Synthesis, structural characterization, photoluminescence and thermal properties of [(Ph3P)2cu(μ-SeC{O}R)2Cu (PPh3)]

Article abstract of DOI:10.1039/b203466b

A number of neutral unsymmetrical dimers [(Ph₃P)₂Cu(μ-SeC{O}R)₂Cu (PPh₃)] (R = C₆H₅ (1), C₆H₄-Me-4 (2), C₆H₄-OMe-4 (3), C₆H₄-Cl-4 (4), C₆H₄-F-4 (5) and CH₃ (6)) were synthesized in moderate yield, and characterized by analytical and spectroscopic techniques including ³¹P NMR. Variable temperature ³¹P NMR experiments appear to indicate that the molecular structure is retained in solution. X-Ray structure determination of 2 and 6 confirmed the molecular geometry of the unsymmetrical dimer. In 2 the two 4-MeO-C₆H₄C=O groups have anti stereochemistry with respect to the Cu₂Se₂ ring while MeC=O groups have syn configuration in 6. A weak C-H...O=C intramolecular hydrogen bond present between the two selenoacetato ligands appears to dictate the syn geometry in 6. This hydrogen bonding is also responsible for the non-planarity of the Cu₂Se₂ ring. These compounds are photoemissive in CH₂Cl₂ solution. Thermogravimetry and pyrolysis experiments in nitrogen atmosphere show that 1-6 are good single source precursors to copper selenide bulk materials. Preliminary experiments show that these compounds are good single source molecular precursors for cubic phase Cu_2-xSe nanoparticles with an average size of 75 ± 15 nm.

Full text of DOI:10.1039/b203466b

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Synthesis, structural characterization, photoluminescence and thermal
properties of [(Ph3P)2Cu(l-SeC{O}R)2Cu(PPh3)]y
Zheng Lu,a Wei Huangb and Jagadese J. Vittala
a
Department of Chemistry, National University of Singapore, 3 Science Drive,
Singapore 117543
Institute of Materials Science and Engineering, 3 Science Link, Singapore 117602
b
Received (in London, UK) 9th April 2002, Accepted 31st May 2002
First published as an Advance Article on the web 12th July 2002
A number of neutral unsymmetrical dimers [(Ph3P)2Cu(m-SeC{O}R)2Cu(PPh3)] (R ¼ C6H5 (1), C6H4-Me-4
(2), C6H4-OMe-4 (3), C6H4-Cl-4 (4), C6H4-F-4 (5) and CH3 (6)) were synthesized in moderate yield, and
characterized by analytical and spectroscopic techniques including 31P NMR. Variable temperature 31P NMR
experiments appear to indicate that the molecular structure is retained in solution. X-Ray structure
determination of 2 and 6 confirmed the molecular geometry of the unsymmetrical dimer. In 2 the two 4-MeO-
=
C6H4C O groups have anti stereochemistry with respect to the Cu2Se2 ring while MeC O groups have syn
=
=
configuration in 6. A weak C–Hꢀ ꢀ ꢀO C intramolecular hydrogen bond present between the two selenoacetato
ligands appears to dictate the syn geometry in 6. This hydrogen bonding is also responsible for the non-
planarity of the Cu2Se2 ring. These compounds are photoemissive in CH2Cl2 solution. Thermogravimetry and
pyrolysis experiments in nitrogen atmosphere show that 1–6 are good single source precursors to copper
selenide bulk materials. Preliminary experiments show that these compounds are good single source molecular
precursors for cubic phase Cu2ꢁxSe nanoparticles with an average size of 75 ꢂ 15 nm.
NC5H4) were found to decompose to a mixture of CuSe and
Introduction
CuSe2 .16 O’Brien et al. have used Cu(Se2CNEt2)2 for Cu2ꢁxSe
Se nanoparticles.17 However, the synthetic procedure of this
precursor involves highly toxic CSe2 .
The chalcogenate chemistry of the main group elements has
continued to attract considerable attention in recent years
due to the fact that the main group chalcogenolates can be
potentially used as single-source precursors for the low-tem-
perature synthesis of semiconductor bulk materials. Further
the bonding between metals and the heavier chalcogens is rela-
tively unexplored as compared to the corresponding sulfur
analogues. For instance, a number of thiocarboxylate metal
complexes have been synthesized and well characterized.1–6
Some of them have been shown to be good single source pre-
cursors for sulfide materials.1,2 For copper thiocarboxylate,
Speier et al. first reported a triphenylphosphine adduct of a
Cu(I) thiocarboxylate complex with an unsymmetrical dimeric
structure4 in 1991. Recently, a number of triphenylphosphine
adducts of Cu(I) thiocarboxylate complexes have been success-
fully isolated in our lab.3 In contrast, to the best of our knowl-
edge, no selenocarboxylate complex of group 11 metals is
known and only a few transition metal selenocarboxylate com-
plexes have been reported.7–10
Recent investigation in our lab on the PPh3:Cu(I):RC{O}Sꢁ
system led to the isolation of structurally diversified com-
pounds ranging from mononuclear, dinuclear to two different
tetranuclear aggregates.3 Each neutral derivative was found to
thermally decompose to a particular phase of Cu2S or Cu2ꢁx
S
for which several crystalline phases are well known.18 This
study naturally gives rise to the following questions: Is there
a
parallel chemistry for selenocarboxylato ligand? Will
RC{O}Seꢁ ligand chemistry exhibit similar structural diver-
sity? Can these be used as single-source precursors for
Cu2Se/Cu2ꢁxSe materials? We wish to address these questions
and hence have undertaken a project on stable neutral copper
selenocarboxylate compounds containing triphenylphosphine.
The compounds synthesized have been characterized by stan-
dard analytical and spectroscopic techniques and the solid
state structures of representative compounds have been deter-
mined by X-ray crystallography. We have also investigated the
photoluminescence properties of these compounds. Thermoa-
nalytical techniques, especially TG, have been employed to
investigate their thermal properties and the formation of the
expected copper selenide was determined by the residual
weight observed. Further it was thought worthwhile to study
the influence of the nature of the substituents at the para posi-
tion of the phenyl ring on the thermal stability, if any, and the
final products of decomposition. We have, therefore, prepared
a few phenyl derivatives of selenocarboxylato ligands and their
complexes with Cu(I). The final products of thermal decompo-
sition obtained in pyrolysis experiments were characterized by
X-ray powder diffraction methods and Scanning Electron
Microscopy. Preparation of nanoparticle Cu2ꢁxSe is also
explored. The results of our findings are discussed in this
paper.
Copper selenide has been widely used in solar cells,11 as an
optical filter,12 and superionic materials.13 In the last few years
many synthetic methods for copper selenide have been
reported. For example, Qian and his coworkers have success-
fully obtained nanocrystalline Cu2ꢁxSe at room temperature
by g-irradiation of copper acetate and sodium selenosulfate.14
On the other hand, the reaction of Cu and Se in n-butylamine
gave crystalline Cu2ꢁxSe.15 Single source routes to copper sele-
nide, on the other hand, are rare. Cu(SePy)4 and Cu(SePy*)4
(where SePy ¼ 2-Se-NC5H4 and SePy* ¼ 3-Me3Si-2-Se-
y Electronic supplementary information (ESI) available: XRPD pat-
terns and SEM images of the residues from pyrolysis of 1–6. See
http://www.rsc.org/suppdata/nj/b2/b203466b/
1122
New J. Chem., 2002, 26, 1122–1129
DOI: 10.1039/b203466b
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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Results and discussion
Sodium selenocarboxylates were prepared by the literature
method from Na2Se and acid chlorides, and were allowed to
react with [(Ph3P)2Cu(NO3)] to get the neutral unsymmetrical
dimer, [(Ph3P)2Cu(m-SeC{O}R)2Cu(PPh3)], as shown below. A
number of derivatives of selenocarboxylates has been prepared
in low to moderate yield (30–63%). Interestingly, this is the
only product isolated by varying the Ph3P:Cu(I):RC{O}Seꢁ
ligand ratio and experimental conditions.
Fig. 2 VT 31P NMR of 1 and PPh3 in 1:1 ratio in CD2Cl2 solution.
it is closer to the coalescent point, which occurs around 270 K
(not shown). Below the coalescence temperature is the slow
exchange region and the two 31P NMR signals start appearing.
On cooling these signals sharpen at 2.07 (Dn1/2 ¼ 61 Hz) and
ꢁ5.47 (Dn1/2 ¼ 15 Hz) at 200 K and precipitation occurs
below 200 K. The integration of the intensities of these two
peaks gives a ratio of 2:1 and appears to support the number
of phosphorus atoms attached to Cu(1) and Cu(2) respectively
in 1.
We were unable to isolate the monomer [(Ph3P)2Cu(Se-
COR)] or symmetrical dimer [(Ph3P)2Cu(SeCOR)2Cu(PPh3)2]
and hence we have investigated whether these compounds exist
in solution by 31P NMR studies. The VT 31P NMR spectra of
a 1:PPh3 mixture shown in Fig. 2 contain a signal at ꢁ1.51
(Dn1/2 ¼ 57 Hz) at 300 K. On cooling to 273 K, the signal
broadens and moves slightly to higher field. At 250 K two
more signals at d 1.91 and ꢁ4.75 appear. The peak at ꢁ2.67
ppm starts broadening and is shifted to ꢁ1.76 at 230 K and
almost disappears at 200 K while the other two signals are
positioned at 2.07 and ꢁ5.47 respectively.
ð1Þ
Several attempts to get a compound with a 1:1 ratio of
Cu(I):PPh3 using [(Ph3P)CuCl] in the same way only gave
the unsymmetrical dimer compound. It is rather surprising
that the selenocarboxylate ligands behave differently from
the thiocarboxylato ligands toward Cu(I) in the presence of
PPh3 . Further, the substitution (whether it is alkyl or aryl)
has no effect on the end product formation. The color of the
compounds varies from dark brown to pale yellow. The com-
pounds are soluble in CH2Cl2 , CHCl3 and THF, but insoluble
in MeCN and MeOH. These aromatic selenocarboxylate com-
plexes are thermally stable in air at room temperature for more
than a month without any observable decomposition except 6,
which can be stored for a long time in the refrigerator below
0 ꢃC.
The carbonyl stretching frequencies of complexes 1–6 which
appear in the region 1617–1657 cmꢁ1 have higher wavenum-
bers than those of the corresponding alkali-metal salts
(1500–1560 cmꢁ1 18,19
but are lower than the corresponding
)
By comparing the chemical shifts, the shape and intensities
of the signals at 2.07 and ꢁ5.47 ppm, it is tempting to conclude
that these are due to the presence of [(Ph3P)2Cu(SeCOPh)2-
Cu(PPh3)], 1, as in Fig. 1 and the signal between these two che-
mical shifts may be due to exchange between 1 and PPh3 at
higher temperature, which is still present at 200 K. However
there is no signal observed in the 31P NMR spectra due to free
PPh3 to support this assignment. The signal at ꢁ5.47 ppm is
relatively sharp as compared to the other signals suggesting
that it may be due to free PPh3 . The peak at d ¼ 2.07 in
Fig. 1 may then be assigned to the symmetrical dimer with
one PPh3 attached to each Cu implying that dissociation
occurred in solution. The relative broadening of this peak
may be due to the quadrupoles of the Cu isotopes. The signal
at ꢁ1.76 ppm at 230 K (in Fig. 2) could then be from the PPh3
attached to tetrahedral Cu(I) of the unsymmetrical dimer. The
reason that it broadens at 200 K is because the exchange of
PPh3 between the inequivalent ends is slowing. However, there
is no evidence observed for the presence of excess PPh3 in
CH2Cl2 solution containing 1 and PPh3 in the ESI-MS experi-
ments and hence this alternative assignment may be ruled out.
The structures of representative compounds 2 and 6 have
been determined by single crystal X-ray diffraction methods
and are described in detail below.
Se-alkyl and aryl esters (1660–1720 cmꢁ1).20,21 Further, they
occur at higher frequencies than in the corresponding thiocar-
boxylates (1553 cmꢁ1).4 However, these fall in the same region,
1610–1645 cmꢁ1
,
observed in [M(SeC{O}R)2(PR03)2]
(M ¼ Ni, Pt, Pd; R ¼ Me, Ph, 2-MeC6H4 , 4-MeC6H4 ;
R0 ¼ Et, Ph).9
VT 31P NMR studies in solution
Variable temperature 31P NMR spectra of 1 are displayed in
Fig. 1. Compound 1 gives a singlet at ꢁ0.66 ppm (Dn1/2
¼
83 Hz), at room temperature, which is as a result of rapid
exchange of PPh3 ligands in CD2Cl2 solution. When the solu-
tion is cooled to 273 K the signal is very broad indicating that
Structure of 2
A perspective view of the molecule is illustrated in Fig. 3.
Selected bond distance and angles are given in Table 1. The
dimer contains one three- and one four-coordinate Cu(I) atom
which are bridged by selenium atoms of two p-methylbenzene-
carboselenoate ligands. The geometry of Cu(1) is distorted tet-
rahedral with two PPh3 ligands attached, while Cu(2) has a
Fig. 1 Variable temperature 31P NMR of 1 in CD2Cl2 solution.
New J. Chem., 2002, 26, 1122–1129
1123

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Fig. 3 Thermal ellipsoid (50% probability) diagram of 2 in which
only the ipso C atoms of the phenyl rings are shown. The hydrogen
atoms are omitted for clarity.
trigonal planar geometry with a PPh3 ligand bnded to it. The
angles around Cu(2) sum up to 359.7(1)ꢃ thus confirming the
planar geometry. The tetrahedral angle around Cu(1) varies
from 98.42(4) to 118.24(4)ꢃ. The structure of 2 is isomorphous
and isostructural to the sulfur analogue.4
Fig. 4 The molecular structure of 6 showing the C–Hꢀ ꢀ ꢀO interac-
tion. Only the ipso C atoms of the phenyl rings are shown for clarity.
˚
shorter than the sum of the van der Waals radii of Cu(I), 2.8
26
A. Further, the two p-methylbenzenecarboselenoato ligands
The Cu(I)–P distances, 2.290(1) and 2.284(1) A, are longer
˚
than the Cu(2)–P(2) distance, 2.241(2) A. The observed differ-
˚
adopt anti stereochemistry in the Cu2Se2 ring. The C–
ence is due to the tetrahedral and trigonal planar geometries
around the Cu(1) and Cu(2) centers respectively. The same is
true for Cu–Se distances also. Cu–Se distances of 2.5390(8)
A and 2.5088(8) A at the tetrahedral Cu(1) atom are also
longer than the Cu(2)–Se(1) and Cu(2)–Se(2) distances
˚
(2.4108(8) and 2.4277(8) A respectively) at the trigonal Cu(2)
=
C{ O}Se plane and the benzene ring are twisted by 3.8(4)
and 9.0(3)ꢃ in the two ligands which are very normal.3,5,6
The structure of 2 in general is quite similar to the sulfur ana-
logue [(Ph3P)Cu(m-SC{O}Ph)2Cu(PPh3)2]4 reported by Speier
et al.
˚
˚
center. The disparity in Cu–Se distances due to tetrahedral
and trigonal geometries is also found in [Ph3Cu(m-SePh)2-
Structure of 6
˚
Cu(PPh3)2]ꢀCH3CN (tetrahedral 2.617(1) and 2.482(1) A and
23
trigonal 2.406(1) and 2.416(1) A). 22 The average Cu(I)–Se dis-
˚
A perspective view of 6 is shown in Fig. 4. Selected bond dis-
tance and angles are given in Table 2. The dimer contains one
three- and one four-coordinate Cu(I) atom which are bridged
by selenium atoms of two selenoacetato ligands similar to 2.
The Cu(I)–P and Cu(I)–Se distances are comparable to those
observed in 2 and other related compounds.22–25 The effect
of the substitution on the Se atom is not pronounced.
There appears to be a C–Hꢀ ꢀ ꢀO hydrogen bond between one
of the methyl hydrogen atoms and the oxygen atom of the
CH3C{O}Seꢁ ligand. The hydrogen bonding parameters are:
˚
tance found in [(Ph3P)2CuIn(SeEt)4] is 2.526 A. The Cu–Se
distances around both the tetrahedral and trigonal copper
˚
atoms are longer than 2.37 A in (Ph4P)2[Cu4(Se4)2.4(Se5)0.6
]
24,25
˚
and 2.325 A in [(Ph4P)4(Cu2Se14)].
The four-membered Cu2Se2 ring is strictly planar
(ScCu2Se2 ¼ 359.99ꢃ, with the mean deviation of the plane
˚
0.0032 A) and approximately assumes the shape of a parallelo-
gram. The Cu2Se2P3 core has approximate C2 symmetry with
the C2-axis passing through Cu(1), Cu(2) and P(3). The
Cu(1)–Se(1)–Cu(2) and Cu(1)–Se(2)–Cu(2) angles, 64.16(2)
and 63.39(2)ꢃ respectively, are comparable to other similar
angles (range, 65.91(3)–67.95(3)ꢃ) observed in the literature.22
The Se–Cu–Se angles (111.86(3) and 119.58(3)ꢃ) are obtuse
and this naturally brings the two Cu atoms closer together.
˚
˚
H(4b)ꢀ ꢀ ꢀO(1) 2.54 A, C(4)ꢀ ꢀ ꢀO(1) 3.480(6) A and C(4)–
H(4b)ꢀ ꢀ ꢀO(1) 168ꢃ. Such C–Hꢀ ꢀ ꢀO interactions have been well
documented by Steiner and Desiraju27,28 and help to influence
ꢃ
˚
Table 2 Selected bond lengths (A) and angles ( ) for compound 6
˚
The Cu(1)ꢀ ꢀ ꢀCu(2) distance, 2.6311(8) A, observed here is simi-
4
˚
lar to 2.62 A in [(Ph3P)Cu(m-SC{O}Ph)2Cu(PPh3)2] and
Bond lengths
Cu(1)–P(1)
Cu(1)–P(2)
Cu(2)–P(3)
Se(1)–Cu(1)
Se(2)–Cu(1)
Se(1)–Cu(2)
2.2983(7) Se(2)–Cu(2)
2.2761(7) Se(1)–C(1)
2.2416(8) Se(2)–C(3)
2.5482(4) C(1)–O(1)
2.5358(4) C(3)–O(2)
2.4278(4) Cu(1)–Cu(2)
2.4339(4)
1.929(3)
1.956(3)
1.200(4)
1.201(4)
2.7043(4)
ꢃ
˚
Table 1 Selected bond lengths (A) and angles ( ) for 2
Bond Lengths
Cu(1)–P(1)
Cu(1)–P(2)
Cu(2)–P(3)
Cu(1)–Se(1)
Cu(1)–Se(2)
Cu(2)–Se(1)
2.290(1)
2.284(1)
2.241(2)
Cu(2)–Se(2)
Se(1)–C(1)
Se(2)–C(9)
2.4277(8)
1.925(5)
1.932(5)
1.211(6)
1.203(6)
2.6311(8)
2.5390(8) O(1)–C(1)
2.5088(8) O(2)–C(9)
2.4108(8) Cu(1)–Cu(2)
Bond Angles
C(1)–Se(1)–Cu(2)
C(1)–Se(1)–Cu(1)
Cu(2)–Se(1)–Cu(1)
C(3)–Se(2)–Cu(2)
C(3)–Se(2)–Cu(1)
Cu(2)–Se(2)–Cu(1)
P(2)–Cu(1)–P(1)
P(2)–Cu(1)–Se(2)
99.4(1)
109.87(9)
65.79(1)
104.06(9)
108.24(9)
65.90(1)
114.67(3)
110.30(2)
P(1)–Cu(1)–Se(2)
105.52(2)
P(2)–Cu(1)–Se(1) 118.51(2)
P(1)–Cu(1)–Se(1) 97.52(2)
Se(2)–Cu(1)–Se(1) 109.04(1)
Bond Angles
P(2)–Cu(1)–P(1)
P(2)–Cu(1)–Se(2)
P(1)–Cu(1)–Se(2)
P(2)–Cu(1)–Se(1)
P(1)–Cu(1)–Se(1)
116.25(5)
Se(1)–Cu(2)–Se(2)
119.58(3)
P(3)–Cu(2)–Se(1)
P(3)–Cu(2)–Se(2)
Se(1)–Cu(2)–Se(2) 116.77(2)
121.63(2)
121.59(2)
98.42(4)
113.14(4)
118.24(4)
99.64(4)
C(1)–Se(1)–Cu(2)
C(1)–Se(1)–Cu(1)
Cu(2)–Se(1)–Cu(1)
C(9)–Se(2)–Cu(2)
C(9)–Se(2)–Cu(1)
Cu(2)–Se(2)–Cu(1)
93.2(2)
109.2(2)
64.16(2)
100.8(2)
104.2(2)
64.39(2)
Se(2)–Cu(1)–Se(1) 111.86(3)
P(3)–Cu(2)–Se(1)
P(3)–Cu(2)–Se(2)
Hydrogen Bond
H(4b)ꢀ ꢀ ꢀO(1)
127.04(5)
113.06(5)
2.54
C(4)ꢀ ꢀ ꢀO(1)
3.480(6)
C(4)–H(4b)ꢀ ꢀ ꢀO(1) 168
1124
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Fig. 6 TGA–DTA of 1.
Fig. 5 Photoluminescence spectra of 1 in CH2Cl2 , excited at l ¼ 280
nm and 350 nm.
Thermal decomposition studies
Thermal analysis experiments were carried out in an N2 atmo-
sphere to prevent the oxidation of the sample. Typical TGA–
DTA curves are shown in Fig. 6 and 7 respectively for 1 and
6. The results have been complied in Table 4.
All the compounds show only endotherms in DTA indicat-
ing that the decomposition of these compounds at any stage is
an endothermic process. The first endotherms in the range
149–174 ꢃC are due to melting followed by decomposition.
Both the DTA peak temperature and inception temperature
of TG weight loss indicate that the alkyl derivative 6 is ther-
mally less stable than the aromatic derivatives. Furthermore,
there is no special trend found due to substituent effects at
the para position.
Although the TG curves show only a single unresolved step
of the weight loss in the temperature region 140–325 ꢃC, the
corresponding DTA curves show two endotherms. This indi-
cates the complexity of the thermal decomposition process.
However it is clear that energy is needed to break the bonds
in these complexes. The percentage of the residual weight at
the end of the thermal decomposition process (inception of
plateau after ca. 300–325 ꢃC) for each compound matches
fairly well with the percentage weight of the residue calculated
for the formation of Cu2Se/Cu2ꢁxSe (Table 4). The side pro-
ducts of decomposition possibly include Ph3P, Ph3PSe, and
(RCO)2Se, based on the assumption that the selenocarboxy-
lates decompose very similarly to thiocarboxylates as discussed
by Hampden-Smith and coworkers.2
the packing in the solid state. The Cꢀ ꢀ ꢀO distance observed in
29
˚
chloroalkyl compounds ranges from 3.32 to 3.59 A, and the
range of Hꢀ ꢀ ꢀO is from 2.0 to 2.8 A. The hydrogen bonding
˚
parameters found in 6 fall in the normal range of C–Hꢀ ꢀ ꢀO
bonding. This weak interaction seems to control the stereo-
chemistry in the Cu2Se2 core. First, the two selenoacetato
ligands have syn geometry instread of the anti configuration
found in 2. Secondly, the Cu2Se2 ring is no longer planar
˚
unlike 2 and has a butterfly conformation. Se(1) is 0.49 A
˚
above the Cu(1)–Cu(2)–Se(2) plane and Se(2) is 0.49 A above
the Cu(1)–Cu(2)–Se(1) plane. In other words, the interplanar
angle between the two Cu2Se planes bisecting the
Cu(1)ꢀ ꢀ ꢀCu(2) axis is 12.7ꢃ. The Cu2Se2P3 core has an approx-
imate m symmetry with the mirror plane passing through
Cu(1) and Cu(2). As a consequence of the C–Hꢀ ꢀ ꢀO interac-
˚
tion, the Cu(1)ꢀ ꢀ ꢀCu(2) distance, 2.704 A, is found to be longer
than the value observed for compound 2.
Photoluminescence studies
Photoluminescence spectra were measured at room tempera-
ture in CH2Cl2 solution. The concentration of the solution is
10ꢁ5 M, and the solid-state experiment was performed by eva-
porating the solution on a quartz plate. Two excitation wave-
lengths, 280 and 350 nm, were used for the experiments in
solution.
A typical spectrum is shown in Fig. 5 for 1. Table 3 lists the
emission maxima observed for all the compounds in CH2Cl2
solution. For excitation at 280 nm, an emission signal at 481
nm is observed while at 350 nm an emission at 560 nm is
observed for 1. Similar shifts have been noted for other com-
pounds too. As seen in Table 3, all the compounds have
absorption peaks at 241–257 nm in the UV-Vis region and
emission in the range 481–492 nm (for 280 nm excitation)
and 560–564 nm (for 350 nm excitation). These may be attrib-
uted to MLCT transition.30
Pyrolysis experiment
Since the amounts used in thermal analysis experiment were in
milligram quantities, sufficient amounts needed for further
characterization of the final residues were obtained by the pyr-
olysis method. In this method, abut 300 mg were heated at
300 ꢃC and 0.5 Torr pressure for 30 min and the final products
Table 3 UV-Vis and photoluminescence spectra for compounds 1–5
l
em/nm
l
abs/nm
(e ꢄ 10ꢁ4
/
Excited at Excited at
350 nm
Medium (T/K) ml molꢁ1 cmꢁ1
)
280 nm
481
1
2
3
4
5
CH2Cl2 (298)
CH2Cl2 (298)
CH2Cl2 (298)
CH2Cl2 (298)
CH2Cl2 (298)
245 (4.30)
560
560
564
560
561
241 (2.97), 259 (2.97) 486
241 (2.66), 264 (2.94) 487
257 (4.84)
244 (3.44)
492
483
Fig. 7 TGA–DTA of 6.
New J. Chem., 2002, 26, 1122–1129
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Table 4 TGA–DTA and pyrolysis results for compounds 1–6
Thermogravimetry analysis
Pyrolysis experimenta
Residual weight (%)
Temperature
Compounds range/ꢃC
Calculated Weight of Phase of
Observed for Cu2Se the residue (%) residue
ICP result for
the value of x JCPDS No.
DTA/ꢃCb
1
2
3
4
5
6
153–310
154–300
165–317
159–325
160–300
141–287
174(ꢁ), 273(ꢁ), 312(ꢁ) 17.1
183(ꢁ), 272(ꢁ), 306(ꢁ) 16.5
196(ꢁ), 280(ꢁ), 303(ꢁ) 18.3
183(ꢁ), 295(ꢁ), 395(ꢁ) 17.2
195(ꢁ), 269(ꢁ), 306(ꢁ) 17.0
149(ꢁ), 257(ꢁ), 286(ꢁ) 18.4
16.1
15.7
15.4
15.3
15.6
17.8
17.0
16.8
16.0
15.7
16.1
16.2
Cubic Cu2Se
Cubic Cu2ꢁxSe
Cubic Cu2Se
Cubic Cu2ꢁxSe
Cubic Cu2Se
Cubic Cu2ꢁxSe
Cubic Cu2Se
Cubic Cu2ꢁxSe
Cubic Cu2Se
Cubic Cu2ꢁxSe
Cubic Cu2Se
Cubic Cu2ꢁxSe
0.10
0.24
0.17
0.10
0.13
0.16
04-0839
06-0680
04-0839
06-0680
04-0839
06-0680
04-0839
06-0680
04-0839
06-0680
04-0839
06-0680
a
b
The samples were heated at 300 ꢃC under 0.5 Torr pressure for 30 minutes. (+) Exthothermic, (ꢁ) endothermic.
were weighed. Again the weight of the residue in the pyrolysis
method matched well with the expected residue weight calcu-
lated for the formation of copper selenides.
particle size distribution of Cu2Se and Cu2ꢁxSe is not clear
at present. This interesting observation will certainly fuel
further investigation.
Characterization of the final products
Cu2ꢁxSe nanoparticles preparation
X-Ray powder diffraction analysis was carried out on the resi-
dues obtained in the pyrolysis experiments. The results are
shown in Table 4. The products are highly crystalline giving
sharp intense signals. The XRD patterns (see ESIy) were com-
pared with known copper selenide patterns from the database
and the JCPDS numbers of matched phases are shown in
Table 4. The entire residue has been found to be a mixture
of the cubic phase of Cu2Se (JCPDS No. 04-0839) and
Cu2ꢁxSe (Berzelianite, JCPDS No. 06-0680) compounds. In
the non-stoichiometric Cu2ꢁxSe compounds, the ratio of x
was experimentally determined to be 0.1–0.24 by ICP experi-
ments. The results are again tabulated in Table 4. There is
no apparent difference in the nature of the final products due
to substituents at the para position of the benzene rings.
SEM images (see ESIy) of the residues were obtained to find
the nature of the final products as well as to investigate the
influence of the substituents on the particle size. Particles with
defined faces as big as 100 mm are seen in 1 and 2. However the
distribution of the particle sizes is not uniform. It varies from 1
mm to 100 mm in 1, 2, 3 and 6. SEM images of the residues
obtained from 4 and 5 show that they fall in a very narrow par-
ticle size distribution (1–20 mm) and the average particle size is
ꢅ5 mm. These samples were obtained from F and Cl substi-
tuted precursors. The exact effect of these substituents on the
Trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO)
capped Cu2ꢁxSe nanoparticles were prepared by dissolving 1
in TOP and injecting into hot TOPO for thermal decomposi-
tion to obtain a dark green powder. XRPD and TEM were
used to characterize the Cu2ꢁxSe nanoparticles. Fig. 8 shows
the X-ray diffraction pattern of the product, which has been
indexed as cubic Cu2ꢁxSe (06-0680). The TEM image is dis-
played in Fig. 9. The average particle size was found to be
75 ꢂ 15 nm as calculated from the TEM images of more than
70 particles which showed particle sizes ranging from 40 to
100 nm (Fig. 10).
Summary
In this report the synthesis of a number of neutral dimers,
[(Ph3P)2Cu(m-SeC{O}R)2Cu(PPh3)], has been described. The
chemistry of the Ph3P:Cu(I):RC{O}Seꢁ system is found to be
different from that of the corresponding sulfur analogue. In
the corresponding thiocarboxylate system, a number of com-
pounds with varying nuclearity and bonding modes have been
observed. However, the unsymmetrical neutral dimer is the
Fig. 8 XRPD for TOP/TOPO capped Cu2ꢁxSe nanoparticles.
Fig. 9 TEM for TOP/TOPO capped Cu2ꢁxSe nanoparticles.
1126
New J. Chem., 2002, 26, 1122–1129

View Article Online
1070, 1026, 998, 927, 878, 768, 742, 689, 670, 624, 517, 487,
435, 419. UV-Vis lmax(log e) ¼ 244 (4.63) nm.
[Cu2(l-SeC{O}C6H4-CH3-4)2(PPh3)3] 2. Dark brown crys-
tals. Yield: 56%. Anal. Calcd. for C70H59Cu2O2P3Se2 (mol
wt 1310.17): C, 64.17; H, 4.54. Found: C, 63.87; H, 5.21%.
1H NMR (CD2CL2): d/ppm. for p-methyselenobenzoato
ligand: 2.23 (6H, CH3), 6.93 (4H, m), 7.60 (4H, o); for PPh3 :
7.08–7.31 (45H). 13C NMR (CDCl3): d/ppm. For p-methysele-
nobenzoato ligand: 21.37 (CH3), 127.80 (C2/6 or C3/5), 128.72
(C2/6 or C3/5), 140.71 (C1), 141.32 (C4), 201.97 (COSe). For
PPh3 : 128.19 (C3 , 3J(P–C) ¼ 8.7 Hz), 129.24 (C4), 133.29
(C1 , 1J(P–C) ¼ 28.4 Hz), 133.91 (C2 , 2J(P–C) ¼ 15.3 Hz).
31P NMR (CDCl3): d/ppm. ꢁ0.69. IR data: cmꢁ1. 3051,
=
1623(C O), 1609, 1597, 1568, 1479, 1457, 1434, 1404, 1303,
1288, 1212, 1194, 1162, 1094, 1070, 1027, 997, 882, 825, 784,
742, 694, 612, 564, 541, 514, 504, 493, 467, 418. UV-Vis
Fig. 10 Histogram of particle sizes of Cu2ꢁxSe nanoparticles from 1.
lmax(log e) ¼ 260 (4.47), 242 (4.48) nm.
only product obtained by changing the Ph3P:Cu(SeC{O}R)
ratios during the preparation. Variable temperature 31P
NMR indicates that the phosphine ligands undergo fast
exchange in solution above 270 K. It appears that the unsym-
metrical dimer dissociates in solution but the Cu2Se2 core is
intact. Weak C–Hꢀ ꢀ ꢀO hydrogen bonding is found to dictate
the sterochemistry of the RCO groups in the Cu2Se2 rings.
These compounds are photoemissive in the region 480–492
nm when excited at 280 nm and ꢅ560 nm when excited at
350 nm in CH2Cl2 solution. Pyrolysis and TG experiments sug-
gest that these neutral dimers can be good single source mole-
cular precursors to Cu2Se/Cu2ꢁxSe bulk materials.
Preliminary experiments show that [(Ph3P)2Cu(m-SeC{O}Ph)2-
Cu(PPh3)] can produce nanoparticles.
[Cu2(l-SeC{O}C6H4-OCH3-4)2(PPh3)3] 3. Greenish yellow
crystals. Yield: 58%. Anal. Calcd. for C70H59Cu2P3Se2 (mol
wt 1342.17): C, 62.64; H, 4.43. Found: C, 62.76; H, 4.50%.
1H NMR d/ppm. (CD2Cl2): For p-methoxyselenobenzoato
ligand: 3.77 (6H, OCH3), 6.63 (4H, m), 7.72 (4H, o); for
PPh3 : 7.12–7.39 (45H). 13C NMR (CDCl3): d/ppm. For p-
methoxyselenobenzoato ligand: 55.20 (OCH3), 112.26 (C3/5),
130.94 (C2/6), 132.06 (C1), 162.26 (C4). For PPh3 : 128.28
(C3 , 3J(P–C) ¼ 8.7 Hz), 129.36 (C4), 133.03 (C1 , 1J(P–
C) ¼ 28.4 Hz), 133.85 (C2 , 2J(P–C) ¼ 15.3 Hz). 31P NMR
(CD2Cl2): d/ppm. ꢁ1.29. IR data: cmꢁ1. 3051, 3000, 2834,
=
1617(C O), 1610, 1592, 1569, 1500, 1479, 1459, 1434, 1412,
1307, 1261, 1246, 1199, 1180, 1155, 1109, 1093, 1070, 1036,
1024, 997, 971, 882, 840, 810, 788, 782, 743, 694, 645, 624,
611, 513, 503, 487, 450, 418. UV-Vis lmax (log e) ¼ 266
(4.47), 241 (4.43) nm.
Experimental
[Cu2(l-SeC{O}C6H4-Ph-Cl-4)2(PPh3)3] 4. Brown crystals.
Yield: 44%. Anal. Calcd. for C68H53Cu2O2P3Se2Cl2 (mol wt
1351.01): C, 60.45; H, 3.95; Cl: 5.25. Found: C, 59.94; H,
3.91; Cl: 5.25%. 1H NMR (CD2Cl2): d/ppm. For p-chlorosele-
nobenzoato ligand: 7.07 (4H, m), 7.59 (4H, o); for PPh3 : 7.12–
7.37 (45H). 13C NMR (CDCl3): d/ppm. For p-chloroseleno-
benzoato ligand: 127.15 (C3/5), 129.88 (C2/6), 137.19 (C1),
141.25 (C4), 200.37 (COSe). For PPh3 : 128.18 (C3 , 3J(P–
C) ¼ 9.8 Hz), 129.32 (C4), 133.11 (C1 , 1J(P–C) ¼ 29.4 Hz),
133.87 (C2 , 2J(P–C) ¼ 14.2 Hz). 31P NMR (CD2Cl2): d/
Sodium, selenium (gray), naphthalene powder, acyl chloride,
copper nitrate, triphenylphosphine, trioctylphosphine oxide
(TOPO, 90%), trioctylphosphine (TOP) were obtained com-
mercially and used as received. THF was dried by refluxing
with sodium metal using benzophenone as indicator and distil-
ling before use and the other solvents were dried by allowing
˚
them to stand over 3 A molecular sieves overnight.
Synthesis
ppm. ꢁ0.63. IR data: cmꢁ1. 3052, 3003, 1629(C O), 1600,
=
The synthetic procedure described in detail for 1 was also used
to prepare compounds 2 to 6.
1582, 1567, 1479, 1434, 1393, 1326, 1306, 1277, 1187, 1158,
1093, 1027, 1012, 998, 973, 877, 838, 743, 723, 693, 628, 617,
551, 517, 487, 424. UV-Vis lmax (log e) ¼ 258 (4.69) nm.
[Cu2(l-SeC{O}Ph)2(PPh3)3] 1. Sodium selenocarboxylate
was prepared in situ by the literature method as follows.31 In
a N2 flushed 2-necked flask Na2Se (0.150 g, 1.2 mmol) and
benzoyl chloride (0.093 mL, 0.8 mmol) were mixed in MeCN
(15 mL) at 0 ꢃC and stirred for an hour. Then the mixture
was filtered and to the filtrate a solution of (Ph3P)2CuNO3
(0.455 g, 0.7 mmol) in MeCN (10 mL) was added, and stirred
for 30 min. The orange precipitate formed was filtered, washed
with MeCN, H2O and Et2O, and recrystallized from a mixture
of CH2Cl2 and Et2O. Yield: 0.43 g (48%). Anal. Calcd. for
C68H55Cu2O2P3Se2 (mol wt 1282.12): C, 63.70; H, 4.32.
[Cu2(l-SeC{O}C6H4-F-4)2(PPh3)3] 5. Bright yellow crystals.
Yield: 63%. Anal. Calcd. for C68H53Cu2O2P3Se2F2 (mol wt
1318.10): C, 61.96; H, 4.05; F: 2.88. Found: C, 61.84; H,
3.68; F: 2.80%. 1H NMR (CD2Cl2) d/ppm. For p-fluoroseleno-
benzoato ligand: 6.79 (4H, m), 7.69 (4H, o); for PPh3 : 7.12–
7.38 (45H). 13C NMR (CDCl3): d/ppm. for p-fluoroselenoben-
zoato ligand: 113.78 (C3/5 , J ¼ 21.8 Hz), 130.92 (C2/6
,
J ¼ 8.7 Hz), 139.34 (C1), 164.74 (C4, J ¼ 251.9 Hz), 200.08
(COSe). For PPh3 : 128.18 (C3 , 3J(P–C) ¼ 9.8 Hz), 129.30
(C4), 133.19 (C1 , 1J(P–C) ¼ 28.4 Hz), 133.89 (C2 , 2J(P–
C) ¼ 14.0 Hz). 31P NMR (CD2Cl2): d/ppm. ꢁ0.73. IR data:
1
Found: C, 63.74; H, 3.96%. H NMR (CD2Cl2): d/ppm. For
selenobenzoato ligand: 7.67 (4H, o); 7.09–7.29 (51H, PPh3
(45H), 4H, o, 2H, p). 13C NMR (CDCl3): d/ppm. For seleno-
benzoato ligand: 127.12 (C2/6 or C3/5), 128.61 (C2/6 or C3/5),
131.05 (C4), 142.94 (C1), 201.50 (COSe). For PPh3 : 128.15
(C3 , 3J(P–C) ¼ 8.7 Hz), 129.30 (C4), 133.14 (C1 , 1J(P–
C) ¼ 28.4 Hz), 133.95 (C2 , 2J(P–C) ¼ 14.2 Hz). 31P NMR
cmꢁ1. 3053, 2923, 1624(C O), 1612, 1587, 1496, 1480, 1434,
=
1403, 1312, 1288, 1221, 1183, 1148, 1095, 1028, 998, 886,
843, 803, 745, 694, 637, 610, 518, 493, 463, 434. UV-Vis lmax
(log e) ¼ 244(4.54) nm.
[Cu2(l-SeC{O}CH3)2(PPh3)3] 6. Grey crystals. Yield: 30%.
Anal. Calcd. for C58H51Cu2O2P3Se2 (mol wt 1157.98): C,
60.16; H, 4.44. Found: C, 59.43; H, 3.97%. 1H NMR (CD2Cl2):
(CD2Cl2): d/ppm. ꢁ0.63. IR data: cmꢁ1. 3051, 1625(C O),
=
1608, 1592, 1572, 1479, 1443, 1434, 1305, 1189, 1164, 1094,
New J. Chem., 2002, 26, 1122–1129
1127

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d/ppm 1.94 (6H, CH3), 7.21–7.40 (45H, (C6H5)3P). 13C NMR
(CDCl3): d/ppm 40.42 (CH3 , J ¼ 14.2 Hz), 204.40 (COSe).
For PPh3 : 128.26 (C3 , 3J(P–C) ¼ 9.8 Hz), 129.39 (C4),
Table 5 Crystal data and refinement details
2
6
1
2
133.30 (C1 , J(P–C) ¼ 27.3 Hz), 133.92 (C2 , J(P–C) ¼ 15.3
Chemical formula
Formula weight
T/K
C
70H59Cu2O2P3Se2
C58H51Cu2O2P3Se2
1157.90
293(2)
Hz). 31P NMR (CD2Cl2): d/ppm. ꢁ1.51. IR data: cmꢁ1
.
1310.08
293(2)
Triclinic
=
3050, 2923, 2854, 1682(C O), 1657, 1586, 1572, 1480, 1434,
1338, 1311, 1183, 1158, 1094, 1028, 998, 931, 850, 743, 694,
584, 565, 516, 431, 422.
Crystal system
Space group
Monoclinic
P21/c
¯
P1
˚
a/A
10.8966(1)
12.9988(2)
22.3564(3)
90.883(1)
101.145(1)
99.031(1)
3064.97(7)
2
19.4676(2)
14.5399(1)
20.0449(2)
˚
b/A
˚
c/A
Nanoparticle preparation
a/ꢃ
b/ꢃ
g/ꢃ
A typical preparation of TOP/TOPO capped metal selenides
has been reported in the literature32 and the procedure is
described as follows. The reaction vessel containing 1 g of
TOPO was heated and stabilized at 220–230 ꢃC and ambient
pressure under nitrogen. 1 (0.3 g) was dissolved in TOP (5
mL) using an ultrasonic bath, then loaded into a 10 mL syr-
inge, then quickly injected into the vigorously stirring hot
TOPO through a rubber septum. The stirring was continued
for an hour at 220–230 ꢃC and then the mixture was allowed
to cool down to 60–70 ꢃC. Methanol was added into the mix-
ture, and a flocculate was formed which was separated by cen-
trifugation. This residue was washed with MeOH 2–3 times
and dried in a vacuum desiccator. The sample for TEM was
prepared by placing a drop of a dilute toluene dispersion of
nanocrystallites on the surface of a grid, waiting for ꢅ1 min,
and then wicking away the solution.
111.690(1)
3
˚
V/A
5272.12(8)
4
Z
Dc/g cmꢁ3
m/mmꢁ1
1.420
2.004
15 098
1.459
2.320
26 569
Reflections
measured
Independent
reflections
Data/restraints/
parameters
GooF
10 055 (Rint ¼ 0.0272)
9260 (Rint ¼ 0.0260)
10 055/0/713
9260/0/607
1.121
0.988
Final R indices
R1 ¼ 0.0560,
wR2 ¼ 0.0872
R1 ¼ 0.0890,
wR2 ¼ 0.0988
R1 ¼ 0.0288,
wR2 ¼ 0.0695
R1 ¼ 0.0416,
wR2 ¼ 0.0769
R indices (all data)
Physico-chemical measurements
CCDC reference numbers 171911 and 171912. See http://
www.rsc.org/suppdata/nj/b2/b203466b/ for crystallographic
data in CIF or other electronic format.
The 13C{1H}, 1H and 31P{1H} NMR spectra were recorded on
a Bruker ACF300 FT-NMR instrument using TMS as internal
reference at 25 ꢃC in CD2Cl2 or CDCl3 . The IR spectra (KBr
pellet) were recorded using a Bio-Rad FT-IR spectrophot-
ometer. UV-Vis spectra were recorded using a Shimadzu
UV-2501PC model spectrophotometer in CH2Cl2 solution.
The elemental analyses were performed in the microanalytical
lab in the Department of Chemistry, National University of
Singapore. Photoluminescence spectra were recorded on a Per-
kin Elmer Luminescence Spectrometer LS50B–50 Hz.
Thermogravimetry analysis was recorded on a SDT 2960
Simultaneous DTA–TGA with a heating rate of 10 ꢃC minꢁ1
under N2 atmosphere. The pyrolysis experiment was carried
out in a self-designed apparatus with a heating rate of 10 ꢃC
minꢁ1 under a mild vacuum of about 0.5 Torr. XRPD was
recorded on a D5005 Bruker AXS X-ray diffractometer, with
a scanning angle range from 2 to 90ꢃ. SEM images were
recorded on a JEOL JSM-T220A Scanning Microscope, with
an accelerating voltage of 20 KV. The TEM image was taken
on a Jeol STEM CXII Electro Microscope using an accelerat-
ing voltage of 100 KV. Copper grids (200 mesh) coated with
amorphous carbon film were purchased from Solid Vision.
Acknowledgements
J. J. V. would like to thank NUS for financial support through
the grant R143-000-084-112. Dr. T. C. Deivaraj of this depart-
ment and Prof. Phil Dean from the University of Western
Ontario, London, Canada are thanked for their interest in this
project.
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1129