Acylselenoureato bis(chelates) of lead: Synthesis, structural characterization and microwave-assisted formation of PbSe nano- A nd microstructures

Article abstract of DOI:10.1039/d0nj01433h

We report the synthesis of three lead(ii) bis(acylselenoureato) complexes (1a-3a), which were prepared by the reaction of the respective acylselenourea compounds ArC(O)NHC(Se)NR₂ [Ar = 4-MeC₆H₄, R = Et (1); 4-ClC₆

Full text of DOI:10.1039/d0nj01433h

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DOI: 10.1039/D0NJ01433H.
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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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Acylselenoureato bis(chelates) of lead: Synthesis, structural
characterization and microwave-assisted formation of PbSe nano-
and microstructures
Received 00th January 20xx,
Accepted 00th January 20xx
Karsten Klauke,a,b Alexa Schmitz,b Ann-Christin Swertz,a Björn B. Beele,a Beatriz Giesen,b Carsten
DOI: 10.1039/x0xx00000x
Schlüsener,b Christoph Janiakb,* and Fabian Mohra,*
We report the synthesis of three lead(II) bis(acylselenoureato) complexes (1a – 3a), which were prepared by the reaction
of the respective acylselenourea compounds ArC(O)NHC(Se)NR2 [Ar = 4-MeC6H4, R = Et (1); 4-ClC6H4, R = nBu (2), iBu (3)]
with Pb(OAc)2. All three compounds were confirmed by NMR spectroscopy, elemental analysis and single crystal X-ray
diffraction. Furthermore, the lead(II) complex 3a was transformed into PbSe nanostructures in a microwave-assisted
synthesis using either a mixture of trioctylphosphine (TOP), octadecene (ODC) and the ionic liquid [BMIm][NTf2], a mixture
of [BMIm][NTf2] and propylene carbonate (PC), pure [BMIm][NTf2] or pure PC. The analyses of the resulting particle
dispersions indicated formation of either small PbSe nanoparticles of 19 nm, PbSe submicro-cubes between 178 nm and
366 nm as well as strongly intergrown structures depending on the stabilizing reagent used during the synthesis. The
prepared PbSe particle suspensions were characterized by powder X-ray diffraction (PXRD), transmission electron
microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectrocopy (EDX) as well as by selected
area electron diffraction (SAED).
towards the development of easy to handle single-source
precursors, allowing a convenient and intrinsic control of the
stoichiometry and reactivity. In this respect, the group of the
late O'Brien reported the use of lead(II) acylselenoureas in the
aerosol-assisted preparation of PbSe thin films as well as the
synthesis of PbSe nanoparticles using trioctylphosphine (TOP)
as stabilizing reagent.6,7,8,9,10
Introduction
As an IV-VI compound semiconductor, lead selenide is one
of the oldest known semiconductor materials which has been
used for more than 100 years for the production of electronic
and optoelectronic components.1 Despite the toxicity of lead
compounds, the excellent thermoelectric and semiconducting
properties of PbSe still make it a very sought-after material for
modern applications such as LEDs,2 photovoltaic cells3 or
photodetectors.4 The aforementioned properties include an
advantageous bulk bandgap of 0.28 eV, high charge carrier
mobilities and a particularly strong quantum confinement due
to relatively large Bohr radii of about 23, 23 and 46 nm for
electron, hole and exciton, respectively.5 Especially the
phenomenon of multi-exciton generation (MEG), which was
first observed in PbSe quantum dots in 2004, has recently
generated great interest.3
Ionic liquids represent an interesting and by now quite
established option for the stabilization of colloidal systems
besides conventional stabilizing agents such as TOP or
hexadecylamine (HDA), which are strongly bound to the
particle surface.11,12,13 Because of this, nanoparticles are
electrosterically stabilized by various directed intermolecular
interactions, or in other words, by a combination of steric and
electrostatic effects. Although a large number of inorganic
nanostructures has been prepared in ionic liquids, including
PbS,14,15,16,17 to our knowledge only very few syntheses of PbSe
nanoparticles using this novel group of compounds have been
described in the literature.18 Therefore, on basis of O’Brien’s
work6,7,8,9,10 and our experience with microwave-assisted
synthesis of various nanoparticles in ionic liquids, we report
herein a study of acylselenoureato lead(II) complexes as
potential precursors compounds for PbSe nanomaterials in
ionic liquids as well as the effect of [BMIm][NTf2] on particle
shape and size compared to TOP-stabilized nanoparticles.
The conventional route to PbSe typically involves chemical
vapour deposition processes using volatile (gaseous) and
highly toxic precursors. Efforts have thus been directed
a. Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie,
Bergische Universität Wuppertal, 42119 Wuppertal, Germany.
b. Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität
Düsseldorf, 40204 Düsseldorf, Germany.
† Electronic Supplementary Information (ESI) available: Thermal analysis, further
PbSe particle characterization, crystallographic solution and refinement details.
CCDC 1586974–1586978. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/x0xx00000x
Results and Discussion
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Synthesis, spectroscopic and structural characterization
[Pb{ArC(O)NC(Se)NR2}2] (1a – 3a) as yellowish, air- and
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moisture stable solids in good yields (SchDeOmI:e101.)1.039/D0NJ01433H
The acylselenourea compounds ArC(O)NHC(Se)NR2 [Ar = 4-
MeC6H4, R = Et (1); 4-ClC6H4, R = nBu (2), iBu (3)] were
prepared following the method by Douglass from the
corresponding acyl chlorides, KSeCN and the secondary amines
in acetone (Scheme 1).19 The compounds were obtained in low
to moderate yields as yellow, crystalline solids.
Characterization by NMR spectroscopy unambiguously
confirmed the identity and purity of the compounds. The
signal for the N-H proton can be observed in the 1H-NMR
spectra as a sharp signal at around 8-9 ppm. The 77Se-NMR
spectra of the compounds feature singlet resonances in the
range of 480 – 500 ppm. The two new acylselenoureas 4-
R
N
NR'2
Se
O
1. KSeCN
2. R'2NH
Se
O
O
Pb(OAc)2
NaOAc
Cl
Pb
NR'2
N
H
R
O
Se
NR'2
R
N
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R = Me, R' = Et (1)
R = Cl, R' = nBu (2)
R = Cl, R' = iBu (3)
R
R = Me, R' = Et (1a)
R = Cl, R' = nBu (2a)
R = Cl, R' = iBu (3a)
Scheme 1. Synthetic methodology for the preparation of the lead(II)
bis(acylselenoureato) complexes (1a – 3a)
ClC6H4C(O)NHC(Se)NR2 [R
=
nBu (2), iBu (3)] were also
The compounds were fully characterized by NMR
spectroscopy and also by X-ray diffraction. From the proton
NMR spectra of the complexes it is apparent that the signal
due to the N-H proton has disappeared, consistent with
deprotonation of the acylselenourea upon coordination to the
metal. The observed signals in the proton NMR spectra of the
complexes are slightly shifted upon deprotonation and
coordination, however, their multiplicity remains unchanged
as expected. Similarly, the observed singlet resonances in the
77Se NMR spectra are shifted upfield by almost 200 ppm in the
lead complexes.
characterized by single crystal X-ray diffraction.
In order to unambiguously confirm the compositions of 1a–3a,
single crystal X-ray diffraction experiments were performed
(Figs. 3 – 5).
Fig. 1. Molecular structure of 2. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms have been omitted for clarity. Selected bond distances [Å] and
angles [°]: Se(1)-C(1) 1.844(2), O(1)-C(2) 1.223(3), C(1)-N(1) 1.408(3), C(2)-N(1) 1.385(3),
C(1)-N(2) 1.324(3); O(1)-C(2)-N(1) 122.91(19), C(2)-N(1)-C(1) 124.62(18), Se(1)-C(1)-N(1)
116.87(15), Se(1)-C(1)-N(2) 125.72(16).
Fig. 2. Molecular structure of 3. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms have been omitted for clarity. Selected bond distances [Å] and
angles [°]: Se(1)-C(1) 1.835(2), O(1)-C(2) 1.216(2), C(1)-N(1) 1.401(3), C(2)-N(1) 1.387(3),
C(1)-N(2) 1.322(3); O(1)-C(2)-N(1) 122.60 (19), C(2)-N(1)-C(1) 122.56(17), Se(1)-C(1)-
N(1) 118.09(14), Se(1)-C(1)-N(2) 124.54 (15).
Fig. 3. Molecular structure of 1a. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms have been omitted for clarity.
The C-O and C-Se bond lengths in these compounds are
consistent with the presence of the expected carbon-
chalcogen double bonds and are similar to those observed in
the molecular structures of other acylselenoureas.20,21 The C-O
and C-Se moieties in the molecules are rotated away from
each other with Se=C-N-C-torsion angles of around 121° (2)
and 127° (3). Similar torsion angles are observed in the related
acylselenoureas
PhC(O)NHC(Se)NnBu2
and
2-
FC6H4C(O)NHC(Se)NiBu2.20,21
The reaction of the acylselenoureas 1-3 with Pb(OAc)2 in
EtOH in the presence of sodium acetate afforded the
corresponding lead(II) bis(acylselenoureato) complexes
Fig. 5. Molecular structure of 2a. Thermal ellipsoids are drawn at the 50% probability
level. Only one part of the disordered nBu chains is shown.
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synthesis with a mixture of octadecene and TOP aVsieswoAlvrteicnletOanlninde
stabilizing reagent. Due to the strong diDpOoIl:e10m.1o03m9/eDn0tNoJ0f1i4o3n3iHc
liquids, [BMIm][NTf2] was added to the solution for efficient
absorption of microwave radiation. As expected, the
corresponding selected area electron diffraction, SAED and
powder pattern confirms the formation of PbSe nanoparticles
with crystal sizes of about 20 nm (Fig. 7b,c), using the Scherrer
equation with a form factor of 1. This agrees well to the
particle size of 19 ± 4 nm, determined from TEM images (Fig.
7a, Fig. S2, ESI†). EDX analysis of the particles also indicates a
ratio of Pb to Se of nearly 1 : 1 and shows no impurities
besides signals of carbon and copper, which originate from the
used TEM-grid.
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Fig. 6 Molecular structure of 3a. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms have been omitted for clarity.
Table 1 Selected bond distances (Å) and angles (°) of compounds 1a-3a.
1a
2a
3a
Pb-Se
Pb-O
Se-Pb-O
O-Pb-O*
Se-Pb-Se*
2.8669(7)
2.414(3)
80.80(8)
150.34(18)
91.18(3)
2.8631(7)
2.429(5)
81.32(11)
149.7(2)
91.49(3)
2.8546(5)
2.418(3)
78.86(7)
150.38(14)
92.08(2)
a)
b)
* Symmetry related atom.
Since the general structural features are quite similar, they
will be discussed together in the following section. The lead
complexes contain two deprotonated acylselenoureas bound
via oxygen and selenium atoms to the metal in a distorted
trigonal bipyramidal arrangement. The two selenium atoms
and the lone pair of the lead atom lie in the equatorial plane
(angle Se-Pb-Se* = 91.5°), whilst the oxygen atoms are located
on the axial positions of the bipyramid (angle O-Pb-O* =
149.7°). There are only three examples for Pb(II)-
acylselenoureato bis(chelates) that have been structurally
characterized. These include [Pb{PhC(O)NC(Se)NEt2}2], [Pb{4-
c)
O2NC6H4C(O)NC(Se)NiBu2}2]
and
[Pb{2-
napC(O)NC(Se)NEt2}2].9,22,23 The Pb-Se and Pb-O bond distances
in complexes 1a-3a of 2.86 Å and 2.4 Å, respectively are very
similar to those found in the other lead acylselenoureato
bis(chelates). Similarly, the bite angle of the acylselenoureato
ligand (i.e. the angle Se-Pb-O) of around 80° is typical for this
class of ligands.
Nanoparticle synthesis
Since we were interested in using these lead compounds as
single-source precursors for PbSe nanomaterials, we initially
examined their thermal behaviour by TGA. As can be seen in
Figure S1 (ESI†), the compounds 1a – 3a undergo clean
decomposition above 170°, the residual mass being consistent
with the formation of PbSe. With this data in hand, we figured
that the preparation of PbSe nanomaterials by thermal
decomposition of these complexes would be feasible under
relatively mild conditions.
d)
Fig. 7 a) TEM image, b) SAED, c) powder pattern with simulation from Crystallography
Open Database (COD ID: 1538746) as red bars (middle) and d) EDX spectrum (bottom)
of PbSe nanoparticles obtained from compound 3a in ODC/TOP with a small amount of
[BMIm][NTf2].
Therefore, following the work of the late Paul O'Brien,
compound 3a was decomposed in a microwave-assisted
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Fig. 8 PXRD of PbSe samples obtained from decomposition of compound 3a: (a)
Compared to the formation of PbSe nano-cubes with an edge
length of 11 + 9 nm, as was previously reported by O'Brien for
the thermal decomposition of a structurally related complex,9
the microwave assisted synthesis led to spherical and slightly
View Article Online
[BMIm][NTf2], 180 °C, 50 W; (b) [BMIm][NTf2], 200 °C, 50 W; (c) [BMIm][NTf2] in PC (50
DOI: 10.1039/D0NJ01433H
vol. %), 200 °C, 50 W; (d) PC, 200 °C, 50 W. The simulated PbSe pattern from
Crystallography Open Database (COD ID: 1538746) is added as red bars in (a).
larger particles. Furthermore, the particles exhibit
a
The corresponding TEM and SEM images (Fig. 9) as well as
EDX spectra (Fig. S3 – S6, ESI†) confirm the previously made
assumptions.
significantly greater degree of aggregation, which might not
necessarily be a result of the synthesis but could also be
related to a different sample preparation used for the TEM
measurements from that used by O'Brien.9 An influence of the
ionic liquid on the particle shape, size or aggregation is rather
unlikely due to the very small amount of [BMIm][NTf2]
compared to that of TOP, but cannot be completely ruled out.
In order to further investigate the capability of
[BMIm][NTf2] to stabilize PbSe nanoparticles and to evaluate
its influence on other factors such as particle growth or
morphology, a total of four additional experiments were
carried out (Table 2). The choice of propylene carbonate as
solvent resulted from to its desirable properties in terms of
microwave absorption and nanoparticle stabilization as well as
its excellent miscibility with ionic liquids.
As expected, the respective powder patterns of the
prepared samples show the formation of cubic PbSe (Figure 8).
However, unlike when using TOP, the diffraction lines are
relatively narrow and sharp, indicating the formation of much
larger particles. The PbSe sample, which was produced in pure
propylene carbonate, displays an intensity increase of the line
at 29.17° from the (200) plane compared to the other lines,
which may suggest a preferential growth along the respective
plane.
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The TEM images show either aggregated particles with an
average size of 178 nm, 186 nm or 366 nm and a mostly cubic
morphology in in the presence of the ionic liquid or
significantly larger and strongly intergrown structures without
the use of [BMIm][NTf2]. Considering the cubic elementary cell
of PbSe, the particle shape is not particularly unusual, as
already shown in previous publications. In all cases, the EDX
spectra confirm the expected ratio of Pb to Se of about 1 : 1. In
addition to these results, SAED analyses also verify the
formation of cubic PbSe (Fig. S3 – S6, ESI†).
Table 2 Parameters of microwave-assisted decomposition of compound 3a
Decomposition parameters
[BMIm][NTf2], 180 °C, 50 W
[BMIm][NTf2], 200 °C, 50 W
Particle size [nm]a
178 ± 90
a
b
c
186 ± 116
[BMIm][NTf2] in PC (50 vol. %), 200 °C, 50 W
PC, 200 °C, 50 W
366 ± 90
b
d
–
a
The particle size distribution results from a size determination of at least 90
particles from the TEM images. b A precise determination of the particle size was
not possible due to the formation of strongly intergrown agglomerates.
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referenced externally against Me4Si (1H and 13CV)ieawnAdrticMle Oen2lSinee
1
2
3
4
Fig. 9 TEM and SEM images of PbSe samples obtained with different decomposition
parameters: (a) [BMIm][NTf2], 180 °C, 50 W; (b) [BMIm][NTf2], 200 °C, 50 W; (c)
[BMIm][NTf2] in PC (50 vol. %), 200 °C, 50 W; (d) PC, 200 °C, 50 W.
77
DOI: 10.1039/D0NJ01433H
( Se). Elemental analyses were carried out by staff of the in-
house facility using an Elementar Vario EL analyser. TGA-DSC
curves were recorded on a Netzsch STA 449 F5 Jupiter system
over a temperature range from 25 to 1000° with a heating rate
of 8 K/min. The powder X-ray diffraction (PXRD) patterns of
the PbSe nanostructures were measured at ambient
temperature on a Bruker D2-Phaser using a flat sample holder
for 1 h with a Cu-K Kα radiation (λ = 1.54182 Å, 35 kV). HR-TEM
imaging was performed on a FEI Tecnai G2 F20 electron
microscope operated at 200 kV accelerating voltage.25 Digital
images were recorded by a Gatan UltraScan 1000P detector.
Samples were prepared using 200 µm carbon-coated copper
grids. The TEM-EDX spectra, high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
images, EDX-mapping and SAED patterns were also recorded
on the FEI Tecnai G2 F20 with an exposure time of 3 min. for
the individual EDX spectra. The spectra always showed traces
of oxygen as a result of the preparation of the grids in air. For
each acquisition, a sample region with a significant amount of
material was placed inside the aperture. The diffraction images
were calibrated with Debye-Scherrer patterns recorded from a
gold reference sample (S106, Plano GmbH, Wetzlar, Germany).
SEM images were aquired on a JEOL JSM-6510 advanced
electron microscope, with a LaB6 cathode at 5-20 keV. The
microscope was equipped with a XFlash 410 silicon drift
detector.
5
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An exemplary STEM-EDX mapping (Fig. 10) of the particles
obtained by decomposition of the lead compound at 200 °C in
[BMIm][NTf2] shows that Pb and Se are both homogeneously
distributed within the particles. This clearly illustrates the
homogeneity of the prepared particles and also confirms their
composition based on a ratio of the two elements of 1 : 1
according to the EDX analyses.
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Synthesis of the acylselenourea compounds
To a solution of KSeCN (3.6 g, 25 mmol) in acetone (200
mL) the corresponding acid chloride (25 mmol) was added.
After ca. 5 minutes at room temperature the secondary amine
(25 mmol) was slowly added to the suspension. After stirring
for a further 10-15 min. at room temperature, the entire
mixture was poured into cold 0.1 M HCl, resulting in
precipitation of the crude product. This was isolated by
filtration and was recrystallised from EtOH or acetone.
4-ClC6H4C(O)NC(Se)NnBu2 (2)
Fig. 10 STEM-EDX mapping of the PbSe particles obtained by decomposition of
compound 3a in [BMIm][NTf2] at 200 °C (white: HAADF-STEM image of the PbSe
particles, red: mapping of Se, orange: mapping of Pb L-shell, yellow: Pb M-shell).
With regard to the microwave-assisted synthesis of PbSe
nanoparticles, the present results show that, on one hand,
compound 3a is very well suited as a precursor compound and,
on the other hand, that under the selected conditions the ionic
liquid is only capable of stabilizing large particles rather than
nano-sized PbSe. Presumably, the weakly coordinating
interactions of the ionic liquid are not sufficient enough to
obtain particles below 100 nm. Therefore, [BMIm][NTf2] is an
ideal additive for the microwave assisted synthesis of PbSe
nanoparticles rather than a stabilizing reagent, due to its very
good absorption properties towards microwave irradiation and
its very weak interaction with the growing particles.
This was prepared as described above using KSeCN, 4-
chlorobenzoyl chloride and di-n-butylamine. Yellow crystals
were obtained in 24 % yield after recrystallisation from
1
acetone. H NMR (CDCl3, 400 MHz):
d = 0.92 (t, J = 7.4 Hz, 3 H,
CH3), 1.01 (t, J = 7.3 Hz, 3 H, CH3), 1.31 (sext., J = 7.4 Hz, 2 H,
CH2), 1.47 (sext., J = 7.3 Hz, 2 H, -CH2), 1.69 (quint., J = 7.5 Hz,
2 H, -CH2), 1.85 (quint., J = 7.4 Hz, 2 H, -CH2), 3.56 (t, J = 7.4
Hz, 2 H, -CH2), 4.09 (t, J = 7.3 Hz, 2 H, -CH2), 7.47 (d, J = 7.6
Hz, 2 H, C6H4), 7.82 (d, J = 7.6 Hz, 2 H, C6H4), 8.61 (s, 1 H, NH).
13C NMR (CDCl3, 150 MHz):
= 13.60 (CH3), 13.77 (CH3), 19.96
-CH2), 20.01 ( -CH2), 28.62 ( -CH2), 29.71 ( -CH2), 53.63 (
CH2), 56.27 ( -CH2), 129.11 (m-C6H4), 129.31 (o-C6H4), 130.96
(ipso-C6H4), 139.39 (CCl), 161.35 (CO), 180.91 (CSe). 77Se NMR
(CDCl3, 76 MHz): = 484.6. Elemental analysis calcd. for
g-
g
b
b
a
a
d
Experimental
General
(g
g
b
b
a-
a
Reagents and solvents (HPLC grade) were obtained from
commercial suppliers and were used as received. Reactions
were carried out under ambient conditions without exclusion
d
C16H23N2ClOSe (373.8): C 51.41, H 6.20, N 7.49 %. Found C
51.58, H 6.11, N 7.54 %.
of air or moisture. Acylselenourea
previously reported.24
1 was prepared as
4-ClC6H4C(O)NC(Se)NiBu2 (3)
This was prepared as described above using KSeCN, 4-
chlorobenzoyl chloride and di-iso-butylamine. Yellow crystals
NMR spectra were recorded on a Bruker Avance 400 or a
Bruker Avance III 600 spectrometer. Chemical shifts are
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were obtained in 71 % yield after recrystallisation from (nonet, J = 6.8 Hz, 2 H, CH), 2.17 (nonet, J = 6.8 VHiezw, A2rticHle,OCnHlin)e,
1
acetone. H NMR (CDCl3, 400 MHz):
d
= 0.9 (d, J = 6.6 Hz, 6 H, 3.61 (d, J = 7.5 Hz, 4 H, CH2), 3.65 (d, J = D7O.2I:H10z.,10439H/,DC0HNJ20),1473.33H6
CH3), 1.06 (d, J = 6.6 Hz, 6 H, CH3), 2.14 (nonet, J = 6.6 Hz, 1 H, (d, J = 8.7 Hz, 4 H, C6H4), 8.07 (d, J = 8.7 Hz, 4 H, C6H4). 13C NMR
CH), 2.43 (nonet, J = 6.7 Hz, 1 H, CH), 3.36 (d, J = 7.1 Hz, 2 H, (CDCl3, 100 MHz): = 20.15 (CH3), 20.57 (CH3), 28.01 (CH),
d
CH2), 3.94 (d, J = 7.3 Hz, 2 H, CH2), 7.46 (d, J = 8.7 Hz, 2 H, 28.45 (CH), 59.97 (CH2), 62.15 (CH2), 128.06 (m-C6H4), 131.06
C6H4), 7.81 (d, J = 8.7 Hz, 2 H, C6H4), 8.43 (s, 1 H, NH). 13C NMR (o-C6H4), 135.69 (ipso-C6H4), 137.27 (CCl), 168.81 (CO), 171.48
(CDCl3, 100 MHz):
d d = 324.2. Elemental analysis
= 19.97 (CH3), 20.02 (CH3), 26.53 (CH), (CSe). 77Se NMR (CDCl3, 76 MHz):
27.38 (CH), 62.58 (CH2), 63.89 (CH2), 129.17 (m-C6H4), 129.25 calcd. for C32H44N4Cl2O2Se2Pb (952.8): C 40.34, H 4.66, N 5.88
(o-C6H4), 130.82 (ipso-C6H4), 139.43 (CCl), 160.72 (CO), 181.67 %. Found C 40.33, H 4.69, N 5.99 %.
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(CSe). 77Se NMR (CDCl3, 76 MHz):
d = 490.4. Elemental analysis Synthesis of PbSe nanoparticles
calcd. for C16H23N2ClOSe (373.8): C 51.41, H 6.20, N 7.49 %.
Found C 51.60, H 6.23, N 7.44 %.
All synthetic experiments were carried out under nitrogen
or argon atmosphere using Schlenk techniques. The ionic liquid
was synthesized according to literature procedures26 by the
reaction of 1-methylimidazole with 1-chlorbutane following an
ion-exchange reaction with LiNTf2. 19.2 mg of complex 3a
were suspended overnight in a mixture of 0.5 ml TOP (1.12
mmol), 0.3 ml octadecene (0.94 mmol) and 30 mg (0.07 mmol)
of the ionic liquid. The resulting suspension was then heated in
a microwave reactor and compound 3a was decomposed at
200 °C (50 W) for 15 minutes.
Synthesis of the lead complexes
Caution! Lead compounds are highly toxic materials.
Appropriate personal protective equipment must be worn and
waste must be disposed off according to local regulations.
To a solution of the acylselenourea in EtOH (10 mL) was
added a solution of Pb(OAc)2 (0.5 equiv.) in water (10 mL). To
this was added a solution of NaOAc (2.5 equiv.) in water (2 mL)
and the resulting mixture was left to stand in a refrigerator for
ca. 24 h. The resulting solid was isolated by filtration and dried.
The product was purified by dissolving the crude material in
CH2Cl2 and filtration through Celite. Addition of hexanes to the
filtrate and cooling afforded the products in crystalline form.
[Pb{4-MeC6H4C(O)NC(Se)NEt2}2] (1a)
Synthesis of PbSe submicrostructures
19.8 mg of complex 3a (corresponding to an amount of 0.5
weight % Pb.) was suspended overnight in a either 1.00 g (2.38
mmol) [BMIm][NTf2], propylene carbonate (9.79 mmol) or
[BMIm][NTf2] in propylene carbonate (50 vol. %). The resulting
suspensions were then heated in a microwave reactor and
compound 3a was decomposed at 200 °C (50 W) for 15
minutes. Additionally, another suspension of the complex in
[BMIm][NTf2] was decomposed for 15 minutes at 180 °C (50
W). For the investigations of the particles, the particle
dispersions were first precipitated with acetonitrile, washed
three times with 1 ml acetonitrile each and were then dried
under vacuum to give a powdery product, that could be
redispersed if needed.
This was prepared as described above using 1 (0.50 mmol)
and Pb(OAc)2 (0.50 mmol). The product was obtained as a
yellowish solid in 78% yield. 1H NMR (CDCl3, 400 MHz):
d = 1.14
(t, J = 7.1 Hz, 6 H, CH3), 1.23 (t, J = 6.9 Hz, 6 H, CH3), 2.41 (s, 6
H, CH3C6H4), 3.81 (q, J = 7.1 Hz, 8 H, CH2), 7.19 (d, J = 8.0 Hz, 4
H, C6H4), 8.03 (d, J = 8.2 Hz, 4 H, C6H4). 13C NMR (CDCl3, 100
MHz):
d = 13.22 (CH3), 14.35 (CH3), 21.52 (CH3C6H4), 45.41
(CH2), 48.72 (CH2), 128.49 (m-C6H4), 129.79 (o-C6H4), 134.52
(ipso-C6H4), 141.35 (p-C6H4), 167.32 (CO), 171.00 (CSe). 77Se
NMR (CDCl3, 76 MHz):
d = 286.6. Elemental analysis calcd. for
X-Ray crystallography
C26H34N4O2Se2Pb (799.7): C 39.05, H 4.29, N 7.01 %. Found C
38.93, H 4.10, N 6.95 %.
Data were collected at 150 K using a Rigaku Oxford
Diffraction Gemini E Ultra diffractometer, equipped with an
EOS CCD area detector and a four-circle kappa goniometer. For
the data collection, the Mo source emitting graphite-
monochromated Mo-Kα radiation (λ = 0.71073 Å) was used.
Data integration, scaling and empirical absorption correction
was carried out using the CrysAlis Pro program package.27 The
structure was solved using Direct Methods and refined by Full-
Matrix-Least-Squares against F2. The non-hydrogen atoms
were refined anisotropically and hydrogen atoms were placed
at idealized positions and refined using the riding model. All
calculations were carried out using ShelXS and ShelXT within
the Olex2 graphical interface.28 Important crystallographic and
refinement details are collected in Table S1, ESI†. The butyl
chains in the structure of the lead complex [Pb{4-
ClC6H4C(O)NC(Se)NnBu2}2] (2a) were found to be disordered
over two positions. Refinement of the occupancy factor of the
two atom positions resulted in an approximate ratio of 60:40.
[Pb{4-ClC6H4C(O)NC(Se)NnBu2}2] (2a)
This was prepared as described above from 2 (0.50 mmol)
and Pb(OAc)2 (0.50 mmol). The product was obtained as pale-
1
yellow solid in 92% yield. H NMR (CDCl3, 600 MHz):
d
= 0.87-
-CH2), 1.31
-CH2), 3.70-
-CH2), 7.35 (d, J = 8.7 Hz, 4 H, C6H4), 8.06 (d, J =
8.7 Hz, 4 H, C6H4). 13C NMR (CDCl3, 150 MHz):
= 13.78 (CH3),
13.84 (CH3), 19.95 ( -CH2), 20.33 ( -CH2), 29.97 ( -CH2), 31.18
-CH2), 51.72 ( -CH2), 54.39 ( -CH2), 128.02 (m-C6H4), 131.03
(o-C6H4), 135.85 (ipso-C6H4), 137.20 (CCl), 169.06 (CO), 176.52
(CSe). 77Se NMR (CDCl3, 76 MHz):
= 301.4. Elemental analysis
0.92 (m, 12 H, CH3), 1.20 (sext., J = 7.5 Hz, 4 H,
(sext., J = 7.5 Hz, 4 H, -CH2), 1.57-1.64 (m, 8 H,
3.72 (m, 4 H,
g
g
b
a
d
g
g
b
(b
a
a
d
calcd. for C32H44N4Cl2O2Se2Pb (952.8): C 40.34, H 4.66, N 5.88
%. Found C 40.51, H 4.72, N 6.03 %.
[Pb{4-ClC6H4C(O)NC(Se)NiBu2}2] (3a)
This was prepared as described above from 3 (0.50 mmol)
and Pb(OAc)2 (0.50 mmol). The product was obtained as a
n
Anisotropic refinement of all of the Bu chain carbon atoms
proved unsuccessful, therefore some atoms could only be
1
yellow solid in 97% yield. H NMR (CDCl3, 600 MHz):
d = 0.88
(d, J = 6.8 Hz, 12 H, CH3), 0.89 (d, J = 6.8 Hz, 12 H, CH3), 2.03
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refined isotropically. The structural data has been deposited
with the Cambridge Crystallographic Data Center under
deposition numbers CCDC 1586974–1586978.†
ARTICLE
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DOI: 10.1039/D0NJ01433H
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Conclusions
4
5
9
In summary, three lead(II) bis(acylselenoureato) complexes (1a
– 3a) were prepared by the reaction of the respective
acylselenoureas and Pb(OAc)2. These complexes were fully
characterized by NMR spectroscopy, elemental analysis and
single crystal diffraction.
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The lead(II) compound 3a was successfully decomposed in
a
microwave-assisted reaction in either a mixture of
TOP/octadecene/[BMIm][NTf2], a mixture of [BMIm][NTf2]/PC,
in pure [BMIm][NTf2] or in pure PC, resulting in the formation
of different PbSe nanostructures. The respective TEM and SEM
analysis showed the formation of nanoparticles, sub-
microstructures or strongly aggregated and intergrown
structures depending on the stabilizing reagent used,
indicating that the ionic liquid [BMIm][NTf2] is rather an ideal
additive than an exceptional stabilizing reagent for the
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work has been supported by the German Science
Foundation (DFG) within the priority program SPP 1708
“Material Synthesis Near Room Temperature” (grant JA
466/31-1/2 for CJ). We also want to thank Dr. Juri Barthel at
the Ernst Ruska-Centre, Forschungszentrum Jülich GmbH (FZ
Jülich), Germany for his technical support at the TEM facility
under project number ER-C A-060 and Retorte GmbH for a
generous donation of elemental selenium.
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