Chelating C4-bound imidazolylidene complexes through oxidative addition of imidazolium salts to palladium(0)

Article abstract of DOI:10.1002/ejic.201100940

Oxidative addition of donor-functionalised 4-iodoimidazolium salts to palladium(0) provides a selective route for the preparation of abnormal chelating N-heterocyclic carbene complexes and enables the introduction of a variety of donor groups. The activation of the C4 position does not necessitate protection of the imidazolium C2 position, thereby leaving this site available for further modification. While metallation of the unsubstituted C2 position of the N-heterocyclic carbene ligand was unsuccessful when palladium was bound to the C4 carbon atom, sequential metallation of first the C2 position, by means of transmetallation, followed by C4-I oxidative addition, afforded a dimetallic complex comprised of two palladium centres bridged by a single NHC ligand. Oxidative addition of 4-iodoimidazolium salts to low-valent palladium(0) provides access to abnormal NHC-palladium complexes without requiring protection of the C2 position. Hence, this site is available for further functionalisation which allows, for example, dimetallic complexes to be prepared.

Full text of DOI:10.1002/ejic.201100940

FULL PAPER
DOI: 10.1002/ejic.201100940
Chelating C4-Bound Imidazolylidene Complexes through Oxidative Addition of
Imidazolium Salts to Palladium(0)
Anneke Krüger,[a] Evelyne Kluser,[b] Helge Müller-Bunz,[a] Antonia Neels,[c] and
Martin Albrecht*[a]
Keywords: N-Heterocyclic carbenes / Palladium / N ligands / Abnormal bonding / Oxidative addition / Chelates
Oxidative addition of donor-functionalised 4-iodoimidazol- of the unsubstituted C2 position of the N-heterocyclic carb-
ium salts to palladium(0) provides a selective route for the
preparation of abnormal chelating N-heterocyclic carbene
complexes and enables the introduction of a variety of donor
groups. The activation of the C4 position does not necessitate
protection of the imidazolium C2 position, thereby leaving
this site available for further modification. While metallation
ene ligand was unsuccessful when palladium was bound to
the C4 carbon atom, sequential metallation of first the C2
position, by means of transmetallation, followed by C4–I oxi-
dative addition, afforded a dimetallic complex comprised of
two palladium centres bridged by a single NHC ligand.
sised through direct or base-assisted C–H activation.[1,4]
Formation of a silver carbene complex for transmetallation
has proven problematic.[5] In order to ensure the exclusive
formation of C4-bound carbenes, the C2 position generally
needs to be substituted by an alkyl or aryl group.[6] An al-
ternative to the protection of the C2 position is the acti-
vation of the C4 position by a halide substituent, which
enables metallation by C–X oxidative addition to a low-
valent metal centre (Scheme 1). Oxidative addition of imid-
azolium salts to transition metal centres is a well-estab-
lished route for the synthesis of normal NHC complexes.[7]
Introduction
Abnormal N-heterocyclic carbenes (NHCs) have remarka-
bly different electronic properties from their normal NHC
analogues,[1] which has led to distinct reactivity patterns
and in some cases enhanced catalytic activity.[2] The features
responsible for the special properties of abnormal NHCs,
i.e. the enhanced donor capacity due to decreased hetero-
atom stabilisation, also implies that the free abnormal carb-
ene is far less stable than its normal counterpart.[3] Abnor-
mal imidazolylidene complexes are therefore mostly synthe-
Scheme 1. Typical synthetic pathways to normal and abnormal imidazolylidene complexes.
[a] School of Chemistry & Chemical Biology, University College
Dublin,
Apart from rendering metallation chemoselective, this pro-
tocol principally also allows the C2 position to be kept un-
protected and available for further functionalisation. For
example, dimetallic systems may become accessible by me-
tallation through C2–H activation. While dimetallic triazol-
diylidene complexes have been explored in depth,[8] related
Belfield, Dublin 4, Ireland
Fax: +353-1716-2501
E-mail: martin.albrecht@ucd.ie
[b] Dept Chemistry, University of Fribourg,
Chemin du Musée 9, 1700 Fribourg, Switzerland
[c] XRD Application Lab, CSEM,
Rue Jaquet-Droz 1, 2002 Neuchâtel, Switzerland
1394
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1394–1402

Chelating C4-Bound Imidazolylidene Complexes
dimetallic carbene/alkenyl complexes derived from imid- single imidazole proton only. Apparently, the size of the
azolium salts are far less known, though they have received iodide nucleus in combination with the bulk of the iPr
increasing attention recently.[9] Dimetallic complexes, espe- group provides sufficient steric congestion to induce re-
cially when they are comprised of two different metals, pro- gioselective alkylation. Potentially chelating, functionalised
vide interesting opportunities for catalysing tandem pro- wingtip groups were introduced by N2-quaternisation of
cesses.[10]
the N1-substituted 4-iodoimidazole derivative with appro-
Expanding on our initial work,[11] we have used C4-io- priately functionalised alkyl halides, thus yielding the ligand
dinated imidazolium salts as NHC precursors for oxidative precursors 1–4 (Scheme 2).[14]
addition to palladium(0) and explored the potential of in-
Oxidative addition of the imidazolium salts 1–4 to the
corporating different chelating donor groups (denoted as palladium(0) centre in Pd(dba)2 yielded abnormal NHC–
“E”) as wingtip substituents, ranging from the relatively palladium(II) complexes 5–8 in unoptimised yields of 18–
hard –NEt2 to a comparably soft –SPh group. Preliminary 60% (Scheme 2). The syntheses were carried out under inert
results also indicate that this approach may be useful for conditions at room temperature in CH2Cl2 or DMSO, but
the synthesis of a variety of homo- and heterodimetallic all the complexes are air- and moisture-stable. Complex 8b
systems.
was obtained by treatment of 8a with NaI at room tempera-
ture.
Results and Discussion
X-ray Crystallographic Analyses
Synthesis
The chelating nature of the ligands in complexes 5–8 was
unambiguously confirmed by single-crystal X-ray diffrac-
tion analyses. The molecular structure of 5 has been re-
ported previously,[11] and the structures of complexes 6, 7
and 8b are depicted in Figure 1. In all three structures the
palladium centre resides in a slightly distorted square-
planar environment comprised of the C,E-bidentate carb-
ene ligand and two halides. The halides in the structures of
6 and 7 were scrambled and their occupancies refined to a
ratio of 7:3 in 6 and 11:9 in 7.[15] In both cases the major
isomer contains the iodide cis to the NHC ligand, and the
minor isomer has a mutual trans arrangement of the NHC
group and iodido ligand. The major isomer represents the
kinetic product and is also expected to be thermodynami-
cally most stable when considering the relative trans influ-
ence (NHC Ͼ SR2 Ͼ NR3 and iodide Ͼ bromide). How-
ever, the differences between I– and Br– may be sufficiently
small to account for the observed solid-state distribution
(cf. solution studies below).
4/5-Iodoimidazole, which is readily accessible through
iodination of imidazole,[12] was used as precursor to the ab-
normal NHC ligands. Selective alkylation of the remote ni-
trogen atom was achieved by using 2-iodopropane,[13] thus
yielding 4-iodo-N-isopropylimidazole. Exclusive alkylation
at N1 was demonstrated by NOESY experiments, which
unambiguously confirmed the proximity of the iPr group
to both residual imidazole protons. In contrast, alkylation
with EtI gave an approximate 3:1 mixture of the two pos-
sible isomers, i.e. N-ethyl-4- and -5-iodoimidazole. The
minor isomer exhibited nuclear Overhauser effects with a
The Ccarbene–Pd–E bite angle in the ethylene-linked che-
lates 6 [93.14(13)°], 7 [93.4(3)°] and 8b [93.34(12)°] is slightly
larger than the corresponding bite angle in 5 [86.7 (3)°],
reflecting the larger flexibility of the palladacycle comprised
of two sp3-hybridised carbon atoms. The imidazolylidene
ring in 6, 7 and 8b is furthermore twisted out of the metal
coordination plane by roughly 30°, while in 5 the imid-
azolylidene and pyridyl rings both form a dihedral angle of
ca. 40° with the metal coordination plane and assume a
puckered conformation.[16]
Because of the halide disorder in complexes 6 and 7, a
sensible bond-length comparison is limited to complexes 5
and 8b. The structure of 5 contained only one isomer, de-
spite the presence of two different halides, whereas halide
scrambling in 8b is irrelevant.[17] The Pd–I(1) bond in 8b
[2.6350(5) Å] is significantly longer than the corresponding
bond in 5 (2.5179 Å) indicating a stronger trans influence
of the soft S(alkyl) ligand in 8b compared with the pyridyl
ligand in 5.[18] In addition, the less pronounced puckering
Scheme 2. Synthesis of abnormal carbene complexes by oxidative
addition of imidazolium salts. Reagents and conditions: (i) Pd-
(dba)2, CH2Cl2, r.t., 12 h; (ii) Pd(dba)2, CH2Cl2, 0 °C to r.t., 2 d;
(iii) Pd(dba)2, DMSO, r.t., 2 d; (iv) Pd(dba)2, DMSO, r.t., 12 h; (v)
NaI, acetone, r.t., 16 h.
Eur. J. Inorg. Chem. 2012, 1394–1402
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1395

A. Krüger, E. Kluser, H. Müller-Bunz, A. Neels, M. Albrecht
FULL PAPER
imity to the metal-bound halide in the cis position (C–H···X
2.86 and 2.69 Å, respectively). A hydrogen bond has pre-
viously been noted between the pyridyl C6–H and the
bromide atom in the cis position in complex 5.[11] Such
short contacts between the pyridyl C6–H and halide in the
cis position have also been identified in related normal
NHC complexes.[23] Selected bond lengths and angles for
complexes 5, 6, 7 and 8b are shown in Table 1.
Table 1. Selected bond lengths [Å] and angles [°] for complexes 5,
6, 7 and 8b.[a]
5[b]
6[b]
7[c]
8b[c]
Pd(1)–C(1)
Pd(1)–E
C(1)–C(2)
1.961(7)
2.098(6)
1.393(10) 1.370(5)
1.989(3)
2.160(3)
1.977(9)
2.281(2)
1.376(13) 1.355(6)
93.34(12)
2.013(4)
2.2865(11)
C(1)–Pd(1)–E 86.7(3)
93.14(13) 93.4(3)
[a] Data for complex 5 from ref.[11] [b] E = N(3). [c] E = S(1).
NMR Spectroscopic Studies
Palladium complex formation was evidenced in solution
by the shift of the high-field C–I carbon signal in the 13C
NMR spectrum. Furthermore, palladation induced an up-
field shift of the signals of the C2–H and C5–H protons (to
δH ≈ 8.3 and 7.2 ppm, respectively) in all complexes.[24] In
[D6]DMSO these signals appear broad in both the 1H
NMR and in particular in the 13C NMR spectra. The pro-
ton and carbon signals due to the ethylene linker and also
the signals of the chelating SPh and NEt2 groups in 6–8
were broad, in contrast to the iPr proton signals, which
were sharp. While this may indicate a degree of fluxionality
in the coordination of the nitrogen and sulfur atoms to the
palladium centre in solution (hemilability), variable-tem-
perature experiments indicated that the signals remained
broad up to 80 °C. Such natural broadening of the signals
may originate from reduced flexibility of the ligand due to
conformationally stabilised hydrogen bonding to halides (cf.
Figure 1. ORTEP representation of 6 (a), 7 (b) and 8b (c). All ther-
mal ellipsoids drawn at the 50% probability level. Hydrogen atoms
and cocrystallised solvent molecules omitted for clarity.
of the ligand in 8b may induce steric repulsion between the X-ray crystallographic section). This hypothesis is further
iPr group and the iodido ligand. The Pd–Ccarbene bond supported by a 0.73 ppm downfield shift and substantial
lengths in all the complexes fall within the range typically broadening of the pyridyl C6–H proton signal in complex
observed for abnormal NHC–palladium complexes 5.
1
[1.99(3) Å][2,4,19] and do not differ from related complexes
In complex 6, two sets of imidazolylidene H NMR sig-
featuring a normal C2 bonding mode of the NHC li- nals are visible in an approximate 5:1 ratio. The minor set
gand.[15,16,20] The Pd–Ccarbene bond in 8b [2.013(4) Å] is is broad and overlaps with the resonances of the major
slightly longer than in 5 [1.961(7) Å], which presumably re- component, except for the low-field resonance of the C2-
flects the different flexibility in the six-membered metallacy- bound proton, which appears at δ = 8.90 ppm for the major
cle (cf. bite angles). The average heterocyclic C(1)–C(2) species and at δ = 8.84 ppm for the minor one. In the 13C
bond length in complexes 5–8 is 1.37(3) Å, which points to NMR spectrum, all resonances are broadened, and the
a predominantly π-conjugated system and suggests vinyl- minor isomer was not detected. The presence of two com-
type bonding of the C4-bound carbene ligand.[21] In normal pounds may be rationalised by halide scrambling (cf. X-ray
NHC complexes the C–C bond length is typically around discussion) or by partial dissociation of the halide trans to
1.33 Å, consistent with a rather localised double bond. In the NHC ligand.[21b,25] In support of the latter, a single
addition, complexes 6 and 7 feature hydrogen bonds be- compound with sharp signals was observed when the spec-
tween the imidazolylidene C5–H, crystallographically lab- trum was recorded in CD3CN solution, indicating that the
elled C(2), and the halide in the cis position [C(2)–H···X coordinating ability of the solvent is relevant. The
2.86 and 2.91 Å, respectively].[22] Furthermore, one of the NCH2CH3 signals are diastereotopic and resonate as two
methylene protons of each NEt group in 6 is in close prox- well-resolved multiplets. Halide substitution by a solvent
1396
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1394–1402

Chelating C4-Bound Imidazolylidene Complexes
molecule thus induces enhanced flexibility of the C,E-bi-
dentate ligand, presumably because of the absence of hydro-
gen bonding between the ligand and a metal-bound halide.
While fluxional behaviour of the six-membered metallacy-
cle cannot be excluded, it is worth noting that the sulfido
complexes 7 and 8 display a single set of resonances, despite
the chirality at the sulfur atom.
Metallation of the Imidazolium C2 Position
Figure 2. ORTEP representation of 10 (50% probability level, hy-
drogen atoms omitted for clarity). Selected bond lengths [Å] and
angle [°]: Pd(1)–C(1) 1.976(4), Pd(1)–N(3) 2.080(3), C(1)–C(2)
1.363(5), C(1)–Pd(1)–N(3) 86.76(14).
The availability of a C2–H unit in complexes 5–8 and the
relatively acidic character of this proton, as deduced from
NMR spectroscopy, prompted us to explore the possibility
of constructing dimetallic complexes by metallation of the
C2 position. In order to further stabilise a potentially C2-
bound palladium centre, a second donor group was intro-
duced onto the NHC precursor. The imidazolium salt 9a
(Scheme 3) was prepared directly from 4/5-iodoimidazole
and (bromomethyl)pyridine according to a known pro-
cedure.[26] Subsequent oxidative addition to Pd(dba)2 af-
forded complex 10 as an air- and moisture-stable solid in
60% yield.
suggests limited flexibility of one picolyl unit in solution
akin to complex 5. The resonance of the C6Ј–H proton of
the coordinated pyridyl ring is broad and shifted downfield
to δH = 8.89 ppm suggesting that the crystallographically
identified C(9)–H···Br–Pd hydrogen bonding motif persists
in solution as was observed for complex 5.
Reaction of complex 10 or any of the complexes 5–8 with
Ag2O did not produce the desired C2-bound silver–carbene
complexes for use in transmetallation reactions.[27] Direct
metallation with Pd(OAc)2 also proved unsuccessful.[28] Af-
ter 16 h in DMSO at 60 °C, the complexes apparently de-
composed. Attempts to deuterate the C2-bound hydrogen
atom in 7 by using D2O similarly failed, even in the pres-
ence of KOH.[29] Inversion of the metallation sequence was
more successful. Thus, palladation of the C2 position of
the imidazolium chloride 9b, obtained from 9a by halide
exchange, was accomplished by treatment with Ag2O and
Scheme 3. Metallation of 9. Reagents and conditions: Pd(dba)2,
CH2Cl2, r.t., 5 d.
The structure of 10 was unambiguously confirmed by X- subsequent transmetallation using [PdCl2(MeCN)2]. This
ray crystallography (Figure 2), which revealed the expected procedure cleanly afforded the pincer complex 11 in 60%
distorted square-planar coordination geometry around the yield (Scheme 4). The formation of 11 was confirmed by the
1
palladium atom and C,N-bidentate chelation of the ligand. disappearance of the C2–H proton signal in the H NMR
Similar to 5, the bicyclic ligand assumes a puckered confor- spectrum, as well as by an upfield shift of the C5–H proton
mation with the imidazolylidene and pyridyl rings twisted signal to δ = 7.82 ppm. No broadening of the imidazolylid-
out of the metal coordination plane by about 40°, and the ene signals was observed in the 13C NMR spectrum, and
bite-angle is slightly more acute than 90°. The structure the carbene carbon resonance was noted at δC = 151.3 ppm.
contains two isomers in an approximately 8:2 ratio due to The proton and carbon signals due to the picolyl moieties
halide scrambling. The major isomer contains the iodide were also sharp, and chelation therefore seems to be rigid.
ligand cis with respect to the NHC ligand and reveals close The inequivalence of the picolyl NCH2 signals is reflected
C–H···X contacts for the imidazolylidene and the pyridyl by a 0.1 ppm shift difference between the two singlets in the
heterocycle through C(2)–H···I(1) and C(9)–H···Br(1) inter- 1H NMR spectrum.
actions, respectively.
Subsequent exposure of 11 to Pd(dba)2 in the presence
In agreement with the NMR analyses of complexes 5–8, of bipyridine (bpy) under reaction conditions similar to
palladation of the precursor 9b to form 10 brings about an those used previously resulted in the formation of 12 and
upfield shift of the imidazolylidene C2–H and C5–H proton [PdCl2(bpy)] (13) in a 1:0.4 ratio (Scheme 4). This ratio did
signals and a shift of the resonance due to the carbon atom not change upon prolonged stirring. Crystallisation
originally bound to iodide. The chemical shift difference of attempts yielded a pure fraction of 13 yet induced signifi-
the inequivalent methylene groups increased from 0.03 ppm cant decomposition of 12. The identity of 13 was confirmed
1
to 0.23 ppm upon palladation, indicative of a change in unambiguously by H NMR spectroscopy and X-ray crys-
chemical environment due to metal coordination of one pic- tallography.[30] Formation of the dinuclear complex 12 is
olyl group only. Similarly, the difference in chemical shift of supported by ESI mass spectrometry, especially through the
the two sets of pyridyl protons became more pronounced characteristic isotope distribution pattern that correlates
and, notably, one set of signals was less sharp. This broad- well with a dipalladium species. While the instability of
ening was also observed in the 13C NMR spectrum and complex 12 precluded its isolation in pure form thus far,
Eur. J. Inorg. Chem. 2012, 1394–1402
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1397

A. Krüger, E. Kluser, H. Müller-Bunz, A. Neels, M. Albrecht
FULL PAPER
Conclusions
Abnormal imidazolylidene–palladium(II) complexes
were successfully synthesised by oxidative addition of iodo-
functionalised imidazolium salts to Pd(dba)2. The com-
plexes are air- and moisture-stable and were fully character-
ised by NMR spectroscopy and X-ray crystallography. Dif-
ferent functionalised wingtip groups, ranging from hard
–NEt2 to soft –SPh, were tolerated, and X-ray crystallogra-
phy and NMR spectroscopy provided evidence for chela-
tion in the solid state and also in solution. The asymmetry
of the bidentate ligand renders the trans positions on the
complex electronically inequivalent and may lead to inter-
esting reactivity of the complex. Since the oxidative ad-
dition protocol does not require the protection of the C2
position, a route to dimetallic complexes has been devised.
The generality of the C2 metallation provides vast opportu-
nities for incorporating a range of different transition met-
als into the dimetallic complex, which may be particularly
attractive for redox processes and for exploring synergistic
potentials, e.g. for inducing catalytic tandem transforma-
tions.
Scheme 4. Sequential metallation at C2 and C4 to form the dimet-
allic complex 12. Reagents and conditions: (i) Ag2O, DMSO/
CH2Cl2, r.t., 4 d, then [PdCl2(MeCN)2], DMSO/CH2Cl2/MeCN,
2.5 h; (ii) Pd(dba)2, 2,2-bipyridine, DMSO/MeCN, r.t., 2 d.
Experimental Section
NMR spectra of the crude reaction mixture were informa-
tive. Metallation of the C4 position of 11 led to an upfield
shift of the signal of the C5-bound hydrogen atom from δH
General Comments: 4-Iodoimidazole, 4-iodo-N-isopropylimidazole,
the imidazolium salt 1 and complex 5 were synthesised as reported
previously.[11–13] All other reagents are commercially available and
= 7.82 to δH = 7.06 ppm. The NCH2 signals appeared as were used as received. Standard Schlenk techniques were used in
two sets of AB doublets at δH = 5.94 and 5.87 ppm (2JH,H
the synthesis of compounds 5–8 and 10–13. Unless otherwise
= 15.4 Hz) and at δH = 5.73 and 5.66 ppm (2JH,H
=
stated, NMR spectra were recorded at 30 °C with Bruker and Var-
ian spectrometers operating at 400, 500 or 600 MHz (1H NMR)
and 100, 125 or 150 MHz [13C{1H} NMR], respectively. Chemical
shifts (δ in ppm, coupling constants J in Hz) were referenced to
residual solvent resonances. Assignments are based on homo- and
heteronuclear shift-correlation spectroscopy. Elemental analyses
were performed by the Microanalytical Laboratory at the Federal
Institute of Technology in Zurich, Switzerland and at the Univer-
sity College Dublin, Ireland.
15.3 Hz). 2D shift-correlation experiments revealed the
presence of five sets of pyridyl signals, one of which was
assigned to the bpy ligand in 13.[30] The remaining four sets
were attributed to the asymmetric bpy ligand and the two
picolyl groups of 12 (Figure 3). The two sets of picolyl sig-
nals remained sharp, suggesting coordination to the C2-
bound rather than to the C4-bound palladium centre. In
agreement with this notion, the change in chemical environ-
ment upon formation of the dimetallic complex induced
only a slight increase in chemical shift difference compared
with the corresponding shifts in 11.
Synthesis of 2: 4-Iodo-N-isopropylimidazole (2.2 g, 9.5 mmol), 2-
bromo-N,N-diethylamine hydrobromide (2.5 g, 9.5 mmol) and
NaHCO3 (1.2 g, 14 mmol) were stirred in refluxing EtOH (30 mL)
for 4 d. The colour of the reaction mixture changed from colourless
to yellow. After cooling to room temp., the solvent was removed in
vacuo, and the residue was dissolved in CH2Cl2 (100 mL) and fil-
tered through Celite. The solution was concentrated to 10 mL and
added to Et2O (100 mL) to precipitate the crude product. Tritura-
tion with acetone yielded 2 as a white hygroscopic solid (1.1 g, 26%
4
yield). 1H NMR ([D6]DMSO, 500 MHz): δ = 9.32 (d, JH,H
=
1.6 Hz, 1 H, Himi), 8.13 (d, 4JH,H = 1.6 Hz, 1 H, Himi), 4.66 (septet,
3JH,H = 6.7 Hz, 1 H, CHMe2), 4.11 (t, JH,H = 6.0 Hz, 2 H,
3
3
NCH2CH2N), 2.69 (t, JH,H = 6.0 Hz, 2 H, NCH2CH2N), 2.45 (q,
3JH,H = 7.0 Hz, 4 H, NCH2CH3), 1.46 [d, JH,H = 6.7 Hz, 6 H,
3
CH(CH3)2], 0.83 (t, 3JH,H = 7.0 Hz, 6 H, NCH2CH3) ppm. 13C{1H}
NMR ([D6]DMSO, 125 MHz): δ = 137.6, 126.4 (2ϫ Cimi), 80.7
(Cimi-I), 52.7 (CHMe2) 51.1 (NCH2CH2N), 48.6 (NCH2CH2N),
46.6 (NCH2CH3), 22.2 [CH(CH3)2], 12.0 (NCH2CH3) ppm.
C12H23BrIN3·0.5H2O (425.15): calcd. C 33.90, H 5.69, N 9.88;
found C 34.63, H 5.69, N 9.88.
1
Figure 3. Section of the 500 MHz H NMR spectrum of the crude
reaction mixture ([D6]DMSO solution) showing 12 and 13 in a
1:0.4 ratio (grey filled circles, grey open circles: py signals; black
filled diamonds: bpy signals of 12; grey filled squares: bpy signals
of 13).
Synthesis of 3: 4-Iodo-N-isopropylimidazole (0.24 g, 1.0 mmol) and
2-chloroethyl phenyl sulfide (0.3 mL, 2 mmol) were stirred at
1398
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1394–1402

Chelating C4-Bound Imidazolylidene Complexes
120 °C for 16 h. During this period, the reaction mixture changed
colour form light yellow to red. Et2O was added, and the formed
precipitate was isolated by decantation. The crude product was
purified by recrystallisation from MeCN and Et2O to yield 3 as a
CH(CH3)2] ppm. 13C{1H} NMR ([D6]DMSO, 125 MHz): δ = 133.2
(br., Cimi), 129.7 (br., Caryl), 129.3, 128.8 (2ϫ Caryl), 124.7 (br.,
Cimi), 51.0 (CHMe2), 47.9 (br., NCH2), 36.5 (br., SCH2), 22.5
[CH(CH3)2] ppm; carbene C and quaternary Caryl signals not re-
solved. C14H18ClIN2PdS (515.15): calcd. C 32.64, H 3.52, N 5.44;
light yellow hygroscopic solid (0.36 g, 88% yield). 1H NMR
4
([D6]DMSO, 500 MHz): δ = 9.45 (d, JH,H = 1.6 Hz, 1 H, Himi), found C 32.48, H 3.38, N 5.15.
8.03 (d, 4JH,H = 1.6 Hz, 1 H, Himi), 7.39–7.33 (m, 4 H, Haryl), 7.26–
Synthesis of 8a: A suspension of 4 (0.30 g, 0.87 mmol) and Pd-
3
7.21 (m, 1 H, Haryl), 4.56 (septet, JH,H = 6.7 Hz, 1 H, CHMe2),
(dba)2 (0.50 g, 0.87 mmol) in DMSO (10 mL) were stirred at room
temp. for 1 d. The solution was filtered through Celite and added
to EtOH (10 mL). The crude product was precipitated from the
solution by the addition of Et2O (100 mL) and isolated by centrifu-
gation. Recrystallisation from hot MeCN (30 mL) yielded 8a as a
yellow solid (120 mg, 30% yield). 1H NMR ([D6]DMSO,
500 MHz): δ = 8.86 (s, 1 H, Himi), 7.3 (br., 1 H, Himi), 4.51 (sept,
3JH,H = 6.6 Hz, 1 H, CHMe2), 4.4 (br., 2 H, NCH2), 2.8 (br., 2 H,
4.32 (t, 3JH,H = 6.6 Hz, 2 H, NCH2CH2N), 3.50 (t, 3JH,H = 6.6 Hz,
3
2 H, NCH2CH2N), 1.41 [d, J H,H = 6.7 Hz, 6 H, CH(CH3)2] ppm.
13C{1H} NMR ([D6]DMSO, 125 MHz): δ = 137.7 (Cimi), 133.9,
129.2, 128.7 (3ϫ Caryl), 126.5 (Cimi), 126.5 (Caryl), 82.2 (Cimi-I),
52.8 (CHMe2) 49.7 (NCH2CH2S), 31.7 (NCH2CH2S), 22.1
[CH(CH3)2] ppm. C14H18ClIN2S·H2O (426.74): calcd. C 39.40, H
4.72, N 6.56; found C 39.39, H 4.36, N 6.27.
3
SCH2), 2.70 (s, 3 H, SMe), 1.42 [d, JH,H = 6.6 Hz, 6 H, CH-
Synthesis of 4: 4-Iodo-N-isopropylimidazole (0.24 g, 1.0 mmol) and
2-chloroethyl methyl sulfide (0.2 mL, 2 mmol) were stirred at 60 °C
for 2 h and then at 80 °C for 16 h. The yellowish reaction mixture
was cooled to room temp., and Et2O (5 mL) was added upon which
a white precipitate immediately formed. The mixture was stirred
for 10 min and then left to settle. The formed precipitate was iso-
lated by decantation. The crude product was purified by recrystalli-
sation from warm MeCN to yield 4 as an off-white solid (81 mg,
(CH3)2] ppm. 13C{1H} NMR ([D6]DMSO, 125 MHz): δ = 132.6
(Cimi), 129.2 (br., Cimi), 123.0 (br., Cimi-Pd), 50.8 (CHMe2), 47.9
(NCH2), 32.4 (SCH2), 22.4 [CH(CH3)2], 22.0 (SMe) ppm.
C9H16ClIN2PdS (453.08): calcd. C 23.86, H 3.56, N 6.18; found C
23.91, H 3.71, N 6.16.
Synthesis of 8b: Complex 8a (80 mg, 0.18 mmol) and an excess of
NaI (0.75 g) were stirred in acetone (100 mL) at room temp. for
16 h. The solvent was removed in vacuo and the residue redissolved
in hot MeCN (20 mL) and filtered. Cooling of the filtrate to –30 °C
1
23% yield). H NMR (CDCl3, 360 MHz): δ = 10.88 (s, 1 H, Himi),
3
7.44 (s, 1 H, Himi), 4.82 (sept, JH,H = 6.8 Hz, 1 H, CHMe2), 4.58
3
3
(t, JH,H = 6.4 Hz, 2 H, NCH2CH2N), 2.98 (t, JH,H = 6.4 Hz, 2
1
yielded orange crystals of 8b (38 mg, 39% yield). H NMR ([D6]-
3
H, NCH2CH2N), 2.28 (s, 3 H, SMe), 1.63 [d, JH,H = 6.8 Hz, 6 H,
DMSO, 500 MHz): δ = 8.86 (s, 1 H, Himi), 7.4 (br., 1 H, Himi), 4.50
CH(CH3)2] ppm. 13C{1H} NMR (CDCl3, 90 MHz): δ = 139.3,
125.7 (2ϫ Cimi), 80.3 (Cimi-I), 54.0 (CHMe2) 49.3 (NCH2CH2S),
33.8 (NCH2CH2S), 23.1 [CH(CH3)2], 15.8 (SMe) ppm.
C9H16ClIN2S (346.66): calcd. C 31.18, H 4.65, N 8.08; found C
31.31, H 4.73, N 8.01.
(sept, 3JH,H = 6.7 Hz, 1 H, CHMe2), 4.4 (br., 2 H, NCH2), 2.8 (br.,
3
2 H, SCH2), 2.77 (s, 3 H, SMe), 1.41 [d, JH,H = 6.7 Hz, 6 H,
CH(CH3)2] ppm. 13C{1H} NMR ([D6]DMSO, 125 MHz): δ = 133.2
(Cimi), 129.2 (Cimi), 51.4 (CHMe2), 48.5 (NCH2), 33.2 (SCH2), 25.1
(SMe), 23.0 [CH(CH3)2] ppm; carbene C signal not resolved.
C9H16I2N2PdS (544.53): calcd. C 19.85, H 2.96, N 5.14; found C
19.4, H 2.82, N 5.00.
Synthesis of 6: A suspension of 2 (0.18 g, 0.39 mmol) in dry CH2Cl2
(15 mL) was cooled to 0 °C and added to Pd(dba)2 (0.23 g,
0.40 mmol). The reaction mixture, which immediately changed col-
our from colourless to deep red, was allowed to reach room temp.
and stirred for 2 d. A yellow precipitate gradually formed. The re-
action mixture was concentrated to 7 mL and filtered through Ce-
lite. The residue was washed with CH2Cl2 and subsequently ex-
tracted with MeCN. The combined MeCN fractions were concen-
trated, and the residue was triturated with Et2O and dried in vacuo
to yield 6 as a yellow solid (38 mg, 18% yield). 1H NMR ([D6]-
DMSO, 500 MHz): δ = 8.90 (s, 1 H, Himi), 7.13 (s, 1 H, Himi), 4.49
Synthesis of 9a: 4-Iodoimidazole (2.9 g, 15 mmol), 2-(bro-
momethyl)pyridine hydrobromide (7.6 g, 30 mmol) and NaHCO3
(3.8 g, 45 mmol) were suspended in EtOH (80 mL) and heated to
reflux for 3 d. The colour of the reaction mixture turned from
colourless to pink within a few minutes. At room temp. the reaction
mixture was filtered and washed with EtOH, and the red solution
was concentrated in vacuo. A precipitate formed, which was iso-
lated by filtration and repeatedly washed with CH2Cl2 and acetone
to yield 9a as an off-white solid (1.7 g, 25% yield). 1H NMR
3
4
(sept, JH,H = 6.6 Hz, 1 H, CHMe2), 4.27 (br., 2 H, NCH2), 3.3,
([D6]DMSO, 500 MHz): δ = 9.59 (d, JH,H = 1.4 Hz, 1 H, Himi),
3
3
3
3.0 (br., 4 H, NCH2CH3), 2.8 (br., 2 H, NCH2), 1.40 [d, JH,H
=
8.59 (d, JH,H = 4.8 Hz, 1 H, Hpy), 8.53 (d, JH,H = 4.8 Hz, 1 H,
6.6 Hz, 6 H, CH(CH3)2], 1.4 (br., 6 H, NCH2CH3) ppm. 13C{1H}
NMR ([D6]DMSO, 125 MHz): δ = 132.4, 124.6 (br., 2ϫ Cimi), 53.2
(NCH2CH3), 51.9 (NCH2), 51.6 (CHMe2), 46.4 (NCH2), 22.5
[CH(CH3)2], 11.7 (NCH2CH3) ppm; carbene C signal not resolved.
HR-MS (ESI+): calcd. for C12H23IN3Pd [M – Br]+ 441.9971; found
441.9990. C12H23BrIN3Pd·0.25Et2O (541.09): calcd. C 28.86, H
4.75, N 7.77; found C 28.77, H 4.55, N 7.82.
4
Hpy), 8.05 (d, JH,H = 1.4 Hz, 1 H, Himi), 7.94–7.87 (m, 2 H, Hpy),
7.53–7.47 (m, 2 H, Hpy), 7.45–7.37 (m, 2 H, Hpy), 5.64 (s, 2 H,
NCH2), 5.61 (s, 2 H, NCH2) ppm. 13C{1H} NMR ([D6]DMSO,
125 MHz): δ = 153.2, 152.8 (2ϫ Cpy), 149.6, 149.5 (2ϫ Cpy), 140.4
(Cimi), 137.5, 137.3 (2ϫ Cpy), 129.4 (Cimi), 123.7, 123.5 (2ϫ Cpy),
122.5, 122.4 (2ϫ Cpy), 81.0 (C–I), 53.9, 53.4 (2ϫ NCH2) ppm.
C15H14BrIN4 (457.11): calcd. C 39.41, H 3.09, N 12.26; found C
39.16, H 3.13, N 12.02.
Synthesis of 7: A solution of 3 (0.31 g, 0.76 mmol) in dry DMSO
(10 mL) was added to Pd(dba)2 (0.44 g, 0.76 mmol) and stirred at
room temp. for 2 d. The reaction mixture was filtered through Ce-
Synthesis of 9b: Compound 9a (1.7 g, 3.6 mmol) was dissolved in
MeOH and filtered through a Dowex ion-exchange resin (1ϫ4
lite, and the filtrate was added to a 1:1 mixture of Et2O/CH2Cl2. chloride form, 100–200 mesh). After removal of the solvent in
The formed precipitate was isolated by centrifugation, washed with
vacuo, 9b was obtained as a light yellow solid (1.1 g, 72% yield).
4
CH2Cl2 and dried in vacuo, thus affording 7 as a yellow solid
1H NMR (CD3OD, 600 MHz): δ = 9.54 (d, JH,H = 1.3 Hz, 1 H,
1
3
3
(0.12 g, 31% yield). H NMR ([D6]DMSO, 500 MHz): δ = 8.89 (s, Himi), 8.58 (d, JH,H = 4.6 Hz, 1 H, Hpy), 8.52 (d, JH,H = 4.6 Hz,
3
1 H, Himi), 8.1–7.7 (m, 2 H, Haryl), 7.55–7.40 (m, 3 H, Haryl), 7.3 1 H, Hpy), 7.91–7.86 (m, 3 H, Himi, Hpy), 7.59 (d, JH,H = 7.8 Hz,
3
3
(br., 1 H, Himi), 4.52 (sept, JH,H = 6.7 Hz, 1 H, CHMe2), 4.3 (br.,
1 H, Hpy), 7.52 (d, JH,H = 7.8 Hz, 1 H, Hpy), 7.45–7.37 (m, 2 H,
2 H, NCH2), 3.2 (br., 2 H, SCH2), 1.42 [d, JH,H = 6.7 Hz, 6 H,
Hpy), 5.65 (s, 2 H, NCH2), 5.62 (s, 2 H, NCH2) ppm. 13C{1H}
3
Eur. J. Inorg. Chem. 2012, 1394–1402
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1399

A. Krüger, E. Kluser, H. Müller-Bunz, A. Neels, M. Albrecht
tion mixture was stirred for 2.5 h, filtered through Celite, and the
FULL PAPER
NMR ([D6]DMSO, 125 MHz): δ = 154.0, 153.6 (2ϫ Cpy), 151.0,
1
150.9 (2ϫ Cpy), 141.4 (t, JD,C = 32.8 Hz, Cimi), 139.2, 138.9 (2ϫ filtrate was concentrated in vacuo. The concentrated solution was
Cpy), 131.1 (Cimi), 125.3, 125.0 (2ϫ Cpy), 124.3, 124.0 (2ϫ Cpy),
79.6 (C–I), 55.7, 55.2 (NCH2) ppm. C15H14ClIN4 (412.66): calcd.
C 43.66, H 3.42, N 13.58; found C 43.36, H 3.35, N 13.29.
added to a 1:1 mixture of Et2O/CH2Cl2 (50 mL). The product pre-
cipitated as a yellow solid, which was isolated by filtration and
dried in vacuo (330 mg, 60% yield). An analytically pure sample
1
was obtained by recrystallisation of 11 from CHCl3 and Et2O. H
Synthesis of 10: A suspension of 9a (0.23 g, 0.50 mmol) and
Pd(dba)2 (0.29 g, 0.50 mmol) in dry CH2Cl2 (15 mL) was stirred at
room temp. for 5 d. A yellow precipitate gradually formed. The
reaction mixture was filtered through Celite, and the residue was
washed with CH2Cl2 and cold MeCN. The product was then ex-
tracted from the residue with warm DMSO and added to a 5:1
mixture of Et2O and CH2Cl2. The formed precipitate was isolated
by centrifugation and dried in vacuo as a yellowish solid (168 mg,
60% yield). 1H NMR ([D6]DMSO, 500 MHz): δ = 9.02 (s, 1 H,
NMR ([D6]DMSO, 600 MHz): δ = 9.41–9.38 (m, 2 H, Hpy), 8.27–
3
8.22 (m, 2 H, Hpy), 8.14 (d, JH,H = 7.6 Hz, 1 H, Hpy) 7.89 (d,
3JH,H = 7.0 Hz, 1 H, Hpy), 7.82 (s, 1 H, Himi), 7.73–7.68 (m, 2 H,
H
py), 5.71 (s, 2 H, NCH2), 5.61 (s, 2 H, NCH2) ppm. 13C{1H}
NMR ([D6]DMSO, 150 MHz): δ = 155.8, 155.7, 152.5, 152.3 (4ϫ
py), 151.3 (Ccarbene), 141.4, 141.3 (Cpy), 127.9 (Cimi), 126.9, 126.6,
C
125.5, 125.3 (4ϫ Cpy), 75.9 (Cimi-I), 54.6, 53.9 (2ϫ NCH2) ppm.
HR-MS (ESI+): calcd. for C15H13IN4Pd [M – Cl]+ 516.8908; found
516.8906. C15H13Cl2IN4Pd·CHCl3 (672.90): calcd. C 28.56, H 2.10,
N 8.33; found C 28.59, H 1.92, N 8.47.
Himi), 8.9 (br., 1 H, Hpy), 8.56–8.50 (m, 1 H, Hpy), 8.20–8.08 (m, 1
H, Hpy) 7.88–7.81 (m, 1 H, Hpy), 7.81–7.74 (m, 1 H, Hpy), 7.66–
7.56 (m, 1 H, Hpy), 7.47–7.41 (m, 1 H, Hpy), 7.40–7.33 (m, 1 H,
Hpy), 7.13 (s, 1 H, Himi), 5.65 (s, 2 H, NCH2), 5.42 (s, 2 H, NCH2)
ppm. 13C{1H} NMR ([D6]DMSO, 125 MHz): δ = 154.0, 151.8,
149.6, 140.4 (br), 137.5 (5ϫ Cpy), 134.6, 128.5 (2ϫ Cimi), 125.2,
125.0 (br., 2ϫ Cpy), 123.6, 122.7 (2ϫ Cpy), 53.9 (br., NCH2), 53.4
(NCH2) ppm; carbene C and pyridyl C6 signals not resolved.
C15H14BrIN4Pd·0.5DMSO (602.59): calcd. C 31.89, H 2.84, N
9.30; found C 31.65, H 2.49, N 9.42.
Synthesis of 12: A suspension of 11 (19 mg, 0.035 mmol), Pd(dba)2
(20 mg, 0.035 mmol) and 2,2Ј-bipyridine (5.5 mg, 0.035 mmol) was
stirred in DMSO (2 mL) at room temp. for 1 d. The reaction mix-
ture was filtered through Celite, and the filtrate was added to a
1:1 mixture of CH2Cl2/Et2O (20 mL). The formed precipitate was
isolated by centrifugation and dried in vacuo. The light yellow
crude product contained 12 and 13 in 2.5:1 molar ratio (20 mg).
3
1H NMR ([D6]DMSO, 600 MHz): δ = 9.48 (d, JH,H = 5.8 Hz, 1
H, HpyA), 9.45 (d, JH,H = 5.7 Hz, 1 H HpyB), 9.1 (br., 1 H, Hbpy),
3
Synthesis of 11: A solution of 9b (0.41 g, 1.0 mmol) in a mixture
of dry CH2Cl2 (35 mL) and DMSO (15 mL) was added to Ag2O
(0.14 g, 0.60 mmol). The reaction mixture was stirred at room
temp. for 4 d protected from light, after which it was filtered and
8.69–8.63 (m, 2 H, Hbpy), 8.42–8.37 (m, 1 H, Hbpy), 8.33–8.27 (m,
1 H, Hbpy), 8.27–8.22 (m, 1 H, HpyA), 8.17–8.12 (m, 1 H, HpyB),
3
8.1 (br., 1 H, Hbpy), 7.93 (d, 1 H, JH,H = 7.6 Hz, HpyA), 7.91–7.87
3
transferred to a suspension of [PdCl2(MeCN)2] (0.26 g, 1.0 mmol) (m, 1 H, Hbpy), 7.86 (d, 1 H, JH,H = 7.6 Hz, HpyB), 7.73–7.63 (m,
in a 1:1 mixture of CH3CN/CH2Cl2 (20 mL). The reaction mixture 1 H, HpyA), 7.68–7.63 (m, 1 H, HpyB), 7.53–7.47 (m, 1 H, Hbpy),
2
immediately became clear and orange, and a white precipitate
started to form slowly, while the solution became yellow. The reac-
7.06 (s, 1 H, Himi), 5.94 (d, 1 H, JH,H = 15.4 Hz, NCH,HA), 5.87
(d, 1 H, 2JH,H = 15.4 Hz, NCH,HA), 5.73 (d, 1 H, 2JH,H = 15.3 Hz,
Table 2. Crystallographic data for complexes 6, 7, 8b and 10.
6
7
8b
10
Colour
Shape
orange
rod
yellow
plate
yellow,
plate
yellow
plate
Crystal size [mm]
Empirical formula
Formula mass
T [K]
Crystal system
Space group
Unit cell
0.38ϫ0.18ϫ0.13
C12H32BrIN3Pd·C2H6OS C14H18ClIN2PdS
600.67
100(2)
monoclinic
P21/c (# 14)
0.05ϫ0.03ϫ0.01
0.20ϫ0.20ϫ0.03
C9H16I2N2Pd·C2H6OS
622.63
173(2)
monoclinic
P21/c (# 14)
0.17ϫ0.14ϫ0.03
C15H14BrIN4Pd
563.51
515.11
100(2)
monoclinic
P21/c (# 14)
100(2)
triclinic
¯
P1 (# 2)
a [Å]
b [Å]
c [Å]
α [°]
12.8889(9)
18.4942(12)
8.6701(6)
90
105.135(1)
90
1995.0(2)
4
2.000
10.0971(5)
8.5516(4)
19.3799(9)
90
96.279(4)
90
1663.35(14)
4
2.057
13.1246(16)
8.3281(6)
17.658(2)
90
105.579(9)
90
1859.1(3)
4
2.224
8.8721(1)
8.9579(2)
10.5343(2)
91.309(1)
92.488(1)
91.114(1)
836.04(3)
2
β [°]
γ [°]
V [Å3]
Z
Dcalcd. [gcm–3]
2.239
26.255
μ [mm–1]
4.590
26.146
4.539
Total reflections
Unique reflections
Rint
47788
6618
0.0315
0.587–0.327
206, 0
0.0375, 0.1032
1.067
8851
2378
0.0415
0.461–0.703
184, 0
0.0479, 0.1287
1.044
23384
5028
0.0615
0.406–0.201
185, 0
0.0340, 0.0782
0.977
23746
3462
0.0624
0.507–0.056
200, 0
0.0287, 0.0808
1.067
Transmittance range
Parameters, restraints
[b]
R,[a] Rw
GOF
Largest hole, peak
–1.606, 2.855
–1.316, 0.823
–1.800, 0.805
–1.269, 1.690
[eÅ–3]
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo| for all I Ͼ 2σ(I). [b] wR2 = {Σw(Fo – Fc ) /Σ[w(Fo4)]1/2}.
2
2 2
1400
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1394–1402

Chelating C4-Bound Imidazolylidene Complexes
2
NCH,HB), 5.66 (d, 1 H, JH,H = 15.3 Hz, NCH,HB) ppm. A and
[5]
[6]
a) A. R. Chianese, B. M. Zeglis, R. H. Crabtree, Chem. Com-
mun. 2004, 2176–2177; b) A. Prades, M. Viciano, M. Sanau,
E. Peris, Organometallics 2008, 27, 4354–4259.
B denote the two picoline residues in 12. HR-MS (ESI+): calcd. for
C25H21Cl2N6Pd2 [M – I]+ 688.9323; found 688.9357.
In some cases, steric constraints enforce an abnormal coordina-
tion mode of imidazolylidenes; for examples, see: a) X. Hu, I.
Castro-Rodriguez, K. Meyer, Organometallics 2003, 22, 3016–
3018; b) A. A. Danopoulos, N. Tsoureas, J. A. Wright, M. E.
Light, Organometallics 2004, 23, 166–168; c) N. Stylianides,
A. A. Danopoulos, N. Tsoureas, J. Organomet. Chem. 2005,
690, 5948–5958; d) C. E. Ellul, M. F. Mahon, O. Saker, M. K.
Whittlesey, Angew. Chem. 2007, 119, 6459; Angew. Chem. Int.
Ed. 2007, 46, 6343–6345; e) C. E. Cooke, M. C. Jennings, R. K.
Pomeroy, J. A. Clyburne, Organometallics 2007, 26, 6059–6062.
P. J. Fraser, W. R. Roper, R. G. A. Stone, A. Gordon, J. Chem.
Soc., Dalton Trans. 1974, 760–764.
a) O. Guerret, S. Sole, H. Gornitzka, M. Teichert, G. Trinquier,
G. Bertrand, J. Am. Chem. Soc. 1997, 119, 6668–6669; b) O.
Guerret, S. Sole, H. Gornitzka, G. Trinquier, G. Bertrand, J.
Organomet. Chem. 2000, 600, 112–117; c) E. Mas-Marza, J. A.
Mata, E. Peris, Angew. Chem. 2007, 119, 3803; Angew. Chem.
Int. Ed. 2007, 46, 3729–3731; d) A. Zanardi, R. Coberan, J. A.
Mata, E. Peris, Organometallics 2008, 27, 3570–3576; e) A.
Zanardi, J. A. Mata, E. Peris, Organometallics 2009, 28, 4335–
4339.
a) P. L. Arnold, S. T. Liddle, Organometallics 2006, 25, 1485–
1491; b) U. J. Scheele, S. Dechert, F. Meyer, Chem. Eur. J. 2008,
14, 5112–5115; c) D. Mendoza-Espinosa, B. Donnadieu, G.
Bertrand, J. Am. Chem. Soc. 2010, 132, 7264–7265; d) R. S.
Ghadwal, H. W. Roesky, M. Granitzka, D. Stalke, J. Am.
Chem. Soc. 2010, 132, 10018–10020; e) Y. Wang, Y. Xie, M. Y.
Abraham, P. Wei, H. F. Schaefer III, P. v. R. Schleyer, G. H.
Robinson, J. Am. Chem. Soc. 2010, 132, 14370–14372.
A. Zanardi, J. A. Mata, E. Peris, J. Am. Chem. Soc. 2009, 131,
14531–14537.
Structure Determination and Refinement of 6, 7, 8b and 10:[17] Suit-
able single crystals were mounted on a Bruker SMART APEX
CCD diffractometer (6) with a D8 goniometer and a graphite-mon-
ochromator (Mo-Kα radiation, λ = 0.71073 Å), on an Agilent Su-
perNova A diffractometer (7 and 10) with a mirror-monochroma-
tor (Cu-Kα radiation, λ = 1.54184 Å), or on a Stoe Mark II-Image
Plate Diffraction System (8b) with a graphite-monochromator
(Mo-Kα radiation, λ = 0.71073 Å). A semi-empirical absorption
correction was applied for 6 by using SADABS[31] and for 8b by
using MULscanABS as implemented in PLATON.[32] An analytical
numeric absorption correction by using a multifaceted crystal
model was applied for 7 and 10.[33] The structures were solved by
direct methods using the program SHELXS-97[34] and refined by
full-matrix least squares on F2 with SHELXL-97. The hydrogen
atoms were included in calculated positions and treated as riding
atoms by using SHELXL-97 default parameters. All non-hydrogen
atoms were refined anisotropically. Crystals of complex 6 contained
one DMSO molecule per complex molecule. Crystals of 8b con-
tained one disordered DMSO molecule per asymmetric unit, which
was refined with occupancies of 0.5 for all participating atoms. In
6, 7 and 10 the iodide and bromide (7 and 10) and the iodide and
chloride (6) were partially occupied. The major component con-
tained the iodide ligand cis to the carbene with a site occupation
factor of 0.708(3) in 6, 0.556(3) in 7 and 0.778(3) in 10. The minor
component contained the iodide trans to the carbene with a site
occupation factor of 0.292(3) in 6, 0.444(3) in 7 and 0.222(3) in 10.
The sum of the site occupation factors of the major and minor
components were constrained to be 1. Further details on data col-
lection and refinement are summarised in Table 2. CCDC-843008, -
843009, -843010, -843011, and -843012 for complexes 6, 7, 8a, 8b,
and 10, respectively, contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[7]
[8]
[9]
[10]
[11]
[12]
E. Kluser, A. Neels, M. Albrecht, Chem. Commun. 2006, 4495–
4497.
a) H. Pauly, E. Arauner, J. Prakt. Chem. 1928, 118, 33–47; b)
H. B. Bensusan, M. S. R. Naidu, Biochemistry 1967, 6, 12–15;
c) K. J. Brunings, J. Am. Chem. Soc. 1947, 69, 205–208.
A. A. Gridnev, I. M. Mihaltseva, Synth. Commun. 1994, 24,
1547–1555.
A. A. D. Tulloch, A. A. Danopoulos, S. Winston, S. Kleinhenz,
G. Eastham, J. Chem. Soc., Dalton Trans. 2000, 4499–4506.
Halide scrambling in carbene–metal complexes has precedents;
see, for example: C.-C. Ho, S. Chatterjee, T.-L. Wu, K.-T.
Chan, Y.-W. Chang, T.-H. Hsiao, H. M. Lee, Organometallics
2009, 28, 2837–2847.
a) A. A. D. Tulloch, A. A. Danopoulos, R. P. Tooze, S. M. Caf-
ferkey, S. Kleinhenz, M. B. Hursthouse, Chem. Commun. 2000,
1247–1248; b) A. A. D. Tulloch, S. Winston, A. A. Dano-
poulos, G. Eastham, M. B. Hursthouse, Dalton Trans. 2003,
699–708.
A crystal structure determination of 8a revealed a 100% occu-
pancy of the iodide position, yet the site of the chloride is par-
tially occupied by iodide (34%), presumably due to partial hal-
ide metathesis with unreacted ligand precursor imidazolium
salt. See CCDC-843010 for crystal structure determination, re-
finement and further crystallographic details.
a) R. F. Hartley, Chem. Soc. Rev. 1973, 2, 163–179; b) the
stronger trans influence of the NHC vs. SR2 is demonstrated
by the different Pd–I bond lengths in 8b: Pd–I1 2.6350(5) Å
trans to SR2, and Pd–I2 2.6962(5) Å trans to NHC.
a) A. Poulain, M. Iglesias, M. Albrecht, Curr. Org. Chem. 2011,
15, 3325–3336; b) M. Heckenroth, A. Neels, M. G. Garnier, P.
Aebi, A. W. Ehlers, M. Albrecht, Chem. Eur. J. 2009, 15, 9375–
9386.
a) V. J. Catalano, A. O. Etogo, Inorg. Chem. 2007, 46, 5608–
5615; b) P. L. Chiu, C. L. Lai, C. F. Chang, C. H. Hu, H. M.
Lee, Organometallics 2005, 24, 6169–6178; c) S. Ahrens, A.
Zeller, M. Taige, T. Strassner, Organometallics 2006, 25, 5409–
[13]
[14]
[15]
Acknowledgments
We thank the Swiss National Science Foundation, the European
Research Council, and the Science Foundation Ireland for generous
financial support.
[16]
[17]
[1] a) P. L. Arnold, S. Pearson, Coord. Chem. Rev. 2007, 251, 596–
609; b) M. Albrecht, Chem. Commun. 2008, 3601–3610; c) O.
Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem.
Rev. 2009, 109, 3445–3478; d) M. Melami, M. Soleilhavoup, G.
Bertrand, Angew. Chem. Int. Ed. 2010, 49, 8810.
[2] a) D. Bacciu, K. J. Cavell, I. A. Fallis, L. Ooi, Angew. Chem.
2005, 117, 5416; Angew. Chem. Int. Ed. 2005, 44, 5282–5284;
b) M. Heckenroth, E. Kluser, A. Neels, M. Albrecht, Angew.
Chem. 2007, 119, 6409; Angew. Chem. Int. Ed. 2007, 46, 6293–
6296; c) A. Krüger, M. Albrecht, in N-heterocyclic Carbenes:
From Laboratory Curiosities to Efficient Synthetic Tools (Ed.:
S. Diez-Gonzalez), RSC, Cambridge, UK, 2011, pp. 134–165.
[3] a) G. Sini, O. Eisenstein, R. H. Crabtree, Inorg. Chem. 2002,
41, 602–604; b) R. Tonner, G. Heydenrych, G. Frenking, Chem.
Asian J. 2007, 2, 1555–1567; c) E. Aldeco-Perez, A. J. Rosen-
thal, B. Donnadieu, P. Parameswaran, G. Frenking, G. Ber-
trand, Science 2009, 326, 556–559; d) G. Ung, G. Bertrand,
Chem. Eur. J. 2011, 17, 8269.
[18]
[19]
[20]
[4] a) P. Mathew, A. Neels, M. Albrecht, J. Am. Chem. Soc. 2008,
130, 13534–13535; b) M. Heckenroth, E. Kluser, A. Neels, M.
Albrecht, Dalton Trans. 2008, 6242–6249.
Eur. J. Inorg. Chem. 2012, 1394–1402
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1401

A. Krüger, E. Kluser, H. Müller-Bunz, A. Neels, M. Albrecht
FULL PAPER
5415; d) T. Scherg, S. K. Schneider, G. D. Frey, J. Schwarz, E.
Herdtweck, W. A. Herrmann, Synlett 2006, 2894–2907; e)
H. M. Lee, C. Y. Lu, C. Y. Chen, W. L. Chen, H. C. Lin, P. L.
Chiu, P. Y. Chen, Tetrahedron 2004, 60, 5807–5825.
[21] The abnormal NHC ligand may formally be divided into a
positively charged amidinium component and an anionic vin-
ylic unit: a) A. Krüger, M. Albrecht, Aust. J. Chem. 2011, 64,
1113–1117; b) A. Krüger, L. J. L. Häller, H. Müller-Bunz, O.
Serada, A. Neels, S. A. Macgregor, M. Albrecht, Dalton Trans.
2011, 40, 9911–9920.
[25]
[26]
a) L. Yang, A. Krüger, A. Neels, M. Albrecht, Organometallics
2008, 27, 3161–6171; b) A. Krüger, A. Neels, M. Albrecht,
Chem. Commun. 2010, 46, 315–317; c) G. Buscemi, M. Basato,
A. Biffis, A. Gennaro, A. A. Isse, M. M. Natile, C. Tubaro, J.
Organomet. Chem. 2010, 695, 2359–2365.
A. M. Magill, D. S. McGuinness, K. J. Cavell, G. J. P. Britov-
sek, V. C. Gibson, A. J. P. White, D. J. Williams, A. H. White,
B. W. Skelton, J. Organomet. Chem. 2001, 617–618, 546–560.
H. M. J. Wang, I. J. B. Lin, Organometallics 1998, 17, 972–975.
W. A. Herrmann, C.-P. Reisinger, M. Spiegler, J. Organomet.
Chem. 1998, 557, 93–96.
S. T. Handy, M. Okello, J. Org. Chem. 2005, 70, 1915–1918.
H. S. Sahoo, D. K. Chand, S. Mahalakshmi, H. Mir, R.
Raghunathan, Tetrahedron Lett. 2007, 48, 761–765.
SADABS, Bruker AXS Inc., Madison, Wisconsin, USA, 2001.
A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13.
[27]
[28]
[22] The site occupancy factor of the atoms has been taken into
account in the calculation of the sum of the van der Waals ra-
dii.
[29]
[30]
[23] S.-T. Liu, C.-I. Lee, C.-F. Fu, C.-H. Chen, Y.-H. Liu, C. Elsev-
ier, S.-M. Peng, J.-T. Chen, Organometallics 2009, 28, 6957–
6962.
[31]
[32]
[33]
1
1
[24] Assignment of the H NMR signals of 5 has been revised: H
NMR ([D6]DMSO, 500 MHz): δ = 9.25 (br., 1 H, Hpy), 8.97
(s, 1 H, Himi), 8.14 (m, 1 H, Hpy), 7.79 (m, 1 H, Hpy), 7.61 (m,
1 H, Hpy), 7.15 (s, 1 H, Himi), 5.58 (s, 2 H, NCH2), 4.51 (sept,
R. C. Clark, J. S. Reid, Acta Crystallogr., Sect. A 1995, 51, 887–
897.
G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
Received: September 6, 2011
[34]
3JH,H = 6.4 Hz, 1 H, CHMe2), 1.39 [d, JH,H = 6.4 Hz, 6 H,
3
CH(CH3)2] ppm.
Published Online: November 10, 2012
1402
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1394–1402