A Zwitterionic Heterobimetallic Gold-Iron Complex Supported by Bis(N-Heterocyclic Imine)Silyliumylidene

Silyliumylidene Ion Stabilized by Two σ-Donating Ni(0)- and Pd(0)-Fragments

A silyliumylidene ion 2 stabilized by two σ-donating Ni(0)- and Pd(0)-fragments was successfully synthesized. Due to the σ-donation of M→Si interactions, 2 presents a pyramidalized cationic silicon center with a localized lone pair. The additional coordination of basic Pd(0) fragment to the mono-Ni(0)-stabilized silyliumylidene 1 results in a higher HOMO level and an unchanged HOMO-LUMO gap and thus, 2 remains highly reactive. Interestingly, the coordination mode at the Si center is closely related to the nature of M-ligands. Indeed, the donor/donor-stabilized silyliumylidene ion 2 has been transformed into a donor/acceptor-stabilized ion 13, featuring a trigonal planar Si center with a vacant orbital, just via a ligand exchange reaction from PCy3/NHC toward PMe3.

Gold(III) Auracycles Featuring C(sp3)-Au-C(sp2) Bonds: Synthesis and Mechanistic Insights into the Cycloauration Step

The direct auration of arenes is a key step in numerous gold-catalyzed reactions. Although reported more than 100 years ago, understanding of its underlying mechanism has been hampered by the difficulties in the isolation of relevant intermediates given the propensity of gold(III) species to undergo reductive elimination. Here, we report the synthesis and isolation of a new family of intriguing zwitterionic [C(sp3)^C(sp2)]-auracyclopentanes, as well as of their alkyl-gold(III) precursors and demonstrate their value as mechanistic probes to study the C(sp2)-Au bond-forming event. Experimental investigations employing Kinetic Isotope Effects (KIE), Hammett plot, and Eyring analysis provided important insights into the formation of the auracycle. The data suggest a SEAr mechanism wherein the slowest step might be the π-coordination between the arene and the gold(III) center, en route to the Wheland intermediate. We also show that these auracyclopentanes can work as catalysts in several gold-promoted transformations.

Unusual nucleophilic reactivity of a dithiolene-based N-heterocyclic silane

While the dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX3 (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, the parallel reaction of 4 with BCl3 results in 10, a dithiolene-substituted N-heterocyclic silane, via the Si-S bond cleavage. Unlike 5, the labile 8 may be readily converted to 9via BI3-mediated cleavage of the Si-N bond. The formation of 5 and 8 confirms that 4 uniquely possesses dual nucleophilic sites: (a) the terminal sulphur atom of the dithiolene moiety; and (b) the backbone carbon of the N-heterocyclic silane unit.

Potent Anticancer Activity of a Dinuclear Gold(I) bis-N-Heterocyclic Imine Complex Related to Thioredoxin Reductase Inhibition in Vitro

A dinuclear gold(I) complex featuring a strongly donating bis-N-heterocyclic imine ligand was synthesised and characterised by different methods, including single crystal X-ray diffraction (SC-XRD) analysis. The compound has been tested for its antiproliferative effects in a panel of human cancer cell lines in vitro, showing highly selective anticancer effects, particularly against human A549 non-small cell lung cancer cells (NSCLC), with respect to non-tumorigenic cells (VERO). The accumulation of the compound in A549 and VERO cells was studied by high-resolution continuum source atomic absorption spectrometry (HRCS-AAS), revealing that the anticancer effects are not particularly related to the different amounts of gold taken up by the cells over 72 h. Enzyme inhibition studies to evaluate the activity of the seleno-enzyme thioredoxin reductase (TrxR) in cancer cell extracts show that the gold(I) compound is a potent inhibitor (IC50=0.567±0.208 μM), while the free ligand is ineffective. This result correlates with the observed compound's selectivity towards A549 cells overexpressing the enzyme.

The Electron-Rich and Nucleophilic N-Heterocyclic Imines on Metal Surfaces: Binding Modes and Interfacial Charge Transfer

The strongly electron-donating N-heterocyclic imines (NHIs) have been employed as excellent surface anchors for the thermodynamic stabilization of electron-deficient species due to their enhanced nucleophilicity. However, the binding mode and interfacial property of these new ligands are still unclear, representing a bottleneck for advanced applications in surface functionalization and catalysis. Here, NHIs with different side groups have been rationally designed, synthesized, and analyzed on various metal surfaces (Cu, Ag). Our results reveal different binding modes depending on the molecular structure and metal surface. The molecular design enables us to achieve a flat-lying or upright configuration and even a transition between these two binding modes depending on the coverage and time. Importantly, the two binding modes exhibit different degrees of interfacial charge transfer between the molecule and the surface. This study provides essential microscopic insight into the NHI adsorption geometry and interfacial charge transfer for the optimization of heterogeneous catalysts in coordination chemistry.

Tetryliumylidene ions in synthesis and catalysis

Tetryliumylidene ions ([R-E:]+), recognised for their intriguing electronic properties, have attracted considerable interest. These positively charged species, with two vacant p-orbitals and a lone pair at the E(ii) centre (E = Si, Ge, Sn, Pb), can be viewed as the combination of tetrylenes (R2E:) and tetrylium ions ([R3E]+), which makes them potent Lewis ambiphiles. Such electronic features highlight the potential of tetryliumylidenes for single-site small molecule activation and transition metal-free catalysis. The effective utilisation of the electrophilicity and nucleophilicity of tetryliumylidenes is expected to stem from appropriate ligand choice. For most of the isolated tetryliumylidenes, electron donor- and/or kinetic stabilisation is necessary. This minireview highlights the developments in tetryliumylidene syntheses and the progress of research towards their reactivity and applications in catalytic reactions.

N-Heterocyclic imine-based bis-gallium(I) carbene analogs featuring a four-membered Ga2N2 ring

A combination of Ga(I) centers as important building blocks and scaffolds containing N-heterocyclic imines gives new insights into low-valent Ga chemistry. In this study, a mixture of LDipNLi (LDip = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene), tBuOK, and Cp*Ga (Cp* = pentamethylcyclopentadienyl) in toluene afforded [LDipN-Ga]2 (1) via salt metathesis. X-ray structure analysis of 1 revealed a four-membered Ga2N2 ring, and DFT studies indicated the presence of a lone pair at each Ga center. In addition, compound 1 demonstrated diverse reactivities towards methyl trifluoromethanesulfonate, diphenyl disulfide, 9,10-phenanthrenequinone, and ECl2 (E = Ge or Sn).

Room Temperature Intermolecular Dearomatization of Arenes by an Acyclic Iminosilylene

A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (-Si^tBu₃) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C-C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Büchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC₆ rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4-bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC₅ rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C-C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF₆ ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC₈ results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Synthesis, Characterization and Reactivity of a σ‐Donating Ni 0 ‐Stabilized Silyliumylidene Ion

The synthesis of a silyliumylidene cation complex 2 stabilized by a Ni⁰‐based donating ligand is reported. Experimental and theoretical studies demonstrate that the highly electrophilic Si^II center is stabilized by a dative Ni→Si σ‐interaction and π‐donations from the amino‐ and Ni‐moieties. Due to the energetically close frontier orbitals localized on the Si and Ni atoms, complex 2 presents a competitive reactivity at Si and Ni sites.

Mesoionic Imines (MIIs): Strong Donors and Versatile Ligands for Transition Metals and Main Group Substrates

We report the synthesis and the reactivity of 1,2,3‐triazolin‐5‐imine type mesoionic imines (MIIs). The MIIs are accessible by a base‐mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O‐stretching frequencies in MII−CO₂ and −Rh(CO)₂Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N‐heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent‐induced fluorine‐specific interactions and undergo C−H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO–LUMO gap compared to other related ligands. MIIs are potentially able to act as both π‐donor and π‐acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.

Chemistry of the Bis(imine)-Based Tetradentate Ligand Stabilized Group 14 E(II) Cations (E=Ge and Sn)

The ambiphilic Ge(II) and Sn(II) cationic species have been reported to be isolated through kinetic or thermodynamic stabilizations. Nonetheless, steric congestion or excessive coordination of donor atoms to the cationic center concurrently disfavors its prompt reactivity. Our research in this field revolves around the utilization of structurally non-rigid bis(imine) based tetradentate supporting ligands for the stabilization of Ge(II) and Sn(II) cationic species. Such E(II) cationic systems have been advantaged due to inherent flexibility present at the ligand backbone allowing disposal of E(II) orbitals through geometric rearrangements for further reactivity. The bifunctionality present in the ligand enables the first examples of Ge(II) bis-monocations. Furthermore, the redox-active nature of the ligand encourages participation in chemical transformations. In this personal account we have provided a detailed discussion of our published work in this direction in the last five years.

Application of ferrocene-bridged N-heterocyclic carbene stabilised bis-phosphinidenes in Sn(II) complexation

Two new bidentate ferrocene-bridged bis(N-heterocyclic carbene-phospinidenes) (bisNHCPs) were successfully isolated by treating 1,1'-bis-(dichlorophosphine)ferrocene with N-heterocyclic carbenes, followed by dechlorination using sodium naphthalenide. The bisNHCPs were used in complexation of various Sn(II) halides and Sn(II) bistriflate (SnX₂ with X = Cl, Br, I, OTf). Transmetalation to a CuCl complex and Sn(II) transfer to a bisimine was performed to investigate the stannyliumylidenes' reactivity.