Cyclometalated gold(iii) complexes: noticeable differences between (N,C) and (P,C) ligands in migratory insertion

Insertion of Molecular Oxygen into a Gold(III)-Hydride Bond

The use of molecular oxygen as an oxidant in chemical synthesis has significant environmental and economic benefits, and it is widely used as such in large-scale industrial processes. However, its adoption in highly selective homogeneous catalytic transformations, particularly to produce oxygenated organics, has been hindered by our limited understanding of the mechanisms by which O2 reacts with transition metals. Of particular relevance are the mechanisms of the reactions of oxygen with late transition metal hydrides as these metal centers are better poised to release oxygenated products. Homogeneous catalysis with gold complexes has markedly increased, and herein we report the synthesis and full characterization of a rare AuIII-H, supported by a diphosphine pincer ligand (tBuPCP = 2,6-bis(di-tert-butylphosphinomethyl)benzene). [(tBuPCP)AuIII-H]+ was found to cleanly react with molecular oxygen to yield a stable AuIII-OOH complex that was also fully characterized. Extensive kinetic studies on the reaction via variable temperature NMR spectroscopy have been completed, and the results are consistent with an autoaccelerating radical chain mechanism. The observed kinetic behavior exhibits similarities to that of previously reported PdII-H and PtIV-H reactions with O2 but is not fully consistent with any known O2 insertion mechanism. As such, this study contributes to the nascent fundamental understanding of the mechanisms of aerobic oxidation of late metal hydrides.

Unlocking the Chain-Walking Process in Gold Catalysis

The successful realization of gold-catalyzed chain-walking reactions, facilitated by ligand-enabled Au(I)/Au(III) redox catalysis, has been reported for the first time. This breakthrough has led to the development of gold-catalyzed annulation reaction of alkenes with iodoarenes by leveraging the interplay of chain-walking and π-activation reactivity mode. The reaction mechanism has been elucidated through comprehensive experimental and computational studies.

Hemilabile P,N-Ligand-Assisted Gold-Catalyzed Heck Reaction of Aryl and Styryl Iodides with Styrenes

A gold-catalyzed Heck reaction of aryl and styryl iodides with styrenes was developed. The hemilabile P,N-ligand-assisted gold-catalyzed C(sp2)-C(sp2) cross-coupling can synthesize stilbenes and bistyryl complexes, with good functional-group tolerance and mild conditions. The elementary organometallic steps of migratory insertion and β-hydride elimination might be involved in this ligand-enabled Au(I)/Au(III)-catalyzed Heck reaction with styrenes.

A Conformationally Restricted Gold(III) Complex Elicits Antiproliferative Activity in Cancer Cells

Diamine ligands are effective structural scaffolds for tuning the reactivity of transition-metal complexes for catalytic, materials, and phosphorescent applications and have been leveraged for biological use. In this work, we report the synthesis and characterization of a novel class of cyclometalated [C^N] Au(III) complexes bearing secondary diamines including a norbornane backbone, (2R,3S)-N2,N3-dibenzylbicyclo[2.2.1]heptane-2,3-diamine, or a cyclohexane backbone, (1R,2R)-N1,N2-dibenzylcyclohexane-1,2-diamine. X-ray crystallography confirms the square-planar geometry and chirality at nitrogen. The electronic character of the conformationally restricted norbornane backbone influences the electrochemical behavior with redox potentials of -0.8 to -1.1 V, atypical for Au(III) complexes. These compounds demonstrate promising anticancer activity, particularly, complex 1, which bears a benzylpyridine organogold framework, and supported by the bicyclic conformationally restricted diaminonorbornane, shows good potency in A2780 cells. We further show that a cellular response to 1 evokes reactive oxygen species (ROS) production and does not induce mitochondrial dysfunction. This class of complexes provides significant stability and reactivity for different applications in protein modification, catalysis, and therapeutics.

Au(III) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 34 Years (1989–2022) of NMR and Single Crystal X-ray Studies

A review paper on Au(III) cyclometallated compounds with 2-arylpyridines (2-phenylpyridine, 2-benzylpyridine, 2-benzoylpyridine, 2-phenoxypyridine, 2-phenylsulfanylpyridine, 2-anilinopyridine, 2-(naphth-2-yl)pyridine, 2-(9,9-dialkylfluoren-2-yl)pyridines, 2-(dibenzofuran-4-yl)pyridine, and their derivatives) and their analogues (2-arylquinolines, 1- and 3-arylisoquinolines, 7,8-benzoquinoline), with 113 references. A total of 554 species, containing κ2-N(1),C(6′)*-Au(III), or analogous moiety (i.e., chelated by nitrogen of the pyridine-like ring and the deprotonated ortho- carbon of the phenyl-like ring) and, thus, possessing a character intermediate between metal complexes and organometallics, studied in the years 1989–2022 by NMR spectroscopy and/or single crystal X-ray diffraction (207 X-ray structures), are described. The compounds for which biological or catalytic activity and the luminescence properties were studied are also quoted.

Mechanism of Alkyne Hydroarylation Catalyzed by (P,C)-Cyclometalated Au(III) Complexes

Over the last 5-10 years, gold(III) catalysis has developed rapidly. It often shows complementary if not unique features compared to gold(I) catalysis. While recent work has enabled major synthetic progress in terms of scope and efficiency, very little is yet known about the mechanism of Au(III)-catalyzed transformations and the relevant key intermediates have rarely been authenticated. Here, we report a detailed experimental/computational mechanistic study of the recently reported intermolecular hydroarylation of alkynes catalyzed by (P,C)-cyclometalated Au(III) complexes. The cationic (P,C)Au(OAc^F)⁺ complex (OAc^F = OCOCF₃) was authenticated by mass spectrometry (MS) in the gas phase and multi-nuclear NMR spectroscopy in solution at low temperatures. According to density functional theory (DFT) calculations, the OAc^F moiety is κ²-coordinated to gold in the ground state, but the corresponding κ¹-forms featuring a vacant coordination site sit only slightly higher in energy. Side-on coordination of the alkyne to Au(III) then promotes nucleophilic addition of the arene. The energy profiles for the reaction between trimethoxybenzene (TMB) and diphenylacetylene (DPA) were computed by DFT. The activation barrier is significantly lower for the outer-sphere pathway than for the alternative inner-sphere mechanism involving C-H activation of the arene followed by migratory insertion. The π-complex of DPA was characterized by MS. An unprecedented σ-arene Au(III) complex with TMB was also authenticated both in the gas phase and in solution. The cationic complexes [(P,C)Au(OAc^F)]⁺ and [(P,C)Au(OAc^F)(σ-TMB)]⁺ stand as active species and off-cycle resting state during catalysis, respectively. This study provides a rational basis for the further development of Au(III) catalysis based on π-activation.

Density Functional Theory Study of Gold-Catalyzed 1,2-Diarylation of Alkenes: π-Activation versus Migratory Insertion Mechanisms

Density functional theory calculations are carried out to better understand the first gold-catalyzed 1,2-diarylation reactions of alkenes reported in the recent literature. The calculations on two representative reactions, aryl alkene/aryl iodide coupling pair (the aryl-I bond is located outside the aryl alkene) versus iodoaryl alkene/indole coupling pair (the aryl-I bond is located in the aryl alkene), confirm that the reaction involves a π-activation mechanism rather than the general migratory insertion mechanism in previously known metal catalysis by Pd, Ni, and Cu complexes. Theoretical results rationalize the regioselectivity of the reactions controlled by the aryl-I bond position (intermolecular or intramolecular) and also the ligand and substituent effects on the reactivity.

Platinum Assisted Tandem P-C Bond Cleavage and P-N Bond Formation in Amide Functionalized Bisphosphine o-Ph2PC6H4C(O)N(H)C6H4PPh2-o: Synthesis, Mechanistic, and Catalytic Studies

The reactions of amide functionalized bisphosphine o-Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂-o (1) with platinum salts are described. Treatment of 1 with [Pt(COD)Cl₂] yielded a chelate complex, [PtCl₂{o-Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂-o}κ²-P,P] (2), which on subsequent treatment with LiHMDS formed a novel 1,2-azaphospholene-phosphine complex [Pt(C₆H₅)Cl{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (3) involving a tandem P-C bond cleavage and P-N bond formation. The same complex 3 on passing dry HCl gas afforded the dichloro complex [PtCl₂{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (5). Complex 2 upon refluxing in toluene or treatment of 1 with [Pt(COD)Cl₂] in the presence of a base at room temperature resulted in the pincer complex [PtCl{o-Ph₂PC₆H₄C(O)N(C₆H₄PPh₂-o)}κ³-P,N,P] (4). Reaction of 1 with [Pt(COD)ClMe] at room temperature also afforded the pincer complex [PtMe{o-Ph₂PC₆H₄C(O)N(C₆H₄PPh₂-o)}κ³-P,N,P] (6). Mechanistic studies on 1,2-azaphospholene formation showed the reductive elimination of LiCl to form a phosphonium salt that readily adds one of the P-C bonds oxidatively to the in situ generated Pt⁰ species to form a chelate complex 3. The analogous palladium complex [PdCl₂{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (7) showed excellent catalytic activity toward N-alkylation of amines with alcohols with a very low catalyst loading (0.05 mol %), and the methodology is very efficient toward the gram-scale synthesis of many N-alkylated amines.

Chelation-assisted C-C bond activation of biphenylene by gold(i) halides

A chelation-assisted oxidative addition of gold(i) into the C-C bond of biphenylene is reported here. The presence of a coordinating group (pyridine, phosphine) in the biphenylene unit enabled the use of readily available gold(i) halide precursors providing a new, straightforward entry towards cyclometalated (N^C^C)- and (P^C)-gold(iii) complexes. Our study, combining spectroscopic and crystallographic data with DFT calculations, showcases the importance of neighboring, weakly coordinating groups towards the successful activation of strained C-C bonds by gold.

Piezo-enhanced activation of dinitrogen for room temperature production of ammonia

The catalytic conversion of nitrogen to ammonia remains an energy-intensive process, demanding advanced concepts for nitrogen fixation. The major obstacle of nitrogen fixation lies in the intrinsically high bond energy (941 kJ mol^-1) of the N≡N molecule and the absence of a permanent dipole in N₂. This kinetic barrier is addressed in this study by an efficient piezo-enhanced gold catalysis as demonstrated by the room temperature reduction of dinitrogen into ammonia. Au nanostructures were immobilized on thin film piezoelectric support of potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) by chemical vapor deposition of a new Au(III) precursor [Me₂Au(PyTFP)(H₂O)]1(PyTFP = (Z)-3,3,3-trifluoro-1-(pyridin-2-yl)-prop-1-en-2-olate) that exhibited high volatility (60 °C, 10^-3mbar) and clean decomposition mechanism to produce well adherent elemental gold films on KNN and Ti substrates. The gold-functionalized KNN films served as an efficient catalytic system for ammonia production with a Faradaic efficiency of 18.9% achieved upon ultrasonic actuation. Our results show that the spontaneous polarization of piezoelectric materials under external electrical fields augments the sluggish electron transfer kinetics by creating instant dipoles in adsorbed N₂molecules to deliver a piezo-enhanced catalytic system promising for sustained activation of dinitrogen molecules.

The interplay of carbophilic activation and Au(I)/Au(III) catalysis: an emerging technique for 1,2-difunctionalization of C-C multiple bonds

Gold complexes have emerged as the catalysts of choice for various functionalization reactions of C-C multiple bonds due to their inherent carbophilic nature. In a parallel space, efforts to realize less accessible cross-coupling reactivity have led to the development of various strategies that facilitate the arduous Au(i)/Au(iii) redox cycle. The interplay of the two important reactivity modes encountered in gold catalysis, namely carbophilic activation and Au(i)/Au(iii) catalysis, has allowed the development of a novel mechanistic paradigm that sponsors 1,2-difunctionalization reactions of various C-C multiple bonds. Interestingly, the reactivity as well as selectivity obtained through this interplay could be complementary to that obtained by the use of various other transition metals that mainly involved the classical oxidative addition/migratory insertion pathways. The present review shall comprehensively cover all the 1,2-difunctionalization reactions of C-C multiple bonds that have been realized by the interplay of the two important reactivity modes and categorized on the basis of the method that has been employed to foster the Au(i)/Au(iii) redox cycle.

Novel approach to benzo-fused 1,2-azaphospholene involving a Pd(II)-assisted tandem P-C bond cleavage and P-N bond formation reaction

New bisphosphine o-Ph2PC6H4C(O)N(H)C6H4PPh2-o (1) (Bala-HariPhos) showed a unique reactivity towards Pd(ii) resulting in a 1,2-azaphospholene complex, involving a tandem P-C bond cleavage, P-N bond formation and cyclization process via the elimination of PhH. Mechanistic details were investigated using NMR spectroscopy, DFT calculations and kinetic data, and by SCXRD analysis. It involves the reductive elimination from a tautomerised complex to form a phosphonium salt followed by oxidative addition.

Striking ligand-disproportionative Cl/aryl scrambling in a simple Au(III) system. Solvent role, driving forces and mechanisms

Aryl rearrangements triggered by Cl- extraction from trans-[AuIII(Rf)2Cl2]- (Rf = C6F3Cl2-3,5), led quickly to a mixture of [Au(Rf)3(solv)], cis-[Au(Rf)2Cl(solv)] and [Au(Rf)Cl2(solv)] (solv = OEt2, OH2). 19F NMR and X-ray diffraction studies led us to identify the species present in solution and the role of the solvent in their formation, while DFT calculations confirm the thermodynamic basis of their evolution. Very different Rf-Rf coupling rates are found from (μ-Cl)2[cis-Au(Rf)2]2 or cis-[Au(Rf)2ClL] species (L = OEt2, NCMe, Cl-) depending on the coordination strength of the ligand or solvent in the fourth position.

Carbon-Phosphorus Coupling from C^N Cyclometalated AuIII Complexes

With the aim of exploiting new organometallic species for cross-coupling reactions, we report here on the AuIII -mediated Caryl -P bond formation occurring upon reaction of C^N cyclometalated AuIII complexes with phosphines. The [Au(C^N)Cl2 ] complex 1 featuring the bidentate 2-benzoylpyridine (CCO N) scaffold was found to react with PTA (1,3,5-triaza-7-phosphaadamantane) under mild conditions, including in water, to afford the corresponding phosphonium 5 through C-P reductive elimination. A mechanism is proposed for the title reaction based on in situ 31 P{1 H} NMR and HR-ESI-MS analyses combined with DFT calculations. The C-P coupling has been generalized to other C^N cyclometalated AuIII complexes and other tertiary phosphines. Overall, this work provides new insights into the reactivity of cyclometalated AuIII compounds and establishes initial structure-activity relationships to develop AuIII -mediated C-P cross-coupling reactions.

Cyclometalated AuIII Complexes for Cysteine Arylation in Zinc Finger Protein Domains: towards Controlled Reductive Elimination

With the aim of exploiting the use of organometallic species for the efficient modification of proteins through C‐atom transfer, the gold‐mediated cysteine arylation through a reductive elimination process occurring from the reaction of cyclometalated Au^III C^N complexes with a zinc finger peptide (Cys₂His₂ type) is here reported. Among the four selected Au^III cyclometalated compounds, the [Au(C^CON)Cl₂] complex featuring the 2‐benzoylpyridine (C^CON) scaffold was identified as the most prone to reductive elimination and Cys arylation in buffered aqueous solution (pH 7.4) at 37 °C by high‐resolution LC electrospray ionization mass spectrometry. DFT and quantum mechanics/molecular mechanics (QM/MM) studies permitted to propose a mechanism for the title reaction that is in line with the experimental results. Overall, the results provide new insights into the reactivity of cytotoxic organogold compounds with biologically important zinc finger domains and identify initial structure–activity relationships to enable Au^III‐catalyzed reductive elimination in aqueous media.

Gold(iii) π complexes

Recent achievements in the preparation, characterization and reactivity of gold(iii) complexes of alkenes, alkynes and arenes are presented and discussed.

Synthesis of a (N,C,C) Au(iii) pincer complex via Csp3–H bond activation: increasing catalyst robustness by rational catalyst design

The synthesis of a (N,C,C) Au(iii) catalyst via C_sp3–H bond activation has been achieved following a rational catalyst design approach.