M. Luísa Ramos

Peroxovanadium(V) complexes of glycolic acid as studied by NMR spectroscopy

Multinuclear ( 1 H, 13 C, 17 O, 51 V) 1D and 2D NMR spectroscopy has been used to characterize the peroxovanadium(V) complexes of glycolic acid in aqueous solution. One 2:2:2 (metal:ligand:peroxo) complex, together with a 1:1:1 and a 2:2:1 species, are found in the pH range 1‐7. The 2:2:2 complex is a monoperoxo (one peroxide unit per vanadium atom) dinuclear species having a V2O3 4 seven-coordinated metal centre. In this structure, the two vanadium atoms are triple bridged, two of those bridges being formed by oxygen atoms of the hydroxyl group of the acid. The 1:1:1 species has a seven-coordinated VO 3 metal centre. Glycolic acid bonds to the vanadium atoms in a bidentate way, through both the carboxylic and the hydroxyl groups. The peroxo groups are bound in the equatorial plane relative to the apical VO and the geometry around each vanadium atom is close to pentagonal bipyramidal. The 2:2:1 complex is similar to the 2:2:2 species, except for one of the vanadium centres, which is now a five-coordinated oxovanadium centre. Three additional complexes are found in very small amounts for some pH and concentration conditions. Further support for the proposal of monoperoxovanadium species is given by UV‐visible spectroscopy results.

Conformational studies of poly(9,9-dialkylfluorene)s in solution using NMR spectroscopy and density functional theory calculations.

Relationships have been obtained between intermonomer torsional angle and NMR chemical shifts ((1)H and (13)C) for isolated chains of two of the most important poly(9,9-dialkylfluorenes), poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6) and the copolymer poly(9,9-dioctylfluorene-co-[2,1,3]benzothiadiazole-4,7-diyl) (F8BT), using DFT calculations. The correlations provide a model for NMR spectral data interpretation and the basis for analysis of conformational changes in poly(9,9-dialkylfluorene-2,7-diyl)s. The correlations obtained for PF2/6 indicate that the (13)C chemical shifts of the aromatic carbons close to the intermonomer connection (C1, C2, and C3) have minimum values at planar conformations (0 degrees and 180 degrees ) and maximum values at 90 degrees conformations. In contrast, the (1)H chemical shifts of the corresponding aromatic ortho protons (Ha and Hb) are greatest for planar conformations, and the minimum values are seen for 90 degrees conformations. For the F8BT copolymer, similar relationships are observed for the (1)H (Ha, Hb, and Hc) aromatic shifts. Considering the aromatic carbons of F8BT, the behavior of C2, C4, C5, and C6 is similar to that found for the PF2/6 carbons. However, C1 and C3 of the fluorene moiety behave differently with varying torsion angle. These are in close proximity to the fluorene-benzothiadiazole linkage and are markedly affected by interactions with the thiadiazole unit such that delta(C1) is a maximum for 180 degrees and a minimum for 0 degrees , whereas delta(C3) is a maximum for 0 degrees and minimum for 180 degrees. We have studied the (1)H and (13)C spectra of the two polymers at temperatures between -50 degrees C and +65 degrees C. The observed changes to higher or lower frequency in the aromatic resonances were analyzed using these theoretical relationships. Fluorescence studies on PF2/6 in chloroform solution suggest there are no significant interchain interactions under these conditions. This is supported by variable-temperature NMR results. Polymer-solvent and polymer intramolecular interactions were found to be present and influence all of the alkylic and one of the aromatic (1)H resonances (Hb). The detailed attribution of the (1)H and (13)C NMR spectra of the two polymers was made prior to the establishment of the relationships between torsion angle and NMR chemical shifts. This was carried out through DFT calculation of the (1)H and (13)C shielding constants of the monomers, coupled with distortionless enhancement by polarization transfer and heteronuclear correlation NMR spectra. Several DFT levels of calculation were tested for both optimization of structures and shielding constants calculation. The B3LYP/6-31G(d,p) method was found to perform well in both cases.

MULTINUCLEAR NMR STUDY OF THE COMPLEXATION OF D-GLUCARIC ACID WITH MOLYBDENUM(VI) AND TUNGSTEN(VI)

Abstract Proton and 13 C NMR spectra of aqueous solutions of sodium molybdate and D -glucaric acid for variable molar ratios and pH values (range 1–9) clearly show the existence of five complexes dominating for specific concentration and pH conditions: a 2:1 metal to ligand complex, a , dominant at low pH (pH ≈ 2) for dilute solutions; two other complexes, b and c , which seem to have 2:2 and 1:1 composition, respectively, and which dominate at low pH for higher concentrations; another 2:1 complex, d , formed at intermediate pH (pH ≈ 4.5–6); still another 2:1 complex, e , formed at higher pH (pH ≈ 6–8.5). From the 1 H and 13 C chemical shifts observed on complexation, the binding sites of D -glucaric acid to the metal are established. From the proton-proton coupling constants the approximate conformation of the bound ligand is determined. The structures proposed on this basis are partially supported by 95 Mo chemical shifts, and the structure changes for the 2:1 complexation with pH are rationalized. Similar results are expected with W(VI) but due to close chemical shifts and exchange phenomena, only a 2:1 complex at low pH is adequately characterized.

NMR, DFT and luminescence studies of the complexation of Zn(II) with 8-hydroxyquinoline-5-sulfonate.

Multinuclear ((1)H, (13)C and (27)Al) magnetic resonance spectroscopy (1D and 2D), DFT calculations and fluorescence have been used to study the complexation of 8-hydroxyquinoline-5-sulfonate (8-HQS) with Al(III). The study combines the high sensitivity of luminescence techniques, the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to provide a detailed understanding of the complexation between the Al(3+) ion and 8-HQS. A full speciation study has been performed and over the concentration region studied, the Al(3+) ion forms complexes with 8-HQS in an aqueous solution in the pH range 2-6. At higher pH, the extensive hydrolysis of the metal limits complexation. Using Jobs method, three complexes were detected, with 1 : 1, 1 : 2 and 1 : 3 (metal : ligand) stoichiometries. These results are in agreement with those previously reported using potentiometric and electrochemical techniques. The geometries of the complexes are proposed based on the combination of NMR results with optimized DFT calculations. All the complexes in aqueous solutions at 25 °C are mononuclear species, and have an approximately octahedral geometry with the metal coordinated to one molecule of 8-HQS and four molecules of water (1 : 1 complex), two molecules of 8-HQS and two molecules of water mutually cis (1 : 2 complex), and to three molecules of 8-HQS in non-symmetrical arrangement (mer-isomer), for the 1 : 3 (metal : ligand) complex. On binding to Al(III), 8-HQS shows a more marked fluorescence than the weakly fluorescent free ligand. In addition, as previously noted, there are marked changes in the absorption spectra, which support the use of 8-HQS as a sensitive optical sensor to detect Al(3+) metal ions in surface waters and biological fluids. These complexes also show potential for applications in organic light emitting diodes (OLEDs).

MULTINUCLEAR NMR STUDY OF COMPLEXATION OF D-GALACTARIC AND D-MANNARIC ACIDS WITH TUNGSTEN(VI) OXOIONS

Abstract The coordination compounds formed between W(VI) and D-galactaric and D-mannaric acids, in aqueous solution, have been studied by 1H, 13C, 17O and 183W NMR spectroscopy. In the pH range 3–8 for D-galactaric acid and 2–10 for D-mannaric acid, the acids are found to form n:n species (mainly 2:2) with tungstate, in which the ligands are bound to the metal by the two carboxylate groups and their adjacent OH groups. Above pH 6.5, a 2:1 species is also formed, in which all the OH functions are coordinated to the metal, the two carboxylate groups remaining free. The formation of symmetrical or asymmetrical species is discussed, taking into account the configuration of the ligands. Structures for the various complexes are formulated.

Density functional theory study of the oxoperoxo vanadium(V) complexes of glycolic acid. Structural correlations with NMR chemical shifts

The DFT B3LYP/SBKJC method has been used to calculate the gas-phase optimized geometries of the glycolate oxoperoxo vanadium(V) complexes [V(2)O(2)(OO)(2)(gly)(2)](2-), [V(2)O(3)(OO)(gly)(2)](2-) and [VO(OO)(gly)(H(2)O)](-). The (51)V, (17)O, (13)C and (1)H chemical shifts have been calculated for the theoretical geometries in all-electron DFT calculations at the UDFT-IGLO-PW91 level and have been subsequently compared with the experimental chemical shifts in solution. In spite of being applied to the isolated molecules, the calculations allowed satisfactory reproduction of the multinuclear NMR solution chemical shifts of the complexes, suggesting that the theoretical structures are probably close to those in solution. The effects of structural changes on the (51)V and (17)O NMR chemical shifts have been analysed using the referred computational methodologies for one of the glycolate complexes and for several small molecules taken as models. These calculations showed that structural modifications far from the metal nucleus do not significantly affect the metal chemical shift. This finding explains why it is possible to establish reference scales that correlate the type of complex (type of metal centre associated with a certain type of ligand) with its typical region of metal chemical shifts. It has also been found that the V[double bond, length as m-dash]O bond length is the dominant geometrical parameter determining both delta(51)V and the oxo delta(17)O in this kind of complex.

Oxoperoxo vanadium(V) complexes of L-lactic acid: density functional theory study of structure and NMR chemical shifts.

Various combinations of density functionals and pseudopotentials with associated valence basis-sets are compared for reproducing the known solid-state structure of [V 2O 2(OO) 2 l-lact 2] (2-) cis . Gas-phase optimizations at the B3LYP/SBKJC level have been found to provide a structure that is close to that seen in the solid state by X-ray diffraction. Although this may result in part from error compensation, this optimized structure allowed satisfactory reproduction of solution multinuclear NMR chemical shifts of the complex in all-electron DFT-IGLO calculations (UDFT-IGLO-PW91 level), suggesting that it is probably close to that found in solution. This combination of approaches has subsequently been used to optimize the structures of the vanadium oxoperoxo complexes [V 2O 3(OO) l-lact 2] (2-) cis , [V 2O 3(OO) l-lact 2] (2-) trans , and [VO(OO)( l-lact)(H 2O)] (-) cis . The (1)H, (13)C, (51)V, and (17)O NMR chemical shifts for these complexes have been calculated and compared with the experimental solution chemical shifts. Excellent agreement is seen with the (13)C chemical shifts, while somewhat inferior agreement is found for (1)H shifts. The (51)V and (17)O chemical shifts of the dioxo vanadium centers are well reproduced, with differences between theoretical and experimental shifts ranging from 22.9 to 35.6 ppm and from 25.1 to 43.7 ppm, respectively. Inferior agreement is found for oxoperoxo vanadium centers, with differences varying from 137.3 to 175.0 ppm for (51)V shifts and from 148.7 to 167.0 ppm for (17)O(oxo) shifts. The larger errors are likely to be due to overestimated peroxo O-O distances. The chosen methodology is able to predict and analyze a number of interesting structural features for vanadium(V) oxoperoxocomplexes of alpha-hydroxycarboxylic acids.

NMR spectroscopy study of the peroxovanadium(V) complexes of L-malic acid

Abstract Continuing the solution speciation studies of peroxovanadium(V) complexes with α-hydroxycarboxylic acids, we now report on the complexes formed with l -malic acid, studied by multinuclear (1H, 13C, 51V) 1D and 2D NMR spectroscopy. The system V(V)– l -malic acid–H2O2 in aqueous solution was found to involve a large number of species: nine major peroxovanadium(V) complexes in the pH range 2–7, the structures of which have been deduced, and eight weaker complexes in very low concentrations. The former are monoperoxo species with the stoichiometries (metal:acid:peroxide) 2:2:2 (two complexes), 2:2:1 (two complexes), 2:1:1 (two complexes) and 1:1:1 (three complexes). The 2:2:2 complexes have seven-coordinated metal centres which are doubly bridged by the hydroxyl oxygen atom of malic acid. In one case, all three functional groups of malic acid are bound to each of the two VO3+ metal centres, whereas in the other the C4 carboxylic group of the ligand is dangling, two possibilities being advanced for the vanadium centres: either one V2O34+ centre or two VO3+ centres. The two complexes of stoichiometry 2:2:1 are the result of degradation (loss of one peroxide unit) of the 2:2:2 species in which l -malic acid is a bidentate ligand; they have seven- and five-coordinated metal centres, respectively, the peroxo and the oxo centres, and the malate groups are equally bidentate. The two complexes of stoichiometry 2:1:1 have presumably V2O42+ metal centres and all three functional groups of l -malic acid are involved in chelation; isomerism is interpreted as due to which of the two non-equivalent V(V) atoms is the peroxo centre. The three 1:1:1 complexes have either seven- or six-coordinated VO3+ metal centres; in one of the complexes, l -malic acid acts as a tridentate ligand, whereas in the other two, the ligand is bidentate.

Multinuclear NMR study of the complexation of D-galactaric and D-mannaric acids with molybdenum(VI)

The coordination compounds formed between MoVI and d-galactaric and d-mannaric acids, in aqueous solution, have been studied by 1H, 13C, 17O and 95Mo NMR spectroscopy, and compared taking into account the configuration of the ligands. In the pH range ca. 2–9, four major complexes are detected (in slow exchange at room temperature). The results point to the following structures: a 4 : 2 complex, in acidic solution, two ligand molecules bridging two cis Mo2O2+5 moieties via all hydroxyl and carboxylate groups; a 2 : 1 complex, in basic solution, having a tetradentate ligand molecule bound to an Mo2O2+5 centre via the four hydroxyl groups; 2 : 2 and 1 : 1 species, all along the pH range, in which the ligands are bound to MoO2+2 centres via the carboxylate and their adjacent OH groups.

Peroxovanadium(V) Complexes of L-Lactic Acid as Studied by NMR Spectroscopy

Over the past few years increasing attention has been paid to the chemistry of peroxovanadium(V) complexes. This interest is mainly due to the important role of these complexes in biological systems and their application in oxidation reactions. Peroxovanadium(V) complexes have been found to have antitumour[1] and insulin mimetic activities,[2,3] and have been studied as functional models for the vanadium haloperoxidase enzymes.[4,5] These enzymes catalyse the oxidation of halides by hydrogen peroxide and are thought to be involved in the biosynthesis of a large number of marine natural products, many of them with potent antifungal, antibacterial, antiviral (e.g. HIV) and antineoplastic properties.[6] In addition, a large variety of oxidation reactions can be efficiently performed by peroxovanadium(V) complexes. These complexes have been shown to hydroxylate benzene and other aromatics, epoxidise and hydroxylate alkenes and allylic alcohols and oxidise sulfides and primary and secondary alcohols.[7] Previously, the vanadium(V) complexes that form with several α-hydroxycarboxylic acids were the object of a multinuclear NMR study carried out by this group.[8210] In view of the interest in peroxovanadium(V) compounds and the need to know their structures in order to fully understand both their chemistry and biochemistry, we have now extended our previous work to the more complex systems involving hydrogen peroxide. This paper deals with the peroxo complexes that form when hydrogen peroxide is added to a mixture of a vanadate(V) salt and -lactic acid in aqueous solution. Our intention was to investigate these species with respect to their number, stoichiometries, structures and stability by NMR spectroscopy. This technique has been intensively used in the study of vanadium(V)/hydrogen peroxide systems, both in the presence and absence of other