Dicopper(I) oxalate complexes as molecular precursors for the deposition of copper compounds

Article abstract of DOI:10.1016/j.jorganchem.2005.04.055

The synthesis, characterization, and thermal decomposition behavior of dicopper(I) oxalato complexes L₄Cu₂(C₂O ₄) (L = CNtBu (2a), CNCMe₂CH₂tBu (2b), CNC ₆H₃Me₂-2,6 (2c)) is reported. 2c can be prepared in a straightforward manner by the reaction of stoichiometric amounts of Cu₂O and oxalic acid with four equivalents of CNC ₆H₃Me₂-2,6, while those complexes with aliphatic isocyanides are better prepared from a copper(I) oxalato complex with alkine capping ligands (Me₃SiCCSiMe₃)₂Cu ₂(C₂O₄) (1) via ligand exchange. Crystallographic and spectroscopic evidence for 2a-c confirms the anticipated dinuclear structure with the oxalate in a μ-1,2,3,4 bridging mode and an essentially σ-character of the terminal isocyanides. In solid form the complexes are stable at room temperature and can be handled in air for some time. Their decomposition was studied by thermal gravimetric analysis coupled with mass spectrometry, and the degradation pathway was shown to depend on the type of isocyanide capping ligand. Decomposition of 2a,b takes place between 150 and 200°C to give CuCN in a clean process that involves isobutene elimination from the terminal ligands, with elimination of (CN)₂ and conversion to elemental copper at higher temperatures. Heating of 2c leads to CuO (and then to Cu₂O) via release of the intact isocyanide, CO ₂, and CO in a well-behaved thermal process around 200-280°C.

Full text of DOI:10.1016/j.jorganchem.2005.04.055

Journal of Organometallic Chemistry 690 (2005) 5255–5263
www.elsevier.com/locate/jorganchem
Dicopper(I) oxalate complexes as molecular precursors
for the deposition of copper compounds
*
Jo¨rg Teichgra¨ber, Sebastian Dechert, Franc Meyer
Institut fu¨r Anorganische Chemie der Georg-August-Universita¨t, Tammannstrasse 4, D-37077 Go¨ttingen, Germany
Received 18 February 2005; received in revised form 15 April 2005; accepted 15 April 2005
Available online 5 July 2005
Abstract
The synthesis, characterization, and thermal decomposition behavior of dicopper(I) oxalato complexes L4Cu2(C2O4)
(L = CNtBu (2a), CNCMe2CH2tBu (2b), CNC6H3Me2-2,6 (2c)) is reported. 2c can be prepared in a straightforward manner by
the reaction of stoichiometric amounts of Cu2O and oxalic acid with four equivalents of CNC6H3Me2-2,6, while those complexes
with aliphatic isocyanides are better prepared from a copper(I) oxalato complex with alkine capping ligands (Me3SiC„CSiMe3)2-
Cu2(C2O4) (1) via ligand exchange. Crystallographic and spectroscopic evidence for 2a–c confirms the anticipated dinuclear struc-
ture with the oxalate in a l-1,2,3,4 bridging mode and an essentially r-character of the terminal isocyanides. In solid form the
complexes are stable at room temperature and can be handled in air for some time. Their decomposition was studied by thermal
gravimetric analysis coupled with mass spectrometry, and the degradation pathway was shown to depend on the type of isocyanide
capping ligand. Decomposition of 2a,b takes place between 150 and 200 ꢀC to give CuCN in a clean process that involves isobutene
elimination from the terminal ligands, with elimination of (CN)2 and conversion to elemental copper at higher temperatures. Heat-
ing of 2c leads to CuO (and then to Cu2O) via release of the intact isocyanide, CO2, and CO in a well-behaved thermal process
around 200–280 ꢀC.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Copper; Dinuclear complexes; Oxalate complexes; Precursor compounds; Thermal analysis
1. Introduction
LCuI(b-diketonate), where the b-diketonate often is
hexafluoroacetylacetonate (hfac) and L is a neutral Le-
wis-base such as a phosphine, alkyne, or alkene, have
shown promising potential as copper CVD precursors
[4,5]. Their deposition reaction involves disproportion-
ation into CuII(b-diketonate)2, the free donor ligand L,
and metallic copper according to Eq. (1), thus limiting
the maximum yield of copper metal to 50%
Copper deposition has become an important and rap-
idly growing area in integrated circuit manufacturing,
since copper is a favorable material for interconnect
structures in microelectronic devices [1]. This has stimu-
lated the search for molecular copper(I) and copper(II)
complexes that may serve as sources for device quality
copper via various deposition techniques [2,3]. In partic-
ular, chemical vapour deposition (CVD) has been stud-
ied intensively for the formation of copper thin films,
and metal–organic CuI compounds of general formula
2 LCuIðb-diketonateÞ
DT
! Cu0 þ CuIIðb-diketonateÞ þ 2 L
ð1Þ
2
On the other hand, CuII precursors such as CuII
(hfac)2 may be used (Eq. (2)) [6], but in this case an
external reducing agent such as H2 is required to get
good quality thin films [3]. Furthermore, conversion to
*
Corresponding author. Fax: +49 551 393063.
E-mail address: franc.meyer@chemie.uni-goettingen.de (F. Meyer).
0022-328X/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.055

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J. Teichgra¨ber et al. / Journal of Organometallic Chemistry 690 (2005) 5255–5263
elemental copper is often inefficient, and in the absence
of H2 large amounts of carbon or oxygen contaminants
are found in the copper metal, due to uncontrolled li-
gand decomposition. Self-reducible CuII complexes with
b-aminoalcoholate ligands have been proposed as alter-
native source reagents with an intrinsic decomposition
pathway, giving ketone or imine fragments depending
on the specific b-aminoalcoholate ligand used [7]
A obvious advantage of this clean and straightfor-
ward conversion is the formation of the reusable ligand
L and non-toxic CO2 as sole byproducts. On the other
hand, dicopper(I) oxalato complexes may also be antic-
ipated to represent useful molecular precursors for the
deposition of CuO or Cu2O rather than elemental cop-
per, if thermal degradation proceeds via other pathways
with elimination of CO instead of CO2, or a mixture of
both. The prospect to switch between distinct decompo-
sition pathways of such LnCu2(C2O4) systems by suit-
able variations of the capping ligands L is particularly
attractive. The present contribution now describes the
synthesis and structural characterization of a series of
dicopper(I) oxalate complexes with isocyanide capping
ligands, and preliminary results on their thermal be-
haviour with respect to copper and copper oxide
deposition.
CuIIðb-diketonateÞ þ H2 ! Cu0 þ 2b-diketone
ð2Þ
2
Despite considerable progress in copper CVD, how-
ever, drawbacks such as poor adhesion of the copper films
onto the TiN diffusion barrier layer when using fluorine-
containing copper precursors [8] have fired the search
for new fluorine-free copper source reagents [3]. In addi-
tion, alternative precursor delivery methods are being
developed in order to circumvent some general limitations
ofconventional CVDprocesses such as the requirement of
high volatility of the metal compound. Recent approaches
comprise, inter alia, spin coating [9] or aerosol-assisted
CVD (AACVD) techniques [10] and, most recently,
reductive copper deposition from supercritical carbon
dioxide (chemical fluid deposition, CFD) [11,12]. Many
organometallic complexes exhibit significant solubility
in CO2, and common CuII(b-diketonate)2 complexes with
either fluorinated or fluorine-free b-diketonate ligands
have already being tested in CFD. Pyrolysis of copper
oxalate hydrate under supercritical CO2 has most recently
been reported to yield 1D copper nanowires [13].
Similarly, the controlled preparation of copper oxide
films (which are known as p-type semiconductors) has
received much attention in view of their applications
in a wide range of technological fields such as electro-
chemical devices, solar cells, coatings, the synthesis of
cuprate superconductors, and their importance in catal-
ysis [14]. While conventionally prepared by, inter alia,
electrodeposition, chemical or thermal oxidation, or
chemical vapor deposition [15], careful regulation of
conditions is required to obtain pure CuO or Cu2O
and alternative approaches are being sought, including
those based on single-source precursors [16].
2. Results and discussion
2.1. Synthesis and spectroscopic characterization
In close analogy to the straightforward synthesis of
complexes L2Cu2(C2O4) where the Lewis base L is an al-
kyne or alkene [17], initial attempts to prepare the
sought-after complexes (RNC)xCu2(C2O4) were carried
out by reacting Cu2O, oxalic acid and the respective iso-
nitrile RNC (R = tBu, CMe2CH2tBu, cyclohexyl,
C6H3Me2-2,6) at room temperature in CH2Cl2 (route
A, Scheme 2). However, this route only worked well
for the aromatic isonitrile to give 2c (R = C6H3Me2-
2,6), while for aliphatic isonitriles (R = tBu, CMe2CH2-
tBu, cyclohexyl) solutions of the reaction mixtures
gradually turned blue, suggesting the unwanted forma-
tion of copper(II) species. In these experiments, the
anticipated type 2 products could still be isolated, but
yields are rather poor. In one case (R = cyclohexyl) pale
blue crystals of the decomposition product 3 could be
obtained in crystalline form, and 3 was identified as
[Cu(CNR)4]2[Cu(C2O4)2] by X-ray crystallography (see
below). Gradual decomposition of type 2 complexes
apparently involves some ligand dismutation.
We thus turned to a ligand exchange reaction starting
from preformed (Me3SiC„CSiMe3)2Cu2(C2O4) (1) [17]
according to route B (Scheme 2). The reaction can be
conveniently followed by IR spectroscopy, revealing effi-
cient displacement of the Me3SiC„CSiMe3 capping li-
gands in 1 by the respective isonitriles to give 2a–c.
The colorless products 2a–c are highly soluble in
CH2Cl2 or CHCl3 solutions and can be obtained in crys-
talline form upon cooling concentrated solutions in
those solvents to ꢀ20 ꢀC. Elemental analyses suggest
the formula (RNC)4Cu2(C2O4) in all cases, indicating
that two isonitrile capping ligands are bonded to each
metal ion. While sensitive to oxygen in solution, the
In a recent report, we described a series of novel
dicopper(I) oxalate complexes stabilized by neutral Le-
wis bases L (L = alkyne, alkene) that are promising
new candidates of self-reducing precursors for copper
deposition [17]. While being reasonably air stable and
highly soluble, these complexes incorporate the oxalate
ligand as a built-in reducing equivalent that ensures a
100% yield of metallic copper upon thermal decomposi-
tion via an internal redox reaction (Scheme 1).
O
O
O
O
∆T
L
Cu
Cu
L
2 Cu0 + 2 L + 2 CO2
Scheme 1.

J. Teichgra¨ber et al. / Journal of Organometallic Chemistry 690 (2005) 5255–5263
5257
Me
Si
Me
Si
3
3
O
OH
O
O
O
O
O
+ 2M e SiCCSiMe
3
3
Cu O
+
2
Cu
Cu
HO
Si
Si
Me
Me
3
1
3
+ 4 CNR
route A
route B
+ 4 CNR
O
O
O
O
RN
NR
NR
C
C
C
Cu
Cu
C
RN
R = tBu (2a)
CMe CH tBu (2b)
2
2
C H Me -2,6 (2c)
6
3
2
Scheme 2. Synthesis of the complexes.
complexes are almost air stable as solids and can thus be
handled in air for a short time.
sured either as KBr pellets or in CH2Cl2, indicating that
structures in the solid state and in solution are the same.
However, ESI mass spectra of CHCl3 solutions of the
complexes do not show the expected molecular ion
peaks but dominant signals for [Cu(CNR)2]+ and minor
peaks characteristic for [Cu(CNR)3]+ and [Cu3(C2O4)-
(CNR)3]+.
The 1H and 13C NMR spectra of 2a–c in CDCl3
consist of well-resolved single resonances for each of
the organic groups of the coordinated isonitriles,
reflecting the high symmetry of the molecules. The ab-
sence of any NMR signals for free isonitrile confirms
that ligands do not dissociate in solution, in accordance
with the IR spectral findings. In the 13C NMR spectra,
resonances of the isonitrile-C are clearly shifted to
higher field compared to the free isonitriles (Table 2),
consistent with an essentially r-donor character of
the ligands [23].
IR spectra of the isolated materials display the char-
acteristic strong mas(C–O) band of the oxalato ligand at
around 1620 cmꢀ1, corroborating its tetradentate bridg-
ing mode (Table 1). The mas(C–O) absorption is shifted
by ꢁ25 cmꢀ1 to lower energy compared to the copper(I)
oxalate complexes with alkyne and alkene capping li-
gands (e.g., 1, Table 1) [17], presumably reflecting the
less pronounced Cu ! L backbonding contribution for
the isocyanides. A band appearing at 1308 cmꢀ1 in the
IR spectra of all complexes 2a–c is tentatively assigned
to ms(C–O). Two m(N„C) vibrations are observed for
the isonitrile ligands, in accordance with a D2h structure
(B1u + B2u). The m(N„C) stretches are shifted hypso-
chromically by ꢁ30 cmꢀ1 (2a,b) or ꢁ20 cmꢀ1 (2c) com-
pared to the free isonitriles, in agreement with what is
usually observed for terminally bound isonitriles that
act as two-electron r-donors towards copper(I) with
only little p-backbonding [18–20]. Values for 2c are close
to those reported for [(p-MeC6H4NC)2Cu(OPh)]2 (2145/
2125 cmꢀ1) [21], while values for 2a,b are similar to
The resonance of the oxalato carbon atoms appears
at ꢁ169 ppm, i.e., at somewhat higher field compared
to the oxalate carbon signal of 1 (171.8 ppm) and to
all other copper(I) oxalate complexes with alkyne or
alkene capping ligands [17]. This is consistent with the
differences in mas(C–O) frequency discussed above.
those of [(C6H11NC)2Cu(dmnpz)]2 (2165/2143 cmꢀ1
;
dmnpz = 3,5-dimethyl-4-nitropyrazolato) [22]. m(N„C)
vibrations of all new complexes are very similar if mea-
Table 2
Selected 13C NMR data for complexes 2a–c
Table 1
Selected IR absorptions for complexes 2a–c
Compound
13C{1H} NMR
13C{1H} NMR
Compound
m(N„C)a
mas(C–O)
d (N„C) (ppm)a
ꢀ
d (OCO) (ppm)
171.8
168.9
1 [17]
2a
2b
1 [17]
2a
2b
–
1642
1617
1621
1622
138.0 (152.5)
139.8 (155.0)
150.4 (167.6)
2180/2151 (2136)
2172/2147 (2130)
2151/2128 (2121)
168.9
169.2
2c
2c
a
a
Values for the free isonitriles in parentheses.
Values for the free isonitriles in parentheses.

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J. Teichgra¨ber et al. / Journal of Organometallic Chemistry 690 (2005) 5255–5263
2.2. Solid state structures
While copper(II) oxalate and its complexes are well
known, only few reports about copper(I) oxalate have
been published. Apart from those L2Cu2(C2O4) com-
plexes reported recently (with L = alkyne, alkene) [17],
no structural data of copper(I) oxalate complexes are
known. Complexes of type Cu2(C2O4)(CNR)2 have been
briefly mentioned in a patent, but no analytical data or
structural information had been given [24]. In view of
the present findings, it seems more likely that these com-
plexes with isonitriles generally have the stoichiometry
Cu2(C2O4)(CNR)4. To confirm the constitution of the
new complexes as deduced from spectroscopy and to
gain further insight into the structural details of cop-
per(I) oxalate species, complexes 2a–c and the partially
oxidized product 3 were analyzed by X-ray crystallogra-
phy. Molecular structures of 2a and 2c are depicted in
Figs. 1 and 2 as examples. The other molecular struc-
tures are depicted in Figures S1 and S2 in the supporting
material.
Fig. 2. ORTEP plot (30% probability thermal ellipsoids) of the
molecular structure of 2c. For the sake of clarity all hydrogen atoms
˚
have been omitted. Selected atom distances (A) and angles (ꢀ): Cu1–O1
2.122(2), Cu1–O20 2.094(2), Cu1–C2 1.874(4), Cu1–C11 1.902(4),
O1–C1 1.249(4), O2–C1 1.257(4), C1–C10 1.559(7), C2–N1 1.163(5),
C11–N2 1.157(4) and O1–Cu1–O20 79.70(9), C2–Cu1–C11 129.1(1),
N1–C2–Cu1 178.0(4), N2–C11–Cu1 177.7(3), O1–C1–O2 125.7(3),
O1–C1–C10 117.9(3), O2–C1–C10 116.3(3). Symmetry transformation
used to generate equivalent atoms: (0) 1 ꢀ x, 1 ꢀ y, 1 ꢀ z.
All copper(I)-complexes 2a–c show a dinuclear struc-
ture with the oxalate in a l-1,2,3,4 bridging mode. Addi-
tionally each copper(I) atom is bound to two isonitrile
ligands with almost linear Cu–C–N arrangement. The
coordination environment for each metal atom is
strongly distorted from tetrahedral due to, inter alia,
the small bite angle imposed by the chelating oxalate.
˚
˚
SiMe3, EtC„CEt) (1.987(1)–2.004(2) A) [17] but
The Cu–O bond lengths in 2a–c (2.081(2)–2.122(2) A)
˚
are approximately 0.1 A longer then those determined
comparable to those found in the bis(l-phenoxo)
previously for [{LCu}2(l-C2O4)] (L = Me3SiC„C-
dicopper(I) compound [(p-MeC6H4NC)2Cu(l-OPh)]2
˚
(2.066(4) and 2.083(4) A) [21]. The Cu–C distances of
˚
about 1.9 A are similar for 2a–c and [(p-MeC6H4NC)2-
˚
Cu(l-OPh)]2 (1.905(7)/1.907(7) A), but a little shorter
than those of [(C6H11NC)2Cu(dmnpz)]2 (1.930(3)/
˚
1.937(4) A) [22]. Even longer Cu–C bonds were deter-
mined for tetrakis(isonitrile) complexes of copper(I),
˚
such as [(MeNC)4Cu]BF4 (1.94–2.00 A) [25] or 3
˚
(1.95(1)–1.98(2) A; see below). Comparison of the NC
bond lengths of the free isonitrile 2,4-Me-C6H4NC and
the isonitrile complex 2c shows no significant elongation
˚
or shortening (2c: 1.163(5) and 1.157(4) A, 2,4-Me-
˚
C6H4NC: 1.160(3) and 1.161(2) A) [26], while a some-
what smaller N„C distance has been reported for
[(p-MeC6H4NC)2Cu(l-OPh)]2 [21]. In contrast to these
findings a distinct lengthening of the C„C bonds con-
comittant with alkyne bending was reported for the
dicopper(I) oxalate [{LCu}2(l-C2O4)] (L = Me3SiC„C-
SiMe3) [17].
The unit cell of 3 consists of tetrahedral tetra-
kis(cyclohexylisonitrile)copper(I) cations and charge
compensating bis(oxalato)cuprate(II) anions with
square planar coordination of the copper(II) center
(Figure S2). Geometric parameters of the cation are
very similar to those reported previously for some
[Cu(CNR)4]+ ions [25,27].
Fig. 1. ORTEP plot (30% probability thermal ellipsoids) of the
molecular structure of 2a. For the sake of clarity all hydrogen atoms
˚
have been omitted. Selected atom distances (A) and angles (ꢀ): Cu1–O1
2.120(2), Cu1–C2 1.902(4), O1–C1 1.257(3), C1–C100 1.551(9), C2–N1
1.164(5) and O1–Cu1–O10 78.9(1), C2–Cu1–C20 123.0(2), Cu1–C2–N1
176.9(3), O1–C1–O1000125.9(4), O1–C1–C100117.1(2). Symmetry trans-
formations used to generate equivalent atoms: (0) x, y, 1 ꢀ z, (00) 1 ꢀ y,
1 ꢀ x, 1 ꢀ z, (000) 1 ꢀ y, 1 ꢀ x, z.

J. Teichgra¨ber et al. / Journal of Organometallic Chemistry 690 (2005) 5255–5263
5259
2.3. Thermal behavior
Thermal gravimetric analysis and differential scan-
ning calorimetry coupled to mass spectrometry (TGA/
MS) was employed in order to evaluate the thermal sta-
bility of the copper(I) oxalate complexes 2a–c and to
investigate their decomposition behavior with increasing
temperature. Several previous studies have dealt with the
thermal decomposition of copper(II) oxalate and its
derivatives: decomposition of Cu(C2O4) Æ 0.25H2O has
been reported to proceed with release of CO2 and step-
wise cation reduction (Cu2+ ! Cu+ ! Cu0) [28], but it
has also been observed that complexes LnCu(C2O4)
(where L is H2O or various N-ligands) thermally decom-
pose without forming any isolable intermediate to give
either Cu2O or CuO, depending on L [29]. Molecular
copper(I) oxalato complexes L2Cu2(C2O4) (L = alkyne,
alkene) have recently been described as promising pre-
cursors for copper deposition, due to their clean conver-
sion to elemental copper via an internal redox reaction
according to Scheme 1 [17]. Since isonitrile ligands might
be more strongly bound to Cu than alkenes and alkynes
and might be more susceptible to ligand fragmentation,
the release of CO, CO2, intact isonitrile CNR, or frag-
ments of the latter could have been expected upon heat-
ing 2a–c. It turns out that the thermal decomposition
pattern of 2c is different from those of 2a and 2b.
The TGA/DSC curves for 2a and 2b depicted in Fig.
3 clearly show major endothermic processes in the
range 150–200 ꢀC that are accompanied with a drastic
weight loss. Mass spectrometric analyses reveal the re-
lease of H2O, CO, CO2, HCN (m/z = 27), and isobutene
(m/z = 56) during these processes. Minor amounts of
CNtBu are observed in the case of 2a, while no intact
CNCMe2CH2tBu can be detected for 2b. The remain-
ing weight percentage at 200 ꢀC of 33.5% (2a) or
23.3% (2b) is in excellent agreement with the theoretical
value expected for Cu2(CN)2 (32.7% or 23.2%, respec-
tively). IR spectra of TGA samples that have only been
heated to 240 ꢀC show the expected C„N vibration for
CuCN at 2168 cmꢀ1 [30,31], and the endothermic peak
at ꢁ472 ꢀC in the DSC curves represents the character-
istic melting point of CuCN [30]. Thermal decomposi-
tion of cupreous cyanide has been studied in quite
some detail in the context of the preparation of cuprate
superconductors, where CuCN was found to be a use-
ful precursor for single-phase YBa2Cu3O7ꢀx [32]. The
thermal behavior of 2a and 2b in the range 200–
700 ꢀC and the mass spectrometric detection of (CN)2
(m/z = 52) during continued heating is consistent with
gradual decomposition of CuCN to yield elemental
copper (theoretical weight values: 23.2% or 16.5% for
2a and 2b, respectively), which was reported to proceed
via various cyclic Cu(CN)x intermediates. However, in
the present case this process is already completed at
ꢁ620 ꢀC, i.e., at a temperature much lower than
Fig. 3. TGA (solid) and DSC (broken) curves for 2a (top) and 2b
(bottom), including selected MS traces; heating rate 5 ꢀC/min.
ꢁ850 ꢀC found for customary cupreous cyanide [30].
Formation of elemental copper as the final product is
confirmed by an endothermic peak at 1086 ꢀC in the
Fig. 4. TGA (solid) and DSC (broken) curves for 2c, including
selected MS traces; heating rate 5 ꢀC/min.

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J. Teichgra¨ber et al. / Journal of Organometallic Chemistry 690 (2005) 5255–5263
DSC curves representing the melting point of the pure
metal (not shown in Fig. 3).
R
R
R
R
O
O
O
O
N
N
N
N
C
C
C
C
The TGA/DSC curves for 2c are shown in Fig. 4. In
the range 200–280 ꢀC a series of endothermic processes
take place, which according to mass spectrometric anal-
yses of the volatile components correspond to the loss of
CO and CO2 (mainly around ꢁ240 ꢀC) and of the intact
isonitrile (m/z = 131, mainly around ꢁ260 ꢀC). The
remaining weight of ꢁ21% at 310 K is quite consistent
with the formation of CuO (expected: 21.5%), and min-
or additional weight loss upon continuous heating to
700 ꢀC under N2 might by due to gradual transforma-
tion into Cu2O (degradation of CuO to give Cu2O is
most rapid above 800 ꢀC and is completed around
1000 ꢀC [33]). Cu2O has also been reported to form
upon thermal decomposition of some dinuclear cop-
per(I) complexes with perfluorinated carboxylates
[Cu2{P(OMe)3}2(l-RFCOO)2] [34].
Cu
Cu
- CO
- CO
- H O
2
R = H, tBu
2
- 2 HCN
∆T
- 4(8)
Cu (CN)
2
2
- (CN)
∆T
2
0
2 Cu
Scheme 3. Major thermal decomposition pathway for 2a and 2b.
3. Conclusions
O
O
O
O
ArN
ArN
NAr
NAr
C
C
C
C
Cu
Cu
A series of new copper(I) oxalato complexes with iso-
cyanide capping ligands has been prepared and fully
characterized. X-ray crystal structures and spectroscopic
data reveal further insight into (still rare) molecular cop-
per(I) oxalato systems and confirm that the terminal iso-
cyanides act as predominant r-donors. Copper(I)
oxalato complexes that bear neutral capping ligands
have recently been considered as well defined and self-
reducing molecular precursors for copper deposition,
since they may potentially decompose with loss of the
capping ligands and release of CO2 to give a 100% yield
of elemental copper via an internal redox reaction [17].
According to combined TGA/DSC-MS data, however,
the present isocyanide complexes degrade in a more
complex way, depending on the specific isocyanide.
Thermal decomposition of 2a and 2b with aliphatic iso-
cyanides that incoporate tBu groups follows the se-
quence given in Scheme 3. Here, elimination of
isobutene generates HCN, leading in a clean process
to CuCN as the initial product. This decomposes further
to elemental Cu, which is completed around 620 ꢀC (see
Fig. 3).
In contrast, isocyanide degradation is not a major
process for 2c, whose predominant decomposition path-
way is sketched in Scheme 4. Generation of HCN is not
favorable from the aromatic isocyanide ligand, which
upon heating is mainly lost as the intact CNAr. In this
case CuO (and Cu2O at higher temperatures) is formed.
The present findings once more underline the need of
proper ligand choice in order to get to the desired prod-
uct in a precursor-based deposition process. Whereas
copper(I) oxalato complexes with alkene or alkyne cap-
ping ligands proved to be promising precursors for cop-
per deposition [17], the isocyanide complexes described
Ar = C H Me -2,6
6
3
2
- CO
- CO
- 4 CNR
2
∆T
CuO / Cu O
2
Scheme 4. Major thermal decomposition pathway for 2c.
in this work are less suitable in this regard, but they lead
to different copper compounds such as CuCN or CuO in
distinct thermal decomposition pathways.
4. Experimental
4.1. General procedures and methods
All manipulations were carried out under an atmo-
sphere of dry nitrogen by employing standard Schlenk
techniques. Solvents were dried according to established
procedures. 1 was synthesized according to the reported
method [17], all other chemicals were used as purchased.
Microanalyses: Analytisches Labor des Anorganisch-
Chemischen Instituts der Universita¨t Go¨ttingen. – IR
spectra: Digilab Excalibur, recorded as KBr pellets or
in solution (NaCl cell). – Mass spectra: Finnigan MAT
95 (FAB-MS, matrix nitrobenzylalcohol) and Finnigan
MAT LCQ (ESI-MS). – NMR spectra: Bruker Avance
500, measured at 300 K; residual proton signal of the sol-
vent as internal chemical shift reference (CDCl3:
dH = 7.24, dC = 77.0 ppm) – TGA/DSC-MS: Netzsch

J. Teichgra¨ber et al. / Journal of Organometallic Chemistry 690 (2005) 5255–5263
5261
STA 449C Jupiter QMS, using an open pan (Al2O3)
which was purged with N2; heating rate 5 ꢀC/min.
(C(CH3)3), 138.0 (C„N), 168.9 (COO). IR (KBr):
2980 m, 2936 w, 2180 s, 2151 s, 1676 w, 1617 vs, 1402
w, 1370 w, 1308 w, 1199 m, 780 w. MS (ESI, CH2Cl2):
m/z (%) = 526 (10) ½Cu3ðoxÞðCNtBuÞþꢂ, 312 (27)
½CuðCNtBuÞþꢂ, 229 (100) ½CuðCNtBuÞþꢂ3. Anal. calcd.
4.2. Synthesis of the complexes
3
2
Route A. Cu2O (0.54 g, 3.7 mmol) is dissolved in
CH2Cl2 (50 ml) and oxalic acid (0.33 g, 2.6 mmol) and
2,6-dimethylphenylisocanide (2.0 g, 15.2 mmol) are
added. The reaction mixture is left stirring for 3 h at
room temperature and any solid residue is then filtered
off. The volume of the resulting clear solution is reduced
to ꢁ20 ml under reduced pressure and stored at ꢀ20 ꢀC.
Colorless crystals of the product 2c gradually form.
They are isolated by filtration, washed with small
amounts of cold (ꢀ30 ꢀC) CH2Cl2, and dried under vac-
uum. Analytical data are identical to those of 2c ob-
tained via route B.
for C22H36Cu2N4O4 (547.64): C 48.25, H 6.63, N
10.23; Found: C 47.49, H 6.58, N 9.94.
Complex 2b. 1H NMR (CDCl3): d = 1.01 (s, 36H,
C(CH3)3), 1.43 (s, 24H, C(CH3)2), 1.56 (s, 8H, CH2).
13C NMR (CDCl3): d = 30.9 (CH3), 31.0 (CH3), 31.8
(C(CH3)3), 53.6 (CH2), 58.2 (C(CH3)2), 139.8
(C„N), 168.9 (COO). IR (KBr): 2955 m, 2906 m,
2172 s, 2147 s, 1621 vs, 1474 m, 1397 w, 1370 m,
1308 m, 1217 m, 1132 w, 780 m, 756 w. MS (ESI,
CHCl3): m/z (%) = 694 (4) ½Cu3ðoxÞðCNCðCH3Þ
2
CH2tBuÞþꢂ, 480 (7) ½CuðCNðCH3Þ CH2tBuÞþꢂ, 341
3
2
3
(100) ½CuðCNðCH3Þ CH2tBuÞþꢂ. Anal. calcd. for
2
2
Route B. 1 (1.0 g, 1.8 mmol) is dissolved in CH2Cl2
or CHCl3 (50 ml) and four equivalents (7.2 mmol)
of the respective isonitrile are added via a syringe at
room temperature. After stirring for 2 h, the volume of
the solution is reduced to ꢁ20 ml under reduced pres-
sure and stored at ꢀ20 ꢀC. Colorless crystals of the
product 2a–c gradually form. They are isolated by filtra-
tion, washed with small amounts of cold (ꢀ30 ꢀC)
CH2Cl2 or CHCl3, and dried under vacuum. Non-
optimzed yields of crystalline material are in the range
25–45 %.
C38H68Cu2N4O4 (771.96): C 59.11, H 8.88, N 7.26;
Found: C 58.47, H 8.60, N 7.23.
Complex 2c. 1H NMR (CDCl3): d = 2.35 (s, 24H,
CH3), 7.02 (d, J = 7.6 Hz, 8H, CHm), 7.15 (t, 7.6 Hz,
4H, CHp). 13C NMR (CDCl3): d = 18.8 (CH3), 126.4,
127.8, 129.1, 135.3, 150.4 (C„N), 169.2 (COO). IR
(KBr): 3056 w, 2989 w, 2922 w, 2151 m, 2128 s, 1619
vs, 1472 w, 1308 m, 1274 w, 1163 w, 1036 w, 780 m,
721 w, 696 w, 494 m. MS (ESI, CHCl3): m/z (%) =
670 (45) ½Cu3ðoxÞðCNC6H3ðCH3Þ Þþꢂ, 325 (100)
2
3
½CuðCNC6H3ðCH3Þ Þþꢂ. Anal. calcd. for C38H36Cu2-
2
2
Complex 2a. 1H NMR (CDCl3): d = 1.38 (s, 36H,
CH3). 13C NMR (CDCl3): d = 30.2 (CH3), 55.5
N4O4 (739.72): C 61.69, H 4.91, N 7.57; Found: C
61.47, H 4.92, N 7.64.
Table 3
Crystal data and refinement details for complexes 2a, 2b, 2c, and 3
2a
2b
2c
3
Formula
C
22H36Cu2N4O4,
C38H68Cu2N4O4,
4CHCl3
1249.52
C38H36Cu2N4O4,
3CHCl3
1097.89
2C28H44CuNþ4 ,
C4CuO28ꢀ, C7H8
1332.14
2.25CH2Cl2
741.71
Mr
Crystal size (mm)
Crystal system
Space group
0.36 · 0.28 · 0.22
Tetragonal
P42/mnm (no. 136)
11.3960(10)
11.3960(10)
14.5846(15)
90
0.37 · 0.21 · 0.15
Monoclinic
P21/c (no. 14)
12.5873(8)
14.0048(7)
17.3091(12)
91.582(5)
3050.1(3)
1.361
0.23 · 0.18 · 0.12
Monoclinic
C2/c (no. 15)
18.7165(16)
13.1684(8)
20.9320(17)
108.145(7)
4902.5(7)
0.42 · 0.31 · 0.24
Monoclinic
C2/c (no. 15)
47.304(2)
12.6338(4)
26.0673(10)
117.601(3)
13805.6(9)
1.282
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢀ)
3
˚
V (A )
1894.1(3)
1.301
qcalcd. (g cmꢀ3
Z
)
1.487
2
764
2
1292
4
2224
8
5640
F(000) (e)
l (Mo Ka) (mmꢀ1
Tmax/Tmin
hkl range
)
1.471
1.261
1.401
0.973
0.7295/0.6411
13, 13, ꢀ17 to 14
2.53–24.78
7502
0.8373/0.6188
14, ꢀ15 to 16, 20
1.62–24.87
16431
0.8171/0.6887
ꢀ21 to 22, ꢀ13 to 15, ꢀ23 to 24
1.92–24.78
13597
0.7843/0.5963
55, 14, 30
1.57–24.81
70083
h range (ꢀ)
Measured reflections
Unique reflections (Rint
)
909 (0.0447)
791
50
5250 (0.0416)
3988
299
4195 (0.0523)
3026
293
11826 (0.0677)
7659
999
Observed reflections I > 2r(I)
Refined parameters
Goodness-of-fit
1.083
1.025
1.010
1.028
R1, wR2 (I > 2r(I))
R1, wR2 (all data)
Residual electron density (e A
0.0384, 0.1032
0.0456, 0.1066
ꢀ0.406/0.196
0.0313, 0.0703
0.0472, 0.0743
ꢀ0.299/0.318
0.0425, 0.0830
0.0698, 0.0900
ꢀ0.473/0.534
0.0860, 0.2115
0.1258, 0.2389
ꢀ1.101/1.662
ꢀ3
˚
)

5262
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2
˚
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Acknowledgments
[11] J.S. Thompson (E.I. Du Pont de Nemours and Company) WO 03/
053895, 2003.
This research was supported by the Georg-August-
Universita¨t Go¨ttingen. We sincerely thank Katrin
Gehrke and Prof. Konrad Samwer (I. Physikalisches
Institut, Universita¨t Go¨ttingen) for TGA/DSC-MS
measurements.
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Appendix A. Supplementary data
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