Synthesis and characterization of volatile liquid cobalt amidinates

Article abstract of DOI:10.1039/b800712h

Three new volatile cobalt amidinate compounds were prepared: Co( ^tBuNC(R)NEt)₂, R = Me, Et and n-Bu. They were characterized by elemental analysis, ¹H NMR, X-ray structure analysis, melting point, vapor pressure, vaporization rate, thermal stability and chemical reactivity. They were found to evaporate cleanly without decomposition. Two of them are liquids at room temperature, allowing for more convenient preparation, handling and purification by distillation. They are highly reactive compounds that have been found to be suitable precursors for vapor deposition of cobalt metal, cobalt nitride and cobalt oxide. A new synthetic method allows for the facile and inexpensive preparation of large quantities of these compounds. The Royal Society of Chemistry.

Full text of DOI:10.1039/b800712h

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www.rsc.org/dalton | Dalton Transactions
Synthesis and characterization of volatile liquid cobalt amidinates†
Zhengwen Li,‡ Don Kuen Lee,§ Michael Coulter, Leonard N. J. Rodriguez and Roy G. Gordon*
Received 15th January 2008, Accepted 7th March 2008
First published as an Advance Article on the web 14th April 2008
DOI: 10.1039/b800712h
Three new volatile cobalt amidinate compounds were prepared: Co(tBuNC(R)NEt)2, R = Me, Et and
n-Bu. They were characterized by elemental analysis, 1H NMR, X-ray structure analysis, melting point,
vapor pressure, vaporization rate, thermal stability and chemical reactivity. They were found to
evaporate cleanly without decomposition. Two of them are liquids at room temperature, allowing for
more convenient preparation, handling and purification by distillation. They are highly reactive
compounds that have been found to be suitable precursors for vapor deposition of cobalt metal, cobalt
nitride and cobalt oxide. A new synthetic method allows for the facile and inexpensive preparation of
large quantities of these compounds.
high reactivity at low deposition temperatures. The precursor
Introduction
should be reactive with reducing agents to deposit pure cobalt
metal, with nitrogen sources to form cobalt nitrides, and with
oxygen precursors to form oxides. It is also desirable to have
a synthetic method that can produce large quantities of the
precursor relatively inexpensively.
In this paper we present two new cobalt precursors that
meet all of these requirements for a cobalt precursor for vapor
deposition. Three new cobalt amidinates were synthesized, two
of which are liquids at room temperature, while the third is a
low-melting solid. A new synthetic method is also presented that
uses inexpensive reactants, rapid reactions at ambient pressure,
and has been used successfully for large-scale preparation. Vapor
pressures and vaporization rates are found to be high enough for
use in vapor deposition at source temperatures below 80 ◦C, at
which temperature the lifetimes are well over one year. The new
precursors have been successfully used in the vapor deposition of
cobalt metal, as well as its nitrides and oxides.22
Thin films of cobalt metal are important in many areas of tech-
nology, including magnetic information storage,1 computer chips2
and catalysis.3 A wide variety of techniques have been used to make
them, including physical vapor deposition methods (PVD), such
as evaporation and sputtering, and chemical methods, including
chemical vapor deposition (CVD) and atomic layer deposition
(ALD). The chemical methods have the potential to produce
highly conformal coatings. As device structures are made smaller
and more complex, this conformality has become a more crucial
property.4
To be used successfully, chemical methods need to start with
precursors that have suitable properties, including volatility, ther-
mal stability and chemical reactivity suitable to make pure films.
Commercially acceptable precursors should be liquids at room
temperature. Liquids are easier than solids to purify, handle, meter
and vaporize. Solid sources tend to be non-reproducible sources of
vapor because of their slow evaporation kinetics, variable surface
areas, and tendency to accumulate non-volatile impurities or
decomposition products on their surfaces. Solids can also shed
particles that may contaminate products.
Results
Synthesis
Some previously used cobalt precursors are solids, such as
cobalt bis(N,Nꢀ-diisopropylacetamidinate),5–7 cobalt bis(N,Nꢀ-di-
tert-butylacetamidinate),6 dicobalt octacarbonyl,8–11 cobalt ace-
tylacetonate12,13 and bis(cyclopentadienyl)cobalt.14,15 Liquid cob-
alt precursors include cyclopentadienylcobalt dicarbonyl,3,15,16
cobalt nitroso tricarbonyl,17–20 and cobalt hydride tris(tri-
alkylphosphite).21 These liquid precursors, however, have only
produced pure cobalt films at high substrate temperatures10,19,21
or in the presence of a plasma,15,16 both of which conditions
preclude making films with high conformality. Thus there is a
need for volatile, thermally stable, liquid cobalt precursors with
The cobalt amidinate compounds were made by two different
methods. The first method, as published5,23 and shown in Scheme 1,
adds an alkyllithium to a carbodiimide to form the lithium salt of
the amidine, which is then reacted with cobalt(II) chloride.
Department of Chemistry and Chemical Biology, Harvard University, 12
Oxford Street, Cambridge, Massachusetts, 02138, USA. E-mail: gordon@
chemistry.harvard.edu; Fax: +1 (617) 495-4923; Tel: +1 (617) 495-4017
† CCDC reference numbers 674856 and 674857. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b800712h
‡ Current address: IBM, Semiconductor R&D Center, Fishkill, New York,
USA.
§ Current address: Samsung Cheil Industries, Inc., Seoul, South Korea.
Scheme 1
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A new preparation of the lithium amidinates is shown in
Scheme 2, in which an alkylnitrile is added to an alkyl chloride in
the presence of iron(III) chloride. The intermediate alkylnitrilium
tetrachloroferrate salt is then reacted with a primary amine, and
then neutralized to release the free amidine.24,25 Alternative routes
utilizing catalysts based on lanthanide26 or samarium27 have been
reported but proved incapable of attaching sterically bulky tert-
butyl sidegroups. A previous literature report28 of N,Nꢀ-di-tert-
butylacetylamidine provided no guidance as to its production.
Fig. 1 Crystal structure of cobalt bis(N,Nꢀ-di-tert-butylacetamidinate) 1.
Scheme 2
The synthesis in Scheme 2 is completed by reacting the lithium
amidinate with CoCl2, as in the last step in Scheme 1.
The products are extracted from the lithium chloride byproduct
with pentane or hexane. Final purification is done by distillation or
sublimation. Identical products were obtained from the methods
in Scheme 1 and Scheme 2.
Characterization
The compounds are dark blue/green, air-sensitive solids or liquids.
Their melting points and vapor pressures are given in Table 1. At
room temperature, compounds 1, 2 and 5 are solids, while 3 and 4
are liquids.
The proton NMR spectra of the products have large shifts
and broad peaks, which shows that they are paramagnetic. The
numbers of peaks are consistent with monomeric structures in
solution.
Fig.
midinate) 5.
2
Crystal structure of cobalt bis(N-tert-butyl-Nꢀ-ethylaceta-
Table 2 Selected bond lengths and bond angles for cobalt amidinate
compounds
The crystal structure of compound 1 was determined by X-ray
crystallography, and it is shown in Fig. 1. A monomeric molecular
structure was also previously determined for compound 2 by X-
ray crystallography.5 The molecular structure of compound 5, by
contrast, is the dimer shown in Fig. 2. This solid-state dimer must
be fairly weakly bound, because it is a monomer in solution,
according to its NMR. Some bond lengths and bond angles are
given in Table 2 and crystallographic parameters are given in
Table 3.
Compound
1
2
5
˚
Bond lengths/A
Co–Nave
2.012(8)
1.320(2)
2.007(2)
2.080(3)
2.035(3)
2.505(3)
1.324(5)
1.323(5)
Co–Nbridge
Co–C
2.444(2)
1.330(3)
N–Cave
N–Cbridge
Bond angles/◦
N(1A)–Co(1)–N(1)
N(1A)–Co(1)–N(2)
N(2)–Co(1)–N(2A)
N(1)–Co(1)–N(2A)
N(1)–Co(1)–N(2)
N–C–N
132.31(9)
137.02(6)
133.68(8)
137.02(7)
65.57(6)
111.2(2)
136.30(12)
130.28(7)
142.81(12)
130.28(7)
65.84(7)
109.23(13)
119.79(13)
119.49(13)
102.19(13)
63.81(13)
112.4(2)
Thermogravimetric analyses of the compounds demonstrated
complete evaporation in single steps without measurable residue,
as shown in Fig. 3.
110.11(11)
N–C–Nbridge
115.3(5)
Table 1 Melting points, vapor pressures and vaporization rates
Melting
Vapor pressure/
Vaporization rate at
Another set of thermogravimetric measurements was taken
at constant temperatures, instead of using a linearly increasing
temperature ramp. These isothermal measurements were done
at a sequence of temperatures spaced by 10 K. At each fixed
temperature, the mass loss was followed until a constant rate of
decrease was reached. The results of these isothermal measure-
ments are plotted in Fig. 4. The data points for each compound fit
point/◦C
mTorr, T/◦C
80 ◦C/nmol min−1 cm−2
1
2
3
4
5
90
84
−21
−17
37
30, 45
30, 40
30, 110
60, 50
30, 70
25
116
6
50
84
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Table 3 Crystallographic parameters
Compound
1
5
Empirical formula
Formula weight
T/K
C20H42CoN4
397.51
C16H34CoN4
341.40
193(2)
193(2)
Crystal system
Space group
Orthorhombic
Aba2
14.528(2)
13.9928(17)
11.8948(17)
90
90
90
2418.0(6)
4
7739
Monoclinic
P2(1)/c
9.945(3)
16.996(5)
22.432(6)
90
91.265(5)
90
3790.8(18)
8
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
Z
3
˚
Fig. 5 Vapor pressure of compound 4.
Reflections collected
20117
log10 P (Torr) = 5.424 − 2151/T (K).
Independent reflections
2860
6681
[Rint = 0.0366]
R1 = 0.0360
wR2 = 0.0887
R1 = 0.0376
wR2 = 0.0898
[Rint = 0.0637]
R1 = 0.0548
wR2 = 0.1282
R1 = 0.0828
wR2 = 0.1382
Thermal stability of the cobalt amidinates was studied by sealing
solutions in deuterobenzene in heavy-walled NMR tubes. The
tubes were then heated in an oven for various periods of time.
Periodically the tubes were cooled to room temperature and their
NMR spectra taken. The amount of un-decomposed compound
was estimated by comparison of the integrated areas under its
NMR peaks, normalized to an internal standard in the solution,
C6D5H. These decomposition studies were repeated at several
temperatures from 100 to 200 ◦C, with the results listed in Table 4.
These half-lives are plotted in Arrhenius form in Fig. 6. The
activation energies for these fits are given in Table 4.
Final R indices
R indices (all data)
Fig. 3 Thermogravimetric analyses of cobalt amidinates. Temperature
ramp rate of 10 K min−1 in a flow of nitrogen at 1 atm, and sample sizes
of about 10 mg.
Fig. 6 Half-lives of cobalt amidinates in benzene solution.
Discussion
Although there is no generally predictive theory of melting points,
one can see that the presence of more flexible groups such as ethyl
and n-butyl tends to lower the melting points, in comparison with
compounds with the more rigid tert-butyl and isopropyl groups.
One way to quantify this flexibility is to count each ethyl group as
one flexible joint, and each n-butyl group as 3 flexible joints.29 In
Fig. 7 we plot the melting points as a function of the total number
of flexible joints in a monomer.
Fig. 4 Vaporization rates of cobalt amidinates measured in 1 atm of
flowing nitrogen gas.
The compounds were found to be volatile. This characteristic
arises from the fact that they are non-polar, so their intermolecular
forces are fairly weak. Also, the steric bulk of the amidinate ligands
is large enough to keep the molecules from polymerizing into non-
volatile polymers.
an Arrhenius temperature dependence to within the experimental
errors.
The vapor pressure of the liquid compound 4 is plotted in Fig. 5.
The values are approximated by the equation:
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Table 4 Half-lives for thermal decomposition of cobalt amidinates in C6D6 solution, in days
100 ◦
C
135 ◦
C
150 ◦
C
170 ◦
C
200 ◦
C
Activation energy/kJ mol−1
1
2
3
4
5
3069
899
101
117
96
195
50
46
47
3.0
78
17
6.5
3.2
1.7
52
17
4.4
1.8
1.5
4.56
1.25
0.44
0.35
0.26
91.0
90.9
79.8
89.8
81.9
distinguish between them. Another factor is steric crowding, which
also may inhibit thermal decomposition. Steric crowding falls in
the same order 1 > 2 > 3 > 4 > 5 as the thermal stability.
Several cobalt complexes formed from arylamidines have been
reported in the literature31–34 and are predominately monomeric as
might be expected from the bulk of the derivatized phenyl groups
used. The internal angle of the amidine moiety is generally seen
to be slightly larger within ligands that are bridging; this effect
is on the order of a 5% change. There is likewise a trend towards
longer cobalt–nitrogen bond lengths in the dimeric structures with
a similar magnitude of change.
Conclusions
Three new cobalt amidinates were synthesized. They were found
to be volatile without decomposition. Two are liquids at room
temperature, which makes them more convenient for synthesis,
handling and vaporization. These properties make them well-
suited to be precursors for chemical vapor deposition and
atomic layer deposition. Their successful use for vapor deposition
of cobalt metal, cobalt nitride and cobalt oxide is reported
elsewhere.22
Fig. 7 Melting points of cobalt amidinates as a function of the total
number of flexible joints in the alkyl groups.
The tert-butyl and isopropyl groups have sufficient steric bulk to
maintain the monomeric structures of compounds 1 and 2. On the
other hand, the smaller steric bulk of the ethyl group in compound
5 allows it to form a dimer in the solid state (although it appears
to be monomeric in solution).
The vaporization rate generally decreases with increasing molec-
ular mass, as shown in Fig. 8 for data taken at 80 ◦C.
Experimental
General considerations
All reactions and manipulations were conducted under a pure
nitrogen atmosphere using either an inert atmosphere box or
standard Schlenk techniques. Tetrahydrofuran (THF), methylene
chloride (CH2Cl2) and pentane were dried using an Innovative
˚
Technology solvent purification system and stored over 4 A
molecular sieves. Butyllithium, tert-butyl chloride, ethylamine,
propionitrile, CoCl2 and FeCl3 were used as received.
The synthetic procedures for Scheme 1 have been published.5
The procedure for a typical synthesis according to Scheme 2 is as
follows:
N-tert-Butyl-Nꢀ-ethylpropionamidine. This compound was
made according to Scheme 2 by reacting propionitrile with
ferric chloride and tert-butyl chloride, followed by addition of
ethylamine.
Fig. 8 Vaporization rates of cobalt amidinates at 80 ◦C as a function of
molecular mass.
0.30 mol (50 g) anhydrous FeCl3 was suspended in 250 mL
dry CH2Cl2. After 2 min, the solution was cooled to −40 ◦C and
21 mL (0.30 mol) of anhydrous propionitrile was added at once
with magnetic stirring. The ferric chloride went into solution and
the colo◦r of the medium turned dark red. The solution was cooled
to −78 C, then anhydrous tert-butyl chloride (33 ml, 0.30 mol)
was added at once. A brown ocher precipitate formed within
a few minutes; presumably it was N-tert-butylacetonitrilium
The thermal stabilities of the compounds fall roughly in the
order 1 > 2 > 3 > 4 > 5. The greater stability of 1 can be
understood because it lacks a hydrogen in the carbons beta to the
nitrogens. According to theoretical calculations,30 beta-hydrogen
transfer from this position to the cobalt atom is one pathway to
thermal decomposition. All the molecules except 1 have the same
number (4) of these labile hydrogens, so this mechanism does not
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Table 5 Theoretical (above) and experimental (below) elemental analyses
tetrachloroferrate. The reaction medium was then maintained at
−78 ◦C. Ethylamine was condensed (13 g, 0.30 mol) into the stirred
reaction mixture; an exothermic reaction ensued. Stirring was
maintained while it warmed to ambient temperature. It was then
cooled to −10 ◦C and poured into 250 mL of 5 M NaOH stirred in
an ice bath. The resulting mixture was extracted twice with CHCl3.
The organic phase was washed twice with 100 ml of water. The
organic solution was dried over MgSO4, and then evaporated to
yield a light yellow liquid. The crude amidine thus obtained was
for cobalt amidinates
Compound
C (%)
H (%)
N (%)
CoC20H42N4 1
CoC16H34N4 2
CoC22H46N4 3
CoC18H38N4 4
CoC16H34N4 5
60.43
60.63
56.29
55.93
62.09
62.39
58.52
58.36
56.29
55.93
10.65
10.65
10.04
10.30
10.90
10.57
10.37
10.66
10.04
10.03
14.09
13.98
16.41
16.12
13.17
12.91
15.16
14.87
16.41
16.11
◦
then purified by distillation (40 C, 0.06 Torr) to give a colorless
◦
1
liquid. Yield 38 g, 81%. H NMR (CDCl3, 25 C, ppm): 0.97 (t,
J = 7.6 Hz, 3H, CCH2CH3), 1.02 (t, J = 7.2 Hz, 3H, NCH2CH3),
1.30 (s, 9H, C(CH3)3), 2.03 (q, J = 7.6 Hz, 2H, CCH2CH3), 3.16
(q, J = 7.2 Hz, 2H, NCH2CH3). 13C NMR (DMSO-d6, 25 ◦C,
ppm): 12. 7 (CCH2CH3), 18.7 (NCH2CH3), 23.3 (CCH2CH3),
29.4 (NC(CH3)3), 42.8 (NCH2CH3), 50.3 (NC(CH3)3), 159.3
(NC(Et)N). HRMS C9H20N2H+: 157.1699 (calc.), 157.1683 (expt).
2 −70.0 (br, 12H), 309.4 (br, 3H), 324.9 (br, 2H);
3 −102.3 (br, 3H), −31.2 (br, 9H), 30.3 (br, 3H), 49.3 (br, 2H),
87.6 (br, 2H), 249.6 (br, 2H), 269.8 (br, 2H);
4 −100.7 (br, 3H), −30.6 (br, 9H), 86.7 (br, 3H), 248.5 (br, 2H),
268.8 (br, 2H);
N-tert-Butyl-Nꢀ-ethylpentanamidine. This was prepared by
the carbodiimide route and◦ then protonated with water for
5 −89.7 (br, 3H), −28.4 (br, 9H), 244.5 (br, 3H), 305.4 (br, 2H).
Elemental analyses are satisfactory for all compounds, as shown
in Table 5.
1
analysis. H NMR (C6D6, 25 C, ppm): 0.80 (t, J = 7.0 Hz, 3H,
C(CH2)3CH3), 1.10 (t, J = 7.2 Hz, 3H, NCH2CH3), 1.16 (m,
J = 6.8 Hz, 2H, CCH2CH2CH2CH3), 1.31 (m, J = 6.6 Hz 2H,
CCH2CH2CH2CH3), 1.42 (s, 9H, NC(CH3)3), 1.77 (t, J = 7.6 Hz,
2H, CCH2(CH2)2CH3), 3.26 (q, J = 7.2 Hz, 2H, NCH2CH3).
13C NMR (C6D6, 25 ◦C, ppm): 13.9 (CCH2CH2CH2CH3), 18.1
(NCH2CH3), 22.8 (CCH2(CH2)2CH3), 29.1 (NC(CH3)3), 29.8
(C(CH2)3CH3), 30.2 (C(CH2)2CH2CH3), 43.3 (NCH2CH3), 50.5
(NC(CH3)3), 156.3 (NC(Bu)N). HRMS C11H24N2H+: 185.2012
(calc.), 185.2010 (expt.).
Acknowledgements
We appreciate the vapor pressure data on compound 4 taken at
the Advanced Thin-Film Technologies Group, Rohm and Haas
Electronic Materials. D. K. L. was supported by a Korea Research
Foundation Grant from the Korean Government (MOEHRD,
KRF-2005-214-C00206). This work was supported in part by the
Rohm and Haas Company, Intel Corporation and the National
Science Foundation.
Bis(N-tert-butyl-Nꢀ-ethylpropionamidinato) cobalt(II) 4. N-
tert-Butyl-Nꢀ-ethylpropionamidine is first reacted with butyl-
lithium and then with cobalt(II) chloride.
A solution of n-butyllithium (1.6 M in hexanes, 81 mL,
0.13 mol) was added dropwise to a solution of N-tert-butyl-
Nꢀ-ethylpropionamidine (20 g, 0.13 mol) in 200 mL of THF at
−78 ◦C. The mixture was warmed to room temperature and stirred
for 4 h. This resultant solution was then added to a solution
of cobalt(II) chloride, CoCl2, (8.4 g, 0.065 mol) in 100 mL of
THF at room temperature. The reaction mixture was stirred for
12 h under nitrogen atmosphere. All volatiles were then removed
under reduced pressure and the resulting solid was extracted with
pentane. The pentane extract was filtered through a pad of Celite.
The pentane was removed under reduced pressure to afford a
dark green oil. A pure dark blue/green liquid compound was
obtained by distillation at 90 ◦C (30 mTorr). Yield 34 g, 71%.
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