Ellipsometric in vitro studies on the activation of complement by human immunoglobulins M and G after adsorption to methylated silicon

Human serum immunoglobulin M (IgM) or human immunoglobulin G (IgG) were adsorbed to dichlorodimethyl silane (DDS) treated silicon. Subsequently, the model surfaces were incubated in normal-, complement factor 1q (C1q)-complement factor B or complement factor 2 (C2)-depleted human sera at 37 degrees C for up to 1.5 h. The serum deposition and binding of selected polyclonal complement antibodies into this layer were then quantified by null ellipsometry. Both types of precoated surfaces bound large amounts of anti-complement factor 3c (anti-C3c), anti-properdin and anti-C3d, after incubation in normal serum. In contrast to IgG coated surfaces, IgM coated surfaces bound no anti-C1q after the serum incubations and no anti-C3c deposition lag time was observed after incubations in EGTA serum. Upon immersions of IgM coated surfaces in the different sera, a rapid complement activation via a C1q factor B, and Ca(2+)-independent, but C2 dependent pathway, was indicated. When IgM was instead immobilized to APTES/glutaraldehyde surfaces, anti-C3c deposition was lower after incubations in EGTA than normal serum. The results suggest that, under the present experimental conditions, human IgM and IgG activate the complement system differently.     

Inhibition of hepatitis C virus serine protease in living cells by RNA aptamers detected using fluorescent protein substrates

Hepatitis C virus is one of the causative agents of non-A non-B hepatitis. Since one of viral proteins, NS3, has serine protease activity indispensable for virus maturation. NS3 serine protease is considered to be a suitable target for anti-HCV reagents. We report an assay of HCV NS3 protease in living cells. We designed peptide substrates bearing one of the sequences of HCV NS3 protease cleavage sites sandwiched with fluorescent proteins CFP and YFP. Substrates were expressed and cleaved efficiently in HeLa cells by cotransfection with HCV NS3 protease. The relationship between the progress of cleavage reaction and the change in fluorescence of the substrate emitted from living cells was confirmed. As a group of candidates for inhibitor of HCV NS3 protease, we chose RNA aptamers, nucleic acid ligands selected from a completely random RNA pool by in vitro selection. We found that 3 classes of aptamers, G9-I, II and III, bound NS3 protease specifically and inhibited cleavage in vitro. We studied the effect of RNA aptamers introduced into HeLa cells. The addition of G9-II RNA in the medium at a concentration of 2.5 micro g/ml reduced cleavage by one-third that of control.     

Structure and function of complement activating enzyme complexes: C1 and MBL-MASPs

The complement system is a major effector arm of the immune defense contributing to the destruction of invading pathogens. There are three possible routes of complement cascade activation: the classical, the alternative and the lectin pathways. The activation of the classical and lectin pathways is initiated by supramolecular complexes, which resemble each other. Each complex has a recognition subunit (C1q in the classical and mannose-binding lectin (MBL) in the lectin pathway), which associates with serine protease zymogens (C1q with C1r and C1s, and MBL with MBL-associated serine proteases: MASP-1, MASP-2) to form the C1 and MBL-MASPs complexes, respectively. As the recognition subunits bind to activator structures, subsequent activation of the serine protease zymogens occurs. The precise structure of the complexes and the exact mechanism of their activation have not been solved, yet. In this review we summarize the recent advances about the structure and function of the individual subcomponents of both complexes achieved by genetic engineering, molecular modeling, physico-chemical and functional studies. Special emphasis will be laid on the serine proteases: the role of the individual domains in the assembly of the C1s-C1r-C1r-C1s tetramer and in the control of the protease activity will be discussed. We will then focus on recent functional models of the supramolecular complexes. The question of how a non-enzymatic signal (the binding of C1q or MBL to activators) can be converted into enzymatic events (activation of serine protease zymogens) will be addressed. The similarities and differences between C1 and MBL-MASPs will also be discussed.     

A Novel Domino Reaction of 1, 1'‐bi‐2‐Naphthol Catalyzed by Copper (II)‐amine Complexes

1,1′‐Bi‐2‐naphthol (1) was oxidized into q‐oxo‐13c‐alkyloxy‐l,13c‐dihydro‐dibenzo [a,kl]‐xanthenes (2–11) with high isolated yields (58–94%) in alcohol solvents under the catalysis of copper(II)‐amine complexes in the presence of oxygen. The conversion of 1 to 2–11 belongs to Domino‐reaction.     

Synthetic possibilities of the 2,4-pentanedione-1,2-diaminoethane system: An overall view

Single condensation of acetylacetone (AcacH) with 1,2-diaminoethane (En) yields the terdentate “half-unit” 7-amino-4-methyl-5-aza-3-hepten-2-one (abbreviated as AEH). In the presence of a metal ion, this ligand leads to the macrocycle 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclo tetradeca-4,6,11,13-tetraene, which can be considered as resulting from the condensation of two molecules of (AcacH) with two molecules of (En). This “half-unit” can also be used to obtain acyclic ligands and complexes made with one (AcacH) molecule and two (En) molecules (1:2) or conversely two (AcacH) molecules and one (En) molecule (2:1). Using reagents other than (AcacH) and (En), this “half-unit” may yield homo and heterodinuclear complexes, macrocyclic compounds and non-symmetrical tetradentate Schiff bases.     

Aluminum complexes of the redox-active [ONO] pincer ligand

A series of aluminum complexes containing the tridentate, redox-active ligand bis(3,5-di-tert-butyl-2-phenol)amine ([ONO]H(3)) in three different oxidation states were synthesized. The aluminum halide salts AlCl(3) and AlBr(3) were reacted with the doubly deprotonated form of the ligand to afford five-coordinate [ONHO(cat)]AlX(solv) complexes (1a, X = Cl, solv = OEt(2); 1b, X = Br, solv = THF), each having a trigonal bipyramidal coordination geometry at the aluminum and containing the [ONHO(cat)](2-) ligand with a protonated, sp(3)-hybridized nitrogen donor. The [ONO] ligand platform may also be added to aluminum through the use of the oxidized ligand salt [ONO(q)]K, which was reacted with AlCl(3) in the presence of either diphenylacetylacetonate (acacPh(2)(-)) or 8-oxyquinoline (quinO(-)) to afford [ONO(q)]Al(acacPh(2))Cl (2) or [ONO(q)]Al(quinO)Cl (3), respectively, with well-defined [ONO(q)](-) ligands. Quinonate complexes 2 and 3 were reduced by one electron to afford the corresponding complexes K{[ONO(sq)]Al(acacPh(2))(py)} (4) and K{[ONO(sq)]Al(quinO)(py)} (5), respectively, containing well-defined [ONO(sq)](2-) ligands. The addition of tetrachloro-1,2-quinone to 1a in the presence of pyridine resulted in the expulsion of HCl and the formation of an aluminum complex with two different redox active ligands, [ONO]Al(o-O(2)C(6)Cl(4))(py) (6). Similar results were obtained when 1a was reacted with 9,10-phenanthrenequinone to afford [ONO]Al(o-O(2)C(14)H(8))(py) (7) or with pyrene-4,5-dione to afford [ONO]Al(o-O(2)C(16)H(8))(py) (8). Structural, spectroscopic and preliminary magnetic measurements on 6-8 suggest ligand non-innocent redox behavior in these complexes.     

Lanthanide complexes based on a 1,7-diaza-12-crown-4 platform containing picolinate pendants: a new structural entry for the design of magnetic resonance imaging contrast agents

We have synthesized a new macrocyclic ligand, N,N'-Bis[(6-carboxy-2-pyridyl)methyl]-1,7-diaza-12-crown-4 (H 2bp12c4), designed for complexation of lanthanide ions in aqueous solution. The X-ray crystal structure of the Gd (III) complex shows that the metal ion is directly bound to the eight donor atoms of the bp12c4 ligand, the ninth coordination site being occupied by an oxygen atom of a carboxylate group of a neighboring [Gd(bp12c4)] (+) unit, while the structure of the Lu (III) analogue shows the metal ion being only eight-coordinate. The hydration numbers obtained from luminescence lifetime measurements in aqueous solution of the Eu (III) and Tb (III) complexes suggest an equilibrium in aqueous solution between a dihydrated ( q = 2), ten-coordinate and a monohydrated ( q = 1), nine-coordinate species. This has been confirmed by a variable temperature UV-vis spectrophotometric study on the Eu (III) complex. The structure of the complexes in solution has been investigated by (1)H and (13)C NMR spectroscopy, as well as by theoretical calculations performed at the DFT (B3LYP) level. The results indicate that the change in hydration number occurring around the middle of the lanthanide series is accompanied by a change in the conformation adopted by the complexes in solution [Delta(lambdalambdalambdalambda) for q = 2 and Lambda(deltalambdadeltalambda) for q = 1]. The structure calculated for the Yb (III) complex (Lambda(deltalambdadeltalambda)) is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the Yb (III)-induced paramagnetic (1)H shifts.     

Bond-selective nitrile insertion into the 2H-azaphosphirene ring system as induced by tetracyanoethylene

Competitive reactions of 2H-azaphosphirene metal complexes 1a-c (M =Cr, Mo, W) with 1-piperidinonitrile and tetracyanoethylene in toluene have been observed at elevated temperatures. For the case of complex 1c, the delta5-1,2-azaphospholene complex 2c (as main product) and the 2H-1,4,2-diazaphosphole complex 3c (as by-product) were separated from the product mixture. At ambient temperature and using 1-piperidinonitrile as solvent, bond and regioselective insertion of 1-piperidinonitrile into the P-N bond of 2H-azaphosphirene metal complexes 1a-c (M = Cr, Mo, W) has been achieved in the presence of tetracyanoethylene (TCNE), yielding 2H-1,4,2-diazaphosphole metal complexes 3a-c, analogous reactions in benzo- or acetonitrile afforded the 2H-1,4,2-diazaphosphole tungsten complexes 3d, e. A preliminary study with the 2H-azaphosphirene tungsten complex 1c and 1-piperidinonitrile as solvent has revealed that substoichiometric amounts of TCNE (0.3 equiv) induce approximately 70% conversion of complex 1c. NMR data of the complexes 2c and 3a-e and the X-ray structure of complex 3c are discussed.     

Chiral Iridium Spiro Aminophosphine Complexes: Asymmetric Hydrogenation of Simple Ketones,Structure, and Plausible Mechanism

The iridium complexes of chiral spiro aminophophine ligands, especially the ligand with 3,5‐di‐tert‐butylphenyl groups on the P atom ( 1c ) were demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of alkyl aryl ketones. In the presence of KOtBu as a base and under mild reaction conditions, a series of chiral alcohols were synthesized in up to 97 % ee with high turnover number (TON up to 10 000) and high turnover frequency (TOF up to 3.7×10⁴ h⁻¹). Investigation on the structures of the iridium complexes of ligands (R)‐ 1a and 1c by X‐ray analyses disclosed that the 3,5‐di‐tert‐butyl groups on the P‐phenyl rings of the ligand are the key factor for achieving high activity and enantioselectivity of the catalyst. Study of the catalysts generated from the Ir‐(R)‐ 1c complex and H₂ by means of ESI‐MS and NMR spectroscopy indicated that the early formed iridium dihydride complex with one (R)‐ 1c ligand was the active species, which was slowly transformed into an inactive iridium dihydride complex with two (R)‐ 1c ligands. A plausible mechanism for the reaction was also suggested to explain the observations of the hydrogenation reactions.     

Crystalline complexes of N,N'-ditritylurea (DTU) and N-tritylurea (NTU) hosts with molecular guests

This paper reports crystalline complexes of the new hosts N,N'-ditritylurea (DTU) and N-tritylurea (NTU) with various uncharged molecular guests. The crystal structures of the following complexes were elucidated by single crystal X-ray diffraction analysis at 115^oK: (I) 1:1 DTU-propanamide — space group C2/c, a=15.839Å, b=9.088Å, c=24.584Å, =111.05^o, Z=4; (II) 1:1 DTU-ethyl N-acetylglycinate — space group P1, a=9.010Å, b=10.800Å, c=19.810 Å, =105.29^o =94.33^o, =93.03^o, Z=2; (III) 2:1 NTU-N, N-dimethylformamide — space group Cc, a=29.614Å, b=8.906Å, c=16.127Å, =121.04^o, Z=4. The three crystal structures are stabilized mainly by a cooperative effect of hydrogen bonding between amide fragments displaced along the shortest axis of each crystal. This interaction occurs between host and guest in complexes I and II, and between host and host in complex III. The latter also represents a cage-type clathrate in which the guest molecules are accommodated in voids between the hydrophobic fragments of four neighboring NTU hosts. On the other hand, complexes of DTU are characterized by a more specific interaction between the two components, each guest molecule being inserted between two adjacent hosts (related by translation) and strongly bound to them via hydrogen bridges. These results illustrate a useful concept in the design of molecular species which can be potential hosts upon crystallization with neutral molecular guests. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82022 (7 pages).     

Does Fibronect in Influence the Determination of Circulating Immune Complexes by Means of a Clq-Enzyme Immunoassay?

Abstract

Fibronect in was shown to bind to solid-phase C1q and to inhibit the binding of aggregated lgG in an C1q-solid-phase enzyme immunoassay in the absence of human serum. In the presence of human serum the determination of aggregated lgG and circulating immune complexes is not influenced by fibronectin in this assay. Therefore, we suggest that fibronectin does not affect the determination of immune complexes in serum specimens by C1q binding assays.     

Molecular Mechanics Study of the Complexes of β-Cyclodextrin with 4-(dimethylamino)benzonitrile and Benzonitrile

Molecular Mechanics calculations with the Tripos Force Field were employed to study the complexation of 4-(dimethylamino)benzonitrile (DMABN) and/or benzonitrile (BN) with -cyclodextrin (CD). The systems studied have 1 : 1 (DMABN : CD and BN : CD), 2 : 2 (DMABN : CD) and 1 : 1 : 2 (DMABN : BN : CD) stoichiometries. Evidence for the formation of such complexes, binding constants and other thermodynamic parameters were extracted from the analysis of the steady state fluorescence measurements performed in a previous work. The Molecular Mechanics study, based on the energy changes upon guest-host approaching, was performed in vacuo and in the presence of water as a solvent. Results show that the driving forces for 1 : 1 complexation are mainly dominated by non-bonded van der Waals host : guest interactions. However, the driving forces for association between 1 : 1 complexes to give 2 : 2 homo- or 1 : 1 : 2 heterodimers are dominated by non-bonded electrostatic interactions. Head-to-head electrostatic interactions between CDs, which are presumably due to the hydrogen bonding formation between secondary hydroxyl groups of CDs, are responsible for most of the stability of the dimers.     

Separation ability and stoichiometry of cyclodextrin complexes

Gas-liquid chromatography has been applied to search relations between selectivity towards isomers and stoichiometry of cyclodextrin complexes. The model tested compounds were: dimethylnaphthalenes and alpha- and beta-pinenes as constitutional isomers; cis/trans decalins, anetholes and isosafroles as diastereomers and as enantiomers (+/-)-alpha-pinenes and (+/-)-camphenes. Experimental retention data are used to confirm a simple theoretical model that allows distinguishing formation of G x CD complexes (1:1) and G x CD2 complexes (1:2). Based on the experimental data, stability constants K were evaluated. It has been found that remarkable selectivity factor alpha may appear both within the range of 1:1 stoichiometry (beta-CD complexes of decalins and of alpha- and beta-pinenes) and 1:2 stoichiometry (alpha-CD complexes with (+/-)-alpha-pinenes and (+/-)-camphenes). Occasionally selectivity arises from a different composition, when one isomer forms a 1:1 stoichiometry complex while another forms a 1:2 complex (dimethylnaphthalenes, cis/trans-anetholes and cis/trans-isosafroles).     

Synthesis, thermal characterization, and antimicrobial activity of lanthanum, cerium, and thorium complexes of amino acid Schiff base ligand

Metal complexes of La(III), Ce(IV), and Th(IV), with the amino Schiff base ligand, [N-(2-hydroxybenzyl)-l-methionine acid](H₃L), were prepared in the presence of triethylamine as a deprotonating agent. All synthesized compounds were identified and confirmed by mass spectra, elemental analyses, molar conductivities, and spectral analyses (UV–Visible, IR, ¹H NMR, and ¹³CNMR). Conductance measurements suggest the non-electrolytic nature and the complexes were isolated in 1:1 ratios. The thermal decomposition of the complexes was discussed in relation to structure. The data from thermogravimetric analysis clearly indicated that the decomposition of the complexes proceeds in four or five steps and the organic part of the complexes decomposed in one or two intermediates. The decomposition of all complexes ended with metal oxide and carbon residue. The Schiff bases and their complexes were screened for their antibacterial (Escherichia coli, Staphylococcus aureus) and antifungal (Aspergillus flavus and Candida Albicans) activities.     

Synthetic approach to substituted cyclopropanes based on the coupling reaction of lithiated chloroalkyloxazolines with Fischer carbene complexes

Regioselective addition of lithiated oxazoline 2a, easily available from 2-(1-chloroethyl)-4,4-dimethyl-2-oxazoline 1a (LDA, THF, -98 degrees C), to alpha,beta-unsaturated Fischer carbene complexes 3 afforded cyclopropylcarbene complexes 4 as sole diastereoisomers. Exposure of carbene complexes 4a-c (M = Cr) to air and sunlight gave cyclopropane carboxylate derivatives 5a-c. A plausible mechanistic explanation is proposed. Moreover, when lithiated oxazoline 2b was generated from 1b in the presence of the carbene complex 3a,b, the oxazolinylcyclopropane carboxylates 6a,b formed as a 1:1 mixture of diastereoisomers. Chiral lithiated oxazoline 2c added regioselectively and diastereoselectively to chromium complexes 3a,b and to tungsten complexes 3d,e, leading, after oxidation of the metal fragment, to esters 7a,b with good diastereoselectivity (dr = 4:1). The reaction of lithiated oxazoline 2d with chromium complex 3b and tungsten complex 3e proceeded less diastereoselectively, furnishing, in both cases, after oxidation, the ester 7c as a 3:2 diastereoselective mixture.     

钒化合物引发丙烯腈聚合──高分子场效应

研究了酸性介质中五价钒氧离子（VO）、强碱性阴离子交换树脂负载钒（V）（BAEV）、硫酸氧钒（VOSO4）、杨梅形聚羧酸氧钒（IV）（APCV）、杨酸形聚亚氨二乙酸氧钒（IV）（APIV）、杨梅形聚得偕亚氨二乙酸氧钒（IV）（APOV）、笼形聚羧酸氧钒（IV）（CPCV）和笼形聚肟偕亚氨二乙酸氧钒（IV）（CPOV）等与硫脲（TU）配住生成活性种并引发丙烯腈（AN）聚合反应。表观聚合速度（Rp）分别是：VO-TU:Rp=2.8X105e-14200/RTC2.2'(AN)c.20(HNO3)c0(V5+)c1.3(TU)BAEV-TU:Rp=1.9X104e-6860/RTc1.2(AN)c1.0'(HNO3)c0.44(PV)c1.0(TU)VOSO4-TU:Rp=0APCV-TU:Rp=2.3X104e-4100/Rtc1.5(AN)c1.5(HNO3)c0.5(PV)c2.0(TU)APIV-TU:RP=2.2X105e-6860/RTc1.0(AN)c2.0(H2SO4)c0.5(PV)c1.5(TU)APOV-TU:RP=1.9X108e-10800/RTc.10(AN)c1.0(HNO3)c0.6(PV)c1.5(TU)CPCV-TU:Rp=9.7X105e-10500/RTc1.0(AN)c1.5(HNO3)c0.5(PV)c0.76(TU)CPOV-TU:Rp=1.0X108e-10500/RTc1.0(AN)c3.0(HNO3)c1.0(PV)c1.5(TU)根据实验结果,认为：（一）钒化合物与硫脲在酸性介质中通过“逐步配位—质子转移”机理产生引发种;（二）钒络合物及其活化后所产生的阳离子自由基（i＝0,1,2,…,n）处于大分子引力场内进行链引发,在某些情况下,原地进行键增长反     

DNA interaction of platinum(II) complexes with 1,10-phenanthroline and extended phenanthrolines

The interaction with DNA of the platinum(II) square planar complexes [Pt(N-N)(py)(2)](2+) (N-N = 1,10-phenanthroline (phen), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), dipyrido[3,2-a:2',3'-c]phenazine (dppz), benzodipyrido[b:3,2-h:2'3'-f]phenazine (bdppz)) has been investigated by means of absorption, circular and linear dichroism spectroscopy, DNA melting, and viscosity. In the presence of excess [DNA] all the complexes intercalate to the double helix. For those with the most extended phenanthrolines the binding mode depends on the [DNA]/[complex] ratio (q); at low q values the substances bind externally to DNA probably self-aggregating along the double helix. When the DNA concentration is large enough, the aggregate breaks up and the complex intercalates within the nucleobases. The complexes self-aggregate, without added DNA, in the presence of a large salt concentration.     

Mesophase behaviour of azobenzene-based angular supramolecular hydrogen-bonded liquid crystals

New types of angular 1:1 hydrogen-bonded supramolecular complexes via hydrogen-bond formation between 4-alkoxyphenylazo benzoic acids (In) and 4-(3?-pyridylazo)-4??-alkoxybenzoates (IIm) with various alkoxy chains (from 6 to 16 carbons) were prepared and investigated for their mesophase behaviour by differential scanning calorimetry (DSC) and polarised-light microscopy (PLM). All prepared homologues were found to be dimorphic, possessing smectic C and nematic mesophases. The formation of 1:1 hydrogen-bonded supramolecular liquid crystals (LCs) complexes was confirmed by FTIR and UV?visible (UV?vis) absorption spectroscopy. The study revealed that nematic transition enhancement (ΔT) decreases with the increase of the alkoxy chain length on the base complement, while it increases with the increase of the chain attached to the acid complement of the complex, that is the stability of the nematic phase is more dependent on the length of the acid component.     

Synthesis and Reactivity of Six‐Membered Oxa‐Nickelacycles: A Ring‐Opening Reaction of Cyclopropyl Ketones

Cyclopropanecarboxaldehyde ( 1 a ), cyclopropyl methyl ketone ( 1 b ), and cyclopropyl phenyl ketone ( 1 c ) were reacted with [Ni(cod)₂] (cod=1,5‐cyclooctadiene) and PBu₃ at 100 °C to give η²‐enonenickel complexes ( 2 a – c ). In the presence of PCy₃ (Cy=cyclohexyl), 1 a and 1 b reacted with [Ni(cod)₂] to give the corresponding μ‐η²:η¹‐enonenickel complexes ( 3 a , 3 b ). However, the reaction of 1 c under the same reaction conditions gave a mixture of 3 c and cyclopentane derivatives ( 4 c , 4 c′ ), that is, a [3+2] cycloaddition product of 1 c with (E)‐1‐phenylbut‐2‐en‐1‐one, an isomer of 1 c . In the presence of a catalytic amount of [Ni(cod)₂] and PCy₃, [3+2] homo‐cycloaddition proceeded to give a mixture of 4 c (76 %) and 4 c′ (17 %). At room temperature, a possible intermediate, 6 c , was observed and isolated by reprecipitation at ?20 °C. In the presence of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr), both 1 a and 1 c rapidly underwent oxidative addition to nickel(0) to give the corresponding six‐membered oxa‐nickelacycles ( 6 ai , 6 ci ). On the other hand, 1 b reacted with nickel(0) to give the corresponding μ‐η²:η¹‐enonenickel complex ( 3 bi ). The molecular structures of 6 ai and 6 ci were confirmed by X‐ray crystallography. The molecular structure of 6 ai shows a dimeric η¹‐nickelenolate structure. However, the molecular structure of 6 ci shows a monomeric η¹‐nickelenolate structure, and the nickel(II) 14‐electron center is regarded as having “an unusual T‐shaped planar” coordination geometry. The insertion of enones into monomeric η¹‐nickelenolate complexes 6 c and 6 ci occurred at room temperature to generate η³‐oxa‐allylnickel complexes ( 8 , 9 ), whereas insertion into dimeric η¹‐nickelenolate complex 6 ai did not take place. The diastereoselectivity of the insertion of an enone into 6 c having PCy₃ as a ligand differs from that into 6 ci having IPr as a ligand. In addition, the stereochemistry of η³‐oxa‐allylnickel complexes having IPr as a ligand is retained during reductive elimination to yield the corresponding [3+2] cycloaddition product, which is consistent with the diastereoselectivity observed in Ni⁰/IPr‐catalyzed [3+2] cycloaddition reactions of cyclopropyl ketones with enones. In contrast, reductive elimination from the η³‐oxa‐allylnickel having PCy₃ as a ligand proceeds with inversion of stereochemistry. This is probably due to rapid isomerization between syn and anti isomers prior to reductive elimination.     

From large 12-membered macrometallacycles to ionic (NHC)2M+Cl- type complexes of gold and silver by modulation of the N-substituent of amido-functionalized N-heterocyclic carbene (NHC) ligands

A series of structurally diverse gold and silver complexes extending from ionic (NHC) 2M(+)Cl(-) (M=Au, Ag) type complexes to large 12-membered macrometallacycles have been prepared by the appropriate modification of the N-substituent of amido-functionalized N-heterocyclic carbenes. Specifically, the ionic, [1-(R)-3-{ N-(t-butylacetamido)imidazol-2-ylidene}]2M(+)Cl(-), (R=t-Bu, i-Pr; M=Au, Ag; 1b, 1c, 2b, 2c) complexes, were obtained in case of the N- t-butyl substituent of the amido-functionalized sidearm while 12-membered macrometallacycles, [1-(R)-3-{N-(2,6-di i-propylphenylacetamido)imidazol-2-ylidene}]2M2, (R=t-Bu, i-Pr; M=Au, Ag; 3b, 3c, 4b, 4c) were obtained in case of the 2,6-di i-propylphenyl N-substituent. These structurally diverse complexes of gold and silver were, however, prepared employing a common synthetic pathway involving the reactions of the imidazolium chloride salts (1a, 2a, 3a, 4a) with Ag2O to give the silver complexes (1b, 2b, 3b, 4b) and which, when treated with (SMe2)AuCl, gave the gold complexes (1c, 2c, 3c, 4c). Detailed density functional theory studies of 1b, 1c, 2b, 2c, 3b, 3c, 4b, and 4c were carried out to gain insight about the structure, bonding, and the electronic properties of these complexes. The NHC-metal interaction in the ionic 1b, 1c, 2b, and 2c complexes is primarily composed of the interaction of the carbene lone pair with the empty p orbital of the metal (5p for Ag and 6p for Au) while the same in the macrometallacyclic 3b, 3c, 4b, and 4c complexes consisted of the interaction of the carbene lone pair with the empty s orbital of the metal (5s for Ag and 6s for Au). The observation of a low energy emission in about the 580-650 nm region has been tentatively assigned to originate from the presence of weak metallophilic interaction in these macrometallacyclic 3b, 3c, 4b, and 4c complexes.