Novel tandem synthesis of bis(μ-NN′-tetrazolate) bridged dinuclear nickel(ii) Schiff base complex via [3 + 2] cyclo-addition at ambient condition

Article abstract of DOI:10.1039/c3dt52398e

A novel bis(μ-NN′-tetrazolate) bridged centrosymmetric dinuclear nickel(ii) Schiff base complex [Ni₂(L)₂(PTZ) ₂]·2H₂O·2CH₃CN (HL is a tridentate Schiff base, 2-((2-(ethylamino)ethylimino)methyl)-6-methoxyphenol and HPTZ is 5-pyrazinyltetrazole) has been synthesized via [3 + 2] cyclo-addition reaction of 2-cyanopyrazine and sodium azide in presence of nickel(ii) acetate tetrahydrate and HL. The structure of the complex is confirmed by single crystal X-ray diffraction analysis. The combination of H-bonding and C-H...π interactions creates a 3⁶-hxl topological supramolecular network. The acetonitrile molecules are encapsulated as a hydrophobic guest within the 2D supramolecular network.

Full text of DOI:10.1039/c3dt52398e

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Novel tandem synthesis of bis(μ-NN’-tetrazolate)
bridged dinuclear nickel(II) Schiff base complex via
[3 + 2] cyclo-addition at ambient condition†
Cite this: Dalton Trans., 2014, 43,
5643
Received 2nd September 2013,
Accepted 6th January 2014
Mithun Das,a Klaus Harmsb and Shouvik Chattopadhyay*a
DOI: 10.1039/c3dt52398e
www.rsc.org/dalton
A novel bis(μ-NN’-tetrazolate) bridged centrosymmetric dinuclear been reported so far, the majority of these methods have
nickel(II) Schiff base complex [Ni2(L)2(PTZ)2]·2H2O·2CH3CN (HL shown drawbacks like drastic reaction conditions, water sensi-
is a tridentate Schiff base, 2-((2-(ethylamino)ethylimino)methyl)- tivity and presence of highly toxic, explosive and volatile hydra-
6-methoxyphenol and HPTZ is 5-pyrazinyltetrazole) has been zoic acids.11 It was shown some time ago that performing the
synthesized via [3 + 2] cyclo-addition reaction of 2-cyanopyrazine reaction on a metal site facilitated the 1,3-dipolar addition.12
and sodium azide in presence of nickel(II) acetate tetrahydrate and More recently, the preparation of a variety of 5-substituted-1H-
HL. The structure of the complex is confirmed by single crystal tetrazoles using various transition metals as catalysts has been
X-ray diffraction analysis. The combination of H-bonding and reported.13 On the other hand, only very few metal tetrazolate
C–H⋯π interactions creates a 36-hxl topological supramolecular complexes containing Schiff bases as blocking ligands exist in
network. The acetonitrile molecules are encapsulated as a hydro- literature. To the best to our knowledge, only two such
phobic guest within the 2D supramolecular network.
examples with two different metal atoms (nickel and platinum)
are known.14 In the present paper, we would like to report the
tandem synthesis of a dinuclear nickel(II) Schiff base complex
bridged by a substituted tetrazolate formed by the reaction
of 2-cyanopyrazine, sodium azide, nickel(II) acetate and a
Schiff base ligand, HL, under stirring condition in methanol.
The X-ray diffraction study has confirmed the structure of the
complex.
The rational design and synthesis of homo- and hetero-
metallic coordination polymers using organic and inorganic
linkers to develop new materials are of continuing interest as
they found uses as single-molecule transistors,1 single-mole-
cule magnets,2 catalysts,3 sensors, storage and separation
devices.4 Among several synthetic schemes, the in situ gene-
ration of tetrazole-based ligands to produce coordination poly-
mers by a hydrothermal process is well-known.5 The chemistry
of tetrazoles and their complexes has received a vast attention
now-a-days.6 The tetrazole functional group has found a wide
range of applications in coordination chemistry as ligands,7 in
medicinal chemistry as a metabolically stable surrogate for a
carboxylic acid group8 and in material science, as high density
energy materials.9 The coordination ability of the tetrazolate
through the four nitrogen atoms allows it to serve as either a
multi-dentate or a bridging building block in supra-molecular
assemblies and this is of particular interest to supramolecular
chemists.10 However, although several synthetic methods have
All starting materials were commercially available, reagent
grade, and used as purchased from Sigma-Aldrich without
further purification.
A methanol solution (20 ml) of N-ethyl-1,2-diaminoethane
(1 mmol, 0.1 ml) and 3-methoxysalicylaldehyde (1 mmol,
152 mg) was refluxed for ca. 1 h. A methanol solution (5 ml) of
2-cyanopyrazine (1 mmol, 105 mg) was then added to it, fol-
lowed by the addition of a methanol solution (5 ml) of nickel(II)
acetate tetrahydrate (1 mmol, 249 mg) with constant stir-
ring. A solution of sodium azide (2 mmol, 130 mg) in metha-
nol–water solution was added to it and the stirring was
continued for additional 2 h. An initial precipitation was fil-
tered off. The crystalline complex started to separate on stand-
ing the resulting solution at room temperature and it was
collected by filtration after ca. 2 days. Red single crystals, suit-
able for X-ray diffraction, were obtained from an acetonitrile–
DMSO mixture. (See notes and references for details).‡
The ligand, HL, was prepared by the 1 : 1 condensation of
3-methoxysalicylaldehyde with the N-ethyl-1,2-diaminoethane
in methanol following the literature method.15 Nickel(II)
aDepartment of Chemistry, Inorganic Section, Jadavpur University,
Kolkata - 700 032, India. E-mail: shouvik.chem@gmail.com; Tel: +91 3324572941
bFachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse,
D-35032 Marburg, Germany
†Electronic supplementary information (ESI) available. CCDC 956491. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52398e
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Scheme 1 Graphical representation of synthetic route and possible reaction pathway for the tandem synthesis of complex 1.
acetate tetrahydrate was then reacted with this tridentate
ligand, followed by 2-cyanopyrazine and sodium azide to
form bis(tetrazolate) bridged dinuclear nickel(II) complex 1
(Scheme 1).
Complex 1 crystallizes in the monoclinic space group P21/c.
The complex features a centrosymmetric bis(µ-NN′-tetrazolate)
bridged dinuclear nickel(II) moiety [Ni2L2(PTZ)2] with two non-
coordinated water and two acetonitrile molecules. A perspec-
tive view of 1 is shown in Fig. 1. Within the dinuclear unit,
each nickel(II) center is six-coordinated and assumes distorted
octahedral geometry. Ni(1) is coordinated by an oxygen atom,
O(8), and two nitrogen atoms, N(3) and N(6), of a deprotonated
tridentate Schiff base (L−) in meridional fashion and two nitro-
gen atoms, N(14) and N(21), of 5-pyrazinyltetrazolate. A nitro-
gen atom, N(22)*, from a symmetry related (2 − x, −y, 2 − z)
(PTZ)− ligand of the centrosymmetric dimer coordinates Ni(1)
to complete its distorted octahedral geometry. It should be
noted that the (µ-NN′) bridging geometry of the (PTZ)− ligand
is considerably asymmetric, which can be seen from the
inequality in the bridging angles (Ni(1)–N(21)–N(22) = 139.8(1)
Fig. 1 Perspective view of complex 1 with selective atom numbering
scheme. The water and acetonitrile molecules have been omitted for
clarity. Selected bond lengths: Ni(1)–N(3) = 2.1499(16), Ni(1)–N(6) =
1.9957(16), Ni(1)–N(14) = 2.1887(15), Ni(1)–N(21) = 2.0628(15), Ni(1)–
N(22)* = 2.0666(15), Ni(1)–O(8) = 2.0152(13). Selected bond angles are
given in Table S1 (ESI†). Symmetry element * = 2 − x, −y, 2 − z.
and Ni(1)*–N(22)–N(21) = 124.6(1)) involving the bridging
nitrogen atoms, as was also observed in similar complex.14
The saturated five membered ring [Ni(1)–N(3)–C(4)–C(5)–N(6)]
has an envelope conformation on C(4) with puckering para-
meters Q(2) = 0.448(2) Å and ϕ(2) = 252.3(2)°.16
The complex shows significant supramolecular interactions
(H-bonding and C–H⋯π interactions17). The H-bonding inter- of H-bonding and C–H⋯π interactions forms a 36-hxl topolo-
actions between centrosymmetric dimer and lattice water gical supramolecular network as shown in Fig. S3 (ESI†).18 The
molecules produce two-dimensional H-bonded network details of supramolecular structure analysis are provided in ESI.†
(Fig. S1 in ESI†). Furthermore, C–H⋯π interactions forms a
In the IR spectrum (Fig. S4, ESI†), a distinct band due
one dimensional array as shown in Fig. S2 (ESI†). Combination to the azomethine (CvN) group at 1641 cm−1 has been
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noticed.19 A broad band around 3473 cm−1 is assigned to the
OH stretching vibration of the water molecule, which is
involved in hydrogen bonding.20 The sharp band due to the
amino NH group appears at 3217 cm−1 21
The peaks at 1444
.
and 1404 cm−1 are due to the tetrazolate moieties which are
consistent with those of other tetrazolate-based metal com-
plexes in previous reports.22 The electronic spectrum (Fig. S5,
ESI†) shows an absorption band at 539 nm and a broad band
at 883 nm with a shoulder peak at 778 nm in DMSO. These
bands are respectively assigned to the following transitions of
an octahedral nickel(II) complex: 3A2g→3T1g(F), 3A2g→3T2g(F)
3
and A2g→1Eg (shoulder band).23 The higher energy d–d band
3A2g→3T1g(P) is obscured by strong ligand to metal charge
transfer transition at 385 nm, which is also characteristics of
transition metal complexes with Schiff base ligands.24 The
emission spectral behavior has been studied at room tempera-
ture using DMSO solution. The complex on excitation at
385 nm shows emissions at 413 nm and 436 nm with exited
state mean lifetimes 1.322 ns and 0.914 ns respectively at
room temperature. It can be attributed to intra-ligand fluo-
rescent 1(π→π*) emissions of the coordinated ligand. The
details of the photoluminescence lifetime data and spectrum
have been depicted in Table S2 (ESI†) and Fig. 2 respectively.
The room temperature magnetic susceptibility measurement
shows that the magnetic moment of the complex is 3.1 BM per
nickel atom, indicating the presence of two unpaired electrons
as expected for octahedral nickel(II) complex.
The present complex exhibits a reduction peak at Epc
=
−0.98 V with a direct oxidation peak at Epa = −0.66 V corres-
ponding to the formation of a quasi reversible Ni(II)/Ni(I)
couple with E1/2 = −0.82 V. The peak separation of this couple
(ΔEp) is 320 mV. A representative cyclic voltammogram is
shown in Fig. S6 (ESI†). The single-electron nature of the vol-
tammogram has been confirmed by the comparison of the
current heights for the complex and that of a simple [Fe-
(bipy)3]2+ complex under identical condition.25 The criteria of
reversibility has been checked by observing the constancy of
the peak–peak separation (ΔEp = Epa – Epc) and the ratio of the
peak heights (ipa/ipc ∼ 1) with variation of scan rates.26 All the
redox signals remain virtually invariant under different scan
rates (0.01–1.0 V s−1) in the temperature range 300–280 K.
Solvent dependent shift and change in electrochemical reversi-
bility of redox couples are not noteworthy.
In the present work, we have reported the synthesis and
characterization of a bis(μ-NN′-tetrazolate) bridged dinuclear
nickel(II) Schiff base complex via [3 + 2] cycloaddition reaction
between metal coordinated azide and 2-cyanopyrazine. The
syntheses of tetrazoles by microwave irradiations or under
solvo-thermal conditions are frequently reported by several
groups. But the tetrazolate bridged transition metal complexes
with Schiff bases as co-ligands are extremely rare in literature.
Fig. 2 (a) Photoluminescence spectrum and (b) time-resolved photo-
luminescence decays of complex 1 in DMSO at room temperature.
[3 + 2] cycloaddition reaction to form tetrazolate moiety. Par-
ticipation of tetrazolate in a (μ-NN′) fashion forms a dinuclear
nickel(II) complex. The facile synthesis of the complex would,
therefore, afford a convenient synthetic route for this type of
complexes and opens up new possibilities for the synthesis of
tetrazolate bridged transition metal complexes with Schiff
base co-ligands.
Work is in progress to improve the yield of the reaction and
to develop more systems based on this strategy to generalize
the concept.
Acknowledgements
We have prepared a bis(tetrazolate) bridged dinuclear nickel(II) This work was supported by the DST, India under FAST Track
complex with Schiff base as the blocking ligand under non- Scheme (order no. SR/FT/CS-118/2010, dated 15/02/2012). We are
hydrothermal and non-microwave reaction condition. During thankful to Dr Saurabh Das, Department of Chemistry, Jadavpur
the course of the reaction, a metal Schiff base complex with University, India for providing facilities for UV-Vis spectral studies
azide as co-ligand is formed and this is then made to undergo and Mr Kallol Bera for fluorescence life-time measurements.
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Notes and references
‡Yield: 330 mg (68%, based on nickel(II) salt). Anal. Calc. for C19H25N9O3Ni
(972.38): C, 46.9; H, 5.2; N, 25.9%. Found: C, 46.8; H, 5.4; N, 25.7%. FT-IR (KBr,
cm−1): 3473 (broad, s, OH), 3217 (m, NH), 1641 (s, CvN), 1444 (s, PTZ), 1403 (m,
PTZ). UV-Vis, λmax (nm) [εmax(dm3 mol−1 cm−1)] (DMSO): 843(76), 778(61),
539(54), 385(8694). Magnetic moment: 3.1 B.M. Elemental analysis (carbon,
hydrogen and nitrogen) was performed using a PerkinElmer 240C elemental
analyzer. IR spectrum in KBr (4000–450 cm−1) was recorded using a PerkinElmer
Spectrum Two FT-IR spectrometer. Electronic spectrum in DMSO (1100–300 nm)
was recorded in a JASCO V-630 spectrophotometer. The magnetic susceptibility
measurements were done with an EG & PAR vibrating sample magnetometer,
model 155 at room temperature and diamagnetic corrections were made using
Pascal’s constants for all the constituent atoms. Fluorescence spectrum was
obtained on SHIMADZU RF-5301PC Spectrofluorophotometer at room tempera-
ture. Electrochemical measurements were performed in DMSO solution under a
dry nitrogen atmosphere in a conventional three-electrode configuration using
Pt disk working electrode, Pt auxiliary electrode and Ag/AgCl reference electrode,
with tetrabutylammonium hexafluorophosphate as supporting electrolyte in the
potential range from −2 to +2 V and were uncorrected for junction contribution.
The value for the Fc–Fc+ couple under our conditions was 0.39 V. Fluorescence
total emission intensity decay measurement was carried out on a fluorescence
lifetime spectrometer with components from Edinburgh Analytical Instruments
(EIA, UK) and EG&G ORTEC (USA) and operated in the time-correlated-single-
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photon-counting mode. Excitation was provided by
a pulsed high-pressure
1.5 atm N-lamp operating at 25 kHz repetition rate, the pulse profile having a
FWHM of 1.3 ns. Slits of bandwidth 16 nm were used in both excitation
and emission channels. Intensity decay profiles were fitted to the sum-of-
exponentials series
where Ai is a factor representing the fractional contribution to the time-resolved
decay of the component with a lifetime of τi. Best fit parameters were estimated
using software supplied by Edinburgh Instruments (UK), implementing a non-
linear least-squares iterative fitting procedure. Mean lifetime was calculated
using the relation
A single crystal having suitable dimensions was used for the X-ray crystallo-
graphic analysis using a ‘Bruker SMART APEX II’ diffractometer equipped with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 100 K. The frames
were integrated with the Bruker SAINT software package using a narrow-frame
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full-matrix least-squares refinement on F2 with 300 variables converged at R1
=
3.67% for the observed data and wR2 = 7.29% for all data. Hydrogen atoms were
placed in their geometrically idealized positions and constrained to ride on their
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Fourier maps and were kept fixed. CCDC reference number 956491.
Crystal data: formula = C38H50N18Ni2O6, formula weight = 972, crystal size
[mm] = 0.17 × 0.18 × 0.23, temperature (K) = 100, crystal system = monoclinic,
space group = P21/c, a = 13.3833(8) Å, b = 9.8759(6) Å, c = 17.7659(11) Å,
β = 110.1600(10)°, Z = 2, dcalc (g cm−3) = 1.465, μ (mm−1) = 0.921, F(000) = 1016,
total reflections = 25 325, unique reflections = 5029, observed data [I > 2σ(I)] =
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