RESUMO
We report the Mn(II) complexes with two pyclen-based ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with acetate pendant arms at either positions 3,6 (3,6-PC2A) or 3,9 (3,9-PC2A) of the macrocyclic fragment. The 3,6-PC2A ligand was synthesized in five steps from pyclen oxalate by protecting one of the secondary amine groups of pyclen using Alloc protecting chemistry. The complex with 3,9-PC2A is characterized by a higher thermodynamic stability [log KMnL = 17.09(2)] than the 3,6-PC2A analogue [log KMnL = 15.53(1); 0.15 M NaCl]. Both complexes contain a water molecule coordinated to the metal ion, which results in relatively high 1H relaxivities (r1p = 2.72 and 2.91 mM-1 s-1 for the complexes with 3,6-PC2A and 3,9-PC2A, respectively, at 25 °C and 0.49 T). The coordinated water molecule displays fast exchange kinetics with the bulk in both cases; the rates (kex298) are 140 × 106 and 126 × 106 s-1 for [Mn(3,6-PC2A)(H2O)] and [Mn(3,9-PC2A)(H2O)], respectively. The two complexes were found to be remarkably inert with respect to their dissociation, with half-lives of 63 and 21 h, respectively, at pH = 7.4 in the presence of excess Cu(II). The r1p values recorded in blood serum remain constant at least over a period of 120 h. Cyclic voltammetry experiments show irreversible oxidation features shifted to higher potentials with respect to [Mn(EDTA)(H2O)]2- (H4EDTA = ethylenediaminetetraacetic acid) and [Mn(PhDTA)(H2O)]2- (H4PhDTA = phenylenediamine-N,N,N',N'-tetraacetic acid), indicating that the PC2A complexes reported here have a lower tendency to stabilize Mn(III). The superoxide dismutase activity of the Mn(II) complexes was tested using the xanthine/xanthine oxidase/p-nitro blue tetrazolium chloride assay at pH = 7.8. The Mn(II) complexes of 3,6-PC2A and 3,9-PC2A are capable of assisting decomposition of the superoxide anion radical. The kinetic rate constant of the complex of 3,9-PC2A is smaller by 1 order of magnitude than that of 3,6-PC2A.
Assuntos
Acetatos/química , Compostos Azabicíclicos/química , Complexos de Coordenação/química , Teoria da Densidade Funcional , Manganês/química , Complexos de Coordenação/síntese química , Humanos , Cinética , Ligantes , Estrutura Molecular , Estereoisomerismo , Superóxido Dismutase/metabolismoRESUMO
We report a detailed study of the photophysical properties of EuIII and TbIII complexes with two ligands based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing either four pyridine-2yl-methyl (L1) or four hydroxyethyl (L2) pendant arms. The [TbL1]3+ and [TbL2]3+ complexes present moderate luminescence quantum yields upon excitation through the ligand bands (φH2O = 7.4 and 21%, respectively). The [EuL2]3+ complex displays a relatively low quantum yield in H2O (φH2O = 1.6%) that increases considerably in D2O (φD2O = 5.3%), which highlights the strong quenching effect of the four ligand O-H oscillators. The emission spectrum of [EuL1]3+ is rather unusual in that it shows a relatively high intensity of the 5D0 â 7F5,6 transitions, which appears to be also related to the distorted D4d symmetry of the coordination polyhedron. Surprisingly, the quantum yield of the [EuL1]3+ complex is very low (φH2O = 0.10%), considering the good protection of the EuIII coordination environment offered by the ligand. Cyclic voltammograms recorded from aqueous solutions of [EuL1]3+ display a reversible curve with a half-wave potential of -620 mV (versus Ag/AgCl), while [EuL2]3+ presents a reduction peak at more negative potential (-1040 mV). Thus, the L1 ligand provides a rather good stabilisation of divalent Eu compared to the L2 analogue, suggesting that the presence of a low-lying ligand-to-metal charge-transfer (LMCT) state might be responsible for the low quantum yield determined for [EuL1]3+. A density functional theory (DFT) study provides very similar energies for the ligand-centered excited singlet (1ππ*) and triplet (3ππ*) states of [EuL1]3+ and [EuL2]3+. The energy of the 9LMCT state of [EuL1]3+ was estimated to be 20 760 cm-1 by using all-electron relativistic calculations based on the DKH2 approach, a value that decreases to 15 940 cm-1 upon geometry relaxation.
RESUMO
We report a series of ligands containing pentadentate 6,6'-((methylazanediyl)bis(methylene))dipicolinic acid binding units that form mono- (H2dpama), di- (mX(H2dpama)2), and trinuclear (mX(H2dpama)3) complexes with Mn2+ containing two coordinated water molecules per metal ion, which results in pentagonal bipyramidal coordination around the metal ions. In contrast, the hexadentate ligand 6,6'-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid (H2bcpe) forms a complex with distorted octahedral coordination around Mn2+ that lacks coordinated water molecules. The protonation constants of the ligands and the stability constants of the Mn2+, Cu2+, and Zn2+ complexes were determined using potentiometric and spectrophotometric titrations in 0.15 M NaCl. The pentadentate dpama2 ligand and the di- and trinucleating mX(dpama)24 and mX(dpama)36 ligands provide metal complexes with stabilities that are very similar to that of the complex with the hexadentate ligand bcpe2, with log ß101 values in the range 10.111.6. Cyclic voltammetry experiments on aqueous solutions of the [Mn(bcpe)] complex reveal a quasireversible system with a half-wave potential of +595 mV versus Ag/AgCl. However, [Mn(dpama)] did not suffer oxidation in the range 0.01.0 V, revealing a higher resistance toward oxidation. A detailed 1H NMRD and 17O NMR study provided insight into the parameters that govern the relaxivity for these systems. The exchange rate of the coordinated water molecules in [Mn(dpama)] is relatively fast, kex298 = (3.06 ± 0.16) × 108 s1. The trinuclear [mX(Mn(dpama)(H2O)2)3] complex was found to bind human serum albumin with an association constant of 1286 ± 55 M1 and a relaxivity of the adduct of 45.2 ± 0.6 mM1 s1 at 310 K and 20 MHz.
RESUMO
We report two macrocyclic ligands containing a 1,10-diaza-18-crown-6 fragment functionalized with either two picolinamide pendant arms (bpa18c6) or one picolinamide and one picolinate arm (ppa18c6(-)). The X-ray structure of [La(ppa18c6)(H2O)](2+) shows that the ligand binds to the metal ion using the six donor atoms of the crown moiety and the four donor atoms of the pendant arms, 11-coordination being completed by the presence of a coordinated water molecule. The X-ray structure of the [Sr(bpa18c6)(H2O)](2+) was also investigated due to the very similar ionic radii of Sr(2+) and Eu(2+). The structure of this complex is very similar to that of [La(ppa18c6)(H2O)](2+), with the metal ion being 11-coordinated. Potentiometric measurements were used to determine the stability constants of the complexes formed with La(3+) and Eu(3+). Both ligands present a very high selectivity for the large La(3+) ion over the smaller Eu(3+), with a size-discrimination ability that exceeds that of the analogous ligand containing two picolinate pendant arms reported previously (bp18c6(2-)). DFT calculations using the TPSSh functional and the large-core pseudopotential approximation provided stability trends in good agreement with the experimental values, indicating that charge neutral ligands derived from 1,10-diaza-18-crown-6 enhance the selectivity of the ligand for the large Ln(3+) ions. Cyclic voltammetry measurements show that the stabilization of Eu(2+) by these ligands follows the sequence bp18c6(2-) < ppa18c6(-) < bpa18c6 with half-wave potentials of -753 mV (bp18c6(2-)), -610 mV (ppa18c6(-)), and -453 mV (bpa18c6) versus Ag/AgCl. These values reveal that the complex of bpa18c6 possesses higher stability against oxidation than the aquated ion, for which an E1/2 value of -585 mV has been measured.