Interconnected trimeric, pentameric, and hexameric metallacycles in a singular silver-adenine framework

Article abstract of DOI:10.1021/ic1010902

This Article describes the synthesis and crystallographic investigation of a silver complex of a modified adenine derivative bearing a nitrile pendant at the N9 position. All three adenine ring nitrogen atoms coordinated to silver ions, while the fourth coordination was achieved at the nitrile functionality, thus resulting in the formation of silver-mediated interconnected trimeric, pentameric, and hexameric metallacyclic rings and helical signatures in two orthogonal directions.

Full text of DOI:10.1021/ic1010902

8
012 Inorg. Chem. 2010, 49, 8012–8016
DOI: 10.1021/ic1010902
Interconnected Trimeric, Pentameric, and Hexameric Metallacycles in a Singular
Silver-Adenine Framework
Ashutosh Kumar Mishra and Sandeep Verma*
Department of Chemistry, Indian Institute of Technology (IIT) Kanpur, Kanpur 208016, India
Received May 31, 2010
This Article describes the synthesis and crystallographic investigation of a silver complex of a modified adenine
derivative bearing a nitrile pendant at the N9 position. All three adenine ring nitrogen atoms coordinated to silver ions,
while the fourth coordination was achieved at the nitrile functionality, thus resulting in the formation of silver-mediated
interconnected trimeric, pentameric, and hexameric metallacyclic rings and helical signatures in two orthogonal
directions.
5
Introduction
adenine analogues are known, including several examples
6
from our group. Our focused investigation of silver-adenine
Nitrogenous nucleobases offer multiple metal binding
possibilities for the generation of novel coordination motifs
and architectures. Such interactions could also be used to
amend classical hydrogen-bonded base-pairing patterns with
the help of metal-mediated base pairing to create unusual
supramolecular architectures. This effect can be further
augmented by the deliberate introduction of metal-binding
functional groups at specified sites, thus leading to interesting
coordination patterns and unique architectures.
interaction has afforded interesting supramolecular motifs
suitable for patterned surface deposition, enhanced lumines-
cence properties, and applications involving fluorescent sensing
1
6,7
for silver ions. We have designed suitably N9-substituted
adenine ligands to achieve hierarchical architectures of incre-
ased complexity (Scheme 1a). In this connection, we recently
reported the formation of a silver metallamacrocyclic hex-
amer, as a result of tetradentate coordination involving
μ-(N1,N3,N7) interaction and an additional coordination via
2
3
9
-Substituted adenine offers three main coordination sites,
6d
an N9 carboxylate group bearing a substituent. To broaden
the scope of this approach, we prepared 3-(6-aminopurin-9-
yl)propionitrile (Scheme 1b) containing an N9-nitrile pen-
dant, and a detailed crystallographic investigation of its silver
complex was performed.
N1, N3, and N7, with N1 and N7 being predominantly used
for metal ion complexation in the mono- or bidentate co-
ordination mode. However, several examples involving the
4
coordination mode that employs all three nitrogen atoms in
*
To whom correspondence should be addressed. E-mail: sverma@iitk.ac.
Experimental Section
in.
(
1
13
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Inorg. Chem. 2002, 41, 6190–6192. (d) Rother, I. B.; Freisinger, E.; Erxleben, A.;
Lippert, B. Inorg. Chim. Acta 2000, 300-302, 339–352.
(6) (a) Purohit, C. S.; Verma, S. J. Am. Chem. Soc. 2006, 128, 400–401.
(b) Purohit, C. S.; Mishra, A. K.; Verma, S. Inorg. Chem. 2007, 46, 8493–8495.
(c) Purohit, C. S.; Verma, S. J. Am. Chem. Soc. 2007, 129, 3488–3489.
(d) Kumar, J.; Verma, S. Inorg. Chem. 2009, 48, 6350–6352.
(h) Amo-Ochoa, P.; Miguel, P. J. S.; Lax, P.; Alonso, I.; Roitzsch, M.; Zamora, F.;
Lippert, B. Angew. Chem., Int. Ed. 2005, 44, 5670–5674. (i) Knobloch, B.; Sigel,
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3) (a) Galindo, M. A.; Houlton, A. Inorg. Chim. Acta 2009, 362, 625–633.
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b) Meiser, C.; Song, B.; Freisinger, E.; Peilert, M.; Sigel, H.; Lippert, B. Chem.—
(7) (a) Verma, S.; Mishra, A. K.; Kumar, J. Acc. Chem. Res. 2010, 43, 79–
91. (b) Mishra, A. K.; Purohit, C. S.; Kumar, J.; Verma, S. Inorg. Chim. Acta
2009, 362, 855–860. (c) Mishra, A. K.; Purohit, C. S.; Verma, S. CrystEngComm
2008, 10, 1296–1298. (d) Pandey, M. D.; Mishra, A. K.; Chandrasekhar, V.;
Verma, S. Inorg. Chem. 2010, 49, 2020–2022. (e) Prajapati, R. K.; Kumar, J.;
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(
pubs.acs.org/IC
Published on Web 08/09/2010
r 2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 8013
Scheme 1. (a) Schematic Representation of the Possible Mode of
Metal Binding Sites in an N9-Adenine Analogue and (b) Chemical
Structure of the Ligand
Table 1. Crystallographic Data for Complex 1
identification code
empirical formula
fw
1
16 3 15 12
C H16Ag N O
934.05
colorless
cryst color
cryst size (mm)
cryst syst
space group
0.28 ꢀ 0.24 ꢀ 0.20
tetragonal
3 1
P4 2 2 (No. 96)
9.6180(18)
9.6180(18)
29.490(8)
90
90
90
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
λ (deg)
3
V (A )
2728.0(10)
2.274
-
3
Dcalcd (mg m
)
Z
4
2.223
1816
-
1
)
(TMS). High-resolution mass spectrometry (HRMS) spectra
μ(Mo KR) (mm
F(000)
for the ligand and metalated complexes were recorded at IIT
Kanpur, Kanpur, India, on a Waters Q-Tof Premier Micro-
mass HAB 213 mass spectrometer. Solvents were evaporated
using a rotary evaporator under reduced pressure. Adenine,
acroylnitrile, and silver nitrate were purchased from Spectro-
chem Pvt. Ltd., Mumbai, India, and potassium carbonate
was purchased from SD Fine Chemicals Pvt. Ltd., Mumbai,
India, andusedwithoutfurtherpurification.Allsolventswere
distilled prior to use using standard procedures. Fluorescence
spectra were recorded on a Varian Luminescence Cary
Eclipse with a 10 mm quartz cell at 25( 0.1 °C, and electronic
spectra were recorded on a Perkin-Elmer-Lambda 20 UV-
visspectrometerwitha10mmquartz cell at room temperature.
A 10 μM stock solution of 1 in a water-dimethyl sulfoxide
2
θ range
2.2-26.0
15 104
2478 [R(int) = 0.0441]
2685
206
R1 = 0.0432, wR2 = 0.1118
1.160
778346
reflns measd
indep reflns
reflns obsd [I > 2σ(I)]
no. of param
final R1, wR2 (obsd data)
GOF (obsd data)
CCDC no.
dissolved in methanol, and to this was added dropwise with
stirring the corresponding silver salt solution in water (1 mol
equiv). The complex started precipitating out with the addition
of the silver salt solution. Stirring was continued for another 1 h.
After that time, the precipitate was carefully filtered to avoid
direct light and washed with water (4 ꢀ 5 mL) and methanol (4 ꢀ
5 mL) to remove any traces of unreacted metal salt and ligand.
The product so obtained was dried under a high vacuum. The
[DMSO; 1:1 (v/v)] system was used to perform the fluores-
cence experiment. The excitation wavelength was 320 nm.
X-ray Analysis and Structure Refinement. Data were collected
on a Bruker SMART CCD4 X-ray diffraction instrument using
þ
þ
yields were almost quantative. HRMS (L þ Ag ). Calcd: m/z
˚
294.9861. Found: m/z 294.9865. HRMS (L þ 2Ag þ 1). Calcd:
graphite-monochromated Mo KR radiation (R = 0.710 A) at
þ
m/z 402.8991. Found: m/z 402.8940. HRMS (L þ 3Ag ). Calcd:
1
program and refined using full-matrix least squares on F
(
00 K. The crystal was solved by direct methods using the SIR92
þ
8
2
m/z 508.7963. Found: m/z 508.9141. HRMS (L þ 4Ag þ 2).
1
Calcd: m/z 617.7171. Found: m/z 617.7957. H NMR (500 MHz,
9
SHELX97). The structure was expanded using Fourier tech-
DMSO-d , 22.8 °C, TMS): δ 3.13-3.16 (t, 2H), 4.49-4.55 (t,
niques. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed at geometrically idealized posi-
tions. Table 1 contains the final refinement parameters for 1. All
of these software packages were the integrated WINGX software
package. CCDC 778346 contains the supplementary crystal-
lographic data for this paper. Copies of this information can be
obtained free of charge upon application to CCDC, 12 Union
Road, Cambridge CB21EZ, U.K. (fax þ44-1223/336-033; e-mail
deposit@ccdc.cam.ac.uk).
6
1
3
2
H), 7.77 (s, 1H), 8.32 (s, 1H), 8.42 (s, 1H). C NMR (125 MHz,
DMSO-d
, 17.7 °C TMS): δ 18.71, 40.83, 118.63, 118.85, 142.67,
49.28, 154.09, 155.94.
6
1
Results and Discussion
NMR Studies. Favorable interaction between the li-
gand and silver ions was evident in H NMR titration
1
experiments. It was observed that the addition of silver
ions caused a downfield shift in the exocyclic amino
protons from 7.25 to 7.82 ppm, while the C2H and C8H
proton singlets were downfield-shifted from 8.14 and
8.16 to 8.34 and 8.46 ppm, respectively (Figure 1). These
observations reflect significant interaction of modified
silver-adenine complex, which results in substantial
chemical shift values. After encouraging solution-phase
studies, crystals of the silver-ligand complex were grown
to investigate the solid-state interactions.
Synthesis. Synthesisof 3-(6-Aminopurin-9-yl)propionitrile (1).
In a round-bottomed flask, adenine (500 mg, 1 equiv) was dis-
solved in 5 mL of DMSO, followed by the addition of potassium
carbonate (30 mg, 1.2 equiv), and the resulting solution was
stirred for 30 min at room temperature. After acroylnitrile (1.22
mL, 0.95 equiv) was added, the reaction was stopped after 30 min.
The compound was extracted with a water-ethyl acetate system
and further purified by column chromatography (75.6% yield).
þ
HRMS [(M þ H) ]. Calcd: m/z 189.0889. Found: m/z 189.0856.
1
H NMR (500 MHz, DMSO-d , 20.9 °C, TMS): δ 3.15 (t, 2H),
6
13
4
.40-4.43(t, 2H), 7.25 (s, 1H), 8.14(s, 1H), 8.15(s, 1H). C NMR
Crystallographic Studies. Crystals suitable for single-
crystal analysis were grown by a slow evaporation tech-
nique from an acetonitrile solution of the ligand and silver
nitrate. Colorless crystals were formed after 1 week. Refine-
ment of the collected data suggested that the silver com-
plex crystallized in the tetragonal system, space group
P4 2 2 (No. 96). A closer inspection of the crystal lattice
(
125 MHz, DMSO-d
6
, 20.6 °C, TMS): δ 18.65, 40.86, 118.50,
19.21, 141.08, 149.15, 153.15, 156.55. Anal. Calcd for C H N : C,
1
5
8
8
6
1.06; H, 4.28; N, 44.66. Found: C, 51.21; H, 4.19; N, 44.60.
Synthesis of a Silver Complex of 1. In a 25 mL round-
bottomed flask, wrapped with aluminum foil, the ligand was
3
1
(
8) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl.
Crystallogr. 1993, 26, 343.
9) Sheldrick, G. M. SHELX97; University of Goettingen: Goettingen,
Germany, 1997.
revealed a tetradentate coordination mode, where the
three ring nitrogen atoms (N1, N3, and N7) and the nitro-
gen atom from the pendant nitrile group were coordinated
(

8
014 Inorganic Chemistry, Vol. 49, No. 17, 2010
Mishra and Verma
þ
þ
Figure 1. Representation of NMR titration data with added amounts of silver nitrate: (a) with 5 equiv of Ag ; (b) with 1 equiv of Ag ; (c) with 0.5 equiv of
Ag ; (d) ligand alone.
þ
Figure 2. Representation of the tetradentate coordination mode in 1.
Crystallographically unique silver ions are highlighted by colored poly-
hedra.
Figure 3. (a) 1D coordination polymeric helices in 1, which run perpen-
dicular to each other along the a and b axes (inset shows the view of the
crystal lattice along the c axis). (b) 1D coordination polymeric helices in 1
along the b axis involving N3 and N10 nitrogen atoms.
to the silver ions. Interestingly, the coordination environ-
ment around the silver ions confirmed the presence of
two crystallographically unique silver ions, labeled as Ag1
and Ag2, as represented by polyhedra of two different
colors (Figure 2).
The crystal lattice, when viewed along the c axis, reveals
the formation of two different types of infinite 1D coordina-
tion polymeric helices tethered by intervening Ag1 ions and
running in directions orthogonal to each other (Figure 3a).
It is possible to identify one silver-tethered supramolecular
helix along the a axis through the participation of N1 and
N3 adenine nitrogen atoms possessing a right-handed screw
axis, while the other one involves N1 and N10 (of nitrile
group) nitrogen atoms and runs along the direction of the b
axis, possessing a left-handed screw axis. There are other
interesting helical features present in the crystal lattice
involving N3 and N10 nitrogen atoms (Figure 3b). The
emergence of such a framework could be ascribed to three
fixed nitrogen centers in the heterocyclic ring, with another
coordinating nitrogen center being contributed by the nitrile
group (designated as N10) at adenine N9 position.
A closer inspection of the lattice organization reveals a
highly unusual occurrence of the formation of three diffe-
rent types of interconnected metallacyclic rings (Figure 4).
The pentameric and hexameric rings are formed with the
help of adenine rings and intervening silver ions, where a
single interaction between silver and N10 nitrile nitrogen
is invoked to achieve closure of the ring. However, the
Ag1 was found to exhibit a tetracoordinated mode via
participation of N1, N3, and N10 nitrogen atoms, from
three different modified adenine-ligand molecules, and a
nitrate counteranion (Figure 2). This arrangement leads
to a distorted trigonal-pyramidal coordination geometry
in which the counteranion occupies the axial position.
˚
The Ag-N bond distances were in the range of 2.23-2.35 A,
whereas the N1-Ag-N10, N1-Ag-N3, and N3-Ag-
N10 bond angles were 97.63(6)°, 121.72(7)°, and 140.62(7)°,
respectively.
On the other hand, Ag2 was stabilized in a rare penta-
coordinated environment, displaying a distorted square-
pyramidal geometry, via coordination from two N7 nitrogen
atoms from two different molecules and three nitrate
counteranions (Figure 2). The two N7 nitrogen atoms
and two counteranions occupy the planar position in a
trans fashion, while the third counteranion occupies the
apical position, with a shorter bond length compared to
the corresponding equatorial bond lengths [2.38(3) and
˚
2
.85(4) A, respectively].

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 8015
Figure 4. (a) Part of the crystal lattice viewed along the a axis showing embedded and interconnected 3-, 5-, and 6-membered metallacycles in 1.
b) Representation of three different types (trimeric, pentameric, and hexameric) of interconnected rings, shown in different colors. Parts of the complex
and hydrogen atoms are omitted for clarity.
(
Figure 5. (a) Hydrogen-bonding interactionbetween the modified adenineand nitrate counteranions, leading to two polymeric chains runningin different
directions. (b) Representation of probable hydrogen-bonding interactions between water molecules present in the crystal lattice with an adjacentnitrate and
the C8H hydrogenatomofthe adeninemoiety(shownwith black dottedlines). Attempts toassign hydrogenatomsonthe watermoleculeswere unsuccessful
during solution of the crystal.
Figure 6. Close proximity of exocyclic amino groups to coordinated
silver ions. Dotted lines shows hydrogen bonding between the N6
hydrogen atom and the nitrate anion. Other hydrogen atoms are omitted
for clarity.
trimeric ring is composed of two silver-nitrile interac-
tions, in addition to a N1-Ag-N3 interaction.
Significant hydrogen-bonding interactions between
Figure 7. Fluorescence spectra of the ligand alone (red line) and in the
presence of Ag (black) in a 1:1 water-DMSO solution.
þ
N6H and CH (C11) of modified adenine with oxygen
2
atoms of the nitrate group could be implicated in the
reinforcement of the overall stability to the crystal lattice
crystal lattice with adjacent nitrate and the C8H hydrogen
atom of the adenine moiety are evident (Figure 5b),
thereby further stabilizing the crystal lattice. Selected
hydrogen-bonding distances and angles are given in Table 2.
As is evident from the solid-state analysis of 1, there is
visible proximity of the silver ions to the exocyclic amino
groups of coordinated adenine moieties (Figure 6). Thus,
the observed prominent NMR shift of an exocyclic amino
group could be attributed to the polarizing effects resulting
(
Figure 5a). It is interesting to observe how these hydro-
gen-bonding interactions afford a boxlike arrangement,
leading to a 1D polymeric chain. The hydrogen-bonding
distances and the angles between N6-H O and C-
3
3 3
˚
H
O were in the range of 2.11-2.36 and 2.38 A and
3
3 3
156.12-167.63° and 139.95°, respectively. Favorable in-
teractions between the water molecules present in the

8
016 Inorganic Chemistry, Vol. 49, No. 17, 2010
Mishra and Verma
Table 2. Selected Hydrogen-Bonding Distances and Angles for Complex 1
Conclusions
#
We have synthesized and investigated the solid-state
structural analysis of silver complex of N9-adenine and
its fluorescence behavior. The presence of two supramo-
lecular helices running orthogonal to each other and the
interconnection of three different types of metallacyclic
rings are the salient features of the crystal lattice. The
possibility of minor chemical modifications in nucleo-
bases for the generation of complex hierarchical struc-
tures will be further evaluated.
D-H
3 3 A
d
H
3 3 3 A
d
D 3 3 3 A
angle
3
N6-H6A O2
3 3
2.11
2.36
2.45
2.36
2.39
2.92(9)
3.20(9)
3.36(11)
3.32(10)
3.19(9)
156
168
166
174
140
3
i
N6-H6B O4
3
3 3
C8-H8 O6
3
3 3
C11-H11A O1
3
3 3
ii
C11-H11B O3
3 3
3
#
1
3
1
1
1
Symmetry of A: (i) /
- z.
2
- y, - /
2
þ x, - /
4
þ z; (ii) - /
2
þ x, /
2
- y,
1
/
4
1
0
from metal ion coordination, in addition to support
from the hydrogen-bonding interaction with nitrate an-
ions, further reinforcing the downfield shift.
Acknowledgment. DST, India, is gratefully acknowl-
edged for financial support (to S.V.). A.K.M. acknowl-
edges CSIR, India, for a Senior Research Fellowship.
Fluorescence Studies. 1 also exhibited weak fluorescence
when compared to the ligand alone. When excited at 320
nm, it showed an enhanced emission maximum at ∼450 nm,
10
Supporting Information Available: X-ray crystallographic
data in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
which could be ascribed to the known phenomenon of d
metal complexes exhibiting enhanced fluorescence without
introducing low-energy metal-centered or charge-separated
excited states into the molecule (Figure 7).
11
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Chem. 1999, 455–460. (b) Guo, X.; Qian, X.; Jia, L. J. Am. Chem. Soc. 2004,
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(d) Yam, V. V.-W.; Lo, K. K.-W. Chem. Soc. Rev. 1999, 28, 323–334.
(
10) (a) L u€ th, M. S.; Willermann, M.; Lippert, B. Chem. Commun. 2001,
058–2059. (b) Szabo, Z. Coord. Chem. Rev. 2008, 252, 2362–2380.
2