B. Mohapatra et al.
Inorganica Chimica Acta 484 (2019) 167–173
unusual associated connectivity of μ
3
-CO
3 2
group through CO trapping
(
Fig. 3b). Some previous reports have documented di, tri and tetra
3 2
nuclear μ -carbonate complexes [35–37]. However, CO was exogen-
ously supplied for activation in most of these studies. IR spectra of L2
and complex 2 were measured in the solid state at ambient tempera-
−
1
ture. The sharp peak observed at 1479 cm
symmetrically bridged μ -carbonate groups [38]. The existence of car-
bonate ligands in complex 2 was additionally confirmed by solid-state
could be ascribed to
3
13
C NMR spectra, where a peak at 167.9 ppm, ascribed to coordinated
carbonate group, was observed (Fig. 4) [15]. Notably, experimental
PXRD patterns of 2 matched nicely with simulated ones, supporting the
resemblance of produced majority materials to single crystals (Figure
S11).
It is proposed that the carbonate anion is formed as a result of at-
mospheric carbon dioxide hydration, as decomposition of acetate ion is
not feasible under the reaction conditions employed. Bermejo and co-
workers have reported that zinc complex of heptadentate Schiff base
ligand captures atmospheric carbon dioxide under basic medium when
reacted with zinc acetate.31 Control reactions performed in the open air
without added ligands did not reveal characteristic carbonate peak in
13
2
C spectrum. From this experiment, it was concluded that CO fixation
is possible in the presence of ligand as well as all reaction conditions.
The purine-purine interaction leads 2D polymeric structure to 3D
polymeric structure (Fig. 5). Interestingly one carbonate ion is co-
Fig. 5. 2D polymeric crystal lattice self-assemble with another layer to form 3D
polymeric structure of complex 2 (Few H-atoms are deleted for clearness) and
purine-purine interactions are shown in the dotted line and methanol molecules
are shown in the center of the polymeric structure.
ordinated to three different Zn atoms to form μ
ion coordinate to Zn atoms to form 1D polymeric bridge structure
Fig. 3b). It is noticed that in complex 2, six metal atoms and two li-
gands L2 form a macrocyclic (M L) ring composed of 30 atoms with a
3
-CO
3
3 3
bridge. The μ -CO
(
2
the atmosphere, afforded corresponding crystalline complexes 1–3,
which possess different structural framework, although they crystal-
lized in different space groups P21/c, P21/n, and C2/c of the mono-
clinic crystal system, respectively, while complex 4 exhibits triclinic
system having space group P-1.Fig. 2.
minimum distance of 4.41 Å between two metal ions and maximum
separation between two zinc atoms is 13.84 Å. N7 nitrogen and Zn atom
gets separated by a bond distance of 2.02 Å and shows a bond angle of
98.48°. The distance between carbonate oxygen and Zn atoms varies
from 1.92 to 1.99 Å.
The asymmetric unit of mononuclear complex 1 consisted of two L1
molecules, two water molecules, and one zinc atom. The zinc atom
coordinated to four oxygen ions derived from two carboxylate ions and
two coordinated water molecules to exhibit a distorted octahedral
geometry (Fig. 1). Synthesis of ligand L3 favours increased binding sites
and also brings changes in supramolecular structures by introducing
exocyclic sulphur ion at C6 position. The asymmetric unit of 3 com-
posed of one ligand L3, two water molecules and one zinc ion (Fig. 1).
Other crystallographic details of these complexes are provided in the
supporting information.
3.1. Morphological study
Samples to study autonomous, solution-phase self-organization of 2
was followed by two different methods (Fig. 6: M1 and M2). In M1,
crystals of complex 2 were dissolved in 60% methanol–water (1 mM),
followed by microscopy study; while in M2 ligand L2 alone was dis-
3 2
solved together with Zn(CH COO) in methanolic solution (ammonia
solution was added to make pH 9) and incubated for 2 hrs, followed by
microscopy study. Spherical morphologies appeared within 2 hrs in M1
sample as confirmed by microscopy (Fig. 6), which upon further in-
cubation of 7 days afforded formation of spherical clusters. Interest-
ingly, spherical morphologies did not form in first 2 hrs of co-incuba-
tion (Fig. 6). However, spherical structures appeared when the solution
was kept in contact with the atmosphere for 7 days. The presence of
zinc ions in the spherical structures were confirmed by energy-dis-
persive X-ray spectroscopy (EDX) (Figure S10). This experiment sug-
Notably, only modified diaminopurine ligand (L2) was successful in
trapping atmospheric CO
L1) did show coordination with zinc to afford a mononuclear complex
and the substitution of C6-amino group with a thiol in L3 afforded zinc
coordination via C6-thiol, albeit without incorporation of CO . Such an
outcome suggests that substitution patterns in the purine framework
could be of significance in tuning CO activation by zinc complexes of
2
in carbonate form (Fig. 3). Modified adenine
(
2
2
modified derivatives, due to changes in pKa causing altered metal ion
coordinating ability of N7 nitrogen [28]. Ligand L2 reacts with zinc
acetate in a closed system to give a discrete dimeric structure 4, having
no µ-carbonate group (Figure S2), which suggest that carbon dioxide
fixation did not occur, due to the absence of atmospheric CO . This is in
2
line with our observations concerning the design of metal–nucleobase
gests that uptake of CO
soft spherical structures.
2
by the ligand lead to the formation of stable
Gross morphology of autonomously assembled complex 2 was
analyzed by scanning electron microscopy (SEM) and atomic force
microscopy (AFM). Spherical properties of these assemblies might be
ascribed to the self-assembly of complex 2. These structures were fur-
ther investigated by high-energy, focused ion-beam (FIB) experiments
to visualize the core of these spherical structures (Fig. 7). These ex-
periments suggested that spherical structures consisted of hollow inner
core, proposing the formation of relatively stable, hollow vesicle-like
structures that were amenable to ion-beam milling (Fig. 7) [39–40].
These spherical morphologies bear resemblance to metal–organic
supramolecular structures leading to novel coordination frameworks
[
29–34].
Notably, the asymmetric unit of complex 2 was composed of two Zn
atoms, one ligand, water and a methanol molecule (Fig. 1). Zn ion
coordinated to four oxygen atoms (three carbonate oxygen and one
water molecule) form a tetrahedral structure. 2 also contained an
171
Mohapatra, Balaram
