Chelating N-heterocyclic Carbene Ligands Research Articles

Synthesis, structures, and photophysical properties of two Cu(I) complexes supported by N-heterocyclic carbene and phosphine ligands

Abstract Two new cationic four-coordinate Cu(I) complexes supported by different chelating N-heterocyclic carbene ligands and the diphosphine ligand bis[2-(diphenylphosphino)phenyl]ether (POP) have been synthesized. The chemical structures of both complexes have been characterized by 1H NMR, 13C NMR, 31P NMR, and mass spectroscopy, and the crystal structure of one complex has been determined by single-crystal X-ray diffraction. Results of theoretical calculations indicate that the lowest energy electronic transitions of these complexes are mainly the metal-to-ligand charge transfer and ligand-to-ligand charge transfer transitions. The complexes in solid state show intense emissions with high photoluminescence quantum yields. The photophysical behavior at 298 and 77K shows that emissions of these complexes at room temperature are thermally activated delayed fluorescence mixed with phosphorescence.

Transition-metal complexes bearing chelating NHC Ligands. Catalytic activity in cross coupling reactions via C[sbnd]H activation

In recent years, the direct functionalization of CH bonds has become an important tool for CC or C–heteroatom cross-coupling reactions. In these reactions, homogeneous catalysts based on transition metals have demonstrated good selectivity for direct CH activation. This process is not only simple but easily leads to molecules and products of high added value through greener and more energy efficient processes. In this context, metallacyclic complexes have been very successful in CH functionalization reactions, especially with the use of transition metal compounds bearing at least one N-heterocyclic carbene (NHC). Thus, herein we highlight some of the most recent advances on the catalytic CC or C–heteroatom (B, N and O) bond formation via CH activation, using metallacyclic transition metal NHC complexes.

Highly β-Selective Cyclopolymerization of 1,6-Heptadiynes and Ring-Closing Enyne Metathesis Reaction Using Grubbs Z-Selective Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts.

It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote exo-cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the β-selectivity for CP further, one could increase the size of the substrates on the monomers and lower the reaction temperature to obtain conjugated polyenes containing up to 95% six-membered rings. Moreover, the physical properties of the resulting polymer were analyzed in detail and compared with those of the conjugated polyenes containing only five-membered rings prepared from the same monomer but with a conventional Grubbs catalyst.

N-Alkylation and N,C-Dialkylation of Amines with Alcohols in the Presence of Ruthenium Catalysts with Chelating N-Heterocyclic Carbene Ligands

A series of new benzimidazolium salts and ruthenium(II) complexes containing chelating N-heterocyclic carbenes (NHCs) functionalized with a benzylic group and an acetal group were prepared. All of the synthesized compounds were characterized by elemental analysis and NMR spectroscopy, and the molecular structures of 2c and 2d were determined by X-ray crystallography. All of the complexes were tested in the alkylation of cyclic amine derivatives with alcohols and showed excellent activity in this reaction. Cyclic amines were alkylated with primary and heteroaromatic alcohols. The Ru–NHC complexes also catalyzed N,C3-dialkylation of cyclic amines.

Blue phosphorescent nitrile containing C^C* cyclometalated NHC platinum(ii) complexes

Since C^C* cyclometalated Pt(II) complexes with N-heterocyclic carbene (NHC) ligands have been identified as potential emitter materials in organic light-emitting devices (OLEDs), very promising results regarding quantum yields, colour and stability have been presented. Herein, we report on four nitrile substituted complexes with a chelating NHC ligand (1-(4-cyanophenyl)-3-isopropyl-1H-benzo[d]imidazole or 4-(tert-butyl)-1-(4-cyanophenyl)-3-methyl-1H-imidazole) and a bidentate monoanionic auxiliary ligand (acetylacetone or dimesitoylmethane). The complexes have been fully characterized including extensive 2D NMR studies (COSY, HSQC, HMBC, NOESY, (195)Pt NMR), three of them also by solid-state structures. Photophysical measurements in amorphous PMMA films and pure emitter films at room temperature reveal the impact of the mesityl groups in the auxiliary ligand, which led to a significant increase of the quantum yield, while the decay lifetimes decreased. The electron withdrawing nitrile groups shift the emission towards blue colour coordinates.

Sulfur containing platinum(ii) complexes with N-heterocyclic carbene ligands obtained by reactions of a hydrosulfido complex

A hydrosulfido platinum(ii) complex with a chelated N-heterocyclic carbene (NHC) ligand was oxidised with O(2) in the presence of excess hydrogen sulfide, to give a linear tetrasulfido complex, and without hydrogen sulfide, to give a thiosulfato-bridged dinuclear complex. The hydrosulfido complex also reacted with an acetato complex containing the chelating NHC platinum unit to afford a trinuclear platinum complex with two triply bridging sulfido ligands showing an equilibrium in solution between two isomers based on the arrangement of the chelating NHC ligands.

Synthesis and Characterization of Di- and Tetracarbene Iron(II) Complexes with Chelating N-Heterocyclic Carbene Ligands and Their Application in Aryl Grignard–Alkyl Halide Cross-Coupling

A series of new and known bis(imidazolium) chloride and bromide salts bridged by either a methylene group (1–8, 10a,b) or an ethylene group (9a,b) and bearing different N substituents (Me, Et, Bn, tBu, Mes) have been reacted with [Fe{N(SiMe3)2}2]2 to yield the four-coordinate iron(II) complexes [LFeX2] (11–20; X = Cl, Br; L = chelating bis(imidazolylidene) ligand). Molecular structures of six of these complexes have been characterized by X-ray crystallography, and selected examples have been characterized by 1H NMR and UV–vis spectroscopy, cyclic voltammetry, Mossbauer spectroscopy, and SQUID magnetometry. In all cases the iron(II) is found in a distorted-tetrahedral environment; it is in the high-spin state and shows large quadrupole splittings in the range 3.67–4.03 mm·s–1 (δ = 0.73–0.81 mm·s–1). Subtleties of the metric parameters depend on the bridging unit between the two imidazolylidene groups, the peripheral N substituents, and the coligand (Cl or Br). In case of rather small (Me, Et) or flexible (...

Platinum(IV) Complexes with Chelating N-Heterocyclic Carbene Ligands

Platinum(II) complexes with one or two chelating bis(NHC) ligands have been oxidized by bromine as well as iodobenzene dichloride to provide the corresponding platinum(IV) carbene complexes in good yields under very mild reaction conditions. All compounds have been fully characterized by 1H and 13C NMR spectroscopy and in three cases also by solid-state structures. The NMR spectra of the novel Pt(IV) complexes show significantly shifted signals compared to their Pt(II) precursor complexes.

Iridium complexes of N-heterocyclic carbenes in C–H borylation using energy efficient microwave technology: influence of structure, ligand donor strength and counter ion on catalytic activity

Bridged and unbridged N-heterocyclic carbene (NHC) ligands were metalated with [Ir(COD)Cl]2 to give iridium(I) mono- and biscarbene substituted catalysts [Ir(COD)NHC(Cl)] and [Ir(COD)(NHC)2][X] (X: I, PF6, BF4, CF3COO, OTf). The prepared NHC-complexes were tested in the C–H borylation reaction of aromatic carbons with bis(pinacolato)diboron (B2pin2) and pinacolborane (HBpin). The use of microwave technology in this study not only facilitates a time efficient screening of a wide range of influences such as ligand σ-donor strength and structural motif as well as the effects of the complex counter ion, but also provides an energy efficient heating source. Catalyst 6TFA, which features a chelating NHC ligand, proved to be most effective catalyst and further investigations with this complex in the borylation of mono- and disubstituted benzene derivatives exploring chemo- and regioselectivity were undertaken.

Chiral, bridged bis(imidazolin-2-ylidene) complexes of palladium

Varieties of chiral, bridged bisimidazolium salts as well as the synthesis of palladium complexes of general formula [ bridge { NC(H) C(H)N ( R ∗ ) C ¯ } 2 PdBr 2 ] with the corresponding chelating N-heterocyclic carbene ligands is reported. This is the first systematic study of chiral bis(imidazolin-2-ylidene)palladium(II) complexes bearing chiral groups on the endocyclic nitrogens. Structural proof of such a chiral palladium(II) complex is presented by way of an X-ray diffraction study of complex 3a.

Bridged Imidazolium Salts Used as Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki-Heck Reaction

A variety of chiral and achiral imidazolium salts is synthesized. Methylene-, ethylene-, propylene- and pyridinyl-bridged bis(imidazolium) halides are used to generate the respective free chelating carbenes. The synthesis of palladium complexes of general formula [cis-CH 2 {NC(H)=C(H)N(R)C} 2 PdX 2 ] with these chelating N-heterocyclic carbene ligands is reported. Structural proofs of complexes 28 and 46 are represented by X-ray diffraction studies. Catalytic applications in the Mizoroki-Heck reaction are presented.

Piano-Stool Iron(II) Complexes as Probes for the Bonding of N-Heterocyclic Carbenes: Indications for π-Acceptor Ability

A series of new piano-stool iron(II) complexes comprising mono- and bidentate chelating N-heterocyclic carbene ligands [Fe(cp)(CO)(NHC)(L)]X have been prepared and analyzed by spectroscopic, electrochemical, crystallographic, and theoretical methods. Selectively substituting the L site with a series of ligands going from carbene to pyridine to CO suggests that CO is the strongest π acceptor, while the behavior of pyridine and carbene is nearly identical. This suggests that in these complexes comprising an electron-rich iron(cp)(carbene) fragment, N-heterocyclic carbenes are not pure σ donors but also moderate π acceptors. Theoretical calculations support this bonding model and indicate charge saturation at the metal as key for π back-bonding to N-heterocyclic carbenes. On the basis of voltammetric measurements, the Lever electrochemical parameter of these carbenes has been determined: EL = +0.29. Systematic substitution of the wingtip groups of the carbene revealed only subtle changes in the electronic p...

A Chelating N-Heterocyclic Carbene Ligand in Organochromium Chemistry

Several chromium complexes supported by a chelating N-heterocyclic carbene ligand have been prepared in an attempt to study their ability to catalyze the polymerization of ethylene. LCrCl3(THF) (3) (L = 1,1‘-methylene-3,3‘-di-2,6-diisopropylphenylimidazole-2,2‘-diylidene (2)) was found to polymerize ethylene when activated by either excess methylaluminoxane (MAO) or excess Et2AlCl. Chloride abstraction from 3 with Na+BARF - (BARF = tetrakis(3,5-di-trifluoromethylphenyl)borate) yielded a dinuclear, chloride-bridged product, [(LCrCl)2(μ-Cl)3]+BARF - (4). In a similar fashion, reaction of 3 with 3.0 equiv of AlMe3 gave the dinuclear, chloride-bridged alkyl cation [(LCrMe)2(μ-Cl)3]+AlX4- (X = Me, Cl) (5), which is isostructural to 4. A triaryl complex was also prepared by reaction of 2 with CrPh3(THF)3, giving LCrPh3 (6) in good yield. Similarly, a series of alkyl and aryl chloride complexes of the form LCrRCl2(THF) (R = Me (7a), Ph (7b), tolyl (7c)) have also been made from the appropriate CrRCl2(THF)3 starting material. Further treatement of 7a with Na+BARF- resulted in the formation of the BARF- salt of complex 5. Complexes 3−6 and 7a−c catalyze the polymerization of ethylene when activated by MAO. Preliminary studies with Cr(II) have shown that LCrX2(THF) (X = Cl (8a), Br (8b)) does not polymerize ethylene upon activation by either MAO or alkylaluminum reagents. Complexes 3−6, 7c, and 8b have been structurally characterized by X-ray diffraction methods.

A New Class of Chelating N-Heterocyclic Carbene Ligands and Their Complexes with Palladium

A new series of chelating N-heterocyclic carbene (NHC) ligands and their complexes with palladium are described. The ligands feature a chelating phenolic unit, thereby expanding the class of available NHC ligands for organometallic catalysis.

Chelating N-heterocyclic carbene ligands in palladium-catalyzed heck-type reactions

A novel synthesis of palladium complexes with chelating N-heterocyclic carbene ligands is reported and direct proof is presented by X-ray diffraction. These catalyst precursors are efficiently applied in palladium-catalyzed Heck-, Suzuki- and alkyne coupling reactions, demonstrating the versatility of our ligand concept in homogeneous catalysis.