Toward absolute asymmetric synthesis of coordination polymers with bidentate sulfide ligands

Article abstract of DOI:10.1016/j.jorganchem.2012.10.035

In search for sulfide-containing coordination polymers that crystallize as conglomerates, five new copper(I) complexes with prochiral sulfide ligands have been prepared and characterized by single crystal X-ray structure determination. Three unsymmetrical sulfides have been used: phenyl propargyl sulfide (Sprop), allyl methyl sulfide (Sally), and 2,5-dithiahexane (SS). In [CuCl(Sprop)] _n (1), layers are formed via π-coordination of propargyl groups to copper(I). In [Cu₂Br₂(Sprop)₄] (2), discrete dimers form with non-coordinating propargyl groups. In [CuCl(Sally)]_n (3), layers are formed via π-coordination of allyl groups to copper(I), but disordered Sally ligands are also found. The mesitylcopper complex [Cu ₄(Mes)₄(Sally)₂] (4) is chiral but discrete. In [Cu₄(Mes)₄(SS)]_n (5), racemic chains are formed by the SS ligand. Three out of five complexes prepared thus form coordination polymers, and all of the five complexes (1-5) exhibit terminal sulfide ligands that could be oxidized selectively when incorporated in an enantiopure polymer. Unfortunately none of 1-5 crystallized as a conglomerate, but whether this reflects an inherent tendency in this system is too early to say.

Full text of DOI:10.1016/j.jorganchem.2012.10.035

Journal of Organometallic Chemistry 724 (2013) 17e22
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Toward absolute asymmetric synthesis of coordination polymers with bidentate
sulfide ligands
*
Theonitsa Kokoli, Susanne Olsson, Per Martin Björemark, Staffan Persson, Mikael Håkansson
Department of Chemistry and Molecular Biology, University of Gothenburg, SE-412 96 Gothenburg, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
In search for sulfide-containing coordination polymers that crystallize as conglomerates, five new cop-
per(I) complexes with prochiral sulfide ligands have been prepared and characterized by single crystal X-
ray structure determination. Three unsymmetrical sulfides have been used: phenyl propargyl sulfide
(Sprop), allyl methyl sulfide (Sally), and 2,5-dithiahexane (SS). In [CuCl(Sprop)]n (1), layers are formed via
Received 18 January 2012
Received in revised form
11 October 2012
Accepted 19 October 2012
p
-coordination of propargyl groups to copper(I). In [Cu2Br2(Sprop)4] (2), discrete dimers form with non-
coordinating propargyl groups. In [CuCl(Sally)]n (3), layers are formed via -coordination of allyl groups
p
Keywords:
Absolute asymmetric synthesis
Chiral
to copper(I), but disordered Sally ligands are also found. The mesitylcopper complex [Cu4(Mes)4(Sally)2]
(4) is chiral but discrete. In [Cu4(Mes)4(SS)]n (5), racemic chains are formed by the SS ligand. Three out of
five complexes prepared thus form coordination polymers, and all of the five complexes (1e5) exhibit
terminal sulfide ligands that could be oxidized selectively when incorporated in an enantiopure polymer.
Unfortunately none of 1e5 crystallized as a conglomerate, but whether this reflects an inherent tendency
in this system is too early to say.
Copper
Coordination polymer
Conglomerate
Sulfide ligand
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
substrates in chiral crystals can give high enantiomeric excesses,
they are limited to special cases. However, if a general reagent could
Homochirality of essential molecules, such as sugars and amino
acids, is one of the fundamental aspects of Life. The origin of such
biomolecular homochirality, and at what stage during evolution
homochirality became dominant, are matters that are still open for
speculation [1]. Although many stereoselective reactions, under the
name of asymmetric synthesis, have become powerful tools in the
quest for enantiopure molecules and materials, they usually
depend on pre-formed optical activity in some form (e.g. in the
substrate, reagent or catalyst) and are thus not very informative
concerning the genesis of molecular optical activity. On the other
hand, the discovery of new spontaneous symmetry-breaking
reactions may provide insight into the origin of biomolecular
homochirality. Reactions that can form enantiomerically enriched
products from achiral precursors, without the intervention of pre-
formed optical activity, constitute examples of absolute asym-
metric synthesis (AAS) [1c,2]. There are only a few genuine exam-
ples of absolute asymmetric syntheses known today, and they often
depend on the transformation (usually photo-chemically) of an
achiral substrate in a chiral crystal [3]. Although AAS using achiral
be obtained as a homochiral batch of enantiopure crystals by total
spontaneous resolution (TSR) [4] a wide range of reactions giving
products from AAS would be feasible [5]. Or, alternatively, if
a general prochiral substrate would form a stereochemically labile
chiral metal complex, then single-colony crystallization could lead
to a homochiral batch of enantiopure crystals. The substrate in such
a homochiral crystal batch could then be transformed to several
different enantiopure products by solid-state reactions with achiral
reagents [6].
We are especially interested in crystallizing chiral polymers and
networks containing prochiral substrates [7]. If such crystals could
be induced to crystallize as a conglomerate, it would be possible to
obtain homochiral crystal batches on a large scale by single-colony
crystallization. In addition to the relevance in connection to the
origin of biomolecular homochirality, there would be several
potential applications for such enantiopure materials, for example
as heterogenous catalysts, as chiral selector phases in separation
and chromatography, or as bio-materials or -sensors. We would like
to induce chiral polymer formation by using unsymmetrical sulfide
ligands; such ligands could be oxidized enantioselectively in
a chiral crystal (Scheme 1) [8]. Such chiral coordination polymers
should be unsoluble in non-coordinating solvents and could thus
be subjected to heterogenous oxidation at elevated temperatures, if
* Corresponding author.
E-mail address: hson@chem.gu.se (M. Håkansson).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.10.035

18
T. Kokoli et al. / Journal of Organometallic Chemistry 724 (2013) 17e22
2.5. Preparation of [Cu4(Mes)4(Sally)2] (4)
A solution of mesitylcopper in THF [11] (0.8 mmol, 0.4 M,
2.0 mL) was stripped of solvent in vacuo and dissolved in 0.10 mL
allyl methyl sulfide with gentle heating. Needle-shaped yellow
crystals were deposited after a few hours at ambient temperature
in 31% yield.
2.6. Preparation of [Cu4(Mes)4(SS)]n (5)
A solution of mesitylcopper in THF [11] (0.8 mmol, 0.4 M,
2.0 mL) was stripped of solvent in vacuo and dissolved in 1.0 mL
toluene wherafter 0.10 mL 2,5-dithiahexane was added. A light
yellow precipitate was formed immediately and the mixture was
heated to give a clear yellow solution from which yellow crystals
were deposited after a few hours in 27% yield.
2.7. X-ray crystallography
Crystal and experimental data are summarized in Table 1. All
crystals were selected and mounted under nitrogen in a glass
capillary at low temperature [9] and transferred in liquid nitrogen
to a Rigaku R-AXIS IIc image plate system. Diffracted intensities
were measured using graphite-monochromated Mo
Ka
ꢀ
(l
¼ 0.71073 A) radiation from a RU-H3R rotating anode operated at
50 kV and 90 mA. Using the R-AXIS IIc detector, 90 oscillation
photos with a rotation angle of 2ꢁ were collected and processed
using the CrystalClear software package. An empirical absorption
correction was applied using the REQAB program under Crystal-
Clear. Crystal and refinement data for compounds 1e5 are
summarized in Table 1. All structures were solved by direct
methods (SIR 97) [13] and refined using full-matrix least-squares
calculations on F2 (SHELXL-97) [14] operating in the WinGX
program package [15]. Anisotropic thermal displacement parame-
ters were refined for all the non-hydrogen atoms. Hydrogen atoms
were included in calculated positions and refined using a riding
model. Figs. 1e5 have been drawn with ORTEP-3 for Windows [15]
under WinGX.
Scheme 1. Enantioselective sulfide oxidation by AAS.
necessary. In this work we set out to investigate if using bidentate
sulfides could aid in polymer formation by bridging.
2. Experimental
2.1. General
All operations were carried out under nitrogen using Schlenk or
low temperature [9] techniques. Solvents were distilled from
sodium/benzophenone shortly prior to use. Copper(I)chloride and
copper(I)bromide were purified according to literature procedures
[10]. Phenyl propargyl sulfide (Sprop) and allyl methyl sulfide
(Sally) were dried with molecular sieves and deoxygenated. Mesi-
tylcopper [11] and 2,5-dithiahexane [12] (SS) were prepared as
previously described.
3. Results and discussion
We reasoned that a soft metal center, such as copper(I), would
be suitable to coordinate the soft sulfide ligands. Halide ligands
could bridge two, three, or even four metal centers and thus assist
in polymer formation [16]. Mesityl ligands can also bridge and have
proved useful in organocopper chemistry [11]. In order to obtain
enantiopure crystals we need to identify conglomerate [17] phases.
However, finding a conglomerate could be a time-consuming
enterprise since we essentially are left with trial-and-error strate-
gies. It is estimated that less than 10% of all chiral compounds form
conglomerates [18], but it must be kept in mind that the statistics is
largely based on organic compounds, and the numbers could be
very different for stereochemically labile coordination compounds.
In this work we have prepared and structurally characterized five
new copper(I) halide and mesityl (Mes) complexes using phenyl
propargyl (Sprop), allyl methyl sulfide (Sally), and 2,5-dithiahexane
(SS) as ligands (Scheme 2 and Table 2). While many metal
complexes having symmetrical SR2 ligands have been previously
reported [19], complexes with unsymmetrical (and thus prochiral)
RSR’ ligands are much rarer. A polymeric copper(I) chloride
complex with 2,5-dithiahexane that exhibits a centrosymmetric
crystal structure is one example [20]. A search in the Cambridge
Structural Database (CSD) reveals only two metal complexes with
2.2. Preparation of [CuCl(Sprop)]n (1)
Copper(I) chloride (0.14 g, 1.4 mmol) was dissolved in 1.0 mL
phenyl propargyl sulfide at ambient temperature. Colorless needles
were obtained after a week at ꢀ15 ꢁC in 60% yield.
2.3. Preparation of [Cu2Br2(Sprop)4] (2)
Copper(I) bromide (0.30 g, 2.1 mmol) was dissolved in 1.0 mL
phenyl propargyl sulfide at ambient temperature. Colorless crystals
were obtained after a few days at ꢀ25 ꢁC in 65% yield.
2.4. Preparation of [CuCl(Sally)]n (3)
Copper(I) chloride (0.14 g, 1.4 mmol) was dissolved in 1.0 mL
allyl methyl sulfide at ambient temperature. Colorless crystals were
obtained after a week at ꢀ80 ꢁC in 75% yield.
the Sally ligand: a ruthenium(II) chloride complex [21] with a
s-
bonded Sally and a rhodium(I) complex [22] with -bonded Sally.
s,p

T. Kokoli et al. / Journal of Organometallic Chemistry 724 (2013) 17e22
19
Table 1
Crystal and refinement data for 1e5.
Complex
1
2
3
4
5
Formula
C9H8CuClS
247.20
Orthorhombic
Pbca
8.8918(15)
10.188(2)
21.641(4)
90
C18H16CuBrS2
439.88
Monoclinic
P21/n
8.966(2)
20.530(4)
9.825(2)
95.930(9)
1798.9(6)
4
1.624
3.660
3.03e26.00
12,081
3374
C8H16Cu2Cl2S2
374.31
Monoclinic
P21/c
6.3196(9)
21.046(4)
10.7575(17)
111.43(1)
1331.9(4)
4
1.867
3.869
3.45e26.00
9123
2586
C44H60Cu4S2
907.20
Monoclinic
C2/c
25.662(6)
8.697(2)
23.505(6)
125.320(6)
4280(2)
4
1.408
2.087
3.18e25.50
13,344
3830
C47H62Cu4S2
945.25
Monoclinic
C2/c
45.392(7)
8.581(1)
24.489(4)
109.881(5)
8970(2)
8
1.400
1.995
2.52e25.00
27,823
7521
Formula weight
Crystal system
Space group
ꢀ
a [A]
ꢀ
b [A]
ꢀ
c [A]
b
[ꢁ]
3
ꢀ
V [A ]
1960.5()
8
1.675
Z
dcalcd [g cmꢀ3
]
m
[mmꢀ1
range [ꢁ]
]
2.652
q
3.58e26.00
12,602
1867
Refls collected
Indep refls
Parameters
113
207
124
233
493
R1 [I > 2
s
(I)]
0.044
0.030
0.055
0.047
0.050
wR2 [all data]
0.083
0.34
ꢀ0.43
0.076
0.89
ꢀ0.62
0.130
1.03
ꢀ1.01
0.106
0.78
ꢀ0.50
0.089
0.44
ꢀ0.61
Max peak [e Åꢀ3
]
Max hole [e Åꢀ3
]
Interestingly, there are no metal complexes with the Sprop ligand
in the CSD.
Crystals of [CuCl(Sprop)]n (1) could be prepared by using neat
phenyl propargyl sulfide as solvent; slow cooling proved efficient. A
crystal structure determination (Fig. 1) reveals formation of layers,
where bridging is afforded both via the anionic and neutral ligands.
The Sprop ligand is bridging by side-on coordination of the C8eC9
triple bond to Cu1. This is reflected in the ligand geometry: the
Fig. 1. ORTEP drawing showing part of a layer in 1. The interaction between Cu1 and
the C8eC9 triple bond is instrumental in building the polymer. Selected bond distances
Fig. 2. ORTEP drawing of the dimer in 2; the alkyne groups do not bridge. Selected
bond distances (A) and angles (ꢁ): C8eC9 1.174(4), C17eC18 1.179(4), Cu1eS1
ꢀ
(A) and angles (ꢁ): C8eC9 1.190(5), Cu1eC8 2.053(3), Cu1eC9 2.054(4), Cu1eS1
ꢀ
2.3159(8), Cu1eS2 2.3085(7), Cu1eBr1 2.4756(6), Cu1eBr1* 2.4939(5), Cu1eCu1*
3.0062(7), C7eC8eC9 179.1(3), C16eC17eC18 179.8(4), S1eCu1eBr1 107.15(2), Br1e
Cu1eBr1* 105.553(13), Cu1eBr1eCu1* 74.447(13).
2.4065(10), Cu1eCl1 2.2614(9), Cu1eCl1* 2.5845(10), C7eC8eC9 168.1(4), Cu1eCl1e
Cu1* 83.58(3), Cl1eCu1eCl1* 96.42(3), Cl1eCu1eS1 107.44(4).

20
T. Kokoli et al. / Journal of Organometallic Chemistry 724 (2013) 17e22
enantiopure crystal, i.e. a conglomerate, can be prepared. Crystals of
1 are however racemic; they belong to the centrosymmetric space
group Pbcn.
While trying to investigate if the CuBr analog of 1 crystallized as
a conglomerate, we instead obtained [Cu2Br2(Sprop)4] (2). This
complex is a discrete dimer (Fig. 2) where none of the triple bonds
in the Sprop ligand are coordinated; the C8eC9 bond distance is
ꢁ
ꢀ
1.174(4) A, while the C7eC8eC9 bond angle is 179.1(3) . That CuBr
is less prone to coordinate alkenes and alkynes than CuCl should
perhaps not come as a surprise; this tendency has been reported
previously [16]. All sulfur atoms in 2 are chirogenic, this is an effect
of coordination to Cu1. The dimer itself is not chiral because two of
the sulfur atoms have the R configuration and two have the S
configuration.
Encouraged by the formation of layers via p-complexation in 1,
we decided to use CuCl with another functionalized sulfide ligand:
allyl methyl sulfide (Sally). This sulfide is extra interesting since the
allyl group is prochiral in itself, which opens up the possibility of
selective attack on one side of the double bond in a chiral crystal. By
using the same simple preparative method as for 1 and 2 (the neat
ligand is used as solvent), we could isolate crystals of [CuCl(Sally)]n
(3). As indicated in Fig. 3, a coordination polymer is indeed formed.
Unfortunately, half of the sulfide ligands (C5eS2eC6eC7eC8) are
disordered, possibly because the allyl group is not coordinated by
copper. The other Sally ligand bridges three copper atoms; the
double bond is coordinated and both electron pairs on S1 are
engaged in bonding to copper. While this is good in the sense that it
assists in formation of a layer, it is unfortunate because S1 is no
longer accessible for oxidation. The low precision in the geomet-
rical data for the disordered Sally ligand precludes a meaningful
comparison between the coordinated and free ligands. Although
there are elements of chirality in the layers in 3 (such as the
coordinated double bond and the S atom in the disordered sulfide),
each layer as well as the whole crystal is centrosymmetric.
Mesitylcopper (CuMes) is a very useful starting material for
a variety of organocopper and -cuprate complexes [11]. The main
reasons are that CuMes is soluble in many organic solvents (most
other homoleptic organocopper complexes form insoluble poly-
mers) and that CuMes is reasonably easy and safe to handle
(alkylcopper complexes may be explosive when isolated and dried).
Mesitylcopper is known to aggregate, and in benzene an equilib-
rium between tetramers and pentamers exists [11]. We decided to
see if Sally could break or bridge such aggregates. As previously, we
used the neat sulfide ligand as solvent and could isolate crystals of
[Cu4(Mes)4(Sally)2] (4). A tetrameric Cu4Mes4 core is retained
(Fig. 4) despite complexation of two Sally ligands by each tetramer.
Fig. 3. ORTEP drawing of the asymmetric unit in 3. Layers extend in the acꢁplane, via
ꢀ
sulfide and chloride bridging. Selected bond distances (A) and angles ( ): C3eC4
1.343(9), Cu2eC3* 2.119(6), Cu2eC4* 2.118(6), C7eC8 1.35(2), Cu1eCl1 2.3307(16),
Cu2eCl2 2.2930(16), Cu1eCl2 2.3629(16), Cu1eS1 2.2733(16).
ꢀ
triple bond is somewhat lengthened to 1.190(5) A and the C7eC8e
C9 bond angle of 168.1(4)ꢁ deviates from the 180ꢁ required by sp
hybridization. These changes in ligand geometry upon coordination
to Cu1 are quite small and indicate that
p back donation is weak, as
expected in copper(I) complexes [15]. That the S atom in itself is
terminal means that it has a free electron pair, which could be used
in a subsequent enantioselective oxygen transfer reaction to form
the corresponding chiral sulfoxide. Such a reaction requires that an
The allyl groups are not coordinated and it seems like
p-
complexation is not sufficient to break up the tetramer. Interest-
ingly, both chirogenic sulfur atoms, in each molecule of 4, have the
same configuration. In Fig. 1 both sulfur atoms have the R config-
uration; the whole tetranuclear complex is thus chiral. Unfortu-
nately, [Cu4(Mes)4(Sally)2] crystallizes in the centrosymmetric C2/c
space group, which means that crystals of 4 are racemic.
All molecules in 4 are chiral; if one could link such molecules
together it should be possible to construct homochiral helices. We
thus decided to replace the weakly coordinating allyl group in Sally
for a second sulfide group, resulting in the 2,5-dithiahexane ligand
(SS). Addition of this ligand to a toluene solution of CuMes resulted
in precipitation of a yellow powder, which could be recrystallized to
give [Cu4(Mes)4(SS)]n (5). A crystal structure determination (Fig. 5)
revealed that we indeed had been successful in linking tetrameric
Cu4Mes4 units by the SS ligand to form polymeric chains. Similar
stacking of Cu4Ar4 units has been previously reported [23]. Unfor-
tunately, the two sulfur atoms in 5 have different configurations so
that racemic chains are formed instead of the desired helices.
Fig. 4. ORTEP drawing of
a discrete tetranuclear aggregate in 4. Selected bond
ꢁ
ꢀ
distances (A) and angles ( ): C21eC22 1.314(7), Cu1eC1 2.069(4), Cu2eC1 2.020(4),
Cu2eC10 2.007(4), Cu1eS1 2.3455(12), Cu1eCu2 2.4220(8), Cu2eCu2* 2.6253(12),
C20eC21eC22 124.0(5), C1eCu1eS1 99.52(12), Cu1eC1eCu2 72.63(15), C19eS1eCu1
109.70(17).

T. Kokoli et al. / Journal of Organometallic Chemistry 724 (2013) 17e22
21
ꢁ
ꢀ
Fig. 5. ORTEP drawing showing how the bridging SS-ligand form chains in 5. Selected bond distances (A) and angles ( ): Cu2eS1 2.3459(9), Cu3eS2 2.3565(9), Cu1eCu2 2.4526(7),
Cu3eCu4 2.4581(7), Cu1eCu4 2.5773(6); C10eCu1eC1 141.28(13); C19eCu2eC10 163.76(13).
Scheme 2. Sulfide ligands used in 1e5.

22
T. Kokoli et al. / Journal of Organometallic Chemistry 724 (2013) 17e22
Table 2
Asymmetry 14 (2003) 185;
(d) K. Soai, T. Shibata, H. Morioka, K. Choji, Nature 378 (1995) 767.
Aggregation and coordination mode in 1e5.
[3] (a) M. Vaida, R. Popovitz-Biro, L. Leiserowitz, M. Lahav, in: V. Ramamurthy
(Ed.), Photochemistry in Organized and Constrained Media, VCH, New York,
1991 (Chapter 6);
Complex
Aggregate
CuX:L
La
(1) [CuCl(Sprop)]n
(2) [Cu2Br2(Sprop)4]
(3) [CuCl(Sally)]n
(4) [Cu4Mes4(Sally)2]
(5) [Cu4Mes4(SS)]n
Layer
Dimer
Layer
Tetramer
Chain
1:1
1:2
1:1
2:1
4:1
1:0
1:0
1:1
2:0
1:0
(b) M. Sakamoto, Chem. Eur. J. 3 (1997) 684;
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a
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Functionalized ligands like allyl methyl sulfide and phenyl
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well suited, due to their lower tendency to form p-complexes. The
2,5-dithiahexane ligand can be used to link mesitylcopper aggre-
gates to form chains or potentially helices. All five complexes (1e5)
exhibit terminal sulfide ligands that may be oxidized selectively
when incorporated in an enantiopure polymer. Unfortunately none
of 1e5 crystallized as a conglomerate, but whether this reflects an
inherent tendency in this system is too early to say. Complexes 1e5
are highly sensitive to air, which could prove to be valuable in
oxidation reactions [24] once a conglomerate has been isolated.
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Acknowledgments
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(b) L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
Financial support from the Swedish Research (VR) is gratefully
acknowledged.
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Appendix. ASupplementary material
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(1991) 347.
CCDC 818801, 818803, 818802, 818800 and 818799 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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