The influence of counter ion and ligand methyl substitution on the solid-state structures and photophysical properties of mercury(ii) complexes with (E)-N-(pyridin-2-ylmethylidene)arylamines

Article abstract of DOI:10.1039/c2dt32283h

Ten neutral monomeric, dimeric and polymeric mercury(ii) complexes of compositions HgX₂L (3, 8), [HgX₂L]₂ (1, 2, 4-6 and 7), [Hg(NO₃)₂L]_n (9) and {[Hg(N ₃)₂L]₂}_n (10) where X = chloride, bromide, iodide, nitrate and azide, and L = (E)-N-(pyridin-2-ylmethylidene) arylamine, are described. Compounds 1-10 were characterized by elemental analyses, and IR and ¹H NMR spectroscopic studies. The solution-state photophysical properties of the complexes are highly dependent on the anions as seen in the fluorescence emission features. Single-crystal X-ray crystallography showed that the molecular complexes can aggregate into larger entities depending upon the anion coordinated to the metal centre. Iodide gives discrete monomeric complexes, chloride and bromide generate binuclear complexes formed through Hg-X-Hg bridges, while nitrate and azide lead to 1D coordination polymers. The significant differences in the observed aggregation patterns of the compounds indicate that the anions exert a substantial influence on the formation of the compounds. A further influence upon supramolecular aggregation is the presence of methyl substituents in L³ and L⁴, which generally enhances the probability of forming supramolecular π?π interactions involving the five-membered C₂N₂Hg chelate rings in their crystal structures. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt32283h

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The influence of counter ion and ligand methyl
substitution on the solid-state structures and
photophysical properties of mercury(II) complexes with
(E)-N-(pyridin-2-ylmethylidene)arylamines†
Cite this: Dalton Trans., 2013, 42, 1905
Tushar S. Basu Baul,*a Sajal Kundu,a Sivaprasad Mitra,a Herbert Höpfl,b
Edward R. T. Tiekink*c and Anthony Lindend
Ten neutral monomeric, dimeric and polymeric mercury(II) complexes of compositions HgX2L (3, 8),
[HgX2L]2 (1, 2, 4–6 and 7), [Hg(NO3)2L]n (9) and {[Hg(N3)2L]2}n (10) where X = chloride, bromide, iodide,
nitrate and azide, and L = (E)-N-(pyridin-2-ylmethylidene)arylamine, are described. Compounds 1–10
were characterized by elemental analyses, and IR and 1H NMR spectroscopic studies. The solution-state
photophysical properties of the complexes are highly dependent on the anions as seen in the fluor-
escence emission features. Single-crystal X-ray crystallography showed that the molecular complexes can
aggregate into larger entities depending upon the anion coordinated to the metal centre. Iodide gives
discrete monomeric complexes, chloride and bromide generate binuclear complexes formed through
Hg–X–Hg bridges, while nitrate and azide lead to 1D coordination polymers. The significant differences
in the observed aggregation patterns of the compounds indicate that the anions exert a substantial
influence on the formation of the compounds. A further influence upon supramolecular aggregation is
the presence of methyl substituents in L3 and L4, which generally enhances the probability of forming
supramolecular π⋯π interactions involving the five-membered C2N2Hg chelate rings in their crystal
structures.
Received 28th September 2012,
Accepted 30th October 2012
DOI: 10.1039/c2dt32283h
www.rsc.org/dalton
such as in the paper industry, paints, cosmetics, preservatives,
thermometers, manometers, energy-efficient fluorescent light
Introduction
The coordination chemistry of mercury(II) has been a focus of bulbs and, to a limited extent, mercury batteries.2 Nonetheless,
attention for many years owing to the significant toxicological reports of mercury compounds are disproportionately scarce
effects exhibited by mercury upon living organisms. Mercury compared with those describing zinc and cadmium, including
has a special affinity for sulphur and nitrogen over oxygen in crystal engineering endeavours. Recent developments in the
when these appear as potential ligator atoms in biochemically crystal engineering of metal–organic coordination polymers
relevant or model compounds.1 Despite their toxicity, have produced many novel materials with various structural
mercury(II) compounds are still being used in various fields, features and properties. Supramolecular structures that
contain mercury(II) seem to have much more in common with
low-valent main group elements than with transition metals,
in part because they tend to form structures with low-coordi-
nate linear or other distorted coordination geometries.3 The
spherical d10 configuration of Hg(II) is associated with a flexi-
ble coordination environment so that the geometries of these
complexes can vary from linear to octahedral or even distorted
hexagonal bipyramidal, and severe distortions from ideal
coordination polyhedra occur easily. Furthermore, due to the
general lability of d10 metal complexes, the formation of
coordination bonds is reversible, which enables metal ions
and ligands to rearrange during the supramolecular assembly
to give highly ordered network structures. Consequently,
mercury(II) can readily accommodate different kinds of
aDepartment of Chemistry, North-Eastern Hill University, NEHU Permanent Campus,
Umshing, Shillong 793 022, India. E-mail: basubaul@nehu.ac.in,
basubaulchem@gmail.com; Fax: +91-3642721000; Tel: +91-3642722626
bCentro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos,
Av. Universidad 1001, 62209 Cuernavaca, Mexico. E-mail: hhopfl@uaem.mx;
Fax: +52 777 3297997; Tel: +52 777 3297997
cDepartment of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia.
E-mail: Edward.Tiekink@gmail.com; Fax: +60 3 7967 4193; Tel: +60 3 7967 6775
dInstitute of Organic Chemistry, University of Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland. E-mail: alinden@oci.uzh.ch; Fax: +41-44 635 6812;
Tel: +41-44 635 4228
†Electronic supplementary information (ESI) available. CCDC 892204–892213.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt32283h
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architectures, and a selection of varying topological types of composition, HgX2L, [HgX2L]2, [Hg(NO3)2L]n and {[Hg-
1D, 2D and 3D polymers is given in ref. 2,4 and 5. (N3)2L]2}n, which have been characterized by means of IR and
Recent attention has also been given to d10 metal com- 1H NMR spectroscopic studies, and for each of 2–10 addition-
plexes containing α-diimine ligands due to their luminescent ally by single-crystal X-ray crystallography; the structure of 1
and electroluminescent properties,6–13 and their lower cost has been reported by others recently.48 The solution-state
compared with the corresponding more commonly used photophysical properties of these compounds are also
rhenium(I),14–17 ruthenium(II)18–20 and osmium(II)21–23 com- reported.
plexes. In this context, the luminescent and electrolumines-
cent properties of alkynylmercury complexes and mercury
bipyridine complexes have been examined.24–28 Weak inter-
Experimental
molecular d10⋯d10 interactions such as Au⋯Au and Hg⋯Hg
contacts can play an important role in materials science and General considerations
crystal engineering,29–36 as their strength is comparable with
Caution! Compounds of mercury are highly toxic.49 Care must be
that of hydrogen bonds and they are often responsible for the
observed optical properties.37–39 The formation of metal⋯
metal and aryl⋯aryl interactions in the solid-state can cause a
red shift in the emission band compared with that in solution
phase.40,41
taken when handling samples, and appropriate disposal pro-
cedures are necessary. All chemicals were used as purchased
without purification: HgCl2, pyridine-2-carboxaldehyde
(Merck), HgBr2, HgI2 (Fine Chemicals), Hg(NO3)2 (Sarabhai
Chemicals), aniline (Sd Fine), o-/m-toluidine (Thomas Bakers)
and p-toluidine (CDH). Solvents were purified by standard pro-
cedures and were freshly distilled prior to use. The (E)-N-
(pyridin-2-ylmethylidene)arylamine derivatives L1–L4 were pre-
pared in situ from pyridine-2-carboxaldehyde and the corres-
ponding aniline. Attempts to prepare crystalline (E)-N-(pyridin-
2-ylmethylidene)arylamine were unsuccessful and in all
instances either an oil or a viscous liquid was isolated. 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 in the
range 4000–400 cm−1 were obtained on a Perkin Elmer Spec-
trum BX series FT-IR spectrophotometer with samples pre-
pared as KBr discs (complex 9 also in Nujol mull). The 1H
NMR spectra were recorded on a Bruker Avance II spectrometer
and measured at 400.13 MHz. The 1H chemical shifts were
referenced to Me4Si set at 0.00 ppm. 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 spectro-
fluorimeter (with the excitation and emission slits fixed at 10
and 20 nm, respectively) and all spectra were corrected for the
instrument response function. Quartz cuvettes of 10 mm
optical path length received 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 compar-
ing the total fluorescence intensity under the whole fluor-
escence spectroscopic range with that of a standard using the
method described elsewhere.50 The relative experimental error
of the measured quantum yield was estimated within 10%.
Solution electrical conductivity measurements were made with
a Wayne Kerr automatic precision bridge 6440B.
The solid-state structures of mercury(II) halide complexes
with N-donor organic ligands (monodentate) have been
thoroughly studied both by spectroscopic methods as well as
by X-ray crystallography, which have demonstrated a broad
variety of binding modes according to the characteristics of
the ligands.42–44 It is well known that mercury(II) is capable of
forming various coordination modes with N,N-donor atoms
when suitable ligands such as bipyridine (bpy), phenanthro-
line (phen) and their derivatives, e.g. 2,9-dimethyl-1,10-phe-
nanthroline (dmph), are employed.45,46 In the case of Hg(bpy)-
I2, the presence of the bpy ligand leads to the formation of
weakly associated dimers through Hg⋯I interactions, i.e. [Hg-
(bpy)I2]2, resulting in five-coordinate metal centres. In con-
trast, with phen and dmph, monomeric structures with dis-
torted tetrahedral coordination geometries of composition Hg-
(phen/dmph)I2 are observed.45 This is as expected because the
presence of bulky phen or dmph ligands inhibits aggregation.
However, for Hg(phen)2Cl2 a six-coordinate complex with a
distorted octahedral geometry was observed.46 A previous
investigation revealed that mercury(II) is capable of forming an
extended polymeric structure with 2-[(E)-2-(3-methylphenyl)-1-
diazenyl]pyridine in which the ligand is monodentate via the
pyridyl nitrogen, while modifying the structure of the ligand
by changing the methyl group on the phenyl ring from the
m- to the p-position resulted in a dimeric structure in which
the 2-[(E)-2-(4-methylphenyl)-1-diazenyl]pyridine ligand is
coordinated in a bidentate mode via the pyridyl nitrogen and
one of the azo nitrogen atoms.47
In order to extend the current knowledge of the structural
chemistry of mercury(II) compounds with N-donor ligands,
attention is now directed towards the systematic synthesis and
structural characterization of HgX2 (X = chloride, bromide,
iodide, nitrate and azide) complexes with (E)-N-(pyridin-2-
ylmethylidene)arylamine derivatives (L1–L4, Chart 1), in order
to analyze comprehensively the competition between anion X
and ligand L for the coordination sites at the mercury(II)
Synthesis of mercury compounds
centre. Herein, we report the self-assembly and resulting struc- HgX2L (3, 8), [HgX2L]2 (1, 2, 4–7), [Hg(NO3)2L]n (9) and {[Hg-
tures of ten mercury(II) complexes (1–10) of varied (N3)2L]2}n (10).
1906 | Dalton Trans., 2013, 42, 1905–1920
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Chart 1 Chemical structures of ligands L1–L4 and the investigated mercury(II) complexes 1–10.
Synthesis of dihalomercury(II) complexes (1–8)
1H, H-4′], 7.50 [m, 4H, H-2,3,5,6], 7.36 [t, 1H, H-4] ppm. The
atom numbering scheme employed is shown in Chart 1.
SYNTHESIS OF [HgBr2L1]2 (2). A similar synthetic procedure to
The methods employed for the preparation of the dihalomer-
cury(II) complexes (1–8) are very similar, so that the preparation that used for 1 was used except that HgCl2 was replaced by
of the dichloride derivative (1) is given in detail as a represen- HgBr2, giving pale-yellow crystals from acetonitrile solution.
tative example.
Yield 46%. M.p. 184–186 °C. Found: C, 26.75; H, 1.80; N,
SYNTHESIS OF [HgCl2L1]2 (1). To a solution of pyridine-2-car- 5.17%. Calc. for C24H20Br4Hg2N4: C, 26.54; H, 1.86; N, 5.16%.
boxaldehyde (0.20 g, 1.86 mmol) in ethanol (5 mL) was added Λm (CH3CN): 4 Ω−1 cm2 mol−1. IR (cm−1): 1646 νasym(C(H)vN);
a solution of aniline (0.17 g, 1.86 mmol) in ethanol (10 mL). 1586, 1480, 1434 ν(CvN)py. 1H-NMR (DMSO-d6): δ 8.88 [s, 1H,
The mixture was stirred at ambient temperature for 30 min. To H-7], 8.73 [d, 1H, H-3′], 8.04 [m, 2H, H-5′,6′], 7.65 [dd, 1H,
this reaction mixture, HgCl2 (0.50 g, 1.84 mmol) in methanol H-4′], 7.40 [m, 5H, H-2,3,4,5,6] ppm.
(20 mL) was added drop-wise under stirring which resulted in
SYNTHESIS OF [HgI2L1]2 (3). A similar synthetic procedure to
the immediate formation of a yellow precipitate. Stirring was that used for 1 was used except that HgCl2 was replaced by
continued for 3 h and then the mixture was filtered. The HgI2, giving pale-yellow crystals from acetonitrile solution.
residue was washed with methanol (3 × 5 mL) and dried in Yield 44%. M.p. 170–172 °C. Found: C, 22.60; H, 1.68; N,
vacuo. The dried solid was dissolved by boiling in acetonitrile 4.47%. Calc. for C12H10HgI2N2: C, 22.62; H, 1.58; N, 4.40%. Λm
(40 mL) and filtered while hot. The filtrate, upon cooling to (CH3CN): 3 Ω−1 cm2 mol−1. IR (cm−1): 1639 νasym(C(H)vN);
room temperature, afforded a yellow crystalline material. Yield 1586, 1487, 1447 ν(CvN)py. 1H-NMR (DMSO-d6): δ 8.87 [s, 1H,
0.35 g (40%). M.p. 184–186 °C. Found: C, 31.70; H, 2.12; N, H-7], 8.66 [d, 1H, H-3′], 8.06 [d, 1H, H-6′], 7.94 [dd, 1H, H-5′],
6.10%. Calc. for C24H20Cl4Hg2N4: C, 31.75; H, 2.22; N, 6.17%. Λm 7.66 [dd, 1H, H-4′], 7.45 [m, 5H, H-2,3,4,5,6] ppm.
(CH3CN): 5 Ω−1 cm2 mol−1. IR (cm−1): 1633 νasym(C(H)vN); 1586,
1487, 1434 ν(CvN)py. H-NMR (DMSO-d6): δ 8.96 [s, 1H, H-7], that used for 1 was used except that aniline was replaced by
8.93 [d, 1H, H-3′], 8.18 [d, 1H, H-6′], 8.13 [dd, 1H, H-5′], 7.72 [dd, o-toluidine, giving pale-yellow crystals from acetonitrile
SYNTHESIS OF [HgCl2L2]2 (4). A similar synthetic procedure to
1
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solution. The crystalline sample contained crystals of two poly- drops of concentrated nitric acid and the resulting solution
morphs. The estimate of the relative proportion of each poly- was diluted with 10 mL of water. This solution was added
morph in the sample was based on visual inspection of the drop-wise to a previously prepared solution of pyridine-2-car-
crystals. The minor product (4a) crystallized in the form of boxaldehyde (0.15 g, 1.40 mmol) and p-toluidine (0.15 g,
needles and the major product (4b) formed as rhombohedral 1.40 mmol) in ethanol (15 mL) which resulted in the immedi-
prisms; needles of 4a were cut for the X-ray crystallographic ate formation of a yellow precipitate. Stirring was continued
analysis. Combined yield 42%. M.p.: 4a; 158–160 °C; 4b; for 3 h and then the mixture was filtered. The residue was
169–170 °C. Found: C, 33.45; H, 2.32; N, 6.07%. Calc. for washed thoroughly with water until the filtrate was pH neutral,
C26H24Cl4Hg2N4: C, 33.36; H, 2.59; N, 5.99%. Λm (CH3CN): then with methanol (3 × 5 mL) and dried in vacuo. The dried
3 Ω−1 cm2 mol−1. IR (cm−1): 1639 νasym(C(H)vN); 1593, 1485, solid was dissolved in boiling acetonitrile (60 mL) and filtered
1440 ν(CvN)py. The IR spectra of 4a and 4b were indistin- while hot. The filtrate, upon cooling to room temperature
1
guishable. H-NMR (DMSO-d6): δ8.87 [d, 1H, H-3′], 8.73 [s, 1H, afforded compound 9 in the form of a yellow crystalline
H-7], 8.13 [m, 2H, H-5′,6′], 7.71 [dd, 1H, H-4′], 7.23 [m, 3H, material. Yield 0.47 g (58%). M.p. 220–221 °C. Found: C, 30.15;
H-,3,5,6], 7.06 [t, 1H, H-4], 2.41 [s, 3H, CH3] ppm.
H, 2.12; N, 10.80%. Calc. for C13H12HgN4O6: C, 29.96; H, 2.32;
SYNTHESIS OF [HgCl2L3]2 (5). A similar synthetic procedure to N, 10.76%. Λm (CH3CN): 7 Ω−1 cm2 mol−1. IR (cm−1) KBr: 1593
that used for 1 was used except that aniline was replaced by νasym(C(H)vN) + ν(CvN)py; 1527, 1381, 1321 ν(NO3), Nujol:
m-toluidine, giving-yellow crystals from acetonitrile solution. 1593 νasym(C(H)vN) + ν(CvN)py; 1527 ν1(NO3), 1334 ν5(NO3).
Yield 42%. M.p. 172–174 °C. Found: C, 33.25; H, 2.52; N, 1H-NMR (DMSO-d6): δ 9.09 [s, 1H, H-7], 8.74 [d, 1H, H-3′], 8.10
5.86%. Calc. for C26H24Cl4Hg2N4: C, 33.36; H, 2.59; N, 5.99%. [m, 2H, H-5′,6′], 7.77 [dd, 1H, H-4′], 7.07 [m, 4H, H-2,3,5,6],
Λm (CH3CN): 4 Ω−1 cm2 mol−1. IR (cm−1): 1638 νasym(C(H)vN); 2.19 [s, 3H, CH3] ppm.
1593, 1487, 1434 ν(CvN)py. 1H-NMR (DMSO-d6): δ 8.91 [s, 1H,
H-7], 8.87 [d, 1H, H-3′], 8.03 [m, 2H, H-5′,6′], 7.66 [t, 1H, H-4′], carboxaldehyde (0.15 g, 1.40 mmol) in ethanol (5 mL) was
7.28 [m, 3H, H-2,5,6], 7.10 [d, 1H, H-4], 2.32 [s, 3H, CH3] ppm. added a solution of p-toluidine (0.15 g, 1.40 mmol) in ethanol
SYNTHESIS OF {[Hg(N3)2L4]2}n (10). To a solution of pyridine-2-
SYNTHESIS OF [HgCl2L4]2 (6). A similar synthetic procedure to (10 mL). The mixture was stirred at ambient temperature for
that used for 1 was used except that aniline was replaced by 30 min and was added drop-wise to a stirred methanolic solu-
p-toluidine, giving pale-yellow crystals. Yield 43%. M.p. tion containing Hg(N3)2 (prepared in situ from the reaction of
230–232 °C. Found: C, 33.50; H, 2.66; N, 5.80%. Calc. for Hg(OAc)2 (0.45 g, 1.40 mmol) in 20 mL methanol with an
C26H24Cl4Hg2N4: C, 33.36; H, 2.59; N, 5.99%. Λm (CH3CN): 5 excess of NaN3 (0.36 g, 5.48 mmol) in 30 mL methanol) which
Ω−1 cm2 mol−1. IR (cm−1): 1639 νasym(C(H)vN); 1593, 1467, resulted in the immediate formation of a yellow precipitate.
1
1434 ν(CvN)py. H-NMR (DMSO-d6): δ 8.94 [s, 1H, H-7], 8.86 The stirring was continued for 3 h and then the mixture was
[d, 1H, H-3′], 8.12 [d, 1H, H-6′], 8.02 [dd, 1H, H-5′], 7.73 [dd, filtered. The residue was washed thoroughly with water, then
1H, H-4′], 7.05 [d, 2H, H-3,5], 7.30 [d, 2H, H-2,6], 2.41 [s, 3H, with methanol (3 × 5 mL) and dried in vacuo. The dried solid
CH3] ppm.
SYNTHESIS OF [HgBr2L4]2 (7). A similar synthetic procedure to hot. The filtrate, upon cooling to room temperature afforded
that used for 1 was used except that HgCl2 and aniline were yellow crystalline material. Yield 0.26 (32%). M.p.
was dissolved in boiling acetonitrile (60 mL) and filtered while
g
replaced by HgBr2 and p-toluidine, respectively, giving pale- 188–190 °C. Found: C, 32.66; H, 2.72; N, 23.42%. Calc. for
yellow crystals. Yield 53%. M.p. 206–208 °C. Found: C, 28.00; C26H24Hg2N16: C, 32.45; H, 2.52; N, 23.30%. Λm (CH3CN):
H, 2.22; N, 5.17%. Calc. for C13H12Br2HgN2: C, 28.03; H, 2.17; 3 Ω−1 cm2 mol−1. IR (cm−1): 1639 νasym(C(H)vN); 1600; 1475,
N, 5.03%. Λm (CH3CN): 3 Ω−1 cm2 mol−1. IR (cm−1): 1639 1447 ν(CvN)py; 2037 νas(N3). 1H-NMR (DMSO-d6): δ 8.70 [d,
1
νasym(C(H)vN); 1593, 1480, 1447 ν(CvN)py. H-NMR (DMSO- 1H, H-3′], 8.66 [s, 1H, H-7], 8.00 [dd, 1H, H-5′], 7.88 [d, 1H,
d6): δ 8.87 [s, 1H, H-7], 8.71 [d, 1H, H-3′], 8.05 [d, 1H, H-6′], H-6′], 7.44 [dd, 1H, H-4′], 7.21 [d, 2H, H-3,5], 7.15 [d, 2H,
7.91 [dd, 1H, H-5′], 7.66 [dd, 1H, H-4′], 7.45 [d, 2H, H-3,5], 7.19 H-2,6], 2.30 [s, 3H, CH3] ppm. Caution! While no incident
[d, 2H, H-2,6], 2.33 [s, 3H, CH3] ppm.
occurred whilst using azide during preparation and isolation, care
SYNTHESIS OF [HgI2L4]2 (8). A similar synthetic procedure to in handling azides must be exercised owing to their potentially
that used for 7 was used except that HgBr2 was replaced by explosive nature.
HgI2, giving pale-yellow crystals. Yield 59%. M.p. 194–196 °C.
Found: C, 24.15; H, 2.02; N, 4.18%. Calc. for C13H12HgI2N2: C,
Quantum chemical calculations
23.98; H, 1.86; N, 4.31%. Λm (CH3CN): 5 Ω−1 cm2 mol−1. IR Quantum chemical calculations were carried out on the
(cm−1): 1639 νasym(C(H)vN); 1586, 1507, 1440 ν(CvN)py. ligands (L1–L4) in order to derive their structural parameters
1H-NMR (DMSO-d6): δ 8.86 [s, 1H, H-7], 8.63 [d, 1H, H-3′], 8.04 since the ligands could not be isolated in a pure crystalline
[d, 1H, H-6′], 7.89 [dd, 1H, H-5′], 7.64 [dd, 1H, H-4′], 7.45 form and characterized crystallographically. Previously, crystals
[d, 2H, H-3,5], 7.23 [d, 2H, H-2,6], 2.29 [s, 3H, CH3] ppm.
of (E)-N-(pyridin-2-ylmethylidene)aniline (L1) were obtained by
SYNTHESIS OF [Hg(NO3)2L4]n (9). The standard preparative sublimation and its crystal structure was reported;51 however,
method was slightly modified for this complex because of the the structure suffers from whole molecule disorder (see discus-
low solubility of Hg(NO3)2 in ethanol. In this case, Hg sion). In view of the unavailability of the pure ligands, the
(NO3)2(0.45 g, 1.40 mmol) was dissolved under heating in five assignment of diagnostically important experimental infrared
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an analytical absorption correction60 was applied for each of 2,
3 and 10. The structures of 7–9 were solved by heavy-atom
Patterson methods61 which revealed the position of the Hg, Br
and I atoms in their respective compounds. All remaining
non-hydrogen atoms were located in a Fourier expansion of
the Patterson solution, which was performed by DIRDIF94.62
The structures of 2, 4–6 and 10 were solved by direct methods
using SHELXS97,63 which revealed the positions of all non-
hydrogen atoms while the structure of 3 was solved by direct
methods using SIR92.64 The non-hydrogen atoms in each
structure were refined anisotropically. All of the H-atoms were
placed in geometrically calculated positions and refined by
using a riding model where each H-atom was assigned an iso-
tropic displacement parameter with a value equal to 1.2Ueq of
its parent atom (1.5Ueq for methyl-H). The refinement of each
Fig. 1 A view of the geometry optimized structure of L2.
bands due to ν(C(H)vN) and ν(CvN)py was also difficult. To
resolve these issues, the geometries of the ligands (L1–L4) were
optimized using the B3LYP level of theory and the B3LYP/
6-31G(d,p) basis set.52 Harmonic frequency calculations were
performed at all stationary points to characterize their nature
and to ensure that the optimized structure corresponded with
a global minimum. A similar method of calculation was suc-
cessfully applied in the structural and spectroscopic character-
structure was carried out on F2 by using full-matrix least-
2
squares procedures, which minimized the function Σw(Fo
−
2 2
Fc ) . For 3 and 9, corrections for secondary extinction were
applied and five and two reflections, respectively, were omitted
owing to poor agreement. The absolute structure parameter of
0.259(7) for 7 indicated that the crystal investigated was an
inversion twin. As is not uncommon for heavy atom structures,
several data sets presented relatively large residual electron
density peaks. Usually, these were located near the mercury
atom, but in the cases of 7, 9 and 10, these were located in
chemically meaningless positions. A comment on the large
residual electron density peak in 2 is appropriate owing to its
great size, i.e. 9.55 e Å−3. The minimum electron density peak
was −1.36 e Å−3 and the next highest peak was 1.00 e Å−3. The
large residual peak was located 1.08 Å from the Hg atom and
attempts were made to resolve its aetiology. The applied
absorption correction was based on face-indexing but when an
empirical absorption correction was applied, the residual per-
sisted. Similarly, the peak and pseudo-symmetry equivalent
remained when the refinement was performed in the P1 space
group; no evidence for twinning was found. While the residual
may be indicative of disorder, no evidence of disorder was
ization of
a
series of 2-hydroxy-5-[(E)-(aryldiazenyl)]-
benzaldehydes and 4-[((E)-1-{2-hydroxy-5-[(E)-2-(aryl)-1-diaze-
nyl]phenyl}methylidene)amino]benzoic acid ligands.53 A rep-
resentative optimized structure, e.g. L2, is shown in Fig. 1
while the other three molecules are given as ESI Fig. S1–S3.†
The optimized geometric parameters for L1–L4 are listed in ESI
Table S1.† The experimental IR frequencies involve anharmo-
nic terms whereas the calculated frequencies are derived from
a harmonic oscillator model. This difference can be corrected
by scaling the calculated values by a factor of 0.9623.53,54 This
information was utilized to interpret and assign the experi-
mental infrared data for 1–10.
X-ray crystallography
Crystals of compounds 2–10 suitable for X-ray crystal-structure found in other parts of the molecule. The largest residual elec-
determination were obtained by slow evaporation of aceto- tron density peak for 3 was also a little high at 5.32 e Å−3. The
nitrile solutions of the respective compounds at room temp- peak is within 0.93 Å of the Hg atom and the next highest
erature. In the case of compound 4, the crystalline sample peak was 0.88 e Å−3. Again, an absorption correction based on
contained crystals of different shapes and full crystal structure face-indexing was applied. Other absorption correction trials
determinations were conducted on each crystal form revealing did not reduce the peak and there was no evidence for twin-
two polymorphs. The measurements for 7–9 were made at low ning or disorder in the structure. The SHELXL97 program63
temperature on a Nonius KappaCCD diffractometer55 with was used for the calculations of 2, 3, and 7–10, while refine-
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and ment and data output of 4–6 were carried out with the
an Oxford Cryosystems Cryostream 700 cooler. The data for 2, SHELXTL-NT program package.65 The data collection and
3 and 10 were recorded on an Agilent Technologies Super Nova refinement parameters are given in Table 1, and views of the
area-detector diffractometer56 using Mo-Kα radiation from a molecular structures are shown in Fig. 4–8.
micro-focus X-ray source and an Oxford Instruments Cryojet
XL cooler, and data for 4–6 were recorded on a Bruker-APEX
diffractometer equipped with a CCD area detector and Mo-Kα
Results and discussion
Synthesis
radiation. Data reduction was performed using HKL Denzo
and Scalepack57 for 7–9, with CrysAlisPro56 for 2, 3 and 10,
and with SAINT58 for 4–6. An empirical absorption correction In convenient one-pot reactions, a systematic series of ten
based on the multi-scan method59 was applied for 4–9 while complexes with the general stoichiometry HgX2L has been
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Table 1 Crystal data and refinement details for 2–10
2
3
4a
4b
Empirical formula
Formula weight
Crystal size (mm)
Crystal morphology
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
24H20Br4Hg2N4
C12H10HgI2N2
636.53
0.10 × 0.15 × 0.15
Prism
160(1)
Monoclinic
P21/n
12.68311(19)
7.13335(10)
15.9101(2)
90
90.8269(13)
90
1439.28(4)
4
2.938
C26H24Cl4Hg2N4
935.47
0.15 × 0.16 × 0.20
Prism
293(2)
Triclinic
ˉ
P1
8.2038(11)
13.8179(19)
14.4494(19)
115.149(2)
99.505(2)
100.406(2)
1403.5(3)
2
C26H24Cl4Hg2N4
935.47
0.15 × 0.21 × 0.22
Prism
293(2)
Triclinic
ˉ
P1
12.0247(16)
14.3877(19)
17.542(2)
78.370(2)
83.453(2)
78.383(2)
2903.2(7)
4
1085.06
0.05 × 0.10 × 0.10
Prism
160(1)
Triclinic
ˉ
P1
8.03383(16)
8.9441(2)
10.0091(3)
106.595(3)
100.732(2)
99.5612(19)
658.57(3)
1
γ (°)
V (Å3)
Z
Dx (g cm−3
)
2.736
17.694
2.214
11.330
2.140
10.954
μ (mm−1
)
14.964
Transmission factors (min, max)
θ range (°)
0.145, 0.358
2.2–32.6
20 442
4394; 0.033
4057
154
0.038
0.100
1.07
9.55, −1.36
0.168, 0.336
3.1–32.4
22 859
4851; 0.035
4367
155
0.027
0.066
1.05
5.32, −1.49
0.210, 0.281
1.7–25.0
13 697
4934; 0.056
3716
327
0.040
0.133
1.04
1.38, −1.08
0.197, 0.290
1.2–25.0
28 274
10 210; 0.057
5153
653
0.038
0.057
0.99
1.19, −0.81
Reflections measured
Independent reflections; Rint
Reflections with I > 2σ(I)
Number of parameters
R(F) [I > 2σ(I) reflns]
wR(F2) (all data)
GOF(F2)
Δρmax, min (e Å−3
)
5
6
7
8
9
10
C26H24Cl4Hg2N4
935.47
0.23 × 0.34 × 0.46
Prism
C26H24Cl4Hg2N4
935.47
0.27 × 0.29 × 0.34
Prism
C26H24Br4Hg2N4
C13H12HgI2N2
650.64
0.22 × 0.25 × 0.25
Prism
C13H12HgN4O6
520.86
0.10 × 0.13 × 0.30
Prism
C26H24Hg2N16
961.79
0.08 × 0.13 × 0.20
Tablet
1113.32
0.10 × 0.12 × 0.18
Prism
293(2)
Monoclinic
P21/c
293(2)
Monoclinic
P21/n
160(1)
Monoclinic
P21
160(1)
Triclinic
ˉ
P1
160(1)
Monoclinic
P21/c
160(1)
Triclinic
ˉ
P1
7.7944(9)
8.4673(10)
21.336(2)
90
95.221(2)
90
7.5731(19)
15.346(4)
12.503(3)
90
97.653(4)
90
8.2201(1)
17.5934(3)
10.0430(2)
90
104.6895(9)
90
7.7545(1)
9.9076(2)
10.9202(2)
76.889(1)
83.930(1)
76.652(1)
793.78(2)
2
10.6101(2)
14.2146(2)
10.3608(2)
90
97.2161(11)
90
6.6753(2)
10.3614(3)
11.5961(4)
71.528(3)
73.693(3)
78.321(3)
724.43(4)
1
1402.3(3)
2
1440.2(6)
2
1404.94(4)
2
1550.22(5)
4
2.216
11.339
2.157
11.041
2.632
16.626
2.722
13.570
2.232
9.968
2.205
10.633
0.078, 0.180
2.6–25.0
12 935
2465; 0.054
2096
0.117, 0.155
2.1–25.0
13 356
2528; 0.068
1885
0.152, 0.213
2.1–30.0
41 875
8003; 0.054
7309
0.033, 0.072
1.9–27.5
17 262
3607; 0.075
3330
0.145, 0.388
2.5–30.0
42 486
4503; 0.080
3755
0.280, 0.721
2.4–30.5
18 011
4055; 0.033
3767
164
0.029
164
0.041
328
0.032
164
0.040
219
0.031
200
0.020
0.063
0.96
0.076
1.06
0.073
1.04
0.105
1.07
0.074
1.04
0.042
1.07
1.07, −1.15
0.72, −1.04
2.36, −2.00
1.23, −4.19
2.04, −1.60
1.02, −0.75
prepared, where X is chloride, bromide, iodide, nitrate or structure for compound 1 was reported, but no spectroscopic
azide, and L is a variously methyl-substituted Schiff base properties were documented.48 These are therefore included
ligand, (E)-N-(pyridin-2-ylmethylidene)arylamine. In alcohol, herein for comparative purposes. The synthesis of polymeric
one equivalent of HgX2 reacts rapidly with one equivalent of L [Hg(NO3)2L]n (9) was conducted in an aqueous ethanol
(generated in situ from pyridine-2-carboxaldehyde and a sub- medium owing to the poor solubility of the mercury precursor
stituted aniline) to give a yellow precipitate which proved to be in absolute ethanol. All mercury complexes are insoluble in
mercury complexes of the formula HgX2L (3, 8), [HgX2L]2 (1, 2, the reaction medium, but can be recrystallized using a large
4–7) and [HgX2L]n (10), see Chart 1. Recently, the X-ray crystal volume of acetonitrile to provide crystals suitable for X-ray
1910 | Dalton Trans., 2013, 42, 1905–1920
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diffraction studies. The results of the crystal structure determi- 200 cm−1 (ν1(NO3) 1527 cm−1, ν5(NO3) 1334 cm−1).70 The
nations of 2–10 are consistent with the chemical and spectro- bidentate coordination mode of the nitrate groups was sub-
scopic analyses, giving clear evidence of the formation of 1 : 1 sequently confirmed by the X-ray crystal structure determi-
adducts between the bidentate N-donors and the correspond- nation (see below). On the other hand, an important
ing HgX2. Complexes 2–10 are all air-stable and behave as non- observation for the IR spectrum of 10 is the presence of a very
electrolytes in acetonitrile solution.
strong band at 2037 cm−1 corresponding to νasym(N3); the brid-
ging nature of N3− is revealed by weak doublet splitting.71,72
The 1H NMR spectra recorded in DMSO-d6 solution dis-
Geometry optimized structures of L1–L4
Although the crystal structure of (E)-N-(pyridin-2-ylmethyli- played the expected signals,73,74 and, therefore, revealed the
dene)aniline(L1) has been reported,51 the geometric para- presence of the ligand skeleton in the respective complexes.
meters could not be used with confidence owing to whole Coupling constants could not be established with certainty
molecule disorder. As anticipated, most of the geometric para- owing to the broad unresolved nature of the signals. The effect
meters in the calculated structures of L1–L4 (ESI Table S1†) are of coordination to mercury(II) upon the 1H NMR chemical
found to be insensitive to the nature and position of the sub- shifts could not be judged in the absence of the NMR data for
stituents. However, the substituents can have a profound effect the ligands, which could only be prepared in situ (see
on the planarity of the molecule. The basic structural frame- Experimental).
work of each of L1–L4 contains one pyridine ring and one aryl
Table 2 summarizes the solution UV-Vis and fluorescence
ring connected through the C(H)vN linkage, and these are properties of complexes 1–10; spectra remained unchanged
planar in the optimized structures of L1, L3 and L4. However, over a period of 25 days. The absorption spectra of all com-
L2 is non-planar as seen in the C6–N2–C7–C8 torsion angle of plexes were recorded in the range 300–450 nm in acetonitrile
41.3°, an observation correlated with steric pressure should a solutions at concentrations of ∼10−5 M. The electronic spectra
planar arrangement be adopted. Molecules L1–L4 exist in the exhibit a coalescence absorption in the range of 325 to 360 nm
trans-isomeric form, as observed by Wiebcke and Mootz.51 The (Fig. 2) and the origin of the band could not be assigned
stretching vibrational frequencies due to C(H)vN (1640 cm−1
)
unambiguously due to the non-availability of data for the free
and ν(CvN)py (1575 cm−1) are found to be almost unchanged ligands. Nevertheless, the absorption is possibly a result of
in L1–L4 and this assignment was used to diagnose the said overlap of intramolecular charge transfer transitions (ε ∼ 104)
bands in the experimental infrared spectra of complexes 1–10. with a weak band due to MLCT transition from Hg(II) → π*
Finally, the Mulliken charges were calculated based on the (ligand), as observed for the cognate systems.75,76
optimised structures. This showed that the charge distribution
on the N1 and N2 atoms of L1 (−0.4562 and −0.4317) and L3 as independent evidence of complexation. In acetonitrile solu-
(−0.4564 and −0.4324) were virtually identical and less than tion, the complexes have broad emission bands at λmax
those calculated for L2 (−0.4582 and −0.4367) and L4 (−0.4567 410 nm along with a shoulder at ∼430 nm within the wave-
The steady-state fluorescence studies have been employed
=
and −0.4349).
length range of 390–550 nm, when they are excited at their
respective absorption maxima (Fig. 3), indicating that the tran-
sitions are charge transfer in nature. In general, the complexes
IR, NMR, UV-Vis and fluorescence spectroscopy
The infrared spectra of complexes 1–10 are very similar and show very low fluorescent quantum yields, which can be attri-
the IR assignments of selected diagnostic bands are given in buted to the heavy atom effect.77,78 The Hg2+ cation and chlor-
the Experimental section. The complexes display a moderately ide anions can quench the fluorescence and result in efficient
intense IR band in the region 1630–1650 cm−1, which is luminescence decay. However, complexes 3, 8 and 10 exhibit a
assigned to the νasym(C(H)vN) stretch of the coordinated
Schiff base ligands.66 In addition, well resolved sharp bands of
Table 2 Photophysical data for complexes 1–10 recorded in acetonitrile
variable intensity observed in the regions 1600–1580,
solution
1490–1475 and 1450–1435 cm−1 are assigned to the coordi-
nated pyridine ring.66–68 Complexes 9 and 10 deserve specific
Electronic absorption
data λmax (nm);
Photoluminescence data
mention. The cited nitrate frequencies in 9 are medium-depen-
dent. Using a KBr matrix, the IR spectrum showed a very
strong band at approximately 1381 cm−1 which is indicative of
the simultaneous presence of ionic and coordinated nitrates;67
it is noted that pressing a KBr pellet can also influence the
nitrate coordination which has been fully discussed in the lit-
erature.69 In addition, the solid-state spectrum of 9 displayed
bands at 1527 and 1321 cm−1, which are indicative of biden-
tate chelating nitrate groups. The assumption of bidentate che-
lating nitrate groups67 was further established from the nitrate
vibrations observed in the Nujol mull spectrum of 9, since the
separation of the two bands ν1 and ν5 is approximately
Complexes (ε[M−1])
λema (nm)
ϕF
1
2
3
4
5
6
7
8
9
10
336 (12 698)
338 (25 190)
331 (33 717)
356 (16 258)
338 (45 680)
348 (10 781)
341 (13 367)
340 (32 679)
360 (29 458)
325 (31 701)
409, 431
410, 432
408, 434
409, 437
409, 430
413, 429
410, 432
411, 433
410, 434
411, 432
0.21
0.19
0.08
0.27
0.19
0.16
0.20
0.02
0.39
0.03
a The low energy wavelength emission appears as a shoulder in all
cases.
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Fig. 2 UV-Vis spectra of complexes 1–10 in acetonitrile (concentration ∼10−5 M).
Fig. 3 Fluorescence spectra of complexes 1–10 in acetonitrile (concentration ∼10−5 M) obtained by excitation at the respective absorption maxima.
very weak emission yield, which is about one order of magni- persistent trend in the series of structures reported herein is
tude lower than for the remaining complexes. It can be pre- the presence of chelating (E)-N-(pyridin-2-ylmethylidene)aryl-
sumed that the significantly weakened emission intensities for amine ligands and the observation that the Hg–N(pyridyl) bond
3 and 8, when compared with the related analogues 1–2 and length is consistently shorter than the Hg–N(imino) bond
6–7, respectively, can be attributed to the competitive quench- length (Δ(Hg–N) = 0.12 to 0.26 Å, except for 8 where it is only
ing effect of the iodide ions.79 The difference in the intensity 0.03 Å). Across the series of structures, the parameters about
of the emission results from the variation of the coordinated the N(2)vC(6) bond do not differ experimentally (Tables 3 and
anions to mercury(II), and indicates that the anions strongly 4), are comparable with the parameters derived from the geo-
affect the fluorescence emission features.25,80
metry optimised structures (ESI Table S1†), and are, therefore,
not discussed further. The five-membered chelate ring in 3 is
planar with the root-mean-square (r.m.s.) deviation for the
fitted atoms being 0.026 Å, and indeed the overall molecule of
L1 is planar as seen in the dihedral angle of 3.19(17)° formed
between the pyridyl and phenyl ring planes. In this structure
and across the series, the Hg–N–C bond angles follow the
same trends with the exo-chelate ring angles being approxi-
mately 8° (N1) and 12° (N2) wider than the endo-chelate ring
angles (Tables 3 and 4).
Molecular structures
The crystal and molecular structures of 2–10 have been deter-
mined in the present study and reveal a variety of structural
motifs; the structure of 1 is available in the literature.48
Herein, the molecular structures will be described in order of
increasing nuclearity. Selected geometric parameters are col-
lected in Tables 3 and 4.
Mononuclear species are found for the diiodido complexes
[HgI2L1] (3) and [HgI2L4] (8), Fig. 4. In each case, the mercury
atom is tetrahedrally coordinated by two iodine atoms and the
nitrogen atoms derived from the chelating ligand. The
A similar coordination geometry pertains in the structure of
8, the r.m.s. deviation of the chelate ring and dihedral angle
between the six-membered rings being 0.074 Å and 4.5(3)°,
1912 | Dalton Trans., 2013, 42, 1905–1920
This journal is © The Royal Society of Chemistry 2013

Table 3 Selected bond lengths (Å) and angles (°) for 1–8 and 10a
1b
2
3
4ab
4bb (molec. A)
4bb (molec. B)
5
6
7b
8
10c
2.232(2)
Hg1–N1
Hg1–N2
Hg1–X1
Hg1–X1′
Hg1–X2
2.322(8)
2.497(7)
2.526(3)
2.896(3)
2.396(3)
2.327(4)
2.507(4)
2.6299(6)
3.0140(6)
2.5203(6)
2.357(3)
2.478(3)
2.6446(3)
2.256(7)
2.337(8)
2.512(8)
2.499(8)
2.474(3)
2.428(3)
3.026(3)
2.983(3)
2.410(3)
2.382(3)
—
1.459(14)
1.467(15)
1.259(11)
1.248(12)
1.444(12)
1.426(13)
70.8(3)
2.362(6)
2.338(6)
2.489(6)
2.507(6)
2.411(2)
2.411(2)
3.155(2)
3.056(2)
2.383(2)
2.389(2)
—
1.455(9)
1.451(10)
1.284(8)
1.275(8)
1.413(8)
1.414(10)
69.3(2)
2.347(6)
2.366(6)
2.516(6)
2.503(6)
2.416(2)
2.393(2)
3.169(2)
3.155(2)
2.372(2)
2.383(2)
—
1.473(9)
1.464(10)
1.281(8)
1.277(8)
1.441(8)
1.438(9)
69.6(2)
2.276(4)
2.354(6)
2.490(6)
2.423(2)
3.011(2)
2.355(3)
2.288(6)
2.295(6)
2.471(5)
2.467(5)
2.5010(7)
2.4994(7)
3.4749(8)
3.6344(8)
2.5779(8)
2.5831(7)
2.404(5)
2.438(5)
2.6698(5)
2.500(4)
2.496(3)
2.125(2)
2.823(2)
2.468(2)
2.4207(14)
2.9837(15)
2.4376(13)
—
—
2.6964(3)
2.6363(5)
Hg1–X2′
C1–C6
—
—
—
—
—
—
—
2.579(2)
1.461(4)
1.478(12)
1.270(12)
1.417(11)
1.475(7)
1.268(7)
1.419(7)
70.44(15)
1.474(5)
1.276(5)
1.421(5)
70.47(11)
1.473(7)
1.265(6)
1.429(6)
71.22(14)
1.477(10)
1.258(9)
1.420(9)
70.3(2)
1.477(9)
1.453(9)
1.278(9)
1.283(8)
1.434(8)
1.418(8)
1.471(9)
1.264(8)
1.418(7)
69.73(18)
N2–C6
1.274(3)
1.417(4)
71.05(8)
158.28(10)
91.82(8)
118.41(9)
80.13(7)
83.85(7)
119.61(8)
N2–C7
N1–Hg1–N2
N1–Hg1–X1
N1–Hg1–X2
N2–Hg1–X1
N2–Hg1–X2
N1–Hg1–X1′
N2–Hg1–X1′
70.3(3)
126.6(2)
118.0(2)
91.69(18)
109.3(2)
84.9(2)
71.13(18)
71.30(18)
144.52(13)
141.55(14)
96.39(13)
100.57(14)
107.83(12)
108.38(12)
111.26(12)
113.72(12)
74.98(13)
74.97(13)
130.20(13)
129.66(12)
—
68.5(3)
69.1(2)
68.4(2)
126.56(11)
118.84(11)
91.40(10)
109.66(11)
84.90(11)
146.02(11)
124.84(8)
100.53(8)
105.79(7)
107.52(7)
—
127.7(2)
103.3(2)
123.3(2)
131.1(2)
108.17(19)
114.8(2)
93.63(19)
91.2(2)
87.3(2)
87.2(2)
158.04(18)
150.6(2)
—
111.21(16)
111.80(18)
117.00(16)
114.40(17)
112.65(15)
114.97(16)
92.82(15)
90.50(15)
85.76(17)
84.46(18)
153.83(16)
151.02(17)
—
104.84(16)
112.21(16)
121.84(16)
113.25(16)
114.50(15)
113.23(16)
94.54(14)
91.02(16)
84.59(17)
84.35(17)
151.28(16)
151.52(17)
—
143.12(11)
105.99(11)
103.76(9)
112.40(9)
83.01(10)
143.45(10)
107.73(16)
113.76(17)
94.10(15)
104.98(15)
87.35(16)
156.50(16)
104.46(13)
114.29(14)
101.71(12)
114.55(12)
—
144.45(19)
—
—
N1–Hg1–X2′
N2–Hg1–X2′
X1–Hg1–X2
—
—
—
—
—
—
—
—
—
—
—
—
84.57(8)
146.87(8)
108.70(9)
—
—
—
—
115.35(10)
114.59(2)
130.455(11)
108.93(9)
125.56(10)
84.28(9)
86.02(9)
—
99.41(9)
92.67(11)
—
130.93(7)
132.77(7)
83.06(6)
85.24(6)
—
91.64(6)
89.79(6)
—
131.56(7)
133.76(7)
83.48(6)
84.14(6)
—
88.40(6)
92.22(7)
—
109.40(5)
138.07(11)
115.66(3)
113.36(3)
80.75(2)
77.54(2)
—
107.78(2)
108.31(2)
—
133.685(17)
X1–Hg1–X1′
82.91(8)
84.768(17)
—
81.50(5)
85.66(7)
—
74.50(10)
X1–Hg1–X2′
X2–Hg1–X1′
—
—
—
—
—
99.15(5)
—
—
—
92.44(9)
156.52(8)
104.66(10)
102.542(19)
90.34(7)
X2–Hg1–X2′
Hg1–X1–Hg1′
—
97.09(8)
—
—
—
—
98.50(5)
—
—
—
78.57(8)
105.50(10)
95.232(17)
94.08(8)
93.96(8)
—
94.53(6)
97.10(7)
—
95.94(7)
96.04(7)
—
94.34(7)
98.70(2)
102.94(2)
—
Hg1–X2–Hg1′
Hg1–N1–C1
—
116.7(6)
—
116.6(3)
—
116.2(3)
—
—
—
101.43(8)
118.24(18)
116.8(6)
118.2(7)
123.9(6)
123.6(7)
109.2(6)
112.8(7)
129.5(6)
121.7(6)
116.9(5)
118.0(6)
124.1(5)
125.4(6)
113.4(5)
110.2(6)
124.1(5)
123.3(5)
118.4(5)
118.4(6)
124.5(5)
123.6(5)
112.0(5)
112.4(6)
127.3(4)
123.8(5)
117.2(3)
116.5(5)
117.5(4)
116.6(4)
123.1(4)
123.0(5)
111.9(4)
111.3(4)
126.5(4)
126.8(4)
114.6(4)
Hg1–N1–C5
Hg1–N2–C6
Hg1–N2–C7
124.3(7)
112.6(6)
125.8(6)
124.4(4)
111.9(3)
126.8(3)
124.7(3)
112.7(3)
125.5(2)
123.3(3)
110.7(3)
128.9(3)
124.9(5)
112.6(5)
125.6(5)
125.9(5)
115.1(4)
122.5(4)
121.81(18)
110.31(18)
125.40(18)
a Compounds 1, 2, 5 and 6 are dinuclear complexes in which the participating molecules are related by a crystallographic centre of inversion. Compounds 4a, 4b and 7 are also dinuclear complexes, but the
participating molecules are crystallographically independent. In the case of compound 4b, the asymmetric unit contains two such dimers (A and B). Analogous atom numbering schemes have been used in all cases.
Therefore, for the sake of clarity, in column 1 only labels for one of the independent molecules are given. Primed atoms indicate the longer Hg⋯X bridging distance to the second unit of the dimer. b Data taken from
ref. 48. c Compound 10 is a one-dimensional polymer with centrosymmetric double azide bridges between adjacent Hg-atoms. There are two symmetry-independent pairs of bridges. One contains atom N3 (X1) and
its inversion-related counterpart, while the other involves N6 (X2) and its counterpart.

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Table 4 Selected bond lengths (Å) and angles (°) for 9
Parameter
Parameter
Hg1–N1
Hg1–N2
Hg1–O1
Hg1–O2
2.218(4)
2.353(3)
2.534(3)
2.620(3)
2.174(3)
2.739(4)
2.755(3)
Hg1–O3i
N1–C1
N1–C5
N2–C6
N2–C7
C1–C6
2.913(3)
1.351(5)
1.340(5)
1.276(5)
1.438(5)
1.473(6)
Hg1–O4
Hg1–O5
Hg1–O2i
Fig. 4 Perspective views of the monomers found in the crystal structures of
compounds [HgI2L1] (3) and [HgI2L4] (8). Displacement ellipsoids are drawn at
the 30% probability level and H atoms are shown as small spheres of arbitrary
radii.
N1–Hg1–N2
N1–Hg1–O2
N1–Hg1–O5
N1–Hg1–O3i
N2–Hg1–O2
N2–Hg1–O5
N2–Hg1–O3i
O1–Hg1–O4
O1–Hg1–O2i
O2–Hg1–O4
O2–Hg1–O2i
O4–Hg1–O5
O4–Hg1–O3i
O5–Hg1–O3i
Hg1–N1–C1
Hg1–N2–C6
Hg1–O1–N3
Hg1–O4–N4
Hg1ii–O2–N3
Hg1–O2–Hgii
73.13(12)
102.81(12)
141.44(14)
80.87(11)
84.53(11)
84.14(12)
148.52(11)
85.03(11)
115.03(10)
92.82(12)
163.04(13)
50.35(12)
71.46(11)
106.81(11)
116.7(3)
N1–Hg1–O1
N1–Hg1–O4
N1–Hg1–O2i
N2–Hg1–O1
N2–Hg1–O4
N2–Hg1–O2i
O1–Hg1–O2
O1–Hg1–O5
O1–Hg1–O3i
O2–Hg1–O5
O2–Hg1–O3i
O4–Hg1–O2i
O5–Hg1–O2i
O2i–Hg1–O3i
Hg1–N1–C5
Hg1–N2–C7
Hg1–O2–N3
Hg1–O5–N4
Hg1ii–O3–N3
87.86(11)
152.22(12)
81.42(11)
125.05(11)
131.90(12)
112.33(11)
49.52(10)
130.57(13)
70.12(9)
105.59(13)
118.94(9)
77.41(12)
78.99(13)
44.93(9)
124.2(3)
125.8(3)
93.8(2)
83.0(3)
112.3(3)
98.6(2)
108.7(3)
97.5(2)
91.0(2)
149.38(16)
Symmetry operators: i: x, −y + 1/2, z + 1/2; ii: x, −y + 1/2, z − 1/2.
Fig. 5 Perspective views of the weakly associated dimers found in the crystal
structures of [HgBr2L1]2 (2) and [HgBr2L4]2 (7). Displacement ellipsoids are
drawn at the 30% probability level and H atoms are shown as small spheres of
arbitrary radii.
respectively. Fig. S4† shows an overlay diagram of molecules 3
and inverted 8 highlighting the similarity in the mode of
coordination of the bidentate ligands. However, the distortions
from the ideal tetrahedral geometry are larger in 3 (range of
tetrahedral angles = 70.47(11) to 124.84(8)°) than in 8 (69.73(18)
to 114.55(12)°). The more symmetrical arrangement in 8
As for 2, the molecules in 7 self-associate but this time
correlates with a more symmetric mode of coordination of L4, through a non-crystallographic centre of inversion. The two
and is seen in the small difference of the Hg–N bond dis- independent entities, containing Hg1 or Hg21, comprise the
tances, Δ(Hg–N), of 0.04 Å. This compares with Δ(Hg–N) = asymmetric unit with the inverted form of the Hg21-contain-
0.12 Å for 3 and correlates with the increase in basicity of the ing molecule virtually superimposable upon that with Hg1
imino-N2 atom in L4 owing to the electron donating nature of (ESI Fig. S5†). This similarity is reflected in the r.m.s. deviation
the methyl group.
of bond lengths and angles of 0.014 Å and 1.49°, respectively.83
It was also possible to characterize crystallographically the A dramatic influence exerted by the methyl substituent of L4 in
dibromido analogues of 3 and 8, i.e. [HgBr2L1]2 (2) and the self-association of 7 is evident from the bridging Hg⋯Br
[HgBr2L4]2 (7), for which notable structural changes were distances in 7, which are significantly longer than in 2, i.e.
observed, Fig. 5. Molecules of 2 are formed by self-association 3.4749(8) and 3.6344(8) Å, and longer than the sum of their
of the mononuclear entity over a centre of inversion via sec- van der Waals radii (3.40 Å).81 The more covalent character of
ondary Hg⋯Br interactions. The Hg–Br1 bond length of the Hg–Br bond can be correlated with the better coordinating
2.6299(6) Å is significantly longer than the terminal Hg–Br2 ability of the L4 ligand compared with L1, as reflected in the
bond (2.5203(6) Å), consistent with the participation of the Br1 shorter Hg–N bond lengths, Table 3, and as discussed above
atom in the bridge to the second Hg atom of the binuclear for the structures of 3 and 8.
molecule. The Hg2Br2 cycle is not symmetric as the bridging
With the structure of [HgCl2L1]2 (1) having being reported
Hg⋯Br distance is 3.0140(6) Å, a value significantly less than previously in the literature,48 the complete series of HgCl2
the sum of the van der Waals radii of mercury and bromine of structures with L1–4 is available for comparison, Fig. 6 and
3.40 Å.81 The resulting Br3N2 donor set defines a coordination Table 3. Compound [HgCl2L2]2 (4) crystallizes in two poly-
geometry intermediate between square pyramidal and trigonal morphic triclinic forms, which were deposited concomitantly
bipyramidal as quantified by the value of τ = 0.32 which com- from the same recrystallization and can be distinguished by
pares to the τ values of 0.0 and 1.0 for ideal square pyramidal their different crystal shapes: needles for the minor product
and trigonal bipyramidal geometries, respectively.82
(4a) and rhombohedral prisms for the major product (4b).
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Fig. 7 Overlay diagram of HgCl2L2 molecules in polymorphic 4a (the Hg1- and
inverted Hg21-containing molecules are illustrated in red and green, respect-
ively) and 4b (the Hg1-, inverted Hg21-, Hg41 and inverted Hg61-containing
molecules are illustrated in blue, pink, light-blue and black, respectively). The
molecules have been aligned to make the five-membered chelate rings
coincident.
4b, with the dihedral angle between the two rings in each of 1,
5 and 6 being 11.7(6), 9.0(3) and 13.2(4)°, respectively.
The relatively large standard uncertainty values associated
with the Hg–N bond lengths across the series of the [HgCl2L]2
structures preclude definitive conclusions about the relative
coordinating abilities of L1–4 molecules, but this can be ascer-
tained indirectly by consideration of the Hg–Cl bond lengths.
Thus, the Hg–Cl1 and Hg⋯Cl1 bond lengths are systematically
shorter and longer, respectively, in the structures involving the
methyl-substituted ligands compared with those incorporating
the unsubstituted ligand, Table 3. In terms of coordination
geometry, the values of τ were generally around 0.30 for the
mercury atoms in 1, 4a (Hg21), 4b and 6 with exceptional
values being found for the Hg1 atom in each of 4a (0.50) and
5 (0.01).
Clear trends are evident from the structural data on the
aforementioned HgX2L species in that when X = I, no evidence
for Hg⋯I bridges was found, but weak Hg⋯X bridges and
clear pairing of mononuclear entities started to appear in
structures with X = Br and were uniformly characterized in the
structures with X = Cl. Also of interest was the observation that
the coordinating ability of the (E)-N-(pyridin-2-ylmethylidene)
arylamine molecules is significantly moderated by the pres-
ence of methyl substituents in the aryl rings and this in turn
reduced the propensity of the Hg⋯X bridge formation.
Next, attention was directed to investigating structures with
X = nitrate (9) and azide (10). In the structure of (10), contain-
ing the pseudo-halide ligand azide, the familiar centrosym-
metric dimer is found, Fig. 8 and Table 3. Here, the bridge
involves the terminal N6 atom and the difference in Hg–N
bond lengths (0.27 Å) indicates a relatively symmetric bridge.
The value of τ is 0.19 indicating a gap in the coordination geo-
metry which is occupied by a weakly associated N3 atom (Hg–
N = 2.823(2) Å cf. the sum of the van der Waals radii of Hg and
N of 3.1081) that serves as a bridge to link the dimeric aggre-
gates into a supramolecular chain along the a-axis, Fig. 8b.
Fig. 6 Perspective views of the dimers found in the crystal structures of the
polymorphic forms of [HgCl2L2]2 (4a and 4b), [HgCl2L3]2 (5) and [HgCl2L4]2 (6).
Displacement ellipsoids are drawn at the 30% probability level and H atoms are
shown as small spheres of arbitrary radii.
In 4a and 4b, two and four independent mononuclear enti-
ties comprise the asymmetric unit, respectively, with the
dinuclear species arising from association via non-crystallo-
graphic centres of inversion. It can be seen from the overlay
diagram, Fig. 7, drawn with Qmol,84 that while the pyridyl resi-
dues are virtually superimposable in the six independent mole-
cules, differences appear in the dihedral angles formed
between this and the tolyl residues (range = 40.9(4) to 70.4(5)°)
and in the relative orientation of the chloride ligands. The
higher calculated density in 4a (2.214 g cm−3) compared with
that of 4b (2.140 g cm−3) is reflected in the higher packing
index83 for 4a (0.670 cf. 0.648), and suggests that the needles
of 4a are thermodynamically more stable.
In each of [HgCl2L1]2 (1),48 [HgCl2L3]2 (5) and [HgCl2L4]2
(6), the dinuclear molecule is generated by the application of a
centre of inversion. The respective (E)-N-(pyridin-2-ylmethyli-
dene)arylamine ligands in these structures are considerably
less twisted than in the sterically congested complexes 4a and
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Fig. 9 Perspective views of [Hg(NO3)2L4]: (a) the asymmetric unit, and (b) the
[Hg(NO3)2L4]n chains found in the crystal structure of compound 9. The displa-
cement ellipsoids for (a) are drawn at the 30% probability level and H atoms are
shown as small spheres of arbitrary radii.
with L1, 2 each containing L2 and L3, and 18 examples of struc-
tures having L4. In each case, L1–4 was present as a chelating
ligand. There are two closely related structures particularly
worthy of special mention. While the structure of [ZnI2L1]86
resembles that of mononuclear [HgI2L1] (3), by contrast, the
structure of [ZnCl2L1]87 is mononuclear compared with binuc-
lear [HgCl2L1]2 (1), a result correlated with the reduced Lewis
acidity of zinc compared with mercury.
Fig. 8 Perspective views of [Hg(N3)2L4]: (a) two centrosymmetrically-related
azide-bridged units, and (b) the {[Hg(N3)2L4]2}n extended chain found in the
crystal structure of compound 10. Displacement ellipsoids for (a) are drawn at
the 30% probability level and H atoms are shown as small spheres of arbitrary
radii.
The final structure to be described is that of the nitrate ana-
logue of the foregoing series, [Hg(NO3)2L4] (9), which differs
considerably in terms of coordination geometry and mode of
association of anions. Within the asymmetric unit, Fig. 9a, the
mercury atom is chelated by the (E)-N-(pyridin-2-ylmethyli-
dene)arylamine and two nitrate ligands, one, containing the
N3 atom, symmetrically and the other (with N4) asymmetri-
cally, as reflected in the disparate Hg–O bond lengths and Hg–
O–N bond angles in the latter, Table 4. The N3-nitrate forms
an additional two interactions with a symmetry-related (glide
operation) mercury atom and serves to link the molecules into
a zig-zag chain along the c-axis, Fig. 9b. The range of Hg–O
bond lengths about the mercury atom spans nearly 0.75 Å, and
the angles range from an acute 44°, formed by two bidentate
nitrate-O atoms, to a wide 163° formed by the O atoms from
two different nitrate ligands, thereby making assignment of a
specific coordination geometry problematic.
Crystal packing
In the absence of strong structure-directing hydrogen bonding,
the crystal structures of 1–10 may be considered as close
packing of the various supramolecular zero- and one-dimen-
sional aggregates. Despite the close similarity of the molecular
structures, with the exception of 148 and 2, none of the struc-
tures are isomorphous. The crystal packing patterns are dis-
cussed in the same order as for the molecular structures, i.e.
generally in order of increasing nuclearity of the molecular
aggregates.
The crystal structure of 3 comprises loosely associated
dimers of 3 held together by π⋯π interactions [3.705(2) Å]
formed between the pyridyl and benzene rings as detailed in
ESI Fig. S6.† The dimers stack in columns along the b-axis
with no specific interactions between them. To a first approxi-
mation, a similar situation pertains in the crystal structure of 8
with an important difference in that the columns of loosely
associated dimers found in 3 are now connected into supramo-
lecular chains along the b-axis by π⋯π interactions occurring
Before describing the salient features of the crystal packing
of 1–10, a few general observations based on the literature
structures containing L1–L4 should be made. A survey of the
Cambridge Crystallographic Database85 revealed 25 structures
1916 | Dalton Trans., 2013, 42, 1905–1920
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Fig. 10 Unit cell contents for 8 viewed in projection down the c-axis. The π⋯π
interactions between the (N1,C1–C5) and (C7–C12)i rings are indicated as
purple dashed lines [inter-centroid distance = 3.739(4) Å, angle of inclination
between the rings = 4.9(3)° for symmetry operation i: 1 − x, 2 − y, 2 − z] and
those formed between the chelate ring, HgN2C2, and (N1,C1–C5)ii shown as
blue dashed lines [inter-centroid distance = 3.703(4) Å, angle of inclination =
4.5(3)° for ii: 2 − x, 2 − y, 2 − z].
between the chelate and pyridyl rings, as illustrated in Fig. 10;
geometric details characterising these interactions are given in
the figure caption. While not often commented upon, intermo-
lecular interactions involving chelate rings, having metalloaro-
matic character,88 interacting with other chelate rings, other
aromatic rings, and as both acceptors and donors of C–H con-
tacts are attracting increasing attention in the supramolecular
chemistry literature.89 The formation of π(HgN2C2)⋯π(pyridyl)
interactions in 8 but not in 3 is correlated with the better co-
ordinating ability of L4 compared with L1, as commented upon
previously.
Fig. 11 Supramolecular layer in the ab-plane in 2. The π⋯π interactions
between the (N1,C1–C5) and (C7–C12)i rings are indicated as purple dashed
lines [inter-centroid distance = 3.820(4) Å, angle of inclination between the
rings = 9.7(3)° for symmetry operation i: −x, 1 − y, −z] and those formed
between the chelate ring, HgN2C2, and (N1,C1–C5)ii are shown as blue dashed
lines [inter-centroid distance = 3.764(3) Å, angle of inclination = 4.4(3)° for ii:
1 − x, 1 − y, −z]. The C6–H6⋯Br1iii contacts are shown as orange dashed lines
[H6⋯Br1iii = 2.92 Å, C6⋯Br1iii = 3.800(6) Å and angle at H6 = 155° for
iii: −x,1 − y,−z].
Just as the transformation of the mononuclear arrangement
in 3 into a binuclear unit in 2 was correlated with the increase
in electronegativity of Br compared with I, the presence of
π(HgN2C2)⋯π(pyridyl) interactions in 2 but not in 3 is due to
the same reason: the electronegativity difference enhances the
metalloaromatic behaviour of the chelate ring. Owing to the
binuclear nature of the molecules in 2, layers mediated by
these interactions are formed in the ab-plane with additional
stabilisation provided by C–H⋯Br interactions, Fig. 11. A
similar layer arrangement is seen in the crystal structure of 7,
ESI Fig. S8,† but the chelate rings are interacting with the C6
rather than the pyridyl ring of L4, and additional stabilization
by methyl-C–H⋯π(pyridyl) interactions is noted.
arranged to allow the chelate rings to self-associate via π⋯π
interactions, see Fig. 12. The resulting aggregates are con-
nected into a supramolecular layer via π⋯π interactions occur-
ring between pyridyl rings. With the foregoing in mind,
perhaps contrary to expectation, the binuclear molecules in 6
aggregate into a three-dimensional architecture via π⋯π inter-
actions occurring between pyridyl and C6 rings as well as
pyridyl-C–H⋯Cl interactions with no evidence for participation
in intermolecular interactions by the chelate rings, see ESI
Fig. S11.†
The supramolecular chains in 10 are consolidated in the
crystal packing by a combination of π⋯π interactions, occur-
ring between pyridyl and C6 rings, as well as C–H⋯N inter-
actions, see ESI Fig. S12.† Finally, and in contrast to the
packing features described thus far, the crystal structure of 9 is
devoid of π⋯π interactions with the supramolecular chains
sustained in a three-dimensional architecture by a network of
C–H⋯O interactions, see ESI Fig. S13.†
The crystal structure of 148 is isomorphous with that of 2.
While on electronic grounds, greater metalloaromaticity is
anticipated in the polymorphic [HgX2L2]2 structures 4a and 4b,
these are not found owing to the significant deviations from
planarity of the L2 ligands which preclude close approach of
the chelate rings. In 4a, the molecules are connected by
pyridyl-C–H⋯Cl and π⋯π interactions, with the latter occur-
ring between pyridyl and tolyl rings, see ESI Fig. S9.† In 4b, the
π⋯π interactions occur exclusively between pyridyl rings, while
the pyridyl-C–H⋯Cl contacts persist, see ESI Fig. S10.† With
no steric hindrance in 5, chelate rings again feature in the
supramolecular assembly. In this case, the molecules are
Conclusions
The synthesis and structural characterization of a series
of mercury complexes with (E)-N-(pyridin-2-ylmethylidene)-
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arylamine (Chart 1), which differ in the location of the substi-
tuent in the aryl ring and in the type of the counter ion, has
been achieved. These assemble to generate four types of
neutral complexes of formulae: zero-dimensional HgX2L (3, 8),
[HgX2L]2 (1, 2, 4–7), and one dimensional [Hg(NO3)2L]n (9) and
{[Hg(N3)2L]2}n (10) depending on the bridging capacity of the
counter ion. The propensity for the formation of self-
assembled binuclear entities over mononuclear species in the
sequence Cl > Br > I and is related to the electronegativity of
X. The influence of the presence (and position) of the methyl
group in the (E)-N-(pyridin-2-ylmethylidene)arylamine ligands
can promote the formation of π⋯π interactions involving the
five-membered chelate rings in supramolecular assembly.
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Acknowledgements
The financial support of the Department of Science & Techno-
logy, New Delhi, India (Grant No. SR/S1/IC-03/2005,TSBB), the
University Grants Commission, New Delhi, India, through
SAP-DSA, Phase-III and Indo-Swiss Joint Research Programme,
Joint Utilisation of Advanced Facilities (Grant No. JUAF 11,
TSBB, AL) are gratefully acknowledged. Support from the Min-
istry of Higher Education, Malaysia, High-Impact Research
scheme
(UM.C/HIR-MOHE/SC/03)
is
also
gratefully
acknowledged.
24 W.-Y. Wong, G.-L. Lu, L. Liu, J.-X. Shi and Z. Lin,
Eur. J. Inorg. Chem., 2004, 2066–2077.
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