Synthesis of β-ketoiminate and β-iminoesterate tungsten (VI) oxo-alkoxide complexes as AACVD precursors for growth of WOx thin films

Article abstract of DOI:10.1016/j.poly.2018.10.035

The tungsten (VI) oxo-alkoxide complexes WO(OR)₃L [R = ^tBu; L = acNac, etNac, tbNac, acNac^Me, acNac^Et] (1–5) and WO(OCH₃)₃(acNac) (6) have been synthesized. The isomeric purity of these complexes depends on the steric bulk of the substituent on the imino nitrogen of the chelating ligand. The thermal properties of the complexes have been evaluated to assess the effect of the β-ketoiminate or β-iminoesterate ligands. WO(OC(CH₃)₃)₃(acNac) (1) has been used as a precursor for aerosol assisted chemical vapor deposition (AACVD) of WO_x thin films at temperatures from 250 to 450 °C. The results of mass spectrometry and thermolysis studies have been used to propose possible precursor decomposition pathways during film deposition.

Full text of DOI:10.1016/j.poly.2018.10.035

Polyhedron 157 (2019) 548–557
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis of b-ketoiminate and b-iminoesterate tungsten (VI)
oxo-alkoxide complexes as AACVD precursors for growth
of WO thin films
x
⇑
Xiaoming Su, Taehoon Kim, 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
The tungsten (VI) oxo-alkoxide complexes WO(OR) L [R = Bu; L = acNac, etNac, tbNac, acNacMe, acNacEt
(
t
]
Article history:
3
Received 31 August 2018
Accepted 13 October 2018
Available online 24 October 2018
3 3
1–5) and WO(OCH ) (acNac) (6) have been synthesized. The isomeric purity of these complexes depends
on the steric bulk of the substituent on the imino nitrogen of the chelating ligand. The thermal properties
of the complexes have been evaluated to assess the effect of the b-ketoiminate or b-iminoesterate
ligands. WO(OC(CH
deposition (AACVD) of WO
3
)
3
)
3
(acNac) (1) has been used as a precursor for aerosol assisted chemical vapor
thin films at temperatures from 250 to 450 °C. The results of mass spectrom-
Dedicated to Prof. William D, Jones.
x
etry and thermolysis studies have been used to propose possible precursor decomposition pathways dur-
ing film deposition.
Keywords:
Tungsten oxide
Chemical vapor deposition
Single source precursors
metal oxo complexes
metal oxide films
Ó 2018 Elsevier Ltd. All rights reserved.
1
. Introduction
Tungsten oxide is an extensively investigated functional mate-
tional CVD in which the precursor is dissolved in a solvent and then
delivered to the reactor as an aerosol [13]. Therefore, AACVD relies
on the solubility of the precursors instead of volatility, which offers
greater flexibility in precursor design [14].
rial for various applications such as electrochromic devices [1,2],
sensing [3,4], catalysis [5–7] and organic electronics [8]. Besides
the advantages of low cost, good chemical stability and mechanical
flexibility, the wide application of tungsten oxide can also be
attributed to its variety of phases (structural complexity) and sto-
ichiometries (compositional complexity), which results in unique
optical, electrical, photocatalytic and magnetic properties [9]. For
example, WO2.5 can be applied as an efficient electron and hole
injection/transport layer in organic light-emitting diodes (OLEDs)
Ideal precursors for CVD would have adequate volatility (or sol-
ubility for AACVD), stability during the transport process to the
reactor, clean decomposition and easy preparation [15]. Utilizing
chelating ligands as a strategy to increase the volatility and stabil-
ity of tungsten oxide precursors has been reported [16–19].
However, these studies are limited to the use of b-diketonate/
b-ketoesterate and aminoalkoxide ligands. The b-ketoiminate/imi-
noesterate ligands are a very attractive class of mixed N,O-biden-
tate ligands for preparation of various metal complexes, which
have been applied in medicinal chemistry [20], polymerization
[21] and MOCVD [22–25]. Apart from easy preparation and good
coordination ability, their steric and electronic properties can be
tuned by altering the substituents on the backbone or the nitrogen.
[
10], while WO2.72 is an efficient catalyst for hydrogenation [5]
and WO2.9 shows great potential in HCHO sensing and elimination
11,12].
Due to the close relationship between physiochemical struc-
tures and materials properties, the ability to control stoichiometry,
degree of crystallinity and morphology of WO to meet the require-
ments for specific application is critical. Among the numerous WO
[
x
Due to the limited application of these ligands in designing WO
precursors, we have synthesized complexes of the type WO
x
x
t
processing techniques including sol–gel, sputtering and thermal
evaporation, chemical vapor deposition finds practical use due to
its capability to deliver large-scale conformal coatings. Aerosol-
assisted chemical vapor deposition (AACVD) is a variant of conven-
3
(OR) L (R = Bu, Me) bearing various b-ketoiminate/b-iminoester-
ate ligands, which include 4-amino-3-penten-2-onate (acNac),
Me
4-(methylamino)-3-penten-2-onate (acNac ), 4-(ethylamino)-
Et
3-penten-2-onate (acNac ), ethyl 3-aminocrotonate (etNac) and
tert-butyl 3-aminocrotonate (tbNac) (see Scheme 1). We then
studied AACVD of WO
x 3 3 3
thin films from WO(OC(CH ) ) (acNac)
⇑
Corresponding author.
E-mail address: lmwhite@chem.ufl.edu (L. McElwee-White).
(1). To the best of our knowledge, this the first reported study in
https://doi.org/10.1016/j.poly.2018.10.035
277-5387/Ó 2018 Elsevier Ltd. All rights reserved.
0

X. Su et al. / Polyhedron 157 (2019) 548–557
549
Scheme 1. Synthesis of complexes 1–6.
which the N,O-bonding analogs of acac chelating ligands have been
used in precursors for chemical vapor deposition of tungsten
oxides.
product was sublimed between 62 and 74 °C (50–100 mTorr) to
obtain yellow solid. Yield: 0.715 g (66.0%). Crystals for X-ray crys-
tallography were grown by cooling a toluene/hexane solution of
1
complex 1 to ꢀ3 °C. H NMR (C
6
D
6
, 25 °C): d 7.13 (b, 1H, NH),
), 1.59 (s, 9H, C
C(NH), J = 1.0 Hz).
), 168.95 (C(NH)), 99.76
), 80.62 (C(CH ), 30.22 ((CH C),
CO), 25.60 (CH C(NH)). Anal. Calc. for
35N: C, 39.47; H, 6.82; N, 2.71. Found: C, 38.23, H, 6.34;
4
.75 (d, 1H, CHCO, J = 1.7 Hz), 1.75 (s, 3H, COCH
CH ), 1.44 (s, 18H, C(CH ), 1.32 (d, 3H, CH
C NMR (C , 25 °C): d 181.26 (COCH
CHCO(CH )), 82.48 (C(CH
0.74 ((CH C), 26.12 (CH
WO
3
2
. Experimental
(
3
)
3
3
)
3
3
1
3
6
D
6
3
2.1. General considerations
(
3
3
)
3
3
)
3
3 3
)
3
3
)
3
3
3
2
All reactions were carried out under an inert atmosphere (N )
5
17
C H
employing conventional glovebox or standard Schlenk techniques.
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 prior to use. Diethyl
ether and tetrahydrofuran were dried using sodium/benzophe-
none, distilled, and stored over activated 3 Å molecular sieves prior
+
N, 2.75%. MS (ESI) m/z Calcd for WO
Found: 556.1634.
5 17
C H35NK [M+K] : 556.1657.
2
.2.2. WO(OC(CH
Compound 2 was synthesized following the same procedure as
used for 1. Starting with WO(OC(CH (1.219 mmol, 0.6003 g)
and etNacH (1.229 mmol, 0.1588 g), a yellow solid (0.5601 g,
4.0%) was obtained after removal of the volatiles under vacuum.
The crude product was sublimed at 52–62 °C (100–140 mTorr) to
3 3 3
) ) (etNac) (2)
3 3 4
) )
to use. Hexamethyldisiloxane, methanol, chloroform-d
bridge Isotopes) and benzene-d (Cambridge Isotopes) were stored
1
(Cam-
6
8
over 4 Å molecular sieves. Diethylamine was distilled over potas-
sium hydroxide and stored over 4 Å molecular sieves. The com-
1
6 6
afford a yellow product. Yield: 0.1425 g (23.5%). H NMR (C D ,
pounds WOCl
4 3 3 4 3 4 2
[26], WO[OC(CH ) ] [26], [WO(OCH ) ] [27],
Me
Et
2
2
5 °C): d 6.41 (b, 1H, NH), 4.66 (d, 1H, CHCO, J = 1.6 Hz), 4.04 (q,
H, CH CH , J = 7.1 Hz), 1.56 (s, 9H, C(CH ), 1.45 (s, 18H, C
), 1.33 (s, 3H, CH C(NH)), 1.06 (t, 3H, CH CH , J = 7.1 Hz).
C NMR (C , 25 °C): d 170.96 (COCH ), 168.89 (C(NH)), 82.98
CHCOCH ), 82.02 (C(CH ), 81.01 (C(CH
1.05 ((CH C), 30.72 ((CH C), 26.19 (CH
). MS (ESI) m/z Calcd for WO
acNacH [28], acNac H [29], acNac H [30], 4-tert-butylamino-3-
penten-2-one (acNac H) [31], etNacH [28] and tbNacH [28] were
tBu
2
3
3 3
)
(
CH
3
)
3
3
3
2
synthesized according to literature procedures with modifications.
All other solvents and reagents were of analytical grade and were
used as received. NMR spectra were recorded on either a Varian
Mercury 300BB (300 MHz) spectrometer or a Varian INOVA 500
spectrometer using residual protons from deuterated solvents for
reference. Thermogravimetric analysis was performed using a TA
Q5000 under an atmosphere of nitrogen at a temperature scan rate
of 10 °C/min to 600 °C. Mass spectrometry was performed on a
ThermoScientific DSQ II with direct insertion probe (DIP) using
chemical ionization (CI) with methane as the reagent gas or elec-
tron ionization (EI) (70 eV) or a Bruker Daltonics Impact II-Qq-
TOF instrument with electrospray as the source of ionization in
positive mode. Elemental analyses were performed by Robertson
Microlit Laboratories (Ledgewood, NJ).
1
3
6
D
6
2
(
3
CH
2
3
)
3
3
)
3
), 60.39 (CH
2 3
CH ),
3
)
3
3
)
3
3
C(NH)), 15.01 (CH -
3
+
2
C
6 18
H
37NK [M+K] : 586.1762.
Found: 586.1781.
2.2.3. WO(OC(CH ) ) (tbNac) (3)
3 3 3
Compound 3 was synthesized following the same procedure as
used for 1. Starting with WO(OC(CH ) ) (1.015 mmol, 0.4998 g)
3
3 4
and tbNacH (1.023 mmol, 0.1608 g), a yellow solid (0.4422 g,
75.7%) was obtained after removal of the volatiles under vacuum.
The crude product was sublimed at 59–62 °C (220–380 mTorr) to
1
afford a yellow product. Yield: 0.1124 g (19.2%). H NMR (C D ,
6 6
2
9
5 °C): d 6.27 (b, 1H, NH), 4.60 (d, 1H, CHCO, J = 1.6 Hz), 1.57 (s,
H, C(CH ), 1.50 (s, 9H, COC(CH ), 1.44 (s, 18H, C(CH ), 1.32
C(NH), J = 0.6 Hz). C{ H} NMR (C , 25 °C): d 171.78
), 168.74 (C(NH)), 84.45 (CHCOC(CH ), 81.55 (C
), 80.90 (C(CH ), 79.83 (OC(CH ), 31.06 ((CH C), 30.74
C), 29.16 ((CH C), 25.78 (CH C(NH)). MS (ESI) m/z Calcd
3
)
3
3
1
)
3
3 3
)
2
2
.2. Synthesis
1
3
(
(
(
(
d, 3H, CH
COC(CH
CH
(CH
3
6 6
D
3
)
3
3 3
)
3 3 3
.2.1. WO(OC(CH ) ) (acNac) (1)
In the glove box, WO(OC(CH ) ) (2.093 mmol, 1.030 g) was dis-
3 3 4
3
)
3
3
)
3
3
)
3
3 3
)
3
)
3
3
)
3
3
solved in diethyl ether (30 mL) in a Schlenk flask and then trans-
ferred to the Schlenk line. 4-Amino-3-penten-2-one (2.108 mmol,
+
for WO
C
6 20
H41NK [M+K] : 615.210. Found: 615.2129.
0
.2090 g) was dissolved in diethyl ether (10 mL). The WO(OC
Me
(
CH solution was cooled to 0 °C in ice bath for 30 min, and
3
3
) )
4
3 3 3
2.2.4. WO(OC(CH ) ) (acNac ) (4)
the solution of 4-amino-3-penten-2-one was then added dropwise.
The reaction mixture was then slowly warmed to room tempera-
ture after stirring for 30 min. The reaction was allowed to stir for
Compound 4 was synthesized following the same procedure as
used for 1. Starting with WO(OC(CH (2.099 mmol, 1.0333 g)
and acNac H (2.105 mmol, 0.2382 g), a yellow solid (0.8806 g,
79.0%) was obtained after removal of the volatiles under vacuum.
3 3 4
) )
Me
1
2 h, then the volatiles were removed under vacuum. The crude

5
50
X. Su et al. / Polyhedron 157 (2019) 548–557
The crude product (4A with traces of 4B) was sublimed at 82–94 °C
100–200 mTorr) to afford a yellow solid, which is a mixture of 4A
and 4B in ꢁ1:3 ratio. Yield of the mixture: 0.5383 g (48.3%). Isomer
plate vibrating at 1.44 MHz and delivered to the reaction chamber
with nitrogen (99.999% purity, Airgas) carrier gas at a flow rate of
1000 sccm. Deposition was conducted for 150 min, during which
the reactor pressure was maintained at 350 Torr and the deposi-
tion temperature was controlled by a radio frequency (RF) heating
system on the susceptor.
(
1
4
A: H NMR (C
6
D
6
, 25 °C): d 4.85 (s, 1H, CHCO), 3.10 (s, NCH
), 1.58 (s, 9H, C(CH ), 1.48 (s, 18H, C(CH ), 1.45 (s,
, 25 °C): d 170.34 (COCH ), 167.26
)), 82.56 (C(CH ), 80.96 (C(CH ),
C), 30.27 ((CH C), 25.07 (CH CO),
, 25 °C): d 4.89 (s,
), 1.77 (s, 3H, COCH ), 1.58 (s, 9H, C
), 1.40 (s, 3H, CH C(NCH
, 25 °C): d 177.57 (COCH ), 168.62 (C(NCH )), 104.03 (CHCO
)), 82.72 (C(CH ), 80.41 (C(CH ), 43.33 (N(CH )), 31.16
C), 30.75 ((CH C), 25.22 (CH CO), 21.92 (CH C(NCH )).
37N (mixture of 4A and 4B): C: 40.69; H:
.02; N: 2.64. Found: C, 40.30; H, 6.77; N, 2.63%.
3
), 1.86
(
3
s, 3H, COCH
3
3
)
3
3 3
)
1
3
H, CH
3
C(NCH
3
)). C NMR (C
6
D
6
3
(C(NCH
3
)), 104.21 (CHCO(CH
9.50 (N(CH )), 30.97 ((CH
1.41 (CH C(NCH )). Isomer 4B: H NMR (C
H, CHCO), 3.41 (s, NCH
), 1.43 (s, 18H, C(CH
3
3
)
3
3
)
3
The elemental composition of WO
x
films was determined using
+
3
2
1
3
3
)
3
3
)
3
3
ULVAC-PHI XPS Al K radiation after 2 min 2 kV Ar sputtering to
a
1
3
3
6
D
6
remove surface contaminants and oxygen was used as an internal
standard set at 530.5 eV [16,33]. The Shirley base line was sub-
tracted before peak fitting. The XPS spectra of the W 4f core level
were fitted into peak doublets with parameters of spin–orbit sep-
3
3
13
(
(
(
(
CH
3
)
3
3
)
3
3
3
)). C NMR
6
C D
6
3
3
CH
(CH
3
3
)
3
3
)
3
3
aration DEP (4f5/2–4f7/2) = 2.18 eV. The intensity ratio of the W
)
3 3
)
3 3
3
3
3
4f7/2 and W 4f5/2 peak doublet was set to 4:3. The morphology of
the films was examined using field emission scanning electron
microscopy (FESEM, FEI Nova NanoSEM 430). The crystallinity
was measured by X-ray diffraction (XRD, Panalytical X’pert Pro).
5 18
Anal. Calc. for WO C H
7
Et
2
.2.5. WO(OC(CH
Compound 5 was synthesized following the same procedure as
used for 1. Starting with WO(OC(CH (1.623 mmol, 0.7988 g)
and acNac H (1.640 mmol, 0.2086 g), a yellow oil (0.5192 g,
8.7%) was obtained after removal of the volatiles under vacuum.
3 3 3
) ) (acNac ) (5)
2.4. Crystallographic structure determination of 1
3 3 4
) )
Et
X-ray Intensity data were collected at 100 K on a Bruker DUO
5
diffractometer using Mo Ka radiation (k = 0.71073 Å) and an APEXII
1
The crude product was distilled at 82–90 °C (100–200 mTorr) to
afford a yellow oil. Yield: 0.2895 g (32.7%). The isomer ratio of 5A
and 5B is ꢁ1:1 and they were not separated from one another.
CCD area detector. Raw data frames were read by the program 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 corrected for Lorentz
and polarization effects and numerical absorption corrections were
applied based on indexed and measured faces.
For the mixture of 5A and 5B: 1H NMR (C
, 25 °C): d 4.85 (s,
CH , J = 7.1 Hz), d
, J = 7.1 Hz), 1.82 (s, 3H, COCH ), 1.75 (s, 3H,
), 1.58 (s, 9H, C(CH ), 1.55 (s, 9H, C(CH ), 1.54 (s, 3H,
C(NC )), 1.53 (s, 3H, CH C(NC )), 1.45 (s, 18H, C(CH ),
.41 (s, 18H, C(CH ), 1.33 (t, NCH CH , J = 7.1 Hz), 1.16 (t, NCH
, J = 7.1 Hz). C NMR (C , 25 °C): d 177.10 (COCH ), 169.24
), 168.09 (C(NC )), 165.60 (C(NC )), 104.01 (CHCO
)), 103.51 (CHCO(CH )), 82.67 (C(CH ), 82.40 (C(CH ),
1.02 (C(CH ), 80.70 (C(CH ), 50.18 (N(CH CH )), 46.09 (N(CH
)), 31.08 ((CH C), 30.97 ((CH C), 30.26
C), 25.32 (CH CO), 25.08 (CH C(NC )),
1.19 (CH C(NC )), 16.39 (N(CH CH CH )). Anal.
Calc. for WO
H, 7.21; N, 2.66%.
6
D
6
1
3
H, CHCO), 4.77 (s, 1H, CHCO), 3.85 (q, NCH
.57 (q, NCH CH
2
3
2
3
3
COCH
CH
3
3
)
3
3
)
3
The structure was solved and refined in SHELXTL2014, 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. The asymmetric unit consists of two crystallographically
independent but chemically equivalent complexes. The amino pro-
ton from both complexes were obtained from a Difference Fourier
map and refined freely. The second complex (W1b) has two disor-
dered regions. In one, the three methyl groups bound to C10b are
refined in two parts. In the second, the full ligand on O3b is refined
in two parts as well. In both cases the site occupation factors were
dependently refined. In the final cycle of refinement, 9941 reflec-
3
H
2 5
3
2
H
5
3 3
)
1
CH
3
)
3
2
3
2
-
1
3
3
6
D
6
3
(
(
COCH
CH
3
2
H
5
2 5
H
3
3
3
)
3
3 3
)
8
3
)
3
3
)
3
2
3
2
-
CH
3
3
)
3
3
)
3
3
3
C), 30.85 ((CH
CO), 21.62 (CH
)), 15.42 (N(CH
3 3
)
(
(CH
)
3 3
3
3
2 5
H
2
3
2
H
5
2
2
3
C
5 19
H39N: C: 41.85; H: 7.21; N: 2.57. Found: C, 41.52;
tions (of which 9078 are observed with I > 2
refine 483 parameters and the resulting R , wR
of fit) were 2.72%, 5.51% and 1.064%, respectively. The refinement
was carried out by minimizing the wR
function using F2 rather
than F values. R is calculated to provide a reference to the conven-
r
2
(I)) were used to
and S (goodness
2
.2.6. WO(OCH
3
)
3
(acNac) (6)
1
Compound 6 was synthesized following the same procedure as
used for 1. Starting with [WO(OCH (0.771 mmol, 0.4998 g) and
acNacH (1.572 mmol, 0.1558 g), a yellow solid (0.4952 g, 82.0%)
was obtained after removal of the volatiles under vacuum. The
3
)
]
4 2
2
1
tional R value but its function is not minimized.
crude product was distilled at 70–74 °C (100–140 mTorr) to afford
1
a yellow oil. Yield: 0.1695 g (28.1%). H NMR (C
6
D
6
, 25 °C): d 7.36
2.5. Thermolysis studies
(
4
b, 1H, NH), 4.68 (s, 3H, OCH
.41 (s, 6H, OCH ), 1.75 (s, 3H, COCH
J = 0.8 Hz). C{ H} NMR (C , 25 °C): d 182.40 (COCH
)), 63.41 (OCH ), 61.77 (OCH
). Anal. Calc. for WO 17N: C: 24.57; H: 4.38;
3
), 4.63 (d, 1H, CHCO, J = 1.9 Hz),
), 1.18 (d, 3H, CH C(NH),
), 171.30
), 26.07
3
3
3
Each compound (complex 1, 2 and pure ligand acNacH) was
placed in a 10 mL headspace vial with a septum and heated in
the GC oven at 250 °C for 40 min. The oven was then cooled to
32 °C. Once cool, the headspace vial was removed from the oven.
1
3
1
6
D
6
3
(
(
C(NH)), 100.87 (CHCO(CH
CH ), 25.55 (CH
3
3
3
3
3
5 8
C H
N: 3.58. Found: C, 24.36; H, 4.21; N, 3.61%.
A 100-lL sample of the headspace gases was taken with a gas-tight
syringe and injected in split mode for GC/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 chro-
matography was conducted on a ThermoScientific Trace GC Ultra.
2
.3. WO materials growth and characterization
x
WO
x
films were deposited onto silicon substrates with native
, n-type, <100>) using a custom-built, verti-
silicon dioxide (Si/SiO
2
cal cold-wall impinging-jet AACVD reactor [32]. Substrates were
cleaned in boiling acetone, ethanol and deionized water for 3 min
each, and then placed on a SiC covered graphite susceptor in the
2.6. Adhesion test
The Scotch tape test was used to evaluate the adhesion of the
reaction chamber under vacuum. Solutions of precursor
1
deposited WO
x
films on native silicon dioxide (Si/SiO
2
, n-type,
TM
(
0.034 M in diglyme) were prepared and loaded into a 10 mL gas-
<100>) substrates. A strip of commercial Scotch tape (Magic ,
3M) was attached to the deposited film and then peeled off. The
film adhered to the substrate and not to the tape. Images of the
ꢀ1
tight Hamilton syringe (4 mL h injection rate). Aerosol was gen-
erated from the precursor solution using a nebulizer with a quartz

X. Su et al. / Polyhedron 157 (2019) 548–557
551
tape and the films before and after the test are in supplementary
information (Fig. S8).
Table 1
Crystal data and structure refinements for 1.
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
a (Å)
17 5
C H35NO W
517.31
100(2) K
0.71073 Å
triclinic
Pꢀı
3
. Results and discussion
3.1. Precursor design
Compounds of the type WO(OR)
3
L (R = alkyl; L = b-diketonate,
CVD precursors [16,17]
10.4335(6)
13.5435(7)
15.4896(8)
95.753(1)
95.922(1)
92.651(1)
b-ketoesterate) have been studied as WO
x
b (Å)
c (Å)
but the research on tungsten complexes containing the structurally
related b-ketoiminate or b-iminoesterate ligands has been very
limited. b-Ketoiminates or b-iminoesterates have been successfully
used as ligands in CVD precursors for metal oxide films including
a
(°)
b (°)
(°)
c
3
V (Å )
2162.6(2)
MgO [34], ZnO [35] and Co
3
O
4
[36]. In contrast to the commonly
Z
D
4
calc (Mg/m3)
1.589
5.364
1032
used b-diketonate ligands, these mixed N/O chelating ligands pro-
vide more flexibility for controlling the physical and thermal prop-
erties of the complexes by tuning the substituents at the imino
functionality or the backbone. Oxo and alkoxide groups have
advantageous features in synthesizing single source metal oxide
precursors. As O-bound ligands, they can serve as the oxygen
source for materials growth. Tungsten-oxygen multiple bonding
in the oxo ligands reduces bond cleavage requirements during
deposition. Bulky tert-butoxide and the smallest alkoxide methox-
ide have been chosen as ligands in this study, to allow exploration
of the influence of steric bulk and mixed N/O chelating ligands.
Absorption coefficient (mmꢀ1)
F(0 0 0)
Crystal size (mm)
Theta range for data collection (°)
Index ranges
0.221 ꢂ 0.095 ꢂ 0.044
1.906–27.499
ꢀ13 ꢃ h ꢃ 13, ꢀ17 ꢃ k ꢃ 17,
ꢀ
20 ꢃ l ꢃ 20
Reflections collected
Independent reflections (Rint
Completeness to theta = 25.242°
Absorption correction
Refinement method
61 410
)
9941 (0.0166)
100.0%
numerical
Full-matrix least-squares on F2
9941/504/483
1.064
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R
R
1
= 0.0272, wR
= 0.0310, wR
2
= 0.0551 [9078]
= 0.0579
3.2. Synthesis
1
2
Extinction coefficient
n/a
Largest difference peak
5.508 and ꢀ3.202
b-Ketoimines and b-iminoesters were synthesized by condensa-
tion reactions between the corresponding b-diketone/b-ketoester
and a suitable amine source according to modified literature proce-
ꢀ3
and hole (e Å
)
P
P
P
P
2
2
) ]/ (w(F 2
2
2
1/2
R
1
=
(||F
o
| ꢀ |F
c
||)/ |F
o
|
wR
w = 1/[
2
= [ [w(F
o
ꢀ F
c
o
) ]]
P
2
2
2
2
1/2
2
2
) + (m*p)2 + n*p]
S = [ [w(F
o
)
ꢀ F
c
) ]/(n-p)]
r
(F
o
2
2
p = [max(F
o
,0) + 2*F
c
]/3, m & n
dures [28,30]. The mononuclear complexes WO(OR)
L = b-ketoiminate or b-iminoesterate) were prepared by addition
3
L (R = alkyl,
are constants
t
of the bidentate ligand to [WO(OMe)
4 2 4
] or WO(O Bu) in ether
(Scheme 1). After reacting at room temperature for 12 h, all vola-
tiles were removed to give the crude product, which can be further
purified by sublimation or distillation. Attempts to synthesize and
Me
Et
3
purify WO(OMe) L complexes where L = acNac or acNac failed,
due to rapid decomposition of the products. The reactivity of b-
ketoiminate ligands towards tungsten oxo alkoxide complexes is
significantly affected by the steric bulk of their imino side chains.
t
tBu
t
The N- butyl compound acNac H does not react with WO(O Bu)
4
or [WO(OMe)
tion time.
4 2
] even at elevated temperature with increased reac-
3.3. X-Ray crystallographic structure determination of 1
Single crystals of complex 1 that were suitable for molecular
structure determination (Table 1) were grown by cooling a concen-
trated solution of 1 in toluene layered with hexane to ꢀ3 °C. There
are two crystallographically independent but chemically equiva-
lent molecules per asymmetric unit. One of the molecules is pre-
sented in Fig. 1. Selected bond angles and lengths are given in
Table 2. Complex 1 exhibits a distorted octahedral geometry
around the metal center with an equatorial plane defined by a
tert-butoxide, the terminal oxo and the bidentate b-ketoiminate
ligand. The other two chemically equivalent tert-butoxide ligands
occupy non-linear axial positions (O5B-W1B-O3B, 163.96(16)°)
bending away from the terminal oxo group. Besides the effect of
steric repulsion, the distortion of normal octahedral geometry is
also influenced by the strong multiple bonding of W–O(oxo),
which is facilitated by the restricted bite angle of the chelating b-
ketoiminate ligand (O2B-W1B-N1B, 79.41(13)°) [17,37].
Fig. 1. A representation of 1 with thermal displacement ellipsoids shown at 40%.
geometry between the oxo ligand and O (O,O-trans) due to the
trans effect of W–O(oxo) [37–39]. Compared with compounds of
Despite the possibility of isomers due to the unsymmetrical
3
the type [WO(OR) (acac)], whose W–O bonds trans to the oxo
nature of the acNac ligand, complex 1 shows an exclusive trans
groups are typically ꢁ2.2 Å [17], the analogous W1B–O2B bond

5
52
X. Su et al. / Polyhedron 157 (2019) 548–557
Table 2
Selected bond distances (Å) and angles (°) for complex 1.
Bond parameter
Bond Parameter
W1B–O1B
W1B–O3B
W1B–O5B
C1B–C2B
O5B–W1B–O3B
O4B–W1B–N1B
O1B–W1B–O2B
1.727(3)
1.897(4)
1.893(3)
1.369(6)
163.96(16)
169.50(13)
165.90(13)
W1B–O2B
W1B–O4B
W1B–N1B
C2B–C3B
O2B–W1B–N1B
O1B–W1B–O4B
O1B–W1B–N1B
2.084(3)
1.880(3)
2.175(3)
1.414(6)
79.41(13)
103.84(13)
86.62(14)
distance (2.084(3) Å) for complex 1 is significantly shorter.
Although solid state pure acNacH appears to be more enaminoke-
tone than enol [40], the C2B-C3B bond of 1 (1.414(6) Å) is notice-
ably longer than the C1B–C2B bond (1.369(6) Å). These bond
lengths are consistent with the enolate being the major resonance
contributor to the overall complex structure.
Scheme 2. Structures of compounds 1–6.
3.4. NMR spectroscopy
In the 1H NMR spectra of 1–6, there are two sets of symmetry
inequivalent tert-butoxide signals in a 2:1 intensity ratio, which
shows that the complexes are all mer isomers. According to the
NMR spectra, only one isomer of compounds 1–3 and 6 was
formed. In contrast, the NMR spectra of 4 and 5 indicate the pres-
ence of two isomers. Due to the unsymmetrical nature of the
chelating ligand, there are two possible mer isomers of [WO
(
3
OR) L] (L = b-ketoiminate or b-iminoesterate) complexes: O,O-
trans and O,N-trans. For compounds 1–3 and 6, the oxygen is trans
to the terminal oxo, as favored by the trans effect of the W-O(oxo)
unit. The O,O-trans geometries of 2, 3 and 6 were inferred from the
crystallographic structure of 1, which indicates the O,O-trans iso-
mer instead of O,N-trans. In contrast to the single isomers of 1–3
and 6, compounds 4 and 5, which are N-alkylated, are formed as
1
a pair of isomers. For the crude product before sublimation,
H
NMR indicates that 4A is the major isomer with the presence of
traces of 4B (<5%). After sublimation, 4B is the major component
of the mixture (4A ꢁ25%, 4B ꢁ75%). These results are consistent
with isomerization of the kinetic product 4A (O,O-trans) to the
thermodynamic product 4b (O,N-trans) as the mixture is heated
during sublimation. For compound 5, which has a somewhat larger
N-alkyl group than 4 (ethyl vs. methyl), both the crude and dis-
tilled product of compound 5 are composed of 5A and 5B in a
roughly 1:1 ratio.
Fig. 2. Thermogravimetric analysis plots for compounds 1–3 and 6.
the ester group when compared with b-diketonates [16]. The
observed residual masses of complexes 1 and 3 are nearly identical
to the calculated residual mass of WO , implying clean decomposi-
3
tion to the oxide. Complexes 2 and 6 have higher residual mass
than the calculated WO3 percentage (Table 3), consistent with
some incorporation of material from the ligands into the final pyr-
olyzed product. The difference between the residual mass and the
For compounds 1–3 and 6, a long-range HAH coupling is
b
a
4
observed:
(
H
is
a
doublet coupled with
H
( JHH ꢄ 1.6 Hz)
calculated WO percentage from 1-3 and 6 demonstrates that tert-
3
1
Scheme 2). This long range coupling is not observed in the
H
butyl groups (1 and 3) provide easier fragmentation and cleaner
NMR spectrum of the free ligand and is attributed to the ‘‘W”
geometry enforced when the bidentate ligand is coordinated with
tungsten [41].
decomposition compared with methyl and ethyl derivatives (2
and 6). The comparison of compound 1 and 6, which only differ
from each other in the steric bulk of alkoxides, demonstrates that
the application of sterically crowded ligands can improve the ther-
mal behavior of precursors [42].
3.5. Thermogravimetric analysis
The thermal properties of compounds 1–3 and 6 were studied
3.6. Mass spectrometry
using thermogravimetric analysis (Fig. 2). All compounds display
stepwise decomposition with major mass loss from the initial
heating stage until 200 °C. Compounds 1–3 bearing large tert-
butoxide ligands show a larger mass loss rate below 150 °C com-
pared with methoxide complex 6, which is consistent with early
loss of tert-butyl groups. Compounds 2 and 3, which have b-imi-
noesterate ligands, start to decompose at lower temperatures than
b-ketoiminate complexes 1 and 6, indicating that b-iminoesterate
ligands can provide easier decomposition pathways. This observa-
tion is similar to our previous finding during the study of com-
Mass spectrometry (MS) has been utilized to study deposition
mechanisms of CVD precursors [43–45]. Careful interpretation of
Table 3
Actual residual mass and the calculated WO
3
percentage for compounds 1–3 and 6.
Residual mass (%)
3
Calculated WO percentage (%)
1
2
3
6
43
49
42
66
45
42
40
59
3
plexes WO(OR) L (R = alkyl; L = b-ketoesterates, b-diketonates)
that b-ketoesterates offer more facile fragmentations involving

X. Su et al. / Polyhedron 157 (2019) 548–557
553
Table 4
Selected fragments observed in positive ion mass spectra of 1–3 and 6.
WO(OR)
3
L
[M-HOR+K/H]+
[M-L]+
[M-OR-R+H/K]+
[M-L-R+H/K)]+
[M-OR-HOR]+
m/z (%)
m/z (%)
m/z (%)
m/z (%)
m/z (%)
t
(acNac) (1)a
(etNac) (2)b
n.o.d
n.o.
n.o.
328.0 (83)
WO(O Bu)
3
3
3
3
466.4 (100)
512.1 (91)
502.2 (77)
360.0 (100)
419.2 (51)
n. o.
419.1 (29)
293.0 (67)
388.3 (27)
456.0 (61)
446.1 (32)
n.o.
363.3 (83)
401.0 (10)
363.1 (44)
n.o.
t
WO(O Bu)
t
b
WO(O Bu)
(tbNac) (3)
WO(OMe)
(acNac) (6)c
Compounds were analyzed by (a) DIPCI, (b) ESI-MS/MS, (c) DIPEI, (d) n.o. = not observed.
mass spectrometry data is required because the ionic fragments
observed in MS are not the neutral species commonly produced
upon thermolysis under film growth conditions [43,44]. Selected
ions observed in positive ion mass spectra of 1–3 and 6 are shown
3.7. Thermolysis studies
Each compound 1 and 2 was heated in an airtight vial and the
volatile decomposition fragments were analyzed by GC–MS. In
comparison with mass spectrometry, thermolysis explores how
precursors decompose when heated and the connection to CVD is
more straightforward. Major fragments and their abundance for
each compound are shown in Scheme 4. The identification of iso-
butylene, tert-butanol and acetone provides insight into the
decomposition mechanism for the tert-butoxide ligand. During
thermolysis, the tert-butoxide ligand may abstract a proton to give
tert-butanol and isobutylene, which converts the alkoxide ligand
into a second oxo moiety. This finding is consistent with the pres-
+
in Table 4. In tandem ESI mass spectrometry for 2 and 3, [M+K] for
+
compound 2 and [M+H] for compound 3 were selected as the
parent ions to undergo further fragmentation. Fragmentation pat-
terns show that decomposition can start with a loss of alkoxide
+
or the chelating ligand. The higher abundance of [M-OR] than
+
[
M-L] suggests more lability of alkoxides than b-ketoiminate/b-
iminoesterate ligands, which is not surprising given the chelating
+
+
nature of the latter. The ions [M-OR-R+H/K] and [M-L-R+H/K)]
observed for compounds 1–3 indicate pathways for dealkylation
of a tert-butoxide ligand to generate a second oxo group, leading
+
+
ence of [M-OR] and [M-OR-R+H/K] ions in mass spectrometry.
The tert-butoxyl radical can also decompose to give acetone and
a methyl radical [46]. The higher abundance of acetone fragments
for compound 1 compared with compound 2 is attributed to
decomposition of the acNac ligand to certain extent. The thermol-
ysis of free acNacH yields acetone as its major fragmentation pro-
duct (99.76%). The presence of ethanol in the decomposition
products from compound 2 shows that the b-iminoesterate ligand
introduces an additional decomposition pathway in which proto-
nation of the ester alkyl-oxygen is followed by a loss of alcohol.
to the formation of tungsten dioxo species as previously observed
t
for the complexes WO(OR)
3
L (R = Np, Bu, C(CH
3
)
2
CF
3 3
, C(CF )
2 3
CH ;
L = b-ketoesterate) [16,17]. The chelating ligand or a tert-butoxide
group can be protonated from an adjacent tert-butoxide function-
ality with a loss of tert-butanol/HL and isobutylene to generate
+
WO
2
(OR)
2
species. For compound 6, [M-OR] can lose MeOH
through protonation of methoxide by the imino proton to generate
+
[
M-OR-HOR] which is illustrated in Scheme 3.
x
3.8. Characterization of WO films
Tungsten oxide films were grown by AACVD on silicon <100>
substrates at 250, 300, 350, and 450 °C using complex 1 as the pre-
cursor. The Scotch tape test demonstrated robust adhesion of WO
films grown at various temperatures on Si substrates (Fig. S8). The
films grown at 250 °C were blue while the films grown at higher
x
Scheme 3. Proposed decomposition mechanism based on the presence of [M-OR-
HOR]+ for compound 6.
Scheme 4. (a) Thermolysis products of compounds 1 and 2; (b) proposed decomposition mechanism.

5
54
X. Su et al. / Polyhedron 157 (2019) 548–557
temperatures also had some yellow tint. According to XPS analysis,
The grain size is significantly reduced with the increase in deposi-
tion temperature.
all the films had good coverage of the substrates with no Si (ꢃ0.1 at
%
) detected in samples grown from 250 to 350 °C and a small
X-ray photoelectron spectroscopy (XPS) was used to determine
the composition of the films grown from precursor 1. The elemen-
tal compositions and stoichiometric values are reported in Fig. 4.
Nitrogen (ꢃ3 at%) was not detected as a significant component of
the deposits at any of the deposition temperatures. The C 1s bind-
ing energies (BE) are within the range of 284.5 ± 0.5 eV, suggesting
that C is free rather than bound to W or O [33,47]. The as-deposited
films grown at 250 °C had an O:W ratio of 2.58. After the films
amount of Si (ꢃ3 at%) detected in samples grown at 450 °C. This
result is consistent with our previous study using b-diketonate/b-
ketoesterate tungsten (VI) oxo-alkoxides in that a small amount
of the substrate elements could be detected in films deposited at
and above 450 °C [33]. The plane-view SEM images of tungsten
oxide films are shown in Fig. 3. The materials grown at 250 °C
are amorphous with no texture observed. The films grown at
+
3
00 °C appear to have scattered large grains, 200–300 nm in size,
were sputtered using 2 kV Ar source for 2 min to remove surface
which indicates a transition from amorphous films to polycrys-
talline structures. As the temperature increases to 350 and
contaminants, a relatively low O:W ratio of 1.46 was observed.
Although an additional 2 min of sputtering only caused the W
and O composition to vary within a small range (<0.6 at%), the
4
50 °C, tungsten oxides with grain-like morphology are deposited.
2
Fig. 3. Plane-view SEM images of samples grown on Si <100> at (a) 250, (b) 300, (c) 350 and (d) 450 °C from precursor 1 dissolved in diglyme using N as carrier gas.
Fig. 4. (a) Overall W, O, C, and N elemental fractions and (b) O:W atomic ratio variation derived from XPS data vs. deposition temperature for samples grown from precursor 1.

X. Su et al. / Polyhedron 157 (2019) 548–557
555
Fig. 5. Deconvolution of W 4f core-level spectra for samples grown from precursor 1 at (a) 450 °C; (b) 350 °C; (c) 300 °C and (d) 250 °C after sputtering. Note: XPS data of as-
deposited WO films are available in supplementary information (Fig. S11).
x
6
+
measurements are consistent with preferential O sputtering at the
deposit surface [48]. There is a rapid increase of O:W ratio to 2.79
when the deposition temperature was elevated to 300 °C. With fur-
ther increase of growth temperature from 300 to 450 °C, the O:W
ratio increases slowly ranging from 2.79 to 2.92. The W 4f core-
level spectra for samples grown at different temperatures were
deconvoluted to determine the oxidation state of tungsten in the
films (Fig. 5). The curves of the W 4f core level spectra for films
grown from 300 to 450 °C could be fitted with two spin–orbit dou-
tures demonstrate a greater preference for the higher valent W
5
+
tungsten state compared with W . The W 4f spectrum for deposits
grown at 250 °C can be deconvoluted into three doublet peaks with
binding energies for W 4f7/2 peak at 35.3 eV, 32.7 eV and 31.1 eV,
6
+
4+
5+
0
which corresponds to W , W or W , and W .
XRD patterns were acquired for the deposited films (Fig. 6). The
patterns from films deposited at 250 and 300 °C are featureless
with only Si reflections from the substrate evident. The diffraction
peaks at 23.1 and 24.0° 2h for films deposited at 350–450 °C are
blets, attributed to W6 (W 4f7/2: 35.5 ± 0.3 eV) and W (W 4f7/2
+
5+
:
assigned to crystalline WO
2h assigned to non-stoichiometric WO2.9 (JCPDS card no. 18-
417). These tungsten oxides can be characterized by their unique
colors. Stoichiometric WO is yellow and sub-stoichiometric WO2.9
3
(JCPDS card no. 20-1324) with 33.7°
3
3.8 ± 0.3 eV) of W atoms [49,50]. Films grown at higher tempera-
1
3
is blue. These deposited films have tints of blue and yellow when
viewed from different angles, which suggests coexistence of both
WO and WO2.9 phases.
3
4
. Conclusion
3
Tungsten (VI) oxo-alkoxide complexes WO(OR) L bearing b-
ketoiminate or b-iminoesterate ligands have been synthesized,
characterized and evaluated as AACVD precursors for deposition
x
of WO films. The steric bulk of the substituent on the imino nitro-
gen of the chelating ligand influences the population of O,O-trans
and O,N-trans isomers of these complexes. Possible mechanisms
for formation of W-O(oxo) bonds by loss of alcohols and HL during
deposition are consistent with the results of mass spectrometry
and thermolysis studies. Non-stoichiometric WO
deposited from compound 1 via AACVD. Films grown at 300–
50 °C show O:W ratios ranging from 2.79 to 2.92, while the films
x
films were
4
grown at 250 °C have a significantly lower O:W ratio. Powder X-ray
diffraction demonstrates that deposits grown at temperatures of
3
50 °C and above contain the crystalline phases of WO
3
and
Fig. 6. XRD patterns for samples grown from precursor 1.
WO2.9. Further growth studies with other complexes (e.g., 2–6)

5
56
X. Su et al. / Polyhedron 157 (2019) 548–557
would help to clarify how the incorporation of nitrogen content is
influenced by different N-containing precursors and deposition
conditions. The electronic and chemical properties of WO films
x
are affected by the concentrations of nitrogen doping, thus control
of the N content could facilitate potential applications in photo-
catalysis and photochromism.
[17] R.O. Bonsu, D.C. Bock, H. Kim, R.Y. Korotkov, K.A. Abboud, T.J. Anderson, L.
2
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x
i
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[(O
(
2
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n
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[
[
[
19] D. Baxter, H.M. Chisholm, S. Doherty, E.N. Gruhn, Chemical vapour deposition
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Acknowledgements
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 Duane Bock, Frank
Schreffler, Nathan Richey and Will Carden for their assistance with
materials characterization.
[
[
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Appendix A. Supplementary data
CCDC 1862469 contains the supplementary crystallographic
data for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data to this article can be found online at
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