Mononuclear and Binuclear Lanthanide Complexes of the Macrocyclic Polyamine Polycarboxylic Ligands H&lt;sub&gt;8&lt;/sub&gt;OTEC and H&lt;sub&gt;8&lt;/sub&gt;OHEC

Schumann, Herbert; Böttger, U.; Zschunke, A.; Weißhoff, Hardy; Ziemer, Burkhard

doi:10.4028/www.scientific.net/msf.315-317.128

Cited by 3 publications

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“…Unfortunately, this chelate is not typical of the polyaminocarboxylate Gd III chelates that are currently used as contrast agents. A better model, namely the binuclear Gd III chelate of the OHEC ligand (OHEC = octaazacyclohexacosane-1,4,7,10,14,17,20,23-octaacetate), [15,16] which is depicted in Figure 1, was investigated in the present study. In this binuclear complex the paramagnetic ions are about 6.5 ä apart.…”

Section: Introductionmentioning

confidence: 99%

Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the Ln^III Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Nicolle¹,

Yerly²,

Böttger³

et al. 2003

Chemistry A European J

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We report the study of binuclear Ln(III) chelates of OHEC (OHEC=octaazacyclohexacosane-1,4,7,10,14,17,20,23-octaacetate). The interconversion between two isomeric forms, which occurs in aqueous solution, has been studied by NMR, UV/Vis, EPR, and luminescence spectroscopy, as well as by classical molecular dynamics (MD) simulations. For the first time we have characterized an isomerization equilibrium for a Ln(III) polyaminocarboxylate complex (Ln(III)=Y, Eu, Gd and Tb) in which the metal centre changes its coordination number from nine to eight, such that: [Ln(2)(ohec)(H(2)O)(2)](2-) r<==>[Ln(2)(ohec)](2-)+2 H(2)O. The variable temperature and pressure NMR measurements conducted on this isomerization reaction give the following thermodynamic parameters for Eu(III): K(298)=0.42+/-0.01, DeltaH(0)=+4.0+/-0.2 kJ mol(-1), DeltaS(0)=+6.1+/-0.5 J K(-1) mol(-1) and DeltaV(0)=+3.2+/-0.2 cm(3) mol(-1). The isomerization is slow and the corresponding kinetic parameters obtained by NMR spectroscopy are: k(298)(is)=73.0+/-0.5 s(-1), DeltaH++(is)=75.3+/-1.9 kJ mol(-1), DeltaS++(is)= +43.1+/-5.8 J K(-1) mol(-1) and DeltaV++(is)=+7.9+/-0.7 cm(3) mol(-1). Variable temperature and pressure (17)O NMR studies have shown that water exchange in [Gd(2)(ohec)(H(2)O)(2)](2-) is slow, k(298)(ex)=(0.40+/-0.02)x10(6) s(-1), and that it proceeds through a dissociative interchange I(d) mechanism, DeltaV( not equal )=+7.3+/-0.3 cm(3) mol(-1). The anisotropy of this oblong binuclear complex has been highlighted by MD simulation calculations of different rotational correlation times. The rotational correlation time directed on the Gd-Gd axis is 24 % longer than those based on the axes orthogonal to the Gd-Gd axis. The relaxivity of this binuclear complex has been found to be low, since 1) only [Gd(2)(ohec)(H(2)O)(2)](2-), which constitutes 70 % of the binuclear complex, contributes to the inner-sphere relaxivity and 2) the anisotropy of the complex prevents water molecules from having complete access to both Gd(III) cages; this decreases the outer-sphere relaxivity. Moreover, EPR measurements for the Gd(III) and for the mixed Gd(III)/Y(III) binuclear complexes have clearly shown that the two Gd(III) centres interact intramolecularly; this enhances the electronic relaxation of the Gd(III) electron spins.

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Section: Introductionmentioning

confidence: 99%

Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the Ln^III Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Nicolle¹,

Yerly²,

Böttger³

et al. 2003

Chemistry A European J

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“…In the OHEC complexes with the chair shape the central atoms as well as the inner-sphere water molecules lie alternately below and above a mean ring plane of the macrocycle, formed by the four propylene bridged nitrogen atoms. [1,3] No such mean plane is found for 1 and 2. Here, the metal ions both sit on the same side, within the boat conformation of the macrocyclic ring.…”

Section: Conformation Of the Macrocycle In Lanthanide Ohec Complexesmentioning

confidence: 70%

“…The fivefold repetition of anti torsion angles in the propylene chains allows for a maximal (27) Ϫ168.67 (35) separation of the two lanthanides, thereby minimizing electrostatic repulsion. While the centrosymmetric conformation of the macrocyclic ring of the lighter lanthanide() OHEC complexes [3] as well as for the corresponding OHEC-amine ligand and its binuclear copper() complex [Cu 2 (OHEC-amine)](SO 4 ) 2 [1] corresponds to the chair shape, for 1 and 2 the ring adopts the boat conformation with a typical pseudo-C 2 symmetry axis. In the OHEC complexes with the chair shape the central atoms as well as the inner-sphere water molecules lie alternately below and above a mean ring plane of the macrocycle, formed by the four propylene bridged nitrogen atoms.…”

Section: Conformation Of the Macrocycle In Lanthanide Ohec Complexesmentioning

confidence: 99%

“…In addition, in aqueous solution the complexes are present in two isomeric forms. [3,4] The doubled NMR signal number of the second isomer obviously reveals an asymmetrical molecule, the geometry of which could not be deduced from the NMR spectroscopic data because of strongly overlapping signals with those of the main isomer.…”

Section: Introductionmentioning

confidence: 97%

“…This model may also explain the intramolecular rearrangements necessary during water exchange in the inner coordination sphere of the complex [ europium(), and lanthanum() with H 8 OHEC have been discussed, as well as the solution structure and dynamics of its diamagnetic Y III complex. [2,3] These complexes are almost isostructural in that they all crystallise in the monoclinic space group P2 1 /n, and the conformation of the OHEC macrocycle as well as the coordination geometry around the metal ions is nearly the same. NMR studies on the yttrium complex revealed that the same centrosymmetric structure is maintained in aqueous solution as is found in the solid state.…”

Section: Introductionmentioning

confidence: 98%

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Structure Comparison of Early and Late Lanthanide(III) Homodinuclear Macrocyclic Complexes with the Polyamine Polycarboxylic Ligand H₈OHEC

et al. 2004

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The solid‐state structures of two new homodinuclear chelate complexes with the late lanthanide(III) ions Yb and Lu, [Na2(Yb2OHEC)].14.5H2O (1), and [Na2(Lu2OHEC)].14.5H2O (2) (H8OHEC = 1,4,7,10,14,17,20,23‐octaazacyclohexacosane‐ 1,4,7,10,14,17,20,23‐octaacetic acid), have been determined by X‐ray crystal structure analysis. Each lanthanide(III) ion is coordinated by eight donor atoms of the ligand and the geometry of the coordination polyhedron approaches a bicapped trigonal prism. These structures are compared with those of the homodinuclear chelate complexes with the same ligand and the mid to early lanthanide(III) ions Gd, Eu, La and also Y. A distinctive structural change occurs across the lanthanide series. The centrosymmetric mid to early lanthanide(III) complexes are all ninefold‐coordinated in a capped square antiprismatic arrangement with a water molecule coordinated in a prismatic position. This structure is maintained in aqueous solution, together with an asymmetric minor isomer. The late lanthanide(III) OHEC complexes not only lack the inner‐sphere water, but the change of coordination sphere also results in a loss of symmetry of the whole complex molecule. The observed change of coordination mode and number of the lanthanide ion may offer a geometric model for the isomerization process in eight‐ and ninefold‐coordinated complex species that are isomers in a possible coordination equilibrium observed by NMR in aqueous solution. This model may also explain the intramolecular rearrangements necessary during water exchange in the inner coordination sphere of the complex [(Gd2OHEC)(H2O)2]2− through a slow dissociative mechanism. Protonation constants of the H8OHEC ligand and complex formation constants of this ligand with GdIII, CaII, CuII and ZnII have been determined by solution thermodynamic studies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

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⁸S paramagnetic centres in molecular assemblies: possible effect of their proximity on the water proton relaxivity

Nicolle

Helm

Merbach

2003

Magnetic Reson in Chemistry

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Mononuclear and Binuclear Lanthanide Complexes of the Macrocyclic Polyamine Polycarboxylic Ligands H<sub>8</sub>OTEC and H<sub>8</sub>OHEC

Cited by 3 publications

References 0 publications

Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the Ln^III Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the Ln^III Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Structure Comparison of Early and Late Lanthanide(III) Homodinuclear Macrocyclic Complexes with the Polyamine Polycarboxylic Ligand H₈OHEC

⁸S paramagnetic centres in molecular assemblies: possible effect of their proximity on the water proton relaxivity

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Mononuclear and Binuclear Lanthanide Complexes of the Macrocyclic Polyamine Polycarboxylic Ligands H<sub>8</sub>OTEC and H<sub>8</sub>OHEC

Cited by 3 publications

References 0 publications

Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the LnIII Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the LnIII Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Structure Comparison of Early and Late Lanthanide(III) Homodinuclear Macrocyclic Complexes with the Polyamine Polycarboxylic Ligand H8OHEC

8S paramagnetic centres in molecular assemblies: possible effect of their proximity on the water proton relaxivity

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Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the Ln^III Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Towards Binuclear Polyaminocarboxylate MRI Contrast Agents? Spectroscopic and MD Study of the Peculiar Aqueous Behavior of the Ln^III Chelates of OHEC (Ln=Eu, Gd, and Tb): Implications for Relaxivity

Structure Comparison of Early and Late Lanthanide(III) Homodinuclear Macrocyclic Complexes with the Polyamine Polycarboxylic Ligand H₈OHEC

⁸S paramagnetic centres in molecular assemblies: possible effect of their proximity on the water proton relaxivity