Self assembly of silver(i) coordination polymers formed through hydrogen bonding with a new ditopic heteroscorpionate ligand

Article abstract of DOI:10.1039/b906047b

The new ditopic heteroscorpionate ligand, (3-methoxy-4-hydroxyphenyl)bis(3, 5-dimethylpyrazolyl)methane (L5v), and four coordination polymers of silver Ag(i) derived therefrom, have been synthesized and characterized by X-ray diffraction, ESI-MS, IR spect

Full text of DOI:10.1039/b906047b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Self assembly of silver(I) coordination polymers formed through hydrogen
bonding with a new ditopic heteroscorpionate ligand†
Guillermo A. Santillan and Carl J. Carrano*
Received 26th March 2009, Accepted 29th May 2009
First published as an Advance Article on the web 2nd July 2009
DOI: 10.1039/b906047b
The new ditopic heteroscorpionate ligand, (3-methoxy-4-hydroxyphenyl)bis(3,5-dimethyl-
pyrazolyl)methane (L5v), and four coordination polymers of silver Ag(I) derived therefrom, have been
synthesized and characterized by X-ray diffraction, ESI-MS, IR spectroscopy and elemental analysis.
-
-
-
-
The presence of different anions (i.e. BF4 , CF3SO3 , NO3 , or SbF6 ) lead to different crystal packing
schemes due to different H-bonding patterns that depend on the relative charge delocalization and/or
shape of the anion.
scorpionate and heteroscorpionate parents to produce different
coordination polymers.14
Introduction
Thus in previous work,15 we have shown that 3- and
4-substituted carboxyphenyl heteroscorpionates (designated L4c
and L3c) coordinate silver(I) only through the pyrazole nitrogens
of the ligand leaving the uncoordinated carboxylic group free to
engage in self-complementary hydrogen bonding (Fig. 1: I or
II) that leads to a variety of solid-state polymers stabilized by
these interactions. To establish the generality of this approach,
herein we describe the preparation and crystal structure of a
new heteroscorpionate ligand (L5v) whose 4-hydroxy-3-methoxy
substitution is expected to lead to analogous self complimentary
H-bonded interactions in the solid-state (Fig. 1: III or IV). This
expectation is only partially realized and we discuss the influence
of the counteranion on the intermolecular interactions and solid-
state structures.
Self assembly of flexible achiral ligands with metal binding sites
has produced a wide variety of interesting solid state structures
that have attracted attention because of possible applications
in materials science.1 Simple complexes of silver(I) have been
extensively utilized as building blocks in this regard for the self
assembly of supramolecular coordination polymers of differing
dimensionality.2 These polymers are of interest not only because
of their remarkable molecular structures but also as a result
of their potential application as gas adsorption,3 antimicrobial,4
conductive material,5 luminescent,6 and magnetic materials.7
In these systems extensive use has been made of the extremely
strong preference displayed by silver(I) towards coordination
to aromatic nitrogen donors over those of the harder oxygen
ligands. Thus the synthesis and reactivity of silver(I) with nitrogen
heterocyclic ligands has been extensively investigated.8 Particu-
larly relevant is the extensive chemistry of silver with different
bis(pyrazolyl)borates and methanes in which the silver atoms are
invariably coordinated solely through nitrogen pyrazoles of the
ligand.9
While most higher order solid-state structures based on coor-
dination polymers utilize stable coordinate covalent bonds,10
a
variety of different non-covalent interactions such as hydrogen
bonding and p–p stacking can also be employed to this end.11
Of these the hydrogen bond is the strongest, most selective and
directional of these non-covalent interactions and thus potentially
an effective organizing interaction for designing and controlling
solid-state structures.12 In particular amino and hydroxyl groups
can be employed as linking units between two coordination
monomers because of their strong tendency towards the formation
of hydrogen bonds with their complement.13 Our approach has
been to make use of ditopic heteroscorpionate ligands, created
by modifications to the original facially coordinate tripodal
Fig. 1 Possible self-complementary hydrogen bonding.
Experimental
Department of Chemistry and Biochemistry, San Diego State University,
San Diego, CA 92182-1030. E-mail: carrano@sciences.sdsu.edu
Materials
† Electronic supplementary information (ESI) available: Crystal structure
showing the silver–pi arene distance interaction. CCDC reference numbers
720110–720114. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b906047b
All chemicals and solvents used during these syntheses were
reagents grade. Bis(3,5-dimethylpyrazolyl)ketone was prepared
according to the procedure described by The´ and Peterson.16
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Ligand synthesis
[Ag2(L5v)2](SbF6)2 (4). This complex was prepared in a
manner analogous to that of complex 1 using silver hex-
afluoroantimonate (AgSbF6) in place of silver tetrafluorobo-
rate (AgBF4). Yield: 43.8 mg (69%). Anal. Calcd (Found) for
[Ag2(L5v)2](SbF6)2·0.5THF, C38H48N8O4.5Sb2F12 Ag2: C, 33.17
(33.35); H, 3.52 (3.71); N, 8.14 (8.32). IR (KBr pellets) n/cm-1:
3420, 1605, 1560, 1519, 1456, 1426, 1390, 1291, 1272, 1245,
1173, 1134, 1030, 860, 820, 806, 798, 779, 702, 636, 606. ESI-
MS (acetonitrile): m/z [Ag2(L5v)2SbF6]+ = 1102 amu. The same
crystal packing was obtained by using silver hexafluorophosphate
AgPF6 instead of hexafluoroantimonate AgSbF6.
(3-Methoxy-4-hydroxyphenyl)bis(3,5-dimethylpyrazolyl) methane
(L5v). Bis(3,5-dimethylpyrazolyl)ketone (2.4 g, 11 mmol) was
placed in a 100 cm3 round-bottomed flask. 4-Hydroxy-3-
methoxybenzaldehyde (1.62 g, 10.8 mmol), CoCl2 (20 mg) and
triethylamine (4 mL) were then added. The mixture was heated
to 110 ◦C for 4 h with vigorous stirring during which time the
mixture turned deep purple and evolved CO2. The flask was
then allowed to cool to room temperature and the solid taken
up in water and filtered. The filtrate was neutralized with 6 M
hydrochloric acid. At ca. pH = 9.3 a pale brown solid began
to precipitate. This solid was collected by filtration, washed with
water and dried in vacuum. Yield: 68%. Anal. Calcd (Found) for
L5v·H2O, C18H24N4O3: C, 62.77 (62.76); H, 7.02 (7.70); N, 16.27
(16.17). 1H NMR (600 MHz, CDCl3) d: 2.20 (s, 6H, CH3), 2.21 (s,
6H, CH3), 3.76 (s, 3H, CH3), 5.85 (s, 2H, PzH), 6.38 (q, 1H, J =
7.8 Hz, ArH), 7.56 (s, 1H, CH), 6.47 (d, 1H, J = 7.7 Hz, ArH),
6.84 (d, 1H, J = 7.6 Hz, ArH). 13C NMR (CDCl3) d: 148.24,
146.65, 145.81, 141.07, 128.59, 120.08, 114.29, 109.71, 106.82,
73.92, 55.96, 13.72, 11.81. FT-IR (KBr) n/cm-1: 3519, 1558, 1530,
1464, 1392, 1250, 1223, 1128, 1034, 812, 794, 756. Melting point:
106 (1) ◦C.
Physical methods
Elemental analyses were performed on all compounds by Numega
laboratories, San Diego, CA. Samples were dried in vacuum prior
to analysis. 1H and 13C NMR spectra were collected on Varian 400
and 600 MHz NMR spectrometers. Chemical shifts are reported
in ppm relative to an internal standard of TMS. IR spectra were
recorded as KBr disks on a ThermoNicolet Nexus 670 FT-IR
spectrometer and are reported in wavenumbers. Electrospray mass
spectra (ESI-MS) were recorded in positive ion mode on an Agilent
6330 Series Ion Trap LC/MS system mass spectrometer equipped
with an ESI source. Samples were dissolved in acetonitrile and
eluted with acetonitrile containing 0.1% formic acid. Agilent 6330
Series software was used for data acquisition and plotting. Isotope
distribution patterns were simulated using the program IsoPro 3.0.
Silver complexes
[Ag(L5v)2]BF4 (1). A tetrahydrofuran solution (4 mL) of
ligand L5v (47.8 mg, 0.146 mmol) was added to a tetrahyrofuran
solution (4 mL) of silver tetrafluoroborate, AgBF4 (14.2 mg,
0.073 mmol), the mixture was covered with aluminium foil and
stirred for 1 min. The solution was centrifuged and layered
with hexanes. Colorless crystals were obtained after 2 d. Yield:
50.88 mg (82%). Anal. Calcd (Found) for [Ag(L5v)2]BF4·0.75THF,
C39H50N8O4.75BF4Ag: C, 51.96 (51.74); H, 5.59 (5.37); N, 12.43
(12.30). IR (KBr pellets) n/cm-1: 3519, 2960, 1601, 1560, 1522,
1465, 1385, 1310, 1252, 1223, 1126, 1084, 1033, 838, 812, 795, 757,
700. ESI-MS (acetonitrile): m/z [Ag(L5v)2BF4H]+ = 848 amu.
The same crystal packing was obtained by using silver perchlorate
AgClO4 in place of silver tetrafluoroborate AgBF4.
X-Ray crystal structure characterization
Crystals of complexes 1–4 were mounted on nylon loops with
paratone oil (Hampton Research), and placed in the cold stream
(240 K) of a Bruker X8 APEX CCD diffractometer operating at
˚
50 kV and 30 mA using Mo Ka radiation (l = 0.71073 A). Data
for the free ligand, L5v, were collected at room temperature. The
structures were solved using direct methods or via the Patterson
function, completed by subsequent difference Fourier syntheses,
and refined by full-matrix least-squares procedures on F2. All
non-hydrogen atoms were refined with anisotropic displacement
coefficients and treated as idealized contributions using a riding
model except where noted. The relatively large residual peaks
found in the final difference map of 4 are associated with the
antimony of the hexafluoroantimonate anion. All software and
sources of the scattering factors are contained in the SHELXTL
6.12 program library (G. Sheldrick, Siemens XRD, Madison,
WI).18 Crystallographic data for ligand L5v and silver complexes
1–4 are listed in Table 1 and selected bond lengths and angles
for all compounds are given in Tables 2 and 3. CCDC reference
numbers 720110–720114.†
[Ag(L5v)2]CF3SO3 (2). This complex was prepared in a
manner analogous to that of complex 1 using silver trifluo-
romethanesulfonate (AgOSO2CF3) in place of silver tetrafluorob-
orate (AgBF4). Yield: 47.5 mg (77%). Anal. Calcd (Found) for
[Ag(L5v)2]CF3SO3, C37H44N8O7SF3Ag: C, 48.85 (48.73); H, 4.88
(4.76); N, 12.32 (12.44). IR (KBr pellets) n/cm-1: 3300, 2975, 1606,
1560, 1519, 1463, 1426, 1390, 1291, 1273, 1244, 1173, 1030, 860,
839, 820, 798, 702, 636, 606.
[Ag(L5v)2]NO3 (3). The ligand L5v (22.2 mg, 0.068 mmol) was
dissolved in a mixture of acetonitrile : methanol (3 mL : 1 mL) and
added to a methanol solution (4 mL) of silver nitrate (5.78 mg,
0.34 mmol). The mixture was covered with aluminium foil and
stirred for 1 min. The colorless solution was left to stand at
room temperature and X-ray quality crystals were obtained after
a period of 6 d. Yield: 18.1 mg (64%). Anal. Calcd (Found) for
[Ag(L5v)2]NO3·0.25H2O, C36H44.5N9O7.25Ag: C, 52.27 (52.37); H,
5.42 (5.25); N, 15.24 (15.38). IR (KBr pellets) n/cm-1: 3316, 2985,
1600, 1559, 1520, 1459, 1385, 1319, 1276, 1239, 1124, 1039, 980,
856, 821, 807, 788, 702, 606.
Solid-state structure of the complexes
Silver complexes
The thermal ellipsoid diagrams for ligand (L5v) and silver com-
plexes [Ag(L5v)2]BF4 (1), [Ag(L5v)2]CF3SO3 (2), [Ag(L5v)2]NO3
(3) and [Ag2(L5v)2](SbF6)2 (4) are depicted in Fig. 2. Select bond
distances and angles for complexes 1–4 are shown in Tables 2
and 3. The reaction of ditopic heteroscorpionate ligand L5v
with silver tetrafluoroborate AgBF4, silver triflate AgCF3SO3,
6600 | Dalton Trans., 2009, 6599–6605
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Table 1 Crystallographic data and parameters for: ligand (L5v)·H2O, [Ag(L5v)2]BF4·THF (1), [Ag(L5v)2]CF3SO3·THF (2), [Ag(L5v)2]NO3 (3) and
[Ag2(L5v)2](SbF6)2·2THF(4)
L5v
1
2
3
4
Molecular formula
FW
T/K
C18H24N4O3
344.41
298(2)
Triclinic
¯
P1
7.8522(2)
9.8917(2)
12.2509(3)
80.3380(10)
77.7700(10)
88.9180(10)
916.62(4)
2
0.087
1.248
368
0.5 ¥ 0.4 ¥ 0.3
-11 → 11
-14 → 14
-18 → 13
2.48–31.77
21 884
C40H52N8O5BF4Ag
919.58
240(2)
Monoclinic
P21/c
9.3894(7)
24.7239(18)
18.8870(13)
90
97.518
90
4346.8(5)
4
0.532
1.405
1904
0.4 ¥ 0.2 ¥ 0.1
-10 → 11
-30 → 26
-23 → 23
2.18–26.22
42 624
C41H52N8O8SF3Ag
981.84
240(2)
Triclinic
¯
P1
12.3951(10)
12.7251(11)
16.9436(14)
77.654(3)
71.812(3)
62.334(3)
2241.1(3)
2
0.568
1.455
1016
0.3 ¥ 0.2 ¥ 0.2
-15 → 15
-16 → 16
-18 → 21
20.2–27.11
41 858
C36H44N9O7Ag
822.67
240(2)
Monoclinic
P21/c
9.5688(4)
15.7555(7)
24.7812(12)
90
91.724(2)
90
3734.4(3)
4
0.600
C44H58N8O6F12Sb2Ag2
1482.22
240(2)
Monoclinic
C2/c
15.0535(4)
17.1969(4)
20.9904(6)
90
92.663(2)
90
5428(2)
4
1.789
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
m/mm-1
r
calc/g cm-3)
F(000)
1.463
1704
1.814
2920
Size/mm3
0.3 ¥ 0.3 ¥ 0.3
-10 → 10
-17 → 17
-27 → 27
2.59–23.26
33 620
0.4 ¥ 0.4 ¥ 0.3
-17 → 17
-20 → 17
-24 → 24
2.56–24.93
48 734
h, k, l ranges collected
q range/◦
Reflections collected
Independent reflections
Parameters
6243
235
8701
532
9732
559
5359
478
4737
336
Data/parameters
26.56
0.0886
0.2163
1.052
16.35
0.0711
0.1507
1.015
17.40
0.0758
0.1319
1.027
11.21
0.0504
0.1057
1.027
14.09
0.0517
0.1217
1.032
R(F)a
Rw(F2)b
GOFc
-3
˚
Largest diff peak and hole/e A
0.422 and -0.323
0.907 and -0.719
0.658 and -0.523
0.485 and -0.334
2.196 and -1.406
ꢀ
ꢀ
ꢀ
ꢀ
a R = [ |DF|/ |Fo|] for [I > 2s(I)]. b Rw = [ w(DF)2/ wFo2] for [I > 2s(I)]. c Goodness of fit on F2.
˚
by four pyrazole nitrogens of the chelating ligand in a pseudo-
tetrahedral geometry with a structural index parameter (t4)19 of
0.64 (complex 1), 0.52 (complex 2) and 0.63 (complex 3). The
dihedral angle between N(1)–Ag(1)–N(3) and N(5)–Ag(1)–N(7)
planes are 81.88◦ in (1), 62.07◦ in (2) and 76.65◦ in (3) all of which
are much closer to the 90◦ angle expected of a tetrahedron, than
the 0◦ expected for the alternate square planar configuration. The
largest N(3)–Ag–N(5) angles varies from 153.77◦ in (1) to 159.56◦
in (2) while that of (3) is intermediary at 158.40◦. The remaining
angles are in the range between 79.46 to 126.43◦. Silver nitrogen
bond lengths are asymmetric with most values falling between
Table 2 Selected bond lengths (A) for: [Ag(L5v)2]BF4·THF (1),
[Ag(L5v)2]CF3SO3·THF (2), [Ag(L5v)2]NO3 (3) and [Ag2(L5v)2]-
(SbF6)2·2THF (4)
1
2
3
4
Ag(1)–N(1)
Ag(1)–N(1)#1
Ag(1)–N(3)
Ag(1)–N(3)#1
Ag(1)–N(5)
Ag(1)–N(7)
2.401(3)
2.260(3)
2.449(3)
2.307(2)
2.368(3)
2.265(3)
2.138(4)
2.138(4)
2.126(4)
2.126(4)
2.277(3)
2.370(3)
2.385(3)
2.344(3)
2.252(3)
2.407(3)
Ag(1)–Ag(1)#1
3.02336(7)
˚
2.26 and 2.44 A.
The crystal packing architecture of (1) consists of self-
complementary hydrogen bonding between hydroxyl groups
Table 3 Selected angles (◦) for: [Ag(L5v)2]BF4·THF (1), [Ag(L5v)2]-
CF3SO3·THF (2), [Ag(L5v)2]NO3 (3) and [Ag2(L5v)2](SbF6)2·2THF·(4)
˚
(av. O ◊ ◊ ◊ O 2.915 A) leading to 1-D chains. The counteranion
1
2
3
4
-
BF4 takes part in several CH ◊ ◊ ◊ F weak hydrogen bonding
interactions that support the assembly by bridging between the
N(3)–Ag(1)–N(1)
N(5)–Ag(1)–N(1)
N(7)–Ag(1)–N(1)
N(5)–Ag(1)–N(3)
N(7)–Ag(1)–N(3)
N(7)–Ag(1)–N(5)
N(1)#1–Ag(1)–Ag(1)#1
N(3)#1–Ag(1)–Ag(1)#1
N(3)–Ag(1)–N(1)#1
82.74(10) 79.46(9) 84.09(10) 172.98(15)
˚
silver polymer chains. The CH ◊ ◊ ◊ F interaction average is 2.55 A
107.80(11) 159.56(9) 112.56(10)
107.35(10) 109.00(9) 110.40(9)
153.77(12) 108.49(9) 158.40(10)
115.41(11) 126.43(9) 104.71(10)
84.98(11) 81.89(9) 82.95(10)
(Fig. 3).
For complex 2, as was the case for 1, the crystal packing
shows the formation of a racemic mixture of 1-D coordination
polymer chiral helices (Fig. 4). However in this case they are
formed, not by self-complementary hydrogen bonding between
the uncoordinated hydroxyl groups, but rather through hydrogen
bonding between hydroxyl groups on adjacent molecules bridged
through an uncoordinated triflate counterion (av. O ◊ ◊ ◊ O separa-
71.29(10)
107.59(10)
172.98(15)
and silver nitrate AgNO3 smoothly yielded complexes with a
ratio of 1Ag : 2L : 1Anion. All these complexes show the same
environment about the silver Ag(I) cation, which is coordinated
˚
tion 2.73 A). The helices are held together by weak p–p interactions
˚
with a distance of 3.8 A.
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Fig. 2 Thermal ellipsoid diagram for silver complexes 1, 2, 3, 4 and ligand L5v at the 50% probability level.
The crystal packing in 3 (Fig. 5) reveals yet a third motif where
the silver monomers are linked by hydrogen bonds between the
hydroxyl groups and the bridging nitrate anion to produce solid-
state dimers. These dimers are also held together by weak hydrogen
bonding between the methyl group C–H and the oxygen from the
linear coordination with a N(1)–Ag(1)–N(3) angle of 173.2◦.
˚
The silver atom separation is 3.023 A, less that the sum of
˚
silver–silver van der Waals radii (3.44 A), indicative of a weak
Ag–Ag interaction.21 The bridging Ag–Ag bonding is interesting
and unusual for poly(pyrazolyl)methane type ligands.22 The Ag–N
˚
˚
methoxy group (av. 3.15 A).
bond distances are symmetrical at 2.13 A. This complex exhibits
˚
The reaction of ditopic heteroscorpionate ligand L5v with
silver hexafluoroantimonate AgSbF6 in tetrahydrofuran produced
complex 4 which is completely different from all the other
complexes isolated in this study in that a 1Ag : 1L rather than a
1Ag : 2L stoichiometry is seen. Compound 4 crystallizes in space
group C2/c and displays crystallographic inversion symmetry.
The structure determination reveals the presence of discrete
dimeric species [Ag2L2](SbF6)2 as depicted in Fig. 2. Each silver is
equatorially surrounded by two pyrazole nitrogens, one each from
two different L5v ligands. The silver atoms exhibit a distorted
cation–p interaction (Ag-centroid distance 3.2 A) between the
silver and aromatic ring of the ligand.
The crystal packing architecture of 4 reveals that the counteran-
ion SbF6 takes part in a series of C–H ◊ ◊ ◊ F hydrogen bonding
interactions that support the assembly by bridging between the
dinuclear complexes. The C–H ◊ ◊ ◊ F “bond” distances is in the
-
˚
range of 2.48 A. In addition to the interactions between the anions
and the silver complex, there is also hydrogen bonding between the
hydroxyl group of the ligand and a tetrahydrofuran of solvation,
but these hydrogen bonds do not contribute to the development
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Fig. 5 Crystal packing diagram for (3) as seen along crystallographic
a axis. Hydrogen bonding bonds (O ◊ ◊ ◊ H ◊ ◊ ◊ O and CH ◊ ◊ ◊ O) are shown as
dotted lines.
Fig. 3 Crystal packing diagram for (1) as seen along crystallographic
a axis. Hydrogen bonding bonds (O ◊ ◊ ◊ H ◊ ◊ ◊ O and CH ◊ ◊ ◊ F) are shown as
dotted lines. Solvent molecules are omitted.
Fig. 6 Crystal packing diagram for (4) as seen along crystallographic
b axis. Hydrogen bonds (O ◊ ◊ ◊ H ◊ ◊ ◊ O and CH ◊ ◊ ◊ F) are shown as dotted
lines.
Results and discussion
Synthesis and characterization
In previous work,15 we showed that Ag(I) binds the ditopic
heteroscorpionates L4c and L3c exclusively through the softer
pyrazole nitrogen donors of the ligand, leaving the carboxylic acid
group uncoordinated and thus free to link the monomeric units
together via self-complementary hydrogen bonding interactions
or hydrogen bonding with solvent molecules or counter ions.
In this study, we sought to determine if such a strategy was
general by changing the hydrogen bonding functional group
from a carboxylic acid to a 4-hydroxy-3-methoxyphenyl moiety.
The reaction of the ditopic heteroscorpionate ligand designated
L5v with silver Ag(I) did indeed produce coordination polymers
either by self-complementary hydrogen bonding between the
hydroxymethoxy moieties or via their hydrogen bonding with
bridging anions. Thus, the reaction of the ligand L5v with silver
tetrafluoroborate (AgBF4) in tetrahydrofuran solvent, the polymer
isolated has a [1Ag : 2L : 1BF4] ratio where the free hydroxyl
group of the ligand engages in intermolecular self-complementary
hydrogen bonding to yield a 1-D coordination polymer. On the
other hand the use of AgCF3SO3 as a starting material results
in a product with similar stoichiometry but the anion triflate
bridges between free hydroxyl groups of the chelating ligands
between adjacent molecules leading to a racemic mixture of chiral
linear coordination polymers. In methanol, the reaction of ligand
L5v with silver nitrate AgNO3 as a metal source also leads to
(1M : 2L : 1Anion) stoichiometry but with the formation of solid-
Fig. 4 (a) Crystal packing diagram for 2 showing the two chiral helices.
Hydrogen bonds (O–H ◊ ◊ ◊ O) are shown as dotted lines. (b) Crystal packing
diagram for two molecules of 2 as seen along the crystallographic a axis;
p–p interactions are shown as dotted lines. Solvent molecules and hydrogen
atoms are omitted for clarity.
of a coordination polymer (Fig. 6). Indeed the hydrogen bonding
by the THF of solvation serves to “cap” the free hydroxyl groups
and prevents them from forming the self-complementary hydrogen
bonding seen in 1.
-
state dimers where the nitrate anion (NO3 ) bridges between
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hydroxyl groups. The situation is different however when silver
hexafluoroantimonate (AgSbF6) is used in tetrahydrofuran where
discrete molecular dimers of [2M : 2L : 2SbF6] stoichiometry are
obtained.
Anion bridging
In our research, we have repeatedly observed that the anions
-
-
nitrate, NO3 , and triflate, CF3SO3 , behave significantly different
-
-
than anions such as perchlorate (ClO4 ), tetrafluoroborate (BF4 ),
hexafluoroantimonate (SbF6 ) etc. in that they invariably form
-
hydrogen bonds that bridge between the carboxylic acid or
hydroxyl groups in the solid-state. That is they tend to break
up the strong self-complementary hydrogen bonding between
these groups found with the non-bonding spherically symmetric
anions. To date only a few examples of nitrate and triflate anion
bridging via hydrogen bonding are known.20 This interaction
implies that the nitrate or triflate hydrogen bonding with the
carboxylate or hydroxyl centers is stronger than that provided by
the corresponding ligand based self-complementary. We attribute
this tendency to a difference in the charge density among the
various anions where the negative charge in nitrate and triflate is
less delocalized compared to non-bonding spherical anions such
-
-
as perchlorate (ClO4 ), tetrafluoroborate (BF4 ) or hexafluoroan-
-
timonate (SbF6 ) (Scheme 1).
Fig. 7 Left: representation of the solid-state polymers produced by
self-complementary hydrogen bonding between L4c ligands in a Ni(II)
complex (upper) and L5v ligands in a Ag(I) complex (lower) with spherical
anions such as perchlorate or tetrafluoroborate. Right: representation of
the corresponding solid-state polymers produced with the non-spherical
triflate or nitrate showing insertion of the anions between the ligands.
Scheme 1 Representation of anions with delocalized charge.
This tendency is well illustrated in our previous work,17 where
we showed that a meso-helical solid-state polymer was formed
via self-complementary hydrogen bonding between carboxylate
groups in a Ni(II) complex with the carboxylate ligand L4c in
the presence of perchlorate. However in the presence of nitrate,
although a helical polymer was still produced, the nitrate ion
disrupted the self-complementarity between ligands and bridged
between the carboxylate units in the solid-state (Fig. 7, top). A
similar situation occurs with the new L5v ligand with Ag(I) as the
cation. Again in the presence of spherically symmetric anions a
1-D solid-state polymer chain is produced by self-complementary
hydrogen bonding while in the presence of triflate the ligand–
ligand hydrogen bonding is broken and the interaction replaced
by hydrogen bonding between the bridging anion and the ligand
functional group (Fig. 7, bottom). We have found many other
examples of this, thus it seems to be a representative phenomenon.
producing chiral silver monomers (tectons) and leaving hydroxyl
groups uncoordinated, free to engage in intermolecular hydrogen
bonding. However the resulting packing contains alternating
chiral monomers forming an overall racemic mixture. Moreover,
the shape and size of the counteranions play an important role
and dictate the crystal packing of these silver complexes. Future
work will involve the demethylation of the L5v ligand to obtain the
potentially “bis-bidentate” catecholate heteroscorpionate ligand.
Acknowledgements
This work was supported in part by NSF Grant CHE-0313865.
The NSF-MRI program Grant CHE-0320848 is also gratefully
acknowledged for support of the X-ray diffraction facilities at San
Diego State University.
Conclusion
Notes and references
The silver(I) chemistry with the ditopic heteroscorpionate ligand
L5v in the solid-state has been explored. This ligand has demon-
strated the ability to yield a series of new silver complex derivatives
showing both discrete and polymeric architectures. This flexible
bidentate ligand is bound asymmetrically to the silver cation,
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