Mixed amido/imido/guanidinato complexes of tantalum: Effects of ligand substitution on thermal properties

Article abstract of DOI:10.1002/ejic.200600653

Mixed amido/imido/guanidinato tantalum complexes [Ta(NR¹R ²){C(NR¹R²)(NR³)₂} ₂(NR⁴)] [R¹,R² = methyl, ethyl; R³ = cyclohexyl (cy), isopropyl; R⁴ = tert-butyl, n-propyl] have been synthesized by the insertion of carbodiimides into tantalum-amide bonds. All compounds have been characterized by means of NMR spectroscopy, mass spectrometry, and CHN analysis. Additionally, single crystal structures of five compounds (4: R¹ = Me, R² = Et, R ³ = iPr, R⁴ = tBu; 5: R¹, R² = Et, R³ = iPr, R⁴ = tBu; 6: R¹, R² = Me, R³ = cy, R⁴ = tBu; 8: R¹, R² = Et, R³ = cy, R⁴ = tBu; 9: R¹, R² = Et, R³ = iPr, R⁴ = nPr) have been determined and relevant structural issues are discussed. The thermal properties of all complexes have been investigated by TGA/DTA measurements. Thus, complexes originating from diisopropylcarbodiimide (iPr-cdi) are significantly more volatile than complexes derived from dicyclohexylcarbodiimide (cy-cdi). In contrast, variations of R¹, R², and R⁴ have only a small impact on the thermal behavior of the complexes. Finally, selected compounds have been pyrolyzed at 600°C, and the decomposition products studied by GC-MS and ¹H NMR spectroscopy. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600653

FULL PAPER
DOI: 10.1002/ejic.200600653
Mixed Amido/Imido/Guanidinato Complexes of Tantalum: Effects of Ligand
Substitution on Thermal Properties
Arne Baunemann,[a] Manuela Winter,[a] Karoly Csapek,[a] Christian Gemel,[a] and
Roland A. Fischer*[a]
Keywords: X-ray diffraction / Chelates / N ligands / Tantalum / Nitrides
Mixed amido/imido/guanidinato tantalum complexes
[Ta(NR1R2){C(NR1R2)(NR3)2}2(NR4)] [R1,R2 = methyl, ethyl; R3
= cyclohexyl (cy), isopropyl; R4 = tert-butyl, n-propyl] have
been synthesized by the insertion of carbodiimides into tan-
talum–amide bonds. All compounds have been characterized
by means of NMR spectroscopy, mass spectrometry, and
CHN analysis. Additionally, single crystal structures of five
compounds (4: R1 = Me, R2 = Et, R3 = iPr, R4 = tBu; 5: R1, R2
= Et, R3 = iPr, R4 = tBu; 6: R1, R2 = Me, R3 = cy, R4 = tBu; 8:
discussed. The thermal properties of all complexes have
been investigated by TGA/DTA measurements. Thus, com-
plexes originating from diisopropylcarbodiimide (iPr-cdi) are
significantly more volatile than complexes derived from dicy-
clohexylcarbodiimide (cy-cdi). In contrast, variations of R1,
R2, and R4 have only a small impact on the thermal behavior
of the complexes. Finally, selected compounds have been py-
rolyzed at 600 °C, and the decomposition products studied
1
by GC-MS and H NMR spectroscopy.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
R1, R2 = Et, R3 = cy, R4 = tBu; 9: R1, R2 = Et, R3 = iPr, R4
=
nPr) have been determined and relevant structural issues are
Introduction
imido tantalum complexes are the most prominent.[2] While
most compounds in this class are liquid and have a suffi-
ciently high vapor pressure, they tend to be chemically and
thermally rather fragile and show problems in terms of
long-term stability at vaporization temperatures as well as
a high sensitivity towards moisture or reactive gases. Thus,
the use of didentate ligands such as hydrazines has been
suggested.[3] In this paper we wish to report on an extension
of this approach by introducing guanidinates as suitable
chelating ligands for mixed amido/imido tantalum com-
plexes. The use of guanidinates has already shown promis-
ing results for the deposition of Ti(C)N or HfO2 thin films
in CVD processes.[4,5] Moreover, we recently published a
communication on the synthesis and characterization of
mixed amido/imido/guanidinato complexes 3–5 and showed
the successful deposition of conductive, almost carbon-free
TaN (C content Ͻ 1%) thin films using compound 4 as a
single source precursor.[6] This is a rather surprising result
considering the fact that less complex amido/imido com-
pounds of tantalum with a considerably lower relative car-
bon content produce films with much higher carbon con-
tents of around 10%.[7] These results encouraged us to carry
out a more detailed examination of the tantalum guanidin-
ato/amido/imido complex system and the impact of substit-
uent variations at different ligand sites on the chemical and
thermal properties of the resulting complexes. Table 1 gives
a summary of all complexes synthesized and characterized
in this paper.
Thin films of tantalum nitride (TaN) play an important
role in the ongoing process of device optimization in micro-
electronics. Conducting TaN films serve as gate electrode
materials in metal oxide semiconductor field-effect transis-
tors as well as diffusion barriers for the inhibition of copper
or silver diffusion into the silicon substrates.[1] A rough lit-
erature survey shows that 492 patents on the production,
processing, and use of TaN films were filed in 2005 alone.
TaN thin films can be deposited by various methods, in-
cluding sputtering processes and gas-phase deposition tech-
niques such as MOCVD (metal organic chemical vapor de-
position), plasma-enhanced CVD (PECVD), or atomic
layer deposition (ALD). These gas-phase-based techniques
make use of binary metal halides, although more recently
metal-organic tantalum coordination complexes have also
been used. Key parameters for precursors in MOCVD or
ALD processes include volatility, thermal decomposition
behavior, chemical reactivity, accessibility, and safety issues.
Several tantalum–ligand systems have been tested for the
deposition of TaN films, amongst which mixed amido/
[a] Lehrstuhl für Anorganische Chemie II – Organometallics and
Materials Chemistry, Ruhr-University Bochum
44870 Bochum, Germany
Fax: +49-234-321-4174
E-mail: roland.fischer@rub.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2006, 4665–4672
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Baunemann, M. Winter, K. Csapek, C. Gemel, R. A. Fischer
FULL PAPER
Table 1. Overview of compounds 3–10 in this publication.
The synthesis of 3–5 has already been described in a pre-
vious communication.[6a] It is noteworthy that no insertion
products could be isolated when sterically encumbering car-
bodiimides such as (tBu)2C2N or (SiMe3)2C2N were used,
even after refluxing the reaction mixtures for 24 h in the
presence of a large excess of the carbodiimides. On the
other hand, reaction with the unsubstituted carbodiimide
(which exists as its tautomer H2NCϵN) did not lead to
isolable products but instead gave an orange solid which is
insoluble in organic solvents. It is likely that transamination
reactions lead to the formation of insoluble, oligomeric tan-
talum amides.
Compound
R1
R2
R3
R4
M.p. [°C]
3[a]
4[b]
5[b]
6
Me
Me
Et
Me
Me
Et
Me
Et
Et
Me
Et
Et
iPr
iPr
iPr
cy
cy
cy
tBu
tBu
tBu
tBu
tBu
tBu
nPr
nPr
232
217
210
210
218
204
195
205
7
8
9
10
Et
Et
Et
Et
iPr
cy
[a] Included for reasons of comparison. [b] Crystal structure has
not been published so far.
Compounds 3–10 can be prepared in high yields (Ͼ80%)
and are white solids with rather high melting points of 195–
230 °C. All compounds are highly soluble in hexane, ben-
zene, or toluene and their elemental analyses are in good
agreement with their suggested molecular formulae. The
mass spectra of 3–10 show differences between the bis(cy-
clohexyl)carbodiimide (cy-cdi) containing compounds and
their (iPr-cdi) based congeners. Thus, the intensities of the
[M+] peaks of 6, 7, 8, and 10 (cy-cdi) are below 1% of the
base peak (if visible at all), while they are above 10% for 3,
4, 5, and 9 (iPr-cdi). As will be shown below, this is consis-
tent with the thermal properties of these compounds, since
cy-cdi based compounds generally show significant decom-
position during evaporation. The most intense peaks for 6,
Results and Discussion
Two basic pathways are known in the literature for the
synthesis of metal guanidinate complexes: (i) salt metathesis
reactions in which sodium or lithium guanidinates are
treated with the metal halides[4,8] and (ii) the insertion of
carbodiimides into metal–amido bonds, which is a more
convenient pathway that does not involve the formation of
LiX or NaX byproducts.[6,9] In the case of tantalum guanid-
inates, previous work from Tin et al. includes the synthesis
of [Ta(NMe2)4{C(NMe2)(NRЈ)2}] (RЈ = cy, iPr) as well as
the preparation of tantalum complexes of the neutral,
protonated guanidine ligand.[10] The large variety of com-
pounds published before 2001 have been summarized in a
recent review.[11]
7, 8, and 10 are observed at m/z 41, 43 and 55 and are due
+
to the cyclohexyl fragments C3H5+, C3H7+, and C4H7
.
The synthesis of the starting compounds [Ta(NR1R2)3-
(NR4)] followed published procedures.[12] At first the inter-
mediate compound [TaCl3(N-nPr)(py)2] (1) was formed by
reaction of TaCl5 with n-propylamine, chlorotrimethylsil-
ane, and pyridine. It should be noted that on scaling up the
NMR Analysis
All NMR assignments are based on two-dimensional
literature procedure it is possible to keep the amount of NMR spectra of the previously published compounds 3–
solvent almost constant by increasing the amount of pyri- 5.[6a] The 1H NMR spectra of 3–10 show rather large differ-
dine, which significantly increases the solubility of 1 in tolu- ences in the exact chemical shifts of the diastereotopic alkyl
ene and thus allows the separation of the product from the substituents of the Ta-amido groups, which also show a hin-
precipitated ammonium salts (see Experimental Section). In dered rotation due to the partial π character of the Ta–N
a second step, this pyridine-stabilized salt was treated with bond. In the case of the dimethyl-substituted amido group
LiNEt2 in order to form [Ta(NEt2)3(N-nPr)] (2). Higher (6), the two singlets are separated by 0.5 ppm and in the
yields of 2 can be obtained when stoichiometric amounts case of the diethyl-substituted amido group (8, 9, and 10),
of lithium amide are added to 1 at –60 °C instead of an the four doublets of quartets (ABX3 spin system) are found
excess at room temperature. The mixed amido/imido tanta- in a range of 1.5 ppm. Of these four multiplets, three are
lum complexes were then treated with the respective car- close to each other while the other is strongly high-field
bodiimides in hexane to give the guanidinato/amido/imido shifted, probably because of the anisotropic magnetic field
complexes 3–10 (Scheme 1).
of the guanidinate π systems. The 1H and 13C NMR spectra
Scheme 1. Synthesis of compounds 3–10.
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 4665–4672

Mixed Amido/Imido/Guanidinato Complexes of Tantalum
FULL PAPER
of 7 (MeEtN substitution) could not be fully assigned due unsymmetrical charge distribution within the delocalized π
to the formation of rotational isomers, which leads to a system of the guanidinato ligand, which is also well re-
mixture of several interconvertible isomers and thus highly flected in a rather large difference in the N–C bond lengths
1
complex solution spectra. The H NMR spectra of the cy- [N(3)–C(2) = 1.308(9), N(4)–C(2) = 1.363(9) Å]. Scheme 2
cdi based complexes 6, 7, 8, and 10 show broad signals for illustrates the mesomeric structures for the metal–guanidin-
the cyclohexyl groups, with only the Cα proton being ato interactions, with structure a describing best the ob-
slightly low-field shifted. The assignment of all 13C NMR served bond lengths in the solid-state structure.
spectra, with the exception of 7, was straightforward and is
therefore not discussed here in detail.
X-ray Analysis
Single crystals of 4, 5, 6, 8, and 9 were obtained by
recrystallization of the compounds from toluene at –30 °C.
Several attempts to obtain crystals of 7 or 10 suitable for
single-crystal X-ray analysis failed. Compounds 4 and 5
crystallize in an orthorhombic crystal system while the crys-
tal systems of 6, 8, and 9 are of lower symmetry, i.e. mono-
clinic for 6 and 8 and triclinic for 9. The molecular struc-
tures of 6 and 9 are displayed in Figure 1 as representatives
of all solid-state structures.[13] Table 2 contains bond
lengths, angles, and dihedral angles for all the above com-
plexes.
The molecular structures of all compounds are very sim-
ilar to each other and to that of compound 3, which has
already been discussed in an earlier publication.[6a] Thus,
only the solid-state structure of 6 will be discussed here in
more detail. The tantalum(V) center in 6 is sixfold coordi-
nated by two didentate guanidinato ligands, one amido, and
one imido group, resulting in a highly distorted octahedron.
The Ta–N(6)–C(18) unit of the imido ligand is almost linear
[170.6(4)°] and the Ta–N(6) bond is relatively short
(1.79 Å), which indicates an almost sp-hybridized imido li-
gand. Due to the strong π-donating effect of this imido li-
gand,[14] the Ta(1)–N(3) bond of the guanidinato ligand op-
posite to the imido group is lengthened [2.437(5) Å], while
the second Ta–N bond in the same guanidinato ligand
Scheme 2. Mesomeric structures for metal–guanidinato interac-
tions.
The second guanidinato ligand [N(1)–C(1)–N(2)] is more
symmetric – the two Ta–N bond lengths are 2.168(6) Å [Ta–
N(1)] and 2.279(5) Å [Ta(1)–N(2)]. Again, the longer Ta–N
bond is found trans to the more basic ligand, i.e. the amido
group. The noncoordinated NR2 groups of the guanidinato
ligands are almost planar, with angular sums around the
nitrogen atoms of 355.1° and 359°, respectively. A rather
weak π-donating effect of the NR2 groups to the guanidin-
ato π-systems is suggested by a relatively large tilt angle of
the C–N–C planes of 45.3° and 40.6°, respectively, which
is obviously an effect of the steric repulsions of the alkyl
substituents. However, an even weaker interaction is found
in [Ta(NMe2)4{C(NMe2)(N-cy)2}],[10a] which shows a tilt
angle of the two CNC planes of more than 80°. Indeed, the
C–N bond length of the exocyclic nitrogen ligand to the
guanidinato core is elongated by 0.03 Å compared to 6.
Analysis of Thermal Properties
The thermal properties of compounds 6–10 were ana-
[Ta(1)–N(4) = 2.123(5) Å] is considerably shorter. Obvi- lyzed by classic melting point determinations, standard TG/
ously, the π-donating effect of the imido group results in an DTA measurements, isothermal analyses, and sublimation
Figure 1. Povray/Ortep drawings of the molecular structures of 6 and 9 in the solid state. All hydrogen atoms have been omitted for
clarity.
Eur. J. Inorg. Chem. 2006, 4665–4672
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4667

A. Baunemann, M. Winter, K. Csapek, C. Gemel, R. A. Fischer
Table 2. Selected bond lengths [Å] and bond and torsion angles [°] for 3,[a] 4, 5, 6, 8, and 9.
FULL PAPER
3
4
5·Et
6
8·Et
9·Et
Ta(1)–N(1)
2.174(4)
2.254(4)
2.396(5)
2.129(4)
2.014(5)
1.785(5)
1.345(7)
1.309(7)
1.320(7)
1.361(7)
1.456(7)
1.463(7)
1.444(7)
1.390(7)
1.442(7)
1.440(8)
1.393(7)
1.444(8)
1.450(7)
59.56(18)
155.36(19)
58.92(16)
162.19(19)
112.0(5)
113.2(5)
122.7(5)
121.9(5)
115.3(5)
120.7(5)
122.8(5)
116.5(5)
170.6(4)
178.5(7)
179.8(9)
176.3(9)
45.5(13)
44.2(13)
2.185(7)
2.251(7)
2.409(7)
2.127(7)
2.002(8)
1.791(8)
1.352(11)
1.318(11)
1.319(11)
1.356(11)
1.453(12)
1.468(13)
1.453(12)
1.382(11)
1.454(12)
1.447(12)
1.391(11)
1.437(13)
1.447(12)
59.4(3)
2.198(8)
2.275(8)
2.408(8)
2.126(8)
2.023(8)
1.790(10)
1.327(13)
1.327(14)
1.323(13)
1.359(13)
1.464(13)
1.465(12)
1.449(14)
1.400(13)
1.464(14)
1.467(16)
1.393(13)
1.444(14)
1.461(14)
58.8(3)
2.168(6)
2.279(5)
2.437(5)
2.123(5)
2.005(5)
1.786(6)
1.389(8)
1.318(9)
1.308(9)
1.363(9)
1.451(8)
1.453(8)
1.457(9)
1.388(9)
1.465(9)
1.443(9)
1.397(8)
1.455(8)
1.459(9)
60.3(2)
2.198(4)
2.269(4)
2.408(4)
2.136(4)
2.008(4)
1.797(4)
1.342(6)
1.319(6)
1.320(6)
1.363(6)
1.457(7)
1.464(6)
1.440(7)
1.400(6)
1.456(7)
1.446(7)
1.393(7)
1.456(8)
1.483(10)
59.14(15)
156.64(17)
58.62(14)
161.69(16)
111.9(4)
112.9(4)
120.3(5)
118.4(5)
120.9(5)
121.1(4)
121.1(4)
116.0(4)
172.3(4)
180.0(7)
166.2(6)
172.4(5)
58.6(6)
2.192(4)
2.250(4)
2.433(4)
2.122(4)
2.013(4)
1.780(4)
1.329(6)
1.325(6)
1.308(6)
1.359(6)
1.471(6)
1.454(7)
1.439(7)
1.388(6)
1.467(6)
1.457(6)
1.393(6)
1.451(7)
1.463(7)
59.38(14)
155.50(16)
58.05(14)
156.78(16)
112.1(4)
113.2(4)
120.0(4)
121.8(4)
116.9(4)
120.6(4)
119.1(4)
118.2(4)
176.2(4)
178.0(5)
163.6(4)
166.8(5)
48.7(8)
Ta(1)–N(2) (opp. amido)
Ta(1)–N(3) (opp. imido)
Ta(1)–N(4)
Ta(1)–N(5) (Ta-amido)
Ta(1)–N(6) (Ta-imido)
N(1)–C(1)
N(2)–C(1)
N(3)–C(2)
N(4)–C(2)
N(5)–C(16)
N(5)–C(17)
N(6)–C(18)
N(7)–C(2)
N(7)–C(11)
N(7)–C(12)
N(8)–C(1)
N(8)–C(3)
N(8)–C(4)
N(1)–Ta(1)–N(2)
N(2)–Ta(1)–N(5)
N(3)–Ta(1)–N(4)
N(3)–Ta(1)–N(6)
N(1)–C(1)–N(2)
N(3)–C(2)–N(4)
C(1)–N(8)–C(3)
C(1)–N(8)–C(4)
C(3)–N(8)–C(4)
C(2)–N(7)–C(11)
C(2)–N(7)–C(12)
C(11)–N(7)–C(12)
Ta(1)–N(6)–C(18)
Ta(1)–N(5)–C(16)–C(17)
C(2)–N(7)–C(11)–C(12)
C(1)–N(8)–C(3)–C(4)
C(3)–N(8)–C(4) plane vs. N(1)–C(1)–N(2)–plane
C(11)–N(7)–C(12) plane vs. N(3)–C(2)–N(4)–plane
154.8(3)
58.1(3)
158.7(3)
58.5(3)
153.7(2)
58.0(2)
161.1(3)
111.0(7)
112.0(8)
120.6(8)
121.4(8)
117.2(8)
122.9(8)
120.7(8)
116.1(8)
175.7(7)
179.4(12)
174.6(11)
169.9 (12)
50.4(18)
45.0(18)
160.6(4)
111.6(9)
112.5(8)
120.6(9)
120.1(8)
118.5(9)
121.4(8)
121.4(8)
117.2(8)
174.9(8)
178.4(14)
179.0(14)
169.9(14)
53.7(14)
42.2(14)
161.4(2)
111.3(6)
113.0(6)
121.0(6)
119.0(5)
115.1(5)
123.0(6)
120.9(6)
115.1(6)
175.7(5)
175.7(10)
169.0(10)
155.5(10)
45.3(10)
40.6(10)
46.2(7)
45.9(6)
[a] The data of 3 have already been published and are only given here for comparison.
experiments under vacuum conditions. The melting points cdi based compounds. However, by changing R1 and R2
of all compounds are relatively high (195–232 °C, see from methyl to ethyl (e.g. 3 to 5) no significant effect on the
Table 1) and cannot be rationalized by the type of ligand volatility is observed. Obviously, the variation of R3 has a
substitution (R1–R4). It can be concluded from the DTA significant effect on the interactions between the molecules
spectra that the melting process is accompanied by the initi- in the solid state, for example by a different packing of the
ation of decomposition reactions. The compounds do not moieties.
melt fully at the documented temperatures but undergo
some sort of waxy transition phase over a temperature
range of approximately 5 °C (vide infra).
The nature of the carbodiimide substituents (cy vs. iPr)
has a big influence on the volatility of the compounds.
Thus, 3, 4, 5, and 9 (iPr-cdi) can be sublimed without any
residues at 120 °C in vacuo (10–4 mbar) whereas 6, 7, 8, and
10 (cy-cdi) do not sublime at all under these conditions.
Isothermal studies at 120 °C underline these results. After
240 min at ambient pressure the weight loss of 3, 4, 5, and
9 is approximately 10% in all cases, while compounds 6 and
7, for example, show a weight loss of only 2.5% (Figure 2).
The mass transport of these complexes is highest for 9
(4.9 µgmin–1) and significantly lower for 6 and 7 (0.93 and
Figure 2. Isothermal analysis of 6, 7, and 9. The spectra were re-
0.59 µgmin–1, respectively). The simplest explanation for
corded at a temperature of 120 °C, 300 sccm nitrogen flow for 4 h.
this observation is the higher molecular masses of the cy- Heating from 25 °C to 120 °C at a rate of 10 °Cmin–1.
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Mixed Amido/Imido/Guanidinato Complexes of Tantalum
FULL PAPER
The residual masses after TG/DTA measurements from sor (300 mg), deuterated benzene was injected into the cool-
1
cy-cdi based complexes are higher than the theoretical ing trap, and the resulting C6D6 solutions analyzed by H
masses (residual masses for 6–8, 10: 29–35%, theoretical NMR spectroscopy and GC-MS measurements. The 1H
content of TaN: 22–24%). This indicates the incorporation NMR spectra revealed the formation of the corresponding
of significant amounts of carbon during the decomposition. free amines for 3–5 (HNMe2, HNMeEt and HNEt2), small
In contrast, the iPr-cdi based complexes 4, 5, and 9 show amounts of tBuNH2 (resulting from the former imido li-
considerably lower residual masses and thus cleaner decom- gand), as well as free carbodiimide. At these very high tem-
position processes (residual masses: 10–12%, theoretical peratures (600 °C) carbodiimide could be released from the
content of TaN: 27–28%).
guanidinato ligand by deinsertion analogous to the behav-
With the exception of 7, all compounds show one or two ior of amidinate complexes.[15] It should be noted that al-
endothermic peaks at temperatures below their melting/de- though these compounds represent the major part of de-
composition points (Figure 3). Due to the fact that no composition products, smaller peaks were also present
weight loss can be observed in any case, a phase change of which could not be unequivocally assigned. GC-MS mea-
1
the solids is possibly responsible for this behavior. This surements confirm these H NMR results and additionally
phase change is not reversible, which means that running a show the presence of small amounts of isopropylamine as
program in which the sample is heated, cooled down, and well as several unidentified species containing the carbodi-
heated again shows endothermic peaks only in the first imide group. Although the relatively large variety of decom-
period of heating. The position of the peaks is strongly de- position products suggests rather complex decomposition
pendent on the nature of the alkyl substituents R1, R2, and mechanisms, preliminary MOCVD experiments of com-
R3. Going from methyl to ethyl for the amido substituents pound 4 as a single-source precursor showed the possibility
R1 and R2 results in a shift of these peaks to higher of carbon-free (atomic concentration Ͻ 1%)[6] TaN deposi-
temperatures (3Ǟ4Ǟ5: 52 °CǞ70 °CǞ110 °C; 6Ǟ8: tion, which requires clean thermal decomposition processes.
112 °CǞ139 °C). The same is true for the variation of R3
from iPr to cy (3Ǟ6: 52 °CǞ112 °C; 5Ǟ8: 110 °CǞ139 °C;
9Ǟ10: 82 °CǞ140 °C). It can be concluded, that the onset
of the temperature for phase changes depends on the mo-
bility of the molecules in the solid state, which supposedly
increases with higher chain lengths and/or size of the sub-
stituents.
Figure 4. Setup for the decomposition experiments of 3–5.
Conclusions
We have presented the synthesis and characterization of
several new guanidinato/amido/imido complexes of the type
[Ta(NR1R2){C(NR1R2)(NR3)2}2(NR4)] {R1,R2 = methyl,
ethyl; R3 = cyclohexyl (cy), isopropyl; R4 = tert-butyl, n-
propyl}. All complexes are structurally very similar, the
bond lengths and angles of the solid-state structures show-
ing only minor variations. TG/DTA measurements and iso-
thermal studies reveal the low volatility of the cy-cdi based
complexes. This is rather unexpected considering the fact
that the structural and chemical behaviors of compounds
3–10 are very similar. Thus, cy-cdi based complexes are, in
general, not suitable for MOCVD purposes due to a sup-
posedly weak precursor transport. However, application of
the compounds derived from cy-cdi in liquid injection
MOCVD is feasible. The choice of substituents of the
amido or imido groups does not seem to have a significant
impact on the thermal behavior of the resulting complexes.
Finally, thermal decomposition experiments of 3–5 clearly
Figure 3. TG/DTA spectrum of 9 and TG curves of 2 and 10. Heat-
ing rate: 5 °Cmin–1.
Decomposition Experiments of 3–5
The nature of the thermal decomposition of potential show that highly complex pyrolysis reactions are taking
MOCVD precursors into the desired materials is crucial. place, although they lead to a clean decomposition of the
Thus, compounds 3–5 were evaporated at 120 °C in vacuo, selected precursors in all cases.
with a constant flow of argon providing a continuous trans-
port of the precursor through a decomposition tube at
600 °C (Figure 4). The decomposition products were sub-
Experimental Section
sequently trapped at –196 °C (liquid nitrogen) in a slightly
modified cooling trap. After full evaporation of the precur- performed on a conventional vacuum/argon line using standard
All manipulations of air- and moisture-sensitive compounds were
Eur. J. Inorg. Chem. 2006, 4665–4672
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4669

A. Baunemann, M. Winter, K. Csapek, C. Gemel, R. A. Fischer
FULL PAPER
Schlenk techniques. Preparations of samples for further analysis
were carried out in argon-filled glove boxes. All solvents were puri-
fied by an MBraun solvent purification system (SPS) and stored
over activated molecular sieves (4 Å). C6D6 was degassed and dried
with activated molecular sieves. Elemental analysis was performed
by the analytical service of our Chemistry Department (CHNSO
Vario EL 1998). 1H and 13C NMR spectra were recorded on a
Bruker Advance DPX-250 spectrometer. EI mass spectra were re-
corded with a Varian MAT spectrometer. Simultaneous TG/DTA
analysis was carried out using a Seiko TG/DTA 6300S11 at ambi-
ent pressure (sample weight Ϸ 10 mg). GC-MS measurements were
carried out on a Shimadzu QP2010 instrument at a column tem-
perature of 290 °C. The tantalum-containing starting compounds,
namely [Ta(NMe2)3(N-tBu)], [Ta(NMeEt)3(N-tBu)], [Ta(NEt2)3(N-
tBu)], and [Ta(NEt2)3(N-nPr)] were synthesized in batches of 15–
40 g following modified literature procedures.[12] Reagents were
purchased from the following companies and used without further
purification: butyllithium (1.6  solution in hexane, Merck), dieth-
ylamine (Ͼ99% purity, Merck), methylethylamine (Ͼ 97% purity,
Fluka), pyridine (Normapur grade, VWR), chlorotrimethylsilane
(98%, Acros), lithium dimethylamide (95%, Aldrich), diisopro-
pylcarbodiimide (99%, careful: very toxic, Acros), and dicyclohex-
ylcarbodiimide (99%, Aldrich). The synthesis and characterization
of 3–5 was described in a previous publication.[6a] For a brief sum-
mary of the crystallographic data see Table 3. CCDC-611058 to
-611062 (4–6, 8, and 9, respectively) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
[TaCl3(N-nPr)(py)2] (1): N-Propylamine (20.2 mL, 14.52 g,
246 mmol) and chlorotrimethylsilane (15 mL, 118 mmol) were
added to a cooled suspension of TaCl5 (20 g, 55.8 mmol) in toluene
(150 mL). The color of the suspension changed to slight yellow-
greenish. After stirring the reaction mixture at room temperature
for 25 min, pyridine was added (25 mL, 24.5 g, 310 mmol). After
24 h, the clear, yellow-green solution was filtered, and the solvents
removed in vacuo. The crude product was recrystallized from hot
toluene (100 °C, ca. 120 mL). Yield: 22.2 g (44.2 mmol, 79% based
on TaCl5). C13H17Cl3N3Ta (502.63): calcd. C 31.13, H 3.39, N 8.38;
found C 31.14, H 3.67, N 8.67. All other analyses (NMR, MS)
were in accordance with the previously published results.
[Ta(NEt2)3(N-nPr)] (2): A suspension of lithium diethylamide
(62 mmol) was prepared in situ by adding 40 mL of a 1.6  BuLi
solution (hexane) to diethylamine (7.5 mL, 62 mmol) at –60 °C. Af-
ter stirring the reaction mixture for 24 h at room temperature, the
resulting suspension was added to a suspension of 1 (10 g,
20.8 mmol) in hexane at –60 °C. The reaction mixture slowly
turned yellow/orange and was filtered after stirring for an ad-
ditional 24 h. The yellow/orange filtrate was concentrated in order
to obtain the dark orange product 2. Yield: 8.38 g (18.4 mmol, 92%
based on 1). C15H37N4Ta (454.43): calcd. C 39.65, H 8.20, N 12.33;
found C 39.87, H 7.80, N 12.49. All other analyses (NMR, MS)
were in accordance with the previously published results.
[Ta(NMe2){(N-cy)2C(NMe2)}2(N-tBu)] (6): A solution of dicyclohex-
ylcarbodiimide (1.52 g, 7.83 mmol) in hexane (20 mL) was added
to a solution of [Ta(NMe2)3(N-tBu)] (1.55 g, 4.03 mmol) in hexane
(40 mL). The temperature of the solution increased to about 45 °C
Table 3. Crystallographic data and structure refinement details for 4, 5, 6, 8, and 9.
4 (CVD82)
5 (CVD72)
6 (CVD101)
8 (CVD76)
9 (CVD99)
Empirical formula
Formula mass
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C27H61N8Ta
678.79
108(2)
orthorhombic
Pbca
10.7124(7)
18.2835(10)
34.0026(18)
90
90
90
6659.8(7)
8
1.354
C30H67N8Ta
720.87
105(2)
orthorhombic
Pbca
11.4691(11)
18.1889(18)
34.854(3)
90
C36H71N8Ta
796.96
113(2)
C42H83N8Ta
881.11
108(2)
monoclinic
P21/c
14.4945(9)
17.4746(8)
21.6152(14)
90
98.182(5)
90
5045.2(5)
4
1.160
2.211
C29H65N8Ta
706.84
111(2)
monoclinic
P21/n
11.0993(12)
19–395(4)
15.960(2)
90
91.702(9)
90
3434.2(9)
4
1.367
3.230
triclinic
¯
P1
10.1139(9)
16.8089(14)
23.796(2)
80.815(7)
88.860(7)
78.771(7)
3916.8(6)
4
1.351
2.841
0.833/0.465
1664
90
90
γ [°]
V [Å3]
7270.8(12)
8
1.317
3.053
0.643/0.589
3008
Z
Density (calcd.) [gcm–3]
µ(Mo-Kα) [mm–1]
Tmax./Tmin.
F(000)
3.328
0.967/0.787
2816
0.733/0.556
1856
0.529/0.298
1472
Crystal size [mm]
θ range [°]
Completeness to θ [%]
Index ranges
0.21×0.06×0.01
2.99–25.00
99.8
–12 Յ h Յ 12
–21 Յ k Յ 19
–40 Յ l Յ 40
48057
5852
5852/0/325
1.393
0.16×0.10×0.07
2.92–25.00
98.2
–13 Յ h Յ 13
–21 Յ k Յ 21
–39 Յ l Յ 40
48479
6292
6292/0/369
1.058
0.18×0.08×0.03
2.60–25.00
99.7
–12 Յ h Յ 12
–19 Յ k Յ 19
–28 Յ l Յ 27
53636
13711
13711/6/811
1.229
0.16×0.11×0.07
3.01–27.56
99.9
–17 Յ h Յ 15
–22 Յ k Յ 19
–28 Յ l Յ 28
50240
11646
11646/0/460
1.113
0.25×0.17×0.11
2.76–25.00
99.8
–13 Յ h Յ 13
–23 Յ k Յ 23
–18 Յ l Յ 18
42514
6019
6019/0/358
1.097
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
R indices [IϾ2σ(I)] R1
wR2
0.0869
0.1086
0.0707
0.1447
0.0662
0.0960
0.0562
0.1119
0.0406
0.0977
R indices (all data) R1
wR2
0.1041
0.1138
0.1197
0.1663
4.998/–1.103
0.0861
0.1016
1.771/–0.725
0.0797
0.1221
3.882/–1.493
0.0440
0.1000
2.954/–1.558
Largest diff. peak/hole [eÅ–3] 2.699/–1.589
4670
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 4665–4672

Mixed Amido/Imido/Guanidinato Complexes of Tantalum
FULL PAPER
during addition; no color change was observed. After stirring the
mixture for 24 h, the solvent was removed in vacuo. Small, white
crystallites suitable for X-ray analysis were obtained by recrystalliz-
ing the crude product from toluene. M.p. 210 °C (uncorrected).
Yield: 2.73 g (3.43 mmol, 85% based on [Ta(NMe2)3(N-tBu)]).
C36H71N8Ta (796.96): calcd. C 45.27, H 8.71, N 17.60; found C
45.22, H 9.00, N 17.83. 1H NMR (25 °C, 250 MHz, C6D6): δ =
1.30–2.10 (m, cyclohexyl), 1.58 [s, 9 H, Ta{NC(CH3)3}], 2.58, 2.62
[2×s, 2 ×6 H, Ta{(N-cy)2C[N(CH3)2]}], 3.2–3.7 (m, 4 H, cyclo-
hexyl, N–C–H) 3.59 [s, 3 H, Ta–N(CH3)2], 4.20 [s, 3 H, Ta–N-
(CH3)2] ppm. 13C NMR (25 °C, 62.5 MHz, C6D6): δ = 26.2, 26.5,
26.6 (2×), 26.7 (3×), 27.1 (2×), 27.2, 27.4 (2×) (cyclohexyl, 3,4-
position), 35.2 [Ta{NC(CH3)3}], 34.3, 34.5, 35.2, 36.3, 36.6, 37.0,
37.1, 37.5 (cyclohexyl, 2-position), 40.3 [Ta{(N-cy)2C[N(CH3)2]}],
49.1, 59.3 [Ta{N(CH3)2}], 55.1, 56.0, 57.3, 57.5 (cyclohexyl, N-C),
63.6 [Ta{NC(CH3)3}], 164.8, 169.3 [Ta{(N-cy)2C(NMe2)}] ppm.
EI-MS (70eV): m/z (%) = 796 (0.5) [M+], 590 (15) [M+ – cy-cdi],
206 (25) [cy-cdi+], 124 (45) [cy-N=C=N–H+], 83 (80) [cy+], 55 (100)
[C4H7+], 43 (70) [C3H7+], 41 (75) [C3H5+].
N=C=N–H+], 83 (81) [cy+], 55 (100) [C4H7+], 43 (70) [C3H7+], 41
(71) [C3H5+].
[Ta(NEt2){(N-iPr)2C(NEt2)}2(N-nPr)] (9): The synthesis of 9 fol-
lowed, in general, the same procedure as for 6 but with [Ta-
(NEt2)3(N-nPr)] (1.83 g, 4.03 mmol) and diisopropylcarbodiimide
(1.01 g, 8.06 mmol). The diimide was used without dissolving it in
hexane. M.p. 195 °C (uncorrected). Yield: 2.39 g (3.38 mmol, 84%
based on [Ta(NEt2)3(N-nPr)]). C29H65N8Ta (706.84): calcd. C
1
49.28, H 9.27, N 15.85; found C 49.28, H 9.17, N 16.10. H NMR
(25 °C, 250 MHz, C6D6):
δ = = 6.9 Hz, 3 H,
0.89 [t, 1J
1
Ta(NCH2CH2CH3)], 0.91, 2×0.96, 1.06, 1.07, 1.47 [6×t, J = 6.7,
2×7.0, 7.4, 6.8, 7.0 Hz, 6×3 H, 4×Ta{(N-iPr)2C[N(CH2CH3)2]}
and 2×TaN[CH2CH3]2], 1.16, 1.23, 1.35, 1.38, 2×1.40, 1.47, 1.58
[8×d, 1J
=
6.4, 6.4, 6.5, 6.0, 2×6.0, 6.4, 6.5 Hz, 24 H,
Ta{[N[CH(CH3)2]]2C(NEt2)}], 1.72 [sextet, 1J = 7.3 Hz, 2 H,
Ta(NCH2CH2CH3)], 2.80–3.15 {m, H, Ta{(N-iPr)2C[N-
8
1
(CH2CH3)2]}}, 3.41, 4.25, 4.59, 4.73 [4×d of q, J = 6.3, 6.0, 6.0,
5.9 Hz, 4 H, TaN(CH2CH3)], 3.76, 3.87, 3.94, 4.10 [4×septet, J =
1
6.5, 6.4, 6.4, 6.5 Hz, 4 H, Ta{{N[CH(CH3)2]}2C(NEt2)}], 4.31, 4.37
[2×d of t, 1J = 6.8, 6.7 Hz, 2 H, Ta(NCH2CH2CH3)] ppm. 13C
NMR (25 °C, 62.5 MHz, C6D6): δ = 12.40 [Ta(NCH2CH2CH3)],
13.6, 14.0, 14.8, 15.7, 17.0 [Ta{(N-iPr)2C[N(CH2CH3)2]} and
TaN(CH2CH3)] 23.5, 24.5, 25.0, 25.2, 25.9, 26.1, 26.5, 28.0
[Ta{[N{CH(CH3)2}]2C(NEt2)}], 28.5 [Ta(NCH2CH2CH3)], 41.4,
42.3, 43.7 (br) [Ta{(N-iPr)2C[N(CH2CH3)2]}], 46.8, 56.7
[TaN(CH2CH3)], 47.2, 47.5, 48.4, 48.8 [Ta{[N{CH(CH3)2}]2C-
(NEt2)}], 61.9 [Ta(NCH2CH2CH3)], 166.5, 170.3 [Ta{N(iPr)2C-
(NEt2)}] ppm. EI-MS (70eV): m/z (%) = 706 (12) [M+], 635 (79)
[M+ – NEt2], 509 (100) [M+ – guanidinate], 466 (29) [M+ – guanid-
[Ta(NEtMe){(N-cy)2C(NEtMe)}2(N-tBu)] (7): The synthesis of 7
followed the same procedure as for 6 but with [Ta(NEtMe)3(N-
tBu)] (1.72 g, 4.03 mmol) and dicyclohexylcarbodiimide (1.52 g,
7.83 mmol). M.p. 218 °C (uncorrected). Yield: 2.77 g (3.30 mmol,
82% based on [Ta(NEtMe)3(N-tBu)]). C39H77N8Ta (839.03): calcd.
C 53.98, H 9.25, N 13.36; found C 53.62, H 9.34, N 13.48. 1H
NMR (25 °C, 250 MHz, C6D6): δ = 0.9–1.1 [3×q, 3×3 H,
2×Ta{(N-cy)2C[N(CH2CH3)Me]} and Ta{N(CH2CH3)Me}], 1.10–
2.10 (m, cyclohexyl), 1.57 [s, 9 H, Ta{NC(CH3)3}], 2.62, 2.63, 2.64
[no assignment possible, most probably Ta{(N-cy)2C[N(CH3)Et]}],
2.9–3.7 [m, cyclohexyl, N–C–H, Ta{(N-cy)2C[N(CH2CH3)Me]}
and Ta{N[CH2CH3]Me}], 4.25 [s, 3 H, Ta{NEt(CH3)}] ppm. 13C
inate – iPr], 407 (calcd. 409) (10) [M+ – iPr – N-Pr (i or n)], [M+
–
guanidinate], 198 (4) [guanidinate+], 141 (15) [N(iPr)2C(NH)+], 126
(5) [iPr-N=C=N-iPr+], 69 (34) [iPr-N–C+], 58 (10) [HN-Pr+], 43
(10) [iPr].
NMR (25 °C, 62.5 MHz, C6D6):
δ = 14.0 [Ta{(N-cy)2C-
[N(CH2CH3)Me]}], 15.2 [Ta{[N(CH2CH3)Me]}], 35.4 [Ta{N-
[C(CH3)3]}], 54.5, 54.6 [Ta{N(CH2CH3)Me} and Ta{NEt(CH3)}],
63.8 [Ta{NC(CH3)3}], 166.1 [Ta{(N-cy)2C(NMeEt)}] ppm; peaks
that cannot be clearly assigned: δ = 26.0, 26.7, 27.1, 27.3 27.5, 24.4,
35.1, 36.6, 37.0, 56.5, 57.4 ppm. EI-MS (70eV): m/z (%) = 576 (3)
[M+ – guanidinate], 265 (10) [guanidinate+], 206 (10) [cy-cdi+], 163
(37) [cy-cdi+ – C3H7], 124 (35) [cy-N=C=N–H+], 83 (67) [cy+], 55
(97) [C4H7+], 43 (100) [C3H7+], 41 (97) [C3H5+].
[Ta(NEt2){(N-cy)2C(NEt2)}2(N-nPr)] (10): The synthesis of 10 fol-
lowed the same procedure as for 6 but with [Ta(NEt2)3(N-nPr)]
(1.83 g, 4.03 mmol) and dicyclohexylcarbodiimide (1.52 g,
7.83 mmol). M.p. 205 °C (uncorrected). Yield: 2.27 g (2.62 mmol,
65% based on [Ta(NEt2)3(N-nPr)]). C41H81N8Ta (866.61): calcd. C
56.80, H 9.41, N 12.92; found C 56.13, H 10.04, N 13.20. 1H NMR
[Ta(NEt2){(N-cy)2C(NEt2)}2(N-tBu)] (8): The synthesis of 8 fol- (25 °C, 250 MHz, C6D6): δ = 0.97, 0.98, 2×1.02, 1.08, 1.09 [6×t,
lowed the same procedure as for 6 but with [Ta(NEt2)3(N-tBu)]
(1.89 g, 4.03 mmol) and dicyclohexylcarbodiimide (1.52 g,
7.83 mmol). M.p. 204 °C (uncorrected). Yield: 3.09 g (3.50 mmol,
coupling constants could not be determined, 18 H,
Ta(NCH2CH2CH3), 3×Ta{(N-cy)2C[N(CH2CH3)2]} and
2×Ta{N[CH2CH3]2}], 1.49 [t, 1J = 7.1 Hz, 3 H, Ta{(N-cy)2C-
87% based on [Ta(NEt2)3(N-tBu)]). C42H83N8Ta (881.11): calcd. C [N(CH2CH3)2]}], 1.73 [m, Ta (NCH2CH2CH3)], 1.2–2.3 (m, cyclo-
57.25, H 9.49, N 12.72; found C 57.41, H 10.18, N 11.96. 1H NMR hexyl), 2.90–3.20 [m, 8 H, Ta{(N-cy)2C[N(CH2CH3)2]}], 3.30–3.80
(25 °C, 250 MHz, C6D6): δ = 0.9–1.1 [m, 12 H, 4 of 6 CH3 groups [m, 5 H, 4×cyclohexyl (N–C–H) and 1×Ta{N(CH2CH3)2}], 4.25–
of Ta{(N-cy)2C[N(CH2CH3)2]} and Ta{N[CH2CH3]2}], 1.40, 1.46 4.50 [m, 3 H, 2×Ta(NCH2CH2CH3) + 1×Ta[N(CH2CH3)2]], 4.62,
[m,
2
of
6
CH3 groups of Ta{(N-cy)2C[N(CH2CH3)2]} 4.76 [2×sextet, 1J = 5.8, 6.0 Hz, 2 H, Ta{N(CH2CH3)2}] ppm. 13
C
and Ta{N[CH2CH3]2}], 1.20–2.20 (m, cyclohexyl), 1.58 [s, 9 H, NMR (25 °C, 62.5 MHz, C6D6): δ = 12.56 [Ta(NCH2CH2CH3)],
Ta{NC(CH3)3}], 2.9–3.2 [m, 8 H, Ta{(N-iPr)2C[N(CH2CH3)2]}],
3.30–3.70 [m, 4 H, cyclohexyl, N-C-H, and 1 H, Ta–N(CH2CH3)],
4.44, 4.66, 5.00 [3×d of q, 1J = 6.5, 6.2, 6.3 Hz, 3×1 H, Ta–
N(CH2CH3)] ppm. 13C NMR (25 °C, 62.5 MHz, C6D6): δ = 13.5,
14.3, 15.7, 15.9 [Ta{(N-cy)2C[N(CH2CH3)2]} and TaN(CH2CH3)]
13.6, 13.9, 14.90 (br), 15.9, 16.8 [Ta{(N-cy)2C[N(CH2CH3)2]}
and TaN(CH2CH3)], 26.5, 2×26.6, 26.7, 26.8, 26.9, 2×27.0,
2×27.1, 27.5, 27.6, 28.5 [12×cyclohexyl (3,4-position),
1×Ta{(NCH2CH2CH3)}], 33.5, 34.8, 35.7, 35.8, 36.6, 2×36.7 (cy-
clohexyl, 2-position), 39.1, 41.5, 42.5, 44.0 (br) [Ta{(N-cy)2-
26.0, 26.5, 26.6 (2×), 26.7, 26.8 (2×), 26.9, 27.2, 27.3, 27.5, 27.6 C[N(CH2CH3)2]}], 47.5 [1×TaN(CH2CH3)2], 55.7 55.9, 56.2, 56.9,
(cyclohexyl, 3,4-position), 34.0, 34.5, 34.6, 35.9, 36.4, 37.1, 37.6,
38.7 (cyclohexyl, 2-position), 35.2 [Ta{NC(CH3)3}], 41.5, 43.0,
43.1 (br), 44.6 (br) [Ta{(N-cy)2C[N(CH2CH3)2]}], 47.5, 58.1
[TaN(CH2CH3)], 55.6, 56.2, 57.5, 58.8 (cyclohexyl, N-C), 64.1
[Ta{NC(CH3)3}], 166.2, 170.2 [Ta{(N-cy)2C(NEt2)}] ppm. EI-MS
58.0, 58.3 [1×TaN(CH2CH3)2, 4×cyclohexyl (N–C)], 62.0
[Ta(NCH2CH2CH3)], 166.6, 170.3 [Ta{N(cy)2C(NEt2)}] ppm.
Small amounts of free cy-cdi were detected in the 13C NMR spec-
trum of this compound. EI-MS (70eV): m/z (%) = 795 (calcd. 794)
(0.5) [M+ – NEt2], 589 (calcd. 588) (0.5) [M+ – guanidinate], 206
(20) [cy-N=C=N-cy+], 82 (61) [cy+], 56 (100), 55 (C4H7+), 43 (60)
[C3H7+], 41 (67) [C3H5+].
(70eV): m/z (%) = 810 (3) [M+ – NEt2], 604 (calcd. 605) (3) [M+
cy-cdi], 206 (27) [cy-cdi+], 163 (48) [cy-cdi+ – C3H7], 124 (44) [cy-
–
Eur. J. Inorg. Chem. 2006, 4665–4672
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4671

A. Baunemann, M. Winter, K. Csapek, C. Gemel, R. A. Fischer
FULL PAPER
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