Novel scorpionate-type triscarbene ligands and their silver and gold complexes

Article abstract of DOI:10.1016/j.jorganchem.2008.09.019

New silver(I) carbene complexes were obtained starting from the N-heterocyclic carbene ligand precursors {[HB(RImH)₃]Br₂} (R = Bn, Mes and t-Bu) and {[HC(MeBImH)₃](BF₄)₃}, by treatment of the imidazolium salt with Ag₂O. Use of the tris-imidazolylborate precursors resulted in stable, well-characterized trimetallic complexes of general formula {Ag₃[HB(RIm)₃]₂}Br, which were successfully employed as carbene transfer reagents in the synthesis of related gold(I) complexes by transmetallation. The silver complexes also proved to be active catalysts of the coupling of aryl iodides with terminal alkynes (the Sonogashira reaction), although related bimetallic silver complexes were found to exhibit enhanced reactivity.

Full text of DOI:10.1016/j.jorganchem.2008.09.019

Journal of Organometallic Chemistry 693 (2008) 3760–3766
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Novel scorpionate-type triscarbene ligands and their silver and gold complexes
Andrea Biffis a, , Giancarlo Gioia Lobbia , Grazia Papini , Maura Pellei , Carlo Santini , Elena Scattolin a,
b
b
b
b,*
*
a
Cristina Tubaro
a
Department of Chemical Sciences, University of Padova, Via Marzolo, 1, 35131 Padova, Italy
Department of Chemical Sciences, University of Camerino, Via S. Agostino 1, 62032 Camerino (MC), Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2008
Received in revised form 10 September
New silver(I) carbene complexes were obtained starting from the N-heterocyclic carbene ligand precur-
sors {[HB(RImH)
lium salt with Ag
trimetallic complexes of general formula {Ag
3
]Br
O. Use of the tris-imidazolylborate precursors resulted in stable, well-characterized
[HB(RIm) }Br, which were successfully employed as car-
2 3 4 3
} (R = Bn, Mes and t-Bu) and {[HC(MeBImH) ](BF ) }, by treatment of the imidazo-
2
2
008
3
3 2
]
Accepted 11 September 2008
Available online 18 September 2008
bene transfer reagents in the synthesis of related gold(I) complexes by transmetallation. The silver com-
plexes also proved to be active catalysts of the coupling of aryl iodides with terminal alkynes (the
Sonogashira reaction), although related bimetallic silver complexes were found to exhibit enhanced
reactivity.
Keywords:
Tris(carbene)borate
Silver(I)
Ó 2008 Elsevier B.V. All rights reserved.
Gold(I)
Imidazole
Catalysis
Sonogashira reaction
1
. Introduction
An extremely useful and versatile class of ligands [1–3] is repre-
known [8,16–26], the use of anionic NHCs is still scarce [27]. Re-
cently, Siebert et al. [28,29] described the synthesis and character-
ization of the first anionic monocarbene imidazol-2-ylidenes, the
3-borane-1-alkylimidazol-2-ylidenes anions and their metal com-
plexes [30]. The synthesis of monoanionic chelating dicarbene
bis(imidazolylidene)borates, and their use as ligands in various
homoleptic and heteroleptic palladium(II), platinum(II), gold(I)
[31] and nickel(II) [32] complexes has been recently described.
The first chelating triscarbene ligand with the topology of Tro-
fimenko’s tris(pyrazolyl)borate, tris(3-methylimidazolin-2-yli-
dene-1-yl)borate, has been introduced in 1995 by Fehlhammer
and co-workers [33], together with its hexacarbene iron(III) and
cobalt(III) complexes [34,35]; very recently, this ligand was found
to form a trinuclear complex with copper(I) centers, which proved
to be an active catalyst for C–N and C–O coupling reactions [36]. In
2005 Smith et al. [37] reported a new synthetic route that allows
for the incorporation of bulkier substituents, and showed that a
bulky tripodal tris(carbene)borate ligand, prepared from 1-tert-
butylimidazole, is cleanly transferred to iron(II) by a magnesium
reagent. Successively they have modified the tris(carbene)borate
class of ligands to incorporate a phenyl group on the boron atom
[38]. These bulky tris(carbene)phenylborate ligands are able to sta-
bilize coordinatively unsaturated cobalt(II) centers [38] and to
form cobalt(III) [39] and high-valent iron imido complexes [40].
More recently, the same authors have prepared and characterized
the manganese(I) tricarbonyl complex of tris(carbene)phenylb-
orate ligands prepared from 1-methylimidazole [41].
sented by N-heterocyclic carbenes (NHCs), first proposed by Wanz-
lick [4] and Öfele [5], and later isolated in the free state by
Arduengo et al. [6]. Their chemical versatility not only implies a
wide variety of structural diversity and coordination modes, but
also a capability to form stable complexes with a large number
of transition metals with different oxidation states.
Polydentate NHCs can provide new complexes with enhanced
catalytic performances and higher stability [7–10]. Most of the
poly-carbenes reported so far are neutral bidentate biscarbene li-
gands coordinated to transition metals such as Pd, Pt, Rh, Ru and
Ir [10]. It is well recognized that chelating bis-NHCs are able to
yield more stable metal complexes with interesting features that
can provide fine tuning of topological properties such as steric hin-
drance, bite angles, chirality and fluxional behavior. In the prepara-
tion of chelating N-heterocyclic carbene complexes several
methods have proved to be efficient, and these have been reviewed
[
1,2,7,8,11–15].
Whereas there are many studies describing the coordination of
pincer N-heterocyclic carbene ligands [10] and a number of un-
charged and flexible nitrogen-anchored poly-NHC ligands are
*
Corresponding authors. Fax: +39 737 637345 (C. Santini).
E-mail addresses: andrea.biffis@unipd.it (A. Biffis), carlo.santini@unicam.it
(
C. Santini).
0
022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.09.019

A. Biffis et al. / Journal of Organometallic Chemistry 693 (2008) 3760–3766
3761
The design of NHCs with a tripod coordination is important be-
These solutions were injected (1
ll) into the spectrometer via a
cause in octahedral complexes the fac-tricoordinate geometry
should stabilise the metal complex, especially when the stability
of the ligand–metal bond is high, while leaving three additional
coordination sites for labile coligands, easily displaceable to yield
mutually cis coordination sites available for catalytic activity. Me-
tal complexes of these ligands should have quite different elec-
tronic properties from those of the tris(pyrazolyl)borates due to
HPLC HP 1090 Series II fitted with an autosampler. The pump deliv-
ered the solutions to the mass spectrometer source at a flow rate of
À1
300
l
l min , and nitrogen was employed both as a drying and
nebulizing gas. Capillary voltages were typically 4000 V and
3500 V for the positive- and negative-ion mode, respectively. Con-
firmation of all major species in this ESI-MS study was aided by
comparison of the observed and predicted isotope distribution pat-
terns, the latter calculated using the ISOPRO 3.0 computer program.
the very strong
r-donor nature of N-heterocyclic carbenes.
In this paper, we have synthesized new tripodal N-heterocyclic
carbene ligand precursors and the related silver(I) carbene com-
plexes (Schemes 1–3). The transmetallation from a silver(I) NHC
complex, synthesized by treatment of the imidazolium salt with
2.2. Synthesis
All reagents were purchased from Aldrich and used without fur-
Ag
2
O, is a useful coordination method in the preparation of chelat-
3
ther purification. The ligand K[HB(Im) ] [46] was prepared in
ing N-heterocyclic carbene complexes [42]; in most cases this pro-
cedure can be carried out under aerobic conditions, and the process
has been successful with a variety of metals such as Au, Cu, Ni, Pd,
Pt, Rh, Ir and Ru [43–45]. In the present paper we have used a sim-
ilar approach to synthesize various gold(I) complexes by transmet-
allation reactions of the starting silver(I) derivatives. Finally, a
preliminary screening of the catalytic potential of these and other
silver(I) NHC complexes in C–C coupling reactions has been carried
out.
accordance with the literature method. The 1-mesitylimidazole
[47] and the 1-tert-butylimidazole [48] were synthesized in accor-
dance with the literature methods and purified by vacuum distilla-
tion. The imidazolium salt {[HB(t-BuImH)
modifying the synthetic procedure previously reported by Smith
et al. [37]. Complexes {Ag [CH (MeIm) (PF (12) and
{Ag [CH (n-BuIm) (PF } (13) were prepared following a litera-
ture procedure [49].
3 2
]Br } (3) was prepared
2
2
2
]
2
6 2
) }
2
2
2
]
2
6 2
)
2
.2.1. {[HB(BnImH)
Benzylbromide (4.395 g, 25.7 mmol) was added to a chloroform
solution (50 mL) of K[HB(Im) ] (2.000 g, 7.9 mmol). The reaction
3 2
]Br } (1)
2
. Experimental
3
mixture was stirred at room temperature for 24 h to give a light
yellow solution and a white precipitate. The salt was removed by
filtration and the volatiles were removed in vacuo to afford a light
2
.1. Material and methods
All syntheses and handling were carried out under an atmo-
yellow oil. The oil was dissolved in a minimum amount of CH
10 mL) and diethyl ether was added to give a white solid that
was collected by filtration, washed with diethyl ether and dried
2 2
Cl
sphere of dry oxygen-free dinitrogen, using standard Schlenk tech-
niques or a glove box. All solvents were dried, degassed and
distilled prior to use. Elemental analyses (C,H,N,S) were performed
in house with a Fisons Instruments 1108 CHNS-O Elemental Ana-
lyser. Melting points were taken on an SMP3 Stuart Scientific
(
1
at reduced pressure. Yield 85%. H NMR (CD
3
OD, 293 K): d 5.44
(
7
2
1
s, 6H, CH ), 7.40–7.44 (m, 15H, CH), 7.54 (d, 3H, 4-CH or 5-CH),
2
1
3
À1
.62 (d, 3H, 4-CH or 5-CH), 9.02 (s, 3H, 2-CH). C NMR (CD
93 K): d 54.03 (CH ), 125.02, 125.45 (4-CH and 5-CH), 129.81,
30.43, 130.53, 135.54 (C
038m (CH), 2525m (BH), 1553m (C = C + C@N). ESI-MS (major po-
3
OD,
Instrument. IR spectra were recorded from 4000 to 400 cm with
a
Perkin–Elmer FT-IR Spectrum100. IR annotations used:
2
À1
6 5
H ), 140.52 (2-CH). IR (cm ): 3103w,
br = broad, m = medium, mbr = medium broad, s = strong,
sh = shoulder, w = weak. 1H and C NMR spectra were recorded
13
3
+
+
1
sitive-ions, CH
3
OH), m/z (%): 243 (100) [HB(BnImH)
3
]
, 566 (40)
OH), m/z
31BBr
on an Oxford-400 Varian spectrometer (400.4 MHz for H and
1
to internal Me
and the chemical shifts are reported in ppm versus Me
+
1
3
1
[
{HB(BnImH)
3
}Br] . ESI-MS (major negative-ions, CH
3
00.1 MHz for C). Chemical shifts for H NMR spectra are relative
À
Si. C NMR spectra were run with 1H decoupling,
13
(
%): 726 (100) [{HB(BnImH)
}Br
3 3
] . Anal. Calc. for C30
H
2 6
N :
4
C, 55.76; H, 4.84; N, 13.00. Found: C, 55.59; H, 4.80; N, 12.88%.
4
Si. NMR
annotations used: br = broad, d = doublet, dd = double doublet,
m = multiplet, s = singlet, sbr = broad singlet. Electrospray mass
spectra (ESI-MS) were obtained in positive- or negative-ion mode
on a Series 1100 MSD detector HP spectrometer, using an acetone
mobile phase. The compounds were added to the reagent grade
methanol to give solutions of approximate concentration 0.1 mM.
2
.2.2. {[HB(MesImH)
A 1 M dichloromethane solution of (CH
added to a dichloromethane solution (40 mL) of mesitylimidazole
2.000 g, 10.7 mmol). The reaction mixture was stirred at reflux
for 24 h, then it was cooled to room temperature and the volatiles
3 2
]Br } (2)
3 2 2
) S:BHBr (3.3 mL) was
(
H
B
H
B
N
N
Bn
N
N
N
N
N
N
N
N
N
N
H
N
H
N
N
Bn
Br
N
Bn
Br
Bn
Bn
B
B
a
N
N
b
Bn
c
N
N
Ag
Au
Bn
Ag
Au
Ag
N
Au
N
Bn
Bn
N
N
N
N
Bn
Bn
K
Bn Br
Bn
N
Bn
N
Br Bn
N
N
N
N
N
N
N
N
K[HB(Im)3]
[HB(BnImH) ]Br , 1
2 2
B
H
B
H
Ag [HB(BnIm) ] Br, 5
Au [HB(BnIm) ] Br, 9
3
3
3 2
3 2
Scheme 1. Preparation of 1, 5 and 9. Reaction conditions: (a) room temperature, benzylbromide, CHCl
Au(SMe )Cl, CH Cl
; (b) room temperature, Ag O, CH Cl ; (c) room temperature,
3 2 2 2
2
2
2
.

3
762
A. Biffis et al. / Journal of Organometallic Chemistry 693 (2008) 3760–3766
H
B
H
B
N
N
R
N
N
N
N
N
N
N
N
N
N
H
N
N
R
N
R
R
R
B
N
N
R
a
N
N
b
Ag
R
c
Au
R
Ag
Au
Ag
N
Br
Au
N
Br
R
R
N
R
R
R
N
R
N
R
Br
Br
R
N
N
N
N
N
N
N
N
[
HB(RImH) ]Br2
B
H
B
H
2
2
3
, R = Mesityl
, R = t-Butyl
Ag [HB(RIm) ] Br
Au [HB(RIm) ] Br
3 3 2
3
3 2
6
, R = Mesityl
10, R = Mesityl
11, R = t-Butyl
7
, R = t-Butyl
Scheme 2. Preparation of 2, 3, 6, 7, 10 and 11. Reaction conditions: (a) reflux, (CH
3
)
2
S:BHBr
2
, CH
Cl
2 2
; (b) room temp, Ag
2
O, CH
2
Cl
2
2 2 2
; (c) room temperature, Au(SMe )Cl, CH Cl .
H
C
N
N
N
Me
N
N
N
N
N
N
N
Me
Me
H
H
a
b
Me
Ag
C
C
3BF4
Me
Ag
Ag
Me
N
N
N
N
Me
N
N
N
N
N
N
Me
N
N
N
Me
N
3
BF
4
C
H
[
HC(BIm)3]
{[HC(MeBImH) ](BF ) }, 4
{Ag [HC(MeBIm) ] (BF ) }, 8
3
4 3
3
3 2
4 3
Scheme 3. Preparation of 4 and 8. Reaction conditions: (a) room temp, (Me
3
O)(BF
4
), CH
2
Cl
2
; (b) room temp, Ag O, CH
2
2
Cl /CH
2
3
CN.
+
were removed in vacuo to afford a brown hygroscopic solid. The so-
lid was purified by re-crystallization from dichloromethane/
BuImH)
3
}Br] . ESI-MS (major negative-ions, CH
3
OH), m/z (%): 624
37BBr C,
À
(100) [{HB(t-BuImH)
3
}Br
3
] . Anal. Calc. for
C
21
H
2 6
N :
diethyl ether (1/2), and dried at reduced pressure to give derivative
46.35; H, 6.85; N, 15.44. Found: C, 46.17; H, 6.83; N, 15.31%.
1
2
3
in 82% yield. H NMR (CDCl , 293 K): d 2.05 (s, 18H, CH
3
), 2.33 (s,
9
H, CH
3
), 6.99 (s, 6H, CH), 7.18 (s, 3H, 4-CH or 5-CH), 8.98 (s, 3H, 4-
2.2.4. {[HC(MeBImH) ](BF } (4)
A solution of 1,1,1-tris(benzimidazol-1-yl)methane [50] (1.3 g,
3.6 mmol) in dichloromethane (15 ml) was added in small portions
3
4 3
)
13
CH or 5-CH), 9.90 (s, 3H, 2-CH). C NMR (CDCl
2
1
3
, 293 K): d 17.75,
1.21 (CH
40.83 (CH). IR (cm ): 3020w (CH), 2512w (BH), 1532s
OH), m/z (%): 285
3
), 120.86, 122.66, 124.45, 126.07, 129.96, 134.44,
À1
3 4
to a dichloromethane suspension (20 mL) of (Me O)(BF ) (1.90 g,
(
(
(
[
C = C + C@N). ESI-MS (major positive-ions, CH
100) [HB(MesImH)
major negative-ions, CH
{HB(MesImH)
3
12.8 mmol). The reaction mixture was stirred at room temperature
for 24 h to give a light yellow solution and a white precipitate. The
product was filtered and recrystallized from a mixture MeOH/
++
+
3
]
, 650 (30) [{HB(MesImH)
OH), m/z (%): 80 (100) [Br] , 810 (30)
3
}Br] . ESI-MS
À
3
À
1
3
}Br
3
] . Anal. Calc. for C36
H43BBr
2
N
6
: C, 59.20; H,
water 95/5. Yield 50%. H NMR (DMSO, 293 K): d 4.14 (s, 9H,
5
.93; N, 11.51. Found: C, 59.07; H, 5.99; N, 11.29%.
CH
CH).
3
), 7.50–8.50 (m, 12H, CHBIm), 10.08 (s, 3H, 2-CH), 10.65 (s, 1H,
1
3
3
C NMR (DMSO, 293 K): d 34.49 (CH ), 73.19, 113.92,
2
.2.3. {[HB(t-BuImH)
A 1 M dichloromethane solution of (CH
3
]Br
2
} (3)
114.64, 128.24, 128.37, 129.06, 132.35 (CH), 143.45 (2-CH). IR
(cm ): 3094m (CH), 1618w, 1579m, 1460m, 1337m, 1259m,
À1
3
)
2
S:BHBr
2
(2.9 mL) was
added to a dichloromethane solution (40 mL) of 1-tert-butylimid-
azole (1.118 g, 9.0 mmol). The reaction mixture was stirred at re-
flux for 24 h, then it was cooled to room temperature and the
volatiles were removed in vacuo to afford a white hygroscopic so-
lid. The solid was purified by re-crystallization from dichlorometh-
ane/diethyl ether (1/2), and dried at reduced pressure to give
1220m, 1034br, 753s. Anal. Calc. for C25
3.73; N, 12.53. Found: C, 42.60; H, 3.39; N, 11.75%.
25 3 12 6
H B F N : C, 44.80; H,
2.2.5. {Ag
Ag O (2.700 g, 11.6 mmol) was added to a dichloromethane
solution (50 mL) of {[HB(BnImH) ]Br } (5.000 g, 7.7 mmol). The
3 3 2
[HB(BnIm) ] Br} (5)
2
3
2
derivative 3 in 78% yield. 1H NMR (CDCl
CH ), 7.32 (d, 3H, 4-CH or 5-CH), 8.42 (d, 3H, 4-CH or 5-CH), 9.75
, 293 K): d 1.71 (s, 27H,
reaction mixture was stirred at room temperature for 12 h. The
resulting suspension was filtered through Celite and the solution
was concentrated under vacuum and treated with diethyl ether
(100 mL) to precipitate a microcrystalline white solid. It was fil-
3
3
1
(
s, 3H, 2-CH). H NMR (DMSO, 293 K): d 1.58 (s, 27H, CH
3
), 7.74
(
d, 3H, 4-CH or 5-CH), 8.01 (d, 3H, 4-CH or 5-CH), 9.09 (s, 3H, 2-
1
3
CH). C NMR (DMSO, 293 K): d 29.89 (CH
1
3
), 59.68 (C(CH
3
3
) ),
tered off, washed with diethyl ether and dried at reduced pressure.
À1
1
22.01, 124.22, 137.61 (CH). IR (cm ): 3100sh, 3065w (CH),
Yield 60%. M.p. 183–185 °C. H NMR (CD
3
OD, 293 K): d 5.41 (s,
2
476w (BH), 1534m (C = C + C@N). ESI-MS (major positive-ions,
12H, CH
2
), 7.02–7.33 (m, 30H, CH), 7.51 (d, 6H, 4-CH or 5-CH),
+
+
1
3 3
CH OH), m/z (%): 192 (100) [HB(t-BuImH) ] , 464 (60) [{HB(t-
7.66 (d, 6H, 4-CH or 5-CH). H NMR (DMSO, 293 K): d 5.26 (s,

A. Biffis et al. / Journal of Organometallic Chemistry 693 (2008) 3760–3766
3763
1
2H, CH
2
), 7.00–7.40 (m, 30H, CH), 7.48 (d, 6H, 4-CH or 5-CH), 7.98
7.50–8.05 (m, 12H, CHBIm), 9.83 (s, 1H, CH). The instability of the
compound in solution prevents the obtainment of a C NMR
13
13
(
d, 6H, 4-CH or 5-CH). C NMR (DMSO, 293 K): d 56.71 (CH
2
),
1
(
21.14, 121.57 (4-CH and 5-CH), 128.38, 129.07, 129.88, 138.92
spectra.
CH), 183.07 (dd, J(C– Ag) = 202 Hz, 1J(C– Ag) = 176 Hz, Ag–
1
109
107
À1
C). IR (cm ): 3105w, 3065w, 3029w (CH), 2434m, 2413m (BH),
2.2.9. {Au
A dichloromethane solution (30 mL) of Au(SMe
0.68 mmol) was added dropwise to a dichloromethane solution
of {Ag [HB(BnIm) Br} (0.31 g, 0.23 mmol). A white precipitate
3 3 2
[HB(BnIm) ] Br} (9)
1
4
556m (C = C + C@N), 669w, 661w 636br, 616sh, 573m, 460m,
31br. ESI-MS (major positive-ions, CH OH), m/z (%): 1290 (100)
BrN12: C, 52.59; H,
2
)Cl (0.20 g,
3
+
[
Ag
3
{HB(BnIm)
3
}
2
] . Anal. Calc. for C60
H56Ag
B
3 2
3
3 2
]
4
.12; N, 12.27. Found: C, 52.84; H, 4.01; N,12.00%.
The mother liquor was evaporated at reduced pressure; the
(AgBr) formed upon addition of the gold(I) salt. The mixture was
stirred at room temperature for 2 h, and then filtered through Cel-
ite. The filtrate was concentrated under vacuum and treated with
diethyl ether (20 mL) to give a white crystalline powder. The solid
was collected by filtration, washed with diethyl ether, and dried
resulting residue was treated with MeOH (0.5 mL), CH
0.5 mL) and diethyl ether (5 mL) to give an off-white solid, which
was filtered and dried at reduced pressure. This second crop of iso-
lated solid was identified as the bis-carbene byproduct {[Ag(Bn I-
m) ]AgBr
}. Yield 30%. 1H NMR (DMSO, 293 K): d 5.30 (s, 8H,
CH ), 7.20–7.34 (m, 20H, CH), 7.54 (s, 4H, 4-CH and 5-CH).
NMR (DMSO, 293 K): d 54.25 (CH ), 122.56 (4-CH and 5-CH),
27.54, 128.00, 128.77, 137.26 (CH), carbene carbon not detected.
2 2
Cl
(
1
2
under vacuum. Yield 77%. M.p. 166–168 °C (dec.). H NMR (DMSO,
2
2
293 K): d 5.37 (s, 12H, CH
CH or 5-CH), 8.05 (d, 6H, 4-CH or 5-CH). C NMR (DMSO, 293 K): d
2
), 7.15–7.30 (m, 30H, CH), 7.51 (d, 6H, 4-
1
3
13
2
C
2
52.98 (CH
2
), 122.15, 122.73 (4-CH and 5-CH), 127.40, 127.76,
À1
1
128.60, 137.03 (CH), 188.68 (Au–C). IR (cm ): 3115w, 3068w
(CH), 2417mbr (BH), 1526m (C = C + C@N), 637br, 577m, 460br,
ESI-MS (major positive-ions, CH
n
3
OH), m/z (%): 604 (100) [Ag(B-
OH), m/z (%): 268
+
2
Im)
2
] . ESI-MS (major negative-ions, CH
3
325w. ESI-MS (major positive-ions, CH
[Au {HB(BnIm) } ] . Anal. Calc. for C60H56Au B
3 3 2 3 2
3
OH), m/z (%): 1558 (100)
BrN12: C, 44.01;
H, 3.45; N, 10.26. Found: C, 43.84; H, 3.29; N, 10.07%
À
+
(
100) [AgBr
2
] .
2
.2.6. {Ag
Ag O (0.695 g, 3.0 mmol) was added to a dichloromethane solu-
tion (50 mL) of {[HB(MesImH) ]Br } (1.462 g, 2.0 mmol). The reac-
3 3 2
[HB(MesIm) ] Br} (6)
2
2.2.10. {Au
A dichloromethane solution (50 mL) of Au(SMe
0.68 mmol) was added dropwise to a dichloromethane solution
of {Ag [HB(MesIm) Br} (0.35 g, 0.23 mmol). A white precipitate
(AgCl) formed upon addition of the gold(I) salt. The mixture was
stirred at room temperature for 2 h, and then filtered through Cel-
ite. The filtrate was concentrated under vacuum and treated with
diethyl ether (20 mL) to give a light brown powder. The solid
was collected by filtration, washed with diethyl ether, and dried
3 3 2
[HB(MesIm) ] Br} (10)
3
2
2
)Cl (0.20 g,
tion mixture was stirred at room temperature for 12 h. The
resulting suspension was filtered through Celite and volatiles were
removed in vacuo to afford a light brown hygroscopic solid. The so-
lid was purified by re-crystallization from chloroform/diethyl ether
3
3 2
]
(
1/2), and dried at reduced pressure to give the colourless deriva-
1
3
tive 6 in 66% yield. M.p. 169–171 °C (dec.). H NMR (CDCl ,
2
6
93 K): d 1.65 (s, 36H, CH
.90 (s, 6H, 4-CH or 5-CH), 7.46 (s, 6H, 4-CH or 5-CH). C NMR
), 122.67, 123.21, 129.04,
3 3
), 2.41 (s, 18H, CH ), 6.77 (s, 12H, CH),
1
3
under vacuum to give the colourless derivative 10. Yield 71%.
1
(
DMSO, 293 K): d 17.52, 21.48 (CH
3
M.p. 192–194 °C. H NMR (CDCl
3
, 293 K): d 1.65 (sbr, 36H, CH
3
),
1
109
1
34.73, 136.70, 137.85 (CH), 188.67 (dd, J(C– Ag) = 207 Hz,
2.42 (sbr, 18H, CH
CH), 7.47 (br, 6H, 4-CH or 5-CH). C NMR (DMSO, 293 K): d
3
), 6.80 (sbr, 12H, CH), 6.90 (br, 6H, 4-CH or 5-
1
107
À1
13
J(C– Ag) = 180 Hz, Ag–C). IR (cm ): 3118w (CH), 2457m (BH),
556m (C = C + C@N), 649br, 635br, 582m, 560w, 539m, 527w.
ESI-MS (major positive-ions, CH OH), m/z (%): 1459 (100)
] . Anal. Calc. for C72 BrN12: C, 56.20;
H, 5.24; N, 10.92. Found: C, 56.02; H, 5.21; N, 10.75%.
1
16.80, 20.79 (CH
137.17 (CH), 190.44 (Au–C). IR (cm ): 3120w (CH), 2470m (BH),
1551br (C = C + C@N), 638m, 629w, 617m, 551m, 541w. ESI-MS
3
), 122.42, 124.03, 128.29, 133.85, 135.98,
À1
3
+
[
Ag
3
{HB(MesIm)
3
}
2
3 2
H80Ag B
(major positive-ions, CH
3
OH), m/z (%): 1726 (100) [Au
] . Anal. Calc. for C72 BrN12: C, 47.89; H, 4.47; N,
9.31. Found: C, 48.01; H, 4.54; N, 9.13%.
3
{HB(Me-
+
sIm)
3
}
2
3 2
H80Au B
2
.2.7. {Ag
Ag O (0.695 g, 3.0 mmol) was added to a dichloromethane solu-
tion (50 mL) of {[HB(t-BuImH) ]Br } (1.088 g, 2.0 mmol). The reac-
3 3 2
[HB(t-BuIm) ] Br} (7)
2
3
2
2.2.11. {Au
A dichloromethane solution (50 mL) of Au(SMe
0.68 mmol) was added dropwise to a dichloromethane solution
of {Ag [HB(t-BuIm) Br} (0.27 g, 0.23 mmol). A white precipitate
3 3 2
[HB(t-BuIm) ] Br} (11)
tion mixture was stirred at room temperature for 12 h. The
resulting suspension was filtered through Celite and volatiles were
removed in vacuo to afford a light brown hygroscopic solid. The so-
lid was purified by re-crystallization from chloroform/diethyl ether
2
)Cl (0.20 g,
3
3 2
]
(AgCl) formed upon addition of the gold(I) salt. The mixture was
stirred at room temperature for 2 h, and then filtered through Cel-
ite. The filtrate was concentrated under vacuum and treated with
diethyl ether (20 mL) to give a light brown powder. The solid
was collected by filtration, washed with diethyl ether, and dried
(
1/2), and dried at reduced pressure to give the colourless deriva-
1
tive 7 in 60% yield. M.p. 175 °C (dec.). H NMR (DMSO, 293 K): d
.63 (s, 54H, CH ), 7.41 (s, 6H, 4-CH or 5-CH), 7.48 (s, 6H, 4-CH
or 5-CH). C NMR (DMSO, 293 K): d 31.50 (CH ), 56.21 (C(CH ),
19.31, 120.52 (4-CH and 5-CH), 182.45 (dd, J(C– Ag) = 197 Hz,
1
3
1
3
3
3 3
)
1
109
1
under vacuum to give the colourless derivative 11. Yield 71%.
1
107
À1
1
J(C– Ag) = 180 Hz, Ag–C). IR (cm ): 3131w (CH), 2456m (BH),
533m (C = C + C@N), 727s, 684w. ESI-MS (major positive-ions,
M.p. 236 °C dec. H NMR (CDCl
3
, 293 K): d 1.82 (s, 54H, CH
3
),
13
1
7.02 (d, 6H, 4-CH or 5-CH), 7.16 (s, 6H, 4-CH or 5-CH). C NMR
+
CH
C
3
OH), m/z (%): 1086 (100) [Ag
BrN12: C, 43.26; H, 5.88; N, 14.41. Found: C, 43.02;
H, 5.68; N, 14.20%.
3
{HB(t-BuIm)
3
}
2
] . Anal. Calc. for
(DMSO, 293 K): d 31.86 (CH
CH and 5-CH), 187.39 (Au–C). IR (cm ): 3137w (CH), 2466br
3
), 57.38 (C(CH
3
)
3
), 118.42, 129.72 (4-
À1
42
H68Ag
3
B
2
(BH), 1558m, 1532m (C = C + C@N). ESI-MS (major positive-ions,
+
CH
3
OH), m/z (%): 1353 (100) [Au
3
{HB(t-BuIm)
}
3 2
] . Anal. Calc. for
2
.2.8. {Ag
Ag O (0.12 g, 0.5 mmol) was added to an dichloromethane/ace-
tonitrile solution (7 mL/3 mL) of {[HC(MeBImH) ](BF } (0.21 g,
.31 mmol). The reaction mixture was stirred at room temperature
3
[HC(MeBIm)
3
]
2
(BF
4
)
3
} (8)
42 3 2
C H68Au B BrN12: C, 41.42; H, 4.59; N, 10.08. Found: C, 41.25;
H, 4.32; N, 10.25%.
2
3
4 3
)
0
2.3. Catalysis
for 5 h. The resulting suspension was filtered through Celite and
volatiles were removed in vacuo to afford a off-white hygroscopic
General procedure for the catalytic tests. An oven-dried
Schlenk tube equipped with a magnetic stirring bar was charged
1
solid. Yield 30%. H NMR (DMSO, 293 K): d 3.91 (s, 9H, CH
3
),

3
764
A. Biffis et al. / Journal of Organometallic Chemistry 693 (2008) 3760–3766
with aryl halide (0.25 mmol), Cs
.2 equiv.) and 0.008 mmol catalyst (10 mol% silver). The tube
was closed with a rubber septum, evacuated and filled with ar-
gon. Phenylacetylene (33 L, 0.3 mmol, 1.2 equiv.) and solvent
2 mL) were subsequently injected, and the tube was placed in
2
CO
3
(97.7 mg, 0.3 mmol,
in which every silver atom is coordinated to two imidazolin-2-yli-
1
dene rings, belonging to two different tricarbene units, each one
1
therefore coordinating in
g
fashion. The resulting complexes 5–
l
7 are cationic, monomeric and organic-soluble. Complexes 5–7
have been used to synthesize the homoleptic gold(I) complexes
(
an oil bath preheated at 110 °C. The reaction mixture was stirred
at 110 °C for 24 h, after which it was cooled to room tempera-
ture and diluted with 5 mL dichloromethane. The resulting sus-
pension was filtered and the solvent was removed under
vacuum to give a brown solid. The reaction yield was deter-
2
9–11 by transmetallation reaction with Au(SMe )Cl in dichloro-
methane solution. Similar types of complexes have been obtained
with bidentate bis(carbene) ligands [54,55]. The formation of tri-
nuclear complexes 5–11 reflects the tripodal nature of the N-het-
erocyclic carbene donor ligands and the tendency of group 11
metal ions to form linear, two-coordinate complexes [16,17,19,56].
1
mined by H NMR spectroscopy.
3.2. Spectroscopy
3
. Results and discussion
The ligands 1–3 and the metal derivatives 5–11 have been char-
3.1. Syntheses
acterized by analytical and spectral data. The infrared spectra car-
ried out on the solid samples showed all the expected bands for the
ligands and the silver(I) or gold(I) moieties: weak absorptions in
The
[HB(BnImH)
tion via alkylation with benzyl bromide of the starting compound
potassium tris-(imidazol-1-yl)borate K[HB(Im) ] (Scheme 1). The
bulky {[HB(MesImH) ]Br (2), and tris(tert-butyl-imidazol-3-
ium-1-yl)hydroborate, {[HB(t-BuImH) ]Br } (3), were synthesized
by a similar synthetic path to that reported for the preparation of
ionic liquid imidazole-based ‘‘boronium” ions [51]. Three equiva-
lents of 1-tert-butylimidazole or 1-mesityl-imidazole were heated
N-heterocyclic
carbene
ligand
precursor
salt,
{
3
]Br } (1), has been synthesized in chloroform solu-
2
À1
the range 3020–3137 cm are due to the azolyl ring C–H stretch-
À1
ings and medium to strong absorptions at 1526–1558 cm are re-
3
lated to ring ‘‘breathing” vibrations. In the infrared spectra of
3
2
}
tris(imidazolyl)borate ligands 1–3 the B–H stretching vibrations
3
2
À1
appear in the range 2476–2525 cm . Interestingly, on going from
the imidazolium salts 1–3 to the carbene complexes 5–7 and 9–11,
a red shift of the m(BH) feature is always observed.
1
13
The H and C NMR data reflect significant changes in going
3 2 2
in refluxing dichloromethane with (CH ) S:BHBr (Scheme 2).
from the imidazolium salts to the carbene complexes. In partic-
Compounds 1–3 are hygroscopic solids; they are soluble in meth-
anol, DMSO and chlorinated solvents and insoluble in water,
diethyl ether and n-hexane. The related compound 4, {[HC(Me-
1
ular the H NMR spectra of the complexes 5–11 showed the dis-
appearance of the diagnostic 2-CH imidazolium resonances in
the 1–4 ligands at 9.02, 9.90, 9.09 and 10.08 ppm, respectively.
The formation of the silver(I) carbene complexes 5–7 was con-
firmed by the appearance of the 2-C resonance of the imida-
zol-2-ylidene as double doublets at 183.07, 188.67 and
BImH)
tris(benzimidazol-1-yl)methane with Meerwein’s salt (Me
Scheme 3). Compound 4 possesses a CH bridge and is therefore
precursor for a neutral triscarbene ligand. The silver(I) complexes
Ag [HB(BnIm) Br} (5) (Scheme 1), {Ag [HB(MesIm) Br} (6),
and {Ag [HB(t-BuIm) Br} (7) (Scheme 2), were prepared in
dichloromethane solution via deprotonation of the imidazolium
salts 1–3 with Ag O. The complexes were obtained analytically
3
](BF
4
)
3
}, was synthesized by simple methylation of 1,1,1-
3
4
O)(BF )
(
1
82.45 ppm, respectively, due to coupling of 2-C with the two
{
3
3
]
2
3
3 2
]
spin 1/2 isotopes of silver. The individual couplings to the two
isotopes of silver were well resolved in the spectra
3
3 2
]
1
109
1
107
(
J(C– Ag) = 197–207 Hz, J(C– Ag) = 176–180 Hz), and they
2
1
07
109
are in the range of the values for
stants found in similar silver(I) N-heterocyclic carbenes
44,52,53]. Unfortunately, the relative instability of complex 8
Ag and
Ag coupling con-
pure in good yields. Remarkably, during the preparation of com-
plex 5 we were able to isolate and identify a mononuclear NHC–sil-
ver(I) complex [52,53] with two N,N-dibenzylimidazolinylidene
[
À
13
13
prevents the obtainment of a C NMR spectrum. In the
C
carbene ligands bearing AgBr2 as counterion (Scheme 4) as
NMR spectra of the gold(I) complexes 9-11 the 2-C resonance
of the imidazol-2-ylidene appeared as singlet in the range
byproduct of the reaction. Similar byproducts were detected also
in the preparation of the other complexes by ESI-MS. We are cur-
rently investigating the mechanism by which such byproducts
1
87.39–190.44 ppm; the chemical shift value for the carbene
carbons is in good agreement with the carbene carbon chemical
shifts of similar dinuclear bis(carbene)gold species [31,54,55].
Mass spectra (ESI-MS) of all the new compounds have been re-
corded (Section 2). The general appearance of the molecular ions
followed by characteristic fragmentations supports the structural
assignments. The structure of the imidazolium salts 1–3 was
confirmed by the presence of peaks at m/z 243, 285 and 192,
2
could form. Reaction of the benzimidazolium salt 4 with Ag O
yields a white solid, identified as the tricationic, trinuclear silver(I)
complex 8. Product 8 is, however, unstable, as indicated by the pro-
gressive darkening of the solid and by the fact that we were unable
to obtain satisfactory elemental analyses, so that this behaviour
prevents the study of the subsequent transfer reaction to gold. Ow-
ing to the tripodal nature of the uninegative carbene ligands, they
+
+
due to the species [HB(RImH)
trospray ionization mass spectra, together with minor peaks at
m/z 566, 650 and 464 due to the aggregates [{HB(RImH)
}Br]+
R = Bn, Mes and t-Bu). The negative ion mode electrospray ion-
3
]
in the positive ion mode elec-
2 3
likely give rise to metallacycles [(L) (l-Ag) ] (L = carbene ligand),
3
(
ization mass spectra of 1–3 were dominated by peaks due to the
À
species [{HB(RImH)
3
3
}Br ]
(R = Bn, Mes and t-Bu). The structure
of the silver(I) complexes 5–7 was confirmed by the presence
in their positive-ion mass spectra of peaks at m/z 1290, 1459
+
and 1086 due to the trinuclear species [Ag
3
{HB(RIm)
3
}
2
] . Analo-
gously the positive ion mode electrospray ionization mass spec-
tra of the gold(I) compounds 9–11 were dominated by peaks due
+
to the trinuclear species [Au
3
{HB(RIm)
3
}
2
] . No fragment ions
were observed for the pure complexes, showing that the silver(I)
and gold(I) complexes were stable under the conditions of the
electrospray ionization and detection.
Scheme 4. Ag(I)–NHCs obtained from reaction media.

A. Biffis et al. / Journal of Organometallic Chemistry 693 (2008) 3760–3766
3765
Scheme 5. General procedure for arylation of phenylacetylene.
3
.3. Catalysis
exclusively observed in DMSO, whereas use of N,N-dimethylform-
amide (DMF) as the reaction solvent yielded exclusively the Sono-
gashira product; the only exception to this behaviour was complex
8, which in DMSO yielded equal amounts of the Sonogashira prod-
uct and of the aryl halide homocoupling product. The reason for
this unexpected reversal in selectivity are currently being investi-
gated. The reaction yields were, however, quite low, even with
the activated 4-iodoacetophenone substrate: best results were ob-
tained with the dicarbene complex 12, which beside 4-iodoaceto-
phenone was also able to activate the less reactive 4-iodoanisole.
For sake of comparison, simple AgI was reported to convert both
substrates with >90% yield in the Sonogashira reaction with phen-
ylacetylene under similar reaction conditions [58].
Recently there has been a surge of interest in the application of
silver salts and complexes as catalysts for fine chemical synthesis
57]. In particular, a recent report highlights the possibility to
[
use such metal species as catalyst in the coupling of aryl halides
with terminal alkynes (the Sonogashira reaction) [58]. This reac-
tion is routinely performed using a palladium-based catalyst
[
59], but efficient palladium-free systems would obviously be
much more interesting for economical reasons, related both to
the higher cost of palladium as well as to the difficulties in remov-
ing the metal from the reaction product. Indeed, it is a common
industrial practice to avoid, whenever possible, the use of Pd cata-
lysts in the last steps of the synthesis of a complex molecule.
We set out to evaluate the potential of our silver(I) triscarbene
complexes as catalysts for the reaction outlined in Scheme 5. The
reaction conditions were chosen according to those found to be
optimal in [59]. For comparison, we also included in the study
the silver(I) dicarbene complexes 12 and 13 (Scheme 6), whose
preparation has been previously reported in the literature [49]. Re-
sults of the catalytic screening of the various complexes are re-
ported in Table 1.
In principle, two side reactions potentially affecting selectivity
can occur in this system beside the Sonogashira reaction, namely
homocoupling of the aryl halide or of the alkyne. Interestingly, it
turned out that in the case of the polycarbene–silver complex cat-
alysts the solvent has a decisive influence in determining the selec-
tivity of the reaction. In fact, aryl halide homocoupling was
4
. Conclusions
In this paper, we have synthesized novel silver(I) carbene com-
plexes starting from the tripodal N-heterocyclic carbene ligand
precursors {[HB(RImH)
]Br } (R = Bn, Mes and t-Bu) and {[HC(Me-
BImH) ](BF }. Use of the tris-imidazolylborate precursors re-
sulted in stable, well-characterized trimetallic complexes of
general formula {Ag [HB(RIm) }Br, which were successfully em-
ployed as carbene transfer reagents in the synthesis of related
gold(I) complexes by transmetallation reaction with Au(SMe )Cl.
3
2
3
4 3
)
3
3 2
]
2
Finally, a preliminary screening of the catalytic potential of these
and other silver(I) NHC complexes in C–C coupling reactions has
been carried out. The silver complexes proved to be active catalysts
of the coupling of aryl iodides with terminal alkynes (the Sono-
gashira reaction), although bis(imidazolinylidene)methanes sil-
ver(I) dicarbene complexes were found to exhibit enhanced
reactivity.
Acknowledgment
This work was supported by University of Camerino (FAR 2007).
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