adsorption–desorption experiments and X-ray photoelectron spectroscopy (XPS). The results revealed that L-cysteine as a bridging linker could facilitate the immobilization of NiAl-LDH crystals onto the surface of functionalized CNTs, thus giving rise to the dispersion-enhanced Ni nanoparticles with a small diameter of about 6.0 nm and narrow size distribution in the range of 3–7 nm. Compared with that prepared by the conventional impregnation method, the as-prepared supported Ni catalyst exhibited superior catalytic performance in liquid phase selective hydrogenation of o-chloronitrobenzene to o-chloroaniline along with the highest yield of 98.1% in 150 min, owing to its higher dispersion as well as the existence of both the electron-rich support and electron-deficient metal species.

Crystal Structure Diversities Based on 4,4′-(2,3,6,7-Tetramethoxyanthracene-9,10-diyl)dibenzoic Acid: From 2D Layer to 3D Net Framework

Five metal–organic coordination complexes with the formulas of {Mn(L)(H2O)2}n (1), {[Cd1.5L1.5(DEF)2]·2DEF}n (2), [Co1.5(L)1.5(H2O)]n (3), {[Co(L)(H2O)2]·4H2O}n (4), and {[Ni(L)(H2O)2]·4H2O}n (5), (H2LOMe = 4,4′-(2,3,6,7-tetramethoxyanthracene-9,10-diyl)dibenzoic acid) based on a new rigid dicarboxylate ligand were synthesized and structurally characterized. Structural analysis reveals that H2LOMe acts as multibidentate bridging linker to connect metal ions and possesses similar coordination modes with terephthalic acid. Complex 1 is a (44)-sql layer incorporating bidentate-bridging H2LOMe and infinite Mn(CO2)2 SBUs. Complexes 2 and 3 have similar 2D (36)-hxl layer topology structure based on bidentate-chelating/bridging H2LOMe ligand and trinuclear hourglass SBUs. Complexes 4 and 5 are isostructural and possess 3D open frameworks based on infinite M-(μ2-H2O) chain. From the viewpoint of crystal structure diversity and comparison, our results further demonstrate that the coordination mode of metal ions and ligand are the vital elements in determining the final crystal structure. Moreover, thermal stabilities of 1–5 and temperature-dependent photoluminescence behaviors of 1 and 2 are discussed.

Highly Potent Bactericidal Activity of Porous Metal‐Organic Frameworks

Recent outbreaks of bacterial infection leading to human fatalities have been a motivational force for us to develop antibacterial agents with high potency and long-term stability. A novel cobalt (Co) based metal-organic framework (MOF) was tested and shown to be highly effective at inactivating model microorganisms. Gram-negative bacteria, Escherichia coli (strains DH5alpha and XL1-Blue) were selected to determine the antibacterial activities of the Co MOF. In this MOF, the Co serves as a central element and an octa-topic carboxylate ligand, tetrakis [(3,5-dicarboxyphenyl)-oxamethyl] methane (TDM(8-) ) serves as a bridging linker. X-ray crystallographic studies indicate that Co-TDM crystallizes in tetragonal space group P$\overline 4$2(1) m with a porous 3D framework. The potency of the Co-TDM disinfectant was evaluated using a minimal bactericidal concentration (MBC) benchmark and was determined to be 10-15 ppm within a short incubation time period (<60 min). Compared with previous work using silver nanoparticles and silver-modified TiO(2) nano- composites over the same time period, the MBC and effectiveness of Co-TDM are superior. Electron microscopy images indicate that the Co-TDM displayed distinctive grain boundaries and well-developed reticulates. The Co active sites rapidly catalyzed the lipid peroxidation, causing rupture of the bacterial membrane followed by inactivation, with 100% recycling and high persistence (>4 weeks). This MOF-based approach may lead to a new paradigm for MOF applications in diverse biological fields due to their inherent porous structure, tunable surface functional groups, and adjustable metal coordination environments.

A comparative study of coordination architectures constructed by di[4-(pyridin-3-yl)pyrimidinyl]disulfide and different metal salts: crystal structures and luminescence properties

Assembly of di[4-(pyridin-3-yl)pyrimidinyl]disulfide (3-ppds) with different metal salts resulted in a variety of coordination polymers that were structurally elucidated. For MnCl2, a 1-D repeated rhomboidal chain structure [MnCl2(3-ppds)2]n (1) was obtained, whereas a 1-D helical chain structure [Zn(NO3)2(3-ppds)]n (2) was built from Zn(NO3)2. A 1-D zigzag chain structure [Cu2(OAc)4(3-ppds)]n (3) was produced from Cu(OAc)2. In all three complexes, the 3-ppds ligand plays the same role as a bis(monodentate) bridging linker but with variations in both C–S–S–C torsion angles and dihedral angles defined by its conjugated heteroaromatic rings (pyrimidine and pyridine). The luminescence properties of the complexes have been evaluated in the solid state.

Syntheses of POM-templated MOFs containing the isomeric pyridyltetrazole

By introducing the isomeric pyridyltetrazole into POMs, four POM-templated MOFs compounds, H[Ag6(2-pytz)4(H2O)4][PW12O40]·6H2O (1), [Ag3(3-pytz)2]2[AgPW12O40] (2), H[KAg7(4-pytz)5][SiW12O40] (3), {H[NaAg7(4-pytz)5(H2O)0.5][GeMo12O40]}2 (4) [pytz = 5-(pyridyl) tetrazolate], were hydrothermally synthesized and characterized by routine methods and single-crystal X-ray crystallography. Due to 2-pytz as a bridging linker and chelator, compound 1 displays a 2D MOF with a void (ca. 19.1 × 8.8 A), in which the Keggin anions as sandwich templates locate in the adjacent two voids. In 2, 3-pytz as a shorter radiating linker generates a compacted 2D MOF, in which the Keggin anions as chain templates direct the formation of a stair-like MOF. In the isostructural compounds 3 and 4, 4-pytz as the bridging and radiating linkers form 3D covalent MOFs with larger voids (ca. 25.3 × 12.5 A), in which the Keggin anions as double-templates occupy the voids in a “shoulder-to-shoulder” mode. In addition, the photoluminescence of the compounds were investigated.

Synthesis, Characterization, and Catalytic Performance of Highly Dispersed Supported Nickel Catalysts from Ni–Al Layered Double Hydroxides

Highly dispersed nickel nanoparticles were successfully prepared by reduction of Ni–Al layered double hydroxide precursor (NiAl-LDH) supported on nitric acid modified carbon nanotubes (HCNTs). The physicochemical properties of materials were investigated by powder X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed decomposition (TPDE), temperature programmed desorption (TPD), transmission electron microscopy (TEM), low-temperature nitrogen adsorption–desorption experiments, and X-ray photoelectron spectroscopy (XPS). The results revealed that in the synthesis process of NiAl-LDH and HCNTs composite, l-cysteine could improve the attachment of NiAl-LDH on the HCNTs as bridging linker and inhibit the growth of NiAl-LDH crystallites, thus leading to the formation of highly dispersed and uniform Ni nanoparticles with the small particle sizes. Compared with HCNTs-supported Ni catalyst prepared without the addition of l-cysteine, this highly dispersed Ni catalyst exhibited en...

Understanding the role of the flexible bridging linker through kinetics and mechanistic study of Pt(ii) amphiphiles derived from a bis(2-pyridylmethyl)amine chelate head group

The substitution of aqua ligands of mononuclear Pt(II) complexes of the general form [Pt(H(2)O)(N,N-bis(2-pyridylmethyl)-N(CH(2))(n)-CH(3); -NC(CH(3))(3); -NH](CF(3)SO(3))(2), n = 1 (bpea); 2 (bppa); 3 (bpba); 5 (bpha), 9 (bpda) -NC(CH(3))(3) (bpbta) and -NH (bpma) by thiourea nucleophiles was investigated under pseudo first-order conditions as a function of concentration and temperature using the stopped-flow technique and UV-vis spectroscopy. The substitution reactions occur via two separate reaction steps, each fitting to a single exponential curve. In the two reaction steps, the thiourea nucleophiles first substitute the coordinated aqua ligand followed by ring opening via dechelation of one of the pyridyl units. The mode of activation for both steps remains associative in nature and the observed rate constants can be fitted to the equation k(obs(1st/2nd)) = k(2(1st/2nd))[Nu]. Appending a primary alkyl hydrocarbon group on the trans-N donor atom of the chelate head group marginally increases the rate of substitution of the aqua leaving group due to the weaker trans-influence of its alkyl amine donor group. However, when a tert-butyl group is the pendant group, reactivity increases by a factor of about two, reiterating the inductive nature of the flow of electron density from the tailing groups towards the Pt(II) metal centres. A comparison of the reactivities of the studied complexes with their dinuclear analogues bridged by alkyl diamines has demonstrated that the electronic effect of the alkyl diamine bridge on the overall reactivity of the multinuclear Pt(II) complexes is weak and insignificant when compared to steric effects due to the constraining bridge.

Additive effect of 1,8-diaminoctane as a bi-functional linker on the two-dimensional array of large gold nanoparticles

1,8-Diaminooctane ligand was investigated as a molecular bridging linker to induce a uniform array of gold nanoparticles (GNPs) (larger than 60 nm) via the lateral immobilization of GNPs on amine-functionalized glass substrate. The effects of array time and ligand amounts were optimized to obtain a uniform and two-dimensional (2D) array of large GNPs. With the increase of ligand dosage, the deposition of GNPs on glass substrate progressively increased, finally leading to a densely packed 2D array of GNPs. Excessive dosage of ligand, however, led to a multilayered array of GNPs on glass substrate.

Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration

Hydrothermal stability is a pertinent issue to address for many industrial applications where percent levels of water can be present at temperatures ranging from subambient to several hundred degrees. Our objective is to understand relative stabilities of MOF materials through experimental testing combined with molecular modeling. This will enable the ultimate design of materials with improved hydrothermal stability, while maintaining the properties of interest. The tools that we have employed for these studies include quantum mechanical calculations based upon cluster models and combinatorial steaming methods whereby a steam stability map was formulated according to the relative stability of different materials. The experimental steaming method allows for high throughput screening of materials stability over a broad range of steam levels as well as in-depth investigation of structural transformations under more highly resolved conditions, while the cluster model presented here yields the correct trends in hydrothermal stability. Good agreement was observed between predicted relative stabilities of materials by molecular modeling and experimental results. Fundamental information from these studies has provided insight into how metal composition and coordination, chemical functionality of organic linker, framework dimensionality, and interpenetration affect the relative stabilities of PCP materials. This work suggests that the strength of the bond between the metal oxide cluster and the bridging linker is important in determining the hydrothermal stability of the PCP. Although the flexibility of the framework plays a role, it is not as important as the metal-linker bond strength. This demonstration of alignment between experimental and calculated observations has proven the validity of the method, and the insight derived herein insight facilitates direction in designing ideal MOF materials with improved hydrothermal stability for desired applications.

Hybrid ZnAl‐LDH/CNTs nanocomposites: Noncovalent assembly and enhanced photodegradation performance

AbstractIn this article, we reported a facile and effective strategy for assembling hybrid ZnAl‐layered double hydroxide/carbon nanotubes (ZnAl‐LDH/CNTs) nanocomposites through noncovalent bonds, for the first time, in the presence of L‐cysteine molecules. The materials have been characterized by powder X‐ray diffraction (XRD), energy dispersive X‐ray spectroscopy (EDS), transmission electron microscopy (TEM), thermogravimetry and differential scanning calorimetry (TG‐DSC), X‐ray photoelectron spectra (XPS) and specific surface area measurement. The results indicate that L‐cysteine as bridging linker plays a key role for enhancing both adhesion and dispersion of LDH nanocrystallites onto the surface of CNTs matrix through the interfacial interaction, and effectively inhibits the in situ growth of LDH crystallites, thus resulting in remarkably reduced LDH crystallite sizes; the Eu(III) fluorescence quenching in intercalated‐Eu(III)complex LDH/CNTs nanocomposite can occur because of the interaction between LDH crystallites and CNTs matrix. Furthermore, it is found that as‐assembled hybrid LDH/CNTs nanocomposites exhibit excellent performance for photodegradation of methyl orange molecules under UV irradiation, which is closely related to the unique hybrid nanostructure and composition of composites. © 2009 American Institute of Chemical Engineers AIChE J, 2010