Palladium Atoms Research Articles

Nature of active palladium sites on nitrogen doped carbon nanofibers in selective hydrogenation of acetylene

The nature of the active sites for palladium supported on nitrogen doped carbon nanofibers (Pd/N-CNFs) catalysts for selective hydrogenation of acetylene to ethylene was examined. Palladium concentrations of 0.05–0.6 wt% were deposited on carbon nanofibers doped by nitrogen atoms (N-CNFs) and characterized by transmission electron microscopy (TEM), CO chemisorption, X-ray photoelectron spectroscopy (XPS), Extended X-ray Absorption Fine Structure (EXAFS) and quantum-chemical calculations. The data indicates that the Pd initially forms highly-dispersed particles. However, decreasing of the Pd concentration on the carbon nanofibers doped by nitrogen atoms below 0.15 wt% led to stabilization of the metal in the atomic state. Porphyrin-like defects with four nitrogen atoms are formed on the surface of the CNFs and strongly interact with palladium atoms, and thus can be sites for stabilization of atomic Pd. The catalytic activity and selectivity of Pd/N-CNFs catalysts depend on the ratio of these two palladium states.

Annealing behavior of Cu-7at.%Pd alloy deformed by cold rolling

The anneal hardening behavior of Cu-7at.%Pd alloy after deformation by cold rolling was investigated by means of hardness, microhardness, electrical conductivity, light optical microscopy (LOM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). In this regard, the cast Cu-7at.%Pd alloy after quenching was deformed with 20% and 40% reductions and isochronally annealed between 100 °C and 700 °C. After annealing up to 400 °C, the deformed samples exhibited atypical anneal hardening behavior in combination with an increase in electrical conductivity. In addition, the recrystallization was substantially retarded by the applied thermo-mechanical treatment which included solution annealing, quenching, cold rolling, and annealing. The microstructural evolution during a low-temperature annealing proved a strong pinning of grain boundaries by solute palladium atoms, indicating that grain boundaries have an important role during the anneal hardening.

Static dipole polarizability of palladium from relativistic coupled-cluster theory

Nonrelativistic and relativistic coupled-cluster calculations extrapolated to the complete basis set limit including excitations up to the quintuple level (CCSDTQP) were carried out to accurately determine the static electric dipole polarizability of the closed-shell palladium atom. The resulting value of $\alpha$ = 26.14(10) a.u. implies that palladium has the smallest dipole polarizability of all known elemental metal atoms due to its unique 4d$^{10}$5s$^0$ configuration. Relativistic effects are found to be already sizeable ($\Delta_R\alpha$= +1.86~a.u.) compared to electron correlation ($\Delta_C\alpha$= +5.06~a.u.), and need to be included for the accurate determination of the dipole polarizability. We also report a value of the second hyperpolarizability to be $\gamma\approx$ 40,000~a.u., but here the coupled-cluster contributions are not yet converged out with respect to higher than quintuple excitations.

Stabilization of Palladium Atoms in Nitrogen-Doped Porphyrin-Like Fragments of Carbon Nanofibers

The paper considers the system of nitrogen-doped carbon nanofibers (N-CNFs) with palladium atoms deposited on their surfaces. The concentration of deposited palladium varied in the interval of 0.05-0.6 wt.%. The state of palladium was studied with the methods of quantum chemistry, electron microscopy, CO adsorption, and EXAFS. Carbon structures that contain porphyrin-like defects with four nitrogen atoms in the graphene layer interact strongly with palladium atoms and therefore can be stabilization centers of atomic palladium.

X-ray absorption study of platinum and palladium atoms in argon matrices: Evidence for platinum in a substitutional site and a short Pd[sbnd]Ar interaction

The Pt L3-edge X-ray absorption spectrum of Pt atoms generated in a hollow cathode sputtering device and trapped in an Ar matrix yielded a PtAr distance of 3.78(4) Å with a coordination number of ca. 12 which confirms the evidence from electronic absorption spectroscopy that Pt atoms occupy a substitutional site in the Ar lattice. These data also yield a van der Waals radius for Pt atoms of 1.90 Å. The Pd K-edge X-ray absorption spectrum of sputtered Pd atoms trapped in an Ar matrix is radically different to that for Pt atoms. The analysis reveals a much shorter PdAr distance of 2.53(3) Å with a low coordination number close to 1, together with an “atomic” EXAFS like spectrum with no significant oscillations indicating the absence of any well defined nearest neighbours. On annealing to 25 K, the 2.53(3) Å interaction essentially disappears to leave the “atomic” spectrum. The featureless “atomic” spectrum is associated with Pd 1S0 atoms in an argon substitutional site, or other site such as a grain boundary with high disorder. The short PdAr distance of 2.53(3) Å is consistent with Pd atoms with a 1S0 atomic ground state in an interstitial octahedral site, the formation of a PdArn exciplex with a short PdAr distance, or the formation of a more formal palladium argon compound such as PdAr2. Although it is not possible to be definitive, the most likely carrier of the short PdAr distance is a PdArn exciplex. What is clear is that this work has identified a short PdAr interaction for the first time.

Synthesis and characterization of a palladium(0) complex with cyclophosphazene bearing two diphenylphosphine ligands and application in Suzuki-Miyaura cross-coupling

This paper describes the synthesis of a cyclophosphazene-based diphenylphosphine ligand and a new Pd(0) complex. Infrared spectroscopy (ATR-IR), ESI+-MS, 31P, 1H and 13C NMR, Raman, WD-XRF, ICP-OES and TGA analysis show the coordination of two palladium atoms per unit of cyclophosphazene. A semiempirical calculation method was employed to find the lowest energy structure among the possible ones and Density Functional Theory (DFT) was used to optimize the found structure and obtain its bond angles, dihedral angles, bond lengths, atomic distances, and to calculate the vibrational spectrum (PBE/def2-TZVP(-f)). The new Pd complex showed activity in Suzuki-Miyaura cross-coupling reactions with halobenzenes and phenylboronic acid, tolerating different functional groups.

Lu5Pd4Ge8 and Lu3Pd4Ge4: Two More Germanides among Polar Intermetallics

In this study, two novel Lu5Pd4Ge8 and Lu3Pd4Ge4 polar intermetallics were prepared by direct synthesis of pure constituents. Their crystal structures were determined by single crystal X-ray diffraction analysis: Lu5Pd4Ge8 is monoclinic, P21/m, mP34, a = 5.7406(3), b = 13.7087(7), c = 8.3423(4) Å, β = 107.8(1), Z = 2; Lu3Pd4Ge4 is orthorhombic, Immm, oI22, a = 4.1368(3), b = 6.9192(5), c = 13.8229(9) Å, Z = 2. The Lu5Pd4Ge8 analysed crystal is one more example of non-merohedral twinning among the rare earth containing germanides. Chemical bonding DFT studies were conducted for these polar intermetallics and showing a metallic-like behavior. Gathered results for Lu5Pd4Ge8 and Lu3Pd4Ge4 permit to described both of them as composed by [Pd–Ge]δ– three dimensional networks bonded to positively charged lutetium species. From the structural chemical point of view, the studied compounds manifest some similarities to the Zintl phases, containing well-known covalent fragment i.e., Ge dumbbells as well as unique cis-Ge4 units. A comparative analysis of molecular orbital diagrams for Ge26– and cis-Ge10– anions with COHP results supports the idea of the existence of complex Pd–Ge polyanions hosting covalently bonded partially polarised Ge units. The palladium atoms have an anion like behaviour and being the most electronegative cause the noticeable variation of Ge species charges from site to site. Lutetium charges oscillate around +1.5 for all crystallographic positions. Obtained results explained why the classical Zintl-Klemm concept can’t be applied for the studied polar intermetallics.

Palladium(II) Polynuclear Complex with β-Mercaptoethanol

A hexanuclear palladium complex with β-mercaptoethanol was synthesized and characterized by various physical methods. A single sulfur atom participates in the coordination and binds two palladium atoms. The hydroxyl group of the ligand is not involved in the complex formation. A crown-shaped hexanuclear cycle without a metal-metal bond is formed.

A comparative study of the coordination of saccharinate, thiosaccharinate and benzisothiozolinate ligands to cyclometalated [Pd(Me2NCH2C6H4-κN,C)(μ-Cl)]2: Molecular structures of [Pd(Me2NCH2C6H4-κN,C)(µ-X)]2 (X = sac, bit and tsac) and [Pd(Me2NCH2C6H4-κN,C)Cl(ampyH-κN)] (ampyH = 2-amino-3-methylpyridine)

Abstract Reactions of [Pd(Me2NCH2C6H4-κ2N,C)(μ-Cl)]2 with two equivalents of sodium saccharinate (Nasac), thiosaccharin (Htsac) or sodium benzisothiozolinate (Nabit) results in the stepwise substitution of the bridging halides to form sequentially [Pd2(Me2NCH2C6H4-κ2N,C)2(μ-Cl)(μ-X)] (X = sac, tsac) and [Pd(Me2NCH2C6H4-κ2N,C)(μ-X)]2 (X = sac, tsac, bit). The molecular structures of all three disubstituted complexes are reported. In each the two metalated ligands bind in a chelate manner adopting a relative anti conformation, while the new ligands bridge the two palladium atoms adopting a relative cis conformation. The local conformation about each palladium differs with small ligand changes. Thus in the sac and bit complexes all nitrogens lie trans to one another, in the tsac complex they are cis. Conformational changes also lead to large differences in the non-bonded Pd…Pd distance which range over 0.5 A. Treatment of [Pd(Me2NCH2C6H4-κ2N,C)(μ-Cl)]2 with two equivalents of 2-amino-3-methylpyridine (ampyH) in the presence of NEt3 affords mononuclear [PdCl(Me2NCH2C6H4-κ2N,C)(ampyH-κ1N)] as a result of “bridge-splitting”, the ampyH ligand binding through the pyridyl-nitrogen and lying approximately perpendicular to the PdCClN2 plane as shown by a crystallographic study.

Single-atom heterogeneous catalysts based on distinct carbon nitride scaffolds

Abstract Carbon nitrides integrating macroheterocycles offer unique potential as hosts for stabilizing metal atoms due to their rich electronic structure. To date, only graphitic heptazine-based polymers have been studied. Here, we demonstrate that palladium atoms can be effectively isolated on other carbon nitride scaffolds including linear melem oligomers and poly(triazine/heptazine imides). Increased metal uptake was linked to the larger cavity size and the presence of chloride ions in the polyimide structures. Changing the host structure leads to significant variation in the average oxidation state of the metal, which can be tuned by exchange of the ionic species as evidenced by X-ray photoelectron spectroscopy and supported by density functional theory. Evaluation in the semi-hydrogenation of 2-methyl-3-butyn-2-ol reveals an inverse correlation between the activity and the degree of oxidation of palladium, with oligomers exhibiting the highest activity. These findings provide new mechanistic insights into the influence of the carbon nitride structure on metal stabilization.

α-Substituted Benzylic Complexes of Palladium(II) as Precursors of Palladium Hydrides

The adoption of a pseudoallylic (η3) form makes palladium benzylic derivatives a class of stabilized palladium alkyls that can undergo β-H elimination reactions in a more controlled way. α-(Pentafluorophenylmethyl)benzyl palladium complexes have been studied, and they decompose by β-H elimination to give palladium hydrides that, depending on the auxiliary ligands, can: (a) transmetalate to another palladium atom and, by reductive elimination, give hydrogenated products; this process is favored for a combination of bridging ligands (i.e., halogens) and low coordinating ligands. (b) Be used as a hydride source and get trapped by a diene to give palladium allylic derivatives. The presence of carbon monoxide does not induce a β-H elimination reaction, and only CO insertion into the Pd–benzyl bond to give acyl derivatives is observed.

Elucidating the Role of the Boronic Esters in the Suzuki-Miyaura Reaction: Structural, Kinetic, and Computational Investigations.

The Suzuki-Miyaura reaction is the most practiced palladium-catalyzed, cross-coupling reaction because of its broad applicability, low toxicity of the metal (B), and the wide variety of commercially available boron substrates. A wide variety of boronic acids and esters, each with different properties, have been developed for this process. Despite the popularity of the Suzuki-Miyaura reaction, the precise manner in which the organic fragment is transferred from boron to palladium has remained elusive for these reagents. Herein, we report the observation and characterization of pretransmetalation intermediates generated from a variety of commonly employed boronic esters. The ability to confirm the intermediacy of pretransmetalation intermediates provided the opportunity to clarify mechanistic aspects of the transfer of the organic moiety from boron to palladium in the key transmetalation step. A series of structural, kinetic, and computational investigations revealed that boronic esters can transmetalate directly without prior hydrolysis. Furthermore, depending on the boronic ester employed, significant rate enhancements for the transfer of the B-aryl groups were observed. Overall, two critical features were identified that enable the transfer of the organic fragment from boron to palladium: (1) the ability to create an empty coordination site on the palladium atom and (2) the nucleophilic character of the ipso carbon bound to boron. Both of these features ultimately relate to the electron density of the oxygen atoms in the boronic ester.

Controlling Ligand Exchange through Macrocyclization.

Ligand exchange at a sterically hindered palladium center was investigated for six different ligands. The palladium atom was coordinated to a tridentate, NNN pincer bis(amido)pyridine macrocycle to produce a square-planar complex, in which an acetonitrile molecule occupies one of the coordination sites. Kinetic studies showed that ligand exchange at the palladium center proceeds through an associative mechanism and, as a consequence, is impeded by the small size of the metallomacrocycle cavity. The ligand-exchange rate on the palladium center between acetonitrile and six different ligands has been investigated and compared to the exchange rate on the corresponding open form. Our results demonstrate that macrocyclization of ligands is a way to modify the rate of guest exchange in a square-planar metal complex.

Reaction of PdCl2 with Diphenylacetylene Revisited: Effect of Solvents and Ball Milling on Cyclization and Crystal Structures of Dimeric Cyclobutenylpalladium Chloride Complexes

AbstractIn an attempt to establish the structures of the product formed in the reaction of PdCl2 and diphenylacetylene in polar organic solvents, the reactions were carried out in alcohols, acetonitrile and tetrahydrofuran at room temperature. The reactions in protic solvents such as primary and secondary alcohols resulted in the cyclic dimerization of diphenylacetylene to yield the corresponding dimeric endo‐4‐alkoxy‐π‐1,2,3,4‐tetraphenylcyclobutenypalladium(II) chloride complexes 1–4 in good yields. In these complexes the tetraphenylcyclobutenyl moieties are coordinated to palladium atoms in η3‐mode and are bridged by two chlorides as confirmed by the single crystal X‐ray analysis of complexes 1, 2 and 4. In all these complexes, the coordination of cyclobutenyl moieties to palladium atoms are similar but a marked difference was observed for the Pd2Cl2 core which is planar in complex 1 but puckered in complexes 2 and 4 resulting in shorter Pd⋅⋅⋅Pd distances. The same reaction under solvent free ball milling condition yielded hexaphenylbenzene by the cyclic trimerization while that in the presence of ethanol and n‐butanol produced complexes 1 and 3, respectively, as major and hexaphenylbenzene as minor product. An efficient catalytic synthesis of hexaphenylbenzene using PdCl2 was developed using ball milling condition which has several advantages over the existing catalytic systems.

Investigating the Reactivity of Single Atom Alloys Using Density Functional Theory

Single atom alloys are gaining importance as atom-efficient catalysts which can be extremely selective and active towards the formation of desired products. They possess such desirable characteristics because of the presence of a highly reactive single atom in a less reactive host surface. In this work, we calculated the electronic structure of several representative single atom alloys. We examined single atom alloys of gold, silver and copper doped with single atoms of platinum, palladium, iridium, rhodium and nickel in the context of the d-band model of Hammer and Norskov. The reactivity of these alloys was probed through the dissociation of water and nitric oxide and the hydrogenation of acetylene to ethylene. We observed that these alloys exhibit a sharp peak in their atom projected d-band density of states, which we hypothesize could be the cause of high surface reactivity. We found that the d-band centers and d-band widths of these systems correlated linearly as with other alloys, but that the energy of adsorption of a hydrogen atom on these surfaces could not be correlated with the d-band center, or the average reactivity of the surface. Finally, the single atom alloys, with the exception of copper–palladium showed good catalytic behavior by activating the reactant molecules more strongly than the bulk atom behavior and showing favorable reaction pathways on the free energy diagrams for the reactions investigated.

Palladium Complexes of N,N'-Bis(2-aminoethyl)oxamide (H2L): Structural (PdIIL, PdII2L2, and PdIVLCl2), Electrochemical, Dynamic 1H NMR, and Cytotoxicity Studies.

The monomeric (PdL·2H2O) and dimeric (Pd2L2·7H2O) palladium(II) complexes of N,N'-bis(2-aminoethyl)oxamide (H2L) were isolated, and their structures were established by single-crystal X-ray diffraction. Both compounds display identical cis-(2Namide + 2Namine) coordination environments of the metal ion. The dimer, representing a combination of two PdL species with an open lateral chelate ring, has an "open clamshell"-like structure. The intramolecular metal-metal separation in Pd2L2 (3.215 Å) is slightly shorter than the sum of the van der Waals radii of the palladium(II) atoms. The dimeric complex is relatively stable to dissociation, and its spectral features in aqueous solutions have been compared to those of the monomeric complex. A 1H NMR spectroscopic study revealed the presence of the dynamic conformational exchange process assigned to a turning of the dimeric molecule "inside out" with an activation energy of 65 kJ/mol. Cyclic voltammetry of PdL in perchlorate-, chloride-, and sulfate-containing electrolytes revealed two-electron oxidation of the palladium center. For the dimeric complex similar, though irreversible, oxidation to the palladium(IV) state was observed in NaCl electrolyte. At the same time, in NaClO4 or Na2SO4 solutions oxidation of Pd2L2 occurs in two distinct steps. The first step is quasi-reversible and can be assigned to the formation of species in an intermediate PdIIIPdIII state. Monomeric palladium(IV) complex PdIVLCl2 was generated via chemical oxidation of PdIIL by peroxodisulfate in the presence of chloride ions and structurally characterized. The related MIIL complexes (M = Pd, Ni, Cu) showed low cytotoxicity in human cancer cell lines AGS (gastric adenocarcinoma) and HCT116 (colorectal carcinoma) with IC50 values from 204 to 525 μM, while the proligand H2L was devoid of antiproliferative activity (IC50 > 1000 μM).

Dichlorido[1,1-(diphenylphosphino)hydrazide]palladium(II) dichloromethane monosolvate

In the title compound, [PdCl2(C24H22N2P2)]·CH2Cl2, the palladium(II) atom is surrounded by a chelating P—N—P ligand Ph2P—N(NH2)—PPh2 containing a hydrazine entity and two chlorido ligands in a distorted square-planar coordination geometry. Beside the complex molecule, the asymmetric unit contains a co-crystallized dichloromethane molecule. The contributions of further disordered solvent molecules were removed from the diffraction data using the SQUEEZE [Spek (2015). Acta Cryst. C71, 9–18] procedure.

Sc5Pd4Si6- crystal structure and 29Si/45Sc solid state MAS NMR spectroscopic investigations.

The silicide Sc5Pd4Si6 was synthesized from the elements by arc-melting. Its structure was refined from single crystal X-ray diffractometer data: Immm, Li5Cu3.75P6 type, a = 397.12(6), b = 945.4(1), c = 1314.4(2) pm, wR2 = 0.0245, 578 F2 values and 29 parameters. The palladium atoms have slightly distorted tetrahedral silicon coordination and a condensation of these PdSi4 tetrahedra leads to a two-dimensional substructure which is condensed via Si2 pairs (234 pm Si-Si), forming the [Pd4Si6]δ- polyanionic network. The Sc5Pd4Si6 structure contains three crystallographically independent scandium sites with coordinations Sc1@Pd4Si8Sc6, Sc2@Pd6Si6Sc3 and Sc3@Pd4Si6Sc4. Sc5Pd4Si6 is a Pauli paramagnet with a low susceptibility of 2.9(5) × 10-5 emu mol-1 at room temperature. The 29Si MAS-NMR spectrum confirms the presence of two crystallographically distinct sites in a 2 : 1 ratio. Likewise, the three crystallographic scandium sites are well-differentiated by three 45Sc MAS NMR signals in the expected 2 : 2 : 1 ratio. Unambiguous assignments could be made based on the comparison of the nuclear electric quadrupolar coupling parameters with predicted values from WIEN2k calculations.

Стабилизация атомов палладия в порфирино-подобных фрагментах углеродных нановолокон, допированных азотом

Стабилизация атомов палладия в порфирино-подобных фрагментах углеродных нановолокон, допированных азотом

Synthesis and structural characterization of well-defined bis(oxamato)palladate(II) precatalysts for Suzuki and Heck reactions

Abstract A family of tetra-n-butylammonium salts of bis(oxamato)palladate(II) complexes of formula (n-Bu4N)2[Pd(Lm)2]·pH2O [m = 1–9; L1 = N-phenyloxamate (pma) and p = 2 (1), L2 = N-2-methylphenyloxamate (2-Mepma) and p = 4 (2), L3 = N-4-methylphenyloxamate (4-Mepma) and p = 2 (3), L4 = N-2,3-dimethylphenyloxamate (2,3-Me2pma) and p = 3 (4), L5 = N-2,4-dimethylphenyloxamate (Me2pma) and p = 4 (5), L6 = N-2,5-dimethylphenyloxamate (2,5-Me2pma) and p = 7 (6), L7 = N-3,4-dimethylphenyloxamate (3,4-Me2pma) and p = 6 (7), L8 = N-3,5-dimethylphenyloxamate (3,5-Me2pma) and p = 5 (8)] have been prepared and characterized by spectroscopic methods. The crystal structure of 1 that has been solved by single crystal X-ray diffraction shares main structural features with that reported for (n-Bu4N)2[Pd(L9)2]·2CH3CONH2 [L9 = N-2,4,6-dimethylphenyloxamate (2,4,6-Me3pma) and CH3CONH2 = acetamide] (see F.R. Fortea-Perez et al. (2016)). Each palladium atom in 1 is four-coordinate with two oxygen and two nitrogen atoms from two fully deprotonated pma ligands building a centrosymmetric square-planar surrounding, the Pd N/Pd O bond lengths being 2.020(3)/2.003(2) A. The bis(oxamato)palladate(II) units in 1 are well separated from each other by the bulky organic cations, the resulting value of the shortest metal–metal separation being 9.56 A. This series of very stable, phosphine-free, complexes with well-defined structures were shown to efficiently catalyze the Heck reaction of aryl halides with olefins in DMF at 80 °C in 3 h, most likely involving Pd(II)/Pd(IV) species in the vinylation reaction of aryl halides, as well as the Suzuki coupling of aryl halides with arylboronic acids in the same solvent under similar conditions. The reaction conditions for both Heck and Suzuki couplings were optimized by varying the amount of the catalyst, and their scope was systematically studied by using different olefins and aryl halides. The influence of the number and positions of the methyl substituents from the phenyloxamate ligand on the catalytic activity is analyzed and discussed in the light of the results obtained.