Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-complexes: Syntheses, spectroscopy, tautomerism, redox potentials, EPR, PXRD and DFT/TDDFT

Article abstract of DOI:10.1016/j.molstruc.2019.127207

The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) react with the copper(II) acetate or nitrate to give the bis[3-(o/p-tolylimino)indole-2-olato-κ²N,O]copper(II) (1 and 2) via lactam (L)- to enol (E)-tautomerism in solution.

Full text of DOI:10.1016/j.molstruc.2019.127207

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Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-
complexes: Syntheses, spectroscopy, tautomerism, redox potentials, EPR, PXRD and
DFT/TDDFT
Mohammed Enamullah, Md Al-Muktadir Zaman, Mortuza Mamun Bindu, Mohammad
Khairul Islam, Mohammad Ariful Islam
PII:
S0022-2860(19)31316-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.127207
MOLSTR 127207
Reference:
To appear in: Journal of Molecular Structure
Received Date: 20 July 2019
Revised Date: 6 October 2019
Accepted Date: 8 October 2019
Please cite this article as: M. Enamullah, M. Al-Muktadir Zaman, M.M. Bindu, M.K. Islam, M.A. Islam,
Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-complexes: Syntheses,
spectroscopy, tautomerism, redox potentials, EPR, PXRD and DFT/TDDFT, Journal of Molecular
Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127207.
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Graphical Abstract
Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-complexes:
Syntheses, spectroscopy, tautomerism, redox potentials, EPR, PXRD and DFT/TDDFT
Mohammed Enamullah*, Md. Al-Muktadir Zaman, Mortuza Mamun Bindu, Mohammad Khairul
Islam, Mohammad Ariful Islam
Department of Chemistry, Jahangirnagar University, Dhaka, Bangladesh
The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) react with the copper(II)
acetate or nitrate to give the bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]copper(II) (1 and 2) via
lactam (L)- to enol (E)-tautomerism in solution.

Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-complexes:
Syntheses, spectroscopy, tautomerism, redox potentials, EPR, PXRD and DFT/TDDFT
Mohammed Enamullah*, Md. Al-Muktadir Zaman, Mortuza Mamun Bindu, Mohammad Khairul
Islam, Mohammad Ariful Islam
Department of Chemistry, Jahangirnagar University, Dhaka, Bangladesh
Abstract
The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) react with the copper(II)
acetate or nitrate to give the bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]copper(II) (1 and 2) via
lactam (L)- to enol (E)-tautomerism in solution. IR spectra show solely the L-tautomer at solid state,
1
while both tautomers in solution. H NMR results indicate a dynamic tautomerism equilibria
between the L- and E-tautomers in solution with varying ratios of ca. 83/17, 60/40, 54/46 (L/E)
within 0.5, 5, 24 h, respectively. Cyclic voltammogram demonstrate two quasi reversible one
electron charge transfer processes for Cu(II)/Cu(I) and Cu(I)/Cu(0) couples in acetonitrile,
o
respectively. PXRD patterns at 5-50 (2
θ
) correspond well to the crystalline nature with various
degrees of crystallinity for the ligands and complexes. X-band EPR spectrum shows an intense
broad band at high field region with isotropic nature. Magnetic moment values (µeff. = 1.56 -1.63
µB) indicate paramagnetic nature of the copper(II)-complexes with one unpaired electron in
methanol. Optimized structures by DFT show the L-form relatively stable than E-form by 19.76
Kcal/mol, while compounds HL2 and 2 are slightly stable than HL1 and 1 by 0.35 and 0.79
Kcal/mol, respectively. Electronic spectra by TDDFT strongly support the experimental results in
methanol.
Keywords: Isatin-Schiff bases; Copper(II)-isatin Schiff bases; Lactam-Enol tautomerism; Redox
potentials; DFT/TDDFT.

1. Introduction
The indoline-2,3-dione (Isatin), a cheap starting material for drug syntheses, and isatin-Schiff
bases are particularly important because of their pharmacological activities including anticancer,
antibacterial, antifungal, antiviral, antileukemic and antiprotozoal agents [1,2,3]. In this
connection, many isatin-Schiff bases and their derivatives were synthesized from condensation
of isatin with arylamine [4], hydrazine [5] semicarbazide [6], thiosemicarbazide [7,8,9,10] and
dithiocarbazate [11], respectively. Indeed, because of biological applications and dependences of
many enzymes reactivities on copper, a large number of copper(II)-isatin Schiff bases complexes
have been reported [7-9,11,12,13].
As part of our recent studies on transition metal complexes with chiral/achiral-Schiff bases
ligands [14-25], we reported syntheses, spectroscopic characterizations, chiroptical properties
and X-ray structural analyses of the enantiopure isatin-Schiff bases of 3-{(R)-(Ar)-ethylimino}-
1
1,3-dihydro-indol-2-one (Ar = C6H5, o-/p-MeOC6H4, p-BrC6H4 and 1-naphthyl) [26]. H NMR
studies demonstrate a time dependent dynamic tautomerism equilibrium between the lactam (L)-
and enol (E)-forms of the isatin-Schiff bases in solution [12b,26]. Whereas, solid state X-ray
structures show solely the lactam-form [26]. Our results further explore several intermolecular
hydrogen bonding interactions and/or short contacts, a common feature of isatin-Schiff bases, at
solid state [4,5,6,10,26]. However, isatin-Schiff bases readily react with the dinuclear [Rh(µ-
O2CMe)(η4-cod)]2 (cod = 1,5-cyclooctadiene) to give the mononuclear [Rh(η4-cod){3-((R)-(Ar)-
ethylimino)-3H-indol-2-olato}] complexes via tautomerism in solution [26].
The present endeavor, in continuation, reports the results of syntheses, spectroscopy, lactam-
enol tautomerism, redox potential, EPR, paramagnetism and PXRD studies on isatin-Schiff
bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) and their complexes bis[3-(o/p-
tolylimino)indole-2-olato-κ2N,O]copper(II) (1 and 2), respectively. Theoretical calculations by
DFT/TDDFT are employed to rationalise the experimental results.
2. Experimental
2.1. Materials and measurements

IR spectra were recorded on 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. Elemental analyses were performed on a VarioEL from
the Elementaranalyensysteme. 1H NMR-spectra were recorded on Bruker Avance 400
spectrometer operating at 400 MHz in CDCl3 or dmso-d6 at 20 °C. ESI mass (positive mode)
spectrum for compound 1 was recorded on Shimadzu LCMS-2020. The isotopic distributions
63 65
patterns for the / Cu containing ions are visible in the mass spectrum. EpsilonTM instruments
(BASi) electrochemical analyser was used for running cyclic voltammograms experiments
containing tetra-N-butyl-ammonium-hexaflorophosphate (TBAP) as supporting electrolyte in
acetonitrile at 25 °C. A three-electrode measurement system was used, which composed of a
platinum disc working electrode, platinum wire auxiliary electrode and Ag/AgCl reference
electrode, respectively. The solution containing the sample and TBAP was deoxygenated with
purging nitrogen gas for 10 minutes 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. The reproducibility of CV pattern was checked out at a scan rate of 0.10 Vs-1 in
acetonitrile. The X-band EPR spectrum was recorded on ECS-EMX (Bruker) spectrometer in
dmso at 120 K using the diphenylpicrylhydrazine (DPPH) as reference compound. The magnetic
moments values (µeff./µB) for the complexes were measured, based on the Evans method [27], in
1
methanol at 20 °C. For measurement, H NMR spectra were run using a sealed coaxial insert
containing TMS in acetone-d6 (0.5%, v/v) as an external standard and Instrument lock.
Cyclohexane in methanol (1.0%, v/v) was used as an internal reference compound, and methanol
peaks (i.e., CH3 and OH) were used for temperature calibration [27c-f].
2.2. Syntheses of the 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2)
An equimolar amount of Indoline-2,3-dione (Isatin) (0.294 g, 2.0 mmol) and o/p-toludine (0.214
g, 2.0 mmol) were dissolved in 10 mL methanol. Added 3-4 drops of concentrated H2SO4 into
this reaction mixture and refluxed the solution for 6-8 hours. Colour of the solution changed
from red-orange to orange-yellow. Afterwards, reduced the volume of the solvent to ca. 50% in
vacuo and left standing the solution for crystallization via. slow evaporation of the solvent at
room temperature. Orange-yellow microcrystals were precipitated out after 2-3 days, filtered off

and washed the crystals with methanol for three times (2 ml in each). Dried the microcrystals in
air for 3-4 days and obtained the Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2).
3-(o-tolylimino)indolin-2-one (HL1): Yield: 0.385 g (82%). IR (KBr, cm–1): 3451vs/br. (νH2O),
3241vs (νN-H), 3067w (νC-HAr), 1748vs (νC=O) and 1615 (νC=N). IR (in CHCl3, cm–1): 3693s,
3688s, 3683s and 3609w (νO-H), 3439s (νN-H), 1746s (νC=O), 1734s (νC−O) and 1619s, 1603sh
(νC=N). IR (ATR, cm–1): 3234vs (νN-H), 3066w (νC-HAr), 1745vs (νC=O) and 1610, 1593vs
1
(νC=N). H NMR (400 MHz, CDCl3): δ = 2.18 (s, 3H, CH3-L, ca. 83%), 2.46 (s, 3H, CH3-E, ca.
17%), 6.53 (d, J = 7.6 Hz, 1H, H14-L), 6.77 (t, J = 7.6 Hz, 1H, H13-L), 6.86 (d, J = 7.6 Hz, 1H, H11-
L), 6.92 (d, J = 8.0 Hz, 1H, H8-L), 6.94 (d, J = 7.0 Hz, 1H, H14-E), 7.10-7.15 (m, 2H, H11,13-E), 7.19
(t, J = 7.2 Hz, 1H, H12-L), 7.26 (t, J = 7.2 Hz, 1H, H6-L), 7.31-3.33 (m, 4H, H6,12-E, H5,7-L), 7.34 (d,
J = 7.6 Hz, 1H, H8-E), 7.59 (t, J = 7.6 Hz, 1H, H7-E), 7.65 (d, J = 7.2 Hz, 1H, H5-E), 8.24 (br, 1H,
OH-E, 20%), 8.46 (br, 1H, NH-L, 80%) (L = Lactam-form and E = Enol-form, for proton
numbering see the Scheme 1).
3-(p-tolylimino)indolin-2-one (HL2): Yield 0.376 g (80%). IR (KBr, cm–1): 3453vs/br. (νH2O),
3261vs (νN-H), 3046w (νC-HAr), 1742vs (νC=O) and 1613vs (νC=N). IR (in CHCl3, cm–1): 3689s,
1
3675s, 3610w (νO-H), 3439s (νN-H), 1745s (νC=O) and 1616s, 1595sh (νC=N). H NMR (400
MHz, CDCl3): δ = 2.32 (s, 3H, CH3-E, ca. 15%), 2.43 (s, 3H, CH3-L, ca. 85%), 6.79 (t, J = 7.6 Hz,
1H, H6-L), 6.83 (t, J = 7.2 Hz, 1H, H5-L), 6.94 (t, J = 8.0 Hz, 4H, H11-12,14-15-E), 6.97 (d, J = 8.0 Hz,
4H, H11-12-L), 7.06 (d, J = 7.6 Hz, 1H, H8-E), 7.14 (t, J = 7.6 Hz, 1H, H8-L), 7.26 (d, J = 8.0 Hz,
3H, H14-15-L+H6-E), 7.33 (t, J = 7.6 Hz, 1H, H7-L), 7.58 (t, J = 7.6 Hz, 1H, H7-E), 7.64 (d, J = 7.2
Hz, 1H, H5-E), 8.41 (br, 1H, OH-E, ca. 15%), 8.66 (br, 1H, NH-L, ca. 85%).
2.3. Syntheses of the bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]copper(II) (1 and 2)
Two equivalents of the Schiff base 3-(o/p-tolylimino)indolin-2-one (HL1 or HL2) (1.0 mmol)
were dissolved in 10 mL methanol and stirred the solution for 10 min at room temperature.
Added one equivalent of copper(II) acetate or nitrate solution (0.5 mmol, dissolved in 10 mL
methanol) into the solution of Schiff base. The solution mixture was then stirred for ca. 8 hours
at room temperature and colour changed from orange-yellow to dark brown. Reduced the volume
of solvent to ca. 70 % in vacuo and left standing this solution for precipitation at room

temperature. After 4-5 days, dark brown precipitate was formed, filtered off and washed two
times with diethyl ether (2 mL in each) followed by methanol. The precipitate was dried for 3-4
days and obtained dark brown microcrystals of bis[3-(o/p-tolylimino)indole-2-olato-
κ2N,O]copper(II) (1 and 2).
Bis[3-(o-tolylimino)indole-2-olato-κ2N,O]copper(II), [Cu(L1)2] (1): Yield: 0.220 g (75%). IR
(ATR, cm–1): 3170, 3065w (νC-Haromatic), 1728vs (νC-O) and 1612, 1591vs (νC=N). MS (ESI+): m/z
(%) = 602 (20) [M(HCO2H)+Na]+, 556 (10) [M+Na]+, 473 (100) [(HL1)2+H]+, 382 (25)
[(HL1)2−C6H4(CH3)+H]+, 361 (10) [(CuL1)(CH3CO2)+2H+H]+, 298 (17) [CuL1]+, 237 (40)
[HL1+H]+ and 208 (12) [CuL1−C6H4(CH3)+H]+ {M = Cu(L1)2 = C30H22N4O2Cu; HL1 =
C15H12N2O}. − C30H22N4O2Cu 3H2O (588.11): Calcd C 61.27, H 4.80, N 9.53; found C 60.90, H
4.62, N 9.41.
Bis[3-(p-tolylimino)indole-2-olato-κ2N,O]copper(II), [Cu(L2)2] (2): Yield 0.210 g (71%). IR (ATR,
cm–1): 3188, 3061w (νC-Haromatic), 1726vs (νC-O) and 1614vs, 1595sh (νC=N).
− C30H22N4O2Cu 3H2O (588.11): Calcd C 61.27, H 4.80, N 9.53; found C 61.00, H 4.65, N 9.35.
2.4. Computational method
To rationalize the experimental results, a through computational procedure was performed with
the Gaussian 09 software [28]. The gas phase equilibrium geometries for both the Lactam (L)-
and Enol (E)-forms for HL1 or HL2 were optimized with DFT using the functional B3LYP and
the basis set LANL2DZ, respectively (Fig. S1). Optimizations were also carried out in
chloroform with Polarization Continuum Model (PCM) at B3LYP/LANL2DZ, respectively. It
has been reported that four-coordinated metal(II)-N^O Schiff bases complexes in tetrahedral to
distorted square-planar geometry exhibit induced chirality at-metal center, and provide two
possible ∆(right)-Cu and Λ(left)-Cu handed isomers (i.e., opposite configuration at-metal center)
along the C2-symmetry of the molecule [15-24]. Thus, we optimized both the ∆-Cu and Λ-Cu
configured structures for 1 or 2 using the functional B3LYP and the basis set 6-31G(d),
respectively (Fig. 2).
For excited state properties, time-dependent density functional theory (TDDFT) was
employed using the functionals B3LYP, M06 and the basis sets SDD, TZVP, 6-31G(d) on the

optimized structures with B3LYP/6-31G(d) or B3LYP/SDD, respectively for 1. Polarization
Continuum Model (PCM) was incorporated using methanol as solvent and 72 excited states
(roots) were considered for calculations (Table S1). The calculated electronic spectra, from
different combinations of the functionals and the basis sets, are similar in nature with little
shifting of the band maxima (λmax) (Figs. 3, S2), strongly support the reliability and validity of
the methods used for calculations. The best fitted calculated spectra to the experimental spectra
are obtained with B3LYP/6-31G(d)//B3LYP/6-31G(d) (Fig. 3). The spectra for both ∆- and Λ-
configured structures of 1 were calculated. TDDFT was further employed on the Lactam- and
Enol-forms of HL1 at B3LYP/LANL2DZ, respectively (using PCM in methanol) (Fig. S3).
Assignments of excited state properties and molecular orbitals (MOs) calculations were carried
out at the same level of theory. The electronic spectra were generated using the program SpecDis
[29] by applying Gaussian band shape with exponential half-width σ = 0.24 eV.
3. Results and discussion
Reaction of indoline-2,3-dione (Isatin) with the o/p-toludine in presence of 3-4 drops of
concentrated H2SO4 under reflux affords the isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one
(HL1 or HL2) in methanol. The isatin-Schiff bases undergo tautomerism between the lactam (L)-
and enol (E)-forms in solution, and hence react with the copper(II) acetate or nitrate to give the
bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]copper(II), {Cu(L1)2] (1) or [Cu(L2)2] (2)} (Scheme
1). ESI mass spectrum shows the molecular ion peaks at m/z 556 ([M+Na]+) for 1 (Fig. 1).
Indeed, the spectrum is dominated by several ions peaks for the protonated Schiff base ligand at
m/z 473 {[(HL1)2+H]+} and 237 ([HL1+H]+), and fragmented complex species at m/z 602
([M(HCO2H)+Na]+), 298 ([CuL1]+) and 208 ([CuL1−C6H4(CH3)+H]+).

Scheme 1. Synthetic route to the bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]copper(II) (1 or 2).
Fig. 1. ESI-mass spectrum for compound 1.

3.1. Vibrational spectra and optimized structures
The vibrational spectra for the isatin-Schiff bases (KBr disc) show main characteristics bands at
3246 (HL1)/3261 (HL2) for νN-H, and at 1748 (HL1)/1742 (HL2) cm−1 for νC=O, suggesting
solely the lactam (L)-tautomer at solid state (Fig. S4, top). The spectra taken in chloroform
solution show bands at ca. 3439 cm-1 (νN-H) and 1745 (νC=O) cm-1 for the lactam (L)-tautomer,
and bands at ca. 3690/3608 cm−1 (νO-H) and 1734 cm-1 (νC-O) for the enol (E)-tautomer (Fig.
S4, bottom). The ratio of peaks areas for L/E-tautomers is ca. 60/40 within ca. 2 h of dissolution
in solution. The results suggest the existences of both the L- and E-tautomers simultaneously in
1
solution (Scheme 1), as evidenced by H-NMR studies (discussed below). The vibrational
spectrum for HL1 (with lactam-form) from optimized structure at B3LYP/LANL2DZ shows the
characteristics bands at 3672, 1728 and 1665 cm-1 due to νN-H, νC=O and νC=N, respectively
(Fig. S5). While the spectrum for HL1 (with enol-form) shows bands at 3654 and 1593 cm−1 due
to νO-H and νC=N, respectively (where νN-H/νC=O bands are absent). As the complexes are
formed via deprotonation of the enolic-proton (resulting from tautomerism in solution), the νN-
H, νC=O and/or νO-H bands are obviously absent in their spectra (Fig. S6). Indeed, the
complexes show strong band at ca. 1728 (1) and 1726 (2) cm-1 for νC-O (Fig. S6). The Schiff
bases further show strong bands at ca. 1615 cm-1 (KBr disc) and ca. 1620, 1603 cm-1 (in CHCl3)
for νC=N, which are found at ca. 1612, 1591 cm-1 (1) and ca. 1614, 1595 cm-1 (2) in complexes.
Several weak bands/shoulders are observed at 2950-3070 cm-1 for νC-H both in the Schiff bases
and complexes (Figs. S4, S6).
Optimized structures (gas phase) for the lactam (L)- and enol (E)-tautomers for HL1 or HL2
show the L-tautomer relatively more stable than E-tautomer by ca. 19.76 Kcal/mol (Fig. S1).
Similarly, optimized structures in chloroform also reveal the L-form more stable by ca. 21.33
Kcal/mol. The optimized structures at B3LYP/6-31G(d) show both the ∆-Cu and Λ-Cu
configured isomers are equi-energetic (i.e., same electronic and thermal free energy) for 1 or 2.
The optimized structure for HL2 (or 2) with p-tolyl group is slightly stable by ca. 0.35 (or ca.
0.79) Kcal/mol than HL1 (or 1) with o-tolyl group due to less steric hindrance (Fig. 2). Some
selected bond lengths and angles for the ligands and complexes are listed in Table 1, which are
comparable to X-ray structural data for HL1 with lactam-form [30]. The computed C8-N2-C9

bond angle (ca. 127.7 °) is higher by ca. 10 ° than the X-ray value (117.7 °) in HL1 due to
existing intermolecular hydrogen bonding interactions and short contacts in the crystals [26,30].
The bond angles are lowered by 5-7 ° in 1 or 2 due to steric constraints experienced in the
coordination sphere. In fact, the two ligands differ in their relative orientations of the aryl- and
lactam-rings, which is reflected by the dihedral angles (
the aryl- and lactam-rings) of 57.1 ° (HL1) and 54.4 ° (HL2) because of uneven steric hindrances
induced by the o- or p-methyl groups on the aryl-ring (Table 1). However, these values are
θ /° = angle between the two planes of
θ
significantly lowered by 11-14 ° than the X-ray value (68.6 ° in HL1) due to intermolecular
hydrogen bonding and short contacts in the crystals as mentioned above. The N2-C8 bond
lengths (1.27-1.29 Å) indicate carbon-nitrogen double bond, while C8-N2-C9 bond angles
(117.7-127.8 °) represent sp2-hybridization of the imino-nitrogen atom (Figs. 2, S1) [26].
Further, the bond angles for O-Cu-O' (ca. 100 °) and N2-Cu-N2' (ca. 104 °) comply well with the
distorted tetrahedral geometry for the complexes.
(HL1, Lactam-tautomer)
(HL2, Lactam-tautomer; +0.35 Kcal/mol)
(1, ∆-Cu isomer)
(2, ∆-Cu isomer; +0.79 Kcal/mol)

Fig. 2. DFT optimized structures for the ligands (HL1 and HL2) at B3LYP/LANL2DZ and
complexes (1 and 2) at B3LYP/6-31G(d), respectively.
Table 1. Some selected bond lengths (Å) and angles (°) in HL1 and HL2.
Bond lengths (Å)/
angles (°)
C7-O
HL1
(X-ray HL1 (cal.) 1
HL2 (cal.) 1
1 (cal.) 2
2 (cal.) 2
structure) a
1.212(2)
1.362(2)
1.412(3)
1.503(3)
1.276(2)
1.431(6)
1.2423
1.4001
1.4114
1.5392
1.2925
1.4125
1.2422
1.4006
1.4107
1.5395
1.2935
1.4121
1.2838
1.3055
1.4100
1.5325
1.2955
1.4259
1.9153
2.0105
122.65
104.13
98.93
1.2833
1.3058
1.4084
1.5342
1.2964
1.4206
1.9138
2.0056
124.13
103.76
100.20
148.17
86.69
C7-N1
C1-N1
C7-C8
C8-N2
N2-C9
Cu-O/O'
Cu-N2/N2'
C8-N2-C9
N2-Cu-N2'
O-Cu-O'
117.7(3)
127.78
127.62
O-Cu-N2'/N2-Cu-O'
N2-Cu-O/N2'-Cu-O'
Dihedral angles (θ
/°) 3
149.00
86.67
68.62
57.12
54.38
66.00
56.29/52.63
a From ref. 30; 1 Calculated at B3LYP/LANL2DZ, 2 at B3LYP/6-31G(d) for ∆-isomer.
3 θ /° = Dihedral angles between the two planes of the aryl- and lactam-rings.
3.2. Experimental and computed electronic spectra (UV-Vis.)
Electronic spectra (UV-Vis.) for the Schiff bases and complexes in methanol are identical in
nature (Fig. 3) and feature several bands/shoulders below 500 nm due to the ligand-centered
n→π*/π→π* (LL) transitions. A weak broad band, in addition, is found at visible region (500-
800 nm) in the complexes (Fig. 3, inset), results from overlapping of metal-ligand (ML) charge
transfer and metal-centered d-d transitions (MM) of copper(II)-core electrons [15,19,26,30].
Excited state properties by TDDFT for the complexes, from different combinations of the
functionals and the basis sets, provide similar spectra with little shifting of the bands maxima
(λmax), which are comparable to the experimental spectra (Figs. 3, S2). The computed spectra for
L- and E-tautomers of isatin-Schiff bases are almost identical and fit well to the experimental
spectra (Fig. S3). Similarly, spectra for ∆- and Λ-configured structures are essentially identical
for 1 or 2. [15,17-23]. However, there are some discrepancies in the bands positions (λmax
values) between experimental and calculated spectra (Figs. 3, S3 and Table 2) due to certain

drawbacks of TDDFT protocol including overestimation of excitation energies, presence of low
energy states with low oscillator strengths, improper description and/or long distance of charge-
transfer processes, etc. [31]. “ In addition, lacking of proper choices of the functionals and basis
sets, gas phase DFT optimizations and limitations of experimental spectra including unavoidable
(a) solute and solvent interactions and (b) dissociation-equilibria as found for the labile
tetrahedral/square-planar complexes in solution [16-19,22-25,32-33] may contribute to these
discrepancies.”
Some selected and simplified assignments on computed spectrum, relevant to the experimental
data, are made based on the orbital and population analyses for 1 (∆-Cu isomer) (Table 2)
[15,19,26,30]. A combined band resulting from metal d-d, metal-ligand (ML) and intra-ligand
(LL) transitions (for example) is found at 593 nm, close to the experimental band/shoulder at ca.
600 nm (Table 2), for HOMO to LUMO and HOMO-14 to LUMO transitions with MOs
contributions of 54 and 14 %, respectively. Frontier molecular orbitals HOMO, HOMO-14 and
LUMO, calculated at B3LYP/6-31g(d)//B3LYP/6-31G(d) in methanol, are shown in Fig. 4. The
HOMO orbital is localized on the π-electrons moieties of the lactam- and aryl-ring plus metal d-
moieties, while the LUMO is localized mainly on the π-moieties of the ary-ring with a very little
d-moieties. The energy gap between these two MOs are very narrow (∆Eg = 2.7x10−4 eV),
involves maximum transitions in the excitations (MOs >54%) [31f]. On the other hand, the
HOMO-14 is localized only on the π-electrons moieties of the lactam-ring (no metal d-moieties),
which transits to LUMO with a high energy gap (∆Eg = 6.10 eV) and involves low transitions in
the excitations (MOs <14%).

Fig. 3. Experimental electronic spectra for HL1 (0.07 mmol/L) and 1 (0.15 mmol/L) in methanol
at 25 oC, and calculated spectrum for 1 (∆-Cu isomer) at B3LYP/6-31G(d)//B3LYP/6-31G(d) in
methanol.
Table 2.
Some selected and simplified assignments on 1 (∆-Cu isomer), calculated at B3LYP/6-
31G(d)//B3LYP/6-31G(d) in methanol.
Oscillator
strength (f)
0.0007
MOs
Assignments c,#
λ/nm a
contributions (%) b
H-23 → L (47), H-16 → L (31)
628
MM, ML
593 (600sh)
535
0.0113
0.0153
0.1319
0.2094
0.0951
0.1431
H-14 → L (14), H → L (54)
H-1 → L+2 (55), H → L+1 (68)
H-3 → L (78)
MM, ML, LL
MM, ML, LL
MM, ML, LL
MM, ML, LL
ML, LL
446 (380sh)
323 (300sh)
282 (245)
269
H-7 → L+2 (45), H-3 → L+2 (10)
H-14 → L (29), H-10 → L (41)
H-12 → L+1 (12), H-10 → L+2 (32)
ML, LL
a
c
Experimental values are in parentheses; b H = HOMO and L = LUMO; MM = metal-centered
(d-d), ML = metal-ligand, LL= ligand-centered transitions; # β-spin MOs are considered.

Fig. 4. Frontier molecular orbitals (HOMO, HOMO-14 and LUMO) for 1 (∆-Cu isomer),
calculated at B3LYP/6-31g(d)//B3LYP/6-31G(d) in methanol (Isovalue 0.02).
3.3. 1H NMR studies: Lactam (L)
Enol (E) tautomerism
1H NMR spectra for the Schiff bases are shown in Figs. 5, 6 and spectral data are summarized
in the experimental section. As mentioned above, isatin-Schiff bases undergo a dynamic
tautomerization equilibrium between the lactam (L)- and enol (E)-tautomers in solution (Scheme
1), and exhibit peaks for both the tautomers simultaneously [26]. Thus, the methyl protons show
two singlets at
in HL2} within ca. 30 min of dissolution in CDCl3, which correspond to the existences of both
L- and E-tautomers simultaneously (Figs. 5, 6). The O- (enol) and N- (lactam) protons
appear as broad signals at 8.24 and 8.46 ppm in HL1 (or at 8.41 and 8.66 ppm in HL2),
respectively. The spectra further show several doublets, triplets and multiplets at the range of
δ 2.18 (85%) and 2.46 (15%) ppm in HL1 {or δ 2.43 (83%) and 2.28 (17%) ppm
H
H
δ
δ
δ
6.50 - 7.65 ppm, associated to the several aromatic protons for both forms (see experimental
section).

To study the dynamic tautomerism equilibrium between the lactam (L)- and enol (E)-forms
1
as a function of time, we run H NMR spectra at different time intervals of HL2 dissolution in
CDCl3 (Fig. 6). The results show that both the tautomers co-exit simultaneously at varying ratios
with dissolution time. Thus, immediately upon dissolution (within ca. 30 min), the major
tautomer is lactam-form (ca. 83%, based on peak integration value) with the minor enol-form
(ca. 17%). The lactam-to-enol ratios (L/E) become ca. 60/40 within 5 h and ca. 54/46 within 24
h of HL2 dissolution in solution. Indeed, no more change is noted upon further standing of this
solution, suggesting an equilibrium has been established with almost equal amount of both
tautomers in solution. IR spectra taken in chloroform also support these results as discussed
above. In accordance with IR spectra at solid state (where solely the lactam-form exits), we
assume the major tautomer to be lactam-form and the minor one to be enol-form immediately
upon dissolution (i.e., within 30 min). However, 1H NMR peaks assignments and percentages of
the lactam- and enol-forms are made based on corresponding peaks integration values for
C
H3/N-H (lactam) and O-H (enol) protons at different time intervals, respectively (Fig. 6).
Fig. 5. 1H NMR spectrum for HL1 in CDCl3 at 20 °C (L/E = Lactam-/Enol-form).

CDCl3
L (CH3)
30 min
L
E+L
E
L
L
L
E
E (CH3)
L (N-H) (83%)
E (O-H) (17%)
E
5 h
L (N-H) (60%)
E (O-H) (40%)

CDCl3
L (CH3)
E (CH3)
24 h
E+L
E
L
E
L
L
L
E
L (N-H) (54%)
E (O-H) (46%)
Fig. 6. 1H NMR spectra for HL2 at different time intervals in CDCl3 at 20 °C (L/E = Lactam-
/Enol-form).
3.4. Reaction of 1 with NaCN
To check the presence of the isatin-Schiff bases in paramagnetic [Cu(L1)2] (1), we conducted
a decomplexation reaction via reduction of Cu(II) to Cu(I) in presence of NaCN in dmso-d6
−
solution [15,21,23]. This reaction results in the formation of diamagnetic [CuI(CN)4]3 and
deprotonated Schiff base anion (L1−Na+) in solution, accompanies a colour change from dark-
brown to light-orange (Scheme 2). 1H NMR spectrum of this solution shows a singlet at
ppm for the methyl protons in L1−, while no peaks for the O-
(enol) and/or N-
protons, as evidenced in the free HL1 (Fig. 5). However, several peaks associated to the
aromatic protons are found at
6.50 - 7.70 ppm in L1−.
δ
= 2.11
H
H (lactam)
δ

Scheme 2. Reaction pathway between [Cu(L1)2] (1) and NaCN in dmso-d6.
3.5. Cyclic voltammetry
Cyclic voltammograms for 1 and 2 were taken at the range of −0.8 to 0.4 V vs. Ag/AgCl at
varying scan rates in acetonitrile at 25 °C, respectively (Figs. 7, S7). The forward scan shows two
strong broad cathodic peaks at ca. − 0.40 (Ic1) and ca. − 0.57 V (I
c2) at scan rate of 0.10 Vs-1,
correspond to two electrons charge transfer processes for [CuII(L)2]0/[CuI(L)2]− and
−
[CuI(L)2]−/[Cu0(L)2]2 couples (L− = deprotonated Schiff base), respectively [15,19,25,32]. On
the other hand, the reverse scan shows two corresponding anodic peaks at ca. 0.03 (
Ia1) and ca.
− 0.45 V (
I
a2), due to two electrons charge transfer processes for [Cu0(L)2]2−/[CuI(L)2]− and
[CuI(L)2]−/[CuII(L)2]0 couples, respectively. A poor reductive peak on the forward scan at 0.1 to
0.3 V, in particular with higher scan rates, is due to the electrolyte [21,32]. For comparison
studies, we run CV for the electrolyte (TBAP) and Schiff base (HL1) plus TBAP solution at the
range of −1.0 to 2.0 V vs. Ag/AgCl at scan rates of 0.10 Vs-1 in acetonitrile at 25 °C, respectively
(Fig. S8). The TBAP solution shows two pairs of redox peaks (
potential (0.5 to 1.0 V) [21,32], while the Schiff base plus TBAP solution shows extra two pairs
of redox peaks ( c1/ a1 and c2/ a2) at relatively higher potential (1.3 to 2.0 V). The results
Ic1'/Ia1' and Ic2'/Ia2') at lower
I
I
I
I
suggest that the observed redox peaks at the range of −0.05 to −0.60 V (Figs. 7, S7) are solely
−
due to [CuII(L)2]0/[CuI(L)2]− and [CuI(L)2]−/[Cu0(L)2]2 couples in complexes solution, while no
peaks at this range in Schiff base (HL1) plus TBAP solution (Fig. S8). Thus CV results indicate
two quasi-reversible one electron charge transfer processes for Cu(II)/Cu(I) and Cu(I)/Cu(0)
couples, respectively as reported for related copper(II)-Schiff bases complexes [15,19,25,32].
However, analyses of voltammograms for 2 at varying scan rates demonstrate that both the

cathodic and anodic peaks become more intense with shifting to higher and lower potentials,
respectively with faster scan rates. Indeed, a linear relationship between the cathodic peak current
(I
c2/µA) and square root of scans rate (ν1/2/Vs-1) (Fig. 8) corresponds to a diffusion-controlled
electrochemical process in acetonitrile.
Potential (V) vs. Ag/AgCl
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
90
60
30
0
0.02 [V/s]
Cu(I)/Cu(0) (Ic2)
0.05 [V/s]
0.10 [V/s]
0.15 [V/s]
0.20 [V/s]
Cu(II)/Cu(I)
(Ic1)
Cu(0)/Cu(I) (Ia2)
Cu(I)/Cu(II) (Ia1)
-30
3
3
Fig. 7. Cyclic voltammograms for 2 (0.5 mmol dm− ) containing TBAP (0.1 mol dm− ) at
varying scans rate (ν/Vs-1) in acetonitrile at 25 °C.

100
80
60
40
20
0
R² = 0.9914
0.00
0.20
0.40
0.60
ν1/2 (Vs-1)
Fig. 8. Plot of peak current (
I
c2/µA) vs. square root of scans rate (ν1/2/Vs-1) for 2 (0.5 mmol
3
3
dm− ) containing TBAP (0.1 mol dm− ) in acetonitrile at 25 °C.
3.6. Powder XRD studies
Powder XRD patterns (Fig. 9) for the Schiff base (HL2) and its complex (2) was recorded at
the range of 5−50 o (2
with various degrees of crystallinity [33,34,35]. Comparision studies of spectral patterns show
the absences of certain intense peaks at low 2
value (<10 o) in HL2, and the presences of several
θ) at ambient temperature, which correspond well defined crystalline nature
θ
new and/or slightly shiftted peaks in the complex. The results suggest coordination of the Schiff
bases to the metal(II) ion, and formation of the complex with different structure (phase).

1200
1000
800
600
400
200
0
HL2
2
5
15
25
35
45
2θ /o
Fig. 9. Powder XRD patterns for the HL2 and 2 at ambient temperature.
3.7. EPR spectra and paramagnetism
X-band EPR spectrum of 2 in dmso at 120 K (Fig. 10) shows an intense broad band at high field
region, having isotropic pattern due to tumbling motion of the molecules [12a,32,33,36]. There
might be some peak broadening effect originated from the dipolar interactions of close
paramagnetic centres. The copper(II)-complexes with one unpaired electron are essentially
paramagnetic, and show the magnetic moment values, µeff. = 1.52 (1) and 1.60
µB (2) in
methanol at 20 °C [19,32,33,36c], close to the spin-magnetic moment for a single electron.

0.6
0.2
2500
2800
3100
3400
3700
G/H
-0.2
-0.6
-1
Fig. 10. X-band EPR spectrum for 2 in dmso at 120 K.
4. Conclusions
The bis[3-(o/p-tolylimino)indole-2-olato-κ o/p-
2N,O]copper(II) are synthesized from 3-(
tolylimino)indolin-2-one via lactam (L)- to enol (E)-tautomerism in solution. Solid state IR
1
spectra show only the L-form, while both L- and E-forms in solution. H NMR results reveal a
dynamic tautomerization equilibrium between the L- and E-forms in solution, which changes
with time. CV results demonstrate two quasi-reversible one electron charge transfer processes for
Cu(II)/Cu(I) and Cu(I)/Cu(0) couples in acetonitrile, respectively. PXRD patterns correspond
well define crystalline nature with various degrees of crystallinity for the ligands and complexes.
X-band EPR spectrum shows an intense broad band at high field region with isotropic nature.
Magnetic moment values (µeff.) indicate paramagnetic nature of the copper(II)-complexes with an
unpaired electron. Optimized structures and excited state properties by DFT/TDDFT correspond
well to the experimental results both in solution and at solid state. The results could be
interesting for pharmacological and/or biological studies of the isatin-Schiff bases and their
copper(II)-complexes.

Acknowledgements
We acknowledge Wazed Miah Science Research Centre (WMSRC) at Jahangirnagar
University, Dhaka, Bangladesh for obtaining elemental, CV and PXRD data. We thank to
Professor C. Janiak, Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-
Universität Düsseldorf, Germany for ESI mass spectra. Our sincere thanks to T. E. Burrow,
Department of Chemistry, University of Toronto, Toronto, ON, Canada for running EPR
spectra. We also grateful to Professor ABP Lever, Department of Chemistry, York
University, Toronto, and the Computecanada.ca (https://ccdb.computecanada.ca/) Ontario,
Canada for using computational resources.
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Highlights
ꢀ 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2)
ꢀ Copper(II)-3-(o/p-tolylimino)indole-2-olate complexes (1 and 2)
ꢀ Lactam-Enol tautomerism in solution
ꢀ Redox potentials
ꢀ DFT/TDDFT calculations