(CAAC)Pd(py) Catalysts Disproportionate to Pd(CAAC)₂

Palladium complexes with one N-heterocyclic carbene (NHC) and a pyridine ancillary ligand are powerful cross-coupling precatalysts. Herein, we report such complexes with a cyclic (alkyl)(amino)carbene (CAAC) ligand replacing the NHC. We find that the alleged reduced form, (CAAC)Pd(py), disproportionates to the (CAAC)2Pd0 complex and palladium nanoparticles. This notwithstanding, they are potent catalysts in the Buchwald–Hartwig amination with aryl chlorides under mild conditions (60 °C). In the presence of dioxygen, these complexes catalyze the formation of diazenes from anilines. The catalytic activities of the NHC- and CAAC-supported palladium(0) and palladium(II) complexes are similar in the cross-coupling reaction, yet the CAAC complexes are superior for diazene formation

A Fluorous Chiral Dirhodium(II) Complex as a Recyclable Asymmetric Catalyst

The chiral fluorous complex tetrakis-dirhodium(II)−(S)-N-(n-perfluorooctylsulfonyl)prolinate has been prepared and used as a catalyst in homogeneous or fluorous biphasic fashion. The catalyst displays good chemo- and enantioselectivity in intermolecular cyclopropanation and C−H bond activation reactions. The catalyst can be simply and thoroughly separated from the reaction mixture and is recyclable

From Au₁₁ to Au₁₃: Tailored Synthesis of Superatomic Di-NHC/PPh₃‑Stabilized Molecular Gold Nanoclusters

Herein, we report a new method to synthesize molecular gold nanoclusters (AuNCs) stabilized by phosphine (PR3) and di-N-heterocyclic carbene (di-NHC) ligands. The interaction of di-NHC gold(I) complexes, with the general formula [(di-NHC)Au2Cl2] with well-known [Au11(PPh3)8Cl2]Cl clusters provides three new classes of AuNCs through a controllable reaction sequence. The synthesis involves an initial ligand metathesis reaction to produce [Au11(di-NHC)(PPh3)6Cl2]+ (type 1 clusters), followed by a thermally induced rearrangement/metal complex addition with the formation of Au13 clusters [Au13(di-NHC)2(PPh3)4Cl4]+ (type 2 clusters). Finally, an additional metathesis process yields [Au13(di-NHC)3(PPh3)3Cl3]2+ (type 3 clusters). The electronic and steric properties of the employed di-NHC ligand affect the product distribution, leading to the isolation and full characterization of different clusters as the main product. A type 3 cluster has been also structurally characterized and was preliminarily found to be strongly emissive in solution

Microgels as Soluble Scaffolds for the Preparation of Noble Metal Nanoparticles Supported on Nanostructured Metal Oxides

An approach for the preparation of noble metal nanoparticles supported on nanostructured metal oxides is described herein. The approach is based on the sequential generation of the noble metal nanoparticles and of a metal oxide phase inside a cross-linked polymer colloid (microgel). By tuning the properties of the employed microgel, the nature and amount of both the noble metal nanoparticles and the metal oxide phase can be independently varied. The resulting composite colloids are colloidally stable and, upon isolation by precipitation and subsequent calcination, produce noble metal nanoparticles dispersed on a crystalline, nanostructured oxide phase. Preliminary catalytic tests provide information on the accessibility of the noble metal nanoparticles and, particularly in the case of gold, result in promising catalytic performances in the aerobic oxidation of alcohols

Dinuclear N-Heterocyclic Dicarbene Gold Complexes in I–III and III–III Oxidation States: Synthesis and Structural Analysis

A series of dinuclear N-heterocyclic bis-dicarbene gold(III) complexes of the general formula [Au2Br4(RIm-Y-ImR)2](PF6)2 (Im = imidazol-2-ylidene; 1b, R = Me, Y = CH2; 2b, R = Me, Y = (CH2)2; 3b, R = Me, Y = (CH2)3; 4b, R = Me, Y = (CH2)4; 5b, R = Cy, Y = CH2; 6b, R = Me, Y = m-xylylene) were successfully synthesized by oxidative addition of bromine to the corresponding dicarbene gold(I) complexes 1a–6a. The stability of the digold(III) complexes depends on the length of the bridge Y between the carbene units. The complex with Y = CH2 undergoes a partial reductive elimination, giving the first example of the mixed-valence gold(I)/gold(III) dinuclear bis-dicarbene complex 1c, together with a minor quantity of the neutral digold(III) mono-dicarbene complex [Au2Br6(MeIm-CH2-ImMe)] (1d). The X-ray crystal structures of complexes 1c,d, 3b, and 6b were determined. Besides complex 3b, the addition of bromine to complex 3a gives complex 3b′, a coordination metallopolymer, formed by an infinite chain of AuBr2 units bridged by the dicarbene ligand

Ligand Effects in Pd-Catalyzed Intermolecular Alkyne Hydroarylations

The use of palladium­(II) catalysts for the synthesis of aryl alkenes by addition of aromatic C–H bonds to alkynes has received a great interest in the literature. The mechanistic features of the reaction have been largely discussed, but no systematic study has been reported so far, particularly for what concerns the role of ligands. In this work, we performed a detailed theoretical study in order to fill this gap. To this extent, three different systems have been considered, with the aim to emphasize how the steric and electronic metal environment affects the catalytic activity and, most notably, steers the reaction selectivity toward the two main products of single and double alkyne insertion into the aromatic C–H bond. Moreover, given the crucial role of the acid media, two acids have been considered, namely, trifluoroacetic acid and tetrafluoroboric acid, to understand the effect of the acid strength and coordinative power on the competition between the different pathways

Ligand Effects in Pd-Catalyzed Intermolecular Alkyne Hydroarylations

The use of palladium­(II) catalysts for the synthesis of aryl alkenes by addition of aromatic C–H bonds to alkynes has received a great interest in the literature. The mechanistic features of the reaction have been largely discussed, but no systematic study has been reported so far, particularly for what concerns the role of ligands. In this work, we performed a detailed theoretical study in order to fill this gap. To this extent, three different systems have been considered, with the aim to emphasize how the steric and electronic metal environment affects the catalytic activity and, most notably, steers the reaction selectivity toward the two main products of single and double alkyne insertion into the aromatic C–H bond. Moreover, given the crucial role of the acid media, two acids have been considered, namely, trifluoroacetic acid and tetrafluoroboric acid, to understand the effect of the acid strength and coordinative power on the competition between the different pathways

A Simple Route to Novel Palladium(II) Catalysts with Oxazolin-2-ylidene Ligands

Novel palladium(II) complexes with oxazolin-2-ylidene ligands have been synthesized via direct reaction of palladium acetate and oxazolium salts, prepared in turn by alkylation of oxazole with methyl iodide or benzylic bromides. The resulting complexes have been characterized and used as catalysts in Heck coupling reactions of aryl bromides, where they exhibit remarkable catalytic activity, higher than that of the closely related bis-imidazolin-2-ylidene and bis-benzothiazolin-2-ylidene complexes

Chelate Palladium(II) Complexes with Saturated <i>N</i>‑Phosphanyl-N-Heterocyclic Carbene Ligands: Synthesis and Catalysis

<i>N</i>-Phosphanyl-N-heterocyclic carbenes (NHCPs) featuring a saturated imidazolin-2-ylidene or tetrahydropyrimid-2-ylidene ring have been synthesized and characterized. The free carbenes exhibit good stability and can be stored in the solid state for months at ambient temperature without decomposition. Contrary to imidazoline-based NHCPs, which decompose by ring opening, <i>N</i>-phosphanyl­tetrahydropyrimid-2-ylidenes isomerize to 2-phosphanyl tetrahydropyrimidines upon heating. The free carbenes are capable of acting as chelating ligands toward palladium­(II), forming very stable mononuclear complexes that have been structurally characterized. The catalytic potential of the complexes has been preliminarily assessed in cross-coupling reactions, most notably in the Suzuki coupling of aryl chlorides, where these complexes display promising activity, and in the copper- and amine-free Sonogashira coupling of aryl bromides

Group 10 Metal Complexes with Chelating Macrocyclic Dicarbene Ligands Bearing a 2,6-Lutidinyl Bridge: Synthesis, Reactivity, and Catalytic Activity

Palladium­(II) and platinum­(II) complexes of the title ligands have been prepared; the two carbene moieties of the ligand coordinate to the metal in <i>cis</i> fashion, while the bridging pyridyl group remains outside the metal coordination sphere but close to the metal center. In this peculiar situation, the pyridyl group can assist the oxidation of the metal center to the +IV oxidation state upon coordination to the metal in the product. Furthermore, the pyridyl group is found to promote the catalytic role of the palladium­(II) complexes in copper- and amine-free Sonogashira reactions