Cage-Shaped Borate Catalysts Bearing Precisely Controlled Lewis Acidity and Their Application in Glycosylations

Cage-shaped borates, whose Lewis acidity can be precisely modulated by the structural attributes of the triphenolic ligands, were employed as catalysts for glycosylation. Each cage-shaped borate displayed distinctive reactivity; thus, screening of the borate catalysts enabled controllable activation of glycosyl fluorides under mild conditions. Practical glycosylation was achieved by fine-tuning the Lewis acidity tailored to the substrate reactivity, thereby providing a versatile method applicable to the synthesis of complex glycans.

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

Reactivity of [AuF3 (SIMes)]: Pathway to Unprecedented Structural Motifs

We report on a comprehensive reactivity study starting from [AuF3 (SIMes)] to synthesize different motifs of monomeric gold(III) fluorides. A plethora of different ligands has been introduced in a mono-substitution yielding trans-[AuF2 X(SIMes)] including alkynido, cyanido, azido, and a set of perfluoroalkoxido complexes. The latter were better accomplished via use of perfluorinated carbonyl-bearing molecules, which is unprecedented in gold chemistry. In case of the cyanide and azide, triple substitution gave rise to the corresponding [AuX3 (SIMes)] complexes. Comparison of the chemical shift of the carbene carbon atom in the 13 C{1 H} NMR spectrum, the calculated SIMes affinity and the Au-C bond length in the solid state with related literature-known complexes yields a classification of trans-influences for a variety of ligands attached to the gold center. Therein, the mixed fluorido perfluoroalkoxido complexes have a similar SIMes affinity to AuF3 with a very low Gibbs energy of formation when using the perfluoro carbonyl route.

Spectroscopic Manifestations and Implications for Catalysis of Quasi-d10 Configurations in Formal Gold(III) Complexes

Several gold +I and +III complexes are investigated computationally and spectroscopically, focusing on the d-configuration and physical oxidation state of the metal center. Density functional theory calculations reveal the non-negligible electron-sharing covalent character of the metal-to-ligand σ-bonding framework. The bonding of gold(III) is shown to be isoelectronic to the formal CuIII complex [Cu(CF3 )4 ]1- , in which the metal center tries to populate its formally unoccupied 3dx2-y2 orbital via σ-bonding, leading to a reduced d10 CuI description. However, Au L3 -edge X-ray absorption spectroscopy reveals excitation into the d-orbital of the AuIII species is still possible, showing that a genuine d10 configuration is not achieved. We also find an increased electron-sharing nature of the σ-bonds in the AuI species, relative to their AgI and CuI analogues, due to the low-lying 6s orbital. We propose that gold +I and +III complexes form similar bonds with substrates, owing primarily to participation of the 5dx2-y2 or 6s orbital, respectively, in bonding, indicating why AuI and AuIII complexes often have similar reactivity.

Gold Teflates Revisited: From the Lewis Superacid [Au(OTeF5 )3 ] to the Anion [Au(OTeF5 )4 ]. Chemistry 2023;29:e202203634. [PMID: 36598847 DOI: 10.1002/chem.202203634] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

A new synthetic access to the Lewis acid [Au(OTeF₅ )₃ ] and the preparation of the related, unprecedented anion [Au(OTeF₅ )₄ ]^- with inorganic or organic cations starting from commercially available and easy-to-handle gold chlorides are presented. In this first extensive study of the Lewis acidity of a transition-metal teflate complex by using different experimental and quantum chemical methods, [Au(OTeF₅ )₃ ] was classified as a Lewis superacid. The solid-state structure of the triphenylphosphine oxide adduct [Au(OPPh₃ )(OTeF₅ )₃ ] was determined, representing the first structural characterization of an adduct of this highly reactive [Au(OTeF₅ )₃ ]. Therein, the coordination environment around the gold center slightly deviates from the typical square planar geometry. The [Au(OTeF₅ )₄ ]^- anion shows a similar coordination motif.

Constructing tunable coordinatively unsaturated sites in Fe-based metal-organic framework for effective degradation of pharmaceuticals in water: Performance and mechanism

Micropollutants are ubiquitously detected in the aqueous environment, which needs to be removed by novel materials effectively. Herein, we synthesized a photo-Fenton catalyst based on MIL-53 (Fe) to effectively degrade sulfadimidine, one of the micropollutants in water. Abundant Lewis acid active sites (54.26 μmol/g) were successfully constructed within the metal cluster using FeCl₃·6H₂O, 1,4-benzene dicarboxylate, and modulators. This study reports a strategy by effectively constructing tunable Lewis acid active sites within the cavities in MIL-53 (Fe) via a facile solvothermal reaction for sixteen micropollutants removal. The photo-Fenton degradation of sulfamethazine was completely removed (∼99%) within only 1 min with a small amount of hydrogen peroxide added. Both theoretical calculation and the experiment results prove that introducing the unsaturated coordinated/lewis acid sites can remarkably reduce the band gap energy and increase the charge-separation efficiency by changing the electron configuration with more distribution asymmetry of structures. The effective degradation of structurally diverse pharmaceuticals with environmentally relevant concentrations was studied by immobilizing MOF-catalyst into a PVDF support. This work advanced the development of effective approaches for emergency contaminants control.

Insights on the Lewis Superacid Al(OTeF5 )3 : Solvent Adducts, Characterization and Properties

Preparation and characterization of the dimeric Lewis superacid [Al(OTeF5 )3 ]2 and various solvent adducts is presented. The latter range from thermally stable adducts to highly reactive, weakly bound species. DFT calculations on the ligand affinity of these Lewis acids were performed in order to rank their remaining Lewis acidity. An experimental proof of the Lewis acidity is provided by the reaction of solvent-adducts of Al(OTeF5 )3 with [PPh4 ][SbF6 ] and OPEt3 , respectively. Furthermore, their reactivity towards chloride and pentafluoroorthotellurate salts as well as (CH3 )3 SiCl and (CH3 )3 SiF is shown. This includes the formation of the dianion [Al(OTeF5 )5 ]2- .

Crystal Growth from Anhydrous HF Solutions of M2+ (M = Ca, Sr, Ba) and [AuF6]-, Not Only Simple M(AuF6)2 Salts

Crystal growth from anhydrous HF solutions of M²⁺ (M = Ca, Sr, Ba) and [AuF₆]⁻ (molar ratio 1:2) gave [Ca(HF)₂](AuF₆)₂, [Sr(HF)](AuF₆)₂, and Ba[Ba(HF)]₆(AuF₆)₁₄. [Ca(HF)₂](AuF₆)₂ exhibits a layered structure in which [Ca(HF)₂]²⁺ cations are connected by AuF₆ units, while the crystal structure of Ba[Ba(HF)]₆(AuF₆)₁₄ exhibits a complex three-dimensional (3-D) network consisting of Ba²⁺ and [Ba(HF)₂]²⁺ cations bridged by AuF₆ groups. These results indicate that the previously reported M(AuF₆)₂ (M = Ca, Sr, Ba) compounds, prepared in the anhydrous HF, do not in fact correspond to this chemical formula. When the initial M²⁺/[AuF₆]⁻ ratio was 1:1, single crystals of [M(HF)](H₃F₄)(AuF₆) were grown for M = Sr. The crystal structure consists of a 3-D framework formed by [Sr(HF)]²⁺ cations associated with [AuF₆]⁻ and [H₃F₄]⁻ anions. The latter exhibits a Z-shaped conformation, which has not been observed before. Single crystals of M(BF₄)(AuF₆) (M = Sr, Ba) were grown when a small amount of BF₃ was present during crystallization. Sr(BF₄)(AuF₆) crystallizes in two modifications. A high-temperature α-phase (293 K) crystallized in an orthorhombic unit cell, and a low-temperature β-phase (150 K) crystallized in a monoclinic unit cell. For Ba(BF₄)(AuF₆), only an orthorhombic modification was observed in the range 80–230 K. An attempt to grow crystals of Ca(BF₄)(AuF₆) failed. Instead, crystals of [Ca(HF)](BF₄)₂ were grown and the crystal structure was determined. During prolonged crystallization of [AuF]₆^– salts, moisture can penetrate through the walls of the crystallization vessel. This can lead to partial reduction of Au(V) to A(III) and the formation of [AuF₄]⁻ byproducts, as shown by the single-crystal growth of [Ba(HF)]₄(AuF₄)(AuF₆)₇. Its crystal structure consists of [Ba(HF)]²⁺ cations connected by AuF₆ octahedra and square-planar AuF₄ units. The crystal structure of the minor product [O₂]₂[Sr(HF)]₅[AuF₆]₁₂·HF was also determined.

Crystal growth from anhydrous HF solutions of M²⁺ (M = Ca, Sr, Ba) and [AuF₆]⁻ (molar ratio 1:2) gave [Ca(HF)₂](AuF₆)₂ and Ba[Ba(HF)]₆(AuF₆)₁₄. These results indicate that the previously reported M(AuF₆)₂ (M = Ca, Sr, Ba) compounds do not correspond to this chemical formula. When the initial Sr²⁺/[AuF₆]⁻ ratio was 1:1, crystals of [Sr(HF)](H₃F₄)(AuF₆) were grown. Single crystals of M(BF₄)(AuF₆) (M = Sr, Ba) were obtained when a small amount of BF₃ was present during crystallization. The crystal structures of [Ba(HF)]₄(AuF₄)(AuF₆)₇, [O₂]₂[Sr(HF)]₅[AuF₆]₁₂·HF, and Ca(BF₄)(AuF₆) byproducts were also determined.

Synthesis, Reactivity, and Bonding of Gold(I) Fluorido-Phosphine Complexes

Gold(I) fluorido complexes with phosphine ligands have been synthesized from their respective iodido precursors. The bonding situation in comparison between complexes bearing phosphines and N-heterocyclic carbenes (NHCs) was explored quantum-chemically, obtaining similar results for both. Calculations of the ¹⁹F NMR chemical shifts match the experimental values well, including the approximately 40 ppm low-field shifts for the phosphine complexes compared to the NHC complexes, in spite of similar negative charges on fluorine. The reactivity of the highly water-sensitive gold(I) fluorido complexes was studied, resulting in substitution at the metal using trimethylsilyl reagents. The compounds studied were characterized using NMR as well as X-ray diffraction methods.

Trifluoromethylation of [AuF3 (SIMes)]: Preparation and Characterization of [Au(CF3 )x F3-x (SIMes)] (x=1-3) Complexes

Trifluoromethylation of [AuF3 (SIMes)] with the Ruppert-Prakash reagent TMSCF3 in the presence of CsF yields the product series [Au(CF3 )x F3-x (SIMes)] (x=1-3). The degree of trifluoromethylation is solvent dependent and the ratio of the species can be controlled by varying the stoichiometry of the reaction, as evidenced from the 19 F NMR spectra of the corresponding reaction mixtures. The molecular structures in the solid state of trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] are presented, together with a selective route for the synthesis of the latter complex. Correlation of the calculated SIMes affinity with the carbene carbon chemical shift in the 13 C NMR spectrum reveals that trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] nicely follow the trend in Lewis acidities of related organo gold(III) complexes. Furthermore, a new correlation between the Au-Ccarbene bond length of the molecular structure in the solid state and the chemical shift of the carbene carbon in the 13 C NMR spectrum is presented.

Unexpected persistence of cis-bridged chains in compressed AuF3

Raman scattering measurements indicate that cis-bridged chains are retained in AuF3 even at a compression of 45 GPa - in contrast to meta-GGA calculations suggesting that structures with such motifs are thermodynamically unstable above 4 GPa. This metastability implies that novel gold fluorides (e.g. AuF2) might be attainable at lower pressures than previously proposed.

Reactivity of VOF3 with N-Heterocyclic Carbene and Imidazolium Fluoride: Analysis of Ligand-VOF3 Bonding with Evidence of a Minute π Back-Donation of Fluoride

Reaction of vanadium(V) oxide trifluoride (VOF₃) and the new "naked" fluoride reagent [(L^Dipp)H][F] (L^Dipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2 H-imidazol-2-ylidene) leads to the isolation of [(L^Dipp)H][VOF₄] (1) where the long sought discrete [VOF₄]^- anion was finally obtained. The neutral [(L^Dipp)VOF₃] (2) complex was synthesized by a similar reaction between VOF₃ and bulky N-heterocyclic carbene (NHC) ligand L^Dipp. In this context, we analyzed, by means of DFT calculations, intermolecular interactions between [(L^Dipp)VOF₃] (2) complexes in the crystal structure and realized that these interactions have a significant effect on the V-F_trans bond length. We further scrutinized ligand bonding within [(L^Dipp)VOF₃] (2) and related complexes, because, in this kind of complexes, a rather short distance between C_NHC and cis-halogen atoms has spurred some discussion about the type of interactions between them. We provide evidence of a minute π back-bonding into NHC ligands, which is larger for chloride [(NHC)VOCl₃] than fluoride [(NHC)VOF₃] complexes, although the fluoride ions are, counterintuitively and to a larger degree, involved in back-bonding than chloride ions. The influence of π back-bonding on V-F_trans and V-F_cis bond lengths was also rationalized. Finally, the hydrolysis of [(L^Dipp)VOF₃] (2) product was studied and [(L^Dipp)H][VO₂F₂] (3) salt was obtained and characterized as the most stable product in this system.