Aminocarboxylate Complexes Research Articles

Reactions of carbonate radical anion with amino-carboxylate complexes of manganese(II) and iron(III)

The reaction kinetics of eight amino-carboxylate complexes of Fe(III) and Mn(II) with carbonate radical anion were studied using the pulse radiolysis method and UV-vis spectroscopy. Difference spectra revealed the formation of Fe(IV) and Mn(III) after reaction with CO3•−. Spectral measurements revealed the first step to be the coordination of carbonate to the metal center. All of these led to the conclusion that the role of coordinated carbonate is essential to the electron transfer process by carbonate radical anion.

Mechanistic insights into boron-catalysed direct amidation reactions.

The generally accepted monoacyloxyboron mechanism of boron-catalysed direct amidation is brought into question in this study, and new alternatives are proposed. We have carried out a detailed investigation of boron-catalysed amidation reactions, through study of the interaction between amines/carboxylic acids and borinic acids, boronic acids and boric acid, and have isolated and characterised by NMR/X-ray crystallography many of the likely intermediates present in catalytic amidation reactions. Rapid reaction between amines and boron compounds was observed in all cases, and it is proposed that such boron-nitrogen interactions are highly likely to take place in catalytic amidation reactions. These studies also clearly show that borinic acids are not competent catalysts for amidation, as they either form unreactive amino-carboxylate complexes, or undergo protodeboronation to give boronic acids. It therefore seems that at least three free coordination sites on the boron atom are necessary for amidation catalysis to occur. However, these observations are not consistent with the currently accepted 'mechanism' for boron-mediated amidation reactions involving nucleophilic attack of an amine onto a monomeric acyloxyboron intermediate, and as a result of our observations and theoretical modelling, alternative proposed mechanisms are presented for boron-mediated amidation reactions. These are likely to proceed via the formation of a dimeric B-X-B motif (X = O, NR), which is uniquely able to provide activation of the carboxylic acid, whilst orchestrating the delivery of the amine nucleophile to the carbonyl group. Quantum mechanical calculations of catalytic cycles at the B3LYP+D3/Def2-TZVPP level (solvent = CH2Cl2) support the proposal of several closely related potential pathways for amidation, all of which are likely to be lower in energy than the currently accepted mechanism.

Syntheses, spectroscopy, X-ray and DFT/TDDFT investigations of Rh(η4-cod)-enantiopure aminocarboxylate complexes

The titled Rh(η4-cod)-enantiopure aminocarboxylate complexes, [Rh(η4-cod) (AA)] {cod = cycloocta-1,5-diene; AA = L-prolinato (1) and D-4-hydroxy-phenylglycinato (2)}, are synthesized from the reaction of enantiopure aminocarboxylate with [Rh(η4-cod) (acetate)]2. Polarimetric measurements confirm the enantiopurity or very similar enantiomeric excess of the complexes in chloroform. Electronic spectra in different solvents exhibit a negative solvatochromism in the solvents of increasing polarity or acceptor number. DSC analyses show an exothermic peak at 269 °C (ΔH = −72.42 kJ M−1) for 2, corresponding to an irreversible phase transformation. Cyclic voltammograms reveal a quasi-reversible one electron transfer for [Rh(η4-cod) (l-prolinate)]/[Rh(η4-cod) (l-prolinate)]+ couple in acetonitrile. Electronic spectra calculated by TDDFT reveal the bands at relatively lower energy (>230 nm) due to combined d–d, MLCT and LLCT transitions, while the bands at higher energy only for LLCT, which are very similar to the experimental results. X-ray structure demonstrates a distorted square-planar geometry with N,O-chelation of the l-prolinate ligand to the Rh(η4-cod)-fragment in 1. The crystal packing is evidenced by charge-assisted intermolecular N–H···O and C–H···O interactions to the non-coordinated carboxylate oxygen atom. Hydrogen bonding leads to the formation of supramolecular chains along the a direction of the unit cell. The interchain packing is governed mostly by van-der-Waals forces and reminiscent of a 'herring-bone' packing.

Oxidation of CrIII aminocarboxylate complexes by hydrous manganese oxide: products and time course behaviour

Environmental context Oxidation of CrIII (trivalent chromium) to CrVI (hexavalent chromium) is of environmental concern because CrVI is a known mutagen and carcinogen. Our results show that hydrous manganese oxide (HMO) is capable of oxidising soluble CrIII complexed with iminodiacetic acid and nitrilotriacetic acid to CrVI at appreciable rates. CrVI production from soluble CrIII organic complexes is therefore expected to occur in natural and engineered systems that contain HMO. Abstract MnIII,IV (hydr)oxides are believed to be the principal oxidants of CrIII in the subsurface. In nearly all previous work on this subject, the CrIII reactant was prepared from inorganic salts (e.g. nitrate, chloride, sulfate). In our present work, CrIII complexes with the synthetic chelating agents iminodiacetic acid (IDA) and nitrilotriacetic acid (NTA) were reacted with hydrous manganese oxide (HMO) over a wide pH range to examine rates of reaction and product distribution. Capillary electrophoresis was used to quantify changes in reactant (CrIII–IDA and CrIII–NTA) and product (CrVI, free IDA and free NTA) concentrations as a function of time. In addition, a small number of experiments were performed using solutions prepared from CrIII alum (KCr(SO4)2·12H2O(s)) as the CrIII reactant. CrIII–IDA and CrIII–NTA were oxidised to CrVI, but rates were considerably lower than those obtained using inorganic CrIII. Within the timescales of our experiments, complete conversion of CrIII–NTA occurred at pH >7, but not under moderately acidic conditions, even when there was a large stoichiometric excess of HMO. MnCl2 addition experiments indicated that the observed reaction inhibition was attributable to MnII generation during the reaction. Our previous work has shown that citric acid, IDA, NTA and ethylenediaminetetraacetic acid solubilise CrIII from amorphous Cr(OH)3(s) at appreciable rates. The results of this study show that HMO is capable of oxidising the resulting soluble CrIII complexes, providing a viable mechanism for CrIII oxidation to CrVI over a wide pH range.

Selective complexation of α-amino acids and simple peptides via their carboxylate groups.

The complexation of anions of selected α-amino acids (alanine, valine, proline, tyrosine) and small peptides (L-alanyl-L-alanine, L-alanyl-L-alanyl-L-alanine, and L-alanyl-L-alanyl-L-alanyl-L-alanine) by the dinuclear nickel(II) complex [LNi2(μ-Cl)]+ (1), where (L)2− represents a 24-membered binucleating hexamine-dithiophenolato ligand, has been investigated. The following complexes were prepared, isolated as perchlorate or tetraphenylborate salts, and characterized by UV/Vis, IR, and CD spectroscopy: [LNi2(μ-L-alaninato)]+ (2), [LNi2(μ-L-valinato)]+ (3), [LNi2(μ-L-prolinato)]+ (4), [LNi2(μ-L-tyrosinato)]+ (5a), [LNi2(μ-D-tyrosinato)]+ (5b), [LNi2(μ-L,D-tyrosinato)]+ (5c), [LNi2(μ-L-alanyl-L-alaninato)]+ (6), [LNi2(μ-(L-alanyl)2-L-alaninato)]+ (7), [LNi2(μ-(L-alanyl)3-L-alaninato)]+ (8). Compounds 4, 5a and 6 were additionally identified by X-ray crystallography. In contrast to unsupported amino carboxylate complexes which typically contain five membered NO chelate rings, the [LNi2]2+ fragment selectively binds the α-amino acids and peptides via μ1,3-bridging carboxylato groups. Coordination of the carboxylato coligands in this way confers dissymmetry on the complexes. The CD spectra of the syn,syn-bridged structures are significantly different from those of the NO chelates, and can distinguish between the two coordination modes. The encapsulation of the peptides increases their solubility in the solvent system MeOH–MeCN by up to two orders of magnitude. This is discussed in terms of the absence of intermolecular hydrogen bonding interactions as indicated in the X-ray structure of 6.

Rates of Water Exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 Molecule and Its Implications for Geochemistry

The ammonium salt of [Fe(4)O(OH)(hpdta)(2)(H(2)O)(4)](-) is soluble and makes a monospecific solution of [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N',N'-tetraacetate). This tetramer is a diprotic acid with pK(a)(1) estimated at 5.7 ± 0.2 and pK(a)(2) = 8.8(5) ± 0.2. In the pH region below pK(a)(1), the molecule is stable in solution and (17)O NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (±2.6) × 10(5) s(-1), ΔH(++) = 46 (±4.6) kJ mol(-1), ΔS(++) = 22 (±18) J mol(-1) K(-1), and ΔV(++) = +1.85 (±0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H(+)] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (±0.8) × 10(5) s(-1). These rates are ∼10(4) times faster than those of the [Fe(OH(2))(6)](3+) ion (k(ex)(298) = 1.6 × 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental ΔH(++) and ΔS(++) values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between <Fe(III)-OH(2)> bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.

Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer catalysts

The synthesis and characterization of optically active amino carboxylate complexes of formula [(η(6)-arene)Ru(Aa)Cl] (arene = C(6)H(6), C(6)Me(6), Aa = amino carboxylate) as well as those of the related trimers [{(η(6)-arene)Ru(Aa)}(3)][BF(4)](3) are reported. Trimerization takes place with chiral self-recognition: only diastereomers equally configured at the metal, R(Ru)R(Ru)R(Ru) or S(Ru)S(Ru)S(Ru), are detected. The crystal structures of the complexes [(η(6)-C(6)H(6))Ru(Pip)Cl] and [{(η(6)-C(6)Me(6))Ru(Pro)}(3)][BF(4)](3) have been determined by X-ray diffraction methods. Both types of complexes catalyse the hydrogen transfer reaction from 2-propanol to ketones with moderate enantioselectivity (up to 68% ee). The enantiodifferentiation achieved can be accounted for by assuming that Noyori's bifunctional mechanism is operating.

Kinetics of Ga(NOTA) Formation from Weak Ga-Citrate Complexes

Gallium complexes are gaining increasing importance in biomedical imaging thanks to the practical advantages of the (68)Ga isotope in Positron Emission Tomography (PET) applications. (68)Ga has a short half-time (t(1/2) = 68 min); thus the (68)Ga complexes have to be prepared in a limited time frame. The acceleration of the formation reaction of gallium complexes with macrocyclic ligands for application in PET imaging represents a significant coordination chemistry challenge. Here we report a detailed kinetic study of the formation reaction of the highly stable Ga(NOTA) from the weak citrate complex (H(3)NOTA = 1,4,7-triazacyclononane-1,4,7- triacetic acid). The transmetalation has been studied using (71)Ga NMR over a large pH range (pH = 2.01-6.00). The formation of Ga(NOTA) is a two-step process. First, a monoprotonated intermediate containing coordinated citrate, GaHNOTA(citrate)*, forms in a rapid equilibrium step. The rate-determining step of the reaction is the deprotonation and slow rearrangement of the intermediate accompanied by the citrate release. The observed reaction rate shows an unusual pH dependency with a minimum at pH 5.17. In contrast to the typical formation reactions of poly(amino carboxylate) complexes, the Ga(NOTA) formation from the weak citrate complex becomes considerably faster with increasing proton concentration below pH 5.17. We explain this unexpected tendency by the role of protons in the decomposition of the GaHNOTA(citrate)* intermediate which proceeds via the protonation of the coordinated citrate ion and its subsequent decoordination to yield the final product Ga(NOTA). The stability constant of this intermediate, log K(GaHNOTA(citrate)*) = 15.6, is remarkably high compared to the corresponding values reported for the formation of macrocyclic lanthanide(III)-poly(amino carboxylates). These kinetic data do not only give mechanistic insight into the formation reaction of Ga(NOTA), but might also contribute to establish optimal experimental conditions for the rapid preparation of Ga(NOTA)-based radiopharmaceuticals for PET applications.

Photochemical decarboxylation and deacetylation of the cobalt(III) aminocarboxylate complexes: The crystal structure of the [Co(CH2CH2CH2NH2)(En)2)]Br2 and [Co(Bipy)(Cl)(Edma)]Cl · 2H2O photoproducts (En is Ethylenediamine, Bipy is 2,2′-bipyridine, and Edma is ethylenediaminemonoacetate)

The crystals of [Co(CH2CH2CH2NH2)(En)2]Br2 (I) and [Co(Bipy)(Cl)(Edma)]Cl · 2H2O (IIa) (IIa) are studied by X-ray diffraction analysis. Compound I is synthesized by the crystallization of the [Co(En)2(Amb)]2+ primary photolysis products. Compound IIa is synthesized from the [Co(Bipy)(Edda)]+ final photolysis products (En is ethylenediamine; Bipy is 2,2′-bipyridine; Edma and Edda are the anions of ethylenediaminemonoacetic and ethylenediamine-N,N′-diacetic acids, respectively; Amb is the 4-aminobutyrate ion). The crystal structure of complex I indicates the contraction of the seven-membered aminobutyrate CoO2CCH2CH2CH2NH2 ring to the five-membered CoCH2CH2CH2NH2 ring by the photoelimination of the CO2 molecule. The formation of the Co(III) complexes with the Edma ligands upon the photolysis of [Co(Bipy)(Edda)]+ is due to successive reactions of contraction of the five-membered aminoacetate rings and hydrolysis of the Co-C bond.

Aminocarboxylate complexes of vanadium(III): Electronic structure investigation by high-frequency and -field electron paramagnetic resonance spectroscopy

Aminocarboxylate complexes of vanadium(III) are of interest as models for biologically and medicinally relevant forms of this interesting and somewhat neglected ion. The V(III) ion is paramagnetic, but not readily suited to conventional EPR, due to its integer-spin ground state (S=1) and associated large zero-field splitting (zfs). High-frequency and -field EPR (HFEPR), however, has the ability to study such systems effectively. Three complexes, all previously structurally characterized: Na[V(trdta)].3H(2)O, Na[V(edta)(H(2)O)].3H(2)O, and [V(nta)(H(2)O)(3)].4H(2)O (where trdta stands for trimethylenediamine-N,N,N',N'-tetraacetate and nta stands for nitrilotriacetate) were studied by HFEPR. All the investigated complexes produced HFEPR responses both in the solid state, and in aqueous solution, but those of [V(nta)(H(2)O)(3)].4H(2)O were poorly interpretable. Analysis of multi-frequency HFEPR spectra yielded a set of spin Hamiltonian parameters (including axial and rhombic zfs parameters: D and E, respectively) for these first two complexes as solids: Na[V(trdta)].3H(2)O: D=5.60 cm(-1), E=0.85 cm(-1), g=1.95; Na[V(edta)(H(2)O)].3H(2)O: D=1.4 cm(-1), E=0.14 cm(-1), g=1.97. Spectra in frozen solution yielded similar parameters and showed multiple species in the case of the trdta complex, which are the consequence of the flexibility of this ligand. The EPR spectra obtained in frozen aqueous solution are the first, to our knowledge, of V(III) in solution in general and show the applicability of HFEPR to these systems. In combination with very insightful previous studies of the electronic absorption of these complexes which provided ligand-field parameters, it has been possible to describe the electronic structure of V(III) in [V(trdta)](-) and [V(edta)(H(2)O)](-); the quality of data for [V(nta)(H(2)O)(3)] does not permit analysis. Qualitatively, six-coordinate V(III) complexes with O,N donor atoms show no electronic absorption band in the NIR region, and exhibit relatively large magnitude zfs (D5 cm(-1)), while analogous seven-coordinate complexes do have a NIR absorption band and show relatively small magnitude zfs (D<2 cm(-1)).

Investigation of the kinetics of aquation of the 1:2 complex between Cr III and nitrilotriacetic acid

Aquation of the 1:2 complex between Cr III and nitrilotriacetic acid (NTA) was monitored using a combination of capillary electrophoresis (CE), ultraviolet–visible (UV–vis) spectrophotometry, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. To our knowledge, this is the first published report of the use of either CE or ATR-FTIR to monitor the kinetics of ligand exchange reactions of Cr III–aminocarboxylate complexes. The aquation products were identified as the 1:1 Cr III complex with NTA and “free” NTA. The 1:1 complex dimerized to form a 2:2 complex in a slower subsequent reaction step. Rates of disappearance of the 1:2 complex were first-order under all experimental conditions. First-order rate constants for aquation, k obs (h −1), measured using all three techniques were similar at equivalent pH conditions, and with values reported previously in the literature. Measured k obs values exhibited a complicated pH dependence with three distinct regions: (i) at pH < 6.5, k obs values increased with decreasing pH, (ii) between pH 6.5 and 8.0, k obs values were relatively constant, and (iii) at 8.0 < pH < 10.0, k obs increased with increasing pH and then leveled off. A kinetic model incorporating five distinct aquation pathways was successfully employed to model the pH dependence of k obs from 0.0 < pH < 10.0. These results show that CE and ATR-FTIR can be used as tools for better understanding ligand exchange processes occurring in aqueous solution.

Micellar-extraction concentration of cobalt and nickel in the form of aminocarboxylate complexes

The paper has investigated extraction of carboxylate, amine, and aminocarboxylate complexes of cobalt and nickel into a phase of a nonionic surface-active substance at the cloud temperature. The technique of atomic-absorption determination of these metals in natural and waste waters has been developed with preliminary micellar-extraction concentration.

Synthesis and X-ray structures of rhenium(I) carbonyl aminoalkoxide and aminocarboxylate complexes

The complexes [Re{MeN(CH 2CH 2O)(CH 2CH 2OH)-κ 3 N, O, O}(CO) 3] ( 1), [Re{N(CH 2CH 2O)(CH 2CH 2OH) 2-κ 3 N, O, O}(CO) 3] ( 2), [Me 3NH] 2[(OC) 3Re{N(CH 2CO 2) 2-κ 3 N, O, O}CH 2CH 2{N(CH 2CO 2) 2-κ 3 N, O, O}Re(CO) 3] ( 3), [Me 3NH] 2[Re 2{μ 2-2,6-(O 2C) 2(C 5H 3N)-κ 3 N, O, O} 2(CO) 6] ( 4) and [Re 2{μ 2-2,6-(OCH 2)(C 5H 3N)(CH 2OH)-κ 2 N, O} 2(CO) 6] ( 5) were synthesized in high yields via the reactions of [Re 2(CO) 10] and Me 3NO with MeN(CH 2CH 2OH) 2, N(CH 2CH 2OH) 3, EDTA, pyridine-2,6-dicarboxylic acid and pyridine-2,6-dimethanol, respectively. Complexes 1– 5 were characterized by IR and 1H NMR spectroscopy, elemental analysis and X-ray crystallography.

Fine-tuning water exchange on GdIII 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.

Estimating the Number of Bound Waters in Gd(III) Complexes Revisited. Improved Methods for the Prediction of q-Values

Two literature computational methods for the prediction of the number of inner-sphere aqua ligands, q, have been applied to a test set of seven Gd(aminocarboxylate) complexes. The first method is based on the hypothesis that q should be proportional to the solvent-accessible surface area of the ligand-complexed Gd ion (Castonguay, L. A., Treasurywala, A. M., Caulfield, T. J., Jaeger, E. P., and Kellar, K. E. (1999) Prediction of q-Values and Conformations of Gadolinium Chelates for Magnetic Resonance Imaging. Bioconjugate Chem. 10, 958). The second method is based on the hypothesis that q-values can be deduced by examining series of steric energy versus ionic radii plots as a function of coordination number (Reichert, D. E., Hancock, R. D., and Welch, M. J. (1996) Molecular Mechanics Investigation of Gadolinium(III) Complexes. Inorg. Chem. 35, 7013). This study identifies deficiencies in these methods and, with respect to the first method, describes some apparent errors. Although neither method was reliable at predicting q, two alternate approaches based on either molecular mechanics strain thresholds or exposed Gd surface area thresholds are shown to predict observed q-values for all Gd aminocarboxylate complexes in the test set.

Ruthenium(III) triazacyclononane dithiocarbamate, pyridinecarboxylate, or aminocarboxylate complexes as scavengers of nitric oxide.

The preparation of a series of [Ru(III)(tacn)(eta(2)-dtc)(eta(1)-dtc)][PF(6)] (tacn = 1,4,7-triazacyclononane; dtc = dimethyldithiocarbamate, diethyldithiocarbamate, pyrrolidinedithiocarbamate, l-prolinedithiocarbamate, l-prolinemethyl ester dithiocarbamate, l-N-methylisoleucinedithiocarbamate) complexes, 5-11, is described. Complex 5 reacts with NO to form the ruthenium nitrosyl complex 12. A series of [Ru(III)(tacn)(pyc)Cl][PF(6)] (pyc = 2-pyridinecarboxylic acid, 2,4- and 2,6-pyridinecarboxylic acid) complexes, 14-16, were prepared along with [Ru(III)(tacn)(mida)][PF(6)] (mida = N-methyliminodiacetic acid), 13, and [Ru(III)(Hnota)Cl], 17, (Hnota = 1-acetic acid-4,7-bismethylcarboxylate-1,4,7-triazacyclononane). Complexes 5-17 were evaluated for use as NO scavengers in an in vitro assay using RAW264 murine macrophage cells. [Ru(III)(tacn)(eta(2)-dtc)(eta(1)-dtc)][PF(6)] complexes 5-11 are very efficient NO scavengers in this assay.

Aminocarboxylate complexes and octreotide complexes with no carrier added 177Lu, 166Ho and 149Pm

Several aminocarboxylate complexes of the “no carrier added” (NCA) radiolanthanides 149Pm, 166Ho and 177Lu were evaluated using our in vitro hydroxyapatite and serum stability model and in vivo in normal CF-1 mice [10]. The aminocarboxylate chelates evaluated with the NCA radiolanthanides for in vitro stability were EDTA, CDTA, DTPA, MA-DTPA and DOTA. In addition, the NCA radiolanthanide complexes with DTPA-octreotide (DTPA-OCT) were synthesized and evaluated, as a model for a peptide conjugated aminocarboxylate complex. The biodistribution studies of the NCA complexes with DTPA, DOTA and DTPA-OCT showed that the in vitro model correctly predicted the in vivo stability of the radiolanthanide complexes, with Ln-DOTA > Ln-DTPA > Ln-DTPA-OCT.

Structural XAFS Investigation of Eu2+and Sr2+Poly(amino carboxylates): Consequences for Water Exchange Rates on MRI-Relevant Complexes

The structure of the Eu2+ and Sr2+ DOTA4- (1,4,7,10-tetraazacyclododecane-1,4,7,10-tertraacetate), DTPA5- (diethylenetriamine-N,N,N‘,N‘ ‘,N‘ ‘-pentaacetate), and ODDA2- (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetate) complexes were characterized using XAFS in the solid state and in aqueous solution. The results show the structural similarity between the highly paramagnetic and MRI-relevant Eu2+ poly(amino carboxylate) complexes with their diamagnetic Sr2+ homologues in each state as well as the overall conservation of the solid-state structure in aqueous solution. The DOTA4- ligand adopts a twisted-square antiprism conformation in aqueous solution to accommodate the large Eu2+ and Sr2+ ions, leading to metal ion-to-coordinated water distances 0.2 Å longer in the [MII(DOTA)(H2O)]2- complexes than in the [MII(DTPA)(H2O)]3- complexes (MII = Eu2+, Sr2+). The different structures adopted by the complexes in aqueous solution were found to be responsible for their different water exchange mechanisms: changing from D (DTPA5-, DOTA4-) for Gd3+ to Id (DTPA5-), I (DOTA4-), and Ia (ODDA2-) for the Eu2+ complexes. Finally, a lower charge density and substantially longer M−Ow distances for the Eu2+ ion explain the 3 orders of magnitude higher water exchange rates observed for the Eu2+ poly(amino carboxylates) over the corresponding Gd3+ complexes. Such high water exchange rates could be valuable in designing more efficient responsive contrast agents for magnetic resonance imaging.

Synthesis and near-IR luminescence properties of neodymium(iii) and ytterbium(iii) complexes with poly(pyrazolyl)borate ligands

We have prepared neodymium(III) and ytterbium(III) complexes from a range of poly(pyrazolyl)borate ligands and studied their near-IR luminescence properties, in particular using solution lifetimes obtained in protonated and deuterated methanol to determine the extent of solvation of the complexes. For the neodymium complexes, the luminescence lifetime in methanolic solutions increases as solvent is excluded from the inner coordination sphere. However, for a given inner sphere coordination number, these complexes have longer luminescence lifetimes than complexes with aminocarboxylate ligands: there are fewer proximate C–H oscillators in the pyrazolylborate complexes compared to the aminocarboxylate complexes, and non-radiative quenching is therefore reduced. This bears out earlier suggestions that long luminescence lifetimes can be obtained by minimising the number of close diffusing X–H oscillators.

Preparation of acetylated pyranoid glycals from glycosyl halides by chromium(II) complexes under aqueous biphasic conditions

Reactions of chromium(II) aminocarboxylate complexes (ethylenediamine-tetraacetate, nitrilo-triacetate, imino-diacetate) with acetylated glycopyranosyl chlorides or bromides in liquid–liquid or solid–liquid biphasic media (such as water–diethylether, water–ethyl acetate or halosugar–water) give the corresponding glycals in 75–97% yield.