N-Functionalised heterocyclic dicarbene complexes of silver: Synthesis and structural studies

Article abstract of DOI:10.1016/j.jorganchem.2007.11.038

A range of new diimidazolium salts, held together by an alkyl unit and bearing alcohol pendant arms on their nitrogen, was synthesized. A short modular reaction pathway leads to the N-heterocyclic carbene (NHC) precursors, differing by the flexibility of

Full text of DOI:10.1016/j.jorganchem.2007.11.038

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Journal of Organometallic Chemistry 693 (2008) 579–583
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Note
N-Functionalised heterocyclic dicarbene complexes of silver:
Synthesis and structural studies
Franßcois Jean-Baptiste dit Dominique a, Heinz Gornitzka b, Catherine Hemmert a,*
a Laboratoire de Chimie de Coordination du CNRS, 205 Route de Narbonne, 31077 Toulouse Cedex 4, France
b Laboratoire d’ He´te´rochimie Fondamentale et Applique´e (UMR-CNRS 5069), Universite´ Paul Sabatier, 118 Route de Narbonne,
31062 Toulouse Cedex 9, France
Received 25 October 2007; received in revised form 21 November 2007; accepted 21 November 2007
Available online 2 January 2008
Abstract
A range of new diimidazolium salts, held together by an alkyl unit and bearing alcohol pendant arms on their nitrogen, was
synthesized. A short modular reaction pathway leads to the N-heterocyclic carbene (NHC) precursors, differing by the flexibility of
the bridge, the steric bulk of substituents in a-position of the OH groups and the anions. Treatment of diimidazolium salts with
Ag2O yields AgI(carbene)2 complexes. The related trimethylene-bridged bis-NHC silver complexes 6 and 7 were crystallised with di-
tosylate and di-hexafluorophosphate anions, respectively. Their X-ray structures revealed dimeric species, involving two ligands with dif-
ferent arrangements around the Ag cations, leading to crossed and parallel conformations.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Imidazolium salt; N-Heterocyclic carbene; Silver(I) complex; X-ray structure
1. Introduction
ents in a-position of the OH functions. We also report
structural studies of two original dimeric silver complexes.
Since the pioneer work on metal N-heterocyclic carbenes
¨
(NHCs) by Ofele and Wanzlick, intensive activity research
2. Results and discussion
has been focused on the organometallic chemistry of this
class of ligands [1–3]. Strong nucleophilic character, low
tendency for ligand dissociation and ability to stabilise
high- and low-valent metals, have made NHC-complexes
attractive targets in contemporary catalysis [4–7]. In the
last decade, a large number of reports described the
unprecedented activity of carbene complexes in important
reactions, such as Pd-catalysed Heck- and Suzuki-coupling,
Co-catalysed ethylene copolymerisation, Ru-catalysed ole-
fin metathesis and Rh-catalysed hydrosilylation [8–18].
We are interested in the use of polydentate bis-NHC
ligands. For this purpose, we describe the preparation of
several diimidazolium salts, bridged by aliphatic chains
and bearing alcohol pendant arms, with different substitu-
2.1. Synthesis of NHC precursors bearing alcohol arms
Alkane-bridged bis-imidazolium salts 2, 3 and 5 were
synthesized in two steps (Scheme 1). The first one corre-
sponds to a ring-opening of an epoxide, a method applied
by Arnold for the synthesis of asymmetric imidazolium
precursors of mixed alkoxide-carbene ligands [19]. Quater-
nisation of the alcohol-substituted imidazoles 1 or 4 by
reaction with one half equivalent of the appropriate dibro-
moalkane or ditosylate diol lead to precursor ligands 2, 3
and 5 in quantitative yield.
Main characteristics in 1H spectra are resonances for the
imidazolium protons (9.08 ppm (2), 9.24 ppm (3) and
9.17 ppm (5)). In 13C spectra, corresponding imidazole
carbon resonances are located at 137.4 ppm (2), 138.4 ppm
(3) and 137.1 ppm (5). FAB-MS spectra obtained from
*
Corresponding author. Fax: +33 5 61553003.
E-mail address: hemmert@lcc-toulouse.fr (C. Hemmert).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.11.038

580
F. Jean-Baptiste dit Dominique et al. / Journal of Organometallic Chemistry 693 (2008) 579–583
n
2X-
N
+
N
+
3
2
N
N
HO
OH
R1
2
R1
CH3CN
4
+
X
X
N
N
n
80°C, 8h
5
1
R2
R1
R2
R2
OH
2: R1 = R2 =CH3, n = 1, X = TsO-, R = 99%
3: R1 = R2 =CH3, n = 1, X = Br-, R = 90%
5: R1 = H, R2 = Ph, n = 3, X = TsO-, R = 99%
1
n = 1, 3
: R1 = R2 =CH3
X = TsO-, Br-
4: R1 = H, R2 = Ph
1
2
1
2
2
3
1
2TsO-
N
3
N
+
N
N
N
+
N
2TsO-
2Br-
N
+
N
N
3
4
4
5
+
+
5
5
6
+
4
N
N
OH HO
OH HO
N
HO
OH
6
6
9
H3C
CH3
9
7
7
7
8
H3C
CH3
3
CH3
CH3
H3C
H3C
Ph
Ph
H
H
8
8
5
2
Scheme 1. Synthesis of alcohol-functionalised diimidazolium salts 2, 3 and 5.
the ditosylate derivatives exhibit peaks corresponding
to [MꢀTsOꢀ]+ (m/z 493 (2) and 617 (5)) and
[Mꢀ2TsOꢀꢀH+]+ (m/z 321 (2) and 445 (5)), respectively.
FAB-MS spectrum of 3 shows only the [MꢀBrꢀ]+ cation
at m/z 401.
2). Ion exchange of complex 6 with NH4PF6 in methanol
afforded complex 7.
1
Notable features in H spectra of complexes 6–9 are the
resonances for alcohol functions (4.84 ppm (6, 7), 5.05 ppm
(8) and 5.84 ppm (9)). Arnold reported that reaction of
alcohol-functionalised imidazolium halides with Ag2O
can lead to deprotonation of C–H and O–H protons, yield-
ing halide free Ag(I)–NHC–alkoxide complexes [19,20]. In
our case, Ag2O is not basic enough to deprotonate the alco-
hol functions; the chemical behaviour of the remaining
acidic H-atom of the OH function is as it is bridged
between the alkoxide and carbene groups, making it less
easy to remove [20].
2.2. Synthesis and characterisation of silver(I)–carbene
complexes
AgI(carbene)2 complexes 6, 8 and 9 were prepared by
deprotonation of alcohol-functionalised bis-imidazolium
salts 2, 3 and 5, with the mild base Ag2O at 50 °C in dry
˚
DMSO or methanol in the presence of 4 A MS (Scheme
2+
-
[AgI(L)]2 [PF6 ]2
7
Complex , R = 92%
NH4PF6
MeOH
2X-
N
+
N
N
+
N
Ag2O
+
-
[AgI(L)]n [X]n
MeOH or DMSO
50°C
OH
HO
[LH2]2+ [2X-]:
Complex:
+
2
3
5
6: [AgI(L)]2+ [TsO-]2, R = 88%
: [LH2]2+ [2TsO-]
: [LH2]2+ [2Br-]
8: [AgI(L)]n+ [Br-]n, n = not defined, R = 56%
: [LH ]2+ [2TsO-]
: [Ag (L)]n [TsO-]n, n = not defined, R = 51%
I
9
2
Scheme 2. Synthesis of silver complexes 6–9.

F. Jean-Baptiste dit Dominique et al. / Journal of Organometallic Chemistry 693 (2008) 579–583
581
In the 13C spectrum of 6 synthesized in methanol, the
carbene resonance appears at two doublets (181.3 ppm)
107
C–Ag
109
with coupling constants (J
= 181.9 Hz,
J
=
C–Ag
209.0 Hz), in the range of reported values [21]. The 13C
spectra of 6, prepared in DMSO, and of 7–9 show only
one doublet or a sharp signal (181.4 ppm (6), 180.2 ppm
(7), 181.9 ppm (8) and 181.4 ppm (9)) for the carbonium
carbon. The lack of silver–carbene coupling indicates the
lability of the Ag–carbene bond in solution [22]. The pres-
ence of PF6ꢀ in complex 7 is confirmed by the 31P spectrum
(septuplet centered at ꢀ144.2 ppm, JPF = 716. 8 Hz). All
the MS/FAB spectra of complexes 6–9 show mass peaks
according to the mono-cationic species [AgIL]+ (m/z 427
for 6–8 and 551 for 9).
Fig. 2. Structure of 7 (50% probability level). H atoms, anions and solvent
molecules are omitted for clarity. Selected bond lengths (A) and angles (°):
Ag(1)–C(1) 2.086(3), Ag(1)–C(18) 2.084(3), Ag(2)–C(9) 2.087(3), Ag(2)–
C(26) 2.093(3), Ag(1). . .Ag(2) 6.305, C(18)–Ag(1)–C(1) 170.96(13), C(9)–
Ag(2)–C(26) 174.29(12).
˚
Crystals suitable for X-ray diffraction studies of 6 and 7
were grown from diffusion of acetone into a solution of 6 in
MeOH and from diffusion of MeOH into a solution of 7 in
acetonitrile. These two compounds involve the samꢀe
ligand, but differ in their anions, TsOꢀ for 6 and PF6
for 7. Although poor quality crystals of complex 6 were
grown in a reproducible manner, the connectivity can be
deduced unequivocally. Both structures showed dinuclear
dimers containing two Ag(I) ions bridged by two NHC
ligands. The 2+ charge is balanced by two non-interacting
anions. Silver NHC complexes show diverse structures in
the solid state, depending on several factors (Ag/L ratio,
nature of the NHC ligand, source of silver, counter ions,
solvent and temperature). As a matter of fact, addition of
a methanolic solution of NH4PF6 to 6, to afford 7, dramat-
ically changes the arrangement of the ligands around the
silver ions (Figs. 1 and 2). Complex 6 crystallizes in the tri-
conformation with dihedral angles C(11)–Ag–Ag–C(1) of
93.86° and C(18)–Ag–Ag–C(28) of 94.53°.
The structure of the ‘‘helical” cation results in a weak
˚
Agꢁ ꢁ ꢁAg interaction (3.074 A), which is shorter than the
˚
sum of the Van der Waals radii (3.40 A) [23]. Compound
7 crystallizes in the monoclinic space group P21/n with
the asymmetric unit containing one dicationic dimer, two
ꢀ
PF6 anions and 3.25 molecules of methanol. Compound
7 forms a ‘‘shell-like” structure with the same orientation
for all alcohol chains, thus creating a solvent pocket with
the OH groups being turned inside (Fig. 2).
The two NHCs units adopt a parallel conformation.
Coordination about each Ag(I) slightly deviates from line-
arity, with classical silver–carbene bonds ranging from
˚
2.084(3) to 2.093(3) A and C–Ag–C bond angles of
ꢀ
170.93(16)° and 174.29(12)° [24]. This little deviation could
arise from the folding up of the molecule imposed by
hydrogen interactions between the alcohol groups and
methanol molecules.
The complexes form a tube in the crystal, which host the
solvent molecules (Fig. 3). This channel like arrangement
allows a high liberty for the methanol molecules in solid
state, leading to high disorder problems. In both structures,
no contact between oxygen atoms of the alcohols and silver
ions occurs. Neither complex shows extended intermolecu-
lar metal–metal interactions. We were not able to determine
clinic space group P1 with one dicationic dimer and two
TsOꢀ anions in the asymmetric unit. The X-ray structure
of 6 (Fig. 1) shows a nearly linear coordination geometry
around the silver atoms with silver–carbene bond distances
˚
˚
between 2.039(11) A and 2.104(11) A and bond angles of
179.0(5)° and 174.9(5)°. Due to the flexibility of this ligand,
the two bis-coordinated silver linkages adopt a crossed
Fig. 1. Structure of 6 (50% probability level). H atoms and anions are
omitted and parts of the molecule are simplified for clarity. Crossed
˚
conformation is depicted on the right side. Selected bond lengths (A) and
angles (°): Ag(1)–C(1) 2.073(12), Ag(1)–C(28) 2.104(11), Ag(2)–C(11)
2.039(11), Ag(2)–C(18) 2.099(11), Ag(1)–Ag(2) 3.074(2), C(28)–Ag(1)–
C(1) 179.0(5), C(11)–Ag(2)–C(18) 174.9(5).
Fig. 3. Crystal packing of 7.

582
F. Jean-Baptiste dit Dominique et al. / Journal of Organometallic Chemistry 693 (2008) 579–583
either anions or solvents employed for crystallisation play
an important role for the obtained helical or parallel con-
formations. Similar structures were precedently obtained
for silver(I) complexes of ether-functionalised bis-carbene
ligands, by two independent groups [25,26]. The flexibility
of this kind of ligands, allowed several conformations which
are probably influenced by crystal-packing and intra- and
inter-molecular forces.
2 H, H3), 7.50 (bs, 2H, H4), 4.84 (s, 2H, OH), 4.13 (bs,
4H, H2), 3.73 (s, 4H, H6), 1.01 (s, 12H, H8,9). H NMR
1
(300 MHz, CD3CN): d = 7.31 (d, 2H, H3, 3J = 1.8 Hz),
7.30 (d, 2H, H4, 3J = 1.8 Hz), 4.19 (t, 4H, H2, 3J = 6.0
Hz), 3.82 (s, 4H, H6), 3.08 (s, 2H, OH), 2.51 (m, 2H,
H1), 1.11 (s, 12H, H9,8). 13C NMR (75 MHz, CD3CN):
d = 181.9 (2C, C5), 124. 1 (2C, C4), 120.3 (2C, C3), 69.4
(2C, C7), 61.4 (2C, C6), 48.2 (2C, C2), 30.6 (1C, C1),
26.5 (4C, C8,9). 31P NMR (121 MHz, DMSO-d6): d =
ꢀ144.2 (septuplet JPF = 716.8 Hz). MS (FAB): m/z = 427
[MꢀPF6ꢀ]+.
3. Conclusion
We have prepared alcohol functionalised diimidazolium
salts with varied bridging arms and different substituents
on the pendant OH arms, using a modulable synthetic
strategy. Corresponding silver–carbene complexes AgI(car-
bene)2 have been synthesized. Two dinuclear Ag(I) com-
plexes have been structurally characterised in the
4.1.3. Structure determinations
Crystal data for 6 and 7. Complex 6: C48H70Ag2N8O10S,
˚
ꢀ
M = 1198.98, triclinic, P1, a = 11.686(3) A, b = 12.574(3)
˚
˚
A, c = 20.967(5) A, a = 102.935(5)°, b = 97.087(5)°,
3
˚
c = 108.452(5)°, V = 2784.9(12) A , Z = 2, T = 173(2) K.
presence of non coordinating anions TsOꢀ and PF6
,
19370 reflections (7857 independent, Rint = 0.0280) were
ꢀ
ꢀ3
˚
collected. Largest electron density residue: 4.127 e A ,
respectively, showing considerable variations in solid state
geometries. We continue to explore these potentially alk-
oxy-functionalised bis-NHC ligands to design new attrac-
tive catalysts.
R1 (for I > 2r(I)) = 0.0962 and wR2 = 0.2424 (all data)
P
P
P
2
with R1 = kFoj ꢀ jFck/ jFoj and wR2 = ( w(Fo
ꢀ
P
Fc ) / w(Fo2)2)0.5. Complex 7: C37.25H69Ag2F12N8O9.75P2,
2 2
M = 1250.68, monoclinic, P21/n, a = 15.6227(3) A, b =
˚
˚
˚
4. Experimental
12.4109(2) A,c = 28.4935(6) A,
b = 100.024(1)°,
V =
3
˚
5440.32(18) A , Z = 4, T = 173(2) K. 46510 reflections
(9165 independent, Rint = 0.0426) were collected. Largest
4.1. N-functionalised heterocyclic dicarbene silver complexes
6 and 7
electron density residue: 0.526 e Aꢀ3, R1 (for I > 2r(I)) =
˚
0.0305 and wR2 = 0.0748 (all data).
4.1.1. Complex 6
A
Data for all structures were collected at low temperature
using an oil-coated shock-cooled crystal on a Bruker-AXS
APEX2 diffractometer with Mo Ka radiation (k =
Schlenk was charged with Ag2O (0.417 mg,
˚
1.8 mmol), 2 (0.600 g, 0.9 mmol), 4 A MS (1.0 g) and
DMSO (15 mL). This mixture was stirred under argon at
50 °C for 14 h, then diluted with dichloromethane
(25 mL) and filtered on a celite bed. On addition of dieth-
ylether (60 mL), a colorless solid precipitates. After filtra-
tion, the desired product was obtained as a colourless
solid. Yield: 0.475 g, 88%. Anal. Calc. for C24H35N4O5-
SAg: C, 48.08; H, 5.88; N, 9.35. Found: C, 47.74; H,
5.61; N, 9.15%. 1H NMR (300 MHz, DMSO-d6):
˚
0.71073 A). The structures were solved by direct methods
(SHELXS-97) [27] and all non-hydrogen atoms were refined
anisotropically using the least-squares method on F2 [28].
Acknowledgements
This work was supported by the Centre National de la
Recherche Scientifique (CNRS). Dr. Bernard Meunier is
acknowledged for fruitful discussions.
3
d = 7.64 (d, 2H,ꢀH3, J = 1.5 Hz), 7.51 (bs, 2H, H4), 7.48
ꢀ
(d, 2H, HArTsO
, ,
3J = 7.8 Hz), 7.12 (d, 2H, HArTsO
3J = 7.8 Hz), 4.84 (s, 2H, OH), 4.13 (t, 4H, H2,
Appendix A. Supplementary material
3J = 5.4 Hz), 3.73 (s, 4H, H6), 2.26 (s, 3H, HCH TsO ),
ꢀ
3
1.01 (s, 12H, H8,9). 13C NMR (75 MHz, DMSO-d6):
CCDC 664306 and 664307 contain the supplementary
crystallographic data for compounds 6 and 7. These data
can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.jorganchem.2007.11.038.
d = 181.3 (pseudod, 2C, C5), 146.3 (2C, CArTsOꢀ), 138.1
(2C, CArTsOꢀ), 128.5 (4C, CArTsOꢀ), 125.9 (4C, CArTsOꢀ),
124.7 (2C, C4), 120.9 (2C, C3), 68.9 (2C, C7), 54.7 (2C,
C6), 47.8 (2C, C2), 31.1 (1C, C1), 27.7 (4C, C8,9), 21.2
(2C, HꢀCH TsO)). MS (FAB): m/z = 427 [MꢀTsOꢀ]+.
3
4.1.2. Complex 7
Ion exchange of 6 (0.71 g, 0.8 mmol) was realised in a
5 mL methanolic solution of NH4PF6 (0.260 mg, 1.60
mmol). After filtration, the colourless solid was obtained.
Yield: 0.42 g, 92%. Anal. Calc. for C17H28N4O2P1F6Ag:
C, 35.62; H, 4.92; N, 9.77. Found: C, 35.50; H, 4.89; N,
9.44%. 1H NMR (300 MHz, DMSO-d6): d = 7.64 (bs,
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