Instituto de Síntesis Química y Catálisis Homogénea

This review covers the advances made in the synthesis of luminescent transition metal complexes containing N-heterocyclic carbene (NHC) ligands. The presence of a high field strength ligand such as an NHC in the complexes gives rise to high energy emissions, and consequently, to the desired blue colour needed for OLED applications. Furthermore, the great versatility of NHC ligands for structural modifications, together with the use of other ancillary ligands in the complex, provides numerous possibilities for the synthesis of phosphorescent materials, with emission colours over the entire visible spectra and potential future applications in fields such as photochemical water-splitting, chemosensors, dye-sensitised solar cells, oxygen sensors, and medicine.

This tutorial review covers the recent advances made in the study of gold complexes containing N-heterocyclic carbene ligands with biological properties. The great stability, ease of modulation of the electronic properties and excellent σ-donating capacity displayed by NHCs allow gold-NHC derivatives to reach high stability in biological media and relatively good internalization into cells and for that they have emerged as excellent potential chemotherapeutics. The new gold-NHC derivatives show superior anticancer activity compared to other standards such as Cisplatin or Auranofin. In addition, the application of gold-NHC complexes in the treatment of other human diseases as antibacterial, antioxidant and antiparasitic agents is reviewed for the first time.

The emission of a bright blue fluorescence is a unique feature common to the vast variety of polymer carbon dots (CDs) prepared from carboxylic acid and amine precursors. However, the difficulty to assign a precise chemical structure to this class of CDs yet hampers the comprehension of their underlying luminescence principle. In this work, we show that highly blue fluorescent model types of CDs can be prepared from citric acid and ethylenediamine through low temperature synthesis routes. Facilitating controlled polycondensation processes, the CDs reveal sizes of 1-1.5 nm formed by a compact network of short polyamide chains of about 10 monomer units. Density functional theory calculations of these model CDs uncover the existence of a spatially separated highest occupied molecular orbital and a lowest unoccupied molecular orbital located at the amide and carboxylic groups, respectively. Photoinduced charge transfer between these groups thus constitutes the origin of the strong blue fluorescence emission. Hydrogen-bond-mediated supramolecular interactions between the polyamide chains enabling a rigid network structure further contribute to the enhancement of the radiative process. Moreover, the photoinduced charge transfer processes in the polyamide network structure easily explain the performance of CDs in applications as revealed in studies on metal ion sensing. These findings thus are of general importance to the further development of polymer CDs with tailored properties as well as for the design of technological applications.

The financial support of C.I.C.Y.T. (Project CTQ2005-08016 and Consolider Ingenio 2010 CSD2006-0003) is greatly acknowledged.

This critical review covers advances in the field of glycerol-derived solvents, with a particular focus on hazardous solvents substitution issues.

Abstract In this review, recent advances in catalytic asymmetric hydrophosphonylation of aldehydes and imines are discussed. We also summarize several proposed mechanisms for the different possibilities of asymmetric induction and the application of this catalytic methodology to the enantioselective synthesis of α‐amino and α‐hydroxy phosphonates. A variety of metal‐based chiral catalysts as well as several organic catalysts have been exploited as suitable systems for the preparation of enantiomerically pure phosphonates. The recent evolution and future trends of those and other catalytic systems are described.

The hydrothiolation of unsaturated carbon-carbon bonds is a practical and atom-economical approach for the incorporation of sulfur into organic frameworks. In recent years, we have witnessed the development of a range of transition-metal-based catalytic systems for the control of the regio- and stereoselectivity. In this Minireview we highlight the mechanistic background behind this transformation so as to help the design of more specific and active organometallic hydrothiolation catalysts.

Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(η(2)-olefin)](2) and RhCl(IPr)(py)(η(2)-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(η(2)-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step.

This article summarises the state of the art of metal complexes in cell imaging, particularly fluorescence microscopy, and presents prospects for the future development of this area. This article combines discussion of, and examples from, both the d- and f-block which have traditionally been considered separately, presenting the important classes of agents in each case, with a general description of their photophysical and cellular behaviour, and comparing and contrasting their properties and applications.

The reactions of [Ir(μ-OMe)(cod)]2 with the N-allyl-substituted benzimidazolium salts 1-methyl-3-(2-propenyl)benzimidazolium iodide (2) and 1,3-di(2-propenyl)benzimidazolium bromide (3) have been found to afford the five-coordinated Ir(I) complexes [IrX(cod)(η2-C-NHC)] (X = I, NHC = 1-methyl-3-(2-propenyl)benzimidazol-2-ylidene, 4; X = Br, NHC = 1,3-di(2-propenyl)benzimidazol-2-ylidene, 5), respectively. The cationic derivative [Ir(cod)(η2:η2-C-NHC)]BF4 (NHC = 1,3-di(2-propenyl)benzimidazol-2-ylidene, 6), which contains an iridium center exclusively coordinated by sp2 carbon atoms, has been prepared by treatment of 5 with AgBF4. The reaction between [Ir(μ-Cl)(cod)]2 and 3 in ethanol, in the presence of excess NaOEt, has allowed the synthesis of the four-coordinate complex [IrBr(cod)(C-NHC)] (NHC = 1,3-di(propyl)benzimidazol-2-ylidene, 7) after deprotonation of 3 and hydrogenation of both N-allyl substituents. The compounds 5, 6, and 7 have been characterized by X-ray diffraction. The neutral complexes 4, 5, and 7 have been tested as catalysts in the transfer hydrogenation of cyclohexanone using 2-propanol as hydrogen source. The catalytic reactions using 4 and 5 have been observed to progress without hydrogenation of the allyl substituents.

A series of alkylammonium-imidazolium chloride salts [RImH(CH2)nNMe2]Cl·HCl (R = Me, t-Bu, Mes; n = 2, 3) have been prepared by alkylation of 1-substituted imidazole compounds with the corresponding chloro-alkyl-dimethylamine hydrochloride. These salts are precursors for the synthesis of a library of rhodium(I) complexes containing amino-alkyl functionalized N-heterocyclic carbene (NHC) ligands with hemilabile character by varying the substituent on the heterocyclic ring and the length of the linker with the dimethylamino moiety. The monodeprotonation of alkylammonium-imidazolium salts with NaH in the presence of [{Rh(μ-Cl)(cod)}2] gave the amino-imidazolium salts [RImH(CH2)nNMe2][RhCl2(cod)]. Further deprotonation with NaH under non anhydrous conditions gave the neutral complexes [RhCl(cod)(RIm(CH2)nNMe2)] in good yields. The abstraction of the chloro ligand by silver salts rendered the cationic complexes [Rh(cod)(κ2C,N-RIm(CH2)3NMe2)][BF4] (R = Me, Mes) by coordination of the NMe2 fragment of the sidearm of the functionalized NHC ligands. The catalytic activity of the rhodium complexes in the hydrosilylation of terminal alkynes using HSiMe2Ph has been investigated with Ph-C≡CH, t-Bu-C≡CH, n-Bu-C≡CH, and Et3Si-C≡CH as substrates. Higher activities were achieved using neutral complexes having small substituents at the heterocyclic ring (R = Me). Excellent selectivities in the β-(Z)-vinylsilane isomer were found in the hydrosilylation of 1-hexyne and predominantly the β-(E) and α-bis(silyl)alkene isomers were obtained in the hydrosilylation of triethylsilylacetylene.

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

Complex [Os(η6-p-cymene)(OH)(IPr)]OTf is an efficient catalyst precursor for the α-alkylation of arylacetonitriles and methyl ketones with alcohols, which works with turnover frequencies between 675 and 176 h–1 for nitriles and between 194 and 28 h–1 for ketones.

A family of glycerol derivatives, consisting of over sixty 1,3-dialkoxy-2-propanols and 1,2,3-trialkoxypropanes, both symmetrically and unsymmetrically substituted at terminal positions, have been synthesized and the possible role of these glycerol derivatives as substitutive solvents has been evaluated through measurements of their physico-chemical properties. The molecular diversity of the derivatives prepared results in significant variations of polarity properties, facilitating the identification of possible candidates for solvent substitution.

The first three-coordinate gold(I) N-heterocyclic carbene (NHC) complexes have been prepared with the nido-carborane diphosphine. The complexes are brightly luminescent and present very high quantum yield values. The carbene ligand is able to modulate the energy of the emissions and, depending upon the substituent, the luminescence changes from blue to green. Theoretical calculations corroborate that the emissions are phosphorescence and arise from charge transfer (LML'CT) transitions from nido-carborane ligand (L) to metal/ligand group "gold(I)-NHC ligand" (ML').

A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]+ (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH3)(cod)(MeImR)]+ (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX(cod)(MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h–1. The complexes [Ir(NCCH3)(cod)(MeImR)]+ (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity.

In this tutorial review, we discuss how recycling and stability advantages alone are frequently not sufficient to justify the effort necessary to immobilize chiral complexes unless additional advantages are found. Nanostructured solids with well-controlled surfaces and pores may act as nanoreactors, hindering or even blocking some of the reaction channels, and hence modifying the stereochemical result of the reaction. The use of support effects to improve or change the enantioselectivity is emerging as an interesting field, whose understanding might allow, in the near future, the design of chiral ligands better adapted to this strategy.

CO2 as feedstock: An air- and moisture-stable iridium(III) catalyst effectively promotes the hydrosilylation of CO2. This reaction leads to silyl formate in a highly selective manner and proceeds efficiently under mild conditions, most likely by an outer-sphere mechanism, as suggested by theoretical calculations.

A large number of interesting organocatalytic enantioselective protocols have been explored and successfully applied in the last decade. Among them, the Henry (nitroaldol) reaction represents a powerful carbon-carbon bond-forming procedure for the preparation of valuable synthetic intermediates, such as enantioenriched nitro alcohols, which can be further transformed in a number of important nitrogen and oxygen-containing compounds. This area of research is still in expansion and a more complex version of this useful process has recently emerged, the domino Michael/Henry protocol, affording highly functionalized cycles with multiple stereogenic centers.

A facile and efficient synthetic route leading to catalytically relevant N-heterocyclic carbene (NHC) gold complexes is described. The method consists of one pot synthesis starting from readily available imidazolium salts and [AuCl(tht)], in the presence of K2CO3. Using the same protocol NHC silver complexes have been synthesised starting from AgNO3.