Manfred Bochmann

Gold catalysis has developed into an increasingly important method in organic synthesis. Especially the potential of gold(I,III) redox systems and gold(III) complexes has more recently become a focus of interest. Although often underestimated or not considered, the ligand trans -influence is a crucial factor in determining the reactivity of gold(III) compounds; it modifies not only bond polarisation, dissociation energies and thermal stability but also controls the nature of the gold frontier orbitals. This Perspective highlights recent insights into the origins and consequences of the ligand trans -influence and its importance for the energetics of gold-catalysed reactions, with special emphasis on the trans -influence in gold(III) hydrides and recent mechanistic insights.

The d⁸ complexes A[Au(C^C)(CN)₂] and A₂[Pt(ĈC)(CN)₂] (where C^C = 4,4′-di-tert-butylbiphenyl-2,2′-diyl and A⁺ = NBu₄⁺, K⁺) were synthesized from [Au(C^C)Cl]₂ and [Pt(C^C)COD], which were themselves obtained from Sn(C^C)ⁿBu₂. These complexes are bright photoemitters but exhibit remarkable differences in the origin of their photoluminescence. The tin complex Sn(C^C)ⁿBu₂ displays blue/white photoluminescence originating from an admixture of transitions. In contrast, the gold complexes exhibit green, long-lived phosphorescence (lifetimes up to 100 μs), which theoretical calculations attribute to ³IL(C^C) transitions. Although the platinum complexes show similar absorption and emission energies, theoretical calculations indicate an admixture with ³MLCT character in their emissive states. This is evidenced by (i) luminescence lifetimes up to 1 order of magnitude shorter than those of the gold complexes under similar conditions; (ii) a greater contribution from fluorescence with respect to phosphorescence in solution, and (iii) reduced susceptibility to ³O₂ quenching, a consequence of the shorter triplet-state lifetime. The combination of water solubility, efficient ISC, and a long-lived triplet state with high sensitivity to dissolved O₂ endows K[Au(C^C)(CN)₂] with excellent catalytic activity in the photo-oxidation of p-bromothioanisole. This result underscores the potential of this class of Au(III) salts for photocatalysis in green solvents.

Carbofunctionalization of alkynes with trifluoromethylsulfonate nucleophiles is a powerful strategy for the synthesis of vinyl triflates with diverse molecular complexity. However, stereoselective protocols are challenging to realize, and the development of novel strategies for controlling the selectivity is highly desirable. In this work, we show that gold complexes bearing the hemilabile MeDalPhos ligand (MeDalPhos = di­(1-adamantyl)-2-dimethylamino-phenylphosphine) catalyze the E-stereoselective carbofunctionalization of internal alkynes using aryl/vinyl iodides and AgOTf as simple starting reagents. Based on the outer-sphere nature of this reaction and the beneficial effect of the MeDalPhos ligand, the Z-selective attack is practically suppressed, leading to an ideal kinetic selectivity. Mechanistic studies, both experimental and theoretical, revealed that the interplay between kinetics and thermodynamics is crucial in determining the final E/Z ratios for each substrate.

The initial reaction steps in the formation of Heck‐type arylated alkenes catalyzed by P^N chelated Au(III) complexes have been studied computationally. Two mechanistic alternatives have been explored: (1) alkene coordination and insertion into a gold‐carbon bond, and (2) nucleophilic attack on a gold(III) alkene adduct. The common starting point, the [(P^N)AuPh(alkene)]²⁺ dication (alkene = C₂H₄, H₂C═CHEt, or H₂C═CEt₂), shows unequal bonding to the olefinic carbons of the 1‐alkenes (P^N = 1,2‐C₆H₄NMe₂(PR₂); R = H, Me, 1‐adamantyl). This polarization increases with steric hindrance and with the inclusion of an OTf^‐ anion in the model. While the reaction pathways are strongly governed by the trans‐influence of the ligand, the effects of steric hindrance in the ligand and alkene are remarkably small. In all cases the nucleophilic attack pathway is energetically favored. However, changing the ligand from a P^N to a P^P chelate, with a strongly electron‐donating ‐PMe₂ donor trans to Ph, sufficiently destabilizes the Au─Ph bond to make alkene insertion competitive. Alkene 1,2‐insertion regiochemistry is always preferred, unlike Pd‐catalyzed Heck reaction that requires a 2,1‐insertion. Based on these results, an alkene insertion pathway en route to Heck‐type olefins can therefore be ruled out.

In zwei vielbeachteten Veröffentlichungen wurde vor kurzem über die Bildung von arylierten Heck‐Olefinen durch Katalyse mit MeDalPhosAuCl/AgOTf (J. Am. Chem. Soc. 2023, 145, 8810) und ihre Cyclisierung zu Tetralinen berichtet (Angew. Chem. Int. Ed. 2023, e202312786). Es wurde behauptet, es handle sich dabei um den erstmaligen Nachweis von Gold‐katalysierten Alken‐Insertionen in Au‐Aryl‐Bindungen, β‐H‐Eliminierung und Kettenwanderung durch Au−H. Wir zeigen hier, dass es sich dabei tatsächlich aber um einen Zweistufenprozess handelt, wobei nur der erste Schritt, die Bildung eines Alkyltriflat‐Esters als primäres organisches Produkt durch die wohlbekannte Alken‐Heteroarylierungssequenz, einen Gold‐Katalysator involviert. Die nachfolgende Umsetzung zu Heck‐Olefinen sowie deren Cyclisierung zu Tetralinen ist klassische, H⁺‐initiierte Carbokationenchemie. Diese Schritte finden auch unter Gold‐freien Bedingungen statt, mit identischen Resultaten. Literaturberichte über neuartige Reaktivität von Gold‐Katalysatoren, wie Kettenwanderung durch das hypothetische [LAuH]²⁺ Dikation, entbehren jeder faktischen Grundlage.

Two recent high‐profile publications reported the formation of Heck‐type arylated alkenes catalysed by MeDalPhosAuCl/AgOTf (J. Am. Chem. Soc. 2023, 145, 8810) and their cyclisation to tetralines (Angew. Chem. Int. Ed. 2023, e202312786). It was claimed that these were the first demonstrations in gold catalysis of alkene insertion into Au‐aryl bonds, β‐H elimination and chain‐walking by Au−H dications. We show here that in fact this chemistry is a two‐stage process. Only the first step, the production of an alkyl triflate ester as the primary organic product by the well‐known alkene heteroarylation sequence, involves gold. The subsequent formation of Heck‐type olefins and their cyclisation to tetralines represent classical H⁺‐triggered carbocationic chemistry. These steps proceed in the absence of gold with identical results. Literature claims of new gold reactivity such as chain walking by the putative [LAuH]²⁺ dication have no basis in fact.

Two recent high-profile publications reported the formation of Heck-type arylated alkenes catalysed by MeDalPhosAuCl / AgOTf (J. Am. Chem. Soc. 2023, 145, 8810) and their cyclisation to tetralines (Angew. Chem. Int. Ed. 2023, e202312786). It was claimed that these were the first demonstrations in gold catalysis of alkene insertion into Au-aryl bonds, β-H elimination and chain-walking by Au-H cations. We show here that in fact this chemistry is a two-stage process. Only the first step, the production of an alkyl triflate ester as the primary organic product by the well-known alkene heteroarylation sequence, involves gold. The subsequent formation of Heck-type olefins and their cyclisation to tetralines represent classical H+-triggered carbocationic chemistry. These steps proceed in the absence of gold with identical results. Literature claims of new gold reactivity such as chain walking by the putative [LAuH]2+ dication have no basis in fact.

A series of carbene‐gold‐acetylide complexes [(BiCAAC)AuCC]_nC₆H_5−n (n = 1, Au1; n = 2, Au2; n = 3, Au3; BiCAAC = bicyclic(alkyl)(amino)carbene) have been synthesized in high yields. Compounds Au1–Au3 exhibit deep‐blue to blue‐green phosphorescence with good quantum yields up to 43% in all media. An increase of the (BiCAAC)Au moieties in gold complexes Au1–Au3 increases the extinction coefficients in the UV–vis spectra and stronger oscillator strength coefficients supported by theoretical calculations. The luminescence radiative rates decrease with an increase of the (BiCAAC)Au moieties. The time‐dependent density functional theory study supports a charge‐transfer nature of the phosphorescence due to the large (0.5–0.6 eV) energy gap between singlet excited (S₁) and triplet excited (T₁) states. Transient luminescence study reveals the presence of both nonstructured UV prompt‐fluorescence and vibronically resolved long‐lived phosphorescence 428 nm. Organic light‐emitting diodes (OLED) are fabricated by physical vapor deposition with 2,8‐bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) as a host material with complex Au1. The near‐UV electroluminescence is observed at 405 nm with device efficiency of 1% while demonstrating OLED device lifetime LT₅₀ up to 20 min at practical brightness of 10 nits, indicating a highly promising class of materials to develop stable UV‐OLEDs.

The syntheses and structures of a series of complexes based on the (CC)-C-?-chelated Au(III) unit ((CC)-C-? = 4,4'-bis(t-butyl) 2,2'-biphenyl-1,1'-diyl) are reported, namely, [{((CC)-C-?)Au(C=(CBu)-Bu-t)(2)}(2)M-2], ((CC)-C-?)Au(C=CR)(C=NXyl), and [{((CC)-C-?)Au(C=CR)(2)}{M(C=NXyl)}] (M = Ag, Cu; R = Bu-t, (C6H4Bu)-Bu-t-4 , C6H4OMe-4; Xyl = 3,5-Me2C6H3). The X-ray structures reveal a broad range of dispositions determined by the different coordination modes of Ag(I) or Cu(I). The complexes are bright photoemitters in the solid state and in poly(methyl methacrylate) (PMMA) films. The photoluminescence is dominated by (IL)-I-3((CC)-C-?) transitions, with indirect effects from the rest of the molecules, as supported by theoretical calculations. This work opens up the possibility of accessing Au(III) carbon-rich anions to construct photoluminescent aggregates.

Carbene-metal-amides (CMAs) are an emerging class of photoemitters based on a linear donor-linker-acceptor arrangement. They exhibit high flexibility about the carbene-metal and metal-amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 10⁶ s^-1. Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin-orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device.