Synthesis, crystal structure, and reactivity of Na5[CuO2](OH)2

Article abstract of DOI:10.1002/1521-3749(200112)627:12<2571::AID-ZAAC2571>3.0.CO;2-3

Na₅[CuO₂](OH)₂ has been obtained as orange single crystals from mixtures of NaOH, Na₂U and Cu₂U in sealed Ag containers. The crystal structure has been refined from X-ray diffraction data (IPDS data, Pnma, Z = 4, a = 607.4(1) pm, b = 891.2(1) pm, c = 1201.0(2) pm, R₁ = 0.03). The characteristic unit is the bent [CuO₂]^3- complex (Z(O-Cu-O) = 170°). The reactivity of Na₅[CuO₂](OH)₂ has been studied by DSC and in situ X-ray diffraction techniques. IR spectroscopy has been used for further characterization. The Madelung Part of the Lattice Energy (MAPLE) has been calculated as well. WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001.

Full text of DOI:10.1002/1521-3749(200112)627:12<2571::AID-ZAAC2571>3.0.CO;2-3

ARTIKEL
Synthesis, Crystal Structure, and Reactivity of Na5[CuO2]!OH)2
Peter Amann and Angela MoÈller*
KoÈln, Institut fuÈr Anorganische Chemie der UniversitaÈt
Received April 12th, 2001.
Abstract. Na5[CuO2]$OH)2 has been obtained as orange sin-
gle crystals from mixtures of NaOH, Na2O and Cu2O in
sealed Ag containers. The crystal structure has been refined
from X-ray diffraction data $IPDS data, Pnma, Z = 4,
a = 607.4$1) pm, b = 891.2$1) pm, c = 1201.0$2) pm, R1 =
0.03). The characteristic unit is the bent [CuO2]3± complex
$Ð$O±Cu±O) = 170°). The reactivity of Na5[CuO2]$OH)2 has
been studied by DSC and in situ X-ray diffraction techni-
ques. IR spectroscopy has been used for further characteri-
zation. The Madelung Part of the Lattice Energy $MAPLE)
has been calculated as well.
Keywords: Copper; Cuprate; Crystal structure; IR spectro-
scopy
Darstellung, Kristallstruktur und ReaktivitaÈt von Na5[CuO2]!OH)2
InhaltsuÈbersicht. Na5[CuO2]$OH)2 wurde aus Gemengen
von NaOH, Na2O und Cu2O in verschlossenen Ag-Contai-
nern in Form oranger Einkristalle erhalten. Die Kristall-
struktur wurde anhand von Einkristalldaten bestimmt
$IPDS-Daten, Pnma, Z = 4, a = 607,4$1) pm, b = 891,2$1) pm,
c = 1201,0$2) pm, R1 = 0,03). Die charakteristische Bauein-
heit, [CuO2]3±, liegt nicht linear vor $Ð$O±Cu±O) = 170°).
Die ReaktivitaÈt von Na5[CuO2]$OH)2 wurde thermoanaly-
tisch $DSC) und mit in-situ-RoÈntgendiffraktometrie unter-
sucht. Anhand von IR-Spektren erfolgte die weitergehende
Charakterisierung. Der Madelunganteil der Gitterenergie,
MAPLE, fuÈr Na5[CuO2]$OH)2 wird ebenfalls diskutiert.
1
Introduction
[7] have been described before, we have modified our
2±
synthesis by introducing additional anions, O2
$Na2O2, for redox reactions) and OH± $NaOH), re-
spectively. The latter reaction has been successful: we
obtained Na5[CuO2]$OH)2 ± the first hydroxide of an
oxocuprate$I).
Several ternary oxides with monovalent copper in the
systems A2O/Cu2O with A = Na±Cs have been re-
ported. Apart from ACuO $Na±Cs) [1] no other
phases of monovalent copper with sodium have been
structurally characterized so far. It is interesting that
only for A = K, Rb, Cs, another structure type in the
copper rich part of the phase diagram, A3Cu5O4 [2],
and an alkaline rich phase, A3[CuO2] $A = K, Rb) [3],
is known. Na3[CuO2] [4] has been reported not to
crystallize isotypic with Na3[AgO2] [5] and A3[CuO2]
[3]. Therefore, we have studied again the sodium rich
part of the phase diagram A2O/Cu2O. We were not
able to obtain any other compound than NaCuO.
Since the system with sodium apparently reacts differ-
ent from the heavier alkaline metals, which may be at-
tested by the fact that a mixed-valent oxide, NaCu2O2
[6], and a divalent copper-hydroxide Na2[Cu$OH)4]
2
Synthesis and Reactivity
Na5[CuO2]$OH)2 has been synthesized from Na2O $from re-
actions of NaOH with sodium metal $98% Riedel-de HaeÈn))
[8], NaOH $p. a. Merck) and Cu2O $p. a. Aldrich) $molar ra-
tio: 3 : 4 : 1). Reactions have been carried out in Ni $99.0%
Goodfellow) and Ag $Degussa) containers under an inert
gas atmosphere $Ar) and sealed in silica glass ampoules. The
mixture has been annealed $600 °C, 5d), then cooled down
with a rate of 25 °C/d to 400 °C $3 d) and finally to 300 °C
$5d).
The mixture of the educts was studied by DSC experi-
ments $STA 409, Netzsch, D) as well. We used self-made,
air-tight silver containers and the following temperature pro-
gram: heating rate 10 K/min, 25 °C ® 600 °C, hold for
10 min, then cool to 200 °C. This cycle was repeated three
times.
* Dr. Angela MoÈller,
Institut fuÈr Anorganische Chemie,
UniversitaÈt zu KoÈln,
Greinstraûe 6,
Fig. 1 shows the principal effects at 285/272 °C, which
correspond to the phase transition a-NaOH « b-
D-50939 KoÈln
E-mail: angela.moeller@uni-koeln.de
Z. Anorg. Allg. Chem. 2001, 627, 2571±2575 Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0044±2313/01/6272571±2575 $ 17.50+.50/0 2571

P. Amann, A. MoÈller
NaOH and at 310/305 °C to the m. p. of NaOH [9], re-
spectively. The third effect at 295/289 °C corresponds
to the eutectic temperature of the binary phase dia-
gram NaOH/Na2O $13 wt-% Na2O) [9]. At 408/384 °C
we observe an additional reversible effect in the DSC.
It is interesting to note that we always detected the
presence of NaCuO by X-ray diffraction afterwards.
Therefore, we investigated the reaction with the pre-
viously mentioned technique in the temperature range
25 °C ® 400 °C ® 25 °C, Fig. 2. In this case we used
a BuÈhler camera $E. BuÈhler, D; H-H diffractometer,
Stoe & Cie, D) which represents a quasi open system.
The whole equipment is set up for protection against
moisture in a dry glove box $Braun, D).
No reflections of Na2O at 400 °C were found. We as-
sume that at somewhat lower temperature the mixture
Na2O/NaOH melts, with the corresponding thermal
effect at 408/384 °C observed in the DSC. From the
phase diagram NaOH/Na2O [9] the melting point
for a composition of 25wt-% Na 2O in NaOH is
approximately 420 °C. For preparative and DSC inves-
tigations we used closed reaction containers and
ended up always with a mixture of NaOH, NaCuO
and Na5[CuO2]$OH)2. From the BuÈhler camera ex-
periments the formation of the latter compound, Na-
CuO and Na2O were not observed by cooling below
the m. p. of NaOH. The reflections of NaOH became
too prominent $by a factor of 10). From these results
we assume that Na5[CuO2]$OH)2 is stable up to
400 °C and is obtained via a peritectic reaction.
Fig. 2 illustrates the principle reflection patterns as
a function of temperature. The reactions occurring in
the system NaOH/Na2O/Cu2O are:
1. 4 NaOH$l) + 3 Na2O$s) + Cu2O$s)
3
Crystal Structure of Na5[CuO2]!OH)2
® 2 Na5[CuO2]$OH)2$s)
$300 °C)
$400 °C)
The crystal structure of Na5[CuO2]$OH)2 has been de-
termined from X-ray diffraction data. For details see
Tab. 1 and 2. Fig. 3 shows a projection of the crystal
structure.
2. Na5[CuO2]$OH)2$s)
® NaCuO$s) + Na2O$s) + 2 NaOH$l)
The characteristic feature of the structure are the
bent [CuO2]3± complexes oriented in channels along
[010]. Angles Ð$O±M±O) of 175°±180° are typical for
[MO2]3± anions with M = Fe, Co, Ni [10, 11], whereas
we observe 170.14$8)° here. The two interatomic dis-
tances, d$Cu±O) are not equal, 178.8$2) pm and
181.7$2) pm. A similar effect upon bending has been
reported for KNa2[NiO2] as well [11].
The next nearest neighbours of the [CuO2]3± com-
plex are Na+, d$Cu±Na) ³ 285.2$1) pm and four hydro-
xide ions, which are orientated towards the monova-
lent metal $d$Cu±HO) = 315pm and 332 pm) in the
Fig. 1 DSC studies of the formation and decomposition of
Na5[CuO2]$OH)2 under inert gas atmosphere in sealed Ag-
containers.
Table 1 Crystallographic data and structure refinement for
Na5[CuO2]$OH)2
Space group
Pnma $No. 62), Z = 4
Lattice constants
single crystal data powder data [19]
a = 607.4$1) pm
b = 891.2$1) pm
a = 607.2$1) pm
b = 891.4$2) pm
c = 1201.0$2) pm c = 1201.3$3) pm
Diffractometer
Radiation
IPDS
STADI P
$Stoe & Cie)
Cu$Ka),
$Stoe & Cie)
Mo$Ka),
k = 71.073 pm
471.8
k = 154.0598 pm
F$000)
Absorption coefficient
Measured reflections
Unique reflections
Measured range
l = 3.48 mm±1
5035
806
±8 £ h £ 8, ±10 £ k £ 11, ±13 £ l £ 15,
2 Hmax = 56.15°
R$int)
0.0307
Absorption correction
X-RED, X-SHAPE [20]
Structure solution and refinement SHELXL-97 [21]
Number of refined parameters
R1
60
0.0223 for 675F o > 4r$Fo),
0.0298 for all data
0.0641
Fig. 2 In situ X-ray diffraction studies of the formation and
decomposition of Na5[CuO2]$OH)2 under inert gas atmos-
phere.
wR2
GooF
0.961
2572
Z. Anorg. Allg. Chem. 2001, 627, 2571±2575

Synthesis, Crystal Structure, and Reactivity of Na5[CuO2]$OH)2
Table 2 Atomic positions, coefficients of the ªanisotropicº and equivalent temperature factors in pm2 for Na5[CuO2]$OH)2
Atom
Site
x
y
z
U11
U22
U33
U23
U13
U12
0
Ueq
Cu
4 c
4 c
8 d
4 c
4 c
4 c
4 c
8 d
8 d
0.00356$5)
0.9138$2)
0.7580$1)
0.0517$2)
0.4363$2)
0.7741$3)
0.1936$3)
0.8691$3)
0.786$5)
0.75
0.52402$3)
149$6)
0.63883$7)
0.0406$1)
210$6)
100$2)
226$8)
236$5)
137$5)
483$11)
93$12)
116$2)
176$6)
93$5)
294$8)
209$6)
110$9)
015
119$2)
0 )
224$4)
138$5)
0 )
0
11$1)
184$3)
3$3)
27$4)
301$4)
0
111$1)
Na1
Na2
Na3
Na4
O1
O2
O3
H
0.750.2706$1)
0.49435$9)
0.75
0.750.7069$1)
0.750.6211$1)
0.750.4104$2)
0.4536$2)
0.490$2)
0±13$5
14$3)
0
0 98$5
22$7)
0
4$3)
0
184$2)
190$3)
132$9)
123$9)
0
112$4)
18$7)
182$09$)11)
161$7)
40$8)
174$7)
134$4)
0.8283$1)
0.871$2)
239$9)
30$6)
7$6)
191$3)
62$59)
Fig. 3 Projection of the crystal structure of Na5[CuO2]$OH)2.
Fig. 4 View along the channels containing [CuO2]3± and
OH± ions in Na5[CuO2]$OH)2. The grey-shaded bonds be-
tween the hydrogen atoms illustrate only the plane perpendi-
cular to the dumb-bell like oxocuprate.
Fig. 5 Connection of crystallographically independent [NaOx]
polyhedra with its equivalents present in Na5[CuO2]$OH)2.
plane perpendicular to the dumb-bell like oxocuprate
anions, Fig. 4. Obviously OH± is nonbonding to Cu+
and only coordinated by Na+. Within the channels
along [010] interatomic distances of d$Cu±Cu) =
448 pm occur, which are similar to those in KNa2-
[NiO2] and Na5[NiO2][CO3] [11, 12]. K5[AuO2]I2 [13]
contains a similar structural segment with channels of
[AuO2]3± complexes with d$Au±Au) = 551 pm and
d$Au±I) = 425pm for the four I ±, defining the perpen-
dicular plane to the dumb-bell like linear oxoaurate
units. The difference to Na5[CuO2]$OH)2 is that in
K5[AuO2]I2 all K+ are coordinated by four I± and two
O2±.
Four crystallographically independent sites are oc-
cupied by sodium. Na1 forms chains along [100] with
its crystallographically equivalent ones. These are con-
nected via triangular trans faces of the distorted octa-
hedra, built of four OH± and two trans O2±, Fig. 5.
Z. Anorg. Allg. Chem. 2001, 627, 2571±2575
2573

P. Amann, A. MoÈller
is the case for the [CuO2]3± assembling d10 configura-
tion. This sequence also holds, if one compares with
Cu2O: d$Cu±O) = 184 pm, mas = 615cm ±1 [15] and the
second exited state progression observed for K3[NiO2]
with d$Ni±O) = 184 pm, ms = 535 cm±1 [11]. The de-
Table 3 Motifs of coordination relationships [22]
Na5[CuO2]$OH)2 with distances in pm
O1
O2
O3 $OH) C. N. ECoN MEFIR
Na1
Na2
2/2
238.8$2)
255.3$2)
4/2
253.8$2)
259.3$2)
6
5.8
108.9
rived force constants for the stretching modes $D¥h
)
Ê
Ê
[16] are 4.2 mdyn/A, 3.4±3.7 mdyn/A and 2.4±
1/2
1/2
2/2
4
3.9
96.5
Ê
2.7 mdyn/A for 177 pm, 180 pm and 184 pm, respec-
290.0$1)
227.5$1)
240.1$2)
242.2$2)
tively. The $NaO) vibrations are commencing from
550 cm±1 towards smaller wavenumbers. In the FIR
region of the spectrum a broad band at 250 cm±1 is ob-
served with a shoulder at 130 cm±1 which might indi-
cate the very weak bending mode for the [O±Cu±O]3±
complex, see also [2].
Na3
Na4
1/1
236.6$2)
1/1
225.3$2)
2/1
244.9$2)
4
4
3.8
3.0
98.2
99.1
2/2
2/1
228.8$2)
229.6$2)
270.5$2)
Cu
1/1
1/1
2
2.0
50.2
181.7$2)
178.8$2)
5
MAPLE calculations [17]
C.N.
6
6
6
The difference between the MAPLE value of the bin-
ary and the pseudo-ternary compounds is less than
±1%, see Tab. 4.
ECoN
MEFIR
5.9
134.1
5.7
133.6
5.7
144.7
Since the [CuO2]3± complex is bent $170.14$8)°),
we have investigated the influence of the interatomic
distance and angle by modifying the Cu2O structure
[18] in the range of d$Cu±O) = 175±185 pm and
Ð$O±Cu±O) = 160±180°. As expected, there is only a
small dependence of the MAPLE value for Cu+ upon
a distortion from 180°. We find that this value is re-
duced by 3 kcal/mol towards a bent bridging angle of
160° and just » +1 kcal/mol for 170°. Therefore, the
bending should occur as a result of the lattice effects
caused by the difference in the anionic part of the
structure, in this case O2± and OH±. The MAPLE con-
tribution for Cu+ increases by 10 to 12 kcal/mol when
the interatomic distance is reduced to 175pm. This
explains very nicely the already observed fact that
MAPLE$M+) for ternary oxometalates $d$M±O) =
177 pm) is approximately 10 to 15kcal/mol larger than
the estimated value from the binary components [11].
For Cu2O the interatomic distance d$Cu±O) equals
184 pm [18].
Initial values in pm: r$Na1) = 106.33, r$Na2) = 98.50, r$Na3) = 99.56,
r$Na4) = 100.13, r$Cu) = 54.07, r$O) = 140
The distorted tetrahedra, [Na4O2$OH)2], are con-
nected only via O2± to zigzag chains along [100]. For
Na2 a similar motif along [010] is observed, but
further bridging to crystallographically equivalent
sodium atoms occurs via both OH± to layers. In con-
trast to all other sodium atoms, Na3 is in this respect
isolated with a C. N. of 2 + 2. For a detailed list of in-
teratomic distances and motifs of coordination see
Tab. 3.
4
IR Spectrum of Na5[CuO2]!OH)2
Selected single crystals of Na5[CuO2]$OH)2 were in-
vestigated by infrared spectroscopy. The two typical
hydoxide modes for a bridging OH± [14] are observed
at 3437 cm±1 and 1085cm±1 for m$OH) and d$NaOH),
respectively. It is interesting to note that the infrared
spectrum confirms that no further hydrogen bonding
is present here. The shortest interatomic distances are
d$Na±OH) = 224±270 pm, d$Cu±HO) = 320 pm and
d$O±HO) = 350±390 pm. Furthermore, the asymmetric
stretching mode of the [O±Cu±O]3± complex is ob-
served at 796 cm±1. Since this complex is not exactly
linear, the symmetric stretching vibration should be
allowed for observation in the infrared spectrum for
C2v symmetry. In fact, we observe a band located
at 617 cm±1. This is about 50 cm±1 lower than in
K3[NiO2] and KNa2[NiO2] with d$Ni±O) = 177 pm
[11]. But it is approximately the value derived from
the vibrational progression in the first excited state of
a linear [NiO2]3± complex with two electrons in the dz2
orbital and an interatomic distance of 180 pm, which
Table 4 Madelung Part of the Lattice Energy $MAPLE) in
kcal/mol for Na5[CuO2]$OH)2
binary
quaternary
D $binary-quaternary)
Na1
Na2
Na3
Na4
Cu
1 ´
2 ´
1 ´
1 ´
1 ´
1 ´
1 ´
117.1a)
117.1a)
117.1a)
117.1a)
147.7b)
465.1c)
465.1c)
118.3d)
114.1
125.0
120.1
115.9
162.5±14.8
465.2
453.9
3.0
±15.8
±3.0
1.2
O1
O2
±0.1
11.2
0.2
O3 $OH) 2 ´
118.2
R
1900.0
1918.1
±18.1 = ±0.95%
a)
average value from 2 MAPLE$Na+$NaOH)) + 3 MAPLE$Na+$Na2O))
MAPLE$Cu+$Cu2O))
b)
c)
d)
average value from 0.5MAPLE$O 2±$Cu2O)) + 1.5MAPLE$O 2±$Na2O))
MAPLE$OH±$NaOH))
2574
Z. Anorg. Allg. Chem. 2001, 627, 2571±2575

Synthesis, Crystal Structure, and Reactivity of Na5[CuO2]$OH)2
Acknowledgement. We like to thank cand.-phys. Eva Bencki-
ser for carrying out some of the preparative work.
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Supplementary Material Available: Details of the structure
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