Palladium(II) complexes of imidazolin-2-ylidene N-heterocyclic carbene ligands with redox-active dimethoxyphenyl or (hydro)quinonyl substituents

Article abstract of DOI:10.1016/j.ica.2011.02.007

Four novel imidazolium salts, precursors to N-heterocyclic carbene (NHC) ligands, with 2,5-dimethoxybenzyl or 2,5-dihydroxybenzyl (i.e., p-hydroquinone) substituents have been prepared. The crystal structure of the hydroquinone-substituted imidazolium sal

Full text of DOI:10.1016/j.ica.2011.02.007

Inorganica Chimica Acta 370 (2011) 374–381
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Palladium(II) complexes of imidazolin-2-ylidene N-heterocyclic carbene
ligands with redox-active dimethoxyphenyl or (hydro)quinonyl substituents
1
⇑
Louise A. Berben, Donald C. Craig, Carolina Gimbert-Suriñach , Andrew Robinson, Kristian H. Sugiyarto ,
Stephen B. Colbran
⇑
School of Chemistry, The University of New South Wales, Sydney NSW 2052, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel imidazolium salts, precursors to N-heterocyclic carbene (NHC) ligands, with 2,5-dimethoxy-
benzyl or 2,5-dihydroxybenzyl (i.e., p-hydroquinone) substituents have been prepared. The crystal struc-
ture of the hydroquinone-substituted imidazolium salt H3L3Br reveals Brꢀꢁ ꢁ ꢁH–O bridged chiral chains of
alternating [H3L3]+ cations and Brꢀ counter-ions parallel to the x-axis. Palladium(II) complexes were
accessible from reactions of the dimethoxyphenyl-substituted imidazolium precursors with palladium(II)
acetate, but not from reactions of imidazolium cations with hydroquinonyl substituents. The crystal
structure of the bis(dimethoxybenzyl)-substituted bis(NHC)Pd complex, cis-[PdBr2(L2)] (2), is described.
Puckering of the bis(NHC) ligand leads to a cleft in which an included molecule of dimethylformamide is
Received 18 October 2010
Received in revised form 31 January 2011
Accepted 1 February 2011
Available online 10 March 2011
Keywords:
Palladium complex
N-heterocyclic carbene ligand
Quinone substituent
Oxidation catalysis
situated. The cleft is closed by one of the dimethoxybenzyl groups which
p-stacks with the dimethyl-
formamide; the other dimethoxybenzyl group points away from the cleft and Pd(II) centre. Reaction of
complex 2 with BBr3 afforded the targeted bis(hydroquinone)-substituted bis(NHC)Pd(II) complex 3
(97% yield) which, in turn, was oxidised by 2,3-dichloro-5,6-dicyano-benzoquinone to the corresponding
p-benzoquinone-substituted bis(NHC)Pd(II) complex 4 (98% yield). The cyclic voltammograms of the
Pd(II) complexes 2–4 reveal waves that are attributed to an admix of the anticipated ligand-centred
and [Pd(C-NHC)2Br2]-centred processes.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
tions of Pd(0) to Pd(II) exhibit unfavourable slow kinetics. Thus
an electron transfer mediator is typically added to provide satisfac-
Palladium(II) catalysed oxidation reactions are well known in
the literature [1]. One prominent example is the Wacker process,
the oxidation of ethylene to give acetaldehyde, which has been
used in industry for about 50 years [2]. Other related representa-
tive examples include the oxidative addition of nucleophiles such
as alcohols or amides to alkenes [3], the 1,4-oxidation of conju-
gated dienes [4] and the allylic oxidation of olefins [5]. Pd(II) spe-
cies have also been used to catalyse the copolymerisation of
alkenes and carbon monoxide to give polyketones [6], oxidation
of alcohols [7], oxidative cross-coupling reactions [8], oxidative
heterocyclizations [9] and even the oxidation of methane [10].
More recently, the hydroxylation of arenes under mild conditions
has also been described [11].
torily fast catalysis (Scheme 1) [1a]. One straightforward modifica-
tion of current Pd(II) complexes for oxidative catalysis would be to
incorporate the redox mediator as integral part of the ligand thus
enhancing electron-transfer and facilitating step B in Scheme 1.
Benzoquinone and its redox derivatives comprise perhaps the
quintessential organic proton-coupled electron transfer system
[12]. Benzoquinones have been used either as the final oxidant or
as an electron-transfer mediator in many of the above mentioned
catalytic oxidations [1,2a,2c,4–6,8,9a,10b,11]. In this paper we de-
scribe the synthesis of new Pd(II) complexes incorporating novel
N-heterocyclic carbene (NHC) ligands adorned by electrochemi-
cally-active hydroquinonyl or quinonyl substituents positioned to
facilitate direct (proton-coupled) electron transfer with the Pd cen-
tre (Scheme 2). Pd(II) complexes of N-heterocyclic carbene (NHC)
based ligands were targeted for their high stability, ready availabil-
ity and high catalytic activities [13]. Even though the use of Pd(II)-
NHC complexes has been mainly restricted to C–C bond formation
reactions such as Mizoroki-Heck, Suzuki-Miyaura or Buchwald-
Hartwig cross-coupling processes [14], there are examples where
they act as highly active and selective catalysts in oxidation reac-
tions of the type shown in Scheme 1 [2b,7b,9c,10a,c,15].
In most of the former cases, the actual oxidants are molecular
oxygen or hydrogen peroxide, which thermodynamics reveal are
well able to re-oxidise the Pd(0) to Pd(II). However, the re-oxida-
⇑
Corresponding authors.
E-mail addresses: c.gimbert@unsw.edu.au (C. Gimbert-Suriñach), s.colbran
@unsw.edu.au (S.B. Colbran).
1
On leave from The Department of Chemistry Education, The State University of
Yogyakarta, Indonesia.
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.007

L.A. Berben et al. / Inorganica Chimica Acta 370 (2011) 374–381
375
HO
OH
Ph
Pd(II)
Pd(0)
Substrate
ETMred
Oxidant
B
C
A
Product
ETMox
Sacrificial reduced
compound
O
CHO
O
O
Ph
Ph
ETM: electron-transfer mediator
Scheme 1. Oxidation reaction catalysed by Pd(II) facilitated by an electron-transfer mediator (ETM). A Wacker-Type oxidation reaction using p-benzoquinone as ETM is given
as an specific example.
RO
OR
OR
Br
N
Br
N
Pd
N
N
O
RO
H3CO
N
O
O
OCH3
Br
N
Br
N
I
2
3
R: CH3 ( ), H ( )
Pd
Pd
N
I
N
N
OCH3
N
N
O
H3CO
1
4
Scheme 2. New (NHC)2Pd(II) complexes with electrochemically-active (hydro)quinonyl substituents.
two hydroxyl groups, each in an adjacent [H3L3]+ cation; the
relevant distances are O1ꢁ ꢁ ꢁBr 3.172 Å and O2ꢁ ꢁ ꢁBr 3.245 Å. The
inter-ion hydrogen bonds lead to chiral chains parallel to the
x-axis; each chain is paired by crystallographic inversion with
one of opposite handedness that runs in the opposing direction.
2. Results and Discussion
2.1. Imidazolium salts
The synthesis of the imidazolium salt precursors HL1I and
H2L2Br2 required for the synthesis of complexes 1–4 is outlined
in Scheme 3. The common precursor 1-(2,5-dimethoxybenzyl)-
imidazole (dbi) was obtained by treating imidazole with NaH in
DMF followed by reaction with 2,5-dimethoxybenzyl bromide.
Stirring dbi with methyl iodide in THF afforded HL1I in excellent
overall yield. Alternatively, dbi was reacted with CH2Br2 in toluene
to give H2L2Br2, the precursor to a bidentate bis(NHC) ligand.
Treatment of suspensions of HL1I or H2L2Br2 in dichloromethane
(DCM) with BBr3, followed by methanol, yielded the hydroqui-
none-substituted NHC-ligand precursors H3L3Br and H6L4Br2,
respectively. The imidazolium salts are all easily handled and crys-
tallised solids.
2.3. Palladium(II) N-heterocyclic carbene complexes
As summarised in Scheme 4, the (dimethoxybenzyl-NHC)Pd(II)
complexes [PdI2(L1)2] (1) and [PdBr2(L2)] (2) were readily prepared
by reaction of Pd(OAc)2 with HL1I and H2L2Br2, respectively, in
DMSO at 60 °C. Compound 1 was obtained as a yellow powder in
a modest 20% yield. The 1H NMR spectrum of 1 exhibits twice
the anticipated number of peaks indicative for two species in solu-
tion. In DMSO-d6, the relative abundance of the two species is
approximately 1:1 whereas in CDCl3 it is 5:2. The 5:2 mixture in
CDCl3 converts to a 1:1 mixture in DMSO-d6. The results agree with
simple interconversion between the cis and trans isomers of 1, a
common process in Pd(II) complexes [16].
2.2. X-ray crystal structure of H3L3Br
A single crystal analysis of H3L3Br was undertaken on an apricot
rod-shaped crystal grown from methanol solution. The crystal
structure and labelling scheme is illustrated in Fig. 1. Significantly,
atoms C1 and O1 appear ideally positioned for chelation to a metal
centre. The bond angle at the benzylic bridge between the imidazo-
lium and hydroquinonyl functional groups, N2–C5–C6, is 110.8(3)°,
slightly greater than a perfect tetrahedral angle. All other bond
lengths and angles are within anticipated ranges. An interesting
feature is hydrogen bonding between the hydroquinone groups
and the bromide ions. Each bromide ion is hydrogen bonded with
Complex 2 was isolated as an off-white powder in 66% yield. It
is surprisingly insoluble in most common lower-boiling point or-
ganic solvents. It is more readily soluble in DMF and DMSO. The
1H NMR spectrum (Fig. S1, Supplementary materials) exhibits
characteristic AA0 doublets at d 6.38 and 6.32 for the non-equiva-
lent protons of the methylene bridge (one proton points toward,
and the other away, from the Pd(II) ion, see Fig. 2b vide infra),
and two characteristic, but broad doublets at d 5.70 and 5.38 for
the prochiral benzylic CH2 protons. The imidazolylidene protons
appear at d 6.74 and 7.14. The broad peaks are indicative for an

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L.A. Berben et al. / Inorganica Chimica Acta 370 (2011) 374–381
H3CO
N
Br
N
H
N
H3CO
1) NaH, DMF
H3CO
CH2Br2, Toluene
OCH3
OCH3
N
N
N
2)
OCH3
N
N
Br
Br
73 %
OCH3
dbi
H3CO
MeI, THF
66 %
H2L2Br2
HO
Br
H3CO
1) BBr3, DCM
2) MeOH
1) BBr3, DCM
2) MeOH
N
N
I
OH
OCH3
N
N
HO
98 %
HL1I
70 %
H3L3Br
N
Br
OH
OH
N
N
98 %
H6L4Br2
N
Br
HO
Scheme 3. Synthesis of ligand precursors HL1I, H2L2Br2, H3L3Br and H6L4Br2.
Fig. 1. View of the crystal structure for H3L3Br showing the labelling scheme and the H-bond interactions between the hydroquinone groups of the [H3L3]+ cation and the
bromide anions (see text); for clarity, 30% thermal ellipsoids are shown and C-bound H-atoms are deleted.
exchange process, which we believe is between the conformers of
the puckered complex.
protons. At higher temperatures, the 1H NMR signals of 3 become
sharper and better defined. In the 1H NMR spectrum of 3 recorded
at 350 K (see Fig. S2, Supplementary materials), and similarly to
complex 2, the AA0 system of the methylene bridge gives rise to
two characteristic doublets at d 6.32 and 6.41. The benzylic CH2
protons are, in this case, magnetically equivalent and show a broad
singlet at d 5.53. Low temperature experiments were not possible
due to the low solubility of the compound in suitable solvents.
To obtain the benzoquinone derivative 4, complex 3 was treated
with two equivalents of 2,3-dichloro-5,6-dicyanobenzoquinone
(DDQ). Complex 4 was isolated in near-quantitative yield (Scheme
4). The IR spectrum confirms the disappearance of the OH band and
the growth of a strong quinone C@O peak at 1658 cmꢀ1. Interest-
ingly, in the 1H NMR spectrum of 4, the bridging methylene CH2
Attempts to prepare hydroquinolyl complexes by direct reac-
tion of the imidazolium salts H3L3Br or H6L4Br2 (Scheme 3) with
Pd(OAc)2, both with and without sodium acetate (1 equiv.) as an
ancillary Brønsted base, proved unsuccessful; a palladium mirror
always formed. However, the hydroquinonyl-substituted complex
3 was obtained in 97% yield by deprotection of the hydroquinonyl
groups in complex 2 using BBr3 to cleave the methoxy groups
(Scheme 4). The IR spectrum of 3 reveals a new broad band at
ꢂ3350 cmꢀ1 characteristic for the OH groups. The 1H NMR spec-
trum of 3 shows only broad signals at 300 K. Of note, the character-
istic methoxy signals of 2 are replaced by a new pair of broad
signals at d 8.55 and 8.84 corresponding to the phenolic (OH)

L.A. Berben et al. / Inorganica Chimica Acta 370 (2011) 374–381
377
H3CO
H3CO
N
OCH3
N
I
Pd(OAc)2, DMSO
Pd
N
N
I
I
OCH3
OCH3
N
N
H3CO
HL1I
, 20 %
1
H3CO
RO
OR
OR
Br
Br
N
N
Br
N
Pd
OCH3
Pd(OAc)2, DMSO
N
N
N
N
RO
N
Br
R: CH3 (2), 66 %
R: H (3), 97 %
1) BBr3, DCM
2) MeOH
H3CO
OCH3
DDQ, acetone
H2L2Br2
4
, 98%
Scheme 4. Synthesis of complexes 1–4.
2.4. X-ray crystal structure of [PdBr2(L2)]ꢁDMF (2ꢁDMF)
Recrystallisation of 2 from dimethylformamide solution under a
diethyl ether atmosphere yielded 2ꢁDMF as clear prisms. There are
two independent molecules of 2 (A and B) and two DMF in the unit
cell. The independent molecules of 2 exhibit near identical geom-
etry and bond parameters (e.g. compare Fig. 2 and Fig. S4) and so
only molecule A will be discussed. The crystal structure of complex
2 is of relatively poor quality in that large thermal ellipsoids are
observed for the ligand L2, particularly for the dimethoxybenzyl
substituents, suggestive for some ligand flexibility within the crys-
tal at 294 K, see Fig. 2a. The Pd(II) ion is bound by bis(NHC) chelate
ligand L2 to form a puckered six-membered ring (defined by PdA,
C1A, N1A, Cmethylene, N3A, C13A) with a boat conformation. The
PdA–CNHC distances are 1.968(27) and 1.939(24) Å. Two cisoid bro-
mo ligands with PdA–Br 2.503(3) and 2.470(4) Å complete the pri-
mary coordination plane of the Pd(II) ion. The maximum deviation
of an atom from the coordination plane defined by PdA and the
four ligand donor atoms is 0.094 Å for PdA, see Fig. 2b. The sum
of the six independent X–PdA–X bond angles is 701°, which com-
pares with 720° for a perfect square planar complex and, therefore,
indicates considerable distortion within the square plane. Of most
note, the C–PdA–C bite angle is 78.6(11)°. To chelate to the Pd(II)
ion, the bis(NHC) ligand L2 has puckered, which creates a cleft that
is illustrated in Fig. 2c. The angles between the carbene ring planes
and the Pd coordination plane are 48.9 and 50.7°. These angles are
similar to those found in related palladium(II) complexes of
Fig. 2. (a) View showing the molecular structure and atom labelling scheme for
complex 2 along with the included molecule of dimethylformamide. (b and c) Views
of complex 2 with dimethoxybenzyl substituents omitted emphasising the planar-
ity of the coordination sphere (b) and the puckering of the bis(NHC) ligand L2 (c).
The red line in (b) is the coordination plane defined by PdA, Br1A, Br2A, C1A and
C13A. For clarity, in all views 30% thermal ellipsoids are shown and H-atoms
(except for the methylene bridge H-atoms in view (b)) are omitted.
methylene-bridged bis-carbene ligands [16].
A molecule of
dimethylformamide lies included within the cleft formed by the
bis-carbene ligand. The formyl group of the dimethylformamide
protons of this compound are magnetically equivalent in contrast
to complexes 2 and 3, whereas the prochiral benzylic protons ap-
pear as two doublets at d 5.15 and 5.80 (see Fig. S3, Supplementary
materials).
is
p
-stacked at ꢂ3.2 Å separation with a dimethoxyphenyl group
(that defined by O1A, O2A) that bends back over and caps the cleft.
The other dimethoxyphenyl group of L2 is orientated away from
the cleft and metal centre.

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L.A. Berben et al. / Inorganica Chimica Acta 370 (2011) 374–381
Fig. 3. Cyclic voltammograms of complexes (a) 2, (b) 3 and (c and d) 4. Conditions: solvent = acetonitrile – 0.1 mol Lꢀ1 [NBun4][PF6]; [complex] = 2.0 mmol Lꢀ1; working
electrode = freshly polished 0.5 mm diameter glassy carbon-disc; scan rate = 100 mV sꢀ1; temperature = 295 K; E1/2 (ferrocenium-ferrocene) = 0.47 V. Note the current scales
differ in (a)–(d).
2.5. Redox chemistry
for 3). The small cathodic peak at +0.26 V in the return sweep
is typical for reduction of
a protonated quinone species
The redox chemistry of complexes 2–4 was probed by cyclic
voltammetry. Cyclic voltammograms of 2 reveal an irreversible
anodic process at +0.8 V followed by a second process at ca.
+1.2 V that exhibits double the current and some chemical revers-
ibility (Fig. 3a). The same processes appear in the cyclic voltammo-
grams of 3 and 4. One possibility to be considered was that these
processes originate from free bromide ion in solution derived from
dissociation or substitution of the bromo ligands. However, the
Brꢀ/Brꢀ3 couple appears at higher potential (at ca. +1.15 V [17])
than that of the first process. Thus the first process must be asso-
ciated with oxidation of the [PdBr2(C-NHC)2] centre as summarised
by Eq. (1). Decomposition of [PdBr2(C-NHC)2]+ within the electro-
chemical timescale (ꢂ1 ms), Eq. (2), would account for the ob-
served irreversibility of the oxidation. The origin of the second
oxidation couple cannot be definitively assigned although, based
on a comparison of the anodic currents for the two consecutive
oxidation processes, it must arise from two-electron oxidation of
a decomposition product of the [PdBr2(C-NHC)2]+ centre.
([PdBr2(C-NHC-q/H+)2]2+ in Eq. (3)) that is generated in the forward
sweep, and is assigned as such.
2þ
½PdBr2ðC-NHC ꢀ hqH Þ ꢃ ð3Þ ¢ ½PdBr2ðC-NHC-q=HþÞ ꢃ
2
2
2
þ 4eꢀ þ 2Hþ
ð3Þ
As already mentioned, complex 4 shares the same oxidative
processes as 2 and 3 (Fig. 3c). Of more interest, however, is the
reduction process at ꢀ0.2 V which is followed by a broad process
at ca. ꢀ0.6 V. The ꢀ0.2 V process is poorly reversible and switching
experiments reveal that it gives rise to a peak +0.3 V in the reverse
scan. The observed cyclic voltammetry is entirely consistent with
that previously observed for reactive quinones [18–21]. The first
reduction corresponds to the benzoquinone-semiquinone radical
anion couple, Eq. (4). The poor chemical reversibility is indicative
for a reaction consuming the semiquinone anion within the elec-
trochemical timescale (ca. ꢂ1 ms). We have previously observed
pendant semiquinone radical anions to bind to a metal centre,
including by displacement of halogen co-ligands [19]. We have also
observed internal electron transfer between semiquinone or
hydroquinonyl substituents and a metal centre leading to ligand
dissociation reactions [21]. Alternatively, reactive semiquinone
radical anions readily abstract a proton from the solvent-electro-
lyte medium and may then disproportionate to afford benzoqui-
none and hydroquinone anion (hqHꢀ) [18–20]. Any of these
processes would account for the poor reversibility and the
diminished current for the semiquinone anion/hydroquinone
dianion couple at ꢀ0.6 V (Eq. (5)). The anodic peak at +0.3 V does
not occur unless the semiquinone/hydroquinone dianion couple
is swept through (see Fig. 3d, inset). Thus, regardless of what the
½PdBr2ðC-NHCÞ ꢃ ð2Þ ! ½PdBr2ðC-NHCÞ ꢃþ þ eꢀ
ð1Þ
ð2Þ
2
½PdBr2ðC-NHCÞ2ꢃþ ! decomposition productðsÞ
2
The cyclic voltammogram of complex 3 shows the aforemen-
tioned anodic processes which overlap and merge with the charac-
teristic anodic peak for the two-electron proton-coupled oxidation
of each of the two hydroquinone substituents to give, overall, a
broad peak at ca. 1.1 V [18] (Fig. 3b). The anodic current of the pro-
cess is entirely consistent with this interpretation (for [com-
plex] = 2.0 mM, the anodic peak current of 3 is 17
lA compared
to 7
lA for 2, i.e. ꢂ2.4
l
A/eꢀ or 3eꢀ in total for 2 and 7eꢀ in total

L.A. Berben et al. / Inorganica Chimica Acta 370 (2011) 374–381
379
intervening chemical step upon formation of the semiquinone
radical is, the +0.3 V anodic peak must arise from oxidation
vacuum (ca. 0.2 mm Hg) over phosphorus pentoxide prior to anal-
ysis. Quoted melting points are uncorrected. (+)ESI mass spectra
were acquired on a VG Quattro mass spectrometer. Cyclic voltam-
metry measurements were performed in a conventional three elec-
trode cell using a computer-controlled Pine Instrument Co. AFCBP1
bipotentiostat as described in detail elsewhere [19a,20a,21].
of
a species with coordinated or protonated hydroquinone
substituent(s).
2ꢀ
½PdBr2ðC-NHC-qÞ2ꢃ ð4Þ þ 2eꢀ ¢ ½PdBr2ðC-NHC-sqꢄꢀÞ2ꢃ
ð4Þ
ð5Þ
2ꢀ
2ꢀ
4ꢀ
½PdBr2ðC-NHC-sqꢄꢀÞ2ꢃ þ 2eꢀ ¢ ½PdBr2ðC-NHC-hq Þ2ꢃ
4.2. Syntheses of imidazolium salts
2.6. Attempted catalysis of Wacker oxidation of styrene
4.2.1. 1-(2,5-Dimethoxybenzyl)-3-methylimidazolium iodide (HL1I)
Imidazole (1.90 g, 28 mmol) was dissolved in anhydrous DMF.
NaH (0.74 g, 30 mmol) was added and the mixture stirred for 2 h
(until the NaH had dissolved). 2,5-Dimethoxybenzylbromide [22]
(4.50 g, 19 mmol) was added and the solution stirred overnight.
Removal of the solvent under vacuum gave tan oil that was ex-
tracted with DCM–water mixture (20:20 mL) and the layers sepa-
rated. The aqueous layer was extracted with DCM (2 ꢅ 20 mL) and
then the combined organic layers were washed with brine (20 mL),
dried over anhydrous magnesium sulfate and the solvent removed.
The resulting yellow oil was purified by flash chromatography
using a gradient elution with ethyl acetate and methanol to give
1-(2,5-dimethoxybenzyl)-3-methylimidazole (dbi) as a yellow oil
(3.01 g, 14 mmol, 73%). dH (CDCl3): 3.75 (3H, s, OCH3), 3.82 (3H,
s, OCH3), 5.10 (2H, s, CH2), 6.60 (1H, s, imidazole), 6.84 (2H, d,
J = 1 Hz, Ar), 6.96 (1H, s, Ar), 7.07 (1H, s, imidazole), 7.58 (1 H, s,
Finally, preliminary assays of compounds 2–4 as catalysts for
the Wacker oxidation of styrene were performed. Using standard
reaction conditions [2c,g], no oxidation product(s) was detected
after 24 h. Addition of common additives such as strong acids
and/or copper co-catalysts did not improve the outcome. The pres-
ence of chloride ion may inhibit Wacker oxidation reactions [2c].
The bromo ligands in compounds 2–4 could similarly kill their cat-
alytic activity.
3. Concluding remarks
A facile route to 2,5-dimethoxybenzylimidazolium and 2,5-
dihydroxybenzylimidazolium (i.e., p-hydroquinone-substituted
imidazolium) salts has been demonstrated. Pd(II) complexes of
NHC ligands with dimethoxybenzyl-substituents are readily avail-
able from the 2,5-dimethoxybenzylimidazolium precursor and
Pd(OAc)2. The direct reaction fails for the p-hydroquinone-substi-
tuted imidazolium salts because the hydroquinone group reduces
the palladium precursor to metallic palladium. However, Pd(II)
complexes of NHC ligands with hydroquinonyl-substituents are
easily obtained from the corresponding dimethoxybenzyl-substi-
tuted (NHC)Pd(II) precursors by cleavage of the methoxy groups
with BBr3. These complexes can be oxidised to afford the corre-
sponding Pd(II) complexes of NHC ligands with quinone substitu-
ents. The crystal structure of [PdBr2(L2)]ꢁDMF (2ꢁDMF) reveals the
complex has a cleft capped by one of the dimethoxybenzyl substit-
uents. In the crystal structure, the capped cleft is occupied by an
included molecule of DMF. Although not explored in detail, this
suggests complex 2 may behave as a small molecule receptor.
Complexes 3 and 4 are expected to exhibit similar structures
and, therefore, similar receptor behaviour. The redox chemistry
of the [Pd(C-NHC-R)2Br2] complexes, 2–4, is an admixture of con-
secutive oxidation processes for the Pd(II) centre and the processes
characteristic for the hydroquinone or benzoquinone substituents.
Unfortunately, complexes 2–4 do not catalyse the oxidation of sty-
rene under Wacker conditions. Comparison with the literature for
similar reactions suggests the absence of activity may be due to the
bromo ligands. Finally, it is worth noting that halide-free deriva-
tives of complexes 2–4 should be readily available and these may
exhibit efficacious catalytic activity in oxidation reactions.
imidazole). m/z (EI-MS) 218 (M+).
A stirred solution of dbi
(0.95 g, 4.4 mmol) and iodomethane (0.4 mL, 6.5 mmol) in THF
(25 mL) was heated to 50 °C for 10 min and stirred at room tem-
perature overnight during which time a white powder precipi-
tated. This was collected and dried under vacuum to give
compound HL1I (1.5 g, 4.3 mmol, 98%). Mp: 82 °C. Anal. Calc. for
C13H17IO2N2: C, 43.35; H, 4.76; N, 7.78. Found: C, 43.39; H, 4.84;
N, 7.70%. dH (CDCl3): 3.80 (3H, s, OCH3), 3.85 (3H, s, OCH3), 4.07
(3H, s, CH3), 5.43 (2H, s, CH2), 6.83 (1H, d, J = 9 Hz, Ar], 6.88 (1H,
dd, J = 9 and 3 Hz, Ar], 7.28 (1H, d, J = 3 Hz, Ar], 7.37 (2H, m, imid-
azole), 10.05 (1H, s, imidazole). m/z (ESI-MS) 233 (MꢀI)+.
4.2.2. 1,10-Bis(2,5-dimethoxybenzyl)-3,30-methylenediimidazolium
dibromide (H2L2Br2)
The method of Gardiner et al. for preparation of a related
bis(carbene) ligand was adapted here [23]. A stirred solution of
dbi (2.70 g, 12 mmol) prepared as described above for HL1I and
dibromomethane (2.09 g, 12 mmol) in toluene (10 mL) was re-
fluxed for 3 days to yield a white powder which was collected,
washed with THF, recrystallised from methanol and dried under
vacuum to afford H2L2Br2 (2.31 g, 3.8 mmol, 63%). Mp 197–
198 °C. Anal. Calc. for C25H30Br2N4O4ꢁ0.5H2O: C, 48.48; H, 5.05; N,
9.05. Found: C, 48.10; H, 5.12; N, 9.38%. dH (300 MHz, CDCl3):
3.74 (6H, s, OCH3), 3.84 (6H, s, OCH3), 5.33 (4H, s, CH2), 6.82 (2H,
d, J = 9 Hz, Ar), 6.86 (2H, dd, J = 9 and 3 Hz, Ar), 7.08 (2H, d,
J = 3 Hz, Ar], 7.39 (2H, s, imidazole), 7.52 (2H, s, imidazole), 9.08
(2H, s, CH2), 11.02 (2H, s, imidazole). m/z (ESI-MS) 449 (Mꢀ2BrH)+.
4. Experimental
4.2.3. 1-(2,5-Dihydroxybenzyl)-3-methylimidazolium bromide
(H3L3Br)
4.1. General methods
Boron tribromide (1.0 mL, 10 mmol) was added to a stirred sus-
pension of HL1I (247 mg, 0.69 mmol) in DCM (25 mL) at 0 °C. After
16 h the solution had changed from colourless to bright pink. The
solvent was removed under vacuum and the resulting oil was
chilled in acetone-liquid nitrogen. Methanol (ca. 25 mL), also
chilled in acetone-liquid nitrogen, was added to the oil, the solu-
tion stirred for 15 min and then the solvent removed. The resulting
apricot powder was treated with methanol (ca. 5 mL) and, upon
cooling to ꢀ18 °C, apricot rod-shaped crystals of H3L3Br formed
(138 mg, 0.48 mmol, 70%). Mp: 180–181 oC (decomp). Anal. Calc.
for C11H13BrN2O2ꢁ0.5H2O: C, 45.22; H, 4.83; N, 9.59. Found: C,
Reactions were routinely performed under a dry dinitrogen
atmosphere using standard Schlenk and cannula techniques. Sol-
vents were distilled from the appropriate drying agent under dini-
trogen atmosphere or taken from a Innovative Technology Pure
Solvent Dispenser prior to use. NMR spectra were recorded on a
Bruker DPX 300 spectrometer. Elemental analyses for C, H and N
were performed in The Campbell Microanalytical Laboratory, Uni-
versity of Otago, New Zealand or in the Microanalytical Unit of the
Research School of Chemistry, Australian National University, Can-
berra, Australia. The samples were dried at 40 °C for 40 h under

380
L.A. Berben et al. / Inorganica Chimica Acta 370 (2011) 374–381
45.34; H, 4.68; N, 9.35%. dH (DMSO-d6) 3.82 (3H, s, CH3), 5.19 (2H, s,
CH2), 6.61 (1H, dd, J = 3 and 9 Hz, Ar), 6.66 (1H, d, J = 3 Hz, Ar), 6.69
(1H, d, J = 9 Hz, Ar), 7.66 (2H, s, imidazole), 8.90 (1H, s, OH), 9.08
(1H, s, OH), 9.33 (1H, s, imidazole). m/z (ESI-MS) 205 (MꢀBr)+.
mixture was cooled down again into an acetone-liquid nitrogen
slush bath and 30 mL of methanol were added slowly. The solvent
was then removed under vacuum to give a brown-orange solid,
which was recrystallised from methanol to yield a white solid
(230 mg, 0.35 mmol, 97%). Mp: 265–267 °C (decomp). Anal. Calc.
for C21H20Br2N4O4Pd: C, 38.33; H, 3.06; N, 8.52. Found: C, 38.24;
H, 3.00; N, 8.40%. dH (T = 350 K, DMSO-d6) 5.53 (4H, s, CH2), 6.32
(1H, d, J = 12.7 Hz, CHAHB), 6.41 (1H, d, J = 12.7 Hz, CHAHB), 6.63
(2H, dd, J = 2.5 and 8.7 Hz, Ar), 6.74 (2H, d, J = 8.7 Hz, Ar), 6.80
(1H, s, imidazole), 7.06 (2H, s, imidazole), 7.57 (2H, J = 2.5 Hz,
Ar), 8.55 (2H, s broad, OH), 8.84 (2H, s broad, OH). m/z (ESI-MS)
578 (MꢀBr)+.
4.2.4. 1,10-Bis(2,5-dihydroxybenzyl)-3,30-methylenediimidazolium
dibromide (H6L4Br2)
Boron tribromide (0.6 mL, 6 mmol) was added to a stirred solu-
tion of H2L2Br2 (0.12 g, 0.20 mmol), in DCM (20 mL) at 0 °C. A
white precipitate formed immediately. After 16 h the solvent was
removed under vacuum and the resulting oily residue was dis-
solved in methanol (ca. 20 mL), which had been chilled in ace-
tone-liquid nitrogen. The methanol was removed to yield a white
solid, H6L4Br2 (109 mg, 0.19 mmol, 98%). Mp: 242 °C (decomp).
Anal. Calc. for C21H22Br2N4O4ꢁH2O: C, 44.38; H, 4.25; N, 9.86.
Found: C, 43.63; H, 4.13; N, 9.51%. dH (DMSO-d6) 5.23 (4H, s,
CH2), 6.59 (2H, s, CH2), 6.64 (2H, dd, J = 3 and 9 Hz, Ar), 6.72 (2H,
d, J = 9 Hz, Ar), 6.73 (2H, d, J = 3 Hz, Ar), 7.79 (2H, s, imidazole),
7.97 (2H, s, imidazole), 8.96 (2H, s, OH), 9.39 (2H, s, imidazole),
9.41 (2H, s, OH). m/z (ESI-MS) 473 (MꢀBr)+.
4.3.4. [1,10-Bis(toluquinonyl)-3,30-methylenediimidazolin-2,20-diylid-
ene]dibromo palladium(II) (4)
A solution of DDQ (27 mg, 0.12 mmol) in acetone (2 mL) was
added to a solution of 3 (40 mg, 0.06 mmol) in acetone (10 mL).
The resulting solution was stirred for 30 min during which time
a pale yellow precipitate formed. The precipitate was collected,
washed with acetone and air-dried to give 4 as a pale yellow solid
(39 mg, 0.059 mmol, 98%). Mp: 218–220 °C (decomp). Anal. Calc.
for C21H16Br2N4O4Pd: C, 38.52; H, 2.46; N, 8.54. Found: C, 38.72;
H, 2.59; N, 8.36%. dH (DMSO-d6) 5.15 (2H, d, J = 18.0 Hz, CHAHB),
5.80 (2H, d, J = 18.0 Hz, CHAHB), 6.12 (2H, s broad, CH2), 6.37 (2H,
s, Ar), 6.81–6.90 (4H, m, Ar), 7.38 (2H, d, J = 3 MHz, imidazole),
4.3. Syntheses of (NHC)Pd(II) complexes
4.3.1. Bis[1-(2,5-dimethoxybenzyl)-3-methylimidazolin-2-ylidene]-
diiodopalladium(II) (1)
7.68 (2H, d, J = 3 MHz, imidazole). m/z (ESI-MS) 654 (M+). mmax
/
A stirred DMSO solution (3 mL) of Pd(OAc)2 (51 mg, 0.22 mmol)
and HL1I (158 mg, 0.46 mmol) was heated to 60 °C for 24 h. The
solvent was removed under vacuum and the residue extracted
with MeCN. After chilling the solution to ꢀ18 °C yellowish solid
precipitated, which was collected, washed with diethyl ether and
air-dried (35 mg, 0.04 mmol, 20%). Mp: 222–224 °C. Anal. Calc.
for C26H32I2N4O4Pd: C, 37.86; H, 3.91; N, 6.79. Found: C, 37.85;
H, 4.07; N, 6.62%. dH (DMSO-d6) shows cis and trans isomers:
3.55, 3.63, 3.67, 3.80 (6H, s, OCH3), 3.75, 3.83 (6H, s, CH3), 5.36
(2H, s, CH2), 5.45 (2H, s, CH2), 6.75–6.88 (6H, m, Ar), 7.04 (2H, m,
imid), 7.28 (2H, m, imid). dC (75 MHz, dmso-d6) shows cis and trans
isomers; 32.21, 32.30 (CH3), 48.44, 48.59 (CH2), 55.72, 55.79, 56.02,
56.19 (OCH3), 112.02, 112.14, 114.40, 114.46, 115.76, 116.01 (Ar),
150.93, 151.08, 153.29, 153.34 (imidazole).
cmꢀ1 (CO) 1657s (KBr disc).
4.4. General procedure for Wacker oxidation reactions
The Pd(II) complex 2, 3 or 4 (0.01 mmol) was dissolved in a mix-
ture of DMF/water (3:1, 1.0 mL) under air. Styrene (0.5 mmol) was
added and the mixture stirred at 60 °C. Reaction progress was
monitored by GC–MS.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.02.007.
4.3.2. [1,10-Bis(2,5-dimethoxybenzyl)-3,30-methylenediimidazolin-
2,20-diylidene] dibromo palladium(II) (2)
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MeCN, from which, after vigorous mixing, an off-white powder
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Ar). m/z (ESI-MS) 634 (MꢀBr)+.
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