Luminescent complexes of heavy metals such as iridium, platinum, and ruthenium play an important role in photocatalysis and energy conversion applications as well as organic light-emitting diodes (OLEDs). Achieving comparable performance from more–earth-abundant copper requires overcoming the weak spin-orbit coupling of the light metal as well as limiting the high reorganization energies typical in copper(I) [Cu(I)] complexes. Here we report that two-coordinate Cu(I) complexes with redox active ligands in coplanar conformation manifest suppressed nonradiative decay, reduced structural reorganization, and sufficient orbital overlap for efficient charge transfer. We achieve photoluminescence efficiencies >99% and microsecond lifetimes, which lead to an efficient blue-emitting OLED. Photophysical analysis and simulations reveal a temperature-dependent interplay between emissive singlet and triplet charge-transfer states and amide-localized triplet states.

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) are among the most nucleophilic (σ donating) and also electrophilic (π-accepting) stable carbenes known to date. These properties allow them to activate a variety of small molecules and enthalpically strong bonds, to stabilize highly reactive main-group and transition-metal diamagnetic and paramagnetic species, and to bind strongly to metal centers, which gives rise to very robust catalysts. The most important results published up to the end of 2013 are briefly summarized, while the majority of this Review focuses on findings reported within the last three years.

A systematic study is presented on the physical and photophysical properties of isoelectronic and isostructural Cu, Ag, and Au complexes with a common amide (N-carbazolyl) and two different carbene ligands (i.e., CAAC = (5R,6S)-2-(2,6-diisopropylphenyl)-6-isopropyl-3,3,9-trimethyl-2-azaspiro­[4.5]­decan-2-ylidene, MAC = 1,3-bis­(2,6-diisopropylphenyl)-5,5-dimethyl-4-keto-tetrahydropyridylidene). The crystal structures of the (carbene)­M(I)(N-carbazolyl) (MCAAC) and (MAC)­M(I)(N-carbazolyl) (MMAC) complexes show coplanar carbene and carbzole ligands and C–M–N bond angles of ∼180°. The electrochemical properties and energies for charge transfer (CT) absorption and emission compounds are not significantly affected by the choice of metal ion. All six of the (carbene)­M­(Cz) complexes examined here display high photoluminescence quantum yields of 0.8–1.0. The compounds have short emission lifetimes (τ = 0.33–2.8 μs) that fall in the order Ag < Au < Cu, with the lifetimes of (carbene)­Ag­(Cz) roughly a factor of 10 shorter than for (carbene)­Cu­(Cz) complexes. Detailed temperature-dependent photophysical measurements (5–325 K) were carried out to determine the singlet and triplet emission lifetimes (τfl and τph, respectively) and the energy difference between the singlet and triplet excited state, ΔE S1–T1. The τfl values range between 20 and 85 ns, and the τph values are in the 50–200 μs regime. The emission at room temperature is due exclusively to E-type delayed fluorescence or TADF (i.e., ). The emission rate at room temperature is fully governed by ΔE S1–T1, with the silver complexes giving ΔE S1–T1 values of 150–180 cm–1 (18–23 meV), whereas the gold and copper complexes give values of 570–590 cm–1 (70–73 meV).

Transition-metal-catalyzed C-H activation has recently emerged as a powerful tool for the functionalization of organic molecules. While many efforts have focused on the functionalization of arenes and heteroarenes by this strategy in the past two decades, much less research has been devoted to the activation of non-acidic C-H bonds of alkyl groups. This Minireview highlights recent work in this area, with a particular emphasis on synthetically useful methods.

Cyclic (alkyl)-and (aryl)-(amino)carbenes (CAACs and CAArCs) are stronger sdonors and p-acceptors than imidazol-2-ylidenes and imidazolidin-2-ylidenes, the well-known Nheterocyclic carbenes (NHCs). Consequently, they form strong bonds with coinage metals, and stabilize both low and high oxidation states. This Focus Review shows that CAACs and CAArCs have allowed for the isolation of copper and gold complexes which were believed to be only transient intermediates. This has not only allowed for a better understanding of the mechanism of known processes but has also led to the development of novel coinage metal-catalyzed reactions.In addition to their role in homogeneous catalysis, CAAC and CAArC coinage metal complexes have recently found applications in medicinal chemistry, as well as in materials science. When possible, the performance of CAAC and CAArC ligands are compared with those of classical NHCs. 2 CONTENTS 1. Introduction 2. Coordination Chemistry of Cyclic (Alkyl)-and (Aryl)-(Amino)carbenes with Coinage Metals 2.1. Synthesis of (CAAC)MCl (M = Cu, Ag, Au) Complexes 2.2. Synthesis of Various Gold(I) and Gold(III) Complexes from (CAAC)AuCl and (CAAC)AuOH 2.3. Synthesis of (CAAC)CuH Complexes 2.4. Synthesis of CAAC Coinage Metal Alkene, Alkyne, and Aryl p-Complexes 2.5. Synthesis of CAAC Coinage Metal PCO Complexes 2.6. Synthesis of CAAC Coinage Metal(0) Complexes 2.7. Synthesis of CAAC Coinage Metal Clusters 2.8. Synthesis of HemilabileCAAC-Gold(I) and (III) Complexes. Oxidative addition at Gold(I) 2.9. Coordination Chemistry of CAAC-6 and BiCAAC 2.10. Synthesis of Saturated Abnormal Carbene and Cyclic (Aryl)(amino)carbene Coinage Metal Complexes 3. Cyclic (Alkyl)(Amino)carbene Coinage Metal Complexes in Catalysis 3.1. Catalytic Cross-Coupling of Two Unsaturated Carbon Centers Leading to Allenes 3.2. Gold Catalyzed Hydroamination of Alkynes and Allenes, and Hydroammoniumation and Methylamination of Alkynes3.3. (CAAC)-Mono-and -Bis-(copper)acetylide Complexes in Catalysis. 3.4. (CAAC) Coinage Metals for the Activation of Small Molecules 3.4.1. (CAAC)CuBH4 Complexes for the Catalytic Hydrolytic Dehydrogenation of BH3NH3 and for the Reduction of CO2 into Formate with H2. 3.4.2. Trinuclear (CAAC) Gold Clusters as Catalysts for the Carbonylation of Amines 3.5. Miscellaneous 3.5.1. (CAAC) Gold and Copper Catalysts for the Hydroarylation of Styrene with Anilines 3.5.2. (CAAC)Gold(I) Complexes in Gold/Palladium Dual Catalysis 3.5.3. (CAAC) Gold(III) Complexes in Migratory Insertion of Carbenes into Au(III)-C Bonds. 3.6. Chiral CAAC Ligands in Enantioselective Catalysis 4. Biological Applications of Cyclic (Alkyl)-and (Aryl)-(Amino)carbene Coinage Metal Complexes 5. Optoelectronic Applications of Cyclic (Alkyl)(amino)carbene Coinage Metal Complexes 6.

Thermolysis of Ru(PPh3)3(CO)H2 with the N-heterocyclic carbene bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene) (IMes) results in C-C activation of an Ar-CH3 bond in one of the mesityl rings of the carbene ligand. Upon addition of IMes to Ru(PPh3)3(CO)H2 at room temperature in the presence of an alkene, C-H bond activation is observed instead. The thermodynamics of these C-C and C-H cleavage reactions have been probed using density functional theory.

Cyclic (alkyl)(amino)carbenes with a six-membered backbone were prepared. Compared to their five-membered analogues, they feature increased % V and enhanced donor and acceptor properties, as evidenced by the observed n → π* transition trailing into the visible region. The high ambiphilic character even allows for the intramolecular insertion of the carbene into an unactivated C(sp)-H bond. When used as ligands, they outcompete the five-membered analogues in the palladium-mediated α-arylation of ketones with aryl chlorides.

A straightforward strategy allows for the synthesis of storable bicyclic (alkyl)(amino)carbenes (BICAACs), which feature enhanced σ-donating and π-accepting properties compared to monocyclic (alkyl)(amino)carbenes (CAACs). Due to the bicyclo[2.2.2]octane skeleton, the steric environment around the carbene center is different from that of CAACs and similar to that observed in classical N-heterocyclic carbenes. The different electronic properties of BICAACs as compared to CAACs allow for ligand exchange reactions not only at a metal center, but also at main group elements.