Syntheses, X-ray structures and CVD studies of trimethylphosphite stabilized silver(I) methanesulfonates

Article abstract of DOI:10.1016/j.jorganchem.2011.03.049

The preparation of {[(MeO)₃P]_n·AgO ₃SCH₃} (n = 1, 2a; n = 2, 2b) is described. The molecular structure of 2a was determined by using X-ray single crystal analysis. Complex 2a contains an Ag₄ rectangular make-up, the centroid of which constitutes an inversion center. Complex 2b was used as precursor in the deposition of silver films using metal organic chemical vapor deposition (MOCVD) technique for the first time. The silver films obtained were characterized using scanning electron microscopy (SEM) and energy-dispersion X-ray (EDX) analysis.

Full text of DOI:10.1016/j.jorganchem.2011.03.049

Journal of Organometallic Chemistry 696 (2011) 2681e2684
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Syntheses, X-ray structures and CVD studies of trimethylphosphite stabilized
silver(I) methanesulfonates
*
Xian Tao, Ke-Cheng Shen, Ying-Zhong Shen
Applied Chemistry Department, School of Material Science & Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation of {[(MeO)3P]n$AgO3SCH3} (n ¼ 1, 2a; n ¼ 2, 2b) is described. The molecular structure of
2a was determined by using X-ray single crystal analysis. Complex 2a contains an Ag4 rectangular make-
up, the centroid of which constitutes an inversion center. Complex 2b was used as precursor in the
deposition of silver films using metal organic chemical vapor deposition (MOCVD) technique for the first
time. The silver films obtained were characterized using scanning electron microscopy (SEM) and
energy-dispersion X-ray (EDX) analysis.
Received 8 February 2011
Received in revised form
26 March 2011
Accepted 29 March 2011
Keywords:
Silver
Ó 2011 Elsevier B.V. All rights reserved.
Methanesulfonates
Precursor
MOCVD
Trimethylphosphite
1. Introduction
There have been some reports of suitable silver complexes with,
or without auxiliary Lewis-base ligands as precursors [11]. Among
The formation of metallic silver films on substrates is of
substantial interest owing to its wide application. For example,
interconnects in microelectronics [1], magnetic materials [2],
bactericidal coatings [3], and surface-conductive flexible polymeric
tapes [4], highly reflective thin-film concentrators/reflectors in
space environments for solar thermal propulsion [5].
them, diverse silver(I)
b-diketonates [12e16] and some silver(I)
carboxylates (phenolates) [17] have been studied as possible Ag-
CVD precursor. However, to date new silver(I) complexes which
can be used as precursors in chemical vapor deposition are still not
sufficiently available, and decomposition mechanism(s) are still
unclear.
There have been several methods to deposit silver thin films
including sputtering [6], thermal evaporation [7], electron-beam
evaporation [8], and chemical vapor deposition (CVD) [9]. Among
these techniques, metal organic chemical vapor deposition
(MOCVD) is one of promising methods because it can easily be
scaled up to deposit high purity films over large areas with high
quality.
In order to search for the new precursors for the growth of silver
films, we reported here the synthesis of trimethylphosphite stabi-
lized silver(I) methanesulfonates which were used as precursors for
the deposition of silver by using MOCVD techniques for the first
time. The single crystal structure of 2a has been determined and
discussed as well.
The key problem in MOCVD is the selection of suitable
precursor. As precursors for CVD, they should have (i) a good
volatility during its evaporation and transportation in the gas
phase; (ii) high purity and suitable thermal decomposition mech-
anism; (iii) suitable thermal stability which means that it could not
decompose during the transportation but decompose easily in the
CVD reactor [10]. Out of this, research for new generation of silver(I)
complexes as precursors has had an enormous impact.
2. Results and discussion
Silver complexes of type {[(MeO)3P]n$AgO3SCH3} (n ¼ 1, 2) were
synthesized by the reaction of [AgO3SCH3] (1) with P(OMe)3 in
stoichiometric ratios in CH2Cl2 as shown in Scheme 1. The
complexes obtained are stable under inert atmosphere for months
at room temperature. On exposure to air, however, they all
decompose during hours or days. The complexes are insoluble in
cold non-polar solvents such as petroleum, whereas they are highly
soluble in polar or unsaturated hydrocarbons such as dichloro-
methane, benzene and tetrahydrofuran. All the products obtained
gave satisfactory elemental analysis results and were characterized
by FT-IR, 1H NMR and 13C{H} NMR, respectively.
* Corresponding author. Tel.: þ86 25 52112904x84765; fax: þ86 25 52112626.
E-mail address: yz_shen@nuaa.edu.cn (Y.-Z. Shen).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.049

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X. Tao et al. / Journal of Organometallic Chemistry 696 (2011) 2681e2684
n P(OMe)3
- 1/2 H2O
- 1/2 CO2
[(MeO)3P]n.AgO3SCH3
Ag2CO3
CH3SO3H + 1/2
AgO3SCH3
1
n = 1, 2a
n = 2, 2b
Scheme 1. Synthesis of complexes 2ae2b.
As we know, the typical group vibration of sulfonates shows the
range of 1350e1050 cmꢀ1 [18]. In the IR spectra of 2ae2b, the
absorptions in the range of 1238e1239 cmꢀ1 and 1205e1208 cmꢀ1
could be attributed to the asymmetric and symmetric vibration of
S]O double bond, which is similar to previous reported [18]. The
absorptions around 1020 cmꢀ1 can be assigned as the stretch
vibration of SeO; which is similar to the reported value [18]. In
complexes 2a and 2b, the PeO stretching in trimethylphosphite
ligands (730 cmꢀ1) shifted to higher frequencies (768e785 cmꢀ1
)
provides good evidence that the occurrence of P atom in trime-
thylphosphite ligands coordinating to silver ion. In 1H NMR spectra,
the proton integration is consistent with the stoichiometries of the
complexes. The protons of CH3 in the trimethylphosphite showed
a doublet at 3.8 ppm in the complexes. The proton of CH3e in
CH3SO3e appeared at 2.9 ppm, which agrees well with previous
reported [18]. In the 13C{H} NMR spectra of 2ae2b the trimethyl-
phosphite carbon resonances are easily distinguished
(51.6e51.7 ppm) from those of CH3e in CH3SO3e (39.0e39.1 ppm).
Complex 2a crystallizes in a monoclinic with space group P2(1)/
n, which is composed of a tetramer [(MeO)3P$AgO3CH3]4, as shown
in Fig. 1. This is different from the reported complex [Ph3PA-
gO3SCF3] which exists as a trimer [19]. Compare with the “cubane-
like” tetramer [(Ph3PAgO3SCH3)4$4CH2Cl2] [18], complex 2a
possesses a discrete tetra-silver entity. Complex 2a contains an Ag4
rectangular make-up, the centroid of which constitutes an inver-
sion center. The four Ag atoms are symmetrically bridged by two
single anchor (O) methanesulfonates on the periphery, and cross-
connected by six O atoms from the remaining two methanesulfo-
nates. This bonding mode was found in [Ag4(OTf)4(dppf)2]
(dppf ¼ 1,10-bis(diphenylphosphino)ferrocene) [20]. The phos-
phorus atoms of P(OMe)3 are approximately vertical, coordinating
to the silver atoms, thereby completing a distorted tetrahedral
coordination arrangement of the silver atoms. As seen more clearly
in the line-drawing of 2a in Fig. 1, the asymmetric unit contains
three fused metallacycles, consisting of one each of AgOSOAgOSO,
AgOAgOSO, AgOAgO. The AgeP bond distances in complex 2a
[2.333(8) Ǻ, 2.324(8) Ǻ] are shorter than the sum of covalent radii of
the P and Ag atoms (2.44 Ǻ) [21] and slightly shorter than that of
Fig. 1. The crystal structure and line-drawing of 2a. The hydrogen atoms are omitted
ꢀ
for clarity. Symmetry operation: ꢀx þ 1, ꢀy þ 2, ꢀz. Selected bond lengths (A) and
angles (ꢁ): Ag(1)eP(1) 2.333(8), Ag(1)eO(8) 2.335(18), Ag(1)eO(9A) 2.412(18), Ag(1)e
O(7) 2.419(17), Ag(2)eP(2) 2.324(8), Ag(2)eO(8) 2.332(19), Ag(2)eO(7) 2.398(17),
Ag(2)eO(10A) 2.405(18), O(7)eS(1) 1.471(17), O(8)eS(2) 1.477(18), O(10)eS(1)
1.454(18), O(9)eS(1) 1.452(19), C(7)eS(1) 1.764(3); P(1)eAg(1)eO(8) 136.2(5), P(1)e
Ag(1)eO(9A) 125.6(5), P(1)eAg(1)eO(7) 128.5(4), O(8)eAg(1)eO(9A) 85.3(7), O(8)e
Ag(1)eO(7) 76.4(6), O(9A)eAg(1)eO(7) 87.3(6), P(2)eAg(2)eO(8) 136.9(5), P(2)e
Ag(2)eO(7) 130.4(4), O(8)eAg(2)eO(7) 76.8(6), P(2)eAg(2)eO(10A) 122.7(5), O(8)e
Ag(2)eO(10A) 84.9(7), O(7)eAg(2)eO(10A) 88.1(6), S(1)eO(7)eAg(2) 128.6(9), S(1)e
O(7)eAg(1) 129.8(9).
other complexes [(Ph3PAgO3SCH3)4$4CH2Cl2] 2.348(2)
Ǻ [18],
[Ag(P(OMe)3)2NO3]2 [2.411(3) Ǻ, 2.412(3) Ǻ] [22].
Thermogravimetry (TG) and differential scanning calorimetry
(DSC) studies are required to optimize the temperature at which
the respective silver precursor should be maintained during the
CVD experiments. Complex 2a, for example, the TG and DSC curves
are shown in Fig. 2. It can be seen from the DSC curve that there is
one continuous endothermic process with two apparent peaks
temperatures at 205 ꢁC and 210 ꢁC and the silver precursor
decomposes in the range of 125e465 ꢁC. Firstly, it may lose two
P(OMe)3 from 125 to 205 ꢁC with corresponding weight loss about
52.92% which is close to the theoretical value. It is very difficult to
know the real thermal decomposition mechanism of methanesul-
fonates. The final percentage of the residue is 26.92%, which is little
higher than the theoretical value of silver (23.91%). The thermal
measurement is in good agreement with the structural analysis.
The surface morphology and composition of the silver films
from MOCVD experiment using 2b were examined by scanning
electron microscopy (SEM) and energy-dispersion X-ray analysis
(EDX). As-prepared films at 480 ꢁC were of two colors; silvery film
on the substrate closer to the precursor inlet, and light brownish
reflective film on the adjacent substrate towards the center of the
reactor. This difference is most probably caused by the temperature
gradient in the reactor, or precursor decomposition as it moves
through the reactor [23]. SEM of the as-deposited silvery films
consists of spherical particles spreading all over the substrate
surface (Fig. 3a). Two kinds of silver particles are visible, 50e60 nm
for small particles and 150e200 nm for larger particles. The
morphology of the light brownish reflective film appears to be
a mixture of spherical particles and cubic particles (Fig. 3b). The size
of the spherical particles and cubic particles were in the range
60e70 nm and 300e350 nm, respectively. EDX analysis of the
silvery film and the light brownish reflective film show the pres-
ence of silver alone. The other light elements, such as C, O, and P,
which might be present as impurities or formed by surface oxida-
tion of silver, are below the detection limit.
In summary, a straightforward synthesis methodology for the
preparation of trimethylphosphite stabilized silver(I) methane-

X. Tao et al. / Journal of Organometallic Chemistry 696 (2011) 2681e2684
2683
EDX analysis were carried out by Hitachi Model S-4800 with
scanning electron microscope and energy dispersive X-ray detector.
Electrical resistivities of films were measured by four-point probe
(HisuperGroup Inc., SDY-5).
3.2. Synthesis
3.2.1. Synthesis of [(MeO)3P$AgO3SCH3]4 (2a)
Trimethylphosphite (0.2482 g, 2.00 mmol) dissolved in 20 mL of
CH2Cl2 was added dropwise into a stirred solution of [CH3SO3Ag]
(0.4059 g, 2.00 mmol) suspended in 20 mL of CH2Cl2 at 0 ꢁC. The
clear solution was obtained by filtration through a pad of celite after
stirring the reaction mixture for 6 h at 0 ꢁC. A white solid was
obtained after removing all the volatiles in oil-pump vacuo, Yield:
0.85 g (94%, based on [CH3SO3Ag]). M.p.: 57 ꢁC dec. Anal. Calc. for
C16H48Ag4O24P4S4: C, 14.69; H, 3.70. Found: C, 14.67; H, 3.68%. 1H
NMR (CDCl3):
d
2.9 (s, 3H, SCH3),
d
3.8 (d, 9H, OCH3, JPH ¼ 13.6 Hz).
13C{H} NMR (CDCl3):
d
39.0 (SCH3),
d
51.6 (JPC ¼ 5.2 Hz, OCH3). IR
Fig. 2. TG and DSC curves of 2b (heating rate 10 ꢁC minꢀ1, nitrogen atmosphere).
(KBr) data (cmꢀ1): 2981 (s), 2934 (m), 2904 (m), 1478 (m), 1444 (s),
1388 (m), 1238 (s), 1208 (s), 1163 (s), 1098 (m), 1020 (vs), 939 (vs),
768 (vs), 531 (s).
sulfonates complexes was described. Complex 2b was tested as
MOCVD precursor for the deposition of silver for the first time. It
has been demonstrated that the complex was a promising candi-
date as precursor for the deposition of silver.
3.2.2. Synthesis of [[(MeO)3P]2$AgO3SCH3] (2b)
Complex 2b was synthesized by following the procedure, only
using [CH3SO3Ag] (0.4059 g, 2.00 mmol) and trimethylphosphite
(0.4964 g, 4.00 mmol). After appropriate work-up, complex 2b was
obtained as a colourless liquid. Yield: 0.83 g (92%, based on
[CH3SO3Ag]). Anal. Calc. for C7H21AgO9P2S: C, 18.64; H, 4.69. Found:
3. Experimental
3.1. General procedures
C, 18.53; H, 4.62%. 1H NMR (CDCl3):
d
2.9 (s, 3H, SCH3),
d
3.8 (d, 18H,
OCH3, JPH ¼ 12.8 Hz). 13C{H} NMR (CDCl3):
d
39.1 (SCH3), 51.7
d
All operations were carried out under an atmosphere of purified
nitrogen with standard Schlenk techniques. The solvent dichloro-
methane (CH2Cl2) was purified by distillation from P2O5 under N2
before use. Compound [AgO3SCH3] (1) was synthesized by the
reaction of methanesulfonic acid with excessive Ag2CO3 in water
[18]. 1H NMR were recorded on a Bruker Advance 300 spectrometer
operating at 300.130 MHz in the Fourier transform mode; 13C{H}
NMR spectra were recorded at 75.467 MHz. Chemical shifts are
(JPC ¼ 3.3 Hz, OCH3). IR (KBr) data (cmꢀ1): 3006 (s), 2952 (m), 2845
(m), 1461 (s), 1328 (m), 1239 (s), 1205 (vs), 1016 (vs), 785 (vs), 553
(s), 523 (s), 462 (m).
3.3. X-ray structure determination
Single crystals of 2a could be obtained by cooling a saturated
dichloromethane solution to ꢀ30 ꢁC. Suitable crystals for X-ray
determination were placed in glue under N2 due to their sensitive
to oxygen and moisture. X-ray structure measurement was per-
reported in
thylsilane (
d units (parts per million) downfield from tetrame-
d
¼ 0.0 ppm) with the solvent as the reference signal (1H
NMR, CDCl3 d ¼ 7.26; 13C{H} NMR, CDCl3 d ¼ 77.55). Infrared spectra
were collected on Bruker Vector 22 in KBr at room temperature.
Elemental analysis was performed on a PerkineElmer 240 C
elemental analyzer. Thermogravimetric studies (TG) and differen-
tial scanning calorimetric (DSC) studies were carried out with the
formed on BRUKER SMART Apex CCD, detector equipped with
ꢀ
graphite monochromatic Mo K
a
radiation (
l
¼ 0.71073 A), at room
temperature. Date collection and processing (cell refinement, data
reduction, and empirical absorption correction) were performed
using the CRYSTALCLEAR program package [24]. The structure was
solved using direct methods and refined by full-matrix least-
squares procedures on F2 (SHELX-97) [25]. All of the non-hydrogen
NETZSCHꢀ1STA 409 PC/PG with
a constant heating rate of
10 ꢁC min under N2 (30 cm3 minꢀ1). Melting points were obse-
rved in sealed capillaries and were uncorrected. SEM images and
Fig. 3. SEM images of silver films deposited on Si wafer by MOCVD from 2b at flow a rate of 20 sccm and growth temperatures of 480 ꢁC (a) silvery film, (b) light brownish reflective
film.

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X. Tao et al. / Journal of Organometallic Chemistry 696 (2011) 2681e2684
Table 1
Crystallographic data for 2a.
Program for Graduate Education (CX10B_100Z), and Funding for
Outstanding Doctoral Dissertation in NUAA (BCXJ10-11) are also
acknowledged.
Formula
C16H48Ag4O24P4S4
1308.14
P2(1)/n
0.3 ꢂ 0.2 ꢂ 0.1
Monoclinic
2
14.969(3)
8.930(18)
16.123(3)
90.00
95.36(3)
90.00
2145.9(7)
2.025
1296
Formula weight
Space group
Crystal size (mm)
Crystal system
Z value
Appendix A. Supplementary material
CCDC 801453 for 2a contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Center via www.
ccdc.cam.ac.uk/data_request/cif.
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
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The average film thickness was about 0.5
of 0.5
m hꢀ1, and the room temperature electrical resistivities of
silvery film and light brownish reflective film were 5 mU cm and
mU cm, respectively.
mm, giving a growth rate
m
7
Acknowledgements
[24] Rigaku/MSC, CRYSTALCLEAR. Rigaku/MSC Inc., The Woodlands, Texas, USA,
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This work was supported by Natural Science Foundation of
Jiangsu Province (BK2007199), National Science and Technology of
Major Project (2009ZX02039101-02), Funding of Jiangsu Innovation