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Non-macrocyclic H4OCTAPA derivatives form highly stable complexes with Bi(iii), but require an appropriate rigid spacer to enhance the inertness of the complex, as demonstrated by radiolabeling studies with 205/206Bi-nuclides.

The combination of two complementary magnetic resonance techniques, integrated by computational procedures, allows detailed solution characterization of vanadium(iv) complexes.

[Abstract] We present a quantitative analysis of the thermodynamic stabilities of Mn(II) complexes, defined by the equilibrium constants (log KMnL values) and the values of pMn obtained as −log[Mn]free for total metal and ligand concentrations of 1 and 10 μM, respectively. We used structural descriptors to analyze the contributions to complex stability of different structural motifs in a quantitative way. The experimental log KMnL and pMn values can be predicted to a good accuracy by adding the contributions of the different motifs present in the ligand structure. This allowed for the identification of features that provide larger contributions to complex stability, which will be very helpful for the design of efficient chelators for Mn(II) complexation. This issue is particularly important to develop Mn(II) complexes for medical applications, for instance, as magnetic resonance imaging (MRI) contrast agents. The analysis performed here also indicates that coordination number eight is more common for Mn(II) than is generally assumed, with the highest log KMnL values generally observed for hepta- and octadentate ligands. The X-ray crystal structure of [Mn2(DOTA)(H2O)2], in which eight-coordinate [Mn(DOTA)]2– units are bridged by six-coordinate exocyclic Mn(II) ions, is also reported. C.P.-I. and D.E.-G. (PID2019-104626GB-I00) and A.R.-R. (Grant PID2019-108352RJ-I00) thank Ministerio de Ciencia e Innovación and Xunta de Galicia (Grant ED431B 2020/52) for generous financial support. R.U.-V. thanks the Xunta de Galicia (Grant ED481A-2018/314) for funding her Ph.D. contract. L.V is indebted to CACTI (Universidade de Vigo) for X-Ray measurements. P.C. acknowledges funding from the National Institute of Diabetes and Digestive and Kidney Diseases (R01DK121789) Xunta de Galicia; ED431B 2020/52 Xunta de Galicia; ED481A-2018/314 Estados Unidos de América. National Institute of Diabetes and Digestive and Kidney Diseases; R01DK121789

International audience; We report a series of four cyclams and cross-bridged cyclams, N-functionalized by one hydroxyethyl arm, which may incorporate additional methyl(s) group(s). The Cu(II) and Zn(II) complexes of these ligands were synthesized and fully characterized. The investigation of the metal complexes in solid-state and in solution was carried via X-ray diffraction, NMR, EPR, absorption spectroscopy and DFT calculations. The influence of N-functionalization on their structural/stereochemical properties, electrochemical behavior and kinetic inertness was carefully studied and identified. N-Functionalization by a hydroxyethyl group did not influence importantly the stereochemical properties of the complexes. However, it stabilized the complexes kinetically and electrochemically. As for N-methylation, it induces significant distortion of the coordination geometry, decreasing the kinetic inertness of the complexes and stabilizing Cu+. Overall, this work expanded the family of cyclam and cross-bridged cyclam metal complexes and provides a detailed analysis of their structural features. The rather large body of data accumulated for this family of complexes provides opportunities to design systems with pre-determined properties for specific applications.

Two regioisomeric Gd(III) complexes with heptadentate AAZTA-like ligands show different hydration state (q = 1 and 2) and astonishingly different thermodynamic stability and dissociation kinetics.

Investigating the relaxation of water 1H nuclei induced by paramagnetic Mn(II) complexes is important to understand the mechanisms that control the efficiency of contrast agents used in diagnostic magnetic resonance imaging (MRI). Herein, a series of potentially hexadentate triazacyclononane (TACN) derivatives containing different pendant arms were designed to explore the relaxation of the electron spin in the corresponding Mn(II) complexes by using a combination of 1H NMR relaxometry and theoretical calculations. These ligands include 1,4,7-triazacyclononane-1,4,7-triacetic acid (H3NOTA) and three derivatives in which an acetate group is replaced by sulfonamide (H3NO2ASAm), amide (H2NO2AM), or pyridyl (H2NO2APy) pendants. The analogue of H3NOTA containing three propionate pendant arms (H3NOTPrA) was also investigated. The X-ray structure of the derivative containing two acetate groups and a sulfonamide pendant arm [Mn(NO2ASAm)]− evidenced six-coordination of the ligand to the metal ion, with the coordination polyhedron being close to a trigonal prism. The relaxivities of all complexes at 20 MHz and 25 °C (1.1–1.3 mM–1 s–1) are typical of systems that lack water molecules coordinated to the metal ion. The nuclear magnetic relaxation profiles evidence significant differences in the relaxivities of the complexes at low fields (, The nature of the donor atoms and particularly the coordination polyhedron affect dramatically the zero field splitting in six-coordinate Mn(II) complexes, which has a strong impact in the relaxivities measured at low magnetic fields.

The discovery of the nephrogenic systemic fibrosis (NSF) and its link with the

The wide range of optical and magnetic properties of lanthanide(III) ions is associated with their intricate electronic structures which, in contrast to lighter elements, is characterized by strong relativistic effects and spin-orbit coupling. Nevertheless, computational methods are now capable of describing the ladder of electronic energy levels of the simpler trivalent lanthanide ions, as well as the lowest energy term of most of the series. The electronic energy levels result from electron configurations that are first split by spin-orbit coupling into groups of energy levels denoted by the corresponding Russell-Saunders terms. Each of these groups are then split by the ligand field into the actual electronic energy levels known as microstates or sometimes mJ levels. The ligand-field splitting directly informs on the coordination geometry and is a valuable tool for determining the structure and thus correlating the structure and properties of metal complexes in solution. The issue with lanthanide complexes is that the determination of complex structures from ligand-field splitting remains a very challenging task. In this paper, the optical spectra-absorption, luminescence excitation, and luminescence emission-of ytterbium(III) solvates were recorded in water, methanol, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF). The electronic energy levels, that is, the microstates, were resolved experimentally. Subsequently, density functional theory calculations were used to model the structures of the solvates, and ab initio relativistic complete active space self-consistent field calculations (CASSCF) were employed to obtain the microstates of the possible structures of each solvate. By comparing the experimental and theoretical data, it was possible to determine both the coordination number and solution structure of each solvate. In water, methanol, and N,N-dimethylformamide, the solvates were found to be eight-coordinated and have a square antiprismatic coordination geometry. In DMSO, the speciation was found to be more complicated. The robust methodology developed for comparing experimental spectra and computational results allows the solution structures of homoleptic lanthanide complexes to be determined.

We have prepared a hetero-tetrametallic assembly consisting of three ytterbium ions coordinated to a central [Ru(bpm)

[Abstract] The limited use of palladium-103 and -109 radionuclides for molecular radiotherapy is surely due to the lack of appropriate ligands capable of fulfilling all criteria required for application in nuclear medicine. Furthermore, the thermodynamic properties of these complexes in solution remain difficult to establish. The challenge is compounded when considering that radiolabeling of compounds for translation to clinical trials requires fast complexation. Thus, the coordination of Pd(II) and 103/109Pd-nuclides is a huge challenge in terms of molecular design and physicochemical characterization. Herein, we report a comprehensive study highlighting TE1PA, a monopicolinate cyclam – already established in nuclear imaging with 64Cu-PET (positron emission tomography) imaging tracers – as a highly relevant chelator for natural Pd and subsequently 109Pd-nuclide. The structural, thermodynamic, kinetic and radiolabeling studies of Pd(II) with TE1PA, as well as the comparison of this complex with three structurally related derivatives, support palladium-TE1PA radiopharmaceuticals as leading candidates for targeted nuclear medicine. R.T. and N.L.B. acknowledge the Ministère de l′Enseignement Supérieur et de la Recherche and the Centre National de la Recherche Scientifique. J.P. is grateful to the Ligue contre le Cancer, the MAC-group (UBO) and the University of Cape Town for her PhD fellowship; C.H.S.D thanks the Technology Innovation Agency (TIA) seed fund implemented through the South African Nuclear Energy Corporation for financial support. C.P.-I. thanks Centro de Supercomputación de Galicia for providing access to computing facilities. L.M.P.L was financially supported by Project LISBOA-01-0145-FEDER-007660 (Microbiologia Molecular, Estrutural e Celular), funded by FEDER funds through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT - Fundação para a Ciência e a Tecnologia

We report a series of four cyclams and cross-bridged cyclams

International audience; The stability constants of Mn2+ complexes with ligands containing a trans-1,2-cyclobutanediamine spacer functionalized with picolinate and/or carboxylate functions were determined using potentiometric titrations (25 °C, 0.1 M KCl). The stability constant of the complex with a hexadentate ligand containing four acetate groups (L14−, log KMnL = 10.26) is improved upon replacing one (L24−, log KMnL = 14.71) or two (L34−, log KMnL = 15.81) carboxylate groups with picolinates. The [Mn(L1)]2− complex contains a water molecule coordinated to the metal ion in aqueous solutions, as evidenced by 1H NMRD studies and 17O chemical shifts and transverse relaxation rates. The 1H relaxivities determined at 60 MHz (3.3 and 2.4 mM−1 s−1 at 25 and 37 °C, respectively) are comparable to those of monohydrated complexes such as [Mn(edta)]2−. The exchange rate of the inner-sphere water molecule (k298ex = 248 × 106 s−1) is slightly lower than that of the edta4− analogue. DFT calculations (M11/def2-TZVP) suggest that the water exchange reaction follows a dissociatively activated mechanism, providing activation parameters in reasonably good agreement with the experimental data. DFT calculations also show that the 17O hyperfine coupling constant A/ℏ is affected slightly by changes in the Mn–Owater distance and the orientation of the water molecule with respect to the Mn–O vector.

Rigid derivatives of the acyclic ligand PDTA4− (H4PDTA = propylenediamine-N,N,N′,N′-tetraacetic acid) were prepared by functionalization of a 1,3-diaminocyclobutyl spacer. The new ligands contain either four acetate groups attached to the central scaffold (H4L1) or incorporate pyridyl (H2L2) or propylamide (H2L3) units replacing two of the carboxylate groups. The ligand protonation constants and the stability constants of their Mn2+ complexes were determined using potentiometric and spectrophotometric titrations. The stability of the [Mn(L1)]2− complex was found to be significantly higher than that of the flexible [Mn(PDTA)]2− derivative (log KMnL = 10.78 and 10.01, respectively). A detailed study of the 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and 17O NMR measurements evidence that the [Mn(L1)]2− and [Mn(L2)] complexes display a hydration equilibrium in solution involving a seven-coordinate species with an inner-sphere water molecule and a six-coordinate species that lacks a coordinated water molecule. As a result the 1H relaxivities of these complexes are somewhat lower than that of [Mn(EDTA)]2− and related systems. The introduction of propylamide groups in [Mn(L3)] shifts the hydration equilibrium to the seven-coordinate species, which results in a 1H relaxivity (r1p = 3.7 mM−1 s−1 at 22 MHz and 25 °C) exceeding that of [Mn(EDTA)]2− (r1p = 3.3 mM−1 s−1 at 22 MHz and 25 °C). The parameters that control the relaxivities in this family of complexes were determined by simultaneous fitting of the experimental 1H NMRD and 17O NMR data (transverse relaxation rates and chemical shifts), with the aid of computational studies performed at the DFT and CASSCF/NEVPT2 levels. These studies provide detailed insight of the parameters that control the efficiency of these relaxation agents at the molecular level.

Electronic supplementary information (ESI) available: For general methods, supplementary details on the preparation and characterization of Ald-ZIFs and TSC-ZIFs, sorption isotherms and kinetics, and regeneration of the adsorbent. See DOI: 10.1039/d1ra02025k [Abstract] Zeolitic imidazolate frameworks (ZIF-8), and their derivatives, have been drawing increasing attention due to their thermal and chemical stability. The remarkable stability of ZIF-8 in aqueous and high pH environments renders it an ideal candidate for the removal of heavy metals from wastewater. In this study, we present the preparation of novel aldehyde-based zeolitic imidazolate frameworks (Ald-ZIF) through the integration of mixed-linkers: 2-methylimidazole (MIM) and imidazole-4-carbaldehyde (AldIM). The prepared Ald-ZIFs were post-synthetically modified with bisthiosemicarbazide (Bisthio) and thiosemicarbazide (Thio) groups, incorporating thiosemicarbazone (TSC) functionalities to the core of the framework. This modification results in the formation of TSC-functionalized ZIF derivatives (TSCZIFs). Thiosemicarbazones are versatile metal chelators, hence, adsorption properties of TSC-ZIFs for the removal of mercury(II) from water were explored. Removal of mercury(II) from homoionic aqueous solutions, binary and tertiary systems in competition with lead(II) and cadmium(II) under ambient conditions and neutral pH are reported in this study. MIM3.5:Thio1:Zn improved the removal efficiency of mercury(II) from water, up to 97% in two hours, with an adsorption capacity of 1667 mg g 1. Desorption of mercury(II) from MIM3.5:Thio1:Zn was achieved under acidic conditions, regenerating MIM3.5:Thio1:Zn for five cycles of mercury(II) removal. TSC-ZIF derivatives, designed and developed here, represent a new class of dynamically functionalized adsorption material displaying the advantages of simplicity, efficiency, and reusability. https://www.rsc.org/suppdata/d1/ra/d1ra02025k/d1ra02025k1.pdf?_ga=2.90571418.1846997019.1635238700-352143482.1630049716

We present two ligands containing a N-ethyl-4-(trifluoromethyl)benzenesulfonamide group attached to either a 6,6'-(azanediylbis(methylene))dipicolinic acid unit (H3DPASAm) or a 2,2'-(1,4,7-triazonane-1,4-diyl)diacetic acid macrocyclic platform (H3NO2ASAm). These ligands were designed to provide a pH-dependent relaxivity response upon complexation with Mn2+ in aqueous solution. The protonation constants of the ligands and the stability constants of the Mn2+ complexes were determined using potentiometric titrations complemented by spectrophotometric experiments. The deprotonations of the sulfonamide groups of the ligands are characterized by protonation constants of log KiH = 10.36 and 10.59 for DPASAm3- and HNO2ASAm2-, respectively. These values decrease dramatically to log KiH = 6.43 and 5.42 in the presence of Mn2+, because of the coordination of the negatively charged sulfonamide groups to the metal ion. The higher log KiH value in [Mn(DPASAm)]- is related to the formation of a seven-coordinate complex, while the metal ion in [Mn(NO2ASAm)]- is six-coordinated. The X-ray crystal structure of Na[Mn(DPASAm)(H2O)]·2H2O confirms the formation of a seven-coordinate complex, where the coordination environment is fulfilled by the donor atoms of the two picolinate groups, the amine N atom, the N atom of the sulfonamide group, and a coordinated water molecule. The lower conditional stability of the [Mn(NO2ASAm)]- complex and the lower protonation constant of the sulfonamide group results in complex dissociation at relatively high pH (

We report the synthesis and characterization of the macrocyclic ligand 2,2'-((2-(3,9-bis(carboxymethyl)-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-yl)ethyl)azanediyl)diacetic acid (H4L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-ray diffraction measurements. Two N atoms of the pyclen moiety in the trans position are protonated in the solid state, together with the exocyclic N atom and one of the carboxylate groups of the ligand. The relaxivity of the Gd3+ complex was found to increase from 6.7 mM-1 s-1 at pH 8.6 to 8.5 mM-1 s-1 below pH ≈ 6.0. Luminescence lifetime measurements recorded from H2O and D2O solutions of the Eu3+ complex evidence the presence of a single complex species in solution at low pH (∼5.0) that contains two inner-sphere water molecules. DFT calculations suggest that the coordination environment of the Ln3+ ion is fulfilled by the four N atoms of the pyclen unit, two oxygen atoms of the macrocyclic acetate groups, and an oxygen atom of an exocyclic carboxylate group. The two inner-sphere water molecules complete coordination number nine around the metal ion. At high pH (∼9.3), the lifetime of the excited 5D0 level of Eu3+ displays a biexponential behavior that can be attributed to the presence of two species in solution with hydration numbers of q = 0 and q = 1. The 1H NMR and DOSY spectra recorded from solutions of the Eu3+ and Y3+ complexes reveal a structural change triggered by pH and the formation of small aggregates at high pH values.

The enantiomers of carboline-based cryptates were successfully resolved by chiral HPLC. These complexes show high configurational stability under harsh conditions and their absolute configuration was determined by comparing theoretical and experimental electronic circular dichroism spectra. The enantiopure cryptates exhibit strong circularly polarized luminescence with a maximum dissymmetry factor glum = 0.25 for the f-f transition 5D0→7F1 (λ = 594 nm) under visible light excitation at λex = 400 nm.

International audience; We report a macrocyclic ligand (H) based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing three acetate pendant arms and a benzyl group attached to the fourth nitrogen atom of the macrocycle. The X-ray structures of the Y and Tb complexes reveal nine coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and three oxygen atoms of the carboxylate pendants. A combination of NMR spectroscopic studies (H, C, and Y) and DFT calculations indicated that the structure of the Y complex in the solid state is maintained in an aqueous solution. The detailed study of the emission spectra of the Eu and Tb complexes revealed Ln-centered emission with quantum yields of 7.0 and 60%, respectively. Emission lifetime measurements indicate that the ligand offers good protection of the metal ions from surrounding water molecules, preventing the coordination of water molecules. The Y complex is remarkably inert with respect to complex dissociation, with a lifetime of 1.7 h in 1 M HCl. On the other hand, complex formation is fast (∼1 min at pH 5.4, 2 × 10 M). Studies using the Y-nuclide confirmed fast radiolabeling since [Y]Y is nearly quantitatively formed (radiochemical yield (RCY) > 95) in a short time over a broad range of pH values from ca. 2.4 to 9.0. Challenging experiments in the presence of excess ethylenediaminetetraacetic acid (EDTA) and in human serum revealed good stability of the [Y]Y complex. All of these experiments combined suggest the potential application of H derivatives as Y-based radiopharmaceuticals.

We report a macrocyclic ligand (H

The stability constants of lanthanide complexes with the potentially octadentate ligand

Fluorine-19 MRI is increasingly being considered as a tool for biomolecular imaging, but the very poor sensitivity of this technique has limited most applications. Previous studies have long established that increasing the sensitivity of (19)F molecular probes requires increasing the number of fluorine nuclei per probe as well as decreasing their longitudinal relaxation time. The latter is easily achieved by positioning the fluorine atoms in close proximity to a paramagnetic metal ion such as a lanthanide(III). Increasing the number of fluorine atom per molecule, however, is only useful in as much as all the fluorine nuclei are chemically equivalent. Previous attempts to achieve this equivalency have focused on designing highly symmetric and rigid fluorinated macrocyclic ligands. A much simpler approach consists of exploiting highly fluxional lanthanide complexes with open coordination sites that have high affinity for phosphated and phosphonated species. Computational studies indicate that Ln(III)-TREN-MAM is highly fluxional, rapidly interconverting between at least six distinct isomers. In neutral water at room temperature, Ln(III)-TREN-MAM binds two or three equivalents of fluorinated phosphonates. The close proximity of the (19)F nuclei to the Ln(III) center in the ternary complex decreases the relaxation times of the fluorine nuclei up to 40-fold. Advantageously, the fluorophosphonate-bound lanthanide complex is also highly fluxional such that all (19)F nuclei are chemically equivalent and display a single (19)F signal with a small LIS. Dynamic averaging of fluxional fluorinated supramolecular assemblies thus produces effective (19)F MR systems.

We report the synthesis of the macrocyclic ligands 3,9-PC2AMH (2,2'-(3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-3,9-diyl)diacetamide) and 3,9-PC2AMtBu (2,2'-(3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-3,9-diyl)bis(N-tert-butyl)acetamide) which contain a pyclen platform functionalized with acetamide or tert-butylacetamide pendant arms at positions 3 and 9 of the macrocyclic unit. The corresponding Co(II) and Ni(II) complexes were prepared, isolated and characterised as potential paramagnetic chemical exchange saturation transfer (paraCEST) agents. The X-ray structures of the Ni(II) complexes reveal six-coordination of the ligands to the metal ion. The Co(II) complex with 3,9-PC2AMtBu shows a similar six-coordinate structure in the solid state, while the Co(II) complex with 3,9-PC2AMH contains a seven-coordinate metal ion, seventh coordination being completed by the presence of an inner-sphere water molecule. The structure of the Co(II) complexes was investigated using 1H NMR spectroscopy and computational methods. The complexes present a seven-coordinate structure in solution, as demonstrated by the analysis of the paramagnetic shifts using density functional theory. Ligand protonation constants and stability constants of the complexes with 3,9-PC2AMH were determined using potentiometric titrations (I = 0,15 M NaCl). The Co(II) complex was found to be more stable than the Ni(II) analogue (log KCoL = 14.46(5) and log KNiL = 13.15(3)). However, the Ni(II) and Co(II) complexes display similar rate constants characterizing the proton-assisted dissociation mechanism. The presence of highly shifted 1H NMR signals due to the amide protons in slow exchange with bulk water results in sizeable CEST signals, which are observed at +67 and +15 ppm for the Co(II) complex with 3,9-PC2AMH and +42 and +7 ppm for the Ni(II) analogue at 25 °C.

[Abstract] Several bifunctional chelators have been synthesized in the last years for the development of new 64Cu-based PET agents for in vivo imaging. When designing a metal-based PET probe, it is important to achieve high stability and kinetic inertness once the radioisotope is coordinated. Different competitive assays are commonly used to evaluate the possible dissociation mechanisms that may induce Cu(II) release in the body. Among them, acid-assisted dissociation tests or transchelation challenges employing EDTA or SOD are frequently used to evaluate both solution thermodynamics and the kinetic behavior of potential metal-based systems. Despite of this, the Cu(II)/Cu(I) bioreduction pathway that could be promoted by the presence of bioreductants still remains little explored. To fill this gap we present here a detailed spectroscopic study of the kinetic behavior of different macrocyclic Cu(II) complexes. The complexes investigated include the cross-bridge cyclam derivative [Cu(CB-TE1A)]+, whose structure was determined using single-crystal X-ray diffraction. The acid-assisted dissociation mechanism was investigated using HClO4 and HCl to analyse the effect of the counterion on the rate constants. The complexes were selected so that the effects of complex charge and coordination polyhedron could be assessed. Cyclic voltammetry experiments were conducted to investigate whether the reduction to Cu(I) falls within the window of common bioreducing agents. The most striking behavior concerns the [Cu(NO2Th)]2+ complex, a 1,4,7-triazacyclononane derivative containing two methylthiazolyl pendant arms. This complex is extremely inert with respect to dissociation following the acid-catalyzed mechanism, but dissociates rather quickly in the presence of a bioreductant like ascorbic acid. The authors thank Ministerio de Ciencia e Innovación (PID2019-104626GB-I00) and Xunta de Galicia (ED431B 2020/52) for generous financial support. R.U.-V. thanks Xunta de Galicia (Grant ED481A-2018/314) for funding her PhD contract. V. P. and R. T. acknowledge the Ministère de l'Enseignement Supérieur et de la Recherche and the Centre National de la Recherche Scientifique. L.V. thanks CACTI (Universidade de Vigo) for X-ray measurements Xunta de Galicia; ED431B 2020/52 Xunta de Galicia; ED481A-2018/314

International audience; Two homometallic ytterbium dimers are prepared and their solution-state photoluminescence and upconversion properties are investigated. Both complexes exhibit two-photon cooperative luminescence upconversion in the visible region (lambda(em) approximate to 510 nm) upon excitation into the near-infrared Yb F-2(5/2)

Typically, Ln(III) complexes are isostructural along the series, which enables studying one particular metal chelate to derive the structural features of the others. This is not the case for [Ln(AAZTA)(H2O)x]− (x = 1, 2) systems, where structural variations along the series cause changes in the hydration number of the different metal complexes, and in particular the loss of one of the two metal-coordinated water molecules between Ho and Er. Herein, we present a 1H field-cycling relaxometry and 17O NMR study that enables accessing the different exchange dynamics processes involving the two water molecules bound to the metal center in the [Gd(AAZTA)(H2O)2]− complex. The resulting picture shows one Gd-bound water molecule with an exchange rate ∼6 times faster than that of the other, due to a longer metal–water distance, in accordance with density functional theory (DFT) calculations. The substitution of the more labile water molecule with a fluoride anion in a diamagnetic-isostructural analogue of the Gd-complex, [Y(AAZTA)(H2O)2]−, allows us to follow the chemical exchange process by high-resolution NMR and to describe its thermodynamic behavior. Taken together, the variety of tools offered by NMR (including high-resolution 1H, 19F NMR as a function of temperature, 1H longitudinal relaxation rates vs B0, and 17O transverse relaxation rates vs T) provides a complete description of the structure and exchange dynamics of these Ln-complexes along the series., Herein, we present a 1H field−cycling relaxometry and 17O NMR study that enables accessing the different exchange dynamics processes involving the two water molecules bound to the metal center in the [Gd(AAZTA)(H2O)2]− complex.

Upconversion nanoparticles have led to various breakthrough applications in solar energy conversion, imaging, and biomedicine. One key impediment is the facilitation of such processes at the molecular scale in solution where quenching effects are much more pronounced. In this work, molecular solution-state cooperative luminescence (CL) upconversion arising from a Yb excited state is explored and the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/ Tb systems is investigated. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with a quantum yield of 2.8 × 10-6 at a low laser power density of 2.86 W/cm2.

Rigid derivatives of the acyclic ligand PDTA4- (H

Upconversion nanoparticles have led to various breakthrough applications in solar energy conversion, imaging, and biomedicine. One key impediment is the facilitation of such processes at the molecular scale in solution where quenching effects are much more pronounced. In this work we explore, for the first time, molecular solution-state cooperative luminescence (CL) upconversion arising from a Yb excited state and investigate the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/ Tb systems. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with an unprecedented quantum yield of 2.8 × 10-6 at a low laser power density of 2.86 W/cm2.

International audience; Two calix[4]arene systems, C234+ and C244+, where 2 corresponds to the number of viologen units and 3-4 corresponds to the number of carbon atoms connecting the viologen units to the macrocyclic core, have been synthesized and led to the formation of [3]pseudorotaxanes when combined with either CB[7] or CB[8]. The [3]pseudorotaxanes spontaneously dissociate upon reduction of the bipyridinium units as the result of intramolecular dimerization of the two face-to-face viologen radical cations. CB[7] and CB[8]-based [2]pseudorotaxanes containing monomeric viologen guest models compounds, MC32+ and MC4+, do not undergo decomplexation and dimerization following electrochemical reduction of their bipyridinium units.

International audience; Herein, we report a comprehensive coordination study of the previously reported ligands cyclam, CB-cyclam, TMC, DMC, and CB-DMC and of their C-functional analogues, cyclam-E, CB-cyclam-E, TMC-E, DMC-E, and CB-DMC-E. This group of ligands includes cyclam, cross-bridged cyclams, their di- or tetramethylated derivatives, and the analogues bearing an additional hydroxyethyl group on one β-N position of the ring. The Cu(II) and Zn(II) complexes of these macrocycles have been highlighted previously for the biological interest, but the details of their structures in the solid state and in solution remained largely unexplored. In particular, we analyzed the impact that adding noncoordinating N-methyl and C-hydroxyethyl functionalities has in the structures of the complexes. All the Cu(II) and Zn(II) complexes were synthesized and investigated using single crystal X-ray diffraction and NMR, electronic absorption, and EPR spectroscopies, along with DFT studies. Dissociation kinetics experiments in acidic conditions and electrochemical studies were also performed. Special attention was paid to analyze the different configurations present in solution and in the solid state, as well as the impact of the C-appended hydroxyethyl group on the coordination behavior. Various ratios of the trans-I, trans-III, and cis-V configurations have been observed depending on the degree of N-methylation and the presence of the ethylene cross-bridge.

The nature of the axial ligand coordinated to the Yb3+ ion in [Yb(DOTAM)]3+ has profound consequences in the magnetic anisotropy and optical properties of the complex, as evidenced by 1H NMR and UV-vis spectroscopies. The pseudocontact shifts of 1H nuclei and the 2F5/2 ← 2F7/2 absorption band were found to be very sensitive to the nature of the axial ligand (MeOH, H2O, MeOH, or F-). The energy levels of the 2F5/2 and 2F7/2 manifolds in [Yb(DOTAM)(X)]3+ (X = MeOH, H2O, or dimethyl sulfoxide (DMSO)) and [Yb(DOTAM)F]2+ complexes were assigned from the analysis of the optical spectra and ab initio calculations based on CASSCF wave functions that considered dynamic correlation through perturbation theory (NEVPT2) and spin-orbit coupling effects. The magnetic anisotropies obtained with ab initio calculations are in good agreement with the experimental values derived from 1H NMR spectral data, though for the [Yb(DOTAM)(H2O)]3+ and [Yb(DOTAM)F]2+ complexes, the explicit inclusion of a few second-sphere water molecules is required to improve the calculated data. Crystal-field calculations show that the observed pseudocontact shifts do not correlate well with the crystal-field parameter B20, as predicted by Bleaney's theory. The change in the sign of the magnetic anisotropy from prolate (X = MeOH, H2O, or DMSO) to oblate in [Yb(DOTAM)F]2+ is related to the relative energies of the 4fz3 orbital and the 4fx3/4fy3 pair, which are affected by the coordination ability of the axial ligand.

We report a detailed structural study of [Ln(AAZTA)]− complexes by using a combination of experimental and theoretical tools. The analysis of the 1H NMR paramagnetic shift of the methyl peak across the series suggests that a structural change occurs between Ho and Er. Chemical exchange saturation transfer experiments were subsequently used to determine the number of coordinated water molecules and their corresponding exchange rates kex at 278 K. The Z-spectra recorded for the Ho(III) complex present two signals that confirm the presence of two coordinated water molecules, which are endowed with rather different exchange rates: 5.8(±3.0) × 103 and 8.1(±0.5) × 104 s−1. On the contrary, the Er(III) and Tm(III) complexes present a single signal in the Z-spectra. The exchange rate of the coordinated water molecule(s) decreases markedly across the series from Gd(III) to Yb(III). DFT calculations support the change in hydration number by the end of the lanthanide series, which is the result of an increased steric compression around one of the coordinated water molecules on decreasing the size of the metal ion. X-ray diffraction studies on the Er(III) complex confirm the presence of a single inner-sphere water molecule and the dodecahedral coordination environment of the metal ion.

Mn(II) has several favorable physicochemical characteristics and a good toxicity profile, which makes it a viable alternative to the Gd(III)-based MRI contrast agents currently used in clinics. Although many studies have been undertaken in the last 10 years, this is a field of investigation still in rapid and continuous development. This review aims to critically discuss the chemical and magnetic properties of Mn(II) compounds relevant as MRI probes, both small complexes and nanosystems containing a large number of metal centers, the possible approaches for optimizing their efficiency by understanding the role of various molecular parameters that control the relaxation processes, and the most important issues related to stability and kinetic inertness.

We present a detailed study of the lanthanide(III) complexes with cyclen-based ligands containing phenylacetamide pendants that incorporate CF3 group(s) at different distances from the metal ion. The complexes exhibit square-antiprismatic coordination in solution, as demonstrated by analysis of the Yb3+-induced paramagnetic shifts and the X-ray structure of the [YbL3] complex. Luminescence lifetime measurements and a detailed 1H and 17O relaxometric characterization confirmed the presence of an inner-sphere H2O molecule. The Tm3+ complexes provide chemical-exchange saturation-transfer response upon saturation at the frequency of the amide protons. A 19F relaxation study provided accurate estimates of the Ln···F distances that were used to rationalize the efficiency of the complexes as 19F magnetic resonance imaging (MRI) probes, which was tested in vitro using MRI phantom studies.

The absorption and emission spectra of three Yb3+ complexes possessing D3, D2, and C2 symmetries were analyzed with the aid of ab initio calculations based on Complete Active Space (CAS) self-consistent field wave functions (CAS(13,7)). The absorption spectra present contributions from both cold and hot bands, involving thermally populated excited sublevels of the 2F7/2 manifold. The high-resolution emission spectrum of the tris-picolinate complex [Yb(DPA)3]3- recorded at 77 K presents four components, while the complexes with macrocyclic ligands show both cold and hot emission bands, resulting in more than four components for the 2F5/2 → 2F7/2 transition. The combined information provided by the absorption and emission spectra allowed to identify most of the crystal field sublevels of the 2F5/2 and 2F7/2 states. The energies of these crystal field components are well-reproduced by the ab initio calculations, with deviations typically lower than 100 cm-1. The crystal field splitting is very sensitive to subtle changes of the Yb3+ coordination environment. The magnetic anisotropy of [Yb(DPA)3]3- obtained with ab initio calculations was found to be extremely sensitive to changes in the twist angle of the upper and lower faces of the tricapped trigonal prismatic coordination polyhedron. Ab initio ligand field theory provides a straightforward chemical justification for the changes in magnetic anisotropy, which are responsible for the observed pseudocontact shifts in the NMR spectra.

We present two novel octadentate cyclen-based ligands bearing one (L1 ) or two (L2 ) phenylacetamide pendants with two CF3 groups either at positions 3 and 5 (L1 ) or 4 (L2 ). The corresponding Gd3+ complexes possess one coordinated water molecule, as confirmed by luminescence lifetime measurements on the EuIII and TbIII analogues. A detailed 1 H and 17 O relaxometric characterization has revealed the parameters that govern the relaxivities of these complexes. The water-exchange rate of the mono-amide derivative GdL1 (kex298 =1.52×106 s-1 ) is faster than that determined for the bis-amide complex GdL2 (kex298 =0.73×106 s-1 ). 1 H and 19 F NMR studies have indicated that the complexes are present in solution almost exclusively as the square-antiprismatic (SAP) isomers. 19 F NMR relaxation studies indicated Gd⋅⋅⋅F distances of 7.4±0.1 and 9.1±0.1 A for GdL1 and GdL2 , respectively. Phantom MRI studies revealed the favorable properties of GdL2 as a dual 1 H/19 F magnetic resonance imaging (MRI) probe, whereas the shorter Gd⋅⋅⋅F distance of GdL1 reduces the signal-to-noise ratio due to the very short transverse relaxation time of the 19 F NMR signal.

Two macrocyclic triamide octadentate chelator scaffolds L1 and L2 were synthesized, characterized by several spectroscopic techniques, and their pKa values were determined by potentiometric titration. Using these ligands, two water soluble paramagnetic Eu(III) complexes, [EuL1(OH2)](NO3)3·H2O (EuL1) and [EuL2(OH2)](NO3)3·H2O (EuL2) were synthesized and characterized in the solid state and in solution. Single crystal X ray diffraction measurement of [EuL1(OH2)](NO3)3.H2O (EuL1) reveals octadentate binding of the ligand to Eu(III) and ninth coordination being completed by an oxygen atom of a solvent molecule (H2O). X-ray diffraction data of [EuL2(OH2)](NO3)3·H2O (EuL2) were severely disordered and hence its chloride analogue [EuL2(DMF)]Cl3·H2OMeOH (EuL2-Cl) was synthesized and characterized using single-crystal X-ray diffraction measurements. The crystal data of [EuL2(DMF)](Cl)3·H2OMeOH (EuL2-Cl) reveal octadentate binding of the ligand to Eu(III), with the ninth coordination being completed by an oxygen atom of a solvent molecule (DMF). Luminescence measurements confirm the presence of a water molecule coordinated to Eu(III) in aqueous solution. The stability of the Eu(III) complexes was investigated using spectrophotometric molar ratio method. Cyclic voltammetry studies obtained from aqueous solutions of the Eu(III) complexes show reversible one electron reduction processes at the glassy carbon electrode with E1/2 = −0.799 V and −0.777 V (versus Ag/AgCl) for the complexes of L1 and L2. © 2018 Elsevier B.V.

Herein, we report a comprehensive coordination study of the previously reported ligands

Herein, we report a comprehensive coordination study of the previously reported ligands cyclam, CB-cyclam TMC, DMC, CB-DMC, and of their C-functional analogues, cyclam-E, CB-cyclam-E, TMC-E, DMC-E and CB-DMC-E. This group of ligands includes cyclam, cross-bridged cyclams, their di- or tetramethylated derivatives and the analogues bearing an additional hydroxyethyl group on one β-N position of the ring. These Cu(II) and Zn(II) complexes of these macrocycles have been highlighted previously for the biological interest, but the details of their structures in the solid state and in solution remained largely unexplored.

We report a series of structurally related Gd(III) complexes designed to modulate the exchange of the coordinated water molecule, which is an important parameter to be controlled to achieve optimal performance of contrast agents for application in magnetic resonance imaging (MRI). The ligands contain a DO3A scafold functionalised with 2′-methoxyphenacyl or 4′-methoxyphenacyl groups (DO3A-oMAP and DO3A-pMAP), a 2′-aminophenacyl group (DO3A-oAnAP) or a 2′,4′-dihydroxyphenacyl moiety (DO3A-DiHAP). The results are compared with those obtained previously for the analogues containing 2′- or 4′-hydroxyphenacyl groups (DO3A-oHAP and DO3A-pHAP, respectively) and the parent system with an unsubstituted acetophenone pendant arm (DO3A-AP). 1H NMR studies performed on the Eu(III) complexes show that ligand functionalisation causes a very minor effect on the relative populations of the SAP and TSAP isomers present in solution, with the SAP isomer representing 70–80% of the overall population. The emission spectra of the Eu(III) complexes confirm the presence of a water molecule coordinated to the metal center and point to similar coordination environments around the metal ion. The analysis of the 1H NMRD profiles and 17O NMR data recorded for the Gd(III) complexes evidences that water exchange is modulated by the ability of peripherical substituents to establish hydrogen bonds with the coordinated and/or second sphere water molecules. DFT calculations were used to model the transition states responsible for the dissociative water exchange mechanism, providing support to the crucial role of hydrogen-bonds in accelerating water exchange.

The wide range of optical and magnetic properties of the lanthanide(III) ions is associated to their intricate electronic structures, which in contrast to lighter elements is characterized by strong relativistic effects and spin-orbit coupling. Nevertheless, computational methods are now capable of describing the ladder of electronic energy levels of the simpler trivalent lanthanide ions, as well as the lowest energy term of most of the series. The electronic energy levels result from electron configurations that are first split by spin-orbit coupling into groups of energy levels denoted by the corresponding Russel-Saunders terms. Each of these groups are then split by the ligand field into the actual electronic energy levels known as microstates or sometimes mJ levels. The ligand field splitting directly informs on coordination geometry, and is a valuable tool for determining structure and thus correlating the structure and properties of metal complexes in solution. The issue with lanthanide complexes is that the determination of complex structures from ligand field splitting remains a very challenging task. In this manuscript, the optical spectra – absorption, luminescence excitation and luminescence emission – of ytterbium(III) solvates were rec-orded in water, methanol, dimethyl sulfoxide and N,N-dimethylformamide. The electronic energy levels, that is the microstates, were resolved experimentally. Subsequently, density functional theory (DFT) calculations were used to model the structures of the solvates and ab initio relativistic complete active space self-consistent field (CASSCF) calculations were employed to obtain the microstates of the possible structures of each solvate. By comparing experimental and theoretical data, it was possible to determine both the coordination number and solution structure of each solvate. In water, methanol and N,N-dimethylformamide the solvates were found to be eight-coordinated and to have a square anti-prismatic coordination geometry. In DMSO the speciation was found to be more complicated. The robust methodology developed for comparing experimental spectra and computational results allows the solution structures of lanthanide complexes to be determined, paving the way for the design of complexes with predetermined properties.