New mixed-donor bidentate ligands based on N-heterocyclic carbene and thione donors

Article abstract of DOI:10.1021/om200629j

A new family of mixed-donor bidentate ligands containing both N-heterocyclic carbene (NHC) and thione functionalities is reported. The imidazolium salt precursors 1,1′-methylene(3-R-imidazol-2-ylidene)(3-R-2H- imidazole-2-thione) [CS^RH] halide (where R = methyl, benzyl; halide = bromide, iodide) have been synthesized. Their coordination to copper(I) salts has been explored, providing the complexes [Cu(CS^R)X]₂ (where R = methyl, benzyl; halide = bromide, iodide). Structural characterization of two of the complexes confirmed the dimeric nature of these complexes in the solid state but revealed different coordination modes for each case (κ¹-C, κ¹-S, μ-S and κ¹-C, κ¹-S). Solutions of the copper complexes slowly react with oxygen under aerobic conditions to form uncoordinated 1,1′-methylene(3-R-2H-imidazol-2-one)(3-R-2H-imidazole-2-thione), SO ^R (where R = methyl, benzyl), where the NHC moiety has been converted into a urea functional group. The complexes were investigated as candidates in copper-catalyzed cross-coupling reactions.

Full text of DOI:10.1021/om200629j

ARTICLE
pubs.acs.org/Organometallics
New Mixed-Donor Bidentate Ligands Based on N-Heterocyclic
Carbene and Thione Donors
Miriam Slivarichova, Ruaa Ahmad, Yu-Ying Kuo, Joshua Nunn, Mairi F. Haddow, Hafiizah Othman, and
Gareth R. Owen*,‡
The School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K.
S Supporting Information
b
ABSTRACT:
A new family of mixed-donor bidentate ligands containing both N-heterocyclic carbene (NHC) and thione functionalities is
reported. The imidazolium salt precursors 1,10-methylene(3-R-imidazol-2-ylidene)(3-R-2H-imidazole-2-thione) [CSRH] halide
(where R = methyl, benzyl; halide = bromide, iodide) have been synthesized. Their coordination to copper(I) salts has been
explored, providing the complexes [Cu(CSR)X]2 (where R = methyl, benzyl; halide = bromide, iodide). Structural characterization
of two of the complexes confirmed the dimeric nature of these complexes in the solid state but revealed different coordination modes
for each case (k1-C, k1-S, μ-S and k1-C, k1-S). Solutions of the copper complexes slowly react with oxygen under aerobic conditions
to form uncoordinated 1,10-methylene(3-R-2H-imidazol-2-one)(3-R-2H-imidazole-2-thione), SOR (where R = methyl, benzyl),
where the NHC moiety has been converted into a urea functional group. The complexes were investigated as candidates in copper-
catalyzed cross-coupling reactions.
Chart 1. Selected Sulfur-Functionalized N-Heterocyclic
Carbene Ligands
’ INTRODUCTION
N-Heterocyclic carbene (NHC) ligands have featured promi-
nently in the chemical literature since the isolation of the first
stable carbene by Arduengo in 1991.1 Since that time, these
strong and robust ligands have been comprehensively explored.2
It is without doubt that they are outstanding supporting ligands
for an extensive range of homogeneous catalytic applications.3,4
Some investigations have focused on carbene ligands containing
additional functional groups based on donors such as phos-
phorus, nitrogen, and oxygen.5 In the last year or so, there has
been a sharp rise of interest in sulfur-based functionalization
(Chart 1).6,7 Many of these bidentate and tridentate ligands
have provided complexes with excellent activities over a range
of catalytic applications.6,7 Despite the large number of sulfur-
functionalized NHC ligands reported, there are to our knowl-
edge no reported examples of NHC-based ligands featuring the
thione functionality.8
Chart 2. Bidentate Ligands Based on Thione and NHC
Donor Groups
Many investigations concerning N-heterocyclic carbene ligands
have focused on methylene-bridged bidentate ligands (CCR)
(Chart 2).9 A number of synthetic methods are available that
allow access to these ligands from their corresponding bis-
imidazolium salt precursors.9,10 In a related area, Williams first
reported the synthesis of bidentate bis-thione ligands (SSR) by
reaction of their corresponding imidazolium salts with elemental
sulfur in the presence of a base.11,12 Given the straightforward
nature of these syntheses, it is somewhat surprising that the
mixed ligand CSR, which contains one each of these functional-
ities, is still unknown.8 In all studies of the methylene-bridged
imidazolium salts to date, both imidazolium groups have been
converted to thione. We were therefore intrigued to determine
whether the synthesis of these mixed ligands was possible.
Herein, we wish to report the straightforward synthesis and
characterization of this new family of mixed bidentate ligand
precursors together with some preliminary copper(I)
complexes.
Received: July 12, 2011
Published: August 11, 2011
r
2011 American Chemical Society
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Figure 1. ORTEP representations of [CSMeH]Br (top, left), [CSMeH]I (top, right), and [CSBnH]Br (bottom). The halide counterions and hydrogen
atoms, with the exception of H(6), have been removed for clarity (thermal ellipsoids drawn at 50% probability level).
Scheme 1. Synthesis of Methylene-Bridged Mixed Bidentate NHC/Sulfur Ligand Precursors from Their Corresponding Bis-
imidazolium Salts
’ RESULTS AND DISCUSSION
Crystals suitable for X-ray diffraction were grown by layering
concentrated DCM solutions with hexane. ORTEP representa-
tions of the three imidazolium cations are presented in Figure 1.
Selected bond lengths and angles are highlighted in Table 1. The
structures revealed similar parameters to the corresponding bis-
imidazolium [CCR(H)2]2+ and bis-thione (SSR) compounds.10,11b
The C(1)ꢀS(1) distances in the three structures range between
1.6753(14) and 1.6846(18) Å and are typical of a imidazole-
thione function.11b The imidazolium and thione functionalities
are orientated in the same direction in [CSMeH]Br, while they
are in opposite directions in [CSMeH]I. This is perhaps related to
the packing of the compound in the crystal lattice rather than any
electronic factors within the cation.
Synthesis of Copper Complexes. The coordination chem-
istry of NHC ligands and their derivatives has been extensively
investigated using a wide range of transition metal centers. In
particular, the coordination to copper has been reported in a
number of examples.1,4,13ꢀ17 We therefore aimed to prepare a
series of copper(I) complexes containing these mixed ligands.
Our initial attempts using Cu2O as a precursor to synthesize our
target complexes were unsuccessful.15 The reactions were fol-
lowed by 1H NMR spectroscopy, revealing incomplete reactivity
of the imidazolium salts even after prolonged periods of time and
Synthesis of Ligand Precursors. The imidazolium salts 1,10-
methylene(3-methylimidazol-2-ylidene)(3-methyl-2H-imidazole-
2-thione) bromide, [CSMeH]Br, and 1,10-methylene(3-benzyli-
midazol-2-ylidene)(3-benzyl-2H-imidazole-2-thione) bromide,
[CSBnH]Br, were readily synthesized by a directly analogous
method to that described by Williams (Scheme 1) using only one
equivalent of sulfur (1/8 S8). The corresponding iodide salts
[CSMeH]I and [CSBnH]I were also prepared via the same
methodology. The extent of these reactions could be followed
by taking aliquots from the reaction mixture and recording their
1H NMR spectra (in DMSO-d6). The solid products [CSRH]X
[R = Me, Bn; X = Br, I] were obtained in high yields as white
powders following standard workup and were fully charac-
terized by spectroscopic and analytical methods. The 1H NMR
spectra confirmed the loss of one of the imidazolium protons by
revealing a new signal in the region between 9.38 and 9.70 ppm,
which integrated for a single proton. The 13C{1H} NMR spectra
showed a characteristic signal confirming the presence of the
CdS group in these new compounds with signals between 163.0
and 163.9 ppm, respectively.
The formation of [CSMeH]Br, [CSBnH]Br, and [CSMeH]I
was further confirmed by X-ray single-crystal diffraction studies.
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harsh conditions. In a typical experiment involving [CSMeH]Br
and Cu2O, signals corresponding to the target compound were
observed albeit with limited conversions (ca. <18%) after 24 h in
refluxing toluene/acetonitrile mixtures. The NMR spectra also
revealed the presence of a second species, which was identified as
As a consequence of this reactivity, we sought an alternative
synthetic methodology to provide our target complexes. The
utilization of copper(I) acetate to prepare monodentate NHC
complexes has previously been reported.16a,17 This reagent has
the benefit that it acts as a base that can deprotonate the imidazolium
proton as well as being the metal to which the NHC coordinates.
By using this protocol, the complexes [Cu(CSMe)X]2 [X = Br
(1); I (2)] and [Cu(CSBn)X]2 [X = Br (3); I (4)] were readily
prepared by addition of one equivalent of the appropriate
imidazolium precursor to an acetonitrile solution of copper(I)
acetate (Scheme 2). The reaction mixtures were stirred for 18 h,
after which time the volume of the solutions were reduced. In
cases where the product did not precipitate, diethyl ether was
added. In the reaction involving [CSMeH]I, the transformation to
complex 2 was slow at room temperature, and so the mixture was
heated to reflux to drive the reaction to completion. The resulting
solids were isolated by filtration and washed to provide the
products as white solids in moderate to high yields and fully char-
acterized by NMR and IR spectroscopy, mass spectrometry, and
elemental analysis. Complexes 1ꢀ4 were not particularly soluble
in many standard organic solvents; however they were reasonably
soluble in polar solvents such as MeCN and DMSO (the solubility
of 3 in MeCN was very low). The complexes were found to be
moderately air sensitive in these solutions (vide infra). Structural
characterization also revealed the dimeric nature of the resulting
complexes, and further details are provided below.
SS .
Me 11b The additional sulfur atom presumably originates from
a reaction between “free” NHC generated in situ and the large
quantity of [CSMeH]Br (the source of sulfur) in the reaction
mixture.
Cazin recently reported an oxygen atom transfer from Cu2O
to a “free carbene”.15d Throughout our investigations we found
no evidence of oxygen transfer when the reactions were carried
out strictly under a nitrogen atmosphere. When the reactions
were performed under aerobic conditions however, a single
1
new product was observed in the H NMR spectrum. The
product was isolated by standard techniques and fully character-
ized by analytical and spectroscopic methods. The new product
was confirmed as 1,10-methylene(3-methyl-2H-imidazol-2-one)-
(3-methyl-2H-imidazole-2-thione), SOMe; see Figure 2 (see
below for further details).
Table 1. Comparison of Structural Data [Bonding Distances
(Å)/Angles (deg)] for [CSMeH]Br, [CSMeH]I, [CSBnH]Br,
and SOMe
[CSMeH]Br [CSMeH]I [CSBnH]Br
SOMe
The 1H NMR spectra of 1ꢀ4, recorded in DMSO-d6,18 were
consistent with the coordination of the new ligands to the copper
centers, which was corroborated by the disappearance of the
characteristic signals corresponding to the imidazolium protons.
The methylene resonances were located between 6.23 and 6.26
ppm, a small upfield chemical shift relative to their respective
ligand precursors. This was suggestive of a bidentate coordina-
tion mode for the new ligands. The coordination of the NHC and
thione donors to the copper centers was confirmed in the
13C{1H} NMR spectra of complexes 1ꢀ4. The spectra revealed
resonances between 182.3 and 183.1 ppm for the carbene carbon
and between 157.5 and 159.0 ppm for the CdS group. These
values are consistent with previously reported copper complexes
containing either of these two donor functionalities.4a,19 All other
spectroscopic and analytical data were consistent with the
formation of [Cu(CSR)X]2 (where R = Me, Bn and X = Br, I).
Structural Characterization of Complexes 1 and 4. In order
to further confirm our assignment of complexes 1ꢀ4, two
complexes were structurally characterized (Figures 3 and 4).
Single crystals of 1 and 4 were obtained from saturated acetoni-
trile solutions of the complexes. Selected bond lengths and angles
are highlighted in Table 2. The crystal structures also revealed
C(1)ꢀS(1)
N(1)ꢀC(1)
C(1)ꢀN(2)
N(2)ꢀC(5)
C(5)ꢀN(3)
N(3)ꢀC(6)
C(6)ꢀN(4)
1.6753(14) 1.677(2)
1.3577(17) 1.362(3)
1.3772(16) 1.374(3)
1.4413(17) 1.434(3)
1.4688(17) 1.468(3)
1.3259(17) 1.331(3)
1.3259(17) 1.328(3)
1.6846(18) 1.6770(13)
1.358(2)
1.369(2)
1.447(2)
1.460(2)
1.333(2)
1.327(2)
1.3622(16)
1.3682(17)
1.4606(16)
1.4416(16)
1.3800(16)
1.3659(16)
N(2)ꢀC(5)ꢀN(3) 110.69(11) 111.66(18) 109.97(14) 112.07(10)
C(1)ꢀN(2)---
N(3)ꢀC(6)
2.6(2)
143.3(2)
151.2(2)
159.4(1)
Figure 2. The product resulting from the reaction of oxygen with a
copper-NHC complex.
Scheme 2. Synthesis of Dinuclear Copper Complexes [Cu(CSMe)X]2 and [Cu(CSBn)X]2
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that the complexes formed dimeric structures in the solid state
involving two copper centers, two CSR ligands, and two halides.
Complex 1 contains a four-membered CuꢀSꢀCuꢀS ring
resembling a parallelogram motif with CuꢀSꢀCu0 angles of
74.960(13)° and SꢀCuꢀS0 angles of 105.039(13)°. A number of
complexes featuring a CuSCuS core have previously been
reported.20,21 The copper centers are approximately equidistant
from the two sulfur atoms, where the Cu(1)ꢀS(1) distance is
2.4458(4) Å and the Cu(1)ꢀS(10) distance is 2.4210(4) Å.
distance is 2.9614(4) Å [cf. ∑r(CuCu) = 2.64 Å].22 This distance
is above the range to be considered as true metallophillic
interactions or d10ꢀd10 metal bonding. In any case, it should
be noted that short Cu Cu distances may be geometrically
3 3 3
imposed in systems containing bridging groups and may not
originate from attractive interactions.23 The dimeric structure
adopts a “zigzag”-type structure where the two CSMe ligands are
located above and below the CuSCuS plane at opposite sides of
the parallelogram (Figure 3). The metal center in 1 adopts a
distorted tetrahedral geometry with angles ranging between
101.66(5)° and 120.62(5)°; the largest angle corresponds to
the C(6)ꢀCu(1)ꢀBr(1) angle. One of the methylene hydrogen
atoms of CSMe points toward the bromine atom, although the
Within the four-membered CuSCuS ring, the Cu(1) Cu(10)
3 3 3
Table 2. Comparison of Structural Data [Bonding Distances
(Å)/Angles (deg)] for Complexes 1 and 4
1
4
Cu(1)ꢀX(1)a
Cu(1)ꢀS(1)
Cu(1)ꢀS(10)b
Cu(1)ꢀC(6)
2.4690(3)
2.4458(4)
2.4210(4)
1.9491(16)
2.9614(4)
1.7044(16)
74.960(13)
105.039(13)
110.77(5)
101.66(5)
120.62(5)
105.649(12)
112.180(13)
111.95(12)
ꢀ11.81(7)
120.41(5)
17.25(7)
2.6119(3)
2.8012(6)
2.3557(6)
1.948(2)
Cu(1) Cu(10)b
3.6609(4)
1.705(2)
3 3 3
C(1)ꢀS(1)
Cu0ꢀSꢀCub
90.025(18)
89.974(18)
96.33(6)
S0ꢀCuꢀSb
S(1)ꢀCu(1)ꢀC(6)
S(10)ꢀCu(1)ꢀC(6)b
C(6)ꢀCu(1)ꢀX(1)a
S(1)ꢀCu(1)ꢀX(1)a
S(10)ꢀCu(1)ꢀX(1)a,b
N(1)ꢀC(5)ꢀN(3)
126.06(6)
120.77(6)
98.745(14)
110.901(16)
111.31(16)
ꢀ22.74(10)
ꢀ144.28(8)
ꢀ13.26(8)
149.6(1)
C(6)ꢀCu(1)ꢀS(1)ꢀC(1)
X(1)ꢀCu(1)ꢀS(1)ꢀC(1)a
X(1)ꢀCu(1)ꢀS(10)ꢀC(10)a,b
C(1)ꢀS(1)ꢀCu(10)ꢀC(60)b
C(6)ꢀCu(1)ꢀS(1)ꢀCu(10)b
147.48(9)
109.01(5)
Figure 3. ORTEP representation of 1, highlighting the zigzag con-
formation of two ligands relative to the CuSCuS plane (ellipsoids drawn
at 50% probability level; hydrogen atoms, with the exception of H5a and
H5b, have been removed for clarity).
ꢀ126.31(6)
a X = Br or I. b Symmetry operator for symmetry generated atoms: ꢀx,
ꢀy, 2ꢀz (for 1) and 1ꢀx, 1ꢀy, 1ꢀz (for 4).
Figure 4. ORTEP representation of 4, highlighting the 14-membered ring (ellipsoids drawn at 50% probability level; hydrogen atoms, with the
exception of H5a and H5b, have been removed for clarity).
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H(5A) Br(1) distance is too large to be classed as a significant
Reactivity of Complexes 1ꢀ4 with Oxygen. Hwang has
previously reported an unusual reaction involving complexes 5
and 7 upon exposure to oxygen. In this transformation the
methylene bridge of the ligand is lost and the ligand undergoes
a CꢀC coupling reaction involving the carbene moieties, leading
to the oxidation of the metal to copper(II).24 It was therefore of
interest to investigate the reactivity of 1ꢀ4 with oxygen
(Scheme 3). When a DMSO-d6 solution of 1 was opened to
air, a slow conversion of the complex to the previously described
3 3 3
interaction [2.8339(2) Å].22 The CuSCuS ring in complex 4 is
different, with a motif resembling a rectangle [cf. CuꢀSꢀCu0
angles 90.025(18)° and SꢀCuꢀS0 angles 89.974(18)°]
(Figure 4). The corresponding Cu(1)ꢀS(1) and Cu(1)ꢀS(10)
distances are 2.8012(6) and 2.3557(6) Å, respectively. The
former distance is too long to be considered a significant
interaction [∑r(CuS) = 2.37 Å],22 and therefore the CSBn ligand
adopts a k1-C, k1-S mode. The dinuclear complex displays a
14-membered-ring structure with a novel conformation constrained
1
compound SOMe was observed in the H NMR spectrum.
by a planar CuSCuS core. In this complex, the Cu(1) Cu(10)
Approximately 11% conversion was observed after heating the
mixture at 80 °C in air for 4 h (see Table S1 in the Supporting
Information for further details). Demetalation and oxygen atom
addition of the CSR ligands25 occur in these copper(I) com-
plexes. Once formed, SOR is eliminated from the copper
coordination sphere. We believe that this is due to a combination
of the lower coordination abilities of this new compound and the
utilization of the strong donor solvents (DMSO and MeCN)
necessary to dissolve the precursors 1ꢀ4. No transformation of 1
to SOMe was observed in the absence of O2 or in the presence of
deoxygenated H2O, leading us to draw the conclusion that O2 is
the source of the oxygen atom. In order to confirm our assump-
tion, complex 1 was placed under an atmosphere of 18O2. The
formation of 18O-labeled compound S18OMe was confirmed by
mass spectrometry (227.1 amu, [S18OMe+H]+).
The chemical transformation of free NHCs to ureas has
previously been recorded albeit in a small number of cases.26
In contrast, the reactivity involving addition of molecular oxygen
across the double bond in the related enetetramines is much
more established.27 To our knowledge, there are no previous
reports of the formation of the urea moiety directly from a
transition metal bound NHC.28 The reactivity was unexpected
given the reputation of NHC ligands to form very strong bonds
to transition metals2,29 and that the coordination of the CSR
ligands would be expected to form strong chelates. Our attempts
to generate the urea functionality from the “free” NHC, gener-
ated in situ, were unsuccessful, and only traces of SOMe (along
3 3 3
distance is 3.6609(4) Å. The longer copperꢀcopper distance is
perhaps expected due to the larger steric demands of the iodide
ligand versus bromide. Additionally, in this structure the methy-
lene group of the CSBn ligand points toward the sulfur atom and
not the halide, as is the case for 1. As a consequence of the
different bonding features of the CuSCuS core within complexes
1 and 4, the ligands have different coordination modes. In
complex 1, CSMe adopts a k1-C, k1-S, μ-S coordination motif,
while in complex 4, CSBn adopts a k1-C, k1-S coordination mode
(Figure 5). Also of note are the Cu(1)ꢀC(6) distances in 1 and
4, which are typical of Cu(I)-NHC compounds [1.9491(16) Å
(1) and 1.948(2) Å (4)].
A number of copper complexes containing methylene-bridged
bis-NHC ligands have also been reported.13,14 As shown in
Figure 6, the k1-C, k1-C coordination mode, forming a dimeric
A-frame-type motif, appears to be most common (5ꢀ9). In the
case of ligands containing large, sterically hindered tert-butyl
groups, a polymeric structure is preferred (10).13b Although
μ-NHC coordination modes have been reported,16 such motifs
are less likely in the methylene-bridged bis-NHC ligands due to
the steric constraints of the ligands. A large number of copper(I)
halide complexes containing thione groups that exhibit a μ-S
coordination mode have been reported.19 These complexes
feature a variety of coordination modes and geometries with
many of the complexes containing either “Cu2(μ-S)2” or “Cu2-
(μ-halide)2” cores dependent on a wide range of parameters.
Scheme 3. Reactivity of [Cu(CSR)X]2 with O2
a
Figure 5. The k1-C, k1-S, μ-S and k1-C, k1-S coordination modes of
CSR (a schematic representation of the ligand has been used, and the
halide substituents have been removed for clarity).
a R = Me, Bn; X = Br, I; solvent = MeCN, DMSO.
Figure 6. Comparison of copper(I) complexes containing methylene-bridged bis-NHC ligands and complexes 1ꢀ4.
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with a slightly larger quantity of SSMe) were formed.30 The
mechanism involved in the conversion of NHC to urea is
currently unclear; however it appears to be mediated by the
copper center. The copper complexes presumably form an
intermediate oxygen complex,31 which is then transferred to
the NHC fragment. A recent structure reported by Pascu and
Whittlesey, which features a tris-copper(I) carbene complex
capped by a single oxygen atom, might be important.14a To
our knowledge there are no other reported examples of the
reactivity of molecular oxygen with copper complexes featuring
either the thione or NHC functionalities.31 Furthermore we have
not observed this transformation in any of our investigations with
other transition metal complexes to date, suggesting that such
reactivity might be limited to copper.
Despite the wide application of N-heterocyclic carbene com-
plexes in catalysis, it was a surprise to us that there are very few
examples of their application in copper-catalyzed cross-coupling
reactions.32 Complexes 1ꢀ4 were tested for their activities in
CꢀN and CꢀS cross-coupling reactions. These complexes
revealed very poor results for cross-coupling of aryl iodides with
amides. The complexes revealed good activities in the cross-
coupling of aryl iodides with thiols, consistent with many
literature reports (see Supporting Information for further
details).33 Several recent investigations have highlighted the high
activities of ligand-free systems under certain conditions.34 Our
control experiments also revealed that ligand-free copper(I) salts
showed comparable results, suggesting that these are indeed the
active species in the cross-coupling reactions.
bis(3-methylimidazol-2-ylidene) dibromide11a (12.9 g, 38.3 mmol) was
dissolved in methanol (150 mL), and sulfur (1.2 g, 38.3 mmol-1/8 S8)
and potassium carbonate (4.0 g, 28.7 mmol) were subsequently added.
The mixture was left to stir under reflux for 24 h and then filtered, the
solvent was evaporated, and the crude product was dissolved in DCM
(100 mL). The mixture was filtered to remove the inorganic salts, and
the solvent was evaporated to provide a white-yellow solid. Yield: 10.4 g,
36.0 mmol, 94%.
1
NMR δ ppm (DMSO-d6): H, 3.47 (s, 3H, NCH3), 3.88 (s, 3H,
NCH3), 6.38 (s, 2H, NCH2N), 7.27 (d, 3JHH = 2.6 Hz, 1H, CHCH),
7.58 (d, 3JHH = 2.6 Hz, 1H, CHCH), 7.73 (m, 1H, CHCH), 7.97 (m, 1H,
CHCH), 9.48 [br, 1H, NC(H)N]; 13C{1H}, 35.1 (CH3), 36.6 (CH3),
57.9 (NCH2N), 117.6 (CHCH), 120.3 (CHCH), 122.3 (CHCH), 124.5
(CHCH), 137.7 [NC(H)N], 163.6 (CdS). IR (cmꢀ1): 3070, 1575,
1551, 1434, 1381, 1244, 1151. MS (ESI)+: 209 [CSMeH]+. Anal. Calcd
for C9H13BrN4S: C 37.38, H 4.53, N 19.37. Found: C 37.54, H 4.64,
N 19.08.
1,10-Methylene(3-methylimidazol-2-ylidene)(3-methyl-
2H-imidazole-2-thione) Iodide, [CSMeH]I. 1,10-Methylene bis-
(3-methylimidazol-2-ylidene) diiodide11a (4.0 g, 9.25 mmol) was dis-
solved in methanol (50 mL), and sulfur (0.3 g, 9.25 mmol-1/8 S8) and
potassium carbonate (1.1 g, 7.60 mmol) were subsequently added. The
mixture was left to stir under reflux for 24 h and then filtered, the solvent
was evaporated, and the crude product was dissolved in DCM (100 mL).
The mixture was filtered to remove the inorganic salts, and the solvent
was evaporated to provide a white-yellow solid. The crude product was
washed with 20 mL of warm THF (45 °C) to provide the product. Yield:
2.0 g, 5.95 mmol, 64%.
1
NMR δ ppm (DMSO-d6): H, 3.46 (s, 3H, NCH3), 3.86 (s, 3H,
NCH3), 6.32 (s, 2H, NCH2N), 7.24 (d, 3JHH = 2.4 Hz, 1H, CHCH),
7.45 (d, 3JHH = 2.4 Hz, 1H, CHCH), 7.70 (m, 1H, CHCH), 7.89 (m, 1H,
CHCH), 9.38 [br, 1H, NC(H)N]; 13C{1H}, 34.6 (CH3), 36.1 (CH3),
57.4 (NCH2N), 117.0 (CHCH), 119.9 (CHCH), 121.8 (CHCH), 124.0
(CHCH), 137.1 [NC(H)N], 163.0 (CdS). IR (cmꢀ1): 3079, 1574,
1552, 1432, 1380, 1242, 1148. MS (ESI)+: 209 [CSMeH]+. Anal. Calcd
for C9H13N4IS: C 32.15, H 3.90, N 16.66. Found: C 32.40, H 3.96,
N 16.31.
’ CONCLUSIONS
In summary we have reported the synthesis of four new ligands
containing both NHC and thione functionalities. These ligands
have been utilized to synthesize dinuclear copper(I) complexes
with either k1-C, k1-S, μ-S or k1-C, k1-S coordination modes.
Demetalation and oxygen atom addition of the CSR ligands occur
in strongly coordinating solvents under aerobic conditions,
implying weak coordination of the NHC function to copper(I)
centers. Such coordination might limit the applicability of such
NHC-based copper complexes in cross-coupling reactions.
1,10-Methylene(3-benzylimidazol-2-ylidene)(3-benzyl-2H-
imidazole-2-thione) Bromide, [CSBnH]Br. 1,10-Methylene bis-
(3-benzylimidazol-2-ylidene) dibromide35 (8.8 g, 17.96 mmol) was dis-
solved in methanol (100 mL). Then sulfur (0.58 g, 17.96 mmol-1/8 S8)
and potassium carbonate (1.24 g, 8.98 mmol) were added. The mixture
was stirred under reflux for 18 h. The crude product was filtered, and the
solvent was evaporated and redissolved in DCM (60 mL). Then the
solution was filtered and evaporated again. The resulting product had an
off-white color. Yield: 6.8 g, 15.42 mmol, 86%.
’ EXPERIMENTAL SECTION
General Remarks. The syntheses of the ligand precursors were
performed in open flasks under ambient conditions, while the syntheses
of the copper complexes were carried out under a nitrogen atmosphere
using standard Schlenk techniques. All chemicals were used as received.
The solvents MeCN and Et2O were dried using a Grubbs alumina
system and were kept in Young’s ampules under N2 over molecular
NMR δ ppm (DMSO-d6): 1H, 5.20 (s, 2H, NCH2C6H5), 5.49 (s, 2H,
NCH2C6H5), 6.40 (s, 2H, NCH2N), 7.29ꢀ7.34 (m, 5H, C6H5
+
3
1H, CHCH), 7.40ꢀ7.44 (m, 5H, C6H5), 7.60 (d, JHH = 2.4 Hz, 1H,
CHCH), 7.85 (m, 1H, CHCH), 7.99 (m, 1H, CHCH), 9.70 [br, 1H,
NC(H)N]; 13C{1H}, 50.6 (NCH2C6H5), 52.6 (NCH2C6H5), 58.2
(NCH2N), 118.5 (CHCH), 119.5 (CHCH), 122.9 (CHCH), 123.4
(CHCH), 128.4 (p-C6H5), 128.5 (o-C6H5), 128.9 (o-C6H5), 129.1
(m-C6H5), 129.3 (p-C6H5), 129.5 (m-C6H5), 135.2 (i-C6H5), 136.9
(i-C6H5), 137.7 [NC(H)N], 163.9 (CdS). IR (cmꢀ1): 3101, 3078,
3011, 1564, 1555, 1448, 1414, 1236, 1155. MS (ESI)+: 361 [CSBnH]+
100%, 803 [(CSBnH)2Br]+ 53%. Anal. Calcd for C21H21BrN4S: C 57.14,
H 4.80, N 12.69. Found: C 57.36, H 4.91, N 12.75.
ꢀ
sieves (4 Å). Deionized and degassed water or MeOH was used to wash
1
the copper complexes. H NMR spectra were recorded at room tem-
perature on a JEOL Lambda 300 spectrometer operating at 300 MHz or
a JEOL ECP 400 spectrometer operating at 400 MHz. 13C{1H} NMR
spectra were recorded at room temperature on a JEOL ECP 400
spectrometer operating at 101 MHz. Electrospray mass spectra (ESI+)
were recorded on a Bruker Daltonics Apex 4e 7.0T FT-MS mass spectro-
meter. Infrared spectra were recorded in the region 4000ꢀ650 cmꢀ1 on
a Perkin-Elmer Spectrum 100 FT-IR spectrometer (solid state). Crystal
structure was determined on Kappa Apex II single-crystal diffract-
ometers. Elemental analyses were performed by the microanalytical
laboratory, School of Chemistry, University of Bristol.
1,10-Methylene(3-benzylimidazol-2-ylidene)(3-benzyl-2H-
imidazol-2-thione) Iodide [CSBnH]I. 1,10-Methylene bis(3-benzy-
limidazol-2-ylidene) diiodide36 (2.7 g, 4.62 mmol) was dissolved in
methanol (60 mL). Then sulfur (0.15 g, 4.62 mmol-1/8 S8) and
potassium carbonate (0.32 g, 2.31 mmol) were added. The mixture
was stirred under reflux for 20 h. The crude product was filtered and
1,10-Methylene(3-methylimidazol-2-ylidene)(3-methyl-
2H-imidazole-2-thione) Bromide, [CSMeH]Br. 1,10-Methylene
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Organometallics
ARTICLE
3
3
evaporated, then extracted into DCM (60 mL) and put in a freezer. After
24 h the white-colored crystals were collected by filtration. Yield: 2.0 g,
4.1 mmol, 89%.
7.32 (d, JHH = 2.2 Hz, 1H, CHCH), 7.58 (d, JHH = 2.2 Hz, 1H,
CHCH), 7.62 (d, 3JHH = 1.6 Hz, 1H, CHCH); 13C{1H}, 35.6 (CH3),
37.5 (CH3), 59.4 (NCH2N), 118.9 (CHCH), 120.8 (CHCH), 121.1
(CHCH), 122.4 (CHCH), 158.4 (CdS), 183.1 (CCu). IR: 3110, 3076,
1568, 1469, 1385, 1371, 1244, 1196. MS (ESI)+: 209 [CSMeH]+ (10%),
271 [CSMeCu]+ (100%), 479 [(CSMe)2Cu]+ (90%), 669 [(CSMe)2Cu2I]+
(15%). Anal. Calcd for C9H12CuN4IS: C 27.11, H 3.03, N 14.05. Found:
C 27.51, H 3.24, N 13.94.
1
NMR δ ppm (DMSO-d6): H, 5.19 (s, 2H, NCH2C6H5), 5.47
(s, 2H, NCH2C6H5), 6.37 (s, 2H, NCH2N), 7.29ꢀ7.34 (m, 5H, C6H5 +
3
1H, CHCH), 7.40ꢀ7.43 (m, 5H, C6H5), 7.52 (d, JHH = 2.2 Hz, 1H,
CHCH), 7.82 (m, 1H, CHCH), 7.94 (m, 1H, CHCH), 9.64 [br, 1H,
NC(H)N]; 13C{1H}, 50.0 (NCH2C6H5), 52.1 (NCH2C6H5), 57.9
(NCH2N), 117.9 (CHCH), 118.9 (CHCH), 122.3 (CHCH), 123.0
(CHCH), 127.9 (overlapping o-C6H5 and p-C6H5), 128.4 (o-C6H5),
128.6 (m-C6H5), 128.9 (p-C6H5), 129.0 (m-C6H5), 134.6 (i-C6H5),
136.3(i-C6H5), 137.1[NC(H)N], 163.3 (CdS). IR (cmꢀ1): 3072, 2982,
1561, 1546, 1433, 1410, 1232, 1140. MS (ESI)+: 361 [CSBnH]+. Anal. Calcd
for C21H21N4IS: C 51.64, H 4.33, N 11.47. Found: C 52.01, H 4.16, N 11.42.
1,10-Methylene[3-methyl-2H-imidazole-2-thione][3-methyl-
2H-imidazol-2-one], SOMe. A flask was charged with copper(I)
oxide (123.7 mg, 0.86 mmol) and 1,10-methylene(3-methylimidazol-2-
ylidene)(3-methyl-2H-imidazole-2-thione) bromide (250.0 mg, 0.86 mmol).
Toluene (100 mL) and acetonitrile (25 mL) were subsequently added to
the mixture. The mixture was heated to reflux for 3 days to provide a
colorless solution with a small amount of red-brown precipitate. After
removing the residue by filtration all volatiles were removed under
vacuum. The residue was subsequently extracted with DCM, filtered
through diatomaceous earth, and layered with an equal volume of
hexane. After one day colorless crystals of SOMe appeared in the flask,
which were isolated by filtration. Yield: 84.5 mg, 0.38 mmol, 44%.
NMR δ ppm (CDCl3): 1H, 3.21 (s, 3H, NCH3), 3.57 (s, 3H, NCH3),
Synthesis of [Cu(CSBn)Br]2 (3). Copper(I) acetate (200.0 mg,
1.63 mmol) was suspended in acetonitrile (30 mL) under a nitrogen
atmosphere, and 1,10-methylene(3-benzylimidazol-2-ylidene)(3-ben-
zyl-2H-imidazole-2-thione) bromide (719.4 mg, 1.63 mmol) was
subsequently added. The mixture was allowed to stir for 18 h at
RT. The mixture was then filtered, and the resulting solid was washed
with MeCN (2 ꢁ 10 mL) to give a white solid. Yield: 700.0 mg,
0.69 mmol, 85%.
NMR δ ppm (DMSO-d6): 1H, 5.23 (s, 2H, NCH2C6H5), 5.26 (s, 2H,
NCH2C6H5), 6.26 (s, 2H, NCH2N), 7.28ꢀ7.34 (m, 8H, C6H5), 7.39
(m, 2H, CHCH), 7.43ꢀ7.47 (m, 2H, C6H5), 7.62 (unresolved d, 1H,
CHCH), 7.66 (unresolved d, 1H, CHCH); 13C{1H}, 51.0 (NCH2-
C6H5), 53.6 (NCH2C6H5), 59.6 (NCH2N), 119.5 (CHCH), 120.1
(CHCH), 121.1 (CHCH), 121.6 (CHCH), 128.5 (3 overlapping
signals, 2 ꢁ p-C6H5 and o-C6H5), 128.8 (o-C6H5), 129.1 (m-C6H5),
129.2 (m-C6H5), 136.5 (i-C6H5), 137.9 (i-C6H5), 158.9 (CdS), 182.9
(CCu). IR (cmꢀ1): 3113, 3090, 3002, 1571, 1446, 1395, 1247, 1225. MS
(ESI)+: 423 [Cu2(CSBn)2Br]+. Anal. Calcd for C21H20BrCuN4S: C
50.05, H 4.00, N 11.12. Found: C 49.96, H 3.85, N 10.73.
3
5.82 (s, 2H, NCH2N), 6.10 (d, JHH = 2.9 Hz, 1H, CHCH), 6.61
Synthesis of [Cu(CSBn)I]2 (4). Copper(I) acetate (69.0 mg, 0.56
mmol) was suspended in acetonitrile (40 mL) under a nitrogen atmo-
sphere, and 1,10-methylene(3-benzylimidazol-2-ylidene)(3-benzyl-2H-
imidazole-2-thione) iodide (274.8 mg, 0.56 mmol) was subsquently
added. The mixture was allowed to stir for 18 h at RT. The mixture was
filtered, the solvent volume was reduced, and diethyl ether (20 mL) was
added. The resulting solid was then isolated by filtration to give a white
solid. Yield: 170.0 mg, 0.15 mmol, 55%.
(d, 3JHH = 2.4 Hz, 1H, CHCH), 6.85 (d, 3JHH = 2.9 Hz, 1H, CHCH),
7.10 (d, 3JHH = 2.4 Hz, 1H, CHCH); 13C{1H}, 30.4 (CH3), 35.0 (CH3),
53.5 (NCH2N), 111.2 (CHCH), 111.8 (CHCH), 117.7 (CHCH), 118.0
(CHCH), 153.3 (CdO), 163.2 (CdS). IR (cmꢀ1): 3124, 1672s
(CdO), 1566, 1466, 1453, 1432, 1400, 1379, 1224, 1184. MS (ESI)+:
225 [SOMeH]+ (25%), 247 [SOMe +Na] + (100%). Anal. Calcdfor C9H12-
N4OS: C 48.20, H 5.39, N 24.98. Found: C 48.03, H 5.33, N 24.36.
Synthesis of [Cu(CSMe)Br]2 (1). Copper(I) acetate (250.0 mg,
2.00 mmol) was suspended in acetonitrile (150 mL) under a nitrogen
atmosphere. 1,10-Methylene(3-methyl-2H-imidazole-2-thione)(3-methyl-
imidazol-2-ylidene) bromide (693.6 mg, 2.40 mmol) was subsequently
added. The mixture was allowed to stir for 18 h at RT and then filtered.
The solvent was reduced, diethyl ether (20 mL) was added, and the
resulting solid was isolated by filtration. The product was then washed
twice with water (2 ꢁ 10 mL) and dried under reduced pressure for
several hours. Yield: 480.0 mg, 0.68 mmol, 68%.
NMR δ ppm (DMSO-d6): 1H, 5.25 (s, 2H, NCH2C6H5), 5.33 (s, 2H,
NCH2C6H5), 6.26 (s, 2H, NCH2N), 7.28ꢀ7.35 (m, 8H, C6H5), 7.38 (d,
3JHH = 1.8 Hz, 1H, CHCH) 7.40 (d, JHH = 2.4 Hz, 1H, CHCH),
3
7.44ꢀ7.48 (m, 2H, C6H5), 7.62 (d, 3JHH =2.4 Hz, 1H, CHCH), 7.64 (d,
3JHH =1.8 Hz, 1H, CHCH); 13C{1H}, 51.1 (NCH2C6H5), 53.3
(NCH2C6H5), 59.7 (NCH2N), 119.7 (CHCH), 120.1 (CHCH),
121.2 (CHCH), 121.5 (CHCH), 128.5 (3 signals, 2 ꢁ p-C6H5 and
o-C6H5), 128.8 (o-C6H5), 129.1 (m-C6H5), 129.2 (m-C6H5), 136.4
(i-C6H5), 137.8 (i-C6H5), 159.0 (CdS), 183.1 (CCu). IR (cmꢀ1):
3117, 3089, 3002, 1585, 1564, 1450, 1435, 1392, 1244, 1229. MS (ESI)+:
361 [CSBnH]+ (58%), 423 [CSBnCu]+ (31%), 783 [(CSBnH)2Cu]+
(100%), 975 [(CSBnH)2CuI]+ (26%). Anal. Calcd for C21H20CuIN4S:
C 45.78, H 3.66, N 10.17. Found: C 45.48, H 3.91, N 9.79.
1
NMR δ ppm (DMSO-d6): H, 3.51 (s, 3H, NCH3), 3.68 (s, 3H,
NCH3), 6.24 (s, 2H, NCH2N), 7.29 (two overlapping signals, 2H,
CHCH), 7.62 (s, 1H, CHCH), 7.69 (s, 1H, CHCH); 13C{1H}, 35.6
(CH3), 37.6 (CH3), 59.5 (NCH2N), 118.9 (CHCH), 120.9 (CHCH),
121.2 (CHCH), 122.4 (CHCH), 157.5 (CdS), 182.3 (CCu). IR: 3113,
3069, 1569, 1469, 1388, 1373, 1248, 1198. MS (ESI)+: 209 [CSMeH]+
(29%), 271 [CSMeCu]+ (59%), 479 [(CSMe)2Cu]+ (100%), 623
[(CSMe)2Cu2Br]+ (79%). Anal. Calcd for C9H12BrCuN4S: C 30.73,
H 3.44, N 15.93. Found: C 31.01, H 3.17, N 15.39.
’ ASSOCIATED CONTENT
Supporting Information. The crystal structure of SOMe
S
b
along with further details of the cross-coupling experiments
together with crystallographic details and structural parameters
for all complexes are available free of charge via the Internet at
http://pubs.acs.org.
Synthesis of [Cu(CSMe)I]2 (2). Copper(I) acetate (100.0 mg,
0.82 mmol) was suspended in acetonitrile (50 mL) under a nitrogen
atmosphere, and 1,10-methylene(3-methylimidazol-2-ylidene)(3-methyl-
2H-imidazole-2-thione) iodide (274.1 mg, 0.82 mmol) was subsequently
added. The mixture was heated to reflux for 18 h and filtered to remove
any unreacted starting material. The solvent was reduced down, diethyl
ether (15 mL) was added, and the product was isolated by filtration.
The product was then washed with water (10 mL). Yield: 151.3 mg,
0.19 mmol, 46%.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: gareth.owen@bristol.ac.uk.
1
Notes
‡G.R.O. is a Royal Society Dorothy Hodgkin Research Fellow.
NMR δ ppm (DMSO-d6): H, 3.54 (s, 3H, NCH3), 3.71 (s, 3H,
NCH3), 6.23 (s, 2H, NCH2N), 7.30 (d, 3JHH = 1.6 Hz, 1H, CHCH),
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Organometallics
ARTICLE
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The authors thank the Royal Society for funding a Royal
Society Dorothy Hodgkin Research Fellowship for G.R.O.
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broad in DMSO-d6, while the corresponding spectra in MeCN-d3 were
relatively sharp. Their poor solubility in MeCN meant that not all
spectra could be obtained by using this solvent. For consistency all data
1
presented in the Experimental Section are for DMSO-d6. Selected H
NMR spectra in MeCN-d3: Complex 1, δ ppm, 3.53 (s, 3H, NCH3),
3.73 (s, 3H, NCH3), 6.15 (s, 2H, NCH2N), 6.94 (d, 3JHH = 2.5 Hz, 1H,
CHCH), 7.04 (d, 3JHH = 1.8 Hz, 1H, CHCH), 7.19 (d, 3JHH = 2.5 Hz,
1H, CHCH), 7.32 (d, 3JHH = 1.8 Hz, 1H, CHCH). Complex 3, 5.28 (m,
4H, NCH2C6H5), 6.18 (s, 2H, NCH2N), 6.98 (s, 1H, CHCH), 7.20
(s, 1H, CHCH), 7.29ꢀ7.35 (m, 12H, C6H5+CHCH).
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