NHC–Pd(II)–Im (NHC=N-heterocyclic carbene,Im=1-methylimidazole) complex catalyzed coupling reaction of arylboronic acids with carboxylic acid anhydrides in water

A well-defined N-heterocyclic carbene (NHC)–palladium chloride–imidazole complex exhibited high catalytic activity in the coupling reaction of arylboronic acids with carboxylic acid anhydrides in pure water under mild conditions. Under the optimal conditions, all reactions gave the desired coupling products in moderate to high yields.     

Oxygen binding to [Pd(L)(L')] (L= NHC, L' = NHC or PR3, NHC = N-heterocyclic carbene). synthesis and structure of a paramagnetic trans-[Pd(NHC)2(η(1)-O2)2] complex

The reactivity of a number of two-coordinate [Pd(L)(L')] (L = N-heterocyclic carbene (NHC) and L' = NHC or PR(3)) complexes with O(2) has been examined. Stopped-flow kinetic studies show that O(2) binding to [Pd(IPr)(P(p-tolyl)(3))] to form cis-[Pd(IPr)(P(p-tolyl)(3))(η(2)-O(2))] occurs in a rapid, second-order process. The enthalpy of O(2) binding to the Pd(0) center has been determined by solution calorimetry to be -26.2(1.9) kcal/mol. Extension of this work to the bis-NHC complex [Pd(IPr)(2)], however, did not lead to the formation of the expected diamagnetic complex cis-[Pd(IPr)(2)(η(2)-O(2))] but to paramagnetic trans-[(Pd(IPr)(2)(η(1)-O(2))(2)], which has been fully characterized. Computational studies addressing the energetics of O(2) binding have been performed and provide insight into reactivity changes as steric pressure is increased.     

[(NHC)CrCl(μ-Cl)(THF)]_2 and(NHC)_2CrCl_2(NHC=1,3-Diisopropyl-4,5-dimethylimidazole-2-ylidene):Syntheses,Structures,and Polymerization Activities

The preparation of divalent chromium N-heterocyclic carbene(NHC,1,3-diisopropyl4,5-dimethylimidazole-2-ylidene) compounds is reported.The reaction of 1:1 molar ratio of NHC with CrCl2 led to an isolation of [(NHC)CrCl(μ-Cl)(THF)]2(1),while that of 2:1 ratio resulted in the formation of(NHC)2CrCl2(2).1 can be considered as an intermediate in the formation of 2 and further converted into 2 by the addition of another equiv.of NHC.The reaction of 2 with CpNa afforded an ion pair compound [(NHC)2CrCp]+[Cp]-(3),indicating a strong coordination ability of NHC supplanting one of the ionic Cr-Cp bonding.In combination of methylalumoxane(MAO) as cocatalyst 1 and 2 both are active for catalyzing ethylene polymerization.     

Synthesis of (NHC)Rh(cod)Cl and (NHC)RhCl(CO)2 complexes – Translation of the Rh- into the Ir-scale for the electronic properties of NHC ligands

Twenty-three different Rh complexes of the (NHC)RhCl(cod) and (NHC)RhCl(CO)₂ type were synthesized from [RhCl(cod)]₂. The electron donating nature of the NHC ligands was changed in a systematic manner. The redox potentials of the various (NHC)RhCl(cod) and the ν(CO) of the various (NHC)RhCl(CO)₂ were determined. A correlation of the Rh redox potentials and the Rh ν(CO), respectively, with the related data from analogous (NHC)IrCl(cod) and (NHC)IrCl(CO)₂ complexes established two linear relationships. The linear regression (R² = 0.993) of the Rh and the Ir redox potentials results in an equation for the redox potential transformation: E_1/2(Ir) = 1.016 · E_1/2(Rh) ? 0.076 V. The linear regression (R² = 0.97) of the Rh and Ir ν_av(CO) results in an equation for the ν_av(CO) transformation: ν_av(CO)Ir = 0.8695 · ν_av(CO)Rh + 250.7 cm^?1. In this manner the Rh and the Ir-scale for the determination of the electron donating properties of NHC ligands are unified.     

Hydrogenation of C?C Multiple Bonds Mediated by [Pd(NHC)(PCy3)] (NHC=N‐Heterocyclic Carbene) under Mild Reaction Conditions

N‐Heterocyclic carbenes do it again! [Pd(SIPr)(PCy₃)] ( 1 ; SIPr=N,N′‐(bis(2,6‐diisopropylphenyl)imidazolidine), which is stable under H₂, was shown to be a highly active catalyst for the hydrogenation of a wide range of alkenes and alkynes. Reactions proceed under mild conditions (relatively low Pd loading, RT, low pressures of H₂) even when sterically hindered alkenes (tri‐ and tetra‐substituted) are used.

     

Mechanism of ketone hydrosilylation using NHC–Cu(I) catalysts: a computational study

The porous MIL-47 material shows a selective adsorption behavior for para-, ortho-, and meta-isomers of xylenes, making the material a serious candidate for separation applications. The origin of the selectivity lies in the differences in interactions (energetic) and confining (entropic). This paper investigates the xylene–framework interactions and the xylene–xylene interactions with quantum mechanical calculations, using a dispersion-corrected density functional and periodic boundary conditions to describe the crystal. First, the strength and geometrical characteristics of the optimal xylene–xylene interactions are quantified by studying the pure and mixed pairs in gas phase. An extended set of initial structures is created and optimized to sample as many relative orientations and distances as possible. Next, the pairs are brought in the pores of MIL-47. The interaction with the terephthalic linkers and other xylenes increases the stacking energy in gas phase (?31.7?kJ/mol per pair) by roughly a factor four in the fully loaded state (?58.3?kJ/mol per xylene). Our decomposition of the adsorption energy shows various trends in the contributing xylene–xylene interactions. The absence of a significant difference in energetics between the isomers indicates that entropic effects must be mainly responsible for the separation behavior.     

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent-Silyliumylidene Cation

The trapping of a silicon(I) radical with N-heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6-iPr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent-silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1-F-2-I_Me-C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X-ray crystallography.     

Chelating alkoxy NHC–Rh(I) complexes and their applications in the arylation of aldehydes

In this study, new rhodium(I) complexes (5 and 6) have been prepared by the reaction of [RhCl(COD)]₂ with a series of imidazolium salts (3 and 4), which were obtained from a chiral amino alcohol. The catalytic activities of these complexes were tested in the arylation of aldehydes. It was found that the synthesized rhodium complexes were highly effective catalysts for the arylation of aldehydes in short reaction times (5 min, TOF = 1193 h⁻¹). However, the obtained ee values (up to 32% ee) remained low. We have proposed a mechanism for the arylation reaction of aldehydes, which is confirmed via ¹⁹F NMR spectroscopy.     

Theoretical study of silicon–sulfur clusters (SiS2)n (n = 1–6)

 The possible geometrical structures and relative stability of (SiS₂) _n (n=1–6) silicon–sulfur clusters are explored by means of density functional theory quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. The electronic structures and vibrational spectra of the most stable geometrical structures of (SiS₂) _n are analyzed by the same method. As a result, the regularity of the (SiS₂) _n cluster growth is obtained, and the calculation may used for predicting the formation mechanism of the (SiS₂) _n cluster. Received: 17 November 1999 / Accepted: 3 November 2000 / Published online: 3 May 2001     

Heats of formation for Fe(CO)n(n=1–4)

The dissociation energies of Fe(CO)_n (n=2–4) are computed using correlation consistent basis sets and the CCSD(T) approach. The dissociation energies are extrapolated to the CBS limit and are corrected for core–valence (CV), scalar relativistic, spin–orbit, zero-point, and thermal effects. Our iron carbonyl bond strengths agree with experiment within the respective error bars. We use our dissociations energies at 298 K to obtain the heats of formation of Fe(CO)_n (n=1–4).     

Chalcogen-Bonding Interactions in Telluroether Heterocycles [Te(CH2)m]n (n=1–4; m=3–7)

The Te ⋅⋅⋅ Te secondary bonding interactions (SBIs) in solid cyclic telluroethers were explored by preparing and structurally characterizing a series of [Te(CH₂)_m]_n (n=1–4; m=3–7) species. The SBIs in 1,7-Te₂(CH₂)₁₀, 1,8-Te₂(CH₂)₁₂, 1,5,9-Te₃(CH₂)₉, 1,8,15-Te₃(CH₂)₁₈, 1,7,13,19-Te₄(CH₂)₂₀, 1,8,15,22-Te₄(CH₂)₂₄ and 1,9,17,25-Te₄(CH₂)₂₈ lead to tubular packing of the molecules, as has been observed previously for related thio- and selenoether rings. The nature of the intermolecular interactions was explored by solid-state PBE0-D3/pob-TZVP calculations involving periodic boundary conditions. The molecular packing in 1,7,13,19-Te₄(CH₂)₂₀, 1,8,15,22-Te₄(CH₂)₂₄ and 1,9,17,25-Te₄(CH₂)₂₈ forms infinite shafts. The electron densities at bond critical points indicate a narrow range of Te ⋅⋅⋅ Te bond orders of 0.12–0.14. The formation of the shafts can be rationalized by frontier orbital overlap and charge transfer.     

Sol–gel synthesis of Zn-thiourea-SiO2 thin films from (EtO)3Si(CH2)3NHC(S)NHPh as molecular precursor

In order to study new and convenient sol–gel syntheses to homogeneous nanocomposites systems based on zinc sulphide clusters embedded in films of silica glass, a thiourea-functionalized silane ((EtO)₃Si(CH₂)₃NHC(S)NHPh, SilTu) was investigated as starting precursor. Electrospray ionization mass spectrometry (ESI-MS), FT-IR and NMR spectroscopies clearly evidence that SilTu coordinates Zn²⁺ ions in solution through the thiourea function. Both zinc acetate (Zn(CH₃COO)₂) and zinc chloride (ZnCl₂) were used as zinc source. The SilTuZn²⁺ complex is also stable under sol–gel conditions. Alcoholic solutions of SilTu, zinc acetate and tetraethoxysilane Si(OEt)₄ (TEOS) were used to prepare the thin films by dip-coating. Transparent, homogeneous and crack-free layers were obtained under annealing up to 600 °C. The samples were investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Beside ZnS formation, the presence of oxygenated zinc species was observed inside the silica matrix.     

Copper(I) Complexes with N-allylazomethines. The Role of π- and σ-Coordination in Structure Formation of 2CuBr·R–CH=N–C3H5 (R = 2-furyl) and CuBr·R–CH=N–C3H5 (R = phenyl)

Crystals of the complexes 2CuBr·C₄H₃(O)–CH=N–CH₂–CH=CH₂ (I) and CuBr·C₆H₅–CH=N–CH₂–CH=CH₂ (II) were obtained from an ethanol solution of CuBr₂ and N-allyl-2-furylaldimine or N-allylbenzaldimine using the alternating current electrochemical procedure. Their crystal structures were determined (DARCH diffractometer, MoK radiation, 1001 (F 2 F) and 911 (F 6 F) reflections; R = 0.029 and 0.063, respectively, for I and II. , -Complex I crystallizes in space group A2/a, a = 13.698(6), b = 17.679(5), c = 10.247(4) , = 114.03(3)°, Z = 8. Crystal data for -complex II: space group P2 ₁ /a, a = 19.016(5), b = 13.876(2), c = 4.0272(9) , = 92.90(2)°, Z = 4. The structure of I involves separate - and -coordinations of copper(I) in the form of planar trigons similar to those in the analogous nonisostructural -complex with copper(I) chloride. The length of the coordinated C=C bond is 1.35(1) . The structure of II consists of infinite (CuBr) _n chains -coordinated to the nitrogen atom of the ligand molecules. The noncoordinated allyl group is disordered.     

Studies on T-Methacrylate Copper(Ⅱ)Com plexes with Im idazole

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C(spn)−X (n=1–3) Bond Activation by Palladium

We have studied the palladium-mediated activation of C(spⁿ)−X bonds (n = 1–3 and X = H, CH₃, Cl) in archetypal model substrates H₃C−CH₂−X, H₂C=CH−X and HC≡C−X by catalysts PdL_n with L_n = no ligand, Cl⁻, and (PH₃)₂, using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C(sp³)−X, to C(sp²)−X, to C(sp)−X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from sp³, to sp², to sp, originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C(spⁿ)−X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst–substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C(sp³)−X to C(sp)−X.     

Synthesis and Crystal Structure of an Indenylnickel(Ⅱ) NHC Complex (Ind)NiBr(1,3-bisbenzylimidazol-2-ylidene)

The complex (Ind)NiBr(1,3-bisbenzylimidazol-2-ylidene) (Ind = indenyl) 1 has been prepared in ca. 65% yield via the reaction of (Ind)2Ni with an equivalent of 1,3-bis-benzylimidazolium bromide in THF/CH2Cl2 at 45 °C, supported by elemental analysis, NMR spectroscopy and X-ray crystal determination. The crystal belongs to the triclinic system, space group P1 with a = 7.072(1), b = 11.264(2), c = 14.541(3) (A), a = 102.21(3),β = 93.44(3), γ = 90.81(3)°, V= 1129.7(4)(A)3, Z = 2, Mr = 502.08, Dc. = 1.476 g/cm3, F(000) = 512,μ = 2.642 mm-1, R = 0.0490and wR = 0.1137 for 3913 observed reflections (I ＞ 2σ(I)). The center nickel atom is coordinated by a bromide ligand, a carbene carbon and three carbon atoms of the five-membered indenyl group to form either a highly distorted square pyramid or a highly distorted square plane.     

Synthesis and Crystal Structure of an Indenylnickel(II) NHC Complex (Ind)NiBr(1,3-bisbenzylimidazol-2-ylidene)

1 INTRODUCTION During the past few decades, indene and its substituted derivatives (Ind) are of special interest in Group IVB complexes owing to their easy coordina- tion modification by substitution[1～7]. The Ind li- gands can be tuned spontaneously to coordinate to the center metal from η1 to η5 via relatively facile “ring slippage”, thus leading to better stabilization of transition states or reaction intermediates[8]. How- ever, their applications in Group 10 metal complexes ha…     

Comparative mass spectrometric investigation of transition metal polymethylcyclopentadienylcarbonyl complexes: II mass spectra of RnC5H5–nMN(CO)3 (R = CH3, n = 0–5; R = t-C(CH3)3, n = 1)

Investigation of π-(CH_3nC₅H_5–nMn(CO)₃ (Me_n-CpMnT) n = 0−5 and t-butyl CpMnT mass spectra showed that Me_nCpMnT molecular ion (M⁺) fragmentation occurs by a simpler scheme than that for Me_nCpReT M⁺ molecular ions. The reason is that MnCp and MnCO bonds are not as strong as the ReCp and ReCO bonds, and the relative “inertness” (compared to Re) of the Mn atom (ion), coordinated to the methylcyclopentadienyl ligand. Variations of M⁺ molecular ion intensity with different values of n are probably due to a complexity of electronic and spatial methyl-carbonyl group interactions in M⁺.     

Synthesis and Structure of Tetraploid (Imidazole) Copper(II) Terephthalate,[Cu(Im)_4](teph)

Introduction　　Imidazoleisofconsiderableinterestasaligandwhichpresentsinmanybiologicalsystems (forexampleinthehistidylresidueofproteins)providingapotentialbindingsiteformetalions .Imidazoleasanunidentateligandcanformcomplexeswithmetalionsthroughitstertiarynitro genatom .Somecomplexesofimidazoleanditsderivativeswithtransition metalionshavebeenreported .1 4　　Thepreparationofthecomplexesofcopper(II)car boxylatewithavarietyofbasicligandsandtheirmagneticandspectralpropertieshavebeenreported .5,…