Non-covalent tetrel bonding interactions in hemidirectional lead(ii) complexes with nickel(ii)-salen type metalloligands

Article abstract of DOI:10.1039/c7nj05148d

Four hetero-dinuclear nickel(ii)/lead(ii) complexes have been synthesized and characterized. Their structures have been confirmed by single crystal X-ray diffraction studies. In each complex, nickel(ii) is placed in the inner N₂O₂ co

Full text of DOI:10.1039/c7nj05148d

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Page 1 of 47
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Non-covalent tetrel bonding interactions in hemidirectional
lead(II) complexes with nickel(II)-salen type metalloligands
a
b
c
c
Sourav Roy, Michael G. B. Drew, Antonio Bauzá, Antonio Frontera and Shouvik
*a
Chattopadhyay
a
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata - 700032, India.
Tel: +9133-2457-2941; E-mail: shouvik.chem@gmail.com
b
School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, United
Kingdom,
c
Department of Chemistry, Universitat de les Illes Balears, Crta de Valldemossa km 7.5, 07122 Palma de
Mallorca (Baleares), SPAIN; E-mail: toni.frontera@uib.es
Abstract: Four hetero-dinuclear nickel(II)/lead(II) complexes have been synthesized and
characterized. Structures have been confirmed by single crystal X-ray diffraction studies. In
each complex, nickel(II) is placed in the inner N O compartment and lead(II) in the outer O O '
2
2
2 2
compartment of the respective Schiff base. In complexes 1, 3 and 4, lead(II) centres are
hemidirectionally coordinated, and thus are well suited for establishing tetrel bonding
interactions which have been investigated thoroughly by means of DFT calculations.
Keywords: Lead(II); Nickel(II); Heteronuclear; Compartmental Schiff base, Tetrel bonding,
Supramolecular interactions.

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Introduction
Although lead is a deadly toxic element and is a dangerous biological poison similar to
1
mercury , its large radius, adoption of different coordination numbers or valences etc leading to
versatile coordination chemistry attracted synthetic inorganic chemists to prepare new lead(II)
2
complexes. Despite the negative functions associated with lead(II) related to pollution and
health, lead containing materials are increasingly used in batteries, non-linear optical materials,
3
ferroelectric materials and semiconductors. Na CaEDTA is a well-known drug used to trap
2
4
lead(II) from human body , and it is a very challenging task currently to develop other chelating
5
ligands for the treatment of lead intoxication. In the present work, salen type N O O' donor
2
2
2
compartmental Schiff bases have been used to trap nickel(II) in the inner N O cores and these
2
2
nickel(II)-salen type metalloligands have been, in turn, used to lock lead(II) in outer O O' cores.
2
2
Salen type ligands are very common in literature due to their ease in synthesis and tendency to
6
,7
ligate a variety of transition metal ions in tetradentate N O cores. 1,3-propane diamine is
2
2
used to prepare these salen type ligands because of the enhanced stability of six member
unsaturated chelate ring (Ni-N-C-C-C-N-Ni). These heteronuclear complexes may have potential
8
9
10
applications in gas separation and encapsulation , hydrometallurgy , metal clusters , material
1
1
12
science , transport and activation of small molecules etc. The valence shell electronic
1
4
10
2
2
2
configuration of lead is 4f 5d 6s 6p . The 6s electron pair remains as inert primarily as a
1
4
result of increasing effective nuclear charge due the presence of imperfectly screening f and
1
0
2
d electrons. High penetrating property of the 6s orbital and relativistic stabilization of 6s
1
3
electrons are also major reasons for lead to show the inert pair effect. The extent to which the

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1
4
lone pair is stereochemically active is currently proving an interesting research topic for the
1
5
theoretical chemists.
In addition to the widely used hydrogen bonding interactions for the construction of
metal organic frameworks (MOFs), other directional interactions such as σ-hole interactions are
1
6,17
becoming interesting players.
It is well-known that the size and electron deficiency of σ–
holes increase with the polarizability of the Lewis acid atom. Strong σ–hole interactions occur in
complexes where a heavy atom of groups IV to VII is covalently bonded to electronegative ones.
These interactions have been widely described for chalcogen, pnictogen and halogen atoms.
However, those of group IV involving tetrel bonding interactions are scarcely to be found in the
1
8
literature, in spite of the fact that it has been shown that they can be markedly strong
(especially in the cases of tin and lead). For instance, the design and synthesis of lead(II) MOFs
1
9,20
based on covalent and non-covalent tetrel bonding have been reported.
In these MOFs,
Pb⋯S and Pb⋯N non-covalent tetrel bonds interconnect the covalently bonded units into
supramolecular assemblies. It has been also shown that tetrel bonds with hemi-directionally
coordinated lead(II) occur with high probability and predictable geometries, thus playing an
important role in the solid-state chemistry of lead. As a matter of fact, the divalent form of lead
Pb(II), presents a special interest for physical chemists due to its unusual coordination
properties, which are highly versatile and flexible. It is characterized by a large variety of
coordination numbers and a structural organisation in two distinct classes (Scheme 1). One is
named holodirected structure, in which the bonds to ligand atoms are directed throughout the
surface of an encompassing sphere. The other is named hemidirected structure, in which the
bonds to ligand atoms are directed throughout a hemisphere and thus exhibit a clear void in

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2
1a
the distribution of bonds to the ligands. It has been shown that Pb(II) complexes with a
variety of ligands are hemidirected for coordination number up to five (n ≤ 5), and holodirected
2
1
for coordination number six and higher (n ≥ 6).
Scheme 1: Schematic representation of the holodirected (a) and hemidirected (b) coordination
modes of lead(II), with indication of the location of the inert lone pair.
1
In the present work, we used three compartmental Schiff bases, H L [N,N'-bis(3-
2
2
methoxysalicylidene)-2,2-dimethylpropane-1,3-diamine],
H2L
[N,N'-bis(3-
3
ethoxysalicylidene)propane-1,3-diamine]
and
H2L
[N,N'-bis(3-ethoxysalicylidene)-2,2-
dimethylpropane-1,3-diamine], to prepare four heterodinuclear nickel(II)/lead(II) complexes,
1
2
2
[
[
(H O)Ni(SCN)L Pb(OAc)]·DMSO (1), [(H O) NiL PbCl ] (2), [(SCN)NiL (OAc)Pb] (3),
2 2 2 2
3
(H O)Ni(SCN)L PbCl] (4). Their structures have been determined by single crystal X-ray
2
diffraction analyses. Interestingly, in three complexes (1, 3 and 4) the lead(II) is hemi-
directionally coordinated (n = 5) and in two of them (1 and 3) the lead(II) participates in non-
covalent tetrel bonding interactions. These forces along with other non-covalent interactions
control the supramolecular architectures and organometallic frameworks observed in their
solid state architecture. The lead(II) presents a coordination mode in which it has a clear void in

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2
1
the distribution of bonds to the ligands as usual in hemi-directional coordination. Such a void
facilitates the approach of electron donors to the lead(II) enabling the formation of a strong
tetrel bond with a predictable geometry. The nature of the tetrel bonds in both structures (1
and 3) was studied by DFT calculations, which showed the presence of the σ-hole at the lead(II)
and the considerable strength of these interactions.
Experimental Section
Nickel(II) thiocyanate tetrahydrate was prepared in the laboratory following the
2
2
literature method. All other materials were commercially available, reagent grade and used as
purchased from Sigma-Aldrich without further purification.
Synthesis
Synthesis of ligands
1
Synthesis of H L [N,N'-bis(3-methoxysalicylidene)-2,2-dimethylpropane-1,3-diamine]
2
A methanol solution (10 mL) of 3-methoxysalicylaldehyde (300 mg, 2 mmol) and 1,3-
1
diaminopropane (0.13 mL, 1 mmol) was refluxed for ca. 1 h to prepare H L . The ligand was not
2
isolated and used directly for the synthesis of complex 1.
2
3
Synthesis of H L [N,N'-bis(3-ethoxysalicylidene)propane-1,3-diamine] and H L [N,N'-bis(3-
2
2
ethoxysalicylidene)-2,2-dimethylpropane-1,3-diamine]
Similarly, methanol solution (10 mL) of 3-ethoxysalicylaldehyde (332 mg, 2 mmol) was
refluxed with 1,3-diaminopropane (0.13 mL, 1 mmol) and 2,2-dimethyl-1,3-diaminopropane

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2
3
(
0.12 mL, 1 mmol) separately, for ca. 1 h to prepare H L and H L respectively. The ligands were
2
2
2
3
not isolated and used directly for the synthesis of complexes 2-3 (H L ) and 4 (H L ).
2
2
Synthesis of complexes
1
Synthesis of [(H O)Ni(SCN)L Pb(OAc)]·DMSO (1)
2
A methanol (10 mL) solution of lead(II) acetate trihydrate (380 mg, 1 mmol) was added
1
to the methanol solution (20 mL) of H L and the resulting solution was stirred for 15 min. A
2
methanol (10 mL) solution of nickel(II) thiocyanate tetrahydrate (250 mg, 1 mmol) was then
added to it. Stirring was continued for about 2h. A few drops of DMSO were then added to the
solution. Single crystals, suitable for X-ray diffraction, were obtained after 3-4 days on slow
evaporation of the solution in open atmosphere.
Yield: 557 mg (68%). Anal. Calc. for C H N NiO PbS (FW = 819.55): C, 36.47; H, 3.86;
2
4
31
3
8
2
-
1
N, 5.55; Found: C, 36.3; H, 3.7; N, 5.6%. FT-IR (KBr, cm ): 1632 (C=N); 2070 (NCS); 1470, 1419
-
1
-1
4
3
3
(
C=O). UV-VIS [λmax(nm)] [εmax(Lmol cm )] (DMF): 268 (1.8×10 ); 293 (3.3×10 ); 363 (3.1×10 );
5
81 (12.2); 813 (15.1).
2
Synthesis of [(H O) NiL PbCl ] (2)
2
2
2
A methanol (10 mL) solution of lead(II) nitrate (332 mg, 1 mmol) was added to the
2
methanol solution (20 mL) of H L and the resulting solution was stirred for 15 min. A methanol
2
(10 mL) solution of nickel(II) chloride hexahydrate (237 mg, 1 mmol) was then added to it.
Stirring was continued for about 2h. Single crystals, suitable for X-ray diffraction, were obtained
after 3-4 days on slow evaporation of the solution in open atmosphere.

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Yield: 496 mg (67%). Anal. Calc. for C H Cl N NiO Pb (FW = 741.24): C, 34.05; H, 3.83;
2
1
28
2
2
6
-
1
N, 3.80; Found: C, 33.9; H, 3.7; N, 3.9%. FT-IR (KBr, cm ): 1624 (C=N). UV-VIS [λmax(nm)]
-1
-1
4
4
3
[
(
εmax(Lmol cm )] (Acetonitrile): 231 (2.2×10 ); 269 (1.04×10 ); 348 (3.6×10 ); 568 (9.11); 814
29.8)
2
Synthesis of [(SCN)NiL (OAc)Pb] (3)
A methanol (10 mL) solution of lead(II) acetate trihydrate (380 mg, 1 mmol) was added
2
to the methanol solution (20 mL) of H L and the resulting solution was stirred for 15 min. A
2
methanol (10 mL) solution of nickel(II) thiocyanate tetrahydrate (250 mg, 1 mmol) was then
added to it. Stirring was continued for about 2h. Single crystals, suitable for X-ray diffraction,
were obtained after 3-4 days on slow evaporation of the solution in open atmosphere.
Yield: 526 mg (70%). Anal. Calc. for C H N NiO PbS (FW = 751.44): C, 38.36; H, 3.62; N,
2
4
27
3
6
-
1
5
.59; Found: C, 38.2; H, 3.5; N, 5.7%. FT-IR (KBr, cm ): 1631 (C=N); 2101 (NCS); 1456, 1228
-
1
-1
3
3
3
(
N=O). UV-VIS [λmax(nm)] [εmax(Lmol cm )] (DMF): 265 (4.4×10 ); 285 (1.9×10 ); 354 (1.8×10 );
5
85 (3.5); 877 (5.2).
3
Synthesis of [(H O)Ni(SCN)L PbCl] (4)
2
A methanol (10 mL) solution of lead(II) nitrate (332 mg, 1 mmol) was added to the
3
methanol solution (20 mL) of H L and the resulting solution was stirred for 15 min. A methanol
2
(10 mL) solution of nickel(II) chloride hexahydrate (237 mg, 1 mmol) was then added to it. After
1
5 min of stirring, methanol (10 mL) solution of sodium thiocyanate (81 mg, 1 mmol) was added

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to it. Stirring was continued for about 2h. Single crystals, suitable for X-ray diffraction, were
obtained after 3-4 days on slow evaporation of the solution in open atmosphere.
Yield: 503 mg (65%). Anal. Calc. for C H ClN NiO PbS (FW =773.92): C, 37.25; H, 3.91;
2
4
30
3
5
-
1
N, 5.43; Found: C, 37.1; H, 3.7; N, 5.5%. FT-IR (KBr, cm ): 1633 (C=N); 2098 (C≡N). UV-VIS
-
1
-1
4
4
3
[
(
λmax(nm)] [εmax(Lmol cm )] (Acetonitrile): 232 (3.4×10 ); 270 (2.1×10 ); 348 (8.1×10 ); 548
34.9); 833 (14.4).
Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed 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 (for complexes 2 and
4
) and DMF (for complexes 1 and 3) were recorded on a Perkin Elmer Lambda 35 UV-visible
spectrophotometer. Steady state photoluminescence spectra in acetonitrile (for complexes 2
and 4) and DMF (for complexes 1 and 3) were obtained in Shimadzu RF-5301PC
spectrofluorometer at room temperature. Time dependent photoluminescence spectra were
recorded using Hamamatsu MCP photomultiplier (R3809) and were analyzed by using IBHDAS6
software. Emissions of complexes are tentatively attributed to the intra-ligand transitions
modified by metal coordination. Intensity decay profiles were fitted to the sum of exponentials
−t
series ( ) ∑i αi
I t =
exp
(
)
τi
, where α was a factor representing the fractional contribution to
i
the time resolved decay of the component with a lifetime of τi. Bi-exponential function was
2
used to fit the decay profile for complexes, with obtaining χ close to 1. The intensity-averaged

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2
i
∑i αiτ
∑i αiτ
life times (τav) were determined from the result of the exponential model usingτav
=
,
i
where α and τi are the pre-exponential factors and excited state luminescence decay time
i
associated with the i-th component, respectively.
X-ray crystallography
Suitable crystals of complexes 1-4 were picked, mounted on a glass fiber and diffraction
intensities were measured with an Oxford Diffraction XCalibur diffractometer equipped with
Mo K radiation (λ =0.71073 Å, 50 kV, 40 mA) at 150 K. Data collection and reduction were
α
2
3
performed with the Oxford diffraction Crysalis software. The structures of the complexes
2
4
were solved by direct methods using the SHELXS-97 program. Non-hydrogen atoms were
refined anisotropically. Hydrogen atoms attached to nitrogen and oxygen atoms were located
2
24
by difference Fourier maps. The structures were refined on F using SHELX2016-6. Other
2
5
26
27
28
programs used included PLATON, DIAMOND, ORTEP and MERCURY.
Hirshfeld surfaces
29
30
The Hirshfeld surface of complexes 1-4 were determined using Crystal Explorer.
Theoretical Methods
The geometries of the complexes included in this study were computed at the M06-
X/def2-TZVP level of theory using the crystallographic coordinates within the TURBOMOLE
2
3
1
program. This level of theory is adequate for studying non-covalent interactions in
organometallic complexes. The basis set superposition error for the calculation of interaction

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3
2
energies has been corrected using the counterpoise method. The “atoms-in-molecules”
3
3
(
AIM) analysis of the electron density has been performed at the same level of theory using
3
4
the AIMAll program.
Results and Discussion
Synthesis
1
,3-diaminopropane was refluxed separately, with 3-methoxysalicylaldehyde and 3-
ethoxysalicylaldehyde, respectively, in 1:2 ratio to form two N O donor compartmental Schiff
2
4
1
2
35
base ligands, H2L and H2L respectively, following the literature method.
3-
ethoxysalicylaldehyde on refluxing with 2,2-dimethyl-1,3-diaminopropane in 2:1 ratio gave
3
,36
1
another N O donor compartmental Schiff base ligand H L. The Schiff base (H L ) on reaction
2
4
2
2
with lead(II) acetate trihydrate, followed by the addition of nickel(II) thiocyanate tetrahydrate
and DMSO respectively gave heteronuclear complex 1. Complex 2 was prepared by reacting
2
Schiff base, H L , with lead(II) nitrate followed by the addition of nickel(II) chloride hexahydrate.
2
2
Complex 3 was prepared by reacting Schiff base, H L with lead(II) acetate trihydrate followed
2
by the addition of nickel(II) thiocyanate tetrahydrate. Similarly, complex 4 was prepared by
3
reacting Schiff base, H L with lead(II) nitrate followed by the addition of nickel(II) chloride
2
hexahydrate and sodium thiocyanate. Formation of all complexes is shown in Scheme 2.
The synthetic procedure has been repeated by adding nickel(II) first followed by the
addition of lead(II). However, same complexes have been formed in all cases with nickel(II)
occupying inner N O and lead(II) occupying outer O O ´ cavities. This is probably because of
2
2
2 2
the larger size of lead(II), the ionic radius of which (133 pm) is much larger compared to that of

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nickel(II) (83 pm). Therefore, it is difficult for lead(II) to be accommodated in compact (and
therefore smaller) inner N O compartment, where smaller nickel(II) fits well. Lead(II) on the
2
2
other hand, fits well in the open (and therefore bigger) outer O O ´ compartment.
2
2
Scheme 2: Preparation of ligands and complexes.
Synthesis and X-ray characterization of several heteronuclear nickel(II)/lead(II)
3
7
complexes could be found in literature (Table 1). Tetrel bonding interactions have not been
explored in any complex. Thus the present work is the first systematic study to explore tetrel
bonding interactions in hemidirected heteronuclear nickel(II)/lead(II) complexes.

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Table. 1: Previously reported X-ray characterized heteronuclear nickel(II)/lead(II) complexes.
Tetrel
Complex
CCDC)
Coordination
mode of lead
Formula
Pb-X (Å)
(
Bonding
Ref
1
Not
37a
AYAKUG
UGENIF
[Pb(OAc)(NiL ) ](OAc)
Holodirected
Hemidirected
-
2
explored
Not
2
37b
37b
[NiL (H O) PbBr ]
-
2
2
2
explored
Not
2
UGENOL [NiL (H O) Pb(NO ) ]
Hemidirected
Hemidirected
Hemidirected
Hemidirected
Hemidirected
Hemidirected
Hemidirected
Hemidirected
Hemidirected
-
2
2
3 2
explored
Not
3
37c
YISROI
[{L Ni}Pb(NC H )Cl]
2
3.423(6)
3.22(2)
3.006(4)
2.976(7)
-
5
5
explored
Not
4
37c
YISRUO
[{L Ni} Pb](H O)(Py)
2 2
explored
Not
5
{
[NiL (py) ]PbCl }(Py)
37d
2
2
XUMGER
explored
Not
6
37e
ECEBAR
[NiL Pb(NO ) ]
3 2
explored
Not
7
37f
PUYLEA
[PbNiL (AcO) ]
2
explored
present
8
[
(H O)Ni(SCN)L Pb(OA
This Work
This Work
This Work
2
-
-
-
3.379(4)
-
c)]·DMSO
9
[(H O) NiL PbCl ]
absent
2
2
2
9
[(SCN)NiL Pb(NO )]
present
3.064(2)
3

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1
0
This Work
-
[(H O)Ni(SCN)L PbCl]
Hemidirected
absent
-
2
1
2
Where, H2L = N,N'-bis(3-methoxysalicylidene)ethane-1,2-diamine; H2L = N,N'-bis(3-
3
ethoxysalicylidene)propane-1,3-diamine; L = tris-((2-hydroxybenzylidene)-aminoethyl)amine;
4
5
L = tris-((-2-hydroxybenzylidene)-aminomethyl)propane; H L = N,N'-bis(salicylidene)propane-
2
6
7
1
,3-diamine; H L = N,N'-bis(3-ethoxysalicylidene)ethane-1,2-diamine; H L = N,N'-dimethyl-
2 2
8
N,N'-ethylene-di(5-bromo-3-formyl-2-hydroxybenzylamine);
methoxysalicylidene)-2,2-dimethylpropane-1,3-diamine],
H2L
=
[N,N'-bis(3-
N,N'-bis(3-
9
H2L
=
10
ethoxysalicylidene)propane-1,3-diamine;
dimethylpropane-1,3-diamine.
H2L
=
N,N'-bis(3-ethoxysalicylidene)-2,2-
Structure description
The X-ray crystal structure determination reveals that both complexes 1 and 4 crystallize
in triclinic space group P , whereas complexes 2 and 3 crystallize in monoclinic space group
C2/c and P2 /n respectively. Details of crystallographic data and refinement details are given in
1
Table 2. Important bond lengths and bond angles are gathered in Tables 3 and S1 {ESI
(Electronic Supplementary Information)} respectively.
The molecular structures of complexes 1-4 are built from isolated hetero-dinuclear units
1
2
2
of [(H O)(SCN)NiL Pb(OAc)] (Fig. 1), [(H O) NiL PbCl ] (Fig. 2), [(SCN)NiL (OAc)Pb] (Fig. 3) and
2
2
2
2
3
[
(H O)(SCN)NiL PbCl] (Fig. 4) respectively. Only complex 1 contains a lattice DMSO molecule.
2
’
1
3
Three N O O donor compartmental Schiff bases (H L -H L ) are used to prepare the complexes
2
2
2
2
2

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in which nickel(II) centres are placed in inner N O cavities and lead(II) centres are placed in
2
2
’
outer O O cavities. Nickel(II) centre in each complex is hexacoordinated and assumes distorted
2
2
octahedral geometry. Lead(II) centres, are pentacoordinated, in complexes 1, 3 and 4. On the
other hand, lead(II) centre is hexacoordinated in complex 2. It is also to be noted that the
central carbon atom, C21, of the propylene link in complex 2 is disordered off the two-fold axis
in two possible positions (C21A and C21B).
The nickel(II) centre, [Ni(1)] has pseudo-octahedral geometry, where two imine nitrogen
i
i
atoms, {N(19) and N(23) [for complexes 1, 3 and 4]; N(19) and N(19 ) { = symmetry
transformation; -x, y, ½-z} [for complex 2]} and two phenoxo oxygen atoms, {O(11) and O(31)
i
[
for complexes 1, 3 and 4]; O(11) and O(11 ) [for complex 2]} of the deprotonated di-Schiff base
i
i
constitute the equatorial plane. In case of complex 2, N(19), N(19 ), O(11) and O(11 ) of the
deprotonated di-Schiff base constitute the equatorial plane, which is almost planar with none
of these atoms giving rise to r.m.s. deviation more than 0.042 Å. The nickel(II) is perforce on the
two-fold axis in the equatorial plane. On the other hand, deviations of the coordinating atoms
from the mean equatorial planes passing through them and that of the nickel(II) centres from
the same planes in complexes 1, 3 and 4 are listed in Table 4.
The axial positions of nickel(II) centres are occupied by nitrogen and/or oxygen atoms:
one oxygen atom, O(1), from a water molecule and one nitrogen atom, N(1), from a
i
i
thiocyanate molecule in complex 1, two oxygen atoms, O(1) and O(1) [ = symmetry
transformation = -x, y, ½-z] in complex 2, a nitrogen atom, N(1), from a thiocyanate and an
oxygen atom, O(1), from an acetate in complex 3, an oxygen atom, O(1), from a water molecule
and a nitrogen atom, N(1), from a thiocyanate in complex 4. All nickel(II)-nitrogen and nickel(II)-

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oxygen bond lengths are comparable to that observed in other complexes with similar
3
8
structures. Phenoxo oxygen atoms {O(11) and O(31) [for complexes 1, 3 and 4]; O(11) and
i
O(11 ) [for complex 2]} of deprotonated Schiff bases also coordinate to lead(II) centres. The
potential donor ethoxy oxygen atoms {O(131) and O(291) [for complexes 1, 3 and 4]; O(131)
i
and O(131 ) [for complex 2]} of the compartmental Schiff bases also coordinate to lead(II), but
at much longer distances to form equatorial planes. All lead(II)-oxygen bond lengths are
3
9
comparable to that observed in other complexes with similar structures.
In case of complex 1, two oxygen atoms of one acetate group coordinates lead(II)
centre in axial positions with different Pb-O(acetate) distances {2.394(4) and 2.737(5) Å}.
Apparently the acetate may be considered as a chelating bidentate ligand. However, the small
°
value of the bite angle, O(2)-Pb(1)-O(3) [50.5(2) ], subtended by the two oxygen atoms, O(2)
and O(3), suggests that the two oxygen atoms share one axial site, i.e. the acetate group is
better thought of as a single entity occupying just one stereochemical site in the coordination
sphere of lead(II) instead of as a bidentate ligand. However, because of the vacancies in the
equatorial plane, the coordination sphere cannot be described as square pyramidal, rather it is
a pentagonal bipyramid with an axial site and an equatorial site unoccupied. In complex 2, two
i
chlorine atoms, Cl(11) and Cl(11 ), coordinate axial positions of lead(II). As can be seen in Fig. 2,
the geometry can best be described as a pentagonal bipyramid with one equatorial site
unoccupied. Complex 3 is unique in the sense that two oxygen atoms {O(10) and O(3)} of one
acetate group bridges nickel(II) and lead (II) {lead(II) by O(3) and nickel(II) through O(1)}.
Another interesting feature is that there is another bond from Pb(1) to the thiocyanate S(1)
from an adjacent molecule (½-x, ½+y, ½-z) at 3.064(2) Å in complex 3 which leads to the

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formation of a chain, as shown in Fig. 5. It is noteworthy that this bond is on the same side of
the equatorial plane as O(1) leading to a very irregular geometry. It may be that the presence of
the two ethoxy groups close to the vacant site in the equatorial plane has precluded further
bonding in that plane. One axial site is occupied by a chlorine atom, Cl(1), in complex 4. The
geometry can best be considered as a pentagonal bipyramid with an axial and an equatorial site
unoccupied (Fig. 4) as in complex 1.
The Ni(1)O(11)O(31)Pb(1) core is almost planar as the dihedral angle is 9.39° [for
i
complex 1] and 6.19° [for complex 4]. In case of complex 2, Ni(1)O(11)O(11 )Pb(1) core is
perforce planar as both metals occupy the twofold axis. The dihedral angle is 24.25° and this
indicates that the Ni(1)O(11)O(31)Pb(1) core is not planar in complex 3. The distance between
the Ni(1) and Pb(1) is 3.536(7) Å [for complex 1], 3.464(2) Å [for complex 2], 3.395(1) Å [for
complex 3] and 3.400(2) Å [for complex 4].

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Fig. 1: Perspective view of complex 1 with selective atom numbering scheme. The solvent
DMSO and hydrogen atoms have been omitted for clarity.
Fig. 2: Perspective view of complex 2 with selective atom numbering scheme. Symmetry
i
element, = -x, y, ½-z. Hydrogen atoms have been omitted for clarity.

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Fig. 3: Perspective view of the asymmetric unit of complex 3 with selective atom numbering
scheme. Hydrogen atoms have been omitted for clarity.

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Fig. 4: Perspective view of complex 4 with selective atom numbering scheme. Hydrogen atoms
have been omitted for clarity.
Fig. 5: Perspective view of 1D chain of complex 3 considering a bond between Pb(1) to S(1) at
a
3
.064(2) Å highlighted in violet colour. Symmetry element, = ½-x, ½+y, ½-z.
Table 2: Crystal data and refinement details of complexes 1-4.
Complex
1
2
3
4
Formula
Formula Weight
Temperature(K)
Crystal system
Space group
a (Å)
C H N NiO PbS
C H Cl N NiO Pb
C H N NiO PbS
C H ClN NiO PbS
2
4
31
3
8
2
21 28
2
2
6
24 27
3
6
24 30
3
5
819.55
150
751.44
150
7
41.24
50
773.92
150
1
Triclinic
P
Monoclinic
Monoclinic
Triclinic
P2 /n
1
C2/c
P
10.5264(6)
11.3184(6)
12.5352(8)
86.021(5)
12.6526(7)
14.8338(8)
13.9646(8)
90
1
4.760(3)
2.3947(15)
4.300(3)
9.8038(8)
b (Å)
1
11.9201(11)
12.8868(8)
95.767(6)
c (Å)
1
α (°)
9
0

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β (°)
γ (°)
Z
71.617(5)
85.853(4)
2
104.762(6)
90
1
10.72(2)
104.841(6)
111.721(8)
2
9
0
4
4
-
3
d (g cm )
1.928
1.969
7.504
calc
2
7
.012
.899
1.947
-
1
μ (mm )
6.821
7.302
F (000)
804
1464
1
440
706
387
420
756
Total Reflections
9927
15411
5
5897
Unique Reflections
Observed data [I > 2 σ
No. of parameters
R (int)
7843
7120
7177
5713
3
2
4369
3750
363
328
1
62
335
0.029
0.058
0
.092
0.075
R1, wR2 (all data)
R1, wR2 [I > 2 σ (I)]
0.0520, 0.0997
0.0452, 0.0958
0.0859, 0.1494
0.0648, 0.1379
0
0
.1076, 0.1766
.0758, 0.1570
0.0893,0.2275
0.0761,0.2061
Table 3: Selected bond lengths (Å) of complexes 1-4.
Complex
Ni(1)-N(19)
Ni(1)-O(11)
Ni(1)-O(31)
Ni(1)-N(23)
Ni(1)-N(1)
1
2
3
4
2.027(5)
2.047(4)
2.038(4)
2.051(4)
2.089(5)
2.164(4)
-
2.032(9)
2.027(7)
2.037(5)
2.043(5)
2.047(7)
2.095(7)
-
2.007(10)
2.030(7)
2.005(10)
2.059(9)
2.022(10)
2.048(12)
-
-
-
-
Ni(1)-O(1W)
Ni(1)-O(1)
-
2.121(8)
2.306(6)
2.621(7)
-
2.084(6)
2.370(5)
2.732(5)
2.294(5)
2.194(9)
2.309(9)
2.684(9)
2.299(9)
Pb(1)-O(11)
Pb(1)-O(131)
Pb(1)-O(31)
2.405(3)
2.696(4)
2.346(4)

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Pb(1)-O(291)
Pb(1)-O(1)
Pb(1)-O(2)
Pb(1)-Cl(1)
Pb(1)-O(3)
2.643(4)
-
2.820(5)
2.698(9)
2.737(5)
-
-
-
2.394(4)
-
-
-
-
-
2.939(3)
-
-
2.637(3)
-
2.293(6)
a
Pb(1)-S(1)
-
-
3.064(2)
-
a
Symmetry element: = -x+½, ½+y, ½-z.
Table 4: Deviation of coordinating atoms from the least square mean plane passing through
them and that of nickel(II) from the same plane in complexes 1, 3 and 4.
Deviations of atoms in Å
Complex
O(11)
O(31)
N(23)
N(19)
Ni(1)
1
0.055(3)
0.012(6)
-0.028(4)
0.020(6)
0.051(4)
0.009(6)
0.020(5) 0.0573(6)
0.018(7) -0.0171(9)
3
4
0.047(10) 0.007(10)
0.046(11) 0.001(12 0.085(2)
Supramolecular Interactions
In complex 1, the water molecule bonded to nickel(II) forms two strong donor hydrogen
bonds. Hydrogen atom, H(1), attached to the oxygen atom, O(1W), forms an intramolecular
hydrogen bond with an oxygen atom, O(2), of an acetate ligand. Dimensions are O(1W)···O(2)
o
2
.744(5) Å, O(1W)-H(1)···O(2) 174(7) , H(1)···O(2) 1.90(2) Å. Another hydrogen atom, H(2),
attached to the oxygen atom, O(1W), forms an intermolecular hydrogen bond with a symmetry

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related (3-x, 1-y, -z) oxygen atom, O(41), of a DMSO molecule. Dimensions are O(1W)···O(41)
o
2
.808(5) Å, O(1W)-H(2)···O(41) 173(7) , H(2)···O(41) 1.96(2) Å. The hydrogen bonding
interactions are shown in Fig. 6.
b
Fig. 6: Perspective view of hydrogen bonding interaction in complex 1. Symmetry element = 3-
x, 1-y, -z.
In complex 2, the water molecules bonded to the nickel(II) form hydrogen bonds to the
chlorine atoms bonded to the lead(II). Hydrogen atom, H(1), attached to oxygen atom, O(1),
forms an intramolecular hydrogen bond with a chlorine atom, Cl(1), bonded to Pb(1).
o
Dimensions are O(1)···Cl(1) 3.161(8) Å, O(1)-H(1)···Cl(1) 160(8) , H(1)···Cl(1) 2.34(4) Å. Another
hydrogen atom, H(2), attached to oxygen atom, O(1), forms an intermolecular hydrogen bond
with a symmetry (x,2-y,1/2+z) related chlorine atom, Cl(1). Dimensions are O(1)···Cl(1) 3.225(8)

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o
Å, O(1)-H(2)···Cl(1) 160(9) , H(2)···Cl(1) 2.41(4) Å. These interactions lead to the formation of 1D
chain shown in Fig. 7.
Fig. 7: Perspective view of 1D chain formed by hydrogen bonding interactions in complex 2.
c
Only relevant atoms have been shown for clarity. Symmetry transformation: = x,2-y,1/2+z.
In complex 3, no significant H-bonding interactions are observed. In complex 4, two
hydrogen bonding interactions are observed. Hydrogen atom, H(1), attached to the oxygen
atom, O(1), forms an intermolecular hydrogen bond with a symmetry (1-x,1-y,-z) related
o
chlorine atom, Cl(1). Dimensions are H(1)···Cl(1) 2.40(2) Å, O(1)-H(1)···Cl(1) 170(4) , O(1)···Cl(1)
3
.256(11) Å. Hydrogen atom, H(2), attached to the oxygen atom, O(1), forms an intramolecular
hydrogen bond to Cl(1), but this is rather weak compared to the intermolecular hydrogen bond.
o
Dimensions are H(2)···Cl(1) 2.88(2) Å, O(1)-H(2)···Cl(1) 113(12) , O(1)···Cl(1) 3.308(12) Å. These
interactions form a supramolecular dimer shown in Fig. 8.

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Fig. 8: Perspective view of hydrogen bonding interactions in complex 4. Only the relevant atoms
d
have been shown for clarity. Symmetry transformation: = 1-x,1-y,-z.
IR and electronic spectra
-
1
-1
-1
Distinct bands due to the azomethine (C=N) groups at 1632 cm , 1628 cm ,1632 cm
-
1
and 1637 cm are routinely noticed in the IR spectra of complexes 1-4 respectively (Figures S1-
4
0
-1
-1
S4) {ESI (Electronic Supplementary Information)}. A strong band at 2069 cm , 2101 cm and
-1
2
097 cm indicate the presence of terminal N-bonded thiocyanate in complexes 1, 3 and 4
4
1
−1
-1
-1
respectively. Broad bands at 3379 cm , 3406 cm and 3499 cm indicate the presence of the
4
2
O-H stretching frequency of water molecule in complexes 1, 2 and 4 respectively. Absorptions
-
1
~
1463 and at 1416 cm are attributed to asymmetric and symmetric (C=O) stretching frequency
4
3
of the acetate group, respectively in complex 1 and 3.

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Electronic spectra of the complexes (Figures S5-S8) {ESI (Electronic Supplementary
Information)} display five bands around 250 nm, 270 nm, 350 nm, 560 nm and 820 nm.
3
3
Absorption bands around 560 nm may be assigned as T (F)← A (F), whereas bands at 820
1
g
2g
3
3
44
3
nm may be assigned as T (F)← A (F) respectively. The higher energy d-d band, T (P)
2
g
2g
1g
3
←
A (F), cannot be observed as it is obscured by a strong charge transfer transition (~350 nm)
2g
4
5
in each case. The UV absorption bands around 270 nm may be assigned to intra ligand n-π*
4
6
47
transitions. Bands around 250 nm may be attributed to π-π* transitions. Electronic spectra
of the ligands are shown in (Figures S9-S11) {ESI (Electronic Supplementary Information)}. All
complexes exhibit fluorescence (Fig. S12). The fluorescence data are listed in Table 5 (without
1
2
3
solvent correction). The fluorescence spectra of H L , H L and H L (Fig. S13) were also
2
2
2
recorded (Excitation wavelengths were 329, 328 and 330 nm respectively, and emission wave
lengths were 440 nm, 440 nm and 442 nm respectively) and compared with that of the
complexes (Table 5). There is a clear shift in emission maxima towards high energy region. The
fluorescence intensity also decreases in complexes probably due to the presence of
*
paramagnetic nickel(II) centres in each case. These are assigned as intra-ligand (π-π )
4
7
fluorescence. Mean lifetimes (г ) of the exited states are listed in Table 5. Decay profiles are
avg
shown in Fig. 9.

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Fig. 9: Lifetime decay profile of complexes 1-4.
Table 5: Photophysical data for complexes 1-4.
2
A1
A2
χ
Excitation Emission
τ
1
(ns)
τ
2
(ns) τavg
ns)
Complex
(
%)
(%)
(
nm)
(nm)
(
1
2
3
4
350
350
350
348
391
390
390
390
67.38 1.17 32.62 5.67
51.43 1.83 48.57 4.78
61.34 2.15 38.66 4.38
64.36 2.84 35.64 5.91
4.3 1.01
3.9 1.09
3.4 1.21
4.5 1.11

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Hirshfeld surfaces
Hirshfeld surfaces of complexes 1-4, mapped over dnorm (range of -0.1 to 1.5 Å), shape
index and curvedness, are illustrated in Fig. 10. Transparent surfaces are used so that molecular
moieties (around which the surfaces are calculated) can be visualised clearly. Red spots on the
dnorm surface (Fig. 10) indicate the interaction between sulphur and hydrogen atoms. O···H,
N···H and Cl···H contacts are also observed in the Hirshfeld surfaces as smaller visible spots of
light colour indicating weaker and longer contact. Distinct spikes in 2D fingerprint plot (Fig. 11)
indicate intermolecular interactions, e.g. S···H/H···S, Cl···H/H···Cl or O···H/H···O interactions etc.
Relative contributions to the Hirshfeld surface area for the various intermolecular contacts in
complexes 1-4 are shown in Fig. 12.
Fig. 10: Hirshfeld surfaces mapped with dnorm (top), shape index (middle) and curvedness
(below) for complexes 1-4.

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Fig. 11: Fingerprint plot: Different contacts contributed to the total Hirshfeld Surface area of
complexes 1-4.

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Fig. 12: Relative contributions to the Hirshfeld surface area for the various intermolecular
contacts in complexes 1-4.
Theoretical study of the supramolecular assemblies
The theoretical study has been focused on the analysis of the remarkable
supramolecular assemblies observed in the solid state of complexes 1 and 3, directing our
attention to the tetrel bonding interactions. Particularly in complex 1, the Pb⋯N interactions
have been analysed that promote the formation of self-assembled dimers in the solid state.
Moreover, in complex 3, the Pb···S interactions have been studied that govern the formation of
infinite 1D chains in its solid state architecture.
Self-complementary Pb⋯N contacts are responsible for the formation of self-assembled
dimers in complex 1 (Fig. 13a) that are interconnected to other dimers by weak van der Waals
interactions. In order to rationalize the donor-acceptor properties of the Pb(II) complex, the