Selenophilicity of copper in selenium-carbon bond formation from selenous acid using Cu(II)/Sn(II) reagent

Article abstract of DOI:10.1080/10426500802040349

A selenium transfer reaction from selenous acid to benzyl and alkyl halides is initiated in the presence of stannous chloride and a catalytic amount of cupric chloride resulting in the formation of the corresponding diorganoselenides and/or diorganodiselenides as the major products as indicated by 1H, 13C, 77Se NMR, and MS. The reaction is characterized by a dual-metal effect at the selenium activation and transfer step. Thus, initial reaction of stannous chloride, cupric chloride, and selenous acid gives rise to α-Cu2Se. Selenium transfer from the latter to the organic halide takes place with additional assistance of stannous chloride. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500802040349

Phosphorus, Sulfur, and Silicon, 183:2943–2955, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802040349
Selenophilicity of Copper in Selenium-Carbon Bond
Formation from Selenous Acid Using Cu(II)/Sn(II) Reagent
Joyanta Choudhury,1 Pradipta Sinha,1 S. Prabhakar,2
M. Vairamani,2 and Sujit Roy1
1Organometallics and Catalysis Laboratory, Chemistry Department,
Indian Institute of Technology, Kharagpur, India
2National Centre for Mass Spectrometry, Indian Institute of Chemical
Technology, Hyderabad, India
A selenium transfer reaction from selenous acid to benzyl and alkyl halides is
initiated in the presence of stannous chloride and a catalytic amount of cupric
chloride resulting in the formation of the corresponding diorganoselenides and/or
1
13 77
diorganodiselenides as the major products as indicated by H, C, Se NMR, and
MS. The reaction is characterized by a dual-metal effect at the selenium activation
and transfer step. Thus, initial reaction of stannous chloride, cupric chloride, and
selenous acid gives rise to α-Cu Se. Selenium transfer from the latter to the organic
2
halide takes place with additional assistance of stannous chloride.
Keywords Bimetallic catalysis; Cu(II)/Sn(II); diorganoselenides; selenium transfer;
selenous acid
INTRODUCTION
By virtue of their electronic and opto-electronic properties, inorganic
selenides are key targets for a large number of investigations in ma-
terials science and in nano-chemistry.1 At the same time, their merit
as synthons or reactive intermediates in organic synthesis remains un-
abated. Metal organoselenolates RSeM are of interest in areas such as
ligand-assisted cluster design, molecular precursors for new materials,
and as selenium transfer reagents.2−4
Received 21 November 2007; accepted 8 March 2008.
We thank Dr. Raja Roy and Mr. R. K. Grover (CDRI, Lucknow, India) for generous
help with 77Se NMR. This work is supported by the CSIR and the DST. The research
fellowship to P. S. and J. C. (UGC) is acknowledged.
Address correspondence to Sujit Roy, Organometallics and Catalysis Laboratory,
Chemistry Department, Indian Institute of Technology, Kharagpur 721302, India.
E-mail: sroy@chem.iitkgp.ernet.in
2943

2944
J. Choudhury et al.
Diorgano(poly)selenides (RSenR, RSenR’) are an important class
of compounds in organic chemistry and are widely used as precur-
sors for various organic transformations. Major selenium transfer
routes to diorganoselenides include the reaction of organic halides,
esters or synthons thereof with (a) metal selenides/polyselenides,5
(b) metal organoselenolates (RSeM),6 and (c) mixed metal selenium
hydrides.7 Other conceptually different strategies involve (a) the reac-
tion of an organometallic reagent with metallic selenium (elemental
grey selenium) or a diorganodiselenide;8 (b) selenium transfer from
N-seleno derivatives;9 and (c) organoselenium transfer using novel se-
lenotungstate and molybdenum-sulfur reagent.10
The use of selenous acid (H2SeO3) and selenium dioxide (SeO2) in or-
ganic synthesis is so far limited to oxidation reactions alone. Examples
include oxidation of ketones, aldehydes, quinolines and allylic hydroxy-
lations of alkenes.11,12 Herein, we present a selenium transfer reaction
directly from selenous acid to benzyl and activated alkyl halides as
organic electrophiles supported by Cu(II)/Sn(II) reagent resulting in
the formation of diorganoselenides/diselenides as the major product.
Mechanistic studies indicate the distinct role of copper, and the initial
formation of a copper(I) selenide, namely α-Cu2Se. Selenium transfer
from the latter to the organic halide takes place with additional assis-
tance of stannous chloride.
RESULTS AND DISCUSSION
Organic Reactivity
Treatment of selenous acid (1 mmol) with stannous chloride dihydrate
(3.2 mmol), 4-methylbenzyl iodide 1a (1 mmol) and a catalytic amount
of cupric chloride dihydrate (0.3 mmol, 30 mol% with respect to selenous
acid) in THF-DMSO (1:2, 3 mL) under argon at ambient temperature
initially resulted in a brown colored suspension. The suspension turned
into a clear yellow solution within 15 h. Aqueous work up afforded the
corresponding ditolyl diselenide 2d as the major product (85% accord-
1
ing to H NMR). The minor products are ditolylselenide 2m and the
triselenide 2t (Scheme 1, Table I). With respect to reaction time and
yields, THF-DMSO resulted as the best solvent as compared to DCM,
THF, MeCN, DMSO, DCM-DMSO, and MeCN-DMSO.
Similar reactions of selenous acid with benzyl bromide, 4-
chlorobenzyl iodide, and 4-nitrobenzyl bromide 1b–1d give the corre-
sponding diorganoselenides 3m–5m and diselenides 3d–5d in varying
ratios (Scheme 1, Table I). Minute traces of RSe3R (3t–5t) could be
1
detected in the H NMR spectrum of the product mixture in certain

Selenophilicity of Copper
2945
R
H2SeO3
X
(Sen)
SnCl2, CuCl2 (cat)
THF-DMSO (1:2)
ambient temp.
R
R
2m (n=1), 2d (n=2), 2t (n=3): R = Me
3m (n=1), 3d (n=2), 3t (n=3): R = H
4m (n=1), 4d (n=2), 4t (n=3): R = Cl
1a: R = Me, X = I
1b: R = H, X = Br
1c: R = Cl, X = I
1d: R = NO2, X = Br
5m (n=1), 5d (n=2), 5t (n=3): R = NO2
SCHEME 1
cases (Scheme 1, Table I). Interestingly, the reaction of 2-nitrobenzyl
bromide 1e led to the formation of the corresponding diorganodise-
lenide 6d and triselenide 6t in a 85:15 ratio (Scheme 2, Table I).
Similarly, 2-carbomethoxybenzyl bromide 1f gave the corresponding
di- and triselenide 7d and 7t in a 86:14 ratio. The coordinating ability
of -NO2 and -CO2Me groups might be influencing the stabilization of
triselenide linkage.13Intramolecular selenium coupling reaction takes
place with α,αꢀ-dibromo-o-xylene 1g yielding 1,4-dihydro-2,3-diselena-
naphthalene 8d (Scheme 2, Table I).
TABLE I Formation of Diorgano Selenides R-(Se)n-R (n = 1, 2, 3) from
the Reaction of Selenous Acid and an Organic Halide (RX) (Schemes
1 and 2)a
R
Products
Product Ratiob
Total Se Conversion (%)c
4-Me-C6H4CH2-
C6H5CH2-
4-Cl-C6H4CH2-
4-NO2-C6H4CH2-
2-NO2-C6H4CH2-
2-MeOC(O)-C6H4CH2-
1,2-[-CH2-C6H4-CH2-]
2m:2d:2t
3m:3d:3t
4m:4d:4t
5m:5d:5t
6d:6t
10:85:5
50:45:5
62:34:4
20:75:5
85:15
37
52
57
41
30
43
38
7d:7t
8d
86:14
100
aReaction conditions: RX (1 mmol), H2SeO3 (1 mmol), SnCl2·2 H2O (3.2 mmol),
CuCl2·2 H2O (0.3 mmol), THF-DMSO (1:2, 3 mL), ambient temp., 15 h.b The ratio is
based on 1H NMR of the product mixture.cBased on isolated yield of the
product/product mixture from two reactions (accuracy 5%).

2946
J. Choudhury et al.
R
Se
Se
6d: R = NO2
7d: R = CO2Me
R
H2SeO3
SnCl2, CuCl2 (cat)
Br
+
R
Se
Se Se
1e: R = NO2
1f: R = CO2Me
R
R
6t: R = NO2
7t: R = CO2Me
H2SeO3
SnCl2, CuCl2 (cat)
Br
Br
Se
Se
8d
1g
SCHEME 2
Spectroscopic Characterization
1H (200 MHz), 13C (50 MHz), and 77Se NMR (52.7 MHz) spectra of the
organoselenides were recorded in CDCl3 as solvent (Table II). Along
with the chemical shifts in 77Se NMR, the number of signals i.e. for
monoselenide and diselenide: one signal, and for triselenide: two sig-
nals with a ratio of 1:2, is used as a clear diagnostic tool. The 77Se
chemical shifts of RSenR (n = 1, 2, 3) range between 200 and 650
ppm, and shift to lower field with increasing n-value. The signals for
the α-hydrogen and α-carbon atoms are observed at 3.0−4.5 and at
TABLE II Selected 1H, 13C, and 77Se NMR chemical shifts (ppm) of
R-(Se)n-R (n = 1, 2, 3)
δ1H of Se-CH2-
δ
13C of Se-C
Se2 Se3
δ
77Sea
R
Se Se2 Se3
Se
Se
Se2
Se3
4-Me-C6H4CH2-
C6H5CH2-
4-Cl-C6H4CH2-
4-NO2-C6H4CH2-
2-NO2-C6H4CH2-
2-MeOC(O)-C6H4CH2-
1,2-[-CH2-C6H4-CH2-]
3.60 3.77 4.17 27.2 32.4 34.5 329 397
3.74 3.85 4.27 27.5 32.5 34.6 330 400
3.67 3.81 4.21 26.7 31.7 33.5 335 406
3.73 3.83 4.27 26.9 31.2 32.7 341 494
—
—
—
—
—
—
—
4.14 4.44
4.21 4.54
—
—
—
30.8 31.6
32.7 34.2
—
—
—
528 558, 622
563 593, 663
443
4.23
—
29.8
—
—
aδ77Se values agree well with those reported in the literature5g,5h

Selenophilicity of Copper
2947
SCHEME 3
23.5−39.0 ppm, respectively. Qualitatively ꢀδ is markedly higher be-
tween di- and triselenide than between mono- and diselenide. All of the
above results fit well with those of previous studies.5c,5g−5h
Mass spectrometric studies of organoselenium compounds are of
continuing interest.14 From the mass spectra (EI, 70 eV) of the
organoselenides, molecular ion and daughter ion peaks could be di-
agnosed easily from the characteristic selenium isotope pattern. The
spectra of dibenzyldiselenides 2d–5d show a uniform skeletal rear-
rangement pattern (Scheme 3). The principal fragmentation involves
cleavage of the Se-Se bond (path-A), H-migration to selenium (path-B),
and loss of SeH radical (path-C). The ease of H-migration over Se-Se
cleavage is strongly pronounced in the case 1,4-dihydro-2,3-diselena-
naphthalene 8d, which results in the formation of ion A. Loss of SeH
radical produces the benzocyclobutane ion B.
Mechanistic Studies
The following results provided further insight into the reactivity of
the Cu(II)/Sn(II) bimetallic reagent system in the present selenium
transfer reactions.

2948
J. Choudhury et al.
1. No reaction takes place either in the absence of the copper salt
or in presence of nickel and palladium salts or complexes such
as NiCl2, NiCl2(PPh3)2, Pd(OAc)2, Pd2(dba)3, and PdCl2(PPh3)2
as catalysts. This emphasizes the special role of copper in the
reaction.
2. Neither allyltributyltin nor allyltrichlorostannane reacts with se-
lenous acid in the presence or absence of copper(II) halides.
The possibility of an organotin intermediate is thereby ruled
out.
3. Selenous acid does not form any complex with copper salts (UV
monitoring). Hence a copper-selenolate intermediate is most unlikely.
4. The mixture which is prepared from selenous acid and stannous
chloride (1:3) does not react with any organic halide in the absence
of a copper halide. The formation of any organoselenyltin species
(RSe-Sn) is thereby ruled out.
5. As noted earlier, the selenium transfer proceeded through a brown-
black suspension, which turned into a clear yellow solution as the re-
action was progressed. The suspension was quickly filtered, washed
with THF and an XRD was recorded (Sample A; Figure 1). In an
independent experiment, the reaction of stannous chloride dihy-
drate (24 mmol), cupric chloride dihydrate (16 mmol) and selenous
acid (4 mmol) in THF resulted in a brown-black precipitate, which
was washed with THF and dried (Sample B, XRD shown in Figure
1). Qualitative analysis of the precipitate indicated the presence
of chloride. The precipitate was washed with aqueous ammonia
till a constant weight was reached (Sample C, Figure 1). Analy-
sis of the XRD spectra clearly allowed us to identify the species
in the three samples. Thus, while sample C corresponds to pure
α-Cu2Se, samples A and B are mixtures of α-Cu2Se and CuCl.15
ICP-AAS analysis of sample C further supports the composition to
be Cu2Se (Anal. calcd. for Cu2Se: Cu, 61.68; Se, 38.32%. Found Cu,
61.83%). It may be pointed out that Xie et al. have recently described
the synthesis of nanocrystalline copper selenides from the reac-
tion of sodium selenosulfate and cuprous iodide under sonochemical
irradiation.16
6. Curiously, when a mixture of sample C (α-Cu2Se) and an organic
halide (1 equiv.) was stirred in THF-DMSO, there was no indica-
tion of product formation. Addition of stannous chloride (1 equiv.
with respect to the organic halide) caused spontaneous reaction
leading to the formation of the corresponding diorganoselenides.
This further augments the bimetallic reactivity of the Cu(II)/Sn(II)
reagent.

Selenophilicity of Copper
2949
FIGURE 1 XRD pattern of Samples A, B, C and of α-Cu2Se and CuCl.15 d-
Values (%) for Sample C: 3.42 (76), 2.06 (100), 1.75 (28), 1.45 (15); for α -Cu2Se:
3.37 (70), 2.06 (100), 1.76 (31), 1.46 (7).
7. In an earlier study,17we have demonstrated that the reaction of the
Cu(II)/Sn(II) reagent with PhSeSePh results in the formation of
Cu2SePh. The latter reacts with an organic halide in the presence
of a tin(II) halide via an intermediate having a Cu-Se-Sn core unit
(Scheme 4).
All of the above experimental results, the known chemistry of stan-
nylcuprates and the established selenophilicity of copper guide us to
indicate the major bond-forming steps (Scheme 5). These include: (a)
formation of α-Cu2Se from CuCl2, SnCl2 and H2SeO3, (b) activation

2950
J. Choudhury et al.
R
SnCl2
+
Se
-SnCl4
R'Br
CuCl 2
+
Cu
Cl
Cu
Cl
R'SeR
Sn
RSeSeR
SCHEME 4
of Cu2Se by SnCl2, (c) activation of the organic halide at the copper
site and concomitant selenium transfer to generate “R-Se-Cu”, and (d)
elimination of the diorganoselenide. A complete elaboration of the above
steps must require model studies, which are presently underway in our
laboratory.
In summary, we have demonstrated a novel ‘copper-directed reac-
tivity’ and dual-metal effect for selenium transfer from selenous acid
to organic electrophiles. The selenophilicity of copper is well known in
literature, especially in ligand-assisted chemistry. In light of these in-
sights, efforts are underway to critically look into the selenium transfer
steps in detail.
EXPERIMENTAL
All reactions were performed under an inert atmosphere of argon.
All the starting materials were > 98% pure according to 1H NMR.
1H and 13C NMR spectra were recorded in CDCl3 using a Brucker-
200 spectrometer. 77Se NMR spectra were obtained in CDCl3 with a
Brucker-300 spectrometer using diphenyl diselenide (δ = 460 ppm)
as external standard. EIMS (70 eV) spectra were recorded using
Cu2Se
RSenR + SnX2Cl2
SnCl2
RX + SeO32- + SnCl2
R
Se
Sn
Se
RX
Cu
Cl
Cu
Cl
Cu
Cl
Cu
X
Sn
Cl
I
II
SCHEME 5

Selenophilicity of Copper
2951
a VG MicroMass 7070H and a VG Autospec M mass spectrome-
ter. FAB/MS spectra were recorded with a JEOL SX 102/DA-6000
mass spectrometer using argon/xenon (6 kV, 10 mA) as the FAB
gas. X-ray powder diffraction data were obtained with a Phillips PW-
1840 instrument using a Cu-Kα target at 40 kV. ICP-AAS analyses
were carried out at Central Glass & Ceramics Research Institute,
Kolkata.
Reaction of 4-Methylbenzyl Iodide with Selenous Acid
A mixture of selenous acid (129 mg, 1 mmol) and stannous chloride
dihydrate (722 mg, 3.2 mmol) was stirred in THF-DMSO (1:2, 3 mL)
under argon at ambient temperature, yielding a pale yellow solution
within 30 min. A solution of 4-methylbenzyl iodide 1a (232 mg, 1 mmol)
and a catalytic amount of cupric chloride dihydrate (51 mg, 0.3 mmol,
30 mol% with respect to selenous acid) in THF-DMSO (1:2, 3 mL) was
added drop wise to the above solution. A brown colored suspension
was observed, which turned into a clear yellow solution within 15 h.
An aqueous solution of ammonium fluoride (15%, 10 mL) was added
to the reaction mixture, the organic layer was extracted with diethyl
ether (3 × 20 mL), washed with water (2 × 10 mL), brine (2 × 10 mL),
and dried over magnesium sulfate. Removal of the solvent followed by
short column chromatography (silica gel 60–120 mesh, SRL, eluent n-
hexane) afforded 69 mg of a product mixture containing 2m:2d:2t in a
10:85:5 ratio (according to 1H NMR).
2d: 1H NMR (CDCl3): δ = 2.25 (s, 6H), 3.77 (s, 4H), 6.99−7.11
(m, 8H); 13C NMR (CDCl3): δ = 21.1, 32.4, 128.8, 129.1, 136.1, 136.9;
77Se NMR (CDCl3): δ = 397.
2m: 1H NMR (CDCl3): δ = 2.25 (s, 6H), 3.60 (s, 4H), 6.99−7.11
(m, 8H); 13C NMR (CDCl3): δ = 21.0, 27.2, 128.8, 129.1, 136.1, 136.6;
77Se NMR (CDCl3): δ = 329; EIMS m/z (rel. intensity): 370 (24), 290
(12), 209 (29), 185 (22), 105 (100), 77 (59).
Reaction of Benzyl Bromide with Selenous Acid
The procedure described for 1a was identically followed using 1b (171
mg, 1 mmol) to afford 102 mg of a product mixture containing 3m:3d:3t
in a 50:45:5 ratio (according to 1H NMR).
3d: 1H NMR (CDCl3): δ = 3.85 (s, 4H), 7.22−7.37 (m, 10H); 13C
NMR (CDCl3): δ = 32.6, 127.2, 128.4, 129.0, 139.0; 77Se NMR (CDCl3):
δ = 400; FABMS m/z (rel. intensity): 342 (51), 181 (49), 154 (81).

2952
J. Choudhury et al.
3m: 1H NMR (CDCl3): δ = 3.74 (s, 4H), 7.22−7.37 (m, 10H); 13C
NMR (CDCl3): δ = 27.5, 127.0, 128.4, 129.0, 139.1; 77Se NMR (CDCl3):
δ = 330; FABMS m/z (rel. intensity): 262 (43), 181 (11), 154 (100).
Reaction of 4-Chlorobenzyl Iodide with Selenous Acid
The procedure described for 1a was identically followed using 1c (253
mg, 1 mmol) to afford 146 mg of a product mixture containing 4m:4d:4t
in a 62:34:4 ratio (according to 1H NMR). Compounds 4m and 4d were
separated by column chromatography.
4d: 1H NMR (CDCl3): δ = 3.81 (s, 4H), 7.11−7.37 (m, 8H); 13C
NMR (CDCl3): δ = 31.6, 128.6, 130.3, 132.9, 137.4; 77Se NMR (CDCl3):
δ = 406; FABMS m/z (rel. intensity): 410 (12), 125 (30).
4m: 1H NMR (CDCl3): δ = 3.67 (s, 4H), 7.11−7.37 (m, 8H); 13C
NMR (CDCl3): δ = 26.7, 128.5, 130.2, 132.4, 137.4; 77Se NMR (CDCl3):
δ = 335.
Reaction of 4-Nitrobenzyl Bromide with
Selenous Acid
The procedure described for 1a was identically followed using 1d (216
mg, 1 mmol) to afford 93 mg of a product mixture containing 5m:5d:5t
in a 20:75:5 ratio (according to 1H NMR).
5d: 1H NMR (CDCl3): δ = 3.83 (s, 4H), 7.19−7.29 (m, 4H), 8.08−8.13
(m, 4H); 13C NMR (CDCl3): δ = 31.2, 123.9, 129.7, 146.5, 146.9; 77Se
NMR (CDCl3): δ = 494.
5m: 1H NMR (CDCl3): δ = 3.73 (s, 4H), 7.19−7.29 (m, 4H), 8.08-8.13
(m, 4H); 13C NMR (CDCl3): δ = 26.9, 123.7, 129.6, 146.3, 146.9; 77Se
NMR (CDCl3): δ = 341; EIMS m/z (rel. intensity): 432 (8), 352 (21), 272
(35), 216 (3), 136 (100), 90 (61), 78 (69).
Reaction of 2-Nitrobenzyl Bromide with Selenous Acid
The procedure described for 1a was identically followed using 1e
(216 mg, 1 mmol) to afford 62 mg of a product mixture containing
bis(2-nitrobenzyl)diselenide 6d and bis(2-nitrobenzyl)triselenide 6t in
a 85:15 ratio (according to 1H NMR).
6d: 1H NMR (CDCl3): δ = 4.14 (s, 4H), 7.16−7.21 (m, 2H), 7.29−7.52
(m, 4H), 7.96−8.01 (m, 2H); 13C NMR (CDCl3): δ = 30.8, 125.6, 128.3,
132.1, 133.4, 135.3, 147.3; 77Se NMR (CDCl3): δ = 528.
6t: 1H NMR (CDCl3): δ = 4.44 (s, 4H), 7.16−7.21 (m, 2H), 7.29−7.52
(m, 4H), 7.96−8.01 (m, 2H); 13C NMR (CDCl3): δ = 31.6, 125.8, 128.4,
132.3, 133.4, 135.3, 147.3; 77Se NMR (CDCl3): δ = 558, 622.

Selenophilicity of Copper
2953
Reaction of 2-Carbomethoxybenzyl Bromide with Selenous
Acid
The procedure described for 1a was identically followed us-
ing 1f (229 mg,
1 mmol) to afford 99 mg of a product
mixture containing bis(2-carbomethoxybenzyl)diselenide 7d and
bis(2-carbomethoxybenzyl)triselenide 7t in a 86:14 ratio (according to
1H NMR).
7d: 1H NMR (CDCl3): δ = 3.82 (s, 6H), 4.21 (s, 4H), 7.05−7.36
(m, 6H), 7.85−7.89 (m, 2H); 13C NMR (CDCl3): δ = 32.7, 52.0, 127.0,
127.9, 130.9, 131.2, 132.0, 142.0, 167.4; 77Se NMR (CDCl3): δ =
563.
1
7t: H NMR (CDCl3): δ = 3.81 (s, 6H), 4.54 (s, 4H), 7.05−7.36 (m,
6H), 7.85−7.89 (m, 2H); 13C NMR (CDCl3): δ = 34.2, 52.7, 127.2,
128.4, 130.3, 132.1, 132.3, 142.0, 167.4; 77Se NMR (CDCl3): δ =
593, 663; FABMS m/z (rel. intensity): 535 (<2), 458 (24), 229 (10),
149 (52).
Reaction of α,α’-Dibromo-o-xylene with Selenous Acid
The procedure described for 1a was identically followed using 1g (264
mg, 1 mmol) to give 1,4-dihydro-2,3-diselena-naphthalene 8d. Isolated
1
yield: 50 mg (19%). H NMR (CDCl3): δ = 4.23 (s, 4H), 7.03−7.17 (m,
8H); 13C NMR (CDCl3): δ = 29.8, 125.9, 126.4, 141.7; 77Se NMR (CDCl3):
δ = 443; EIMS m/z (rel. intensity): 262 (2), 183 (68), 103 (22), 83
(100).
Preparation of Sample B and Sample C
Stannous chloride dihydrate (1.81 g, 8 mmol) was added in portions to a
solution of selenous acid (0.516 g, 4 mmol) in THF (25 mL) with stirring
at ambient temperature and under argon atmosphere. A red colored
precipitate was formed. To the above was added a mixture of stannous
chloride dihydrate (3.61 g, 16 mmol) and cupric chloride dihydrate (2.73
g, 16 mmol) in THF (30 mL). The resulting brown-black precipitate was
further stirred for 1 h, the solid was filtered under argon, washed with
THF (10 × 10 mL) and diethylether (5 × 10 mL) and finally dried under
vacuum (Sample B; yield 1.71 g).
Sample B (1.5 g) was further washed with 25% aqueous ammonia
solution (10 × 7 mL), followed by distilled water (5 × 10 mL), and
ethanol (5 × 10 mL) and dried under vacuum until a constant weight
was reached (Sample C; yield 0.310 g).

2954
J. Choudhury et al.
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