Europium Analogue Research Articles

Structure, Thermal Properties and Photoluminescence of Terbium and Dysprosium Mixed‐Ligand Complexes with Dimethyl‐N‐Trichloroacetylamidophosphate and 2,2’‐Bipyridine

AbstractConsidering the growing interest in lanthanide‐based multifunctional materials two mixed‐ligand complexes [Ln(L)3bipy] (Ln=TbIII, DyIII) with carbacylamidophosphate (CAPh) type ligand – dimethyl‐N‐trichloroacetylamidophosphate (HL) and additional bidentate ligand 2,2’‐bipyridine (bipy) were obtained and studied by X‐ray diffraction study, thermal gravimetric analysis, and luminescence spectroscopy. The obtained complexes are monomeric and isostructural with earlier reported europium analog and crystallizes in the monoclinic system (space group C2/c). They are stable during long‐term storage for at least a couple of years as well as thermally stable up to nearly 200 °C. Both complexes exhibit f–f luminescence upon excitation into the 2,2’‐bipyridine absorption band, being emitters of green (TbIII) and yellow (DyIII) light. The luminescence intensity of the obtained complexes decreases constant exposure to UV radiation for several hours and then stays constant for at least till 32 h.

Lanthanide-based complexes as efficient physiological temperature sensors

A new molecular thermometric sensor based on the terbium(III) complex [C2mim][Tb(fod)4] (C2mim – 1-methyl-3-ethylimidazolium, fod− - tetrakis-6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate), doped with 0.015% of its europium(III) analogue (1, [C2mim][Tb(fod)4]0.99985:[C2mim][Eu(fod)4]0.00015), was prepared and its thermochromic behaviour evaluated from ambient temperature up to 75 °C, including in the physiological range (35–45 °C). It was found that the intensity ratio of the 5D4→7F5 (TbIII) and 5D0→7F2 (EuIII) transitions is correlated with temperature having three different linear regimes. Visual colorimetry allowed the evaluation of the temperature in different ranges from green at ambient temperature, to yellow and finally red at higher temperatures. The TbIII complex emission intensity is extremely sensitive to small temperature variations, particularly between 25 and 35 °C, were it reaches only 40% of the initial intensity.Confinement of the dopped TbIII tetrakis-complex in the organic polymeric matrix poly(methylmethacrylate) (PMMA) induced higher thermal stability in 1, together with a strong temperature dependence of the most intense emissive transition of the TbIII complexes. The photoluminescence quantum yield of polymer-lanthanide hybrid materials increased significantly compared with that of 1. Under 366 nm irradiation, the hybrid material presents a green colour at 25 °C that evolves to yellow at 30 °C and to a white tone at 35 °C.

A Walk Across the Lanthanide Series: Trend in Affinity for Phosphate and Stability of Lanthanide Receptors from La(III) to Lu(III).

The trend in affinity of two 1,2-hydroxypyridinonate lanthanide(III) receptors-LnIII-2,2-Li-HOPO and LnIII-3,3-Gly-HOPO (LnIII = LaIII, PrIII, NdIII, SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII, TmIII, YbIII, and LuIII)-for phosphate across the series was investigated by luminescence spectroscopy via competition against the central europium(III) analog. Regardless of the ligand, the rare earth receptors display a steep and continuous increase in affinity for their phosphate guest across the series, with the later lanthanides displaying the highest affinity for the oxyanion. This trend mirrors that of the stability of the lanthanide receptors, which also increases significantly and continuously from LaIII to LuIII. For these two ligands, the ionic radius of a rare earth, a parameter directly linked to its Lewis acidity, correlates strongly with its affinity for anions, regardless of whether that anion is the one coordinating it (in this case the 1,2-hydroxypyridinonate ligand) or the guest targeted by the lanthanide receptor (in this case phosphate). These observations are indicative of a lack of steric hindrance for coordination of phosphate. Advantageously, increased efficacy of the lanthanide receptor comes with increased stability. The remarkably high stability of LuIII-2,2-Li-HOPO, combined with its high affinity for phosphate, makes it a particularly promising candidate for translational application to medical or environmental sequestration of phosphate since the higher stability will further reduce the risk of the rare earth leaching during anion separation. The unusually large difference in stability between lanthanide complexes (the LuIII complex of 2,2-Li-HOPO is at least 7 orders of magnitude more stable than the LaIII one) bodes well for potential applications in rare earth separation.

Photoluminescence of Visible and NIR-Emitting Lanthanide-Doped Bismuth-Organic Materials.

A bismuth-organic compound containing 2,2':6'2"-terpyridine (terpy) and 2-thiophenecarboxylate (TC), of the general formula (terpy)Bi(κ2 -TC)3 ⋅0.47 H2 O (BiOM-1), has been synthesized under hydrothermal conditions. Addition of a lanthanide nitrate solution to the reaction mixture led to statistical replacement of the bismuth centers, and yielded isomorphous lanthanide containing compounds Bi1-x Lnx OM-1 (Ln=Nd, Sm, Eu, Tb, Dy, Er, and Yb) that showed bismuth and/or ligand sensitized lanthanide-centered emission, and the first example of NIR emission from a lanthanide doped BiOM. The structure was determined by single-crystal X-ray diffraction, and the level and uniformity of lanthanide ion incorporation into the bismuth host was determined by ICP-OES and electron microprobe analysis. For the visible emitters, lifetime data and quantum yields are presented. A high efficiency of sensitization was calculated for the europium analog (50.1 %), showing significant improvement over previously reported europium thiophenecarboxylates. These novel materials may provide strategies to address concerns over the long-term sustainability of the rare earth elements, especially relating to optical devices.

Crystallographic and Spectroscopic Characterization of Americium Complexes Containing the Bis[(phosphino)methyl]pyridine-1-oxide (NOPOPO) Ligand Platform.

The crystal structures of americium species containing a common multifunctional phosphine oxide ligand, reported for its ability to extract f elements from acidic solutions, namely, 2,6-[Ph2P(O)CH2]2C5H3-NO, L, were finally determined after over three decades of separations studies involving these species and their surrogates. The molecular compounds Am(L)(NO3)3, Am 1:1, and [Am(L)2(NO3)][2(NO3)], Am 2:1, along with their neodymium and europium analogues, were synthesized and characterized using single-crystal X-ray crystallography, attenuated total reflectance Fourier transform infrared spectroscopy, and luminescence spectroscopy to provide a comprehensive comparison with new and known analogous complexes.

PH-Responsive Lanthanide Complexes Based on Reversible Ligation of a Diphenylphosphinamide

Ln-dpp-DO3A-based complexes [dpp is a pendant diphenylphosphinamide moiety and DO3A is 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane] exhibit pH-responsive reversible ligation of the phosphinamide for both the gadolinium(III) and europium(III) analogues. pKa values were 8.1 (±0.1) and 7.8 (±0.1) for Gd.1 and Gd.2, respectively. The relaxivities (20 MHz, 298 K) of the gadolinium(III) analogues were r1 = 7.9 mM(-1) s(-1) (Gd.1) and r1 = 8.2 mM(-1) s(-1) (Gd.2) in acidic media, corresponding to a hydration state q = 2; in basic media, deprotonation and coordination of the phosphinamide occurs, with r1 = 5.4 mM(-1) s(-1) (Gd.1) and r1 = 4.4 mM(-1) s(-1) (Gd.2) corresponding to q = 1. Sensitized luminescent emission was observed from the europium(III) analogues following excitation at λex = 270 nm. The hydration state values of the europium(III) analogues were consistent with those of the gadolinium(III) complexes, i.e., q = 1 and 2 in basic and acidic media, respectively. The ratio of the emission intensities of the ΔJ = 1 and 2 bands enables concentration-independent reporting of the pH. Excited-state pKa values were 8.3 (±0.1) and 8.5 (±0.1) for Eu.1 and Eu.2, respectively.

Controlled intracellular self-assembly of gadolinium nanoparticles as smart molecular MR contrast agents

Herein we developed a new “smart” Gd-based MR contrast agent (i.e., 1) which is susceptive to furin, a protease overexpressed in tumor. Under the action of furin, 1 condenses to form dimers (1-Ds) and the latter self-assemble into gadolinium nanparticles (Gd-NPs). Relaxivity of 1-D is more than 2 folds of those of 1 and magnevist at 1.5 T, and 1.4 folds of that of 1 at 3 T. Intracellular condensation of 1 in furin-overexpressed MDA-MB-468 cells was proven with direct two-photon laser microscopy (TPLM) fluorescence imaging of the cells incubated with the europium analog of 1 (i.e., 2). Intracellular Gd-NPs of 1 were uncovered and characterized for the first time. MRI of MDA-MB-468 tumors showed that 1 has enhanced MR contrast within the tumors than that of its scrambled control 1-Scr.

The basis for the molecular recognition and the selective time-gated luminescence detection of ATP and GTP by a lanthanide complex

Two molecular probes for the detection of nucleotides are presented. The positively charged [Tb-DOTAm-Phen]3+ responds only to purine nucleotides and selectively detects GTP and ATP over GDP/GMP and ADP/AMP, respectively. The neutral Eu-DOTA-Phen complex, on the other hand, is selective for purines over pyrimidines but does not distinguish between tri-, di- and monophosphate nucleotides. Both probes possess long luminescence lifetimes that readily enable time-gated experiments and complete removal of any autofluorescence background. The decrease of the Stern–Volmer constants with temperature is indicative of a static mechanism of quenching in agreement with the formation of a stacked phenanthridine–purine complex. The selectivity of the probes for purines over pyrimidines is due to a higher binding affinity and quenching ability of the more extended purines for the phenanthridine chromophore. The selectivity of the positively charged terbium complex for the tri- over the di- and mono-phosphate nucleotides is due to weak electrostatic interactions with the negatively charged phosphates of ATP and GTP, as evidenced by the lack of any selectivity for ATP/ADP/AMP or GTP/GDP/GMP by the neutral europium complex. Both the selective terbium complex and its non-responsive europium analog can be used concomitantly in the same solution as neither probe affects the response of the other. Simultaneous use of the two chemosensors enables more accurate ratiometric detection of ATP to ADP or GTP to GDP conversion and continuous monitoring of kinase reactions.

Redox-active Magnetic Resonance Imaging Contrast Agents: Studies with Thiol-bearing 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetracetic Acid Derivatives

The synthesis and structure-activity relationships of a homologous series of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid gadolinium(III) complexes bearing thiol-terminated alkyl side chains from three to nine carbons in length are reported. The observed binding with human serum albumin (HSA) of the compounds having C-3 through C-7 side chain lengths was inhibited by homocysteine in a manner consistent with single-site binding. The observed binding with HSA of the compounds having C-8 and C-9 side chain lengths was only partly inhibited by homocysteine, consistent with multisite binding. The binding affinity of the C-7 compound could be related to the HSA oxidation state. 2D 1H-1H NMR TOCSY provided evidence of covalent binding of the europium analog of the C-6 compound to HSA-Cys34. The longitudinal water-proton MRI relaxivities of the gadolinium complexes at 7 T increased upon binding to HSA. On the basis of these results, the C-6 and C-7 compounds were identified as promising redox-sensitive MRI contrast agents.

Optically Active Homoleptic Bis(phthalocyaninato) Rare Earth Double-Decker Complexes Bearing Peripheral Chiral Menthol Moieties: Effect of π−π Interaction on the Chiral Information Transfer at the Molecular Level

With the view to creating novel sandwich-type phthalocyaninato rare earth complexes toward new applications in material science and catalysis, d- and l-enantiomers of a series of optically active homoleptic bis(phthalocyaninato) rare earth double-deckers with four chiral menthol moieties at the peripheral positions of the phthalocyanine ligand, M(Pc*)(2) [Pc* = 2(3),9(10),16(17),23(24)-tetrakis(2-isopropyl-5-methylcyclohexoxyl)phthalocyanine; M = Eu, Y, Lu] (1-3), have been designed and prepared by treating (d)- or (l)-4-(2-isopropyl-5-methylcyclohexoxyl)-1,2-dicyanobenzene with the corresponding M(acac)(3).nH(2)O (acac = acetylacetonate) in the presence of the organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in refluxing n-pentanol. For the purpose of comparative study, heteroleptic bis(phthalocyaninato) europium analogues (d)- and (l)-Eu(Pc)(Pc*) (4) as well as the unsubstituted homoleptic bis(phthalocyaninato) europium counterpart Eu(Pc)(2) (5) were also prepared. The novel synthesized bis(phthalocyaninato) rare earth double-deckers have been characterized by a wide range of spectroscopic methods including MS, (1)H NMR, IR, and electronic absorption spectroscopic measurements in addition to elemental analysis. In contrast to the CD silent monomeric metal-free 2(3),9(10),16(17),23(24)-tetrakis(2-isopropyl-5-methylcyclohexoxyl)phthalocyanine, observation of the CD signal in the N absorption region of 4 reveals the significant effect of intramolecular pi-pi interaction on intensifying the asymmetrical perturbation of the chiral menthol units onto the phthalocyanine chromophore, which results in successful chiral information transfer from menthol moieties to the phthalocyanine chromophore at a molecular level in the heteroleptic double-decker compound 4 despite the lack of CD signal in the Soret and Q absorption regions of the phthalocyanine ligand. This is further supported by the optical activity of homoleptic bis(phthalocyaninato) rare earth double-deckers M(Pc*)(2) (1-3), as revealed by the CD signals even in the Soret and Q absorption regions according to the CD spectroscopic result, indicating the intensified asymmetrical perturbation of the chiral menthol units onto the phthalocyanine chromophores along with the increase in the chiral menthol substituent number from 4 to 1-3. The present result at the molecular level is helpful for understanding the chiral information transfer mechanism at the supermolecular level. In addition, the electrochemical properties of bis(phthalocyaninato) rare earth complexes have also been comparatively investigated by cyclic voltammetry and differential pulse voltammetry.

Trapping of Anionic Organic Radicals by (TpMe2)2Ln (Ln = Sm, Eu)

Stoichiometric reaction of [ Sm(Tp(Me2))2 ], 1, with a variety of reducible ketone- and quinone-type substrates gave thermally stable, isolable radical anions/ketyls in moderate to good yields. Thus reaction with benzophenone gave [Sm(Tp(Me2))2(OCPh2)], 2, with fluorenone [Sm(Tp(Me2))2(eta1-OC13H8)], 3, and di-tert-butylparaquinone [Sm(Tp(Me2))2(eta1-OC6H2(tBu)2O)], 4, each of which was structurally characterized. In the case of the less-hindered benzoquinone, an unimetallic semiquinone [Sm(Tp(Me2))2(OC6H4O)], 5, could be isolated, although it was unstable with respect to formation of the dimetallic complex [Sm(Tp(Me2))2]2(mu-OC6H4O), 6. Compound 6 was structurally characterized, as was its anthraquinone analogue [Sm(Tp(Me2))2]2(mu-OC14H8O), 7. When the analogous reaction was carried out between the less-reducing [Eu(Tp(Me2))2] and benzoquinone, only the europium analogue of the semiquinone 5, [Eu(Tp(Me2))2(OC6H4O)], 8, could be isolated. The use of the sterically hindered 3,5-di-tert-butyl-o-benzoquinone allowed isolation of [Sm(Tp(Me2))2(DTBSQ)], 9.

Structural Trends in Alkaline Earth and Rare Earth Metal 3,5‐Diisopropylpyrazolates

AbstractA series of 3,5‐diisopropylpyrazolates of magnesium, calcium, and strontium in addition to a divalent europium analog continues our investigation on the differences and similarities between the alkaline earth and divalent rare earth metal elements. The alkane elimination reaction between dibutylmagnesium and excess 3,5‐diisopropylpyrazole (iPr2pzH) in toluene afforded the dinuclear magnesium pyrazolate [{Mg(iPr2pz)2(iPr2pzH)}2] (1). [{Ca(iPr2pz)2(iPr2pzH)2}2] (2) and [{Eu(iPr2pz)2(iPr2pzH)2}2] (3) were obtained as the product of redox‐transmetallation/ligand‐exchange reactions between an excess of the corresponding metal, 2 equiv. of iPr2pzH, and Hg(C6F5)2 in toluene. This reaction with Sr metal led to the isolation of a unique byproduct, [Hg3(iPr2pz)4(C6F5)2] (4). High‐temperature direct metallation of barium with iPr2pzH afforded [{Ba(iPr2pz)2(py)3}2] (5) upon treatment of the hydrocarbon‐insoluble homoleptic [{Ba(iPr2pz)2}n] with pyridine (py). X‐ray crystal structure analyses revealed dinuclear structures for compounds 1–3 and 5 with μ‐ηn:ηm bridging pyrazolato ligands, including the new μ‐η1:η5 linkage, and further stabilization of 1–3 by intramolecular hydrogen bonding between the terminal pyrazole donor and terminal 1 or bridging pyrazolato ligands in 2 and 3. The trinuclear 4 displays the first example of a complex with an open, triangular Hg3 unit. It has μ‐η1:η1 bridging pyrazolato ligands and terminal pentafluorophenyl groups.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Magnesium Based Ternary Metal Hydrides Containing Alkali and Alkaline‐Earth Elements

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Role of Donor and Secondary Interactions in the Structures and Thermal Properties of Alkaline-Earth and Rare-Earth Metal Pyrazolates

The addition of neutral coligands to reduce the aggregation and improve the volatility of potential heavy alkaline-earth metal chemical vapor deposition (CVD) precursors has typically resulted in liberation of the coligand upon heating. A new series of dinuclear alkaline-earth and rare-earth metal pyrazolates, bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)calcium] (1), bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)strontium] (2), and bis[bis(3,5-di-tert-butylpyrazolato)bis(tetrahydrofuran)barium] (3), have been obtained from our previous donor-free oligonuclear complexes [{M(3,5-tBu2pz)2}n] (5, M = Ca, n = 3; 6, M = Sr, n = 4; 7, M = Ba, n = 6) by treatment with tetrahydrofuran (THF). Compounds 1-3, as well as the europium analogue bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)europium(II)] (4), can also be prepared by direct reaction of the metals and pyrazole in THF and anhydrous liquid ammonia. Recrystallization from hexane led to single crystals of 2-4, while the powder diffraction pattern of 1 revealed it to be isostructural with the previously published bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)ytterbium(II)] (8), providing important insight into differences and similarities between the two groups of metals. Detailed structural analysis of the compounds reveals secondary interactions including pi-bonding and agostic interactions, which are considered essential in stabilizing the metal complexes. The direct comparison of structural features and thermal properties (as evaluated by thermogravimetric analysis and sublimation studies) of the donor-free oligonuclear and the donor-containing dinuclear species offers a better understanding of the role of donors and secondary interactions.

Toward Templated Metal−Organic Frameworks: Synthesis, Structures, Thermal Properties, and Luminescence of Three Novel Lanthanide−Adipate Frameworks

Three novel praseodymium-adipate frameworks were synthesized hydrothermally. GWMOF-3 ([Pr(2)(adipic acid)(3)(H(2)O)(4)].adipic acid.4H(2)O) and GWMOF-6 ([Pr(2)(adipic acid)(3)(H(2)O)(2)].4,4'-dipyridyl) formed three-dimensional structures, whereas GWMOF-4 ([Pr(2)(adipic acid)(3)(H(2)O)(2)].H(2)O) produced a more dense, two-dimensional topology. Single-crystal X-ray and powder diffraction, IR spectroscopy, fluorescence spectroscopy, thermogravimetric analysis, and elemental analysis were employed to characterize all samples. GWMOF-6 represents an innovative step forward in metal-organic framework synthesis where a neutral molecular species not used in the construction of the framework is utilized as a structure-directing agent, or template. Furthermore, this template molecule (4,4'-dipyridyl) is shown to sensitize the fluorescence of lanthanide metal centers in a europium analogue of GWMOF-6.

Sr6Mg7H26, a new saline high-pressure hydride with Ba6Zn7F26-type structure

Sr6Mg7H26 and its deuteride have been synthesized at high pressure (25 kbar) and temperature (900 K) from mixtures of the corresponding binary hydrides (deuterides). Structure refinements on X-ray and neutron powder diffraction data show a monoclinic Ba6Zn7F26 type structure (space group I2/m, a=14.254(2) Å, b=5.6711(4) Å, c=11.575(1) Å, β=90.59(1)°, deuteride, T=293 K). The four magnesium atoms have nearly octahedral deuterium coordinations with average bond distances Mg–D=1.97, 1.97, 1.96 and 1.85 Å. The three strontium sites have 10- and 11-fold deuterium coordination with average bond distances Sr–D=2.66 (CN=10), 2.71 (CN=11) and 2.62 Å (CN=10). The latter are longer by ∼0.07 Å, on the average, than the corresponding Eu–D distances in the europium analogue, thus underlining the structural analogy between Sr- and Eu-based metal hydrides.

Synthesis, structures, and magnetic and optical properties of a series of europium(III) and gadolinium(III) complexes with chelating nitronyl and imino nitroxide free radicals.

This paper reports the synthesis, structures, and magnetic and optical properties of a series of gadolinium(III) (1a-4a) and europium(III) (1b-4b) complexes with nitronyl or imino nitroxide radicals. The crystal structures of compounds 1a and 1b consist of [Ln(III)(radical)(2)(NO(3))(3)] entities in which the gadolinium(III) (1a) or europium(III) ion (1b) is 10-coordinated to two nitronyl nitroxide radicals and three nitrato ligands. The crystal structures of compounds 2a-4a and 2b-4b consist of [Ln(III)(hfac)(3)(radical)] entities in which the gadolinium(III) (2a-4a) or europium(III) ion (2b-4b) is 8-coordinated to one nitronyl (2a and 2b) or one imino (3a, 4a and 3b, 4b) nitroxide radical and three hexafluoroacetylacetonato ligands. The gadolinium(III) complexes (1a-4a) are isostructural with their europium(III) analogues (1b-4b). The magnetic properties of the gadolinium complexes were studied. Along the series 1a-4a only compound 2a exhibits a ferromagnetic Gd(III)-radical coupling (J(Gd-rad) = +1.7 cm(-1)), while for the others this coupling is antiferromagnetic (1a: J(Gd-rad1) = -4.05 cm(-1) and J(Gd-rad2) = -0.80 cm(-1); 3a: J(Gd-rad) = -2.6 cm(-1); 4a: J(Gd-rad) = -1.9 cm(-1)). The first full luminescence spectra of lanthanide complexes with free radical ligands are reported between 650 and 1200 nm. The rich vibronic structure in luminescence and absorption spectra indicates that several excited states define the absorption spectra between 400 and 800 nm. Qualitative trends can be established between magnetic ground state properties and the energies and fine structure of the title compounds.

Synthesis, X-ray structure and spectroscopic studies of new praseodymium(III) six-coordinate complexes with 3-halo-4-methoxy-2,6-lutidine N-oxide: PrCl3(XCH3OC7H7NO)3 where X=Cl, Br and I

The X-ray structure of a new PrCl3(BrCH3OC7H7NO)3 compound has been determined and described in monoclinic space group P21/n with four molecules per unit cell of dimensions: a=8.232, b=23.733, c=16.763 Å and β=90.75°. The Pr3+ ion is six-coordinated with three Cl ions and three oxygen ions of the N-oxide group of the 3-bromo-4-methoxy-2,6-lutidine moieties. The molecules are linked through hydrogen bonds of the type C–H⋯Cl and length 3.591 Å. The chloro and iodo derivatives of the lutidine N-oxide have also been synthesized and their properties are compared to those of the bromo complex. The FTIR and FT Raman spectra as well as electron absorption and emission spectra have been recorded and analyzed in terms of site symmetry and energy level scheme of praseodymium(III) ion. In addition, the europium analogue was used as a model system to study the ligand to metal energy transfer.

Ligand-to-metal charge-transfer quenching of the Eu 3+( 5D 1) state in europium-doped tris(2,2,6,6-tetramethyl-3,5-heptanedionato)gadolinium (III)

Rate constants for the decay of the Eu 3+( 5D 1) state in dilute Eu 3+-doped tris(2,2,6,6-tetramethyl-3,5-heptanedionato)gadolinium(III) are reported for a temperature range 50–315 K. A temperature-independent contribution of 1.3×10 6 s −1 is attributed to carbonyl mediated 5D 1→ 5D 0 relaxation. A temperature-dependent contribution, that has been parameterized as k CT= Ae − E a/ kT with E a=2540±140 cm −1 and A=1.3×10 12 s −1 is attributed to quenching of 5D 1 by a low-lying ligand-to-metal charge-transfer state (CTS). The height of the activation barrier, E a, relative to the previously reported height for charge-transfer quenching of the Eu 3+( 5D 0) state in the pure europium analog (J. Phys. Chem. 100 (1996) 9216) is discussed in terms of mechanistic implications. Specifically, these data strongly suggest that quenching of 5D 1 by the charge-transfer state is mediated by the 5D 0 state with a novel, indirect mechanism.

A holmium complex of a macrocyclic ligand (DOTA) and its isostructural europium analogue

The holmium and europium complexes of the tetraanion of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (H4DOTA), namely, sodium aqua(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetato)-holmium(III) tetrahydrate, Na[Ho(C 16 H 24 N 4 O 8 )(H 2 O)].4H 2 O, and sodium aqua(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetato)europium(III) tetrahydrate, Na[Eu(C 16 H 24 N 4 O 8 )(H 2 O)].4H 2 O, show nine-coordination around the metal ion, with a square antiprismatic coordination geometry involving four N- and four O-donor atoms of the DOTA ligand, and a coordinated water molecule in a capping position.