(μ-O,O′)-nitrito bridged 3-D coordination frameworks of M2+ (Mn Co, Zn) with mab and jsm topology

Article abstract of DOI:10.1016/j.jssc.2017.07.031

Four new nitrito-bridged coordination polymers of formulation [Zn(NO₂)₂(H₂O)₂]_n (1) and [M(bipy)(NO₂)₂]_n [M = Zn (2), Co (3), Mn (4)] were synthesized under solvothermal condition using N,N-dimethylformamide (DMF) as a reducing solvent. Theoretical calculation suggests in situ reduction of nitrate to nitrite with high spontaneity and without any thermodynamic barrier (ΔG = ?20.4?kcal/mol). All the complexes were characterized by elemental analysis, FT-IR spectroscopy, UV–vis spectroscopy, Raman Spectroscopy, TG analysis as well as by single crystal X-ray diffraction methods. TD-DFT approach has been employed to assign the electronic transitions occurring in the cobalt(II) complex (3) and were compared with those observed experimentally. This work also describes the preliminary studies on heterogeneous oxidation of cyclohexene by 2&3 in acetonitrile medium using TBHP.

Full text of DOI:10.1016/j.jssc.2017.07.031

Author’s Accepted Manuscript
(
μ-O,O′)-nitrito bridged 3-D coordination
2+
frameworks of M (Mn Co, Zn) with mab and jsm
topology
Rima Paul, Ankur K. Guha, Lakshi Saikia,
Sanchay J. Bora
www.elsevier.com/locate/yjssc
PII:
S0022-4596(17)30291-8
http://dx.doi.org/10.1016/j.jssc.2017.07.031
YJSSC19881
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received date: 25 May 2017
Revised date: 4 July 2017
Accepted date: 27 July 2017
Cite this article as: Rima Paul, Ankur K. Guha, Lakshi Saikia and Sanchay J
2+
Bora, (μ-O,O′)-nitrito bridged 3-D coordination frameworks of M (Mn Co, Zn
with mab and jsm topology, Journal of Solid State Chemistry
http://dx.doi.org/10.1016/j.jssc.2017.07.031
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2
+
(
-O,O)-nitrito bridged 3-D coordination frameworks of M (Mn Co, Zn)
with mab and jsm topology
a
b
c
a
Rima Paul, Ankur K. Guha, Lakshi Saikia and Sanchay J. Bora*
a
b
c
Department of Chemistry, Pandu College, Guwahati-781012, Assam, India
Department of Chemistry, Gauhati University, Guwahati-781014, Assam, India
Materials Science Division, North-East Institute of Science and Technology, Jorhat-785006,
Assam, India
Tel: +91-361-2570450, E-mail:sanchay.bora@gmail.com
Abstract
Four new nitrito-bridged coordination polymers of formulation [Zn(NO ) (H O) ] (1) and
2
2
2
2 n
[
M(bipy)(NO ) ] [M= Zn (2), Co (3), Mn (4)] were synthesized under solvothermal condition
2 2 n
using N,N-dimethylformamide (DMF) as a reducing solvent. Theoretical calculation suggests in
situ reduction of nitrate to nitrite with high spontaneity and without any thermodynamic barrier
(G = -20.4kcal/mol). All the complexes were characterized by elemental analysis, FT-IR
spectroscopy, UV-vis spectroscopy, Raman Spectroscopy, TG analysis as well as by single
crystal X-ray diffraction methods. TD-DFT approach has been employed to assign the electronic
transitions occurring in the cobalt(II) complex (3) and were compared with those observed
experimentally. This work also describes the preliminary studies on heterogeneous oxidation of
cyclohexene by 2 & 3 in acetonitrile medium using TBHP.
Keywords: Nitrito-bridged coordination polymer, solvothermal synthesis, topological analysis,
isostructurality, TD-DFT
1
. Introduction
The fascinating structural diversities of inorganic coordination polymers (CPs) [1-6],
coupled with their specific functionality have made them excellent candidates for various
potential applications[7-13] such as heterogeneous catalysis, gas and liquid adsorption,
nonlinear optics, magnetism, and molecular recognition. These polymeric complexes are
often generated from relatively simple subunits, wherein the nature of the metal ion and
the coordinating ability of the inorganic/organic ligands (linkers) play an important role in

the self assembly process. An effective strategy to assemble these functional CPs is to use
different polyatomic ligands which include N- or O-coordinating molecules to connect the
-
metal ions or clusters (nodes) [14-17]. In this context, the nitrite group (NO ) is regarded
2
as unique among the short ligands because of the number of ways in which it can
coordinate to the metal ion [18-21]. It possesses diverse bridging capabilities and have
been employed as inorganic linkers to construct mono-, di- or poly-nuclear coordination
-
polymers with different dimensionalities. For example, as an ambidentate ligand, NO2
can coordinate either through N- or O-atom, while as a bidentate ligand, it can chelate or
bridge between the metal centers. The chelation or bridge is possible either through N-
and O- atoms or through O-atoms only. However, to the best of our knowledge, only a
handful of 2-D or 3-D polymeric compounds containing 1,3-nitrito bridges have been
reported so far. Most of them were constructed using paramagnetic metal ions with a view
of examining their detail magnetic properties [22, 23]. No reports have been made with
such coordination polymers showing catalytic properties. In addition, aromatic N-donor
auxiliary ligands that can compete with the parent ligand in coordination may also be
introduced to bring about substantial structural tuning of the polymer [24-26]. The rod
like linear N,N-donor ligand, 4,4-bipyridine (bipy) is often used in the construction of
multi-dimensional CPs involving 1-D ladder [27-30], molecular bilayer [31,32], square
or rectangular grid [33-36] frameworks etc for their unique functional behaviour. In this
study, we have synthesized four new nitrito- bridged coordination polymers and their
catalytic activity towards cyclohexene oxidation has been explored.
2
. Experimental

Materials used in this work were obtained from commercial sources and used without further
purification. Zn(NO ) .6H O (Merck, India), 4,4′-bipyridine (Sigma Aldrich, USA),
3
2
2
Co(NO ) .6H O (Merck, India). C,H,N analyses were done using a Perkin-Elmer 2400 Series II
3
2
2
CHNS/O analyzer. While cobalt was estimated gravimetrically by precipitating out cobalt as
Hg[Co(NCS) ] following standard procedure, zinc was determined via complexometric
4
titrations. Thermogravimetric (TG) data were obtained using a Mettler Toledo TGA/DSC1
e
STAR system in the 40-800C range. Thermal decompositions of all the complexes were
-1
studied under N atmosphere at a heating rate of 10°C min . FT-IR spectra were recorded on a
2
-1
Perkin-Elmer RX1 spectrophotometer in the mid-IR region (4000 to 450 cm ) for KBr pellets.
The UV-vis-NIR (240-2600 nm) diffuse-reflectance spectra were obtained using a Hitachi U-
4
100 spectrophotometer. Raman spectra were recorded on a Horiba Jobin Vyon, Model LabRam
-1
HR system in the range 50-1800 cm . Room temperature magnetic susceptibilities were
measured at 300K on a Sherwood Mark 1 Magnetic Susceptibility Balance by Evans Method.
2
.1. Preparation of [Zn(NO ) (H O) ] 1
2 2 2 2 n
0
.297 g (1 mmol) of zinc nitrate hexahydrate Zn(NO ) .6H O was dissolved in 10mL
3 2 2
DMF placed in a stainless steel vessel, which was sealed and placed in programmable
oven. The mixture was heated at 120°C at 5°C/min and held at that temperature for 24hr
followed by further cooling at room temperature. The resulting colourless crystals were
filtered, washed with methanol and then dried in a vacuum desiccator over fused CaCl2.
Yield:68%. Anal. Calcd. for N H O Zn : H, 2.05%; N, 14.50%; Zn, 33.82%. Found: H,
4
8
12
2
-
1
2
2
1
.29%; N, 14.88%; Zn, 33.01%. IR spectral data (KBr disc, cm ): 3431(br), 2924(m),
851(m), 2793(w), 2496(w), 2363(m), 1595(s), 1458(m), 1342(s), 1256(w), 1103(m),
026(m), 986(w), 897(w), 802(s), 604(m). [s, strong; m, medium; w, weak; br, broad ].

2
.2. Preparation of [Zn(bipy)(NO ) ] 2
2 2 n
0
.297 g (1 mmol) of zinc nitrate hexahydrate Zn(NO ) .6H O and 4,4'-bipyridine (C H N )
3 2 2 10 8 2
(0.312g, 2mmol) was dissolved in 10mL DMF placed in a stainless steel vessel, which was
sealed and placed in programmable oven. The mixture was heated at 120°C at 5°C/min and held
at that temperature for 24hr followed by further cooling at room temperature. The resulting
colourless crystals were filtered, washed with methanol and then dried in a vacuum desiccator
over fused CaCl . Yield: 75%. Anal. calcd. for C H N O Zn: C, 38.33%; H, 2.57%; N, 17.89%;
2
10
8
4
4
Zn, 20.85%. Found: C, 37.79%; H, 2.13%; N, 18.21%; Zn, 19.96%. IR spectral data (KBr disc,
-1
cm ): 3422(br), 3055(w), 2922(m), 2849(m), 2762(w), 2369(m), 2344(m), 1969(w), 1611(s),
1
6
2
0
558(s), 1389(s), 1350(s), 1221(w), 1090(w), 1015(w), 988(w), 870(w), 831(m), 777(s), 737(w),
37(s), 573(w), 527(m).
.3. Preparation of [Co(bipy)(NO ) ] 3
2
2 n
.291 g (1mmol) of cobalt nitrate hexahydrate Co(NO ) .6H O and 4,4
'-bipyridine
3
2
2
(
C H N ) (0.312g, 2mmol) was dissolved in 10mL DMF placed in a stainless steel
1
0
8
2
vessel, which was sealed and placed in programmable oven. The mixture was heated at
20°C at 5°C/min and held at that temperature for 24hr followed by further cooling at
1
room temperature. The resulting rod-like pink crystals were filtered, washed with water
and then dried in a vacuum desiccator over fused CaCl . Yield: 82%. Anal. Calcd. For
2
C H N O Co: C, 39.11%; H, 2.63%; N, 18.24%; Co, 19.18%. Found: C, 39.66%; H,
1
0
8
4
4
-
1
2
3
1
6
.11%; N, 18.93%; Co, 18.70%. eff = 5.02 BM. IR spectral data (KBr disc, cm ):
420(br), 2922(m), 2859(m), 2766(w), 2363(w), 2344(w), 1967(w), 1609(s), 1560(s),
389(s), 1348(s), 1221(w), 1090(w), 1013(m), 988(w), 870(w), 831(s), 779(s), 739(w),
35(s), 575(w), 527(s).

2
.4. Preparation of [Mn(bipy)(NO ) ] 4
2 2 n
0
.251 g (1mmol) of manganese nitrate tetrahydrate Mn(NO ) .4H O and 4,4'-bipyridine
3 2 2
(
C H N ) (0.312g, 2mmol) was dissolved in 10mL DMF placed in a stainless steel
10 8 2
vessel, which was sealed and placed in programmable oven. The mixture was heated at
20°C at 5°C/min and held at that temperature for 24hr followed by further cooling at
1
room temperature. The resulting light brown coloured crystals were filtered, washed with
water and then dried in a vacuum desiccator over fused CaCl . Yield: 68%. Anal. Calcd.
2
For C H N O Mn: C, 39.12%; H, 2.64%; N, 18.49%. Found: C, 38.10%; H, 2.05%; N,
1
0
8
4
4
-
1
1
2
1
7.56%; IR spectral data (KBr disc, cm ): 3408(br), 2954(w), 2841(w), 2706(w),
374(w), 1603(s), 1533(w), 1414(m), 1391(m), 1352(m), 1221(m), 1140(w), 1076(w),
045(w), 1004(m), 864(w), 816(s), 752(m), 734(m), 658(w), 629(s), 498(m).
2
.5 Computational Details
The complex has been fully optimized at B3LYP [37, 38] level of theory using 6-
11++G** basis set. Analytical frequency calculation has been performed at the same
3
level of theory to characterize the nature of the stationary state. All molecules are found
to be in local minimum with all real frequencies. There is a very good agreement
between the calculated geometrical parameters with the experimental X-ray data. The
UV-Vis spectra has been calculated at the same level of theory using methanol as the
solvent medium by employing polarizable continuum model (PCM) [39].
calculations are performed using Orca [40] suite of program.
All
2
.6 Catalysis
In a typical methodology, 2 mmol of cylcohexene was taken in 10 ml of acetonitrile in a 25 ml
double necked round bottom flask fitted with water condenser to which catalyst was added at a

time. Then 2 equivalent of aqueous TBHP was added to the reaction mixture slowly. The
progress of the reaction was monitored by Gas Chromatograph (ThermoFischer Trace 1300) by
taking out reaction mixture through syringe at regular interval.
2
.7 X-ray Crystallographic Procedures
Suitable crystals of compounds 1-4 for single crystal X-ray diffraction work were obtained by
solvothermal process. In each case the chosen crystal was mounted on a glass fiber for intensity
data collection at room temperature using graphite monochromatised Mo-K radiation on a
Bruker SMART CCD Diffractometer [41]. Crystal structures were solved by the direct methods
(SHELXS) and refined by full-matrix least squares techniques (SHELXL) using SHELX-97 [42]
from within WinGX [43]. The hydrogen atoms of aqua ligands in 1 were located in difference
Fourier maps and refined with isotropic atomic displacement parameters. In order to avoid
distortion in one of the coordinated water molecule (O6), the following lengths were restrained
using SHLXL DFIX instructions. The bond lengths, O6−H6A and O6−H6B were restrained to
be 0.99 Å and 0.86 Å respectively with the e.s.d. value of 0.02 for each. On the other hand, the
hydrogen atoms of 2 and 3 are placed in calculated positions using a riding model of refinement.
Crystallographic and structure refinement details are listed in Table 1. The structural diagrams
were drawn using ORTEP-III [44] for Windows [45], PLUTON [46], Diamond [47] and
Mercury [48].
Powder X-ray diffraction patterns in the 3−60
Philips X’Pert PRO instrument using Cu-K radiation (1.5418 Å) at a scan rate of 0.5 sec
0.5 per step at 40 KV/30 mA. The calculated diffraction patterns assuming Bragg-
 2-range were recorded on a

(
 2
Brentano geometry were obtained from results of single crystal structure analyses using
the computer program PowderCell [49].

Table 1. Crystallographic and structure refinement data.
Details
[Zn(NO ) (H O) ]
2 n
1
[Zn(bipy)(NO2)2]n
2
[Co(bipy)(NO2)2]n
3
[Mn(bipy)(NO2)2]n
4
2
2
2
Empirical formula
Formula weight
Temperature
H N O Zn
C H N O Zn
C H N O Co
C H N O Mn
2
4
2
6
20 16
8
8
2
20 16
8
8
2
20 16
8
8
193.41
627.15
614.27
606.29
Crystal system, space 298(2)K
298(2)K
298(2)K
298(2)K
group
a /Å
b /Å
c /Å
Monoclinic, P2 /c
Tetragonal, P4 2 2 Tetragonal, P4 2 2 Tetragonal, P4 2 2
1 1 1 1 1 1
1
8.680(7)
7.186(5)
9.276(6)
90.00(0)
97.51(5)
90.00(0)
573.6(7)
7.9719(2)
7.9719(2)
17.7528(7)
90.0(0)
7.9805(3)
7.9805(3)
17.5253(8)
90.0(0)
8.1510(6)
8.1510(6)
17.8407(13)
90.0(0)

/°
/°
/°
Volume/Å

90.0(0)
90.0(0)
90.0(0)

90.0(0)
90.0(0)
90.0(0)
3
1128.21(6)
2, 1.846
2.194
1116.16(8)
2, 1.828
1.555
1185.32(15)
2, 1.699
1.130
Z, C3 alculated density/ 4, 2.216
-
gcm
4.250
Absorption coefficient/ 376
632
620
612
-
1
mm
F(000)
Crystal size/ mm
0.31 × 0.29 × 0.16
3.60 to 30.07
-12≤h≤10,
0.43 × 0.27 × 0.18
2.80 to 30.07
-11≤h≤11, -
10≤k≤10,
0.37 × 0.21 × 0.11
3.61 to 29.90
-11≤h≤10,-7≤k≤11, -10≤h≤11, -4≤k≤11,
0.35 × 0.25 × 0.13
3.72 to 29.99
-
range
for
data 10≤k≤10,
-20≤l≤24
-17≤l≤25
collection/º
-12≤l≤13
-25≤l≤18
6121/1608
6532
/1724
Index ranges
5133/1622
6194/1647
[R(int)=0.0250]
[R(int)=0.0197]
[
R(int)=0.0906]
[R(int)=0.0252]
Full-matrix LS on Full-matrix LS on
2
2
Reflections
unique
Refinement method
collected/ Full-matrix LS on Full-matrix LS on
F
F
2
2
F
F
1608 / 0 / 89
1.116
1724 / 0 / 89
1.182
1622 / 0 / 101
1.031
0.0452 / 0.1644
0.000
1647 / 0 / 89
1.105
0.0284/ 0.0757
0.000
0.478 and -0.428
1450029
Data
/
restraints
/
0.0271/ 0.0779
0.000
0.0286 / 0.0862
0.000
parameters
Goodness-of-fit on F
*
2
0.586 and -0.511
1450008
0.377 and -0.634
1532392
†
Final R indices [I>2(I)] 2.178 and -4.718
R1/ wR2
Shift /esd) max
Largest diff. peak and
1532391
(
-
3
hole /e.Å
CCDC Number
†
2
2 2
c
2 2
1/2
2
2 2
c
2 1/2
wR2 = { [ w(F –F ) ] /  [ w(F ) ] } ; R1 =  | |F | – |F | | /  |F *GooF = S = { [w(F - F ) ] / (n-p) }
o
o
o
c
o
o

3
. Results and Discussion
3
.1 Syntheses and properties
Crystals of the polymeric complexes, 1-4 have been obtained using nitrate salts of the respective
-
-
metal via solvothermal synthesis using DMF as the solvent. The conversion of NO to NO ions
3
2
in theses complexes may be ascribed to the reducing property of DMF at elevated temperature.
Pastoriza-Santos and co-worker had demonstrated the ability of DMF for the reduction of Ag(I)
to Ag(0) even at room temperature [50]. Such a redox reaction could be possible for other metals
as well while dealing with a solution of the metal in DMF. The reducing property of formamide
was practiced in the syntheses of ‘Ni(NO ) ’ complexes from Ni(NO ) salts as described by Vo
2
2
3 2
-
et al. [23]. In the present work, DMF reduces NO ions and oxidises itself to the corresponding
3
acid, which is believed to be taking place via a mechanism (Scheme 1) similar to the earlier
report. TD-DFT calculation also supports the spontaneity of the reaction with G value of -20.4
-
kcal/mol and suggests that there is no thermodynamic barrier for the formation of the NO ion,
2
which will bind effectively to the metal fragment during complexation.
Scheme 1
All these complexes are insoluble in water and other common organic solvents. They are
non-hygroscopic and compositionally stable in air at room temperature for extended period of
time.

3
3
.2 Crystal Structures
.2.1 [Zn(NO ) (H O) ] 1
2
2
2
2 n
Single crystal X-ray diffraction analysis reveals that 1 crystallizes in monoclinic P2 /c space
1
2
+
group. The asymmetric unit consists of two crystallographically independent Zn cations [Zn(1)
-
&
Zn(2)], two NO ligands and two coordinated water molecules. The crystallochemical formula
2
2
1
(
CCF) [51] of this species can be written as A B M , where B corresponds to nitrito ligand and
2 4 4
M corresponds to aqua ligand. As shown in Fig. 1(a), the equatorial plane of the nearly
-
octahedral Zn(1) coordination is constructed by four oxygen atoms from different bridging NO2
ligands that are connecting four nearby Zn(1) centers and generate layers parallel to the bc-plane.
-
The remaining axial sites of the octahedron are once again occupied by two bridging NO2
rd
ligands that extends in 3 dimension relating two neighboring hexacoordinated Zn(2) centers
whose equatorial planes are being made of four aqua oxygen atoms. Thus, the Zn(1) atoms can
be considered as six-connected (6-c) nodes while the Zn(2) atoms as two-connected nodes (2-c),
and the whole 3-D framework can be rationalized as a 6-c uninodal mab underlying net [Fig. 1
4
10
(
b)] with point symbol 4 .6 .8 (analysis was performed with ToposPro [52] program). A
perspective view of the polymeric extension of complex 1 in 3-D is presented in Fig. 2 (a). The
Zn(1)−O (equatorial) distances ranging from 2.070(2) to 2.099(2) Å (Table S1 in SI) in 1 are of
comparable magnitude to those of similar Ni(1)−O distances ranging from 2.075(3) to 2.083(3)
Å in Ni(NO ) [23]. Deviations of the L−M−L angles in both the coordination spheres from those
2
2
2
+
expected for an octahedron (90) speak about the distorted nature of the octahedron around Zn
centers. Further, the polymeric structure of 1 is stabilized by both intra- and inter-molecular
hydrogen bonding interactions as shown in the Fig. 2 (b). These hydrogen bridges lead to an
overall supramolecular network architecture having intricate patterns that can be rationalized by

invoking the graph set theory [53]. The trans aqua ligands in 1 along with the nitrito-O atoms
2
connect via hydrogen bonds to form supramolecular chains which can be described as C (12) in
2
2
2
graph set notation. Hydrogen bonded rings of descriptors R (3) and R (5) having periodic
1
2
repetitions are also observed in the crystal structure of 1.
Figure 1 a) An ORTEP presentation of polymer
labelled b) a fragment of the six-connected mab underlying net observed in the crystal
structure of
1, atoms of the asymmetric units are
1
.
Figure 2 a) Perspective view along the b-axis showing the extension of polymeric
structure in 3-D b) intra- and inter- molecular hydrogen bridges present in the crystal
structure of 1.

3
.2.2 [M(bipy)(NO ) ] [M = Zn for 2; Co for 3 and Mn for 4]
2 2 n
2
+
2+
Single crystal X-ray diffraction analysis indicates that the complexes
2
[Zn ],
3 [Co ]
2
+
and
structure of the Ni analogue [23]. All these complexes are differing solely with respect
to the metal ion and hence, crystal structure description of only has been given in detail.
This Zn-CP crystallizes in tetragonal space group P4 2 2. The asymmetric unit consists of
4 [Mn ] are isomorphous and isostructural with the previously reported crystal
2
+
2
1
1
-
one half of the metal cation, one NO anion and one half of the bipy molecule (Fig. 3).
2
2
The CCF of
2
is given as AB , where B corresponds to nitrito and bipy ligands. Each
3
2
+
2+
Zn ion is bridged with six neighbouring Zn ions via four 1,3-nitrito bridges and two
bipy ligands giving rise to a robust 3-D coordination architecture [Fig. 4(a)]. TOPOS
analysis suggests a uninodal 6-c jsm underlying net [Fig. 4(b)] with point symbol
1
0
4
(
5 .6 .7). All the 6-connected nodes present in the structure corresponds to Zn atoms
which are linked with two bipy and four nitrito ligands. The natural tiling for the jsm net
is isohedral [Fig. 5(a)]., i.e. the net is formed by only one kind of tile with eight vertices,
2
2
3
four faces, face symbol with ring type [5a .5b ] and volume 141.03Å . A slightly
2
+
distorted octahedral geometry around the Zn center can be understood from the
observed bonds angles being deviated from the expected value of 90°. The equatorial M–
O distances for the complex were found to be 2.175(2)Å and 2.157(2)Å. A bit shorter M–
N distances [2.105(2)Å and 2.117(2)Å] compared to the equatorial M−O distances
suggest that the octahedral geometry is little compressed along the axial direction. These
values are pretty close to those observed for the isostructural species [Ni(bipy)(NO ) ]
2
2 n
[
23], being [2.124(5)Å and 2.126(5)Å] and [2.051(8)Å and 2.063(7)Å] for M–O and M–
N distances respectively. Quite interestingly, the polymeric extension along the

crystallographic c-axis through the nitrito-bridges may be viewed as double-stranded
-
helical chains as illustrated in Fig. 5(b). The trans bridging NO ligands are coordinated
2
2
+
to the Zn ion with a significantly twisted conformation having a dihedral angle of
89.1(2)°, indicating that they are almost perpendicular to each other. In fact, this
conformational twist may be responsible for the formation of double helical chains in the
-
crystal structure of the compound in the solid state. It is important to note that the NO2
anion in 1-4 functions as a bridging spacer via both the O-atoms. Considering the
coordination abilities of the nitrite anion, this bridging (-O,O-nitrito) coordination mode
2
+
in these M complexes may be viewed as rare and rather unique. Structural similarity
among these three complexes can be evaluated by using a method developed by Fabian
and Kalman [54]. The observed unit cell similarity index (
elongation value (ϵ) for the pairs of structures ( 0.0196, ϵ = 0.020;
    are quite close to zero, indicating
the isomorphous nature of the complexes.
) and also the mean
2
&
3
,
3 & 4,

ϵ


ϵ
Figure 3 ORTEP diagram with 50% probability ellipsoid showing the metal coordination
of the polymeric species
2
.

Figure 4
a) A representation of
2
showing an extended 3-D structure via the
-
coordination of Zn(II) ions with NO and bipy ligands b) fragment of the six-connected
2
jsm net.
Figure 5
a) The isohedral tiling of jsm net b) Double stranded helical structure of 2
3.3 Powder X-ray diffraction behaviour
All the observed peaks in the experimental powder patterns of 2-4 are seen in the powder pattern
calculated on the basis of structural results obtained from single-crystal data. A comparison of
experimental PXRD patterns along with the calculated pattern of 2 is shown in Fig. 6. The close
similarity between observed and calculated XRD patterns suggests the purity of crystalline bulk
sample and also the veracity of the structure described by us. Powder XRD data with hkl indices

for the prominent lines have been determined by comparing the two patterns. Furthermore,
nearly super-imposable PXRD patterns of the complexes 2-4 confirm their structural analogy.
Figure 6
Experimental powder XRD patterns of 2-4 with the simulated pattern of 2.
3
3
.4Spectral Properties
.4.1 FT-IR and Raman spectroscopy
The Infrared and Raman spectra of the crystalline solids, 1-4 have been measured and are
given together with tentative assignments of the bands in Table 2. Coincidence of Raman
and Infrared frequencies has been observed for all the four compounds. The characteristic
intra-ligand vibrations of NO group [55-59] viz. sym(N-O) stretching and (NO2) bending
2
-
1
modes of the compounds were observed at expected positions, being 1352-1389 cm and
-
1
7
74-777 cm respectively. The skeletal vibration mode, (M-ONO) stretching were found at

relatively low frequencies and are shifted from one another. This frequency shift can
probably be explained on the basis of Badger’s rule [60-62] that relates the shifting
vibrational frequencies with metal-ligand bond lengths. That is, increase in bond strength
leads to corresponding frequency increase and vice versa. The M−Ligand(-ONO) bond
distances and their corresponding vibrational frequencies of 1-4 are summarized in Table
3
. On the other hand, the vibrations related to the bipy ligand are spreaded over the whole
spectra, are similar to those of free bipy itself and slightly shifted in the solid compounds
to a higher frequency.
Table 2. FT-IR and Raman frequencies of 1-4 with tentative assignment of bands.
-1
Frequency (cm )
Assignment
1
2
3
4
R
IR
-
-
R
126
777
IR
-
-
R
130
775
IR
-
-
R
117
774
IR
-
-
1
67
74
(M-ONO)
(NO2)
7
1
358
-
-
-
-
-
-
-
-
1355
403
661
1026
1086
1244
1315
1526
1630
1389
1352
401
659
1023
1084
1243
1314
1524
1627
1389
1353 1352
 (N-O)
sym
-
-
-
-
-
-
-
-
-
-
396
659
-
ring breathing
ring breathing
ring breathing
637
1015
1090
1221
1350
1558
1611
635
1013
1090
1221
1348
1560
1609
658
1019 1004
1084 1076 (C-C), (C-N), (C-H)
1241 1221
1312 1352
1523 1533
1625 1603
(C-H)
ring breathing, (C-H)
(C-C), (C-N)
(C-C)
-
R, Raman; IR, Infrared.
-1
Table 3. Observed M-O vibrational frequencies (cm ) and their bond distances (Å) for the 1-4
Compound
(M-ONO) sym. stretch
M−O bond lengths
[
[
[
[
Zn(NO ) (H O) ] 1
167 (Zn-ONO)
130(Co-ONO)
126(Zn-ONO)
2.070(2)
2.157(2)
2.175(2)
2.229(2)
2
2
2
2 n
Co(bipy)(NO ) ] 3
2
2 n
Zn(bipy)(NO ) ] 2
2
2 n
Mn(bipy)(NO ) ] 4
2
2 n
1
17 (
Mn-ONO)

3.4.2 UV-vis spectroscopy
The TD-DFT calculated UV-vis spectra of the cobaltous compound (
3
), is in very good
agreement with the experimental solid sate spectra (Fig. 7). The peak positions are almost
similar to that of the experimental values; however, there is a slight decrease in the
intensity of the calculated spectra. The peak at 490 nm (oscillator strength f = 0.032) is
assigned to the transition from HOMO-4 to LUMO+1 (Fig. 8). The HOMO-4 of the
complex contains the metal d orbital while the LUMO+1 consists of * orbitals of NO2
xy
ligand. The transition at 990 nm (oscillator strength f = 0.022) is mainly assigned to the
transition from HOMO-2 having contribution from metal d orbital to LUMO+1. The
yz
peak at 1220 nm (oscillator strength f = 0.057) is mainly assigned to the HOMO to
LUMO transition. The HOMO of the complex consists of the metal d orbital with
zx
nonbonding electrons at the O-atoms of the NO group while the LUMO is mainly the

*
2
orbital centered at the NO group.
2
Figure 7
The observed (black) and TD-DFT calculated (green) UV-vis spectra of 3.

Figure 8 Frontier Molecular Orbitals (FMOs) of 3 with the assigned electronic transitions.
3
.5 Thermogravimetric analysis
TG analysis (Fig. 9) shows that compound 1 undergoes a combined weight loss of 35.03% for
the expulsion of two aqua ligands along with a molecule of NO which is close to the calculated
loss of 34.19% occurring at ~195 C. In the subsequent step, the species releases one NO2
molecule (Calc. wt loss = 23.83%, Obs. wt loss = 25.01%) leaving ZnO as the final residue.
Compounds 2 & 4 undergo one-step decomposition at ~260 C with the weight loss of 72.86%
(Calculated value = 74.0%) and 79.96% (Calculated value = 78.21%) to produce ZnO and Mn3-
O respectively. In case of 3, the release of organic matters (Calc. wt loss = 73.9%, Obs. wt loss
4
=
73.56%) took place in an unidentified manner and the final product was identified as Co O .
3 4
While the gaseous products have not been experimentally identified by us, the observed PXRD
pattern confirms the identity of the residue [Fig. 3 (a-c) in ESI]. This pathway is however

tentative because it has been proposed only on the basis of observed weight losses. Nevertheless,
the TG results clearly suggest that the polymeric species 1-4 can be thermolysed to produce their
respective metal oxides as pure products at relatively low temperature.
Figure 9 Thermogravimetric curves obtained for compounds 1-4 under N atmosphere.
2
3
.6 Catalytic properties
To access the catalytic properties, the polymers 2 and 3 were employed in the oxidation
of cyclohexene (S) using TBHP as oxidant in acetonitrile (Scheme 2) medium. Results of these
reactions indicate that maximum conversions of cyclohexene were obtained at 8 hrs (48.8%
conv.) and 10 hrs (62.4% conv.) for the catalysts 2 & 3 respectively and are presented in Fig. 10.
Oxidation of cyclohexene in presence of 2 produces cyclohexene epoxide (I) as the major
product and 2-cyclohexen-1-one (II), 2-cyclohexenol (III) and cyclohexane-1,2-diol (IV) in small
yields which are evidently identified using GC by comparison with standards. On the other hand
with catalyst 3, the selectivity towards the product II & III were found to be more compared to
the rest. Reusability of compound 2 as a catalyst has been checked (Fig. 4 ESI). The catalyst was

recovered by centrifugation, washed with methanol to remove the contaminants, dried properly
th
and reused in the catalytic run. The similar procedure was performed upto 4 run and it was
observed that the activity was decreased however, a little bit cyclohexane oxide selectivity was
increased.
Scheme 2
(
a)
(b)
Figure 10 Cyclohexene conversion with respect to time using (a) 2 and (b) 3 at 60 °C.

4
Conclusion
Four new nitrito-bridged coordination polymers,
salts using DMF as the reducing solvent. The in-situ reduction of NO to NO (using
-
1-4 were constructed from nitrate (NO )
3
-
-
3
2
DMF) during the syntheses of these CPs was established by theoretical calculations. Two
crystallographically as well as compositionally distinct Zn(II) ions in
1 are bridged
-
through 1,3-NO anions generating a 3-D polymeric structure and can be rationalized as
2
a 6-c uninodal mab underlying net. In the crystal structures of 2-4, the bridging nitrito
ligands are twisted in such a way that it results in double stranded helical chains. These
isostructural CPs are extending in 3-D with a uninodal 6-c jsm underlying net. Oxidation
of cyclohexene in presence of
2 and 3 shows maximum conversion of 48.8% and 62.4%
producing cyclohexene epoxide and 2-cyclohexen-1-one respectively as the major
products.
Acknowledgements
Financial support from Department of Science and Technology, India (FASTTRACK
Grant No. SB/FT/CS-047/2013) is gratefully acknowledged. The authors also thank
Department of Instrumentation and USIC, Gauhati University, Guwahati and Department
of Chemistry, Jadavpur University, Kolkata for providing with the X-ray diffraction data.
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Graphical abstract
New nitrito-bridged coordination polymers of formulation [Zn(NO ) (H O) ] and
2
2
2
2 n
[
M(bipy)(NO ) ] [M= Zn, Co, Mn] were synthesized under solvothermal condition.
2 2 n
-
-
Theoretical calculations established that there is in-situ reduction of NO to NO (using
3
2
DMF) during the syntheses of these CPs. The catalytic activity for oxidation of
cyclohexene was found to be very good.