Unexpected multiple bond cleavage and rearrangement of organosulfide ligands in the presence of Cu(ii) assisted by solvothermal and solvothermal-microwave conditions

Article abstract of DOI:10.1039/c0dt00973c

An unprecedented in situ multiple bond cleavage of S-S, S-C(sp²) and C-N in the pyrimidinedisulfide (pym₂S₂) ligand is observed by the reaction of CuCl₂·2H₂O with this ligand under solvothermal and solvothermal-microwave conditions. In this process the formation of the compound [Cu^II(μ-Cl)(Cl)L]₂, where L represents the new ligand (L = 2-(pyrimidin-2-ylamino)-1,3-thiazole-4- carbaldehyde), is observed. This ligand has been further isolated and X-ray characterized. The similar reaction carried out under solvothermal-microwave conditions gives, in addition to the latter compound, the complex {9·[Cu(pym₂S₃)(μ-Cl)(Cl)]₂· [Cu(pym₂S₂)(μ-Cl)(Cl)]₂}. Coordination of a pyrimidinetrisulfide ligand (pym₂S₃) is reported for the first time. This work represents an illustrative example of the novel synthetic perspectives attainable via solvothermal-microwave procedures. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0dt00973c

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PAPER
Unexpected multiple bond cleavage and rearrangement of organosulfide
ligands in the presence of Cu(II) assisted by solvothermal and
solvothermal-microwave conditions†
Salome´ Delgado,*a Almudena Gallego,a Oscar Castillob and Fe´lix Zamora*a
Received 6th August 2010, Accepted 18th October 2010
DOI: 10.1039/c0dt00973c
An unprecedented in situ multiple bond cleavage of S–S, S–C(sp2) and C–N in the pyrimidinedisulfide
(pym2S2) ligand is observed by the reaction of CuCl2·2H2O with this ligand under solvothermal and
solvothermal-microwave conditions. In this process the formation of the compound [CuII(m-Cl)(Cl)L]2,
where L represents the new ligand (L = 2-(pyrimidin-2-ylamino)-1,3-thiazole-4-carbaldehyde), is
observed. This ligand has been further isolated and X-ray characterized. The similar reaction carried
out under solvothermal-microwave conditions gives, in addition to the latter compound, the complex
{9·[Cu(pym2S3)(m-Cl)(Cl)]2·[Cu(pym2S2)(m-Cl)(Cl)]2}. Coordination of a pyrimidinetrisulfide ligand
(pym2S3) is reported for the first time. This work represents an illustrative example of the novel
synthetic perspectives attainable via solvothermal-microwave procedures.
novelmaterials. Insome cases metal–sulfur containingcompounds
Introduction
are useful as structural models of biological systems.7 Although
the in situ cleavage of S–S or both S–S and S–C(sp2) bonds
The design and synthesis of novel coordination architectures with
in situ generated ligands (especially those that are inaccessible by
direct preparation) is a topic of great interest in coordination and
organic chemistry for both discovering new organic reactions and
understanding their mechanism.1
in dithiodipyridine ligands have been recently observed under
solvothermal conditions in the presence of Co2+ or Cu2+ ions,6d,8
the cleavage of S–S, S–C(sp2) and N–C(sp2) bonds has not been
reported.
A large number of structures of coordination compounds
reported to date have been prepared under hydro- or solvothermal
conditions by in situ metal/ligand reactions in the presence of
transition-metal ions. In few cases unexpected chemical, structural
and/or compositional changes in the organic ligands occur during
the reaction process.2 Microwave and solvothermal-microwave-
assisted synthesis might dramatically reduce the reaction time
and this simple and energy-efficient process has become a rapidly
developing synthetic method. Although microwave is fairly com-
mon for organic syntheses,3 examples of coordination compounds
prepared by this procedure are still very scarce.4
On the other hand, organosulfides exhibit versatile binding
modes in their coordination to metal ions.5 In addition, they
show a rich redox chemistry6 based on both oxidative formation
and reductive cleavage of the disulfide bonds, which has been
explored as an attractive route towards functional ligands and
In this paper, we deal with the study of the reaction between
CuCl2 and bis(2-pyrimidyl)disulfide (pym2S2) under solvother-
mal and solvothermal-microwave-assisted conditions. The adjust-
ment of the hydrothermal and solvothermal-microwave synthetic
conditions leads to an unprecedented in situ multiple bond
cleavage of S–S, S–C(sp2) and C–N in the pyrimidinedisul-
fide (pym2S2) ligand leading to [CuII(m-Cl)(Cl)L]2 dimer forma-
tion, where L represents a novel 2-(pyrimidin-2-ylamino)-1,3-
thiazole-4-carbaldehyde ligand. The complex {9·[Cu(pym2S3)(m-
Cl)(Cl)]2·[Cu(pym2S2)(m-Cl)(Cl)]2} has also been isolated when the
process was carried out under specific solvothermal-microwave
conditions. This work describes an example of the potential
use of novel synthetic perspectives attainable via solvothermal-
microwave procedures.
Experimental
aDepartamento de Qu´ımica Inorga´nica, Universidad Auto´noma de Madrid,
28049, Madrid, Spain. E-mail: salome.delgado@uam.es, felix.zamora@
uam.es
Materials
All chemicals were of reagent grade and were used as commer-
cially obtained. The bis(2-pyrimidyl)disulfide ligand was prepared
according to the published procedure.9 FTIR spectra (KBr
pellets) were recorded on a Perkin-Elmer 1650 spectrophotometer.
Elemental analyses were performed on a Perkin-Elmer 240 B
bDepartamento de Qu´ımica Inorga´nica, Facultad de Ciencia y Tecnolog´ıa,
Universidad del Pa´ıs Vasco, Apartado 644, E-48080, Bilbao, Spain. E-mail:
oscar.castillo@ehu.es
† CCDC reference numbers 781619–781621. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00973c
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microanalyzer. ESI-MS spectra were recorded on a mass spec-
trometer with a QTOF analyzer mode QSTAR pulsar (Applied
Biosystems). The microwave oven used for synthesis was an
Ethos Synth Lab Station, Milestone Inc. equipped with a PRO-
16 rotor (Milestone Inc.). The multimode microwave has a twin
magnetron (2 ¥ 800 W, 2.45 GHz) with a maximum delivered
power of 1000 W. During the experiments, time, temperature and
power were controlled with the “easyWAVE” software package.
An advanced fiber-optic temperature sensor (ATC-FO, Milestone
Inc.) allowed direct monitoring of the internal temperature; the
fibre-optic probe was inserted into a thermowell of the reference
vessel. This thermowell warrants optimal thermal exchange with
the reaction mixture, yet isolates the probe from physical contact
with the media inside the vessel.
dried in air (1: 0.044 g 40% yield, based on Cu; 2: 0.005 8% yield,
based on Cu).
Anal. Calc. for 2 C16H12Cl4Cu2N8S5.8: C, 24.92%, H, 1.57%, N,
14.53%, S, 24.12%. Found: C, 25.01%, H, 1.60%, N, 14.47%, S,
24.09%. IR selected data (KBr, cm-1): 3063 (w), 1571 (s), 1552 (s),
1384 (vs), 1170 (m), 820 (m), 761 (m), 745 (m), 639 (m).
X-Ray data collection and structure determination†
Crystal data were collected on a Bruker SMART 6 K CCD
diffractometer (Mo-Ka radiation for compounds 1 and 2, and
Cu-Ka radiation for compound 3). Data reduction and cell
refinement were carried out using the DENZO and SCALE-
PACK10 programs. The structure was solved by standard Patterson
methods11 and refined by full-matrix least-squares methods based
on F2 using the SHELXTL-PLUS,12 SHELXL-9713 and WinGX14
programs. The refinement of the structural model of 2 indicates
the existence of a random mixture of bis(2-pyrimidyl)disulfide
and bis(2-pyrimidyl)trisulfide. Two substantial maxima in the
Magnetic measurement was performed on a Quantum Design
SQUID susceptometer covering the temperature range 5.0–300 K
at a magnetic field of 1000 G.
Preparations
˚
difference Fourier map were found at a distance of ca. 2.0 A
Solvothermal synthesis of [CuII(l-Cl)(Cl)L]2 (1). A mixture of
CuCl2·2H2O (0.088 g, 0.52 mmol) and pym2S2 (0.115 g, 0.52 mmol)
in 26 mL of CH2Cl2–CH3CN (1 : 1), was fully stirred at room
temperature for 30 min, sealed in a 30 mL Teflon-lined autoclave
and heated at 90 ◦C for 20 h. Then, the autoclave was slowly cooled
(2.5 ◦C h-1) to ambient temperature.
Green crystals of 1 were obtained from solution, after the filtrate
was allowed to stand in a refrigerator (0 ◦C) for 2 weeks, washed
with Et2O and dried in air (0.090 g, 51% yield, based on Cu)
together with crystals of elemental sulfur (crystal data for S8:
˚
between them and ca. 1.8 A from atoms C1 and C6. The
constrained refinement of the disulfide and trisulfide ligands
indicated that only 10% of bis(2-pyrimidyl)disulfide is present in
the compound. The sum of the occupancy parameters of the two
terminal ligands was constrained to be 1.0. The bond distances
within the aromatic rings of compound 2 were restrained to be
similar in bis(2-pyrimidyl)disulfide and bis(2-pyrimidyl)trisulfide
ligands.
All non-hydrogen atoms were refined anisotropically in com-
pounds 1 and 3. The mentioned disorder in compound 2 did not
allow the anisotropic refinement of the carbon and nitrogen atoms.
All hydrogen atoms were included in geometrically calculated
positions and refined isotropically according to the riding model.
Crystal data, data collection and refinement parameters for
compounds 1–3 are summarized in Table 1.
˚
˚
orthorhombic, space group Fddd, a = 10.464(4) A, b = 12.851(5) A,
c = 24.51(1) A, V = 3296(2) A ).
3
˚
˚
Anal. Calc. for 1 C16H12Cl4Cu2N8O2S2: C, 28.20%, H, 1.78%,
N, 16.45%, S, 9.41%. Found: C, 27.58%, H, 1.99%, N, 16.61%,
S, 9.36%. IR selected data (KBr, cm-1): 1661 (vs), 1653 (vs), 1603
(vs), 1561 (vs), 1529 (vs), 1496 (s), 1442 (vs), 1408 (vs), 1225 (s),
1165 (s), 891 (w), 786 (m), 627 (m), 527 (m).
When a solution of 1 in CH3CN was treated with the
Chelex resin for 2 h, the 2-(pyrimidin-2-ylamino)-1,3-thiazole-4-
carbaldehyde ligand, compound 3, was isolated and characterized.
1H NMR (300 MHz. DMSO-d6): d 12.52 (s, 1H, NH), 9.98 (s,
1H, aldehyde), 8.75 (d, J = 4.5 Hz, 2H, pyrimidin), 8.37 (s, 1H,
thiazole), 7.17 (t, J = 4.5 Hz, 1H, pyrimidin). IR selected data
(KBr, cm-1): 1642 (vs), 1601 (vs), 1579 (vs), 1550 (vs), 1517 (vs),
1452 (vs), 1442 (vs), 1399 (vs), 1229 (s), 1167 (vs), 816 (s), 697 (s),
660 (m), 627 (s).
Results and discussion
The treatment of CuCl2·2H2O with pyrimidinedisulfide (pym2S2)
◦
in CH2Cl2–CH3CN under solvothermal conditions (90 C, 20 h)
leads to the formation of a green crystalline solid charac-
terized as [CuII(m-Cl)(Cl)L]2 (L = 2-(pyrimidin-2-ylamino)-1,3-
thiazole-4-carbaldehyde) (1). The same reaction carried out under
solvothermal-microwave conditions (90 ◦C, 1 h) gives com-
pound 1 together with {9[Cu(pym2S3)(m-Cl)(Cl)]2·[Cu(pym2S2)(m-
Cl)(Cl)]2} (2) (Scheme 1). Similar results are obtained when the
reaction proceeds under milder (60 ◦C, 1 h) or stronger (90 ◦C,
3–20 h) solvothermal-microwave-assisted conditions. All these
processes produce pale-yellow orthorhombic crystals of elemental
Solvothermal microwave synthesis of {9·[Cu(pym2S3)(l-
Cl)(Cl)]2·[Cu(pym2S2)(l-Cl)(Cl)]2} (2). CuCl2·2H2O (0.054 g,
0.32 mmol) and pym2S2 (0.071 g, 0.32 mmol) in 16 mL of CH2Cl2–
CH3CN (1 : 1) were placed in a 70 mL Teflon leaned vessel, sealed
and sited into a microwave oven equipped with a rotor. The
multimode microwave has a twin magnetron (2 ¥ 800 W, 2.45 GHz)
with a maximum delivered power of 1000 W. The temperature was
increased from room temperature to 90 ◦C in 20 min (microwave
power was set at 350 W) and kept for 1 h. Then, the vessel was
slowly cooled down to room temperature in 9 h. Green crystals of
1 were obtained from the filtrate after it was allowed to stand in a
refrigerator (0 ◦C) for 2 weeks, together with deep green crystals
of 2, which were manually separated, then washed with EtO2 and
2-
sulfur S8 as a by-product and formation of SO4 anions was
inferred from the solutions analysis.
The crystal structure of compound 1 consists of centrosym-
metric double chloro-bridged copper(II) dimeric entities. The in
situ formed 2-(pyrimidin-2-ylamino)-1,3-thiazole-4-carbaldehyde
molecule is coordinated to the metal centre through its N1 and N2
atoms to establish a six-membered chelating ring. The value of the
bite angle is 86.67(7)◦. Two bridging and one terminal chlorido
anions are also coordinated to the metal centre and complete
its CuCl3N2 coordination environment. The bridging chlorido
848 | Dalton Trans., 2011, 40, 847–852
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Table 1 Single-crystal data and structure refinement details
1
2
3
Formula
Mr
C16H12Cl4Cu2N8O2S2
681.34
C16H12Cl4Cu2N8S5.8
771.11
C8H6N4OS
206.23
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
˚
a/A
9.8830(5)
9.6562(5)
12.5052(7)
112.968(3)
1098.79(10)
2
2.059
0.15 ¥ 0.10 ¥ 0.05
Blue
676
2.648
0.0375
26 132
2845
2406
154
1.071
11.9670(6)
6.8657(4)
16.9169(8)
110.541(3)
1301.55(12)
2
1.984
0.21 ¥ 0.10 ¥ 0.09
Blue
765.5
2.553
0.0589
20 691
3681
3144
171
1.170
5.4675(4)
21.7393(16)
8.3532(5)
120.377(4)
856.56(10)
4
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
D
calc/Mg m-3
1.599
Crystal size/mm
Color
0.10 ¥ 0.08 ¥ 0.05
Colourless
424
3.124
0.0325
10 249
1612
1554
127
F(000)
m/mm-1
Rint
Reflns. collected
Reflns. independent
Reflns.unique [I > 2s(I)]
Parameters
GOF/Sa
1.116
0.0313/0.0840
0.0323/0.0852
R1/wR2[I ≥ 2s(I)]b
R1/wR2[all data]b
0.0248/0.0600
0.0349/0.0651
0.0475/0.1147
0.0576/0.1189
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a S = [ w(|Fo| - |Fc|)2/(Nobs - Nparam)]1/2
.
b R1 = (|Fo| - |Fc|)/ |Fo|; wR2 = w(Fo - Fc2)2/ w|Fo|2]1/2
.
2
Scheme 1 Schematic synthetic procedures.
anions are coordinated to the two copper atoms in an asymmetric
fashion with significantly different bond lengths: 2.2751(6) and
Fig. 1 View of dimeric entity in compound 1, showing the atom labelling
˚
scheme.
2.5754(6) A, caused by the active Jahn–Teller distortion of the
copper(II) ion. A drawing of the dimer structure showing the
labelling scheme is given in Fig. 1. Selected bond lengths and
angles are reported in Table 2.
The penta-coordination of Cu(II) is very common and usually
presents either a square-pyramidal or a trigonal-bipyramidal
geometry (or any of the distorted intermediate geometries). The
geometry of the CuCl3N2 chromophore can be quantitatively
characterised using the parameter t, as defined by Addison et al.15
The calculated value of t = 0.46 (relative to one for a regular
trigonal bipyramid and zero for a square pyramid) indicate that
the coordination geometry around the metal centre, although
intermediate between the two ideal polyhedrons, is closer to
an elongated tetragonal pyramid with chlorido anions placed
alternatively on equatorial and apical positions. As a result, the
geometry of the complex consists of two square pyramids sharing
one base-to-apex edge, with parallel basal planes.16 The copper
atom is displaced from the mean equatorial plane towards the
◦
˚
Table 2 Selected bond distances (A) and angles ( ) for compound 1
Cu1–N1
Cu1–N2
Cu1–Cl1
Cu1–Cl2
Cu1–Cl1i
1.977(2)
2.079(2)
2.5754(6)
2.3163(6)
2.2751(6)
N1–Cu1–N2
N1–Cu1–Cl1
N1–Cu1–Cl2
N1–Cu1–Cl1i
N2–Cu1–Cl1
N2–Cu1–Cl2
N2–Cu1–Cl1i
Cl1–Cu1–Cl2
Cl1–Cu1–Cl1i
Cl1i–Cu1–Cl2
Cu1–Cl1–Cu1i
86.67(7)
91.61(5)
89.02(5)
178.43(5)
104.22(5)
150.90(5)
93.32(5)
104.66(2)
89.92(2)
90.23(2)
90.08(2)
Symmetry code: (i) -x + 1, -y, -z.
˚
corresponding axial chlorido bridging atoms by 0.287 A.
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◦
ial bonds distances [Cu–Cl1i: 2.7545(10) and Cu–S2/S2A:
˚
Table 3 Selected bond distances (A) and angles ( ) for compound 2
˚
2.6616(11)/2.689(10) A] substantially longer than the equatorial
Cu1–Cl1
pym2S3
Cu1–N1
Cu1–N3
Cu1–S2
2.2760(9)
Cu1–Cl2
pym2S2
Cu1–N1A
Cu1–N3A
2.2705(10)
˚
˚
ones (ca. 2.27 A for chlorido atoms and 2.05–2.12 A for the ni-
trogen atoms, respectively). The pym2S3 ligand is fac-coordinated
to the metal centre through three donor positions: N1, S2 and
N3. Although this coordination mode generates two equivalent
and robust five membered chelate rings for pym2S3, in the case
of pym2S2 a strained four membered chelate ring is present.
This could be the reason for Cu–N bond distances established
with the pym2S2 ligand (ca. 2.12 A) being longer than with
pym2S3 (ca. 2.06 A). The bridging chlorido anions are coordinated
asymmetrically with significantly different bond lengths: 2.2760(9)
2.050(4)
2.062(4)
2.6616(11)
2.115(12)
2.119(14)
2.689(10)
2.7545(10)
85.65(3)
Cu1–S2A
Cu1–Cl1i
Cl1–Cu1–Cl2
94.97(4)
Cl1–Cu1–Cl1i
Cl12–Cu1–Cl1i
pym2S2
93.44(3)
pym2S3
N1–Cu1–N3
N1–Cu1–S2
N1–Cu1–Cl1
N1–Cu1–Cl2
N1–Cu1–Cl1i
N3–Cu1–S2
N3–Cu1–Cl1
N3–Cu1–Cl2
N3–Cu1–Cl1i
S2–Cu1–Cl1
S2–Cu1–Cl2
S2–Cu1–Cl1i
88.8(2)
N1A–Cu1–N3A
N1A–Cu1–S2A
N1A–Cu1–Cl1
N1A–Cu1–Cl2
N1A–Cu1–Cl1i
N3A–Cu1–S2A
N3A–Cu1–Cl1
N3A–Cu1–Cl2
N3A–Cu1–Cl1i
S2A–Cu1–Cl1
S2A–Cu1–Cl2
S2A–Cu1–Cl1i
88.1(8)
82.4(6)
173.6(7)
89.6(7)
98.6(6)
63.6(6)
87.0(5)
175.0(4)
91.3(5)
90.73(19)
111.7(2)
154.8(2)
˚
84.58(13)
177.17(17)
86.87(16)
96.40(13)
85.12(10)
89.12(11)
171.07(10)
94.78(10)
93.37(3)
˚
˚
and 2.7545(10) A. Within the Cu(m-Cl)2Cu core, the Cu–Cu
˚
distance is 3.7040(9) A, and the Cl–Cu–Cl and Cu–Cl–Cu bond
angles are 85.65(3)◦ and 94.35(3)◦, respectively.
The supramolecular crystal structure is based on an intricate
network of S ◊ ◊ ◊ S contacts (3.26 A), weak C–H ◊ ◊ ◊ Cl hydrogen
bonds and parallel face-to-face interactions among the pyrimidine
rings: interplanar distance of 3.17 A and lateral offset of 2.27 A
for pym2S3.
The formation of 1 implies cleavage of the S–S, S–C(sp2)
and N–C(sp2) bonds in the pym2S2 ligand. Inspection of all the
solutions of the solvothermal processes by means of ESI-MS
spectrometry show two peaks at m/z 223.01 and 159.07, assigned
to the monoprotonated cations of the pym2S2 and 2,2¢-bipym
ligands, respectively, and three peaks at m/z 252.96, 284.93 and
317.08 assigned to radical cations [Cu(pym2S)]∑+, [Cu(pym2S2)]∑+
and [Cu(pym2S3)]∑+, respectively. These data, together with the
˚
86.71(4)
179.01(3)
˚
˚
Symmetry code: (i) -x, -y, -z. pym2S3 = bis(2-pyrimidyl)trisulfide;
pym2S2 = bis(2-pyrimidyl)disulfide.
Within the Cu(m-Cl)2Cu core, the Cu–Cu distance of
3.4387(5) A is close to the average value for chlorido bridged
copper(II) dimers (3.476 A) in CSD.17 The Cu2Cl2 core corresponds
to an almost ideal square as reflected from the Cl–Cu–Cl and Cu–
Cl–Cu bond angles of 89.92 and 90.08◦, respectively. Moreover,
the small Cu–Cl–Cu angle plays an important role in determining
the magnetic coupling within the dimer.
˚
˚
2-
presence of S8 and SO4 as by-products, suggest an intricate
mechanism. Technical limitations, inherent to the solvothermal
process, do not allow performing a step-by-step analysis of the
whole process. However, it seems likely that in a first step the
homolytic rupture of the S–S and S–C(sp2) bonds, and subsequent
formation of free-radical intermediates: [2-pymS-S]∑, [2-pymS]∑
and [2-pym]∑, could take place, followed by self-recombination
of these radicals. Additionally, it is probably that part of the
S∑ radicals self-recombine to give the observed S8. This is the
The dimeric entities assemble together by means of N–H ◊ ◊ ◊ Cl
hydrogen bonds to give rise to 1D supramolecular chains which
are further connected through weak C–H ◊ ◊ ◊ N/O hydrogen bonds
to provide the overall 3D cohesiveness to the crystal structure. No
evidence of Cl ◊ ◊ ◊ Cl contacts has been observed.
The crystal structure of compound 2 is closely related to that
of compound 1. It consists of centrosymmetric double chlorido-
bridged copper(II) dimeric entities by using the in situ generated
pym2S3 ligand as both terminal and blocking ligand. The similar
coordination capability of the disulfide and trisulfide analogous
allows the randomly partial substitution of the pym2S3 ligand for
the initial pym2S2 molecule, with 90 and 10% occupation rates,
respectively (Fig. 2). Selected bond lengths and angles are reported
in Table 3.
2-
first time that the oxidation of dithiodipyrimidines to SO4 has
been reported. However, similar desulfurisation processes have
already been reported for closely related pyridine-4-thiol18 and
dithiodipyridine4d ligands.
The cleavage of the N–C(sp2) bond of the in situ gener-
ated pyrimidine-2-thione should take place. The presence of
pyrimidine-2-thione to get the formation of 1, has been shown
in the reaction between CuCl2 and pyrimidine-2-thione under
The CuCl3N2S chromophore of copper(II) ion exhibits the
typical tetragonally elongated octahedron, with two trans ax-
Fig. 2 View of dimeric entity in compound 2 showing the ligand disorder.
850 | Dalton Trans., 2011, 40, 847–852 This journal is The Royal Society of Chemistry 2011
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◦
similar solvothermal conditions (CH2Cl2–CH3CN, 90 C, 20 h),
where 1 is obtained together with the polymer [Cu(pymH)Cl]n.
We have also verified that H2O appears to be required for
formation of compound 3.19 Thus, we carried out the same
reactions using anhydrous solvents with Schlenk techniques. In
that case, compound 1 was not observed, but [Cu(pymH)Cl]n
together with S8 was obtained. Likewise, the need of Cu2+ to
produce the cleavage of the N–C(sp2) bond has been demonstrated,
because the analogous reaction carried out between NiCl2·6H2O
◦
and pym2S2 (CH2Cl2–CH3CN, 90 C, 20 h) does not lead to the
formation of the analogous compound. Instead, the preliminary
X-ray diffraction measurements indicate that crystal structure
consists of dimer [Ni2Cl4(pym2S)(pym2S3)] complexes.20
Finally, the new organic ligand, compound 3, has been isolated
from 1 by treating a CH3CN solution of this compound with
a Chelex resin for 2 h. Structural X-ray diffraction studies were
performed on single crystals obtained from slow evaporation of
a CH3CN–MeOH solution. All bond distances and angles are
within their usual values.21 The lack of steric hindrance between the
thiazole and pyrimidine rings allows a planar conformation of the
molecule (Fig. 3). It is worth noting that the free molecule presents
a syn-conformation of the thiazole ring with respect to the central
amine group, in contrast to what happens in compound 1 where the
chelation to the metal centre stabilises the anti-conformation of the
2-(pyrimidin-2-ylamino)-1,3-thiazole-4-carbaldehyde molecule.
Fig. 4 Thermal dependence plot of cM vs. T for compound 1. Solid line
corresponds to the best theoretical fit.
S = 1/2 ions (0.75 cm3 K mol-1 for g = 2.0). When cooling
down the sample, the cMT product remains constant down to
150 K and below this temperature it shows a continuous increase
reaching a maximum of ca. 0.93 cm3 K mol-1 at 11 K. Below
11 K, cMT decreases to reach a value of ca. 0.88 cm3 K mol-1 at
5 K. This behaviour indicates that complex 1 presents predominant
ferromagnetic interactions, as suggested by the increase in cMT
observed at low temperatures. The decrease observed at low tem-
peratures can be attributed to the presence of antiferromagnetic
interactions between the S = 1 dimers at low temperatures and/or
to a zero field splitting (ZFS) of the S = 1 ground spin state.
Taking into account the crystal structure of compound 1, we
have fitted the magnetic susceptibility data to a simple ferromag-
netic S = 1/2 dimer.22 A Weiss constant (H) was introduced into the
temperature term to account for both intermolecular effects and
zero-field splitting. This model reproduces very satisfactorily the
magnetic data of complex 1 in the whole temperature range with
the following set of parameters: g = 2.128(2), J = 9.9(2) cm-1, and
H = -1.28(1) K (solid line in Fig. 3). As expected (see below), the
intradimer magnetic coupling is ferromagnetic and the interdimer
one is much weaker and antiferromagnetic.
Fig. 3 View of compound 3.
Although the S–S bond cleavage in organosulfur ligands is
easy under solvothermal conditions, the C–S bond cleavage is
much less common and it has been only recently reported,4c,18
and the N–C(sp2) bond cleavage in this kind of ligand is here
reported for the first time. These results open new perspectives
towards the development of alternative ways for the synthesis
of organic ligands. Under solvothermal-microwave conditions,
together with 1, compound 2 is isolated. Its formation could be
explained from self-recombination of the in situ generated pymS∑
There have been several attempts to establish magneto–
structural correlations in dinuclear dichlorido-bridged Cu(II)
complexes, from the pioneering work of Willett and co-workers23
and Hatfield and co-workers24 that related the magnetic coupling
with the Cu–Cl–Cu angle (a) and with the a/R ratio, respectively
(where R is the Cu–Cl bridge distance), to the more recent studies
of Rodr´ıguez et al.,25 Mrozinski and co-workers26 and Julve and
co-workers.27 All these correlations indicate that the exchange
coupling constant J depends on the value of the Cu–Cl–Cu
bridging angle, a, as well as on the bond length of the axial
(longer) Cu–Cl bond, R in SP geometry. In complex 1, an a/R
∑
and pymS2 radicals. It is worth noting that compound 2 was
not detected under solvothermal conditions. It seems likely that
formation of compound 2 requires microwave irradiation. Varying
reaction times in the solvothermal microwave conditions give rise
to a mixture of 1 and 2.
Although the mechanism has yet to be investigated in detail, the
present result is the first example of a combination of cleavage of
both S–S, S–C(sp2) and C(sp2)–N bonds in organosulfur ligands.
The thermal variation of the molar magnetic susceptibility vs.
the temperature (cMT) for complex 1 is shown in Fig. 4. At room
temperature, the value for cMT is ca. 0.85 cm3 K mol-1 which is
slightly higher than that expected for two non-interacting Cu(II)
◦
value of 35.0 A-1 is measured, for which the empirical correlation
predicts23 an antiferromagnetic interaction with a J value of ca.
-9 cm-1. This value is in contradiction to the experimental one (J =
+9.9 cm-1).
˚
However, these parameters are not the only factors playing an
important role in determining the magnetic coupling. The different
types of arrangement of the two copper(II) polyhedral have a
great influence on the magnetic behaviour of such complexes.
The global arrangement of the two square pyramids gives rise
to three types of geometries: (a) square pyramids sharing one
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 847–852 | 851
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base-to-apex edge but with parallel basal planes (Type-I), (b)
square pyramids sharing one base-to-apex edge with the two
bases nearly perpendicular to one another (Type-II) and (c) square
pyramids sharing a basal edge with coplanar basal planes (Type-
III). Complex 1 belongs to the Type-I class of compounds and in
this case the extent of this magnetic coupling is decided by the small
structural deviations from the ideal square Cu2Cl2 core. Recently,
Mrozinski and co-workers26 established a theoretical correlation
between the magnetic coupling and both parameters (a and R)
showing that for small a values and short Cu–Cl distances the
magnetic coupling is ferromagnetic. These structural parameters
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in compound 1 (90.1◦ and 2.58 A), that predict a moderately
˚
strong ferromagnetic coupling, are very close to those reported
for the [Cu2Cl2(Mebta)6](ClO4)2 (Mebta = 1-methylbenzotriazole)
compound, which has a similar J value of 10.4 cm-1 (92.9◦ and
26
˚
2.55 A).
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Conclusions
In summary, the results shown here are an interesting example
of the synthetic potential of the reactions carried out un-
der solvothermal and solvothermal-microwave conditions using
organosulfur ligands in the presence of redox active metal ions.
An unprecedented in situ multiple bond cleavage of S–S, S–
C(sp2) and C–N in the pym2S2 ligand is produced by reaction
with CuCl2. In fact, the C–N bond cleavage for these types of
organosulfur ligands is reported here for the first time. This
process has enabled a novel organic compound, 2-(pyrimidin-
2-ylamino)-1,3-thiazole-4-carbaldehyde to be isolated. Moreover,
the in situ formed ligand coordinates to Cu(II) ion leading to the
formation of the dimetallic complex [CuII(m-Cl)(Cl)L]2 (L = 2-
(pyrimidin-2-ylamino)-1,3-thiazole-4-carbaldehyde). In addition,
the analogous reaction carried out under solvothermal microwave
conditions leads to the subsequent Cu(II) dimetallic complex and
to the {9[Cu(pym2S3)(m-Cl)(Cl)]2·[Cu(pym2S2)(m-Cl)(Cl)]2} com-
pound. Preliminary results, suggest that the analogous reactions
carried out with Ni(II), instead of Cu(II), proceed only through the
S–S bond breakage and subsequent rearrangement leading to the
complex [Ni2Cl4(pym2S)(pym2S3)].
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could generate a hydroxy intermediate which undergoes dehydrogena-
tive oxidation.
Acknowledgements
Financial support by the MEC (projects no. MAT2007-66476-
C02-02, MAT2008-05690/MAT, FP6-029192) and Comunidad de
Madrid (project S-2009-MAT-1467) is gratefully acknowledged.
We thank Prof. T. Torres for helpful discussions.
20 Preliminary crystallographic data for [Ni2Cl4(pymS)(pymS3)] (obtained
from a twinned specimen): monoclinic, space group P21/◦n, a =
˚
˚
˚
8.923(4) A, b = 25.271(7) A, c = 10.998(5) A, b = 102.3(6) , V =
˚
2423(6) A.
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852 | Dalton Trans., 2011, 40, 847–852
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