Low-valent Palladium Research Articles

Cuboctahedral Pd13 as a spherical aromatic noble metal core: insights from a ligand-protected [Pd13(Tr)6]2+ cluster.

Low-valent palladium nanoparticles are efficient species promoting catalytic activity and selectivity in a number of chemical reactions. Recently, an atom-centered cuboctahedral Pd13 motif has been characterized as a ligand-protected [Pd13(Tr)6]2+ cluster featuring a 1s2 superatomic shell structure. In this report, we describe the ligand-cluster of and endohedral-cage interaction in [Pd13(Tr)6]2+, which accounts for a favorable situation in the overall cluster. In addition, the spherical aromatic properties of the cluster were evaluated to understand the behavior of the ligand-protected Pd13 cluster core. Our results indicate a sizable interaction towards carbon-based ligands in an overall spherical aromatic cluster featuring a long-range shielding cone. Thus, [Pd13(Tr)6]2+ is rationalized as the first ligand-protected palladium cluster to date exhibiting spherical aromatic properties, serving as a stable building block for molecule-based materials or as a dopant in porous carbon materials.

Enantioselective Double Carbonylation Enabled by High-Valent Palladium Catalysis.

Palladium-catalyzed carbonylation reactions are efficient methods for synthesizing valuable molecules. However, realizing a carbonylation with excellent yield and chemo-, regio-, and enantioselectivities by classical low-valent palladium catalysis is highly challenging. Herein, we describe an enantioselective carbonylation reaction using a high-valent palladium catalysis strategy and employing a chiral sulfoxide phosphine (SOP) ligand. This double aminocarbonylation reaction begins with the formation of a carbamoylpalladium(II) species, which undergoes enantioselective oxidative addition with a cyclic diaryliodonium salt to generate a palladium(IV) intermediate, followed by a second CO insertion and reductive elimination. The mechanism has been illustrated with experimental and computational studies.

Low Valent Palladium Clusters: Synthesis, Structures and Catalytic Applications

As one of the most common transition metal catalysts, mononuclear palladium catalysts have been developed systematically and a widely recognized catalytic mechanism featured with a Pd0/PdII catalytic cycle for a palladium center has been proposed. However, recent research works have revealed that palladium clusters may also be catalysts or catalytic intermediates involved in the catalytic processes. This major discovery brings new perspectives for a deep and comprehensive understanding of the Pd‐catalyzed reaction mechanisms. Combining with the latest advances in the field of organometallic chemistry, we here highlight the synthesis, structures, and catalytic reactivities of palladium cluster compounds. This review discusses several well‐defined synthetic strategies and diverse palladium cluster‐based catalytic systems in detail, to provide insights into the rational design of promising palladium cluster catalysts and the effective search for appropriate synthetic methods.

Low valent palladium benzoquinone complexes bearing different spectator ligands. The versatile coordinative capability of benzoquinone

The synthesis of some benzoquinone palladium complexes bearing different spectator ligands was carried out and the hapticity of the coordinated olefin was inferred from the features of their 1H and 13C NMR spectra. It was shown that benzoquinone coordinates either η2 and η4 and that its coordinative choice is not easily predictable, although the η2 coordination seems to be predominant in the presence of rigid ancillary ligands. The coordinative capability of benzoquinone was tested by means of thermodynamic and kinetic reference reactions and its slightly enhanced inertness with respect to the isofunctional naphthoquinone was assessed. Finally, the re-crystallization by slow diffusion at low temperature of diethylether in a dichloromethane solution of the complex [Pd(η2-bq)(TTbQ-Me)] (bq = benzoquinone, TTbQMe = 8-t-Butylsulfanyl-2-methyl-quinoline) allows the separation of the dimer [Pd2(η2-bq)(TTbQ-Me)2] whose solid state structure was resolved.

Carbocyclization of unsaturated thioesters under palladium catalysis

An intramolecular thioester–olefin cross-coupling reaction for the preparation of cyclic ketone structures from unsaturated thioesters is described. This method capitalizes on the unique reactivity of thioesters with low-valent palladium catalysts and copper(I) salts to form acyl-metal species. Trapping of such intermediates with alkene functional groups generates the corresponding exo-methylene cycloalkanone products. Unsaturated thioester substrates, which can be accessed using a suite of modern synthetic methods, can now be regarded as precursors to complex carbocyclic derivatives.

Ruthenium−Terpyridine Complexes with Multiple Ethynylpyrenyl or Ethynyltoluyl Subunits: X-ray Structure, Redox, and Spectroscopic Properties

Several heteroleptic and homoleptic ruthenium-terpyridine complexes bearing two and four ethynylpyrenyl or ethynyltoluyl residues have been prepared from complexes carrying reactive bromo functions. Cross-coupling promoted by low-valent palladium(0) on these preformed complexes has advantageously been used to prepare the target complexes. The structure of a bis-terpyridine complex carrying four ethynylpyrenyl subunits was determined by single-crystal X-ray diffraction, showing a distorted octahedral geometry around the metal center, with the ethynylpyrenyl fragment being slightly tilted (about 5 degrees) from the terpyridine plane. The molecular packing is characterized by intermolecular pi...pi interaction within dimers. The counteranions and the solvent molecules are entrapped in well-defined channels spanning along the a-axis. The complexes are redox active with a Ru oxidation overlapping the pyrene oxidation and two well-defined ligand-centered reduction processes. Pyrene reduction is irreversible and strongly cathodic. The new multichromophoric complexes are luminescent both in solution and in rigid matrix at 77 K, with room-temperature lifetimes and quantum yields significantly larger than those of [Ru(terpy)2]2+. At room temperature, the toluyl-substituted complexes are triplet metal-to-ligand charge-transfer (3MLCT) emitters, whereas for the pyrene-grafted complexes pyrene-centered emission is observed. For the latter complexes, the energy gap, DeltaTT, between higher 3MLCT levels and lower ligand-centered (3pipi*, 3LC) levels is in the 640-730 cm(-1) range, which results in the interstate dynamics at the basis of the observed luminescent behavior. At 77 K, for the pyrene-grafted complexes, the emission reveals features that are tentatively ascribed to intraligand interactions involving the pyrene and terpyridine units.

Formation of low-valence palladium species in the course of oxidation of alcohols with palladium(II) tetraaqua complex

The formation of low-valence palladium species Pd n 2+ (n ≥ 2) in the course of oxidation of aliphatic C1-C4 alcohols with oxygen in the presence of palladium(II) tetraaqua complex in aqueous solution was proved UV-spectrophotometrically by the absorption band with a maximum at 312 or 316 nm depending on particular alcohol.

Palladium-catalyzed oxidation of lower aliphatic alcohols with oxygen in the presence of aromatic nitriles in aqueous medium

The kinetics of oxidation of lower aliphatic alcohols (C1–C4) to the corresponding carbonyl compounds with oxygen in the presence of palladium(II) tetraaqua complex and aromatic nitriles (benzonitrile, phenylacetonitrile, and o-tolunitrile) in aqueous medium (c = 0.01 M) at 65°C under atmospheric pressure were studied. A probable reaction mechanism and kinetic equation were proposed. Aromatic nitriles were found to stabilize decomposition of low-valence palladium species, ensuring activation of molecular oxygen and subsequent oxidation of alcohols.

A new synthesis for thermolabile low-valent palladium complexes by electron transfer reactions from nickel(0) to palladium(II) compounds

The nickel(0) complex [Ni(bpy)(cod)] (bpy: 2,2′-bipyridine, cod: cycloocta-1,5-diene) was used as a mild reducing reagent for the synthesis of the extremely reactive low-valent palladium complexes [Pd 2X 2(cod) 2] ( 1: X = Cl, 2: X = Br), Pd(cod) 2 ( 3) and Pd(norbornene) 3 ( 4). The X-ray analysis of 1 showed that the two [Pd(cod)(Cl)] moieties are only connected by a short Pd(I)–Pd(I) bond (bond length: 2.5379(4) Å) with the chloride ions as monodentate ligands. The X-ray structure of 3 which is also known to be an extremely reactive compound could be determined by X-ray diffraction. As expected, the Pd(0) centre is surrounded by the two cod ligands to form a PdC 4 tetrahedron with typical Pd–C bond lengths. The crystal structure of 3 shows it to be very similar to the closely related complexes M(cod) 2 (M: Ni, Pt). The X-ray structure of 4 displays that the Pd(0) centre is in a trigonal planar environment of the three olefin groups. According to 1H NMR measurements the complexes have the same structure in solution as found in the solid state.

Palladium–(N‐Heterocyclic Carbene) Hydrogenation Catalysts

Low-valent palladium compounds with N-heterocyclic carbenes (NHCs) formed from [Pd(NHC)(η2-alkene)2] (see example) are amazingly selective catalysts for the hydrogenation of several alkynes to Z alkenes. One such complex, [Pd{1,3-(2,6-diethylphenyl)imidazol-2-ylidene}], has been employed for the selective (95 %) hydrogenation of 1-phenyl-1-propyne to (Z)-1-phenyl-1-propene.

Cis-[Ru(2,2':6',2' '-terpyridine)(DMSO)Cl(2)]: useful precursor for the synthesis of heteroleptic terpyridine complexes under mild conditions.

[Ru(II)(terpy)(DMSO)Cl(2)] complexes were synthesized as a 5/1 mixture of cis and trans isomers, and their reactivities with CO and with substituted 2,2':6',2' '-terpyridine (terpy) moieties have been investigated. The structure of a trans isomer and its CO adduct have been unambiguously assigned by spectroscopy and X-ray diffraction. The [Ru(terpy)(terpy-Br)](2+) complex prepared either from the cis-[Ru(II)(terpy)(DMSO)Cl(2)] or from the cis-[Ru(II)(terpy-Br)(DMSO)Cl(2)] precursor appeared to be reactive in cross-coupling reactions promoted by low-valent palladium(0) and is an attractive target for the stepwise synthesis of polynuclear complexes bearing vacant coordination sites (terpy-Br for 4'-bromo-2,2':6',2' '-terpyridine). Several bipyridine, phenanthroline, and bipyrimidine complexes were prepared this way and their optical and redox properties determined and discussed.

Zero-valent palladium complexes with monodentate nitrogen sigma-donor ligands.

The facile synthesis and isolation of the stable zero-valent complexes [Pd(L)2(maleic anhydride)] with σ-donor N ligands (L=ammonia, aniline, diethylamine, and pyridine) is remarkable in light of the fact that according to the HSAB principle, low-valent palladium compounds with donor groups such as simple amines should be difficult to prepare. The X-ray crystal structure of the pyridine complex is shown (Pd gold, O orange, N blue).

The First Stable Mononuclear Silyl Palladium Hydrides

The reaction of tertiary silanes with the low valent palladium complex [(mu-dcpe)Pd]2 affords equilibrium mixtures with mononuclear silyl palladium hydrides. These complexes have been characterized by NMR spectroscopy and, in one case, by X-ray crystallography. The silyl palladium hydride complexes rapidly interchange silicon and hydride coordination environments in solution which give rise to extraordinary temperature-dependent kinetic isotope effects for the fluxional process. An intermediate 2eta-Si-H complex is proposed for the interchange.

Stabilization of Electron-Dense Palladium−Hydrido Complexes in Solid-State Hydrides

Inelastic neutron scattering, supported by IR and Raman spectroscopy, has been used to observe a decreasing Pd−H bond strength in a series of four low-valent palladium−hydrogen complexes in the solid-state hydrides: K2[Pd(II)H4], Li2[Pd(0)H2], Na2[Pd(0)H2], NaBa[Pd(0)H3], and Ba2[Pd(0)H4]. Further, self-consistent linear muffin-tin orbital calculations of the electronic structures describe the palladium−hydrido complexes in a sdn hybridization with a decreasing Pd−H bond order as an increasing number of antibonding orbitals must be utilized. A strong support to the bonding from the cations via easily polarizable H- is, however, necessary to explain the stability of the systems. The effect of the large polarizability of hydrogen can in this respect be compared with the more conventional “back-bonding” to ligand orbitals for stabilizing a formal low-valent oxidation state by distributing electron density away from the central atom.

Low-valence palladium complexes: Stoichiometric reactions and catalysis

New data on the structure and reactivity of palladium clusters are surveyed. The mechanisms of stoichiometric and catalytic reactions of the palladium cluster complexes with alkenes, alcohols, aldehydes, formic acid, CO, and phenol are discussed.

Cleavage of Bis(thiophosphinyl)disulfanes, R2P(S)SSP(S)R2 (R = Et, Ph), by a Low-Valent Palladium Dimer

Cleavage of Bis(thiophosphinyl)disulfanes, R2P(S)SSP(S)R2 (R = Et, Ph), by a Low-Valent Palladium Dimer

A simple and efficient synthesis of 2,- 3-, or 4-(2-nitrophenyl)pyridine derivatives via palladium catalyzed ullmann cross-coupling reaction

The Ullmann cross-coupling reaction of halopyridines with 2-bromonitirobenzenes was successffully catalyzed by low valent palladium [Pd(PPh 3) 4, PdCl 2(PPh 3) 2, PdCl 2] to afford (2-nitrophenyl)pyridine derivatives in good to excellent yield.

Palladium-catalyzed reaction of 5-methylene-1,3-dioxolan-2-ones. A new access to and reactivity of oxatrimethylenemethane-palladium

The oxidative addition of 5-methylene-1,3-dioxolan-2-ones (1) to a low-valent palladium complex and the subsequent decarborylation has been studied as an access to oxatrimethylenemethane palladium (OTMM-Pd) intermediates in a catalytic process. The reaction of 5,5-dimethyl-4-methylene-1,3-dioxolan-2-one (1a) with norbornene in the presence of Pd(PPh 3 ) 4 gave 3-(2-methylpropanoyl)tricyclo[3.2.1.0 2.4 ]octane (3a) via addition and isomerization

Oxidative Addition of Chlorobenzene on an Iron-Cyclopentadienyl Cation to a Low-Valent Palladium Complex

Abstract The oxidative addition of the coordinated chlorobenzene in [CpFe(C6H5Cl)][PF6] to Pd(PPh3)4 proceeds under THF-reflux conditions to give [CpFe{η6-[PdCl(PPh3)2]C6H5}][PF6]. X-Ray analysis of this complex revealed that the phenyl ring is σ-bound to Pd and π-bound to Fe, and that the palladium center takes the cis-configuration. This complex reacts with CO / MeOH, CO / HNEt2, styrene, and methyl acrylate to give methyl benzoate, N,N-diethylbenzamide, stilbene, and methyl cinnamate, respectively, after photolysis.

ChemInform Abstract: Synthesis of Higher Unsaturated Sulfides Using Palladium Complexes.

AbstractDie Palladium‐katalysierte Umsetzung der Thiole (I) mit Butadien (II) führt zu Gemischen der Sulfide (III)‐(VIII).