Syntheses, spectroscopy, tautomerism, induced chirality and molecular structure of 3-(o/p-tolylimino)indolin-2-one and bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]-Δ/Λ-zinc(II)

Article abstract of DOI:10.1016/j.ica.2019.119245

The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) undergo tautomerization from lactam (L)- to enol (E)-form in solution, which in turn reacts with zinc(II) nitrate to give bis[3-(o/p-tolylimino)indole-2-olato-κ²N,O]-Δ/Λ-zinc

Full text of DOI:10.1016/j.ica.2019.119245

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Research paper
Syntheses, spectroscopy, tautomerism, induced chirality and molecular struc-
ture of 3-(o/p-tolylimino)indolin-2-one and bis[3-(o/p-tolylimino)indole-2-ola-
2
to-κ N,O]-Δ/Λ-zinc(II)
Mohammed Enamullah, Mohammad Khairul Islam, Simon-Patrick Höfert,
Christoph Janiak
PII:
S0020-1693(19)31457-4
https://doi.org/10.1016/j.ica.2019.119245
ICA 119245
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
21 September 2019
29 October 2019
31 October 2019
Please cite this article as: M. Enamullah, M.K. Islam, S-P. Höfert, C. Janiak, Syntheses, spectroscopy, tautomerism,
induced chirality and molecular structure of 3-(o/p-tolylimino)indolin-2-one and bis[3-(o/p-tolylimino)indole-2-
2
olato-κ N,O]-Δ/Λ-zinc(II), Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.119245
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Syntheses, spectroscopy, tautomerism, induced chirality and molecular structure of 3-(o/p-
2
tolylimino)indolin-2-one and bis[3-(o/p-tolylimino)indole-2-olato- N,O]-/-zinc(II)
a,*
b
Mohammed Enamullah , Mohammad Khairul Islam, Simon-Patrick Höfert, Christoph Janiak
a
Department of Chemistry, Jahangirnagar University, Dhaka-1342, Bangladesh. E-mail:
enamullah@juniv.edu
b
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität,
Universitätsstr. 1, D-40225 Düsseldorf, Germany.

Graphical Abstract
Syntheses, spectroscopy, tautomerism, induced chirality and molecular structure of 3-(o/p-
2
tolylimino)indolin-2-one and bis[3-(o/p-tolylimino)indole-2-olato- N,O]-/-zinc(II)
a,*
Mohammed Enamullah , Mohammad Khairul Islam, Simon-Patrick Höfert and Christoph
Janiak b
Isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one undergo tautomerism between the lactam (L)-
and enol (E)-forms in solution, which in turn reacts with zinc(II) nitrate to give bis[3-(o/p-
2
tolylimino)indole-2-olato-κ N,O]-/-zinc(II) (1 and 2).

Highlights
2





Bis[3-(o/p-tolylimino)indole-2-olato- N,O]-/-zinc(II)
Induced chirality ( and ) at-metal center
Lactam-Enol tautomerism
Hirshfeld surfaces analyses
DFT/TDDFT calculations

Abstract
The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) undergo
tautomerization from lactam (L)- to enol (E)-form in solution, which in turn reacts with zinc(II)
2
nitrate to give bis[3-(o/p-tolylimino)indole-2-olato-κ N,O]-/-zinc(II) (1 and 2). IR spectra for
1
the Schiff bases suggest solely the lactam-form at solid state, while both the forms in solution. H
NMR spectra show the peaks associated to both the lactam- and enol-forms in solution.
Molecular structure for HL1 explores two symmetry independent molecules (A and B) in an
asymmetric unit due to intermolecular O···H and N···H hydrogen bonding interactions. The
molecule crystallises with monoclinic P2 /n space group. Supramolecular analyses suggest that
1
the packing in the crystals is organized, in addition to hydrogen bonds, by a few intra- and inter-
molecular C-H… contacts. Studies of Hirshfeld surfaces using CrystalExplorer strongly support
1
the quantitative analyses by supramolecular packing in the crystals. H NMR spectra for 1 or 2
show two sets of peaks for each proton, correspond to the -Zn and -Zn configured isomers at
1
a ratio of ca. 65:35, resulting from induced chirality at-metal centre. H NMR spectra taken at
different time intervals rule out any possibility of interconversion between the isomers in
solution, that is, the same ratio of the isomers exists at solid state. These results indicate
preferential formation one isomer over another (i.e., -Zn or -Zn) both in solution and at solid
state. Optimized structures and excited state properties by DFT/TDDFT correspond well to the
experimental spectra.
Keywords: Induced chirality ( vs. ); -Zn and -Zn isomers; Zinc(II)-isatin Schiff bases;
Lactam- and Enol-tautomerism; DFT/TDDFT.
1
. Introduction
The transition metal(II)-complexes with chiral/achiral-asymmetric bidentate ligands in
tetrahedral to distorted square-planar geometry exhibit induced chirality at-metal center and
provide the right - and left ()-handed configured isomers (i.e., opposite configuration at
metal) along the C -symmetry of the molecule [1-2]. Use of an achiral ligand A^B gives the
2

isomers of non-planar -M(A^B) and -M(A^B) configured complexes. The enantiopure
2
2
ligands (R or S-A^B) provide the diastereomeric pairs of -M(R-A^B) and -M(R-A^B) {or
2
2

-M(S-A^B) and -M(S-A^B) }, while the racemic ligands (R/S-A^B) gives all four
2
2
diastereomers. It has well been documented that the non-covalent interactions acting within the
metal-ligands co-ordination sphere lead to a free energy difference between the -M and -M
isomers and hence one of the isomers is thermodynamically and/or kinetically preferred [3-13].
The phenomenon can efficiently control by ligand design and/or chirality, metal ion selection
and solute-solvent interactions, respectively [3-13]. In addition, the inter- and intra-molecular
interactions such as hydrogen bonds, … and CH… contacts in the crystal packing direct the
induced chirality very selective way and provide the -M and/or -M isomers [9,13].
We have investigated the phenomenon in details using the four-coordinated, non-planar and
C -symmetric transition metal(II)-complexes with chiral Schiff bases ligands, [M(R- or S-
2
N^O) ] (M = Fe, Co, Ni, Cu, Zn; N^O = deprotonated Schiff bases anion = salicylal- or
2
naphthal-diminate) both in solution and at solid state [3-13]. Our results unequivocally
demonstrate the preferred formation of the -R-M or -S-M isomer and vice versa (i.e., -S-M
or -R-M) as evidenced by X-ray structural analyses, electronic/vibrational circular dichroism
1
(
ECD/VCD), H NMR studies, thermal analyses and DFT/TDDFT calculations, respectively.
The solid-state X-ray structures determinations demonstrate that the co-ordination of enantiopure
R or S-ligands (or racemic R/S-ligands) to the metal ion diastereoselectively gives the
enantiopure crystals with absolute configuration of Λ-R-M or -S-M (or racemic crystals with
Λ/-R/S-M or /Λ-R/S-M configuration) and vice versa. Though the induced chirality is
quantitative at solid-state, it becomes rather complicated in solution, where a dynamic
interconversion equilibrium prevails between Δ-R-M and Λ-R-M (or Δ-S-M and Λ-S-M)
isomers. Use of racemic ligands show an equilibrium between the Λ/-R/S-M and /Λ-R/S-M
isomers and vice versa. However, if the equilibrium process is considerably slow, it can easily be
1
followed by running variable time and temperature dependent H NMR spectra for the
diamagnetic complexes, respectively. Thus, the equilibrium shifts from a single-isomer of Λ/-
R/S-M (i.e., 100% within 10 min of complex dissolution) to a ratio of 33:67 (/Λ-R/S-M:Λ/-
R/S-M within 40 min) or 46:54 (within 36 h) for the racemic zinc(II)-(R/S)-aminoalcohol based
Schiff bases complexes [3]. In contrary, similar studies result in no shifts of equilibrium with

time for enantiopure zinc(II)-(R or S)-naphthaldimine complexes [6]. Indeed, variable
1
temperature (VT) H NMR studies on the later complexes show temperature-dependent shifts of
o
o
the equilibrium with a ratio of 87:13 (-S-M:-S-M) at 50 C to 73:27 at 35 C in solution [6].
Indeed, studies of VT electronic circular dichroism (ECD) spectra reveal the parallel results in
Co/Zn(II)-(R or S)-salicylal-/naphthal-dimine complexes in solution [6,11].
We have reported two examples of induced chirality at-metal center, so far, in Zn(II)-
complexes with aciral Schiff bases ligands, which provide both - and -configured complexes
in a single crystals [9,13]. The achiral Schiff-bases, N-2-(pyridyl)salicylaldimine react with the
zinc(II) salts to provide the enantiomorphous chiral at-metal centered bis[N-2-
2
(
pyridyl)salicylaldiminato- N,O]--zinc(II), which crystallizes as a racemic conglomerate
via spontaneous resolution [9]. The - and -configured complexes assemble in P (right)- and M
left)-handed 4 - and 4 -helical chains via weak intermolecular CH···O hydrogen bonds.
(
1
3
Further, only the same - or -configured molecules are combined into a helical chain, and only
the chains of the same P- or M-handedness are combined with the corrugated van-der-Waals
surfaces through weak interactions to form the homochiral crystals [9]. Another example of
induced - and -chirality is reported in bis[N-2-(R-pyridyl)-2-oxo-1-naphthaldiminato-
2
N,O]Λ- and Δ-zinc(II), (R = H, 4/6-CH ) by achiral Schiff bases ligands [13]. The
3
supramolecular packing analysis exhibit strong  interactions resulting an inversion-
symmetric complexes pair with opposite Λ- and Δ-configurations at-metal center.
In view of further investigations along induced - and Λ-chirality, we have synthesized the
isatin-Schiff bases and their complexes with transition metal ions [14,15]. The isatin-Schiff
bases readily react with the copper(II) ion to give the bis[3-(o/p-tolylimino)indole-2-olato-
2
4
κ N,O]copper(II) [15]. Similar reaction with the dinuclear [Rh(µ-O CMe)( -cod)] (cod = 1,5-
2
2
4
cyclooctadiene) provides the mononuclear [Rh( -cod){3-((R)-(Ar)-ethylimino)-3H-indol-2-
olato}] [14]. The present endeavor, in continuation, reports the results of syntheses,
spectroscopy, tautomerism, induced chirality and X-ray molecular structure of the isatin-Schiff
bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) and bis[3-(o/p-tolylimino)indole-2-olato-
κ2N,O]-/-zinc(II) (1 and 2), respectively. The optimized structures and excited state
properties by DFT/TDDFT are employed to get detailed insights into experimental results.

2
. Experimental
2
.1. Materials and measurements
UV-Vis. spectra were obtained with the Shimadzu UV 1800 spectrophotometer in methanol at
2
5 °C. Nicolet iS10 (Thermo Scientific) spectrometer was used to run IR spectra at solid state
(
KBr discs) and in CHCl at ambient temperature. Elemental analyses were performed on a
3
1
VarioEL from the Elementaranalyensysteme. H NMR-spectra were recorded on Bruker Avance
4
00 spectrometer operating at 400 MHz in CDCl at 20 °C.
3
2
.2. Syntheses of the Schiff bases (HL1 and HL2)
An equimolar amount of indoline-2,3-dione (isatin) and o/p-toludine were dissolved in 10 mL
methanol. Three drops of concentrated H SO was added into this mixture solution, which was
2
4
then refluxed for 6-8 hours. Colour of the solution changed to orange-yellow. Afterwards,
reduced the volume of the solution to ca. 50% in vacuo and left standing the solution for
crystallization via. slow evaporation of solvent at room temperature. Orange-yellow micro
crystals were formed after 2-3 days, filtered off and washed the crystals with methanol (2 ml in
each) for three times. Dried the product in air for 3-4 days and obtained micro crystals of the
Schiff bases, 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2). X-ray quality single crystals for
HL1 are obtained by slow evaporation of concentrated methanol solution after 4-5 days.
2
.2.1. 3-(o-tolylimino)indolin-2-one (HL1):
–
1
Yield: 8.520 g (82%). IR (KBr, cm ): 3454br (νH O), 3259s (νN-H), 3184, 3087, 2914w (νC-H),
2
–
1
1
3
3
2
7
742, 1724vs (νC=O) and 1613, 1593vs (νC=N). IR (CHCl , cm ): 3693, 3683, 3609w (νO-H),
3
–
1
439s (νN-H), 1746s, 1734sh (νC=O) and 1619s, 1603sh (νC=N). IR (ATR, cm ): 3234w (νN-H),
1
066w (νC-H), 1744, 1717vs (νC=O) and 1610, 1593vs (νC=N). H NMR (400 MHz, CDCl ):  =
3
.18 (s, 3H, CH -L, 93%), 2.38 (s, 3H, CH -E, 17%), 6.52 (d, J = 7.6 Hz, 1H, H -L), 6.67 (t, J =
3
3
14
.6 Hz, 1H, H -L), 6.87 (d, J = 7.6 Hz, 1H, H -L), 6.97 (d, J = 7.8 Hz, 1H, H -L), 7.00 (m, 1H,
1
3
11
8
H -E), 7.15 (m, 2H, H
-E), 7.19 (t, J = 7.2 Hz, 1H, H -L), 7.27 (t, J = 7.2 Hz, 1H, H -L), 7.32
12 6
1
4
11,13
(
d, J = 7.6 Hz, 2H, H6,12-E+2H, H -L), 7.33 (d, J = 7.5 Hz, 1H, H -E), 7.56 (t, J = 7.6 Hz, 1H, H -
5,7 8 7
E), 7.62 (d, J = 7.5 Hz, 1H, H -E), 9.35 (br, 1H, OH-E, 8%), 9.62 (br, 1H, NH-L, 92%) (L/E=
5
lactum-/enol-form; for proton numbering see the Scheme 1).

2
.2.2. 3-(p-tolylimino)indolin-2-one (HL2):
–
1
Yield 9.10 g (83%). IR (KBr, cm ): 3451br (νH O), 3241s (νN-H), 3067s, 2920w (νC-H), 1748vs,
2
–
1
1
720s (νC=O) and 1612vs, 1594sh (νC=N). IR (CHCl , cm ): 3689, 3676, 3610w (νO-H), 3439s
3
–
1
(
νN-H), 1745s (νC=O) and 1616s, 1595sh (νC=N). IR (ATR, cm ): 3252w (νN-H), 3167, 3022w
1
(
νC-H), 1740, 1722vs (νC=O) and 1609, 1591vs (νC=N). H NMR (400 MHz, CDCl ):  = 2.28 (s,
3
3
H, CH -E, ca. 15%), 2.43 (s, 3H, CH -L, ca. 85%), 6.79 (t, J = 7.6 Hz, 1H, H -L), 6.83 (t, J = 7.2
3 3 6
Hz, 1H, H -L), 6.95 (d, J = 8.0 Hz, 1H, H -L), 6.98 (d, J = 8.0 Hz, 4H, H11-12,14-15-L), 7.06 (m, 1H,
5
8
H -E), 7.13 (t, J = 7.6 Hz, 1H, H -E), 7.26 (d, J = 8.0 Hz, 4H, H11-12,14-15-E), 7.33 (t, J = 7.2 Hz, 1H,
8
6
H -L), 7.57 (t, J = 7.6 Hz, 1H, H -E), 7.63 (d, J = 7.6 Hz, 1H, H -E), 8.91 (br, 1H, OH-E, 18%) and
7
7
5
9
.19 (br, 1H, NH-L, 82%).
2
.3. Syntheses of the complexes (1 and 2)
Two equivalents of isatin-Schiff base 3-(o/p-tolylimino)indolin-2-one (HL1 or HL2) (0.236 g,
1
mmol) were dissolved in 10 ml of methanol and stirred the solution for 10 min. Added one
equivalent of Zn(II) nitrate (0.131 g, 0.5 mmol), dissolved in 10 mL methanol, into this solution.
Afterwards, two equivalents of NaHCO (dissolved in 5 ml methanol) poured into the mixture
3
solution and refluxed the solution. An orange-yellow precipitate was started in the solution with
2
-3 hours of reflux. After 24 hours reflux, cooled the reaction mixture to room temperature, and
left standing this solution for complete precipitation. Filtered off the precipitate and washed two
times with methanol (2 mL in each) followed by diethyl ether (2 mL in each), respectively. The
2
orange-yellow micro crystals of bis[3-(o/p-tolylimino)indole-2-olato-κ N,O]zinc(II) (1 or 2)
o
were obtained after drying the products in oven at 40 C for 24 hours.
2
2
.3.1. Bis[3-(o-tolylimino)indole-2-olato-κ N,O]zinc(II), (1):
–
1
Yield: 0.215 g (73%). IR (KBr, cm ): 3067, 2923w (νC-H), 1733s (νC-O) and 1633, 1593, 1584vs
–
1
1
(
νC=N). IR (ATR, cm ): 3065, 2930w (νC-H), 1734s (νCO) and 1634, 1591vs (νC=N). H NMR
(
400 MHz, CDCl ):  = 2.17 (s, 3H, CH -, ca. 85%), 2.20 (s, 3H, CH -, ca. 15%), 6.54 (d, J =
3
3
3
8
.0 Hz, 1H, H -), 6.77 (t, J = 7.6 Hz, 1H, H -), 6.86 (d, J = 7.6 Hz, 1H, H -), 6.90 (d, J = 8.0
1
4
13
11
Hz, 1H, H -), 6.91 (d, J = 8.0 Hz, 1H, H -), 7.17 (m, 1H, H -), 7.18 (t, J = 7.2 Hz, 1H, H -),
8
14
11
12
7
.26 (t, J = 7.6 Hz, 1H, H -), 7.28-7.32 (m, 4H, H -, H
-), 7.35 (d, J = 7.6 Hz, 1H, H -),
6
7
6,12,13
8

7
.59 (t, J = 7.6 Hz, 1H, H -), 7.65 (d, J = 7.6 Hz, 1H, H -), 7.83 (br, 1H, H -) (for proton
7
5
5
numbering see the Scheme 1).  C H N O Zn3H O (589.96): Calcd C 61.08, H 4.78, N 9.50;
3
0
22
4
2
2
found C 60.80, H 4.60, N 9.30.
.3.2. Bis[3-(p-tolylimino)indole-2-olato-κ N,O]zinc(II), (2)}:
2
2
–
1
Yield 0.210 g (71%). IR (KBr, cm ): 3058, 2923w (νC-H), 1735s (νCO) and 1637, 1594, 1580vs
(
νC=N). H NMR (400 MHz, CDCl ):  = 2.31 (s, 3H, CH -, 45%), 2.43 (s, 3H, CH -, ca. 55%),
1
3
3
3
6
7
.26 (m, 2H, H11,15-), 6.79 (t, J = 7.6 Hz, 1H, H -), 6.83 (t, J = 7.6 Hz, 1H, H -), 6.91 (t, J =
11
15
.6 Hz, 1H, H -), 6.97 (d, J = 8.0 Hz, 2H, H12,14-), 7.11 (t, J = 7.6 Hz, 1H, H -), 7.14 (t, J = 7.6
6
6
Hz, 1H, H -), 7.26 (d, J = 8.0 Hz, 2H, H12,14-), 7.33 (t, J = 7.6 Hz, 1H, H -), 7.58 (m, 1H, H -
7
7
8

), 7.64 (d, J = 7.6 Hz, 1H, H -), 7.87 (d, J = 8.0 Hz, 1H, H -), 7.99 (br, 1H, H -) (for proton
5
8
5
numbering see the Scheme 1).  C H N O Zn3H O (589.96): Calcd C 61.08, H 4.78, N 9.50;
3
0
22
4
2
2
found C 60.70, H 4.67, N 9.25.
.4. Single crystal X-ray diffraction
Single crystal of HL1 was carefully selected under a polarizing microscope and mounted on a
2
loop. Data collection: Bruker APEX II CCD diffractometer with graphite- or multi-layer mirror
mono-chromated Mo-K radiation (λ = 0.71073 Å). Data collection and cell refinement with
α
APEX2 [16] data reduction with SAINT (Bruker) [17]. Structure analysis and refinement: The
structure was solved by direct methods (SHELXS-97) [18]; refinement was done by full-matrix
2
least squares on F using the SHELXL-97 program suite, empirical (multi-scan) absorption
correction with SADABS (Bruker) [19]. All non-hydrogen positions were refined with
anisotropic temperature factors. The hydrogen atoms for aromatic CH were positioned
geometrically (CH = 0.95 Å) and refined using a riding model (AFIX 43) with U (H) = 1.2U .
iso
eq
The hydrogen atoms for CH were positioned geometrically (CH = 0.98 Å) and refined using a
3
riding model (AFIX 137) with U (H) = 1.5U . The hydrogen atoms for N-H were found and
iso
eq
refined free with U (H) = 1.5U . The tolyl rings are disordered over two positions with about
iso
eq
equal occupancy. The carbon atoms of these disordered tolyl rings have only been refined
isotropically. Details of X-ray data and refinements are listed in Table 1. Graphics were drawn
with the DIAMOND (Version 4.4) [20]. The structural data have been deposited with the

Cambridge Crystallographic Data Center (CCDC-numbers 1922815). These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif.
Table 1. Crystal data and structure refinement for HL1.
Empirical formula
C H N O
15 12 2

1
M (g mol )
236.27
Crystal size (mm)
Temperature (K)
θ range (°)
0.23 × 0.23 × 0.23
140
1.6 to 27.7
h; k; l range
±19; ±12; 21, +22
monoclinic
Crystal system
Space group
a, b, c (Å)
P2 /n (14)
1
15.1636(14), 9.9282(9), 16.9957(15)

(deg.)
107.400(4)
2441.6(4)
8
3
V (Å )
Z
Dcalc [g/cm ]
-3
1.285

1
F(000)/μ (mm )
992/0.082
1.000/0.845
18751, 0.037
5628
Trans. (max/min)
Reflect. collected, R (int)
Independent reflections
Obs. reflections [I > 2 (I)]
Data / restraints / parameters
4108
5628/0/323
0.37/0.23
0.0538/0.1352
0.0785/0.1515
1.026

3
Largest diff. peak and hole ( /e Å )
R /wR [I > 2(I)]
R /wR (all reflect.)
Goodness-of-fit on F2 b
a
1
2
a
1
2
 a R = [(║F ││F ║) /  │F │]; wR = [ [w(F F ) ]/[w(F ) ]] .
2
2 2
2 2 1/2
1
o
c
o
2
o
c
o
 b Goodness-of-fit = [ [w(F F ) ] /(np)] .
2
2 2
1/2
o
c
2
.5. Computational method
A through computational procedure was performed with the Gaussian 09 software [21] to get
further insight and to rationalize the experimental results. As mentioned above, C -symmetrical
2
[
M(N^O) ] (N^O = deprotonated Schiff base anion) complexes with distorted tetrahedral
2
geometry exhibit induced chirality at-metal center and provide the (right)- and (left)-handed

isomers [5-12]. Thus, we optimized two possible isomers having -Zn and -Zn configured
structures at B3LYP/6-31G(d) for 1 or 2, respectively (Fig. 1). The excited state properties by
TDDFT were employed with different combinations of the functionals B3LYP, cam-B3LYP and
m06, and the basis sets TZVP or SDD on the optimized structures for both the -Zn and -Zn
isomers, respectively for 2 [6-7]. Electronic spectra for the -Zn and -Zn isomers for 2 were
calculated incorporating Polarization Continuum Model (PCM) using methanol as solvent. 72
excited states (roots) were considered for calculations. A simple and simplified assignments on
the excited state properties and molecular orbitals (MOs) calculations were made at the same
level of theory. The spectra were generated using the program SpecDis [22] by applying
Gaussian band shape with exponential half-width σ = 0.33 eV.
3
. Results and discussion
Reaction of the indoline-2,3-dione (Isatin) with the o/p-toludine under reflux affords the
isatin-Schiff bases of 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2). The Schiff bases undergo
tautomerism between the lactam (L)- and enol (E)-forms in solution, which in turn react with the
zinc(II) nitrate in presence of NaHCO under reflux to give the bis[3-(o/p-tolylimino)indole-2-
3
2
olato-κ N,O]-/-zinc(II) (1 or 2) (Scheme 1).

2
Scheme 1: Syntheses of the bis[3-(o/p-tolylimino)indole-2-olato- N,O]-/-zinc(II) (1 or 2).
3
.1. Vibrational spectra
Vibrational spectra of the isatin-Schiff bases (as KBr disc) show very strong bands at 3259
−
1
−1
(
HL1)/3241 (HL2) cm for N-H, and at 1742, 1724 (HL1)/1748, 1720 (HL2) cm for C=O,
correspond to the lactam (L)-form at solid state (Fig. S1) [14-15]. The isatin-Schiff bases
undergo tautomerization between the lactam- and enol-forms (Scheme 1) and exhibit bands for
both the forms simultaneously in solution [14-15]. To check this, we run the spectra in
−
1
chloroform solution, which show weak bands at ca. 3690, 3610 cm (νO-H) corresponding to
-1
-1
the enol-form, while the bands at ca. 3439 cm (νN-H) and ca. 1743, 1728 cm (C=O)
corresponding to the lactam-form (Fig. S2). The ratio of the bands areas for the lactam to enol
1
forms is ca. 3:2, comparable to the H NMR results in solution (discussed below). The spectra
-1
-1
further show very strong bands at ca. 1612, 1593 cm (or ca. 1618, 1602 cm in CHCl ) for
3

-1
νC=N, which shift to ca. 1635, 1595 cm upon co-ordination to the metal ion in 1 or 2. The
spectra for the complexes (1 and 2), taken as KBr disc (Fig. S1), display a strong broad band at
-1
ca. 3450 cm may be due to the νH O (moisture from KBr) or to νN-H and/or νO-H bands from
2
unreacted Schiff bases (if any in the sample). To resolve the issue, we run ATR-IR spectra for
the complexes (Fig. S3), which show no bands for the νH O, νN-H and/or νO-H. The results
2
suggest that there is no free Schiff base in the sample. The spectra further show a weak band at
-1
−1
1
734 cm for the C-O and very strong bands at 1634, 1591 cm for the νC=N (Fig. S3).
However, since the complexes are formed via deprotonation of the enolic-proton (resulting from
tautomerism in solution), the νN-H and νO-H (or C=O) bands are obviously absent in their
spectra (Figs. S1 and S3). The ATR spectra for the Schiff bases (Fig. S3) reveal no band for the
νH O (as observed in Fig. S1, taken as KBr disc), rather display weak band at ca. 3234
2
-1
(
HL1)/3252 (HL2) cm for the N-H, while very strong bands at 1744, 1717 (HL1)/1740, 1722
-1
−1
(
HL2) cm for the C=O and at 1610, 1593 (HL1)/1609, 1591 (HL2) cm for the νC=N. The
overall results of the vibrational spectra strongly indicate the formation of the complexes as
depicted in the reaction Scheme 1.
3
.2. X-ray molecular structure and packing analyses for HL1:
X-ray molecular structure for HL1 is shown in Fig. 1, and some selected bond lengths and
angles are listed in Table 2. The molecule crystallises with monoclinic P2 /n space group. There
1
are two symmetry independent molecules (A and B) in the asymmetric unit due to the
intermolecular hydrogen-bonding interactions (Fig. 1). The molecules A and B differ most
significantly in their relative orientations of the aryl- and lactam-rings, which is reflected by the
angles between the two planes of the aryl- and lactam-rings ( /). In molecule A, the two rings

are rotated by 69.9/70.0  ( ) with respect to one another, while the two rings are approximately

perpendicular to one another with an angle of 88.1/86.5  ( ) in molecule B (Table 2). This

difference in orientations arises from the intermolecular hydrogen-bonding interactions and C-
H… contacts in the crystals (Figs. 1 and 2). The values of bond lengths C2-N2/C17-N4
(1.2762/1.2729 Å) indicate the carbon-nitrogen double bond character, while the bond angles
2
C2-N2-C9 (117.74/119.85 ) and C17-N4-C24 (119.07/117.58 ) represent a sp -hybridization of
the imino-nitrogen atom. Indeed, the observed bond lengths and angles are comparable to the
optimized structures by DFT (Table 2), and also to the X-ray results for the related chiral isatin-

Schiff bases ligands [14]. The X-ray results clearly elucidate the structure of HL1 to be the
lactam-form.
Fig. 1. Molecular structures within the crystal of HL1, showing two symmetry-independent
molecules (A and B) in the asymmetric unit, and the hydrogen-bonding interactions between A
and B.
Table 2. Some selected bond lengths (Å) and angles () in HL1.
Bond
lengths Molecule
A
Bond lengths Molecule
B
HL1
HL2
1 (cal.)2
2 (cal.)2
(HL1) (exptl.) (Å)/angles ()
(HL1) (exptl.) (cal.)1
(cal.)1
1.2422
1.4006
1.4107
1.5395
1.2935
1.4121
(Å)/angles ()
C1-O1
1.2114(25)
1.3619(21)
1.4127(28)
1.5303(27)
1.2762(20)
1.4136(57)/1.
C16-O2
C16-N3
C23-N3
C16-C17
C17-N4
N4-
1.2161(26)
1.3554(22)
1.4092(27)
1.5198(28)
1.2729(20)
1.4038(50)/1.
4728(71)
1.2423
1.4001
1.4114
1.5392
1.2925
1.4125
1.2869
1.3038
1.4115
1.5424
1.2921
1.4286
1.2864
1.3043
1.4081
1.5458
1.2949
1.4170
C1-N1
C8-N1
C1-C2
C2-N2
N2-C9A/C9B
4
672(60)
C24A/C24B
Zn-O1/O1'
Zn-N2/N2'
C2-N2-
C9A/C9B
N2-Zn-N2'
O1-Zn-O1'
O1-Zn-N2'/N2-
Zn-O1'
1.9398
2.0637
123.07
1.9398
2.0637
126.49
117.74/119.85 C17-N4-
C24A/C24B
119.07/117.58 127.78
127.62
120.15
127.49
119.72
126.68
131.22
115.15
N2-Zn-O1/N2'-
Zn-O1'
86.88
86.88



 a
 b
69.91/70.01
88.10/86.51
57.12
54.38
69.95
86.56
48.12
77.66

1
2
a
b
Calculated from optimized structure at B3LYP/LANL2DZ [15];
Calculated from optimized structure at B3LYP/6-31G(d) for -Zn isomer;


/ = Angles between the two planes of the aryl- and lactam-rings.
/ = Dihedral angles between the two planes formed by the chelating donor atoms with
the metal ion (i.e., O1-Zn-N2 and O1'-Zn-N2').
Packing analyses for HL1 explore some intermolecular hydrogen bonding interactions and
CH··· contacts, a common feature found in the isatin-Schiff bases at solid state [14,23,24, 25].
There are three hydrogen bonding interactions between the H3(N3) and O1 [H3-O1 2.37 Å] {or
i
i
i
H1(N1) and O2 [H1-O2 2.41 Å]}, H3(N3) and N2 [H3-N2 2.38 Å] {or H1(N1) and N4 [H1-
i
i
i
N4 2.35 Å]} and H22(C22) and O1 [H22-O1 2.48 Å] {or H7(C7) and O2 [H7-O2 2.44 Å]},
respectively. Details of the hydrogen bonds are given in Tables 3 (Fig.1). Packing in the crystals,
in addition to the hydrogen bonds, is further organized by a few CH··· contacts [26], whereas
there are no  contacts [27] even in the presences of extended aromatic rings. Thus, there are a
few intra- and inter-molecular CH··· contacts, respectively in the crystals (Figs. 2, S6 and
Table S1).
Fig. 2. Intramolecular C-H··· contacts along with N-H···O and C-H···O hydrogen bonding (as
dashed orange lines) in HL1. Details of these contacts are given in Tables S1 (Cg = ring
centroid).

Table 3. Hydrogen bonding parameters in HL1
D—H···A
D—H (Å)
H···A (Å)
D···A (Å)
D—H···A (º)
N3—H3···O1
N3—H3···N2
N1—H1···O2i
N1—H1···N4i
C7—H7···O2i
0.81 (3)
0.81 (3)
0.88 (2)
0.88 (2)
0.95
2.37 (2)
2.38 (3)
2.41 (2)
2.35 (2)
2.44
3.046 (2)
3.064 (2)
3.135 (2)
3.123 (2)
3.201 (2)
3.221 (3)
141 (2)
143 (2)
140 (2)
147 (2)
136
C22—H22···O1 0.95
2.48
135
Symmetry code: (i) x-1/2, -y+1/2, z-1/2.
3
.3. Hirshfeld surfaces analyses
Studies of Hirshfeld surface using the CrystalExplorer [28] strongly support the quantitative
analyses of the intermolecular interactions and contacts resulting from supramolecular packing in
the crystals [29]. Indeed, the graphical presentation of the Hirshfeld surface with 2D fingerprint
plot (Fig. 3a) is an overlay of different contributions resulting from close interactions in the
crystals. The plot illustrates O···H and N···H hydrogen bonds as two sharp tails pointing to the
lower left, respectively. The upper sharp one represents to the H-bond donor and lower one to the
acceptor [2b]. The plot, in addition, reveals two wings at lower middle position for CH….
contacts. Hirshfeld surface mapped with dnorm property (Fig. 3b) [2c] clearly shows the hydrogen
bonding interactions and CH…. contacts as discussed in packing analyses above. The red
circles represent the closest contacts (i.e., shorter than the Van der Waals contact), while blue
represent the most distant contacts (i.e., longer than the Van der Waals contact) on the dnorm
surface (Fig. 3b). Breakdown of the Hirshfeld surfaces (fingerprint plot and dnorm property) into
the contributions from C···C (···, 1.0%), C···H (C–H···, 31.3%), O···H (10.8%), N···H
(4.6%) and H···H (50.4%) interactions, respectively are shown Fig. S7.

Fig. 3 (a) Graphical presentation of the Hirshfeld surface with 2D fingerprint plot with
characteristic features due to CH···O (red arrows), CH···N (blue arrows) and CH···
(
green arrows) interactions (d and d are the distances from the surface to the nearest atom
i e
interior and exterior to the surface, respectively), (b) Hirshfeld surface mapped with dnorm
property (the red spots represent the closest contacts and blue the most distant interactions.
3
.4. DFT optimized structures
The optimized structures at B3LYP/6-31G(d) (Fig. 4) demonstrate both the -Zn and -Zn
configured isomers are of equi-energetic for 1 or 2. However, compound 2 is relatively stable
than 1 by ca. 2.12 Kcal/mol due to less steric hindrance experienced by the p-tolyl group in the
co-ordination sphere. The calculated bond lengths and angles for 1 or 2 and isatin-Schiff bases
(HL1 or HL2) are comparable to the X-ray values for HL1 with the lactam-form (Table 2). The
angles ( /) between the two planes of the aryl- and lactam-rings are significantly smaller (i.e.,

5
6
7.1/54.4  in HL1/HL2 or 69.95/48.12 in 1/2) in compare to the X-ray values in HL1 (i.e., ca.
9.9/70.0  in A or ca. 88.1/86.5  in B) due to the presence of intermolecular hydrogen bonds
and CH…. contacts in the crystal, as mentioned above [14]. These interactions are obviously
absent in the optimization structures at gas phase. The interactions in the crystal packing squeeze
the bond angles C2-N2-C9A/C9B (ca. 117.7/119.9  in A) and C17-N4-C24A/C24B (ca.
1
19.1/117.6  in B) in comparison to those of the calculated values (ca. 127.8/127.6  in

HL1/HL2 or ca. 123.1/126.5  in 1/2). However, the dihedral angles ( ) between two planes
formed by the chelating donor atoms with the metal ion (i.e., planes O1-Zn-N2 and O1'-Zn-N2')
provide quantitative measure of the geometry for the complexes [4,6,11,12]. Thus, the dihedral
angles ( ) values of 86.6  (1) and 77.7  (2) explore the distorted tetrahedral geometry for the
complexes.
(2, -isomer; ca. 2.12 Kcal/mol)
(1, -isomer)
Fig. 4. DFT optimized structures for 1 and 2, calculated at B3LYP/6-31G(d), respectively.
3
.5. Electronic and computed spectra
Electronic spectra for the isatin-Schiff bases and their complexes in methanol are identical
(Fig. 5), and feature strong bands below 340 nm with absorption maxima (max) at 248 nm { =
3
-1
-1
3
-1
-1
2
1629 (HL1), 34035 dm mol cm (2)} and at 292 nm { = 4462 (HL1), 7079 dm mol cm
*
*
(
2)} due to the intra-ligand  and n (LL) transitions, respectively [3,6-7,9,13]. The
relatively weak broad band at 350-500 nm with absorption maximum at 414 nm { = 2552
3
-1
-1
(
HL1), 4257 dm mol cm (2)} is attributed to the intra-ligand charge transfer (LLCT)
transitions. The excited state properties by TDDFT using different combinations of the
functionals and the basis sets, respectively, provide identical electronic spectra (Fig. S4). These
results strongly support the reliability and validity of the methods employed as well as the
electronic properties of the complex. However, the calculated electronic spectra are comparable
to the experimental one with little shifting of the band maxima (Fig. 5 and Table 4). Indeed, the
spectra for -Zn and -Zn configured isomers are essentially identical (Fig. S5).

A few selected and simplified assignments on the computed spectra are made based on the
molecular orbitals (MOs) and population analyses and compared to the experimental data (Table
4
). A combined band resulting from MM, ML and LL transitions is found at 481 nm, close to the
experimental band at ca. 414 nm, for HOMO to LUMO+1 and HOMO-1 to LUMO transitions
with MOs contributions of 49 and 48 %, respectively (Fig. 6 and Table 2). The HOMO, LUMO
and LUMO+1 orbitals are mainly localized on the -electrons clouds of both the lactam- and
aryl-ring, while the HOMO1 is localized on the -electrons clouds of the lactam-ring only (Fig.
6
). Although the metal d-d electrons transitions, for the present diamagnetic closed shell Zn-
complex, are not allowed, however, there are a bit of d-electrons clouds in the
HOMO/HOMO1. This is because the metal-d MO has same symmetry as the ligand MOs,
provides a very small degree of electrons clouds to the metal-d MO (Fig. 6). Indeed, a very
insignificant d-electrons clouds in the LUMO/LUMO+1 represent a tiny bit of back donation
again due to same symmetry of the metal-d MO and ligand MOs. As a result, excited state
properties potentially provide a very little d-d contributions in addition to the ligand centred -
transitions (Fig. 6 and Table 4).
7
6
5
4
3
2
1
0
2 (calcd.)
HL2 (exptl.)
2
(exptl.)
200
300
400
500
600

/nm
o
Figure 5. UV-Vis. spectra for HL2 (0.015 mmol/L) and 2 (0.08 mmol/L) in MeOH at 25 C, and
calculated spectrum for 2 (-Zn isomer) at b3lyp/tzvp//b3lyp/6-31g(d)) in methanol using PCM.

Fig. 6. Frontier molecular orbitals (HOMO, HOMO1, LUMO, and LUMO+1) for 2 (-Zn
isomer), calculated at b3lyp/tzvp//b3lyp/6-31g(d) in methanol (Isovalue 0.02).
Table 4.
Assignments on calculated spectra for 2 (-isomer) at b3lyp/tzvp//b3lyp/6-31g(d)) in
methanol using PCM.
max/nm 1
Assignments 3
Oscillator
strength (f)
0.0406
MOs
contributions (%) 2
4
81 (414)
H → L+1 (49), H-1 → L (48)
MM, ML, LL
MM, ML, LL
MM, ML, LL
MM, ML, LL
ML, LL
3
48
0.4517
0.0944
0.3340
1.1001
0.2593
H-3 → L (38), H-2 → L+1 (49)
H-9 → L (39), H-8 → L+1 (47)
H-13 → L+1 (25), H-10 → L (35)
H-11 → L+1 (14), H-1 → L+5 (14)
3
14 (292)
49 (248)
35
2
2
2
05
H-3 → L+7 (9), H-2 → L+4 (24)
ML, LL
1
2
3
Experimental values are in parentheses; H = HOMO and L = LUMO; MM = metal-centered
(d-d), ML = metal-ligand, LL= ligand-centered transitions bands.

3
.6. Lactam (L)- and Enol (E)-tautomers
1H NMR spectrum of HL2 is shown in Fig. 7, and spectral data are summarized in the
experimental section. The isatin-Schiff bases undergo a dynamic tautomerization between the
lactum (L)- and enol (E)-forms (Scheme 1) and show peaks for both the forms simultaneously in
1
solution. The ratio of lactam- to enol-form varies with time as evidenced by variable time H
NMR studies in solution [14-15]. Thus, the methyl protons show two singlets at δ 2.18 (ca. 90%)
and 2.38 (ca. 10%) ppm in HL1 and at 2.43 (ca. 85%) and 2.28 (ca. 15%) ppm in HL2,
correspond to the lactam- and enol-forms, respectively. The phenolic O-H (enol-form) and amine
N-H (lactam-form) protons show broad signals at δ 9.35 and 9.63 ppm in HL1 and 8.91 and 9.19
ppm in HL2, respectively (Fig. 7). In accordance with the solid-state X-ray structure for HL1
with lactam-form (discussed above), we assume the major tautomer to be the lactam-form and
the minor one to be the enol-form immediately upon dissolution. Peak assignments and their
percentages are made based on the peak integration values for the methyl and N-H (Lactam)/O-H
(Enol) protons, respectively. The results are in good accord with the vibrational studies showing
both forms in solution, as discussed above.
1
o
Fig. 7. H NMR spectrum of the 3-(p-tolylimino)indolin-2-one (HL2) in CDCl at 20 C (L and
3
E represent the lactam- and enol-forms, respectively).

 -Zn and -Zn isomers

1
H NMR spectra for 1 and 2 in CDCl (Fig. 8) show the peaks associated to the co-ordinated
3
Schiff bases ligands to the metal ion. As mentioned above, the enol-form of ligands reacts with
the metal ion via deprotonation of the enolic proton and forms the complexes in solution
(Scheme1). Thus, the broad peaks found for N-H (lactam) and O-H (enol) protons (i.e., at δ
9
.63/9.35 ppm in HL1 or 9.19/8.91 ppm in HL2, Fig. 7) are absent in the spectra (Fig. 8). Rather,
the spectra show two sets of peaks for each proton in accordance with the existences of the two
possible -Zn and -Zn configured isomers with a ratio of ca. 65:35 (Δ:Λ or :). The amounts
of the isomers were calculated based on integration values of the most downfield aromatic peaks,
respectively. Labelling of the peaks as - or -isomer may be in reversed way. Indeed, spectra
taken at different time intervals show no significant changes of this ratio within 45 min, 4 h and
2
4 h of complex dissolution in CDCl , respectively. Hence, the results rule out any
3
interconversion between the two isomers in solution, that is, same ratio of the isomers (ca. 65:35)
exists at solid state. Or, a rapid equilibrium between the two isomers has been established
immediately upon dissolution, provides a ratio of ca. 65:35, irrespective of the ratio at solid state.
However, the results indicate preferred formation of the -Zn or -Zn isomer both in solution
and at solid state, based on the thermodynamic and/or kinetic stability, as evidenced in the
analogous Zn(II)-Schiff bases complexes [3,6-7,9,13]. In fact, comparable results of
diastereomers at a ratio of ca. 77:23 (Δ:Λ or :) within 10 min, 4 h and 20 h of complex
dissolution, respectively were reported for the zinc(II)-(S)-naphthaldimine complexes [6]. Thus,
1H NMR results strongly support the formation of the -Zn and -Zn configured isomers
resulting from induced chirality at-metal centre by the N,O-chelate ligands in distorted
tetrahedral Zn(II)-isatin Schiff bases complexes.

(
1)
(
2)
1
2
Fig. 8. H NMR spectra of the bis[3-(o/p-tolylimino)indole-2-olato- N,O]-/-zinc(II) (1 and
o
2
) in CDCl at 20 C (labelling of the peaks as - or -isomer may be in reversed way).
3
4
. Conclusion
The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) undergo tautomerization
from lactam (L)- to enol (E)-form which in turn reacts with the zinc(II) nitrate to give the bis[3-
2
(
o/p-tolylimino)indole-2-olato-κ N,O]-/-zinc(II) (1 and 2). Vibrational spectra show the bands
for νN-H and νO-H (or C=O) in HL1 or HL2, which are absent in 1 or 2, indicate the formation
of the complexes. Spectra for HL1 or HL2 show solely the lactam-form at solid state, while both
1
forms in solution. H NMR studies reveal both the lactam- and enol-tautomers in solution.

Molecular structure determination for HL1 explore two symmetry independent molecules (A and
B) in an asymmetric unit due to intermolecular hydrogen-bonding interactions and C-H…
contacts. Studies of Hirshfeld surfaces using the CrystalExplorer strongly support quantitative
1
analyses by supramolecular packing in the crystals. H NMR spectra indicate formation of the -
Zn and -Zn configured isomers (with one preferential formation) resulting from induced
chirality at-metal centre in 1 or 2. Optimized structures and excited state properties by
DFT/TDDFT correspond well to the experimental results.
Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
Declaration of interests “None”
On behalf of all authors
Mohammed Enamullah
Acknowledgement

We acknowledge Wazed Miah Science Research Centre (WMSRC) at Jahangirnagar
University, Bangladesh for obtaining elemental and H NMR data. Our sincere thanks to
Professor ABP Lever, Department of Chemistry, York University, Toronto for many useful
discussions on spectral analyses by DFT/TDDFT and the computecanada.ca
(https://ccdb.computecanada.ca/) Ontario, Canada for using their computational resources.
References
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