C–S bond cleavage by cobalt: synthesis, characterization and
crystal structure determination of
1,2-di-(o-salicylaldiminophenylthio)ethane and its Co(III)
product with C–S bond cleaved fragments
Gudneppanavar Rajsekhar a, Chebrolu P. Rao a,*, Pauli K. Saarenketo b,
Erkki Kolehmainen b, Kari Rissanen b
a
Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
b
Department of Chemistry, University of Jyvaskyla, Fin 40351, Finland
Abstract
1,2-Di-(o-salicylaldiminophenylthio)ethane reacts with Co(II) salts to form a complex with oxidative cleavage of the C–S bond,
to result in the formation of a Co(III) complex of the cleaved ligands.
Keywords: 1,2-Di-(o-salicylaldiminophenylthio)ethane; Oxidative cleavage of C–S bond; Crystal structure of Co(III) complex; Activation by Co(II)
salts
1. Introduction
Cleavage of C–S bond is a first step in hydrodesulfurization (HDS) process. This is generally achieved by
activating the C–S bond using transition metal ions. The
chemistry of thio-containing complexes of group 9
metals has been studied extensively because of their
relevance to HDS process and their potential utility as
heterogeneous catalysts [1]. While the role of cobalt in
such activation was least explored, the same by Rh and
Ir was somewhat better understood in the literature.
Herein we report a case where Co(II) salts bring an
oxidative cleavage of C–S bond in the presence of dioxygen. The reaction between Co(II) and 1,2-di-(o-salicylaldiminophenylthio)ethane (L1 ) yielded in a Co(III)
complex where L1 is cleaved at C–S bond and the resultant fragments (L2 and L3 ) were still bound to the
metal center. On the other hand, similar reactions carried out between Ni(II) and L1 or Zn(II) and L1 resulted
in complexes where L1 remain intact and no C–S bond
cleavage is observed. Earlier studies reported the formation of a neutral Co(II) product [2a] and/or a cationic
complex of Co(III) [2b,2c,2d] using L1 , but the studies
were not supported by any NMR or crystal structure
determination. None of these reports refer to any activation of the organic portion by cobalt. Thus our studies
clearly demonstrate the cleavage of C–S bond by simple
Co(II) system, although C–S bond cleavages were reported with mixed transition metal oxide systems particularly in zeolites and mesoporous materials [3].
2. Results and discussion
The precursor ligand, L1 was synthesised as reported
earlier [2d], and was characterized by analytical, spectral
[4] and single crystal X-ray diffraction [5] (XRD) by us.
Molecular weight of L1 was confirmed based on the
molecular ion peak in FAB mass spectra. Reaction
mixture of 1:1 ratio of CoðacetateÞ2 4H2 O and L1 taken
in dichloromethane/methanol was initially allowed to
react under argon atmosphere and then followed by
650
Scheme 1. Reactions of Co(II) salts with L1 .
under O2 atmosphere. During this, the reaction mixture
went through a colour change from orange red to dark
brown. The reaction was monitored in solution by UV–
Vis spectra as a function of time. It was noticed that
when the reaction was maintained under argon atmosphere, the orange red complex remains stable, and
when the reaction was purged with O2 , it quickly converted to a dark brown Co(III) complex with the
cleavage of L1 as shown in Scheme 1. Thus under argon
atmosphere, the reaction mixture exhibited a ligand!metal charge transfer band at 425 nm in the
absorption spectra and this band was found to grow in
intensity as a function of time and the peak is reminiscent of Co(II) center. When this reaction mixture was
bubbled with O2 , the spectra exhibited a very broad
band which upon deconvolution represent two least resolved bands positioned at 440 and 475 nm, respectively
with almost equal absorbance, indicating the formation
of a more positive Co(III) center favouring better charge
transfer from thiolate moiety. Similarly the band observed at 300 nm under argon atmosphere shifts to 285
nm when purged with O2 . Even the absorption spectrum
measured from the dichloromethane solution of the
isolated product exhibited a spectrum that is exactly
identical to that obtained from the reaction mixture
after O2 purge.
The final product, ½CoðIIIÞðL2 ÞðL3 Þ was characterized by analytical, spectral [6] and single crystal XRD
[7]. The molecular ion peak observed at m/z 541 in the
FAB mass spectrum corresponds to the molecular
weight of the complex. This product exhibited a diamagnetic 1 H NMR spectrum supporting the presence of
Co(III) and also the presence of two ligands (L2 ; L3 )
formed by C–S bond cleavage of the precursor, L1 , as
noticed from the two sets of resonances appeared for
imine proton (8.922, 8.792 ppm) as well as for the aromatic protons (6.460–8.463 ppm) [6]. The peak corresponding to the –S–CH2 –CH2 –S– (3.207 ppm) of L1 was
disappeared from the spectrum of [Co(III)(L2 )(L3 )], instead there exists two multiplets (5.598–5.802 ppm)
corresponding to –S–HC@CH2 -moiety present in L3
due to the cleavage of C–S bond and hence results in the
formation of a thiophenolate ligand L2 . The FTIR
spectrum of the complex ½CoðIIIÞðL2 ÞðL3 Þ exhibited a
strong band that was observed at 1525 cm1 assignable
to the C@C stretching in the –S–HC@CH2 moiety
present in L3 .
Structure of the precursor ligand, L1 , shown in Fig.
1(a), possesses a center of symmetry and exhibits a helical structure, where the molecule was stabilized
through a 6-atom intramolecular H-bond interaction of
Fig. 1. (a) Molecular structure of L1 . (b) Molecular structure of
[Co(III)(L2 )(L3 )]. Hydrogen atoms are omitted for clarity in both (a)
) and angles (°): Co1–O1b 1.894(2),
and (b). Selected bond lengths (A
Co1–N9a 1.913(3), Co1–O1a 1.914(2), Co1–N9b 1.926(2), Co1–S16b
2.227(9), Co1–S16a 2.238(9), C17b–C18b 1.302, C18b–S16a 5.555,
Co1–C17b 3.252, Co1–C18b 4.302, S16b–Co1–S16a 86.07, S16a–Co1–
N9a 88.25, N9a–Co1–O1b 84.93, O1b–Co1–O1a 87.89, O1a–Co1–N9b
86.25, N9b–Co1–S16b 89.12, N9a–Co1–N9b 177.94, S16a–Co1–O1a
175.63, S16b–Co1–O1b 175.64, S16b–Co1–N9a 90.72, S16a–Co1–O1b
94.07, N9a–Co1–O1a 95.81, O1b–Co1–N9b 95.24, O1a–Co–S16b
92.28, N9b–Co–S16a 89.69.
651
the type Nazomethine H–Ophenolic and was reinforced by
a very weak 9-atom H-bond interaction of the type
S H–Ophenolic [8]. The corresponding metric data of
the hydrogen bonds is given in [8], where D and A refers
to the donor and acceptor of hydrogen respectively.
The crystal structure of the reaction product,
½CoðIIIÞðL2 ÞðL3 Þ shown in Fig. 1(b) exhibits the presence of two ligands, L2 (dianionic) and L3 (monoanionic), both acting as tridentate ones by extending ONS
core. This resulted in the formation of one 5-membered
and another 6-membered chelates to give distorted octahedral CoðONSÞ2 core. Selected bond lengths and
bond angles are given in Fig. 1. The metric data fully
supported the cleavage of L1 in order to form the eth) and Co–Sthiolate
ylene moiety C17b–C18b (1.302 A
) bonds. Even, a
(2.227 A) and Co–Sthioether (2.238 A
similar reaction carried out between CoCl2 6H2O and
L1 resulted in a product whose composition and crystal
structure were found to be same as that of
½CoðIIIÞðL2 ÞðL3 Þ. Based on FAB mass and 1 HNMR
studies, it was found that similar reaction carried out
using CoðSO4 Þ2 7H2 O also resulted in the same product and thereby exhibited C–S bond cleavage.
However, the reactions carried out with Ni(II) and
Zn(II) using L1 did not exhibit any C–S bond cleavage,
rather resulted in complexes of the type ½NiðIIÞðL1 Þ and
½ZnðIIÞðL1 Þ where the L1 acted as dianionic hexadentate
ligand as understood by establishing their crystal
structures. A typical structure in case of ½ZnðIIÞðL1 Þ
complex [9] is shown in Fig. 2, however, all the structural details and relevant comparisons of these and related complexes will find a place in a full paper. Though
the reaction of Co(II) salts exhibited C–S bond cleavage
as reported in this communication, the earlier workers
could not identify this in the absence of any NMR data
Fig. 2. Molecular structure of [Zn(III)(L1 )]. Hydrogen atoms are
): Zn(1)–O(1) 1.980(3),
omitted for clarity. Selected bond lengths (A
Zn(1)–O(34) 2.009(3), Zn(1)–N(9) 2.095(3), Zn(1)–N(26) 2.101(4),
Zn(1)–S(16) 2.582(2), Zn(1)–S(19) 2.676(2).
or crystal structure determination. Thus, in these reactions, the oxidation of Co(II) to Co(III) is associated
with the cleavage of L1 to give bound L2 and L3 as
shown in Scheme 1.
Acknowledgements
CPR acknowledges the financial support from the
Council of Scientific and Industrial Research and Department of Science and Technology, New Delhi and
CDRI Lucknow for FAB mass spectral measurements.
GR acknowledges the SRF fellowship from CSIR.
References
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[4] Anal. Cald for (L1 ) C28 H24 N2 O2 S2 : C, 69.39; H, 4.99; N, 5.78; S,
13.23. Found: C, 69.82; H, 4.91; N, 5.38; S, 12.74; mp 189–190 °C;
IR (KBr, cm1 ); t 3000, 1612, 1573, 1561, 1468, 1279, 1181, 907,
753; kmax /nm, (=mol1 L cm1 ); 351(26 900), 272(52 100),
234(54 800); 1 H NMR (500 MHz, DMSO-d6 , d ppm): 3.207 (s,
4H, S–CH2–CH2–S), 6.948–7.651 (m, 16H, Ar–H), 8.913 (s, 2H, –
CH@N), 12.984 (s, 2H, –OH); 13 C NMR (500 MHz, DMSO-d6 , d
ppm): 30.6 (S–CH2 –CH2 –S), 163.0 (–CH@N), 116.6–160.2 (Ar–
C);FABMS: m/z : 485 ð½Mþ , 70%).
[5] Crystal data for the precursor, (L1 ): Single crystals were grown
from the solution of the product dissolved in a mixture of
CHCl3 :C2 H5 OH (8:2 v/v). C28 H24 N2 O2 S2 , T ¼ 173(2)K, monoclinic, space group P 21 =c (No. 14), a ¼ 10:716ð1Þ, b ¼ 16:811ð1Þ,
,
3 ,
Z ¼ 4,
c ¼ 13:097ð1Þ A
b ¼ 104:23°,
V ¼ 2287:0ð3Þ A
F ð0 0 0Þ ¼ 1016, 12 704 reflections measured of which 4014 were
independent on F 2 , final R indices [I > 2rðIÞ]: R1 ¼ 0:0634,
652
wR2 ¼ 0:1619, R indices (all data): R1 ¼ 0:0744, wR2 ¼ 0:1712,
solved using SIR92 and refined using SHELXL-97.
[6] Anal. Cald for. ½CoðIIIÞðL2 ÞðL3 Þ, CoC28 H21 N2 O2 S2 : C, 62.22; H,
3.92; N, 5.18; S, 11.86. Found: C, 62.32; H, 3.75; N, 5.00; S, 11.31;
mp > 250 °C; IR (KBr, cm1 ); t 1604, 1573, 1525, 1435, 1249,
1177, 1143, 927, 747; kmax /nm, (=mol1 L cm1 ); 475(10 800),
440(10 700), 285(57 800); 1 H NMR (500 MHz, DMSO-d6 , d ppm):
5.598–5.802 (s, 3H, -CH@CH2 ), 6.460–8.463 (m, 16H, Ar–H),
8.792 (s, 1H, –CH@N), 8.922 (s, 1H, –CH@N); 13 C NMR ( 500
MHz, DMSO-d6 , d ppm): 113.5–166.0 (Ar–C and –CH@CH2 ),
162.2 (–CH@N), 159.4 (–CH@N); FABMS: m/z: 541 (½Mþ , 85%).
[7] Crystal data for ½CoðIIIÞðL2 ÞðL3 Þ: Single crystals were obtained by
diffusing diethyl ether slowly into a saturated solution of the
product in dichloromethane. C28 H21 CoN2 O2 S2 , T ¼ 150ð2Þ K,
monoclinic, Cc (No. 9), a ¼ 15:279ð1Þ, b ¼ 12:807ð1Þ, c ¼ 12:191
, b ¼ 94:51°, V ¼ 2378:1ð3Þ A
3 , Z ¼ 4, F ð0 0 0Þ ¼ 1112,
ð1Þ A
16 743 reflections measured of which 3542 were independent on
F 2 , final R indices [I > 2rðIÞ]: R1 ¼ 0:0276, wR2 ¼ 0:0624, R
indices (all data): R1 ¼ 0:0303, wR2 ¼ 0:0636, solved using SIR92
and refined using SHELXL-97.
,
[8] H-bond data for (L1 ): Nazomethine H–Ophenolic . (d(D–H) 0.84 A
, d(D A) 2.680(5) A
, < ðDHAÞ 146.7°) and
d(H A) 1.94 A
, d(H A) 2.97 A
, d(D A)
S H–Ophenolic (d(D–H) 0.84 A
, < ðDHAÞ 141.9°). The D and A refer to donor and
3.666(5) A
acceptor of hydrogen respectively.
[9] Crystal data for ½ZnðIIÞðL1 Þ: Single crystals were obtained by
concentrating the reaction mixture. C28 H22 N2 O2 S2 Zn, T ¼ 173ð2Þ
, V ¼
K, tetragonal, P41 (No. 76), a ¼ 11:383ð1Þ, c ¼ 18:347ð1Þ A
3 , Z ¼ 4, F ð0 0 0Þ ¼ 1128, 8877 reflections measured of
2377:3ð3Þ A
which 3546 were independent on F 2 , final R indices [I > 2rðIÞ]:
R1 ¼ 0:0375, wR2 ¼ 0:0702, R indices (all data): R1 ¼ 0:0477,
wR2 ¼ 0:0737, solved using SIR92 and refined using SHELXL-97.