Metal‐Ion Sensing Europium(III) Complexes with Bidentate Phosphine Oxide Ligands Containing a 2,2′‐Bipyridine Framework

Novel Eu^III complexes with bidentate phosphine oxide ligands containing a bipyridine framework, i.e., [3,3′‐bis(diphenylphosphoryl)‐2,2′‐bipyridine]tris(hexafluoroacetylacetonato)europium(III) ([Eu(hfa)₃(BIPYPO)]) and [3,3′‐bis(diphenylphosphoryl)‐6,6′‐dimethyl‐2,2′‐bipyridine]tris(hexafluoroacetylacetonato)europium(III) ([Eu(hfa)₃(Me‐BIPYPO)]), were synthesized for lanthanide‐based sensor materials having high emission quantum yields and effective chemosensing properties. The emission quantum yields of [Eu(hfa)₃(BIPYPO)] and [Eu(hfa)₃(Me‐BIPYPO)] were 71 and 73%, respectively. Metal‐ion sensing properties of the Eu^III complexes were also studied by measuring the emission spectra of Eu^III complexes in the presence of Zn^II or Cu^II ions. The metal‐ion sensing and the photophysical properties of luminescent Eu^III complexes with a bidentate phosphine oxide containing 2,2′‐bipyridine framework are demonstrated for the first time.     

X-ray structure and temperature dependent luminescent properties of two bimetallic europium complexes

The structural, luminescent and temperature dependent luminescent properties of two homodinuclear europium complexes bridged by 2,2′-bipyrimidine (bpm) are reported. β-Diketonate ligands 4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione (tfa) and 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (tta) are used as capping ligands resulting in complexes of the form [Eu(tfa)₃]₂bpm (1) and [Eu(tta)₃]₂bpm (2). All Eu^III ions are eight coordinate with six O atoms from the β-diketones and two N atoms from the polyazine bridging ligand. Excitation of the β-diketonate ligands tfa or tta at ca. 340 nm in toluene solutions results in the characteristic Eu^III emission in the visible region of the spectrum. The emission intensity and lifetime associated with the Eu^III centers decrease as the temperature of the solution is increased. Lifetime measurements are fit to a monoexponential while the temperature dependent lifetime data is fit to an Arrhenius-type equation. Evaluation of the data in comparison to data obtained from the monometallic Eu^III analogs reveal very similar photoluminescent properties. This suggests little electronic communication between Eu^III ions via the polyazine bpm bridging ligand.     

Crystal structures and DNA-binding properties of PrIII complexes derived from 8-hydroxyquinoline-2-carboxaldehyde and three aroylhydrazines

Pr^III and three synthesized ligands, 8-hydroxyquinoline-2-carboxaldehyde-(benzoyl)hydrazone, 8-hydroxyquinoline-2-carboxaldehyde-(2′-hydroxybenzoyl)hydrazone, and 8-hydroxyquinoline-2-carboxaldehyde-(isonicotinyl)hydrazone, respectively, can form binuclear Pr^III complexes with 1?:?1 metal-to-ligand stoichiometry and nine-coordination at Pr^III indicated by X-ray crystal structural analyses. Ligands are dibasic tetradentate, binding to Pr^III through the phenolate oxygen, nitrogen of quinolinato unit, the C=N of methylene, and ^–O–C=N– enolized and deprotonated from O=C–NH– of the aroylhydrazone side chain. One DMF binds orthogonally to the ligand plane from one side to the metal ion, while another DMF and a bidentate nitrate simultaneously bind from the other. Dimerization of the monomeric unit occurs through the phenolate oxygen leading to a central planar four-membered (PrO)₂ ring. The crystal structures are similar to each other and to other nine-coordinate lanthanide complexes with geometry of distorted edge-sharing mono-capped square-antiprism of [LnL(NO₃)(DMF)₂]₂ (Ln = La^III, Nd^III, Sm^III, Eu^III, Tb^III, Dy^III, Ho^III, and Er^III) except for Yb^III with eight-coordinate Yb^III center with distorted edge-sharing dodecahedron of [YbL(NO₃)(DMF)]₂, derived from 8-hydroxyquinoline-2-carboxaldehyde and aroylhydrazines. The ligands and Pr^III complexes can bind to calf thymus DNA through intercalation with binding constants at 10⁵ M^?1 and probably be used as potential antitumor drugs.     

Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand

This work demonstrates sign reversal of large circularly polarized luminescence (CPL) signal based on the hinge‐like twisting motion of a bidentate ligand, 3,3‐bis(diphenylphosphoryl)‐2,2‐bipyridine (BIPYPO), in a cis–trans isomerization of chiral europium(III) complexes. X‐ray diffraction analysis revealed that twisting motion of BIPYPO provides s‐cis and s‐trans geometries of a chiral Eu^III complex containing either tris[3‐(trifluoromethylhydroxymethylene)‐(+)‐camphorate] (D ‐ 1 ) or tris[3‐(heptafluoropropylhydroxymethylene)‐(+)‐camphorate] (D ‐ 2 ). The s‐cis Eu^III complexes show eight‐coordinate geometry around the Eu^III ion, in which the chelate between the phosphoryl oxygen and the Eu^III ion forces the s‐cis geometry of BIPYPO. In contrast, the phosphorus–nitrogen interaction provides a conformational lock for the s‐trans geometry of the BIPYPO ligand, inducing a quasi‐seven‐coordinate Eu^III complex. The difference in coordination geometry causes the sign change of the CPL signals between the s‐cis and s‐trans isomers, whereby the s‐cis and s‐trans isomers of Eu^III complexes exhibit the positive and negative CPL signals, respectively, for the ⁵D₀→⁷F₁ transition. The proportion of the s‐trans‐D ‐ 1 against s‐cis‐D ‐ 1 increases upon changing the solvent from [D₃]acetonitrile to [D₆]acetone, inducing a sign change of the CPL signals. The complexes D ‐ 1 and D ‐ 2 show a biexponential decay with two different lifetimes, suggesting two emitting species, that is, the s‐cis and s‐trans isomers of Eu^III complexes. In both cases, the proportions of the longer lifetime components (τ₁) decrease and instead the shorter lifetime components (τ₂) increase upon changing the solvent from [D₃]acetonitrile to [D₆]acetone.     

Luminescence of Eu3+ and Tb3+ Complexes of Two Macrobicyclic Ligands Derived from a Tetralactam Ring and a Chromophoric Antenna

Two macrobicyclic ligands derived from an 18‐membered tetralactam ring and 2,2′‐bipyridine or 2,6‐bis(pyrazol‐1‐yl)pyridine moieties, 1 and 2 , respectively, form stable complexes with Gd^III, Eu^III, and Tb^III ions in aqueous solution. The ligand‐based luminescence is retained in the Gd^III cryptates, whereas this radiative deactivation is quenched in the Eu^III and Tb^III cryptates by ligand‐to‐metal energy transfer, resulting in the usual metal‐centered emission spectra. Singlet‐ and triplet‐state energies, emission‐decay lifetimes, and luminescence yields were measured. [Tb⊂ 1 ]³⁺ cryptate shows a long luminescence lifetime (τ=1.12 ms) and a very high metal luminescence quantum yield (Φ=0.25) in comparison with those reported in the literature for Tb³⁺ complexes sensitized by a bipyridine chromophore. By comparison to [Ln⊂ 1 ]³⁺, [Ln⊂ 2 ]³⁺ presents markedly lower luminescence properties, due to worse interaction between the 2,6‐bis(pyrazol‐1‐yl)pyridine unit and the metal ion. Moreover, the luminescent metal and the triplet ligand energy levels of [Eu⊂ 2 ]³⁺ do not match. The effects of H₂O molecules coordinated to the metal centre and of thermally activated decay processes on nonradiative deactivation to the ground‐state are also reported.     

Comparative Investigation into the Complexation and Extraction Properties of Tridentate and Tetradentate Phosphine Oxide-Functionalized 1,10-Phenanthroline Ligands toward Lanthanides and Actinides

Two new phosphine oxide-functionalized 1,10-phenanthroline ligands, tetradentate 2,9-bis(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-BPPhen, L¹ ) and tridentate 2-(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-MPPhen, L² ), were synthesized and studied comparatively for their coordination with trivalent actinides and lanthanides. The complexation mechanisms of these two ligands toward trivalent f-block elements were thoroughly elucidated by NMR spectroscopy, UV/vis spectrophotometry, fluorescence spectrometry, single-crystal X-ray diffraction, solvent extraction, and theoretical calculation methods. NMR titration results demonstrated that 1 : 1 and 1 : 2 (metal to ligand) lanthanides complexes formed for L¹ , whereas 1 : 1, 1 : 2 and 1 : 3 lanthanide complexes formed for L² in methanol. The formation of these species was validated by fluorescence spectrometry, and the corresponding stability constants for the complexes of Nd^III with L¹ and L² were determined by using UV/vis spectrophotometry. Structures of the 10-coordinated 1 : 1-type complexes of Eu L¹ (NO₃)₃ and [Eu L² (NO₃)₃(H₂O)] Et₂O in the solid state were characterized by X-ray crystallography. In solvent-extraction experiments, L¹ exhibited extremely strong extraction ability for both Am^III and Eu^III, whereas L² showed nearly no extraction toward Am^III or Eu^III due to its high hydrophilicity. Finally, the structures and bonding natures of the complex species formed between Am^III/Eu^III and L¹/L² were analyzed in DFT calculations.     

Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes

A series of tridentate benzimidazole‐substituted pyridine‐2‐carboxylic acids have been prepared with a halogen, methyl or alkoxy group in the 6‐position of the benzimidazole ring, which additionally contains a solubilising N‐alkyl chain. The ligands form neutral homoleptic nine‐coordinate lanthanum, europium and terbium complexes as established from X‐ray crystallographic analysis of eight structures. The coordination polyhedron around the lanthanide ion is close to a tricapped trigonal prism with ligands arranged in an up–up–down fashion. The coordinated ligands serve as light‐harvesting chromophores in the complexes with absorption maxima in the range 321–341 nm (ε=(4.9–6.0)×10⁴ M ^?1 cm^?1) and triplet‐state energies between 21 300 and 18 800 cm^?1; the largest redshifts occur for bromine and electron‐donor alkoxy substituents. The ligands efficiently sensitise europium luminescence with overall quantum yields ( ) and observed lifetimes (τ_obs) reaching 71 % and 3.00 ms, respectively, in the solid state and 52 % and 2.81 ms, respectively, in CH₂Cl₂ at room temperature. The radiative lifetimes of the Eu(⁵D₀) level amount to τ_rad=3.6–4.6 ms and the sensitisation efficiency η_sens= (τ_rad/τ_obs) is close to unity for most of the complexes in the solid state and equal to approximately 80 % in solution. The photophysical parameters of the complexes correlate with the triplet energy of the ligands, which in turn is determined by the nature of the benzimidazole substituent. Facile modification of the ligands makes them promising for the development of brightly emissive europium‐containing materials.     

Excitation Energy-Transfer Processes in the Sensitization Luminescence of Europium in a Highly Luminescent Complex

The excitation energy transfer (EET) pathways in the sensitization luminescence of Eu^III and the excitation energy migration between the different ligands in [Eu(fod)₃dpbt] [where fod=6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione and dpbt=2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine], exhibiting well-separated fluorescence excitation and phosphorescence bands of the different ligands, were investigated by using time-resolved luminescence spectroscopy for the first time. The data clearly revealed that upon the excitation of dpbt, the sensitization luminescence of Eu^III in [Eu(fod)₃dpbt] was dominated by the singlet EET pathway, whereas the triplet EET pathway involving T₁(dpbt) was inefficient. The energy migration from T₁(dpbt) to T₁(fod) in [Eu(fod)₃dpbt] was not observed. Moreover, upon the excitation of fod, a singlet EET pathway for the sensitization of Eu^III luminescence, including the energy migration from S₁(fod) to S₁(dpbt) was revealed, in addition to the triplet EET pathway involving T₁(fod). Under the excitation of dpbt at 410 nm, [Eu(fod)₃dpbt] exhibited an absolute quantum yield for Eu^III luminescence of 0.59 at 298 K. This work provides a solid and elegant example for the concept that singlet EET pathway could dominate the sensitization luminescence of Eu^III in some complexes.     

New advances in the synthesis of tripyridinophane macrocycles suitable to enhance the luminescence of Ln(III) ions in aqueous solution

A series of four new 18-membered hexaaza macrocyclic ligands bearing three endocyclic pyridine units and acetate or methylenephosphonate pendant arms has been prepared. The new synthetic procedure is based on the use of amine and diamine precursors incorporating masked carboxylate or phosphonate functions and on an efficient sodium template effect which controls the crucial macrocyclization step (yields of macrocyclization reactions: 62–88%). This procedure appears as a suitable alternative compared to the classical Richman-Atkins methodology generally used for the preparation of this class of macrocycles. As demonstrated with the Eu^III and Tb^III complexes derived from two ligands, these tripyridinophane chelators form luminescent and stable mononuclear Ln^III complexes in aqueous solution at physiological pH. In such a medium, Tb^III complexes exhibit a brightness of 1700 (λ_exc?=?279?nm) and 3000 (λ_exc?=?268?nm)?M^?1?cm^?1.     

Syntheses, spectroscopic analysis, and structural determination of two novel nine-coordinate mononuclear Na4[EuIII(Dtpa)(H2O)]2 · 11.5H2O and binuclear (NH4)4[EuIII(Dtpa)]2 · 10H2O complexes

Two novel rare-earth metal complexes, namely, mononuclear Na₄[Eu^III(Dtpa)(H₂O)]₂ · 11.5H₂O (I) and binuciear (NH₄)₄[Eu^III(Dtpa)]₂ · 10H₂O (II) (H₅Dtpa = diethylenetriamine-N,N,N??,N??,N??-pentaacetic acid), have successfully been synthesized and characterized by infrared spectrum, UV-Vis spectrum, fluorescence spectrum, thermal analysis, and single-crystal X-ray diffraction techniques. Since these two Eu(III) complexes have different counterions, causing different coordination environment, fluorescence spectrum analysis displays different fluorescence properties. X-ray diffraction reveals that the coordination polyhedra of both complexes adopt pseudo-D _3h tricapped trigonal prismatic conformation. However, I is a nine-coordinate mononuclear complex and crystallizes in the monoclinic crystal space group P2₁/n and II is a nine-coordinate binuciear complex and crystallizes in the triclinic crystal space group $P\bar 1$ . In addition, II has two independent binuciear structural units, [Eu(1)₂(Dtpa)₂] and [Eu(2)₂(Dtpa)₂]. Along the yz plane both [Eu(1)₂(Dtpa)₂] and [Eu(2)₂(Dtpa)₂] form a 1D chain structure, respectively. Further, along the y axis linking of each other forms a 2D planar structure.     

Lanthanide(III) Complexes of Bis‐semicarbazone and Bis‐imine‐Substituted Phenanthroline Ligands: Solid‐State Structures,Photophysical Properties,and Anion Sensing

Phenanthroline‐based hexadentate ligands L¹ and L² bearing two achiral semicarbazone or two chiral imine moieties as well as the respective mononuclear complexes incorporating various lanthanide ions, such as La^III, Eu^III, Tb^III, Lu^III, and Y^III metal ions, were synthesized, and the crystal structures of [ML¹Cl₃] (M=La^III, Eu^III, Tb^III, Lu^III, or Y^III) complexes were determined. Solvent or water molecules act as coligands for the rare‐earth metals in addition to halide anions. The big Ln^III ion exhibits a coordination number (CN) of 10, whereas the corresponding Eu^III, Tb^III, Lu^III, and Y^III centers with smaller ionic radii show CN=9. Complexes of L², namely [ML²Cl₃] (M=Eu^III, Tb^III, Lu^III, or Y^III) ions could also be prepared. Only the complex of Eu^III showed red luminescence, whereas all the others were nonluminescent. The emission properties of the Eu derivative can be applied as a photophysical signal for sensing various anions. The addition of phosphate anions leads to a unique change in the luminescence behavior. As a case study, the quenching behavior of adenosine‐5′‐triphosphate (ATP) was investigated at physiological pH value in an aqueous solvent. A specificity of the sensor for ATP relative to adenosine‐5′‐diphosphate (ADP) and adenosine‐5′‐monophosphate (AMP) was found. ³¹P NMR spectroscopic studies revealed the formation of a [EuL²(ATP)] coordination species.     

Comparing the 2,2′‐Biphenylenedithiophosphinate Binding of Americium with Neodymium and Europium

Advancing our understanding of the minor actinides (Am, Cm) versus lanthanides is key for developing advanced nuclear‐fuel cycles. Herein, we describe the preparation of (NBu₄)Am[S₂P(^tBu₂C₁₂H₆)]₄ and two isomorphous lanthanide complexes, namely one with a similar ionic radius (i.e., Nd^III) and an isoelectronic one (Eu^III). The results include the first measurement of an Am?S bond length, with a mean value of 2.921(9) Å, by single‐crystal X‐ray diffraction. Comparison with the Eu^III and Nd^III complexes revealed subtle electronic differences between the complexes of Am^III and the lanthanides.     

Tetranuclear Heterometallic {Zn2Eu2} Complexes With 1‐Naphthoate Anions: Synthesis,Structure and Photoluminescence Properties

A series of new tetranuclear heterometallic Zn^II‐Eu^III complexes have been synthesized, that is, (bpy)₂Zn₂Eu₂(naph)₁₀ ( 1 ), (bpy)₂Zn₂Eu₂(naph)₈(NO₃)₂ ( 2 ), and (phen)₂Zn₂Eu₂(naph)₈(NO₃)₂ ( 3 ), and other ones, where naph^? is the 1‐naphthoate anion, bpy=2,2′‐bipyridyl, and phen=1,10‐phenanthroline. The solid‐phase complexes consist of large supramolecular ensembles due to stacking interactions between the aromatic ligands. Photoluminescence (PL) measurements were carried out to study PL spectra, lifetimes and quantum yields (QY) of the synthesized complexes at different temperatures. The external QY for the solid phases of complexes under UV excitation was found to exceed 20 %. It has been shown that partial replacement of naphthoate ligands in the coordination environment of Eu³⁺ by NO₃^? anions influences the PL properties. To investigate the behavior of these complexes in solvent, we dissolved complex 3 in MeCN, put it on a transparent glass as a substrate, and studied the PL properties at room temperature.     

Energy transfer in luminescent complexes of EuIII and TbIII with homo- and copolymers of acrylic acid and alkyl methacrylates

The influence of the composition and structure of a macromolecular ligand on the efficiency of energy transfer in complexes of Eu^III and Tb^III with ligands based on acrylic acid has been studied. It has been found that a decrease in the size of an alkyl group of methacrylic esters and a low content of acrylic moieties of Eu^III complexes with copolymers increase the efficiency of Eu^III concentration quenching. Insertion of noncoordinative alkyl methacrylate into the polymeric chain results in an increase in the efficiency of energy transfer from Tb³⁺ to Eu³⁺.Translated fromIzvestiya Akademii Nauk. Seriya Klrimicheskaya, No. 6, pp. 1425–1428, June, 1996.     

Crystal Structures of the Lanthanum(III), Europium(III), and Terbium(III) Cryptates of Tris(bipyridine) Macrobicyclic Ligands

The crystal structures of the La^III, Eu^III, and Tb^III complexes of macrobicyclic [bpy.bpy.bpy] ligands, [La³⁺ ? 1 ]3 Cl^? ( = 3- La), [Tb³⁺ ? 1 ]3 Cl^? ( = 3- Tb), and [Eu³⁺ ? 2 ]3 C1^? ( = 3- Eu), have been determined. They confirm the cryptate nature of these species, the cations being bound to the eight N-sites of the ligand. The macrobicycle presents two open faces, thus allowing additional coordination of two species, Cl^? ions or H₂O molecules, to the bound cations. These data provide structural support for the photophysical studies of the luminescent properties of the Eu^III and Tb^III cryptates, which indicated residual coordination of H₂O molecules.     

Modulation of the Lifetime of Water Bound to Lanthanide Metal Ions in Complexes with Ligands Derived from 1,4,7,10‐Tetraazacyclododecane Tetraacetate (DOTA)

A series of di‐ and tetraamide derivatives of DOTA were synthesized, and their lanthanide(III) complexes were examined by multinuclear ¹H‐, ¹³C‐, and ¹⁷O‐NMR spectroscopy, and compared with literature data of similar, known complexes (Table). All ligands formed structures similar to the parent [Ln^III(DOTA)]^? complexes, with four N‐atoms and four O‐atoms from DOTA and one O‐atom from the inner‐sphere water molecules. Interestingly, the lifetimes τ_M of the inner‐sphere, metal‐bound water molecules vary widely, ranging from nano‐ to milliseconds, depending on the identity of the pendent amide side chains. In general, positively charged [Ln^III(DOTA‐tetraamide)]³⁺ complexes display the longest residence times (high τ_M values), while complexes with additional charged functional groups on the extended amides display much smaller τ_M values, even when the side groups are not directly coordinated to the central Ln³⁺ ions. The design of novel [Ln^III(DOTA‐tetraamide)]³⁺ complexes with a wide, tunable range of τ_M values is of prime importance for the application of fast‐responding, paramagnetic chemical‐exchange‐saturation‐transfer (PARACEST) imaging agents used for the study of physiological and metabolic processes.     

Syntheses,structural determination,and binding studies of nine-coordinate multinuclear (mnH)2[EuIII(egta)]2·6H2O and binuclear (mnH)4[EuIII2(dtpa)2]·6H2O

Two lanthanide complexes, (mnH)₂[Eu^III(egta)]₂·6H₂O (1) (H₄egta = ethyleneglycol-bis-(2aminoethylether)-N,N,N′,N′-tetraacetic acid) and (mnH)₄[Eu^III₂(dtpa)₂]·6H₂O (2) (H₅dtpa = diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid), have been synthesized and characterized by FT-IR spectroscopy, thermal analysis, and single-crystal X-ray diffraction. X-ray diffraction reveals that 1 is multinuclear nine-coordinate and crystallizes in the monoclinic crystal system with space group C2/c. The obtained cell dimensions are a = 38.513(3)?Å, b = 13.5877(8)?Å, c = 8.7051(5)?Å, β = 99.6780(10)°, and 4490.6(5)?ų. Each methylamine (mnH⁺) cation in 1, through hydrogen bonds, connects three adjacent [Eu^III(egta)]^? anions. The [Eu^III(egta)]^? anions connect one another forming a 1-D multinuclear zigzag chain structure along the c-axis. Complex 2 is nine-coordinate binuclear structure with tricapped trigonal prismatic conformation and crystallizing in the monoclinic crystal system, but with space group P2₁/n. The obtained cell dimensions are a = 9.9132(8)?Å, b = 24.1027(18)?Å, c = 10.7120(10)?Å, β = 109.1220(10)°, and 2418.2(3)?ų. For 2, there are two kinds of methylamine cations (mnH⁺) connecting [Eu^III₂(dtpa)₂]^4? complex anions and lattice waters through hydrogen bonds, leading to formation of a 2-D ladder-like layer structure.     

Kinetics and mechanism of reduction of 2,8-dimethyl-1,9-diphenyl-3,7-diaza-2,7-nonadiene-1,9-dione dioximatocopper(III) ([CuIII A]+) and 4,6,9-trimethyl-5,8-diaza-4,8-dodecadiene-2,3,10,11-tetraone 3,10-dioximatocopper(III) ([CuIIIB]+) by p-methoxyphenol, 1,2-dihydroxybenzene, 1,4-dihydroxybenzene and some of its derivatives

The kinetics of reduction of two copper(III)-imine-oxime complexes, [Cu^IIIA]⁺ and [Cu^IIIB]⁺, (H₂A and H₂B=2,8-dimethyl-1,9-diphenyl-3,7-nonadiene-1,9-dione dioxime and 4,6,9-trimethyl-5,8-diaza-4,8-dodecadiene-2,3,10,11-tetraone 3,10-dioxime respectively) by hydroquinone (H₂Q), 2-methylhydroquinone (MH₂Q), 2-chlorohydroquinone (ClH₂Q), catechol (H₂Cat) and p-methoxyphenol (pMHP) have been examined in aqueous acidic solution. Under fixed reaction conditions, the kinetics display first-order dependence on each oxidant and reductant. The pH-dependence is complex for the reduction of [Cu^IIIA]⁺, since both the copper(III) complex and the reductants undergo protonation–deprotonation equilibria. In the lower pH range, the second-order rate constant, k ₂, decreases with increasing pH. In the higher pH range, k ₂ increases with increasing pH. In the lower pH range the most important oxidant is [Cu^IIIHA]²⁺, whereas, in the higher pH range the most important reactants are deprotonated reductants. However for H₂Cat, as was observed before, two reaction pathways seem to operate in the high pH range. In one pathway, HCat^? seems to be involved; whereas, in the other pathway Cat^2? seems to be the reactive species. Doubly deprotonated catechol, Cat^2?, is very unlikely to be formed at pH ≤ 5. It was therefore necessary to invoke a strong interaction between [Cu^IIIA]⁺ and HCat^? followed by loss of the second proton. The pH dependence for the reduction of [Cu^IIIB]⁺ is less complex. Thus H₂Q and MH₂Q showed no pH dependence up to pH ～ 4.60, whereas ClH₂Q, pMHP and H₂Cat displayed an inverse first-order dependence on [H⁺]. Observed rate constants showing first-order dependence and inverse first-order dependence on [H⁺] correlate reasonably well with those calculated using the Marcus equation. The reaction path involving Cat^2? is believed to proceed by an inner-sphere mechanism. The agreement between the calculated and observed values for the [Cu^IIIA]⁺ complex is lower than was found for the [Cu^IIIA₁]⁺(A₁=3,9-diethyl-4,8-diaza-3,8-undeca-2,10-dionedioxime). It seems that the replacement of methyl groups in the latter complex by phenyl groups in the former complex causes both electronic and steric effects, and both effects seem to retard electron transfer. The electronic effect is readily seen in the decrease of the reduction potential of [Cu^IIIA]⁺ (E ⁰=1.09 V) compared to the reduction potential of [Cu^IIIA₁]⁺(E ⁰=1.16 V) and thus making the former a weaker oxidant. The self-exchange rate constant (5 × 10⁵ M ^?1 s^?1) estimated for complexes with type H₂A ligands seem to work well for complexes with type H₂B ligands. This situation is supported by the findings of a fairly constant value for the self-exchange rate constant for Cu ^III/II –peptide complexes with varying substituents.     

Two Isostructural Layered Lanthanide(III) Complexes: Syntheses,Structures, Magnetic and Luminescent Properties

Two N'-(2-hydroxybenzylidene)pyridine N-oxide-carbohydrazide (H₃L)-based coordination complexes with the formula [Ln₂(DMF)₂(OAc)₂(HL)₂]_n (Ln = Dy for 1 and Eu for 2 ) were solvothermally synthesized. Crystal structures, thermal stabilities, magnetic and luminescent properties of the two complexes were fully investigated. Both complexes are isomorphic two-dimensional layers with centrosymmetric {Ln₂} subunits extended by doubly deprotonated HL^2– connectors. Complex 1 with highly anisotropic Dy^III spin exhibits slightly frequency-dependent magnetic relaxations under zero dc field with an effective energy barrier of ca. 6.84 K. Eu^III-based complex 2 displays only one weak fluorescent emission around 532 nm with the absence of characteristic emission of Eu^III ion. These results provide helpful hints of the hydrazide Schiff-functionalized organic ligands on the function modulations of the resulting Ln complexes.     

Luminescent europium(III) complexes of tripodal heptadentate N7 ligands containing three imidazole groups

Luminescent Eu^III complexes with tripodal heptadentate N₇ ligands containing three imidazole groups, [Eu^III(H₃L^2-H)(ac)](ClO₄)₂·H₂O (1), [Eu^III(H₃L^2-Me)(ac)](ClO₄)₂·2EtOH (2), and [Eu^III(H₃L^4-Me)(ac)](ClO₄)₂·H₂O (3), were synthesized and characterized, where H₃L^2-H, H₃L^2-Me, and H₃L^4-Me are the tripodal ligands derived from the 1:3 condensation of tris(2-aminoethyl)amine and either 4-formylimidazole, 2-methyl-4-formylimidazole, and 4-methyl-5-formylimidazole, respectively, and ac denotes an acetate ion. Single-crystal X-ray analyses revealed that each Eu^III ion is coordinated by a tripodal heptadentate N₇ ligand and two oxygen atoms of the acetate ion as a bidentate ligand. The complexes displayed sharp emission bands based on the f-f transitions by excitation at 261 nm in acetonitrile. The emission intensities increased in the order 1 < 2 < 3 in acetonitrile, while the emission spectra were quenched in aqueous solution due to the partial dissociation of the acetate ion and tripodal ligand.