Synthesis and characterization of Pt(II) and Au(I) complexes with N-oxy-heterocyclic carbene ligands (NOHCs)

Two N-alkyloxy-N'-phenylimidazolium proligands and the corresponding platinum(II) cyclometalated N-alkyloxyimidazol-2-ylidene complexes with β-diketonate auxiliary ligands, [(CNOHC^C*)Pt(L∩L)] (L∩L = acetyacetonate (acac) or 1,3-bis(2,4,6-trimethylphenyl)-propane-1,3-dionato (mesacac)) were synthesized and fully characterized. In addition, a Au(I) monocarbene complex was synthesized, isolated and characterized. Solid-state structures of two cyclometalated platinum(II) NOHC complexes and the Au(I) NOHC complex were obtained providing structural proof.

N-Alkoxyimidazolylidines (NOHCs): nucleophilic carbenes based on an oxidized imidazolium core

We report the first preparation of N-alkoxyimidazolylidene (NOHC), a nucleophilic carbene based on an oxidized imidazolium core. The Arduengo-type analogous carbene center shows the most upfield ¹³C NMR shift compared to common NHCs. The obtained gold(I) complex of the carbene follows the ¹³C NMR upfield trend and shows the marked influence the alkoxy substituents. Similarly, the ⁷⁷Se and ¹⁵N NMR shifts of a range of NOHC-selenium adducts show increased σ-donation and decreased π-back donation in the bonding with the nucleophile. This extension of the NHC family provides altered electronic properties for the use of such carbenes as ligands or catalysts.

Tris(pentafluoroethyl)difluorophosphorane: A Versatile Fluoride Acceptor for Transition Metal Chemistry

Fluoride abstraction from different types of transition metal fluoride complexes [Ln MF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C2 F5 )3 PF2 to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2 F5 )3 PF3 ]- ) is reported. (C2 F5 )3 PF2 reacted with trans-[Ni(iPr2 Im)2 (ArF )F] (iPr2 Im=1,3-diisopropylimidazolin-2-ylidene; ArF =C6 F5 , 1 a; 4-CF3 -C6 F4 , 1 b; 4-C6 F5 -C6 F4 , 1 c) through fluoride transfer to form the complex salts trans-[Ni(iPr2 Im)2 (solv)(ArF )]FAP (2 a-c[solv]; solv=Et2 O, CH2 Cl2 , THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh3 , solvent coordination was suppressed and the complexes trans-[Ni(iPr2 Im)2 (PPh3 )(C6 F5 )]FAP (trans-2 a[PPh3 ]) and cis-[Ni(iPr2 Im)2 (Dipp2 Im)(C6 F5 )]FAP (cis-2 a[Dipp2 Im]) (Dipp2 Im=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were isolated. Fluoride abstraction from [(Dipp2 Im)CuF] (3) in CH2 Cl2 or 1,2-difluorobenzene led to the isolation of [{(Dipp2 Im)Cu}2 ]2+ 2 FAP- (4). Subsequent reaction of 4 with PPh3 and different carbenes resulted in the complexes [(Dipp2 Im)Cu(LB)]FAP (5 a-e, LB=Lewis base). In the presence of C6 Me6 , fluoride transfer afforded [(Dipp2 Im)Cu(C6 Me6 )]FAP (5 f), which serves as a source of [(Dipp2 Im)Cu)]+ . Fluoride abstraction of [Cp2 TiF2 ] (7) resulted in the formation of dinuclear [FCp2 Ti(μ-F)TiCp2 F]FAP (8) (Cp=η5 -C5 H5 ) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand.

2-Mercaptoimidazolium halides: structural diversity, stability and spontaneous racemisation

The complementarity of experiment and theory unravelled structural features, potential polymorphism, moisture and temperature dependent stability and the racemisation behaviour of 2-mercaptoimidazolium salts.

Surface Enrichment in Equimolar Mixtures of Non-Functionalized and Functionalized Imidazolium-Based Ionic Liquids

For equimolar mixtures of ionic liquids with imidazolium‐based cations of very different electronic structure, we observe very pronounced surface enrichment effects by angle‐resolved X‐ray photoelectron spectroscopy (XPS). For a mixture with the same anion, that is, 1‐methyl‐3‐octylimidazolium hexafluorophosphate+1,3‐di(methoxy)imidazolium hexafluorophosphate ([C₈C₁Im][PF₆]+[(MeO)₂Im][PF₆]), we find a strong enrichment of the octyl chain‐containing [C₈C₁Im]⁺ cation and a corresponding depletion of the [(MeO)₂Im]⁺ cation in the topmost layer. For a mixture with different cations and anions, that is, [C₈C₁Im][Tf₂N]+[(MeO)₂Im][PF₆], we find both surface enrichment of the [C₈C₁Im]⁺ cation and the [Tf₂N]⁻ (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO)₂Im]⁺ and [PF₆]⁻ are depleted from the surface. We propose that the observed behavior in these mixtures is due to a lowering of the surface tension by the enriched components. Interestingly, we observe pronounced differences in the chemical shifts of the imidazolium ring signals of the [(MeO)₂Im]⁺ cations as compared to the non‐functionalized cations. Calculations of the electronic structure and the intramolecular partial charge distribution of the cations contribute to interpreting these shifts for the two different cations.

Synthesis and crystal structures of 2-bromo-1,3-di-methyl-imidazolium iodides

Attempts at direct bromination of 1,3-di-methyl-imidazolium salts were futile. The title compounds, 2-bromo-1,3-di-methyl-imidazolium iodide chloro-form 0.33-solvate, C5H8BrN2+·I-·0.33CHCl3, 2-bromo-1,3-di-methyl-imidazolium iodide di-chloro-methane hemisolvate, C5H8BrN2+·I-·0.5CH2Cl2, and 2-bromo-1,3-di-methyl-imidazolium iodide hemi(diiodide), C5H8BrN2+·I-·0.5I2, were obtained by methyl-ation of 2-bromo-1-methyl-imidazole. They crystallized as CHCl3, CH2Cl2 or I2 solvates/adducts. The Br atom acts as a σ-hole to accept short C-Br⋯I inter-actions. C-H⋯I hydrogen bonds are observed in each structure.

Synthesis and crystal structures of non-symmetric 1,3-di(alkyloxy)imidazolium salts

A series of non-symmetric 1,3-di(alkyloxy)imidazolium salts were synthesized by stepwise alkylation of 1-hydroxyimidazole-3-oxide. The quaternary salts were subsequently functionalized by bromination in 2-position. A 2-azidoimidazolium salt and an imidazoline-2-thione were prepared exemplarily. Crystal structures of two 2-bromo-1-alkyloxy-3-methyloxyimidazolium tribromides and a mercury(II)-thione complex have been determined by X-ray diffraction.

2-(2-Azabicyclo[3.2.1]octa-3,6-dien-2-yl)-1,3-dimethoxyimidazolium hexafluoridophosphate

The title salt, C12H16N3O2+·PF6−, was obtained by the dipolar cycloaddition of norbornadiene to 2-azido-1,3-dimethoxyimidazolium hexafluoridophosphate. The methoxy groups attached to the imidazolium ring of the cation adopt ananticonformation [displacements of the C atoms from the ring plane = 1.386 (4) and −1.404 (3) Å]. In the crystal, weak inter-ionic C—H...F contacts are observed. The structure was refined as a two-component twin. Positional disorder of the fluorine atoms of the PF6anion was observed, the occupancy ratio being 0.562 (16):0.438 (16).

Synthesis and reactions of 2-azido-1,3-di(benzyloxy)imidazolium hexafluoridophosphate

2-Azido-1,3-di(benzyloxy)imidazolium hexafluoridophosphate was obtained from the corresponding 2-bromo compound by reaction with sodium azide. Cycloaddition of the 2-azido compound with norbornene and norbornadiene gave the respective tricyclic aziridine and bicyclic azaoctadiene. Addition of triphenylphosphane yielded the phosphazide which upon heating eliminated dinitrogen to afford the phosphazene. The crystal structures of five compounds were determined by X-ray diffraction.

1,3-Dimethoxy-2-methylimidazolium bis(trifluoromethanesulfonyl)imide

The title molecular salt, C6H11N2O2+·C2F6NO4S2−, was obtained by the methylation of 1-hydroxy-2-methylimidazole 3-oxide and subsequent ion metathesis. In the crystal, C—H...O=S hydrogen bonds and O...π interactions are observed.

Crystal structure of bis(1,3-dimethoxyimidazolin-2-ylidene)silver(I) hexafluoridophosphate, N-heterocyclic carbene (NHC) complex

The title salt, [Ag(C₅H₈N₂O₂)₂]PF₆, was obtained by deprotonation and metalation of 1,3-di­meth­oxy­imidazolium hexa­fluorido­phosphate using silver(I) oxide in methanol. The C—Ag—C angle in the cation is 178.1 (2)°, and the N—C—N angles are 101.1 (4) and 100.5 (4)°. The meth­oxy groups adopt an anti conformation. In the crystal, anions (A) are sandwiched between cations (C) in a layered arrangement {C…A…C}_n stacked along [001]. Within a C…A…C layer, the hexafluoridophosphate anions accept several C—H⋯F hydrogen bonds from the cationic complex.

From unsuccessful H2-activation with FLPs containing B(Ohfip)3 to a systematic evaluation of the Lewis acidity of 33 Lewis acids based on fluoride, chloride, hydride and methyl ion affinities

From hard to soft: The ion affinities of a large set of 33 Lewis acids towards hard and soft bases were examined with a unified isodesmic approach.

1,3-Di(alkoxy)imidazolium-based Ionic Liquids: Improved Synthesis and Crystal Structures

A new and convenient synthetic pathway to 1,3-di(alkoxy)imidazolium bis(trifluoromethylsulfonyl)amides and novel 1,3-di(alkoxy)imidazolium tetrachloroferrates was developed. As an intermediate isolation step of the respective hexafluorophosphates was required in previously reported preparations, they suffered from low overall yields and additional expense. The use of FeCl3/HCl resulted in substantially improved yields and allows one-pot preparations with good scalability. Results of single-crystal X-ray structure determination of the new tetrachloroferrate salts are discussed.