Metal-organic chains constructed from pre-organized N2S2 donor ligands

Article abstract of DOI:10.1016/j.poly.2009.05.073

Three new N₂S₂ donor ligands 1,1′-((2-(2-(phenylthio)phenylthio)phenyl)methylene)bis(3,5-R-1H-pyrazole), R = H (L^H), R = Me (L^Me), R = i-Pr (L^i-Pr) have been prepared and characterized. These bifuncti

Full text of DOI:10.1016/j.poly.2009.05.073

Polyhedron 29 (2010) 361–371
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Metal-organic chains constructed from pre-organized N2S2 donor ligands
*
Marcello Gennari, Irene Bassanetti, Luciano Marchiò
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Università degli Studi di Parma, viale G. P. Usberti 17/a, I 43100 Parma, Italy
a r t i c l e i n f o
a b s t r a c t
Three new N2S2 donor ligands 1,10-((2-(2-(phenylthio)phenylthio)phenyl)methylene)bis(3,5-R-1H-pyra-
zole), R = H (LH), R = Me (LMe), R = i-Pr (Li-Pr) have been prepared and characterized. These bifunctional
ligands incorporate two distinct chelate donor systems, by virtue of the presence of bispyrazole and bis-
thioether functions. The preferred conformation of these ligands is such that the N2 and S2 donor moieties
may be oriented in opposite directions, thus favoring the formation of molecular chains when treated
with AgBF4. The X-ray structures of Ag(I) complexes show that, depending on the steric hindrance present
Article history:
Available online 7 June 2009
Keywords:
C-centered N2S2 ligands
Ag(I) complexes
Molecular chains
on the pyrazole rings, these ligands behave as
bridging tridentate (when R = Me, i-Pr). Interestingly, [Ag(LH)]BF4 crystallizes in the chiral space group
P41, with the molecular chain that is folded around the 41 screw axis.
j l bridging tetradentate (when R = H), or j
4-SSNN- 3-SNN-
l
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
In the present work, three different N2S2 donor ligands were
synthesized: LH, LMe, and Li-Pr. The N2 donor system is derived from
a bis(pyrazole)methane moiety. A bisthioether function attached
to its central carbon atom provides the S2 donor system. These
ditopic ligands adopt a conformation that makes them suitable to
build molecular chains when treated with metal ions since the
N2 and S2 donor groups point in nearly opposite directions. Fur-
thermore, the three ligands differ by the steric hindrance posi-
tioned on the pyrazole ring in the form of H, Me or i-Pr groups,
and this has structural consequences on the overall polynuclear
structure. To investigate the suitability of these ligands in the con-
struction of molecular chains, Ag(I) complexes were prepared by
treating AgBF4 with LH, LMe or Li-Pr. The molecular structures of
the Li-Pr ligand and of the [Ag(LH)]BF4, [Ag(LMe)(Me2CO)]BF4ꢀMe2CO,
and [Ag(Li-Pr)(Me2CO)]BF4 complexes are described. Interestingly,
the [Ag(LH)]BF4 complex crystallizes in the P41 chiral space group
due to the formation of a homochiral molecular chain having the
shape of a right-handed helix. A conformational study on a model
ligand was performed in order to evaluate the preferred ligand
conformation prior to the interaction with the metal center.
As far as the silver complexes are concerned, many other exam-
ples can be found in the literature reporting 1D polymeric struc-
tures obtained by using nitrogen donor ligands. In some cases,
these 1D structures are generated by ligands that act as a bridge
between two metal centers thus forming a coordination polymer
[16,17], whereas in other cases the building blocks are represented
by metal complexes that interact with neighboring ones by means
Supramolecular chemistry is an active and ever growing field of
investigation where elegant structures are spontaneously gener-
ated from smaller building blocks [1,2]. The components of these
assemblies are in many cases represented by an anionic or neutral
part in the form of an organic ligand, and by a cationic part repre-
sented by a metal center. The ligands can be specifically synthe-
sized in order to incorporate the desired donor atoms in the
preferred geometric arrangements. In addition, the different coor-
dination polyhedra, characteristic of opportunely selected metal
ions, contribute to the shape of the resulting coordination poly-
mers [3–5] that can be defined as metal organic frameworks or me-
tal organic polyhedra [6,7]. These assemblies exhibit peculiar
properties such as magnetism and luminescence [8]. Furthermore,
according to the porous properties of many of these systems, they
have also found application in the field of catalysis in confined
space [9,10], and in the constructions of gas storage devices at
the molecular and nano-scale levels [11–13].
The stability of the supramolecular architectures depends on
the affinity between the ligand donor set and the metal ions,
and, more specifically, it is crucially dependent on the capacity of
the binding sites of the ligand to satisfy the stereochemical
requirement of the metal center. There is in fact a net thermody-
namic advantage during the generation of the supramolecular
structure when the ligands display a pre-organized donor set in
which the metal ion can easily settle [14,15].
of weaker forces such as hydrogen bonds,
p
–
p
stacking, or C–Hꢀꢀꢀ
p
interactions [18]. By a judicious choice of ligands, these molecular
chains can be chiral, either because they form helical structures
[19–22], or because a chiral ligand has been specifically employed
[23].
* Corresponding author. Tel.: +39 0521 905424; fax: +39 0521 905557.
E-mail address: marchio@unipr.it (L. Marchiò).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.05.073

362
M. Gennari et al. / Polyhedron 29 (2010) 361–371
Na2CO3,
DMF, 90oC
R=H, Me, i-Pr
CoCl ,
ΔT
2
LH, R=H
Me, L=Me
LiPr, R= i-Pr
L
Scheme 1. Synthesis of the ligands described in this work.
using a chiral and enantiopure electrophile, this procedure can also
afford chiral enantiopure ligands [27]. Alternatively, chiral ligands
can be generated starting from bis(pyrazolyl)methane functional-
ized with chiral groups [28].
2. Results and discussion
2.1. Synthesis
In the second synthetic route, bis(pyrazolyl)ketones are treated
with aldehydes or ketones bearing the desired donor groups that
are to be attached to the bispyrazole platform [29–32]. This proce-
dure has yielded N2O and N2S donor ligands, which were mainly
investigated for biomimetic purposes [33,34]. Also with this proce-
dure, enantiopure ligands can be prepared in a straightforward
way by employing enantiopure pyrazoles [35,36].
The ligands reported in this work were prepared according to
the second procedure, by treating bis(pyrazolyl)ketones with
2-(2-(phenylthio)phenylthio)benzaldehyde (Scheme 1). These li-
gands exhibit a N2S2 donor set and provide the geometric environ-
ment for the encapsulation of a metal center in a distorted
tetrahedral arrangement. To test this hypothesis, two models
based on the LMe ligand and Cu(I) or Ag(I) were taken as a bench-
mark, and their geometry were optimized [37]. In both model com-
plexes, the metal geometry is distorted trigonal planar achieved by
two nitrogen atoms and by the sulfur atom located at the end of
There are two main synthetic routes that lead to C-centered
heteroscorpionate tripodal ligands based on the bispyrazole scaf-
fold. The first method consists of the deprotonation of the methy-
lene bridge of bis(pyrazolyl)methane with a strong base. The
generated carbanion can then be treated with specific electro-
philes. This procedures has been developed for the production of
various tripodal ligands among which are the bis(pyrazolyl)car-
boxylate/bis(pyrazolyl)thiocarboxylate [24,25], and the hybrid
bis(pyrazolyl)cyclopentadienyl [26] ligands. In addition, when
Fig. 1. Optimized geometry (b3lyp/lanl2dz) of the model complexes [Cu(LMe)]+ and
0
0
[Ag(LMe)]+. Relevant geometric parameters: Cu–N, 2.049 and 2.010 AÅ; Cu–S, 2.399 ÅA;
0
N–Cu–0N, 96.0°; N–Cu–S 126.8°; N–Cu–S, 135.8°. Ag–N, 2.367 and 2.271 ÅA; Ag–S,
2.644 ÅA; N–Ag–N, 85.3°; N–Ag–S 123.8°; N–Ag–S, 137.9°.
Fig. 2. Ortep drawing of Li-Pr with thermal ellipsoids at the 30% probability level.

M. Gennari et al. / Polyhedron 29 (2010) 361–371
363
τ
τ
δ
τ
Scheme 2. Stereochemical descriptors (a), and torsion angles varied during the conformational studies (b).
Table 1
Summary of X-ray crystallographic data for Li-Pr, 1, 2 and 3.
Li-Pr
1
2
3
Empirical formula
Formula weight
Color, habit
Crystal size, mm
Crystal system
Space group
a (Å)
C37H44N4S2
608.88
colorless, prism
0.45 ꢁ 0.10 ꢁ 0.10
monoclinic
P21/c
20.758(2)
8.218(1)
21.153(2)
90
C25H20AgBF4N4S2
635.25
colorless, block
0.65 ꢁ 0.25 ꢁ 0.15
tetragonal
P41
9.138(1)
9.138(1)
30.539(4)
90
C35H40AgBF4N4O2S2
807.51
colorless, block
0.48 ꢁ 0.23 ꢁ 0.11
orthorhombic
Pca21
27.664(2)
8.836(1)
15.110(1)
90
C40H50AgBF4N4OS2
861.64
colorless, block
0.45 ꢁ 0.40 ꢁ 0.37
orthorhombic
Pbca
20.781(1)
18.131(1)
22.205(1)
90
b (Å)
c (Å)
a
(°)
b (°)
100.57(1)
90
3547.2(6)
4
90
90
2550.1(5)
4
90
90
3693.5(6)
4
90
90
8366.4(7)
8
c
(°)
V (Å3)
Z
T (K)
293(2)
1.140
0.180
173(2)
1.655
1.006
200(2)
1.452
0.716
293(2)
1.368
0.635
q
l
(calc) (Mg/m3)
(mmꢂ1
)
h Range (°)
Number of reflections
Absolute structure parameters
Goodness-of-fit (GoF)
1.00–26.00
36 977/3814
–
2.23–26.99
26 687/4742
0.05(4)
1.47–28.02
42 998/7522
ꢂ0.01(1)
1.007
1.75–28.00
96 601/9977
ꢂ
1.009
1.036
1.004
R1
0.0693
0.0691
0.0280
0.0597
wR2
0.2052
0.1635
0.0517
0.1040
P
P
P
P
R1
=
||Fo|ꢂ|Fc||/ |Fo|, wR2 = [ [w(F2o ꢂ Fc2)2]/ [w(F2o)2]]1/2, w = 1/[
r
2(Fo2) + (aP)2 + bP], where P = [max(Fo2, 0) + 2F2c ]/3.
the bisthioether arm (Fig. 1). The ligand can accommodate both
metal ions, but by doing so it faces a certain degree of distortion
of the aromatic ring attached to the bispyrazole moiety. In addi-
tion, this ligand conformation brings the CHcentral and the aromatic
CHortho in close proximity, which could possibly constitute an addi-
tional destabilizing factor. In order to investigate the coordinative
behavior of these ligands, Ag(I) complexes were prepared by treat-
ing equimolar amounts of ligands and AgBF4. The complexes could
be crystallized by stratifying pentane over an acetone solution of
each complex.
Table 2
Selected bond lengths (Å) and angles (°) for 1, 2 and 3.
1
Ag–N(21)
Ag–N(22)
Ag–S(13)0
Ag–S(14)0
2.355(7)
2.297(6)
2.607(2)
2.625(2)
N(21)–Ag–N(22)
N(21)–Ag–S(13)0
N(21)–Ag–S(14)0
N(22)–Ag–S(13)0
N(22)–Ag–S(14)0
S(13)0–Ag–S(14)0
84.9(2)
125.2(2)
111.2(2)
128.4(2)
137.0(2)
75.52(6)
2
Ag–N(21)
Ag–N(22)
Ag–S(14)00
Ag–O1s
2.307(2)
2.253(2)
2.4324(6)
2.550(2)
N(21)–Ag–N(22)
N(21)–Ag–O1s
N(21)–Ag–S(14)00
N(22)–Ag–O1s
N(22)–Ag–S(14)00
O1s–Ag–S(14)00
86.74(6)
107.34(7)
123.96(5)
94.55(7)
142.99(5)
94.82(5)
Table 3
List of conformation descriptors for compounds 1–3.
Complex
s
(1) (°)
121.9(3)a
d (°)
3
Li-Pr
1
2
79
51
82
89
Ag–N(21)
Ag–N(22)
Ag–S(14)000
Ag–O1s
2.252(4)
2.321(4)
2.427(1)
2.737(4)
N(21)–Ag–N(22)
N(21)–Ag–O1s
86.6(2)
96.7(1)
124.2(1)
91.3(2)
146.1(1)
98.1(1)
ꢂ130.3(7)b
121.6(2)a
130.0(5)a
N(21)–Ag–S(14)000
N(22)–Ag–O1s
3
N(22)–Ag–S(14)000
O1s–AgꢂS(14)000
a
Conformers with torsion angles of opposite values are present in the structure
(non-chiral space groups).
b
0
00
000
Only this conformer is present the structure (chiral space group P41).
= y; ꢂx; z ꢂ 1/4, = x; y + 1; z, = ꢂx + ½; y + ½; z.

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M. Gennari et al. / Polyhedron 29 (2010) 361–371
2.2. Molecular structures
this respect, both conformational enantiomers are present in the
unit cell, one with
(1) = ꢂ121.9(3)° and the other one with
(1) = 121.9(3)° (Table 3). An additional stereochemical descriptor
s
L
i-Pr ligand. The molecular structure is reported in Fig. 2. The two
s
pyrazole rings are in an anti conformation, and this arrangement is
probably adopted to avoid the electronic repulsion between the
nitrogen lone pairs. This is a common feature of ligands based on
the bis(pyrazolyl)methane scaffold [38]. In addition, a weak CHꢀꢀꢀN
interaction between N(21) and the hydrogen of C(72) of an i-Pr
group (3.196(5) Å) contributes to the stabilization of this pyrazole
arrangement. Another feature worth of mentioning is represented
by the conformation of the thioether arm with respect to the pyr-
azole rings that can be better described by introducing the torsion
useful to define the ligands conformations is the dihedral angle (d)
between one of the pyrazole rings and the II phenyl group (Scheme
2a).
[Ag(LH)]BF4. (1). Selected bond lengths and angles are reported
in Table 2, whereas the molecular structure and crystal packing
are depicted in Figs. 3 and 4. The ligand behaves as
j l
4-SSNN-
bridging tetradentate, with N2 chelated on a metal center and a
second metal linked to the bisthioether functions that point
approximately in opposite directions with respect to the N2 donor
system. The metal adopts a severely distorted tetrahedral geome-
try and the distortion from the ideal geometry is a consequence
of the small bite angles of the N2 (85°) and S2 (75°)0 chelate groups.
angle
s
(1)-[C(33)–C(23)–C(13)–N(11)], which is ꢂ121.9(3)° in the
present case. The C(33)–H is thus pointing directly above the nitro-
gen atom of one pyrazole (d[C(33)ꢀꢀꢀN(12)] = 2.850(4) Å) giving rise
to a weak CH–
p
interaction that can stabilize the phenyl ring of the
The Ag–N bond distances (2.297(6) and 2.355(7) ÅA) are consider-
0
thioether arm in the current position. Furthermore, the steric
repulsion between the C(33)–H and C(13)–H forces away these
two groups, and may be an additional factor contributing to the
resulting ligand arrangement. This has important consequences
on the structures of the metal complexes prepared with these li-
gands, since we may put forward the hypothesis of a pre-organiza-
tion of the free ligand in a way that the N2 and S2 donor systems
are oriented in different directions. This is likely responsible for
the polymeric molecular structures of the Ag(I) complexes de-
scribed below.
ably smaller than the Ag–S ones (2.607(2) and 2.625(2) ÅA). The li-
gand adopts the same conformation of the thioether arm found
It is worth noting that the asymmetry of the bisthioether group
and the present ligand conformation implies that Li-Pr is chiral,
though we may speak of a conformational chirality, and as stereo-
descriptor we can use the s(1) angle defined above (Scheme 2a). In
Fig.ꢂ3. Ortep drawing of 1 with thermal ellipsoids at the 30% probability level. The
BF4 anions were removed for clarity. Symmetry codes: 0 = y; ꢂx; 3/4 + z ꢂ 1, 00 = ꢂy;
Fig.ꢂ4. Crystal packing of 1 projected along the a axis. The hydrogen atoms and the
000
x; 1/4 + z, = ꢂx; ꢂy; 1/2 + z.
BF4 counter ion were removed for clarity.

M. Gennari et al. / Polyhedron 29 (2010) 361–371
365
for Li-Pr with the torsion angle
s
(1)-[C(33)–C(23)–C(13)–N(11)] of
in the present complex the ligand acts as
j
3-SNN-
l
bridging tri-
ꢂ130.3(7)°. This preferred conformation of the thioether moiety
with respect to the bispyrazole scaffold leads to the formation of
molecular chains that are parallel to the c crystallographic axis.
Furthermore, since the compound crystallizes in the chiral space
group P41, only one ligand conformation is present in the structure,
with molecular chains that are homochiral (Fig. 4). These chains
have the shape of a right-handed helix folded around the c axis,
dentate, bound to a metal center with the bis(pyrazole)moiety
and bridging another metal center with the S(14) sulfur atom.
The S(13) atom does not take part in the metal coordination. The
presence of the acetone molecule ligated to the metal (d[Ag–
O(1s)] = 2.550(2) Å] completes the distorted tetrahedral geometry
of the metal. The metal–nitrogen and metal–sulfur bond distances
are significantly smaller than the metal–oxygen distance, attesting
the weak nature of the latter interaction. The presence of certain
steric hindrance in the form of methyl groups on the pyrazole rings
causes the second phenyl ring (II) of the thioether arm to adopt a
conformation that prevents the coordination of the S(13) atom to
the metal center, Scheme 3b. The conformation of ring I is analo-
gous to that of the previous structures, and since the space group
and the helix pitch corresponds to the length of the c axis
0
(ꢃ30.5 ÅA), whereas the displacement of the silver atoms from the
0
helix center is ꢃ1 AÅ. It is likely that, during the crystallization pro-
cess, a racemic conglomerate [39,40] formed and that both enan-
tiomorphic space groups were present (P41 and P43). However,
inspection of the morphology of the crystals did not permit to indi-
viduate the enantiomorphic pairs as the complex gave rise to long
needle-like crystals with poorly defined faces [41]. Similar exam-
ples of racemic conglomerates formed when using an achiral pyr-
idine-thioether ligand were reported. Also in those cases, the
formation of the helical structures depends on the conformation
adopted by the ligand when bound to the metal [20,42].
is a non chiral one, both conformers are present with
s(1)-
[C(33)–C(23)–C(13)–N(11)] of 121.6(2)°. The pyrazole ring and
II form an angle of 82°, whereas in 1, where there are no bulky
groups on the pyrazoles, this angle is 51° (Table 3). The formation
of molecular chains is determined by the coordination of the S(14)
atom to a second metal. These chains present the phenyl residues
oriented on the opposite side with respect to the coordinated ace-
tone molecules, and the pyrazole rings are interposed between
these two donor groups (Fig. S4). The chains are homochiral, built
by ligands of the same conformations, and chains of opposite chi-
rality are present in the unit cell (Fig. 6).
[Ag(LMe)(Me2CO)]BF4ꢀMe2CO (2). Selected bond lengths and an-
gles are reported in Table 2, whereas the molecular structure and
crystal packing are depicted in Figs. 5 and 6. Differently from 1,
[Ag(Li-Pr)(Me2CO)]BF4ꢀMe2CO (3). The molecular structure of the
asymmetric unit of this complex is similar to that of 2 (Fig. 7).
The metal presents a trigonal planar geometry achieved by the
bis(pyrazole) moiety and by a bridging sulfur atom (S(14)). Also
for this complex, there is a weak Ag–O interaction that derives
from an acetone molecule. However, the Ag–O1s distance in the
present complex is ꢃ0.2 Å longer than that found in 2 (Table 2).
For this reason, the metal geometry can be described as trigonal
planar slightly distorted toward the tetrahedral one. The confor-
Fig.ꢂ5. Ortep drawing of 2 with thermal ellipsoids at the 30% probability level. The
BF4 anions were removed for clarity. Symmetry codes: 0 = x; y + 1; z, 00 = x; y ꢂ 1; z,
Fig. 6. Portionꢂof the crystal packing of 2 projected along the c axis. The hydrogen
atoms, the BF4 counter ion, and the solvate acetone molecules were removed for
clarity.
000
= x; y ꢂ 2; z.

366
M. Gennari et al. / Polyhedron 29 (2010) 361–371
Scheme 3. Different pyrazole substituents determine different ligand conformations and modes of coordination.
mation of the I phenyl ring of the thioether arm is analogous to
that of the previous complexes, with (1)-[C(33)–C(23)–C(13)–
lar chains contain alternating ligands of opposite chirality. The
crystal packing is also similar to that of 2, even though in the pres-
ent complex there are no additional acetone molecules of crystal-
lization (Figs. 8 and S5).
s
N(11)] of ꢂ130.0(5)°. The steric hindrance on the pyrazole rings
is determined by i-Pr residues and this reproduces an effect similar
to the methyl groups in 2 to the overall structure. In fact, the S(13)
atom is not bound to the metal and the II phenyl ring of the thio-
ether arm is nearly perpendicular to the pyrazole plane (Scheme 3).
Differently from the previously described complexes, the molecu-
2.3. Conformational studies
Conformational studies [37] were performed in order to inves-
tigate the energetics relative to the mobility of the pyrazole rings
and of the thioether group of the ligands. For this purpose, a model
complex was used in which the –II-S-III fragment was replaced by
a methyl group, Scheme 2b. The calculations were performed by
varying two torsion angles, one that rotates the thioether group
with respect to the pyrazole rings (s1), and the other one that
Fig.ꢂ7. Ortep drawing of 3 with thermal ellipsoids at the 30% probability level. The
Fig. 8. Portion of the crystal packing of 3 projected along the c axis. The hydrogen
atoms and the BF4ꢂ counter ion were removed for clarity. The chains are composed
of alternating ligand conformers.
0
BF4 anions were removed for clarity. Symmetry codes: = 1/2 ꢂ x; 1/2 + y; z,
00
000
= 1/2 ꢂ x; y ꢂ 1/2; z, = x; y ꢂ 1; z.

M. Gennari et al. / Polyhedron 29 (2010) 361–371
367
Fig. 9. Rigid potential energy surface scan (b3lyp/6-31G) obtained by varying s(1) and s(2) torsion angles as defined in Scheme 2b. Molecular structures of relevant
conformations are also reported.
rotates one of the pyrazole rings (
s
2). A picture describing this rigid
scenario, as it requires a minor energetic expenditure. Extending
these results to the LH, LMe and Li-Pr ligands, it can be then proposed
that, prior to their interaction with a metal center, the ligands are
in either one of the 1–3 conformations. When a metal center ap-
proaches the ligands, it becomes chelated by the pyrazole rings
and the sulfur atom(s) is(are) available for bridging on a second
metal center on the opposite side. The results presented here are
to be taken as a qualitative picture of the possible conformations
potential energy surface scan is depicted in Fig. 9. It can be noted
that there are very high-energy peaks, and these correspond to
unrealistic conformations where the C–H and –S–CH3 groups are
in very close proximity. This occurs because at every calculation
step the molecular geometry was not optimized (rigid scan). For
all of the other regions, the maximum energy difference
is ꢃ80 kJ/mol, and there appear preferential pathways for the con-
formational mobility. In particular, the trajectory that connects the
1–3 conformations is characterized by the rotation of the pyrazole
occurring in solution for the real ligand systems LH, LMe, and Li-Pr
.
In fact, the different length of the bisthioether arm and the bulky
groups on the pyrazole rings may considerably alter the shape of
the potential energy surface.
ring (
s2), but with the sulfur atom that points towards the CHcentral.
Conformation 1 is similar to that observed for the structure of Li-Pr
,
with the pyrazole rings in an anti disposition, whereas in 3 one of
the pyrazoles exchanges an interaction with the CHphenyl in the
ortho position. The normalized energy profile of this conforma-
tional change is depicted in Fig. S6. The saddle point (structure 2)
lies ꢃ30 kJ/mol above the 1 and 3 minima. On the other hand,
the trajectory that connects the 4 and 5 conformations lies 80 kJ/
mol above the one that connects the 1–3 conformations. Along this
4–5 path, the molecule rotates the pyrazole ring, but a destabiliz-
ing factor is represented by the repulsive interaction between the
sulfur atom and one of the nitrogen atoms. Two other structural
modifications can be envisaged by inspection of Fig. 9, and are
those linking the 1–4 and 3–5 conformations. These correspond
to the rotation of the thioether group with respect to the bis(pyra-
zole) moiety. The normalized energy profile of the 1–4 trajectory
shows that saddle point 4 is ꢃ80 kJ/mol above the minima, and
ꢃ50 kJ/mol higher than saddle point 2 (Fig. S6).
As far as the silver complexes are concerned, the 1H NMR spec-
tra recorded in CD3CN or (CD3)2CO at variable temperature suggest
that a dynamic behavior involves at least one of the two pyrazole
rings. In fact, the substituents of LMe and Li-Pr of one pyrazole group
exhibit coalescence at ꢃ250 K for LMe and 280 K for Li-Pr (Fig. 10).
The reason for this behavior is not easily interpretable, but may
be related to the rotation of the pyrazole ring along the Ccentral–N
vector. This process determines the rupture of the Ag–N bond
and consequently a modification of the silver geometry. In support
of this hypothesis are the molecular structures of 1–3, where it is
evident that one of the Ag–N bond distances is significantly longer
than the other one by 0.05 Å (1 and 2) and 0.07 Å (3). Furthermore,
the longest Ag–N distance is associated with the pyrazole on the
same side of the sulfur atom and for which there is a mismatch be-
tween the directionality of the nitrogen lone pair and the silver
atom (Fig. 11). At the same time, the N–Ag–S bond angle opposite
to the fluxional Ag–N bond tends towards linearity (128° in 1, 143°
in 2 and 146° in 3) further supporting the lability of the Ag–N bond
[43]. Assuming this fluxional behavior, the silver coordination var-
ies between the distorted tetrahedral and distorted T-shape.
From these calculations, it is evident that the thioether group is
preferentially oriented towards the hydrogen atom of CHcentral
,
hence on the opposite direction with respect to the N2 donor
atoms, whereas the rotation of the pyrazole ring is a more viable

368
M. Gennari et al. / Polyhedron 29 (2010) 361–371
Fig. 10. Variable temperature 1H NMR spectra of 2 (a) and 3 (b) recorded in CD3CN. The assignment of the signals was performed by means of COSY experiments performed at
240 and 320 K (supinfo). The asterisks denote fluxional Me or i-Pr protons. # = impurities (CD2HCN and acetone).

M. Gennari et al. / Polyhedron 29 (2010) 361–371
369
thioether function is not stabilizing enough to promote a coordina-
tion of this group on the same metal, and the ligands adopt a dif-
ferent conformation that is suitable for the generation of
molecular chains.
The coordination properties of LH, LMe, and Li-Pr also depend on
the steric hindrance located on the pyrazole rings. In fact, when
unsubstituted pyrazoles are present (as in LH), the ligand acts as
j
4-SSNN-
l
bridging tetradentate, whereas, when Me or i-Pr groups
are present (LMe, and Li-Pr) these ligands behave as 3-SNN-
j
l
bridg-
ing tridentate. This is a consequence of the conformation adopted
by the rings that compose the thioether arm in the presence of dif-
ferent pyrazole substituents. We are currently investigating the
coordination capabilities of these ligands with other transition
metals that exhibit different coordination polyhedra in order to
evaluate the effect of the metal geometry on the resulting supra-
molecular assembly.
4. Experimental
Bis(pyrazolyl)ketones [30–32] were prepared as reported in the
literature. 2-(phenylthio)benzenethiol [45] and 2-(2-(phenyl-
thio)phenylthio)benzaldehyde [46] were prepared by slight modi-
fications of reported procedures. All reagents and solvents were
commercially available. Solvents were dried and distilled before
use. 1H NMR spectra were recorded on a Bruker Avance 300 spec-
trometer using standard Bruker pulse sequences. Chemical shifts
are reported in ppm referenced to residual solvent protons (CDCl3,
CD2Cl2, CD3CN, (CD3)2CO). Infrared spectra were recorded from
4000 to 700 cmꢂ1 on a Perkin–Elmer FT-IR Nexus spectrometer
equipped with a Thermo-Nicolet microscope. Elemental analyses
(C, H, N) were performed with a Carlo Erba EA 1108 automated
analyzer.
Fig. 11. Comparison of the Ag environment and Ag–N bond distances in 1–3.
4.1. Synthesis of 2-(phenylthio)benzenethiol
3. Conclusions
A solution of thiophenol (4 mL, 39.0 mmol) in cyclohexane
(10 mL) was added to a solution of n-BuLi (1.6 M in hexane,
54 mL, 86.4 mmol) and N,N,N0,N0-tetramethylethylenediamine
(TMEDA, 13 mL, 86.7 mmol) in cyclohexane (60 mL) at 0 °C under
nitrogen. The reaction was stirred for 30 min at 0 °C and for 6 h
at room temperature. A solution of diphenyl disulfide (9.5 g,
43.6 mmol) in cyclohexane (90 mL) was added at 0 °C. After being
stirred for 30 min at 0 °C, the mixture was stirred overnight at
room temperature and quenched with 15% HCl(aq) (175 mL). The
organic and water layers were separated and the latter was ex-
tracted with diethyl ether (3 ꢁ 50 mL). The organic phases were
mixed and dried with anhydrous Na2SO4. Solvents were removed
under vacuum. The solid was recrystallized from a CH2Cl2:EtOH
mixture, yielding a colorless crystalline solid (4 g, 18.4 mmol,
47%). 1H NMR (300 MHz, CDCl3): 4.29 (s, 1H, SH), 7.1–7.6 (m, 9H,
CH). Anal. Calc. for C12H10S2 (218.02): C, 66.01; H, 4.62. Found: C,
66.10; H, 4.44%.
The molecular structure of Li-Pr and of the silver complexes with
LH, LMe, and Li-Pr show that these N2S2 ligands are capable of gener-
ating molecular chains according to the presence of the N2 bispy-
razole and the S2 bisthioether groups. The conformational study
performed on a simplified model ligand suggests that the thioether
function is preferably oriented towards the CHcentral and hence
points approximately in the opposite direction with respect to
the N2 system. These calculations also show that the rotation of
one pyrazole group along the Ccentral–N vector requires a minimum
expenditure of energy (ꢃ30 kJ/mol). This is in accordance with the
formation of the molecular structures of the silver complexes here
reported when starting from a ligand that usually displays the
bis(pyrazole) moiety in an anti conformation, as in the case of
L
i-Pr. Additional proof of this conformational preference can be ob-
tained by inspecting the values of (1) introduced in Scheme 2,
s
which vary in the narrow range of 122–130° in the reported molec-
ular structures (Table 3). This suggests that this conformation actu-
ally corresponds to an energy minimum. Bispyrazole ligands
functionalized with phenolate or thiophenolate groups attached
on the central carbon atom were previously described [36,44].
When deprotonated, these ligands behave as N2O or N2S donors
when treated with a metal center such as zinc(II), hence adopting
a conformation that is apparently in contrast with that of the li-
gands reported here. In fact, the destabilizing factor that may arise
from the CHcentral–CHphenyl repulsion is compensated by the favor-
able interaction between the phenoxo/thiophenoxo groups with
the metal. Nevertheless, when complexed in their neutral form,
the phenyl rings of these ligands adopt a conformation that is anal-
ogous to the structures described here. In LH, LMe, and Li-Pr the
4.2. Synthesis of 2-(2-(phenylthio)phenylthio)benzaldehyde
To
a stirred solution of 2-(phenylthio)benzenethiol (2 g,
9.2 mmol) and anhydrous Na2CO3 (1.3 g) in dry DMF (5 mL) under
nitrogen at 90 °C, 2-chlorobenzaldehyde (0.9 mL, 7.99 mmol) was
slowly added. The mixture was stirred at 90 °C overnight. After
cooling, the reaction mixture was poured into water (50 mL) and
extracted with diethyl ether (50 mL). The extract was washed with
water (2 ꢁ 50 mL), dried with anhydrous Na2SO4, and concentrated
under vacuum. The resulting oil was triturated with hexane in an
ultrasound bath for 20 min, filtered and dried under vacuum to af-
ford a pale white solid (1.85 g, 5.7 mmol, 72%). 1H NMR (300 MHz,

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M. Gennari et al. / Polyhedron 29 (2010) 361–371
CDCl3): 7.11–7.51(m, 12H, CH Ph), 7.94 (d, J = 7.5 Hz, 1H, CH Ph),
10.39 (s, 1H, CHO). Anal. Calc. for C19H14OS2 (322.05): C, 70.77;
H, 4.38. Found: C, 70.53; H, 4.51%.
Anal. Calc. for C37H44N4S2 (608.90): C, 72.98; H, 7.28; N, 9.20.
Found: C, 73.07; H, 7.11; N, 9.12%.
4.6. Synthesis of [Ag(LH)]BF4
4.3. Synthesis of 1,10-((2-(2-(phenylthio)phenylthio)phenyl)-
methylene)bis(1H-pyrazole) (LH)
LH (100 mg, 0.227 mmol) and AgBF4 (44 mg, 0.227 mmol) were
mixed in acetone (20 mL), and the colorless solution was stirred for
1 h. The solvent was then removed under vacuum and the white
solid was washed with diethyl ether in an ultrasound bath. The
mixture was filtered affording a white solid (45 mg, 0.091 mmol,
40%). The compound was recrystallized by stratifying pentane over
an acetone solution of the complex, yielding colorless crystals. IR
(cmꢂ1): 3133w, 3051w, 2969w, 1689w, 1621w, 1566w, 1511w,
1434m, 1396m, 1287m, 1052s, 1021s, 746s, 690m, 626m, 520m.
1H NMR (300 MHz, (CD3)2CO): 6.47 (t, J = 2.4 Hz, 1H, CH pz), 6.98
(d, J = 8.1 Hz, 1H, CH Ph), 7.21 (m, 4H, CH Ph), 7.46 (m, 8H, CH
Ph), 7.68 (d, J = 1.8 Hz, 2H, CH pz), 8.10 (d, J = 2.7 Hz, 2H, CH pz),
8.69 (s, 1H, CH). Anal. Calc. for C25H20AgBF4N4S2 (635.26): C,
47.27; H, 3.17; N, 8.82. Found: C, 47.76; H, 3.02; N, 8.45%.
2-(2-(Phenylthio)phenylthio)benzaldehyde
(875 mg,
2.71
mmol) and CoCl2ꢀ6H2O (5 mg, 0.021 mmol) were added to
bis(1H-pyrazol-1-yl)ketone (565 mg, 3.48 mmol) in a Schlenk flask
under nitrogen. The mixture was heated at 130 °C and vigorously
stirred. After 1 h, the green mixture was cooled to room tempera-
ture. Diethyl ether (50 mL) and water (50 mL) were then added,
and the organic phase was separated and washed with brine (2 x
50 mL), dried with anhydrous Na2SO4 and filtered. The volatiles
were removed under vacuum yielding a pale oil. The compound
was recrystallized in hot methanol (5 mL) and the solution was
left to rest at 0°. Colorless micro-crystals were isolated and vacuum
dried (LMe
,
907 mg, 2.06 mmol, 76%). 1H NMR (300 MHz,
CDCl3): 6.26 (t, J = 2.0 Hz, 2H, CH pz), 6.86 (dd, J = 7.5 Hz,
J = 1.6 Hz, 1H CH Ph), 6.99–7.40 (m, 14H, CH Ph, CH pz), 7.62 (d,
J = 1.7 Hz, CH pz), 8.20 (s, 1H, CH bridge). Anal. Calc. for
C25H20N4S2 (440.59): C, 68.15; H, 4.58; N, 12.72. Found: C, 67.86;
H, 4.21; N, 12.41%.
4.7. Synthesis of [Ag(LMe)(Me2CO)]BF4ꢀMe2CO
L
Me (100 mg, 0.201 mmol) and AgBF4 (40 mg, 0.205 mmol) were
mixed in acetone (15 mL), and the colorless solution was stirred for
1 h. The solvent was then concentrated under vacuum and a white
solid was precipitated after addition of pentane. The solid was fil-
tered and vacuum dried (138 mg, 0.187 mmol, 93%). The com-
pound was recrystallized by stratifying pentane over an acetone
solution of the complex, yielding colorless crystals. IR (cmꢂ1):
3062w, 2985w, 2914w, 1708w, 1692m, 1577m, 1442m, 1419m,
1248w, 1051s, 1030s, 876m, 753s, 689m, 521m. 1H NMR
(300 MHz, CD3CN): 1.95 (s, 6H, CH3), 2.37 (s, 6H, CH3), 6.03 (s,
2H, CH pz), 6.88 (d, J = 7.2 Hz, 1H, CH Ph), 7.22 (m, 12H, CH Ph),
7.72 (s, H, CHcentral). Anal. Calc. for C35H40AgBF4N4O2S2 (807.52):
C, 52.06; H, 4.99; N, 6.94. Found: C, 51.67; H, 4.52; N, 6.68%.
4.4. Synthesis of 1,10-((2-(2-(phenylthio)phenylthio)phenyl)-
methylene)bis(3,5-dimethyl-1H-pyrazole) (LMe
)
2-(2-(Phenylthio)phenylthio)benzaldehyde
(900 mg,
2.79
mmol) and CoCl2ꢀ6H2O (5 mg, 0.021 mmol) were added to
bis(3,5-dimethyl-1H-pyrazol-1-yl)ketone (670 mg, 3.07 mmol) in
a Schlenk flask under nitrogen. The mixture was heated at 90 °C
and vigorously stirred. After 2 h, the dark blue mixture was cooled
to room temperature. CHCl3 (50 mL) and water (50 mL) were then
added. The organic layer was separated and washed with water
(2 ꢁ 50 mL), dried with anhydrous Na2SO4 and concentrated under
vacuum yielding a pale oil, which was purified by column chroma-
tography (Silica gel, gradient of elution from hexane/ethyl acetate
18:1 to pure ethyl acetate). The oil was then triturated with hexane
in an ultrasound bath, filtered and dried under vacuum, affording a
white powder (LMe, 790 mg, 1.59 mmol, 57%). 1H NMR (300 MHz,
CDCl3): 2.11 (s, 6H, CH3), 2.20 (s, 6H, CH3), 5.82 (s, 2H, CH pz),
6.99–7.14 (m, 5H, CH Ph), 7.31 (m, 8H, CH Ph), 7.86 (s, 1H, CH
bridge). Anal. Calc. for C29H28N4S2 (496.69): C, 70.13; H, 5.68; N,
11.28. Found: C, 69.96; H, 5.79; N, 11.36%.
4.8. Synthesis of [Ag(Li-Pr)(Me2CO)]BF4
Li-Pr (123 mg, 0.202 mmol) and AgBF4 (40 mg, 0.205 mmol)
were mixed in acetone (15 mL), and the colorless solution was stir-
red for 1 h. The solvent was then concentrated under vacuum and a
white solid was precipitated after addition of pentane. The solid
was filtered and vacuum dried (147 mg, 0.164 mmol, 81%). The
compound was recrystallized by stratifying pentane over an ace-
tone solution of the complex, yielding colorless crystals. IR
(cmꢂ1): 2963w, 2876w, 1693 m, 1544w, 1462 m, 1441 m, 1380w,
1363w, 1303w, 1276w, 1226 m, 1048s, 821 m, 758s, 648 m,
528 m. 1H NMR (300 MHz, CD3CN): 1.11 (m, 24H, 8CH3), 2.65 (s
br, 2H, CH i-Pr), 3.24 (m, J = 6.9 Hz, 2H, CH i-Pr), 6.24 (s, 2H, CH
pz), 6.56 (s br, 1H, CH Ph), 7.13 (m, 2H, CH Ph), 7.28 (m, 10H, CH
Ph), 7.91 (s, 1H, CH). Anal. Calc. for C40H50AgBF4N4OS2 (861.66):
C, 55.76; H, 5.85; N, 6.50. Found: C, 55.34; H, 5.39; N, 6.17%.
4.5. Synthesis of 1,10-((2-(2-(phenylthio)phenylthio)phenyl)-
methylene)bis(3,5-diisopropyl-1H-pyrazole) (Li-Pr
)
2-(2-(Phenylthio)phenylthio)benzaldehyde
(770 mg,
2.39
mmol) and CoCl2ꢀ6H2O (4 mg, 0.017 mmol) were added to
bis(3,5-diisopropyl-1H-pyrazol-1-yl)ketone (910 mg, 2.75 mmol)
in a Schlenk flask under nitrogen. The mixture was heated at
150 °C and vigorously stirred. After 4 h, the dark green mixture
was cooled to room temperature. CHCl3 (50 mL) and water
(50 mL) were then added, and the organic layer was separated
and washed with water (2 ꢁ 50 mL), dried with anhydrous Na2SO4
and concentrated under vacuum yielding a dark oil, which was
purified by column chromatography (Silica gel, hexane:ethyl
acetate 9:1). The resulting oil was recrystallized from hexane
(2–3 mL) at room temperature, yielding a crystalline white solid
(Li-Pr, 640 mg, 1.05 mmol, 44%). 1H NMR (300 MHz, CDCl3): 0.92
(d, J = 6.9 Hz, 6H, CH3), 0.99 (d, J = 6.9 Hz, 6H, CH3), 1.20 (d,
J = 6.9 Hz, 12H, CH3), 2.91 (hept, J = 6.9 Hz, 2H, CH i-Pr), 3.15 (hept,
J = 6.9 Hz, 2H, CH i-Pr), 5.91 (s, 2H, CH pz), 6.95 (m, 1H, CH Ph), 7.08
(m, 4H, CH Ph), 7.19–7.32 (m, 8H, CH Ph), 8.00 (s, 1H, CH bridge).
4.9. X-ray crystallography
A summary of data collection and structure refinement for Li-Pr
1–3 are reported in Table 1. Single crystal data were collected with
a Bruker AXS Smart 1000, Mo K : k = 0.71073 Å. The unit cell
parameters were obtained using 60 -frames of 0.5° width and
,
a
x
scanned from three different zones of reciprocal lattice. The inten-
sity data were integrated from several series of exposures frames
(0.3° width) covering the sphere of reciprocal space [47]. An
absorption correction was applied using the program SADABS [48]
with min. and max. transmission factors of: 0.777–1.000 (Li-Pr),
0.506–1.000 (1), 0.828–1.000 (2), 0.808–1.000 (3), and 0.834–
1.000 (3). The structures were solved by direct methods (SIR97

M. Gennari et al. / Polyhedron 29 (2010) 361–371
371
[49]) and refined on F2 with full-matrix least squares (SHELXL-97
[50]), using the WINGX software package [51]. Non-hydrogen atoms
were refined anisotropically for all compounds, and the hydrogen
Pellinghelli, Dalton Trans. (1999) 3537–3539. An alternative method
developed for the preparation of bis(pyrazolyl)carboxylate ligands was
reported, see: A. Beck, B. Weibert, N. Burzlaff, Eur. J. Inorg. Chem. (2001)
521–527.
ꢂ
[25] A. Otero, J. Fernandez-Baeza, A. Antinolo, F. CarrilloHermosilla, J. Tejeda, A.
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Sanchez-Barba, M. Sanchez-Molina, S. Franco, M.I. Lopez-Solera, A.M.
Rodriguez, Inorg. Chem. 46 (2007) 8475.
atoms were placed at their calculated positions. The BF4 anion
in 2 and 3 was statically disordered in two positions with site occu-
pancy factors of 0.56/0.44 (2), and 0.67/0.33 (3). Graphical material
was prepared with ORTEP3 for Windows [52] and MERCURY 2.0 [53]
programs.
Acknowledgement
[28] L. Peters, N. Burzlaff, Polyhedron 23 (2004) 245.
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This work was supported by the Università degli Studi di Parma
(Parma, Italy).
[33] T.C. Higgs, C.J. Carrano, Inorg. Chem. 36 (1997) 291.
Appendix A. Supplementary data
[34] T.C. Higgs, D. Ji, R.S. Czernuszewicz, B.F. Matzanke, V. Schunemann, A.X.
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M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C.
Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G.
Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M.
Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J.
Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas,
D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G.
Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong,
C. Gonzalez, J.A. Pople, Gaussian Inc., Wallingford CT, 2004.
CCDC 729091, 729092, 729093 and 729094 contain the supple-
mentary crystallographic data for Li-Pr, and 1–3. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2009.05.073.
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