Two unusual mixed-valent trinuclear CuII2Cu I complexes containing copper(I) tribromide dianion as bridging ligand: Identification of an unprecedented doubly hydrogen-bonded water dimer

Article abstract of DOI:10.1039/c0ce00784f

Two very rare mixed-valent Cu^II₂Cu^I trinuclear complexes, [Cu₂(Br-L¹)₂CuBr ₃]·2H₂O (1) and [Cu₂(L²) ₂CuBr₃]·2CH₃

Full text of DOI:10.1039/c0ce00784f

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PAPER
Two unusual mixed-valent trinuclear CuII CuI complexes containing copper(I)
tribromide dianion as bridging ligand: Identification of an unprecedented
doubly hydrogen-bonded water dimer†
2
a
Apurba Biswas, Rajat Saha and Ashutosh Ghosh
b
a
*
Received 28th October 2010, Accepted 5th January 2011
DOI: 10.1039/c0ce00784f
Two very rare mixed-valent CuII2CuI trinuclear complexes, [Cu2(Br–L1)2CuBr3]$2H2O (1) and
[Cu2(L2)2CuBr3]$2CH3OH (2) have been synthesized using CuBr2 and the tridentate reduced Schiff-
base ligands HL1 (2-[(2-dimethylamino-ethylamino)-methyl]-phenol) and HL2 (2-[1-(2-dimethylamino-
ethylamino)-ethyl]-phenol), respectively. The complexes have been characterized by X-ray structural
analyses. In both complexes, the deprotonated tridentate reduced Schiff base forms a di-m-phenoxo
bridge between the two CuII centers and the CuBr32ꢀ anion acts as an additional bridge between them to
result in the trinuclear structures. During complex formation, ligand HL1 has been brominated at the
five position of the aromatic ring but HL2 did not undergo any such reaction. In complex 1 an
unprecedented doubly hydrogen-bonded water dimer, which is considered as an important transition
state for the interchange of hydrogen atoms within the water dimer, has been identified. The dimensions
of this water dimer agree quite well to those obtained from theoretical calculations.
The mixed-valent CuI/CuII coordination compounds are
Introduction
gaining popularity due to their great biological importance for
their structural application in certain metallo-enzymes and
interesting electronic properties,9 as well as the good electrical
conductivity10 and bistability of the compounds.11 However,
there are great difficulties in controlling the final product with
two different oxidation states, which is reflected in the literature
of mixed-valent copper coordination compounds.12–14 As
a result, the number of mixed-valent copper complexes are
limited and among them there is only one example of mixed-
valent CuI/CuII coordination compounds where a copper(I) tri-
halide dianion acts as a bridging ligand between two CuII ions in
a trinuclear structure.14 The compound was prepared by using
a resorcinol-substituted trimethylethylenediamine ligand. The
tridentate reduced Schiff base ligands derived from the mono-
condensation of diamines with salicylaldehyde has a very similar
structure to that of this resorcinol-substituted trimethylethyle-
nediamine ligand. Therefore, it is of interest to find out whether
such ligands can also lead to this type of rare mixed-valence
species.
Recently, much attention has been focused on water dimers1
since the water dimer appears to be an important species in the
atmosphere,1f,2 where it may influence the energy balance of the
earth. A supramolecular water dimer was first identified by
Pimentel and co-workers long ago by infrared spectroscopy.3
Later with the help of various other experimental techniques,4–6 it
has been clearly established that the water dimer is linear with
one water acting as a proton acceptor and the other as a proton
donor. Theoretical calculations also supported the linear struc-
ture.7 On the other hand, study of water dimers with the help of
X-ray crystal structure analysis in restricted environments such
as organic and inorganic host lattices are very rare.8 Moreover,
the few water dimers which have been fully characterized by
crystallography in the solid state are in conformity with the near-
linear structure as proposed by the spectral evidences and theo-
retical calculations. To our knowledge, until now identification
of a doubly hydrogen-bonded water dimer is unprecedented.
In this paper we report the synthesis, and crystal structures of
two mixed-valent CuI/CuII coordination compounds, [Cu2-
(Br–L1)2CuBr3]$2H2O (1) and [Cu2(L2)2CuBr3]$2CH3OH (2)
obtained from the reduced Schiff base ligands 2-[(2-dimethyla-
mino-ethylamino)-methyl]-phenol (HL1) and 2-[1-(2-dimethyla-
aDepartment of Chemistry, University College of Science, University of
Calcutta, 92, A.P.C. Road, Kolkata, 700 009, India. E-mail: ghosh_59@
yahoo.com
bDepartment of Physics, Jadavpur University, Jadavpur, Kolkata, 700 032,
India
mino-ethylamino)-ethyl]-phenol
(HL2)
respectively.
In
† Electronic supplementary information (ESI) available. CCDC
reference numbers 793723 (1) and 793724 (2). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0ce00784f
compound 1, an unprecedented doubly hydrogen-bonded water
dimer which has been considered from theoretical calculation as
5342 | CrystEngComm, 2011, 13, 5342–5347
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one of the intermediates for the interchange of hydrogen atoms
within the water dimer, is identified.
a dynamic atmosphere of dinitrogen (flow rate ¼ 30 cm3 minꢀ1).
The samples were heated in an alumina crucible at a rate of 5 ꢁC
minꢀ1. Electrochemical studies were done using a PAR 273
potentiostat. The measurements were performed at 300 K in
Experimental
acetonitrile solutions containing 0.2 M TEAP and 10ꢀ3
M
Materials
complex deoxygenated by bubbling with nitrogen. The working,
counter, and reference electrodes used were a platinum wire,
a platinum coil, and a SCE, respectively. The X-band EPR
spectra were recorded using a Bruker E300 automatic spec-
trometer on solid samples at 293 K and frozen acetonitrile
solutions at 77 K.
All the chemicals were of reagent grade and used without further
purification.
Synthesis of the reduced Schiff base ligands 2-[(2-dimethylamino-
ethylamino)-methyl]-phenol (HL1) and 2-[1-(2-dimethylamino-
ethylamino)-ethyl]-phenol (HL2)
The Schiff base ligand has been synthesized by refluxing a solu-
tion of salicylaldehyde (0.52 mL, 5 mmol) and N,N-dimethyle-
thylenediamine (0.54 mL, 5 mmol) in methanol (30 mL) for one
hour.15 The solution was cooled to 0 ꢁC and solid sodium
borohydride (210 mg, 6 mmol) was added slowly to this meth-
anolic solution with stirring. After completion of the addition,
the resulting solution was acidified with concentrated HCl (5 mL)
and then evaporated to dryness.15 The reduced Schiff base ligand
HL1 was extracted from the solid mass with methanol and this
methanol solution (ca. 20 mL) was used for preparation of the
complexes. HL2 was synthesized in the same way as HL1 using 2-
hydroxyacetophenone (0.60 mL, 5 mmol) instead of salicy-
laldehyde.
Crystallographic data collection and refinement
Suitable single crystals of each complex were mounted on
a Bruker SMART diffractometer equipped with a graphite
ꢀ
monochromator and Mo-Ka (l ¼ 0.71073 A) radiation. The
structures were solved using the Patterson method by using the
SHELXS97. Subsequent difference Fourier synthesis and least-
square refinement revealed the positions of the remaining non-
hydrogen atoms. Non-hydrogen atoms were refined with
independent anisotropic displacement parameters. Hydrogen
atoms bonded to carbon were placed in idealized positions and
their displacement parameters were fixed to be 1.2 times larger
than those of the attached non-hydrogen atoms. The hydrogen
atoms of the water molecule were located from a difference map
and kept fixed during refinement. Successful convergence was
indicated by the maximum shift/error of 0.001 for the last cycle of
the least squares refinement. All calculations were carried out
using SHELXS 97,16 SHELXL 97,17 PLATON 99,18 ORTEP-
3219 and WinGX system Ver-1.64.20 Data collection and struc-
ture refinement parameters and crystallographic data for the two
complexes are given in Table 1. The selected bond lengths and
bond angles are summarized in Table 2.
Synthesis of [Cu2(Br–L1)2CuBr3]$2H2O (1) and [Cu2(L2)2CuBr3]$
2CH3OH (2)
An extracted methanol solution of HL1 as prepared above was
added to a solution of CuBr2 (1.672 g, 7.5 mmol) in methanol
(20 mL). The mixture was stirred for 1 h and filtered. The filtrate
was kept undisturbed at room temperature. Deep brown crystals
of 1 suitable for X-ray diffraction were obtained after 24 h. The
deep brown coloured single crystals of complexes 2 were
obtained in the same way using HL2 instead of HL1.
Complex 1. (Yield: 1.920 g; 76%) Anal. Calc. for
C22H36Br5Cu3N4O4: C, 26.14; H, 3.59; N, 5.54%. Found: C,
26.13; H, 3.51; N, 5.62%. lmax (nm), [3max (dm3 molꢀ1 cmꢀ1)]
Table 1 Crystal data and structure refinement of complexes 1 and 2
1
2
(acetonitrile), 685 (332), 422(2870); IR: n(N–H), 3155 cmꢀ1
n(C–N), 1595 cmꢀ1
,
Formula
M
C22H36Br5Cu3N4O4
1010.72
Monoclinic
C2/c (No. 15)
22.979(10)
10.157(4)
15.902(7)
90
118.944(4)
90
3248(2)
4
2.067
8.131
1960
0.043
6068
2239
1508
0.0624, 0.2024
296
C26H46Br3Cu3N4O4
909.02
,
Crystal system
Space group
Monoclinic
C2/c (No. 15)
23.267(3)
11.0415(16)
15.727(2)
90
122.907(2)
90
3392.1(8)
4
1.780
5.433
1816
0.047
12417
3359
1873
0.0712, 0.2423
293
Complex 2. (Yield: 1.613 g; 71%) Anal. Calc. for
C26H46Br3Cu3N4O4: C, 34.35; H, 5.10; N, 6.16%. Found: C,
34.31; H, 5.15; N, 6.09%. lmax (nm), [3max (dm3 molꢀ1 cmꢀ1)]
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
(acetonitrile), 650 (335), 422(3231); IR: n(N–H), 3149 cmꢀ1
n(C–N), 1593 cmꢀ1
,
a (ꢁ)
b (ꢁ)
g (ꢁ)
.
3
ꢀ
V/A
Z
Physical measurements
Dc/g cmꢀ3
m/mmꢀ1
F(000)
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed using a Perkin-Elmer 240C elemental analyzer. IR
spectra in KBr (4500–500 cmꢀ1) were recorded using a Perkin-
Elmer RXI FT-IR spectrophotometer. Electronic spectra in
acetonitrile (1200–350 nm) were recorded in a Hitachi U-3501
spectrophotometer. Thermal analysis (TG-DTA) was carried out
on a Mettler Toledo TGA/SDTA 851 thermal analyzer in
R(int)
Total reflections
Unique reflections
I > 2s(I)
R1, wR2
T/K
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ꢁ
ꢀ
Table 2 Bond distances (A) and angles ( ) for complexes 1 and 2
Infrared and electronic spectra
All four compounds show a moderately strong, sharp peak due
to N–H stretching vibration at 3155, and 3149 cmꢀ1 for the
complexes 1, and 2, respectively, which shows that an imine
group of the Schiff base is reduced. The reduction of the imine
group is also very clearly indicated by the absence of the strong
band due to imine vibration which appears in the region of 1620–
1650 cmꢀ1 for the complexes of the corresponding unreduced
Schiff bases.21
1
2
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–N(2)
1.959(7)
1.994(10)
2.047(10)
1.947(8)
2.701(2)
2.392(2)
2.346(4)
91.1(4)
165.5(4)
76.6(3)
85.5(4)
157.5(4)
101.7(4)
92.3(2)
101.5(3)
102.2(3)
97.8(2)
106.96(8)
119.19(7)
121.62(14)
119.19(7)
1.965(6)
2.007(9)
2.048(9)
1.947(7)
2.6818(16)
2.3810(16)
2.420(7)
90.2(3)
162.7(3)
77.4(3)
86.5(4)
157.5(3)
99.9(3)
94.05(17)
100.2(2)
103.3(2)
99.32(17)
106.02(7)
118.47(5)
123.06(10)
118.47(5)
Cu(1)–O(1)a
Br(1)–Cu(1)
Br(1)–Cu(2)
Br(2)–Cu(2)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
O(1)–Cu(1)–O(1)a
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–O(1)a
N(2)–Cu(1)–O(1)a
Br(1)–Cu(1)–O(1)
Br(1)–Cu(1)–N(1)
Br(1)–Cu(1)–N(2)
Br(1)–Cu(1)–O(1)a
Cu(1)–Br(1)–Cu(2)
Br(1)–Cu(2)–Br(2)
Br(1)–Cu(2)–Br(1)a
Br(2)–Cu(2)–Br(1)a
The electronic spectra of these two compounds are recorded in
acetonitrile solution. The electronic spectra show single absorp-
tion bands at 685 and 650 nm for compounds 1 and 2, respec-
tively. The positions of these bands are of typical d–d transitions
in the square-pyramidal CuII surroundings.22 At a higher energy
region, the ligand to metal charge transfer bands were located at
422 nm for both the complexes.
Description of structures of complexes 1 and 2
Both complexes consist of mixed valent CuII2CuI trinuclear units
a
Symmetry Operation: 1ꢀx, y, 0.5ꢀz for complex 1 and 2.
which sit on
a twofold axis. The structure [Cu2(Br–
L1)2CuBr3]$2H2O (1) is shown in Fig. 1 together with the atomic
numbering scheme.
The asymmetric unit consists of a CuII atom, a ligand and half
of a copper(I) tribromide unit. The copper(II) atom is five-coor-
dinate being bonded equatorially to three donor atoms of one
ligand L1 together with a bridging oxygen atom O(1)a (a ¼ 1ꢀx, y,
0.5ꢀz) from a second ligand L1 that forms a di-m-phenoxo bridge
between the two CuII centers. A copper(I) tribromide unit acts as
an additional bridge between the copper(II) centers by coordi-
nating weakly to the axial position of each copper(II) and thus
forms the mixed-valent CuII2CuI trinuclear entity. Bond lengths
Results and discussion
Synthesis of the complexes
The condensation of N,N-dimethylethylenediamine in 1 : 1
molar ratio with salicylaldehyde and 2-hydroxyacetophenone
afforded the Schiff bases, 2-[(2-dimethylamino-ethylimino)-
methyl]-phenol and 2-[1-(2-dimethylamino-ethylimino)-ethyl]-
phenol, respectively, which on reduction with sodium
borohydride readily produced the reduced Schiff bases, HL1 and
HL2 (Scheme 1). Complexes 1 and 2 were prepared by reacting
a methanol solution of copper(II)bromide and the ligands (HL1)
and (HL2), respectively, in a 3 : 2 molar ratio.
in the CuII coordination sphere are Cu(1)–O(1) 1.959(7) A,
ꢀ
ꢀ
ꢀ
Cu(1)–N(1) 1.994(10) A and Cu(1)–N(2) 2.047(10) A to the
chelating terdentate ligand L1 and Cu(1)–O(1a) (a ¼ 1ꢀx, y,
ꢀ
0.5ꢀz) 1.947(8) A to the bridging oxygen in the equatorial plane.
ꢀ
The axial Cu(1)–Br(1) 2.701(2) A to the copper(I) tribromide unit
is rather long as is expected for CuII ion with Jahn–Teller
distortion. The deviations of the coordinating atoms O(1), N(1),
N(2), O(1a) from the least-square mean plane through them are
ꢀ
0.052(8), ꢀ0.046(9), 0.042(11) and ꢀ0.047(8) A, respectively. The
ꢀ
deviation of copper(II) from the same plane is 0.2988(14) A in
the direction of the coordinated bromide Br(1) of CuBr32ꢀ. The
Addison parameter (s)23 of the penta-coordinated CuII is 0.13
Scheme 1 Formation of the complexes.
Fig. 1 The structure of 1 with ellipsoids at 30% probability.
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indicating that the geometry is nearly square pyramidal with
a slight distortion towards being trigonal bipyramidal. The
CuBr3 unit is trigonal planar with Cu–Br bond lengths and Br–
Cu–Br bond angles typical of the well known and characterized
surprising as CuBr2 is an effective reagent for the bromination of
activated aromatics.28,29 The aryl bromination reaction produces
cuprous bromide and hydrogen bromide as side-products
(according to: Ar–H + 2CuBr2 ¼ Ar–Br + H–Br + 2CuBr).
However, rather surprisingly bromination of the ligand (L2) did
not take place in the case of complex 2 indicating that substitu-
tion of the H-atom of the aldehyde by a methyl group prevents
the ligand from bromination i.e. this bromination is very much
susceptible to the constitution of the ligands. It should also be
noted that the CuBr32ꢀ ion is also formed in complex 2 although
copper(I) anion,24–26 CuBr3 : Cu(2)–Br(1) 2.392(2) A and Cu(2)–
2ꢀ
ꢀ
ꢁ
ꢀ
Br(2) 2.346(4) A and :Br(1)–Cu(2)–Br(2) 119.19(7) and
:Br(1)–Cu(2)–Br(1a) (a ¼ 1ꢀx, y, 0.5ꢀz) 121.62(14)ꢁ.
The molecular structure of 2, [Cu2(L2)2CuBr3]$2CH3OH is
equivalent to that of 1 as shown in Fig. 2 together with the atomic
numbering scheme. The copper(II) atom is bonded in a similar
fashion to that in 1 with three equatorial bonds to the ligand L2,
another one to an oxygen atom of a second ligand that forms the
phenoxo bridge and one axial bond to a bromine Br(1) of
a bridging copper(I) tribromide unit. The deviations of the
coordinating atoms O(1), N(1), N(2), O(1a) from the least-square
mean plane through them are 0.026(6), ꢀ0.023(8), 0.021(9) and
2ꢀ
no bromination takes place. Formation of the CuBr3 ion
without any bromination has also been observed previously.27
Although the mechanism of the formation of cuprous bromide is
not certain in these cases, the solvent perhaps is responsible for
its generation.
ꢀ
ꢀ0.024(6) A, respectively. The deviation of copper(II) from the
ꢀ
same plane is 0.3252(11) A towards the axially coordinated
Hydrogen bonding in the complexes and identification of a doubly
hydrogen-bonded water dimer in 1
bromide ion Br(1). The overall geometry around copper(II) is
nearly square pyramidal with Addison parameter (s) ¼ 0.08.
Bond lengths in the CuII coordination sphere are Cu(1)–O(1)
A very interesting feature of complex 1 is that there is a doubly
hydrogen-bonded water dimer as shown in Fig. 3.
ꢀ
ꢀ
ꢀ
1.965(6) A, Cu(1)–N(1) 2.007(9) A and Cu(1)–N(2) 2.048(9) A to
the terdentate ligand L2 with Cu(1)–O(1a) (a ¼ 1ꢀx, y, 0.5ꢀz)
The two water molecules O(2W) and O(2Wb) (b ¼ 0.5ꢀx,
1.5ꢀy, 1ꢀz) arrange themselves to form a non-linear doubly
hydrogen-bonded water dimer [O(2W)–H(1W2)/O(2Wb)]. The
geometrical parameter of the water dimer is provided in Table 3.
ꢀ
1.947(7) A and Cu(1)–Br(1) 2.6818(16) A to the copper(I) tri-
ꢀ
bromide unit—all are very similar to those in 1. The bond lengths
2ꢀ
in the trigonal planar CuBr3 are: Cu(2)–Br(1) 2.3810(16) and
ꢀ
ꢀ
Cu(2)–Br(2) 2.420(7) A. The Br–Cu–Br bond angles are:
:Br(1)–Cu(2)–Br(2) 118.47(5)ꢁ and :Br(1)–Cu(2)–Br(1a) (a ¼
The O/O distance in the (H2O)2 dimer is 2.93(5) A. For
comparison, the corresponding values in regular ice, in liquid
1ꢀx, y, 0.5ꢀz) 123.06(10)ꢁ.
water, and in the vapor phase are 2.74, 2.85, and 2.98 A,
ꢀ
It should be noted that the [CuBr3]2ꢀ ion is well documented in
the literature. However, there are only two previous examples in
which the CuBr32ꢀ anion acts as a bridge between two copper(II)
centres. Between these two examples, one is a polymeric chain
compound, (1,4,8,11-tetraazacyclotetradecane)copper(II) tri-
respectively.30 The value in the linear water dimer which has been
ꢀ
determined previously by X-ray crystal structure is 2.84 A. To
our knowledge no doubly hydrogen-bonded water dimer has
been characterized by X-ray structural analysis. However, the
dimensions of the doubly hydrogen-bonded water dimer have
been proposed by a variety of density functional theories and the
bromocuprate(I)27 and the other is
a discrete trinuclear
compound14 that has a very similar structure to that of 1 and 2.
Interestingly, this reported trinuclear compound was formed
with a resorcinol-substituted trimethylethylenediamine ligand
(L) which is very similar to those used for complexes 1 and 2 and
the ligand has been brominated at the five-position of the
aromatic ring during complex formation with CuBr2, just like the
present complex 1. The bromination of the ligands is not
ꢀ
calculated dimensions (the O/O separation, 2.917 A, H(1W)/
ꢁ
ꢀ
O distance, 2.37 A and :O–H(1W)/O is 115 ) are in good
agreement with the values found in the present compound
(Table 3).
The amine hydrogen atom, H(1) of the trinuclear unit forms an
intermolecular hydrogen bond, N(1)–H(1)/Br(1c) (c ¼ 1ꢀx,
2ꢀy, 1ꢀz) with the coordinated bromide (Br(1c)) of a neigh-
ꢀ
boring molecule with N(1)/Br(1) 3.357(9) A, H(1)/Br(1)
ꢁ
ꢀ
2.54 A and :N(1)–H/Br(1) 149 to result in a zig-zag 1D
supramolecular structure (Fig. 4). The trinuclear units of
complex 2 are also connected by the H–bond, N(1)–H(1)/Br(1d)
d
ꢀ
( ¼ 1ꢀx, 2ꢀy, ꢀz) with dimensions, N(1)/Br(1) 3.352(8) A,
ꢁ
ꢀ
H(1)/Br(1) 2.52 A and :N(1)–H/Br(1) 152 to form a 1D
chain as in 1 (Fig. S1†). However, unlike 1 the solvent molecule is
methanol in 2.
Fig. 2 The structure of 2 with ellipsoids at 30% probability.
This journal is ª The Royal Society of Chemistry 2011
Fig. 3 Hydrogen bonding water dimer in compound 1.
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a
ꢁ
ꢀ
Table 3 Hydrogen bonding distances (A) and angles ( ) for the complexes 1 and 2
:D–H–A (ꢁ)
ꢀ
D–H (A)
ꢀ
ꢀ
Complex
D–H/A
A/H (A)
D/A (A)
1
O(2W)–H(1W2)/O(2W)b
N(1)–H(1)/Br(1)c
0.85
0.91
0.91
2.29
2.54
2.52
2.93(5)
3.357(9)
3.352(8)
132
149
152
2
N(1)–H(1)/Br(1)d
a
Symmetry Operation: b 0.5ꢀx, 1.5ꢀy, 1ꢀz, c 1ꢀx, 2ꢀy, 1ꢀz, d 1ꢀx, 2ꢀy, ꢀz.
Fig. 5 The cyclic voltammogram of 1 in acetonitrile solution (scan rate
100 mV sꢀ1).
oxidative responses around 1.2 V may be due to the CuII/CuIII
couple.
Fig. 4 Hydrogen bonding polymeric structure of compound 1.
Thermal analysis
EPR studies
The simultaneous TG-DTA curves for complex 1 reveal that
upon heating it starts to lose water molecules at 80 ꢁC and
becomes dehydrated at ca. 110 ꢁC in a single step. The observed
weight loss (3.9%) corroborates the loss of two water molecules
(calcd. 3.5%). The compound does not show any color change on
deaquation. Complex 2 starts to lose meꢁthanol molecules at ca.
X-band EPR spectra were recorded for complexes 1 and 2 in
solid state at room temperature and at 77 K. The spectra at 293 K
show only one broad signal. This type of spectra unfortunately
gives no information about the electronic ground state of the
Cu(II) ion present in the complexes. At 77 K both the complexes
are EPR-silent, suggestive of strong antiferromagnetic coupling
between the two Cu(II) centres as is found for many similar
phenoxo-bridged copper(II) dimers.31 Considering the large
bridging angles (>102ꢁ) in both complexes strong antiferro-
magnetic coupling is expected.32 The EPR spectra of frozen
acetonitrile solutions at 77 K of both the complexes show four-
line spectrum which is indicative of localized unpaired electrons
in the Cu(II) centre i.e. of Class I mixed valence complex (Fig. S4
and S5†). However, these signals seem to appear from the break-
up of the trinuclear compound, at least partially upon dissolution
in acetonitrile as was also found in the reported similar
complex.14
ꢁ
55 C and becomes desolvated at ca. 80 C in a single step. The
observed weight loss (7.1%) corroborates the loss of two meth-
anol molecules (calcd. 7.0%). Both the complexes start to
ꢁ
decompose at 150 C. (Fig. S2 and S3†)
Electrochemical study
The cyclovoltammograms of the copper complexes 1 and 2 in
acetonitrile solution display irreversible reductive responses (Epc
at ꢀ1.11 and ꢀ1.10 V, respectively) during cathodic scanning,
attributed to the CuII/CuI couple. During the anodic potential
scan, both the complexes show an oxidative response at ca. ꢀ0.08
V with a very narrow width and high peak current and another
quasi-reversible oxidative response at 1.12 and 1.17 V for 1 and 2,
respectively. On reversing the potential scan just after completion
of the peak, the corresponding cathodic peak is observed at 0.90
and 0.92 V for complexes 1 and 2, respectively (Fig. 5). The
response at ꢀ0.08 V is typical of the anodic stripping of copper.
Therefore, it may be inferred that both the copper(II) species
undergo reduction to their respective CuI complexes which
subsequently undergo disproportionation to Cu0 and Cu2+. The
Conclusions
Two unusual mixed-valent trinuclear CuII2CuI complexes con-
taining a copper(I) tribromide dianion as a bridging ligand have
been synthesized using CuBr2 and two tridentate reduced Schiff-
base ligands HL1 and HL2. In both cases, a portion of CuBr2 was
2ꢀ
reduced in situ to CuBr3 which acted as a bridging ligand
between the two CuII centers generating the very rare trinuclear
CuII2CuI species. During complex formation, ligand HL1 has
been brominated at the five position of the aromatic ring but HL2
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the H-atom of the aldehyde by a methyl group prevents the
ligand from bromination. In compound 1, a doubly hydrogen-
bonded water dimer, which is considered as one of the interme-
diates for the interchange of hydrogen atoms within the water
dimer from theoretical calculations, has been identified. To our
knowledge, to date there is no crystallographic evidence in the
literature for the existence of such a water dimer. Thus, the
present example should be valuable for verification of the results
of theoretical calculations regarding water dimers.
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Acknowledgements
We thank CSIR, the Government of India for Junior Research
Fellowship to A. Biswas, Sanction No. [09/028(0717)/2008-
EMR-I], and DST FIST for financial support for the X-ray
diffraction facility. We are also thankful to Prof. Shyamal
Kumar Chattopadhyay, Bengal Engineering and Science
University for cyclic voltammetry experiments.
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