Switching from 4 + 1 to 4 + 2 zinc coordination number through the methyl group position on the pyridyl ligand in the geometric isomers bis[N-2-(4/6-methyl-pyridyl)salicylaldiminato-κ2N,O]zinc(II)

Article abstract of DOI:10.1016/j.ica.2014.11.020

The Schiff bases N-2-(4/6-methyl-pyridyl)salicylaldimine (HL) which are geometric isomers differing in the para- or ortho-position of the methyl group to the pyridyl nitrogen atom react with zinc(II) acetate to give the bis-ligand chelate complexes bis[N-

Full text of DOI:10.1016/j.ica.2014.11.020

Inorganica Chimica Acta 427 (2015) 103–111
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Switching from 4 + 1 to 4 + 2 zinc coordination number through
the methyl group position on the pyridyl ligand in the geometric
2
isomers bis[N-2-(4/6-methyl-pyridyl)salicylaldiminato-
j
N,O]zinc(II)
Mohammed Enamullah a, , Mohammad Abdul Quddus , Mohammad Abdul Halim ,
⇑
a
b
a
c
c,
⇑
Mohammad Khairul Islam , Vera Vasylyeva , Christoph Janiak
a
Department of Chemistry, Jahangirnagar University, Dhaka 1342, Bangladesh
b
Bangladesh Institute of Computational Chemistry and Biochemistry, 38 Green Road West, Dhaka 1205, Bangladesh
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität, Universitätsstr. 1, D-40225 Düsseldorf, Germany
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2014
Received in revised form 24 October 2014
Accepted 20 November 2014
Available online 13 December 2014
The Schiff bases N-2-(4/6-methyl-pyridyl)salicylaldimine (HL) which are geometric isomers differing in
the para- or ortho-position of the methyl group to the pyridyl nitrogen atom react with zinc(II) acetate
2
to give the bis-ligand chelate complexes bis[N-2-(4/6-methyl-pyridyl)salicylaldiminato-
4-methyl or para (1), 6-methyl or ortho (2)}. The zinc complexes feature an N
the two salicylaldiminato moieties with one or two additional weak Zn–N(pyridyl) contacts to give a
+ 1 or 4 + 2 coordination in 1 or 2, respectively. The difference in metal coordination is traced to com-
petitive alternative C–Hꢀ ꢀ ꢀ interactions of the non-coordinated methyl-pyridyl ring in compound 1
which are absent in 2. Instead, the weakly zinc-coordinated methyl-pyridyl rings in 1 and 2 are at the
same time engaged in
j
N,O]zinc(II)
{
2 2
O chromophore from
4
Keywords:
Schiff base ligands
Zinc(II)–Schiff base complexes
Variable zinc coordination number
p
p
ꢀ ꢀ ꢀ interactions. The latter compound features two C–Hꢀ ꢀ ꢀO contacts. DFT calcu-
p
lation produces the similar structural features for 1 and 2. The excited state properties calculated by
p
ꢀ ꢀ ꢀ
p
interactions
interactions
C–Hꢀ ꢀ ꢀ
p
TDDFT reveal that the complexes have distinctive ligand–ligand (LL) and metal-to-ligand (ML) charge
transfer bands. Hirshfeld surface analysis also discloses similar C–Hꢀ ꢀ ꢀ
p
and C–Hꢀ ꢀ ꢀO interactions in
crystals 1 and 2, respectively.
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
[14–16]]. In the cases of enzymes and zinc finger proteins, the
cation is usually tetrahedrally coordinated [17] leading to a defined
asymmetric geometry involving the zinc atom in their active site.
Further, the variable coordination leads the chemical properties
of the zinc ion to be either metabolically active or inert [14d–e].
The phenomenon also plays a key role for catalytic turnover via
structural rearrangement of the metal sites, and affords strong
interaction with a variety of substrates [14f].
Zinc is highly versatile in its coordination geometry and num-
ber. Zinc centers can be pseudo-tetrahedrally four-coordinated
1–7], five-coordinated [7–11], and octahedrally [3,6,7,10,12–14]
[
to trigonal prismatic six-coordinated [3]. In all coordination modes
the zinc atom can also adapt to different degrees of distortion
from the ideal tetrahedral, square-pyramidal [10] or trigonal–
bipyramidal [9] to octahedral coordination polyhedron. Because
of the relevance of Zn in enzymes and proteins this is of fundamen-
tal importance. The flexibility and adoptability of variable coordi-
nation geometry at the zinc ion is essential for the enzyme or
protein function as the molecular basis for the numerous biological
effects [14]. Zinc complexes are highly abundant in biological
systems, being essential for the functionalities of a number of
metalloproteins, expressing both catalytic and structural roles
We are investigating the coordination chemistry of modified
Schiff base ligands and have recently given attention to achiral
and in particular chiral N,O-chelate Schiff base ligands (HL) of
the salicylaldimine and naphthaldimine type, and their complexes
4
4
with transition metal ions such as [Rh(
g -cod)(L)], [Rh(g -cod)
+
(HL)] and [Cu/Ni(L)
2
] [18–25]. Solid state X-ray analyses show
that the deprotonated Schiff base anion (L ) coordinates to
ꢁ
4
the Rh(
g -cod)-fragment as six-membered N,O-chelate in dis-
torted-tetrahedral geometry. More recently, we reported the syn-
theses, diastereoselectivity, molecular structures, solution CD and
DFT studies on chiral four-coordinated, distorted-tetrahedral
⇑
Corresponding authors.
E-mail addresses: enamullah@juniv.edu (M. Enamullah), mahalim@mun.ca
[
2
M(R/S-N^O) ] {M = Cu or Zn , N^O = deprotonated Schiff base
(
M.A. Halim), janiak@uni-duesseldorf.de (C. Janiak).
http://dx.doi.org/10.1016/j.ica.2014.11.020
020-1693/Ó 2014 Elsevier B.V. All rights reserved.
0

1
04
M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
ꢁ
ligand = L } [26–28]. The R- or S-ligand chirality induces metal-
centered - or -chirality at the metal atom in these C -symmetric
complexes.
(d, JHH = 6.0 Hz, 1H, H
6.9 Hz, JHH = 1.8, 1.2 Hz, 1H, H
HH = 1.8, 1.5 Hz, 1H, H ), 8.38 (d, JHH = 5.1 Hz, 1H, H
1H, HCN) and 13.53 (s, 1H, OH).
6
0
), 7.18 (s, 1H, H
), 7.52 (dd, JHH = 7.8, 7.5 Hz,
), 9.46 (s,
3
), 7.42 (ddd, JHH = 7.5,
K
D
2
0
4
J
5
6
The present paper utilizes the Schiff base ligands N-2-(4/
6
-methyl-pyridyl)salicylaldimine [29–31] and describes the effect
of the pendant isomeric para- or ortho-methyl pyridyl group in
2
.1.2. N-2-(6-methyl-pyridyl)salicylaldimine (HL2)
the Schiff base ligands (HL1 and HL2) on the zinc coordination
ꢁ1
Yield: 2.550 g (85%). IR (KBr, cm ):
m = 3053, 2978w (C–H),
2
polyhedra in bis[N-2-(4/6-methyl-pyridyl)salicylaldiminato-
j
N,O]
+
1
612vs (C@N), and 1578s (C@C). ESI-MS: m/z 213 (100) [M+H] .
zinc(II) (1, 2) (Scheme 1). Structural and photophysical properties
of the complexes are also studied by DFT (Density Functional
Theory) and TDDFT (Time Dependent Density Functional Theory)
calculations.
1
H NMR (300 MHz, CDCl
.5 Hz, JHH = 0.9 Hz, 1H, H
d, JHH = 7.5 Hz, 1H, H ), 7.14 (d, JHH = 7.8, 7.5 Hz, JHH = 1.8,
), 7.41 (dt, JHH = 7.8 Hz, JHH = 1.2 Hz, 1H, H ), 7.53
), 7.68 (t, JHH = 7.5,
), 9.46 (s, 1H, HCN) and 13.59 (s, 1H, OH).
3
): d = 2.61 (s, 3H, CH
3
), 6.97 (t, JHH
=
7
5
0
), 7.05 (d, JHH = 8.1 Hz, 1H, H
0
3
), 7.11
(
0
6
1
.5 Hz, 1H, H
5
0
4
(
dd, JHH = 7.5, 7.8 Hz, JHH = 1.5, 1.7 Hz, 1H, H
.8 Hz, 1H, H
3
7
4
2
. Experimental and computational methods
FT-IR-spectra were recorded on Nicolet iS10 (Thermo Scientific)
2.2. General procedure to synthesize the complexes
spectrometer as KBr discs at ambient temperature. Electronic spec-
tra were obtained with Shimadzu UV 1800 spectrophotometer in
CHCl at 25 °C. Elemental analyses were performed on a VarioEL
3
Two equivalents of N-2-(4-methyl-pyridyl)salicylaldimine
(HL1) (0.425 g, 2.00 mmol) dissolved in 10 mL of ethanol were
added into 10 mL of an ethanol solution of Zn(O CCH ) ꢀ2H O
from Elementar. Thermal analysis was performed on a Shimadzu
2
3 2
2
DSC-60 differential scanning calorimeter (DSC), heating range at
(0.219 g, 1.00 mmol). Into this solution two equivalents of NaHCO3
(dissolved in 5 mL of ethanol) were added and the mixture refluxed
for 24 h. The color changes from bright-yellow to orange-yellow.
Reduced the volume of solvent to 50% in vacuo and left standing
this solution for crystallization via slow evaporation of solvent at
ꢁ1 1
3
03–553 K and rate at 10 K min . H NMR-spectra were recorded
on a Bruker Avance DPX 200 spectrometer operating at 300 MHz
1
(
H) at 20 °C with calibration against the residual protonated
solvent signal of CDCl (d 7.25 ppm). MS spectra were taken on
Thermo-Finnigan TSQ 700. Isotopic distributions patterns for
3
room temperature. Light orange-yellow crystals of bis[N-2-(4-
6
4/66/68
2
Zn(II)-containing ions are clearly visible in the mass spectra.
methyl-pyridyl)salicylaldiminato-
j
N,O]zinc(II) (1), suitable for
X-ray measurement, were obtained within one week. Filtered off,
washed the crystals two times with ethanol (3 ml), and dried in
vacuo at 30 °C. The same procedure was followed for synthesis of
2.1. General procedure to synthesize the Schiff bases
2
using the Schiff base HL2.
Salicylaldehyde (1.730 g, 14.18 mmol) was dissolved into 10 mL
of methanol, 2–3 drops of H SO added and the solution stirred for
0 min. An equimolar amount of 2-amino-4-methyl-pyridyl
1.531 g, 14.18 mmol) was added into this solution. The reaction
mixture was then refluxed for 6 h, and the color turned to bright
yellow. The solvent was evaporated to 50% in vacuo, and the
remaining solution left standing for crystallization by slow solvent
evaporation at room temperature. Crystals were formed within
2
4
2
2.2.1. Bis[N-2-(4-methylpyridyl)salicylaldiminato-j N,O]zinc(II) (1)
1
(
ꢁ
1
Yield: 0.262 g (76%). – IR (KBr, cm ):
H-C), 1610vs (C@N), and 1586vs (C@C). – H NMR (300 MHz,
CDCl ): d = 2.08 (s, 3H, CH ), 6.72 (t, JHH = 7.5 Hz, 1H, H ), 6.96 (d,
HH = 7.5 Hz, 1H, H ), 6.99 (d, JHH = 6.5 Hz, 1H, H ), 7.06 (s, 1H,
), 7.39–7.44 (m, 2H, H ,5), 8.20 (d, JHH = 5.1 Hz, 1H, H ), and
m = 3069, 3050, 3024w
1
(
0
5
3
3
J
H
9
3
0
0
6
0
3
4
6
+
.42 (s, 1H, HCN). – MS (EI, 70 eV): m/z (%) = 486 (40) [M] , 366
4
–5 d, filtered off and washed three times with methanol (3 mL
+
+
+
(
20) [M–C
6 4
H (OH)(CHN)] , 275 (100) [M–L1] , 212 (20) [HL1] ,
in each). The bright yellow crystals were dried in air for 2 d and
analyzed as N-2-(4-methyl-pyridyl)salicylaldimine (HL1). Com-
pound N-2-(6-methyl-pyridyl)salicylaldimine (HL2) was prepared
following same procedure by using 2-amino-6-methyl-pyridyl.
+
and 78 (50) [C
6
H
5 4
N] . – C26
22 4 2
H N O Zn (487.87). Anal. Calc. for C,
4.01; H, 4.55; N, 11.48. Found: C, 63.41; H, 4.71; N, 11.72.
2
2.2.2. Bis[N-2-(6-methylpyridyl)salicylaldiminato-
j
N,O]zinc(II) (2)
ꢁ
1
Yield: 0.288 g (75%). – IR (KBr, cm ):
(H–C), 1615vs (C@N), and 1591vs (C@C). – H NMR (300 MHz,
CDCl ): d = 2.34 (s, 3H, CH ), 6.74 (t, JHH = 7.5 Hz, 1H, H ), 6.95 (d,
HH = 8.0 Hz, 1H, H ), 7.01 (d, JHH = 7.5 Hz, 1H, H ), 7.05 (d,
HH = 7.8 Hz, 1H, H ), 7.32 (t, JHH = 7.8 Hz, 1H, H ), 7.45 (d,
HH = 7.5 Hz, 1H, H ), and 9.37 (s, 1H,
m = 3065, 3044, 3020w
1
2
.1.1. N-2-(4-methyl-pyridyl)salicylaldimine (HL1)
ꢁ1
Yield: 2.450 g (81%). IR (KBr, cm ):
m
= 3053, 2978w (C–H),
3
3
0
5
+
1
615vs (C@N), and 1570s (C@C). ESI-MS: m/z 213 (100) [M+H] .
J
J
J
3
0
0
6
1
H NMR (300 MHz, CDCl
3
): d = 2.43 (s, 3H, CH
3
), 6.99 (dt, JHH
=
5
0
4
7
.5 Hz, JHH = 0.9 Hz, 1H, H
5
0
), 7.05 (d, JHH = 8.0 Hz, 1H, H
3
0
), 7.07
3
), 7.55 (t, JHH = 7.5 Hz, 1H, H
4
4
3
5
6
6
'
R
H
2
N
5'
4'
N
N
R
R
CHO
N
Zn(O CCH ) ·2H O
2
3 2
2
N
N
O
Zn
O
3
'
MeOH / H+
h, reflux
NaHCO , EtOH
3
OH
OH
N
N
6
24h, reflux
HL1: R = 4-CH
HL2: R = 6-CH
3
3
R
[
2
ZnL ]
: R = 4-CH
: R = 6-CH
1
3
3
only
and
2
Scheme 1. Synthetic route to bis[N-2-(4/6-methyl-pyridyl)salicylaldiminato-j
2N,O]zinc(II) (1, 2).

M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
105
Table 1
(k = 1.54178 Å) for 2, multilayer mirror system,
data collection with Apex2 [32], cell refinement and data reduction
with SAINT [33], experimental absorption correction with SADABS
x
- and /-scan;
Crystal data and details of structure refinements.
Compound
1
2
[
34]. Structure Analysis and Refinement: The structures were solved
Empirical formula
C
26
H
22
N
4
O
2
Zn
C
26 22 4 2
H N O Zn
ꢁ
1
M (g mol
Crystal size (mm )
T (K)
)
487.85
487.85
by direct methods using SHELXS-97; refinement was done by full-
matrix least squares on F using the SHELXS-97 program suite [35].
All non-hydrogen positions were refined with anisotropic displace-
ment parameters. Hydrogen atoms were positioned geometrically
3
2
0.036 ꢂ 0.076 ꢂ 0.200 0.032 ꢂ 0.037 ꢂ 0.062
296(2)
95(2)
h range (°) (completeness)
h; k; l range
Crystal system
Space group
a (Å)
1.74–25.09 (99.1%)
ꢁ10, 9; ±12; ꢁ14, 12
4.00–64.99 (98.1%)
–26, 27; ±10; ꢁ13, 10
monoclinic
C2/c (No. 15)
23.566(3)
8.2495(9)
11.8437(13)
90
110.380(5)
90
2158.4(4)
4
and refined using riding models with Uiso(H) = 1.2 Ueq(CH, CH
and Uiso(H) = 1.5 Ueq(CH ). Crystal data and details on the structure
refinement are given in Table 1. Graphics were drawn with
DIAMOND [36], analyzes on the supramolecular C–Hꢀ ꢀ ꢀ and
-stacking, as well as the CꢁHꢀꢀꢀO interactions were done with
PLATON for Windows [37].
2
)
triclinic
P 1ꢀ (No. 2)
3
9.0723(12)
10.6337(16)
12.1638(18)
89.461(5)
74.742(5)
87.643(5)
1131.1(3)
2
1.432
1.117
504
0.745 / 0.656
b (Å)
c (Å)
p
p–p
a
(°)
b (°)
(°)
c
3
V (Å )
2
.4. Computational methods
Z
D
l
calc (g cm 3
ꢁ
)
1.501
1.842
1008
0.753 / 0.543
(Mo K
a
) (mmꢁ1)
All calculations were performed with the GAUSSIAN 09 software
F(000)
Maximum/minimum
transmission
Reflections collected
Independent reflections
int
(R )
package [38]. The initial geometry of the complexes was generated
from the X-ray structures and then vibrational frequencies were
calculated at B3LYP/SDD level of theory. The absence of imaginary
frequencies confirmed that the stationary points correspond to
minima on the Potential Energy Surface. Molecular orbitals were
also computed at same level of theory. For calculating the excited
state properties, time-dependent density functional theory
(TD-DFT) was applied with CAM-B3LYP/6-31 + G(d,p), M06/6-
31 + G(d,p) and B3LYP/SDD level of theories incorporating PCM
14,393
3990 (0.0239)
5890
5890 (0.0296)
Data/restraints/parameters
Maximum/minimum
3990/0/300
1.562/ꢁ0.373
1798/0/151
0.480/ꢁ0.387
D
q
ꢁ
3 a
(
e Å
)
(I)] b
R
1
/wR
2
[I > 2
r
0.0368/0.0946
0.0420/0.0977
1.030
0.0326/0.0842
0.0350/0.0854
1.075
b
R1/wR2 (all data)
Goodness-of-fit (GOF) on F
2
(
Polarization Continuum Model) using chloroform as a solvent.
c
For TDDFT calculation, 50 excitation states were considered (Tables
S1, S2). However, B3LYP/SDD showed the best agreement with the
a
b
c
Largest difference peak and hole.
P
P
||)/ |F
P
P
P [w(F
2
2 2
2)2]]1/2.
R
1
= [ (||F
o
| ꢁ |F
c
o
|]; wR
2
= [ [w(F
2
o
ꢁ F
c
) ]/
o
experimental UV–Vis results. For disclosing C–Hꢀ ꢀ ꢀ
p
and C–Hꢀ ꢀ ꢀO
2
2
1/2
Goodness-of-fit = [ [w(F
o
ꢁ F
c
) ]/(n ꢁ p)]
.
interactions in both crystals, Hirshfeld surface analysis [39a–b]
has been performed by CrystalExplorer software [39c].
+
6 4
HCN). – MS (EI, 70 eV): m/z (%) = 486 (45) [M] , 366 (25) [M–C H
3
. Results and discussion
+
+
+
(
[
OH)(CHN)] , 275 (100) [M–L2] , 212 (30) [HL2] , and 78 (60)
C H N] . – C26H N O Zn (487.87). Anal. Calc. for C, 64.01; H,
5 4 22 4 2
+
The Schiff base ligands N-2-(4/6-methyl-pyridyl)salicylaldimine
4
.55; N ,11.48. Found: C, 63.55; H, 4.75; N, 11.32.
(HL) react with zinc(II) acetate to give bis[N-2-(4/6-methyl-pyri-
2
dyl)salicylaldiminato-
presence of NaHCO under reflux for 24 h (Scheme 1).
j
N,O]zinc(II) (4-CH
3
, 1; 6-CH
3
, 2) in the
2.3. X-ray crystallography
3
Suitable single crystals were carefully selected under a polariz-
3
.1. Spectroscopy and analyzes
ing microscope. Data collection: Compound 1 or 2: Bruker APEX2
CCD diffractometer (with microfocus tube) at 296 ± 2 K; Mo K
radiation (k = 0.71073 Å) for 1, and at 95 ± 2 K; Cu K radiation
a
ꢁ1
Vibrational spectra show a very strong band at 1610–1615 cm
a
(m
C@N) for the azomethine group. A strong band found at 1575–
ꢁ1
ꢁ1
1
595 cm and weak band at 2970–3080 cm are assigned to the
m
C@C and
m
C–H, respectively. The frequency for the azomethine
3
3
2
2
2
1
1
6
2
8
4
0
6
2
8
4
0
HL1
2 m
ꢁ1
group is calculated at 1640 cm (Fig. S1), while
appear at 1557–1623 and 3040–3145 cm , respectively (Fig. S1).
MS spectra show the parent ion peak at m/z 213 [HL + H] (HL1 or
HL2) and 486 [M] (1 or 2). The spectra are further dominated by
several ion peaks for [M–C
and [C H N] species, respectively.
Experimental electronic spectra of the complexes (Fig. 1, S2,
Table 2) show several bands/shoulders below 380 nm with absorp-
tion maxima at 350 (1 or 2), 308 (1)/311 (2), 270 (1 or 2) and 243
(1)/240 (2) nm, due to ligand (LL) only transitions for the azome-
thine group [1–10,18–26,29–31]. The free Schiff bases show the
identical bands/shoulders below 380 nm. However, the complexes
further show a strong broad band at 380–500 nm with absorption
maxima at 415 (1)/417 (2) nm, assigned to the metal-to-ligand
m
C@C and
m
C–H
LL
LL
ꢁ1
3
4
5
6
7
8
m
m
m
m
m
m
+
+
+
+
+
5
H
4
N] , [M–C
6 4
H (OH)(CHN)] , [M–L] ,
+
5 4
ML
10 m
2 m
15 m
8 m
24 m
1
1
3
5
8
1
2
6 m
6 m
7 m
44 m
18 m
2
40
290
340
390
440
490
540
(
ML) charge transfer transitions. The calculated excited state
λ/nm
properties of both complexes are presented in Fig. 2 (Table 3).
Due to several transitions, a straightforward assignment of the
computed UV–Vis absorption spectra is challenging. Only selected
Fig. 1. Electronic spectra of compounds HL1 (4.01 ꢂ 10 mM) and 1 (8.00 ꢂ 10ꢁ2 mM)
ꢁ2
3
at different time interval in CHCl at 25 °C.

1
06
M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
50000
40000
30000
20000
10000
0
and 260 nm (Fig. 1, S2) indicates the existing equilibria among dif-
ferent ionic species in solution.
1
In H NMR spectra (Fig. 3), the methyl and imine protons appear
as a singlet at d 2.43 (HL1), 2.61 (HL2), 2.08 (1), 2.34 ppm (2), and
9
2
1
.46 (HL1 or HL2), 9.42 (1), 9.37 ppm (2), respectively [1–10,18–
6,31]. The phenolic proton shows a singlet at d 13.53 (HL1), and
3.59 ppm (HL2), which is absence in the complexes. The aromatic
H6 proton is found at relatively low field as a doublet at d 8.38 ppm
HL1) and 8.20 ppm (1) due to strong inductive effect of the adja-
(
cent pyridine nitrogen atom [31a]. Similarly, the methyl protons
shift to relatively low field in HL2 or 2 (d 2.61 or 2.34 ppm) in
comparison to those in HL1 or 1 (d 2.43 or 2.08 ppm). The spectra
further show several aromatic protons peaks at the range of
240
300
360
420
480
540
600
6
.70–7.80 ppm.
Wavelength (nm)
Fig. 2. Calculated electronic spectra of compounds 1 (solid line) and 2 (dash line) at
B3LYP/SDD level of theory in chloroform.
3.2. Thermally induced structural phase transformation
It is well documented that the transition metal complexes
incorporating the N,O-chelate ligands exhibit thermally induced
structural phase transformation from a low temperature crystal-
line phase (distorted tetrahedral/square planar) into a high tem-
perature isotropic liquid phase (regular tetrahedral/square
planar) [27,28,40]. The differential scanning calorimetry (DSC)
and simplified assignments related to the experimental data are
discussed here. The calculated LL and ML bands (very strong)
appear with absorption maxima at 234 (1 or 2), 309 (1)/313 (2),
and 408 (1)/417 (2) nm, respectively, which are in good accordance
with those of the experimental results. The time dependent elec-
tronic spectra (Fig. 1, S2) reveal the successive decomposition of
the complexes in chloroform solution, a common feature of the
labile distorted tetrahedral Zn(II)-complexes. The LL bands become
more intense and the ML bands completely disappear with time.
Indeed, the presence of several isosbestic points at 375, 320, 300,
analyses of heating curves (Fig. 4) show exothermic peaks at
ꢁ1
2
39 °C (
D
H = ꢁ46.69 kJ mol ) for 1 and 267 °C (
D
H = ꢁ49.98
ꢁ1
kJ mol ) for 2, which correspond to a phase transformation from
crystalline phase to isotropic liquid phase. The absence of any peak
on respective cooling curves indicates an irreversible phase
Table 2
3
Electronic spectral data of HL1–HL2 and 1–2 in CHCl at 25 °C.
Entitya
Assignments
LL: kmax/nm (
e
max) b
ML: kmax/nm (
e
max)b
HL1 (4.01 ꢂ 10ꢁ2)
350 (32361), ꢃ320sh, 306 (35353), ꢃ277sh, 272 (28197), 237 (35491)
350 (17093), ꢃ335sh, ꢃ315sh, 308 (39389), ꢃ277sh, ꢃ270sh, 243 (26487)
351 (24753), 322 (23391), 310 (23810), ꢃ277sh, 270 (20509), 236 (24991)
350 (16774), ꢃ335sh, ꢃ317sh, 311 (26117), ꢃ277sh, 270 (17902), 240 (16947)
–
ꢁ
2
1
(8.00 ꢂ 10
)
415 (20576)
–
417 (10737)
HL2 (3.82 ꢂ 10ꢁ2)
2
2
(5.23 ꢂ 10ꢁ
)
a
Concentration in mM.
b
ꢁ1
ꢁ1
e
in mM cm .
Table 3
Selected calculated absorption properties of compounds 1 and 2 at B3LYP/SDD level of theory in chloroform.
Wavelength (nm)
Excitation energies
Oscillator strengths
MO Contributions a
Assignments a
Compound 1
450.09
418.08
408.06
398.31
343.77
327.20
323.78
313.54
308.89
306.25
2.7546
2.9656
3.0384
3.1128
3.6066
3.7893
3.8293
3.9543
4.0139
4.0485
0.0046
0.268
H ? L(99%)
H-1 ? L(97%)
H ? L + 1(97%)
H-1 ? L + 1(97%)
H-5 ? L(36%), H-4 ? L(53%),
H-2 ? L(92%), H-4 ? L(4%)
H-5 ? L + 1(24%), H-4 ? L+(55%)
H-5 ? L(12%), H-3 ? L(82%),
H-4 ? L(8%), H-2 ? L(82%)
H-5 ? L(45%), H-4 ? L(39%), H-3 ? L(7%)
ML/LL
ML/LL
ML/LL
ML/LL
ML/LL
LL
LL/ML
ML/LL
LL
0.2809
0.0288
0.0018
0.0101
0.0258
0.3965
0.478
0.1758
ML/LL
Compound 2
436.14
434.29
420.47
417.38
345.54
345.02
316.71
315.93
313.22
312.21
2.8427
2.8548
2.9487
2.9705
3.5881
3.5935
3.9148
3.9244
3.9583
3.9712
0.0275
0.0013
0.258
0.3264
0.0029
0.0079
0.0763
0.1077
0.4434
0.1681
H-1 ? L + 1(21%), H ? L(78%)
H-1 ? L(41%), H ? L + 1(57%)
H-1 ? L(57%), H ? L + 1(41%)
H-1 ? L + 1(78%), H ? L(21%)
H-5 ? L + 1(36%), H-4 ? L(46%), H-2 ? L(16%)
H-5 ? L(43%), H-4 ? L + 1(41%), H-2 ? L + 1(12%)
H-5 ? L + 1(23%), H-2 ? L(74%)
H-5 ? L(26%), H-3 ? L(3%), H-2 ? L(67%)
H-3 ? L(92%)
ML/LL
ML/LL
ML/LL
ML/LL
ML/LL
ML/LL
ML/LL
ML/LL
LL
H-3 ? L + 1(91%)
LL
a
H = HOMO; L = LUMO; ML = metal–ligand transition; LL = ligand only transition.

M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
107
Fig. 3. 1H NMR spectra of HL1 and 1 in CDCl
at 20 °C.
3
5
0
Table 4
o
1
72 C
Measure of distortions from tetrahedral to square planar metal geometry in 1 and 2.
a
b
R
Compound
h/°
s
tet-sq = h /90°
s
4
5
0
100
150
T/ C
200
250
300
-4-CH
-6-CH
3
1
2
86.53(8)
81.92(5)
0.96
0.91
0.81
0.78
o
3
-
5
a
Calculated with Diamond according to Scheme 2 [36].
See Table 5 for the two largest angles
b
a and b.
-
-
-
10
15
20
(
1)
(2)
o
2
67 C
o
2
39 C
Fig. 4. DSC analysis curves of compounds 1 and 2.
transformation. However, the cooling curve shows
a minor
endothermic peak at 172 °C, due to the ligand resulting from
decomposition of 2.
Scheme 2. Assessment of deviation between tetrahedral and square-planar geom-
etry by the dihedral angle h or the stet-sq index.
For a more quantitative assessment, the degree of distortion
from tetrahedral to square planar can be expressed by the dihedral
angle h between the two planes formed by the donor atoms with
the metal atom, that is, N1–M–O1 and N2–M–O2 (Table 4). This
dihedral angle will be 90° for a tetrahedral geometry and 0° for a
square planar geometry (not considering the imminent distortion
induced by the chelate ring formation) [41]. Furthermore, this
angle h can be normalized through division by 90° (h/90°) to give
3.3. Solid state and optimized structure
X-ray structure determinations for 1, and 2 reveal that two
molecules of N^O-chelate ligands form a pseudo-tetrahedral
N O -coordination sphere around the zinc atom in distorted tetra-
2 2
hedral geometry (Fig. 5). These Zn–O/N bond lengths and bond
angles (Table 5) are as expected from reported literatures for the
similar Zn(II)-N^O-chelate complexes [1,2,14,26]. The calculated
bond distances by density functional theory (DFT) are nearly same
as the experimental values (Table 5). However, some calculated
bond angles are slightly larger (1–6°) compared to the crystal
structures. An additional weak coordination of one or two pyri-
dyl-nitrogen atoms to the zinc atom extends the coordination
number to 4 + 1 or 4 + 2. The Zn–Npyridyl bonds are substantially
longer than the Zn–Nimine bonds (Table 5). Similar trend is also
observed for the calculated Zn–Npyridyl bonds. At first sight, the
position of the methyl group on the pyridine ligand affects the
pyridyl coordination to zinc in the isomeric complexes.
an index stet-sq = 1.00 for tetrahedral and 0.00 for square planar
geometry (Scheme 2).
Also, a geometry index
4
s for four-coordinate complexes with
s
4
= [360° ꢁ (
Addison and Reedijk’s five-coordinate
being the two largest angles in the four-coordinate species. The
values of will range from 1.00 for a perfect tetrahedral geometry,
a
+ b)]/141° has been proposed [42], inspired by
s
5
index [43], with and b
a
s
4
since 360–2(109.5) = 141, to zero for a perfect square planar geom-
etry, since 360–2(180) = 0. Intermediate structures, including tri-
gonal pyramidal and seesaw, fall within the range of 0–1.00 [36].
For two chelate ligands, as in the present bis-bidentate Schiff base
complexes, it is however, better to take the dihedral angle h or its
normalization index
s
tet-sq = h/90°. The geometry index
s
4
will not
We can further reason the difference in the zinc coordination
from an analysis of the solid-state supramolecular interactions, in
correctly assess a tetrahedral geometry because of the already
imminent distortion induced by the chelate ring formation. With
chelate ligands the two largest angles will often inevitable be lar-
particular the non-classical hydrogen bonds C–Hꢀ ꢀ ꢀ
p
[44] and
C–Hꢀ ꢀ ꢀO (Table 6) [45] as well as
pꢀꢀꢀ
p
interactions [46]. C–Hꢀ ꢀ ꢀ
p
ger than 109.5°, hence,
s
4
< 1, even if the dihedral planes are per-
interactions are present in compound 1 and include the methyl
group and an aryl-C–H of the non-coordinated pyridyl ring
(Fig. 6a). 2D fingerprint plot from the Hirshfeld surface calculation
fectly perpendicular. Both measures of distortion are listed in
Table 4 and support the notion of a close to tetrahedral structure
for zinc(II) complexes.
for compound 1 also confirms that C–Hꢀ ꢀ ꢀ
p interactions are

1
08
M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
Fig. 5. Crystal structures (left) and optimized structures (right) of 1 (top) and 2 (bottom) (50% thermal ellipsoids). Symmetry label, i = ꢁx, y, 1.5 ꢁ z. Optimized structures are
computed at B3LYP/SDD level of theory.
Table 5
Selected bond lengths (Å) and angles (°) in 1 and 2.
Experimental
Calculated
Experimental
Calculated
1
Zn1–O2
Zn1–O1
Zn1–N1
Zn1–N3
Zn1–N2
1.9209(19)
1.952(2)
1.997(2)
2.006(2)
2.750(2)
1.9336
1.9752
2.0331
2.0097
2.7883
O2–Zn1–O1
O2–Zn1–N1
O1–Zn1–N1
O2–Zn1–N3
O1–Zn1–N3
N1–Zn1–N3
110.46(9)
118.57(8)
93.09(8)
97.79(8)
108.77(9)
127.67(8)
115.51
112.63
91.43
97.17
110.36
130.88
2a
Zn1–O1
Zn1–N1
Zn1–N2
1.9754(14)
2.0011(18)
2.714(2)
1.9752
2.0147
2.7999
O1–Zn1–O1i
O1–Zn1–N1i
O1–Zn1–N1
108.12(9)
108.85(6)
93.46(7)
108.82
110.90
92.84
i
N1 –Zn1–N1
141.87(10)
139.12
a
Symmetry operations: i = ꢁx, y, ꢁz + 3/2.
Table 6
Intermolecular hydrogen bonding interactions with lengths (Å) and angles (°) in 1 and 2.
Compound 1
a
b
d(H\plane)c
2.96
2.74
2.70
2.92
C–Hꢀ ꢀ ꢀ
p
d(Hꢀ ꢀ ꢀCt)
3.00
2.88
2.72
2.93
<(C–Hꢀ ꢀ ꢀCt)
118
156
167
124
ii
C13–H13Bꢀ ꢀ ꢀring(Zn–N3–C20–C10–C19–C14–O2)
v
C15–H15ꢀ ꢀ ꢀring(C1–C6)
iii
C23–H23ꢀ ꢀ ꢀring(N2–C8–C12)
iii
C26–H26Aꢀ ꢀ ꢀring(N4–C21–C25)
i
C26–H26Bꢀ ꢀ ꢀring(C14–C19)
2.86
2.82
138
d
Compound 2
C–Hꢀ ꢀ ꢀO
d(C–H)
0.95
0.95
d(Hꢀ ꢀ ꢀO)
2.49
2.47
d(Cꢀ ꢀ ꢀO)
3.298(3)
3.299(3)
<(CHO)
143
146
iii
C7–H7ꢀ ꢀ ꢀO1
iii
C9–H9ꢀ ꢀ ꢀO1
a
b
c
Symmetry operations: i = ꢁ1 + x, y, z; ii = x, –1 + y, z; iii = –x, –y, 1 ꢁ z; v = 1 + x, y, z.
Ct = centroid of specified ring.
Perpendicular distance of H to ring plane.
d
Symmetry operations: iii = x, –y, z + 1/2.

M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
109
Fig. 6. Presentation of (a) the C–Hꢀ ꢀ ꢀ
p interactions in 1 and (b) the C–Hꢀ ꢀ ꢀO interactions (in orange) in 2. See Table 6 for details and for symmetry operations. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1
10
M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
Fig. 7. Hirshfeld 2D fingerprint plots for compounds 1 (left) and 2 (right). di and de are the distance from the surface to the nearest atom interior and exterior to the surface,
respectively.
Fig. 8.
pꢀ ꢀ ꢀp interaction(s) of the zinc-coordinated methyl-pyridyl ring(s) with their centroid–centroid contacts in 1 (left) and 2 (right) (in 2 the perpendicular oriented
ligands are symmetry related to the horizontal ones). See Tables S3, S4 and Figs. S3, S4 in Supporting Information for full details.
present in the crystal (Fig. 7, left). The wings at the upper left
de > di) and lower right (di > de) are recognized as characteristic
of C–Hꢀ ꢀ ꢀ interactions, where the points on the upper left surface
correspond to C–H donor and points on the lower right surface cor-
respond to interactions are absent in 2.
The latter features two C–Hꢀ ꢀ ꢀO contacts instead (Fig. 6b, Table 6).
The chelating C–Hꢀ ꢀ ꢀO contacts demand an almost co-planar orien-
tation of the ligand ring planes and thereby orient the nitrogen
atom of the methyl-pyridyl ring towards the zinc atom for a weak
4. Conclusions
(
p
The coordination number 4 in distorted tetrahedral zinc com-
plexes with the Schiff base ligands N-2-(4/6-methyl-pyridyl)sali-
cylaldiminato can be extended to five (in 1) or six (in 2) through
a long contact to the methyl-pyridyl nitrogen atom. The experi-
mental and computational analyses reveale that the supramolecu-
p
acceptor [47]. C–Hꢀ ꢀ ꢀ
p
lar packing is organized by weak C–Hꢀ ꢀ ꢀ
interactions in 1 and by two C–Hꢀ ꢀ ꢀO and ꢀ ꢀ ꢀ
In both compounds the zinc-coordinated methyl-pyridyl ring is
engaged in interactions while the non-coordinated methyl-
ꢀ ꢀ ꢀ
pyridyl ring in 1 is involved in the C–Hꢀ ꢀ ꢀ interactions. We sug-
gest a synergistic effect of the weak Znꢀ ꢀ ꢀN coordination by reduc-
ing the -electron density and consequently the repulsion.
ꢀ ꢀ ꢀ
interactions appear to be compet-
p
and a few
p interactions in 2.
p
ꢀ ꢀ ꢀ
p
p
coordination. Absence of C–Hꢀ ꢀ ꢀ
p interactions in 2 is also con-
firmed by Hirshfeld 2D fingerprint as there is no characteristic
wings appeared in the plot (Fig. 7, right). Here 2D fingerprint plot
shows two sharp (short) features near 1.4, 1.1 (di, de), correspond-
ing to the closest C–Hꢀ ꢀ ꢀO contact [47].
p p
p
p
p p
An analysis of the
p
ꢀ ꢀ ꢀ
p
interaction in compounds 1 and 2
On the other hand, the C–Hꢀ ꢀ ꢀ
p
reveals that the weakly zinc-coordinated methyl-pyridyl ring(s)
in both compounds is at the same time also engaged in inter-
ꢀ ꢀ ꢀ
actions (Fig. 8). The number of interactions found by PLATON is
itive in strength to compete against the weak Znꢀ ꢀ ꢀN coordination.
p
p
pꢀ ꢀ ꢀp
somewhat larger for 2 than for 1 (see Tables S3 S4, and Figs. S3, S4
Acknowledgements
in Supporting Information). In summary, we conclude that the
strength of and energy gain from the C–Hꢀ ꢀ ꢀ
competitive and superior to the formation of a weak Znꢀ ꢀ ꢀN bond.
We also conclude that interaction and weak Zn–N coordina-
ꢀ ꢀ ꢀ
tion may be a synergistic effect in that the Zn coordination lowers
the -electron density, thereby decreasing the repulsion [46].
ꢀ ꢀ ꢀ
p
interactions in 1, is
Ministry of Science and Technology (MOST), Dhaka, Bangladesh
is acknowledged for financial support under the project 2012/13.
We thank the Alexander von Humboldt Foundation (AvH) for a
fellowship to ME. We acknowledge the Wazed Miah Science
Research Centre at Jahangirnagar University, Dhaka, Bangladesh
p
p
p
p p

M. Enamullah et al. / Inorganica Chimica Acta 427 (2015) 103–111
111
for obtaining elemental data. For allocating computational
resource and time, we are also thankful to Professor Raymond
Poirier, Department of Chemistry, Memorial University, Canada,
and the Atlantic Computational Excellence Network (ACEnet).
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