Tunable Ligands Research Articles

An Original L‐shape, Tunable N‐Heterocyclic Carbene Platform for Efficient Gold(I) Catalysis

AbstractThe synthesis and characterization of original NHC ligands based on an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their use as tunable ligands for efficient gold‐catalyzed C−N, C−O, and C−C bond formations. High activity, regio‐, chemo‐, and stereoselectivities are obtained for hydroelementation and domino processes, underlining the excellent performance (TONs and TOFs) of these IPy‐based ligands in gold catalysis. The gold‐catalyzed domino reactions of 1,6‐enynes give rise to functionalized heterocycles in excellent isolated yields under mild conditions. The efficiency of the NHC gold 5Me complex is remarkable and mostly arises from a combination of steric protection and stabilization of the cationic AuI active species by ligand 1Me.

An Original L-shape, Tunable N-Heterocyclic Carbene Platform for Efficient Gold(I) Catalysis.

The synthesis and characterization of original NHC ligands based on an imidazo[1,5-a]pyridin-3-ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their use as tunable ligands for efficient gold-catalyzed C-N, C-O, and C-C bond formations. High activity, regio-, chemo-, and stereoselectivities are obtained for hydroelementation and domino processes, underlining the excellent performance (TONs and TOFs) of these IPy-based ligands in gold catalysis. The gold-catalyzed domino reactions of 1,6-enynes give rise to functionalized heterocycles in excellent isolated yields under mild conditions. The efficiency of the NHC gold 5Me complex is remarkable and mostly arises from a combination of steric protection and stabilization of the cationic AuI active species by ligand 1Me .

A High‐Voltage Molecular‐Engineered Organic Sensitizer–Iron Redox Shuttle Pair: 1.4 V DSSC and 3.3 V SSM‐DSSC Devices

AbstractThe development of high voltage solar cells is an attractive way to use sunlight for solar‐to‐fuel devices, multijunction solar‐to‐electric systems, and to power limited‐area consumer electronics. By designing a low‐oxidation‐potential organic dye (RR9)/redox shuttle (Fe(bpy)33+/2+) pair for dye‐sensitized solar‐cell (DSSC) devices, the highest single device photovoltage (1.42 V) has been realized for a DSSC not relying on doped TiO2. Additionally, Fe(bpy)33+/2+ offers a robust, readily tunable ligand platform for redox potential tuning. RR9 can be regenerated with a low driving force (190 mV), and by utilizing the RR9/Fe(bpy)33+/2+ redox shuttle pair in a subcell for a sequential series multijunction (SSM)‐DSSC system, one of the highest known three subcell photovoltage was attained for any solar‐cell technology (3.34 V, >1.0 V per subcell).

A High-Voltage Molecular-Engineered Organic Sensitizer-Iron Redox Shuttle Pair: 1.4 V DSSC and 3.3 V SSM-DSSC Devices.

The development of high voltage solar cells is an attractive way to use sunlight for solar-to-fuel devices, multijunction solar-to-electric systems, and to power limited-area consumer electronics. By designing a low-oxidation-potential organic dye (RR9)/redox shuttle (Fe(bpy)33+/2+ ) pair for dye-sensitized solar-cell (DSSC) devices, the highest single device photovoltage (1.42 V) has been realized for a DSSC not relying on doped TiO2 . Additionally, Fe(bpy)33+/2+ offers a robust, readily tunable ligand platform for redox potential tuning. RR9 can be regenerated with a low driving force (190 mV), and by utilizing the RR9/Fe(bpy)33+/2+ redox shuttle pair in a subcell for a sequential series multijunction (SSM)-DSSC system, one of the highest known three subcell photovoltage was attained for any solar-cell technology (3.34 V, >1.0 V per subcell).

Design and Synthesis of Tunable Ligands with 4,4′-Bipyridyl as an Electron-Accepting Unit and Their Rhenium Complexes

We have designed and prepared a series of rhenium complexes with 4,4′-bipyridyl as an electron-accepting unit. Electrochemical studies revealed that oxidation and reduction of these complexes occurred on the rhenium center and the 4,4′-bipyridyl skeleton, respectively. The redox potentials of the rhenium complexes are controllable by tuning the substituents on the 4,4′-bipyridyl skeleton and the coordinating groups to the rhenium center. We have also examined the reactivity of the rhenium complexes in electrochemical and photochemical CO2 reduction.

Pushing Chemical Boundaries with N-Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry.

N-Heterocyclic olefins (NHOs) have gone from the topic of a few scattered (but important) reports in the early 1990s to very recently being a ligand/reagent of choice in the far-reaching research fields of organocatalysis, olefin and heterocycle polymerization, and low oxidation state main group element chemistry. NHOs are formally derived by appending an alkylidene (CR2) unit onto an N-heterocyclic carbene (NHC), and their pronounced ylidic character leads to high nucleophilicity and soft Lewis basic character at the ligating carbon atom. These olefinic donors can also be structurally derived from imidazole, triazole, and thiazole-based heterocyclic carbenes and, as a result, have highly tunable electronic and steric properties. In this Account, we will focus on various synthetic routes to imidazole-2-ylidene derived NHOs (sometimes referred to as deoxy-Breslow intermediates) followed by a discussion of the electron-donor ability of this structurally tunable ligand group. It should be mentioned that NHOs have a close structural analogy with Breslow-type intermediates, N-heterocyclic ketene aminals, and β-azolium ylides; while these latter species play important roles in advancing synthetic organic chemistry, discussion in this Account will be confined mostly to imidazole-2-ylidene derived NHOs. In addition, we will cover selected examples from the literature where NHOs and their anionic counterparts, N-heterocyclic vinylenes, are used to access reactive main group species not attainable using traditional ligands. Added motivation for these studies comes from the emerging number of low coordinate main group element based compounds that display reactivity once reserved for precious metal complexes (such as H-H and C-H bond activation). Moreover, NHOs are versatile precursors to new mixed element (P/C and N/C), and potentially bidentate, ligand constructs of great potential in catalysis, where various metal oxidation states and coordination environments need to be stabilized during a catalytic cycle. The most active area of recent growth for NHOs is their use as nucleophiles to promote efficient organocatalytic transformations, including transesterification, carbonyl reduction, and the conversion of CO2 into value added products. Polyesters have also been generated through the NHO-promoted ring-opening polymerization of lactones, and the highly tunable nature of NHO organocatalysts allows for the rapid screening and enhancement of catalytic performance. Therefore, the growing utility of NHOs in the realm of organic and polymer chemistry can be viewed as evidence of the widespread impact of N-heterocyclic olefins on the chemical community. It is hoped that through this Account others will join this flourishing research domain and that the rapid recent growth of NHO chemistry is sustained for the foreseeable future.

Cover Feature: Tunable Ligand Effects on Ruthenium Catalyst Activity for Selectively Preparing Imines or Amides by Dehydrogenative Coupling Reactions of Alcohols and Amines (Chem. Eur. J. 52/2017)

Cover Feature: Tunable Ligand Effects on Ruthenium Catalyst Activity for Selectively Preparing Imines or Amides by Dehydrogenative Coupling Reactions of Alcohols and Amines (Chem. Eur. J. 52/2017)

Tunable Ligand Effects on Ruthenium Catalyst Activity for Selectively Preparing Imines or Amides by Dehydrogenative Coupling Reactions of Alcohols and Amines

Selective dehydrogenative synthesis of imines from a variety of alcohols and amines was developed by using the ruthenium complex [RuCl2 (dppea)2 ] (6 a: dppea=2-diphenylphosphino-ethylamine) in the presence of catalytic amounts of Zn(OCOCF3 )2 and KOtBu, whereas the selective dehydrogenative formation of amides from the same sources was achieved by using another ruthenium complex, [RuCl2 {(S)-dppmp}2 ] [6 d: (S)-dppmp=(S)-2-((diphenylphosphenyl)methyl)pyrrolidine], in the presence of catalytic amounts of Zn(OCOCF3 )2 and potassium bis(trimethylsilyl)amide (KHMDS). Our previously reported ruthenium complex, [Ru(OCOCF3 )2 (dppea)2 ] (8 a), was the catalyst precursor for the imine synthesis, whereas [Ru(OCOCF3 )2 {(S)-dppmp}2 ] (8 d), which was derived from the treatment of 6 d with Zn(OCOCF3 )2 and characterized by single-crystal X-ray analysis, was the pre-catalyst for the amide formation. Control experiments revealed that the zinc salt functioned as a reagent for replacing chloride anions with trifluoroacetate anions. Plausible mechanisms for both selective dehydrogenative coupling reactions are proposed based on a time-course study, Hammett plot, and deuterium-labeling experiments.

Smart N-Heterocyclic Carbene Ligands in Catalysis.

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

NeoPHOX - a structurally tunable ligand system for asymmetric catalysis.

A synthesis of new NeoPHOX ligands derived from serine or threonine has been developed. The central intermediate is a NeoPHOX derivative bearing a methoxycarbonyl group at the stereogenic center next to the oxazoline N atom. The addition of methylmagnesium chloride leads to a tertiary alcohol, which can be acylated or silylated to produce NeoPHOX ligands with different sterical demand. The new NeoPHOX ligands were tested in the iridium-catalyzed asymmetric hydrogenation and palladium-catalyzed allylic substitution. In both reactions high enantioselectivities were achieved, that were comparable to the enantioselectivities obtained with the up to now best NeoPHOX ligand derived from expensive tert-leucine.

Pre-reduction of the Phillips CrVI/SiO2 catalyst by cyclohexene: A model for the induction period of ethylene polymerization

The pre-reduction of the industrially relevant CrVI/SiO2 Phillips catalyst by cyclohexene at room temperature was investigated by means of operando XANES, Diffuse Reflectance UV–Vis and transmission FT-IR spectroscopies. It was found that cyclohexene efficiently reduces the surface chromates sites (CrVI) at room temperature to give mainly divalent chromium sites and aldehyde by-products, which are rapidly converted in ester species. These by-products remain in strong interaction with the reduced chromium sites, defining a complex and tunable ligand sphere, which controls the entrance of incoming molecules, including ethylene. Unlike the CrVI/SiO2 catalyst, the cyclohexene-reduced Cr/SiO2 catalyst polymerizes ethylene already at room temperature with no induction time. The reactivity of cyclohexene with the CrVI species mimics that of ethylene during the induction period on CrVI/SiO2 catalyst, simplified by the absence of the further polymerization step. Hence, the results discussed in this work are potentially useful to understand what happens during the induction period in the presence of ethylene.

Development and Testing of Diglycolamide Functionalized Mesoporous Silica for Sorption of Trivalent Actinides and Lanthanides.

Sequestration of trivalent actinides and lanthanides present in used nuclear fuel and legacy wastes is necessary for appropriate long-term stewardship of these metals, particularly to prevent their release into the environment. Organically modified mesoporous silica is an efficient material for recovery and potential subsequent separation of actinides and lanthanides because of its high surface area, tunable ligand selection, and chemically robust substrate. We have synthesized the first novel hybrid material composed of SBA-15 type mesoporous silica functionalized with diglycolamide ligands (DGA-SBA). Because of the high surface area substrate, the DGA-SBA was found to have the highest Eu capacity reported so far in the literature of all DGA solid-phase extractants. The sorption behavior of europium and americium on DGA-SBA in nitric and hydrochloric acid media was tested in batch contact experiments. DGA-SBA was found to have high sorption of Am and Eu in pH 1, 1 M, and 3 M nitric and hydrochloric acid concentrations, which makes it promising for sequestration of these metals from used nuclear fuel or legacy waste. The kinetics of Eu sorption were found to be two times slower than that for Am in 1 M HNO3. Additionally, the short-term susceptibility of DGA-SBA to degradation in the presence of acid was probed using (29)Si and (13)C solid-state NMR spectroscopy. The material was found to be relatively stable under these conditions, with the ligand remaining intact after 24 h of contact with 1 M HNO3, an important consideration in use of the DGA-SBA as an extractant from acidic media.

Highly tunable fluorinated trispyrazolylborates [HB(3-CF3-5-{4-RPh}pz)3](-) (R = NO2, CF3, Cl, F, H, OMe and NMe2) and their copper(I) complexes.

The ethene and carbon monoxide adducts of copper(I) with seven trispyrazolylborate ligands ([HB(3-CF3-5-{4-RPh}pz)3](-); R = NO2 (4a), CF3 (4b), Cl (4c), F (4d), H (4e), OMe (4f) and NMe2 (4g)) were synthesized and characterized. The ligands were synthesized from their corresponding pyrazoles and sodium tetrahydridoborate and were obtained as solvent adducts of their sodium salts after workup. When the pyrazole with the most electron-withdrawing substituent (R = NO2) is used the asymmetric ligand [HB(3-CF3-5-{4-NO2Ph}pz)2(3-{4-NO2Ph}-5-CF3pz)](-) (4a') is formed as the major product. Copper(I) complexes with ethene or CO as a co-ligand were prepared in good yields and were structurally characterized using (1)H NMR, (13)C NMR and infrared spectroscopy. Single crystal X-ray crystallography analyses revealed the structures of Na4a', Na4b, four copper ethene complexes and four copper carbonyl complexes. The structures of the copper(I) complexes show Cu(I) ions in pseudo-tetrahedral coordination environments consisting of three nitrogen atoms of the trispyrazolylborate ligand and the carbonyl or η(2)-coordinated ethene ligands, with nearly identical coordination environments around the Cu(I) ion. The compound [Na(4a')(H2O)]n crystallizes as one-dimensional chains with intermolecular NaO2N interactions. The sodium ions were found in severely distorted octahedral geometries with three nitrogen atoms from the trispyrazolylborate ligand, one aqua ligand and two oxygen atoms from the nitro group of an adjacent molecule. The compound [Na2(4b)2(μ-H2O)2] crystallizes as a centrosymmetric water-bridged dimer: two five-coordinate square-pyramidal sodium ions each are coordinated facially by three nitrogen atoms from a trispyrazolylborate ligand and two bridging water ligands. Below the base of the pyramidal structure one intermolecular and two intramolecular NaF short contacts are present. The (1)H and (13)C NMR spectra of the copper-ethene complexes show signals for the ethene ligands in the range of 4.84-4.96 ppm and 84.9-86.8 ppm respectively. The infrared spectra of the carbonyl complexes show CO stretching frequencies in the range of 2096-2120 cm(-1). Both the NMR signals for the ethene ligands and infrared signals for the carbonyl ligands were found to show good correlations with the Hammett σp parameters of the substituents on the phenyl rings of the ligands.

Picolinamides as effective ligands for copper-catalysed aryl ether formation: structure-activity relationships, substrate scope and mechanistic investigations.

The use of picolinic acid amide derivatives as an effective family of bidentate ligands for copper-catalysed aryl ether synthesis is reported. A fluorine-substituted ligand gave good results in the synthesis of a wide range of aryl ethers. Even bulky phenols, known to be very challenging substrates, were shown to react with aryl iodides with excellent yields using these ligands. At the end of the reaction, the first examples of end-of-life Cu species were isolated and identified as Cu(II) complexes with several of the anionic ligands tested. A preliminary mechanistic investigation is reported that suggests that the substituents on the ligands might have a crucial role in determining the redox properties of the metal centre and, consequently, its efficacy in the coupling process. An understanding of these effects is important for the development of new efficient and tunable ligands for copper-based chemistry.

775: Synergistic targeting to HER2-positive breast cancer via nanocapsules with tunable ligand density and magnetic targeting

775: Synergistic targeting to HER2-positive breast cancer via nanocapsules with tunable ligand density and magnetic targeting

Copper ionic liquids: Tunable ligand and anion chemistries to control electrochemistry and deposition morphology

A multi-technique investigation was performed on three copper-based ionic liquids to elucidate the influence of coordinating ligands and charge-balancing anions on the electrochemical properties of the materials. Galvanostatic cycling of Cu(OHCH2CH2NH2)6(BF4)2 (Cu1) in 1-butyl-3-methyl-imidazolium hexafluorophosphate gave partially reversible plating of copper that was consistent with cyclic voltammetry data (collected using an ionic liquid-based reference electrode verified with measurements of ferrocene, cobaltocene, and lithium). Scanning electron microscopy also showed pitting in the copper-coated surface of the electrode that was consistent with the stripping wave observed by cyclic voltammetry. Potentiostatic deposition in neat Cu1 showed significant dendrite formation. The substitution of the OHCH2CH2NH2 ligands of Cu1 with stronger coordinating NH(CH2CH2OH)2 in Cu(NH(CH2CH2OH)2)6(BF4)2 (Cu2) resulted in the complete suppression of both copper stripping and dendrite formation. Substitution of the BF4- anions of Cu2 with CF3SO3- in Cu(NH(CH2CH2OH)2)6(CF3SO3)2 (Cu3) shifted the copper deposition 0.1V more negative and produced slightly larger spherical particles (1.5μm versus 5μm). The results suggested that while the anion composition influenced particle size, and the metal–ligand bond strength helped control particle morphology, both factors affected the electrochemical properties including the plating and stripping of copper.

ChemInform Abstract: “Click”‐BINOLs: A New Class of Tunable Ligands for Asymmetric Catalysis.

AbstractA novel family of BINOL‐derived ligands bearing triazole moieties is prepared.

Fabrication of Ringlike and Disclike Nano-Structures with Surface-Enhanced Raman Scattering Activity

Functional gold nanoparticles with tunable surface ligands were used to assemble ringlike and disclike nano-structures. The adjustable formation mechanism was deduced, and important assembling processes happened such as evaporation, cooling, Marangoni flow and drying. With its hydrophobic nature, butanethiol monolayer-protected gold nanoparticles were arranged to form ringlike nano-assemblies. Well-defined disclike nano-structures were obtained as a result of the change of hydrophilic-hydrophobic property and the aggregation of carboxyl group functionalized gold nanoparticles induced by tris(2-aminoethyl)amine. Surface-enhanced Raman spectroscopy (SERS) activities of nano-assemblies were examined. These nano-assemblies showed strong SERS enhancement occurred with the increase of localized surface plasmon resonance. The efficient technique will open a new avenue for fabricating organized nanoparticle structures in a convenient manner. It may be potentially applied for the biosensor due to SERS activities of those structures.

Camphor-annelated imidazolines with various N1 and C2 pendants as tunable ligands for nitroaldol reactions

Starting from (1R,2S,3R)-camphordiamine and (hetero)aromatic imidates and orthoformate, nine new camphor-annelated NH-imidazolines were synthesized. Subsequent N-modification was carried out via methylation, acylation, benzoylation, and sulfonylation. Two regioisomers were usually isolated with the ratios reflecting the structure of the starting NH-imidazoline and the electrophile used. All of the successfully prepared N1- and C2-substituted camphor-annelated imidazolines were applied to the asymmetric version of a Cu(II)-catalyzed Henry reaction. The electronic effects of both N1- and C2-pendants on the chemical and asymmetric outcomes of the nitroaldol reaction have been studied and discussed.

'Click'-BINOLs: A New Class of Tunable Ligands for Asymmetric Catalysis

A new class of easily tunable 1,2,3-triazole-BINOL ligands, 'click'-BINOLs, have been synthesized from readily available alkynes and several azido-BINOL derivatives using the powerful Huisgen [3+2] cycloaddition 'click' approach. The activity of these ligands in asymmetric Lewis acid catalysis has been explored for the first time in the diethylzinc addition to aldehydes. The <i>C</i> ₂-symmetric ligand <b>1d</b> showed an interesting catalytic behavior, which suggests the non-innocent participation of the triazole units. Thus, good enantioselectivities (up to 86% ee) were obtained by both the right selection of the substitution pattern at the triazole ring and the fine tuning of the reaction conditions.