Sandra Gago

Preparation and photophysical characterisation of Zn–Al layered double hydroxides intercalated by anionic pyrene derivatives

Zn–Al hydrotalcite-like compounds intercalated by 1,3,6,8-pyrenetetrasulfonate (PTS), 1-pyrenesulfonate (PS) and 1-pyrenecarboxylate (PC) anions were synthesised by an ion-exchange procedure. The materials were characterised by powder X-ray diffraction at different temperatures, thermogravimetric analysis, FTIR, 13C{1H} CP/MAS NMR and photoluminescence techniques. In the fully hydrated states, the interlayer distances are 13.7 A for Zn–Al–PTS, 18.9 A for Zn–Al–PS, and 24.8 A for Zn–Al–PC. These can be ascribed to a monolayer arrangement for intercalated PTS anions and bilayer arrangements for the 1-pyrenyl derivatives. The samples exhibit different thermal decomposition pathways, and in the case of Zn–Al–PTS the removal of physisorbed and interlayer water leads to a change in the orientation of the organic anion with respect to the hydroxide layers. The structural transformation is fully reversible upon hydration. The photophysical characterisation of the bulk materials was based on the determination of their emission and fluorescence excitation spectra, and the fluorescence lifetimes. From the steady-state (monomer and “excimer-like” bands) and time-resolved (triple exponential decays) data, evidence for the presence of a structure with similar characteristics to pyrene dimer together with monomer (by comparison with the emission of dilute solutions of PS and PC), pre-associated and (possibly) dynamic excimer species could be presented.

Synthesis and catalytic properties of molybdenum(VI) complexes with tris(3,5-dimethyl-1-pyrazolyl)methane.

The complex [MoO(2)Cl{HC(3,5-Me(2)pz)(3)}]BF(4) (1) (HC(3,5-Me(2)pz)(3) = tris(3,5-dimethyl-1-pyrazolyl)methane) has been prepared and examined as a catalyst for epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (TBHP) as the oxidant. For reaction of cis-cyclooctene, epoxycyclooctane is obtained quantitatively within 5 h when water is rigorously excluded from the reaction mixture. Increasing amounts of water in the reaction mixture lead to lower activities (without affecting product selectivity) and transformation of 1 into the trioxidomolybdenum(VI) complex [{HC(3,5-Me(2)pz)(3)}MoO(3)] (4). Complex 4 was isolated as a microcrystalline solid by refluxing a suspension of 1 in water. The powder X-ray diffraction pattern of 4 can be indexed in the orthorhombic Pnma system, with a = 16.7349(5) Å, b = 13.6380(4) Å, and c = 7.8513(3) Å. Treatment of 1 in dichloromethane with excess TBHP led to isolation of the symmetrical [Mo(2)O(4)(μ(2)-O){HC(3,5-Me(2)pz)(3)}(2)](BF(4))(2) (2) and unsymmetrical [Mo(2)O(3)(O(2))(2)(μ(2)-O)(H(2)O){HC(3,5-Me(2)pz)(3)}] (3) oxido-bridged dimers, which were characterized by single-crystal X-ray diffraction. Complex 2 displays the well-known (Mo(2)O(5))(2+) bridging structure where each dioxidomolybdenum(VI) center is coordinated to three N atoms of the organic ligand and one μ(2)-bridging O atom. The unusual complex 3 comprises dioxido and oxidodiperoxo molybdenum(VI) centers linked by a μ(2)-bridging O atom, with the former center being coordinated to the tridentate N-ligand. The dinuclear complexes exhibit a similar catalytic performance to that found for mononuclear 1. For complexes 1 and 2 use of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate and N-butyl-3-methylpyridinium tetrafluoroborate as solvents allowed the complexes to be completely dissolved, and in each case the catalyst and IL could be recycled and reused without loss of activity.

Dynamics of short as compared with long poly(acrylic acid) chains hydrophobically modified with pyrene, as followed by fluorescence techniques.

New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol(-1) and 450,000 g mol(-1), respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio I(E)/I(M) decreases with the increase of pH, oppositely to the increase in the I(E)/I(M) ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pK(a) values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small- and long-sized PAA polymers, i.e., the size of the polymer matters.

Picosecond Dynamics of Dimer Formation in a Pyrene Labeled Polymer

A commercial poly(acrylic acid) (PAA, M(n) = 450 000 g mol(-1)) has been labeled with high levels (9.1 mol %) of pyrene by reaction with 1-pyrenylmethylamine in the presence of boric acid. The modified polymer was found to display an unusual photophysical behavior closely resembling that found for pyrene in constrained environments. The appearance of a band at longer wavelengths in the excitation spectra when collected at the long wavelength emission band was attributed to the spectra of a dimer. The emission of this dimer was particularly enhanced in dioxane:water solvent mixtures with low dioxane content. From time-resolved fluorescence measurements in the picosecond time domain, two decay components were obtained: a fast decay (4-10 ps) at short wavelengths, which becomes a rising component at longer wavelengths, and a second exponential (2-4 ns) related to the emission of the relaxed dimer. Time-resolved emission spectra were seen to change with time, revealing the emission contribution of two species. This is one of the first reports where the dynamics of dimer formation in a pyrene derivative have been observed.

Effect of β-cyclodextrin on the chemistry of 3′,4′,7-trihydroxyflavylium

The pH dependence of the network of chemical reactions involving 3′,4′,7-trihydroxyflavylium was studied and all the rate and equilibrium constants were determined in the presence and absence of β-cyclodextrin. The β-cyclodextrin increases the rate constant of the flavylium cation hydration to give the hemiketal and decreases the reverse rate constant (de-hydration). The quantum yield of the flavylium cation formation from irradiation of the trans-chalcone increases from 0.023 to 0.055, respectively, in the absence and presence of β-cyclodextrin. NMR spectral characterization of the inclusion complex between 3′,4′,7-trihydroxyflavylium and β-cyclodextrin was performed by ROESY and suggests a structure in which the hydroxyl substituents at positions 3′ and 4′ interact with the internal surface of the β-cyclodextrin.

Investigation of Layered Double Hydroxides Intercalated by Oxomolybdenum Catecholate Complexes

Oxomolybdenum(VI) complexes of 3,4-dihydroxybenzoic acid (3,4-H 2dhb) have been incorporated into layered double hydroxides (LDHs) by treatment of the LDH-nitrate (Zn-Al, Mg-Al) or LDH-chloride (Li-Al) precursors with aqueous or water/ethanol solutions of the complex (NMe 4) 2[MoO 2(3,4-dhb) 2].2H 2O at 50 or 100 degrees C. The texture and chemical composition of the products were investigated by elemental analysis and scanning electron microscopy (SEM) with coupled energy dispersive spectroscopy (EDS). Microanalysis for N and EDS analysis for Cl showed that at least 90% of nitrate or chloride ions were replaced during the ion exchange reactions. The final Mo content in the materials varied between 6.5 and 11.6 wt %. Mo K-edge EXAFS analysis, supported by IR, Raman, UV-vis, and (13)C{ (1)H} CP/MAS NMR spectroscopic studies, showed the presence of cointercalated [MoO 2(3,4-dhb) 2] ( m- ) and [Mo 2O 5(3,4-dhb) 2] ( m- ) complexes in proportions that depend on the type of LDH support and the reaction conditions. The binuclear bis(catecholate) complex, with a Mo...Mo separation of 3.16 A, was the major species intercalated in the Zn-Al and Li-Al products prepared using only water as solvent. The X-ray powder diffraction (XRPD) patterns of all the Mo-containing LDHs showed the formation of an expanded phase with a basal spacing around 15.4 A. High-resolution synchrotron XRPD patterns were indexed with hexagonal unit cells with a c-axis of either 30.7 (for Li-Al-Mo LDHs) or 45.9 A (for a Zn-Al-Mo LDH). Fourier maps ( F obs) calculated from the integrated intensities extracted from Le Bail profile decompositions indicated that the binuclear guest species are positioned such that the Mo --> Mo vector is parallel to the host layers, and the overall orientation of the complex is perpendicular to the same layers. The thermal behavior of selected materials was studied by variable-temperature XRPD, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC).

Contrasting pKa Shifts in Cucurbit[7]uril Host–Guest Complexes Governed by an Interplay of Hydrophobic Effects and Electrostatic Interactions

Cucurbit[7]uril inclusion complexes with guests bearing dimethylamino groups show the expected upward pKa shifts, whereas their diethylamino counterparts display a decrease in pKa due to the preferential stabilization of the unprotonated form. These results identify the diethylamino group as the substituent of choice to avoid receptor-assisted protonation of guest molecules and present new evidence for the role of the hydrophobic effect as a driving force in cucurbituril complexation.

Cobalt(III) sepulchrate complexes: application as sustainable oxidative catalysts

The application of cobalt sepulchrate (sep) complexes as active and robust homogeneous catalysts is reported here for the first time, as well as the crystal structure of the [Co(sep)]2(SO4)3·10H2O compound. The reaction chosen to investigate the catalytic performance of these catalysts was the oxidation of styrene due to the importance of its products in various industrial applications. Sustainable experimental conditions were selected, using H2O2 as the oxidant and ionic liquids as solvents. A similar catalytic activity was found using complexes containing different anions: [Co(sep)]Cl3 and [Co(sep)]2(SO4)3. Different kinetic profiles were found using different ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate or tetrafluoroborate) or acetonitrile as solvents. Complete conversions were achieved after 24 h of reaction. Benzaldehyde was the main product formed and in some cases the only one observed. The homogeneous cobalt catalysts using an ionic liquid as solvent could be recycled for consecutive cycles without significant loss of activity. In addition, the stability of the cobalt sepulchrate complex was confirmed by 1H NMR after catalytic use.

Unidirectional switching between two flavylium reaction networks by the action of alternate stimuli of acid and base.

The introduction of an ester group in the flavylium core allowed the reversible conversion between two different flavylium compounds each one exhibiting its own reaction network. An unidirectional switching cycle between 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyrylium and 2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium was achieved by means of alternate acid and base stimuli. Addition of base to a methanolic solution of the ester derivative gives rise to the trans-chalcone of the parent carboxylic acid, which upon acidification of the solution forms the respective flavylium cation. This species esterifies under very acidic conditions to restore the original methyl ester derivative. The chemical reaction networks of both compounds were fully characterized from their thermodynamic and kinetic aspects, by a series of pH jumps followed by UV-vis absorption and emission spectroscopy, stopped flow and (1)H NMR. The crystal structure of the trans-chalcone of the ester derivative was unveiled showing a supramolecular structure involving hydrogen bonding.

Heterometallic complexes involving iron(II) and rhenium(VII) centers connected by μ-oxido bridges

The reaction of FeI(2) with two equivalents of AgReO(4) in acetonitrile leads to yellow, crystalline Fe(ReO(4))(2)(CH(3)CN)(3) (1), and the treatment of 1 with four equivalents of CpFe(CO)(2)CN gives orange, crystalline Fe(ReO(4))(2)[CpFe(CO)(2)(CN)](4) (2). Compound 2 can also be prepared in one step by the reaction of FeI(2), AgReO(4) and CpFe(CO)(2)CN in dichloromethane. The structure of 1 consists of infinite chains in which alternating {Fe(CH(3)CN)(4)} and {Fe(ReO(4))(2)(CH(3)CN)(2)} units are linked by perrhenate anions to form a (-ReO(2)-O-Fe(ReO(4))(2)(CH(3)CN)(2)-O-ReO(2)-O-Fe(CH(3)CN)(4)-O-)(n) molecular wire. The structure of 2 shows a monomeric iron complex with a slightly distorted octahedral coordination environment consisting of four organometallic complexes coordinated in the equatorial plane via the cyanide groups and two monodentate perrhenates in the corresponding apical positions. Both compounds were further characterised in the solid state by IR spectroscopy, thermogravimetric analysis and magnetic susceptibility measurements. The magnetic data indicate that 1 behaves as a ferrimagnetic chain with 3D ordering below 8 K due to inter-chain interactions. Compound 2 has antiferromagnetic interactions within the Fe(ReO(4))(2)[CpFe(CO)(2)(CN)](4) clusters.