Redox-Switchable Complexes Based on Nanographene-NHCs

A series of rhodium and iridium complexes with a N‐heterocyclic carbene (NHC) ligand decorated with a perylene‐diimide‐pyrene moiety are described. Electrochemical studies reveal that the complexes can undergo two successive one‐electron reduction events, associated to the reduction of the PDI moiety attached to the NHC ligand. The reduction of the ligand produces a significant increase on its electron‐donating character, as observed from the infrared spectroelectrochemical studies. The rhodium complex was tested in the [3+2] cycloaddition of diphenylcyclopropenone and methylphenylacetylene, where it displayed a redox‐switchable behavior. The neutral complex showed moderate activity, which was suppressed when the catalyst was reduced.

N-heterocyclic Carbene Gold Complexes Active in Hydroamination and Hydration of Alkynes

Until the year 2000, gold compounds were considered catalytically inert. Subsequently, it was found that they are able to promote the nucleophilic attack on unsaturated substrates by forming an Au–π-system. The main limitation in the use of these catalytic systems is the ease with which they decompose, which is avoided by stabilization with an ancillary ligand. N-heterocyclic carbenes (NHCs), having interesting s-donor capacities, are able to stabilize the gold complexes (Au (I/III) NHC), favoring the exploration of their catalytic activity. This review reports the state of the art (years 2007–2022) in the nucleophilic addition of amines (hydroamination) and water (hydration) to the terminal and internal alkynes catalyzed by N-heterocyclic carbene gold (I/III) complexes. These reactions are particularly interesting both because they are environmentally sustainable and because they lead to the production of important intermediates in the chemical and pharmaceutical industry. In fact, they have an atom economy of 100%, and lead to the formation of imines and enamines, as well as the formation of ketones and enols, all important scaffolds in the synthesis of bioactive molecules, drugs, heterocycles, polymers, and bulk and fine chemicals.

Synthesis of Ionic Imprinted Membrane (IIM) Based on Sulfonated Polyeugenol for Selective Transport of Gold (III) Metal Ions from Motherboard Waste

Ionic imprinted membrane (IIM) was successfully synthesized using sulfonated polyeugenol, a derivative of eugenol as a functional polymer, with PVA as a base membrane and PEGDE as a crosslinker. IIM Au(III) is a membrane with an Au(III) ion template. This study aimed to determine the pH effect of a feed phase for selective transport of IIM Au(III), comparing it with a non-imprinted membrane (NIM) against Au3+ metal ions from motherboard waste. It also aimed to compare the membrane selectivity of Au3+ metal ions to Cu2+ metal ions, which are also found in motherboard waste. Gold samples were prepared using H(AuCl4) standard and leaching solutions from motherboard waste. The leaching of the motherboard used aqua regia and the assistance of a microwave to accelerate the leaching process. The optimum transport of Au3+ metal ions was when using IIM Au(III) at pH 3. This proved that the presence of a template affected IIM Au(III) to recognize Au(III) ions. IIM Au(III) showed higher selectivity than NIM, as evidenced by the percentage in the receiving phase of the Au3+ metal ions, which was more significant than the Cu2+ metal ions from the motherboard leaching solution.

Synthesis, Characterization, and Biological Activity of Water-Soluble, Dual Anionic and Cationic Ruthenium-Arene Complexes Bearing Imidazol(in)ium-2-dithiocarboxylate Ligands

An efficient synthetic protocol was devised for the preparation of five cationic ruthenium-arene complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands from the [RuCl₂(p-cymene)]₂ dimer and 2 equiv of an NHC·CS₂ zwitterion. The reactions proceeded cleanly and swiftly in dichloromethane at room temperature to afford the expected [RuCl(p-cymene)(S₂C·NHC)]Cl products in quantitative yields. When the [RuCl₂(p-cymene)]₂ dimer was reacted with only 1 equiv of a dithiolate betaine under the same experimental conditions, a set of five bimetallic compounds with the generic formula [RuCl(p-cymene)(S₂C·NHC)][RuCl₃(p-cymene)] was obtained in quantitative yields. These novel, dual anionic and cationic ruthenium-arene complexes were fully characterized by various analytical techniques. NMR titrations showed that the chelation of the dithiocarboxylate ligands to afford [RuCl(p-cymene)(S₂C·NHC)]⁺ cations was quantitative and irreversible. Conversely, the formation of the [RuCl₃(p-cymene)]^- anion was limited by an equilibrium, and this species readily dissociated into Cl^- anions and the [RuCl₂(p-cymene)]₂ dimer. The position of the equilibrium was strongly influenced by the nature of the solvent and was rather insensitive to the temperature. Two monometallic and two bimetallic complexes cocrystallized with water, and their molecular structures were solved by X-ray diffraction analysis. Crystallography revealed the existence of strong interactions between the azolium ring protons of the cationic complexes and neighboring donor groups from the anions or the solvent. The various compounds under investigation were highly soluble in water. They were all strongly cytotoxic against K562 cancer cells. Furthermore, with a selectivity index of 32.1, the [RuCl(p-cymene)(S₂C·SIDip)]Cl complex remarkably targeted the erythroleukemic cells vs mouse splenocytes.

Redox-Switchable Cycloisomerization of Alkynoic Acids with Napthalenediimide-Derived N-Heterocyclic Carbene Complexes

Two naphthalene‐diimide (NDI) bis‐imidazolium salts have been used as N‐heterocyclic carbene (NHC) precursors for the preparation of NDI‐functionalized complexes of rhodium and iridium of general formula [MCl(NDI‐NHC)(COD)] (M=Rh, Ir; NDI‐NHC=NDI‐functionalized NHC ligand). Comparison of the IR spectra of the complexes [IrCl(NDI‐NHC)(CO)₂] and their related one‐ and two‐electron reduced forms, reveal that each one‐electron reduction produces a decrease of the average ν(CO) of 9–10 cm⁻¹, indicating a significant enhancement of the electron‐richness of the metal. The [MCl(NDI‐NHC)(COD)] complexes were tested in the catalytic cycloisomerization of alkynoic acids. The one‐electron reduced forms showed greatly enhanced activities. For the cyclization of 5‐hexynoic acid, the two‐electron reduction of the ligand produced further enhancement of the catalytic activity, therefore showing that the catalyst can switch between three redox species with three distinct catalytic activities.

Main Avenues in Gold Coordination Chemistry

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.

Stabilization of the Pd–NHC framework with 1,2,4-triazol-5-ylidene ligands toward decomposition in alkaline media

New NHC ligands containing a base-ionizable RNH substituent at the C3 atom of the 1,2,4-triazole ring provide superior stability of the Pd–NHC bond against cleavage in strong alkaline media.

Polysulfone Influence on Au Selective Adsorbent Imprinted Membrane Synthesis with Sulfonated Polyeugenol as Functional Polymer

An ionic imprinted membrane (IIM) was synthesized using sulfonated polyeugenol, derived from eugenol, as its functional polymer and polysulfone as its base membrane for the selective adsorption of Au(III). This study aims to determine the adsorption of Au(III) metal ions using IIM compared with the non-imprinted membrane (NIM) and to figure out the membrane selectivity towards Au(III) in mixed solutions of Au/Cd, Au/Cu, and Au/Fe. IIM has a pore size of 0.767 μm while the non-imprinted membrane (NIM) has a pore size of 0.853 μm. The best adsorption result was obtained in the variation of the membrane with the addition of 3.84 g of polysulfone that had pores according to the size of Au. The selectivity results of the Au/Cd mixture solution in NIM and IIM were 17.802 and 36.265. In the mixture of Au/Cu, the NIM and IIM selectivity was 2.386 and 6.886, and in the mixed solution of Au/Fe, the selectivity of NIM and IIM was 0 and 8.489. Thus, the selectivity of IIM towards Au is bigger than NIM.

N-Heterocyclic Carbenes: A Door Open to Supramolecular Organometallic Chemistry

The field of metallosupramolecular chemistry is clearly dominated by the use of O-, N-, and P-donor Werner-type polydentate ligands. These molecular architectures are of high interest because of their wide range of applications, which include molecular encapsulation, stabilization of reactive species, supramolecular catalysis, and drug delivery, among others. Only recently, organometallic ligands have allowed the preparation of a variety of supramolecular coordination complexes, and the term supramolecular organometallic complexes (SOCs) is gaining space within the field of metallosupramolecular chemistry. While the early examples of SOCs referred to supramolecular architectures mostly containing bisalkenyl, diphenyl, or bisalkynyl linkers, the development of SOCs during the past decade has been boosted by the parallel development of multidentate N-heterocyclic carbene (NHC) ligands. The first examples of NHC-based SOCs referred to supramolecular assemblies based on polydentate NHC ligands bound to group 11 metals. However, during the last 10 years, several planar poly-NHC ligands containing extended π-conjugated systems have facilitated the formation of a large variety of architectures in which the supramolecular assemblies can contain metals other than Cu, Ag, and Au. Such ligands are Janus di-NHCs and trigonal-planar tris-NHCs-most of them prepared by our research group-which have allowed the preparation of a vast range of NHC-based metallosupramolecular compounds with interesting host-guest chemistry properties. Although the number of SOCs has increased in the past few years, their use for host-guest chemistry purposes is still in its earliest infancy. In this Account, we describe the achievements that we have made during the last 4 years toward broadening the applications of planar extended π-conjugated NHC ligands for the preparation of organometallic-based supramolecular structures, including their use as hosts for some selected organic and inorganic guests, together with the catalytic properties displayed by some selected host-guest inclusion complexes. Our contribution describes the design of several Ni-, Pd-, and Au-based metallorectangles and metalloprisms, which we used for the encapsulation of several organic substrates, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The large binding affinities found are ascribed to the incorporation of two cofacial panels with large π-conjugated systems, which provide the optimum conditions for guest recognition by π-π-stacking interactions. We also describe a series of digold(I) metallotweezers for the recognition of organic and inorganic substrates. These metallotweezers were used for the recognition of "naked" metal cations and polycyclic aromatic hydrocarbons. The recognition properties of these metallotweezers are highly dependent on the nature of the rigid connector and of the ancillary ligands that constitute the arms of the tweezer. A peculiar balance between the self-aggregation properties of the tweezer and its ability to encapsulate organic guests is observed.

Acid Responsive Hydrogen-Bonded Organic Frameworks

A porous hydrogen-bonded organic framework (HOF) responsive to acid was constructed from a hexaazatrinaphthylene derivative with carboxyphenyl groups (CPHATN). Precise structures of both 1,2,4-trichlorobenzene solvate [CPHATN-1(TCB)] and activated HOF with permanent porosity (CPHATN-1a) were successfully determined by single-crystalline X-ray diffraction analysis. Permanent porosity of CPHATN-1a was evaluated by gas sorption experiments at low temperature. CPHATN-1a also shows significant thermal stability up to 633 K. Its crystals exhibit a rich photochemistry thanks to intramolecular charge-transfer and interunit proton-transfer reactions. Femtosecond (fs) experiments on crystals demonstrate that these events occur in ≤200 fs and 1.2 ps, respectively. Moreover, single-crystal fluorescence microscopy reveals a shift of the emission spectra most probably as a result of defects and a high anisotropic behavior, reflecting an ordered crystalline structure with a preferential orientation of the molecular dipole moments. Remarkably, CPHATN-1a, as a result of the protonation of pyradyl nitrogen atoms embedded in its π-conjugated core, shows reversible vapor acid-induced color changes from yellow to reddish-brown, which can be also followed by an ON/OFF of its emission. To the best of our knowledge, this is the first HOF that exhibits acid-responsive color changes. The present work provides new findings for developing stimuli responsive HOFs.

A stable Cerberus tris(maloNHC) and its coinage metal complexes

The first stable trisanionic Cerberus-type tris(maloNHC) was synthesized and incorporated into two different zwitterionic coinage metal complexes (Au(i) and Ag(i)).

Synthesis of topologically constrained naphthalimide appended palladium(ii)–N-heterocyclic carbene complexes – insights into additive controlled product selectivity

Pd(ii)–NHC catalyzed regioselective heteroannulation of o-haloanilines and arylethynyl-trimethylsilane to yield indoles and additive controlled switchable product selectivity has been demonstrated.

Impact of nuclearity and topology on the single molecule magnet behaviour of hexaazatrinaphtylene-based cobalt complexes

A hexaazatrinaphtylene-based transition metal complex that exhibits single molecule magnet behaviour is reported herein. This study reveals the influence of both nuclearity and topology on the magnetic properties of hexaazatrinaphtylene-based complexes.

N-Heterocyclic germylenes and stannylenes of the type [Fe{(η 5-C5H4)NR}2E] with bulky alkyl substituents

The 1,1′-diaminoferrocene derivatives [Fe(η 5-C5H4–NHAd)2] (Ad=2-adamantyl) and [Fe(η 5-C5H4–NHtBu)2] were investigated in terms of their suitability for the synthesis of N-heterocyclic tetrylenes of the type [{Fe(η 5-C5H4–NR)2}E] (E=Ge, Sn). The synthesis of these target compounds was easily achieved with R=tBu, but failed with R=Ad. In the latter case, the stannylene was not sufficiently stable for isolation and decomposed to the aminoiminoferrocene derivative [Fe(η 5-C5H4–NHAd)(η 5-C5H4–N=Ad′)] (Ad′=adamant-2-ylidene). Attempts to synthesise [{Fe(η 5-C5H4–NAd)2}Ge] afforded intractable material, from which the unusual compound [μ 2-{Fe(η 5-C5H4–NAd)2}Ge2(μ 3-O)(GeCl2)] was obtained by serendipity. It contains GeO, stabilised by adduct formation with GeCl2 and the target germylene. [Fe(η 5-C5H4–NHtBu)2], [Fe(η 5-C5H4–NHAd)2], [Fe(η 5-C5H4–NHAd)(η 5-C5H4–N=Ad′)], [μ 2-{Fe(η 5-C5H4–NAd)2}Ge2(μ 3-O)(GeCl2)] and [{Fe(η 5-C5H4–NtBu)2}E] (E=Ge, Sn) were structurally characterised by single-crystal X-ray diffraction.