Determining the Impact of Ligand and Alkene Substituents on Bonding in Gold(I)-Alkene Complexes Supported by N-Heterocyclic Carbenes: A Computational Study

Going for gold - the chemistry of structurally authenticated gold(I)-ethylene complexes

Gold coordination chemistry and catalysis involving unsaturated hydrocarbons such as olefins have experienced a remarkable growth during the last few decades. Despite the importance, isolable and well-characterized molecules with ethylene, the simplest and the most widely produced olefin, on gold are still limited. This review aims to cover features of, and strategies utilized to stabilize, gold-ethylene complexes and their diverse use in chemical transformations and homogeneous catalytic processes. Isolable and well-authenticated gold-ethylene complexes are important not only for structural, spectroscopic, and bonding studies but also as models for likely intermediates in gold mediated reactions of alkenes and gold-alkene species observed in the gas phase. There has also been development on AuI/III catalytic cycles. Nitrogen based ligands have been the most widely utilized ligand supports thus far for the successful stabilization of gold-ethylene adducts. Gold has a bright future in olefin chemistry and with ethylene.

Rationalizing the influence of α-cationic phospholes on π-catalysis

The physical factors behind the experimentally observed high activity of gold(I)-catalysts having an α-cationic phosphole as a ligand have been computationally explored. To this end, the gold(I)-catalysed hydroarylation reactions of phenylacetylene and mesitylene involving both neutral and cationic phosphole as well as phosphine ligands have been quantitatively analyzed in detail with the help of the activation strain model of reactivity in combination with the energy decomposition analysis method. It is found that the cationic phosphole ligands induce a dramatic change in both the geometry and the electronic structure of the initially formed π-complex which significantly enhances its electrophilicity. This results in an enhancement of the key π(mesitylene) → π*(LAu-acetylene complex) molecular orbital interaction which is the main factor responsible for the activating effect of these cationic ligands.

A dicoordinate gold(I)-ethylene complex

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold. Besides, the bonding situation of this species has been investigated by means of state-of-the-art Density Functional Theory (DFT) calculations indicating that π-backdonation plays a minor role compared with tricoordinate analogues.

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold.

Internal and External Influences on Stability and Ligand Exchange Reactions in Bromido[3-ethyl-4-aryl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) Complexes

The ligand scrambling reaction of gold(I) complexes is a phenomenon occurring primarily in L–Au^I–X (L = phosphine, N-heterocyclic carbene (NHC), and thiol; X = halide and thiol) complexes and has been observed among others for e.g., the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) complex (7a), which underwent ligand rearrangement in aqueous solutions. In this study, we investigated the influence of substituents on the 4-aryl ring of the related (NHC)Au^IBr complexes (1a–9a) in terms of the conversion to the [(NHC)₂Au^I]⁺ (1b–9b) and [(NHC)₂Au^IIIBr₂]⁺ (1c–9c) species. Furthermore, the influence of external factors such as solvent, temperature, concentration, and presence of halides (Cl^–, Br^–, and I^–) or hydroxyl ions was studied to gain a deeper understanding of the ligand rearrangement reaction. The substituent on the 4-aryl ring has a marginal impact on the scrambling reaction. Out of the investigated organic solvents (dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol (EtOH), methanol (MeOH), and acetonitrile (ACN)), only ACN separates single complex molecules. In all other solvents, relatively stable ((NHC)Au^IBr)₂ dimers are present. The addition of water to ACN solutions forces the formation of such dimeric units, starting the transformation to [(NHC)₂Au^I]⁺ and [(NHC)₂Au^IIIBr₂]⁺. The rate-determining step is the release of Br^– from a T-shape intermediate because an excess of KBr terminates this reaction. Furthermore, it is obvious that only single molecules react with halides. The aurophilic interactions between two (NHC)Au^IBr molecules are too strong in the presence of water and largely impeded reaction with halides. As a single molecule, the reaction with Cl^– (e.g., in a 0.9% NaCl solution) is notable, while I^– even leads to a fast and quantitative conversion to (NHC)Au^II and finally to [(NHC)₂Au^I]⁺.

Internal and external factors of the ligand scrambling reaction in (NHC)Au^IBr complexes were investigated by the means of HPLC. The data represent the impact of substituents, temperature, solvent, concentration and addition of halides on the conversion to the respective [(NHC)₂Au^I]⁺ and the oxidation to the [(NHC)₂Au^IIIBr₂]⁺ complexes.

N-Heterocyclic Carbene Gold(I) Complexes: Mechanism of the Ligand Scrambling Reaction and Their Oxidation to Gold(III) in Aqueous Solutions

N-Heterocyclic carbene (NHC) gold(I) complexes offer great prospects in medicinal chemistry as antiproliferative, anticancer, and antibacterial agents. However, further development requires a thorough understanding of their reaction behavior in aqueous media. Herein, we report the conversion of the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ((NHC)Au^IBr, 1) complex in acetonitrile/water mixtures to the bis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ([(NHC)₂Au^I]⁺, 7), which is subsequently oxidized to the dibromidobis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(III) ([(NHC)₂Au^IIIBr₂]⁺, 9). By combining experimental data from HPLC, NMR, and (LC-)/HR-MS with computational results from DFT calculations, we outline a detailed ligand scrambling reaction mechanism. The key step is the formation of the stacked ((NHC)Au^IBr)₂ dimer (2) that rearranges to the T-shaped intermediate Br(NHC)₂Au^I–Au^IBr (3). The dissociation of Br^– from 3 and recombination lead to (NHC)₂Au^I–Au^IBr₂ (5) followed by the separation into [(NHC)₂Au^I]⁺ (7) and [Au^IBr₂]⁻ (8). [Au^IBr₂]⁻ is not stable in an aqueous environment and degrades in an internal redox reaction to Au⁰ and Br₂. The latter in turn oxidizes 7 to the gold(III) species 9. The reported ligand rearrangement of the (NHC)Au^IBr complex differs from that found for related silver(I) analogous. A detailed understanding of this scrambling mechanism is of utmost importance for the interpretation of their biological activity and will help to further optimize them for biomedical and other applications.

By means of experimental data from HPLC and (LC-)MS in combination with DFT calculations, we present a detailed mechanism for the ligand scrambling reaction of (NHC)Au^IBr to the corresponding [(NHC)₂Au^I]⁺ complex and the oxidation to the [(NHC)₂Au^IIIBr₂]⁺ species in aqueous solutions.