Synthesis, molecular structures and solution NMR studies of N-heterocyclic carbene-amine silver complexes

Article abstract of DOI:10.1016/j.jorganchem.2008.04.030

The synthesis of the Boc-protected 1-(2-aminoethyl)-3-methylimidazolium salts [BocNHCH₂CH₂ImMe]X [2]X (X = I, PF₆) and their straightforward transformation into [NH₂CH₂CH₂ImMe]X [3]X is reported. The reaction between [2]X and Ag₂O leads to the formation in the solid state of three different bonding motifs: a biscarbene salt [(NHC-NHBoc)₂Ag]PF₆ ([4]PF₆, NHC-NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-ylidene), a tetranuclear complex [Ag(NHC-NHBoc)₂]₂[Ag₂I₄], (5), and a polymeric silver staircase [(NHC-NHBoc)₂-Ag₄-I₄]_n, (6) composed of Ag₄I₄ clusters. The same reaction carried out with [3]I showed that a primary silver mono-NHC-NH₂ carbene complex of the type [(NHC-NH₂)AgI] (7) is likely to form but it is unstable in solution. The solid state molecular structures of [4]PF₆, 5 and 6 were determined by X-ray diffraction analysis, whereas PGSE NMR experiments were employed to investigate the hydrodynamic dimension of the imidazolium salts and silver complexes and, consequently, to gain information on the level of aggregation in solution. PGSE NMR studies were complemented by NOE NMR investigations in order to obtain information on anion-cation relative orientation within aggregates.

Full text of DOI:10.1016/j.jorganchem.2008.04.030

Journal of Organometallic Chemistry 693 (2008) 2579–2591
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, molecular structures and solution NMR studies of N-heterocyclic
carbene–amine silver complexes
a
a
b,
a
Luigi Busetto a, , M. Cristina Cassani , Cristina Femoni , Alceo Macchioni , Rita Mazzoni ,
*
*
Daniele Zuccaccia b
a Dipartimento di Chimica Fisica ed Inorganica, viale Risorgimento 4, I-40136 Bologna, Italy
b Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto 8, I-06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of the Boc-protected 1-(2-aminoethyl)-3-methylimidazolium salts [BocNHCH2CH2ImMe]X
[2]X (X = I, PF6) and their straightforward transformation into [NH2CH2CH2ImMe]X [3]X is reported. The
reaction between [2]X and Ag2O leads to the formation in the solid state of three different bonding
motifs: a biscarbene salt [(NHC–NHBoc)2Ag]PF6 ([4]PF6, NHC–NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imi-
dazolin-2-ylidene), a tetranuclear complex [Ag(NHC–NHBoc)2]2[Ag2I4], (5), and a polymeric silver ‘‘stair-
case” [(NHC–NHBoc)2–Ag4–I4]n, (6) composed of Ag4I4 clusters. The same reaction carried out with [3]I
showed that a primary silver mono-NHC–NH2 carbene complex of the type [(NHC–NH2)AgI] (7) is likely
to form but it is unstable in solution. The solid state molecular structures of [4]PF6, 5 and 6 were deter-
mined by X-ray diffraction analysis, whereas PGSE NMR experiments were employed to investigate the
hydrodynamic dimension of the imidazolium salts and silver complexes and, consequently, to gain infor-
mation on the level of aggregation in solution. PGSE NMR studies were complemented by NOE NMR
investigations in order to obtain information on anion–cation relative orientation within aggregates.
Ó 2008 Elsevier B.V. All rights reserved.
Received 27 March 2008
Received in revised form 23 April 2008
Accepted 23 April 2008
Available online 30 April 2008
Keywords:
Imidazolium salts
N-Heterocyclic carbene ligands
Silver complexes
PGSE
NOE NMR
1. Introduction
as one of the most important classes of compound used for a great
number of transition metal mediated catalytic reactions [3].
For several years within our group one research line has dealt
with the study of carbene ligands in diiron complexes, including
heteroatom substituted (Fischer type) alkylidenes, in both bridging
and terminal coordination modes [1]. In the course of these stud-
ies, in 1995, we found that the reaction of diiron l-alkylidene com-
plexes such as [Fe2{l-C(CN)SMe2}(l-CO)(CO)2(Cp)2][SO3CF3] with
ethylenediamine results in the cleavage of the C–S and C–C bonds
at the l-alkylidene carbon yielding to [Fe2{CN(H)(CH2)2N(H)}
(l-CO)(CO)2(Cp)2] (Eq. (1)) in which an N-heterocyclic carbene li-
gand is bound to an iron metal centre [2].
The heterocyclic carbene moiety offers great possibilities for
fine-tuning the ligand structure, and, thereby, the catalytic proper-
ties, through the introduction of appropriate substituents at the
nitrogen atoms or at the 5-membered ring carbon atoms. The com-
bination of ligand donor units with very different coordination
properties in a di- or polydentate spectator ligand may permit a
stereoelectronic control of the reactivity at the remaining coordi-
nation sites in a transition metal complex. Consequently, a strong
motivation is leading to design polydentate NHC carbene ligands
functionalized with a neutral or anionic donor group linked by
an organic spacer [4]. Moreover, taking advantage of the ability
of N-heterocyclic carbenes to form strong r-bonds with the metal,
it can serve as an anchoring point for the immobilization of the me-
tal complexes onto solid supports [5].
Therefore several hybrid ligands based on NHC have been re-
ported including those having an N-atom at the other terminus
(NHC–N) in the form of a pyrazolyl [6], pyridine [7], oxazoline
[8] sterically demanding imine [9] and amido/amine groups [10].
With regard to the NHC–amine case we were interested in expand-
ing the library of this type of ligands focusing our attention on the
NHC–NH2 system that, as far as we know, has been firstly obtained
by Douthwaite et al. by slow hydrolysis of NHC–imino complexes
of palladium [9e].
+
H
CN
Fe
SMe
2
O
C
N
C
NH
NH
2
2
CO
Cp
ð1Þ
CO
Cp
N
CO
Fe
Fe
Fe
+
H
-HCN, -H , -SMe
2
Cp
Cp
C
O
C
O
Recently our interest in this area has been renewed since complexes
containing the N-heterocyclic carbene (NHCs) ligands have emerged
* Corresponding authors. Tel.: +39 051 2093694; fax: +39 051 2093690.
E-mail address: busetto@ms.fci.unibo.it (L. Busetto).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.04.030

2580
L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
We herein report on the synthesis of the Boc-protected 1-(2-
der to avoid competitive oligomerization side reactions, we tried to
modify the synthetic protocol reacting 1-methylimidazole with the
N-protected, N-Boc-bromoethylamine. However, in this case, we
found that no reaction occurs at room temperature whilst by heat-
ing the reaction mixture the only result is the decomposition of the
latter reagent. Consequently our first step was that of preparing, by
modifying a synthetic procedure recently described in a Canadian
patent [17] the starting molecule (2-imidazol-1-yl-ethyl)-carbamic
acid tert-butyl ester hereafter abbreviated as BocNHCH2CH2Im, 1.
The N-quaternization of 1 with an excess of MeI led, in quanti-
tative yields, to the methylimidazolium salt [BocNHCH2CH2Im-
Me]I, [2]I that readily underwent to an anion exchange with
AgPF6 to afford [2]PF6. The subsequent thermolytic deprotection
of the –NH2 functionality of neat [2]I and [2]PF6 derivatives
cleanly gave the 1-(2-aminoethyl)-3-methylimidazolium salts
[NH2CH2CH2ImMe]X [3]I and [3]PF6 although for the latter harsher
conditions were required (Scheme 2).
aminoethyl)-3-methylimidazolium salts [BocNHCH2CH2ImMe]X
[2]X (X = I, PF6), their straightforward transformation into
[NH2CH2CH2ImMe]X [3]X together with the syntheses and charac-
terization of new silver NHC–amine complexes. The X-ray molecu-
lar structural studies of three silver carbene complexes show
considerable differences depending on the coordinating or only
weakly coordinating nature of the counterions [11]: when X = I,
the tetranuclear complex of the type [Ag(NHC–NHBoc)2]2[Ag2I4]
(5, NHC–NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-yli-
dene) is obtained but on standing in solution it gradually affords
a polymeric silver ‘‘staircase” of the type [(NHC–NHBoc)2–Ag4–
I4]n, (6) composed of Ag4I4 clusters. On the contrary when X = PF6
the crystal structure consists of two independent and stable bis
carbene [(NHC–NHBoc)2Ag]PF6 molecules [4]PF6 in a 1:1 enantio-
meric mixture. In order to gain a better understanding of the dy-
namic behaviour, level of aggregation and cation–anion relative
orientation present in solution, the imidazolium salts [2]X and sil-
ver complexes were studied by PGSE (Pulsed Gradient Spin Echo)
[12] and NOE [13] NMR techniques. Diffusion measurements for
NHC-complexes are still scarce [14,15] and, as far as we know,
the information herein reported represent the first data for NHC–
Ag complexes. Finally the reactivity of [3]I toward Ag2O is also pre-
sented and discussed.
Compounds 2 and 3 are very viscous air-stable liquids. Unlike 2,
the deprotected [3]I is insoluble in THF and chlorinated solvents,
partially soluble in acetonitrile and completely soluble in DMSO
and water, whereas [3]PF6 is completely soluble only in water.
Assignments for 1H and 13C chemical shifts have been unambigu-
ously confirmed by gCOSY, gHSQC and HMBC experiments. With
the exception of the spectra in D2O that, as often observed, did
not show the downfield peak for the imidazolium proton [18],
the NCHN resonance was found in the interval d 9.70–8.70 (ca. d
137 for the corresponding NCHN carbon). The imidazole backbone
protons (CHimid) invariably appear as broad singlets for 2, while in
the deprotected case [3]I, probably due to strong intermolecular
interactions favoured by the polar solvents used (CD3CN, DMSO-
d6), the appearance of the signals is dependent on the concentra-
tion and in dilute samples a doublets of doublets multiplicity with
2. Results and discussion
2.1. Ligand synthesis
Inspired by the recent work of Song et al. [16] in which a new
functionalized ionic liquid, 1-(2-aminoethyl)-3-methylimidazo-
lium hexafluorophosphate [NH2CH2CH2ImMe][PF6] was synthe-
3
4
resonances at 7.74 and 7.71 with JH,H = 2.0 and JH,H = 1.6 Hz can
sized
from
1-methylimidazole
and
2-bromoethylamine
be observed.
hydrobromide, we decided to exploit this synthetic procedure to
prepare the NHC–amine precursor. However, we found that the
synthesis is thwarted by competitive oligomerization of the amine
that invariably give a mixture of several products as depicted in
Scheme 1. The major, ethanol soluble product, 3-methylimidazo-
lium bromide (A), can be easily separated from the ethanol-insol-
uble B, C and D salts (see also Section 4).
In addition, considering what previously reported by Douthwa-
ite et al. [10d], namely that the synthesis of the imidazolium salt
NHC-precursors by reaction of N-alkyl imidazoles with haloge-
nalkylamines requires protection of the amine functionality in or-
2.2. Synthesis and characterization of silver aminocarbene complexes
The use of the silver base Ag2O has been found a very advanta-
geous method in order to trap the carbene molecule and has re-
cently been comprehensively reviewed [19,20]. The imidazolium
[2]I was hence treated with a slurry of Ag2O in dichloromethane
in a 2:1 molar ratio and stirred at room temperature for 2 h in
the dark and under argon. The resulting grey suspension was fil-
tered and the volatiles removed under reduced pressure to afford
Br-
Br-
Br
NH2. HBr
H
NH2.HBr
N
N
N
N
N
N
+
+
+
MeCN, 80 ˚C, 4 h
A, 72.5%
B, 20.5%
Br-
NH
N
+
NH2. HBr
N
C, 6.5%
Br-
NH
NH2. HBr
N
+
N
H
N
D, 0.5%
Scheme 1. Reaction between 1-methylimidazole and 2-bromoethylamine hydrobromide.

L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
2581
I-
-
PF6
+
NHBoc
NHBoc
N
NHBoc
i
ii
N
N
N
N
N
+
[2]PF6
[2]I
1
iii
iv
X-
+
NH2
N
N
[3]I, [3]PF6
Scheme 2. Reagents and conditions: (i) MeI(excess), THF, rt, overnight; (ii) AgPF6, CH2Cl2, 4 h; (iii) 185 °C, 30 min under argon; (iv) 210 °C, 3 h under argon.
in quantitative yields the silver complex [4][AgI2] as a white solid
that was analyzed by different spectroscopic techniques (Eq. (2)).
N
N
N
N
NHBoc
NHBoc
Ag
CH2Cl2
r.t., 2 h
[AgI2]
2[2]I + Ag2O
ð2Þ
[4][AgI2]
In the 1H NMR spectrum in CDCl3 the complete disappearance of the
high frequency peak for the imidazolium proton was coupled with
the appearance in the 13C NMR spectrum of a sharp singlet at d
184.9 assigned to a Ag–Ccarbene carbon (no 13C–107Ag/109Ag coupling
was observed); the CHimid carbons at d 121.7 and 121.4 ppm, are,
like the Ag–Ccarbene chemical shift, perfectly in keeping with the val-
ues reported in the literature for other imidazol-2-ylidene–silver
complexes [19,21]. The electrospray ionization mass spectrometry
analysis in methanol indicated the presence in solution of the silver
salt [Ag(NHC–NHBoc)2]+[AgI2]ꢀ with peaks at 557 (100) m/z for
[M]+ (C22H38AgN6O4) and at 361 (100) m/z for [AgI2]ꢀ with the ob-
served isotopic distribution in perfect agreement with the calcu-
lated one. The carbonyl stretching frequency (mCO
) of the
carbamate group appeared at 1716 cmꢀ1 in the IR spectrum in
THF whilst the IR-Microscopy showed a strong N–H stretching
absorption at 3339 cmꢀ1 and a mCO at 1701 cmꢀ1. The silver complex
is completely soluble in halogenated solvents, THF, acetone but
insoluble in diethyl ether and petroleum ether.
Colourless crystals suitable for an X-ray crystal structure deter-
mination were obtained from a double layer acetone/hexane at
ꢀ20 °C. As illustrated in Fig. 1 (selected bonds and angles are indi-
cated in Table 1) the crystal structure consists of a [Ag2I4]2ꢀ frag-
ment sandwiched between two identical [Ag(NHC–NHBoc)2]+
complexes (5) and it is analogous to the one reported by Chen
and Liu [22].
The similarities are found in the quasi-planar geometry showed
by the imidazolic rings, connected through Ag(1), as well as in their
almost linear arrangement as indicated by the C–Ag(1)–C angle of
170.9(3)°. Furthermore, the Ag(1)–C bonds are 2.081(7) and
2.077(9) Å, slightly shorter but in line with the ones previously ob-
served [21]. The Ag–I distances within the [Ag2I4]2ꢀ anion, namely
2.6902(9) Å for I(2)–Ag(2), 2.7871(9) Å for I(1#1)–Ag(2) and
Fig. 1. ORTEP diagram (30% probability thermal ellipsoids) of 5 (for sake of clarity
hydrogen atoms have been omitted).
2.8390(9) Å for I(1)–Ag(2), are also comparable. There are, how-
ever, major differences with regard to the Ag–Ag bond distances,
the intermolecular interactions and the crystal packing. As a mat-
ter of fact the Ag(1)–Ag(2) contact connecting the complex to the
anionic fragment is 3.2873(10) Å long, and a second long distance
of 3.2421(14) Å is found between the two Ag(2) atoms of the latter,
while the values Chan and Liu report are 3.042 and 2.9643(16) Å,
respectively. This might be due to the fact that, unlike theirs, the
structure we present displays important intermolecular hydrogen

2582
L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
Table 1
Selected bond lengths (Å) and angles (°) for 5, 6 and [4]PF6
5
6
[4]PF6
Ag(1)–C(21)
Ag(1)–C(1)
Ag(1)–Ag(2)
Ag(2)–I(2)
Ag(2)–I(1#1)
Ag(2)–I(1)
Ag(2)–Ag(2#2)
Ag(2#2)–I(1)
C(1)–N(2)
C(1)–N(5)
C(4)–N(5)
C(3)–C(4)
C(3)–N(2)
C(21)–Ag(1)–C(1)
C(21)–Ag(1)–Ag(2)
C(1)–Ag(1)–Ag(2)
I(2)–Ag(2)–I(1#1)
I(2)–Ag(2)–I(1)
I(1#1)–Ag(2)–I(1)
I(2)–Ag(2)–Ag(2#2)
I(1#1)–Ag(2)–Ag(2#2)
I(1)–Ag(2)–Ag(2#2)
I(2)–Ag(2)–Ag(1)
2.077(7)
2.081(7)
I(1)–Ag(4#2)
I(1)–Ag(4)
I(2)–Ag(4)
I(2#1)–Ag(3)
I(2)–Ag(3)
Ag(3)–C(1)
Ag(4)–I(1#2)
Ag(4)–Ag(3)
Ag(4)–Ag(4#2)
C(1)–N(1)
C(1)–N(2)
C(2)–N(2)
2.882(2)
2.919(2)
2.816(3)
2.855(2)
2.868(2)
2.167(16)
2.882(2)
3.132(3)
3.117(3)
1.31(2)
1.32(2)
1.32(2)
1.37(3)
1.37(2)
64.99(6)
88.47(6)
124.6(5)
100.96(6)
121.05(9)
114.52(10)
58.08(6)
56.93(6)
Ag(1)–C(1)
Ag(1)–C(17)
C(1)–N(5)
C(1)–N(2)
N(2)–C(3)
C(3)–C(4)
C(4)–N(5)
N(5)–C(6)
C(1)–Ag(1)–C(17)
N(5)–C(1)–N(2)
N(5)–C(1)–Ag(1)
N(2)–C(1)–Ag(1)
C(1)–N(2)–C(3)
C(1)–N(2)–C(7)
C(3)–N(2)–C(7)
C(4)–C(3)–N(2)
C(3)–C(4)–N(5)
C(1)–N(5)–C(4)
C(1)–N(5)–C(6)
C(4)–N(5)–C(6)
2.062(7)
2.088(6)
1.348(8)
1.374(8)
1.378(9)
1.300(11)
1.393(9)
1.454(9)
3.2873(10)
2.6902(9)
2.7871(9)
2.8390(9)
3.2421(14)
2.7871(9)
1.354(8)
1.348(9)
1.373(9)
1.327(11)
1.360(9)
170.9(3)
91.48(18)
90.99(19)
123.97(3)
124.56(3)
109.63(3)
167.71(4)
55.57(2)
177.8(2)
103.4(5)
130.7(5)
125.7(5)
110.6(6)
125.5(5)
123.6(6)
107.7(7)
107.4(6)
110.9(6)
123.9(6)
125.2(6)
C(2)–C(3)
C(3)–N(1)
Ag(4#2)–I(1)–Ag(4)
Ag(4)–I(2)–Ag(3)
C(1)–Ag(3)–I(2)
I(2)–Ag(4)–I(1)
I(2)–Ag(4)–Ag(4#2)
I(1#3)–Ag(4)–Ag(4#2)
I(1#2)–Ag(4)–Ag(4#2)
I(1)–Ag(4)–Ag(4#2)
54.07(2)
100.49(3)
bonds that build an arrangement of infinite [Ag(NHC–NHBoc)2]+
chains and, consequently, weaken the Ag(1)–Ag(2) interactions.
The elongation of the Ag–Ag distance within the [Ag2I4]2ꢀ anion
seems to be another consequence of the network assembled by
the intermolecular interactions. More specifically, hydrogen bonds
involve the intermolecular NHꢁ ꢁ ꢁO@C donorꢁ ꢁ ꢁacceptor carbamic
couples, with values between 2.890(8) and 2.821(8) Å.
In order to obtain suitable crystals for X-ray diffractions
different crystallization attempts where necessary. During these
procedures, and already after the first crystallization from ace-
tone–hexane, we noticed the presence of a white solid material
only soluble in CDCl3 warmed at 65 °C. The NMR spectra run on
the heterogeneous sample at room temperature showed one single
set of resonances identical to those observed for the crude material
[4][AgI2] whereas the NMR of the same sample made homoge-
neous after heating at 65 °C, revealed the presence of a second,
identical but higher frequency shifted, set of signals attributed to
a second NHC–Ag species 6. The relative intensity of these new res-
onances increased with the number of re-crystallization steps:
after two steps the presence of 6, estimated by NMR of ca. 8%
raised up to ca. 50% after four times.
tetrahedral coordination to Ag(3) if kept into account. The other
Ag atom, Ag(4), indeed possesses this last type of coordination,
being completely internal to the cluster structure and bonded to
four I atoms through contacts whose lengths vary from 2.816(3)
to 2.919(2) Å. There is also an additional, fairly long Ag(4)–Ag(4)
contact [3.117(3) Å]. The wide range observed for the Ag–I bond
distances [2.816(3)–2.919(2) Å] is comparable to what found in
the literature for analogous compounds [22]. The relatively long
Ag–Ag distances observed [3.117(3) and 3.132(3) Å] suggest that
they are quite weak and the ribbon stair frame is mainly stabilized
by the multiple bridging iodides. These distances are less than the
sum of the van der Waals radii of 3.44 Å for silver [24], but are in
the range observed for ligand-unsupported AgI–AgI distances
(2.80–3.30 Å) [25]. As far as the iodine atoms are concerned, they
all are tri-coordinated to Ag atoms and the main difference stands
in their position: I(1) is internal to the Ag4I4 cluster unit and has
also got an additional forth long contact with Ag(3), I(2) is periph-
eral. The carbene molecules are positioned onto both sides of the
ribbon stair structure, bonded to every peripheral Ag atoms. The
rings protrude outward along the polymeric AgI chain and the X-
ray crystal packing pattern (Fig. 2b) showed that although they
are not stacked, they lay on parallel planes distant 3.362 Å from
each other. The 5-membered rings are planar and all the bond
lengths in the ring are, within the experimental errors, very close
to those observed for other similar complexes [9f]. The Ag–C bond
distance of 2.167(16) Å is somewhat longer than those of known
silver carbene complexes (2.052–2.090 Å) [26], but identical to that
observed for a similar staircase-type structure [2.161(9) Å] [22].
With regard to the carbamic moiety, the X-ray analysis showed a
positional disorder hence in the solid state it can be found in two
different regions of the unit cell, one occupied for about 69%, the
other for the remaining 31%. This is reasonable if we consider that
the carbamic moiety is actually the most external fraction of the
structure, nevertheless the disorder is partially controlled by inter-
molecular N–Hꢁ ꢁ ꢁO hydrogen bonds [2.480 Å in the prevailing iso-
mer, 2.794 Å in the minor one].
During one of these re-crystallization steps we obtained crystals
whose morphology appeared different from 5 and whose structure,
shown in Figs. 2a and 2b (selected bonds and angles are indicated
in Table 1), represents a rare example of multinuclear, polymeric
silver complex with a staircase-type structure composed of Ag4I4
cluster fragments that aggregate to give an infinite ribbon stair
structure, to which the carbene units are coordinate on both sides
via C–Ag bonds [22,23].
The perspective view shown in Fig. 2b highlights the complex
step structure of the AgI polymer, which propagates along the b
axis.
The unit cell contains an independent carbene molecule, two
independent silver atoms [Ag(3) and Ag(4)] and two independent
iodine atoms [I(1) and I(2)]. The silver atom Ag(3) shows a quasi-
planar trigonal coordination, if we exclude the long Ag(3)–Ag(4)
interaction [3.132(3)], being directly bonded to the carbenic C1
atom [2.167(16) Å] and to two I(2) atoms with bond lengths equal
to 2.855(2) and 2.868(2) Å (Fig. 2b). There is also a forth long con-
tact of 3.306 Å between Ag(3) and I(1) that would give a pseudo
The reaction of [2]PF6 with Ag2O in a 2:1 molar ratio, under ba-
sic PTC (phase transfer catalyst)/OHꢀ conditions in the presence of
[NBu4]PF6 and NaOH [19b,27] formed the simple biscarbene salt
[4]PF6 (Eq. (3)) in 71% yield (NMR).

L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
2583
Fig. 2a. ORTEP diagram (30% probability thermal ellipsoids) of 6.
and their torsion angle is about 39°. Similarly to what found for
5 and 6, the 5-membered rings are planar and the bond lengths
comparable to those observed for other similar complexes [9e].
The Ag–C bond distances 2.062(7) and 2.088(6) Å are shorter than
those found in 6 but comparable with 5 and totally in agreement
with what reported for other analogous cationic biscarbene com-
plexes [20a,21]. The C(1)–Ag(1)–C(17) angle is close to linearity
with an angle of 177.8(2)°.
N
N
N
N
PTC/OH-
NHBoc
NHBoc
Ag
PF6
2[2]PF6 + Ag2O
CH2Cl2/H2O, r.t., 48 h
ð3Þ
We finally studied the reactivity of [NH2CH2CH2ImMe]I, [3]I to-
ward Ag2O under a variety of conditions. Our investigations started
carrying the reaction between [3]I and Ag2O in a 2:1 ratio in two
parallel experiments: (i) in a flask containing anhydrous DMSO-
d6 and 4 Å molecular sieves with the filtered, colourless reaction
mixture successively transferred into a NMR tube and (ii) by direct
addition of all the reagents in a NMR tube. In both cases the anal-
ysis of the NMR spectra indicated that [3]I rapidly reacts with Ag2O
to give a single silver-containing product: in the 1H NMR the peak
at d 9.10 disappears and the resonances of the imidazole CHs back-
bone are shifted upfield from 7.74 and 7.71 to 7.46 and 7.42,
respectively, while nothing can be said on the fate of the NH2
group, whose resonance is never observed even in the starting
proligand. In the 13C NMR spectra the original resonance at d
136.9 for the NCHN carbon is replaced by a singlet at d 179.8, anal-
ogous to the Ag–Ccarbene found for the silver complexes [4][AgI2].
However continuous monitoring by NMR showed that this product
is rather unstable and rapidly decomposes: the solution turned yel-
low and a silver mirror together with a brown powdery solid
coated the NMR tube whilst the proton and carbon NMR spectra
showed that the peaks assigned to the silver adduct weaken in
intensity while the resonances of the starting imidazolium salt
reappear. When the same reaction was carried in bulk, the black
suspension stirred for 2 h was filtered on a celite pad and after
addition of acetonitrile a light brown precipitate was obtained.
[4]PF6, 71%
Complex [4]PF6 was completely characterized (see Section 4). The
1H and 13C NMR spectra are not significantly different from those
of [4][AgI2] but unlike this latter the carbene carbon signal at d
180.3 appears as a broad doublet with C–107/109Ag coupling con-
stant of 185.4 Hz [19b,27].
Suitable crystals for X-ray diffraction of [4]PF6 have been ob-
tained by slow evaporation from a CDCl3 solution at room temper-
ature. The crystal packing and structure of [4]PF6 ꢁ 2CDCl3 are,
respectively, shown in Figs. 3 and 4, respectively (selected bonds
and angles are indicated in Table 1), and the latter consists of
two independent silver biscarbene molecules in a 1:1 enantiomeric
mixture. Within the same molecule, the two carbenes coordinated
to the same silver atom are absolutely identical and carry the car-
bamic chain on the same side of the 5-membered ring with respect
to the silver position. Furthermore, the cis position of the substitu-
ents allows the enantiomers to face one another in pairs, with the
carbene units not staggered but laying on parallel planes distant
about 3.219 Å from each other. The space between the two groups
of isomers is filled by the anion and solvent molecules (vide infra).
The sterical hindrance caused by the carbamic chains, which are
not linear but bent inwards within the same space region, causes
the two 5-membered rings to adopt a non-coplanar geometry

2584
L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
Fig. 2b. ORTEP diagram (30% probability thermal ellipsoids) of 6. For sake of clarity hydrogen atoms have been omitted.
Fig. 3. Crystal packing of [4]PF6 (30% probability thermal ellipsoids). For sake of
Fig. 4. ORTEP diagram (30% probability thermal ellipsoids) of [4]PF6. For sake of
clarity hydrogen atoms have been omitted.
clarity hydrogen atoms have been omitted.

L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
2585
The ESI-MS(+) spectrum of the freshly filtered solution showed
peaks for the [(M+Na)]+ cation at 382 (100) and 384 (90) m/z and
peaks at 366 (42) 368 (38) m/z that we tentatively attributed to
the presence in solution of the neutral species (NHC–NH2)AgI (7,
NHC–NH2 = 1-(2-aminoethyl)-3-methyl-imidazolin-2-ylidene) that
in the ESI-MS conditions easily lose the NH2 fragment.
The values of V+H measured for [4][AgI2] at low concentration (en-
try 9) are not consistent with its formulation. It can be noted that VH+
is practically identical to that of [2]I (entry 6) and about half of that
of [4]PF6 (entry 14). These observations suggest that the cationic sil-
ver iodide [4][AgI2] undergoes a NHC/I exchange between two silver
centres (Eq. (4)) leading to the formation of neutral mono-NHC silver
iodide in diluted solution via a mechanism similar to that suggested
by Wang and Lin [19b].
2.3. PGSE and NOE NMR studies
The imidazolium salts [2]X (X = I, PF6) and the silver complexes
[4][AgI2] and [4]PF6 were investigated through NOE and PGSE NMR
experiments.
N
N
N
N
N
N
NHBoc
NHBoc
NHBoc
1H and 19F-PGSE NMR experiments were carried out for [2]X,
[4][AgI2] and [4]PF6 in CD2Cl2, using tetrakis-(trimethylsilyl) silane
(TMSS) as internal standard. PGSE measurements allowed the
Ag
[AgI2]
Ag
I
2
translational self-diffusion coefficients (Dt) for both cationic (Dt+
)
and anionic (Dꢀt ) moieties to be determined (Table 2). From the mea-
sured self-diffusion coefficients (Dt), the average hydrodynamic ra-
dius (rH) of the diffusing particles was derived taking advantage of
the Stokes–Einstein equation, Dt = kT/cpgrH, where k is the Boltzman
constant, T is the temperature, c is a numerical factor and g is the
solution viscosity. Dt data were treated taking all the methodological
precautions described in our recent paper [28]. In this regard
although the ionic impurity [NBu4]+ present in [4]PF6 do not have
any influence in the determination of the counter anion position
(vide infra), it certainly does with respect to the self aggregation ten-
dency hence, the hydrodynamic volumes VꢀH are only reported for the
entries 1–5 but not for entries 14–16 as the latter data would repre-
sent average VꢀH values resulting from an ion-pair association among
the PFꢀ6 anion with the two [NBu4]+ and [4]+ cations.
From the average hydrodynamic radii of the aggregates, as-
sumed to be spherical, their volumes V+H and VHꢀ were obtained.
V+H of [2]PF6 spans from 421 Å3 (entry 1) to 1394 Å3 (entry 5) while
VꢀH is included between 276 and 1242 Å3 (entries 1–5). In [2]I, VH+
stays in the range of 460–2112 Å3 (entries 6–8). For the silver com-
plex [4][AgI2], V+H spans from 457 Å3 (entry 9) to 776 Å3 (entry 13).
On the other hand the fact that no sensible variation of V+H is ob-
served for complex [4]PF6 on varying the concentration (values of
847–960 Å3, entries 14–16) suggests that, as already reported in
the literature for analogous studies [28,29] the presence of quadru-
polar aggregates is negligible, therefore further confirming that the
ionic impurities present do not have any influence with regards to
the diffusional data relative to the cation.
[4][AgI2]
ð4Þ
In order to have an idea of the level of aggregation, V+H and VHꢀ
can be compared with the volume of the ion pair (V0H) obtaining
the cationic (N+) and anionic (Nꢀ) aggregation numbers (Table 2)
[29]. We have recently shown that only for molecules not having in-
lets V0H is well-described by the van der Waals volume (VVdW) of the
ion pair, easily obtainable from X-ray or theoretical data [30]. In
other cases, it is preferable to use the hydrodynamic volume mea-
sured at very low concentration or extrapolated at infinite dilution
(V0H). This procedure, requiring time-consuming PGSE experiments,
can be avoided combining diffusion NMR and conductometric mea-
surements or even by a single diffusion measurement if the volume
of one ionic fragment is known or can be well-described by VVdW
[31]. Imposing VVdW = 62 Å3 for PF6ꢀ in [2]PF6, a volume of 392 Å3
was determined for [2]+ from the PGSE measurement at the lowest
concentration (Table 2, entry 1). Consequently, V0H of [2]PF6, derived
by adding the volumes of the single ions, was 454 Å3 (entry 1). VH0 of
[2]I, [4][AgI2] and [4]PF6 were properly obtained combining VVdW of
Iꢀ, Ag+, and PFꢀ6 with the volume of [2]+. It is important to outline
that for [2]I and [4]PF6 the observed VHs for the most diluted solu-
tions are consistent with the V0s calculated according to the proce-
dure above-described (entries 6 and 14).
The aggregation tendency of compounds can be evaluated look-
ing at the trends of N+ and Nꢀ (Table 2) or V+H with the concentra-
Table 2
Diffusion coefficients (1010Dt, m2 sꢀ1), hydrodynamic radii (rH, Å), hydrodynamic volumes (VH, Å3) and aggregation number for cation (N+) and anion (Nꢀ) of compounds [2]X
(X = I, PF6), [4][AgI2] and [4]PF6 in CD2Cl2 as a function of concentration (C, mM)
Dt+
Dtꢀ
rH+
rHꢀ
VH+
VHꢀ
N+
Nꢀ
C
1
2
3
4
5
6
7
8
[2]PF6 (V0 = 454 Å3)
12.6
11.3
10.9
7.10
5.58
11.6
10.4
2.64
11.6
11.4
11.1
10.6
9.5
15.3
11.5
11.1
7.37
5.84
–
–
–
–
–
–
–
–
–
–
–
4.65
4.79
4.92
6.43
6.93
4.79
5.11
7.96
4.78
4.83
4.90
4.98
5.70
5.87
5.92
6.12
4.04
4.72
4.82
6.23
6.67
–
–
–
–
–
–
–
–
–
–
–
421
276
440
469
1012
0.9
1.0
1.1
2.4
3.1
1.1
1.3
0.6
1.0
1.0
2.2
2.7
–
–
–
–
–
–
–
–
–
–
–
0.14
2.3
8.7
126
237
1.0
460
499
1113
1394
460
560
2112
457
471
492
517
776
847
869
960
1242
[2]I (V0 = 426 Å3)
–
–
–
–
–
–
–
–
–
–
–
14
5.0
458
0.01
0.08
0.72
2.3
b
9
[4][AgI2] (V0 = 441 Å3)
b
b
b
b
10
11
12
13
14
15
16
10.5a
2.3
22.4
120.5
[4]PF6 (V0 = 853 Å3)c
9.29
8.68
7.41
1.0
1.0
1.1
a
Saturated solution.
b
c
Since compound [4][AgI2] undergoes the equilibrium described in Eq. (4) (vide infra), N cannot be properly defined.
Contamined with [NBu4]PF6.

2586
L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
tion: as expected, N+/ꢀ and VH+ increase with the concentration. Ion
pairing is the main aggregative process below 14 mM (N+ little devi-
ates from 1) for [2]PF6 and [2]I salts whereas at higher concentra-
tion, aggregation of ion pairs occurs: at ca. 100 mM, [2]PF6 is
prevalently present as ion quadruples (entry 4), at ca. 250 mM, it
forms hexapoles (entry 5), finally [2]I at still higher concentration
(ca. 450 mM) appears to form decapoles (entry 9). On the other hand,
the silver complex [4]PF6 does not exhibit any tendency to form
aggregates higher than ion pairs also at the highest concentration
investigated (entry 16) [32].
The anion–cation relative orientation in solution for [2]PF6 and
[4]PF6 was determined by detecting dipolar interionic interactions
in the 19F,1H-HOESY NMR spectra (Figs. 5 and 6). NOE data were
treated taking into account that the volumes of the cross peaks
are proportional to (nInS/nI + nS) where nI and nS are the number
of equivalent I and S nuclei, respectively (Table 3) [33]. In the
19F,1H-HOESY NMR spectrum of [2]PF6, strong contacts were ob-
served between F-atoms of the counterion and H(1) and H(7) pro-
tons. Medium contacts were detected with H(2), H(5) and H(6)
resonances while the anion did not show any interaction with pro-
tons H(3) and H(4) of the imidazolium ring. This pattern of NOE
contacts indicates that PFꢀ6 is mainly located close to the H(1) of
the imidazolium ring. From this position the anion has the possibil-
ity to interact with H(2) methyl protons and H(5) and H(6) methy-
lene protons. The presence of a strong interaction between the
anion and the acidic NH proton (Table 3) indicates the establishment
of a hydrogen bond between a fluorine of the anion and the proton
H(7) (Fig. 5). An analogous relative anion–cation orientation was de-
duced for 2-pyridylmethyl imidazolium salts from computational
and NOE experiments [14].
Fig. 6. A section of 19F,1H-HOESY NMR spectrum (376.65 MHz, 296 K, CD2Cl2) of
[4]PF6. On the right the column projection is reported. ** Denote the residue of non-
deuterated solvent.
Table 3
Relative NOE intensities treated taking into account that the volumes of the NOE cross
peaks are proportional to (nInS/nI + nS) and determined by arbitrarily fixing at 1 the
intensity of the NOE(s) between the anion resonances and H(2) resonances
[2]PF6
[4]PF6
H(1)
H(2)
H(3)
H(4)
H(5)
H(6)
H(7)
H(8)
3.56
1
–
–
1
1.22
2.84
1.53
1.31
4.20
0.31
–
1.22
1.04
1.94
0.16
Differently from what observed for the related imidazolium salt,
the 19F,1H-HOESY NMR spectrum of [4]PF6 indicates that the coun-
terion interacts with all protons of the cationic moieties (Fig. 6 and
Table 3). In particular, the anion strongly interacts with NH and
H(4) imidazolium ring protons. Medium intensity contacts are
detected with H(3), H(5) and H(6) resonances while the anion
shows weak interactions with H(8) protons of the Boc group. The
quantitative NOE analysis does not allow a single anion–cation
orientation explaining all the data found. In the solid state, four
anion–cation orientations are present (Fig. 7); in two of them the
Fig. 5. 19F,1H-HOESY NMR spectrum (376.65 MHz, 296 K, CD2Cl2) of [2]PF6. On the
right the column projection is reported. * Denotes the H2O resonance.** Denotes the
residue of non-deuterated solvent.

L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
2587
being appropriately dried and degassed were stored in ampoules
under argon on 4 Å molecular sieves. The prepared derivatives
were characterized by elemental analysis and spectroscopic meth-
ods. The IR spectra were recorded with a FT-IR spectrometer Per-
kin–Elmer Spectrum 2000. The IR spectra on the crystals were
recorded with a Nexus 470 instrument configured with a Nicolet
Continuum microscope and a MCT liquid nitrogen cooled detector;
the spectra were collected in the Near Normal-Reflection-Absorp-
tion mode. The NMR spectra were recorded using Varian Gemini
XL 300 (1H, 300.1; 13C, 75.5 MHz), Varian MercuryPlus VX 400
(1H, 399.9; 13C, 100.6 MHz), Varian Inova 600 (1H, 599.7, 13C,
150.8 MHz) instruments. The spectra were referenced internally
to residual solvent resonances, and were recorded at 298 K for
characterization purposes; full 1H and 13C NMR assignments were
done, when necessary, by gCOSY, gHSQC and HMBC NMR experi-
ments using standard Varian pulse sequences. ESI-MS analysis
were performed by direct injection of methanol solutions of the
metal complexes using a WATERS ZQ 4000 mass spectrometer. Ele-
mental analyses were performed on a ThermoQuest Flash 1112
Series EA Instrument. The reagents 1-methylimidazole, 2-bromo-
ethylamine hydrobromide, imidazole (ImH), Ag2O were used as
purchased from Aldrich; MeI was distilled under argon and stored
on molecular sieves; NaH (60% dispersion in mineral oil, Aldrich)
was washed several time with petroleum ether and the resulting
free-flow white powder was stored under Argon; 2-t-Boc-amin-
oethylbromide [carbamic acid 2-bromoethyl-t-butyl ester] was
prepared according to literature procedures [34]; sodium imidazo-
lide (NaIm) was prepared in large quantities by reacting imidazole
with NaH in THF. Petroleum ether (Etp) refers to a fraction of b.p.
60–80 °C. Column chromatography were carried out on silica gel
previously heated at about 200 °C while a slow stream of a dry
nitrogen was passed through it [35]. Melting points were taken
with a Stuart Scientific Melting point apparatus SMP3 and were
uncorrected.
Fig. 7. A view of four anion position (AHB, A, B and BHB) present in the solid state for
compound [4]PF6. The Boc groups is omitted for clarity.
anion stays close to the Ag atom (A and AHB, Ag–P distances rang-
ing around 5.6 Å) whereas in the other two, the anion locates close
to imidazolium ring (B and BHB, Ag–P distances ranging around
7.8 Å). In AHB and BHB positions an hydrogen bond between the
fluorine of the counterion and the NH groups is present (N–F dis-
tances ranging around 3.2 Å). The observation of strong PFꢀ6 /NH
and PFꢀ6 /H(4) NOEs in the 19F,1H-HOESY NMR spectrum (Fig. 6)
suggests that AHB and BHB orientations are prevalently present in
solution (Fig. 7).
4.2. Reaction between 1-methylimidazole and 2-bromoethylamine
hydrobromide
According to literature procedure [16] a mixture of 1-methylim-
idazole (2.00 g, 0.024 mol) and 2-bromoethylamine hydrobromide
(5.00 g, 0.024 mol) in 15 mL of acetonitrile was heated with stir-
ring at 80 °C for 4 h. After removal of the solvent under vacuum
the residue was recrystallized from ethanol to afford a white solid.
The NMR and ESI-MS analyses carried out on (i) the crude material
before recrystallization from ethanol, (ii) the ethanol mother li-
quors and (iii) the crystallized white solid, revealed the presence
of 3-methylimidazolium bromide A (soluble in ethanol, 72.5%
NMR), the target molecule B (20.5%) together with C (6.5%) and
3. Conclusions
We have developed a simple and selective method for the prep-
aration of the Boc-protected [BocNHCH2CH2ImMe]X [2]X, (X = I,
PF6) and its straightforward transformation into the unprotected
1-(2-aminoethyl)-3-methylimidazolium salts [NH2CH2CH2Im-
Me]X, [3]X. The reaction of [2]X with Ag2O led to the silver com-
plexes 4–6 whose structures in the solid state and in solution
confirmed the fascinating structural diversity of NHC–silver com-
plexes and the importance of the role played by the counterion.
NMR studies indicate that also in solution the latter has a key role:
with a weakly coordinating anion (X = PF6) the biscarbene salt
[(NHC–NHBoc)2Ag]PF6, [4]PF6, forms whereas when X = I an equi-
librium between a similar complex having AgIꢀ2 as counterion and
a mono-carbene silver iodide (NHC–NHBoc)AgI establishes.
Studies on the chemistry of the reported silver complexes as
carbene transfer reagents are currently in progress and will be re-
ported at a later date.
D
(0.5%) (see Scheme 1). Characterization of A: 1H NMR
(399.9 MHz, D2O): d = 8.68 (s, 1H, NCHN), 7.50 (s, 2H, CHimid),
3.95 (s, 3H, NCH3), NH not observed; ESI-MS (MeOH, m/z): 83
(100) [M]+; 79 (100), 81 (97) [M]ꢀ. Characterization of B, C, D: 1H
NMR (399.9 MHz, D2O): d = 8.94, 8.89, 8.84 (NCHN), 7.66, 7.57,
7.48 (CHsimid), 3.96, 3.94 (NCH3), 4.70–4.30 (m,CH2), 3.80–2.80
(m, CH2); ESI-MS (MeOH, m/z): 126 (100) [CH3ImCH2CH2NH2]+,
169
(88)
[CH3ImCH2CH2NHCH2CH2NH2]+,
212
(20)
[CH3ImCH2CH2NHCH2CH2NHCH2CH2NH2]+; 79 (100), 81 (97) [M]ꢀ.
4. Experimental
4.3. Synthesis of BocNHCH2CH2Im (1)
4.1. Materials and procedures
This product was synthesized using a modified literature meth-
od [17]. In a 250 mL flask 2.53 g (0.028 mol) of NaIm was reacted
with 6.25 g (0.028 mol) of N-Boc-bromoethylamine in THF (ca.
50 mL). The reaction suspension was stirred overnight at room
temperature and the precipitated NaBr removed by filtration. Puri-
fication of the crude material by column chromatography on silica
All reactions were carried out under Argon using standard
Schlenk techniques. Solvents were dried and distilled under nitro-
gen prior to use, dimethyl sulfoxide was dried over CaH2 and dis-
tilled under reduced pressure; the deuterated solvents used after

2588
L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
gel (eluting with CH2Cl2:MeOH:NH4OH 100:5:1–100:5:2) gave a
white solid identified as 1 (2.21 g, 37%). 1H NMR (399.9 MHz,
CDCl3): d 7.45 (s, 1H, NCHN), 7.06 (s, 1H, CHimid), 6.91 (s, 1H,
for C6H12IN3: C, 28.46; H, 4.74; N, 16.60. Found: C, 28.50; H,
4.77; N, 16.90%.
3
CHimid), 4.62 (br s, 1H, NH), 4.07 (t, 2H, JH,H = 6.0 Hz, NCH2), 3.42
4.7. Synthesis of 1-(2-aminoethyl)-3-methylimidazolium
hexafluorophosphate [3]PF6
(m, 2H, CH2NHBoc), 1.42 (s, 9H, CH3).13C-{1H}NMR (100.6 MHz,
CDCl3): d 155.8 (C@O), 137.1 (NCHN, Im), 129.2 (CHimid), 118.8
(CHimid), 79.6 (Cq, t-Bu), 46.4 (NCH2), 41.3 (CH2NHBoc), 28.1
(CH3). IR (THF, cmꢀ1) 1714 (vs, mCO). ESI-MS (MeOH, m/z): 234
(100) [M+Na]+.
Neat 1-(2-BocNH-ethyl-)-3-methylimidazolium hexafluoro-
phosphate[2]PF6 0.150 g (0.40 mmol) was heated under Argon at
210 °C for 3 h. The oil changed from colourless to orange to yield
0.108 g of [3]PF6 in quantitative yield. 1H NMR (399.9 MHz, D2O):
d 8.91 (s, 1H, NCHN), 7.63 (s, 1H, CHimid), 7.55 (s, 1H, CHimid),
4.62 (m, 2H, ImCH2), 3.95 (s, 3H, NCH3), 3.59 (m, 2H, CH2NH2).
ESI-MS (MeOH, m/z): 126 (100) [M]+, 145 (100) [M]ꢀ. Anal. Calc.
(%) for C6H12F6N3P: C, 26.57; H, 4.46; N, 15.50. Found: C, 26.95;
H, 4.77; N, 15.79%.
4.4. Synthesis of 1-(2-BocNH-ethyl-)-3-methylimidazolium iodide [2]I
To a solution of 1 (0.380 g, 1.80 mmol) dissolved in THF (ca.
10 mL) an excess of MeI (0.5 mL) was added. The reaction was
stirred overnight and the solvent removed under vacuum to yield
0,623 g (98%) of
a
yellow oil identified as [2]I. 1H NMR
4.8. 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-ylidene silver iodides
[4][AgI2], 5, 6
(399.9 MHz, CDCl3): d 9.70 (s, 1H, NCHN), 7.45 (s, 1H, CHimid),
3
7.39 (s, 1H, CHimid), 5.78 (t, 1H, JH,H = 5.8 Hz, NH), 4.55 (t, 2H,
3JH,H = 5.6 Hz, NCH2), 4.07 (s, 3H, NCH3), 3.68 (m, 2H, CH2NHBoc),
1.40 (s, 9H, CH3). 13C-{1H}NMR (100.6 MHz, CDCl3): d 156.2
(C@O), 137.0 (CH, NCHN), 123.1 (2CH, CHimid), 79.9 (Cq, t-Bu),
46.7 (NCH2), 40.2 (CH2NH), 37.2 (NCH3), 28.6 (CH3, t-Bu).
1H NMR (399.9 MHz, CD3CN): d 8.70 (s, 1H, NCHN), 7.42 (s, 1H,
CHimid), 7.37 (s, 1H, CHimid), 5.78 (br s, 1H, NH), 4.25 (t, 2H,
3JH,H = 5.6 Hz, NCH2), 3.85 (s, 3H, NCH3), 3.45 (m, 2H, CH2NHBoc),
1.36 (s, 9H, CH3). 1H NMR (399.9 MHz, D2O): d 7.52 (s, 1H, CHimid),
To a solution of [2]I 0.467 g (1.32 mmol) in CH2Cl2 (ca. 10 mL)
stirred in a Schlenck, Ag2O 0.158 g (0.68 mmol) was added. The
suspension was stirred for 2 h, filtered and the solvent removed
under vacuum to give 0.600 g of a white solid identified as
[4][AgI2]. 1H NMR of 4 (399.9 MHz, CDCl3): d 7.00 (s, 1H, CHimid),
3
6.91 (s, 1H, CHimid), 4.34 (t, 2H, JH,H = 6.0 Hz, NCH2), 3.89 (s, 3H,
NCH3), 3.56 (t, 2H, 3JH,H = 6.0 Hz, CH2NH), 1.40 (s, 9H, CH3), the res-
onance for the NH proton was not observed.13C-{1H}NMR
(100.6 MHz, CDCl3): d 184.9 (C–Ag), 157.0 (C@O), 121.7 (CHimid),
121.4 (CHimid), 79.8 (Cq, t-Bu), 50.9 (NCH2), 41.4 (CH2NH), 39.0
(NCH3), 28.4 (CH3). IR (THF, cmꢀ1) 1716 (vs, mCO); IR (refl./abs.,
cmꢀ1): 3339 (s, mNH), 3160, 3134, 3119, 3098 (m, @CH), 3001,
2976, 2930 (broad vs, –CH), 1701 (vs, mCO). ESI-MS (MeOH, m/z):
557 (100), 558 (27), 559 (97), 560 (25) [C22H38AgN6O4]+; 361
(100), 363 (90) [AgI2]ꢀ. M.p. = 189 °C dec. Colourless, suitable crys-
tals for X-ray diffraction corresponding to 5 have been obtained
from a double layer acetone/hexane at ꢀ20 °C.
3
7.48 (s, 1H, CHimid), 4.30 (t, 2H, JH,H = 6.0 Hz, NCH2), 3.92 (s, 3H,
NCH3), 3.54 (m, 2H, CH2NHBoc), 1.39 (s, 9H, CH3). IR (THF, cmꢀ1
)
1708 (vs, mCO). ESI-MS (MeOH, m/z): 226 (100) [M]+, 127 (100)
[M]ꢀ. Anal. Calc. for C11H20IN3O2: C, 37.40; H, 5.70; N, 11.90.
Found: C, 37.80; H, 5.90; N, 12.20%.
4.5. Synthesis of 1-(2-BocNH-ethyl-)-3-methylimidazolium
hexafluorophosphate [2]PF6
In a 100 mL two-neck flask 0.162 g (0.46 mmol) of [2]I were dis-
solved in 5 mL of CH2Cl2. Solid AgPF6 (0.127 g, 0.50 mmol) was
added and after stirring the reaction mixture for 4 h the AgI formed
was separated by filtration on a celite pad. The solvent was re-
moved under vacuum to give 0.160 g (94%) of [2]PF6 as a colourless
oil. 1H NMR (399.9 MHz, CDCl3): d 8.70 (s, 1H, NCHN), 7.26 (s, 1H,
CHimid), 7.20 (s, 1H, CHimid), 5.35 (br s, 1H, NH), 4.31 (t, 2H,
3JH,H = 5.6 Hz, NCH2), 3.93 (s, 3H, NCH3), 3.55 (m, 2H, CH2NHBoc),
1.39 (s, 9H, CH3). 13C-{1H}NMR (100.6 MHz, CDCl3): d 156.4
(C@O), 136.6 (CH, NCHN), 123.0 (2CH, CHimid), 79.7 (Cq, t-Bu),
49.8 (NCH2), 40.3 (CH2NH), 36.4 (NCH3), 28.2 (CH3). IR (THF,
cmꢀ1) 1714 (vs, mCO). ESI-MS (MeOH, m/z): 226 (100) [M]+, 145
(100) [M]ꢀ. Anal. Calc. for C11H20F6N3O2P: C, 35.58; H, 5.40; N,
11.32. Found: C, 35.60; H, 5.70; N, 11.55%.
During the re-crystallization phases we noticed the formation
of increasing amounts of an insoluble white product soluble in
hot CDCl3. The 1H NMR presented two sets of signals: one attrib-
uted to [4][AgI2] and one to a new complex 6. 1H NMR of 6
(399.9 MHz, CDCl3, 313 K): d 7.37 (s, 1H, CHimid), 7.30 (s, 1H,
3
CHimid), 4.56 (t, 2H, JH,H = 5.6 Hz, NCH2), 4.04 (s, 3H, NCH3), 3.66
3
(t, 2H, JH,H = 5.6 Hz, CH2NH), 1.38 (s, 9H, CH3), the resonance for
the NH proton was not observed. Attempts to run an overnight
13C-{1H}NMR spectra at 313 K led to decomposition of the silver
complex. Complex 6 (colourless crystals obtained from a double
layer acetone/hexane at ꢀ20 °C) was also characterized by X-ray
diffraction. The elemental analyses of [4][AgI2] on the bulk mate-
rial always gave erratic results probably due the presence of trace
amounts of 6.
4.6. Synthesis of 1-(2-aminoethyl)-3-methylimidazolium iodide [3]I
4.9. Synthesis of bis[1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-
ylidene]silver hexafluorophosphate [4]PF6
Neat 1-(2-BocNH-ethyl-)-3-methylimidazolium iodide [2]I
0.220 g (0.62 mmol) was heated under Argon at 185 °C for
30 min. The oil changed colour from pale yellow to dark orange
to yield 0.157 g (quantitative yield) of [3]I. 1H NMR (399.9 MHz,
D2O): d 7.58 (s, 1H, CHimid), 7.53 (s, 1H, CHimid), 4.47 (t, 2H,
A mixture of 0.132 g (0.36 mmol) of [2]PF6 in 5 mL of dichloro-
methane, 0.042 g (0.18 mmol) of Ag2O, 100 mg (2.5 mmol) of
NaOH in 3 mL of H2O (1.8 M) and 23 mg (0.06 mmol) of [NBu4]PF6
as a phase transfer catalyst was stirred for 48 h. The organic layer
was separated from the aqueous layer and filtered to remove solid
by-products. Addition of hexane precipitated the crude product,
however despite repeated purifications by recrystallization from
dichloromethane/hexane the silver complex [4]PF6 resulted always
contamined by the phase transfer catalyst. After four crystalliza-
3
3JH,H = 6.0 Hz, NCH2), 3.95 (s, 3H, NCH3), 3.38 (t, 2H, JH,H = 6.0 Hz,
CH2NH2). 1H NMR (399.9 MHz, DMSO-d6): d 9.10 (s, 1H, NCHN),
3
4
7.74 (dd, 1H, JH,H = 2.0 Hz, JH,H = 1.6 Hz, CHimid), 7.71 (dd, 1H,
4
3
3JH,H = 2.0 Hz, JH,H = 1.6 Hz, CHimid), 4.24 (t, 2H, JH,H = 5.2 Hz,
3
ImCH2), 3.85 (s, 3H, NCH3), 3.07 (t, 2H, JH,H = 5.6 Hz, CH2NH2).
13C-{1H}NMR (100.6 MHz, DMSO-d6): d 136.9 (CH, NCHN), 123.5
(CHimid), 122.5 (CHimid), 49.6 (NCH2), 40.1 (CH2NH2), 35.8 (NCH3).
ESI-MS (MeOH, m/z): 126 (100) [M]+, 127 (100) [M]ꢀ. Anal. Calc.
tions 0.101 g of
a mixture composed of [4]PF6 (0.13 mmol,
Y = 71%, calculated from the NMR spectra) and [NBu4]ꢀ
(0.03 mmol) was obtained. 1H NMR (399.9 MHz, CDCl3): d 7.13 (s,

L. Busetto et al. / Journal of Organometallic Chemistry 693 (2008) 2579–2591
2589
1H, CHimid), 7.02 (s, 1H, CHimid), 5.42 (br s, 1H, NH), 4.23 (t, 2H,
3JH,H = 6.0 Hz, NCH2), 3.86 (s, 3H, NCH3), 3.50 (m, 2H, CH2NH),
1.35 (s, 9H, CH3). 13C-{1H}NMR (100.6 MHz, CDCl3): d 180.0 (br
[DMSO]. To a solution of [3]I (610 mg, 2.41 mmol) in DMSO
(4 mL) stirred in a Schlenck, Ag2O 308 mg (1.33 mmol) was added.
After stirring the reaction mixture for 2 h the black suspension was
filtered on a celite pad later washed with 2 ꢂ 2 mL of DMSO. The
addition of 10 mL of acetonitrile led to the formation of a light
brown precipitate that after being further washed with acetonitrile
(2 ꢂ 5 mL) was dried under vacuum to give 0.650 g of a light brown
solid containing an impure silver complex tentatively identified as
[1-(2-aminoethyl)-3-methyl-imidazolin-2-ylidene]silver(I) iodide
(7). The ESI-MS(+) (MeOH) analysis of the freshly filtered solution
showed peaks for the [(C6H12AgIN3+Na)]+ cation at 382 (100)
1
d, JC-107/109Ag = 185.4 Hz), 156.2 (C@O), 122.5 (CHimid), 122.0
(CHimid), 79.4 (Cq, t-Bu), 51.4 (NCH2), 41.5 (CH2NH), 38.6 (NCH3),
28.3 (CH3). IR (CH2Cl2, cmꢀ1) 1712 (vs, mCO); IR (refl./abs., cmꢀ1):
3304 (s, NH), 2966 (broad vs, CH), 1680 (vs, mCO). ESI-MS (MeOH,
m/z): 557 (100), 558 (27), 559 (97), 560 (25) [M]+; 145 (100) [M]ꢀ.
Colourless, suitable crystals for X-ray diffraction have been obtai-
ned by slow evaporation from a CDCl3 solution at room tempera-
ture. The crystals were removed from the vial with a small amount
of mother liquor and immediately coated with silicon grease.
and 384 (90) m/z and peaks at 366 (42) 368 (38) m/z for [(C6H10
-
AgIN2+Na)]+ (no peaks were observed in the ESI-MS(ꢀ) spectrum).
4.10. Reaction of [3]I with Ag2O
4.11. Single-crystal X-ray diffraction studies of 5, 6 and [4]PF6
[DMSO-d6]: (a) An NMR tube was charged with [3]I (35.0 mg,
0.14 mmol), Ag2O (18.0 mg, 0.077 mmol) and DMSO-d6 (0.6 mL).
The tube was sealed under argon and the resulting suspension
was monitored at room temperature by an NMR spectrometer.
The 1H NMR spectrum showed the existence of a silver complex
after 5 min. – 1H NMR (399.9 MHz, DMSO-d6): d 7.46 (s, 1H,
Crystal data and collection details for 5, 6 and [4]PF6 ꢁ 2CDCl3
are reported in Table 4. The diffraction experiments were carried
out on a Bruker APEX II diffractometer equipped with a CCD detec-
tor operating at 50 kV and 30 mA, using Mo Ka radiation. An
empirical correction was applied using SADABS [36]. The structure
was solved by direct methods and refined by full-matrix least
squares on F2 using SHELXL97 [37]. The non-hydrogen atoms were
subjected to anisotropic refinement, the disordered ones were split
in two positions using distance and anisotropic displacement
parameter restraints. Hydrogen atoms were added in calculated
positions, which were not refined but continuously updated with
respect to their carbon atoms, and were given a fixed isotropic
thermal parameters.
3
CHimid), 7.42 (s, 1H, CHimid), 4.09 (t, 2H, JH,H = 5.6 Hz, NCH2), 3.81
(s, 3H, NCH3), 2.90 (t, 2H, 3JH,H = 5.6 Hz, CH2NH2), NH2 not observed;
13C-{1H}NMR (100.6 MHz, DMSO-d6):
d 179.8 (C–Ag), 123.1
(CHimid), 122.1 (CHimid), 54.1 (NCH2), 43.1 (CH2NH2). (b) In a paral-
lel experiment in a round bottom flask [3]I (104.0 mg, 0.41 mmol)
was treated with Ag2O (53.0 mg, 0.23 mmol), DMSO-d6 (1 mL)
and activated 4 Å molecular sieves. After stirring for two hours in
the absence of light, part of the solution was transferred with a
filtering cannula in a NMR tube. After flame-sealing the tube, the
1H NMR spectrum showed the resonances described above together
with those of the starting [3]I. Moreover as time passed by, a silver
mirror together with a brown powdery solid coated the NMR tube
whilst the proton and carbon NMR spectra showed that the peaks
assigned to the silver adduct weakened in intensity.
4.12. NOE measurements
The 1H-NOESY [38] NMR experiments were acquired by the
standard three-pulse sequence or by the PFG version [39]. Two-
dimensional 19F.1H-HOESY NMR experiments were acquired using
Table 4
Crystal Data for 5, 6 and [4]PF6 ꢁ 2CDCl3
Compound
5
6
[4]PF6 ꢁ 2CDCl3
Formula
Formula weight
T (K)
C44H76Ag4I4N12O8
1840.25
296(2)
C22H38Ag4I4N6O4
1389.66
298(2)
C48H78D2Ag2Cl12F12N12O8P2
1884.32
298(2)
k (Å)
0.71073
0.71073
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
P1
Monoclinic
C2/c
46.28(2)
4.513(2)
18.828(10)
90
92.872(6)
90
3927(4)
Monoclinic
P2(1)/n
12.2213(14)
33.636(4)
19.440(2)
90
90.113(2)
90
7991.3(16)
4
ꢀ
9.3688(15)
10.5290(17)
16.916(3)
101.691(2)
91.257(2)
93.741(2)
1629.5(5)
1
a (°)
b (°)
c (°)
Cell volume (Å3)
Z
4
Dcalc (g cmꢀ3
)
1.875
3.130
888
2.350
5.142
2576
1.566
1.011
3808
l (mmꢀ1
F(000)
)
Crystal size (mm)
h Limits (°)
Index ranges
0.15 ꢂ 0.10 ꢂ 0.08
1.98–25.00
ꢀ11 6 h 6 11, ꢀ12 6 k 6 12,
ꢀ20 6 l 6 20
15604
0.12 ꢂ 0.12 ꢂ 0.08
2.17–25.00
ꢀ54 6 h 6 54, ꢀ5 6 k 6 5,
ꢀ22 6 l 6 22
17109
0.25 ꢂ 0.25 ꢂ 0.10
1.21–25.00
ꢀ14 6 h 6 14, ꢀ40 6 k 6 40,
ꢀ23 6 l 6 23
75615
Reflections collected
Independent reflections [Rint
]
5718 [0.0462]
99.6
5718/0/333
1.002
3471 [0.0973]
99.9
3474/216/264
1.037
14082 [0.0759]
99.9
14082/384/866
1.073
Completeness to h = 25.00° (%)
Data/restraints/parameters
Goodness-of-fit on F2
R1 (I > 2r(I))
0.0441
0.0810
0.0549
wR2 (all data)
0.0900
0.2284
0.1695
Largest difference in peak and hole
0.684 and ꢀ0.611
1.805 and ꢀ1.570
0.855 and ꢀ0.696
(e Åꢀ3
)

2590
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the standard four-pulse sequence or the modified version [40]. The
number of transients and the number of data points were chosen
according to the sample concentration and to the desired final dig-
ital resolution. Semi-quantitative spectra were acquired using a 2 s
relaxation delay and 800 ms mixing times.
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ꢀ
ꢁ
I
I0
d
2
ln ¼ ꢀðcdÞ Dt D ꢀ
G2
ð5Þ
3
where I = intensity of the observed spin echo, I0 = intensity of the
spin echo without gradients, Dt = diffusion coefficient, D = delay be-
tween the midpoints of the gradients, d = length of the gradient
pulse, and c = magnetogyric ratio.
The shape of the gradients was rectangular, their duration (d)
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the standard deviation of m by performing experiments with dif-
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standard deviation of approximately 3–4% in the hydrodynamic ra-
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Acknowledgements
The authors wish to thank the University of Bologna, the Minis-
tero della Industria, Università e Ricerca (MIUR), Programmi di Ric-
erca Scientifica di Notevole Interesse Nazionale, Cofinanziamento
MIUR 2003-04, for financial support.
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