Synthesis, X-ray molecular structure analysis, and study on ligand scrambling reactions of new thiolatogold(I) complexes with various phosphines

Hetero- and homoleptic binuclear gold(I)-thiolate and -halide complexes - ligand exchange kinetics and supramolecular structures

Heteroleptic and homoleptic binuclear Au(I) complexes [Au2(μ-PAnP)(SPh)(X)] (X = Cl^- or Br^-), [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(SPhCO2H)2] (SPh = benzenethiolate and SPhCO₂H = 4-thiolatobenzoic acid) containing the bridging diphosphine, 9,10-bis(diphenylphosphino)anthracene (PAnP), were synthesized and characterized by single crystal X-ray diffraction. [Au2(μ-PAnP)(SPh)2] exists as a monomer in its crystals but [Au2(μ-PAnP)(SPhCO2H)2] polymerizes into zig-zag chains via intermolecular hydrogen bonding. [Au2(μ-PAnP)(SPh)(Cl)] forms cyclophane-like dimers of C_i symmetry in crystals via intermolecular aurophilic interactions (Au-Au distance = 3.3081(5) Å). Recrystallization of [Au2(μ-PAnP)(SPh)(Br)] invariably led to crystals composed of [Au2(μ-PAnP)(SPh)(Br)] and [Au2(μ-PAnP)(Br)2]. Despite the chemically different P atoms in the heteroleptic [Au2(μ-PAnP)(SPh)(Cl)] and [Au2(μ-PAnP)(SPh)(Br)], solutions of the complexes show only a single signal in their ³¹P{¹H} NMR spectra at room temperature which resolved into two singlets of equal intensity at 183 K. Identical signals which show the same thermal behavior were observed in solutions of [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(X)2] in 1 : 1 molar ratios, indicating that there are three exchanging species, [Au2(μ-PAnP)(SPh)(X)], [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(X)2], in solution. A solution of [Au2(μ-PAnP)(Cl)2] and [Au2(μ-PAnP)(Br)2] in 1 : 1 molar ratio shows two singlets, implying that the exchange is not due to the dissociation of either PAnP or halide ligands, but rather it involves the exchange of the thiolate and the halide ligands (SPh^- ↔ X^-). A mixture of [(PPh3)Au(SPh)] and [(PPh3)Au(Cl)] (1 : 1 molar ratio) showed only one signal in its room temperature ³¹P{¹H} NMR spectrum, indicating that the ligand exchange can happen intermolecularly. Self-exchange of SPh^- ligands is possible as the room temperature ³¹P NMR spectrum of a mixture of [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(SPhCO2H)2] displayed only one signal. The rate constants of the exchange were determined by fitting the line shapes of the ³¹P NMR signals at different temperatures. The activation energies (E_as), obtained from Arrhenius plots, for the SPh^- ↔ Cl^- and SPh^- ↔ Br^- exchange are 36.9 ± 0.7 and 33.7 ± 1.0 kJ mol^-1, respectively. The activation enthalpy and activation entropy, obtained from Eyring plots, for the SPh^- ↔ Cl^- and SPh^- ↔ Br^- exchange are 35.0 ± 0.7 kJ mol^-1 and -25.7 ± 3.2 J K^-1, and 32.0 ± 1.0 kJ mol^-1 and -21.8 ± 4.7 J K^-1, respectively. Based on the kinetic results, two possible mechanisms were proposed for the reactions.

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.

Coinage metal coordination chemistry of stable primary, secondary and tertiary ferrocenylethyl-based phosphines

Ferrocene-based phosphines constitute an important auxiliary ligand in inorganic chemistry. Utilizing the (ferrocenylethyl)phosphines (FcCH2CH2)3-nHnP (Fc = ferrocenyl; n = 2, 1; n = 1, 2; n = 0, 3) the synthesis of a series of coordination complexes [(FcCH2CH2)3-nHnPCuCl]4 (n = 2, 1-CuCl; n = 0, 3-CuCl), [(FcCH2CH2)2HPCuCl] (2-CuCl), {[(FcCH2CH2)H2P]2AgCl}2 (1-AgCl), [(FcCH2CH2)2HPAgCl] (2-AgCl), [(FcCH2CH2)3PAgCl]4 (3-AgCl), [(FcCH2CH2)3PM(OAc)]4 (M = Cu, 3-CuOAc M = Ag, 3-AgOAc), [(FcCH2CH2)3-nHnPAuCl] (n = 1, 2-AuCl; n = 0, 3-AuCl), via the reaction between the free phosphine and MX (M = Cu, Ag and Au; X = Cl, OAc), is described. The reaction between the respective phosphine with a suspension of metal-chloride or -acetate in a 1 : 1 ratio in THF at ambient temperature affords coordinated phosphine-coinage metal complexes. Varying structural motifs are observed in the solid state, as determined via single crystal X-ray analysis of 1-CuCl, 3-CuCl, 1-AgCl, 3-AgCl, 3-CuOAc, 3-AgOAc, 2-AuCl and 3-AuCl. Complexes 1-CuCl and 3-CuCl are tetrameric Cu(i) cubane-like structures with a Cu4Cl4 core, whereas silver complexes with primary and tertiary phosphine reveal two different structural types. The structure of 1-AgCl, unlike the rest, displays the coordination of two phosphines to each silver atom and shows a quadrangle defined by two Ag and two Cl atoms. In contrast, 3-AgCl is distorted from a cubane structure via elongation of one of the ClAg distances. 3-CuOAc and 3-AgOAc are isostructural with step-like cores, while complexes 2-AuCl and 3-AuCl reveal a linear geometry of a phosphine gold(i) chloride devoid of any aurophilic interactions. All of the complexes were characterized in solution by multinuclear (1)H, (13)C{(1)H} and (31)P NMR spectroscopic techniques; the redox chemistry of the series of complexes was examined using cyclic voltammetry. This class of complexes has been found to exhibit one reversible Fe(ii)/Fe(iii) oxidation couple, suggesting the absence of electronic communication between the ferrocenyl units on individual phosphine ligands as well as between different phosphines on the polymetallic cores.

First EXAFS studies on aurophilic interactions in solution

EXAFS has been used to directly show the existence of Au...Au interactions in dissolved Au(I) complexes for the first time; the information has been used to understand the optical properties of these materials.