Bis-imidazolium Salt Research Articles

3,3′-Dimethyl-1,1′-methylenediimidazolium tetrachlorocobaltate(II)

The title compound, (C9H14N4)[CoCl4], a methylene-bridged bis-imidazolium salt containing a tetrachlorocobaltate anion, is one of the first examples where an alkyl-bridged bis-imidazolium compound could be structurally characterized. Short C-H...Cl contacts between the imidazolium C-H bonds and the Cl atoms of the counter-anion build up a three-dimensional network and indicate that the C-H bonds are strongly polarized.

Irreversible cleavage of a carbene–rhodium bond in Rh- N-heterocyclic carbene complexes: implications for catalysis

Despite the generally accepted belief that carbene–metal bonds are strong and do not dissociate, the reaction of Rh- N-heterocyclic carbene complexes with triphenylphosphine in dichloroethane was determined to take place via cleavage of the Rh–carbene bond. The products of this reaction are Wilkinson’s catalyst and a bisimidazolium salt derived from reaction between dichloroethane and two equivalents of the carbene. The implications of this reaction for catalysis are significant since the carbene complex shows lower activity than Wilkinson’s catalyst in hydrogenation reactions. In non-halogenated solvents, the catalyst shows higher stability, such that the rate of exchange with free phosphine could be measured, and was determined to be ca. 10 times slower than in Wilkinson’s catalyst.

Tetrafluoroborate anion BF bond activation—unusual formation of a nucleophilic heterocyclic carbene:BF 3 adduct

Tetrafluoroborate anion boronfluorine bond activation, yielding a boron trifluoride-nucleophilic heterocyclic carbene (NHC) adduct, is reported. Bis-[2-(2-(trifluoroborane)-3-methylimidazolin-2-yliden-1-yl)ethyl]ether ( 4) was obtained as a by-product from the reaction of di-silver-bis-[bis-(2-(3-methylimidazolin-2-yliden-1-yl)ethyl)ether] di-tetrafluoroborate ( 3) and ZrCl 4. The Ag I(NHC)(BF 4) complex was prepared from bis-(2-(3-methylimidazolium-1-yl)ethyl)ether diiodide ( 2) and silver(I) oxide; spectroscopic data suggests that it is a dinuclear species. Compounds 2– 4 were spectroscopically characterised, the structures of the NHC:BF 3 adduct and parent bis-imidazolium salt being confirmed by single crystal X-ray studies. The geometry of 4 is similar to that of previously reported NHC:BF 3 adducts. Heating the NHC:BF 3 adduct in dimethyl-d 6 sulfoxide (d 6-DMSO) in the presence of CsF leads to complete hydrogen/deuterium substitution of the imC 4,5 positions. It was found that the H/D substitution was readily effected at room temperature on the free carbene bis-(2-(3-methylimidazolin-2-yliden-1-yl)ethyl)ether. Prolonged heating was required to observe a similar H/D substitution in 3, preliminary to its decomposition, and (2-(3-methylimidazolium-1-yl)ethyl)ether tetrafluoroborate was found to be resistant to substitution.

Mercury(ii) partitioning from aqueous solutions with a new, hydrophobic ethylene-glycol functionalized bis-imidazolium ionic liquidThis work was presented at the Green Solvents for Catalysis Meeting held in Bruchsal, Germany, 13–16th October 2002.

A room temperature ionic liquid containing a bis-imidazolium cation incorporating a short ethylene-glycol spacer, 1,1′-[1,2-ethanediylbis(oxy-1,2-ethanediyl)] bis[3-methyl-1H-imidazolium-1-yl]bis(trifluoromethanesulfonyl)imide, has been prepared from the corresponding chloride salt, and the X-ray crystal structure of the low-melting hexafluorophosphate salt has been determined. The crystal structure reveals the ether linkage to be quite flexible and to participate in strong C2–H⋯O hydrogen bonds leading to asymmetry. The crystal structure of the bis-imidazolium salt incorporating a decyl-spacer, 1,1′-[1,10-decyl]bis[3-methyl-1H-imidazolium-1-yl] hexafluorophosphate, has also been determined and displays an all-trans (symmetric) conformation except at the beta carbon positions where a characteristic kink is observed. Introducing the ethylene-glycol functionality dramatically increases the distribution ratio of mercury ions, but not caesium, from aqueous solution to the hydrophobic ionic liquid, and from basic solution. This is the first example of pH dependent partitioning and stripping of mercury from ionic liquid/aqueous two-phase systems. The crystal structure of the related mercury(II) carbene complex, obtained from the reaction of mercury(II) acetate with 1,1′-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl)]bis[3-methyl-1H-imidazolium-1-yl] tosylate, containing a three-ether spacer, in acetonitrile, reveals the possibility of a carbene extraction mechanism.

Bidentate N-heterocyclic carbene complexes of Group 13 trihydrides and trihalides

Treatment of the potentially chelating bis-carbene, 1,2-ethylene-3,3′-di-tert-butyl-diimidazole-2,2′-diylidene, EtIBut, with [MH3(NMe3)], M = Al, Ga, In, in a 1 ∶ 1 or 1 ∶ 2 stoichiometery led to good yields of the metal rich 2 ∶ 1 adducts, [(MH3)2(μ-EtIBut)]. These complexes have been spectroscopically characterised and the X-ray crystal structures of two (M = Al or In) obtained. In contrast, the 1 ∶ 1 or 1 ∶ 2 reactions of EtIBut with MX3, M = In or Tl, X = Br or Cl, led to the 1 ∶ 1 adducts, [MX3(EtIBut)]. The X-ray crystal structure of one of these, [InBr3(EtIBut)], shows the ligand to act in a chelating mode, taking up equatorial positions of an trigonal bipyramid. The reactions of EtIBut with MCl3, M = Al or Ga, led to decomposition and in the aluminium reaction the major product of the decomposition process was the bis-imidazolium salt, [EtIButH2][Cl]2, which has also been crystallographically characterised. Several related reactions are described.

1,1′-(Pyridine-2,6-diyl)bis(3-benzyl-2,3-dihydro-1H-imidazol-2-ylidene), a new multidentate N-heterocyclic biscarbene and its silver(I) complex derivative

Abstract 1,1′-(Pyridine-2,6-diyl)bis(3-benzyl-2,3-dihydro-1 H -imidazole-2-ylidene) ( 5 ) is a new polydentate N -heterocyclic biscarbene, stable at −20°C in THF solution and obtained by deprotonation of the corresponding bisimidazolium salt 4 . Reaction of 5 with silver(I) triflate yields the silver(I) complex 7 .

First chelated chiral N-heterocyclic bis-carbene complexes

[formula: see text] First chiral bidentate bis-carbene complex of Pd(II) prepared from a bis-imidazolium salt derived from (S)-2,2'-bromomethyl-1,1'-binaphthyl exists in both cis- and trans-square planar geometry. These and the corresponding trans-Ni(II) complexes are remarkably resistant to high temperature, air, water, and silica gel chromatography. The Pd complexes catalyze Heck reactions.

Palladium-bisimidazol-2-ylidene complexes as catalysts for general and efficient Suzuki cross-coupling reactions of aryl chlorides with arylboronic acids

A series of bisimidazolium salts 1– 6 were synthesized and evaluated as precursors to bisimidazol-2-ylidene ligands in palladium-catalyzed Suzuki cross-coupling reactions with aryl chlorides and arylboronic acids. The bisimidazolium salt 6 was found to be superior over imidazolium and other bisimidazolium salts affording high yields of biaryl products employing a wide variety of substrates.