Bimetallic catalysis involving dipalladium(I) and diruthenium(I) complexes

A one-pot reduction route to bimetallic manganese 1,8-naphthyridine complexes

Reaction of the dinucleating ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (MeL) with the MnI and MnII precursors MnBr(CO)5 and MnCl2 resulted in the formation of the monometallic complexes [MnBr(CO)3(MeL)] (1) and [MnCl2(MeL)] (3). In both cases, formation of bimetallic manganese complexes could be achieved by reduction with KC8, yielding the carbonyl-bridged complex [Mn2(CO)6(MeL)] (2) and the helicate complex [Mn2(MeL)2] (4), respectively. EPR results demonstrate that 4 represents a novel, weakly antiferromagnetically coupled homovalent dimer (J = -0.85 cm-1). The two formally Mn0 ions are both high spin (S = 3/2) and exhibit a zero-field splitting of ≈1 cm-1, suggesting reduction of the complex is substantially ligand centered, and may be better described as a MnII complex coupled to two open shell singlet ligands [MnII2(MeL2-)2]. X-ray crystallography, UV-Vis spectroscopy and DFT analysis support this finding.

Utilising a Proton-Responsive 1,8-Naphthyridine Ligand for the Synthesis of Bimetallic Palladium and Platinum Complexes

We present four proton-responsive palladium and platinum complexes, [MCl2 (R PONNHO)] (M=Pd, Pt; R=i Pr, t Bu) synthesised by complexation of PdCl2 or PtCl2 (COD) with the 1,8-naphthyridine ligand R PONNHO. Deprotonation of [MCl2 (tBu PONNHO)] switches ligand coordination from mono- to dinucleating, offering a synthetic pathway to bimetallic PdII and PtII complexes [M2 Cl2 (tBu PONNO)2 ]. Two-electron reduction gives planar MI -MI complexes [M2 (tBu PONNO)2 ] (M=Pd, Pt) containing a metal-metal bond. In contrast to the related nickel system that forms a metallophosphorane [Ni2 (tBu PONNOPONNO)], an unusual phosphinite binding mode is observed in [M2 (tBu PONNO)2 ] containing close phosphinite-naphthyridinone P⋅⋅⋅O interactions, which is investigated spectroscopically, crystallographically and computationally. The presented proton-responsive and structurally-responsive R PONNHO and bimetallic R PONNO complexes offer a novel platform for future explorations of metal-ligand and metal-metal cooperativity with palladium and platinum.

Changes in ligand coordination mode induce bimetallic C-C coupling pathways

Carbon-carbon coupling is one of the most powerful tools in the organic synthesis arsenal. Known methodologies primarily exploit monometallic Pd⁰/Pd^II catalytic mechanisms to give new C-C bonds. Bimetallic C-C coupling mechanisms that involve a Pd^I/Pd^II redox cycle, remain underexplored. Thus, a detailed mechnaistic understanding is imperative for the development of new bimetallic catalysts. Previously, a Pd^II-Me dimer (1) supported by L1, which has phosphine and 1-azaallyl donor groups, underwent reductive elimination to give ethane, a Pd^I dimer, a Pd^II monometallic complex, and Pd black. Herein, a comprehensive experimental and computational study of the reactivity of 1 is presented, which reveals that the versatile coordination chemistry of L1 promotes bimetallic C-C bond formation. The phosphine 1-azaallyl ligand adopts various bridging modes to maintain the bimetallic structure throughout the C-C bond forming mechanism, which involves intramolecular methyl transfer and 1,1-reductive elimination from one of the palladium atoms. The minor byproduct, methane, likely forms through a monometallic intermediate that is sensitive to solvent C-H activation. Overall, the capacity of L1 to adopt different coordination modes promotes the bimetallic C-C coupling channel through pathways that are unattainable with statically-coordinated ligands.

Towards Dual-Metal Catalyzed Hydroalkoxylation of Alkynes

Poly (vinyl ethers) are compounds with great value in the coating industry due to exhibiting properties such as high viscosity, soft adhesiveness, resistance to saponification and solubility in water and organic solvents. However, the main challenge in this field is the synthesis of vinyl ether monomers that can be synthetized by methodologies such as vinyl transfer, reduction of vinyl phosphate ether, isomerization, hydrogenation of acetylenic ethers, elimination, addition of alcohols to alkyne species etc. Nevertheless, the most successful strategy to access to vinyl ether derivatives is the addition of alcohols to alkynes catalyzed by transition metals such as molybdenum, tungsten, ruthenium, palladium, platinum, gold, silver, iridium and rhodium, where gold-NHC catalysts have shown the best results in vinyl ether synthesis. Recently, the hydrophenoxylation reaction was found to proceed through a digold-assisted process where the species that determine the rate of the reaction are PhO-[Au(IPr)] and alkyne-[Au(IPr)]. Later, the improvement of the hydrophenoxylation reaction by using a mixed combination of Cu-NHC and Au-NHC catalysts was also reported. DFT studies confirmed a cost-effective method for the hydrophenoxylation reaction and located the rate-determining step, which turned out to be quite sensitive to the sterical hindrance due to the NHC ligands.

Recent advances of dinuclear nickel- and palladium-complexes in homogeneous catalysis

In this highlight, we provide a current perspective of synthetic methodology development catalyzed by dinuclear Ni- and Pd-complexes in the past decade. The new catalytic reactivities of dinuclear Ni- and Pd-complexes are discussed.

The surprisingly facile formation of Pd(i)-phosphido complexes from ortho-biphenylphosphines and palladium acetate

The widely-used ortho-biphenylphosphine ligands SPhos and RuPhos not only undergo facile orthometallation with palladium acetate, yielding strained, four-membered dimeric palladacycles but more surprisingly, in the presence of alcoholic solvents, along with the less encumbered analogue MePhos, yield unusual dinuclear Pd(i) complexes, in which the Pd-centers are bridged by both a phosphide ligand and by the arene of a coordinated phosphine donor.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal-support interaction, and metal-reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Dinuclear Nickel(I) and Palladium(I) Complexes for Highly Active Transformations of Organic Compounds

In typical catalytic organic transformations, transition metals in catalytically active complexes are present in their most stable valence states, such as palladium(0) and (II). However, some dimeric monovalent metal complexes can be stabilized by auxiliary ligands to form diamagnetic compounds with metal–metal bonding interactions. These diamagnetic compounds can act as catalysts while retaining their dimeric forms, split homolytically or heterolytically into monomeric forms, which usually have high activity, or in contrast, become completely deactivated as catalysts. Recently, many studies using group 10 metal complexes containing nickel and palladium have demonstrated that under specific conditions, the active forms of these catalyst precursors are not mononuclear zerovalent complexes, but instead dinuclear monovalent metal complexes. In this mini-review, we have surveyed the preparation, reactivity, and the catalytic processes of dinuclear nickel(I) and palladium(I) complexes, focusing on mechanistic insights into the precatalyst activation systems and the structure and behavior of nickel and palladium intermediates.

Buchwald–Hartwig amination using Pd(i) dimer precatalysts supported by biaryl phosphine ligands

We report the synthesis of air-stable Pd(i) dimer complexes featuring biaryl phosphine ligands.

Detection of Palladium(I) in Aerobic Oxidation Catalysis

Palladium(II)-catalyzed oxidation reactions exhibit broad utility in organic synthesis; however, they often feature high catalyst loading and low turnover numbers relative to non-oxidative cross-coupling reactions. Insights into the fate of the Pd catalyst during turnover could help to address this limitation. Herein, we report the identification and characterization of a dimeric PdI species in two prototypical Pd-catalyzed aerobic oxidation reactions: allylic C-H acetoxylation of terminal alkenes and intramolecular aza-Wacker cyclization. Both reactions employ 4,5-diazafluoren-9-one (DAF) as an ancillary ligand. The dimeric PdI complex, [PdI (μ-DAF)(OAc)]2 , which features two bridging DAF ligands and two terminal acetate ligands, has been characterized by several spectroscopic methods, as well as single-crystal X-ray crystallography. The origin of this PdI complex and its implications for catalytic reactivity are discussed.

CuBr-ZnI2 Combo-Catalysis for Mild CuI-CuIII Switching and sp2 C-H Activated Rapid Cyclization to Quinolines and Their Sugar-Based Chiral Analogues: A UV-Vis and XPS Study

An unprecedented CuBr-ZnI₂ combo-catalyzed mild Cu¹-Cu^III switching activation of sp² C-H of highly electron-rich arenes is reported. Anilines, aldehydes, and terminal alkynes were rapidly coupled together at ambient temperature to construct a ubiquitous quinoline framework through cyclization of the C≡C bond. This smart solvent-free strategy was exploited for the direct synthesis of valuable 4-substituted, 2,4-disubstituted, and thermally labile sugar-based chiral quinolines in good yields. In contrast to the frequently used imine-alkyne cyclization reaction, this uncommonly mild Cu^I-Cu^III combo-catalysis for a rapid three-component cyclization is expected to proceed through the formation of a flexible propargyl amine intermediate, which provides a Cu^I-procatalyst for rapid sp² C-H activation with cyclization involving transient Cu^III species. The in situ generation of transient Cu^III species was confirmed through online ultraviolet-visible spectroscopy (UV-vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS) analyses.

Dinuclear Pd(I) complexes with bridging allyl and related ligands

There are many important synthetic methods that utilize palladium catalysts. In most of these reactions, the palladium species are proposed to exist exclusively in either the Pd(0) or Pd(II) oxidation states. However, in the last decade, dinuclear Pd(I) complexes have repeatedly been isolated from reaction mixtures previously suggested to involve only species in the Pd(0) and Pd(II) oxidation states. As a consequence, in order to design improved catalysts there is considerable interest in understanding the chemistry of dinuclear Pd(I) complexes. A significant proportion of the known dinuclear Pd(I) complexes are supported by bridging allyl or related ligands such as cyclopentadienyl or indenyl ligands. This review provides a detailed account of the synthesis, electronic structure and stoichiometric reactivity of dinuclear Pd(I) complexes with bridging allyl and related ligands. Additionally, it describes recent work where dinuclear Pd(I) complexes with bridging allyl ligands have been detected in catalytic reactions, such as cross-coupling, and discusses the potential implications for catalysis.

Enhancing catalytic activity towards Heck-reaction by using 4,5,9,10-tetra(arylimino)pyrenylidenyldipalladium tetrachlorides

A series of binuclear 4,5,9,10-tetra(arylimino)pyrenylidenyldipalladium(ii) tetrachloride complexes (Pd1–Pd4) was synthesized and characterized by FT-IR and NMR spectroscopy as well as elemental analysis.

Synthesis of a cyclometalated 1,3,5-triphenylpyrazole palladium dimer and its activity towards cross coupling reactions

A phosphine free 1,3,5-triphenylpyrazole acetate-bridged palladacycle was prepared from simple commercially available starting materials.

Activation of C–H bonds of thiosemicarbazones by transition metals: synthesis, structures and importance of cyclometallated compounds

Transition metals (Pd^II, Pt^II, Ru^II, Rh^IIIand Ir^III) have induced activation of C–H bonds of thiosemicarbazones and yielded mono-, di-, tri- and tetra-nuclear complexes.

Insight into the efficiency of cinnamyl-supported precatalysts for the Suzuki-Miyaura reaction: observation of Pd(I) dimers with bridging allyl ligands during catalysis

Despite widespread use of complexes of the type Pd(L)(η(3)-allyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd(I) dimers of the form (μ-allyl)(μ-Cl)Pd2(L)2 has been underexplored. Here, the relationship between the monomeric and the dimeric compounds is investigated using both experiment and theory. We report an efficient synthesis of the Pd(I) dimers (μ-allyl)(μ-Cl)Pd2(IPr)2 (allyl = allyl, crotyl, cinnamyl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) through activation of Pd(IPr)(η(3)-allyl)Cl type monomers under mildly basic reaction conditions. The catalytic performance of the Pd(II) monomers and their Pd(I) μ-allyl dimer congeners for the Suzuki-Miyaura reaction is compared. We propose that the (μ-allyl)(μ-Cl)Pd2(IPr)2-type dimers are activated for catalysis through disproportionation to Pd(IPr)(η(3)-allyl)Cl and monoligated IPr-Pd(0). The microscopic reverse comproportionation reaction of monomers of the type Pd(IPr)(η(3)-allyl)Cl with IPr-Pd(0) to form Pd(I) dimers is also studied. It is demonstrated that this is a facile process, and Pd(I) dimers are directly observed during catalysis in reactions using Pd(II) precatalysts. In these catalytic reactions, Pd(I) μ-allyl dimer formation is a deleterious process which removes the IPr-Pd(0) active species from the reaction mixture. However, increased sterics at the 1-position of the allyl ligand in the Pd(IPr)(η(3)-crotyl)Cl and Pd(IPr)(η(3)-cinnamyl)Cl precatalysts results in a larger kinetic barrier to comproportionation, which allows more of the active IPr-Pd(0) catalyst to enter the catalytic cycle when these substituted precatalysts are used. Furthermore, we have developed reaction conditions for the Suzuki-Miyaura reaction using Pd(IPr)(η(3)-cinnamyl)Cl which are compatible with mild bases.

Room temperature C-H bond activation on a [Pd(I)-Pd(I)] platform

Heteroatom assisted C-H bond activation of 2-substituted naphthyridines with solvated Pd(I)2(CH3CN)6(BF4)2 provides bi- and trinuclear Pd(II) cyclometalated complexes, which provide valuable insight into potential intermediates in palladium catalyzed direct arylation reactions.

Versatile reactivity of Pd-catalysts: mechanistic features of the mono-N-protected amino acid ligand and cesium-halide base in Pd-catalyzed C–H bond functionalization

The C–H functionalization strategies, complexity in Pd-catalyzed chemical transformations, unprecedented Pd-clustering, base (Cs-halide) and weakly coordinated amino acid ligand effects.

An unexpected semi-hydrogenation of a ligand in the complexation of 2,7-bispyridinyl-1,8-naphthyridine with Ru3(CO)12

A novel formate-bridged diruthenium complex [(bpnp-H₃)Ru₂(μ-HCOO)(CO)₄] was prepared, in which the ligand bpnp was partially hydrogenated during the complexation of the ligand with Ru₃(CO)₁₂ in the presence of water.