Synthesis, characterization, and calculated electronic structure of the crystalline metal-organic polymers [Hg(SC6H4S)(en)] n and [Pb(SC6H4S)(dien)]n

Article abstract of DOI:10.1021/ic201779a

The reaction of Hg(OAc)₂ with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg(SC₆H₄S)(en)] _n, while the reaction of Pb(OAc)₂ with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [P

Full text of DOI:10.1021/ic201779a

Article
pubs.acs.org/IC
Synthesis, Characterization, and Calculated Electronic Structure of
the Crystalline Metal−Organic Polymers [Hg(SC6H4S)(en)]n and
[Pb(SC6H4S)(dien)]n
Dayna L. Turner,†,⊥ Kevin H. Stone,‡,∥ Peter W. Stephens,‡ Aron Walsh,§ Mayank Pratap Singh,†
and Thomas P. Vaid*,†
†Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
‡Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, United States
§Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Claverton Down,
Bath BA2 7AY, U.K.
S
* Supporting Information
ABSTRACT: The reaction of Hg(OAc)2 with 1,4-benzene-
dithiol in ethylenediamine at 80 °C yields [Hg(SC6H4S)-
(en)]n, while the reaction of Pb(OAc)2 with 1,4-benzenedi-
thiol in diethylenetriamine at 130 °C yields [Pb(SC6H4S)-
(dien)]n. Both products are crystalline materials, and structure
determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal−organic
polymers with -M-SC6H4S- repeat units. Diffuse reflectance UV−visible spectroscopy indicates band gaps of 2.89 eV for
[Hg(SC6H4S)(en)]n and 2.54 eV for [Pb(SC6H4S)(dien)]n, while density functional theory (DFT) band structure calculations
yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-
benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant
electrical conductivity as a pressed pellet. In the case of [Pb(SC6H4S)(dien)]n calculations indicate fairly flat bands and therefore
low carrier mobilities, while the conduction band of [Hg(SC6H4S)(en)]n does have moderate dispersion and a calculated
electron effective mass of 0.29·me. Hybridization of the empty Hg 6s orbital with SC6H4S orbitals in the conduction band leads to
the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors.
inorganic components of these polymers have been studied
separately as electrically conducting molecules: the 1,4-
benzenedithiolate linker is probably the most-studied entity
in single-molecule conductivity investigations,12−15 and, more
recently, the single-molecule conductivity of small metal-
chalcogenide clusters has been reported.16 The polymers
[Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n can be viewed
as infinite arrays of metal atoms linked by 1,4-benzenedithio-
late, or, alternatively, as the smallest possible HgS or PbS units
linked by phenylene bridges. The physical characterization and
band-structure calculation of these simple polymers give insight
into their electronic structure and suggest modifications to
achieve better electrical conductivity in new metal-chalcoge-
nide-organic network materials.
INTRODUCTION
■
Hybrid organic−inorganic materials have been receiving
increasing attention because they offer structures and properties
not attainable in purely organic or inorganic materials. The most
prominent class of hybrid materials is metal−organic frame-
works (MOFs),1−5 which often have very high porosities and
are being examined as materials for gas storage,6 separation,7
and sensing.8 The vast majority of well-defined hybrid organic−
inorganic materials (including MOFs) are electrical insulators
and are diamagnetic or paramagnetic at room temperature, and
hybrid materials with interesting electrical or magnetic proper-
ties are therefore ripe for further investigation.9 In known hybrid
materials there is generally poor electronic communication
between metal centers across the organic bridges, and one
reason may be the large electronegativity difference in the
metal−oxygen and metal−nitrogen linkages that form those
networks. We have been studying hybrid materials that contain
instead metal−sulfur and metal−selenium linkages in an effort
to find semiconducting materials.10,11
EXPERIMENTAL SECTION
■
General Synthetic Procedures and Materials. Reactions were
carried out under N2 in degassed ethylenediamine or degassed
diethylenetriamine; the products are air-stable for periods of at least a
few hours, and the workup and subsequent handling was done without
protection from air, although the products were then stored in a
nitrogen-filled glovebox. Reagents were purchased from commercial
The compounds reported in this paper, [Hg(SC6H4S)(en)]n
and [Pb(SC6H4S)(dien)]n (SC6H4S is 1,4-benzenedithiolate,
en is ethylenediamine, and dien is diethylenetriamine), are one-
dimensional polymers, and therefore represent particularly
simple examples of the metal-chalcogen-organic hybrid
materials that we have been investigating. The organic and
Received: August 17, 2011
Published: December 2, 2011
© 2011 American Chemical Society
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Article
suppliers and used as received. The reagent 1,4-benzenedithiol was
prepared from 1,4-bis(isopropylthio)benzene,17 which was reduced by
electrical conductivity at 60 °C. However, both compounds
were completely amorphous and could not be structurally
characterized. The use of ethylenediamine or diethylenetri-
amine as a solvent, as described below, leads to crystalline
products.
sodium in liquid ammonia to yield 1,4-benzenedithiol.18
[Hg(SC6H4S)(en)]n. Mercury acetate (224 mg, 0.703 mmol) and
1,4-benzenedithiol (100 mg, 0.703 mmol) were combined in 10 mL of
ethylenediamine. The suspension was heated to 80 °C for 16 h and
then cooled to room temperature. The pale yellow precipitate was
isolated by filtration and washed with methanol and ether. Yield of
[Hg(SC6H4S)(en)]n: 218 mg, 77%. IR (Nujol, cm−1): 3324 (m), 3272
(m), 1582 (s), 1125 (w), 1096 (s), 1084 (w), 1012 (s), 974 (s), 946
(s), 824, (s), 666 (w). Anal. Calc. for C8H12N2S2Hg: C, 23.97; H, 3.02;
N, 6.99. Found: C, 23.16; H, 1.79; N, 5.39.
[Pb(SC6H4S)(dien)]n. Lead(II) acetate trihydrate (133 mg, 0.351
mmol) and 1,4-benzenedithiol (50 mg, 0.352 mmol) were combined
in 12 mL of diethylenetriamine. The suspension was heated to 130 °C
for 16 h and then cooled to room temperature. The bright yellow solid
was isolated by filtration and washed with methanol and then
diethylether. Yield of [Pb(SC6H4S)(dien)]n: 144 mg, 91%. IR (Nujol,
cm−1): 3356 (w), 3326 (w), 3192 (m), 3107 (w), 1570 (w), 1316 (w),
1227 (w), 1139 (w), 1095 (s), 1006 (w), 990 (w), 899 (s), 867 (w),
818 (m), 668 (w). Anal. Calcd for C10H17N3S2Pb: C, 26.66; H, 3.80;
N, 9.33. Found: C, 26.63; H, 3.77; N, 9.01.
Room-temperature solid-state conductivity of pressed powders was
measured as described previously,19 in a two-electrode configuration
on a lab-built apparatus that consists of a Delrin block with a 6.35 mm
diameter cylindrical hole and two 6.35 mm copper cylindrical contacts.
The material to be studied was inserted between the two contacts
within the Delrin block and a mass of 3.85 kg was placed on the top
contact, thus applying 12 bar pressure.
Diffuse reflectance UV−visible−NIR spectra were obtained on a
Shimadzu UV-3600 spectrometer with a Harrick Praying Mantis
accessory. A Spectralon disk was used as the reflectance standard.
Percent reflectance was converted to absorbance by the formula abs =
log10(100/%R).
Synchrotron X-ray Diffraction. The crystal structures of both
materials were solved from high resolution synchrotron X-ray powder
diffraction data collected at beamline X16C at the National
Synchrotron Light Source at Brookhaven National Laboratory at
ambient temperature. Both materials were loaded into thin walled glass
capillaries, illuminated with X-rays of ∼0.7 Å wavelength, and spun
during data collection for improved powder averaging. The diffracted
beam was analyzed with a Ge(111) crystal and detected by a NaI
scintillation counter. Structures were indexed, solved by simulated
annealing, and Rietveld refined using the TOPAS-Academic
software.20
Computational Techniques. Electronic structure calculations
were performed for [Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n
at the level of density functional theory (DFT)21,22 within the
generalized gradient approximation (the PBEsol exchange-correlation
functional)23 as implemented in the all-electron code FHI-AIMS.24,25
Periodic boundary conditions in three dimensions were employed to
represent the infinite solid.26 A well converged “tight” numerical basis
set was used to construct the electronic wave function, with scalar
relativistic effects treated at the scaled-ZORA level of theory.27
Comparison of the calculated electronic structure of a Hg(II)
compound, HgO, arising from this approach with results from a
previous DFT study using a plane-wave basis set28 showed excellent
agreement, and reproduced the measured X-ray photoemission
spectra.29 The equilibrium (T = 0 K) structural parameters for both
[Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n were determined
through minimization of the quantum mechanical forces and stresses
to within 0.005 eV/Å.
The synthesis of [Hg(SC6H4S)(en)]n is similar to that of
[Pb3(SC6H4S)3(en)2]n,10 [Cd(en)3][Cd(SC6H4S)2],11 and
[Zn(en)3][Zn(SC6H4S)2]11 reported previously. The reaction
of Hg(OAc)2 with 1,4-benzenedithiol in ethylenediamine at
80 °C yields a pale yellow precipitate of [Hg(SC6H4S)(en)]n.
The synthesis of [Pb(SC6H4S)(dien)]n is quite similar to that
of [Pb3(SC6H4S)3(en)2]n, with the substitution of diethylenetri-
amine for ethylenediamine as the solvent. The reaction of
Pb(OAc)2·3H2O with 1,4-benzenedithiol in diethylenetriamine
at 130 °C yields the bright yellow solid [Pb(SC6H4S)(dien)]n.
Both compounds are stable in air over short periods: the reaction
workups were performed without protection from air, while the
isolated products were stored in a nitrogen-filled glovebox.
Crystal Structures. The powder diffraction pattern of
[Hg(SC6H4S)(en)]n was indexed to a c-centered monoclinic
unit cell with lattice parameters of a = 13.147 Å, b = 7.121 Å,
c = 11.630 Å, and β = 102.61°. Peak absences suggested space
group Cc or C2/c. Initial structure solution by simulated
annealing supported C2/c, with the Hg atom on a 2-fold axis
and the center of the benzenedithiolate on an inversion center.
Subsequent Rietveld refinement showed that this description is
correct. In the refinement (Rietveld fit shown in Supporting
Information, Figure S1), the benzenedithiolate was treated as a
regular hexagon with the sulfurs constrained along the axis, with
two internal degrees of freedom for the C−C and C−S bond
lengths. The Hg atom and the ethylenediamine were treated as
a single z-matrix with the N and C atoms restrained within the
range of literature bond distances and angles. Crystal data is
given in Table 1, along with lattice parameters calculated by
DFT (see below).
Table 1. Crystal Data, Including Experimental and
Calculated (DFT) Lattice Parameters for
[Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n
formula
Mr
C8H12N2HgS2
400.91
C10H17N3PbS2
450.61
space group
Z
C2/c (Hall −C2yc)
4
2.506 g cm−3
synchrotron, λ = 0.70014 Å
3° to 36° (0.003°)
8.364%
P1 (Hall -P1)
̅
2
Dx
2.217 g cm−3
synchrotron, λ = 0.69702 Å
3° to 40° (0.01°)
4.678%
radiation
data range
Rwp
Rexp
6.385%
2.68%
RBragg
χ2
2.615%
1.872%
1.716
3.049
lattice
parameters
experimental
DFT
experimental
DFT
a
b
c
13.1472(4)
7.1208(3)
11.6303(4)
90
13.078
7.121
11.617
90
7.3106(3) Å
7.292
10.293
10.288
61.18
86.28
85.84
10.2825(4) Å
10.2809(4) Å
61.186(3) deg
86.117(3) deg
86.053(3) deg
α
β
γ
RESULTS AND DISCUSSION
■
102.606(2)
90
101.57
90
Synthesis. The compounds [Hg(SC6H4S)]n and [Pb-
(SC6H4S)]n (without any donor ligands) were reported in
1975.30 [Hg(SC6H4S)]n (yellow) was synthesized in 2,4-lutidine,
and [Pb(SC6H4S)]n (brown) was synthesized in a water−
methanol mixture. [Hg(SC6H4S)]n did display a small amount of
The structure of [Hg(SC6H4S)(en)]n consists of one-
dimensional polymers of Hg(en) linked by SC6H4S units, as
shown in Figure 1. The polymers run along the [101]
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vs 5.037%, with the same number of refined atomic coordinates.
There is no “smoking gun” proving that the triclinic model is
correct, but we regard the improvement in quality of fit as
compelling. The triclinic lattice parameters listed in Table 1 span
a close approximation to the original monoclinic lattice (a′ = b +
c, b′ = b − c, c′ = a→ a′ = 17.701 Å, b′ = 10.465 Å, c′ = 7.311 Å,
α′ = 89.94°, β′ = 94.55°, γ′ = 90.01°).
Figure 1. Structure of [Hg(SC6H4S)(en)]n determined by powder X-
ray diffraction.
Powder diffraction data has limited ability to resolve the exact
configuration of the dien moiety wrapped around the Pb atom,
and indeed even restrained refinements had unacceptably close
contacts between adjacent molecules. Accordingly, the Pb-
diethylenetriamine configuration determined by DFT calcu-
lations (see below) was used as a rigid body in the refinement
(Rietveld fit shown in Supporting Information, Figure S2).
The structure of [Pb(SC6H4S)(dien)]n is more complex than
that of [Hg(SC6H4S)(en)]n. The coordination sphere of Pb in
[Pb(SC6H4S)(dien)]n is shown in Figure 3. Each Pb atom is
crystallographic direction. There are no significant interchain
interactions: the shortest interchain S−S distance is 3.671(5) Å
and the shortest interchain Hg−S distance is 5.088(4) Å.
Within a polymer chain the Hg−S bond distance is 2.382(3) Å,
slightly longer than the average of 2.34 Å for 48 reported
Hg(SR)2 complexes.31
Structure solution for [Pb(SC6H4S)(dien)]n was less
straightforward. The diffraction pattern was originally indexed
to a c-centered monoclinic unit cell with lattice parameters of
a
mono = 17.703 Å, bmono = 10.465 Å, cmono = 7.310 Å, and βmono =
94.55°. A structure with benzenedithiol on an inversion center
and Pb on a 2-fold axis in space group C2/c gives a fair
approximation to the measured diffraction pattern. However, in
this case, the Pb atom coordinated by diethylenetriamine
cannot be accommodated on a 2-fold axis. This implies either a
partially disordered dien molecule in C2/c, or a solution in Cc
with every atom on a general position, that is, no crystallo-
graphic symmetries aligned with molecular point symmetries.
The Cc solution (Z = 4 formula units in the unit cell, Z′ = 1 in
the irreducible volume) had slightly better refined profile Rwp
factor, but refinements of the dien group were not stable.
Furthermore, there were conspicuous problems visible in the
comparison of calculated and experimental data, as can be seen
in Figure 2 (top).
Figure 3. Coordination sphere of Pb in [Pb(SC6H4S)(dien)]n.
coordinated by the three N atoms of a dien molecule. There are
four sulfur atoms within a distance of Pb that indicates some
interaction, with Pb−S distances of 2.96(1), 3.07(2), 3.27(1),
and 3.65(2) Å. If the two shorter Pb−S interactions at 2.96 and
3.07 Å are considered to be regular covalent bonds (shown as
solid lines in Figure 3) and the two longer Pb−S interactions at
3.27 and 3.65 Å are considered to be Pb←S dative bonds
(shown as dashed lines in Figure 3), then a bonding scheme
that includes only the Pb−S regular covalent bonds yields the
one-dimensional polymer shown in Figure 4. Those chains run
Figure 4. One-dimensional chains in [Pb(SC6H4S)(dien)]n.
along the [-111] crystallographic direction. If both the short
and the long Pb−S interactions are considered, then chains of
corner-sharing Pb2S2 parallelograms are also present, as shown
in Figure 5. In Figure 5 the 4-sulfur coordination sphere of each
Pb is evident, with the two dative Pb−S bonds oriented
“forward” and “backward” on alternating Pb atoms. The corner-
sharing Pb2S2 parallelogram chains run along the [100]
crystallographic direction.
Figure 2. Comparison of a limited region of Rietveld fits for
[Pb(SC H S)(dien)] in the Cc space group (top) and P1 space group
̅
6
4
n
(bottom). Experimental (black dots) versus calculated (red line)
diffracted intensity, also showing peak markers and difference plots.
The observation of a number of features where the computed
peak positions did not exactly match the observed values led us
to suspect a small distortion to a lower, triclinic symmetry.
Indeed, it was found that a solution in a triclinic cell of half the
volume (Z = 2, Z′ = 1) gave a slightly improved fit: Rwp = 4.678%
Spectroscopy, Band Gap, and Conductivity. Figure 6
shows the diffuse reflectance UV−visible spectrum of [Hg-
(SC6H4S)(en)]n, and Figure 7 shows the diffuse reflectance
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determination of the band gap from the spectrum of
[Hg(SC6H4S)(en)]n somewhat uncertain, and its band gap
may be somewhat less than 2.89 eV.
Because of the relatively large band gaps for the two
compounds, any electrical conductivity of the undoped
materials was expected to be small. Simple two-point
measurements of pressed powders of [Pb(SC6H4S)(dien)]n
and [Hg(SC6H4S)(en)]n were performed. The conductivity of
both compounds is less than 1 × 10−12 Ω−1cm−1, the limit of
detection of our experimental setup.
Electronic Structure Calculations and Their Relation-
ship to the Structure and Properties of [Hg(SC6H4S)-
(en)]n and [Pb(SC6H4S)(dien)]n. The relatively simple one-
dimensional structures of [Hg(SC6H4S)(en)]n and [Pb-
(SC6H4S)(dien)]n make them good materials in which to
examine correlations between structure/composition and
electronic structure. We therefore performed DFT band
structure calculations on both [Hg(SC6H4S)(en)]n and [Pb-
(SC6H4S)(dien)]n.
The crystallographically determined structures of [Hg-
(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n were used as
starting points for a structure optimization in the DFT
calculations. The crystallographic space group of each was
maintained, while the lattice parameters (and, of course, atomic
positions) were allowed to optimize. In both cases the
calculated structures were very similar to the crystallographic
ones. In Table 1 the crystallographic and calculated lattice
parameters of both compounds can be compared. Table 2 gives
Figure 5. Corner-sharing Pb2S2 parallelograms in [Pb(SC6H4S)-
(dien)]n, with short Pb−S interactions shown as solid bonds and long
(dative) Pb−S interactions shown as dashed bonds.
Table 2. Crystallographic and Calculated Bond Distances
[Å] for [Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n
experimental
calculated
Hg−S
Hg−N
Pb−S1 (axial)
Pb−S1b (axial)
Pb−S1 (equatorial)
Pb−S1b (equatorial)
2.382(3)
2.566(9)
2.96(1)
3.07(2)
3.27(1)
3.65(2)
2.432
2.496
2.876
3.095
3.334
3.402
Figure 6. Diffuse reflectance UV−visible spectrum of [Hg(SC6H4S)(en)]n.
the selected bond distances from the crystal structure and
calculations for both [Hg(SC6H4S)(en)]n and [Pb(SC6H4S)-
(dien)]n.
The calculated electronic band structure and density of states
for [Pb(SC6H4S)(dien)]n and [Hg(SC6H4S)(en)]n are plotted
in Figures 8 and 9, respectively. The band gap calculated for
[Hg(SC6H4S)(en)]n is 2.24 eV and for and [Pb(SC6H4S)-
(dien)]n is 2.10 eV. The calculated values are 0.65 and 0.44 eV
smaller, respectively, than those determined by diffuse
reflectance spectroscopy; DFT-calculated band gaps are
typically lower than the experimental values.32 Additionally, as
mentioned above, the band gap of [Hg(SC6H4S)(en)]n may in
fact be somewhat lower than the 2.89 eV determined by diffuse
reflectance spectroscopy.
In [Pb(SC6H4S)(dien)]n sulfur states, and, to a lesser extent,
carbon states, make the major contribution to the top of the
valence band and carbon states dominate the bottom of the
conduction band, as can be seen in Figure 8. In Figure 10 the
electron density associated with the highest occupied and
lowest unoccupied states (at the T point of the Brillouin zone;
points in the Brillouin zone are defined here33) of [Pb-
(SC6H4S)(dien)]n are shown. The electron density around the
SC6H4S fragment at the valence band edge bears a striking
Figure 7. Diffuse reflectance UV−visible spectrum of [Pb(SC6H4S)-
(dien)]n.
UV−visible spectrum of [Pb(SC6H4S)(dien)]n. Extrapolation
of the absorbance edge to the baseline indicates a band gap of
2.89 eV for [Hg(SC6H4S)(en)]n and 2.54 eV for [Pb(SC6H4S)-
(dien)]n. The absorption onset for [Hg(SC6H4S)(en)]n is more
gradual than that of [Pb(SC6H4S)(dien)]n, which makes the
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Figure 8. Calculated electronic band structure and density of states of
[Pb(SC6H4S)(dien)]n. The highest occupied state is set to 0 eV.
Figure 10. Calculated electron density (square of the electronic wave
function) associated with the highest occupied (bottom) and lowest
unoccupied (top) states at the T point of the Brillouin zone for
[Pb(SC6H4S)(dien)]n.
Figure 9. Calculated electronic band structure and density of states of
[Hg(SC6H4S)(en)]n. The highest occupied state is set to 0 eV.
−
resemblance to the HOMO of an isolated SC6H4S− dianion,
as determined by a simple Huckel calculation (see Figure 12).
̈
Figure 11. Calculated electron density (square of the electronic wave
function) associated with the highest occupied (bottom) and lowest
unoccupied (top) states at the M point of the Brillouin zone for
[Hg(SC6H4S)(en)]n.
Similarly, the electron density around the SC6H4S fragment at
the conduction band edge resembles the LUMO of an isolated
−SC6H4S− dianion, as shown in Figure 12. Thus it appears that
the electronic structure of [Pb(SC6H4S)(dien)]n can be fairly
well described as the sum of the ionic components Pb(dien)2+
bands at the T point are too flat to extract a meaningful
electron or hole effective mass.
−
and −SC6H4S−, with the SC6H4S− HOMO and LUMO states
dominant at the band edges. The Pb states in the calculated
band structure are also consistent with the presence of Pb2+:
there are filled Pb 6s states near the bottom of the upper
valence band (at about −8 eV) and Pb 6p states in the
conduction band.34 Because the states at both the valence and
In some hybrid inorganic−organic materials that contain a
semiconducting component, the electron or hole movement
occurs only through the inorganic component, while the
organic component is a passive insulator. The prime example of
such materials are the ME(da)0.5 (M = Cd, Zn; E = S, Se, Te;
da = diamine such as ethylenediamine) layered com-
pounds.35−39 In [Pb(SC6H4S)(dien)]n there are purely
inorganic corner-sharing Pb2S2 parallelogram chains, as shown
−
the conduction band edges are mainly due to SC6H4S− units,
which do not interact significantly with each other, the band
dispersions in [Pb(SC6H4S)(dien)]n are quite narrow; the
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semiconductors, and indicates that electron conduction may be
feasible in this class of materials. For comparison, the isotropic
electron effective mass in HgS is 0.10·me.47 Any free electrons
in [Hg(SC6H4S)(en)]n will therefore have fairly good mobility
along the polymer chains, and the low conductivity of
[Hg(SC6H4S)(en)]n is due to the very low carrier concen-
tration in this wide-bandgap, undoped semiconductor.
CONCLUSIONS
■
In [Pb(SC6H4S)(dien)]n the electronic states at the band edges
are essentially the HOMO and LUMO of the −SC6H4S−
dianion, so the band gap of [Pb(SC6H4S)(dien)]n is largely
determined by the HOMO−LUMO gap of the SC6H4S
organosulfur fragment. In contrast, in [Hg(SC6H4S)(en)]n
the empty Hg 6s orbital is a major component of the
conduction band edge. The Hg 6s orbital hybridizes with the
SC6H4S fragment in the conduction band, leading to moderate
band dispersion and, theoretically, reasonable electron mobility
along the polymer chains. However, the hybridization of the Hg
6s orbital with the SC6H4S fragment leads to a somewhat larger
band gap for [Hg(SC6H4S)(en)]n. Hybrid materials with higher
conductivity are likely to be obtained with organic components
with smaller HOMO−LUMO gaps, generally leading to smaller
band gaps in the hybrid materials. The selection of metal will
also be critical, and should be chosen such that unoccupied
metal orbitals will have energies near the conduction band edge
and/or occupied metal orbitals will have energies near the
valence band edge.
Figure 12. HOMO (left) and LUMO (right) of the −SC6H4S−
dianion, calculated by the Huckel method.
̈
in Figure 5. Those chains run along the [-111] direction. An
examination of the bands at the R point, which corresponds to
the [-111] direction, shows no significant band dispersion, nor
a larger contribution from Pb at the band edges. Thus it seems
that extended polymers of an inorganic component within a
hybrid material do not necessarily lead to semiconductivity
through those inorganic polymers. That conclusion is
consistent with observations by us and others in systems that
contain purely inorganic networks (if dative bonds are
included). For example, the compound [Pb3(SC6H4S)3(en)2]n
contains Pb−S infinite two-dimensional networks,10 while
[Tl2(SC6H4S)]n and [Tl2(SeC6H4Se)]n contain Tl−S and
Tl−Se infinite two-dimensional networks,40 yet none of these
compounds has significant electrical conductivity at room
temperature. The simple metal thiolates Cd(SPh)2,41 Pb-
(SPh)2,42 Pb(SCH2CH2S),43 and AgSR44 (R = n-alkyl or aryl)
all contain infinite M-S networks, yet none are reported to be
semiconducting. Similarly, cadmium thiolate clusters linked by
RS− (R = aryl) anions to form infinite networks are known,45,46
but no electrical conductivity has been reported for any of those
materials.
ASSOCIATED CONTENT
* Supporting Information
■
S
Rietveld refinement plots for [Hg(SC6H4S)(en)]n and [Pb-
(SC6H4S)(dien)]n, DFT-calculated structures for [Hg-
(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n as text (in CIF
format) in the SI PDF, and CIFs for the structures of
[Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n determined by
X-ray diffraction. This material is available free of charge via the
Internet at http://pubs.acs.org.
In [Hg(SC6H4S)(en)]n the main atomic contributions to the
top of the valence band are from sulfur and carbon (see Figure
9), as in [Pb(SC6H4S)(dien)]n. The electron density associated
with the highest occupied state (at the M point) is shown in
Figure 11, and the electron density around the SC6H4S
AUTHOR INFORMATION
Corresponding Author
*E-mail: tpvaid@ua.edu.
■
resembles the Huckel-calculated HOMO of the −SC6H4S−
Present Addresses
̈
⊥Novus International, 20 Research Park Drive, St. Charles,
dianion, just as in [Pb(SC6H4S)(dien)]n. In contrast, the states
near the conduction band edge (at M) are quite different from
those in [Pb(SC6H4S)(dien)]n. In Figure 9 the states at the
band edge include significant contributions from carbon, sulfur,
and mercury, in stark contrast to [Pb(SC6H4S)(dien)]n, where
carbon dominates the states at the conduction band edge (see
Figure 8). The (empty) electron density on mercury at the
conduction band edge at the M point is visible in Figure 11,
right panel. The electron density consists, essentially, of the Hg
6s orbital, which is empty in Hg2+, perturbed to the side of the
atom opposite the en ligand.
MO 63304.
∥Materials Sciences Division, Lawrence Berkeley Lab, Berkeley,
CA 94720.
ACKNOWLEDGMENTS
■
This work was supported by the University of Alabama.
Computations were performed on the University of Bath’s
High Performance Computing Facility, and facilitated by
membership of the UK's HPC Materials Chemistry Con-
sortium, which is funded by EPSRC (Grant No. EP/F067496).
Use of the National Synchrotron Light Source, Brookhaven
National Laboratory, was supported by the U.S. Department of
Energy, Office of Basic Energy Sciences, under Contract No.
DE-AC02-98CH10886.
The presence of the empty Hg 6s atomic orbital leads to
hybridization between the metal and organic components in
[Hg(SC6H4S)(en)]n, and thereby some moderate band
dispersion in its conduction band, especially around the M
point. The M point of the Brillouin zone corresponds to the
polymer chain orientation in real space. Fitting the conduction
band dispersion around the M point yields an averaged electron
effective mass of 0.29·me, which falls into the range of typical
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