Role of non-covalent interactions in the supramolecular architectures of mercury(ii) diphenyldithiophosphates: An experimental and theoretical investigation

Article abstract of DOI:10.1039/d0nj05709f

The vital role of non-covalent interactions in the supramolecular organization of two mercury(ii) complexes with sulfur donor ligands have been investigated. These new complexes of mercury(ii) diphenyldithiophosphates were synthesized in purely aqueous media and characterized, which corresponds to [Hg(L¹)₂] (1) and [Hg(L²)₂]_n(2), {L¹=O,O-bis(4-tert-butylphenyl) phosphorodithioate and L²=O,O-bis(3,4-dimethylphenyl) phosphorodithioate}. These complexes have been characterized by NMR (¹H,¹³C and³¹P), IR, UV-visible, TGA and elemental analysis. The molecular structure of1and2belong to the triclinic and monoclinic system of the space group P1? andP2₁/c, respectively, as revealed by single crystal diffraction analysis. Interestingly, complex1consists of a monomer unit while complex2consists of a polymer unit with an additional Hg?S bond. The supramolecular network including hydrogen bonds, spodium bonds and CH?π interactions in both the complexes have been identified and confirmed using theoretical analysis. A thorough analysis of the Hirshfeld surface has also been carried out to examine the intermolecular contacts in the crystal structure. Bader's Quantum Theory of Atoms-in-Molecules (QTAIM) has been applied extensively in order to acquire the quantitative and qualitative insight into the physical nature of weak non-covalent interactions. Additionally, the non-covalent interactions reduced density gradient (NCI-RDG) methods support nicely the presence of such non-covalent intermolecular interactions.

Full text of DOI:10.1039/d0nj05709f

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Role of non-covalent interactions in the
supramolecular architectures of mercury(II)
diphenyldithiophosphates: An experimental and
theoretical investigation†
Cite this: New J. Chem., 2021,
45, 2249
a
b
Pretam Kumar,
Snehasis Banerjee,
Anu Radha,a Tahira Firdoos,a
c
a
Subash Chandra Sahoo
and Sushil K. Pandey
*
The vital role of non-covalent interactions in the supramolecular organization of two mercury(II) complexes
with sulfur donor ligands have been investigated. These new complexes of mercury(II) diphenyldithio-
phosphates were synthesized in purely aqueous media and characterized, which corresponds to [Hg(L1)2]
(1) and [Hg(L2)2]n (2), {L1
= = O,O-bis(3,4-
O,O-bis(4-tert-butylphenyl) phosphorodithioate and L2
dimethylphenyl) phosphorodithioate}. These complexes have been characterized by NMR (1H, 13C and 31P),
IR, UV-visible, TGA and elemental analysis. The molecular structure of 1 and 2 belong to the triclinic and
%
monoclinic system of the space group P1 and P21/c, respectively, as revealed by single crystal diffraction
analysis. Interestingly, complex 1 consists of a monomer unit while complex 2 consists of a polymer unit
with an additional Hgꢀꢀ ꢀS bond. The supramolecular network including hydrogen bonds, spodium bonds
and CHꢀꢀꢀp interactions in both the complexes have been identified and confirmed using theoretical
analysis. A thorough analysis of the Hirshfeld surface has also been carried out to examine the
intermolecular contacts in the crystal structure. Bader’s Quantum Theory of Atoms-in-Molecules (QTAIM)
has been applied extensively in order to acquire the quantitative and qualitative insight into the physical
nature of weak non-covalent interactions. Additionally, the non-covalent interactions reduced density
gradient (NCI-RDG) methods support nicely the presence of such non-covalent intermolecular interactions.
Received 23rd November 2020,
Accepted 29th December 2020
DOI: 10.1039/d0nj05709f
rsc.li/njc
to covalent interactions, these play a vital role in crystal
engineering, anion transport, anion-sensing, anion-recognition
Introduction
Supramolecular chemistry deals with various non-covalent inter- chemistry, protein folding, enzyme inhibition, drug–receptor
actions, such as hydrogen bonding, C–Hꢀ ꢀ ꢀp, pꢀ ꢀ ꢀp, van der interactions etc.6–9 These non-covalent interactions are noticed
Waals forces, ion–ion interactions, dipole–dipole interactions, to control the biological and chemical processes both in vivo and
ion–dipole interactions etc.1–4 These non-covalent interactions in vitro.10–12 The nature of non-covalent forces control biological
show the tendency to self-assemble in the solid-state through systems significantly because these are specific without conferring
secondary bonds to form supramolecular structures. Generally, as much severity as covalent forces.
these bonds are not strong enough to survive in solution, but
Very recently, the attractive attraction between any elements
depict amazing effects in the building and designing of a crystal of the post-transition metal group 12 and electron rich atoms
structure.5 Though non-covalent interactions are weak compared (Lewis bases or anions) has been termed as the Spodium bond
(SpB).13 The SpB has become a new member of the s-hole
family of interactions. The mercury complexes of sulfur donor
ligands show the tendency to self-assemble in the solid-state
through secondary bonds and to form supramolecular
a Post Graduate Department of Chemistry, University of Jammu,
Baba Saheb Ambedkar Road, Jammu Tawi-180006, India.
E-mail: kpsushil@rediffmail.com
b Govt. College of Engineering and Leather Technology, Salt Lake Sector-III,
structures.5,14 The chemistry of mercury complexes bearing
sulfur donor ligands has a remarkable deal of interest over
the past decade owing to the potential application of such
compounds to function as dynamic sites in various biological
activities with diverse geometries of variable nuclearity and
structural diversity.15–21 Mercury is known to have a large
radius and high thiophilicity.22 The thiophilic nature of
Block-LB, Kolkata-700106, India
c Department of Chemistry & Centre of Advanced Studies in Chemistry, Panjab
University, Chandigarh-160014, India
† Electronic supplementary information (ESI) available. CCDC 2026504 and
2026505 contain the supplementary crystallographic data for complexes 1 and
2, respectively. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d0nj05709f
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mercury is due to the metal–ligand binding affinities that are phenyl) phosphorodithioate (NaL2) ligands were prepared in quan-
qualitatively rationalized in terms of the hard–soft acids and bases titative yield by following a procedure already published by us.39,40
(HSAB) theory.23,24 Mercury can adopt different coordination Since the synthesized ligands are moisture sensitive, standard
modes when binding with sulfur donor ligands; the linear, Schlenk techniques were used carefully to exclude the moisture
trigonal planar and tetrahedral array being the most throughout the experimental manipulations.
common.25 The mercury derivatives with sulfur donor ligands
Physical measurements
form supramolecular structures, which can be either cyclic (or
quasi-cyclic) dimers or polymeric arrangements.5,14,26–32 Reactions IR spectra were recorded on a Shimadzu FT-IR spectrophotometer
of HgCl2 with 1,1-dithiolate ligands tend to form polynuclear as KBr pallets in the range of 4000–400 cmꢁ1. UV-visible spectra
complexes.18,30,33,34 It remains still unanswered whether the were obtained on a PG spectrophotometer; model T-90. Multi-
mercury dithiophosphate arrangements, like monomers, nuclear (1H, 13C and 31P) NMR spectra were recorded on a Bruker
dimers or polymers, are formed in the initial reaction process Avance III (400 MHz) spectrometer. All chemical shifts are
or during crystal growth experiments.
reported in ppm (parts per million) relative to their corresponding
In the recent past, the Hirshfeld surface (HS) analysis has external standards (SiMe4 for 1H, 13C and 85% H3PO4 for 31P) and
emerged as an attempt to define the space occupied by a all the coupling constants (J values) in Hz as absolute values.
molecule in a crystal to partition the crystal electron density Elemental analyses (C, H and S) were conducted using the
into molecular fragments.35–38 HS is a base tool that represents Elemental Analyser Vario EL-III. A thermal advantage PerkinElmer
a unique approach to the crystal structure prediction and order thermal analyzer was used to carry out thermo-gravimetric analy-
to examine the intermolecular contacts in a crystal lattice. sis (TGA) under an atmosphere of nitrogen gas and at a heating
Hence, the intermolecular interaction analysis using HS should rate of 10 1C minꢁ1 over a temperature range of 30–950 1C.
be considered by the supramolecular chemist to build multi-
Synthesis
dimensional molecular architectures.
In the present work, we have synthesized and structurally
Hg[L1]2 (1). To the stirred aqueous solution (B30 mL) of
characterized two new mercury(II) complexes, [Hg(L1)2] (1) and HgCl2 (0.16 g, 0.59 mmol), an aqueous solution (B30 mL) of
[Hg(L2)2]n (2), with substituted diphenyldithiophosphate ligands NaL1 (0.50 g, 1.20 mmol) in a 100 mL round bottom flask was
[NaL1 = sodium O,O-bis(4-tert-butylphenyl) phosphorodithioate added dropwise using a dropping funnel. A white precipitate
and NaL2 = sodium O,O-bis(3,4-dimethylphenyl) phosphoro- was formed immediately upon addition and the suspension
dithioate]. These complexes were successfully crystallized and was stirred for further 20 minutes. After stirring, the contents
analyzed by X-ray diffraction analysis. In this study, we have were filtered using a sintered glass disk (G-4) to obtain the
focused in particular on the determination of recurrent supra- residue of the complex Hg[S2P{OC6H4(4-(CH3)3C)}2]2 as a pale
molecular motifs that emerge from their non-covalent inter- yellow solid, which was dried for two hours in vacuo using an oil
actions and compared the intermolecular interactions, packing bath of temperature 45–50 1C. The yield of the desired product
motifs and Hirshfeld surface plots of the structures in combination was obtained B0.47g, 81% (based on mercury), m.p. 153–
with theoretical DFT calculations. Non-covalent interactions 154 1C. The complex could be crystallized by dissolving pale
(H-bond, SpB and C–Hꢀ ꢀ ꢀp) in the supramolecular assembly yellow powder in DMSO, which gave colorless single crystals in
of both complexes were explored energetically using theoretical one week suitable for X-ray analysis. Elem. anal. calcd (%) for
DFT calculations to understand clearly the presence of inter-
C40H52O4P2S4Hg (987.57): C, 48.64%; H, 5.31%; S, 12.99%.
molecular hydrogen bonding. These interactions were also Found: C, 48.60%; H, 5.27%; S, 12.95%. IR (KBr pellet, cmꢁ1):
analyzed using several computational tools, including Bader’s 1364.23 [v(P)–O–C], 933.08 [vP–O–(C)], 630.04 [vPQS], 554.27
‘‘atoms-in-molecules’’ (AIM) and Reduced Density Gradient [vP–S]. UV-Vis, lmax (nm), [emax (L molꢁ1 cmꢁ1)] (CH2Cl2): 229;
(NCI-RDG) methods that support nicely the presence of such 264; 360. 1H NMR (CDCl3, ppm): 1.34 (s, 36H, CH3 of C(CH3)3);
non-covalent intermolecular interactions.
7.27 (d, 8H, J = 8.8 Hz, H2,6); 7.39 (d, 8H, J = 8.8 Hz, H3,5); 13C
NMR (CDCl3, ppm): 31.43 [C of methyl group of C(CH3)3]; 34.52
[tert C of C(CH3)3]; 121.02 (C2,6); 126.62 (C3,5); 148.47 (C4);
148.89 (C1); 31P NMR (CDCl3, ppm): 93.02 (s) (Scheme 1).
[Hg[L2]2]n (2). A similar protocol was applied for the synthesis of
complex 2. NaL2 (0.50 g, 1.39 mmol) and HgCl2 (0.18 g, 0.66 mmol)
Experimental
Materials
The ligands were prepared in toluene, which was procured from were used. A white precipitate was formed and the reaction mixture
commercial sources. Toluene was purified and dried according to was stirred for 20 minutes. Crystallization of complex 2 was achieved
standard procedures. Doubly distilled water was used in the using a similar procedure to that used for complex 1. Yield: B0.44g,
synthetic procedures for the complexes. All chemicals used in 76% (based on mercury), m.p. 147–148 1C. Elem. Anal. Calcd. (%) for
synthetic procedures were of analytical grade and were purchased C32H36O4P2S4Hg (875.38): C, 43.90%; H, 4.14%; S, 14.65%. Found: C,
from Sigma-Aldrich and used as received. Caution! Mercury com- 43.86%; H, 4.11%; S, 14.60%. IR (KBr pellet, cmꢁ1): 1492.92
pounds are toxic and appropriate precautions were taken in [v(P)ꢁOꢁC], 961.03 [vPꢁOꢁ(C)], 652.01 [vPꢁS]asym
,
556.13
handling these compounds. Sodium O,O-bis(4-tert-butylphenyl) [vPꢁS]sym. UV-Vis, lmax (nm), [emax (L molꢁ1 cmꢁ1)] (CH2Cl2): 228;
phosphorodithioate (NaL1) and sodium O,O-bis(3,4-dimethyl- 268; 360. 1H NMR (CDCl3, ppm): 2.25 (s, 12H, CH3 bonded to C3);
2250 | New J. Chem., 2021, 45, 2249--2263
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to partition the crystal electron density into molecular
fragments.37,38 The molecular HS in the crystal structure is
constructed considering the electron distribution calculated as
the sum of spherical atom electron densities. For a given crystal
structure the set of spherical atomic electron densities and the
asymmetric unit in the crystal structure has a unique49 Hirsh-
feld surface. For each point on that isosurface, two distances
are defined i.e., di and de; di is the distance from the point to the
nearest nucleus internal to the surface and de is the distance
from the nucleus external to the surface. The normalized
contact distance denoted by dnorm, based on both di and de,
and the van der Waals (vdw) radii of the atom, given by eqn (1),
facilitate the identification of the regions of particular impor-
tance to various molecular interactions.36 The combination of
de and di in the form of a two-dimensional fingerprint plot50
provides a summary of the molecular contacts in the crystal.
The HS is mapped with dnorm, and the two-dimensional finger-
print plots presented in this work were generated using Crystal-
Explorer package ver. 17.5 software.51 The coordinates of atoms
were imported from CIF files. In the case of the polymeric
structure (complex 2), the Hirshfeld surface (HS) was drawn for
the asymmetric unit using very high resolution. The molecular
HS was mapped with dnorm using a red-white-blue color
scheme, whereas red indicates shorter contacts, white indicates
contacts around the vdW separation and blue indicates longer
contacts.
Scheme 1 Ring labeling for NMR spectroscopic assignments of com-
plexes (a) 1 and (b) 2.
2.40 (s, 12H, CH3 bonded to C4); 7.02 (d, 4H, J = 8.4 Hz, H6); 7.10 (s,
4H, H2); 7.27 (d, 4H, J = 8.2 Hz, H5); 13C NMR (CDCl3, ppm): 19.21
(CH3 bonded to C4); 19.92 (CH3 bonded to C3); 118.85 (C6);
122.61(C2); 130.24 (C4), 134.22 (C5), 138.07 (C3), 149.24 (C1); 31P
NMR (CDCl3, ppm): 88.73 (s) (Scheme 1).
X-ray crystallography
Suitable single crystals of complexes 1 and 2 were used for data
collection using a SuperNova, a single source at far offset, and a
HyPix3000 diffractometer, equipped with graphite-mono-
chromatic Mo Ka radiation (l = 0.71073 Å). Using the program
Olex2,41 the structures of complexes 1 and 2 were solved with
the ShelXT42 structure solution program using intrinsic phas-
ing and refined with the ShelXL43 refinement package using
Least Squares minimization on F2. All non-hydrogen atoms
were refined anisotropically.44–47 The details of the crystallo-
graphic data and refinements of the complexes have been given
in Table 1. The reported crystal structural pictures for both the
complexes were drawn with the program Mercury, version
4.3.0.48
À
Á
À
Á
di ꢁ rvi dw
de ꢁ rvedw
dnorm
¼
þ
(1)
rvi dw
rvedw
Computational details
All calculations were performed with Gaussian 09.52 To
Hirshfeld surface
The Hirshfeld surface (HS)35,36 emerged from a unique attempt know the exact position of hydrogen the atoms of both
to define the space occupied by a molecule in a crystal molecules were optimized in their ground spin state B3LYP
Table 1 Crystal data and structural refinement for complexes 1 and 2
Complex
CCDC
1
2
2026504
2026505
Chemical formula
Mr
Crystal system, space group
Temperature (K)
a, b, c (Å)
a, b, g (1)
V (Å3)
C40H52HgO4P2S4
987.58
Triclinic, P1
C32H36HgO4P2S4
875.38
Monoclinic, P21/c
130
%
293
10.2272 (2), 12.2502 (2), 17.0456 (3) 14.6282 (4), 11.2271 (2), 22.4201 (7)
81.444 (2), 85.958 (2), 87.659 (2)
2105.55 (7)
2
90, 106.821 (3), 90
3524.56 (17)
4
Z
m (mmꢁ1
)
3.97
4.73
Crystal size (mm)
Tmin, Tmax
0.16 ꢂ 0.12 ꢂ 0.08
0.12 ꢂ 0.08 ꢂ 0.06
0.220, 1.000
51266, 7697, 5394
0.047
0.648
0.039, 0.071, 1.05
7697
0.471, 1.000
No. of measured, independent and observed [I 4 2s(I)] reflections 27664, 8474, 7437
Rint
0.067
0.627
0.036, 0.078, 1.08
8474
(sin y/l)max (Åꢁ1
)
R[F2 4 2s(F2)], wR(F2), S
No. of reflections
No. of parameters
472
2.00, ꢁ2.33
396
1.06, ꢁ1.10
Drmax, Drmin (e Åꢁ3
)
Computer programs: CrysAlis PRO 1.171.39.46 (Rigaku OD, 2018), ShelXT (Sheldrick, 2015), SHELXL (Sheldrick, 2015), Olex2 (Dolomanov et al.,
2009).
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density functional starting from their crystallographic geo- Results and discussion
metries. Here, Los Alamos Effective Core Potential lanL2DZ
basis set was employed for the Hg atom. On the other hand,
Synthesis
The substituted diphenyldithiophosphate ligands were synthesized
by the reaction of substituted phenols, phosphorous pentasulfide
and triethylamine in a 4 : 1 : 2 molar ratio followed by the addition
of sodium metal to convert them into their sodium salts. The
reaction of the sodium salts of the ligands with HgCl2 in a 2 : 1
molar ratio in water is quite facile and gave complexes 1 and 2 in
fairly good yield. The complexes were yellow powder and soluble in
chloroform. The formation of the complexes is shown in Scheme 2
while the plausible mechanism of the reaction is given in
Scheme 3.
the split-valence 6-31G(d) basis set was applied for the
other atoms.
The topological features derived from Bader’s theory of atoms
in molecules (AIM) approach was applied to understand the
electron-density features like charge density (r) and Laplacian of
2
charge density (r r) using ADF2014.10. The recently developed
Reduced Density Gradient (RDG) based NCI (non-covalent inter-
actions) index calculations were applied for real-space visualiza-
tion of both attractive (van der Waals and hydrogen-bonding)
and repulsive (steric) interactions based on the properties of the
electron density. Herein, the single-point calculations were
based on the structure obtained from X-ray studies and in these
Description of the structures
structures, the hydrogen atom positions were optimized before Determination of X-ray crystallography reveals that complex 1 is
computation. in the monomeric unit while complex 2 is in the polymeric unit
To calculate the hydrogen bond energy the following and belongs to the triclinic and monoclinic crystal systems with
latest formula was proposed by Emamian et al.,53 derived at space groups P1 and P21/c, respectively. The molecular struc-
%
the very accurate CCSD(T)/jul-cc-pVTZ level including BSSE tures of complexes 1 and 2 (asymmetric unit) show the selective
correction as well as SAPT2+(3)dMP2/aug-cc-pVTZ level, this atom numbering scheme and thermal displacement ellipsoids
allows the bond energy to be decomposed into physically drawn at the 50% probability level as shown in Fig. 1. The
meaningful components to shed light on the electronic nature details of the crystallographic data and refinements of both the
of the considered hydrogen bond interactions:
complexes are given in Table 1. Selected bond lengths and bond
angles for complexes 1 and 2 are listed in Tables S1 and S2
(ESI†). In the molecular structure of complex 1, the mercury
atom (Hg1) is coordinated by two ligands (L1) wherein one of
Hydrogen bond energy (kcal molꢁ1) = ꢁ223.08 ꢂ rBCP(a.u.)
+ 0.7423
Scheme 2 Synthetic route for complexes 1 and 2.
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Scheme 3 Plausible mechanism for complexes 1 and 2.
Fig. 1 Molecular structures of complexes (a) 1 and (b) 2 (monomeric unit), showing the selective atom numbering scheme and thermal displacement
ellipsoids drawn at the 50% probability level.
the dithiophosphate ligands acts as a chelating ligand (biden- with (2.8347 (10) Å, Hg1–S1 and 2.3875 (9) Å, Hg1–S2) mercury–
tate) and bonds to the mercury atom by its two sulfur atoms sulfur bond lengths. This bidentate sulfur ligand forms a four-
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membered chelate ring upon complexation with two sulfur (4) and 102.98 (4) respectively. The geometry around the
atoms. The other dithiophosphate ligand acts in a monodentate phosphorous atom is a distorted tetrahedron. All coordinate
mode to the mercury atom via one of the sulfur atoms (2.3591 (9) Å, bond lengths and bond angles are within the range of
Hg1–S3) and the other sulfur atom (S4) is free. Thus, the values previously observed for the mercury(II) polymer of
mercury ion is coordinated with three sulfur atoms to produce dithiophosphates.30,31
an approximately T-shape geometry around the mercury with
The monomeric nature of 1 is probably owing to the presence
S3–Hg1–S2, S3–Hg1–S1 and S2–Hg1–S1 bond angles of 166.15 of a bulky tert-butyl group, which causes steric hindrance thus
(3), 113.75 (3) and 78.46 (3). The T-shape geometry represents forcing the complex to stay in the monomeric form. On the other
the most extreme case of trigonal distortion54,55 in mercury hand, the polymeric form in the case of 2 is because of the
sulfur complexes. The distorted trigonal planar geometry is presence of less bulky methyl groups.
assumed to be responsible for mercury(II) detoxification in
Supramolecular interactions in the solid-state structures
living organisms because it is the most stable form, followed
by a symmetrical dicoordinate S–Hg–S configuration.56,57 The The solid-state structures of the complexes show intermolecular
geometry around the phosphorus atom is a distorted tetra- forces that lead to supramolecular structures through several
hedron, as the chelate mode ligand in which P1–S1, P1–S2, non-covalent interactions such as hydrogen bonding (Hꢀ ꢀ ꢀH,
P1–O1 and P1–O2 bond lengths are 1.9471(14), 2.0408(14), Cꢀ ꢀ ꢀH, CHꢀ ꢀ ꢀO, etc.), Sꢀ ꢀ ꢀC, Hgꢀ ꢀ ꢀS, C–Hꢀ ꢀ ꢀp and cationꢀ ꢀ ꢀp
1.588(3) and 1.587(3) Å and the monodentate mode ligand interactions. The sulfur-based mercury complexes show the
in which P2–S3, P2–S4, P2–O3 and P2–O4 bond lengths are tendency to self-assemble in the solid-state through secondary
2.0590 (14), 1.9106 (14), 1.590 (3) and 1.595 (3) Å, respectively.
contact and to form supramolecular structures.5,14 Complexes 1
In the monomeric unit of complex 2, one mercury atom and 2 show supramolecular architectures through H-bond, SpB
(Hg1) is surrounded by two ligands (L2), one of the ligands acts and C–Hꢀ ꢀ ꢀp interactions. The non-covalent interactions present
as a chelating ligand and binds to the mercury atom by its two in both the complexes are listed in Tables 2–4.
sulfur atoms with a bond length of Hg1–S1 = 2.8868 (12) Å and
In the solid-state structure of complex 1, the monomeric
Hg1–S2 = 2.4267 (12) Å mercury–sulfur bonds. The other ligand unit is interlinked to each other via strong self-complementary
bonds to the mercury atom via one of the sulfur atoms (S3) with hydrogen bonds associated with the CHꢀ ꢀ ꢀO interactions.
a bond length Hg1–S3 equal to 2.3907 (11) Å and the other O2ꢀꢀꢀH16 shows hydrogen bonding interaction of distance
sulfur atom (S4) is free. The free sulfur atom (S4) bonds to the 2.713 Å which gives rise to the dimeric unit as shown in
neighbouring mercury atom with a bond length of Hg1—S4i Fig. 3. Also, the dimeric unit is stabilized through C–Hꢀ ꢀ ꢀp
2.7283 (11), so the second ligand acts as a bridging ligand interactions via H19CꢀꢀꢀCg(1) [Cg(1) = center of mass of the ring
between the neighbouring mercury atoms giving rise to an C(1)–C(6)] with a distance of 3.395 Å.
infinite polymer. The monomeric units of complex 2 are
The monomeric unit in the case of complex 1 also forms a
arranged to give rise to an infinite polymeric structure along one-dimensional chain through intermolecular Cꢀ ꢀ ꢀH and
the direction of the b axis to form a polymer (Fig. 2b). The C–Hꢀ ꢀ ꢀp interactions via C30ꢀ ꢀ ꢀH2 and H26ꢀ ꢀ ꢀCg(2) [Cg(2) =
coordination geometry around the mercury atom found a center of mass of the ring C(12)–C(16)] having distances 2.871 Å
distorted tetrahedron shown in Fig. 2a with S3–Hg1–S2, S3– and 4.267 Å as shown in Fig. 4. Another monomeric unit
Hg1–S4i, S3–Hg1–S1, S2–Hg1–S4i, S2–Hg1–S1 and S4i –Hg1–S1, interacts with the neighbouring moiety to form a dimeric
bond angles of 157.98 (4), 102.62 (4), 107.05 (4), 97.01 (4), 77.74 structure through CHꢀ ꢀ ꢀO (2.621 Å, H6ꢀ ꢀ ꢀO4 and 2.537 Å,
H6ꢀ ꢀ ꢀO3) interactions as shown in Fig. 5a. Other secondary
interactions such as S3ꢀ ꢀ ꢀC22, S3ꢀ ꢀ ꢀHg1 (SpB) and S2ꢀ ꢀ ꢀC25 in
complex 1 having distances 3.453 Å, 3.197 Å and 3.476 Å aid the
formation of the dimeric unit as shown in Fig. 5b. Also, the
dimeric unit is stabilized through Hgꢀ ꢀ ꢀp SpB interactions via
Table 2 Relevant hydrogen bonding parameters in complexes 1 and 2 (Å, 1)
D–H Hꢀ ꢀ ꢀA Dꢀ ꢀ ꢀA D–Hꢀ ꢀ ꢀA
D–Hꢀ ꢀ ꢀA
(Å)
(Å)
(Å)
(1)
Symmetry
Complex 1
C16—H16ꢀ ꢀ ꢀO2
C2—H2ꢀ ꢀ ꢀC30
C6—H6ꢀ ꢀ ꢀO4
C6—H6ꢀ ꢀ ꢀO3
0.93 2.71
0.93 2.87
0.93 2.62
0.93 2.54
3.43
3.69
3.14
3.37
134.3
147.7
115.91
148.50
ꢁx, 2 ꢁ y, 1 ꢁ z
x, ꢁ1 + y, z
ꢁx, 1 ꢁ y, 1 ꢁ z
ꢁx, 1 ꢁ y, 1 ꢁ z
Complex 2
C7—H7BꢀꢀꢀC3
0.96 2.90
3.60
3.58
2.82
3.50
131.34
139.09
123.04
139.60
ꢁx, 1 ꢁ y, 1 ꢁ z
x, 1.5 ꢁ y, ꢁ1/2 + z
x, 1.5 ꢁ y, ꢁ1/2 + z
—
Fig. 2 (a) Distorted tetrahedral geometry around Hg1 of complex 2; and
(b) perspective view of the extended structure of complex 2 along the b
axis, hydrogen atoms are omitted for clarity.
C21—H21ꢀ ꢀ ꢀC21 0.93 2.82
C21—H21ꢀꢀꢀH21 0.93 2.20
C15—H15Cꢀ ꢀ ꢀO2 0.96 2.72
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Table 3 Geometrical parameters for the C–Hꢀ ꢀ ꢀp interactions in com-
plexes 1 and 2
important hydrogen bonding intermolecular interaction
present in the polymer chain is CHꢀ ꢀ ꢀO where H15C interacts
with the O2 of the neighbouring moiety and helps to form a
supramolecular architecture having an intermolecular bond
distance of 2.723 Å as shown in Fig. 6. The Hꢀ ꢀ ꢀH intermole-
cular interaction also plays a pivotal role in forming a supra-
molecular polymer chain in the crystal design in which
H21ꢀ ꢀ ꢀH21 interacts with a distance of 2.203 Å as shown in
X–Hꢀ ꢀ ꢀCg
X–Cg (Å) Hꢀ ꢀ ꢀCg (Å) X–Hꢀ ꢀ ꢀCg (1) Symmetry
Complex 1
C19—H19CꢀꢀꢀCg1 3.89
3.40
4.27
114.52
128.91
ꢁx, 2 ꢁ y, 1 ꢁ z
x, ꢁ1 + y, z
C26—H26ꢀ ꢀ ꢀCg2 4.91
Complex 2
C7—H7Bꢀ ꢀ ꢀCg4 3.64
3.08
3.43
118.32
94.79
ꢁx, 1 ꢁ y, 1 ꢁ z
C7—H7Aꢀ ꢀ ꢀCg5 3.64
ꢁx, 1 ꢁ y, 1 ꢁ z Fig. S4 (ESI†). The solid-state polymeric structure of complex 2
represents C–Hꢀ ꢀ ꢀp interactions that play a crucial role in the
crystal packing in a unique way. The C–Hꢀ ꢀ ꢀp interactions are
associated with C7—H7Bꢀ ꢀ ꢀCg(4) [Cg(4) = center of mass of the
Table 4 Geometrical parameters for the Hgꢀ ꢀ ꢀS, Hgꢀ ꢀ ꢀp and Cꢀ ꢀ ꢀS inter-
actions in complexes 1 and 2
ring C(1)–C(6)] having a distance of 3.082 Å (Fig. 7) and
C7—H7Aꢀ ꢀ ꢀCg(5) [Cg(5) = center of mass of the ring C(1)–
C(6)] having a distance of 3.429 Å (Fig. S5, ESI†).
Hgꢀ ꢀ ꢀS/Hgꢀ ꢀ ꢀp/Cꢀ ꢀ ꢀS Hgꢀ ꢀ ꢀS (Å) Hgꢀ ꢀ ꢀp(Å) Cꢀ ꢀ ꢀS (Å) Symmetry
Complex 1
NMR, IR and electronic spectra
Hg1ꢀ ꢀ ꢀS3
3.19
—
—
—
3.52
—
—
—
3.45
3.47
1 ꢁ x, 1 ꢁ y, 1 ꢁ z
1 ꢁ x, 1 ꢁ y, 1 ꢁ z
1 ꢁ x, 1 ꢁ y, 1 ꢁ z
1 ꢁ x, 1 ꢁ y, 1 ꢁ z
Hgꢀ ꢀ ꢀp
Complexes 1 and 2 were deduced from common analytical
methods such as elemental analysis, NMR, IR and UV-visible
spectroscopy. The details are given in the Experimental section.
NMR (1H, 13C and 31P) spectroscopy was employed to precisely
identify the new structures in solution. The experimental
section provides the numerical NMR, IR and UV-visible data,
while the NMR and IR spectra are entrenched in Fig. S6–S13 in
the ESI.†
C22ꢀ ꢀ ꢀS3
C25ꢀ ꢀ ꢀS2
—
—
The spectral data of 1H NMR show the characteristic proton
resonances of the corresponding substituted phenyl protons.
The splitting patterns of peaks in the spectra of both the
complexes were found to be reliable with the structure. In
complex 1, the chemical shift of the CH3 protons of 4-tert-
butyl exhibits a singlet and was observed at 1.34 ppm. The
aromatic protons of the phenyl group exhibit doublets and were
observed at 7.27 and 7.39 ppm. In complex 2, the CH3 protons
of 4-methyl and CH3 protons of 3-methyl were observed as a
singlet at 2.25 and 2.40 ppm. The aromatic protons of the
phenyl group exhibit one singlet at 7.10 ppm and two doublets
at 7.02 and 7.27 ppm.
The 13C NMR chemical shifts for carbon nuclei of the aryl
group (except the carbon atom bonded to an oxygen atom)
displayed their resonance in the region of 118.85–148.47 ppm.
The chemical shift for the carbon atom bonded to the oxygen
atom was found at 148.89 ppm for 1 and 149.24 ppm for 2. The
chemical shifts for the carbon atoms of tert-butyl and CH3
groups of complex 1 displayed their resonance at 31.43 and
Fig. 3 Supramolecular dimer of complex 1 generated through CHꢀꢀꢀO and
C–Hꢀꢀꢀp interactions. Only the relevant hydrogen atoms are shown for clarity.
Symmetry transformation = ꢁx, 2 ꢁ y, 1 ꢁ z, where O2ꢀꢀꢀH16 = 2.713 Å,
H19CꢀꢀꢀCg(1) = 3.395 Å [Cg(1) = center of mass of the ring C(1)–C(6)].
Hg1 and Cg(3) [Cg(3) = center of mass of the ring C(21)–C(26)] 34.32 ppm. In complex 2, the carbon atoms of the CH3 group
having distances 3.521 Å shown in Fig. S1 in the ESI.†
In the case of complex 2, the monomeric unit combines to
displayed their resonance at 19.21 and 19.92 ppm.
31P NMR spectra (proton-decoupled) of both the complexes
form an infinite polymer chain as shown in Fig. 2b. X-ray displayed a single resonance in each case. The appearance of a
crystallography reveals that complex 2 exists in the polymeric single peak in the spectra clearly indicates the chemical
form. This polymeric unit in Fig. 2b interlinked with the equivalent nature of the phosphorus atoms.58 The 31P NMR
neighbouring moiety to form supramolecular structures. The spectra were observed as a single peak at 93.02 ppm for
Cꢀ ꢀ ꢀH interactions in the solid state structure of complex 2 are complex 1 and 88.73 ppm for complex 2.
associated with the H7B of the molecule which interacts with
IR spectra of both the complexes are in fine agreement with
C3 in a neighbouring moiety having a bond distance of 2.898 Å those of their resultant crystal structures and the tentative
and H21 interacts with C21 (2.820 Å) helping to form a assignments were made based on pertinent literature
supramolecular polymeric chain, Fig. S2 and S3 (ESI†). Another reports.5,30,31 In both the complexes, the strong intensity bands
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Fig. 4 Molecular chain of complex 1 generated through intermolecular Cꢀ ꢀ ꢀH and C–Hꢀ ꢀ ꢀp interactions. Only the relevant hydrogen atoms are shown
for clarity. Symmetry transformation = x, ꢁ 1 + y, z, where C30ꢀ ꢀ ꢀH2 = 2.871 Å, H26ꢀ ꢀ ꢀCg(2) = 4.267 Å [Cg(2) = center of mass of the ring C(12)–C(16)].
Fig. 5 Supramolecular dimer of complex 1 generated through (a) CHꢀ ꢀ ꢀO
Fig. 6 Supramolecular polymer chain of complex 2 generated through
(H6ꢀ ꢀ ꢀO4 and H6ꢀ ꢀ ꢀO3); (b) S3ꢀ ꢀ ꢀC22, S3ꢀ ꢀ ꢀHg1 and S2ꢀ ꢀ ꢀC25 interactions.
Only the relevant hydrogen atoms are shown for clarity. Symmetry trans-
formation a = ꢁx, 1 ꢁ y, 1 ꢁ z, where H6ꢀ ꢀ ꢀO4 = 2.621 Å and H6ꢀ ꢀ ꢀO3 =
2.537 Å. And symmetry transformation b = 1 ꢁ x, 1 ꢁ y, 1 ꢁ z, where
S3ꢀ ꢀ ꢀC22 = 3.453 Å, S3ꢀ ꢀ ꢀHg1 (SpB) = 3.197 Å and S2ꢀ ꢀ ꢀC25 = 3.476 Å.
CHꢀ ꢀ ꢀO interactions. Only the relevant hydrogen atoms are shown for
clarity, where H15Cꢀ ꢀ ꢀO2 = 2.723 Å.
intensity near 264 nm in complex 1 and 268 nm in complex 2
may be attributed to the n - p* transition of the ligand.30 One
absorption band at very high intensity in the region of 229 nm in 1
at 1364.23 cmꢁ1 in 1 and 1492.92 cmꢁ1 in 2 due to v((P)–O–C) and 228 nm in 2 may be attributed to the p - p* transitions of the
stretching vibrations and at 933.08 in 1 and 961.03 cmꢁ1 in 2 due ligands. The low energy bands for both complexes are associated
to v(P–O–(C)) stretching vibrations were observed. In complex 1, with the HOMO to LUMO electronic transitions. The difference in
sharp to medium intensity bands at 630.04 and 554.27 cmꢁ1 the energy gap between the HOMO and LUMO can be used to
indicate the presence of [vPQS] and [vP–S] vibrations. In predict the stability and reactivity of the molecules. Molecules
complex 2, the bands present at 652.01 and 556.13 cmꢁ1 indicate with large HOMO–LUMO gaps are generally stable and unreactive,
the presence of [vP–S]asym and [vP–S]sym vibrations.
while those with small gaps are generally reactive.59,60 In complex
The electronic (UV-visible) spectra of complexes 1 and 2 in 1, the energy of the LUMO is 0.87 eV and the HOMO is
CH2Cl2 solutions in the range of 200–600 nm at room temperature ꢁ2.49 eV and in the case of complex 2, the LUMO is 0.50 eV
are presented in Fig. 8. There is no band for the d–d transition, all and the HOMO is ꢁ2.05 eV. The energy gaps between the two
the bands absorb in the ultraviolet region, a very low-intensity frontier orbitals are found to be 3.37 eV in 1 and 2.55 eV in 2.
band at 360 nm in both the complexes may be attributed to The isosurfaces of the HOMO and LUMO along with their
ligand-to-metal charge transfer (LMCT). A broad band of low energies are shown in Fig. 9.
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Fig. 7 Supramolecular chain of complex 2 generated through C–Hꢀ ꢀ ꢀp interactions. Only the relevant hydrogen atoms are shown for clarity. Symmetry
transformation = ꢁx, 1 ꢁ y, 1 ꢁ z, where H7Bꢀ ꢀ ꢀCg4 = 3.082 Å [Cg(4) = center of mass of the ring C(1)–C(6)].
Fig. 9 Isosurface plots of the HOMO–LUMO energy gap diagram of
complexes 1 and 2 obtained at the DFT/B3LYP/LanL2DZ level of theory.
Fig. 8 UV-visible spectrum for complexes (a) 1 and (b) 2.
Hirshfeld surface analysis
packing. The Hirshfeld surfaces of complexes 1 and 2 are
To quantify the non-covalent interactions in the solid-state illustrated in Fig. 10, showing the surfaces that have been
structure of both the complexes, in this investigation, the mapped over a dnorm range of ꢁ0.3596 to 1.7250 Å, and a de
contacts responsible for building supramolecular assemblies
are evaluated. Hirshfeld surface (HS)35–38 analysis has become a
of 1.0131 to 2.7440 Å. The surfaces are shown as transparent to
allow the visualization of a molecular moiety, in a similar
standard tool for the analysis of various intermolecular con- orientation for both the structures, around which they were
tacts and is typically used for characterization of hydrogen- calculated. In the case of polymeric structure 2, the HS was
bonded molecules or discrete complexes.61–67 Whereas its drawn for the asymmetric unit. The individual bonding inter-
application to polymeric structures is much less common due actions are strictly identified separately on the dnorm surfaces in
to some complicating factors.68 The pattern of the intermole- Fig. 10, where regions of most important interactions are
cular interactions of the solid-state structures of both com- indicated with arrows. The spots summarize the information
plexes encouraged us to investigate and quantify the
contribution of the non-covalent interactions in the crystal
presented in the intermolecular bonding table (Tables 2–4)
effectively, with the deep-red surfaces are an indication of very
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complexes 1 and 2) than its Hꢀ ꢀ ꢀC counterparts (5.7% and
8.5% in complexes 1 and 2). Thus, the sum of Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC
contributes 12.3% and 18.0% for complexes 1 and 2 of total HS
area of the molecule. The Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC interactions represented
by the spikes in the bottom left and right region (di + de) E
2.87 Å in 1 and (di + de) E 2.898 Å in complex 2. The point in the
region (di + de) E 2.87 Å in complex 1 represents the atom
numbering C30 of the molecule that interact with the H2 atom
of a neighbouring moiety to result in building a 1D supra-
molecular chain through Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC interactions and 1D
chain of intermolecular CHꢀ ꢀ ꢀp interactions (Fig. 4). The
(di + de) E 2.898 Å in complex 2 is associated with the Cꢀ ꢀ ꢀH
interactions in solid-state structure that build a supramolecular
polymer chain (Fig. S2, ESI†). The Sꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀS interaction
comprises 13.9% for the complex 1 and in 17.7% for complex 2,
which are evidenced by two distinct spikes in the bottom left
and right region (di + de) E 2.86 Å in complex 1 and (di + de) E
2.82 Å in complex 2; where Sꢀ ꢀ ꢀH contribute largely i.e., 7.9%,
(1) and 10.5% (2) compared to its Hꢀ ꢀ ꢀS counterpart 6.1% (1)
and 7.2% (2).
Fig. 10 Hirshfeld surface of complexes (a) 1 and (b) 2 mapped with dnorm
(left) and de (right). Regions of most important interactions in dnorm are
indicated with arrows.
The proportions of the Oꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀO interactions comprise
6.9% for complex 1 and 5.9% for complex 2 of the total
Hirshfeld surface area. The Oꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀO interactions are also
short intermolecular contacts and other light-colored depres- evidenced by two distinct spikes of almost equal length in the
sions on the dnorm surface are an indication of weaker and 2D FP plots; where (di + de) E 2.713 Å for complex 1 while 2.72 Å
longer contacts may be due to hydrogen or other than for complex 2. The dnorm values of complex 1 show hydrogen
hydrogen bonds. The intermolecular interactions involved bonding interactions wherein O2 of monomer unit interact
within the molecular structures are also visible on the two- with H16 of a neighbouring moiety that gives rise to the dimeric
dimensional fingerprint (2D FP) plots49,50 and can be decom- unit (Fig. 3). The dnorm values of complex 2 depict O2 inter-
posed to analyze the individual contributions of molecular action with H15C that helps to form a supramolecular polymer
interactions related to each molecular structure. FP plots are chain (Fig. 6). In the Oꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀO interactions, the Oꢀ ꢀ ꢀH
the key tools for the computation of the density of molecular contribute more [3.7% (1) and 3.2% (2)] than its counter parts
contacts present in a crystal system. A representation of the Hꢀ ꢀ ꢀO interaction [3.2% (1) and 2.7% (2)]. The Oꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀO
separation of contribution from various interaction types, interactions in complex 1 are represented by spikes illustrates
which overlap in the full fingerprint, is given in Fig. 11. The that O2 and H16 of one molecule interact with H16 and O2 of
combination of di and de in the form of 2D FP provides an another molecule through hydrogen bonding; leading to the
outline of intermolecular contacts in the crystals, where one formation of supramolecular dimer of the complex. The pro-
molecule acts as a donor (de 4 di) and the other as an acceptor portions of Hꢀ ꢀ ꢀH contacts contributes to the total Hirshfeld
(de o di).
surface area 59.0% and 52.2% in complexes 1 and 2, respec-
The intermolecular interactions in both the complexes like tively. This much of contribution shows that dominating inter-
HꢀꢀꢀH, Hꢀ ꢀ ꢀC/Cꢀ ꢀ ꢀH, Hꢀ ꢀ ꢀS/Sꢀ ꢀ ꢀH, Hꢀ ꢀ ꢀO/Oꢀ ꢀ ꢀH and Hgꢀ ꢀ ꢀS/ actions in both the complexes and their fingerprint plots
Sꢀ ꢀ ꢀHg atoms can be seen in the Hirshfeld surface that are spread to di = de = 2.24 Å in 1 and 2.20 Å in 2.
marked in Fig. 10. The other interactions such as Hgꢀ ꢀ ꢀp and
The inspection of the other atom contacts pointed out that
CHꢀ ꢀ ꢀp are also associated with the Hgꢀ ꢀ ꢀC and Cꢀ ꢀ ꢀH inter- there are also specific features of interatomic contacts such as
actions. In complex 1, the deep red spots in the dnorm surface are in complex 1, the Cꢀ ꢀ ꢀS/Sꢀ ꢀ ꢀC, Hgꢀ ꢀ ꢀC/Cꢀ ꢀ ꢀHg, Hgꢀ ꢀ ꢀS/Sꢀ ꢀ ꢀHg,
indicative of the Hꢀ ꢀ ꢀO and Oꢀ ꢀ ꢀH interactions and the inter- Cꢀ ꢀ ꢀO/Oꢀ ꢀ ꢀC, Sꢀ ꢀ ꢀS, Hgꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀHg, Oꢀ ꢀ ꢀO and Cꢀ ꢀ ꢀC contribute
atomic distances between them are in the range of 2.713 Å that 2.4%, 1.7%, 1.4%, 1.4%, 0.5%, 0.2%, 0.1% and 0.1% of the total
actually helps to form a dimeric unit of the complex; generated HS area. The Cꢀ ꢀ ꢀS/Sꢀ ꢀ ꢀC, Hgꢀ ꢀ ꢀC/Cꢀ ꢀ ꢀHg and Hgꢀ ꢀ ꢀS/Sꢀ ꢀ ꢀHg
through hydrogen bonding interactions as shown in Fig. 3. In contacts help for forming a dimer unit (Fig. 5b). In complex
complex 2, the deep red spot in the dnorm surface is indicative of 2, the other interatomic contacts are Hgꢀ ꢀ ꢀS/Sꢀ ꢀ ꢀHg, Cꢀ ꢀ ꢀO/
Hgꢀ ꢀ ꢀS interaction, which contribute to form a polymeric system Oꢀ ꢀ ꢀC, Sꢀ ꢀ ꢀO/Oꢀ ꢀ ꢀS, Hgꢀ ꢀ ꢀO/Oꢀ ꢀ ꢀHg, Cꢀ ꢀ ꢀS/Sꢀ ꢀ ꢀC, Cꢀ ꢀ ꢀC and
(asymmetric unit is drawn in the HS analysis).
Sꢀ ꢀ ꢀS, which contribute 2.1%, 1.5%, 1.5%, 0.6%, 0.3%, 0.2%
The Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC molecular interactions appear as two dis- and 0.1% in the total HS area included as other interactions in
tinct spikes of almost equal length in the 2D FP plots in the the relative contribution. The Cꢀ ꢀ ꢀH interactions are associated
region 2.81 Å o (di + de) o 2.89 Å, where the portion of Cꢀ ꢀ ꢀH with C–Hꢀ ꢀ ꢀp interaction, because the solid-state structures are
interactions has larger contribution (6.6% and 9.5% in stabilized through C–Hꢀ ꢀ ꢀp interaction as shown in Fig. 3, 4
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Fig. 11 Fingerprint plots of complexes 1 and 2 showing the percentages of contacts to the total Hirshfeld surface area of molecules.
and 7. The presence of C–Hꢀ ꢀ ꢀp interactions in the solid-state the formation of HgS (calculated mass loss 77% and 73%). The
structures is evident because of the bright orange spot and second event occurs between the temperature range 465–900 1C
consequently a large shade on the de surface above the p-cloud with a mass loss of 7% in 1 and 25% in 2 (calculated mass loss
of the aryl ring specifies the existence of C–Hꢀ ꢀ ꢀp interaction 23% and 27%) corresponding to the thermolysis of HgS into
and clearly visible on the de surfaces. From Hirshfeld surface mercury vapour and sulfur.69 The lack of compatibility in 1
analysis, it is not possible to individuate the Hꢀ ꢀ ꢀC/Cꢀ ꢀ ꢀH between experimental and calculated mass losses is due to the
contacts for hydrogen bond and C–Hꢀ ꢀ ꢀp interactions. Fig. 12 onset of HgS decomposition and overlap of the complex
contains the relative percentages of contributions for a variety decomposition.18,69,70 In case of 2, the experimental mass loss is
of contacts in the complexes 1 and 2.
in excellent agreement with the calculated values.
Thermogravimetric analysis
Theoretical study on non-covalent interactions
The thermal behaviors of the complexes were monitored under N2 Hydrogen bonding interactions in both complexes are charac-
atmosphere over the temperature range 30–950 1C at a heating terized by AIM analysis, which gives an orange sphere as BCP
rate of 10 1C minꢁ1. The experimental traces of both the (Fig. 13 and 15). The existence of the hydrogen bonds is also
complexes are plotted in Fig. S14 and S15 in the ESI.† Thermo- well supported by the RDG plots and NCI surfaces, as shown in
gravimetric analysis of complexes 1 and 2 reveals no significant Fig. 13–15.
mass loss in the region of 30–140 1C that indicates lack of water.
Here, we computed the reduced density gradient (RDG)
The thermal decomposition occurs in two endothermic events. to represent the deviation from a homogeneous electron
The first event occurring in the temperature range between 200– distribution. As presented in the following eqn (2), r is
465 1C with a mass loss of 93% in 1 and 75% in 2 associated with the gradient operator and |rr| is the electronic density
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Fig. 12 Relative contributions of various intermolecular interactions to
the Hirshfeld surface area in complex 1 (top) and complex 2 (bottom).
Fig. 14 NCI RDG isosurfaces of the dimers of investigated complex 1 as
shown in Fig. 3–5 computed at B3LYP/Lanl2DZ/6-31G(d) level; (a) and (b)
represent Fig. 3; (c) represents Fig. 4; (d) represents Fig. 5a; and (e)
represents Fig. 5b.
gradient mode. Now, it is a helpful approach to explore and
visualize different kinds of non-covalent interactions (NCIs)
in real space such as both intra- and intermolecular weak
interactions like hydrogen bonds and van der Waals forces.
Fig. 13 AIM analyses and NCI isosurfaces of the dimers of investigated complex 1 showing (a) intermolecular hydrogen bonding and (b) C–Hꢀ ꢀ ꢀp
interaction as shown in Fig. 3. Bond critical points are represented by orange spheres at B3LYP/Lanl2DZ/6-31G(d) level.
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Fig. 15 AIM analyses and NCI isosurfaces of the polymers of investigated complex 2 as shown in Fig. 6 and 7 computed at B3LYP/Lanl2DZ/6-31G(d)
level; (a) and (b) represent Fig. 6; and (c) represent Fig. 7.
Therefore, the NCI index is used to investigate NCIs in the (usually steric effect). Values near zero indicate weak vdW
investigated complex.
interactions. The color of RDG (s) vs. sign (l2)r and the
isosurfaces have the same meaning; blue color signifies hydrogen
bonds; green, van der Waals forces; and red, steric hindrance.
The darker the corresponding color, the stronger is the
interaction. Both the complexes show a very similar RDG
scatter plot. As shown in Fig. 13 the existence of hydrogen
bond is clear from the scattered plot with negative sign(l2)r for
complex 1.
The green region in Fig. 13 clearly characterizes the hydro-
gen bonding in the solid-state of the complex 1. This is also
supported from interaction energy. The results show that
complex 1 is highly stabilized by two weak H-bond (r(BCP) =
0.0031 a.u.). Applying the formula as discussed in the computa-
tional section, the calculated hydrogen bond energy is below ca.
1 kcal molꢁ1 for two hydrogen bonds.
1
jrrj
s ¼
(2)
1
3
4
2
3
r
2ð3p Þ
The color mapped isosurfaces and corresponding scatter
diagrams of RDG versus sign (l2)r for the investigated complex
in dimer forms are shown in Fig. 13. As discussed, the results
characterized the hydrogen bonds via colored isosurfaces
according to the values of sign (l2)r. The sign of l2 is used to
distinguish bonded (l2 o 0) and non bonded (l2 4 0) interactions,
whereas the electron density is an indicator of the bonding
strength. Large negative values of sign (l2)r indicates of
attractive interactions (such as hydrogen bonding), whereas,
if sign (l2)r is large and positive, the interaction is nonbonding
Fig. 16 AIM analyses and NCI isosurfaces of investigated complex 2 characterizing additional Hg–S bond. Bond critical points are represented by orange
spheres at B3LYP/Lanl2DZ/6-31G(d) level.
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The presence of non-covalent interaction in complex 1 as
stated in Fig. 3–5 are very clear from the RDG isosurface plot as
shown in Fig. 14. And in complex 2 the non-covalent interactions
as stated in Fig. 6 and 7 are very clear from the RDG isosurface
plot as given in Fig. 15.
NJC
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Also, the existence of an additional Hg–S bond along with
NCI is shown in Fig. 16.
´
13 A. Bauza, I. Alkorta, J. Elguero, T. J. Mooibroek and
Concluding remarks
A. Frontera, Angew. Chem., Int. Ed., 2020, 59, 17482–17487.
14 I. Haiduc and D. S. Sowerby, Polyhedron, 1996, 15,
2469–2521.
In summary, two mercury(II) complexes with substituted diphenyl-
dithiophosphate ligands have been synthesized and structurally
characterized. In the solid-state structure of mercury(II) complexes,
we see a unique combination of non-covalent interactions such as
H-bond, SpB and CHꢀꢀꢀp interactions that contributes to self-
assembly process. These interactions clearly indicate their impor-
tance in the crystal packing frameworks by the formation of
extended supramolecular networks. Analysis of these interactions
is examined with the Hirshfeld surface analyses and DFT calcula-
tions to confirm the existence of non-covalent interactions and
further corroborated with Bader’s ‘‘atoms-in-molecules’’ (AIM) and
Reduced Density Gradient (NCI-RDG) methods that support nicely
the presence of such non-covalent intermolecular interactions.
´
´
15 P. Fernandez, A. Sousa-Pedrares, J. Romero, J. A. Garcıa-
´
´
Vazquez, A. Sousa and P. Perez-Lourido, Inorg. Chem., 2008,
47, 2121–2132.
16 A. Manceau, C. Lemouchi, M. Rovezzi, M. Lanson,
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20 M. J. Cox and E. R. T. Tiekink, Rev. Inorg. Chem., 1997, 17,
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Conflicts of interest
21 R. A. Howie, E. R. T. Tiekink, J. L. Wardell and
S. M. S. V. Wardell, J. Chem. Crystallogr., 2009, 39, 293–298.
22 D. Riccardi, H. B. Guo, J. M. Parks, B. Gu, A. O. Summers,
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The authors declare no conflict of interest.
Acknowledgements
P. K. is thankful to Council of Scientific and Industrial Research
(CSIR), India for award of a Senior Research Fellowship [Sanction
No. 09/100(0212)/2018-EMR-I]. SCS thanks to DST-FIST for single
crystal X-ray facility at PU. We gratefully acknowledge to the
Department of Chemistry, University of Jammu, Jammu, for
providing NMR, IR, UV-visible and TGA facilities.
´
´
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