Volatility and high thermal stability in mid- to late-first-row transition-metal diazadienyl complexes

Article abstract of DOI:10.1021/om200626w

Treatment of MCl₂ (M = Cr, Mn, Fe, Co, Ni) with 2 equiv of lithium metal and 1,4-di-tert-butyl-1,3-diazadiene (^tBu2DAD) in tetrahydrofuran at ambient temperature afforded Cr(^tBu2DAD) ₂ (38%), Mn(^tBu2DAD)₂ (81%), Fe( ^tBu2DAD)₂ (47%), Co(^tBu2DAD)₂ (36%), and Ni(^tBu2DAD)₂ (41%). Crystal structure determinations revealed monomeric complexes that adopt tetrahedral coordination environments and were consistent with ^tBu2DAD radical anion ligands. To evaluate the viability of M(^tBu2DAD)₂ (M = Cr, Mn, Fe, Co, Ni) as potential film growth precursors, thermogravimetric analyses, preparative sublimations, and solid-state decomposition studies were performed. Mn( ^tBu2DAD)₂ is the most thermally robust among the series, with a solid-state decomposition temperature of 325 °C, a sublimation temperature of 120 °C/0.05 Torr, and a nonvolatile residue of 4.3% in a preparative sublimation. Thermogravimetric traces of all complexes show weight loss regimes from 150 to 225 °C with final percent residues at 500 °C ranging from 1.5 to 3.6%. Thermolysis studies reveal that all complexes except Mn(^tBu2DAD)₂ decompose into their respective crystalline metal powders under an inert atmosphere. Mn(^tBu2DAD)₂ may afford amorphous manganese metal upon thermolysis.

Full text of DOI:10.1021/om200626w

ARTICLE
pubs.acs.org/Organometallics
Volatility and High Thermal Stability in Mid- to Late-First-Row
Transition-Metal Diazadienyl Complexes
†
‡
†
§
,†
Thomas J. Knisley, Mark J. Saly, Mary Jane Heeg, John L. Roberts, and Charles H. Winter*
†
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States
‡
SAFC Hitech, 1429 Hilldale Avenue, Haverhill, Massachusetts 01832, United States
§
SAFC Hitech, Power Road, Bromborough, Wirral CH62 3QF, U.K.
S Supporting Information
b
ABSTRACT: Treatment of MCl (M = Cr, Mn, Fe, Co, Ni) with 2 equiv of lithium metal and 1,4-di-tert-
2
tBu2
tBu2
butyl-1,3-diazadiene ( DAD) in tetrahydrofuran at ambient temperature afforded Cr( DAD)2
tBu2
tBu2 tBu2
2 2 2
tBu2
(
(
38%), Mn( DAD) (81%), Fe( DAD) (47%), Co( DAD) (36%), and Ni( DAD)2
41%). Crystal structure determinations revealed monomeric complexes that adopt tetrahedral
coordination environments and were consistent with
viability of M( DAD) (M = Cr, Mn, Fe, Co, Ni) as potential film growth precursors, thermogravi-
tBu2
DAD radical anion ligands. To evaluate the
tBu2
2
tBu2
metric analyses, preparative sublimations, and solid-state decomposition studies were performed. Mn( DAD) is the most
2
thermally robust among the series, with a solid-state decomposition temperature of 325 °C, a sublimation temperature of
1
20 °C/0.05 Torr, and a nonvolatile residue of 4.3% in a preparative sublimation. Thermogravimetric traces of all complexes show
weight loss regimes from 150 to 225 °C with final percent residues at 500 °C ranging from 1.5 to 3.6%. Thermolysis studies reveal
that all complexes except Mn( DAD) decompose into their respective crystalline metal powders under an inert atmosphere.
Mn( DAD) may afford amorphous manganese metal upon thermolysis.
tBu2
2
tBu2
2
’
INTRODUCTION
surface reactive sites. Once all accessible surface reactive sites
have become occupied, the surface is saturated, no further
reactions can occur, and unreacted precursor and reaction
byproducts are removed from the reactor with an inert gas
purge. At this point, a second precursor vapor is passed over the
substrate, which reacts with the surface-adsorbed metal pre-
cursors to produce the desired thin film material. To conclude a
growth cycle, a second inert gas purge is then performed to
remove additional byproducts and excess precursor. The
growth rate per cycle generally remains constant, allowing the
film thickness to be dependent only upon the number deposi-
tion cycles.
Thin films containing the mid- to late-first-row transition
1
,2
metals have many important existing and future applications.
In the microelectronics area, copper has replaced aluminum as
the interconnect material in integrated circuits due to its lower
1
resistivity and higher resistance to electromigration. Ultrathin
(
2ꢀ8 nm) manganeseꢀsiliconꢀoxygen layers have been pro-
posed as replacements for existing nitride-based copper diffu-
3
sion barrier layers in future devices. Since copper does not
nucleate well on SiO and other surfaces, it is difficult to
1
2
deposit copper metal into the surface features of microelec-
tronics substrates. Accordingly, there has been considerable
interest in the formation of seed layers of metals such as
chromium, cobalt, and others which adhere better to substrates4,
and upon which copper films can be subsequently grown.
Nickel and nickel silicide (NiSi) are important contact materials
in device fabrication. Magnetoresistive random access memory
devices require the growth of thin, conformal layers of magnetic
metals such as nickel, cobalt, and iron. Microelectronics device
ALD precursors must combine high thermal stability, high
reactivity toward a second reagent to afford the desired thin film
material, and enough volatility to permit vapor transport at
9
temperatures of generally <200 °C. Thermal decomposition of
5
the precursor within the deposition chamber would lead to loss
of the self-limited ALD growth mechanism and uncontrolled
chemical vapor deposition (CVD) like film growth. Among the
first-row transition elements, ALD processes are best developed
6
dimensions are scheduled to reach 22 nm by 2012, and existing
deposition processes will soon not be able to provide the
required level of thickness control and conformality, especially
2
for copper metal, and precursors have incorporated amidinate,
β-diketonate, β-ketoiminate, β-diketonate, alkoxide, and chlo-
ride ligands. Reducing agents have included molecular hydro-
gen, main-group-metal alkyls, alkylsilanes, alcohols containing
1
,7
in high aspect ratio features. The atomic layer deposition
(ALD) film growth method is well-suited for nanoscale film
2
growth, since it affords inherently conformal coverage and
subnanometer film thickness control, due to its self-limited
growth mechanism. In a typical ALD process, a metal pre-
β-hydrogen atoms, formic acid, and zinc metal. In addition,
plasma activation has been used to get around the low reactivities
8
cursor is transported in the vapor phase by an inert carrier gas
into the reaction chamber, where it chemisorbs upon substrate
Received: July 12, 2011
Published: August 25, 2011
r 2011 American Chemical Society
5010
dx.doi.org/10.1021/om200626w
|Organometallics 2011, 30, 5010–5017

Organometallics
ARTICLE
Chart 1. Possible Forms of R-Diimine Ligands
Table 1. Crystal Data and Data Collection Parameters for 1
and 3ꢀ5
1
3
4
5
formula
fw
C
20
H
40CrN
4
C
20
H
40FeN
4
C
20
H
40CoN
4
C
20
H
40NiN
4
388.56
392.41
395.49
395.27
space group
a (Å)
P1
Pnma
Pnma
Pnma
11.1969(10) 18.3813(8) 18.2363(8)
14.0988(13) 13.4195(6) 13.3810(6)
18.1617(7)
13.3739(5)
9.2280(4)
b (Å)
2
b,m
c (Å)
14.4001(14) 9.2799(4)
86.857(2)
9.2410(4)
of the precursors toward many of the reducing agents.
ALD
precursors for nickel metal growth have employed amidinate,
alkoxide, and cyclopentadienyl ligands, and reducing core-
agents have included ammonia plasma, hydrogen plasma, and
R (deg)
β (deg)
γ (deg)
82.672(3)
88.077(3)
2
o,5
3
molecular hydrogen.
Growth of cobalt metal films by ALD
V (Å )
2250.5(4)
4
2289.05(17) 2254.99(17) 2241.41(15)
has employed precursors containing cyclopentadienyl, amidi-
Z
4
4
4
1
0
nate, and carbonyl ligands. Reducing coreagents include
ammonia plasma, hydrogen plasma, and nitrogen plasma.
T (K)
λ (Å)
100(2)
0.71073
1.147
0.518
6.65
100(2)
0.71073
1.139
0.668
2.96
100(2)
0.71073
1.165
0.770
2.62
100(2)
0.71073
1.171
0.875
3.02
1
1
ALD growth of iron metal on aerogels was claimed, although
no details were given. Manganese and chromium metal films
have not been grown by ALD. Existing ALD precursors for
copper, nickel, and cobalt generally suffer from low thermal
stability, low reactivity toward reducing agents, or a combina-
ꢀ
3
Fcalcd (g cm )
ꢀ
1
μ (mm
)
a
R(F) (%)
a
R
w
(F) (%)
R(F) = ∑||F
for I > 2σ(I).
16.83
7.78
6.64
2
o
7.72
2
,5,10
a
2 2
2 2 1/2
tion of both.
Use of plasmas can overcome the low
o
| ꢀ |F
c
||/∑|F
o
|; R
w
(F) = [∑w(F
ꢀ F
c
) /∑w(F
o
) ]
,
reactivities toward reducing agents, but plasma reactors are
more complex than thermal reactors and plasmas can introduce
undesired damage to the films.
heating, and may serve as precursors in ALD and other film
growth procedures that require molecular precursors.
Nitrogen ligands are of special interest in the design of ALD
precursors, since the metalꢀnitrogen bonds are often highly
reactive. However, the challenge is to identify nitrogen ligands
that can also lead to volatile and highly thermally stable com-
’
RESULTS AND DISCUSSION
0
0
plexes. Recently, 1,4-diaza-1,3-butadienes (RNdCR CR dNR,
Synthesic Aspects. Treatment of anhydrous metal(II) chlo-
rides (MCl , M = Cr, Mn, Fe, Co, Ni) with 2 equiv of 1,4-di-tert-
butyl-diaza-1,3-butadiene and 2 equiv of lithium metal afforded
complexes of the formula M( DAD) as purple (1), black (2),
brown (3), blue (4), and dichroic red/green (5) crystalline
powders (eq 1). Crystalline samples of 1ꢀ5 were subsequently
obtained by either sublimation or crystallization in hexane at
0
R = alkyl, aryl, R = H, alkyl) have been investigated as ligands,
especially for the mid- to late-first-row transition metals.
2
12ꢀ16
tBu2
These ligands are redox noninnocent and can exist in three
2
13ꢀ16
distinct forms (Chart 1).
Form A is formally dianionic and
has a short carbonꢀcarbon distance and long carbonꢀnitrogen
distances, form B is a monoanionic, delocalized radical anion with
intermediate carbonꢀcarbon and carbonꢀnitrogen distances,
and form C is neutral with a long carbonꢀcarbon distance and
short carbonꢀnitrogen distances. These ligands can form square-
ꢀ
23 °C. The synthetic procedure is a modification of previous
routes to transition-metal complexes containing 1,4-diaza-1,3-
13ꢀ16
butadiene ligands.
copper(II) or copper(I) complexes containing
since all reactions afforded copper powders. The compositions of
It was not possible to prepare any
13
tBu2
planar to distorted-tetrahedral complexes with chromium(II),
16
DAD ligands,
1
4
15
16
manganese(II), iron(II), cobalt(II), and nickel(II) ions.
Our long-term research objective is to develop new ALD
precursors and processes that afford thin films of high-purity
chromium, manganese, iron, cobalt, nickel, and copper metal
thin films. These processes should obey the self-limited ALD
growth mechanism over the widest possible temperature range
to meet future microelectronics and other manufacturing
requirements. To meet these goals, highly volatile precursors
with the highest possible thermal stabilities are required. 1,4-
Diaza-1,3-butadiene-derived ligands form four-coordinate com-
plexes with the mid- to late-transition-metal ions in the +2
1
ꢀ5 were determined by a combination of analytical and
spectroscopic techniques and by X-ray crystal structure determi-
nations of 1 and 3ꢀ5. Complex 5 is the only diamagnetic species
in the series and revealed tert-butyl and imino hydrogen atom
1
resonances at δ 1.93 and 8.95, respectively, in the H NMR
spectrum in benzene-d . In the infrared spectra of 1ꢀ5, the
6
carbonꢀnitrogen stretching frequencies were observed between
ꢀ
1
1
716 and 1698 cm . Solid-state magnetic moments for 1ꢀ4
were 2.83, 3.85, 2.88, and 1.75 μ , respectively. Very similar
B
values were measured in benzene solution using the Evans
method, suggesting similar molecular structures in the solid state
and solution. The magnetic moments for 1ꢀ4 are very close to
those expected for high-spin M(II) ions that are antiferromag-
netically coupled to two unpaired electrons of radical anion
1
3ꢀ16
oxidation state,
and the nature of the nitrogen and carbon
substituents should be easily modified to adjust volatility,
thermal stability, and reactivity. Herein we report the synthesis,
structure, volatility, thermal stability, and thermal decomposi-
tion of a series of chromium(II), manganese(II), iron(II),
cobalt(II), and nickel(II) complexes that contain 1,4-di-tert-
tBu2
DAD ligands (Chart 1, form B). Analogous magnetic cou-
pling is well-established in transition-metal complexes containing
1,4-diaza-1,3-butadiene radical anion ligands with various alkyl
and aryl substituents.
tBu2
butyl-diaza-1,3-butadienyl ( DAD) ligands. These com-
12ꢀ16
15e
16e
plexes sublime at low temperatures, have high solid-state
decomposition temperatures, decompose to the metals upon
Complexes 3 and 5 have been
16g
previously reported, and 4 was studied theoretically.
The
5
011
dx.doi.org/10.1021/om200626w |Organometallics 2011, 30, 5010–5017

Organometallics
ARTICLE
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1
Cr(1)ꢀN(1)
Cr(1)ꢀN(2)
Cr(1)ꢀN(3)
Cr(1)ꢀN(4)
Cr(1)ꢀC(1)
Cr(1)ꢀC(2)
1.924(3)
1.924(2)
1.928(3)
1.934(2)
2.351(3)
2.361(3)
C(1)ꢀC(2)
C(11)ꢀC(12)
C(1)ꢀN(1)
C(2)ꢀN(2)
C(11)ꢀN(3)
C(12)ꢀN(4)
1.395(4)
1.337(5)
1.360(4)
1.356(4)
1.386(4)
1.367(4)
N(1)ꢀCr(1)ꢀN(2)
91.79(11) N(3)ꢀCr(1)ꢀN(4)
82.91(11)
89.69(19)
90.38(18)
N(1)ꢀCr(1)ꢀN(3) 127.13(11) Cr(1)ꢀN(1)ꢀC(1)
N(1)ꢀCr(1)ꢀN(4) 118.60(11) Cr(1)ꢀN(2)ꢀC(2)
N(2)ꢀCr(1)ꢀN(3) 123.99(11) Cr(1)ꢀN(3)ꢀC(11) 112.1(2)
N(2)ꢀCr(1)ꢀN(4) 114.94(11) Cr(1)ꢀN(4)ꢀC(12) 112.2(2)
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Figure 1. Perspective view of 1 with thermal ellipsoids at the 50%
probability level.
3
ꢀ5
3
4
5
MꢀN(1)
1.952(1)
1.956(1)
1.953(1)
1.393(2)
1.397(2)
1.339(2)
1.341(2)
1.347(1)
1.929(1)
1.936(1)
1.931(1)
1.393(2)
1.403(2)
1.334(2)
1.332(2)
1.335(2)
1.919(1)
1.916(1)
1.917(1)
1.401(2)
1.407(2)
1.326(2)
1.326(2)
1.330(1)
MꢀN(2)
MꢀN(3)
C(1)ꢀC(2)
C(9)ꢀC(9)
C(1)ꢀN(1)
C(2)ꢀN(2)
C(9)ꢀN(3)
0
N(1)ꢀMꢀN(2)
N(1)ꢀMꢀN(3)
N(2)ꢀMꢀN(3)
N(3)ꢀMꢀN(3)
MꢀN(1)ꢀC(1)
MꢀN(2)ꢀC(2)
MꢀN(3)ꢀC(9)
84.72(5)
84.65(6)
83.51(5)
122.74(3)
123.28(3)
85.00(5)
122.93(4)
123.26(4)
84.77(6)
123.93(3)
123.62(3)
83.65(5)
0
110.18(10)
109.97(9)
110.07(6)
110.76(11)
110.61(11)
110.87(8)
112.33(10)
112.61(10)
112.55(7)
Figure 2. Perspective view of 3 with thermal ellipsoids at the 50%
probability level.
solution-state magnetic moment reported for 3 in benzene
15e
solution was 2.88 μB, which is very similar to the values of
.88 and 2.68 μ that were observed herein for the solid-state and
and 5 are isostructural with 3, and their perspective views are
contained in the Supporting Information.
2
B
1
benzene solution magnetic moments, respectively. The H NMR
Complexes 1 and 3ꢀ5 adopt mononuclear structures, with
spectrum previously reported for 5 exactly matches that reported
distorted-tetrahedral geometry about the metal centers. In
1
6e
herein.
3
ꢀ5, the planes of the C N ligand cores are constrained by
2
2
symmetry to be orthogonal. Complex 1 crystallizes with two
independent molecules in the unit cell; both molecules are
identical within experimental uncertainty, and only data for the
molecule containing Cr(1) are presented herein. Complexes 3ꢀ5
are isostructural. The metalꢀnitrogen bond lengths (1, 1.924-
(
2)ꢀ1.934(2) Å;3, 1.952(1)ꢀ1.956(1) Å; 4, 1.929(1)ꢀ1.936(1)
Å; 5, 1.916(1)ꢀ1.919 Å) fall into a narrow range. The metalꢀ
nitrogen bond distances in 1 are considerably shorter than
those found in Cr(2,6-iPr C H NdCHC(Me)dNC H -2,6-
X-ray Crystal Structures of 1ꢀ5. The X-ray crystal structures
of 1 and 3ꢀ5 were determined to establish their solid-state
configurations. Despite multiple attempts, high-quality crystals
of 2 could not be obtained, although a low-resolution X-ray
structure revealed a monomeric, tetrahedral complex with a
molecular arrangement similar to those of 3ꢀ5. Experimental
crystallographic data are summarized in Table 1, selected bond
lengths and angles are given in Tables 2 and 3, and perspective
views of 1 and 3 are presented in Figures 1 and 2. Complexes 4
2
6
3
6
3
1
3a
iPr2)2 (2.019(1), 2.030(1) Å)
and Cr(2,6-iPr C H Nd
2 6 3
1
3b
CHCHdNC H -2,6-iPr ) (2.030(4), 2.035(5) Å) but are simi-
6
3
2 2
lar to the values observed in Cr (2,6-iPr C H NdCHCHdNC H -
2
2
6
3
6 3
13c
2,6-iPr ) (1.914(2), 1.913(2) Å). The metalꢀnitrogen bond
2
2
distances in 3 are similar to those of Fe(C F NdC(Me)
6
5
15c
C(Me)dNC F ) (1.962(2), 1.962(2) Å) but are shorter than
6
5 2
those found in Fe(2,6-iPr C H NdC(Me)C(Me)dNC H -2,
2
6
3
6
3
5
012
dx.doi.org/10.1021/om200626w |Organometallics 2011, 30, 5010–5017

Organometallics
ARTICLE
Table 4. Sublimation Temperature, Melting Point, Solid
State Decomposition Temperature, Percent Recovery, and
Percent Nonvolatile Residue for 1ꢀ5
sublimation
solid-state
temp
dec temp recovery nonvolatile
complex (°C/0.05 Torr) mp (°C)
(°C)
(%)
residue (%)
1
2
3
4
5
85
120
115
115
115
95ꢀ97
295
325
260
235
230
96.7
95.0
96.1
94.7
92.3
3.2
4.3
3.4
5.2
6.9
155ꢀ157
132ꢀ134
174ꢀ175
184ꢀ185
1
5d
6
-iPr ) (1.988(9)ꢀ2.077(8) Å). Fe(C F NdC(Me)C(Me)d
2
2
6 5
NC F ) is proposed to contain two diazadienyl radical anion
Figure 3. Comparison of the solid-state decomposition temperatures of
6 5 2
2
+
0
ligands, based upon the carbonꢀcarbon and carbonꢀnitrogen
1ꢀ5 versus the M /M redox couple.
1
5c
distances of the ligand core. In contrast, one of the ligand
core carbonꢀcarbon distances in Fe(2,6-iPr C H NdC(Me)
radical anion. These assignments are also consistent with the
magnetic moment data described above. The situation with 1
is a little more complex, since one ligand has a carbonꢀcarbon
distance of 1.337(5) Å and carbonꢀnitrogen distances of
1.386(4) and 1.367(4) Å. The other ligand is bent toward the
chromium ion and has a carbonꢀcarbon distance of 1.395(4) Å
and carbonꢀnitrogen distances of 1.360(4) and 1.356(4) Å. The
2
6 3
C(Me)dNC H -2,6-iPr ) is about 0.08 Å longer than the other,
6
3
2 2
suggesting more neutral character in one ligand (Chart 1,
1
5d
form C). The cobaltꢀnitrogen distances in 4 are similar to
those of Co(C F NdC(Me)C(Me)dNC F ) (1.931(3),
6
5
6 5 2
1
6c
1
.932(3) Å).
The nickelꢀnitrogen bond lengths in 5
6 5 6 5 2
compare well with those of Ni(C F NdC(Me)C(Me)dNC F )
1.9173(18), 1.9165(17) Å) and Ni(2,6-Me C H NdCHC-
16c
(
(
bond distances of the latter ligand are very similar to those of
2
6 3
16a
tBu2
Me)dNC H -2,6-Me ) (1.921(1)ꢀ1.948(1) Å)
but are
3ꢀ5 and support assignment as a
DAD radical anion. In the
6
3
2 2
shorter than those found in Ni(2,6-iPr C H NdCHC(Me)d
former ligand, only the carbonꢀcarbon bond length differs signi-
ficantly within experimental error from that of the latter ligand.
Hence, there may be a slightly higher amount of charge localiza-
tion on the nitrogen atoms in the former ligand, but any differences
in the structural data are small and are at the edge of experimental
2
6 3
1
6a
NC H -2,6-iPr ) (1.963(2)ꢀ1.999(2) Å). Interestingly, the
6
3
2 2
1
6d
X-ray crystal structure of a polymorph of 5 was reported
and has nickelꢀnitrogen bond lengths of 1.906ꢀ1.941 Å, with an
average value of 1.923 Å.
The average metalꢀnitrogen bond lengths follow the order 3
uncertainty. The magnetic moment data for 1 described above
tBu2
(
1.954 Å) > 4 (1.932 Å) > 1 (1.928 Å) > 5 (1.917 Å), even
are consistent with the presence of two
DAD radical anion
4
though the ionic radii of the metal ions decrease in the order
ligands that are antiferromagnetically coupled to a high-spin d
tBu2 18a
Cr(II) (0.80 Å) > Fe(II) (0.78 Å) > Co(II) (0.745 Å) > Ni(II)
Cr(II) ion. Complexes of the formula FeCl
3
(
DAD),
DAD) have been struc-
ligand core carbonꢀcarbon
1
7
tBu2
18b
tBu2
18c
(
0.69 Å). Hence, the chromiumꢀnitrogen distances in 1 are
CoCl
turally characterized and have C N
2 2
2
(
DAD), and NiBr (
2
shorter than expected, on the basis of the ionic radius of the
Cr(II) ion relative to those of the other ions. Closer inspection of
and carbonꢀnitrogen distances of 1.455ꢀ1.510 and 1.247ꢀ1.275
Å, respectively. The carbonꢀcarbon distances in these complexes
are close to those expected for a single bond, and the carbonꢀ
nitrogen distances are close to those expected for a double bond.
Hence, these ligands are consistent with neutral form C in Chart 1
and are distinct from the radical anion ligand type B observed in 1
and 3ꢀ5.
tBu2
1
reveals that there are two types of
DAD ligands. The ligand
that contains N(1) and N(2) has the C(1) and C(2) atoms tilted
toward the chromium atom, with chromiumꢀcarbon distances
of 2.351(3) and 2.361(3) Å. In contrast, the ligand containing
N(3) and N(4) forms a planar CrN C ring and has chro-
2
2
miumꢀcarbon distances of 2.758 and 2.767 Å. For comparison
tBu2
the metalꢀcarbon distances associated with the
DAD core
Volatility and Thermal Stability. The volatility and thermal
stability of 1ꢀ5 were studied by preparative sublimation, ther-
mogravimetric analyses, and melting point/solid-state thermal
decomposition experiments to assess their potential for use as
ALD precursors. Sublimation data, melting points, and solid-
state decomposition temperatures for 1ꢀ5 are given in Table 4.
In preparative sublimations, 0.5ꢀ1.0 g samples were sublimed at
0.05 Torr and the temperature was adjusted so that the sublima-
tion was complete in less than 5 h. In previous work, we have
established that these preparative sublimation temperatures
approximate the temperatures required for the vapor phase
carbon atoms of 3ꢀ5 range between 2.706 and 2.727 Å and the
metal ions are coplanar with the N C rings. The chromiumꢀ
2
2
carbon interactions in the ligand in 1 containing C(1) and C(2)
appear to reflect the additional empty d orbitals associated
4
with the d Cr(II) ion. The modified bonding to one of the
ligands in 1 may alleviate interligand tert-butyl crowding by a
small amount, thereby allowing chromiumꢀnitrogen bond
lengths slightly shorter than expected on the basis of the ionic
radius of the Cr(II) ion.
It is well-established that the carbonꢀcarbon and carbonꢀ
19
nitrogen bond lengths within the C N ligand cores offer a
delivery of precursors in our ALD reactors. Under these
conditions, the sublimed recoveries of 1ꢀ5 were g92.3% with
nonvolatile residues of e6.9%. The high air sensitivity of 1ꢀ5
limited the ability to obtain higher sublimed recoveries, since
product isolation had to be conducted in an inert atmosphere
drybox. Additionally, exposure to trace amounts of air during
2
2
1
3ꢀ16
reliable tool for distinguishing among forms AꢀC in Chart 1.
In 3ꢀ5, the carbonꢀcarbon and carbonꢀnitrogen bond lengths
fall into the narrow ranges of 1.393ꢀ1.407 and 1.326ꢀ1.347 Å,
tBu2
respectively. These values are between those expected for
single and double bonds and are diagnostic of the
DAD
5
013
dx.doi.org/10.1021/om200626w |Organometallics 2011, 30, 5010–5017

Organometallics
ARTICLE
Figure 5. X-ray diffraction pattern of chromium metal powder obtained
upon thermolysis of 1.
aF ti g1 u0 r e° C4 /. mT i hn e. rmogravimetric analysis traces of 1ꢀ5 from 50 to 550 °C
As described above, solid-state decompositions of 1ꢀ5
afforded shiny metallic foils as products. To verify the formation
of the metals, preparative-scale solid-state thermolyses were
carried out on 1ꢀ5 as described in the Experimental Section and
the residues were analyzed by X-ray powder diffraction. X-ray
diffraction analyses demonstrated that 1 and 3ꢀ5 afford crystal-
line, shiny gray-black powders of the metals. Figure 5 shows the
X-ray diffraction pattern of the chromium metal thermolysis
product from 1, which matches the JCPDS 06-0694 reference
pattern for chromium metal. The X-ray diffraction patterns of the
powders derived from 3ꢀ5 are contained in the Supporting
Information. The thermolysis product of 2 was also a shiny
metallic powder. The X-ray diffraction pattern of the thermolysis
product obtained under conditions similar to those used for the
thermolysis of 1 and 3ꢀ5 showed weak reflections that were
sample loading may have led to higher nonvolatile residues. The
solid-state decomposition temperatures were determined vi-
sually by monitoring sealed-glass capillary tubes containing a
few milligrams of 1ꢀ5 and then noting the temperatures at which
metal foils began to appear. Previous work from our growth with
ALD precursors containing tris(pyrazolyl)borate or amidinate
ligands has shown that solid-state decomposition temperatures
are generally very close to the upper limit of self-limited ALD
19
growth in plots of growth rate versus deposition temperature
and are thus very useful. The solid-state decomposition tem-
perature of 2 is the highest at 325 °C, while that of 5 is the lowest
at 230 °C. Interestingly, a plot of the solid-state decomposition
2+
0
22
temperatures of 1ꢀ5 versus the M f M reduction
consistent with Mn O (JCPDS 04-0732). Since it was
3
4
2
0
potentials is linear (Figure 3), suggesting that decomposition
possible that the Mn O formed upon oxidation of manganese
3
4
tBu2
might occur through transfer of an electron from the
DAD
metal by residual oxygen or water in the argon used in the
thermolysis experiment, the thermolysis of 2 was repeated under
a vacuum of 0.05 Torr at 375 °C for 1 h. X-ray diffraction spectra
of the resulting gray-black metallic powder did not show any
reflections, suggesting an amorphous product. Treatment of the
powders resulting from the thermolysis of 2 with 30% aqueous
hydrogen peroxide led to vigorous reaction and gas evolution.
Similar reactivities were observed for the powders resulting from
thermolysis under flowing argon and under vacuum. For com-
parison, manganese metal powder reacted in a similarly vigorous
manner with 30% aqueous hydrogen peroxide, whereas commercial
Mn O powder was inert under the same conditions. The com-
radical anion ligands to the metal ions. The failure to produce
copper(II) or copper(I) complexes noted above likely arises
from immediate reduction of the copper ions to copper metal,
2+
due to the large positive reduction potentials of these ions (Cu ,
+
20
0
.342 V; Cu , 0.521 V).
Thermogravimetric analyses (TGA) were performed on 1ꢀ5
to understand their volatilities and thermal stabilities
Figure 4). These analyses were carried out with an instrument
(
that was contained in a high-purity nitrogen-filled glovebox to
minimize decomposition arising from exposure to air. Com-
plexes 1ꢀ5 have similar TGA traces with single-step weight
losses occurring between 150 and 225 °C. The residues upon
reaching 500 °C were all e3.6%. Complex 2 is the most air-
sensitive compound in the series, and its TGA traces always
showed 10ꢀ20% weight losses between 50 and 150 °C that
were presumably due to reaction with ambient oxygen or water,
in spite of multiple runs and utmost care to maintain a high-
purity nitrogen atmosphere.
3
4
mercial Mn O powder was crystalline and indexed as Hausmannite
3
4
(JCPDS 24-0734) and is thus in a different crystalline form from the
powder described above that was obtained upon thermolysis of 2.
However, the reactivities of both forms of Mn O toward 30%
3
4
hydrogen peroxide should be similar. Hence, it is possible that
thermolysis of 2 affords amorphous manganese metal powder.
Evaluation of Precursor Properties. Complexes 1ꢀ5 sub-
lime at 85 (1) and 115ꢀ120 °C (2ꢀ5) with low nonvolatile
residues, have high solid-state decomposition temperatures, and
are highly reactive toward ambient atmosphere. Additionally, 1
and 3ꢀ5 decompose to the metals upon thermolysis, and 2 may
afford manganese metal upon thermolysis. These complexes thus
have useful properties for applications as film growth precursors
by ALD and CVD. While our specific interest is in the growth of
metal thin films by ALD, 1ꢀ5 may also be useful in the ALD and
CVD growth of oxide, nitride, and other types of thin films.
Vapor pressure measurements were carried out on 2 and 5
21
using a previously reported method and apparatus. Details of
these analyses are found in the Supporting Information. The
vapor pressure of 2 obeys the equation log P (mTorr) = 12.753
10
ꢀ
3631/T (K), whereas the equation for 5 is log10 P (mTorr) =
3.983 ꢀ 3986/T (K). The vapor pressures of 2 and 5 at 115 °C
1
are 2.48 and 5.13 Torr, respectively. The vapor pressures of 1, 3,
and 4 should be close to those of 2 and 5, since the preparative
sublimation temperatures of 1ꢀ5 are similar.
5
014
dx.doi.org/10.1021/om200626w |Organometallics 2011, 30, 5010–5017

Organometallics
ARTICLE
Complexes 1 and 3ꢀ5 decompose thermally between 230 and
solution was stirred for 6 h. This solution was then added dropwise by
cannula over a 30 min period to a stirred suspension of anhydrous
chromium(II) chloride (0.365 g, 2.970 mmol) in tetrahydrofuran
(40 mL). The resultant deep purple solution was stirred for 6 h at
ambient temperature. The volatile components were then removed
under reduced pressure, and the resultant dark purple powder was
dissolved in toluene (50 mL). The solution was filtered through a 1 cm
pad of Celite on a coarse glass frit, and toluene was then removed under
reduced pressure. Dark purple crystals of 1 were obtained by sublimation
2
95 °C to afford the metals, and 2 decomposes at 325 °C possibly
to afford manganese metal. Since 1ꢀ5 evaporate with low residues
between 85 and 120 °C, they are highly likely to be useful CVD
precursors to films of the metals. ALD precursors must be
thermally stable at the film growth temperatures, or the self-
limited ALD growth mechanism is lost and CVD-like growth
8
,9
occurs. Complexes 1ꢀ5 also have properties that may be useful
in ALD film growth, especially since they have high thermal
decomposition temperatures compared to other available pre-
cursors for each metal. For example, the amidinate complex
ꢀ
1
at 85 °C/0.05 Torr (0.442 g, 38%): mp 95ꢀ97 °C; IR (Nujol, cm
)
1
704 (w), 1628 (w), 1538 (w), 1246 (m), 1209 (s), 1132 (m), 1104 (m),
2
p
1034 (m); μeff = 2.83 and 2.84 μ in the solid state and in benzene
B
Ni(iPrNC(Me)NiPr) decomposes at about 180 °C, in com-
2
solution, respectively. Anal. Calcd for C H CrN : C, 61.82; H, 10.38;
2
0
40
4
parison to a solid-state decomposition temperature of 235 °C for
nickel complex 5. The amidinate complexes Fe(tBuNC-
N, 14.42. Found: C, 61.71; H, 10.06; N, 14.37.
Preparation of Bis(1,4-di-tert-butyl-1,3-diazabutadienyl)-
manganese(II) (2). In a fashion similar to the preparation of 1,
(
Me)NtBu) and Co(iPrNC(Me)NiPr) exhibited single-step
2
2
weight loss events in the TGA traces but had 12 and 9%
2
treatment of anhydrous MnCl (0.371 g, 2.970 mmol) in tetrahydrofuran
nonvolatile residues, respectively, upon reaching 225 (Fe(tBuN-
tBu2
(
40 mL) with a solution of Li DAD (prepared from 1,4-di-tert-butyl-
2p
C(Me)NtBu) ) and 200 °C (Co(iPrNC(Me)NiPr) ). These
2
2
1,3-diazabutadiene (1.000 g, 5.940 mmol) and freshly cut lithium metal
nonvolatile residues are higher than those observed in the TGA
traces of 1ꢀ5 (<3.6%), again suggesting that our new complexes
have higher thermal decomposition temperatures than the
analogous amidinate complexes. The increased thermal stabili-
ties of 1ꢀ5 could allow wider temperature ranges of self-limited
ALD film growth, relative to amidinate and other precursors with
lower decomposition temperatures. ALD growth studies using
(
0.042 g, 6.000 mmol) in tetrahydrofuran (20 mL)) for 6 h at ambient
temperature afforded 2 (0.942 g, 81%) as black crystals upon sublima-
ꢀ1
tion at 120 °C/0.05 Torr: mp 155ꢀ157 °C; IR (Nujol, cm ) 1716 (m),
1
7
610 (m), 1558 (w), 1364 (s), 1254 (s), 1210 (s), 1007 (m), 929 (m),
1
59 (s); H NMR (C D , 23 °C, δ) 8.06 (s, broad, CH), 1.10 (s, very
6
6
broad, C(CH
3
)
3
); μeff = 3.85 and 3.85 μ
B
in the solid state and in
40MnN : C, 61.36;
H, 10.30; N, 14.31. Found: C, 60.99; H, 9.96; N, 14.01.
benzene solution, respectively. Anal. Calcd for C20
H
4
1
ꢀ5 are ongoing and will be reported separately.
Preparation of Bis(1,4-di-tert-butyl-1,3-diazabutadienyl)-
iron(II) (3). In a fashion similar to the preparation of 1, treatment of
’
EXPERIMENTAL SECTION
anhydrous FeCl
2
(0.377 g, 2.970 mmol) in tetrahydrofuran (40 mL)
with Li DAD (prepared from 1,4-di-tert-butyl-1,3-diazabutadiene
1.000 g, 5.940 mmol) and freshly cut lithium metal (0.042 g, 6.000
tBu2
General Considerations. All manipulations werecarried out under
(
argon using either Schlenk or glovebox techniques. Tetrahydrofuran was
distilled from sodium benzophenone ketyl, and hexane was distilled from
P O . Lithium metal was obtained from Acros Organics. Anhydrous
mmol) in tetrahydrofuran (20 mL)) for 6 h at ambient temperature
afforded 3 (0.544 g, 47%) as dark brown crystals upon sublimation at
2
5
ꢀ
1
1
(
(
10 °C/0.05 Torr: mp 132ꢀ134 °C; IR (Nujol, cm ) 1703 (w), 1606
w), 1525 (w), 1359 (s), 1254 (s), 1208 (s), 1022 (m), 1002 (m), 926
m), 762 (s); μeff = 2.88 and 2.68 μ in the solid state and in benzene
transition-metal chlorides (CrCl
obtained from Strem Chemicals Inc. and used as received. Manganese
metal powder and Mn were obtained from Aldrich Chemical Co.
2 2 2 2 2
, MnCl , FeCl , CoCl , and NiCl ) were
3 4
O
B
2
3
24
solution, respectively. Anal. Calcd for C H FeN : C, 61.22; H, 10.27;
N, 14.16. Found: C, 61.39; H, 10.03; N, 14.16.
Preparation of Bis(1,4-di-tert-butyl-1,3-diazabutadienyl)-
cobalt(II) (4). In a fashion similar to the preparation of 1, treatment of
2
anhydrous CoCl (0.386 g, 2.970 mmol) in tetrahydrofuran (40 mL)
with Li DAD (prepared from 1,4-di-tert-butyl-1,3-diazabutadiene
(1.000 g, 5.940 mmol) and freshly cut lithium metal (0.042 g, 6.000
mmol) in tetrahydrofuran (20 mL)) for 6 h at ambient temperature
afforded 4 (0.418 g, 36%) as dark blue crystals upon sublimation at
NiCl CH CN and 1,4-di-tert-butyl-1,3-diazabutadiene were pre-
pared according to literature procedures.
20 40
4
2
3
3
1
13
1
H and C{ H} NMR spectra were obtained at 400 and 100 MHz,
respectively, in benzene-d and were referenced to the residual proton
6
13
and the C resonances of the solvent. Infrared spectra were obtained
using Nujol as the medium. Magnetic moments were determined in the
solid state using a Johnson Mathey magnetic susceptibility apparatus and
in benzene solution using the Evans method. Melting points were
determined on a Thermo Scientific Mel-Temp 3.0 melting point
apparatus and are uncorrected. X-ray-quality crystals of 1 and 3ꢀ5 were
grown from hexane at ꢀ23 °C. Preparative sublimations and solid-state
decomposition temperatures were determined using previously de-
tBu2
25
ꢀ
1
110 °C/0.05 Torr: mp 173ꢀ174 °C; IR (Nujol, cm ) 1698 (m), 1605
(m), 1527 (m), 1362 (s), 1260 (s), 1210 (s), 1008 (s), 933 (m), 763 (s);
μ
eff = 1.75 and 1.83 μ
respectively. Anal. Calcd for C20
Found: C, 60.84; H, 10.01; N, 14.29.
in the solid state and in benzene solution,
B
19e
scribed procedures. Thermogravimetric analyses were performed in
a nitrogen-filled glovebox on a TA Instruments Q500 device equipped
with an evolved gas analysis furnace with samples heated at a rate of
H
40CoN : C, 60.74; H, 10.19; N, 14.17.
4
Preparation of Bis(1,4-di-tert-butyl-1,3-diazabutadienyl)-
nickel(II) (5). In a fashion similar to the preparation of 1, treatment of
NiCl CH CN (0.507 g, 2.970 mmol) in tetrahydrofuran (40 mL) with
Li DAD (prepared from 1,4-di-tert-butyl-1,3-diazabutadiene (1.000
g, 5.940 mmol) and freshly cut lithium metal (0.042 g, 6.000 mmol) in
tetrahydrofuran (20 mL)) for 6 h at ambient temperature afforded 5
(0.482 g, 41%) as dichroic red/green crystals upon sublimation at
10 °C/min. Elemental analyses were performed by Midwest Microlab,
Indianapolis, IN. Powder X-ray diffraction data were acquired on a
Rigaku RU200B diffractometer with a Cu KR rotating anode. Crystalline
phases were identified by comparison of the experimental patterns with
the powder diffraction files of the International Center of Diffraction
Data using the Jade 5.0 software package.
2
tBu2
3
3
ꢀ1
Preparation of Bis(1,4-di-tert-butyl-1,3-diazabutadienyl)-
chromium(II) (1). A 100 mL Schlenk flask, equipped with a magnetic
stir bar and a rubber septum, was charged with 1,4-di-tert-butyl-1,3-
diazabutadiene (1.000 g, 5.94 mmol) and tetrahydrofuran (20 mL). To
this stirred solution at ambient temperature was slowly added freshly cut
lithium metal (0.042 g, 6.000 mmol), and the resultant dark brown
110 °C/0.05 Torr: mp 184ꢀ185 °C; IR (Nujol, cm ) 1715 (w),
1
1625 (w), 1547 (w), 1493 (s), 1264 (s), 1212 (s), 934 (m), 764 (s); H
NMR (C
6
D
6
, 23 °C, δ) 8.95 (s, 2H, CH), 1.93 (s, 18H, C(CH
C{ H} NMR (C , 23 °C, ppm) 129.88 (s, CH), 64.61 (s,
C(CH ), 30.61 (s, C(CH ). Anal. Calcd for C20 Ni: C,
60.77; H, 10.20; N, 14.85. Found: C, 60.89; H, 9.88; N, 14.61.
3 3
) );
1
3
1
6 6
D
)
)
3
H N
40 4
3
3
3
5
015
dx.doi.org/10.1021/om200626w |Organometallics 2011, 30, 5010–5017

Organometallics
ARTICLE
Solid-State Thermolyses of 1ꢀ5. Thermolysis experiments
were performed on analytically pure samples of 1ꢀ5 to assess their
solid-state thermal decomposition products. A 20 cm long, 2.5 cm
diameter quartz tube, equipped with female 24/40 joints on each end,
was fitted with two flow control valves that were attached to male 24/40
joints. A 6 cm long, 1 cm diameter glass vial was charged with 1.00 g of
the sample in a glovebox. The vial was placed in the center of the quartz
tube. This apparatus was placed into a tube furnace, and a 50 sccm flow
of argon was established. The sample was then heated to 500 °C for 1 h
and was cooled to room temperature under an argon flow. Subsequently,
the powder residues were collected from the inside of the quartz tube
and subjected to X-ray powder diffraction analyses as described in
the text.
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each frame. The frames were recorded for 3ꢀ5 s. APEX-II and
27
SHELX software were used in the collection and refinement of the
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(
’
ASSOCIATED CONTENT
4
S
Supporting Information. CIF files giving X-ray crystal-
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lographic data for the structure determinations of 1 and 3ꢀ5 and
figures giving perspective views of 4 and 5, X-ray powder
diffraction spectra of the thermolysis products of 1ꢀ5, and vapor
pressure determination reports for 2 and 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
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(
’
AUTHOR INFORMATION
J. A. C.; Lauhoff, G. Rep. Prog. Phys. 2008, 71, 056501. (c) Shiratsuchi, Y.;
Yamamoto, M.; Bader, S. D. Prog. Surf. Sci. 2007, 82, 121–160.
Corresponding Author
(7) International Technology Roadmap for Semiconductors, http://
*E-mail: chw@chem.wayne.edu.
www.itrs.net/.
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ACKNOWLEDGMENT
We are grateful to the U.S. National Science Foundation
(d) Niinist €o , L. Curr. Opin. Solid State Mater. Sci. 1998, 3, 147–152.
(e) Ritala, M. Appl. Surf. Sci. 1997, 112, 223–230. (f) Suntola, T. Thin
Solid Films 1992, 216, 84–89.
(Grant No. CHE-0910475) and SAFC Hitech for support of this
research.
(
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25–145.
10) (a) Kim, J.-M.; Lee, H.-B.-R.; Lansalot, C.; Dussarrat, C.;
1
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