Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

DOI: 10.1039/b703656f

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Dalton Transactions p. 2696 - 2706 (2007)

Update date:2022-08-04

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Authors:

Alcalde, Ermitas

Ceder, Rosa M.

Lopez, Cristina

Mesquida, Neus

Muller, Guillermo

Rodriguez, Sandra

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Article abstract of DOI:10.1039/b703656f

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b·2Cl were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b·2Br or a six-step sequence for propylene dications 7a,b·2Br in ≥ 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]⁺ (25-26) complexes with the same ligand bridge were synthesized. [PdX₂bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)] ₂ and -bis(carbene)(AgX)₂ complexes in 1: 1 or in 0.5: 1 ratio leads to binuclear compounds [Pd₂Cl₂(allyl) ₂(-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]⁺ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]⁺ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)] PF₆ complexes were inactive as catalytic precursors in the allylic substitution reaction. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703656f

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PAPER

www.rsc.org/dalton | Dalton Transactions

Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines)

with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands

and silver and palladium complexes†‡

Ermitas Alcalde,*^aRosa M. Ceder,^bCristina Lo´pez,^bNeus Mesquida,^aGuillermo Muller*^band

Sandra Rodr´ıguez^a

Received 12th March 2007, Accepted 13th April 2007

First published as an Advance Article on the web 30th April 2007

DOI: 10.1039/b703656f

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-

trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the

nature of the alkylidene arm. For a methylene arm, simple dications 5a,b·2Cl were obtained directly.

The higher homologue counterparts were conveniently prepared by general multistep routes following a

ﬁve-step sequence for ethylene dications 6a,b·2Br or a six-step sequence for propylene dications 7a,b·2Br

in ≥ 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare

palladium complexes (17–20) with N-heterocyclic carbenes via transmetallation from well-deﬁned silver

compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the

palladium species two [Pd(allyl)(bis-oxazoline)]⁺(25–26) complexes with the same ligand bridge were

synthesized. [PdX₂bisL] complexes appeared in solution as mixtures of species, mononuclear with cis-

or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]₂and l-bis(carbene)(AgX)₂

complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd₂Cl₂(allyl)₂(l-bis-carbene)]

(19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]⁺(20a,20b) compounds, respectively. The

preparation of analogous [Pd(allyl)(bis-oxazoline)]⁺complexes showed the formation of one of the four

possible isomers. [Pd(allyl)(bis-oxazoline)]PF₆complexes were inactive as catalytic precursors in the

allylic substitution reaction.

imidazolium salt is performed by the use of a suitable base.

However, a widely employed synthetic strategy introduced by

1. Introduction

Wang and Lin¹²involves the use of silver–NHC complexes as air

stable reagents for carbene transfer reactions in mild conditions.¹³

So, the preparation of bis(imidazolium) salts as intermediates to

obtain polydentate NHC ligands is an attractive ﬁeld of research.

One of the main features of these salts is the nature of the bridging

moiety. The use of a substituted aromatic ring generates different

coordination possibilities depending on the ring substitution, 1,2-

1, 3- or 1,4-.

The study of the coordination features of 1,2-xylyl or similar 1,2-

cyclohexyl bis(carbene) ligands has been reported, and cis-^14,15,16

and trans-¹⁷geometry of the dihalo palladium complexes has been

observed. The meta-xylyl bis(imidazolium) salts could lead to a

coordination in a bidentate or arene-tethered fashion depending

on the overall chain length. Metallation of the central C–H proton

has also been observed giving a pincer ligand, similar to those

obtained when 2,6-substituted pyridine is used as the aromatic

ring.^17,18

Transition metal complexes bearing N-heterocyclic carbene

(NHC) ligands have been known for many years. The ﬁrst

complexes were obtained by deprotonation of imidazolium salts^1,2

and by double bond splitting of electron-rich oleﬁns of the 1,3-

substituted imidazol-2-ylidene moiety.³In the last few years, after

the isolation of stable free carbenes,⁴the number of NHC ligands

described has regularly increased with the emergence of different

applications.⁵In particular, transition metal complexes containing

the new NHC ligands showed excellent performance in a large

number of catalytic processes.^6,7The nature of the metal–carbon

bond in the NHC carbenes has been discussed recently.⁸Between

the valuable properties of the complexes are the higher thermal

stability and the lower sensitivity towards oxygen and moisture.

The relatively easy preparation of the carbene ligands starting

from imidazolium salts allowed their systematic study either as

monodentate, polydentate or mixed donor systems.^9,10,11

Imidazolium salts are frequently used as direct precursors for

metal N-heterocyclic carbene complexes. Deprotonation of the

In this paper we report the synthesis of

a group of

bis(imidazolium) salts containing 1,3-alkylidene-2,4,6-trimethyl-

benzene linkers, where the alkyl arm could be a methylene

(5a,b·2Cl), ethylene (6a,b·2Br) or propylene (7a,b·2Br) fragment

and the preparation of the silver carbene complexes of the shorter

and longer arms, methylene (15a,b) and propylene (16a,b) frag-

ments. We also discuss the coordination features of the potentially

bidentate 1,3-(CH₂ImR)₂-2,4,6-trimethylbenzene bis(carbene)

ligand with palladium, since the use of the trimethylbenzene unit

^aLaboratori de Qu´ımica Orga`nica, Facultat de Farma`cia, Universitat de

Barcelona, Avda. Joan XXIII s/n, E-08028, Barcelona, Spain

^bDepartament de Qu´ımica Inorga`nica, Universitat de Barcelona, Mart´ı

i Franque`s 1-11, E-08028, Barcelona, Spain. E-mail: ealcalde@ub.edu,

guillermo.muller@qi.ub.es

† The HTML version of this article has been enhanced with colour images.

‡ Electronic supplementary information (ESI) available: Fig. S1–S16 (ESI

mass spectra and NMR spectra of selected compounds). See DOI:

10.1039/b703656f

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in the bridge could make metallation processes difﬁcult. As the

geometry and nuclearity of the palladium carbene complexes

obtained was not evident, analogous bis(oxazoline) and carbene-

oxazoline ligands¹⁹were coordinated to support our conclusions.

Well-deﬁned [bis(oxazoline)(allyl)palladium] complexes were in-

active as catalytic precursors in the allylic substitution reaction.

yield. An efﬁcient malonic-ester acid synthesis protocol applied

to bis(chlorobenzylic) derivative 1 afforded dimalonate 11, and

subsequent hydrolysis gave dimalonic acid 12, which underwent

decarboxylation to give diacetic acid 13. This was subsequently

reduced to dipropan-1-ol 14 which was converted to the key bis(3-

bromopropyl) intermediate 3.

2.2 Synthesis of silver–carbene complexes

2. Results and discussion

Silver carbene complexes display a wide variety of structures

depending on several factors.¹³In solid state they tend to

dimerize–oligomerize via silver–silver interactions or bridging

ligands like halide or polycarbenes.^22,23,24Since the discovery

of the transmetallation reaction,¹²they have regularly been

used as intermediate species to generate transition-metal–carbene

bonds because the reactions proceed under mild conditions and

without the use of strong bases. So, silver dinuclear complexes,

[1,3((CH₂)_nImMeAgCl)₂]-2,4,6-Me₃C₆H] (15–16) were obtained

by reacting the corresponding imidazolium salt and Ag₂O in

CH₂Cl₂(Scheme 3). The white complexes are stable in the solid

state in the dark, but on standing in solution, the formation

of insoluble oligomeric species was observed and was faster

with the complexes containing the N-mesitylene substituent. The

analytical data and NMR spectra of the complexes are compatible

with the expected simple halide and carbene coordination to

silver. Crystal structure determination for analogous meta-xylyl

complexes, where R = Me, has been reported.²⁵

2.1 Preparation of the imidazolium precursors

Dicationic ligand precursors 5b·2Cl, 6a,b·2Br and 7a,b·2Br

were prepared following standard protocols for quaternizing N-

substituted imidazoles.¹⁹Dications 5a·2Cl and 5b·2Cl have been

described by Cristau et al.²⁰and Trudell,²¹respectively. Reac-

tion of bis(haloalkyl)-1,3,5-trimethylbenzenes 1, 2 or 3 with N-

substituted imidazoles 4a or 4b under neutral conditions produced

the targeted dications 5b·2Cl, 6a,b·2Br and 7a,b·2Br in yields

ranging from 76% to 98% (Scheme 1).

Scheme 1

From 2,4-bis(chloromethyl)-1,3,5-trimethylbenzene 1, both di-

cations 5a·2Cl²⁰and 5b·2Cl²¹were prepared directly, whereas

multistep routes were necessary to obtain the bis(bromoalkyl)

intermediates 2 and 3 (Scheme 2). The synthesis of the key

intermediate 2 proceeded in four-steps starting from compound

1 in 74% overall yield, by transformations to known compound

8¹⁹and hydrolysis to the corresponding diacetic acid 9, fol-

lowed by reduction of this with NaBH₄/I₂to bis(hydroxyethyl)

derivative 10 in excellent yield. Using conc. HBr, the desired

bis(bromoethyl) key compound 2 was obtained. Following a ﬁve-

step sequence, preparation of bis(bromopropyl) key compound

3 was carried out from bis(chloromethyl)arene 1 in 68% overall

Scheme 3

The proton NMR spectra of the complexes at room temperature

showed well-deﬁned signals. The main differences with the spec-

trum of the corresponding bis(imidazolium) salts are the absence

of the 2H-imidazolium proton and the high ﬁeld shift of the

4H and 5H-imidazolium hydrogens. The protons of each of the

methylene groups of the alkyl bridge appeared as single signals

coupled between them as expected. The ¹³C{ H} NMR spectrum

of complex 15a was recorded to compare with published results,

the carbene resonance appeared at 179.4 ppm in CDCl₃and in

pincer silver complexes at 179.8 or 181.1 ppm.^5,22

2.3 Synthesis and characterisation of the palladium(II) carbene

complexes

To explore the coordination features of the 1,3-(CH₂ImR)₂-2,4,6-

trimethylbenzene moiety as a potential bidentate ligand we tried

to obtain the [PdCl₂(bis-carbene)] complex. Using either the direct

reaction of palladium acetate with the imidazolium salt 5 or

the carbene transfer from the silver complex 15a to a palladium

Scheme 2

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dichloro complex with a labile ligand the same white precipitate

17 was obtained (Scheme 4).

Scheme 5

in mild conditions as white solids sparingly soluble in common

organic solvents but soluble enough to obtain acceptable proton

and ¹³C{ H} spectra. Analytical data were disappointing but

Scheme 4

mass spectra (ESI+) gave the expected pattern for the [M–Cl]⁺

and [M–2Cl]²⁺fragments of both compounds without signiﬁcant

contamination (see ESI‡).

As reported for similar complexes^14,15,17compound 17 has

limited solubility in several common solvents and unfortunately

the H-NMR spectrum obtained showed a single group of very

The signals at 181.18 and 182.6 ppm of the ¹³C{ H} spectrum

of 19a and 19b could be assigned to the carbenic carbon in

good agreement with published results which were 175–215 or

173–177 ppm.^26,27The difference of chemical shifts between the

terminal allylic carbons was high, and the signals appear at 71.76

and 48.5 ppm for 19a and at 70.87 and 49.23 ppm for 19b. The

proton 1D NMR spectrum of 19a showed one set of singlets

for the different types of hydrogen atoms. The allyl ligand gives

rise to the expected two syn- and two-anti-protons in agreement

with the asymmetric structure of the complex. Since the plane

deﬁned by the carbene ligand adopts a perpendicular disposition

with respect to the coordination plane,²⁶two isomers of the allyl

complex depending on the position of the central allylic methyl

with respect to the different N-substituents could be obtained.

The 2D NOESY experiment showed a fast interchange of the

syn and anti allylic protons with the second isomer present in

an almost undetectable amount in the 1D spectrum (see ESI‡),

where the exchange mechanisms involved are the pseudorotation

of the whole allyl ligand and the g –g –g isomerization process.

The exchange between syn/anti allyl methylene proton pairs was

selectively observed with one pair that could be considered cis to

the carbene atom according to the strong trans-inﬂuence of the

carbene ligand.²⁶The proton NMR spectrum of 19b showed some

interesting features: the signals of the para-methyl group and the

methine proton of the mesitylene ligand appeared duplicated, and

four signals could be assigned to the ortho-methyl groups of the

same ligand. Furthermore, one syn and one anti proton of the

set of four signals of the allylic ligand of each isomer appeared

duplicated. So, in the compound 19b the rotation around the N-

mesitylene bond was restricted at room temperature and the two

allylic isomers present in the same amount were observed. The

proton signals of the spacer group appeared as singlets, as did the

methine hydrogens of the imidazole skeleton.

broad signals, although they were in the positions expected for

the coordinated carbene. The analytical data was disappointing

but the mass spectra showed the correct mass distribution for the

[PdCl(bis-carbene)]⁺fragment and a major signal pertaining to the

“Pd-bis(carbene)” cation. Ions with a ratio palladium : bidentate

ligand different to 1 : 1 were not observed. The ¹³C{ H} spectrum

showed the expected group of signals but duplicated, the minor

set sharp and the major set broad signals suggesting the presence

of at least two different species in solution. The carbenic carbon

appeared at 170.18 (sharp) and 169.60 (broad) ppm.

Therefore, since metallation products were not detected, all data

point towards mononuclear species, probably mixtures of trans-

and the two conformations of the cis-complexes (Scheme 4). The

assignment of a trans- or cis-geometry is not evident, central ortho-

xylyl groups, cis-,¹⁵trans- or dinuclear species¹⁷were determined by

X-ray crystallography. With central meta-xylyl groups it is even less

evident. Furthermore, in a rigid environment the proton signals

of the methylene linkers would be different in both geometries, so

it is unhelpful in supporting geometric assignations.

To obtain additional information about the coordination

fashion of the bis(carbene) ligand, the transmetallation reaction

between 15a and [PdClMe(COD)] was performed. The white

product obtained (18), scarcely soluble in common organic

solvents, showed analogous erratic analytical data and very broad

proton NMR signals. A signal at 0.55 ppm could be associated

to the methyl group bonded to palladium and the mass spectrum

(ESI+) presents the fragment [PdMe(bis-carbene)]⁺at 419.12 uma.

No new information about the geometry of the compounds can

be deduced.

Allyl complexes [{PdCl(g³-2-CH₃C₃H₄)}₂(l-carbene)] (19) and

[Pd(g³-2-CH₃C₃H₄)(carbene)]PF₆(20). The methodology of

transmetallation using the silver carbene complex proposed by

Lin et al.¹²was used to obtain allyl palladium complexes. This

methodology has previously been used successfully.^26–28Changing

the stoichiometry of the allyl palladium precursor it was possible

to obtain well-deﬁned neutral complexes (19) where the carbene

operates in a monodentate fashion, or cationic allyl carbene

complexes that can be cis-mononuclear species or oligomers

(20) (see Scheme 5). Compounds 19a and 19b were obtained

Slightly coloured solids 20 were obtained when the preparation

of cationic mononuclear allyl complexes with a bidentate coordi-

nation mode of the bis(carbene) ligand (Scheme 5) was attempted

using the same preparative method that led to compounds 19.

However, proton NMR spectra of the complexes in acetone

showed very broad signals where it was not possible to observe

the allylic protons. Only the analytical data of compound 20b,

which was obtained using a preformed allyl cationic complex, gave

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reasonable results but when the dinuclear compound [PdCl(allyl)]₂

in the presence of NH₄PF₆was used to prepare 20b, a signal at

8.6 ppm was observed in the ¹H NMR spectrum in the range of the

2H-imidazolium proton in the imidazolium salts. Furthermore,

in the MS (ESI+) spectrum a fragment was observed at 339.14

uma corresponding to a [M + H]²⁺stoichiometry, coherent with a

dicationic allyl palladium compound with one arm of the former

bis(carbene) coordinated and the second arm converted into the

original imidazolium cation (see ESI‡). Metallation reactions

cannot be discarded but no clear evidence suggests that kind

of process. The dynamic behaviour of this kind of bis(carbene)

ligand bridge, even when the unit is ﬁxed with a Pd–carbon

bond in pincer systems,^{14,15,17,29,30}makes it difﬁcult to conclude

clearly about the real capacity to act as a bidentate ligand in

a cis-coordination mode. It was not possible to obtain suitable

crystals to perform X-ray diffraction studies. For this reason we

used the analogous available bis(oxazoline) ligands in order to

obtain a clear conﬁrmation of the coordination admitted by the

1,3-dimethylene-2,4,6-trimethylbenzene bridge.

The mixture of the bis(oxazoline) (S,S)-21d with [PdCl₂(COD)]

in CHCl₃led to the substitution of COD (Scheme 7). The proton

NMR spectrum of the solid obtained (24) showed broad signals

with the chemical shifts and with some of the splittings of the

diastereotopic protons of the methylene linkers expected for the co-

ordinated bis(oxazoline). The spectrum also showed the presence

of at least two different products. The methyl protons of the iso-

propyl group appear as two doublets compatible with a mononu-

clear trans-complex or polynuclear oligomers. Mass spectrometry

(ESI+) points to the presence of fragments containing one or two

bis(oxazoline) with one or two palladium centres. Similar results

were observed by Danopoulos with 1,2-xylyl dicarbene ligands.¹⁷

2.4 Synthesis and characterisation of the allylic palladium(II)

bis(oxazoline) complexes (25c, 25d)

Scheme 7

In order to compare the suitability of the 1,3-dimethylene-

2,4,6-trimethylbenzene bridge in bis(oxazoline) ligands analo-

gous to the bis(carbene) described previously, we used two

bis[(oxazolinyl)methyl]-2,4,6-trimethylbenzene compounds (21c,

(S,S)-21d) already reported by us (Scheme 6).¹⁹These ligands

could be useful to conﬁrm the coordination in a bidentate fashion

giving either cis- or trans-mononuclear complexes.

The reaction of [Pd(g -2-Me-C₃H₄)(l-Cl)]₂with the appropri-

ate bis(oxazoline) ligand (L = 21c, (S,S)-21d) in the presence

of ammonium hexaﬂuorophosphate salt, afforded ionic allylic

monometallic palladium square-planar complexes of general

formula [Pd(g -2-Me-C₃H₄)(L)]PF₆(25c,d) (Scheme 8), where

the coordination of the bis(oxazoline) to the palladium is in a

bidentate cis-fashion. Studies of the coordination chemistry of

bis(oxazolinyl)benzene derivatives, which differ in the relative

position of the two oxazoline moieties on the phenyl group,

showed that the proximity of two nitrogen atoms in the ortho-

bis(oxazoline) facilitates chelate coordination, while for meta-

and para-bis(oxazoline) ligands only bridging coordination is

observed.³²

Scheme 6

Scheme 8

The preparation of the bis(oxazoline) ligands was performed

starting from the diacetonitrile intermediate. After mixing with

the proper aminoalcohol, the condensation and ring closing took

place in one step. Similar methodology can be used to prepare the

analogous ligands but with longer spacers. As shown in Scheme 6,

bis[(oxazolyl)ethyl]-2,4,6-trimethylbenzene 23 was prepared from

dipropanoic acid 13, via N-acylation giving compound 22, and this

was subsequently converted to bis(oxazoline) 23 in 36% overall

yield.

Similar bis[(oxazolyl)methyl]-4,6-dimethylbenzene pincer lig-

ands obtained in the same way have been previously described.

The reaction with RhCl₃led to cyclometallated mononuclear

pentacoordinated rhodium(III) complexes and in some cases to

trans-[Rh(II)Cl₂(bisox)] compounds.³¹

The complexes were fully characterised by the usual techniques.

The ¹H NMR spectrum of the allylic palladium complex 25c

showed a symmetrical structure in solution, with only one kind

of oxazolinyl, mesityl and methylene protons. The syn and anti

protons of the methylallyl group were also equivalent. The protons

of both methylene groups, those associated with the mesityl group

and with the oxazolinyl group are diastereotopic and appear as

an AB pattern at room temperature in 400 MHz instruments.

However, the methyl groups of the oxazoline appeared as a

single signal. The 2D NOESY spectrum enables the allyl and

the bis(oxazoline) protons to be correlated by NOE contacts.

There are close contacts between the syn and anti protons and

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between the syn and the methyl protons of the allyl ligand, and also

between the ortho methyl and methylene groups of the mesitylene

and between the methyl and methylene groups of the oxazolinyl

fragment. But the most interesting interligand NOE interactions

occur between the anti allyl protons and the ortho methyl protons

of the mesitylene and between syn protons and the allyl methyl

with the methyl groups of the oxazoline. All these data suggest

the structure of the single isomer in which the plane containing

the allylic group and that containing the mesityl group must be

nearly perpendicular one with respect to each other with the ortho

methyl of the mesityl and anti allyl protons in close contact, exactly

like the conformer A (Fig. 1). Preliminary molecular mechanics

calculations also point in this direction when the four possible

conformations are compared. Fig. 1 depicts the two conformations

coherent with the NMR data.

mechanism could probably be different.²⁹MS (ESI+ and FAB)

data of both complexes gave the mass of the molecular allyl

cation.

We tested the catalytic activity of complex (S,S)-25d in

the allylic alkylation of the model substrates, rac-3-acetoxy-

1,3-diphenyl-1-propene (cinammyl acetate) and (E)-3-acetoxy-1-

phenyl-1-propene (1,3-diphenyl acetate) using dimethyl malonate

as nucleophile, under basic conditions, in dichloromethane as

a solvent and at room temperature, but the system was not

active after 24 h. To understand these results, we synthesized

the analogous complex [(1,3-Ph₂-C₃H₃)Pd((S,S)-21d)]⁺, usually

accepted as the Pd intermediate under catalytic conditions.³⁴

We prepared [(1,3-Ph₂-C₃H₃)Pd((S,S)-21d)]BF₄by the reaction of

[Pd(1,3-Ph₂-C₃H₃)(l-Cl)]₂, with AgBF₄and the oxazoline ligand

(S,S)-21d in THF (eqn (1)). Puriﬁcation by precipitation from an

acetone solution with diethyl ether gave the desired complex as

an orange solid (26). The insolubility of the complex avoided the

obtainment of a useful ¹H NMR spectrum at room temperature.

Evidence of the proposed monometallic complex in which the

oxazoline ligand is bidentate comes from elemental analysis and

the molecular ion ([(1,3-Ph₂-C₃H₃)Pd((S,S)-21d)]⁺) in the MS

(FAB positive spectrum). So, the steric hindrance of the more

stable conformer of the bis(oxazoline) complexes (Fig. 1) could

be responsible for the lack of activity in the catalytic reaction

probably in the oxidative addition step.

Fig. 1 Different bidentate coordination modes of the bis(oxazoline)

ligands.

(1)

In the ¹H NMR spectrum of complex (S,S)-25d containing the

(S,S)-21d oxazoline a single isomer was detected, contrasting with

the results obtained with allylic palladium complexes containing

bis(oxazolinyl)benzene.³³Owing to the loss of C₂symmetry of

the ligand upon coordination, the two oxazoline fragments are

not equivalent in a cis-environment and therefore each of the anti

and syn allyl protons exhibit different chemical shifts. All four

methylene protons of the oxazoline moiety appeared separately as

2.5. Synthesis and characterisation of the silver and palladium(II)

oxazoline–carbene complexes

Mixed bidentate carbene–oxazoline ligands have been reported,³⁵

and since we recently obtained oxazolinyl-imidazolium salts

containing the same mesityl bridge¹⁹it was possible to check

the behaviour of the ligand precursor 27. To obtain analogous

palladium complexes (20) we used the same strategy based

on the carbene transfer from the silver N-heterocyclic carbene

complex (Scheme 9). Stirring the imidazolium salt 27 with Ag₂O

in dichloromethane at room temperature for 3 h afforded the

silver–NHC complex 28. The proton NMR spectra at room

temperature showed relatively sharp signals and the absence of

the 2H-imidazolium hydrogen. The CH₂protons of the two linkers

appeared as singlets.

triplets (J_gem∼ J_HH). Four signals were observed for the methyls of

the isopropyl groups. In order to elucidate the solution structure

of this complex, the NOESY experiment was especially useful

because it allowed us a complete correlation between protons. One

of the allyl syn protons was assigned on the basis of the NOESY

experiment, since in the monodimensional H NMR this proton

is not distinguished because it is overlapped with the methylene

signals of the bridge. The NOE contacts observed were similar to

those described for complex 25c. Furthermore, anti allyl protons

interact with the aromatic CH proton and one syn proton showed

a contact with one methyl of an isopropyl group. These data were

also consistent with the same conformation A (Fig. 1) similar to

that found in 25c. The NOESY spectrum also reveals exchange

signals of the syn and anti allyl protons of (S,S)-25d with a minor

isomer which due to its low abundance cannot be characterised

from the ¹H NMR spectrum. Both allyl complexes showed

exchange signals between the pairs of methylene protons of the

linkers and the oxazoline according to a complete interconversion

of the coordination sphere with respect to the plane deﬁned by the

phenyl linker. Miecznikowski et al. studied similar interchanges in

the methylene linkers of Pd pincer complexes, although here the

Scheme 9

The silver complex 28 was reacted with [PdCl₂(NCPh)₂] and [(g -

CH₃C₃H₄)PdCl]₂to yield the corresponding palladium complexes.

The solution of the reaction with the dichloropalladium complex

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contains a mixture of different compounds similar to those

observed with the bisoxazoline or biscarbene ligands. The solid

obtained when CH₂Cl₂solutions of the silver adduct and [Pd(g -

environment makes metallation reactions difﬁcult although the

steric crowding prevents easy access to the metallic centre.

2-Me-C₃H₄)(l-Cl)]₂were stirred for 6 h at room temperature

gave the correct mass spectrum of 29 (ESI(+)-MS, m/z 486.3

uma), although no reproducible analytical data were obtained.

The proton NMR spectrum in CDCl₃showed very broad signals

but in acetone it was possible to observe a group of well-deﬁned

signals where the two protons of the methylene linkers and the

two methyl substituents of the oxazoline ring appeared separately.

Therefore, it was assumed that the mononuclear complex 29 where

the carbene–oxazoline ligand is coordinated in a bidentate fashion

was present in the solution mixture.

5. Experimental

General methods

All compounds were prepared under

a puriﬁed nitrogen

atmosphere using standard Schlenk and vacuum-line techniques.

The solvents were puriﬁed by standard procedures and distilled

under nitrogen. [Pd(g -2-Me-C₃H₄)(l-Cl)]₂,³⁶[Pd(g -1,3-Ph₂C₃H₃)-

(l-Cl)]₂,³⁷1-mesityl-1H-imidazole 4b,³⁸3,3^ꢀ-[(2,4,6-trimethyl-1,3-

phenylene)bis(methylene)]bis(1-methyl-1H -imidazol-3-ium)

dichloride 5a·2Cl,²⁰2,2^ꢀ-(2,4,6-trimethyl-1,3-phenylene)diaceto-

nitrile 8,¹⁹1,3-bis-[(4,4-dimethyl-4,5-dihydrooxazol-2-yl)methyl]-

2,4,6-trimethylbenzene 21c,¹⁹1,3-bis{[(4S)-4-isopropyl-4,5-dihy-

drooxazol-2-yl]methyl}-2,4,6-trimethylbenzene (S,S)-21d,¹⁹and

1-[3-(4,4-dimethyl-4,5-dihydro-2-oxazolyl)methyl-2,4,6-trimethyl-

benzyl]-3-methylimidazolium bromide 27¹⁹was prepared as pre-

viously described. 2,4-Bis(chloromethyl)-1,3,5-trimethylbenzene

1, 1-methyl-1H-imidazole 4a, 2-amino-2-methyl-1-propanol were

purchased from commercial sources. Melting point: CTP-MP

4. Conclusions

In summary, new simple bis(imidazolium)-based ligand precursors

containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer were

prepared using efﬁcient multi-step synthetic routes. Depending on

the length of the alkylidene side chain, imidazolium-based sub-

strates lead to different coordination possibilities, either bidentate

fashion or through the tethering of the phenyl ring. The general

synthetic protocols applied to these simple imidazolium-based

frameworks should be adapted to a variety of chiral frameworks,

thereby developing their catalytic performance in asymmetric

reactions.

300 hot-plate apparatus with ASTM 2C thermometer. The H

and ¹³C, NMR spectra were recorded on either a Varian XL-500

and Mercury-400 MHz or Varian Gemini 200 and 300 MHz,

(standard SiMe₄) spectrometers in CDCl₃unless otherwise cited.

Chemical shifts in ppm were reported downﬁeld from standards.

The two-dimensional experiments were generally carried out with

a Bruker DMX500 or a Varian XL-500 instrument. IR spectra

were recorded on the following spectrometers: FT-IR Avatar

330 and FTIR Nicolet 5700. MS (ESI+ or FAB) spectra were

obtained with a Fisons V6-Quattro or an Agilent LC/MSD-TOF

spectrometer. Organometallic samples were introduced with 1%

formic acid. Mass spectra (CI or EI at 70 eV) were obtained

using a Hewlett-Packard spectrometer (HP-5989A model). TLC:

Merck precoated silica gel 60 F254 plates or Merck neutral

aluminium oxide 60 F254 plates using UV light (254 nm) as a

visualizing agent and/or H₂PtCl₂3% aq./KI 10% aq. (1 : 1)

or KMnO₄ethanolic solution. Column chromatography was

performed on silica gel 60 ACC 35–70 lm Chromagel (SDS) or

neutral aluminium oxide 90 activity II-III (Merck). The routine

GC analyses were performed on a Hewlett-Packard 5890 Series

II gas chromatograph (50-m Ultra 2 capillary column) with a

FID detector. Elemental analyses were carried out by the Serveis

From these bis(imidazolium) salts it was possible to obtain

silver carbene complexes which could be used in transmetallation

reactions.

We also studied the coordination mode of the bis(carbene)

ligand when the spacer is the shorter 1,3-methylene-2,4,6-

trimethylbenzene moiety. In a square planar arrangement around

simple palladium species, mixtures of mononuclear and bin-

uclear compounds could be formed. When a cis geometry is

imposed using allyl ligands, the preparation of neutral complexes

where the bis(carbene) ligand coordinates in a monodentate

fashion was achieved without any problem (19). However, when

it was attempted to coordinate the bis(carbene) ligands in a

bidentate fashion it was not possible to characterize clearly

the species obtained in solution. No decomposition of the allyl

compounds was observed and also no evidence of cyclometal-

lation reactions was noted. The preparation of analogous well-

deﬁned bis(oxazoline) allyl complexes (25) showed that the 1,3-

methylene-2,4,6-trimethylbenzene spacer is compatible with a

cis-coordination, but this is subject to fast dynamic processes,

probably due to the interchange between isomers as depicted in

Fig. 1, and to the complete interconversion of the coordination

sphere with respect to the plane deﬁned by the phenyl linker.

The bis(oxazoline) complex (S,S)-25d was tested as catalytic

precursor in the allylic substitution reaction carried out in mild

conditions.³³The result using rac-cinammyl acetate and (E)-3-

acetoxy-1-phenyl-1-propene was negative, probably due to the

crowding of the palladium centre. However, palladium complexes

with similar bis(carbene) ligands showed activity in Heck or

Suzuki coupling reactions although they were performed at high

temperature.^14,15Therefore the coordination of the bis(carbene)

or bis(oxazoline) ligands containing the 1,3-methylene-2,4,6-

trimethylbenzene spacer in a bidentate fashion in a square planar

Cientiﬁcotecnics of the Universitat Rovira i Virgili in an Eager

1108 microanalyzer.

Synthesis of bis(imidazolium) salts

2,2^ꢀ-(2,4,6-Trimethyl-1,3-phenylene)diacetic acid 9. To 40 mL

of water was added concentrated sulfuric acid (35 mL). When the

solution had cooled to about 50 C, the diacetonitrile 8 (3.50 g,

◦

17.65 mmol) was added and the mixture was reﬂuxed for 12 h. The

resulting mixture was cooled to room temperature and poured into

ice-water (500 mL). The resulting suspension was ﬁltered and dried

◦

under vacuum at 40 C t_◦o give diacid 9 (4.10 g, 98% yield) as a

white solid. Mp 232–234 C. ¹H NMR (400 MHz, CD₃OD): d =

2.33 (s, 3H, CH₃), 2.34 (s, 6H, CH₃), 3.77 (s, 4H, CH₂COOH), 6.96

(s, 1H) ppm. ¹³C NMR (100.6 MHz, CD₃OD): d = 16.5 (CH₃),

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The Royal Society of Chemistry 2007

Dalton Trans., 2007, 2696–2706 | 2701

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−1

20.5 (CH₃), 36.3 (CH₂COOH), 130.8 (CH), 131.3, 136.8, 137.4,

(C O) cm . EI-MS: m/z (%): 157 (100), 185 (70), 213 (67), 287

•

175.6 (COOH) ppm. IR (KBr): m = 1686 (C O), 2950 (COO–

(65), 305 (81), 418 (41) [M⁺−46], 446 (54) [M⁺−18], 464 (4)

•

H) cm⁻¹. EI-MS: m/z (%): 91(41), 145 (53) [M⁺−89], 191 (100)

[M⁺].

•

[M⁺−45], 236 (66) [M⁺], 237 (9) [M⁺+ 1].

2,2^ꢀ-(2,4,6-Trimethyl-1,3-phenylene)dimalonic acid 12. To a

solution of ester 11 (21.39 g, 46.05 mmol) in ethanol (400 mL)

was added a solution of NaOH (27.63 g, 690.75 mmol) in ethanol

(200 mL) under an argon atmosphere. The reaction mixture was

reﬂuxed for 12 h. The resulting suspension was concentrated under

reduced pressure to obtain a white solid, which was dissolved in

water (200 mL). The resulting solution was acidiﬁed with 5N HCl

and extracted with EtOAc (3 × 300 mL). The organic layers were

dried (anhydrous Na₂SO₄), ﬁltered and concentrated to dryness

to o_◦btain acid 12 (16.22 g, 99% yield) as a white solid. Mp 202–

204 C. ¹H NMR (300 MHz, CD₃OD): d = 2.43 (s, 6H, CH₃), 2.47

(s, 3H, CH₃), 3.46 (d, J = 7.3 Hz, 4H, CH₂), 3.67 (t, J = 7.3 Hz,

2H, CH), 6.98 (s, 1H) ppm. ¹³C NMR (75.4 MHz, CDCl₃): d =

16.3 (CH₃), 20.4 (CH₃), 30.0 (CH₂), 53.0 (CH), 131.6 (CH), 134.5,

2,2^ꢀ-(2,4,6-Trimethyl-1,3-phenylene)diethanol 10. To a suspen-

sion of NaBH₄(3.01 g, 79.57 mmol) in dry THF (85 mL) was

added a solution of iodine (8.08 g, 31.84 mmol) in dry THF

(20 mL), at 0 ^◦C under an argon atmosphere. The resulting

solution was heated at reﬂux temperature and then the acid 9

(3.76 g, 15.92 mmol) was added. The reaction mixture was stirred

at reﬂux temperature for 21 h. The resulting mixture was cooled to

room temperature and then methanol was added until an orange

solution was formed. This solution was stirred for an additional

30 min and was concentrated under reduced pressure to give an

orange residue, which was diluted in aqueous 20% KOH solution

(450 mL) and stirred at room temperature for 12 h. The aqueous

solution was extracted with CH₂Cl₂(3 × 200 mL) and the organic

layers were dried (anhydrous Na₂SO₄), ﬁltered and concentrated

to dryness to give alcohol 10 (3.27 g, 99% yield) as a white solid.

136.1, 136.6, 173.0 (C O) ppm. IR (KBr): m = 1702 (C O), 3008

(COO–H) cm⁻¹. EI-MS: m/z (%): 145 (65), 205 (100), 246 (31),

◦

•

Mp 92–94 C. H NMR (400 MHz, CD₃OD): d = 2.33 (s, 6H,

CH₃), 2.39 (s, 3H, CH₃), 2.98 (t, J = 7.4 Hz, 4H, CH₂), 3.64 (t, J =

7.4 Hz, 4H, CH₂OH), 6.87 (s, 1H) ppm. ¹³C NMR (100.6 MHz,

CD₃OD): d = 15.6 (CH₃), 20.2 (CH₃), 34.3 (CH₂), 62.0 (CH₂OH),

131.0 (CH), 133.7, 135.4, 136.1 ppm. IR (KBr): m = 1043 (C–O),

264 (49) [M⁺−88].

3,3^ꢀ-(2,4,6-Trimethyl-1,3-phenylene)dipropanoic acid 13. Acid

12 (16.22 g, 46.04 mmol) was heated at melting temperature for

12 h. The resulting brown residue was basiﬁed with a saturated

aqueous solution of K₂CO₃. The resulting solution was washed

with ethyl acetate (3 × 250 mL). The aqueous layer was acidiﬁed

with 5N HCl and extracted with EtOAc (3 × 300 mL). The organic

layers were dried (anhydrous Na₂SO₄), ﬁltered and concentrated

under reduced pressure t_◦o give acid 13 (9.88 g, 81% yield) as a

beige solid. Mp 158–160 C. ¹H NMR (300 MHz, CD₃OD): d =

2.39 (s, 6H, CH₃), 2.42 (s, 3H, CH₃), 2.50 (t, J = 7.3 Hz, 4H,

CH₂COOH), 3.08 (t, J = 7.3 Hz, 4H, CH₂), 6.95 (s, 1H) ppm.

¹³C NMR (75.4 MHz, CD₃OD): d = 15.2 (CH₃), 19.9 (CH₃), 26.5

•

3290 (O–H) cm⁻¹. EI-MS: m/z (%): 91 (22), 133 (39), 160 (37) [M⁺

•

−48], 177 (100) [M⁺−31], 208 (37) [M⁺], 209 (5) [M⁺+ 1].

2,4-Bis(2-bromoethyl)-1,3,5-trimethylbenzene 2. To alcohol 10

(3 g, 13.60 mmol) was added 48% hydrobromic acid (11 mL,

206.02 mmol), at 0 ^◦C. The resulting mixture was heated at reﬂux

temperature for 3 h. The reaction mixture was cooled and water

(100 mL) was added. The resulting solution was extracted with

CH₂Cl₂(3 × 100 mL). The organic layers were dried (anhydrous

Na₂SO₄), ﬁltered and concentrated under reduced pressure to

obtain bromide 2 (3.81 g, 82% yield) as a beige solid. Mp 142–

144 ^◦C. ¹H NMR (400 MHz, CDCl₃): d = 2.31 (s, 6H, CH₃), 2.33

(s, 3H, CH₃), 3.22 (t, J = 7.4 Hz, 4H, CH₂), 3.37 (t, 4H, J = 7.4 Hz,

CH₂Br), 6.90 (s, 1H) ppm. ¹³C NMR (100.6 MHz, CDCl₃): d =

15.5 (CH₃), 19.9 (CH₃), 29.7 (CH₂), 34.2 (CH₂Br), 130.6 (CH),

134.0, 134.9, 135.2 ppm. EI-MS: m/z (%): 91 (30), 115 (29), 129

(CH₂), 36.6 (CH₂COOH), 131.2 (CH), 134.9, 136.4, 176.9 (C O)

−1

ppm. IR (KBr): m = 1739 (C O), 2968 (COO–H) cm . EI-MS:

•

m/z (%): 145 (65), 205 (100) [M⁺−59], 246 (30) [M⁺−18], 264

•

(48) [M⁺−48].

3,3^ꢀ-(2,4,6-Trimethyl-1,3-phenylene)dipropan-1-ol 14. To

suspension of NaBH₄(1.43 g, 37.85 mmol) in dry THF (40 mL)

was added a solution of iodine (3.84 g, 15.14 mmol) in dry THF

(10 mL), at 0 ^◦C under an argon atmosphere. The resulting solution

was heated at reﬂux temperature and then the acid 13 (2 g,

7.57 mmol) was added. The resulting mixture was reﬂuxed for

21 h. The reaction mixture was cooled to room temperature and

then methanol was added until an orange solution was formed.

This solution was stirred for an additional 30 min and was

concentrated under reduced pressure to give an orange residue,

which was diluted in aqueous 20% KOH solution (200 mL) and

stirred at room temperature for 12 h. The resulting solution was

extracted with CH₂Cl₂(3 × 150 mL). The organic layers were

dried (anhydrous Na₂SO₄), ﬁltered and evaporated to dryness to

give alcohol 14 (1.76 g, 98% yield) as a white solid. Mp 128–130 ^◦C.

¹H NMR (300 MHz, CD₃OD): d = 1.70–1.76 (m, 4H, CH₂), 2.31

(s, 6H, CH₃), 2.35 (s, 3H, CH₃), 2.75 (t, J = 6.8 Hz, 4H, Ar–CH₂),

3.71 (t, J = 6.8 Hz, 4H, CH₂OH), 6.84 (s, 1H) ppm. ¹³C NMR

(75.4 MHz, CD₃OD): d = 15.4 (CH₃), 20.0 (CH₃), 27.3 (Ar–CH₂),

33.6 (CH₂), 63.1 (CH₂OH), 130.9 (CH), 134.0, 134.6, 137.7 ppm.

IR (KBr): m = 1058 (C–O), 3376 (O–H) cm⁻¹. EI-MS: m/z (%): 71

•

(43), 145 (69), 159 (62), 239 (79) [M⁺−95], 241 (81) [M⁺−93],

•

253 (100) [M⁺−81], 255 (93) [M⁺−79], 334 (50) [M⁺], 335 (8)

•

[M⁺+ 1], 336 (27) [M⁺+ 2].

Tetraethyl 2, 2^ꢀ-(2,4,6-trimethyl-1,3-phenylene)dimalonate 11.

To a solution of diethyl malonate (13.9 mL, 92.10 mmol) and

K₂CO₃(31.82 g, 230.25 mmol) in dry acetonitrile (350 mL)

was added under an argon atmosphere a solution of 2,4-

bis(chloromethyl)-1,3,5-trimethylbenzene 1 (10 g, 46 mmol) in dry

acetonitrile (200 mL). The resulting mixture was reﬂuxed for 48 h.

The reaction mixture was cooled to room temperature, ﬁltered

and the solution was evaporated to dryness to yield dimalonate

11 (21.18 g, 99% yield) as a yellow solid. Mp. 58–60 ^◦C. ¹H NMR

(300 MHz, CDCl₃): d = 1.18 (t, J = 7.0 Hz, 12H, CH₃), 2.25 (s, 6H,

CH₃), 2.26 (s, 3H, CH₃), 3.30 (d, J = 7.6 Hz, 4H, CH₂), 3.55 (t, J =

7.6 Hz, 2H, CH), 4.12 (q, J = 7.0 Hz, 8H, CH₂), 6.79 (s, 1H) ppm.

¹³C NMR (75.4 MHz, CDCl₃): d = 13.9 (CH₃), 16.0 (CH₃), 20.1

(CH₃), 28.7 (CH₂), 51.6 (CH), 61.4 (OCH₂), 130.6 (CH), 132.9,

135.2, 135.5, 169.2 (C O) ppm. IR (NaCl): m = 1283 (C–O), 1733

2702 | Dalton Trans., 2007, 2696–2706

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•

(36), 133 (93), 147 (86) [M⁺−89], 191 (100) [M⁺−45], 236 (64)

General procedure for preparation of bis(imidazolium) salts

5b·2Cl, 6b·2Br and 7b·2Br. Halides 1, 2 or 3 (1 equiv) and 1-

mesityl-1H-imidazole 4b (1 equiv) were dissolved in dry DMF

and heated to 100 ^◦C under an argon atmosphere for 12 h and the

solvent was then removed under vacuum. The solids obtained were

washed several times with dry acetone and used without further

puriﬁcation. The yields were not optimized.

•

[M⁺], 237 (12) [M⁺+ 1].

2,4-Bis(3-bromopropyl)-1,3,5-trimethylbenzene 3. To alcohol

14 (6.34 g, 26.84 mmol) was added 48% hydrobromic acid

(21.5 mL, 402.70 mmol), at 0 ^◦C. The resulting mixture was

heated at reﬂux temperature for 12 h. The reaction solution was

cooled and then water (100 mL) was added. The resulting mixture

was extracted with CH₂Cl₂(3 × 250 mL). The organic layers

were dried (anhydrous Na₂SO₄), ﬁltered and concentrated under

reduced pressure to obtain bromide 3 (8.56 g, 88% yield) as a

brown oil. Mp 58–60 ^◦C. ¹H NMR (300 MHz, CDCl₃): d = 1.97–

2.05 (m, 4H, CH₂), 2.30 (s, 6H, CH₃), 2.31 (s, 3H, CH₃), 2.80

(t, J = 6.8 Hz, 4H, Ar–CH₂), 3.53 (t, J = 6.8 Hz, 4H, CH₂Br),

6.87 (s, 1H) ppm. ¹³C NMR (75.4 MHz, CDCl₃): d = 15.3 (CH₃),

19.9 (CH₃), 28.8 (Ar–CH₂), 32.3 (CH₂), 34.0 (CH₂Br), 130.1 (CH),

3,3^ꢀ-[(2,4,6-Trimethyl-1,3-phenylene)bis(methylene)]bis[1-(2,4,6-

trimethylphenyl)-1H-imidazol-3-ium] dichloride 5b·2Cl. The

above procedure was followed using 2,4-bis(chloromethyl)-

1,3,5-trimethylbenzene 1 (1.08 g, 4.98 mmol) and 1-mesityl-1H-

imidazole 4b (1.85 g, 9.95 mmol) and dry DMF (10 mL). The

product was obtained as a solid with 98% yield. Mp 94–96 ^◦C.

¹H NMR (400 MHz, DMSO-d₆): d = 1.95 (s, 12H, CH₃), 2.18 (s,

6H, CH₃), 2.31 (s, 6H, CH₃), 2.47 (s, 3H, CH₃), 3.26 (t, 4H, J =

7.4 Hz, CH₂), 4.46 (t, 4H, J = 7.4 Hz, CH₂-Imi), 7.12 (s, 4H),

7.24 (s, 1H), 7.93 (Imi), 8.20 (Imi), 9.50 (Imi) ppm. ¹³C NMR

(100.6 MHz, DMSO-d₆): d = 15.9 (CH₃), 17.0 (CH₃), 20.0 (CH₃),

20.7 (CH₃), 30.2 (CH₂), 48.3 (CH₂-Imi), 123.8 (Imi), 128.9 (Imi),

129.4 (CH), 130.3 (CH), 131.2, 131.5, 134.4, 135.4, 136.2, 137.6

(Imi), 140.4 ppm. ESI(+)-MS: m/z (%): 273.4 (100) [M]²⁺, 626.7

(11) [M + Br]⁺. Anal. Calcd for C₃₇H₄₆Br₂N₄·H₂O: C, 61.33; H,

6.68; N, 7.73; found: C, 61.41; H, 7.01; N, 7.49.

•

133.8, 134.2, 135.7 ppm. EI-MS: m/z (%): 147 (29), 253 (100) [M⁺

•

−109], 255 (98) [M⁺−107], 362 (25) [M⁺], 363 (4) [M⁺+ 1], 364

•

(12) [M⁺+ 2].

General procedure for preparation of bis(imidazolium) dibromides

6a·2Br and 7a·2Br. Bromides 2 or 3 were dissolved in 1-methyl-

1H-imidazole 4a and heated at reﬂux temperature for 30 min,

under argon atmosphere. After cooling to room temperature and

reduction of the volume, the residue was ground several times

with dry acetone. The resulting solid was ﬁltered and used without

further puriﬁcation. The yields were not optimized.

3,3^ꢀ-(2,4,6-Trimethyl-1,3-phenylenediethane-2,1-diyl)bis[1-(2,4,6-

trimethyl)-1H-imidazol-3-ium] dibromide 6b·2Br. The above pro-

cedure was followed using 2,4-bis(2-bromoethyl)-1,3,5-trimethyl-

benzene 2 (2.50 g, 7.48 mmol) and 1-mesityl-1H-imidazole 4b

(2.86 g, 15.33 mmol) and dry DMF (15 mL). The product was

obtained as a hygroscopic foam with 97% yield. Mp 94–96 ^◦C.