Synthesis, Crystal Structure, and Selected Properties of [Au(S2CNH2)2]SCN: A Precursor for Gold Macro-Needles Consisting of Gold Nanoparticles Glued by Graphitic Carbon Nitride

Article abstract of DOI:10.1002/chem.201805913

A new preparation route is developed for the synthesis of needle-like crystals of [Au(S₂CNH₂)₂]SCN, which avoids disproportionation of the Au^I salt used as a starting material. In the crystal structure, the two crystallographically independent Au^III centers are in a square-planar environment of two S₂CNH₂ ligands. The Hirshfeld surface analysis reveals the presence of noncovalent intermolecular S???S interactions, which are essential for the spatial arrangement of the molecules. Density functional theory (DFT) calculations including dispersion and damping corrections result in a unit cell volume very close to the value determined experimentally. Thermal decomposition in an inert atmosphere generates black needles with lengths of up to 500 μm. X-ray powder diffraction and pair distribution function analyses demonstrate that the needles are composed of nanosized crystals with a volume-weighted average domain size of 20(1) nm. According to results of X-ray photoemission experiments, the black needles are covered by a nitrogen-rich carbon nitride with composition near (CN)₂N. ¹³C solid-state NMR investigations indicate that two different carbon species are present, with signals corresponding well to heptazine units as in melon and triazine units as in poly(triazin imide) type compounds. Scanning transmission electron microscopy tomography evidences that the needles are composed of slightly elongated nanoparticles.

Full text of DOI:10.1002/chem.201805913

A Journal of
Accepted Article
Title: Synthesis, Crystal Structure and Selected Properties of
[Au(S2CNH2)2]SCN: A precursor for gold macro-needles
consisting of gold nanoparticles glued by graphitic carbon nitride
Authors: Wolfgang Bensch, Christoph Ludwig Teske, Anna-Lena
Hansen, Richard Weihrich, Lorenz Kienle, Marius Kamp,
Kasper Pieter van der Zwan, Jürgen Senker, Carsten Dosche,
and Gunther Wittstock
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201805913
Link to VoR: http://dx.doi.org/10.1002/chem.201805913
Supported by

Chemistry - A European Journal
10.1002/chem.201805913
Synthesis, Crystal Structure and Selected Properties of
Au(S CNH ) ]SCN: A precursor for gold macro-needles consisting
[
2
2 2
of gold nanoparticles glued by graphitic carbon nitride
a
a
b
c
Christoph Ludwig Teske , Anna-Lena Hansen , Richard Weihrich , Lorenz Kienle ,
c
d
d
e
Marius Kamp , Kasper Pieter van der Zwan , Jürgen Senker , Carsten Dosche ,
e
a,*
Gunther Wittstock , Wolfgang Bensch
a
Institute of Inorganic Chemistry, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 2, D-
4118 Kiel Germany), E-mail: wbensch@ac.uni-kiel.de
2
b
Institute for Materials Research Management, University of Augsburg, Sigma Technopark,
D-86135 Augsburg
c
Institute for Material Science, Christian-Albrechts-Universität zu Kiel, Kaiserstr. 2, D-24143
Kiel
d
Inorganic Chemistry III, University of Bayreuth, Universitätsstr. 30, D-95447 Bayreuth
e
Physical Chemistry, University of Oldenburg, D-26111 Oldenburg
Keywords: Gold Dithiocarbamate, Gold Nanoparticles, Pair Distribution Function, X-
Ray diffraction, Transmission Electron Tomography
1
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Abstract
For the synthesis of needle-like crystals of [Au(S CNH ) ]SCN a new preparation
2
2 2
route was developed avoiding disproportionation of the Au(I) salt which was used as
starting material. In the crystal structure the two crystallographically independent
Au(III) centers are in a square-planar environment of two S CNH ligands. The
2
2
Hirshfeld surface analysis reveals the presence of non-covalent intermolecular SS
interactions which are essential for the spatial arrangement of the molecules. Density
functional theory (DFT) calculations including dispersion and damping corrections
results in a unit cell volume very close to the experimentally determined value.
Thermal decomposition in inert atmosphere generates black needles with lengths up
to 500 µm. X-ray powder diffraction and pair distribution function analysis
demonstrate that the needles are composed of nano-sized crystals with volume
weighted averaged domain size of 20(1) nm. According to results of X-ray
photoemission experiments the black needles are covered by a nitrogen-rich carbon
13
nitride with composition near (CN) N. C solid state NMR investigations indicate that
2
two different carbon species with signals corresponding well to heptazine units like in
melon and triazine units like in poly(triazin imide) typ compounds are present.
Scanning transmission electron microscopy tomography evidences that the needles
are composed of slightly elongated nanoparticles.
2
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Introduction
Dithiocarbamates are organic molecules (S CNR , R = more or less bulky organic
2
2
groups), which are versatile ligands forming complexes with all transition metal
[
1]
cations and with many main group elements. Several transition metal cations form
complexes with these ligands in a wide range of oxidation states like e.g. W and
[
2]
Mo. Dithiocarbamates often act as chelating ligands leading to formation of two
nearly equal M-S bonds. In many complexes these molecules are monodentate,
which is observed if sterical demands hinder the second bond formation to the metal
center, or if the electronic structure of the metal center cannot accept the lone
electron pair of the second S atom. Other coordination modes are also observed,
which are less common than the two mentioned above. Dithiocarbamates are widely
[
3]
[4, 5]
[6]
used in agricultural industry
as fungicides
or pesticides , but also as
[
7, 8]
vulcanization accelerators.
transition metal cations form different fluid media.
dithiocarbamate complexes were applied as single source precursors for the
These compounds are also used for the removal of
[
9-11]
In the last few years,
[
12-16]
preparation of metal sulfides by thermal decomposition
or for the generation of
[
17-20]
sulfide films.
[
21]
The first Au(I) dithiocarbamate compounds were reported by Akerström (1959
),
and after this publication the chemistry of such compounds was rapidly developed.
[
22-26]
A common structural feature of these Au(I) compounds are intermolecular
AuAu interactions thus forming dimers or linear chains, which are characterized by
alternating short (2.5 – 2.8 Å) and long (3 – 3.1 Å) Au-Au distances. Recently we
reported on preparation, crystal structure, and selected properties of
3
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10.1002/chem.201805913
[
27]
mono(dithiocarbamato)-gold(I), AuS CNH .
During a long-time experiment to
2
2
recrystallize this compound from an acid aqueous solution partial disproportionation
occurred, and several crystals of the new compound [Au(S CNH ) ]SCN were
2
2 2
obtained. [Au(S CNH ) ]SCN is related to a number of well characterized
2
2 2
[
28-41]
bis(dithiocarbamato)-gold(III) complexes
with the general composition [Au(-
S CNR ) ]X, X = halide or complex anions. The bromide containing compound [Au(-
2
2 2
[
42]
S CNH ) ]Br has been prepared earlier
, but besides spectroscopic data no
2
2 2
structural information or detailed properties were mentioned. Here we report the
synthesis of the new bis(dithiocarbamato)-Au(III)-thiocyanate compound
[
Au(S CNH ) ]SCN, the crystal structure, the Hirshfeld surface analysis, the
2 2 2
properties of the thermal decomposition products including nitrogen sorption, X-ray
13
15
diffraction and pair distribution function analysis, X-ray photoemission, C and
MAS NMR spectroscopy, and Scanning Transmission Electron Microscopy.
N
Experimental Section
Preparation and Properties: [Au(S CNH ) ]SCN can be synthesized in an alcohol-
2
2 2
+
water-acetone mixture by oxidation of Au with thiuram disulfide (C H N S ) in the
2
4
2 4
presence of a large excess of NH SCN. A possible reaction pathway is formulated in
4
eq. 1:
K[Au(CN) ] + C H N S + NH SCN → [Au(S CNH ) ]SCN + NH CN + KCN
(1)
2
2
4
2
4
4
2
2 2
4
Warning: Handling cyanides is generally very hazardous because of their toxicity
and the possible release of HCN! Working under the fume hood and observance of
the appropriate safety regulations is absolutely obligatory.
4
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Preparation of [Au(S CNH ) ]SCN: A successful progress of the reaction is strongly
2
2 2
associated with the composition of the solvent, the used gold compound, and the pH
value. After several attempts the following synthetic approach yielded the best result:
i.)
In a flat evaporating dish ( ≈13 cm) 0.288 g K[Au(CN) ] (1 mmol) + 7.62 g
2
NH SCN (0.1 mol) were dissolved in 4 mL H O + 150 mL methanol.
4
2
ii.)
iii.)
After stirring for some minutes, ca. 5 drops of formic acid were added giving a
pH ≈ 4.
A solution of 0.184 g C H N S (1 mmol) dissolved in 20 mL acetone was
2
4
2 4
added.
The solution became instantly light yellow and at later stages turned to deep orange-
yellow, indicating the formation of an Au(III) salt. The solvent was allowed to
evaporate in the fume hood at ambient temperature. The crystalline product was
suspended in water, filtered by suction through a Hirsch funnel, successively washed
with several mL of water, ca. 2 mL of acetone, and finally with ca. 2 mL of CS . For
2
final purification the sample was dissolved in methanol (solubility s ≈ 0.26 – 0.3 g/100
mL), and a corresponding portion of NH SCN (ca. 0.04 – 0.05 g/100 mL, dissolved in
4
1
-2 mL MeOH) was added. The solvent was allowed to evaporate (vide supra). The
final precipitate was suspended in water, filtered by suction, washed with water, and
dried in a desiccator over P O . Red-orange crystals developed (Figure S1). Thiuram
2
5
[
28]
[43]
disulfide (C H N S ) was prepared by oxidation
of NH S CNH
with a slight
2
4
2
4
4
2
2
excess of potassium hexacyanoferrate(III), K [Fe(CN) ], in an aqueous solution.
3
6
[
44]
K[Au(CN) ] was prepared from metallic gold following the procedure reported in.
2
[
Au(S CNH ) ]SCN is stable in ambient atmosphere at room temperature. It is
2 2 2
moderately soluble in alcohol and poorly in acetone. The percent composition data
(standard deviation in brackets) were determined by combustion analysis with a
5
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CHNS-Rapid-Element-Analyser (Heraeus GmbH) using thiourea as standard:
-1
[
Au(S CNH ) ]SCN (439.38 g·mol ): C 8.3(2) (calcd. 8.2); H 0.8(1) (calcd. 0.9 ); N
2 2 2
9
.4(3) (calcd. 9.6); S 36.5(4) (calcd. 36.5).
X-ray Powder Diffraction
In house PXRD patterns were measured in transmission geometry on a STOE Stadi
P, equipped with a Dectris Mythen 1K detector using monochromatic CuK radiation
α
(λ = 1.5406 Å).
High Energy Synchrotron Radiation Scattering
Experiments were performed at Petra III, beamline P02.1 at DESY, Germany (λ =
0
.2074 Å, 60 keV), equipped with a PerkinElmer XRD 1621 2D detector. For XRPD
the sample to detector distance (SDD) was 1113 mm, for total scattering experiments
PDF) 345 mm. The data were collected for 120 s (XRPD) and 300 s (PDF). The
(
-1
Qmax value of the PDF analysis was chosen as 22.89 Å . The instrumental
-1
contribution to the dampening of the PDF (Qdamp = 0.0215 Å ) was refined using LaB
6
as reference. For background subtraction an empty capillary was measured under
the same experimental conditions. Rietveld Refinement was performed using TOPAS
[45]
academic version 6.
Total scattering data were processed using PDFgui and
[
46]
PDFgetX3.
Single Crystal X-ray Investigation
A STOE Imaging Plate Diffraction System (IPDS-1) with graphite monochromatized
Mo-K radiation (λ = 0.7107 Å) was used to collect single crystal X-ray diffraction data
α
of [Au(S CNH ) ]SCN at 200(2) K. The absorption correction was applied using X-
2
2 2
[
47]
[48]
RED
and X-Shape
after the correction of the raw data for Lorentz and
[
49]
polarization effects. The SHELXL-2014 program package
was used to solve the
2
crystal structure by Direct Methods, and the refinement was performed against F . All
6
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atomic positions, except those of H atoms, were refined with anisotropic
displacement parameters. The hydrogen atoms were located in difference Fourier
maps. They were positioned geometrically and were refined with isotropic
displacement parameters applying a riding model, with a distance of N-H = 0.88 Å,
and U (H) = 1.2 U (N). Technical details of the data acquisition and selected
iso
eq
refinement results are summarized in Table S1. The final atomic coordinates as well
as the equivalent isotropic displacement parameters are listed in Table S2. Table S3
shows the list of the shortest inter-atomic distances, selected bonding angles, and
geometric parameters of possible hydrogen bonding interactions. Further details of
the
crystal
structure
investigations
may
be
obtained
from
the
Fachinformationszentrum Karlsruhe D-76344 Eggenstein-Leopoldshafen, Germany
on quoting the depository number CCDC 1881359, E-mail: crysdata@fiz-
karlsruhe.de
Further Characterizations
For thermal analysis a STA-429 instrument from Netzsch was used. The DTA-TG
investigations were carried out in N atmosphere (purity: 4.8; crucible: sintered Al O )
2
2
3
-1
at a flow rate of 75 mL∙min applying different heating rates and final temperatures
according to:
-1
a) 1, 2, 6 and 8 K∙min in the temperature range of T = 25 to 470 °C.
-1
b) 4 K∙min in the temperature range of T = 25 to 600 °C.
-1
c) 1 K∙min in the temperature range of T = 25 to 175 °C.
-1
d) 1 K∙min in the temperature range of T = 25 to 710 °C.
7
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Corrections for buoyancy and current effects were applied during data evaluation.
The instrument was calibrated with standard reference samples from Netzsch. The
residues were recovered from the crucibles for X-ray powder diffraction
investigations.
The N sorption measurements were carried out with a BelSorpMax system. The
2
sample was degassed at T = 200 °C for 16 h in vacuum. The nitrogen purity was 5.0.
Density functional calculations were performed with the pseudopotential method as
[
50-53]
implemented in the VASP code (version 5.4.1).
The framework of density
functional theory (DFT) was applied within the generalized gradient approximation
[
54]
(GGA) as parametrized by Perdew-Burke-Enzerhof (PBE).
The calculations were
-4
performed by constant volume and global optimizations. They converged to 10 eV
with normal („accurate) and high global accuracy, and k-point nets of 25225 to 38115
points. Grimme’s D3 correction was applied for accounting of dispersion interactions.
[
55, 56]
Scanning Transmission electron microscopy (STEM) investigations were performed
2
with a Tecnai F30 STwin G with 300 kV acceleration voltage. A special high angle
tilting holder is used to acquire a linear tilt series with a step size of 1° and a
maximum angle of 70°. By using an algorithm called filtered back projection real
space reconstruction, a three-dimensional model is calculated.
Scanning electron microscopy (SEM) images were taken using a Philips ESEM XL
3
0 electron microscope.
13
The C MAS NMR spectrum was recorded at room temperature on a Bruker Avance
III HD 400 spectrometer operating at 9.3 T resulting in Larmor frequencies of 400.133
8
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1
13
15
MHz ( H) and 100.625 MHz ( C). The N MAS NMR experiment was carried out on
room temperature at a Bruker Avance II 300 spectrometer operating at 7.1 T resulting
1
15
in Larmor frequencies of 300.153 MHz ( H) and 30.417 MHz ( N). The sample was
13
15
filled into zirconia rotors with diameters of 3.2 mm ( C) and 7 mm ( N), respectively,
13
and mounted into a commercial triple-resonance MAS probe (Bruker). The C and
1
5
N MAS NMR spectra were referenced indirectly with respect to tetramethylsilane
TMS) and nitromethane, respectively, using adamantane and glycine as secondary
(
13
references. The MAS spinning frequencies were set to 12.5 kHz ( C) and 5 kHz
15
(
N). Both MAS spectra were acquired using a crosspolarization (CP) transfer via
13
15
the proton bath with contact times of 3 ms ( C) and 7 ms ( N) and nutation
1
3
15
[57]
frequencies of nut = 104 kHz ( C) and  = 62.5 kHz ( N).
During acquisition
sequence with nut
nut
[
58]
proton broadband decoupling was applied using a spinal-64
0 kHz.
=
7
X-ray photoemission investigations were performed with an Escalab 250Xi (Thermo
Fisher) XPS spectrometer XR 6, Al K monochromated X-ray source (1486.6 eV), X-

ray spot size 500 µm, magnetic lens mode, in-lens charge compensation. Ar ion
sputtering was carried out with a MAGCIS dual beam argon ion/argon cluster source
(energy 6000 eV, raster size 2 mm, 120 sec). Survey spectra were collected with a
band pass of 100 eV (step width: 1 eV, dwell time: 10 ms, 5 scans). High resolution
spectra were recorded with a band pass of 10 eV, a step size of 0.02 eV, a dwell time
of 50 ms and 10 scans (for C 1s, N 1s, O 1s) or 5 scans (for Au 4f) . For sample
preparation an in-house build powder sample holder was used. Acquisition and data
analysis were performed by using the Thermo Fisher instrument control and data
9
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handling software Avantage, version 5.952. Details on the electron energy and
instrument calibration are given in the Supporting Information.
Results and Discussion
Synthetic considerations
Crystals of [Au(S CNH ) ]SCN were first discovered, after several weeks, during an
2
2 2
[
27]
experiment to recrystallize AuS CNH2
from an acid aqueous solution, as
2
mentioned above. The formation of Au(III) is due to the fact that Au(I) complexes tend
0
3+ [59]
to disproportionate in aqueous media forming Au and Au
, despite the very low
solubility of compounds like AuS CNH . Hence, the formation of [Au(S CNH ) ]SCN
2
2
2
2 2
may be rationalized by the formal chemical reaction given in eq. 2:
0
3+
3
AuS CNH → 2Au + [Au (S CNH ) ]SCN + H S.
(2)
2
2
2
2 2
2
The generated metallic gold particles could be clearly identified in the center of the
AuS CNH crystal druses of the sample (Figure 1.)
2
2
1
0
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Figure 1. SEM image of a crystal of [Au(S CNH ) ]SCN (a) with attached poly crystals of
2
2 2
AuS CNH (b). Magnified inset: deposition of metallic gold (c).
2
2
The disproportionation reaction of AuS CNH is certainly not an applicable method to
2
2
synthesize a phase-pure sample of the title compound. However, synthesis of Au(III)
dithiocarbamates using thiuram disulfide, as oxidizing agent, was reported in
[
28, 30, 32]
literature.
Reduction of the disulfide and scission of the S-S bond results in
[
60]
incorporation of the dithiocarbamate molecules into the metal coordination sphere.
The chelating dithiocarbamate anion additionally stabilizes the higher oxidation state
Au(III). Actually, in this context, one cannot expect that only the reaction according to
eq. (1) is relevant, and additional competing reactions may take place as well, which
[
28, 32]
was already discussed in literature (e.g., Ref.
). Hence, during our early
experiments, we occasionally detected considerable contaminations with elemental
S, metallic Au, and further impurities not investigated in more detail. Therefore,
numerous experiments were necessary with variations of the precursors and
alternating mixtures of solvents to prevent the competing reactions, as far as
possible, which led to the above described procedure.
Description of the crystal structure
The new compound [Au(S CNH ) ]SCN crystallizes in the triclinic space group Pī
2
2 2
(see Table S1) with two unique Au atoms located on special positions, while all other
independent atoms are on general positions. The structure analysis shows that the
compound comprises of two crystallographically independent ionic units
+
-
[
Au(S CNH ) ] (abbreviated as “Au1” and “Au2”) and SCN , respectively. The Au(III)
2 2 2
centers are coordinated to two dithiocarbamate units, in the S,S´-bidentate mode
Au-S = 2.333(2) – 2.345(2) Å), in an square planar arrangement (Figure 2).
Comparison of the geometric parameters of the two independent cations reveals, that
(
1
1
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the dithiocarbamate moieties of “Au2” are significantly different from those of “Au1”
with respect to the N-C-S angles (Table S3). We note that the Au-S bond lengths are
+
[29-38]
very similar to those in corresponding cations [Au(S CNR ) ]
, where Au-S
bonds range from 2.290 to 2.357 Å, and the geometry of the dithiocarbamate groups
Table S3) is comparable with analogous cations containing the dithiocarbamate
2
2 2
(
group (ibid). Au(III) is situated in the center of the square planar [AuS ] moiety with
4
D2h symmetry, and the diagonal angles are S-Au-S = 180°. The latter is in
2
accordance with the low-spin (intraorbital) dsp -hybrid state of Au(III). The
thiocyanate anions are not strictly linear (S-C-N angle = 177.7°, Table S3).
-
Deviations from the linear geometry are a common feature of the SCN anion and
known from other compound in the literature, for instance, from other coinage metal
[
61]
+
thiocyanates.
The two crystallographically independent [Au(S CNH ) ] cations
2 2 2
are inclined relative to the (100) plane (Figure 2, angle of inclination: αAu1 = 48.09(3)°
and αAu2 = 50.09(4)°, respectively). They stack along [100] to form polymeric chains
(
Figure S3), held together by secondary Au-S interactions (Au – S = 3.578(2) Å and
1 4
Au – S = 3.566(2) Å, Table S3). These separations are smaller than the sum (Σr
vdW
2
2
=
3.9 Å) of the recommended van der Waals radii of Au and S (with rvdW(Au) = 2.1Å
[62]
and rvdW(S) = 1.8 Å
). Thus, the Au(III) centers adopt a distorted octahedral
environment of S. The angle of mutual inclination (αAu1-Au2 = 4.22(8) °) between the
planes through “Au1” and “Au2” accounts for the slight difference between these
secondary bonds. The Au–Au separation of 3.9104(3) Å corresponds to 0.5×a.
1
2
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Figure 2. View of the arrangement of cations and anions in the structure of the title
compound.
[
63, 64]
Between cations and anions strong, moderate, and very weak hydrogen bonds
(N-H•••N and N-H•••S, Table S3) are observed accomplishing the spatial
+
-
arrangement of [Au(S CNH ) ] and SCN in the crystal structure. The shorter
2
2 2
hydrogen bonds (range: 2.073 - 2.485 Å) connect the cations “Au1” and “Au2” via the
-
SCN anions. The very weak H-bonds (range: 2.918 - 3.131 Å) are responsible for the
direct intermolecular linkage between “Au1” and “Au2” (Figure S2). However, these
hydrogen bridges are strongly bent (see Table S3), and therefore, a priori, it is not
clear how essential they are for the structure.
As mentioned above, it is noticeable that the dithiocarbamate moiety of “Au1” and
“
Au2” differ significantly in the angles N-C-S (with < N(2)-C(2)-S(3) = 125.4(5)° and <
N(2)-C(2)-S(4) = 122.9(6)°, for “Au2”) whereas for “Au1” both < N-C-S angles are ≈
25° (see Table S3). This might be due to the fact that N(2) is involved in three
hydrogen bonds, of which two asymmetrically lay almost in the same plane with
1
(iii)
(iii)
“
Au2” (N(2)-H(3)•••S(1) and N(2)-H(3)•••S(5) , (Figure S3 and Table S3).
1
3
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Hirshfeld surface Analysis and DFT calculations
[
65-70]
A calculation of the Hirshfeld surface of the ionic units (Crystal Explorer 17
)
yields a more detailed picture of the interionic contacts (Figure 3, left). The Hirshfeld
surface shows the distance from a point on the surface to the nearest atom outside
the surface, d , and the distance, d, from a point on the surface to the nearest atom
e
i
within the surface. The normalized contact distance, dnorm, is calculated according to
vdW
vdW
vdW
vdW
vdW
vdW
dnorm = (d – ri ) / ri
+ (d – r
e
) / re
(were ri ; re
= van der Waals radii of
i
e
the atoms). The plot of the Hirshfeld surface shows red areas (Figure 3, left)
indicating distances shorter than the sum of vdW radii and white to blue coded areas,
which mark distances longer than the sum of vdW radii. The so-called fingerprint plot
can be regarded as the 2D representation of the 3D Hirshfeld surface displaying
binning (d, d ) pairs (Figure 3, right).
i
e
1
4
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Figure 3. Hirshfeld surfaces of “Au1” (top, left) and “Au2” (bottom, left) and the
corresponding fingerprint plots (right). Color coding: red indicates distances shorter than the
sum of vdW radii; white to blue: distances are longer than the sum of vdW radii.
The Hirshfeld surface of the two cations is similar with only slightly differing volumes
3
3
of the molecules (“Au1”: 184.5 Å ; “Au2”: 188.1 Å ). But differences of intermolecular
interactions are visible analyzing decomposed fingerprint plots in detail. Both
fingerprint plots show two spikes which are caused by N-HS interactions at longer
d (d : ca. 1.5 Å, d  0.85 Å) and by N-HN at shorter d /d values. For “Au1” these
e
e
i
e
i
two interactions amount to 21.3 % of all intermolecular interactions, and it is lower for
Au2” (18.6 %). For “Au1” intermolecular SS interactions are seen as a spike-like
feature for d /d at ca. 1.65 Å (26.3 % of all interactions), while for “Au2” these
“
e
i
interactions (18.4 %) seem to be less pronounced at d /d at  1.8 Å. Some AuS
e
i
interactions (“Au1”: 4.5 %; “Au2”: 4.1 %) are also located in this d /d region. SH-N
e
i
bonding occurs at d  1.3 – 1.9/d  1.9 – 2.3 Å (“Au1”: 14.3 %; “Au2”: 11 %). The
e
i
-
differing orientation of the cations with respect to the SCN anion results in
significantly different contributions of CS/SC interactions: “Au1” = 2.8/4.6 %,“Au2”:
6
.5/10.7 %. Finally, HH (“Au1”: 7.1 %; “Au1”: 6.7 %) and NH-N (“Au1”: 1.8 %;
“Au1”: 2.0 %) interactions contribute to the overall arrangement and stability of the
1
5
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cations and anion. The gap of interactions seen in the fingerprint plot of “Au2” (d: 1.6
i
–
1.9 Å; d : 1.25 – 1.5 Å) is mainly caused by missing NH-N and HH interactions.
e
We note that non-covalent chalcogen-chalcogen interactions are not unusual and are
[
71]
frequently observed in organic compounds containing S, Se or Te.
For understanding the importance of van der Waals interactions DFT structure
optimizations without and with additional correction for dispersion were performed
revealing that significant contributions of van der Waals interactions stabilize the
structure in the title compound (Figure 4).
-
-
-
0
1
2
PBE
PBE+D3
PBE+D3+BJ
E/eV
-
-
-
3
4
5
400
450
500
550
V/A³
600
650
700
Figure 4. Energy-volume optimizations without (PBE) and with correction for dispersion
according to Grimme (PBE+D3) and Becke-Johnson-Potential (BJ).
Neglecting dispersion interactions, as given within standard GGA functionals like
PBE, a cell volume is predicted that exceeds the experimentally known value of 492
A³ by 20%. In line with results from calculations on dispersion interactions, e.g. in
[
50]
white nanorod and black phosphorous
, this error is corrected to a good
1
6
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approximation by the addition of Grimme´s D3 correction. In the present case
applying D3 reduces the total electronic binding energy by approximately 300 kJ/mol
(
unit cell content) resulting from dispersion interactions. The best coincidence of
experimentally found and predicted cell volumes is obtained when Becke-Johnson
BJ) damping is added. The obtained energy contribution of dispersion interaction is
(
then approx. 400 kJ/mol.
Thermal Decomposition and Investigation of the Decomposition Products
Depending on the heating rate, thermal decomposition of [Au(S CNH ) ]SCN starts in
2
2 2
the range of T ≈ 150 (1 K/min) to 176 °C (8 K/min). Because of a poor separation of
the mass steps (Δm) in the TG-curve an unambiguous assignment of the volatile
components emitted at distinct temperature intervals is not possible. Obviously,
several decomposition processes interfere with each other. Thermal degradation is
almost complete at 600 °C, and assuming formation of elemental Au, the theoretical
mass loss of Δmtheor = 55.2% matches well with Δmexp = 53.3% and 54.62%, obtained
with two different samples. The SEM images of residues (Figure 5, bottom) show that
the needle-like morphology of the crystals (Figure 5, top) is preserved with lengths >
5
00 µm. However, the shape of the needles is distorted and they are partially bent.
Observed with the naked eye the residues are black (samples from a, b,
Experimental Section), but in the optical microscope the needle-like crystals appear
gold-colored (Figure S4).
1
7
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Figure 5. SEM images: pristine crystals of [Au(S CNH ) ]SCN (top). Morphology of the
2
2 2
residues after thermal decomposition (bottom) at T = 600 °C.
f
The PXRD patterns of the residues, after thermal decomposition, revealed that these
particles consist of nano crystalline gold. The reflections show a significant
characteristic broadening (Figure S5, red pattern). Further chemical analysis of the
residues showed that the samples contain small amounts of C, N and H. The
percentage depends on the final temperature T during the thermal degradation (e.g.
f
4
60 °C ≤ T ≤ 600 °C). The ratio N:C lies in the range of 1.8 to 1.4. A ratio of N:C ≈
f
1
.6 is characteristic of condensed triazine and heptazine containing compounds like
[
72]
[73]
[74]
melem C N (NH )
2 3
, melon
or polytriazine imid (PTI).
Formation of melem
6
7
can be ruled out because this material is crystalline and one would expect
characteristic reflections in the X-ray powder pattern (see below). In addition, melem
is soluble in hot water, but the black material covering the nano gold particles could
not be dissolved in any usual solvent. N:C = 1.5 corresponds to C N , or melon
2
3
(
C N H (NH ) , respectively, N:C ≈ 1.33 matches C N . Hence, increasing T leads
18 24 3 2 3 3 4 f
[
72]
to decreasing nitrogen content (see e.g. postulated condensation of melamine ).
1
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13
C Solid State NMR investigations
Disregarding the relative high hydrogen content in such compounds, we firstly
assume tentatively, the black covering on the nano gold needles is a mixed polymer
13
of melon and PTI type compounds. C solid-state NMR measurements support this
hypothesis (see Figure S6). The shifts of 164 and 157 ppm correspond well to
[
73]
[72]
heptazine units present in melon
and melem.
As the resonance at 164 ppm is
significantly more pronounced and the stoichiometry of heptazine units requires a 1:1
ratio of the two peaks, it is likely that also compounds containing triazine groups like
[
74]
[72]
13
in PTI
and melamin
are present as well. The latter exhibit typical
C
resonances around 165 ppm, which overlap with the corresponding heptazine peak
given the experimental resolution. The fact that the signals are broad and
overlapping most likely is a result from the attachment to the gold surface and the
15
poor crystallinity of the film. This is also consistent with the N NMR spectroscopic
measurement, which did not yield any spectral intensity. We attribute this to the
[
75]
locally changed susceptibility at the metal carbon nitride interface.
the condensation proceeds via elimination of NH , and ends with carbon nitride C N
4
Furthermore,
3
3
(
N:C ≈ 1.33) as postulated. The samples decomposed at lower temperatures (T <
f
6
00°C) contain some Au S. With increasing T the portion Au S/nano gold shifts
2 f 2
totally in favor to nano gold. At T > 700°C starts the formation of microcrystalline
f
gold, which can be noticed from PXRD (e.g. Figure S5, black pattern).
X-ray photoelectron spectroscopy analysis
The XPS spectra of the C1s and N1s regions are displayed in Figure 6. The main C
1
s contribution is located at 288.45 eV, and two signals with significantly lower
intensity are observed at 284.91 and 286.27 eV.
1
9
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Figure 6. High resolution C 1s (top) and N 1s (bottom) spectra of the decomposition product.
Red line: fit; black dots: experimental data, top bar are residuals expanded by a factor of 2.
In the N 1s region, two peaks are seen with binding energies of 398.84 and 400.24
eV. The background on the high binding energy (E ) side of the C 1s and N 1s region
B
2
0
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*
indicates shake-up features due to  transitions of aromatic systems. The
composition of the film determined by integration of the N 1s and C 1s signals is
roughly C N , which is in good agreement with the elemental analysis (C N ).
2
3
2
3.2
In one of the earliest XPS studies of carbon nitrides, 27 films prepared in different
ways were investigated. Two different types of carbon nitrides could be identified: N
rich films (average N:C ratio: 1.29(15)) with C 1s at 287.7(2) eV and N poor films
(N:C ratio: 0.40(17)) with a binding energy of 286.1(2) eV. The corresponding N 1s
binding energies were 398.5(2) and 400.0(3) eV. The most intense C 1s signal at
2
84.6 eV was interpreted as adventitious carbon and surface carbon without a bond
[
71]
to nitrogen due to reaction of nitrogen with O and/or CO from air.
The assumption
2
that CO reacts with nitrogen of a C-N network is highly unlikely because normal
laboratory air should contain CO in the lowest ppm range. In addition, CO is a
chemically relatively inert molecule, and one cannot assume a reaction with nitrogen.
Since this first report, many investigations and characterizations were published with
[
77-93]
differing XPS results.
Selected data are summarized in Table S4 giving the
binding energies for the C 1s and N 1s signals. Intriguingly, in most spectra the signal
of C 1s at around 288 eV dominates the C 1s region, but there are also spectra
where this peak exhibits the lowest intensity. The most intense N 1s signal ranges
from 398.0 to 400.06 (Table S4). Assuming the presence of a mixture of s-triazine
and tri-s-triazine (heptazine) motifs on the surface of the nano gold needles, the C 1s
signal at the highest binding energy is caused by the N-C=N group which is the most
3
abundant species. The signal at 286.27 eV may be due to sp -coordinated carbon
located on the defective surface of the film, and/or carbon containing contaminants.
Finally, the peak at 284.91 eV may be due to adventitious hydrocarbons on the
2
1
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2
surface of the film. For the N 1s peaks the most probable assignments are sp -
hybridized nitrogen (C=N-C, 398.84 eV) and N-(C ) or N-H (400.24 eV). Hence, the
3
XPS analysis reveals that the film deposited on the gold nanoparticles is most
probably a nitrogen-rich carbon nitride with composition near (CN) N.
2
Nitrogen sorption investigation
Nitrogen sorption investigation of the sample obtained at T = 460 °C shows textural
porosity with the adsorption/desorption branches indicating slit pores generated by
the arrangement of the nanoparticles (Figure 7). The specific surface area derived
2
from the BET method is approximately 19 m /g.
Figure 7. Nitrogen adsorption and desorption curves measured on a samples obtained at
decomposition temperature of 460 °C.
High energy X-ray synchrotron scattering experiments
High energy synchrotron scattering experiments were performed to evaluate the
nano-structure of residues after thermal decomposition. To calculate the
corresponding pair distribution function (PDF), additional scattering measurements
2
2
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were performed at higher Q range. All reflections could be indexed in agreement with
gold (ISCD # 64701), but discrepancies of intensities of different reflections became
obvious during Rietveld refinement. Additionally, an anisotropic broadening of the
reflections was observed, which is most likely caused by the anisotropic nature of the
pristine compound, causing a preferred growth direction of gold nanoparticles during
thermal decomposition. The volume weighted averaged domain size was determined
to be 20(1) nm, which is in very good accordance with results of PDF analysis (Table
S5). To elucidate the anisotropic nature of gold nanocrystallites, different models
were utilized to account for anisotropic domain shapes. First, a geometric approach
[
94]
by Ectors et. al. using an ellipsoidal model.
The z-axis of the ellipsoid was placed
parallel to [111] direction, since this was determined to be the preferred growth
[
94]
direction of the gold nanoparticles.
The second model is based on analyzing the
volume weighted domain size (L ) of different cubic reflection families (h00, hk0,
vol
[
95]
hkl). The results agree very well with each other and are given in Table S6.
With
both models the anisotropically broadened reflections are very well modeled and the
fitted intensity deviations could be fully taken into account. Both models provide an
identical picture of gold nanoparticles: gold domains of ~ 20 nm with an elongation
along the [111] axis (Figure 8).
c
2
3
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Figure 8. Rietveld refinement fit of XRPD data measured at P02.1, DESY. All main
reflections are in agreement with cubic gold. Anisotropic peak broadening, caused by
[
94]
ellipsoid domain shapes, was modeled using the approach by Ectors et. al.
Additionally, the domain size distribution was determined using the whole powder
[
96, 97]
pattern modeling (WPPM) approach established by Scardi and Leoni.
A cuboid
model was assumed for WPPM analysis, in accordance with hkl-dependent domain
size analysis (Figure 9). The results reveal a rather narrow distribution of nano
domains. In-house X-ray powder patterns showed slight asymmetric broadening of
the (200) reflection towards the (111), giving rise to the assumption that a certain
amount of stacking faults may be present in the samples as reported by e.g. Longo
[
98]
and Martorana.
Nevertheless, high energy synchrotron measurements revealed
the presence of small amounts of nano-crystalline Au S. The reflections of this
2
additional phase slightly overlap with the reflections of metallic gold causing the
observed asymmetry.
2
4
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Figure 9. LogNormal domain size distribution calculated based on Rietveld refinement of
high energy PXRD data collected at P02.1, DESY. The idealized gold particle was modelled
based on analyzing hkl-dependent domain sizes displaying the same elongation along [111]c
as in the ellipsoid model depicted in Figure 8.
[
99]
Additional stacking fault analysis after Warren
on the basis of (111) and (200)
reflection positions to calculate the fault density did not provide any substantial
evidence that notable amounts of stacking faults are present in the sample. Likewise,
the analysis of the PDF revealed no major deviations from an idealized Au model,
neither at short nor long range order (Figure 10). The results of a Rietveld–like least
squares fitting of the model PDF are given in Table S6.
2
5
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Figure 10. PDF obtained from total scattering experiments at beamline P02.1, DESY (λ =
.2074 Å) in comparison with a fitted model PDF based on fcc gold. The inset shows a zoom
0
into the short range part of the PDF (radial distance 2 – 15 Å).
Tomography by STEM
A STEM overview image of a porous needle structure is presented in Figure 11 a),
the highlighted area is magnified in Figure 11 b). STEM tomography is performed to
verify the morphology of the Au structures three-dimensionally. In this procedure, a
series of STEM images are merged by an algorithm to create a reconstruction model
that represents the morphology and gives the pore distribution of the porous Au
needles. The reconstruction results are visualized by the respective iso-surfaces of
Au in Figure 11 c) and d), the full volume is represented along x- and y-direction,
respectively. For a detailed presentation, a video file is available in the supporting
information that underlines the three-dimensional character of the Au structure. The
2
6
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average diameter of the individual nanoparticles is estimated to be 30(20) nm
standard deviation in parentheses) in average with a slight elongation, which
(
coincides with the result estimated from high energy X-ray synchrotron scattering.
The Bragg reflection intensity on concentric rings of the electron diffraction pattern
(Figure S7) is in good agreement with the structure of crystalline interconnected cubic
gold nanoparticles.
Figure 11. An overview STEM image a) and a magnified STEM image b) of the inset is
showed. A STEM tomography reconstruction with color coded Au iso-surfaces is shown b)
along the x- and c) along the y-direction. The elongated Au nanoparticles are interconnected
and form a porous Au structure.
Conclusions
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The title compound represents an exception among the already known related gold
compounds with the general composition [Au(S CNR ) ]X where R is a more or less
2
2 2
bulky organic group, while R = H in the present sample. As a consequence, N-H•••S
and N-H•••N hydrogen bonds as well as non-covalent intermolecular SS
interactions are observed which significantly contribute to the overall arrangement
and stability of the cations and anion in the structure. Gold particles with average size
of coherently scattering domains of 20 nm are formed during the thermal
decomposition of the title compound. The formation of such nanosized gold particles
was never reported before and we assume that the thermal decomposition behavior
was not investigated accordingly using [Au(S CNR ) ]X compounds. However, we
2
2 2
strongly presume that the thermal decomposition reaction is linked to the special
chemical composition ([Au(S CNH ) ]SCN) allowing the intermediate formation of
2
2 2
different graphitic carbon nitrides during thermal degradation. Tentatively, we believe
-
the SCN anions play an important role for the formation of the C-N-H compounds. It
was postulated in the literature that the condensation of melamine yields graphitic
[
72]
carbon nitrides.
J. v. Liebig synthesized melamine by heating KSCN in presence
of NH Cl [100], which is equivalent of using NH SCN. Hypothetically, similar chemical
4
4
reactions during the thermal decomposition of the title compound are conceivable,
like e.g.:
2
[Au(S CNH ) ]SCN → 2Au + 3S↑ + „2NH SCN” + 2CS ↑ + (CN) ↑ + S↑
(3)
2
2 2
4
2
2
We note: On the one hand, there is no experimental evidence that the decomposition
proceeds according to formal equation 3. On the other hand, provided that actually
NH SCN or its decomposition products (e.g. cyanamide, H N-C≡N) develop during
4
2
the thermal decomposition in N atmosphere, the formation of graphitic carbon
2
2
8
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nitrides is easily understood by studying the schematic diagram of the ammonium
[
101]
thiocyanate pyrolysis process, as indicated in a recently published article.
The
thermal decomposition of NH SCN in inert gas atmosphere and its product formation
4
are reported in detail. According to this study melamine together with other solid and
gaseous products are developed in the first step in the temperature interval of 220 –
[
72, 102]
3
00 °C). Melamine undergoes the above mentioned condensation.
Finally,
between 460 and 600 °C the black “glue” is generated on the surface of the nano
gold particles consisting of graphitic carbon nitrides which suppress formation of
larger gold particles. The graphitic carbon nitrides “will be almost completely
[
102, 103]
decomposed until the temperature rises to 700 °C”.
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgements
Financial support by the State of Schleswig-Holstein and DFG (INST 184/144-1
FUGG for University of Oldenburg) is gratefully acknowledged. We thank Inke Jeß for
the measurements of the single crystal data and Hennig Lühmann for the SEM
images.
2
9
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