Another silver complex of 1,3-dibenzylimidazol-2-ylidene: Solution and solid-state structures

Article abstract of DOI:10.1016/j.jorganchem.2009.02.030

The reaction of 1,3-dibenzylimidazolium bromide (Bn₂Im · HBr) with silver(I) oxide yields the dinuclear compound {(Bn₂Im)AgBr}₂ as determined with X-ray crystallography. The structure turned out to differ significantly fro

Full text of DOI:10.1016/j.jorganchem.2009.02.030

Journal of Organometallic Chemistry 694 (2009) 2217–2221
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Another silver complex of 1,3-dibenzylimidazol-2-ylidene:
Solution and solid-state structures
Joris Berding a, Huub Kooijman b, Anthony L. Spek b, Elisabeth Bouwman a,
*
a Coordination and Bio-Inorganic Chemistry, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
b Bijvoet Centre for Biomolecular Research, Crystal and Structural Chemistry, Department of Chemistry, Faculty of Science, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2009
Received in revised form 10 February 2009
Accepted 24 February 2009
Available online 6 March 2009
The reaction of 1,3-dibenzylimidazolium bromide (Bn2Im ꢀ HBr) with silver(I) oxide yields the dinuclear
compound {(Bn2Im)AgBr}2 as determined with X-ray crystallography. The structure turned out to differ
significantly from the mononuclear complex (Bn2Im)2AgBr that was reported during our studies. It
appears that the various structures are a result of different crystallization conditions. A new classification
of solid-state structures of these and other silver(I) complexes containing monodentate N-heterocyclic
carbenes is presented.
Keywords:
Carbene ligands
Silver
Ó 2009 Elsevier B.V. All rights reserved.
Solid-state structures
NMR study
2NHCꢀHX + Ag2O ! 2(NHC)AgX + H2O
ð1Þ
1. Introduction
In the past few years we have been interested in the synthesis of
nickel(II)–NHC complexes, which we prepare with the aid of sil-
ver(I)–NHC intermediates. During our studies the unusual structure
of [(NHC)2AgBr], in which NHC is 1,3-dibenzylimidazol-2-ylidene,
was reported [7], whereas with the same NHC and the same syn-
thetic procedure we had obtained the compound {(NHC)AgBr}2. In
this paper, we report the synthesis and crystal structure of this
compound, and we attempt to rationalize the formation of the dif-
ferent complexes.
In recent years the study and application of N-heterocyclic carb-
enes (NHCs) has increased rapidly, most notably as spectator li-
gands in various homogeneous catalysts [1]. As these ligands are
strong
r-donors, they form stable transition-metal NHC complexes
with strong metal–carbon bonds. Many synthetic methods leading
to these useful carbene complexes have been investigated. One of
these methods is the generation of a silver(I)–NHC complex, fol-
lowed by transfer of the carbene to another transition metal [2].
Such a reaction has successfully been applied to a variety of metals,
including rhodium, iridium, gold, palladium, and nickel [2], and
may lead to NHC complexes that cannot be obtained via other syn-
thetic routes, including the addition of the free carbene to a metal
salt. Furthermore, Ag–NHC complexes have been used as catalysts
[3,4], and as antimicrobial agent [5].
2. Results and discussion
2.1. Synthesis of silver complex
The ligand precursor 1,3-dibenzylimidazolium bromide (Bn2-
Im ꢀ HBr, 1) was obtained by the alkylation of N-benzylimidazole
with benzyl bromide in hot 1,4-dioxane. In accordance with the
general reaction for the generation of silver–NHC complexes (Eq.
(1)) and the literature procedure [7], 1 was reacted with 0.5 equiv.
of silver(I) oxide in dichloromethane. This reaction yielded an off-
white solid product, which was assumed to be compound 2
(Scheme 1) [7]. The 1H NMR spectrum of this solid product in
CD2Cl2 lacked the downfield NCHN signal, indicating successful
carbene formation, while the 13C NMR spectrum showed the Ag–
C resonance at 182 ppm. No 13C–107/109Ag coupling was observed,
consistent with the fluxional behavior known for other Ag(I)–
NHC complexes [6]. The elemental analysis, however, revealed a
Three common approaches towards the synthesis of Ag–NHC
complexes are: (1) the reaction of a free NHC with silver salts,
(2) the reaction of azolium salts with silver salts under basic
phase-transfer conditions and (3) the reaction of azolium salts
with silver bases. The latter method, in which Ag2O is used as a
base (Eq. (1)) is now by far the most commonly employed [2]. Soon
after the first report in 1998 [6], the Ag2O route was recognized for
its attractive features, such as its stability towards air and the tol-
erance towards other reactive hydrogen atoms.
* Corresponding author. Tel.: +31 71 5274550; fax: +31 71 5274451.
E-mail address: bouwman@chem.leidenuniv.nl (E. Bouwman).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.02.030

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J. Berding et al. / Journal of Organometallic Chemistry 694 (2009) 2217–2221
Scheme 1. Reaction of imidazolium halide with Ag2O, yielding 2 or 3(4).
clear 1:1:1 ligand to silver to bromide ratio, in disagreement with
the reported structure 2. To elucidate the identity of this new com-
pound 3, the solid-state structure was determined.
ole ring is rotated with respect to the Ag2Br2-plane by 36.82(12)°. A
weak stacking interaction of the aromatic phenyl side groups is ob-
served in the crystal structure with a center-to-center distance of
3.90 Å, with the center of one ring directly above the quaternary
carbon of the other ring (see Supporting Information). Several sim-
ilar {(NHC)AgX}2 structures have been reported in the literature
(see Fig. 2).
Apart from the mononuclear compound 2, in which two NHC li-
gands are bound to one silver ion (Scheme 1), Newman et al. ob-
tained the dinuclear {(Bn2Im)AgCl}2 (4) with a structure similar
to 3 [7], with only minor differences in bond lengths and angles;
e.g., the carbene–silver distances are 2.090(2) and 2.102(2) Å,
respectively. Only the distance between the silver center and the
closest halide is significantly different due to the different van
der Waals radii of Brꢁ and Clꢁ. The two silver–carbene bond
lengths in 2 of 2.116(4) and 2.117(4) Å are slightly longer than
those in 3. The silver–bromide distance of 2.89 Å in 2 is intermedi-
ate between the two Ag–Br distances in 3 and it was suggested that
2.2. X-ray crystal structure determination of silver complex 3
Single crystals suitable for X-ray diffraction were obtained by
slow diffusion of diethyl ether into a chloroform solution of the sil-
ver compound at room temperature. The complex has a dinuclear
solid-state structure (3), as depicted in Fig. 1. In the structure of 3,
two (Bn2Im)AgBr moieties are present around an inversion center
to give a dinuclear species with side-on AgꢀꢀꢀBr interaction. The
Ag2Br2 4-membered ring is planar with an Ag–Ag distance of
4.0296(6) Å. The two bridging bromide ions are asymmetrically
bound with Ag–Br distances of 2.4913(5) and 3.0415(5) Å. The
Ag1–C2 distance is comparable to those in other silver–carbene
complexes [8]. The silver center is shifted slightly out of the trigo-
nal [CBr2] coordination plane, while the plane through the imidaz-
Fig. 1. ORTEP projection of 3 with displacement ellipsoids drawn at the 50% probability level. The molecule is located on a crystallographic inversion center. Symmetry
transformation used to generate equivalent atoms: ꢁx, 1 ꢁ y, 1 ꢁ z. Selected bond distances (Å) and angles (°): Ag1–C2 2.102(2); Ag1–Br1 2.4913(5); Ag1–Br1a 3.0415(5);
N1–C2 1.355(3); N3–C2 1.471(4); C4–C5 1.349(4); Ag1–Ag1a 4.0296(6); Br1–Ag1–C2 159.30(6); Br1a–Ag1–C2 112.77(6); Br1–Ag1–Br1a 87.05(2); Ag1–Br1–Ag1a 92.95(2);
N1–C2–N3 104.21(18).

J. Berding et al. / Journal of Organometallic Chemistry 694 (2009) 2217–2221
2219
2 is better described as ionic complex [(NHC)2Ag]+Brꢁ [7]. A silver
chloride complex containing the ligand 1,3-bis(2-pyridyl-
methyl)imidazole-2-ylidene does indeed yield the ionic complex
[L2Ag]+Clꢁ in which two silver ions are in close contact (Ag–
Ag = 3.65 Å) [9].
2.3. NMR studies
Two distinct solid-state structures of silver halide complexes
with the ligand Bn2Im were described in the previous section. It
was decided to investigate the structures of these compounds in
solution. An attempt was undertaken to monitor the formation of
the silver complex in solution. Thus, a 2:1 mixture of 1 and Ag2O
was taken into deuterated dichloromethane and the NMR spec-
trum of the mixture was recorded within 5 min. This spectrum al-
ready showed full conversion of the ligand precursor to the silver
complex. Repeating this procedure at 243 K allowed for the reac-
tion to be monitored. However, no peaks other than those of the
precursor and a single silver complex could be observed in the
1H and 13C NMR spectra (see Supporting Information). After full
conversion was reached, the 1H and 13C NMR spectra were identi-
cal to spectra of complex 3. After standing in the dark for several
hours, colorless crystals formed in the NMR tube. Elemental anal-
ysis of these crystals revealed a 1:1 ligand to silver ratio, consistent
with 3. Some of the crystals were redissolved in CD2Cl2; the NMR
spectra confirmed that the crystals are identical to the bulk com-
pound in solution. Repetition of the NMR experiment in deuterated
DMSO yielded comparable 1H and 13C NMR spectra.
In addition, a silver complex with the non-coordinating anion
tetraphenylborate (BPhꢁ4 ) was prepared, presuming that the ionic
complex [(Bn2Im)2Ag]BPh4 would be formed. This complex was
prepared in hot DMSO, as the reaction did not proceed well in
dichloromethane. The 13C NMR spectrum (see Supporting Informa-
tion) showed an unresolved doublet at 180 ppm, with a separation
of about 190 Hz, consistent with coupling constants reported for
other ionic Ag(I)–NHC complexes [10]. The fact that this ionic spe-
cies shows Ag–C coupling, while compound 3 does not, is in agree-
ment with compound 3 being a non-ionic species in solution.
The 13C NMR spectrum of the chloride complex 4 is reported to
show an Ag–C resonance at 151 ppm [7]. In our hands, however,
repeating the kinetic NMR study described above for the reaction
between Bn2ImꢀHCl and Ag2O, the Ag–C signal was observed as a
singlet at 181 ppm. We therefore conclude that, in our experi-
ments, the solution structures of silver complexes obtained from
Bn2Im ꢀ HCl and Bn2Im ꢀ HBr are very similar, in contrast to what
has been reported earlier.
Fig. 2. Classification of various silver NHC species discussed in this paper.
and functional groups), the halide, the solvent and the tempera-
ture. For instance, imidazole- and benzimidazole-based carbenes
with short alkyl-chain substituents yield complexes of type B in
the solid state [6,13], while those bearing long alkyl chains often
result in complexes of type A2 [14]. The exact geometry of this di-
mer A2 can vary and a number of different structures has been re-
ported, including the Ag2X2 4-membered ring of 3 and 4 and a
dimer formed by Ag–Ag interaction [8,15]. A study to determine
the influence of the solvent showed that an increase in solvent
polarity shifts the solution equilibrium from A to B for (IMes)AgCl
(IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene)
[8].
Coordination polymers derived from A (An, n > 3) and dimers de-
rived from B (B2) have been reported in the solid state, but in solu-
tion are most likely in equilibrium with A and B [3,13,16,17]. The
1,3-bis(2-pyridylmethyl)imidazol-2-ylidene silver complex men-
tioned above, is an example of a type C02 structure [9].
The current classification does not include complexes with non-
coordinating anions that will adopt a type C0 structure [18–20], or
silver clusters [9,16,21]. One example is known with
a
[{(NHC)2Ag}3I]2+2Iꢁ structure, in which the NHC has pendant pyr-
idine groups, that may be considered intermediate between struc-
ture C and C0 [22]. Chelating NHC ligands are not included
explicitly, but are expected to form similar species.
Type C0 is not likely to be a major product in the solid state with
potentially coordinating halides and only few solid-state structures
of this type have been reported with bulky ligands [4]. We argue
that in the structure reported by Wang et al. (CCDC code: PENJIC)
the iodide counter ion may be considered weakly bound to the sil-
ver center at 3.36 Å, as this is shorter than the sum of the van der
Waals radii of silver and iodine of 3.70 Å [23].
Even though we were unable to find kinetic evidence in our
NMR experiments, we assume that structure A is the direct out-
come of the reaction between the azolium salt and Ag2O in solu-
tion. The other structures are most likely generated
subsequently, due to the fluxional behavior between the ionic
and neutral complexes observed in solution for most of the Ag–
NHC compounds [6], and the known equilibrium between AgX2ꢁ
and AgX + Xꢁ [2]. For instance, type C may be obtained by separa-
tion of AgX from type B. The fact that with the ligand Bn2Im both
type A2 (3 and 4) and type C (2) structures have been characterized
in the solid state, demonstrates that the ligand and the halide are
not the only factors determining the solid-state structure. The only
distinction in the reaction conditions of the synthesis of 2 and 3 is
the length of the reaction time. Whereas 3 was obtained after 2 h,
compound 2 was isolated after 15 h of stirring. This seems to sug-
gest that the dimeric 3 is the kinetic product, while three-coordi-
nate 2 is thermodynamically favored under the conditions used.
However, in our hands, repeating the synthesis with 18 h stirring
yielded a solid which again analyzed as complex 3. Therefore, we
ascribe the fact that it was possible to isolate 3 in crystalline form,
to a subtle difference in crystallization conditions. The observation
that different crystallization conditions may lead to different crys-
tal structures of Ag(I)–NHC complexes has been noted before [14].
In addition, very recently another example of a type C solid-state
structure was reported, obtained from a tetra-ether substituted
The mass spectra of 3 and the crystals obtained from the NMR
experiment described above both show one peak at m/z 605, indi-
cating a [(Bn2Im)2Ag]+ moiety. It has been shown, however, that
mass spectrometry cannot be used to unambiguously distinguish
(ligand)AgX from [Ag(ligand)2]+ [11].
2.4. Structural considerations
In a recent review seven types of solid-state structures obtained
from the reaction between Ag2O and monodentate azolium salts
are described [2]. For the current discussion, we propose another
classification, shown in Fig. 2 [12].
Structure types A and An (n = 2,3,. . .) are characterized by a 1:1
ligand to silver ratio and coordination of both the ligand and the
halide to the same metal center. Structure types B and B2 show
the same ratio; however, in this case two ligands and two halides
are bound separately over two metal centers. Structure types C, C0
and C02 have a ligand to silver ratio of 2:1.
The solid-state structure of a silver NHC complex will depend
heavily on the nature of the ligand (type of NHC, bulk of ligand,

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J. Berding et al. / Journal of Organometallic Chemistry 694 (2009) 2217–2221
imidazolium bromide [24]. In that case, however, the parent silver
complex analyzed as a 1:1 complex and AgBr is reported to be
formed at the bottom of the crystallization tube upon crystalliza-
tion by slow evaporation of a dichloromethane solution. Unfortu-
nately, no NMR data are given for the crystalline compound.
Several attempts to obtain the 2:1 complex by using different
crystallization conditions were undertaken, including the use of
different solvents, the absence and presence of light and a low tem-
perature. In all cases, however, crystals that formed analyzed (by
elemental analysis) as the 1:1 complex. Moreover, stirring a sam-
ple of 3 in THF at 70 °C for 14 h, followed by precipitation with
diethyl ether, did not change the composition of the compound.
In conclusion, it appears that the isolation of 2 with the type C
structure should be regarded as serendipitous.
570 (w), 482 (m) cmꢁ1. MS (ESI): m/z 249 ([MꢁBr]+, 100%). Anal.
Calc. for C17H17BrN2: C, 62.02; H, 5.20; N, 8.51. Found: C, 61.21;
H, 5.23; N, 8.44%.
3.3. Preparation of bis(l-bromido(1,3-dibenzylimidazol-
2-ylidene)silver(I)) (3)
1,3-Dibenzylimidazolium bromide (0.32 g, 1.0 mmol) and sil-
ver(I) oxide (0.14 g, 0.5 mmol) were mixed in dichloromethane
(10 mL) and stirred in the absence of light at room temperature
for 2 h. The resulting solution was filtered with the aid of a mem-
brane filter and concentrated in vacuo. Addition of diethyl ether
gave an off-white precipitate which was isolated by filtration,
washed with diethyl ether and dried in vacuo. Single crystals suit-
able for X-ray diffraction were grown by slow diffusion of diethyl
ether into a chloroform solution at room temperature.
2.5. Concluding remarks
Yield: 0.65 g (75%). 1H NMR (300 MHz, CDCl3, 300 K): d = 7.36
(m, 6H, CHarom), 7.23 (m, 4H, CHarom), 6.92 (s, 2H, CHIm), 5.30 (s,
4H, NCH2). 13C NMR (300 MHz, CDCl3, 300 K): d = 181.9 (Ag–C),
135.4 (Cq) 129.1 (C-Ar), 128.7 (C-Ar), 127.9 (C-Ar), 121.4 (CHIm),
In summary, an Ag(I)–NHC complex was synthesized starting
from 1,3-dibenzylimidazolium bromide, following a common pro-
cedure. This complex displays a solid-state structure that is signif-
icantly different from a structure that was reported during our
studies. It appears that the difference originates at the crystalliza-
tion procedure and we believe that our 1:1 ligand to silver complex
is the major product after the first isolation. With this in mind, we
recommend the use of elemental analysis, rather than mass spec-
trometry, for the determination of the composition of the crude
product. In addition, NMR experiments indicate that the formation
of the silver complex is fast and evidence was found that in solu-
tion the complex is present as a neutral species. Furthermore, a no-
vel classification of the solid-state structures of monodentate NHC
silver complexes is introduced.
55.8 (NCH2). IR (neat):
m = 3127 (w), 1492 (w), 1452 (m), 1356
(w), 1226 (m), 1152 (w), 1027 (w), 752 (s), 700 (s), 662 (m), 582
(m) cm–1. MS (ESI): m/z 605 ([L2Ag]+, 100%). Anal. Calc. for
C17H16AgBrN2: C, 46.82; H, 3.70; N, 6.42, Found: C, 46.85; H,
3.70; N, 6.48%.
3.4. X-ray crystallographic structure determination of 3
ꢀ
C34H32Ag2Br2N4, triclinic, space group P1, a = 8.1258(10) Å, b =
10.0156(10) Å, c = 10.5681(10) Å,
= 85.19(2)°, V = 794.43(17) Å3, Z = 2, MW = 872.18, Dcalc
1.8231(4) Mg mꢁ3. X-ray data were collected with a Nonius Kap-
paCCD diffractometer on rotating anode (T = 150 K, Mo K radia-
a = 89.427(12)°, b = 68.000(16)°,
c
=
a
0
3. Experimental
tion, k = 0.71073 ÅA, h(max) = 27.5°, 19,982 reflections measured).
The structure was solved by Patterson methods (DIRDIF) [27] and
refined with SHELXL-97 [28]. Hydrogen atoms were introduced at
calculated positions and refined riding on their carrier atoms. Con-
vergence was reached at R = 0.0236 for 3474 reflections with I >
2r(I), wR2 = 0.0628 for 3625 reflections, S = 1.045. Illustrations
and structure validation were done with PLATON [29]. Crystallo-
graphic data may be obtained as CIF files (see Appendix A).
3.1. General procedures
All chemicals were obtained from commercial sources and used
as received. Solvents were reagent grade and used without further
purification, except for 1,4-dioxane which was distilled from CaH2
and stored on molecular sieves under argon. NMR spectra were ob-
tained on a Bruker DPX300 spectrometer and are referenced
against TMS. IR spectra were recorded on a Perkin–Elmer Paragon
1000 FT-IR spectrophotometer. C,H,N determinations were per-
formed on a Perkin–Elmer 2400 Series II analyzer. Electrospray
mass spectra were recorded on a Finnigan TSQ-quantum instru-
ment using an electrospray ionization technique (ESI-MS), using
water/acetonitrile solutions. Imidazolium bromide 1 has been re-
ported before [7,25,26]; however, a full characterization was never
reported.
Acknowledgments
We thank Prof. J. Reedijk for continuous support of our work
and valuable discussions. This work has been supported financially
by the National Graduate Research School Combination NRSC-
Catalysis, a joint activity of the graduate research schools NIOK,
HRSMC and PTN and by the Council for Chemical Sciences of the
Netherlands Organization for Scientific Research (A.L.S.).
3.2. Preparation of 1,3-dibenzylimidazolium bromide (1)
Appendix A. Supplementary data
Under argon, a mixture of N-benzylimidazole (3.96 g, 25 mmol)
and benzyl bromide (5.13 g, 30 mmol) in 30 mL dry 1,4-dioxane
was stirred at 100 °C for 4 h. The resulting two-phase system
was cooled and the layers were separated. The bottom layer was
washed two times with 1,4-dioxane and once with diethyl ether.
Drying in vacuo yielded a clear, colorless oil that crystallized very
slowly.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.02.030.
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