Nickel‐Triad Complexes of a Side‐on Coordinating Distannene

NHC adducts of the stannylene Trip₂Sn (Trip=2,4,6‐triisopropylphenyl) were reacted with zero‐valent Ni, Pd, and Pt precursor complexes to cleanly yield the respective metal complexes featuring a three‐membered ring moiety Sn‐Sn‐M along with carbene transfer onto the metal and complete substitution of the starting ligands. Thus the easily accessible NHC adducts to stannylenes are shown to be valuable precursors for transition‐metal complexes with an unexpected Sn? Sn bond. The complexes have been studied by X‐ray diffraction and NMR spectroscopy as well as DFT calculations. The compounds featuring the structural motif of a distannametallacycle comprised of a [(NHC)₂M⁰] fragment and Sn₂Trip₄ represent rare higher congeners of the well‐known olefin complexes. DFT calculations indicate the presence of a π‐type Sn–Sn interaction in these first examples for acyclic distannenes symmetrically coordinating to a zero‐valent transition metal.     

Synthesis and reactivity of copper(I) complexes containing a bis(imidazolin-2-imine) pincer ligand

The new pincer ligand 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine (TL(tBu)) has been prepared in high yield from 2,6-bis(hydroxymethyl)pyridine (1) and 1,3-di-tert-butylimidazolin-2-imine (3). Reaction of TL(tBu) with [Cu(MeCN)4]PF6 affords the highly reactive copper(I) complex [(TL(tBu))Cu]PF6, [5]PF6, which forms the stable copper(I) isocyanide complexes [6a]PF6 (nu(CN) = 2179 cm(-1)) and [6b]PF6 (nu(CN) = 2140 cm(-1)) upon addition of tert-butyl or 2,6-dimethylphenyl isocyanide, respectively. For the cations 6a and 6b, DFT calculations reveal ground-state electronic structures of the type [(TL(tBu)-kappaN(1):kappaN(2))Cu(CNR)] with tricoordinate geometries around the copper atoms. Exposure of [5]PF6 to the air readily leads to trapping of atmospheric CO2 to form the square-planar complex [(TL(tBu))Cu(HCO3-kappaO)]PF6, [7]PF6, with the bicarbonate ligand adopting a rarely observed monodentate coordination mode. In chlorinated solvents such as dichloromethane or chloroform, [5]PF(6) rapidly abstracts chloride by reductive dechlorination of the solvent to yield [(TL(tBu))CuCl]PF6, [8]PF6 quantitatively. Reaction of TL(tBu) with copper(I) bromide or chloride affords complexes 9a and 9b, respectively, for which X-ray diffraction analysis, low-temperature NMR experiments and DFT calculations reveal the presence of a kappa(2)-coordinated ligand of the type [(TL(tBu)-kappaN(1):kappaN(2))CuX]. In solution, complex 9b undergoes slow disproportionation forming the mixed-valence copper(II)/copper(I) system [(TL(tBu))CuCl][CuCl2], [8]CuCl2 with a linear dichlorocuprate(I) counterion.     

Synthesis,cytotoxicity and antibacterial studies of symmetrically and non‐symmetrically benzyl‐ or p‐cyanobenzyl‐substituted N‐Heterocyclic carbene–silver complexes

From the reaction of 1H‐imidazole ( 1a ), 4,5‐dichloro‐1H‐imidazole ( 1b ) and 1H‐benzimidazole ( 1c ) with p‐cyanobenzyl bromide ( 2 ), symmetrically substituted N‐heterocyclic carbene (NHC) [( 3a–c )] precursors, 1‐methylimidazole ( 5a ), 4,5‐dichloro‐1‐methylimidazole ( 5b ) and 1‐methylbenzimidazole ( 5c ) with benzyl bromide ( 6 ), non‐symmetrically substituted N‐heterocyclic carbene (NHC) [( 7a–c )] precursors were synthesized. These NHC? precursors were then reacted with silver(I) acetate to yield the NHC‐silver complexes [1,3‐bis(4‐cyanobenzyl)imidazole‐2‐ylidene] silver(I) acetate ( 4a ), [4,5‐dichloro‐1,3‐bis(4‐cyanobenzyl)imidazole‐2‐ylidene] silver(I) acetate ( 4b ), [1,3‐bis(4‐cyanobenzyl)benzimidazole‐2‐ylidene] silver(I) acetate ( 4c ), (1‐methyl‐3‐benzylimidazole‐2‐ylidene) silver(I) acetate ( 8a ), (4,5‐dichloro‐1‐methyl‐3‐benzylimidazole‐2‐ylidene) silver(I) acetate ( 8b ) and (1‐methyl‐3‐benzylbenzimidazole‐2‐ylidene) silver(I) acetate ( 8c ) respectively. The four NHC‐precursors 3a–c, 7c and four NHC–silver complexes 4a–c and 8c were characterized by single crystal X‐ray diffraction. The preliminary antibacterial activity of all the compounds was studied against Gram‐negative bacteria Escherichia coli, and Gram‐positive bacteria Staphylococcus aureus using the qualitative Kirby‐Bauer disc‐diffusion method. All NHC–silver complexes exhibited medium to high antibacterial activity with areas of clearance ranging from 4 to 12 mm at the highest amount used, while the NHC‐precursors showed significantly lower activity. In addition, all NHC–silver complexes underwent preliminary cytotoxicity tests on the human renal‐cancer cell line Caki‐1 and showed medium to high cytotoxicity with IC₅₀ values ranging from 53 ( ± 8) to 3.2 ( ± 0.6) µM. Copyright © 2010 John Wiley & Sons, Ltd.     

Zur Reaktion der Alkin‐Kupfer(I)‐Komplexe [CuX(S‐Alkin)] (X = Cl,Br, I; S‐Alkin = 3,3,6,6‐Tetramethyl‐1‐thia‐4‐cycloheptin) mit den Phosphanen PMe3 und Ph2PCH2CH2PPh2 (dppe)

Organometallic Compounds of Copper. XVIII. On the Reaction of the Alkyne Copper(I) Complexes [CuX(S‐Alkyne)] (X = Cl, Br, I; S‐Alkyne = 3,3,6,6‐Tetramethyl‐1‐thiacyclohept‐4‐yne) with the Phosphanes PMe₃ and Ph₂PCH₂CH₂PPh₂ (dppe) The alkyne copper(I) halide complexes [CuX(S‐Alkyne)]_n ( 2 ) ( 2 a : X = Cl, 2 b : X = Br, 2 c : X = I; S‐Alkyne = 3,3,6,6‐tetramethyl‐1‐thiacyclohept‐4‐yne; n = 2, ∞) add the phosphanes PMe₃ and Ph₂PCH₂CH₂PPh₂ (dppe) to form the mono‐ and dinuclear copper compounds [(S‐Alkyne)CuX(PMe₃)] ( 6 ) ( 6 a : X = Cl, 6 b : X = Br) and [(S‐Alkyne)CuX(μ‐dppe)CuX(S‐Alkyne)] ( 7 a : X = Cl, 7 b : X = Br, 7 c : X = I), respectively. By‐product in the reaction of 2 a with dppe is the tetranuclear complex [(S‐Alkyne)Cu(μ‐X)₂Cu(μ‐dppe)₂Cu(μ‐X)₂Cu(S‐Alkyne)] ( 8 ). In case of the compounds 7 prolonged reaction times yield the alkyne‐free dinuclear copper complexes [Cu₂X₂(dppe)₃] ( 9 ) ( 9 a : X = Cl, 9 b : X = Br, 9 c : X = I)). X‐ray diffraction studies were carried out with the new compounds 6 a , 6 b , 7 b , 8 , and 9 c .     

EPR/Spin‐trapping study of free radical intermediates in the photolysis of trifluoromethyl ketones with initiators

Photolysis of trifluoromethyl ketones (TFMKs) 1a–1e versus the non‐fluorinated ketones 2a–2b in the presence of radical initiators by electron paramagnetic resonance spectroscopy has been studied for the first time. The transient radicals generated after irradiation of the ketones were identified by trapping with 2‐methyl‐2‐nitrosopropane (MNP) and 2,4,6‐tri‐tert‐butylnitrosobenzene (TTBNB) as spin traps. TTBNB is a powerful, particularly useful spin trap in these kinds of processes producing anilino and nitroxyl spin adducts due to the ambivalent reactivity on the N and O atoms. In the presence of t‐butylperoxide, short‐chain TFMKs, such as 1,1,1‐trifluoroacetone (1d) and hexafluoroacetone (1e), give rise to detection of the elusive trifluoromethyl radical. In contrast, long‐chain TFMKs did not provide clues to prove formation of the trifluoromethyl radical but instead to radicals derived by abstraction of one α‐methylene proton to the carbonyl. Although TFMKs are quite stable to photodegradation in the absence of initiator, methyl ketone 2b and phenyl ketone 3 produce radicals resulting from abstraction of a γ‐hydrogen to the carbonyl group. Copyright © 2010 John Wiley & Sons, Ltd.     

CH–π and CF–π Interactions Lead to Structural Changes of N‐Heterocyclic Carbene Palladium Complexes

The role of CH–π and CF–π interactions in determining the structure of N‐heterocyclic carbene (NHC) palladium complexes were studied using ¹H NMR spectroscopy, X‐ray crystallography, and DFT calculations. The CH–π interactions led to the formation of the cis‐anti isomers in 1‐aryl‐3‐isopropylimidazol‐2‐ylidene‐based [(NHC)₂PdX₂] complexes, while CF–π interactions led to the exclusive formation of the cis‐syn isomer of diiodobis(3‐isopropyl‐1‐pentafluorophenylimidazol‐2‐ylidene) palladium(II).     

A Comparison of the Stability and Reactivity of Diamido‐ and Diaminocarbene Copper Alkoxide and Hydride Complexes

The mononuclear N‐heterocyclic carbene (NHC) copper alkoxide complexes [(6‐NHC)CuOtBu] (6‐NHC=6‐MesDAC ( 1 ), 6‐Mes ( 2 )) have been prepared by addition of the free carbenes to the tetrameric tert‐butoxide precursor [Cu(OtBu)]₄, or by protonolysis of [(6‐NHC)CuMes] (6‐NHC=6‐MesDAC ( 3 ), 6‐Mes ( 4 )) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2 , the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring‐opening reactions, the latter affording [(6‐MesDAC)Cu(OC(O)CMe₂C(O)N(H)Mes)(CNMes)] ( 6 ). The intermediacy of [(6‐MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu₂O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6‐MesDAC)CuH] by reaction of 1 with Et₃SiH resulted instead in migratory insertion to generate [(6‐MesDAC‐H)Cu(P(p‐tolyl)₃)] ( 9 ) upon trapping by P(p‐tolyl)₃. Migratory insertion was also observed during attempts to prepare [(6‐Mes)CuH], with [(6‐Mes‐H)Cu(6‐Mes)] ( 10 ) isolated, following a reaction that was significantly slower than in the 6‐MesDAC case. The longer lifetime of [(6‐Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi‐reduction of alkynes and hydrosilylation of ketones.     

Expanded‐Ring N‐Heterocyclic Carbenes Efficiently Stabilize Gold(I) Cations,Leading to High Activity in π‐Acid‐Catalyzed Cyclizations

A series of six‐ and seven‐membered expanded‐ring N‐heterocyclic carbene (er‐NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er‐NHC)AuX] (X^?=BF₄^?, NTf₂^?, OTf^?) were generated in solution. According to their ¹³C NMR spectra, the ionic character of the complexes increases in the order X^?=Cl^?<NTf₂^?<OTf^?<BF₄^?. Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er‐NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au⁺] cations. A comparative study has been carried out of the catalytic activities of five‐, six‐, and seven‐membered carbene complexes [(NHC)AuX], [(Ph₃P)AuX], [(Me₂S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD‐Dipp)Au]BF₄ (1,3‐bis(2,6‐diisopropylphenyl)‐3,4,5,6‐tetrahydrodiazepin‐2‐ylidene gold(I) tetrafluoroborate) catalyst bearing seven‐membered‐ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol % catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations.     

Alkoxydiaminophosphine Ligands as Surrogates of NHCs in Copper Catalysis

A family of phosphine ligands containing a five-membered ring similar to the popular N-heterocyclic carbene ligands and an alkoxy third substituent has been developed. These alkoxydiaminophosphine ligands (ADAP) can be generated in one pot and reacted with a copper(I) source leading to the high yield isolation of complexes [(ADAP)CuX]₂ (X=Cl, Br). The dinuclear nature of these compounds has been established by means of X-ray studies and DOSY experiments. A screening of the catalytic properties of these complexes toward carbene-transfer reactions from diazocompounds to C−H bonds (alkane, arene), olefins or N−H bonds, as well as in CuAAC or nitrene transfer reactions have shown a performance at least similar, if not better, than their (NHC)CuCl analogues, opening a new window in copper catalysis with these readily tunable ADAP ligands.     

Enantioselective Alkynylation of Trifluoromethyl Ketones Catalyzed by Cation‐Binding Salen Nickel Complexes

Cation‐binding salen nickel catalysts were developed for the enantioselective alkynylation of trifluoromethyl ketones in high yield (up to 99 %) and high enantioselectivity (up to 97 % ee). The reaction proceeds with substoichiometric quantities of base (10–20 mol % KOt‐Bu) and open to air. In the case of trifluoromethyl vinyl ketones, excellent chemo‐selectivity was observed, generating 1,2‐addition products exclusively over 1,4‐addition products. UV‐vis analysis revealed the pendant oligo‐ether group of the catalyst strongly binds to the potassium cation (K⁺) with 1:1 binding stoichiometry (K_a=6.6×10⁵ m ^?1).     

Theoretical insights into the absorption and emission properties of blue luminescent copper(I) complexes based on the pyrazolyl‐pyridine ligands

The ground geometrical and electronic structures, charge transfer (CT) behaviors, absorption, and emission properties of the three copper(I) complexes [Cu(pypz)(POP)]⁺ (1) , [Cu(pympz)(POP)]⁺ (2) , and [Cu(pytfmpz)(POP)]⁺ (3) (pypz=1‐(2‐pyridyl)pyrazole, pympz=3‐methyl‐1‐(2‐pyridyl)pyrazole, and pytfmpz=3‐trifluoromethyl‐1‐(2‐pyridyl)pyrazole), have been investigated using density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT). The vertical absorption energies of the all copper(I) complexes are well reproduced by TD‐DFT calculations based on the CT amount calculations. The triplet emission properties of the all copper(I) complexes were correctly evaluated at BMK/LANL2DZ/6‐31G* level of theory. In addition, the thermally activated delayed fluorescence properties of 1–3 were discussed in detail based on the spatial separation of the HOMO and LUMO and vertical excited energies. These theoretical insights should be expected to provide some guides for the design and synthesis of efficient luminescent copper(I) complexes. © 2014 Wiley Periodicals, Inc.     

An Abnormal N‐Heterocyclic Carbene–Carbon Dioxide Adduct from Imidazolium Acetate Ionic Liquids: The Importance of Basicity

In the reaction of 1‐ethyl‐3‐methylimidazolium acetate [C₂C₁Im][OAc] ionic liquid with carbon dioxide at 125 °C and 10 MPa, not only the known N‐heterocyclic carbene (NHC)–CO₂ adduct I , but also isomeric aNHC‐CO₂ adducts II and III were obtained. The abnormal NHC‐CO₂ adducts are stabilized by the presence of the polarizing basic acetate anion, according to static DFT calculations and ab initio molecular dynamics studies. A further possible reaction pathway is facilitated by the high basicity of the system, deprotonating the initially formed NHC‐CO₂ adduct I , which can then be converted in the presence of the excess of CO₂ to the more stable 2‐deprotonated anionic abnormal NHC–CO₂ adduct via the anionic imidazolium‐2,4‐dicarboxylate according to DFT calculations on model compounds. This suggests a generalizable pathway to abnormal NHC complex formation.     

Systematic Modulation of the Fluorescence Brightness in Boron‐Dipyrromethene (BODIPY)‐Tagged N‐Heterocyclic Carbene (NHC)–Gold–Thiolates

Five different highly fluorescent boron‐dipyrromethene (BODIPY)‐tagged N‐heterocyclic carbene NHC–gold halide complexes were synthesized. The substitution of the halogeno ligand by 4‐substituted aryl thiolates leads to a decrease in the brightness of the complexes. This decrease depends on the electronic nature of the thiols, being most pronounced with highly electron‐rich thiols (4‐R=NMe₂). The brightness of the gold thiolates also depends on the distance between the sulfur atom and the BODIPY moiety. The systematic variation of the electron density of [(NHC–bodipy)Au(SC₆H₄R)] (via different R groups) enables the systematic variation of the fluorescence brightness of an appended BODIPY fluorophore. Based on this and supported by DFT calculations, a photoinduced electron‐transfer quenching appears to be the dominant mechanism controlling the brightness of the appended BODIPY dye.     

Synthesis and Structures of Arene Ruthenium (II)–NHC Complexes: Efficient Catalytic α‐alkylation of ketones via Hydrogen Auto Transfer Reaction

A panel of six new arene Ru (II)‐NHC complexes 2a‐f , (NHC = 1,3‐diethyl‐(5,6‐dimethyl)benzimidazolin‐2‐ylidene 1a , 1,3‐dicyclohexylmethyl‐(5,6‐dimethyl)benzimidazolin‐2‐ylidene 1b and 1,3‐dibenzyl‐(5,6‐dimethyl)benzimidazolin‐2‐ylidene 1c ) were synthesized from the transmetallation reaction of Ag‐NHC with [(η⁶‐arene)RuCl₂]₂ and characterized. The ruthenium (II)‐NHC complexes 2a‐f were developed as effective catalysts for α‐alkylation of ketones and synthesis of bioactive quinoline using primary/amino alcohols as coupling partners respectively. The reactions were performed with 0.5 mol% catalyst load in 8 h under aerobic condition and the maximum yield was up to 96%. Besides, the different alkyl wingtips on NHC and arene moieties were studied to differentiate the catalytic robustness of the complexes in the transformations.     

Sulfonated N‐heterocyclic carbine‐silver (I) complexes: Synthesis,characterisation and biological evaluation

The synthesis, characterisation and biological activity of water‐soluble Ag(I)‐NHC complexes, general formula Na[(NHC)AgCl] where NHC is a sulfonated and sterically hindered N‐heterocyclic carbene, is reported. The Ag‐NHC complexes (2a–e) were synthesised by reacting the corresponding sulfonated NHC ligands with Ag₂O in the presence of NaCl or NaBr in methanol/water (1:1) solution. Synthesised silver (I)‐N‐heterocyclic carbene complexes have been characterised by NMR, micro‐analysis and HRMS spectroscopic methods. The IC₅₀ values of these complexes were determined by a proliferation BrdU enzyme‐linked immunosorbent assay (ELISA) against HeLa (human cervix carcinoma), HT29 (human adenocarcinoma) and L929 (mouse fibroblast) cell lines. These complexes have been highlighted as promising and original platforms for building new types of metalodrug. All new water‐soluble Ag(I) complexes demonstrated remarkable cytotoxic activity against HeLa, HT29 and L929 cell lines.     

Chemistry surrounding monomeric copper(I) methyl, phenyl, anilido, ethoxide, and phenoxide complexes supported by N-heterocyclic carbene ligands: reactivity consistent with both early and late transition metal systems

Monomeric copper(I) alkyl complexes that possess the N-heterocyclic carbene (NHC) ligands IPr, SIPr, and IMes [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] react with amines or alcohols to release alkane and form the corresponding monomeric copper(I) amido, alkoxide, or aryloxide complexes. Thermal decomposition reactions of (NHC)Cu(I) methyl complexes at temperatures between 100 and 130 degrees C produce methane, ethane, and ethylene. The reactions of (NHC)Cu(NHPh) complexes with bromoethane reveal increasing nucleophilic reactivity at the anilido ligand in the order (SIPr)Cu(NHPh) < (IPr)Cu(NHPh) < (IMes)Cu(NHPh) < (dtbpe)Cu(NHPh) [dtbpe = 1,2-bis(di-tert-butylphosphino)ethane]. DFT calculations suggest that the HOMO for the series of Cu anilido complexes is localized primarily on the amido nitrogen with some ppi(anilido)-dpi(Cu) pi-character. [(IPr)Cu(mu-H)]2 and (IPr)Cu(Ph) react with aniline to quantitatively produce (IPr)Cu(NHPh)/dihydrogen and (IPr)Cu(NHPh)/benzene, respectively. Analysis of the DFT calculations reveals that the conversion of [(IPr)Cu(mu-H)]2 and aniline to (IPr)Cu(NHPh) and dihydrogen is favorable with DeltaH approximately -7 kcal/mol and DeltaG approximately -9 kcal/mol.     

Triptycene‐Based Chiral and meso‐N‐Heterocyclic Carbene Ligands and Metal Complexes

Based on 1‐amino‐4‐hydroxy‐triptycene, new saturated and unsaturated triptycene‐NHC (N‐heterocyclic carbene) ligands were synthesized from glyoxal‐derived diimines. The respective carbenes were converted into metal complexes [(NHC)MX] (M=Cu, Ag, Au; X=Cl, Br) and [(NHC)MCl(cod)] (M=Rh, Ir; cod=1,5‐cyclooctadiene) in good yields. The new azolium salts and metal complexes suffer from limited solubility in common organic solvents. Consequently, the introduction of solubilizing groups (such as 2‐ethylhexyl or 1‐hexyl by O‐alkylation) is essential to render the complexes soluble. The triptycene unit infers special steric properties onto the metal complexes that enable the steric shielding of selected areas close to the metal center. Next, chiral and meso‐triptycene based N‐heterocyclic carbene ligands were prepared. The key step in the synthesis of the chiral ligand is the Buchwald–Hartwig amination of 1‐bromo‐4‐butoxy‐triptycene with (1S,2S)‐1,2‐diphenyl‐1,2‐diaminoethane, followed by cyclization to the azolinium salt with HC(OEt)₃. The analogous reaction with meso‐1,2‐diphenyl‐1,2‐diaminoethane provides the respective meso‐azolinium salt. Both the chiral and meso‐azolinium salts were converted into metal complexes including [(NHC)AuCl], [(NHC)RhCl(cod)], [(NHC)IrCl(cod)], and [(NHC)PdCl(allyl)]. An in situ prepared chiral copper complex was tested in the enantioselective borylation of α,β‐unsaturated esters and found to give an excellent enantiomeric ratio (er close to 90:10).     

Homogeneous oxidative coupling catalysts: stoichiometry and product characterization of the oxidation of copper(I) complexes [(Pyr) n CuX]4 ( n = 1 or 2, Pyr = pyrrolidine,X = Cl,Br or I) by dioxygen in aprotic media

Copper(I) halides react quantitatively with pyrrolidine (Pyr) in dioxygen-free methylene chloride or nitrobenzene to form tetranuclear copper(I) complexes [(Pyr) _n CuX]₄O₂; n = 1 or 2, X = Cl, Br or I. These complexes are very soluble and completely reduce dioxygen to a dioxo bridging ligand with stoichiometry, Δ[Cu(I)]/Δ[O₂] = 4.0. Analytical and cryoscopic data establish the formation of discrete tetranuclear products. The stable oxo solids [(Pyr) _n CuX]₄O₂ mimic the tyrosinase copper protein and are homogeneous oxidative coupling catalysts for phenols. Electronic transition spectra in the near infrared for [(Pyr) _n CuX]₄O₂ are explained as charge transfer from a minimum of three halo ligands to a copper(II) site, (LMCT). Therefore, one can expect a higher molar absorptivity when X = I, as in the [(Pip) _n CuX]₄O₂ system. However, molar absorptivity is about half of expected. The lower absorptivity explains the step structure of [(Pyr) _n CuI]₄O₂, in which only two copper(II) sites, instead of four as in [(Pyr) _n CuX]₄O₂, X = Cl or Br, have three iodo ligands per copper center. In addition to infrared and electronic spectra for [(Pyr) _n CuI]₄O₂, EPR bands are observed, and it is concluded that two different types of copper(II) sites are present as represented in scheme 1.     

Structural Diversity of Copper(I)–N‐Heterocyclic Carbene Complexes; Ligand Tuning Facilitates Isolation of the First Structurally Characterised Copper(I)–NHC Containing a Copper(I)–Alkene Interaction

The preparation of a series of imidazolium salts bearing N‐allyl substituents, and a range of substituents on the second nitrogen atom that have varying electronic and steric properties, is reported. The ligands have been coordinated to a copper(I) centre and the resulting copper(I)–NHC (NHC=N‐heterocyclic carbene) complexes have been thoroughly examined, both in solution and in the solid‐state. The solid‐state structures are highly diverse and exhibit a range of unusual geometries and cuprophilic interactions. The first structurally characterised copper(I)–NHC complex containing a copper(I)–alkene interaction is reported. An N‐pyridyl substituent, which forms a dative bond with the copper(I) centre, stabilises an interaction between the metal centre and the allyl substituent of a neighbouring ligand, to form a 1D coordination polymer. The stabilisation is attributed to the pyridyl substituent increasing the electron density at the copper(I) centre, and thus enhancing the metal(d)‐to‐alkene(π*) back‐bonding. In addition, components other than charge transfer appear to have a role in copper(I)–alkene stabilisation because further increases in the Lewis basicity of the ligand disfavours copper(I)–alkene binding.     

Highly Enantioselective Copper(I)‐Catalyzed Conjugate Addition of Terminal Alkynes to 1,1‐Difluoro‐1‐(phenylsulfonyl)‐3‐en‐2‐ones: New Ester/Amide Surrogates in Asymmetric Catalysis

A highly enantioselective copper‐catalyzed conjugate alkynylation of monoactivated enones, namely 1,1‐difluoro‐1‐(phenylsulfonyl)‐3‐en‐2‐ones, is described. The reaction products are obtained with good yields and excellent enantioselectivities (from 92 to 99% ee). The β‐alkynylated difluoro(phenylsulfonyl) ketones can be converted into the corresponding β‐alkynylated difluoro‐ and trifluoromethyl ketones, esters and amides. This is the first example on the use of 1,1‐difluoro‐1‐(phenylsulfonyl)‐3‐en‐2‐ones as substrates in an enantioselective reaction, which have been shown to be new ester/amide surrogates.