5-(2-Pyridil)-1H-tetrazole complexes with Mo(iv) and W(iv) cyanides

Article abstract of DOI:10.1039/C6DT04908G

The reactions of tetrazole ligand derived from 2-cyanopyridine with K₃Na[Mo(CN)₄O₂]·6H₂O and K₃Na[W(CN)₄O₂]·6H₂O in water-ethanol solution result in isolation of two new complexes with the following formulae: (PPh₄)₂[Mo(CN)₃O(pdt)]·2H₂O (1) (Hpdt = 5-(2-pyridil)-1H-tetrazole) and (PPh₄)₂[W(CN)₃O(pdt)]·3H₂O (2). The complexes were characterized by elemental analysis, single crystal X-ray structure measurements, IR and UV-Vis spectroscopy, cyclic voltammetry measurement as well as thermogravimetric measurement. The X-ray crystal structure measurements indicated the presence of two coordination isomers in a single crystal. The pyridine nitrogen occupies both trans and/or cis position to the M═O bond, which is explained by similar pK of pyridine and tetrazole nitrogen. The DTG measurements indicate the stabilization of tetrazole upon coordination.

Full text of DOI:10.1039/C6DT04908G

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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5-(2-pyridil)-1H-tetrazole complexes with Mo(IV) and W(IV)
cyanides.
A. Mrowieca, A. Jurowskaa, M. Hodorowicza and J. Szklarzewicza
Received 00th January 20xx,
Accepted 00th January 20xx
The reactions of tetrazole ligand derived from 2-cyanopyridine with K3Na[Mo(CN)4O2]∙6H2O and K3Na[W(CN)4O2]∙6H2O in
water-ethanol solution result in isolation of two new complexes of formulae: (PPh4)2[Mo(CN)3O(pdt)]∙2H2O (1) (pdt = 5-(2-
pyridil)-1H-tetrazole) and (PPh4)2[W(CN)3O(pdt)]∙3H2O (2). The complexes were characterized by elemental analysis, single
crystal X-ray structure measurements, IR and UV-Vis spectroscopy, cyclic voltammetry measurement as well as
thermogravimetric measurement. The X-ray crystal structure measurements indicate the presence of two coordination
isomers in a single crystal. The pyridine nitrogen ocupies both trans and/or cis position to the M=O bond, which is
explained by similar pK of pyridine and tetrazole nitrogen. The DTG measurements indicate the stabilization of tetrazole
upon coordination.
DOI: 10.1039/x0xx00000x
www.rsc.org/
The metal complexes themselves were studied relatively
late. Most of the papers describe the synthesis and the
structure, without analysing other complex physicochemical
properties. The structure of the complexes with tetrazole
Introduction
Tetrazoles are one of the most intensively studied nitrogen
rich compounds in the last decade. This is not only connected
with their application as modern primary explosives, but also
they found numerous applications in medicine (for example as
the acid substituent exchanging carboxylic group and thus
spreading the application of known pharmaceuticals),
biochemistry, material science, optronic devices, sensors etc.1
Due to a variety of coordination modes, they started to be
utilized in the synthesis of materials of desired properties. The
coordination to a metal centre was studied at the very
beginning by one of the most important synthesis method
which is a cyclisation, in the mild conditions, between organic
nitriles and the azide anion in the presence of d-electron
metals.2 The most important was the yield of the synthesis and
the efficient removing of metal (mostly zinc, but also copper)
to recover the organic ligand – tetrazole.3 In our previous work
we have shown, that described in the literature methods do
not always work as well as described previously.4 Nowadays,
many methods of the synthesis of tetrazoles have been
described, most of them were discovered to decrease the cost
of the synthesis, decrease the temperature of the reaction (to
avoid explosion) and to get the good yield of the synthesis. The
use of lithium as a catalyst is an unique achievement,5 lastly
less expensive Fuller’s earth was also applied as the catalyst.6
ligands is very important because many of them form very
interesting metal-organic frameworks (MOF), 2D and 3D
structures which were identified and described.7 The
coordination to the metal centre may also increase tetrazole’s
thermal stability and its biochemical accessibility, which results
in a wider use of tetrazoles. Tetrazoles might be also
coordinated to the metal centre either by the reaction of the
metal centre with tetrazole ligands, by the presence (very
often as the catalyst of the reaction) of metal ions during
tetrazole’s synthesis or by the reaction of azido complex with
organic nitrile.2,8 The 1H-tetrazoles have a relatively low pKa
value, for example for 5-(2-pyridil)-1H-tetrazole pKa = 4.9, thus
they coordinate to most of the d-electron metals.9 If the metal
is present during the synthesis of the complex, the complex
can be isolated and characterised, but very often it is
decomposed without the isolation of pure tetrazoles.3,10
Very little is known of the reaction’s mechanism of the
tetrazole’s coordination to the metal centre and about the role
of metal in the tetrazole’s synthesis. Most of the studied
tetrazoles are these with the monodentate mode of the
coordination, for the MOF synthesis dinitriles with CN groups
in para or meta position are mostly used. The chelate, N-N
donor ligands are relatively less studied. In order to analyse
the sequence of ligand donor atoms coordinated to the metal
centre, we described here the 5-(2-pyridil)-1H-tetrazole.
As a coordination probe, we used tetracyanido complexes of
Mo(IV) and W(IV) for which the mechanism and the sequence
of the ligand exchange is well known.11 The 5-(2-pyridil)-1H-
tetrazole (pdt) has two nitrogen donor atoms, the tetrazole
nitrogen with pKa = 4.9 and the pyridine nitrogen with
†Electronic supplementary information (ESI) available: Selected bond lengths and
angles for compound 1 and 2. TGA and IR spectra for 1 and 2. CCDC 1523849-1523850.
For ESI and crystallographic data in CIF or another electronic format see DOI:
10.1039/x0xx00000x
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pKa = 5.25.9 Very similar pKa of both nitrogen atoms should Synthesis of (PPh4)2[Mo(CN)3O(pdt)]·2H2O (1)
. Tetrazole
result in the formation of two different coordination modes of ligand (0.134 g, 1 mmol) was added to K3Na[Mo(CN)4O2]∙6H2O
pdt in formed [M(CN)4O(pdt)]2- (M = Mo or W) ions, as the (0.490 g, 1 mmol), dissolved in the mixture of water (30 mL)
coordination sphere of metal is not symmetrical. We used the and ethanol (10 mL). The pH was lowered to 9.0 with
single X-ray crystal structure determination to describe the hydrochloric acid (4.0 M). The resulting dark green solution
coordination modes of tetrazoles, but we also presented other was treated with a solid (PPh4)Br (0.5 g, 1 mmol). After one
physicochemical properties of the isolated complexes.
day, the dark green crystals (1) were filtered off, washed with
water and dried in the air at the room temperature. Anal.
Calcd. for C57H48MoN8O3P2: C, 64.08; H, 4.70; N, 10.65%.
Found: C, 64.95; H, 4.71; N, 10.15%.
Experimental
Synthesis of (PPh4)2[W(CN)3O(pdt)]·3H2O (2). The synthetic
Materials and methods
procedure was analogous to the
1, however instead of
K3Na[Mo(CN)4O2]∙6H2O, K3Na[W(CN)4O2]∙6H2O (0.568 g,
1
K3Na[Mo(CN)4O2]∙6H2O and K3Na[W(CN)4O2]∙6H2O were
synthesized according to the literature methods.12 All other
chemicals were of analytical grade (Aldrich) and were used as
supplied. Microanalyses of carbon, hydrogen and nitrogen
were performed using Elementar Vario MICRO Cube elemental
analyser. IR spectra were recorded on a Nicolet iS5 FT-IR
spectrophotometer. The electronic absorption spectra were
mmol) was used. Anal. Calcd. for C57H48WN8O3P2: C, 60.06; H,
4.33; N, 9.83 %. Found: C, 59.38; H, 4.05; N, 9.65 %.
Crystallography
Crystallographic data collection and structure refinement.
recorded
on
Shimadzu
UV-3600
UV-VIS-NIR
Crystals of
1 and 2 suitable for X-ray analysis were selected
spectrophotometer equipped with a CPS-240 temperature
controller. Diffuse reflectance spectra were measured in BaSO4
pellets with BaSO4 as a reference on Shimadzu 2101PC
equipped with an ISR-260 integrating sphere attachment.
Cyclic voltammetry measurements were carried out in MeCN
(acetonitrile) with [Bu4N]PF6 (0.1 M) as the supporting
electrolyte, using Pt working and counter and Ag/AgCl as
from the materials prepared as described in the Synthetic
procedures. The crystal data, details of data collection and
structure refinement parameters are summarized in the Table
1. Intensity data for
1 and 2 were collected on a Super Nova
diffractometer using monochromatic Mo Kα radiation,
λ = 0.71073 Å. The positions of all atoms were determined by
direct methods. All nonhydrogen atoms were refined
anisotropically using weighted full-matrix least-squares on F2.
Hydrogens were included in the structure at idealized
positions. The structure was solved using SIR-97 and refined by
SHELXL-2014 program.14
reference electrodes on an AUTOLAB/PGSTAT 128
N
Potentiostat/Galvanostat. E1/2 values were calculated from the
average anodic and cathodic peak potentials, E1/2 = 0.5(Ea + Ec).
The redox potentials were calibrated versus ferrocene (0.440 V
versus SHE), which was used as an internal potential standard
for measurements in organic solvents to avoid the influence of
a liquid junction potential; the final values were reported
versus the standard hydrogen electrode (SHE). DTG
measurements were performed on a TGA/SDTA 851e Mettler
Toledo Microthermogavimeter under Ar atmosphere in the 25-
Table 1 Crystal data and structure refinement parameters for 1 and
2.
1
2
Empirical formula
C57H48MoN8O3P2
1050.91
C57H48WN8O3P2
1144.30
oC
range
with
scan
speed
Formula weight
700
10 oC/min.
Crystal size (mm)
0.20 × 0.20 × 0.10
Triclinic
0.30 × 0.20 × 0.10
triclinic
Crystal system
Space group
P
1
P
1
Synthetic procedures
a (Å)
13.365(5)
13.484(5)
16.718(5)
70.768(5)
88.204(5)
63.024(5)
-18 to 18
-18 to 18
-23 to 23
2510.1(15)
2
13.3538(6)
13.5304(5)
16.6261(7)
71.257(4)
88.524(4)
62.823(4)
-18 to 18
-18 to 18
-22 to 13
2504.4(2)
2
b (Å)
Synthesis of 5-(2-pyridil)-1H-tetrazole (pdt). Tetrazole ligand
c (Å)
derived from 2-cyanopyridine was synthesized according to
the modified literature method.13 To
α (°)
a solution of 2-
β (°)
cyanopyridine (2.18 mL, 20 mmol) in DMF (30 mL) NaN3 (1.47
g, 20 mmol), NH4Cl (1.21 g, 20 mmol) and LiCl (0.32 g, 7.5
mmol) were added. The mixture was heated in the oil-bath at
γ (°)
h
k
o
130 C for 180 minutes. The resulting precipitate was filtered
l
V (Å3)
off and washed with water, which caused dissolution of the
precipitate. The pH of the filtrate was lowered to 6.5 with
hydrochloric acid (4.0 M). Than the solution was extracted 3
times with 20 mL of CHCl3. Evaporation of CHCl3 resulted in the
formation of solid, but for synthesis the CHCl3 solution was
used as well.
Z
T (K)
125(3)
120(1)
Dx (g/cm3)
1.514
1.394
Reflections measured
Independent reflections
40286
23371
13585 [R(int) = 0.0526]
1.198
12615 [R(int) = 0.0480]
1.227
2
Goodness-of-fit on F
2 | J. Name., 2012, 00, 1-3
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Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.1003, wR2 = 0.1828
R1 = 0.1398, wR2 = 0.1972
R1 = 0.0416, wR2 = 0.0784
R1 = 0.0556, wR2 = 0.0828
W (2) complexes resembles slightly distorted octahedral with
equatorial plane formed by three cyano (C1, C2, C3) and one
nitrogen (N8 pyridine for A and N4 tetrazole for B) atom of
bidentate tetrazole (pdt) ligand. In complex 1 two coordination
isomers are observed simultaneously in the structure, the cis
one for component A: three cyano ligands and nitrogen atom
(N8) of pyridine ring are occupying equatorial positions (the
O1A–Mo1A–N8 angle is 94.69°), whereas oxo ligand and
nitrogen atom from tetrazole ring (N4) are occupying axial
positions (the O1A–Mo1A–N4 angle is 165.52°). In the case of
component B of
1 the way of the coordination is reversed
Results and discussion
(isomer trans) with respect to component A (N4 tetrazole at
equatorial and N8 pyridine at axial positions). In both isomers
Crystal Structures. The asymmetric parts of the unit cells of
the complexes and with adopted atomic numbering
1
2
of
1 cyano ligands are displaced from the equatorial plane
scheme are shown in the Fig. 1. Both compounds crystallize in
triclinic space group P-1 and are isostructural. They have
similar stoichiometry, so the asymmetric part of the unit cells
away from the oxo ligand and the average O1–Mo–C angle is
102.92°. It is the first situation in literature where the same
ligand has different coordination – in cis and in trans position
to oxo ligand in one crystal. The two different coordination
modes of tetrazole (cis and trans to M=O bond) seem to be a
not a case, as in both Mo and W complexes this phenomenon
was found. For both Mo and W tetracyanides, it is well
recognized, that ligand trans to M=O bond is substituted at
least order of magnitude faster than cyano ligands. Thus, we
expected that the tetrazole nitrogen should be found in this
position (as this nitrogen is more basic, pKa = 4.9 versus 5.25
for the pyridine nitrogen). The difference in basicity of both
nitrogens is not high, which allows both isomers to be formed.
It is worth to mention that the ratio of both isomers is
different for Mo than for W analogue, which indicates that
additional factors must influence the isomers’ ratio. This is
probably caused by the fact that the rate constant for the
ligand substitution for W is a little lower than for Mo
tetracyanide, while formation quotient is substantially (order
of magnitude) higher for W than for Mo analogue.
is composed of: [M(CN)3O(pdt)]2- anion (where M = Mo for
and M = W for ) where pdt = 5-(2-pyridil)-1H-tetrazole
(Scheme 1), two PPh4 counterions, and additionally two and
three water molecules for and , respectively. Selected bond
distances and bond angles are listed in the Table S1 (ESI†)
1
2
+
1
2
.
Scheme 1 Structure of 5-(2-pyridil)-1H-tetrazole, hydrogen atoms
are omitted for clarity.
In complex
in cis position to W=O bond (Fig. 1 bottom). The geometry
around W atom is listed in the Table S1 (ESI . In the
2 the nitrogen atom (N8) of the pyridine ring is
†
)
1
molybdenum atom is located above the square base of the
octahedron (0.4465(43) Å for Mo1A and 0.5377(53) Å for
Mo1B) formed by equatorial atoms C1, C2A (C2B), C3 and N8
(N4), in direction to the axial oxygen atom O1A (O1B). In case
of compound
2 the tungsten W1A atom is located 0.3743(19)
Å above the base of polyhedron defined by C1-C2-C3-N8
atoms. Providing analogous value for W1B component is
impossible due to the inability to model the disorder of oxo
(O1) and cyano (C2) ligands resulting from the low ratio of the
complementary disordered component (12%).
An average metal–C distance is 2.193 Å for
. These distances are typical for complexes with bidentate
N,N-donating ligands. The metal–Ooxo (the oxo ligand in axial
1 and 2.157 Å
for
2
position) bond distance (Mo for 1: isomer A = 1.698 Å and
Fig. 1 Single-crystal X-ray structure of [Mo(CN)3O(pdt)]2- (top) and
B
=
1.656 Å, W for 2: A = 1.717 Å) is typical for
[W(CN)3O(pdt)]2- (bottom), hydrogen atoms are omitted for clarity.
15
cyanooxometalates and is significantly closer to the central
metal(IV) ion than to four cyano groups. It is worth noticing
that metal–Ooxo bond is relatively short and can be treated
rather as the triple bond (Mo≡O ≈ 1.65 Å) according to the
Cotton classification.16 All interatomic bond distances of
In both structures, central atom (Mo for
1 and W for 2)
shows static disorder and was refined in two complementary
components A and B (percentage ratio of A/B = 44/54 for Mo
whereas A/B = 88/12 for W). The geometry around Mo (1) and
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metal(IV) ion=O are comparable with other characterized responsible for self-assembly of the complex molecules in
cyanooxocomplexes of molybdenum(IV) and tungsten(IV) with layers (Fig. 2 bottom) and stabilization of 3D structure. The
mono- and bidentate ligands (avg. 1.68 Å).15
In both structures the pyridine group of the pdt ligand and 7.270
shortest intermolecular metal...metal distances are 7.372 Å
for and respectively. The
Å
1
2,
(atoms C5-C6-C7-C8-C9-N8) forms a plane twisted to the tetraphenylphosphonium cations propagate along the [100]
tetrazole plane (C4-N4-N5-N6-N7) by angle 8.79(10)° and and [001] directions and are stabilized by π...π stacking. Chains
6.63(2)°, for
pdt ligand in both structures are well established, although layers of [Mo(CN)3O(pdt)]2- anions parallel to the ac plane.
displacement ellipsoids of atoms C5-C6-C7-C8-C9-N8 of are
1 and 2
, respectively. Generally, all atoms of the of [PPh4]+ cations related by translation along b axis separate
1
Table 3 The hydrogen bond lengths in 1 and 2.
significantly larger than other atoms in this structure. This
shows a quite significant conformational freedom of the ligand
H...Cg [Å]
1
X-H...Cg [°]
X...Cg [Å]
resulting from the static disorder of the structure
Packing analysis of the crystal structures of
1.
C12-H12...Cg2 [-x,1-y,1-z]
2.72
2.88
2.65
149
131
150
3.563(8)
3.573(7)
3.502(7)
1
and 2 at the
C31-H31...Cg3 [1-x,1-y,1-z]
C48-H48...Cg4 [-x,1-y,1-z]
supramolecular level reveals that the complex molecules are
held together by three hydrogen C-H...π bondings (Fig. 2 top)
and several hydrogen interactions which are summarized in
the Table 2. The X-H...π contact can be classified by type IV,
according to Malone.17
2
C(12)-H(12)...Cg4 [1-x,1-y,1-z]
C(43)-H(43)...Cg3 [1-x,1-y,1-z]
C(56)-H(56)...Cg1 [2-x,1-y,1-z]
2.76
2.84
2.70
145
132
151
3.580(4)
3.552(5)
3.558(5)
1: Cg1: C16-C17-C18-C19-C20-C21, Cg2: C40-C41-C42-C43-C44-C45, Cg3: C46-C47-C48-C49-
C50-C51, Cg4: N4-N5-N6-N7-C4;
2: Cg1: C28-C29-C30-C31-C32-C33, Cg2: C46-C47-C48-C49-C50-C51, Cg3: C10-C11-C12-C13-
C14-C15, Cg4: N4-N5-N6-N7-C4
IR Spectra. IR spectra of
(ESI . The νMo=O and νW=O band position is observed at 950
and at 948 cm-1 in respectively. For [Mo(CN)3O(LL-κ2-
and 2
1 and 2 are presented in the Fig. S1
†)
1
N,N)]n- complexes (where LL = 1,10-phenanthroline, 2,2’-
bipyridine,
2-(aminomethyl)pyridine, Schiff base ligands based on
2-acetylpyridine and ethanolamine, 2-acetylpyridine and
methylamine or 2-acetylpyridine and butylamine)18-21 the νMo=O
band position is observed in the range 958 – 973 cm-1. The
νW=O band position in
2 is the lowest among all known W(IV)
complexes with N,N-donating ligands and it is similar to νW=O
band position in W(IV) complexes with N,O-donating ligands.
The complex with tetrazole ligand is placed between
(PPh4)2[W(CN)3O(pic)] and (PPh4)2[W(CN)3O(quinal)].15 In the
C≡N stretching region, two bands located at 2098 and 2111
cm-1 for and at 2089 and 2106 cm-1 for
1 2 are observed. These
bands are connected with stretching vibrations of three cyano
ligands. The bands number, as well as bands positions, is
characteristic for complexes of this class. In other regions, the
+
bands typical for PPh4 cation, water molecules and tetrazole
Fig. 2 Intramolecular C-H...π interactions (top), the hydrogen bonds
with omitted the H atoms that do not participate in the hydrogen
bonds (middle) and the hydrogen bonds network with omitted
cations and the hydrogen atoms for clarity (bottom) in 1.
ligand are observed.
UV-Vis Spectra. The UV-Vis absorbance spectra in the solid
state and in organic solvents of
1 and 2 are presented in the
Fig. 3. The bands with the lowest energy are attributed to
Table 2 π...π bond lengths in 1 and 2.
d-d transitions and are located in reflectance spectra at 673
π...π [Å]
1
shift [Å]
nm for
(metal-to-ligand charge-transfer) transitions, which are located
at 472 nm for and 513 nm for , respectively. In the
ultraviolet part (at λ < 360 nm) of the reflectance and solution
spectra of and , the bands are attributed to transitions
1 and at 599 nm for 2. They are overlapped with MLCT
Cg1...Cg2 [1+x,-1+y,1+z]
Cg2...Cg1 [-1+x,1+y,-1+z]
3.985(4)
3.984(4)
2
1.688
2.168
1
2
Cg1...Cg2 [x,y,1+z]
Cg2...Cg1 [x,y,-1+z]
3.945(3)
3.945(3)
2.086
1.695
1
2
+
within the PPh4 cations, tetrazole ligand and charge-transfer
transitions between metal (molybdenum or tungsten) and
ligands.
The lattice of water molecules participates in many
hydrogen bonds of C—H∙∙∙O, O—H∙∙∙O and O—H∙∙∙N type
(Table 3) linking pairs of complex molecules from two separate
layers (O2 and O3) (Fig. 2 middle). All these interactions are
4 | J. Name., 2012, 00, 1-3
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Table 4 Band positions of
solvents.
1
and
2
and ET parameter for various
1
2
ET [kcal·mol-1
39.1
]
λ [nm]
445
454
454
471
472
463
457
447
449
437
431
λ [nm]
503
506
506
523
505
510
503
498
495
483
478
Solvent
CHCl3
C2H4Cl2
CH2Cl2
Me2CO
DMF
41.9
41.1
42.2
43.8
DMSO
MeCN
iso-PrOH
n-BuOH
EtOH
45.0
46.0
48.6
50.2
51.9
MeOH
55.5
Fig. 3 UV-Vis reflectance spectra of 1 (top) and 2 (bottom) in BaSO4
after Kubelka-Munk transformation.
For compounds
1 and 2 the solvatochromism of the MLCT
transition is observed. In the Fig. 4 the visible part of the
spectrum, corrected to the absorbance = 1.0 (for the most
intense MLCT band) is presented.
Fig. 5 The correlations of the band position of
1
(top) and 2
(bottom) versus the solvent ET parameter for various solvents.
Cyclic voltammetry measurements. The results of cyclic
voltammetry measurements for and are presented in the
Fig. 6. For , at low scan speeds one oxidation and reduction
1
2
1
Fig. 4 UV-Vis spectra of 1 (top) and 2 (bottom) in different solvents,
spectra were adjusted to A = 1.0 for the most intense MLCT band.
peak 1 is observed at E1/2 = 0.407V. The peak-to-peak
separation (at 50 mV/s, 88 mV) indicates the one electron
process. The anodic and cathodic peak separation slightly
increases with the increasing scan speed, which may indicate
that the reversibility of this process is not full. However, the
peak separation increase is not as high as it should be for the
typical quasi-reversible redox process. The observed peak 1 is
connected with oxidation of Mo(IV) to Mo(V), reversibility of
this process is rather rare among [Mo(CN)3O(LL)]n- type
complexes (where LL denote bidentate organic ligand) and
only several complexes with such properties were described in
literature.24 Probably, the presence of the tetrazole ring
A
roughly linear correlation between λmax and the
Reichardt parameter (ET) is observed for hydroxylic, non-
hydroxylic and alkyl halogenated solvents, as presented in the
Fig. 5. Table 4 presents band positions of
1 and 2 and ET
parameter for various solvents. Very similar spectral behaviour
has also been observed for all other Mo(IV) and W(IV)
complexes [M(CN)3O(LL)]n- (M = Mo, W).22,23
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stabilizes the high oxidation state of molybdenum, allowing
the redox process to be reversible.
Thermogravimetry measurements. The thermogram of
The increase of scan speed results in the formation of the polycrystalline samples of 2 (W) (Fig. S2 bottom, ESI†) shows
new cathodic peak 2 (see Fig. 6 top) at ca. 0.507 V (at that dehydration takes place in the one separate step with Tmax
o
200mV/s). The lack of anodic peak indicates the irreversible = 88 C (Δm found 3,77 %, calculated for two water molecules
oxidation of Mo(V) to Mo(VI), but the very small shift of the Δm = 3,16 %). Then, the complex is thermally stable up to
peak position versus scan speed suggests that the oxidation 200 oC, at which starts the total decomposition of the complex
+
process is in fact reversible. Thus the lack of the anodic peak with PPh4 , 3CN- and the tetrazole’s ligand release. Found for
should be interpreted as the reduction of Mo(VI) (after its this process Δm = 71,8 %, calculated Δm = 72,3 %. The
formation) in chemical reaction following the redox oxidation. dehydrated sample was found to be much more thermally
Such process is observed for the first time for this class of stable than other complexes of [M(CN)3O(LL)]n- type25 and
Mo(IV) complexes, and it seems that the presence of the indicates that the tetrazole ligand is greatly stabilized by its
tetrazole ligand is responsible for the possibility of the coordination to the tungsten(IV) centre. In the contrary to 2,
observation of Mo(V)/Mo(VI) redox couple. The presence of the molybdenum analogue (Fig. S2 top, ESI†) starts to
the peak 2 at high scan speeds may be also responsible for the decompose much more easily. At Tmax = 73 oC the dehydration,
observed slight anodic and cathodic potential dependence on while at Tmax = 142 oC 3 HCN molecule release is observed. The
scan speed.
The assignment of the peaks in
for complex
processes are not well resolved, however calculated for both
1
are confirmed by the data decomposition Δm = 10,3 %, calculated Δm = 10,8 %. Almost
+
2
presented in the Fig. 6 (bottom). The peak 1 can together with cyano ligands, release of PPh4 and tetrazole
o
be attributed to W(IV)/W(V) couple, and is observed at ligand are observed at Tmax = 313 C. The calculated total Δm
+
E1/2 = 0.051V while cathodic peak 2 is observed at 0.121 V. This (for 2 water molecules, 3 HCN, tetrazole and 2 PPh4 ) was
is typical for Mo and W cyano complexes which redox found as 88,5 %, calculated Δm = this process Δm = 89,2 %. The
potential of W is much lower than this for Mo, for the similar decomposition temperature was found to be similar to Ni
type of complexes. In the contrary to complex 1
, the cathodic complexes (328 oC) with tetrazoles, claimed as potential
peak for W(IV)/W(V) redox couple becomes visible at higher energetic materials26 and much higher than for Cu(II)
scan speeds, while the second cathodic peak is visible also at complexes with 5-(2-pyridil)-1H-tetrazole and its derivatives.27
low scan speeds. The only slight peaks position dependence on All presented data indicate, that tetrazole ligand do not
the scan speed indicates that all observed redox processes are decompose so easily upon the coordination, thus the
reversible. In general, observed cyclic voltammograms for
and
1
coordination is an effective method for stabilization of highly
2
are similar, also the shift of the peak 2 versus 1 is similar energetic molecules of tetrazoles.
to both salts.
Conclusions
The first representatives of tetrazole complexes with
Mo(IV) and W(IV) were synthesized and characterized. 5-(2-
pyridil)-1H-tetrazole coordinates to the metal centre as the
bidentate ligand, but due to the low difference in pKa of the
pyridine and the tetrazole nitrogen, the formation of two
isomers, with the pyridine nitrogen in trans or cis position to
the M=O bond, is observed. This phenomenon could not be
observed for symmetric complexes, where cis and trans
position cannot be distinguished. The most surprising was the
finding, that cis and trans isomers coexists in the single crystal,
both for Mo and W analogues, forming chains of alternately cis
and trans isomers.
The coordination of the tetrazole to the metal centre
resulted in the increase of the thermal stability. This is not a
common effect, as for Cu complexes of 5-(2-pyridil)-1H-
o
tetrazole the temperature of the decomposition is ca. 100 C
lower than for Mo(IV) and W(IV) complexes,27 described
complexes could be rather compared with Ni ones, still
attributed to the potentially high energetic materials.26
Moreover, the cyclic voltammetry measurements indicate the
possibility of the isolation of Mo(V) and W(V) analogues,
probably the tetrazole ring stabilizes the high oxidation state
of the metal centre. The strong solvatochromism of MLCT
transition, as well as other described properties, make
Fig. 6 Cyclic voltammogram of
1
(top) and
2
(bottom) in 0.1 M
Bu4NPF6, MeCN as electrolyte.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
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