Coordination compounds of N,N′-olefin functionalized imidazolin-2-ylidenes

A New Class of Task‐Specific Imidazolium Salts and Ionic Liquids and Their Corresponding Transition‐Metal Complexes for Immobilization on Electrochemically Active Surfaces

Adding to the versatile class of ionic liquids, we report the detailed structure and property analysis of a new class of asymmetrically substituted imidazolium salts, offering interesting thermal characteristics, such as liquid crystalline behavior, polymorphism or glass transitions. A scalable general synthetic procedure for N‐polyaryl‐N’‐alkyl‐functionalized imidazolium salts with para‐substituted linker (L) moieties at the aryl chain, namely [LPh_mIm^HR]⁺ (L=Br, CN, SMe, CO₂Et, OH; m=2, 3; R=C₁₂, PEG_n; n=2, 3, 4), was developed. These imidazolium salts were studied by single‐crystal X‐ray diffraction (SC‐XRD), NMR spectroscopy and thermochemical methods (DSC, TGA). Furthermore, these imidazolium salts were used as N‐heterocyclic carbene (NHC) ligand precursors for mononuclear, first‐row transition metal complexes (Mn^II, Fe^II, Co^II, Ni^II, Zn^II, Cu^I, Ag^I, Au^I) and for the dinuclear Ti‐supported Fe‐NHC complex [(OPy)₂Ti(OPh₂ImC₁₂)₂(FeI₂)] (OPy=pyridin‐2‐ylmethanolate). The complexes were studied concerning their structural and magnetic behavior via multi‐nuclear NMR spectroscopy, SC‐XRD analyses, variable temperature and field‐dependent (VT‐VF) SQUID magnetization methods, X‐band EPR spectroscopy and, where appropriate, zero‐field ⁵⁷Fe Mössbauer spectroscopy.

Dinuclear Gold(I) Complexes Bearing N,N'-Allyl-Bridged Bisimidazolylidene Ligands

A novel N,N'-allyl-bridged bisimidazolium salt and a novel dinuclear Ag(I) and a Au(I) NHC complex are reported. Both metallacyclic complexes have a twisted structural shape due to the rigid allylic system and form two different isomers relating to the position of the double bonds. The allyl-group shows photoisomerisation, but no reactivity towards bases for the additional coordination of Pd(II).

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Synthesis, characterization, and catalytic application in aldehyde hydrosilylation of half-sandwich nickel complexes bearing (κ1-C)- and hemilabile (κ2-C,S)-thioether-functionalised NHC ligands

Ni complexes bearing thioether-functionalised NHCs; hemilabile ligands for catalytic aldehyde hydrosilylation.

Smart N-Heterocyclic Carbene Ligands in Catalysis

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

Cyclopentadienyl nickel(ii) N,C-chelating benzothiazolyl NHC complexes: synthesis, characterization and application in catalytic C–C bond formation reactions

CpNi(ii) N,C-chelating benzothiazolyl NHC complexes have been synthesized. They are efficient catalysts for the homo-coupling of benzyl bromide in the presence of MeMgCl at r.t. with good function group tolerance.

Half-sandwich nickel complexes with ring-expanded NHC ligands – synthesis, structure and catalytic activity in Kumada–Tamao–Corriu coupling

We report the synthesis and catalytic properties of novel [Ni(Cp)(X)(NHC)] complexes with ring-expanded NHC ligands of six-, seven-, and eight-membered heterocycles.

Novel imidazolium and imidazolinium salts containing the 9-nickelafluorenyl anion – synthesis, structures and reactivity

Investigation of the properties of carbene complexes is one of the most important fields of modern coordination chemistry.

N, N'-Olefin functionalized bis-imidazolium gold(I) salt is an efficient candidate to control keratitis-associated eye infection

Keratitis treatment has become more complicated due to the emergence of bacterial or fungal pathogens with enhanced antibiotic resistance. The pharmaceutical applications of N-heterocyclic carbene complexes have received remarkable attention due to their antimicrobial properties. In this paper, the new precursor, 3,3′-(p-phenylenedimethylene) bis{1-(2- methyl-allyl)imidazolium} bromide (1a) and its analogous PF₆ salt (1b) were synthesized. Furthermore, silver(I) and gold(I) -N-heterocyclic carbene (NHC) complexes [Ag₂LBr₂/Au₂LBr₂; 2a/3a], [(Ag₂L₂)(PF₆)₂/(Au₂L₂)(PF₆)₂; 2b/3b] were developed from their corresponding ligands. All compounds were screened for their antimicrobial activities against multiple keratitis-associated human eye pathogens, including bacteria and fungi. Complexes 2a and 3a showed highest activity, and the effectiveness of 3a was also studied, focusing eradication of pathogen biofilm. Furthermore, the structures of 1a, 2a and 3b were determined using single crystal X-ray analysis, 2b and 3a were optimized theoretically. The mechanism of action of 3a was evaluated by scanning electron microscopy and docking experiments, suggesting that its target is the cell membrane. In summary, 3a may be helpful in developing antimicrobial therapies in patients suffering from keratitis-associated eye infections caused by multidrug-resistant pathogens.

Synthesis, structural characterization, and reactivity of late transition-metal complexes bearing linked cyclopentadienyl–carboranyl ligands

Late transition-metal complexes bearing linked cyclopentadienyl/indenyl–carboranyl ligands were synthesized and their reactivities were examined. Reaction of Li2[Me2C(L)(C2B10H10)] (L = C5H4, C9H6, Me2NCH2CH2C5H3) with MCl2(PPh3)2 in Et2O afforded [η5:σ-Me2C(C5H4)(C2B10H10)]M(PPh3) (M = Co (4), Ni (5)), [η5:σ-Me2C(C9H6)(C2B10H10)]M(PPh3) (M = Co (6), Ni (7)), and [η5:σ-Me2C(Me2NCH2CH2C5H3)(C2B10H10)]Ni(PPh3) (8). Treatment of 4 or 5 with 2,6-dimethylphenylisocyanide, N-heterocyclic carbene (NHC), PCy3, or 1,2-bis(diphenylphosphino)ethane (dppe) gave the corresponding PPh3 displacement complexes [η5:σ-Me2C(C5H4)(C2B10H10)]M(2,6-Me2C6H3NC) (M = Co (9), Ni (10)), [η5:σ-Me2C(C5H4)(C2B10H10)]M[1,3-(2,6-i-Pr2C6H3)2C3N2H2] (M = Co (11), Ni (12)), [η5:σ-Me2C(C5H4)(C2B10H10)]Ni(PCy3) (13), or {[η5:σ-Me2C(C5H4)(C2B10H10)]Co}2(dppe) (14), respectively. These complexes were characterized by various spectroscopic techniques and elemental analyses. The molecular structures of 4–14 were further confirmed by single-crystal X-ray analyses.

Nickel and palladium complexes of enolatefunctionalised N-heterocyclic carbenes

The reaction of chloroethyltrimethylsilylether with 1-methylimidazole furnishes an ionic liquid that undergoes methanolysis to crystalline 2-hydroxyethylimidazolium chloride (crystal structure presented). Conversion to defined hydroxyethylimidazol-2-ylidene nickel complexes failed, but was accomplished with 1-methyl-3-acetophenyl-imidazolium bromide. The bis(NHC⋂O−) nickel(II) chelate is formed, rather than a methallylnickel monochelate, but with nickelocene a monochelate NiCp complex was detected. The bulky 1-(2,6-diisopropylphenyl)-3-(2’-phenyl-enolato)-imidazol-2-ylidene allylpalladium chloride was obtained in pure form. Attempts to generate catalysts for ethylene oligomerization by in situ techniques have failed so far whereas P⋂O− ligands, comparable by the P-C diagonal relationship, provide active catalysts.

Half-sandwich NHC-nickel(II) complexes as pre-catalysts for the fast Suzuki coupling of aryl halides: a comparative study

Cationic half-sandwich nickel complexes of general formula [Ni(NHC)(NCMe)(eta(5)-C5R5)](PF6) [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) a, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) b; R = H, Me] were prepared from the reaction of their neutral homologues [Ni(NHC)Cl(eta(5)-C5R5)] with 1 equiv. of KPF6 in acetonitrile at room temperature. The new cationic complexes [Ni(IPr)(NCMe)(eta(5)-C5Me5)](PF6) 3a, [Ni(IMes)(NCMe)(eta(5)-C5Me5)](PF6) 3b and [Ni(IMes)(NCMe)(eta(5)-C5H5)](PF6) 4b were obtained in high yield and were fully characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, elemental analyses, and in the case of 3a by a single-crystal X-ray diffraction study. The neutral analogue of 3a, [Ni(IPr)Cl(eta(5)-C5Me5)] 1a was also structurally characterized. Their geometries were compared and no significant structural differences were observed. Nevertheless solution NMR spectroscopy established that the acetonitrile ligand of the cationic species is labile in solution. This results in the absence of any rotational significant barrier about the nickel-carbene carbon bond at ambient temperature in solution in the sterically congested cationic complexes 3a and 3b, in contrast to their neutral analogues 1a and [Ni(IMes)Cl(eta(5)-C5Me5)] 1b. The neutral and the cationic complexes catalyzed the cross-coupling of phenylboronic acid with aryl halides in the absence of co-catalysts or reductants. Surprisingly, the neutral or cationic nature of the complexes proved to have almost no influence on the reaction yields and rates. However, complexes bearing the bulky electron-rich pentamethylcyclopentadienyl ligand were much more active than those bearing the cyclopentadienyl ligand, and TOF of up to 190 h(-1), a high rate for nickel(II) complexes under similar conditions, were observed with these species.

Heterocyclic carbenes: synthesis and coordination chemistry

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of beta-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.