Organotin dithiocarbamates: Single-source precursors for tin sulfide thin films by aerosol-assisted chemical vapor deposition (AACVD)

Article abstract of DOI:10.1021/cm301660n

A series of diorganotin complexes of dithiocarbamates [Sn(C ₄H₉)₂(S₂CN(RR′) ₂)₂] (R, R′ = ethyl (1); R = methyl, R′ = butyl (2); R, R′ = butyl (3); R = methyl, R′ = hexyl (4); and [Sn(C₆H₅)₂(S₂CN(RR′) ₂)₂] (R, R′ = ethyl (5); R = methyl, R′ = butyl (6); R, R′ = butyl (7); R = methyl, R′ = hexyl (8) were synthesized. Single-crystal X-ray structures of 2, 3, and 8 were determined. Thermogravimetric analysis (TGA) showed single-step decomposition for the complexes 1, 3, and 5-8, and double-step decomposition for the complexes 2 and 4 between 195 C and 325 C. Complexes 1-4 were used as single-source precursors for the deposition of SnS thin films by aerosol-assisted chemical vapor deposition (AACVD) at temperatures from 400 C to 530 C. Orthorhombic SnS thin films were deposited from all four complexes at all deposition temperatures. The films were characterized by UV-vis spectroscopy, powder X-ray diffraction (p-XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and also electrical resistivity measurements. Published 2013 by the American Chemical Society.

Full text of DOI:10.1021/cm301660n

Article
pubs.acs.org/cm
Organotin Dithiocarbamates: Single-Source Precursors for Tin
Sulfide Thin Films by Aerosol-Assisted Chemical Vapor Deposition
(AACVD)
Karthik Ramasamy,†,§ Vladimir L. Kuznetsov,‡ Kandasamy Gopal,†,# Mohammad A. Malik,†
James Raftery,† Peter P. Edwards,‡ and Paul O’Brien*,†
†The School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom
‡Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, United
Kingdom
S
* Supporting Information
ABSTRACT: A series of diorganotin complexes of dithio-
carbamates [Sn(C4H9)2(S2CN(RR′)2)2] (R, R′ = ethyl (1); R
= methyl, R′ = butyl (2); R, R′ = butyl (3); R = methyl, R′ =
hexyl (4); and [Sn(C6H5)2(S2CN(RR′)2)2] (R, R′ = ethyl (5);
R = methyl, R′ = butyl (6); R, R′ = butyl (7); R = methyl, R′ =
hexyl (8) were synthesized. Single-crystal X-ray structures of 2,
3, and 8 were determined. Thermogravimetric analysis (TGA)
showed single-step decomposition for the complexes 1, 3, and
5−8, and double-step decomposition for the complexes 2 and
4 between 195 °C and 325 °C. Complexes 1−4 were used as single-source precursors for the deposition of SnS thin films by
aerosol-assisted chemical vapor deposition (AACVD) at temperatures from 400 °C to 530 °C. Orthorhombic SnS thin films were
deposited from all four complexes at all deposition temperatures. The films were characterized by UV−vis spectroscopy, powder
X-ray diffraction (p-XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy
(EDX), and also electrical resistivity measurements.
KEYWORDS: diorganotin dithiocarbamate, single source precursors, tin sulfide, thin films, aerosol-assisted chemical vapor deposition
higher growth rate. In particular, CVD using single-source
precursors possess some potential intrinsic advantages, such as
improved air/moisture stability of the precursor, excluding
prereactions and lower toxicity.30 Accordingly, various single-
source precursors for the deposition of tin sulfide thin films
have been investigated recently; examples include thiolates
{Sn(SCH2CF3)4, Sn(SPh)4, and Sn(SCy)4},13,31 dithiolates
{Sn(SCH2CH2S)2},32 dithiocarbamates {Sn(S2CNEt2)4},33
and organotin unsymmetrical dithiocarbamates, {RnSn[S2CN-
(C4H9)CH3]4−n, where R = Me, Bu, or Ph and n = 1, 2, or 3.34
However, all these precursors require H2S as a sulfur source to
deposit tin sulfide.32 This behavior was attributed to a facile
disulfide (RS-SR) elimination pathway. Subsequent molecular
orbital calculations and an extensive literature search revealed
that the general class of [M(SR)4] molecules (where M is a
Group IV element) suffer significant distortions from
tetrahedral geometry, because of noncovalently bonded S···S
interactions.29 This distortion and cis-annular interaction
promotes this disulfide elimination pathway. Recently, we
have studied the deposition behavior of tribenzyltin chloride−
thiosemicarbazone compounds by aerosol-assisted chemical
INTRODUCTION
■
Tin sulfide is a IV−VI semiconductor and exists in three main
forms, including SnS, SnS2, and Sn2S3.1 It has attracted
particular attention as a low-toxicity solar energy absorber,2,3
in holographic recording,4,5 and for infrared detection.6 The
band gaps of SnS, SnS2, and Sn2S3 are 1.3 eV, 2.18 eV, and 0.95
eV, respectively.7−9 All three forms of tin sulfides exhibit
semiconducting properties; however, SnS has attracted the
most attention, because of its electronic bandgap, which lies
midway between those of elemental silicon and GaAs.9 It has
been reported that, depending on the tin content, SnS may be a
p-type or n-type conductor and may also change its
conductivity upon heat treatment.10 SnS2 is an n-type
semiconductor, and Sn2S3 has highly anisotropic conduction.10
Many methods have been used to prepare tin sulfide thin
films, including spray pyrolysis,11 dip deposition,12 chemical
vapor deposition,13−16 thermal evaporation,17 chemical bath
deposition,18−20 chemical vapor transport,21 atomic layer
deposition,22 and melt growth.23 In addition, a variety of
methods have been employed to prepare tin sulfide
nanostructures, including hydrothermal methods,24 solvother-
mal methods,25 the hot injection method,26 the aqueous
solution method,27 and polyol methods.28,29
Received: May 30, 2012
Revised: December 6, 2012
Published: January 8, 2013
Chemical vapor deposition (CVD) methods have the
attraction of being able to deposit relatively large area films at
Published 2013 by the American Chemical
Society
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Chemistry of Materials
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a
Table 1. Crystallinity Data for Compounds 2, 3, and 8
parameter
2
3
8
formula
C20H42N2S4Sn
557.49
C52H102N4S8Sn2
1277.24
C28H42N2S4Sn
653.63
formula weight, M
appearance
colorless plates
monoclinic, C2/c
colorless block
monoclinic, P21/n
colorless block
monoclinic, C2/c
system and space group
unit-cell dimensions
a
21.936(3) Å
7.164(1) Å
21.873(2) Å
127.45°
20.345(5) Å
9.597(5) Å
34.350(5) Å
91.27°
27.282(9) Å
17.766(6) Å
18.671(6) Å
96.77°
b
c
β
V
2728.8(6) Å3
6705(4) Å3
8987.3(5) Å3
Z
4
4
12
density, D
1.357 Mg/m3
180(2) K
8178/3198
0.0626
1.265 Mg/m3
100(2) K
28488/14787
0.0286
1.449 Mg/m3
100(2) K
6019/4943
0.0297
temperature, T
reflections collected/unique reflections
Rint
for [I > 2σ(I)]
R1
0.0378
0.0601
0.0318
0.0927
0.0339
0.0821
wR2
for all data
R1
0.0715
0.0482
0.0447
0.0857
1.644 and −0.607 e A−3
wR2
0.0647
0.555 and −0.610 e A−3
0.0755
1.533 and −1.168 e A−3
largest diff. peak and hole
goodness of fit, GOF
0.763
1.016
1.054
a
CCDC Nos. 806740 (for 2), 806738 (for 3), and 806739 (for 8) contain the supplementary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
vapor deposition (AACVD).16 In the search for new single-
electrodes were used for the fabrication of the ohmic contacts. Raman
spectra were recorded using a Perkin−Elmer Model 400F Raman
spectrometer.
source precursors to deposit high-quality tin sulfide thin films
without the use of H2S, here, we report the synthesis and
characterization of symmetric and unsymmetric diorganotin
dithiocarbamates. Single-crystal X-ray structures of tin com-
plexes 2, 3, and 8 were determined. Complexes 1−4 were used
as single-source precursors for the deposition of tin sulfide thin
films by AACVD. Parkin et al. demonstrated that the
unsymmetric organotin dithiocarbamates could deposit tin
sulfide films by APCVD with H2S gas.34 Therefore, our interest
is in the development of symmetric and unsymmetric
diorganotin dithiocarbamates as precursors for the deposition
of tin sulfide thin films by AACVD without any external sulfur
source or co-reagent.
X-ray Crystallography. Single-crystal X-ray diffraction (XRD)
data for the compounds were collected using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å) on a Bruker APEX diffractometer.
The structure was solved by direct methods and refined by full-matrix
least-squares35 on F2. All non-H atoms were refined anisotropically. H
atoms were included in calculated positions, assigned isotropic thermal
parameters, and allowed to ride on their parent C atoms. All
calculations were carried out using the SHELXTL package.36 The
details pertaining the data collection of the crystals are given in Table
1.
Deposition of Films by AACVD. In a typical deposition, 0.20 g of
the precursor was dissolved in 10 mL of toluene in a two-necked 100
mL round-bottom flask with a gas inlet that allowed the carrier gas
(argon) to pass into the solution to aid the transport of the aerosol.
This flask was connected to the reactor tube by a piece of reinforced
tubing. The argon flow rate was controlled by a Platon flow gauge.
Seven borosilicate glass substrates (∼1 cm × 3 cm and 1 mm thick)
were placed inside the reactor tube, which is placed in a Carbolite
furnace. The precursor solution in a round-bottom flask was kept in a
water bath above the piezoelectric modulator of a PIFCO ultrasonic
humidifier (Model No. 1077). The aerosol droplets of the precursor
thus generated were transferred into the hot wall zone of the reactor
by carrier gas. Both the solvent and the precursor were evaporated and
the precursor vapor reached the heated substrate surface, where
thermally induced reactions and film deposition occurred.
EXPERIMENTAL SECTION
■
Author: All preparations were performed under an inert atmosphere of
dry nitrogen using standard Schlenk techniques. All reagents were
purchased from Sigma−Aldrich Chemical Company and used as
received. Solvents were distilled prior to use. Mass spectra were
recorded on a Kratos Concept 1S instrument. Infrared spectra were
recorded on a Specac single-reflectance attenuated total reflectance
(ATR) instrument (4000−400 cm−1, resolution of 4 cm−1). Elemental
analysis was performed by the microanalytical laboratory at the
1
University of Manchester. H and 119Sn NMR spectra were recorded
in CDCl3 solution (unless specified) on a Bruker spectrometer
Characterization of Thin Films. XRD studies were performed on
a Bruker AXS D8 diffractometer, using Cu Kα radiation. The samples
were mounted flat and scanned over a range of 20°−80° in a step size
of 0.05° with a count rate of 9 s. Films were carbon-coated using an
Edwards Model E306A coating system before carrying out SEM and
EDX analyses. SEM analysis was performed using a Philips XL 30FEG
system, and EDX was carried out using a DX4 instrument.
Synthesis of [Sn(C4H9)2(S2CN(C2H5)2)2] (1). Complex 1 was
synthesized by a literature method,37 sodium N,N-diethyldithiocarba-
operating at 400.0 MHz. The chemical shifts are referenced with
1
respect to tetramethylsilane (for H) and tetramethyltin (for 119Sn).
All the 119Sn NMR spectra were recorded under broadband decoupled
conditions. TGA measurements were carried out by a Seiko SSC/S200
model under a heating rate of 10 °C min−1 under nitrogen. The
electrical conductivity and Hall coefficient were measured in the
temperature range of 77−340 K by the van der Pauw technique using
an Ecopia Hall Measurement System (HMS-3000) with a custom-built
cryogenic cooling system and a 0.55 T magnet. Soldered indium
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Scheme 1. Synthesis of Compounds 1−8
mate (3.0 g, 1.3 mmol) and di-n-butyltin dichloride (2.02 g, 0.65
mmol) were dissolved in ethanol (200 mL) and stirred for 30 min,
during which time a fine sodium chloride powder precipitated. The
reaction mixture was filtered and the ethanol removed from the filtrate
by vacuum distillation. The resulting white solid was recrystallized
from ethanol chloroform (1:1) to give colorless crystalline plates. Yield
= 4.2 g (60.0%). Mp: 57 °C. IR (vmax/cm−1): 2953(w), 2924(w),
1483(s), 1456(m), 1417(s), 1353(m), 1298(m), 1253(s), 1205(s),
and 1138(s). Elemental analysis: Calc. for C18H38N2S4Sn: C, 40.8; H,
7.2; N, 5.3; S, 24.2; Sn, 22.4%. Found: C, 39.9; H, 7.5; N, 5.1; S, 22.8;
Sn, 21.4%. 1H NMR (δ/ppm): 0.86 (6H, t, J = 8, Bu CH3), 1.22 (12H,
t, J = 8, CH3), 1.37 (4H, m, Bu CH2), 1.85 (4H, m, Bu CH2), 1.99
(4H, m, Bu CH2), 3.80 (8H, q, NCH2); 119Sn NMR (δ/ppm):
−336.2(s).
Synthesis of [Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2). Complex 2 was
synthesized by dissolving sodium hydroxide (2.3 g, 5.7 mmol),
methylbutylamine (5.0 g, 5.7 mmol) in ethanol (100 mL) and stirring
for 30 min, followed by the addition of CS2 (3.47 mL, 5.7 mmol) and
stirring for 1 h. A solution of di-n-butyltin dichloride (8.7 g, 2.85
mmol) in ethanol (25 mL) was added dropwise and stirred for 30 min,
during which time fine sodium chloride powder precipitated. The
reaction mixture was filtered and the solvent was evaporated from the
filtrate by vacuum distillation to leave an oily product. Colorless blocks
of crystals were formed after leaving the resulting product in a
2869(w), 1493(s), 1426(m), 1391(w), 1296(m), 1221(m), and
1148(s), Elemental analysis: Calc. for C26H54N2S4Sn: C, 48.7; H,
8.5; N, 4.4; S, 20.0; Sn, 18.5%. Found: C, 48.6; H, 9.3; N, 4.3; S, 18.8;
Sn, 18.8%. 1H NMR (δ/ppm): 0.86 (6H, t, J = 8, Bu CH3), 0.88 (12H,
t, J = 8, Bu CH3), 1.28 (8H, m, CH2), 1.35 (4H, m, CH2), 1.63 (8H,
m, CH2), 1.84 (4H, m, CH2), 1.97 (4H, m, CH2), 3.71 (8H, t, J = 8,
NCH2); 119Sn NMR (δ/ppm): −338.4(s).
Synthesis of [Sn(C4H9)2(S2CN(CH3)(C6H13))2] (4). Complex 4
was synthesized by following the method given for complex 2, but
using methylhexylamine (6.5 g, 5.7 mmol) as the amine source.
Colorless oil, yield = 7.8 g (44.3%). IR (vmax/cm−1): 2953(w),
2926(w), 2865(w), 1489(s), 1467(m), 1441(m), 1395(s), 1299(m),
1253(m), and 1204(s). Elemental analysis: Calc. for C24H50N2S4Sn: C,
46.9; H, 8.2; N, 4.6; S, 21.0; Sn, 19.4%. Found: C, 47.4; H, 8.8; N, 4.5;
1
S, 19.5; Sn, 19.1%. H NMR (δ/ppm): 0.65 (6H, t, J = 8, Hex CH3),
1.05 (6H, t, J = 8, Bu CH3), 3.13 (6H, t, J = 8, NCH2) 3.58 (4H, t, J =
8, NCH2); 119Sn NMR (δ/ppm): −340.4(s).
Synthesis of [Sn(C6H5)2(S2CN(C2H5)2)2] (5). Complex 5 was
synthesized by following the method given for complex 1, but using
diphenyltin dichloride (4.4 g, 0.65 mmol) as the tin source. White
solid, yield = 5.4 g (74%). Mp: 150 °C. IR (vmax/cm−1): 2956(w),
2923(w), 2868(w), 1493(s), 1426(s), 1369(w), 1297(w), 1221(m),
1148(m), and 985(w). Elemental analysis: Calc. for C22H30N2S4Sn: C,
46.4; H, 5.3; N, 4.9; S, 22.5; Sn, 20.8%. Found: C, 46.4; H, 5.5; N, 4.9;
S, 22.4; Sn, 20.9%. 1H NMR (δ/ppm): 1.19 (12H, q, CH3), 3.69 (8H,
t, J = 8, NCH2), 7.27 (5H, m, ArH) 7.76 (5H, m, ArH); 119Sn NMR
(δ/ppm): −501.5(s).
refrigerator after a month. Yield = 8.2 g (51.5%). Mp: 41 °C. IR (vmax
/
cm−1): 2954(w), 2922(w), 2868(w), 1485(s), 1462(m), 1431(m),
1389(s), 1293(m), 1248(m), 1204(s), and 1138(s). Elemental
analysis: Calc. for C20H42N2S4Sn: C, 43.1; H, 7.6; N, 5.0; S, 23.0;
Sn, 21.3%. Found: C, 43.3; H, 8.6; N, 4.9; S, 21.7; Sn, 21.5%. 1H NMR
(δ/ppm): 0.86 (6H, t, J = 7.2, Bu CH3), 1.17 (6H, t, J = 7.2, Bu CH3),
1.29 (4H, m, CH2), 1.35 (4H, m, CH2), 1.63 (4H, m, CH2), 1.84 (4H,
m, CH2), 1.97 (4H, m, CH2), 3.32 (6H, s, NCH3), 3.78 (4H, t, J = 8,
NCH2); 119Sn NMR (δ/ppm): −339.5(s).
Synthesis of [Sn(C6H5)2(S2CN(CH3)(C4H9))2] (6). Complex 6 was
synthesized by following the method given for complex 2, but using
diphenyltin dichloride (9.7 g, 2.85 mmol) as the tin source. White
solid, yield = 6.7 g (39.7%). Mp: 133 °C. IR (vmax/cm−1): 2955(w),
2924(w), 1485(s), 1418(m), 1354(w), 1264(s), 1207(m), 1139(m),
1073(m), and 995(w). Elemental analysis: Calc. for C24H34N2S4Sn: C,
48.2; H, 5.7; N, 4.7; S, 21.5; Sn, 19.9%. Found: C, 48.3; H, 5.9; N, 4.6;
Synthesis of [Sn(C4H9)2(S2CN(C4H9)2)2] (3). Complex 3 was
synthesized by following the method given for complex 2, but using di-
n-butylamine (7.3 g, 5.7 mmol) as the amine source. White solid, yield
= 7.4 g (40.2%). Mp: 50 °C, IR (vmax/cm−1): 2955(w), 2929(w),
1
S, 21.2; Sn, 20.2%. H NMR (δ/ppm): 0.99 (6H, t, J = 8, Bu CH3),
1.39 (4H, m, Bu CH2), 1.73 (4H, m, Bu CH2), 3.36 (6H, s, NCH3),
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Figure 1. Molecular structures of (a) 2, (b) 3, and (c) 8 are shown with 50% ellipsoidal probability level. All the H atoms on carbon frames are
removed for clarity. Selected bond lengths (Å) and bond angles (°) for 2: Sn1−S1, 2.526(1); Sn1−S2, 2.940(1); Sn1−C7, 2.130(3); S1−C1,
1.736(3); S2−C1, 1.697(4); and C7−Sn1−C7*, 137.63(2); S1−Sn1−S1*, 83.93(5); S2−Sn1−S2*, 145.68(3); S2−C1−S1, 119.6(2). Selected
bond lengths (Å) and bond angles (°) for 3: Sn1−S1, 2.507(1); Sn1−S2, 2.894(1); Sn1−S3, 2.524(1); Sn1−S4, 3.029(1); Sn1−C19, 2.152(3);
Sn1−C23, 2.133(3); S1−C1, 1.754(3); S2−C1, 1.698(3); S3−C10, 1.740(3); S4−C10, 1.690(3); and C19−Sn1−C23, 129.13(1); S1−Sn1−S3,
82.23(3); S2−Sn1−S4, 147.85(3); S2−C1−S1, 119.15(2); S3−C10−S4, 119.79(2); for 8: Sn1−S1, 2.498(1); Sn1−S2, 2.983(1), Sn1−S3, 2.505(1);
Sn1−S4, 2.933(1); Sn1−C17, 2.128(3); Sn1−C23, 2.135(3); S1−C1, 1.745(4); S2−C1, 1.692(4); S3−C9, 1.741(4); S4−C9, 1.699(4); and C17−
Sn1−C23, 136.89(15); S1−Sn1−S3, 83.87(3); S2−Sn1−S4, 145.30(3); S2−C1−S1, 120.1(2); S3−C9−S4, 119.5(2).
3.78 (4H, t, J = 8, NCH2), 7.41 (5H, m, ArH), 7.93 (5H, m, ArH);
119Sn NMR (δ/ppm): −505.8(s).
1221(m). Elemental analysis: Calc. for C28H42N2S4Sn: C, 51.5; H,
6.5; N, 4.3; S, 19.6; Sn, 18.2%. Found: C, 51.9; H, 6.7; N, 4.3; S, 19.0;
Sn, 18.2%. 1H NMR (δ/ppm): 0.96 (6H, t, J = 8, Hex CH3), 1.37 (8H,
m, Hex CH2), 1.75 (4H, m, Hex CH2), 3.36 (6H, s, NCH3), 3.78 (4H,
t, J = 8, NCH2), 7.41 (5H, m, ArH), 7.95 (5H, m, ArH); 119Sn NMR
(δ/ppm): −506.8(s).
Synthesis of [Sn(C6H5)2(S2CN(C4H9)2)2] (7). Complex 7 was
synthesized by following the method given for complex 2, but using di-
n-butylamine (7.3 g, 5.7 mmol) as the amine source and diphenyltin
dichloride (9.7 g, 2.85 mmol) as the tin source. White solid, yield = 8.2
g (56.6%). Mp: 97 °C. IR (vmax/cm−1): 2955(w), 2929(w), 1492(s),
1426(m), 1369(w), 1298(m), 1262(m), and 1220(m). Elemental
analysis: Calc. for C30H46N2S4Sn: C, 52.9; H, 6.9; N, 4.1; S, 18.8; Sn,
RESULTS AND DISCUSSION
■
1
17.4%. Found: C, 52.9; H, 7.3; N, 4.1; S, 18.4; Sn, 17.5%. H NMR
Synthesis. Two synthetic procedures have been employed
for the synthesis of diorganotin dithiocarbamate complexes.
The first method involves the reaction of a diorganotin
dichloride, R2SnCl2 (R = n-Bu or Ph) with sodium N,N-
diethyldithiocarbamate in a 1:2 stoichiometric ratio in ethanol
to give mononuclear complexes 1 and 5 in quantitative yields
(see Scheme 1). The second method involves the in situ
synthesis of dithiocarbamates and then reaction with
diorganotin dichloride, R2SnCl2 (R = n-Bu or Ph) in a
(δ/ppm): 0.99 (12H, t, J = 8, Bu CH3), 1.39 (8H, m, Bu CH2), 1.78
(8H, m, Bu CH2), 3.75 (8H, t, J = 8, NCH2), 7.41 (5H, m, ArH), 7.92
(5H, m, ArH); 119Sn NMR (δ/ppm): −506.3(s).
Synthesis of [Sn(C6H5)2(S2CN(CH3)(C6H13))2] (8). Complex 8
was synthesized by following the method given for complex 2, but
using methylhexylamine (6.5 g, 5.7 mmol) as the amine source and
diphenyltin dichloride (9.7 g, 2.85 mmol) as the tin source. White
solid, yield = 5.8 g (47%). Mp: 120 °C. IR (vmax/cm−1): 2956(w),
2928(w), 1492(s), 1426(s), 1390(w), 1297(m), 1261(m), and
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stoichiometric ratio of 2:1. It also produces mononuclear
complexes 2−4 and 6−8 in quantitative yields (see Scheme 1).
Complexes 2 and 4 are colorless viscous oils, whereas the
other complexes are white solids; all of them are soluble in all
common organic solvents. We were able to crystallize complex
2 from viscous oil after leaving it refrigerated at 5 °C for a
month. Compounds 1−8 show a single resonance in their 119Sn
NMR spectra in a narrow chemical shift window ranging from
−336.0 ppm to −340.0 ppm for compounds 1−4 (di-n-butyltin
derivatives) and −501.0 ppm to −506.0 ppm for compounds
5−8 (diphenyltin derivatives), indicating the presence of only
one type of Sn atom in the structure. These chemical shifts are
the signature of diorganotin dithiocarbamate complexes that
contain a single Sn atom with a 2C, 2S coordination
environment in solution.38−40
Figure 2. TGA of [Sn(C4H9)2(S2CN(C2H5)2)2] (1) (black squares,
Single-Crystal X-ray Structures of 2, 3, and 8.
Complexes 2, 3, and 8 also have been characterized by
single-crystal X-ray analysis. Their molecular structures and
selected bonding parameters are given in Figure 1. The
structure determination for 2 was difficult; at low temperatures,
crystals cracked, possibly because of a phase change, and final
data were collected at 180 K on a relatively small crystal.
Application of absorption corrections did not improve the
refinement, and the thermal ellipsoids on the alkyl substituents
are quite large. Despite attempts, no better solutions were
found. The molecular structures of all three compounds are
isostructural, but each has different substituents, either on tin or
in the dithiocarbamate moiety. The structure of 2 is discussed
here as an example. The structure determination reveals that
the diorganotin bis(dithiocarbamate) adopts a well-known
discrete structure.38−40 The asymmetric unit of 2 contains half
of the molecule, with one Sn atom (Sn1) sitting on a special
position along a 2-fold axis. The tin is involved in four covalent
bonds (2C and 2S), having a distorted tetrahedral geometry.
The other S atom on each dithiocarbamate moiety (S2 and
S2*) is involved in a weaker bonding interaction with tin along
the tetrahedral faces (Figure 1a). Accordingly, two types of Sn−
S distances are seen. The comparatively shorter Sn1−S1 and
Sn1−S1* (2.526(1) Å) distances correspond to Sn−S covalent
bonds; whereas, the Sn1−S2 and Sn1−S2* (2.940(1) Å)
distances correspond to the Sn−S weaker bonding. The sum of
the van der Waals radii of tin and sulfur is 3.97 Å.41 In view of
this and also in view of literature precedents,38−40 it can be
concluded that a coordinative interaction exists between Sn1
and S1 and S2, with each dithiocarbamate moiety shown
chelating an isobidentate coordination mode. Consequently,
the Sn atom is six-coordinated with a 2C, 4S coordination
environment, and the geometry around the Sn atom can be
described as a skewed trapezoidal bipyramid (see Figure 1a).42
Thermogravimetric analysis (TGA). Thermogravimetric
analysis gives information about the decomposition and/or
volatility of the complexes. TGA (25−500 °C; heating rate of
10 °C min−1 under nitrogen) of the tin complexes (1 and 3)
showed close to single-step decomposition, with weight loss
occurring between 212 °C and 298 °C and between 214 °C
and 294 °C, respectively (see Figure 2). The solid residue for 1
(27.8%) is in good agreement with calculated value of 28.4%
for SnS. Similarly, for 3, the final residue (20.4%) is close to
that which is expected for SnS (23.4%). Complex 2
decomposes in two steps between 198 °C and 274 °C, finally
leading to 36.1%, which corresponds to the calculated value
(37.1%) for SnS2. The solid undergoes a second decomposition
between 274 °C and 329 °C, with a total residue weight of
□
○
), [Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2) (red circles, ), [Sn-
△
(C4H9)2(S2CN(C4H9)2)2] (3) (blue triangles,
(C4H9)2(S2CN(CH3)(C6H13))2] (4) (golden orange diamonds,
under a N2 atmosphere.
), and [Sn-
◇
)
8.9%. Complex 4 decomposes in two steps; the first step,
between 220 °C and 303 °C, gives 30.2%, which is in good
agreement with the value for SnS2 (29.7%). The residue further
decomposed at temperatures between 303 °C and 325 °C, with
a residue weight of 19.0%. Further decomposition of SnS2 has
previously been noted and, in these cases, could be associated
with oxygen leakage into the reactor.
Figure 3 shows a TGA graph of the diphenyl derivative of tin
complexes. Complexes 5, 6, 7, and 8 showed single-step
Figure 3. TGA of [Sn(C6H5)2(S2CN(C2H5)2)2] (5) (black squares,
□
○
), [Sn(C6H5)2(S2CN(CH3)(C4H9))2] (6) (red circles, ), [Sn-
△
(C6H5)2(S2CN(C4H9)2)2] (7) (blue triangles,
(C6H5)2(S2CN(CH3)(C6H13))2] (8) (golden orange diamonds,
under a N2 atmosphere.
), and [Sn-
◇
)
decomposition with weight loss occurring within the ranges of
242−300 °C, 242−318 °C, 247−321 °C, and 247−327 °C,
respectively. The residue amounts of 26.5% for 5 agree well
with the calculated value of 26.4% for SnS, and those for 6
(25.2%), 7 (20.1%), and 8 (21.5%) show agreement with the
calculated values for SnS (25.1%, 22.0%, and 23.0%,
respectively). We presume that these complexes follow nearly
the same decomposition pathway as that reported for similar
dithiocarbamate complexes.43,44
Tin Sulfide Thin Films from Dibutyltin Dithiocarbamate
Complexes. Thin films of tin sulfides were deposited on glass
substrates using toluene solution of precursors 1−4 by
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Chemistry of Materials
Article
AACVD. The film deposition temperature was varied over a
range of 400−530 °C at an argon flow rate of 140 sccm.
Substrate temperatures below 400 °C were not enough to
initiate deposition. Using precursor 1 at 400 °C, a dense,
brownish yellow adherent film was deposited, whereas at 450
and 500 °C, dark black films were deposited. Precursor 2
deposited dark brownish films at 400−500 °C and black
adherent films were deposited at 530 °C. Precursor 3 deposited
black adherent uniform films at all temperatures, whereas
precursor 4 deposited brownish films at 400 and 450 °C and
black films at 500 °C.
The XRD pattern of films deposited on a glass substrate at
temperatures of 400−500 °C from a toluene solution of
precursor 1 is shown in Figure 4. The diffraction peaks could be
Figure 5. XRD pattern of SnS films deposited from [Sn-
(C4H9)2(S2CN(CH3)(C4H9))2] (2) on a glass substrate: (a) 400
°C, (b) 450 °C, (c) 500 °C, and (d) 530 °C. The asterisk symbol (*)
denotes the SnO2 phase.
Rietveld refinements of powder XRD patterns confirm the
presence of orthorhombic SnS as a main phase for films
deposited from precursors 1−4 with a small amount of SnO2
phase. Figure 6 displays a representative refinement for the film
deposited at 530 °C from precursor 2. The calculated
composition for this film is 89.6 wt % orthorhombic SnS and
10.4 wt % SnO2 phases, with lattice parameter values of a =
11.197(5) Å, b = 3.997(2) Å, and c = 4.283(1) Å for
orthorhombic SnS. The refinement data for films deposited
Figure 4. XRD pattern of SnS films deposited from [Sn-
(C4H9)2(S2CN(C2H5)2)2] (1) on a glass substrate: (a) 400 °C, (b)
450 °C, and (c) 500 °C. The asterisk symbol (*) denotes the SnO2
phase.
indexed to the orthorhombic herzenbergite-SnS (ICDD File
Card No. 033-1375). The relative highest intensity peak was
observed at 2θ = 31.49°, corresponding to the (111) plane of
orthorhombic SnS. The major diffraction peaks could be
indexed as the (120), (021), (101), (131), and (141)
reflections of SnS. The relatively lower intensity peak at 2θ =
33.50° was observed, which could be indexed as a SnO2 phase.
The XRD pattern of films deposited from precursor 2 at
temperatures between 400 °C and 530 °C is shown in Figure 5.
The highest intensity diffraction peak could be indexed as the
(111) plane of SnS. Similarly, the XRD pattern of films
deposited using precursors 3 and 4 could also be confirmed as
being orthorhombic SnS, with the highest intensity reflection
being from the (111) plane. The intensity of peak at 2θ =
33.50° is 40% (400 °C), 8.1% (450 °C), and 1.3% (500 and
530 °C) from films deposited using precursor 3, and 12.9%
(400 °C), 6.6% (450 °C), and 3.8% (500 °C) from films
deposited using precursor 4, relative to the (111) plane of the
SnS phase.
Figure 6. Results of the Rietveld refinement of the XRD pattern of
SnS films deposited from [Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2) on
glass substrate at 530 °C [orthorhombic SnS phase: 89.6 wt %, space
group Pnma, a = 11.197(5) Å, b = 3.997(2) Å, c = 4.283(1) Å;
tetragonal SnO2 phase: 10.4 wt %, space group P42/mnm, a = b =
4.760(6) Å, c = 3.189(7) Å; Rwp = 0.102, GOF = 3.01]. The calculated
positions of reflections are shown as tick marks (black for SnS and red
for SnO2). The difference curve is shown underneath the main plot.
271
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Chemistry of Materials
Article
from precursor 1 at 400 °C exhibit a phase composition of 75.9
wt % SnS and 25.0 wt % SnO2, with lattice parameters of a =
11.198(4) Å, b = 4.000(4) Å, and c = 4.285(2) Å. The presence
of SnO2 could be due to the aerial oxidation of tin precursors,
since samples were handled in open air.
The formation of SnS films in our AACVD experiment
indicate the facile reduction of metal center Sn(IV) to Sn(II) in
an argon atmosphere.13,31 Similar observations have also been
reported by Barone et al. during the APCVD experiment using
Sn(IV) thiolates as precursors in the presence of H2S. The
authors reported that the stream of H2S facilitates the reduction
of Sn(IV) to Sn(II).31
has been adopted from ref 45. SnS forms a double-layered
structure at room temperature. Each Sn(II) ion is bonded to six
S atoms in a distorted octahedral geometry. Tin is separated
with sulfur through three shorter (ca. 2.7 Å) and three longer
(ca. 3.4 Å) distances in an octahedron coordination. In the
double-layer structure of SnS, each layer connected with longer
Sn−S bonds. It undergoes a transformation from a GeS-type
structure to a TlI-type structure above 605 °C with continuous
movement of Sn and S along the [100] direction.46 Tin
disulfide forms the PbI2 layered structure in which Sn(IV) is in
the middle of a perfect octahedron. Each Sn(IV) ion bonded
with six S atoms by ca 2.6 Å. In SnS2 layers, SnS6 building unit
stacked on top of one other along the crystallographic c-axis by
weak van der Waals forces. There have been more than 70
polytypes of SnS2 reported. Sn2S3 is a mixed-valence compound
with ribbon structure. The Sn(II) ion coordinated with sulfur in
a trigonal bipyramidal geometry through Sn−S distances of ca.
2.6 and 2.7 Å. Sn(IV) sites in the ribbon are octahedrally
coordinated with Sn−S distances of ca. 2.45−2.60 Å.
The crystal structure of orthorhombic SnS, SnS2, and Sn2S3
in Figure 7 gives a clear picture about the bonding around
The morphology of films deposited using a toluene solution
of precursors 1−4 was characterized by scanning electron
microscopy (SEM). The SEM images in Figures 8 and 9 show
the morphology of films deposited at 400 °C as consisting of a
bundle of sheets, consisting of small spherical crystallites with
sheets ∼25−30 μm in size and sheets 25−30 and 10−15 μm in
size at temperatures of 450 and 500 °C from precursor 1.
Similarly, the sheetlike crystallites were deposited from
precursors 2, 3, and 4 with considerable variation in size.
This gives an idea that variation in the alkyl group on the
external coordination zone does not have a profound effect on
the decomposition. The morphology of crystallites is different
from the films deposited using dithiol, dithiolates, and
thiosemicarbazone complexes of Sn(IV) by AACVD.13,16
However, spherical crystallites with a sheetlike morphology
have been observed in the low-pressure chemical vapor
deposition (LPCVD) growth of SnS films.47 This indicates
that the aerosols of precursor solution does not play a major
role in the morphology of SnS films.
Figure 7. Local Sn environments in (a) SnS, (b) SnS2, and (c) Sn2S3;
Sn···S connectivity in (d) SnS, (e) SnS2, and (f) Sn2S3. [Color legend:
gray, Sn(II); violet, Sn(IV); and yellow, S.] Crystal structure data were
obtained from the Inorganic Crystal Structure Database (ICSD)
database.
The composition of tin and sulfides was analyzed by
quantitative energy-dispersive X-ray (EDX) analysis. The
Sn(II) and Sn(IV) ions in different stoichiometries of tin
sulfide. The single-crystal X-ray structure of orthorhombic SnS
Figure 8. SEM images of SnS films deposited on glass substrates from [Sn(C4H9)2(S2CN(C2H5)2)2] (1) at (a) 450 °C and (b) 500 °C; SEM images
of SnS films deposited on glass substrates from [Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2) at (c) 450 °C and (d) 500 °C.
272
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Chemistry of Materials
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Figure 9. SEM images of SnS films deposited on glass substrates from [Sn(C4H9)2(S2CN(C4H9)2)2] (3) at (a) 450 °C and (b) 500 °C; SEM images
of SnS films deposited on glass substrates from [Sn(C4H9)2(S2CN(CH3)(C6H13))2] (4) at (c) 450 °C and (d) 500 °C.
EDX analysis confirmed the tin:sulfur stoichiometry of films
(percentage basis) as 53:47 (400 °C), 56:44 (450 °C), and
58:43 (500 °C) from tin sulfide films deposited using precursor
1; 51:49 (400 °C), 53:47 (450 °C), 56:44 (500 °C), and 59:41
(530 °C) from films deposited using precursor 2; 51:49 (400
°C), 55:45 (450 and 500 °C), and 56:44 (530 °C) from
precursor 3; and 55:45 (400 and 450 °C) and 59:41 (530 °C)
from precursor 4. The variation in atomic percentage of tin and
sulfur in tin sulfide films grown on a glass substrate at different
temperatures from a toluene solution of precursors clearly
indicates sulfur-deficient films from all precursors. Furthermore,
it showed the presence of a small amount of carbon
contamination on the films.
The optical band gap of the deposited tin sulfide films can be
Figure 10. Square of the absorption coefficient (αhν)2, as a function of
photon energy (hν), for SnS film deposited from [Sn(C4H9)2(S2CN-
(CH3)(C4H9))2] (2) at 400 °C on a glass substrate.
estimated from the vis-IR spectra. The recorded spectra closely
resembled those reported in the literature for SnS.7 The optical
bandgap of the deposited films is influenced by two factors: the
inherent band gap of the material and a tail due to disorder.
Bulk SnS has both direct (1.3 eV) and indirect (1.1 eV) band
gaps. The band gap of the films was measured from the
dependence of absorption coefficient (α) on the photon energy
(hv). In the present case, a plot of (αhv)2 versus hv is linear,
indicating the direct band nature of the films. Extrapolating the
linear portion of the curve onto the x-axis gives the bandgap for
the film (see Figure 10). The evaluated band-gap values are as
follows: 1.31 eV (400 °C), 1.36 eV (450 °C), and 1.33 eV (500
°C) for [Sn(C4H9)2(S2CN(C2H5)2)2] (1); 1.39 eV (400 °C),
1.21 eV (450 °C), and 1.35 eV (500 and 530 °C) for
[Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2); 1.21 eV (400 °C),
1.22 eV (450 °C), 1.58 eV (500 °C), and 1.71 eV (530 °C) for
[Sn(C4H9)2(S2CN(C4H9)2)2] (3); and 1.3 eV (400 °C), 1.38
eV (450 °C), and 1.47 eV (500 °C) for [Sn(C4H9)2(S2CN-
(C4H9)2)2] (4). The band gap was found to increase as the
deposition temperature increased. It is well-known that the
energy band gap of a semiconductor is affected by the residual
strain, defects, charged impurities, disorder at the grain
boundaries, and also particle size confinement. In addition,
the tensile strain will result in a decrease of band gap due to the
elongated lattice whereas a compressional strain increases the
band gap due to the compressed lattice of the film.48 However,
contribution from SnO2 and carbon impurities to the higher-
band-gap value of SnS films, in some cases, is not ruled out.
Raman Measurements. Raman spectral studies on selected
films deposited using precursors 1−3 at 450 and 530 °C
confirms the presence of SnS (see Figure 11). The character-
istic Raman bands are seen at the wavenumbers of 95, 161, 190,
and 218 cm−1, which agree well with previous observations at
95, 160, 189, and 219 cm−1 for single-crystal SnS.49,50 The
smaller shift in the observed band position in the present case
may be caused by a strain between SnS films and glass
substrates. These lines could be assigned to the longitudinal
and transverse optic modes of SnS. It is well-known that the
Raman lines at 95, 190, and 218 cm−1 correspond to the Ag
phonon modes of SnS and that at 161 cm−1 corresponds to the
B2g phonon mode of SnS.
Electrical Measurements. Figure 12 displays the temper-
ature variations of electrical resistivity of films deposited on
glass substrates from [Sn(C4H9)2(S2CN(C2H5)2)2] (1) at 500
°C and from [Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2) at 530 °C
and 450 °C. The electrical resistivity of all films exhibits
semiconducting behavior with an activation energy at room-
temperature region varying from 0.233 eV for the film
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Chemistry of Materials
Article
applications. We have investigated the photoconductive
response of tin sulfide films deposited from precursors 1−4
at 500 °C by measuring I−V curves using a 633-nm laser lamp.
I−V curves of the difference between resistivity values recorded
under illumination shown in Figure 13. It is seen that the
Figure 11. Raman spectra of SnS films deposited from [Sn-
△
(C4H9)2(S2CN(C4H9)2)2] (1) at 530 °C (red triangles, ) and
□
[Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2) at 450 °C (black squares, ),
both on glass substrates.
Figure 13. Current−voltage characteristics of SnS films under dark
■
conditions (black solid squares, ) and illuminated conditions (red
●
solid circles, ) (4.6 mW; 633-nm laser lamp) for films deposited at
500 °C from [Sn(C4H9)2(S2CN(C2H5)2)2] (1). Inset show photo-
generated current for SnS films deposited from [Sn(C4H9)2(S2CN-
□
(C2H5)2)2] (1) (black open squares, ) and [Sn(C4H9)2(S2CN-
△
(CH3)(C4H9))2] (2) (red open triangles, ).
gradient of the illuminated I−V curves higher than that of the
I−V curve under dark condition. This suggests that the
conductivity of the tin sulfide films become high under
illumination that is due to photoinduced carriers. I−V
measurement also showed linear data without any rectification
behavior, which indicates the absence of a Schottky barrier at
Ag/SnS contacts. The observed photosensitivities of tin sulfide
films deposited at 500 °C from the dibutyl derivative of tin
precursors are 0.4% for 1, 2.1% for 2, 1.2% for 3, and 1.0% for
4.
Figure 12. Temperature variation of electrical resistivity of films
deposited on glass substrates from [Sn(C4H9)2(S2CN(C2H5)2)2] (1)
●
at 500 °C (blue circles, ), from [Sn(C4H9)2(S2CN(CH3)(C4H9))2]
▲
(2) at 530 °C (red triangles, ), and from [Sn(C4H9)2(S2CN(CH3)-
■
(C4H9))2] (2) at 450 °C (green squares, ).
CONCLUSIONS
deposited from precursor 1 to 0.109 and 0.064 eV for films
deposited from precursor 2 at 530 and 450 °C, respectively, as
shown in the inset in Figure 12. These activation energies
correspond well with previously reported values for SnS
films51,52 and indicate that all three films are extrinsic
semiconductors. The room-temperature carrier concentration,
mobility, and electrical resistivity values of films are summarized
in Table 2. Hall effect measurements confirmed that all films
are p-type semiconductors with hole concentration in the range
of (0.4−8.6) × 1018 cm−3 and hole mobility of 0.9−1.4 cm2 V−1
s−1. P-type conduction in SnS is believed to originate from high
concentration of Sn vacancy sites.
■
A series of diorganotin complexes of dithiocarbamates,
including [Sn(C4H9)2(S2CN(RR′)2)2] (R, R′ = ethyl (1); R
= methyl, R′ = butyl (2); R, R′ = butyl (3); R = methyl, R′ =
hexyl (4); and [Sn(C6H5)2(S2CN(RR′)2)2] (R, R′ = ethyl (5);
R = methyl, R′ = butyl (6); R, R′ = butyl (7); R = methyl, R′ =
hexyl (8) have been synthesized. The structures of 2, 3, and 8
were determined by single-crystal X-ray crystallography.
Thermogravimetric analysis (TGA) showed single-step decom-
position of complexes 1, 3, and 5−8 and double-step
decomposition of complexes 2 and 4 at temperatures between
195 °C and 325 °C. Complexes 1−4 were used as single-source
precursors for the deposition of tin sulfide thin films by aerosol-
assisted chemical vapor deposition (AACVD) at temperatures
of 400−530 °C. Complexes 1−4 primarily deposited the
orthorhombic SnS phase. Rietveld analysis on the powder
diffraction patterns appears to indicate the presence of some
SnO2. The films consist of sheetlike crystallites. The measured
bandgap of the films varied between 1.2 eV and 1.7 eV. Distinct
Raman bands at 95, 161, 190, and 218 cm−1 confirmed the
presence of the orthorhombic SnS phase. The results of SnS
films deposited from precursors 1−4 are given in Table 3. The
highest photosensitivity (2.1%) was observed from films
deposited using precursor 2 at 500 °C.
The photoconductive response of semiconductor materials is
a prerequisite for a material to be considered for solar cell
Table 2. Electrical Properties of Tin Sulfide Films Deposited
from Precursor 1 at 500 °C and Precursor 2 at 530 and 450
°C
deposition
precursor temp (°C)
carrier conc, p
resistivity, ρ
(Ω cm)
mobility, μ
(× 1018 cm−3
)
(cm2 V−1 s−1
)
1
2
2
500
530
450
4.13 × 1017
3.60 × 1018
8.64 × 1018
10.8
1.58
0.83
1.4
1.1
0.87
274
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Chemistry of Materials
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(5) Radot, M. Rev. Phys. Appl. 1977, 18, 345.
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(°C)
band gap
(eV)
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(2θ) = 33.5° (%)
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Sn
S
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[Sn(C4H9)2(S2CN(C2H5)2)2] (1)
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175−180.
400
1.31
16.4
53
47 bundle of
sheets
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450
500
1.36
1.33
16.3
1.1
56
58
44 sheets
43 bundle of
sheets
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[Sn(C4H9)2(S2CN(CH3)(C4H9))2] (2)
400
450
1.39
1.21
7.3
3.4
51
53
49 sheets
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I. P. J. Mater. Chem. 2001, 11, 464−468.
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sheets
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Vap. Deposition 1998, 4, 222−225.
500
530
1.35
1.35
5.3
2.9
56
59
44 bundle of
sheets
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41 bundle of
sheets
[Sn(C4H9)2(S2CN(C4H9)2)2] (3)
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400
450
500
530
1.21
1.22
1.58
1.71
40
8.1
51
55
55
56
49 sheets
45 sheets
45 sheets
44 sheets
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1.3
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400
450
500
1.3
12.9
55
55
59
45 bundle of
sheets
1.38
1.47
6.6
45 bundle of
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ASSOCIATED CONTENT
* Supporting Information
Crystallographic information files (CIF) for complexes 2, 3,
and 8 as electronic copies. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: paul.obrien@manchester.ac.uk.
■
(29) Xu, Y.; Al-Salim, N.; Bumby, C. W.; Tilley, R. D. J. Am. Chem.
Soc. 2009, 131, 15990.
Present Addresses
(30) Malik, M. A.; Afzaal, M.; O’Brien, P. Chem. Rev 2010, 110,
§Center for Materials for Information Technology, The
University of Alabama, Tuscaloosa, AL 35487, USA.
#Department of Chemistry, Indian Institute of Technology-
Bombay, Powai, Mumbai, 400076, India.
4417−4446.
(31) Barone, G.; Hibbert, T. G.; Mahon, M. F.; Molloy, K. C.; Price,
L. S.; Parkin, I. P.; Hardy, A. M. E.; Field, M. N. J. Mater. Chem. 2001,
11, 469−473.
(32) Parkin, I. P.; Price, L. S.; Hibbert, T. G.; Molloy, K. C. J. Mater.
Chem 2001, 11, 1486−1490.
Notes
The authors declare no competing financial interest.
(33) Barone, G.; Chaplin, T.; Hibbert, T. G.; Kana, A. T.; Mahon, M.
F.; Molloy, K. C.; Worsley, I. D.; Parkin, I. P.; Price, L. S. J. Chem. Soc.,
Dalton Trans., 2002, 1085−1092.
ACKNOWLEDGMENTS
■
K.R. is grateful to ORS and The University of Manchester for
financial support. The authors also thank EPSRC, UK for the
grants to POB that have made this research possible. K.G.
gratefully acknowledges the European Commission for a Marie
Curie International Incoming Fellowship (EC Contract No:
PIIF-GA-2007-220035-CaCoPhos).
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