A comparative study on dinuclear and multinuclear Ni(II), Pd(II), and Pt(II) complexes of a thiolato-functionalized, benzannulated N -heterocyclic carbene ligand

Article abstract of DOI:10.1021/ic400672z

Dimeric thiolato-bridged Ni(II) and Pt(II) NHC complexes 2 and 4 have been synthesized from ligand precursor A through a combined and in situ deprotonation/hydrolysis protocol of a thioester-functionalized benzimidazolium salt in the presence of the respective metal salts. Reactivity studies of 2 and 4, and their previously reported Pd(II) analogue 1a toward either Me ₃OBF₄, NaOH, or Na₂S·9H₂O revealed clear differences. Complex 2 decomposed when treated with Me ₃OBF₄. On the other hand, its reaction with aqueous NaOH solution in the presence of NaBF₄ yielded trinuclear [Ni ₃S₃O] complex 6, which possesses an interesting [Ni ₃S₃] triangle with a capping μ₃-oxido ligand. Pt(II) analogue 4 was converted to the tetranuclear [Pt ₄S₄] macrocycle 5 when treated with Me₃OBF ₄, in analogy to the result from 1a, while no defined products could be isolated when 4 was treated with either NaOH or Na₂S· 9H₂O. Pd(II) analogue 1a reacted with Na₂S·9H ₂O to give the tripalladium [Pd₃S₃S] complex 7 bearing a capping μ₃-sulfido ligand.

Full text of DOI:10.1021/ic400672z

Article
pubs.acs.org/IC
A Comparative Study on Dinuclear and Multinuclear Ni(II), Pd(II), and
Pt(II) Complexes of a Thiolato-Functionalized, Benzannulated
N‑Heterocyclic Carbene Ligand
Dan Yuan and Han Vinh Huynh*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
S
* Supporting Information
ABSTRACT: Dimeric thiolato-bridged Ni(II) and Pt(II)
NHC complexes 2 and 4 have been synthesized from ligand
precursor A through a combined and in situ deprotonation/
hydrolysis protocol of a thioester-functionalized benzimidazo-
lium salt in the presence of the respective metal salts.
Reactivity studies of 2 and 4, and their previously reported
Pd(II) analogue 1a toward either Me3OBF4, NaOH, or
Na2S·9H2O revealed clear differences. Complex 2 decomposed
when treated with Me3OBF4. On the other hand, its reaction
with aqueous NaOH solution in the presence of NaBF4 yielded trinuclear [Ni3S3O] complex 6, which possesses an interesting
[Ni3S3] triangle with a capping μ3-oxido ligand. Pt(II) analogue 4 was converted to the tetranuclear [Pt4S4] macrocycle 5 when
treated with Me3OBF4, in analogy to the result from 1a, while no defined products could be isolated when 4 was treated with
either NaOH or Na2S·9H2O. Pd(II) analogue 1a reacted with Na2S·9H2O to give the tripalladium [Pd3S3S] complex 7 bearing a
capping μ3-sulfido ligand.
strates the unusual reactivity of Meerwein’s salts as a metal-free
halido-abstracting agent.5a
INTRODUCTION
■
Donor-functionalized N-heterocyclic carbenes (NHCs) have
become the focus of metal carbene chemistry, because they give
access to complexes with diverse structures, enhanced
stabilities, and versatile catalytic applications.1 Most commonly,
substituents containing N-, O-, and P-donor groups have been
introduced at the N atoms of NHCs.2 We have devoted our
studies to the exploration of less-common S-functionalized
NHCs, which can be mainly categorized as thioether-, thiolato-,
thiophene-, sulfonate-, and sulfoxide-NHCs.3,4 Among these,
the soft and electron-rich thiolato function forms the strongest
M−S bond, and also gives rise to a diverse coordination
chemistry, because of its tendency to bridge metal centers,
which particularly attracted our interest.5
Reported syntheses of NHC-thiolato complexes require the
handling of either air-sensitive free thiols or metal(0)
precursors.6 To circumvent this, we have developed a one-
step approach to thiolato-bridged Pd(II)-NHC dimers, which
involves the direct reaction of thioester-functionalized azolium
salts A/B with Pd(OAc)2 (see Scheme 1). The thioester-
functionalized azolium salts function as synthetic equivalents of
otherwise air-sensitive thiol-NHCs, which are generated in situ
by a combined deprotonation and hydrolysis step.5 In this
method, the handling of air-sensitive materials can be avoided,
and the synthetic sequence is also shortened. The resulting
complexes 1a/b with a [Pd2S2] core showed interesting
reactivities toward AgO2CCF3, thiolates, and the strong
electrophile Me3OBF4. In the latter two cases, tetranuclear
[Pd4S4] molecular squares were isolated, which also demon-
In order to investigate whether the in situ generation of
thiolato-NHCs from thioester-NHCs has general application,
we explored the syntheses of isoelectronic and isostructural
Ni(II) and Pt(II) analogues of 1a5a starting from ligand
precursor A. Herein, we report different synthetic routes to
these metal-NHC dimers and a comparative reactivity study of
the respective Ni(II), Pd(II), and Pt(II) NHC complexes
toward Me3OBF4, NaOH, and Na2S, which gave rise to a series
of multinuclear complexes with different architectures and
metal centers highlighting the structural diversity of thiolato-
NHC complexes.
RESULTS AND DISCUSSION
■
Synthesis of the Ni(II) Dimeric Complex. Following our
methodology on synthesizing benzimidazolin-2-ylidene com-
plexes of Ni(II),7 benzimidazolium salt A bearing a thioester
function was heated with Ni(OAc)2 in molten [N(n-Bu)4]Br
under vacuum at 120 °C (see Scheme 2). In analogy to Pd(II),
deprotonation and in situ hydrolysis of the thioester group
occurred, giving rise to the formation of the dinuclear Ni(II)-
NHC complex 2 with bridging thiolato chelates. The dinuclear
complex 2 was isolated as a red solid, which is soluble in
chlorinated solvents, CH3CN, dimethyl sulfoxide (DMSO), and
dimethylformamide (DMF), but insoluble in diethyl ether and
Received: March 19, 2013
© XXXX American Chemical Society
A
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Scheme 1. Syntheses of Thiolato-Bridged Dimeric Pd(II) NHC complexes 1a/b.5
Scheme 2. Synthesis of Thiolato-Bridged Dimeric Ni(II)
Benzimidazolin-2-ylidene Complex 2
1
hexane. The formation of 2 is corroborated by its H NMR
1
spectrum, in which both the downfield H NMR signal for the
acidic NCHN proton and the singlet due to the methyl-
thioester group of ligand precursor A are absent. Furthermore,
the benzylic protons become diastereotopic upon coordination,
giving rise to two doublets with a coupling constant of 2J = 15.8
Figure 1. Molecular structure of the compound 2 showing 50%
probability ellipsoids; H atoms are omitted for clarity. Selected bond
lengths and bond angles: Ni1−C1, 1.873(6) Å; Ni1−S1, 2.1740(17)
Å; Ni1−S2, 2.2329(16) Å; Ni1−Br1, 2.2979(10) Å; Ni2−C17,
1.892(5) Å; Ni2−S2, 2.1799(16) Å; Ni2−S1, 2.2398(15) Å; Ni2−
Br2, 2.3266(10) Å; C1−Ni1−S1, 92.66(17)°; S1−Ni1−S2, 78.88(6)°;
S2−Ni1−Br1, 98.38(5)°; Br1−Ni1−C1, 91.24(17)°; C1−Ni1−S2,
169.37(18)°; S1−Ni1−Br1, 167.53(6)°; C17−Ni2−S2, 92.61(17)°;
S2−Ni2−S1, 78.61(6)°; S1−Ni2−Br2, 97.19(5)°; Br2−Ni2−C17,
92.13(16)°; C17−Ni2−S1, 170.45(17)°; S2−Ni2−Br2, 168.65(5)°;
Ni1−S1−Ni2, 82.69(6)°; Ni1−S2−Ni2, 82.72(6)°. NiCS2Br/NHC
dihedral angle = 58.3(2)°, 55.8(1)°.
Hz centered at 6.71 and 5.61 ppm, respectively. Similarly, the
diastereotopy of the methylene groups of the bridges results in
four pseudo-triplets/doublets in the range of 5.05−1.53 ppm.
The carbene signal at 176.1 ppm is shifted downfield compared
to that of 1a (cf. 175.4 ppm),5a which may be due to the lower
effective nuclear charge of Ni(II), compared to Pd(II). A signal
at m/z = 733 in the positive-ion FAB mass spectrum assignable
to the [M − Br]+ fragment also supports the formation of 2.
Single crystals of 2 suitable for X-ray analysis were obtained
by slow evaporation of a concentrated CH2Cl2 solution. The
molecular structure depicted in Figure 1 shows that complex 2
is isostructural to its Pd analogue 1a.5a Each Ni(II) center is
coordinated by one carbene, two bridging thiolato donors, and
one terminal bromido ligand in a square planar fashion. The
lengths of the two Ni−Ccarbene bonds are 1.873(6) and 1.892(5)
Å, respectively, which are shorter than those in 1a [cf. 1.977(4)
and 1.997(4) Å].5a The Ni−S bonds trans to the carbenes
[2.2329(16) and 2.2398(15) Å] are longer than those trans to
the bromido ligands [2.1740(17) and 2.1799(16) Å], because
of the stronger trans influence of the NHCs. The dihedral
angles between the [NiCSBr2] coordination plane and NHC
planes amount to 58.3(2)° and 55.8(1)°, respectively. As
observed for 1a,5a complex 2 is also bent with a similar hinge
angle of 117.43(6)°.
Synthesis of Pt(II) Dimeric Complexes. Since Pt(OAc)2
is not as easily available as Pd(OAc)2 or Ni(OAc)2, the
synthesis of the thiolato-functionalized Pt(II)-NHC complex
was first attempted by reacting ligand precursor A with PtBr2
and NaOAc as an external base at 80 °C.8 However, no reaction
occurred, and mainly starting materials were recovered. When
the temperature was increased to 160 °C, some intractable
white solids with a strong odor formed.
To circumvent this problem, a two-step protocol involving
precoordination of the thiolato-donors prior to NHC
generation was attempted.9 Thus, ligand precursor A was first
reacted with PtBr2 in boiling CH3CN (see Scheme 3) yielding
an orange precipitate poorly soluble in DMF. Washing of the
solid with DMF afforded complex 3 as a yellow powder, which
1
is slightly better soluble in DMSO. Its H NMR spectrum
recorded in DMSO-d6 shows two broad singlets narrowly
spaced at 10.15 and 10.08 ppm in an approximate 1:1 ratio
indicative of benzimidazolium NCHN protons. The three
methylene groups also give rise to three broad signals, and
coupling could not be resolved. Unfortunately, low-temperature
NMR experiments were hampered by the DMSO-d6 solvent
and the generally poor solubility of the complex. Partial
solvolysis of complex 3 may have occurred upon dissolution in
DMSO. The replacement of one bromido ligand with the
coordinating solvent would lead to inequivalent benzimidazo-
lium moieties, which is also supported by 13C NMR
spectroscopy. Furthermore, it is noteworthy that the signal
corresponding to the thioester group of A is absent, indicating
that hydrolysis took place in wet CH3CN, and coordination of
the resulting thiolato donor to Pt(II) is thus conceivable. Since
the electron-rich thiolato-donors tend to bridge two metal
centers, a dinuclear complex with a [Pt2S2] core and two
benzimidazolium moieties can be anticipated. This proposal
was supported by a signal at m/z = 1244 in the ESI mass
spectrum with the correct isotopic pattern for the [M − Br +
DMSO]+ fragment, which is also consistent with the
aforementioned solvolysis.
X-ray diffraction (XRD) analysis on single crystals obtained
by diffusing acetone into a DMSO solution provided
convincing evidence for the identity of 3. The molecular
B
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Scheme 3. Synthesis of Thiolato-Bridged Dimeric Pt(II) Complexes 3 and 4
structure depicted in Figure 2 shows that the thiolato functions
have indeed formed and are bridging two Pt(II) centers. The
Single crystals of 4 were obtained by slow evaporation of a
CH2Cl2 solution. Figure 3 depicts the molecular structure of
Figure 3. Molecular structure of the compound 4·CH2Cl2, showing
50% probability ellipsoids; H atoms, phenyl rings of N-substituents,
and the CH2Cl2 molecule are omitted for clarity. Selected bond
lengths and bond angles: Pt1−C1, 1.973(7) Å; Pt1−S1, 2.2887(17) Å;
Pt1−S2, 2.3593(18) Å; Pt1−Br1, 2.4711(8) Å; Pt2−C17, 2.004(8) Å;
Pt2−S2, 2.2996(17) Å; Pt2−S1, 2.3508(18) Å; Pt2−Br2, 2.4698(8);
C1−Pt1−S1, 90.3(2)°; S1−Pt1−S2, 80.32(6)°; S2−Pt1−Br1,
96.00(5)°; Br1−Pt1−C1, 93.3(2)°; C1−Pt1−S2, 170.6(2)°; S1−
Pt1−Br1, 174.84(5)°; C17−Pt2−S2, 90.0(2)°; S2−Pt2−S1,
80.27(6)°; S1−Pt2−Br2, 94.87(5)°; Br2−Pt2−C17, 94.9(2)°; C17−
Pt2−S1, 170.2(2)°; S2−Pt2−Br2, 174.84(5)°; Pt1−S1−Pt2,
90.03(6)°; Pt1−S2−Pt2, 89.55(6)°. PtCS2Br/NHC dihedral angle =
55.8(2)°, 51.7(2)°.
Figure 2. Molecular structure of the compound 3·2DMSO, showing
50% probability ellipsoids; H atoms and the DMSO molecule are
omitted for the sake of clarity. Selected bond lengths and bond angles:
Pt1−S1, 2.2802(16) Å; Pt1−Br1, 2.4619(6) Å; Pt1−Br2, 2.4767(7) Å;
Pt1−S1A, 2.2823(14) Å; S1−Pt1A, 2.2824(14); S1−Pt1−Br1,
92.18(4)°; Br1−Pt1−Br2, 93.48(2)°; Br2−Pt1−S1A, 91.51(4)°; S1−
Pt1−S1A, 83.03(6)°; Br2−Pt1−S1, 172.99(4)°; S1A−Pt1−Br1,
174.45(4)°; Pt1−S1−Pt1A, 96.97(6)°.
square planar geometry at each Pt(II) is completed by two
terminal bromido ligands. The two benzimidazolium moieties
remain pendant and balance the overall charge of complex 3.
The [Pt2S2] core is planar with the two benzimidazolium
groups located at opposite sides. All bond parameters are in the
expected range and do not require further comments.
the thiolato-bridged Pt(II) NHC complex 4, which is
isostructural to the Pd(II) and Ni(II) analogues 1a and 2,
respectively. The Pt−carbene bonds of 1.973(7) and 2.004(8)
Å are similar to those in 1a,5a but longer than those in 2 (see
Table 1). The Pt−S bonds trans to the carbenes [2.3593(18)
and 2.3508(18) Å] are expectedly longer than those trans to the
The desired carbene complex 4 was finally synthesized by
treatment of the [Pt2S2] complex 3 with Ag2O in CH3CN at 75
°C (see Scheme 2), since no reaction occurred at ambient
temperature. Complex 4 was isolated as a yellow powder, which
is soluble in CHCl3, CH2Cl2, CH3CN, DMF, and DMSO. Its
formation is corroborated by a peak at m/z = 1045 in the
positive ESI mass spectrum corresponding to the [M − Br +
Table 1. Comparison of Selected Parameters Observed in
the Isostructural Complexes 1a, 2, and 4
1
CH3CN]+ fragment. Its H NMR spectrum is similar to the
1a5a
2
4
Pd(II) and Ni(II) analogues 1a and 2, respectively, showing
diastereotopic methylene protons and similar splitting patterns.
The carbene carbon atoms resonate at 161.5 ppm in the 13C
NMR spectrum recorded in DMSO-d6, which is significantly
upfield shifted compared to those of 1a5a and 2 in CDCl3 (cf.
1a, 175.4 ppm; 2, 176.1 ppm), which is consistent with the
highest effective nuclear charge of Pt(II) in this series.8
13
C
(ppm)
175.4
176.1
161.5
carbene
M−Ccarbene bond
1.977(4),
1.997(4)
1.873(6),
1.892(5)
1.973(7),
2.004(8)
length (Å)
dihedral angle (°)
hinge angle (°)
55.07(9),
54.21(9)
58.3(2),
55.8(1)
55.8(2),
51.7(2)
119.26(3)
117.43(6)
134.84(3)
C
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Scheme 4. Synthesis of Trinuclear and Tetranuclear NHC Complexes
1
Figure 4. H NMR spectrum of 5 in the range of 6.50−2.20 ppm. Signals marked with an asterisk (*) are due to solvents.
bromido ligands [2.2887(17) and 2.2996(17) Å]. The latter are
comparable with the Pt−S bonds in 3. The dihedral angles
between the [PtCS2Br] coordination planes and the NHC
planes amount to 55.8(2)° and 51.7(2)°, respectively, which
are again similar to the Pd(II) and Ni(II) analogues. Different
from the planar [Pt2S2] complex 3, carbene complex 4 is bent
with a hinge angle of 134.84(3)°, which is significantly larger
than those of its analogues 1a and 2 (see Table 1).
was more promising (see Scheme 4), and tetranuclear complex
5 was isolated as an off-white precipitate, which is soluble in
DMSO but only sparingly soluble in DMF.
The formation of 5 is supported by ESI MS, which reveals a
predominant peak at m/z = 1004 with isotopic intervals of 0.5
in the positive mode corresponding to the [M − 2BF4]2+
fragment. Similar to the Pd(II) analogue 1a′,5a its H NMR
1
spectrum shows 11 signals for all the methylene groups of the
two pairs of inequivalent thiolato−NHC ligands in the range of
6.16−2.35 ppm. Accidental overlap of two methylene groups
led to a broad signal at 5.72 ppm (Figure 4). Consistent with
this, two carbene signals are observed at 148.8 and 147.1 ppm
in the 13C NMR spectrum. Attempts to obtain single crystals
for X-ray analysis met with limited success.
Synthesis of Multinuclear Group 10 NHC−Thiolato
Complexes. Previously, we reported the preparation of
macrocyclic [Pd4S4] complexes via the treatment of dimeric
Pd(II) complexes 1a and 1b with Me3OBF4, AgBF4, or NaSiPr,
respectively.5 In analogy, the possibility of synthesizing bigger
Ni and Pt aggregates from their respective dimers 2 and 4 was
explored. However, the reaction of Ni complex 2 with either
AgBF4 or Me3OBF4 in CH2Cl2 led to severe decomposition,
and downfield signals corresponding to benzimidazolium salts
Since both AgBF4 and Me3OBF4 react too harshly with 2,
milder bromide-abstracting reagents were sought. Thus, two
equiv of NaSiPr, generated in situ from NaOH (in 6.25 M
aqueous solution) and HSiPr, were used to treat 2 in the
presence of NaBF4. Analysis of the reaction mixture revealed
that halido abstraction indeed occurred. However, the resulting
1
appeared in the H NMR spectra of the products. Apparently,
the Ni−Ccarbene bonds were cleaved upon reaction. On the
other hand, the reaction of Me3OBF4 with the Pt(II) complex 4
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Figure 5. Molecular structure of the compound 6·CH2Cl2, showing 50% probability ellipsoids ((left) top view and (right) side view). H atoms,
phenyl rings of N-substituents (left), N-substituents (right), and the CH2Cl2 molecule are omitted for the sake of clarity. Selected bond lengths and
bond angles: Ni1−C1, 1.860(5) Å; Ni1−O1, 1.874(4) Å; Ni1−S1, 2.1868(15) Å; Ni1−S1B, 2.1966(15) Å; S1−S1B, 4.3380(25); C1−Ni1−S1,
93.98(16)°; C1−Ni1−S1B, 102.06(16)°; S1B−Ni1−O1, 81.79(4)°; O1−Ni1−S1, 82.05(5)°; S1−Ni1−S1B, 163.49(5)°; C1−Ni1−O1,
175.67(18)°; Ni1−O1−Ni1A, 91.0(2)°. NiCS2O/NHC dihedral angle = 47.5(2)°. Ni3S3/Ni2O dihedral angle = 54.1(1)°.
Ni(II)−NHC fragments did not aggregate with the external
isopropylthiolato ligands. Instead, the trinuclear complex 6
bearing an interesting cationic [Ni3S3O] triangle formed as a
result of reorganization and concurrent capture of an oxygen
atom. The complex charge is balanced by one BF4
counteranion.
Apparently, the thiols were not involved in this reaction, and
the O atom is likely to originate from the aqueous NaOH
solution. It is anticipated that the bromido are replaced by
hydroxido ligands in a first step, and subsequent deprotonation
of the intermediate Ni-hydroxo acid afforded a trinuclear
species with a central μ3-O2− ligand. It is worth noting that the
addition of sodium isopropylthiolate to the Pd(II)-imidazolin-
2-ylidene analogue 1b gave rise to a tetrapalladium macrocycle
with bridging isopropylthiolato ligands, which nicely demon-
strates the very different reactivity of dinickel complex 2 and its
Pd(II) analogues 1a/b.
1.892(5) Å], and the same applies to the Ni−S bonds of
2.1868(15) and 2.1966(15) Å [cf. 2.1799(16)/2.2398(15) Å
for 2]. Finally, the dihedral angles between NiCS2O and NHC
planes are 47.5(2)°.
To further explore this interesting reactivity, dimeric complex
2 was treated with Na2S·9H2O and NaBF4 in CH3CN or
CH3CN/H2O, for the purpose of preparing the sulfido
analogue of 6. However, the majority of the staring materials
were reisolated, and electrospray ionization−mass spectroscopy
(ESI MS) analysis of the latter reaction revealed that 6 formed
as the major product. The preferred binding of the hard O
atom over the soft S atom in this system is very consistent with
the HSAB theory.
−
To compare the reactivity of 2 to its higher analogues further,
1a and 4 were treated with either NaOH or Na2S·9H2O,
respectively. The Pd(II) dimer 1a did not react with NaOH and
NaBF4 under various conditions (e.g., excess reagents,
increased reaction temperature and prolonged reaction time).
On the other hand, the reaction of 1a with Na2S·9H2O in the
presence of NaBF4 and CH3CN/H2O proceeded more easily,
and the expected trinuclear [Pd3S3S] complex 7 was isolated in
the yield of 68% (see Scheme 4), as supported by ESI MS,
which shows a base peak at m/z = 1152 for the [M − BF4]+
complex cation. Similar to 6, only one set of signals is observed
A control reaction without isopropyl thiol supported this
hypothesis, and the reaction of 2 with aqueous NaOH solution
and excess NaBF4 led indeed to the isolation of 6 in a better
yield of 64%, in comparison to 40% from the reaction with the
thiols (Scheme 4). Complex 6 is soluble in CH3CN, DMSO,
and DMF, sparingly soluble in CH2Cl2 and CHCl3, but
insoluble in less-polar solvents such as hexane and diethyl ether.
Its positive-mode ESI spectrum shows a base peak at m/z = 993
1
in the H NMR spectrum of 7. Two doublets resonate at 6.10
1
corresponding to the [M − BF4]+ fragment. In the H NMR
and 5.90 ppm, respectively, which are assignable to the benzylic
protons, with a coupling constant of 2J(H−H) = 15.8 Hz. Four
other pseudo-doublets/triplets are observed in the range of
5.12−2.36 ppm, corresponding to the methylene protons of the
sulfur chelate. In its 13C NMR spectrum, one downfield signal
at 179.5 ppm is observed for the carbene carbon.
spectrum, only one set of ligand signals is observed, which
points to a symmetrical molecule. Five pseudo-doublets/triplets
resonating in the range of 6.24−1.71 ppm are assigned to the
six diastereotopic methylene protons, and consistent with the
symmetry, one carbene signal is observed at 179.6 ppm.
Single crystals of 6 were grown from CH2Cl2, and the
molecular structure determined by XRD analysis is shown in
Figure 5. There is one six-membered [Ni3S3] equilateral
triangle with an equal side length of 4.3380(25) Å. Above the
center of the triangle, one oxido ligand bridges three Ni(II)
centers, capping the triangular plane. The dihedral angle
between the [Ni3S3] triangle and each of the Ni2O planes is
54.1(1)°. Besides the μ3-oxido ligand, each Ni(II) center is
coordinated by one chelating NHC-thiolato ligand and one
additional sulfur donor from a neighboring NHC in an
essentially square planar manner. Within 3σ, the Ni−Ccarbene
bonds [1.860(5) Å] are similar to those in 2 [1.873(6) and
Surprisingly, Pt(II) dimer 4 did not react with either NaOH
or Na2S·9H2O under various reaction conditions.
CONCLUSION
■
With the isolation of complex 2 and 4, we have confirmed that
the syntheses of dimeric thiolato-bridged NHC complexes via a
combined and in situ deprotonation/hydrolysis pathway of a
thioester-functionalized azolium salt can be extended to Ni(II)
and Pt(II). The general methodology of masking an NHC−
thiol ligand via its thioester-functionalized azolium ligand
precursor is likely to find more applications in the coordination
E
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Ar−H), 8.20 (d, 3J(H,H) = 9.0 Hz, 1 H, Ar−H), 7.93 (s, 2 H, Ar−H),
7.66−7.39 (m, 14 H, Ar−H), 5.80 (br-s, 4 H, CH2Ph), 5.33 (br-s, 4 H,
NCH2), 3.26 (br-s, 4 H, CH2S). 13C{1H} NMR (75.47 MHz, DMSO-
d6): 143.1, 143.0 (s, NCHN), 134.2, 134.16, 131.9, 131.7, 131.3, 129.3,
129.1, 128.7, 127.1, 114.3 (s, Ar−C), 50.4 (s, CH2Ph), 48.5 (d,
chemistry of NHC/thiolato chelators. Reactivity studies of the
aforementioned dimeric complexes and their Pd(II) analogue
1a toward either Me3OBF4, NaOH, or Na2S·9H2O gave
different results. Ni(II) dimer 2 decomposed when treated with
Me3OBF4, while Pt(II) analogue 4 reacted in a similar way to
Pd(II) dimer 1a, leading to tetranuclear [Pt4S4] macrocycle 5.
The reaction of 2 and aqueous NaOH in the presence of NaBF4
yielded a new trinuclear Ni(II) complex 6 with a [Ni3S3O]
triangle via capture of an μ3-oxido ligand. In contrast, the
reaction of Pd(II) dimer 1a with NaOH was sluggish, while the
treatment of 1 with Na2S·9H2O yielded a new trinuclear Pd(II)
species [Pd3S3S] with an capping μ3-sulfide ligand. On the
other hand, the Pt(II) analogue 4 did not lead to the isolation
of any product with either NaOH or Na2S·9H2O. All complexes
have been fully characterized and some of their molecular
structures determined by X-ray diffraction (XRD) analyses.
Research in our laboratory is currently ongoing to extend our
methodology to transition metals beyond group 10. Studies are
also underway to explore the preparation of higher NHC/
thiolato aggregates by simple halido abstraction as an entry to
metallo-based supramolecular chemistry.
2
3J(C,Pt) = 42 Hz, NCH2), 30.2 (d, J(C,Pt) = 55 Hz, CH2S). Anal.
Calcd for C32H32Br4N4Pt2S2: C, 30.83%; H, 2.59%; N, 4.49%. Found:
C, 30.60%; H, 2.17%; N, 4.36%. MS (ESI): m/z = 1244 [M − Br]+.
Dimeric Pt(II) Complex 4. Complex 3 (374 mg, 0.3 mmol) and
Ag2O (70 mg, 0.3 mmol) were suspended in CH3CN (10 mL) and
heated at 75 °C overnight. The mixture was cooled and filtered over
Celite to remove the precipitate. After the solvent of the filtrate was
removed in vacuo, the resulting solid was redissolved in CH2Cl2 (10
mL) and passed through a short plug of silica gel. Removal of the
solvent afforded the product as a yellow solid (172 mg, 0.16 mmol,
1
53%). H NMR (500 MHz, DMSO-d6): δ 7.86 (d, 3J(H,H) = 5.0 Hz,
2 H, Ar−H), 7.57−7.52 (m, 6 H, Ar−H), 7.38−7.28 (m, 10 H, Ar−
H), 6.48 (d, 2J(H,H) = 15.8 Hz, 2 H, NCHHPh), 5.68 (d, 2J(H,H) =
15.8 Hz, 2 H, NCHHPh), 5.03 (ps-d, 2 H, NCHH), 4.31 (ps-d, 2 H,
NCHH), 3.76 (ps-d, 2 H, CHHS), 2.24 (ps-t, 2 H, CHHS). 13C{1H}
NMR (125.77 MHz, DMSO-d6): 161.5 (s, NCN), 136.7, 133.7, 133.5,
129.1, 128.3, 127.9, 124.3, 124.2, 112.6, 112.3 (s, Ar−H), 51.2 (s,
NCH2Ph), 50.4 (s, NCH2), 23.1 (s, CH2S). Anal. Calcd for
C32H30Br2N4Pt2S2: C, 35.43%; H, 2.79%; N, 5.17%. Found: C,
35.40%; H, 2.73%; N, 4.79%. MS (ESI): m/z = 1045 [M − Br +
CH3CN]+.
EXPERIMENTAL SECTION
■
Tetranuclear Pt(II) Complex 5. A mixture of 4 (54 mg, 0.05
mmol) and Me3OBF4 (22 mg, 0.15 mmol) in dry CH2Cl2 (5 mL) was
heated under reflux overnight. The resulting mixture was filtered and
the product was obtained as an off-white powder (17 mg, 0.008 mmol,
General Considerations. Unless otherwise noted, all operations
were performed without taking precautions to exclude air and
moisture, and all solvents and chemicals were used as-received.
Thioester-functionalized benzimidazolium salt A and dimeric Pd(II)
complex 1a have been synthesized according to a reported
procedure.5a 1H, 13C, and 19F NMR spectra were recorded on a
Bruker ACF 300 spectrometer or AMX 500 spectrophotometer, and
the chemical shifts (δ) were internally referenced to the residual
solvent signals relative to tetramethylsilane (1H, 13C) or externally to
CF3CO2H (19F NMR). Electrospray ionization−mass spectroscopy
(ESI MS) spectra were measured using a Finnigan MAT LCQ
spectrometer. Elemental analyses were performed on an Elementar
Vario Micro Cube elemental analyzer at the Department of Chemistry,
National University of Singapore.
1
30%). H NMR (500 MHz, DMSO-d6): δ 8.02 (d, 3J(H,H) = 5.0 Hz,
3
2 H, Ar−H), 7.75 (d, J(H,H) = 5.0 Hz, 4 H, Ar−H), 7.54−7.29 (m,
2
30 H, Ar−H), 6.14 (d, J(H,H) = 15.1 Hz, 2 H, CHHPh), 5.78 (d,
2J(H,H) = 15.1 Hz, 2 H, CHHPh), 5.72 (br s, 4 H, CH2Ph), 5.44 (ps-
d, 2 H, NCHH), 5.25 (ps-d, 2 H, NCHH), 5.10 (ps-t, 2 H, NCHH),
4.92 (ps-t, 2 H, NCHH), 4.05 (ps-d, 2 H, CHHS), 2.94 (ps-t, 2 H,
CHHS), 2.60 (ps-t, 2 H, CHHS), 2.36 (ps-d, 2 H, CHHS). 13C{1H}
NMR (125.77 MHz, DMSO-d6): 148.8, 147.1 (s, NCN), 135.5,
135.48, 133.8, 133.6, 133.1, 129.4, 129.1, 128.8, 128.4, 127.7, 127.2,
125.2, 125.16, 124.9, 113.1, 113.0, 112.8, 112.5 (s, Ar−H), 51.0, 50.6
(s, CH2Ph), 50.3, 49.1 (s, NCH2), 33.9, 31.6 (s, CH2S). 19F{1H} NMR
(282.37 MHz, DMSO-d6): −72.28 (s, 10BF4), −72.33 (s, 11BF4). Anal.
Calcd for C64H60B2Br2F8N8Pt4S4: C, 35.21%; H, 2.77%; N, 5.13%.
Found: C, 35.13%; H, 3.04%; N, 4.79%. MS (ESI): m/z = 1004 [M −
2BF4]2+.
Dimeric Ni(II) Complex 2. A mixture of salt A (117 mg, 0.3
mmol), Ni(OAc)2 (53 mg, 0.3 mmol) and [N(n-Bu)4]Br (0.5 g) was
dried under vacuum at 60 °C for 3 h. After slowly heating to 120 °C,
the solid melted and the color changed to dark red. The mixture was
stirred at this temperature under vacuum overnight. After cooling to
ambient temperature, H2O (20 mL) and CH2Cl2 (10 mL) were added
to the resulting solid. The organic phase was separated, washed with
H2O (3 × 20 mL), dried over Na2SO4, and passed through a short
plug of silica gel. Removal of the solvent in vacuo afforded the product
as a red solid (81 mg, 0.10 mmol, 68%). 1H NMR (500 MHz, CDCl3):
δ 7.61 (d, 3J(H,H) = 7.6 Hz, 4 H, Ar−H), 7.38−7.28 (m, 8 H, Ar−H),
7.24−7.13 (m, 6 H, Ar−H), 6.71 (d, 2J(H,H) = 15.8 Hz, 2 H,
NCHHPh), 5.61 (d, 2J(H,H) = 15.8 Hz, 2 H, NCHHPh), 5.02 (ps-t, 2
H, NCHH), 4.78 (ps-d, 2 H, NCHH), 3.14 (ps-d, 2 H, CHHS), 1.56
(ps-t, 2 H, CHHS). 13C{1H} NMR (125.77 MHz, CDCl3): 176.1 (s,
NCN), 136.0, 135.0, 134.0, 128.8, 128.0, 127.9, 123.04, 123.02, 111.1,
109.9 (s, Ar−H), 52.7 (s, NCH2Ph), 49.4 (s, NCH2), 25.6 (s, CH2S).
Anal. Calcd for C32H30Br2N4Ni2S2: C, 47.34%; H, 3.72%; N, 6.90%.
Found: C, 47.47%; H, 3.75%; N, 6.62%. MS (FAB): m/z = 733 [M −
Br]+.
Trinuclear Ni(II) Complex 6. A mixture of complex 2 (41 mg,
0.05 mmol), NaOH (6.25 mol/L aqueous solution, 16 μL, 0.1 mmol),
and NaBF4 (6 mg, 0.05 mmol) was stirred in CH3CN (5 mL) at 70 °C
overnight. The mixture was allowed to cool to ambient temperature
and filtered over Celite. The solvent of the filtrate was removed under
vacuum, and the resulting red solid was redissolved in CH3CN (5 mL)
and filtered over Celite again. After the CH3CN was removed in vacuo,
the red solid was washed with CH2Cl2 (3 × 3 mL) to give complex 6.
The CH2Cl2 solution was left for slow evaporation to obtain red
1
crystals as a second crop of complex 6 (23 mg, 0.02 mmol, 64%). H
NMR (500 MHz, DMSO-d6): δ 7.75−7.62 (m, 12 H, Ar−H), 7.42−
2
7.28 (m, 15 H, Ar−H), 6.24 (d, J(H,H) = 15.8 Hz, 3 H, CHHPh),
5.78 (m, 6 H, CHHPh + NCHH), 5.34 (ps-d, 3 H, NCHH), 1.94 (ps-
t, 3 H, CHHS), 1.71 (ps-d, 2 H, CHHS). 13C{1H} NMR (125.77
MHz, DMSO-d6): 179.6 (s, NCN), 136.7, 134.6, 134.5, 129.3, 128.6,
127.9, 123.9, 123.7, 111.7, 111.3 (s, Ar−H), 51.1 (s, CH2Ph), 49.9 (s,
NCH2), 28.8 (s, CH2S). 19F{1H} NMR (282.37 MHz, DMSO-d6):
−72.28 (s, 10BF4), −72.33 (s, 11BF4). Anal. Calcd for
C48H45BF4N6Ni3OS3·3DMSO: C, 49.31%; H, 4.83%; N, 6.39%.
Found: C, 49.23%; H, 4.36%; N, 6.59%. MS (ESI): m/z = 993 [M
− BF4]+.
Dinuclear Pt(II) Complex 3. Salt A (391 mg, 1 mmol) and PtBr2
(355 mg, 1 mmol) were dissolved in CH3CN (5 mL) and heated
under reflux overnight. The resulting orange solid was collected by
filtration and washed with DMF until the color turned yellow.
Complex 3 was obtained after washing with diethyl ether (3 × 5 mL)
and drying under vacuum. The DMF solution was left to stand and the
second batch of the product was isolated by collecting the yellow
precipitate and washing it with diethyl ether (3 × 5 mL) (374 mg, 0.3
Trinuclear Pd(II) complex 7. Complex 1a5a (30 mg, 0.033 mmol)
was treated with Na2S·9H2O (8 mg, 0.033 mmol) and NaBF4 (4 mg,
0.033 mmol) in CH3CN (5 mL) and H2O (1 mL) at 70 °C for one
day. After cooling to ambient temperature, the mixture was filtered
1
mmol, 60%). H NMR (300 MHz, DMSO-d6): δ 10.15 (br-s, 1 H,
3
NCHN), 10.08 (br-s, 1 H, NCHN), 8.29 (d, J(H,H) = 9.0 Hz, 1 H,
F
dx.doi.org/10.1021/ic400672z | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Selected X-ray Crystallographic Data for Complexes 2, 3·2DMSO, 4·CH2Cl2, and 6·CH2Cl2
2
3·2DMSO
4·CH2Cl2
6·CH2Cl2
formula
C32H30Br2N4Ni2S2
C32H32Br4N4Pt2S2·2C2H6OS
C33H32Br2Cl2N4Pt2S2·CH2Cl2
C48H45BF4N6Ni3OS3·CH2Cl2
formula wt, Fw
color, habit
cryst size [mm]
temp [K]
cryst syst
space group
a [Å]
811.96
1402.81
1169.65
1165.95
red, block
yellow, rod
0.46 × 0.26 × 0.10
223(2)
colorless, block
red, block
0.12 × 0.10 × 0.10
296(2)
0.26 × 0.10 × 0.10
100(2)
0.16 × 0.10 × 0.04
100(2)
orthorhombic
monoclinic
P2(1)/n
orthorhombic
rhombohedral
P2(1)2(1)2(1)
Fdd2
R3c
9.7788(4)
13.0288(7)
7.8627(5)
21.6471(12)
90
18.1205(9)
13.6577(12)
b [Å]
15.2048(6)
49.575(3)
13.6577(12)
c [Å]
21.1848(8)
15.1791(8)
45.745(4)
α [deg]
90
90
90
β [deg]
90
107.2210(10)
90
90
90
γ [deg]
V [Å3]
90
90
120
3149.9(2)
2118.1(2)
2
13635.7(12)
7389.7(11)
Z
4
16
6
Dc [g cm−3
]
1.712
2.199
2.279
1.572
radiation used
μ [mm−1
Mo Kα
Mo Kα
Mo Kα
Mo Kα
]
3.895
10.605
10.854
1.430
θ range [deg]
1.65−27.49
7215
1.64−27.49
4860
1.80−27.50
7820
1.94−27.49
2856
no. of unique data
max., min transmn
final R indices [I > 2σ(I)]
R indices (all data)
goodness-of-fit on F2
0.6968, 0.6522
R1 = 0.0508, wR2 = 0.1051
R1 = 0.0828, wR2 = 0.1156
0.989
0.4169, 0.0847
R1 = 0.0389, wR2 = 0.0947
R1 = 0.0483, wR2 = 0.0991
1.043
0.4099, 0.1647
R1 = 0.0295, wR2 = 0.0726
R1 = 0.0321, wR2 = 0.0737
1.030
0.9450, 0.8034
R1 = 0.0486, wR2 = 0.1235
R1 = 0.0521, wR2 = 0.1261
1.121
peak/hole [e Å−3
]
0.576/−0.526
1.516/−1.997
2.069/−1.141
0.0914/−0.842
over Celite and the solvent of the filtrate was removed. The resulting
yellow solid was redissolved in CH3CN (5 mL) and filtered over Celite
again. After the solvent was removed in vacuo, the yellow solid was
redissolved in MeOH (5 mL) and filtered. Complex 7 was obtained as
a yellow solid after removal of the solvent (19 mg, 0.015 mmol, 68%).
1H NMR (500 MHz, DMSO-d6): δ 7.78−7.73 (m, 6 H, Ar−H), 7.60−
7.58 (m, 6 H, Ar−H), 7.41−7.36 (m, 15 H, Ar−H), 6.10 (d, 2J(H,H)
= 15.8 Hz, 3 H, CHHPh), 5.90 (d, 2J(H,H) = 15.8 Hz, 3 H, CHHPh),
5.12 (ps-d, 2 H, NCHH), 4.13 (ps-t, 3 H, NCHH), 2.36 (ps-d, 2 H,
CHHS). The other signal due to CHHS overlaps with the residue
signal of d6-DMSO. 13C{1H} NMR (125.77 MHz, DMSO-d6): 179.5
(s, NCN), 135.8, 133.6, 133.2, 128.9, 128.2, 127.5, 123.9, 112.2, 111.7
(s, Ar−H), 51.3 (s, CH2Ph), 50.4 (s, NCH2), 30.1 (s, CH2S). 19F{1H}
NMR (282.37 MHz, DMSO-d6): −72.28 (s, 10BF4), −72.33 (s, 11BF4).
Anal. Calcd for C48H45BF4N6Pd3S4: C, 46.48; H, 3.66; N, 6.78. Found:
C, 46.40; H, 3.80; N, 6.79%. MS (ESI): m/z = 1152 [M − BF4]+.
X-ray Diffraction Studies. X-ray data for 2, 3·2DMSO, 4·CH2Cl2,
and 6·CH2Cl2 were collected with a Bruker AXS SMART APEX
diffractometer, using Mo Kα radiation at 296(2) K (for 2), 223(2) K
(for 3·2DMSO), or 100(2) K (for 4·CH2Cl2 and 6·CH2Cl2) with the
SMART suite of Programs.10 Data were processed and corrected for
Lorentz and polarization effects with SAINT,11 and for absorption
effect with SADABS.12 Structural solution and refinement were carried
out with the SHELXTL suite of programs.13 The structure was solved
by direct methods to locate the heavy atoms, followed by difference
maps for the light, non-hydrogen atoms. All non-hydrogen atoms were
generally given anisotropic displacement parameters in the final model.
All H atoms were put at calculated positions. A summary of the most
important crystallographic data is given in Table 2.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chmhhv@nus.edu.sg.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National University of Singapore for
financial support (No. WBS R-143-000-410-112). Technical
support from staff at the CMMAC of our department is
appreciated. In particular, we thank Ms. Geok Kheng Tan and
Prof. Lip Lin Koh for determining the X-ray molecular
structures.
REFERENCES
■
(1) (a) Kuhl, O. Chem. Soc. Rev. 2007, 36, 592. (b) Normand, A. T.;
̈
Cavell, K. J. Eur. J. Inorg. Chem. 2008, 2781.
(2) (a) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2006, 25,
5927. (b) Jahnke, M. C.; Pape, T.; Hahn, F. E. Eur. J. Inorg. Chem.
2009, 1960.
(3) (a) John, A.; Ghosh, P. Dalton Trans. 2010, 39, 7183.
(b) Bierenstiel, M.; Cross, E. D. Coord. Chem. Rev. 2011, 255, 574.
(4) Yuan, D.; Huynh, H. V. Molecules 2012, 17, 2491 and references
therein.
(5) (a) Yuan, D.; Huynh, H. V. Organometallics 2010, 29, 6020.
(b) Yuan, D.; Huynh, H. V. Dalton Trans. 2011, 40, 11698.
(6) (a) Sellmann, D.; Geipel, F.; Heinemann, F. W. Z. Anorg. Allg.
Chem. 2001, 627, 1034. (b) Cabeza, J. A.; del Rio, I.; San
́
chez-Vega, M.
G.; Suarez, M. Organometallics 2006, 25, 1831. (c) Iwasaki, F.; Yasui,
́
ASSOCIATED CONTENT
M.; Yoshida, S.; Nishiyama, H.; Shimamoto, S.; Matsumura, N. Bull.
Chem. Soc. Jpn. 1996, 69, 2759. (d) Holm, S. C.; Rominger, F.; Straub,
B. F. J. Organomet. Chem. 2012, 719, 54.
■
S
* Supporting Information
Crystallographic data for 2, 3·2DMSO, 4·CH2Cl2, and
6·CH2Cl2 as CIF files. This material is available free of charge
via the Internet at http://pubs.acs.org.
(7) (a) Huynh, H. V.; Holtgrewe, C.; Pape, T.; Koh, L. L.; Hahn, E.
Organometallics 2006, 25, 245. (b) Huynh, H. V.; Wong, L. R.; Ng, P.
S. Organometallics 2008, 27, 2231.
G
dx.doi.org/10.1021/ic400672z | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(8) (a) Han, Y.; Huynh, H. V.; Tan, G. K. Organometallics 2007, 26,
4612. (b) Ku, R.; Huang, J.; Cho, J.; Kiang, F.; Reddy, K. R.; Chen, Y.;
Lee, K.; Lee, J.; Lee, G.; Peng, S.; Liu, S. Organometallics 1999, 18,
2145.
(9) Huynh, H. V.; Meier, N.; Pape, T.; Hahn, F. E. Organometallics
2006, 25, 3012.
(10) SMART, Version 5.628; Bruker AXS, Inc.: Madison, WI, 2001.
(11) SAINT+, Version 6.22a; Bruker AXS, Inc.: Madison, WI, 2001.
(12) Sheldrick, G. W. SADABS, Version 2.10; University of
Gottingen: Gottingen, Germany, 2001.
̈
̈
(13) SHELXTL, Version 6.14; Bruker AXS, Inc.: Madison, WI, 2000.
H
dx.doi.org/10.1021/ic400672z | Inorg. Chem. XXXX, XXX, XXX−XXX