Ag(I)-based tectons for the construction of helical and meso-helical hydrogen-bonded coordination networks

Article abstract of DOI:10.1021/ic701919r

The new ligand 4-(3-(2-pyridyl)pyrazol-1-ylmethyl)benzoic acid (L) has been prepared and characterized. This bifunctional ligand incorporates both a chelating region, with two nitrogen donors, suitable for chelating to soft transition metal ions, and a self-complementary hydrogen-bonding region which can facilitate intermolecular association of ligands or ligand-based complexes. X-ray structural analysis of the ligand shows it to adopt a one-dimensional helical polymeric structure, with adjacent ligands hydrogen bonded to each other. Reaction of L with silver(I) salts (AgOTf (1, 1·1.5H ₂O), AgNO₃ (2), AgPF₆ (3·CH ₃OH), and AgClO₄ (4·CH₃OH)) results in the formation of complexes with 2:1 stoichiometries. X-ray structural analysis of these complexes shows that, in each case, one-dimensional chain structures are obtained where chiral AgL₂ tectons are hydrogen bonded together, either directly or mediated by anions or solvent. Structures with either helical or meso-helical structures are observed.

Full text of DOI:10.1021/ic701919r

Inorg. Chem. 2008, 47, 592−601
Ag(I)-Based Tectons for the Construction of Helical and meso-Helical
Hydrogen-Bonded Coordination Networks
David A McMorran*
Department of Chemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand
Received September 28, 2007
The new ligand 4-(3-(2-pyridyl)pyrazol-1-ylmethyl)benzoic acid (L) has been prepared and characterized. This
bifunctional ligand incorporates both a chelating region, with two nitrogen donors, suitable for chelating to soft
transition metal ions, and a self-complementary hydrogen-bonding region which can facilitate intermolecular association
of ligands or ligand-based complexes. X-ray structural analysis of the ligand shows it to adopt a one-dimensional
helical polymeric structure, with adjacent ligands hydrogen bonded to each other. Reaction of L with silver(I) salts
(AgOTf (1, 1‚1.5H2O), AgNO3 (2), AgPF6 (3‚CH3OH), and AgClO4 (4‚CH3OH)) results in the formation of complexes
with 2:1 stoichiometries. X-ray structural analysis of these complexes shows that, in each case, one-dimensional
chain structures are obtained where chiral AgL2 tectons are hydrogen bonded together, either directly or mediated
by anions or solvent. Structures with either helical or meso-helical structures are observed.
Introduction
of metal ion and donor atom, such as palladium(II) or silver-
(I) and pyridine nitrogen donors, have become very familiar
in the literature. However, incorporation of less labile metal
ions, such as ruthenium(II), present problems using this
‘metal ions as glue’ approach.
As interest in the field of metallosupramolecular chemistry
continues to grow, so too has the range of strategies
concerning how to deliberately incorporate transition metal
ions into supramolecular systems. For much of its develop-
ment, the concept of using transition metal ions as ‘glue’ to
assemble suitable organic fragments into a whole has been
a dominant theme in metallosupramolecular synthesis.1 By
taking advantage of the unique geometrical and electronic
properties of certain transition metal ions, a vast array of
structures with fascinating structural and functional properties
has been reported.2 An underlying paradigm in the construc-
tion of these systems is that they are formed by the self-
assembly of suitable components, or tectons, (transition metal
ions and ligands) and that the covalent bonds formed between
the metal ion and the ligand donor atom(s) during the
assembly process are labile enough that a self-correction of
the assembly is possible. For this reason, some combinations
More recently a second theme has emerged, that of using
noncovalent intermolecular interactions to assemble suitable
transition metal ion containing fragments into a supramo-
lecular structure. Such an approach overcomes the problems
associated with nonlabile metal ion-ligand bonds by using
the metal complex as a whole as the tecton for the assembly
process. While various suitable intermolecular interactions
might be imagined, in practice the one which has been most
explored is hydrogen bonding.3 The strength and, in par-
ticular, the directionality of hydrogen bonds make them an
ideal way of rationally assembling supramolecular systems,
as is clearly seen in biology and exemplified in the structure
of DNA. In order to maximize these properties of hydrogen
bonds, however, two factors must be considered, the relative
orientations of the hydrogen bond donor(s) and acceptor(s)
and also the numbers of donors and acceptors. With this in
* E-mail: davidm@chemistry.otago.ac.nz.
(1) (a) Steel, P. J. Chem. N.Z. 2003, 67, 57-60. (b) Holliday, B. J.; Mirkin,
C. A. Angew. Chem., Int. Ed. 2001, 40, 2022-2043. (c) Swiegers, G.
F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483-3537. (d) Baxter, P.
N. W. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L.,
Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J.-M., Eds.;
Pergammon Press: New York 1996; Vol. 9, pp 166-211.
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You, C. C.; Saha-Moller, C. R. Chem. Soc. ReV. 2004, 33, 133-146.
(c) Janiak, C. Dalton Trans. 2003, 2781-2804. (d) Siedel, S. R.; Stang,
P. J. Acc. Chem. Res. 2002, 35, 972-983. (e) Dinolfo, P. H.; Hupp,
J. T. Chem. Mater. 2001, 13, 3113-3125.
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Champness, N. R. Cryst. Eng. Comm. 2005, 7, 1-19. (b) Kitagawa,
S.; Uemura, K. Chem. Soc. ReV. 2005, 34, 109-119. (c) Roesky, H.
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592 Inorganic Chemistry, Vol. 47, No. 2, 2008
10.1021/ic701919r CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/08/2007

Construction of Hydrogen-Bonded Coordination Networks
Scheme 1 Bifunctional Ligand, L, Showing the Orthogonal Binding
Sites
mind, various reliable synthons, such as carboxylic acids,
oximes, and amides have been explored in the area of organic
crystal engineering.4 Since the early elucidation of the
concept,5 a number of workers have reported the assembly
of transition metal complex tectons in which such hydrogen-
bonding synthons, remote from the metal binding site of
the ligand, assemble either in self-complementary fashion
or to other organic species to form infinite metallosupramo-
lecular systems with various structures.3 In particular, a
number of reports of such systems using nicotinic acid and
its derivatives have appeared.6 It is noteworthy, however,
that few of these explore ligands with chelating donor
sites, thereby limiting some of the inherent geometrical
information imparted by the transition metal ion, which can
be lost due to the flexibility in orientation of the coordinated
ligands.
While this new approach has led to the preparation of
various types of metallosupramolecular systems, one par-
ticular type of structure, the helix, which has featured as a
favorite target of metallosupramolecular chemistry since its
inception,7 has been relatively elusive. Helices are of interest
because of their inherent chirality, and both discrete helicates
and helical polymeric structures have received a great deal
of attention.8 In the ‘metal ions as glue’ approach, helices
have been prepared by the wrapping of organic ligands with
multiple metal binding sites around metal ions lying on a
screw axis,9 and much effort has been devoted to the
development of such ligands.10 In contrast, reports of helices
assembled using hydrogen bonding are much less common.11
One approach has been to use hydrogen bonding to assemble
the organic ligands which wrap around the metal ions.12
However, most of the reported examples appear to have
resulted serendipitously from intermolecular OH‚‚‚halogen13
or CH‚‚‚halogen14 interactions. Instances where more reliable
synthons have been deliberately employed are rare.
The current work reports the synthesis and characterization
of the new bifunctional ligand 4-(3-(2-pyridyl)pyrazol-1-
ylmethyl)benzoic acid, L, which has been designed with two
orthogonal binding sitessa metal ion chelating site and a
remote hydrogen-bonding surface (Scheme 1). Ligands based
on the 2-pyridyl-pyrazole chelating system have been shown
by us15 and others16 to both form stable metal complexes
and to have high structural diversity due to the ready
elaboration of the pyrazole ring by various organic fragments.
Reaction of this new ligand with silver(I) salts gives
complexes with 2:1 ligand to metal ratios. As expected, the
Ag(I) ions bind exclusively to the softer N donors, rather
than the harder COOH donors which instead, in all cases,
facilitate the formation of infinite one-dimensional polymeric
structures by the assembly of AgL2 tectons. Importantly,
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Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97, 2005-2062.
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J.-M., Eds.; Pergammon Press: New York 1996; Vol. 9, pp 213-
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Qiao, Y.-Q.; He, J.-R.; Pan, M.; Kang, B.-S.; Su, C.-Y. Cryst. Growth
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C.-X.; Araki, T.; Ueda, I. Inorg. Chem. Commun. 2003, 6, 165-167.
(h) Tuna, F.; Hamblin, J.; Clarkson, G.; Errington, W.; Alcock, N.
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J.; Sumby, C. J. Inorg. Chem. Commun. 2002, 5, 323-327. (j)
Caradoc-Davies, P. L.; Hanton, L. R. Chem. Commun. 2001, 1098-
1099.
(9) Khatua, S.; Stoeckli-Evans, H.; Harada, T.; Kuroda, R.; Bhattacharjee,
M. Inorg. Chem. 2006, 45, 9619-9621.
(10) Constable, E. C. Prog. Inorg. Chem. 1994, 42, 67-138.
(11) (a) Xiao, D.-R.; Wang, E.-B.; An, H.-Y.; Li, Y.-G.; Xu, L. Cryst.
Growth Des. 2007, 7, 506-512. (b) Enamullah, M.; Sharmin, A.;
Hasegawa, M.; Hoshi, T.; Chamayou, A.-C.; Janiak, C. Eur. J. Inorg.
Chem. 2006, 2146-2154. (c) Zhang, Y.; Jianmin, L.; Nishiura, M.;
Deng, W.; Imamoto, T. Chem. Lett. 1999, 1287-1288. (d) Radford,
J. D.; Vittal, J. J.; Wu, D. Angew. Chem., Int. Ed. 1998, 37, 1114-
1116.
(12) (a)Telfer, S. G.; Kuroda, R. Chem.-Eur. J. 2005, 11, 57-68. (b) Telfer,
S. G.; Sato, T.; Kuroda, R. Angew. Chem., Int. Ed. 2004, 43, 581-
584.
(13) (a) Lonnon, D. G.; Colbran, S. B.; Craig, D. C. Eur. J. Inorg. Chem.
2006, 1190-1197. (b) Pradeep, C. P.; Zacharias, P. S.; Das, S. K.
Eur. J. Inorg. Chem. 2005, 3405-3408.
(14) (a) Prabhakar, M.; Zacharias, P. S.; Das, S. K. Inorg. Chem. 2005,
44, 2585-2587. (b) Balamurugan, V.; Hundal, M. S.; Mukherjee, R.
Chem.-Eur. J. 2004, 10, 1683-1690. (c) Balamurugan, V.; Jacob, W.;
Mukherjee, J.; Mukherjee, R. Cryst. Eng. Comm. 2004, 6, 396-400.
(15) (a) McMorran, D. A.; Steel, P. J. Inorg. Chem. Commun. 2003, 6,
43-47. (b) McMorran, D. A.; Steel, P. J. Chem. Commun. 2002,
2120-2121.
(16) Argent, S. P.; Adams, H.; Riis-Johannessen, T.; Jeffery, J. C.; Harding,
L. P.; Clegg, W.; Harrington, R. W.; Ward, M. D. Dalton Trans. 2006,
4996-5013 and references therein.
Inorganic Chemistry, Vol. 47, No. 2, 2008 593

McMorran
UNITYINOVA spectrometer at 298 K, referenced to the internal
solvent signal. IR spectra were recorded on a Perkin-Elmer
Spectrum BX FT-IR spectrometer using KBr discs. Microanalyses
were performed at the Campbell Microanalytical Laboratory at the
University of Otago. Mass Spectra were collected on a Bruker
micrO-TOF-Q spectrometer.
Scheme 2. Assembly of Chiral Tectons to Give Helical or
meso-Helical Polymeric Structures
Caution: Although no problems were encountered in this work,
transition metal perchlorates are potentially explosiVe. They should
be prepared in small amounts and handled with care.
4-(3-(2-Pyridyl)pyrazol-1-ylmethyl)benzoic acid (L). 4-Bro-
momethylbenzoic acid (1.00 g, 4.65 mmol) and 3-(2-pyridyl)-
pyrazole (675 mg, 4.65 mmol) were refluxed together in a mixture
of benzene (15 mL), 40% NaOH(aq) (15 mL) and tetrabutylam-
monium hydroxide (40% solution, 5 drops) for ca. 18 h. The
solution was then cooled, the layers separated, and the aqueous
layer washed with ethyl acetate (20 mL). The aqueous layer was
acidified to pH 6 with concd HCl and then extracted with ethyl
acetate (3 × 20 mL). This organic layer was washed with water (2
× 10 mL) and brine (1 × 10 mL) and dried over MgSO4. Removal
of the solvent gave a golden oil which solidified on standing to a
pale yellow solid. Recrystallization from hot ethyl acetate/ethanol
1:1 gave a pale yellow crystalline solid. Yield 53%. mp 207-208
°C. Anal. Calcd for C16H13N3O2: C, 68.81; H, 4.69; N, 15.04.
Found: C, 68.98; H, 4.85; N, 15.32. 1H NMR (CD3OD, 300 MHz)
δ 5.54 (s, 2H, CH2), 6.97 (d, 1H, He), 7.37 (m, 1H, Hb), 7.38 (d,
2H, Hg), 7.82 (d, 1H, Hf), 7.90 (td,1H, Hc), 8.01 (m, 1H, Hd), 8.02
(d, 2H, Hh), 8.56 (d, 1H, Ha). Selected IR (KBr, cm-1): νmax 3458
(OH), 1699 (CdO), 1603, 1493, 1272 (C-O).
[Ag(L)2]OTf (1). AgOTf (13.8 mg, 0.054 mmol) was dissolved
in methanol (1 mL) and added to L (30.0 mg, 0.108 mmol) in
methanol (8 mL) to give a clear, pale brown solution. Slow
evaporation of the solution gave the product as colorless crystals.
Yield 67%. mp >240 °C. Anal. Calcd for C33H26N6O7AgSF3: C,
48.60; H, 3.21; N, 10.30. Found: C, 48.72; H, 3.22; N, 10.38. 1H
NMR (CD3OD, 300 MHz): δ 5.26 (s, 2H, CH2), 7.01 (d, 2H, Hg),
7.09 (d, 1H, He), 7.38 (dd, 1H, Hb), 7.56 (d, 2H, Hh), 7.99 (m, 2H,
Hc,Hd), 8.09 (d, 1H, Hf), 8.18 (d, 1H, Ha). Selected IR (KBr, cm-1):
νmax 3142 (OH), 1727, 1696 (CdO), 1603, 1501, 1296 (C-O),
1223 (OTf-), 1153 (OTf-), 1107 (OTf-), 1025 (OTf-). Slow
evaporation of an acetonitrile/methanol solution of 1 gave a small
number of crystals, which were determined by X-ray crystal-
lography to have the composition [AgL2]OTf‚1.5H2O (1‚1.5H2O).
[AgL2]NO3 (2‚H2O). Complex 2‚H2O was prepared from AgNO3
as above to give a white crystalline solid. Yield 66%. mp 203-
204 °C. Anal. Calc for C32H28N7O8Ag: C, 51.48; H, 3.78; N, 13.12.
Found: C, 51.49; H, 3.66; N, 13.34. 1H NMR (CD3OD, 300
MHz): δ 5.31 (s, 2H, CH2), 7.09 (m, 3H, He, Hg), 7.40 (dd, 1H,
Hh), 7.63 (d, 2H, Hh), 7.98 (m, 2H, Hc, Hd), 8.09 (d, 1H, Hf), 8.24
(d, 1H, Ha). Selected IR (KBr, cm-1): νmax 3447 (OH), 1700 (Cd
O), 1603, 1492, 1383 (NO3-), 1270 (C-O).
these tectons possess axial chirality due to the way in which
the ligands coordinate to the metal ion. In two of the five
complexes structurally characterized, 1-dimensional helical
chains are found in which AgL2 tectons of the same chirality
are hydrogen bonded together, either directly or mediated
by anions. The resulting overall structures contain both M
(left handed) and P (right handed) helical chains. In the other
three structures, AgL2 tectons of alternating chirality are
hydrogen bonded together, either directly or mediated by
solvent molecules, to give achiral meso-helical chains. While
the helix (a three-dimensional transformation of a circle) is
a very common structural motif in metallosupramolecular
chemistry, as discussed above, the meso-helix (a three-
dimensional transformation of a lemniscate or figure-eight)
is very much less common.17 However, both are equally valid
ways of assembling the AgL2 tectons, with, on the one hand,
the tectons segregated according to their chirality in the
overall structure whereas, on the other, tectons of both
chiralities are accommodated into the one structural motif
(Scheme 2).
The current examples are, to our knowledge, the first
metal-containing meso-helical structures to be prepared using
hydrogen bonding. A related organic example, the hydrogen-
bonded meso-helical structure of m-phenylene dioxamic acid
diethyl ester, has been reported.18
Experimental Section
General. The ligand precursors 4-bromomethylbenzoic acid19
and 3-(2-pyridyl)pyrazole20 were prepared by published procedures.
All other chemicals were purchased commercially and used as
1
received. H NMR spectra were recorded on a 300 MHz Varian
(17) (a) Burchell, T. J.; Puddephatt, R. J. Inorg. Chem. 2006, 45, 650-
659. (c) Luan, X.-J.; Cai, X.- H.; Wang, Y.-Y.; Li, D.-S.; Wang, C.-
J.; Liu, P.; Hu, H.-M.; Shi, Q.-Z.; Peng, S.-M. Chem. Eur. J. 2006,
12, 6281-6289. (d) Son, S. U.; Park, K. H.; Kim, B. Y.; Chung, Y.
K. Cryst. Growth Des. 2003, 3, 507-512. (e) Cai, Y.-P.; Zhang, H.-
X.; Xu, A.- W.; u, C.-Y.; Chen, C.-L.; Liu, H.- Q.; Zhang, L.; Kang,
B.-S. J. Chem. Soc. Dalton Trans. 2001, 2429-2434. (f) Plasseraud,
L.; Maid, H.; Hampel, F.; Saalfrank, R. W. Chem. Eur. J. 2001, 7,
4007-4011. (g) Ratilainen, J.; Airola, K.; Fro¨lich, R.; Nieger, M.;
Rissanen, K. Polyhedron 1999, 18, 2265-2273. (h) Airola, K.;
Ratilainen, J.; Nyro¨nen, T.; Rissanen, K. Inorg. Chim. Acta 1998, 277,
55-60.
(18) Blay, G.; Ferna´ndez, I.; Pedro, J. R.; Ruiz-Garc´ıa, R.; Carmen Munoz,
M.; Cano, J.; Carrasco, R. Eur. J. Org. Chem. 2003, 1627-1630.
(19) Schuster, M. C.; Mann, D. A.; Buchholz, T. J.; Johnson, K. M.;
Thomas, W. D.; Kiessling, L. L. Org. Lett. 2003, 5, 1407-1410.
(20) Amoroso, A. J.; Cargill Thompson, A. M.; Jeffrey, J. C.; Jones, P. L.;
McCleverty, J. A.; Ward, M. D. J. Chem. Soc. Chem. Commun. 1994,
2751-2752.
[Ag(L)2]PF6 (3‚CH3OH). Complex 3‚CH3OH was prepared as
for 1 to give a colorless crystalline solid. Yield 67%. mp 222-225
°C. Anal. Calcd for C33H30N6O5AgPF6: C, 46.94; H, 3.58; N, 9.95.
Found: C, 47.11; H, 3.48; N, 10.06. 1H NMR (CD3OD, 300
MHz): δ 5.26 (s, 2H, CH2), 7.01 (d, 2H, Hg), 7.09 (d, 1H, He),
7.38 (dd, 1H, Hb), 7.56 (d, 2H, Hh), 7.99 (m, 2H, Hc,Hd), 8.09 (d,
1H, Hf), 8.19 (d, 1H, Ha). Selected IR (KBr, cm-1): νmax 3448
(OH), 1698 (CdO), 1603, 1493, 1273 (C-O), 847, 560 (PF6-).
ES-MS (CH3CN): 667.1105 [AgL2]+, 386.0228 [AgL]+ (100%),
280.1104 [L]+.
[Ag(L)2]ClO4 (4‚CH3OH). Complex 4‚CH3OH was prepared as
for 1 to give a colorless crystalline solid. Yield 66%. mp > 230
°C. Anal. Calcd for C33H30N6O9AgCl: C, 49.67; H, 3.79; N, 10.52.
594 Inorganic Chemistry, Vol. 47, No. 2, 2008

Construction of Hydrogen-Bonded Coordination Networks
Table 1. Crystal Data for L and 1-4‚CH3OH
L
1
1‚1.5H2O
2
3‚CH3OH
4‚CH3OH
formula
fw
cryst syst
a (Å)
b (Å)
c (Å)
C16H13N3O2
279.29
orthorhombic
7.4682(2)
16.0485(5)
22.0992(7)
90
C33H26AgF3N6O7S C16.5H15Ag0.5F1.5N3O4.75S0.5 C16H13Ag0.5N3.5O3.5 C33H30AgF6N6O5P C33H30AgClN6O9
815.53
monoclinic
14.568(3)
19.028(4)
13.933(3)
90
121.002(7)
90
3310.5(11)
1.636
429.78
monoclinic
19.0056(17)
18.7539(16)
13.4069(12)
90
131.387(3)
90
3585.2(5)
1.592
364.23
orthorhombic
13.3534(6)
16.6898(8)
26.1459(18)
90
90
90
5827.0(6)
1.661
843.47
797.95
triclinic
11.6894(19)
12.967(2)
13.407(2)
85.429(3)
64.351(3)
65.069(3)
1648.1(5)
1.608
triclinic
12.1546(7)
13.0571(7)
13.4179(8)
72.723(2)
63.402(2)
63.561(2)
1692.23(17)
1.655
R (deg)
â (deg)
γ (deg)
V (Å3)
90
90
2648.66(14)
1.401
Pbca
F
calcd/g cm-1
space group
Cc
4
C2/c
8
Fdd2
16
P1h
2
P1h
2
Z
8
µ (mm-1
F(000)
)
0.095
1168
0.748
0.700
0.756
0.728
0.758
1648
1744
2960
852
812
dimens (mm3) 0.53 × 0.26 × 0.15 0.28 × 0.23 × 0.05 0.40 × 0.20 × 0.10
0.69 × 0.48 × 0.21 0.68 × 0.18 × 0.18 0.35 × 0.20 × 0.09
T (K)
86(2)
89(2)
89(2)
86(2)
86(2)
89(2)
no. measured
82 729
27 969
22 514
39 865
32 312
28 654
data/restraints/ 4842/0/191
params
1.024
final R indices R1 ) 0.0430
4262/6/460
4300/9/251
5366/1/215
9029/0/476
7428/43/484
GOF on F2
1.078
1.064
1.049
1.248
1.045
R1 ) 0.0472
wR2 ) 0.1135
R1 ) 0.0556
wR2 ) 0.1169
R1 ) 0.0677
wR2 ) 0.1904
R1 ) 0.0847
wR2 ) 0.2095
R1 ) 0.0170
wR2 ) 0.0466
R1 ) 0.0172
wR2 ) 0.0468
R1 ) 0.0306
wR2 ) 0.882
R1 ) 0.0365
wR2 ) 0.0980
R1 ) 0.0810
wR2 ) 0.2220
R1 ) 0.0956
wR2 ) 0.2355
[I > 2σ(I)]
R indices
(all data)
wR2 ) 0.1120
R1 ) 0.0538
wR2 ) 0.1215
Scheme 3. Synthesis of La
Found: C, 49.43; H, 3.65; N, 10.50. 1H NMR (CD3OD, 300
MHz): δ 5.24 (s, 2H, CH2), 6.99 (d, 2H, Hg), 7.08 (d, 1H, He),
7.36 (dd, 1H, Hb), 7.55 (d, 2H, Hh), 7.98 (m, 2H, Hc,Hd), 8.07 (d,
1H, Hf), 8.17 (d, 1H, Ha). Selected IR (KBr, cm-1): νmax 3462
(OH), 1690 (CdO), 1603, 1500, 1281 (C-O), 1099 (ClO4-).
Crystallography. Colorless crystals of L were grown by slow
evaporation of a methanol solution. Colorless crystals of 1, 3‚CH3-
OH, and 4‚CH3OH were grown by slow evaporation of a methanol
solution of the complex, while crystals of 3 were grown by slow
evaporation of an acetonitrile solution. While the crystals of 1 were
of relatively poor quality, attempts to solve the structure using the
alternative space group C2/c were unsuccessful. In 1‚1.5H2O, the
triflate anion was disordered about a crystallographic center of
inversion and was modeled satisfactorily with 50% contributions
from each of the two orientations. Crystals of 4‚CH3OH were also
of poor quality, and while the cation and the methanol solvate
a Hydrogen atom labeling (as used in 1H NMR spectral assignment) and
ring labeling (as in X-ray structure discussion) are shown.
on carbon and COOH oxygen atoms were included in calculated
positions with isotropic displacement parameters 1.2 and 1.5 times
the isotropic equivalent of their carrier atoms, respectively.
Hydrogen atoms on solvent water and methanol molecules were
found from the difference map where possible and refined ap-
propriately. Packing diagrams were prepared using Mercury version
1.4.2.26
-
molecule were well resolved, the ClO4 anion, which was disor-
dered about a crystallographic center of inversion, could not be
satisfactorily resolved. The crystal data, data collection, and
refinement parameters are listed in Table 1. All measurements were
made with a Bruker ApexII diffractometer using graphite-mono-
chromated Mo KR (λ ) 0.71073 Å) radiation. Intensities were
corrected for Lorentz and polarization effects21 and for absorption
using SADABS.22 The structures were solved by direct methods
using SHELXS23 or SIR97,24 running within the WinGX package,25
and refined on F2 using all data by full-matrix least-squares
procedures with SHELXL-97.23 All non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen atoms
Results and Discussion
The new ligand L was prepared in reasonable yield from
4-bromomethylbenzoic acid and 3-(2-pyridyl)pyrazole by a
phase-transfer-catalyzed alkylation reaction (Scheme 3) and
1
was characterized by microanalysis, H NMR and infrared
spectroscopies, and X-ray crystallography. The IR spectrum
showed characteristic peaks for the COOH group at 1699
and 1272 cm-1.27 The ligand was poorly soluble in most
organic solvents but was relatively soluble in methanol.
Ligand L was reacted with AgOTf, AgNO3, AgPF6, and
AgClO4 in a 1:2 metal/ligand stoichiometry in methanol
(21) (a) SAINT V4, Area Detector Control and Integration Software;
Siemens Analytical X-ray Systems Inc.: Madison, WI, 1996. (b)
Otwinowski, Z.; Minor, W. In Methods in Enzymology: Macromo-
lecular Crystallography Part A; Carter, C. W., Jr., Sweet, R. M., Eds.;
Academic Press: New York, 1997; Vol. 276, pp 307-326.
(22) Sheldrick, G. M. SADABS, Program for Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(26) Mercury: Visualization and analysis of crystal structures; Macrae,
C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Shields, G. P.;
Taylor, R.; Towler, M.; van de Streek, J. J. Appl. Crystallogr. 2006,
39, 453-457.
(27) Nakamoto, M. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Part B; Wiley-Interscience: New York,
1997.
(23) Sheldrick, G. M. SHELXS and SHELXL; Institu¨t fu¨r Anorganische
Chemie der Universita¨t: Go¨ttingen, Germany, 1996.
(24) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115-119.
(25) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
Inorganic Chemistry, Vol. 47, No. 2, 2008 595

McMorran
Figure 2. (a) View of one of the helical chains (M helix) formed by L in
the solid state. (b) View of two adjacent, intertwined helical chains (M
helix in blue, P helix in green).
Figure 1. View of the X-ray crystal structure of L. Thermal ellipsoids
are drawn at the 50% probability level.
Table 2. Selected Distances (Å) and Angles (deg) for L and
1-3‚CH3OH
solutions. In all cases, colorless crystalline solids, 1, 2‚H2O,
3‚CH3OH, and 4‚CH3OH, were obtained in good yields from
slow evaporation of the reaction solvent. Microanalyses
confirmed that, in each case, the 2:1 ligand-to-metal ratio
was conserved in these complexes. IR spectroscopy showed
that the carboxylic acid group was protonated, and so not
involved in metal binding, in each case, with peaks at 1727,
1696, and 1296; 1700 and 1270; 1698 and 1273; and 1690
and 1281 cm-1 for 1, 2‚H2O, 3‚CH3OH, and 4‚CH3OH,
respectively. The observation of two CdO peaks for 1 (and
possibly two C-O peaks, with the second obscured by the
OTf- peaks) suggests two different types of COOH group
in the molecule. This is found to be consistent with the X-ray
L
1
1‚1.5H2O
2
3‚CH3OH
Bond Distances (Å)
Ag1-N2
Ag1-N3
Ag1-N5
Ag1-N6
2.355(7)
2.308(8)
2.379(8)
2.274(8)
2.358(4)
2.266(4)
2.4212(8)
2.2604(8)
2.4150(12)
2.2535(12)
2.4357(14)
2.2471(12)
Bond Angles (deg)
112.2(3)
133.5(3)
136.7(3)
138.5(3)
N2-Ag1-N5
N2-Ag1-N6
N3-Ag1-N5
N3-Ag1-N6
N2-Ag1-N2A
N2-Ag1-N3A
N3-Ag1-N3A
112.47(4)
116.71(4)
115.55(4)
165.22(4)
122.55(18) 114.86(4)
119.30(15) 109.16(3)
157.1(2)
176.04(5)
1
Plane Distances (Å)
3.54
structure of 1 (see Discussion below). H NMR spectra for
A···D
3.38
3.54
1 and 2-4 showed similar chemical shifts for each of the
peaks of the pyridine and pyrazole rings: the small differ-
ences between these and positions of the related peaks in
the spectrum of L are consistent with expected coordination-
induced shifts.15 Much larger shifts are observed for the
protons on benzoic acid rings, ranging between 0.31 and 0.46
ppm upfield compared to free L. The number of peaks and
the observed peak shifts suggest that a single complex
predominates in solution. The observed shifts in the A ring
peaks are consistent with these rings being involved in
intramolecular π-π stacking interactions in an AgL2 species,
with a structure similar to that found for 3‚CH3OH (see later).
ES-MS spectra of 4 in dilute acetonitrile solution show peaks
Plane Angles (deg)
A-D
B-C
A-C
A-F
C-F
D-F
E-F
85.40
6.86
2.45
76.11
70.47
76.18
82.28
10.65
87.64
17.99
82.09
8.68
3.60
83.11
162.91 12.25
88.96 83.39
12.52
83.88
88.03
84.29
5.25
87.68
8.79
and the benzoic acid (A) ring are almost orthogonal with an
angle between the rings of 88.96° (Table 2).
Each ligand molecule is associated with two others via
hydrogen bonds between the pyridyl ring nitrogen and the
OH group of the benzoic acid ring (Figure 2a, Table 3). This
type of hydrogen-bond interaction has been shown to be
+
corresponding to AgL2 , AgL+, and L+ species, supporting
favored over the alternative R2 (8)28 COOH dimer.29 These
2
the suggestion that the complexes maintain their integrity in
solution.
hydrogen-bonding interactions, which mirror the way in
which the ligand is anticipated to behave in the metal
complexes (with the nitrogen donors chelating to the metal
rather than hydrogen bonding), reveal the molecule to adopt
a chiral conformation in the solid state (Scheme 4).30
The ligand L crystallized in the orthorhombic space group
Pbca with the asymmetric unit containing a single L
molecule (Figure 1). The nitrogen donor atoms of the
chelating region are transoid, as is commonly found in free
ligands of this type. The angle between the pyridyl (C) ring
and the pyrazolyl (B) ring is 162.91°, while the pyridyl ring
(28) Etter, M. C. Acc. Chem. Res. 1990, 23, 120-126.
(29) Steiner, T. Acta Crystallogr., Sect. B 2001, B57, 103-106.
596 Inorganic Chemistry, Vol. 47, No. 2, 2008

Construction of Hydrogen-Bonded Coordination Networks
Table 3. Hydrogen Bond Dataa
d(D···A)/Å
(D-H···A)/deg
L
O(1)-H(1)···N(3)#1
2.6569(10)
171.4
1
O(1)-H(1)···O(5)
O(3)-H(3)···O(2)#2
2.687(9)
2.639(9)
160.6
154.0
1‚1.5H2O
2.727(7)
2.582(7)
O(6)-H(6C)···O(2)
O(1)-H(1)···O(6)#3
171(10)
165.2
2
O(1)-H(1)···O(3)
2.6574(9)
169.8
3‚CH3OH
2.5847(17)
2.6282(17)
2.7677(18)
O(1)-H(1B)···O(50)
O(3)-H(3B)···O(4)#4
O(50)-H(50A)···O(2)#5
173(2)
178(2)
170(2)
a Symmetry transformations: #1, -x + 1, y - 1/2, -z + 1/2; #2, x -
1/2, y + 1/2, z - 1; #3, -x + 1, -y - 1, -z; #4, -x + 1, -y + 1, -z +
2; #5, -x, -y + 1, -z + 2.
Scheme 4. Enantiomeric Forms of L, as Found in the Solid State
Figure 3. View of the X-ray crystal structure of 1 (∆ configuration).
Hydrogen atoms, except those involved in hydrogen bonding interactions,
are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability
level.
The ligands adopt conformations similar to that observed
in the free ligand. The pyridyl and pyrazole rings are
approximately coplanar, with B ring-C ring angles of 12.25°
and 8.79°. In each case, however, these rings are almost
orthogonal to their respective A rings, with A ring-C ring
angles of 83.39° and 87.68°. Further, the B ring-C ring
planes of each ligand are almost orthogonal to each other,
with an angle between the C rings of 83.88° and an angle
between the A rings of 85.40°. Stabilizing this orientation
of the ligands are intramolecular π-π stacking interactions
between, in each case, the A ring of one ligand and the C
ring of the other, with an intercentroid distance of 3.88 Å.
The nature of the ligand conformations give the AgL2 tecton
an axis of chirality, bisecting the N1-Ag-N6 and N3-Ag-
N4 angles.
The triflate forms a hydrogen-bonding interaction to the
COOH group of one of the ligands (Table 3, Figure 3). This
COOH group also acts as the acceptor in a hydrogen bond
to the COOH group on an adjacent AgL2 tecton. The COOH
group on the other ligand is only involved in an intertecton
hydrogen bond. Thus, the structure contains two different
types of COOH group, consistent with the IR spectrum of
1. Each tecton in the chain has the same chirality, so 1 forms
one-dimensional helical chains in the crystal, propagating
parallel to the intersection of the (-1, 1, 1) and (1, 1, 0)
planes (Figure 4a) with a helical pitch of 15.27 Å. Helical
chains of the same chirality are arranged in sheets lying in
the (2, 0, -1) plane with a closest distance between chains
of 8.40 Å. At this distance there can be no interchain
interactions by which the helicity of one chain is directed
Each ligand molecule forms hydrogen bonds to two others
of the same chirality, generating helical chains of ligands
which run parallel to the b axis, with a helical pitch of 16.049
Å. Consideration of the packing of the one-dimensional
helices shows that helices of opposite chirality intertwine.
These are held together by strong π-π stacking interactions,
running parallel to the a axis, between benzoic acid rings of
adjacent chains; the intercentroid distance is 3.76 Å and the
angle between the ring planes is 2.33° (Figure 2b).
Slow evaporation of a methanol solution of 1 gave crystals
suitable for X-ray analysis. 1 crystallizes in the monoclinic
space group Cc with the asymmetric unit containing one Ag
atom, two coordinated L ligands and one triflate anion
(Figure 3). The Ag atom is coordinated by the pyridine
nitrogens (Npy) of each ligand, with Ag-Npy distances of
2.274(8) and 2.308(8) Å and also more weakly by the
pyrazole nitrogens (Npz), with Ag-Npz distances of 2.355-
(7) and 2.379(8) Å. Weaker binding of the pyrazole donors
in ligands of this type has been reported previously.15 Also,
the bonding of the pyrazole nitrogens to the Ag atom is
significantly out of the pyrazole ring plane, with Ag-N-
centroid angles of 163.20° and 164.65°. Again, this has been
previously noted.15 The Npy-Ag-Npy bond angle is 138.5°,
giving a geometry best described as distorted tetrahedral.
(30) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; Wiley-Interscience: New York, 1994.
Inorganic Chemistry, Vol. 47, No. 2, 2008 597

McMorran
Figure 4. (a) View of one of the 1-dimensional hydrogen-bonded helical
chains of 1 (P helix) with ligands colored to emphasize the helical structure.
(b) View of the packing of the helical chains of 1 in three dimensions,
looking down the a axis (left) and perpendicular to the a axis (right). Sheets
of M helical chains are in green, while the adjacent sheet of P helical chains
are in gray. Triflate anions are orange.
Figure 5. Perspective view of the X-ray crystal structure of 1‚1.5H2O (Λ
configuration). Hydrogen atoms, except those involved in hydrogen-bonding
interactions, are omitted for clarity. Only one orientation of the disordered
triflate anion is shown. Atoms XA are related to atoms X by the symmetry
transformation [-x + 1, y, -z + 1/2]. Thermal ellipsoids are drawn at the
30% probability level.
by the helicity of its neighbors. The adjacent sheet, which
contains helical chains of the opposite chirality, lies parallel
to the first but with the chains in the second lying at an angle
of 77.1° to those in the first. The second sheet slots into
grooves in the first sheet (Figure 4b) with the chains in
adjacent sheets being held together by π-π stacking interac-
tions between A rings in one sheet and B ring-C ring planes
in the other, with an intercentroid distance of 3.60 Å. It
appears that it is these interactions which direct the respective
helicities of the chains. Subsequent sheets are offset such
that the apparent channels in the structure do not propagate
through the structure.
Slow evaporation of a solution of 1 in methanol and
acetonitrile also gave a small number of crystals suitable for
X-ray analysis, along with a large amount of amorphous
material. One of these was selected for data collection and,
surprisingly, a different structure was found to that of 1,
despite the new complex still having a 1:2 Ag/L ratio. The
new molecule, 1‚1.5H2O, crystallizes in the monoclinic space
group C2/c with the asymmetric unit containing one L ligand,
half a silver atom, one and one-half water molecules and
half a triflate anion, which is disordered (Figure 5). As was
found for 1, the silver atom in 1‚1.5H2O is chelated by the
ligand, again with the Ag-Npz bond significantly longer than
the Ag-Npy bond (Table 2). The Npy-Ag-Npy bond angle
is 157.1(2)°, meaning that the structure has a very distorted
tetrahedral structure. The ligand adopts the bent conformation
observed previously and the angle between the pyridine rings
is 76.18° but, in contrast to 1, the two ligands in 1‚1.5H2O
coordinate to the silver such that their A rings are on the
same side of the silver and, in fact, form an intramolecular
π-π stacking interaction with each other, with an intercen-
troid distance of 3.79 Å. The angle between the OH bonds
of the two COOH groups is 73.7°. Again, the way in which
the ligands wrap around the silver atom gives the AgL2 tecton
an axis of chirality.
Figure 6. (a) View of one of the one-dimensional hydrogen-bonded meso-
helical chains of 1‚1.5H2O with ligands colored to emphasize the meso-
helical structure. (b) View of the packing of adjacent meso-helical chains
of 1‚1.5H2O in three dimensions, looking down the ac diagonal (left) and
down the c axis (right). Full-occupancy water molecules are red. Triflate
anions and half-occupancy water molecules are omitted for clarity.
this extended hydrogen-bonding system from the ligand
COOH group is only present for half the COOH groups. A
second, symmetry-related water molecule also hydrogen
bonds to the COOH group and these two then hydrogen bond
The COOH group of the ligand is hydrogen bonded to a
water molecule, which is, in turn, hydrogen bonded to a
second water molecule (Figure 6a, Table 3). This second
water molecule is itself hydrogen bonded to the triflate anion.
However, due to the disordered nature of the triflate and the
fact that the second water molecule only has 50% occupancy,
to the COOH group of a second AgL2 unit, giving an R4 -
4
(12) hydrogen-bonding motif (Figure 6a). This so-called
598 Inorganic Chemistry, Vol. 47, No. 2, 2008

Construction of Hydrogen-Bonded Coordination Networks
Figure 8. (a) View of one of the one-dimensional hydrogen-bonded helical
chains of 2 (M helix) with ligands colored to emphasize the helical structure.
(b) View of the packing of the helical chains of 2 in three dimensions,
looking down the c axis (left) and down the ab diagonal (right). Sheets of
P helical chains are in green, while the adjacent sheet of M helical chains
are in gray. Nitrate anions are red.
Figure 7. View of the X-ray crystal structure of 2 (Λ configuration).
Hydrogen atoms, except those involved in hydrogen-bonding interactions,
are omitted for clarity. Atoms XA are related to atoms X by the symmetry
transformation [-x, -y, z]. Thermal ellipsoids are drawn at the 50%
probability level.
and the intercentroid distance is 3.68 Å. The angle between
the OH bonds of the two COOH groups is 62.19°.
One of the OH groups hydrogen bonds to the nitrate anion
(Table 3) and that same nitrate oxygen atom is further
hydrogen bonded to the OH group of a second AgL2. Thus,
the bifurcating nitrates assemble identical AgL2 tectons into
one-dimensional helical chains, which propagate parallel to
the crystallographic ab diagonal (Figure 8a). The helical pitch
is 10.687 Å. Helical chains of the same chirality form sheets
which lie in the (0, 0, 1) plane, with interchain Ag‚‚‚Ag
distances of 10.687 Å. Sheets of helical chains, of alternate
chirality, in turn stack upon each other, with the chains in
one sheet at 77.3° to those in the next (Figure 8b). The sheets
are held together by π-π stacking interactions between A
rings in one chain and C rings in the next: the angle between
the rings is 9.21°, and the intercentroid distance is 3.94 Å.
Slow evaporation of methanol solutions of hexafluoro-
phosphate 3 and perchlorate 4 gave colorless crystals. X-ray
analysis found the structures of the two compounds to be
the same. The following describes the structure of 3‚CH3-
OH, but the description equally applies to 4‚CH3OH. The
complex crystallizes as a methanol solvate in the triclinic
space group P1h with the asymmetric unit containing one Ag
atom, two L ligands, two half PF6- anions, and one molecule
of methanol (Figure 9). The Ag atom again adopts a distorted
tetrahedral geometry (Table 2). The ligand again adopts a
bent conformation, with the pyridyl and pyrazolyl rings again
approximately coplanar, and these planes approximately
orthogonal to the A rings. The angle between the pyridine
rings of each ligand is 88.03°, and the two almost parallel
A rings form a π-π stacking interaction, with an intercen-
troid distance of 3.96 Å. The angle between the OH bonds
of the two COOH groups is 77.6°. Again, due to the way
that the ligands wrap around the Ag atom, each AgL2 tecton
is axially chiral.
expanded dimer motif has been reported previously.31 The
second AgL2 tecton is related to the first by a glide reflection
and so has opposite chirality to the first. The assembly of
AgL2 tectons of alternating Λ and ∆ configurations, mediated
by the hydrogen-bonded water molecules, generates an
infinite one-dimensional meso-helical chain which runs
parallel to the crystallographic c axis. The distance between
successive silver atoms in the chain is 18.84 Å, and the pitch
of the meso-helix is 13.41 Å.
Consideration of the packing shows that the meso-helical
chains lie in interdigitated sheets parallel to the (2, 0, 0) plane
with strong π-π stacking interactions between the B ring-C
ring planes of ligands in adjacent chains (intercentroid
distance of 3.68 Å) (Figure 6b). Further, chains in adjacent
sheets stack upon each other in such a way as to generate
undulating channels, within which the triflate anions and the
half-occupancy water molecules reside. These water mol-
ecules form bridging hydrogen-bonding interactions between
water molecules in adjacent meso-helical chains, thereby
generating the three-dimensional structure.
Slow evaporation of an acetonitrile solution of 2‚H2O gave
large colorless crystals suitable for X-ray analysis. The
complex crystallized in the orthorhombic space group Fdd2,
with the asymmetric unit containing half an Ag atom, one
L ligand, and half a nitrate anion (Figure 7). The Ag atom
again adopts a distorted tetrahedral geometry (Table 2) with
the ligands adopting a bent conformation and the angle
between the two pyridyl rings coordinated to the Ag atom
being 82.09°. The two ligands wrap around the metal to form
a chiral tecton, stabilized by π-π stacking interactions
between the benzoic acid ring of one ligand and the pyridine
and pyrazole rings of the other; the angle between the B
ring-C ring plane and the A ring of the other ligand is 21.18°
-
In this case, the PF6- (or ClO4 ) anion is not involved in
any hydrogen-bonding interactions with the COOH groups
of the ligands. Instead, a molecule of methanol is incorpo-
(31) Dunitz, J. D. Chem. Eur. J. 1998, 4, 745-746.
rated into an R4 (12) hydrogen-bonding motif with COOH
4
Inorganic Chemistry, Vol. 47, No. 2, 2008 599

McMorran
Figure 11. Overlay of the AgL2 tectons of 1‚1.5H2O (red), 2 (green),
and 3‚CH3OH (blue).
crystallographic a axis. The distances between consecutive
Ag atoms in the chain are very similar, being 17.925 Å across
the R4 (12) hydrogen bond and 17.768 Å across the R2 (8)
4
2
Figure 9. View of the X-ray crystal structure of 3‚CH3OH (∆ configu-
ration). PF6- anions and hydrogen atoms, except those involved in hydrogen-
bonding interactions, are omitted for clarity. Thermal ellipsoids are drawn
at the 50% probability level.
hydrogen bond. The pitch of the meso-helix is 12.15 Å.
Consideration of the packing in the structure shows that
the meso-helical chains form sheets which lie in the (0, 2,
-2) plane, with adjacent chains interacting via strong π-π
stacking interactions between B ring-C ring planes (Figure
10b). The intercentroid distance is 3.67 Å. These sheets, in
turn, stack with chains in adjacent sheets running parallel to
each other, again in contrast to 1 and 3.
Conclusion
The new ligand L has been prepared, and complexes
formed between it and silver(I) salts have been prepared and
structurally characterized. The ligand adopts a chiral con-
formation in the solid state, which leads it to form one-
dimensional helical chains with OH-Npy hydrogen bonds
joining successive molecules. When chelated to silver, this
chiral conformation is maintained and a distorted tetrahedral
coordination geometry is observed in all cases. This is in
spite of the known plasticity of the Ag(I) coordination
geometry. The pyridine nitrogens are more strongly bound
than the pyrazole ones; however, the pyrazole donors play a
crucial role in the structural nature of the complexes because,
by chelating to the silver, the orientation of the hydrogen-
bonding synthons (the benzoic acid groups) is more rigor-
ously defined. An overlay of the structures of 1‚1.5H2O, 2,
and 3‚CH3OH (Figure 11) shows that there is marked
similarity between them, in particular between 1‚1.5H2O and
3‚CH3OH (those incorporated into meso-helical structures)
where, in each case, A ring-A ring π-π stacking interac-
tions stabilize the structure. 2 is slightly different, due to its
adopting A ring-BC ring plane π-π stacking interactions.
Overall, the chelation of the Ag(I), in concert with the
intramolecular π-π stacking interactions, make the AgL2
tecton a more general and reliable structural motif.
Figure 10. (a) View of one of the one-dimensional hydrogen bonded meso-
helical chains of 3‚CH3OH with ligands colored to emphasize the meso-
helical structure. (b) View of the packing of adjacent meso-helical chains
of 3‚CH3OH in three dimensions, viewed in the (0, -1, 1) plane (left) and
down the a axis (right). Methanol molecules are blue.
groups from adjacent AgL2 tectons, such that pairs of
methanol molecules link pairs of AgL2 tectons (Figure 10a,
Table 3). This is very similar to the ‘extended dimer’ motif
observed in 1‚1.5H2O. In this case, however, the second
COOH group in each AgL2 unit forms an R2 (8) hydrogen
2
bond directly to a COOH group on an adjacent AgL2 unit.
Thus, two different types of hydrogen-bonding motif act to
link AgL2 tectons of alternating Λ and ∆ configurations into
meso-helical one-dimensional chains, propagating along the
1H NMR spectra of the complexes suggest that the tectons
are stable in solution and, based on the observed changes in
chemical shifts upon coordination of L, are likely to adopt
600 Inorganic Chemistry, Vol. 47, No. 2, 2008

Construction of Hydrogen-Bonded Coordination Networks
a structure similar to that observed in the X-ray crystal
structure of 3‚CH3OH. This is supported by the ES-MS
spectrum of 3.
In two respects, then, the current complexes represent what
can be termed reliable tectons for the synthesis of solid-
state structures. The chelating nature of the ligands, in
combination with the intramolecular π-π stacking interac-
tions leads, in all cases but one, to the formation of distorted
tetrahedral AgL2 tectons in which the hydrogen-bonding
synthons are in a relatively constant orientation. The
hydrogen bonding of these tectons together is then controlled
by this orientation and by the nature of the anion chosen;
spherical anions lead to meso-helical structures in which the
anions do not play a role, whereas nonspherical anions
generate structures in which they mediate the hydrogen
bonding of the tectons into helical chains.
The tetrahedral coordination geometry, combined with the
bent conformation of the ligands, precludes the formation
of discrete [AgL2] dimers. Rather, X-ray crystal structures
of the silver(I) complexes show that the axially chiral AgL2
tectons assemble to form infinite helical or meso-helical
chains via hydrogen-bonding interactions between the COOH
synthon incorporated into L and either solvent or anions or,
in one case, between the COOH synthons themselves. It has
been found that anions can be divided into two types in terms
of their hydrogen-bonding nature. So-called spherical anions,
-
-
-
such as ClO4 , BF4 , and PF6 , have been found to be poor
at hydrogen bonding, whereas nonspherical anions, such as
It seems likely that the structural reliability observed in
these silver-based tectons might be extended to similar
tectons containing other transition metal ions. Such studies
on these, and systems with the related ligand 4-(3-(2-pyridyl)-
pyrazol-1-ylmethyl)benzoic acid, are under way.
-
OTf- and NO3 , have been found to be much more readily
incorporated into hydrogen bonds.3f This is consistent with
what has been found in the current study. In 1 and 2, the
anion mediates the hydrogen bonds which join the AgL2
tectons into helical chains, whereas in 3‚CH3OH and 4‚CH3-
OH, the anions play no role in the hydrogen-bonding arrays,
with solvent molecules instead incorporated. In the case of
1‚1.5H2O, while the OTf- anions are not involved in the
hydrogen bonds forming the meso-helical chains, they are
involved in the hydrogen-bonding interactions which link
adjacent meso-helicies.
Acknowledgment. Assoc Prof Lyall Hanton is thanked
for helpful discussions, and Lisa Bucke is thanked for her
assistance in preparing some of the diagrams.
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC701919R
Inorganic Chemistry, Vol. 47, No. 2, 2008 601