Thermolabile noble metal precursors: (NO)[Au(NO3)4], (NO)2[Pd(NO3)4], and (NO) 2[Pt(NO3)6]

Article abstract of DOI:10.1002/anie.201106107

Oxidizing noble metals: Pure N₂O₅ can oxidize elemental gold and palladium. The complex nitrates obtained by these reactions, (NO)[Au(NO₃)₄], (NO)₂[Pd(NO₃) ₄], and also the first nitrate of tetravalent platinum, (NO) ₂[Pt(NO₃)₆] (see picture for the anion: Pt-gray, N-green, O-blue), have the potential to act as precursors owing to their high thermolability. The thermal decomposition of (NO)[Au(NO ₃)₄] was elucidated in detail by several methods. Copyright

Full text of DOI:10.1002/anie.201106107

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DOI: 10.1002/anie.201106107
Noble Metal Precursors
Thermolabile Noble Metal Precursors: (NO)[Au(NO3)4],
(NO)2[Pd(NO3)4], and (NO)2[Pt(NO3)6]**
Mathias S. Wickleder,* Frauke Gerlach, Steffen Gagelmann, Jçrn Bruns, Mandus Fenske, and
Katharina Al-Shamery*
Noble metals play a crucial role in various fields of
application, for example, as optical and electrical devices
and as heterogeneous catalysts. For most of these applica-
tions, the metals have to be provided in a well-defined form to
address the desired properties. For example, catalytic activity
is usually strongly surface-dependent and the production of
nanoparticles of the metals is often mandatory.[1–5] A second
example is the use of metals in electronic devices as conductor
pathways, which requires well-defined depositions of the
metals.[6,7] Owing to this significant impact, several physical
and chemical processes have been developed to allow for the
deposition of noble metals in different shapes. Among these,
chemical vapor deposition (CVD) is the most common
chemical procedure, and numerous precursors are known,
especially for the element gold.[8–12] The drawback of CVD
techniques is that organometallic compounds are usually
used, leading to significant carbon contamination of the
deposited metals.[13–15] Recently we have shown that the
nitrylium nitrate (NO2)[Au(NO3)4] is a suitable thermolabile
precursor for gold; this species was obtained from the
reaction of elemental gold with N2O5.[16] The use of nitrogen
oxides for the preparation of anhydrous nitrates can be traced
back to the 1970s when Addison et al. reported first inves-
tigations, mainly based on N2O4.[17] The advantage of
(NO2)[Au(NO3)4] as a precursor is that it does not contain
carbon or chlorine, and that its decomposition leads only to
volatile products besides the metal. Moreover, the nitrate can
be dissolved in N2O5, so that deposition of the precursor for
example on surfaces is easy to perform. Interestingly, this
precursor can not only be decomposed thermally but also
using an electron beam, which allows the simple “writing” of
gold structures.[17,18] These results led us to investigate
whether noble metals can generally be transformed into
thermolabile nitrates using N2O5. In the course of these
investigations, we were able to prepare the palladium and
platinum compounds (NO)2[Pd(NO3)4] and (NO)2[Pt(NO3)6],
and furthermore a new gold nitrate, (NO)[Au(NO3)4].
Red single crystals of (NO)2[Pd(NO3)4] and yellow single
crystals of (NO)[Au(NO3)4], both exhibiting a plate-shaped
habit, form when the respective metals react with N2O5 at
room temperature under inert conditions (Figure 1a,b). It is
Figure 1. Microscope images of single crystals of the complex nitrates
(NO)2[Pd(NO3)4] (left), (NO)[Au(NO3)4] (middle), and (NO)2[Pt(NO3)6]
(right).
imperative that the N2O5 is very pure, because no reaction
occurs if the N2O4 content in the anhydride is too high. The
difference to the preparation of the nitrylium compound
(NO2)[Au(NO3)4][16] that we have described earlier is that
much less N2O5 is used here. Unexpectedly, elemental
platinum does not react with N2O5 under these conditions,
even if the metal is provided as fine powder. Therefore we
attempted to apply various platinum compounds as starting
material, and we finally succeeded by using H2[Pt(OH)6],
which was prepared according to a literature procedure.[19]
This reaction afforded, again at room temperature, light
yellow plates of (NO)2[Pt(NO3)6] (Figure 1c). The nitrylium
nitrates are extremely hygroscopic and have to be handled
under the strict exclusion of moisture. Furthermore, the
decomposition of (NO)2[Pt(NO3)6] begins as soon as the
compound is removed from the reaction mixture.
In the crystal structure of (NO)2[Pd(NO3)4], the Pd atom
is coordinated in a square-planar manner by four monoden-
À
tate nitrate groups (Figure 2, left). The Pd O distances are
uniformly at about 201 pm and the O-Pd-O angles deviate not
[*] Prof. Dr. M. S. Wickleder, Dr. F. Gerlach, S. Gagelmann, J. Bruns,
Dr. M. Fenske, Prof. Dr. K. Al-Shamery
À
Carl von Ossietzky University of Oldenburg, Institute for Pure and
Applied Chemistry
Carl-von-Ossietzky Strasse 9–11, 26129 Oldenburg (Germany)
E-mail: mathias.wickleder@uni-oldenburg.de
Katharina.Al.Shamery@uni-oldenburg.de
more than 28 from the ideally expected 908. The N O
distances within the nitrate groups are significantly enlarged
for those oxygen atoms that are bonded to the Pd atom.
According to the site symmetry of the palladium atom, the
complex [Pd(NO3)4]2À anions show inversion symmetry (Ci)
and are located on the mid points of the unit cell edges
[**] Financial support of the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, and the Stiftung Nordwestmetall
is gratefully acknowledged. We thank Wolfgang Saak for the
collection of the X-ray data.
(Figure 2, right). Charge balance is achieved by the NO+
[20]
À
cations, which have a typical N O distance of 109.4 pm.
The structure of the anionic complex [Au(NO3)4]À in
(NO)[Au(NO3)4] is nearly identical to its palladium analogue.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106107.
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Figure 2. The [Pd(NO3)4]2À complex (left) and the crystal structure of
(NO)2[Pd(NO3)4] (right). Ellipsoids are set at 75% probability. Selected
bond lengths [pm] and angles [8]: Pd1–O21/O31 201.0(1)/201.1(1),
N2–O21/O22/O23 131.4(2)/123.1(2)/122.8(2), N3–O31/O32/O33
131.7(2)/124.0(2)/121.5(2), N1–O1 104.9(2); O21-Pd1-O31 87.97(6)/
92.03(6).
Figure 4. The [Pt(NO3)6]2À complex (left) and the crystal structure of
(NO)2[Pt(NO3)6] (right). Ellipsoids are set at 75% probability. Selected
bond lengths [pm] and angles [8]: Pt1–O11/O21/O31 199.9(2)/
200.9(2)/200.3(2), N1–O11/O12/O13 134.2(3)/121.0(3)/121.6(4),
N2–O21/O22/O23 135.0(3)/120.2(3)/122.3(4), N3–O31/O32/O33
133.8(3)/121.1(3)/121.8(3), N4–O4: 102.9(4); O11-Pt1-O21 101.14(7)/
78.86(7), O11-Pt1-O31 100.93(7)/79.07(7), O21-Pt1-O31 101.56(7)/
78.44(7).
It shows also Ci symmetry and the square-planar coordination
of the gold atoms is even closer to the ideal case with regard to
À
the Au O distances (200 pm) and the O-Au-O angles, which
are essentially 908 (Figure 3, left). The strong asymmetry of
resulting {PtO6} octahedron is remarkably distorted with
respect to the O-Pt-O angles, which deviate in part more than
108 from the ideal value of 908. Also in this compound there is
a significant bond length variation within the nitrate groups,
with the larger values found for the platinum-coordinated
oxygen atoms. The complex [Pt(NO3)6]2À ions show inversion
symmetry according to the 2d site of the Pt atom, and the
NO+ ions are located on a general crystallograhic site, thus
allowing a clear differentiation to be made between the N and
À
the O atom (N O: 102.9 pm).
The thermal decomposition of the complex noble metal
nitrates as monitored by differential thermal analysis (DTA)/
thermogravimetric analysis (TG) measurements show that
the metals form in any case. (NO)2[Pd(NO3)4] decomposes in
a three-step process, with the first step already complete at
1688C. It can be attributed to the formation of Pd(NO3)2
according to the observed mass loss of 41.9% (calcd. 42.4%).
The second step starts immediately after the first and leads to
PdO at 2988C. The latter decomposes between 602 and 7348C
and gives elemental palladium, as shown by its XRD pattern.
For (NO)2[Pt(NO3)6], the thermal analysis was carried out on
samples containing N2O5 to avoid decomposition of the
compound prior to the measurement. Thus, the first step of
the observed decomposition is correlated to the release of
adhesive N2O5. Nevertheless, it seems to be certain that at
least one intermediate is formed that is further degraded,
leading to a platinum oxide (most reliably PtO2), which finally
leads to elemental platinum at a temperature of 5508C,
according to X-ray analysis.
The decomposition of (NO)[Au(NO3)4] is already com-
plete at about 2408C. The DTA/TG measurements show one
quite strong but relatively broad signal ranging from 140 to
1908C and two significantly weaker effects with maxima at
200 and 2158C (Figure 5). The broad signal shows a shoulder
indicating that the observed peak is in fact a superposition of
two effects. Indeed, a DSC measurement of the compound
made these two peaks visible, and the maxima could be found
Figure 3. The [Au(NO3)4]À complex (left) and the crystal structure of
(NO)[Au(NO3)4] (right). Ellipsoids are set at 75% probability. Selected
bond lengths [pm] and angles [8]: Au–O11/O21 200.3(7)/200.1(7),
N1–O11/O12/O13 136(1)/124(1)/122(1), N2–O21/O22/O23 134(1)/
121(1)/123(1), N3–O3 113(3); O11-Au-O21 90.0(3)/89.9(3).
À
the nitrate ions with respect to the N O distances is also
observed. The main difference in the crystal structures of the
palladium and the gold nitrate is the arrangement of the
complex anions with respect to each other, which is caused by
the different charge and the reduced number of charge-
balancing NO+ cations in (NO)[Au(NO3)4]. In contrast to the
palladium compound, the NO+ cations are located on special
sites in the sense that a center of inversion is situated at the
À
mid points of the N O bonds (2b site of space group P21/n).
This position makes the nitrogen and the oxygen atom of the
NO+ dumbbells undistinguishable (Figure 3, right). It should
be noted that (NO)[Au(NO3)4] is strongly related to its
nitrylium congener (NO2)[Au(NO3)4] where the NO2+ cation
is located at the same position (with the nitrogen atom at the
2b site) as the NO+ cation here.
The platinum(IV) nitrate (NO)2[Pt(NO3)6] is the first
example of a tetravalent platinum compound containing
a complex oxoanion. Six nitrate groups are coordinated as
À
monodentate ligands with the Pt O distances falling in the
narrow range of 199.9 to 200.9 pm (Figure 4, left). The
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O2. This region matches quite well the strong peaks observed
in the DSC and DTA measurements. The third region in the
TDS measurements is dominated by the signal for O2 (m/z =
32) and is related to the weak peaks observed in the DSC
curve. With respect to these findings, we assume that the
decomposition of (NO)[Au(NO3)4] occurs via an oxide
intermediate that readily decomposes further to yield ele-
mental gold.
This assumption is also corroborated by XPS measure-
ments (XPS = X-ray photoelectron spectroscopy) that were
performed during the precursor decomposition (Figure 6).
The spectrum at 3508C after complete decomposition of the
Figure 5. Thermal behavior of (NO)[Au(NO3)4]: DSC and DTA data
(upper part) show two closely associated strong signals, and two weak
signals slightly below and above 2008C. According to thermal desorp-
tion measurements (TDS; lower part) the first step is dominated by
the release of nitrogen oxides, while for the second step additionally
O2 is observed. Oxygen is also the dominant species for the last step.
TDS curves show signals below 1008C as the sample was prepared in
liquid N2O5. A strong H2O signal (m/z=18) is attributed to contami-
nation during sample preparation.
at 156 and 1698C. The weak peaks are also seen in the DSC
curve, with maxima at 193 and 2138C. To investigate the
decomposition behavior in more detail and to elucidate the
precursor potential of (NO)[Au(NO3)4], we performed ther-
mal desorption measurements (TDS) of samples that were
deposited onto a silicon wafer as a solution in N2O5. The ion
signals upon heating can be divided in three regions. The first
is clearly attributed to the release of solvent N2O5 and shows
mainly ions with m/z at 30 (NO), 28 (N2), and 44 (N2O).
Furthermore a signal at m/z = 18 indicates that some H2O is
formed, which is most likely due to the formation of HNO3
during the sample preparation. The second region is associ-
ated with the precursor decomposition and shows the release
of NO and N2O and, after a small delay, also the occurrence of
Figure 6. Temperature-dependent XPS on (NO)[Au(NO3)4], indicating
the changes in energy for the 4f7/2 and 4f5/2 states of gold. The regions
of strong changes are marked as red spectra and by arrows in the left
diagram. Lower part: the spectra before and after heating of the
precursor; the observed 2p signal for silicon did not change during the
measurement and served as a standard during data handling.
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device. After unit cell determination, the reflection intensities were
collected.[22] For (NO)2[Pd(NO3)4] and (NO)[Au(NO3)4], powder
diffraction measurements were performed. For that purpose, finely
grounded samples were filled in glass capillaries, which were sealed
and measured with CuKa radiation using the STADI P powder
diffractometer. The data were handled with the software package
for the STOE powder diffraction system (WIN XPOW 3.01, Stoe & Cie,
Darmstadt, Germany, 1996). Owing to the above-mentioned decom-
position, no powder pattern of (NO)2[Pt(NO3)6] could be obtained.
DTA/TG measurements: The thermal decomposition of the
nitrates was investigated using a TGA/SDTA851e apparatus (Mettler-
Toledo GmbH, Schwerzenbach, Switzerland). (NO)2[Pd(NO3)4] or
(NO)[Au(NO3)4] (ca. 10 mg) was filled in corundum crucibles in
a glove box. For the platinum compound, measurements were
performed on samples containing some amounts of N2O5 to avoid
decomposition prior to the measurement. All of the samples were
heated with a rate of 4 KminÀ1 up to 9008C. The collected data were
processed using the software of the analyzer (STARe V8.1, Mettler-
Toledo, Schwerzenbach, 2004).
DSC measurements: For (NO)[Au(NO3)4], DSC measurements
were performed in a DSC-204 PhoenixTM cell (Netsch GmbH, Selb,
Germany) using a TASC 414/3A controller. For the measurement, the
sample (4 mg) was prepared in corundum crucibles equipped with
perforated lids. The heating rate was 5 KminÀ1 and the data were
handled using the program of the device (Netzsch Instrumental
Software Version 3.5).
TDS measurements: The precursor decomposition was carried
out by heating the silicon wafer, which was coated with a solution of
(NO)[Au(NO3)4] in N2O5, with steps of 15 K up to 3508C. Debuting
gases were characterized by a quadrupole mass spectrometer (Pfeiffer
QMS/QMA-200). The data were handled with the spectrometer
software (Quadstar 422).
precursor clearly shows the expected bonding energies of 84.0
and 87.7 eV for the Au4f7/2 and 4f5/2 states of elemental
gold.[23] The temperature region between 110 and 1408C is
dominated by the Au4f7/2 and 4f5/2 signals at 85.7 and 86.4 eV,
respectively, which can be assigned to a gold oxide from the
findings of the TDS measurements. When compared to
literature data, this gold oxide is most probably Au2O3,[23]
but there are also some reports claiming bonding energies for
oxidic AuI species in the same region.[24] However, reliable
data are not available because Au2O is still elusive. The
spectra at temperatures before the precursor decomposition
starts show energies at 86.5 and 90.2 eV, which can be
attributed to 4f7/2 and 4f5/2 states of Au3+ in the precursor
compound. It has to be emphasized that small intensities of
the gold oxide and of elemental gold are already present in
the low-temperature spectra. Obviously some decomposition
of the compound is not avoidable during the preparation of
the sample for the measurement. Nevertheless, the three
different measurements (DSC/DTA/TG, TDS, and XPS) are
highly congruent and present a clear view of the thermal
behavior of (NO)[Au(NO3)4]. It shows that even for gold, the
decomposition proceeds via an oxide as intermediate. How-
ever, in contrast to the palladium and platinum compound,
this oxide decomposes immediately to elemental gold.
Presently we are investigating whether (NO)[Au(NO3)4] can
also be used in electron-beam-induced decomposition reac-
+
tions in analogy to its NO2 congener, and also whether
mixtures of the gold compound and it palladium or platinum
analogues are suitable for decomposition to give defined alloy
structures.
X-ray photoelectron spectroscopy (XPS). The spectra of a pre-
cursor-coated silicon wafer were obtained at steps of ca. 10 K (in
accordance with the observed TDS date) in an ultrahigh vacuum
chamber (base pressure of 3 ꢀ 10À10 hPa). The sample was irradiated
by Mg Ka1,2 photons (kinetic energy 1253,6 eV) generated by
a SPECS XR-50 X-ray gun. The emitted electrons were analyzed
by a hemispheric energy analysator (Leybold EA-10) and detected
with an electron multiplier system (Leybold).
Experimental Section
N2O5: Pure N2O5 was prepared by dehydration of fuming HNO3 with
P4O10 according to Ref. [21]. During the dehydration, a constant
stream of ozone (ca. 5% in flowing O2) was passed through the
apparatus, and the resulting N2O5 was directly condensed into the
reaction flasks.
Received: August 29, 2011
Published online: January 27, 2012
(NO)2[Pd(NO3)4]: Palladium powder (0.053 g) was filled into
a 250 mL glass flask. The flask was cooled to À708C and N2O5 in
excess was condensed onto the metal powder. The flask was sealed
and allowed to warm up to room temperature. During that time, the
metal reacted under formation of a red-brown solution and a brown
gas (NO2). From the solution, well-developed orange-red single
crystals separate over the course of five days. By purging with dry N2,
remaining N2O5 as well as other volatile reaction products were
removed and the product was transferred to a glove box.
(NO)[Au(NO3)4]: The same procedure as described for
(NO)2[Pd(NO3)4] was repeated using gold powder (0.050 g). The
metal dissolved rapidly, and within three days yellow plate-like single
crystals grew, which were also handled in a glove box after unreacted
N2O5 and other volatile products had been removed with N2.
(NO)2[Pt(NO3)6]: H2[Pt(OH)6] was prepared according to
Ref. [19], and the compound (109 mg) was reacted with N2O5 as
described above. Yellow single crystals started to grow shortly after
the reaction has finished. It turned out that the crystals decompose
readily when the remaining N2O5 was removed completely. Therefore
some N2O5 was kept in the flask for storage purposes.
Keywords: analytical methods · nitrates · noble metals ·
photoelectron spectroscopy
.
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[22] Measurements: STOE IPDS I (for Au and Pd), BRUKER k-
APEX2 (for Pt), l(MoKa) = 71.073 pm, T= 153 K; Computing:
structure solutions by direct methods, full-matrix least-square
refinements, programs SHELXS-97 and SHELXL-97 (G. M.
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with X-RED 1.22 and X-SHAPE 1.06 (Stoe, Darmstadt 2001
and 1999); Crystal data: (NO)[Au(NO3)4]: Yellow block (0.55 ꢀ
0.38 ꢀ 0.37 mm3), monoclinic, P21/n, Z = 2, a = 827.01(9), b =
726.38(9), c = 909.20(10) pm, b = 111.907(10)8, 506.74(10) ꢃ3,
1 = 3.133 gcmÀ3, 2qmax. = 52.348, 6208 reflections, 1000 unique
reflections (Rint. = 0.1064), m = 146.11 cmÀ1, min./max. transmis-
sion = 0.0459/0.0745, 89 parameters, R1 = 0.0342, wR2 = 0.0618
for all reflections, max./min. residual electron density = 1.620/
À1.495 eÀ ꢃÀ3
.
(NO)2[Pd(NO3)4]: Red plate (0.40 ꢀ 015 ꢀ
0.07 mm3), monoclinic, P21/c, Z = 2, a = 806.93(9), b =
747.31(5), c = 917.08(10) pm, b = 99.372(13)8, 545.64(9) ꢃ3, 1 =
2.523 gcmÀ3, 2qmax. = 56.508, 7773 reflections, 1266 unique reflec-
tions (Rint. = 0.0514), m = 18.80 cmÀ1, min./max. transmission =
0.5316/0.8839, 97 parameters, R1 = 0.0215, wR2 = 0.0446 for all
reflections, max./min. residual electron density = 0.468/
À0.678 eÀ ꢃÀ3
. (NO)2[Pt(NO3)6]: Yellow block (0.41 ꢀ 0.23 ꢀ
0.21 mm3), monoclinic, P21/c, Z = 2, a = 711.38(2), b =
934.96(3), c = 1156.68(4) pm, b = 107.559(2)8, 733.48(4) ꢃ3, 1 =
2.840 gcmÀ3
, 2qmax. = 56.608, 12740 reflections, 3041 unique
reflections (Rint. = 0.0596), m = 97.09 cmÀ1, min./max. transmis-
sion = 0.1097/0.2350, 134 parameters, R1 = 0.0258, wR2 = 0.0734
for all reflections, max./min. residual electron density = 4.068/
À2.172 eÀ ꢃÀ3. Further details on the crystal structure investiga-
tion may be obtained from the Fachinformationszentrum
Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax:
(+ 49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on
quoting the depository numbers CSD-423407 (NO)[Au(NO3)4],
423406 (NO)2[Pd(NO3)4], and 423405 (NO)2[Pt(NO3)6].
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Angew. Chem. Int. Ed. 2012, 51, 2199 –2203
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