Studies on phenylmercury(II) complexes of nitrogen-sulfur ligands: Synthesis, spectral, structural characterization, TD-DFT and photoluminescent properties

Article abstract of DOI:10.1016/j.poly.2013.08.032

The reactions of phenylmercury(II) acetate with N-phenyl-5-(pyridin-4-yl)- 1,3,4-thiadiazol-2-amine (Hppt) (1) and potassium N(4-methylpiperazine)-1- carbodithioate [K(mpcdt)] yielded [PhHg(ppt)] (2) and [PhHg(mpcdt)] (3). The complexes have been characte

Full text of DOI:10.1016/j.poly.2013.08.032

Polyhedron 65 (2013) 170–180
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Studies on phenylmercury(II) complexes of nitrogen–sulfur ligands:
Synthesis, spectral, structural characterization, TD-DFT and
photoluminescent properties
a
a
c
a,
⇑
b
a,
⇑
A. Bharti , P. Bharati , Ram Dulare , M.K. Bharty , Dheeraj K. Singh , N.K. Singh
a
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
Department of Physics, Banaras Hindu University, Varanasi 221005, India
Department of Chemistry, P.G. College, Ghazipur, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of phenylmercury(II) acetate with N-phenyl-5-(pyridin-4-yl)-1,3,4-thiadiazol-2-amine
Hppt) (1) and potassium N(4-methylpiperazine)-1-carbodithioate [K(mpcdt)] yielded [PhHg(ppt)] (2)
Received 1 April 2013
Accepted 4 August 2013
Available online 20 August 2013
(
1
and [PhHg(mpcdt)] (3). The complexes have been characterized by elemental analyses, IR, UV–Vis,
H
13
and C NMR spectroscopy. The ligand Hppt (1)- and complexes 2 and 3 crystallize in the triclinic, mono-
ꢀ
clinic and orthorhombic system, space group P1, C 2/c and Pbca, respectively. The crystal X-ray studies
Keywords:
revealed that complexes 2 and 3 both form a one dimensional metal–organic structure, with a linear
N–Hg–Ph and S–Hg–Ph core respectively. The most noteworthy features in complex 2 is that the ligand
bound phenylmercury cation is stabilized via intramolecular as well as intermolecular weak Hgꢀ ꢀ ꢀN sec-
Phenylmercury(II) complexes
Thiadiazole complex
Carbodithioate complex
Secondary interactions
Hydrogen bonding
ondary interactions. Crystal structure of complex 2 is also stabilized via weak
p
ꢀ ꢀ ꢀ
is stabilized by intermolecular/intramolecular Hgꢀ ꢀ ꢀS
p interactions. The ligands Hppt and [K(mpcdt)] exhibit green emissions and
p
and C–Hꢀ ꢀ ꢀ
p interac-
tions. The crystal structure of complex
interactions and C–Hꢀ ꢀ ꢀ
3
Supramolecular architecture
complex 2 shows photoluminescence due to the presence of Hgꢀ ꢀ ꢀN interactions, whilst complex 3 does
not show photoluminescence because Hgꢀ ꢀ ꢀS interactions show quenching behaviour in the solid state.
The solution state photoluminescence properties of complex 2 indicate that intermolecular as well as
intramolecular Hgꢀ ꢀ ꢀN interactions persist even in very dilute solution. The geometrical optimization
of Hppt (1), [K(mpcdt)] and complexes 2 and 3 was calculated in the gas phase using density functional
theory (DFT) with the B3LYP hybrid functional, and was used to predict their molecular properties. The
electronic excitation energies and intensities of the six lowest-energy spin allowed transitions were cal-
culated using time dependent density functional theory (TD-DFT).
Ó 2013 Elsevier Ltd. All rights reserved.
1
. Introduction
Organomercury(II) cations have a high affinity for the sulfur do-
carbodithioates have been found to be extremely versatile groups
for the construction of supramolecular arrays, exhibiting lumines-
cence properties and they are also used in dye sensitized solar cells
[7]. Phenylmercury(II) carbodithioate complexes show a bulkiness
of the pendant group in the carbodithioate part of the compound,
which plays an important role in the construction of the supramo-
lecular framework through Hgꢀ ꢀ ꢀS interactions. Weak intermolecu-
lar as well as intramolecular Hgꢀ ꢀ ꢀN secondary interactions and
other non-covalent interactions can determine the solution state
luminescent properties of organometallic compounds [8]. The thi-
ohydantoin ring coordinates to the phenylmercury cation mainly
via its deprotonated N–H group, resulting in a very short Hgꢀ ꢀ ꢀN
distance, while both intermolecular and intramolecular Hgꢀ ꢀ ꢀS
distances are rather long [9]. Two types of ligands were chosen
to stabilize the organomercury(II) cation. The first of these was
N-phenyl-5-(pyridine-4-yl)-1,3,4-thiadiazol-2-amine because it is
a conjugate system possessing a nitrogen donor and its complex
may be stabilized via intermolecular as well as intramolecular
nor atoms present in proteins, peptides and amino acids [1–3].
Hg(II) has an extremely high affinity for complexation with sulfhy-
dryl groups in the blood and tissues of humans where it is bound
by cysteine-containing peptides and proteins. Metal complexes
with nitrogen–sulfur rich ligands are very interesting from the
viewpoint of their electrical conductivity, molecular magnetism,
electrochemical properties, biological processes and optoelectronic
properties [4]. Phenylmercury(II) compounds have gained signifi-
cant attention because of their importance in the preparation of
other organometallics as intermediates in organic chemistry and
their relevance to mercury detoxification [5,6]. Phenylmercury(II)
⇑
Corresponding authors. Tel.: +91 542 6702452.
E-mail addresses: mkbharty@bhu.ac.in (M.K. Bharty), singhnk_bhu@yahoo.com
(
N.K. Singh).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.032

A. Bharti et al. / Polyhedron 65 (2013) 170–180
171
Hgꢀ ꢀ ꢀN interactions, capable of enhancing photoluminescent prop-
erties. The other ligand, N(4-methylpiperazine)-1-carbodithioate,
has no conjugate system, it is a sulfur donor and the complex
may be stabilized via intermolecular and intramolecular Hgꢀ ꢀ ꢀS
interactions, which exhibit photoluminescent quenching proper-
ties. In view of the above, we report herein the synthesis, spectral,
X-ray crystal data, time dependent density functional theory, elec-
trostatic potential mapping and photoluminescent studies of the
phenylmercury(II) complexes [PhHg(ppt)] (2) and [PhHg(mpcdt)]
The reaction mixture was stirred continuously for 40 min and the
separated white solid potassium N (4-methylpiperazine)-1-car-
bodithioate [K(mpcdt)] was filtered, washed with EtOH and air
dried. Yield: 70%; m.p. 205 °C. Anal. Found: C, 33.50; H, 5.13; N,
0
13.08; S, 29.80%. Calc. for C
6 11 2 2
H N S K (215.00): C, 33.48; H, 5.11;
ꢁ1
N, 13.02; S, 29.76%. IR (KBr,
m
cm ):
m
(C@N) 1469,
), 4.38 (s, 3H,
, d, ppm): 200.10 (C@S), 154.05 (C–N),
m(C@S) 970.
1
H NMR (DMSO-d
6
, d, ppm): 2.20–3.06 (m, 8H, CH
2
1
3
CH
3
). C NMR (DMSO-d
6
51.80–54.25 (CH
288.
2
3
), 45.25 (CH ). UV–Vis [kmax, DMSO, nm]: 268,
(3), derived from N-phenyl-5-(pyridine-4-yl)-1,3,4-thiadiazol-2-
amine and N(4-methylpiperazine)-1-carbodithioate, respectively.
2
. Experimental
2.4. Synthesis of [PhHg(ppt)] (2)
2.1. Materials and methods
A methanol–chloroform solution of Hppt (0.254 g, 1 mmol) was
added to a methanol–chloroform solution of phenylmercury(II)
acetate (0.336 g, 1 mmol) and the solution was stirred slowly for
Commercial reagents were used without further purification
and all experiments were carried out in an open atmosphere at
room temperature. Phenyl isothiocyanate, 4-methylpiperazine,
4
h. The resulting clear yellow solution was filtered off and kept
for crystallization at room temperature. Single crystals suitable
for X-ray analysis were obtained on slow evaporation of the sol-
vent over 21 days. Yield: 55%; m.p. 215 °C. Anal. Found: C, 42.91;
2
isonicotinic acid hydrazide (Sigma–Aldrich), CS (SD Fine Chemi-
cals) and KOH (Qualigens) were used as received. The solvents
were dried and distilled before use following the standard proce-
dures. Carbon, hydrogen, nitrogen and sulfur contents were esti-
mated on an Elementar Vario EL III Carlo Erbo 1108. The
electronic spectra were recorded on a SHIMADZU 1700 UV–Vis
spectrophotometer. IR spectra were recorded in the 4000–
H, 2.60; N, 10.51; S, 6.59%. Calc. for C19
H
14HgN
4
S (531.00): C,
(C@N),
6
, d, ppm): [8.86
ꢁ1
4
2.93; H, 2.63; N, 10.54; S, 6.02%. IR (KBr, cm ): 1484m
m
1
6
85 (pyridine), 826 (phenyl). H NMR (DMSO-d
(
2H), 8.19 (2H)] pyridine; [7.60 (2H), 7.37 (1H), 7.11 (2H)] phenyl.
13
C NMR (DMSO-d
C(5,6)] pyridine; [138.60 C(8), 129.44 C(10,12), 120.63 C(11),
20.11 C(9,13)] phenyl; [174.44 C(1), 150.72 C(2)] thiadiazole car-
bons of Hppt. UV–Vis [kmax, DMSO, nm]: 254, 263, 351 (Scheme 1).
6
, d, ppm): [146.05 C(4,7), 138.81 C(3), 123.30
ꢁ1
4
00 cm region as KBr pellets on a Varian Excalibur 3100 FT-IR
1
13
spectrophotometer. H and C NMR spectra were recorded in
DMSO-d on a JEOL AL300 FT NMR spectrometer using TMS as an
internal reference.
1
6
2
.2. Synthesis of N-phenyl-5-(pyridine-4-yl)-1,3,4-thiadiazol-2-amine
2.5. Synthesis of [PhHg(mpcdt)] (3)
(
Hppt) (1)
A solution of [K(mpcdt)] (0.215 g, 1 mmol) in MeOH (10 ml)
was added to a MeOH–CHCl solution (10 ml) of phenylmercury(II)
3
A mixture of isonicotinic acid hydrazide (2.74 g, 20 mmol) and
phenyl isothiocyanate (2.4 ml, 20 mmol) in benzene was refluxed
for 8 h at 80 °C. On cooling the mixture, a white precipitate of 1-
isonicotinoyl-4-phenyl thiosemicarbazide was obtained which
was filtered off, washed with water and diethyl ether, air dried
and crystallized from ethanol. The above prepared 1-isonicoti-
noyl-4-phenyl thiosemicarbazide (2.72 g, 10 mmol) was added
acetate (0.336 g, 1 mmol). This mixture was stirred continuously
for 5 h at room temperature. The resulting clear yellow solution
was filtered off and kept for crystallization. White single crystals
of 3 suitable for X-ray analysis were obtained by slow evaporation
of the above solution over a period of 22 days. Yield: 50%; m.p.
3
C
60 °C. Anal. Found: C, 31.72; H, 4.50; N, 6.21; S, 14.18%. Calc. for
16HgN (453.00): C, 31.78; H, 3.53; N, 6.18; S, 14.12%. IR (m
2 4
slowly to 10 ml conc. H SO and stirred for 2 h at low temperature.
12
H
2
S
2
The mixture was poured over crushed ice and the precipitated so-
lid was filtered off, washed twice with cold water and dried. Yellow
crystals of Hppt (1) suitable for X-ray analysis were obtained by
slow evaporation of the methanol solution over 15 days. Yield:
ꢁ1
1
cm , KBr): 1464
m
(C–N), 912
m
2
(C–S), 443
), 4.10 (s, 3H, CH
aromatic protons). C NMR (DMSO-d , d, ppm): 202.10 (C@S),
28.32–154.36 (phenyl ring), 51.80–54.25 (CH ), 45.25 (CH
Scheme 2). UV–Vis [kmax, DMSO, nm]: 258, 297.
m
(Hg–S). H NMR (CDCl
3
,
d, ppm): 2.32–2.52 (m, 8H, CH
3
) 7.11–7.67 (m, 5H,
1
3
6
1
(
2
3
)
8
C
1
5%. Anal. Found: C, 61.35; H, 3.96; N, 22.20; S, 12.65%. Calc. for
13
10 4
H N
S (254.31): C, 61.34; H, 3.93; N, 22.02; S, 12.58%; m.p.
ꢁ1
40 °C; IR (KBr, cm ): 3003m
dine), 826 (phenyl). H NMR (DMSO-d
m
(NH), 1484m
, d, ppm): [8.86 (2H), 8.19
2H)] pyridine; [7.60 (2H), 7.37 (1H), 7.11 (2H)] phenyl; 4.84
m(C@N), 685 (pyri-
1
6
(
(
2
.6. X-ray crystallography
1
3
6
1H, NH). C NMR (DMSO-d , d, ppm): [146.42 C(4,7), 140.01
C(3), 122.93 C(5,6)] pyridine; [141.58 C(8), 129.30 C(10,12),
22.93 C(11), 122.07 C(9,13)] phenyl; [174.62 C(1), 153.88 C(2)]
thiadiazole carbons of Hppt. UV–Vis [kmax, DMSO, nm]: 319, 334,
50. The synthesis of the ligand Hppt has been reported [10] but
Crystals suitable for X-ray analyses of compounds 1, 2 and 3
were grown at room temperature. The crystal data were collected
on an Oxford Gemini diffractometer equipped with a CrysAlis CCD
1
3
software using a graphite monochromated Mo K
a (k = 0.71073 Å)
its characterization by single crystal X-ray was not available.
radiation source at 293 K for 1, 2 and 3. A multi-scan absorption
correction was applied to the X-ray data collection for all the com-
pounds. The structures were solved by direct methods (SHELXS-08)
0
2
.3. Synthesis of potassium N (4-methylpiperazine)-1-carbodithioate
2
[K(mpcdt)]
and refined against all data by full matrix least-square on F using
anisotropic displacement parameters for all non-hydrogen atoms.
All hydrogen atoms were included in the refinement at geometri-
cally ideal positions and refined with a riding model [11]. The MER-
CURY package and the ORTEP-3 for Windows program were used for
generating the structures [12,13].
0
The potassium N (4-methylpiperazine)-1-carbodithioate was
prepared by the addition of CS (1.5 ml, 20 mmol) dropwise to a
suspension of 4-methylpiperazine (2.20 ml, 20 mmol) in methanol
20 ml) in the presence of potassium hydroxide (1.2 g, 20 mmol).
2
(

1
72
A. Bharti et al. / Polyhedron 65 (2013) 170–180
O
O
H
N
H
N
N
NH2
PhNCS
Benzene,Reflux
H
N
H
N
S
N
Conc.H SO
2
4
NaOH
N
5
4
S
PhHgOAc
9
H
N
N
8
N
3
S
1
1
0
2
MeOH-CHCl3
N
N
Hg
11
7
6
N
N
13
12
Scheme 1. Synthesis of N-phenyl-5-(pyridine-4-yl)-1,3,4-thiadiazol-2-amine (1) and its PhHg(II) complex 2.
S
CS , KOH(Alc.)
2
N
H C
3
N
H C
3
N
NH
0
-5 ºC, MeOH
SK
MeOH-CHCl
3
PhHgOAc
S
Hg
N
S
N
H C
3
Scheme 2. Synthesis of potassium N-(4-methylpiperazine)-1-carbodithioate (mpcdt) and its PhHg(II) complex 3.
3
. Results and discussion
.1. General aspects
The ligand
3.2. Spectroscopy
3
The IR spectrum of the ligand N-phenyl-5-(pyridine-4-yl)-1,3,4-
thiadiazol-2-amine (Hppt) (1) in KBr shows absorption bands at
ꢁ1
N-phenyl-5-(pyridine-4-yl)-1,3,4-thiadiazol-2-
3003, 1484, 1070 and 829 cm due to the stretching modes of
N–H, C@N, N–N and C–S, respectively. In the complex [PhHg(ppt)]
(2), the disappearance of the NH band and the appearance of a new
amine was synthesized by the cyclization of 1-isonicotinoyl-4-phe-
nyl thiosemicarbazide in the presence of strong acid followed by
careful neutralization with NaOH solution at the low temperature
of 0–5 °C. The methanol–chloroform solution of the ligand N-phe-
nyl-5-(pyridine-4-yl)-1,3,4-thiadiazol-2-amine (Hppt) (1) reacts
with phenylmercury(II) acetate to form a yellow coloured clear
solution which gives colourless crystals of the complex [PhHg(ppt)]
ꢁ
1
band at 485 cm due to an M–N stretching vibration [14] indi-
cates that the phenylmercury cation is bonded through the depro-
tonated NH group of the ligand. The IR spectrum of potassium
0
N (4-methylpiperazine)-1-carbodithioate in KBr shows absorp-
tions due to the stretching modes of
m
(C–N) and
m(C@S) at 1469
ꢁ1
(2), whereas a methanol–chloroform solution of phenylmercury(II)
and 970 cm , respectively. Whilst in the complex [PhHg(mpcdt)]
ꢁ
1
acetate on reaction with a methanolic suspension of potassium
N (4-methylpiperazine)-1-carbodithioate yields a colourless clear
solution which forms single crystals of the complex [PhHg(mpcdt)]
(3), the appearance of a m(C@S) band at 912 cm indicates that
the carbodithioate sulfur is involved in bonding with the phenyl-
0
1
mercury cation. All the compounds show well resolved H NMR
1
(3). The ligands and complexes are stabilized via various types of
signals. The H NMR spectrum of Hppt shows a signal at d
interactions, such as C–Hꢀ ꢀ ꢀ , hydrogen bonding and weak Hgꢀ ꢀ ꢀS
p
4.84 ppm due to the NH proton. The protons due to the phenyl ring
appear as multiplets between 7.60 and 7.11 ppm, and the pyridine
ring between 8.86 and 8.19 ppm. The signals at d 174.62 and
or Hgꢀ ꢀ ꢀN secondary interactions. All the compounds are air-stable
1
13
and were characterized by elemental analyses, IR, H and C NMR
spectroscopy, and their structural elucidation is achieved by single
crystal X-ray diffraction techniques. Complexes 2 and 3 are insolu-
ble in methanol, ethanol and chloroform but are soluble in DMSO,
and they melt at 215 and 360 °C, respectively.
1
3
153.88 ppm in the C NMR spectrum are attributed to the >C–S
(C1) and >C@N (C2) carbons, respectively, of the thiadiazole ring.
1
In the H NMR spectrum of [PhHg(ppt)] (2), the disappearance of
NH proton at 4.84 ppm shows bonding of the deprotonated

A. Bharti et al. / Polyhedron 65 (2013) 170–180
173
Table 1
TD-DFT calculated electronic transitions for the complex [PhHg(ppt)] (2).
3.3. Electronic absorption and photoluminescent properties
Excitation
energy
Wavelength
(nm)
Oscillator
strength
Composition
Major
contribution
The experimentally observed absorption bands of the com-
plexes [PhHg(ppt)] (2) and [PhHg(mpcdt)] (3) have been explained
with the help of TD-DFT calculations. The vertical excitation ener-
gies, oscillator strength and tentative nature of the transitions ob-
tained at the TD-DFT level have been presented in Tables 1 and 2.
The optimized geometries of the ligands Hppt and [K(mpcdt)], and
their complexes [PhHg(ppt)] (2) and [PhHg(mpcdt)] (3), are shown
in Fig. 1. Since each absorption line in a TD-DFT spectrum is due to
several single excitations, a depiction of the transition character is
generally not straightforward. Calculations indicate that the com-
plex [PhHg(ppt)] (2) shows six bands at 642, 434, 365, 355, 271
and 268 nm (experimentally observed at 254, 263 and 351 nm in
DMSO solution). Energy bands calculated at 642 and 434 nm with
oscillator strengths (f) 0.7559 and 0.8519 are due to HOMO-
ꢁ1 ? LUMO and HOMO ? LUMO+1 electronic excitations, and
are attributed to electron transfer from the nitrogen atom of the li-
gand to the d-orbital of metal ion. The other two bands calculated
at 365 and 355 nm with oscillator strengths (f) 0.6425 and 0.6812,
respectively, are assigned to the charge transfer transitions from
(
eV)
⁄
⁄
⁄
1.9292
2.8043
2.8522
3.2103
3.3943
3.4849
4.1938
4.3480
4.5591
4.6221
642.66
442.12
434.70
386.21
365.27
355.77
295.64
285.16
271.95
268.24
0.75599
0.60031
0.85190
0.76996
0.64257
0.68121
0.77377
0.99711
0.74963
0.76864
HOMOꢁ1 ? LUMO
HOMO ? LUMO+3
HOMO ? LUMO+1
HOMOꢁ2 ? LUMO
p
p
p
?
?
?
p
p
p
⁄
n ?
p
HOMOꢁ1 ? LUMO+1 LMCT
HOMOꢁ1 ? LUMO+3 LMCT
⁄
⁄
HOMOꢁ1 ? LUMO+4 n ?
HOMOꢁ1 ? LUMO+5 n ?
p
p
HOMOꢁ3 ? LUMO
HOMOꢁ4 ? LUMO
LLCT
LLCT
Table 2
TD-DFT calculated electronic transitions for the complex [PhHg(mpcdt)] (3).
Excitation
energy
Wavelength
(nm)
Oscillator
strength
Composition
Major
contribution
(
eV)
⁄
⁄
3.9679
4.3922
4.7364
4.8279
4.8780
312.47
282.28
261.77
256.81
254.17
0.75951
0.58086
0.56893
0.68312
0.70516
HOMO ? LUMO
p
p
?
?
p
p
the coordinated nitrogen atom and p-electron clouds of the aro-
matic ring to the metal d-orbital. Additionally, the absorptions cal-
culated at 271 and 268 nm with oscillator strengths (f) 0.7496 and
HOMO ? LUMO+1
HOMOꢁ1 ? LUMO
LLCT
⁄
HOMO+1 ? LUMO+2 n ?
p
0
.7686, respectively, are suggested to be ligand-to-ligand (LLCT)
HOMOꢁ3 ? LUMO
ILCT
and intraligand (ILCT) charge transfer transitions, which are listed
in Table 1 and Fig. 2. In the case of the complex [PhHg(mpcdt)] (3),
the electronic absorption spectrum displayed three bands at 312,
261 and 254 nm (experimentally observed at 258 and 297 nm in
DMSO solution). Quantum chemical calculations reveal that the
first lower energy band, calculated at 312 nm with an oscillator
strength (f) of 0.5903, can be assigned to the HOMO ? LUMO elec-
tron transfer from the coordinated sulfur atoms of the carbodithio-
ate ligand with a slight intermingling of the metal d-electrons to
the aromatic ring. The remaining lower energy bands, calculated
at 261 and 254 nm having oscillator strengths of (f) 0.5689 and
0.7051, respectively, arise from ligand to ligand (LLCT) and intral-
igand (ILCT) charge transfers (Table 2 and Fig. 3). The calculated
and observed bands are in good agreement for both complexes.
nitrogen with the phenylmercury(II) cation in complex 2. The 13C
NMR spectrum of complex 2 shows various signals for carbon
atoms, of which the signals at d 174.44 and 150.72 ppm are due
1
to the >C–S and >C@N carbons, respectively. The H NMR spectrum
of [K(mpcdt)] exhibits three signals at d 2.20, 3.06 and 4.38 ppm for
1
the methylene and methyl protons of the piperazine ring. In the H
1
3
and C NMR spectra of the complex [PhHg(mpcdt)] (3), the
appearance of signals in the aromatic region suggests complexa-
tion of the carbodithioate ligand with the phenylmercury(II) ion.
An upfield shift of 2.0 ppm in the CS
2
carbon as compared to the
free carbodithioate ligand indicates Hg–S bonding in complex 3.
(
a)
(b)
(
c)
(d)
Fig. 1. (a) Optimized geometry of Hppt. (b) Optimized geometry of [PhHg(ppt)]. (c) Optimized geometry of K[mpcdt]. (d) Optimized geometry of [PhHg(mpcdt)].

1
74
A. Bharti et al. / Polyhedron 65 (2013) 170–180
Fig. 2. Electronic transitions of selected molecular orbitals for complex 2. An isosurface value of 0.02 e/Å3 was used for the orbitals.
Fig. 3. Electronic transitions of selected molecular orbitals for complex 3. An isosurface value of 0.02 e/Å3 was used for the orbitals.

A. Bharti et al. / Polyhedron 65 (2013) 170–180
175
ꢁ
5
Fig. 4. (a) UV–Vis spectra of [PhHg(ppt)] (2) and [PhHg(mpcdt)] (3) at 1 ꢂ 10 M concentration in DMSO solution. (b) The solution state photoluminescent emission spectra
ꢁ
3
ꢁ5
of complex 2 at various (1 ꢂ 10 –1 ꢂ 10 M) concentrations in DMSO solution. (c) Solid state photoluminescent emission spectra of Hppt and [PhHg(ppt)] (2). (d) Solid
state photoluminescent emission spectrum of the ligand [K(mpcdt)].
ꢁ
3
ꢁ5
The photoluminescent properties of the ligands Hppt and
K(mpcdt)] and their phenylmercury(II) complexes were examined
(1 ꢂ 10 –1 ꢂ 10 M) resulted in slight blue shift [15] and a de-
[
crease in intensity at various concentrations due to weakening of
in the solid state at room temperature (30 °C). The ligands Hppt
and [K(mpcdt)] displayed a photoluminescent emission at 566
and 530 nm upon excitation with a 355 and 268 nm laser source.
The main chromosphere of the ligand Hppt is the aromatic five-
membered thiadiazole ring, and its conjugation degree is further
enhanced by the pyridine and phenyl rings. For Hppt, the strong
emission is ascribed to an interligand charge transfer transition
the
p p interactions, but the luminescent properties persist even
ꢀ ꢀ ꢀ
in very dilute solution (Fig. 4) due to the existence of intermolecu-
lar as well as intramolecular Hgꢀ ꢀ ꢀN secondary interactions. The
green emission of Hppt, [K(mpcdt)] and complex 2 in the solid
state implies that these compounds may be potentially applicable
as materials for light emitting diode devices [16].
3.4. Electrostatic potential mapping
from the HOMO (
localized on the thiadiazole ring, which is further supported by
the existence of interactions in the crystal structure. The
p) residing on the phenyl ring to the LUMO (p)
In order to validate and to understand the complex formation of
pꢀ ꢀ ꢀp
the ligands with phenyl mercury, molecular electrostatic potential
ESP) mappings were performed. The molecular electrostatic po-
complex [PhHg(ppt)] (2), on excitation with a 355 nm laser source
at room temperature, resulted in a slight blue shift (4 nm) com-
pared to the free ligand Hppt. This is presumably due to a slight de-
(
tential (ESP) at a point r in the space around a molecule is given
by (in atomic units):
crease of the
p electron density on the thiadiazole ring when the
Z
0
ligand coordinates to the phenylmercury cation. Complex 2 exhib-
its a slightly lower luminescent property as compared to the ligand
Hppt, with respect to intensity and energy. The origin of the solid
state luminescence emissions of the ligand is suggested to be
mainly due to interligand charge transfer transitions (ILCT), but
in its complex this is mainly from ligand to metal charge transfer
transitions (LMCT). Complex 3 shows no photoluminescence,
which indicates that quenching behaviour is observed in this com-
plex. The solution state photoluminescence properties of complex
X
Z
A
q
ðrÞðdr Þ
VðrÞ ¼
ꢁ
0
jR
A
ꢁ rj
jr ꢁ rj
0
A A
where Z is the charge on nucleus A located at R and q(r ) is the
electronic density. The first term in the expression represents the
effect of the nuclei and the second represents that of the electrons.
The electrostatic potential correlates with the dipole moment, elec-
tronegativity, partial charges and the site of chemical reactivity of
the molecule. It provides a visual method to understand the relative
polarity of a molecule. The electron density isosurface on to which
2
in DMSO indicates that increasing the dilution of solution

1
76
A. Bharti et al. / Polyhedron 65 (2013) 170–180
Fig. 5. Electrostatic potential plotted at the van der Waals surfaces for Hppt, [K(mpcdt)] and their complexes 2 and 3, calculated at the B3LYP level of theory. Red regions –
most negative electrostatic potential; blue regions – most positive electrostatic potential; green regions – zero potential. (Colour online.)
Table 3
Crystallographic data for compounds 1, 2 and 3.
Parameters
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
T (K)
C
13
H
10
N
4
S
C
19
H
14HgN
4
S
C
12 2 2
H16HgN S
254.31
monoclinic
C2/c
531.00
monoclinic
P2 /n
293(2)
0.71073
5.0490(9)
17.817(3)
18.873(3)
90.00
95.451(3)
90.00
1690.0(5)
4
453.00
orthorhombic
Pbca
293(2)
0.71073
7.5586(3)
14.7254(7)
26.2893(12)
90
90
90
1
293(2)
k, Mo K
a (Å)
a
(Å)
0.71073
27.245(2)
5.5866(7)
16.5438(13)
90.00
104.945(6)
90.00
3649.7(5)
8
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
2926.1(2)
8
2.057
Z
q
l
3
calc. (g/cm )
1.389
0.252
2.087
9.239
ꢁ
1
(mm
)
10.786
F(000)
1056
1008
1712
Crystal size (mm)
h range for data collections (°)
Index ranges
0.35 ꢂ 0.25 ꢂ 0.17
3.08–29.03
ꢁ36 6 h 6 28
0.27 ꢂ 0.25 ꢂ 0.23
2.17–28.47
ꢁ6 6 h 6 6
ꢁ18 6 k 6 23
ꢁ25 6 l 6 16
4272
0.23 ꢂ 0.21 ꢂ 0.19
2.77–32.02
ꢁ9 6 h 6 611
ꢁ21 6 k 6 21
ꢁ39 6 l 6 39
5076
ꢁ
ꢁ
6 6 k 6 7
22 6 l 6 22
2883
No. of reflections collected
No. of independent reflections (Rint
)
2124
3116
2816
No. of data/restrains/parameters
2883/0/163
1.045
0.0614, 0.1146
0.0408, 0.1075
0.274, ꢁ0.161
4272/0/179
1.009
0.0326, 0.0833
0.0636, 0.1256
0.924, ꢁ0.645
5076/0/155
1.000
0.0442, 0.1386
0.1020, 0.1704
1.952, ꢁ1.492
2
Goodness-of-fit on F
a
a
b
b
R
R
1
, wR
, wR
2
[(I > 2
(all data)
r
(I)]
1
2
Largest difference in peak/hole (e Åꢁ3)
P
P
a
R
R
1
=
||F
o
| – |Fc|| |F
o
|
P
2
P
1=2
b
2
2
2
2
o
2
¼ ½ wðjF j ꢁ jF jÞ = wjF j ꢃ
o
c
:
the electrostatic potential surface has been mapped is shown in
Fig. 5 for the ligands and their complexes with phenyl mercury.
Such surfaces depict the size, shape, charge density and site of
chemical reactivity of the molecules. The different values of the
electrostatic potential at the surface are represented by different
colours; red regions – the most negative electrostatic potential, blue
regions – the most positive electrostatic potential, and green re-
gions – zero potential. The potential increases in the order

A. Bharti et al. / Polyhedron 65 (2013) 170–180
177
Table 4
Interatomic distances (Å) and angles (°) for Hppt (1).
Expt.
Bond length (Å)
Calc.
Expt.
Calc.
Bond angles (°)
C(2)–S(1)–C(1)
N(2)–C(1)–N(1)
N(2)–C(1)–S(1)
N(1)–C(1)–S(1)
C(1)–N(2)–N(3)
N(3)–C(2)–C(3)
N(3)–C(2)–S(1)
C(3)–C(2)–S(1)
S(1)–C(2)
S(1)–C(1)
N(1)–C(1)
N(1)–C(8)
N(3)–C(2)
N(3)–N(2)
C(1)–N(2)
C(3)–C(2)
1.741(16)
1.735(14)
1.345(19)
1.400(19)
1.298(19)
1.364(18)
1.317(19)
1.463(2)
1.7762
1.7552
1.3639
1.4061
1.2983
1.3520
1.3140
1.4641
87.18(7)
85.98
119.72(13)
112.92(11)
127.36(12)
113.08(12)
122.75(15)
113.40(12)
123.84(11)
120.20
110.78
125.68
112.87
121.34
114.08
121.45
Fig. 6. Ortep diagram of Hppt (1) showing the atom numbering scheme. The
thermal ellipsoids are plotted at the 30% probability level.
Table 5
Interatomic distances (Å) and angles (°) for [PhHg(ppt)] (2).
Expt. Calc.
Bond length (Å)
Expt.
Calc.
Bond angles (°)
Hg(1)–C(14) 2.065(13) 2.0320 C(14)–Hg(1)–N(1)
173.0(4)
87.2(6)
112.3(12) 110.08
106.8(8)
123.8(9)
170.16
85.60
Hg(1)–N(1)
S(1)–C(2)
S(1)–C(1)
N(2)–C(1)
N(2)–N(3)
N(3)–C(2)
2.130(10) 2.1110 C(2)–S(1)–C(1)
1.730(14) 1.7727 C(1)–N(2)–N(3)
1.750(14) 1.8296 C(1)–N(1)–Hg(1)
1.320(18) 1.3595 C(8)–N(1)–Hg(1)
1.343(16) 1.3279 C(15)–C(14)–Hg(1) 122.4(10) 121.09
1.472(4) 1.3098 C(19)–C(14)–Hg(1) 122.2(10) 121.04
92.94
138.25
Table 6
Interatomic distances (Å) and angles (°) for [PhHg(mpcdt)] (3).
Expt. Calc. Expt.
Bond length (Å)
Calc.
Bond angles (°)
2.0578 C(12)–Hg(1)–S(1)
2.395(17) 2.7000 C(12)–Hg(1)–S(2)
Hg(1)–C(12) 2.070(7)
165.9(17) 164.06
Hg(1)–S(1)
Hg(1)–S(2)
S(1)–C(1)
S(2)–C(1)
N(1)–C(1)
N(1)–C(2)
N(3)–C(5)
122.2(2)
65.7(5)
96.3(2)
77.5(2)
146.36
63.08
86.25
3.007(2)
1.746(7)
1.691(6)
1.327(8)
1.481(8)
2.8100 S(1)–Hg(1)–S(2)
1.7428 C(1)–S(1)–Hg(1)
1.7434 C(1)–S(2)–Hg(1)
86.36
1.3435 C(11)–C(12)–Hg(1) 121.4(5)
120.70
117.98
120.08
1.3980 C(7)–C(12)–Hg(1)
119.6(6)
119.9(4)
1.463(11) 1.3896 S(2)–C(1)–S(1)
Fig. 7. Showing N–Hꢀ ꢀ ꢀN hydrogen bonding and C–Hꢀ ꢀ ꢀ
p interactions leading to the
supramolecular structure in 1.
red < orange < yellow < green < blue. While the negative electro-
static potential corresponds to an attraction of a proton by the con-
centrated electron density in the molecule (colours in shades of
red), the positive electrostatic potential corresponds to repulsion
of a proton by atomic nuclei in regions where low electron density
exists and the nuclear charge is incompletely shielded (colours in
shades of blue). These ligands contain nitrogen and sulfur atoms
and the shape of the electrostatic potential surface is influenced
by the charge density distributions in the molecules with sites close
to the nitrogen and sulfur showing regions of the most negative
electrostatic potential. In the case of the ligands, the N and S atoms
are electronegative, so they show the most negative electrostatic
potential (very close to red colour). However, in the case of the com-
plexes, a significant amount of charge is redistributed (Fig. 5). In the
complex [PhHg(ppt)] (2) the electron density is distributed over the
molecule homogeneously, while in complex [PhHg(mpcdt)] (3) the
electron density is mainly concentrated on the nitrogen and sulfur
atoms of the carbodithioate ligand and the central part of the phen-
ylmercury cation.
Fig. 8. Ortep diagram of [PhHg(ppt)] (2) showing the atom numbering scheme. The
thermal ellipsoids are plotted at the 30% probability level.
Table 7
Hydrogen bond parameters [Å and °] in Hppt (1).
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
<(DHA)
Symmetry equivalent operators
N1–H1ꢀ ꢀ ꢀN2A
0.859
0.861
2.061
2.047
2.917
2.903
174.07
173.55
2 ꢁ x, ꢁ1 ꢁ y, 1 ꢁ z
2 ꢁ x, ꢁ1 ꢁ y, 1 ꢁ z
N1A–H1Aꢀ ꢀ ꢀN2

1
78
A. Bharti et al. / Polyhedron 65 (2013) 170–180
Fig. 11. Ortep diagram of [PhHg(mpcdt)] (3) showing the atom numbering scheme.
The thermal ellipsoids are plotted at the 30% probability level.
Table 3 and selected bond distances and bond angles in Tables
Fig. 9. ‘‘Head-to-tail’’ dimeric structure for 2 displaying weak intramolecular and
intermolecular Hgꢀ ꢀ ꢀN secondary interactions.
4–6. Hydrogen bond parameters for Hppt (1) are given in Table 7.
3.6.1. Crystal structure description of Hppt (1)
3.5. Computational details
Fig. 6 shows an ORTEP diagram of the ligand Hppt (1) together
with the atom numbering scheme, and selected bond distances and
bond angles are listed in Table 4 along with theoretically predicted
data at B3LYP level of theory. The molecular structure of Hppt (1)
shows that the dihedral angles between the thiadiazole–pyridine
and thiadiazole–phenyl rings are 2.05° and 5.06° respectively, indi-
cating that these pairs are almost coplanar to each other. Bond
lengths and angles in the pyridine and thiadiazole rings are nor-
mal. The endocyclic S(1)–C(1) (1.735(14) Å) and S(1)–C(2)
(1.741(16) Å) bond lengths are comparable and are found to be
longer than a typical carbon sulfur double-bond, S@C (1.56 Å)
[22], which indicates that 1-isonicotinoyl-4-phenyl thiosemicarba-
zide is converted to the corresponding 1,3,4-thiadiazole. This is
further supported by the N(2)–C(1) and N(3)–C(2) bond lengths
of 1.317(19) and 1.298(19) Å, which come in the range of N@C
double bonds (1.27 Å) [23]. The structure is stabilized by an inter-
molecular N–Hꢀ ꢀ ꢀN interaction between NH of the amine group
and the nitrogen of the thiadiazole ring. The values found for the
N–Hꢀ ꢀ ꢀN interaction are close to the bond distances and bond an-
gles reported earlier [24,25]. In the crystal of Hppt (1), inversion di-
mers are formed via a pair of N–Hꢀ ꢀ ꢀN hydrogen bonds. There are
All the computational calculations were performed using the
GAUSSIAN 09 programming package [17]. The geometrical optimiza-
tion of the ligands and complexes 2 and 3 were calculated in the
gas phase using density functional theory (DFT) with the B3LYP hy-
brid functional for predicting molecular properties [18]. B3LYP is a
hybrid functional consisting of Becke’s exchange functional, the
Lee–Yang–Parr correlation functional and a HF exchange term. It
is well known fact that DFT, using a non-local and gradient cor-
rected functional, performs as equally well as other correlated
methods, such as calculations at the MP2 level of theory [19]. In
studies reported recently [20], DFT calculations with the B3LYP
method showed a nice correlation with experimental results and
several meaningful conclusions were drawn from those calcula-
tions. In the present study, the Lanl2dz basis set [21] was em-
ployed for the heavy Hg atom and the 6-311++g(d, p) basis set
for the C, H, N and S atom containing molecules in all the calcula-
tions. The global minima on the potential energy surface were con-
firmed by the real harmonic vibrational wavenumbers calculated
for each calculated molecular geometry. The optimized structures
of the complexes at DFT level were used for molecular orbital anal-
yses and vertical electronic spectra calculations. The electronic
excitation energies and intensities of the six lowest-energy spin
allowed transitions were calculated using the time dependent den-
sity functional theory (TD-DFT). In addition to this, the electrostatic
potential mapping surfaces were also plotted for a more clear pre-
sentation regarding the charge distributions.
two C–Hꢀ ꢀ ꢀ
p
interactions (3.366 Å) occurring between CH of a phe-
electrons of the thiadiazole ring (Fig. 7).
nyl ring and
p
3.6.2. Crystal structure description of [PhHg(ppt)] (2)
Fig. 8 shows an ORTEP diagram of the complex [PhHg(ppt)] (2)
together with the atom numbering scheme, and it shows a monode-
protonated ligand bonded to PhHg(II) via the thiadiazole nitrogen
atom. The environment around Hg(II) is almost linear, having a
C(14)–Hg–N(1) bond angle of 172.99(4)° and Hg–N(1) bond length
of 2.130(10) Å. The dihedral angle between the thiadiazole and pyr-
idine rings is found to be 7.06°, indicating that both rings are almost
planar, whilst the dihedral angle of 9.87° between the phenyl and
3
.6. Crystal structure description
The crystallographic data and structural refinement details for
Hppt (1), [PhHg(ppt)] (2) and [PhHg(mpcdt)] (3) are given in
Fig. 10. (a) Showing the C–Hꢀ ꢀ ꢀ
p interaction occurring between a hydrogen of a phenyl ring and p electrons of the thiadiazole ring: (b) weak pꢀ ꢀ ꢀp stacking between
pyridine–thiadiazole and phenyl–thiadiazole ring centroids in [PhHg(ppt)] (2).

A. Bharti et al. / Polyhedron 65 (2013) 170–180
179
Fig. 12. Two intermolecular C–Hꢀ ꢀ ꢀ
p
interactions occurring between a hydrogen atom of a piperazine ring and p electrons of a phenyl ring in [PhHg(mpcdt)] (3).
with the experimental data (Table 6). The complex [PhHg(mpcdt)]
3) contains a monodeprotonated ligand which is coordinated to
(
the metal via the dithio sulfur atom, and the geometry around
Hg(II) is linear. The coordination environment around Hg(II) is ful-
filled by the ipso-C atom of the phenyl group and the S(1) atom of
the carbodithioate ligand. The C–S bond distances of 1.746(7) and
1
.691(6) Å (Table 6) in complex 3 agree well with those in other re-
lated compounds, being intermediate between C–S single (1.82 Å)
and C@S double (1.56 Å) bond distances [22]. The dihedral angle
between the piperazine ring and phenyl ring attached to mercury
is found to be 63.61° indicating that the phenyl ring is oriented to-
wards the piperazine ring. The Hg–S(1) bond length is 2.395(17) Å,
whilst that of Hg–S(2) is 3.007(2) Å, which suggests a weaker inter-
actions between Hg and S(2), thereby reflecting the propensity of
Hg(II) to exist in a linear coordination. The proximity of the atom
S(2) is partly responsible for the deviation from the ideal linear
geometry, as seen in the (Ph)C–Hg–S(1) bond angle of
Fig. 13. ‘‘Head-to-tail’’ dimeric structure for 3 displaying weak intramolecular and
intermolecular Hgꢀ ꢀ ꢀS secondary interactions.
1
65.91(17)°. The bond length and angle are in close agreement
with those reported for other similar compounds, with a greater
deviation from linearity than in complex 3 [7,28]. This may be
attributed to the somewhat weaker intermolecular Hgꢀ ꢀ ꢀS second-
thiadiazole rings again suggests that both the rings are also copla-
nar. The dihedral angle between the thiadiazole ring and the phenyl
ring attached to mercury is 54.50°, suggesting that the phenyl ring
is oriented towards the thiadiazole ring. In complex 2, there is a
ary interaction. There are two intermolecular C–Hꢀ ꢀ ꢀ
p interactions
(3.265 and 3.318 Å) occurring between CH of piperazine and
p
electrons of a phenyl ring (Fig. 12). The crystal structure is further
stabilized by weak intermolecular and intramolecular Hgꢀ ꢀ ꢀS inter-
actions occurring between the dithio sulfur atom and Hg(II)
(Fig. 13). Within the carbodithioate moiety, the C–N bond length
weak intramolecular Hgꢀ ꢀ ꢀNthia interaction at
a distance of
2
.771(9) Å. In addition, complex 2 also contains an intermolecular
Hgꢀ ꢀ ꢀNPy secondary interaction at a distance of 2.821(2) Å. Both
Hgꢀ ꢀ ꢀN distances are longer than the covalent bond length, but
are well within the sum of the van der Waals’ radii of the respective
elements (rvdw(Hg) = 1.73–2.00 Å and rvdw(N) = 1.55 Å). Intermolec-
ular and intramolecular Hgꢀ ꢀ ꢀN interactions stabilize the structure
of the complex (Fig. 9) [26]. The crystal packing of complex 2 shows
of 1.326(3) Å indicates substantial delocalization of the
density [29].
p-electron
4. Conclusions
a weak
p
ꢀ ꢀ ꢀ
p
interaction (3.974 Å) between the phenyl ring and the
This paper reports on the syntheses and crystal structures of two
novel complexes, [PhHg(ppt)] (2) and [PhHg(mpcdt)] (3), of N-phe-
nyl-5-(pyridin-4-yl)-1,3,4-thiadiazol-2-amine (Hppt) (1) and
thiadiazole ring of a nearby molecule (Fig. 10a) [27]. The geometry
and bonding parameters agree with those of other related phenyl-
mercury complexes [7,28]. In addition, the structure is stabilized
0
potassium N (4-methylpiperazine)-1-carbodithioate [K(mpcdt)]. It
by a C–Hꢀ ꢀ ꢀ
and -electrons of a thiadiazole ring of another unit of the complex
Fig. 10b). To get a clear picture of the binding site and geometry we
p
interaction (3.516 Å) between CH of a phenyl ring
has been observed that the replacement of an aliphatic or aromatic
hydrocarbon moiety by a heteroaromatic group such as pyridine
makes the ligand able to bind with PhHg(II) thiadiazole via new sec-
ondary Hgꢀ ꢀ ꢀN interactions. PhHg(II) carbodithioate involves a new
secondary Hgꢀ ꢀ ꢀS interaction. The crystal structures of the ligand 1
p
(
have performed geometry optimization for complex 2 at the B3LYP
density functional theory level. The computed optimized structure
of 2 corroborates the geometrical parameters obtained from the
crystal structure analysis (Table 5). The results are therefore
strongly supported by the present theoretical predictions.
and complexes 2 and 3 are stabilized by C–Hꢀ ꢀ ꢀ
p
interactions. In
addition, complex 2 is stabilized by weak
pꢀ ꢀ ꢀ
p interactions. The so-
lid state photoluminescent properties indicated that the presence
of Hgꢀ ꢀ ꢀS interactions quenches the chances of photoemission,
whereas a Hgꢀ ꢀ ꢀNPy intermolecular interaction retains or increases
the probability. The electronic excitation energies and intensities
of the lowest-energy spin allowed transitions were calculated using
time dependent density functional theory (TD-DFT) and they were
found to match the experimental results, with just slight deviations.
3.6.3. Crystal structure description of [PhHg(mpcdt)] (3)
Fig. 11 shows the ORTEP diagram of the complex [PhHg(mpcdt)]
(3) together with the atom numbering scheme. The geometry opti-
mization of complex 3 at the B3LYP level of theory revealed that
the bond distances and bond angles are in very good agreement

1
80
A. Bharti et al. / Polyhedron 65 (2013) 170–180
The electrostatic potential mapping surfaces plotted, regarding the
charge distributions, indicated a redistribution of charge from the
ligand to the metal in the complexes.
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Acknowledgements
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2008) 466.
A. Bharti thanks CSIR, New Delhi for the award of a JRF and Dr.
M.K. Bharty is thankful to the Department of Science and Technol-
ogy, New Delhi, India for the award of a Young Scientist Project
[
[
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[
[
Appendix A. Supplementary data
(
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Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth,
P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman,
J.V. Ortiz, J. Cioslowski, D.J. Fox, GAUSSIAN 09, Gaussian, Inc., Wallingford, CT,
2009.
CCDC 786907, 786908, and 875786 contains the supplementary
crystallographic data for Hppt (1), [PhHg(ppt)] (2) and
[
PhHg(mpcdt)] (3), respectively. 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.
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