A cationic NCN pincer platinum(II) aquo complex with a bis(imidazolinyl) phenyl ligand: Studies toward its synthesis and asymmetric Friedel-Crafts alkylation of indoles with nitroalkenes

Article abstract of DOI:10.1021/om200714z

Studies toward the synthesis and characterization of a cationic NCN pincer platinum(II) aquo complex with a chiral bis(imidazolinyl)phenyl (Phebim) ligand are reported. During these studies, the new symmetrical Phebim-H ligands 1c,d and unsymmetrical hybr

Full text of DOI:10.1021/om200714z

Article
pubs.acs.org/Organometallics
A Cationic NCN Pincer Platinum(II) Aquo Complex with a
Bis(imidazolinyl)phenyl Ligand: Studies toward its Synthesis and
Asymmetric Friedel−Crafts Alkylation of Indoles with Nitroalkenes
Xin-Qi Hao, Yan-Xiao Xu, Ming-Jun Yang, Li Wang, Jun-Long Niu, Jun-Fang Gong,*
and Mao-Ping Song*
Department of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Zhengzhou University, Zhengzhou
450052, People’s Republic of China
S
* Supporting Information
ABSTRACT: Studies toward the synthesis and character-
ization of a cationic NCN pincer platinum(II) aquo complex
with a chiral bis(imidazolinyl)phenyl (Phebim) ligand are
reported. During these studies, the new symmetrical Phebim-
H ligands 1c,d and unsymmetrical hybrid oxazoline−imidazo-
line ligands 6a−c, as well as the related neutral symmetrical
NCN pincer Pt(II) complexes 2b−d and 3a−d and unsym-
metrical NCN′ pincers 7a,b and 8c have been isolated and
characterized. Despite considerable attempts, only one of the
expected cationic chiral NCN pincer Pt(II) complexes (4), which contains water as an exchangeable neutral ligand for the Pt(II)
center, was obtained in pure form from the reaction of the neutral Pt−I complex 3a with AgOTf in wet CH2Cl2 at room
temperature. In contrast, the cationic Pt(II) pyridine complexes 5a,c could be easily prepared in high yields from the reaction of
the neutral Pt−Cl complexes 2a,c with AgOTf in the presence of pyridine in CH2Cl2 at room temperature. The molecular
structures of the neutral symmetrical Pt(II) complexes 2c,d and 3c,d and the unsymmetrical Pt(II) complex 7b as well as the
cationic Pt(II) aquo complex 4 have been determined by X-ray single-crystal analysis. The cationic complexes 4 and 5a,c have
been applied to the asymmetric Friedel−Crafts alkylation of indoles with nitroalkenes. Among them, the aquo complex 4 was
found to be effective under the given reaction conditions. With a catalyst loading of 5 mol %, a variety of the nitroalkylated
indoles were produced in good yields with moderate to good enantioselectivities (up to 83% ee).
complexes also displayed excellent enantioselectivities in the
hydrogenation (up to 98% ee) and transfer hydrogenation of
ketones (up to 97% ee)7a,b and the trans-selective cyclo-
propanations with tert-butyl α-diazoacetates (up to 99% ee)7c
as well as in the direct alkynylation of aldehydes with terminal
alkynes (up to 95% ee).7d In contrast, the Pt and especially the
Pd Phebox complexes showed relatively inferior potential of
stereocontrol. For example, the cationic Pt complexes have
been used as stoichiometric controllers for the alkylation of
aldimines with up to 82% ee4a and as catalysts for the aldol
reaction of isocyanides and aldehydes with up to 75% ee.4b The
Pd complex was found to exhibit moderate stereoselectivity in
asymmetric Suzuki−Miyaura coupling for the synthesis of
axially chiral biaryl compounds (up to 49% ee)3b and the
cationic Pd-Phebox complexes catalyzed the Michael reaction
of 2-cyanocarboxylates with various Michael acceptors to form
adducts, with the highest ee being only 34%.3a
INTRODUCTION
■
Organo-transition-metal pincer complexes with anionic triden-
tate 1,3-bis(2′-oxazolinyl)phenyl (Phebox) have emerged as a
versatile chiral pincer complex family since the first studies
independently reported by Denmark, Richards, and Nishiyama
in 1997.1,2 In these complexes, the Phebox ligands coordinate
to the metal center via N atoms of the two oxazolines and the
central phenyl carbanion in a meridional fashion, which gives
them the name NCN pincer complexes. The chiral moiety in
the Phebox skeletons can be easily introduced and tuned by
using various optically active amino alcohols, providing ample
opportunities to fine tune the catalytic activities and particularly
stereoselectivities of the complexes. Therefore, a variety of
metal−Phebox complexes, including those of Pd,1,3 Pt,4 Rh,5
Ir,5d Ni,3c,6 Ru,7 and Fe,8 have been prepared and used in
asymmetric catalysis, with the majority of the later work being
carried out by the research group of Nishiyama. Among these
metal−Phebox complexes, the Rh complexes have been
extensively investigated and exhibited high performance in
asymmetric catalytic reactions, such as conjugate reduction of
α,β-unsaturated esters with hydrosilanes (up to 98% ee)5b,d and
reductive aldol reaction of acrylates and aldehydes with
hydrosilanes (up to 96% ee for anti products).5c,d The Ru
On the other hand, 2-imidazoline is a structural analogue of
2-oxazoline and, similar to the case for the oxazolines, the
chirality in the imidazolines can also be easily introduced and
Received: August 2, 2011
Published: January 23, 2012
© 2012 American Chemical Society
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Chart 1. Synthesis of the Phebim-H Ligands as Well as the Neutral Pincer Pt(II), Pd(II), and Ni(II) Complexes via the C−H
Activation of the Ligands
Scheme 1. Synthesis of the Neutral Pincer Pt(Phebim)Cl and Pt(Phebim)I Complexes
tuned through using different chiral amino alcohols. Moreover,
imidazoline ligands have demonstrated unique advantages of
further tuning electronic and conformational properties of the
ligands by the proper choice of substituent on the additional
nitrogen atom. As a result, the imidazoline-derived catalyst
systems were often superior to those of the corresponding
oxazolines in terms of the yields and enantioselectivities of the
catalysis products.9,10 For example, in the enantioselective
Friedel−Crafts alkylation reaction of indole with ethyl 3,3,3-
trifluoropyruvate the m-phenylene-bis(imidazoline)/Cu(II)
complex gave the product in a 97% yield with 93% ee even
with a catalyst loading of 0.5 mol %, while the corresponding
bis(oxazoline)/Cu(II) complex afforded a 87% yield with 77%
ee with a catalyst loading of 10 mol % under the same reaction
conditions.10a Similar phenomena were also observed in the
Cu(I)-catalyzed three-component reaction of aldehydes,
amines, and aliphatic alkynes for the synthesis of optically
active propargylamines, where the bis(imidazoline)pyridine/
Cu(I) outperformed the related bis(oxazoline)pyridine
(Pybox)/Cu(I) in both the yield and enantioselectivity of the
product (93% yield with 98% ee vs 33% yield with 82% ee).10b
Following our interest in the metallacycles as well as their
applications, we have synthesized a series of neutral NCN
pincer Pt(II), Pd(II),11 and Ni(II)12 complexes (Chart 1) with
anionic tridentate 1,3-bis(2′-imidazolinyl)phenyl ligands (ab-
breviated as Phebim). It was hoped that Phebim would provide
a more flexible ligand scaffold and also be a valuable addition to
the known Phebox. In fact, it was found that the Phebim-H
ligands did exhibit reactivities different from Phebox-H ligands
toward direct C−H activation for the preparation of the related
pincer Pt, Pd, and Ni complexes, with the activation of the
former being more facile. Unfortunately, attempts to get pure
samples of the cationic Pt(II) aquo complexes as potential
chiral Lewis acid catalysts from the reaction of the neutral
Pt(Phebim)Cl complexes with Ag(I) salts proved to be
unsuccessful. Nonetheless, the formed crude cationic species
was found to be an effective catalyst for the enantioselective
Friedel−Crafts alkylation between indole and trans-β-nitro-
styrene with moderate stereoselectivity (55% ee).11b In a
continuation of this study, herein we report in detail our studies
on the synthesis of the cationic pincer Pt(II) aquo complex
with the Phebim ligand. Investigations on its application to the
asymmetric Friedel−Crafts alkylation of indoles with nitro-
alkenes are also presented.
RESULTS AND DISCUSSION
■
Synthesis of Bis(imidazoline) Ligands and the Neutral
Pincer Pt(Phebim)Cl and Pt(Phebim)I Complexes as Well
as the Cationic Pincer Pt(II) Complexes. Following the
procedure previously reported by us,11 the chiral bis-
(imidazoline) ligands (Phebim-H) 1a−d and the corresponding
neutral pincer Pt(II) chloride complexes 2a−d could be
successfully obtained (Scheme 1). Then, replacement of the
chloride ligand in the Pt(Phebim)Cl complex 2a by an
exchangeable neutral ligand such as H2O to form the cationic
metal−aquo complex was attempted. Although this seemed to
be very facile for the related Pt(Phebox)Cl complexes,4 the
chloride abstraction on 2a with commonly used Ag(I) salts
such as AgBF4, AgOTf, and AgSbF6 under various conditions
could not afford the separable, pure cationic aquo complex. In
addition, when the cationic species obtained in this way was
used in the asymmetric Friedel−Crafts alkylation between
indole and trans-β-nitrostyrene in toluene at room temperature,
it was found that the reproducibility of the catalytic results was
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not good, giving the product in high yields with the
enantioselectivities in the range of 40−64% ee. Therefore, a
way to get the pure samples of the cationic aquo complexes
must be found. Considering that AgI precipitates more easily
than AgCl, we are thinking that the corresponding Pt(Phebim)I
complexes 3 (Scheme 1), which can be prepared from the
chlorides by the halogen exchange reaction, may be better
precursors for the Pt(II) aquo complexes. Thus, the Pt−Cl
complexes 2a−d were treated with an excess of KI in acetone−
MeOH at room temperature for 12 h and the iodides 3a−d
were readily isolated in 76−83% yields (in the case of 3d, the
yield was 54% based on the ligand 1d).
Scheme 3. Formation of the Neutral Pt(Phebim)Cl Complex
2d from the Reaction of the Pt(Phebim)I Complex 3d with
AgOTf
With the expected iodides in hand, the iodide abstraction on
3a was first investigated due to its higher yield from ligand 1a
via complex 2a. The reaction was carried out with complex 3a
and 2 equiv of AgOTf in wet CH2Cl2 (CH2Cl2/H2O = 9.5 mL/
0.5 mL) at room temperature protected from light (Scheme 2).
which led to the unsuccessful isolation of the pure cationic Pt−
OH2 complexes due to the existence of substantial Pt−Cl
complexes for 3b,c and isolation of the pure Pt−Cl complex for
3d. In addition, it should be mentioned that the reactions of 3,
particularly those of 3b−d with AgOTf, may also give some
Pt−OTf complexes, since the Pt−OTf complexes with Phebox
ligands could be produced from the reaction of Pt(Phebox)Cl
complexes with AgOTf.4a,b Therefore, 3d was treated with
AgBF4 in wet acetone (acetone/H2O = 10 mL/0.1 mL) at
room temperature and acetone instead of CH2Cl2 was used as
solvent in the workup to avoid the formation of the Pt−Cl and
Pt−OTf complexes. After removal of AgI by repeated filtration
through Celite and evaporation of solvent, a yellow solid was
Scheme 2. Synthesis of the Cationic Pincer Pt(II) Aquo
Complex 4 from the Pt(Phebim)I Complex 3a
1
obtained. Disappointingly, the H NMR spectrum of the solid
exhibited many more signals for the phenyl and cyclohexyl
protons than expected though it had some obvious differences
from that of starting complex 3d. Purification of the solids by
recrystallization from acetone was unsuccessful, and recrystal-
lization from CH2Cl2/n-hexane gave the chloride complex 2d
again.
On the other hand, it was found that the cationic Pt(II)
pyridine complexes 5a,c could be easily prepared in high yields
from the reaction of the neutral Pt−Cl complexes 2a,c with
AgOTf in the presence of pyridine in CH2Cl2 at room
temperature (Scheme 4). Different from the case for the
After removal of AgI by repeated filtration through Celite with
CH2Cl2 as eluent, evaporation of solvent, and recrystallization
from CH2Cl2/n-hexane, light yellow needles were obtained.
The NMR spectra, including 1H and 13C NMR of the obtained
needles, showed the essentially pure sample and some obvious
difference from those of complex 3a in the signals due to the
CH(CH3)2 proton (broad singlet at δ 2.43 ppm vs mutiplet at δ
2.87−2.83 ppm) and the ortho-metalated carbon atom (155.1
ppm vs 166.4 ppm), although the signal due to the metal-
1
coordinated water protons was not observed in the H NMR
spectrum. Elemental analysis of C, H, N, and S contained in the
sample was consistent with the expected formula, and
furthermore X-ray single-crystal analysis (vide infra) unambig-
uously confirmed that the needles were the cationic metal−
aquo complex 4. The result with complex 3a seemed quite
inspiring; however, the following synthesis of the correspond-
ing cationic Pt−OH2 complexes from 3b−d by the procedure
similar to that for 3a proved to be problematic. In the case of
complexes 3b,c, the pure samples of the cationic complexes
could not be isolated. Also, in the case of 3d, it was very
surprising for us to find that the red crystal is the neutral
chloroplatinum complex 2d when we obtained its X-ray crystal
structure (vide infra). The yield of 2d produced in this way was
very high (92% based on 3d, Scheme 3). We guessed that the
reaction of 3d with AgOTf first gave the related cationic
species, which seized chloride from CH2Cl2 used in the reaction
or the workup, producing the chloride complex 2d. On the
basis of the above results, it seemed reasonable to think that
iodide abstraction of 3a−d by AgOTf actually proceeded with
ease. However, the formed cationic Pt−OH2 complexes had a
tendency to capture chloride from CH2Cl2 and the tendency
depended much on the substituents on the imidazoline rings,
Scheme 4. Synthesis of the Cationic Pincer Pt(II) Pyridine
Complexes 5 from the Pt(Phebim)Cl Complexes 2
cationic Pt−OH2 complexes with Phebox4 or Phebim ligands,
the cationic Pt−Py complexes 5 can be purified by
chromatography on silica gel. The facile preparation of cationic
Pt−Py complexes further indicated that the halide abstraction
on complexes 2 and 3 by AgOTf was really effective. The two
complexes 5a,c with a loading of 5 mol % were applied to the
asymmetric Friedel−Crafts alkylation between indole and trans-
β-nitrostyrene in toluene. Unfortunately, the formation of the
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Scheme 5. Synthesis of the Hybrid Oxazoline−Imidazoline Ligands and the Related Unsymmetrical, Neutral Pincer Pt(II)
Chloride and Iodide Complexes
expected adduct was not observed at 0 °C after 36 h or at room
temperature after 24 h.
that for 3a). This made it impossible to investigate the
enantioselectivities of the cationic complexes with different
chiral substituents on the imidazoline rings in the asymmetric
reactions to optimize the catalyst. On the other hand, the
related cationic Pt(Phebox) complexes could be easily
produced from the neutral Pt(Phebox)Cl complexes.4 Con-
sequently, we were interested to see how the neutral,
unsymmetrical pincer Pt(II) complexes with hybrid oxazo-
line−imidazoline ligands would behave. Also, it was hoped that
the corresponding cationic aquo complexes, which are
structurally different, particularly when the chiral substituents
on the imidazoline/oxazoline rings are different from those of
complex 4, could be successfully isolated in order to get the
optimal catalyst for the assigned asymmetric reaction. For this
purpose, three chiral and unsymmetrical NCN′ pincer ligand
precursors 6a−c (Scheme 5) were synthesized. The synthetic
procedure was similar to that for the symmetrical bis-
(imidazoline) ligands as shown in Chart 1. First, reaction of
isophthalyl chloride with L-valinol or L-phenylglycinol readily
afforded the corresponding known bis(amido alcohols). Then
treatment of the bis(amido alcohols) with excess SOCl2 for a
shorter time (2 h), followed by the reaction with a smaller
amount of amine (1.5 equiv relative to the bis(amido alcohols))
in comparison with preparation of the bis(imidazoline) ligands
(6 h and 2.2 equiv of amine) could afford the hybrid ligands
6a−c. Among these, the pure samples of ligand 6c were isolated
in a 33% yield (based on the bis(amido alcohol)), while the
ligands 6a,b might contain some bis(imidazoline) ligands seen
All of the new ligands and Pt(II) complexes were well
characterized by elemental analysis (HRMS for ligands) and 1H
NMR, 13C NMR, and IR spectra. The 1H NMR and 13C NMR
spectra of the new compounds are consistent with a C2-
symmetric environment. Also, the chloride complexes 2a−d as
well as the corresponding iodide complexes 3a−d exhibited
similar NMR spectra in terms of peak shapes and chemical
shifts. In comparison with the free ligands 1a−d, the
disappearance of the singlet corresponding to the central aryl
proton located ortho to both imidazoline rings in the 1H NMR
spectra of the complexes confirmed the occurrence of C-
platination. Additionally, the signals of central aryl protons for
the complexes were strongly shifted upfield, while those of the
imidazoline rings as well as the N−Cy protons shifted
downfield due to the N−Pt coordinations and C−Pt bond
formation in the complexes. Some evidence for the formation
of the pincer platinum complexes could also be obtained from
comparison of the 13C NMR spectra of the complexes with
those of the corresponding Phebim-H ligands. For example, the
resonances due to the carbon atom of imidazolinyl CN
appeared in the range of 165−167 ppm and those due to the C-
2 atom of central aryl appeared around 130 ppm in the ligands,
while in the platinum complexes, the corresponding resonances
obviously shifted downfield. The former were observed in the
range of 173−175 ppm and the latter in the range of 165−168
ppm. Finally, the NMR spectra of the cationic Pt−OH2
complex 4 exhibited similar phenomena in comparison with
those of the Phebim-H ligand 1a. Surprisingly, in the 1H NMR
spectra of the cationic Pt−Py complexes 5a,c the imidazoline-
NCH protons as well as the i-Pr and Ph protons showed
obvious upfield shifts relative to those of Phebim-H ligands
1a,c.
1
from their H NMR spectra. Nonetheless, the pure, unsym-
metrical NCN′ pincer Pt(II) chloride complexes 7a,b were
readily isolated in yields around 50% after heating 6a,b with
K2PtCl4 in refluxing HOAc for 36 h. Frustratingly, efforts to
acquire pure cationic aquo complexes from the reaction of 7a,b
with AgOTf in wet CH2Cl2 at room temperature were also
1
Synthesis of the Hybrid Oxazoline−Imidazoline
Ligands and the Related Unsymmetrical, Neutral Pincer
Pt(II) Chloride and Iodide Complexes. We did not expect
that it was difficult for us to get the pure samples of the cationic
Pt−OH2 complexes with Phebim ligands from the neutral
Pt(Phebim)X (X = Cl and I) complexes since only one such
complex (4) was successfully obtained (from 3a) despite many
attempts on 3b−d (the corresponding pure Pt−OH2
complexes could not be isolated by the procedure similar to
unsuccessful. Also, H NMR analysis of the obtained solids
revealed a complex mixture of species. Finally, the unsym-
metrical NCN′ pincer Pt(II) iodide complex 8c, which has a
chiral substituent and imidazoline ring identical with those of
complex 3a, was prepared in a 40% yield (based on the ligand
6c) by treatment of the ligand 6c with K2PtCl4, followed by the
reaction with KI. The reaction of 8c with AgOTf in wet acetone
1
afforded a yellow solid, whose H NMR spectrum exhibited
broad, nonconclusive signals. Furthermore, it was found that in
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Figure 1. Molecular structures of the neutral symmetrical Pt(Phebim)Cl complexes 2c·H2O (left) and 2d·CH2Cl2 (right). Hydrogen atoms and
solvent molecules are omitted for clarity.
Figure 2. Molecular structures of the neutral symmetrical Pt(Phebim)I complexes 3c (left) and 3d·(n-butane) (right). Hydrogen atoms and solvent
molecule are omitted for clarity.
Figure 3. Molecular structures of the neutral unsymmetrical Pt−Cl complex 7b (left) and the cationic symmetrical [Pt(Phebim)(OH2)](OTf)
complex 4 (right). Hydrogen atoms and the anion are omitted for clarity.
the presence of this solid the asymmetric Friedel−Crafts
alkylation between indole and trans-β-nitrostyrene in toluene at
0 °C gave the expected adduct in 46% yield with 46% ee after
36 h. On the other hand, the reaction of 8c with AgBF4 in wet
acetone also gave a yellow product which was essentially pure
from its 1H NMR spectrum. The structure of this new product
could not be identified, due to its poor solubility in common
organic solvents. In addition, the obtained product was found
to be ineffective for the alkylation between indole and trans-β-
nitrostyrene. The unsuccessful isolation of the pure cationic
complex from 8c was somewhat surprising, considering the
successful synthesis from the closely related complex 3a.
The unsymmetrical NCN′ pincer ligand precursor 6c and the
three NCN′ Pt(II) complexes 7a,b and 8c were well
unsymmetrical species were obviously more complicated due to
the dissymmetry of their structures. For example, the 1H NMR
spectra of 6a−c exhibited a set of four signals, including one
singlet, two doublets, and one triplet (or multiplet) for the four
protons on the central aryl ring. For the Pt(II) complexes 7a,b
and 8c, the singlet at about δ 8 ppm corresponding to the
central aryl proton located ortho to both imidazoline and
oxazoline rings disappeared due to the occurrence of C-
platination and the remaining three protons appeared in three
different positions. The unsymmetrical structures of the ligand
6c and the three Pt(II) complexes were more intuitively
indicated by the 13C NMR spectra, which showed two
resonances at about δ 179 and 173 ppm (164.7 and 162.7
ppm for 6c) for the oxazolinyl and imidazolinyl CN,
respectively, as well as six resonances for the central aryl ring. In
addition, the resonances due to the i-Pr (or Ph) groups on the
oxazoline and imidazoline rings appeared at different positions.
Thus, altogether six resonances were observed for the two i-Pr
1
characterized by elemental analysis (HRMS for 6c) and H
NMR, 13C NMR, and IR spectra. In comparison with those of
the symmetrical NCN ligand precursors and the Pt(II)
complexes, the 1H NMR and 13C NMR spectra of these
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a
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Complexes 2c·H2O, 2d·CH2Cl2, 3c, 3d·(n-butane), 4, and 7b
2c·H2O
2d·CH2Cl2
3c
3d·(n-butane)
4
7b
Pt−C
1.935(12)
2.039(9)
2.032(10)
2.404(3)
80.3(4)
1.916(3)
1.937(7)
2.030(8)
2.005(8)
2.6695(8)
79.6(5)
1.932(8)
2.016(6)
2.031(6)
2.6739(7)
78.8(3)
1.878(9)
1.999(8)
1.998(9)
2.154(7)
80.8(4)
1.921(5)
2.033(4)
2.016(4)
Pt−N(1)
Pt−N(3)
Pt−X
C−Pt−N(1)
C−Pt−N(3)
N−Pt−N
C−Pt−X
N(1)−Pt−X
N(3)−Pt−X
2.020(3)
b
2.020(3)
2.3829(10)
79.37(13)
79.79(13)
159.14(12)
179.03(11)
99.77(8)
2.3808(12)
79.6(2)
b
78.5(4)
79.5(5)
80.4(3)
79.7(4)
79.5(2)
158.8(4)
178.1(3)
101.6(3)
99.6(3)
159.1(3)
179.8(5)
100.3(2)
100.6(2)
159.3(3)
178.5(2)
100.35(17)
160.4(4)
177.8(4)
100.1(3)
99.5(3)
158.91(19)
174.78(15)
97.79(13)
103.30(15)
b
101.06(9)
100.36(19)
b
a
For complexes 2c,d and 7b, X = Cl; for complexes 3c,d, X = I; for complex 4, X = O. N(3) should be N(2).
groups (or eight for Ph) in the 13C NMR spectra. Finally, it
should be mentioned that, similar to the case for the
symmetrical pincer Pt(Phebim)X complexes, the signals of
central aryl protons in the 1H NMR spectra for the
unsymmetrical Pt(II) complexes were shifted strongly upfield,
while those of the N-aryl or Cy protons on the imidazoline ring
shifted downfield relative to those of the corresponding free
ligands. A similar downfield shift of the resonances due to the
two CN (δ 178.7 and 172.9 ppm vs 164.7 and 162.7 ppm) as
well as the ortho-metalated carbon atom (δ 164 ppm vs about
130 ppm) was also observed in the 13C NMR spectra when
complex 8c was compared with ligand 6c.
Pt(Phebox)Cl complex4a and Pt(Phebim)Cl complex 2c (175°
vs 178−179°). Finally, it is found that there are several types of
intermolecular hydrogen bonds in the crystals of these
complexes. Also, it is worthy of notice that the intermolecular
C−H···Pt hydrogen bonds13 are present in the crystals of
symmetrical Pt−Cl complex 2c and Pt−I complex 3c (Figure 4
Molecular Structures of the Neutral and Cationic
Pincer Pt(II) Complexes. The molecular structures of the
neutral symmetrical Pt−Cl complexes 2c·H2O and 2d·CH2Cl2,
the Pt−I complexes 3c and 3d·(n-butane), the unsymmetrical
Pt−Cl complex 7b, and the cationic symmetrical Pt−OH2
complex 4 were determined by X-ray single-crystal analysis.
The molecules are shown in Figures 1−3, respectively. Selected
bond lengths and angles are collected in Table 1. The main
structural features of the six pincer Pt(II) complexes are almost
identical. The metal atom in each complex adopts a typical
distorted-square-planar geometry with the ligand coordinating
to the Pt(II) center in a tridentate manner and the chloride,
iodide, or water occupying the fourth coordination site. All of
the bond angles around the metal center in these six complexes
are very similar, except for a smaller C−Pt−X angle in complex
7b. The two C−Pt−N angles in each complex are around 80°,
which are typical for metal pincers comprising two fused five-
membered-ring metallacycles and reflect the relative steric
strain of the system. The cyclohexyl groups in the Pt(II)
complexes 2c,d, 3c,d, and 4 take chair conformations. In
comparison with the related cationic Pt(Phebox)(OH2)
complex, the cationic complex Pt(Phebim)(OH2) (4) shows
comparable bond lengths and angles for the metal coordination
sphere, except for a shorter Pt−C bond (1.878(9) Å vs 1.96(3)
Å).4a The Pt−C and Pt−N bond lengths in the cationic
complex 4 are shorter than those of the neutral Pt complexes
2c,d, 3c,d, and 7b. In addition, the Pt−C, Pt−Cl, and Pt−
N(oxazoline) bond lengths in the neutral unsymmetrical Pt−Cl
complex 7b are similar to those found in the related neutral
symmetrical Pt(Phebox)Cl complex,4a while the Pt−C, Pt−Cl,
and Pt−N(imidazoline) bond lengths are slightly shorter than
those of the symmetrical Pt(Phebim)Cl complex 2c (Table 1).
The shorter Pt−N(imidazoline) bond length is likely due to the
reduced trans influence of the oxazoline ligand compared to
that of the imidazoline. The C−Pt−Cl angle in complex 7b is
found to be smaller than those in the related symmetrical
Figure 4. Crystal-packing view of 2c·H2O. Non-hydrogen-bonding H
atoms are omitted for clarity.
and Figure S1 in the Supporting Information for 3c).
Additionally, there also exist hydrogen bonds between the
chlorine atom and water molecules in the crystal of 2c, which
contribute to the stability of the molecule.
Catalytic Studies. The metal- and organo-catalyzed
asymmetric Friedel−Crafts alkylation reactions have received
considerable interest in recent years, and significant progress
has been achieved in the field.14,15 Among them, those of
indoles have played a central role, due to their potential
applications in the synthesis of pharmacologically and agro-
chemically active molecules with an indole framework.14−16 In
particular, asymmetric Friedel−Crafts alkylation of indoles with
nitroalkenes as electrophiles is quite attractive, since nitro-
alkenes are highly electrophilic and the nitro group in the
products can be further transformed. A variety of chiral
catalysts, including organocatalysts17 such as thiourea com-
pounds and phosphoric acids as well as Lewis acid catalysts
derived from chiral ligands and metal salts10c,18 such as
bis(oxazoline)/Zn(II) and bis(oxazoline)/Cu(II) complexes,
have been successfully applied to the reactions, giving catalysis
products with high levels of enantioselectivities. Nevertheless,
the enantioselectivities are not always high enough. For
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Table 2. Asymmetric Friedel-Crafts Alkylation of Indoles with Nitroalkenes Catalyzed by Cationic Pincer Pt(II) Aquo Complex
a
4
b
c
entry
R1
R2
Ar
yield (%)
ee (%)
e
d
d
d
1
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
99
84
43
73
86
60
81
93
89
77
70
61
69
61
72
58
79
70
65
trace
69 (S)
78 (S)
81 (S)
65
2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
f
3
4
5
6
7
p-FC6H4
p-ClC6H4
p-BrC6H4
p-CH3OC6H4
p-CH3OC6H4
Ph
67
71
83
e
8
e
65
9
Br
58
10
11
12
13
14
15
16
17
18
19
Br
Ph
70
Br
p-FC6H4
p-ClC6H4
p-BrC6H4
p-CH3OC6H4
Ph
73
Br
57
Br
62
Br
57
CH3O
CH3O
CH3O
CH3O
CH3O
H
34
p-FC6H4
p-ClC6H4
p-BrC6H4
p-CH3OC6H4
p-CH3OC6H4
36
39
46
64
e
20
a
Reactions were performed with indole (0.75 mmol) and nitroalkene (0.15 mmol) in 2 mL of toluene in the presence of 5 mol % of the cationic
b
c
d
pincer Pt complex 4 at 0 °C for 36 h under N2. Isolated yields. Detected by chiral HPLC. The absolute configuration was assigned to be S by
comparison of optical rotation with that in ref 18a. Reaction was performed at 25 °C for 24 h. Reaction was performed at −10 °C for 36 h.
e
f
example, in the room-temperature reactions of indoles with
nitroalkenes catalyzed by Lewis acids such as the axially chiral
imine ligands derived from [1,1′]binaphthalenyl-2,2′-diamine/
Zn(OTf)2 complexes (10 mol %),19a 9,9′-biphenanthryl-10,10′-
bis(oxazoline)/Zn(OTf)2 complexes (10 mol %),19b thiazo-
line−oxazoline ligand/Zn(OTf)2 complexes (5 mol % at −20
°C),19c and imidazoline-aminophenol/CuOTf complexes (10
mol % in combination with 2 equiv of 1,1,1,3,3,3-hexafluoro-2-
propanol),19d the highest enantioselectivities obtained were
67%, 79%, 76%, and 85% (74% in the absence of 1,1,1,3,3,3-
hexafluoro-2-propanol), respectively. Thus, the development of
highly efficient catalysts is still desirable. On the other hand, we
notice that the use of chiral platinum complexes as catalysts for
F−C alkylations remains rare. To the best of our knowledge,
only two reports have been reported concerning intra- or
intermolecular asymmetric F−C-alkylation-type reactions
between electron-rich (hetero)aromatic compounds and
unactivated alkenes in the presence of chiral cationic platinum
complexes.20 Also, there were no reports on the F−C alkylation
of indoles with nitroalkenes catalyzed by chiral platinum
complexes. Therefore, the potential of the obtained pure, chiral
cationic pincer Pt(II) aquo complex 4 in this reaction was
evaluated (Table 2). In the beginning the addition of indole to
trans-β-nitrostyrene in toluene, which was the most often used
solvent for metal-catalyzed alkylation of indoles with nitro-
alkenes, was quickly surveyed. It was found that the reaction
using 5 mol % of complex 4 as the catalyst gave the product in
an almost quantitative yield with 69% ee at 25 °C after 24 h
(entry 1). In constrast, in the presence of 10 mol % of
Zn(OTf)2 or Cu(OTf)2 complex with the same ligand (ligand
1a), racemic product was isolated under similar reaction
conditions (data not shown in Table 2). When the reaction
temperature was lowered from 25 to 0 °C, a decreased yield of
84% was obtained with 5 mol % of complex 4 after 36 h, while
the enantioselectivity could be improved to 78% ee (entry 2). A
further decrease in temperature (0 °C to −10 °C) led to a
drastic drop in the yield (43% vs 84%), albeit with a further
enhancement in enantioselectivity (81% vs 78% ee) (entry 3).
As a result, 0 °C was considered to be the appropriate reaction
temperature. Then the alkylation was extended to indole or
indole derivatives with various aromatic nitroalkenes. Overall,
the reactions of indole (65−83% ee) with nitroalkenes gave
enantioselectivities better than those of 5-bromo (57−73% ee)
or 5-methoxyindole (34−64% ee) with the corresponding
nitroalkenes under the same reaction conditions (entries 4−8
vs entries 9−19). In terms of the substituent effect of the
aromatic nitroalkenes on the enantioselectivities, it seemed to
be difficult to establish a clear trend. For example, in the case of
indole or 5-methoxyindole, the nitroalkene which had a p-
CH3OC6H4 moiety afforded the highest enantioselectivities
compared with those containing a Ph, p-F, p-Cl, or p-BrC6H4
moiety under the same reaction conditions (entry 7 vs entries 2
and 4−6 and entry 19 vs entries 15−18), while in the case of 5-
bromoindole, the nitroalkene having a p-FC6H4 moiety gave
the highest enantioselectivity (entry 11 vs entries 10 and 12−
14). Finally, it should be pointed out that the reaction of N-
methylindole with trans-β-nitrostyrene was almost completely
restrained (entry 20).
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598, 533 cm−1. HRMS (positive ESI; m/z): [M + Na]+ calcd for
C36H32N2NaO4 579.2260, found 579.2252; [M + K]+ calcd for
C36H32KN2O4 595.1999, found 595.2069.
CONCLUSIONS
■
In conclusion, the synthesis and characterization of the cationic
chiral NCN pincer Pt(II) aquo complex with the bis-
(imidazolinyl)phenyl (Phebim) ligand have been investigated.
Despite many attempts, only one such complex has been
obtained. Also, during the investigation, the new symmetrical
Phebim-H ligands, unsymmetrical hybrid oxazoline−imidazo-
line ligands, related neutral symmetrical and unsymmetrical
pincer Pt−Cl and Pt−I complexes, and cationic Pt pyridine
complexes have been isolated and characterized. The obtained
pure cationic pincer Pt(II) aquo complex was found to be an
effective catalyst for the asymmetric Friedel−Crafts alkylation
of indoles with nitroalkenes with a loading of 5 mol %,
providing the nitroalkylated indoles in good yields and
moderate to good enantioselectivities (up to 83% ee).
1,3-Bis((S,S)-1-cyclohexyl-4,5-diphenyl-4,5-dihydro-1H-imidazol-
20
2-yl)benzene (1d). Yield: 56%, white solids. Mp: 245−246 °C. [α]D
1
= +192° (c 0.119, CH2Cl2). H NMR (400 MHz, CDCl3): δ 8.31 (s,
1H, central Ar2 H), 7.97 (dd, J = 1.5, 7.7 Hz, 2H, central Ar4,6 H), 7.64
(t, J = 7.7 Hz, 1H, central Ar5 H), 7.40−7.35 (m, 12H, Ph H), 7.33−
7.29 (m, 8H, Ph H), 4.91 (d, J = 6.4 Hz, 2H, NCH), 4.55 (d, J = 6.4
Hz, 2H, NCH), 3.63−3.57 (m, 2H, NCy H), 1.73−1.59 (m, 6H, Cy
H), 1.51−1.35 (m, 6H, Cy H), 1.09−0.96 (m, 4H, Cy H), 0.93−0.83
(m, 4H, Cy H). 13C NMR (100 MHz, CDCl3): δ 166.7, 146.1, 144.4,
132.3, 130.4, 128.9, 128.8, 128.7, 128.6, 127.24, 127.19, 126.5, 126.1,
78.8, 69.8, 56.8, 33.7, 30.2, 26.1, 25.4, 25.2. IR (KBr): ν 3430, 3060,
3028, 2929, 2854, 1738, 1617, 1573, 1489, 1448, 1385, 1341, 1270,
1163, 1110, 1077, 1032, 995, 763, 700 cm−1. HRMS (positive ESI, m/
z): [M + H]+ calcd for C48H51N4 683.4114, found 683.4105.
2,6-Bis((S)-4-benzyl-1-cyclohexyl-4,5-dihydro-1H-imidazol-2-yl)-
phenylchloroplatinum(II) (2b). Yield: 54%, orange solids. Mp: >260
°C. [α]D20 = +177° (c 0.068, CH2Cl2). 1H NMR (400 MHz, CDCl3):
δ 7.37 (d, J = 7.4 Hz, 6H, Ph H and central Ar3,5 H), 7.24 (t, J = 7.4
Hz, 4H, Ph H), 7.18 (t, J = 7.1 Hz, 2H, Ph H), 7.08 (t, J = 7.8 Hz, 1H,
central Ar4 H), 4.55−4.51 (m, 2H, NCH), 4.08−4.03 (m, 2H, NCy
H), 3.73 (app t, J = 10.2 Hz, 2H, NCHH), 3.61 (dd, J = 4.3, 9.9 Hz,
2H, NCHH), 3.56 (dd, J = 3.1, 13.4 Hz, 2H, CHHPh), 2.88 (dd, J =
8.7, 13.4 Hz, 2H, CHHPh), 1.86−1.83 (m, 6H, Cy H), 1.68−1.61 (m,
4H, Cy H), 1.50−1.30 (m, 8H, Cy H), 1.15−1.09 (m, 2H, Cy H). 13C
NMR (100 MHz, CDCl3): δ 173.2, 165.9, 137.8, 132.7, 129.9, 128.2,
126.2, 125.7, 121.1, 61.7, 54.7, 49.1, 40.5, 31.5, 30.8, 25.5, 25.4, 25.2.
IR (KBr): ν 3429, 3199, 3024, 2927, 2851, 1628, 1564, 1515, 1433,
1401, 1319, 1276, 1179, 1084, 889, 731, 700 cm−1. Anal. Calcd for
C38H45ClN4Pt (788.33): C, 57.90; H, 5.75; N, 7.11. Found: C, 57.58;
H, 5.82; N, 6.95.
2,6-Bis((S)-1-cyclohexyl-4-phenyl-4,5-dihydro-1H-imidazol-2-yl)-
phenylchloroplatinum(II) (2c). Yield: 23%, orange solids. Mp: >260
°C. [α]D20 = +220° (c 0.077, CH2Cl2). 1H NMR (400 MHz, CDCl3):
δ 7.52 (d, J = 7.8 Hz, 2H, central Ar3,5 H), 7.35 (d, J = 7.7 Hz, 4H,
Ph2,6 H), 7.26 (t, J = 7.5 Hz, 4H, Ph3,5 H), 7.20−7.14 (m, 3H, Ph4 H
and central Ar4 H), 5.24 (dd, J = 4.5, 11.2 Hz, 2H, NCH), 4.26−4.16
(m, 4H, NCy H and NCHH), 3.73 (dd, J = 4.5, 9.8 Hz, 2H, NCHH),
1.98−1.87 (m, 8H, Cy H), 1.74−1.71 (m, 2H, Cy H), 1.54−1.40 (m,
8H, Cy H), 1.16−1.10 (m, 2H, Cy H). 13C NMR (100 MHz, CDCl3):
δ 173.9, 166.9, 143.1, 132.5, 128.4, 127.2, 126.7, 126.0, 121.1, 63.5,
54.9, 54.4, 31.8, 30.7, 25.5, 25.4, 25.1. IR (KBr): ν 3452, 2928, 2853,
1561, 1512, 1452, 1428, 1313, 1258, 1171, 1074, 1012, 892, 731, 696
cm−1. Anal. Calcd for C36H41ClN4Pt·H2O (778.29): C, 55.56; H, 5.57;
N, 7.20. Found: C, 55.48; H, 5.80; N, 6.93.
EXPERIMENTAL SECTION
■
General Procedures. Solvents were dried by standard methods
and freshly distilled prior to use if needed. Chiral amino alcohols21 and
nitroalkenes22 were prepared according to the literature methods. All
other chemicals were used as purchased. Melting points were
measured on an XT4A melting point apparatus and are uncorrected.
Infrared spectra were obtained with a Bruker VECTOR 22
1
spectrophotometer. H and 13C NMR spectra were recorded on a
Bruker DPX-400 spectrometer with TMS as an internal standard. Mass
spectra were measured on an 1100 LC/MSD Trap XCT System ESI
spectrometer. HRMS were determined on a Waters Q-Tof Micro MS/
MS System ESI spectrometer. Elemental analyses were measured on a
Thermo Flash EA 1112 elemental analyzer. Optical rotations were
recorded on a Perkin-Elmer 341 polarimeter.
Synthesis of Bis(imidazoline) Ligands 1a−d and the Neutral
NCN Pincer Chloroplatinum(II) Complexes 2a−d. The ligands
1a−d and the corresponding complexes 2a−d were prepared
according to the procedure reported by us.11b Reaction of isophthalyl
chloride with chiral amino alcohols in the presence of Et3N in THF at
room temperature easily afforded the corresponding bis(amido
alcohols), which were treated with excess thionyl chloride, followed
by cyclohexylamine. After basic workup with 10% NaOH and
purification, the 1,3-bis(2′-imidazolinyl)benzenes 1a−d were obtained.
Then the ligands 1a−d reacted with K2PtCl4 in refluxing HOAc to
produce the neutral NCN pincer chloroplatinum complexes 2a−d.
The analytical data of the new ligands 1c,d and complexes 2b−d are
given as follows.
1,3-Bis((S)-1-cyclohexyl-4-phenyl-4,5-dihydro-1H-imidazol-2-yl)-
20
1
benzene (1c). Yield: 43%, oil. [α]D = −35° (c 0.178, CH2Cl2). H
NMR (400 MHz, CDCl3): δ 7.80 (s, 1H, central Ar2 H), 7.65 (dd, J =
1.1, 7.5 Hz, 2H, central Ar4,6 H), 7.52 (t, J = 7.7 Hz, 1H, central Ar5
H), 7.35−7.34 (m, 8H, Ph H), 7.28−7.24 (m, 2H, Ph H), 5.16 (dd, J =
8.9, 11.3 Hz, NCH), 3.96 (app t, J = 10.5 Hz, 2H, NCHH), 3.41−3.33
(m, 4H, NCHH and NCy H), 1.81−1.64 (m, 6H, Cy H), 1.55−1.52
(m, 4H, Cy H), 1.43−1.35 (m, 4H, Cy H), 1.16−0.95 (m, 6H, Cy H).
13C NMR (100 MHz, CDCl3): δ 166.0, 144.6, 131.9, 129.7, 128.6,
128.5, 128.0, 127.0, 126.6, 66.8, 55.2, 52.7, 31.5, 30.2, 25.6, 25.3, 25.2.
IR (KBr): ν 3423, 3220, 3030, 2931, 2856, 1711, 1657, 1564, 1492,
1449, 1400, 1268, 1084, 998, 888, 817, 759, 700 cm−1. MS (positive
ESI; m/z): 531.5 [M + H]+. HRMS (positive ESI; m/z): [M + H]+
calcd for C36H43N4 531.3488, found 531.3487.
2,6-Bis((4S,5S)-1-cyclohexyl-4,5-diphenyl-4,5-dihydro-1H-imida-
zol-2-yl)phenylplatinum(II) Chloride (2d). The pure sample of 2d for
characterization was obtained from the reaction of the neutral
iodoplatinum complex 3d with an Ag(I) salt such as AgOTf or
AgBF4. After the reaction at room temperature, removal of AgI by
repeated filtration through Celite with CH2Cl2 or acetone as eluent,
evaporation of solvent, and recrystallization from CH2Cl2/n-hexane
afforded red crystals of 2d. It was believed that the reaction produced
the cationic complex first and then the cationic species easily
abstracted chloride from CH2Cl2 used in the reaction or the workup
to give the neutral chloroplatinum 2d. The yield of 2d was 92% from
20
the reaction of 3d with AgOTf (based on 3d). Mp: >290 °C. [α]D
=
−41° (c 0.150, CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.66 (d, J =
7.9 Hz, 2H, central Ar3,5 H), 7.34−7.19 (m, 21H, central Ar4 H and Ph
H), 4.98 (d, J = 4.4 Hz, 2H, NCH), 4.81 (d, J = 4.4 Hz, 2H, NCH),
4.40−4.34 (m, 2H, NCy H), 1.94−1.91 (m, 2H, Cy H), 1.84−1.77 (m,
4H, Cy H), 1.70−1.57 (m, 4H, Cy H), 1.51−1.24 (m, 6H, Cy H),
0.99−0.88 (m, 4H, Cy H). 13C NMR (100 MHz, CDCl3): δ 174.2,
167.5, 143.05, 143.0, 132.7, 129.0, 128.6, 128.1, 127.4, 126.4, 126.3,
126.2, 121.1, 74.4, 71.4, 56.4, 33.6, 31.8, 25.8, 25.7, 25.0. IR (KBr): ν
3448, 2930, 2855, 1636, 1562, 1508, 1494, 1452, 1431, 1384, 1350,
N,N′-Bis((1S,2R)-2-hydroxy-1,2-diphenylethyl)isophthalamide.
Yield: 90%, white solids. Mp: 240 °C. [α]D20 = +131° (c 0.129, THF).
1H NMR (400 MHz, DMSO-d6): δ 8.72 (d, J = 9.1 Hz, 2H, NH), 7.94
(s, 1H, Ar H), 7.68 (d, J = 7.7 Hz, 2H, Ar H), 7.44−7.41 (m, 9H, Ph H
and Ar H), 7.30−7.15 (m, 12H, Ph H), 5.44 (d, J = 4.9 Hz, 2H, OH),
5.12 (t, J = 8.7 Hz, 2H, CHOH), 4.93−4.90 (m, 2H, CHNH). 13C
NMR (100 MHz, DMSO-d6): δ 165.0, 143.8, 141.5, 134.9, 129.7,
128.6, 128.2, 128.0, 127.80, 127.76, 127.6, 127.5, 127.21, 127.17,
127.0, 126.9, 126.6, 74.7, 59.3. IR (KBr): ν 3312, 3061, 3030, 2925,
1649, 1580, 1531, 1497, 1452, 1350, 1270, 1192, 1092, 1041, 756, 700,
1310, 1270, 1168, 759, 699 cm− 1
. Anal. Calcd for
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C48H49ClN4Pt·CH2Cl2 (997.40): C, 59.01; H, 5.15; N, 5.62. Found: C,
59.26; H, 5.24; N, 5.43.
General Procedure for the Synthesis of Oxazoline−Imidazo-
line NCN′ Ligand Precursors 6a−c. Treatment of isophthalyl
chloride with L-valinol or L-phenylglycinol in the presence of Et3N
gave the corresponding bis(amido alcohols) according to the
procedure reported by us.11b Then the obtained chiral bis(amido
alcohol) (2.0 mmol) was reacted with excess thionyl chloride (6 mL)
at reflux for 2 h. Excess thionyl chloride was evaporated. The residue
was dissolved in dry diethyl ether (and filtered to remove insoluble
impurities, if any). To this solution was added dry triethylamine (1.6
mL, 12 mmol) and p-toluidine or cyclohexylamine (3 mmol). After the
mixture was stirred for 4 h at room temperature, 10% NaOH (8.3 mL)
was added and the mixture stirred for another 6 h. The aqueous layer
was extracted with dichloromethane, and the organic layer was washed
with brine, dried over Na2SO4, and evaporated. The residue was
purified by preparative TLC on silica gel plates with EtOAc/petroleum
ether/Et3N (1/1/0.05) as eluent to give an oil, which is believed to be
mainly imidazoline-containing amido chloride with some expected
oxazoline−imidazoline NCN′ ligand precursors. The amido chloride in
the oil was cyclized to oxazoline by treatment with NaOH (160 mg, 4
mmol) in CH3OH/THF (3 mL/12 mL) at reflux for 12 h. After
cooling, filtration, and evaporation of solvent, the NCN′ ligand
precursors 6a−c were obtained.
General Procedure for the Synthesis of Neutral NCN Pincer
Iodoplatinum(II) Complexes 3a−d. A mixture of chloroplatinum
complexes 2a−c (0.2 mmol) and potassium iodide (332 mg, 2 mmol)
were stirred in methanol/acetone (1/1 v/v, 20 mL) at room
temperature for 12 h. The reaction mixture was filtered through
Celite, and solvent was removed under reduced pressure. Then the
residue was purified by preparative TLC on silica gel plates with
dichloromethane as eluent to give the iodoplatinum complexes 3a−c
as orange solids. For preparation of the iodo complex 3d, the chloro
complex 2d, which was obtained by flash column chromatography on
silica gel with dichloromethane as eluent after the reaction of
bis(imidazoline) ligand 1d with K2PtCl4, directly reacted with KI
without further purification and characterization.
2,6-Bis((S)-1-cyclohexyl-4-isopropyl-4,5-dihydro-1H-imidazol-2-
20
yl)phenyliodoplatinum(II) (3a). Yield: 76%. Mp: >260 °C. [α]D
=
+144° (c 0.092, CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.43 (d, J =
7.8 Hz, 2H, central Ar3,5 H), 7.14 (t, J = 7.8 Hz, 1H, central Ar4 H),
4.30−4.28 (m, 2H, NCH), 4.21−4.15 (m, 2H, NCy H), 3.73−3.64
(m, 4H, NCH2), 2.87−2.83 (m, 2H, CH(CH3)2), 2.01−1.82 (m, 8H,
Cy H), 1.76−1.72 (m, 2H, Cy H), 1.65−1.56 (m, 4H, Cy H), 1.47−
1.35 (m, 4H, Cy H), 1.23−1.13 (m, 2H, Cy H), 0.90 (d, J = 7.0 Hz,
6H, CH3CHCH3), 0.68 (d, J = 6.8 Hz, 6H, CH3CHCH3). 13C NMR
(100 MHz, CDCl3): δ 172.7, 166.4, 132.0, 125.6, 121.5, 66.0, 54.8,
45.2, 31.9, 31.0, 30.6, 25.6, 25.4, 25.2, 18.0, 13.7. IR (KBr): ν 3429,
2931, 2853, 1562, 1511, 1453, 1427, 1314, 1257, 1173, 1014, 757, 695
cm−1. Anal. Calcd for C30H45IN4Pt (783.69): C, 45.98; H, 5.79; N,
7.15. Found: C, 45.70; H, 5.95; N, 6.90.
1-((S)-4-Isopropyl-4,5-dihydrooxazol-2-yl)-3-((S)-4-isopropyl-1-p-
tolyl-4,5-dihydro-1H-imidazol-2-yl)benzene (6a). The ligand con-
1
tained some impurities. Thereofre, only the H NMR spectrum was
1
determined. H NMR (400 MHz, CDCl3): δ 8.21 (s, 1H, central Ar2
H), 7.92 (d, J = 7.8 Hz, 1H, central Ar H), 7.47 (d, J = 7.7 Hz, 1H,
central Ar H), 7.29−7.24 (m, 1H, central Ar H), 6.94 (d, J = 8.2 Hz,
2H, NAr H), 6.67 (d, J = 8.2 Hz, 2H, NAr H), 4.40−4.34 (m, 1H,
oxazoline NCH), 4.14−4.04 (m, 4H, imidazoline and oxazoline H),
3.59 (app t, J = 7.5 Hz, 1H, oxazoline OCHH), 2.23 (s, 3H, CH3),
1.94−1.89 (m, 1H, CH(CH3)2), 1.84−1.79 (m, 1H, CH(CH3)2), 1.04
(d, J = 6.8 Hz, 3H, CH3CHCH3), 0.99 (d, J = 6.8 Hz, 3H,
CH3CHCH3), 0.96 (d, J = 6.8 Hz, 3H, CH3CHCH3), 0.90 (d, J = 6.7
Hz, 3H, CH3CHCH3).
2,6-Bis((S)-4-benzyl-1-cyclohexyl-4,5-dihydro-1H-imidazol-2-yl)-
20
phenyliodoplatinum(II) (3b). Yield: 83%. Mp: >260 °C. [α]D
=
+166° (c 0.072, CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.40 (d, J =
7.0 Hz, 6H, Ph H and central Ar3,5 H), 7.26 (t, J = 7.4 Hz, 4H, Ph H),
7.19 (t, J = 7.2 Hz, 2H, Ph H), 7.14 (t, J = 7.8 Hz, 1H, central Ar4 H),
4.65−4.61 (m, 2H, NCH), 4.08−4.05 (m, 2H, NCy H), 3.69−3.61
(m, 4H, NCH2), 3.58 (dd, J = 3.0, 13.2 Hz, 2H, CHHPh), 2.74 (dd, J
= 9.2, 13.2 Hz, 2H, CHHPh), 1.86−1.69 (m, 10H, Cy H), 1.51−1.34
(m, 8H, Cy H), 1.16−1.06 (m, 2H, Cy H). 13C NMR (100 MHz,
CDCl3): δ 173.3, 166.8, 137.7, 132.2, 129.9, 128.2, 126.2, 125.7, 121.6,
63.1, 54.6, 48.9, 40.3, 31.8, 30.6, 25.6, 25.3, 25.1. IR (KBr): ν 3427,
2927, 2852, 1566, 1515, 1433, 1400, 1320, 1274, 1178, 1083, 1012,
702 cm−1. Anal. Calcd for C38H45IN4Pt·0.2CH2Cl2 (896.77): C, 51.16;
H, 5.10; N, 6.25. Found: C, 51.30; H, 5.38; N, 5.97.
1-((S)-4-Phenyl-4,5-dihydrooxazol-2-yl)-3-((S)-4-phenyl-1-p-tolyl-
4,5-dihydro-1H-imidazol-2-yl)benzene (6b). The ligand also con-
tained some impurities, and only the 1H NMR spectrum was
determined. 1H NMR (400 MHz, CDCl3): δ 8.08 (s, 1H, central
Ar2 H), 7.87 (d, J = 7.8 Hz, 1H, central Ar H), 7.54 (d, J = 7.7 Hz, 1H,
central Ar H), 7.43−7.26 (m, 11H, Ph H and central Ar H), 7.00 (d, J
= 8.1 Hz, 2H, NAr H), 6.77 (d, J = 8.1 Hz, 2H, NAr H), 5.47 (dd, J =
6.0, 13.6 Hz, 1H, oxazoline NCH), 5.33 (dd, J = 8.5, 10.7 Hz, 1H,
imidazoline NCH), 4.47 (app t, J = 10.3 Hz, 1H, imidazoline NCHH),
3.95 (app t, J = 9.0 Hz, 1H, oxazoline OCHH), 3.92−3.85 (m, 2H,
oxazoline OCHH and imidazoline NCHH), 2.25 (s, 3H, CH3).
1-((S)-4-Isopropyl-4,5-dihydrooxazol-2-yl)-3-((S)-1-cyclohexyl-4-
isopropyl-4,5-dihydro-1H-imidazol-2-yl)benzene (6c). Yield: 33%
2,6-Bis((S)-1-cyclohexyl-4-phenyl-4,5-dihydro-1H-imidazol-2-yl)-
20
phenyliodoplatinum(II) (3c). Yield: 78%. Mp: >260 °C. [α]D
=
+217° (c 0.058, CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.57 (d, J =
7.9 Hz, 2H, central Ar3,5 H), 7.29−7.17 (m, 11H, Ph H and central Ar4
H), 5.44 (dd, J = 3.6, 11.1 Hz, 2H, NCH), 4.26−4.21 (m, 2H, NCy
H), 4.16 (app t, J = 10.5 Hz, 2H, NCHH), 3.67 (dd, J = 3.6, 9.8 Hz,
2H, NCHH), 1.91−1.88 (m, 8H, Cy H), 1.73−1.70 (m, 2H, Cy H),
1.57−1.37 (m, 8H, Cy H), 1.17−1.07 (m, 2H, Cy H). 13C NMR (100
MHz, CDCl3): δ 174.4, 168.0, 143.4, 132.0, 128.5, 127.1, 126.5, 126.1,
121.5, 65.1, 54.9, 32.0, 30.7, 25.6, 25.4, 25.2. IR (KBr): ν 3433, 2934,
2857, 1629, 1568, 1517, 1457, 1432, 1398, 1335, 1309, 1201, 1057,
1018, 728 cm−1. Anal. Calcd for C36H41IN4Pt (851.73): C, 50.77; H,
4.85; N, 6.58. Found: C, 50.60; H, 4.99; N, 6.35.
20
based on the related bis(amido alcohol), oil. [α]D = −66° (c
1
0.074, CH2Cl2). H NMR (400 MHz, CDCl3): δ 8.05 (s, 1H, central
Ar2 H), 8.02 (d, J = 7.8 Hz, 1H, central Ar H), 7.58 (d, J = 7.6 Hz, 1H,
central Ar H), 7.44 (t, J = 7.7 Hz, 1H, central Ar H), 4.41 (app t, J =
8.0 Hz, 1H, oxazoline OCHH), 4.16−4.07 (m, 2H, oxazoline NCH
and OCHH), 3.94−3.87 (m, 1H, imidazoline NCH), 3.51 (app t, J =
10.2 Hz, 1H, imidazoline NCHH), 3.27−3.19 (m, 2H, imidazoline
NCHH and NCy H), 1.91−1.82 (m, 2H, CH(CH3)2), 1.76−1.67 (m,
3H, Cy H), 1.57−1.42 (m, 4H, Cy H), 1.17−1.01 (m, 9H, Cy H and
CH3CHCH3), 0.94 (d, J = 6.2 Hz, 3H, CH3CHCH3), 0.92 (d, J = 6.3
Hz, 3H, CH3CHCH3). 13C NMR (100 MHz, CDCl3): δ 164.7, 162.7,
131.7, 130.8, 129.4, 128.4, 128.2, 127.9, 72.6, 70.2, 69.1, 55.1, 46.7,
33.3, 32.8, 31.3, 30.1, 25.5, 25.32, 25.28, 18.85, 18.78, 18.0, 17.9. IR
(KBr): ν 3428, 2931, 2861, 1652, 1619, 1577, 1449, 1396, 1358, 1308,
1268, 1245, 1174, 1071, 987, 913, 813, 706 cm−1. HRMS (positive ESI,
m/z): [M + H]+ calcd for C24H36N3O 382.2858, found 382.2772.
General Procedure for the Synthesis of the Neutral NCN′
Pincer Chloroplatinum(II) Complexes 7a−c and Iodoplatinum-
(II) Complex 8c. A mixture of oxazoline−imidazoline NCN′ ligand
precursors 6a−c (0.2 mmol) and K2PtCl4 (91 mg, 0.22 mmol) in dry
HOAc (60 mL) were refluxed for 36 h. After cooling and
concentration in vacuo, the residue was purified by flash column
2,6-Bis((4S,5S)-1-cyclohexyl-4,5-diphenyl-4,5-dihydro-1H-imida-
zol-2-yl)phenylplatinum(II) Iodide (3d). Yield: 54% based on the
20
bis(imidazoline) ligand 1d, dark red solids. Mp: >290 °C. [α]D
=
−90° (c 0.102, CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.69 (d, J =
7.8 Hz, 2H, central Ar3,5 H), 7.35−7.21 (m, 21H, central Ar4 H and Ph
H), 5.13 (d, J = 4.0 Hz, 2H, NCH), 4.74 (d, J = 4.0 Hz, 2H, NCH),
4.38−4.33 (m, 2H, NCy H), 1.88−1.75 (m, 6H, Cy H), 1.68−1.57 (m,
4H, Cy H), 1.52−1.22 (m, 6H, Cy H), 0.99−0.85 (m, 4H, Cy H). 13C
NMR (100 MHz, CDCl3): δ 174.6, 168.3, 143.5, 142.9, 132.2, 129.1,
128.6, 128.1, 127.3, 126.4, 126.24, 126.16, 121.5, 76.0, 71.6, 56.3, 33.5,
31.9, 25.8, 25.6, 25.0. IR (KBr): ν 3450, 2926, 2853, 1738, 1636, 1563,
1495, 1431, 1384, 1347, 1309, 1271, 1171, 1057, 1007, 759, 697 cm−1.
Anal. Calcd for C48H49IN4Pt (1003.92): C, 57.43; H, 4.92; N, 5.58.
Found: C, 57.19; H, 4.98; N, 5.34.
843
dx.doi.org/10.1021/om200714z | Organometallics 2012, 31, 835−846

Organometallics
Article
chromatography on silica gel with dichloromethane as eluent to give
the chloroplatinum complexes 7a−c. Then without characterization,
the obtained 7c directly reacted with KI in a way similar to that of
NCN chloroplatinum complex 2d to afford the NCN′ iodoplatinum
complex 8c, while complexes 7a,b were further purified by
recrystallization from CH2Cl2/petroleum ether and characterized.
2-((S)-4-Isopropyl-4,5-dihydrooxazol-2-yl)-6-((S)-4-isopropyl-1-p-
tolyl-4,5-dihydro-1H-imidazol-2-yl)phenylchloroplatinum(II) (7a).
through Celite again. After removal of CH2Cl2 in vacuo and
recrystallization from CH2Cl2/n-hexane, light yellow needles of pure
Pt(II) aquo complex 4 were obtained. The cationic Pt(II) pyridine
complexes 5a,c were prepared similarly by the reaction of Pt−Cl
complexes 2a,c with AgOTf (1 equiv) in the presence of pyridine (1
equiv) in CH2Cl2 at room temperature for 24 h. After removal of AgCl
by repeated filtration through Celite with CH2Cl2 as eluent and
evaporation of CH2Cl2, the residue was purified by preparative TLC
on silica gel plates with CH2Cl2/MeOH (10/1 for 5a and 40/3 for 5c,
respectively) as eluent to give 5a,c as yellow solids.
20
Yield: 52%, yellow solids. Mp: >290 °C. [α]D = +159° (c 0.045,
1
CH2Cl2). H NMR (400 MHz, CDCl3): δ 7.26 (d, J = 6.4 Hz, 3H,
central Ar H and NAr H), 7.16 (d, J = 8.1 Hz, 2H, NAr H), 6.86 (t, J =
7.8 Hz, 1H, central Ar4 H), 6.51 (d, J = 7.8 Hz, 1H, central Ar H),
4.72−4.70 (m, 2H, oxazoline OCH2), 4.45−4.41 (m, 2H, oxazoline
and imidazoline NCH), 4.19 (app t, J = 10.6 Hz, 1H, imidazoline
NCHH), 3.95 (dd, J = 5.2, 10.0 Hz, 1H, imidazoline NCHH), 2.96−
2.90 (m, 2H, CH(CH3)2), 2.42 (s, 3H, CH3), 0.96 (d, J = 6.2 Hz, 3H,
CH3CHCH3), 0.95 (d, J = 6.4 Hz, 3H, CH3CHCH3), 0.87 (d, J = 6.8
Hz, 3H, CH3CHCH3), 0.79 (d, J = 6.8 Hz, 3H, CH3CHCH3). 13C
NMR (100 MHz, CDCl3): δ 178.4, 172.0, 162.7, 137.8, 136.6, 131.1,
130.1, 127.7, 127.5, 126.0, 125.8, 120.9, 71.3, 66.9, 66.2, 54.8, 29.6,
28.7, 21.0, 18.6, 18.3, 13.8, 13.7. IR (KBr): ν 3751, 3443, 2957, 2925,
2867, 1602, 1573, 1534, 1500, 1461, 1409, 1330, 1151, 1109, 1003,
934, 822, 732 cm−1. Anal. Calcd for C25H30ClN3OPt (619.06): C,
48.50; H, 4.88; N, 6.79. Found: C, 48.31; H, 4.85; N, 6.44.
Data for the cationic Pt−OH2 complex 4 are as follows. Yield: 62%.
20
1
Mp: >260 °C. [α]D = +93° (c 0.05, CH2Cl2). H NMR (400 MHz,
CDCl3): δ 7.36 (d, J = 7.9 Hz, 2H, central Ar3,5 H), 7.12 (t, J = 7.9 Hz,
1H, central Ar4 H), 4.23−4.13 (m, 4H, NCH and NCy H), 3.76 (app
t, J = 10.7 Hz, 2H, NCHH), 3.67 (dd, J = 5.0, 10.1 Hz, 2H, NCHH),
2.43 (br s, 2H, CH(CH3)2), 1.97−1.86 (m, 8H, Cy H), 1.77−1.73 (m,
2H, Cy H), 1.66−1.54 (m, 4H, Cy H), 1.48−1.36 (m, 4H, Cy H),
1.25−1.14 (m, 2H, Cy H), 0.91 (d, J = 6.9 Hz, 6H, CH3CHCH3), 0.74
(d, J = 6.8 Hz, 6H, CH3CHCH3). 13C NMR (100 MHz, CDCl3): δ
171.2, 155.1, 132.6, 125.6, 122.2, 64.9, 55.0, 45.1, 31.7, 30.9, 30.4, 25.5,
25.4, 25.2, 18.1, 13.9. IR (KBr): ν 3386, 2950, 2842, 1630, 1568, 1519,
1435, 1400, 1293, 1232, 1167, 1033, 731, 640 cm−1. Anal. Calcd for
C31H47F3N4O4PtS (823.87): C, 45.19; H, 5.75; N, 6.80; S, 3.89.
Found: C, 45.18; H, 5.89; N, 6.62; S, 3.61.
Data for the cationic Pt−Py complex 5a are as follows. Yield: >99%.
2-((S)-4-Phenyl-4,5-dihydrooxazol-2-yl)-6-((S)-4-phenyl-1-p-tolyl-
4,5-dihydro-1H-imidazol-2-yl)phenylchloroplatinum(II) (7b). Yield:
51%, yellow solids. Mp: 285 °C. [α]D20 = +299° (c 0.037, CH2Cl2). 1H
NMR (400 MHz, CDCl3): δ 7.44 (d, J = 7.6 Hz, 2H, NAr H), 7.38−
7.20 (m, 13H, Ph and NAr H and central Ar H), 6.92 (t, J = 7.7 Hz,
1H, central Ar4 H), 6.62 (d, J = 7.7 Hz, 1H, central Ar H), 5.46−5.41
(m, 2H, imidazoline and oxazoline NCH), 5.08 (app t, J = 9.3 Hz, 1H,
oxazoline OCHH), 4.79 (dd, J = 5.4, 9.0 Hz, 1H, oxazoline OCHH),
4.58 (app t, J = 10.4 Hz, 1H, imidazoline NCHH), 4.11 (dd, J = 4.6,
10.0 Hz, 1H, imidazoline NCHH), 2.43 (s, 3H, CH3). 13C NMR (100
MHz, CDCl3): δ 179.6, 173.3, 164.5, 142.2, 139.9, 138.3, 136.5, 131.3,
130.4, 128.7, 128.6, 128.3, 128.2, 127.72, 127.69, 127.1, 126.9, 126.7,
126.1, 121.1, 79.3, 65.5, 64.7, 63.8, 21.2. IR (KBr): ν 3423, 3181, 2923,
1596, 1568, 1521, 1491, 1457, 1406, 1323, 1296, 1237, 1150, 1109,
20
1
Mp: 134−135 °C. [α]D = +205° (c 0.095, CH2Cl2). H NMR (400
MHz, CDCl3): δ 8.81 (d, J = 4.2 Hz, 2H, Py H), 8.06 (t, J = 7.1 Hz,
1H, Py H), 7.74 (t, J = 6.1 Hz, 2H, Py H), 7.50 (d, J = 7.6 Hz, 2H,
central Ar3,5 H), 7.26 (t, J = 7.8 Hz, 1H, central Ar4 H), 4.24−4.21 (m,
2H, NCy H), 3.87 (s, 4H, NCH and NCHH), 3.71−3.63 (m, 2H,
NCHH), 1.95−1.93 (m, 8H, Cy H), 1.78−1.75 (m, 2H, Cy H), 1.64−
1.61 (m, 4H, Cy H), 1.46−1.43 (m, 4H, Cy H), 1.25−1.18 (m, 2H, Cy
H), 1.02−1.00 (m, 2H, CH(CH3)2), 0.64 (d, J = 6.4 Hz, 6H,
CH3CHCH3), 0.58 (d, J = 6.4 Hz, 6H, CH3CHCH3). 13C NMR (100
MHz, CDCl3): δ 173.3, 163.6, 151.5, 139.3, 133.1, 127.0, 126.4, 123.7,
64.1, 55.2, 45.4, 31.6, 31.1, 31.0, 25.45, 25.37, 25.1, 17.8, 14.0. IR
(KBr): ν 3074, 2931, 2856, 1735, 1604, 1569, 1522, 1436, 1449, 1385,
1356, 1332, 1272, 1222, 1150, 1031, 893, 762, 732, 705, 637 cm−1. MS
(positive ESI; m/z): 735.4 [M − OTf]+, 656.5 [M − OTf − Py]+.
Anal. Calcd for C36H50F3N5O3PtS·0.4C6H14 (919.43): C, 50.16; H,
6.10; N, 7.62. Found: C, 49.85; H, 6.03; N, 7.15.
1019, 980, 912, 764, 736, 695 cm−1
. Anal. Calcd for
C31H26ClN3OPt·0.2CH2Cl2 (704.08): C, 53.22; H, 3.78; N, 5.97.
Found: C, 53.50; H, 3.71; N, 5.72.
Data for the cationic Pt−Py complex 5c are as follows. Yield: 84%.
2-((S)-4-Isopropyl-4,5-dihydrooxazol-2-yl)-6-((S)-1-cyclohexyl-4-
isopropyl-4,5-dihydro-1H-imidazol-2-yl)phenyliodoplatinum(II)
(8c). Yield: 40% based on the oxazoline−imidazoline ligand 6c, orange
20
1
Mp: 136−137 °C. [α]D = +87° (c 0.110, CH2Cl2). H NMR (400
MHz, CDCl3): δ 7.65 (d, J = 8.0 Hz, 2H, central Ar3,5 H), 7.55 (dd, J =
1.4, 6.3 Hz, 2H, Py H), 7.51−7.47 (m, 1H, Py H), 7.37 (t, J = 8.0 Hz,
1H, central Ar4 H), 7.15−7.11 (m, 2H, Py H), 7.05 (t, J = 7.4 Hz, 4H,
Ph H), 6.88−6.84 (m, 2H, Ph H), 6.78 (d, J = 7.2 Hz, 4H, Ph H), 4.73
(dd, J = 8.4, 11.9 Hz, 2H, NCH), 4.34−4.26 (m, 4H, NCy H and
NCHH), 3.61 (dd, J = 8.4, 10.4 Hz, 2H, NCHH), 2.05−2.02 (m, 2H,
Cy H), 1.95−1.89 (m, 6H, Cy H), 1.77−1.73 (m, 2H, Cy H), 1.61−
1.42 (m, 8H, Cy H), 1.20−1.09 (m, 2H, Cy H). 13C NMR (100 MHz,
CDCl3): δ 174.3, 164.9, 150.1, 141.7, 137.1, 133.3, 128.8, 127.9, 127.0,
126.2, 125.3, 123.9, 64.2, 55.2, 54.6, 31.5, 31.2, 25.4, 25.3, 25.0. IR
(KBr): ν 3062, 3029, 2930, 2854, 1606, 1567, 1518, 1495, 1466, 1450,
1384, 1350, 1318, 1271, 1222, 1149, 1030, 1012, 893, 755, 733, 697,
637 cm−1. MS (positive ESI; m/z): 803.4 [M − OTf]+, 724.4 [M −
OTf − Py]+. Anal. Calcd for C42H46F3N5O3PtS (952.99): C, 52.93; H,
4.87; N, 7.35. Found: C, 52.45; H, 5.14; N, 6.93.
20
1
solids. Mp: >290 °C. [α]D = +131° (c 0.045, CH2Cl2). H NMR
(400 MHz, CDCl3): δ 7.47 (d, J = 7.7 Hz, 1H, central Ar H), 7.34 (d, J
= 7.6 Hz, 1H, central Ar H), 7.17 (t, J = 7.7 Hz, 1H, central Ar4 H),
4.77 (dd, J = 4.5, 9.0 Hz, 1H, oxazoline OCHH), 4.64 (app t, J = 9.3
Hz, 1H, oxazoline OCHH), 4.49−4.45 (m, 1H, oxazoline NCH),
4.32−4.27 (m, 1H, imidazoline NCH), 4.22−4.15 (m, 1H, NCy-H),
3.75 (app t, J = 10.6 Hz, 1H, imidazoline NCHH), 3.69 (dd, J = 4.5,
10.1 Hz, 1H, imidazoline NCHH), 3.04−2.97 (m, 1H, CH(CH3)2),
2.86−2.81 (m, 1H, CH(CH3)2), 1.91−1.86 (m, 4H, Cy H), 1.77−1.74
(m, 1H, Cy H), 1.63−1.55 (m, 2H, Cy H), 1.46−1.38 (m, 2H, Cy H),
1.23−1.16 (m, 1H, Cy H), 0.95 (d, J = 7.2 Hz, 3H, CH3CHCH3), 0.91
(d, J = 7.2 Hz, 3H, CH3CHCH3), 0.72 (d, J = 7.0 Hz, 3H,
CH3CHCH3), 0.70 (d, J = 7.1 Hz, 3H, CH3CHCH3). 13C NMR (100
MHz, CDCl3): δ 178.7, 172.9, 164.0, 131.4, 127.7, 126.7, 126.4, 122.0,
71.5, 68.0, 66.1, 55.0, 45.4, 31.9, 31.2, 30.8, 29.6, 25.6, 25.4, 25.2, 18.5,
17.9, 13.6, 13.5. IR (KBr): ν 3432, 2930, 2859, 1598, 1572, 1527,
1495, 1459, 1415, 1384, 1330, 1266, 1245, 1157, 1001, 936 cm−1. Anal.
Calcd for C24H34IN3OPt·0.8CH2Cl2 (770.48): C, 38.66; H, 4.66; N,
5.45. Found: C, 38.79; H, 4.66; N, 5.18.
General Procedure for the Catalytic Enantioselective
Friedel−Crafts Reaction of Indoles and Nitroalkenes. To a
dried Schlenk tube were added the cationic pincer Pt(II) aquo
complex 4 (6.2 mg, 5 mol %), nitroalkene (0.15 mmol), and toluene
(1 mL) under a nitrogen atmosphere. The solution was stirred at room
temperature for 10 min and cooled to 0 °C, and then indole (0.75
mmol) and toluene (1 mL) were added. After the mixture was stirred
at 0 °C for 36 h, the solvent was removed under vacuum and the
residue was chromatographed on a silica gel column with petroleum
ether/EtOAc as eluent to afford the alkylation product.
Procedure for the Synthesis of the Cationic NCN Pincer
Platinum(II) Complexes 4 and 5a,c. In a flask protected from light,
the neutral iodoplatinum 3a (235 mg, 0.3 mmol) and silver
trifluoromethanesulfonate (154 mg, 0.6 mmol) were stirred in
CH2Cl2/H2O (19/1 v/v, 10 mL) at room temperature for 12 h.
The reaction mixture was filtered through Celite. Evaporation of the
solvent gave a residue, which was redissolved in CH2Cl2 and filtered
X-ray Diffraction Studies. Crystals of 2c and 3c were obtained by
recrystallization from CH2Cl2/acetone/petroleum ether, those of 2d
844
dx.doi.org/10.1021/om200714z | Organometallics 2012, 31, 835−846

Organometallics
Article
Table 3. Summary of Crystallographic Data for Complexes 2c·H2O, 2d·CH2Cl2, 3c, 3d·(n-butane), 4, and 7b
2c·H2O
2d·CH2Cl2
3c
3d·(n-butane)
4
7b
formula
C36H43ClN4OPt
778.28
C49H51Cl3N4Pt
997.38
C36H41IN4Pt
851.72
C52H59IN4Pt
1062.02
C31H47F3N4O4PtS
823.88
C31H26ClN3OPt
687.09
Mr
temp (K)
wavelength (Å)
cryst syst
space group
cryst size (mm)
a (Å)
291(2)
296(2)
291(2)
293(2)
296(2)
293(2)
0.710 73
monoclinic
P21
0.710 73
0.710 73
monoclinic
C2
0.710 73
0.710 73
0.710 73
orthorhombic
P212121
orthorhombic
P212121
orthorhombic
P212121
orthorhombic
P212121
0.20 × 0.14 × 0.14 0.31 × 0.23 × 0.20 0.20 × 0.20 × 0.20 0.20 × 0.16 × 0.16 0.20 × 0.10 × 0.10 0.20 × 0.20 × 0.18
6.1016(12)
25.623(5)
10.717(2)
90
14.6070(9)
15.8303(9)
18.8636(11)
90
49.081(10)
6.2623(13)
10.855(2)
90
14.769(3)
16.384(3)
18.930(4)
90
6.3569(6)
22.374(2)
24.541(3)
90
10.6347(4)
12.1232(4)
20.6502(6)
90
b (Å)
c (Å)
α (deg)
β (deg)
96.53(3)
90
90
92.40(3)
90
90
90
90
γ (deg)
V (Å3)
90
90
90
90
1664.6(6)
2
4361.9(4)
4
3333.4(12
4
4580.6(16)
4
3490.3(6)
4
2662.36(15)
4
Z
Dcalcd (g cm−3)
μ (mm−1)
θ range (deg)
index range
1.553
1.519
1.697
1.540
1.568
1.714
4.329
3.440
5.166
3.776
4.135
5.400
1.59−26.00
0 ≤ h ≤ 7
−31 ≤ k ≤ 31
−13 ≤ l ≤ 13
5989
2.51−25.50
−17 ≤ h ≤ 17
−19 ≤ k ≤ 19
−21 ≤ l ≤ 22
33 381
2.45−26.00
−60 ≤ h ≤ 60
−7 ≤ k ≤ 7
−13 ≤ l ≤ 13
18 269
1.64−27.56
−18 ≤ h ≤ 19
−21 ≤ k ≤ 0
−24 ≤ l ≤ 24
18 522
2.46 - 25.50
−7 ≤ h ≤ 7
−27 ≤ k ≤ 26
−29 ≤ l ≤ 29
26 747
3.22 - 26.37
−9 ≤ h ≤ 13
−14 ≤ k ≤ 15
−25 ≤ l ≤ 16
8342
no. of data collected
no. of unique data
5696
8075
6507
10 320
6488
5025
final R indices (I > 2σ(I))
R1
0.0470
0.1311
0.0254
0.0577
0.0471
0.0910
0.0498
0.1003
0.0508
0.0987
0.0318
0.0355
wR2
R indices (all data)
R1
0.0508
0.0348
0.0573
0.0674
0.1035
0.0452
wR2
0.1375
0.0616
0.0978
0.1098
0.1182
0.0363
F(000)
peak/hole (e Å−3)
780
2008
1664
2120
1656
1344
1.282/−1.041
0.709/−0.524
0.966/−0.769
0.974/−1.055
0.503/−0.404
1.621/−0.825
and 3d from CH2Cl2/n-hexane, those of 4 from CH2Cl2/petroleum
ether, and those of 7b from CHCl3/n-hexane at ambient temperature.
The data for 2c and 3d were collected on a Rigaku-IV imaging plate
area detector, those of 2d and 4 on a Bruker SMART APEX-II CCD
diffractometer, those of 3c on a Rigaku Saturn 724 CCD
diffractometer, and those of 7b on an Oxford Diffraction Gemini E
diffractometer with graphite-monochromated Mo Kα radiation (λ =
0.710 73 Å). The structures were solved by direct methods using the
program SHELXS-97, and the non-hydrogen atoms were refined
anisotropically on F2 by the full-matrix least-squares technique, which
used the SHELXL-97 crystallographic software package.23 The
hydrogen atoms were included but not refined. Details of the crystal
structure determination of Pt(II) complexes 2c·H2O, 2d·CH2Cl2, 3c,
3d·(n-butane), 4, and 7b are summarized in Table 3.
AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +86 371 67783012. E-mail: mpsong@zzu.edu.cn (M.-
P.S.); gongjf@zzu.edu.cn (J.-F.G).
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (No. 21072177) for financial support of this work.
REFERENCES
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ASSOCIATED CONTENT
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* Supporting Information
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