Comparison of Ligand Architecture on Vapor Deposition Precursors: Synthesis, Characterization, and Reactivity of Volatile Cadmium Bis-Amidinate Complexes

Article abstract of DOI:10.1021/acs.inorgchem.0c03307

The lack of low-temperature (<200 °C) and easy-to-handle vapor deposition precursors for cadmium has been a limitation for cadmium chalcogenide ALD. Here, the cadmium amidinate system is presented as a scaffold for vapor deposition precursor design because the alkyl groups can be altered to change the properties of the precursor. Thus, the molecular structure affects the precursor stability at elevated temperature, onset of volatility, and reactivity. Cadmium bis-N,N-diisopropylacetamidinate (1) was synthesized and evaluated for its thermal stability, volatility, and reactivity-properties relevant to ALD precursors. Compounds 2, cadmium bis-N,N-diisopropyltertertiarybutylamidinate, and 3, cadmium bis-N,N-diisopropylbutylamidinate, are analogous to 1 and were synthesized by substituting the alkyl group on the bridging carbon during amidinate synthesis. All three compounds are volatile under reduced pressure, and thermal stability studies showed 1 and 3 to be stable at 100 °C in solution for days to weeks, while 2 decomposed at 100 °C within 24 h. Solution phase reactivity studies show 1 to be reactive with thiols at room temperature in a stoichiometric manner. No reactivity with either bis-silyl sulfides or alkyl sulfides was observed up to 110 °C over more than 3 days. Overall, the cadmium amidinate compounds presented here show potential as precursors in ALD/CVD processing, which can contribute to research critical for semiconductor processing.

Full text of DOI:10.1021/acs.inorgchem.0c03307

pubs.acs.org/IC
Article
Comparison of Ligand Architecture on Vapor Deposition Precursors:
Synthesis, Characterization, and Reactivity of Volatile Cadmium Bis-
Amidinate Complexes
Michael J. Foody, Matthew S. Weimer, Harish Bhandari, and Adam S. Hock*
Cite This: Inorg. Chem. 2021, 60, 6191−6200
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: The lack of low-temperature (<200 °C) and easy-
to-handle vapor deposition precursors for cadmium has been a
limitation for cadmium chalcogenide ALD. Here, the cadmium
amidinate system is presented as a scaffold for vapor deposition
precursor design because the alkyl groups can be altered to change
the properties of the precursor. Thus, the molecular structure
affects the precursor stability at elevated temperature, onset of
volatility, and reactivity. Cadmium bis-N,N-diisopropylacetamidi-
nate (1) was synthesized and evaluated for its thermal stability,
volatility, and reactivity−properties relevant to ALD precursors.
Compounds 2, cadmium bis-N,N-diisopropyltertertiarybutylamidi-
nate, and 3, cadmium bis-N,N-diisopropylbutylamidinate, are
analogous to 1 and were synthesized by substituting the alkyl
group on the bridging carbon during amidinate synthesis. All three compounds are volatile under reduced pressure, and thermal
stability studies showed 1 and 3 to be stable at 100 °C in solution for days to weeks, while 2 decomposed at 100 °C within 24 h.
Solution phase reactivity studies show 1 to be reactive with thiols at room temperature in a stoichiometric manner. No reactivity
with either bis-silyl sulfides or alkyl sulfides was observed up to 110 °C over more than 3 days. Overall, the cadmium amidinate
compounds presented here show potential as precursors in ALD/CVD processing, which can contribute to research critical for
semiconductor processing.
iminopyrrolidinate served to block the potential reaction
pathway for β-hydrogen elimination during deposition; thus
precursor modification can influence reaction mechanism.3
Expanding the number of available precursors and under-
standing their synthetic properties is crucial for developing
ALD processing. Cadmium chalcogenide thin films are a class
of II−VI semiconductors that would benefit from ALD
precursor development.4
Cadmium chalcogenide (CdE, E = S, Se, or Te) thin-films
are direct band gap semiconductors widely used in photo-
voltaic devices.5 They have also attracted interest in the
quantum dot literature due to readily modifiable properties.6
The most common precursor for growing cadmium-based
films by ALD is dimethyl cadmium (CdMe2). CdMe2 has some
favorable properties for ALD: not only is it highly reactive but
it is also a volatile liquid that can be delivered at room
INTRODUCTION
■
Atomic layer deposition (ALD) is a chemical vapor deposition
(CVD) technique that is used to fabricate thin films by
separating, both physically and temporally, two self-limiting
half-reactions between a surface of a film and a vapor-phase
precursor. The benefits of ALD include precise thickness
control, the ability to cover high aspect ratio surfaces
uniformly, and pinhole-free coverage.1 However, the efficacy
of an ALD process is largely dependent on the properties of
the available vapor precursors. A useful ALD precursor must
have self-limiting reactivity over a suitable reactor temperature
window, be volatile at a reasonable delivery temperature, and
be stable at those delivery temperatures for long periods of
time.
Metal−organic (MO) compounds are well suited to meet
the requirements of successful ALD precursors due to their
robust reactivity with protonated surfaces. The ligands of MO-
compounds can be synthetically modified to enhance their
useful ALD-properties. For example, modification of a ligand
from being monodentate to bidentate has been shown to
increase a precursor’s thermal stability in some cases.2 In
another study, Zaera et al., showed that by substituting a tert-
butyl group for a sec-butyl group in Cu(I)-N-sec-butyl-2-
Received: November 10, 2020
Published: April 14, 2021
https://doi.org/10.1021/acs.inorgchem.0c03307
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 6191−6200
6191

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 1. Two Synthetic Routes Towards Amidinate Synthesis
temperature.7 In proton transfer reaction mechanisms
(common in ALD), CdMe2 reacts with a protonated surface
(*E-H) to produce methane as a byproduct, which does not
etch the film or substrate and generates a surface cadmium
bond (*E-CdMe). However, CdMe2 is both highly toxic and
pyrophoric, which poses serious safety risks. It is also limited to
certain systems, as it has been observed to react in
uncontrolled ways, etching and desorbing surface metal ions,
most notably in binary metal chalcogenide systems (CdZnS).8
Metal amidinate complexes have generated interest over the
past decade as reactive, volatile, MO-precursors.9 Lithium
amidinate compounds can be generated either through
deprotonation of an amidine or through nucleophilic attack
by a carbanion (RLi, BrMgR, etc.) to the electrophilic carbon
of a carbodiimide (Scheme 1). Recently, Gordon et al.
synthesized calcium amidinate complexes directly from Ca0
and an amidine.10 The availability of three different alkyl
substitution positions allows for many different modifications
to the ligand. Additionally, it is possible to synthesize
asymmetric amidinates through a Ln3+ catalytic coupling
reaction from an alkyl amine and acetonitrile.11,12 Changing
the alkyl lithium salt used to attack the carbodiimide can have
the effect of varying the backbone alkyl groups, which directly
impacts the bite angle of the ligand.13 Varying the steric bulk of
the ligand has been shown to influence the structure and
properties of a compound.14
from Alfa Aesar. S(SiMe3)2 was prepared using previously reported
methods.19 1H (300 MHz) and 13C (75.4 MHz) NMR spectra were
recorded on a Bruker Avance 300 MHz instrument at ambient
temperature, unless otherwise stated. Chemical shifts were referenced
1
to internal H or 13C solvent peaks. Benzene-d6 was purchased from
Cambridge Isotopes, and toluene-d8 was purchased from Sigma-
Aldrich; both were degassed and dried over 4 Å molecular sieves for a
minimum of 48 h prior to use.
Synthesis involving cadmium requires extra safety precautions due
to the toxicity of cadmium. All cadmium waste was collected in a
waste stream separated from other lab waste. All glassware was acid
washed and soaked in a base bath prior to being recirculated into the
general lab glassware. Handling of cadmium containing compounds
was done with and abundance of caution in an atmospheric glovebox
designated for cadmium based synthesis. XL nitrile gloves were worn
over glovebox gloves during cadmium handling. Compounds 1−3
readily turn yellow and decompose when exposed to atmosphere;
however, they are not pyrophoric.
Synthesis. Cd(amd)2 (1). A solution of N,N′-diisopropylcarbodii-
mide (2.220 g, 17.6 mmol) was cooled to −30 °C in 60 mL of diethyl
ether, and MeLi (11.73 mL, 1.5 M, 17.6 mmol) was added dropwise.
The reaction was stirred at room temperature for 1 h and then cooled
to −30 °C before CdBr2 (2.395 g, 8.8 mmol) was added. The reaction
turned gray/white as it was stirred at room temperature for 12 h. The
precipitate was removed via filtration through Celite, and the solvent
was removed under reduced pressure. The solid was resuspended in
diethyl ether and filtered through Celite for a second time. The
solution was concentrated and kept at −30 °C overnight. 2.7 g (72%
yield based on CdBr2) of white crystalline solid was isolated after
crystallization. Sublimation can be performed at 20 mTorr and 95 °C.
Purity was confirmed by NMR and elemental analysis. The
complexity and broadening of the 1H NMR spectrum at room
temperature suggests the compound to be in equilibrium between the
monomer and dimer. Variable Temperature NMR experiments
support this hypothesis, and a crystal structure supports this
Gordon et al. were the first to synthesize a series of metal
amidinates and evaluate them as ALD precursors.15 Since then,
metal amidinates have been used in ALD for a wide variety of
materials.16−18 This work presents the synthesis and vapor
deposition properties of three cadmium bis-amidinate
compounds: the first cadmium amidinate species of their
kind reported in the literature. These precursors meet the
criteria for viable use as vapor phase precursors, which is of
critical importance to development in semiconductor process-
ing.
1
observation by showing 1 to be a dimer. Mp: 112 °C. H NMR
(C7D8, 25 °C, ppm): 3.60 (br, 4H), 1.87, 1.78, 1.71 (br, 6H −
mixture of monomer and dimer), 1.26, and 1.16 (br, 24H). Elemental
Analysis: Found C 48.70, H 8.92, N 14.17. Calculated C 48.67, H
8.68, N 14.19.
Cd(tbu-amd)2 (2). 2 was synthesized in the same manner as 1 by
substituting tBuLi for MeLi. 2 can be sublimed to purity (66 °C, 25
mTorr). Crystals can be grown from pentane. Yield prior to
EXPERIMENTAL SECTION
■
1
General Comments. All chemical manipulations were carried out
using standard Schlenk and glovebox techniques in a nitrogen
atmosphere. Pentane, THF, toluene, and diethyl ether were dried by
passing through two columns (one neutral alumina and one
copper(II) oxide-Q5 column), storing over 4 Å molecular sieves,
and testing periodically with sodium benzophenone ketyl for O2 or
H2O. Benzophenone ketyl was prepared in THF by treatment of
benzophenone with Na0, which reacts to make a deep purple solution
of ketyl (a radical anion). In the presence of either H2O or O2, the
ketyl oxidizes to benzophenone and the solution becomes clear. Celite
and molecular sieves were kept above 150 °C for a minimum of 5
days and were heated to 160 °C under reduced pressure overnight
prior to use. N,N′-diisopropylcarbodiimide (99%), triphenylsilane-
thiol (99%), tetramethylethylenediamine (99%) (TMEDA), and
diethylsulfide (98%) were used as received from Sigma-Aldrich.
CdBr2 (98%), methyl lithium (1.57 M in diethyl ether), tBuLi (1.6 M
in pentane), and nBuLi (2.5 M in pentane) were used as received
purification was 8.1 g (92%). Mp: 78 °C. H NMR (C6D6, 25 °C,
ppm): 4.30 (m, 4H), 1.33 (s, 18H), 1.17 (d, 24H). 13C NMR (C6D6,
25 °C, ppm): 172.41 (Cq), 45.55 (CH), 39.80 (Cq), 30.34 (CH3),
27.93 (CH3). Elemental Analysis: Calculated C 55.16, H 9.68, N 11.7.
Found C 53.88, H 9.66, N 11.41.
Cd(nbu-amd)2 (3). 3 was synthesized in the same manner as 1 by
substituting nBuLi for MeLi. Yield without purification was 750 mg
(54%). Crystals can be grown from pentane at −30 °C from a
concentrated solution of 3. 3 can also be sublimed to purity at 95 °C
1
at 50 mTorr. Mp: 69 °C. H NMR (C6D6, 25 °C, ppm): 3.69 (br,
4H), 2.25 (br, 4H), 1.52 (br, 4H), 1.29 (tr, 6H), 1.16 (d, 24H), 0.86
(m, 4H). 13C NMR (C6D6, 25 °C, ppm): 143.0 (Cq), 45.55 (CH),
30.63 (CH2), 26.91 (CH3), 25.86 (CH2), 23.24 (CH2), 13.75 (CH3).
Elemental Analysis: Calculated C 55.16, H 9.68, N 11.7. Found C
55.05, H 9.40, N 11.82.
Cd(amd)(SSiPh3) Synthesis (4). 4 was generated by dissolving 1
(615.1 mg, 1.6 mmol) in 10 mL of toluene and adding one equivalent
6192
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 2. Synthesis of 1
of triphenylsilanethiol (430 mg, 1.6 mmol) at room temperature.
After being stirred for 1 h, volatiles were removed to yield a white
powder. 4 was identified by 1H and 13C NMR analysis; however,
analytically pure compound was unable to be isolated via extraction,
crystallization, or vacuum sublimation. Details can be found in the SI
along with an H NMR spectrum of 4. Mp: 88 °C. Observed: H
NMR (C6D6, 25 °C, ppm): 8.04 (br, 6H), 7.20 (br, 9H), 3.36 (br,
2H), 1.45 (s, 6H), 0.97 (d, 12H). 13C NMR (C6D6, 25 °C, ppm):
154.0 (Cq − amidinate), 136.01 (CH − phenyl), 127.72 (CH −
phenyl), 127.48 (CH − phenyl), 46.1 (CH − isopropyl), 25.05 (CH3
− amidinate), 16.8 (CH3 − isopropyl).
days. Crystals of 1 were suspended in Paratone oil and examined
under a microscope with a polarizing filter. A colorless block sized ca.
0.10 × 0.10 × 0.20 mm was chosen and mounted on the end of a glass
fiber by flash-freezing the crystal in Paratone at 150 K, where the data
were recorded on a Bruker APEX 2 diffractometer in the fixed-chi
geometry. Omega scans were collected (30 s, 0.5 width). The data
collection and integration were performed in the APEX 2 software
package. After SADABS was used to correct for absorption, the space
group was determined to be C2/c and the initial solution obtained
using XS. The structures for both 1 and 5 were refined using
ShelXe.20
1
1
Cd(SSiPh3)2TMEDA Synthesis (5). 1 (215 mg, 0.5 mmol) was
dissolved in toluene with tetramethylethylenediamine (TMEDA) (63
mg, 0.5 mmol), and triphenylsilanethiol (301 mg, 1.1 mmol) was
added at room temperature. The reaction was allowed to stir for 30
min, at which time, volatile components were removed under reduced
pressure to produce a light peach solid. The product was washed with
diethyl ether to remove excess amidine byproduct; yield: 250 mg
(57%). The structure and purity was evaluated with 1H and 13C NMR
analysis as well as an X-ray crystal structure. X-ray quality crystals
were grown by dissolving the crude solid in 5 mL of toluene and
reducing the temperature to −30 °C for 10 days. X-ray structure
analysis shows 5 crystallizes with 0.5 toluene molecules per unit cell,
which can be removed by lengthy exposure to vacuum (>ca. 10−3
Single crystals of 5 were grown by storing saturated toluene
solution of 5 at −30 °C for 1−2 days. The crystals were suspended in
Paratone oil and examined under a microscope with a polarizing filter.
A colorless block sized ca. 0.25 × 0.28 × 0.35 mm was chosen and
mounted on the end of a glass fiber by flash-freezing the crystal in
Paratone at 150 K, where the data were recorded on a Bruker APEX 2
diffractometer in the fixed-chi geometry. Omega scans were collected
(10 s, 0.5 width). The data collection and integration were performed
in the APEX 2 software package. After SADABS was used to correct
for absorption, the space group was determined to be P1 and the
initial solution obtained using XS. Full refinement of the structure
proceeded uneventfully for the main Cd molecule. Toluene was found
to be disordered in two orientations across the special position and
was refined by fixing the total occupancy of the site to 0.5 for each
molecule, fixing each toluene core to a rigid hexagon, and restraining
the thermal parameters of the toluene molecules. The methyl group
on one fragment was not located in the difference map (largest peak/
hole: 1.188 and −0.431 e−/Å3), so it was omitted from the final
refinement and thus the 0.5 C in the unit formula. Since the material
had not encountered any benzene, it is believed that the toluene is
rotationally disordered.
Stability Testing. The general method for stability testing
involved dissolution of a sample of about 50 mg of a cadmium
precursor in about 1 mL of C7D8, and the reaction mixture was loaded
into a J. Young tube wrapped with aluminum foil. The tube was
heated to 100 °C in an oil bath, and NMR spectra were taken
periodically. The results are discussed below.
Thermogravimetric Analysis and Differential Scanning
Calorimetry. TGA and DSC were performed using a TA
Instruments SDT Q600 inside of an atmospheric glovebox. In a
standard TGA/DSC analysis, 5−20 mg of compound was loaded into
an alumina pan and placed on the weighing arm of the instrument.
The arm was then inserted into an oven with a flow of ultrahigh purity
N2 (20 mL/min) flowing through the oven to remove any vaporized
compound. A standard experiment consisted of heating a sample at a
rate of 5 °C/min from room temperature until vaporization was
complete.
1
Torr). Mp: 222 °C. H NMR (C6D6, 25 °C, ppm): 8.00 (d, 12H),
7.20 (t, 18H), 1.6 (s, 12H),1.50 (br, 4H). 13C NMR (C6D6, 25 °C,
ppm): 140.2, 135.9, 130.3, 56.3, 46.4. Elemental Analysis: Calculated
C 62.16, H 5.71, N 3.45. Found C 61.85, H 5.38, N 3.62.
Crystallographic information available in Table S2 of the SI.
Variable Temperature 1HNMR for Cd(amd)2 (1). Temperature
manipulation was performed using a heating coil in the path of the
cooling gas. The gas (nitrogen) was cooled by flowing through liquid
nitrogen. Increasing the flow of cold nitrogen gas allowed for cooling,
while the heating coil provided stabilization. The VT 1HNMR
experiment involved dissolving 50 mg of Cd(amd)2 in 1 mL toluene-
d8. After the room temperature spectrum was taken, the temperature
was decreased at a rate of 2 °C/min, with concurrent increase in gas
flow every 20 °C. Spectra were taken at 10 °C intervals after a
stabilization time of 10 min had elapsed. After the low temperature
limit was reached, the temperature was allowed to return to room
temperature at a rate of 2 °C/min and stabilize at 292 K (19 °C) for
30 min. At this point, the heating coil was used to introduce heat to
the sample at a rate of 2 °C/min. Again, spectra were taken at 10 °C
intervals after a stabilization time of 10 min had elapsed. Temperature
accuracy was checked in advance by methanol over the range of 240−
290 K and ethylene glycol over the range of 290−350 K.
1
The VT HNMR peaks for 1 in its monomer and dimer states are
1
given here. The dimeric peaks were observed at −48 °C. H NMR
(C7D8, −48 °C, ppm): 3.72 (m, 4H), 3.52 (m, 4H), 1.82 (s, 6H),
1.72 (s, 6H), 1.48 (d, 12H), 1.33 (d, 12H), 1.26 (d, 12H), 1.21 (d,
12H). The monomeric peaks were observed at 52 °C. 1H NMR
(C7D8, 52 °C, ppm): 3.57 (m, 4H), 1.71 (s, 6H), 1.06 (d, 24H).
X-ray Structure Determinations. For 1, single crystals were
grown by storing saturated pentane solution of 1 at −30 °C for 1−2
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Cadmium bis-N,N-
diisopropylactetamidinate, Cd(amd)2 (1), was synthesized
through nucleophilic attack by a methyl anion to the
6193
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
electrophilic carbon of N,N′-diisopropylcarbodiimide (DIC) to
generate the lithium N,N'-diisopropylactetamidinate. Addition
of CdBr2 generated the cadmium amidinate species 1 (Scheme
2). 1 was purified via extraction with pentane followed by
recrystallization from pentane at −30 °C, which afforded X-ray
quality crystals in 92% yield.
Table 2. Selected Interatomic Distances in 1
Atoms
Distance (Å)
Cd(1) and Cd(1A)
Cd(1)−N(3)
Cd(1)−N(4)
Cd(1)−N(1)
Cd(1)−N(2)
3.1793(3)
2.2311(17)
2.2422(18)
2.2671(17)
2.2838(17)
An X-ray crystal structure showed 1 crystallizes as a dimer
(Figure 1). The structural analysis of 1 showed a monoclinic
The length between Cd(1) and Cd(1A) in 1 indicates no
cadmium bonding interaction as the cadmium centers are
brought into proximity through the bridging amidinates. There
are two different amidinate environments in this complex: two
bridging μ2 acetamidinate ligands and two terminal κ2
acetamidinate ligands. The bond angles for this crystal
structure can be found in Table S2 of the SI.
At room temperature, 1HNMR analysis of 1 showed two sets
of coalescing amidinate peaks. Variable temperature (VT)
1HNMR showed a single set of amidinate peaks at ≥305 K (32
°C) and two sets of amidinate peaks at ≤280 K (7 °C). The
1
changing HNMR peak splitting at different temperatures is
shown in Figure 2. There are two possible causes for this:
either 1 is in equilibrium between a dimer and two dissociated
monomers, or the two different amidinate ligands are
exchanging without disassociation. In the second case, if the
two different amidinate ligand were exchanging intramolecu-
larly at the fast exchange limit, we would expect to see the
peaks of the two amidinate environment average into one set
of peaks at elevated temperatures. This is not observed.
Instead, the methyl peaks corresponding to the iPr and
amidinate shift upfield compared to the corresponding peaks of
dimeric 1 at the slow exchange limit. The low temperature
spectrum of 1 corresponds to the dimer configuration,
supported by the dimeric crystal structure and number of
proton environments. These peaks can be separated into two
different bonding environments of the amidinates, one set for
the bridging amidinates and the other for the terminal
amidinates. When taken to higher temperature, > 310 K (37
°C), the dimer transitions into monomers and the two sets of
amidinate peaks coalesce into a single set of peaks, shifted
upfield, since the amidinate ligand proton environments are
symmetric and the monomer molecule has a C2 rotational axis
of symmetry. These data support the hypothesis of a dimer to
monomer transition, as one would expect the number of peaks
to increase with increasing temperature if ligand dissociation
occurred (before likely converging again); a μ2 amidinate is no
longer symmetric and would have roughly twice the number of
peaks that are observed at elevated temperatures or would have
peak positions converged at the average field strength of the
individual constituents. The peak splitting at the slow exchange
limit (Δv) and the temperature of coalescence (TC) can be
estimated by these data and used to derive the energy barrier
(ΔG‡) between the proposed monomer/dimer equilibrium
using a derivation of the Eyring equation22
Figure 1. A crystal structure of 1 shows the cadmium amidinate exists
in a dimer in the solid state.
Table 1. Crystallographic Data for Structural Analysis
1
5
chem formula
fw
C32H68Cd2N8
789.74
Monoclinic
C2/c
C45.5H49.5CdN2S2Si2
857.07
Triclinic
P1
cryst syst
space group
temp (K)
Unit Cell Dimensions
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
150(2)
149(2)
23.7757(8)
9.6550(3)
18.2640(6)
90°
109.9750(10)o
90°
3940.4(2)
4
1.331
10.5382(8)
14.1976(11)
15.8843(12)
73.4890(10)o
71.3420(10)o
89.3690(10)o
2150.4(3)
2
V (Å3)
Z
ρcalcd (g cm−3
)
1.324
0.694
μ (mm−1
)
1.109
range of 2θ (deg)
λ (Å)
crystal size (mm3)
reflections collected
goodness of fit of F2
1.82−28.36
0.71073
0.20 × 0.10 × 0.10
38128
2.35−30.66
0.71073
0.35 × 0.28 × 0.25
13219
1.519
1.048
Ä
É
Å
Å
Å
Å
Ñ
Ñ
Ñ
Ñ
Ñ
T
Δv
i
j
j
j
y
z
z
z
kcal
mol
c
ΔG‡ = 4.575 × 10−3
T 9.972 + log
Å
crystalline system with C2/c space group (Table 1). This is in
keeping with the observed penchant of amidinate species to
dimerize as previously reported by Gordon et al.15 The
interatomic distance between the two cadmium centers in 1 is
3.1793(3) Å (Table 2). This distance is longer than the bond
between the cadmium centers of diaryldicadmium (Ar′2Cd2)
reported by Power et al.21 In Ar′2Cd2, a dicadmium bond
forms with a reported Cd−Cd bond length of 2.6257(5) Å.
CÅ
Å
Ñ
Ñ
Ñ
Å
(1)
(2)
k
{
Å
Ç
Ñ
Ö
KC = πΔv/ 2 ≈ 2.22Δv
The temperature of coalescence was determined to be 305 K
(32 °C) and the peak splitting of the methyl group on the
acetamidinate ligand at the slow-exchange temperature was
measured to be 48.6 Hz. These values are used to calculate an
6194
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 2. VT 1H NMR experiments show the fast-exchange limit at 305 K (32 °C) and slow-exchange limit at 280 K (7 °C). The energy barrier
(ΔG‡) for the proposed monomer/dimer equilibrium is 15.0 kcal/mol.
Scheme 3. Synthesis of 2 and 3
energy barrier of 15.0 kcal/mol. For comparison, the energy
barrier of the monomer/dimer equilibrium of AlMe3/Al2Me6
has been measured at 20.2 kcal/mol in the vapor phase.23,24
On the basis of the comparatively low energy barrier and the
6195
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 4. Proposed H/D Exchange in 1
Figure 3. TGA (blue) and DSC (red) of 1 (a), 2 (b), and 3 (c). Alteration of steric bulk of ligands can be used to change volatility.
volatility of 1, we propose that 1 exists in equilibrium between
its monomeric and dimeric states. Dimeric and trimeric metal
N,N′-diisopropylacetamidinates have been observed for Mg-
(II), Mn(II), Fe(II), Cu(I), and Ag(I).14,15,25 Equation 2
calculates a rate of exchange at 168 Hz at the slow-exchange
limit.
Two analogous cadmium amidinates were synthesized by
changing the alkyl lithium reagents that reacted with DIC
(Scheme 3). When MeLi was replaced by tBuLi or nBuLi,
Cd(tbu-amd)2 (2) and Cd(nbu-amd)2 (3) were isolated in
good yield (90% for 2, 55% for 3). Both compounds are white
crystalline solids that readily decompose when exposed to
atmosphere. Both 2 and 3 can be purified through vacuum
sublimation and/or recrystallization in pentane. 1HNMR
analysis of 2 and 3 shows a single set of amidinate peaks
suggesting these compounds are monomeric in solution at
ambient temperature (22 °C) (Figures S2 and S3).
Thermal Stability. CVD/ALD precursors must be
thermally stable at a temperature with sufficient volatility to
qualify as viable precursor candidates. Precursor cylinders are
often heated to maintain a uniform vapor delivery pressure and
a precursor must withstand being held at elevated temperatures
throughout a deposition process without decomposition.26
The thermal stabilities of 1-3 were evaluated by dissolving the
compound in deuterated toluene (tol-d8) and heating the
solution in a sealed J. young tube to 100 °C. Periodic room
temperature 1HNMR analysis was used to monitor the
compounds.
from a zirconium amidinate complex by deprotonation.27
Overall, 1 appears to meet the thermal stability requirements
for ALD.
When 2 was heated to 100 °C, the clear solution quickly
developed a yellow/orange color that intensified over the
course of several hours until dark precipitate was observed.
1HNMR analysis revealed two new sets of isopropyl −CH
peaks, four sets of isopropyl −CH3 doublets, and a new tert-
butyl singlet that shifted 0.11 ppm from the starting material.
The likely decomposition pathway is reductive elimination of
the tert-butylamidinate ligands by cadmium to yield a
hydrazine byproduct. Gordon et al. has observed this
decomposition pathway for tris-(N,N′-diisopropylacetamidi-
nate) ruthenium.28 The exact product cannot be resolved from
the 1HNMR spectrum alone (Figure S5 and discussion in SI);
1
2 can be sublimed at 66 °C (25 mTorr) to yield a HNMR
pure compound. Overall, 2 was evaluated to have poor thermal
stability properties above its melting point limiting its use to
low temperature ALD processes.
3 showed no signs of decomposition when dissolved in tol-
d8 and heated to 100 °C for 15 days, during which no physical
1
changes were observed and the HNMR spectrum remained
unchanged. Under a vacuum (50 mTorr), 3 sublimes at 95 °C
without any remaining decomposition products. We evaluate 3
as having favorable thermal stability and volatility properties
for ALD/CVD processes.
Volatility Characterization. Dual TGA/DSC measure-
ments were taken of 1−3 to evaluate their volatility properties.
TGA measures the weight of a compound as a function of
temperature and DSC measures the heat flow of a sample as a
function of temperature in reference to a standard heat flow.
Dual TGA/DSC offers insight into phase changes (change in
heat flow with no mass loss), vaporization (endothermic event
coupled with mass loss), and decomposition (exothermic
event) among other physical or chemical changes.
1 was shown to be stable 100 °C for well over a month
1
without decomposition. No change in the HNMR spectrum
and no physical changes to the solution were observed.
However, the peak corresponding to the acetamidinate methyl
group lessened in intensity and slowly disappeared over the
course of the month. At the same time, the aryl residual peak
for tol-d8 peaks increased. The changing peaks suggest a
hydrogen−deuterium (H/D) exchange as shown in Scheme 4
(Figure S4). Sita et al. previously reported activating the C−H
bond in the methyl group of an amidinate bond with Ph3SiLi
TGA of 1 shows a single step mass loss resulting in 99.41%
vaporization (Figure 3a). DSC shows a sharp endothermic
6196
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
a
Table 3. Thermal Stability and Phase Change Properties of Cd Precursors
b
c
d
d
c
d
Compound
Thermal Stability
Melting Point
Melting Point
Onset of Volatilization
Temperature at Final Mass Loss
Mass Loss
1
2
3
100 °C for over 30 days
Decomposes above 78 °C
100 °C for over 15 days
112 °C
78 °C
Not observed
82 °C
71 °C
134 °C
120 °C
160 °C
195 °C
158 °C
223 °C
99.41%
80.08%
87.53%
69 °C
a
b
c
All observations made at atmospheric pressure. Observed in solution with toluene. Measured with benchtop melting point instrument.
Observed by TGA/DSC.
d
Scheme 5. Reactivity of 1 with Triphenylsilylthiol
peak at 134 °C, which along with the TGA mass loss suggests a
volatilization temperature at 134 °C at atmospheric pressure.
DSC of 2 shows a broad endothermic peak at 82 °C without
mass loss. We propose this is the melting point of 2, as it is
near the melting point observed on our benchtop melting
point apparatus (78 °C). However, just above the observed
melting point, 2 begins to turn yellow in its sealed capillary
tube suggesting decomposition. The TGA mass loss of 2 has
80.02%, and a metallic black residue is observed in the TGA
weigh pan at the end of the measurement. 2 likely decomposes
during the TGA ramp sequence to cadmium metal and
decomposed ligand, which partially vaporizes to cause the mass
loss. The atomic weight percent of Cd in 2 is calculated to be
23.5%. If 2 was completely decomposed, we would expect a
mass loss of 76.5%, which is near the 80.02% mass loss
observed in Figure 2b. We propose 2 begins decomposing after
melting, and while some of 2 is vaporized intact, the majority
decomposes to Cd metal and ligand byproducts. A second
endothermic event in the DSC curve is observed at 120 °C, but
it is not clear if this is caused by vaporization of 2 or
vaporization of decomposed byproducts of 2.
Figure 3c shows an endothermic event at 71 °C for 3
without an accompanying weight loss. This is interpreted as its
melting point, which is consistent with the 69 °C melting point
observed on our benchtop melting point apparatus. 3 is
vaporized at 160 °C with 87.5% mass loss. The 12.5% residual
mass is a gray, solid powder suggesting decomposition is
occurring at elevated temperatures. However, under a vacuum
3 vaporizes at 95 °C (50 mTorr), nearly the same conditions
as 1. The sample analyzed in Figure 3c was also sublimed
completely prior to analysis without decomposed byproduct.
Although 3 has some slight decomposition at elevated
temperatures at atmospheric pressure, this can be avoided at
the low pressure environments used in ALD/CVD processing.
The low melting point of 3 (71 °C) is favorable for ALD/CVD
processing because liquid precursors distribute heat better than
solid precursors allowing for a more uniform vapor delivery
pressure.
Overall 1−3 have favorable volatility properties for ALD/
CVD processing, although thermal stability issues for 2 and 3
may limit the temperature window in which they can be used.
2, in particular, is not predicted to be useful at temperatures
above 80 °C in ALD, due to the onset of compound
decomposition. These results are summarized in Table 3.
Reactivity. The reactivity of 1 with thiols was established
by treating a solution of 1 in toluene with triphenylsilylthiol
(HSSiPh3). HSSiPh3 was selected as a sterically bulky thiol as
an analogue of a surface thiol. Thus, the reaction of 1 with
HSSiPh3 is a proxy for the first half-cycle of ALD growth of
CdS: the reaction of 1 with a protonated, *SH, surface species.
One equivalent of HSSiPh3 was allowed to react with 1 at
room temperature yielding the heteroleptic cadmium com-
1
pound (4) (Scheme 5). 4 was observed by H and 13C NMR
(Figure S6). The three-coordinate cadmium structure
displayed in Scheme 5, 4, was not isolated and likely exists
with bridging sulfides to form a four coordinate, tetrahedral
complex. Silylthiolate cadmium compounds have been
previously observed to form dimers via bridging thiolates.29
Further evidence of reaction is shown via 1HNMR that shows a
single set of amidinate peaks that are shifted from 1 as well as
the presence of aryl peaks (Figure S6) shifted from HSSiPh3,
6197
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
and there is no evidence to support disproportionation back to
1 at room temperature.
temperature processes; however, initial reactivity screening at
110 °C suggests sulfur sources that undergo silyl and alkyl
transfer to the amidinate to be less robust than proton transfer
with 1.
When two equivalents of HSSiPh3 were allowed to react
with 1 in the presence of TMEDA, compound 5 was isolated
as an off-white powder. When 1 was treated without TMEDA,
no clearly identifiable compound was isolated likely due to
uncontrolled polymerization. TMEDA has been used by Sita et
al. to isolate Cd(SSiMe3)2(TMEDA) and prevent decom-
position into CdS.30 Compound 5 was heated to 100 °C for 12
h with no decomposition to CdS observed. A crystal structure
revealed 5 to be monomeric, and it crystallized in the P-1 space
group as a triclinic structure with eight toluene molecules in
each unit cell (Table 2). The bonding distances and angles of 5
were compared to a dimeric cadmium bis-thiolate [Cd(SSi-
(OtBu)3)2]2 reported by Becker et al., where they reported a
mean terminal Cd−S bond length of 2.408 Å.27 This is in
agreement with the Cd−S bond length of 5 being 2.4435 Å.
The Cd−N bond lengths are 2.3803(13) Å and 2.4035(13) Å
with a N−Cd−N angle of 78.74(5)o. The S−Cd−S bond angle
was found to be 152.001(1)o forming pseudolinear S−Cd−S
bonds that are distorted by the steric bulk of the triphenylsilyl
groups and the σ-donating TMEDA (Figure 4).
CONCLUSIONS
■
Three new homoleptic cadmium bis-amidinate compounds, 1,
2, and 3, were synthesized and evaluated, with respect to
volatility, thermal stability, and reactivity, as potential ALD
precursors. 1 was synthesized in high yields and isolated as a
crystalline dimer. In solution, we propose 1 to exist in
equilibrium between its monomer and dimer structures at
room temperature. In addition, at elevated temperatures, above
310 K (37 °C), in solution, 1 exists as a symmetric monomer.
1
VT HNMR analysis showed a fast exchange limit of 305 K
(32 °C) and a slow exchange limit of 280 K (7 °C). Analysis
using the Eyring Equation suggests an energy barrier of 15.0
kcal/mol. 1 was determined to be thermally stable in solution
when held at 100 °C for over a month. TGA/DSC analysis
showed 99.41% mass loss through vaporization of 1 with no
decomposition. 1 was found to be reactive with 1- and 2-
equivalents of HSSiPh3 to form new Cd−S containing
compounds 4 and 5, although 5 was obtained only when the
reaction occurred in the presence of a molar equivalent of
TMEDA. No reactivity of 1 with S(SiMe3)2 or Et2S was
observed at 110 °C over 3 days. Overall, the volatility, thermal
stability, and reactivity of 1 make it a promising candidate as a
cadmium ALD precursor.
2 and 3 were synthesized as analogous compounds to 1 by
altering the alkyl lithium salt used during synthesis. 2 was
found to decompose when heated in solution at more than 80
°C; however, it could be sublimed under a vacuum (70 mTorr)
at 60 °C without decomposition. 3 showed no signs of thermal
decomposition in solution when held at 100 °C for 1 week and
had the lowest observed melting point of 70 °C. TGA/DSC
analysis of 3 showed some decomposition product as a solid
gray residue remaining in the oven pan. Overall, 2 and 3 have
potential as precursors for ALD processing; however, the
temperature processing conditions are expected to be limited
to temperatures ≤80 °C for 2 and ≤160 °C for 3 as estimated
from TGA/DSC.
In conclusion, this work has presented the first cadmium
amidinate species in the literature. Synthetic modeling
indicates that it has properties advantageous to existing ALD
precursor chemistry. Through synthetic modification, these
properties can be altered, and the relationship between the
structure and function of these potential precursors has been
explored.
Figure 4. Crystal structure of 5 shows a monomeric cadmium
dithiolate product that is stabilized with TMEDA.
1 was tested for reactivity with bis-trimethylsilyl sulfide
(S(SiMe3)2) and diethyl sulfide (Et2S): two volatile sulfur
sources previously used in ALD/CVD processes.31,32 S-
(SiMe3)2 is also used as a sulfur source for solution phase
CdS nanocrystal growth with cadmium carboxylate precur-
sors.33 After being heated to 110 °C for more than 3 days, no
1
change to the starting materials was observed by HNMR for
both S(SiMe3)2 and Et2S (Scheme 6). This observation does
not disqualify S(SiMe3)2 and Et2S from use in higher
Scheme 6. 1 Does Not React with S(SiMe3)2 or Et2S
6198
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
(8) Bakke, J. R.; Bent, S. F. Formation of CdxZn1-xS Films for
Photovolatic Buffer Layers by Atomic Layer Deposition. ECS Trans.
2009, 25 (4), 9−14.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03307.
■
sı
(9) Lim, B. S.; Rahtu, A.; Gordon, R. G. Atomic Layer Deposition of
Transition Metals. Nat. Mater. 2003, 2 (11), 749−754.
(10) Kim, S. B.; Yang, C.; Powers, T.; Davis, L. M.; Lou, X.; Gordon,
R. G. Synthesis of Calcium(II) Amidinate Precursors for Atomic
Layer Deposition through a Redox Reaction between Calcium and
Amidines. Angew. Chem., Int. Ed. 2016, 55 (35), 10228−10233.
(11) Forsberg, J. H.; Spaziano, V. T.; Balasubramanian, T. M.; Liu,
G. K.; Kinsley, S. a; Duckworth, C. a; Poteruca, J. J.; Brown, P. S.;
Miller, J. L. Use of Lanthanide(III) Ions as Catalysts for the Reactions
of Amines with Nitriles. J. Org. Chem. 1987, 52 (11), 1017−1021.
(12) Li, Z.; Barry, S. T.; Gordon, R. G. Synthesis and Character-
ization of copper(I) Amidinates as Precursors for Atomic Layer
Deposition (ALD) of Copper Metal. Inorg. Chem. 2005, 44 (6),
1728−1735.
1HNMR spectra of synthetic products, H/D exchange,
1HNMR of thermal decomposition product of 2, and
crystallographic coordinates for 1 and 5 with details
(PDF)
Accession Codes
CCDC 2042666 and 2042670 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(13) Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, V. G.
Synthesis and Structures of Mono- and Bis(amidinate) Complexes of
Aluminum. Organometallics 1997, 16 (24), 5183−5194.
(14) Sadique, A. R.; Heeg, M. J.; Winter, C. H. Monomeric and
Dimeric Amidinate Complexes of Magnesium. Inorg. Chem. 2001, 40
(25), 6349−6355.
(15) Lim, B. S.; Rahtu, A.; Park, J.-S.; Gordon, R. G. Synthesis and
Characterization of Volatile, Thermally Stable, Reactive Transition
Metal Amidinates - Inorganic Chemistry (ACS Publications). Inorg.
Chem. 2003, 42 (24), 67−68.
(16) Seppälä, S.; Niinistö, J.; Blanquart, T.; Kaipio, M.; Mizohata, K.;
Räisänen, J.; Lansalot-Matras, C.; Noh, W.; Ritala, M.; Leskelä, M.
Heteroleptic Cyclopentadienyl-Amidinate Precursors for Atomic
Layer Deposition (ALD) of Y, Pr, Gd, and Dy Oxide Thin Films.
Chem. Mater. 2016, 28 (15), 5440−5449.
(17) McCarthy, R. F.; Weimer, M. S.; Haasch, R. T.; Schaller, R. D.;
Hock, A. S.; Martinson, A. B. F. VxIn(2-x)S3 Intermediate Band
Absorbers Deposited by Atomic Layer Deposition. Chem. Mater.
2016, 28 (7), 2033−2040.
(18) Guo, Z.; Li, H.; Chen, Q.; Sang, L.; Yang, L.; Liu, Z.; Wang, X.
Low-Temperature Atomic Layer Deposition of High Purity, Smooth,
Low Resistivity Copper Films by Using Amidinate Precursor and
Hydrogen Plasma. Chem. Mater. 2015, 27 (17), 5988−5996.
(19) So, J.-H.; Boudjouk, P.; Hong, H. H.; Weber, W. P.
Hexamethyldisilathiane; 1992; Vol. 29.
(20) Hubschle, C. B.; Sheldrick, G. M.; Dittrich, B. ShelXle: A Qt
̈
Graphical User Interface for SHELXL. J. Appl. Crystallogr. 2011, 44
(6), 1281−1284.
AUTHOR INFORMATION
Corresponding Author
Adam S. Hock − Department of Chemistry, Illinois Institute of
Technology, Chicago, Illinois 60616, United States;
orcid.org/0000-0003-1440-1473; Email: ahock@iit.edu
■
Authors
Michael J. Foody − Department of Chemistry, Illinois Institute
of Technology, Chicago, Illinois 60616, United States;
orcid.org/0000-0003-0963-5689
Matthew S. Weimer − Department of Chemistry, Illinois
Institute of Technology, Chicago, Illinois 60616, United
States; orcid.org/0000-0002-5969-6415
Harish Bhandari − Radiation Monitoring Devices, Inc.,
Watertown, Massachusetts 02472, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03307
Funding
The authors received funding from the United States
Department of Defense, STTR Army Grant No. W909MY-
13-C-0034.
Notes
The authors declare no competing financial interest.
(21) Zhu, Z.; Brynda, M.; Wright, R. J.; Fischer, R. C.; Merrill, W.
A.; Rivard, E.; Wolf, R.; Fettinger, J. C.; Olmstead, M. M.; Power, P.
P. Synthesis and Characterization of the Homologous M-M Bonded
Series Ar′MMAr′ (M = Zn, Cd, or Hg; Ar′ = C6H 3−2,6-(C6H3−
2,6-Pri2)2) and Related Arylmetal Halides and Hydride Species. J.
Am. Chem. Soc. 2007, 129 (35), 10847−10857.
(22) Zimmer, K. D.; Shoemaker, R.; Ruminski, R. R. Synthesis and
Characterization of a Fluxional Re(I) Carbonyl Complex Fac-
[Re(CO)3(dpop′)Cl] with the Nominally Tri-Dentate Ligand
dipyrido(2,3-a:′,2′-j)phenazine (Dpop′). Inorg. Chim. Acta 2006,
359 (5), 1478−1484.
(23) Berthomieu, D.; Bacquet, Y.; Pedocchi, L.; Goursot, A.
Trimethylaluminum Dimer Structure and Its Monomer Radical
Cation: A Density Functional Study. J. Phys. Chem. A 1998, 102
(40), 7821−7827.
(24) O’Brien, R. J.; Ozin, A. A Gas-Phase Raman Study of
Trimethylaluminum and Trimethylboron Monomers. J. Chem. Soc. A
1971, 1136−1138.
REFERENCES
■
(1) George, S. M. Atomic Layer Deposition: An Overview. Chem.
Rev. 2010, 110 (1), 111−131.
(2) Matero, R.; Ritala, M.; Leskelä, M.; Jones, A. C.; Williams, P. A.;
Bickley, J. F.; Steiner, A.; Leedham, T. J.; Davies, H. O. Atomic Layer
Deposition of ZrO2 Thin Films Using a New Alkoxide Precursor. J.
Non-Cryst. Solids 2002, 303, 24−28.
(3) Chen, B.; Coyle, J. P.; Barry, S. T.; Zaera, F. Rational Design of
Metalorganic Complexes for the Deposition of Solid Films: Growth of
Metallic Copper with Amidinate Precursors. Chem. Mater. 2019, 31
(5), 1681−1687.
(4) Miikkulainen, V.; Leskela, M.; Ritala, M.; Puurunen, R. L.
Crystallinity of Inorganic Films Grown by Atomic Layer Deposition:
Overview and General Trends. J. Appl. Phys. 2013, 113 (2), 21301.
(5) Kamat, P. V.; Tvrdy, K.; Baker, D. R.; Radich, J. G. Beyond
photovolatics: semiconductor nanoarchitectures for liquid-junction
solar cells. Chem. Rev. 2010, 110, 6664−6688.
(25) Lim, B. S.; Rahtu, A.; Gordon, R. G. Atomic Layer Deposition
of Transition Metals. Nat. Mater. 2003, 2 (11), 749−754.
(26) Ritala, M.; Leskela, M. Chapter 2: Atomic Layer Deposition.
Handbook of Thin Film Materials Vol. 1: Deposition and Processing of
Thin Films. Academic Press 2002; pp 103−159.
(6) Alivisatos, a. P. Semiconductor Clusters, Nanocrystals, and
Quantum Dots. Science 1996, 271 (5251), 933−937.
(7) Bakke, J. R.; Jung, H. J.; Tanskanen, J. T.; Sinclair, R.; Bent, S. F.
Atomic Layer Deposition of CdS Films. Chem. Mater. 2010, 22 (16),
4669−4678.
6199
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200

Inorganic Chemistry
pubs.acs.org/IC
Article
(27) Zhang, Y.; Kissounko, D. A.; Fettinger, J. C.; Sita, L. R.
Chemoselective “on-Site” functionalization of Group 4 Metal
Acetamidinates. Organometallics 2003, 22 (1), 21−23.
(28) Gordon, R. G.; Li, H.; Aaltonen, T.; Lim, B. S.; Li, Z. Synthesis
and Characterization of Ruthenium Amidinate Complexes as
Precursors for Vapor Deposition. Open Inorg. Chem. J. 2008, 2 (1),
11−17.
(29) Wojnowski, W.; Becker, B.; Walz, L.; Peters, K.; Peters, E.-M.;
von Schnering, H. G. Contributions To The Chemistry of Silicon-
Sulfur Compounds - LX.* Synthesis, Molecular Structure and
Properties of The Dimeric Cadmium(II) Bis(Tri-Tert-Butoxysilane-
thiolate) {[(t-C4H9O)3SiS]2Cd}2. Polyhedron 1992, 11 (6), 607−
612.
(30) Babcock, J. R.; Zehner, R. W.; Sita, L. R. A Heterocumulene
Metathesis Route to Cd[ESiMe3]2 and Passivated CdE (E = S and
Se) Nanocrystals. Chem. Mater. 1998, 10 (8), 2027−2029.
(31) Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.;
Kim, C. J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick
Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015,
520 (7549), 656−660.
(32) Liu, H.; Chen, L.; Zhu, H.; Sun, Q. Q.; Ding, S. J.; Zhou, P.;
Zhang, D. W. Atomic Layer Deposited 2D MoS2 Atomic Crystals:
From Material to Circuit. Nano Res. 2020, 13 (6), 1644−1650.
(33) García-Rodríguez, R.; Hendricks, M. P.; Cossairt, B. M.; Liu,
H.; Owen, J. S. Conversion Reactions of Cadmium Chalcogenide
Nanocrystal Precursors. Chem. Mater. 2013, 25 (8), 1233−1249.
6200
https://doi.org/10.1021/acs.inorgchem.0c03307
Inorg. Chem. 2021, 60, 6191−6200