Spectroscopic evidence of chirality in tetranuclear Cu(II)-Schiff base complexes, catalytic potential for oxidative kinetic resolution of racemic benzoin

Article abstract of DOI:10.1080/24701556.2020.1852425

Two chiral Schiff base ligands 2-((1-hydroxy-3-phenylpropan-2-ylimino)methyl)-6-methoxyphenol (L¹H₂) and 2-(4-hydroxy-3-isopropylbut-1-enyl)-6-methoxyphenol (L²H₂) have been synthesized by the condensation of l-phenylalaninol/l-valinol and o-vanillin (2-hydroxy-3-methoxy benzaldehyde). A tetranuclear homometallic Cu(II) complex [Cu₄(L¹H)₂(L¹)₂] (ClO₄)₂ (C1) and a hexanuclear heterometallic complex [Cu₄(L²)₄Na₂(DMF)₂(H₂O)] (ClO₄)₂ (C2) have been synthesized with the ligands. Both the complexes possess cubane like Cu₄O₄ core with interesting structural variations and inherit the chirality of their corresponding ligands. The catalytic potential of the complexes has been explored for the oxidative kinetic resolution of racemic benzoin. The electronic, optical and chiroptical properties of the ligands and the complexes have been studied by DFT and TD-DFT calculations.

Full text of DOI:10.1080/24701556.2020.1852425

Inorganic and Nano-Metal Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsrt21
Spectroscopic evidence of chirality in tetranuclear
Cu(II)-Schiff base complexes, catalytic potential for
oxidative kinetic resolution of racemic benzoin
Dipali Sadhukhan , Prithwi Ghosh & Susanta Ghanta
To cite this article: Dipali Sadhukhan , Prithwi Ghosh & Susanta Ghanta (2020): Spectroscopic
evidence of chirality in tetranuclear Cu(II)-Schiff base complexes, catalytic potential for
oxidative kinetic resolution of racemic benzoin, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2020.1852425
To link to this article: https://doi.org/10.1080/24701556.2020.1852425
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INORGANIC AND NANO-METAL CHEMISTRY
https://doi.org/10.1080/24701556.2020.1852425
Spectroscopic evidence of chirality in tetranuclear Cu(II)-Schiff base complexes,
catalytic potential for oxidative kinetic resolution of racemic benzoin
Dipali Sadhukhana , Prithwi Ghoshb
and Susanta Ghantaa,c
b
aDepartment of Chemistry, Indian Institute of Technology, Kharagpur, West Midnapur, West Bengal, India; Department of Botany, Narajole
Raj College, Narajole, Paschim Medinipur, West Bengal, India
ABSTRACT
ARTICLE HISTORY
Received 13 July 2020
Accepted 18 October 2020
Two chiral Schiff base ligands 2-((1-hydroxy-3-phenylpropan-2-ylimino)methyl)-6-methoxyphenol
(L1H2) and 2-(4-hydroxy-3-isopropylbut-1-enyl)-6-methoxyphenol (L2H2) have been synthesized by
the condensation of L-phenylalaninol/L-valinol and o-vanillin (2-hydroxy-3-methoxy benzaldehyde).
A tetranuclear homometallic Cu(II) complex [Cu4(L1H)2(L1)2] (ClO4)2 (C1) and a hexanuclear heter-
ometallic complex [Cu4(L2)4Na2(DMF)2(H2O)] (ClO4)2 (C2) have been synthesized with the ligands.
Both the complexes possess cubane like Cu4O4 core with interesting structural variations and
inherit the chirality of their corresponding ligands. The catalytic potential of the complexes has
been explored for the oxidative kinetic resolution of racemic benzoin. The electronic, optical and
chiroptical properties of the ligands and the complexes have been studied by DFT and TD-DFT
calculations.
KEYWORDS
Chiral complexes;
asymmetric catalysts; kinetic
resolution; enantioselectiv-
ity; TD-DFT calculation
GRAPHICAL ABSTRACT
Two chiral tetranuclear Cu(II) complexes have been synthesized from chiral Schiff base ligands.
There are structural, spectroscopic and computational evidence of chirality of the complexes. The
complexes show moderate activity toward enantioselective kinetic resolution of racemic benzoin
with a maximum of 63% enantiomeric excess.
Abbreviations: OKR: oxidative kinetic resolution; TEMPO: 2,2,6,6-tetramethylpiperidin-1-oxyl; ee:
enantiomeric excess; DMF: dimethyl formamide; DFT: density functional theory; TD-DFT: time
dependent density functional theory
Introduction
In current synthetic chemistry, there is a growing interest of
synthesize chiral transition metal complexes, excellent
catalysts for many enantioselective reactions such as
enantiomeric epoxidation of unfunctionalized olefins,
hydroxilations, oxidations, epoxidations, aziridinations,
the development of new chiral ligands and their metal com-
plexes as efficient catalysts for asymmetric transformation.[1]
After the pioneering work of Jacobsen and Katsuki[2] chiral
salen-type Schiff base ligands have been extensively used to
CONTACT Dipali Sadhukhan
Bengal, India; Susanta Ghanta
Bengal 721302, India.
dipali_juchem@yahoo.co.in
susanta.ghanta82@gmail.com
Department of Chemistry, Indian Institute of Technology, Kharagpur, West Midnapur, West
Department of Chemistry, Indian Institute of Technology, Kharagpur, West Midnapur, West
Supplemental data for this article can be accessed at the publisher’s website.
c Preset address - Department of Chemistry, National Institute of Technology, Agartala, Tripura, India
ß 2020 Taylor & Francis Group, LLC

2
D. SADHUKHAN ET AL.
cyclopropanation, Diels–Alder cyclization, kinetic resolution hexahydrate was prepared by treatment of copper carbonate
of racemic allenes, etc.[3]
(E.Merck, India) with 60% perchloric acid (E.Merck, India)
Use of chiral metal catalysts for the kinetic resolution of
racemic secondary alcohols is a very important synthetic
process as the enantiopure alcohols are the potential inter-
mediates for several natural products, chiral ligands and bio-
logically active compounds.[4] Enantiomerically pure
secondary alcohols are usually synthesized by the enzymatic
kinetic resolution of secondary alcohols through acylation/
deacylation,[5] non-enzymatic kinetic resolution,[6] and
enantioselective reductions of ketones.[7] Among the various
kinetic resolution techniques, the most privileged method is
the oxidative kinetic resolution (OKR) as molecular oxygen
is used as the sole stoichiometric oxidant and water is
formed as the only by-product. Although a number of chiral
metal complexes of Ru,[8] Pd,[9] Mn,[10] V[11] and Ir[12] have
been used for the OKR of racemic secondary alcohols, those
are expensive catalysts. Copper being a cheaper and more
abundant metal, chiral Cu(II) complexes[13] can be econom-
ically competent catalysts, although not very widely
explored. In the present report, we have investigated the
catalytic role of some chiral Schiff base-Cu complexes in the
aforementioned oxidative kinetic resolution of racemic ben-
zoin and to investigate the extent of chiral induction.
followed by the slow evaporation on the steam bath. It was
then filtered through a fine glass-frit and preserved in a
CaCl2 desiccator for further use.
Caution! Perchlorate salts are potentially explosive and
should be used in less quantity and with much care
although no such problem was encountered during the pre-
sent experiments.
Physical measurements
The FT-IR spectra of the compounds were recorded on a
Perkin Elmer RX I FT-IR spectrometer with KBr pellets in
the range 4000–400 cmꢀ1. The electronic spectra were
recorded at 300 K on a Shimadzu UV 2450 UV–Vis spec-
trometer using reagent grade methanol as solvent in the
1
range 800–200 nm. H and 13C-NMR spectra of the Schiff
base ligands, benzoin and benzil were recorded on Bruker
400 MHz FT-NMR spectrometer using tetramethylsilane as
internal standard in CDCl3 solvent. The CD spectra of the
samples were recorded with a Jasco J 815 CD spectrometer.
Herein we have synthesized chiral Schiff base ligands by
the condensation of enantiopure amino alcohols (L-phenyl
alaninol and L-valinol) and an aldehyde o-vanillin (2-
hydroxy-3-methoxybenzaldehyde). The ligands were used to
synthesize Cu(II) complexes that retain the chirality of the
ligand. The ligands and the complexes were characterized by
FT-IR and UV–Visible and circular dichroism spectroscopy.
The structures of the complexes were established by single
crystal X-ray diffraction. Analysis of the catalytic efficiency
Crystallographic data collection and structure
refinements
The X-ray diffraction data were collected on a Bruker Apex
II Kappa CCD diffractometer containing area detector and
graphite
monochromator
with
MoKa
radiation
(k ¼ 0.71069 Å) at 293 K. Data collection and data reduction
were performed with SHELX[15] programmes. The structures
were solved by direct method using the program SIR92[16]
of the complexes for kinetic resolution of racemic benzoin and refined with the program CRYSTALS.[17] All non-
has been investigated. To obtain a deeper understanding of
the optical and chiroptical properties of molecules, density
functional theory (DFT) and time dependent density func-
tional theory (TD-DFT) calculations are necessary[14] as
those methods are reliable tools for the assignment and
interpretation of electronic transitions involved in the
optical properties. Hence in the present work, the structures
of the ligands were optimized at the level of DFT in the
relevant solvent phase. Time dependent density functional
theory (TD-DFT) calculations complemented with quantum
mechanics/molecular mechanics (QM/MM) studies have
also been performed for the ligands and the complexes to
understand the origins of the absorption and circular
dichroism spectra.
hydrogen atoms were refined anisotropically by full-matrix
least-squares based on F. All other H atoms were generated
geometrically and were included in the refinement in the
riding model approximation. The crystallographic data for
the complexes are summarized in Table S8.
Theoretical calculations
The DFT and TD-DFT calculations have been performed
using the Gaussian 09 program package.[18] The structures
of the ligands were optimized at the B3LYP level of the-
ory.[19] The various vertical Franck–Condon electronic tran-
sitions were calculated at the TD-DFT level of theory using
[20]
ꢁ
6-31 g basis set.
The optimization calculations have been
done by applying the self-consistent reaction field (SCRF)
under the polarizable continuum model (c-pcm) including
methanol as the solvent.[21] We have considered first fifty
excitations for TD-DFT calculation for both ligands and
complex molecules. The ground state optimized structures
were considered for all excited state calculations for the
ligands (L1H2 and L2H2) while for the complexes (C1 and
C2), the initial structures were taken from the X-ray diffrac-
tion CIF files. CAM-B3LYP method is employed for TD-
Material and methods (experimental and
theoretical)
Material
All solvents were of reagent grade and used after distillation.
L-phenyl alanine, L-valine, sodium borohydride, iodine,
potassium hydroxide, sodium sulfate, o-vanillin, sodium
azide, benzoin and TEMPO were purchased from local
chemical suppliers and used as received. Copper perchlorate DFT calculation for both complexes.

INORGANIC AND NANO-METAL CHEMISTRY
3
Syntheses of the ligands and the complexes
4.59; N, 3.45. Found: C, 50.24; H, 4.56; N, 3.49. FT-IR (KBr)
ꢀ cmꢀ1: 3054w, 3024w, 2934w, 2830w, 1635s, 1601s, 1557m,
1539m, 1520w,1506w,1489w, 1467s, 1454s, 1446s, 1373w,
1360w, 1322w, 1295w, 1250s, 1220s, 1084s, 1048w, 973m,
853w, 776w, 737m, 703m, 652w, 624m, 586w, 503w, 470w,
454w, 443w. UV–Vis (MeOH) kmax nm (e Lmolꢀ1cmꢀ1):
681(d-d transition), 378 (15,400), 281 (51,400), 236 (88,300),
209 (84,000).
Synthesis of the ligands: 2-(3-benzyl-4-hydroxybut-1-enyl)-
6-methoxyphenol (L1H2) and 2-(4-hydroxy-3-isopropylbut-
1-enyl)-6-methoxyphenol (L2H2)
The ligands L1H2 and L2H2 were synthesized in two steps
following the procedure depicted in Scheme 2.
In the first step, L-phenyl alanine (for L1H2)/L-valine (for
L2H2) were reduced to L-phenyl alaninol/L-valinol according
to the published procedure.[22] In the second step, L-phenyl
alaninol (10 mmol, 1.51 g)/L-valinol (10 mmol, 1.03 g) were
mixed with o-vanillin (10 mmol, 1.52 g) in 50 mL methanol
and the mixture was refluxed at 65 ꢂC for 6 h. Yellow solu-
tions of L1H2 and L2H2 were obtained. The L1H2 solution
on vacuum evaporation resulted in a yellow powder which
was recrystallized from methanol. Yellow crystals of the lig-
and L1H2 were obtained by slow evaporation of the metha-
nolic solution in 2 days. L2H2 was obtained as a viscous
oily liquid.
Synthesis of the complex [Cu4(L2)4Na2(DMF)2(H2O)]
(ClO4)2 (C2)
Cu(ClO4)2. ꢃ H2O (0.5 mmol, 0.187 g) was dissolved in
10 mL methanol. Methanolic solution of L2H2 (0.5 mmol,
10 mL) was added into it while stirring at room temperature
(ꢄ30 ꢂC). Aqueous solution of NaN3 (1.5 mmol, 0.09 g) was
added drop wise into the reaction mixture. The solution was
1
stirred for = h. The green solution was filtered and kept for
2
slow evaporation at room temperature. Green solid precipi-
tated upon evaporation for 1 week. The precipitate was dis-
solved in 20 mL 1:1 DMF-MeOH mixture and the solution
was settled for slow evaporation at RT. Green block shaped
crystals appeared after three weeks. Yield 126 mg (63%).
Anal. Calc. for [C58H84Cl2Cu4N6Na2O23]: C, 43.42; H, 5.28;
N, 5.24. Found: C, 43.45; H, 5.26; N, 5.26. FT-IR (KBr) ꢀ
cmꢀ1: 3650–3100w, 2960m, 2872m, 1640s, 1587m, 1554m,
1468s, 1447s, 1430m, 1404m, 1326s, 1256s, 1219s, 1170m,
1082s, 1052s, 979m, 882m, 735s, 666m, 587m, 518m. UV–Vis
(MeOH) kmax nm (e Lmolꢀ1cmꢀ1): 685 (d-d transition), 378
(15,400), 277 (55,700), 233 (100,600), 207 (94,700).
L1H2: Yield: 185 mg (65%). 1H-NMR (ppm, 400 MHz,
CDCl3) 8.08 (1H,s, H7), 7.262–7.228 (3H,m, H14–16),
7.190–7.138 (3H,m, H4–6), 6.896–6.751 (2H,m, H13,17), 3.928
(3H,s, H1–3), 3.903–3.753 (2H,m, H9,10), 3.538–3.487 (1H,m,
H8), 3.016–2.982 (1H,m, H11), 2.896–2.841 (1H,m, H12)
(Figure S5). 13C-NMR (ppm, 100 MHz, CDCl3) d 166.2 (C8),
152.1 (C2), 148.6 (C7), 137.8 (C12), 129.6 (C14, 16), 128.6
(C13, 17), 126.6 (C6), 123.3 (C15), 118.3 (C3), 118.1 (C5),
114.2 (C4), 72.9 (C10), 66.0 (C9), 56.2 (C1), 39.2 (C11)
(Figure S6) (Scheme 3a). FT-IR (KBr, cmꢀ1): 3439w, 3220w,
2928w, 2854w, 1638s, 1508m, 1497m, 1458m, 1248s, 1216s,
1162m, 1090s, 1050m, 1013w, 965w, 911w, 848w, 789w,
752m, 718m, 702m, 605w, 558w, 467w. UV–Vis (MeOH)
kmax nm (e Lmolꢀ1cmꢀ1): 330 (2900), 262 (14,400),
220 (69,200).
General procedure for AOKR
L2H2: Yield: 243 mg (89%).1H-NMR (400 MHz, CDCl3, The catalyst was dissolved in 10 mL solvent. TEMPO was
ppm): 8.33 (1H,s, H7), 6.97–6.87 (2H,m, H4,6), 6.79–6.76
(1H,m, H5), 3.9 (3H,s, H1–3), 3.89–3.82 (2H,m, H9,10),
3.78–3.73 (1H,m, H8), 1.98–1.93 (1H,m, H11), 0.95 (6H, d,
J ¼ 8 Hz, H12–17) (Figure S7). 13C-NMR (ppm, 100 MHz,
CDCl3) d 166.0 (C8), 152.9 (C2), 148.8 (C7), 123.2 (C6),
118.3 (C5), 117.9 (C4), 114.0 (C3), 64.6 (C10), 56.2 (C1), 30.1
(C9), 19.9 (C11), 18.4 (C12, 13) (Figure S8) (Scheme 3b). FT-
IR (MeOH, cmꢀ1): 3629–3020w, 2942–2847w, 1630s, 1465s,
1390s, 1404s, 1248s, 1176–1142m, 1083m, 1022m, 961m,
896m, 840m, 770m, 720s, 657m, 614m, 544m. UV–Vis
(MeOH) kmax nm (e Lmolꢀ1cmꢀ1): 418 (9600), 245–294
(43,000–25,700), 222 (79,800).
added in it and stirred for 5 min. Benzoin (1 mmol, 212 mg)
was then added and the reaction was continued employing
O2 atmosphere (using O2 baloon), varying temperature and
reaction time. After completion, the reaction mixture was
diluted with ethyl acetate and washed with dilute HCl fol-
lowed by H2O. The organic layer was dried over anhydrous
Na2SO4, concentrated under vacuum, and the resulting resi-
due was purified by silica gel column chromatography (hex-
ane:ethylacetate 20:1) to separate the benzil and the
unreacted benzoin successively. The enantiomeric excess (%
ee) of recovered benzoin was determined by HPLC using a
Daicel ChiralPAK AS-H column (MeOH, 1 mL/min,
254 nm): tR (major, 12.303 min), tR (minor, 13.118 min).
Benzoin: 1H-NMR (ppm, 400 MHz, CDCl3) d 7.84–7.82
(2H, m), d 7.46–7.42 (1H, m), d 7.34–7.30 (2H, m), d
7.27–7.18 (4H, m), d 5.88–5.86 (1H, d, J ¼ 6 Hz), d
4.47–4.46 (1H, d, J ¼ 6 Hz) (Figure S9). 13C-NMR (ppm,
100 MHz, CDCl3) d 194.8, 129.8, 125.0, 124.6, 124.5, 123.7,
72.1 (Figure S10).
Synthesis of the complex [Cu4(L1H)2(L1)2] (ClO4)2 (C1)
Cu(ClO4)2. ꢃ H2O (0.5 mmol, 0.187 g) was dissolved in
15 mL methanol. Solid L1H2 (0.5 mmol, 0.142 g) was added
into it while stirring at room temperature (ꢄ35 ꢂC).
Aqueous solution of NaN3 (1.5 mmol, 0.09 g) was added
drop wise into the reaction mixture. The solution was
Benzil: 1H-NMR (ppm, 400 MHz, CDCl3) d 7.99–7.97
(4H, m), d 7.68–7.64 (2H, m), d 7.54–7.50 (4H, m) (Figure
S11). 13C-NMR (ppm, 100 MHz, CDCl3) d 190.5, 130.8,
1
stirred for = h. The green solution was filtered and kept for
2
slow evaporation at room temperature. On the next day,
green prismatic single crystals appeared. Yield 166 mg
(82%). Anal. Calc. for [C68H74Cl2Cu4N4O22]: C, 50.28; H, 128.9, 125.8, 124.9 (Figure S12).

4
D. SADHUKHAN ET AL.
Figure 1. The asymmetric unit of C1 (a) and C2 (b). Hydrogen atoms are omitted for clarity.
in C1 indicates very small distortion from ideal SP geometry
Results and discussions
but the SP environment is more distorted in C2 as evident from
the s values 0.29, 0.27, 0.31 and 0.37 for Cu1–Cu4, respectively.
In C1, the basal plane around Cu1 and Cu3 is formed by the
Crystal structure of [Cu4(L1H)2(L1)2] (ClO4)2 (C1) and
[Cu4(L2)4Na2(DMF)2(H2O)] (ClO4)2 (C2)
The asymmetric unit of the complexes C1 and C2 are illus- imine nitrogen, phenoxo oxygen, l3-hydroxo oxygen from one
ligand (L1)2‾ and l-phenoxo oxygen from a second intercon-
trated in Figure 1a,b, respectively. Selected bond lengths and
angles of C1 are listed in Tables S1 and S2, respectively, and
necting ligand (HL1)‾, the methoxo oxygens from the respective
that of C2 are listed in Tables S3 and S4, respectively.
The asymmetric unit of both complexes consists of a
Cu4O4 core where four copper atoms are bridged by two l3-
hydroxo (O4, O25) and two l-phenoxo oxygens (O27, O69)
in C1 but by four l3-hydroxo oxygens (O1, O4, O7 and O12)
in C2 in an approximately cubic array of alternate copper and
oxygen atoms occupying the vertices of a cube. In C1, among
the twelve edges, two connecting Cu2–O27 and Cu4–O69 are
missing and Cu2 and Cu4 being at the open corners maintain
a large distance from each other (Cu2ꢅꢅꢅCu4 ꢆ 3.8 Å) while the
oxygen bridged CuꢅꢅꢅCu distances are within 3.2–3.4Å. Hence,
the tetranuclear unit forms an open or pseudo-cubane struc-
ture of Cu4O4 core (Figure 2a) whereas complex C2 features a
full cubane of 4 þ 2 type[23] with four short distances of
approximately 3.1 Å between Cu1ꢅꢅꢅCu2, Cu1ꢅꢅꢅCu3,
Cu2ꢅꢅꢅCu4, Cu3ꢅꢅꢅCu4 and 2 long distances of approximately
3.3 Å between Cu1ꢅꢅꢅCu4, Cu2ꢅꢅꢅCu3 (Figure 2b).
interconnecting ligands occupy the apical positions. The square
pyramids around Cu2 and Cu4 are formed by equatorially coor-
dinated imine nitrogen, hydroxo oxygen, l-phenoxo oxygen
from one ligand (HL1)‾ and l3-hydroxo oxygen from another
ligand (L1)2‾ and apical water molecule. The basal Cu–O/N
bond lengths are within the range of 1.88–2.08 Å whereas the
apical Cu–O bonds are more elongated (2.25–2.45 Å) owing to
Jahn–Teller effect. If the hydroxo oxygens O4 and O25 are con-
sidered to be l3–bridging and the elongated Cu1–O4 and
Cu3–O25 bonds are taken into account, the polyhedra around
Cu1 and Cu3 turn to be a highly distorted octahedra (or pseudo
octahedra) as the trans angles between the axial ligands
(<O4–Cu1–O72 ¼ 143.48ꢂ and < O25–Cu3–O30 ¼ 143.42ꢂ) are
highly deviated from the ideal value (180ꢂ). In C2, the square
pyramid around each Cu is formed by imine nitrogen, l-phenoxo
oxygen and three l3-hydroxo oxygens from three different
ligands. The apical Cu–(l3)O distance (2.25–2.35 Å) is longer
than the two basal Cu–(l3)O components (1.88–1.99 Å) indicat-
ing Jahn–Teller distortion. All these bond lengths are in good
agreement with those previously reported in the literature.[25] The
Naþ ions in the outer sphere of C2 are equatorially coordinated
by l-phenoxo and methoxo oxygens from two adjacent ligands.
One DMF and one H2O complete the octahedral coordination
sphere of Na1 whereas one apical position of Na2 is coordinated
by a DMF molecule resulting in a highly distorted SP geometry
(s ¼ 0.39). The equatorial coordination plane around Na1 is
In C1, two of the phenoxo oxygens (O4 and O25) being
involved in Cu-Cu bridging, have lost the potential to
coordinate to any other metal ion outside the pseudo-
cubane core whereas in C2, the phenoxo oxygens (O2, O5,
O8 and O11) being exempted from building the cubane
core, gets involved in the formation of outer compartments
along with methoxo oxygens (O3, O6, O9 and O10) where
two Naþ ions are accommodated.
Each Cu(II) ion lies in a penta coordinated NO4 square pyr-
amidal environment in both complexes. The Addison param-
eter[24] s ¼ 0.00, 0.11, 0.04 and 0.08 for Cu1–Cu4, respectively, almost perpendicularly oriented with respect to that of Na2.

INORGANIC AND NANO-METAL CHEMISTRY
5
Figure 2. Structural representation of the Cu4O4 pseudo cubane core of C1 (a) and cubane core of C2 (b).
Figure 3. The hydrogen bonding features in C1. Hydrogen atoms and perchlorate molecules not involved in hydrogen bonding are omitted for clarity.
The tetranuclear [Cu4(L1H)2(L1)2] unit of C1 isꢀdicationic coordination of the imine nitrogen atom to the metal cen-
as two of the ligands are monobasic and two ClO4 ions are ter.[26,27] The characteristic stretching band for perchlorate
anion appeared in the region 1084 and 1082 cmꢀ1, respect-
present in the lattice to counterbalance the charge of the
complex. One of the perchlorate ions interconnects the
coordinated H2O ligands of adjacent tetranuclear units
ively, for C1 and C2. Ligand coordination to the metal cen-
ter is substantiated by weak bands appearing at 454 and
443 cmꢀ1 in C1 which are mainly attributed to Cu–N
stretching bands in the complex. The Cu–N stretching bands
in C2 and the Cu-O stretching bands in C1 and C2 are
beyond the limit of the instrument.
through
hydrogen
bonds:
O48–H482ꢅꢅꢅO98
and
O90–H902ꢅꢅꢅO97 (Table S5).
Thus, an infinite one dimensional supramolecular chain
is formed by interconnected tetranuclear cationic complex
ꢀ
units mediated by ClO4 ions (Figure 3).
UV–Visible spectroscopy
Fourier transform infrared spectroscopy
The electronic spectra of the ligands and the complexes
(Figure 4) were analyzed in MeOH solution (10ꢀ5 M) in the
region 200–800 nm. The spectrum of the free L1H2 ligand
The IR spectra of the complexes (C1 and C2) were analyzed
in comparison with that of the respective free ligands L1H2
and L2H2 in 4000–400 cmꢀ1 region (Figures S1–S4). The
ligands showed broad O–H stretching bands around 3229
(L1H2) and 3240 (L2H2) cmꢀ1 and C–H stretching bands
around 2945 (L1H2) and 2930 (L2H2) cmꢀ1. In the spectrum
of the free ligand, imine stretching bands appeared at 1638
(L1H2) and 1630 (L2H2) cmꢀ1, but in the respective com-
ꢁ
shows several p!p transitions at kmax 330, 262 and 220 nm.
In case of free L2H2 several p!p transitions appear at
ꢁ
ꢁ
294–245 and 222 nm. An n!p transition band is observed
at 418 nm in L2H2 which is not prominent in L1H2; possibly
overwhelmed by the p!p transitions, as L1H2 contains
ꢁ
plexes the characteristic imine bands were observed slightly greater number of conjugated p systems than L2H2. In the
shifted 1635 (C1) and 1640 (C2) cmꢀ1 indicating the spectrum of both the complexes (C1 and C2) LMCT band

6
D. SADHUKHAN ET AL.
Figure 4. UV–Visible spectra (MeOH, 10ꢀ5 M) and CD spectra (MeOH, 10ꢀ3 M) of the ligands (black line) and the complexes (blue line). The d!d transition bands
of C1 and C2 are presented as inset.
ꢁ
ꢁ
appears at 378 nm. p!p transition bands are red shifted to corresponds to the n!p transition of its absorption spec-
281 and 236 nm in C1 and to 277 and 233 nm in C2.
A high energy CT band appears in the absorption spec-
trum of the complexes upon coordination of the ligands to
metal center. Hence the sharp peaks at 207 nm in both the
complexes can be attributed to ligand!ligand charge trans-
fer transition (LLCT).[28] The d!d transition bands of
much weaker intensity appear around 681 nm in C1 and
685 nm in C2 and are characteristics of square pyramidal
Cu (II) metal ion (inset in Figure 4).[29]
trum. L2H2 shows a negative CD signal at 280 nm corre-
ꢁ
sponding to p!p transition. Complex C1 exhibits two
negative signals at 365 and 584 nm that correspond to the
LMCT and d!d transitions, respectively. C2 exhibits LMCT
and d!d transition bands at 390 nm and 490 nm, respect-
ꢁ
ively, along with a p!p transition signal at 280 nm.
Computational analysis
In order to understand the transitions involved in the
UV–Visible and circular dichroism spectra of the ligands
and the complexes, DFT and TD-DFT were calculated.[30]
The simulated spectral transitions along with the experimen-
tal transitions are listed in Table S6.
Circular dichroism
The circular dichroism spectra of the ligands and the com-
plexes have close resemblance with their respective absorp-
tion spectra (Figure 4). In methanolic solution (10ꢀ3 M)
The calculated UV–Visible and CD spectra of the ligands
L1H2 shows a distinct negative signal at 330 nm that are displayed in Figure 5.

INORGANIC AND NANO-METAL CHEMISTRY
7
Figure 5. The TD-CAM-B3LYP calculated UV–Visible and CD spectra of the ligands (black line) and the complexes (red line).
A good overall agreement has been found between the ligand charge transfer (LLCT). The MO diagram for the
experimental and the simulated spectra. The B3LYP com- LMCT transition of C1 is shown in Figure 7.
In the simulated absorption spectrum of C2 (Figure 5),
three peaks are observed, the most intense peak is centered
at 280 nm, a shoulder is observed around 374 nm and a very
weak peak appears around 554 nm. The lowest energy exci-
tation is for the d to d transition of Cu metal as observed in
the corresponding MO diagram shown in Figure 7. The
puted HOMO and LUMO frontier molecular orbitals
(FMOs) of both L1H2 and L2H2 are shown in Figure 6
which explains the origin of transitions. For L1H2 the p !
ꢁ
p
transitions were calculated around 312, 266 and 213 nm
attributed to HOMO to LUMO, HOMO-1 to LUMO and
HOMO to LUMO þ 3 excitations, respectively.
The computed absorption bands in L2H2 appear around
ꢁ
other two excitations are due to ligands p ! p transitions.
ꢁ
In the simulated CD spectra of both the complexes a distinct
negative signal appears around 250 nm.
310, 256 and 213 nm due to p ! p transitions from
HOMO to LUMO, HOMO-1 tuo LUMO and HOMO to
LUMO þ 1, respectively. The simulated CD spectra for both
ligands show a negative signal around 250 nm. The qualita-
tive features of the simulated CD spectra are in good agree-
ment with the experimental spectra, the blue shift of the
band positions can be attributed to approximations in both
the density functional and treatment of solvent effect.[31] For
TD-DFT calculation of the complexes we ignored the anions
and considered the bipositive complex cations only, hence
an overall spin multiplicity five has been taken into account.
Oxidative kinetic resolution of racemic benzoin
Enantiopure benzoin is usually synthesized by (i) enzymatic
hydrolysis such as benzaldehyde lyase (BAL) or benzoylfor-
mate decarboxylase (BFD) enzyme,[32] (ii) multistep enantio-
selective benzoin condensations by using polycyclic
triazolium salt or its analogues as catalysts.[33] Our effort is
to synthesize enantioenriched benzoin by a single step and
For C1 a very intense absorption band appeared at 355 nm non-enzymatic process. In this article, we report our initial
accompanied by a much weaker band around 586 nm findings regarding chiral Schiff base-Cu(II) complex cata-
(Figure 5). The lowest energy band is an overlap of ligand lyzed oxidative kinetic resolution of racemic benzoin. ( )
to metal charge transfer (LMCT) and d!d transitions Benzoin was reacted with 2,2,6,6-tetramethylpiperidin-1-oxyl
whereas the intense band at 355 nm is due to the ligand to (TEMPO) and the enantiopure chiral complexes C1 and C2

8
D. SADHUKHAN ET AL.
Figure 6. Calculated highest occupied and lowest unoccupied molecular orbitals of L1H2 (left) and of L2H2 (right).
in the presence of molecular oxygen. The oxidized product benzil was detected and the unreacted benzoin was recov-
benzil is obtained and the unreacted benzoin was recovered ered almost quantitatively which suggests that no side reac-
in enantiomerically enriched form (Scheme 1). To optimize tion is taking place due to TEMPO. The S(þ) enantiomer of
the reaction conditions we have varied the percentage of racemic benzoin was oxidized faster to the corresponding
TEMPO and the catalysts, solvent, temperature and reaction benzil and the slower-reacting R(-) enantiomer was recov-
time as listed in Table S7.
ered in enantiomerically enriched form (Figures S13–S15).
We have performed solvent screening based on the solu- The configuration of the recovered benzoin was determined
bility of the catalysts (C1 and C2). Acetonitrile is found to by comparing the optical rotation value with the literature
value.[13] The reaction conditions, as mentioned in entry 17
be the most suitable solvent in terms of ready solubility of
the catalysts as well as enantioselectivity. Oxidation of ben- were found to be the optimum for a moderate (63%)
zoin not at all occurs at room temperature even after pro- enantiomeric excess using C1 as the catalyst. At similar con-
longed reaction time. Conversion and selectivity increase ditions using C2 as the catalyst we obtained 45% excess of
upon refluxing the reaction mixtures in the respective sol- the R(-) enantiomer of benzoin. From the previously
vents. No oxidation product was detected without TEMPO. reported mechanism of oxidative kinetic resolution of sec-
When the oxidation was carried out only with TEMPO and ondary alcohols[34] it can be anticipated that the substrate
without the catalysts a trace amount of oxidized product and the oxidant bind to the metal center in intermediate

INORGANIC AND NANO-METAL CHEMISTRY
9
Figure 7. The Frontier molecular orbitals for the LMCT transition in C1 (left) and in C2 (right).
Scheme 1. Oxidative kinetic resolution of ( ) benzoin
Scheme 2. Synthetic route of the Schiff base ligands

10
D. SADHUKHAN ET AL.
Scheme 3. The proton and carbon numbering scheme of (a) L1H2 and (b) L2H2
steps. All CuII centers in C1 and C2 are penta-coordinated.
Hence, they have one vacant coordination site to bind with
the substrate (benzoin) and oxygen. Moreover, in both com-
plexes, the oxo-bridged Cu-Cu distance lies between 3.1 and
3.4 Å which is favorable for substrate-oxygen interaction.
Now, in C1 the Cu4O4 cubane core is open (Figure 2),
hence, both benzoin and oxygen can easily approach the Cu
center to bind with. Moreover, Cu2 and Cu4 have labile
H2O ligand at their axial coordination site which can leave
during the reaction and provide two vacant sites for both
substrate and oxygen to coordinate at the same metal center
and the reaction is highly favored. In case of C2, the cubane
core is closed (Figure 2); hence the Cu centers are difficult
for the reactants to approach. The bulky Na ions also dis-
favor this process. Thus, the lower efficacy of C2 compared
to C1 is justified from the structural point of view.
ORCID
Dipali Sadhukhan
Prithwi Ghosh
Susanta Ghanta
https://orcid.org/0000-0001-8176-9222
https://orcid.org/0000-0001-5243-751X
http://orcid.org/0000-0002-7095-5107
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The authors have no conflict of interest.

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