Synthesis and characterization of chiral recyclable dimeric copper(ii)-salen complexes and their catalytic application in asymmetric nitroaldol (Henry) reaction

Article abstract of DOI:10.1039/c3cy00638g

Six new chiral tridentate ditopic ligands with ONO donors possessing different linkers (either achiral or chiral) were synthesized. The characterization of these ligands was accomplished by IR, UV/Vis, NMR, mass spectrometry and optical rotation. These li

Full text of DOI:10.1039/c3cy00638g

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Synthesis and characterization of chiral recyclable
dimeric copper(II)–salen complexes and their
catalytic application in asymmetric nitroaldol
(Henry) reaction†
Cite this: Catal. Sci. Technol., 2014,
4, 411
Anjan Das,ab Rukhsana I. Kureshy, ab P. S. Subramanian,ab Noor-ul H. Khan,ab Sayed
*
H. R. Abdiab and Hari C. Bajajab
Six new chiral tridentate ditopic ligands with ONO donors possessing different linkers (either achiral or
chiral) were synthesized. The characterization of these ligands was accomplished by IR, UV/Vis, NMR,
mass spectrometry and optical rotation. These ligands have been treated with a series of metal ions viz.,
Cu(II), Cu(I), Co(III) and Zn(II), affording varieties of new chiral metal complexes, which have been
characterized thoroughly using different analytical and spectroscopic methods. All the complexes were
screened for catalytic asymmetric nitroaldol reaction using benzaldehyde as a model substrate. The
reaction conditions were optimized and 79% yield with good enantioselectivity (88%) was achieved at RT
with the in situ generated catalyst having a piperazine linker and (1R,2S)-2-amino-1,2-diphenylethanol
collar in combination with cupric acetate as the metal source. By applying other aromatic and aliphatic
aldehydes, similar yields of β-nitroalcohols with improved enantioselectivities (up to 93%) were achieved.
The catalytic system worked very well for up to four cycles with retention of activity and
enantioselectivity of β-nitroalcohols.
Received 27th August 2013,
Accepted 21st October 2013
DOI: 10.1039/c3cy00638g
www.rsc.org/catalysis
for the nitroaldol reaction at very low temperatures and at
high catalysts loading. Therefore, further work is required to
Introduction
The asymmetric nitroaldol reaction is
a
powerful and
address these issues and also the issue of catalyst recyclability
in order to ease the high cost of chiral catalysts, thereby
making this strategy industrially more acceptable. Recently,
we have reported the chiral macrocyclic salen and [H4]salen
complexes of copper(II) salts as very efficient catalysts for the
asymmetric nitroaldol reaction of aldehydes with nitromethane
to give excellent enantioselectivity and diastereoselectivity.11 In
line with our continued interest in developing recyclable
catalysts, here we have synthesized a series of new chiral
recyclable tridentate ONO donor dimeric ligands derived
from different achiral and chiral linkers viz., piperizine,
homopiperazine, trigol, (R)- and (S)-binol with (1R,2S)-2-
amino-1,2-diphenylethanol and their respective complexes
with Cu(II). At first these complexes were generated in situ
with the aim of evaluating the influence exerted by each
ligand on the catalytic activity and enantioselectivity of the
nitroaldol product. The best ligand, which is L2 in the present
case, was then complexed in situ with several other metal ions
viz., Cu(I), Co(III) and Zn(II) to find the most suitable metal
complex for the asymmetric nitroaldol reaction. To further
refine the results and understand the structure of the active
catalyst, Cu(II) complexes of ligands L1–L6 were synthesized
and characterized by CD, magnetic moment and EPR
economically viable tool for the synthesis of β-nitroalcohols.1
Asymmetric nitroaldol reaction generally requires the use of a
chiral transition metal complex derived from Co(II),2 Mg(II),3
Zn(II),4 Cr(III),5 Cu(II),6 rare earth metals7 or organocatalysts8
in order to get high product yield and enantioselectivity.
Chronologically, Shibasaki and coworkers7a–c demonstrated
the first bifunctional lanthanum–lithium chiral binaphthoxide
complex as an efficient catalyst for the asymmetric nitroaldol
reaction. This was followed by Trost et al.'s novel family of
dinuclear zinc complexes.4a,b Over the period various chiral
ligands like BOX9 and salen-type C2-symmetric ligands10 have
hogged the limelight as “privileged chiral ligands” in the
enantioselective nitroaldol reaction. However, most of these
complexes have shown good to excellent enantioselectivities
a Discipline of Inorganic Materials and Catalysis, Central Salt and Marine
Chemicals Research Institute (CSIR-CSMCRI), Bhavnagar, 364 002, India.
E-mail: rukhsana93@yahoo.co.in; Fax: +91 0278 2566970
b Academy of Scientific and Innovative Research (AcSIR), CSIR-CSMCRI,
Bhavnagar, Gujarat 364021, India
† Electronic supplementary information (ESI) available: 1H-NMR data, optical
rotation value and HPLC profiles of nitroaldol product. See DOI: 10.1039/
c3cy00638g
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Table 1 Screening of ligands for the asymmetric nitroaldol reactiona
investigations. Among these, the complex originating from L2
with copper(II) acetate was found to be the best catalyst to give
the nitroaldol product in high yield and ee up to 93%, and is
explored in detail in the present manuscript.
Entry
Ligand
Yieldb (%)
eec (%)
Results and discussion
1
2
3
4
5
6
L1
L2
L3
L4
L5
L6
55
67
60
45
35
30
45(S)
70(R)
40(R)
36(R)
12(R)
25(R)
Chiral ligands L1–L6 were synthesized conveniently by the
reaction of (1R,2S)-2-amino-1,2-diphenylethanol with different
bis-aldehydes in high yield according to Scheme 1.
The ligands L1–L4 possess metal binding chiral domains
at terminal coordinating sites with achiral linkers, whereas
L5 and L6, are composed with chiral motifs both in the termi-
nal and linker regions. 1H-NMR shows only one singlet
observed at 1.26–1.60 and 7.81–8.00 ppm for t-Bu protons
and azomethine proton, respectively, and the MS spectra
confirming the dimeric structure together support the forma-
tion of C2 symmetry of salen ligands L1–L6. Similarly, the
phenolic –OH proton also appeared as singlet for L1–L6 at
13.46–13.67 ppm, confirming the dimeric C2 symmetric struc-
ture for all the ligands. After the successful synthesis and
characterization of dimeric ligands L1–L6, first we screened
the ligands (10 mol%) with cupric acetate as metal source in
the asymmetric nitroaldol reaction of benzaldehyde as a
model substrate with nitromethane in dichloromethane at
RT for 30 h and the results are depicted in Table 1. In the
first set of screening in situ generated complexes derived
from the ligands L1–L4, we altered the achiral linker (methy-
lene, piperizine, homopiperazine and trigol) at 5,5′-positions
of the salen unit by fixing the aminoalcohol functionality
a
All the reactions were carried out on a scale of 0.5 mmol of
Isolated after column flash chromatography.
Determined by HPLC using chiral column OD.
b
the aldehyde.
c
originating from (1R,2S)-2-amino-1,2-diphenylethanol. Among
all the ligands used, ligands L1 and L2 were found to be more
active than the rest. However, L2 was found to the best both
in terms of yield and enantioselectivity (Table 1, entry 2). The
consideration of ligands L5 and L6 for this reaction was based
on our past experience that an additional element of chirality
originating from (R)-binol/(S)-binol in the catalytic concoc-
tion improves the enantioselectivity remarkably.12 But, in the
present case the presence of additional chirality was counter-
productive and the ligand L2 was still the best (Table 1, entry 2).
Having identified the L2 for its superior catalytic activity,
the ligand L2 was allowed to react with several other metal
source, such as ZnEt2, Co(OAc)2, CuBr and CuCl2·2H2O to
generate the active catalyst in order to look for the possibility
of further improving the yield and enantioselectivity of the
Scheme 1 Synthesis of chiral ligands with (1R,2S)-2-amino-1,2-diphenylethanol with various bis-aldehydes. (i) Bis-aldehyde, (1R,2S)-2-amino-1,2-
diphenylethanol, dry MeOH, RT. (ii) Piperazine bis-aldehyde, (1R,2S)-2-amino-1,2-diphenylethanol, dry MeOH, RT. (iii) Homo-piperazine bis-aldehyde,
(1R,2S)-2-amino-1,2-diphenylethanol, dry MeOH, RT. (iv) (R)-Binol, (1R,2S)-2-amino-1,2-diphenylethanol, dry DCM + MeOH, RT. (v) Trigol bis-aldehyde,
(1R,2S)-2-amino-1,2-diphenylethanol, dry MeOH, RT. (vi) (S)-Binol, (1R,2S)-2-amino-1,2-diphenylethanol, dry DCM + MeOH, RT.
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Table 3 Optimization of asymmetric nitroaldol reaction of various
β-nitroalcohol (Table 2). The results clearly show the suitability
of copper metal precursors Cu(OAc)2·H2O, CuCl2·2H2O and
CuBr over ZnEt2 and Co(OAc)2 where poor enantioselectivities
(<5% ee) were obtained with moderate to good yield. Among
the copper sources used, Cu(OAc)2·H2O in combination with
the ligand L2 gave the best catalytic performance and hence
was used for the further optimization of reaction conditions.
Solvent is known to greatly influence the catalytic perfor-
mance of the nitroaldol reaction. Accordingly, different
solvents like CH2Cl2, CHCl3, CH3CN, CH3OH and THF
(Table 3, entry 1–5) were screened at RT, where THF was
found to be best for getting higher ee (84%, entry 5). Next,
the catalyst loading variation from 2–15 mol% (Table 3,
entries 6–8) suggests 5 mol% loading is optimum at RT,
which remained optimum (entry 7) over the temperature
range studied from −10° C–RT (Table 3, entry 9–12). The data
in Table 3 revealed that 5 mol% ligand in THF as solvent at
RT are the optimum reaction parameters to get best catalytic
activity and enantioselectivity (Table 3, Entry 7).
aldehydes with nitromethanea
Entry Ligand (mol%) Solvents Temp. (°C) Yieldb (%) eec (%)
1
2
3
4
5
6
7
8
(10)
(10)
(10)
(10)
(10)
(2)
(5)
(15)
(5)
CH2Cl2
MeOH
CH3CN RT
RT
RT
67
56
57
60
70
65
78
79
60
50
50
65
70
56
67
68
84
80
88
86
87
88
89
86
CHCl3
THF
THF
THF
THF
THF
THF
THF
THF
RT
RT
RT
RT
RT
0
9
10
11
12
(5)
(10)
(10)
−5
−10
10
a
All the reactions were carried out with 0.5 mmol scale of the
Isolated after column flash chromatography.
Determined by HPLC using chiral column OD, AD, OD-H, AD-H.
b
benzaldehyde.
c
After achievement of promising results from benzaldehyde
under the optimized reaction conditions, a series of aldehydes
(both aromatic and aliphatic) were screened (Table 4). In
general, substrates irrespective of electron donating or with-
drawing group on the phenyl ring attached to the aldehyde
functional group gave products with very good to excellent ee
in 30 h. This catalytic protocol also worked well with aliphatic
aldehydes viz., n-hexanal and cyclohexanal.
Table 4 Variation of different substrates under the optimized reaction
conditionsa
Entry
R
Yieldb (%)
eec (%)
It has been observed that among all the combinations
of ligand and metal, the ligand L2 and cupric acetate was
identified as the best catalytic system. Nevertheless, all the
complexes C1–C6 incorporating the ligands L1–L6 and
cupric acetate were prepared and characterized by different
techniques, such as IR, UV-visible spectroscopy, CD and EPR
spectral study.
1
2
3
4
5
6
7
8
C6H5
2-F–C6H5
4-F–C6H5
2-Cl–C6H5
2-MeO–C6H5
3-MeO–C6H5
4-MeO–C6H5
3-NO2–C6H5
4-NO2–C6H5
2-Br–C6H5
4-Br–C6H5
n-Hexanal
Cyclohexanal
4-OH–C6H5
4-CH3CONH–C6H5
78
67
70
70
68
72
75
80
82
64
65
67
68
62
70
88
78
85
90
93
90
89
75
78
88
89
90
91
82
86
9
The IR spectra for all the ligands show a characteristic
stretching band attributable to νCN in the range of
1620–1632 cm−1 is found shifted to 10–15 cm−1 to lower wave
number in their respective metal complexes indicating their
coordination with Cu(II) metal centre. All these complexes
were characterized using positive ion MS spectra. Ligands
10
11
12
13
14
15
a
All the reactions were carried out with 0.5 mmol scale of
L1–L6 behave very similarly and depict
a characteristic
b
the aldehyde.
Isolated after column flash chromatography.
c
Determined by HPLC using chiral column OD, OD-H, AD, AD-H, IC.
Table 2 Screening of metal salts for the asymmetric nitroaldol
reactiona
ionization peak with one hydrogen ion adduct, while the
respective binuclear complexes are obtained with one or two
sodium ion adducts; such observation is a well known
phenomenon in LC-mass spectrometry.13
All the binuclear Cu(II) complexes are soluble in THF. The
electronic spectrum for the ligands L1–L6 and their
complexes with cupric acetate in THF are quite similar. Fig. 1
represents the UV-vis spectra recorded for complexes C2, C4,
C5 and C6 embedded with their respective ligand spectra. All
these ligands show a symmetrical narrow band at 340 nm
found to be red shifted to 400 nm in the respective Cu(II)
complexes, which may be assigned to the ligand to metal
Entry
Ligand
Metal source
Yieldb (%)
eec (%)
1
2
3
4
5
L2
L2
L2
L2
L2
Cu(OAc)2·H2O
ZnEt2
Co(OAc)2
CuBr
67
82
54
56
45
70
3
5
65
54
CuCl2·2H2O
a
All the reactions were carried out with 0.5 mmol scale of the
b
c
aldehyde. Isolated after column flash chromatography. Determined
by HPLC using chiral column OD.
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effects. Generally chirality at the metal center might result in
“Δ” or “Λ” based on their stereochemical chromophoric
arrangements.14 Since complexes C1–C6 are binuclear in nature
the possible combination for the “chirality at metal” would be
ΔΔ, ΔΛ or ΛΛ. The CD pattern obtained for C1 and C2 clearly
show negative and positive Cotton effects in their respective
d–d transitions. Hence it is obvious to assume that these
complexes possess predominantly ΔΔ or ΛΛ and governs the
configuration of the nitroaldol product (Table 1, entries 1, 2).
But in the cases of C3 and C4, the respective CD patterns attrib-
utable to d–d band being almost flat, the spectra show that
these complexes possess equivalent mixture of both conforma-
tional isomers ΔΛ or ΔΔ ≈ ΛΛ, which might have cancelled
each other. With the metal centered chirality thus being a key
factor for any catalytic reaction, the observation from Table 1
supports that C1 and C2 due to their dominance in ΔΔ or ΛΛ
show higher enantioselectivity in the nitroaldol product, while
C3 and C4 are less enantioselective, due to the presence of both
geometrical isomers i.e., ΔΛ or ΔΔ ≈ ΛΛ.
Fig. 1 UV–visible spectra of 0.1 M solution of ligand using THF at RT:
(a) L2 and C2; (b) L3 & C3; (c) L5 & C5; (d) L6& C6.
charge transfer of non-bonding lone pair of the phenolate
oxygen to the d-orbitals of the Cu(II), i.e., LMCT and affirms
the formation of complexes of the ligands with cupric ace-
tate. The broad spectral feature centred at 630 nm in the visi-
ble region obtained for almost all the complexes may be
attributed to the d–d transition. The position of the d–d band
supports the tetrahedrally distorted square planar geometry
around Cu(II) centres.
Magnetic moment studies
The ligand being ditopic, all six complexes are composed of
two copper(II) ions and hence to understand intermetallic
magnetic interactions, and their impact on the catalytic
efficiency, we have determined the magnetic moment using
Evans’ method in solution state.15 Accordingly, the magnetic
moments for C1–C6 were obtained in the range 1.72 to 1.78 BM
per Cu(II) ion, falling well within the range reported for mono-
meric species,16 indicate that in the dimeric structure, all
Cu(II) ions are well separated from each other and are
non-interacting. This observation suggests that the interme-
tallic linkers in the dimeric system are not favouring the M–M
exchange interaction.
In addition to the magnetic moment measurement further
confirmation was established using solution state EPR
spectra. Since the complex C2 gave higher catalytic yield and
enantioselectivity, we were encouraged to investigate the
geometry of the Cu(II) centres in the bimetallic system.
Accordingly, the EPR spectrum for complex C2 was recorded
using liquid nitrogen at 77 K in THF and is depicted in Fig. 3.
A typical four line hyperfine feature corresponding to the
interaction of the unpaired electron (63Cu and 65Cu) with the
The complexes C1–C4 possess similar chiral components,
while the complexes C5 and C6 differ and possess an addi-
tional chiral spacer, binol. Binol is known for its axial chirality.
Hence systematic circular dichroism (CD) spectral investigation
was carried out for the ligands and their respective complexes
and are presented in Fig. 2. The CD pattern for ligand L1 and
L2 are similar, while their respective metal complexes C1 and
C2 are opposite to each other. This may be due to opposite
stereochemical arrangements with respect to their ligands,
which make the metal centre chiral and cause their metal
centered d–d transitions with negative and positive Cotton
Fig. 2 CD spectra for (a) L1 and C1, (b) L2 and C2, (c) L3 and C3, (d) L4
and C4 recorded in THF. Conc. 1 × 10−4 M.
Fig. 3 X-band EPR spectrum recorded for complex C2 in THF solvent,
frozen at 77 K with liquid nitrogen.
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nuclear spin I = 3/2 attributable to −3/2, −1/2, +1/2, +3/2 transi-
tions (δ = 1) is generally observed for monomeric species.
Similarly a weak signal attributable to ΔMs = 2 transition
in the half field region is the characteristic peak generally
observed for dinuclear Cu(II) system. The absence of the
ΔMs = 2 transition in the present case thus rules out the
possibility of any Cu–Cu dimeric exchange interactions in the
binuclear system although the complex is binuclear in nature.
Further, the Cu(II) d9 system in complex C2 illustrates charac-
teristic four line peaks as shown in Fig. 3, which is attribut-
able to monomeric species. In the present case, among the
four lines observed three are resolved and the fourth line is
merged with perpendicular feature. The EPR parameters
g∥ = 2.2160, A∥ = 148 G, g⊥ = 2.0042 derived from the spectra
suggest that the geometry at the Cu(II) centre might be a
tetrahedrally distorted square planar geometry, which is very
much in parallel with the d–d band observed in the UV-vis
spectra. The position of the EPR signal and the absence of
ΔMs = 2 resonance strongly suggested that the molecule,
although binuclear in nature, still possesses a non-interacting
M–M association. This observation supporting the mono-
meric behaviour matches well with the magnetic moment
determined above.
Since both the active sites in the catalytic system worked
separately for producing enantioselectivity as well as activity,
for the evaluation of a probable mechanism of the catalytic
nitroaldol reaction, a single unit was considered, which has
been shown in Scheme 2. Based on the EPR results a tetrahe-
drally distorted square planar geometry complex structure
was considered for the prediction of the probable mecha-
nism. The aldehyde was coordinated to the vacant d orbital
of the copper through the lone pair of the oxygen forming a
penta-coordinate transition state (T1), thereby increasing the
electrophilicity of the carbonyl group. The coordination
number of the copper centre may be extended to six from five
upon further addition of an active nucleophile, nitronate ion,
forming a transition state (T2), where the nitronate ion
attacks the activated aldehyde to give the nitroaldol product.
To investigate the reason behind the preferential forma-
tion of the R-nitroaldol product, a probable transition state
involving complex C2, 2-MeO-benzaldehyde and nitromethane
was generated and energy minimized by using ChemDraw
12.0(3D) (Scheme 3). From the TS it is clear that the nitronate
Scheme 3 Energy minimized probable transition state favouring the
formation of (R)-β-nitroalcohol.
ion attacks the carbonyl group of the aldehyde from the Si
face, favouring the R product.
In order to find out the dependence of both yield and
enantioselectivity on time, a model reaction of benzaldehyde
with nitromethane was carried with the optimal nitroaldol reac-
tion parameters and both the yield and enantioselectivity were
plotted against time (Fig. 4). From Fig. 4 it is clear that initially
up to 6 h there was a rapid increase in the yield (up to 60%)
with slight variation in enantioselectivity. On further increasing
the reaction time up to 30 h, there was very slow increase of
the yield observed with almost constant enantioselectivity.
Recyclability study of the complex C2
After the completion of the nitroaldol reaction of the benzal-
dehyde with nitromethane, the solvent was completely evapo-
rated under reduced pressure and was dried. The product
and unreacted substrate were extracted by using non-polar
solvent (hexane). Then the isolated catalyst was washed four
times with hexane and was dried for 3–4 h under vacuum.
The recovered catalyst was used straightaway (without further
addition of the metal salt or ligand) for subsequent catalytic
cycles. In the case of polar substrates (Table 4, entries 14, 15),
the catalysts were separated from the reaction mixtures by
passing through a silica pad using EtOAc : hexane (1 : 1) as
solvent. The performance of the catalyst remained stable over
five catalytic cycles (Fig. 5). From the recycling experiments it
is evident that the complex C2 is stable during the course of
the asymmetric nitroaldol reaction, confirmed by the IR spectra
(Fig. 6) which matched well with virgin catalyst C2, suggesting
Scheme
2
A
probable mechanism for the catalytic asymmetric
Fig. 4 Variation of yield and ee vs. time of nitroaldol reaction of
nitroaldol reaction.
benzaldehyde with nitromethane.
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spectrophotometer. Mass analyses were performed using pos-
itive electron spray ionization (ESI+) technique on a Waters Q
Tof-micro mass spectrometer for all these complexes upon
dissolving in CH3CN solvent. 1H and 13C spectra were
recorded on a Bruker Avance II 500 MHz FT-NMR spectrome-
ter. Chemical shifts for proton resonances are reported in
ppm (δ) relative to tetramethylsilane. Electron spin resonance
spectra were recorded using Bruker X-band electron paramag-
netic resonance spectrometer and the DPPH was used as filed
marker in the EPR spectra. The CD spectra were recorded on
a JASCO 815 Spectrometer. The formation of nitroaldol was
determined by HPLC (Shimadzu SCL-10AVP) using Chiracel
columns (AD, OD, OD-H).
Fig. 5 Recyclability study of the catalytic system using benzaldehyde
and nitromethane as model substrates.
Typical procedure for the synthesis of ligand L1–L6
Various dialdehydes namely, (S)-5,5′-(1,1′-binaphthyl-2,2'-
diylbis(oxy))bis(methylene)bis(3-tert-butyl-2-hydroxybenzaldehyde)/
(R)-5,5′-(1,1′-binaphthyl-2,2′-diylbis(oxy))bis(methylene)bis(3-
tert-butyl-2-hydroxybenzaldehyde), 5,5′-(piperazine-1,4-diylbis
(methylene))bis(3-tert-butyl-2-hydroxybenzaldehyde) and trigol
bis-aldehydes were synthesized by the reported procedures12
and were taken (1 mmol) in 5 ml THF. The solutions were
added to a solution of (1R,2S)-2-amino-1,2-diphenylethanol
(2 mmol) and the resulting mass was stirred for 5 h at room
temperature (checked by TLC). After the completion of the
reaction, the solvent was completely removed under reduced
pressure on a rotary evaporator to give chiral salen ligands
L1–L6 in high yield.
Fig. 6 IR spectra of fresh and recycled catalyst.
that no major structural changes had taken place during the
course of post-catalytic workup procedure.
Conclusions
Characterization data of ligands L1–L6
Six chiral ditopic ligands and their respective bimetallic Cu(II)
complexes were synthesised and characterized. All these
ligands were screened for their catalytic activity in asymmetric
nitroaldol reaction. The ligand L2 in combination with cupric
acetate was found to be the catalytically most suitable, and
was chosen for further fine tuning of the catalytic studies to
obtain the best results. Although we have tested various
metals, cupric acetate generates the most active catalytic
system with ligand L2, giving high yield and enantiomeric
excess. The CD spectra indicates that the complexes C1 and
C2 gain metal centred chirality and are dominated by ΔΔ > ΛΛ
or ΔΔ < ΛΛ, leading to asymmetrically enhanced catalysis.
However the other complexes show flat CD patterns with
respect to d–d transitions, indicating the existence of a ΔΔ ≈ ΛΛ
situation. The magnetic properties and the EPR spectra
together support the existence of non-interacting Cu–Cu dimers
in the case of complex C2.
L1. Yellow solid; yield 95%; [α]D20 = −15.44 (c 1, CHCl3); 1H
NMR (CDCl3, 500 MHz, TMS): δ = 13.47 (br. s, 2H), 8.00
(s, 2H), 7.10–7.37 (m, 24H), 6.61 (s, 2H), 5.04 (d, J = 5 Hz, 2H),
4.44 (d, J = 5 Hz, 2H), 3.72 (s, 2H), 1.60 (s, 18H);13C NMR
(125 MHz, CDCl3): δ = 29.3, 34.8, 40.2, 78.36, 80.22, 127.2,
127.9, 128.0, 128.1, 129.9, 130.1, 130.5, 137.2, 139.4, 140.1,
158.5, 166.6 ppm. IR (KBr) ν: 3428, 3060, 3030, 3000, 2954,
2908, 2873, 2707, 1955, 1882, 1806, 1753, 1627, 1442 cm−1.
Anal. calcd. for C51H54N2O4C, 80.71; H, 7.17; N, 3.69; found:
C, 80.68; H, 7.19; N, 3.68. LC-MS: m/z 759 [M + H]+.
1
L2. Yellow solid; yield 85%; [α]D20 = −25.54 (c 1, CHCl3); H
NMR (CDCl3, 200 MHz, TMS): 13.52 (s, 2H), 8.07 (s, 2H),
7.36–7.15 (m, 24H), 5.06, 5.03 (d, J = 7 Hz, 2H), 4.49–4.46
(d, J = 7 Hz, 2H), 3.35 (s, 4H), 2.37 (s, 8H) 1.41 (s, 18H);
13C NMR (125 MHz, CDCl3) 166.32, 158.60, 139.12, 138.32,
136.22, 130.65, 130.51, 127.50, 126.90, 126.76, 126.22, 117.22,
79.85, 78.10, 60.62, 52.42, 34.72. IR (KBr) ν: 3426, 3062, 3030,
2952, 2875, 2812, 1884, 1808, 1628, 1447, 1384 cm−1. Anal.
calcd. for C56H64N4O4 C, 78.47; H, 7.53; N, 6.54; found:
C, 78.45; H, 7.52; N, 6.55. LC-MS: m/z 857 [M + H]+.
Experimental section
General
1
L3. Yellow solid; yield 85%; [α]D20 = −12.56 (c 1, CHCl3), H
All the chemicals were purchased from Aldrich & Co. IR
spectra were recorded using KBr pellets (1% w/w) on a
Perkin-Elmer Spectrum GX FT-IR spectrophotometer. Elec-
tronic spectra were recorded on a Shimadzu UV 3101PC
NMR (CDCl3, 200 MHz, TMS): δ ppm = 13.67 (s, 2H), 8.08
(s, 2H), 7.32–7.19 (m, 24H), 5.01–5.00 (d, J = 2 Hz, 2H),
5.49–5.48 (d, J = 2 Hz, 2H), 3.53 (s, 4H), 2.70–2.63 (m, 8H),
1.79 (s, 2H), 1.49 (s, 18H); 13C NMR (125 MHz, CDCl3)
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166.37, 159.62, 140.13, 139.36, 137.20, 130.68, 130.56, 128.51,
127.99, 127.79, 127.20, 118.23, 79.89, 78.16, 61.65, 53.43,
34.74, 29.27. IR (KBr) ν: 3370, 3062, 3030, 2952, 2916, 2871,
2246, 1954, 1881, 1807, 1745, 1629, 1447, 1388 cm−1. Anal.
calcd. for C57H66N4O4 C, 78.59; H, 7.64; N, 6.43; found:
C, 78.56; H, 7.65; N, 6.45. LC-MS: m/z 872 [M + H]+.
C3: IR (KBr) ν: 3430, 2956, 2924, 2359, 1955, 1890, 1735,
1625, 1569, 1435, 1382 cm−1. [α]D20 = 15.58 (c 1, CHCl3).
LC-MS: m/z 1015 [Cu2L3 + Na]+. CD (THF) λmax (nm) (Δε):
294.43(−18.63), 397.92(+16.83). UV/Vis (THF): λmax (ε) = 672.36,
388.22 nm. Anal. calcd. for (C57H62N4O4): C, 68.86; H, 6.29;
N, 5.64; found: C, 68.76; H, 6.21; N, 5.62.
L4. Yellow solid; yield 90%; [α]D20 = −5.42 (c 1, CHCl3); H
C4: IR (KBr) ν: 3433, 3030, 2910, 2863, 1954, 1623, 1566,
1536, 1422, 1388 cm−1. [α]D20 = 26.52 (c 1, CHCl3). LC-MS: m/z
1065 [Cu2L4+Na]+. CD (THF) λmax (nm) (Δε): 290.32(+4.73),
391.51(−13.32). UV/Vis (THF): λmax (ε) = 604.44, 385.63 nm.
Anal. calcd. for (C58H64N2O8): C, 66.71; H, 6.18; N, 2.68;
found: C, 66.72; H, 6.12; N, 2.62.
1
NMR (CDCl3, 200 MHz, TMS): δ ppm = 13.65 (s, 2H), 8.06
(s, 2H), 7.35–7.22 (m, 24H), 5.04–5.01(d, J = 6 Hz, 2H),
4.48–4.45 (d, J = 6 Hz, 2H), 4.39 (s, 4H), 3.61–3.57 (m, 12H),
1.42 (s, 18H); 13C NMR (125 MHz, CDCl3) 166.48, 159.58,
140.13, 139.43, 137.34, 129.86, 129.26, 129.15, 128.69, 128.07,
127.55, 127.22, 118.26, 80.19, 73.17, 70.62, 69.13, 34.86,
29.31. IR (KBr) ν: 3440, 2921, 1632, 1539, 1455, 1385 cm−1.
Anal. calcd. for C58H68N2O8C, 75.62; H, 7.44; N, 3.04; found:
C, 75.64; H, 7.42; N, 3.02. LC-MS: m/z 921 [M + H]+.
C5: IR (KBr) ν: 3433, 3058, 3029, 2949, 2861, 2694, 1948,
1806, 1741, 1619, 1535, 1503, 1454 cm−1. [α]D20 = 32.25 (c 1,
CHCl3). LC-MS: m/z 1224 [Cu2L5 + 2Na]+. CD (THF) λmax (nm)
(Δε): 327.80 (−0.44), 356.57 (−18.92), 414.12 (−19.64), 691.67
(−4.14). UV/Vis (THF): λmax (ε) = 631.82, 389.15 nm. Anal.
calcd. for (C72H64N2O6): C, 73.26; H, 5.47; N, 2.37; found:
C, 73.18; H, 5.45; N, 2.36.
1
L5. Yellow solid; yield 90%; [α]D20 = −17.24 (c 1, CHCl3), H
NMR (CDCl3, 200 MHz, TMS): δ = 13.46 (s, 2H), 7.81 (d, J = 9 Hz,
2H), 7.73 (d, J = 8 Hz, 2H), 7.58 (s, 2H), 7.36 (m, 8H),
7.31(s, 2H), 7.15–7.25 (m, 14H), 7.05–7.07 (m, 4H), 6.88
(s, 2H), 6.20 (s, 2H), 5.00 (d, J = 7 Hz, 2H), 4.80 (s, 4H), 4.38
(d, J = 7 Hz, 2H), 1.26 (s, 18H). 13CNMR (50 MHz, CDCl3):
30.30, 30.69, 72.64, 117.86, 122.83, 125.78, 127.13, 128.28,
129.47, 129.65, 131.17, 131.31, 132.11, 134.37, 135.74, 139.49,
162.03. IR (KBr) ν: 3431, 3059, 3031, 2954, 2921, 2868, 1950,
1805, 1627, 1592, 1502, 1452 cm−1. Anal. calcd. for C72H68N2O6C,
81.79; H, 6.48; N, 2.65; found: C, 81.76; H, 6.46; N, 2.68. LC-MS:
m/z 1058 [M + H]+.
C6: IR (KBr) ν: 3432, 3057, 3028, 2948, 2860, 2693, 1947,
1805, 1740, 1618, 1534, 1502, 1453 cm−1. [α]D20 = 12.12 (c 1,
CHCl3). CD (THF) λmax (nm) (Δε): 324.74 (−36.06), 326.35
(−25.87), 408.00 (−23.02), 680.68 (−5.30). UV/Vis (THF): λmax
(ε) = 630, 389 nm. Anal. calcd. for (C72H64N2O6): C, 73.26;
H, 5.47; N, 2.37; found: C, 73.16; H, 5.42; N, 2.35.
Magnetic moment determination
1
L6. Yellow solid; yield 90%; [α]D20 = −27.12 (c 1, CHCl3), H
Magnetic susceptibility measurements were carried out
as per the procedure reported by Evans. The inner tube
(~2.5 mm i.d.) was filled with the known concentration of
sample solution in THF + tert-butyl alcohol, while the outer
tube was filled with THF + tert-butyl alcohol. A paramagnetic
shift observed in a TMS resonance line was used to calculate
χM using eqn (1).
NMR (CDCl3, 200 MHz, TMS): δ = 13.45 (s, 2H), 7.80 (d, J = 9 Hz,
2H), 7.72 (d, J = 8 Hz 2H), 7.57 (s, 2H), 7.35 (m, 8H), 7.30(s, 2H),
7.14–7.24 (m, 14H), 7.04–7.06 (m, 4H), 6.87 (s, 2H), 6.20 (s, 2H),
5.01 (d, J = 7 Hz, 2H), 4.81 (s, 4H), 4.37 (d, J = 7 Hz, 2H),
1.25(s, 18H). 13C NMR (50 MHz, CDCl3): 30.30, 30.68, 72.63,
117.85, 122.82, 125.77, 127.12, 128.27, 129.47, 129.64, 131.16,
131.30, 132.10, 134.37, 135.74, 139.46, 162.03. IR (KBr) ν: 3431,
3059, 3031, 2954, 2921, 2868, 1950, 1805, 1627, 1592, 1502,
1452 cm−1. Anal. calcd. for C72H68N2O6C, 81.79; H, 6.48; N, 2.65;
found: C, 81.75; H, 6.47; N, 2.64. LC-MS: m/z 1058 [M + H]+.
χM = 3Δf/2πν + χ0 + χ0(d0 − ds)/m
(1)
where f = frequency separation between the TMS lines, fm =
frequency at which the proton resonance is studied, and m =
mass of the substance. The magnetic moment was calculated
from χM using eqn (2).
Characterization data of metal complexes (C1–C6)
C1: IR (KBr) ν: 3405, 3061, 3029, 2953, 2908, 2872, 2616, 1952,
1881, 1806, 1707, 1621, 1532, 1491, 1420 cm−1. [α]D20 = −35.42
(c 1, CHCl3). LC-MS: m/z 881 [Cu2L1 + H]+. CD (THF) λmax (nm)
(Δε): 360.74 (−14.90), 433.70 (−16.43), 563.98 (+5.53), 675.71(−4.48).
μeff = √ χMT
(2)
where T stands for temperature in K.
UV/Vis (THF): λmax (ε)
= 625, 385 nm. Anal. calcd. for
(C51H50N2O4): C, 69.45; H, 5.71; N, 3.18; found: C, 69.15; H, 5.66;
N, 3.16.
Acknowledgements
C2: IR (KBr) ν: 3436, 3029, 2927, 2802, 1812, 1620, 1533,
1421 cm−1. [α]D20 = 29.25 (c 1, CHCl3). LC-MS: m/z 979 [Cu2L2 +
H]+, LC-MS: m/z 996 [Cu2L2 + H2O]+. CD (THF) λmax (nm) (Δε):
347.86 (+14.62), 415.66 (+24.30), 590.40 (−4.90), 693.82 (+9.97).
UV/Vis (THF): λmax (ε) = 630, 390 nm. Anal. calcd. for
(C56H60N4O4): C, 68.62; H, 6.17; N, 5.72; found: C, 68.56; H,
6.10; N, 5.68.
Anjan Das and RIK are thankful to DST and CSIR-Indus
Magic Project CSC0123 for financial assistance. Anjan Das is
thankful to UGC for awarding SRF and to AcSIR for Ph.D
registration. Authors are also grateful to Analytical Discipline
and Centralized Instrument Facility of CSMCRI for providing
instrumental facilities.
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K. Waich, W. Skranc, D. Mink and H. Griengl, Angew. Chem.,
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