Heterobimetallic copper-barium complexes for deposition of composite oxide thin films

Article abstract of DOI:10.1039/b823024b

Heterobimetallic molecular precursors [Ba(dmap)₄Cu ₄(OAc)₆·THF] (1) and [Ba(dmap)₄Cu ₄(TFA)₆·THF] (2) [dmap = N,N- dimethylaminopropanolate, OAc = acetate and TFA = trifluoroacetate] for th

Full text of DOI:10.1039/b823024b

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Heterobimetallic copper–barium complexes for deposition of composite
oxide thin filmsw
Asif Ali Tahir,a Muhammad Mazhar,*a Mazhar Hamid,a Matthias Zellerb
and Allen D. Hunterb
Received (in Gainesville, FL, USA) 24th December 2008, Accepted 16th March 2009
First published as an Advance Article on the web 2nd April 2009
DOI: 10.1039/b823024b
Heterobimetallic molecular precursors [Ba(dmap)4Cu4(OAc)6ÁTHF] (1) and
[Ba(dmap)4Cu4(TFA)6ÁTHF] (2) [dmap = N,N-dimethylaminopropanolate, OAc = acetate and
TFA = trifluoroacetate] for the deposition of barium–copper composite oxide thin films, were
prepared by the interaction of Ba(dmap)2 with Cu(OAc)2 for 1 and Cu(TFA)2 for 2, in THF.
Both heterobimetallic complexes were characterized by melting point, elemental analysis, FT-IR
spectroscopy, mass spectrometry and single crystal X-ray diffraction. X-Ray crystallography
shows that complex 1 crystallizes in the orthorhombic space group P212121 with the cell
dimensions a = 11.2621(11) A, b = 18.2768(17) A and c = 24.541(2) A, while complex 2
crystallizes in the monoclinic space group C2/c with a = 23.9288(14) A, b = 19.8564(12) A,
c = 25.5925(15) A and b = 112.4390(10)1. Thermal gravimetric analysis shows that both
complexes 1 and 2 undergo controlled thermal decomposition at 450 1C and 400 1C, respectively,
to give mixed metal oxide composite thin films. Scanning electron microscopy (SEM), energy
dispersive X-ray (EDX) and X-ray powder diffraction (XRD) analyses of the thin films suggest
the formation of good quality crystalline thin films of BaCuO2–CuO composites from
both 1 and 2, with average grain sizes of 105 to 175 nm and 110 to 205 nm, respectively.
is to bind suitable organic ligands to the metal centers. Due to
Introduction
the inherent different nature of the metal ions present in
The discovery of high Tc YBa2Cu3Ox and other ceramic
superconductors1,2 has intensified research into the
superconductors, ligands have to be chosen based on their
suitability for each metal. For example, a copper(II) ion will
development of new copper and barium precursors for
bind well with oxygen or nitrogen donor atoms in an
solution route processing towards these materials.3 The
essentially covalent fashion while barium(II) and yttrium(III)
commonly used chemical method for the production of thin
ions have a poor affinity towards neutral nitrogen donor
films of superconductor materials is via either chemical vapor
atoms, but, due to their ionic nature, a relatively good affinity
deposition (CVD) or via sol–gel processes.4 The CVD method
towards negatively charged donor atoms such as oxides or
alkoxides.6
has been shown to be able to produce good quality films, but it
is often limited by the unavailability of precursor compounds
Keeping these requirements for the synthesis of materials by
with sufficient volatility.5 In an alternative approach the
AACVD of mixed metal ceramic materials in mind, we have
corresponding precursor compounds that contain all the
focused our research on two areas: the design and synthesis of
desired components are mixed together in solution and are
discrete molecular precursors for ceramic superconductors
then coated onto the substrate via spin coating or a dip coating
and their subsequent use for the deposition of thin films by
process. Another promising technique for the deposition of
aerosol assisted chemical vapour deposition. Following our
thin films of ceramic materials is ultrasonic aerosol assisted
previous work directed towards the design of heterobimetallic
complexes for mixed metal oxide systems,7,8 our strategy was
chemical vapour deposition (AACVD) which provides the
possibility of better control of the deposition conditions. The
therefore to employ bifunctional ligands which contain both a
most important requirements of all the solution techniques are
neutral and a negatively charged coordinating site so that they
that all components should be highly soluble in the solvent of
are able to bind to two different types of metal ions. An
choice, e.g. a standard organic solvent. One way to improve
example of such a ligand is the bifunctional amino alcohol
the solubility of the metal compounds in such organic solvents
ligand N,N-dimethylamino-1-propanol (dmapH). In our
previous investigation we found that it can coordinate in a
a Department of Chemistry, Quaid-I-Azam University,
Islamabad 45320, Pakistan. E-mail: mazhar42pk@yahoo.com;
Fax: +92(0)5190642241; Tel: +92(0)5190642106
variety of coordination modes and did often allow for
the formation of highly soluble molecular precursors.9,10
b STaRBURSTT-Cyberdiffraction Consortium @ YSU & Department
of Chemistry, Youngstown State University, 1 University Plaza,
Youngstown, OH 44555-3663, USA
Application of this ligand to the copper–barium system
yielded two promising mixed metal precursors, which were
both obtained in yields around 90% under mild reaction
w CCDC reference numbers 724118 and 724119. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b823024b
conditions at room temperature. Reaction of Ba(dmap)2
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with Cu(OOCCH3)2 and Cu(OOCCF3)2 gave crystalline
the disordered fluorine atoms were restrained to be isotropic
within a standard deviation of 0.1. The highest peak for
compound 2 has a value of 2.85 electrons per cubic A and is
located 0.03 A from the barium atom.
[Ba(dmap)4Cu4(OAc)6ÁTHF] (1) and [Ba(dmap)4Cu4(TFA)6Á
THF] (2) [dmapH
= N,N-dimethylaminopropanol and
OAc = acetate and TFA = trifluoroacetate]. Both complexes
1 and 2 are soluble in THF and toluene under the experimental
conditions of AACVD and both deposit homogeneous good
quality thin films of barium copper oxides that may
find possible uses in industrial and other technological
applications.
The phase and crystallinity of deposited films were
characterized using a Bruker AXS D8 Advance X-ray
diffractometer with primary monochromatic high-intensity
CuKa (l = 0.154184 nm) radiation using a PSD detector.
A range of 2y (from 10.00 to 75.00) was scanned, to cover
the diffraction patterns of the oxide phase. The percentage
compositions of both the phases were determined using the
‘‘PANanalytical X’Pert PRO’’ XRD software which calculates
the percentage composition of each phase based on the
2y value and intensity of the standard ICDD pattern and the
residual XRD pattern. Surface morphology of thin films was
studied by using a field emission gun scanning electron
microscope (Leo 1530 VP) at an accelerating voltage of
5 kV, and a working distance of 6 mm.
Experimental section
All manipulations were carried out under an inert atmosphere
of dry nitrogen using Schlenk tube and glovebox techniques.
Solvents were rigorously dried and distilled using standard
methods. Barium metal, Cu(OAc)2, lithium metal, trifluoro-
acetic acid, N,N-dimethylamino-1-propanol and all other
reagents were purchased from Aldrich Chemicals. Melting
points were recorded on a Mitamura Riken Kogyo (MT-D)
apparatus and are uncorrected. Elemental analyses were
performed using a CHN Analyzer LECO model CHNS-932.
FAB mass spectra were recorded using a JMS-HX-1100 mass
spectrometer of Jeol, Japan. FT-IR spectra were recorded with
a Bio-Red Excalibur FT-IR model FTs 300MX spectrometer
from KBr discs.
Synthesis of Ba(dmap)4Cu4(OAc)6ÁTHF
Barium metal (0.50 g, 3.64 mmol) was added to a stirred
solution of dmapH (1.5 g, 14.59 mmol) in 25 ml of dry toluene.
The mixture was stirred for 2 h until all pieces of barium were
consumed. Cu(OAc)2 (2.90 g, 14.59 mmol) was then added
with stirring to the above solution of Ba(dmap)2 and the
mixture was stirred for 12 h to obtain a deep blue solution,
which was filtered through a cannula to remove any traces of
solid residue. The reaction mixture was evaporated to dryness
under vacuum, and the solid was redissolved in 10 ml of THF
and placed in a refrigerator at À10 1C for crystallization. Blue
coloured crystals were obtained after 3 days in 85% yield
based on barium metal used. Mp: 110 1C; analysis: calc. for
C36H74BaCu4N4O17 C, 35.22; H, 6.03; N, 4.56; found C, 35.75;
H, 6.45; N, 4.70%. FT-IR/cmÀ1: 2977w, 2927w, 2363w,
2344w, 1582s, 1405s, 1336s, 1193w, 1089m, 1021w, 947m,
869m, 839s, 677m, 619m, 537w, 376w. FAB+ m/z: 1156
[M À THF]+, 1034 [M À 2OAc]+, 955 [M À 2dmap]+, 896
[M À (dmap)2(OAc)]+, 613 [M/2]+, 512 [M/2 À dmap]+, 389
[M/2 À (OAc)2(dmap)]+, 287 [M/2 À (OAc)2(dmap)2]+, 100
Controlled thermal analyses of the complexes were carried
out using a Perkin Elmer thermogravimetric analyzer TGA7
with a computer interface. The measurements were carried out
in alumina crucibles under an atmosphere of flowing nitrogen
gas (25 ml minÀ1) at a heating rate of 10 1C minÀ1
.
Single crystal diffraction data of complexes 1 and 2
were collected on a Bruker AXS SMART APEX CCD
diffractometer at 100(2) K using monochromatic MoKa
radiation with an O scan technique. The unit cells were
determined using SMART11 and SAINT+12 and the data
were corrected for absorption using SADABS in SAINT+.
The structures were solved by direct methods and refined by
full matrix least squares against F2 with all reflections using
SHELXTL.13 Refinement of extinction coefficients was found
to be insignificant. All non-hydrogen atoms were refined
anisotropically.
[dmap
À
H]+, 58 [OAc
À
H]+; TGA: 20–165 1C
(11.85% wt. loss); 165–289 1C (45.96% wt. loss); 289–412 1C
(residue 31.10%).
In the structure of 1 a solvate THF molecule is disordered
around a general site with an occupancy ratio of 0.75(2) to
0.25(2). The two moieties were restrained to have the same
geometries within a standard deviation of 0.02, the atoms were
restrained to be isotropic within a standard deviation of 0.1,
and the atoms within a ring were restrained to have similar
anisotropic components within a standard deviation of 0.03
(SIMU and DELU restraints in SHELXTL).
Synthesis of Ba(dmap)4Cu4(TFA)6ÁTHF
Barium metal (0.50 g, 3.64 mmol) was added to a stirred
solution of dmapH (1.5 g, 14.59 mmol) in dry THF (10 ml).
The mixture was stirred for 2 h until all pieces of barium were
consumed. Cu(TFA)2 (2.90 g, 14.59 mmol) was prepared by
the reaction of 1.83 g (14.59 mmol) of Cu(OMe)2 with 1.66 g
(14.59 mmol) of trifluoroacetic acid in 15 ml THF. The THF
solution of Cu(TFA)2 was added dropwise to the stirring
solution of Ba(dmap)2 and after 5 h the deep blue solution
was filtered through a cannula to remove any traces of solid
residue. The reaction mixture was evaporated to dryness under
vacuum, and the solid was redissolved in 5 ml of THF and
light blue coloured crystals were obtained after 3 days with
95% yield based on barium metal used. Mp: 195 1C; analysis:
calc. for C36H56N4F18BaCu4O17 C, 26.94; H, 3.43; N, 3.69;
In 2 a THF molecule is disordered around a two fold axis
over two sites, one located directly on the two fold axis, the
other around it. The occupancy ratio of the two sites is 0.44(1)
to 0.46(1). The C–C bond distances in the disordered
molecules were restrained to be 1.543 A, the O–C distances
to be 1.426 A. Three of the CF3 groups are disordered each
over two positions with occupancy ratios of 0.60(1) to 0.40(1),
0.43(2) to 0.57(2) and 0.48(1) to 0.52(1). The C–F and F–F
distances were restrained to be each the same for the
disordered moieties within a standard deviation of 0.02, and
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found C, 27.04; H, 3.66; N, 3.68%. FT-IR/cmÀ1: 2918w,
2872w, 2749w, 2539w, 1660s, 1449s, 1385w, 1199s, 1137m,
1039m, 1045s, 107s, 942m, 834s, 800s, 726m, 619m, 570w,
521w. FAB+ m/z: 1484 [M À THF]+, 1452 [M À dmap]+,
1412 [M À dmap]+, 1343 [M À (dmap)(THF)]+, 1307 [M À
(dmap)2]+, 1244 [M À Cu(dmap)2]+, 1000 [M/2 + 2TFA]+,
975 [M/2 + 2dmap]+, 922 [M À BaCu2(TFA)2(dmap)]+, 896
[M/2 + Ba]+, 843 [M/2 + C5H12N]+, 801 [M/2 + OC2H4]+,
mixed-metal complexes involving alkaline-earth metals and
the copper(II) ion. Indeed, several such mixed-metal complexes
have been obtained readily by using the bdmap ligand.18
Two heterobimetallic Cu–Ba complexes were readily synthe-
sized by simple synthetic procedures involving the mixing of
the reactants in the proper molar ratios. Thus complexes
Ba(dmap)4Cu4(OAc)6ÁTHF (1) and Ba(dmap)4Cu4(TFA)6Á
THF (2) were prepared by reacting appropriate amounts
of tetrameric N,N-dimethylaminopropanolato Ba(II) with
2 equivalents of Cu(OAc)2 and Cu(TFA)2 in THF at room
temperature. The complexes are soluble in common organic
solvents such as THF, toluene and alcohols and are stable in
air under ambient conditions.
772 [M/2
+
O]+, 723 [Cu2(dmap)2(TFA)2]+, 699
[Ba(TFA)5]+, 687 [Ba(dmap)3(TFA)2]+, 619 [BaCu(dmap)3-
(TFA)]+, 557 [Ba(dmap)3(TFA)]+, 508 [BaCu(dmap)3]+, 443
[Ba(dmap)3]+, 342 [Cu(dmap)(TFA)]+, 239 [Ba(dmap)3]+,
207 [CuO2(TFA)]+, 166 [Cu(dmap)]+, 136 [Ba]+, 102
[dmap]+; TGA: 195–223 1C (9.06% wt. loss); 223–269 1C
(32.5% wt. loss); 269–400 1C (residue 29.74%).
Both complexes 1 and 2 were characterized by their melting
point, elemental analysis, FT-IR, TGA and single crystal
X-ray diffraction. The FT-IR spectra display absorption
bands characteristic of the carboxylate ligands that appear
at 1582 and 1660 cmÀ1 for the asymmetric nas(CO2) vibration
and at 1405–1449 cmÀ1 due to the symmetric vibration
for 1 and 2, respectively. The difference of the values D =
nas(CO2) À ns(CO2) is greater than 150 cmÀ1 suggesting that
the carboxylate ligands are bridging between two metal
centers.19 In the case of complex 2, very intense sharp absorp-
tion bands at 1199 and 1137 cmÀ1 correspond to the C–F
vibrations20 which are not observed in the case of complex 1.
Additional sharp bands due to the coordinated Lewis base are
present which are shifted to lower frequencies with respect to
those of the free ligand. The FAB mass spectra of both
complexes 1 and 2 show the molecular ion peaks after
the removal of the solvent molecules. The mass spectra of
both the complexes show a well defined peak pattern, which
reveals the stability of the complexes under experimental
conditions.
Deposition of thin films by AACVD
Thin films from complexes 1 and 2 were deposited on
commercially available soda glass slides by using a self
designed ultrasonic aerosol assisted chemical vapor deposition
assembly.14 The glass substrate was cleaned prior to use by
ultrasonically washing with distilled water, acetone and finally
with ethyl alcohol. In a typical deposition, 25 ml of a 0.02 M
solution in toluene of 1 and in THF of 2 were added to a
round-bottomed flask. Air at a flow rate of 150 ml minÀ1 was
used as the carrier gas, and the flow rate was controlled by an
L1X linear flow meter. Substrate slides (2 cm  2 cm) were
placed inside the reactor tube and then heated up to 450 1C for
1 and 400 1C for 2. The aerosols of the precursor solution were
formed by keeping the round-bottomed flask in a water bath
above the piezoelectric modulator of an ultrasonic humidifier.
The generated aerosol droplets of the precursor were
transferred into the hot wall zone of the reactor by the carrier
gas. The reactor was placed in a tube furnace. In the case of
complex 2 the carrier gas was passed through water before the
aerosol flask to avoid the formation of barium fluoride.
Alternatively, the addition of 2 ml of water in the precursor
solution was also effective but some precipitates were left
behind at the end of the deposition which decreased the
barium content in the deposited films.
Molecular structures of complexes 1 and 2
The molecular structures of complexes 1 and 2 are shown in
Fig. 1 and Fig. 2, respectively. Selected material data, bond
distances and angles are listed in Table 1 and Table 2. The
Result and discussion
Lewis acid–base adduct formation to produce heterobimetallic
coordination complexes in good yield under mild conditions
is a very promising method to design molecules for the
deposition of thin films of a desired material.15 This type of
synthetic strategy aims to coordinatively saturate each metal
center by the use of chelating ligands e.g. a carboxylate, a
functionalized alcohol etc., which enhance their stability,
solubility, volatility and their shelf life. We have recently
reported that the amino alcohol N,N-dimethylaminoethanol
(dmaeH) can, as a ligand, bring two very different metal
centers such as copper(II) and lanthanide metals together
through binding via both the amino and the alkoxo site.16,17
Since alkaline-earth metal ions behave quite similarly to the
lanthanides due to their highly electropositive nature,
the ligand bis(dimethylamino)-1-propanol (bdmap) was
anticipated to be also a useful ligand for the synthesis of
Fig. 1 Molecular structure of Ba(dmap)4Cu4(OAc)6 and coordination
sphere of Ba(1) in 1. Ellipsoids are drawn at the 30% probability level.
The solvent (THF) molecule is not shown for clarity.
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known Cu–O bonds involving the dmap ligand.21,22 The
Cu–Cu separations in the dicopper units, Cul–Cu2
=
2.93(11) A, and Cu3–Cu4 = 2.921 (7) A, are comparable to
those of previously reported dinuclear copper complexes.23
The geometry around the copper centers in each dicopper unit
is distorted square pyramidal and the CuO4N core is made up
from two bridging oxygen atoms of an acetate or TFA ligand
(in 1 and 2, respectively), from two triply bridged oxygen
atoms of the dmap ligands and the remaining site is occupied
by a coordinating nitrogen atom of the dmap ligand. The
Cu–N bond lengths in both complexes 1 and 2 are in the range
of 2.02(3)–2.07(6) A which is slightly shorter than reported for
similar complexes.24
The barium atom which is sandwiched between the two
dicopper units has a coordination number of eight with a
distorted square antiprismatic geometry. The coordination
environment around the barium atom is made up by four
bridging OAc/TFA ligands and four triply bridging oxygen
atoms of the dmap ligands. The Ba–Oacetate bond
distances range from 2.69(5)–2.73(2) A which are slightly
shorter than the Ba–Odmap bond distances that range from
2.77(2)–2.91(2) A.
Fig. 2 Molecular structure of Ba(dmap)4Cu4(TFA)6 and coordina-
tion sphere of Ba(1) in 2. Ellipsoids are drawn at the 50% probability
level. Three of the six CF3 groups are disordered each over two
positions. The solvent (THF) molecule is not shown for clarity.
Barium–oxygen bond distances in reported compounds
span a considerable range, 2.60–3.00 A.25,26 For example, in
the unit cell of the Y-123 superconductors (YBa2Cu3O7Àx),
each BaO4 layer is sandwiched by a Cu4O2 layer and a Cu4O4
layer.27 The Ba–O distances between the Ba atom and the
oxygen atoms in the Cu4O4 and Cu4O2 layers are much longer,
2.915 A, than the Ba–O distances in the BaO4 layer
(2.750(3) A). By sharing of the oxygen atom from the bdmap
ligand between two copper and one barium centers and having
molecules in both complexes are closely related and consist of
two dicopper units with the formulas [Cu2(m-dmap)2(m-OAc)2]
in complex 1 and [Cu2(m-dmap)2(m-TFA)2] in complex 2. The
barium ions are sandwiched between these two dicopper units.
In the dicopper unit, the two copper(II) ions are bridged by two
oxygen atoms from the two dmap ligands with normal Cu–O
bond distances (1.96(2) A), and are comparable with the
Table 1 Crystal data and structure refinement for compounds 1 and 2
Empirical formula
Moiety formula
Formula weight
Solvent
C36H74BaCu4N4O17
C68H104Ba2Cu8F36N8O33
C32H66BaCu4N4O16, C4H8O
1226.49
THF
2[C32H66BaCu4F18N4O16], C4H8O
3028.66
THF
Cryst. habit, color
Temperature
Block, blue
100(2) K
Block, blue
100(2) K
Crystal system
Space group
Unit cell dimensions
Orthorhombic
P212121
a = 11.2621(11) A
b = 18.2768(17) A
c = 24.541(2) A
Monoclinic
C2/c
a = 23.9288(14) A
b = 19.8564(12) A
c = 25.5925(15) A
b = 112.4390(10)1
11239.3(12)
Volume/A3
5051.3(8)
Z
4
1.613
2.491
4
1.790
2.302
Density (calculated)/Mg mÀ3
Absorption coefficient/mmÀ1
F(000)
2504
6000
Crystal size
y range for data collection
Index ranges
0.55 Â 0.50 Â 0.42 mm3
1.39 to 28.281
À15 r h r 15,
À22 r k r 24,
À23 r l r 32
34659
0.40 Â 0.35 Â 0.30 mm3
1.38 to 28.281
À31 r h r 31,
À26 r k r 26,
À28 r l r 34
50829
Reflections collected
Independent reflections
Absorption correction
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I 4 2s(I)]
R indices (all data)
12431
Multi-scan
0.351 and 0.228
12431/150/619
1.119
R1 = 0.0629, wR2 = 0.1668
R1 = 0.0680, wR2 = 0.1709
9.961 and À1.407 e AÀ3
13878
Multi-scan
0.501 and 0.447
13878/255/791
1.079
R1 = 0.0374, wR2 = 0.0961
R1 = 0.0463, wR2 = 0.1069
2.847 and À0.863 e AÀ3
Largest diff. peak and hole
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Table 2 Selected bond distances and angles for complexes 1 and 2
Bond distances/A for 1
2.692(5)
Bond distances/A for 2
2.693(2)
Bond angles/1 for 1
Bond angles/1 for 2
Ba(1)–O(9)
Ba(1)–O(11)
Ba(1)–O(8)
Ba(1)–O(6)
Ba(1)–O(14)
Ba(1)–O(4)
Ba(1)–O(3)
Ba(1)–O(13)
Cu(1)–O(4)
Cu(1)–O(7)
Cu(1)–O(3)
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–Cu(2)
Cu(2)–O(3)
Cu(2)–O(5)
Cu(2)–O(4)
Cu(2)–N(1)
Cu(2)–O(2)
Cu(3)–O(13)
Cu(3)–O(10)
Cu(3)–O(14)
Cu(3)–N(3)
Cu(3)–O(15)
Cu(3)–Cu(4)
Cu(4)–O(12)
Cu(4)–O(14)
Cu(4)–O(13)
Cu(4)–N(4)
Cu(4)–O(16)
Ba(1)–O(8)
Ba(1)–O(9)
Ba(1)–O(6)
Ba(1)–O(11)
Ba(1)–O(14)
Ba(1)–O(2)
Ba(1)–O(13)
Ba(1)–O(1)
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–O(5)
Cu(1)–N(4)
Cu(1)–O(4)
Cu(1)–Cu(2)
Cu(2)–O(2)
Cu(2)–O(1)
Cu(2)–O(7)
Cu(2)–N(3)
Cu(2)–O(3)
Cu(3)–O(13)
Cu(3)–O(14)
Cu(3)–O(10)
Cu(3)–N(2)
Cu(3)–O(15)
Cu(3)–Cu(4)
Cu(4)–O(14)
Cu(4)–O(13)
Cu(4)–O(12)
Cu(4)–N(1)
Cu(4)–O(16)
O(9)–Ba(1)–O(11)
O(9)–Ba(1)–O(8)
O(9)–Ba(1)–O(6)
O(9)–Ba(1)–O(14)
O(9)–Ba(1)–O(4)
O(9)–Ba(1)–O(3)
O(9)–Ba(1)–O(13)
O(4)–Cu(1)–O(7)
O(4)–Cu(1)–O(3)
O(4)–Cu(1)–N(2)
O(4)–Cu(1)–O(1)
O(3)–Cu(2)–O(5)
O(3)–Cu(2)–O(4)
O(3)–Cu(2)–N(1)
O(3)–Cu(2)–O(2)
O(13)–Cu(3)–O(10)
O(13)–Cu(3)–O(14)
O(13)–Cu(3)–N(3)
O(13)–Cu(3)–O(15)
O(12)–Cu(4)–O(14)
O(12)–Cu(4)–O(13)
O(12)–Cu(4)–N(4)
O(12)–Cu(4)–O(16)
152.26(14)
106.84(17)
79.62(15)
65.43(13)
121.18(14)
84.64(14)
88.91(13)
153.5(2)
75.03(19)
97.7(2)
102.1(2)
154.74(19)
75.16(19)
96.3(2)
101.49(19)
153.90(19)
75.94(18)
96.4(2)
104.28(19)
152.98(18)
93.75(19)
90.1(2)
O(9)–Ba(1)–O(8)
O(9)–Ba(1)–O(6)
O(9)–Ba(1)–O(11)
O(9)–Ba(1)–O(14)
O(9)–Ba(1)–O(2)
O(9)–Ba(1)–O(13)
O(9)–Ba(1)–O(1)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(5)
O(1)–Cu(1)–N(4)
O(1)–Cu(1)–O(4)
O(2)–Cu(2)–O(1)
O(2)–Cu(2)–O(7)
O(2)–Cu(2)–N(3)
O(2)–Cu(2)–O(3)
O(13)–Cu(3)–O(14)
O(13)–Cu(3)–O(10)
O(13)–Cu(3)–N(2)
O(13)–Cu(3)–O(15)
O(14)–Cu(4)–O(13)
O(14)–Cu(4)–O(12)
O(14)–Cu(4)–N(1)
O(14)–Cu(4)–O(16)
90.37(7)
90.63(8)
156.82(7)
69.57(7)
123.36(6)
89.78(6)
78.87(7)
76.14(9)
155.11(9)
97.38(11)
101.04(9)
75.78(9)
150.02(9)
97.47(10)
105.82(9)
75.56(9)
151.28(9)
97.39(10)
103.58(9)
75.48(9)
151.04(9)
97.54(10)
105.60(9)
2.697(5)
2.707(5)
2.718(5)
2.834(4)
2.837(5)
2.851(4)
2.856(4)
1.963(5)
1.969(5)
2.001(5)
2.071(6)
2.174(5)
2.9348(11)
1.966(5)
1.968(5)
1.992(5)
2.060(6)
2.153(5)
1.958(5)
1.972(5)
1.990(5)
2.053(6)
2.153(5)
2.9289(10)
1.976(4)
1.980(4)
2.004(5)
2.052(6)
2.182(5)
2.714(2)
2.720(2)
2.731(2)
2.779(2)
2.803(2)
2.848(2)
2.910(2)
1.957(2)
1.980(2)
1.993(2)
2.035(3)
2.180(2)
2.9053(5)
1.965(2)
1.988(2)
1.988(2)
2.020(3)
2.202(2)
1.970(2)
1.982(2)
2.013(2)
2.021(3)
2.188(2)
2.8958(5)
1.965(2)
1.990(2)
2.002(2)
2.019(3)
2.177(2)
106.30(19)
two distinct sets of Ba–O bonds, the molecular structures of
complexes 1 and 2 display sandwich-like features similar to
those found in the YBa2Cu3O7Àx structure. The geometry of
the BaCu4 units can be best described as two triangles sharing
a corner.
All the metal centers are connected with each other due to
the bridging nature of the ligands. The oxygen atoms of dmap
ligands function as m3-bridges between each two copper and
one barium atom. This type of bridging of three metal centers
by the oxygen atom of an amino alcohol ligand has been
observed previously.28 Four out of the six acetate ligands
are bridging between copper and barium atoms while the
remaining two are bridging between the two copper centers
in the dicopper units at the apex of the molecule.
Thermal decomposition of complexes 1 and 2
The thermal behavior of complexes 1 and 2 has been examined
by thermogravimetric analysis, performed under an
atmosphere of flowing nitrogen gas (25 ml minÀ1) with heating
rates of 10 1C minÀ1. The TGA/DTA curve of complex 1
shows three stages of weight loss (Fig. 3a). The first step begins
at 20 1C and is completed at 165 1C with a weight loss of
11.85%. The second step starts at 165 1C and is completed at
289 1C giving a maximum weight loss of 47.15%. The last
weight loss starts at 289 1C and is completed at 412 1C
resulting in a residue amounting to 31.10% of the initial
weight. The residual weight (31.10%) is in good agreement
with the expected composition for BaCu4O5 (31.82%). In
contrast the TGA/DTA curve of complex 2 (Fig. 3b) shows
Fig. 3 Thermogravimetric plot showing loss in weight with increase
in temperature (a) for complex 1 and (b) for complex 2.
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two stages of weight loss. The first step begins at 200 1C and is
completed at 269 1C with a maximum weight loss of 42.10%.
The second one starts at 269 1C and is completed at 400 1C,
resulting in a residue amounting to 29.74% of the initial
weight which is slightly less, but close to the expected
composition for BaCu4O5 (32.06%), indicating that the
complexes decompose quantitatively to composite oxides.
The IR spectra of the TGA residues also prove the removal
of carboneous matter from the organic groups leaving a
final residue corresponding to composite oxides. In the
complexes 1 and 2 each metal centre is coordinatively
saturated by the oxygen atoms of the chelating
carboxylate and dmap ligands thus eliminating the need for
additional oxygen to form oxides. Thus the new robust
precursors are suitable for deposition of clean barium
copper oxide thin films at the relatively low temperatures of
400 and 450 1C.
The X-ray diffractograms of the thin films deposited
from complexes 1 and 2 (Fig. 4) indicate the formation of
composites of two different types of oxide phases.
The composite oxides exist as highly crystalline phases with
mass ratios of 49 to 51% for BaCuO2 and CuO, respectively,
This corresponds to a mole ratio of 1 : 3 in favor of CuO, or an
altogether formula BaCu4O5, which is in excellent agreement
with the values obtained in the TGA experiment. CuO29 found
after both reactions is in the form of tenorite with the mono-
clinic space group C2/c with cell parameters of a = 4.684 A,
b = 3.425 A, c = 5.129 A, and b = 99.471, while the
Fig. 5 SEM images of thin films deposited from complex 1 at 450 1C
(a) at low and (b) at high resolution.
30
barium–copper oxide BaCuO2 has a body centered cubic
lattice system with the space group Im-3m. The XRD peak
pattern also closely matches that of another barium–copper
oxide Ba2Cu3O5Àx31 with the assigned tetragonal space group
P42/mcm, but on the basis of peak intensities and 2y values the
of BaCO3.32 It has been reported that fluorine containing
precursors in the formation of superconductor thin films have
the advantage of inhibiting the formation of BaCO3 by
intermediate formation of BaF2, which is easier to decompose
than BaCO3 without deteriorating the quality of the YBCO
during firing.33,34
matching with BaCuO2 is more likely than Ba2Cu3O5Àx
.
In the XRD patterns the peaks at 39.101 correspond to both
BaCuO2 and CuO. The peaks corresponding to CuO are
marked in Fig. 4 while all other peaks correspond to BaCuO2.
There is no evidence of fluorine contamination in sample (b).
IR spectra of the TGA residues do not show any C–O
stretching vibration bands, indicative of the absence of
carbonates. For complex 2 the presence of the fluorinated
ligands might have played a role in inhibiting the formation
The films exhibit a good adhesion to the substrate as verified
by the ‘‘Scotch Tape’’ test and are stable towards air and
moisture. Scanning electron microscopic (SEM) images
(Fig. 5 and Fig. 6) of an oxide thin film deposited from
complexes
1 and 2 show smooth and compact film
morphologies with homogenously dispersed particles. The
mean average particle sizes are 105–175 nm and 110–205 nm
for material obtained from complex 1 and 2, respectively, and
the grains are well defined with clear boundaries. The high
resolution SEM micrographs (Fig. 5b and Fig. 6b) also show
that the film deposited from complex 2 is denser and more
uniform than that from complex 1. An energy dispersive X-ray
analysis of the thin films deposited from precursors 1 and 2
indicates that the metallic ratio of Ba : Cu in both the
complexes is 1 : 4 which also supports the even distribution
of both the phases.
Conclusions
The present work is focused on the easy preparation of single
source heterobimetallic complexes, having a high solubility
and stability, which decompose to homogeneously dispersed
Fig. 4 X-Ray diffractograms of oxides obtained from (a) complex 1
and (b) complex 2. 3 indicates the peaks originated from CuO, all other
peaks are related to the BaCuO2 phase.
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Acknowledgements
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