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1. Aromatic character of [Au ₁₃ ] ⁵⁺ and [MAu ₁₂ ] ^4+/6+ (M = Pd, Pt) cores in ligand protected gold nanoclusters – interplay between spherical and planar σ-aromatics

   https://doi.org/10.1039/C9CP04477A  

   Fedik, Nikita; Boldyrev, Alexander I.; Muñoz-Castro, Alvaro (November 2019, Physical Chemistry Chemical Physics)

   The most characteristic feature of planar π-aromatics is the ability to sustain a long-range shielding cone under a magnetic field oriented in a specific direction. In this article, we showed that similar magnetic responses can be found in σ-aromatic and spherical aromatic systems. For [Au 13 ] 5+ , long-range characteristics of the induced magnetic field in the bare icosahedral core are revealed, which are also found in the ligand protected [Au 25 (SH) 18 ] − model, proving its spherical aromatic properties, also supported by the AdNDP analysis. Such properties are given by the 8-ve of the structural core satisfying the Hirsch 2( N + 1) 2 rule, which is also found in the isoelectronic [M@Au 12 ] 4+ core, a part of the [MAu 24 (SR) 18 ] 2− (M = Pd, Pt) cluster. This contrasts with the [M@Au 12 ] 6+ core in [MAu 24 (SR) 18 ] 0 (M = Pd, Pt), representing 6-ve superatoms, which exhibit characteristics of planar σ-aromatics. Our results support the spherical aromatic character of stable superatoms, whereas the 6-ve intermediate electron counts satisfy the 4 N + 2 rule (applicable for both π- and σ-aromatics), showing the reversable and controlled interplay between 3D spherical and 2D σ-aromatic clusters. 

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2. Isolation and reactivity of a gold( i ) hydroxytrifluoroborate complex stabilized by anion-π ⁺ interactions

   https://doi.org/10.1039/D1CC04105C  

   Zhou, Jiliang; Litle, Elishua D.; Gabbaï, François P. (October 2021, Chemical Communications)

   A 9,9-dimethylxanthene-based ligand substituted at the 4- and 5-positions by a phosphine and a xanthylium unit, respectively, has been prepared and converted into an AuCl complex, the structure of which reveals an intramolecular Au–Cl⋯π + interaction. This new ligand platform was also found to support the formation of an unprecedented hydroxytrifluoroborate derivative featuring a “hard/soft” mismatched Au– μ (OH)–BF 3 motif. Despite its surprising stability, this gold hydroxytrifluoroborate complex is a remarkably potent carbophilic catalyst which readily activates alkynes, without activator. 

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3. Elucidating the active sites for CO ₂ electroreduction on ligand-protected Au ₂₅ nanoclusters

   https://doi.org/10.1039/c8cy01099d  

   Austin, Natalie; Zhao, Shuo; McKone, James R.; Jin, Rongchao; Mpourmpakis, Giannis (January 2018, Catalysis Science & Technology)

   Using density functional theory (DFT) calculations, we investigated the electrochemical reduction of CO 2 and the competing H 2 evolution reaction on ligand-protected Au 25 nanoclusters (NCs) of different charge states, Au 25 (SR) 18 q ( q = −1, 0, +1). Our results showed that regardless of charge state, CO 2 electroreduction over Au 25 (SR) 18 q NCs was not feasible because of the extreme endothermicity to stabilize the carboxyl (COOH) intermediate. When we accounted for the removal of a ligand (both –SR and –R) from Au 25 (SR) 18 q under electrochemical conditions, surprisingly we found that this is a thermodynamically feasible process at the experimentally applied potentials with the generated surface sites becoming active centers for electrocatalysis. In every case, the negatively charged NCs, losing a ligand from their surface during electrochemical conditions, were found to significantly stabilize the COOH intermediate, resulting in dramatically enhanced CO 2 reduction. The generated sites for CO 2 reduction were also found to be active for H 2 evolution, which agrees with experimental observations that these two processes compete. Interestingly, we found that the removal of an –R ligand from the negatively charged NC, resulted in a catalyst that was both active and selective for CO 2 reduction. This work highlights the importance of both the overall charge state and generation of catalytically active surface sites on ligand-protected NCs, while elucidating the CO 2 electroreduction mechanisms. Overall, our work rationalizes a series of experimental observations and demonstrates pathways to convert a very stable and catalytically inactive NC to an active electrocatalyst. 

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4. η2 Coordination of Electron-Deficient Arenes with Group 6 Dearomatization Agents

   https://doi.org/10.1021/acs.organomet.0c00277  

   Smith, J. A.; Simpson, S. R.; Westendorff, K. S.; Weatherford-Pratt, J.; Myers, J. T.; Wilde, J. H.; Dickie, D. A.; Harman, W. D. (June 2020, Organometallics)

   null (Ed.)

   The exceptionally π-basic metal fragments {MoTp(NO)(DMAP)} and {WTp(NO)(PMe3)} (Tp = tris(pyrazolyl)borate; DMAP = 4-(N,N-dimethylamino)pyridine) form thermally stable η2-coordinated complexes with a variety of electron-deficient arenes. The tolerance of substituted arenes with fluorine-containing electron withdrawing groups (EWG; −F, −CF3, −SF5) is examined for both the molybdenum and tungsten systems. When the EWG contains a π bond (nitriles, aldehydes, ketones, ester), η2 coordination occurs predominantly on the nonaromatic functional group. However, complexation of the tungsten complex with trimethyl orthobenzoate (PhC(OMe)3) followed by hydrolysis allows access to an η2-coordinated arene with an ester substituent. In general, the tungsten system tolerates sulfur-based withdrawing groups well (e.g., PhSO2Ph, MeSO2Ph), and the integration of multiple electron-withdrawing groups on a benzene ring further enhances the π-back-bonding interaction between the metal and aromatic ligand. While the molybdenum system did not form stable η2-arene complexes with the sulfones or ortho esters, it was capable of forming rare examples of stable η2-coordinated arene complexes with a range of fluorinated benzenes (e.g., fluorobenzene, difluorobenzenes). In contrast to what has been observed for the tungsten system, these complexes formed without interference of C–H or C–F insertion. 

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5. Unconventional mechanism and selectivity of the Pd-catalyzed C–H bond lactonization in aromatic carboxylic acid

   https://doi.org/10.1038/s41467-022-27986-6  

   Xu, Li-Ping; Qian, Shaoqun; Zhuang, Zhe; Yu, Jin-Quan; Musaev, Djamaladdin G. (January 2022, Nature Communications)

   Abstract The search for more effective and highly selective C–H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C–H oxidation reactions. Here, we reveal the stepwise intramolecular S_N2 nucleophilic substitution mechanism with the rate-limiting C–O bond formation step for the Pd(II)-catalyzed C(sp³)–H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C–O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O₂oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η³-(π-benzylic)–Pd and K⁺–O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η³-(π-benzylic)–Pd and η²-(π-phenyl)–Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K⁺cation in the ortho-C–H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp³)–H bond lactonization in the presence of C(sp²)–H. 

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