Zn(II) formate bpy Coordination polymer 2016

Research paper
Synthesis, characterization and molecular structure of a zinc(II)
formate-2,20-bipyridine mono-dimensional coordination polymer.
Comparison with other 2,2-bipyridine coordination compounds
Jean Ngoune
a,
⇑
, Jean Jacques Anguile
b
, Justin Nenwa
c
, Golngar Djimassinga
d
, Claudio Pettinari
e,
⇑
,
Eleuterio Álvarez
f
, Luciano Pandolfo
g
a
Department of Chemistry, The University of Dschang, PO Box 67, Dschang, Cameroon
b
Department of Chemistry, The University of Sciences & Techniques of Masuku, PO Box 238, Franceville, Gabon
c
Department of Inorganic Chemistry, The University of Yaoundé 1, PO Box 812, Yaoundé, Cameroon
d
Department of Chemistry, Mongo Polytechnique University Institute (IUPM), Chad
e
Scuola di Farmacia, Università di Camerino, via S. Agostino 1, 62032 Camerino (MC), Italy
f
Instituto de Investigaciones Quimicas (IIQ), Consejo Superior de Investigaciones Cientificas (CSIC), Universidad de Sevilla, Avda. Américo Vespucio 49, Isla de La Cartuja, 41092
Sevilla, Spain
g
Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy
article info
Article history:
Received 26 June 2016
Received in revised form 3 August 2016
Accepted 4 August 2016
Available online 4 August 2016
Keywords:
Zinc(II) coordination polymer
Formate complexes
2,2
0
-Bipyridine
X-ray single crystal structure
Thermal analysis
abstract
Hydrated zinc(II) formate reacts with 2,2
0
-bipyridine (2,2
0
-bpy) to form a novel zig-zag mono-dimen-
sional coordination polymer (1D CP) [{Zn(
l -OCHO)(OCHO)(2,2
0
-bpy)(H
2
O)}H
2
O]
n
,1, which was struc-
turally characterized by a single-crystal X-ray diffraction study. Title compound crystallizes in the
orthorhombic system, Pbca space group, [a= 6.7651(8), b= 17.4876(19), c= 23.180(3)]. Each zinc ion
displays a distorted octahedral geometry due to the coordination of a monotopic formate, one molecule
of water and the two nitrogen atoms of 2,2
0
-bpy, while the octahedral scheme is completed by the coor-
dination of a ditopic formate connecting two zinc ions according to a syn-anti configuration, thus gener-
ating the polymeric structure. Moreover, a crystallization water molecule acts as a supramolecular bridge
connecting formate ions belonging to neighboring, symmetry equivalent, 1D CPs through quite strong
H-bonds, thus leading to an overall waved 2D open framework. Thermal analyses show significant weight
losses corresponding to the elimination of water molecules followed by the decomposition of both for-
mate groups and 2,2
0
-bipyridine.
Ó2016 Elsevier B.V. All rights reserved.
1. Introduction
The design and synthesis of coordination polymers (CPs) has
undergone explosive growth because of their intriguing structures
and their possible applications as functional materials in various
fields such as gas sorption, storage, separation and purification,
catalysis, ion exchange or sensor devices [1–15]. A large variety
of metal ions or oligonuclear metal fragments have been employed
as nodes to obtain CPs, through the (self)-assembly with polytopic
linkers. Particularly, Zn ions, having versatile coordination proper-
ties [16–19], have been employed to obtain a large number of com-
plexes showing interesting characteristics, as photoluminescence
[20–22] and have generated a large number of interesting CPs,
being MOF-5, having the Zn
4
( l
4
-O) oxocluster as node, the most
famous example [1,2,5,6,20]. Beside suitable dinitrogen ligands
(bipyridine, imidazolate, pyrazolate, triazolate, etc.) the most
employed linkers to generate CPs are bicarboxylates, but some of
us have evidenced that also monocarboxylates ions, endowed with
a large variety of bridging coordination modes may act as good
linkers, connecting two or three metal ions and generating
1- and 2-D CPs [17,23–31].
Very recently a chiral flexible MOF [Zn
3
(bpydc)
2
(HCOO)
2
]
H
2
ODMF containing the N,O-donor 2,2
0
-bipyridyl-5,5
0
-dicarboxy-
late (bpydc) has been reported which shows unexpected and
uncommon stepwise N
2
and CO
2
adsorption [32]. Here we report
the synthesis, the thermal behavior and the single-crystal (SC)
XRD molecular structure determination of a novel 1D coordination
polymer, based on the mononuclear Secondary Building Unit (SBU)
[Zn(
l -OCHO)(OCHO)(2,2
0
-bpy)(H
2
O)]H
2
O, obtained by exploiting
http://dx.doi.org/10.1016/j.ica.2016.08.005
0020-1693/Ó2016 Elsevier B.V. All rights reserved.
⇑
Corresponding authors.
E-mail addresses: [email protected],[email protected]
(J. Ngoune), [email protected] (C. Pettinari).
Inorganica Chimica Acta 453 (2016) 263–267
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica

the bridging ability of the formate ion and the chelating behavior of
2,2
0
-bipyridine (2,2
0
-bpy).
2. Experimental section
2.1. Materials and methods
All the reactions and manipulations were carried out in air by
using laboratory grade toluene as solvent. All the commercially
available reagents were used as purchased from Aldrich and Alfa
Aesar. The IR spectrum was recorded with a Perkin-Elmer Spec-
trum 100 FT-IR spectrometer in the range 4000–650 cm
1
. The
1
H NMR spectrum was recorded on a Mercury Plus Varian 400
NMR spectrometer (400 MHz) using Me
4
Si as external standard.
Elemental analysis (C, H, N) were carried out on a LECO TruSpec
CHN instrument. Thermal analyses were performed with a Per-
kin-Elmer Simultaneous Thermal Analyzer (STA) 6000 instrument.
2.2. Crystallographic data collection and structure determination
Compound 1crystallizes as nicely shaped colorless crystals, one
of which, coated with dry perfluoropolyether, was glued on the tip
of a glass fiber and, in a cold nitrogen stream [T = 173(2) K]
mounted on top of a goniometer head. Data collection was per-
formed on Bruker-Nonius X8APEX-II CCD diffractometer, using
monochromatic radiation k(Mo, K
a
) = 0.71073 Å. A total of 33,674
intensity data were collected in the
x -scan mode for one hemi-
sphere in the 1.76°<h< 26.40°range. The data collected were
reduced using the program SAINT [33] and corrected for Lorentz-
polarization and for absorption effects by multi-scan method using
the program SADABS [34]. The structure was solved by direct
methods (SIR-2004) [35] and refined against all F
2
data by full-
matrix least-squares techniques (SHELXTL-6.12) [36]. All the
non-hydrogen atoms were refined with anisotropic displacement
parameters. The hydrogen atoms attached to carbon atoms were
included from calculated positions and refined riding on their
respective carbon atoms with isotropic displacement parameters,
while the water hydrogen atoms were located on a Fourier differ-
ence map and refined isotropically with a shelxl DFIX restraint of
0.90 Å for the OAH distances. X-ray crystallographic data collec-
tion and structure refinements are reported in the Table 1 while
the supplementary crystallographic data have been deposited at
the Cambridge Crystallographic Data Centre with deposition num-
ber CCDC 970035. Moreover, relevant selected bond lengths and
bond angles are reported in Table S1 (Supplementary Information)
while the atomic coordinates and isotropic displacement parame-
ters are listed in Table S2 and relevant H-bonds parameters are
reported in Table S3. Molecular graphics were generated by using
Mercury 3.8 program [37,38]. Color codes for all molecular graph-
ics: light gray (Zn), blue (N), red (O), gray (C), white (H).
2.3. Synthesis of [{Zn(
l -OCHO)(OCHO)(2,2
0
-bpy)(H
2
O)}H
2
O]
n
,1
Dihydrate zinc formate (0.384 g, 2.0 mmol) was suspended in
15 mL of toluene and 2,2
0
-bpy (0.32 g, 2.1 mmol) was added under
stirring. The mixture was vigorously stirred at room temperature
for 24 h obtaining a colorless solid that was filtered, washed with
5 mL of toluene and dried in the air at room temperature. Colorless
crystals suitable for a SCXRD determination were obtained by slow
evaporation of a dimethylformamide solution of the dried
precipitate.
1. Yield: 0.555 g, 80%. Elem. Anal.: Calcd for C
12
H
14
N
2
O
6
Zn: C,
41.46; H, 4.06; N, 8.06, Found: C, 41.78; H, 3.79 N, 8.23. IR absorp-
tion: 3274(br), 3162(br), 2843(w), 1605(s), 1564(s), 1475(w), 1439
(s), 1397(w), 1373(m), 1340(br), 1323(br), 1248(w), 1156(w), 1062
(vw), 1021(w), 766(br), 735(br).
1
H NMR (400 MHz, Acetone d
6
): d:
7.51 [m, 2H, 2,2
0
-bpy], 7.53 [m, 2H, 2,2
0
-bpy], 7.81 [m, 2H,
2,2
0
-bpy], 8.21 [d, 2H, 2,2
0
-bpy], and 9.06 [s, 1H, HCOO ].
3. Results and discussion
The reaction of dihydrate zinc formate with 2,2
0
-bpy in toluene
at room temperature leads to [{Zn(
l -OCHO)(OCHO)(2,2
0
-bpy)
(H
2
O)}H
2
O]
n
,1, in high yield. Compound 1is an air-stable colorless
solid, slightly soluble in some common organic solvents such as
acetone, chloroform, dimethylsulfoxide, dimethylformamide. The
FT-IR spectrum shows medium intensity broad bands, centered
at 3274 and 3162 cm
1
due to coordinated and crystallization
water, while strong signals at 1605 and 1564 cm
1
are assigned
to two different
m
CO
stretchings [39].
1
H NMR spectrum exhibits
three multiplets at d7.51, 7.53, 7.81 and a doublet at 8.21 ppm
assigned to coordinated 2,2
0
-bpy, while one singlet at d9.06 ppm
is assigned to the formate H. The results of the thermal analyses
carried out by gradually heating compound 1until 450 °C under
aN
2
flux, are shown in Fig. 1. The TGA curve evidences three dis-
tinct weight losses in the approximate range 60–250 °C, with three
corresponding endothermic process (DTA diagram). The two losses
at ca.60°C and ca.80°C correspond to the release of crystallization
and coordinated water molecules (calc. 4.52%, exp. 5.15%, each).
The resulting uncharacterized compound is stable until about
180 °C where it starts the third weight loss (ca. 40%) due to the
decomposition of the formate and 2,2
0
-bpy ligands.
The molecular structure of compound 1(Fig. 2) has been deter-
mined by a SCXRD study showing 1crystallizing in the orthorhom-
bic crystal system, Pbca space group.
The structure analysis reveals that each zinc ion is octahedrally
coordinated by a water molecule [Zn1AO2 2.058(3) Å], one bipyr-
idine molecule, almost symmetrically chelating in the common N,
N
0
-bidentate mode [Zn1AN1 2.131(3), Zn1AN2 2.161(3) Å], (being
the two pyridyl rings slightly twisted around the C7AC8 bond with
a dihedral angle of ca. 4°), and three oxygen atoms pertaining to
Table 1
Crystal data and structure refinement for 1.
Empirical formula C
12
H
14
N
2
O
6
Zn
Formula weight 347.62
Temperature 173(2) K
Wavelength 0.71073 Å
Crystal system Orthorhombic
Space group Pbca
Unit cell dimensions a = 6.7651(8) Å
a =90°
b = 17.4876(19) Å b=90°
c = 23.180(3) Å
c =90°
Volume 2742.3(6) Å
3
Z8
Density (calculated) 1.684 Mg/m
3
Absorption coefficient 1.821 mm
1
F(0 0 0) 1424
Crystal size 0.34 0.14 0.10 mm
3
Theta range for data collection 1.76–26.40°
Index ranges 56h68, 21 6k612,
28 6l623
Reflections collected 33,674
Independent reflections 2810 [R(int) = 0.0886]
Completeness to theta = 26.40°99.6%
Absorption correction Semi-empirical from equivalents
Max. and min. transmission 0.8389 and 0.5765
Refinement method Full-matrix least-squares on F
2
Data/restraints/parameters 2810/4/202
Goodness-of-fit on F
2
1.086
Final Rindices [I > 2sigma(I)] R1 = 0.0394, wR2 = 0.0801
Rindices (all data) R1 = 0.0643, wR2 = 0.0916
Largest diff. peak and hole 0.801 and 0.444 e.Å
3
R1= R ||F
o
||F
c
||/ R |F
o
|, wR2=[ R (w(F
o
2
F
c
2
)
2
)/ R (w(F
o
2
)
2
)]
1/2
.
264 J. Ngoune et al. / Inorganica Chimica Acta 453 (2016) 263–267

three distinct formate ions. Actually, while O5AC2AO4 carboxylate
coordinates to Zn1 in a monotopic fashion, [Zn1AO4 2.081(2) Å],
O1AC1AO3 behaves as a ditopic ligand, joining Zn1 to the symme-
try equivalent Zn1
i
according to a syn-anti bridging geometry
[Zn1AO1 2.083(2), Zn1
i
AO3 2.228(2) Å], thus generating a bent
ZnAOCHOAZn conformation [20,39,40]. The coordination scheme
is completed by the interaction Zn1AO3
iii
. Thus, the overall coordi-
nation geometry resembles that found in the chiral 3D coordina-
tion polymer [Zn(OCHO)
2
(N
2
H
8
C
10
)]
n
(N
2
H
8
C
10
= 4,4
0
-bipyridine)
[40]. Due to the ditopic behavior of O1AC1AO3, [Zn(
l -OCHO)
(OCHO)(2,2
0
-bpy)(H
2
O)H
2
O] self-assemble in a series of parallel
zig-zag CPs, [where the closest ZnZn separation is 5.4745(7)Å]
one of which is shown in Fig. 3.
Interestingly, coordinated and crystallization water molecules,
coming from hydrated zinc formate and, likely, from the atmo-
spheric moisture, play a relevant role in the crystal packing of 1,
being involved in some strong H-bonds [41]. Actually, as shown
in Fig. 4, coordinated water acts as a strong H-donor toward for-
mate O1 [O2O1
i
2.653(3) Å, O2AH2O2O1
i
162(4)°] and O6
pertaining to the crystallization water molecule [O2O6
iii
2.612
(4) Å, O2AH1O2O6
iii
174(4)°], that, in turn acts a strong H-donor
toward O4 pertaining to the monotopic formate ion [O6O4 2.755
Fig. 1. TGA (blue) and DTA (red) diagrams for compound 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
article.)
Fig. 2. Molecular structure of 1with a partial atom labeling scheme.
Fig. 3. View down the crystallographic baxis of one CP. 2,2
0
-bpy H atoms are
omitted for clarity.
Fig. 4. Strong H-bonds (light-blue dashed lines) connecting water molecules and
oxygens from formates in one 1D CP of compound 1. 2,2
0
-bpy H atoms are omitted
for clarity. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
J. Ngoune et al. / Inorganica Chimica Acta 453 (2016) 263–267 265

(4) Å, O6AH2O6O4 177(4)°], likely reinforcing the 1D CP
assembly.
Moreover, the crystallization water molecules cover another
relevant role, having strong H-bond interactions with O5 formate
pertaining to neighboring CPs, [O6O5
iv
2.699(4) Å,
O6AH1O6O5
iv
170(4)°]. Thus, the overall network of this com-
pound results as waved 2D open frameworks, perpendicular to
the crystallographic bc plane, one of which is shown in Fig. 5.
By examining the CAO distances of carboxylate ions, the rele-
vant role played by H-bonds in compound 1is further highlighted.
As matter of fact, while it is expected that, for the ditopic
O1AC1AO3 formate, the C1AO1 and C1AO3 bond distances are
very similar, according to a good delocalization of the negative
charge of this ion, for monotopic O4AC2AO5 you would expect
that C2AO5 was shorter than C2AO4. The measured bond lengths
[C2AO4 1.255(4) and C2AO5 1.253(5)] are instead very similar
and, incidentally, very similar to C1AO1 and C1AO3 distances
[1.260(4) and 1.242(4) Å, respectively], and this feature is very
likely related to the involvement of O5 into the above discussed
very strong H-bond [41]. This supramolecular interaction has likely
the effect to make the C2AO5 bond overall comparable to the
C2AO4, with a good electron delocalization also in this carboxylate
fragment.
Finally, parallel 2D supramolecular open networks generate the
crystal packing shown in Fig. 6.
A further point worth to be considered concerns the use of
2,2
0
-bpy and formate ion to generate Zn complexes and related
CPs. 2,2
0
-bpy is a well-known, largely used ligand, normally acting
in the symmetrically chelating N,N
0
-bidentate mode [42,43]. The
searching and retrieving information from the Cambridge Struc-
tural Database (CSD version 5.37 update Feb 2016) reveals that
in literature are present 568 structurally validated Zn complexes
containing at least one molecule of 2,2
0
-bpy, while only 158 con-
tain also two carboxylate moieties. Moreover, only 32 of them cor-
respond to CPs, all having di-, tri- or tetracarboxylates as linkers.
The formate anion has been scarcely employed to build CPs
likely due to the fact that it may easily decomposed in drastic
solvothermal conditions: 70 derivatives containing the Zn(HCOO)
2
fragment are reported in the CSD, 57 correspond to CPs and the
OACHAO moiety acts as linker in 48 species only. Moreover, a
large number of these species corresponds to Zn/ammonium oxa-
late mixed salts or Zn oxalate containing clathrate molecules. By
considering both ligands (formate and 2,2
0
-bpy) the CSD returns
only two compounds (indicated as A [44] and B [45] in Fig. 7) con-
taining Zn coordinating one formate anion and one 2,2
0
-bpy mole-
cule, beside another carboxylate species and, in both cases, the
formate ion was serendipitously obtained by in situ solvothermal
decomposition of DMF employed as reaction solvent. On the other
hand, the CSD returns only six distinct metal complexes where at
least one 2,2
0
-bpy molecule and two formate ligands are present,
namely Mn [46],Cu[47,48] and Rh [49–51]. Only a Mn [Mn
(HCOO)
2
(2,2
0
-bpy)] and a Cu [47] {[Cu(HCOO)
2
(2,2
0
-bpy)HCOOH]}
derivatives have a structure someway resembling that of 1.
More specifically, in [Mn(HCOO)
2
(2,2
0
-bpy)] both formate ions
display a ditopic behavior, connecting Mn
II
ions according to an
anti-anti geometry, thus generating a 3D CP. The structure of
[Cu(HCOO)
2
(2,2
0
-bpy)]HCOOH is much more similar to that of 1,
Fig. 5. View down the crystallographic baxis of compound 1evidencing the
H-bond interactions connecting neighboring parallel CPs (magenta dashed lines).
2,2
0
-bpy H atoms are omitted for clarity. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. View down the crystallographic aaxis of the crystal packing of compound 1.
Different colors indicate different 1D CPs running perpendicular the bc plane.
Fig. 7. The sole two structurally validated Zn complexes containing both formate
and 2,2
0
-bibyridine ligands (evidenced in red). (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this
article.)
266 J. Ngoune et al. / Inorganica Chimica Acta 453 (2016) 263–267

one formate anion acting as a monotopic ligand, while the second
one connects two Cu
II
ions in an anti-anti geometry. By this way, a
1D CP is formed and in Fig. 8 the schematic structures of 1and
[Cu(HCOO)
2
(2,2
0
-bpy)]HCOOH are shown, evidencing the resem-
blance between the two structures. In both cases 2,2
0
-bpy chelates
the metal ion and one formate is bonded in a monodentate way,
while the second formate is responsible of the polymeric assembly.
The differences consist in one water molecule coordinated to Zn,
which thus reaches its usual hexacoordination, while in the case
of copper the pentacoordination is achieved, and in the syn-anti
geometry of ditopic formate in the case of 1against the anti-anti
geometry present in the copper derivative.
4. Conclusions
We have here reported the synthesis, the thermal and spectro-
scopic characterization and the structural analysis of a 2,2
0
-bipyri-
dine-containing zinc(II) CP, [{Zn(
l -OCHO)(OCHO)(2,2
0
-bpy)(H
2
O)}
H
2
O]
n
,1, by simply mixing hydrated zinc bis-formate with 2,2
0
-bpy
in toluene. In this zig-zag 1D CP Zn ions are connected each-other
by one formate anion in syn-anti geometry, while the second for-
mate ion is coordinated to Zn in a monodentate way and is
involved into some strong H-bonds connecting parallele 1D CPs
forming waved 2D supramolecular networks. Noteworthy, despite
polycarboxylates and even monocarboxylates have largely
employed in the synthesis on a plethora of Zn CPs, formate anion
has been rarely employed for this purpose. On the other hand,
2,2
0
-bpy has been largely employed as zinc chelating ligand
[42,43], but at the present Zn bis(formate)(2,2
0
-bpy) derivatives
are not present in the CSD. Thus, compound 1is not only the first
‘‘Zn(HCOO)
2
(2,2
0
-bpy)” coordination polymer, but also the first spe-
cies containing both these ligands.
Acknowledgments
We would like to thank Dr. Corrado Di Nicola of the School of
Pharmacy, the University of Camerino, Italy, for assistance with
some analysis and their interpretations. Financial support from
the University of Camerino, the Junta de Andalucía (Grant 2009/
FQM-4826) and CSIC (Grant PIF/2008-017-1) is gratefully
acknowledged.
Appendix A. Supplementary data
Crystallographic data of compound 1have been deposited at
the Cambridge Crystallographic Data Center, CCDC 970035.These
data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2016.08.005.
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Fig. 8. Comparison of the schematic structures of 1(left) and [Cu(HCOO)
2
(2,2
0
-
bpy)]HCOOH (right). Crystallization water (in 1) and formic acid (in [Cu
(HCOO)
2
(2,2
0
-bpy)]HCOOH) are omitted for clarity.
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