Quantum chemical calculations of stability constants: study of ligand effects on the relative stability of Pd(II)�Cpeptide complexes

Replacement of [Pd(H₂O)₄]²⁺ by cis-[Pd(en)(H₂O)₂]²⁺, [PdCl₄]^2?, and [Pd(NH₃)₄]²⁺ on the hydrolytic cleavage of the Ace-Ala-Lys-Tyr-Gly?CGly-Met-Ala-Ala-Arg-Ala peptide is theoretically investigated by using different quantum chemical methods both in the gas phase an in water solution. First, we carry out a series of validation calculations on small Pd(II) complexes by computing high-level ab initio [MP2 and CCSD(T)] and Density Functional Theory (B3LYP) electronic energies while solvent effects are taken into account by means of a Poisson-Boltzmann continuum model coupled with the B3LYP method. After having assessed the actual performance of the DFT calculations in predicting the stability constants for selected Pd(II)-complexes, we compute the relative free energies in solution of several Pd(II)?Cpeptide model complexes. By assuming that the reaction of the peptide with cis-[Pd(en)(H₂O)₂]²⁺, [Pd(Cl)₄]^2?, and [Pd(NH₃)₄]²⁺ would lead to the initial formation of the respective peptide-bound complexes, which in turn would evolve to afford a hydrolytically active complex [Pd(peptide)(H₂O)₂]²⁺ through the displacement of the en, Cl^?, and NH₃ ligands by water, our calculations of the relative stability of these complexes allow us to rationalize why [Pd(H₂O)₄]²⁺ and [Pd(NH₃)₄]²⁺ are more reactive than cis-[Pd(en)(H₂O)₂]²⁺ and [PdCl₄]^2? as experimentally found.     

Theoretical insights into the relative bonding of normal and abnormal N‐heterocyclic carbenes in [PdCl2(NHCR)2] and [PdCl2(NHCR)(aNHCR)] (R = H,Ph, Mes)

Quantum chemical insights into normal Pd‐C2(NHC^R) and abnormal Pd‐C5(aNHC^R) bonding, dominated by dispersion interactions in N‐hetereocyclic carbene complexes [PdCl₂(NHC^R)₂] ( I , R = H; II , R = Ph; III , R = Mes (2,4,6‐trimethyl)phenyl)) and [PdCl₂(NHC^R)(aNHC^R] ( IV , R = H; V , R = Ph; VI , R = Mes) have been investigated at DFT and DFT‐D3(BJ) level of theory with particular emphasis on the effects of the noncovalent interactions on the structures and the nature of Pd‐C2(NHC^R) and Pd‐C5(aNHC^R) bonds. The optimized geometries are good agreement with the experimental values. The Pd‐C bonds are essentially single bond. Hirshfeld charge distributions indicate that the abnormal aNHC^R carbene ligand is relatively better electron donor than the normal NHC^R carbene ligand. The C2 atom has larger %s contribution along Pd‐C2 bond than the C5 atom along Pd‐C5 bond. As a consequence the Pd‐C2(NHC^R) bonds are relative stronger than the Pd‐C5(aNHC^R) bonds. Thus, the results of natural hybrid orbital analysis support the key point of the present study. Calculations predict that for bulky substituent (R = Ph, Mes) at carbene, the Pd‐C2(NHC^R) bond is stronger than Pd‐C5(aNHC^R) bond due to large dispersion energy in [PdCl₂(NHC^R)₂] than in [PdCl₂(NHC^R)(aNHC^R)]. However, in case of non‐bulky substituent with small and almost equal contribution of dispersion energy, the Pd‐C2(NHC^R) bond is relative weaker than Pd‐C5(aNHC^R) bond. The bond dissociation energies are dependent on the R substituent, the DFT functional and the inclusion of dispersion interactions. Major point of this study is that the abnormal aNHCs are not always strongly bonded with metal center than the normal NHCs. Effects of dispersion interaction of substituent at nitrogen atoms of carbene ligand are found to play a crucial role on estimation of relative bonding strengths of the normal and abnormal aNHCs with metal center. © 2016 Wiley Periodicals, Inc.     

A study of palladium recovery with thiourea from nitric acid solutions

Interaction in the system Pd(II)-HNO₃-SC(NH₂)₂ was studied. It was shown that the precipitates formed, which have various compositions and solubilities, contain palladium(II) and thiourea conversion products. Elemental, X-ray diffraction, and atomic-adsorption analyses and IR and electronic absorption spectroscopies were applied to demonstrate the possibility of virtually quantitative precipitation of palladium(II) under certain conditions in the form of a poorly soluble precipitate Pd₂(CN)₄ · H₂O.     

Influence of Pd precursors on the catalytic performance of Pd–H4SiW12O40/SiO2 in the direct oxidation of ethylene to acetic acid

The effects of palladium precursors (PdCl₂, (NH₄)₂PdCl₄, Pd(NH₃)₂Cl₂, Pd(NO₃)₂ and Pd(CH₃COO)₂) on the catalytic properties in the selective oxidation of ethylene to acetic acid have been investigated for 1.0 wt% Pd–30 wt% H₄SiW₁₂O₄₀/SiO₂. The structures of the catalysts were characterized using X-ray diffraction, N₂ adsorption, H₂-pulse chemical adsorption, infrared spectrometry of the adsorbed pyridine, H₂ temperature-programmed reduction and X-ray photoelectron spectroscopy. The present study demonstrates that the different palladium precursors can lead to the significant changes in the dispersion of palladium. It is found that Pd dispersion decreases as follows: PdCl₂ > (NH₄)₂PdCl₄ > Pd(NO₃)₂ > Pd(NH₃)₂Cl₂ > Pd(C₂H₃O₂)₂, which is nearly identical to the catalytic activity. This indicates that the dispersion of palladium plays an important role in the catalytic activity. Furthermore, density of Lewis (L) and Brönsted (B) acid sites are also strongly dependent on the palladium precursors. It is also demonstrated that an effective catalyst should possess a well combination of Brönsted acid sites with dispersion of palladium.     

(CH3NH3)2PdCl4: A Compound with Two‐Dimensional Organic–Inorganic Layered Perovskite Structure

The synthesis of previously unknown perovskite (CH₃NH₃)₂PdCl₄ is reported. Despite using an organic cation with the smallest possible alkyl group, a 2D organic–inorganic layered Pd‐based perovskites was still formed. This demonstrates that Pd‐based 2D perovskites can be obtained even if the size of the organic cation is below the size limit predicted by the Goldschmidt tolerance‐factor formula. The (CH₃NH₃)₂PdCl₄ phase has a bulk resistivity of 1.4 Ω cm, a direct optical gap of 2.22 eV, and an absorption coefficient on the order of 10⁴ cm^?1. XRD measurements suggest that the compound is moderately stable in air, an important advantage over several existing organic–inorganic perovskites that are prone to phase degradation problems when exposed to the atmosphere. Given the recent interest in organic–inorganic perovskites, the synthesis of this new Pd‐based organic–inorganic perovskite may be helpful in the preparation and understanding of other organic–inorganic perovskites.     

Spectroscopic evidence for a delocalized excited state in the magnus salts

The polarized single-crystal spectra of the Magnus salts [Pt(NH₃)₄][PtCl₄], [Pd(NH₃)₄] [PtCl₄], [Pt(NH₃)₄] [PdCl₄], and [Pd(NH₃)₄][PdCl₄] have been obtained in the quartz ultraviolet and visible regions. These results provide strong evidence for a delocalized excited state being associated with the intense ultraviolet transition found in the spectra of the Magnus salts. One thus has available an important new criterion for the evaluation of the theoretical treatments of these systems.     

X-ray structure and theoretical studies on a palladium(II) Schiff base complex

Treatment of N,N′-bis(salicylidene)-1,2-cyclohexanediamine (H₂L) with PdCl₂ in the presence of triethylamine afforded [Pd(N₂O₂)]. Recrystallization in chloroform and acetonitrile (1?:?1) gave suitable crystals for X-ray crystallography. The solid-state structure shows that the environment around palladium is square planar. The structural parameters of the molecule obtained by density functional theory (DFT) calculation in the gas phase and by X-ray diffraction are compared. The Pd(II) Schiff base complex adopts planar geometry by DFT calculation. The coordination site structural parameters, which are obtained from geometry optimization calculation, are close to those from X-ray crystallographic data. The spectral properties such as vibrational frequencies, chemical shifts, electronic excitation and the natural bond orbital analyses of Pd(Salen) are calculated, analyzed and compared with experimental data.     

Transfer of the chelating ligands in the systems [LAuCl2]+-[PdCl4]2−: Synthesis and structure of the salt (NH4)0.20[(Bipy)AuCl2]1.04[(Bipy)PdCl2]0.96[AuCl4]0.76[PdCl4]0.24

The formation of binary complex salts containing gold(III) in the cation and palladium(II) in the anion in the systems [(Bipy)AuCl₂]⁺-[PdCl₄]^2? occurs by transfer of the N,N-electron-donating chelating ligand bipyridine and the chloride ligands between the gold-containing cation and the palladium-containing anion. The resulting neutral salt [(Bipy)PdCl₂] crystallizes together with the anion [AuCl₄]^? from acetonitrile-water (1 : 1-1 : 2, v/v) to give the complex salt (NH ₄ ⁺ )_0.20[(Bipy)AuCl ₂ ⁺ ]_1.04[(Bipy)PdCl₂]_0.96[AuCl ₄ ^? ]_0.76PdCl ₄ ^2? ]_0.24 with a total Au : Pd ratio of 3 : 2. The ammonium cation is formed from acetonitrile upon its hydrolysis most likely catalyzed by Pd complexes. Quantum-chemical calculations were performed to study the transfer of the chelating ligand theoretically.     

Different ways of coordination for aminoalkyldiphosphonic acids in palladium(II) complexes

Complexation in the systems K₂PdCl₄-3-amino-1-hydroxypropane-1,1-diyldiphosphonic acid (AHPDP) and K₂PdCl₄-1-aminoethane-1,1-diyldiphosphonic acid (AEDP) was studied using pH-potentiometry, spectrophotometry, and ³¹P NMR spectroscopy. It was found that AHPDP in equimolar complexes is coordinated by Pd(II) in a bidentate fashion through two phosphonate O atoms forming a six-membered chelate [O,O]-ring, while AEDP is coordinated through the N atom and one phosphonate O atom forming a five-membered chelate [N,O]-ring. In 1: 2 complexes of Pd(II) with AEDP, the second AEDP molecule is coordinated by Pd(II) in a similar way to form a second five-membered [N,O]-ring. The latter complexes show cis-trans and syn-anti isomerism. The structural characteristics of the corresponding isomers were calculated at the DFT level of theory and their stabilities were compared.     

Zum Mechanismus des thermischen Abbaus von Ammonium- und Ammin-Chlorokomplexen des Rutheniums,Rhodiums, Palladiums und Platins

On the Mechanism of the Thermal Decomposition of Ammonium and Ammin Chloro Complexes of Ruthenium, Rhodium, Palladium and Platinum The thermolysis of noble metal compounds was investigated by a combination of independent physical (DTA/TG, high-temperature Guinier-Simon-technique) and preparative methods, characterization of the educts, intermediats and products by Guinier X-ray patterns and IR spectroscopy:

• (a) (NH₄)₂[PtCl₆], (NH₄)₂[PtCl₄], [Pt(NH₃)₄][PtCl₄], [Pt(NH₃)₄]Cl₂, and trans-[Pt(NH₃)₂Cl₂];
• (b) (NH₄)₂[PdCl₄], [Pd(NH₃)₄]Cl₂, trans-[Pd(NH₃)₂Cl₂], and [Pd(NH₃)₄][PdCl₄];
• (c) (NH₄)₂[RhCl₅(H₂O)], (NH₄)₃[Rh₂Cl₉], K₂[RhCl₅(H₂O)], and K₂[RhCl₅(NH₃)];
• (d) (NH₄)₂[RuCl₅(NH₃)] and (NH₄)₄[Ru₂Cl₁₀O]

. The most important step of the mechanism of the thermal decomposition is the ?internal”? redox reaction between the noble metal cation and the nitrogen atom of the ammine ligand.     

Crystal structure of diammine(malonato)palladium(II)

Diammine(malonato)palladium(II) is synthesized by the reaction of [Pd(NH₃)₄](NO₃)₂ with malonic acid, and its crystal structure is determined by single crystal X-ray diffraction. Distorted coordination square of the Pd(II) atom is formed by two N atoms of two ammonia molecules and two O atoms of bidentate malonate ligand. The average Pd-N and Pd-O distances are 2.018(7) ? and 2.014(2) ?, respectively. The molecules are stacked in such a way that the planes of coordination squares are parallel with the Pd...Pd distances between the nearest neighbors in a stack of 4.039 ?.     

Theoretical study of the protonation of square-planar palladium(II) complexes. Assessment of basis set and correlation effects

The protonation of the [Pd(H)₂(Cl)(NH₃)]^– and [Pd(H)₂(NH₃)₂] taken as models of anionic and neutral square-planard ⁸ palladium complexes is investigated through SCF, MP2, MP4, CASSCF and CASPT2 calculations, using various basis sets on the metal and the ligands. It is shown that correlation effects, mainly those associated with the covalent character of the metal hydrogen and metal ligand bonds, are important. The importance of diffuse functions on the ligands, especially for the anionic system, is stressed.     

Identification of Pt(II) and Pd(II) stereoisomeric complexes with aminobutyric acid by NMR and IR spectroscopy

Complexes of Pd(II) with aminobutyric acid AmH = NH₂CH(CH₂CH₃)COOH, namely, trans-[Pd(AmH)₂Cl₂] with monodentate (via the NH₂ group) AmH ligands and cis-, trans-Pd(Am)₂ with bidentate (via NH₂ and COO groups) ligands have been synthesized for the first time. Elemental analysis and IR and NMR spectroscopy were used to identify the synthesized compounds. The NMR spectra of the Pd(II) complexes were interpreted by comparing them with the NMR spectra of the analogous complexes of Pt(II). For Pt(II) and Pd(II) complexes with aminobutyric acid used as examples, an approach to identification of diastereomer bis-aminoacid complexes in specimens with racemic aminoacids by NMR spectroscopy is demonstrated.     

Metal Complexes for the Vinyl Addition Polymerization of Norbornene: New Compound Classes and Activation Mechanism with B(C6F5)3/AlEt3

Classes of mainly nickel(II) and palladium(II) complexes are comparatively presented in their norbornene polymerization activity to vinyl polynorbornene when activated with methylalumoxane, MAO, tris(pentafluorophenyl)borane/triethylaluminum, B(C₆F₅)₃/AlEt₃ or even B(C₆F₅)₃ alone. Classes include Ni and Pd complexes with α-dioxime ligands, salts with [PdCl₄]²⁻ and [Pd₂Cl₆]²⁻ units, dinuclear Ni and Pd complexes with multidentate Schiff-base ligands, polynuclear Ni- and Cr/Ni-carboxylate cage complexes, and dihalo(bisphosphane) Ni and Pd complexes. The study of activation mechanism by ³¹P- and ¹⁹F-NMR together with X-ray structural data points to the formation of PdCl₂ units and “naked” Pd²⁺ cations as highly active species.     

炭载Pd-Fe合金催化剂的制备及电催化氧还原活性

研究了用NH4Cl作配位剂的配位还原法来制备的Pd-Fe/C催化剂,发现由于NH4Cl能与Pd形成配合物,使Pd Cl2的还原电位负移,与Fe Cl3的还原电位接近,从而在低温下制备得到了高合金化程度的Pd-Fe/C催化剂。XPS表征结果表明:Pd与Fe形成合金后,Pd的电荷密度的减少,增加了Pd0的含量。因此,得到的Pd-Fe/C催化剂对氧还原的电催化活性比用相同方法制得的Pd/C催化剂高,而且该催化剂对甲醇氧化没有电催化活性。     

Effect of mechanicochemical treatment on the activity of K<Subscript>2</Subscript>PdCl<Subscript>4</Subscript> in the heterogeneous catalytic hydrochlorination of acetylene

A heterogeneous catalyst for the hydrochlorination of acetylene using gaseous HCl was obtained by prior mechanical activation of K₂PdCl₄ powder in an atmosphere of acetylene or propylene. Active sites are formed during the mechanical treatment in the surface layers of the catalyst, which are Pd(II) complexes with a coordination vacancy.     

Synthèse et analyse radiocristallographique de la solution solide K2PdCl4–xBrx (0 < x < 4)

Preparation and crystal structure determination (by X-ray single crystal measurements) of the K₂PdCl_2.81Br_1.19, K₂PdCl_2.64Br_1.36, K₂PdCl_2.41Br_1.59, K₂PdCl_1.73Br_2.27, and K₂PdCl_0.99Br_3.01 compounds are reported. All these compounds crystallise in the same centrosymmetric space group P4/mmm. The tetragonal cell (a ≈ 7.3 Å; c ≈ 4.2 Å) contains one Pd atom, which lies on the C4 axis. The square-planar 〚PdX₄〛^2– ions (X = Cl, Br) are disordered. The Pd–X distances (2.348(2), 2.356(2), 2.362(2), 2.383(1), and 2.422(2) Å) have intermediate values between those previously observed for Pd–Cl (2.313 Å) in the K₂PdCl₄ compound and Pd–Br (2.444 Å) in the K₂PdBr₄ compound.     

Catalytic Synthesis of 2-Methyl-1,4-Naphthoquinone (Vitamin K3) Over Silica-Supported Aminomethyl Phosphine-Ru(II), Pd(II), and Co(II) Complexes

Ru(II), Pd(II), and Co(II) complexes of the free ditertiary aminomethylphosphine ligand, N,N-bis(diphenylphosphinomethyl)aminopropyltriethoxysilane [(EtO)₃Si(CH₂)₃ N(CH₂PPh₂)₂] (DIPAPTES), and its SiO₂-DIPAPES have been synthesized under a nitrogen atmosphere using Schlenk techniques. All the complexes were used as catalysts for the oxidation of 2-methyl naphthalene (2MN) to give 2-methyl-1,4-naphthoquinone (vitamin K₃, menadione, 2MNQ) in the presence of hydrogen peroxide as a clean and cheap oxidant. The catalytic synthesis of vitamin K₃ was investigated using both homogeneous catalysis with free complexes and heterogeneous catalysis with silica-supported complexes. [(DIPAPTES)PdCl₂] and its silica-supported form showed the best catalytic activity for the selective oxidation of 2-methyl naphthalene to 2-methyl-1,4-naphtoquinone compared to the other metal complexes. 2MNQ yield reached 52.26% with the 2MN conversion of 90.52% using complex [(DIPAPTES)PdCl₂] and 58.59% with the 2MN conversion of 99.56% using the silica supported [SiO₂(DIPAPES)PdCl₂] complex for 1 h. Recycling was investigated for the silica-supported Pd(II) complex and compared with the classical production of vitamin K₃.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.     

乙基桥联有序介孔有机硅负载Pd(Ⅱ)有机金属催化剂用于水介质Barbier反应

以PPh2C2H4-Si(OEt)3和(EtO)3Si-C2H4-Si(OEt)3为混合硅源,在表面活性剂作用下共缩聚制备有序介孔有机硅杂化材料,再络合Pd(Ⅱ)离子得到固载化Pd(Ⅱ)非均相催化剂.在水介质Barbier反应中,所制备的Pd(Ⅱ)-PPh2-PMO(Et)具有与均相Pd(PPh3)2Cl2催化剂相当的催化活性,主要归因于高比表面积、有序介孔结构,有利于提高Pd(Ⅱ)活性位分散度,减少传质阻力,同时乙基修饰孔壁增强表面疏水性,有利于有机分子在孔道内的扩散和活性位上的吸附,导致高催化活性,而且可重复使用,显示了良好的工业应用前景.     

Kinetics and mechanism of the oxidation of thiourea by chlorine dioxide

The stoichiometry and kinetics of the oxidation of thiourea (SC(NH₂)₂) by chlorine dioxide (ClO₂) have been studied by uv-vis spectrophotometry using conventional and stopped-flow mixing techniques at 25.0 ± 0.1°C, pH 0.3–4.8. In high acid and initial 10:1 molar ratio of thiourea to chlorine dioxide, thiourea is oxidized relatively rapidly to dithiobisformamidine ion ((NH₂)₂CSSC(NH₂)₂²⁺), which slowly decomposes to thiourea, sulfur, and cyanamide (NCNH₂). In high acid and excess ClO₂, thiourea is oxidized to relatively stable formamidine sulfinic acid ((NH) (NH₂)CSO₂H). In high acid and molar ratios of ClO₂ to thiourea of 5:1 and higher, some oxidation to formamidine sulfonic acid ((NH) (NH₂)CSO₃H) occurs. At lower acidity, along with Cl^?, the major ClO₂ reduction product, byproduct sulfate is detected and, at pH < 3, ClO₂^?, also, appears. Kinetics data were collected for high excess thiourea with varying pH. The [ClO₂]-time curves are straight lines with negative slopes that increase in magnitude with increasing [thiourea]. The dependence on [thiourea] is first-order; the dependence on [ClO₂] is zero-order for 90% of reaction. With decreasing pH, the rate increases and the disappearance of ClO₂ becomes autocatalytic. Studies of the effects of reaction products on the rate of reaction lead to the conclusion that autocatalysis at low pH is due to the greater reactivity of HClO₂ compared with ClO₂^?. A 10-step mechanism incorporating a slow one-electron transfer from thiourea to ClO₂ to generate the (NH) (NH₂)CS · radical and subsequent more rapid reactions has been constructed and implemented in a computer simulation which provides a reasonably accurate fit to the observed kinetics curves. © 1993 John Wiley & Sons, Inc.