Transition metal bonding functions and their applications in catalytic adsorptions and reactions: Part II. Micromechanism of CO chemisorption and a new concept of σ-π coordination

A micromechanism of CO adsorption and a new concept of σ-π coordination on transition metal are proposed in this article. Based on experimental facts, we assume CO 5σ- and/ or CO 1 π interacts with the representative M.O.s of the metal valence band, ψ(Mi, Vs) and ψ(Mi, Vd), to form the bonding M.O. group and antibonding M.O. group. The bonding group is located below the Fermi level (E_f), in which some M.O.s are much more characteristic of metal orbitais (denoted as M-CO σ-bondings) while some M.O.s exhibit slight metal orbital characteristics, which belong to the excited valence M.O.s of adsorbed CO, conventionally assigned as adsorbed CO 5σ, CO 1 π and CO 4σ. The calculated data indicate that the peak positions of adsorbed CO 5σ, CO 1 π and CO 4σ are significantly higher than their corresponding M.O.s in the gaseous CO molecule, i.e. adsorbed CO is in an excited (or activated) state. The total energy generated (ΔE) from adsorbed CO 5σ, CO 1 π and CO 4σ can be used as a qualitative parameter for characterizing the ability for CO dissociation. On the other hand, the antibonding empty M.O. group of M-CO is located above the E_f, which exhibits some characteristics of metal d orbitais. The hybridization of CO 2π with dπ- orbitais in the Vs, Vd bands and dπ orbitais of the antibonding M.O. group of M-CO bondings results in the formation of unoccupied M.O.s with CO 2π-M dπ character. These M.O.s plus those unoccupied M.O.s without CO 2π-M dπ character contribute the adsorbate-derived resonances, located 3-5 eV above E_F and observed by Inverse Photo-Emission (IPE) difference spectra. We have used orbital overlap integrals of S(CO 5σ, dσ, Vd) and S(CO 2π, dπ, Vd) to characterize the relative competitive abilities for hybridization of CO 5σ and CO 2π with d orbitais. The calculated results show that CO 5σ possesses a stronger ability to hybridize d orbitals in the Vd band than does CO 2π-, thus the peaks of adsorbate-induced empty levels are shifted farther from the d band when the competitive hybridizing factor [CHF=S(CO 5σ, dσ, Vd)/S(CO 2π, dπ, Vd)] is increased. The calculated data demonstrate that the peak positions of CO adsorbate-derived resonances of Cu, Ni, Pd and Pt metals, observed by IPE difference spectra, are in good parallel with their CHF values. Moreover, the values of CHE also demonstrate that CO σ-bonding stimulates d electrons to transfer upward from the d band to the Vs band, where much more CO 2π-M dπ character exists. We propose here a new concept of d back-donation, i.e. d electrons transfer from the occupied d band to the unoccupied M.O.s exhibiting CO 2π-M dπ character in the Vs and Vd bands, which weakens the π bond of C-O and simultaneously strengthens the M-C bond; these phenomena have been confirmed by IR spectroscopy and EELS. The d back-donation is represented by the B bonding function. The calculations of A, B and AB bonding functions indicate that the AB bonding function of CO adsorption on Cu is significantly smaller than that on Ni, Pd and Pt, so that CO adsorbtion is weak on Cu and is strong on Ni, Pd and Pt. Our micromechanism and our new concept of σ-π coordination provide a unified interpretation of various CO adsorption electronic spectra from below to above the E_F, i.e. from occupied orbitals to empty orbitals; and a unified interpretation of the adsorbate vibration spectra measured by EELS and IR spectroscopy. The advantages of our new concept have been discussed and compared with the conventional concepts of Blyholder and CO 2π-derived resonances.     

X-ray and filtered UV photoelectron spectroscopic study of CO absorbed on Ni(100) and (111) surfaces

The 4σ, 1π and 5σ orbitals, and possibly the 2π^* orbital, of CO adsorbed on (100) and (111) nickel surfaces, have been detected using both XPS and filtered UPS techniques. The 3σ level was detected only by XPS at ≈ 29 eV with a full width half-maximum of ≈ to 12 eV. The Cls and Ols binding energy shifts exhibit systematic differences between the two surfaces, being larger on the (111) surface.     

相转移法制备PdMo催化剂及其氧还原活性

以四丁基氢氧化铵作为相转移剂,以硼氢化钠为还原剂,利用相转移法在二氯甲烷中制备了一系列不同比例的Pd_xMo/C(Pd/Mo的原子比x=1、2、3、4、5)催化剂。透射电镜(TEM)图像显示,Pd_x Mo/C是呈2～4 nm的圆形颗粒,尺寸均匀、分散性良好。X射线衍射(XRD)结果表明,加入第二组元Mo后,Pd的晶格发生扩张,调节了 Pd的几何结构。此外,X射线光电子能谱(XPS)结果表明,相对于Pd/C,Pd_4Mo/C的Pd3d_(5/)2结合能负移了 0.50 eV,说明电负性较大的Pd从Mo吸电子,电子结构发生改变。氧还原反应(ORR)结果表明,不同比例的Pd_xMo/C催化剂活性均优于Pd/C,其中当x=4时,ORR活性最佳,其起始电位和半波电位分别为0.876和0.813 V,高于商业Pt/C的0.870和0.810 V。此外,在经过3 h的运行之后电流密度仍保留82.9%,与商业Pt/C相比具有明显的优势。     

Nitric oxide—heme binding model for nitrosylhemoglobin from its magnetic properties

A simple orbital model of the binding of NO to the heme iron in nitrosylhemoglobin is presented that fits the experimentally measured g factors and spin distribution. The NO π^☆ orbitals are split by approximately 4000 cm^?1 X hc such that the lower singly occupied member lies 8000 cm^?1 X hc above the filled Fe d orbitals. Binding of Fe to the N of NO is primarily of two comparable in-plane (xz) types, dσ—pσ with 23% d_z² character and dπ—pπ with 25% d_xz character. There is relatively less of the out-of-plane dπ—pπ and it involves almost no d_yz character.     

Two isomers of Pd2(S2NC7H4)4

The dichloromethane solvates of the isomers tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁴N:S;κ⁴S:N‐dipalladium(II)(Pd—Pd), (I), and tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁶N:S;κ²S:N‐dipalladium(II)(Pd—Pd), (II), both [Pd₂(C₇H₄NS₂)₄]·CH₂Cl₂, have been synthesized in the presence of (o‐isopropylphenyl)diphenylphosphane and (o‐methylphenyl)diphenylphosphane. Both isomers form a lantern‐type structure, where isomer (I) displays a regular and symmetric coordination and isomer (II) an asymmetric and distorted structure. In (I), sitting on an centre of inversion, two 1,3‐benzothiazole‐2‐thiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the other two benzothiazolethiolate units are bonded to the same Pd atoms by, respectively, a Pd—S and a Pd—N bond. In (II), three benzothiazolethiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the fourth benzothiazolethiolate unit is bonded to the same Pd atoms by, respectively, a Pd—S bond and a Pd—N bond.     

Barium as Honorary Transition Metal in Action: Experimental and Theoretical Study of Ba(CO)+ and Ba(CO)−

Ba(CO)⁺ and Ba(CO)^? have been produced and isolated in a low‐temperature neon matrix. The observed C?O stretching wavenumber for Ba(CO)⁺ of 1911.2 cm^?1 is the most red‐shifted value measured for any metal carbonyl cations, indicating strong π backdonation of electron density from Ba⁺ to CO. Quantum chemical calculations indicate that Ba(CO)⁺ has a ²Π reference state, which correlates with the ²D(5d¹) excited state of Ba⁺ that comprises significant Ba⁺(5d_π¹)→CO(π* LUMO) backbonding, letting the Ba(CO)⁺ complex behave like a conventional transition‐metal carbonyl. A bonding analysis shows that the π backdonation in Ba(CO)⁺ is much stronger than the Ba⁺(5d_σ/6s)←CO(HOMO) σ donation. The Ba⁺ cation in the ²D(5d¹) excited state is a donor rather than an acceptor. Covalent bonding in the radical anion Ba(CO)^? takes place mainly through Ba(5d_π)←CO^?(π* SOMO) π donation and Ba(5d_σ/6s)←CO^?(HOMO) σ donation. The most important valence functions at barium in Ba(CO)⁺ cation and Ba(CO)^? anion are the 5d orbitals.     

cis‐μ‐Chloro‐μ‐p‐toluene­thiol­ato‐bis­[chloro­(triethyl­phos­phine)­palladium]

The title compound, cis‐[Pd₂Cl₃(C₇H₇S)(C₆H₁₅P)₂], has bridging chloro and aryl­thiol­ato groups, with the phosphines being trans to the bridging chloro group. The four‐membered metallocyclic Pd₂ClS ring is unexpectedly non‐planar, with a dihedral angle of 133.8 (1)° between the PdCl₂SP coordination planes. Principal dimensions include Pd—Cl_t 2.316 (3) and 2.329 (3), Pd—Cl_b 2.442 (3) and 2.432 (3), Pd—S 2.280 (3) and 2.282 (3), and Pd—P 2.233 (3) and 2.236 (3) Å (where Cl_t and Cl_b are terminal and bridging chloro ligands, respectively).     

Semiempirical configuration interaction calculations of XPS shake-up satellites in Ni(CO)4

INDO/CI calculations were used to analyze the C1s and O1s shake-up spectra of nickel tetracarbonyl, Ni(CO)₄. The satellite structure in both cases is dominated by excitations from metal–ligand bonding (2Π_b) to metal–ligand antibonding (2Π_a) orbitals and by excitations within the core-ionized CO molecule, Π_CO—Π*_CO. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 649–657, 1998     

UV photoelectron spectroscopy of matrix-isolated and condensed CO and N2

UV photoelectron spectra of matrix-isolated and condensed CO and N₂ have been measured. The emission from the 4σ, 1π and 5σ orbitals show similar relaxation shifts of about 1 eV. This shift is 0.3 eV larger for the species isolated in xenon matrices. The width of the peaks is smaller by 0.3 eV in the Xe matrix in comparison with the pure solids but remains larger than those measured for the gas phase.     

Electronic Modulation of Palladium in Metal Phosphide Nanoparticles for Chemoselective Reduction of Halogenated Nitrobenzenes

Tuning the electronic property of a transition metal plays an important role in the selective catalysis. Herein, the control synthesis of (Pd_xNi_y)‐P nanoparticles is reported. The binding energy of Pd3d_5/2 as a function of x/y ratio is well tunable from 335.3 to 335.9 eV. The composition‐induced electronic modulation was correlated with the selective catalysis of (Pd_xNi_y)‐P in the reduction of halogenated nitrobenzenes. The electro‐deficiency of Pd helped to improve the selectivity. The amorphous (Pd₃₈Ni₂₆)P₃₆/C performed an exceptional selectivity in comparison with other related (Pd‐Ni)‐P/C, Pd₃₈Ni₂₆/C, and Pd/C. Various halogenated nitrobenzenes (chlorides, bromides, and iodide) were tolerant and the corresponding halogenated anilines were obtained in high yields. This work provides some clues for the rational design of bimetallic phosphides with covalent interactions to boost the catalysis.     

Density functional theory study of p‐chloroaniline adsorption on Pd surfaces and clusters

The adsorption mode of aromatic molecules on transition metal surfaces plays a key role in their catalytic transformation. In this study, by means of density functional theory calculations, we systematically investigate the adsorption of p‐chloroaniline on a series of Pd surfaces, including stepped surfaces, flat surfaces, and clusters. The adsorption energies of p‐chloroaniline on these substrates [Pd(221), Pd(211), Pd(111), Pd(100), Pd₁₃‐icosahedral, Pd₁₃‐cubo‐octahedron, Pd₅₅] are ?1.90, ?2.13, ?1.70, ?2.11, ?2.53, ?2.65, ?2.23 eV, respectively. Benzene ring is adsorpted on catalyst rather than amine group in p‐chloroaniline molecular. A very good linear relationship is further found between the adsorption energies of p‐chloroaniline and the d‐band center of both Pd surfaces and clusters. The lower of d‐band center of Pd models, the stronger adsorption of p‐chloroaniline on catalysts. In addition, the frontier molecular orbital and density of states analysis explain the adsorption energy sequence: cluster Pd₁₃ > stepped Pd(221) surface > flat Pd(111) surface. © 2014 Wiley Periodicals, Inc.     

Interaction of NO molecules with Pd clusters: Ab initio density–functional study

The adsorption of NO molecules on small Pd_n (n = 1?6) clusters has been studied using first‐principles density‐functional theory. Three adsorption sites were considered: vertex (on–top), bridge, and hollow. Adsorption is strong, ranging from 2 to 3 eV. In all cases NO adsorbs in a bent configuration. Calculated shifts in N–O bond vibration frequencies (with anharmonic corrections) agree very well with available experimental data. In contrast to metallic Pd surfaces, adsorption of NO on palladium clusters causes considerable changes in geometry around adsorption site because palladium d‐orbitals rehybridize to maximize the overlap with NO orbitals (mainly the antibonding π*). Thus, the overall energetic effect of NO adsorption is the result of two competing processes: lowering of the total energy through tighter bonding with NO and rising the energy due to cluster deformation. The Pd_n–NO bond creation is governed by electron transfer from Pd–d orbitals into the NO π*. As a result, the Pd cluster becomes locally demagnetized (with total magnetic moment of 1 μ_B located at Pd atoms not connected to NO) and the NO molecule is activated: the N–O bond length is increased and the vibration frequency is redshifted. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Preparation and characterization of Pd modified CeO2 nanoparticles for photocatalytic degradation of dye

The aim of this work was to investigate the structural and optical properties of bare cerium dioxide (CeO₂) and Pd-doped CeO₂ (0.5, 1.0, 1.5 and 2.0 wt%) photocatalysts prepared by a combination of homogeneous precipitation and the impregnation method. X–ray diffraction analysis indicated that all samples were composed of the cubic fluorite phase of CeO₂. Scanning electron micrographs revealed that all samples provided mostly spherical morphology with high agglomeration and estimated particle sizes ranging from 10 to 20 nm in diameter. The XPS core-level spectra of Pd species after incorporating 2.0 wt% Pd–doped CeO₂ showed double peaks with binding energies of Pd3d_5/2 and Pd3d_3/2 corresponding to the Pd²⁺ oxidation state. The results from diffuse reflectance UV–visible spectroscopy showed that doping with Pd increased the absorbance onset of CeO₂ to a longer wavelength, while the band gap decreased from 3.0 eV to 2.8 eV with 2.0% Pd doping concentration. This was likely due to the creation of impurity levels of Pd²⁺ inside the conduction and valence bands of CeO₂. The photoluminescence spectra (PL) indicated that the emission peak intensity of CeO₂ decreased in the presence of Pd²⁺ dopant in CeO₂. This was associated with a decrease in the electron–hole recombination rate for electronically-excited. Photocatalytic activity for methyl orange dye degradation under visible light irradiation of 1.0 wt% Pd–doped CeO₂ was determined as the optimal doping level with photocatalytic activity 5 times higher than that of bare CeO₂ photocatalyst.     

Synthesis of palladium-tin carbonylphosphine clusters and X-ray study of the Pd3Sn2(acac)4(CO)2(PPh3)3 cluster

It has been shown for the first time that the reaction of bi-valent tin acetyl-acetonate with palladium carbonylphosphine clusters, Pd₄(CO)₅(PPh₃)₄ (I), Pd₄(CO)₅(PEt₃)₄ (II) and Pd₃(CO)₃(PPh₃)₄ (III), results in the formation of heterometal pentanuclear clusters of general formula Pd₃Sn₂(acac)₄(CO)₂(PR₃)₃; R  Ph (IV), Et (V). X-ray analysis of Pd₃Sn₂(acac)₄(CO)₂(PPh₃)₃ at 20°C (λ(Mo), 4396 reflections, space group P2₁/n, Z = 4, R = 0.037) shows that IV in the form of the crystalline hydrate, Pd₃Sn₂(acac)₄(CO)₂(PPh₃)₃ · χH₂O (χ ∼ 1), contains a distorted “propeller”-shaped Pd₃Sn₂ metal frame with PdSn distances of 2.679–2.721(1) Å; two short PdPd bonds, 2.708 and 2.720(1) Å, bridged by μ₂-CO ligands, and an elongated central Pd(1)Pd(2) bond of 2.798 Å. Sn atoms have distorted octahedral coordination, the dihedral angles formed by Pd₃ moieties and two Pd₂Sn triangles are 127.6 and 106.5°; and the angle between Pd₂Sn moieties is 126.0°.     

Theoretical Study of Pd11Si6 Nanosheet Compounds Including Seven‐Coordinated Si Species and Its Ge Analogues

Nanosheet compounds Pd₁₁(SiiPr)₂(SiiPr₂)₄(CNtBu)₁₀ ( 1 ) and Pd₁₁(SiiPr)₂(SiiPr₂)₄(CNMes)₁₀ ( 2 ), containing two Pd₇(SiiPr)(SiiPr₂)₂(CNR)₄ plates (R=tBu or Mes) connected with three common Pd atoms, were investigated with DFT method. All Pd atoms are somewhat positively charged and the electron density is accumulated between the Pd and Si atoms, indicating that a charge transfer (CT) occurs from the Pd to the Si atoms of the SiMe₂ and SiMe groups. Negative regions of the Laplacian of the electron density were found between the Pd and Si atoms. A model of a seven‐coordinated Si species, that is, Pd₅(Pd?SiMe), is predicted to be a stable pentagonal bipyramidal molecule. Five Pd atoms in the equatorial plane form bonding overlaps with two 3p orbitals of the Si atom. This is a new type of hypervalency. The Ge analogues have geometry and an electronic structure similar to those of the Si compounds. But their formation energies are smaller than those of the Si analogues. The use of the element Si is crucial to synthesize these nanoplate compounds.     

Alloyed Tetranuclear Metal Chains of Pd4−nPtn (n=0–3) Scaffolded by a New Linear Tetraphosphine Containing a PNP Bridge

A new linear tetraphosphine containing a PNP phosphazane bridge, rac-bis[(diphenylphosphinomethyl)phenylphosphino]phenylamine (rac-dpmppan), was synthesized and utilized to support a series of Pd/Pt mixed metal tetranuclear chains, [Pd_4−nPt_n(μ-rac-dpmppan)₂(XylNC)₂](PF₆)₂ (XylNC=xylyl isocyanide; n=0: Pd₄ ( 1 ), 1: PtPd₃ ( 2 ), 2: PtPd₂Pt ( 3 ), 2: Pt₂Pd₂ ( 4 ), 3: Pt₂PdPt ( 5 )), in which the number and positions of additional Pt atoms were successfully controlled depending on the respective synthetic procedures using transformations from 1 to 3 through 2 and from 4 to 5 by redox-coupled exchange reactions. The ³¹P{¹H} NMR and ESI mass spectra and X-ray diffraction analyses revealed almost identical tetranuclear structures, with slight contraction of metal-metal bonds according to incorporation of Pt atoms. The electronic absorption spectra of 1 – 5 exhibited characteristic bands at 635–510 nm with an energy propensity depending on the number and positions of Pt centres, which were assigned to HOMO (dσ*σσ*) to LUMO (dσ*σ*σ*) transition by theoretical calculations. The present results demonstrated that the electronic structures of Pd/Pt mixed-metal tetranuclear complexes are finely tuned as orbital-overlapping alloyed metal chains by atomically precise Pt incorporation in the Pd₄ chain.     

Homoleptic Platinum(II) and Palladium(II) Organothiolates and Phenylselenolates: Solvothermal Synthesis,Structural Determination,Optical Properties,and Single‐Source Precursors for PdSe and PdS Nanocrystals

Homoleptic d⁸‐metal organothiolates and phenylselenolates [M(EC₆H₅)₂]_∞ (E=S, M=Pt 1 , M=Pd 2 , M=Ni 5 ; E=Se, M=Pt 3 , M=Pd 4 ) were prepared as crystalline solids under solvothermal conditions. Their structures were solved using powder X‐ray diffraction data. In each case, the EC₆H₅ (E=S, Se) ligand binds to two metal ions (M=Pt, Pd, and Ni) to form chain‐like structures with planar (in 1 ) or zig‐zag (in 2 , 3 , 4 , 5 ) conformations. The [M(SR)₂]_∞ complexes (M=Pt, R=4‐tert‐butylphenyl 6 ; R=2‐naphthyl 8 ; R=4‐nitrophenyl 10 and M=Pd, R=4‐tert‐butylphenyl 7 ; R=2‐naphthyl 9 ; R=4‐nitrophenyl 11 ) were prepared under similar solvothermal conditions. Based on the XPS binding energies and elemental analyses, complexes 6 , 7 , 8 , 9 , 10 , 11 have the same [M(SR)₂]_∞ formulation as 1 and 2 . The cyclic complex [Pd₆(SCH₃)₁₂] 12 was prepared as a crystalline solid by solvothermal annealing treatment of the amorphous precipitate. A chain‐like polymer structure is proposed for both [Pd(SC₁₂H₂₅)₂]_∞ 13 and [Pd(SC₁₆H₃₃)₂]_∞ 14 ; these polymeric chains self‐assemble to give layer‐like structures. Solid‐state diffuse reflectance spectra reveal that the optical band gap E_g (eV) of complexes 1 , 6 , 8 , 10 and of 2 , 7 , 9 , 11 are in the range of 2.10–3.00 eV and 2.10–2.63 eV, respectively, and 5 has the lowest E_g value (1.72 eV). Heating solid samples of 4 and 13 under solvothermal conditions afforded phase‐pure Pd₁₇Se₁₅ and PdS nanocrystals, respectively. Field‐effect transistors fabricated with a drop‐cast thin film made from Pd₁₇Se₁₅ nanocrystals prior treated with an ethanolic solution of 1‐hexadecanethiol displayed ambipolar charge transporting properties with hole and electron mobility being 7×10^?2 cm² V^?1 s^?1 and 6×10^?2 cm² V^?1 s^?1, respectively.     

n→π*-Übergänge in Dicarbonylverbindungen Der Einfluss der n,n- und π*, π*-Wechselwirkung

By CNDO-CI calculations we have found that dicarbonyl compounds exhibit only two n → π* transitions in the visible or near UV. region, instead of four as expected from simpler MO-models. The dominant features of the long-wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest π* orbitals. In general we distinguish between three cases:

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CASSCF study of bonding in NiCO and FeCO

A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the σ/π interactions are checked by examining the validity of the CASSC calculation to describe the molecule with a particular choice of the active space. The calculation results substantiate that the stability of MCO is determined by a balance between π donation from the metal 3d_π to the CO 2π and repulsion between the metal σ electrons and the CO 5σ lone pair and, at the same time, emphasizes the importance of the synergistic σ/π interactions between the metal and the CO group. The relative importance of σ/π interactions depends on the nature of the metal. In the case of NiCO, it is the π donation from Ni 3d_π to CO 2π that makes the largest contribution to the formation of the Ni CO bond, while in the case of FeCO, it is the correlation of σ electrons that holds the metal and CO together. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 221–231, 1999     

Oxidation of CO Catalyzed by a Cu Cluster: Influence of an Electric Field

Adsorption ability and reaction rate are two essential parameters that define the efficiency of a catalyst. Herein, we implement density functional theory (DFT) and report that CO can be oxidized by a pyramidal Cu cluster with an associated reaction barrier E_b=1.317 eV. In this case, our transition state calculations reveal that the barrier can be significantly lowered after superimposing a negative electric field. Moreover, when the field intensity corresponds to F=?0.010 au, the magnitude of E_b=0.698 eV is equivalent to—or lower than—those of typical catalysts such as Pt, Rh, and Pd. The superimposition of a positive field is found to enhance the release of the nascent CO₂ molecule. Our study demonstrates that small Cu clusters have better adsorption ability than the corresponding flat surface while the field can be used to enhance the purification of the exhaust gas.