Azidoaurates of the alkali metals

Article abstract of DOI:10.1002/zaac.200600062

Synthesis, crystal structure as well as IR- and Raman-spectroscopic properties of the new ternary azidoaurates(III) A[Au(N₃)₄] (A = K, Rb, Cs) are reported. The translucent orange-red compounds were prepared by reaction of the respective binary azides with HAuCl₄ in aqueous solutions at room temperature. The crystal structures determined by single crystal X-ray diffraction methods are monoclinic (C2/c (No. 15), K/Rb/Cs[Au(N₃)₄]: a = 1049.30(9) pm / 1033.93(10) pm / 1011.9(8) pm, b = 1047.52(8) pm / 1064.20(11) pm / 1093.8(8) pm, c = 776.97(7) pm / 809.79(8) pm / 858.4(6) pm, β = 91.220(5)° / 90.322(5)° / 92.72(5)°) and provide the first examples for azidoaurates with mono-atomic cations. The unusual reduction of the lattice parameter a with increasing cation size in the isotypic series is discussed in detail. IR-and Raman-spectroscopic measurements show strongly covalent gold-nitrogen contacts, a bonding situation which is further supported by quantum chemical calculations and ELF-analyses.

Full text of DOI:10.1002/zaac.200600062

DOI: 10.1002/zaac.200600062
Azidoaurates of the Alkali Metals
Semih Afyonb, Peter Höhna,*, Marc Armbrüstera, Alexey Baranova, Frank R. Wagnera, Mehmet Somerb, and
Rüdiger Kniepa
a Dresden, Max-Planck-Institut für Chemische Physik fester Stoffe
b Sariyer-Istanbul/Turkey, Koc¸ University
Received February 23rd, 2006.
Professor Hans-Georg von Schnering zum 75. Geburtstag gewidmet
Abstract. Synthesis, crystal structure as well as IR- and Raman-
spectroscopic properties of the new ternary azidoaurates(III)
A[Au(N3)4] (A ϭ K, Rb, Cs) are reported. The translucent orange-
red compounds were prepared by reaction of the respective binary
azides with HAuCl4 in aqueous solutions at room temperature. The
crystal structures determined by single crystal X-ray diffraction
methods are monoclinic (C2/c (No. 15), K/Rb/Cs[Au(N3)4]: a ϭ
and provide the first examples for azidoaurates with mono-atomic
cations. The unusual reduction of the lattice parameter a with inc-
reasing cation size in the isotypic series is discussed in detail. IR-
and Raman-spectroscopic measurements show strongly covalent
gold-nitrogen contacts, a bonding situation which is further sup-
ported by quantum chemical calculations and ELF-analyses.
1049.30(9) pm
/
1033.93(10) pm
/
1011.9(8) pm,
b
ϭ
1047.52(8) pm / 1064.20(11) pm / 1093.8(8) pm, c ϭ 776.97(7) pm /
809.79(8) pm / 858.4(6) pm, β ϭ 91.220(5)° / 90.322(5)° / 92.72(5)°)
Keywords: Azides; Crystal structure; Vibrational spectroscopy;
ELF
Introduction
prepare binary alloys, e.g. NaAu2 [9]. In this study, the in-
fluence of cation size on the crystal structure, the shape of
the azidoaurate anions, and the spectroscopic properties
have been investigated in detail.
The binary azides of alkali metals represent ionic com-
pounds containing symmetric azide anions [1]. Depending
on the cation size, they crystallize in distorted NaCl (LiN3
[2], NaN3 [2]) or CsCl (KN3 [2], RbN3 [2], CsN3 [2]) type
structures. Especially NaN3 is of technical interest in such
diverse fields as nitrogen sources, detonators, and biocides.
In contrast, binary gold azides (e.g. AuIN3, AuIII(N3)3) are
hitherto unknown and are expected to be highly unstable.
The first gold azide described as AuN3 and prepared from
NaN3 and AuCl3 was reported in 1898 [3], and the first
publication on sodium gold azide appeared in 1957 [4], but
no further information on structure and properties has been
given. Stable azidoaurate complexes [AuIII(N3)4]Ϫ and
[AuI(N3)2]Ϫ, both containing non-centrosymmetric azide
groups, have been first reported in compounds with large
complex cations (e.g. [As(C6H5)4]ϩ [5]) and later on in com-
pounds stabilized by crystal water (Na[Au(N3)4] · 2H2O [6]),
but even these compounds are potentially explosive. Or-
ganic gold azides (e.g. (C6H5)3PAuI[N3] [7]) are often used
in photolysis chemistry as precursors for gold clusters [8].
Mixtures of alkali azides with gold have also been used to
Experimental Section
Due to the potentially explosive nature of the compounds
under investigation, all handlings were carried out under
extreme caution. It was especially important to prevent
shock, heat, and intense radiation, as well as handling the
products with metallic objects.
Binary azides, KN3, RbN3, and CsN3, were synthesized from NaN3
(Roth, 99.9 %, recrystallized in water) and the corresponding car-
bonates A2CO3 (A ϭ K, Rb, Cs, all Alfa, 99.9 %) by ion-exchange
methods in water. The resin column (l ϭ 775 mm, лi ϭ 14.2 mm)
was filled with acidic cation-exchange resin (Merck), and ion-ex-
change was performed with a 5 ml·minϪ1 flow rate of HN3. The
white product was collected after evaporation of the solvent.
Alkali metal azidoaurates, A[Au(N3)4] (A ϭ K, Rb, Cs; in the
further text referred to as: A ϭ A[Au(N3)4], K ϭ K[Au(N3)4], Rb ϭ
Rb[Au(N3)4], Cs ϭ Cs[Au(N3)4]), were synthesized by reaction of
the binary azides with HAuCl4 (Alfa). The starting materials were
dissolved in water or ethanol, the solutions were mixed with a slight
excess of the alkali azide according to (1) and kept for 12 Ϫ 24
hours in a slightly tilted petri dish. Temperature treatment experi-
ments were performed using either a heat plate or an ice bath in
the range of 273 K Ϫ 323 K. Orange-red, brittle and often twinned
crystals which are highly explosive when exposed to pressure, heat,
or intense radiation were obtained during evaporation of the sol-
* Dr. Peter Höhn
Max-Planck-Institut für Chemische Physik fester Stoffe
Nöthnitzer Str. 40
D-01187 Dresden/Deutschland
FAX ϩ49-351-4646-2260
e-mail: hoehn@cpfs.mpg.de
Z. Anorg. Allg. Chem. 2006, 632, 1671Ϫ1680
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1671

S. Afyon, P. Höhn, M. Armbrüster, A. Baranov, F. R. Wagner, M. Somer, R. Kniep
vent. ACl was removed mechanically or by the procedure of re-
detonate under radiation, the laser power used in the Raman
experiment did not exceed 10 mW. The FT-IR spectra were regis-
tered from PE pellets with a Bruker IFS 66/S spectrometer.
crystallization in ethanol or isopropanol. Crystal quality was im-
proved after re-crystallization. Small single crystals in the shape of
translucent orange-red needles and suitable for X-ray single crystal
diffraction experiments could also be obtained by immediate re-
moval of the solution after the formation of the first A crystals.
Electronic structure calculations
To investigate the nature of the chemical bond between Au
and the azide N3 ion in the azidoaurates A, quantum
HAuCl4 ϩ 5 AN3 Ǟ A[Au(N3)4] ϩ 4 ACl ϩ HN3
(1)
Ϫ
chemical DFT calculations have been performed for the free
azide anion and for [Au(N3)4]Ϫ in its singlet ground state
using the ADF program package [12]. While the structure
of the free azide has been optimized, the structure of the
complex anion was taken from the experimental data. For
inclusion of relativistic effects the scalar relativistic ZORA
[13] scheme has been employed. The exchange correlation
potential LDA and all electron triple zeta basis sets with 2
sets of polarization functions (TZ2P) have been used
throughout.
Due to the highly explosive nature of the title compounds, the con-
tent of N and O in the bulk material could not be determined
quantitatively by carrier gas hot-extraction technique. In prelimi-
nary experiments with sample amounts below 2 mg, the heavy ex-
plosions threatened to damage not only the crucibles used for the
analysis, but also the analyzer itself.
Suitably sized single crystals of A were glued to the tips of glass
fibres for X-ray diffraction intensity data collection on a Rigaku
AFC7 Mercury CCD automatic diffractometer with MoKα radi-
ation using a graphite-monochromator and the φ/ω scan technique.
The isotypic crystal structures were solved from single crystal data
using SHELXS-97 [10] and refined using the SHELXL software
package [11].
Results and Discussion
IR-spectroscopy and Raman-spectroscopy were used to analyze
and investigate the vibrational behavior of the alkali metal azidoau-
rates. The FT-Raman spectra were recorded from powdered
samples using a Bruker RFS 100/S spectrometer (Nd:YAG-Laser,
1064 nm, 200 mW). Due to the tendency of the azidoaurates to
Crystal structure
The isotypic alkali metal azidoaurates(III) A (Figure 1)
crystallize in the monoclinic space group C2/c (No. 15, K/Rb/
Cs: a ϭ 1049.30(9) pm / 1033.93(10) pm / 1011.9(8) pm, b ϭ
Table 1 Crystallographic data for
K[Au(N3)]4
K[Au(N3)]4
non-standard-setting
Rb[Au(N3)]4
Cs[Au(N3)]4
Temperature of Measurement
Crystal system
293(2)
monoclinic
Space group
a [pm]
b [pm]
c [pm]
C 2/c (No. 15)
1033.93(10)
1049.30(9)
1047.52(8)
776.97(7)
91.220(5)
853.82(12)
4
1049.30(9)
1047.52(8)
776.97(7)
88.780(5)
853.82(12)
4
1011.9(8)
1093.8(8)
858.4(6)
92.72(5)
949.1(12)
4
1064.20(11)
809.79(8)
90.322(5)
891.00(15)
4
β [°]
Cell volume [109 pm3]
Z
ρcalcd [g cmϪ3
]
3.144
3.144
3.359
3.485
Crystal color, shape
orange-red, needle
Crystal size [mm3]
0.02 x 0.02 x 0.4
17702
31.25
0.02 x 0.02 x 0.4
17702
31.25
0.02 x 0.02 x 0.4
21924
27.49
Rigaku AFC7 Mercury CCD
0.71069
graphite-monochromator
φ/ω scan
0.02 x 0.02 x 0.4
19268
31.13
µ(MoKα) [mmϪ1
2θ range [°]
Diffractometer
]
Radiation, Wavelength
Monochromator
Scan mode
Measured reflections
Reflections for refinement
Observed reflections [Fo>4σ(Fo)]
Rint
3269
1195
1090
0.0277
3269
1195
1090
0.0277
3010
837
747
0.0494
3504
1341
970
0.0554
Structure solution method
Program for structure solution
Program for structure refinement
Number of Parameters
Direct methods
SHELXS-97 [10]
SHELXL-97 [11]
66
66
1.129
66
1.129
66
1.112
2
Goodness-of-fit on F
1.114
R values [Fo > 4σ(Fo)]
0.0443, 0.1095
0.0492, 0.1192
3.06/Ϫ2.24
0.0443, 0.1095
0.0492, 0.1192
3.06/Ϫ2.24
0.0390, 0.0937
0.0455, 0.0996
2.84/Ϫ1.76
0.0668, 0.1459
0.0938, 0.1712
4.33/Ϫ1.71
R values (for all data)
Residual electron density [e x 10Ϫ6 pmϪ3
]
Further details of the crystal structure investigation are available from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-
Leopoldshafen (Germany), on quoting the depository number CSD-416489, CSD-416488, and CSD-416487, respectively, the name of the
authors, and citation of the paper.
1672
www.zaac.wiley-vch.de
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 1671Ϫ1680

Azidoaurates of the Alkali Metals
Table 2 Atomic coordinates and equivalent isotropic displace-
ment parameters /104 pm2 for a) K[Au(N3)4], b) K[Au(N3)4] (non-
standard setting), c) Rb[Au(N3)4], and d) Cs[Au(N3)4].
Table 3 Anisotropic displacement parameters /104 pm2 for a)
K[Au(N3)4], b) Rb[Au(N3)4], and c) Cs[Au(N3)4].
a)
a)
Atom U11
U22
U33
U23
U13
U12
Atom
Site
Wyckoff
x
y
z
Ueq
Au1
K1
0.0389(4)
0.0298(3)
0.0346(13) 0.0507(15)
0.0472(3)
0.000
0.000
Ϫ0.0110(2)
Ϫ0.0119(14)
Ϫ0.019(5)
Ϫ0.012(3)
Ϫ0.010(5)
Ϫ0.027(4)
Ϫ0.015(4)
Ϫ0.031(5)
0.000
0.000
0.074(2)
0.043(4)
0.045(4)
0.073(6)
0.047(4)
0.048(5)
0.046(5)
Au1
K1
Au
K
N
N
N
N
N
N
4e
4e
8f
8f
8f
8f
8f
8f
0.5000
0.5000
0.00771(4)
0.4116(3)
0.1522(7)
0.1196(7)
0.0954(9)
0.3801(8)
0.2785(7)
0.1766(9)
0.2500
0.2500
0.0388(3)
0.0533(8)
0.054(2)
0.052(2)
0.065(3)
0.057(2)
0.0455(17)
0.065(3)
N1a
N1b
N1c
N2a
N2b
N2c
0.029(4)
0.034(4)
0.042(4)
0.026(3)
0.029(3)
0.054(5)
0.097(7)
0.056(4)
0.078(6)
0.088(6)
0.079(6)
0.095(7)
Ϫ0.007(4)
0.000(3)
Ϫ0.018(4)
0.003(4)
0.006(4)
0.002(5)
Ϫ0.007(3)
Ϫ0.007(3)
Ϫ0.011(4)
0.006(3)
0.005(3)
0.003(4)
N1a
N1b
N1c
N2a
N2b
N2c
0.3798(9)
0.2817(8)
0.1872(9)
0.1371(9)
0.1082(8)
0.0939(10)
0.1779(14)
0.1075(13)
0.0417(15)
0.3169(15)
0.3722(11)
0.4208(14)
b)
b)
Atom U11
U22
U33
U23
U13
U12
Atom
Site
Wyckoff
x
y
z
Ueq
Au1
Rb1
N1a
N1b
N1c
N2a
N2b
N2c
0.0344(4)
0.0610(10) 0.0327(8)
0.0288(4)
0.0447(4)
0.0396(8)
0.117(10)
0.055(6)
0.069(7)
0.093(8)
0.070(7)
0.085(8)
0.000
0.000
0.0135(3)
0.0149(7)
0.031(6)
0.015(4)
0.015(5)
0.038(6)
0.020(5)
0.036(6)
0.000
0.000
Au1
K1
Au
K
N
N
N
N
N
N
4e
4e
8f
8f
8f
8f
8f
8f
0.5000
0.5000
0.00771(4)
0.4116(3)
0.1522(7)
0.1196(7)
0.0954(9)
0.3801(8)
0.2785(7)
0.1766(9)
0.2500
0.2500
0.0388(3)
0.0533(8)
0.054(2)
0.052(2)
0.065(3)
0.057(2)
0.0455(17)
0.065(3)
0.046(6)
0.039(5)
0.064(7)
0.053(6)
0.045(6)
0.051(6)
0.028(5)
0.034(5)
0.045(6)
0.026(5)
0.032(5)
0.054(6)
Ϫ0.018(5)
0.003(4)
Ϫ0.018(5)
0.002(5)
0.006(4)
0.009(5)
0.005(4)
0.008(3)
0.006(5)
Ϫ0.004(4)
0.000(4)
0.001(4)
N1a
N1b
N1c
N2a
N2b
N2c
0.3798(9)
0.2817(8)
0.1872(9)
0.1371(9)
0.1082(8)
0.0939(10)
0.3221(14)
0.3925(13)
0.4583(15)
0.1831(15)
0.1278(11)
0.0792(14)
c)
c)
Atom U11
U22
U33
U23
U13
U12
Atom
Site
Wyckoff
x
y
z
Ueq
Au1
Cs1
0.0447(5)
0.0317(4)
0.0320(6)
0.036(7)
0.049(8)
0.078(11)
0.039(7)
0.037(7)
0.065(10)
0.0614(6)
0.0507(8)
0.104(12)
0.067(9)
0.088(12)
0.17(2)
0.000
0.000
0.0136(4)
0.0168(6)
0.035(9)
0.025(6)
0.029(8)
0.000
0.000
0.0652(9)
0.062(8)
0.043(6)
0.055(8)
0.053(8)
0.048(7)
0.079(11)
Au1
Rb1
N1a
N1b
N1c
N2a
N2b
N2c
Au
Rb
N
N
N
N
N
N
4e
4e
8f
8f
8f
8f
8f
8f
0.5000
0.5000
0.00888(4)
0.41365(12)
0.1498(8)
0.1190(8)
0.0971(9)
0.3848(9)
0.2822(8)
0.1806(10)
0.2500
0.2500
0.0359(3)
0.0444(4)
0.057(3)
0.049(2)
0.063(3)
0.064(3)
0.043(2)
0.059(3)
N1a
N1b
N1c
N2a
N2b
N2c
Ϫ0.004(8)
Ϫ0.005(7)
Ϫ0.004(9)
Ϫ0.021(10) 0.051(11)
Ϫ0.005(6) 0.001(6)
Ϫ0.025(10) 0.006(10)
0.020(7)
0.005(6)
Ϫ0.007(8)
Ϫ0.001(7)
0.008(6)
0.011(9)
0.3811(10)
0.2841(9)
0.1915(10)
0.1388(10)
0.1085(9)
0.0954(10)
0.3215(15)
0.3941(13)
0.4615(14)
0.1814(17)
0.1314(12)
0.0889(13)
0.060(9)
0.092(14)
d)
Table 4 Selected distances /pm and angles /° in a) K[Au(N3)4], b)
Rb[Au(N3)4], and c) Cs[Au(N3)4].
Atom
Site
Wyckoff
x
y
z
Ueq
Au1
Cs1
Au
Cs
N
N
N
N
N
N
4e
4e
8f
8f
8f
8f
8f
8f
0.5000
0.5000
0.00867(7)
0.41442(11)
0.1470(12)
0.1188(12)
0.0987(16)
0.3885(14)
0.2850(12)
0.1860(17)
0.2500
0.2500
0.0456(3)
0.0489(4)
0.066(4)
0.052(3)
0.073(5)
0.087(6)
0.048(3)
0.079(5)
a)
b)
c)
N1a
N1b
N1c
N2a
N2b
N2c
0.3838(15)
0.2923(12)
0.2023(15)
0.1346(15)
0.1076(13)
0.0943(17)
0.326(2)
0.3996(17)
0.469(2)
0.174(3)
0.1378(18)
0.101(2)
d(AuϪN1a)
d(AuϪN2a)
204.1(8)
202.3(8)
120.5(12)
113.5(12)
176.3(10)
119.0(11)
114.4(12)
172.3(11)
291.1(10)
322.2(11)
307.3
202.6(9)
203.0(9)
120.4(13)
113.6(12)
175.9(11)
120.5(13)
114.3(13)
171.4(11)
307.3(10)
328.1(11)
316.3
204.2(12)
202.2(14)
118.8(18)
113.3(18)
175.9(18)
120.3(18)
113.5(20)
173.5(18)
323.0(15)
339.1(20)
330.9
d(N1aϪN1b)
d(N1bϪN1c)
Є(N1aϪN1bϪN1c)
d(N2aϪN2b)
d(N2bϪN2c)
Є(N2aϪN2bϪN2c)
d(AϪNx)min
1047.52(8) pm / 1064.20(11) pm
/ 1093.8(8) pm, c ϭ
776.97(7) pm / 809.79(8) pm / 858.4(6) pm, β ϭ 91.220(5)° /
90.322(5)° / 92.72(5)°). Table 1 contains crystallographic
and refinement data; positional and displacement param-
eters are given in Tables 2 and 3. Selected distances and
angles are given in Table 4. Discussion of the unusual re-
duction of the lattice parameter a with increasing cation
size is facilitated by transforming the unit cell of
K[Au(N3)4] to a non-standard setting (a ϭ 1049.30(9) pm,
b ϭ 1047.52(8) pm, c ϭ 776.97(7) pm, β ϭ 88,780(5)°) using
the matrix (-1 0 0, 0 -1 0, 0 0 1). The atomic positions are
transformed to the new setting by (Ϫx, y, z).
d(AϪNx)max
d(AϪNx)ave
The AuϪN distances within the complex anions (K: d ϭ
202.3 (8) pm Ϫ 204.1 (8) dave ϭ 203.2 pm; Rb: d ϭ 202.6
(9) pm Ϫ 203.0 (9) pm, dave ϭ 202.8 pm ; Cs: d ϭ 202.2
(14) pm Ϫ 204.2 (12) pm, dave ϭ 203.2 pm) are in the
range observed in other azidoaurate(III) compounds
([As(C6H5)4][Au(N3)4]: d ϭ 203.2 pm [5]; [Me4N][Au(N3)4],
dave ϭ 202.5 pm [14]). In contrast, in organic azido gold(I)
compounds, the AuϪN distances are significantly longer
((C6H5)3PAuI[N3]: d ϭ 210.0 pm [7]). In all compounds the
angle AuϪN1aϪN1b is nearly constant within the standard
deviation (K: 115.6(6)°, Rb: 116.3(7)°, Cs: 116.9(11)°,
Predominant structural features in A are complex azido-
aurate anions [AuIII(N3)4]Ϫ (Figure 2). The gold centre is
located within a (distorted) tetragonal planar coordination
sphere built up by the inner N species of the azide ions.
Z. Anorg. Allg. Chem. 2006, 1671Ϫ1680
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1673

S. Afyon, P. Höhn, M. Armbrüster, A. Baranov, F. R. Wagner, M. Somer, R. Kniep
Figure 1 The crystal structure of A[Au(N3)4] (A ϭ K, Rb, Cs). Complex anions [Au(N3)4]Ϫ and coordination polyhedra AN10 are emphasized.
The polyhedra AN10 are conneted along [001] via rectangular N4 interfaces (green); the polyhedra chain in the center is shown transparent.
113.3(18) pm) and N2bϪN2c (K: d ϭ 114.4(12) pm,
Rb: d ϭ 114.3(13) pm, Cs: d ϭ 113.5(20) pm) respectively,
but in the range observed in other azidoaurates
([As(C6H5)4][Au(N3)4]: d ϭ 121.2 pm / 113.6 pm [5]). In the
binary alkali metal azides, the NϪN distances in the sym-
metric azide anion are 117 Ϫ 118 pm [1], the average
NϪN distances in the azidoaurates are within the same re-
gion (K: dave ϭ 116.9 pm, Rb: dave ϭ 117.2 pm, Cs: dave
ϭ
116.5 pm). The NϪNϪN angles in the azide ions are
slightly smaller than 180°, but in a range expected for non-
centrosymmetric azides (K: 176.3(10)°/172.3(11)°, Rb:
175.9(11)°/171.4(11)°, Cs: 175.9(18)°/173.5(18)°; [As(-
C6H5)4][Au(N3)4]: 174.4°/175.8° [5]). Similar values of
NϪNϪN angles down to 170° have been found for azides
which are covalently bonded to a metal or an organic sub-
stituent [15]. The coordination sphere of the azide ions 1
and 2 in A consists of 3 (azide ion 2: N2a: Au; N2c: 2x A)
and 4 (azide ion 1: N1a: Au, A; N1c: 2x A) metal species
forming highly distorted (psi-) tetrahedra. By this, the coor-
dination spheres of the azide ions are significantly different
compared with the binary alkali metal azides which are sur-
rounded by 8 cations in the form of a distorted tetragonal
prism.
Figure
2
The complex azidoaurate anions [Au(N3)4]Ϫ in
A[Au(N3)4] (A ϭ K, Rb, Cs) along [102] (top) and [010] (bottom)
indicating the AuN4 plane (red) and the orientation of the azide
ions.
Up to now, two different types of complex azidoaurate
ions have been observed: in compounds with large
cations, the azidoaurate ions show C4h symmetry (e.g.
[As(C6H5)4][Au(N3)4] [5]), whereas in all alkali metal azi-
doaurates the complex anion has C2 symmetry.
whereas the angle Au-N2a-N2b (K: 119.9(7)°, Rb:
119.9(8)°, Cs: 123.2(12)°) increases with increasing cation
size. Both symmetry independent azide ions are clearly non-
centrosymmetric with NϪN distances towards the gold
contact N1aϪN1b (K: d ϭ 120.5(12) pm, Rb: d ϭ
120.4(13) pm, Cs: d ϭ 118.8(18) pm) and N2aϪN2b (K:
d ϭ 119.0(11) pm, Rb: d ϭ 120.5(13) pm, Cs: d ϭ
120.3(18) pm) being significantly larger as N1bϪN1c (K:
d ϭ 113.5(12) pm, Rb: d ϭ 113.6(12) pm, Cs: d ϭ
The alkali metal cations are coordinated by 10 azide ions
(Figure 3, K: d ϭ 291.1(10) pm Ϫ 322.2(11) pm, dave
307.3 pm; Rb: d ϭ 307.3(10) pm Ϫ 328.1(11) pm, dave
316.3 pm; Cs: d ϭ 323.0(15) pm Ϫ 339.1(20) pm, dave
ϭ
ϭ
ϭ
330.9 pm); the AϪN distances are longer compared with
the corresponding binary azides (KN3: d(KϪN)
296.3(3) pm [1], RbN3: d(RbϪN) ϭ 310(3) pm [1], CsN3:
ϭ
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Z. Anorg. Allg. Chem. 2006, 1671Ϫ1680

Azidoaurates of the Alkali Metals
Figure 3 Detail of the polyhedra chain AN2(N4/2)2 in A[Au(N3)4]
(A ϭ K, Rb, Cs). The rectangular N4 interfaces are emphasized
(green).
d(CsϪN) ϭ 328(2) pm [1]) but in these compounds the co-
ordination number of the cation is only 8. The alkali metal
cations in A form 1D zig zag chains along [001] (d(KϪK) ϭ
430.4(3) pm, d(RbϪRb) ϭ 444.7(1) pm, d(CsϪCs) ϭ
468.3(3) pm), the angle AϪAϪA is increasing from K to
Cs: (K: 129.03(1)°, Rb: 131.17(1)°, Cs: 132.87(1)°). The
AN10 polyhedra are connected along the chain via two rec-
tangular N4 interfaces, which are built up exclusively from
N species without Au contacts. These interfaces are tilted
with respect to the connecting line AϪA: K: 7.316°, Rb:
4.842°, Cs: 2.707°. The azide anions are coordinated by A
either from only within one AϪA chain (N2) or from two
neighbouring AϪA chains (N1).
The unit cell parameters of the compounds A provide
another interesting feature: whereas the unit cell volume
steadily increases from K to Cs, the lattice parameter a is
decreasing. To explain this feature, it is advisable to use a
non-standard setting (β < 90°) for the unit cell of
K[Au(N3)4], thereby transforming the atomic parameters to
values comparable to the Rb and Cs compounds. The mon-
oclinic angle β is increasing from 88.780(5)° in K[Au(N3)4]
to 90.322(5)° in Rb[Au(N3)4], and 92.72(5)° in Cs[Au(N3)4],
respectively. The x, y, z parameters of A and Au do not
change significantly within standard deviation, but the posi-
tion of the N species, especially N1aϪN1c, adapts in depen-
dence of the cation species. The effects on the crystal struc-
tures are shown in Figure 4. The angle η between the AuN4
plane of the complex anion and the plane of the AϪA zig
zag chain is decreasing with increasing cation size (K:
69.731°, Rb: 66.733°, Cs: 60.862°). The same is observed
for the angle ϕ between the connecting line N1cϪN1cЈ and
the A plane (K: 64.73°, Rb: 61.51°, Cs: 56.12°), whereas the
changes for N2cϪN2cЈ are only minor (K: 37.00°, Rb:
37.06°, Cs: 35.67°; not shown in Figure 4).
Figure
4
Effect of cation size on the crystal structure of
A[Au(N3)4] (A ϭ K, Rb, Cs). The effects of the displacement K Ǟ
Cs (arrows) are indicated. The angle η between the AuN4 plane
(red) of the complex anion and the plane of the A Ϫ A zig zag
chain (grey) is decreasing with increasing cation size. The same is
observed for the angle ϕ between the connecting line N1c Ϫ N1cЈ
(blue) and the A plane (grey).
discrete [Au(N3)4]Ϫ unit. The assignment of the normal
modes is supported by Normal Coordinate Analysis (NCA)
and PED (Potential Energy Distribution) [16] calculations.
The point group (C2) analysis performed on [Au(N3)4]Ϫ
predicts a total of 33 fundamentals distributed as follows:
Γvib ϭ 17 A (RE, IR)ϩ 16 B (RE, IR).
According to the selection rules both A and B species are
Raman allowed and IR active. The Raman and infrared
spectra of the title compounds depicted in Figure 5 and
wave numbers given in Table 5 show great similarities both
in their band intensities and the wave numbers. Slight differ-
ences in the IR spectra, especially around 1300Ϫ1475 cmϪ1
are due to the presence of combination modes (Table 5).
Regarding the hitherto reported data on binary [17Ϫ19]
and ternary azido metalates [5, 7, 20, 21], as well as the
results of the present NCA and PED calculations, the vi-
brational spectra of the A[Au(N3)4] (A ϭ K, Rb, Cs) series
can be divided into four well-separated wave number re-
gions:
1. 2100Ϫ1000 cmϪ1
stretches,
,
resulting from ν(NϪNϪN)
2. 700Ϫ550 cmϪ1, originating in and out of plane (o.o.p.)
deformations δ(NϪNϪN) and γ(NϪNϪN),
3. ν(AuϪN) valence frequencies around 400 cmϪ1
,
Vibrational spectroscopy
4. ν < 300 cmϪ1, dominated by the δ(NϪAuϪN) and
δ(AuϪNϪN) deformations, as well as τ (AuϪNϪN) and
τ(NϪNϪN) torsions.
The vibrational spectra of the title compounds have been
measured and interpreted based on the C2 symmetry of a
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S. Afyon, P. Höhn, M. Armbrüster, A. Baranov, F. R. Wagner, M. Somer, R. Kniep
• the marked loss of the Raman intensity of the symmetric
stretch of the free azide on complexation and its low fre-
quent shift with respect to the free anion.
Both findings are in good agreement with the measured
bond lengths d(NϪN) (Table 4). In fact, the two “shorter-
bonded” outermost atoms (-NϪNoϪNo) experience an in-
crease in their bond order, while the situation for the longer
inner (-NiϪNiϪN) bonds is quite the contrary. This behav-
iour is best reflected in the (NϪN) force constants listed in
Table 6, with f(NoϪNo) ϭ 15.44 N·cmϪ1 and f(NiϪNi) ϭ
10.18 N·cmϪ1 for the outermost and the inner (NϪN)
bonds, respectively. In the NCA, a value of foϪi ϭ 1.5
N·cmϪ1 has been used for the (NoϪNo) / (NiϪNi) bond
interaction, taken as arithmetic average of the well-known
HϪNϭNϭN (foϪi ϭ 1.3 N·cmϪ1, [22]) Ϫ with the asym-
metric azide group Ϫ and the symmetric [NϭNϭN]Ϫ
(foϪi ϭ 1.8 N·cmϪ1, [17]).
The force constant f(NoϪNo) is remarkably higher than
that for the free azide ion (f(NϪN) ϭ 13.15 N·cmϪ1 [17])
but significantly lower than found for N2 (f(NϪN) ϭ 22.39
N·cmϪ1 [17]). Likewise, the force constant for the inner
(NiϪNi) bonds ranks between double (f ϭ 13.15 N·cmϪ1
)
and single bond (f ϭ 5.30 N·cmϪ1 [17]). Using the latter as
value for the (NϪN) single bond one obtains after Siebert
[23] for the (NϪN) bond orders N ϭ 2.40 (NoϪNo) and
1.69 (NiϪNi), respectively, meaning that the bonding situ-
ation in the tetraazidoaurate(III) anion can be described
with a presence of enforced and weakened double bonds,
.. ..
i.e.: [Au(N
N
N)4].
In plane bending motions of the azido groups are re-
corded as weak bands between 693 cmϪ1 and 641 cmϪ1
both in Raman and IR, while the respective out of plane
mode is only observable in the IR spectrum at ca. 580 cmϪ1
,
.
A
detailed assignment, including the low frequency
Figure 5 Raman and IR (Ϫ) spectra of the compound series
A[Au(N3)4].
T ϭ Transmittance, I ϭ Intensity in arbitrary units, * ϭ combina-
tions.
(ν < 300 cmϪ1) δ(NϪAuϪN) and δ(AuϪNϪN) bending
modes and τ(AuϪNϪN), τ(NϪNϪN) torsions is given in
Table 5.
Of special interest are the gold-nitrogen stretching fre-
quencies and (AuϪN) force constants. In all A[Au(N3)4]
compounds the symmetric νs(AuϪN) mode was registered
as the most intense Raman band around 410 cmϪ1 (i.e.
416 cmϪ1 (K), 410 cmϪ1 (Rb), 408 cmϪ1 (Cs)). The corre-
sponding antisymmetric valence modes νas(AuϪN) are lo-
cated as very strong IR absorptions at 429 cmϪ1 (K) ,
426 cmϪ1 (Rb), and 422 cmϪ1 (Cs), respectively. The re-
maining two (AuϪN) stretches give rise to only weak bands
around 400 cmϪ1, alternatively in IR and Raman. The ob-
served (AuϪN) wave numbers harmonize very well with
those reported for [Au(N3)4]Ϫ (C4h symmetry) in the coordi-
nation compound [(Ph)4As][Au(N3)4] [7] (Table 5).
Based on results of the NCA, the best fit for the (AuϪN)
frequencies was achieved when the initial value (f ϭ 2.00
N·cmϪ1) for (AuϪN) force constant was refined to
f(AuϪN) ϭ 2.32 N·cmϪ1. Unfortunately, no literature data
on f(AuϪN) are available yet, enabling a comparative state-
ment on the strength of the (AuϪN) bonding. For a rough
estimation, a force constant comparison between f(AuϪN)
The vibrational spectra of the free azide anion with the
symmetry Dϱh exhibit three bands stemming from the sym-
metric and antisymmetric stretches at 2036 cmϪ1 (Σu, IR),
1344 cmϪ1 (Σg, RE), respectively, and the double degener-
ate deformation at 647 cmϪ1 (Πu, IR) [17, 18]. The
[Au(N3)4]Ϫ complex comprises two pairs of symmetry
equivalent azide groups vibrating in or out of phase with
respect to each other. In this sense, the band groups be-
tween 2030Ϫ2080 cmϪ1 and 1300Ϫ1200 cmϪ1 represent
clearly the stretching modes of the azide moieties, the for-
mer originally being the antisymmetric stretch (Σu, IR)
which became Raman active due to symmetry reduction to
C2 in the complex. Hereby, two details are worth to be men-
tioned:
• the strong Raman intensity of the originally exclusively
IR active antisymmetric stretch in the azidoaurate com-
plex and its slight but significant shift to higher wave
numbers,
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Azidoaurates of the Alkali Metals
Table 5 Vibrational wave numbers /cmϪ1 of [Au(N3)4]Ϫ in A[Au(N3)4] (A ϭ K, Rb, Cs, [Ph4As]) with assignment and relative Intensities;
v ϭ very, s ϭ strong, m ϭ medium, w ϭ weak, sh ϭ shoulder.
ϩ
* ϭ [Au(N3)4]Ϫ in [7] has C4h symmetry. ϭ Raman intensities are estimated.
K[Au(N3)4]
R
Rb[Au(N3)4]
R
Cs[Au(N3)4]
R
[Ph4As][Au(N3)4] [7]*,ϩ
Assignment
IR
IR
IR
R
IR
117 b,m
176 sh
117 b,m
118 b,m
δ(AuϪNϪN),
τ(NϪNϪN)
172 w
188 w
219 w
219 w
220 w
δ(NϪAuϪN)
234 m
238 mw
238 mw
236 w
325 ms
339 vs
398 mw
Lattice
modes
399 sh
410 vs
396 sh
408 vs
399 sh
429 vs
580 w
688 sh
400 w
426 vs
579 w
399 sh
402 mw
414 vs
ν(AuϪN)
416 vs
422 vs
568 w
576 w
427 vs
575 w
γ(NϪNϪN)
δ(NϪNϪN)
682 w
693 sh
680 w
693 sh
674 w
692 w
676 w
683 s
693 w
720 w
1268 vs
692 vw
1267 vs
691 w
399ϩ325ϭ724
ν(-N؊NϪN)
1270 vs
1250 s
1276 vw
1284 vw
1324 w
1282 vw
1332 w
1261 vw
1282 w
Combinations
2030Ϫ692ϭ1338
1377 w
1464 m
1372 vw
1385 s
682ϩ693ϭ1375
691ϩ692ϭ1383
2036Ϫ580ϭ1456
2032 sh
2061 m
2035 w
2070 m
2032 w
2063 m
2036 vs
2078 s
2030 vs
2077 s
2030 vs
2075 sh
2027 s
ν(-NϪN؊N)
2049 m
and the f(AuϪCl) ϭ 2.1 N·cmϪ1 [17] in the covalent
[AuCl4]Ϫ might be useful. The fact that f(AuϪN) ϭ 2.32
N·cmϪ1 > f(AuϪCl) indicates that the (AuϪN) bond in the
title compounds is strong and predominantly covalent.
Table 6 List of the internal force constants used for the NCA. For
simplification, the numerical values of same type of force constants
which are not symmetry equivalent were equated. The structure
parameters for [Au(N3)4]Ϫ are taken from the Cs compound. The
average difference between the calculated and experimental wave
numbers was less than 0.9 % for the stretching modes, and around
2.5 % for the low frequency deformations and torsions. The units
of the force constants for bonds (bond interactions) and angles
(torsions) are given in N·cmϪ1 and N·cm, respectively. For distinc-
tion, N atoms of the azide group are numbered as follows:
Au(-NaϪNbϪNc).
Quantum chemical calculations and ELF
For a bonding analysis in position space we employed the
topological analysis of the electron density (AIM method)
[24] and of the electron localization function ELF [25]. Both
quantities have been calculated from the ADF all electron
wavefunction using program DGrid [26] on a uniform equi-
distant grid with a mesh size of 2.6 pm. Numerical topo-
logical analysis of these data has been performed using pro-
gram Basin [27]. Electronic basin populations are calculated
by integration of the total electron density within the bas-
ins. Numerical results are given in Table 7. In order to eluci-
date the changes within the azide ligand due to AuϪN in-
teraction, the same analysis has been done for the free azide
anion with an optimized distance of d(NϪN) ϭ 118.7 pm.
The classical ’valence bond formula’ for N3Ϫ implies two
lone pairs for each terminal N atom and double bonds be-
Internal coordinate,
force constant f
bond (d) /angle(α), torsion (τ)
d(NbϪNc)
d(NaϪNb)
d(AuϪNa)
α1(NaϪNbϪNc)
α2(NaϪAuϪNaЈ)
α3(AuϪNaϪNb)
τ1(ϪNaϪNbϪNc)
τ2(ϪAuϪNaϪNb)
(NaϪNb)/(NbϪNc) interaction
(AuϪNa)/(NbϪNc) interaction
f(NoϪNo) ϭ 15.44
f(NiϪNi) ϭ 10.08
f(AuϪN) ϭ 2.32
fα1 ϭ 0.71
fα2 ϭ 1.11
fα3 ϭ 0.67
fτ1 ϭ 0.04
fτ2 ϭ 0.07
fo-i ϭ 1.50
fAu-i ϭ 0.24
NaЈ is pendant of Na from the symmetry equivalent azide unit.
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S. Afyon, P. Höhn, M. Armbrüster, A. Baranov, F. R. Wagner, M. Somer, R. Kniep
Table 7 Numerical results of the topological analysis of the elec-
tron density (AIM charges) and ELF. For the “ELF controlled ato-
mic charges” (Z being the order number) NϪN disynaptic basin
populations have been equally distributed between the nitrogen
atoms, while the NϪAu disynaptic basin has been fully attributed
to N1a, N2a.
Ϫ
Atom, feature
[Au(N3)4]Ϫ
N3
ELF analysis
(basin populations q/eϪ)
Au Ϫ N1a, N2a
lp.: N1a, N2a / Nt
0.739, 0.705
3.979, 4.000
Ϫ
4.644
bond: N1a Ϫ N1b, N2a Ϫ N2b / Nt Ϫ Nm
bond: N1b Ϫ N1c, N2b Ϫ N2c / Nm Ϫ Nt
2.663, 2.770
3.867, 3.777
3.165
3.165
Ϫ
Figure 6 ELF 0.80-localization domains for free N3
.
lp.: N1c, N2c / Nt
N1c, N2c / Nt
1.824, 2.078
2.232, 1.969
4.644
core: N1a, N2a / Nt
N1b, N2b / Nm
N1c, N2c / Nt
2.111, 2.107
2.112, 2.114
2.106, 2.107
2.111
2.117
2.111
ing regions. A similar situation is known from the N2 mol-
ecule [28].
core: Au
77.499
Ϫ
Sum azide
21.633, 21.627
164.019
21.957
Ϫ
Atomic charges can be calculated using density basins of
the AIM method and also from ELF basin populations. In
the latter case the electronic populations of disynaptic bas-
ins were equally partitioned among the two atoms. This is
still an arbitrary choice, since although the atom types are
the same, Nt (terminal) and Nm (middle) are clearly different
species, which explains the rather different atomic charges
for Nt and Nm resulting from that choice. In comparison,
the AIM charges are much more similar and negative for
both atom types, which we feel is more realistic. This differ-
ence can be understood by realizing that the density basin
for Nm contains significantly more than just the half of the
disynaptic ELF basin. Although the values differ signifi-
cantly, both methods give the same trend, namely that the
middle Nm atom is attributed less electrons than the ter-
minal ones. This fits the classical expectations.
Sum [Au(N3)4]Ϫ
ELF controlled atomic charges ZϪqELF
Au
ϩ1.501
Ϫ
N1a, N2a / Nt
N1b, N2b / Nm
N1c, N2c / Nt
Ϫ1.161, Ϫ1.197
ϩ1.623, ϩ1.613
Ϫ1.096, Ϫ1.043
Ϫ1.338
ϩ1.718
Ϫ1.338
Sum azide
Ϫ0.634, Ϫ0.627
Ϫ1.021
Ϫ0.957
Ϫ
Sum [Au(N3)4]Ϫ
AIM charges ZϪqAIM
Au
ϩ0.825
Ϫ
N1a, N2a / Nt
N1b, N2b / Nm
N1c, N2c / Nt
Ϫ0.300, Ϫ0.291
Ϫ0.106, Ϫ0.108
Ϫ0.055, Ϫ0.066
Ϫ0.412
Ϫ0.133
Ϫ0.412
The sum of all ELF basin populations (15.6 eϪ) in the
valence region significantly deviates from 16, since each N
atom does not exactly possess 2 electrons in the 1st atomic
shell but about 2.1 eϪ. This is a feature already known from
atomic shell structure studies [29] and is not due to bonding
effects or errors of integration.
Sum azide
Ϫ0.461, Ϫ0.465
Ϫ1.027
Ϫ0.957
Ϫ
Sum [Au(N3)4]Ϫ
tween the N atoms, but due to cylindrical symmetry of the
molecule, two separate lone pairs for each terminal nitrogen
atom cannot be formed.
The bonding situation for the complex anion [Au(N3)4]Ϫ
is characterized by the bond alternation within each of the
azide ligands and the interaction of one terminal N atom
with Au. The bond alternation is well displayed by a change
of ELF basin populations for disynaptic basins. While the
sum of charges in disynaptic basins of each azide ligand
(6.5 eϪ) only slightly increases compared to free azide
(6.3 eϪ), their populations strongly differentiate according
to 2.7 eϪ/2.8 eϪ (long d(N1aϪN1b)/long d(N2aϪN2b)) and
3.9 eϪ/3.8 eϪ (short d(N1bϪN1c/d(N2cϪN2c))), respec-
tively. Assuming a linear dependence between the NϪN
bond orders and the disynaptic basin populations [30], the
Ϫ
Topological analysis of ELF for free N3 reveals two
types of basins in the valence region (Figure 6). One mono-
synaptic basin with a ring attractor occurs at the nearest
neighbour opposed side of the terminal Nt atoms. This
corresponds to the lone pair feature of the bonding situ-
ation. Disynaptic basins with point attractors between the
N atoms signify covalent bonding interactions. The occur-
rence of a point attractor along the interconnection line for
double- bonded atoms is a known feature of the ELF rep-
resentation, which does not exhibit σϪπ separation as seen
in the MO representation. The electronic basin occupations
are 4.6 eϪ and 3.2 eϪ for each of the monosynaptic and the
disynaptic basins, respectively. Thus, with respect to the
classical picture, the ELF basin populations of the lone pair
region have gained 0.6 eϪ in excess at the cost of the bond-
observed changes of about
20 % in basin populations
(taking the double bond of free azide as the reference) result
in bond orders of about 1.7 and 2.4 for the long and the
short bond, respectively. This is in good agreement with the
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Azidoaurates of the Alkali Metals
(N3)0.6Ϫ results for both azide species. A quite similar effec-
tive charge according to (N3)0.5Ϫ is obtained from AIM ba-
sin populations. The reason for the difference between both
populations is, that the NϪAu disynaptic ELF basin is not
completely included in the AIM basin of the corresponding
N atom, but also in the AIM basin of Au. All that shows
that the charge transfer from Au to azide has not fully
taken place giving about [Au1.50ϩ(N30.63Ϫ)4]Ϫ and
[Au0.83ϩ(N30.46Ϫ)4]Ϫ, respectively for each method (Table 7),
which is another indication for the covalent interaction
NϪAu.
Conclusion
A new class of alkali metal azidoaurates A[Au(N3)4] (A ϭ
K, Rb, Cs) has been prepared from binary alkali metal az-
ides and HAuCl4 and their crystal structures are solved
from X-ray single crystal data. The unusual reduction of
the lattice parameter a with increasing cation size has been
shown to be due to a shearing movement of the complex
anions in the crystal structure. Force constants for the
NϪN bonds in the azide ions were derived from the meas-
ured and fully assigned vibrational bands in IR and Raman
spectra. Analysis of chemical bonding for the isolated com-
plex anion on the basis of topological analysis of ELF re-
veals significant covalent bonding interaction NϪAu. As a
consequence of this interaction a cumulene-type stereo-
chemistry of the lone pairs can be observed. For the azide
species NϪN bond orders of 1.7 and 2.4 calculated on the
basis of ELF basin populations agree very well with those
derived from vibrational spectra. All these data point to the
weak/strong double bond scenario instead of the single
bond/triple bond one in the coordinated azide species. Both
crystallographic and vibrational spectroscopic data as well
as quantum chemical calculations and ELF analyses con-
firm the presence of complex azidoaurate(III) anions
[Au(N3)4]Ϫ in these compounds.
Figure 7 ELF for [Au(N3)4]Ϫ in different representations. Right
side: 0.80-localization domains (brown) and two N-Au bonding ba-
sins (blue and purple) for the symmetry independent part of the
molecule. Left side: yellow 0.8385- and orange 0.93-localization do-
mains displaying the cumulene-type arrangement of lone pairs. The
strongly structurized 5th shell of Au is indicated by the green 0.63-
localization domain.
bond orders (1.7, 2.4) derived from normal coordinate
analysis of the vibrational data.
The interaction of Au with one side of the azide ligand
causes the lone pair type basin with its ring attractor to
split into two, one of which becoming a disynaptic basin
according to a covalent donor-acceptor bond NϪAu. An-
other strong indication of the covalent bond nature is the
strong structuring of the 5th atomic shell of gold due to
participation of Au(5d) functions in directed bonding inter-
actions [31]. The stereochemistry being fixed at the coordi-
nated side of the azide, it is fixed on the non-coordinated
side as well: two lone pair type basins occur whose attrac-
tors are located in a plane perpendicular to the attractors
on the coordinated side (Figure 7). The occurrence of two
lone pair type basins on each side of the azide ligand and
the cumulene-like stereochemistry clearly point to the long-
short double bond scenario instead of a single bondϪtriple
bond scenario, since in the latter case one lone pair type
basin on the terminal N and three basins on the gold side
of the molecule should be seen. Again, this corroborates
the double bond scenario deduced from vibrational studies.
Concerning AuϪN bonding it is remarkable that the elec-
tronic population (0.7 eϪ) of the donor-acceptor bond
strongly deviates from that of its lone pair neighbor
(4.0 eϪ). The sum of both nearly equals the lone pair basin
population for free azide (4.6 eϪ). On the contrary, at the
terminal side of the ligand significantly reduced lone pair
basin populations are found (4.0 eϪ). This loss overcompen-
sates all small gains in electronic populations compared to
free azide such that after summing up all populations (the
AuϪN bond has been fully counted for N) a charge of
Acknowledgement. The authors wish to acknowledge Anja Völzke
and Ulrike Schmidt for chemical analyses, Dr. Horst Borrmann,
Steffen Hückmann, and Dr. Yurii Prots for X-ray investigations.
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