41 research outputs found
Unconventional Approaches in Coordination Chemistry and Organometallic Reactivity
This contribution highlights a number of approaches developed by coordination and organometallic chemists that go beyond mainstream inorganic reactivity. A few of these strategies have been known for decades and are fundamental tools in synthesis and catalysis, while others are more recent and still belong to a niche. Through selected examples, we show herein how transmetalation, metal exchange, metal cooperativity, and catalytic transformation of metal complexes provide unique new opportunities to expand the reactivity arsenal of inorganic systems for synthetic and technological applicationsL.S. acknowledges financial support from the Spanish State Research Agency for the grant PID2019-109111RB-I00 and the Spanish Multi-MetDrugs network (RED2018-102471-T) for fruitful discussions. This work was performed under the Severo Ochoa Centres of Excellence Program of the Spanish State Research Agency, Grant No. CEX2018-000867-S (DIPC)
HIV-1 Very Low Level Viremia is Associated with Virological Failure in HAART-treated Patients
Structural and electronic characterization of nano-sized inorganic materials by X-ray Absorption Spectroscopies
MONITORING OF VIROLOGICAL AND PHARMACOLOGICAL CEREBROSPINAL FLUID PARAMETERS IN HAART-TREATED HIV-POSITIVE PATIENTS IN THE CLINICAL SETTING
Supramolecular bulky phosphines comprising 1,3,5-triaza-7-phosphaadamantane and Zn(salphen)s: structural features and application in hydrosilylation catalysis
Supramolecular, photophysical and catalytic properties of zn(salphen) based complexes and materials
Los Salen [N, N'-bis(salicilideno)etilendiamina] son ligandos tetradentados lineales, sistemas N2O2-coordinados, que se obtienen de la reacción de condensación entre la diamina y los precursores(sustituidos)del salicilaldehído. Los complejos metálicos de salen combinan las ventajas típicas tales como la facilidad de la variación en las características estéricas y electrónicas, y el uso de estrategias modulares que permiten la manipulación de diferentes fragmentos de los sistemas de ligandos. Los complejos metálicos de salen son versátiles y han demostrado ser catalizadores eficaces para muchas conversiones asimétricas, incluyendo (ep)oxidaciones, reacciones de apertura de anillo de epóxidos y polimerizaciones estéreo-selectivas. Últimamente, los ligandos de salen con puentes de fenilo ("salphens" o "salophens", Esquema 1) y estructuras relacionadas, han empezado a llamar la atención debido a sus ventajas particulares con respecto a sus análogos de salen; estasestructuras representan sistemas de ligandos π-conjugados con propiedades fotofísicas ajustables, y son más rentables que los correspondientes ligandos de salen (quirales). Por estas razones, los sistemas salphen tienen un excelente potencial como pilares básicos en la ciencia de materiales, entre otras aplicaciones. Esquema 1. Representación esquemática de un ligando salphen que muestra el bolsillo de unión del metal N2O2y las diversas sustituciones posibles.También debe tenerse en cuenta que la geometría rígida alrededor del centro metálico, dictada por el ligando salphen, puede ser utilizada para manipular las propiedades tales como, el carácter ácido de Lewis del metal y por lo tanto se puede aplicar eficazmente para aumentar la reactividad del complejo resultante. Por lo tanto, el trabajo presentado en esta tesis se ha centrado en el estudio de propiedades de auto-ensamblaje y en el desarrollo de nuevas aplicaciones para los complejos de Zn(salphen).Los dos primeros capítulos (2 y 3) presentan estudios completos sobre el auto-ensamblaje de diferentes complejos de Zn(salphen) multinucleares, centrándose en el tipo y la intensidad de las interacciones involucradas.En el Capítulo 2 se describe la síntesis y caracterización de un complejo macrocíclico de tetra-Zn(salphen), utilizando una aproximación modelo del metal. El complejo macrocíclico de Zn muestra un fuerte auto-ensamblaje en comparación con una serie de compuestos de referencia/modelo.El proceso de auto-ensamblaje está mediado por la formación de unidades diméricas intermoleculares de Zn(salphen) que se mantienen unidos a través de puentes μ2-fenoxo (Esquema 2). Los experimentos de valoración competitiva UV-vis/fluorescencia sugieren que un aumento molecular del número de unidades de Zn(salphen) eselresponsable de una formación fuerte de agregados supramoleculares, y que la desagregación está acompañada por un fuerte efecto hipercrómico. El fuerte auto-ensamblajedel complejo macrocíclico de tetra-Zn(salphen) estátambién soportado por estudios de RMN, espectrometría de masas y estudios de microscopía electrónica de transmisión (TEM). Esquema 2.A la izquierda la estructura del complejo macrocíclico detetra-Zn(salphen), a la derecha, la naturaleza dimérica de auto-ensamblaje mostradapor las unidades estándar de Zn(salphen).El capítulo 3 representa un estudio sobre el auto-ensamblaje extremadamente fuerte de un complejo de bis-Zn(salphen) tanto en solución, así como en una interfase sólido-líquido. Las moléculas de bis-Zn(salphen) han demostrado auto-organizarse en agregados oligoméricos extremadamente estables(Figura 1), que no pueden ser disociados por la adición de ligandos donantes. Este comportamiento en la interfase líquido-sólido ha demostrado estar en excelente acuerdo con los estudios de soluciones: valoraciones competitivas con UV-Vis y fluorescencia, dispersión de luz dinámica (DLS) y microscopía electrónica de transmisión (TEM). El análisis a nivel DFT llevado a cabo para el complejobis-Zn(salphen) muestra que opera un modo de unión inusual, y también racionaliza la alta estabilidad de las estructuras auto-ensambladas provocadas por motivos de coordinaciónoligomérica (ZnO)ndentro delconjunto (oligomérico). Este modo de coordinación es sorprendentemente diferente en comparación con los análogos mononucleares de Zn(salphen),los cuales forman estructuras diméricas que tienen una unidad central típica Zn2O2(Esquema 2 a la derecha). Por lo tanto, la alta estabilidad de la estructura multinuclear representa una gran promesa para el desarrollo de materiales de auto-ensamblaje estables con potencial para la obtención de nuevos materiales opto-electrónicos. Figura 1.A la izquierda,la estructura de los complejos de bis-Zn(salphen).En el centro, la imagen STM del agregado oligomérico; a la derecha, el modo de auto-ensamblaje del conjunto pentamérico muestra las cadenas oligoméricas (Zn-O)n entre las unidades de bis-Zn(salphen), (optimización en DFT).El capítulo 4reporta la síntesis de una nueva clase de complejos de Zn(II) de bases de Schiff que incorporan una unidad de fenazina en la cadena principal (designados como compuestos de "salphenazina"). Los complejos de salphenazina muestran propiedades fotofísicas interesantes, en comparación con los complejos de salen y salphen típicos, sin dejar de ser fácilmente sintetizados y ajustables. A partir de los datos de UV-Vis experimentales y teóricos, las bandas de absorción han sido asignadas a transiciones-* que están directamente influenciadas por el tipo de sustituyente presente en el armazón de la salphenazina (Figura 2). Por lo tanto, a través de una elección sensata de estos grupos, es posible obtener sistemas de propiedades fotofísicas deseados. Los Complejos de Zn(salphenazina) también han demostrado que tienen potencial aplicación en las células solares orgánicas de moléculas pequeñas (smOSCs), debido a su alta estabilidad y propiedades electrónicas adecuadas contra el aceptor C60. A pesar de que la eficiencia del dispositivo todavía no es competitiva, los complejos de Zn(salphenazina) muestran el potencial sintético para una mejora significativa sobre la base de la construcción modular de estos complejos foto-activos. Figura2. Estructura del complejo de Zn(salphenazina) (a la izquierda), y una transición-* seleccionada, representada por mapas de densidad electrónica diferencial EDDM (a la derecha).El capítulo 5reporta un estudio DFT detallado del mecanismo de reacción de la adición de expansión de anillo de CO2 a epóxidos, catalizada por un sistema binario compuesto por un complejo Zn(salphen) y NBu4X(X = Br, I). La reacción catalítica se ha estudiado en detalle describiendo sus tres pasos principales: la apertura del anillo del epóxido, la etapa de inserción de CO2 y un final cierre del anillo para la formación de carbonatos cíclicos de cinco miembros. Se examinó la etapa de apertura de anillo estableciendo la preferencia para la apertura del mismo en el carbono del metileno (C) o en el carbono del metino (C). También se exploró la inserción de CO2 y el cierre del anillo para seis distintos epóxidos sustituidos, resultando ser significativamente más difícil en comparación con la etapa de apertura del anillo. Los resultados combinados han ayudado a explicar anteriores observaciones experimentales y limitaciones del catalizador. Esquema 3.La adición de expansión de anillo de CO2 con epóxidos (substratos 1-6 usados en esta investigación) para la generación de carbonatos cíclicos de cinco miembros, usando el catalizador binario compuesto por Zn(salphen) (Cat) y NBu4X (X = I, Br).“Salen” ligands are relatively planartetradentate, N2O2-coordinating ligand systems which are obtained by the condensation reaction of readily available diamine and (substituted) salicylaldehyde precursors. Metallosalen complexes combine a number of advantages such as ease of variation in steric and electronic features, and the use of modular strategies that enable manipulation of different fragments of the ligand systems. Metal-derived salen complexes are versatile and they have proven to be effective catalysts for many asymmetric conversions including (ep)oxidations, epoxide ring-opening reactions and stereo-selective polymerizations. Lately, phenyl-bridged salen ligands (designated as “salphens” or “salophens”, Scheme 1) and related structures have started to raise the attention of the scientific community due to their particular advantages over their salen analogues; they represent π-conjugated ligand systems with tuneable photophysical properties, and are more cost-effective than the corresponding (chiral) salen ligands. For these reasons, such salphen systems have excellent potential as building blocks in material science amongst other applications. Scheme 1.Schematic drawing of a salphen ligand showing the N2O2 metal binding pocket and the various possibilities for substitution thereof.It should also be noted that the rigid geometry around the metal centre, dictated by the salphen ligand, can be used to manipulate properties such as the Lewis acid character of the metal and therefore it can be effectively applied to increase the reactivity, substrate binding properties and self-assembly behaviour of the resulting complex. Thus, the work reported in this thesis has principally focused on the study of novel self-assembly features and patterns, and on the development of other new application ofZn(salphen) complexes.The first two chapters (2 and 3) present detailed studies on the self-assembly of different multinuclear Zn(salphen)s focusing on the type and the strength of the interactions involved. In Chapter 2the synthesis and characterization of a tetra-Zn(salphen) macrocycleusing a metal template approachis described. The macrocyclic Zn complex shows strong self-assembly in comparison to a number of reference/model compounds. The self-assembly process is mediated by the formation of intermolecular dimericZn(salphen) units that are held together via μ2-phenoxo bridges (Scheme 2). Competitive UV-Vis/fluorescence titration experiments suggest that a molecular increase of the number of Zn(salphen) units is responsible for the formation of more stable supramolecular aggregates, and de-aggregation is accompanied by a strong hyperchromic effect. The strong self-assembly behaviour of the tetra-Zn(salphen) macrocyclewas further supported by NMR, mass spectrometry and transmission electron microscopy (TEM) studies. Scheme 2. On the left, line drawing of the tetra-Zn(salphen) macrocycle; on the right,a dimeric self-assembled Zn(salphen) complex showing the involved ZnO coordinative bonds.Chapter 3represents a study on the extremely strong self-assembly of a bis-Zn(salphen) complex both in solution as well as at a solid-liquid interface. The bis-Zn(salphen) molecules are shown to self-organize into extremely stable, edge-on orientated oligomeric aggregates (Figure 1), which could not be dissociated by addition of (small amounts of) donating ligands. This behaviour at the liquid-solid interface has demonstrated to be in excellent agreement with solution studies: competitive UV-Vis and fluorescence titrations, dynamic light scattering (DLS) and transmission electron microscopy (TEM) experiments are also in line with a highly increased stability for the assembled state of these bis-Zn(salphen)s. DFT analysis carried out for thebis-Zn(salphen) shows that an unusual binding mode is operative, and also rationalizes the very high stability of the self-assembled structures provoked by oligomeric (ZnO)n coordination motifs within the (oligomeric) assembly. This coordination mode is strikingly different when compared with mononuclear Zn(salphen) analogues that form dimeric structures having a typical Zn2O2 central unit (Scheme 2 on the right). The high stability of the multinuclear structure therefore holds great promise for the development of stable self-assembled monolayers (SAMs) with potential for new opto-electronic materials. Figure 1.On the left, line drawing of the bis-Zn(salphen) complexes. In the central part, section of the STM image of an oligomeric aggregate; on the right, the most likely self-assembly mode of a pentameric assembly showing the (Zn-O)noligomeric chains between the individual bis-Zn(salphen) units (DFT optimization).Chapter 4reports the synthesis of a new class of Zn(II) Schiff base complex incorporating a phenazine unit in the backbone and is designated as “Zn(salphenazine)”. These Zn(salphenazine) complexes show interesting photophysical properties compared to typical salen and salphen complexes while remaining easily synthesized and tuneable. From the experimental and theoretical UV-Vis data, the absorption bands have been assigned to -* transitions which are directly influenced by the type of substituents present in the salphenazine scaffold (Figure 2). Therefore, by a judicious choice of these groups, it is possible to obtain systems with desired photophysical properties. Zn(salphenazine) complexes have also shown to have potential application in small molecule Organic Solar Cells (smOSCs) due to their high stability and adequate electronic properties against the acceptor C60. Even though the efficiency of the device is not competitive yet, Zn(salphenazine)s show the synthetic potential for significant improvement based on the modular construction of these photo-active complexes. Figure 2. Schematic drawing of a Zn(salphenazine) complex (on the left) and a selected -* transition represented with electron density difference map EDDM (on the right). Scheme 3.The ring-expansion addition of CO2 to epoxides (substrates 1-6 used in this investigation) generating five-membered cyclic carbonates using a binary catalyst composed of a Zn(salphen) component (cf., Cat) and NBu4X (X = I, Br).Chapter 5reports a detailed DFT study of the reaction mechanism of the ring-expansion addition of CO2to epoxides catalyzed by a binary catalyst comprising a Zn(salphen) complex and co-catalytic NBu4X (X=Br, I). The catalytic reaction has been studied in detail describing the three main steps involved: the epoxide ring-opening, the CO2 insertion step and a final ring-closurestep to afford the five-membered cyclic carbonate products. The ring-opening step of the process was examined and the preference for opening at the methylene (C) or methine carbon (C) was established. Also, the CO2 insertion and the ring-closing steps have been explored for six differently substituted epoxides and proved to be significantly more challenging compared with the ring-opening step. The combined results have helped to explain earlier experimental observations and catalyst limitations, and are useful for improved catalyst design and development
NMR spectroscopy: a potent tool for studying monolayer-protected metal nanoclusters
Monolayer protected metal clusters are currently in the focus of interest both for fundamental reasons and for their use in possible applications. In the past two decades the interest was mainly focused on the evolution of the structrue and properties as the clusters grow in size. The field profited tremednously from mass spectrometry and X-ray structure analysis. For future applications of monolayer protected clusters other properties like the interaction of the clusters with molecules will become important. Also, it has been realized more recently that these monolayer protected clusters are rather dynamic, which calls for techniques able to address this property. By discussing selected examples we demonstrate the power of nuclear magnetic resonance (NMR) spectroscopy to study the structure and the dynamics of clusters and their interacion with molecules (sensing). NMR spectroscopy is an abundant technique and has become very sophisticated. Future work in the field of monolayer protected clusters may greatly profit from this. We believe that NMR spectroscopy, although not yet used much in the field of monolayer protected clusters, has the potential to become a key technique complementary to mass spectrometry and X-ray structure determination
Metal Complexes of Oxadiazole Ligands: An Overview
Oxadizoles are heterocyclic ring systems that find application in different scientific disciplines, from medicinal chemistry to optoelectronics. Coordination with metals (especially the transition ones) proved to enhance the intrinsic characteristics of these organic ligands and many metal complexes of oxadiazoles showed attractive characteristics for different research fields. In this review, we provide a general overview on different metal complexes and polymers containing oxadiazole moieties, reporting the principal synthetic approaches adopted for their preparation and showing the variety of applications they found in the last 40 years
Silver migration between Au<sub>38</sub> (SC<sub>2</sub>H<sub>4</sub> Ph)<sub>24</sub> and doped Ag<sub>x</sub>Au<sub>38−x</sub>(SC<sub>2</sub>H<sub>4</sub>Ph)<sub>24</sub> nanoclusters
A fast redistribution of metal atoms occurs upon mixing the AgxAu38−x and Au38 nanoclusters in solution, as observed by mass spectrometry. Physical separation of AgxAu38−x and Au38 species by a dialysis membrane prohibits the metal migration, which suggests that collisions between the reacting clusters are at the origin of the observation