Oxoanionic noble metal compounds from fuming nitric acid: The palladium examples Pd(NO3)2 and Pd(CH3SO3)2

Article abstract of DOI:10.1002/chem.201405355

The oxidation of elemental palladium at 100?°C in a mixture of fuming nitric acid and a pyridine-SO₃ complex leads to the anhydrous nitrate Pd(NO₃)₂ (monoclinic, P2₁/n, Z = 2, a = 469.12(3) pm, b = 593.89(3) pm, c = 805.72(4) pm, ?2 = 105.989(3)?°, V = 215.79(2) A??³). The Pd²⁺ ions are in square-planar coordination with four monodentate nitrate groups which are connected to further palladium atoms, leading to a layer structure. The reaction of elemental palladium with a mixture of fuming nitric acid and methanesulfonic acid at 120?°C leads to single crystals of Pd(CH₃SO₃)₂ (monoclinic, P2₁/n, Z = 2, a = 480.44(1) pm, b = 1085.53(3) pm, c = 739.78(2) pm, ?2 = 102.785(1)?°, V = 376.254(17) A??³). Also in this structure the Pd²⁺ ions are in square-planar coordination with four monodentate anions; however, the connection to adjacent palladium atoms leads to a chain-type structure. The thermal decomposition of the compounds has been investigated by means of DSC/TG measurements. Furthermore, IR and Raman spectra have been recorded, and an assignment of the observed vibrational frequencies has been carried out based on theoretical investigations.

Full text of DOI:10.1002/chem.201405355

DOI: 10.1002/chem.201405355
Full Paper
&
Anhydrous Pd(NO3)2 and Pd(CH3SO3)2
Oxoanionic Noble Metal Compounds from Fuming Nitric Acid:
The Palladium Examples Pd(NO3)2 and Pd(CH3SO3)2
Jçrn Bruns, Thorsten Klꢀner, and Mathias S. Wickleder*[a]
Abstract: The oxidation of elemental palladium at 1008C in
a mixture of fuming nitric acid and a pyridine-SO3 complex
leads to the anhydrous nitrate Pd(NO3)2 (monoclinic, P21/n,
Z=2, a=469.12(3) pm, b=593.89(3) pm, c=805.72(4) pm,
b=105.989(3)8, V=215.79(2) ꢁ3). The Pd2+ ions are in
square-planar coordination with four monodentate nitrate
groups which are connected to further palladium atoms,
leading to a layer structure. The reaction of elemental palla-
dium with a mixture of fuming nitric acid and methanesul-
fonic acid at 1208C leads to single crystals of Pd(CH3SO3)2
(monoclinic, P21/n, Z=2, a=480.44(1) pm, b=1085.53(3)
pm, c=739.78(2) pm, b=102.785(1)8, V=376.254(17) ꢁ3).
Also in this structure the Pd2+ ions are in square-planar co-
ordination with four monodentate anions; however, the con-
nection to adjacent palladium atoms leads to a chain-type
structure. The thermal decomposition of the compounds has
been investigated by means of DSC/TG measurements. Fur-
thermore, IR and Raman spectra have been recorded, and an
assignment of the observed vibrational frequencies has
been carried out based on theoretical investigations.
Introduction
with low-temperature ferromagnetic ordering of the com-
pound. Interestingly this type of coordination seems to be fa-
vored with disulfate groups as ligands, as can be seen from
the hydrogendisulfate Pd(HS2O7)2, which is also paramagnet-
ic.[5] Up to now, analogous platinum compounds could not be
obtained and simple “binary” platinum sulfates are still lacking.
However, in the presence of suitable counterions, elemental
platinum can be oxidized by oleum to its tetravalent state in
form of the complex anion [Pt(S2O7)3]2ꢀ.[6] The metal atom is
coordinated by three chelating disulfate ions, a structural motif
that we have observed for a large number of other tetravalent
central atoms, such as Si, Ti, Ge, and Sn.[7] Another important
focus of our research is the reaction with nitric acid and its an-
hydride N2O5. The latter in particular is an extreme strong oxi-
dizer, and even elemental gold dissolves readily in N2O5. This
reaction leads to (NO2)[Au(NO3)4] or (NO)[Au(NO3)4].[8] As we
have shown, these nitrates are interesting precursors for the
structuring of elemental gold. Similar compounds could be
also obtained for palladium and platinum, namely the complex
nitrates (NO)2[Pd(NO3)4] and (NO)2[Pt(NO3)6].[8] In contrast to
such nitrato-metalates, binary nitrates of the noble metals are
not known up to now; only the hydrate Pd(NO3)2·2H2O has
been reported.[9] We have now succeeded in the preparation
of the first anhydrous noble metal nitrate Pd(NO3)2 by using
a reaction of elemental palladium with fuming nitric acid. This
acid is obviously both a strong oxidizer and a suitable solvent
for the crystallization of the nitrate. The addition of a small
amount of SO3 (in form of its pyridine complex) to the reaction
mixture was thought as a scavenger for H2O that might form
in the reaction. The successful use of fuming nitric acid as a sol-
vent forced us to also investigate the formation of other oxo-
anionic palladium compounds from that solvent. In course of
these attempts we were able to prepare the methanesulfonate
Over the last years, we have explored reactions of noble
metals and noble-metal compounds with highly concentrated
mineral acids and their anhydrides under harsh conditions.
Under these conditions, unexpected reactivities were frequent-
ly observed, and noble metal compounds with unusual struc-
tures and properties have been obtained. The gold sulfate
Au2(SO4)2 is a striking example. The latter forms in the reaction
of Au(OH)3 in concentrated sulfuric acid and contains metal–
4+
metal-bonded Au2 dumbbells.[1] Another interesting example
is the reaction of elemental platinum with H2SO4 at elevated
temperature. Usually platinum is considered to be inert against
sulfuric acid, but it turned out that under the conditions ap-
plied, the red platinum(III) sulfate Pt2(SO4)2(HSO4)2 forms. This
sulfate also contains a metal–metal bond in the dumbbell-
6+
shaped Pt2 ion.[2] The lighter platinum congener palladium
cannot be oxidized to an oxidation state higher than +II
under these conditions and instead the sulfate PdSO4 is found
in various modifications.[3] Even with neat SO3 no high valent
palladium can be gained. However, the reaction of palladium
with SO3 afforded a highly remarkable compound, namely the
disulfate Pd(S2O7).[4] This disulfate exhibits Pd2+ ions in unusual
octahedral coordination leading to paramagnetic behavior,
[a] J. Bruns, Prof. Dr. T. Klꢀner, Prof. Dr. M. S. Wickleder
Carl von Ossietzky Universitꢁt Oldenburg
Institut fꢀr Chemie
Carl-von-Ossietzky Strasse 9–11
26129 Oldenburg (Germany)
Fax: (+49)441-798-3352
E-mail: mathias.wickleder@uni-oldenburg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405355.
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Pd(CH3SO3)2 when excess methanesulfonic acid was added to
the reaction mixture. This compound is the first binary noble
metal methanesulfonate reported to date. Furthermore, apart
from the gold complexes M[Au(CH3SO3)4] (M=Li, Na, Rb), it is
only the second noble metal methanesulfonate at all.[10] This is
remarkable because methanesulfonic acid is a very common
electrolyte for electroplating, and is also used for such process-
es involving noble metals.[11] The two palladium compounds
have been not only characterized by means of single-crystal X-
ray diffraction, but also by thermal analyses, and vibrational
spectroscopy supported by quantum chemical calculations.
Figure 1. Structure and atom labeling for the [Pd(NO3)4] unit in Pd(NO3)2.
The four nitrate anions are related by inversion symmetry at the Pd site. El-
lipsoids are set at 75% probability. Selected bond lengths [pm] and angles
[8] (theoretical values in italics): Pd1ꢀO1 200.5(1)/204.3, Pd1ꢀO2 202.5(1)/
209.4, N1ꢀO1 127.8(2)/128.3, N1ꢀO2 129.3(2)/128.4, N1ꢀO3 120.7(2)/122.4,
O1-Pd1-O2 87.94(4)/89.3, O2-Pd1-O1 92.06(4)/91.6, O1-N1-O2 117.2(1)/118.6,
O1-N1-O3 119.8(1)/119.1, O2-N1-O3 124.0(1)/122.3 (For a full list, see the Sup-
porting Information, Table S3).
Results and Discussion
Crystal structures
The anhydrous nitrate Pd(NO3)2 crystallizes with monoclinic
symmetry (Table 1). The Pd2+ cation is located on the special
Wyckoff site 2b of space group P21/n, bearing inversion sym-
metry. Four oxygen atoms surround it in a square-planar
manner with the distances PdꢀO being 200.5(1) and 202.5(1)
pm, respectively (Figure 1). The angles O-Pd-O of 87.94(4)8 and
92.06(4)8 show that there is only a slight deviation from ideal
values. The observed values for the bond lengths and angles
are in good accordance with the values found for the compa-
rable ternary palladium nitrates M2[Pd(NO3)4] (M=Na, K, Rb, Cs,
NO).[8,12] The oxygen atoms belong to four monodentate ni-
trate groups and each nitrate group is connected to one fur-
ther Pd2+ ion leading to infinite layers according to the Niggli
Table 1. Crystal data and structure refinement for Pd(NO3)2 and
Pd(CH3SO3)2.
Formula
Pd(NO3)2
Pd(CH3SO3)2
Formula weight [gmolꢀ1
Temperature [K]
Crystal system
Space group
a [pm]
]
230.42
153(2)
296.59
120(2) K
formula 2[Pd(NO3)4/2] (Figure 2). Within the nitrate group, the
1
Pd2+ bonded oxygen atoms show significantly longer NꢀO dis-
tances (127.8(2) and 129.3(2) pm) than the respective distance
to the remaining non-coordinating oxygen atom (120.7(2) pm).
The observed distances match very well the respective data
obtained for theoretical investigations (see caption of
Figure 1). These have been performed on the fragment
[(NO3)3Pd(NO3)Pd(NO3)3]3ꢀ as a cut-off of the three-dimensional-
ly linked crystal structure. This fragment is used as a suitable
model for the bidentate bridging nitrate group. The infinite
monoclinic
P21/n (No. 14)
469.12(3)
593.89(3)
805.72(4)
105.989(3)
215.79(2)
2
monoclinic
P21/n (No. 14)
480.44(1)
1085.53(3)
739.78(2)
102.785(1)
376.25(2)
2
2.618
30.01
288
0.18ꢃ0.08ꢃ0.06
6.782 to 80.5588
ꢀ8ꢁhꢁ8
0ꢁkꢁ19
0ꢁlꢁ13
15247
b [pm]
c [pm]
b [8]
Volume [ꢁ3]
Z
1calc [mgmꢀ1 m3]
3.546
42.58
216
Absorption coefficient [cmꢀ1
F(000)
]
Crystal size [mm3]
0.05ꢃ0.02ꢃ0.02
2q range for data collection 8.64 to 80.548
Index ranges
ꢀ8ꢁhꢁ8
ꢀ10ꢁkꢁ10
ꢀ14ꢁlꢁ14
12002
Reflections collected
Independent reflections
Completeness to q=40.278
Absorption correction
Refinement method
1368 (Rint =0.0354) 3994 (Rint =0.0224)
1.000
0.999
empirical
multi-scan
Full-matrix least-
squares on F2
3994/0/54
1.070
Full-matrix least-
squares on F2
1368/0/44
1.051
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I^2s (I)]
R1 =0.0159,
wR2 =0.0316
R1 =0.0288,
wR2 =0.0346
0.0077(11)
R1 =0.0150;
wR2 =0.0416
R1 =0.0161;
wR2 =0.0421
–
Final R indexes (all data)
Extinction coefficient
Largest diff. peak/hole/eꢀ/ꢁꢀ3 0.50/ꢀ0.63
Ratio of non-meroedric twin
individuals [%]
0.54/ꢀ102
47:53
Figure 2. Infinite layers according to 2[Pd(NO3)4/2] in the structure of
Pd(NO3)2. The layers are oriented parallel to the (ꢀ101) plane of the unit cell.
1
Deposition number
425998 (see ICSD)
963236 (see CCDC)
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Figure 5. Infinite chains according to the Niggli formula 1[Pd(CH3SO3)4/2] in
1
the structure of Pd(CH3SO3)2.
ꢀ
Figure 3. View of the layers shown in Figure 2 along the [010] direction. The
layers are held together only by weak interactions.
The oxygen atoms belong to four monodentate CH3SO3
anions, which are connected to one further Pd2+ ion. In con-
trast to the structure of Pd(NO3)2, this connection leads now to
a chain structure according to 1[Pd(CH3SO3)4/2] (Figure 5).
1
layers of nitrate linked Pd2+ cations are oriented parallel to the
Those oxygen atoms of the anion that are connected to a palla-
dium cation show significantly longer SꢀO bonds (149.09(8)
and 150.02(8) pm) compared to the uncoordinated oxygen
atom (143.54(9) pm). This finding is expected and in line with
the observation for the [Au(CH3SO3)4]ꢀ complex, which shows
even a distance of about 154 pm for the gold-coordinated
oxygen atom.[10] Furthermore, the observed distances are well
reproduced by theory (cf. caption of Figure 4). For these calcu-
lations the [(CH3SO3)2Pd(CH3SO3)2Pd(CH3SO3)2]2ꢀ fragment has
been chosen as a model. It is essentially a part of the
¯
(101) plane of the unit cell (Figure 3). The layers are held to-
gether by weak interactions only. The shortest distances be-
tween palladium atoms and oxygen atoms of adjacent layers
are clearly above 350 pm.
Similar to the structure of Pd(NO3)2, in the monoclinic meth-
anesulfonate Pd(CH3SO3)2 the Pd2+ ion is also located on a spe-
cial position with inversion symmetry of the space group P21/n
(Wyckoff site 2a) and coordinated in a square planer manner
by oxygen atoms (Figure 4). The PdꢀO distances show typical
values of 201.16(8) and 202.05(8) pm and the [PdO4] moiety is,
with respect to the angles O-Pd-O of 94.17(3) and 85.83(3)8,
slightly more distorted compared to the structure of Pd(NO3)2.
11[Pd(CH3SO3)4/2
]
chain. These 1[Pd(CH3SO3)4/2
]
chains in
1
Pd(CH3SO3)2 are oriented along the [100] direction and they
are related by the glide plane n that is perpendicular to the
[010] direction at y=1= [010] (Figure 6). The chains are ar-
4
ranged in a typical densest rod-packing fashion, and there are
obviously only weak interactions between the chains. These
might be attributed to hydrogen bridges with CH3 groups as
donor groups and the non-coordinated oxygen atoms of sulfo-
nate groups as acceptors. Even if this type of hydrogen bridge
is certainly very weak, they are believed to be important for
the assembling of neutral building blocks in the solid state.[13]
The shortest donor–acceptor distances are found between C1
Figure 4. Structure and atom labeling of the [Pd(CH3SO3)4] unit in
Pd(CH3SO3)2. Ellipsoids are set at 75% probability. Selected bond lengths
[pm] and angles [8] (theoretical values in italics): Pd1ꢀO11 200.16(8)/203.2,
Pd1ꢀO12 202.05(8)/206.5, S1ꢀO11 150.02(8)/152.3, S1ꢀO12 149.09(8)/152.1,
S1ꢀO13 143.54(9)/147.1, S1ꢀC1 174.98(11)/179.6, O1-Pd1-O2 85.83(3)/87.8,
O2-Pd1-O1 94.17(3)/90.8, O11-S1-O12 107.47(5)/112.4, O11-S1-O13 113.30(5)/
115.7, O11-S1-C1 106.03(5)/102.8, O12-S1-O13 114.77(5)/110.7, O12-S1-C1
103.68(5)/105.7, O13-S1-C1 110.81(5)/108.7 (for a full list, see the Supporting
Information, Table S3).
Figure 6. The 1[Pd(CH3SO3)4/2] chains in Pd(CH3SO3)2, which are arranged in
a rod-packing fashion along the [100] direction. They are related by the
1
glide plane n of the monoclinic unit cell.
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46.7%, in accordance with the loss of one molecule of N2O5
per formula unit. As intermediate the oxide PdO is formed, as
has been proven by XRD measurements. The degradation of
Pd(NO3)2 matches quite well our observations during the de-
composition of (NO)2[Pd(NO3)4]. For this compound, we postu-
lated the intermediary formation of anhydrous Pd(NO3)2, which
then is further decomposed to PdO at a temperature of 1688C,
in accordance with the findings presented here. The final step
is the decomposition of PdO, yielding elemental palladium.
This decomposition occurs between 640 and 7608C (Tmax
=
7408C) accompanied by an experimental mass loss of 7.3%
(calcd. 6.9%). The nature of the residue as elemental palladium
has also been confirmed by XRD techniques (Supporting Infor-
mation, Figure S6).
Figure 7. Weak hydrogen bonds between the 1[Pd(CH3SO3)4/2] chains in
Pd(CH3SO3)2. The donor–acceptor distance for the bridges emphasized by
dashed lines is 344.3(1) pm.
1
The thermal decomposition of Pd(CH3SO3)2 has been moni-
tored in both a flowing stream of nitrogen and oxygen, respec-
tively, in the temperature range from 25 to 10508C (Figure 9).
In both cases, the first mass loss observed can be attributed to
some adhesive CH3SO3H that could not be removed complete-
ly during the sample preparation. The decomposition of the
methanesulfonate occurs nearly in the same temperature
region, independent of the nature of the gas stream (Table 3).
In both cases, two closely related signals are observed in the
DSC curve. However, while under N2 atmosphere the decom-
position is endothermic, the process is clearly exothermic
under O2 conditions. The first decomposition is accompanied
by an experimental mass loss of 28.4%, the second by about
and O13 at 344.3(1) pm (Figure 7), that is, very weak according
to Jeffrey’s nomenclature.[14]
Thermal behavior
According to the DSC/TG measurement, the thermal decompo-
sition of Pd(NO3)2 occurs in a two-step process (Figure 8). A
slight mass loss of about 3% at the beginning of the thermal
treatment can be attributed to remains of the solvent (n-
hexane) that was used during sample preparation. Pd(NO3)2
starts to decompose with an onset temperature of 1658C
(Table 2). This first decomposition step ends at 2758C (Tmax
=
2478C) and is accompanied by an experimental mass loss of
Figure 8. DSC/TG diagram of the thermal decomposition of Pd(NO3)2. Up to
1508C there is a small mass loss, which can be attributed to the evaporation
of adhesive solvent.
Table 2. Experimental and calculated mass loss of the thermal decompo-
sition of Pd(NO3)2 with confirmed intermediary and final products.
Step
Tonset/Tmax/Tend
Mass loss (exp.)
[%]
Mass loss (calc.)
[%]
Product
1
2
S
165/247/275
640/740/760
46.7
7.3
54.0
46.9
6.9
53.8
PdO
Pd
Pd
Figure 9. DSC/TG analysis of Pd(CH3SO3)2 in N2 (top) and O2 (bottom) atmos-
phere, respectively. In both cases there is a significant mass loss which can
be attributed to the removal of adhesive CH3SO3H.
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Table 3. Experimental and calculated mass loss of the thermal decompo-
sition of Pd(CH3SO3)2 with proved final product.
N2 stream
Step
Onset/Tmax/offset Mass loss (exp.) Mass loss (calc.) Product
[%]
[%]
1
2
S
230/265/270
270/290/310
28.4
33.6
62.0
64.1[a]
Pd
O2 stream
Step
Onset/Tmax/offset Mass loss (exp.) Mass loss (calc.) Product
[%]
[%]
1
2
3[b]
S
225/255/270
270/280/310
820/849/890
27.9
34.2
Pd
Pd
Pd
62.1
64.1[a]
[a] For the calculation the mass of the sample after removal of adhesive
solvent is set to 100%. [b] Prior to the decomposition of PdO this oxide
forms in the reaction of Pd in the oxygen stream.
Figure 10. IR spectrum (top) and Raman spectrum (bottom) of Pd(NO3)2.
33.6% (N2 atmosphere; both scaled to the solvent-free starting
material at 2108C). The decomposition product in both cases
is elemental palladium. For the investigations under oxygen at-
mosphere, the mass of the sample increases in the tempera-
ture region between 495 and 6208C. This is due to the forma-
tion of PdO, which again decomposes at higher temperature
to elemental palladium. To test this assumption, we have per-
formed a comparable measurement of palladium metal in an
O2 stream. We actually found the same signature as discussed
before (Supporting Information, Figure S5). In any case, the for-
mation of palladium metal as the final decomposition residue
has been confirmed by XRD measurements.
It is remarkable that in both cases the decomposition of
Pd(CH3SO3)2 leads directly to elemental palladium, in contrast
to the findings for Pd(NO3)2, where PdO is the primary degra-
ꢀ
dation compound. Obviously the CH3SO3 anion is a suitable
Figure 11. IR spectrum (top) and Raman spectrum (bottom) of Pd(CH3SO3)2.
reductant during the decomposition.
Vibrational spectroscopy
NꢀO vibrations involving the palladium-coordinated oxygen
atoms are at about 1300 cmꢀ1 for the asymmetric and be-
tween 1000 and 1200 cmꢀ1 for the symmetric modes. Howev-
er, these latter modes are already affected by PdꢀO vibrations
of the [PdO4] moiety. The PdꢀO vibrations became even domi-
nant for the bands observed between 1000 and 850 cmꢀ1. At
energies below 800 cmꢀ1, the vibration modes are essentially
stamped by the in-plane and out-of-plane bending modes of
the nitrate group. The measuring range of the Raman spec-
trum allows also for the detection of several lattice modes,
which occur between 119 and 441 cmꢀ1. One additional band
in the IR spectrum at 2197 cmꢀ1 is not assignable. We believe
that trace residues of nitric acid cause the presence of some
NO species, which have the strongest vibration around this
value.[16]
The vibrational spectra of Pd(NO3)2 and Pd(CH3SO3)2 are quite
complex (Figures 10 and 11). To assure a reasonable assign-
ment of the observed bands, we used the calculations on
the fragments [(NO3)3Pd(NO3)Pd(NO3)3]3ꢀ and [(CH3SO3)2-
Pd(CH3SO3)2Pd(CH3SO3)2]2ꢀ, respectively. As expected, most of
the observed bands are combination modes also involving the
[PdO4] moieties. However, some of the vibrational modes can
be attributed quite clearly. The observed bands and their as-
signments are collected in Table 4.
For Pd(NO3)2, the vibration bands with the highest energies
are observed around 1500 cmꢀ1. According to the DFT calcula-
tions, these bands can be clearly attributed to N=O stretching
vibrations involving the non-coordinating oxygen atoms of the
nitrate group. This energy is clearly above the value for a bare
ꢀ
NO3 ion (1340 cmꢀ1), for example in anilinium nitrate,[15] and
The IR and the Raman spectra show a large number of
mainly characteristically resolved bands for the methane-
sulfonate as well (Figure 11, Table 5). The band distribution of
demonstrates the double bond character expected for the
third oxygen atom of the bidentate bridging nitrate ion. The
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and 1150 cmꢀ1 the SꢀO stretching modes are seen. They can
be separated into those vibrations that involve terminal non-
coordinated oxygen atoms (1398–1432 cmꢀ1) and those that
involve palladium-coordinated oxygen atoms (1155–
1332 cmꢀ1). The latter are influenced by PdꢀO modes of the
[PdO4] fragment, similar to the findings for Pd(NO3)2. Also in
line with the observations for the nitrate, the bands between
894 and 1025 cmꢀ1 are clearly dominated by PdꢀO vibrations.
Another strong band occurs at about 770 cmꢀ1 in both spectra,
which can be assigned to the SꢀC stretching modes. Finally,
Table 4. IR frequencies and their assignment for Pd(NO3)2.[a]
IR frequency [cmꢀ1
]
Raman[b] frequency [cmꢀ1
]
Assignment
560
593
673
686
760
787
w
m
m
sh
s
d (NO3)out-of-plane
636
708
751
780
852
976
1006
1021
w
w
m
w
w
w
m
s
d (NO3)in-plane
m
n (NꢀO)+n (PdꢀO)
947
sh
ꢀ
the deformation vibrations of the CH3SO3 anion are found be-
1011
1075
1170
1263
1296
1465
1498
1532
s
m
s
sh
w
sh
sh
sh
tween 509 and 613 cmꢀ1. Low-energy bands between 89 and
426 cmꢀ1 in the Raman spectrum are due to lattice vibrations.
There is one band at 1673 cmꢀ1, which could not be assigned
reliably.
s
s
ns (NꢀOcoord.
)
1202
w
m
w
m
s
nas (NꢀOcoord.
)
1313
1496
1512
1538
n (N=Ounc
)
Conclusion
[a] s=strong, m=medium, w=weak, sh=shoulder. [b] In the Raman
spectrum, there are additionally some lattice vibrations in the range be-
tween 119 and 441 cmꢀ1 visible (cf. Figure 10).
We have demonstrated for the first time that fuming nitric acid
might be a suitable reactant and also a solvent for the prepara-
tion of oxoanionic noble metal compounds. This is of out-
standing importance because these type of compounds have
been only scarcely investigated owing to lacking synthetic
methods for their preparation. The reported representatives
are the palladium compounds Pd(NO3)2 and Pd(CH3SO3)2 exhib-
iting a layer and a chain structure, respectively. We are con-
vinced that the preparation route will lead to further noble
metal compounds of this kind in the future.
Table 5. IR frequencies and their assignment for Pd(CH3SO3)2.[a]
IR frequency [cmꢀ1
]
Raman[b] frequency [cmꢀ1
]
Assignment
509
587
779
w
m
s
573
613
774
s
m
s
d (SO3)
n (SꢀC)
894
971
985
m
m
m
Experimental Section
987
1025
m
s
n (SꢀO)+n (PdꢀO)
Synthesis of Pd(NO3)2
Pd (0.055 g (0.52 mmol), Hereaus, Hanau, Germany) and the pyri-
dine-SO3 complex (0.095 g, 0.60 mmol), Merck, Darmstadt, Germa-
1155
1256
1332
1398
1432
2936
3025
m
s
m
w
w
1262
w
nas (SꢀOcoord.
)
ny) were loaded into
a thick-walled glass ampoule (length
250 mm, diameter 20 mm, thickness of the tube wall 2 mm).
Fuming nitric acid (1.5 mL; Merck, Darmstadt, Germany) was
added. The ampoule was torch-sealed, placed into a tube furnace,
and heated up to 1008C. The temperature was maintained for 30 h
and finally decreased to 258C via 150 h. A large number of yellow
crystals were obtained, and the yield was quantitative with respect
to the initial palladium. The product was handled under strictly
inert conditions. Caution! Fuming nitric acid is a strong oxidizer,
which needs careful handling. During and even after the reaction
the ampoule might be under remarkable pressure and contains ni-
trous fumes. It is mandatory to cool down the ampoules with
liquid nitrogen prior to opening.
1418
2937
3026
nas (SꢀOunc.
ns (CꢀH)
)
s
s
nas (CꢀH)
[a] s=strong, m=medium, w=weak, sh=shoulder. [b] In the Raman
spectrum, there are additionally some lattice vibrations in the range be-
tween 89 and 426 cmꢀ1 visible (cf. Figure 11).
the IR spectrum fits very well to those of recently presented
ternary
gold
methanesulfonates
Li[Au(CH3SO3)4]
and
Rb[Au(CH3SO3)4],[10] the only previous examples of precious
metal methanesulfonates. The findings for previously reported
alkaline earth methanesulfonates of sodium and cesium also
fall in the narrow range of the results for Pd(CH3SO3)2. Each
spectrum reveals significant strong signals at high wavenum-
bers in the IR as well as in the Raman spectra. The bands of
highest energy are found at 3025 and 2936 cmꢀ1 in the IR and
3026 cmꢀ1 and 2937 cmꢀ1 in the Raman spectrum. With re-
spect to our calculations and in accordance with reported
data, they can be clearly attributed to the asymmetric and
symmetric CꢀH stretching modes. At energies between 1450
Synthesis of Pd(CH3SO3)2
Elemental palladium (100 mg, 0.94 mmol), Hereaus, Hanau, Germa-
ny), was loaded into a thick-walled glass ampoule (length 250 mm,
diameter 20 mm, thickness of the tube wall=2 mm). After adding
fuming nitric acid (1 mL; Merck, Darmstadt, Germany) and metha-
nesulfonic acid (2 mL; Sigma Aldrich, Steinheim, Germany) the am-
poule was torch-sealed, placed into a block resistance furnace, and
heated up to 1208C within 24 h. The temperature was maintained
for 72 h, additionally the temperature was decreased to 258C in
a two-step process, first within 79 h to 508C and finally to 208C
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within 50 h. Additionally, the ampoule was left unchanged for crys-
tallization at room temperature for 70 days. Finally, a large number
of yellow crystals were obtained, and the yield was quantitative
with respect to the initial palladium. The crystals are extremely
moisture sensitive. Therefore the crystals have to be handled
under strictly inert conditions. Caution! Fuming nitric acid and
methanesulfonic acid are strong oxidizers which need careful han-
dling. During and even after the reaction the ampoules might be
under remarkable pressure. It is mandatory to cool down the am-
poules by liquid nitrogen prior to opening.
ble using a fine paper towel. For a measurement under N2 atmos-
phere, 9.0 mg of the sample were filled in a corundum crucible; for
investigations in an O2 stream, 4.4 mg of Pd(CH3SO3)2 were treated
in the same way. The crucibles were heated at a rate of 10 Kminꢀ1
up to 10508C. The data were base-line-corrected and the charac-
teristic point was taken from the DTA and DSC curves, respectively.
As confirmation of the assumed oxidation on palladium during the
course of the decomposition reactions of Pd(CH3SO3)2 under O2,
we heated elemental palladium in an O2 stream with rate of
10 Kminꢀ1 up to 10508C. The respective diagram is given in the
Supporting Information, Figure S5. The collected data were pro-
cessed using the software of the analyzer.[20]
X-ray crystallography
Several single crystals were transferred into inert oil (AB128333,
ABCR, Karlsruhe, Germany). A suitable crystal was prepared, mount-
ed onto a glass needle (diameter 0.1 mm) and immediately placed
into a stream of cold N2 (120 K) inside the diffractometer (k-APEX
II, Bruker, Karlsruhe, Germany). After unit-cell determination, the re-
flection intensities were collected. Structure solution and refine-
ments were done with the SHELX program package.[17]
X-ray powder diffraction
The adhesive nitric acid of the crude Pd(NO3)2 product was washed
away with dried n-hexane in a glove box. The dried bulk material
was ground and checked by means of X-ray powder diffraction
techniques on a sample prepared in a glass capillary (diameter
0.5 mm). The measurement was performed with the help of the
powder diffractometer STADIP (Stoe & Cie) using Cu-Ka radiation
(l=154.06 pm). The diffraction data were processed with the
WinXPow 2007 program package. The lattice parameters obtained
from the diffractogram are: a=469.2(2) pm, b=594.5(3) pm, c=
805.3(4) pm, b=105.85(4)8, V=216.1(2) ꢁ3. The experimental and
the calculated diffractogram of Pd(NO3)2 are shown in the Support-
ing Information, Figure S3. Apart from some very small reflections
at low 2q values, both diffractograms match quiet well. The data
were processed with the WINXPOW program.[21]
The crystal structure of Pd(NO3)2 could be solved in the monoclinic
space group P21/n (no. 14). The heavy atom positions were deter-
mined by SHELXS-97 using direct methods.[17] Further atoms could
be successfully located by difference Fourier techniques during re-
finement with SHELXL-97.[17] An absorption correction was applied
to the data using the program SADABS-2012/1.[18] Finally the struc-
ture and model refined to R1 =0.0288 and wR2 =0.0346 for all
data. Selected bond lengths are presented in the Supporting Infor-
mation, Table S4. For Pd(NO3)2, further details of the crystal struc-
ture investigation may be obtained from the Fachinformationszen-
trum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax:
(+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, or at http://
www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting
the CSD number 425998. CCDC 963236 (Pd(CH3SO3)2) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
For Pd(CH3SO3)2, the mother liquor could only be removed me-
chanically by wiping on a fine paper towel. Washing of the crystals
and bulk material with other even non polar solvents leads to de-
composition. Nevertheless, the adhesive acid could be removed
sufficiently, so that the bulk material of Pd(CH3SO3)2 could be
ground and filled in a glass capillary (diameter 0.5 mm) for XRD
powder diffraction. The diffraction data were collected using the
STADIP diffractometer (Stoe & Cie) using Cu-Ka radiation (l=
154.06 pm) and processed with the WinXPow 2007 program pack-
age.[21] The lattice parameters obtained from the diffractogram are:
a=480.2(1) pm, b=1091.2(2) pm, c=749.1(2) pm, b=102.66(2)8,
V=382.9(2) ꢁ3. Besides several well-indexed sharp reflections for
Pd(CH3SO3)2, we only find small reflections for some unreacted pal-
ladium (Supporting Information, Figure S4). The data were pro-
cessed with the WINXPOW program.[21]
The crystal structure of Pd(CH3SO3)2 could be solved in the mono-
clinic space group P21/n (no. 14). The heavy-atom positions were
determined by SHELXS-2013 using direct methods.[17] Further
atoms could be successfully located by difference Fourier tech-
niques during refinement with SHELXL-2013.[17] The investigated
crystal was calculated as non-meroedric twin and the ratio of the
two individuals was found to be 47%/53%. An absorption correc-
tion was applied to the data using the program TWINABS-2012/
1.[19] Finally the structure and model refined to R1 =0.0161 and
wR2 =0.0421 for all data. Selected bond lengths are given in the
Figure captions (cf. Figures 1 and 4) and in the Supporting Infor-
mation, Table S4. Further details of the crystal structure investiga-
tion may be obtained from the Fachinformationszentrum Karls-
ruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: (+49)7247–
808–666; e-mail: crysdata@fiz-karlsruhe.de, or at http://www.fiz-
karlsruhe.de/request for deposited data.html) on quoting the CSD
number 425998.
Vibrational spectroscopy
The Raman spectra were measured (spectrometer FRA106, Bruker,
Karlsruhe, Germany) on a number of selected crystals in a small
glass tube. For the measurements of the IR spectra, a sample of
the respective compound was mounted in a glove box to the
sample holder of a FTIR spectrometer (PlatinumATR, Tensor 27,
Bruker) and immediately measured. Previously, the nitrate was
washed with n-hexane, while the methanesulfonate was cleaned
mechanically by wiping with a soft paper towel. The IR data were
processed with the OPUS 2.0.5 program.[22] The assignment of the
observed vibrations was carried out based on theoretical investiga-
tions, which were performed for characteristic cut-offs of the solid
state structures.
Thermal analysis
The investigation of the thermal behavior was performed using
a TGA/DSC apparatus (TGA/DSC1, Mettler Toledo GmbH, Schwer-
zenbach, Switzerland) and a TGA/SDTA device (TGA/SDTA851e,
Mettler-Toledo GmbH, Schwerzenbach, Switzerland). In a flow of
dry nitrogen, Pd(NO3)2 (about 11 mg) washed with n-hexane was
placed in a corundum crucible and heated at a rate of 10 Kminꢀ1
up to 10508C. For Pd(CH3SO3)2, a sample was dried as far as possi-
Theoretical investigations
For the calculations, two different fragments of the crystal struc-
tures were chosen. In the case of the nitrate we extracted
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ꢂ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
a [(NO3)3Pd(NO3)Pd(NO3)3]3ꢀ, which should clearly reveal the charac-
teristics of the bidentate bridging nitrate group. For Pd(CH3SO3)2,
we selected a [(CH3SO3)2Pdꢀ(CH3SO3)2Pd(CH3SO3)2]2ꢀ anion with two
bidentate bridging CH3SO3 groups. A full geometry optimization
for both fragments has been performed within density functional
theory (DFT) using the B3LYP functional for exchange and correla-
tion. A 6-31+G(d) basis set for S, O, C, and H was used, while pseu-
dopotentials were adopted for Pd (PP-cc-pVDZ). Throughout the
study, the Gaussian09 program package was used.[23]For the com-
parison with the experimental data, we extracted the data for the
respective bridging anions. The calculations were done without
symmetry restricꢀtions, thus in case of the [PdO4] moieties and the
bridging CH3SO3 groups we calculated more distances than found
experimentally. For comparison, the very similar theoretical values
have been merged.
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Financial support of the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. J.B. is thankful for a stipend of the
Stiftung der Metallindustrie im Nordwesten. The authors want
to thank Dr. Marc Schmidtmann for the collection of the X-ray
data. Furthermore we thank Mrs. Regina Stçtzel and Prof. Clau-
dia Wickleder (both University of Siegen) for measuring the
Raman spectra. The simulations were performed at the HPC
Cluster HERO, located at the University of Oldenburg (Germa-
ny) and funded by the DFG through its Major Research Instru-
mentation Programme (INST 184/108-1 FUGG) and the Ministry
of Science and Culture (MWK) of the Lower Saxony State.
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Keywords: X-ray diffraction · methanesulfonate · nitrate ·
palladium · theoretical investigations
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Received: September 21, 2014
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Published online on November 27, 2014
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ꢂ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim