Synthesis, structural characterisation and stereochemical investigation of chiral sulfur-functionalised N-heterocyclic carbene complexes of palladium and platinum

Article abstract of DOI:10.1002/chem.201204320

Palladium and platinum complexes containing a sulfur-functionalised N-heterocyclic carbene (S-NHC) chelate ligand have been synthesised. The absolute conformations of these novel organometallic S-NHC chelates were determined by X-ray structural analyses and solution-phase 2D ¹H-¹H ROESY NMR spectroscopy. The structural studies revealed that the phenyl substituents on the stereogenic carbon atoms invariably take up the axial positions on the Pd-C-S coordination plane to afford a skewed five-membered ring structure. All of the chiral complexes are structurally rigid and stereochemically locked in a chiral ring conformation that is either (R_s,S,R)-λ or (S_s,R,R)-δ in both the solid state and solution. Copyright

Full text of DOI:10.1002/chem.201204320

DOI: 10.1002/chem.201204320
Synthesis, Structural Characterisation and Stereochemical Investigation of
Chiral Sulfur-Functionalised N-Heterocyclic Carbene Complexes of
Palladium and Platinum
Deepa Krishnan, Sumod A. Pullarkat, Meiyi Wu, Yongxin Li, and Pak-Hing Leung*[a]
Abstract: Palladium and platinum com-
plexes containing a sulfur-functional-
ised N-heterocyclic carbene (S-NHC)
chelate ligand have been synthesised.
The absolute conformations of these
novel organometallic S-NHC chelates
were determined by X-ray structural
tural studies revealed that the phenyl
substituents on the stereogenic carbon
atoms invariably take up the axial posi-
tions on the Pd-C-S coordination plane
to afford a skewed five-membered ring
structure. All of the chiral complexes
are structurally rigid and stereochemi-
cally locked in a chiral ring conforma-
tion that is either (Rs,S,R)-l or
(Ss,R,R)-d in both the solid state and
solution.
Keywords: carbenes · nitrogen het-
erocycles · palladium · platinum ·
sulfur
1
analyses and solution-phase 2D H–1H
ROESY NMR spectroscopy. The struc-
Introduction
and steric properties of the sulfur atom, it is possible to
attain an optically active and configurationally stable sulfur-
functionalised metallacycle.[10] Thus, our interest lies in the
development of a stereochemically rigid five-membered
chiral C,S chelate NHC (Figure 1), keeping in mind the pos-
sibility that the transmission of chirality from the chiral
carbon atom will occur through the sulfur moiety onto the
neighbouring coordination site, which would afford a higher
degree of asymmetric induction.
Since the isolation of the first stable crystalline N-heterocy-
clic carbenes (NHCs) by Arduendo et al.,[1] a rapid develop-
ment of NHCs as an interesting class of ligands in homoge-
nous catalysis has occurred.[2] Transition-metal complexes in-
corporating NHCs in a chelating mode, such as in a metalla-
cycle, are relatively less prone to reductive elimination than
monodentate NHCs, due to the stability imposed by the che-
late ring.[3] Furthermore, chelation also provides conforma-
tional rigidity in chiral variants. This is of critical importance
in their application as catalysts in asymmetric synthesis.
Hence, several groups have focused on the synthesis of func-
tionalised chelating E-NHCs (E: P,[4] N,[4a,5] or O[6]) associat-
ed with hemilabile donors and their potential applications in
various metal-catalysed reactions.[2h,7] In comparison with
those for other donor-functionalised NHCs, the synthetic
protocols for S-NHC metallacycle complexes are quite rare,
although the ability of these complexes in various catalytic
transformations has been demonstrated.[8] An important
aspect of S-NHC metallacycles is that, upon chelation to
metal, the sulfur atom exhibits stereogenicity, which can
augment the stereocontrol provided by the backbone chirali-
ty in an asymmetric-synthesis scenario. However, in most
cases, this stereogenicity on the sulfur atom is not configura-
tionally stable and tends to undergo a relatively facile py-
Figure 1. The skeletal structure of the five-membered C,S chelate ring.
As an extension of our on-going studies on the develop-
ment and application of metallacycles in asymmetric synthe-
sis,[11] we became interested in the structural behaviour of
hetero-bidentate NHC metal complexes. Such behaviour in
the PdII/PtII complexes of S-NHCs in solution has not been
much explored, especially with relation to the stereochemi-
cal rigidity in chiral variants. We report herein the synthesis
of chiral and configurationally stable sulfur-containing NHC
complexes of PdII and PtII. The complexes reported in the
current study have been prepared from the (R)-3-[(tert-
ACHTUNGTRENNUNGramACHTUNGTRENNUNG
idal inversion.[9] Apparently, by changing the electronic
butylthio)
ium bromide ligand (2; Scheme 1), and the resultant dia-
stereomeric metallacycles 4 and 5 have been resolved by
fractional crystallisation. A detailed structural study in both
the solid state and solution further revealed critical factors
that impact on the stability and configurational rigidity in
this class of metallacycles.
ACHTUNGTREN(NUGN phenyl)methyl]-1-(1-phenylethyl)-1H-imidazol-3-
[a] Dr. D. Krishnan, Dr. S. A. Pullarkat, M. Wu, Dr. Y. Li,
Prof. P. H. Leung
G
ACHTUNGTRENNUNG
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University, 21 Nanyang Link
Singapore 637371 (Singapore)
E-mail: pakhing@ntu.edu.sg
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FULL PAPER
stereoACHTNUTRGNEUNGgenic centres at C12 and
S1 in the Pd-C-S coordination
plane are shown in Figure 2.
The separation of the two iso-
mers, (Rs,S,R)-4 and (Ss,R,R)-4,
was based on their difference in
solubility. A single crystallisa-
tion of the diastereomeric mix-
ture by slow diffusion of diethyl
ether into an acetonitrile solu-
tion at room temperature af-
forded the less soluble diaster-
eomeric complex (Rs,S,R)-4 as
yellow crystals in 85% yield
and >98% de (according to the
1H NMR data), with [a]D = +
100 (c=0.2 in CH3CN). The re-
maining mother liquor was
evaporated to half the volume
and seeded with the pure crys-
tals of (Rs,S,R)-4 to promote
further resolution. After subse-
quent fractional crystallisation,
the mother liquor was enriched
in diastereomer (Ss,R,R)-4. Un-
Scheme 1. Synthesis of five-membered S-NHC complexes. COD: cycloocta-1,5-diene.
In general, the five-membered organometallic ring under-
goes rapid transformation between the two non-equivalent l
and d conformations in solution. Hence, to gain a better un-
derstanding about the stereochemical influence of these S-
NHC metallacycles in catalytic scenarios, we have studied
the conformational rigidity of S-NHC-bearing PdII/PtII com-
plexes in solution and in the solid state by means of single-
crystal X-ray diffraction and 1H NMR, 2D ROESY NMR
and variable-temperature NMR spectroscopy techniques.
Results and Discussion
Synthesis of PdII and PtII complexes: The racemic thioether-
imidazolium bromide 2 was prepared in a two-step proce-
dure, as shown in Scheme 1. The a-bromo compound 1 was
readily available from the reaction of substituted chiral im-
ACHTUNGTRENNUNGidACHTUNGTRENNUNGazole and benzaldehyde with thionyl bromide in dichloro-
Figure 2. The possible ring conformations of complex 4. The palladium
centres, which should appear in the middle of the dashed line between
the sulfur and carbon atoms, have been omitted for clarity.
methane at À508C under a nitrogen atmosphere. Compound
1 was converted into 2 by subsequent nucleophilic substitu-
tion with the sodium salt of tert-butylsulfide at À108C. Com-
plexes 4 and 5 were synthesised by the transmetallation
method[12] with the addition of Ag2O (0.55 equiv) to 2 under
exclusion of light to yield 3. The reactions of silver carbene
fortunately, single crystals of (Ss,R,R)-4 suitable for X-ray
analysis were not obtained from the variety of solvent sys-
tems tested. However, according to 1H NMR data, the de
value of the (Ss,R,R)-4 complex was >98%, with [a]D = +
214 (c=0.2 in CH3CN). The absolute configuration of com-
plex (Rs,S,R)-4 was determined by single-crystal X-ray dif-
fraction analysis. The solution structures of (Rs,S,R)-4 and
(Ss,R,R)-4 were determined by 2D 1H–1H ROESY NMR
spectroscopy studies.
complex 3 with ACHTUNGTRENNUNG[PdCl2ACHTUNGTERN(NUGN CH3CN)2] and [PtCl2COD] gave the
neutral compounds 4 and 5, respectively, in moderate yields.
Stereochemical investigation of palladium complex 4: The
palladium compound 4 exists as a 1:1 mixture of two stable
diastereomers, (Rs,S,R)-4 and (Ss,R,R)-4. The eight possible
ring conformations by virtue of the two newly formed
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5469

P.-H. Leung et al.
phenyl substituent to adopt an axial disposition, which led
to the l-ring conformation. Furthermore, the d conforma-
tion of (Rs,S,R)-4 has both substituents in equatorial posi-
tions (Figure 2), and the enantiomer (Ss,R,R)-4 has the d
conformation as a consequence of the R configuration at the
C12 position and the Ph substituent adopting an axial trans
À
À À
disposition. The C13 C12 S1 C19 torsion angle between
the Ph substituent at the a-carbon atom and the tBu sub-
À
stituent at the S1 position is 112.248. The longer Pd1 Cl2
À
bond (2.362 ꢁ) compared to the Pd1 Cl1 bond (2.317 ꢁ) re-
flects the strong trans-influence effect of the carbene. The
complex exhibits a distorted square–planar geometry, with a
À
À
Ccarbene Pd1 bond length of 1.984 ꢁ and a decreased C1
À
Pd1 S1 bond angle of 84.408. Moreover, for bidentate com-
À À
À À
plex (Rs,S,R)-4, the M S C bond angles of Pd1 S1 C12
À
À
and Pd1 S1 C19 were found to be 99.218 and 110.228, re-
spectively, values that are smaller than the observed angle
for tetrahedral geometry, and this presumably accounts for
the steric factors.
Figure 3. Molecular structure of complex (Rs,S,R)-4. Thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms, except for
HACHTUNGTRENNUNG(C12) and H(C2) are omitted for clarity.
1
In solution, the H NMR spectroscopic characterisation of
(Rs,S,R)-4 was assisted by a combination of 1H and 2D
1H–1H ROESY NMR spectroscopy experiments. The ring
conformation of the five-membered S-NHC palladacycle
1
The molecular structure of (Rs,S,R)-4 was studied by
was deduced from NOE data obtained from the 2D H–1H
single-crystal structural analysis (Figure 3), and selected
bond lengths and angles are listed in Table 1. The X-ray dif-
fraction study revealed a l conformation for complex
(Rs,S,R)-4 in the solid state, in which the phenyl substituent
ROESY NMR spectrum. The 2D 1H–1H ROESY NMR
spectrum of (Rs,S,R)-4 in (CD3)2SO is shown in Figure 4.
Generally, for the S absolute configuration at the a-carbon
(C12) chiral centre, the five-membered S-NHC palladacycle
tends to adopt the l conformation. The NOE signal A for
the interaction between the tBu (Me20/Me21/Me22) protons
and the H12 atom is clearly recorded. On other hand, the
strong interactions of the a-chiral H12 proton with the
Ph(a) (D) and H11 protons (E) are also recorded. Other ex-
pected NOE interactions, namely Me3–H2 (B) and Me3–
H10 (C), are also observed.
Table 1. Selected bond lengths and angles in complex (Rs,S,R)-4.
Bond
Length [ꢁ]
Bonds
Angle [8]
À
À
À
Pd1 C1
1.984(13)
2.266(4)
2.317(4)
2.362(4)
1.355(19)
1.360(19)
1.496(2)
1.520(2)
1.527(2)
1.0000
1.444(2)
1.5092(18)
1.8368(14)
1.8633(15)
1.0000
N1 C1 N2
104.68(12)
124.53(12)
112.73(12)
117.06(10)
84.40(4)
107.37(9)
180.10
180.10
À
À
À
Pd1 S1
C1 N2 C12
À
À
À
Pd1 Cl1
N2 C12 C13
À
À
À
Pd1 Cl2
N2 C1 Pd1
À
À
À
C1 N1
C1 Pd1 S1
Complex (Rs,S,R)-4 remained locked in the l-ring confor-
mation in solution. The possible ring inversion certainly in-
À
À
À
C1 N2
N2 C12 S1
À
À
À
C2 N1
S1 C12 H12
À
volves a Pd S1 bond cleavage prior to inversion. Subse-
À
À
À
C2 C4
N2 C12 H12
quent ring closure would lead to inversion of configuration
at the S1 position because the substituents on the S1 and a-
C12 atoms adopt the preferred trans disposition. Therefore,
complex (Rs,S,R)-4 in the l conformation is able to undergo
ring inversion, which presumably proceeds through cleavage
À
À À
C2 C3
C12 S1 Pd1
99.21(5)
À
À À
C2 H2
C12 S1 C19
102.32(7)
112.24(9)
170.913(14)
172.83(4)
95.71(4)
À
À
À
C12 N2
C13 C12 S1
À
À
À
C12 C13
Cl1 Pd1 S1
À
À
À
C12 S1
Cl1 Pd1 Cl2
À
À
À
C19 S1
C1 Pd1 Cl1
À
of the Pd S1 bond, followed by ring inversion and reforma-
À
À À
C12 H12
Pd1 S1 C19
110.22(6)
À
tion of Pd S1 bond with concomitant inversion of configura-
tion at the S1 position to give (Ss,S,R)-4 with the d-ring con-
formation (Figure 2). Hence, the presence of the other con-
former of (Ss,S,R)-4 with the Ph substituent in the equatorial
position within the C,S chelate ring should be detected in
attached at the a-carbon atom (C12) invariably adopts the
axial position below the Pd-C-S coordination plane. The
driving force for the phenyl substituent at the C12 position
adopting the axial disposition is attributed to steric repulsion
between the tBu and Ph substituents in the chelate ring.
Upon chelation, the S1 atom becomes a stereogenic centre
with a predominantly R configuration. The selectivity can be
explained by the lower steric interactions due to the relative
trans disposition of the Ph(a) and tBu groups. The ring con-
formation in (Rs,S,R)-4 is a consequence of the S configura-
tion at the C12 position and the preference for the attached
1
the 2D H–1H ROESY NMR spectrum. The absence of any
correlations between the Ph(a) protons and the tBu (Me20/
1
Me21/Me22) protons in the 2D H–1H ROESY NMR spec-
trum established the absence of the d conformer in solution.
Thus, these spectroscopic results confirm that the five-mem-
bered S-NHC palladacycle (Rs,S,R)-4 exists predominantly
in the stereochemically locked l-ring conformation in both
the solid state and in solution.
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Sulfur-Functionalised N-Heterocyclic Carbene Complexes of Palladium and Platinum
FULL PAPER
NMR spectrum of (Ss,R,R)-4 in
(CD3)2SO is shown in Figure 5.
In this spectrum NOE signal
A accounts for the interaction
between the tBu (Me20/Me21/
Me22) protons and H12 atom.
The strong NOE signals (D and
E) are due to the interaction
between the H12 atom and the
Ph(a) and H11 protons. Other
expected NOE interactions for
Me3–H2 (B) and Me3–H10 (C)
are also observed in the spec-
trum. However, the absence of
key correlations between the
tBu (Me20/Me21/Me22) pro-
tons and the Ph(a) protons in
the 2D 1H–1H ROESY NMR
spectrum clearly and complete-
ly rules out the existence of
(Ss,R,R)-4 in the l conforma-
tion, which has both the tBu
and Ph(a) groups in equatorial
positions, as shown in Figure 2.
Hence, the above NMR spec-
troscopic investigations con-
firmed that the complex
1
Figure 4. The 2D H–1H ROESY NMR spectrum of (Rs,S,R)-4 in (CD3)2SO.
As mentioned earlier, suita-
ble single crystals could not be
obtained for the diastereomeric
complex (Ss,R,R)-4 in the array
of solvents tested. Generally,
the chiral orthopalladated com-
plex tends to adopt the d con-
formation if the chiral carbon
atom has an R absolute config-
uration. As a consequence, the
Ph group on the stereogenic
carbon atom (C12) invariably
takes the axial position above
the Pd-C-S coordination plane,
as illustrated in Figure 2. In sol-
ution, the 1H NMR spectro-
scopic
characterisation
of
a
(Ss,R,R)-4 was assisted by
combination of 1H and 2D
1H–1H ROESY NMR spectro-
scopy experiments. The ring
conformational behaviour of
the five-membered tBuS-NHC
palladacycle was deduced from
NOE data obtained from the
1
2D H–1H ROESY NMR spec-
trum. The 2D 1H–1H ROESY
1
Figure 5. The 2D H–1H ROESY NMR spectrum of (Ss,R,R)-4 in (CD3)2SO.
Chem. Eur. J. 2013, 19, 5468 – 5475
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5471

P.-H. Leung et al.
(Ss,R,R)-4 exists as a single isomer in solution and remains
stereochemically locked in the d (R) configuration at the
C12 position, with the Ph(a) substituent adopting an axial
trans disposition.
1
Variable-temperature H NMR data for complex (Ss,R,R)-
4, in the temperature ranges from 23 to À308C (Figure 6;
extensive precipitation of the complex occurred below
À308C) and from 23 to 758C were collected by using a
Figure 7. Molecular structure of complex (Rs,S,R)-5. Thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms, except for H-
AHCTUNGTRENN(GUN C12) and H(C4), are omitted for clarity.
dium complex, the organoplatinum five-membered ring
adopts the l conformation with the S1 atom pointing above
the Pt-C-S coordination plane, with comparable bond
lengths and angles (Table 2). Similarly, the Ph(a) substituent
on the C,S chelate ring is also axially disposed below the co-
Figure 6. The variable-temperature NMR spectrum of (Ss,R,R)-4 in
CD3CN.
[D3]acetonitrile solution of (Ss,R,R)-4. In both experiments,
at the aforementioned temperatures, only one sharp reso-
nance was observed for the tBuS protons, which shows that
complex (Ss,R,R)-4 is not fluxional in solution. Hence, it is
clear that the occurrence of a process involving ring inver-
Table 2. Selected bond lengths and angles in complex (Rs,S,R)-5.
Bond
Length [ꢁ]
Bonds
Angle [8]
À
À
À
Pt1 C1
1.975(5)
2.245(13)
2.362(14)
2.314(13)
1.348(6)
1.368(7)
1.472(7)
1.492(8)
1.523(8)
1.473(7)
1.507(7)
1.873(6)
1.846(5)
1.0000
N1 C1 N2
104.5(5)
124.7(5)
113.0(4)
116.4(4)
85.60(14)
106.4(4)
180.30
À
sion along with pyramidal sulfur inversion through Pd S1
À
À
À
Pt1 S1
C1 N2 C12
bond dissociation–association is certainly absent for complex
(Ss,R,R)-4. As a result, the sulfur atom, upon coordination
to the palladium(II) centre, is stereochemically locked with
the tBu group at the preferred axial position and the lone
pair at the equatorial position.
À
À
À
Pt1 Cl1
N2 C12 C13
À
À
À
Pt1 Cl2
N2 C1 Pt1
À
À
À
C1 N1
C1 Pt1 S1
À
À
À
C1 N2
N2 C12 S1
À
À
À
C4 N1
S1 C12 H12
À
À
À
C4 C11
N2 C12 H12
180.30
À
À À
C4 C5
C12 S1 Pt1
99.57(18)
102.3 (3)
112.5(4)
89.54(5)
89.38(5)
174.90(15)
À
À À
Stereochemical investigations of platinum complex 5: The
platinum(II) complex 5 also contains a 1:1 mixture of the
two diastereomers, (Rs,S,R)-5 and (Ss,R,R)-5. A single crys-
tallisation of the diastereomeric mixture from acetonitrile/
diethyl ether afforded the less soluble diastereomeric com-
plex (Rs,S,R)-5 as colourless crystals in 70% yield and
>98% de (according to the 1H NMR data) with [a]D = +
100 (c=0.2 in CH3CN). The remaining mother liquor was
evaporated to half the volume and seeded with the pure
crystals of (Rs,S,R)-5 to promote further resolution of re-
maining compound. Successive fractional crystallisations
were carried out until the mother liquor was enriched in the
diastereomer (Ss,R,R)-5.
C12 N2
C12 S1 C19
À
À
À
C12 C13
C13 C12 S1
À
À
À
C19 S1
Cl1 Pt1 S1
À
À
À
C12 S1
Cl1 Pt1 Cl2
À
À
À
C12 H12
C1 Pt1 Cl1
ordination plane because of the strong repulsive interaction
between the tBu group and the C12 substituents, which pre-
vents the free interchange of the former between the axial
and equatorial positions. Therefore, in the solid state, the al-
ternative d-ring conformation, with the Ph(a) group at the
equatorial position in the Pt-C-S coordinating plane would
be highly disfavoured. The chiral S1 atom has the R configu-
ration upon coordination to platinum and the higher trans
effect of the carbene towards the sulfur atom is reflected in
Colourless single crystals of (Rs,S,R)-5 were recrystallised
from an acetone/diethyl ether mixture at room temperature.
The molecular structure is presented in Figure 7. As expect-
ed, the platinum atom adopts a distorted square–planar co-
ordination geometry. Similar to that in the (Rs,S,R)-4 palla-
À
À
the longer bond length of Pt Cl1 (2.362 ꢁ) than that of Pt
Cl2 (2.314 ꢁ). Crystal data for compounds (Rs,S,R)-4 and
(Rs,S,R)-5 are listed in Table 3.
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Chem. Eur. J. 2013, 19, 5468 – 5475

Sulfur-Functionalised N-Heterocyclic Carbene Complexes of Palladium and Platinum
FULL PAPER
Table 3. Crystal data for compounds (Rs,S,R)-4 and (Rs,S,R)-5.
(Ss,R,R)-5 is present as a single isomer and remains stereo-
chemically locked in the d (R) conformation in both the
solid state and in solution.
4
5
empirical formula
formula weight
T [K]
C22H26Cl2N2PdS
527.81
103(2)
C22H26Cl2N2PtS
616.50
103(2)
l [ꢁ]
0.71073
0.71073
Conclusion
crystal system
space group
a [ꢁ]
orthorhombic
P2(1)2(1)2(1)
10.4613(3)
11.5237(3)
18.8138(5)
2268.06(11)
4
1.546
1.156
1072
0.38ꢂ0.26ꢂ0.24
2.07 to 40.47
À19 to 17, À17 to 20,
À31 to 34
57020
orthorhombic
P2(1)2(1)2(1)
10.4741(2)
11.5386(2)
18.7933(3)
2271.29(7)
4
1.803
6.515
1200
0.12ꢂ0.08ꢂ0.06
2.07 to 30.66
À8 to 14, À16 to 9,
À26 to 23
15190
In conclusion, five-membered chiral tBuS-NHC metallacy-
cles were synthesised and their ring conformations were de-
termined with X-ray crystallographic and solution-phase 2D
1H–1H ROESY NMR studies. From the results obtained, it
was confirmed that both the PdII and PtII complexes are
structurally rigid and stereochemically locked in chiral ring
conformations, namely either (Rs,S,R)-l or (Ss,R,R)-d in
both the solid state and in solution. The catalytic applica-
tions of these complexes are currently being evaluated.
b [ꢁ]
c [ꢁ]
V [ꢁ3]
Z
Dcalcd [mgmÀ3
]
m [mmÀ1
(000)
]
F
A
crystal size [mm3]
q range [8]
hkl range
N
Nind (Rint
)
14052 (0.0333)
99.6
6936 (0.0256)
99.5
completeness to
q=40.478 [%]
max. and min. trans-
mission
Experimental Section
0.7688 and 0.6677
0.6959 and 0.5086
6936, 0, 258
General methods: Reactions involving moisture-sensitive compounds
were carried under an inert atmosphere of purified nitrogen by using
Schlenk line techniques in oven-dried glassware. All solvents were distil-
led prior to use and reagents were used as received. 1H NMR and
13C NMR (400/100 MHz and 500/125 MHz) spectra were recorded in
CDCl3, CD3CN, and (CH3)2SO solutions by using Bruker Avance
DPX400 and DPX500 spectrometers, respectively. Unless stated other-
wise, all NMR spectroscopic experiments were performed at room tem-
perature (300 K). High-resolution mass spectra were obtained by using a
Water Q-Tof Premier, with ESI mode. Melting points were determined
on a SRS-Optimelt MPA-100 apparatus and are uncorrected. Optical ro-
tations were measured for the specified solution in a 0.1 dm cell at 208C
with a Perkin–Elmer model 341 polarimeter. Crystal data for complexes
(Rs,S,R)-4 and (Rs,S,R)-5 were collected on a Bruker X8 CCD diffractom-
eter.
data, restraints, param- 14052, 0, 257
eters
GOF on F2
1.061
1.142
R1
G
0.0286
0.0614
0.001(13)
0.0267
0.0633
À0.001(8)
wR2(F2)
absolute structure pa-
rameters
The 2D 1H–1H ROESY NMR spectrum of complex
(Rs,S,R)-5 is similar to that of the (Rs,S,R)-4 palladium com-
plex. The presence of only one resonance for each chemical-
ly non-equivalent proton clearly indicates the existence of a
single geometrical isomer in solution. Once again, the ab-
sence of NOE correlations between the Ph(a) protons and
Synthesis of (R)-3-[bromo
azol-3-ium bromide (1): A 2m solution of (R)-1-(1-phenylethyl)-1H-imid-
azole (1 g, 5.814 mmol) and a 2m solution of benzaldehyde (1.23 g,
ACHTUNGTREN(NUNG phenyl)methyl]-1-(1-phenylethyl)-1H-imid-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
11.628 mmol) were added simultaneously to a stirred 1m solution of
SOBr2 (0.957 g, 4.651 mmol) in dichloromethane at À508C under an
argon atmosphere. The reaction mixture was slowly warmed to room
temperature and allowed to stir for 8 h. Evaporation of the reaction mix-
ture followed by tituration with diethyl ether gave the desired crude
product. The crude product was purified by column chromatography on
the tBu protons in the 2D H–1H ROESY NMR spectrum
1
rules out the existence of (Ss,S,R)-5 with the d-ring confor-
mation in solution. Hence, for the S absolute configuration
at the a-carbon chiral centre, the five-membered platinacy-
cle remained in the l conformation in both the solid state
and in solution.
Unfortunately, single crystals suitable for X-ray crystallog-
raphy could not be obtained for the diastereomeric complex
(Ss,R,R)-5 from the array of solvents tested. However, ac-
1
silica (eluent: chloroform/methanol, 40:1). Yield: 1.58 g (67%); H NMR
(500 MHz, (CH3)2SO): d=11.25 (s, 1H; NCHN), 8.56 (s, 1H; NCHBr),
7.81–7.83 (m, 2H, NCH=CHN); 7.47–7.48 (m, 1H; Ar H), 7.28–7.44 (m,
9H; BrCHAr H, Ar H), 5.87–5.90 (q, JH–H =7.0 Hz, 1H; NCHCH3),
2.04–2.06 ppm (d, 3H; NCHCH3);13C NMR (100 MHz, CDCl3): d=137.0,
136.9, 136.8, 134.8, 134.8, 130.8, 130.8, 129.8, 129.8, 129.7, 129.4, 129.4,
128.1, 128.1, 127.0, 126.9, 121.0, 120.8, 120.7, 61.0, 60.9, 58.5, 21.4,
21.2 ppm; HRMS: calcd for C18H18BrN2Br [MÀBr]+: 341.06; found:
341.07.
cording to H NMR data, the diastereomeric excess of com-
plex (Ss,R,R)-5 was >98%, with [a]D = +190 (c=0.2 in
CH3CN). The 2D 1H–1H ROESY NMR was recorded in
(CD3)2SO to allow the assignment of the absolute stereo-
chemistry and conformation of the complex and was shown
to be exactly the same as the (Ss,R,R)-4 spectrum. The five-
membered platinacycle adopts the d conformation if the
chiral centre has an R absolute configuration. The phenyl
group on the stereogenic carbon atom invariably takes up
the axial position above the C,S chelate ring. The absence of
an NOE signal due to interactions between the tBu (Me20/
Me21/Me22) and Ph(a) protons indicates that the complex
Synthesis of (R)-3-[(tert-butylthio)ACTHNUTRGNEUGN(phenyl)methyl]-1-(1-phenylethyl)-
1H-imidazol-3-ium bromide (2; diastereomers (R,R)- and (R,S)-2 in a
1:1 ratio): Sodium-2-methylpropane-2-thiolate (0.267 g, 2.381 mmol) was
added to a stirred solution of 1 (1 g, 2.381 mmol) in dry dichloromethane
(10 mL) at À108C. The reaction mixture was warmed to room tempera-
ture and stirred for another 4 h. On completion of the reaction, as moni-
tored by TLC, the reaction mixture was filtered and the filtrate was
evaporated. This was followed by tituration with diethyl ether to yield
the desired crude product. The crude product was purified by column
chromatography by using silica gel (eluent: chloroform/methanol, 100:3)
to obtain diastereomers (R,R) and (R,S)-2 in a 1:1 ratio. Yield: 0.82 g
Chem. Eur. J. 2013, 19, 5468 – 5475
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5473

P.-H. Leung et al.
(80%); 1H NMR (500 MHz, (CD3)2SO): d=11.55 (s, 1H; NCHN), 11.48
(s, 1H; NCHN) 7.82 (s, 2H), 7.78 (s, 1H; NCHS), 7.73 (s, 1H; NCHS),
7.57–7.59 (m, 3H), 7.31–7.40 (m, 15H), 7.14–7.15 (m, 2H), 5.78–5.83 (q,
X-ray crystallographic studies: Crystal data for complexes (Rs,S,R)-4 and
(Rs,S,R)-5 were collected on a Bruker X8 CCD diffractometer with MoKa
radiation (graphite monochromator). SADABS absorption corrections
were applied. All non-hydrogen atoms were refined anisotropically; hy-
drogen atoms were introduced at calculated positions and refined riding
on their carrier atoms. CCDC 913651 ((Rs,S,R)-4) and 913652 ((Rs,S,R)-5)
contain the supplementary crystallographic 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.
J
H–H =7.0 Hz, 1H; NCHCH3), 5.69–5.73 (q, JH–H =6.5 Hz, 1H; NCHCH3),
2.05–2.06(d, JH–H=7.0 Hz, 3H; NCHCH3), 2.02–2.03(d, JH–H=7.0 Hz, 3H;
NCHCH3), 1.41 (s, 9H; SCCH3), 1.40 ppm (s, 9H; SCCH3); 13C NMR
(100 MHz, CDCl3): d=137.8, 137.5, 137.5, 135.8, 135.8, 129.5, 129.4,
129.3, 129.3, 129.3, 129.2, 126.8, 126.7, 126.6, 121.4, 121.1, 120.8, 120.7,
64.7, 64.7, 60.4, 60.2, 47.4, 30.9, 30.8, 21.4, 21.3 ppm; HRMS: calcd for
C22H27N2SBr [MÀBr]+: 351.19; found: 351.19.
Synthesis of bis{1-[(tert-butylthio)
ACHTUNGTRENNUNG
2,3-dihydro-1H-imidazol-2-yl}silver
AHCTUNGTRENNUNG
Acknowledgements
(0.308 g, 1.327 mmol) was added to a solution of 2 (0.82 g, 1.894 mmol) in
CH2Cl2 (10 mL) and the suspension was stirred for 4 h at room tempera-
ture in the dark. The mixture was filtered through a small celite plug and
We thank Nanyang Technological University for support of this research
work and also funding a research scholarship to D.K.
1
concentrated to afford the crude product. Yield: 0.865 g (85%); H NMR
(500 MHz, CDCl3): d=7.59–7.60 (m, 2H; NCHS), 7.27–7.34 (m, 13H),
7.20–7.24 (m, 5H), 7.01–7.02 (m, 2H), 6.79–6.80 (m, 2H), 5.72–5.75 (t,
J
H–H =7.0 Hz, 2H; NCHCH3), 1.83–1.87 (m, 6H; NCHCH3), 1.32 (s, 9H;
[1] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991,
113, 361–363.
SCCH3), 1.29 ppm (s, 9H; SCCH3); HRMS: calcd for C44H52AgN4S2Br
[MÀBr]+: 807.28; found: 807.30.
[2] a) F. E. Hahn, M. C. Jahnke, Angew. Chem. 2008, 120, 3166–3216;
Angew. Chem. Int. Ed. 2008, 47, 3122–3172; b) P. L. Arnold, S. Pear-
son, Coord. Chem. Rev. 2007, 251, 596–609; c) N. T. S. Phan, M. Van
Der Sluys, C. W. Jones, Adv. Synth. Catal. 2006, 348, 609–679;
d) J. C. Garrison, W. J. Youngs, Chem. Rev. 2005, 105, 3978–4008;
e) E. Peris, R. H. Crabtree, Coord. Chem. Rev. 2004, 248, 2239–
2246; f) R. B. Bedford, C. S. J. Cazin, D. Holder, Coord. Chem. Rev.
2004, 248, 2283–2321; g) A. C. Hillier, G. A. Grasa, M. S. Viciu,
H. M. Lee, C. L. Yang, S. P. Nolan, J. Organomet. Chem. 2002, 653,
69–82; h) W. A. Herrmann, Angew. Chem. 2002, 114, 1342–1363;
Angew. Chem. Int. Ed. 2002, 41, 1290–1309; i) I. P. Beletskaya, A. V.
Cheprakov, Chem. Rev. 2000, 100, 3009–3066.
Synthesis of palladium complex 4: PdCl2ACTHNUTRGEN(UNG CH3CN)2 (0.138 g, 0.537 mmol)
was added to a stirred solution of 3 (0.4 g, 0.537 mmol) in dry acetonitrile
(15 mL) under an argon atmosphere. The mixture was stirred in the dark
overnight at room temperature, filtered through celite and concentrated.
The residue was triturated with methanol to afford (Rs,S,R)-4 and
(Ss,R,R)-4 as a yellow solid (0.150 g, 51%). The diastereomers were sepa-
rated with an acetonitrile/diethyl ether system.
ACHTUNGTRENNUNG(Rs,S,R)-4: Yellow crystals; yield: 0.065 g; m.p: 216–2188C; [a]D = +100
(c=0.2 in CH3CN); 1H NMR (400 MHz, (CD3)2SO): d=7.82 (s, 1H),
7.51–7.56 (m, 5H), 7.48–7.50 (m, 1H), 7.45–7.46 (m, 1H), 7.39–7.42 (m,
4H), 7.31–7.35 (m, 1H), 6.92 (s, 1H; NCHS), 1.84–1.86 (d, JH–H =6.8 Hz,
[3] O. Kꢃhl, Chem. Soc. Rev. 2007, 36, 592–607.
[4] a) A. A. Danopoulos, N. Tsoureas, S. A. Macgregor, C. Smith, Or-
3H; NCHCH3), 1.24 ppm (s, 9H; SCACHTUNGTRNEUNG
(CH3)3); 13C NMR (100 MHz,
(CD3)2SO): d=155.8, 141.4, 135.4, 129.7, 129.4, 128.6, 128.1, 126.6, 126.2,
121.3, 120.3, 65.2, 56.8, 55.9, 29.1, 19.4 ppm; HRMS: calcd for
C22H27Cl2N2PdS [MÀ2Cl]+: 457.09; found: 457.09.
ACHUTNGRENgNUG anACHUTNGTRENoNGUN metallics 2007, 26, 253–263; b) C. C. Lee, W. C. Ke, K. T. Chan,
C. L. Lai, C. H. Hu, H. M. Lee, Chem. Eur. J. 2007, 13, 582–591;
c) J. Wolf, A. Labande, J. C. Daran, R. Poli, J. Organomet. Chem.
2006, 691, 433–443; d) F. E. Hahn, M. C. Jahnke, T. Pape, Organo-
metallics 2006, 25, 5927–5936; e) J. Y. Zeng, M. H. Hsieh, H. M.
Lee, J. Organomet. Chem. 2005, 690, 5662–5671; f) L. D. Field, B. A.
Messerle, K. Q. Vuong, P. Turner, Organometallics 2005, 24, 4241–
4250; g) P. L. Chiu, H. M. Lee, Organometallics 2005, 24, 1692–
1702; h) H. M. Lee, J. Y. Zeng, C. H. Hu, M. T. Lee, Inorg. Chem.
2004, 43, 6822–6829; i) C. L. Yang, H. M. Lee, S. P. Nolan, Org. Lett.
2001, 3, 1511–1514; j) N. Tsoureas, A. A. Danopoulos, A. A. D. Tul-
loch, M. E. Light, Organometallics 2003, 22, 4750–4758; k) F. E.
Hahn, A. R. Naziruddin, A. Hepp, T. Pape, Organometallics 2010,
29, 5283–5288; l) A. R. Naziruddin, A. Hepp, T. Pape, F. E. Hahn,
Organometallics 2011, 30, 5859–5866.
ACHTUNGTRENNUNG(Ss,R,R)-4: Yellow powder; yield: 0.075 g; m.p: 234–2368C; [a]D = +214
1
(c=0.2 in CH3CN); H NMR (400 MHz, CD3CN): d=7.56–7.59 (m, 3H),
7.54 (s, 1H), 7.51–7.53 (m, 1H), 7.48–7.50 (m, 1H), 7.42–7.46 (m, 4H),
7.35–7.39 (m, 1H), 7.26–7.27 (d, 1H), 6.96–6.97 (d, 1H), 6.45 (s, 1H;
NCHS), 1.82–1.83 (d, JH–H =6.8 Hz, 3H; NCHCH3), 1.52 ppm (s, 9H;
SCCH3); 13C NMR (100 MHz, CDCl3): d=157.7, 140.5, 136.0, 130.8,
130.3, 129.3, 128.8, 128.0, 127.0, 122.3, 120.7, 66.6, 57.5, 56.9, 29.9,
21.1 ppm; HRMS: calcd for C22H27Cl2N2PdS [MÀ2Cl]+: 457.09; found:
457.09.
Synthesis of platinum complex 5: This complex was prepared in
a
manner analogous to that described for 4, by 3 (0.4 g, 0.537 mmol) in dry
dichloromethane (10 mL) and PtCl2COD (0.4 g, 0.537 mmol) under an
argon atmosphere to yield a mixture of (Rs,S,R)-5 and (Ss,R,R)-5 (0.153 g,
55%) as a yellow powder. The diastereomers were separated with an
acetonitrile/diethyl ether system.
[5] a) M. Y. Chiang, Y. X. Li, D. Krishnan, P. Sumod, K. H. Ng, P. H.
Leung, Eur. J. Inorg. Chem. 2010, 1413–1418; b) M. C. Jahnke, T.
Pape, F. E. Hahn, Eur. J. Inorg. Chem. 2009, 1960–1969; c) M. V. Ji-
mꢄnez, J. J. Pꢄrez-Torrente, M. I. Bartolomꢄ, V. Gierz, F. J. Lahoz,
L. A. Oro, Organometallics 2008, 27, 224–234; d) N. Stylianides,
A. A. Danopoulos, N. Tsoureas, J. Organomet. Chem. 2005, 690,
5948–5958; e) L. H. Gade, V. Cꢄsar, S. Bellemin-Laponnaz, Angew.
Chem. 2004, 116, 1036–1039; Angew. Chem. Int. Ed. 2004, 43, 1014–
1017; f) V. Cꢄsar, S. Bellemin-Laponnaz, L. H. Gade, Organometal-
lics 2002, 21, 5204–5208; g) D. S. McGuinness, K. J. Cavell, Organo-
metallics 2000, 19, 741–748.
[6] a) N. A. Jones, S. T. Liddle, C. Wilson, P. L. Arnold, Organometallics
2007, 26, 755–757; b) P. L. Arnold, M. Rodden, C. Wilson, Chem.
Commun. 2005, 1743–1745; c) P. L. Arnold, M. Rodden, K. M.
Davis, A. C. Scarisbrick, A. J. Blake, C. Wilson, Chem. Commun.
2004, 1612–1613; d) H. Clavier, L. Coutable, L. Toupet, J. C. Guille-
min, M. Mauduit, J. Organomet. Chem. 2005, 690, 5237–5254;
e) A. W. Waltman, R. H. Grubbs, Organometallics 2004, 23, 3105–
3107; f) P. L. Arnold, A. C. Scarisbrick, A. J. Blake, C. Wilson,
Chem. Commun. 2001, 2340–2341.
ACHTUNGTRENNUNG(Rs,S,R)-5: Colourless crystals; yield: 0.060 g; m.p: 302–3048C; [a]D = +
100 (c=0.2 in CH3CN); 1H NMR (400 MHz, CD2Cl2): d=7.77–7.78 (d,
1H), 7.48–7.54 (m, 6H), 7.43–7.46 (m, 1H), 7.36–7.41 (m, 4H), 7.30–7.34
(m, 1H), 6.80 (s, 1H; NCHS), 1.85–1.87 (d,
NCHCH3), 1.19 ppm (s, 9H; SC
d=149.2, 141.1, 134.8, 130.7, 130.2, 129.4, 128.8, 127.5, 127.0, 121.8, 118.0,
67.0, 57.7, 56.0, 29.4, 20.6 ppm; HRMS: calcd for C22H27Cl2N2PtS
[MÀ2Cl]+: 546.15; found: 546.17.
ACHTUNGTRENNUNG(Ss,R,R)-5: Colourless crystal; yield: 0.080 g; m.p: 294–2988C; [a]D = +
190 (c=0.2 in CH3CN; H NMR (500 MHz, (CD3)2SO): d=7.53–7.54 (m,
3H), 7.48–7.51 (m, 3H), 7.42–7.52 (m, 2H), 7.37–7.40 (m, 4H), 7.34–7.36
(m, 1H), 6.88 (s, 1H; NCHS), 1.77–1.79 (d,
NCHCH3), 1.39 ppm (s, 9H; SC
(CD3)2SO): d=146.8, 140.3, 135.9, 130.0, 129.9, 129.0, 128.4, 127.5, 126.8,
122.2, 119.7, 65.2, 56.1, 56.0, 28.9, 21.1 ppm; HRMS: calcd for
C22H27Cl2N2PtS [MÀ2Cl]+: 546.15; found: 546.17.
JH–H =6.8 Hz, 3H;
AHCTUNGTRENNUNG
1
JH–H =7.0 Hz, 3H;
AHCTUNGTRENNUNG
5474
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Chem. Eur. J. 2013, 19, 5468 – 5475

Sulfur-Functionalised N-Heterocyclic Carbene Complexes of Palladium and Platinum
FULL PAPER
[7] a) A. T. Normand, K. J. Cavell, Eur. J. Inorg. Chem. 2008, 2781–
2800; b) E. A. B. Kantchev, C. J. OꢅBrien, M. G. Organ, Angew.
Chem. 2007, 119, 2824–2870; Angew. Chem. Int. Ed. 2007, 46, 2768–
2813.
Basso, M. R. Meneghetti, R. A. Konrath, R. Burrow, M. Horner, Or-
ganometallics 1997, 16, 2386–2391; c) J. Dupont, N. R. Basso, M. R.
Meneghetti, Polyhedron 1996, 15, 2299–2302; d) E. W. Abel, S. K.
ˇ
Bhargava, K. Kite, K. G. Orrell, V. Sik, B. L. Williams, Polyhedron
[8] a) M. Bierenstiel, E. D. Cross, Coord. Chem. Rev. 2011, 255, 574–
590; b) H. V. Huynh, Y. X. Chew, Inorg. Chim. Acta 2010, 363,
1979–1983; c) H. V. Huynh, D. Yuan, Y. Han, Dalton Trans. 2009,
7262–7268; d) C. Gandolfi, M. Heckenroth, A. Neels, G. Laurenczy,
M. Albrecht, Organometallics 2009, 28, 5112–5121; e) C. Fliedel, G.
Schnee, P. Braunstein, Dalton Trans. 2009, 2474–2476; f) D. S.
McGuinness, J. A. Suttil, M. G. Gardiner, N. W. Davies, Organome-
tallics 2008, 27, 4238–4247; g) S. J. Roseblade, A. Ros, D. Monge, M.
Alcarazo, E. ꢆlvarez, J. M. Lassaletta, R. Fernꢇndez, Organometal-
lics 2007, 26, 2570–2578; h) J. Wolf, A. Labande, J. C. Daran, R.
Poli, Eur. J. Inorg. Chem. 2007, 5069–5079; i) H. V. Huynh, C. H.
Yeo, G. K. Tan, Chem. Commun. 2006, 3833–3835; j) A. Ros, D.
Monge, M. Alcarazo, E. ꢆlvarez, J. M. Lassaletta, R. Fernꢇndez, Or-
ganometallics 2006, 25, 6039–6046; k) H. Seo, H. Park, B. Y. Kim,
J. H. Lee, S. U. Son, Y. K. Chung, Organometallics 2003, 22, 618–
620; l) N. Matsumura, J. Kawano, N. Fukunishi, H. Inoue, M. Yasui,
F. Iwasaki, J. Am. Chem. Soc. 1995, 117, 3623–3624; m) D. Sell-
mann, W. Prechtel, F. Knoch, M. Moll, Organometallics 1992, 11,
2346–2348; n) C. Fliedel, A. Sabbatini, P. Braunstein, Dalton Trans.
2010, 39, 8820–8828; o) J. Iglesias-Sigꢃenza, A. Ros, E. Dꢈez, A.
Magriz, A. Vꢇzquez, E. ꢆlvarez, R. Fernꢇndez, J. M. Lassaletta,
Dalton Trans. 2009, 8485–8488.
1982, 1, 289–298; e) E. W. Abel, M. Booth, K. G. Orrell, J. Chem.
Soc. Dalton Trans. 1980, 1582–1592; f) G. Yoshida, H. Kurosawa, R.
Okawara, J. Organomet. Chem. 1976, 113, 85–89; g) R. J. Cross,
I. G. Dalgleish, G. J. Smith, R. Wardle, J. Chem. Soc. Dalton Trans.
1972, 992.
[10] J. Dupont, A. S. Gruber, G. S. Fonseca, A. L. Monteiro, G. Ebeling,
R. A. Burrow, Organometallics 2001, 20, 171–176.
[11] a) Y. H. Huang, S. A. Pullarkat, S. Teong, R. J. Chew, Y. X. Li, P. H.
Leung, Organometallics 2012, 31, 4871–4875; b) C. Xu, G. J. H.
Kennard, F. Hennersdorf, Y. X. Li, S. A. Pullarkat, P. H. Leung, Or-
ganometallics 2012, 31, 3022–3026; c) Y. H. Huang, S. A. Pullarkat,
Y. X. Li, P. H. Leung, Chem. Commun. 2010, 46, 6950–6952;
d) Y. H. Huang, S. A. Pullarkat, Y. X. Li, P. H. Leung, Inorg. Chem.
2012, 51, 2533–2540; e) K. Chen, Y. X. Li, S. A. Pullarkat, P. H.
Leung, Adv. Synth. Catal. 2012, 354, 83–87; f) Y. H. Huang, R. J.
Chew, Y. X. Li, S. A. Pullarkat, P. H. Leung, Org. Lett. 2011, 13,
5862–5865.
[12] H. M. J. Wang, I. J. B. Lin, Organometallics 1998, 17, 972–975.
Received: December 5, 2012
Revised: January 21, 2013
Published online: February 28, 2013
[9] a) A. Albinati, J. Eckert, P. Pregosin, H. Rꢃegger, R. Salzmann, C.
Stçssel, Organometallics 1997, 16, 579–590; b) J. Dupont, N. R.
Chem. Eur. J. 2013, 19, 5468 – 5475
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5475