The influence of anion, ligand geometry and stoichiometry on the structure and dimensionality of a series of AgI-bis(cyanobenzyl)piperazine coordination polymers

Article abstract of DOI:10.1039/c4ce00816b

Two new bis-(cyanobenzyl)piperazine ligands have been prepared, and have been used to prepare five Ag(i) coordination polymers which have been structurally characterised. Ligand N,N′-bis((4-cyanophenyl)methyl) piperazine L1 gave two structurally related two-dimensional coordination polymers poly-[Ag₂(L1)(CF₃SO₃)₂] 1 and poly-[Ag₂(L1)(CF₃SO₃)₂(C ₃H₆O)_1.5] 2 when reacted with silver trifluoromethanesulfonate in tetrahydrofuran and acetone, respectively. Compounds 1 and 2 display similar ligand geometries but form different extended networks, as a result of the inclusion of coordinating solvent molecules and the resulting influence on the flexible metal geometry. Reaction of N,N′-bis((3-cyanophenyl)methyl)piperazine L2 with silver ions gave rise to three new structurally characterised species. Complex poly-[Ag ₂(L2)(C₃H₆O)₂]₂(PF ₆) 3 forms a one-dimensional chain structurally related to 1. Complex poly-[Ag₂(L2)(C₃H₈O)₂(CF ₃SO₃)₂] 4 formed under similar conditions to 3, except with the inclusion of a coordinating anion which links equivalent chains to those found in 3 into a two-dimensional sheet. Finally, complex poly-[AgL2]CF₃SO₃5 was formed simply by altering the reaction stoichiometry, producing a three-dimensional PtS network containing linear, acentric anion channels. Analysis of the structures reveals that the ligand geometries are largely consistent, leading to the extended structures being more influenced by the nature of the anions and solvent molecules.

Full text of DOI:10.1039/c4ce00816b

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The influence of anion, ligand geometry and
stoichiometry on the structure and dimensionality
of a series of AgI-bis(cyanobenzyl)piperazine
coordination polymers†
Cite this: CrystEngComm, 2014, 16,
6459
a
a
a
ab
*
*
Lianna J. Beeching, Chris S. Hawes, David R. Turner and Stuart R. Batten
Two new bis-(cyanobenzyl)piperazine ligands have been prepared, and have been used to prepare five Ag(I)
coordination polymers which have been structurally characterised. Ligand N,N′-bis((4-cyanophenyl)methyl)piperazine
L1 gave two structurally related two-dimensional coordination polymers poly-[Ag2(L1)(CF3SO3)2] 1 and
poly-[Ag2(L1)(CF3SO3)2(C3H6O)1.5] 2 when reacted with silver trifluoromethanesulfonate in tetrahydrofuran
and acetone, respectively. Compounds 1 and 2 display similar ligand geometries but form different
extended networks, as a result of the inclusion of coordinating solvent molecules and the resulting
influence on the flexible metal geometry. Reaction of N,N′-bis((3-cyanophenyl)methyl)piperazine L2 with
silver ions gave rise to three new structurally characterised species. Complex poly-[Ag2(L2)(C3H6O)2]2(PF6) 3
forms a one-dimensional chain structurally related to 1. Complex poly-[Ag2(L2)(C3H8O)2(CF3SO3)2] 4
formed under similar conditions to 3, except with the inclusion of a coordinating anion which links
equivalent chains to those found in 3 into a two-dimensional sheet. Finally, complex poly-[AgL2]CF3SO3 5
was formed simply by altering the reaction stoichiometry, producing a three-dimensional PtS network
containing linear, acentric anion channels. Analysis of the structures reveals that the ligand geometries are
largely consistent, leading to the extended structures being more influenced by the nature of the anions
and solvent molecules.
Received 18th April 2014,
Accepted 9th May 2014
DOI: 10.1039/c4ce00816b
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thermodynamically favoured product amongst many possible
kinetic side-products.4 For studies investigating the formation
Introduction
The synthesis and structural characterisation of coordination
polymers has continued to expand as an area of extremely
active research in recent years.1 Interest in applications such
as gas sorption and sieving, molecular separation, catalysis
and optical properties have brought the study of these materials
to the forefront of modern coordination and materials chemistry.2
Central to the rational design of such materials is the notion
of molecular self-assembly and, more crucially, the inherent
reversibility in the formation of metal–ligand bonds.3 Often,
the design of supramolecular materials can be governed almost
entirely by careful choice of reaction conditions, taking
advantage of bond reversibility to favour the formation of one
of crystalline metal–organic assemblies, Ag(I) is often the
metal ion of choice, as its soft Lewis acid character favours
reversibility, while a lack of strongly preferred coordination
geometries leads to
architectures.5
a range of interesting coordination
A relatively common fragment in organic chemistry, and
particularly in pharmaceuticals,6 piperazine has seen use as a
bridging ligand in coordination polymer synthesis, alongside
the related compounds hexamethylenetetramine and DABCO
(1,4-diazabicyclo[2.2.2]octane).7 Furthermore, the N-substituted
heterotopic derivatives of piperazine, commonly containing
pyridyl, amidopyridyl or phosphonate groups, are becoming
increasingly popular.8 Interestingly, cyclic alkyl amines such
as piperazine possess a low energy barrier to stereochemical
inversion, with the result that interconversion between isomers
a School of Chemistry, Monash University, Clayton, VIC 3800, Australia.
E-mail: chris.hawes@monash.edu, stuart.batten@monash.edu
b Department of Chemistry, Faculty of Science, King Abdulaziz University,
Jeddah, Saudi Arabia
of N-substituted cyclic amines can readily take place,9
a
property which may further lend itself to the self-assembly
process. Crucially, the coordinating ability of the piperazine
nitrogen atoms is likely to be dependent on the method of
functionalisation; conjugation between the nitrogen atom
† Electronic supplementary information (ESI) available: Powder X-ray diffraction
plots for L1, L2 and compounds 1–5, additional structural diagrams. CCDC
997441–997447. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4ce00816b
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Reflectance (ATR) sampler. Bulk phase purity of all crystalline
materials was confirmed with X-ray powder diffraction patterns
recorded with a Bruker X8 Focus powder diffractometer
operating at Cu Kα wavelength (1.5418 Å), with samples
mounted on a zero-background silicon single crystal stage.
Scans were performed at room temperature in the 2θ range
5–55° and compared with predicted patterns based on single
crystal data (ESI†).
X-ray crystallography
Refinement and structural information is presented in Table 1.
Data collections for all compounds except for 1 and 4 were
performed on a Bruker APEX-II diffractometer, using graphite-
monochromated Mo Kα (λ = 0.71073 Å) radiation, while the
diffraction data for 1 and 4 were collected at the Australian
Synchrotron on the MX1 beamline, operating at 17.4 keV
(λ = 0.7108 Å) with data collections conducted using BluIce
control software.12 Data for compounds 1 and 4 were collected
with a 360° scan in Φ, and the crystals were then reoriented
and a second 360° scan in Φ was conducted, and the data
from both collections merged in order to provide sufficient
completeness. Corrected anomalous dispersion values were
calculated where necessary using Brennan and Cowan data.13
Diffraction data for L1, L2, 2, 3 and 5 were processed using
the Bruker SAINT suite of programs,14 with multi-scan absorption
correction carried out with SADABS,15 while the remaining
diffraction data were processed, reduced and corrected with
the XDS software suite.16 All structures were solved using
direct methods with SHELXS and refined on F2 using all data
by full matrix least-squares procedures with SHELXL-97 within
OLEX-2.17–19 Non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were included in
calculated positions with isotropic displacement parameters
1.2 times the isotropic equivalent of their carrier atoms. The
Fig. 1 Structure of ligands L1 and L2.
and adjacent amido or aryl functionality would be expected
to prevent metal coordination from taking place through
these centres.
To date, the majority of functionalised piperazine ligands
reported have featured pyridyl functional groups, with fewer
studies reporting the use of other functionalities.10 As
well as amines and nitrogen heterocycles, nitrile functional
groups are also an excellent soft Lewis basic platform
for coordinating to Ag(I) in non-coordinating solvents, and
heterotopic nitrile-containing ligands are known to form
intricate metallosupramolecular architectures on reaction with
Ag(I).11 Herein we report the synthesis of two new cyanobenzyl
piperazine ligands (Fig. 1), containing both amine and nitrile
nitrogen donors. These ligands represent particularly interesting
and uncommon examples of divergent heterotopic ligands,
being derived from a combination of very long and very short
potential bridging distances by virtue of the closely separated
piperazine nitrogen atoms and substantial distance between
the nitrile groups. We have employed these new ditopic ligands
to prepare a series of Ag(I) coordination polymers, whose
structures are heavily influenced by the nature and geometry
of the donor ligands.
functions minimized were Σw(Fo − Fc ), with w = [σ2(Fo2) +
2
2
2
aP2 + bP]−1, where P = [max(Fo)2 + 2Fc ]/3.
Synthesis of N,N′-bis((4-cyanophenyl)methyl)piperazine L1
Piperazine (1.0 g, 11.6 mmol) was added to potassium
carbonate (6.4 g, 46.5 mmol), potassium iodide (20 mg,
0.1 mmol) and 4-(chloromethyl)benzonitrile (3.7 g, 24.7 mmol)
in 50 mL acetonitrile and the mixture was heated to reflux
overnight under a nitrogen atmosphere. On cooling, the
mixture was filtered, and the filtrate was concentrated to
approximately 10 mL volume in vacuo to give a small batch
of colourless crystals (150 mg). The precipitate, comprising
a mixture of the product and inorganic salts, was added
to 20 mL water, stirred for 10 minutes, and filtered to yield
3.14 g of white powder, which gave identical analysis to the
crystalline material. Combined yield 3.29 g (10.4 mmol; 90%);
m.p. 210–214 °C; found C, 76.10; H, 6.35; N, 17.80; C20H20N4
requires C, 75.91; H, 6.38; N, 17.71%; δH (400 MHz, CDCl3)
2.49 (s, 8H), 3.57 (s, 4H), 7.46 (d, 4H, 3J1 = 8.3 Hz), 7.62
(d, 4H, 3J1 = 8.2 Hz); δC (100 MHz, CDCl3) 53.10, 62.38, 110.91,
118.92, 129.49, 132.11, 144.10; m/z (ES+) 317.1 ([M + H+],
Experimental
Materials and methods
All reagents, solvents and starting materials were purchased
from Sigma Aldrich, Alfa Aesar or Merck and were used as
received. NMR spectra were recorded on a Bruker AVANCE
1
spectrometer operating at 400 MHz for H and 100 MHz for
13C nuclei, with all samples dissolved in CDCl3 and peaks
referenced to the residual solvent peak. Melting points were
recorded in air on an Electrothermal melting point apparatus,
and are uncorrected. Mass spectrometry was carried out using
a Micromass Platform II ESI-MS instrument, with samples
dissolved in dichloromethane. Microanalysis was performed
by Campbell Microanalytical Laboratory, University of Otago,
New Zealand. Infrared spectra were obtained using an Agilent
Cary 630 spectrometer equipped with an Attenuated Total
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calculated for C20H21N4+ 317.2); νmax(ATR)/cm−1 2954w, 2807m,
2219m, 1604m, 1453m, 1343m, 1290m, 1258m, 1156m, 1008s,
841s sh, 806s sh.
N, 6.70; C20H20N4F12P2Ag2 requires C, 29.22; H, 2.45; N, 6.82%,
consistent with loss of lattice acetone molecules; νmax(ATR)/cm−1
3067w br, 2236m, 1581m, 1445m sh, 1341m, 1297s sh, 1141m,
1033s, 858s, 807s, 692s.
Synthesis of N,N′-bis((3-cyanophenyl)methyl)piperazine L2
Synthesis of poly-[Ag2(L2)(C3H8O)2(CF3SO3)2] 4
Piperazine (210 mg, 2.4 mmol) was added to potassium carbonate
(2.7 g, 19.5 mmol) and 3-(bromomethyl)benzonitrile (1.0 g,
5.1 mmol) in 40 mL acetonitrile and the mixture was heated
to reflux overnight under a nitrogen atmosphere. On cooling,
the mixture was filtered and concentrated to approximately
5 mL volume on a rotary evaporator, yielding pale yellow
crystals, which were filtered and dried in air. Yield 550 mg
(1.7 mmol; 73%). m.p. 120–122 °C; found C, 75.92; H, 6.41;
N, 17.69; C20H20N4 requires C, 75.91; H, 6.38; N, 17.71%;
δH (400 MHz, CDCl3) 2.48 (s, 8H), 3.54 (s, 4H), 7.42 (t, 2H,
3J1 = 7.8 Hz), 7.53–7.57 (m, 4H, overlapping), 7.66 (s, 2H);
δC (100 MHz, CDCl3) 53.01, 61.98, 112.42, 118.94, 129.04,
130.81, 132.41, 133.33, 140.02; m/z (ES+) 339.1 ([M + Na+],
To L2 (20 mg, 63 μmol) in 5 mL acetone was added silver
trifluoromethanesulfonate (64 mg, 250 μmol) in
5 mL
acetone. The mixture was quickly filtered and allowed to
stand, giving a fine white precipitate within several minutes.
The mixture was sealed and allowed to stand for one week,
giving colourless crystals which rapidly lost crystallinity when
removed from the solution. Yield 48.4 mg (51 μmol; 81%);
m.p. 188–193 °C (decomp.); found C, 30.38; H, 2.78; N, 6.36;
C22H20N4O6F6S2Ag2·2H2O requires C, 30.50; H, 2.79; N, 6.47%,
consistent with loss of lattice acetone and uptake of water on
standing in air; νmax(ATR)/cm−1 3435m br, 2256s, 1638m, 1462w,
1246s, 1158s, 1023s, 898w, 979m.
calculated for C20H20N4Na+ 339.2), 317.1 ([M
+
H+],
calculated for C20H21N4+ 317.2); νmax(ATR)/cm−1 2957w, 2834w,
2774m, 2615m, 2222w, 1457m sh, 1340m sh, 1282m sh,
1126m, 1003m, 828m, 792s sh, 684s sh.
Synthesis of poly-[AgL2]CF3SO3 5
To L2 (20 mg, 63 μmol) in 5 mL acetone was added silver
trifluoromethanesulfonate (16 mg, 63 μmol) in 5 mL acetone.
The resulting solution was enclosed in darkness and left
at room temperature to crystallise. Colourless crystals were
filtered after 48 hours and dried in air. Yield 10.7 mg
(19 μmol; 29%); m.p. 185–188 °C; found C, 43.94; H, 3.57;
N, 9.65; C42H40N8O6F6S2Ag2 requires C, 43.99; H, 3.52; N, 9.77%;
νmax(ATR)/cm−1 2834w, 2236w, 1441w, 1264s, 1145s, 1028m sh,
803m, 742m, 662m.
Synthesis of poly-[Ag2(L1)(CF3SO3)2] 1
To a solution of L1 (10 mg, 32 μmol) in 5 mL tetrahydrofuran
was added silver trifluoromethanesulfonate (15 mg, 59 μmol)
dissolved in 5 mL tetrahydrofuran. The solution was filtered
and the filtrate allowed to stand in a sealed vial for one week.
The colourless crystals formed were filtered and dried in air.
Yield 15.5 mg (19 μmol; 58%); m.p. 218–221 °C (decomp.);
found C, 31.94; H, 2.51; N, 6.50; C22H20N4O6F6S2Ag2 requires
C, 31.83; H, 2.43, N, 6.75%; νmax(ATR)/cm−1 2250m, 1608w,
1466m, 1297s, 1226s, 1165m, 1020s, 858s, 807s.
Results and discussion
Structures of L1 and L2
Compounds L1 and L2 were formed by reaction of piperazine
with 4-chloromethyl- or 3-bromomethylbenzonitrile, respectively.
Each compound was crystallised from hot acetonitrile, and the
structures were determined crystallographically, where the
diffraction data were solved and refined in the triclinic space
Synthesis of poly-[Ag2(L1)(CF3SO3)2(C3H6O)1.5] 2
To a solution of L1 (10 mg, 32 μmol) in 5 mL acetone was
added silver trifluoromethanesulfonate (15 mg, 59 μmol) in
5 mL acetone. The resulting colourless solution was enclosed
in darkness and left to stand at room temperature. The resulting
colourless crystals were filtered and dried after one week.
Yield 9.7 mg (11 μmol; 33%); m.p. 216–220 °C (decomp);
found C, 31.07; H, 2.68; N, 6.43; C22H20N4O6F6S2Ag2·H2O
requires C, 31.15; H, 2.61; N, 6.60%, consistent with loss
of lattice acetone molecules and uptake of water on standing
in air; νmax(ATR)/cm−1 3443w br, 2237w, 1460w, 1287s, 1163s,
1036s, 983w, 860m, 815m, 759w.
¯
group P1 in both cases. L1 and L2 were observed to display
very similar structures in the solid state (Fig. 2); both adopt
the expected chair conformation for the central piperazine ring
with trans-diequatorial orientation of the pendant cyanobenzyl
groups. Both structures contain half of one molecule within
the asymmetric unit, with inversion centres located at the
centre of the piperazine rings.
Similarities are also observed in the extended structures of
L1 and L2. For both compounds, the primary intermolecular
contacts are offset face-to-face π–π interactions between adjacent
cyanophenyl groups, with mean interplanar distances of 3.451(2)
and 3.635(2) Å, respectively (ESI,† Fig. S2). In the structure of L1,
an additional intermolecular interaction is observed, in the
form of a reciprocating pair of C–H⋯N interactions between
the nitrile groups and ortho hydrogen atoms of two adjacent
cyanophenyl groups, with C⋯N distance 3.572(2) Å and
C–H⋯N angle 167.68(9)°.20 No other significant intermolecular
Synthesis of poly-[Ag2(L2)(C3H6O)2]2(PF6) 3
To a solution of L2 (20 mg, 63 μmol) in 10 mL of acetone was
added 64 mg (250 μmol) of silver hexafluorophosphate
dissolved in 10 mL of acetone, and the mixture was filtered and
sealed. The colourless crystals which formed within several
hours were filtered and dried after 2 days. Yield 24.1 mg
(46 μmol; 36%); m.p. 276–292 °C; found C, 29.46; H, 2.59;
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Fig. 2 Structures of L1 (left) and L2 (right) with labelling scheme for
the unique heteroatoms. Hydrogen atoms omitted for clarity.
Fig.
3
Structure of
1
with partial heteroatom labelling scheme.
Hydrogen atoms omitted for clarity. Symmetry codes used to generate
equivalent atoms: i) 1 − x, y, 1 − z; ii) 1 − x, 1 − y, −z; iii) −x, 1 − y, −z.
interactions were obvious in the structures of L1 or L2, with
both forming densely packed molecular arrangements.
the weak nature of triflate as a ligand and the bridging
coordination mode.
As would be expected by steric considerations, the piperazine
rings in 1 display a chair conformation, with silver ions
occupying the axial positions of the two crystallographically
equivalent amine sites. Emanating from the equatorial positions,
the benzonitrile arms adopt a similar geometry to that
observed in the structure of the free ligand, with both nitriles
coordinating to silver ions, with the result that each molecule
of L1 connects four silver ions. The extended structure of 1 is
a two-dimensional sheet, in which chains of L1-linked silver
ions are extended into a second dimension by bridging through
triflate oxygen atoms (Fig. 4). Adjacent layers associate by
slight overlap of parallel phenyl ring π-systems, with a mean
interplanar distance of 3.420(3) Å, and are further supported
by interdigitation of triflate CF3 groups (ESI,† Fig. S3).
Synthesis of Ag(I) complexes of L1 and L2
The syntheses of complexes 1–5 were each achieved by
combining solutions of the appropriate ligand and metal salts
in acetone or tetrahydrofuran solutions. Each of the crystalline
phases were recovered from the sealed solutions within several
days. It must be noted that, while the lability of the coordination
bonds is beneficial to the self-assembly process, such a property
can also promote the formation of mixed crystalline phases,
and can lead to instability of the solid materials on extended
drying. As such, the identities of the bulk phase materials
were primarily confirmed by X-ray powder diffraction (ESI†),
as well as infrared spectroscopy and elemental analysis. The
conditions for each synthesis were subsequently optimised to
avoid the formation of mixed phases. All silver-containing
species were prepared and stored in the dark, however no
strong light sensitivity was observed.
Structure of poly-[Ag2(L1)(CF3SO3)2(C3H6O)1.5] 2
When ligand L1 was reacted with silver trifluoromethane-
sulfonate in acetone rather than tetrahydrofuran, a different
crystalline material, poly-[Ag2(L1)(CF3SO3)2(C3H6O)1.5] 2 was
isolated after standing for several days, and the X-ray diffraction
data were solved and refined in the monoclinic space group C2/c.
The asymmetric unit was found to contain two non-equivalent
Structure of poly-[Ag2(L1)(CF3SO3)2] 1
Ligand L1 was combined with two equivalents of silver
trifluoromethanesulfonate in tetrahydrofuran, and the resulting
mixture was allowed to stand for several days, yielding
colourless single crystals which were isolated by filtration.
Analysis of the material by single crystal X-ray diffraction
yielded data which were solved and refined in the triclinic
¯
space group P1, revealing a compound of the formula
poly-[Ag2(L1)(CF3SO3)2] 1. The asymmetric unit of 1 contains
one Ag(I) ion, a trifluoromethanesulfonate anion and half
of an L1 molecule, with the remaining symmetry equivalent
atom sites generated by an inversion centre within the piperazine
ring. Each silver ion displays a highly distorted sawhorse-type
four-coordinate geometry, and is coordinated by two equivalent
triflate oxygen atoms, one piperazine nitrogen atom and one
nitrogen atom from a nitrile group (Fig. 3). The coordinating
triflate oxygen atoms bridge between two symmetry-related
silver ions to generate a four-membered ring. The Ag–O bonds,
at 2.5978(13) and 2.5306(14) Å, are considerably longer
than the Ag–N bonds (2.243(2) and 2.285(2) Å for nitrile
and amine coordination, respectively), consistent with both
Fig. 4 Extended structure of 1 viewed perpendicular to the two-
dimensional network. Hydrogen atoms omitted for clarity.
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silver ions, one complete molecule of L1, two triflate anions
and acetone molecules disordered over three unique sites
with a total occupancy of 1.5 molecules per asymmetric unit.
As was the case in complex 1, L1 coordinates to four silver
ions through amine and nitrile donors. Ag1 is best described
as having a 3 coordinate, T-shaped geometry, with strong
bonds to nitrile, amine and triflate oxygen donors, giving
bond lengths of 2.216(5), 2.281(5) and 2.491(5) Å, respectively
(Fig. 5). In addition, a weak interaction to an oxygen atom of
a second triflate anion is observed, with a Ag–O distance of
2.728(6) Å.
The Ag1 coordination sphere is capped by an interaction
with a phenyl ring C9–H9 group engaged in a possible agostic
interaction,21 defined by a relatively long Ag–H distance of
2.85 Å and a C–H⋯Ag angle 98.7°. The more strongly bound
triflate anion of the two involved in bonding to Ag1 bridges to
Ag2 in a μ2-κO:κO′ fashion, where the bond to Ag2 is somewhat
longer, at 2.636(4) Å. The coordination sphere of Ag2 is
completed by one nitrile and one piperazine amine group
(Ag–N bond lengths 2.220(5) and 2.284(5) Å, respectively),
and one oxygen atom from a coordinating acetone molecule
disordered over two positions. Further interaction is provided
by a second disordered acetone molecule, which provides a
weak interaction to either of two symmetry-related Ag2 sites
(Ag2–O50 distance 2.744(11) Å).
Although the geometry and coordination pattern of the
L1 ligand is similar to that observed in the structure of 1,
the extended structure of 2 displays several key differences.
Firstly, the linear chains of L1-linked silver ions present in
1 are prohibited from forming in 2, due to a mismatch in
geometry of the two silver sites caused by the presence of
coordinating acetone molecules. This results in an inclined
orientation of the two L1 units connected to each silver ion,
rather than the parallel orientation required for chain formation
(Fig. 6). The divergence of adjacent L1 molecules gives rise
to a two-dimensional sheet when extended through L1 links
only. These networks are further linked by the bridging triflate
ions, which join parallel pairs of sheets, giving the bilayer
Fig. 6 (top) Extended structure of compound 2. Hydrogen atoms,
solvent disorder and acetone carbon atoms omitted for clarity.
(bottom) Topological representation of the bilayer sheet structure of 2.
L1 nodes coloured blue, silver nodes coloured grey, with green
linkages representing triflate bridges.
structure shown in Fig. 6. Surprisingly, no strong π–π interactions
were observed in the structure, either within the networks or
between adjacent layers, which associate with interdigitation
of both terminal triflate groups and acetone ligands. Both
elemental analysis and infrared spectroscopy showed the loss
of lattice acetone molecules and association of atmospheric
water on standing in air.
Structure of poly-[Ag2(L2)(C3H6O)2]2(PF6) 3
Single crystals of poly-[Ag2(L2)(C3H6O)2]2(PF6) 3 were formed
by reaction of ligand L2 with silver hexafluorophosphate in
acetone. Analysis of the structure by single crystal X-ray
diffraction provided a structure model in the triclinic space
¯
group P1, in which the asymmetric unit contains half of an
L2 molecule, one silver ion, two coordinating acetone molecules
and a non-coordinating hexafluorophosphate anion. The silver
ion displays an approximately tetrahedral geometry, with
coordination to piperazine amine and nitrile groups at Ag–N
distances 2.254(2) and 2.231(3) Å, respectively. As expected, the
coordinating acetone molecules are comparatively weakly bound,
with Ag–O bond lengths of 2.464(2) and 2.452(2) Å. As was the
case with the previous structures, the piperazine ligand itself
exists in a chair conformation, with an inversion centre located
within the six-membered ring. The pendant alkyl arms occupy
equatorial positions of the piperazine nitrogen sites, while
coordination to the four equivalent silver ions takes place
Fig. 5 Structure of 2 with partial labelling scheme. Solvent disorder
and hydrogen atoms omitted for clarity. Symmetry codes used to
generate equivalent atoms: i) +x, 1 − y, −1/2 + z; ii) +x, 2 − y, 1/2 + z;
iii) 1/2 − x, −1/2 + y, 1/2 − z.
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with axial geometry from the amines, as well as through both
nitrile groups (Fig. 7).
functionality, as seen in the structures of 1 and 2. The
diffraction data for 4 were solved and refined in the triclinic
¯
When extended through the bridging L2 molecules,
compound 3 forms a one-dimensional chain in which each
unique half of an L2 molecule forms a loop with an adjacent
fragment via two silver ions, as shown in Fig. 7. Despite the
silver ion geometry in 3 being comparable to the acetone-
coordinated fragment in 2, the divergent orientation of the
ligand axes around each silver ion, which gave rise to higher
dimensionality in 2, is offset by the bent geometry of the
coordination sites of ligand L2, which allows the formation of
a lower dimensional assembly. Although the geometry of the
phenyl groups within each chain prohibits any intramolecular
π–π interaction, adjacent chains associate by way of parallel
offset face-to-face interactions, with mean interplanar distance
3.575(3) Å. The combination of these interactions with the
chain propagation direction gives a series of layers parallel to
the crystallographic ac plane, with adjacent sheets separated
by interstitial layers of hexafluorophosphate anions (ESI†).
space group P1, and revealed a structure of the formula
poly-[Ag2(L2)(C3H8O)2(CF3SO3)2]. The asymmetric unit of 4
contains one silver ion in a sawhorse geometry, where the
trans positions are occupied by a piperazine amine and a
nitrile group, with bond distances 2.288(2) and 2.208(3) Å,
respectively. The equatorial sites are filled by two triflate
oxygen atoms, at Ag–O distances 2.594(3) and 2.536(2) Å, and
a weakly associated acetone molecule, with Ag–O distance
2.983(2) Å. As was observed in the structure of 2, the triflate
ions bridge between two silver ions in a μ2-κO:κO′ fashion,
forming an 8-membered metallocycle, shown in Fig. 8.
Consistent with the structures observed for compounds 1–3,
each of the four nitrogen donor sites in L2 is occupied by
equivalent silver ions. Inspection of the extended structure of
4 reveals several other similarities to the previous structures.
When extended through L2 linkages alone, a one-dimensional
chain equivalent to that observed in 3 results. However, where
the coordination sphere of 3 was completed by acetone molecules,
the presence of bridging triflate anions in 4 provides links into
a second dimension. The resulting two-dimensional sheet structure
is reminiscent of the structure of 1, with slight variation in both
ligand geometry and anion bridging mode (Fig. 9). However,
Structure of poly-[Ag2(L2)(C3H8O)2(CF3SO3)2] 4
Following the elucidation of the structure of 3, containing a
non-coordinating anion, ligand L2 was reacted with silver
trifluoromethanesulfonate in acetone, with the expectation of
additional dimensionality provided by a potential bridging
Fig. 8 Structure of 4 with partial atom labelling scheme. Hydrogen
atoms omitted for clarity. Symmetry codes used to generate equivalent
atoms: i) −x, 1 − y, 1 − z; ii) −x, 2 − y, 1 − z.
Fig.
7 (top) Structure of 3 with partial atom labelling scheme.
Hydrogen atoms omitted for clarity. Symmetry codes used to generate
equivalent atoms: i) 2 − x, −y, −z. (bottom) Extended structure of 3
showing two adjacent 1-dimensional chains. Hydrogen atoms omitted
for clarity.
Fig. 9 Extended structure of 4 viewed perpendicular to the two-
dimensional sheet. Hydrogen atoms omitted for clarity.
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whereas the sheets in 1 closely associated with one another
through multiple π–π interactions, no particular attractive
interactions are observed between the layers of 4, where
acetone molecules and anion trifluoromethyl groups protrude
towards the inter-layer regions. On removal from the mother
liquor, the crystals of 4 were observed to rapidly lose single
crystallinity, most likely by displacement of these interstitial
acetone molecules. Powder X-ray diffraction showed loss
of bulk crystallinity immediately on drying in air, with only
broad and low-intensity peaks observed corresponding to a
poorly crystalline desolvated phase, which we were unable to
further characterise crystallographically. Similar to the case
in 2, elemental analysis on the amorphous air-dried sample of
4 showed loss of lattice acetone molecules and uptake of
atmospheric water.
When extended through the divergent Ag-L2 linkages, the
structure of 5 takes the form of a three-dimensional polymer.
Considering both the ligand and metal ion as four-connected
nodes, the network is described by the PtS topology (Fig. 11).
The nodal geometries involved are well matched to the PtS
network, as the L2 nodes, visualised at the centroid of the
piperazine ring, adopt a distorted square planar connectivity,
while the silver ion nodes connect in a distorted tetrahedral
fashion. The network is somewhat distorted by the large
variations in internodal distance, caused by the substantially
different bridging distances imparted by L2.
Interestingly, the presence of the non-coordinating triflate
counterions in 5 gives rise to a series of anion channels
within the structure, oriented parallel to the crystallographic
[1,0,1] vector. Although the cationic network itself displays
additional symmetry in the form of a twofold axis and
corresponding inversion centre located within the piperazine
ring, the extra symmetry is broken by the ordering of the
anions, whose directionality within the channels enforces an
overall noncentric structure. When the synthesis was repeated
using the higher symmetry anion hexafluorophosphate, an
equivalent network was formed, however a high quality structure
Structure of poly-[Ag(L2)]CF3SO3 5
In an attempt to increase the dimensionality of the
polymeric Ag-L2 adducts, ligand L2 was reacted with silver
trifluoromethanesulfonate in a 1 : 1 ratio. Colourless crystals
deposited overnight, which were analysed by single crystal
X-ray diffraction. The diffraction data, solved in the monoclinic
space group Cc, revealed an asymmetric unit containing one
complete molecule of L2 coordinating through each nitrogen
atom to a single silver ion, with a non-coordinating triflate
counterion. The geometry of the silver ion is best described
as highly distorted tetrahedral, where the four coordination
sites are filled by the four unique nitrogen donors from L2
(Fig. 10). Each of the Ag–N bond lengths falls in the range
2.315(3)–2.421(4) Å, with slightly shorter bonds being provided
by the amine donors compared to the nitriles. The geometry
of the L2 molecule is comparable to the aforementioned
examples, where the chair conformation is adopted with axially
coordinated silver ions, however each cyanobenzyl group in 5
is twisted about the N–CH2 bonds in a trans orientation to the
silver ions. This arrangement, in contrast to the approximately
cis oriented species displayed previously, imparts a greater
divergence on the overall assembly, and prevents the formation
of complementary loops leading to one-dimensional features
as observed in the previous structures.
Fig. 11 (top) Topological representation of the PtS network within 5
viewed slightly offset from the [1,1,0] vector, where blue nodes represent
L2 units and grey nodes represent Ag ions. (bottom) Extended structure
of 5 viewed along the [1,0,1] vector, parallel to the anion channels.
Fig. 10 Structure of 5 with partial labelling scheme. Hydrogen atoms
omitted for clarity. Symmetry codes used to generate equivalent atoms:
i) +x, 1 − y, −1/2 + z; ii) 1/2 − x, −1/2 + y, +z; iii) −1/2 + x, 1/2 − y, +z.
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model was unable to be elucidated from the single crystal data.
We ascribe this difficulty to incommensurate ordering of the
anion channels, which align with the primitive c axis, preventing
cell indexing. Instead, the identity of the structure was confirmed
using X-ray powder diffraction, which showed the material to
be identical in the bulk phase to compound 5 (ESI†).
a simple low-dimensional structure. One possible origin of
this conformational change is an increase in steric crowding
around the coordination sphere by incorporation of a second
coordinating piperazine, forcing a ligand rearrangement. Such
a coordination mode was not observed in structures 1–4, which
instead featured anion or solvent coordination to complete
the silver coordination spheres.
Discussion
Conclusions
Close inspection of the structures of compounds 1–5 reveals
several interesting trends. The presence of the recurring
looped chain structural motif in 1, 3 and 4 suggests a clear
preference for the entropically favoured low-dimensionality
assembly in the first instance. Despite the differing ligand
geometries, the structures of 1 and 4 are very closely related,
and effectively differ only by the trifluoromethanesulfonate
bridging mode and the presence of a weakly bound acetone
molecule in 4. Using tetrahydrofuran as the solvent for the
synthesis of 1 prevented the addition of coordinating acetone,
observed in 2–4, the presence of which altered the steric
environment in 2 sufficiently to force the formation of a
different extended structure.
We have prepared and characterised two new bis-cyanophenyl
piperazine ligands L1 and L2, and investigated their coordination
behaviour in the presence of Ag(I) ions. Ligand L1 produced
two related two-dimensional polymers, poly-[Ag2(L1)(CF3SO3)2] 1
and poly-[Ag2(L1)(CF3SO3)2(C3H6O)1.5] 2, when reacted with
silver trifluoromethanesulfonate in tetrahydrofuran or acetone,
respectively. The two structures displayed very similar ligand
geometries, however they differed in the coordination of solvent
molecules to the Ag(I) ion in compound 2, forcing a reorientation
of the L1 donor atoms around the coordination sphere. This
small change caused a significant difference to the nature of
the extended network between 1 and 2.
A structural dependence on reaction stoichiometry was
not observed for L1, despite numerous attempts to generate
an analogous network to 5, instead only giving phases 1 and
2 at metal : ligand ratios from 2 : 1 to 1 : 2. However, as
described above, altering the stoichiometry for silver triflate and
hexafluorophosphate adducts of L2 had the effect of forming
a higher dimensional assembly, where the silver coordination
sphere was saturated entirely by L2 binding sites.
The similarities in ligand geometry and binding mode
within all five structures can be highlighted by examination
of the Ag–Ag distances resulting from bridging through
piperazine and through the two nitrile arms. In all cases, L1
and L2 display the same coordination habit, linking two silver
ions in a trans-diaxial mode through the piperazine ring and
two additional silver ions through the nitrile arms. The Ag–Ag
distances across the piperazine bridges are approximately constant
throughout all structures, falling in the range 5.79–5.99 Å.
The Ag–Ag distances for silver ions linked by benzonitrile
arms display some variation between L1 and L2, as expected,
with Ag–Ag distances in the range 21.24–21.48 Å for 1 and 2
and 19.28–19.76 Å for 3–5. Allowing for the discrepancy caused
by the different phenyl substitution patterns between L1 and
L2, these values show remarkable consistency, given the variability
in the extended structures.
The divergence of the L2 geometry in 5 compared to the
other structures is revealed by examining the orientation of
the flexibly linked cyanobenzyl groups. In structures 1–4, a
convergent-type geometry conducive to the formation of
dinuclear loops is adopted by the L1 and L2 ligands, which
can be rationalised by the small Ag–N–CH2–C torsion angles,
all of which fall in the range 40–60° (ESI,† Fig. S1). The opposite
case is observed in 5, where the cyanobenzyl groups are oriented
in the opposite direction to the nearby piperazine-bound silver
ions (θ = 176.9–177.9°), effectively preventing the formation of
Three coordination polymers were prepared from L2, by
reaction with silver trifluoromethanesulfonate or hexafluoro-
phosphate in acetone. The complexes poly-[Ag2(L2)(C3H6O)2]2(PF6)
3 and poly-[Ag2(L2)(C3H8O)2(CF3SO3)2] 4 were structurally related
materials, where the one-dimensional chains present in 3 were
linked into a two-dimensional layer by incorporation of the
bridging trifluoromethanesulfonate anion in 4. Finally, complex
poly-[Ag(L2)]CF3SO3 5 was prepared by altering the stoichiometry
of the reaction between L2 and silver trifluoromethanesulfonate.
Complex 5 displays a three-dimensional PtS topology and contains
one-dimensional anion channels in which the noncentric
ordering of the trifluoromethanesulfonate species imposes lower
crystallographic symmetry on the overall assembly. The structural
features present in complexes 1 to 5 were rationalised by
examination of the key structural feature of the ligand geometry,
especially the torsion angle describing the twist of the pendant
aromatic arms relative to the conserved Ag–N(piperazine) bonding
arrangement. The variations in coordination tendency between
the anions and solvents employed in the study were also
explored.
Acknowledgements
This work is supported by the Science and Industry Endowment
Fund. Portions of this work were undertaken on the MX1
Macromolecular Crystallography beamline at the Australian
Synchrotron, Victoria, Australia. SRB and DRT acknowledge
the Australian Research Council for fellowships.
Notes and references
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