Diastereoselection and induced chirality at-metal in distorted-tetrahedral bis[(R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-κ2 N,N]-Δ/Λ-copper(PF6)2

Article abstract of DOI:10.1080/00958972.2020.1795841

The enantiopure Schiff bases (R or S)-1-(p-R₁-C₆H₄)-N-(phenyl(pyridin-2-yl)methylene)ethanamine (R₁ = H and OCH₃) are synthesized from reaction between the 2-benzoyl-pyridine and chiral-amine (R or S)-1-(p-R₁-C₆H₄)ethylamine, respectively. These Schiff bases react with the copper(II) acetate to give distorted-tetrahedral bis[(R or S)-1-(p-R₁-C₆H₄)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-κ² N,N]-Δ/Λ-Cu(PF₆)₂ in the presence of NH₄PF₆. A conformational change from cis- to trans-isomer occurs in solution and thereby, the copper(II) coordinates to the solely trans-form to provide the complexes. Cyclic voltammograms demonstrate two quasi-reversible one electron charge transfer processes for the Cu²⁺/Cu⁺ and Cu⁺/Cu⁰ couples in acetonitrile. ECD spectra reveal expected mirror-image relationships and indicate enantiopurity or enantiomeric excess of the R or S-ligands and their associated complexes in solution. Comparisons of experimental and simulated ECD spectra suggest induced chirality at-metal center, which diastereoselectively provides the Δ-Cu-R or Λ-Cu-S diastereomer as major product in solution. DFT/TDDFT optimized structures and excited state properties correspond well to the experimental results.

Full text of DOI:10.1080/00958972.2020.1795841

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Diastereoselection and induced chirality at-metal
in distorted-tetrahedral bis[(R or S)-1-(p-R1-C6H4)-N-
(phenyl(pyridin-2-yl)methylene)ethanamino-κ2
N,N]-∆/Λ-copper(PF6)2
Mohammed Enamullah , Mohammad Anwar Hossain , Mortuza Mamun
Bindu , Mohammad Khairul Islam & Mohammad Al-Moktadir Zaman
To cite this article: Mohammed Enamullah , Mohammad Anwar Hossain , Mortuza Mamun Bindu ,
Mohammad Khairul Islam & Mohammad Al-Moktadir Zaman (2020): Diastereoselection and
induced chirality at-metal in distorted-tetrahedral bis[(R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-
yl)methylene)ethanamino-κ2 N,N]-∆/Λ-copper(PF6)2 , Journal of Coordination Chemistry, DOI:
10.1080/00958972.2020.1795841
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1795841
Diastereoselection and induced chirality at-metal in
distorted-tetrahedral bis[(R or S)-1-(p-R1-C6H4)-N-
(phenyl(pyridin-2-yl)methylene)ethanamino-
j2N,N]-D/K-copper(PF6)2
Mohammed Enamullah, Mohammad Anwar Hossain, Mortuza Mamun Bindu,
Mohammad Khairul Islam and Mohammad Al-Moktadir Zaman
Department of Chemistry, Jahangirnagar University, Dhaka, Bangladesh
ABSTRACT
ARTICLE HISTORY
Received 30 March 2020
Accepted 22 June 2020
The enantiopure Schiff bases (R or S)-1-(p-R1-C6H4)-N-(phenyl(pyri-
din-2-yl)methylene)ethanamine (R1 ¼ H and OCH3) are synthe-
sized from reaction between the 2-benzoyl-pyridine and chiral-
amine (R or S)-1-(p-R1-C6H4)ethylamine, respectively. These Schiff
bases react with the copper(II) acetate to give distorted-tetrahe-
dral bis[(R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-yl)methylene)e-
thanamino-j2N,N]-D/K-Cu(PF6)2 in the presence of NH4PF6. A
conformational change from cis- to trans-isomer occurs in solution
and thereby, the copper(II) coordinates to the solely trans-form to
provide the complexes. Cyclic voltammograms demonstrate two
quasi-reversible one electron charge transfer processes for the
Cu2þ/Cuþ and Cuþ/Cu0 couples in acetonitrile. ECD spectra reveal
expected mirror-image relationships and indicate enantiopurity or
enantiomeric excess of the R or S-ligands and their associated
complexes in solution. Comparisons of experimental and simu-
lated ECD spectra suggest induced chirality at-metal center, which
diastereoselectively provides the D-Cu-R or K-Cu-S diastereomer
as major product in solution. DFT/TDDFT optimized structures
and excited state properties correspond well to the experimen-
tal results.
KEYWORDS
Copper(II)-chiral-N,N-chelate
Schiff bases; induced
chirality at-metal and
diastereoselection; chiropti-
cal properties; cis-to-trans
interconversion; DFT/TDDFT
CONTACT Mohammed Enamullah
enamullah@juniv.edu
Department of Chemistry, Jahangirnagar University,
Dhaka, Bangladesh
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1795841.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
M. ENAMULLAH ET AL.
1. Introduction
Tetrahedral or distorted-tetrahedral transition metal(II)-complexes with chiral-N,O-che-
late Schiff base ligands are of continued interest in the context of syntheses, chiropti-
cal properties, diastereoselection and induced chirality at-metal center [1–15]. The
chiral N,O-chelate Schiff base ligands upon coordination to the metal(II) ion induce
metal-centered chirality and provide the right (D)- and left (K)-handed diastereomers
along C2-symmetry of the molecules [16–19]. The noncovalent interactions acting
within the metal–ligand framework result in a free energy difference between the two
diastereomers and hence, diastereoselectively prefer one over another (i.e. D vs. K)
both in solution and at solid-state. The structure and/or chirality of the ligand, metal
ion selection, solute–solvent interaction, temperature and time, counter anion, crystal
solvent and crystallization protocol efficiently control the induced chirality and diaster-
eoselection phenomenon [1–10].
In this connection, we have reported distorted-tetrahedral [M(R or S-NꢀO)2] (M ¼ Fe,
Co, Ni, Cu, Zn; NꢀO ¼ salicylaldiminato/OH-naphthaldiminato) with preferred formation
of D-M-R-NꢀO or K-M-S-NꢀO and vice versa (i.e. K-M-R-NꢀO or D-M-S-NꢀO), as studied
by solid-state X-ray structures of the enantiopure crystal [1–10]. However, in some
cases, both diastereomers (D and K) co-exist in a single enantiopure crystal [1, 6, 12].
Indeed, both diastereomers appear simultaneously in solution and result in a dynamic
diastereomeric equilibrium, as followed by variable-time and -temperature dependent
1H NMR and ECD spectra, respectively [2, 3, 6–9]. Time-dependent 1H NMR studies
reveal a single diastereomer within 10 min of complex dissolution, which shifts to a
diastereomeric mixture of about 33:67 (within 40 min) or 46:54 (within 36 h) for D/K-(R
or S)-zinc(II)-aminoalcohol based Schiff base complexes in solution [9]. Similar studies
demonstrate an equilibrium with diastereomeric mixture of about 77:23 within 10 min
of complex dissolution, which remains unchanged even after 24 h for bis[f(S)-N-

JOURNAL OF COORDINATION CHEMISTRY
3
Scheme 1. Synthetic route of the formation of bis[(R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-yl)me-
thylene)ethanamino-j2N,N]D/K-Cu(PF6)2.
(phenyl)ethyl-2-oxo-1-naphthaldiminato-j2N,Og]-D/K-Zn [3]. Variable-temperature 1H
NMR studies on the latter complex also exhibit a shift of equilibrium from a diastereo-
meric ratio of about 87:13 at ꢁ50 ꢀC to 73:27 at 35 ꢀC. Similar results of equlibrium
shifting were examined by variable-temperature ECD spectra in solution [2, 3].
However, our studies were limited to the metal(II)-complexes containing the chiral
N,O-chelate Schiff bases including salicylaldimine/OH-naphthaldimine and their deriva-
tives [1–10]. In view of further investigation on the phenomenon, we have planned to
synthesize the chiral N,N-chelate Schiff bases and their copper(II)-complexes. Indeed,
these types of chiral N,N-chelate Schiff bases react with the metal ions to provide
chiral-metal catalysts for enantioselective hydrogenation/hydrosilylation and success-
fully used for conformational analyses of diastereoisomers [20–26].
In the present endeavor, we report syntheses of the chiral N,N-chelate Schiff bases
(R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamine and their com-
plexes bis[(R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]D/
K-copper(II)(PF6)2 (Scheme 1). The Schiff bases and complexes are characterized by
1
elemental, IR, UV–Vis, ECD, H-NMR spectroscopy and cyclic voltammetry. A thorough
computational procedure was employed by DFT/TDDFT to rationalize the experimental
results and to examine the chiroptical properties in detail. Finally, combined studies of

4
M. ENAMULLAH ET AL.
experimental and computational results are considered to get insight into diastereose-
lection and metal entered chirality (D vs. K) induced by the R- or S-ligand in
the complexes.
2. Experimental
2.1. Materials and methods
Vibrational spectra were recorded on a Nicolet iS10 (Thermo Scientific) spectrometer
as KBr discs at ambient temperature. Electronic spectra were obtained with the
Shimadzu UV 1800 spectrophotometer in methanol at 25 ꢀC. 1H NMR-spectra were
recorded on a Bruker Avance 400 spectrometer operating at 400 MHz in CDCl3 at
20 ꢀC. ECD spectra were run with the JASCO spectropolarimeter (J810) in methanol at
about 25 ꢀC. EpsilonTM instruments (BASi) electrochemical analyzer was used for run-
ning cyclic voltammogram experiments containing tetra-N-butyl-ammonium-hexafluor-
ophosphate (TBAP) as supporting electrolyte in acetonitrile at 25 ꢀC. A three-electrode
measurement system was used, which composed of a platinum disc working elec-
trode, platinum wire auxiliary electrode and Ag/AgCl reference electrode, respectively.
The solution containing the sample and TBAP was deoxygenated with purging nitro-
gen gas for 10 min prior to use. Working electrode was cleaned with supplied solution
and dried prior to use in each scan to avoid any contamination caused by deposition
of the metal. Reproducibility of CV patterns was checked at a scan rate of 0.10 Vsꢁ1 in
acetonitrile. The molar conductance was measured using a Mettler Toledo Fivego
(Model F3) conductivity meter in dimethylformamide (DMF) at 25 ꢀC.
2.2. General procedure to synthesize the Schiff bases, (R or S)-1-(p-R1-C6H4)-N-
(phenyl(pyridin-2-yl)methylene)ethanamine (R or S-L)
The 2-benzoyl-pyridine (366 mg, 2.0 mmol) and 3–4 drops of concentrated H2SO4 were
dissolved in 10 mL methanol, stirring the solution for about 10 min. An equimolar
amount of chiral-amine (R or S)-1-(C6H5)ethylamine (242 mg, 2.0 mmol), dissolved in
5 mL methanol, was poured into this solution and continued to reflux for about 12 h.
The color changed from brown–orange to orange–yellow. The solvent was evaporated
in vacuo to obtain orange-yellow semi-liquid products of (R or S)-1-(C6H5)-N-(phenyl(-
pyridin-2-yl)methylene)ethanamine (R-L1 or S-L1). The products were washed with n-
hexane three times (2 mL in each) and dried in open air for several days. The same
procedure was followed to synthesize the orange-yellow semi-liquid products of (R or
S)-1-(p-OCH3C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamine (R-L2 or S-L2) using (R
or S)-1-(p-OCH3C6H4)ethylamine.
2.2.1. (R)-1-(C6H5)-N-(phenyl(pyridin-2-yl)methylene)ethanamine (R-L1)
1
ꢂ
Yield: 0.425 g (74%). H NMR (400 MHz, CDCl3) for cis-isomer, d (ppm): 1.43 (d, JHH
¼
¼
ꢂ
6.8 Hz, 3H, CH3), 4.78 (q, JHH ¼ 6.8 Hz, 1H, CH), 7.10 (m, 3H, Ar-H), 7.27 (d, JHH
6.8 Hz, 2H, Ar-H), 7.34 (m, 3H, Ar-H), 7.46 (m, 1H, Ar-H), 7.53 (m, 2H, Ar-H), 8.19 (d, JHH
¼ 7.6 Hz, 1H, Ar-H), 8.67 (d, JHH ¼ 4.4 Hz, 1H, Ar-H), 8.81 (d, JHH ¼ 4.4 Hz, 1H, Ar-H). For
ꢂ
ꢂ
trans-isomer, d (ppm): 1.65 (d, JHH ¼ 6.8 Hz, 3H, CH3), 4.35 (q, JHH ¼ 6.8 Hz, 1H, CH),

JOURNAL OF COORDINATION CHEMISTRY
5
7.37 (m, 3H, Ar-H), 7.47 (d, JHH ¼ 6.8 Hz, 2H, Ar-H), 7.50 (d, JHH ¼ 7.6 Hz, 1H, Ar-H), 7.52
(d, JHH ¼ 8.0 Hz, 2H, Ar-H), 7.62 (t, JHH ¼ 7.2 Hz, 1H, Ar-H), 7.93 (dt, JHH ¼ 7.6, 1.2 Hz,
1H, Ar-H), 8.07 (d, JHH ¼ 7.6 Hz, 1H, Ar-H), 8.09 (d, JHH ¼ 7.6 Hz, 2H, Ar-H), 8.76 (d, JHH
ꢂ
¼ 4.4 Hz, 1H, Ar-H) ( cis- to trans-isomer ratio about 20:80, calculated from integration
values for corresponding CH and CH3 peaks, respectively).
2.2.2. (S)-1-(C6H5)-N-(phenyl(pyridin-2-yl)methylene)ethanamine (S-L1)
Yield 0.400 g (70%). H NMR (400 MHz, CDCl3) for cis-isomer, d (ppm): 1.42 (d, JHH
1
ꢂ
¼
ꢂ
6.8 Hz, 3H, CH3), 4.71 (q, JHH ¼ 6.8 Hz, 1H, CH), 7.09 (m, 3H, Ar-H), 7.26 (m, 2H), 7.34
(m, 3H, Ar-H), 7.41 (m, 1H, Ar-H), 7.52 (m, 2H, Ar-H), 7.95 (m, 1H, Ar-H), 8.58 (m, 1H, Ar-
ꢂ
ꢂ
H), 8.82 (m, 1H, Ar-H). For trans-isomer, d (ppm): 1.59 (d, JHH ¼ 6.8 Hz, 3H, CH3), 4.29
(q, JHH ¼ 6.8 Hz, 1H, CH), 7.35 (m, 3H, Ar-H),7.42 (d, JHH ¼ 7.6 Hz, 2H, Ar-H), 7.50 (d, JHH
¼ 7.6 Hz, 1H, Ar-H), 7.52 (d, JHH ¼ 7.6 Hz, 2H, Ar-H), 7.62 (t, JHH ¼ 7.6 Hz, 1H, Ar-H),
7.93 (t, JHH ¼ 7.6 Hz, 1H, Ar-H), 8.06 (d, JHH ¼ 7.6 Hz, 1H, Ar-H), 8.08 (d, JHH ¼ 7.6 Hz,
ꢂ
2H, Ar-H), 8.75 (d, JHH ¼ 4.4 Hz, 1H, Ar-H) ( cis- to trans-isomer ratio about 17:83, calcu-
lated from integration values for corresponding CH and CH3 peaks, respectively).
2.2.3. (R)-1-(p-OCH3C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamine(R-L2)
1
ꢂ
Yield: 0.450 g (71%). H NMR (400 MHz, CDCl3) for cis-isomer, d (ppm): 1.51 (d, JHH
¼
ꢂ
6.8 Hz, 3H, CH3), 3.81 (s, 3H, OCH3), 4.65 (q, JHH ¼ 6.4 Hz, 1H, CH), 6.88 (dd, JHH ¼ 8.4,
2.8, 2H, Ar-H17,19), 7.20 (d, JHH ¼ 7.6 Hz, 1H, Ar-H8), 7.30 (dd, JHH ¼ 8.8, 2.8 Hz, 2H, Ar-
H2,16), 8.20 (d, JHH ¼ 7.6 Hz, 1H, Ar-H20), 7.62 (t, JHH ¼ 7.6 Hz, 1H, Ar-H4), 8.58 (d, JHH
¼ 4.4 Hz, 1H, Ar-H12), 7.52 (m, 1H, Ar-H9), 7.64 (d, JHH ¼ 7.6 Hz, 1H, Ar-H10), 7.64 (d,
JHH ¼ 7.6 Hz, 1H, Ar-H11), 8.07 (d, JHH ¼ 7.6 Hz, 1H, Ar-H3), 8.80 (d, JHH ¼ 4.4 Hz, 1H,
ꢂ
Ar-H5). For trans-isomer, d (ppm): 1.53 (d, JHH ¼ 6.8 Hz, 3H, CH3), 3.82 (s, 3H, OCH3),
ꢂ
4.48 (q, JHH ¼ 6.4 Hz, 1H, CH), 6.88 (dd, JHH ¼ 8.4, 2.8, 2H, Ar-H17,19), 7.16 (d, JHH
¼
7.6 Hz, 1H, Ar-H2), 7.20 (d, JHH ¼ 7.6 Hz, 1H, Ar-H8), 7.30 (dd, JHH ¼ 8.8, 2.8 Hz,1H, Ar-
H20), 7.50 (m, 2H, Ar-H4,12), 7.77 (2dt, JHH ¼ 7.6, 1.6 Hz, 1H, Ar-H9), 7.93 (dt, JHH ¼ 8.0,
1.6 Hz, 1H, Ar-H10), 7.77 (2dt, JHH ¼ 7.6, 1.6 Hz, 1H, Ar-H11), 8.10 (d, JHH ¼ 7.6 Hz, 2H,
ꢂ
Ar-H3,16), 8.76 (d, JHH ¼ 4.4 Hz, 1H, Ar-H5) ( cis- to trans-isomer ratio about 48:52, cal-
culated from integration values for corresponding CH and CH3 peaks, respectively)
(see Scheme 2 for hydrogen atoms numbering).
1
2.2.4. Simulated H NMR for the R-L2 (with GIAO method at B3LYP/6-311G (2d,p))
For cis-isomer, d (ppm): 1.04, 1.12, 1.70 (3H, CH3), 3.88, 3.90, 4.03 (3H, OCH3), 5.20 (1H,
CH), 7.06, 7.27 (2H, Ar-H17,19), 7.30 (1H, Ar-H8), 7.60 (1H, Ar-H16), 7.62 (1H, Ar-H2),
7.64 (1H, Ar-H9), 7.80 (1H, Ar-H4), 7.87 (1H, Ar-H10), 7.91 (1H, Ar-H11), 8.24 (1H, Ar-H3),
8.36 (1H, Ar-H20), 8.97 (1H, Ar-H12), 9.38 (1H, Ar-H5). For trans-isomer, d (ppm): 1.25,
1.28, 1.82 (3H, CH3), 3.89, 3.91, 4.05 (3H, OCH3), 4.92 (1H, CH), 7.09, 7.24 (2H, Ar-
H19,17), 7.20 (1H, Ar-H2), 7.49 (1H, Ar-H8), 7.58 (1H, Ar-H20), 7.66 (1H, Ar-H4), 7.71 (1H,
Ar-H12), 7.88 (1H, Ar-H9), 7.91 (1H, Ar-H11), 7.95 (1H, Ar-H10), 7.97 (1H, Ar-H3), 8.20
(1H, Ar-H16), 9.31 (1H, Ar-H5) (see Scheme 2 for hydrogen atoms numbering).

6
M. ENAMULLAH ET AL.
Scheme 2. Hydrogen atoms numbering.
2.2.5. (S)-1-(p-OCH3C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamine (S-L2)
1
ꢂ
Yield: 0.435 g (69%). H NMR (400 MHz, CDCl3) for cis-isomer, d (ppm): 1.51 (d, JHH
¼
ꢂ
6.8 Hz, 3H, CH3), 3.81 (s, 3H, OCH3), 4.65 (q, JHH ¼ 6.4 Hz, 1H, CH), 6.88 (dd, JHH ¼ 8.4,
2.8 Hz, 2H, Ar-H17,19), 7.30 (dd, JHH ¼ 8.8, 2.8 Hz, 1H, Ar-H2), 7.20 (d, JHH ¼ 7.6 Hz, 1H,
Ar-H8), 7.30 (dd, JHH ¼ 8.8, 2.8 Hz, 1H, Ar-H16), 8.20 (d, JHH ¼ 8.0 Hz, 1H, Ar-H20), 8.58
(d, JHH ¼ 4.4 Hz, 1H, Ar-H12), 7.61 (t, JHH ¼ 7.6 Hz, 1H, Ar-H4), 7.52 (m, 1H, Ar-H9), 7.65
(d, JHH ¼ 6.8 Hz, 1H, Ar-H10), 7.65 (d, JHH ¼ 6.8 Hz, 1H, Ar-H11), 8.07 (d, JHH ¼ 7.6 Hz,
ꢂ
1H, Ar-H3), 8.81 (d, JHH ¼ 4.4 Hz, 1H, Ar-H5). For trans-isomer, d (ppm): 1.53 (d, JHH
¼
ꢂ
6.8 Hz, 3H, CH3), 3.82 (s, 3H, OCH3), 4.48 (q, JHH ¼ 6.4 Hz, 1H, CH), 6.88 (dd, JHH ¼ 8.4,
2.8 Hz, 2H, Ar-H17,19), 7.16 (d, JHH ¼ 7.6 Hz, 1H, Ar-H2), 7.20 (d, JHH ¼ 7.6 Hz, 1H, Ar-
H8), 8.10 (d, JHH ¼ 7.6 Hz, 1H, Ar-H16), 7.30 (dd, JHH ¼ 8.8, 2.8 Hz, 1H, Ar-H20), 7.49 (m,
2H, Ar-H4,12), 7.77 (2dt, JHH ¼ 7.6, 1.6 Hz, 2H, Ar-H9,11), 7.92 (dt, JHH ¼ 7.6, 1.6 Hz, 1H,
ꢂ
Ar-H10), 8.10 (d, JHH ¼ 7.6 Hz, 1H, Ar-H3), 8.75 (d, JHH ¼ 4.4 Hz, 1H, Ar-H5) ( cis- to
trans-isomer ratio about 56:44, calculated from integration values for corresponding
CH and CH3 peaks, respectively; see Scheme 2 for hydrogen atoms numbering).
2.3. General procedure to synthesize the copper(II)-Schiff bases complexes
Two equivalents of Schiff base (R or S)-1-(C6H5)-N-(phenyl(pyridin-2-yl)methylene)e-
thanamine (R or S-L1) (286 mg, 1.00 mmol) were dissolved in 10 mL methanol; this
solution was poured into one equivalent of copper(II)-acetate (127 mg, 0.50 mmol) dis-
solved in 5 mL methanol. The reaction mixture was stirred for about 12 h at 40 ꢀC and
the color turned from light yellowish-green to deep green. For anion exchange, two
equivalents of NH4PF6 were added into this solution and continued stirring for 30 min.
The volume of solvent was reduced to about 50% and the solution was left standing
for precipitation at room temperature, filtered off and the green filtrate was collected
and dried in vacuo at 40 ꢀC. The dried green product was washed with n-pentane 3
times (2 mL in each) and dried again in vacuo at 40 ꢀC. The green product was dis-
solved in 5 mL dichloromethane, filtered off and the filtrate dried in vacuo at 40 ꢀC fol-
lowed by nitrogen flash to obtain deep green microcrystals of bis[(R or S)-1-(C6H5)-N-
(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]Cu(II)(PF6)2, [Cu(R or S-L1)2](PF6)2.
The same procedure was followed to synthesize bis[(R or S)-1-(p-OCH3C6H4)-N-(phenyl(-
pyridin-2-yl)methylene)ethanamino-j2N,N]Cu(II)(PF6)2, [Cu(R or S-L2)2](PF6)2 using the R
or S-L2 (316 mg, 1.00 mmol) except stirring the reaction mixture for about 6 h at room

JOURNAL OF COORDINATION CHEMISTRY
7
temperature. No precipitate was formed upon left standing the concentrated (volume
reduced to ca. 50%) solution to room temperature.
2.3.1. Bis[(R)-1-(C6H5)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]copper(II)
(PF6)2, [Cu(R-L1)2](PF6)2, (Cu-R-L1)
Yield 300 mg (65%). IR (ATR, cmꢁ1): 3418br v(H2O), 3063, 3034, 2981, 2938w v(C-H),
1622, 1593, 1570vs v(C ¼ N), 1525sh v(C ¼ C), 839vs v(PF6). C40H36CuN4(P2F12) (926.23):
Calcd. C 51.87, H 3.92, N6.05; found C 50.76, H 4.22, N 5.85%. Km (DMF) ¼ 215 S cm2
molꢁ1 (0.5 mmol).
2.3.2. Bis[(S)-1-(C6H5)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]copper(II)
(PF6)2, [Cu(S-L1)2](PF6)2, (Cu-S-L1)
Yield 280 mg (60%). IR (ATR, cmꢁ1): 3420br v(H2O), 3063, 3032, 2980, 2934w v(C-H),
1622, 1593, 1570vs v(C ¼ N), 1525sh v(C ¼ C), 841vs v(PF6).
2.3.3. Bis[(R)-1-(p-OCH3C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]
copper(II)(PF6)2, [Cu(R-L2)2](PF6)2, (Cu-R-L2)
Yield 300 mg (61%). IR (ATR, cmꢁ1): 3427br v(H2O), 3061, 2982, 2936w v(C-H), 1611,
1589, 1576vs v(C ¼ N), 1514s v(C ¼ C), 843vs v(PF6). C42H40CuN4O2(P2F12) (986.28):
Calcd. C 51.15, H 4.09, N 5.68; found C 50.27, H 4.32, N 5.35%. Km (DMF) ¼ 230 S cm2
molꢁ1 (0.5 mmol).
2.3.4. Bis[(S)-1-(p-OCH3C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]
copper(II)(PF6)2, [Cu(S-L2)2](PF6)2, (Cu-S-L2)
Yield 312 g (63%). IR (ATR, cmꢁ1): 3431br v(H2O), 3061, 2980, 2938w v(C-H), 1610, 1591,
1572vs v(C ¼ N), 1514s v(C ¼ C), 843vs v(PF6). Km (DMF) ¼ 220 S cm2 molꢁ1 (0.5 mmol).
2.4. Decomplexation of Cu-R-L2 with NaCN and isolation of free R-L2
Cu-R-L2 of 30 mg was dissolved in 5 mL methanol and an excess amount of NaCN was
added into this solution. The solution was stirred for 30 min and the color changed from
deep brown to orange–yellow, then filtered off and the filtrate was dried in vacuo at 40 ꢀC
for several hours. The dried product was dissolved into 5 mL dichloromethane, filtered off
and the filtrate dried in vacuo. The product was dissolved again into 5 mL n-pentane, fil-
tered off and dried in vacuo at 40 ꢀC to obtain orange–yellow semi-liquid products of R-L2.
1
2.4.1. H NMR (400 MHz, CDCl3)
ꢂ
ꢂ
For cis-isomer, d (ppm): 1.51 (d, JHH ¼ 7.2 Hz, 3H, CH3), 3.81 (s, 3H, OCH3), 4.64 (q,
JHH ¼ 6.4 Hz, 1H, CH), 6.88 (dd, JHH ¼ 8.4, 2.8, 2H, Ar-H), 7.19 (d, JHH ¼ 7.6 Hz, 1H, Ar-
H), 7.31 (dd, JHH ¼ 8.8, 2.8 Hz, 2H, Ar-H), 7.52 (d, JHH ¼ 8.0 Hz, 2H, Ar-H), 7.61 (d, JHH
¼
7.6 Hz, 1H, Ar-H), 7.92 (dt, JHH ¼ 7.6, 1.6 Hz, 1H, Ar-H), 8.06 (d, JHH ¼ 7.6 Hz, 1H, Ar-H),
8.09 (d, JHH ¼ 7.2 Hz, 2H, Ar-H), 8.75 (d, JHH ¼ 4.4 Hz, 1H, Ar-H). For trans-isomer, d
ꢂ
ꢂ
(ppm): 1.53 (d, JHH ¼ 7.2 Hz, 3H, CH3), 3.82 (s, 3H, OCH3), 4.48 (q, JHH ¼ 6.4 Hz, 1H,
CH), 6.87 (dd, JHH ¼ 8.4, 2.8 Hz, 2H, Ar-H), 7.16 (d, JHH ¼ 8.0 Hz, 1H, Ar-H), 7.19 (d, JHH
¼
7.6 Hz, 1H, Ar-H), 7.28 (m, 1H, Ar-H), 7.49 (t, JHH ¼ 10.4 H, 2H, Ar-H), 7.63 (t, JHH ¼ 6.4 Hz,

8
M. ENAMULLAH ET AL.
Figure 1. Optimized structures for the diastereomeric pair D-Cu-S-L2 and K-Cu-S-L2, calculated at
B3LYP/6-31G(d), respectively.
2H, Ar-H), 7.76 (2dt, JHH ¼ 8.0, 1.6 Hz, 1H, Ar-H), 8.19 (d, JHH ¼ 8.0 Hz, 1H, Ar-H), 8.56 (d,
ꢂ
JHH ¼ 4.4 Hz, 1H, Ar-H), 8.79 (d, JHH ¼ 4.4 Hz, 1H, Ar-H) ( cis- to trans-isomer ratio about
55:45, calculated from integration values for corresponding CH and CH3 peaks,
respectively).
2.5. Computational method
Computational procedure was performed on the Schiff bases and complexes with
Gaussian 09 [27]. The gas phase equilibrium geometries for the Schiff bases (R- or S-L1
and R- or S-L2) with both the cis- and trans-forms were optimized with DFT using the
functional B3LYP and the basis set LANL2DZ, respectively (Supporting Information
Figure S1). The optimized structures were used as input file to obtain the simulated
1H NMR spectra for the cis- and trans-isomers for R-L2 using the GIAO method at
B3LYP/6-311G (2d,p) with polarization continuum model (PCM) in chloroform as solv-
1
ent, respectively. Assignments of protons peaks for experimental H NMR spectra were
estimated based on these simulated spectra and the data are listed in the
Experimental section. The four-coordinate metal(II)-complexes with asymmetric chiral
N,N-chelate type Schiff base ligands in tetrahedral or distorted-tetrahedral geometry
exhibit induced chirality at-metal, and hence provide two possible diastereomers of
D-Cu and K-Cu along C2-symmetry of the molecule [1–10, 15–18]. Thus, we optimized
the diastereomeric pairs of D-Cu-R-L1/K-Cu-R-L1, D-Cu-S-L1/K-Cu-S-L1, D-Cu-R-L2/
K-Cu-R-L2 and D-Cu-S-L2/K-Cu-S-L2 at B3LYP/6-31G(d), respectively (Figure 1 and
Supporting Information Figure S2). For optimization, we used both the cis- and trans-
forms of the ligands to coordinate to the copper(II), and hence, to form the complexes
as depicted in the reaction Scheme 1. However, only the complexes designed with the
trans-form were succeeded to optimize with reasonable minimum energy (Figure 1
and Supporting Information Figure S2). The simulated electronic spectra (UV–Vis and
ECD) for the diastereomeric pairs D-Cu-R-L2/K-Cu-R-L2 and D-Cu-S-L2/K-Cu-S-L2 were

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 2. Experimental UV–Vis spectra for R-L2 (0.06 mM) and Cu-R-L2 (0.8 mM) in methanol at
25 ꢀC. Simulated spectra for D-Cu-R-L2 at B3LYP/6-31G(d) using PCM in methanol (Gaussian band
shape with exponential half-width, r ¼ 0.33 eV).
obtained by TDDFT at B3LYP/6-31G(d) (Figures 2 and 3 and Supporting Information
Figure S4). The PCM using methanol as solvent was incorporated and the 72 excited
states (roots) were considered for calculations (Supporting Information Table S1). The
simulated UV–Vis and ECD spectra for K-Cu-R-L2 were also obtained at different com-
binations of the functionals and the basis sets such as B3LYP/6-31G(d), M06/6-31G(d)
and M06/DEF2SVP, respectively. The results show almost identical spectra with little
shifted bands maxima (Supporting Information Figures S5 and S6), strongly supporting
the reliability and validity of the methods used. The best fit to the experimental spec-
tra was at B3LYP/6-31G(d) (Figures 2 and 3). Assignments of excited state properties
and molecular orbitals (MOs) calculations were carried out at the same level of theory.
The electronic spectra (UV–Vis and ECD) were generated using the program SpecDis
[28] by applying Gaussian band shape with exponential half-width r ¼ 0.33 eV.
3. Results and discussion
3.1. Synthesis and characterization of the Schiff bases and complexes
Reaction of 2-benzoyl-pyridine with (R or S)-1-(p-R1-C6H4)ethylamine under reflux affords
the enantiopure Schiff bases (R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethan-
amine fR1 ¼ H (R- orS-L1), OCH3 (R- orS-L2)g, respectively. These Schiff bases readily react
with copper(II) acetate in the presence of NH4(PF6) to provide bis[(R or S)-1-(p-R1-C6H4)-N-
(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]copper(II)(PF6)2, [Cu(R- or S-L)2](PF6)2 fR1
¼ H (Cu-R-L1 or Cu-S-L1), OCH3 (Cu-R-L2 or Cu-S-L2)g, respectively (Scheme 1). Vibrational
spectra of the complexes (Supporting Information Figure S3) show the main characteristic
band at 1610–1570 cmꢁ1 for the m(C ¼ N) and the band at 839–843 cmꢁ1 for the counter

10
M. ENAMULLAH ET AL.
Figure 3. Experimental ECD spectra for Cu-R-L2 and Cu-S-L2 (6.70 and 10.12 mM at 800–420 nm;
1.82 and 2.23 mM at 420–280 nm) in methanol at 25 ꢀC (experimental Dex6 at 800–420 nm and
Dex3 at 420–280 nm). Simulated ECD spectra for D-Cu-R-L2/K-Cu-R-L2 and D-Cu-S-L2/K-Cu-S-L2 at
B3LYP/6-31G(d) with PCM in methanol.
anion m(PF6). The values of molar conductance (Km ¼ 215-230 S cm2 molꢁ1) indicate 1:2
electrolytic nature of the complexes in DMF [29].
1
3.2. Experimental and simulated H NMR spectra
1H NMR spectra of the Schiff bases in CDCl3 (Figure 4) are essentially identical for each
enantiomeric pair and the spectral data are listed in the Experimental section. The

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 4. 1H NMR spectrum for (R)-1-(p-OCH3C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamine
(R-L2) in CDCl3 at 20 ꢀC (c/t stands for the cis-/trans-isomer).
spectra show two sets of peaks for each proton, associated to the cis- and trans-iso-
mers of the Schiff base in solution [22]. The ratios of cis- to trans-isomer are estimated
based on the integration values of corresponding methyl (CH3) and/or methane (CH)
protons peaks. The cis- to trans-isomer ratios are about 20:80 (R-L1), 17:83 (S-L1), 48:52
(R-L2) and 56:44 (S-L2), and the different ratios represent a dynamic isomerization
equilibrium of the Schiff base in solution. The methyl protons show two doublets at
d ¼ 1.42–1.51 and 1.53–1.65 ppm (JHH ¼ 6.8 Hz) for the cis- and trans-isomers, respect-
ively. The methine proton attached to the chiral-carbon atom shows two quartets at a
significant distance at d ¼ 4.29–4.48 and 4.65–4.78 ppm (JHH ¼ 6.8 Hz) for the trans-
and cis-isomers, respectively. Assignment of the quartet in the cis-isomer at relatively
downfield (by ca. 0.30 ppm) is based on the fact that the methine proton moves
toward the imino-/pyridyl-nitrogen atoms and experiences significant inductive effect
than in the trans-isomer. The methoxy protons show two sharp singlets at about
d ¼ 3.81 and 3.82 ppm for the cis- and trans-isomers, respectively. The proton adjacent
to the pyridyl nitrogen atom (Ar-H5) reveals two doublets at the most downfield at
about d ¼ 8.75 and 8.80 ppm for the trans- and cis-isomers, respectively. However, the
remaining aromatic protons show several peaks in pairs at the range of
d ¼ 6.88–8.81 ppm (Figure 4). The simulated 1H NMR spectra for both the cis- and
trans-isomers of R-L2 are obtained using the GIAO method at B3LYP/6-311G (2d,p)
with PCM in chloroform as solvent. The spectral data are listed in the Experimental
section. Assignments of the proton peaks including the cis- and trans-isomers are
made based on these simulated spectra. The simulated spectra also show the methine
proton peaks in the cis- and trans-isomers of R-L2 at a distance by about 0.30 ppm
(d ¼ 5.20 and 4.92 ppm), in accord with the experimental results discussed above.
3.3. Optimized structures
The optimized structures (gas phase) for the Schiff bases (R- or S-L1 and R- or S-L2)
with both cis- and trans-forms reveal that the cis-isomers are relatively more stable

12
M. ENAMULLAH ET AL.

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 5. Experimental ECD spectra for (a) R-L2 and S-L2 in chloroform and (b) Cu-R-L2 and Cu-S-
L2 (6.70 and 10.12 mM at 800–450 nm; 1.82 and 2.23 mM at 450–280nm) in methanol at 25 ꢀC
(cell path-length: 1.0 mm).
than trans-isomers by 3.15-4.11 kcal/mol (Supporting Information Figure S1). This might
be due to inductive effect between the methine hydrogen and imino-/pyridyl nitrogen
atoms that stabilizes the cis-form rather than the trans-form, as evidenced by relatively
downfield shift of the methine proton peak (in cis-form) both in experimental and
1
simulated H NMR spectra (discussed above). The optimized structures (gas phase) for
the diastereomeric pairs D-Cu-R-L1/K-Cu-R-L1, D-Cu-S-L1/K-Cu-S-L1, D-Cu-R-L2/K-Cu-R-
L2 and D-Cu-S-L2/K-Cu-S-L2 (Figure 1 and Supporting Information Figure S2) show

14
M. ENAMULLAH ET AL.
Figure 6. The frontier molecular orbitals HOMO, HOMO-10 and LUMO for D-Cu-R-L2, calculated at
B3LYP/6-31G(d), for b-spin consideration.
that the K-Cu (for R or S-L1) and D-Cu (for R or S-L2) configured complexes are slightly
more stable by 0.95-1.06 and 0.38-0.87 kcal/mol, respectively. Optimizations further reveal
that complexation is only possible via coordination of the trans-isomer to the copper(II),
as shown in Scheme 1. However, some selected bond lengths and angles in the opti-
mized structures are listed in Table 1, comparable to the X-ray results reported for the
related Co/Ru(II)-chiral-N,N Schiff bases complexes [23, 24]. The values of the imine bond
length (i.e. N2-C6/N20-C60 ¼ 1.27–1.30 Å) indicate the carbon–nitrogen double bond char-
acter, while bonds angles for C6-N2-C13/C60-N20-C130 (120-122ꢀ) represent the sp2-hybrid-
ization of the imino-nitrogen atom [23, 24, 30]. The values of dihedral angles (h ¼ 61–63ꢀ)
between the two coordinating planes N1–Cu–N2 and N10–Cu–N20 (Table 1) comply well
with the distorted-tetrahedral geometry for the complexes.
3.4. Experimental and simulated electronic spectra (UV–Vis and ECD)
UV–Vis spectra for the Schiff base (R-L2) and complex (Cu-R-L2) in methanol show a
very strong common band below about 400 nm (kmax ¼ 342 nm), due to the intra-lig-
ꢂ
ꢂ
and p!p /n!p transitions (LL; Figure 2). The spectrum for the complex shows a
moderate band at 450–550 nm (kmax ¼ 501 nm), attributed to the metal-ligand charge-
transfer (MLCT) transitions. The spectrum further shows two broad bands at visible
region at 550–1100 nm (kmax ¼ 687 and 950 nm) for the d-d (MM) transitions for the
copper(II)-core electrons [1, 8, 10, 14, 31, 32]. The electronic circular dichroism (ECD)
spectra for the free Schiff bases S-L2 and R-L2 (Figure 5(a)) and the complexes Cu-S-L2
and Cu-R-L2 (Figure 5(b)) show several bands with opposite Cotton effects, associated

JOURNAL OF COORDINATION CHEMISTRY
15
Table 2. Some selected and simplified assignments on simulated spectrum for D-Cu-R-L2, calcu-
lated at B3LYP/6-31G(d) with PCM in methanol.
k/nma
Oscillator strength (f)
MOs contributions (%)b
Assignmentsc,d
992 (950)
855 (687)
472 (501)
447
343 (342)
294
0.0031
0.0087
0.0144
0.0628
0.0689
0.1247
H-10!L, H-8!L
H-1!L, H!L
LM
MM, LM
MM, ML/LM, LL
MM, ML, LL
ML/LM, LL
ML, LL
H-4!L, H-1!L þ 1
H-1!L þ 2, H!L þ 2
H-7 !L þ 2, H-6!L þ 1
H-14 !L, H-8!L þ 1
aExperimental values are in parentheses;
bH ¼ HOMO and L ¼ LUMO;
cMM ¼ metal-centered (d-d), ML/LM ¼ metal-ligand/ligand-metal, LL ¼ ligand-centered transitions;
db-spin MOs are considered.
to different electronic transitions found in the UV–Vis spectra. Observed mirror-image
relationships indicate the enantiopurity or enantiomeric excess of the S- and R-ligands
and their associated complexes Cu-S-L2 and Cu-R-L2 in solution, respectively. The com-
parisons of ECD spectra between the free Schiff bases (Figure 5(a)) and the complexes
(Figure 5(b)) reveal several bands at 320-800 nm in the complexes, and hence, confirm
formation of the complexes. ECD spectra for Cu-R-L2/Cu-S-L2 are characterized by the
following series of bands (sign and strength): about 600 nm (þ/ꢁ, weak), 490 nm
(ꢁ/þ, weak), 335 nm (þ/ꢁ, strong) and 300 nm (ꢁ/þ, very strong), respectively.
The experimental and simulated UV–Vis spectra are almost similar with little shifted
bands maxima (Figure 2 and Supporting Information Figure S5). The simulated UV–Vis
spectra for the diastereomeric pair D-Cu-R-L2/K-Cu-R-L2 or D-Cu-S-L2/K-Cu-S-L2 are
essentially identical (Supporting Information Figure S4). The simulated ECD spectra for
the diastereomeric pair D-Cu-R-L2/K-Cu-R-L2 or D-Cu-S-L2/K-Cu-S-L2 show expected
mirror-image relationships with opposite Cotton effects (Figure 3). Selected and simpli-
fied assignments on simulated UV–Vis spectra are made based on orbital and popula-
tion analyses for D-Cu-R-L2 at the same level of theory (Table 2) [1, 8, 10, 14]. The
band at 992 nm with oscillator strength 0.0031 (f) is assigned for the ligand p-moiety
2
(i.e. HOMO-8 and HOMO-10) to the metal-dz (LUMO) transitions. Another band at
2
855 nm (f ¼ 0.0087) results from combination of the metal-dz and ligand p-moiety (i.e.
2
HOMO and HOMO-10) to the metal-dz (LUMO) transitions. There are also intense
bands on the simulated spectra which are close to the experimental bands (Figure 2,
Table 2). The frontier molecular orbitals HOMO, HOMO-10 and LUMO are shown in
Figure 6. The HOMO orbital is mainly localized on the pyridyl-p and metal-dxy electron
2
moieties, while LUMO is localized on the metal-dz electrons. The energy gap between
the HOMO and LUMO is very narrow (i.e. DEg ¼ 0.96 kcal/mol) and thus involves a sig-
nificant contribution to the excitation protocol.
3.5. ECD spectra and absolute configuration at-metal (K vs. D)
It has well been documented that the overall nature of the chiral-metal complexes is
reflected on their ECD spectra in solution. Spectral analyses of the d-d transitions
bands in the visible region predict the absolute configuration at-metal center (i.e. K
vs. D) in the complexes. Comparisons of experimental and simulated ECD spectra
determine the absolute configuration resulting from diastereoselection and induced

16
M. ENAMULLAH ET AL.
Scheme 3. Reaction pathway for decomplexation of Cu-R-L2 with NaCN in MeOH.
chirality at-metal by the coordinated ligands (S or R-L2) [1–8]. Hence, the experimental
spectrum for Cu-R-L2 or Cu-S-L2 shows best fit to the simulated spectrum for the dia-
stereomer D-Cu-R-L2 or K-Cu-S-L2 (Figure 3). These results conclude that Cu-R-L2 or
Cu-S-L2 corresponds to the diastereomeric excess of D-Cu-R-L2 or K-Cu-S-L2 (i.e. major
product) in solution. It has been reported that a dynamic diastereomeric equilibrium
between the two diastereomers prevails in solution [2, 3, 6–9]. Further, the spectrum
for Cu-R-L2 or Cu-S-L2 shows the best fit to the simulated spectrum for the other dia-
stereomer D-Cu-S-L2 or K-Cu-R-L2 (Figure 3). These results are parallel to the preferred
formation of D-M-R or K-M-S (i.e. major products) in the bis[(R or S)-N-(1-(p-
OCH3C6H4)ethyl)salicylaldiminato-j2N,O]-D/K-M(II) (M ¼ Cu [8] and Fe [11]), as evi-
denced by experimental and simulated ECD/VCD spectra in solution.
3.6. Decomplexation of Cu-R-L2 with NaCN and cis-trans interconversion
We employed an indirect method to prove the existence of the Schiff base in the
paramagnetic Cu(II)-complexes and hence, conducted a decomplexation reaction of
Cu-R-L2 with NaCN in methanol and isolated the free Schiff base (Scheme 3, details in
Experimental section). The reaction results in the formation of the diamagnetic
[CuI(CN)4]3- and R-L2 via reduction of the Cu(II) to Cu(I) accompanies a color change
1
from deep-green to light-orange (Scheme 3) [1, 5, 31, 32]. H NMR spectrum of R-L2
(Figure 7) (isolated via decomplexation, see subsection 2.4) shows two sets of peaks
for each proton, indicating the presence of both cis- and trans-isomers in solution. The
methyl protons show two doublets at d ¼ 1.51 and 1.53 ppm (JHH ¼ 7.2 Hz), while the
methine proton shows two quartets at d ¼ 4.48 and 4.64 ppm (JHH ¼ 6.8 Hz). The
methoxy protons show two sharp singlets at d ¼ 3.81 and 3.82 ppm. The ratio of the
1
cis- to trans-isomer is about 55:45. These results are similar to the H NMR spectrum of
R-L2 (as described in subsection 3.2), strongly suggesting the presence of the Schiff
base in the copper(II)-complexes.
1
We further run H NMR spectrum (Figure 8) of in situ reaction mixture of Cu-R-L2
and NaCN in dmso-d6 (in NMR tube), which also shows two sets of peaks for each pro-
ton and indicates both the cis- and trans-isomers of the Schiff base in solution. Here,
the methyl protons show two doublets at d ¼ 1.36 and 1.38 ppm (JHH ¼ 6.8 Hz) and
the methoxy protons show two singlets at d ¼ 3.70 and 3.72 ppm. The methine proton
shows two quartets at d ¼ 4.51 and 4.65 ppm (JHH ¼ 6.8 Hz) with cis-to trans-isomer
ratio of about 8:92. In comparison to almost equal amounts of cis- to trans-isomer (i.e.
ca. 48:52 or 55:45) in the free R-L2 in solution (discussed above), the present ratio indi-
cates the trans-isomer as the major product (ca. 92%) in the reaction mixture. The
results indicate coordination of copper(II) to the solely trans-form of the Schiff base

JOURNAL OF COORDINATION CHEMISTRY
17
Figure 7. 1H NMR spectrum for R-L2 (isolated via decomplexation of Cu-R-L2 with NaCN) in CDCl3
at 20 ꢀC (c/t stands for cis-/trans-isomer).
Figure 8. 1H NMR spectrum of the in situ reaction mixture of Cu-R-L2 and NaCN in DMSO-d6 at
20 ꢀC (c/t stands for cis-/trans-isomer).
(resulting from cis- to trans-interconversion during complexation reaction) and hence,
provide the complexes as depicted in Scheme 1. The results further suggest that the
trans-isomer undergoes interconversion to the cis-isomer (ca. 8%) immediately upon
its discharge via decomplexation in solution (Scheme 3). Finally, with complete conver-
sion, the isolated free R-L2 shows almost equal amounts of cis- to trans-isomer (ca.
55:45) in solution, as discussed above.
3.7. Cyclic voltammetry
Cyclic voltammograms for Cu-R-L2 were run at the range of ꢁ1.30 to 1.00 V (vs. Ag/
AgCl) at varying scan rates in acetonitrile at 25 ꢀC, respectively (Figure 9). The forward
scans exhibit two weak cathodic peaks at about ꢁ0.62 (Ec1) and about ꢁ0.93 V (Ec2)
at a scan rate of 0.10 Vsꢁ1, which become more populated at faster scan rates. These
peaks are referred to two electron charge transfer processes for the [Cu-R-L2]2þ/[Cu-R-
L2]þ and [Cu-R-L2]þ/[Cu-R-L2]0 couples (L2 ¼ Schiff base ligand), respectively. The
reverse scans show two corresponding strong anodic peaks at about ꢁ0.19 (Ea2) and
about þ0.45 V (Ea1) for [Cu-R-L2]0/[Cu-R-L2]þ and [Cu-R-L2]þ/[Cu-R-L2]2þ couples,

18
M. ENAMULLAH ET AL.
Figure 9. Cyclic voltammograms for Cu-R-L2 (0.5 mM); TBAP (0.1 M) at varying scan rates (m/Vs-1)
in acetonitrile at 25 ꢀC.
respectively. The relatively weak cathodic peaks on the forward scans reflect instability
of the reduced species. For comparison, CV for the Schiff base (R-L2) was run under
the same experimental conditions (Supporting Information Figure S7), which shows
two pairs of redox peaks at about þ0.80 to þ1.20 V (EcTBAP/EaTBAP) for the electrolytes
(TBAP) [5, 14] and two pairs of redox peaks at about þ1.30 to þ2.00 V (Ec1L2/EaL2 and
Ec2L2/EaL2) for the Schiff base. The results suggest that the observed peaks at lower
potential (i.e. at þ0.50 to ꢁ1.30 V) are exclusively due to the [Cu-R-L2]2þꢀ [Cu-R-L2]þ
and [Cu-R-L2]þꢀ [Cu-R-L2]0 couples in solution (Figure 9), since the Schiff base shows
no peaks at this range (Supporting Information Figure S7). Thus, the CV results
unequivocally demonstrate two quasi-reversible one electron charge transfer processes
for the Cu2þ/Cuþ and Cuþ/Cu0 couples in acetonitrile, respectively [14, 31, 33, 34]. In
addition, analyses of voltammograms at varying scan rates show that both the cath-
odic and anodic peaks become more populated and shift to lower and higher poten-
tials, respectively, with faster scan rates. Indeed, plot for the anodic to cathodic peak
current ratios versus square root of the scan rates (i.e. Ia2/Ic2 vs. m1/2 [Vsꢁ1]) exhibits a
linear increase, and plot for the potential differences between the two anodic peaks
versus square root of the scan rates (i.e. DEa ¼ Ea2 - Ea1 [V] vs. m1/2 [Vsꢁ1]) remains
unchanged with faster scan rates (Supporting Information Figure S8), suggesting a dif-
fusion-controlled electrochemical process in solution [1, 14, 31, 33, 34].
4. Conclusion
The enantiopure Schiff bases (R or S)-1-(p-R1-C6H4)-N-(phenyl(pyridin-2-yl)methylene)e-
thanamine react with copper(II) acetate to give distorted-tetrahedral bis[(R or S)-1-(p-
R1C6H4)-N-(phenyl(pyridin-2-yl)methylene)ethanamino-j2N,N]D/K-Cu(PF6)2 in the pres-
ence of NH4PF6. 1H NMR spectra (experimental and simulated) of the Schiff bases
reveal two sets of peaks for each proton, resulting from cis- and trans-isomers in solu-
tion. A conformational change from cis- to trans-isomer occurs in solution and the

JOURNAL OF COORDINATION CHEMISTRY
19
copper(II) coordinates to the solely trans-form to provide the complexes. CV results
demonstrate two quasi-reversible one electron charge transfer processes for the Cu2þ
/
Cuþ and Cuþ/Cu0 couples in acetonitrile. ECD spectra show several bands with oppos-
ite Cotton effects (mirror-image), indicating enantiopurity or enantiomeric excess of
the R or S-ligands and their associated complexes in solution. Analyses of experimental
and simulated ECD spectra suggest induced chirality at-metal, which diastereoselec-
tively prefers formation of the D-Cu-R or K-Cu-S diastereomeric solution. These results
are of particular interest to synthesize the chiral-at-metal complexes and to use them
as potential catalysts in a stereoselective way.
Acknowledgements
We acknowledge Wazed Miah Science Research Centre (WMSRC) at Jahangirnagar University,
Dhaka, Bangladesh for obtaining CV data. Our sincere thanks to Professor W. Houry and Mr. V.
Bhandari, Department of Biochemistry, Faculty of Medicine, University of Toronto, for obtaining
€
ECD spectra. We thank Professor C. Janiak, Institut fur Anorganische Chemie und
€
€
Strukturchemie, Heinrich-Heine-Universitat Dusseldorf, Germany for many useful discussions. We
are also grateful to Professor A.B.P. Lever, Department of Chemistry, York University, Toronto,
and the Computecanada.ca (https://ccdb.computecanada.ca/) Ontario, Canada for using compu-
tational resources.
Disclosure statement
No potential conflict of interest was reported by the authors.
References
[1] M. Enamullah, A.K.M. Royhan Uddin, G. Pescitelli, R. Berardozzi, G. Makhloufi, V. Vasylyeva,
A.-C. Chamayou, C. Janiak. Dalton Trans., 43, 3313 (2014).
€
ꢀ
[2] G. Pescitelli, S. Ludeke, A.-C. Chamayou, M. Marolt, V. Justus, M. Gorecki, L. Arrico, L. Di
Bari, M.A. Islam, I. Gruber, M. Enamullah, C. Janiak. Inorg. Chem., 57, 13397 (2018).
[3] M. Enamullah, G. Makhloufi, R. Ahmed, B. Alif Joy, M.A. Islam, D. Padula, H. Hunter, G.
Pescitelli, C. Janiak. Inorg. Chem., 55, 6449 (2016).
[4] M. Enamullah, M.A. Quddus, M.R. Hasan, G. Pescitelli, R. Berardozzi, G. Makhloufi, V.
Vasylyeva, C. Janiak. Dalton Trans., 45, 667 (2016).
[5] M. Enamullah, M.A. Quddus, M.R. Hasan, G. Pescitelli, R. Berardozzi, G.J. Reiß, C. Janiak.
Eur. J. Inorg. Chem., 2015, 2758 (2015).
€
[6] M. Enamullah, V. Vasylyeva, M.A. Quddus, M.K. Islam, S.-P. Hofert, C. Janiak.
CrystEngComm, 20, 4724 (2018).
€
[7] A.-C. Chamayou, S. Ludeke, V. Brecht, T.B. Freedman, L.A. Nafie, C. Janiak. Inorg. Chem. ,
50, 11363 (2011).
€
[8] A.-C. Chamayou, G. Makhloufi, L.A. Nafie, C. Janiak, S. Ludeke. Inorg. Chem., 54, 2193
(2015).
[9] M. Enamullah, V. Vasylyeva, C. Janiak. Inorg. Chim. Acta, 408, 109 (2013).
[10] M. Enamullah, M.K. Islam. J. Coord. Chem., 66, 4107 (2013).
€
€
[11] M. Enamullah, K.S. Banu, M.A. Hossain, U. Kokc¸am-Demir. J. Mol. Struct., 1199, 126947
(2020).
[12] M. Enamullah, M.A. Islam, B.A. Joy, B. Dittrich, G.J. Reiss, C. Janiak. Inorg. Chim. Acta, 482,
935 (2018).
€
[13] M. Enamullah, M.K. Islam, S.-P. Hofert, C. Janiak. Inorg. Chim. Acta, 501, 119245 (2020).

20
M. ENAMULLAH ET AL.
[14] M. Enamullah, M.A. Quddus, M.M. Rahman, T.E. Burrow. J. Mol. Struct., 1130, 765 (2017).
[15] N. Kordestani, H.A. Rudbaria, G. Bruno, S. Rosario, J.D. Braun, D.E. Herbert, O. Blacque, I.
Correia, M. Al-M. Zaman, M.M. Bindu, C. Janiak, M. Enamullah. Dalton Trans., 49, 8247
(2020).
[16] (a) A. von Zelewsky. Stereochemistry of Coordination Compounds, p. 69, Wiley, Chichester
(1996);(b) H. Amouri, M. Gruselle. Chirality in Transition Metal Chemistry: Molecules,
Supramolecular Assemblies and Materials, p. 40 Wiley, Chichester (2008).
[17] (a) C. Evans, D. Luneau. J. Chem. Soc., Dalton Trans., 83 (2002); (b) K.A. Jensen. Inorg.
Chem., 9, 1 (1970);(c) T. Akitsu, Y. Einaga. Acta Crystallogr. C, 60, m640 (2004).
[18] (a) H. Sakiyama, H. Okawa, N. Matsumoto, S. Kida. Bull. Chem. Soc. Jpn., 64, 2644 (1991);
(b) H. Sakiyama, H. Okawa, N. Matsumoto, S. Kida. J. Chem. Soc., Dalton Trans., 2935
(1990).
ꢁ
[19] (a) H. Okawa, M. Nakamura, S. Kida. Inorg. Chim. Acta, 120, 185 (1986); (b) T. Akitsu, Y.
Einaga. Polyhedron, 24, 2933 (2005);(c) T. Akitsu, Y. Einaga. Polyhedron, 24, 1869 (2005).
[20] I. Bernal, W. Ries, H. Brunner, D.K. Rastogi. J. Organomet. Chem., 290, 353 (1985).
[21] G. Zassinovich, R. Bettella, G. Mestroni, N. Bresciani-Pahor, S. Geremia, L. Randaccio. J.
Organomet. Chem., 370, 187 (1989).
[22] H. Brunner, B. Reiter, G. Riepl. Chem. Ber., 117, 1330 (1984).
[23] D.L. Davies, J. Fawcett, R. Krafczyk, D.R. Russell. J. Organomet. Chem., 545–546, 581
(1997).
[24] I. Bernal, G.M. Reisner, H. Brunner, G. Riepl. Inorg. Chim. Acta, 103, 179 (1985).
[25] H. Brunner, D.K. Rastogi. Inorg. Chem., 19, 891 (1980).
ꢀ
ꢂ
ꢀ
[26] D. Gutierrez, S. Bernes, G. Hernandez, O. Portillo, G.E. Moreno, M. Sharma, P. Sharma, R.
ꢀ
Gutierrez. J. Coord. Chem., 68, 3805 (2015).
[27] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P.
Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers,
K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C.
Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.
Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski,
€
G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman,
J.V. Ortiz, J. Cioslowski. D.J. Fox. Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT
(2009).
€
[28] T. Bruhn, A. Schaumloffel, Y. Hemberger, G. Bringmann. Chirality, 25, 243 (2013).
[29] (a) H.E.L. Hamdani, M.E.L. Amane. J. Mol. Struct., 1184, 262 (2019);(b) J.M. Seco, M.J.G.
Garmendia, M. Quiros. J. Coord. Chem., 55, 345 (2002).
[30] M. Enamullah, A.K.M. Royhan Uddin, G. Hogarth. J. Coord. Chem., 65, 4263 (2012).
[31] M. Enamullah, M. Al-M. Zaman, M.M. Bindu, M.K. Islam, M.A. Islam. J. Mol. Struct., 1201,
127207 (2020).
[32] M. Enamullah, A.K.M. Royhan Uddin. J. Bangladesh Chem. Soc., 22, 44 (2009).
[33] M. Enamullah, M.A. Islam, A.-C. Kautz, C. Janiak. J. Coord. Chem., 71, 2557 (2018).
[34] M. Enamullah, M.A. Islam, B.A. Joy, G.J. Reiss. Inorg. Chim. Acta, 453, 202 (2016).