Accessing Chelating Extended Linker Bis(NHC) Palladium(II) Complexes: Sterically Triggered Divergent Reaction Pathways

Electrochemical Synthesis of Gold-N-Heterocyclic Carbene Complexes

An electrochemical synthesis of gold(I)-N-heterocyclic carbene (Au-NHC) complexes has been developed. The electrochemical methodology uses only imidazolium salts, gold metal electrodes, and electricity to produce these complexes with hydrogen gas as the only by-product. This high-yielding and operationally simple procedure has been used to produce eight mononuclear and three dinuclear Au-NHC complexes. The electrochemical procedure facilitates a clean reaction with no by-products. As such, Au-NHC complexes can be directly transferred to catalytic reactions without work-up or purification. The Au-NHC complexes were produced on-demand and tested as catalysts in a vinylcyclopropanation reaction. All mononuclear Au-NHC complexes performed similarly to or better than the isolated complexes.

Effect of methylene versus ethylene linkers on structural properties of tert-butyl and mesityl bis(imidazolium) bromide salts

The crystal structures of ligand precursor bis­(imidaozolium) salts 1,1′-methyl­enebis(3-tert-butyl­imidazolium) dibromide monohydrate, C₁₅H₂₆N₄⁺·2Br⁻·H₂O or [^tBuNHC₂Me][Br]₂·H₂O, 1,1′-(ethane-1,2-di­yl)bis­(3-tert-butyl­imidazolium) dibromide dihydrate, C₁₆H₂₈N₄⁺·2Br⁻·2H₂O or [^tBuNHC₂Et][Br]₂·2H₂O, 1,1′-methyl­enebis[3-(2,4,6-tri­methyl­phen­yl)imidazolium] dibromide dihydrate, C₂₅H₃₀N₄²⁺·2Br⁻·2H₂O or [^MesNHC₂Me][Br]₂·2H₂O, and 1,1′-1,1′-(ethane-1,2-di­yl)bis­[3-(2,4,6-tri­methyl­phen­yl)imidazolium] dibromide tetra­hydrate, C₂₆H₃₂N₄²⁺·2Br⁻·4H₂O or [^MesNHC₂Et][Br]₂·4H₂O, are reported. At 293 K, [^tBuNHC₂Me][Br]₂·H₂O crystallizes in the P2₁/c space group, while [^tBuNHC₂Et][Br]₂·2H₂O crystallizes in the P2₁/n space group at 100 K. At 112 K, [^MesNHC₂Me][Br]₂·2H₂O crystallizes in the ortho­rhom­bic space group Pccn while [^MesNHC₂Et][Br]₂·4H₂O crystallizes in the P2₁/c space group at 100 K.

The crystal structures of ligand precursor bis­(imidazolium) salts 1,1′-methyl­enebis(3-tert-butyl­imidazolium) dibromide monohydrate, C₁₅H₂₆N₄⁺·2Br⁻·H₂O or [^tBuNHC₂Me][Br]₂·H₂O, 1,1′-(ethane-1,2-di­yl)bis­(3-tert-butyl­imidazolium) dibromide dihydrate, C₁₆H₂₈N₄⁺·2Br⁻·2H₂O or [^tBuNHC₂Et][Br]₂·2H₂O, 1,1′-methyl­enebis[3-(2,4,6-tri­methyl­phen­yl)imidazolium] dibromide dihydrate, C₂₅H₃₀N₄²⁺·2Br⁻·2H₂O or [^MesNHC₂Me][Br]₂·2H₂O, and 1,1′-(ethane-1,2-di­yl)bis­[3-(2,4,6-tri­methyl­phen­yl)imidazolium] dibromide tetra­hydrate, C₂₆H₃₂N₄²⁺·2Br⁻·4H₂O or [^MesNHC₂Et][Br]₂·4H₂O, are reported. At 293 K, [^tBuNHC₂Me][Br]₂·H₂O crystallizes in the P2₁/c space group, while [^tBuNHC₂Et][Br]₂·2H₂O crystallizes in the P2₁/n space group at 100 K. At 112 K, [^MesNHC₂Me][Br]₂·2H₂O crystallizes in the ortho­rhom­bic space group Pccn while [^MesNHC₂Et][Br]₂·4H₂O crystallizes in the P2₁/c space group at 100 K. Bond distances and angles within the imidazolium rings are generally comparable among the four structures. All four bis­(imidazolium) salts co-crystallize with one to four mol­ecules of water.

Cationic Charge-Appended Abnormal Carbenes: Synthesis and Study of Electronically Modified Abnormal N-Heterocyclic Carbenes

A range of palladium complexes featuring electronically modified, imidazole-based abnormal N-heterocyclic carbene (aNHC) ligands have been prepared in the hopes of accessing a new class of cationic aNHC ligands electronically distinct from normal NHCs and aNHCs. These palladium complexes represent the first examples of transition metal-ligated aNHC complexes featuring a cationic moiety adjacent to the abnormal carbene center. It was anticipated that these design principles could facilitate electron transfer between the imidazolinylidene and the cationic heterocycle, thus reducing the electron density at the abnormal carbene center. However, this case study suggests that greater conformational restrictions that allow for heterocycle coplanarity are necessary to achieve significant electron transfer and enable access to a new class of cationic charge-appended aNHCs with unique electronic properties.