Volume 45 Number 27 21 July 2016 Pages 10853–11200
Dalton
Transactions
An international journal of inorganic chemistry
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ISSN 1477-9226
PAPER
Lisa McElwee-White et al.
Synthesis and evaluation of 2--diketonate and -ketoesterate tungsten(VI)
oxo-alkoxide complexes as precursors for chemical vapor deposition of
WOx thin films
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Cite this: Dalton Trans., 2016, 45,
10897
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Synthesis and evaluation of κ2-β-diketonate
and β-ketoesterate tungsten(VI) oxo-alkoxide
complexes as precursors for chemical vapor
deposition of WOx thin films†
Richard O. Bonsu,‡a Duane C. Bock,‡a Hankook Kim,‡b Roman Y. Korotkov,c
Khalil A. Abboud,a Timothy J. Andersonb and Lisa McElwee-White*a
Reactions of [WO(OR)4]x (x = 1, 2) complexes with bidentate ligands (LH = acacH, tbacH, dpmH, tbpaH)
afforded complexes 1–13: [WO(OCH3)3(acac) (1); WO(OCH2CH3)3(acac) (2); WO(OCH(CH3)2)3(acac) (3);
WO(OCH3)3(tbac) (4); WO(OCH2CH3)3(tbac) (5); WO(OCH(CH3)2)3(tbac) (6); WO(OCH2CH3)3(dpm) (7);
WO(OCH(CH3)2)3(dpm) (8); WO(OCH2C(CH3)3)3(acac) (9); WO(OCH2C(CH3)3)3(tbac) (10); WO(OCH2C(CH3)3)3(dpm) (11); WO(OCH2C(CH3)3)3(tbpa) (12); WO(OC(CH3)3)3(tbac) (13)]. The synthesis is facilitated
by the lability of the bridging ligands of the [WO(OR)4]2 complexes in solution, which provides a pathway
for exchange of L with an alkoxide ligand. Thermogravimetric analysis and the conditions for sublimation
or distillation of 1–13 demonstrate that they have sufficient vapor pressure and thermal stability for
Received 19th March 2016,
Accepted 29th April 2016
DOI: 10.1039/c6dt01078d
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volatilization in a conventional Chemical Vapor Deposition (CVD) reactor. High solubility in hydrocarbon
and ether solvents establishes that the complexes are also potential candidates for Aerosol-Assisted
Chemical Vapor Deposition (AACVD). AACVD from 10 on ITO or bare glass resulted in growth of continuous, dense and amorphous thin films of substoichiometric WOx between 250–350 °C and nanorods of
W18O49 above 350 °C.
Introduction
Thin films of tungsten oxide are attractive materials for applications in organic and hybrid electronic devices. Notable are
anodic and cathodic charge extraction layers in Organic Light
Emitting Diodes (OLEDs),1,2 anodic layers in near-infrared
absorbing dye-based photovoltaics,3–6 electrochromic layers in
smart windows,7–9 energy storage compartments in photovoltaics, 10 photoanodes for photoelectrocatalysis, 11 and
a
Department of Chemistry, University of Florida, Gainesville, FL, 32611-7200 USA.
E-mail: [email protected]
b
Department of Chemical Engineering, University of Florida, Gainesville, FL,
32611-6005 USA
c
Arkema Inc., 900 First Ave., King of Prussia, PA, 19406 USA
† Electronic supplementary information (ESI) available: Synthesis of 11a.
Crystallographic structure determination of 11a. 1H NMR and 13C NMR spectra
of complexes 1–13 and 19. Molecular geometries of 11 and 11a. Table of bond
distances, bond angles, atomic coordinates, and equivalent isotropic displacement parameters for 1, 10, 11, and 11a. Thermal behaviours of 1, 2, and 5–13
based on TG measurements. Relative abundances for positive ion DIPCI mass
spectra of complexes 1, 2, and 10–13. Deconvoluted XPS spectra of films grown
from 10 at 200 °C and 500 °C. Selected properties of 1–13. CCDC
1454825–1454828. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6dt01078d
‡ These authors made equal contributions to the work.
This journal is © The Royal Society of Chemistry 2016
components in chemical and biological sensing devices.12,13
The versatility of WOx for applications is derived from its high
work function, visible light transparency, electrical and mechanical robustness, large surface area-to-volume ratio of vapor
phase deposits and electrochromism. Although WO3 itself contains a d0 transition metal in its fully oxidized W6+ state, the
capability of chemical vapor deposition (CVD) to produce substoichiometric WOx material with partial occupation of W in
lower oxidation states (W5+ and W4+) offers the possibility of
controlling the electrical properties of the material.14,15
Deposition by CVD can provide access to conformal, highly
uniform and robust layers of WOx thin films for application in
a variety of organic and inorganic devices.16–21 Precursors possessing high volatility are required for CVD processes that volatilize the precursors through evaporation or sublimation.22–24
However, in aerosol-assisted chemical vapor deposition
(AACVD), solutions of the precursors are aerosolized, replacing
the need for volatility in the precursor itself with a requirement for solubility and stability in a suitably volatile organic
solvent.16,25–28 Injection of the precursor aerosol mist into the
reactor chamber not only promotes higher mass transport of
the precursor leading to high material growth rate but can
allow the deposition reaction to run at moderate temperatures
that are cost effective.25,28,29
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Although the oxo-alkoxide complexes [WO(OR)4]n have been
used as precursors for CVD growth of WOx materials, they are
air- and water-sensitive.30,31 In our hands, NMR samples of
WO(OtBu)4 were observed to decompose in solution after
2 hours of exposure to air. Fluorinated derivatives of
WO(OR)4, such as WO(OCH2CF3)4,32 WO[OC(CH3)2CF3]4 and
WO[OC(CF3)2CH3]4 33,34 have been reported to serve as precursors for growth of WOx films and nanostructures but there
is still incentive to prepare and test additional precursors with
greater air- and water-stability and whose syntheses do not
require fluorinated starting materials.
Compounds of the type [WO(OR)3L] (R = tBu, iPr, L = acac,
hfac) have been previously synthesized and used for growth of
electrochromic WOx materials by LPCVD.31 The thermal behavior and vapor pressure of two closely related WO(OtBu)3L
precursors possessing β-ketoesterate ligands (L = methyl
acetoacetate and ethyl acetoacetate) were examined in 2008.35
In both instances, the WO(OR)3L complexes were readily
obtained from the corresponding monomeric oxo alkoxide
complex [WO(OtBu)4]. Precursors of the type [WO(OR)3L] (L =
acac, dpm, tbac) with R groups that are less sterically bulky
than isopropyl have not been reported. Due to the interest in
continued development of precursors for deposition of high
quality WOx thin films for electronic and optoelectronic device
applications, we have synthesized a series of [WO(OR)3L] complexes with less bulky alkoxide ligands (R = Me, Et, iPr, Np, L =
acac, dpm, tbac, tbpa) and prescreened them as potential precursors for deposition of WOx materials. CVD experiments
with WO(OCH2C(CH3)3)3(tbac) (10) resulted in growth of WOx
films and nanostructures, with the morphology of the deposited material depending on the growth temperature.
Experimental details
General procedures
All reactions were carried out under an atmosphere of dry
nitrogen using either glovebox or standard Schlenk techniques. All chemicals used were of reagent grade. Methylene
chloride and toluene (Fisher Scientific) were purified using an
MBraun MB-SP solvent purification system and stored over
activated 3 Å molecular sieves for at least 48 h prior to experiments. Diethylether and tetrahydrofuran were dried using
sodium/benzophenone, distilled, and stored over activated 3 Å
molecular sieves for at least 48 h prior to experiments. Hexamethyldisiloxane, acetylacetone (Hacac), tert-butylacetylacetonate (Htbac), methanol, ethanol, isopropanol, neopentanol
(Sigma-Aldrich), dipivaloylmethane (Hdpm, Matrix Scientific)
and benzene-d6 (Cambridge Isotopes) were stored over 4 Å
molecular sieves for at least 48 h before use. tert-Butylpivaloylacetate (Htbpa) was synthesized and characterized as reported
in the literature.36 WCl6, anhydrous diethylamine (SigmaAldrich) and anhydrous ammonia (AirGas) were used as
received. WOCl4 37 (14), WO[OC(CH3)3]4 37 (15), [WO(OCH3)4]2
(16), [WO(OCH2CH3)4]2 (17) and [WO(OCH(CH3)2)4]2 38 (18)
were synthesized and characterized according to literature
10898 | Dalton Trans., 2016, 45, 10897–10908
Dalton Transactions
procedures. Compound 19 was prepared following the same
route as compounds 16–18. NMR spectra were recorded on a
Varian Mercury 300BB (300 MHz) spectrometer using residual
protons from deuterated solvents for reference. Elemental analyses were performed at the University of Florida or by Complete Analysis Laboratory Inc. (Parsippany, NJ). The mass
spectrometry was conducted on a Thermo Scientific Trace GC
DSQ equipped with 70 eV electron ionization (EI), direct insertion probe (DIP) using chemical ionization (CI) with methane
as the reagent gas or an Agilent 6200 ESI-TOF mass spectrometer using DART-TOF mode of operation.
WO(OCH3)3(acac) (1). In the glovebox, [WO(OCH3)4]2 (16,
1.54 mmol, 1.00 g) was transferred into a 50 mL Schlenk flask
containing a stir bar. THF (30 mL) was added and stirred to
produce a clear yellow solution. Acetylacetone (3.09 mmol,
0.320 mL, 0.312 g) was added dropwise to the stirring solution
of 16 in THF. A reflux condenser was fitted to the Schlenk
flask and the entire reaction set-up was moved to the Schlenk
line and refluxed for 3 h. The solution became clear and
brighter, approaching a yellowish-orange color. Volatiles were
removed under dynamic vacuum and the yellowish-orange
solid residue was kept under vacuum for about 3 h to dry. The
product was sublimed at 60–65 °C (100–150 mTorr) to afford
the product as yellowish-white crystals in 77% yield (0.86 g).
1
H NMR (C6D6, 25 °C): δ 5.03 (s, 1H, COCHCO), 4.68 (s, 3H,
OCH3), 4.45 (s, 6H, OCH3), 1.66 (s, 3H CH3), 1.60 (s, 3H, CH3).
13
C{1H} NMR (C6D6, 25 °C): δ 194.60 (OC(CH3)CH), 186.80
(OC(CH3)CH), 104.68 (CHC(CH3)O), 65.80 (OCH3), 63.65
(OCH3), 27.65 (CH3), 26.43 (CH3). Anal. Calcd for WO6C8H16:
C, 24.51; H, 4.11. Found: C, 24.63; H, 3.96%. Crystals for single
crystal X-ray crystallographic determination were obtained
from the sublimate.
WO(OCH2CH3)3(acac) (2). Compound 2 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH2CH3)4]2 (17, 1.32 mmol, 1.00 g) and Hacac (2.63 mmol,
0.27 mL, 0.263 g), a yellowish-orange moist semi-solid was
obtained after removal of the volatiles under vacuum. This
residue was sublimed at 60–65 °C (150–200 mTorr) to afford
an orange semi-solid product in 63% yield (0.72 g). 1H NMR
(C6D6, 25 °C): δ 5.15 (s, 1H, COCHCO), 5.00 (q, 2H, OCH2CH3,
J = 7.0 Hz), 4.73 (dq, 2H, OCH2CH3, J = 10.8, 7.0 Hz), 4.68 (dq,
2H, OCH2CH3, J = 10.8, 7.0 Hz), 1.73 (s, 3H, CH3), 1.66 (s, 3H,
CH3), 1.28 (t, 3H, CH3CH2O, J = 7.0 Hz), 1.19 (t, 6H, CH3CH2O,
J = 7.0 Hz). 13C{1H} NMR (C6D6, 25 °C): δ 193.27 (OC(CH3)CH),
186.22 (OC(CH3)CH), 103.78 (CHC(CH3)O), 74.25 (OCH2CH3),
71.55 (OCH2CH3), 27.02 (CH3), 25.92 (CH3), 18.61 (CH3CH2O),
17.60 (CH3CH2O). Anal. Calcd for WO6C11H22: C, 30.43; H,
5.11. Found: C, 27.64; H, 4.67%.
WO[OCH(CH3)2]3(acac) (3). Compound 3 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH(CH3)2)4]2 (18, 1.15 mmol, 1.00 g) and Hacac (2.29 mmol,
0.25 mL, 0.23 g) a yellow liquid residue was obtained after
removal of the volatiles. This residue was vacuum distilled at
60–65 °C (150–200 mTorr) to afford a yellowish-orange liquid
in 53% yield (0.58 g). 1H NMR (C6D6, 25 °C): δ 5.35 (sp, 1H,
OCH(CH3)2, J = 6.2 Hz), 5.16 (s, 1H, COCHCO), 5.15 (sp, 2H,
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OCH(CH3)2, J = 6.2 Hz), 1.74 (s, 3H, CH3), 1.69 (s, 3H, CH3),
1.37 (d, 6H, (CH3)2CHO, J = 6.6 Hz), 1.30 (d, 6H, (CH3)2CHO,
J = 6.2 Hz), 1.20 (d, 6H, (CH3)2CHO, J = 6.2 Hz). 13C{1H} NMR
(C6D6, 25 °C): δ 192.84 (OC(CH3)CH), 186.40 (OC(CH3)CH),
103.82 (CHC(CH3)O), 81.47 (OCH(CH3)2), 78.59 (OCH(CH3)2),
27.34 (CH3), 26.39 (CH3), 25.70 ((CH3)2CHO), 25.50
((CH3)2CHO), 24.50 ((CH3)2CHO). Anal. Calcd for WO6C14H28:
C, 35.31; H, 5.93. Found: C, 35.19; H, 6.01%.
WO(OCH3)3(tbac) (4). Complex 4 was synthesized following
the same procedure as used for 1. Starting with [WO(OCH3)4]2
(16, 1.54 mmol, 1.00 g) and Htbac (3.08 mmol, 0.52 mL,
0.50 g) a yellowish-orange liquid residue was obtained after
removal of the volatiles under vacuum. The crude product
was vacuum distilled at 70–75 °C (200–250 mTorr) to afford
a yellowish-orange oil in 68% (0.95 g) yield. 1H NMR
(C6D6, 25 °C): 4.90 (s, 1H, CHC(CH3)O), 4.80 (s, 3H, OCH3),
4.39 (s, 3H, OCH3), 1.72 (s, 3H, CH3), 1.32 (s, 9H, C(CH3)3).
13
C{1H} NMR (C6D6, 25 °C): δ 181.81 (C(CH3)CH), 173.40
(COC(CH3)3CH), 92.82 (CHCOC(CH3)), 82.74 ((CCH3)3), 65.79
(OCH3), 63.21 (OCH3), 28.69 ((CH3)3C), 25.46 (CH3). Anal.
Calcd for WO7C11H22: C, 29.35; H, 4.93. Found: C, 30.09;
H, 5.03%.
WO(OCH2CH3)3(tbac) (5). Complex 5 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH2CH3)4]2 (17, 1.32 mmol, 1.00 g) and Htbac (2.63 mmol,
0.45 mL, 0.43 g) a yellowish-orange liquid residue was obtained
after removal of the volatiles under vacuum. The crude liquid
was vacuum distilled from 60 to 65 °C (150–200 mTorr) to
afford a yellowish-orange oil in 51% yield (0.66 g). 1H NMR
(C6D6, 25 °C): δ 4.94 (s, 1H, CHC(CH3)O), 4.93 (q, 2H,
OCH2CH3, J = 7.0 Hz), 4.92 (dq, 2H, OCH2CH3, J = 10.8, 7.0
Hz), 4.87 (dq, 2H, OCH2CH3, J = 10.8, 7.0 Hz), 1.73 (s, 3H,
CH3), 1.35 (s, 9H, C(CH3)3), 1.27 (t, 3H, CH3CH2O, J = 7.0 Hz),
1.18 (t, 6H, CH3CH2O, J = 7.0 Hz). 13C{1H} NMR (C6D6,
25 °C): δ 182.40 (C(CH3)CH), 173.81 (CO(CH3)3CH), 92.76
(CHCO(CH3)), 82.69 (C(CH3)3), 74.74 (OCH2), 71.77 (OCH2),
29.12 ((CH3)3C), 25.93 (CH3), 19.17 (CH3CH2O), 18.49 (CH3CH2O).
Anal. Calcd for WO7C14H28: C, 34.16; H, 5.73. Found: C, 34.12;
H, 5.73%.
WO[OCH(CH3)2]3(tbac) (6). Complex 6 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH(CH3)2)4]2 (18, 1.32 mmol, 1.00 g) and Htbac (2.29 mmol,
0.40 mL, 0.37 g) a yellowish-orange liquid residue was obtained
after removal of the volatiles under vacuum. The crude liquid
was vacuum distilled at 60–65 °C (150–200 mTorr) to afford a
yellowish-orange oil in a yield of 0.68 g (55%). 1H NMR (C6D6,
25 °C): δ 5.24 (sp, 1H, OCH(CH3)2, J = 6.2 Hz), 5.10 (sp, 2H,
OCH(CH3)2, J = 6.2 Hz), 4.97 (s, 1H, CHCO(CH3)), 1.73 (s, 3H,
CH3), 1.39 (s, 9H, C(CH3)3), 1.39 (d, 6H, (CH3)2CHO, J = 5.7
Hz), 1.33 (d, 6H, (CH3)2CHO, J = 5.9 Hz), 1.22 (d, 6H,
(CH3)2CHO, J = 6.2 Hz). 13C{1H} NMR (C6D6, 25 °C): δ 182.24
(C(CH3)CH), 173.46 (CO(CH3)3CH), 91.95 (CHCO(CH3)), 82.02
(C(CH3)3), 80.98 (OCH(CH3)2), 77.78 (OCH(CH3)2), 28.91
((CH3)3C), 25.81 (CH3), 25.72 ((CH3)2CHO), 25.17 ((CH3)2CHO),
24.54 ((CH3)2CHO). Anal. Calcd for WO7C17H34: C, 38.22; H,
6.41. Found: C, 38.09; H, 6.54%.
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WO(OCH2CH3)3(dpm) (7). Complex 7 was synthesized following the same procedure as used for 1. Starting with
[WO(OCH2CH3)4]2 (17, 1.32 mmol, 1.00 g) and Hdpm
(2.63 mmol, 0.58 mL, 0.51 g), an orange liquid was obtained
after removal of volatiles under vacuum. The crude liquid was
vacuum distilled at 70–80 °C (300–350 mTorr) to afford a clear
orange liquid in a yield of 0.8 g (60%). 1H NMR (C6D6, 25 °C):
δ 5.83 (s, 1H, CHC(CH3)O), 4.98 (q, 2H, OCH2CH3, J = 7.0 Hz),
4.72 (dq, 2H, OCH2CH3, J = 10.8, 7.0 Hz), 4.68 (dq, 2H,
OCH2CH3, J = 10.8, 7.0 Hz), 1.28 (t, 3H, CH3CH2O, J = 7.0 Hz),
1.16 (t, 6H, CH3CH2O, J = 7.0 Hz), 1.13 (s, 9H, C(CH3)3), 1.07
(s, 9H, C(CH3)3). 13C{1H} NMR (C6D6, 25 °C): δ 203.22
(OCC(CH3)3CH), 196.40 (OCC(CH3)3CH), 94.30 (CHCOC(CH3)3),
73.19 (OCH2CH3), 71.02 (OCH2CH3), 41.25 (C(CH3)3), 40.78
(C(CH3)3), 28.39 (C(CH3)3), 27.97 (C(CH3)3), 18.87 (CH3CH2O),
17.72 (CH3CH2O). Anal. Calcd for WO6C17H34: C, 39.40; H, 6.61.
Found: C, 39.18; H, 6.66%.
WO[OCH(CH3)2]3(dpm) (8). Complex 8 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH(CH3)2)4]2 (18, 1.15 mmol, 1.00 g) and Hdpm
(2.29 mmol, 0.50 mL, 0.44 g), an orange liquid was obtained
after removal of volatiles under vacuum. The crude liquid was
vacuum distilled at 70–80 °C (250–300 mTorr) to afford a clear
orange liquid in a yield of 0.72 g (55%). 1H NMR (C6D6, 25 °C):
δ 5.83 (s, 1H, CHC(CH3)O), 5.35 (sp, 1H, OCH(CH3)2 J =
6.2 Hz), 5.04 (sp, 2H, OCH(CH3)2, J = 6.2 Hz), 1.38 (d, 6H,
(CH3)2CHO, J = 6.6 Hz), 1.32 (d, 6H, (CH3)2CHO, J = 6.2 Hz),
1.18 (d, 6H, (CH3)2CHO, J = 6.2 Hz), 1.15 (s, 9H, C(CH3)3),
1.09 (s, 9H, C(CH3)3). 13C{1H} NMR (C6D6, 25 °C): δ 202.70
(OCC(CH3)3CH), 196.64 (OCC(CH3)3CH), 94.36 (CHCOC(CH3)3),
80.25 (OCH(CH3)2), 77.27 (OCH(CH3)2), 41.40 (C(CH3)3), 41.04
(C(CH3)3), 28.72 (C(CH3)3), 28.11 (C(CH3)3), 25.98 ((CH3)2CHO),
25.44 ((CH3)2CHO), 24.62 ((CH3)2CHO). Anal. Calcd for
WO5C21H42: C, 42.87; H, 7.20. Found: C, 42.78; H, 7.28%.
WO[OCH2C(CH3)3]3(acac) (9). Complex 9 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH2C(CH3)3)4]2 (19, 0.910 mmol, 1.00 g) and Hacac
(0.910 mmol, 0.10 mL, 0.09 g), a bright yellow-orange
liquid was obtained after removal of volatiles under vacuum.
The liquid residue was vacuum distilled at 60–65 °C
(200–250 mTorr) to afford a yellow-orange oil in 52% (0.53 g)
yield. 1H NMR (C6D6, 25 °C): δ 5.11 (s, 1H, CHC(CH3)O), 4.78
(s, 2H, OCH2), 4.47 (d, 2H, OCH2, J = 10.5 Hz), 4.43 (d, 2H,
OCH2, J = 10.6 Hz), 1.74 (s, 3H, CH3), 1.68 (s, 3H, CH3), 1.06
(s, 9H, C(CH3)3), 0.97 (s, 18H, C(CH3)3). 13C{1H} NMR (C6D6,
25 °C): δ 193.89 (OC(CH3)CH), 186.34 (OC(CH3)CH), 104.36
(CHC(CH3)O), 89.06 (OCH2), 87.09 (OCH2), 35.47 (C(CH3)3),
35.21 (C(CH3)3), 26.43, 26.41 (CH3)3C). Anal. Calcd for
WO6C21H42: C, 42.87; H, 7.20. Found: C, 42.88; H, 7.16%.
WO[OCH2C(CH3)3]3(tbac) (10). Complex 10 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH2C(CH3)3)4]2 (19, 0.910 mmol, 1.00 g) and Htbac
(0.910 mmol, 0.20 mL, 0.18 g), a yellow-orange moist semisolid was obtained upon removal of volatiles under vacuum.
The residue was sublimed at 55–60 °C (150–200 mTorr) to
yield a pale orange waxy crystalline solid in a yield of 0.62 g
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(55%). 1H NMR (C6D6, 25 °C): δ 4.95 (s, 1H, CHC(CH3)O), 4.75
(s, 2H, OCH2), 4.47 (d, 2H, OCH2, J = 10.6 Hz), 4.43 (d, 2H,
OCH2, J = 10.6 Hz), 1.75 (s, 3H, CH3), 1.42 (s, 9H, OC(CH3)3),
1.06 (s, 9H, C(CH3)3), 0.98 (s, 18H, C(CH3)3). 13C {1H} NMR
(C6D6, 25 °C): δ 182.36 (C(CH3)CH), 173.79 (COC(CH3)3CH),
92.80 (CHCO(CH3)3), 89.78 (OCH2), 86.80 (OCH2), 82.59
(OC(CH3)3), 35.23 (C(CH3)3), 29.20 (C(CH3)3), 27.39 ((CH3)3C),
27.33 (CH3). Anal. Calcd for WO7C24H48: C, 44.67; H, 7.50.
Found: C, 44.49; H, 7.32%. Crystals for single crystal X-ray crystallographic determination were obtained from the sublimate.
WO[OCH2C(CH3)3]3(dpm) (11). Complex 11 was synthesized
following the same procedure as used for 1. Starting with
[WO(OCH2C(CH3)3)4]2 (19, 0.910 mmol, 1.00 g) and Hdpm
(0.910 mmol, 0.20 mL, 0.18 g), the yellow-cream dense residue
obtained after removal of volatiles was sublimed at 90–100 °C
(300–350 mTorr) to yield a cream solid in a yield of 0.62 g
(55%). 1H NMR (C6D6, 25 °C): δ 5.83 (s, 1H, CHC(CH3)O), 4.67
(s, 2H, OCH2), 4.38 (d, 2H, OCH2, J = 10.6 Hz), 4.35 (d, 2H,
OCH2, J = 10.6 Hz), 1.12 (s, 9H, (CH3)3), 1.08 (s, 9H, (CH3)3),
1.00 (s, 9H, C(CH3)3, 0.89 (s, 18H, C(CH3)3. 13C{1H} NMR
(C6D6, 25 °C): δ 203.23 (OCC(CH3)3CH), 196.58 (OCC(CH3)3CH),
94.73 (CHCOC(CH3)3), 87.79 (OCH2), 86.34 (OCH2), 41.62
(C(CH3)3), 41.14 (C(CH3)3), 34.99 (C(CH3)3CH2O), 34.87
(C(CH3)3CH2O), 28.89 ((CH3)3C), 28.36 ((CH3)3C), 27.11
(CH3)3CCH2O), 27.01 (CH3)3CCH2O). Anal. Calcd for
WO6C26H52: C, 48.45; H, 8.13. Found: C, 48.14; H, 8.40%. Crystals for X-ray crystallographic study were grown by cooling a
toluene solution of 11 to −3 °C.
WO[OCH2C(CH3)3]3(tbpa) (12). Complex 12 was synthesized
following the same procedure as used for 1. Starting with
WO(OCH2C(CH3)3)4 (19, 1.79 mmol, 0.984 g) and Htbpa
(1.80 mmol, 0.360 g), the yellow-white solid obtained
after removal of volatiles was sublimed at 90–100 °C
(200–250 mTorr) to yield a white solid in a yield of 0.456 g
(39%). 1H NMR (C6D6, 25 °C): δ 5.27 (s, 1H, CHC(CH3)O), 4.79
(s, 2H, OCH2), 4.55 (d, 2H, OCH2, J = 10.5 Hz), 4.44 (d, 2H,
OCH2, J = 10.5 Hz), 1.45 (s, 9H, COOC(CH3)3), 1.16 (s, 9H,
COC(CH3)3), 1.10 (s, 9H, C(CH3)3), 1.03 (s, 18H, C(CH3)3).
13
C{1H} NMR (C6D6, 25 °C): δ 191.90 (OCC(CH3)3CH), 174.68
(OCC(CH3)3CH), 89.64 (CHCOC(CH3)3), 88.96 (OCH2), 86.67
(OCH2), 82.60 (OC(CH3)3), 40.45 (C(CH3)3), 35.61 (C(CH3)3CH2O),
35.27 (C(CH3)3CH2O), 29.17 ((CH3)3C), 29.11 ((CH3)3C), 27.43
((CH3)3CH2O), 27.33 ((CH3)3CH2O). Anal. Calcd for WO7C26H52:
C, 47.28; H, 7.94. Found: C, 47.28; H, 8.10%.
WO[OC(CH3)3]3(tbac) (13). In the glovebox, WO[OC(CH3)3]4
(15, 2.03 mmol, 1.00 g) was dissolved in THF (40 mL) inside a
100 mL Schlenk flask and tbacH, (2.03 mmol, 0.337 mL) dissolved in THF (20 mL), was placed inside an addition funnel
fitted to the Schlenk flask. The Schlenk flask was placed in an
ice bath for 30 min. The THF solution of Htbac was added
dropwise to the stirring solution of 15 in the Schlenk flask.
After the addition, the yellowish mixture was kept in the ice
bath for additional 30 min, after which the mixture was
allowed to warm to room temperature and stirred for the next
12 h. Volatiles were removed under vacuum to obtain a pale
yellow waxy solid which was sublimed overnight between
10900 | Dalton Trans., 2016, 45, 10897–10908
Dalton Transactions
55–70 °C (350 mTorr) to afford white needles in a yield of
0.46 g (39%). 1H NMR (C6D6, 25 °C): δ 5.03 (s, 1H, CHC(CH3)
O), 1.74 (s, 3H, CH3), 1.54 (s, 9H, COC(CH3)3), 1.47 (s, 18H,
OC(CH3)3), 1.43 (s, 9H, OC(CH3)3). 13C{1H} NMR (C6D6, 25 °C):
δ 181.53 (C(CH3)CH), 174.07 (C(CH3)CH), 90.45 (CHCOC(CH3)),
81.44 ((CCH3)3), 80.98 ((CCH3)3), 30.98 ((CH3)3C), 30.32
((CH3)3C)), 28.62 ((CH3)3C)), 25.78 (CH3). Anal. Calcd for
WO7C19H38: C, 41.68; H, 7.00. Found: C, 41.49; H, 7.21%.
WO(OCH2C(CH3)3)4 (19). In the glovebox, WOCl4 (14,
8.77 mmol, 3.00 g) was transferred into a 500 mL Schlenk flask
charged with a stir bar. THF (60 mL) was slowly added and
stirred into an orange solution. Neopentyl alcohol (35.1 mmol,
3.10 g) was dissolved in a total of 20 mL THF and added dropwise to the solution of 14 in THF. An immediate change in
color from orange to pale green was observed. A dry ice condenser was fitted to the Schlenk flask and the entire setup was
transferred to a Schlenk line in a fume hood. The apparatus
was purged with argon, after which the condenser was filled
with dry ice and acetone and allowed to sit for 20 min. To the
stirring mixture of 14 and neopentyl alcohol in THF, anhydrous diethylamine (10.0 mL, 4.5 equiv.) was added dropwise
from a syringe. A vigorous reaction ensued and was
accompanied by formation of a white precipitate, which
caused the reaction medium to become viscous. The mixture
was stirred for 30 min after addition of the diethylamine. The
volatiles were removed under vacuum to obtain a whitish
residue, which was transferred into the glovebox. The residue
was dissolved in toluene, stirred and filtered under Celite to
yield a pale yellow filtrate. The filtrate was transferred to the
Schlenk line and the solvent was removed under vacuum. The
product was kept under vacuum overnight to obtain 19 as a
whitish-cream solid. Yield 3.82 g, 79%. 1H NMR (C6D6, 25 °C):
4.40 (s, 8H, CH2), 0.98 (s, 36H, CH3). 13C{1H} NMR (C6D6,
25 °C): δ 86.29 (OCH2), 34.34 (C(CH3)3), 26.29 (C(CH3)3). MS
(DART-TOF) m/z calcd for [WO5C20H45]+ 549.2776, Found
549.2780.
Crystallographic structure determination of 1, 10 and 11
X-Ray intensity data for 1, 10 and 11 were collected at 100 K on
a Bruker DUO diffractometer using Mo Kα radiation (λ =
0.71073 Å) and an APEXII CCD area detector. Raw data frames
were read by program SAINT39,40 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 absorptions corrections were
applied based on indexed and measured faces.
The structure of 1 was solved and refined in SHELXTL6.1,39
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
of 1, 2692 reflections (of which 2128 are observed with I >
2σ(I)) were used to refine 141 parameters and the resulting R1,
wR2 and S (goodness of fit) were 1.15%, 2.39% and 1.088,
respectively. The refinement was carried out by minimizing
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the wR2 function using F2 rather than F values. R1 is calculated
to provide a reference to the conventional R value but its function is not minimized.
The structure of 10 was solved and refined in
SHELXTL6.1,39 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 W
complex has two disordered regions. The alkyl group on O2 is
completely disordered and refined in two parts. Atoms C1–C5
are refined against atoms C1′–C5′ along with their protons. In
the second disorder, atoms C21–C25 and C21′–C25′, except
C22 which is common to both parts, make up the disordered
parts. In the final cycle of refinement of 10, 6565 reflections
(of which 5004 are observed with I > 2σ(I)) were used to refine
292 parameters and the resulting R1, wR2 and S (goodness of
fit) were 2.50%, 4.98% and 1.005, respectively. The refinement
was carried out by minimizing the wR2 function using F2
rather than F values. R1 is calculated to provide a reference to
the conventional R value but its function is not minimized.
The structure of 11 was solved and refined in
SHELXTL2013,40 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 chemically equivalent but crystallographically independent molecules. For molecule A, four
t-butyl groups on C2a, C7a, C11a and C23a are disordered. For
molecule B, C2b, C11b, C15b and C23b are also disordered. All
disordered regions were refined in two parts with their site
occupation factors adding up to one, for each one respectively.
A handful of residual electron density peaks were observed in
the final refinement cycles but are attributed to ghost peaks
after checking into the correctness of absorption corrections
applied. In the final cycle of refinement of 11, 14 158 reflections
(of which 10 570 are observed with I > 2σ(I)) were used to refine
567 parameters and the resulting R1, wR2 and S (goodness of
fit) were 4.27%, 9.10% and 1.075, respectively. The refinement
was carried out by minimizing the wR2 function using F2 rather
than F values. R1 is calculated to provide a reference to the conventional R value but its function is not minimized.
Thermolysis of 10
In the glove box, complex 10 (0.124 g, 0.201 mmol) was placed
in a Schlenk tube. This tube was attached via Tygon tubing to
a U tube. This apparatus was placed on a Schlenk line. A flow
of nitrogen gas was introduced to the Schlenk tube through a
needle. The gas flow rate was monitored by a bubbler attached
to the opposing outlet of the U tube. The U tube was placed in
a liquid nitrogen bath. A sand bath was used to heat the
Schlenk tube to 250 °C for 30 minutes. Complex 10 formed a
black product within the Schlenk tube. A white product was
collected in the U tube. The U tube was removed from the liquid
nitrogen bath and allowed to warm for a minute. CDCl3 (2 mL)
was added to the U tube to dissolve the thermolysis products.
This solution was used for 1H NMR and GC-MS analysis.
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Paper
WOx materials growth and characterization
Solutions of complex 10 (0.034 M in diglyme) were prepared
in a nitrogen filled glovebox. The aerosol of the solution was
generated by using a nebulizer with a quartz plate vibrating
at 1.44 MHz, and delivered to the deposition system with
nitrogen (99.999% purity, Airgas) carrier gas at a flow rate of
1000 sccm. ITO covered borosilicate glass substrates sitting
on a SiC coated graphite susceptor were heated to 200–550 °C
by coupling the susceptor to a radio frequency power supply.
During the tungsten oxide growth, the chamber pressure
was maintained at 350 Torr for a total deposition time of
150 min.
The elemental compositions of tungsten oxide samples
were determined by X-ray photoelectron spectroscopy (XPS,
Perkin-Elmer PHI 5100) after 10 min Ar+ sputtering to clean
the surface. The crystallinities and morphologies were
measured by X-ray diffraction (XRD, Panalytical X’pert Pro)
and field emission scanning electron microscope (FESEM, FEI
Nova NanoSEM 430). Atomic force microscopy (AFM, Veeco
Dimension 3100) was used to measure the surface roughness.
The details of the growth conditions and characterizations are
reported elsewhere.33,34
Results and discussion
Precursor design
Design of single-source precursors for AACVD of metal oxide
thin films has emphasized complexes bearing terminal oxo,
alkoxide, β-diketonate and related ligands because these
ligands could be varied to impart the high volatility or solubility needed for use in CVD. Additionally, decomposition of
alkoxide ligands can provide mechanistic pathways for incorporation of oxygen into the deposited films via cleavage to
oxo moieties.41 Terminal oxo ligands in the precursor itself
provide this source of oxygen for materials growth with fewer
bond cleavage steps required.
The chelating ligands acetylacetonate, dipivaloylmethanate, tert-butylacetylacetonate and tert-butylpivaloylacetate
ensure that no empty coordination site is available for intermolecular interactions that can give rise to oligomer formation, and provide improved stability to oxygen and
water.16,42 The use of chelating ligands has proven to be
effective for isolating monomeric compounds bearing alkoxide groups less bulky than tert-butoxide, which would oligomerize in the absence of the chelator.38 Moreover, the
presence of sterically bulky chelates (acac, tbac, dpm and
tbpa) in the coordination sphere of the metal could induce
volatility43 or facile reactivity and ligand lability.44 Reactivity
of the β-diketonate and β-ketoesterate ligands with the polymeric tungsten(VI) alkoxide molecule [WO(OR)4]n is engineered to break symmetry in the resulting monomeric
molecule WO(OR)3L. The highly unsymmetrical WO(OR)3L
target precursors could thus possess sufficient solubility and
volatility to achieve acceptable materials growth under moderate reaction conditions in an AACVD reactor.
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Synthesis
The alkoxide-bridged dimers [WO(OR)4]2 (16–18) and the
monomeric complexes WO(OtBu)4 (15) and WO(ONp)4 (19)
react with β-diketones and β-ketoesters (Hacac, Htbac, Hdpm
and Htbpa) under reflux for 3 h to form the mononuclear complexes WO(OR)3L (1–13; R = methyl, ethyl, isopropyl, neopentyl, L = acac, dpm, tbac, tbpa) as shown in Schemes 1 and 2.
Complex 13 could also be synthesized from 15 and Htbac
under much milder conditions by addition at 0 °C and then
stirring at room temperature for 12 h. Syntheses of 1–8 were
facilitated by the dynamic behavior of dimers 16–18 in solution, which can be observed at room temperature.38 The fluxionality of the bridging alkoxides allows facile ligand exchange
with the bidentate β-diketonate and β-diketoesterate ligands
under reflux. This preparatory route offers a simple workup
that affords clean products in high yields. The only byproducts
are alcohols that are sufficiently volatile to be removed under
vacuum with the reaction solvent.
The pure products were obtained by either sublimation or
vacuum distillation depending on the nature of the crude
product. Compounds 1, 10, 11, 12 and 13 were isolated as
solids with yellow-orange, pale orange, light brown and
whitish colors, respectively. Compound 2 was a moist orange
semi-solid that solidifies upon freezing (−3 °C). The rest of the
complexes were yellow-orange liquids of low viscosity, with
exception of 9 that was slightly viscous.
Reaction of 19 with 4.5 equiv. of Hdpm under conditions
similar those used for synthesis of 11 resulted in formation of
a mixture of 11 and the dioxo tungsten(VI) complex WO2(dpm)2
(11a). The presence of 11a as a side product suggests scission
of an alkoxide C–O bond to form the second dioxo ligand in
Scheme 1
Synthesis of 1–8.
Scheme 2
Synthesis of 9–13.
10902 | Dalton Trans., 2016, 45, 10897–10908
11a, which was identified by X-ray crystallography (ESI,
Fig. S26 and S28†). The cleavage process could be facilitated by
steric repulsions among the ligands. The proportion of 11a in
the product mixture was observed to generally increase with
increasing amounts of Hdpm in the reaction mixture (to
8 equiv.) and increasing the reaction time to 2 days under reflux
(Fig. S26†), although complete conversion to 11a could not be
achieved by further extension of the reaction time to 4 days.
NMR spectroscopy of 1–13
The 1H NMR and 13C NMR spectra of 1–13 contained resonances with splitting patterns and intensities consistent with
the structural assignments. In the 1H NMR spectrum of each
compound, there are two sets of symmetrically inequivalent
alkoxide signals in a 2 : 1 intensity ratio. The integrations of all
of the 1H NMR spectra confirm that only one β-diketonate/
β-ketoesterate ligand is coordinated to the tungsten. Further,
all the spectra displayed symmetry inequivalence between the
substituents on the chelating β-diketonate/β-ketoesterate
ligands, which is consistent with their assignment as the mer
isomers.
X-Ray crystallographic structure determination of 1, 10 and 11
The crystalline samples obtained by sublimation of 1 and 10
contained single crystals that were suitable for crystallographic
data collection. The single crystals of 11 were grown from
toluene solution by cooling. Fig. 1–3 contain the molecular
structures of 1, 10 and 11, respectively, and selected bond
length and angles can be found in Table 1. The crystal structure of 11 contained two independent molecules, one of which
is shown in Fig. 3 (see Fig. S27, ESI† for the geometry of the
second independent molecule of 11). The crystal and structure
refinement data are in Table 2.
The coordination geometries of 1, 10 and 11 in the solid
state adopt the anticipated monomeric structures. The three
structures show distorted octahedral geometry about the W(VI)
center. In the context of the distorted octahedral structure of
1, 10 and 11, the terminal oxo, an alkoxide and the chelating
β-diketonate ligands are defined to occupy equatorial positions, which are almost in plane with the W. The two symmetrically equivalent alkoxide ligands occupy the two axial
Fig. 1 Displacement ellipsoids diagram of the molecular geometry of 1
drawn at 40% probability.
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diketonate ligands were determined to be 80.68(6)°, 80.78(8)°
and 80.35(16)° for 1, 10 and 11 respectively (Table 1), and are
typical of β-diketonate moieties coordinated to a d0 transition
metal in a pseudo-octahedral environment.46 The trans influence exerted by the oxo moiety on the β-diketonate or β-ketoesterate ligand is quite pronounced. As a result, the W–O bonds
trans to the oxo ligands are lengthened to 2.2087(16) Å, 2.2454(19)
Å and 2.217(4) Å for 1, 10 and 11, respectively (Table 1), and
are each an average of 0.2 Å longer than other the W–O bond
to the β-diketonate or β-ketoesterate ligand.
Fig. 2 Displacement ellipsoids diagram of the molecular geometry of
10 drawn at 40% probability.
Fig. 3 Displacement ellipsoids diagram of the molecular geometry of
11 drawn at 40% probability.
positions but are not quite linear with O–W–O bond angles of
161.78(7)°, 159.90(8)° and 161.8(8)° in 1, 10 and 11, respectively (Table 1). The alkoxide ligands are bent away from the
oxo but towards the β-diketonate/β-diketonate ligand.
A similar distortion has been observed in other d0 transition metal oxo complexes. Angular distortion of the axial
H–Mo–H bonds away from the oxo ligands in cis-[MoO2H4]2−
has been studied by EHT and attributed to strong interaction
of the metal orbitals in the x–y plane with the p-orbitals of
oxygen, which optimizes W–O(oxo) π-bonding.45 Strengthening
of the W–O(oxo) π-bonding is also facilitated by the small bite
angles of the β-diketonate ligands. The bite angles of the beta-
Table 1
Mass spectrometry
Table 3 highlights the abundant ions observed in the positive
ion chemical ionization mass spectra of 1, 2 and 9–13. The low
abundances of the molecular ions (2–7%) are consistent with
extensive fragmentation in the gas phase following ionization
(see Table S2, ESI†). Fragmentation tendencies during mass
spectrometry have been used to model the gas phase portions
of precursor decomposition during CVD, although care must
be taken in interpretation.47,48 Complexes 1, 2 and 11 exhibit
similar fragmentation pathways, in which net loss of alkoxide
generates the [M − OR]+ species as the base peak. For the less
sterically crowded 1 and 2, these [M − OR]+ ions undergo subsequent reaction with molecular ions to generate the dinuclear
species [M + (M − OR)]+. The analogous [M + (M − OR)]+ ion is
formed in small quantities from the more hindered neopentoxide complex 9. For complexes 10–13, the [M − OR]+ ions are
evident in the spectra but the [M + (M − OR)]+ ions could not
be detected because their m/z values fall outside the range of
the instrument.
An alternative pathway evident in Table 3 involves initial
loss of β-diketonate/β-ketoesterate fragments to yield the ion
[M − L]+, which can be observed for 1, 2 and 9–13. For 1, 2 and
9, the [M − L]+ ion proceeds to react with starting material,
resulting in small amounts of M + [M − L]+, as shown in
Table S2 (ESI†). For 10–12, the m/z value of the M + [M − L]+
ion falls outside the instrument range and thus could not be
detected. However, the M + [M − L]+ ion is not present for the
tert-butoxide compound 13, although its m/z value would lie
within the range of the instrument.
Comparison of the behavior of acac complexes 1, 2 and 9
provides information on the preferred sites for gas phase protonation and ligand loss as the steric demand of the alkoxides
Selected bond distances (°) and angles (°) for compounds 1, 10 and 11
Bond parameter
1
Bond parameter
10
Bond parameter
11
W1–O1
W1–O2
W1–O3
W1–O4
W1–O5
W1–O6
O5–W1–O6
O2–W1–O3
O1–W1–O2
O2–W1–O4
1.7175(16)
2.0450(16)
2.2087(16)
1.8723(16)
1.8793(15)
1.8807(15)
161.78(7)
80.68(6)
95.54(7)
164.55(7)
W1–O1
W1–O4
W1–O6
W1–O2
W1–O3
W1–O5
O3–W1–O5
O4–W1–O6
O1–W1–O4
O2–W1–O4
1.699(2)
2.021(2)
2.2454(19)
1.860(2)
1.897(2)
1.882(2)
159.90(9)
80.78(8)
93.91(9)
165.9(9)
W1A–O1A
W1A–O4A
W1A–O6A
W1A–O2A
W1A–O3A
W1A–O5A
O3A–W1A–O5A
O4A–W1A–O6A
O1A–W1A–O4A
O2A–W1A–O4A
1.702(5)
2.008(4)
2.217(4)
1.866(4)
1.877(5)
1.877(5)
161.8(3)
80.35(16)
94.2(2)
164.58(18)
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Table 2
Dalton Transactions
Crystal data and structure refinements for 1, 10 and 11
Identification code
1
10
11
Formula
Formula weight
Temperature
Crystal system
Space group
a(Å)
b(Å)
c(Å)
α/β/γ(°)
Z
Volume
Density (calculated)
Crystal size (mm3)
Absorption coefficient
F(000)
Index range
C8H16O6W
392.06
100(2) K
Monoclinic
P21/n
7.3458(5)
12.0008(8)
13.3320(9)
90/97.5390(10)/90
4
2108.0(2) Å3
2.235 mg m−3
0.09 × 0.09 × 0.03
9.921 mm−1
744
−9 ≤ h ≤ 9, −14 ≤ k ≤ 15,
−17 ≤ l ≤ 17
2.29 to 27.50°.
2692 [R(int) = 0.0205]
100.0%(theta = 25.50)
1.088
2692/0/141
Analytical
R1 = 0.0115, wR2 = 0.0239 [2128]
R1 = 0.0180, wR2 = 0.0267
0.496 and −0.579 e Å−3
C23H46O6W
618.45
100(2) K
Monoclinic
P21/n
9.7653(5)
17.3223(9)
17.2212(9)
90/101.5039(10)/90
4
2854.6(3) Å3
1.439 mg m−3
0.180 × 0.079 × 0.047
4.081 mm−1
1256
−12 ≤ h ≤ 12, −22 ≤ k ≤ 21,
−22 ≤ l ≤ 18
1.685 to 27.499°.
6565 [R(int) = 0.0490]
100.0%(theta = 25.242)
1.005
6565/0/292
Integration
R1 = 0.0250, wR2 = 0.0498
R1 = 0.0447, wR2 = 0.0547
1.184 and −0.729 e Å−3
C26H52O6W
644.52
100(2) K
Monoclinic
P21/c
19.3583(14)
19.5078(14)
16.7970(12)
90/103.5832(14)/90
8
6165.8(8) Å3
1.389 mg m−3
0.368 × 0.232 × 0.102
3.779 mm−1
2640
−25 ≤ h ≤ 25, −25 ≤ k ≤ 25,
−21 ≤ l ≤ 21
1.082 to 27.499°.
14 158 [R(int) = 0.0398]
100.0% (theta = 25.242)
1.075
14 158/0/567
Analytical
R1 = 0.0427, wR2 = 0.0910 [10 570]
R1 = 0.0679, wR2 = 0.1158
3.333 and −2.696 e Å−3
Theta range for data coll.
Independent reflections
Completeness to theta
GOF on F2
Data/restraints/param.
Abs. corr.
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff peak and hole
R1 = ∑(||Fo| − |Fc||)/∑|Fo|. wR2 = [∑[w(Fo2 − Fc2)2]/∑(w(Fo2)2)]1/2. S = [∑[w(Fo2)2 − Fc2)2]/(n − p)]1/2. w = 1/[σ2(Fo2) + (m × p)2 + n × p], p = [max(Fo2,0) +
2 × Fc2]/3, m & n are constants.
Table 3
Selected relative abundances (m/z) of ions in the positive ion DIPCI mass spectra of 1, 2, 9, 10, 11, 12, and 13
WO(OR)3(L)
[M − OR]+
m/z (%)
[M + (M − OR)]+
m/z (%)
[M − L]+
m/z (%)
[HOR + H]+
m/z (%)
[R]+
m/z (%)
[HL + H]+
m/z (%)
WO(OMe)3(acac) (1)
WO(OEt)3(acac) (2)
WO(ONp)3(acac) (9)
WO(ONp)3(tbac) (10)a
WO(ONp)3(dpm) (11)
WO(ONp)3(tbpa) (12)b
WO(OtBu)3(tbac) (13)
361 (100)
389 (100)
473 (14)
351 (31)
557 (100)
573 (55)
502 (14)
753 (23)
823 (28)
1033 (4)
n. o.
n. o.
n. o.
n. o.
293 (31)
335 (39)
461 (2)
461 (46)
461 (9)
461 (20)
419 (13)
n. o.b
n. o.
89 (28)
89 (58)
89 (34)
n. o.
n. o.
n. o.
n. o.
71 (3)
71 (100)
71 (17)
71 (100)
57 (100)
101 (5)
101 (3)
101 (100)
159 (18)
185 (10)
n. o.
n. o.
a
Complex 10 also yields a large peak for a ligand fragment at m/z 57 (72%). See ESI for detailed data. b n. o. = not observed.
increases. The base peak for complexes 1 and 2 is [M − OR]+,
which suggests protonation at an alkoxide oxygen followed by
loss of the alcohol to generate [M − OR]+. However, due to
increased bulk of the neopentoxide ligands in compound 9,
the diketonate ligand is preferentially protonated over the alkoxide by the chemical ionization and the dominant pathway is
loss of the acac ligand to produce [HL + H]+ as the base peak.
In compound 11, where the acac ligand is the sterically hindered dpm, protonation of the acac ligand is also less accessible and therefore the loss of the neopentoxide ligand becomes
predominant. Complexes bearing ketoesterate ligands (10, 12
and 13) show significant alkoxide fragmentation to afford the
alkyl cation as their base peaks. This suggests a facile pathway
for conversion of an alkoxide ligand to a second oxo moiety, a
process we have previously noted in the mass spectra of the
10904 | Dalton Trans., 2016, 45, 10897–10908
related oxo alkoxide complexes WO[OC(CH3 )2 CF 3] 4 and
WO[OC(CF3 )2 CH 3 ] 4 .33,49
Thermolysis of 10
Volatile decomposition fragments of compound 10 were isolated and analyzed using 1H NMR and GC-MS following
thermolysis. Neopentanol and the free β-ketoester ligand were
identified as the major components in the 1H NMR spectrum
of the trapped volatile products by comparison to authentic
samples. The identification of neopentanol was confirmed by
detection of mass spectral fragmentation patterns that match
authentic spectra in the NIST database.50 A proposed mechanism for the formation of neopentanol by thermolysis of 10
(Scheme 3) follows a pathway similar to that reported for
thermal decomposition of Ti(ONp)4.51
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Scheme 3
Paper
Proposed decomposition mechanism for 10.
Mass spectra of the thermolysis products also contained
tert-butyl acetoacetate, as identified by comparison to the NIST
database.50 Loss of β-ketoesterate ligands and formation of
acetylketene have been observed in the thermolysis of the
related compound Ti(OiPr)2(tbac)2.52 Mass spectra of thermolysis products from 10 are consistent with the decomposition
chemistry of Ti(OiPr)2(tbac)2, as the molecular ion for acetylketene was detected along with fragment ions from the
β-ketoesterate ligand.
Grown WOx material properties
The tungsten oxide materials grown at 200, 300, 400, and
500 °C from complex 10 appear to be blue in color, as expected
for substoichiometic WO3−x.53 These samples are shown in
plan-view and cross-sectional SEM images in Fig. 4. Films were
grown at low deposition temperatures (200–500 °C), with a
transition in growth habit from dense polycrystalline layers to
nanorod bundles as the growth temperature was raised to
500 °C. The film thicknesses are 80 (±6) and 538 (±9) nm for
film samples grown at 200 and 300 °C, respectively. The textured samples grown at 400 and 500 °C have vertical heights of
522 (±26) nm and 2.23 (±0.4) μm, each. The surface roughness
measured by AFM showed the dense polycrystalline films have
smooth surfaces with RMS roughness values of 7.8 (±0.2) and
6.0 (±0.6) nm. However, the textured sample grown at 400 °C
exhibited a higher roughness of 37.0 (±5.6) nm, and the roughness could not be measured for the nanorod sample grown at
500 °C.
XRD patterns were acquired for each of the films to better
understand the growth habit variations (Fig. 5). The scans for
Fig. 4 Plan-view (upper) and cross-sectional (lower) SEM images of
films grown on ITO/glass at (a) 200, (b) 300, (c) 400, and (d) 500 °C
from precursor 10 dissolved in diglyme in N2 carrier gas. Note not all
images are the same scale.
This journal is © The Royal Society of Chemistry 2016
Fig. 5 XRD patterns for samples grown from precursor 10 solution
under N2 at 4 temperatures.
the continuous films grown at the two lowest temperatures
reveal only a single sharp reflection assigned to ITO. A broad
hump at lower values of 2Θ is also evident, consistent with the
growth of amorphous WOx film. The samples grown at 400
and 500 °C, however, show two additional diffraction peaks
corresponding to the W18O49 monoclinic phase.54 The diffraction peak at 23.5 2Θ° matches a d-spacing of 3.78 Å, and
matches well the lattice parameter in the [010] direction. The
growth of highly textured films at high temperature is often
associated with increased surface mobility of adsorbed reactants, leading to increased crystallinity. A previous study
reports that W18O49 grown by CVD at high temperatures
afforded nanorod structures.33
To better understand the stoichiometry of the grown films,
XPS measurements were performed and the results are shown
in Fig. 6. The O 1s peaks have consistent binding energy (BE)
values in the small range 530.7–530.8 eV regardless of growth
temperature. These peaks correspond to oxygen bound to
tungsten of mixed oxidation state, likely W4+ and W6+.55,56 The
W 4f5/2 and 4f7/2 peak intensities decrease at low binding energies as deposition temperature increases. The XPS peaks of
samples grown at 200 and 500 °C were deconvoluted and compared after Shirley baseline subtraction (Fig. S30†). The W
peaks of sample grown at 200 °C show low BE peaks at 31.4
and 33.4 eV, which correspond to tungsten carbide,57–59 and
Fig. 6 XPS O 1s, C 1s, and W 4f5/2 & 4f7/2 doublet peaks for materials
grown from precursor 10 at different temperatures.
Dalton Trans., 2016, 45, 10897–10908 | 10905
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Paper
high BE peaks at 34.8 and 37.3 eV, which may match to
W6+.60–62 Meanwhile, the W peaks of sample grown at 500 °C
were deconvoluted to minor peaks at BEs of 32.4 and 33.7 eV,
which suggest W4+,63–65 and major peaks at BEs of 35.6 or 37.7
eV, which correspond to W6+,61,66,67 thus suggesting increasing
tungsten oxidation state as deposition temperature increases.
The XPS results were used to estimate overall film composition
(Fig. 7). The results show that the tungsten level decreases as
deposition temperature increases. The oxygen atom fraction
initially increases with temperature increase (200–350 °C);
however it decreases slightly at higher temperatures
(400–550 °C) along with increased carbon content. The stoichiometric value (O : W atomic ratio) of tungsten oxide (WOx)
increases as deposition temperature increases, and this may
correlate to the change in material crystallinity from amorphous to W18O49 monoclinic nanorods (O : W = 2.72).
Carbon was detected in samples grown both at the lowest
growth temperature (200 °C) and ≥400 °C. The carbon contamination at high temperatures is primarily attributed to the
larger surface area provided by the nanorod structures. Precursor and solvent decomposition to afford carbon contamination
has previously been detected in samples grown from other
tungsten oxo alkoxide precursors using the same solvent.33,34
Previously reported XPS measurements on deposited films,
however, showed the samples were essentially carbon-free with
maximum carbon contamination (≤3 at%) detected for just a
few samples. The C 1s peaks in Fig. 7 also show different BE
depending on the growth temperature. The sample grown at
200 °C has its C 1s peak at 283.0 eV, while the samples grown
Dalton Transactions
at 400 and 500 °C have their main peaks at 283.9 and 384.1 eV.
The peak at low BE of 283.0 eV corresponds to carbon in tungsten carbide,63,68 and peaks at BE of 284.0 eV are close to free
carbon.63,69 Thus, the carbon contamination in samples grown
at 200 °C is bound to tungsten, and carbon impurities on
samples grown at high temperatures mostly exist on the
surface. The formation of tungsten carbide under certain
growth conditions is consistent with the low carbon content
that is occasionally observed in the combustion analyses of
these compounds.
Conclusion
Reaction of the dinuclear species [WO(OR)4]2 with β-diketonate/β-ketoesterate ligands L generated mononuclear complexes of the type WO(OR)3L, that display sufficient solubility
and stability in volatile organic solvents to permit rapid growth
of WOx materials by AACVD. The facile synthesis of the complexes is facilitated by the fluxional motion of [WO(OR)4]2 in
solution, which provides a pathway for exchange of alkoxide
with L. Additionally, the low molecular symmetry of WO(OR)3L
resulted in several liquid precursors (3–9) that possess
sufficient vapor pressure for facile gas phase mass transport.
Growth of WOx by AACVD was demonstrated for precursor 10.
Characterization of materials deposited in the temperature
range 200 to 550 °C by XRD indicates growth of an amorphous
film at lower temperature transitioning to bundled nanorods
of monoclinic W18O49 as temperature was elevated. The amorphous substoichiometric WOx grown at 200 °C contained a
small amount of C (∼5 atomic%) as evidenced by observation
of an XPS peak with binding energy consistent with W–C.
Growth between 200 and 300 °C resulted in continuous,
dense, pure and amorphous thin films of substoichiometric
WOx. Increase in oxidation of the films upon growth above
400 °C coincides with crystallization of W18O49 nanorods, a
phenomenon that increased surface area and adsorbed C.
Acknowledgements
The authors thank the National Science Foundation for
support under the GOALI grant CHE-1213965. KAA wishes to
acknowledge the National Science Foundation and the University of Florida for funding of the purchase of the X-ray
equipment.
Notes and references
Fig. 7 (a) Film composition from XPS measurements and (b) O : W
atomic ratio variation with deposition temperature from complex 10 dissolved in diglyme.
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