Polynuclear architectures with di- and tricarbene ligands

Article abstract of DOI:10.1021/om200591d

The 1,3,5-triimidazolium-substituted benzene cation in H ₃-1(PF₆)₃ reacts with Hg(OAc)₂ to yield the cylinder-like trinuclear hexacarbene complex [Hg₃(1) ₂](PF₆)₆. Rea

Full text of DOI:10.1021/om200591d

ARTICLE
pubs.acs.org/Organometallics
Polynuclear Architectures with Di- and Tricarbene Ligands
Arnab Rit, Tania Pape, and F. Ekkehardt Hahn*
Institut f€ur Anorganische und Analytische Chemie, Westf€alische Wilhelms-Universit€at M€unster, Corrensstrasse 30,
D-48149 M€unster, Germany
S Supporting Information
b
ABSTRACT: The 1,3,5-triimidazolium-substituted benzene
cation in H3-1(PF6)3 reacts with Hg(OAc)2 to yield the cylinder-
like trinuclear hexacarbene complex [Hg3(1)2](PF6)6. Reaction
of imidazole with 1,4-dibromobenzene gives 1,4-bis(1-imidazo-
lyl)benzene (2), which after dialkylation with ethyl bromide and
salt metathesis yields the new diimidazolium salt H2-3(PF6)2.
Two equivalents of H2-3(PF6)2 react with 2 equiv of Ag2O to give
the dinuclear AgI tetracarbene complex [Ag2(3)2](PF6)2, which
after transmetalation with [AuCl(SMe2)] yields the AuI tetra-
carbene complex [Au2(3)2](PF6)2. In contrast to the formation of
the tetracarbene complexes of type [M2(3)2]2+ (M = AgI, AuI),
the diimidazolium salt H2-3(PF6)2 reacts with Hg(OAc)2 to yield the dinuclear HgII dicarbene complex [Hg2(OAc)2(3)](PF6)2,
where each mercury center is coordinated by one carbene carbon atom and one acetate oxygen atom. The crystal structure of
[Hg2(OAc)2(3)](PF6)2 reveals that the complex forms a coordination polymer via intra- and intermolecular acetate bridges
between HgII centers.
1. INTRODUCTION
N-heterocyclic carbenes have emerged as an important class of
compounds over the last two decades, due to their utility as
ligands in organometallic chemistry1 and as nucleophiles and
bases in organocatalysis.2 NHC complexes have been used in
different fields such as homogeneous catalysis3 and materials
science4 and as biologically active compounds.5 Very recently,
poly-NHC ligands have been employed as building blocks for the
construction of three-dimensional metallosupramolecular struc-
tures of type [A]4+ (Figure 1) and related assemblies.6 Due to the
high stability of selected MꢀCNHC bonds compared to MꢀN/O
bonds, it is conceivable that such three-dimensional organome-
tallic frameworks (OMFs) featuring only MꢀCNHC bonds could
develop into superior types of host molecules for the encapsula-
tion and catalytic transformation of selected substrates in com-
parison to related structures built from Werner-type metal
complexes.
In addition to the structures of type [A]4+, molecular rectangles
of types [B]4+ and [C]4+ featuring a mixed donor set of rigid
ditopic benzobis-NHC ligands and 4,40-bipyridine or diphosphine
ligands7aꢀc and related structures have also been described.7d,e
Even macrocyclic ligands featuring NHC donor groups have been
Figure 1. Selection of metallosupramolecular structures featuring
MꢀCNHC bonds.
used for the construction of three-dimensional metallosupramo-
lecular structures.8
Even though a mercury(II) complex was among the first com-
plexes to be isolated bearing N-heterocyclic carbenes,9 mercury(II)
NHC complexes belong to the less studied derivatives among the
large variety of NHC metal complexes known today. Complexes
of the type [HgII(NHC)2](ClO4)2 were first prepared by the
reaction of imidazolium perchlorates with Hg(OAc)2.9 Almost
all mercury(II) NHC complexes synthesized thereafter have
been prepared from an azolium starting material and Hg(OAc)2
Received: July 5, 2011
Published: August 29, 2011
r
2011 American Chemical Society
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Scheme 1. Preparation of the Diimidazolium Salts H2-3(Br)2
and H2-3(PF6)2
Figure 2. Examples of mononuclear ([D]2+) and dinuclear ([E]4+)
HgII NHC complexes.
Figure 3. Di- and triimidazolium salts.
consistent with previous observations upon anion exchange.16
The imidazolium C4-H and C5-H protons were observed as
using a one-pot synthesis.10ꢀ14 Most of these mercury(II) NHC
complexes are mononuclear with two NHC donors coordinated
to a Hg center in a linear arrangement such as that found in [D]2+
(Figure 2),10ꢀ14 although some dinuclear HgII NHC complexes
of type [E]4+ have been reported.12
pseudotriplets due to coupling of the C5-H/C4-H protons (3JH,H
=
1.9 Hz) and the C2-H proton (4JH,H = 1.5 Hz). The imidazolium
C2-H proton was also observed as a pseudotriplet (4JH,H
=
Apart from complexes of type [E]4+, polynuclear mercury(II)
NHC complexes are largely unknown. We became interested in
the synthesis of such polynuclear HgII complexes from in situ
generated poly-NHC ligands. Herein we report the synthesis and
characterization of a trinuclear HgII complex from the tricarbene
precursor H3-1(PF6)3. In addition, we studied the coordination
chemistry of the dicarbene ligand obtained from H2-3(PF6)2
with AgI, AuI, and HgII (Figure 3).
1.5 Hz), but this resonance is not well resolved due to the low
4JH,H coupling constant. The 13C{1H} NMR spectrum of
H2-3(PF6)2 features the imidazolium C2 signal at δ 137.0 ppm,
which is a typical value for this resonance.1c,6a,6b The salt H2-
3(PF6)2 was also identified by ESI mass spectrometry (positive
ions), showing peaks at m/z 413.1319 (calcd 413.1324) and
134.0870 (calcd134.0844) forthemolecularions[H2-3(PF6)]+ and
[H2-3]2+, respectively.
Single crystals of H2-3(PF6)2 suitable for an X-ray structure
determination have been grown by slow diffusion of diethyl ether
into a saturated acetonitrile solution of the compound at ambient
temperature. Figure 4 depicts the molecular structure of H2-3(PF6)2.
The asymmetric unit contains a half of the formula unit of the
compound related to the other half by a crystallographic inver-
sion center. Important bond parameters in the diimidazolium
cation fall in the range observed previously for imidazolium
cations.1c,6b,16 In the solid-state structure of H2-3(PF6)2, the
PF6ꢀ counterions are engaged in nonclassical hydrogen bonding16
2. RESULTS AND DISCUSSION
2.1. Synthesis of Imidazolium Salts. We have previously
described the preparation of the triimidazolium salt6 H3-1(PF6)3,
which we intended to use for the preparation of trinuclear
mercury(II) complexes. 1,4-Bis(1-imidazolyl)benzene (2) was
obtained from the copper(I)-catalyzed reaction of imidazole with
1,4-dibromobenzene using the reported protocol.15 The diimi-
dazolium salt H2-3(Br)2 was synthesized by dialkylation of 2 with
ethyl bromide and was obtained as a colorless solid in an excellent
yield of 88% (Scheme 1). The compound H2-3(Br)2 can be
converted into H2-3(PF6)2 by reaction with NH4PF6. Methanol
proved the ideal solvent for this metathesis reaction, since the
starting materials H2-3(Br)2 and NH4PF6 and the byproduct
NH4Br are soluble in MeOH, whereas the desired salt H2-3(PF6)2
is insoluble in MeOH and can thus be isolated by a simple
filtration.
interactions with the ring protons H7 and H3, with H7 F2 and
3 3 3
H3 F3 distances of 2.464 and 2.304 Å, respectively (Figure 4).
3 3 3
The orientation of the imidazolium units with respect to the
central phenyl ring appears to be influenced by these nonclassical
hydrogen bonds, and the upfield shift of the imidazolium
NꢀCHꢀN proton of H2-3(PF6)2 compared to H2-3(Br)2 is
most likely caused by the lack of hydrogen bonds involving the
NꢀCHꢀN proton in H2-3(PF6)2.
2.2. Synthesis of a Trinuclear Mercury(II) Hexacarbene
Complex. For the synthesis of a trinuclear mercury(II) hexacar-
bene complex, 2 equiv of the triimidazolium salt H3-1(PF6)3 was
reacted with 3 equiv of Hg(OAc)2 to give [Hg3(1)2](PF6)6 in
60% yield (Scheme 2). The reaction was carried out in refluxing
1
The H NMR spectrum of H2-3(Br)2 in [D6]DMSO shows
the characteristic resonance for the imidazolium C2-H protons at
δ 10.10 ppm. The corresponding signal was observed slightly
upfield at δ 8.97 ppm for H2-3(PF6)2 (in CD3CN), which is
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Figure 5. Molecular structure of the hexacation [Hg3(1)2]6+ (50%
probability ellipsoids) in [Hg3(1)2](PF6)6 5CH3CN. Hydrogen atoms
3
and solvent molecules have been omitted for clarity, and only the first
atom of each of the N-ethyl substituents is depicted. Selected bond
lengths (Å) and angles (deg): HgꢀCNHC = 2.050(10)ꢀ2.072(11);
CNHCꢀHgꢀCNHC = 173.6(4)ꢀ177.9(4), NꢀCNHCꢀN = 104.7(9)ꢀ
106.7(10).
Figure 4. Molecular structure of H2-3(PF6)2 (ball and stick model)
showing the nonclassical hydrogen bonding interaction of the PF6
ꢀ
counterions with protons H3 and H7 (H3 F3 = 2.304 Å and
3 3 3
H7 F2 = 2.464 Å). Hydrogen atoms, except for those engaged in
3 3 3
out of the plane of the central phenyl ring. However, the NHC
ring planes are not oriented perpendicularly with respect to the
central phenyl ring; instead, all NHC donor planes are rotated in
an anticlockwise direction from an imaginary perpendicular
orientation with respect to the central phenyl. The resulting
torsion angles between the NHC planes and the plane of the
central phenyl ring range from ꢀ51.322(15)° to ꢀ65.066(15)°.
These torsion angles are large when compared to the equivalent
angles observed for trinuclear complexes of the same tricarbene
ligand with coinage metals (AgI, CuI, AuI),6 resulting in an increased
distance of 4.978 Å between the centroids of two phenyl rings.
The dihedral angles between two imidazolin-2-ylidene units
coordinated to the same mercury(II) ion are 40.26, 42.44, and
44.35°. The coordination geometry around the mercury(II)
centers is almost linear, with the CNHCꢀHgꢀCNHC bond angles
in the range 173.6(4)ꢀ177.9(4)°. These values, like the
HgꢀCNHC bond distances (range 2.050(10)ꢀ2.072(11) Å), fall
in the range previously reported for complexes of the type
[Hg(NHC)2]2+.10ꢀ14 The three mercury atoms form a distorted
nonclassical hydrogen bonds and H1/H1*, have been omitted for clarity.
Selected bond lengths (Å) and angles (deg): N1ꢀC1 = 1.326(2),
N2ꢀC1 = 1.337(2); N1ꢀC1ꢀN2 = 108.68(15).
Scheme 2. Synthesis of the Trinuclear HgII Hexacarbene
Complex [Hg3(1)2](PF6)6
CH3CN and took 4 days for completion. The reaction time can
be shortened by the addition of an external base such as NaOAc.
The use of the hexafluorophosphate salt is beneficial here, as
[Hg3(1)2](PF6)6 is insoluble in water and residual Hg(OAc)2
can thus be removed by washing with water. Complex
[Hg3(1)2](PF6)6 was isolated as an air-stable white solid which
exhibits good solubility in organic solvents such as CH3CN,
acetone, and DMF, whereas it is less soluble in solvents such as
MeOH, CH2Cl2, and Et2O.
triangle with nonbonding Hg Hg distances in the range
3 3 3
6.00ꢀ6.29 Å.
2.3. Synthesis of Dinuclear AgI and AuI Tetracarbene
Complexes. Basic Ag2O was selected for the deprotonation of
the imidazolium C2 carbon atoms in H2-3(PF6)2. The Ag2O
method works for most imidazolium salts, giving the silver(I)
NHC complexes in high yield.17 Reaction of equimolar amounts
of H2-3(PF6)2 and Ag2O gave the disilver(I) tetracarbene
complex [Ag2(3)2](PF6)2 (Scheme 3). The reaction was com-
plete after 20 h at 55 °C in acetonitrile and gave [Ag2(3)2](PF6)2
as a slightly light sensitive white solid in 93% yield.
Formation of [Hg3(1)2](PF6)6 was indicated by the disap-
pearance of the imidazolium C2 proton resonance in the
1H NMR spectrum together with the appearance of a resonance
at δ 174.3 ppm for the HgII-bound carbene carbon atom in the
Formation of the carbene complex was detected by NMR
spectroscopy. While the signal of the acidic imidazolium proton
of H2-3(PF6)2 (δ 8.97 ppm) was absent in the 1H NMR
spectrum, a new resonance at δ 180.6 ppm for the carbene
carbon atom appeared in the 13C{1H} NMR spectrum of
[Ag2(3)2](PF6)2. No 13Cꢀ207/209Ag coupling8a,18 was observed,
which could be attributed to exchange processes which can take
place in solutions of AgIꢀNHC complexes.19
13C{1H} NMR spectrum.11,14 The remainder of the H and
1
13C{1H} resonances for the complex are in the range previously
described for mercury(II) dicarbene complexes of the type
[Hg(NHC)2]2+.9,10
The molecular structure of the complex was established by an
X-ray diffraction analysis. Crystals of [Hg3(1)2](PF6)6 5CH3CN
3
were grown by slow diffusion of diethyl ether into a saturated
acetonitrile solution of the compound at ambient temperature.
The structure analysis confirmed the formation of a trinuclear
complex where each mercury(II) ion is coordinated by two
carbene carbon atoms from two different ligands (Figure 5). For
both tricarbene ligands the planes of the NHC donors are rotated
In the 1H NMR spectrum of [Ag2(3)2](PF6)2, the imidazole
C4-H and C5-H proton signals were observed as doublets with a
3J coupling constant of 1.9 Hz, whereas these protons gave rise to
pseudotriplets in the imidazolium salt H2-3(PF6)2, due to
coupling of these protons with the C2 imidazolium proton. This
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Scheme 3. Preparation of the Dinuclear Tetracarbene
Complexes [M2(3)2](PF6)2 (M = AgI, AuI)
Figure 6. Molecular structure of the dication [Au2(3)2]2+ (50% prob-
ability ellipsoids) in [Au2(3)2](PF6)2. Hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg):
Au1ꢀC1 = 2.034(8), Au1ꢀC17 = 2.015(7), Au2ꢀC10 = 2.026(8),
Au2ꢀC26 = 2.041(8); C1ꢀAu1ꢀC17 = 173.6(3), C10ꢀAu2ꢀC26 =
171.8(3), N1ꢀC1ꢀN2 = 105.7(7), N3ꢀC10ꢀN4 = 104.9(6),
N5ꢀC17ꢀN6 = 104.1(6), N7ꢀC26ꢀN8 = 106.3(7).
Crystals of [Au2(3)2](PF6)2 were grown by slow diffusion of
diethyl ether into a saturated solution of the complex in acetonitrile.
The X-ray structure analysis (Figure 6) confirmed the conclusions
drawn from the MS data. The AuꢀCNHC bond lengths
(2.015(7)ꢀ2.041(8) Å) fall in the typical range for [Au(NHC)2]+
complexes.6,18 The CNHCꢀAuꢀCNHC bond angles (171.8(3)
and 173.6(3)°) deviate only slightly from linearity. The dihedral
angles between two imidazolin-2-ylidene donors coordinated to
the same gold(I) ions measure 9.81 and 6.34°. Only few digold(I)
tetracarbene complexes of the structural type found in [Au2(3)2]2+
have been described. Among those are the dinuclear complexes
with two naphthalene bridged di-NHC ligands21 and two 1,3-
phenylene-bridged triazolin-5-ylidenes.22
observation can also be taken as an indication for the formation
of a carbene complex. Additional NMR spectroscopic data for
[Ag2(3)2](PF6)2 fall in the range of those in previous reports for
[Ag(NHC)2]+ complexes.6,17 The HR-ESI mass spectrum
(positive ions) of [Ag2(3)2](PF6)2 shows a signal at m/z
374.0575 (calcd 374.0575) corresponding to the molecular
fragment [Ag2(3)2]2+ as the peak of highest intensity, which
indicates the formation of a dinuclear tetracarbene complex.
Formation of monocarbene silver(I) complexes of the type
[Ag2(L)2(3)] was not observed during the synthesis of
[Ag2(3)2](PF6)2, although dinuclear complexes of this type have
The two phenyl rings in [Au2(3)2]2+ are not oriented in a
coplanar fashion. They are rotated in opposite directions from an
imaginary parallel orientation. This behavior brings one portion
from each of the two phenyl rings (atoms C8 and C9 of the top
phenyl ring and atoms C24 and C25 of the bottom phenyl ring;
Figure 6) closer together (C8ꢀC24 = 3.882 Å and C9ꢀC25 =
3.853 Å), whereas the other parts (C5, C6 and C21, C22) of the
twophenyl ringsmove apart from each other (C5ꢀC21 = 7.254 Å
and C6ꢀC22 = 7.285 Å). The complex cations [Au2(3)2]2+ form
an infinite layered structure via nonclassical hydrogen-bonding
interactions between the hexafluorophosphate anions and the
been obtained in reactions of diimidazolium salts with Ag2O.20
A
disilver(I) tetracarbene complex similar to [Ag2(3)2](PF6)2 has
also been obtained via the Ag2O method from a naphthalene
bridged diimidazolium salt.21
[Au2(3)2]2+ cations and π π interactions between phenyl
Reaction of 1 equiv of [Ag2(3)2](PF6)2 with 2 equiv of
[AuCl(SMe2)] at ambient temperature gives an excellent yield
of 89% of the transmetalation product [Au2(3)2](PF6)2 as a
white powder (Scheme 3). Related transmetalation reactions of
polynuclear polycarbene complexes with retention of the overall
complex structure have been described.6 NMR spectroscopy did
not unequivocally demonstrate the formation of the gold(I)
carbene complex, as the 13C{1H} NMR spectrum showed
the resonance for the carbene carbon atom only marginally
shifted at δ 182.1 ppm compared to the equivalent resonance
for [Ag2(3)2](PF6)2 (δ 180.6 ppm). The same is true for all
additional NMR spectroscopic data, which are essentially identical
for [Au2(3)2](PF6)2 and [Ag2(3)2](PF6)2. Formation of
[Au2(3)2](PF6)2 was finally confirmed by ESI mass spectrometry,
which showed peaks for the molecular fragments [Au2(3)2(PF6)]+
(m/z 1071.2054) and [Au2(3)2]2+ (m/z 463.1189).
3 3 3
rings from neighboring cations (Figure 7). In this layered
structure, the two phenyl rings from two different [Au2(3)2]2+
cations are arranged in a coplanar fashion with an intermolecular
transannular separation of 4.005 Å. In addition, one PF6ꢀ anion
links four [Au2(3)2]2+ cations via nonclassical hydrogen bonds.
2.4. Synthesis of a Dinuclear Mercury(II) Dicarbene Com-
plex. Almost all mercury(II) NHC complexes described contain
the Hg(NHC)2 unit,11ꢀ14 where each mercury center is coordi-
nated by two NHC donors. We have now found that the
deprotonation of the imidazolium groups in H2-3(PF6)2 with
Hg(OAc)2 leads to the formation of a different type of mercury-
(II) NHC complex, [Hg2(OAc)2(3)](PF6)2, where each mer-
cury atom is coordinated by one NHC and one acetate donor
instead of two NHC donors (Scheme 4). The literature contains
only one other example of this type of mercury coordination
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Figure 7. Perspective view of the coordination polymer obtained from [Au2(3)2]2+ and PF6ꢀ anions (ball and stick model). Hydrogen atoms, except for
those involved in nonclassical hydrogen-bonding interactions, have been omitted for clarity.
Scheme 4. Synthesis of the Dinuclear Mercury(II) Complex
[Hg2(OAc)2(3)](PF6)2
Figure 8. Molecular structure of the dication [Hg2(OAc)2(3)]2+ (50%
probability ellipsoids) in [Hg2(OAc)2(3)](PF6)2 1.5CH3OH. Hydro-
3
gen atoms and noncoordinated solvent molecules have been omitted for
chemistry,12a where the [HgII(OAc)(NHC)]+ complex is an
intermediate in the reaction leading to the [Hg(NHC)2]2+
complex. In the reaction of H2-3(PF6)2 with Hg(OAc)2
[Hg2(OAc)2(3)](PF6)2 is the final reaction product and no
dinuclear tetracarbene complex of type [Hg2(3)2]4+ featuring
Hg(NHC)2 units has been observed.
clarity. Selected bond lengths (Å) and angles (deg): Hg1ꢀC1 =
2.052(7), Hg1ꢀO22 = 2.069(5), Hg1ꢀO21 = 2.589(7), Hg1 O25 =
3 3 3
2.715(5), N1ꢀC1=1.343(9), N2ꢀC1=1.337(9), O22ꢀC22 = 1.304(10),
O23ꢀC22 = 1.233(10), Hg2ꢀC9 = 2.017(8), Hg2ꢀO24 = 2.031(5),
N3ꢀC9 = 1.335(10), N4ꢀC9 = 1.363(9), O24ꢀC24 = 1.315(9),
O25ꢀC24
=
1.217(9); C1ꢀHg1ꢀO22
=
171.5(2), N1ꢀC1ꢀ
N2 = 106.8(6), C9ꢀHg2ꢀO24 = 174.5(3), N3ꢀC9ꢀN4 = 105.5(7).
The reaction of 1 equiv of H2-3(PF6)2 with 2.3 equiv of
Hg(OAc)2 in DMF (100 °C, 36 h) was found to be optimal for
the preparation of [Hg2(OAc)2(3)](PF6)2. The reaction of H2-
3(PF6)2 with less than 2 equiv of Hg(OAc)2 always gave a
mixture of products containing mainly [Hg2(OAc)2(3)](PF6)2
along with H2-3(PF6)2. The need to use more than 2 equiv of
Hg(OAc)2 indicates the high tendency of H2-3(PF6)2 to yield
[Hg2(OAc)2(3)](PF6)2 rather than the tetracarbene complex
[Hg2(3)2](PF6)4, which would only require the use of equimolar
amounts of H2-3(PF6)2 and Hg(OAc)2. [Hg2(OAc)2(3)](PF6)2
was isolated as a slightly hygroscopic white solid which is soluble
in solvents such as acetonitrile, acetone, and DMF and almost
insoluble in dichloromethane, THF, or MeOH.
imidazole C4 and C5 atoms appear as doublets with a 3J coupling
constant of 1.9 Hz at δ 8.06 and 8.14 ppm, respectively. The
remaining 1H and 13C{1H} NMR spectroscopic data for
[Hg2(OAc)2(3)](PF6)2 fall in the range previously reported
for similar compounds.10ꢀ14 The MALDI mass spectrum of
[Hg2(OAc)2(3)](PF6)2 exhibits peaks at m/z = 1076, 930, and
785 for the molecular fragments [Hg2(OAc)2(3)(PF6)2]+,
[Hg2(OAc)2(3)(PF6)]+, and [Hg2(OAc)2(3) ꢀ H]+, respectively.
The connectivity in complex [Hg2(OAc)2(3)](PF6)2 was
unambiguously established by an X-ray diffraction analysis of
single crystals grown by slow diffusion of diethyl ether into a
acetonitrile/methanol solution of the complex at room tem-
perature. The X-ray diffraction analysis of [Hg2(OAc)2(3)]-
[Hg2(OAc)2(3)](PF6)2 was characterized by NMR spectros-
copy, mass spectrometry, elemental analysis, and single-crystal
1
X-ray crystallography. The H NMR spectrum, as expected,
(PF6)2 1.5CH3OH (Figure 8) shows that each mercury(II)
3
shows no resonances for the C2-H imidazolium protons of H2-
3(PF6)2, and the 13C{1H} NMR spectrum exhibits a new signal
at δ 163.9 ppm for the carbene carbon atom. The resonance at δ
177.2 ppm, which falls in the range reported for the mercury(II)-
bound carbene carbon atoms,11,13,14 was assigned to the acetate
(C(O)O) carbon atom on the basis of 2D correlation spectra. In
the 1H NMR spectrum the signals for the protons bound to the
ion is coordinated by an acetate oxygen atom and a carbene carbon
atom in an almost linear fashion (angles CNHCꢀHgꢀOacetate
=
171.5(2) and 174.5(3)°). The HgꢀCNHC (range 2.017(8) and
2.052(7) Å)11,12 and HgꢀOacetate (2.069(5)ꢀ2.031(5) Å)12a
bond lengths fall in the ranges previously reported for such
bonds. In addition, Hg1 is coordinated by a methanol molecule
(Hg1ꢀO21 = 2.589(7) Å) and the second oxygen atom of the
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Figure 9. Perspective drawing (ball and stick model) of the 1D coordination polymer formed by the dication [Hg2(OAc)2(3)]2+. Hydrogen atoms and
solvent molecules have been omitted for clarity.
The reasons for the stability of a complex featuring [Hg(OAc)-
(NHC)]+ coordination are not completely clear at this time. We
assume that a combination of steric and electronic parameters
leads to the observed stability. Electronically, the presence of
the acetate anion leads to a complex with a reduced 1+ charge in
comparisonto the classical [Hg(NHC)2]2+ complexes. In addition,
the formation of a polymer made up from [Hg2(OAC)2(3)]2+
building blocks does contribute to the stability of the complex.
3. CONCLUSIONS
We have shown that di- and tricarbene ligands in the presence
of linearly coordinated metal ions (AgI, AuI, and HgII) can
be utilized as building blocks for the generation of metallosu-
pramolecular architectures. Such structures, exclusively held to-
gether by MꢀCNHC bonds, could serve as a new type of molecular
host for the encapsulation of selected substrates or anions. The
trinuclear hexacarbene complex [Hg3(1)2](PF6)6 was obtained
from the salt H3-1(PF6)3 and mercury(II) acetate. The cylinder-
like complex cation [Hg3(1)2]6+, however, does not possess a
cavity large enough for the encapsulation of multiatom substrates.
Anion encapsulation was also not detected, which most likely is
caused by the use of the large PF6ꢀ anions. The dicarbene ligand 3
obtained from the diimidazolium salt H2-3(PF6)2 reacts with AgI
ions to give the dinuclear tetracarbene complex [Ag2(3)2](PF6)2,
Figure 10. View of the supramolecular 1D helical chain of [Hg2(OAc)2-
which undergoes transmetalation with [AuCl(SMe2)] to yield
the AuI tetracarbene complex [Au2(3)2](PF6)2. In contrast to
this, the diimidazolium salt H2-3(PF6)2 reacts with Hg(OAc)2
under formation of complex [Hg2(OAc)2(3)](PF6)2, featuring
the rare Hg(OAc)(NHC) coordination. The dinuclear complex
[Hg2(OAc)2(3)](PF6)2 forms a helical coordination polymer in
the solid state via intra- and intermolecular acetate bridges
between the mercury atoms.
(3)]2+ cations.
acetate anion coordinating to Hg2 (Hg1 O25 = 2.715(5) Å).
3 3 3
The two imidazolin-2-ylidene rings are not coplanar with the
phenyl ring. They are rotated out of the phenyl ring plane by
about 80°.
The [Hg2(OAc)2(3)]2+ dications form indefinite helical chains
in the solid state by an intermolecular interaction of oxygen atom
O23 with atom Hg2 of a neighboring dinuclear dication
4. EXPERIMENTAL SECTION
(Hg2 O23* = 2.581(6) Å). The resulting helical polymer
3 3 3
is depicted in Figure 9. This interaction leads to HgII ions
with differing coordination numbers (CN = 4 for Hg1, CN = 3
for Hg2).
General Procedures. All reactions were carried out under an
argon atmosphere using standard Schlenk techniques or in a glovebox.
Glassware was oven-dried at 130 °C. Solvents were freshly distilled by
standard procedures prior to use. 1H and 13C{1H} NMR spectra were
recorded on Bruker AC 200, Bruker AVANCE I 400, and Bruker
AVANCE III 400 spectrometers. Chemical shifts (δ) are expressed in
ppm downfield from tetramethylsilane using the residual protonated
solvent as an internal standard. All coupling constants are expressed in
The acetate anions act as intramolecular (O24ꢀC24ꢀO25)
and intermolecular (O22ꢀC22ꢀO23) bridges. The resulting
intramolecular and intermolecular Hg Hg distances were
3 3 3
found to be 5.387 and 5.094 Å, respectively. An interesting
feature is the helical arrangement of the dications in the solid
state (Figure 10). The hexafluorophosphate counterions main-
tain nonclassical hydrogen-bonding interactions with cations
1
hertz and are only given for 1Hꢀ H couplings unless mentioned
otherwise. Mass spectra were obtained with MicroTof (Bruker Dal-
tonics, Bremen), Quattro LCZ (Waters-Micromass, Manchester, U.K.),
Bruker Reflex IV, and Varian MAT 212 spectrometers. The compounds
H3-1(PF6)3,6b 1,4-bis(1-imidazolyl)benzene,15 and [AuCl(SMe2)]23
from neighboring helical chains with the H F distances
3 3 3
ranging from 2.30 to 2.50 Å thereby holding two adjacent helical
chains together.
6398
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Organometallics
ARTICLE
413.1324), 239.1284 (calcd for [H2-3 ꢀ Et]+ 239.1297), 134.0870
(calcd for [H2-3]2+ 134.0844).
Synthesis of [Hg3(1)2](PF6)6. Samples of H3-1(PF6)3 (0.120 g,
0.15 mmol) and Hg(OAc)2 (0.96 g, 0.30 mmol) were suspended in
CH3CN (10 mL). The reaction mixture was heated under reflux for
4 days. Subsequently, the mixture was cooled to ambient temperature and
the solvent was removed in vacuo. The solid residue was washed with
water (3 ꢁ 5 mL) and dried in vacuo. Analytically pure [Hg3(1)2](PF6)6
was obtained after repeated recrystallization from acetonitrile/diethyl
ether. Yield: 0.099 g (0.045 mmol, 60%). 1H NMR (400 MHz,
Figure 11. Assignment of NMR resonances.
were prepared as described. Imidazole, 1,4-dibromobenzene, N,N-
dimethylglycine hydrochloride, ethyl bromide, NH4PF6, Ag2O, and
Hg(OAc)2 were purchased from commercial sources and were used as
received without further purification. For assignment of the NMR
resonances, see Figure 11. In accord with previous observations,
consistent micronalytical data could not be obtained for most of the
complexes containing PF6ꢀ anions, due to the large amount of fluorine
present. The purity of these compounds was established by a full NMR
spectroscopic (1H, 13C) and HRMS characterization.
3
CD3CN): δ 7.93 (s, 3H, Ar H), 7.79 (d, J = 1.97 Hz, 3H, imidazole
3
3
H4/H5), 7.66 (d, J = 1.97 Hz, 3H, imidazole H4/H5), 4.53 (q, J =
7.25 Hz, 6H, NCH2), 1.65 ppm (t, 3J = 7.25 Hz, 9H, CH3). 13C{1H}
NMR (100 MHz, CD3CN): δ 174.3 (NCN), 140.6 (Ar CN), 126.7
(imidazole C4/C5), 125.9 (imidazole C4/C5), 125.3 (Ar CH), 48.7
(NCH2), 16.1 ppm (CH3). HRMS (ESI, positive ions): m/z 951.0903
(calcd for [Hg3(1)2(PF6)4]2+ 951.0886), 585.7382 (calcd for [Hg3(1)2-
(PF6)3]3+ 585.7375), 403.0623 (calcd for [Hg3(1)2(PF6)2]4+ 403.0625).
Synthesis of [Ag2(3)2](PF6)2. A sample of H2-3(PF6)2 (0.112 g,
0.2 mmol) was dissolved in 10 mL of CH3CN, and to this solution was
added Ag2O (0.051 g, 0.22 mmol). The resulting suspension was heated
to 55 °C for 20 h with the exclusion of light. After cooling, the suspension
obtained was filtered slowly through Celite to obtain a clear solution.
The filtrate was concentrated to 3 mL, and diethyl ether (20 mL) was
added. This led to the precipitation of a white solid. The solid was
collected by filtration, washed with diethyl ether, and dried in vacuo.
Synthesis of 1,4-Bis(1-imidazolyl)benzene (2).15. A mixture
composed of CuI (0.78 g, 4.1 mmol), N,N-dimethylglycine hydrochlor-
ide (1.12 g, 8.02 mmol), K2CO3 (11.1 g, 80.3 mmol), 1,4-dibromoben-
zene (4.72 g, 20.0 mmol), and imidazole (3.4 g, 49.9 mmol) in 50 mL of
DMSO was heated to 110 °C for 48 h. The reaction mixture was then
partitioned between water and ethyl acetate. The organic layer was
separated, and the aqueous layer was extracted with ethyl acetate. The
combined organic phases were washed with brine and dried over
MgSO4, and all solvents were removed in vacuo. The residue was loaded
on a silica gel column and eluted with CH2Cl2/CH3OH (10/1) to give
compound 2 as a white solid. Yield: 2.14 g (10.2 mmol, 51%). 1H NMR
(400 MHz, CDCl3): δ 7.87 (s, 2H, NCHN), 7.51 (s, 4H, Ar CH),
7.29 (s, 2H, imidazole H4/H5), 7.22 ppm (s, 2H, imidazole H4/H5).
13C{1H} NMR (100 MHz, CDCl3): δ 136.4 (Ar CN), 135.5 (NCN),
130.9 (imidazole C4/C5), 123.8 (imidazole C4/C5), 118.1 (Ar CH)
ppm. MS (EI, 20 eV): m/z (%) 210 (100) [2]+. Anal. Calcd for 2: C,
68.55; H, 4.80; N, 26.66. Found: C, 68.45; H, 4.85; N, 26.61. The
spectroscopic data match those reported in ref 15.
1
Yield: 0.097 g (0.093 mmol, 93%). H NMR (200 MHz, CD3CN):
δ 7.66 (s, 4H, Ar H), 7.53 (d, 2J = 1.89 Hz, 2H, imidazole H4/H5), 7.45
2
3
(d, J = 1.89 Hz, 2H, imidazole H4/H5), 4.30 (q, J = 7.35 Hz, 4H,
NCH2), 1.54 ppm (t, 3J = 7.35 Hz, 6H, CH3). 13C{1H} NMR (50 MHz,
CD3CN): δ 180.6 (NCN), 141.1 (Ar CN), 126.1 (Ar CH),
123.4 (imidazole C4/C5), 123.0 (imidazole C4/C5), 48.4 (NCH2),
17.4 ppm (CH3). HRMS (ESI, positive ions): m/z 374.0575 (calcd for
[Ag2(3)2]2+ 374.0575).
Synthesis of [Au2(3)2](PF6)2. A solution of [Ag2(3)2](PF6)2
(0.052 g, 0.05 mmol) in acetonitrile (10 mL) was treated with solid
[AuCl(SMe2)] (0.030 g, 0.1 mmol). During the addition a white solid
precipitated from the reaction mixture. The reaction mixture was stirred
at ambient temperature for 12 h and then slowly filtered to obtain a clear
filtrate. The filtrate was added slowly to diethyl ether (20 mL). Upon this
addition a white solid precipitated, which was collected by filtration,
washed with diethyl ether, and dried in vacuo without heating to give
[Au2(3)2](PF6)3 as a colorless solid. Yield: 0.054 g (0.045 mmol, 90%).
Synthesis of 1,4-Bis(3-ethyl-1-imidazolium)benzene Di-
bromide (H2-3(Br)2). A Schlenk flask was charged with 1,4-bis-
(1-imidazolyl)benzene (2; 1.13 g, 5.4 mmol) and an excess of ethyl
bromide (3.53 g, 32.0 mmol). To this mixture was added DMF (10 mL),
and the reaction mixture was heated to 95 °C for 3 h. During this time a
white compound precipitated, which was filtered off, washed with
diethyl ether (2 ꢁ 10 mL), and dried in vacuo to give H2-3(Br)2 as a
2
1H NMR (400 MHz, CD3CN): δ 7.80 (s, 4H, Ar H), 7.55 (d, J =
1
white solid. Yield: 2.035 g (4.75 mmol, 88%). H NMR (200 MHz,
1.99 Hz, 2H, imidazole H4/H5), 7.49 (d, 2J = 1.99 Hz, 2H, imidazole
H4/H5), 4.41 (q, 3J = 7.36 Hz, 4H, NCH2), 1.61 ppm (t, 3J = 7.36 Hz,
6H, CH3). 13C{1H} NMR (100 MHz, CD3CN): δ 182.1 (NCN), 139.2
(Ar CN), 125.5 (Ar CH), 122.4 (imidazole C4/C5), 122.2 (imidazole
C4/C5), 47.0 (NCH2), 16.1 ppm (CH3). HRMS (ESI, positive ions):
m/z 1071.2054 (calcd for [Au2(3)2(PF6)]+ 1071.2030), 463.1189
(calcd for [Au2(3)2]2+ 463.1192).
[D6]DMSO): δ 10.10 (pseudo-t, 2H, NCHN), 8.47 (pseudo-t, 2H,
imidazole H4/H5), 8.17 (s, 4H, Ar H), 8.14 (pseudo-t, 2H, imidazole
H4/H5), 4.32 (q, 3J = 7.27 Hz, 4H, NCH2), 1.54 ppm (t, 3J = 7.27 Hz,
6H, CH3). H2-3(Br)2 was not further characterized but was immediately
used for the synthesis of H2-3(PF6)2.
Synthesis of 1,4-Bis(3-ethyl-1-imidazolium)benzene Bis-
(hexafluorophosphate) (H2-3(PF6)2). The bromide salt was con-
verted to H2-3(PF6)2 by adding a solution of NH4PF6 (1.78 g,
10.9 mmol) in methanol (8 mL) to a methanolic solution of H2-3(Br)2
(2.035 g, 4.75 mmol in 20 mL of methanol). The white hexafluoropho-
sphate salt H2-3(PF6)2 precipitated immediately. The precipitated solid
was collected by filtration, washed with small portions of cold methanol
and diethyl ether, and dried in vacuo. Yield: 1.94 g (3.47 mmol, 73%).
1H NMR (400 MHz, CD3CN): δ 8.97 (pseudo-t, 2H, NꢀCH-N), 7.88
(s, 4H, Ar H), 7.84 (pseudo-t, 2H, imidazole H4/H5), 7.65 (pseudo-t,
Synthesis of [Hg2(OAc)2(3)](PF6)2. To a mixture of H2-3(PF6)2
(0.112 g, 0.2 mmol) and Hg(OAc)2 (0.147 g, 0.46 mmol) was added
DMF (4 mL), and the mixture was heated to 100 °C for 36 h. After the
mixture was cooled to ambient temperature, the solvent was stripped in
vacuo and the remaining residue was washed with a small amount of
H2O. The solid residue was eaxtracted with acetone (5 mL), and the
liquid phase was filtered slowly through a short pad of Celite. The filtrate
was collected, and removal of the solvent gave [Hg2(OAc)2(3)](PF6)2
as a white powder. Single crystals suitable for an X-ray diffraction
study were obtained by slow diffusion of diethyl ether into a solution
of [Hg2(OAc)2(3)](PF6)2 in acetonitrile/methanol. Yield: 0.172 g
(0.16 mmol, 80%). 1H NMR (400 MHz, [D6]acetone): δ 8.14
3
2H, imidazole H4/H5), 4.32 (q, J = 7.34 Hz, 4H, NCH2), 1.57 ppm
3
(t, J = 7.34 Hz, 6H, CH3). 13C{1H} NMR (100 MHz, CD3CN):
δ 137.0 (NCN), 135.8 (Ar CN), 125.5 (Ar CH), 124.3 (imidazole
C4/C5), 122.9 (imidazole C4/C5), 46.7 (NCH2), 15.3 ppm (CH3).
HRMS (ESI, positive ions): m/z 413.1319 (calcd for [H2-3(PF6)]+
3
3
(d, J = 1.9 Hz, 2H, imidazole H5), 8.09 (s, 4H, Ar H), 8.06 (d, J =
6399
dx.doi.org/10.1021/om200591d |Organometallics 2011, 30, 6393–6401

Organometallics
ARTICLE
1.9 Hz, 2H, imidazole H4), 4.64 (q, 3J = 7.3 Hz, 4H, NCH2), 1.91 (s, 6H,
with hydrogen atoms on calculated positions, R = 0.0425, Rw = 0.0972,
Rall = 0.0755, Rw,all = 0.1112. The asymmetric unit contains one
molecule of [Au2(3)2](PF6)2.
3
acetate CH3), 1.63 ppm (t, J = 7.3 Hz, 6H, CH3). 13C{1H} NMR
(100 MHz, [D6]acetone): δ 177.2 (CdO), 163.9 (NCN), 140.0 (Ar
CN), 128.5 (Ar CH), 125.9 (imidazole C5), 125.4 (imidazole C4), 47.8
(NCH2), 21.9 (acetate CH3), 16.5 ppm (CH3). MS-MALDI (DCTB,
positive ions): m/z 1076 [Hg2(OAc)2(3)(PF6)2]+, 930 [Hg2(OAc)2-
(3)(PF6)]+, 785 [Hg2(OAc)2(3) ꢀ H+]+, 527 [Hg2(OAc)2(3) ꢀ
Hg(OAc)]+. Anal. Calcd for [Hg2(OAc)2(3)](PF6)2: C, 22.32; H, 2.25;
N, 5.21. Found: C, 21.49; H, 2.24; N, 5.25.
X-ray Crystal Structure Determinations. X-ray diffraction data
were collected at T = 153(2) K with a Bruker AXS APEX CCD dif-
fractometer equipped with a rotating anode using monochromated Mo
Kα radiation (λ = 0.710 73 Å). Diffraction data were collected over the
full sphere and were corrected for absorption. Structure solutions were
found with the SHELXS-9724 package using direct methods and were
refined with SHELXL-9724 against |F2| using first isotropic and later
anisotropic thermal parameters for all non-hydrogen atoms (for excep-
tions, see the descriptions of the individual molecular structures).
Hydrogen atoms were added to the structure models on calculated
positions.
H2-3(PF6)2. Crystals suitable for an X-ray diffraction study were
obtained by diffusion of diethyl ether into a saturated solution of H2-
3(PF6)2 in acetonitrile: C16H20N4F12P2, M = 558.30, colorless crystal,
0.22 ꢁ 0.07 ꢁ 0.05 mm3, monoclinic, space group P21/n, Z = 2,
a = 8.5008(4) Å, b = 12.3583(6) Å, c = 10.4088(5) Å, β = 98.2340(10)°,
V = 1082.23(9) Å3, Fcalcd = 1.713 g cmꢀ3, Mo Kα radiation (λ =
0.710 73 Å), μ = 0.317 mmꢀ1, ω and j scans, 12 311 measured
intensities (5.1° e 2θ e 60.0°), semiempirical absorption correction
(0.934 e T e 0.984), 3148 independent (Rint = 0.0158) and 2788
observed intensities (I g 2σ(I)), refinement of 155 parameters against
|F2| of all measured intensities with hydrogen atoms on calculated
positions, R = 0.0524, Rw = 0.1468, Rall = 0.0572, Rw,all = 0.1521. The
asymmetric unit contains a half formula unit of H2-3(PF6)2 related to
the other half by a crystallographic inversion center.
[Hg2(OAc)2(3)](PF6)2 1.5CH3OH. Crystals suitable for an X-ray dif-
3
fraction study were obtained by diffusion of diethyl ether into a acetonitrile/
methanol solution of the complex [Hg2(OAc)2(3)](PF6)2: C21.5H27.5N4-
Hg2F12O5.5P2, M = 1121.09, colorless crystal, 0.15 ꢁ 0.10 ꢁ 0.07 mm3,
monoclinic, space group P21/c, Z = 4, a = 17.5961(8) Å, b = 13.4202(6) Å,
c = 14.5061(7) Å, β = 104.9020(10)°, V = 3310.3(3) Å3, Fcalcd
=
2.249 g cmꢀ3, Mo Kα radiation (λ = 0.710 73 Å), μ = 9.469 mmꢀ1, ω
and j scans, 33 877 measured intensities (2.4° e 2θ e 60.0°), semiempi-
rical absorption correction (0.331 e T e 0.557), 9634 independent (Rint
=
0.0305) and 6714 observed intensities (I g 2σ(I)), refinement of
438 parameters against |F2| of all measured intensities with hydrogen atoms
on calculated positions, R = 0.0499, Rw = 0.1336, Rall = 0.0776, Rw,all
=
0.1518. The asymmetric unit contains one dication [Hg2(OAc)2(3)]2+, two
PF6ꢀ ions, and 1.5 molecules of methanol, one of which is coordinated to
Hg1 while the methanol half-molecule in the asymmetric unit is disordered.
In addition, one of the methyl groups of one ethyl substituent of the
dicarbene ligand is disordered. No hydrogen atoms were added to the
structure model for the disordered molecules or groups.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files giving X-ray crystal-
b
lographic data for H2-3(PF6)2 [Hg3(1)2](PF6)6 5CH3CN,
[Au2(3)2](PF6)2, and [Hg2(OAc)2(3)](PF6)2 1.5CH3OH. This
material is available free of charge via the Internet at http://pubs.
acs.org.
3
3
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: fehahn@uni-muenster.de.
[Hg3(1)2](PF6)6 5CH3CN. Crystals suitable for an X-ray diffraction
3
study were obtained by diffusion of diethyl ether into a saturated
solution of [Hg3(1)2](PF6)6 in acetonitrile: C52H63N17Hg3F36P6, M =
2397.74, colorless crystal, 0.12 ꢁ 0.10 ꢁ 0.05 mm3, orthorhombic, space
group Iba2, Z = 8, a = 30.443(2) Å, b = 18.8901(15) Å, c = 29.089(2) Å,
’ ACKNOWLEDGMENT
We thank the Deutsche Forschungsgemeinschaft (SFB 858)
for financial support. A.R. thanks the NRW Graduate School of
Chemistry, M€unster and WWU M€unster for a predoctoral grant.
V = 16729(2) Å3, Fcalcd = 1.902 g cmꢀ3, Mo Kα radiation (λ =
3
0.710 73 Å), μ = 5.733 mmꢀ1, ω and j scans, 83 789 measured inten-
sities (2.5° e 2θ e 55.8°), semiempirical absorption correction
(0.546 e T e 0.762), 19 954 independent (Rint = 0.0536) and 16 357
observed intensities (I g 2σ(I)), refinement of 991 parameters against
|F2| of all measured intensities with hydrogen atoms on calculated
positions, R = 0.0533, Rw = 0.1429, Rall = 0.0697, Rw,all = 0.1551. The
asymmetric unit contains one molecule of [Hg3(1)2](PF6)6. Five of
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a = 11.4827(10) Å, b = 11.7727(10) Å, c = 15.0753(13) Å, α =
80.1770(10)°, β = 72.3180(10)°, γ = 84.0760(10)°, V = 1910.3(3) Å3,
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semiempirical absorption correction (0.650 e T e 0.859), 9656
independent (Rint = 0.0351) and 6620 observed intensities (I g 2σ(I)),
refinement of 509 parameters against |F2| of all measured intensities
6400
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