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A critical analysis of the lanthanide induced paramagnetic shift (LIS) data for several series of Ln3+ complexes of C3 symmetry in terms of structural changes, crystal-field effects and/or variation of hyperfine constants along the lanthanide series was undertaken using a combination of the two-nuclei and three-nuclei techniques together with the classical one-nucleus technique. The crystal-field independent two-nuclei technique to study the isostructurality of a series of lanthanide complexes, is usefully complemented by the three nuclei shift ratio method, which is based exclusively on the experimental shift data, requiring no knowledge of B02, 〈Sz〉 or Cj values. However, this later method cannot provide quantitative values for Fi and Gi. The combined use of the three methods was found to be a powerful analytical tool of the solution structure of lanthanide complexes. Isostructurality of whole series of complexes, either with no change of the Fi, Gi and B02 parameters (L5 and L6), or with changes of the Fi and B02 parameters (L7 and L8), is clearly defined by the combination of the two first methods. In these cases, the three-nuclei method sometimes fully supports such an isostructurality (L6, L8), but in other cases, due to the high structural sensitivity of its α and β parameters, it is able to detect small, unnoticed, structural changes in the complexes of L5 and L7. Clear structural changes, involving the Fi, Gi and B02 parameters, are observed for the series of complexes of (L9), where the three methods agree, involving hydration and carboxylate coordination changes. More subtle structural changes, involving the internal dynamics of the bound ligands, are proposed in other cases (L1–L4). These could also result from a magnification, by the present graphical analysis, of the breaks expected from the gradual structural changes along the series due to the lanthanide contraction.

The redox behavior of the potential oral insulin mimetic compound bis[3-hydroxy-1,2-dimethyl-4-pyridinonate]oxovanadium(IV), VO(dmpp) 2 , was studied in aqueous solution, under aerobic conditions and at different pH values and metal-to-ligand ratios, as well as the speciation of its V(V) oxidation products. Dissolution of this compound in water originates a violet solution with pH 4.7, which is partially oxidized to two V(V) species, VO 2 L and [VO 2 L 2 ] − , detected through their 51 V NMR signals at −485 and −504 ppm. However, the VO(dmpp) 2 compound still remains to some extent in solution, as confirmed by EPR Spectroscopy. High pH values favor the oxidation of the V(IV) complex in solution, whereas at low pH the V(IV) species are predominant. Aqueous solutions containing vanadate–Hdmpp mixtures spontaneously yield V(IV) species, reflecting the reducing capacity of the ligand. This reduction process is favored by ligand excess and low pH. For a ligand excess of fivefold or higher, the [VO 2 L 2 ] − complex, the only species present in solution, produces, at pH 2 (H 2 O)] and [VO(H 2 O) 5 ] 2+ , as detected by EPR. At higher vanadate–hdmpp ratios [VO(dmpp)(H 2 O) 2 ] + is also produced. The V(V) species formed in solution were studied through a parallel 51 V and 1 H NMR study of the vanadate–Hdmpp system, as a function of metal/ligand ratio and pH. The p K a values of the two main mononuclear V(V) octahedral complexes, VO 2 L and [VO 2 L 2 ] − (L=dmpp), were determined from the pH dependence of their 51 V and 1 H NMR chemical shifts. For VO 2 L, p K a values of 5.88±0.03 and 3.30±0.07 were assigned, respectively, to protonation of [VO 2 (dmpp)(OH)(H 2 O)] − at the coordinated hydroxo ligand and at the ligand nitrogen atom. For [VO 2 L 2 ] − , the three p K a values obtained, 3.96±0.06, 5.62±0.03, and 2.9±0.4, were attributed, respectively, to protonation of [VO 2 (dmpp) 2 ] − at the nitrogen atom of one of the ligands, at one oxygen of the VO 2 + core, and at its resulting VOH group, with formation of the VO 3+ core. The stability constants of these two complexes (log K 1 =10.48±0.05; log K 2 =5.25±0.04; log β 2 =15.73±0.05) were obtained from 51 V NMR data, at pH 8.2, ionic strength 0.1 M, by changing the metal/ligand ratio at constant metal concentration.

The linear graphical analysis of the LIS NMR data available for the axially symmetric complexes [Ln(DOTA)]- (M and m isomers), [Ln(DOTP)]5- (pH 10.0, 7.0 and 3) and [Ln(DOTEA)]3+ using the classical crystal field dependent method and a crystal field independent method were compared. As the second method provides ratios of geometric structural terms G rather than G values, the effect of lanthanide contraction was reduced. Thus, the large breaks in plots observed for all nuclei of those systems using the classical method are still present in the plots of the second method, only in a few of the nuclei and much reduced. This shows that the large breaks at the middle of the lanthanide series present in plots of the classical method as well as the anomalies often present for those plots for the Tm and Yb ions are mostly due to changes of the crystal field coefficient A2Â 0 along the lanthanide series, while both the hyperfine coupling constants and the ratios of geometric terms also change as a result of the lanthanide contraction, leading to small breaks at the middle of the lanthanide series. Analysis of the proton shifts of [Ln(DOTP)] complexes at pH 10, 7 and 3 indicates that protonation of the complexes results in a decrease on the crystal field coefficient. The dipolar shift ratios and absolute shifts obtained were also interpreted in terms of the structural models for those complexes in solution and their available X-ray crystal structures. http://www.sciencedirect.com/science/article/B6TG5-45S49G7-1/1/7fee3ca7ba8b89327bec4ab48419d290

Copper(II) complexes with a series of bibracchial lariat ethers are described. Independently of the nature of the counterion present (nitrate or perchlorate), the lariat ether N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L1) always forms mononuclear complexes, whereas the lariat ethers N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) and N,N'-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6 (L3) only give binuclear compounds. The X-ray crystal structure of [CuL1](ClO4)2 shows a seven-coordinated copper(II) ion in a distorted (axially compressed) pentagonal-bipyramidal geometry. The X-ray crystal structure of [Cu2(L3-2H)](ClO4)2 confirms the binuclear nature of the compound with both metal ions having identical coordination environments and each one placed out of the crown hole but efficiently encapsulated by the corresponding pendant arm; each copper(II) ion is five-coordinated with an intermediate geometry between trigonal-bipyramidal and square-pyramidal ([tau]=0.40). The EPR spectra in frozen solution samples are in accordance with a stable coordinate pattern for the metal centre of ligand L1, yielding a rhombic distorted complex with axial compression in solution, in agreement with the X-ray crystal structure of [CuL1](ClO4)2. For the binuclear complexes of L2 and L3, the Cu(II) centres in solution can be distorted from their tetragonally elongated structures via interaction with ethanol and/or the nitrate counterion, leading to more than one species. http://www.sciencedirect.com/science/article/B6TG5-433PCM9-T/1/7815e8fe94b2b6b2129e76252c3517d0

The interaction between ZnCl2 and La(DTPA) (DTPA = diethylenetriamine-N,N,N′,N″,N″-penta-acetate), La(DTPA-PA2) (DTPA-PA2 = DTPA-bis(propylamide) ) or La(DTPA-GlucA2) (DTPA-GlucA2 = DTPA-bis(glucamide)) has been studied using 13C and 139La NMR. Extensive transmetallation occurs, which is assisted by interaction between the ionic species formed. It is shown that these interactions involve mainly the DTPA backbone of the ligands. The sugar side-chains of La(DTPA-GlucA2) have no significant affinity for the metal ions. With the aid of 11B NMR it is demonstrated that borate does interact with the polyhydroxy side-chains. The stoichiometries of the various borate ester species derived from DTPA-GlucA2 and its Ln(III) complexes are investigated by recording the corresponding Job plots. A major species has a borate moiety linking the two glucitol side-chains under the formation of a macrocyclic compound. The stability constants of the occurring borate ester species as determined by 11B NMR are reported.

The paramagnetic 13 C NMR relaxation rate enhancements and the 1 H induced chemical shifts of a series of organic molecules, caused by various paramagnetic metal complexes and a nitroxide radical (TEMPOL), were measured in aqueous solution. These NMR perturbations were used to study and model the mechanisms of their non-covalent interactions. The paramagnetic metal complexes showed a varying degree of binding specificity, in contrast to the non-specific interactions of the nitroxide radical. Weak and basically non-specific binding was observed for the neutral DTPA–bis(amide) complexes, possibly due to hydrophobic interactions, whereas the single negatively charged DOTA and DOTP–MB complexes showed weak specific interactions with ammonium functions. The strongest and most specific interactions occurred between the negatively charged Ln(DOTP) chelates and the protonated linear and macrocyclic amines. In the case of Ln(DOTP)–ADA, the 1 H induced shifts and 13 C spin-lattice relaxation rates were fitted to the theoretical equations, yielding a geometry for the adduct where the ammonium group interacts with the Ln-unbound negatively charged oxygen(s) of one phosphonate group. Two Ln(DOTP) molecules appear to be able to sandwich the diprotonated tetraazamacrocyclic amine CY. In the polyhydroxyammonium compound MEG, the strong electrostatic interaction is assisted by hydrogen bonding of hydroxyl groups to the Ln-unbound phosphonate oxygens of DOTP. A comparison of the strong pH dependences found for the paramagnetic NMR effects of the Ln(DOTP) chelates on the 1 H and 13 C nuclei of CY and MEG clearly indicated the dominance of the electrostatic interactions in both cases. Considering the organic molecules used as good models of side-chains of amino-acid residues at the surface of proteins, the observed interactions allow specific probing of protein surfaces using NMR methods.

95Mo and 13C NMR spectra of aqueous solutions containing molybdate and either DTPA or TTHA at pH values between 2.0 and 9.5 reflect the presence of two complexes of 1:1 (MoL) and 2:1 (Mo2L) stoichiometries for both ligands. All complexes give one broad 95Mo signal at 60–70 ppm relative to free molybdate, with linewidth 380–890 Hz which increases when the pH decreases. The 95Mo NMR parameters indicate that the metal center of the complexes consists of the MoO3 species, which is confirmed by the presence of two 17O NMR signals in the intensity ratio of 1:2 from the corresponding oxo ligands which are trans to the nitrogen and oxygen atoms of the ligand IDA-type moities that bind the metal center. The intensities and multiplicities of the 13C signals of the complexes indicate that in the MoL complexes the metal center binds one of the terminal IDA-type moieties of the ligands, whereas in the Mo2L complexes two metal centers bind the two terminal IDA-type moieties, yielding a very symmetric species. The intensities of the 95Mo and 13C signals give the pH dependence of the concentration of the free ligand, L, free molybdate, Mo, and the MoL and Mo2L complexes. In general, for both 1:1 and 2:1 solution stoichiometries used, the free ligand and the two complexes are present between pH 2.0 and about 8, Mo only appears above pH 6 and at pH 9 only L and Mo occur. The pH dependence of the 13C complexation shifts reflects the protonation state and sites of the ligand in the complexes. The binding of the MoO3 center to two nitrogen atoms and one carboxylate oxygen of EDDA leads to two diastereomeric pairs of this complex, which are reflected in the presence of two sets of proton signals. Those spectra were assigned using two-dimensional COSY and J-resolved spectra. The vicinal coupling constants obtained were used to define the structures of the species present. Two-dimensional exchange spectra yielded the exchange mechanisms which operate in solution.

1,4,7-Triazacyclononane- N , N ′, N ″-tris(methylene-phosphonate monoethylester) (NOTPME) and its complexes with the trivalent lanthanide cations and divalent calcium, magnesium, zinc and cadmium have been examined by potentiometry and NMR. The 31 P NMR spectra of the lanthanide-NOTPME complexes show single resonances characteristic of symmetrical chelated species at low concentrations (below 0.1 mM) but multiple resonances at higher concentrations. This tendency of the neutral complexes to form unsymmetrical, perhaps oligomeric, species is dependent upon the size of the Ln 3+ and the solution pH. The alkaline earth and divalent transition metal cation complexes do not show this same behavior. The stability constants of the Ln(NOTPME) complexes were found to be considerably smaller than their respective Ln(NOTA) complexes.

A comparison of the paramagnetic shifts caused on the 7Li+ NMR resonance by the shift reagents Ln(PPP)27−, Ln(TTHA)3−, Ln(NTA)23−, Ln(DPA)33− and Ln(HCA)33− (where Ln3+=Dy3+ or Tm3+, and PPP5−, TTHA6−, NTA3−, DPA2− and HCA2− represent the ligands triphosphate, triethylenetetraminehexacetate, nitrilotriacetate, dipicolinate, and chelidamate, respectively) was carried out under the same ionic strength conditions. Dy(PPP)27−, Tm(PPP)27− and Dy(TTHA)3− induce, in decreasing order, the largest Li+ shifts. However, those produced by Dy(PPP)27− and Tm(PPP)27− are highly pH dependent. It was found by 7Li+ (and 23Na+) NMR spectroscopy that up to seven Li+ (or Na+) ions saturate all the binding sites on Dy(PPP)27− at pH 7.5. The larger Li+ shifts observed at higher pH are due to an increased electrostatic interaction between the shift reagent and Li+ ions as a result of deprotonation of the triphosphate ligands. The Li+ shifts induced by Dy(PPP)27− are smaller in the presence of Na+, K+, Ca2+ and Mg2+. The relative order of cation competition in Dy(PPP)27− is Ca2+ ⩾ Mg2+ > Li+ > Na+ ⩾ K+. 31P NMR studies indicate that divalent cations mainly compete with Dy3+ for the triphosphate ligand via a scrambling mechanism while monovalent cations compete for the Li+ binding sites in the second coordination sphere of the shift reagent. Dy(TTHA)3− is the most promising shift reagent (among those tested) for 7Li+ NMR studies since it interacts weakly with Li+ and yet it produces relatively large shifts which are virtually independent of pH and are less sensitive to the presence of monovalent and divalent cations. Application of Dy(PPP)27−, Tm(PPP)27− and Dy(TTHA)3− to the study of Li+ transport in human red blood cells by 7Li+ NMR spectroscopy is briefly discussed.