Bis(β-ketoiminate) dioxo tungsten(VI) complexes as precursors for growth of WOx by aerosol-assisted chemical vapor deposition

Article abstract of DOI:10.1016/j.poly.2019.05.013

Bis(β-ketoiminate) dioxo tungsten(VI) complexes WO₂L₂ (L = acNac, acNac^Me, acNac^Et) have been synthesized and characterized. The thermal behaviors and possible decomposition mechanisms of these dioxo compounds were studied by TGA, MS and thermolysis. The performance of WO₂(acNac^Me)₂ as an AACVD precursor was evaluated at growth temperatures of 250–550 °C using nitrogen as the carrier gas. The elemental compositions of the as-deposited and sputtered tungsten oxide films were determined by XPS. The morphology and stoichiometry of the as-deposited films were studied by SEM and XRD.

Full text of DOI:10.1016/j.poly.2019.05.013

Polyhedron 169 (2019) 219–227
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Bis(b-ketoiminate) dioxo tungsten(VI) complexes as precursors for
growth of WOx by aerosol-assisted chemical vapor deposition
⇑
Xiaoming Su, Persi Panariti, Khalil A. Abboud, Lisa McElwee-White
Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
a r t i c l e i n f o
a b s t r a c t
Bis(b-ketoiminate) dioxo tungsten(VI) complexes WO2L2 (L = acNac, acNacMe, acNacEt) have been synthe-
Article history:
Received 17 March 2019
Accepted 7 May 2019
Available online 16 May 2019
sized and characterized. The thermal behaviors and possible decomposition mechanisms of these dioxo
compounds were studied by TGA, MS and thermolysis. The performance of WO2(acNacMe
)2 as an AACVD
precursor was evaluated at growth temperatures of 250–550 °C using nitrogen as the carrier gas. The ele-
mental compositions of the as-deposited and sputtered tungsten oxide films were determined by XPS.
The morphology and stoichiometry of the as-deposited films were studied by SEM and XRD.
Ó 2019 Elsevier Ltd. All rights reserved.
Keywords:
Tungsten oxide
Aerosol-assisted chemical vapor deposition
Single source precursors
Metal oxo complexes
Metal oxide films
1. Introduction
requirements and thus allows more flexibility in precursor design
[10]. In this work we explore the use of nonvolatile tungsten oxo
In recent years, much attention has been given to transition
metal oxides (TMOs) as a result of their unique structural, physical
and chemical properties. Tungsten oxide (WOx) is one of the
technically appealing multifunctional TMOs with a broad variety
of stoichiometries, structures and morphologies depending on
processing methods and conditions, which enables applications
in various fields such as electrochromic devices [1,2] and organic
electronics [3]. The compositional diversity of WOx originates from
the existence of various Magnéli phases (2.625 ꢀ x ꢀ 2.92, e.g.,
complexes to grow WOx by AACVD.
Single source metal–organic CVD (MOCVD) precursors for the
growth of tungsten oxide include both homoleptic and heteroleptic
complexes. Homoleptic precursors are typically alkoxides or ary-
loxides, e.g., W(OEt)5 and W(OC6H3F2-3,4)6, which can be either
fluorinated or nonfluorinated [8,11]. There is a wider range of
heteroleptic candidates that contain various ligands such as oxo,
aminoalkoxide, alkoxide and b-diketonate. The incorporation of
oxo groups into the tungsten complex takes advantage of existing
strong metal–oxygen bonds, which reduces the need for bond
cleavage steps and facilitates the precursor decomposition process
during material growth. Monooxo complexes of the type WO(OR)4
(R = alkyl) and WO(OR)3L (R = alkyl, L = aminoalkoxide, b-diketo-
nate/b-ketoesterate, b-ketoiminate/b-iminoesterate) have been
characterized and examined as MOCVD precursors; while the
study of dioxo tungsten CVD precursors is more limited [12–18].
Due to the interest in the N,O-chelating b-ketoiminate/
b-iminoesterate ligands, which have tunable steric and electronic
properties, we have reported use of the tungsten oxo complexes
WO(OR)3L (R = alkyl; L = b-ketoiminate/iminoesterate) as AACVD
precursors for the growth of WOx [15]. Probable decomposition
mechanisms for these monooxo complexes were proposed but it
is not known whether analogous well-defined decomposition
channels are available for bis(b-ketoiminate) dioxo tungsten
complexes. Herein, we report the synthesis and characterization
W32O84, W3O8, W18O49, W20O58, and W25O73) as well as the ease
of generating oxygen defects [4]. Besides chemical composition,
features such as crystalline phases, crystallinity and nanostruc-
tures can all affect the properties of the material [5,6].
Chemical vapor deposition (CVD) is a process of depositing solid
materials by chemical reaction of precursors near or on the sub-
strate surface. This bottom up fabrication method not only can
deliver large-scale conformal coatings even on patterned sub-
strates, but also provides the possibility of controlling composition,
doping and morphology [7–9]. Use of traditional CVD volatilization
techniques such as bubblers requires the precursors to have suffi-
ciently high vapor pressures for efficient delivery. In contrast, aero-
sol-assisted chemical vapor deposition (AACVD), in which
precursor solutions are volatilized as an aerosol generated by an
ultrasonic transducer, does not have stringent precursor volatility
of tungsten dioxo complexes WO2(acNac)2 (1), WO2(acNacMe
)
2
⇑
(2) and WO2(acNacEt)2 (3) as well as a demonstration of AACVD
growth of tungsten oxide from 2.
Corresponding author.
E-mail address: lmwhite@chem.ufl.edu (L. McElwee-White).
https://doi.org/10.1016/j.poly.2019.05.013
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

220
X. Su et al. / Polyhedron 169 (2019) 219–227
obtained. 1H NMR (500 MHz, 25 °C, CDCl3): major isomer A
2. Experimental
2.1. General considerations
(ꢁ90%): d 5.48 (s, 2H, CHCO), 4.06 (m, 1H, NCHH), 3.96 (m, 1H,
NCHH), 2.16 (s, 6H, COCH3), 2.10 (s, 6H, CH3CN), 1.35 (t, 6H, NCH2-
CH3, J = 7.1 Hz). Isomer B (ꢁ10 %, partial spectrum): d 5.42 (s, 2H,
CHCO), 4.26 (m, 1H, NCHH), 2.18 (s, 3H, COCH3), 2.10 (s, 3H, CH3-
CN), 1.31 (t, 6H, NCH2CH3, J = 7.2 Hz). 13C{1H} (500 MHz, 25 °C,
CDCl3) major isomer A: d 181.74 (CO), 171.93 (CN), 107.04 (CHCN),
52.24 (NCH2CH3), 25.66 (COCH3), 21.82 (CNCH3), 13.69 (NCH2CH3).
MS (ESI) m/z Calc. for WO4C12H21N2 [M+H]+: 469.1321. Found:
Methylene chloride, hexane and toluene (Fisher Scientific) were
purified using an MBraun MB-SP solvent purification system and
stored over activated 3 Å molecular sieves. Diethyl ether and
tetrahydrofuran were dried using sodium/benzophenone, distilled,
and stored over activated 3 Å molecular sieves before use. Hexam-
ethyldisiloxane, methanol, chloroform-d1 (Cambridge Isotopes)
and benzene-d6 (Cambridge Isotopes) were stored over 4 Å molec-
ular sieves. Triethylamine was distilled over potassium hydroxide
and stored over 4 Å molecular sieves. The compounds WOCl4
[19], WO2Cl2(dme) [20], acNacH [21], acNacMeH [22] and acNacEtH
[23] were synthesized according to literature procedures. All other
solvents and reagents were of analytical grade and were used as
received. NMR spectra were recorded on a Varian INOVA 500 spec-
trometer using residual protons from deuterated solvents for refer-
ence. Thermogravimetric analysis was run using a TA Q5000 under
nitrogen at a temperature scan rate of 10 °C/min to 600 °C. Mass
spectrometry was performed on an Agilent Technologies 6210
Time of Flight instrument using direct analysis in real time. Ele-
mental analyses were performed by Robertson Microlit Laborato-
ries (Ledgewood, NJ).
469.1323. FTIR (solid sample): 929 cmÀ1 (W@O,
886 cmÀ1 (W@O,
(asym)).
m(sym)),
m
2.3. Film deposition and characterization
Deposition of WOx was carried out using a custom-built, verti-
cal cold-wall impinging-jet AACVD reactor [24]. Silicon substrates
with native silicon dioxide (Si/SiO2, n-type, (100)) were cleaned
in boiling acetone, methanol and deionized water for 3 min each,
and then placed on a SiC covered graphite susceptor in the reaction
chamber under vacuum. A 0.025 M solution of 2 in diglyme was
prepared and transferred into a 10 mL gastight Hamilton syringe.
The precursor solution was introduced into a nebulizer with a
quartz plate vibrating at 1.44 MHz by a syringe pump with an
injection rate of 4 mL hÀ1. The generated aerosol was delivered to
the reaction chamber with nitrogen (1000 sccm, 99.999% purity,
Airgas) carrier gas. During deposition (150 min), the reactor pres-
sure was maintained at 350 Torr and the growth temperature
was controlled by a radio frequency (RF) heating system on the
susceptor.
2.2. Synthesis
2.2.1. WO2{HC[MeC(O)][MeC(NH)]}2 (1)
In the glovebox, 1.01 g (2.68 mmol) WO2Cl2(dme) was sus-
pended in 40 mL THF in a Schlenk flask. Then NEt3 (1.80 mL,
5 eq.) was mixed with 0.532 g (5.37 mmol) acNacH, and 10 mL
THF, which was transferred to an addition funnel. The funnel was
attached to the Schlenk flask and transferred to the Schlenk line.
The WO2Cl2(dme)/THF mixture was cooled in a dry ice/acetone
bath. The ligand/NEt3 solution was added to the flask dropwise.
The resulting mixture was then left to warm to room temperature
and stir for 12 h. Volatiles were removed under vacuum and the
flask was transferred to the glovebox. The product was extracted
with THF and filtered through Celite giving a yellow solution. The
solvent was removed in vacuo. The crude product was washed with
hexane. Yellow solid (0.41 g, 37%) was obtained. 1H NMR
(500 MHz, 25 °C, CDCl3): d 7.62 (b, 2H, NH), 5.35 (s, 2H, CHCO),
2.15 (s, 6H, COCH3), 2.05 (s, 6H, CH3CN). 13C{1H} (500 MHz, 25 °C,
CDCl3): d 185.24 (CO), 172.47 (CN), 102.29 (CHCN), 27.03 (COCH3),
25.86 (CNCH3). MS (ESI) m/z Calc. for WO4C10H18N2 [M+H]+:
413.0694. Found: 413.0765. FTIR (solid sample): 933 cmÀ1
Elemental compositions of the deposits were determined using
ULVAC-PHI XPS Al Ka radiation. O 1s (530.5 eV) or adventitious C
1s (284.8 eV) was used as an internal standard [13,25]. The XPS
spectra of the W 4f core level were deconvoluted with parameters
of spin–orbit separation
DEP (4f5/2–4f7/2) = 2.18 eV and the W 4f5/2-
to-W 4f7/2 peak ratio = 3:4. The crystallinities and morphologies
were measured using field emission scanning electron microscopy
(FESEM, FEI Nova NanoSEM 430) and X-ray diffraction (XRD, Pan-
alytical X’pert Pro).
2.4. Crystallographic structure determination of 2
X-ray Intensity data were collected at 100 K on a Bruker DUO
diffractometer using Mo K
a radiation (k = 0.71073 Å) and an
APEXII CCD area detector. Raw data frames were read by the pro-
1
gram SAINT and integrated using 3D profiling algorithms. The
resulting data were reduced to produce hkl reflections and their
intensities and estimated standard deviations. The data were cor-
rected for Lorentz and polarization effects and numerical absorp-
tion corrections were applied based on indexed and measured
faces.
(W@O, m m(asym)).
(sym)), 892 cmÀ1 (W@O,
2.2.2. WO2{HC[MeC(O)][MeC(NMe)]}2 (2)
Compound 2 was synthesized following the same procedure as
used for 1. Starting with 1.00 g (2.65 mmol) WO2Cl2(dme) and
0.613 g (5.42 mmol) acNacMeH, yellow solid (0.82 g, 70%) was
obtained. 1H NMR (500 MHz, 25 °C, CDCl3): d 5.52 (s, 2H, CHCO),
3.64 (s, 6H, NCH3), 2.11 (s, 6H, COCH3), 2.11 (s, 6H, CH3CN). 13C
{1H} (500 MHz, 25 °C, CDCl3): d 182.27 (CO), 173.06 (CN), 106.90
(CHCN), 45.57 (NCH3), 25.68 (COCH3), 22.78 (CNCH3). MS (ESI) m/
z Calcd for WO4C12H21N2 [M+H]+: 441.1007. Found: 441.1024.
Anal. Calc. for WO4C12H20N2: C, 32.75; H, 4.58; N, 6.36. Found: C,
The structure was solved and refined in SHELXTL2014 [26], using
full-matrix least-squares refinement. The non-H atoms were
refined with anisotropic thermal parameters and all of the H atoms
were calculated in idealized positions and refined riding on their
parent atoms. In the final cycle of refinement, 3477 reflections
(of which 3371 are observed with I > 2r(I)) were used to refine
178 parameters and the resulting R1, wR2 and S (goodness of fit)
were 0.95%, 2.29% and 1.073, respectively. The refinement was car-
ried out by minimizing the wR2 function using F2 rather than F val-
ues. R1 is calculated to provide a reference to the conventional R
value but its function is not minimized.
32.46; H, 4.63; N, 6.13%. FTIR (solid sample): 929 cmÀ1 (W@O,
(sym)), 889 cmÀ1 (W@O,
(asym)).
m
m
2.2.3. WO2{HC[MeC(O)][MeC(NEt)]}2 (3)
2.5. Thermolysis studies
Compound 3 was synthesized following the same procedure as
used for 1. Starting with 0.497 g (1.32 mmol) WO2Cl2(dme) and
0.338 g (2.65 mmol) acNacEtH, yellow solid (0.16 g, 24%) was
The sample (complex 2 or the protonated free ligand acNacMeH)
was placed in a 10 mL headspace vial which was heated in the GC

X. Su et al. / Polyhedron 169 (2019) 219–227
221
oven at 250 °C for 30 min and then cooled to 35 °C. Once cool, the
vial was removed from the oven and placed in a sand bath until
sampled. A 100-lL sample of the headspace gases was taken with
a gas-tight syringe and injected in split mode for GC/EI-MS. Mass
spectrometry was conducted on a ThermoScientific (San Jose, CA)
DSQ II using electron ionization (70 eV) and an ion source at
250 °C. Gas chromatography was conducted on a ThermoScientific
Trace GC Ultra.
metal salt of the ligand, which was then added to the WO2Cl2(dme)
solution. The desired dioxo product formed but the yield was very
low and the product contained noticeable impurities. It has been
reported that lithium salts of the b-ketoimines dimerize easily
and the lithium halide is relatively soluble in organic solvents
[30,31]. The dimerization of the ligand salt and the relatively good
solubility of the byproduct (alkali halide) are not conducive to the
formation of the dioxo product, which can result in low yields.
3.3. X-ray crystallographic structure determination of 2
3. Results and discussion
Single crystals of complex 2 were grown by cooling a concen-
trated solution of 2 in THF to À3 °C. The crystal refinement data
are shown in Table 1 and the selected bond lengths and angles
are summarized in Table 2.
3.1. Precursor design
One of the primary requirements for single source precursors is
to incorporate all elements of the desired deposits within the com-
plex. Terminal oxo, alkoxide and other oxygen containing ligands
such as b-diketonate and aminoalkoxide are commonly used in
designing single source precursors to grow metal oxides [27]. In
order to maximize precursor properties, an approach of utilizing
heteroleptic ligand sets is widely adopted. The precursor design
process can be considered as selecting suitable types of ligands
and fine-tuning ligand structures to tailor electronic and steric
properties.
The molecular structure (Fig. 2) confirms that WO2(acNacMe)2 is
a cis-dioxo complex that adopts a significantly distorted octahedral
geometry around a W(VI) core coordinated by two bidentate b-
ketoiminate ligands. The W@O length is within the expected range
for cis-[WO2]2+ complexes [32,33]. The two enolate oxygens are
trans to the terminal oxo groups. In order to minimize the compe-
tition for available empty metal d orbitals, a weaker
p-donor is
favored in the trans position of the terminal oxo [34,35]. The two
imino nitrogens are bent away from the terminal oxo groups with
a relatively small N1AW1AN2 bond angle (161.06(5)°) deviating
from the ideal octahedral geometry as a consequence of steric
repulsion. It is worth mentioning that the two oxo groups have dif-
ferent effects on the imino group. For a specific imino group in the
chelating ligand, the oxo group that is aligned for overlap with the
Prior studies of WO(OR)3L (R = tBu, Me, L = b-ketoiminate, b-
iminoesterate) incorporated terminal oxo, alkoxide and chelating
mixed N/O ligands into the precursor [15]. The study of decompo-
sition mechanisms of these complexes indicates that the chelating
ligand or a tert-butoxide group can be protonated from an adjacent
tert-butoxide functionality with a loss of tert-butanol or HL and
isobutylene to generate WO2(OR)2 or WO2(OR)L species. Replacing
the alkoxides of WO2(OR)L or WO2(OR)2 with bidentate ligands
gives WO2L2 moieties which can be more stable due to their coor-
dinative saturation. Furthermore, the incorporation of the second
oxo group reduces the need for bond cleavage during deposition
compared with the mono oxo complexes; thus, faster growth rates
may be achieved.
p
conjugated system has greater influence than the other oxo func-
tionality that is orthogonal to the -system. For the imino group
containing atom N2, for example, the oxo ligand (W1@O2) aligned
with the b-ketoiminate -system (N2AC8AO4) has a stronger
p
p
repulsion on it leading to an increased bond angle (O2AW1AN2,
99.63(5)°) compared with the other terminal oxo functionality
(O1AW1AN2, 91.68(5)°).
The two chelating b-ketoiminates have the same bite angle of
78.57(4)° (O3AW1AN1 and O2AW1AN2), which is in agreement
with related literature [36,37]. Consistent with previous study of
the b-ketoiminate ligand in WO(OtBu)3(acNac), the C1AC2 bond
(1.423(2) Å) is ca. 0.5 Å longer than the C2AC3 bond (1.378(2) Å)
indicating the enolate instead of the enaminate being the major
resonance contributor to the overall complex structure.
Complexes of the type WO2L2 (L = bidentate ligands) have been
explored in photodeposition of WO3 (L = acetylacetonate) [28],
atomic layer deposition (L = amidinate) of WO3 [29], and oxygen
transfer chemistry (L = 4-aryl-pent-2-enolate) [30]. To the best of
our knowledge, this type of precursor has not been studied in
depositing tungsten oxide by AACVD. Therefore, WO2L2 (L = acNac,
acNacMe, acNacEt) complexes have been synthesized and studied as
AACVD precursors for the growth of tungsten oxide.
3.4. NMR spectroscopy
3.2. Synthesis
For octahedral cis-dioxo bis-bidentate metal complexes, asym-
metric bidentate ligands such as b-ketoiminates allow the forma-
tion of three isomeric enantiomer pairs (Fig. 3). For O,O-isomers
and N,N-isomers, the two bidentate ligands within one molecule
are symmetrical, thus only one set of NMR signals will be shown.
For the O,N-isomers, the two chelating ligands are not chemically
equivalent, and the corresponding NMR spectra will show two sets
of ligand signals with equal intensities. 1H NMR spectra of 1 and 2
only show one set of ligand peaks, indicating the formation of
Complexes 1–3 were successfully synthesized by the reaction of
2 equiv of the appropriate ligand with WO2Cl2(dme) in the pres-
ence of excess NEt3 (Fig. 1). The product was extracted with THF
and the insoluble triethylamine hydrochloride salt was removed
by filtration. Removal of volatiles under vacuum gave the desired
product.
In other less successful synthetic efforts, the b-ketoimine was
reacted with a strong base such as NaH or LiNMe2 to form the alkali
Fig. 1. Synthesis of complexes 1–3.

222
X. Su et al. / Polyhedron 169 (2019) 219–227
Table 1
Crystal data and structure refinements for 2.
Empirical formula
Formula weight
T (K)
C12H20N2O4W
440.15
100(2)
k (Å)
0.71073
monoclinic
P21/n
a = 8.1901(3) Å
b = 15.1072(5)
Å
Crystal system
Space group
Unit cell dimensions
a
= 90°
b = 91.9780
(10)°
c = 11.6076(4) Å
1435.35(9)
4
2.037
8.059
c = 90°
V (Å3)
Z
DCalc (Mg mÀ3
)
Absorption coefficient (mmÀ1
F(0 0 0)
)
848
Crystal size (mm)
h range for data collection
Index ranges
0.276 Â 0.083 Â 0.034
2.213–28.054°
À10 ꢀ h ꢀ 10, À19 ꢀ k ꢀ 19,
À15 ꢀ l ꢀ 15
Reflections collected
30 079
Independent reflections
Completeness to h = 25.000°
Absorption correction
Refinement method
3477 [Rint = 0.0143]
100.0%
Fig. 2. Molecular structure of 2. Thermal ellipsoids are drawn at the 50% probability
level.
none
full-matrix least-squares on F2
3477/0/178
slow mass loss until 400 °C. With the increased steric bulk of the
alkylimino functionality, the onset temperature of fast decomposi-
tion slightly decreases and the initial rate of mass loss slightly
increases, suggesting more facile reactivity induced by larger
alkylimino groups. The TGA residual masses of all three com-
pounds match the calculated tungsten trioxide percentage indicat-
ing that all compounds decomposed to WO3 upon heating
(Table 3).
Data/restraints/parameters
Goodness-of-fit on F2
1.073
Final R indices [I > 2
R indices (all data)
r(I)]
R1 = 0.0095, wR2 = 0.0229 [3371]
R1 = 0.0101, wR2 = 0.0232
n/a
Extinction coefficient
Largest difference peak and hole (e ÅÀ3
)
0.469 and À0.229
P
P
P
P
P
R1
=
(||Fo| À |Fc||)/ |Fo|, WR2 = [ [w(Fo2 À Fc2)2]/ (w(Fo2)2]]1/2
.
S = [ [w(Fo2)2 À Fc2)2]/(n-p)]1/2. w = 1/[
r
2(Fo2) + (m * p)2 + n * p].
p = [max(Fo2,0) + 2 * Fc2]/3, m & n are constants.
3.6. Mass spectrometry
Table 2
Selected bond distances (Å) and angles (°) for complex 2.
The fragmentation pattern of tungsten dioxo complexes 1–3
was studied using direct analysis in real time mass spectrometry
(DART-MS). Although the CVD process is most likely to generate
neutral species instead of ions and the fragmentation pattern
may vary with the ionization method, fragmentation tendencies
during MS have shown correlation with decomposition pathways
during CVD [38]. Major ions and their relative abundances are pro-
vided in Table 4.
Bond parameter
Bond parameter
W1AO1
W1AO3
W1AN1
C1AC2
1.7323(11)
2.1079(10)
2.1334(13)
1.423(2)
W1AO2
W1AO4
W1AN2
C2AC3
1.7319(11)
2.0971(10)
2.1323(13)
1.378(2)
1.378(2)
166.54(5)
78.57(4)
91.68(5)
166.79(5)
80.72(5)
C8AC9
1.422(2)
C7AC8
O1AW1AO2
O2AW1AO4
O1AW1AN1
O2AW1AN2
O3AW1AN1
N1AW1AN2
102.38(5)
89.90(5)
100.58(5)
99.63(5)
80.72(5)
161.06(5)
O1AW1AO4
O3AW1AO4
O1AW1AN2
O2AW1AO3
O4AW1AN2
O4AW1AN1
The abundance of the molecular ions [M+H]+ for 2 and 3 is low
(3–6%) compared with that of compound 1 (24%), indicating the
greater lability of 2 and 3 introduced by alkyl functionality
attached to the imino nitrogen. The base peaks for all three com-
pounds correspond to protonated ligand [HL+H]+. In the cases of
the less bulky ketoiminate complexes 1 and 2, a small amount of
[M+HL+H]+ was detected. For compound 3, the ligand apparently
loses the ethyl substituent at the nitrogen atom to give rise to a
small quantity of [HLÀEt]+ (9%). The formation of [HLÀEt]+
matches the previous observation that a large alkylimino group
can induce facile fragmentation. Though [MÀL]+ was not directly
observed, the dinuclear ion [M+(MÀL)]+, which can be formed by
the reaction of [MÀL]+ and the original molecule by bridging of
oxo groups, was detected for 1 and 2. Analogous dimeric species
[M+(M+H)]+ were also observed for 1 and 2. The absence of these
dinucluear species for 3 is probably due to higher fragility or larger
steric hindrance introduced by relatively large ketoiminate ligands.
The presence of ligand species and [M+(MÀL)]+ suggests a possible
decomposition pathway by protonation of one ketoiminate ligand
and subsequent loss of HL, consistent with the thermolysis study.
84.31(5)
either the O,O-isomer or the N,N-isomer. Considering the fact that
the O,O-isomer is favored by the trans effect of W@O and the crys-
tallographic study of 2, it can be inferred that only the O,O-isomer
is formed for 1 and 2.
In case of compound 3, there are two sets of ligand peaks with
different intensities demonstrating the formation of one major
symmetric isomer (O,O-isomer or N,N-isomer) and a small amount
of the other symmetric isomer. Formation of the N,N-isomer of 3 is
probably enabled by the electron repulsion between the sub-
stituents on the nitrogen and the oxo groups. The two protons of
the methylene group attached to the nitrogen are diastereotopic
(Ha and Hb in Fig. 3) and give rise to two different multiplets in
the 1H NMR spectrum.
3.5. Thermogravimetric analysis
The TGA analysis of all three complexes 1–3 was carried out
with the heating rates controlled at 10 °C minÀ1. The results are
shown in Fig. 4. All three compounds show multistep decomposi-
tion with an initial major mass loss below 300 °C and a subsequent
3.7. Thermolysis study
The complex 2 was heated in an airtight vial and the gaseous
byproducts were analyzed using GC–MS. Major fragments and

X. Su et al. / Polyhedron 169 (2019) 219–227
223
Fig. 4. Thermogravimetric analysis plots for compounds 1–3.
Table 3
Experimental residual mass and the calculated WO3 percentage for compounds 1–3.
Residual mass
(%)
Calculated residual mass for WO3 (%)
WO2(acNac)2 (1)
54
56
54
56
53
50
WO2(acNacMe
)
2
(2)
(3)
WO2(acNaciPr
)
2
transferred into the AACVD reaction chamber by a nitrogen flow.
The color of the as-deposited films from 2 varies with deposition
temperature: purple (250 °C), purple and yellow (350 °C), and hazy
black (450 and 530 °C).
Fig. 6 shows the plane-view SEM images of the as-deposited
films from 2. The morphology of these films varies considerably
depending on the deposition temperature. No obvious microstruc-
tures were observed for films grown at 250 °C. As the temperature
increases to 350 °C, packed nanorods with an average diameter of
ꢁ45 nm were observed. Longer and thinner nanorods oriented in
random directions were grown at 450 °C. Bundled nanorods were
synthesized when the growth temperature was further elevated
to 530 °C. The transition of morphology from featureless films to
highly textured materials as the growth temperature increases is
consistent with our previous AACVD experiments using other sin-
gle source precursors [13,15].
Fig. 3. Possible isomeric enantiomer pairs of WO2L2.
The composition of the as-deposited and sputtered films depos-
ited from precursor 2 was characterized by X-ray photoelectron
spectroscopy. The sputtering process took 2 min for each film using
a 2 kV Ar+ source. The spectra of the as-synthesized films were cal-
ibrated using the adventitious C 1s peak at 284.8 eV. For the sput-
tered films, the C 1s spectra had a low signal to noise ratio, thus the
O 1s peak at 530.5 eV which corresponds to tungsten-bonded oxy-
gen was used as reference [13,15]. The atomic percentage of W, O,
C, and N as well as the O:W ratio are reported in Fig. 7. A small
amount of nitrogen was detected before (<6 at%) and after sputter-
ing (<4 at%). The binding energy (BE) of N 1s matches the binding
energy of organic nitrogen species, excluding the formation of
tungsten nitride [39]. For films grown at 250 and 350 °C, the
amount of carbon impurities was decreased after Ar+ bombard-
ment, while the atomic percentage of carbon (ꢁ60%) did not
change appreciably for films grown at 450 and 530 °C, indicating
substantial carbon incorporation during deposition at high temper-
atures. The carbon contamination could arise from both solvent
and ligand decomposition [40]. For the b-ketoiminate ligands
their normalized abundance are summarized in Figs. 2–10a. The
major decomposition byproducts detected by GC–MS are the pro-
tonated chelating ligand HL, N-methylpropan-2-imine and ace-
tone. Pyrolysis of acNacMeH under the same conditions generated
acetone as the dominant decomposition product. The presence of
HL suggests a decomposition pathway in which one ketoiminate
is protonated by an adjacent proton and then HL dissociates to gen-
erate intermediate 4 (Fig. 5b). The small quantities of other frag-
ments could be products of subsequent reactions of the
ketoimine and N-methylpropan-2-imine such as hydrolysis and
further decomposition.
3.8. Characterization of WOx films
Complex 2 was utilized as a precursor to grow tungsten oxide
films on native silicon dioxide (Si/SiO2, n-type, (100) by AACVD
at 250, 350, 450, and 530 °C respectively. Aerosol of precursor
solution in diglyme was generated by a nebulizer and was then

224
X. Su et al. / Polyhedron 169 (2019) 219–227
Table 4
Selected fragments observed in positive ion DART mass spectra of 1–3.
WO(OR)3L
[M+H]+
m/z (%)
[M+MÀL]+
m/z (%)
[M+M+H]+
m/z (%)
[HL+H]+
m/z (%)
[M+HL+H]+
m/z (%)
1
2
3
413.0 (24)
441.1 (3)
469.1 (6)
726.0 (30)
768.1 (3)
n. o.a
825.1 (3)
881.2 (7)
n. o.a
100.0 (100)
114.1 (100)
128.1 (100)
512.2 (5)
554.1 (7)
n. o.a
a
n. o. = not observed or trace amounts less than 1%.
Fig. 5. (a) Thermolysis products of compounds 2; (b) proposed decomposition mechanism.
Fig. 6. Plane-view SEM images of samples grown at (a) 250, (b) 350, (c) 450 and (d) 530 °C from precursor 2 dissolved in diglyme using N2 as carrier gas.

X. Su et al. / Polyhedron 169 (2019) 219–227
225
Fig. 7. XPS atomic percentage of W, O, C, and N of (a) as-synthesized films and (b) sputtered films; O:W atomic ratio of (c) as-synthesized films and (d) sputtered films grown
from precursor 2.
High resolution O 1s, C 1s and W 4f spectra of the as-grown
films were analyzed to further study the chemical nature of the
deposits (Fig. S1). Major carbon impurities are identified as amor-
phous carbon and organic CAO (285.9 0.3 eV). Traces of C@O
(289.0 0.4 eV) were detected for films grown at 350 and 450 °C.
Deconvolution of O 1s peaks confirms that oxygen is bonded to
either carbon or tungsten. The XPS spectra of the W4f core levels
were fitted into two sets of doublets with spin–orbit separation
of 2.18 eV through the full range of deposition temperature. The
BE of W 4f7/2 at 35.7 0.2 eV and 34.5 0.4 eV are assigned to
W6+ and W5+ respectively. The amount of W6+ increased as the
growth temperature changed from 250 to 450 °C. Deconvolution
of O 1s spectra of the sputtered materials shows that C@O was
removed effectively. Compared with the presence of only W6+
and W5+ in non-sputtered films, the films after Ar+ bombardment
contained tungsten in lower oxidation states such as W4+ and W0
(Fig. S2). This observation is consistent with the reduction of
tungsten oxides upon sputtering caused by the preferential
removal of oxygen [42]. Sputtering is a complicated process which
results in both chemical and morphological change of the material.
The tendency towards the preferential removal of certain elements
Fig. 8. XRD patterns of the as-synthesized tungsten oxide products.
can be influenced by sputtering parameters as well as the
composition, microstructure and surface topography of the films.
which have alkyl groups attached to the nitrogen, incorporation of
nitrogen content may increase the amount of carbon impurities. It
has been reported that addition of water vapor during deposition
process can reduce carbon impurities when using bis(diketonate)
metal complexes as precursors, thus it should be possible to opti-
mize the film composition by altering deposition conditions [41].
The O:W ratio shows a general upward trend as temperature
increases. The O:W ratio of sputtered films was significantly lower
than the corresponding value before sputtering. It has been
reported that the ion beam bombardment of tungsten oxide can
cause surface reduction by preferential removal of oxygen [42].
Therefore, caution needs to be exercised when conducting depth
profiling of reducible transition metal oxides such as tungsten
oxides.
The crystallinity and phase of the as-synthesized deposits were
studied by X-ray diffraction (XRD) analysis (Fig. 8). Films grown at
250 °C were amorphous, exhibiting only a peak for the silicon sub-
strate. In contrast, the materials synthesized at 350–530 °C show a
strong diffraction peak at 23.3° 2h which is assigned to the (0 1 0)
plane of monoclinic WO3. The films grown at 550 °C also have a
diffraction peak at 48.0° 2h, which is indexed to the (0 4 0) facet
of monoclinic WO3 (ICCD card number 72-0677).

226
X. Su et al. / Polyhedron 169 (2019) 219–227
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In summary, the bis(b-ketoiminate) dioxo tungsten(VI) com-
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Mass spectrometry was performed at the Mass Spectrometry
Research and Education Center at University of Florida, supported
by grant NIH S10 OD021758-01A1. We thank Hang Lu, Will Carden
and Taehoon Kim for their assistance with materials
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