Versatile coordination patterns in the reaction system of N-benzoyl-N′-(2-pyridyl)thiourea with CuCl2. Their reaction conditions, systematic isolation and crystal structures

Article abstract of DOI:10.1039/b206257g

A versatile reaction system; N-benzoyl-N′-(2-pyridyl)thiourea (L) with CuCl₂, has been studied under conditions of control of the relative concentration of the components (L or CuCl₂). Therefore, both converse approaches were employed: one is dropwise addition of L to CuCl₂ in ethanol solution, from which a dark-brown needle-like crystal, ∞¹{[CuClL′][Cl]} 1, was obtained, which is a chloride-bridged one-dimensional Cu(II) polynuclear complex coordinated by the oxidative cyclization product L′ generated from L (L′ = 2-benzoylimino-[1,2,4]thiadiazolo-[2,3-a(]pyridine), and the second approach is conversely dropping CuCl₂ into L in acetone solution; at first, a pale-yellow reduced copper(I) mononuclear complex, [CuL₂Cl] 2, was formed, which exhibits a planar molecular configuration with a regular three coordination geometry, and then a red bis-sulfur-bridged copper(II) dinuclear complex, {[CuLCl₂]₂} 3, appeared subsequently. The crystal structures of these three products have been determined by X-ray diffraction. From the presented results the coordination and redox reaction pathways in the L-CuCl₂ system have been clearly revealed.

Full text of DOI:10.1039/b206257g

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Versatile coordination patterns in the reaction system of
N-benzoyl-N0-(2-pyridyl)thiourea with CuCl2 . Their reaction
conditions, systematic isolation and crystal structures
Gang Li, De-Ji Che,* Zi-Feng Li, Yu Zhu and Da-Peng Zou
Department of Chemistry, Zhengzhou University, Henan 450052, China.
E-mail: chedj@public2.zz.ha.cn
Received (in Montpellier, France) 28th June 2002, Accepted 28th August 2002
First published as an Advance Article on the web 17th October 2002
A versatile reaction system; N-benzoyl-N0-(2-pyridyl)thiourea (L) with CuCl2 , has been studied under
conditions of control of the relative concentration of the components (L or CuCl2). Therefore, both converse
approaches were employed: one is dropwise addition of L to CuCl2 in ethanol solution, from which a
1
dark-brown needle-like crystal, {[CuClL0][Cl]} 1, was obtained, which is a chloride-bridged one-dimensional
1
Cu(II) polynuclear complex coordinated by the oxidative cyclization product L0 generated from L (L0 ¼
2-benzoylimino-[1,2,4]thiadiazolo-[2,3-a]pyridine), and the second approach is conversely dropping CuCl2 into
L in acetone solution; at first, a pale-yellow reduced copper(I) mononuclear complex, [CuL2Cl] 2, was formed,
which exhibits a planar molecular configuration with a regular three coordination geometry, and then a red
bis-sulfur-bridged copper(II) dinuclear complex, {[CuLCl2]2} 3, appeared subsequently. The crystal structures
of these three products have been determined by X-ray diffraction. From the presented results the coordination
and redox reaction pathways in the L–CuCl2 system have been clearly revealed.
It has been shown that the reaction systems of thiourea and its
derivatives with the Cu(II) ion are complicated and versatile as
a result of the ready redox between them in combination with
the flexible coordination of copper and the bridging capability
of thioureas. Therefore, there have been a number of reports
on such systems in which a variety of Cu(I) and Cu(II) mono-
nuclear and polynuclear complexes with one or more co-
ordination geometries usually coexist within a system.1–3
However, as far as we are aware, systematic isolation and
structural studies of various products in a particular system
are still very limited.
[CuL2Cl] 2, were isolated systematically. In particular, we
reported the synthesis and partial characterization of the poly-
mer 1 in a preliminary communication.7 Here we present a
more complete characterization of these complexes, reveal
the nature of Cu(II)–thioureas interactions and demonstrate
the redox and coordination pathways in the system.
Experimental
General details
Even though extensive research has been undertaken over
the past decades, even now many interesting works continue
to be published on studies of the equilibrium and redox
kinetics of copper(II)–thiourea complexes,3 of the crystal struc-
tures of a family of new copper(I) cyanide complexes of
thiourea and substituted thioureas,4 and of a copper(II) com-
plex on a thiourea-functionalized silica gel.5 However, what
is worthy of special mention is their biological importance as
models of the copper-sulfur interaction in many biomolecules
such as copper metallothioneins.6
All reagents and solvents used in this study were commercially
available and were used as received. Solvents were dried by
conventional procedures prior to use. The C, H and N analyses
were carried out on a MOD 1106 analyzer. Infrared spectra in
the range 400–4000 cmꢀ1 were recorded on a Shimadzu IR-435
instrument using KBr pellets. Proton NMR spectra were
obtained with a Bruker DPX-400 spectrometer and calibrated
¨
against the solvent signals (CD3CN, dH 1.95).
In regard to such versatile reaction systems, we believe that
control of the relative concentration between reaction compo-
nents may be crucial. Therefore, a ‘‘dropping method’’ was
adopted, namely drop-by-drop addition of a ligand to a metal
solution, or reversely adding metal to ligand solution, such
that the reactions proceed under the condition of one compo-
nent (ligand or metal) in excess, and the other under control.
By this means, we investigated the complex system of N-
benzoyl-N0-(2-pyridyl)thiourea (L) with copper(II) chloride in
ethanol or acetone solution. From this system three Cu(II)
and Cu(I) coordination compounds: a chloride-bridged one-
dimensional polynuclear copper(II) coordination complex,
Synthesis of N-benzoyl-N0-(2-pyridyl)thiourea (L)
L was prepared using a similar procedure described in the lit-
erature8 by the reaction of benzoyl chloride with KSCN in
anhydrous acetone to produce benzoyl isothiocyanate, fol-
lowed by condensation with 2-aminopyridine. The product
was recrystallized from acetone. Yield: about 83%, m.p.
149 ꢁC. Found: C, 60.94; H, 4.22; N, 16.12. Calcd for
C13H11N3OS: C, 60.7; H, 4.28; N, 16.34%. 1H NMR (400
MHz, CD3CN): d 7.2–7.7(m, 4H, 2, 3, 4 & 5 pyridyl protons),
7.8–8.8 (m, 5H, 20, 30, 40, 50 & 60 phenyl protons), 9.8 (br, NH).
IR (KBr, cmꢀ1) 3000–3300 (N–H); 1680 [–C(O)–NH–carbo-
nyl]; 1500–1600 (pyridine C=N, C=C); 1430–1490 (–N–C=S);
1330 (amide-II); 1260 (N=C=S); 1150 (C=S).
1{[CuClL0][Cl]} 1, a bis-sulfur-bridged copper(II) dinuclear
1
complex, {[CuLCl2]2} 3, and a reduced copper(I)–L complex,
DOI: 10.1039/b206257g
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Reaction of CuCl2 with L
collection. A total of 40 4.00ꢁ oscillation frames with an expo-
sure time of 30.0 min. per frame were collected.
For crystal {[CuLCl2]2} 3, a red prismatic crystal with
dimensions of 0.40 ꢂ 0.15 ꢂ 0.10 mm3 was used for data collec-
tion. A total of 18 10.00ꢁ oscillation frames with an exposure
time of 45.0 min. per frame were collected.
CCDC reference numbers 182319–182321 for compounds
1–3, respectively. See http://www.rsc.org/suppdata/nj/b2/
b206257g/ for crystallographic files in CIF or other electronic
format.
The reaction products in the CuCl2–L system were synthesized
and isolated by the ‘‘dropping method’’ under two sets of
conditions.
Route A: To CuCl2 in ethanol was added dropwise an etha-
nol solution of L at a speed of about one drop per 2 h at room
temperature for several days until a dark-brown needle-like
crystal of 1{[CuClL0][Cl]}, 1, was formed. Details of the
1
synthesis and the characterization of complex 1 have been
described previously.7
Route B: To an acetone solution (50 ml) of L (0.02 mmol),
CuCl2 (0.01 mmol) in acetone (20 ml) was added dropwise at
the same speed as above. At first, a pale-yellow prismatic crys-
tal, [CuL2Cl] 2, began to appear within several days, isolated in
23% yield based on CuCl2 . Anal. found: C, 50.78; H, 3.50; N,
13.65. Calcd for CuC26H22O2N6ClS2 : C, 50.89; H, 3.59; N,
13.70%. IR (KBr, cmꢀ1): 3070–3217 (N–H); 1716 [–C(O)–
NH–carbonyl]; 1526–1606 (pyridine C=N, C=C); 1431 (phen
ring); 1328 (amide-II); 1265 (C=S); 1150 (N=C=S). After two
or more weeks, a second red prismatic crystal, {[CuLCl2]2}
3, formed simultaneously, isolated in 34% yield based on
CuCl2 . Anal. found: C, 40.07; H, 3.12; N, 10.69. Calcd for
CuC13H11OSN3Cl2 : C, 39.84; H, 2.81; N, 10.73%. IR (KBr,
cmꢀ1): 3439–3037 (N–H); 1667 (carbonyl C=O); 1187 (N–
C=S). Both crystals were of adequate size and quality for
X-ray studies and easily separated manually.
Results and discussion
Syntheses by the ‘‘dropping method’’
Despite the complexity and versatility of the copper(II)–
thiourea reaction systems, we think that the key to the investi-
gation of such systems lies in control of the relative concentra-
tion between the reaction components. Therefore, both
converse approaches were employed: one is dropwise addition
of L to copper(II) chloride in ethanol or acetone solution, and
the other is conversely dropping copper(II) chloride into L in
the same solvent. In the first approach a sufficient amount of
CuCl2 exists in the solution to oxidize the additive ligand L
and further coordinate to the in situ generated oxidative
product L0, forming
a
dark-brown needle-like com-
pound, 1{[CuClL0][Cl]} 1, which is a chloride-bridged one-
1
dimensional polynuclear copper(II) complex coordinated by
the oxidative cyclization product L0 (Schemes 1, 2 and
Fig. 1). In the second approach additive CuCl2 is almost
completely reduced to Cu(I), which can be coordinated further
to the excess L in the solution, yielding a pale-yellow reduced
copper(I) mononuclear complex, [CuL2Cl] 2 (Scheme 2 and
Fig. 2), and only in the presence of a lower concentration of
L can the additive CuCl2 be not completely reduced; in this
case, the ligand L can effectively bridge the CuCl2 to form a
red bis-sulfur-bridged copper(II) dinuclear complex,
{[CuLCl2]2} 3 (Scheme 2 and Fig. 3).
It follows that by using the dropping method to control the
relative concentrations of the components (namely stoichio-
metric control), the reactions take place in a regular sequence
and the different products are formed under different concen-
tration conditions; consequently, the reaction pathways are
revealed in more detail and in this system we see that the
thiourea L plays a triple role: acting as a reductant for Cu(II)
and complexing agent for reduced copper(I) and forming a
polynuclear complex through its oxidized derivative L0. In
addition, the dropping method allowed the reactions to pro-
ceed under mild conditions favoring of the formation of single
crystals suitable for X-ray studies. The selection of the solvents
was made for the sake of the growth of quality crystals; the
X-ray quality crystals of 1 were grown in ethanol solution,
and in acetone for 2.
X-Ray crystallography
Crystal data and experimental details are given in Table 1.
Data were collected on a RAXIS-IV area detector diffract-
ometer with graphite monochromated Mo-Ka radiation
˚
(l ¼ 0.7101 A). The structures were solved by direct methods
and expanded using Fourior techniques. The data were cor-
rected for Lorentz and polarization effects. The non-hydrogen
atoms were refined anisotropically and hydrogen atoms were
included but not refined. All calculations were performed by
using the teXsan crystallographic software package of Molecu-
lar Structure Corporation.9
For crystal 1{[CuClL0][Cl]} 1, a dark-brown prismatic
1
crystal with dimensions of 0.60 ꢂ 0.10 ꢂ 0.10 mm3 was used
for data collection. A total of 28 3.00ꢁ oscillation frames with
an exposure time of 20.0 min. per frame were collected.
For crystal [CuL2Cl] 2, a pale-yellow prismatic crystal
with dimensions of 0.4 ꢂ 0.3 ꢂ 0.10 mm3 was used for data
Table 1 Crystallographic data for 1–3
1
2
3
Formula
M
C13H9ON3SCl2Cu C26H22N6O2ClS2Cu C13H11N3OCl2SCu
389.55
613.62
391.76
Crystal system Orthorhombic
Monoclinic
Cc(#9)
Triclinic
Space group
˚
Pbca(#61)
16.801(3)
6.951(1)
¯
P1(#2)
Molecular structures
a/A
10.761(3)
36.62(1)
7.511(9)
9.674(4)
11.085(6)
7.751(3)
97.63(3)
102.76(3)
108.18(2)
751.81
2
One-dimensional polynuclear Cu(II) complex, 1{[CuClL0]-
[Cl]} 1. As can be seen in Fig. 1, compound 1 is a
˚
b/A
1
˚
c/A
24.335(6)
a/ꢁ
b/ꢁ
g/ꢁ
116.186(6)
3
˚
U/A
2841.86
8
2556.18
4
Z
T/K
m/cmꢀ1
No. reflect.
291(1)
20.59
2523
293(1)
11.17
2100
293(1)
19.46
2022
No. obsd refect. 2033 [I > 2.00s(I )] 2075 [I > 1.00s(I )] 2001 [I > 1.00s(I )]
Rint
Ra
0.00
0.062
0.097
0.00
0.055
0.050
0.00
0.073
0.066
b
Rw
a
b
2
R ¼ S[|Fo| ꢀ |Fc|]/S|Fo|. Rw ¼ [Sw(Fo ꢀ Fc2)2/Sw(Fo2)2]1/2
.
Scheme 1 Oxidative cyclization of L.
1630
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Fig. 1 Perspective view of the zigzag chain structure of the poly-
1
1
nuclear complex {[CuClL0][Cl]} 1 with atom labeling scheme.
Fig. 2 Perspective view of compound [CuL2Cl] 2 with atom labeling
scheme. The hydrogen atoms are omitted for clarity.
one-dimensional copper(II) coordination compound, consist-
ing of an infinite zigzag (Cu–Cl)n chain linked by single
Cu–Cl–Cu0 bridges. Each copper(II) center is coordinated to
two bridging and one terminal chloride ions, as well as a
heterocyclic thiadiazole nitrogen atom, displaying a distorted
tetrahedral geometry. The resulting chain unit has symmetrical
angles [Cl(1)–Cu–S(1), 117.83(7)ꢁ; Cl(1)–Cu–S(2), 119.42(7)ꢁ
and S(1)–Cu–S(2), 122.74(6)ꢁ]. The three donor atoms together
with the central copper(I) ion are perfectly coplanar. The Cu–
S(1)–C(8) angle of 111.3ꢁ indicates that the thiourea sulfur is
bound to Cu(I) primarily via a lone pair of electrons in a
non-bonding sp2 sulfur orbital, probably containing some con-
tribution of the p electrons of the S–C bond as well, since the
0
˚
Cl–Cu–Cl bridging distances of 2.313 A, a Cuꢃ ꢃ ꢃCu separation
0
ꢁ
˚
of 3.52 A and a bridging Cu(1)–Cl–Cu(1) angle of 99.07(7)
(Table 2).10 The terminal Cu–Cl(2) distance [2.247(2) A] is
˚
˚
˚
shorter than the bridging one (2.313 A), as expected. The
S(1)–C(8) distance of 1.696(6) A agrees very well with those in
related compounds,11 being intermediate between 1.82 A for a
C–S single bond and 1.56 A for a C=S double bond. The
ligand L0 coordinated to the copper center in the chain is the
oxidative cyclization product formed via the elimination of
both protons from the precursor H2L at the amide nitrogen
(N3) and thioamide nitrogen (N2); its complete hydrogen
molecular structure and coordination mode are unequivocally
displayed in Fig. 1. The chains run along the crystallographic b
axis and the ligands are near planar (mean deviation 0.094),
being disposed parallel and alternate to each other both in
the intra- and inter-chains, forming continuous p stacks
throughout the crystal (Fig. 1).
˚
12
˚
use of the sp2 orbital and electron pair is the most common
bonding mode of thiourea to Cu(I) ion.13 The bond distances
in L are normal as in other similar thioureas.11
Bis-sulfur-bridged Cu(II) dinuclear complex, {[CuLCl2]2} 3.
The molecular structure of 3 shows the complex to be a bis-sul-
fur-bridged copper(II) compound (Fig. 3). Selected geometrical
details are listed in Table 4. Each copper(II) ion is four-coordi-
nate with bonds from two terminal chloride ions and two brid-
ging sulfur atoms originating from ligands L, displaying a
distorted tetrahedral geometry. The two copper(II) ions and
the bridging sulfur atoms form a strictly planar Cu2S2 core
Reduced Cu(I) mononuclear complex, [CuL2Cl] 2. The mole-
cular structure of 2 is illustrated in Fig. 2. Selected bond dis-
tances and angles are listed in Table 3. 2 is a complex of the
reduced copper(I) ion with the ligand L, exhibiting a planar
˚
˚
with two short [2.257(2) A] and two longer Cu–S [2.595(2) A]
molecular configuration with
a butterfly geometry. The
distances. The core also contains a short Cuꢃ ꢃ ꢃCu separation
copper(I) center displays a regular three-coordination with
similar bond distances [Cu–S(1), 2.223(2); Cu–S(2), 2.225(2)
˚
and Cu–Cl, 2.296(2) A], and approximately the same bond
0
˚
of 2.95 A, a narrow bridging Cu(1)–S–Cu(1) angle of
74.4(6)ꢁ, and a wide S(1)–Cu–S(1)0 angle of 105.59(6)ꢁ. Both
terminal chloride ions are coordinated to the copper(II) ion at
˚
different distances: Cu(1)–Cl(1), 2.257(2) A and Cu(1)–Cl(2),
˚
2.337(2) A.
˚
Table 2 Selected bond distances (A) and angles (deg) for 1
Cu(1)–Cl(1)
Cu(1)–Cl(10)
Cu(1)–Cl(2)
2.313(2)
2.319(2)
2.247(2)
2.083(5)
1.356(8)
1.381(8)
1.350(8)
98.37(7)
122.87(8)
113.39(8)
99.8(2)
S(1)–N(1)
S(1)–C(6)
O(1)–C(7)
C(7)–C(8)
N(2)–C(6)
N(2)–C(5)
1.758(6)
1.744(7)
1.241(9)
1.476(9)
1.334(8)
1.368(8)
˚
Table 3 Selected bond distances (A) and angles (deg) for 2
Cu(1)–N(2)
N(3)–C(6)
N(3)–C(7)
Cu(1)–Cl(1)
Cu(1)–S(1)
2.296(2)
2.223(2)
2.225(2)
1.696(6)
117.83(7)
119.42(7)
122.74(6)
111.3(2)
N(1)–C(8)
N(2)–C(8)
1.352(7)
1.371(6)
1.423(6)
1.213(6)
Cu(1)–S(2)
S(1)–C(8)
N(1)–C(7)
O(1)–C(7)
N(1)–C(5)
Cl(1)–Cu(1)–Cl(10)
Cl(1)–Cu(1)–Cl(2)
Cl(10)–Cu(1)–Cl(2)
Cl(1)–Cu(1)–N(2)
Cl(10)–Cu(1)–N(2)
Cl(2)–Cu(1)–N(2)
Cu(1)–Cl(1)–Cu(10)
110.8(2)
110.4(2)
99.07(7)
Cl(1)–Cu(1)–S(1)
Cl(1)–Cu(1)–S(2)
S(1)–Cu(1)–S(2)
Cu(1)–S(1)–C(8)
C(7)–N(1)–C(8)
O(1)–C(7)–N(1)
S(1)–C(8)–N(1)
S(1)–C(8)–N(2)
129.4(4)
122.5(5)
124.2(4)
120.2(4)
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˚
Table 4 Selected bond distances (A) and angles (deg) for 3
Cu(1)–Cl(1)
Cu(1)–Cl(2)
Cu(1)–S(1)
Cu(1)–S(10)
S(1)–C(8)
C(6)–C(7)
2.257(2)
2.337(2)
2.257(2)
2.595(2)
1.692(6)
1.486(8)
113.70(7)
105.03(7)
102.36(6)
118.47(7)
109.90(7)
105.59(6)
74.41(6)
115.6(5)
N(1)–C(7)
N(1)–C(8)
N(2)–C(8)
N(2)–C(9)
O(1)–C(7)
1.397(8)
1.371(7)
1.333(7)
1.405(7)
1.225(7)
Cl(1)–Cu(1)–Cl(2)
Cl(1)–Cu(1)–S(1)
Cl(2)–Cu(1)–S(1)
Cl(10)–Cu(1)–S(1)
Cu(20)–Cu(1)–S(1)
S(1)–Cu(1)–S(10)
Cu(1)–S(1)–Cu(10)
N(2)–C(9)–N(3)
Cu(1)–S(1)–C(8)
Cu(10)–S(1)–C(8)
S(1)–C(8)–N(2)
N(1)–C(8)–N(2)
N(1)–C(8)–S(1)
O(1)–C(7)–N(1)
O(1)–C(7)–C(6)
N(1)–C(7)–C(6)
100.6(2)
115.0(2)
120.1(5)
117.8(5)
122.0(5)
120.4(6)
121.5(6)
118.2(6)
Interpretation on the reaction pathways
Of particular interest is that four kinds of reactions occur in
the system, including oxidative cyclization, chloride-bridging
coordination, bis-sulfur-bridging coordination and reduced
copper(I) coordination.
In the redox process, the Cu(II) was reduced to Cu(I) by
L and L was oxidized to an unexpected cyclization product,
2-benzoylimino-[1,2,4]thiadiazolo[2,3-a]pyridine (labeled as
L0); its molecular structure was clearly revealed in 1 (Fig. 1).
The oxidative cyclization of N-(2-pyridyl) thioureas by Br2
in CH3COOH or PCl5 in POCl316 has been reported. The oxi-
dative cyclization of N-(2-pyridinyl)benzenecarbothiamide and
N-(2-pyridinyl)ketothioacetamide by copper(II) ions, as well as
the coordination of the oxidized products with copper(II) or
copper(I) ions, have also been invesitigated by Fuller17 and
Iliopoulos18 and their colleagues, but no definite products have
been isolated and characterized.
Fig. 3 Perspective view of dinuclear complex {[CuLCl2]2} 3 with
atom labeling scheme. The hydrogen atoms are omitted for clarity.
By analogy with previous reports the reaction of L with a
large excess of copper(II) chloride in acetone or alcohol solu-
tion may result in the formation of two forms of the oxidative
cyclization products L0: 2-benzoylimino-[1,2,4]thiadiazolo-
[2,3-a]pyridine (a), or its thiadiazolo-[2,3-a]pyridium (b), as
shown in Scheme 1. If L0 is coordinated in its neutral form
(a), the copper center would be in the divalent state, if in the
cation form (b), the copper would be coordinated in its mono-
valent state, therefore, it is necessary to determine the coordi-
nated form of L0 in 1. The main difference between both forms
lies in the elimination of one or two protons from the precur-
sor L, namely, deciding on the presence or absence of a proton
on the amide nitrogen atom N(3) in L0. On the basis of our
experiments L0 should be thiadiazolo-[2,3-a]pyridine rather
than its pyridium ion: in the molecular structure of L0 the
amide nitrogen atom N(3) is deprotonated (Fig. 1), and thus
1
IR and H NMR spectroscopy
IR spectra data are summarized in Table 5. The vibrational
stretches of the –NH groups in the ligand L give rise to a multi-
ple absorption band around 3300 cmꢀ1, which disappears in
complex 1, giving evidence of the loss of the N–H proton upon
oxidative cyclization (Scheme 1). The vibrational bands at
1340, 1250 and 860 cmꢀ1 observed in the free ligand L can
be attributed to the n(C=S) vibration;14 two frequencies
(1340, 1250 cmꢀ1) show a slightly negative shift in complexes
2 and 3. The frequency of 860 cmꢀ1 is absent in 1 and weak
in 2 and 3. These observations indicate the involvement of
the C=S group in the cyclization and coordination, which
are in reasonable agreement with the structure results.
1
In the H NMR spectrum of the free ligand L four signals
for the protons on the pyridyl ring were observed clearly at
7.2–7.7 and five signals for the protons on the phenyl ring at
d 7.8–8.8, while the NH resonances (D2O, exchangeable) were
observed at d 9.8 as a broad singlet.15,16 For the complex 1 the
1H NMR signals arising from pyrido (d 7.5–7.7) and phenyl
(d 8.0–8.9) protons show similar patterns to those assigned
to the corresponding protons of L. However, it is worth noting
that no coupling between 1H and N nuclei could be detected in
1, as a consequence of the deprotonation of the N–H bonds
upon the oxidative cyclization (Scheme 1).
˚
the N(3)–C(6) bond length (1.356 A) approaches that of a dou-
ble bond,19 in the IR spectrum the n(N–H) vibration, observed
at 3300 cmꢀ1 in its precursor L, is absent, and the NH reso-
1
nances at d 9.8 in the H NMR spectrum cannot be detected
in L0. Consequently, these results allow us to establish the
coordination of the thiadiazolo[2,3-a]pyridine to the copper(II)
ion in 1.
In order to confirm the reaction pathways we have further
examined the reaction directly between copper(II) chloride
Table 5 Major IR bands (cmꢀ1) of L and complexes
Complex
n(CH), d(NH)
n(C=O)
n(C=N), n(C=C)
n(phen, ring)
n(C=S)
n(N=C=S)
L
3000, 3300
3000
1680
1620
1716
1667
1500–1600
1500–1600
1606
1440
1460
1431
1448
1340, 1250, 860
1680, 1320–1280
1328, 1265, 807
1314, 1245, 856
1160
1160
1150
1158
Polymer 1
Complex 2
Dimer 3
3070, 3217
3000–3480
1614
1632
New J. Chem., 2002, 26, 1629–1633

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Scheme 2 Reaction pathways in CuCl2–L system.
and L0, the latter was synthesized from the oxidation of L by
References
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yet unambiguously observed the formation of 1, which was
easily formed by the dropping method as described above; this
suggests that the reaction may not proceed firstly in the oxida-
tion of L to L0 and then coordination of L0 to copper(II), the
mechanism may be more complicated and still remains to be
explored.
But it seems of interest that the oxidative cyclization product
L0, as its precursor H2L, still acts as a ligand to participate in
the copper(II) coordination through its thiadiazole nitrogen
atom. Such heterocyclic nitrogen coordinated compounds are
comparatively rare.
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atoms to form a bis-sulfur-bridged copper(II) complex 3 (Fig. 3),
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per(I) complex 2 (Fig. 2).
1
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types of reactions occur in the system. On the basis of the pre-
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between the reaction components may be an excellent method
to make the reaction proceed towards a certain direction and
under mild conditions. According to the molecular structures
of the products and conditions of the formation reactions,
the reaction pathways in the system were revealed.
´
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Acknowledgement
We thank the Henan Science and Technology Committee for
financial support.
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