Fine-tuning water exchange on Gd(III) poly(amino carboxylates) by modulation of steric crowding

In the objective of optimizing water exchange rate on stable, nine-coordinate, monohydrated Gd(III) poly(amino carboxylate) complexes, we have prepared monopropionate derivatives of DOTA4- (DO3A-Nprop4-) and DTPA5- (DTTA-Nprop5-). A novel ligand, EPTPA-BAA(3-), the bisamylamide derivative of ethylenepropylenetriamine-pentaacetate (EPTPA5-) was also synthesized. A variable temperature 17O NMR study has been performed on their Gd(III) complexes, which, for [Gd(DTTA-Nprop)(H2O)]2- and [Gd(EPTPA-BAA)(H2O)] has been combined with multiple field EPR and NMRD measurements. The water exchange rates, k(ex)(298), are 8.0 x 10(7) s(-1), 6.1 x 10(7) s(-1) and 5.7 x 10(7) s(-1) for [Gd(DTTA-Nprop)(H2O)]2-, [Gd(DO3A-Nprop)(H2O)]- and [Gd(EPTPA-BAA)(H2O)], respectively, all in the narrow optimal range to attain maximum proton relaxivities, provided the other parameters (electronic relaxation and rotation) are also optimized. The substitution of an acetate with a propionate arm in DTPA5- or DOTA4- induces increased steric compression around the water binding site and thus leads to an accelerated water exchange on the Gd(III) complex. The k(ex) values on the propionate complexes are, however, lower than those obtained for [Gd(EPTPA)(H2O)]2- and [Gd(TRITA)(H2O)]- which contain one additional CH(2) unit in the amine backbone as compared to the parent [Gd(DTPA)(H2O)]2- and [Gd(DOTA)(H2O)]-. In addition to their optimal water exchange rate, [Gd(DTTA-Nprop)(H2O)]2- has, and [Gd(DO3A-Nprop)(H2O)]- is expected to have sufficient thermodynamic stability. These properties together make them prime candidates for the development of high relaxivity, macromolecular MRI contrast agents.     

Characterization of a soluble molecular magnet: unusual magnetic behavior of cyano-bridged Gd(III)-Cr(III) complexes with one-dimensional and nanoscaled square structures

Two cyano-bridged Gd(III)-Cr(III) complexes [Gd(urea)(4)(H(2)O)(2)](2)[Cr(CN)(6)](2) (1) and ([Gd(capro)(2)(H(2)O)(4)Cr(CN)(6)].H(2)O)(n)(2) (capro represents caprolactam) have been synthesized and characterized structurally and magnetically. Complex 1 has a tetranuclear Gd(2)Cr(2) square structure, in which two cis-CN(-) ligands of each [Cr(CN)(6)] link two [Gd(urea)(4)(H(2)O)(2)] groups and in turn, two [Gd(urea)(4)(H(2)O)(2)] link two [Cr(CN)(6)] in a cis fashion. Complex 2 is composed of 1D chains with alternating [Gd(capro)(2)(H(2)O)(4)] and [Cr(CN)(6)] moieties connected by the trans-CN(-) ligands of [Cr(CN)(6)]. The dehydration of 2 at 120 degrees C generates a new complex, [Gd(capro)(2)(H(2)O)(2)Cr(CN)(6)] (2'). Magnetic studies show the existence of antiferromagnetic Gd(III)-Cr(III) interaction in these complexes. On the basis of the tetranuclear model, the magnetic susceptibilities of 1 have been analyzed giving the intermetallic magnetic coupling constant of -0.36 cm(-1). Complex 2' exhibits a ferrimagnetic order below 2.1 K. Interestingly, 2' is quite soluble in water, and slow evaporation of the solution gives the hydrated complex 2. Therefore, 2' is a soluble molecular magnet, and this significant behavior implies potential applications. Isothermal magnetization measurements of 2' and other cyano-bridged Gd(III)-Cr(III) molecular magnets show unusual field-induced metamagnetic behavior from the ferrimagnetic ground state to the ferromagnetic state. Field dependence of magnetization of the cyano-bridged Gd(III)-Cr(III) complexes shows unusual field-induced metamagnetic behavior from the ferrimagnetic ground state to the ferromagnetic state.     

Molecular architectures for trimetallic d/f/d complexes: magnetic studies of a LnCu2 core

Five new trinuclear Cu-Ln-Cu cluster complexes have been prepared by a one-pot reaction using H3bcn (tris- N,N',N'-(2-hydroxybenzyl)-1,4,7-triazacyclononane) and Ln = La(III), Nd(III), Gd(III), Dy(III), and Yb(III) where the d- and f-block metal ions are in close proximity desirable for magnetic studies. The [LnCu2(bcn)2]ClO4.nH2O complexes possess the same stoichiometry as the previously reported [LnNi2(bcn)2]ClO4.nH2O and [LnZn2(bcn)2]ClO4.nH2O systems. Additionally, the solid state structures of the LnCu2 complexes appear to be isostructural to the LnNi2 and LnZn2 species as determined by their nearly superimposable IR spectra. The similarities in the structures of the [LnTM2(bcn)2]ClO4.nH2O series, where TM = Zn(II), Ni(II), and Cu(II), allow for direct comparison of their magnetic exchange. An empirical approach, removing first-order anisotropic contributions determined from the analogous [LnZn2(bcn)2]ClO4.nH2O was used to study the d/f/d spin interactions in the [LnCu2(bcn)2]ClO4.nH2O complexes. A ferromagnetic exchange was determined where Ln = Gd(III), Dy(III), or Yb(III) and an antiferromagnetic exchange for Ln = Nd(III), identical to the magnetic exchange observed for the [LnNi2(bcn)2]ClO4.nH2O complexes. An exchange integral of 3.67 cm(-1) for the trimetallic [GdCu2(bcn)2]ClO4.3H2O species was determined using a modified spin Hamiltonian. The [Cu(Hbcn)] and the [Cu3(Hbcn)2](ClO4)2 building blocks of the larger coaggregated d/f/d species were also synthesized, and their structures are reported.     

Gadolinium(III) complexes of mono- and diethyl esters of monophosphonic acid analogue of DOTA as potential MRI contrast agents: solution structures and relaxometric studies

Two new macrocyclic DOTA-like chelates containing one phosphonate pendant arm were synthesised as potential contrast agents for MRI (magnetic resonance imaging). The chelates bind to the lanthanide(III) in an octadentate manner, via four nitrogen atoms, three carboxylate and one phosphonate oxygen atoms. Solution structures of [Ln(do3ap(OEt2))(H(2)O)] and [Ln(do3ap(OEt))(H(2)O)](-) were studied using (31)P and (1)H NMR spectroscopy and SAP (square-antiprismatic)/TSAP (twisted square-antiprismatic) isomerism was observed. Depending on the nature of the lanthanide(III) ion, the lanthanide(III) complexes of H(4)do3ap(OEt) are present in solution as up to four different diastereoisomers observable with NMR. The TSAP isomer is the most abundant at the beginning of the lanthanide series and, with a decrease of the ionic radius of lanthanide(III) ions, both TSAP and SAP forms were observed. A second interconversion (SAP<-->TSAP') becomes important at the end of the series (TSAP' means the TSAP species without a coordinated water molecule). The remaining axial coordination site is occupied by one water molecule for the Gd(3+)-complex. The calculated fraction of the TSAP isomer in the gadolinium(III) complexes increases in the order [Gd(DOTA)(H(2)O)](-) < [Gd(do3ap(OEt2))(H(2)O)] < [Gd(do3ap(OEt))(H(2)O)](-) < [Gd(do3ap)(H(2)O)](2-). Gadolinium(III) complexes of phosphorus-containing chelates, generally, have the advantage of a relatively fast water exchange rate due to a greater sterical demand of the phosphorus acid moiety and of the presence of the second-sphere water shell, which also contributes to the overall relaxivity. The [Gd(do3ap(OEt2))(H(2)O)] and [Gd(do3ap(OEt))(H(2)O)](-) complexes were studied by variable-temperature (17)O NMR and (1)H NMRD. The experimental data were evaluated simultaneously with commonly used equations based on Solomon-Bloembergen-Morgan approximation, extended by a contribution of the second coordination sphere. The water exchange rates were found to be strongly dependent on the TSAP/SAP isomeric ratio and the overall charge of the complex: the monoanionic [Gd(do3ap(OEt))(H(2)O)](-) complex with TSAP molar fraction equal to 0.36 has the water exchange rate of 20 x 10(6) s(-1) (tau(M) = 50 ns) while neutral [Gd(do3ap(OEt2))(H(2)O)] complex with TSAP molar fraction 0.28 has an exchange rate equal to 4.4 x 10(6) s(-1) (tau(M) = 227 ns).     

Family of carboxylate- and nitrate-diphenoxo triply bridged dinuclear Ni(II)Ln(III) complexes (Ln = Eu, Gd, Tb, Ho, Er, Y): synthesis, experimental and theoretical magneto-structural studies, and single-molecule magnet behavior

Seven acetate-diphenoxo triply bridged M(II)-Ln(III) complexes (M(II) = Ni(II) and Ln(III) = Gd, Tb, Ho, Er, and Y; M(II) = Zn(II) and Ln(III) = Ho(III) and Er(III)) of formula [M(μ-L)(μ-OAc)Ln(NO(3))(2)], one nitrate-diphenoxo triply bridged Ni(II)-Tb(III) complex, [Ni(μ-L)(μ-NO(3))Tb(NO(3))(2)]·2CH(3)OH, and two diphenoxo doubly bridged Ni(II)-Ln(III) complexes (Ln(III) = Eu, Gd) of formula [Ni(H(2)O)(μ-L)Ln(NO(3))(3)]·2CH(3)OH have been prepared in one pot reaction from the compartmental ligand N,N',N"-trimethyl-N,N"-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H(2)L). Moreover, Ni(II)-Ln(III) complexes bearing benzoate or 9-anthracenecarboxylate bridging groups of formula [Ni(μ-L)(μ-BzO)Dy(NO(3))(2)] and [Ni(μ-L)(μ-9-An)Dy(9-An)(NO(3))(2)]·3CH(3)CN have also been successfully synthesized. In acetate-diphenoxo triply bridged complexes, the acetate bridging group forces the structure to be folded with an average hinge angle in the M(μ-O(2))Ln bridging fragment of ~22°, whereas nitrate-diphenoxo doubly bridged complexes and diphenoxo-doubly bridged complexes exhibit more planar structures with hinge angles of ~13° and ~2°, respectively. All Ni(II)-Ln(III) complexes exhibit ferromagnetic interactions between Ni(II) and Ln(III) ions and, in the case of the Gd(III) complexes, the J(NiGd) coupling increases weakly but significantly with the planarity of the M-(O)(2)-Gd bridging fragment and with the increase of the Ni-O-Gd angle. Density functional theory (DFT) theoretical calculations on the Ni(II)Gd(III) complexes and model compounds support these magneto-structural correlations as well as the experimental J(NiGd) values, which were found to be ~1.38 and ~2.1 cm(-1) for the folded [Ni(μ-L)(μ-OAc)Gd(NO(3))(2)] and planar [Ni(H(2)O)(μ-L)Gd(NO(3))(3)]·2CH(3)OH complexes, respectively. The Ni(II)Dy(III) complexes exhibit slow relaxation of the magnetization with Δ/k(B) energy barriers under 1000 Oe applied magnetic fields of 9.2 and 10.1 K for [Ni(μ-L)(μ-BzO)Dy(NO(3))(2)] and [Ni(μ-L)(μ-9-An)Dy(9-An)(NO(3))(2)]·3CH(3)CN, respectively.     

Lanthanide complexes of new nonadentate imino-phosphonate ligands derived from 1,4,7-triazacyclononane: synthesis, structural characterisation and NMR studies

The polyamino ligand 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane (1) has been used to synthesise two new ligands by Schiff-base condensation with methyl sodium acetyl phosphonate to give ligand L and methyl sodium 4-methoxybenzoyl phosphonate to give ligand L1 in the presence of lanthanide ion as templating agent to form the complexes [Ln(L)] and [Ln(L1)](Ln = Y, La, Gd, Yb). Both ligands L and L1 have nine donor atoms comprising three amine and three imine N-donors and three phosphonate O-donors and form Ln(III) complexes in which the three pendant arms of the ligands wrap around the nine-coordinate Ln(III) centres. Complexes with Y(III), La(III), Gd(III) and Yb(III) have been synthesised and the complexes [Y(L)], [Gd(L)] and [Gd(L1)] have been structurally characterised. In all the complexes the coordination polyhedron about the lanthanide centre is slightly distorted tricapped trigonal prismatic with the two triangular faces of the prism formed by the macrocyclic N-donors and the phosphonate O-donors. Interestingly, given the three chiral phosphorus centres present in [Ln(L)] and [Ln(L1)] complexes, the three crystal structures reported show the presence of only one diastereomer of the four possible. 1H, 13C and 31P NMR spectroscopic studies on diamagnetic [Y(L)] and [La(L)] and on paramagnetic [Yb(L)] complexes indicate the presence in solution of all the four different diastereomers in varying proportions. The stability of complexes [Y(L)] and [Y(L1)] in D2O in both neutral and acidic media, and the relaxivity of the Gd(III) complexes, have also been investigated.     

Synthesis, complexation and water exchange properties of Gd(III)-TTDA-mono and bis(amide) derivatives and their binding affinity to human serum albumin

With the objective of tuning the lipophilicity of ligands and maintaining the neutrality and stability of Gd(III) chelate, we designed and synthesized two bis(amide) derivatives of TTDA, TTDA-BMA and TTDA-BBA, and a mono(amide) derivative, TTDA-N-MOBA. The ligand protonation constants and complex stability constants for various metal ions were determined in this study. The identification of the microscopic sites of protonation of the amide ligand by 1H NMR titrations show that the first protonation site occurs on the central nitrogen atom. The values of the stability constant of TTDA-mono and bis(amide) complex are significantly lower than those of TTDA and DTPA, but the selectivity constants of these ligands for Gd(III) over Zn(II) and Cu(II) are slightly higher than those of TTDA and DTPA. On the basis of the water-exchange rate values available for [Gd(TTDA-BMA)(H2O)], [Gd(TTDA-BBA)(H2O)] and [Gd(TTDA-N-MOBA)(H2O)]-, we can state that, in general, the replacement of one carboxylate group by an amide group decreases the water-exchange rate of the gadolinium(III) complexes by a factor of about three to five. The decrease in the exchange rate is explained in terms of a decreased steric crowding and charge effect around the metal ion when carboxylates are replaced by an amide group. In addition, to support the HSA protein binding studies of lipophilic [Gd(TTDA-N-MOBA)(H2O)]- and [Gd(TTDA-BBA)(H2O)] complexes, further protein-complex binding was studied by ultrafiltration and relaxivity studies. The binding constants (KA) of [Gd(TTDA-N-MOBA)(H2O)]- and [Gd(TTDA-BBA)(H2O)] are 8.6 x 10(2) and 1.0 x 10(4) dm3 mol(-1), respectively. The bound relaxivities (r1(b)) are 51.8 and 52 dm3 mmol(-1) s(-1), respectively. The KA value of [Gd(TTDA-BBA)(H2O)] is similar to that of MS-325 and indicates a stronger interaction of [Gd(TTDA-BBA)(H2O)] with HSA.     

Oligonuclear 3d-4f complexes as tectons in designing supramolecular solid-state architectures: impact of the nature of linkers on the structural diversity

Heteronuclear cationic complexes, [LCuLn]3+ and [(LCu)2Ln]3+, were employed as nodes in designing high-nuclearity complexes and coordination polymers with a rich variety of network topologies (L is the dianion of the Schiff base resulting from the 2:1 condensation of 3-methoxysalycilaldehyde with 1,3-propanediamine). Two families of linkers have been chosen: the first consists of exo-dentate ligands bearing nitrogen-donor atoms (bipyridine (bipy), dicyanamido (dca)), whereas the second consists of exo-dentate ligands with oxygen-donor atoms (anions derived from the acetylenedicarboxylic (H2acdca), fumaric (H2fum), trimesic (H3trim), and oxalic (H2ox) acids). The ligands belonging to the first family prefer copper(II) ions, whereas the ligands from the second family interact preferentially with oxophilic rare-earth cations. The following complexes have been obtained and crystallographically characterized: [LCu(II)(OH2)Gd(III)(NO3)3] (1), [{LCu(II)Gd(III)(NO3)3}2(mu-4,4'-bipy)] (2), 1infinity[LCu(II)Gd(III)(acdca)(1.5)(H2O)2].13H2O (3), 2infinity[LCu(II)Gd(III)(fum)(1.5)(H2O)2].4H2O.C2H5OH (4), 1infinity[LCu(II)Sm(III)(H2O)(Hfum)(fum)] (5), 1infinity[LCu(II)Er(III)(H2O)2(fum)]NO3.3H2O (6), 2infinity[LCu(II)Sm(III)(fum)(1.5)(H2O)2].4H2O.C2H5OH (7), [{(LCu(II))2Sm(III)}2fum2](OH)2 (8), 1infinity[LCu(II)Gd(III)(trim)(H2O)2].H2O (9), 2infinity[{(LCu(II))2Pr(III)}(C2O4)(0.5)(dca)]dca.2H2O (10), [LCu(II)Gd(III)(ox)(H2O)3][Cr(III)(2,2'-bipy)(ox)2].9H2O (11), and [LCuGd(H2O)4{Cr(CN)6}].3H2O (12). Compound 1 is representative of the whole family of binuclear Cu(II)-Ln(III) complexes which have been used as precursors in constructing heteropolymetallic complexes. The rich variety of the resulting structures is due to several factors: 1) the nature of the donor atoms of the linkers, 2) the preference of the copper(II) ion for nitrogen atoms, 3) the oxophilicity of the lanthanides, 4) the degree of deprotonation of the polycarboxylic acids, 5) the various connectivity modes exhibited by the carboxylato groups, and 6) the stoichiometry of the final products, that is, the Cu(II)/Ln(III)/linker molar ratio. A unique cluster formed by 24 water molecules was found in crystal 11. In compounds 2, 3, 4, 9, and 11 the Cu(II)-Gd(III) exchange interaction was found to be ferromagnetic, with J values in the range of 3.53-8.96 cm(-1). Compound 12 represents a new example of a polynuclear complex containing three different paramagnetic ions. The intranode Cu(II)-Gd(III) ferromagnetic interaction is overwhelmed by the antiferromagnetic interactions occurring between the cyanobridged Gd(III) and Cr(III) ions.     

Syntheses, crystal structures, and magnetic properties of [Ln(III)2(Succinate)3(H2O)2].0.5H2O [Ln = Pr, Nd, Sm, Eu, Gd, and Dy] polymeric networks: unusual ferromagnetic coupling in Gd derivative

Lanthanide-organic coordination polymeric networks of [Ln(III)2(suc)3(H2O)2].0.5H2O [suc = succinate dianion, Ln = Pr (1), Nd (2), Sm (3), Eu (4), Gd (5), and Dy (6)] have been synthesized and characterized by single-crystal X-ray diffraction analyses. The structural determination reveals that complexes are isomorphous, all crystallizing in monoclinic system, space group I2/a(.) The complexes possess a 3D architecture with Ln ion in a nine-coordination geometry attained by eight oxygen atoms from succinate and one oxygen atom from an aqua ligand. Low-temperature magnetic study indicates that ferromagnetic interaction is present in case of Gd(III) and Dy(III). Antiferromagnetic interaction is observed for the rest of the complexes. Density functional theory calculations are performed which support the existence of a superexchange ferromagnetic coupling in Gd(III) ions, whereas classical crystal field model has been applied to study the complexes 1, 2, 3, and 6.     

Synthesis,characterization and the anticancer activity of six lanthanides(III) complexes with 5,7-dihalogenated-8-quinolinol and 2,2’-bipyridine derivatives

Transition Metal Chemistry - Six new lanthanide(III) complexes, namely [GdIII(QL1)3(BL1)] (Gd1), [GdIII(QL1)3(BL2)] (Gd2), [LuIII(QL2)2(BL3)(NO3)] (Lu1), [LuIII(QL2)2(BL2)(NO3)] (Lu2),...     

Polynuclear and polymeric gadolinium acetate derivatives with large magnetocaloric effect

Two ferromagnetic μ-oxo(acetate)-bridged gadolinium complexes [Gd(2)(OAc)(2)(Ph(2)acac)(4)(MeOH)(2)] (1) and [Gd(4)(OAc)(4)(acac)(8)(H(2)O)(4)] (2) and two polymeric Gd(III) chains [Gd(OAc)(3)(MeOH)](n) (3) and [Gd(OAc)(3)(H(2)O)(0.5)](n) (4) (Ph(2)acacH = dibenzoylmethane; acacH = acetylacetone) are reported. The magnetic studies reveal that the tiny difference in the Gd-O-Gd angles (Gd···Gd distances) in these complexes cause different magnetic coupling. There exist ferromagnetic interactions in 1-3 due to the presence of the larger Gd-O-Gd angles (Gd···Gd distances), and antiferromagnetic interaction in 4 when the Gd-O-Gd angle is smaller. Four gadolinium acetate derivatives display large magnetocaloric effect (MCE). The higher magnetic density or the lower M(W)/N(Gd) ratio they have, the larger MCE they display. Complex 4 has the highest magnetic density and exhibits the largest MCE (47.7 J K(-1) kg(-1)). In addition, complex 3 has wider temperature and/or field scope of application in refrigeration due to the dominant ferromagnetic coupling. Moreover, the statistical thermodynamics on entropy was successfully applied to simulate the MCE values. The results are quite in agreement with those obtained from experimental data.     

Crystal Structures and Magnetic Properties of Novel [Ln(III)Cu(II)(4)] (Ln = Gd, Dy, Ho) Pentanuclear Complexes. Topology and Ferromagnetic Interaction in the Ln(III)-Cu(II) Pair

The first pentanuclear complexes of formula {Dy[Cu(apox)](2)[Cu(apox)(H(2)O)](2)}[ClO(4)](3).7H(2)O (1), {Ho[Cu(apox)][Cu(apox)(H(2)O)](3)}[PF(6)](3).4.5H(2)O (2), {Gd[Cu(apox)](2)[Cu(apox)(H(2)O)](2)}[ClO(4)](3).7H(2)O (3) and {Gd[Cu(apox)][Cu(apox) (H(2)O)](3)}[PF(6)](3).4.5H(2)O (4) (H(2)apox = N,N'-bis(3-aminopropyl)oxamide) have been synthesized. The crystal structures of complexes 1 and 2 have been determined by X-ray diffraction methods. Complexes 3 and 4 are isostructural with 1 and 2, respectively. Crystallographic data are as follows: 1 and 3, monoclinic, space group C2/c and Z = 4, with a = 14.646(6) ?, b = 29.496(7) ?, c = 16.002(7) ?, and beta = 111.76(2) degrees for 1 and a = 14.523(6) ?, b = 29.441(6) ?, c = 15.925(8) ?, and beta = 111.90(4) degrees for 3; 2 and 4, triclinic, P&onemacr;, and Z = 2, with a = 14.346(2) ?, b = 14.454(2) ?, c = 18.107(4) ?, alpha = 90.95(2) degrees, beta = 110.75(2) degrees, and gamma = 106.77(2) degrees for 2 and a = 14.365(6) ?, b = 14.496(5) ?, c = 18.172(7) ?, alpha = 91.27(3) degrees, beta = 110.74(3) degrees, and gamma = 106.67(3) degrees for 4. A tripositive ion is present in these structures, the electroneutrality being achieved by three uncoordinated perchlorate (1) or hexafluorophosphate (2) anions. The lanthanide cations are eight-coordinate with a pseudo-square-antiprismatic environment formed by carbonyl oxygen atoms from two [Cu(apox)] and two Cu(apox)(H(2)O)] (1) and one [Cu(apox)] and three [Cu(apox)(H(2)O)] (2) bidentate ligands. The temperature dependence of the magnetic susceptibility of complexes 1-4 was investigated in the range 1.8-300 K. The ligand-field effect, as well as the mixing of the free-ion states in Dy(III) and Ho(III), make extremely difficult the analysis of the overall antiferromagnetic interaction which is observed for complexes 1 and 2. The magnetic susceptibility data for complexes 3 and 4 have shown that the ground-state spin for the [Gd(III)Cu(II)(4)] unit is S = 11/2, the Gd(III)-Cu(II) interaction being ferromagnetic with an interaction parameter J(GdCu) = 0.85 cm(-)(1) (the interaction Hamiltonian is of the form H = -JS(A).S(B)). The field dependence of the magnetization at 2 K of 3 and 4 confirms the nature of the ground state and of the Gd(III)-Cu(II) interaction. The influence of the topology and of the type of bridging ligand on the nature and magnitude of the magnetic interaction in the Gd(III)-Cu(II) pair is analyzed and discussed in light of available magnetostructural data.     

Physicochemical characterization of the dimeric lanthanide complexes [en{Ln(DO3A)(H2O)}2] and [pi{Ln(DTTA)(H2O)}2]2-: a variable-temperature 17O NMR study

The Gd(III) complexes of the two dimeric ligands [en(DO3A)2] {N,N'-bis[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-10-yl-methylcarbonyl]-N,N'-ethylenediamine} and [pi(DTTA)2]8- [bisdiethylenetriaminepentaacetic acid (trans-1,2-cyclohexanediamine)] were synthesized and characterized. The 17O NMR chemical shift of H2O induced by [en{Dy(DO3A)}2] and [pi{Dy(DTTA)}2]2- at pH 6.80 proved the presence of 2.1 and 2.2 inner-sphere water molecules, respectively. Water proton spin-lattice relaxation rates for [en{Gd(DO3A)(H2O)}2] and [pi{Gd(DTTA)(H2O)}2]2- at 37.0 +/- 0.1 degrees C and 20 MHz are 3.60 +/- 0.05 and 5.25 +/- 0.05 mM(-1) s(-1) per Gd, respectively. The EPR transverse electronic relaxation rate and 17O NMR transverse relaxation time for the exchange lifetime of the coordinated H2O molecule and the 2H NMR longitudinal relaxation rate of the deuterated diamagnetic lanthanum complex for the rotational correlation time were thoroughly investigated, and the results were compared with those reported previously for other lanthanide(III) complexes. The exchange lifetimes for [en{Gd(DO3A)(H2O)}2] (769 +/- 10 ns) and [pi{Gd(DTTA)(H2O)}2]2- (910 +/- 10 ns) are significantly higher than those of [Gd(DOTA)(H2O)]- (243 ns) and [Gd(DTPA)(H2O)]2- (303 ns) complexes. The rotational correlation times for [en{Gd(DO3A)(H2O)}2] (150 +/- 11 ps) and [pi{Gd(DTTA)(H2O)}2]2- (130 +/- 12 ps) are slightly greater than those of [Gd(DOTA)(H2O)]- (77 ps) and [Gd(DTPA)(H2O)]2- (58 ps) complexes. The marked increase in relaxivity (r1) of [en{Gd(DO3A)(H2O)}2] and [pi{Gd(DTTA)(H2O)}2]2- result mainly from their longer rotational correlation time and higher molecular weight.     

Synthesis, structure and magnetic properties of a novel family of heterometallic nonanuclear Na(I)2Mn(III)6Ln(III) (Ln = Eu, Gd, Tb, Dy) complexes

The structures and magnetic properties of four isomorphous nonanuclear heterometallic complexes [Na(2){Mn(3)(III)(μ(3)-O(2-))}(2)Ln(III)(hmmp)(6)(O(2)CPh)(4)(N(3))(2)]OH·0.5 CH(3)CN·1.5H(2)O are reported, where Ln(III) = Eu (1), Gd (2), Tb (3) and Dy (4), H(2)hmmp = 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol. Complexes 1-4 were prepared by the reactions of hmmpH(2) with a manganese salt and the respective lanthanide salt together with NaO(2)CPh and NaN(3). Single-crystal X-ray diffraction analyses reveal that the six Mn(III) and one Ln(III) metal topology in the aggregate can be described as a bitetrahedron. The two peripheral [Mn(III)(3)(μ(3)-O(2-))](7+) triangles are each bonded to a central Ln(III) ion with rare distorted octahedral geometry. The magnetic properties of all the complexes were investigated using variable temperature magnetic susceptibility and both antiferromagnetic and ferromagnetic interactions exist in the [Mn(III)(3)(μ(3)-O(2-))](7+) triangle. Weak ferromagnetic exchange between the Ln(III) and Mn(III) ions has been established for the corresponding Gd derivative. The Gd, Tb and Dy complexes show no evidence of slow relaxation behaviour above 2.0 K.     

Homotrinuclear lanthanide(III) arrays: assembly of and conversion from mononuclear and dinuclear units

The reactions of potentially hexadentate H2bbpen (N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-pyridylmethyl)-ethylenediamine, H2L1), H2(Cl)bbpen (N,N'-bis(5-chloro-2-hydroxybenzyl)-N,N'-bis(2-pyridylmethyl)ethylenediamine, H2L2), and H2(Br)bbpen (N,N'-bis(5-bromo-2-hydroxybenzyl)-N,N'-bis(2-pyridylmethyl)ethylenediamine, H2L3) with Ln(III) ions in the presence of a base in methanol resulted in three types of complexes: neutral mononuclear ([LnL(NO3)]), monocationic dinuclear ([Ln2L2(NO3)]+), and monocationic trinuclear ([Ln3L2(X)n(CH3OH)]+), where X = bridging (CH3COO-) and bidentate ligands (NO3-, CH3COO-, ClO4-) and n is 4. The formation of a complex depends on the base (hydroxide or acetate) and the size of the respective Ln(III) ion. All complexes were characterized by infrared spectroscopy, mass spectrometry, and elemental analyses; in some cases, X-ray diffraction studies were also performed. The structures of the neutral mononuclear [Yb(L1)(NO3)], dinuclear [Pr2(L1)2(NO3)(H2O)]NO3.CH3OH and [Gd2(L1)2(NO3)]NO3.CH3OH.3H2O, and trinuclear [Gd3(L3)2(CH3COO)4(CH3OH)]ClO4.5CH3OH and [Sm3(L1)2(CH3COO)2(NO3)2(CH3OH)]NO3.CH3OH.3.65H2O were solved by X-ray crystallography. The [LnL(NO3)] or [Ln2L2(NO3)]+ complexes could be converted to [Ln3L2(X)n(CH3OH)]+ complexes by the addition of 1 equiv of a Ln(III) salt and 2-3 equiv of sodium acetate in methanol. The trinuclear complexes were found to be the most stable of the three types, which was evident from the presence of the intact monocationic high molecular weight parent peaks ([Ln3L2(X)n]+) in the mass spectra of all the trinuclear complexes and from the ease of conversion from the mononuclear or dinuclear to the trinuclear species. The incompatibility of the ligand denticity with the coordination requirements of the Ln(III) ions was proven to be a useful tool in the construction of multinuclear Ln(III) metal ion arrays.     

Interaction Between the Low Molecular Mass Components of Blood Serum and the Vanadium(III)–2,2′-Bipyridine System

The complex species formed between vanadium(III)?C2,2??-bipyridine (Bipy) and the small blood serum bioligands lactic (HLac), oxalic (H₂Ox), citric (H₃Cit) and phosphoric (H₃PO₄) acids were studied in aqueous solution by means of electromotive forces measurements emf(H) at 25?°C and 3.0?mol?dm^?3 KCl as the ionic medium. The data were analyzed using the least-squares computational program LETAGROP, taking into account the hydrolytic products of vanadium(III) and the binary complexes formed. Formation of the complexes [V(Bipy)(Lac)]²⁺, [V(Bipy)(Lac)₂]⁺, [V(OH)₂(Bipy)(Lac)] and [V₂O(Bipy)₂(Lac)₂]^? were observed in the vanadium(III)?CBipy?CHLac system. Also, the species [V(Bipy)(HOx)]²⁺, [V(Bipy)(Ox)]⁺, [V(OH)(Bipy)(Ox)], [V(OH)₂(Bipy)(Ox)]^? and [V(OH)₃(Bipy)(Ox)]^2? were found in the vanadium(III)?CBipy?CH₂Ox system, the complexes [V(Bipy)(HCit)]⁺, [V(Bipy)(Cit)], [V(OH)(Bipy)(Cit)]^? and [V(OH)₂(Bipy)(Cit)]^2? were found in the vanadium(III)?CBipy?CH₃Cit system, and the species [V(Bipy)(H₂PO₄)]²⁺ and [V(Bipy)(HPO₄)]⁺ were detected in the vanadium(III)?CBipy?CH₃PO₄ system. The stability constants of these complexes were determined.     

Ferromagnetic NiII-GdIII interactions in complexes with NiGd, NiGdNi, and NiGdGdNi cores supported by tripodal ligands

Dinuclear [(NiL)Gd(hfac)(2)(EtOH)](H(3)L = 1,1,1-tris(N-salicylideneaminomethyl)ethane, Hhfac = hexafluoroacetylacetone), trinuclear [(NiL)(2)Gd(NO(3))], and tetranuclear [(NiL)Gd(CH(3)CO(2))(2)(MeOH)](2) complexes, were prepared by treating [Ni(HL)] with [Gd(hfac)(3)(H(2)O)(2)], Gd(NO(3))(3).6H(2)O, and Gd(CH(3)CO(2))(3).4H(2)O, respectively, in the presence of Et(3)N. All the complexes show that ferromagnetic interactions occur between the Ni(II) and Gd(III) ions.     

Unexpected aggregation of neutral, xylene-cored dinuclear GdIII chelates in aqueous solution

We have synthesized ditopic ligands L(1), L(2), and L(3) that contain two DO3A(3-) metal-chelating units with a xylene core as a noncoordinating linker (DO3A(3-) = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate; L(1) = 1,4-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(2) = 1,3-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(3) = 3,5-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzoic acid). Aqueous solutions of the dinuclear Gd(III) complexes formed with the three ligands have been investigated in a variable-temperature, multiple-field (17)O NMR and (1)H relaxivity study. The (17)O longitudinal relaxation rates measured for the [Gd(2)L(1-3)(H2O)(2)] complexes show strong field dependence (2.35-9.4 T), which unambiguously proves the presence of slowly tumbling entities in solution. The proton relaxivities of the complexes, which are unexpectedly high for their molecular weight, and in particular the relaxivity peaks observed at 40-50 MHz also constitute experimental evidences of slow rotational motion. This was explained in terms of self-aggregation related to hydrophobic interactions, pi stacking between the aromatic linkers, or possible hydrogen bonding between the chelates. The longitudinal (17)O relaxation rates of the [Gd(2)L(1-3)(H2O)(2)] complexes have been analysed with the Lipari-Szabo approach, leading to local rotational correlation times tau(1)(298) of 150-250 ps and global rotational correlation times tau(g)(298) of 1.6-3.4 ns (c(Gd): 20-50 mM), where tau(1)(298) is attributed to local motions of the Gd segments, while tau(g)(298) describes the overall motion of the aggregates. The aggregates can be partially disrupted by phosphate addition; however, at high concentrations phosphate interferes in the first coordination sphere by replacing the coordinated water. In contrast to the parent [Gd(DO3A)(H2O)(1.9)], which presents a hydration equilibrium between mono- and dihydrated species, a hydration number of q = 1 was established for the [Ln(2)L(1-3)(H2O)(2)] chelates by (17)O chemical shift measurements on Ln = Gd and UV/Vis spectrophotometry for Ln = Eu. The exchange rate of the coordinated water is higher for [Gd(2)L(1-3)(H2O)(2)] complexes k(ex)(298) = 7.5-12.0 x 10(6) s(-1)) than for [Gd(DOTA)(H2O)](-). The proton relaxivity of the [Gd(2)L(1-3)(H2O)(2)] complexes strongly decreases with increasing pH. This is related to the deprotonation of the inner-sphere water, which has also been characterized by pH potentiometry. The protonation constants determined for this process are logK(OH) = 9.50 and 10.37 for [Gd(2)L(1)(H2O)(2)] and [Gd(2)L(3)(H2O)(2)], respectively.     

Die Chelatbildung von N-Tris(2-aminoethyl)amin-N′,N′,N″,N″,N‴,N‴-hexaessigsäure (H6TTAHA) und N-(Pyrid-2-yl-methyl)ethylendiamin-N,N′,N′-triessigsäure (H3PEDTA) mit Gadolinium(III) – Synthesen,Komplexstabilität und NMR-Relaxivität

Chelate Formation of N-Tris(2-aminoethyl)amine-N′,N′,N″,N″,N?,N?-hexaacetic Acid (H₆TTAHA) and N-(Pyrid-2-yl-methyl)ethylenediamine-N,N′,N′-triacetic Acid (H₃PEDTA) with Gadolinium(III) – Syntheses, Stability Constants, and NMR-Relaxivities The chelate formation of N-tris(2-aminoethyl)amine-N′,N′,N″,N″,N?,N?-hexaacetic acid (H₆TTAHA) and N-(pyrid-2-yl-methyl)ethylenediamine-N,N′,N′-triacetic acid (H₃PEDTA) with gadolinium(III) has been studied potentiometrically in aqueous solution at 25°C and μ = 0.1 (KCl). [Gd(TTAHA)]^3?: 1gβ_M/ML = 19.0; {H[Gd(TTAHA)]}^2?: 1gK_H/MHL = 8.30; [Gd(PEDTA)]: 1gβ_M/ML = 15.56. Both 1 : 1 gadolinium(III) complexes were isolated as Na₂H[Gd(C₁₈H₂₄N₄O₁₂)] · 3.5 H₂O and [Gd(C₁₄H₁₆N₃O₆)] · 3 H₂O, respectively. Their ¹H-NMR relaxivities [1 · mmol^?1 · s^?1] ({H[Gd(TTAHA)]}^2?: 9.5; [Gd(PEDTA)]: 8.8) offer promising applications for ¹H-NMR imaging.     

Preparation, crystal structures, and magnetic features for a series of dinuclear [Ni(II)Ln(III)] Schiff-base complexes: evidence for slow relaxation of the magnetization for the Dy(III) derivative

A series of dinuclear [Ni(II)Ln(III)] Schiff-base complexes (using a Schiff-base dicompartmental ligand derived from o-vanillin [H(2)valpn = 1,3-propanediylbis(2-iminomethylene-6-methoxy-phenol)]) with Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and a hydroxo-bridged tetranuclear [Ni(II)Yb(III)] are reported. The crystal structures have been solved for 10 dinuclear complexes revealing four arrangements for the dinuclear units, which are modulated by the coordinated solvent molecules and the nitrato-anion interactions. The magnetic behaviors have been investigated, and the nature of the Ni(II)-Ln(III) exchange interaction has been emphasized by comparison with the behavior of the related [Zn(II)Ln(III)] derivatives. This allowed for establishing that the interaction within these compounds is antiferromagnetic with the 4f ions of the beginning of the Ln series and turns ferromagnetic from Gd(III) toward the end of the series. AC susceptibility investigations clearly show the occurrence of slow relaxation processes of the magnetization close to 2 K for the dinuclear [Ni(II)Dy(III)] complex.