Synthesis, structures, and spectroscopic properties of Hg(II) complexes of bidentate NN and tridentate NNO Schiff-base ligands

Article abstract of DOI:10.1080/00958972.2014.901508

Reactions of HgX₂ (X = Cl, N₃, NO₃) with (E)-2-methoxy-N-(pyridin-2-ylmethylene)aniline (L¹) and (E)-4-methoxy-N-(pyridin-2-ylmethylene)aniline (L²) in ethanol gave two monomers, [HgL¹(Cl)<

Full text of DOI:10.1080/00958972.2014.901508

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Synthesis, structures, and
spectroscopic properties of Hg(II)
complexes of bidentate NN and
tridentate NNO Schiff-base ligands
Tushar S. Basu Baula, Sajal Kundua, Herbert Höpflb, Edward R.T.
Tiekinkc & Anthony Lindend
a Centre for Advanced Studies in Chemistry, North-Eastern Hill
University, NEHU Permanent Campus, Shillong, India
b Centro de Investigaciones Químicas, Universidad Autónoma del
Estado de Morelos, Cuernavaca, Mexico
c Department of Chemistry, University of Malaya, Kuala Lumpur,
Click for updates
Malaysia
d Department of Chemistry, University of Zurich, Zurich,
Switzerland
Accepted author version posted online: 05 Mar 2014.Published
online: 08 Apr 2014.
To cite this article: Tushar S. Basu Baul, Sajal Kundu, Herbert Höpfl, Edward R.T. Tiekink & Anthony
Linden (2014) Synthesis, structures, and spectroscopic properties of Hg(II) complexes of bidentate
NN and tridentate NNO Schiff-base ligands, Journal of Coordination Chemistry, 67:6, 1061-1078,
DOI: 10.1080/00958972.2014.901508
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 6, 1061–1078, http://dx.doi.org/10.1080/00958972.2014.901508
Synthesis, structures, and spectroscopic properties of Hg(II)
complexes of bidentate NN and tridentate NNO Schiff-base
ligands
TUSHAR S. BASU BAUL*†, SAJAL KUNDU†, HERBERT HÖPFL‡,
EDWARD R.T. TIEKINK*§ and ANTHONY LINDEN¶
†Centre for Advanced Studies in Chemistry, North-Eastern Hill University, NEHU Permanent
Campus, Shillong, India
‡Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernavaca,
Mexico
§Department of Chemistry, University of Malaya, Kuala Lumpur, Malaysia
¶Department of Chemistry, University of Zurich, Zurich, Switzerland
(Received 3 January 2014; accepted 18 February 2014)
Depending on the formation of Hg–O bonds, monomeric (when present) or polymeric (absent)
structures are formed in new Hg(Schiff-base)X2(solvent) compounds.
Reactions of HgX2 (X = Cl, N3, NO3) with (E)-2-methoxy-N-(pyridin-2-ylmethylene)aniline (L1)
and (E)-4-methoxy-N-(pyridin-2-ylmethylene)aniline (L2) in ethanol gave two monomers,
[HgL1(Cl)2] (1) and [HgL2(NO3)2(DMSO)] (5), and three coordination polymers, {[HgL1(N3)2]2·Hg
(N3)2}n (2), [HgL2(Cl)2]n (3), and [HgL2(NO3)2]n·nCH3CN (4). Compounds 1–5 were characterized
by elemental analysis, IR, NMR spectroscopy, and single-crystal X-ray diffraction. The common
feature of monomeric 1 and 5 is the presence of intra- and intermolecular Hg–O bonds. In the
absence of these, polymeric structures arise as a result of azide, chloride, and nitrate bridging in 2,
3, and 4, respectively. Fluorescent properties of 1–5 were also investigated.
*Corresponding authors. Email: basubaul@nehu.ac.in, basubaulchem@gmail.com (T.S. Basu Baul);
Edward.Tiekink@gmail.com (E.R.T. Tiekink)
© 2014 Taylor & Francis

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T.S. Basu Baul et al.
Keywords: Synthesis; Mercury(II) compounds; N,N-donor ligands; Photophysical properties; Crystal
structures
1. Introduction
Schiff bases and their metal complexes have distinctive coordination and structural proper-
ties [1–5]. Schiff-base compounds of transition metals are important components for genera-
tion of supramolecular aggregates through formation of coordination bonds, hydrogen
bonding, and other intermolecular interactions, thereby creating a variety of supramolecular
architectures including high-dimensional frameworks [6–9]. X-ray crystallographic studies
performed on a variety of mercury(II) compounds derived from Schiff-base ligands, viz.
HgLX2, [HgLX2]2, [HgLX2]n, and {[HgLX2]2}n with L = variously substituted (E)-N-(pyri-
din-2-ylmethylene)arylamine and X = halides or pseudohalides or nitrates, revealed that
mercury(II) compounds are promising candidates for diverse supramolecular assemblies
[10, 11]. Although the global topology (monomer, dimer, polymer, etc.) of the resulting
compound is determined by the interplay between Hg2+, X−, and L, in the solid state, the
crystal structure is frequently controlled by weaker intermolecular interactions such as π–π
and CH–π contacts.
Herein, we report syntheses and solid-state structures of five new mercury(II) compounds
derived from (E)-2-methoxy-N-(pyridin-2-ylmethylene)aniline (L1) and (E)-4-methoxy-N-
(pyridin-2-ylmethylene)aniline (L2), which have been prepared from HgX2 (X = Cl, N3, and
NO3) (scheme 1) and then characterized by spectroscopic methods and single-crystal X-ray
diffraction. The structural characterization shows formation of mononuclear or polymeric
compounds. The coordination numbers range from 4 to 8, but are not systematically corre-
lated to L1/L2 or X or solvent.
2. Experimental
2.1. Materials, methods, and instruments
Caution! Mercury compounds are highly toxic [12]. Care must be taken when handling
samples, and appropriate disposal procedures are necessary. All chemicals were used as
purchased: HgCl2, NaN3, pyridine-2-carboxaldehyde (Merck), Hg(NO3)2 (Sarabhai Chemi-
cals), Hg(OAc)2, o-anisidine, p-anisidine (Sd fine). (E)-2-methoxy-N-(pyridin-2-ylmethyl-
ene)aniline (L1) and (E)-4-methoxy-N-(pyridin-2-ylmethylene)aniline (L2) were generated
in situ from pyridine-2-carboxaldehyde and the corresponding anisidine. Solvents were
purified by standard procedures and freshly distilled prior to use. Melting points were
recorded in capillary tubes on a Scanca apparatus and are uncorrected. Elemental analyses
were performed using a Perkin Elmer 2400 series II instrument. IR spectra from 4000 to
400 cm−1 were obtained on a Perkin Elmer Spectrum BX series FT-IR spectrophotometer
1
with samples as KBr disks. H NMR spectra were recorded on a Bruker Avance II spec-
trometer and measured at 400.13 MHz, with chemical shifts referenced to Me4Si set at δ
0.00 ppm; the numbering scheme is as shown in scheme 1. Steady-state absorption spectra
were recorded at ambient temperature in acetonitrile (spectroscopy grade, Merck) solution
on a Perkin-Elmer model Lambda25 absorption spectrophotometer. Fluorescence spectra
were obtained on a Hitachi model FL4500 spectrofluorimeter (with excitation and emission

Hg(II) Schiff-base complexes
1063
slits fixed at 10 and 20 nm, respectively). All spectra were corrected for instrument response
function. Quartz cuvettes of 10 mm optical path length from Perkin Elmer, USA (part No.
B0831009), and Hellma, Germany (type 111-QS), were used for measuring absorption and
fluorescence spectra, respectively. Fluorescence quantum yields (ϕf) were calculated by
comparing the total fluorescence intensity over the whole fluorescence spectroscopic range
with that of a standard using the method described elsewhere [10]. The relative experimen-
tal error of the measured quantum yield was estimated within 10%. Solution electrical con-
ductivity measurements were made in acetonitrile with a Wayne Kerr automatic precision
bridge 6440B.
2.2. Synthesis of mercury compounds
2.2.1. Synthesis of [HgL1(Cl)2] (1). To a solution of pyridine-2-carboxaldehyde (0.13 g,
1.21 mM) in ethanol (5 mL) was added a solution of o-anisidine (0.15 g, 1.21 mM) in etha-
nol (5 mL). After stirring the reaction mixture at ambient temperature for 30 min, a solution
of HgCl2 (0.33 g, 1.21 mM) in methanol (20 mL) was added dropwise, which resulted in
formation of a yellow precipitate. Stirring was continued for 3 h and then the mixture was
filtered. The solid residue was washed with methanol (3 × 5 mL), dried in vacuo and then
dissolved in 40 mL of hot acetonitrile. The solution was filtered while hot and upon cooling
to room temperature, a yellow crystalline material was obtained. Yield 0.35 g (40%). M.p.
172–173 °C. Anal. Calcd for C13H12Cl2HgN2O (MW 483.74): C, 32.28; H, 2.50; N, 5.79.
Found: C, 32.60; H, 2.62; N, 5.80%. Λm (CH3CN): 6 Ω−1cm2 M−1. IR (cm−1): 1623w,
1588s: ν(C=Nimine + C=Npy), 1563w, 1492s, 1475w, 1462m, 1438w: ν(C = Npy), 1368m,
1286w, 1269m, 1250 versus, 1178w, 1156w, 1156m, 1117 versus, 1047w, 1021m, 1011w,
1
972w, 958w, 942w, 903w, 774 versus, 759w, 744w, 638w, 617w, 537w, 508w. HNMR
(DMSO-d6): 9.23 [s, 1H, H-7], 8.79 [d, 1H, H-3′], 8.19 [m, 2H, H-5′,6′], 7.84 [t, 1H, 4′],
7.60 [d, 1H, H-4], 7.43 [t, 1H, H-6], 7.10 [m, 2H, H-3,5], 4.0 [s, 3H, OCH3] ppm.
2.2.2. Synthesis of {[HgL1(N3)2]2·Hg(N3)2}n (2). Note: While no incident occurred while
using azide during preparation and isolation, care in handling azides must be exercised
owing to their potentially explosive nature. To a solution of pyridine-2-carboxaldehyde
(0.25 g, 2.33 mM) in ethanol (5 mL) was added a solution of o-anisidine (0.28 g, 2.32 mM)
in ethanol (5 mL). The mixture was stirred at ambient temperature for 30 min and then
added dropwise to a methanolic solution containing HgN3, which was prepared in situ from
Hg(OAc)2 (1.11 g, 3.48 mM) in methanol (50 mL) and NaN3 (0.68 g, 10.46 mM) in 40 mL
of the same solvent. Immediate formation of a yellow precipitate occurred. Stirring was
continued for 4 h and then the mixture was filtered. The residue was washed thoroughly
with water, then with methanol (3 × 5 mL) and dried in vacuo. The dried solid was dis-
solved in 60 mL of hot acetonitrile and filtered while hot. Upon cooling to room tempera-
ture, a yellow crystalline material was obtained. Yield 0.39 g (56%). M.p. 145–146 °C.
Anal. Calcd for C26H24Hg3N22O2 (MW 1278.39): C, 24.43; H, 1.89; N, 24.10. Found:
C, 24.66; H, 1.80; N, 23.92%. Λm (CH3CN): 5 Ω−1cm2 M−1. IR (cm−1): 2037 versus:
νas(N3), 1623s: ν(C=Nimine), 1589m, 1496m, 1471w, 1458w: ν(C=Npy), 1434w, 1371m,
1287w, 1272w, 1253s, 1193w, 1149w, 1119 versus, 1053w, 1021m, 1011w, 972w, 952w,
1
932w, 901w, 773m, 754w, 637w, 535w, 505w. HNMR (DMSO-d6): 8.94 [s, 1H, H-7],

1064
T.S. Basu Baul et al.
8.73 [d, 1H, H-3′], 7.98 [m, 2H, H-5′,6′], 7.59 [t, 1H, 4′], 7.33 [d, 1H, H-4], 7.28 [t, 1H,
H-6], 6.98 [m, 2H, H-3,5], 3.92 [s, 3H, OCH3] ppm.
2.2.3. Synthesis of [HgL2(Cl)2]n (3). A similar synthetic procedure as for 1 was used
except that o-anisidine was replaced by p-anisidine, giving yellow crystals after recrystalli-
zation from acetonitrile. Yield 0.30 g (49%). M.p. 192–193 °C. Anal. Calcd for
C13H12Cl2HgN2O (MW 483.74): C, 32.28; H, 2.50; N, 5.79. Found: C, 32.35; H, 2.42; N,
6.07%. Λm (CH3CN): 7 Ω−1cm2 M−1. IR (cm−1): 1621w: ν(C=Nimine), 1596s, 1505 versus,
1473w, 1434w: ν(C=Npy), 1307m, 1251 versus, 1171m, 1029 versus, 835s, 776s, 537m.
1HNMR (DMSO-d6): 9.05 [s, 1H, H-7], 8.93 [d, 1H, H-3′], 8.13 [d, 2H, H-5′,6′], 7.75
[t, 1H, 4′], 7.65 [d, 2H, H-2,6], 7.00 [d, 2H, H-3,5], 3.85 [s, 3H, OCH3] ppm.
2.2.4. Synthesis of [HgL2(NO3)2]n·nCH3CN (4). Hg(NO3)2 (0.60 g, 1.46 mM) was dis-
solved under heating in five drops of concentrated nitric acid and the resulting solution was
diluted with water (10 mL). This mixture was added dropwise to a solution containing pyri-
dine-2-carboxaldehyde (0.155 g, 1.45 mM) and p-anisidine (0.179 g, 1.45 mM) in ethanol
(10 mL), which resulted in the immediate formation of a yellow precipitate. Stirring was
continued for 3 h and then the mixture was filtered. The residue was washed thoroughly
with water until the filtrate was pH neutral, then with methanol (3 × 5 mL) and dried in
vacuo. The dried solid was dissolved in 60 mL of hot acetonitrile and filtered while hot.
The filtrate upon cooling to room temperature afforded 4 in the form of a pale yellow crys-
talline material. Yield 0.50 g (56%). M.p. 150–151 °C. Anal. Calcd for C15H15HgN5O7
(MW 577.90): C, 31.18; H, 2.62; N, 12.12. Found: C, 31.20; H, 2.42; N, 11.90%. Λm
(CH3CN):
9
Ω−1cm2 M−1. IR (cm−1) KBr: 1623w: ν(C=Nimine), 1594s, 1559m,
1507s, 1496m, 1447w: ν(C=Npy), 1384 versus, 1306w: ν(NO3), 1257 versus, 1172m,
1117w, 1018m, 984m, 831s, 776m, 743w, 612w, 539w, 518w; Nujol: 1593
ν(C=Nimine + C=Npy), 1527 ν1(NO3), 1314 ν5(NO3). 1HNMR (DMSO-d6): 9.35 [d, 1H,
H-3′], 8.93 [s, 1H, H-7], 8.40 [t, 1H, H-5′], 8.23 [d, 1H, 6′], 8.03 [t, 1H, H-4′], 7.52 [d, 1H,
H-2,6], 6.97 [d, 2H, H-3,5], 3.84 [s, 3H, OCH3] ppm.
2.2.5. Synthesis of [HgL2(NO3)2(DMSO)] (5). Crystallization of 4 in DMSO and chloro-
form (1:4, v/v) afforded pale yellow crystals of 5 in 56% yield. M.p. 165–166 °C. Anal.
Calcd for C15H18HgN4O8S: C, 29.28; H, 2.95; N, 9.11. Found: C, 29.50; H, 3.08; N,
9.10%. Λm (CH3CN): 9 Ω−1cm2 M−1. IR (cm−1) KBr: 1621w: ν(C=Nimine), 1595m:
ν(C=Npy), 1579w, 1559w, 1507m, 1384 versus: ν(NO3), 1320w, 1308w, 1254 versus,
1
1170m, 1158w, 1103w, 1021s: ν(S=O), 948m, 831s, 781m, 639w, 537w. H-NMR (DMSO-
d6): 9.34 [d, 1H, H-3′], 8.95 [s, 1H, H-7], 8.39 [t, 1H, H-5′], 8.29 [d, 1H, 6′], 8.00 [t, 1H,
H-4′], 7.55 [d, 1H, H-2,6], 7.02 [d, 2H, H-3,5], 3.84 [s, 3H, OCH3], 3.46 [s, 6H,
(CH3)2SO)] ppm.
2.3. X-ray data collection and structure determination
Crystals suitable for single-crystal X-ray diffraction analysis were obtained from a solvent
mixture of acetone/acetonitrile (1), acetonitrile (2–4), and a solvent mixture of dimethylsulf-
oxide and chloroform (5) by slow evaporation of the solvent at room temperature. The

Hg(II) Schiff-base complexes
1065
crystal for 1 was investigated at room temperature on a Bruker APEX diffractometer
equipped with a CCD area detector (Mo Kα radiation with λ = 0.71073 Å, graphite mono-
chromator). Data for 2 and 3 were recorded at low temperature on an Agilent Technologies
SuperNova area-detector diffractometer [13] using Mo-Kα radiation (λ = 0.71073 Å) from a
micro-focus X-ray source and an Oxford Instruments Cryojet XL cooler. Measurements for
4 and 5 were made at low temperature on a Nonius KappaCCD diffractometer [14] with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and an Oxford Cryosystems
Cryostream 700 cooler. Data reduction was performed with SAINT [15] for 1, with Cry-
sAlisPro [13] for 2 and 3, and HKL Denzo and Scalepack [16] for 4 and 5. Intensities were
corrected for Lorentz and polarization effects. An empirical absorption correction based on
the multi-scan method [17] was applied for 1, 4, and 5; the data for 3 were treated with an
empirical absorption correction using spherical harmonics [13], and for 2 an analytical
absorption correction [18] was employed. Relevant data collection and refinement parame-
ters are given in table 1 and perspective views of the structures are shown in figures 2, 4,
5(a), 6(a), and 7(a). The structures of 1, 4, and 5 were solved by heavy-atom Patterson
methods [19] which revealed the position of the mercury atom. The structures of 2 and 3
were solved by direct methods using SHELXS97 [20], giving the positions of all
non-hydrogen atoms. In 1, the asymmetric unit consists of two independent mononuclear
molecules. The asymmetric unit in 4 contains one chemical repeat unit of the Hg-complex
polymer plus one molecule of MeCN. Refinement of each structure was carried out on F2
2 2
using full-matrix least-squares procedures, which minimized Σw(Fo2 − Fc ) . The
non-hydrogen atoms in each structure were refined anisotropically. All hydrogen atoms
were placed in geometrically calculated positions and refined using a riding model, where
each hydrogen atom was assigned a fixed isotropic displacement parameter with a value of
1.2Ueq of its parent atom (1.5Ueq for any methyl group). A correction for secondary extinc-
tion was applied for 4 and 5. Three reflections for 1 and one reflection for each of 4 and 5
were omitted from the final refinement, because the intensities were considered to be
extreme outliers. The structure of 5 was refined as an inversion twin in which the major
twin fraction was 0.859(1). With the exception of the positive residual electron density
peaks, table 1, in 3 (1.18 Å from Cl2) and 4 (1.24 Å from C2), the maximum and minimum
residual electron density peaks in the final difference maps were located in the vicinity of a
mercury(II) atom. Refinement and data output of 1 were carried out with the SHELXTL-
NT program package [20], while the SHELXL97 program [21] was used for the calcula-
tions of 2–5. All crystallographic figures were drawn using SHELXTL-NT [20] or
DIAMOND [22].
3. Results and discussion
3.1. Syntheses
Reactions for formation of the mercury Schiff base compounds 1, 3, and 4 were carried out
in methanol or ethanol using 1 : 1 stoichiometric ratios. In either solvent, one equivalent of
HgX2 (X = Cl, NO3) reacts rapidly with one equivalent of L (generated in situ from pyridine-
2-carboxaldehyde and o/p-anisidine) to give a yellow precipitate, from which mercury(II)
compounds, [HgL1(Cl)2] (1), [HgL2(Cl)2]n (3), and [HgL2(NO3)2]n·nCH3CN (4), were iso-
lated. The combination of 4 with DMSO resulted in the formation of [HgL2(NO3)2(DMSO)]
(5). Earlier, it was demonstrated that reaction of one equivalent of Hg(N3)2 with one

1066
T.S. Basu Baul et al.

Hg(II) Schiff-base complexes
1067
equivalent of L, where L = (E)-4-methyl-N-(pyridin-2-ylmethylene)aniline, gives a yellow
product {[HgL(N3)2]2}n [10]. Now, a compound of composition {[HgL1(N3)2]2·Hg(N3)2}n
(2) was isolated from a reaction of L1 with an excess of Hg(N3)2. Compound 2 is trinuclear
and can be considered as an adduct formed between a dimeric [HgL1(N3)2]2 complex and Hg
(N3)2. Compounds 1–5 are air-stable and behave as non-electrolytes in acetonitrile solution
(see Section 2.2). The structures and geometries of 1–5 are detailed in scheme 1. The results
of the crystal structure determinations of 1–5 (see Section 3.3) are consistent with the
chemical and spectroscopic analyses.
3.2. IR, NMR, UV–vis and fluorescence spectroscopy
The infrared spectroscopic assignments of selected bands for 1–5 are given in Section 2.2.
All compounds display a moderately intense band at 1590–1620 cm−1, which is characteris-
tic for ν(C=Nimine) of metal-coordinated Schiff base ligands [10, 11, 23]. In addition, well-
resolved sharp bands of variable intensity at 1600–1580, 1490–1475, and 1450–1435 cm−1
were assigned to metal-coordinated pyridine rings [10, 11, 23–25]. The azide and nitrate
compounds 2, 4, and 5 have additional features. Compound 2 has a very strong band at
2037 cm−1 corresponding to νas(N3) [26, 27]. A similar observation was recently noted for
{[HgL(N3)2]2}n, where L = (E)-4-methyl-N-(pyridin-2-ylmethylene)aniline [10]. Complex 4
displays a strong band at 1384 cm−1, indicative of the simultaneous presence of non-coordi-
nated and coordinated nitrates [24, 28]. The KBr spectrum of 4 also displayed bands at
1507 and 1306 cm−1, indicative of bidentate chelating nitrates. The bidentate chelating
nitrates [24] were further confirmed from the spectrum of 4 run in Nujol mull, with bands
at 1527 cm−1 for ν1(NO3) and at 1314 cm−1 for ν5(NO3), corresponding to a difference of
Δ(NO3) = 213 cm−1 [29]. A similar observation was made recently for an analogous mer-
cury compound, [HgL(NO3)2]n, which has also been characterized crystallographically [10].
In addition to bands for nitrate groups, 5 displayed a strong band at 1021 cm−1, attributed
to ν(S=O) of DMSO coordinated to mercury(II). This value is 35 cm−1 lower than that
reported for free DMSO (1055 cm−1) and is thus indicative of a terminal (end-on) O-mon-
ocoordination. For S-bonded DMSO, an increase in frequency would be expected [30, 31].
Thus, IR spectroscopic data provided substantial evidence in support of the structures which
were subsequently confirmed by single crystal X-ray crystallography (see Section 3.3).
1H NMR spectra for 1–5 in DMSO-d6 solution displayed the expected signals, which cor-
relate well with the hydrogen atoms present in the molecules. Coupling constants could not
be established with certainty, owing to the broad unresolved nature of the signals. The NMR
assignments for the compounds presented in the Experimental section are based on splitting
patterns of the signals and by comparing the data with those in previous reports [32, 33].
Table 2. Photophysical data for 1–5 in acetonitrile.
Photoluminescence data
Electronic spectroscopic data
λ
Compounds
max (nm); (ε[M−1 cm−1])
λ
em (nm)a
ϕF
1
2
3
4
5
359 (12,096)
346 (22,109)
378 (17,937)
382 (11,390)
382 (9305)
413, 427, 502
413, 429
412, 430
411, 431
412, 429
0.11
0.08
0.10
0.12
0.09
aThe long wavelength emission appears as a shoulder in all cases.

1068
T.S. Basu Baul et al.
0.6
0.4
0.2
0.0
(a)
(b)
1
2
3
4
5
1
2
3
4
5
320
360
400
440
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 1. (a) UV–vis (concentration 10−3 M) and (b) fluorescence spectra (concentration ~10−5 M) obtained by
excitation at the respective absorption maxima (table 2) of 1–5 in acetonitrile.
Table 2 summarizes the UV–vis and fluorescent properties of 1–5. The electronic spectra
exhibit a strong absorption at 350–380 nm [figure 1(a)], which is possibly a result of over-
lap of intramolecular charge transfer transitions (ε ~104) with a weak MLCT transition from
Hg(II) → π* (ligand) [10, 11]. In acetonitrile, compounds display broad emission bands at
λmax = 410 nm along with a shoulder at ~430 nm, except for an additional band at 502 nm
for 1, when they are excited at their respective absorption maxima [figure 1(b)], indicating
the charge transfer nature of the transitions [10, 11]. Compounds 1–5 show very low fluo-
rescent quantum yields (ϕF), which can be attributed to the heavy-atom effect [34, 35] and
the ϕF values were of the same order of magnitude as observed recently for related systems
[10, 11]. No appreciable change in ϕF was noticed as a result of variation of the substituents
in the ligands from 2-OCH3 (1 and 2) to 4-OCH3 (3–5) or variation of the anion (chloride,
azide, and nitrate).
3.3. Single-crystal X-ray diffraction studies
The crystal structures of 1–5 were established by X-ray crystallography which revealed the
presence of monomeric 1, 3, and 5 and polymeric 2 and 4 species in the solid state. Two inde-
pendent monomers comprise the asymmetric unit of 1; these monomers are similar to a first
approximation [figure 2(a) and (b)]; geometric parameters for the first independent molecule of
1 are collated in table 3. The mercury(II) atom is five coordinate, being bound by a tridentate
L1 ligand which chelates the metal atom via the nitrogen atoms while simultaneously forming
a Hg–O bond; the coordination environment is completed by two chloride ligands. As
observed in related structures [10, 11], the Hg–N(pyridyl) bond length is shorter than the
Hg–N(imine) bond. The chelate ring is planar in both molecules, exhibiting r.m.s. deviations of
0.016 and 0.029 Å, respectively. However, this planarity does not extend over the entire L1 as
seen in the dihedral angles formed between the outer six-membered rings of 32.2(5)° and 29.5
(4)°, respectively, there being a twist about the imine-N–C(methoxyphenyl) bond; the C6–N2–
C7–C8 and C26–N22–C27–C28 torsion angles are 29.3(12)° and 23.0(11)°, respectively. This
difference between the independent molecules is highlighted in the overlay diagram [36]
[figure 3(a)]. Similar ligand conformations are found in 2–5. Based on τ = 0.23, the trigonality

Hg(II) Schiff-base complexes
1069
Figure 2. Perspective views of the two crystallographically independent monomers (a) and (b) found in the crystal
structure of [HgL1(Cl)2] (1). Displacement ellipsoids are drawn at the 30% probability level and hydrogen atoms
are shown as small spheres of arbitrary radii.
Table 3. Selected bond lengths (Å) and angles (°) for 1 and 3.a
1
3
1
3
Hg(1)–Cl(1)
Hg(1)–Cl(2)
Hg(1)–N(1)
Hg(1)–N(2)
Hg(1)–Cl(2′)
C(1)–C(6)
2.374(2)
2.446(2)
2.324(6)
2.473(6)
–
1.449(12)
1.259(10)
1.406(10)
2.4061(12)
2.5597(11)
2.260(4)
2.557(4)
2.7874(11)
1.454(7)
Cl(2)–Hg(1)–N(1)
Cl(2)–Hg(1)–N(2)
N(1)–Hg(1)–N(2)
Cl(1)–Hg(1)–Cl(2′)
Cl(2)–Hg(1)–Cl(2′)
N(1)–Hg(1)–Cl(2′)
N(2)–Hg(1)–Cl(2′)
Hg(1)–Cl(2)–Hg(1′′)
Hg(1)–N(1)–C(1)
Hg(1)–N(1)–C(5)
Hg(1)–N(2)–C(6)
Hg(1)–N(2)–C(7)
91.02(18)
114.90(15)
69.1(2)
–
–
–
–
–
118.4(6)
122.3(6)
113.6(6)
124.2(5)
98.89(10)
100.07(9)
69.99(13)
95.09(4)
100.77(2)
88.32(10)
151.92(9)
128.57(5)
119.1(3)
121.8(3)
110.7(3)
128.0(3)
N(2)–C(6)
N(2)–C(7)
1.287(6)
1.422(6)
Cl(1)–Hg(1)–Cl(2)
Cl(1)–Hg(1)–N(1)
Cl(1)–Hg(1)–N(2)
125.35(8)
139.01(17)
105.06(17)
106.62(4)
153.09(10)
96.77(9)
aSymmetry operations for primed and doubly primed atoms = 1 − x, ½ + y, 1½ − z, and 1 − x, −½ + y, 1½ − z, respectively.
index [37], compared with 0.0 and 1.0 for ideal square pyramidal and trigonal bipyramidal
geometries, respectively, the coordination geometry about atom Hg1 is based on a square pyra-
mid. The comparable value for the second molecule is 0.02, suggesting a more regular coordi-
nation geometry. The greater distortion for atom Hg1 is traced to a secondary Hg1⋯Cl21i

1070
T.S. Basu Baul et al.
Figure 3. (a) Overlay diagram of the two independent molecules in [HgL1(Cl)2] (1) (red image contains Hg1)
superimposed so that the HgN2C2 rings are overlapped. (b) Illustration of the secondary Hg⋯Cl bonding [black
dashed line, 3.309(2) Å] connecting the two independent molecules as well as the C–H⋯Cl hydrogen bond [green
dashed line; H⋯Cl = 2.79 Å]. (c) View in projection down the a-axis of the unit cell contents. The C–H⋯Cl hydro-
gen bonds [C5–H5⋯Cl21i: H5⋯Cl21i = 2.82 Å, C5⋯Cl21i = 3.502(10) Å, angle at H5 = 131° for i: −1 + x, −1 + y,
z; C25–H25⋯Cl2ii: H25⋯Cl2ii = 2.79 Å, C25⋯Cl2ii = 3.467(9) Å, angle at H25 = 130° for ii: 1 + x, 1 + y, z] and
π‒π interactions [Cg(N21,C21–C25)⋯Cg(N21,C21–C25)iii = 3.484(5) Å for iii: 1 − x, 2 − y, −z; Cg(Hg1,N1,N2,C1,
C6)⋯Cg(N1,C1–C5)iv = 3.744(5) Å, angle of inclination = 7.9(4)° for iv: ½ + x, y, ½−z] are shown as green and
purple dashed lines, respectively (see http://dx.doi.org/10.1080/00958972.2014.901508 for color version).

Hg(II) Schiff-base complexes
1071
interaction of 3.309(2) Å (symmetry operation i: −1 + x, −1 + y, z) between independent mole-
cules as shown in figure 3(b). Often molecules related to 1 dimerize about a center of inversion
via Hg⋯Cl bridges to form binuclear arrangements [10, 11]. In the case of 1, the presence of
Hg–O bonds reduced the Lewis acidity of mercury(II) and only a single and weak Hg⋯Cl
contact between the molecules is formed. Also highlighted in figure 3(b) is the presence of a
pyridyl-C–H⋯Cl hydrogen bond [H⋯Cl = 2.79 Å]. In the crystal structure of 1, there are two
such pyridyl-C–H⋯Cl interactions and these, coupled with the Hg⋯Cl interaction, link the
molecules into an array in the bc-plane. A 3-D architecture arises from π–π interactions along
the a-direction between centrosymmetrically related N21-pyridyl rings, and between N21-pyri-
dyl and Hg1N2C2 chelate rings [figure 3(c)]. The latter π–π interactions where a chelate ring
defines a π-system are known [38–40] and indeed, have been seen in related structures [10,
11].
The structure of {[HgL1(N3)2]2·Hg(N3)2}n (2) is best described as comprising a Hg(N3)2
unit and a centrosymmetric dimer of [HgL1(N3)2]2 with links provided by azido-N atoms
(figure 4). The Hg2 atom in Hg(N3)2 is located on a center of inversion and is bound by
two terminally bound azido-N9 atoms, two μ2-azido-N6 atoms where atom N6 bridges
Figure 4. Perspective view of the polymeric chain found in the crystal structure of {[HgL1(N3)2]2·Hg(N3)2}n (2).
Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted. Singly, doubly,
and triply primed atoms are related by the symmetry operations 1 − x, −y, 1 − z, 1 + x, y, z, and 2 − x, −y, 1 − z,
respectively.
Table 4. Selected bond lengths (Å) and angles (°) for 2.a
Hg(1)–N(1)
Hg(1)–N(2)
Hg(1)–N(3)
Hg(1)–N(6)
Hg(1)–N(3′)
Hg(1)–O(1)
Hg(2)–N(6)
Hg(2)–N(9)
Hg(2)–N(5′)
2.402(3)
2.453(3)
2.184(3)
2.138(3)
2.806(3)
2.926(2)
2.710(3)
2.069(3)
2.756(3)
N(1)–Hg(1)–O(1)
N(2)–Hg(1)–N(3)
N(2)–Hg(1)–N(6)
N(2)–Hg(1)–N(3′)
N(2)–Hg(1)–O(1)
N(3)–Hg(1)–N(6)
N(3)–Hg(1)–N(3′)
N(3)–Hg(1)–O(1)
N(6)–Hg(1)–N(3′)
N(6)–Hg(1)–O(1)
N(3′)–Hg(1)–O(1)
N(6)–Hg(2)–N(9)
N(6)–Hg(2)–N(5′)
N(9)–Hg(2)–N(5′)
120.51(8)
92.00(10)
119.65(11)
141.38(8)
58.11(7)
137.83(12)
77.25(10)
94.72(9)
90.67(10)
81.62(10)
158.05(8)
94.56(13)
87.87(10)
90.25(11)
N(1)–Hg(1)–N(2)
N(1)–Hg(1)–N(3)
N(1)–Hg(1)–N(6)
N(1)–Hg(1)–N(3′)
68.71(9)
112.62(9)
105.01(11)
81.31(9)
aSymmetry operation for primed atoms = 1 − x, −y, 1 − z.

1072
T.S. Basu Baul et al.
Figure 5. (a) View down the a-axis of the unit cell contents of [HgL2(Cl)2]n (3) showing the weakly associated
extended chain. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted.
The primed and doubly primed atoms are related by the symmetry operations 1 − x, ½ + y, 1½ − z, and 1 − x, −½ +
y, 1½ − z, respectively. (b) A view of the supramolecular layer in the ab-plane sustained by C–H⋯Cl hydrogen
bonds [C8–H8⋯Cl2i: H8⋯Cl2i = 2.73 Å, C8⋯Cl2i = 3.543(5) Å, angle at H5 = 144° for i: 2 − x, ½ + y, 1½ − z] and
π‒π interactions [Cg(N1,C1–C5)⋯Cg(C7–C12)ii,iii = 3.631(3) Å and 3.738(3) Å, angle of inclination = 2.2(2)° in
each case, for ii: 2 − x, −½ + y, −1½ − z; and iii: 2 − x, ½ + y, 1½ − z] are shown as green and purple dashed lines,
respectively (see http://dx.doi.org/10.1080/00958972.2014.901508 for color version).

Hg(II) Schiff-base complexes
1073
Figure 6. (a) Perspective view of a portion of the polymeric chain illustrated to show the coordination geometry
about the mercury(II) atom found in the crystal structure of [HgL2(NO3)2]n·nCH3CN (4); the acetonitrile molecule
is omitted. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are shown as small
spheres of arbitrary radii. Primed atoms are related by the symmetry operation −x, −½ + y, ½ − z. (b) The helical
coordination polymer mediated by tetradentate nitrate anions. (c) A view in projection down the b-axis of the unit
cell contents. The supramolecular layers are sustained by C–H⋯O interactions [C2–H2⋯O7i: H2⋯O7i = 2.50 Å,
C2⋯O7i = 3.413(7) Å, angle at H2 = 162°; C6–H6⋯O6i: H6⋯O6i = 2.59 Å, C6⋯O6i = 3.515(6) Å, angle at H6 =
165° for i: ½ + x, y, ½ − z] shown as orange dashed lines. The acetonitrile molecules, shown in space-filling mode,
are connected to the layers by methyl-C–H⋯O(nitrate) interactions [C14–H142⋯O2ii: H142⋯O2ii = 2.56 Å,
C14⋯O2ii = 3.381(9) Å, angle at H142 = 141° for ii: ½ + x, ½ − y, −z] (see http://dx.doi.org/10.1080/
00958972.2014.901508 for color version).

1074
T.S. Basu Baul et al.
across to Hg1, and two azido-N5 atoms where the azido-N3 atom at the other end provides
the bridge between Hg1 of the centrosymmetric dimer, hence this ligand is μ3-bridging. In
the dimer, Hg1 and Hg1′ are connected by a pair of end-on bridging azides, via N3 and N3′
(the N5 atoms of these azides form bridges to the Hg2 atoms in the Hg(N3)2 units), the sec-
ond azido-N6 is bound end-on and bridges to an adjacent Hg2, and tridentate L1 coordi-
nates as found in 1. Hg1 and Hg2 exist in octahedral coordination geometries, at least to a
first approximation. For Hg1, two of the bond lengths are considerably longer than the oth-
ers, e.g. the central core is trapezoidal as the Hg1–N3,N3′ bond lengths differ by about 0.6
Å, and the Hg–O1 distance is also long (table 4). Further distortions arise as a result of the
acute chelate angles. While the angles around Hg2 are close to 90° two of the Hg–N9 bond
lengths are considerably shorter, by over 0.7 Å, than the remaining four bonds. The result
of the azide-mediated bridging is the formation of a coordination polymer aligned along the
a-axis. Chains stack with no specific intermolecular interactions between them.
The next structure to be described is that of [HgL2(Cl)2]n (3), where the methoxy substi-
tuent is no longer proximate to the mercury(II) atom so that a Hg–O1 bond is not formed.
Instead, the mercury(II) atom increases its coordination number leading to a square pyrami-
dal coordination geometry (τ = 0.02, Cl2 in the axial position) by forming a significant inter-
molecular Hg1⋯Cl2 interaction (2.7874(11) Å for symmetry operation 1 − x, ½ + y, 1½ − z)
resulting in a helical supramolecular chain along the b-axis [figure 5(a)]. The chains are
connected by C8–H⋯Cl2 hydrogen bonds as well as π–π interactions between pyridyl and
phenyl rings to form a supramolecular array in the ab-plane [figure 5(b)]. Layers stack with
no specific intermolecular interactions between them.
The mercury(II) atom in polymeric [HgL2(NO3)2]n·nCH3CN (4) [figure 6(a)] is eight
coordinate being chelated by L2, and coordinated by six nitrate oxygen atoms. The N3-
nitrate chelates in an asymmetric mode with the Hg–O3 bond length being 0.6 Å shorter
than Hg–O2 (table 5). The N4-nitrate anion bridges two mercury(II) ions, simultaneously
chelating both with O5 forming two Hg–O bonds. Reflecting the higher denticity of the
Table 5. Selected bond lengths (Å) and angles (°) for 4 and 5.a
4
5
4
5
Hg(1)–O(2)
Hg(1)–O(3)
Hg(1)–O(5)
Hg(1)–O(7)
Hg(1)–N(1)
Hg(1)–N(2)
C(1)–C(6)
N(2)–C(6)
N(2)–C(7)
Hg(1)–O(x)
Hg(1)–O(6′)
2.757(5)
2.157(4)
2.731(4)
2.587(4)
2.200(5)
2.398(4)
1.487(8)
1.272(7)
1.416(7)
2.924(4)
2.725(4)
2.623(3)
2.599(3)
2.393(3)
2.712(3)
2.259(3)
2.431(3)
1.473(5)
1.277(5)
1.417(4)
2.229(3)
–
O(3)–Hg(1)–O(5)
O(3)–Hg(1)–O(7)
O(3)–Hg(1)–N(1)
O(3)–Hg(1)–N(2)
O(5)–Hg(1)–O(7)
O(5)–Hg(1)–N(1)
O(5)–Hg(1)–N(2)
O(7)–Hg(1)–N(1)
O(7)–Hg(1)–N(2)
N(1)–Hg(1)–N(2)
O(2)–Hg(1)–O(x)
O(3)–Hg(1)–O(x)
O(5)–Hg(1)–O(x)
O(7)–Hg(1)–O(x)
N(1)–Hg(1)–O(x)
N(2)–Hg(1)–O(x)
O(2)–Hg(1)–O(6′)
O(5)–Hg(1)–O(6′)
N(1)–Hg(1)–O(6′)
O(x)–Hg(1)–O(6′)
80.86(14)
85.10(15)
167.09(17)
118.92(17)
47.92(12)
103.97(15)
90.15(15)
89.37(16)
129.66(14)
73.41(16)
100.03(12)
84.22(14)
115.21(11)
68.28(12)
82.89(14)
149.00(12)
69.77(13)
159.46(11)
81.22(14)
45.00(11)
74.57(11)
119.40(10)
138.07(10)
95.54(10)
49.40(11)
102.67(11)
160.11(11)
82.30(11)
144.86(10)
72.93(10)
129.49(10)
80.66(10)
95.82(11)
83.68(11)
140.24(11)
99.58(10)
–
O(2)–Hg(1)–O(3)
O(2)–Hg(1)–O(5)
O(2)–Hg(1)–O(7)
O(2)–Hg(1)–N(1)
O(2)–Hg(1)–N(2)
O(3)–Hg(1)–O(6′)
O(7)–Hg(1)–O(6′)
N(2)–Hg(1)–O(6′)
50.81(15)
116.33(13)
135.80(14)
132.82(16)
82.62(15)
90.24(15)
113.22(13)
110.32(13)
49.04(9)
77.18(11)
121.49(10)
89.17(10)
83.31(10)
–
–
–
–
–
–
ax = 5′ for 4 and x = 8 for 5. Symmetry operation for primed atoms = −x, −½ + y, ½ − z.

Hg(II) Schiff-base complexes
1075
Figure 7. (a) Perspective view of the monomer found in the crystal structure of [HgL2(NO3)2(DMSO)] (5). Dis-
placement ellipsoids are drawn at the 50% probability level and hydrogen atoms are shown as small spheres of
arbitrary radii. (b) A view in projection down the a-axis of the unit cell contents. The π‒π interactions [Cg(N1,C1–
C5)⋯Cg(C7–C12)i = 3.763(2) Å, angle of inclination = 2.05(19)° for i: 1 − x, 1 − y, 1 − z] are shown as purple
dashed lines. The C–H⋯O interactions [C3–H3⋯O1ii: H3⋯O1ii = 2.60 Å, C3⋯O1ii = 3.508(5) Å, angle at H3 =
160°; C2–H2⋯O4iii: H2⋯O4iii = 2.52 Å, C2⋯O4iii = 3.463(5) Å, angle at H2 = 173°; C6–H6⋯O2iii: H6⋯O2iii
=
2.42 Å, C6⋯O2iii = 3.252(5) Å, angle at H6 = 146°; C14–H142⋯O4iv: H142⋯O4iv = 2.51 Å, C14⋯O4iv = 3.477(6)
Å, angle at H142 = 169°; C15–H153⋯O4v: H153⋯O4v = 2.50 Å, C15⋯O4v = 3.400(6) Å, angle at H153 = 152°.
Symmetry operations ii: 1 + x, 1 + y, z; iii: 1 − x, 1 − y, 1 − z; iv: −x, −y, −z; v: −1 + x, y, z] are shown as orange
dashed lines (see http://dx.doi.org/10.1080/00958972.2014.901508 for color version).
N4-nitrate anion, the Hg–O bond lengths are long and span 2.587(4)–2.924(4) Å. The
disparity in the bond lengths about the mercury(II) atom, along with the acute chelate
angles, leads to a highly distorted coordination geometry. One description is based on a
square anti-prism with one face (distinctly rectangular, defined by the O2, O3, O5′, and O6
atoms) being twisted about 7° with respect to the other (distinctly trapezoidal, defined by
O5, O7, N1, and N2). The result of the tetradentate and bridging N4-nitrate ligand is the

1076
T.S. Basu Baul et al.
Scheme 1. The ligands L1–L2 and the investigated compounds 1–5.
formation of a helical coordination polymer along the b-axis as illustrated in figure 6(b).
When viewed down the direction of the polymer, the global crystal packing can be
described as undulating layers of polymers stacked along the c-axis [figure 6(c)] and aceto-
nitrile solvent molecules lie in the interlayer region. Connections within layers are of the
type methyl-C–H⋯O(nitrate), as are those between the solvent and layer.
Recrystallization of 4 from DMSO yielded 5, formulated as [HgL2(NO3)2(DMSO)], which
is monomeric and features O-bound DMSO [figure 7(a)]. Both nitrates are chelating with
Hg–O bond lengths ranging from 2.393(3) to 2.712(3) Å, i.e. within the range of comparable
bonds in 4 (table 5). The L2 ligand is also chelating and the Hg–N1, N2 bond lengths are
longer than those in 4, reflecting more tightly bound nitrates and also the strong Hg–O8 bond
of 2.229(3) Å involving the DMSO oxygen atom. With three acute angles provided by the
chelating ligands, not surprisingly the coordination polyhedron is highly distorted. One
description is based on a capped octahedron. The axial angle of 160.11(11) is subtended by
atoms O5 and N2 atom, the approximate basal plane is defined by atoms O2, O3, O8, and
N1, leaving O7 to occupy the triangular face defined by atoms O5, O8, and N1. In the crystal
packing, a pair of π–π interactions occurring between the pyridyl and phenyl rings of centro-
symmetrically related molecules leads to the formation of loosely associated dimeric units.
The dimeric aggregates stack in columns along the a-axis and are connected into a 3-D archi-
tecture by C–H⋯O interactions [figure 7(b)] involving nitrate-O as acceptors, as for 4, the
methoxy-O as an acceptor, and L2- and DMSO-methyl-H as donors.
4. Conclusion
Five neutral [HgL1(Cl)2] (1), {[HgL1(N3)2]2·Hg(N3)2}n (2), [HgL2(Cl)2]n (3),
[HgL2(NO3)2]n·nCH3CN (4), and [HgL2(NO3)2(DMSO)] (5) compounds of bidentate NN
and tridentate NNO donor Schiff-base ligands have been synthesized. The compounds were

Hg(II) Schiff-base complexes
1077
characterized by spectroscopic and single crystal X-ray crystallographic techniques. The
crystal structures revealed the presence of monomeric (1, 3, and 5) and polymeric (2 and 4)
species in the solid state. The appearance of monomeric 1 and 5 is correlated with the addi-
tional coordination, intra- and intermolecularly, by oxygen which reduces the Lewis acidity
of the mercury(II) centre. In the absence of additional Hg–O bonds, polymeric structures
arise via bridging azide, chloride, and nitrate in 2, 3, and 4, respectively.
Supplementary material
CCDC 970664–970668 contains the supplementary crystallographic data for 1–5. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; Fax: +44 1223 336 033; or E-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
The financial support of the University Grants Commission, New Delhi, India (F. No. 42-
396/2013 (SR) TSBB) and the Indo-Swiss Joint Research Programme, Joint Utilisation of
Advanced Facilities [grant number JUAF 11, TSBB, AL] are gratefully acknowledged. Sup-
port from the Ministry of Higher Education, Malaysia, and the University of Malaya, for
High Impact Research grant [UM.C/HIR-MOHE/SC/03] is also gratefully acknowledged.
The authors gratefully acknowledge access to the X-ray facilities in the Chemical Research
Center at the Autonomous State University of Morelos (CIQ-UAEM).
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