Anion recognition using dimetallic coordination complexes

This article describes advances made over the past 3 years in anion recognition using coordination complexes, with a specific focus on dimetallic architectures that utilize a bridging mechanism. The formation of coordination complexes is a relatively straightforward method of constructing fluorescent and colorimetric chemosensors and imaging agents, and a particularly effective way to develop indicator displacement assays that operate in water. These assays are likely to find increased application in various aspects of analytical and environmental chemistry, as well as biomedical imaging and drug discovery. Significant progress in phosphoesterase mimics has been made, and concomitant with the increased mechanistic insight, is the discovery of a catalyst that cleaves phosphodiesters with poor O-alkyl leaving groups. Also discussed is a macrocyclic coordination complex whose shape and supramolecular function is pH-dependent.     

Bimodal, dimetallic lanthanide complexes that bind to DNA: the nature of binding and its influence on water relaxivity

UV-visible titrations, (1)H NMRD experiments and molecular docking studies show that emissive anthraquinone appended dimetallic lanthanide complexes bind to DNA. The strength of binding and the observed relaxivity behaviour depend on the nature of the substituted anthraquinone core.     

Development of a ratiometric time-resolved luminescence sensor for pH based on lanthanide complexes

Time-resolved luminescence bioassay technique using lanthanide complexes as luminescent probes/sensors has shown great utilities in clinical diagnostics and biotechnology discoveries. In this work, a novel terpyridine polyacid derivative that can form highly stable complexes with lanthanide ions in aqueous media, (4′-hydroxy-2,2′:6′,2′′-terpyridine-6,6′′-diyl) bis(methylenenitrilo) tetrakis(acetic acid) (HTTA), was designed and synthesized for developing time-resolved luminescence pH sensors based on its Eu³⁺ and Tb³⁺ complexes. The luminescence characterization results reveal that the luminescence intensity of HTTA–Eu³⁺ is strongly dependent on the pH values in weakly acidic to neutral media (pK_a = 5.8, pH 4.8–7.5), while that of HTTA–Tb³⁺ is pH-independent. This unique luminescence response allows the mixture of HTTA–Eu³⁺ and HTTA–Tb³⁺ (the HTTA–Eu³⁺/Tb³⁺ mixture) to be used as a ratiometric luminescence sensor for the time-resolved luminescence detection of pH with the intensity ratio of its Tb³⁺ emission at 540 nm to its Eu³⁺ emission at 610 nm, I_540 nm/I_610 nm, as a signal. Moreover, the UV absorption spectrum changes of the HTTA–Eu³⁺/Tb³⁺ mixture at different pHs (pH 4.0–7.0) also display a ratiometric response to the pH changes with the ratio of absorbance at 290 nm to that at 325 nm, A_290 nm/A_325 nm, as a signal. This feature enables the HTTA–Eu³⁺/Tb³⁺ mixture to have an additional function for the pH detection with the absorption spectrometry technique. For loading the complexes into the living cells, the acetoxymethyl ester of HTTA was synthesized and used for loading HTTA–Eu³⁺ and HTTA–Tb³⁺ into the cultured HeLa cells. The luminescence imaging results demonstrated the practical utility of the new sensor for the time-resolved luminescence cell imaging application.     

Thermolysis of lanthanide dithiocarbamate complexes

Polycrystalline lanthanide sulfide materials were formed at low temperatures using a single-source precursor based on the lanthanide dithiocarbamate complex. The synthesis temperatures are generally lower than standard solid state preparations, avoid toxic sulfurizing gases and provide a convenient route to prepare lanthanide chalcogenide nanoparticles. Depending on the reaction conditions and oxophilicity of the lanthanide, the sulfide material was formed with oxidized products including oxysulfides, oxysulfates and the oxide.     

Polymerization of lanthanide acrylonitrile complexes

The molecular complexes of some lanthanides scandium (Sc3+), yttrium (Y3+), lanthanum (La3+), gadolinium (Gd3+), cerium (Ce3+) and ytterbium (Yb3) have been studies in dimethyl formamide (DMF) spectrophtometrically equilibrium constants (K), molar extintion coefficient (epsilon), energy of transition (E) and free energy (delta G*) were calculated. The polymerization of acrylonitrile has been studied and investigated in the presence of Sc3+, Y3+, La3+, Gd3+, Ce3+, and Yb3+ ions. The IR spectra of the formed AN-M (III) Br3 polymer complexes show the absence of the C identical to N band and the presence of two new bands corresponding to NH2 and OH groups. Magnetic moment values and the thermal stabilities of homopolymer and the polymer complexes were studied by means of thermogravimetric analysis and the activation energies for degradation were calculated.     

Thermodynamics, structure and properties of polynuclear lanthanide complexes with a tripodal ligand: insight into their self-assembly

Self-assembly processes between a tripodal ligand and Ln(III) cations have been investigated by means of supramolecular analytical methods. At an equimolar ratio of components, tetranuclear tetrahedral complexes are readily formed in acetonitrile. The structural analysis of the crystallographic data shows a helical wrapping of binding strands around metallic cations. The properties of this series of highly charged 3D compounds were examined by using NMR spectroscopy and optical methods in solution and in the solid state. In the presence of excess metal, a new trinuclear complex was identified. The X-ray crystal structure elucidated the coordination of metallic cations with two ligands of different conformations. By varying the metal/ligand ratio, a global speciation of this supramolecular system has been evidenced with different spectroscopic methods. In addition, these rather complicated equilibria were successfully characterised with the thermodynamic stability constants. A rational analysis of the self-assembly processes was attempted by using the thermodynamic free energy model and the impact of the ligand structure on the effective concentration is discussed.     

Two isostructural triboluminescent lanthanide complexes

(4‐Di­methyl­amino­pyridine)­tris(2,2,6,6‐tetra­methyl­heptane‐3,5‐dionato)­terbium(III), [Tb(C₁₁H₁₉O₂)₃(C₇H₁₀N₂)], and its samarium analogue, [Sm(C₁₁H₁₉O₂)₃(C₇H₁₀N₂)], are isostructural. Their polar space group is consistent with observed second harmonic generation and with the involvement of piezoelectric charging in their intense triboluminescence properties, which are of interest for the development of damage sensors in composite materials. The metals display irregular seven‐coordination by one substituted pyridine and three chelating diketonate ligands, bond lengths to Tb being shorter than those to Sm.     

Polymetallic lanthanide complexes with PAMAM-naphthalimide dendritic ligands: luminescent lanthanide complexes formed in solution

Generation 3 PAMAM dendrimers functionalized with 2,3-naphthalimide chromophoric groups on the end branches were synthesized, and the formation of Eu3+ polymetallic complexes was investigated. The luminescence properties of these complexes upon binding were fully characterized. On addition of Eu3+ to the dendrimer solution, lanthanide luminescence appears. The formation of a luminescent species corresponding to a dendrimer:lanthanide ratio of 1:8 was determined by luminescence batch titration and indicated by the maximum of Eu3+ emission. This indicates an overall average coordination number of 7.5 around each lanthanide metal cation. This is the first report of such characterization in the literature. Luminescence lifetimes indicate that the metal cation is well protected from nonradiative deactivation by the dendritic structure. Despite the limited efficiency of the sensitization of Eu3+, the absolute quantum yield being 0.0006, the good protection of the eight lanthanide cations bound in the dendrimer structure and the high absorptivity leads to the red emission from Eu3+ that is easily observed in solution under irradiation with 354 nm UV light.     

Luminescent lanthanide complexes as analytical tools in anion sensing, pH indication and protein recognition

Although lanthanide complexes are recently used in luminescence labeling of bio-targets, this review focuses on sensing profiles of synthetic and biological lanthanide complexes. Rational design and combinatorial screening approaches toward synthetic lanthanide complexes applicable as luminescent sensing materials are described. Iron-carrying transferrin and ferritin proteins further form lanthanide complexes working as pH indicators and protein recognition reagents.     

Fine-tuning the pH trigger of self-assembly

The creation of smart, self-assembling materials that undergo morphological transitions in response to specific physiological environments can allow for the enhanced accumulation of imaging or drug delivery agents based on differences in diffusion kinetics. Here, we have developed a series of self-assembling peptide amphiphile molecules that transform either isolated from molecules or spherical micelles into nanofibers when the pH is slightly reduced from 7.4 to 6.6, in isotonic salt solutions that simulate the acidic extracellular microenvironment of malignant tumor tissue. This transition is rapid and reversible, indicating the system is in thermodynamic equilibrium. The self-assembly phase diagrams show a single-molecule-to-nanofiber transition with a highly concentration-dependent transition pH. However, addition of a sterically bulky Gd(DO3A) imaging tag on the exterior periphery shifts this self-assembly to more acidic pH values and also induces a spherical micellar morphology at high pH and concentration ranges. By balancing the attractive hydrophobic and hydrogen-bonding forces, and the repulsive electrostatic and steric forces, the self-assembly morphology and the pH of transition can be systematically shifted by tenths a pH unit.     

Thermodynamic parameters governing the self-assembly of head-head-head lanthanide bimetallic helicates

The heterobitopic ligands L ABX (X=1, 2, 3, 4 or 5), differing only by a Cl or NEt(2) substituent, have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln2(L ABX)3]6+. The percentage of HHH (head-head-head) isomer, in which each of the three ligand strands coordinates to the same lanthanide ion with the same coordination unit, is deciding the ability of the ligands to selectively form heterobimetallic complexes containing one luminescent and one magnetic or two different luminescent ions. It deviates significantly from the statistical value of 25 % and ranges from 6-20 % for L AB2 complexes to 93-96 % for L AB4 complexes. The equilibrium between HHT (head-head-tail) and HHH isomers has been investigated in detail for homobimetallic helicates (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Lu) by means of variable temperature NMR and thermodynamic parameters have been determined. The equilibrium is characterized by small values of DeltaH and DeltaS, which vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DeltaG almost constant. The results are interpreted in terms of differences in interstrand stacking, ion-dipole interactions and metal-metal repulsion.     

Solution structure of chiral lanthanide complexes

The determination of solution structure of small to medium size chiral lanthanide complexes through paramagnetic NMR and circular dichroism is briefly reviewed. The main focus is on ytterbium as the rare earth, because of its negligible contact contribution to the hyperfine shift and of its intense CD spectrum in the near IR. The structures discussed contain various stereogenic elements: classical chiral centres, atropisomeric axes, slowly interconverting conformations, which gives rise to a manifold of situations to be identified, classified, and characterised through spectroscopic tools. The fallout of these structural properties are in enantioselective catalysis, in molecular recognition, or even in biomedicine, on account of the role of Gd³⁺ complexes as MRI contrast agents. Moreover, the information encoded in the NMR and CD spectra of Ln³⁺ complexes may be used to extract original data on the solution stereochemistry of organic molecules used as ligands. The first part summarises some basic theoretical aspects, with special emphasis onto those which have practical consequences in the experimental design. A discussion of selected applications can be found in the second part.     

Discriminating between lanthanide ions: self-assembly of heterodimetallic triple-stranded helicates

A bis-terdentate segmental ligand has been designed which self-assembles with lanthanide ions of different size to yield heterodimetallic triple-stranded helicates.     

Some lanthanide metal complexes of n-isonicotinamidosalicylaldimine

Trivalent lanthanide complexes of the type K[ML ₂] whereM=La(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III) and Dy(III) and H₂ L=N-isonicotinamidosalicyladimine, have been prepared and characterised. The nephelauxetic ratio (β), covalency (δ) and bonding parameter (b ²) of K[NdL ₂] have been calculated. Infrared spectral studies reveal that N-isonicotinamidosalicylaldimine acts as a dibasic tridentate ligand. A coordination number six has been proposed for the lanthanide metal ions.     

19F NMR based pH probes: lanthanide(III) complexes with pH-sensitive chemical shifts

Experimental measurements and theoretical analysis of magnetic properties, structural dynamics and acid-base equilibria for several lanthanide(III) complexes with tetraazacyclododecane derivatives as 19F NMR chemical shift pH probes are presented; pKa values vary between 6.9 and 7.7, with 18 to 40 ppm chemical shift differences between the acidic and basic forms for Ho(III) complexes possessing T1 values of 10 to 30 ms (4.7-9.4 T, 295 K).     

Oxidative addition of aryl halides: routes to mono- and dimetallic nickel amino-bis-phosphinimine complexes

Oxidative addition of an aryl-halide to Ni(COD)(2) in the presence of an equivalent of amino-bis-phosphinimine ligand affords complexes of the form [HN(CH(2)CH(2)N=PPh(3))(2)Ni-Ar][X] (Ar = C(6)H(4)F, C(6)H(5), X = Cl, Br) while the analogous reactions with 2 equivalents of Ni yield the amido-bridged complexes N(CH(2)CH(2)N=PPh(3))Ni(2)Br(3) and N(1,2-C(6)H(4)N=PPh(3))Ni(2)Br(3).     

Coordinated lanthanide metal complexes ofN-benzoylglycine hydrazide

Summary N-benzoylglycine hydrazide (BzGH) reacts with trivalent lanthanide metal ions forming complexes of the type [Ln(BzGH)₂Cl(H₂O)₂]Cl₂·nH₂O, where Ln=La^III, Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, Tb^III, Dy^III, or Y^III;n=1 or 2. The structures of the complexes have been studied by conductance, magnetic, electronic, i.r.,¹H n.m.r. and¹³C n.m.r. spectral techniques. The nephelauxetic ratio, the bonding parameter, Sinha's parameter and the covalency angular overlap parameter have been calculated from the electronic spectra of Pr^III, Nd^III and Sm^III complexes. Seven-coordination is proposed in the Nd^III complex. The i.r. and¹H n.m.r. spectral data suggest bidentate BzGH in all the complexes.     

pH driven self-assembly of a ternary lanthanide luminescence complex: the sensing of anions using a beta-diketonate-Eu(III) displacement assay

The synthesis and the photophysical evaluation of a novel pH dependent lanthanide luminescent self-assembly in water between a cyclen based europium complex and a beta-diketonate is described and its use as a luminescent sensor in displacement assays for anions such as acetate, bicarbonate and lactate, where the Eu(III) emission was quenched upon anion recognition.