A series of hydrogen bond mediated dinuclear nickel(II) complexes with reduced Schiff base ligands: An insight into the nature of their short intermolecular hydrogen bonds

Article abstract of DOI:10.1016/j.poly.2020.114374

A series of isostructural centrosymmetric hydrogen bonded dimeric nickel(II) complexes of general formula, [Ni₂(HL)₂(DMSO)₂(H₂O)₂]X {where H₂L = N₂O₄ donor reduced Sch

Full text of DOI:10.1016/j.poly.2020.114374

Journal Pre-proofs
A series of hydrogen bond mediated dinuclear nickel(II) complexes with re-
duced Schiff base ligands: An insight into the nature of their short intermolecular
hydrogen bonds
Saikat Mirdya, Michael G.B. Drew, Akash Kumar Chandra, Abhisek Banerjee,
Antonio Frontera, Shouvik Chattopadhyay
PII:
S0277-5387(20)30031-0
DOI:
https://doi.org/10.1016/j.poly.2020.114374
POLY 114374
Reference:
To appear in:
Polyhedron
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Accepted Date:
20 November 2019
15 January 2020
Please cite this article as: S. Mirdya, M.G.B. Drew, A.K. Chandra, A. Banerjee, A. Frontera, S. Chattopadhyay, A
series of hydrogen bond mediated dinuclear nickel(II) complexes with reduced Schiff base ligands: An insight into
the nature of their short intermolecular hydrogen bonds, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly.
2
020.114374
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A series of hydrogen bond mediated dinuclear nickel(II) complexes
with reduced Schiff base ligands: An insight into the nature of their
short intermolecular hydrogen bonds
a
b
a
Saikat Mirdya , Michael G. B. Drew, Akash Kumar Chandra , Abhisek
a,*
c,*
a,*
Banerjee , Antonio Frontera , Shouvik Chattopadhyay
aDepartment of Chemistry, Inorganic Section, Jadavpur University, Kolkata-700032, India. Tel: +033-
2
4572941, e-mail: shouvik.chem@gmail.com
bSchool of Chemistry, The University of Reading, P. O. Box 224, Whiteknights, Reading
RG6 6AD, UK
cDepartament de Química, Universitat de les Illes Balears, Crta de Valldemossa km 7.5, 07122 Palma de
Mallorca (Baleares), SPAIN, e-mail: toni.frontera@uib.es
Abstract
A series of isostructural centrosymmetric hydrogen bonded dimeric nickel(II) complexes
of general formula, [Ni (HL) (DMSO) (H O) ]X {where H L = N O donor reduced Schiff base
2
2
2
2
2
2
2
4
1
ligand, [H L =(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol) and
2
2
H L =(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)] and X=counter
2
anion}, are synthesized and characterized by elemental analysis, infrared and electronic spectra.
The structures of all complexes have been confirmed by single crystal X-ray diffraction analysis.
Each complex forms O···H···O hydrogen bonded dimer in the solid state, where the O···O
distance is very short, ranging from 2.424(3)-2.437(3) Å. The hydrogen bonds are very strong as

can be seen from the DFT estimated energy of these hydrogen bonds. Non-covalent interaction
plot (NCI plot) index has been used to characterize them.
1
. Introduction
The interaction energy of conventional hydrogen bonds basically depends on their
geometric features like length and linearity. However, the difference in the pK values (where
a
pK is the hegative logarithm of K ) of the heavy atoms sharing the proton is also important [1].
a
a
1
The ΔH of formation of the water dimer is ∼5 kcal/mol in the gas phase. However, the HO–
H···OH hydrogen bond in water is weaker due to the large difference between the pK values of
2
a
+
the donor group and the conjugate acid of the acceptor group (15.7 for H O and−1.7 for H O )
2
3
[2]. In the gas and solid state phases, hydrogen bonds between hetero atoms with similar pKa
−
1
values can be very short and strong (up to 30 kcal mol ) [3-6]. When groups of equal pK
a
establish an hydrogen bond, three different situations different situations may occur (see Figure
). The classical hydrogen bond where the hydrogen atom is covalently bonded to one of both
1
oxygen atoms (dipole···dipole interaction, see Figure 1a). As the O···O distance is progressively
shortened, the energy barrier lowers until it reaches the lower vibrational energy levels, thus
leading to a low barrier hydrogen bond (LBHB, see Figure 1b). In this case, the hydrogen atom
can move freely between the two oxygen atoms with its average position is the center (high
covalent character) [7-8]. Further shortening leads to a single-well hydrogen bond with the
highest covalent character and interaction energy of the three situations.

Figure 1: Energy diagrams for OH···H hydrogen bonds between groups of identical pKa.
a)Standard hydrogen bond with O···O distance d(O···O) ≥ 2.8 Å. (b) Low barrier hydrogen
(
bond of d(O···O)~2.55 Å (average position in the center). (c)Single-well hydrogen bond with
d(O···O) ~2.3 Å.
In this manuscript, we report the synthesis and X-ray characterization of a series of
mononuclear nickel(II) complexes that form O···H···O hydrogen bonded dimers in the solid
state where the O···O distance is very short, ranging from 2.424(3)-2.437(3) Å. We have
analyzed the hydrogen bonds using DFT calculations and the non-covalent interaction plot (NCI
plot) index to distinguish between the three possible situations represented in Figure 1.
Keywords: Dinuclear, nickel, reduced Schiff base, hydrogen bond, DFT.
2
. Experimental Section
Materials
Nickel(II) thiocyanate tetrahydrate was prepared in our laboratory following the literature
method [9]. All other materials are commercially available, reagent grade and used as purchased
from Sigma-Aldrich without further purification.

Caution!!! Although no problems were encountered in this work, perchlorate salts in presence of
organic ligands are potentially explosive. Only a small amount of the materials should be
prepared and they must be handled with care.
2
2
2
.1 Synthesis
.1.1. Synthesis of Schiff base ligands
a
.1.1.1. Synthesis of H L [N,N'-bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine],
2
H2Lb [N,N'-bis(3-ethoxysalicylidene)-2,2-dimethyl-1,3-propanediamine], H Lc [N,N'-bis(3-
2
methoxysalicylidene)-1,3-propanediamine] and
H2Ld [N,N'-bis(3-ethoxysalicylidene)-1,3-
propanediamine]
A methanol solution (10 mL) of 2,2-dimethyl-1,3-propanediamine (1 mmol, 0.12 mL)
was mixed separately with of 3-methoxysalicylaldehyde (2 mmol, 304 mg) and 3-
ethoxysalicylaldehyde, (2 mmol, 332 mg) and the resulting solutions were allowed to reflux for
a
ca. 2 h to synthesize two hexadentate N O donor Schiff base ligands, H L [N,N'-bis(3-
2
4
2
b
ethoxysalicylidene)2,2-dimethyl-1,3-propanediamine] and H L [N,N'-bis(3-ethoxysalicylidene)-
2
c
2
,2-dimethyl-1,3-propanediamine] respectively. Other two hexadentate Schiff bases, H L [N,N'-
2
d
bis(3-methoxysalicylidene)-1,3-propanediamine] and H L [N,N'-bis(3-ethoxysalicylidene)-1,3-
2
propanediamine] were synthesized following the similar procedure as mentioned above except
1
,3-propanediamine (1 mmol, 0.11 mL) was used as the diamine instead of 2,2-dimethyl-1,3-
propanediamine.
2
.1.2. Synthesis of the reduced Schiff base ligands

2
.1.2.1.
Synthesis
of
H2L1 [(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-
methoxyphenol)]
a
The prepared Schiff base solution, H L was cooled to 0°C, and solid sodium borohydride
2
(4 mmol, 150 mg) was then added gently to this solution with constant stirring. The solution was
further acidified with glacial acetic acid (10 mL) and placed under reduced pressure in a rotary
evaporator (~60°C). The residue was dissolved in water (15 mL) and extracted with
dichloromethane (15 mL) using a separating funnel. Finally the dichloromethane part was dried
using anhydrous sodium acetate to get the desired hexadentate N O donor reduced Schiff base
2
4
1
ligand, H L . The ligand was not purified and used directly for the synthesis of complex 1.
2
2
.1.2.2.
Synthesis
of
H2L2 [(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-
ethoxyphenol)]
A potential hexadentate N O donor reduced Schiff base was synthesized following the
2
4
1
b
similar procedure as used for Schiff base, H L , except H L was used as the Schiff base
2
2
derivative. It was not purified and was used directly for the synthesis of complex 2.
3
2
.1.2.3. Synthesis of H L [(1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol)]
2
A potential hexadentate N O donor reduced Schiff base was synthesized following the
2
4
c
same procedure as described above, except H L was used as the Schiff base derivative. It was
2
not purified and was used directly for the synthesis of complex 3.
4
2
.1.2.4. Synthesis of H L [(1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)]
2

A potential hexadentate N O donor reduced Schiff base was synthesized following the
2
4
d
same procedure as described above, except H L was used as the Schiff base derivative. It was
2
not purified and was used directly for the synthesis of complex 4.
2
2
.1.3. Synthesis of complexes
1
.1.3.1. Synthesis of [Ni (HL ) (DMSO) (H O) ](ClO ) ∙2H O (1)
2
2
2
2
2
4 2
2
A methanol solution of nickel(II) perchlorate hexahydrate, (0.99 mmol, 257 mg), was
1
added into the methanol solution of a reduced Schiff base ligand, H L , with constant stirring and
2
colour of the solution turned into dark green. Few drops of DMSO were added and the resulting
solution was kept for crystallization. Dark green single crystals, suitable for X-ray diffraction,
were obtained after 3-4 days on slow evaporation of the solution in open atmosphere.
Yield: 350 mg, (~54%, based on Ni). Anal. Calc. for C H N NiO10.5SCl (FW = 636.77): C,
2
3
38
2
-1
4
2.87; H, 5.74; N, 4.35%. Found: C, 42.8; H, 5.8; N, 4.3%. FT-IR (KBr, cm ): 3400 (νO-H), 3270
3
-1
-1
2
(
νN-H), 2990-2850 (νC-H). UV-Vis,  max (nm), [εmax (dm mol cm )] (DMSO), 730 (1.01x10 ),
2
3
3
6
25 (2.00x10 ), 355 (2.65x10 ), 280 (5.6 x10 ).
2
2
.1.3.2. Synthesis of [Ni (HL ) (DMSO) (H O) ](ClO ) (2)
2
2
2
2
2
4 2
Complex 2 was synthesized following the similar procedure as that for complex 1 except
2
1
that the reduced Schiff base ligand, H L , was used instead of H L . Dark green coloured
2
2
crystalline complex suitable for X-ray diffraction, started to separate from the solution after 3-4
days on standing at room temperature and was collected by filtration.
Yield: 343 mg, (~ 53%, based on Ni). Anal. Calc. for C H ClN NiO S (FW = 655.82): C,
2
5
41
2
20
-1
5
3.5; H, 5.8; N, 4.2%. Found: C, 53.4; H, 5.7; N, 4.3%. FT-IR (KBr, cm ): 3395 (νO-H), 3280

3
-1
-1
2
(
νN-H), 2990-2865 (νC-H). UV-Vis,  max (nm), [εmax (dm mol cm )] (DMSO), 720 (0.89x10 ),
2
3
4
6
20 (1.80x10 ), 355 (4.84x10 ), 274 (9.38x10 ).
3
2
.1.3.3. Synthesis of [Ni (HL ) (DMSO) (H O) ](NCS) ∙2CH OH (3)
2
2
2
2
2
2
3
A methanol solution of nickel(II) thiocyanate tetrahydrate (1.01 mmol, 250 mg) was
3
added into the methanol solution of a reduced Schiff base ligand, H L , with constant stirring and
2
colour of the solution turned into dark green. Few drops of DMSO were added and the resulting
solution was kept for crystallization. Dark green single crystals, suitable for X-ray diffraction,
were obtained after 2-3 days on slow evaporation of the solution in open atmosphere.
Yield: 392 mg, (~ 60%, based on Ni). Anal. Calc. for C H N NiO S (FW = 654.48): C, 44.0;
2
4
41
3
8 3
-1
H, 6.2; N, 6.4%. Found: C, 43.7; H, 6.2; N, 6.6%. FT-IR (KBr, cm ): 3390 (νO-H), 3260 (νN-H),
3
-1
-1
3
012-2840 (νC-H), 2045 (νNCS). UV-Vis,  max(nm), [εmax (dm mol cm )] (DMSO), 718
2
3
2
4
(
1.40x10 ), 622 (2.78x10 ), 355 (4.34x10 ), 280 (9.49x10 ).
4
2
.1.3.4. Synthesis of [Ni (HL ) (DMSO) (H O) ](ClO ) (4)
2
2
2
2
2
4 2
Complex 4 was synthesized following the similar procedure as that for complex 3 except
4
3
that the reduced Schiff base ligand, H L , was used instead of H L . Dark green coloured
2
2
crystalline complex suitable for X-ray diffraction, started to separate from the solution after 2-3
days on standing at room temperature and was collected by filtration.
Yield: 344 mg, (~54%, based on Ni). Anal. Calc. for C H N NiO SCl (FW = 627.75): C,
2
3
37
2
10
-1
4
3.9; H, 5.9; N, 4.4%. Found: C, 43.8; H, 5.8; N, 4.5%. FT-IR (KBr, cm ): 3420 (νO-H), 3265
3
-1
-1
2
(
νN-H), 2995-2870 (νC-H). UV-Vis,  max (nm),[εmax (dm mol cm )] (DMSO), 755 (0.58x10 ),
2
3
3
6
35 (0.93x10 ), 360 (3.8x10 ), 271 (8.86x10 ).

2
.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were carried out using a Perkin
-1
Elmer 240C elemental analyzer. IR spectra in KBr (4500-500 cm ) were recorded with a Perkin
Elmer Spectrum Two spectrophotometer. Electronic spectra in acetonitrile (200-800nm) were
recorded on a Perkin Elmer Lambda 35 UV-visible spectrophotometer.
2
.3. X-ray crystallography
Suitable single crystals of all the complexes were used for data collection using a ‘Bruker
D8 QUEST area detector’ diffractometer equipped with graphite-monochromated Mo Kα
radiation ( = 0.71073 Å). The molecular structures were solved by direct methods and refined
2
by full-matrix least squares on F using the SHELXL-16/6 package [10]. Non-hydrogen atoms
were refined with anisotropic thermal parameters. Hydrogen atoms attached to nitrogen and
oxygen atoms were located by difference Fourier maps and refined with distance constraints.
There were two molecules in the asymmetric unit in complexes 1 and 2. In complex 3, it proved
impossible to locate the thiocyanate anion due to excessive disorder and the SQUEEZE option
was applied via the PLATON program [11] to account for the electron density. The calculation
showed 174 electrons in the voids which would allow for one thiocyanate anion and a methanol
solvent molecule in the asymmetric unit. This program was also used for the cavity in complex 2.
All four structures contained very short intramolecular OH∙∙∙O distances in the range 2.424(3)-
2
.437(3) Å. It was assumed that it was most likely that the hydrogen atom occupied a central
position between the oxygen atoms in these structures and distance constraints were applied
accordingly such that the hydrogen was so placed. Other hydrogen atoms bonded to oxygen and
nitrogen atoms were refined with distance constraints but those bonded to carbon were placed in

their geometrically idealized positions and constrained to ride on their parent atoms. Disorder
was also modelled in the perchlorate anions and the attached ethanol molecules in 2 and 4.
Multi-scan empirical absorption corrections were applied to the data using the program
SADABS [11]. A summary of the crystallographic data has been given in Table 1. Programs
used: SHELX-2016/6[10], PLATON [11], WINGX [12], ORTEP [13] and MERCURY [14].
Selected bond lengths for all four complexes are given in Tables 2. Selected bond angles of all
complexes are given in Table S2 (see supporting information, SI).
Table 1: Crystal data and refinement details of complexes 1-4.
Complex
Formula
1
2
3
4
C H N NiO S+
2
1
33
2
6
C H N NiO10.5SCl
C H ClN NiO S
2
3
38
2
25 41
2
20
C H N NiO SCl
2
3
37
2
10
NCS anion + 2 methanol
solvent molecules
Formula
Weight
6
36.77
655.82
500.24
627.75
Crystal System
Space group
a(Å)
Triclinic
Triclinic
Monoclinic
Monoclinic
C2/c
P
P
P2 /n
1
13.044(11)
14.575(13)
12.344(3)
14.130(3)
13.422(1)
15.229(4)
22.317(2)
15.316(11)
b(Å)

c(Å)
α(°)
β(°)
γ(°)
V(Å3)
Z
17.002(15)
83.19(2)
68.10(4)
88.22(15)
2978.0(5)
2
20.995(4)
80.25(5)
78.99(5)
65.07(5)
3243.6(12)
2
13.651(2)
(90)
16.547(13)
(90)
106.16(3)
(90)
91.15(3)
(90)
2680.4(6)
2
5655.0(7)
4
-3
d(calc) [gcm ]
1.418
1.343
1.240
0.835
1060
1.475
0.910
2640
-
1
µ [ mm ]
0.866
0.796
F(000)
1336
1384
Total
8
1257
75605
11546
28688
4778
34336
5009
Reflections
Unique
9
526
266
Reflections
Observed data
8
7735
3638
4135
[I > 2 σ(I)]
R(int)
0.032
0.070
0.090
0.042
*
R1, wR2 (all
0.0546, 0.1517
0.1170, 0.2655
0.0676, 0.1439
0.0706, 0.1861

data)
R1, wR2 ([I >
0
.0470, 0.1429
0.0772, 0.2120
1.047. -0.958
0.0469, 0.1222
0.512, -0.614
0.0594, 0.1747
1.520, -0.950
2
σ(I)]
Residual
Electron
Density
1
.423, -0.815
-
3
(
eÅ )
2
2 2
2 1/2
*
R1 = Σ||F |-|F ||/Σ|F | and wR2 = Σw(|F | -|F | ) /Σw|F | )
o c o o c o
2
.4 Theoretical Methods
The energies of the complexes reported in this study were computed at the B3LYP-
D/def2-SVP level of theory using the crystallographic coordinates by means of the Gaussian-09
program [15]. The Grimme’s dispersion [16] correction has also been used since it is adequate
for the evaluation of non-covalent interactions. The basis set superposition error for the
calculation of interaction energies has been corrected using the counterpoise method [17]. The
NCI index and NCI plot [18] isosurfaces have been used to characterize the non-covalent
interactions. They correspond to both favorable and unfavorable interactions, as differentiated by
the sign of the second density Hessian eigen value and defined by the isosurface color. The color
scheme is a red-yellow-green-blue scale with red for ρ+ (repulsive) and blue for ρ−
cut
cut
(attractive), whereas yellow and green isosurfaces correspond to weak repulsive and weak
attractive interactions, respectively [19].

3
. Results and discussion
3
.1. Synthesis
Two moles of 3-alkoxysalicylaldehydes (3-methoxy and 3-ethoxy) have been used to
condense separately with 2,2-dimethyldiaminopropane and 1,3-diaminopropane in methanol
a
b
c
d
medium to prepare four Schiff bases, H L , H L , H L and H L . These Schiff bases were then
2
2
2
2
reduced with NaBH in methanol under constant stirring at 0°C to prepare four respective
4
1
2
3
4
reduced Schiff bases, H L , H L , H L and H L respectively following the literature method
2
2
2
2
[20].
Scheme 1: Synthetic route to ‘reduced Schiff base’ ligands.
These reduced Schiff bases on then reaction with nickel(II) perchlorate hexahydrate or
nickel(II) thiocyanate in methanol and on adding few drops of DMSO to the individual reaction
mixture gave rise to complexes 1-4.

Scheme 2: Synthetic route to complexes 1 (R =Me, R =Me), 2 (R =Me, R =Et), 3 (R =H,
1
2
1
2
1
R =Me) and 4 (R =H, R =Et). Non coordinated counter anions have not been shown here.
2
1
2
Use of reduced Schiff base ligands seems to be crucial in forming these types of
hydrogen bonded dimeric complexes, as the non-reduced Schiff bases were unable to produce
such complexes. Many synthetic inorganic chemistry groups have prepared many homo and
heteronuclear complexes with the corresponding non-reduced Schiff base ligands, but did not
obtain complexes similar to complexes 1-4. This is probably due to the greater flexibility of the
reduced Schiff bases compared to their non-reduced precursors. .
3
3
.2. Description of structures
1
2
.2.1.[Ni (HL ) (DMSO) (H O) ](ClO ) ∙H O (1), and [Ni (HL ) (DMSO) (H O) ](ClO ) (2)
2
2
2
2
2
4 2
2
2
2
2
2
2
4 2
X-ray crystal structure determinations reveal that complexes 1 and 2 crystallize in
triclinic space group, P with two molecules in the asymmetric unit. Potential N O donor
2
4
1
2
hexadentate reduced Schiff bases, H L and H L , have been used to prepare these complexes
2
2
respectively. Each complex actually possesses a dinuclear moiety formed via strong hydrogen
bonds. Each structure consists of two independent centrosymmetric dimers (A and B) with
equivalent geometry. Views of the A dimers of complexes 1 and 2 are given in Figures 2 and 3
respectively. The structure of dimer B of each complex is very similar to the structure of dimer A
(Figures S1 and S2 in SI).

Figure 2: Perspective view of dimer A of complex 1 with selective atom-numbering scheme.
Non co-ordinated perchlorate ions have been omitted for clarity. Only the hydrogen atoms
between the phenolic oxygen atoms are shown.
In each dimeric unit, nickel(II) centers are hexa-coordinated and adopt distorted
octahedral geometry. Each nickel(II) center, {Ni(1) of A and Ni(2) of B} is equatorially
coordinated by two amine nitrogen atoms, {N(1) and N(2) for Ni(1) and N(3) and N(4) for
Ni(2)}, two phenoxo oxygen atoms, {O(1) and O(2) for Ni(1) and O(7) and O(8) for Ni(2), of
the reduced Schiff base units. The axial sites are coordinated by two oxygen atoms, {O(3) and
O(4) for Ni(1) and O(9) and O(10) for Ni(2)} of coordinated water and DMSO molecule. Each
of the reduced Schiff bases are dibasic in character, but loses only one proton to form
mononegative anion, which, in turn, connects with another such moiety via strong hydrogen
bonding to form the symmetric dimeric structure. The deprotonations of the hydroxyl group are
well reflected in the nickel(II)-oxygen bond lengths (Table 2). The deprotonated hydroxyl groups
are bound strongly (shorter bond length) with nickel(II) centers compared with the non-

deprotonated hydroxyl groups [21]. The saturated six membered chelate rings, Ni(1)-N(1)-C(9)-
C(10)-C(13)-N(2) and Ni(1)-N(1)-C(10)-C(11)-C(14)-N(2), in dimer A of complexes 1 and 2
respectively represent individual chair conformations with puckering parameters [22], q(2) =
0
.583(4) Å, φ = 168(2) , and q(2) = 0.589(8) Å, φ = 199(7) respectively. Similarly the
saturated six membered chelate rings, Ni(2)-N(3)-C(32)-C(33)-C(36)-N(4) and Ni(2)-N(3)-
C(35)-C(36)-C(39)-N(4), in dimer B of complexes 1 and 2 respectively also represent
individual chair conformations with puckering parameters, q(2) = 0.563(7) Å, φ = 180(3) and
q(2) 0.563(7) Å, φ = 180(3) respectively.
Figure 3: Perspective view of dimer A of complex 2 with selective atom-numbering scheme.
Non co-ordinated counter anions are not shown. Hydrogen atoms are omitted for clarity apart
from those between the phenolic oxygen atoms.
3
4
3
.2.2. [Ni (HL ) (DMSO) (H O) ](NCS) ∙2CH OH (3) and [Ni (HL ) (DMSO) (H O) ](ClO ) (4)
2 2 2 2 2 2 3 2 2 2 2 2 4 2
X-ray crystal structure determination reveals that complexes 3 and 4 crystallize in

monoclinic space groups P2 /n and C2/c respectively. Potential N O donor hexadentate reduced
1
2
4
3
4
Schiff bases, H L and H L , have been used to prepare complexes 3 and 4 respectively. Each
2
2
complex contains one centrosymmetric dinuclear moiety formed via strong hydrogen bonds. The
structures of complexes 3 and 4 are jointly shown in Figure 4. The geometry around each nickel(II)
centers is quite similar to that described for complexes 1-2 . Both the nickel(II) centers are hexa-
coordinated and adopt distorted octahedral geometry. Each nickel(II) center is equatorially
coordinated by two amine nitrogen atoms, {N(1) and N(2)} and two phenoxo oxygen atoms, {O(1)
and O(2)}, of the reduced Schiff base unit. The axial sites are coordinated by two oxygen atoms,
{
O(3) and O(4)}, of coordinated water and DMSO molecule respectively. The saturated six
membered chelate rings, Ni(1)-N(1)-C(9)-C(10)-C(11)-N(2), Ni(1)-N(1)-C(10)-C(11)-C(12)-
N(2), of complexes 3 and 4 respectively represent individual chair conformations with puckering
parameters, q(2) = 0.610(6) Å, φ = 3(9) and q(2) = 0.589(5) Å, φ = 182(2) respectively.
Figure 4: Perspective view of complexes 3 (left) and 4 (right) with selective atom-numbering
scheme. Non co-ordinated perchlorate ions are not shown. Hydrogen atoms are omitted for
clarity apart from those between the phenolic oxygen atoms.

Table 2: Selected bond lengths (Å) of complexes 1-2 (both dimer A and B) and complexes 3-4.
Complex
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-O(3)
Ni(1)-O(4)
Ni(1)-N(1)
Ni(1)-N(2)
Ni(2)-O(8)
Ni(2)-O(9)
Ni(2)-O(10)
Ni(2)-N(3)
Ni(2)-N(4)
Ni(2)-O(7)
1
2
3
4
2.079(3)
2.071(3)
2.101(3)
2.080(3)
2.096(3)
2.102(4)
2.081(3)
2.077(3)
2.103(4)
2.095(3)
2.095(3)
2.078(3)
2.085(4)
2.086(5)
2.087(5)
2.074(5)
2.100(7)
2.092(6)
2.076(4)
2.089(5)
2.077(4)
2.084(5)
2.094(4)
2.080(4)
2.076(2)
2.096(3)
2.090(2)
2.101(3)
2.089(2)
2.092(3)
2.085(3)
2.087(3)
2.097(3)
2.096(3)
2.090(3)
2.103(3)
-
-
-
-
-
-
-
-
-
-
-
-
3
.3. Supramolecular interactions in solid state
The details of hydrogen bonding interactions in complexes 1-4 are given in Table 4.
Complexes 1 and 2 show similar kinds of hydrogen bonding interactions. Three hydrogen atoms,
H(1O), H(3A) and H(3B), available in the ligand part in each complex are involved in effective

hydrogen bonding. A hydrogen atom, H(1O), attached to a phenoxy oxygen atom, O(1), forms a
strong intermolecular hydrogen bonding interaction with symmetry related phenoxy oxygen
atom, O(2), which leads to the formation of a dimeric unit, {Ni} . Two remaining hydrogen
2
atoms, H(3A) and H(3B), attached to oxygen atom, O(3), are involved in intermolecular
hydrogen bonding interactions with symmetry related alkoxy oxygen atoms, O(6) and O(5),
respectively. Similar kinds of hydrogen bonding interactions are also observed in dimer B unit in
each complex (Table 4). Pictorial representation of hydrogen bond interactions in dimer A of
complexes 1 and 2 are shown in Figures 5 and 6.
Figure 5: Perspective view of intermolecular hydrogen bonding interactions in dimer A of
complex 1. Only the relevant hydrogen atoms have been shown for clarity. Symmetry
a
transformation: =1-x, 1-y, 1-z.
However in complexes 3 and 4, a hydrogen atom, H(1O) attached to a phenoxy oxygen
atom, O(1), is hydrogen bonded with its adjacent symmetry related phenoxy oxygen atom, O(2)
to form a dimeric unit. Additionaly in complex 3, two other hydrogen atoms, H(3A) and H(3B),

attached to oxygen atom, O(3), are involved in intermolecular hydrogen bonding interactions
with symmetry related alkoxy oxygen atoms, O(5) and O(6), respectively. The hydrogen
bonding interactions in complexes 3 and 4 are shown in Figure S4 and S5 (supporting
information) respectively.
Figure 6: Perspective view of intermolecular hydrogen bonding interaction in dimer A of
complex 2. Coordinated dmso molecules have been omitted for clarity. Only the relevant
b
hydrogen atoms are shown here. Symmetry transformation: =1-x, -y, 1-z.
Table 4: Hydrogen bond distances (Å) and angles (◦) for the complex 1-4.
Complex
D–H···A
D-H
H···A
1.22(3)
1.99(4)
D···A
 D-H···A
165(6)
O(1)-H(1O)···O(2)a
O(3)-H(3A)···O(6)a
1.23(3)
0.85(5)
2.426(4)
2.804(5)
1
163(4)

O(3)-H(3B)···O(5)a
O(8)-H(8O)···O(7)e
O(10)-H(10A)···O(12)e
O(10)-H(10B)···O(11)e
O(1)-H(1O)···O(2)b
O(3)-H(3A)···O(6)b
O(3)-H(3B)···O(5)b
O(8)-H(8O)···O(7)f
O(9)-H(9A)···O(12)f
O(9)-H(9B)···O(11)f
O(1)-H(1O)···O(2)c
O(3)-H(3A)···O(5)c
O(3)-H(3B)···O(6)c
O(1)-H(1O)···O(2)d
O(3)-H(31)···O(5)d
0.86(4)
1.20(3)
0.85(4)
0.84(3)
1.22(6)
0.86(4)
0.85(6)
1.21(6)
0.85(4)
0.84(6)
1.23(4)
0.84(3)
0.84(4)
1.23(7)
0.83(3)
1.95(4)
1.23(3)
1.95(4)
1.96(2)
1.21(6)
2.05(6)
2.00(6)
1.23(6)
2.02(4)
1.97(7)
1.21(3)
1.94(3)
1.93(4)
1.25(7)
2.06(4)
2.777(5)
2.424(4)
2.767(4)
2.762(4)
2.424(6)
2.821(8)
2.825(8)
2.427(6)
2.831(6)
2.793(6)
2.433(3)
2.759(4)
2.766(4)
2.437(4)
2.838(5)
162(4)
172(3)
161(4)
160(4)
174(9)
150(8)
163(5)
168(5)
162(8)
167(6)
169(3)
165(4)
175(5)
159(5)
154(6)
2
3
4

a
b
D = donor; H = hydrogen; A = acceptor, Symmetry transformations: =1-x, 1-y, 1-z, = 1-x, -y,
c
d
e
f
1
-z, = 2-x,1-y,1-z, = 3/2-x,3/2-y,1-z, = 1-x, 2-y, 2-z, = -x,1-y,2-z.
These hydrogen bonded dimeric units are further stabilized by strong C-H   
interactions. The details of C-H    interactions are listed in Table 5. However no significant
··· interactions are found in any of the complexes. Complex 1 shows significant C-H   

interactions involving the hydrogen atom, H(1B), attached to a carbon atom, C(1), with suitable
symmetry related phenyl ring, C(38)-C(39)-C(40)-C(41)-C(42)-C(43). Again two hydrogen
atoms, H(22B) and H(23C) attached to two different methyl carbon atoms, C(22) and C(23),
respectively are involved in intramolecular C-H    interactions with two separate phenyl
rings, C(2)-C(3)-C(4)-C(5)-C(6)-C(7) and C(15)-C(16)-C(17)-C(18)-C(19)-C(20), respectively.

Figure 7: Perspective view of significant C-H∙∙∙ interactions in complex 1 with selective atom
numbering scheme. Only relevant atoms are shown in the figure for clarity. Symmetry
g
transformation: = x, y, -1+z.
Moreover two similar hydrogen atoms, H(45B) and H(46B), attached to two different
methyl carbon atoms, C(45) and C(46), respectively of another subunit show similar type of
intramolecular C-H    interactions with two separate phenyl rings, C(25)-C(26)-C(27)-
C(28)-C(29)-C(30) and C(38)- C(39)-C(40)-C(41)-C(42)-C(43), respectively. The hydrogen
bonding interactions in complex 1 are shown in Figure 7.
Complexes 2 and 3 exhibit similar types of intermolecular C-H    interactions with
phenyl rings. Two hydrogen atoms, H(24B) and H(25C), in complex 2 and H(20A) and H(21C)
in complex 3, attached to the respective carbon atoms, C(24), C(25) and C(20) and C(21)
respectively, participate in intermolecular C-H    interactions with the phenyl rings, C(3)-
C(4)-C(5)-C(6)-C(7)-C(8), C(16)-C(17)-C(18)-C(19)-C(20)-C(21), C(2)-C(3)-C(4)-C(5)-C(6)-C(7)
and C(13)- C(14)-C(15)-C(16)-C(17)-C(18), respectively. The hydrogen bonding interactions in
complexes 2 and 3 are shown in Figure 8.

Figure 8: Perspective view of intramolecular C-H··· stacking interactions in complexes 2 (left)
and 3 (right). Only relevant hydrogen atoms are shown in the figure for clarity.
Table 5: Geometric features (distances in Å and angles in °) of the C-H··· interactions obtained
for complexes 1-4.
Complexes
C-H···Cg(ring)
H···Cg(Å)
C-H···Cg
C···Cg (Å)
C(1)-H(1B)···Cg(1)g
C(22)-H(22B)···Cg(2)
C(23)-H(23C)···Cg(3)
C(45)-H(45B)···Cg(4)
2.65
2.92
2.88
2.80
165
155
140
144
3.585(6)
3.814(8)
3.667(9)
3.626(7)
1

C(46)-H(46C)···Cg(1)
C(24)-H(24B)···Cg(4)
C(25)-H(25C)···Cg(5)
C(20)-H(20A)···Cg(4)
C(21)-H(21C)···Cg(5)
C(11)-H(11A)···Cg(4)h
C(22)-H(22C)···Cg(5)
C(23)-H(23B)···Cg(4)
2.75
2.79
2.81
2.74
2.62
2.62
2.68
2.91
157
156
145
151
152
146
146
152
3.649(6)
3.683(12)
3.648(14)
3.608(6)
3.500(6)
3.463(6)
3.523(7)
3.787(8)
2
3
4
g
h
Symmetry transformation: = x, y, -1+z, = 3/2-x,-1/2-y,1/2-z.
For complex 1: Cg(1) = Centre of gravity of the ring [C(38)- C(39)-C(40)-C(41)-C(42)-C(43)]
Cg(2) = Centre of gravity of the ring [C(2)- C(3)-C(4)-C(5)-C(6)-C(7)]
Cg(3) = Centre of gravity of the ring [C(15)- C(16)-C(17)-C(18)-C(19)-C(20)]
Cg(4) = Centre of gravity of the ring [C(25)- C(26)-C(27)-C(28)-C(29)-C(30)]
For complex 2: Cg(4) = Centre of gravity of the ring [C(3)- C(4)-C(5)-C(6)-C(7)-C(8)]
Cg(5) = Centre of gravity of the ring [C(16)- C(17)-C(18)-C(19)-C(20)-C(21)]
For complex 3: Cg(4) = Centre of gravity of the ring [C(2)- C(3)-C(4)-C(5)-C(6)-C(7)]
Cg(5) = Centre of gravity of the ring [C(13)- C(14)-C(15)-C(16)-C(17)-C(18)]
For complex 4: Cg(4) = Centre of gravity of the ring [C(3)- C(4)-C(5)-C(6)-C(7)-C(8)]
Cg(5) = Centre of gravity of the ring [C(14)- C(15)-C(16)-C(17)-C(18)-C(19)]
In complex 4, H(22C) and H(23B), attached to the methyl carbon atom, C(22) and C(23),
respectively, participate in intramolecular C-H    interactions with two separate phenyl
rings, C(14)-C(15)-C(16)-C(17)-C(18)-C(19) and C(3)-C(4)-C(5)-C(6)-C(7)-C(8), respectively

while another hydrogen atom, H(11A), attached to a carbon atom, C(11), is involved in
intermolecular C-H    interaction with proper symmetry related phenyl ring, C(3)-C(4)-
C(5)-C(6)-C(7)-C(8) to form a supramolecular chain (Figure 9).
Figure 9: Perspective view of a supramolecular chain through intermolecular C-H··· stacking
interactions in complex 4. Only relevant atoms are shown in the figure for clarity. Symmetry
h
transformation: = 3/2-x,-1/2-y,1/2-z.
3
.4. Hirshfeld surfaces analysis
The Hirshfeld surface emerged from an attempt to delineate the space covered by a
molecule in a crystal for the purpose of dividing the crystal electron density into molecular
fragments [23]. The Hirshfeld surfaces of four complexes are mapped over dnorm (range -0.1 Å to
1
.5 Å), shape index and curvedness (Figure 10). Red spots appear on the Hirshfeld surfaces
mapped with dnorm represent the dominant interactions within the complex in solid state. Various
intermolecular interactions are summed up effectively in the spots with the large circular
depressions (deep red) noticeable on the dnorm surfaces indicates the dominance of hydrogen
bonding interactions and other weak interactions. The 2D fingerprint plot of Hirshfeld surfaces
for all complexes and the comparative contributions of different interactions overlapping in full

fingerprint plots are provided in Figure S5. In the 2D fingerprint plot intermolecular interactions
appear as distinct spikes. Complementary regions are observable in the 2D fingerprint plots
where one molecule act as donor (d > d ) and the other as an acceptor (d < d ). The relative
e
i
e
i
percentages of intermolecular interactions of four complexes are shown in Figure S5 (see SI).
Figure 10: Hirshfeld surfaces mapped with dnorm (left column), shape index (middle) and
curvedness (right column) of complexes 1-4.
3
.5. DFT results
The X-ray structures of complexes 1–4 reveal that they crystallize forming dicationic
dimers where two strong hydrogen bonds are formed between the phenoxy oxygen atoms of the
ligands (Figures 2-4). The O···O distance is shorter than 2.44 Å and both O atoms are of
identical pK . Therefore, these hydrogen bonds have strong covalent character and could
a

therefore be examples of LBHBs or single-well hydrogen bonds. The nature of these hydrogen
bonds has therefore been analyzed using B3LYP-D/def2-SVP calculations. The energy profile of
complex 1 has been computed as a representative model. The plot of the energy profile is shown
in Figure 11 and shows that the hydrogen bonds correspond to LBHBs with a barrier of 2.36
kcal/mol for the synchronous movement of both hydrogen atoms in opposite directions.
Therefore the barrier for each hydrogen bond is only 1.18 kcal/mol, in agreement with the
short O···O distance (2.426(4) Å in the model used for the calculations). These results suggest
that in complexes 1–4 the hydrogen atoms can move freely between the two oxygen atoms and
on average are positioned at the centre of the O···O vector which is consistent with their
placement in the structure refinement.
Figure 11: Energy diagram obtained for complex 1 at the B3LYP-D/def2-SVP level of theory.
The energies are relative to the maximum. In the calculation the two hydrogen atoms were
moved in opposite directions to maintain the inversion center of symmetry.
The other additional hydrogen bonds between the coordinated water molecules O(3) and
the methoxy groups of the ligand, O(5) and O(6) (see blue dashed lines in Figure 12) have also

been analyzed. The dimerization energy is very large(ΔE = –107.7 kcal/mol) due to the
1
contribution of the strong LBHBs and the conventional O–H···O bonds that are expected to be
also strong due to the enhanced acidity of these protons due to the coordination of the water
molecule to the nickel(II) metal center. In an effort to estimate the contribution of the LBHBs, a
theoretical model has been used in which the water molecules are eliminated (Figure 12b). As a
result, the conventional hydrogen bonds are not established and the interaction energy drops to
ΔE = –73.3 kcal/mol, that corresponds to the contribution of both LBHBs and also some
2
additional van der Waals interactions established between the bulk of both molecules. This large
interaction energy confirms the strong covalent nature of these hydrogen bonds. This result is
agreement with previous investigations in hydrogen bonds between hetero atoms with identical
pK values where the ΔH of formation can approach 30 kcal/mol [4]. Furthermore, the difference
a
between ΔE andΔE provides a rough estimation of the four O–H···O bonds established by the
1
2
coordinated water molecules and methoxy groups (–8.6 kcal/mol per hydrogen bond).
Figure 12: Theoretical models used to evaluate the LBHBs in complex 1 (a) and a model
complex where the two coordinated water molecules have been eliminated (b). Distances in Å.

Finally, non-covalent interaction plot (NCI plot) index has been made to further
characterize the hydrogen bonds. It allows a direct assessment of host-guest complementarity
and the extent to which non-covalent interactions stabilize a complex. Figure 13 shows the NCI
plot obtained for the self-assembled dimer extracted from the solid state of complex 1. The NCI
index indicates that the LBHBs are covalent bonds, since no isosurface is found between the H
and O atoms. The O–H···O hydrogen bonds involving the coordinated water molecules are
characterized by small and intense blue isosurfaces, thus supporting the strong nature of the
interactions. Several green isosurfaces are also present between both monomeric units, thus
revealing the existence of additional weak interactions that also stabilize the assembly.
Figure 13: NCI plot of the self-assembled dimer of complex 1. The gradient cut-off is sꢀ=ꢀ0.35
au, and the color scale is −0.04ꢀ<ꢀρꢀ<ꢀ0.04 au.
3
.6. IR and electronic spectra