Influences of changes in multitopic tris(pyrazolyl)methane ligand topology on silver(I) supramolecular structures

Article abstract of DOI:10.1021/ic034039r

The reactions between silver tetrafluoroborate and the ligands 1,2,4,5-C₆H₂[CH₂OCH₂C (pz)₃]₄ (L1, pz = pyrazolyl ring), o-C₆H₄[CH₂OCH₂C (pz)₃]₂ (L2), and m-C₆H₄[CH₂OCH₂C (pz)₃]₂ (L3) yield coordination polymers of the formula {C₆H_6-n[CH₂OCH₂C (pz)₃]_n(AgBF4)_m}∞ (n = 4, m = 2, 1; n = 2, ortho substitution, m = 1, 2; meta substitution, m = 2, 3). In the solid state, L2 molecules dimerize by a pair of C-H...π interactions, forming an arrangement that resembles the tetratopic ligand L1. In the solid-state structure of 1, each silver atom is κ²-bonded to two tris(pyrazolyl)methane units from different ligands with the overall structure a polymer made up from 32-atom macrocyclic rings formed by bonding tris(pyrazolyl)methane groups from nonadjacent positions on the central arene rings to the same two silver atoms. In 2, each silver is bonded to two tris(pyrazolyl)methane units in the same κ²-κ² fashion as with 1, forming a polymer chain. The chains are organized into dimeric units by strong face-to-face π-π stacking between the central arene rings making bitopic L2 act as half of tetratopic L1. The chains in both structures are organized by weak C-H...F hydrogen bonds and π-π stacking interactions into very similar 3D supramolecular architectures. The structure of 3 contains three types of silvers with the overall 3D supramolecular sinusoidal structure comprised of 32-atom macrocycles. Infrared studies confirm the importance of the noncovalent interactions. Calculations at the DFT (B3LYP/6-31G*) level of theory have been carried out on L2 and also support C-H...π interactions. Electrospray mass spectral data collected from acetone or acetonitrile show the presence of aggregated species such as [(L)Ag₂(BF₄)]⁺ and [(L)Ag₂]²⁺, despite the fact that ¹H NMR spectra of all compounds show that acetonitrile completely displaces the ligand whereas acetone does not.

Full text of DOI:10.1021/ic034039r

Inorg. Chem. 2003, 42, 3751−3764
Influences of Changes in Multitopic Tris(pyrazolyl)methane Ligand
Topology on Silver(I) Supramolecular Structures
,†
Daniel L. Reger,* Radu F. Semeniuc,† Ioan Silaghi-Dumitrescu,‡ and Mark D. Smith†
Department of Chemistry and Biochemistry, UniVersity of South Carolina,
Columbia, South Carolina 29208, and Facultatea de Chimie s¸i Inginerie Chimicaˇ,
UniVersitatea Babes¸-Bolyai, Cluj-Napoca, Romania
Received January 15, 2003
The reactions between silver tetrafluoroborate and the ligands 1,2,4,5-C H [CH OCH C(pz)3]4 (L1, pz ) pyrazolyl
6
2
2
2
ring), o-C H [CH OCH C(pz)3]2 (L2), and m-C H [CH OCH C(pz)3]2 (L3) yield coordination polymers of the formula
6
4
2
2
6
4
2
2
{C H6-n[CH OCH C(pz)3]n(AgBF4)m}∞ (n ) 4, m ) 2, 1; n ) 2, ortho substitution, m ) 1, 2; meta substitution,
6
2
2
m ) 2, 3). In the solid state, L2 molecules dimerize by a pair of C−H‚‚‚π interactions, forming an arrangement
that resembles the tetratopic ligand L1. In the solid-state structure of 1, each silver atom is κ2-bonded to two
tris(pyrazolyl)methane units from different ligands with the overall structure a polymer made up from 32-atom
macrocyclic rings formed by bonding tris(pyrazolyl)methane groups from nonadjacent positions on the central arene
rings to the same two silver atoms. In 2, each silver is bonded to two tris(pyrazolyl)methane units in the same
κ2−κ2 fashion as with 1, forming a polymer chain. The chains are organized into dimeric units by strong face-to-
face π−π stacking between the central arene rings making bitopic L2 act as half of tetratopic L1. The chains in
both structures are organized by weak C−H‚‚‚F hydrogen bonds and π−π stacking interactions into very similar
3D supramolecular architectures. The structure of 3 contains three types of silvers with the overall 3D supramolecular
sinusoidal structure comprised of 32-atom macrocycles. Infrared studies confirm the importance of the noncovalent
interactions. Calculations at the DFT (B3LYP/6-31G*) level of theory have been carried out on L2 and also support
C−H‚‚‚π interactions. Electrospray mass spectral data collected from acetone or acetonitrile show the presence of
aggregated species such as [(L)Ag2(BF )]+ and [(L)Ag2]2+, despite the fact that 1H NMR spectra of all compounds
4
show that acetonitrile completely displaces the ligand whereas acetone does not.
Introduction
area is the synthesis and characterization of coordination
polymers showing a number of infinite 1-D, 2-D, and 3-D
structures with specific geometries and properties.2 The key
features that determine the overall structures of coordination
polymers are the ligand topicity (i.e. the positions of
coordinating groups), flexibility or rigidity of the linker
groups joining the coordination sites, and the stereochemical
preferences of the coordinated metal ion.1b-f,3 The role of
noncovalent interactions is also recognized as providing
further organization into more complex networks. A wide
Investigations into the architecture of supramolecular
compounds formed by the self-assembly processes have been
ongoing in the last two decades.1 A growing field in this
* To whom correspondence should be addressed. Reger@
mail.chem.sc.edu.
† University of South Carolina.
‡ Universitatea Babes¸-Bolyai.
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10.1021/ic034039r CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/15/2003
Inorganic Chemistry, Vol. 42, No. 12, 2003 3751

Reger et al.
variety of these interactions, some relating to the anions3a-d,4
and the solvent,2g,3b,5 was found to have an impact on the
crystal packing of several compounds. Important examples
are strong6 and weak3e,7 hydrogen bonds, π-π stacking,8
X-H‚‚‚π interactions (X ) O, N, C),9 and interhalogen
interactions.10
Chart 1. Tris(pyrazolyl)methane
How these organizational features determine supra-
molecular structures still needs to be clearly elucidated.11,12
Our efforts in this area are based on the chemistry of metal
complexes of tris(pyrazolyl)methane ligands (Chart 1),13
potentially tripodal, neutral ligand sets isoelectronic to the
more heavily studied tris(pyrazolyl)borate ligands.14 We have
recently reported substantial improvements in the prepara-
tions of tris(pyrazolyl)methane ligands15 and developed
chemistry of them where the central methine carbon atom
can be fuctionalized with groups other than a hydrogen
atom.15e Using this chemistry, we have prepared multitopic
ligands of the general formula C6H6-n[CH2OCH2C(pz)3]n (n
) 2, 4, pz ) pyrazolyl ring).16 We have reported that the
reaction of the three isomers ortho-, meta-, and para-
C6H4[CH2OCH2C(pz)3]2 with [Cd2(thf)5](BF4)4 leads to the
formation of coordination polymers of the formula {C6H4[CH2-
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3752 Inorganic Chemistry, Vol. 42, No. 12, 2003

Multitopic Tris(pyrazolyl)methane Ligands
Chart 2. Possible Modes of Coordination of Tris(pyrazolyl)methane
Chart 3
Units to Silver(I) Metal Centers
OCH2C(pz)3]2Cd}(BF4)2}n, each with a different supra-
molecular structure even though they had the same octahedral
coordination environment about the cadmium.16a We have
also shown that metal complexes of these ligands form 2-D
and 3-D supramolecular structures organized by a multitude
of noncovalent interactions, such as weak hydrogen bonds16a-d
and intermolecular16b,f or intramolecular16d,f π-π stacking
and C-H‚‚‚π interactions.16b,f,g In addition, in some cases,
the metal complexes of these ligands undergo an interesting
double π-π/C-H‚‚‚π interaction.16f This versatility is a
consequence of the presence of the aromatic pyrazolyl rings
and linking arene rings that can act as acceptors in
C-H‚‚‚π interactions or participate in π-π stacking.
Another factor is the hydrogen atoms within the pyrazolyl
rings are acidic enough to be involved in weak hydrogen
bonds. In addition, the tris(pyrazolyl)methane units can act
as (a) κ3 tripodal, (b) κ2 bonded to a single metal with the
third pyrazolyl not coordinated, and (c) κ1-κ2 bonded
bridging two metals (Chart 2).
These bonding features were anticipated for ligands that
were designed to use the versatile bonding properties of tris-
(pyrazolyl)methane units coupled with a semirigid linker,
intermediate between rigid ligands1c,d and more flexible
ligands3f-h used by others. The supramolecular structures of
rigid ligand complexes are dominated by covalent inter-
actions, and noncovalent forces have little effect upon
network topology.1d,e This organization with rigid ligands
allows a better prediction of the overall structure, shape, and
porosity of the resulting array, and therefore, they have been
widely used. In contrast, the use of flexible ligands in the
construction of supramolecular arrays has been less system-
atic, due to the lack of predictability of the structural outcome
of the covalent and/or supramolecular network. The semirigid
ligands used in our studies can adjust their shape to maximize
covalent and noncovalent interactions, and the overall
structure of the supramolecular array will be the resultant
of the interplay among all forces. The rigid central arene
ring and the tris(pyrazolyl)methane units offer structure to
the system, while the flexible ether-based linkages offer
versatility to the system and improve the solubility of the
coordination polymers.
The tetratopic ligand L1 has four tris(pyrazolyl)methane units
linked to a central arene ring via four ether-based sidearms
in the 1,2,4,5-positions. As can be seen in Chart 3, both L2
and L3 can be considered as “half” of L1, each containing
two identical sidearms in the 1,2- and 1,3-positions, respec-
tively, connected to a central arene ring. This investigation
is (to the best of our knowledge) the first systematic study
on how designed changes in ligand toplogy with systems
consisting of semirigid ligands will influence the outcome
of the covalent and noncovalent networks. We have com-
municated the structure of 1 previously.16b
Experimental Section
General Procedure. All operations were carried out under a
nitrogen atmosphere using standard Schlenk techniques and a
Vacuum Atmospheres HE-493 drybox. All solvents were dried and
1
distilled prior to use. The H NMR spectra were recorded on a
Varian AM300 spectrometer using a broad-band probe. Proton
chemical shifts are reported in ppm vs internal Me4Si. IR spectra
were recorded on a Nicolet 5DXBO FTIR spectrometer. Electro-
spray ionization mass spectrometry data were obtained on a
MicroMass QTOF spectrometer. Clusters assigned to specific ions
show appropriate isotopic patterns as calculated for the atoms
present. Elemental analyses were performed by Robertson Microlit
Laboratories (Madison, NJ). 2,2,2-Tris(1-pyrazoyl)ethanol, HOCH2C-
(pz)3,15c,e and the ligands L1,16f L2, and L316a were prepared by
following literature methods.
{1,2,4,5-C6H2[CH2OCH2C(pz)3]4(AgBF4)2}∞ (1). 1,2,4,5-C6H2-
[CH2OCH2C(pz)3]4, L1 (0.22 g, 0.20 mmol), was dissolved in thf
(25 mL). This solution was added dropwise to a solution of silver
tetrafluoroborate, AgBF4 (0.078 g, 0.40 mmol), in dry thf (15 mL)
under an inert atmosphere. A white precipitate appeared as the
mixture was stirred for 2 h. The thf was removed by cannula
filtration; the white precipitate washed with thf (2 × 10 mL) and
then vacuum-dried to afford 0.226 g (73.8%) of solid identified as
In an attempt to elucidate the intimate processes that lead
to the formation of supramolecular species based on these
ligands and to analyze the competition between covalent and
noncovalent interactions, we report here the results of
reactions of AgBF4 with ligands L1, L2, and L3 (Chart 3).
1
{C6H2[CH2OCH2C(pz)3]4Ag2(BF4)2)}∞. H NMR (acetone-d6): δ
Inorganic Chemistry, Vol. 42, No. 12, 2003 3753

Reger et al.
Table 1. Crystallographic Data and Structure Refinement for {1,2,4,5-C6H2[CH2OCH2C(pz)3]4(AgBF4)2}∞ (1), {o-C6H4[CH2OCH2C(pz)3]2(AgBF4)}∞
(2), and {m-C6H4[CH2OCH2C(pz)3]2(AgBF4)2}∞ (3)
1
2
3
empirical formula
fw
C62H66Ag2B2F8N28O4
1656.79
C32H35AgBF4N12O2.50
822.40
C31H33Ag2B2F8N13O4
1041.06
temp, K
173(2)
173(2)
190(2)
crystal system
space group
a, Å
triclinic
monoclinic
C2/c
28.606(2)
monoclinic
P2/c
10.9226(7)
P1h
11.4424(19)
b, Å
11.972(2)
14.9439(11)
13.1641(8)
c, Å
13.962(2)
17.2648(13)
27.1563(18)
R, deg
106.838(3)
92.871(3)
100.803(3)
1787.1(5)
90
90
95.519(2)
90
3886.6(4)
â, deg
109.778(2)
90
6945.0(9)
γ, deg
V, Å3
Z
1
8
4
D(calcd), Mg m-3
abs coeff, mm-1
F(000)
1.539
0.637
842
1.573
0.654
3352
1.779
1.102
2072
cryst size, mm
reflcns collcd
indpdt reflcns
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
0.56 × 0.32 × 0.18
0.36 × 0.16 × 0.06
0.10 × 0.08 × 0.04
12 727
22 836
18 040
6285 [R(int) ) 0.0200]
6285/0/480
1.008
R1 ) 0.0374, wR2 ) 0.0977
R1 ) 0.0455, wR2 ) 0.1005
7104 [R(int) ) 0.0366]
7104/0/475
1.004
R1 ) 0.0402, wR2 ) 0.0901
R1 ) 0.0607, wR2 ) 0.0973
5603 [R(int) ) 0.0993]
5603/6/599
1.013
R1 ) 0.0527, wR2 ) 0.0868
R1 ) 0.1054, wR2 ) 0.1067
7.75, 7.66 (d, d, J ) 1.5 Hz, J ) 2.4 Hz, 12,12 H, 3,5-H pz), 7.40
(s, 2 H, C6H2), 6.53 (dd, J ) 1.6, 2.4 Hz, 12 H, 4-H pz), 4.99 (s,
8 H, OCH2C(pz)3), 4.43 (s, 8 H, OCH2Ph). ES+/MS (m/z):
[C54H54N24O4BF4Ag2]+ calcd 1403.2900; found 1403.2877. Anal.
Calcd for C54H54Ag2B2F8N24O4: C, 43.46; H, 3.65. Found: C,
43.21; H, 3.48.
{o-C6H4[CH2OCH2C(pz)3]2(AgBF4)}∞ (2). This compound was
prepared as above for 1 using o-C6H4[CH2OCH2C(pz)3]2, L2 (0.295
g, 0.50 mmol), and AgBF4 (0.0975 g, 0.50 mmol) to yield 0.282 g
(72%) of solid identified as {o-C6H4[CH2OCH2C(pz)3]2(AgBF4)}∞.
1H NMR (acetone-d6): δ 7.88, 7.55 (d, d, J ) 1.5 Hz, J ) 2.7 Hz,
6, 6 H, 3,5-H pz), 7.30-7.25 (m, 4 H, C6H4), 6.53 (dd, J ) 1.7,
2.7 Hz, 6 H, 4-H pz), 5.22 (s, 4 H, OCH2C(pz)3), 4.54 (s, 4 H,
OCH2Ph). ES+/MS (m/z): [C30H30N12O2Ag]+ calcd 697.1666;
found 697.1677. Anal. Calcd for C30H30AgBF4N12O2: C, 45.88;
H, 3.85. Found: C, 45.56; H, 4.12.
ization effects. Analysis of each data set showed negligible crystal
decay during data collection. Empirical absorption corrections were
applied to each data set (SADABS).18 The reported final unit cell
parameters are based on the least-squares refinement of all
reflections from each data set with I > 5σ(I). The structures were
solved by a combination of direct methods and difference Fourier
syntheses and refined by full-matrix least-squares against F2
(SHELXTL).18 Non-hydrogen atoms in each structure were refined
with anisotropic displacement parameters; hydrogen atoms were
placed in idealized positions and included as riding atoms. 1
crystallizes in the triclinic space group P1h. The centroid of the arene
ring of the ligand lies on an inversion center. The asymmetric unit
contains one Ag atom, half a ligand, one BF4- counterion, and two
acetonitrile molecules of crystallization. Systematic absences in the
intensity data for 2 indicate the monoclinic space groups Cc or
C2/c, the latter of which was confirmed. The asymmetric unit
-
consists of a silver atom, an ortho-C30H30N12O2 ligand, a BF4
{m-C6H4[CH2OCH2C(pz)3]2(AgBF4)2}∞ (3). This compound
was prepared as above for 1 using m-C6H4[CH2OCH2C(pz)3]2, L3
(0.295 g, 0.50 mmol), and AgBF4 (0.195 g, 1.0 mmol) to yield
0.341 g (70%) of solid identified as {m-C6H4[CH2OCH2C(pz)3]2-
counterion and half of a diethyl ether molecule located on a 2-fold
axis of rotation. Systematic absences in the intensity data for 3
were consistent with the monoclinic space groups Pc and P2/c,
the latter of which was confirmed. The asymmetric unit contains
three crystallographically independent Ag atoms: Ag1, which
resides on a 2-fold rotational axis; Ag2, on a general position; Ag3,
located on a center of symmetry. One complete meta-C30H30N12O2
ligand and a nitromethane molecule of crystallization are located
on general positions. Three independent BF4- counterions are also
present, all of which are rotationally disordered about 2-fold axes.
A total of six restraints were used in modeling the anion disorder.
Crystal data and refinement results for 1-3 are presented in Table
1, and ORTEP diagrams of the asymmetric unit for each (Figures
S2-S4) are in the Supporting Information.
1
(AgBF4)2}∞. H NMR (acetone-d6): δ 7.75, 7.64 (d, d, J ) 1.5
Hz, J ) 2.7 Hz, 6, 6 H, 3,5-H pz), 7.32-7.14 (m, 4 H, C6H4), 6.53
(dd, J ) 1.7, 2.7 Hz, 6 H, 4-H pz), 5.21 (s, 4 H, OCH2C(pz)3),
4.65 (s, 4 H, OCH2Ph). ES+/MS (m/z): [C30H30N12O2Ag]+ calcd
697.1666; found 697.1681. Anal. Calcd for C30H30Ag2B2F8N12O2:
C, 36.77; H, 3.09. Found: C, 37.01; H, 2.98.
X-ray Structural Studies. Crystals for compounds 1 and 2 were
grown from acetonitrile by vapor phase diffusion of diethyl ether,
and crystals for compound 3, from nitromethane using the same
method of crystallization. Suitable colorless single crystals of 1-3
were selected and mounted onto thin glass fibers. Low-temperature
X-ray intensity data were measured on a Bruker SMART APEX
CCD-based diffractometer (Mo KR radiation, λ ) 0.710 73 Å).
After preliminary crystal quality and unit cell parameter determi-
nation, a full sphere (1) or hemisphere (2, 3) of raw frame data
was collected. Raw data frame integration was performed with
SAINT+,17 which also applied corrections for Lorentz and polar-
Results
Synthesis of Complexes. The reactions between silver
tetrafluoroborate and ligands L1, L2, and L3 yield three new
coordination polymers as shown in the equations. All
(17) SAINT+ Version 6.02a; Bruker Analytical X-ray Systems, Inc.:
(18) Sheldrick, G. M. SHELXTL Version 5.1; Bruker Analytical X-ray
Madison, WI, 1998.
Systems, Inc.: Madison, WI, 1997.
3754 Inorganic Chemistry, Vol. 42, No. 12, 2003

Multitopic Tris(pyrazolyl)methane Ligands
Figure 1. Structure of L1 and its association in chains supported by
intramolecular and intermolecular C-H‚‚‚π interactions: carbon, yellow;
nitrogen, blue; oxygen, red; hydrogen, gray. C-H‚‚‚π interactions are shown
as red dotted lines.
Figure 2. Formation of dimers and chains in L2 by the means of
C-H‚‚‚π interactions pictured as red dotted lines.
compounds are white powders that precipitated out from the
reaction mixture and are air stable, showing only slight
decomposition under daylight after several weeks. They are
soluble in acetone, acetonitrile, and nitromethane but in-
soluble in halogenated solvents, water, or alcohols.
is centrosymmetric, orienting the 1,4 and 2,5 pairs of tris-
(pyrazolyl)methane units on opposite sides of the arene ring.
Intermolecular C-H‚‚‚π interactions exist between a CH
group located on a pyrazolyl ring and the π cloud from
another pyrazolyl ring on a second molecule, shown as red
dotted lines in Figure 1. The C(51)-H(51)‚‚‚π contact is
2.97 Å (with C-centroid distance of 3.60 Å) with the
corresponding angle of 126°. Two of these interactions
associate adjacent L1 ligands forming chains that run
approximately along the a axis in the [110] plane.
n{C6H2[CH2OCH2C(pz)3]4} + 2nAgBF4 f
{C6H2[CH2OCH2C(pz)3]4(AgBF4)2}n
1
n{o-C6H4[CH2OCH2C(pz)3]2} + nAgBF4 f
{o-C6H4[CH2OCH2C(pz)3]2(AgBF4)}n
2
The metrical parameters for the individual molecules of
L2 were described elsewhere,16a but for comparative purposes
with L1 additional information is shown in Figure 2. A
double C-H‚‚‚π interaction between two molecules, where
hydrogen atoms from a methylene group next to a central
arene ring orient above the centroid of the other central arene
ring, indicated by the red dotted lines, organizes L2 into
dimers. The C-H‚‚‚centroid distances are 2.70 Å (with
C-centroid distances of 3.49 Å), with the corresponding
angles of 138°. These dimers form chains in the [110] plane
and bisect the angle formed by a and b axes via a second
pair of C-H‚‚‚π interactions between adjacent dimer units,
the distances being of 2.96 and 3.88 Å respectively, with
the corresponding C-H‚‚‚centroid angles of 171° (red dotted
lines). These associations show that L2 can be considered
as half of L1 not only with respect to composition and
position of sidearms but also with respect to the structures
in the solid state. The first type of C-H‚‚‚π interaction in
L2 connects two ortho ligands into a 1,2,4,5 dimeric unit
that resembles the four arms ligand L1. The second
C-H‚‚‚π interaction for this dimer, (L2)2, associates it into
the same basic type of chains observed for L1.
n{m-C6H4[CH2OCH2C(pz)3]2} + 2nAgBF4 f
{m-C6H4{CH2OCH2C(pz)3]2(AgBF4)2}n
3
1
The H NMR spectra of the solids 1-3 in CD3CN show
that the acetonitrile completely replaces the ligand; the
spectrum of the compound in CD3CN is the same as the free
1
ligand in this solvent. However, H NMR spectra of the
compounds in deuterated acetone are clearly different from
the free ligand, showing the coordination of the ligand to
the silver(I) in this solution. Although the X-ray structure
shows that in the solid state the pyrazolyl rings are non-
equivalent (vide infra), the NMR spectra of the coordination
polymers of silver show equivalent rings, presumably
because of fast exchange of the ligands and metals on the
NMR time scale.
X-ray Structures of Ligands. The structure of the
tetratopic ligand 1,2,4,5-C6H2[CH2OCH2C(pz)3]4 (L1; see
Figure S1 and Tables S1 and S2 in the Supporting Informa-
tion for details), Figure 1, consists of discrete molecules,
and within each tris(pyrazolyl)methane unit the orientation
of the three pyrazolyl rings is a propeller arrangement. The
adjacent arms of the ligand are oriented above and below
the arene ring plane, with one arm twisted away from the
other. This orientation is sustained by an intramolecular
C-H‚‚‚π interaction between a methylene hydrogen atom
next to the arene ring on the arm that is twisted away and a
pyrazolyl ring on the adjacent arm. The hydrogen atom is
oriented over the pyrazolyl ring at a distance of 2.94 Å and
the corresponding carbon atom at a distance of 3.67 Å, the
value of the corresponding C-H‚‚‚π angle being 132°. These
values are typical for C-H‚‚‚π interactions.9 The molecule
X-ray Structures of Silver(I) complexes. Selected bond
lengths and angles are listed in Table 2. The asymmetric
-
unit of 1 contains one Ag atom, half a ligand, one BF4
counterion, and two acetonitrile molecules of crystalliza-
tion. As can be seen in Figure 3, each silver atom is κ2-
bonded to two tris(pyrazolyl)methane units from different
ligands, with each unit having one pyrazolyl ring that is not
coordinated to a silver. The silver is in a distorted tetrahedral
geometry, with the restraints imposed by the “bite” angle
of the tris(pyrazolyl)methane unit lowering the angles
Inorganic Chemistry, Vol. 42, No. 12, 2003 3755

Reger et al.
Table 2. Selected Bonds (Å) and Angles (deg) for 1-3
Compound 1
Ag-N(11)
Ag-N(21)
Ag-N(51)
Ag-N(61)
2.403(2)
2.274(2)
2.322(2)
2.315(2)
N(11)-Ag-N(21)
N(11)-Ag-N(51)
N(11)-Ag-N(61)
N(21)-Ag-N(51)
N(21)-Ag-N(61)
N(51)-Ag-N(61)
80.60(8)
99.03(7)
138.74(9)
144.78(10)
120.33(9)
83.03(8)
Compound 2
Ag-N(11)
Ag-N(21)
Ag-N(41)
Ag-N(51)
2.283(2)
2.341(2)
2.353(2)
2.271(2)
N(11)-Ag-N(21)
N(11)-Ag-N(51)
N(11)-Ag-N(41)
N(21)-Ag-N(41)
N(21)-Ag-N(51)
N(41)-Ag-N(51)
83.12(8)
142.48(8)
124.05(8)
99.36(8)
123.53(9)
80.76(8)
Compound 3
Ag(1)-N(11)′
Ag(1)-N(11)
Ag(1)-N(21)
Ag(1)-N(21)′
Ag(2)-N(31)′
Ag(2)-N(41)
Ag(2)-N(51)
Ag(3)-N(61)
Ag(3)-N(61)′
2.361(7)
2.361(7)
2.372(7)
2.372(7)
2.262(7)
2.323(7)
2.329(8)
2.133(7)
2.133(7)
N(11)′-Ag(1)-N(11)
N(11)′-Ag(1)-N(21)
N(11)-Ag(1)-N(21)
N(11)′-Ag(1)-N(21)′
N(11)-Ag(1)-N(21)′
N(21)-Ag(1)-N(21)′
N(31)′-Ag(2)-N(41)
N(31)′-Ag(2)-N(51)
N(41)-Ag(2)-N(51)
N(61)-Ag(3)-N(61)′
103.2(3)
141.2(2)
76.6(2)
176.6(2)
141.2(2)
127.7(4)
155.1(2)
112.1(2)
81.2(2)
Figure 5. All noncovalent interactions in 1, including the intrastrand
C-H‚‚‚π interaction of the noncoordinated pyrazolyl ring.
atoms next to the arene ring, with a H-centroid distance of
2.97 Å and with the corresponding C-centroid distance of
3.65 Å. These values, again, are typical for C-H‚‚‚π
interactions.9
180.0(2)
N(11)-Ag-N(21) and N(51)-Ag-N(61) to 80.60(8) and
83.02(8)°, respectively (Table 2).
There are two types of forces that organize the polymer
strands into a supramolecular structure. As shown in Figure
5 as blue dotted lines, each BF4- makes two C-H‚‚‚F weak
hydrogen bonds. The C-H‚‚‚F interactions, made between
adjacent strands, are 2.40 and 2.48 Å, respectively, with the
corresponding angles of 156 and 151°. While not strong
interactions (the sum of van der Waals radii of fluorine and
hydrogen atoms being 2.54 Å),19 the bonds are close to linear.
As stated for other types of weak interactions (e.g. weak
C-H‚‚‚O hydrogen bonds), bonds that are close to linear
indicate a substantial interaction between the F and H atoms,
even in cases where the distances are close to the sum of
van der Waals radii.20 As also shown in Figure 5, there are
two π-π stacking interactions between strands, shown with
black double dotted lines. The first interaction involves
noncoordinated pyrazolyl rings that are oriented away from
the polymeric strands and is between the same strands as
those involved in the weak C-H‚‚‚F hydrogen bond. The
second involves two pyrazolyl rings coordinated to silver
atoms and organizes strands oriented in the opposite diagonal
direction to those joined by the first bonding interactions.
In both cases the two pairs of pyrazolyl rings are displaced
with centroid-centroid distances of 4.06 and 4.11 Å,
respectively, and the rings are parallel with a perpendicular
distance between the rings of 3.55 Å in the first case and
3.45 Å in the second case. This slippage of aromatic rings
involved in π-π interactions is observed frequently, and 3.60
and 3.48 Å are short distances for this type of interaction.8,21
In this configuration, an important contribution to the
The overall structure is a polymer made up of rings formed
by two ligands bonding tris(pyrazolyl)methane groups from
nonadjacent positions on the arene rings to the same two
silver atoms, Figure 4. The arene groups link these 32-atom
macrocycles into polymer chains. The holes formed by the
rings are partially occupied by two of the pyrazolyl rings
that are not coordinated to the silver. The orientation of these
rings is supported by C-H‚‚‚π interactions between the
pyrazolyl rings occupying the holes and methylene hydrogen
Figure 3. Coordination environment around the silver(I) atom in 1.
(19) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Rowland, R. S., Taylor,
R. J. Phys. Chem. 1996, 100, 738.
(20) (a) Steiner, T.; Desiraju, G. R. Chem. Commun. 1998, 891. (b) Steiner,
T.; Lutz, B.; van der Maas, J.; Schreurs, A. M. M.; Kroon, J.; Tamm
M. Chem. Commun. 1998, 171.
(21) (a) Wu, H.-P.; Janiak, C.; Rheinwald, G.; Lang, H. J. Chem. Soc.,
Dalton Trans. 1999, 183.
Figure 4. 1-D covalent network of 1: carbon, yellow; nitrogen, blue;
oxygen, red; hydrogen, gray; silver, purple. C-H‚‚‚π interactions are shown
as red dotted lines.
3756 Inorganic Chemistry, Vol. 42, No. 12, 2003

Multitopic Tris(pyrazolyl)methane Ligands
Figure 6. (a) One polymeric chain of 2. Every second nonbonded pyrazolyl
ring makes a C-H‚‚‚π interaction with a CH2 group. (b) Two strands of 2,
organized in polymeric dimers through a π-π stacking interaction between
the central arene rings.
attractive forces arises from a pronounced π-σ attraction
interaction of a C-H σ bond with the adjacent aromatic π
cloud.8
The distance between two silver atoms in the 32-atom
metallomacrocycles is 9.31 Å and between two equivalent
silver atoms from adjacent macrocycles in the polymer chain
is 14.93 Å. The corresponding Ag‚‚‚Ag‚‚‚Ag angles are 87
and 93°, respectively. Thus, as best seen in Figure 4, the
silver atoms within the chain form an almost perfect series
of rectangles. The closest distance between two silvers atoms
from two chains involved in π-π stacking is 6.93 Å.
Compound 1 also has two acetonitrile molecules of
solvation (not bonded to silver) located in different positions,
one being in the “π-π stacking region” and another lying
above the central arene ring, Figure 5. There is a C-H‚‚‚N
hydrogen bond (blue dotted line in Figure 5) between the
acetonitrile molecules, with a H‚‚‚N distance of 2.54 Å (C-N
distance of 3.44 Å) and a corresponding C-H‚‚‚N angle of
156°. Also organizing the solvent molecules in the crystal
is an interaction of the triple bond of one of the acetonitrile
molecules with a hydrogen atom located on a methylene
group next to the central arene ring (red dotted line), with a
C(3)-H(3b)‚‚‚π distance of 2.90 Å (C‚‚‚π distance of 3.62
Å) with the corresponding angle of 132°.
Compound 2 has an environment around the silver atom
similar to that of 1. As can be seen in Figure 6a, two
pyrazolyl rings from two different ligands chelate the silver
atoms in a distorted tetrahedral arrangement that is, again,
strongly influenced by the “bite” angle of the tris(pyrazolyl)-
methane unit. The N(11)-Ag-N(21) and N(41)-Ag-N(51)
angles are 83.12(8) and 80.76(8)°, respectively, very close
to the corresponding angles in 1 (Table 2). Each bitopic
ligand bonds two silver atom in this κ2-κ2 fashion forming
a polymer chain that runs along the b axis of the unit cell.
There are also noncoordinated pyrazolyl rings, one from each
unit, oriented away from the strand. The central arene rings
are situated on the same side of the polymeric chain, in
contrast with the cadmium analogue16a where the central
arene ring was found at alternate sides along the strand. Half
of the noncoordinated pyrazolyl rings are oriented toward a
Figure 7. All noncovalent interactions in 2: (a) π-π stacking and
C-H‚‚‚π interactions between two polymeric strands; (b) weak C-H‚‚‚F
interactions between two strands.
methylene group adjacent to the central arene ring. This
orientation is supported by a C-H‚‚‚π interaction (pictured
as red dotted line in Figure 6a) between this pyrazolyl ring
and one hydrogen atom from the methylene group, with a
short H(78b)-centroid distance of 2.60 Å and with the
corresponding C(78)-centroid distance of 3.29 Å.9
The strands are organized into dimers, shown in Figure
6b, through face-to-face π-π stacking between the central
arene rings, with a perpendicular distance between planes
of 3.57 Å. The presence of the two adjacent electron-
withdrawing sidearms induces opposite polarization in each
pair of central arene rings, making the stacking more
effective. Also, it leads to a parallel displacement of the rings;
therefore, the centroid-centroid distance is 3.99 Å. The
distance between the silver atoms in the strand is 14.94 and
8.73 Å within a dimer. The corresponding Ag‚‚‚Ag‚‚‚Ag
angles within the dimer are both 90°. The π-π stacking
interactions between the central arene rings cause 2 to form
dimeric strands that appear very similar to those of 1, Figure
4.
These dimeric chains are organized further into a 3-D
architecture by three additional types of noncovalent inter-
actions. Figure 7 shows the additional noncovalent inter-
actions that influence the extended structure of 2. First, the
dimeric strands are arranged in sheets via a face-to-face π-π
stacking between two pyrazolyl rings coordinated to silver,
Inorganic Chemistry, Vol. 42, No. 12, 2003 3757

Reger et al.
Figure 8. Comparison between the extended structures of 1 and 2: (a) crystal packing of four dimeric strands of 2; (b) crystal packing of four strands of
1. a indicates the π-π stacking region, organizing the dimers/strands into sheets; the C-H‚‚‚F hydrogen bonds are oriented in the opposite diagonal direction
to those involved in π-π stacking.
black double dotted lines in Figure 7a. The shortest distance,
C(51)-C(51′), is 3.42 Å and the longest, N(51)-C(53′) is
3.72 Å, with a centroid-centroid distance of 3.55 Å, showing
a strong interaction between dimers. This arrangement is also
supported by several C-H‚‚‚π interactions, shown in Figure
7a as red dotted lines. First, the pyrazolyl rings involved in
π-π stacking have their C(52)-H(52) bond directed toward
another pyrazolyl ring bonded to silver, the H‚‚‚centroid
distances being 2.75 Å (with a C-centroid distance of 3.61
Figure 9. Coordination environment of Ag(1) and Ag(2). Only essential
atoms are shown.
Å) with a corresponding angle of 151°. Second, the
uncoordinated pyrazolyl ring not involved in the intrachain
C-H‚‚‚π interaction mentioned above is pointed toward a
of the pyrazolyl rings are coordinated to one of the silvers.
coordinated pyrazolyl ring from an adjacent strand, the
Figure 9 shows the environment of Ag(1) and Ag(2). As in
H(33)‚‚‚centroid distances being 2.86 Å (with a C-centroid
the structures of 1 and 2, the environment around Ag(1) is
distance of 3.74 Å) with a corresponding angle of 154°.
a distorted tetrahedron, with two pyrazolyl rings from two
These sheets are arranged in a 3-D architecture by
tris(pyrazolyl)methane units located in different ligands
C-H‚‚‚F weak hydrogen bonds. As shown in Figure 7b (blue
bonded in a κ2-κ2 fashion. The bond lengths and angles (see
Table 2) are close to the values observed for 1 and 2, as
-
dotted lines), each BF4 ion makes two short interactions
between a methylene group and a pyrazolyl ring situated on
expected. The third pyrazolyl ring from each of the tris-
adjacent strands, with distances of 2.40 and 2.37 Å,
(pyrazolyl)methane units bonded to Ag(1) is κ1-bonded to a
respectively, with the corresponding angles of 138 and 165°.
second silver atom, Ag(2). The coordination sphere of each
Figure 8a depicts the packing of four dimeric strands of
Ag(2) is completed by κ2-bonding by two pyrazolyl rings
2, together with the counterions, viewed down the polymer
from a tris(pyrazolyl)methane unit located in a different
chain of the strands; Figure 8b provides an analogous view
for compound 1. In these orientations, the π-π stacking
interactions and C-H‚‚‚π interactions between dimer/
ligand. The environment about Ag(2) is a flattened trigonal
pyramid (sum of the N(41)-Ag(2)-N(51) bond angles )
348.42°), with a significant distortion caused by the restricted
polymeric units are arranged vertically (indicated by a in
angle (81.2(2)°) of the κ2-bonded ligand.
the Figure). The C-H‚‚‚F interactions are between the
The coordination around the third silver atom (Ag(3)) is
pictured in the center of Figure 10. The remaining pyrazolyl
rings from two tris(pyrazolyl)methane units κ2-bonded to Ag-
(2), located in different ligands, are coordinated to Ag(3),
with the N(61)-Ag(3) distance being 2.133(7) Å and the
corresponding angle 180°. Furthermore, two pyrazolyl rings
coordinated to Ag(2) are oriented toward Ag(3), each
centroid-Ag(3) distance being 3.15 Å with the correspond-
ing angle of 180°. Several studies on arene-silver inter-
actions, including a statistical analysis based on the existing
strands oriented in the opposite diagonal direction than those
involved in π-π stacking. Although the orientation of the
elongated axis of the dimer/polymeric units is different, the
supramolecular organization is surprisingly similar.
In contrast to compounds 1 and 2, where the ratio of tris-
(pyrazolyl)methane units to silver is 2:1, the ratio expected
for a simple coordination polymer, this ratio is 1:1 in
m-C6H4[CH2OCH2C(pz)3]2(AgBF4)2, 3. Compound 3 con-
tains three different types of silvers, tetrahedral Ag(1),
trigonal Ag(2), and linear Ag(3), in a ratio of 1:2:1, and all
3758 Inorganic Chemistry, Vol. 42, No. 12, 2003

Multitopic Tris(pyrazolyl)methane Ligands
rings (centroid-centroid ) 4.06 Å). The π-π stacking is
in the [010] direction, the distances between equivalent silver
atoms located in adjacent strands being equal with the value
of the b axis, 13.1641(8) Å.
The 2-D sheets formed by the π-π stacking are parallel,
with the counterions located between the sheets and located
near the maxima points of the sinus, with the π-π stacking
organizing the vertical pairs, Figure 13. The BF4- counterions
presumably organize these sheets into a 3-D array, but since
the anions were found to be disordered, detailed information
on individual interactions is not available.
Figure 10. Coordination around the Ag(3) atom, with two ligands closing
a macrocycle. Hydrogen atoms not shown.
structures in the Cambridge Structural Database,22 reported
a range of Ag-centroid distances of 2.89-3.37 Å.
Infrared Studies. Solid-state structures of the ligands and
all three compounds reported here revealed the presence of
three types of noncovalent interactions between the ligand
molecules or the polymeric strands of 1-3, manifested by
close contacts between several atoms. These types of
interactions can also be studied by IR spectroscopy. Classic
hydrogen bonding (X-H‚‚‚Y)6,7 involves two electron-
withdrawing atoms, one having a hydrogen atom attached
and the second carrying lone electron pairs, and there must
be a significant charge transfer from the proton acceptor (Y)
to the proton donor (X-H). The formation of a hydrogen
bond is followed by a lengthening of the donor X-H bond
with a concomitant red shift in the ν(X-H) frequency. Many
structural data and theoretical studies9 have extended the
concept of hydrogen bonding to the X-H‚‚‚π (X ) C, N,
O) type interaction. In contrast to the classical hydrogen
bond, frequently the formation of such a hydrogen bond is
followed by a shortening of the donor X-H bond with a
concomitant blue shift in the X-H frequency.23 This
interaction, first called “anti-H bond” and then, more
accurately, improper H-bond,23c has the same features as the
classic H-bond; there is a charge transfer from the proton
acceptor (i.e. the π cloud originated from an aromatic ring)
followed by a stabilization of the “complex”, due to the
dispersion and electrostatic quadrupole-quadrupole inter-
actions.23
Since the solid-state structures indicate that all the
compounds reported here present both classic and improper
hydrogen bonds, IR spectra of the compounds were used to
confirm the presence of both of these noncovalent inter-
actions. The bands in the vibrational spectra of the com-
pounds have been partially assigned. The assignments are
listed in Table 3 and were made by comparisons to literature
data24 and the spectrum of the parent HC(pz)3 ligand. There
are two regions of interest, 4000-2800 and 1750-400 cm-1.
The IR spectra were recorded in solution (CDCl3) only for
the free ligands, due to the insolubility of the metal
Overall from left to right in Figure 10, the polymeric
chains of 3 are formed in the following manner: one tris-
(pyrazolyl)methane unit from a bitopic ligand is κ2-bonded
to Ag(1) and κ1-bonded to Ag(2). The other tris(pyrazolyl)-
methane unit in this ligand is κ2-bonded to Ag(2′) and κ1-
bonded to Ag(3). A tris(pyrazolyl)methane unit in a second
ligand bonds κ2 to the Ag(2) that is κ1-bonded by the first
ligand and κ1 to Ag(3), that is also κ1-bonded by the first
ligand. The second tris(pyrazolyl)methane unit in this second
ligand bonds κ1 to Ag(2′), which is κ2-bonded by the first
ligand and bonds κ2 to Ag(1′) to continue the polymer chain.
This arrangement generates the following sequence of silver
atoms along the polymer strand: Ag(1)-Ag(2)-Ag(3)-Ag-
(2)-Ag(1). There are twice as many Ag(2) as Ag(1) or Ag-
(3). The structure of 3 can also be regarded as an infinite
chain of 32-atom metallomacrocycles, containing two Ag-
(2) and two ligands. The Ag(3) “short circuits” the large
metallomacrocycles into two 22-atom smaller metallomac-
rocycles, and the large metallomacrocycles are bridged into
polymer chains by Ag(1).
As pictured in Figure 11, the overall shape of the
coordination polymer is sinusoidal with Ag(1) at the maxima;
the “wavelength”smeasured between every other Ag(1)s
is 33.17 Å. The same distance is found between every other
Ag(3) and between every fifth Ag(2). The distance between
any consecutive Ag(1)‚‚‚Ag(2) is 4.90 Å and between any
consecutive Ag(2)‚‚‚Ag(3) is 5.20 Å. Supporting this struc-
ture are C-H‚‚‚π interactions between a hydrogen from a
methylene group next to the central arene ring and the π
cloud from a pyrazolyl ring that is κ1-bonded to Ag(2). The
H‚‚‚centroid and C-centroid distances are 2.87 and 3.56 Å
respectively, with the corresponding angle of 128°.
An important structural feature is that the tris(pyrazolyl)-
methane units of each ligand are oriented on the same side
of the central arene ring, placing these rings, in an alternate
fashion, on the outside of the strands. Face-to-face, π-π
stacking of these central arene rings, with several short C-C
distances at 3.58 Å, organize the polymeric strands into a
2-D array, Figure 12. The presence of the two electron-
withdrawing sidearms in the 1- and 3-positions induces a
polarization in the arene ring, making the stacking more
effective, and also leads to a parallel displacement of the
(23) (a) Hobza, P. Phys. Chem. Chem. Phys. 2001, 3, 2555. (b) Reimann,
B.; Buchhold, K.; Vaupel, S.; Brutschy, B.; Havlas, Z.; Spirko, V.;
Hobza, P. J. Phys. Chem. A 2001, 105, 5560. (c) Hobza, P.; Havlas,
Z. Chem. ReV. 2000, 100, 4253. (d) Muller-Dethlefs, K.; Hobza, P.
Chem. ReV. 2000, 100, 143. (e) Cubero, E.; Orozco, M.; Hobza, P.;
Luque, F. J. J. Phys. Chem. A 1999, 103, 6394. (f) Hobza, P.; Spirko,
V.; Havlas, Z.; Buchhold, K.; Reimann, B.; Barth, H. D.; Brutschy,
B. Chem. Phys. Lett. 1999, 299, 180. (g) Hobza, P.; Spirko, V.; Selzle,
H. L.; Schlag, E. W. J. Phys. Chem. A 1998, 102, 2501.
(22) (a) Mascal, M.; Kerdelhue, J. L.; Blake, A. J.; Cooke, P. A. Angew.
Chem., Int. Ed. 1999, 39, 1968. (b) Mascal, M.; Kerdelhue, J. L.; Blake,
A. J.; Cooke, P. A.; Mortimer, R. J.; Teat, S. J. Eur. J. Inorg. Chem.
2000, 485.
(24) (a) Kuznetsov, M. L.; Dementiev, A. I.; Krasnoshchoikov, S. V. J.
Mol. Struct. (THEOCHEM) 1998, 453, 17. (b) Diaz, G. F.; Campos,
M. V.; Klahn, A. H. O. Vib. Spectrosc. 1995, 9, 257.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3759

Reger et al.
Figure 11. Structure of 3. In this orientation Ag(1) is positioned at the maxima of the sinusoidal chain.
Figure 12. π-π stacking between two central arene rings in 3: left, view of π-π sacking region down to the polymer strands; right, view nearly perpendicular
to the macrocyclic rings formed by two ligands.
-
Figure 13. Crystal packing of 3, showing two sets of two stacked strands together with the disordered BF4 counterions: left, view along sheets; right,
view perpendicular to sheets.
complexes in solvents other than acetone, acetonitrile, and
nitrometanessolvents presenting strong vibrations in the
regions of interest. The IR spectra in the solid state were
recorded using crystals from the same batch used in crystal
structure determinations for both ligands and complexes (L3
has not been crystallized). Also, for comparison purposes,
we recorded the spectra of noncrystalline samples of the three
complexes, using the white powders obtained from the
reaction mixtures.
As can be seen in Table 3, in solution, all ligands show
three vibrations in the first region, at 3155, 2985, and 2902
cm-1, corresponding to the C-H vibrations originated from
the pyrazolyl ring, central arene ring, and methylene groups,
respectively. In the solid state, multiple bands appear in these
regions for all compounds studied. Although some bands
appear at higher wavenumbers for all six compounds (see
Table 3), it is difficult to assume that this is the blue-shift
effect of the improper hydrogen bond, although we cannot
rule out this possibility. More studies on additional com-
pounds will be needed to definitively settle this issue. In
addition, compound 1 has two strong vibrations at 3639 and
3557 cm-1, due to the hydrogen bonding of the acetonitrile
molecules of solvation, also observed in the crystallographic
studies. Free acetonitrile has two vibrations at 3653 cm-1
and 3618 cm-1 that correspond to C-H‚‚‚N hydrogen bonds
in solution, and the red shifts of the crystalline samples of
3760 Inorganic Chemistry, Vol. 42, No. 12, 2003

Multitopic Tris(pyrazolyl)methane Ligands
Table 3. Infrared Data
L1
L2
L3
soln
cryst
soln
cryst
soln
oil
1 (cryst)
3639 s
2 (cryst)
3 (cryst)
assgnts
ν(CH3) (CH3CN)
3630 sh
3557 s
3158 sh
3148 s
3134 s
3155 s
3165 wm
3152 m
3130 s
3155 m 3155 ms
3123 s
3155 m 3157 m
3114 w 3129 s
3108 m
3159 sh
3150 s
3160 s
3147 s
ν(CH) (pyrazole)
3118 m
3107 sh
2956 m
2937 s
2985 s
2985 m 3068 m
3043 w
3014 w
2901 m 2940 ms
2927 m
2901 m
2883 m
1643 m 1610 br
1602 w
1561 w 1541 w
1518 m 1517 m
2984 m 2948 m
3024 w
3070 w
3064 w
3031 wm
2971 w
2958 w
2926 m
2900 m
1609 br
3020 w
ν(CH) (central arene ring)
ν(CH2) (sidearms)
2927 s
2902 m 2892 w
2886 w
2902 m 2929 m
2887 m
2977 w
2940 w
2928 wm
2900 w
1630 m
2928 wm
2885 m
2869 w
2866 w
1646 m 1616 br
1602 w
1560 m 1540 w
1517 ms 1517 s
1480 m
1426 m 1459
1424
1646 m 1631 m
1602 w 1610 w
1560 m 1555 w
1518 m 1515 ms
1471 s 1471 m
1425 m 1451
1425
1618 br
arene and pz ring str
arene and pz ring str
1560 w
1518 ms
1472 w
1458
1560 w
1522 s
1481 w
1465
1561 s
1523 m
1473 w
1458
1471 s
1471 s
1482 m
1426 m 1454
1421
δ(CH2) scissor
1438
1431
1428
1422
1336 ms 1329 s
1336 s
1333 s
1322 s
1336 s 1331 s
1337 s
1327 s
1225 m
1211 ms
1200 ms
1343 s
1327 s
1218 ms
1211 ms
1205 ms
1335 s
pz ring str
1216 m 1227 m
1202 ms 1202 s
1216 m 1222 m
1202 ms 1201 s
1216 m 1237 sh
1202 s 1203 s
1229 m
1212 ms
1204 ms
δ(C(pz)3)
1094
1092 s
1070-1023
948 s
1094 s
1093 s
1068-1024
949 s
1094 s 1092 s
1106-1048 br 1104-1032 br 1108-1036 br see text
1062-1037
949 s
957 ms
951 ms
521 m
958 ms
947 ms
521 ms
967 w
952 ms
521 ms
substituted aromatic ring
out-of-plane deformations
ν4(F2) (BF4
-
)
14 and 61 cm-1, respectively, are an indication of stronger
C-H‚‚‚N hydrogen bond between the acetonitrile molecules
in the solid.
counterion of Td symmetry presents four modes of vibration,
all of them active in the Raman and only two in the IR. In
the crystal, the symmetry of the anion is lowered; thus, the
forbidden bands ν1(A1) and ν2(E) can appear in the IR spectra
of compounds. The strong ν3(F2) characteristic vibration
The δ(CH2) scissor vibrations prove useful for observing
C-H‚‚‚π interactions because the bands were well separated
from others in the compounds. The presence of C-H‚‚‚π
interactions should lead to the observation of higher fre-
quency vibrations.25 In solution, the free ligands show one
band at 1426 cm-1. In the crystalline state of the ligands
and the metal complexes we observe a band in this same
region and, in addition, a second band ca. 30 cm-1 higher
than the first; see Table 3. Crystallographic studies showed
that some of the methylene groups are involved in
C-H‚‚‚π interactions and some are not, leading to the
expectation of two separate vibrations, with the bands for
the CH2 groups involved in C-H‚‚‚π interactions at higher
frequencies than the for the CH2 groups not involved in
C-H‚‚‚π interactions. Although the infrared data do support
the C-H‚‚‚π interactions, the shifted values are close and
cannot be used to judge the relative strength of the inter-
actions. For 1, the only complex where two methylene groups
are involved in two different C-H‚‚‚π interactions in the
crystalline state, two different δ(CH2) vibrations were found
at 1458 and 1438 cm-1, together with the free δ(CH2)
vibration at 1422 cm-1.
- 25
(around 1070 cm-1 for the free BF4 ) was used by others
-
to determine the behavior of BF4 in several compounds.26
However, in the compounds reported here the free ligands
present strong vibrations in the same region, at 1093 cm-1
for ν(CH2-O-CH2) and multiple bands in the interval
1068-1023 cm-1 for substituted aromatic ring. The overlap
of these vibrations with the ν3(F2) vibration of the BF4- anion
leads to the appearance of a very broad peak in the region
of 1110-1030 cm-1 for all three compounds, making the
assignments impossible. However, the ν4(F2) vibration of the
-
BF4 anion can also be used in gaining information about
the participation of the counterion in weak C-H‚‚‚F
hydrogen bonds. In compounds where the anion is not
interacting (e.g. KBF4), this vibration appears as a broad,
weak peak around 533 cm-1,25 while in our crystalline
complexes this vibration was identified at 521 cm-1 as a
sharp, strong peak (see Figure S5). This change in strength
and the red shift of 12 cm-1 show that the fluorine atoms
are involved in hydrogen bonds. It is important to note that
in the IR spectra of amorphous powder samples of 1 and 2
(the precipitate obtained after the thf reaction and prior to
The characteristic vibrations of the anion showed its
-
involvement in weak C-H‚‚‚F hydrogen bonds. The BF4
(26) (a) Jesih, A. J. Fluorine Chem. 2000, 103, 25. (b) von Barner, J. H.;
Andersen, K. B.; Berg, R. W. J. Mol. Liq. 1999, 83, 141. (c) Mu, S.;
Kan, J. Synth. Met. 1998, 98, 51.
(25) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 3rd ed.; Wiley-VCH: New York, 1978.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3761

Reger et al.
Figure 15. Optimum structure of the in-plane•up confomer of L1 in the
gas phase calculated at the DFT (B3LYP/6-31G*) level of theory.
A complete geometry optimization has been performed
on the in-plane•up class observed in the solid-state structure
at the DFT (B3LYP/6-31G*) level of theory.28 The optimum
structure is shown in Figure 15. One relevant feature of this
conformation is the orientation of the CH2-O-CH2 moiety
of one arm toward a pyrazolyl ring located on the adjacent
arm. Interestingly, the H‚‚‚pyrazolyl ring distances fall within
the range of C-H‚‚‚π contacts. Given this result, it is not
surprising that we observe C-H‚‚‚π interaction between the
methylene and pyrazolyl groups in the solid-state structures.
Electrospray Ionization Mass Spectrometry Studies
(ES-MS). ES-MS spectra for all three compounds were
recorded in acetone and acetonitrile. For complexes 1 and
2, the spectra are similar in both solvents. ES-MS of 1 in
acetone shows two single-charged species, corresponding to
[(L1)Ag]+ at m/z 1121 and to [(L1)Ag2(BF4)]+ at m/z 1405.
A double-charged species was observed at m/z 659, corre-
sponding to [(L1)Ag2]2+. In acetonitrile, the same three peaks
were observed, although in acetone the most abundant species
was the double charged peak at m/z 659, while in acetonitrile
the most abundant peak was the single-charged species at
m/z 1121 corresponding to [(L1)Ag]+. For 2, the ES-MS
experiment in both acetone and acetonitrile revealed a peak
at m/z 697, corresponding to the [(L2)Ag]+ species, along
with other peaks at lower values corresponding to silver-
solvent adducts. For 3, the ES-MS spectra were solvent
dependent. In acetonitrile only one peak at m/z 697 was
observed, corresponding to [(L3)Ag]+ species. In acetone,
although the most abundant peak also corresponded to [(L3)-
Ag]+ species at m/z 697, a second peak was observed at m/z
893 corresponding to [(L3)Ag2(BF4)]+ species.
Figure 14. Classes of conformers for L2. From left to right they are noted
as “in-plane•up”, “up•up”, and “up•down”.
crystallization), the ν4(F2) vibration was identified as a small
and broad peak at 529 cm-1, close to the value for the
noninteracting anion. These values, together with the intensity
increase of the peak for the crystalline samples, show that
-
in 1 and 2 the BF4 moiety is not involved in hydrogen
bonding in the powder state but is in the crystalline state.
Surprisingly, the ν4 band in the powder sample of 3 appeared
at 522 cm-1 as a sharp peak, presumably indicating that in
this case there are some C-H‚‚‚F interactions even in the
powder obtained after the thf reaction.
Theoretical Studies. A conformational analysis on L2 has
been carried out using the Spartan 02 package.27 Molecular
orbital calculations at the PM3 semiempirical level reveal
the existence of three classes of conformers (noted as up•up,
up•down and in-plane•up with respect to the orientation
of the sidearms) for this ligand. Figure 14 shows the variation
of the enthalpy of formation (in gas phase) with the C16-
C27-C26-O1 dihedral (defining the rotation of the pyra-
zolyl fragment around the arene-carbon bond). These data
suggest that the most favored conformer is the up•down
and it is separated by ca. 18-20 kJ/mol from the up•up
and in-plane•up conformers. The lowest energy conformer
in gas phase differs from the one observed in the solid
state.16a The associations between the L2 molecules in dimers
via C-H‚‚‚π interactions, as discussed above, prevents the
up•down conformer and leads to the observed in-plane•up
conformer. The C-H‚‚‚π interactions overcomes the ca. 20
kJ/mol difference between the favored up•down class of
conformers and the in-plane•up conformer found in solid
state. In contrast, in the case of L1 there is no intermolecular
C-H‚‚‚π interaction and the observed up•down orientation
of 1,2- and 4,5-positions of the sidearms in the solid-state
structure matches that predicted by theory. Furthermore, this
up•down orientation found in the solid state of L1 favors
the intramolecular C-H‚‚‚π interaction between a methylene
hydrogen next to the central arene ring on one arm and a
pyrazolyl ring on the adjacent arm (see Supporting Informa-
tion, Figure S1, and also ref 16b).
Discussion
Given that the bitopic ligands L2 and L3 can each be
considered half of the tetratopic ligand L1, comparisons of
the supramolecular structures of the silver(I) coordination
polymers formed by each provides an indication of the
relative organizing power of the covalent and noncovalent
interactions. The polymeric strand of 1 (Figure 4a) and the
dimeric polymer unit formed by the association between two
(27) Spartan ’02; Wavefunction, Inc.: Irvine, CA, 2002.
3762 Inorganic Chemistry, Vol. 42, No. 12, 2003
(28) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

Multitopic Tris(pyrazolyl)methane Ligands
strands of 2 (Figure 6b) are very similar, not only with
respect to the silver environment but in the overall arrange-
ment of the organic ligands. In 1, the polymer chain is formed
by linking macrocyclic rings containing two four-coordinate
silvers by the central arene ring. The same is true for 2,
except in this case the “rings” are joined by the π-π stacked
central arene rings of two polymer strands. This π-π
stacking in 2 essentially converts two of the bitopic ligands
L2 into an analogue of the tetratopic ligand L1. In both
compounds, two uncoordinated pyrazolyl rings located in
the center of the rings are involved in intrastrand C-H‚‚‚π
interactions. This interaction is stronger in 2 (2.60 Å) than
in 1 (2.97 Å) because in the latter case the pyrazolyl ring
involved in the interaction has steric contacts with other rings
located across the macrocycle, whereas with 2 these inter-
actions are removed because the pyrazolyl rings across the
“ring” are in a different plane.
with a different overall organization. The macrocycle is
formed by the coordination of two ligands to two different
types of silver atoms, Figure 10. One tris(pyrazolyl)methane
unit from one sidearm of the ligand is coordinated in a κ2
fashion to Ag(2). The other tris(pyrazolyl)methane unit from
the second sidearm is κ1 coordinated to the second Ag(2),
thus closing the macrocycle, but is also κ2 bonded to Ag(1)
to form the polymer chain. The major difference is that two
of the pyrazolyl rings that are noncoordinated in 1 are bonded
to Ag(3), forming a connection across the larger, 32-member
ring. In these structures, L3 cannot be regarded as the half
of L1.
These three structures show the structurally adaptiVe
nature of this family of ligands. These semirigid ligands have
the organizational features of containing multiple tris-
(pyrazolyl)methane units, each of which can show a variety
of coordination modes, and contain functional groups that
can enter into noncovalent forces such as weak hydrogen
bonds, intermolecular and/or intramolecular π-π stacking,
and C-H‚‚‚π interactions. The ortho orientation of the
sidearms in L1 and L2 with a metal such as silver(I) that
prefers low coordination numbers favors the formation of
macrocycles, even at the expense of leaving potential donor
atoms noncoordinated. For L3, all of the possible coordina-
tion sites in the ligands are bonded to silver(I), and these
covalent interactions dominate the structure. Both the flex-
ibility of the ligand sidearms and the organization features
of the ligands contribute to the highly organized structures
of these complexes.
Infrared studies have clearly been able to substantiate the
C-H‚‚‚F weak hydrogen bonding and C-H‚‚‚π interactions
identified in the solid-state structures. The ν4(F2) vibration
of the BF4- anion was most useful in identifying participation
of the counterion in weak C-H‚‚‚F hydrogen bonds.
Crystalline samples of all three complexes show a sharp,
strong peak at 521 cm-1, a peak that is weak in simple salts
of the anion and in powder samples of 1 and 2. Surprisingly,
a sharp ν4 band at 522 cm-1 was observed in the powder
sample of 3, indicating C-H‚‚‚F interactions even in the
powder state.
The δ(CH2) scissor vibrations were most useful in studying
the C-H‚‚‚π interactions. In solution the free ligands show
only one band at 1426 cm-1, but in the solid state we observe
a band in this same region and, in addition, a second band
ca. 30 cm-1 higher than the first. These bands confirm
C-H‚‚‚π interactions observed in the crystallographic stud-
ies. The gas-phase calculations of the in-plane•up orientation
of the L2 conformation also show a C-H‚‚‚π interaction of
the CH2-O-CH2 moiety of one arm with a pyrazolyl ring
located on the adjacent arm.
The two structures are also very similar with respect to
Ag‚‚‚Ag distances and angles. As expected, the distances
between two equivalent silver atoms in a strand are almost
equal, but surprisingly the Ag‚‚‚Ag distance across a
macrocycle in 1 and the analogous Ag‚‚‚Ag distance within
a dimer are almost equal. Also, the corresponding
Ag‚‚‚Ag‚‚‚Ag angles are similar and near 90° in both 1 and
2. The striking similarity between the structures of 1 and 2
in the organization of the strands (in the case of 1) and
dimeric strands (in the case of 2) into a 3D network by means
of π-π stacking, C-H‚‚‚π interactions, and weak hydrogen
bonds causes other close silver atom distances to be similar
in the two complexes. Figure 8 pictures four dimers (Figure
8a) and strands (Figure 8b) viewed down the polymer chain.
The 3-D structures of both compounds are dominated by
similar π-π stacking interactions between strands or dimers,
-
indicated by the a, and C-H‚‚‚F interactions (each BF4
anion involved in two interactions in both compounds!)
between the strands oriented in the opposite diagonal
direction than those involved in π-π stacking. A difference
between the structures is that the elongated axis of the dimer
strands in 2 are rotated 90° with respect to the strands in 1,
such that the central arene rings are oriented horizontally in
1 while in 2 they are oriented vertically. In these structures,
L2 can be regarded as the half of L1 and the noncoValent
interactions oVercome the geometrical differences between
these two ligands leading to Very similar supramolecular
structures.
Although L3 can also be considered as half of L1, the
structures and formulas of the silver complexes are different.
For 1 and 2 the ratio of tris(pyrazolyl)methane units/silver
is 2:1 and all silvers are four-coordinate with one noncoor-
dinated pyrazolyl ring/tris(pyrazolyl)methane unit. For 3 the
ratio is 1:1 and all pyrazolyl rings are bonded to one of the
three different types of silvers, leading to a very different
supramolecular structure. Clearly, the covalent forces that
favor all of the pyrazolyl rings bonding to silver force a very
different structure on 3 when compared to 1. Interestingly,
although the overall structure and formula are different, there
is still a macrocycle present in the structure of 3 containing
the same number of atoms, 32, as the macrocycle in 1 but
The ES-MS data collected from acetone or acetonitrile
show the presence in solution of aggregated species related
to those observed in solid state. All peaks correspond to one
ligand and one or two silver ions with and without counter-
ions. The similarity of ES-MS spectra of the compounds in
1
acetone and acetonitrile is surprising because the H NMR
spectra of all compounds show that acetonitrile completely
displaces the ligand whereas in deuterated acetone all
Inorganic Chemistry, Vol. 42, No. 12, 2003 3763

Reger et al.
resonances are shifted from the free ligand, showing the
coordination of the ligand to silver(I). This means that the
same aggregates that exist in solution (acetone) are formed
when a solvent (acetonitrile) that can displace the tris-
(pyrazolyl)methane ligand is removed in the electrospray
process.
rings are coordinated to a metal, yielding a 1:1 tris-
(pyrazolyl)methane units/silver stoichiometry. The driving
force of strong coordinate covalent bonds, apparently ener-
getically not favored for tris(pyrazolyl)methane units bonded
ortho on an arene ring, dominates the structural differences.
IR studies support the observations of C-H‚‚‚π and
C-H‚‚‚F hydrogen bonds in the crystalline samples but not
in solution or in the powder samples of 1 and 2. The
importance of the organizational power of C-H‚‚‚π inter-
actions in these systems is supported by DFT calculations.
Despite the fact that solution NMR studies show that the
ligand is displaced by acetonitrile but not acetone, the ES-
MS of 1 and 2 are similar in both solvents. For 3, a peak for
a bimetallic cation, [(L3)Ag2(BF4)]+, is only observed in
acetone.
Conclusion
A combination of X-ray crystallography, IR spectroscopy,
DFT calculations, and ES/MS studies has been used to
investigate the solid-state and solution properties of silver-
(I) complexes formed from closely related semirigid, mul-
titopic ligands, one of which is tetratopic and the other two
bitopic analogues that can each be viewed as possessing half
of the bonding properties of the first. The covalent and
supramolecular structures of the silver(I) complexes of
tetratopic L1 and ortho-bitopic L2 are surprisingly similar,
where π-π stacking interactions between the central phenyl
rings of the bitopic ligand form a tetratopic unit that acts
the same as L1. The other noncovalent forces that organize
the supramolecular structures are nearly identical, a surprising
result given the large differences in the systems. In contrast,
the meta-bitopic ligand L3 forms a silver complex with
different stoichiometry and structure, although the same types
of noncovalent forces organize the supramolecular structure.
The differences are driven by the fact that the tris(pyrazolyl)-
methane units in the sterically hindered ortho-ligands L1 and
L2 prefer κ2-bonding to silver with the third pyrazolyl ring
noncoordinated, yielding a coordination polymer with 2:1
tris(pyrazolyl)methane units/silver metal stoichiometry. In
contrast, with less hindered L3 a structure forms in which
each tris(pyrazolyl)methane unit is κ1-κ2 bonded to the
silvers, a bonding arrangement we have observed pre-
viously.16c,g In this bonding arrangement all of the pyrazolyl
Acknowledgment. The authors thank the National Sci-
ence Foundation (Grant CHE-0110493) for support. The
Bruker CCD single-crystal diffractometer was purchased
using funds provided by the NSF Instrumentation for
Materials Research Program through Grant DMR 9975623.
The NSF (Grant CHE-9601723) and NIH (Grant RR-02425)
have supplied funds to support NMR equipment, and the
NIH (Grant RR-02849) has supplied funds to support mass
spectrometry equipment at the University of South Carolina.
Supporting Information Available: X-ray crystallographic files
in CIF format for L1 and 1-3, a figure with the crystallographic
numbering scheme for L1, a table with crystallographic data and
structure refinement, a table with selected bond lengths and angles,
ORTEP diagrams of the asymmetric unit for 1-3, and part of the
infrared spectra of 2 in the crystalline and powder states. This
material is available free of charge via the Internet at http://
pubs.acs.org.
IC034039R
3764 Inorganic Chemistry, Vol. 42, No. 12, 2003