Optical, Electrochemical, and Catalytic Properties of the Unsaturated Host Pd3(dppm)3(CO)2+ and Pd4(dppm)4(H)2+ 2 Clusters: An Overview

This paper presents an overview of the optical, photophysical, and photochemical properties including UV-visible and luminescence spectra in solution at 298 and 77 K, along with electrochemical, and catalytic behavior under reduction conditions (for both thermally and electrochemically assisted systems) of the tri- and tetranuclear Pd₃(dppm)₃(CO)²⁺ and Pd₄(dppm)₄(H)²⁺ ₂ clusters (dppm=bis(diphenylphosphino)methane). This review is also complemented with relevant information about their syntheses, molecular and electronic structures supported from computer modeling, EHMO and DFT calculations, and their host-guest behavior with anions and neutral molecules, in relation with their observed reactivity.     

Generation, characterization, and electrochemical behavior of the palladium-hydride cluster [Pd3(dppm)3(mu3-CO)(mu3-H)]+ (dppm=Bis(diphenylphosphinomethane)

Addition of formate on the dicationic cluster [Pd(3)(dppm)(3)(mu(3)-CO)](2+) (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+). This new palladium-hydride cluster has been characterised by (1)H NMR, (31)P NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. The unambiguous identification of the capping hydride was made from (2)H NMR spectroscopy by using DCO(2) (-) as starting material. The mechanism of the hydride complex formation was investigated by UV/Vis stopped-flow methods. The kinetic data are consistent with a two-step process involving: 1) host-guest interactions between HCO(2) (-) and [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) a reductive elimination of CO(2). Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH(4) to [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) electrochemical reduction of [Pd(3)(dppm)(3)(mu(3)-CO)](2+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) followed by an addition of one equivalent of H(+). Based on cyclic voltammetry, evidence for a dual mechanism (ECE and EEC; E=electrochemical (one-electron transfer), C=chemical (hydride dissociation)) for the two-electron reduction of [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd(3)(H(2)PCH(2)PH(2))(3)(mu(3)-CO)(mu(3)-H)](n) model clusters were performed by using DFT at the B3 LYP level. Upon one-electron reductions, the Pd--Pd distance increases from a formal single bond (n=+1), to partially bonding (n=0), to weak metal-metal interactions (n=-1), while the Pd--H bond length remains relatively the same.     

One‐Electron Oxidation of [M(PtBu3)2] (M=Pd,Pt): Isolation of Monomeric [Pd(PtBu3)2]+ and Redox‐Promoted C−H Bond Cyclometalation

Oxidation of zero‐valent phosphine complexes [M(P^tBu₃)₂] (M=Pd, Pt) has been investigated in 1,2‐difluorobenzene solution using cyclic voltammetry and subsequently using the ferrocenium cation as a chemical redox agent. In the case of palladium, a mononuclear paramagnetic Pd^I derivative was readily isolated from solution and fully characterized (EPR, X‐ray crystallography). While in situ electrochemical measurements are consistent with initial one‐electron oxidation, the heavier congener undergoes C?H bond cyclometalation and ultimately affords the 14 valence‐electron Pt^II complex [Pt(κ²_PC‐P^tBu₂CMe₂CH₂)(P^tBu₃)]⁺ with concomitant formation of [Pt(P^tBu₃)₂H]⁺.     

High Nuclearity Bimetallic Rhodium–Palladium Carbonyl Cluster Complexes. Synthesis and Characterization of Rh6(CO)16[Pd(PBu t 3)]3 and Rh6(CO)16[Pd(PBu t 3)]4

The reaction of Rh₄(CO)₁₂ with Pd(PBu ^t ₃)₂ yielded the high nuclearity bimetallic hexarhodium-tripalladium cluster complex Rh₆(CO)₁₆[Pd(PBu ^t ₃)]₃, 10, in 11% yield. Compound 10 was converted to the hexarhodium-tetrapalladium cluster Rh₆(CO)₁₆[Pd(PBu ^t ₃)]₄, 11, in 62% yield by reaction with an additional quantity of Pd(PBu ^t ₃)₂. Both compounds were characterized crystallographically. Structurally, both compounds consist of an octahedral cluster of six rhodium atoms with sixteen carbonyl ligands analogous to that of the known compound Rh₆(CO)₁₆. Compound 10 also contains three Pd(PBu ^t ₃) groups that bridge three Rh–Rh bonds along edges of the Rh₆ octahedron to give an overall D₃ symmetry to the Rh₆Pd₃ cluster. Compound 11 contains four edge bridging Pd(PBu ^t ₃) groups distributed across the Rh₆ octahedron to give an overall D_2d symmetry to the Rh₆Pd₄ cluster. Each Rh–Pd connection in both compounds contains a bridging carbonyl ligand that helps to stabilize the bond between the Pd(PBu ^t ₃) groups and the Rh atoms. Both compounds can be regarded as Pd(PBu ^t ₃) adducts of Rh₆(CO)₁₆.     

On the Oxidation State of Cu2O upon Electrochemical CO2 Reduction: An XPS Study

The encouraging selectivity of copper oxides for the electroreduction of CO₂ into ethylene and alcohols has led to a vivid debate on the possible relation between their operando (sub-)surface oxidation state (i. e. fully reduced or partially oxidized) and this distinct reactivity. The high roughness of the Cu oxides used in previous studies on this matter adds complexity to this controversy and motivated us to prepare quasi-planar Cu₂O thin films that displayed a CO₂ reduction selectivity similar to that of oxide-derived copper catalysts reported in previous studies. Most importantly, when the post-mortem thin films were transferred for characterization in an air-free environment, X-ray photoelectron spectroscopy measurements confirmed their complete reduction in the course of the CO₂ reduction reaction. Thus, our results indicate that the selectivity of the Cu oxides featured in previous studies stems from their enhanced roughness, highlighting the importance of controlled sample transfer upon post-mortem characterization with ex situ techniques.     

Electrochemical characterization of Co(II) and Pd(II) phthalocyanines carrying diethoxymalonyl and carboxymethyl substituents

In this study, electrochemical behaviors of Co(II) and Pd(II) phthalocyanines carrying tetrakisdiethoxymalonyl and Pd(II) phthalocyanine carrying tetrakiscarboxymethyl substituents at the peripheral positions are investigated by cyclic voltammetry and applied potential chronocoulometry techniques. Cyclic voltammetric studies show that, while Pd(II) phthalocyanines carrying diethoxymalonyl and carboxymethyl substituents give up to three common phthalocyanine ring reductions, Co(II) phthalocyanine carrying diethoxymalonyl substituents gives a metal-centered oxidation and a metal-centered reduction and three ligand-centered reduction and a ligand-centered oxidation processes. First reduction processes of both the PdPc complexes have shoulders. This different voltammetric behaviors of Pd(II) phthalocyanines carrying carboxymethyl and diethoxymalonyl substituents results from interaction of this distinctive substituents with the phthalocyanine ring π electron system and interaction with the different solvent systems. Observation of the splitting of the first reduction process of Pd(II) phthalocyanines carrying diethoxymalonyl and carboxymethyl substituents suggests the aggregation of the complex. Very small diffusion coefficient of the complexes with respect to Co(II) phthalocyanine also confirms the existence of the aggregation of the complex during the electrochemical studies. Effects of the substituents and the solvent media are clearly observed from the differences of the voltammograms of Pd(II) phthalocyanines carrying diethoxymalonyl and carboxymethyl substituents in DMSO and THF solvent media, respectively. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 1, pp. 36–43. The text was submitted by the authors in English.     

Titanium Carbide (Ti3C2Tx) MXene Ornamented with Palladium Nanoparticles for Electrochemical CO Oxidation

Titanium carbide (Ti₃C₂T_x) MXene possesses various unique physicochemical and catalytic properties. However, the electrochemical CO oxidation performance is not yet addressed experimentally. Herein, Ti₃C₂T_x (T_X=OH, O, and F) ordered and exfoliated two-dimensional nanosheets ornamented with semi-spherical palladium nanoparticles (2.5 Wt. %) with an average diameter of (10±1 nm) (denoted as Pd/Ti₃C₂T_x) is rationally designed for the electrochemical CO oxidation. The fabrication process is based on the selective chemical etching of Ti₃AlC₂ and delamination under sonication to form Ti₃C₂Tx nanosheets that are used as a substrate and reducing agent for supporting in situ growth of Pd nanoparticles via impregnation with Pd salt. Interestingly, Pd-free Ti₃C₂T_x displayed inferior CO oxidation activity, while Pd/Ti₃C₂T_x enhanced the CO oxidation activity substantially. This is attributed to the combination of outstanding physicochemical properties of Ti₃C₂T_x and the catalytic merits of Pd nanoparticles.     

Unexpected reaction of the unsaturated cluster host and catalyst [Pd3(mu3-CO)(dppm)3]2+ with the hydroxide ion: spectroscopic and kinetic evidence of an inner-sphere mechanism

The title cluster, [Pd(3)(mu(3)-CO)(dppm)(3)](2+) (dppm=bis(diphenylphosphino)methane), reacts with one equivalent of hydroxide anions (OH(-)), from tetrabutylammonium hydroxide (Bu(4)NOH), to give the paramagnetic [Pd(3)(mu(3)-CO)(dppm)(3)](+) species. Reaction with another equivalent of OH(-) leads to the zero-valent compound [Pd(3)(mu(3)-CO)(dppm)(3)](0). From electron paramagnetic resonance analysis of the reaction medium using the spin-trap agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), the 2-tetrahydrofuryl or methyl radicals, deriving from the tetrahydrofuran (THF) or dimethyl sulfoxide (DMSO) solvent, respectively, were detected. For both [Pd(3)(mu(3)-CO)(dppm)(3)](2+) and [Pd(3)(mu(3)-CO)(dppm)(3)](+), the mechanism involves, in a first equilibrated step, the formation of a hydroxide adduct, [Pd(3)(mu(3)-CO)(dppm)(3)(OH)]((n-1)+) (n=1, 2), which reacts irreversibly with the solvent. The kinetics were resolved by means of stopped-flow experiments and are consistent with the proposed mechanism. In the presence of an excess of Bu(4)NOH, an electrocatalytic process was observed with modest turnover numbers (7-8). The hydroxide adducts [Pd(3)(mu(3)-CO)(dppm)(3)(OH)]((n-1)+) (n=1, 2), which bear important similarities to the well-known corresponding halide adducts [Pd(3)(mu(3)-CO)(dppm)(3)(mu(3)-X)](n) (X=Cl, Br, I), have been studied by using density functional theory (DFT). Although the optimised geometry for the cluster in its +2 and 0 oxidation states (i.e., cation and anion clusters, respectively) is the anticipated mu(3)-OH form, the paramagnetic species, [Pd(3)(mu(3)-CO)(dppm)(3)(OH)](0), shows a mu(2)-OH form; this suggests an important difference in electronic structure between these three species.     

氧化铈对Pd催化剂氧化活性和热稳定性的影响

氧化铈对Pd催化剂氧化活性和热稳定性的影响;钯催化剂;催化氧化     

Direct Oxidation of β‐Aryl Substituted Aldehydes to α,β‐Unsaturated Aldehydes Promoted by an o‐Anisidine–Pd(OAc)2 Co‐catalyst

An o‐anisidine‐Pd(OAc)₂ catalytic system for the direct co‐catalytic Saegusa oxidation of β‐aryl substituted aldehydes to α,β‐unsaturated aldehydes has been developed. The use of o‐anisidine in place of (S)‐diphenylprolinol made the process more simply and cost‐effective. The process not only features the use of unmodified aldehydes rather than enol silyl ethers, but also gives moderate to good yields (44–72 %).     

Synthesis of large palladium clusters. The preparation of Pd38(CO)28(PR3)12 (R = Et,Bun) and Pd34(CO)24(PEt3)12

Several methods for the synthesis of the Pd₃₈(CO)₂₈L₁₂ cluster (L = PEt₃) by treatment of Pd₁₀(CO)₁₂L₆ with CF₃COOH-Me₃NO, CF₃COOH-H₂O₂, Pd(OAc)₂-Me₃NO, and Pd₂(dba)₃ mixtures (dba is dibenzylideneacetone) were proposed. The tri-n-butylphosphine analog, Pd₃₈(CO)₂₈(PBu₃)₁₂, was synthesized by the reaction of Pd₁₀(CO)₁₄(PBu₃)₄ with Me₃NO. The reaction of Pd₄(CO)₅L₄ with Pd₂(dba)₃ yields clusters with an icosahedral packing of the metal atoms, Pd₃₄(CO)₂₄L₁₂ and Pd₁₆(CO)₁₃L₉.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 167–170, January, 1995.     

The Pd3(dppm)3(CO)2+ cluster: an efficient electrochemically assisted Lewis acid catalyst for the fluorination and alcoholysis of acyl chlorides

The dicationic palladium cluster Pd3(dppm)3(CO)2+ (dppm = bis(diphenylphosphino)methane) reacts with acid chlorides RCOCl (R = n-C6H13, t-Bu, Ph) to afford quantitatively the chloride adduct Pd3(dppm)3(CO)(Cl)+ and the acyl cation RCO+ as the organic counterpart. The dicationic reactive cluster can be reformed by electrolyzing the chloride complex with a copper anode leaving CuCl as a byproduct. The combination of these two reactions provides an electrocatalytic way to form the acylium from the acid chloride. Indeed, in CH2Cl2, 0.2 M NBu4PF6, or NBu4BF4, the electrolysis of the acid chloride in the presence of a catalytic amount of the cluster (1%) gives in good yields the acid fluoride RCOF, arising from the coupling of the acylium with a F(-) issued from the fluorinated supporting electrolyte. Alternatively, in CH2Cl2 or 0.2 M NBu4ClO4, by operating with an alcohol R'OH as the nucleophile, the electrolysis gives the ester RC(O)OR' as the only final product.     

Supported Pd–Cu Bimetallic Nanoparticles That Have High Activity for the Electrochemical Oxidation of Methanol

Monodisperse bimetallic Pd–Cu nanoparticles with controllable size and composition were synthesized by a one‐step multiphase ethylene glycol (EG) method. Adjusting the stoichiometric ratio of the Pd and Cu precursors afforded nanoparticles with different compositions, such as Pd₈₅–Cu₁₅, Pd₅₆–Cu₄₄, and Pd₃₉–Cu₆₁. The nanoparticles were separated from the solution mixture by extraction with non‐polar solvents, such as n‐hexane. Monodisperse bimetallic Pd–Cu nanoparticles with narrow size‐distribution were obtained without the need for a size‐selection process. Capping ligands that were bound to the surface of the particles were removed through heat treatment when the as‐prepared nanoparticles were loaded onto a Vulcan XC‐72 carbon support. Supported bimetallic Pd–Cu nanoparticles showed enhanced electrocatalytic activity towards methanol oxidation compared with supported Pd nanoparticles that were fabricated according to the same EG method. For a bimetallic Pd–Cu catalyst that contained 15 % Cu, the activity was even comparable to the state‐of‐the‐art commercially available Pt/C catalysts. A STEM‐HAADF study indicated that the formation of random solid‐solution alloy structures in the bimetallic Pd₈₅–Cu₁₅/C catalysts played a key role in improving the electrochemical activity.     

Size Dependence of [n]Cycloparaphenylenes (n=5–12) in Electrochemical Oxidation

The oxidation processes of [n]cycloparaphenylenes ([n]CPPs) (n=5–12) were systematically investigated by cyclic and rotating disk electrode voltammetry. All CPPs underwent pseudo‐reversible two‐electron oxidation irrespective of ring size, forming the corresponding radical cations and then dications. The results were in sharp contrast to those observed for linear oligoparaphenylenes, which only undergo one‐electron oxidation. The difference in the first and second oxidation potentials in the CPP oxidation was affected by the ring size and became more significant as the decrease of CPP size. In other words, while the first oxidation from neutral CPP to the radical cation occurred faster as the size of CPP becomes smaller, the second oxidation from the radical cation to dication exhibited opposite size dependence.     

Stoichiometric and catalytic activation of the alpha- and beta-2,3,4-tri-O-acetyl-5-thioxylopyranosyl bromide inside the cavity of the Pd3(dppm)3(CO)2+ cluster

The title cluster (Pd(3)(2+)) exhibits a pronounced affinity for Br(-) ions to form the very stable Pd(3)(Br)(+) adduct. Upon a 2-electron reduction, a dissociative process occurs generating Pd(3)(0) and eliminating Br(-) according to an ECE mechanism (electrochemical, chemical, electrochemical). At a lower temperature (i.e. -20 degrees C), both ECE and EEC processes operate. This cluster also activates the C-Br bond, and this work deals with the reactivity of Pd(3)(2+) with 2,3,4-tri-O-acetyl-5-thioxylopyranosyl bromide (Xyl-Br), both alpha- and beta-isomers. The observed inorganic product is Pd(3)(Br)(+) again, and it is formed according to an associative mechanism involving Pd(3)(2+).Xyl-Br host-guest assemblies. In an attempt to render the C-Br bond activation catalytic, these species are investigated under reduction conditions at two potentials (-0.9 and -1.25 V vs SCE). In the former case, the major product is Xyl-H, issued from a radical intermediate Xyl(*) abstracting an H atom from the solvent. Evidence for Xyl(*) is provided by the trapping with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) and DMPO (5,5'-dimethylpyrroline-N-oxyde). In the second case, only one product is observed, 3,4-di-O-acetyl-5-thioxylal, which is issued from the Xyl(-)() intermediate anion.