Hyperconjugative aromaticity and protodeauration reactivity of polyaurated indoliums

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

Nonplanar Aromaticity of Dinuclear Rare-Earth Metallacycles

While the concept of metalla-aromaticity has well been extended to transition organometallic compounds in diverse geometries, aromatic rare-earth organometallic complexes are rare due to the special (n - 1)d0 configuration and high-lying (n - 1)d orbitals of rare-earth centers. In particular, nonplanar cases of rare-earth complexes have not been reported so far. Here, we disclose the nonplanar aromaticity of dinuclear scandium and samarium metallacycles characterized by various aromaticity indices (nucleus-independent chemical shift, isochemical shielding surface, anisotropy of induced current density, and isomerization stabilization energy). Bonding analyses (Kohn-Sham molecular orbital, adaptive natural density partitioning, multicenter bond indices, and principal interacting orbital) reveal that three delocalized π orbitals, predominantly contributed by the 2-butene tetraanion ligand, result in the formation of six-electron conjugated systems. Guided by these findings, we predicted that the lutetium and gadolinium analogues of dinuclear rare-earth metallacycles should be aromatic, which have been verified by the successful synthesis of real molecules. This work extends the concept of nonplanar aromaticity to the field of rare-earth metallacycles and illuminates the path for designing and synthesizing various rare-earth metalla-aromatics.

Syn-Aminoauration versus Anti-Aminoauration of Alkynes in Au(I)/Au(III) Catalysis: Understanding the Origin of Selectivity

There is no experimental evidence of whether such gold-catalyzed aminoauration reactions follow the anti- and/or syn-pathway, and hence, to understand the origin of the selectivity in Au(I)- and Au(III)-catalyzed reactions of alkynes, a thorough mechanistic study was performed using DFT methods. The NBO and ASM analyses provided significant information about the structure-stability-reactivity of the pathway-determining states (PDS). This study further reveals that the oxidation states and geometries of gold, the steric bulk, and the dihedral angles of the PDS direct the mechanistic pathways and control the turnover frequency.

Hyperconjugative Aromaticity-Based Circularly Polarized Luminescence Enhancement in Polyaurated Heterocycles

Hyperconjugative aromaticity (HA) frequently appears in metalla-aromatics, but its effect on photophysical properties remains unexplored to date. Herein, we reveal two different HA scenarios in nearly isostructural triaurated indolium and benzofuranylium compounds. The biased HAs show a discernible effect on the spatial arrangement of metal atoms and thus tailor metal parentage in frontier orbitals and the HOMO-LUMO energy gap. Theoretical calculations and structural analyses demonstrate that HA not only influences the degree of electron delocalization over the trimetalated aromatic rings but also affects π-coordination of Au(I) and intercluster aurophilic interaction. Consequently, the triaurated benzofuranylium complex shows better photoluminescence performance (quantum yield up to 49.7%) over the indolium analogue. Furthermore, four pairs of axially chiral bibenzofuran-centered trinuclear and hexanuclear gold clusters were purposefully synthesized to correlate their HA-involved structures with the chiroptical response. The triaurated benzofuranylium complexes exhibit strong circular dichroism (CD) response in solution but CPL silence even in solid film. In contrast, the hexa-aurated homologues display strong CD and intense CPL signals in both aggregated state and solid film (luminescence anisotropy factor glum up to 10-3). Their amplified chiroptical response is finally ascribed to the dominant intermolecular exciton couplings of large assemblies formed through the HA-tailored aggregation of hexanuclear compounds.

Stereoelectronic Modulation of a Single-Molecule Junction through a Tunable Metal-Carbon dπ-pπ Hyperconjugation

Conjugated molecules play a critical role in the construction of single-molecule devices. However, most conventional conjugated molecules, such as hydrocarbons, involve only a pπ-pπ conjugation of light elements. While the metal d-orbitals can introduce abundant electronic effects to achieve novel electronic properties, it is very scarce for the charge transport study of dπ-pπ conjugated pathways with a metal involved. Here, we employed the single-molecule break junction technique to investigate the charge transport through dπ-pπ conjugated backbones with metal-carbon multiple bonds integrated into the alternative conjugated pathways. The involved dπ-pπ conjugation not only supports high conductivity comparable to that of conjugated hydrocarbons but also significantly enhances the tunable diversity in electronic properties through the metal-induced secondary interaction. Specifically, the introduction of the metal brings an unconventionally stereoelectronic effect triggered by metal-carbon dπ-pπ hyperconjugation, which can be tuned by protonation taking place on the metal-carbon multiple bonds, collectively modulating the single-molecule rectification feature and transmission mechanism. This work demonstrates the promise of utilizing the diverse electronic effect of metals to design molecular devices.

Helical Trilayer Nanographenes with Tunable Interlayer Overlaps

The synthesis of well-defined nanocarbon multilayers, beyond the bilayer structure, is still a challenging goal. Herein, two trilayer nanographenes were synthesized by covalently linking nanographene layers through helicene bridges. The structural characterization of the trilayer nanographenes revealed a compact trilayer-stacked architecture. The introduction of a furan ring into the helicene linker modulates the interlayer overlap and π-conjugation of the trilayer nanographenes, enabling the tuning of the interlayer coupling, as demonstrated by optical, electrochemical, and theoretical analyses. Both synthesized trilayer nanographenes are rigid chiral nanocarbons and show a chirality transfer from the helicene moiety to the stacked nanographene layers. These helical trilayer nanographenes reported here represent the covalently linked multilayer nanographenes rather than bilayer ones, showing the tunable multilayer stacking structure.

Mechanistic Insights into Multisilver-Mediated Synergistic Activation of Terminal Alkynes

Synergistic effect extensively exists in multimetal-involved catalytic or mediated processes of group 11 metals due to their remarkable metallophilic interactions. Herein, we present a multiple synergism model for alkynyl substrates and conduct theoretical investigations on various multimetallic bonding modes and the corresponding synergistic activations. We computationally screen nine alkynyl multisilver coordination modes and sequence their reactivity shown in an intramolecular nucleophilic addition reaction by the trend of active μ₄-η¹η¹η²η² and μ₃-η¹η¹η² to the relatively inert μ₂-η¹η². The transition-state (TS) stabilization of the high-nuclearity mode mainly comes from the significant negative interaction energies between Ag_n and the substrate based on the distortion/interaction analysis. Energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) analysis further reveals the charge-accepting reservoir effect of the polysilver moiety and the orbital match between the alkynyl group and specific spatial arrangement of silver atoms to account for this efficient activation. In addition, tests on different ligands coordinated to silver atoms show a correlation of the ligand conformation adjustability with the reactivity of the alkynyl unit, and the accommodable η² activation unit embodies a lower deformation energy than the other homonuclear synergistic modes. Privileged multiple synergistic models have been further evidenced based on on-bench experiments by isolating trisilver and tetrasilver alkynyl complexes. This study not only systematically evaluates the multimetallic synergism of different coordination modes in alkyne activation but also provides a guidance for the future design of multimetallic catalysts.

Guiding the High-Yield Synthesis of NHC-Ligated Gold Nanoclusters by 19F NMR Spectroscopy

Optimizing the synthesis of atomically precise metal nanoclusters by virtue of molecular tools is highly desirable but quite challenging. Herein we report how ¹⁹F NMR spectroscopy can be used to guide the high-yield synthesis of N-heterocyclic carbene (NHC)-stabilized gold nanoclusters. In spite of little difference, ¹⁹F NMR signals of fluoro-incorporated NHCs (^FNHC) are highly sensitive to the tiny change in their surrounding chemical environments with different N-substituents, metals, or anions, thus providing a convenient strategy to discriminate species in reaction mixtures. By using ¹⁹F NMR, we first disclosed that the one-pot reduction of ^FNHC-Au-X (X is halide) yields multiple compounds, including cluster compounds and also a large amount of highly stable [Au(^FNHC)₂]⁺ byproduct. The detailed quantitative ¹⁹F NMR analyses over the reductive synthesis of NHC-stabilized Au nanoclusters reveal that the formation of the di-NHC complex is deleterious to the high-yield synthesis of NHC-stabilized Au nanoclusters. With the understanding, the reaction kinetic was then slowed by controlling the reduction rate to achieve the high yield of a [Au₂₄(^FNHC)₁₄X₂H₃]³⁺ nanocluster with a unique structure. The strategy demonstrated in this work is expected to provide an effective tool to guide the high-yield synthesis of organic ligand-stabilized metal nanoclusters.

Pro-oxidant response and accelerated ferroptosis caused by synergetic Au(I) release in hypercarbon-centered gold(I) cluster prodrugs

Medicinal applications of gold complexes have recently attracted attention due to their innovative antitumor mechanisms. In this work, two hypercoordinated carbon-centered gold clusters PAA4 and PAA5 are quantitatively synthesized by an intramolecular 6-exo-dig cyclization of polymetalated precursors. The on-bench and in vitro experimental studies demonstrate that the characteristic hypercarbon-tetragold(I) multi-center bonding in PAA4 and PAA5 not only guarantees their stability under common physiological conditions, but also facilitates a glutathione (GSH)-triggered prompt and synergetic release of active Au(I) ions in the GSH-overexpressed and acidic microenvironment of human bladder cancer EJ cells. The instantly massive release of coordination unsaturated Au(I) ions causes the efficient inhibition of thioredoxin reductases and then induces a rapid pro-oxidant response, consequently causing the occurrence of accelerated ferroptosis of EJ cells. As a result, these hypercarbon-centered gold(I) cluster prodrugs show high cytotoxicity to bladder cancer cell lines and thus exhibit a significant inhibition effect towards bladder tumors in vivo. Correlation of the synergetic domino dissociation of carbon-polymetal multi-center bonding in metal clusters with the accelerated ferroptosis of cancer cells provides a strategy for metallo-prodrugs and opens a broader prospect for the biological application of metal cluster compounds.

Structural Control and Chiroptical Response in Intrinsically Tetra- and Pentanuclear Chiral Gold Clusters

Controlling the synthesis of chiral metal clusters in the aspects of nuclearity number, metal-metal interaction, and spatial arrangement of metal atoms is crucial for establishing the correlation of detailed structural factors with chiroptical activity. Herein, a series of enantiopure gold complexes with nuclearity numbers ranging from 2 to 5 were constructed and structurally characterized. On the basis of the annulation reaction between two aurated μ₂-imido nucleophilic units with various aldehydes, we finely adjusted the metal-metal interaction and torsion angles of a characteristic tetranuclear metal cluster by introducing different substituents into the resulting imidazolidine dianionic chiral skeleton. Further structural investigations, contrast experiments, and time-dependent density functional theory calculations confirmed that the chiroptical response of the acquired asymmetric metal clusters was mainly affected by the geometrically twisted arrangement of metal atoms. Finally, the tetranuclear gold cluster compound with the shortest intermetallic interaction and the largest torsion angle of a Au₄ core showed the highest absorption anisotropy factor up to 2.2 × 10^-3. In addition, the correlation of structural factors with the stability of chiral gold clusters was thoroughly evaluated by monitoring the CD, UV-vis, and NMR spectra at elevated temperatures. Insight into the relationship between the structural factors with the chiroptical property and stability of chiral gold clusters in this work will help us to design and achieve more stable chiral metal clusters and stimulate their practical applications in chiroptical functional materials.

Rational designs of structurally similar TADF and HLCT emitters with benzo- or naphtho-carbazole units as electron donors

Structurally similar D–A type molecules with the combination of benzo- or naphtho-carbazole units as electron donors and tunable electron acceptors with different electron-withdrawing ability are designed to realize HLCT and TADF emissions.

A new strategy for hyperconjugative antiaromatic compounds utilizing negative charges: a dibenzo[b,f]silepinyl dianion

Herein we propose a new strategy for hyperconjugative antiaromatic compounds utilizing negative charges and design the 5,5-diphenyldibenzo[b,f]silepinyl dianion (pseudo 16π-electron system) in which negative hyperconjugation occurs between the anionic π-cloud and the σ*(Si-Ph) orbital. Essentially, reduction of the dibenzo[b,f]silepin with lithium readily generated a dilithium salt of the dibenzosilepinyl dianion, and its hyperconjugative antiaromaticity has been evidenced by the upfield shifts of ¹H NMR signals and theoretical calculations, including large NICS_zz values and ACID plots.

Tin Metal Cluster Compounds as New Third-Order Nonlinear Optical Materials by Computational Study

It is quite appealing but challenging to predict and synthesize new nonlinear optical (NLO) materials with exceptional performance. Herein, the different Sn₄ cluster core structures and third-order NLO properties are studied through electronic structure, excited hole-electron, bonding character, and aromaticity analysis. As a result, Sn₄ clusters with ring core structure (Sn₄-R) not only have the smallest E_gap, the largest UV-vis response intensity, but also the strongest third-order NLO response in our work. As proved by natural bond orbitals' (NBO) analysis, electron localization function (ELF), and adaptive natural density partitioning (AdNDP), the Sn₄⁴⁺ has two in-plane four center-two electron (4c-2e) Sn-Sn σ-bonds, resulting in a good delocalization. For the first time, delocalization of metal cluster cores in tin clusters that is beneficial to the third-order NLO response is proposed, which provides a new guidance to design and prepare third-order NLO materials.

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation

Activation of atmospherically abundant dinitrogen (N₂) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N₂ activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N₂ activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N₂ activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N₂ activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N₂ activation chemistry, highlighting great potential of FLP for N₂ activation and functionalization.

Aromaticity Survival in Hydrofullerenes: The Case of C66 H4 with Its π-Aromatic Circuits

The isolated-pentagon rule (IPR) is a determining structural feature that accounts for hollow fullerene stabilization and properties related to C_n (n≥60) cages. The recent characterization of an unprecedented non-IPR hydrofullerene, C_2v C₆₆ H₄ , bearing two heptagons with adjacent fused-pentagon motifs, largely dismisses this feature. Herein, employing DFT calculations, the ¹³ C NMR spectroscopy and aromatic behavior of C_2v C₆₆ H₄ are explored. The results show the presence of three π-aromatic circuits at the bottom boat section of C₆₆ H₄ , indicating the unique features of this hydrofullerene in comparison to those of pristine C₆₀ . In addition, under specific orientations of the external field, certain π-aromatic circuits are enabled, resulting in a more aromatic fullerene than that of C₆₀ , but lower than that of the spherical aromatic C₆₀ ^6- fulleride. Notably, under a field aligned with the saturated carbon atoms, nonaromatic characteristics are exposed. This reveals that spherical-like cages can involve a complex magnetic response that heavily depends on the orientation of the applied field.

Tuning the hyperconjugative aromaticity in Au(III)-substituted indoliums

As a fundamental concept in chemistry, aromaticity has been extended from traditional organics to organometallics. Similarly, hyperconjugative aromaticity (HCA) has also been developed from main group to transition metal systems through the hyperconjugation of the substituents. However, it remains unclear that how the oxidation state of transition metal in the substituents affects the HCA. Herein, we demonstrate via density functional theory calculations that HCA could disappear in indoliums when the Au(i) substituents are changed to the Au(iii) ones. By tuning the ligand or cis-trans isomerization, HCA could be regained or enhanced in indoliums containing Au(iii) substitutents.

Probing hyperconjugative aromaticity in 2H-pyrrolium and cyclopentadiene containing group 9 transition metal substituents: bridged carbonyl ligands can enhance aromaticity

Bridged carbonyls can enhance hyperconjugative aromaticity of group 9 transition metal disubstituted 2H-pyrrolium and cyclopentadiene.

Reaction Mechanisms on [3 + 2] Cycloaddition of Azides with Metal Carbyne Complexes: Significant Effects of Aromaticity, Substituent, and Metal Center

Density functional theory calculations were used to investigate the reaction mechanisms on [3 + 2] cycloaddition reactions of azides with metal carbyne complexes. Our results reveal that the formation of a 1,4-metallatriazole regioisomer is a kinetically favorable process in comparison with the formation of 1,5-metallatriazole. Aromaticity plays an important role in stabilizing the products in these reactions. Further analyses show that the electron-donating ligand on metal centers or the electron-withdrawing group on the azide could accelerate the [3 + 2] cycloaddition reaction. All of these findings could be useful for experimental chemists to develop "click reactions" in organometallic chemistry.

Polynuclear organometallic clusters: synthesis, structure, and reactivity studies

This feature article highlights our recent advances in the controllable synthesis of carbon-centered polynuclear organometallic clusters: from synthesis to transformation, reactivity and mechanism.