Self-assembly of a tetranuclear CoIII-metallacycle from the reaction of a bis(benzene-o-dithiolato) ligand with CoII and subsequent aerial oxidation

Article abstract of DOI:10.1002/ejic.200500862

The bis(benzene-o-dithiol) ligand [(HS)₂-2,3-C₆H ₃-CH₂-C₆H₃-2,3-(SH)₂] (H₄-1) reacts, after deprotonation with Li₂CO₃, with CoCl₂·6H

Full text of DOI:10.1002/ejic.200500862

FULL PAPER
DOI: 10.1002/ejic.200500862
Self-Assembly of a Tetranuclear CoIII-Metallacycle from the Reaction of a
Bis(benzene-o-dithiolato) Ligand with CoII and Subsequent Aerial Oxidation
F. Ekkehardt Hahn,*[a] Thorsten Kreickmann,[a,b] and Tania Pape[a]
Keywords: Cobalt / S ligands / Metallacycles / Self-assembly
The bis(benzene-o-dithiol) ligand [(HS)2-2,3-C6H3–CH2–
C6H3-2,3-(SH)2] (H4-1) reacts, after deprotonation with
Li2CO3, with CoCl2·6H2O. Aerial oxidation in methanol gives
the tetranuclear metallacycle Li4[Co4(1)4]. The X-ray struc-
ture analysis of (PNP)4[Co4(1)4] (7) reveals a cyclic structure
in which each of the bis(benzene-o-dithiolato) ligands forms
a bridge between two cobalt centers. Two differently coordi-
nated cobalt atoms (syn and anti) are observed in the tetranu-
clear complex.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
gand C reacts with TiIV to give a triple-stranded helicate
with a parallel ligand orientation.
Introduction
The combination of polydentate organic ligands and
transition metals has led to supramolecular complexes.[1]
Metal-directed self-assembly has been used to form dif-
ferent supramolecular architectures like helicates,[2] boxes,[3]
squares,[4] molecular containers,[5] and other structural mo-
tifs.[6] Among those, metal helicates have attracted special
interest owing to their presence in nature.[2] The first triple-
stranded helicate [Fe2(RA)3] (RA = rhodotorulic acid),
containing exclusively oxygen donor functions from hydro-
xamate groups, was isolated and characterized by Raymond
et al.[7] The early helicate chemistry, however, was domi-
nated by nitrogen donor ligands like oligobipyridines.[2,8]
Later, triple-stranded helical complexes with catecholato
donor groups were prepared.[2,9] The coordination chemis-
try of dicatechol ligands has been reviewed.[10] Double-
stranded helical structures built from ligands with an un-
symmetrical spacer (directional ligands) have been re-
ported.[11] Even the triple-stranded helicate of a cate-
cholato/aminophenolato ligand has been described.[12]
Much less is known about helicates containing benzene-
o-dithiolato or other dithiolene donor groups. We have de-
scribed a method for the o-functionalization of benzene-
1,2-dithiol, which gives 2,3-dimercaptobenzoic acid[13] or
2,3-dimercaptobenzyl alcohol.[14] These building blocks
were used to synthesize bis(benzene-o-dithiol) ligands of
type A,[15,16] B,[16] and the benzene-o-dithiol/catechol ligand
C[17] (Scheme 1). Non-helical mono- and dinuclear com-
plexes have been obtained with ligands A and B, while li-
Scheme 1. Ligands containing the benzene-o-dithiol donor group.
However, no dinuclear triple-stranded helicates have been
obtained so far from bis(benzene-o-dithiol) ligands such as
A and B. Albrecht et al. have shown that methylene-bridged
dicatecholato[18] and catecholato/aminophenolato[12] li-
gands are capable of forming triple-stranded helicates with
TiIV and GaIII. In our search for helicates built exclusively
from sulfur donors, we have therefore prepared the novel
methylene-bridged bis(benzene-o-dithiol) ligand H4-1
(Scheme 1). Here we present the preparation of ligand H4-
1 and its coordination chemistry with CoIII
.
Results and Discussion
Ligand H4-1 was prepared as depicted in Scheme 2. 1,2-
Bis(isopropylmercapto)benzene (2) was synthesized as de-
scribed previously.[14,16] Deprotonation of 2 with nBuLi/
TMEDA in hexane yielded the organolithium compound 3.
Compound 3 was not isolated but immediately transformed
into 2,3-bis(isopropylmercapto)benzaldehyde (4) by reac-
tion with N,N-dimethylformamide followed by an acidic
[a] Institut für Anorganische und Analytische Chemie, Westfälis-
che Wilhelms-Universität Münster,
Wilhelm Klemm Straße 8, 48149 Münster, Germany
Fax: + 49-251-833-3108
E-mail: fehahn@uni-muenster.de
[b] NRW Graduate School of Chemistry,
Münster, Germany
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F. E. Hahn, T. Kreickmann, T. Pape
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Scheme 2. Synthesis of the bis(benzene-o-dithiol) ligand H4-1.
workup. Reaction of equimolar amounts of 3 and 4 in hex-
ane yielded the alcohol 5 after acidification. Alcohol 5 was
deoxygenated with hypophosphorous acid in the presence
of a catalytic amount of iodine in acetic acid.[19] Removal of
the S-protection groups in ligand precursor 6 with sodium/
naphthalene in THF liberated ligand H4-1, which was iso-
lated after hydrolysis in approximately 85% yield (relative
to 2).
Reaction of equimolar amounts of H4-1 and
CoCl2·6H2O in methanol at ambient temperature in the
presence of two equivalents of Li2CO3 yielded a brown
solution. We assume that this solution contained CoII com-
plexes with benzene-o-dithiolato ligands. Upon aerial oxi-
dation, the brown solution immediately turned deep blue,
which indicates the formation of a CoIII–benzene-o-dithiol-
Figure 1. ESI mass spectrum (negative ions) of (PNP)4[Co4(1)4] (7).
ato complex. Addition of PNPCl [PNP+ = bis(triphenyl- units from different ligands in a square-planar fashion. The
phosphoranylidene)ammonium] led to the precipitation of Co–S and C–S distances in 7 are normal and match those
a blue solid, which was isolated and dried. The composition reported for the mononuclear complex (AsPh3Me)-
of the blue solid was established by ESI mass spectroscopy [Co(S2C6H3CH3)2].[20] Cobalt(iii) complexes with electron-
(Figure 1, negative ions). No peaks for a double-stranded poor benzene-o-dithiolato ligands [S2C6Cl4 or S2C6H2-
complex of composition [Co2(1)2]4– are observed. Instead, (CN)2] have been reported to dimerize.[21] In such dimers
three peaks are found at m/z = 351, 648, and 1241. These the [Co(bdt)2]– (bdt = substituted benzene-o-dithiolate)
are assigned to the ions [Co4(1)4]4–, {(PNP)[Co4(1)4]}3–, and units lie parallel to one another such that the cobalt atom
{(PNP)2[Co4(1)4]}2–. These assignments were corroborated of each [Co(bdt)2]– unit is directly opposite a sulfur atom
by matching isotope patterns. Thus, the reaction of ligand belonging to the second unit. This arrangement leads to
H4-1 with CoCl2 followed by aerial oxidation and precipi- the formation of two new Co–S bonds and a tetragonal-
tation with PNPCl does not yield a dinuclear complex, as pyramidal coordination environment for each cobalt atom.
was expected, but instead a tetranuclear complex of compo-
sition (PNP)4[Co4(1)4] (7).
No such extra Co–S interactions are observed in the
anion [Co4(1)4]4–. The [Co4(1)4]4– anions are well separated
To establish the molecular structure of the [Co4(1)4]4– in the solid state, and no intramolecular Co–Co (shortest
anion, crystals suitable for an X-ray analysis were grown by separation about 9 Å) and Co–S (shortest separation about
slow vapour diffusion of pentane into a solution of 7 in 7 Å) or intermolecular Co–Co (shortest separation longer
dichloromethane. The molecular structure of tetraanion than 9 Å) and Co–S (shortest separation longer than 7 Å)
[Co4(1)4]4– is shown in Figure 2. The complex anion con- contacts are found.
tains four [Co(S2C6H3R)2] units arranged in a cyclic fash-
In contrast to the recently reported dinuclear nickel com-
ion. Each 14– ligand bridges two cobalt atoms. Each of the plex with an ethylene bridged bis(benzene-o-dithiolato) li-
cobalt atoms is coordinated by two benzene-o-dithiolato gand (Scheme 1, B),[16] the tetranuclear complex (PNP)4-
536
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Eur. J. Inorg. Chem. 2006, 535–539

Self-Assembly of a Tetranuclear CoIII-Metallacycle
FULL PAPER
O···O distance 2.85 Å). In addition, possible repulsion be-
tween these oxygen atoms is also compensated by coordina-
tion to hard lithium cations. This coordination is not pos-
sible and was not observed with the soft sulfur donor
atoms. In a hypothetical dinuclear complex with benzene-
o-dithiol ligands, steric interaction between the sulfur atoms
in the 2-position would occur owing to the longer C–S
bonds (1.76 Å versus 1.35 Å for the C–O bonds) and the
larger intraring C–C–S angles (119° versus 114° for the C–
C–O angles). On the basis of these parameters, the S···S
distance in a dinuclear complex [Co2(1)2]2– would be
around 2.2 Å, which is far too short for a nonbonding inter-
action (typically S–S bond lengths are 2.0 Å). Conse-
quently, strong repulsion between the sulfur atoms must oc-
cur in the hypothetical [Co2(1)2]2– complex in which two
cobalt atoms are coordinated in a square-planar fashion by
two CH2-bridged bdt ligands. In order to avoid this un-
favourable repulsion, one benzene-o-dithiolato group of
each 14– ligand is rotated by about 90° around the Ar–CH2
axis, which, in turn, leads to the formation of the tetranu-
clear metallacycle.
Figure 2. Molecular structure of the [Co4(1)4]4– anion of 7. Hydro-
gen atoms and cations have been omitted for clarity. Selected ran-
ges for bond lengths [Å] and bond angles [°]: Co–S 2.157(2)–
2.176(2), C–S 1.746(6)–1.769(6), S–Co–S (intra ligand) 91.08(7)–
91.84(7), Co–S–C 104.4(2)–105.9(2).
[Co4(1)4] contains two metal centers with a slightly different
coordination environment. Two cobalt atoms (Co1 and
Co3) are coordinated by benzene-o-dithiolato units and the
alkyl substituents are arranged in anti positions, while the
other two cobalt atoms (Co2 and Co4) are coordinated by
benzene-o-dithiolato units that have their alkyl substituents
arranged in a syn fashion. This leads to two different C2
axes along the two metal–metal connection lines Co1–Co3
and Co2–Co4 (Figure 3). The distances between the equiva-
lent cobalt atoms are 9.661 Å (Co1–Co3) and 15.933 Å
(Co2–Co4). A large internal cavity is formed by this ar-
rangement. Two benzene rings of the PNP+ cations partly
fill this cavity without any interaction with the metallacycle.
Complex 7 is, to the best of our knowledge, the first metall-
Scheme 3. Hypothetical complex [Co2(1)2]2– (left) and triple-
osupramolecular assembly that contains two differently co- stranded helicate (right).
ordinated metal centers even though a ligand with two iden-
tical donor sites has been used.
Experimental Section
If not stated otherwise, all manipulations were carried out under
dry argon by using standard Schlenk techniques. Hexane, tetrahy-
drofurane, and N,N,NЈ,NЈ-tetramethylethylenediamine (TMEDA)
were freshly distilled from sodium/benzophenone prior to use. The
starting material 1,2-bis(isopropylmercapto)benzene (2) was pre-
pared as described before.[16]
2,3-Bis(isopropylmercapto)benzaldehyde (4): A sample of n-butyl-
lithium (12.16 mL of a 2.5 m solution in hexane) was added drop-
wise to a solution of TMEDA (4.56 mL, 30.4 mmol) and 1,2-bis-
(isopropylmercapto)benzene (2, 6.9 g, 30.4 mmol) in hexane
(100 mL) at 0 °C. After 30 min, the ice bath was removed, and the
Figure 3. View along the Co1–Co3 axis showing the syn (Co2, Co4)
and anti (Co1, Co3) arrangement of the substituted benzene-o-di-
thiolato ligands.
Although a tetranuclear complex [Co4(1)4]4– is entropi- stirring was continued for 3 h at ambient temperature, which re-
sulted in the formation of an off-white slurry. Dry N,N-dimethyl-
formamide (3.53 mL, 45.6 mmol) was added, and the solution was
stirred for 12 h. Water (100 mL) and hydrochloric acid (37%,
20 mL) were added, and the aqueous solution was extracted with
diethyl ether (3×50 mL). The combined organic layers were dried
with MgSO4, and the solvent was removed in vacuo. Column
chromatography (SiO2, Et2O/petroleum ether, 1:3) gave a yellow oil
(7.33 g, 28.8 mmol, 95%). 1H NMR (CDCl3, 300.1 MHz): δ = 10.8
(s, HC=O), 7.67 (dd, 1 H, Ar-H), 7.49–7.35 (m, 2 H, Ar-H), 3.54–
cally less favorable than two dinuclear complexes [Co2-
(1)2]2–, the larger compound is exclusively formed. We be-
lieve that this occurs because of a strong steric interaction
between the two sulfur atoms in the 2-positions of the aro-
matic rings that would occur in a hypothetical dinuclear
complex [Co2(1)2]2– (Scheme 3, left). Albrecht et al. pre-
pared a dicatechol ligand, with the same topology as H4-
1, that forms a dinuclear triple-stranded helicate with TiIV
(Scheme 3, right).[18] In this complex, no repulsion between
3
3
3.32 (m, 2 H, SCH), 1.39 (d, J = 7.0 Hz, 6 H, CH3), 1.23 (d, J =
the oxygen atoms in the 2-positions is observed (intraligand 7.0 Hz, 6 H, CH3) ppm. 13C NMR (CDCl3, 75.5 MHz): δ = 193.4
Eur. J. Inorg. Chem. 2006, 535–539
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537

F. E. Hahn, T. Kreickmann, T. Pape
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(C=O), 146.5, 139.7, 136.4, 131.4, 129.2, 124.3 (Ar-C), 40.9 (SCH),
36.0 (SCH), 23.0 (CH3), 22.6 (CH3) ppm. GC/MS (EI, 70 eV): m/z
(%) = 254 (66) [M]+, 211 (10) [M – C3H7]+, 179 (5) [M – SC3H7]+,
41 (100) [C3H5]. C13H18OS2 (254.40): calcd. C 61.38, H 7.13, S
25.20; found C 61.47, H 7.36, S 24.87.
2 H, Ar-H), 4.18 (s, 2 H, CH2), 3.84 (s, 2 H, SH), 3.81 (s, 2 H,
SH) ppm. 13C NMR (CDCl3, 75.5 MHz): δ = 141.5, 134.7, 133.2,
131.5, 129.7, 128.3 (Ar-C), 43.3 (CH2) ppm. IR (KBr): ν˜ = 3050 (s,
Ar-H), 2894 (m, C–H), 2525 (m, S–H), 1443, 717 (C–H) cm–1. MS
(EI, 70 eV): m/z (%) = 296 (76) [M]+, 263 (85) [M – SH]+, 230 (100)
[M – 2SH]+, 197 (47) [M – 3SH]+. C13H12S4 (295.98): calcd. C
52.67, H 4.08, S 43.25; found C 53.22, H 3.88, S 43.82.
Bis[2,3-bis(isopropylmercapto)phenyl]methanol (5): A sample of n-
butyllithium (3.00 mL of a 2.5 m solution in hexane) was added
dropwise to a solution of TMEDA (1.15 mL, 7.50 mmol) and 1,2-
bis(isopropylmercapto)benzene 2 (1.7 g, 7.50 mmol) in hexane
(30 mL) at 0 °C. After 30 min, the ice bath was removed, and the
stirring was continued for 3 h at ambient temperature, which re-
sulted in the formation of an off-white slurry. 2,3-Bis(isopropylmer-
capto)benzaldehyde (4, 1.91 g, 7.50 mmol) dissolved in hexane
(10 mL) was added dropwise, and the solution was stirred for 12 h.
Acidification with hydrochloric acid (37%) yielded a yellow precipi-
tate, which was isolated by filtration, washed with hexane, and
dried in vacuo to give 5 as a white powder (3.46 g, 7.20 mmol,
(PNP)4[Co4(1)4] (7): A solution of H4-1 (50 mg, 0.169 mmol) and
Li2CO3 (25 mg, 0.34 mmol) in methanol (10 mL) was added to a
solution of CoCl2·6H2O (40 mg, 0.169 mmol) in methanol (10 mL).
The mixture was stirred for 12 h, exposed to air for 10 min, and
filtered. Addition of bis(triphenylphosphoranylidene)ammonium
chloride (PNPCl) (97 mg, 0.169 mmol) to the filtrate gave a blue
precipitate of 7, which was isolated by filtration, washed with meth-
anol, and dried in vacuo (110 mg, 0.031 mmol, 74%). MS (ESI,
negative ions): m/z (%) = 351.0 [Co4(1)4]4–, 648 [(Co4(1)4)4–
+
PNP+]3–, 1241 [(Co4(1)4)4– 2 PNP+]2–. C196H152N4P8S16Co4
+
1
(3559.7): calcd. C 66.13, H 4.30, N 1.57, S 14.41; found C 65.77,
H 4.42, N 1.48, S 14.28.
96%). H NMR (CDCl3, 300.1 MHz): δ = 7.26–7.18 (m, 4 H, Ar-
H), 7.01 (s, 1 H, CH–OH), 7.00 (dd, 2 H, Ar-H), 3.66 (sept, 2 H,
SCH), 3.50 (sept, 2 H, SCH), 1.37 (d, 12 H, CH3), 1.24 (d, 12 H,
CH3) ppm. 13C NMR (CDCl3, 75.5 MHz): δ = 148.3, 144.9, 131.4,
128.7, 126.3, 124.5 (Ar-C), 72.0 (CH–OH), 39.1 (SCH), 36.1
(SCH), 25.5 (CH3), 25.1 (CH3) ppm. IR (KBr): ν˜ = 3457 (br., O–
H), 3050 (s, Ar-H), 2960, 2923, 2863 (s, CHMe2), 1557 (m, Ar-
C=C), 1443 (s, CH) cm–1. MS (EI, 70 eV): m/z (%) = 480 (64)
[M]+, 437 (73) [M – C3H7]+, 405 (100) [M – SC3H7]+. C25H36OS4
(480.16): calcd. C 62.45, H 7.55, S 26.68; found C 62.51, H 7.47, S
26.44.
X-ray Crystallographic Study of 7:
A suitable crystal of 7
(0.15×0.10×0.08 mm) was mounted on a Bruker AXS APEX dif-
fractometer equipped with a rotating molybdenum anode (λ =
0.71073 Å), cooling device, and graphite monochromator. (PNP)4-
¯
[Co4(1)4], C196H152N4P8S16Co4, M = 3559.7, triclinic P1, a =
18.473(7), b = 19.797(7), c = 24.497(9) Å, α = 80.724(7), β =
89.026(7), γ = 79.787(7)°, V = 8701(6) Å3, Z = 2, ρcalcd.
=
1.359 g cm–3, μ = 0.696 mm–1. 54025 Structure factors (–19 Յ h Յ
19, –21 Յ k Յ 21, –26 Յ l Յ 26, 2θ range 2.1–55.0°) were collected
at 123(2) K. An empirical absorption correction was applied (Tmin
= 0.903, Tmax = 0.947) before merging gave 22725 unique intensities
(Rint = 0.069). Structure solution with SHELXS[22] and subsequent
refinement of 2053 parameters against F2 of 22725 unique inten-
sities [13118 observed intensities I Ͼ 2σ(I)] using SHELXL[23] with
anisotropic thermal parameters for all non-hydrogen atoms. Hydro-
gen atoms were added to the structure model on calculated posi-
tions. Final residues R = 0.0591, wR = 0.1186. CCDC-284858 con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Bis[2,3-bis(isopropylmercapto)phenyl]methane (6): Iodine (80 mg,
0.31 mmol) and bis[2,3-bis(isopropylmercapto)phenyl]methanol (5,
1.50 g, 3.10 mmol) were dissolved in concentrated acetic acid
(60 mL). Hypophosphorous acid (1.60 mL, 50% solution by weight
in water) was added, and the solution was stirred at 60 °C for 24 h.
Water (100 mL) was added at ambient temperature, and the aque-
ous phase was extracted with hexane (3×50 mL). The combined
organic layers were washed with saturated NaHCO3 solution
(2×20 mL) and dried with MgSO4, and the solvent was removed
in vacuo. Column chromatography (SiO2, Et2O/hexane, 1:5) gave 6
as a white powder (1.41 g, 3.04 mmol, 98%). 1H NMR (CDCl3,
300.1): δ = 7.20–7.15 (m, 4 H, Ar-H), 6.80–6.75 (dd, 2 H, Ar-H),
4.72 (s, 2 H, CH2), 3.54 (m, 4 H, SCH), 1.45 (d, 12 H, CH3), 1.30
(d, 12 H, CH3) ppm. 13C NMR (CDCl3, 75.5 MHz): δ = 149.1,
147.4, 134.3, 130.9, 128.6, 126.6 (Ar-C), 42.5 (CH2), 41.3 (SCH),
38.1 (SCH), 25.5 (CH3), 25.1 (CH3) ppm. IR (KBr): ν˜ = 3048 (Ar-
H), 2960, 2923, 2863 (s, CHMe2), 1443 (s, CH2) cm–1. MS (EI,
70 eV): m/z (%) = 464 (8) [M]+, 421 (5) [M – C3H7]+, 389 (100)
[M – SC3H7]+, 357 (53) [M – S2C3H7]+. C25H36S4 (464.17): calcd.
C 64.60, H 7.81, S 27.59; found C 64.47, H 7.87, S 27.72.
Acknowledgments
This work was financially supported by the Deutsche Forschungs-
gemeinschaft and the NRW International Graduate School of
Chemistry.
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Self-Assembly of a Tetranuclear CoIII-Metallacycle
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