Synthesis, Characterization, and Thermal Properties of N -alkyl β-Diketiminate Manganese Complexes

Article abstract of DOI:10.1021/acs.inorgchem.7b02476

A series of N,N′-dialkyl-β-diketiminato manganese(II) complexes was synthesized and characterized by single crystal X-ray diffraction, UV-vis and FTIR spectroscopy, and then assayed for volatility, thermal stability, and surface reactivity relevant to vap

Full text of DOI:10.1021/acs.inorgchem.7b02476

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis, Characterization, and Thermal Properties of N‑alkyl
β‑Diketiminate Manganese Complexes
Madelyn M. Stalzer, Tracy L. Lohr,* and Tobin J. Marks*
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
S
* Supporting Information
ABSTRACT: A series of N,N′-dialkyl-β-diketiminato manganese(II) complexes was
synthesized and characterized by single crystal X-ray diffraction, UV−vis and FTIR
spectroscopy, and then assayed for volatility, thermal stability, and surface reactivity
relevant to vapor-phase film growth processes. Bis(N,N′-dimethyl-4-amino-3-penten-
2-imine) manganese(II), 1, and bis(N-N′-diisopropyl-4-amino-3-penten-2-imine)
manganese(II), 2, specifically, emerge as the most promising candidates, balancing
volatility (sublimation temperatures < 100 °C at 100 mTorr) with coordinative
unsaturation and reactivity, as revealed by rapid release of ligand in the presence of a
silica surface. Good correlation is observed between buried volume calculations and
relative surface reactivity data, indicating that metal availability resulting from
sterically open ligand alkyl substituents increases surface reactivity. The thermal
stability, volatility, and reactivity exhibited by these compounds render them
promising precursors for the growth of manganese oxide films via vapor-phase growth processes.
Table 1. Manganese Precursors Evaluated for ALD Growth
Processes
INTRODUCTION
■
Thin manganese oxide films are of interest for a plethora of
applications, ranging from energy storage as battery cath-
odes,1−5 to layered materials for ultracapacitors6,7 due to their
rich redox chemistry, low cost, and earth abundance. Note that
there is an increasing need for high-quality thin films in the
development of these microsystems. To this end, many
techniques have been developed for the synthesis of manganese
oxide thin films, including solution-based processes (electro-
deposition,8,9 sol−gel,10 dip-coating11,12) and vapor-phase
processes (chemical vapor deposition,13,14 molecular beam
epitaxy,15,16 pulsed layer deposition17). Of particular interest
are chemical vapor deposition (CVD) and related atomic layer
deposition (ALD) processes, which provide high levels of
control over film thickness, tunability through metal−organic
precursor choice, conformal coating (they are not line-of-sight
growth processes), and scalability. Additionally, ALD offers self-
limiting surface reactivity that provides smooth, layer-by-layer
film growth (<1 nm roughness)18 under relatively mild
conditions (<300 °C) and with atomic precision. Both CVD
and ALD processes have been adopted industrially in the
microelectronics industry,19−21 making them ideal for high-
throughput thin film syntheses.
compound
ALD process
reference
a
Mn(thd)3
yes
yes
yes
yes
no
no
no
yes
no
no
35
b
Mn(CpEt)2
26
c
CpMnMe(CO)3
Mn2(CO)10
Mn(tBu-MeAMD)2
23
13, 25
33, 34
34
d
e
Mn2(μ-iPr-MeAMD)2(η2-iPr-MeAMD)2
f
Mn(tBu2COCNtBu)2
31
g
[Mn(MetBuCOCNtBu)2]2
31
h
Mn(tBuNNCHCHNtBu)2
30
i
Mn(NMe2NNCHCHNtBu)2
30
j
Mn(tBu2DAD)2
no
36
a
b
thd = 2,2,6,6-tetramethyl-3,5-heptanedione. CpEt = ethylcyclopen-
c
d
tadiene. Cp = cyclopentadiene. tBu-MeAMD = N,N′-di-tert-
e
butylacetamidinate. iPr-MeAMD = N,N′-diisopropylacetamidinate.
f
tBu2COCNtBu = 3-((tert-butyliminomethyl)-2,2,4,4-tetramethylpen-
g
tan-3-oxide. MetBuCOCNtBu = 1-(tert-butylimino)-2,3,3-trimethyl-
h
butan-2-oxide. tBuNNCHCHNtBu = 1,5-di-tert-butyl-1,2,5-triazapen-
i
tadiene. NMe2NNCHCHNtBu = 1-tert-butyl-5-dimethylamino-1,2,5-
j
triazapentadiene. DAD = 1,4-di-tert-butyl-1,3-diazadiene.
development of designer ALD precursors, although these
studies have primarily focused on growth of metallic films.
One class of ligands that lends itself well to vapor deposition
processes is the β-diketiminate ligand (NacNac, Figure 1). β-
Diketiminates provide a high level of ligand tunability, in both
the inner and outer coordination sphere, and have been
A number of modalities are known for the vapor-phase
growth of manganese oxide films by CVD/ALD using a variety
of commercially available precursors (Table 1). The effects of
substrate,22 coreactant,1,23,24 reaction temperature,25,26 and
annealing conditions27,28 have all been investigated, resulting
in promising degrees of control over manganese oxide (MnOx)
composition (MnO, Mn3O4, Mn2O3, MnO2, etc.) after
parameter optimization. Significant advances have also been
made by Winter29−32 and Gordon33,34 (Table 1) in the
Received: September 29, 2017
© XXXX American Chemical Society
A
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Scheme 1. General Synthetic Route to Mn(NacNac)2
Compounds 1−4
Figure 1. β-Diketonate (AcacH), β-diketiminate (NacNacH) ligand
precursors, and example of a dimeric manganese complex resulting
from extreme steric hindrance of N-Ar substituents.20
previously shown to induce volatility in alkali earth metals and
lanthanides.29 They are closely analogous to, but more sterically
encumbered than, acetylacetonate derivatives (e.g., thd in Table
1) that are currently used for the growth of manganese oxide
films.
compounds 2−4 exhibit good solubility in all common
hydrocarbon and etheric solvents. Purification by vacuum
sublimation (see Table 3 for conditions) is also possible and
was performed for all compounds. Note that 1 is extremely air/
moisture sensitive, but this sensitivity decreases as the steric
bulk around the metal increases, such that complexes 3 and 4
are fairly stable under atmospheric conditions for hours in the
solid state before decomposition is observed (compared to
seconds for 1). Compounds 1−4 were characterized by UV−vis
and FTIR spectroscopies (see the Supporting Information,
Figures S1 and S2, respectively) and elemental analysis, and
single crystals suitable for X-ray diffraction were grown from
concentrated pentane (compounds 2−4) or toluene (com-
pound 1) solutions at −40 °C (Figure 2). While these reactions
as described provide modest yields of 1−4 (18−60%) that are
less than the large amounts (5−10 g) of material typically
needed for optimizing film deposition processes, these reactions
are easily scalable and in this work have been successfully scaled
up to the multigram scale.
Single crystal X-ray diffraction analysis confirms the
monomeric nature of bis(β-diketiminate) manganese(II)
complexes 1−4 (Figure 2, Table 2). For all four compounds,
it will be seen that the β-diketiminate ligands act as bidentate
ligands and coordinate in a N,N′-chelating fashion, as indicated
by both the atomic connectivity and electron delocalization-
induced planarity of the NCCCN backbone in the crystal
structures. Other ligand bond lengths and angles are similar to
those of previously reported, related non-manganese com-
pounds.37 While all structures exhibit some distortion in the
tetrahedral coordination geometry due to bidentate bonding of
the ligand that is well-known for four-coordinate Mn(II),52−54
compound 1 is by far the most distorted, with a dihedral angle
of 72.5°. Complexes 2−4 display modest distortion, with
dihedral angles of 89.0°, 87.9°, and 85.8°, respectively. For the
methyl variant, compound 1, higher symmetry is also observed
in the crystal structure, since it crystallizes in the orthorhombic
Pbcn space group, while the rest of the series, compounds 2−4,
crystallize in P21/n (see Table S1 for additional crystallographic
data and refinement parameters). This is attributed to the
decreased degrees of freedom for the methyl substituent
compared to the branched isopropyl, tert-butyl, and cyclohexyl
derivatives. The average Mn−N bond lengths increase as 1
(2.087(1) Å) < 2 (2.103(6) Å) < 4 (2.125(1) Å) ∼ 3 (2.129(2)
Å), resulting from incrementally increased crowding around the
metal center (Table 2). These values are slightly elongated
compared to known N-aryl β-diketiminato manganese(II)
halide dimers [(2,6-iPr2C6H3)NacNacMnCl]2 Mn−N =
2.082(1) Å43 and [(2,6-iPr2C6H3)NacNacMnI]2 Mn−N =
2.067(2) Å,42 again, likely reflecting the increased steric
repulsion of a second β-diketiminate within the primary
coordination sphere. For related bis(amidinato) manganese(II)
While there is a wealth of knowledge about the chemistry
and reactivity of β-diketiminate coordination complexes,37−39
there are surprisingly few reported examples for manga-
nese.40−45 Those previously described utilize either R′ = R″
= aryl or N-aryl substituents (Figure 1) to electronically or
sterically stabilize the manganese(II) center. In the case of
bulky N-2,6-iPr2C6H3 (N-Dipp) substituents, attempts to
produce bis(diketiminato) complexes fail due to steric
crowding at the metal center, and instead form dimeric
diketiminato manganese μ-halido complexes (Figure 1).42
Additionally, while N-aryl β-diketiminates have been widely
studied, they are undesirable for use as vapor deposition
precursors due to potential π−π packing interactions in the
solid state that often decrease volatility.46 Alkyl N-substituted β-
diketiminates, however, can be leveraged to increase both
volatility and metal center reactivity due to their generally lower
steric demands. In the case of metallic copper film deposition
for example, extensive studies have shown that copper alkyl β-
diketiminates are highly competent for copper ALD.47−49 First-
row transition metal alkyl N-substituted β-diketiminates
compounds are well-known in the literature, although the
manganese congeners have remained heretofore unre-
ported.50,51 Thus, the synthesis and characterization of N-
alkyl β-diketiminato manganese compounds presents an
attractive approach for the vapor-phase growth of manganese
oxide containing films.
With the aim of providing new precursors for manganese
oxide film growth processes, the synthesis of new volatile and
thermally stable β-diketiminate manganese complexes was
pursued. This work presents a systematic study of a series of
N-alkyl β-diketiminate manganese(II) complexes. Along with
their synthesis and characterization, thermogravimetric analysis
and solution-phase reactivity data indicate promising perform-
ance for vapor deposition processes.
RESULTS AND DISCUSSION
■
The synthesis of β-diketimine ligands was performed according
to general literature procedures (see the Supporting Informa-
tion for full synthetic details). Metalation was achieved by first
deprotonating the corresponding N-alkyl β-diketimine at 0 °C
n
with BuLi, followed by addition of the resulting Li-ligand
solution to a suspension of MnCl2 in tetrahydrofuran or diethyl
ether (Scheme 1). Pure products are easily obtained by
crystallization from pentane or toluene, in low to moderate
yields (18−60%). Compound 1 is poorly soluble in aliphatic
hydrocarbons, but readily dissolves in toluene and ether, while
B
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Figure 2. Single crystal X-ray structures for (A) complex 1; (B) complex 2; (C) complex 3; (D) complex 4. Thermal ellipsoids at 50% probability.
Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) for Complexes 1−4
Slight puckering of the manganese out of the ligand plane is
observed for complexes 2 and 4 (0.294 and 0.317 Å out of
plane, respectively), which is well-documented in metal β-
diketiminato complexes where there is repulsion between N-
substituents and the metal.39 This puckering of the metal center
is insufficient to disrupt the expected trends evident in the
buried metal center calculations (vide infra). In the case of
compounds 2 and 4, hydrogen atoms on the N-isopropyl and
cyclohexyl γ-carbons appear to approach the metal center
(2.80−2.88 Å), resulting in its subsequent dislocation. Whether
these hydrogens are oriented due to weak interaction with the
metal center or geometric packing preferences of the 2° alkyl
substituted ligands is unclear.
1
Mn1−N1
2.0873(11)
91.51(4)
Mn1−N2
N1−Mn−N1′
2.0861(10)
108.08(6)
N1−Mn−N2
N2−Mn−N2′
2
110.64(6)
Mn1−N1
Mn1−N3
N1−Mn1−N2
N1−Mn1−N3
N2−Mn1−N3
3
2.1010(6)
2.0948(6)
96.93(2)
Mn1−N2
Mn1−N4
N3−Mn1−N4
N1−Mn1−N4
N2−Mn1−N4
2.0978(6)
2.1155(7)
96.82(3)
120.21(2)
119.93(2)
110.84(2)
112.80(2)
Mn1−N1
Mn1−N3
N1−Mn1−N2
N1−Mn1−N3
N2−Mn1−N3
4
2.1282(18)
2.1256(18)
99.32(7)
Mn1−N2
Mn1−N4
N3−Mn1−N4
N1−Mn1−N4
N2−Mn1−N4
2.1321(18)
2.1309(18)
99.29(7)
The effects of these structural distortions are also seen in the
electronic characterization of 1−4. An Evans’ method
analysis57,58 of complexes 1−4 yields μeff = 5.84−5.95 μB (see
Table 3 for exact values), all in excellent agreement with the
114.26(7)
114.22(7)
113.83(7)
116.78(7)
Table 3. Physical and Volatility Characterization Data for
Complexes 1−4
Mn1−N1
Mn1−N3
N1−Mn1−N2
N1−Mn1−N3
N2−Mn1−N3
2.1264(12)
2.1295(12)
94.12(5)
Mn1−N2
Mn1−N4
N3−Mn1−N4
N1−Mn1−N4
N2−Mn1−N4
2.1246(12)
2.1194(12)
94.27(5)
sublimation
a
melting
point (°C)
TGA onset
(°C)
compound
μeff
% Vbur
temp (°C)
110.36(5)
116.41(5)
115.87(5)
126.53(5)
1
2
3
4
5.84
5.95
5.94
5.85
22.2
32.0
36.7
42.8
75
90
180
109
200
190
212
263
280
326
110
125
compounds, however, very similar Mn−N bond lengths are
observed [{PhC(NAr)2}2Mn] = 2.113(3) Å (Ar =
2,6-iPr2C6H3)55 and [{MesC(NiPr)2}2Mn] = 2.085(7) Å.56
a
At 100 mTorr.
C
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value μeff = 5.92 μB expected for high spin S = 5/2 Mn2+. UV−
visible spectroscopy of the yellow/orange compounds (Figure
S1) reveals two main absorption features centered at 311−315
and 380−390 nm, both attributable to ligand-centered charge
transfer-to-metal excitations, in agreement with previous
reports.59 For compounds 1 and 3, the band around 315 nm
is of higher intensity (104 cm−1 M−1) with lower absorption for
the second feature (103 cm−1 M−1), while compounds 2 and 4
follow the opposite trend, with more intense red-shifted
features. These differences can be attributed to displacement of
the metal center out of the ligand plane, resulting in decreased
metal−ligand orbital overlap for compounds 2 and 4, and
subsequent shifting of the ligand orbital energies. Comparable
phenomena are well-known for the Q peaks in structurally
similar metalloporphyrin systems.60
In an attempt to quantify the steric congestion around the
metal centers as a function of N-alkyl substituent, buried
volume calculations were performed.61 Buried volume (Vbur)
has been proposed as a more accurate descriptor for metal
accessibility, specifically for catalytic systems, compared to
classical cone angle measurements as it is generalizable to
virtually any ligand environment.62 Recent studies have also
used such computations to gain insight into coordination
sphere bulk relative to surface reactivity for molybdenum ALD
precursors.63 Calculations and steric maps were computed
using SambVca 2.0 software, and were performed using atomic
coordinates from the single crystal X-ray diffraction data, with a
radius of 7 Å and mesh size of 0.1 Å. This radius was chosen
such that the entire coordination sphere around the manganese
was considered. Hydrogen atoms were not included in the
calculation. The resulting Vbur values are shown in Table 3, and
as expected, 1 exhibits the most open metal site by far, with Vbur
= 22.2%. For substituents R = iPr, tBu, and Cy, the Vbur = 32.0,
36.7, and 42.8%, respectively. Note that these buried volumes
agree very well with expected trends based on increasing sterics
from 1° < 2° < 3° alkyl substituents, except for R = Cy, which
deviates due to the bulkiness of the cyclic C6 rings wrapping
around the metal center. Comparing the 2° alkyl substituents
(isopropyl and cyclohexyl), this additional congestion also
explains the increased air/water stability of compound 4 over 2
(vide supra), since the large hydrophobic ligands would be
expected to impede diffusion of water or oxygen to the reactive
metal center.
Thermogravimetric analysis (TGA) of compounds 1−4 was
next used to evaluate the candidacy of these compounds for
film vapor deposition processes. Measurement of TGA profiles
in a glovebox reveals a single mass loss indicative of smooth
volatilization, without evidence of any decomposition processes
and <5% residue (Figure 3A; onset temperatures are listed in
Table 3). Onset temperatures were extrapolated from the
tangential intersection of the initial mass and midpoint of the
mass loss plot, using the manufacturer’s Pyris software.
Compound 1 exhibits the lowest onset temperature, beginning
at ∼212 °C; the onset temperature then increases linearly with
molecular weight for this series of compounds (Figure 3B).
Note that this also correlates well with the increased
temperature required for sublimation as molecular weight
increases (Table 3). Measurement of melting points in sealed
capillaries for compounds 1−4 (Table 3) reveals a less intuitive
trend, where 2 exhibits the lowest melting temperature,
resulting from a combination of poor crystal packing (low
density) and low molecular weight (for calculated densities
from crystal packing, see Table S1). Compound 1 exhibits the
Figure 3. (A) TGA profiles of complexes 1−4. (B) Correlation of
volatilization onset temperature with molecular weight.
highest melting point, attributable to its highly ordered crystal
packing and fewer degrees of vibrational freedom. Note also
that all compounds are stable up to 200 °C, with no obvious
sign of decomposition, based on heating compounds in a sealed
capillary for 1 h. For 2, quantitative sublimation of 0.5 g of
material was successfully performed. The sublimed material was
then recrystallized and the resulting single crystals had an
identical unit cell as 2, confirming that no decomposition or
other degradative process had occurred. On the basis of these
results, this series of β-diketiminate compounds displays
promising volatility and thermal stability for vapor deposition
processes.
Once volatility and thermal stability were established for this
series of manganese β-diketiminate complexes, solution-phase
reactivity as a model for ALD-like film growth processes was
1
probed using H NMR. Recent reports utilize solution-phase
reactivity, in conjunction with spectroscopic characterization
tools, to elucidate operative mechanisms in ALD processes,64
and here, this technique was employed to inform relative rates
of surface reactivity (Scheme 2). In a typical experiment, in a
glovebox, the yellow/orange manganese precursor was
combined with high surface area silica (400 m2/g) in a J-
Young NMR tube and hexamethylbenzene was added as an
internal standard, all in C6D6. The mixture was then vigorously
agitated and the protonolytic release of ligand monitored as a
function of time (Scheme 2, Figure 4). Over time, the silica
turned bright yellow and the supernatant solution became
colorless, visually indicating manganese chemisorption on the
surface. Note that, in the case of 1 (Figure 4), ligand
concentration initially increases, but then begins to decrease
over time. It is hypothesized that this results from physisorption
of the free Me2NacNacH ligand imine/amine functionalities on
the weakly acidic surface (via hydrogen bonding) as it is
produced from the protonolytic reaction of compound 1 with
the surface (Scheme 2). This was confirmed by a control
experiment which found that, with only free ligand and silica,
the concentration of free Me2NacNacH decreased over time as
it adsorbed onto the surface. While any rate data for reaction of
compound 1 is obscured by ligand consumption, the reaction
solution becomes colorless within 2 h, significantly more
rapidly than for compound 2. For the remaining compounds,
the apparent rate of ligand release, given by t1/2, increases in the
order of 4 < 3 < 2 (Figure 4). Comparing the half-lives with the
buried volume calculations (vide supra), excellent agreement is
observed, such that less buried volume correlates with increased
surface reactivity for 2 (Vbur = 32.0%, t1/2 = 19 min), 3 (Vbur
=
36.7%, t1/2 = 31 min), and 4 (Vbur = 42.8%, t1/2 = 230 min).
Note that the reactivity of 4 appears to decrease significantly
more than expected, possibly reflecting the slower diffusion of
D
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Scheme 2. Proposed Reaction of 1 with Silica Surface, Analogous to the First (A) and Second (B) Half-Cycles of a Putative ALD
Film Growth Process
solution-phase reactivity with ALD processes, the facile
reactivity of compound 1 suggests the greatest potential for
vapor-phase processes. Although physisorption of free ligand
could in principal cause carbon/nitrogen contamination of
films, the elevated temperatures of deposition will likely
minimize this physisorptive phenomenon. Complexes 2 and 3
also exhibit good reactivity, and, due to the easier handling than
1, are posed as potentially applicable to synthesis of manganese
oxide films via vapor-phase or solution-phase growth processes.
Next, the oxidation/hydroxyl regeneration half-cycle of
Scheme 2 was investigated. To this end, the chemisorbed
products of the complexes 1 and 2 with silica (1/SiO2 and 2/
SiO2) were isolated by filtration, washed with pentane, and
dried in vacuo. Exposure to water results in an immediate color
change from yellow to tan. Quantification of released ligand for
Figure 4. Solution-phase reaction of compounds 1−4 with silica in
C6D6.
this step was attempted, but was obscured by the aqueous
hydrolysis of the β-diketimine in the presence of excess water.
X-ray photoelectron spectroscopy (XPS) was next applied to
the more sizable Cy2NacNacH molecule away from the surface.
Thus, buried volume calculations serve as a good predictor of
protonolytic surface reactivity. In terms of correlating this
Figure 5. Mn 2p, O 1s, and Mn 3s XPS spectra of (A) 1/SiO2 and (B) 2/SiO2.
E
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probe the surface species on the hydrolyzed powders (Figure
5). Overlapping binding energies complicate the assignment of
manganese oxidation states based solely on binding energy;
however, the Mn 3s splitting energy is known to correlate with
oxidation state. Thus, for Mn−O species, Mn 3s splitting
energies fall between 5.9 −5.7 eV for Mn(II), 5.6−5.2 eV for
Mn(III), and less than 5.0 for Mn(IV).65,66 Analysis of the
present tan powders by XPS (Figure 5) reveals ΔE3s of 5.8 and
6.0 eV for 1/SiO2 and 2/SiO2, respectively, indicating the
presence of predominantly Mn(II) species on the surface.
Additionally, the presence of the “shake up” peak in the Mn
2p3/2 spectra around 645 eV is consistent with the assignment
of Mn(II) species.67−70 The presence of the extra feature at
654.7 eV in the Mn 2p1/2 peak of 2/SiO2 as well as the
additional higher energy 531.9 eV peak in the O 1s spectrum
may indicate the presence of small amounts of a higher
oxidation state Mn species on the surface. This may reflect
partial oxidation during hydrolysis due to the slower reaction of
the bulkier O−Mn−iPrNacNac environment compared to that
of the methyl analogue. The exact nature of the manganese
oxygen species (e.g., Mn-O-Mn, Mn-OH) cannot be
unambiguously delineated from the O 1s peak due to
overlapping Si-O features.71 Some nitrogenous species (<2%,
Figures S11 and S12) are also observed by XPS, likely resulting
from ligand physisorption to the surface. The binding energy of
these peaks (399 eV) agrees well with known energies for imine
N species, suggesting free ligand binding to the surface.72,73
Slightly increased nitrogenous species are observed for 1/SiO2
(1.2%) than for 2/SiO2 (0.8%), reflecting the strong free ligand
physisorption of 1 to the silica surface that was observed by 1H
NMR in the kinetic experiments. Note that, as discussed above,
ligand physisorption, and subsequent nitrogen contamination,
should be remedied by the elevated temperatures of typical
vapor deposition processes. These results prove the com-
petency of both complexes 1 and 2 for solution-phase reactivity
analogous to that of ALD half-cycles.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02476.
■
S
Crystallographic data and structure refinement details for
1−4 (Table S1); UV−visible spectra of 1−4 (Figure S1);
FTIR spectra of 1−4 (Figure S2); buried volume maps
for 1−4 (Figure S3); time-lapsed 1H NMR spectra of 1−
4/SiO2 (Figures S4−S7); selected XPS peaks for SiO2,
1/SiO2, and 2/SiO2 (Table S2); survey and N 1s XPS
spectra of SiO2, 1/SiO2, and 2/SiO2 (Figures S8−S12);
bond lengths and bond angles for the X-ray crystal
structure of 1 (Tables S3 and S4); bond lengths and
bond angles for the X-ray crystal structure of 2 (Tables
S5 and S6); bond lengths and bond angles for the X-ray
crystal structure of 3 (Tables S7 and S8); bond lengths
and bond angles for the X-ray crystal structure of 4
(Tables S9 and S10) (PDF)
Accession Codes
CCDC 1576501−1576504 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing 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.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: t-marks@northwestern.edu (T.J.M.).
*E-mail: tracy.lohr@northwestern.edu (T.L.L.).
ORCID
Tobin J. Marks: 0000-0001-8771-0141
Notes
The authors declare no competing financial interest.
■
The chemistry of β-diketiminate manganese(II) compounds
has been expanded through the synthesis and characterization
of a series of N-alkyl variants, which were assessed for their
potential as vapor deposition precursors. Structural and
electronic characterization by single crystal X-ray diffraction
as well as UV−vis reveals that steric factors are the main driver
in observed chemical/physical trends, i.e., increased air/water
stability, metal puckering from the ligand plane, and dihedral
angles in the coordination sphere. All of these new compounds
exhibit a single, clean volatilization event in TGA profiles, with
onset temperatures scaling linearly with molecular weight, and
good thermal stability observed up to 200 °C, proving that β-
diketiminates induce volatility in manganese compounds. In
addition to volatility and thermal stability, the solution-phase
reactivity in models for ALD half-cycles was explored, and for
the first half-cycle of protonolytic chemisorption on silica,
reactivity is found to correlate well with buried volume
computation, though ligand diffusion may have some effect
on apparent rates. Hydrolysis of 1/SiO2 and 2/SiO2 mimicks
the second ALD half-cycle, and clean Mn(II)-O surface species
were observed by XPS, in good agreement with NMR results.
Thus, preliminary evaluations were utilized to identify likely
candidates for vapor deposition of manganese oxide films from
a series of N-alkyl β-diketiminates, demonstrating the viability
of this ligand set for manganese vapor deposition precursors.
ACKNOWLEDGMENTS
■
This work was supported as part of the Center for
Electrochemical Energy Science, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Basic Energy Sciences (Award No. DE-AC02-
06CH11357). This work made use of the Keck-II facility of
Northwestern University’s NUANCE Center, which has
received support from the Soft and Hybrid Nanotechnology
Experimental (SHyNE) Resource (NSF ECCS-1542205); the
MRSEC program (NSF DMR-1121262) at the Materials
Research Center; the International Institute for Nanotechnol-
ogy (IIN); the Keck Foundation; and the State of Illinois,
through the IIN. This work also made use of the IMSERC at
Northwestern University, which has received support from the
Soft and Hybrid Nanotechnology Experimental (SHyNE)
Resource (NSF NNCI-1542205); the State of Illinois and
International Institute for Nanotechnology (IIN); and the NSF
(NSF CHE-9871268).
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