A 2D Zinc Coordination Polymer Built from the Mono-deprotonated 4,4′-Azobis(3,5-dimethyl-1H-pyrazole) Ligand

Article abstract of DOI:10.1002/zaac.201800286

4,4′-Azobis(3,5-dimethyl-1H-pyrazole) (H₂azbpz) in its mono-deprotonated form as a pyrazole-pyrazolate ligand was assembled together with zinc(II) into the two-dimensional coordination polymer [Zn(Hazbpz)NO₃]·1.25DMF with sql-a topology, constituted by the dinuclear {Zn₂(μ-pz)₂(Hpz)₂}²⁺ secondary building unit. The μ₃- bridging mode of the ligand is in analogy to bridging modes observed for 4-(4-pyridyl)pyrazolates ligands. After the removal of the DMF solvent molecules, ethanol can be adsorbed up to a maximum uptake of 276 mg·g^–1 at p/p₀ = 0.9 in an S-shaped adsorption isotherm, corresponding to two ethanol molecules per [Zn(Hazbpz)NO₃] formula unit. The desorption isotherm reveals that only one EtOH is desorbed until p/p₀ = 0.4 and the other one remains hydrogen-bonded in the framework until very low pressures.

Full text of DOI:10.1002/zaac.201800286

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800286
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
A 2D Zinc Coordination Polymer Built from the Mono-
deprotonated 4,4Ј-Azobis(3,5-dimethyl-1H-pyrazole) Ligand
Simon Millan,[a] Beatriz Gil-Hernández,[b] Emrah Hastürk,[a] Alexa Schmitz,[a] and Christoph Janiak*[a]
Dedicated to Prof. Dr. Werner Uhl on the Occasion of his 65 Birthday
Abstract. 4,4Ј-Azobis(3,5-dimethyl-1H-pyrazole) (H
mono-deprotonated form as a pyrazole-pyrazolate ligand was as-
sembled together with zinc(II) into the two-dimensional coordination corresponding to two ethanol molecules per [Zn(Hazbpz)NO
polymer [Zn(Hazbpz)NO ]·1.25DMF with sql-a topology, constituted unit. The desorption isotherm reveals that only one EtOH is desorbed
by the dinuclear {Zn (μ-pz)
- bridging mode of the ligand is in analogy to bridging modes ob-
2
azbpz) in its DMF solvent molecules, ethanol can be adsorbed up to a maximum
–
1
uptake of 276 mg·g at p/p
0
= 0.9 in an S-shaped adsorption isotherm,
] formula
3
3
2+
2
2
(Hpz)
2
}
0
secondary building unit. The until p/p = 0.4 and the other one remains hydrogen-bonded in the
μ
3
framework until very low pressures.
served for 4-(4-pyridyl)pyrazolates ligands. After the removal of the
[
8]
Introduction
or catalytic activity. The simplest bipyrazole representatives
[5b]
4
,4Ј-bipyrazolyl
and
3,3Ј,5,5Ј-tetramethyl-4,4Ј-bipyr-
Coordination polymers are of interest due to their intriguing
[5a]
azolyl
are supplemented by bipyrazoles with different
[1]
topologies and their wide-spreading properties. Structures of
one- and two-dimensional coordination polymers have ad-
ditional weak interactions (e.g. hydrogen bonds, π–π stacking,
halogen bonds) besides the polymer-defining coordinative
metal–ligand bonds.[ In this context pyrazole are interesting
ligands with a pyridinic and a pyrrolic nitrogen atom next to
each other. Pyrazole ligands feature a multifaceted coordina-
tion and supramolecular chemistry with interesting catalytic,
magnetic, and optical properties.[ Furthermore, bipyrazole li-
gands are predestined to construct coordination networks with
transition metals as outlined in a recent review by Pettinari
et al.[ Bipyrazole ligands can either be used in their non-
deprotonated forms as neutral bridging ligands or in their de-
protonated forms as anionic bridging ligands. In their neutral
forms bipyrazoles build coordination frameworks related to
spacer groups between the pyrazole rings or substituents on
position 3 and 5 of the pyrazole ring in order to obtain a large
variety of coordination networks.[ Additionally, bipyrazoles
can be used together with carboxylates in the construction of
a multitude of coordination networks.[ On the other hand,
the incorporation of mono-deprotonated bipyrazolate, that is,
pyrazole-pyrazolate ligands, into coordination compounds is
7,9]
2]
10]
[
11]
rather scarce. Examples are a dinuclear nickel complex
and
3]
a ruthenium complex.[ An early example for a coordination
network containing a pyrazole-pyrazolate ligand is an euro-
pium MOF, which was synthesized from the metal in the melt
of the ligand, containing neutral and mono-anionic 3,3Ј,5,5Ј-
tetramethyl-4,4Ј-bipyrazolyl.[
12]
4]
13]
Coordination networks built from a pyrazole-pyrazolate and
an additional dicarboxylate ligand include a cobalt-coordina-
tion polymer containing the mono-deprotonated ligand 4,4Ј-
methylenebis(3,5-dimethyl-1H-pyrazole) and a piperazine-
based dicarboxylate[ and three cobalt(II) or zinc(II) MOFs
4
,4Ј-bipyridines with the adjacent N-H functions interacting
[5]
with hydrogen accepting guest molecules and anions. Their
doubly deprotonated forms – the bipyrazolates – build ther-
mally and chemically stable metal-organic frameworks through
strong metal-ligand bonds between the metal cation and the
pyrazolate anion.
14]
synthesized from mono-deprotonated 3,3Ј,5,5Ј-tetramethyl-
15]
4
,4Ј-bipyrazole and 4,4Ј-biphenyldicarboxylate.[
Herein we present [Zn(Hazbpz)NO ]·1.25DMF as the first
3
Metal-bipyrazolates show interesting properties like flexibil-
coordination polymer with the bipyrazole ligand 4,4Ј-
ity upon gas adsorption,[ tunable water sorption properties,
6]
[7]
azobis(3,5-dimethyl-1H-pyrazole) (H azbpz) (Scheme 1) and
2
investigated its sorption properties towards water and ethanol.
*
Prof. Dr. C. Janiak
E-Mail: janiak@uni-duesseldorf.de
[
[
a] Institut für Anorganische Chemie und Strukturchemie
Heinrich-Heine Universität Düsseldorf
40204 Düsseldorf, Germany
Results and Discussion
b] Departamento de Química
Facultad de Ciencias de La Laguna, Sección Química
Universidad de La Laguna
4,4Ј-Azobis(3,5-dimethyl-1H-pyrazole) (H azbpz) was syn-
2
38206, La Laguna, Tenerife, Spain
thesized in four steps starting from acetylacetone (1). H azbpz
2
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800286 or from the au-
thor.
was first described by Hüttel et al. in 1955 during their studies
on nitro- and nitrosopyrazoles via a Mills coupling reaction,
Z. Anorg. Allg. Chem. 0000, ᭿,0–0
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Figure 1. Extended asymmetric unit of [Zn(Hazbpz)NO
3
]·1.25DMF
(
50% thermal ellipsoids, symmetry transformations: (i) –x, –y+2, –
z+1; (ii) x–1/2, –y + 3/2, z + 1/2; (iii) x + 1/2, –y + 3/2, z–1/2.). The
disordered DMF molecule is omitted for clarity. Hydrogen bond
(
dashed orange line) distances /Å and angle /°: N–H 0.92(5), H···O
1.85(5), N···O 2.698(5), D–H···A 153(5).
2
Scheme 1. Reaction scheme for the synthesis of H azbpz.
but to the best of our knowledge was never used for the con-
struction of coordination compounds.
{
3
Zn (μ-pz) (Hpz) (NO ) } with
.588(8) Å (Figure 2).
a
Zn···Zn distance of
2
2
2
3 2
[16]
A solvothermal reaction between zinc nitrate and H azbpz
2
in dimethylformamide (DMF) and ethanol (EtOH) at 80 °C re-
sulted in plate-shaped yellow single crystals. The representa-
tive nature of the selected crystals with respect to the bulk was
Table 1. Selected bond lengths /Å and angles /° in [Zn(Hazbpz)NO
.25DMF.
3
]·
1
verified by positive matching of the simulated and experimen- Zn–N1
tal powder X-ray diffractograms (Figure S1, Supporting Infor-
1.969(3)
1.977(3)
2.020(3)
2.023(4)
111.78(14)
N1–Zn–O1
N2 –Zn–O1
112.44(15)
118.50(15)
113.26(14)
106.89(15)
92.35(14)
i
i
Zn–N2
ii
Zn–O1
mation). The single-crystal X-ray analysis revealed a coordina-
Zn–N6
N1–Zn–N6
ii
i
ii
N2 –Zn–N6
tion polymer with formula [Zn(Hazbpz)NO ]·1.25DMF. The
i
ii
3
N1–Zn–N2
O1–Zn–N6
product crystallizes in the monoclinic space group P2 /n with
one zinc(II) ion, one mono-deprotonated ligand Hazbpz , one
1
Symmetry transformations: (i) –x, –y+2, –z+1; (ii) x–1/2, –y + 3/2,
z + 1/2.
–
nitrate ligand, and 1.25 DMF molecules in the asymmetric unit
(Figure 1). The N–H function of the non-deprotonated pyr-
azole group forms a hydrogen bond to the carbonyl function
of one DMF molecule. The other DMF molecule is only a
quarter occupied and highly disordered as it sits on a special
position (inversion center) of higher symmetry than the DMF
molecule itself has (Figure S6, Supporting Information). The
solvent content is also underpinned by the thermogravimetric
analysis (TGA) and elemental analysis data (Figure 7, Experi-
mental Section).
The zinc(II) ion is tetrahedrally coordinated by three nitro-
gen atoms and one nitrate oxygen atom. Two nitrogen atoms
belong to two bridging pyrazolates (N1 and N2) and the third
one belongs to nitrogen N6 from the neutral pyrazole group.
Selected bond lengths and angles are given in Table 1. Each
mono-deprotonated pyrazole-pyrazolate ligand bridges be-
Figure 2. {Zn
2 2 2 3 2
(μ-pz) (Hpz) (NO ) } as the dinuclear building block
(
the hydrogen atoms at the methyl groups are omitted for clarity).
–
The mono-deprotonated Hazbpz ligand is perfectly planar
tween three zinc atoms through its non-hydrogen-carrying and links the square planar 4-c secondary building units to
[
17]
nitrogen atoms in the heterocycles (Figure 1). Two κN1:κN2- form a two dimensional sql-a (= fes) net (Figure 3).
The
bridging pyrazolates (pz), two coordinated pyrazoles (Hpz), dimensions of the rhomboid grid are 12.0ϫ12.0 Å2 with
and two terminal nitrato ligands form the inversion- angles of 70° and 110° measured between the centers of the
symmetric dinuclear secondary building unit (SBU) dinuclear Zn SBU.
2
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Figure 3. Section of the packing diagram of 2D-[Zn(Hazbpz)NO
.25DMF with the DMF molecules omitted for clarity.
3
]·
Figure 5. Section of the packing diagram of two adjacent two-dimen-
sional nets of [Zn(Hazbpz)NO ]·1.25DMF. DMF molecules are omit-
1
3
ted for clarity; symmetry relation between layer A (yellow) and layer
B (grey): x + 1, y, z or x, y, z + 1.
The square planar 4-c SBU can be topologically represented
as a rhomboid building block linked by the azo-functionalities
Figure 4). The [Zn(Hazbpz)NO ] layers stack in an AB fash-
(
3
ion with a symmetry relation of [x+1, y, z] or [x, y, z + 1]
between the nets and the dinuclear “Zn ”-SBUs of the adjacent
2
grids sitting in the middle of the openings (Figure 5 and Fig-
ure 6). No significant supramolecular interactions could be
identified between the layers.
Figure 6. Topological sql-a representation of two adjacent AB-stacked
2D layers.
deprotonated
ligand 4,4Ј-methylenebis(3,5-dimethyl-1H-
pyrazole) and 1,4-bis((3-carboxyphenyl)methyl)piperazine
Figure S7, Supporting Information),[
14]
our compound
(
[
Figure 4. Topological sql-a representation of
Zn(Hazbpz)NO ] layer with the {Zn (μ-pz) (Hpz) (NO
yellow rhomboids linked by the azo-functionalities.
a
single
Zn(Hazbpz)NO ]·1.25DMF is, to the best of our knowledge,
3
[
3
2
2
2
3 2
) }-SBU as
the only coordination polymer assembled from the
2+
{
M (μ-pz) (Hpz) } entity (Figures S8–S10, Supporting In-
2
2
2
2
+
Analogous dinuclear metal units {M (μ-pz) (Hpz) } can formation).
2
2
2
be found with M = Zn and Co.[
18]
Pettinari et al. noted
It is interesting to note the similarities of the coordination
–
that with an excess of 1H-pyrazole, the compound behavior of the mono-deprotonated ligand Hazbpz and 4-(4-
[
Zn(μ-pz)(Hpz)(CH COO)] , built from the aforementioned pyridyl)pyrazolates (Hpypz) (Figure S5, Supporting Infor-
3 2
dinuclear units, is formed. The dinuclear molecules transform mation). The latter are an extension of the intensively studied
with thermal treatment into the polymeric [Zn(μ-pz) ]-form. 1,2,4-triazolate system.[ Turner et al. have synthesized the
19]
2
[20]
This transformation was not observed for the 3,5-dimethylpyr- directly related framework [Zn (pypz) Cl ]·2EtOH,
Baruah et al. found that the utilization of is identical in its topology to [Zn(Hazbpz)NO ]·1.25DMF.
DMF favored the formation of the dinuclear pyrazolato- Chen et al. used these cationic [Zn(pypz)] sql-a sheets with
bridged complexes in contrast to monomeric pyrazole com- [Zn (btc) (H O) ] pillars to obtain a three-dimensional po-
plexes in reactions of zinc acetate, 1H-3,5-dimethylpyrazole rous MOF with a high methane storage capacity.[
which
2
2
2
azole analog.[
18b]
3
+
8–
2
4
2
2
21]
and aromatic carboxylic acids.[
18c]
Besides the cobalt
The TGA data of [Zn(Hazbpz)NO ]·1.25DMF as-synthe-
3
coordination polymer by Batten et al., built from the mono- sized (a.s) show a two-step weight loss up to 200 °C, which
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corresponds to a total weight loss of 19.9% (Figure 7). The shows a large hysteresis with a very slow desorption below
–1
first step of 4.0% in the range of 100–150 °C is assigned to p/p = 0.4 or around 140 mg·g , where about one EtOH mol-
0
the disordered and not hydrogen-bonded quarter-occupied ecule per [Zn(Hazbpz)NO ] formula unit would be present.
3
DMF molecule (4.2% theoretically) and the second step with This means that only one EtOH is desorbed until p/p = 0.4
0
15.9% in the range of 150–210 °C to the hydrogen-bonded and the other one remains hydrogen-bonded in the framework
DMF molecule (16.1% theoretically). After the sample was until very low pressures. Under the chosen measurement times,
degassed at 150 °C for 12 h under vacuum (10– mbar) the the desorption curve did not close. The last desorption data
5
TGA curve of the activated sample shows that the solvent mol- point at p/p = 0.01 indicates the beginning desorption of the
0
ecules had been fully removed (Figure 5). Solvent-depleted second hydrogen-bonded ethanol molecule.
[
Zn(Hazbpz)NO ] is thermally stable up to 340 °C.
3
Conclusions
The novel two-dimensional coordination polymer
[
Zn(Hazbpz)NO ]·1.25DMF with sql-a topology was structur-
3
ally characterized. The compound is based on the bipyrazole
ligand H azbpz, which was until now not used in the construc-
2
tion of coordination networks. The vapor sorption characteris-
tics for water and ethanol with the S-shaped adsorption iso-
therm of ethanol and a pronounced hysteresis confirm the
hydrophobic nature of the material. In its mono-deprotonated
version Hazbpz– is
a
pyrazole-pyrazolate ligand and
mimics the coordination behavior of 1,2,4-triazolates and
-(4-pyridyl)pyrazolates. We hypothesize that by careful ad-
4
justment of the synthesis conditions this coordination behavior
of bipyrazoles with a N–H hydrogen-bond donor function di-
]·1.25DMF as-synthesized rectly adjacent to the metal subunits can be very useful in the
Figure 7. TGA curve of [Zn(Hazbpz)NO
3
–
1
(a.s.) and degassed (deg.) with a heating rate of 5 K·min in a N
2
construction of a wide set of novel coordination polymers.
atmosphere.
Since the included solvent molecules could be removed, the
sorption properties of degassed (deg.) [Zn(Hazbpz)NO3]
Experimental Section
The chemicals used were obtained from commercial sources. No fur-
ther purification was carried out. CHN analysis was performed with a
Perkin–Elmer CHN 2400 (Perkin–Elmer, Waltham, MA, USA). IR
spectra were recorded with a Bruker Tensor 37 IR spectrometer
towards H O and EtOH vapors were tested (Figure 8).
2
–
1
3 2
[
Zn(Hazbpz)NO ]-(deg.) adsorbs about 51 mg·g H O at
p/p0
per
= 0.9, which equals about one H O molecule
2
[Zn(Hazbpz)NO3]
formula unit. For ethanol
1
13
(
Bruker Optics, Ettlingen, Germany) with a KBr unit. H and C spec-
[
Zn(Hazbpz)NO ]-(deg.) shows an S-shaped adsorption iso-
3
tra were measured with a Bruker Advance DRX-300. ESI-MS spectra
were recorded with a Thermo Quest Ion Trap API mass spectrometer
Finnigan LCQ Deca. Thermogravimetric analysis (TGA) was done
therm with the main step between 0.5 and 0.7 p/p and a maxi-
0
–
1
mum uptake of 276 mg·g at p/p = 0.9, representing about
0
two EtOH molecules per formula unit. The desorption curve with a Netzsch TG 209 F3 Tarsus (Netzsch, Selb, Germany) in the
range from 25 to 1000 °C, equipped with an Al -crucible and apply-
2 3
O
–
1
ing a heating rate of 5 K·min in a nitrogen atmosphere. The powder
X-ray diffraction pattern (PXRD) was obtained with a Bruker D2
Phaser powder diffractometer with a flat silicon, low background sam-
α
ple holder, at 30 kV, 10 mA for Cu-K radiation (λ = 1.5418 Å). The
water and ethanol sorption isotherms were measured at 293 K using a
Quantachrome VSTAR vapor sorption analyzer. The sample was acti-
vated with a Quantachrome FloVac Degasser at 150 °C for 12 h under
–
5
a vacuum of 10 mbar.
Single Crystal X-ray Structure: Suitable crystals were carefully se-
lected under a polarizing microscope, covered in protective oil and
mounted on a 0.05 mm cryo loop. Data collection: Bruker Kappa
APEX2 CCD X-ray diffractometer (Bruker AXS Inc., Madison, WI,
USA) with microfocus tube, Mo-K
α
radiation (λ = 0.71073 Å), multi-
[
22]
layer mirror system, ω-scans; data collection with APEX2,
cell re-
experimen-
Structure analysis and re-
finement: The structure was solved by direct methods using
[
22]
finement with SMART and data reduction with SAINT,
[
23]
tal absorption correction with SADABS.
[
24]
Figure 8. Water (black) and ethanol (blue) sorption isotherms at 293 K SHELXL2016,
refinement was done by full-matrix least-squares
2
3
for [Zn(Hazbpz)NO ]-(deg.).
on F using the SHELX-97 program suite. Crystal data and details on
Z. Anorg. Allg. Chem. 0000, 0–0
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4
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
the structure refinement are given in Table 2. Graphics were drawn
3 2 2
Zn(NO ) ·4H O (60.2 mg, 0.23 mmol) dissolved in 3 mL of EtOH in
a 50 mL Pyrex vessel. After 4 d at 80 °C in an isothermal oven, the
product was thoroughly washed with a 10 v/v% DMF/EtOH mixture.
with Diamond.[
25]
Table 2. Crystal structure and refinement details of [Zn(Hazbpz)NO
3
]·
[
Zn(Hazbpz)NO
short drying in air. Yield: 76.0 mg (75%). [Zn(Hazbpz)NO
4.25Zn: calcd. C 37.88, 5.03,
found: C 38.06, H 5.11, N 25.58%. [Zn(Hazbpz)NO
Zn: calcd. C 34.85, H 3.80, N 28.45%; found: C 34.97, H
.86, N 27.78%.
3
]·1.25DMF was obtained by suction filtration and
]·1.25DMF,
26.50%;
]-(degassed),
1.25DMF.
3
[
Zn(Hazbpz)NO
3
]·1.25DMF
NO)
C
13.75
H
21.75
N
8.25
O
H
N
3
Empirical formula
[C10
H
13
N
7
O
3
Zn]·1.25(C H
3 7
/g·mol–1
Crystal system
Space group
10 13 7 3
C H N O
3
M
r
436.01
monoclinic
P2 /n
1
Crystal size /mm
Temperature /K
a /Å
b /Å
0.05ϫ0.05ϫ0.01
150
Supporting Information (see footnote on the first page of this article):
PXRD patterns; NMR, IR spectra; SBUs of mono-deprotonated bipyr-
azoles with dicarboxylates from the literature; symmetry transforma-
tion between two adjacent nets.
10.7845 (12)
13.7644 (14)
13.8068 (14)
104.715 (6)
1982.3 (4)
4
1.28
904
0.750/0.680
c /Å
β /° 3
V /Å
Acknowledgements
Z
μ /mm–1
The work was supported by the Federal German Ministry of Education
and Research (BMBF) under grant-# 03SF0492C (Optimat).
F(000)
Max./min. transmission
Measured, indep., observed re-
flections
21901, 2993, 2337 [I Ͼ 2σ(I)]
Keywords: Coordination polymer; Bipyrazole; Zinc; Pyr-
azole-pyrazolate; Ethanol sorption; Hysteresis
R
int
0.053
2993 / 1 / 256
0.834, –0.370
0.0433, 0.1173, 1.086
0.0614, 0.1261, 1.092
Data / restraints / parameters
a)
–3
Max./min. Δρ /e·Å
2
b)
R, wR(F ), S [I Ͼ 2σ (I)]
2
b)
R, wR(F ), S [all data]
References
a) Largest difference peak and hole. b) R
1
= [Σ(|F
o c o
| – |F |)/Σ|F |];
2
2 2
2 2 1/2
2
wR
2
= [Σ [w(F
o
–F
c
) ]/Σ[w(F
o
) ]] . Goodness-of-fit S = [Σ [w(F –
o
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2
4,4Ј-Azobis(3,5-dimethyl-1H-pyrazole) (H azbpz): 3-(2-(3,5-Di-
methyl-1H-pyrazolyl)hydrazone)pentan-2,4-dione (2.01 g, 9.05 mmol)
and hydrazine monohydrochloride (0.740 g, 10.8 mmol, 1.2 equiv.)
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night. The yellow precipitate was filtered off and washed thoroughly
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mixture to afford H
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2
azbpz as a yellow powder. After drying for 48 h
–1
obtained. 1H NMR (300 MHz, [D
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13
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]DMSO): δ = 2.37, 12.36.
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Journal of Inorganic and General Chemistry
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5
Received: July 1, 2018
Published online: ᭿
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© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
S. Millan, B. Gil-Hernández, E. Hastürk, A. Schmitz,
C. Janiak* ......................................................................... 1–7
A 2D Zinc Coordination Polymer Built from the Mono-depro-
tonated 4,4Ј-Azobis(3,5-dimethyl-1H-pyrazole) Ligand
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
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© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim