Water adsorbing silver-adenine interpenetrated framework

Article abstract of DOI:10.1039/c2ce06625d

We describe the synthesis and properties of a novel silver complex of an adenine derivative bearing a sulfonate group. Adenine and sulfonate coordination with silver ions eventually leads to the formation of entangled networks with an embedded catenated framework and double helical assemblies. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ce06625d

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COMMUNICATION
Water adsorbing silver–adenine interpenetrated framework†
a
b
b
Jitendra Kumar, Prakash Kanoo, Tapas Kumar Maji and Sandeep Verma
a
*
*
Received 5th December 2011, Accepted 10th February 2012
DOI: 10.1039/c2ce06625d
that adenine, containing a sulfonate group, will provide an interesting
topology with the combination of ring imino nitrogen–Ag interac-
tions and weaker Ag–sulfonate interactions.‡
We describe the synthesis and properties of a novel silver complex of
an adenine derivative bearing a sulfonate group. Adenine and
sulfonate coordination with silver ions eventually leads to the
formation of entangled networks with an embedded catenated
framework and double helical assemblies.
The colorless crystals of 1 (9-SA: silver complex) were grown
within two weeks by layering an acetonitrile solution of silver nitrate
or silver triflate over an aqueous solution of 9-SA ligand. X-Ray
crystallographic studies showed that the complex crystallized in
monoclinic space group C2/c. The structure of the supramolecular
lattice of 1 [C7H12AgN5O5S]N is a three-dimensional solid having
entangled networks. The asymmetric unit consists of one silver ion
neutralized by the monoanionic form of 9-SA ligand, along with two
water molecules (Fig. 1). The geometry around the silver ion is dis-
torted tetrahedral which comprises two ring imino nitrogens, one
sulfonate oxygen and one water molecule with further weak Ag–O
The rational design of inorganic–organic hybrid materials with
entangled supramolecular architecture of individual motifs has
attracted a great deal of attention because of the topological influ-
ences over the bulk properties of the material and their potential
applications.1 Diverse arrays of entangled supramolecular assemblies
which include polycatenanes, polyrotaxanes, knots and Borromean
rings have been documented.2 Even in biomacromolecules, such
entangled architectures have been observed e.g. catenated circular
DNA3 and protein chainmail.4
ꢀ
interaction of 2.824(6) A with another oxygen of the sulfonate group
as shown in Fig. 1b.
We have reported the formation of novel 3D silver–adenine
frameworks with modified adenine derivatives owing to the presence
of multiple metal-binding sites in their skeleton.5 Our investigation
has revealed that N9-alkyl substituents generate metallaquartets,5a–c
a carboxyethyl substituent affords a silver–adenine hexameric
framework5d whereas a cyanoethyl substituent results in the forma-
tion of an interconnected metalated framework.5e Thus, the N9-
substituent has a significant influence over the structural outcome. As
an extension, we decided to incorporate sulfonic group at the N9-
position and synthesize 2-(N9-adeninyl)ethanesulfonic acid [9-SA] as
shown in Scheme 1 for exploiting its coordination for generating
more complex structures. The coordination aspects of sulfonate
grouping with silver ions have been extensively studied due to the
interesting functional properties imparted by the soft and flexible
silver–sulfonate interactions.6 Silver(I) cation is known for having
variable coordination spheres with distorted coordination geometries
mainly due to its d10 electronic configuration.7 Hence it is expected
The 9-SA anion behaves as a tridentate ligand in which adenine
renders bidentate coordination mode whereas pendant sulfonate
grouping exhibits weak monodentate coordination mode to silver
ions. The coordination of adenine nitrogens namely N1 and N7 to
silver ions resulted in a 1D polymeric chain permeating this structure
along the c-axis (Fig. 1c) which eventually replaces the complemen-
tary hydrogen bonding schemes found for the adenine nucleobase
usually in the absence of metal ions where the H-bonding interaction
between the Watson–Crick face and the Hoogsteen face results in
adenine ribbons.8 Further confirmation of such polymeric assembly
was evident from the analysis of HRMS data which clearly show the
peaks of detectable intensity corresponding to [M + Ag]+, [2M +
Ag]+, [2M + 2Ag–H]+ and [3M + 2Ag–H]+ in ES+ mode where ‘M’
denotes the neutral 9-SA molecule (see ESI†).
Interestingly, the lattice consists of two sets of such polymeric
chains (rendered with different colours) which are interrelated with
a center of inversion (as shown in Fig. 1c) and sulfonate grouping is
projected in the opposite direction connecting other polymeric chains
of the same orientation through weak Ag–Osulfonate coordination
aDepartment of Chemistry, Indian Institute of Technology Kanpur,
Kanpur-208016, UP, India. E-mail: sverma@iitk.ac.in; Tel: +91 512 259
7643
6
ꢀ
(2.75 A) which is in accordance with the reported literature. The
bChemistry and Physics of Materials Unit (CPMU), Jawaharlal Nehru
Center for Advanced Scientific Research (JNCASR), Bangalore 560064,
India. E-mail: tmaji@jncasr.ac.in; Fax: +91 80 2208 2766; Tel: +91 80
2208 2826
† Electronic supplementary information (ESI) available: Summary of
structural refinement parameters, crystallographic information in.cif
format, a hydrogen bonding table and additional representations of the
crystal lattice as figures for complex 1. CCDC reference number
787582. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2ce06625d
Scheme 1 Synthetic scheme for 9-SA.
3012 | CrystEngComm, 2012, 14, 3012–3014
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closest silver–silver distance between polymeric chains is found to be
ꢀ
3.62 A.
The crystal lattice of 1 is shown in Fig. 2a. The opposite orientation
of sulfonate grouping from the two sets of polymeric chains leads to
a highly interesting entangled structure and the solvent water guest
molecules sit inside the small cavities. The catenated segment of the
lattice consists of two pentameric rings as represented with different
color codes and each pentameric ring is contributed by two different
polymeric chains of the same orientation and the ring closure was
invoked by Ag–Osulfonate coordination as shown in Fig. 2b. The
crystal lattice can also be dissected to visualize an embedded double
helical structure running along the a-axis with a pitch length of 11.12
ꢀ
A as a consequence of adenine and Ag–Osulfonate coordination
(Fig. 2c). A schematic representation of the lattice is given in Fig. 3a
which clearly shows the entangled structure due to the presence of
two sets of networks in the lattice. Thus the incorporated sulfonate
moiety plays an important role in giving an unusual supramolecular
lattice apart from the adenine nucleobase.
The crystal lattice is further reinforced with significant hydrogen
bonding interaction rendered by the exocyclic amino group and non-
coordinated sulfonate oxygen atoms. Closer inspection revealed that
one of the NH2 hydrogen interacts with sulfonate oxygen O2 (d ¼
Fig. 1 (a) ORTEP diagram of 1 at 35% probability level; (b) coordi-
nation around silver ions in 1 (Ag–O1a ¼ 2.753(7), Ag–O2a ¼ 2.824(6),
ꢀ
2.01 A) belonging to the same set of polymeric chain whereas other
ꢀ
hydrogen interacts with O3 of the sulfonate group (2.24 A) belonging
Ag–N1b ¼ 2.205(6), Ag–N7 ¼ 2.195(6), Ag1–O1W ¼ 2.724(17) A;
ꢀ
to the different set as represented in Fig. 3b. These interactions
probably encourage network interpenetration and also interlinking of
both independent frameworks together.
symmetry code: a ¼ ½ + x, ½ + y, z and b ¼ x, ꢀy, ½ + z); (c) two
different sets of silver-mediated polymeric chains of opposite orientation
running along the c-axis in 1 (hydrogen atoms and solvent molecules are
removed for clarity, color code: C—grey; O—red; N—blue; S—yellow
and Ag—dark brown).
The dehydrated complex 1 was subjected to solvent vapour
adsorption study at RT with different solvents of varying size and
ꢀ
polarity. The profiles recorded for H2O (kinetic diameter, 2.68 A),
ꢀ
ꢀ
MeOH (4.0 A) and EtOH (4.3 A) vapour using a BELSORP-aqua3
analyzer are shown in Fig. 4. It appears that 1 is highly selective
towards H2O as no uptake was noticed with MeOH and EtOH
vapour. The H2O adsorption isotherm shows a two-step profile with
slow but gradual uptake at low pressures. The 1st step uptake,
54 mL gꢀ1 up to P/P0 z 0.7, corresponds to ꢁ0.85 mol of H2O per
formula unit of 1. This initial 1st step adsorption can be attributed to
the central part of the pore where O2W water molecules are hydrogen
bonded with –SO3 group. After P/P0 z 0.7 a sudden uptake was
observed leading to a final uptake volume of 90 mL gꢀ1 that
Fig. 2 (a) Interpenetrated networks in 1 (view along the a-axis); (b) part
of the lattice as the catenated structure and (c) Highlighted portion in (a)
revealed an embedded helical structure (view along a- and c-axes,
respectively).
Fig. 3 (a) Schematic view of entanglement in 1 (view close to the c-axis)
and (b) hydrogen bonding interaction in 1, interconnecting two different
sets of polymeric chains.
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Synthesis of 9-SA silver complex (1): The colorless crystals of 1 (9-SA:
silver complex) were grown by layering an acetonitrile solution of silver
nitrate over an aqueous solution of 9-SA and crystals were obtained
within two weeks. The same complex was also obtained by using silver
triflate and silver perchlorate as confirmed by X-ray crystallography.
HRMS characterization of complex 1 was carried out in both ESI(+) and
ESI(ꢀ) modes which shows the polymeric structure (Table S1†).
Elemental analysis (C7H10AgN5O4S as monohydrate): calculated C,
22.84; H, 2.74; N, 19.02; found C, 22.78; H, 2.51; N, 18.33%.
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corresponds to a total 1.4 mol of H2O per formula unit of 1. The
desorption curve does not retrace the adsorption curve and a prom-
inent hysteresis was observed which indicates strong interaction of the
H2O molecules with pore surfaces. The bE0 value, which reflects
adsorbate–adsorbent affinity, calculated using DR equation is found
to be 5.8 kJ molꢀ1 and also suggests strong interaction of H2O
molecules with 1. The exclusion of larger molecules like MeOH and
EtOH is probably because of the smaller channel dimension
compared to the kinetic diameter of the adsorbate molecules.
In conclusion, we have synthesized a silver-complex with entangled
networks showing selective water vapour adsorption over methanol
or ethanol. Thus, the notion of creating hierarchical structures with
interesting topological preferences and properties as a result of minor
chemical modification of the adenine nucleobase has been realized.
We thank Single Crystal CCD X-ray facility at IIT-Kanpur, CSIR,
for S. P. Mukherjee Fellowship (J.K.). This work is supported by
Council for Scientific and Industrial Research, India (SV).
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Notes and references
‡ General: 1H and 13C NMR spectra were obtained on a JEOL-DELTA2
500 model spectrometer operating at 500 MHz and 125 MHz, respec-
tively. High resolution mass spectra were obtained on a WATERS HAB
213 machine, Department of Chemistry, IIT-Kanpur, India.
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Synthesis of 2-(N9-adeninyl)ethanesulfonic acid: 9-(2-bromoethyl)
adenine9 (1.5 g, 1.0 eq.) and Na2SO3 (940 mg, 1.2 eq.) were suspended in
15 mL of an ethanol : water (2 : 1) mixture and refluxed at 80 ꢂC till the
completion of reaction as monitored by TLC analysis. The solution was
cooled down to room temperature and acidified with 1 N HCl to pH 5
and dried. The residue was dissolved in 10 mL of dimethyl formamide
and stirred for 10 min and filtered to remove excess of NaCl. The DMF
layer was evaporated and the residue was washed with 10 mL of methanol
and dried which afforded the title compound as an off white powder (1.1
g, yield 73%). HRMS: [M + H]+ calculated: 244.0504, found: 244.0508;
[M + Na]+ calculated: 266.0324, found 266.0327. mp > 275 ꢂC; 1H NMR
(500 MHz, DMSO-d6, 25 ꢂC, TMS): d (ppm) 2.97 (t, 2H, CH2), 4.36 (t,
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NMR (125 MHz, DMSO-d6, 25 ꢂC, TMS): d (ppm) 50.86, 119.06, 141.85,
149.81, 152.73, 156.28.
3014 | CrystEngComm, 2012, 14, 3012–3014
This journal is ª The Royal Society of Chemistry 2012