New octahedral Ta(V) hydrazido-substituted compounds for atomic layer deposition: Syntheses, X-ray diffraction structures of TaCl(NMe2)3[N(TMS)NMe2] and Ta(NMe2)4[N(TMS)NMe2], and fluxional behavior of the amido and hydrazido ligands in solution

Article abstract of DOI:10.1016/j.poly.2010.02.022

The synthesis and structural characterization of new tantalum(V) compounds containing a single hydrazido(I) ligand are reported. Hydrazinolysis of TaCl(NMe₂)₄ using trimethylsilyl(dimethyl)hydrazine affords the compound TaCl(NMe₂)₃[N(TMS)NMe₂] in essentially quantitative yield. Metathetical replacement of the chloride ligand in TaCl(NMe₂)₃[N(TMS)NMe₂] by LiNMe₂ gives the all-nitrogen coordinated compound Ta(NMe₂)₄[N(TMS)NMe₂]. VT ¹H NMR studies support the existence of low-energy pathways involving rotation about the Ta-N bonds of the ancillary amido and hydrazido ligands in both hydrazido-substituted compounds. X-ray crystallographic analyses confirm the octahedral disposition about the tantalum metal in TaCl(NMe₂)₃[N(TMS)NMe₂] and Ta(NMe₂)₄[N(TMS)NMe₂] and the presence of an η²-hydrazido(I) ligand. Preliminary data using Ta(NMe₂)₄[N(TMS)NMe₂] as an ALD precursor for the preparation of tantalum nitride and tantalum oxide thin films are presented.

Full text of DOI:10.1016/j.poly.2010.02.022

Polyhedron 29 (2010) 1754–1759
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
New octahedral Ta(V) hydrazido-substituted compounds
for atomic layer deposition: Syntheses, X-ray diffraction structures of
TaCl(NMe2)3[N(TMS)NMe2] and Ta(NMe2)4[N(TMS)NMe2],
and fluxional behavior of the amido and hydrazido ligands in solution
Shih-Huang Huang a, Tero Pilvi b, Xiaoping Wang c, Markku Leskelä b, Michael G. Richmond a,
*
a Department of Chemistry, University of North Texas, Denton, TX 76203, USA
b Department of Chemistry, University of Helsinki, Helsinki, Finland
c Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and structural characterization of new tantalum(V) compounds containing a single hydraz-
ido(I) ligand are reported. Hydrazinolysis of TaCl(NMe2)4 using trimethylsilyl(dimethyl)hydrazine affords
the compound TaCl(NMe2)3[N(TMS)NMe2] in essentially quantitative yield. Metathetical replacement of
the chloride ligand in TaCl(NMe2)3[N(TMS)NMe2] by LiNMe2 gives the all-nitrogen coordinated com-
pound Ta(NMe2)4[N(TMS)NMe2]. VT 1H NMR studies support the existence of low-energy pathways
involving rotation about the Ta–N bonds of the ancillary amido and hydrazido ligands in both hydrazi-
do-substituted compounds. X-ray crystallographic analyses confirm the octahedral disposition about
the tantalum metal in TaCl(NMe2)3[N(TMS)NMe2] and Ta(NMe2)4[N(TMS)NMe2] and the presence of
Received 15 December 2009
Accepted 15 February 2010
Available online 18 February 2010
Keywords:
Tantalum-amido compounds
Tantalum-hydrazido compounds
ALD
an
preparation of tantalum nitride and tantalum oxide thin films are presented.
Ó 2010 Elsevier Ltd. All rights reserved.
g
2-hydrazido(I) ligand. Preliminary data using Ta(NMe2)4[N(TMS)NMe2] as an ALD precursor for the
X-ray crystallography
1. Introduction
single-source precursors may be systematically ‘‘tuned” through li-
gand modification, leading to metal–nitride barrier layers that pos-
sess the desirable properties of superior adhesion, low resistivity,
and uniform coverage. Device longevity is greatly extended when
high-quality diffusion barriers are present since they prevent dele-
terious diffusion of the Cu interconnect into the silicon dielectric
foundation.
Our interest in metal–nitride films stems from the ongoing
industrial need for alternative single-source precursors that lend
themselves to the construction of TaN films under mild reaction
conditions. A pivotal hurdle associated with the manufacturing
process of TaN films is the formal reduction of the Ta(V) precursor
to the Ta(III) oxidation state required by TaN, which has largely
been achieved using NH3 in a dual capacity as a carrier gas and
reducing agent in the ALD process [3]. In an effort to facilitate
the Ta(V) ? Ta(III) reduction, several groups have synthesized
new hydrazido-substituted derivatives as precursors for TaN thin
films [2c,g,h,4]. Here the presence of an electron-rich hydrazido li-
gand(s) is expected to lower the deposition temperature and pro-
mote the needed two-electron reduction of the tantalum metal
relative to those precursors possessing simple amido and imido
substituents. Herein we report our data on the synthesis and struc-
tural characterization of the mono-hydrazido compounds TaCl(N-
Me2)3[N(TMS)NMe2] and Ta(NMe2)4[N(TMS)NMe2]. The VT NMR
The fabrication of advanced complementary metal-oxide-semi-
conductor (CMOS) architectures for integrated circuit (IC) applica-
tions beyond the 45 nm mode remains a critical pursuit of the
microelectronics industry [1]. An accompanying mandate in the
miniaturization of electronic devices is the need to replace the tra-
ditional silicon-based dielectric barrier with an alternative barrier
component. While many different materials have been examined
as part of this challenge, early metal–nitride barriers based on Ti,
Nb, and Ta have received extensive attention [2], with atomic layer
deposition (ALD) being the technique of choice for the manufacture
of these alternative barrier layers. The preparation of ULSI devices
that contain TiN, NbN, and TaN barrier layers by ALD are best pur-
sued through tailored metal-nitrogen precursors that avoid the use
of corrosive halide ligands and flammable and toxic reducing
agents required in the self-limiting growth step. The absence of
halide and carbon ligands in the primary coordination sphere of
an all-nitrogen containing metal precursor should facilitate the
elimination of some of the commonly found contaminates in the
terminal metal–nitride film. Moreover, the intrinsic reactivity of
* Corresponding author. Tel.: +1 940 565 3548; fax: +1 940 565 4318.
E-mail address: cobalt@unt.edu (M.G. Richmond).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.02.022

S.-H. Huang et al. / Polyhedron 29 (2010) 1754–1759
1755
behavior of these compounds and preliminary ALD results employ-
ing the latter all-nitrogen coordinated compound are also
described.
slowly to room temperature and stirred for additional 24 h, after
which the solution was filtered and the residue extracted with
3 ꢁ 30 mL portions of pentane. The combined pentane solutions
were concentrated to ca. 10 mL and then placed in the freezer
at ꢀ30 °C to furnish ultimately 0.58 g (95% yield) of Ta(NMe2)4-
2. Experimental
[N(TMS)NMe2] as
a
pale yellow solid. 1H NMR (toluene-d8,
298 K): d 0.293 (s, 9H, TMS), 2.48 (s, 6H, Me2NN), 3.25 (bs, 18H,
2.1. General
Me2N). EI-MS: m/e 487 [M]+ and 444 [M-NMe2]+.
The TaCl(NMe2)4 and (TMS)NHNMe2 employed in the synthe-
sis of the hydrazido-substituted compounds reported here were
prepared from Ta(NMe2)5 and Me2NNH2, respectively, according
to published procedures [5–7] with the Ta(NMe2)5 starting mate-
rial synthesized from TaCl5 and LiNMe2 employing the modifica-
tion of Rothwell and co-workers [8]. The TaCl5 was purchased
from Pressure Chemical, and the chemicals LiNMe2 (95%),
Me2NNH2, and Me3SiCl employed in our study were obtained
from Aldrich Chemical. The latter two liquids were distilled from
an appropriate drying agent prior to use, and the LiNMe2 was
stored in the drybox (HE Series double box) and used without fur-
ther purification. All syntheses and sample handling were per-
formed in the drybox given the extreme sensitivity of the title
tantalum compounds to moisture and oxygen. The reaction sol-
vents were distilled from an appropriate drying agent under ar-
gon using Schlenk techniques and stored in Schlenk storage
vessels equipped with high-vacuum Teflon stopcocks [9]. The
benzene-d6 and toluene-d8 NMR solvents were >99% in deuterium
and were distilled from sodium/benzophenone by bulb-to-bulb
distillation prior to their use. Multiple attempts to secure accept-
able combustion analyses on the title compounds were made
(Schwarzkopf and Galbraith Laboratories) but the samples consis-
tently gave low C and H data.
2.4. X-ray diffraction structures
Single crystals of TaCl(NMe2)3[N(TMS)NMe2] and Ta(NMe2)4-
[N(TMS)NMe2] suitable for diffraction analysis were grown in the
drybox at ꢀ30 °C from a pentane solution containing each com-
pound. The X-ray quality crystals were collected from Paratone
oil at ca. ꢀ20 °C using a nylon CryoLoop and rapidly transferred
under the cold nitrogen stream of the diffractometer to the goni-
ometer without excessive deterioration. The X-ray data for both
compounds were collected on an APEX II CCD-based diffractome-
ter at 100 K. The frames were integrated with the available APEX
2
[10] software package using a narrow-frame algorithm [11], and
the structures were solved and refined using the SHELXTL program
package [12]. The Ta(NMe2)4[N(TMS)NMe2] crystal exists as a
two-fold rotation twin about the crystallographic a-axis. The
twinning of Ta(NMe2)4(N(TMS)NMe2) represents a rare example
of a reticular polyholohedry twin, whose origin is traced to the
fact that the c-axis length in the orthorhombic crystal is close
to the multiples of the a and b axes [ratio 2.095:1.00 and
3.024:1.00]. The resulting twin lattice maintains the point group
symmetry but a different orientation of the individual lattice
[13]. The crystal structure of Ta(NMe2)4[N(TMS)NMe2] was solved
initially by using averaged intensity data from both domains.
Structure refinement used the unmerged data (HKL5 option in
SHELXL97) with the twin ratio converging to 0.63:0.37 for the two
domains in the final structure refinement. Each molecular struc-
ture was solved by direct methods with all nonhydrogen atoms
refined anisotropically and then checked by using PLATON [14].
Table 1 summarizes the X-ray processing and data collection
parameters.
The reported 1H NMR spectra were recorded at 500 MHZ on a
Varian VXR-500 spectrometer with the reported chemical shifts
referenced against the residue protons of the NMR solvent. The re-
ported mass spectral data (EI spectra) were collected at the UBC
mass spectrometry facility that is equipped to handle highly
air- and moisture-sensitive samples. The EI mass spectra were
recorded on a Kratos MS50 double-focusing mass spectrometer
configured with a MASPEC data system. The samples were handled
under dry nitrogen and introduced via a direct insertion probe,
employing a source temperature of 120 °C and an ionization
energy of 70 eV.
2.5. Atomic layer deposition (ALD) equipment
2.2. Synthesis of TaCl(NMe2)3[N(TMS)NMe2] from TaCl(NMe2)4 and
(TMS)NHNMe2
ALD experiments were carried out in a flow-type horizontal
hot-wall ALD-reactor (F-120 by ASM Microchemistry Ltd., Hel-
sinki, Finland) and the pressure during the deposition was 5–
10 mbar. The Ta(NMe2)4[N(TMS)NMe2] precursor was evaporated
from open crucibles kept at 90 °C. Ammonia (AGA Gas Ab,
99.998%) was introduced into the reactor through a mass flowme-
ter, a needle valve, and a solenoid valve, with the flow rate of
ammonia kept maintained at 10 sccm (standard cubic centimetre
per minute). Nitrogen (>99.999%, NITROX UHPN 3000 N2-genera-
tor) was used as a carrier and purging gas. Films were deposited
onto as-received Si(1 0 0)-substrates measuring 5 ꢁ 5 cm2. The
applied growth temperature range was 275–300 °C. A few oxide
films were grown for comparison, and water was employed as
the oxygen source. The water was held in an external reservoir
at room temperature and led into the reactor through needle
and solenoid valves. The growth temperature for the Ta2O5 films
was 200–250 °C.
To
a
medium-walled Carius tube was charged 1.10 g
(2.80 mmol) of TaCl(NMe2)4, after which 40 mL of toluene was
added via syringe. The solution was next treated with excess
(TMS)NHNMe2 (2.00 g; 15.1 mmol) and the vessel sealed and
heated at ca. 60 °C for 48 h. Upon cooling, the volatiles were re-
moved under vacuum to afford a yellow-brown solid that was then
taken up in ca. 15 mL of pentane and filtered prior to recrystalliza-
tion at ꢀ30 °C. TaCl(NMe2)3[N(TMS)NMe2] was isolated by filtra-
tion at 0 °C as a yellow solid in 73% yield (98 mg). 1H NMR
(toluene-d8, 298 K): d 0.292 (s, 9H, TMS), 2.60 (s, 6H, Me2NN),
3.39 (bs, 18H, Me2N). EI-MS: m/e 479/481 [M]+ consistent with
chlorine isotopomers and 444 [M–Cl]+.
2.3. Synthesis of Ta(NMe2)4[N(TMS)NMe2] from
TaCl(NMe2)3[N(TMS)NMe2] and LiNMe2
The thickness, roughness, and density of the films were evalu-
ated by X-ray reflectometry (XRR) using a Bruker D8 Advance X-
ray diffractometer. Crystallinity of the films was studied using a
Pananalytical X’pert Pro MPD X-ray diffractometer in grazing inci-
dence mode (GIXRD).
To 0.60 g (1.3 mmol) of TaCl(NMe2)3[N(TMS)NMe2] in 100 mL of
pentane at ꢀ30 °C was added a slight excess of LiNMe2 (84 mg,
1.6 mmol) in a single portion. The solution was allowed to warm

1756
S.-H. Huang et al. / Polyhedron 29 (2010) 1754–1759
Table 1
X-ray crystallographic data and processing parameters for TaCl(NMe2)3[N(TMS)NMe2] and Ta(NMe2)4[N(TMS)NMe2].
Compound
TaCl(NMe2)3[N(TMS)NMe2]
Ta(NMe2)4[N(TMS)NMe2]
CCDC No.
Crystal system
Space group
a (Å)
b (Å)
c (Å)
737 251
monoclinic
P2(1)/n
10.1898(4)
12.2418(5)
15.6803(6)
99.808(1)
1927.4(1)
737 252
orthorhombic
Pbca
14.196(1)
9.8437(8)
29.762(2)
b (°)
V (Å3)
4159.0(6)
formula
C11H33ClN5SiTa
C13H39N6SiTa
Formula weight
Formula units per cell (Z)
Dcalc (Mg/m3)
k (Å)
479.91
4
1.654
0.71 073
488.54
8
1.560
0.71 073
l
(mmꢀ1
)
5.901
5.348
Absorption correction
F (0 0 0)
semi-empirical from equivalents
952
semi-empirical from equivalents
1968
Crystal size (mm)
Absorption correction factor
Total reflections
0.31 ꢁ 0.25 ꢁ 0.16
0.4445/0.2579
22 833
0.27 ꢁ 0.14 ꢁ 0.08
0.6804/0.3241
10 398
Independent reflections
4090
10 341
Data/restraints/parameters
4090/0/183
0.0105
0.0268
1.065
0.620 and ꢀ0.406
10 398/0/204
0.0274
0.0347
0.919
1.249 and ꢀ1.036
a
R1 (I P 2
r
(I)]
b
wR2
Goodness-of-fit (GOF) on F2
Largest difference in peak and hole (e/Å3)
P
P
a
R1
R2
¼
kF0j ꢀ jFck= jFoj:
ꢄ
ꢂ
ꢃꢅ
1=2
ꢀ
ꢁ
h
i
2
P
P
2
F2o ꢀ F2c
=
wðF2o Þ
b
w
0.29, 2.60, and 3.39 in a 3:2:6 ratio for the methyl groups associ-
ated with TMS, hydrazido, and amido moieties, respectively. The
broadened amido resonance (full width at half maximum = 59 Hz)
and the observation of only two methyl singlets for the amido and
hydrazido ligands signal the rapid rotation of these groups about
the Ta–N bonds.
A VT NMR study of TaCl(NMe2)3[N(TMS)NMe2] was performed
in toluene-d8 in order to more thoroughly study the fluxional
NMR behavior. Selected 1H NMR spectra over the temperature
range 293–193 K are shown in Fig. 1. Not shown in these spectral
traces is the TMS resonance whose frequency is independent of
3. Results and discussion
3.1. Synthesis, spectroscopic data, and X-ray diffraction structure of
TaCl(NMe2)3[N(TMS)NMe2]
Treatment of TaCl(NMe2)4 with a measured excess of trimethyl-
silyl(dimethyl)hydrazine at room temperature in pentane fur-
nishes the hydrazido(I) compound TaCl(NMe2)3[N(TMS)NMe2] in
>95% yield after recrystallization from pentane at ꢀ30 °C. The
accompanying by-product formed in this reaction is Me2NH which
is readily removed under vacuum during the work-up procedure.
The hydrazinolysis reaction employed in the synthesis of TaCl(N-
temperature (Dd < 0.05). Lowering the temperature to 243 K pro-
Me2)3[N(TMS)NMe2] is depicted in Eq (1)1
.
motes the growth of a second amido resonance at d 3.10 and the
coalescence of the methyl groups belonging to the hydrazido li-
gand. Distinct hydrazido methyl groups become evident as the
temperature is further lowered and the slow-exchange limit is
Me
NMe2
Ta
NMe2
Me2N
Me2N
Me2N
Me2N
(TMS)NHNMe2
in pentane
N
N
Me
Ta
Cl
NMe2
-Me2NH
TMS
Cl
ð1Þ
TaCl(NMe2)3[N(TMS)NMe2] exists as a yellow solid that is fairly
air- and moisture-sensitive and readily soluble in toluene and pen-
tane solvents. The product was characterized by mass spectrome-
try, 1H NMR spectroscopy, and X-ray diffraction analysis. The EI
mass spectrum of TaCl(NMe2)3[N(TMS)NMe2] revealed a weak
set of m/e peaks at 479/481 for the molecular ion associated with
the two chlorine isotopomers and a m/e peak at 444 for the species
[M–Cl]+ consistent with the formulated structure. The room tem-
perature 1H NMR spectrum in C6D6 revealed three singlets at d
1
We have shown the TaCl(NMe2)4 in Eq. (1) in its monomeric form based on the
recent report by Xue (Ref. [6]) but also point out that the dimeric form of this
compound has been determined earlier by Chisholm (Ref. [5]). Details concerning the
monomer-dimer equilibrium are found in the former reference.
Fig. 1. VT 1H NMR spectra of TaCl(NMe2)3[N(TMS)NMe2] in toluene-d8 from d 4.00
to 1.70 at (a) 193 K, (b) 223 K, (c) 243 K, (d) 273 K, and (e) 293 K.

S.-H. Huang et al. / Polyhedron 29 (2010) 1754–1759
1757
donor ligands and the hydrazido and chlorine groups are counted
as 3e and 1e donor groups, respectively. While this simplistic view
is certainly valid in the case of a hydrazido ligand that contains two
pyramidal nitrogen atoms, the planar amido-like N(4) hydrazido
atom adds dimension to the bonding situation. Here a donation
of up to twelve electrons may be expected from the three NMe2
groups and the amido-like N(4) hydrazido atom. The sum of the
angles about the N(1), N(2), and N(3) centers is ca. 360°, a feature
commonly displayed by planar amido ligands where both
r and p
manifolds contribute to metal–nitrogen bonding [18]. The
Ta–N(amido) bond lengths range from 1.974(2) Å [Ta(1)–N(2)] to
2.003(1) Å [Ta(1)–N(1)] and are comparable to those distances in
[TaCl(NMe2)3]2 [5], TaCl(NMe2)4 [6] and TaCl(NMe2)3[(iPrN)2CNH-
Pri] [19]. The 2.5499(4) Å bond length exhibited by the Ta(1)–
Cl(1) bond is unremarkable with respect to those Ta–Cl distances
reported for the monochloro Ta(V) compounds TaCl(NMe2)4 [6],
TaCl(NMe2)3[N(TMS)2] [20], TaCl(NMe2)3(SiPh2But) [21]. Structur-
ally characterized compounds containing an ancillary trimethyl-
silyl(dimethyl)hydrazido(I) ligand are few, and a check of the
Cambridge Structural Database (version 5.30, May 2009 update)
reveals only eight examples. Of the eight, six are tantalum based
and derive from recent work published by the Fischer and Sunder-
meyer groups [2g,4b]. The hydrazido ligand in TaCl(NMe2)3-
[N(TMS)NMe2] is bound to the tantalum center in an asymmetric
side-on or g2 fashion. The Ta(1)–N(4) bond distance [1.999(1) Å]
is 0.283 Å shorter than the Ta(1)–N(5) vector [2.282(1) Å] but in
keeping with those bond length trends found in Ta(NBut)-
(NMe2)2[N(TMS)NMe2] [2g], TaCl3[N(TMS)NMe2]2 [4b], and
Ta(NBut)(NMe2)[N(TMS)NMe2]2 [4b]. As mentioned earlier, the
Fig. 2. Thermal ellipsoid plot of the molecular structure of TaCl(NMe2)3-
[N(TMS)NMe2] at the 50% probability level. Selected bond distances (Å) and angles
(°) are: Ta(1)–N(1) = 2.003(1), Ta(1)–N(2) = 1.974(1), Ta(1)–N(3) = 1.992(1),
Ta(1)–N(4) = 1.999(1), Ta(1)–N(5) = 2.282(1), Ta(1)–Cl(1) = 2.5499(4), N(2)–Ta(1)–
N(1) = 116.30(6),
N(4)–Ta(1)–N(5) = 38.89(5),
N(4)–Ta(1)–N(1) = 111.14(5),
N(2)–Ta(1)–N(5) = 93.39(5), N(3)–Ta(1)–Cl(1) = 176.77(4).
approached. The two hydrazido methyls recorded at d 2.84 and
2.07 at 243 K, of which the latter is partially obscured by the tolu-
ene solvent (d 2.09), are in agreement with the weigthed-average
N(4) hydrazido atom is trigonal planar [
R of the angles at
chemical shift of these groups at room temperature. The
D
Gà value
N(4) ꢂ 357°]. This represents an ideal geometry for the N(4) atom
in the crowded coordination environment about the Ta(1) center
because pyramridalization of this nitrogen will lead to unfavorable
van der Waals’ contacts between the TMS group and the ligands
syn to the N(4) moiety.
for the equilibration of the hydrazido methyl groups may be esti-
mated as 10.5(5) kcal/mol based on a the separation frequency of
the participant resonances and a coalescence temperature of
243 K [15].2 The three discernible NMe2 singlets at d 3.05, 3.49,
and 3.60 at 193 K confirm that the rotation of the amido groups
about the Ta–N bonds is still rapid and in keeping with the earlier
observations of Chisholm et al. [5]. The equilibration of the hydraz-
3.2. Synthesis, spectroscopic data, and X-ray diffraction structure of
Ta(NMe2)4[N(TMS)NMe2]
ido methyl groups is likely initiated by an g2
coordination mode of the hydrazido ligand [16]. The resulting five-
coordinate TaCl(NMe2)3[
1-N(TMS)NMe2] intermediate could un-
?
g1 change in the
Metathetical replacement of the lone chloride ligand in TaCl(N-
Me2)3[N(TMS)NMe2] with LiNMe2 occurs in pentane to yield Ta(N-
Me2)4[N(TMS)NMe2] as an extremely air- and moisture-sensitive
pale yellow solid as the sole observable product (Eq. (2)). In princi-
ple, the substitution of the chloride group in TaCl(NMe2)4 by the
hydrazide anion [N(TMS)NMe2]ꢀ should also furnish the all-nitro-
gen coordinated product Ta(NMe2)4[N(TMS)NMe2], and this alter-
native reaction was examined during the early phases of this
work. Treatment of TaCl(NMe2)4 with [N(TMS)NMe2][Li], the latter
which was prepared in situ in pentane using freshly titrated EtLi, is
far from clean and gives a mixture of TaCl(NMe2)3[N(TMS)NMe2]
(22%), Ta(NMe2)4[N(TMS)NMe2] (29%), and Ta(NMe2)5 (27%) as as-
sessed by 1H NMR spectroscopy using 4,40-di-tert-butylbiphenyl as
an internal standard.3 The presence of Ta(NMe2)5 implies a formal
transfer of an amido group during the course of the reaction presum-
ably through a ligand redistribution sequence involving a dimeric
species. Ample precedents exist for related redistributions [5,6,22].
Given the problematic work up of such a ternary mixture, this pro-
tocol for the preparation of Ta(NMe2)4[N(TMS)NMe2] was not pur-
sued further.
g
dergo a Berry pseudorotation, which would lead to complete scram-
bling of all of the groups about the tantalum center. Support for a
coordinatively flexible hydrazido ligand is underscored by the recent
report of Wadephol and Gade, where shallow barriers for bridge-to-
linear exchanges of hydrazido ligands at Zr(IV) and Hf(IV) derivatives
have been demonstrated by a combination of experimental and com-
putational methodologies [17]. The g2
?
g1 transformation would
be facilitated by the donation of electrons from the TMS-substi-
p
tuted hydrazido nitrogen to the tantalum center, thus avoiding the
formation of a coordinatively unsaturated intermediate.
The solid-state structure of TaCl(NMe2)3[N(TMS)NMe2] was
determined via X-ray crystallography. Fig. 2 shows the thermal
ellipsoid plot of the molecular structure and confirms the six-coor-
dinate geometry about the tantalum center and the presence of an
g
2-hydrazido group and three amido ligands, the latter which ex-
hibit a fac disposition about the tantalum octahedron. On cursory
inspection, TaCl(NMe2)3[N(TMS)NMe2] contains 18e and is coord-
inatively saturated if the three amido groups are viewed as 3e
2
Here the free energy of activation has been computed using the Eyring equation:
3
D
Gà = RTc[ln(kBTc/hkc)], where the physical constants kB and h have their normal
We also believe that small amounts of TaCl2(NMe3)3 and [TaCl2(NMe3)3]2 are also
meanings and Tc and kc represent the coalescence temperature and the rate constant
at coalescence. The value for kc has been estimated as a function of according to
the equation: kc = (0.5)1/2
formed during the reaction but the insolubility of these species hampers a
quantitative mass balance of the reaction products. See Ref. [5] for a discussion on
the synthesis and solubility issues related to [TaCl2(NMe3)3]2.
D
m
pDm.

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S.-H. Huang et al. / Polyhedron 29 (2010) 1754–1759
Fig. 3. VT 1H NMR spectra of Ta(NMe2)4[N(TMS)NMe2] in toluene-d8 from d 3.80 to
2.20 at (a) 193 K, (b) 223 K, (c) 253 K, and (d) 293 K.
Fig. 4. Thermal ellipsoid plot of the molecular structure of Ta(N-
Me2)4[N(TMS)NMe2] at the 50% probability level. Selected bond distances (Å) and
angles (°) are: Ta(1)–N(1) = 2.064(2), Ta(1)–N(2) = 2.060(2), Ta(1)–N(3) = 2.006(2),
Ta(1)–N(4) = 1.985(2), Ta(1)–N(5) = 2.314(2), Ta(1)–N(6) = 2.013(2), N(4)–Ta(1)–
N(3) = 111.16(9), N(3)–Ta(1)–N(6) = 115.18(8), N(4)–Ta(1)–N(5) = 95.50(8), N(6)–
Ta(1)–N(5) = 38.59(7), N(2)–Ta(1)–N(1) = 178.81(9).
Me
Me
NMe2
Ta
NMe2
Ta
Me2N
Me2N
Me2N
Me2N
LiNMe2
N
N
N
N
in pentane
Me
Me
-LiCl
TMS
TMS
planes containing the C(1)–N(1)–C(2) and C(3)–N(2)–C(4) atoms.
The mean distance of 2.062 Å for the axial Ta(1)–N(1) and Ta(1)–
N(2) vectors is 0.066 Å longer than the mean bond distance associ-
ated with the equatorial Ta(1)–N(3) and Ta(1)–N(4) amido groups.
The idealized structure of Ta(NMe2)4[N(TMS)NMe2] may be recon-
ciled with the solution NMR behavior where a mirror plane coinci-
dent with the equatorial nitrogen atoms and the tantalum metal
generates equivalent hydrazido methyl groups. The Ta(1)–N(5)
[2.314(2) Å] and Ta(1)–N(6) [2.013(2) Å] bond distances are com-
parable to those distances reported for TaCl(NMe2)3[N(TMS)NMe2]
and the hydrazido(1) derivatives prepared by Fischer and Sunder-
meyer [2g,4b].
Cl
NMe2
ð2Þ
The EI mass spectrum of Ta(NMe2)4[N(TMS)NMe2] exhibited a
weak molecular ion at m/e 487 [M+] along with a peak at m/e
444 [M-NMe2]+ for the loss of a single amido group. Ta(NMe2)4-
[N(TMS)NMe2] exhibits three resonances in the 1H NMR spectrum
at d 0.293, 2.48, and 3.25 in a 3:2:8 integral ratio that are
assignable to the TMS, hydrazido, and the amido methyl groups,
respectively. Fig. 3 shows selected VT 1H NMR traces of Ta(NMe2)4-
[N(TMS)NMe2] where the fluxional behavior of the amido groups is
readily revealed. The existence of a rapid g2
?
g1 transformation
of the hydrazido moiety in Ta(NMe2)4-[N(TMS)NMe2], while likely,
cannot be established with certainty due to the presence of
chemical equivalent NMe2 groups in solution that are normal to
the N4 plane containing the hydrazido and equatorial amido
ligands. Qualitatively, the VT changes recorded for Ta(NMe2)4-
[N(TMS)NMe2] parallel the NMR behavior already described for
the corresponding monochloro species. The NMe2 resonance
belonging to the hydrazido ligand is temperature invariant but
lowering the temperature to 193 K leads to the clear separation
of the four amido groups that appear as three separate 6H singlets
at d 3.10, 3.30, and 3.58, along with two 3H singlets at d 3.34 and
3.54. The observation of two distinct methyl groups confirms the
restricted rotation about the Ta–N bond of one of the amido moi-
eties. While the identity of this particular amido ligand remains
3.3. Preliminary ALD results on the preparation of TaN and Ta2O5 thin
films employing Ta(NMe2)4[N(TMS)NMe2]
The ability of Ta(NMe2)4[N(TMS)NMe2] to serve as a single-
source precursor for the deposition of TaN thin films was initially
explored using ammonia as a carry gas and reducing agent. The ini-
tial ALD experiments conducted at 300 °C confirmed the unex-
pected decomposition of Ta(NMe2)4[N(TMS)NMe2] and no
noticeable TaN film formation. These studies signaled the limited
stability of Ta(NMe2)4[N(TMS)NMe2] at elevated temperatures,
and this fact was subsequently verified by controlled sublimations
where residues on the order of 25–40% were found over the tem-
perature range of 125–200 °C and ca. 0.1 Torr. Additional ALD
experiments conducted at 250 °C and 225 °C led only to the
decomposition of Ta(NMe2)4[N(TMS)NMe2]. Given the limited
stability of Ta(NMe2)4[N(TMS)NMe2] and the fact that ammonia
activation requires deposition temperatures at or above 300 °C
[2j,3], further experiments on the preparation of tantalum nitride
films were abandoned. Suitable modification of Ta(NMe2)4-
[N(TMS)NMe2] using more highly substituted amido and hydrazi-
do ligands should afford more robust Ta(V) derivatives capable of
surviving the required ALD deposition conditions.
equivocal, the
D
Gà value for rotation of the methyl groups associ-
ated with this amido moiety may be estimated as ca.
11.1(5) kcal/mol based on a the separation frequency of the partic-
ipant resonances and a coalescence temperature of 253 K [15].
Fig. 4 shows the molecular structure of Ta(NMe2)4[N(TMS)-
NMe2] and confirms the all-nitrogen coordination environment
about the Ta(1) center. To our knowledge, this represents the first
structurally characterized paradigm of an octahedral monohydraz-
ido(I) compound that also contains four amido ligands in the pri-
mary coordination sphere of the central metal. The N(1) and N(2)
amido ligands are situated trans to each other based on the bond
angle of 178.81(9) ° subtended at the metal. The methyl groups
associated with each of the axial amido ligands are staggered on
the basis of the dihedral angle of 50.9(1) ° that is defined by the
Finally, the deposition of Ta2O5 thin films was also investigated
at a silicon substrate at 200 °C. Heating Ta(NMe2)4[N(TMS)NMe2]
at 90 °C and H2O at 25 °C led to the formation of amorphous
Ta2O5 thin films, as shown by the featureless grazing incidence

S.-H. Huang et al. / Polyhedron 29 (2010) 1754–1759
1759
Office of Science, under Contract No. DE-AC05-00OR22725 man-
aged by UT Battelle, LLC. We thank Prof. David M. Hoffman for pro-
viding us with a copy of the 1H NMR spectrum of Ta(NMe2)5 and
Dr. Yun Ling (UBC) for recording the EI mass spectra of TaCl(N-
Me2)3[N(TMS)NMe2] and Ta(NMe2)4[N(TMS)NMe2].
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Financial support from the Robert A. Welch Foundation (Grant
B-1093-MGR) and LAM Semiconductor is greatly appreciated. X.
Wang acknowledges the support by the U.S. Department of Energy,
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