Structural variability in Ag(I) and Cu(I) coordination polymers with thioether-functionalized bis(pyrazolyl)methane ligands

Article abstract of DOI:10.1021/ic201338w

We present here two ligand classes based on a bis(pyrazolyl)methane scaffold functionalized with a rigid (-Ph-S-Ph) or flexible (-CH ₂-S-Ph) thioether function: L^RPhS (R = H, Me) and L ^RCH₂S (R = H, Me, iPr). The X-ray molecular structures of Ag(I) and Cu(I) binary complexes with L^RPhS or L^RCH ₂S using different types of counterions (BF₄^-, PF₆^-, and CF₃SO₃^-) are reported. In these complexes, the ligands are N₂ bound on a metal center and bridge on a second metal with the thioether group. In contrast, when using triphenylphosphine (PPh₃) as an ancillary ligand, mononuclear ternary complexes [M(L)PPh₃]⁺ (M = Cu(I), Ag(I); L = L^RPhS, L^RCH₂S) are formed. In these complexes, the more flexible ligand type, L^RCH₂S, is able to provide the N₂S chelation, whereas the more rigid L^RPhS ligand class is capable of chelating only N₂ because the thioether function preorganized, as it did in the coordination polymers, to point away from the metal center. Rigid potential-energy surface scans were performed by means of density functional theory (DFT) calculations (B3LYP/6-31+G) on the two representative ligands, L^HPhS and L^HCH₂S. The surface scans proved that the thioether function is preferably oriented on the opposite side of the bispyrazole N₂ chelate system. These results confirm that both ligand classes are suitable components for the construction of coordination polymers. Nevertheless, the methylene group that acts as a spacer in L^HCH₂S imparts an inherent flexibility to this ligand class so that the conformation responsible for the N₂S chelation is energetically accessible.

Full text of DOI:10.1021/ic201338w

ARTICLE
pubs.acs.org/IC
Structural Variability in Ag(I) and Cu(I) Coordination Polymers with
Thioether-Functionalized Bis(pyrazolyl)methane Ligands
Irene Bassanetti and Luciano Marchiꢀo*
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universitꢀa degli Studi di Parma, parco Area delle
Scienze 17/a, I 43124 Parma, Italy
S Supporting Information
b
ABSTRACT: We present here two ligand classes based on a
bis(pyrazolyl)methane scaffold functionalized with a rigid (-Ph-
S-Ph) or flexible (-CH2-S-Ph) thioether function: LRPhS (R =
H, Me) and LRCH2S (R = H, Me, iPr). The X-ray molecular
structures of Ag(I) and Cu(I) binary complexes with LRPhS or
LRCH2S using different types of counterions (BF4ꢀ, PF6ꢀ, and
CF3SO3ꢀ) are reported. In these complexes, the ligands are N2
bound on a metal center and bridge on a second metal with the
thioether group. In contrast, when using triphenylphosphine
(PPh3) as an ancillary ligand, mononuclear ternary complexes
[M(L)PPh3]+ (M = Cu(I), Ag(I); L = LRPhS, LRCH2S) are formed. In these complexes, the more flexible ligand type, LRCH2S, is
able to provide the N2S chelation, whereas the more rigid LRPhS ligand class is capable of chelating only N2 because the thioether
function preorganized, as it did in the coordination polymers, to point away from the metal center. Rigid potential-energy surface
scans were performed by means of density functional theory (DFT) calculations (B3LYP/6-31+G) on the two representative
ligands, LHPhS and LHCH2S. The surface scans proved that the thioether function is preferably oriented on the opposite side of the
bispyrazole N2 chelate system. These results confirm that both ligand classes are suitable components for the construction of
coordination polymers. Nevertheless, the methylene group that acts as a spacer in LHCH2S imparts an inherent flexibility to this
ligand class so that the conformation responsible for the N2S chelation is energetically accessible.
’ INTRODUCTION
Many examples of Ag(I) coordination polymers in which the
nature of the donor ligand may lead to the formation of different
frameworks with tailored structures and functions have been
reported in the literature.18ꢀ26 In addition to classical metalꢀ
donor coordination bonds, silver is apt to give rise to metallo-
philic (AgꢀAg)27 and σ or π organometallic (AgꢀC) interac-
tions with aromatic moieties.28ꢀ32
Multitopic bis(pyrazolyl)methane ligands are currently being
studied for the design of coordination polymers, and the choice
of the spacer between two or three bis-pyrazolyl donor functions
influences the structure of the resulting molecular architectures.33ꢀ38
The spacer can be either rigid, as in aromatic groups,35 or more
flexible, as in alkylidene chains,36 and the different conforma-
tional degrees of freedom have important consequences on the
coordination properties of these ligands. In fact, the aromatic
spacer group, when compared to alkylidene chains, allows better
control over the formation of polymeric structures.35 The group
attached to a bis(pyrazolyl)methane scaffold can also be func-
tionalized with carboxylate moieties, and the formation of silver
polymeric structures has been achieved by the direct interaction
of this group with the metal atom or by the formation of bridging
hydrogen bonds.39
The rational design of coordination polymers has produced a
wide variety of interesting solid-state structures that are useful in
the development of new functional materials.1ꢀ4 The possible
applications of these various 1- to 3-dimensional systems can be
grouped into diverse fields such as catalysis,5 gas adsorption6
(gas separation7 and gas storage8,9), molecular recognition,10
luminescence,11,12 magnetism,13,14 and drug delivery.15 The ability
to design multitopic organic ligands that can bind the metal
center in a predictable way, as determined by their preferred
conformation, is crucial to the development of coordination
architectures. In this respect, the ligands may be defined as rigid
or flexible according to the high or low energy barrier, respec-
tively, that separates two or more ligand conformers.16 Specific
ligands can also be designed to be configurationally rigid, and
they can impart peculiar electronic properties upon binding to a
metal.17 The choice of the metal atom as the node in the
construction of coordination polymers is also important because
its stereoelectronic requirements, that is, coordination numbers
and geometry, heavily influence the overall shape of the resulting
polymer. Ag(I) is a fashionable metal node for the construction
of coordination polymers, and it has been used extensively for this
purpose. The lack of a crystal-field effect allows for the occurrence
of different coordination geometries, but the most common are
the linear, trigonal-planar, T-shaped, and tetrahedral geometries.
Received: June 22, 2011
Published: October 03, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Scheme 1. Ligand Synthesis
The syntheses of these ligands were performed under inert gas (N2)
using Schlenk techniques. The solvents were dried and distilled before
use. All other reagents and solvents were commercially available. H
1
NMR spectra were recorded on a Bruker Avance 300 spectrometer using
standard Bruker pulse sequences. Chemical shifts are reported in parts
per million (ppm) and referenced to residual solvent protons (CDCl3,
CD3CN, (CD3)2CO, CD2Cl2). Infrared spectra were recorded from
4000 to 700 cmꢀ1 on a Perkin-Elmer FT-IR Nexus spectrometer
equipped with a Thermo-Nicolet microscope. Elemental analyses (C, H,
and N) were performed with a Carlo Erba EA 1108 automated analyzer.
The synthesis of binary complexes [M(L)]+ was performed by mixing
ꢀ
equimolar amounts of [Cu(CH3CN)4]BF4 or Ag(I) salts (anions: BF4
,
PF6ꢀ, and CF3SO3ꢀ) and the appropriate ligand, whereas the [M(L)-
PPh3]+ ternary complexes were prepared by mixing equimolar amounts
of metal salts, ligand, and PPh3. These syntheses were performed in
acetone or in acetonitrile, and the details are provided in the Supporting
Information together with the crystallization conditions.
X-ray Crystallography. Single-crystal data were collected on a
Bruker Smart 1000 and a Bruker Smart APEXII area-detector diffracto-
meters (Mo Kα; λ = 0.71073 Å). Cell parameters were refined from the
observed setting angles and detector positions of selected strong
reflections. Intensities were integrated from several series of exposure
frames that covered the sphere of reciprocal space.45 A multiscan absorption
correction was applied to the data using the program SADABS.46 The
structures were solved by direct methods (SIR9747,48 and SIR200448)
and refined with full-matrix least-squares (SHELXL-97),49 using the
Wingx software package.50 Graphical material was prepared with the
Mercury 2.051 program. CCDC 828865ꢀ828884 contain the supple-
mentary crystallographic data for this paper.
DFT Calculations. Rigid potential-energy surface (PES) scans were
performed for the LHCH2S and LHPhS ligands starting from the ligand
conformations found in the X-ray structures of complexes [Ag(LHPhS)]n-
(BF4)n and [Ag(LHCH2S)]n(BF4)n, respectively. The calculations were
performed using the gradient-corrected hybrid density functionals
B3LYP52,53 and the 6-31+G basis set.54,55 In particular, the PES scan
was performed for LHCH2S by rotating three molecular fragments: (1)
one pyrazole ring (360° rotation about the Ccentral-Npz bond: 15 steps,
24°), (2) the CH2 linker (360° rotation about the Ccentral-CH2 bond: 15
steps, 24°), and (3) the peripheral phenyl ring (180° rotation about the
S-Ph bond: 10 steps, 18°). The PES scan for LHPhS was performed by
the following rotations: (1) one pyrazole ring (360° rotation about the
Ccentral-Npz bond: 15 steps, 24°), (2) the phenyl ring linker (360°
rotation about the Ccentral-Ph bond: 15 steps, 24°), and (3) the peripheral
phenyl ring (180° rotation about the S-Ph bond: 10 steps, 18°). To gain
insights on the different binding abilities of the two ligand classes
(LRCH2S and LRPhS), we optimized the geometries of two coordina-
tion isomers for each model complex, [Ag(LHCH2S)PH3]+ and
[Ag(LHPhS)PH3]+. The isomers were differentiated by the presence
or absence of the thioether coordination to the metal center. The PH3
ancillary ligand was used instead of the PPh3 one, which is present in
some reported X-ray structures, to save computational resources. The
optimizations of the geometries were performed with the B3LYP density
functional, with the 6-31+G(d) basis set for C, H, N, S, and P and with
the SDD valence basis set and MWB28 effective core potentials for
Ag.56ꢀ58 Vibrational frequencies were calculated at the same theoretical
level to ensure that the stationary points were true minima. All the
calculations were performed with the Gaussian 03 software.59
In our previous work, we used a ditopic N2S2 heteroscorpio-
nate ligands based on a bis(pyrazolyl)methane system that was
functionalized with a bis-thioether function to prepare Ag(I)
complexes. The spacer between the N2 and S2 donor systems was
rigid (Ph group), and this resulted in the opposite orientation of
the N2 and S2 chelate system so that coordination polymers were
preferentially formed with Ag(I). The steric hindrance of the
pyrazole rings also exhibited a considerable influence on the donor
ability of the ligand, because the presence of iPr groups hindered
the metal coordination of one of the thioether moieties.40
In the present work, we wish to present two ligand classes
based on the bis(pyrazolyl)methane system functionalized with a
rigid (-Ph-S-Ph) and flexible (-CH2-S-Ph) thioether function:
LRPhS (R = H, Me) and LRCH2S (R = H, Me, iPr) (Scheme 1).
The molecular structures of Ag(I) and Cu(I) complexes with
ꢀ
LRPhS or LRCH2S using different types of counterions (BF4
,
PF6ꢀ, and CF3SO3ꢀ) are reported, and they show how the
flexibility or rigidity of the thioether group influences the
formation of polymeric structures. Ternary complexes using
triphenylphosphine (PPh3) as an ancillary ligand of the type
[M(LRPhS)PPh3] and [M(LRCH2S)PPh3] (M = Cu(I) and
Ag(I)) are also described, and they more clearly show the
different conformational behaviors of the two ligand classes. In
addition, density functional theory (DFT) calculations are
employed to investigate the energetics pertaining to the mobility
of the pyrazole rings and the two thioether groups in LHPhS and
LHCH2S. These calculations show that, in LHPhS, the thioether
function is preferably oriented toward the CHcentral and is
therefore on the opposite side of the N2 system (rigid ligand).
At variance, LHCH2S is characterized by a greater inherent
flexibility that allows for N2S chelation and the bridging behavior
between two metal centers.
’ EXPERIMENTAL SECTION
1,10-((2-(Phenylthio)phenyl)methylene)bis(1H-pyrazole) (LHPhS),
10-((2-(phenylthio)phenyl)methylene)bis(3,5-dimethyl-1H-pyrazole)
(LMePhS), 1,10-(2-(phenylthio)ethane-1,1-diyl)bis(1H-pyrazole) (LHCH2S),
10-(2-(phenylthio)ethane-1,1-diyl)bis(3,5-dimethyl-1H-pyrazole)
(LMeCH2S), and 10-(2-(phenylthio)ethane-1,1-diyl)bis(3,5-diisopropyl-
1H-pyrazole) (LiPrCH2S) were synthesized as previously described.41ꢀ44
’ RESULTS AND DISCUSSION
The synthesis of ligands is described in Scheme 1. The synthetic
route to obtain C-centered functionalized bis(pyrazolyl)methane
ligands is based on solid-state reaction between bis(pyrazolyl)-
ketones and aldehydes bearing the desired sulfur donor group
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Inorganic Chemistry
ARTICLE
Table 1. Summary of X-ray Crystallographic Data for [Ag(LHPhS)]n(BF4)n, [Cu(LHPhS)(CH3CN)]2(BF4)2, and
[Ag(LHPhS)]2(PF6)2 CH2Cl2
3
[Ag(LHPhS)]n(BF4)n
[Cu(LHPhS)(CH3CN)]2(BF4)2
[Ag(LHPhS)]2(PF6)2 CH2Cl2
3
empirical formula
formula weight
color, habit
crystal size, mm
crystal system
space group
a, Å
C38H32Ag2B2F8N8S2
1054.20
C42H38B2Cu2F8N10S2
1047.64
C40H36Ag2Cl2F12N8P2S2
1340.37
colorless, block
0.47 ꢁ 0.32 ꢁ 0.13
monoclinic
P21/a
colorless, block
0.20 ꢁ 0.12 ꢁ 0.05
monoclinic
C2/c
colorless, block
0.19 ꢁ 0.17 ꢁ 0.12
triclinic
P1
21.368(1)
10.170(1)
21.555(2)
90
26.105(3)
8.080(1)
21.356(2)
90
10.160(2)
10.620(2)
12.736(2)
78.688(3)
73.156(3)
71.562(3)
1239.4(4)
1
b, Å
c, Å
α, deg.
β, deg.
118.776(1)
90
99.832(2)
90
γ, deg.
V, Å3
4105.7(6)
4
4438.4(9)
4
Z
T, K
293(2)
190(2)
293(2)
F (calc), Mg/m3
μ, mmꢀ1
θ range, deg.
no.of rflcn/unique
GOF
1.705
1.568
1.796
1.132
1.131
1.240
2.16 to 28.37
56570/10270
1.044
1.58 to 27.05
18698/4837
1.006
1.68 to 27.10
14504/5407
1.004
R1a
0.0554
0.0521
0.0391
wR2a
0.1314
2
0.0773
2
0.0813
a R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|, wR2 = [∑w(Fo ꢀ Fc2)2/∑w(Fo2)2]1/2, w = 1/[σ2(Fo ) + (aP)2 + bP], where P = [max(Fo ,0) + 2Fc2]/3.
2
ꢁ
that were to be attached to the bis(pyrazolyl)methane scaffold.
The N2S donor set of the ligand described in this work is
generated by treating substituted bis(pyrazolyl)ketones with
(phenylthio)benzaldehyde and (phenylthio)acetaldehyde using
CoCl2 hydrate as a catalyst and heating at 110 °C overnight.
According to the nature of the thioether arm on the carbon
center linking the two pyrazole rings, the ligands can be divided
in two classes: the first class exhibits a phenyl group that links the
bis(pyrazolyl)methane scaffold to the thioether moiety (LRPhS),
whereas the second class presents a methylene group as a
linker (LRCH2S). Moreover, within each ligand class, the steric
hindrance on the pyrazole ring increased when hydrogen atoms
at positions 3 and 5 were substituted with methyl or isopropyl
groups. We have employed Ag(I) and Cu(I) cations to investi-
gate the different coordinative behaviors of the two ligand classes.
The complexes were prepared by treating equimolar amounts of
ligands and [Cu(MeCN)4]BF4 or Ag(I) salts (AgBF4, AgPF6,
and AgCF3SO3). Different counteranions were employed for the
preparation of the Ag(I) complexes to study their possible
influence on the resulting supramolecular assembly.60
ring for Ag(2) {d[Ag(2)ꢀC(116)] = 2.773(6) Å} and by the
peripheral phenyl ring for Ag(1) {d[(Ag(1)ꢀC(24)] =
ꢁ
2.921(4) Å} (Figure 1). The absence of steric hindrance on
the pyrazole rings allows for the approach of the Ag(2) atom
over one of the pyrazole rings. The central phenyl ring of the ligand
is oriented as previously found for bis(pyrazolyl) functionalized
ligands.39,40,61ꢀ67 This donor-set disposition favors the forma-
tion of metalꢀorganic chains, and it is also the preferred con-
formation adopted by the ligands that exhibit a phenyl ring as a
spacer between the bis(pyrazolyl) moiety and the thioether
group (vide infra).
The molecular structure of [Ag(LHPhS)]2(PF6)2 is dinuclear,
with the silver atom in a distorted T geometry (Figure 2). The
donor atoms are represented by the N2 chelate system of one
ligand and by the S(13) and N(21) atoms of a symmetry-related
ligand. Within the N2 chelate group, the AgꢀN(22) bond
ꢁ
distance (2.238(3) Å) is significantly shorter than that of Agꢀ
ꢁ
N(21) (2.428(3) Å). This fact is most likely a consequence of a
weak π-interaction that the silver atom exchanges with the sym-
0
ꢁ
metry related N(21) atom (2.691(3) Å). The evidence that both
nitrogen atoms interact with the metal is derived from the location
of the silver atom above the ideal trigonal plane formed by the
N(21), N(22), and S(13)0 donor atoms and directed toward the
Molecular Structures with the LHPhS Ligand. Details of the
data-collection parameters and other crystallographic informa-
tion are given in Table 1, whereas experimental bond lengths and
angles are provided in Table 2. [Ag(LHPhS)]n(BF4)n exhibits a
chain-like structure that is formed by the N2 coordination on a
metal center and by the bridging of the thioether group on a
second metal. Two different types of silver atoms define the
asymmetric unit. Both metal atoms are in a distorted T-shaped
geometry (this is more pronounced for Ag(1) than for Ag(2)),
which is distorted toward the tetrahedral geometry as a conse-
quence of an AgꢀC interaction that originates by one pyrazole
N(21)0 atom (symmetry code = 1ꢀx; 1ꢀy; ꢀz). As far as the
0
ligand conformation is concerned, the central phenyl ring exhibits a
conformation that is very close to that of [Ag(LHPhS)]n(BF4)n. The
C(11) carbon atom of one pyrazole ring exchanges CꢀH
π
3 3 3
interactions with the central phenyl ring of the symmetry-related
ꢁ
ligand d[C(11)ꢀC1t] = 3.622(4) Å, C1t: phenyl-ring centroid.
The dinuclear complex [Cu(LHPhS)(CH3CN)]2(BF4)2 is
obtained from the reaction between [Cu(CH3CN)4]BF4 and
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Inorganic Chemistry
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[Ag(LHPhS)]n(BF4)na, [Cu(LHPhS)(CH3CN)]2(BF4)2, and
[Ag(LHPhS)]2(PF6)2
[Ag(LHPhS)]n(BF4)n
Ag(1)ꢀN(21)
Ag(1)ꢀN(22)
Ag(1)ꢀS(16)
Ag(1)ꢀC(24)
2.241(4) Ag(2)ꢀN(24)0
2.384(4) Ag(2)ꢀN(25)0
2.492(1) Ag(2)ꢀS(13)
2.921(4) Ag(2)ꢀC(116)
2.398(3)
2.316(4)
2.529(1)
2.773(6)
N(25)0ꢀAg(2)ꢀN(24)0 83.0(1)
N(21)ꢀAg(1)ꢀN(22) 87.7(1)
N(21)ꢀAg(1)ꢀS(16) 171.1(1) N(25)0ꢀAg(2)ꢀS(13) 130.0(1)
N(22)ꢀAg(1)ꢀS(16) 86.7(1)
Figure 2. Molecular drawing of [Ag(LHPhS)]2(PF6)2. The hydrogen
atoms are omitted for clarity. C1t, phenyl ring centroid, symmetry code 0 =
1ꢀx; 1ꢀy; ꢀz.
N(24)0ꢀAg(2)ꢀS(13) 117.36(9)
[Ag(LHPhS)]2(PF6)2
AgꢀN(21)
AgꢀN(22)
2.428(3) AgꢀS(13)00
2.522(1)
2.691(3)
137.8(1)
143.23(8)
75.05(9)
2.238(3) AgꢀN(21)00
N(21)ꢀAgꢀN(22)
N(21)ꢀAgꢀS(13)00
N(21)ꢀAgꢀN(21)00
82.4(1)
N(22)ꢀAgꢀN(21)00
119.69(7) N(22)ꢀAgꢀS(13)00
84.1(1)
N(21)00ꢀAgꢀS(13)00
[Cu(LHPhS)(CH3CN)]2(BF4)2
1.944(3) N(14)ꢀCuꢀN(22)
2.025(3) N(21)ꢀCuꢀN(22)
2.028(3) N(14)ꢀCuꢀS(13)000
2.335(1) N(21)ꢀCuꢀS(13)000
122.3(1) N(22)ꢀCuꢀS(13)000
CuꢀN(14)
113.0(1)
93.9(1)
CuꢀN(21)
CuꢀN(22)
110.0(1)
105.4(1)
111.0(1)
CuꢀS(13)000
N(14)ꢀCuꢀN(21)
a Symmetry codes: 0 = 1/2+x; 1/2ꢀy; z, 00 = 1ꢀx; 1ꢀy; ꢀz, 000 = 1ꢀx; y;
1/2ꢀz.
Figure 3. Molecular drawing of [Cu(LHPhS)(CH3CN)]2(BF4)2,
ꢀ
above, and a portion of the crystal packing, below. The BF4 anions
are omitted for clarity. CꢀH π interactions are represented by
3 3 3
dashed bonds (below). Symmetry codes 0 = 1ꢀx; y; 1/2ꢀz, 00 = 1ꢀx;
yꢀ1; 1/2ꢀz.
the pyrazole rings also adopt a different conformation from that
of [Ag(LHPhS)]n(BF4)n and [Ag(LHPhS)]2(PF6)2 because their
nitrogen lone pairs are oriented on the same side of the
CꢀHcentral. Obviously, this ligand conformation is unsuitable
for the N2S chelation because the CꢀHcentral is interposed
between the N2 and S donor atoms. From a pictorial point of
view, the overall shape of the molecule resembles that of a cone
delimited by alternate pyrazoles (those with N(12) and N(22))
and the peripheral phenyl rings. Two pyrazole rings on the
ꢀ
Figure 1. Molecular drawing of [Ag(LHPhS)]n(BF4)n. The BF4
anions and the hydrogen atoms are omitted for clarity.
LHPhS. The metal shows a distorted tetrahedral geometry at-
tained by the N2 chelate system of one ligand, the thioether
group of the second ligand, and an acetonitrile molecule
(Figure 3). The phenyl ring acting as spacer is rotated approxi-
mately 60° with respect to the previous structures. The result of
ꢁ
opposite side of the cone face each other at a distance of 3.9 Å
(measured from the centroids of the penta-atomic rings) with the
this arrangement is the S atom pointing toward the CꢀHcentral
,
minimum distance between symmetry-related N(21) and N(11)
ꢁ
even though it is located at a noninteracting distance. Moreover,
atoms (3.782(4) Å). The dinuclear units form pillars in which the
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Inorganic Chemistry
ARTICLE
Table 3. Summary of X-ray Crystallographic Data for [Ag(LHCH2S)]n(BF4)n, [Ag(LiPrCH2S)]n(PF6)n (CH3)2CO, and
3
[Ag(LiPrCH2S)]n(CF3SO3)n 2CH2Cl2
3
[Ag(LHCH2S)]n(BF4)n
[Ag(LiPrCH2S)]n(PF6)n (CH3)2CO
[Ag(LiPrCH2S)]n(CF3SO3)n 2CH2Cl2
3
3
empirical formula
formula weight
color, habit
crystal size, mm
crystal system
space group
a, Å
C14H14AgBF4N4S
465.03
C55H82Ag2F12N8OP2S2
1441.09
C29H42AgCl4F3N4O3S2
865.46
colorless, block
0.33 ꢁ 0.24 ꢁ 0.14
monoclinic
P21/c
colorless, block
0.26 ꢁ 0.17 ꢁ 0.11
monoclinic
Cc
colorless, block
0.21 ꢁ 0.17 ꢁ 0.09
monoclinic
C2/c
8.901(1)
17.268(2)
11.510(1)
90
19.316(1)
17.068(1)
21.141(2)
90
23.477(2)
12.839(1)
26.057(2)
90
b, Å
c, Å
α, deg.
β, deg.
102.979(1)
90
102.624(2)
90
106.500(1)
90
γ, deg.
V, Å3
1723.9(3)
4
6801.4(8)
4
7531(1)
Z
8
T, K
293(2)
293(2)
200(2)
F (calc), Mg/m3
μ, mmꢀ1
θ range, deg.
no.of rflcn/unique
GOF
1.792
1.407
1.527
1.335
0.758
0.980
2.17 to 30.51
27964/5172
1.003
1.61 to 26.86
42584/14619
1.001
1.63 to 26.75
21169/7955
1.002
R1a
0.0380
0.0387
0.0493
wR2a
0.1094
2
0.0769
2
0.0612
2
a R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|, wR2 = [∑w(Fo ꢀ Fc2)2/∑w(Fo2)2]1/2, w = 1/[σ2(Fo ) + (aP)2 + bP], where P = [max(Fo ,0) + 2Fc2]/3.
stacked pyrazole rings point inside the cone of another molecular
[Cu(LMeCH2S)]n(BF4)n, the CuꢀN separation varies in the
ꢁ
unit. These pillars are held together by two types of CꢀH
π
relatively narrow range of 1.958(2)ꢀ2.053(2) Å, and the CuꢀS
3 3 3
ꢁ
interactions that occur through (1) the C(11) pyrazole carbon
bond distance is 2.17 Å for both compounds. On the other hand,
atom and the peripheral phenyl ring, d[C(11)ꢀC(103)00] =
the silver complexes [Ag(LHCH2S)]n(BF4)n, [Ag(LMeCH2S)]n-
(BF4)n, [Ag(LMeCH2S)]n(CF3SO3)n, and [Ag(LMeCH2S)]n(PF6)n
present a greater variability with respect to the AgꢀN bond
00
ꢁ
ꢁ
3.629(5) Å, and d[C(11)ꢀC(133) ] = 3.738(6) Å, and (2)
through the central and peripheral phenyl rings, d[C(123)00ꢀ
0
00
0
ꢁ
ꢁ
ꢁ
C(43) ] = 3.773(6) Å, and d[C(123) ꢀC(53) ] = 3.657(6) Å,
distances, which are in the 2.203(2)ꢀ2.404(2) Å range. As for
0
00
symmetry codes = 1ꢀx; y; 1/2ꢀz, = 1ꢀx; yꢀ1; 1/2ꢀz.
Adjacent pillars are oriented in an antiparallel fashion, and they
are aligned along the b crystallographic axis (Figure 3).
the copper complexes, the AgꢀS distance is found in a more
ꢁ
definite range of approximately 2.41 Å for all complexes (Table 4
and Supporting Information, Table S2).
The Ag(I) and Cu(I) complexes with the LHPhS ligand show a
certain structural diversity when they exhibit dinuclear or poly-
nuclear structures. Nevertheless, the invariant feature appears to
be the inability of LHPhS to chelate in the N2S fashion. This
ligand, in fact, behaves like an N2 chelate with pyrazole rings and
bridges to a different metal ion with the sulfur atom.
The propensity of the ligand to bridge between two metal
centers with the N2 and S donor systems leads to the formation of
zigzag chains that run parallel to the c crystallographic axis
(Figure 4). To a different degree of strength, the chains interact
with or are close to each other because of a partial stack of two
symmetry-related pyrazole rings in the range of 3.612(4)ꢀ3.921-
Molecular Structures with the LRCH2S Ligand (R = H, Me,
iPr). It is interesting to note that the complexes [Ag(LHCH2S)]n-
(BF4)n, [Ag(LMeCH2S)]n(BF4)n, [Ag(LMeCH2S)]n(CF3SO3)n,
ꢁ
(6) Å. This supramolecular interaction between the molecular
chains forms a motif that may be described as perpendiculꢀar
intersectꢀing rectangles, which are occupied by the anions (BF4
,
[Ag(LMeCH2S)]n(PF6)n, [Cu(LHCH2S)]n(BF4)n, and [Cu(LMe
-
CF3SO3 or PF6ꢀ) (see Figure 4). Moreover, the view of the
structure along the a crystallographic axis reveals how these
molecular chains are organized in layers that are parallel to the bc
crystallographic plane. We therefore infer that the counterion has
little influence on the supramolecular aggregation of these com-
plexes. The conformation of the central thioether arm influences
the mode of interaction of the chains; in fact, the molecular
structures of [Cu(LHCH2S)]n(BF4)n, [Ag(LHCH2S)]n(BF4)n, and
[Ag(LMeCH2S)]n(PF6)n can be described by the conformation A
CH2S)]n(BF4)n exhibit very similar crystal structures (Table 3)
despite the presence of different pyrazole ring substituents,
metals or counter-anions. Also the supramolecular interactions
responsible of the crystal packing are, from a qualitative point of
view, identical. The molecular structures of these compounds will
therefore be described together (Figures 4 and Supporting Informa-
tion, Figures S1ꢀS5). The metal adopts a distorted trigonal-planar
geometry, with the N2-chelating ligand attached to a metal center
and bridging with the thioether sulfur atom on a symmetry-related
metal ion. An inspection of the coordination bond distances
indicates that, in the copper complexes [Cu(LHCH2S)]n(BF4)n and
depicted in Scheme 2, whereas the structures of [Cu(LMe
-
CH2S)]n(BF4)n, [Ag(LMeCH2S)]n(BF4)n, and [Ag(LMeCH2S)]n-
(CF3SO3)n are represented by the conformation B. The torsion
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Table 4. Selected Bond Lengths (Å) and Angles (deg) for
[Ag(LHCH2S)]n(BF4)n, [Ag(LiPrCH2S)]n(PF6)n, and
[Ag(LiPrCH2S)]n(CF3SO3)n
a
[Ag(LHCH2S)]n(BF4)n
AgꢀN(21)
AgꢀN(22)
AgꢀS(13)0
2.203(2)
2.404(2)
2.4069(7)
N(21)ꢀAgꢀN(22)
N(21)ꢀAgꢀS(13)0
N(22)ꢀAgꢀS(13)0
86.43(7)
162.25(6)
104.43(5)
[Ag(LiPrCH2S)]n(PF6)n
Ag(1)ꢀN(21)
Ag(1)ꢀN(22)
Ag(1)ꢀS(16)00
Ag(2)ꢀN(24)
Ag(2)ꢀN(25)
Ag(2)ꢀS(13)
2.272(3)
2.289(4)
2.419(1)
2.268(3)
2.289(3)
2.429(1)
N(21)ꢀAg(1)ꢀN(22)
N(21)ꢀAg(1)ꢀS(16)00
N(22)ꢀAg(1)ꢀS(16)00
N(24)ꢀAg(2)ꢀN(25)
N(24)ꢀAg(2)ꢀS(13)
N(25)ꢀAg(2)ꢀS(13)
81.8(1)
136.19(8)
141.85(9)
84.6(1)
135.17(8)
140.24(9)
[Ag(LiPrCH2S)]n(CF3SO3)n
AgꢀN(21)
AgꢀN(22)
AgꢀS(13)000
AgꢀO(24)
2.288(3)
2.229(3)
2.433(1)
2.556(3)
N(22)ꢀAgꢀN(21)
N(22)ꢀAgꢀS(13)000
N(21)ꢀAgꢀS(13)000
N(22)ꢀAgꢀO(24)
N(21)ꢀAgꢀO(24)
O(24)ꢀAgꢀS(13)000
86.2(1)
130.25(9)
137.90(9)
98.1(1)
105.8(1)
90.71(7)
a Symmetry codes: 0 = x; 1/2ꢀy; 1/2+z, 00 = x+1/2; y+1/2; z, 000 = ꢀx+1/
2, y+1/2, ꢀz+1/2.
Scheme 2. Depiction of the Conformations A and B, Which
Describe the Intramolecular and Supramolecular Contacts of
the Molecular Chains of Cu(I) and Ag(I) Complexes with the
LRCH2S Ligands
Figure 4. Crystal packing of [Ag(LHCH2S)]n(BF4)n. View along the
a axis (above), and along the c axis (below). The BF4ꢀ anions and the
0
hydrogen atoms are omitted for clarity. Symmetry code = 1ꢀx; ꢀy;
1ꢀz.
angle (τ) involving the orientation of the thioether group with
respect to the stacked pyrazole ring can be adopted as a
stereochemical descriptor for discriminating between these two
conformers. The τ values of the A and B conformations differ by
approximately 120°. In all of these structures, the peripheral
phenyl ring is always positioned above one of the pyrazole rings,
thereby forming a partial π stack, with minimum distances in the
ꢁ
range 3.205(6)ꢀ3.531(3) Å.
The complexes [Ag(LiPrCH2S)]n(PF6)n and [Ag(LiPrCH2S)]n-
(CF3SO3)n form zigzag chains that are analogous to those of the
previously described complexes (Figures 5 and 6). Nevertheless,
the presence of the iPr groups as substituents on the pyrazole
rings in place of the H or Me groups hinders the approach of the
pyrazole rings of symmetry-related molecules. Hence, the chains
do not form the intersecting rectangles observed in the previously
described complexes. In [Ag(LiPrCH2S)]n(PF6)n, the chains are
organized in layers that are parallel to the ab crystallographic
plane by interacting through the iPr and phenyl moieties, and the
chains belonging to different planes are perpendicular to each other
(see Figure 5). In contrast to the molecular structures reported
thus far when using the LRCH2S ligands, [Ag(LiPrCH2S)]n-
(CF3SO3)n exhibits the silver atom in a distorted trigonal-
pyramidal geometry as a consequence of the coordination of
and LRCH2S, we have employed triphenylphosphine (PPh3) as
an ancillary ligand for Cu(I) and Ag(I). Mixtures of equimolar
amounts of LRPhS and LRCH2S ligands, PPh3, and AgBF4 or
[Cu(CH3CN)4]BF4 result in the formation of easily recovered
[M(L)PPh3]BF4 ternary complexes from the reaction mixture.
The molecular structures of the Cu(I) complexes (Table 5) are
reported in Figures 7 and 8 and in Supporting Information,
Figures S6ꢀS8. The presence of PPh3 hinders the formation of
coordination polymers so that all of the resulting complexes are
mononuclear. The most striking difference between the com-
plexes exhibiting the two different classes of ligands is the role
played by the thioether group. In fact, in complexes [Cu-
(LHPhS)PPh3]BF4 and [Cu(LMePhS)PPh3]BF4, the metal is
in a distorted trigonal-planar geometry and is bound by the N2
chelate ligand and PPh3. The thioether group adopts the same
ꢀ
ꢁ
the CF3SO3 anion, d[AgꢀO(24)] = 2.556(3) Å (Figure 6).
Molecular Structures with the LRPhS and LRCH2S Ligands
(R = H, Me) and Triphenylphosphine. To further investigate
the coordinative behavior of the two classes of ligands, LRPhS
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Figure 6. Molecular structure and crystal packing of [Ag(LiPrCH2S)]n-
(CF3SO3)n. The hydrogen atoms and the solvent of crystallization are
omitted for clarity. Symmetry code 0 = 1/2ꢀx; 1/2+y; 1/2ꢀz.
Figure 5. Molecular structure and crystal packing of [Ag(LiPrCH2S)]n-
ꢀ
(PF6)n. The PF6 anions, the hydrogen atoms, and the solvent of
crystallization are omitted for clarity. Green and brown spheres are the
spacefill representations of the silver atoms, which depict the differently
oriented molecular chains. Symmetry code 0 = 1/2+x; 1/2+y; z.
conformation already described for the coordination polymers
that were constructed with these ligands in the absence of
PPh3 (Scheme 3b). In contrast, [Cu(LHCH2S)PPh3]BF4,
[Cu(LMeCH2S)PPh3]BF4, and [Cu(LiPrCH2S)PPh3]BF4 show
the metal in a distorted tetrahedral geometry, and the ligands
employ the N2S donor set. In the present case, the thioether
moiety is in a different conformation than that exhibited in the
coordination polymers because the sulfur points to the same
metal center of the N2 donor system (Scheme 3d). In all the
complexes, the CuꢀN and CuꢀP bond distances are in the
Figure 7. Molecular drawing of [Cu(LHPhS)PPh3]BF4. Thermal ellip-
soids are drawn at the 30% probability level. The hydrogen atoms are
omitted for clarity.
ꢁ
ꢁ
1.999(2)ꢀ2.065(3) Å and 2.161(6)ꢀ2.1863(7) Å ranges, respec-
tively. In the [Cu(LHCH2S)PPh3]BF4, [Cu(LMeCH2S)PPh3]BF4,
and [Cu(LiPrCH2S)PPh3]BF4 complexes, the CuꢀS distance is
number of crystals with hexagonal prismatic shapes are always
present among the differently shaped prismatic crystals of the
[Ag(L)PPh3]BF4 ternary complexes. The molecular structure
derived from the X-ray diffraction analysis of the hexagonal
crystals reveals that the compound is [Ag(PPh3)4]BF4 (Support-
ing Information, Figure S12), which is isostructural with the
previously reported complex [Ag(PPh3)4]PF6.68 Given the difficul-
ties encountered during the recrystallization of the Ag(I) com-
plexes, only the molecular structures of [Ag(LHPhS)PPh3]BF4,
[Ag(LMeCH2S)PPh3]BF4, and [Ag(LiPrCH2S)PPh3]BF4 could
be obtained (Supporting Information, Figures S9ꢀS11), and they
ꢁ
in the narrow range of 2.4801(8)ꢀ2.4860(6) Å (Table 6 and
Supporting Information, Table S5). Thus, the increase in the
steric hindrance on the pyrazole rings when a hydrogen atom is
substituted for an iPr group appears to neither affect the overall
geometry of these complexes nor prevent the coordination of a
relatively bulky ligand such as PPh3.
The crystallization of the Cu(I) complexes reported here always
affords colorless crystals of homogeneous shape. In contrast,
during the crystallization of the Ag(I) complexes, a considerable
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Table 5. Summary of X-ray Crystallographic Data for
[Cu(LHPhS)PPh3]BF4 1/2(C3H6O), [Cu(LHCH2S)-
3
PPh3]BF4
[Cu(LHCH2S)PPh3]BF4
3
[Cu(LHPhS)PPh3]BF4
37H31BCuF4N4PS
1/2(C3H6O)
empirical formula
formula weight
color, habit
crystal size, mm
crystal system
space group
a, Å
C
C33.5H32BCuF4N4O0.5PS
712.01
745.04
colorless, block
0.10 ꢁ 0.08 ꢁ 0.05
monoclinic
P21/n
colorless, block
0.17 ꢁ 0.10 ꢁ 0.08
triclinic
Figure 8. Molecular drawing of [Cu(LHCH2S)PPh3]BF4 1/2
(C3H6O). Thermal ellipsoids are drawn at the 30% probability level.
The hydrogen atoms are omitted for clarity.
3
P1
13.953(4)
15.533(4)
16.860(5)
90
11.641(1)
11.854(1)
13.121(1)
79.685(1)
77.861(1)
70.546(1)
1657.3(2)
2
b, Å
c, Å
Scheme 3. Description of the Coordinative Behavior of the
Two Ligand Classes: Coordination Polymers (a, c) and
Mononuclear Complexes Formed in Presence of Ancillary
Ligands (b, d)
α, deg.
β, deg.
106.95(1)
90
γ, deg.
V, Å3
3495(2)
4
Z
T, K
190(2)
200(2)
F (calc), Mg/m3
μ, mmꢀ1
θ range, deg.
no.of rflcn/unique
GOF
1.416
1.427
0.785
0.825
1.67 to 26.00
36407/6857
0.775
1.60 to 23.26
14162/4761
1.006
R1a
0.0490
0.0242
wR2a
0.0509
0.0627
a R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|, wR2 = [∑w(Fo ꢀ Fc2)2/∑w(Fo2)2]1/2, w =
2
1/[σ2(Fo ) + (aP)2 + bP], where P = [max(Fo ,0) + 2Fc2]/3.
2
2
Table 6. Selected Bond Lengths (Å) and Angles (deg) for
[Cu(LHPhS)PPh3]BF4, [Cu(LHCH2S)PPh3]BF4 1/2
3
(C3H6O)
can be easily related to the corresponding Cu(I) complexes. In
fact, in [Ag(LHPhS)PPh3]BF4, the metal is in a trigonal-planar
geometry achieved by the N2 chelate ligand and PPh3, with a
thioether group that is not bound to the metal center. In contrast,
the molecular structures of [Ag(LMeCH2S)PPh3]BF4, and [Ag-
(LiPrCH2S)PPh3]BF4 present the metal in a distorted tetrahedral
geometry, and the LMeCH2S and LiPrCH2S ligands behave as
N2S tridentate. On average, the coordination bond distances
[Cu(LHPhS)PPh3]BF4
CuꢀN(21)
CuꢀN(22)
CuꢀP
2.000(3)
2.010(3)
2.175(1)
N(21)ꢀCuꢀN(22)
N(21)ꢀCuꢀP
92.0(1)
136.95(9)
131.08(9)
N(22)ꢀCuꢀP
[Cu(LHCH2S)PPh3]BF4 1/2(C3H6O)
3
CuꢀN(21)
CuꢀN(22)
CuꢀS(13)
CuꢀP
2.030(2)
2.037(2)
2.4860(6)
2.1656(5)
N(21)ꢀCuꢀN(22)
90.55(6)
130.80(5)
126.13(4)
90.63(5)
88.81(5)
118.15(2)
ꢁ
of these silver complexes are more than 0.2 Å longer than those of
the copper complexes, which is congruent with the larger size of
the silver cation.
N(21)ꢀCuꢀP
N(22)ꢀCuꢀP
N(21)ꢀCuꢀS(13)
N(22)ꢀCuꢀS(13)
PꢀCuꢀS(13)
The molecular structures of these ternary complexes provide
further evidence that the ligands with the methylene group as a
spacer are coordinatively flexible. In contrast, the ligands with the
phenyl ring as a spacer can be considered coordinatively rigid.
Accordingly, their predominant conformation in the complexes
presented thus far comprises the N2 and S donor systems that are
oriented in nearly opposite directions.
Conformational Studies. Conformational studies were per-
formed to investigate the energetics relative to the mobilities of
the pyrazole rings and thioether groups of the ligands. In a previous
work, we investigated the coordination properties of a ligand
system composed of a bis(pyrazolyl)methane system functionalized
with a -SPhSPh bis-thioether group. To save computational
resources, the peripheral -Ph-S-Ph moiety was replaced by a
methyl group during the calculations.40 In the present work, rigid
PES scans are performed using the two real systems LHPhS and
LHCH2S as benchmarks for the investigation of the flexibility of
the two ligand classes. The calculations are performed by varying
three torsion angles; τ1 rotates one of the pyrazole rings, τ2
rotates the spacer between the thioether and bis(pyrazolyl)-
methane moieties (Ph or CH2), and τ3 rotates the peripheral
phenyl ring. Because the calculations are performed without
optimization at every step (rigid PES scan), there are unrealistic
conformations in which the phenyl, pyrazole or thioether groups
collide with one another. These conformations are characterized
by very high energy and will not be discussed further.
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of this ligand class, and its energy is at least 30 kJ/mol greater
than that of 1a. This is a modest value in absolute terms, but an
inspection of the PES scans indicates that this conformation is
surrounded by regions of higher energy. Therefore, it is possible
that the conformational modification derived from the N2S
chelation would be associated with a destabilization that rules
out this mode of binding for the LHPhS ligand.
The LHCH2S ligand should be characterized by a greater
flexibility when compared to the LRPhS ligand class. This is
evident when examining the PES scan of LHCH2S, because the
regions of low energy are more extensive than those of the LHPhS
PES scan. The conformation of LHCH2S that exhibits the
minimum energy is 1b reported in Figure 9. This conformation
is characterized by (1) pyrazole rings oriented in an antiparallel
fashion (τ1 = 57.9°), (2) the methylene bridge and the central
methine proton in a staggered conformation with the thioether
that points toward the methine proton (τ2 = 321.1°), and (3) the
peripheral phenyl ring nearly staked over one of the pyrazole
rings (τ3 = 288.1°). On the basis of the analysis of the PES scan,
the rotation of the phenyl ring is the least energetically demand-
ing (∼20 kJ/mol), and the most stable conformation presumably
attains a slight stabilization by the partial π-stack of the pyrazole
and phenyl rings. Also, the rotation of the pyrazole ring requires a
limited expenditure of energy: ∼33 kJ/mol for a 360° rotation of
τ1 when τ2 = 321.1° and τ3 = 288.1°. The rotation of the
thioether group requires slightly more energy, but it is still
energetically a relatively inexpensive process: ∼52 kJ/mol for a
360° rotation of τ2 when τ1 = 57.9° and τ3 = 288.1°. Further-
more, by a close inspection of the various energy profiles of
LRCH2S provided in Figure 9 and the Supporting Information, it
is evident that there are a number of conformations that are very
close in energy; they can be grouped into two main reference
structures: one corresponds to the minimum of the PES scan, as
previously described (1b in Figure 9), whereas the other is still
characterized by a staggered conformation of the methylene and
methine protons but with the sulfur atoms pointing to the
opposite side of the central methine proton (2b in Figure 9).
This latter conformation, apart from the rotation of one pyrazole
ring, is the one adopted by the ligand in the ternary complexes in
the presence of PPh3.
Figure 9 also includes the experimental geometric parameters
τ1 and τ2 that were derived by the X-ray structural analyses of the
complexes obtained with the two ligand systems. Because the
PES was derived for the free ligands, the energies associated with
the coordination to the metal ion are expected to result in a
considerable modification of the two surfaces. Nevertheless, it is
helpful to visualize where the τ1-τ2 geometric parameters of the
complexes are positioned on the two free-ligand PESs. For both
systems, it is evident that the τ1 experimental values are localized
in two regions (∼60 and ∼300°), and these are associated with
the pyrazole rings oriented in a favorable position to chelate the
metal ions.69 As far as the τ2 value is concerned, for LRPhS it
varies in a narrow range at approximately 0°, which is associated
with a geometry very close to that of 1a. For LRCH2S, however,
there are three regions in which τ2 varies, but they can be reduced
to two regions because τ2 values of approximately 60 and 300°
arise by the centrosymetrically related molecules in the crystal
lattice (ligand N2 chelates and bridges with S). In particular, they
refer to enantiomerically related fragments in which the periph-
eral phenyl ring is oriented over one or the other pyrazole rings.
The other region where the experimental τ2 values are clustered
(∼180°) is related to the PPh3 complexes in which the ligands
Figure 9. Rigid PES scan of the LHPhS and LHCH2S ligands. For both
ligand classes, the conformations corresponding to the potential N2 and
N2S coordination behavior are indicated by the f and \ symbols,
respectively. The conformations adopted by the ligands in the poly-
nuclear [M(L)]n and mononuclear [M(L)PPh3]BF4 complexes are
indicated with purple and magenta triangles, respectively.
n+
The conformation of LHPhS that exhibits the minimum
energy is 1a reported in Figure 9. This conformation is char-
acterized by the pyrazole ring that orients the nitrogen lone pair
on approximately the same side of the molecule (τ1 = 295.6°)
and by the thioether group that points toward the methine
proton (τ2 = 49.2°). Because of the arrangement of the phenyl
spacer group, the peripheral phenyl ring does not interact with
any other molecular fragment (τ3 = 342.1°). This lack of
interaction has consequences for the energies involved in the
free rotation of this group because only 14 kJ/mol are required
when τ1 = 295.6° and τ2 = 49.2°. The 360° rotation of the
pyrazole ring (τ1) requires 24 kJ/mol when τ2 = 49.2° and τ3 =
342.1°. In comparison, the rotation of the thioether group
requires at least 90 kJ/mol (with τ1 = 295.6° and τ2 = 49.2°).
Furthermore, an inspection of all PES scans reported in the
Supporting Information reveals how the pyrazole rotation is an
energetically relatively inexpensive process, whereas the rotation
of the phenyl ring acting as a spacer usually implies high energy
barriers. In particular, the regions of the PES scans characterized
by minor energy are those that are associated with the conforma-
tion in which the thioether points toward the central methine
proton (τ2 range of = 25.2ꢀ73.2°). Structure 2a corresponds to
the ligand conformation that would allow the N2S coordination
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In particular, the sulfur coordination requires a destabilizing
approach between the CHcentral and the CHortho hydrogen
atoms.
’ CONCLUSIONS
Rigid ligand systems are those usually characterized by one
stable conformation or configuration that exhibits a definite
orientation of the donor atom’s lone pairs that are involved in
metal binding. If the stereoelectronic properties of the metal
match this preferred orientation, the result is usually the forma-
tion of a stable metal complex. Nevertheless, interesting situa-
tions arise when the lone-pair directionality and metal
requirements are mismatched. In particular, when an unusual
geometry is forced on a metal center, the result is an increase in
the energetic state of the system (entatic state).70,71 This concept
was originally developed for metal sites in biological systems,
such as metallo-proteins, but it has a general valence and has been
recently exploited for the construction of metalꢀorganic frame-
works to enhance gas-sorption properties72 or to force a metal
ion to adopt unusual spin states.17 In contrast, flexible ligand
systems are characterized by a number of conformations that are
similar in energy and may be adopted by the ligand in response to
the metal presence or external stimuli.16 Flexible systems have
also found applications in the realization of dynamic coordina-
tion polymers that are subject to structural modification when
interacting with guest molecules.73
In the present work, the coordination properties and con-
formational rigidity/flexibility of two ligand classes were inves-
tigated. These ligands are relevant for the construction of
coordination polymers and they are characterized by the N2S
mixed donor set derived from a bis(pyrazolyl)methane system
and a thioether function. However, the spacer between these two
donor systems exhibits a considerable influence on the coordina-
tion properties of the ligands. The phenyl group in LRPhS
renders these systems more rigid from a conformational point
of view, and in all of the reported molecular structures the phenyl
spacer is always approximately parallel to the Ccentral-Npz bond.
The geometric consequences are such that the bis(pyrazolyl)-
methane moiety is oriented in approximately the opposite direction
of the thioether group, and polynuclear complexes are usually
formed. In contrast, the LRCH2S ligand class is inherently more
flexible, because two thioether arrangements are energetically
accessible that are associated with the following binding modes:
the N2S chelation, and the and N2 chelation on a metal and the
bridging on a second metal with the sulfur atom. Nevertheless, as
attested by the molecular structures of the [M(LRCH2S)]+
binary complexes, the N2 chelation and S-bridging on a second
metal is the preferred conformation adopted by the LRCH2S
ligand class. In fact, molecular chains are invariably formed for these
type of complexes, and they present very similar crystal packing
when R = H or Me. When the steric hindrance of the pyrazole rings
is increased (R = iPr), the supramolecular interaction that is
observed between the chains of the less-encumbered ligands are
no longer displayed, even though the chains are still formed.
The experimental evidence pertaining to the different binding
modes of the two ligand classes are confirmed by computational
studies on two representative ligands: LHCH2S and LHPhS.
According to these calculations, the free rotation of the thioe-
ther group in LHCH2S and LHPhS requires ∼50 kJ/mol and
∼90 kJ/mol, respectively, confirming the more flexible nature of
LHCH2S when compared to LHPhS.
Figure 10. Energy differences (kJ/mol) between the coordination
isomers of the two model complexes, [Ag(LHPhS)PH3]+ and [Ag-
(LHCH2S)PH3]+ (B3LYP/6-31+G(d)-SDD). Selected coordination
ꢁ
bond distances (Å) are reported.
behave as a N2S donor. The preferred conformations of the free
ligand (1b and 2b) present the pyrazole rings oriented in an
antiparallel fashion, whereas the nitrogen lone pairs converge to
the metal center in the complexes. As previously noted, this
conformation determines the localization of the experimental
geometry on PES regions of relatively high energy.
Bearing this consideration in mind, it is nevertheless evident
that the LRPhS ligand class is conformationally more rigid and
preorganized than the LRCH2S class. In fact, the general and
therefore expected behavior of the LRPhS ligands is the N2
chelation on a metal and possibly the bridging interaction of the
thioether group on a second metal. The presence of the phenyl
group as a spacer precludes the N2S chelation mode for this ligand
class. In contrast, the LRCH2S class usually N2 chelates and bridges
with the thioether group on a second metal, but it can also act as a
N2S donor on the same metal ion. The energy expenditure to drive
the thioether and the bis(pyrazolyl)methane systems to chelate the
same metal ion is compensated by the metalꢀsulfur interaction
energy, which is evidently not sufficient for the LRPhS system.
This compensation is further evidenced by inspecting the energy
profiles of the conformational isomers of the two model complexes,
[Ag(LHPhS)PH3]+ and [Ag(LHCH2S)PH3]+ (Figure 10). For
[Ag(LRCH2S)PH3]+, the energy difference between the two
isomers is only 2.6 kJ/mol, which confirms that there is not a
significant energy gain derived by the coordination of the thioether
group when a metal ion is present. In contrast, for [Ag(LHPhS)-
PH3]+, the isomer with the N2S chelate ligand lies 17.7 kJ/mol
above that of the isomer with the N2 bidentate ligand. An
inspection of the optimized molecular structure of this complex
indicates that the sulfur coordination implies a certain ligand strain.
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ARTICLE
The different coordinative behavior between the two ligand
classes is more clearly evidenced in the ternary complexes
[M(L)PPh3]+ (M = Cu(I) or Ag(I); L = LRCH2S or LRPhS),
where the presence of PPh3 hinders the formation of polymeric
structures and favors the formation of mononuclear complexes.
In these types of complexes, the LRCH2S ligands act as a N2S
chelate that yields a distorted tetrahedral metal geometry,
whereas the LRPhS ligands employ only the bis(pyrazolyl)methane
system for the metal coordination and the thioether group is
oriented as in the binary complexes but does not contribute to
the metal binding.
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’ ASSOCIATED CONTENT
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S
Supporting Information. Synthesis of Ag(I) and Cu(I)
b
complexes. Crystallographic information files (CIF). Cartesian
coordinates of the optimized structures (B3LYP/SDD-6-31+G-
(d)) of the two conformational isomers of the model complexes
[Ag(LHCH2S)PH3]+ and [Ag(LHPhS)PH3]+. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: marchio@unipr.it.
’ ACKNOWLEDGMENT
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This work was supported by the Universitꢀa degli Studi di
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