Synthesis, structural diversity, and ligand-transfer potential of (carbene)copper(I) complexes

Article abstract of DOI:10.1002/hlca.200800406

Copper(I) complexes comprising different N-heterocyclic carbene ligands were prepared via in situ deprotonation and metallation. Depending on the wingtip groups on the carbene ligand (alkyl vs. aryl, chelating or monodentate), a variety of different struc

Full text of DOI:10.1002/hlca.200800406

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Helvetica Chimica Acta – Vol. 92 (2009)
Synthesis, Structural Diversity, and Ligand-Transfer Potential of
(Carbene)copper(I) Complexes
by Galmari Venkatachalama), Marion Heckenrotha), Antonia Neelsb), and Martin Albrecht*a)
a
´
) Department of Chemistry, University of Fribourg, Chemin du Musee 9, CH-1700 Fribourg
(phone: þ 41-263008786; fax: þ 41-263009738; e-mail: martin.albrecht@unifr.ch)
b
ˆ
) CSEM Centre Suisse dꢀElectronique et de Microtechnique, Rue Jaquet-Droz 1, CH-2002 Neuchatel
Copper(I) complexes comprising different N-heterocyclic carbene ligands were prepared via in situ
deprotonation and metallation. Depending on the wingtip groups on the carbene ligand (alkyl vs. aryl,
chelating or monodentate), a variety of different structural motifs were identified, such as a trigonal
planar geometry (alkyl wingtips) and an unprecedented see-saw-type structure (pyridinyl wingtip
groups). While aryl wingtip groups increase the stability of the complexes, alkyl substituents induce rapid
demetallation in the presence of moisture. The reactivity of these complexes was used to establish a
carbene-transfer protocol, which is illustrated by the formation of new cyclic thiourea compounds
(transfer to sulfur) and new (carbene)ruthenium(II) complexes (transfer to ruthenium). This suggests
that (carbene)copper(I) complexes could become valuable alternatives to (carbene)silver(I) complexes
for synthesizing (carbene)metal systems via transmetallation.
Introduction. – Copper(I) complexes containing N-heterocyclic carbene (NHC)
ligands have been demonstrated to be highly useful for catalysis [1]. While in some
cases, defined complexes have been employed as catalyst precursors [2], often, the
active species has been reported to be generated in situ by adding the 1H-imidazolium
salt and a base along with a copper(I) source [3]. This latter method provides some
potential pitfalls, e.g., for catalyst optimization, as the assumed bonding mode and
surmised coordination geometry may be incorrect1). In addition, it is widely known
that, in copper-catalyzed reactions in particular, subtle structural differences in the
catalytically active species may induce considerably different activities [5]. Hence,
insights into structural properties and selective reactivity patterns of [CuI(carbene)]
complexes may be very instructive for further catalyst development.
Various methodologies have been developed for the preparation of [CuI(carbene)]
complexes. Most frequently2), a route via free carbenes has been applied including
either isolation of the free carbene before copper addition or utilization of an in situ
procedure, avoiding manipulations with the potentially sensitive free carbene. The
metal precursor, CuX, strongly influences the Cu/carbene stoichiometry in the formed
complexes. Ionic homoleptic [Cu(carbene)2]X systems are generally obtained with
copper salts that comprise only weakly coordinating anions such as TfOꢀ, BF4ꢀ, or PF6ꢀ
1
)
)
For example, abnormal carbene bonding or formation of an ion-pair complex may occur [4].
Alternative routes have been used for the synthesis of [CuI(carbene)] complexes, including the N-
alkylation of 1H-imidazolylcopper complexes, the use of Cu2O for direct metallation, and
transmetallation from [AgI(carbene)] complexes. For key examples, see [6].
2
ꢁ 2009 Verlag Helvetica Chimica Acta AG, Zꢂrich

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[7]. Homoleptic complexes are also obtained when a transmetallation protocol from
[AgI(carbene)] complexes is used [6c][6d]. With coordinating anions (X ¼ halide,
AcOꢀ), monocarbene complexes of the type [CuX(carbene)] are usually formed [8].
While these considerations are valid in general, allowing for a rational synthesis of
simple [Cu(carbene)] complexes (alkyl or aryl wingtip groups at the carbene),
prediction of the product composition becomes increasingly difficult when the carbene-
ligand wingtips comprise functional groups. For example, coordinating phosphine
donors were found to stabilize a rare complex comprising a carbene bound to two
different CuI-centers in what is probably best described as a 3-center-2-electron bond
[9]. Meyer and co-workers produced, depending on the steric bulk at the carbene
wingtips, T-shaped and tetrahedral [CuI(carbene)] complexes from a tripodal tris-
carbene ligand comprising a central amine as additional donor site [10]. A similar
perturbation of the typically linear Cu coordination into distorted tetrahedral and T-
shaped geometries has been observed when hard alkoxide or softer pyridine donor
groups are attached to the carbene [6b][11].
In the framework of our research program directed towards the development of
new types of NHC complexes [4b][12], we report here on the coordination ability of a
small but diverse set of NHC ligands comprising variable degrees of steric bulk and
donor functionality at the wingtip groups. Metallation of these 1H-imidazolium salts
gave rise to a variety of known as well as unprecedented structural motifs in
[CuI(carbene)] chemistry. Some of these complexes proved to be useful carbene-
transfer agents. Hence, [CuI(carbenes)] may become valuable precursors as alternative
to the more frequently used silver route for carbene transfer, for example when
hygroscopic substrates are involved.
Results and Discussion. – Synthesis of the Complexes. Deprotonation of the 1H-
imidazolium precursors 1 – 4 with stoichiometric quantities of KOtBu and in situ
metallation afforded the [CuI(carbene)] complexes 5 – 8 in moderate to excellent yields
(Scheme 1)3). The carbene complexes 6 and 7 are stable in air for several weeks, while
gradual decomposition was observed for complexes 5 and 8 comprising only alkyl
wingtip groups. In the absence of an inert atmosphere, the decomposition was
accelerated in solution (t1/2 ca. 24 h) as compared to solid-state degradation (several
days). According to NMR spectroscopy, the corresponding 1H-imidazolium salt was
the major product from this decomposition, suggesting hydrolysis as the predominant
reaction pathway. Hence, moisture seems of bigger concern than O2 when storing such
[CuI(carbene)] complexes.
The complexes 5 – 8 were characterized in solution and in the solid state. In solution,
NHC metallation is indicated by the low-field resonance of the metal-bound carbene
ligand (d(C) 176 – 180), characteristic of Cu complexes with N-heterocyclic carbenes
[7][8]. Notably, the bulky Ar* wingtip groups in complexes 6 and 7 prevent rotation
about the CuꢀC(carbene) bond, as evidenced by the presence of two nonequivalent
isopropyl Me resonances in the 13C-NMR spectra of these complexes (Ar* ¼ 2,6-
iPr2C6H3). In complex 8, the methylene protons appear as an AB d located at d(H) 6.34
and 5.83 (2J ¼ 13.5 Hz), which is a direct consequence of the restricted flexibility of the
3
)
All our attempts to install the copper center by direct metallation with Cu2O failed, see [11].

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Scheme 1. Synthesis of the [CuI(Carbene)] Complexes 5 – 8
CuꢀC(carbene) bonds in the 12-membered dimetallacycle. Due to the rigid carbene
coordination, one methylene proton is oriented towards the metal center, while the
other one points outwards.

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Solid-State Structures. All complexes 5 – 8 were analyzed by single-crystal X-ray
diffraction. The molecular structures illustrate the high structural diversity in
[CuI(carbene)] chemistry.
The coordination geometry in complex 5 is distorted trigonal, a coordination mode
that is only rarely observed in [CuI(carbene)] complexes (Fig. 1) [11]. While, typically,
linear structures XꢀCuꢀcarbene were found, we assume that the trigonal coordination
in complex 5 is facilitated by the relatively small wingtip groups as compared to mesityl
(¼2,4,6-trimethylphenyl) and Ar* groups (see 6, Fig. 2). The Cu-center and the I-
ligand are located on a crystallographic axis of rotation. The sum of the angles around
the Cu-center is, therefore, exactly 360.08. Notably, the CuꢀI bond (Cu(1)ꢀI(1)
i
2.7623(7) ꢃ) is rather long, perhaps due to the steric bulk imposed by the Pr wingtip
groups of the carbene ligands. The two carbene heterocycles are twisted out of the
trigonal coordination plane by approximately 578, which results in a propeller-like
structure where one blade is represented by the I-ligand.
Fig. 1. Thermal-ellipsoid plot of complex 5 (50% probability, H-atoms omitted). Selected bond lengths
[ꢃ]: Cu(1)ꢀC(1) 1.930(3), Cu(1)ꢀI(1) 2.7623(7), and C(2)ꢀC(3) 1.350(5); selected bond angles [8]:
C(1)ꢀCu(1)ꢀC(1a) 141.80(17), C(1)ꢀCu(1)ꢀI(1) 109.10(8), and N(1)ꢀC(1)ꢀN(2) 103.7(2);
N(1)ꢀC(1)ꢀCu(1)ꢀI(1) 59.3(3) and N(2)ꢀC(1)ꢀCu(1)ꢀI(1) ꢀ 124.8(2).
Complex 6, comprising a sterically more demanding Ar* wingtip as opposed to the
iPr group in 5, displays the linear geometry as expected for homoleptic [Cu(carbene)2]
complexes that were synthesized from [Cu(NCMe)4]PF6 precursors (Fig. 2) [7]. The
Cu-center resides on a crystallographic inversion center. As a consequence, the two
carbene heterocycles are crystallographically related. They adopt a virtually coplanar
orientation, as illustrated by the torsion angle between the N(1)ꢀC(1) bond and the
C(1a)ꢀN(2a) bond of 1.2(5)8. The two Ar* wingtip groups appear to efficiently shield
the Cu-coordination sphere in 6, thus preventing coordination of iodide as in 5.
Substitution of the Me wingtip in 6 by a potentially chelating pyridinyl substituent
as in 7 has a marked influence on the coordination geometry (Fig. 3,a). While the
carbene ligands remain in a pseudo-trans arrangement (CꢀCuꢀC 168.52(19)8), the
short CuꢀNpy distances, 2.310(4) and 2.439(4) ꢃ, suggest that the pyridinyl N-atoms
weakly coordinate to the Cu-center. Pyridinyl bonding results in a distorted see-saw-

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Fig. 2. Thermal-ellipsoid plot of the cationic portion of complex 6 (50% probability, H-atoms omitted,
i
disorder in one of the Pr groups not shown). Selected bond lengths [ꢃ]: Cu(1)ꢀC(1) 1.877(4), and
C(2)ꢀC(3) 1.336(6); selected bond angles [8]: C(1)ꢀCu(1)ꢀC(1a) 180.0(4) and N(1)ꢀC(1)ꢀN(2)
103.3(3); N(1)ꢀC(1) ··· C(1a)ꢀN(2a) 1.2(5).
type structure (disphenoidal geometry). To the best of our knowledge, this structural
motif is unprecedented in (carbene)copper chemistry4). The distortion originates from
a strong deviation of the N-atoms from an ideal 908 angle with respect to the CꢀCuꢀC
axis. For example, the C(2)ꢀCu(1)ꢀN(1) angle is 77.67(17)8, while
C(22)ꢀCu(1)ꢀN(1) is significantly larger, 110.42(15)8. No H-atoms were found that
could be in a location suitable for completing a possible trigonal bipyramidal or even an
octahedral geometry, either via agostic or conventional H-bonding to the Cu-center.
Notably, the two heterocycles of each ligand are virtually coplanar (NpyꢀCpy
ꢀNNHCꢀCNHC 0.88 and 4.38 for the two ligands, resp.). Such an arrangement may
favor CuꢀCNHC p bonding [14]. However, the mutual torsion of the two carbene
heterocycles is 618 (cf. angle f in Fig. 3,b), while p-bond interactions of the carbene
with metal d orbitals would be largest for 08 or 908 torsion angles. Hence, only a small p
contribution is assumed for the CuꢀCNHC bonds in complex 7.
The structure of complex 8b reveals again a quasi linear Cu-coordination geometry
(CꢀCuꢀC 171.5(3)8; Fig. 4). Since the dicarbene ligand is too constrained for trans
chelation to a metal center, a dimeric structure is formed in which both carbene units of
a ligand bind to different Cu-centers. This macrocyclic structural motif is reminiscent of
that observed for related silver(I) complexes comprising this type of dicarbene ligand
[15]. In the crystal structure of 8b, the Cu-centers and the two dicarbene ligands are
symmetry-related. Presumably as a consequence of the bridging character of the
dicarbene ligand, the CuꢀC bond length is considerably stretched (average 2.09 ꢃ) and
4
)
A related strongly distorted bis(pyridinyl-carbene)-type structure was reported for a palladium
complex [13].

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Fig. 3. a) ORTEP Plot of the cationic portion of complex 7 (50% probability; noncoordinating PF6
anion, co-crystallized Et2O, and H-atoms omitted). Selected bond lengths [ꢃ]: Cu(1)ꢀC(2) 1.903(5),
Cu(1)ꢀC(22) 1.889(4), Cu(1)ꢀN(1) 2.310(4), Cu(1)ꢀN(4) 2.439(4), C(3)ꢀC(4) 1.337(7), and
C(23)ꢀC(24) 1.337(6); selected bond angles [8]: C(2)ꢀCu(1)ꢀC(22) 168.52(19), C(2)ꢀCu(1)ꢀN(1)
77.67(17), C(2)ꢀCu(1)ꢀN(4) 114.7(2), C(22)ꢀCu(1)ꢀN(1) 110.42(15), C(22)ꢀCu(1)ꢀN(4) 75.19(15),
and N(1)ꢀCu(1)ꢀN(4) 82.67(16); N(2)ꢀC(2) ··· C(22)ꢀN(6) 114.4(5). b) View of the molecular
structure of 7. The perspective along the C(2)ꢀCu(1)ꢀC(22) vector illustrates the coplanar arrangement
of the pyridinyl and the N-heterocyclic carbene heterocycles for each ligand residue (only Cipso of the Ar*
wingtip groups are shown).
constitutes the longest CuꢀCNHC bond observed thus far. Unlike in monodentate
complexes such as 6, in which the two carbene residues are mutually coplanar, the two
heterocyles in 8b are significantly twisted with a 44.98 torsion angle for the
N(1)ꢀC(1) ··· C(7a)ꢀN(3a) system. Since a 458 angle minimizes any p-bond overlap
[14], the long CuꢀCNHC distances (2.076(9) and 2.112(8) ꢃ, resp., cf. 1.88 – 1.93 ꢃ in
complexes 5 – 7) possibly reflect s-only bonds, which are preserved due to the
conformational rigidity of the metallamacrocycle. The separation of the two Cu-centers
(Cu 3.2268(7) ꢃ) does not suggest significant metal – metal interactions in 8.
Carbene-Transfer Ability. Carbene-ligand transfer from a metal to an acceptor site
has wide synthetic utility. We initially probed the ability to transfer the carbene ligand
from the Cu-center in complexes 5 – 8 to elemental sulfur. When starting from complex
5, substitution of the Cu-center by sulfur and formation of the cyclic thiourea species 9
occurred within 4 h at room temperature (Scheme 2). The carbene transfer was highly
selective and afforded compound 9 in essentially quantitative yields. Product
characterization was facilitated by comparison with an authentic sample prepared
from the free carbene and sulfur [16]. Notably, tetrasubstituted thioureas are known to
be useful precursors for free carbenes [17]. Products identical to 9 were observed when
a disulfide such as MeSSMe was used as acceptor in the presence of 5; however, the
reaction proceeded less selectively, and significant amounts of impurities were formed.

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Fig. 4. ORTEP Plot of the cationic portion of complex 8b (30% probability; noncoordinating anions, co-
crystallized solvent molecule, and H-atoms omitted). Selected bond lengths [ꢃ]: Cu(1)ꢀC(1) 2.076(9),
Cu(1)ꢀC(7a) 2.112(8), C(2)ꢀC(3) 1.332(12), and C(5)ꢀC(6) 1.321(13); selected bond angles [8]:
C(1)ꢀCu(1)ꢀC(7a) 171.5(3), N(1)ꢀC(1)ꢀN(2) 104.1(7), and N(3)ꢀC(7)ꢀN(4) 107.6(7);
N(1)ꢀC(1) ··· C(7a)ꢀN(3a) ꢀ 44.9(11) and C(1)ꢀCu(1) ··· Cu(1a)ꢀC(7) 33.5(4).
Scheme 2. Carbene Transfer to Sulfur
While the carbene transfer was efficiently accomplished with complex 5 comprising
alkylated NHC N-atoms, demetallation was slightly slower with the monoarylated
carbene 6 (giving 10, see Scheme 2), and it essentially failed with the chelate-stabilized
complex 7. Even upon prolonged heating in 1,2-dichloroethane, no substitution of the
Cu-center by sulfur was noted. Apparently, strong chelation increases the stability not
only with respect to hydrolysis but also for ligand-transfer reactions. Interestingly, the
dinuclear complex 8 did not undergo a clean carbene transfer, and mixtures of products
were obtained according to IR and NMR analyses. Mass spectrometry of a purified

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sample showed isotope patterns that are diagnostic for Cu-complexes. This result
suggests that the formed product still contains Cu-ions, perhaps due to the presence of
two, potentially chelating thiourea residues in the anticipated product.
A related transfer of the carbene ligand was also accomplished by using a metal
center as acceptor. Upon exposure of complex 5 to [RuCl2(p-cymene)]2 (p-cymene ¼
1-methyl-4-(1-methylethyl)benzene), the new [RuII(carbene)] complex 11 was formed
in excellent yields under mild reaction conditions (408, 4 h; Scheme 3). The spectral
properties of this complex are similar to those of closely related (carbene)ruthenium
complexes [18] and feature a carbene resonance at d(C) 172.
Scheme 3. Carbene Transfer to Ruthenium(II)
The successful use of [CuI(carbene)] complexes for transmetallation may become
particularly attractive in cases in which the frequently used procedure via [AgI(car-
benes)] [19] is impractical. A general applicability of this transmetallation protocol will
strongly depend on the accessibility of a wide range of [Cu(carbene)] precursors and on
the versatility of carbene transfer to a variety of (transition) metals.
Conclusions. – Structural analysis of a number of new copper(I) complexes with
NHC ligands revealed a rich coordination chemistry and provided structural motifs that
have been rare or even unprecedented thus far. Such a structural diversity due to
(weak) chelation of wingtip groups may play an important role in the activity and
selectivity of CuI-mediated catalytic transformations. Our results further indicated that
low steric shielding of the metal by wingtip substituents increases the reactivity of the
carbene – copper bond. Based on these findings, a transmetallation method was
established as an alternative to the more popular (carbene)silver(I) route. Selective
carbene-ligand transfer may find application in synthesis as well as perhaps in analytics.
This work has been financially supported by the Federal Commission (scholarship to G. V.), the
Swiss National Science Foundation, and the European Research Council through a starting grant. M. A.
gratefully acknowledges an Assistant Professorship of the Alfred Werner Foundation.
Experimental Part
General. All reactions involving copper(I) salts were carried out under Ar with Schlenk techniques.
THF and CH2Cl2 were dried by passage through solvent-purification columns. The 1H-imidazolium salts
1 – 4 [20] and [Cu(MeCN)4]PF6 were prepared according to literature procedures [21], all other
chemicals are commercially available and were used as received. IR Spectra: Mattson-5000-FT-IR
1
instrument; MeCN solns.; ˜n in cmꢀ1. H- and 13C{1H}-NMR Spectra: Bruker spectrometer at 360 and
1
90 MHz, resp., unless specified otherwise; chemical shifts d in ppm rel. to residual solvent H or 13C
resonances, coupling constants J in Hz; assignments based either on distortionless enhancement of
polarization transfer (DEPT) experiments or on homo- and heteronuclear shift-correlation spectroscopy.

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ESI-MS: Bruker-4.7-T-Bio-APEX-II instrument; in MeCN; in m/z. Elemental analysis were performed
by the Microanalytical Laboratory of the ETH Zꢂrich, Switzerland.
Bis[1,3-dihydro-1-methyl-3-(1-methylethyl)-2H-imidazol-2-ylidene]iodocopper (5). A mixture of
1H-imidazolium salt 1 (504 mg, 2.0 mmol), KOtBu (224 mg, 2.0 mmol), and CuI (194 mg, 1.0 mmol) in
dry THF (40 ml) was stirred at r.t. for 12 h. The suspension was filtered through a plug of Celite, and the
solvent was evaporated. The crude product was redissolved in THF (2 ml) and precipitated by addition of
Et2O (20 ml). This precipitation procedure was repeated twice: 5 (257 mg, 59%). Off-white solid.
Crystals suitable for X-ray crystal-structure determination were obtained by slow diffusion of pentane
into a THF soln. of 5. 1H-NMR (CD2Cl2, 360 MHz): 6.95 (s, 2 Him); 6.92 (s, 2 Him); 4.90 (sept., 3J ¼ 6.8,
3
2 Me2CH); 3.85 (s, 2 MeN); 1.48 (d, J ¼ 6.8, 2 Me2CH). 13C-NMR (CD2Cl2, 90 MHz): 182.3 (CꢀCu);
121.6 (Cim); 116.6 (Cim); 53.8 (Me2CH); 38.2 (MeN); 23.8 (Me2CH). ESI-MS: 311.2 ([M ꢀ I]þ). Anal.
calc. for C14H24CuIN4 (438.82): C 38.32, H 5.51, N 12.77; found: C 37.99, H 5.31, N 12.15.
Bis{1-[2,6-bis(1-methylethyl)phenyl]-1,3-dihydro-3-methyl-2H-imidazol-2-ylidene}copper(1þ)
Hexafluorophosphate(1ꢀ) (1:1) 6. To a soln. of 1H-imidazolium salt 2 (250 mg, 0.67 mmol) in THF
(40 ml) was added KOtBu (75 mg, 0.67 mmol) and [Cu(MeCN)4]PF6 (125 mg, 0.335 mmol). The mixture
was stirred at r.t. for 12 h and subsequently filtered through a plug of Celite. The solvent was evaporated
and the residue washed with Et2O (2 ꢁ 25 ml) and then precipitated from THF (2 ml) by addition of Et2O
(20 ml): 6 (210 mg, 91%). White solid. Crystals suitable for X-ray crystal-structure determination were
3
obtained by slow diffusion of Et2O into a CH2Cl2 soln. of 6. 1H-NMR (CDCl3): 7.41 (t, J ¼ 7.7, 2 arom.
H); 7.21 (s, 2 Him); 7.20 (d, 3J ¼ 7.7, 4 arom. H); 6.83 (s, 2 Him); 3.30 (s, 2 MeN); 2.27 (sept., 3J ¼ 6.8,
3
4 Me2CH); 1.03 (d, J ¼ 6.8, 4 Me2CH). 13C-NMR (CDCl3): 178.5 (CꢀCu); 146.1, 134.6, 130.3, 124.0 (4
arom. C); 123.2 (Cim); 122.7 (Cim); 37.3 (MeN); 28.1 (Me2CH); 24.5 (Me2CH); 24.2 (Me2CH). ESI-MS:
547.3 ([M ꢀ PF6]þ). Anal. calc. for C32H44CuF6N4P (693.24): C 55.44, H 6.40, N 8.08; found: C 55.17, H
6.35, N 7.91.
Bis{1-[2,6-bis(1-methylethyl)phenyl]-1,3-dihydro-3-(pyridin-2-yl-kN1)-2H-imidazol-2-ylidene}-
copper(1þ) Hexafluorophosphate(1ꢀ) (1:1) 7. To a soln. of 1H-imidazolium salt 3 (200 mg, 0.52 mmol)
in THF (40 ml), solid KOtBu (58 mg, 0.52 mmol) and [Cu(MeCN)4]PF6 (97 mg, 0.26 mmol) were added,
and the mixture was stirred at r.t. for 6 h. The mixture was then filtered through a plug of Celite, and the
solvent was evaporated. The crude product was dissolved in a minimum amount of CH2Cl2 and
precipitated by addition of Et2O: 7 (100 mg, 47%). Yellow solid. Crystals suitable for a single-crystal
structure determination were obtained by slow diffusion of Et2O into a CH2Cl2 soln. of 7. 1H-NMR
(CDCl3): 7.99 (d, 3J ¼ 1.8, 2 Him); 7.92 – 7.83 (m, 6 Hpy); 7.32 (t, 3J ¼ 7.7, 2 arom. H); 7.19 – 7.16 (m, 2 Hpy);
7.09 (d, 3J ¼ 7.7, 4 arom. H); 7.05 (d, 3J ¼ 1.8, 2 Him); 2.39 (sept., 3J ¼ 6.8, 4 Me2CH); 1.05 (d, 3J ¼ 6.8, 12 H,
Me2CH); 0.62 (d, 3J ¼ 6.8, 12 H, Me2CH). 13C-NMR (CDCl3): 183.2 (CꢀCu); 150.0 (CpyꢀN); 147.2 (Cpy);
145.2 (2 Co (Ar)); 140.2 (Cpy); 135.3 (Cipso (Ar)); 130.0 (C (Ar)); 125.2 (Cim); 123.7 (C(Ar)); 123.3 (Cpy);
117.3 (Cim); 112.9 (Cpy); 28.1 (Me2CH); 24.1 (Me2CH); 23.5 (Me2CH). ESI-MS: 673.3 ([M ꢀ PF6]þ).
Anal. calc. for C40H46CuF6N6P (818.27) · 0.25 CH2Cl2: C 57.51, H 5.58, N 10.00; found: C 57.82, H 6.03, N
9.96.
Bis{m-{1,1’-methylenebis[1,3-dihydro-3-(1-methylethyl)-2H-imidazol-2-ylidene]}}dicopper(1þ)
Hexafluorophosphate(1ꢀ) (1:2) (8a). A soln. of NH4PF6 (0.41 g, 2.5 mmol) in H2O (5 ml) was added to
bis[1H-imidazolium] salt 4 (0.20 g, 0.41 mmol) dissolved in H2O (5 ml). Immediately a precipitate
formed, which was separated by filtration, washed with H2O (2 ꢁ 2 ml) and Et2O (2 ꢁ 5 ml), and
extracted into MeCN (6 ml). All volatiles were evaporated, thus affording the corresponding bis[1H-
imidazolium] bis(hexafluorophosphate) as a white solid (0.22 g, quant.). This salt was suspended in dry
MeCN (5 ml), and [Cu(MeCN)4]PF6 (150 mg, 0.40 mmol) was added, followed by KOtBu (148 mg,
1.3 mmol). The mixture was stirred for 16 h and then filtered. Addition of Et2O (20 ml) to the filtrate
induced precipitation. The solid was collected, washed with Et2O, and dried in vacuo: 8a (201 mg, 56%).
White powder. 1H-NMR (CD3CN, 360 MHz): 7.43 (s, 4 Him); 7.19 (s, 4 Him); 6.34 (A (d) of AB, 2J ¼ 13.5,
2 H, NCH2N); 5.83 (B (d) of AB, 2J ¼ 13.5, 2 H, NCH2N); 4.33 (sept., 3J ¼ 6.7, 4 Me2CH); 1.33 (d, 3J ¼ 6.7,
12 H, Me2CH); 1.04 (d, 3J ¼ 6.7, 12 H, Me2CH). 13C-NMR (CD3CN, 100 MHz): 176.7 (CꢀCu); 122.5
(Cim); 120.0 (Cim); 64.6 (NCH2N); 55.1 (Me2CH); 23.8 (Me2CH).
Bis{m-{1,1’-methylenebis[1,3-dihydro-3-(1-methylethyl)-2H-imidazol-2-ylidene]}}dicopper(1 þ)
Hexafluorophosphate(1ꢀ) Tetrafluoroborate(1ꢀ) (1:1:1) (8b). As described for 8a, with 4 (0.96 g,

Helvetica Chimica Acta – Vol. 92 (2009)
1043
2.0 mmol), AgBF4 (0.78 g, 4.0 mmol) instead of NH4PF6, KOtBu (0.58 g, 5.2 mmol), and [Cu-
(MeCN)4]PF6 (0.74 g, 2.0 mmol): 8b (1.32 g, 85%). Gray solid. Spectroscopic data: identical to those
of 8a.
General Procedure for Carbene Transfer to Sulfur. To a soln. of (carbene)copper(I) in CHCl3
(20 mm) was added elemental sulfur (5 mol-quiv.), and the suspension was stirred at r.t. for 16 h. The
mixture was filtered through Celite and the filtrate was concentrated to give the cyclic thiourea product as
an off-white solid in essentially quant. yield.
1,3-Dihydro-1-methyl-3-(1-methylethyl)-2H-imidazol-2-thione (9): IR (MeCN): 1224 (n(C¼S)).
1H-NMR (CDCl3): 6.72 (s, 1 Him); 6.70 (s, 1 Him); 5.08 (sept., 3J ¼ 6.8, 1 Me2CH); 3.61 (s, MeN); 1.35 (d,
3J ¼ 6.8, Me2CH). 13C-NMR (CDCl3): 161.1 (C¼S); 117.9 (Cim); 112.6 (Cim); 48.8 (Me2CH); 34.7 (MeN);
21.8 (Me2CH). Anal. calc. for C7H12N2S (156.25): C 53.81, H 7.74, N 17.93; found: C 53.91, H 7.58, N 17.80.
1,3-Dihydro-1-[2,6-bis(1-methylethyl)phenyl]-3-methyl-2H-imidazol-2-thione (10): IR: 1226
1
3
3
(n(C¼S)). H-NMR (CD2Cl2, 500 MHz): 7.50 (t, J ¼ 7.8, 1 arom. H); 7.30 (d, J ¼ 7.8, 2 arom. H); 6.98
(d, 3J ¼ 2.3, 1 Him); 6.76 (d, 3J ¼ 2.3, 1 Him); 3.77 (s, MeN); 2.45 (sept., 3J ¼ 6.9, 2 Me2CH); 1.25 (d, 3J ¼ 6.9,
Me2CH); 1.08 (d, 3J ¼ 6.9, Me2CH). 13C-NMR (CD2Cl2, 125 MHz): 146.75, 133.32, 130.91, 124.91 (4 arom.
C); 120.03 (Cim); 119.74 (Cim); 36.58 (MeN); 28.88 (Me2CH); 24.62, 23.68 (2 Me2CH); C¼S not
observed. ESI-HR-MS (pos.): 297.13990 ([M þ Na]þ, C16H22N2NaSþ; calc. 297.13959).
Dichloro-[1,3-dihydro-1-methyl-3-(1-methylethyl)-2H-imidazol-2-ylidene][(1,2,3,4,5,6-h)-1-methyl-
4-(1-methylethyl)benzene]ruthenium (11). A soln. of [RuCl2(p-cymene)]2 (122 mg, 0.2 mmol) in CH2Cl2
(10 ml) was added to a soln. of 5 (88 mg, 0.2 mmol) in CH2Cl2 (5 ml) (!white precipitate). The mixture
was stirred at 408 for 4 h and subsequently filtered through Celite. All volatiles were evaporated, and the
residue was washed with Et2O (3 ꢁ 5 ml): 11 (84 mg, 96%). Dark orange solid. 1H-NMR (CDCl3): 7.06 (s,
1 Him); 7.00 (s, 1 Him); 5.4 (br. s, 2 arom. H); 5.30 (sept., 3J ¼ 7.0, Me2CHN); 5.1 (br. s, 2 arom. H); 3.99 (s,
MeN); 2.94 (sept., 3J ¼ 6.8, Me2CHC); 2.08 (s, Me); 1.44 (d, 3J ¼ 7.0, Me2CHN); 1.26 (d, 3J ¼ 6.8,
Me2CHC). 13C-NMR (CDCl3): 172.1 (CꢀRu); 124.2 (Cim); 118.6 (Cim); 107.9 (arom. C); 98.96 (arom. C);
52.3 (Me2CHN); 39.5 (MeN); 30.7 (Me2CHC); 24.9 (Me2CHN); 22.9 (Me2CHC); 18.6 (Me2CHN); 2
arom. CH not resolved. ESI-MS: 395.5 ([M ꢀ Cl]þ). Anal. calc. for C17H26Cl2N2Ru (430.05): C 47.44, H
6.09, N 6.51; found: C 47.45, H 6.01, N 6.46.
Crystal-Structure Determinations (Table). Suitable single crystals were mounted on a Stoe-Mark-II-
Imaging-Plate diffractometer system (Stoe & Cie, 2002) equipped with a graphite monochromator. Data
collection was performed at ꢀ 1008 by using MoKa radiation (l 0.71073 ꢃ) with a nominal crystal-to-
detector distance of 70 mm (for 6), 100 mm (for 7 and 8b), and 135 mm (for 5), resp. All structures were
solved by direct methods with the program SHELXS-97 and refined by full-matrix least squares on F2
with SHELXL-97 [22]. The H-atoms were included in calc. positions and treated as riding atoms with
SHELXL-97 default parameters. All non-H-atoms were refined anisotropically, except for partially
occupied solvent atoms, which were refined isotropically. For structure 5 and 8b, a semiempirical
absorption correction was applied with MULscanABS as implemented in PLATON03 [23].
Crystals of complex 7 contained half a molecule Et2O in the asymmetric unit, which showed
pronounced disorder. Therefore, the SQUEEZE option in PLATON03 was used to calculate the
potential solvent-accessible volume (190 ꢃ3, containing ca. 29 electrons). One Et2O molecule (42
electrons) per unit cell was hence included in all further calculations. A disordered region of electron
density was also found in crystals of 8b and was removed using the SQUEEZE routine (676 ꢃ3, ca. 197
electrons). One molecule of Et2O per complex molecule was therefore included. Due to this disorder
problem, the anions (PFꢀ6 and BFꢀ4 each half occupied) and parts of the ligand were refined with the same
thermal values. Further details on data collection and refinement parameters are collected in the Table.
CCDC 707733-707736 contain the supplementary crystallographic data for 5 – 8. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

1044
Helvetica Chimica Acta – Vol. 92 (2009)
Table. Crystallographic Data and Structure Refinement of Complexes 5 – 8
5
6
7
8b
Color, shape
Size [mm]
Empirical formula
colorless block
colorless rod
yellow block
colorless rod
0.45 ꢁ 0.35 ꢁ 0.25 0.45 ꢁ 0.40 ꢁ 0.35 0.50 ꢁ 0.35 ꢁ 0.30 0.35 ꢁ 0.30 ꢁ 0.30
C14H24CuIN4
C32H44CuF6N4P
C40H46CuF6N6P ·
0.5 C4H10O
856.40
C26H40BCu2F10N8P ·
C4H10O
897.64
Mr
438.81
693.22
T [K]
Crystal system
Space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
223(2)
223(2)
monoclinic
P21/c (No. 14)
9.5806(8)
13.2996(17)
14.3559(11)
90
105.623(9)
90
1761.6(3)
2
173(2)
173(2)
monoclinic
C2/c (No. 15)
13.8629(17)
10.5896(9)
13.0641(17)
90
108.029(10)
90
1823.7(4)
4
triclinic
monoclinic
C2/c (No. 15)
21.9106(14)
14.3407(9)
14.0843(10)
90
115.929(5)
90
3980.0(5)
4
¯
P1 (No. 2)
10.5582(7)
13.2927(10)
15.7373(12)
102.112(6)
96.793(6)
96.490(6)
2122.6(3)
2
a [8]
b [8]
g [8]
V [ꢃ3]
Z
Dcalc [g cmꢀ3
]
1.598
2.887
1.307
0.724
1.340
0.617
1.498
1.192
m [mmꢀ1] (MoKa)
Total reflections
10686
13730
27538
17488
Unique reflections 2453
3344
11389
3756
Rint
0.0679
0.0480
0.0469
0.0394
Transm. range
No. parameters
No. restraints
Ra), Rwb)
G.o.f.
0.317 – 0.646
95
0
0.0378, 0.0973
1.015
–
204
6
–
353
2
0.606 – 0.694
164
16
0.0926, 0.2792
1.274
0.0585, 0.1515
0.951
0.0856, 0.2340
1.119
Largest peak,
ꢀ 1.743, 1.113
ꢀ 0.804, 0.838
ꢀ 2.154, 1.572
ꢀ 2.262, 1.886
hole [e ꢃꢀ3
]
b
a) R1 ¼ S jj F0 jꢀj Fc jj /S j F0 j for all I > 2s(I). ) wR2 ¼ [Sw(F20 ꢀ Fc2 )2/S(w(F40 )]1/2
.
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Received November 11, 2008