Nucleophically transformed N-heterocyclic nitriles trapped by cyanooxomolybdates(IV): Crystallographic and spectroscopic study

Article abstract of DOI:10.1016/j.poly.2012.07.006

The reaction of K₃Na[Mo(CN)₄O₂] ·6H₂O with 2,3-dicyanopyrazine or 2-pyridinecarbonitrile in aqueous media results in isolation of two new compounds of formulae (PPh ₄)₂[Mo(CN)₃O(pzac)]·2H₂O (Hpzac = 3-carbamoyl-2-pyrazinecarboxylic acid) and (PPh₄) ₂[Mo(CN)₃O(pynccn)]·3H₂O·EtOH (Hpynccn = 2-pyridineiminocarbonitrile) respectively. X-ray single crystal structure measurements as well as physicochemical measurements confirm transformations of 2,3-dicyanopyrazine to 3-carbamoyl-2-pyrazinecarboxylic acid and 2-pyridinecarbonitrile to 2-pyridineiminocarbonitrile. All complexes are characterized by elemental analysis, IR and UV-Vis spectroscopy. Metal-assisted ligand transformations are studied spectrophotometrically in the pH range 8.5-11.5. Two different pathways of nitrile reactivity are shown and discussed.

Full text of DOI:10.1016/j.poly.2012.07.006

Polyhedron 45 (2012) 229–237
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Nucleophically transformed N-heterocyclic nitriles trapped by
cyanooxomolybdates(IV): Crystallographic and spectroscopic study
⇑
Anna Ryniewicz, Monika Tomecka, Janusz Szklarzewicz , Dariusz Matoga, Wojciech Nitek
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of K Na[Mo(CN) O ]ꢀ6H O with 2,3-dicyanopyrazine or 2-pyridinecarbonitrile in aqueous
3
4
2
2
Received 27 March 2012
Accepted 7 July 2012
Available online 20 July 2012
media results in isolation of two new compounds of formulae (PPh
4
2
) [Mo(CN)
3
2
O(pzac)]ꢀ2H O (Hpzac =
3
-carbamoyl-2-pyrazinecarboxylic acid) and (PPh
4
)
2
[Mo(CN)
3
O(pynccn)]ꢀ3H OꢀEtOH (Hpynccn = 2-
2
pyridineiminocarbonitrile) respectively. X-ray single crystal structure measurements as well as physico-
chemical measurements confirm transformations of 2,3-dicyanopyrazine to 3-carbamoyl-2-pyrazinecarb-
oxylic acid and 2-pyridinecarbonitrile to 2-pyridineiminocarbonitrile. All complexes are characterized by
elemental analysis, IR and UV–Vis spectroscopy. Metal-assisted ligand transformations are studied spec-
trophotometrically in the pH range 8.5–11.5. Two different pathways of nitrile reactivity are shown and
discussed.
Keywords:
Molybdenum(IV)
2
2
,3-Dicyanopyrazine
-Pyridinecarbonitrile
Nitrile reactivity
Crystal structure
Ó 2012 Elsevier Ltd. All rights reserved.
1
. Introduction
Aromatic nitriles such as 2,3-dicyanopyrazine and its deriva-
on nitrogen and thus affects the hydrolysis [11]. This is probably
the only possible mechanism for simple nitriles that do not contain
any other donor atoms. However, it is interesting to study more
complicated systems, where a competition between donor atoms
is possible. It must be stressed, that d-electron metal complexes
with nitriles are used also in stabilization of nitriles by their coor-
dination, this is important for example in utilization of nitriles as
dyes [10].
tives have been found to be very important not only due to their
reactivity but also their possible applications as OLED components
[
1], in supramolecular chemistry, and as spacer ligands in the for-
mation of metal-organic frameworks (MOFs) [1–3]. They are very
strong electron acceptors and thus can be applied as second-order
nonlinear optics materials [2,4]. Derivatives of 2,3-dicyanocarbo-
nitrile have also been found to be biologically active, for example
they exhibit antibrochospastic activity [5], inhibit the activity of
p38 MAP kinase [6], cyclin-dependent kinases and show antiprolif-
erative activity [7]. Due to the presence of chromophore groups,
they have been used for the preparation of azaphthalocyanines
and applied in photodynamic therapy, non-linear optics, as dyes
etc. [8,9]. The properties of cyano derivatives can be easily modi-
fied by their coordination to a metal center [10].
On the other hand it is well known that nitriles undergo hydro-
lysis to carboxylic acids both in acidic and basic solutions and that
this process is accelerated even by six orders of magnitude in the
presence of d-electron metals. Thus it is very important to under-
stand the role of metal in this process, as well as to understand the
influence of metal on electron properties of nitriles demonstrated
by their spectra and reactivity. It has been believed that the accel-
eration of nitrile hydrolysis is induced by the coordination of a ni-
trile nitrogen to a metal center that changes the electron density
In our previous studies on a hydrolysis of 2-cyanopyrazine as-
sisted by the presence of Mo(IV) center [12] it has been found that
in aqueous-ethanolic solutions it is possible to isolate, with a high
yield, a complex with monodentate 2-cyanopyrazine as a ligand.
The ligand is coordinated not via the nitrile ligand, but through a
heteroaromatic nitrogen atom. It has also been possible to isolate
the next transient species with hydrolyzed 2-cyanopyrazine, the
2-pyrazinecarboxylic acid, still coordinated in a monodentate mode
with the same nitrogen donor atom. After prolonged reaction time,
the most stable complex with bidentate 2-pyrazinecarboxylate
bonded through N,O donor atoms has been isolated. All those com-
plexes were characterized by single X-ray crystal structures as well
1
13
as by elemental analysis, H and C NMR, IR and UV–Vis spectra. All
these results indicate that the role of a metal is much more compli-
cated than suggested earlier in the literature, and simple coordina-
tion of a metal to nitrile nitrogen may be not so important in nitrile
hydrolysis. In the presence of the excess of KCN it has also been
possible, for the first time in this class of compounds, to introduce
a cyanide to an organonitrile group with a formation of an
nitrile coordinated to the molybdenum(IV) atom in a bidentate
mode. The resulting complex with the -iminonitrile has been
a-imino-
⇑
Corresponding author.
E-mail address: szklarze@chemia.uj.edu.pl (J. Szklarzewicz).
a
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.006

2
30
A. Ryniewicz et al. / Polyhedron 45 (2012) 229–237
isolated and characterized by X-ray single-crystal structure [13]. It
has been found that the Mo(IV) center, based on [Mo(CN) O ]
4 2
4 2 3 2
2.3. Synthesis of (PPh ) [Mo(CN) O(pynccn)]ꢀ3H OꢀEtOH (2)
4ꢁ
precursors, is very useful in the study of complexation mechanisms
due to both its limited possibility of ligand coordination as well as
the ability to indicate a ligand donor atom first coordinated to the
Mo center [14]. It also allows insight into whether the complexes
isolated are formed on the kinetic or thermodynamic way [15]. This
greatly facilitates the study of reaction mechanisms.
K
3
Na[Mo(CN)
4
O
2
]ꢀ6H
2
O (0.50 g, 1.0 mmol) was dissolved in
water (25 ml) and 2-pyridinecarbonitrile (0.15 g, 1.4 mmol) in eth-
anol (5 ml) was added with stirring followed by dropwise addition
of 3 M HCl until pH = 11.5 was reached. The resultant dark blue
solution was treated with PPh Cl (0.40 g, 1.1 mmol). After several
4
days dark green crystals (2) were filtered off, washed with water
and dried in air at room temperature. Yield: 0.070 g (12%). Anal.
In this work we focus on 2,3-dicyanopyrazine and 2-pyridine-
carbonitrile alkaline hydrolysis assisted by the presence of
Calc. for C60
C, 65.90; H, 4.83; N, 6.96%.
6 5 2
H56MoN O P (1): C, 65.57; H, 5.14; N, 7.65. Found:
nꢁ
ꢁ
[
Mo(CN)
4 2
O(L)] (where L = OH or H O, n = 2 or 3) complex ions
in aqueous-ethanolic mixtures. The products of the reactions,
2
ꢁ
2ꢁ
[
Mo(CN)
3
O(pzac)] or [Mo(CN)
3
O(pynccn)] ions (where Hpzac =
2
.4. Crystallographic data collection and structure refinement
2
,3-pyrazinedicarboxylatomonoamide and Hpynccn = pyridinecy-
+
anoimine) are isolated as PPh
turally and physicochemically.
4
salts and characterized both struc-
The crystal of 1 suitable for X-ray analysis was isolated from the
material prepared as described in Section 2.1. Intensity data were
collected on a Nonius Kappa CCD diffractometer using graphite
2
. Experimental
.1. Materials and methods
Na[Mo(CN)
monochromated Mo Ka radiation, k = 0.71073 Å. The crystal data,
details of data collection and structure refinement parameters
are summarized in Table 1. The positions of all atoms were deter-
mined by direct methods. All non-hydrogen atoms were refined
2
2
K
3
4
O
2
]ꢀ6H
2
O was synthesized according to pub-
anisotropically using weighted full-matrix least-squares on F .
lished methods [16]. All other chemicals were of analytical grade
Aldrich) and were used as supplied. Microanalysis of carbon,
Hydrogen atoms H5AA, H5BB, H6AA, H6BB, H6A and H6B were
identified on difference Fourier maps and refined with geometrical
restrains. All other hydrogen atoms were included in the structure
at idealized positions.
The structure was solved using SIR-97 and refined by SHELXL pro-
gram [18,19].
Single crystals of 2 suitable for X-ray diffraction measurements
were obtained from the filtrate in the synthesis described in Sec-
tion 2.2. The crystal structure of 2 was measured several times
on different crystals coming from different synthetic conditions
and from recrystallized samples. In all cases the structures refine-
ments led to high R factors (ca. 20%) in spite of the fact that there
were no problems with crystal quality (such as for example with
mosaicity). In all cases the same complex formula and the same an-
ion structure (within experimental error) were found. Since the
crystal data were consistent with elemental analysis and spectro-
(
hydrogen and nitrogen were performed using Elementar Vario MI-
CRO Cube elemental analyzer. Solid samples for IR spectroscopy
were compressed as KBr pellets and the IR spectra were recorded
on a Bruker EQUINOX 55 FT-IR spectrophotometer. The electronic
absorption spectra were recorded with Shimadzu UV-3600 UV–
Vis–NIR spectrophotometer equipped with a CPS-240 temperature
controller. Diffuse reflectance spectra were measured in BaSO
lets with BaSO as a reference on Shimadzu 2101PC equipped with
an ISR-260 integrating sphere attachment.
]ꢀ6H
The reaction of K Na[Mo(CN) O with nitriles (2,3-dicy-
anopyrazine and 2-pyridinecarbonitrile) was studied in H O–EtOH
4
pel-
4
3
4
O
2
2
2
mixture (2:1 v/v) at pH 8.5, 9.5, 10.5 and 11.5 (± 0.1) at T = 313 K.
The complex to ligand molar ratio was 1:10 with Mo(IV) concen-
ꢁ3
3
tration equal to 4 ꢀ 10 mol/dm . The pH was stabilized with a
universal buffer [17]. The pH values were measured using Elme-
tron CP-401 pH meter. The complex was dissolved in the required
buffer, thermostated for 5 min and ligand EtOH solutions (also
thermostated) of required concentration were added prior to mea-
surements using automatic pipettes with ±0.0001 ml accuracy. The
spectra were measured on Shimadzu UV-3600 UV–Vis–NIR spec-
trophotometer equipped with a CPS-240 temperature controller
in 1-cm quartz cuvettes. Since band intensities in the UV part are
much higher than in the visible region, the reacting mixtures were
transferred to 1-cm cuvettes for measurements in the visible re-
gion, and parts of these mixture were diluted 38 times for UV mea-
surements. The spectra were measured interchangeably in both
cuvettes.
Table 1
Crystal data and structure refinement parameters for 1.
1
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
C
57
H
48MoN
6 6 2
O P
1070.89
0.40 ꢂ 0.25 ꢂ 0.05
triclinic
P1ꢀ
8.004(5)
13.432(5)
25.022(5)
85.277(5)
86.881(5)
77.785(5)
ꢁ10 to 10
ꢁ17 to 17
ꢁ32 to 32
2618
b (Å)
c (Å)
a
(°)
b (°)
c
h
k
l
(°)
2.2. Synthesis of (PPh
4
2
) [Mo(CN)
3
2
O(pzac)]ꢀ2H O (1)
K
3
Na[Mo(CN)
4
O
2
]ꢀ6H O (1.05 g, 2.19 mmol) was dissolved in
2
3
V (Å )
Z
water-ethanol mixture (60 ml, 2:1 v/v) and 2,3-dicyanopyrazine
0.28 g, 2.7 mmol) in ethanol (10 ml) was added. The pH of the
2
T (K)
293
1.358
(
D
c
(Mg/m3)
mixture was lowered from initial 12.7 to 9.2 with 3 M HCl. The
dark violet mixture was heated to 50 °C for 30 min and evaporated
to dryness. The residue was washed with acetone, dissolved in
Reflections measured
Reflections unique
Restrains
36769
11667
657
water with the addition of ca. 10% excess of (PPh
dark green solid was filtered off, washed with water and dried in
air at room temperature. Yield: 0.10 g (4.3%). Anal. Calc. for
4
)Cl. The resulting
Reflections observed [I > 2
R indices [I > 2r(I)]
R
r(I)]
9946
0.0310
0.0859
1.072
wR
2
C
57
H
48
N
6
6
O P
2
Mo: C, 63.93; H, 4.52; N, 7.85%. Found: C, 63.19; H,
S
4
.41; N, 7.92%.

A. Ryniewicz et al. / Polyhedron 45 (2012) 229–237
231
scopic data, we present here the anion structure and we discuss it
only roughly as an the indication of iminonitrile formation.
3
. Results and discussion
3.1. Crystal structures
The crystal structure of 1 contains [Mo(CN)
3
O(pzac)]2– anion,
+
two PPh
4
cations and two water molecules. The selected bond
lengths and angles are listed in Table 2 whereas the anion structure
is presented in Fig. 1. The organic ligand coordinates in a bidentate
mode to afford a distorted octahedral geometry with three cyano li-
gands and nitrogen atom of pzac in equatorial positions as well as
2
oxo ligand and carboxylate oxygen (O ) occupying axial positions.
The anion is strongly distorted: e.g. the O2–Mo1–O1 angle is
64.30° and the angles C–Mo1–O1, O1–Mo1–N4 are 98.90° and
1
9
1.97°, respectively. A similar type of deformation is observed for
nꢁ
3
all known up until now Mo(IV) complexes [Mo(CN) O(LL)]
(n = 1 or 2) [20]. The average Mo1–C and C„N bond distances
are 2.171 and 1.135 Å, respectively and are very similar to those
O(pic)]2 anion, where the bond
ꢁ
found in the structure of [Mo(CN)
3
lengths are 2.161 and 1.148 Å, respectively [21]. Cyano ligands are
not all bound with the same strength, the Mo1–C2 bond is the
shortest (2.151 Å), and this bond shortening is probably caused
by its trans location to the Mo1–N4 bond and therefore the longest
IV
bond C2–N2 is observed. This indicates significant Mo ? CN
p-
ꢁ
back-bonding and the CN ligand exerts a relatively large struc-
Fig. 1. Single-crystal X-ray structure of [Mo(CN)
3
O(pzac)]2 showing the atom
labeling scheme and 50% displacement ellipsoids.
ꢁ
tural trans effect in this complex by competing with the weaker
N-donor pyrazine ligand for
p
-electron density [22]. The coordina-
ꢁ
tion of pzac ligand to molybdenum center is close to that found
for analogous Co and Ni complexes [Co(tren)(pzac)](ClO
tren = tris-(2-aminoethyl)amine) and [Ni(pzac) (H O) ] [23,24].
The C9–O2 and C9–O3 distances in 1 are equal to 1.276 and
.219 Å, respectively, which corresponds well to those found in
Ni(pzac) (H O) ] with distances 1.272 and 1.230 Å, respectively
24]. Similarly C8–O4 and C8–N6 distances are equal to 1.237
4
)Cl
amido group of [Ag(pzac)]
26].
The crystal packing is presented in Fig. 2. The cations and
n
are 1.227 and 1.332 Å, respectively
(
2
2
2
[
1
[
[
anions are mixed between each other and, contrary to many
2
2
2
nꢁ
[
Mo(CN)
3
O(LL)] ion salts, no separated layers of anions can be
isolated.
and 1.314 Å in 1 and 1.235 and 1.312 Å in the Ni complex, respec-
The water molecules and anions are involved in a complicated
network of hydrogen bonds, as presented in Fig. 3. One water mol-
ecule O6, forms a strong hydrogen bond to the carbonyl oxygen O3
with a relatively short distance of 2.782 Å and to N6 with a dis-
tance of 3.130 Å. The second water molecule O5 forms hydrogen
bonds with O1 (2.975 Å) and N2 (2.860 Å) from another anion, thus
forming a bridge between two anions. Second type of inter-anion
supramolecular bridge is formed by hydrogen bonds between
tively. There are several known silver complexes both with 2,3-
ꢁ
dicyanopyrazine and pzac [25,26]. In [Ag(pzac)]
n
both oxygen
atoms of a carboxyl group are involved in the coordination to the
metal center, thus C–O distances are very similar and equal to
1
.239 and 1.251 Å. The adjacent C@O and C–N distances in the
Table 2
0
amine and carbonyl groups. The N6–O4 distance is equal to
Selected bond lengths (Å) and angles (°) in 1.
2
.949 Å. Similar hydrogen interactions between amine groups of
Bond lengths
Bond angles
adjacent anions were found in the silver salt with almost identical
N–O distance (2.947 Å) [26].
The described network of hydrogen bonds generates circles
involving four adjacent anions with the radius of 6.558 Å. Each cir-
cle is composed in such a way that planes formed by C(8)–N(6)–
O(4 )–C(8 )–N(6 )–O(4) atoms are parallel to each other with the
interplane distance of 5.524 Å. All anions are thus involved in a
two-dimensional layer, there are no interactions between such
separated layers. In the silver salt of pzca , no such interactions
were observed [26]. This is a result of the PPh
that makes the three dimensional network less rigid.
The anion structure of 2 is presented in Fig. 4, the estimated
bond lengths and angles are given in Table 3.
The anion structure indicates that the nitrile group was con-
verted into the corresponding iminonitrile, similarly as it was
found in our previous paper for 2-pyrazinecarbonitrile [13]. The or-
ganic ligand is coordinated in a similar mode (with imine and pyr-
idine nitrogen atoms coordinated to the Mo center) and the anion
structure is typical as for the other [Mo(CN)
Mo1–O1
Mo1–O2
Mo1–C1
Mo1–C2
Mo1–C3
Mo1–N4
N1–C1
N2–C2
N3–C3
N4–C4
C4–C5
C5–N5
C6–N5
C6–C7
N4–C7
C6–C8
1.6681(13)
2.1619(12)
2.176(2)
2.151(2)
2.187(2)
2.1689(16)
1.134(2)
1.136(2)
1.134(2)
1.348(2)
1.369(3)
1.337(2)
1.339(3)
1.397(2)
1.351(2)
1.517(3)
1.512(3)
1.276(2)
1.219(2)
1.237(2)
1.314(2)
C2–Mo1–O1
C2–Mo1–O2
C2–Mo1–C1
C2–Mo1–C3
O1–Mo1–O2
O1–Mo1–N4
O1–Mo1–C1
O1–Mo1–C2
O1–Mo1–C3
O2–Mo1–N4
O2–Mo1–C1
O2–Mo1–C3
N4–Mo1–C1
N4–Mo1–C3
C1–Mo1–C3
Mo1–C1–N1
Mo1–C2–N2
Mo1–C3–N3
101.75(7)
93.94(6)
89.61(7)
86.60(7)
164.30(6)
91.97(7)
97.16(7)
101.75(7)
97.80(7)
72.33(6)
82.68(6)
83.13(6)
88.59(6)
91.65(6)
165.02(7)
178.18(19)
175.93(17)
178.17(19)
0
0
0
ꢁ
+
4
cation presence,
C7–C9
O2–C9
O3–C9
O4–C8
N6–C8
nꢁ
3
O(LL)] anions. The

2
32
A. Ryniewicz et al. / Polyhedron 45 (2012) 229–237
Fig. 2. Crystal packing in 1.
Table 3
o
Selected bond lengths (Å) and angels ( ) in 2.
Bond lengths
Bond angles
Mo–C3
Mo–O
2.1151
1.8199
1.9946
1.9858
2.1326
2.3138
1.1593
0.9792
1.2087
1.1829
1.3971
1.6355
C3–Mo–O
C3–Mo–C2
C3–Mo–N4
C3–Mo–C1
C3–Mo–N5
O–Mo–C2
O–Mo–N4
O–Mo–C1
O–Mo–N5
C2–Mo–N4
C2–Mo–C1
97.22
90.94
85.68
165.55
81.32
105.06
105.31
96.64
Mo–C2
Mo–N4
Mo–C1
Mo–N5
C3–N3
C2–N2
N4–C9
C1–N1
C9–C10
C10–N6
173.36
149.63
89.39
Fig. 3. The hydrogen bonds network in 1. The cations and hydrogen atoms are
omitted for clarity.
3.2. IR spectra
Spectral data of 1 and 2, presented in Figs. 5 and 6, are consis-
tent with the proposed complex composition and with the X-ray
structures. The Mo@O band positions in and (959 and
bond lengths and angles are very similar to that found in 1 but
since its structure was determined with high R factors we do not
compare in details the structure properties.
m
1
2
ꢁ
1
9
27 cm , respectively) are in the range typical for analogous
nꢁ
[
Mo(CN)
3
O(LL)] complexes, where LL can be OO, NO or NN-donor
ligands [20]. For 1 its position is the closest to that in complex with
ꢁ1
the picolinic acid (NO donating as in 1) ligand (950 cm ) [21]. For
complex 2, low energy of the
m
Mo@O band suggests relatively strong
O(pnccn)]2 com-
ꢁ
bonding of pyridinecyanoimine. In the [Mo(CN)
plex, with analogous 2-pyrazinecyanoimine ligand, the
3
Mo O
m @
ꢁ1
band is observed at 932 cm [13]. Such a low position of this band
in 2 additionally supports the cyanoimine formation.
In the region characteristic for CN stretching vibrations (2100–
ꢁ
1
2
200 cm ) bands attributed to cyano ligands and nitrile group can
be observed. In salt 1 only bands of this first origin are observed as
all ligand nitrile groups were hydrolyzed to carboxylic or imine
groups. For compound 1
m
CN bands are located at 2101 and
ꢁ
1
2
113 cm . For the closest analogue, complex with NO donating
ꢁ1
picolinic acid, bands are observed at 2097 and 2110 cm
[
2
19]. For compound 2
092 cm , in the range typical for NN donating ligands. Addition-
m
CN bands are observed at 2109 and
ꢁ1
ꢁ1
ally a third band at 2166 cm is observed for 2. This band can be
assigned to the stretching vibrations of a nitrile substituent in the
pynccn ligand; the position of this band clearly indicates that the
ꢁ
O(pzac)]2ꢁ showing the atom
Fig. 4. Single-crystal X-ray structure of [Mo(CN)
3
labeling scheme and 50% displacement ellipsoids.
nitrile group is not coordinated to the metal.

A. Ryniewicz et al. / Polyhedron 45 (2012) 229–237
233
ꢁ1
ꢁ1
1
677 cm in Hpzac to 1690 cm
in 1) is consistent with the
structure and indicates the presence of a pendant carbonyl oxygen
of the amide group [27]. The significant change of a position of
ꢁ
1
v
(C@O) band in the amide group (from 1677 to 1690 cm ) can
ꢁ
be a result of both coordination of pzac ligand to the molybdenum
center (leading to charge distribution changes on the ligand) and
formation of strong intermolecular hydrogen bonds with NH
group of an adjacent anion (see Fig. 3).
2
In the other regions, the most characteristic bands typical for
+
ꢁ1
ꢁ1
PPh
4
cation (at 1100 cm and at 997 cm ) can be also observed
for both compounds.
3.3. UV–Vis spectra
The UV–Vis absorbance spectra of 1 and 2 are presented in Fig. 7.
+
In the UV part, the spectra are dominated by the bands of PPh
4
cat-
ions while the visible parts are dominated by the intense MLCT
metal-to-ligand charge-transfer) transitions overlapping with d–
(
Fig. 5. IR spectrum of 1 in KBr.
d transitions at the low energy end of the spectra. For both com-
pounds 1 and 2 the solvatochromism of the MLCT transitions is ob-
served, as presented in Fig. 7. The most intense MLCT bands show a
batochromic shift from 522 (in EtOH) to 544 nm (in DMSO) for 1
and from 472 (in EtOH) to 482 nm (in DMSO) for 2. The analogous
spectral behavior with the solvatochromism of MLCT bands has also
nꢁ
been found for all other Mo(IV) complexes [Mo(CN)
3
O(LL)] [20].
The reflectance spectra of 1 and 2 are presented in Fig. 8. The
lowest energy bands are observed at 505 nm for 1 and at 636 nm
for 2. These bands are almost at the same position as in solution
and are solvatochromic (Fig. 7). Moreover, they are relatively in-
tense, compared to the UV bands (see Fig. 7) what indicates their
nꢁ
CT (charge-transfer) character. For [Mo(CN)
3
O(LL)]
complexes
the lowest energy bands in reflectance spectra were attributed to
d–d transitions but this is not the case for the studied salts. The
CT bands of MLCT origin (metal-to-ligand CT) are typically observed
nꢁ
for all [Mo(CN)
3
O(LL)] complexes in the visible part of the spec-
tra. However, it has been found that the lowest energy band of d–
d character overlaps with the closest CT band [20]. In salts 1 and
2
the observed solvatochromism of the lowest energy bands indi-
Fig. 6. IR spectrum of 2 in KBr. The upper right corner represents the 2000–
cates mostly their CT character obscuring less intense d–d bands.
ꢁ
1
2
300 cm part of the spectrum.
3.4. Cyclic voltammetry measurements
The cyclic voltammetry measurements carried out in DMSO are
presented in Figs. 9 and 10 for 1 and 2, respectively. The measure-
ments indicate that salt 1 undergoes irreversible oxidation at
0
.57 V. However, when the measurement range is limited to the
oxidation peak, it can be seen that oxidation potential is merely
dependent on the scan speed (in the 20–1000 mV/s range), what
is typical rather for a quasi-reversible than an irreversible oxida-
tion. On the other hand, no reduction wave is observed at all scan
speeds. Such a behavior is indicative for an irreversibility caused by
the chemical reaction following oxidation of the complex and not
by the complex itself.
The cyclic voltammogram of 2 is presented in Fig. 12. The salt
0
1
undergoes reversible oxidation/reduction at E = 0.300 V with
=2
the peak separation of 0.12 V at 100 mV/s scan speed. The observed
redox processes both for 1 and 2 can be attributed to a Mo(IV/V)
redox couple. For 1 the reduction wave observed at ꢁ1.17 V can
be attributed to a ligand-centered process, probably involving the
amide group. For 2 such a reduction wave is not observed since
the amide group is not present in this compound.
Fig. 7. UV–Vis spectra of 1 (upper) and 2 in DMSO and EtOH.
In the C@O stretching vibration region, only one strong band at
3
.5. The reaction of Mo(IV) with nitriles studied in at different pH
ꢁ
1
1
1
690 cm is observed for 1. The disappearance of the band at
ꢁ1
733 cm characteristic for Hpzac (attributed to the carboxylic
To study the formation of
1 and 2 in the reaction of
oxygen) and a small shift of the C@O band of an amide group (from
K
3
Na[Mo(CN) O with nitriles, the spectroscopic studies in
4
O
2
]ꢀ6H
2

2
34
A. Ryniewicz et al. / Polyhedron 45 (2012) 229–237
ꢁ
[
Mo(CN)
above [Mo(CN)
4
O(H
2
O)]2 ion prevails (pK = 9.70) and at pH = 10.5 and
O(OH)]3 is in excess [28,29].
ꢁ
4
The spectral changes observed during the reaction of Mo(IV)
with 2,3-dicyanopyrazine at different pH are presented in Figs.
1
1 and 12.
The UV part shows bands associated with 2,3-dicyanopyrazine,
this is a result of not only their much more intensity but also higher
concentration of the ligand compared to the complex (25:1 molar
ratio). In general, the character of spectral changes seems to be
independent on pH. With reaction time, the band at 228 nm de-
creases, the band at 276 nm remains almost unaltered whereas
the increase of absorption at 300 nm is observed. Not well resolved
isosbestic point at 218 nm is observed at all pH studied, whereas
another isosbestic point, at 258 nm, is observed at all pH values ex-
cept pH = 8.5. After 2 h the reaction seems to be completed. The
complicated spectral changes are connected with the nitrile hydro-
lysis and it seems that 2,3-dicyanopyrazine, after 2 h (at pH = 9.5
and above) is totally converted into a final product. Since the ligand
to complex ratio is 25:1, the conversion of this ligand has to be a
catalytical process. In Fig. 13 it can also be seen that the rate of
the nitrile hydrolysis depends strongly on pH. At pH = 8.5 the slow-
est spectral changes are observed, whereas at pH > 8.5 the reaction
is accelerated. This is a typical behavior for a basic nitrile hydrolysis.
In the visible part of the spectra (Fig. 12) the changes are much
more complicated. In this spectral region only the bands of molyb-
denum complexes can be observed, as the nitrile and the products
4
Fig. 8. UV–Vis reflectance spectra of 1 (dotted line) and 2 (solid line) in BaSO after
Kubelka–Munk transformation.
of its hydrolysis do not absorb in this region. Since the Mo(IV) com-
nꢁ
plexes [Mo(CN)
4
OL] (where L is a monodentate ligand) exhibit
only d–d transitions (above 500 nm) of low intensity [20], bands
of such complexes cannot be clearly observed in Fig. 12 due to
the low concentration of the initial complex. However, complexes
nꢁ
of [Mo(CN)
3
O(LL)] type have intense CT bands in the visible re-
gion and their formation can be easily studied under reaction con-
ditions. What is more important, such complexes can be formed at
pH < 9.7, where the labile aqua form of the Mo(IV) complex is dom-
inant. This is an explanation of spectral changes presented in
Fig. 12. At pH = 8.5 formation of the final complex 1 can be easily
seen, whereas with increasing pH smaller amount of the complex
is formed. The observed spectral changes in Fig. 12 are much more
complicated since the hydrolysis of 2,3-dicyanopyrazine leads to
several products that can bind to the molybdenum center in a
bidentate mode. For example, the band at ca. 420 nm, observed
at the beginning of the reaction, can be attributed to the molybde-
num complex with 2-monocarbox-3-cyanopyrazinemonoamine.
The considerably different pathway is observed for 2-pyridine-
carbonitrile, as illustrated in Fig. 13 in the UV part of the electronic
spectra. The spectral changes indicate very little ligand conversion
to the final product. This can be interpreted as a different way of
ligand transformation. If the hydroxyl group could attack the ni-
trile group, activated by its coordination to the molybdenum cen-
ter, the conversion into amide should occur catalytically and the
total conversion of the nitrile into a corresponding amide and/or
Fig. 9. Cyclic voltammogram of 1 in 0.1 M Bu
upper left corner voltammograms recorded at different scan speeds are shown.
4 6
NPF DMSO as electrolyte. In the
2
-pyridinecarboxylic acid should be observed, similarly as it was
for 2-dicyanopyrazine. Contrary to this, however, the amount of
the ligand transformed is very small and it correlates with the
2
ꢁ
amount of the [Mo(CN)
amount of the resulting [Mo(CN)
3
O(pynccn)]
formed. Moreover, the
O(pynccn)]2 remains very low
ꢁ
3
4 6
Fig. 10. Cyclic voltammogram of 2 in 0.1 M Bu NPF DMSO as electrolyte. The
even at pH = 8.5 and decreases with the pH increase. The first step
of the reaction is the coordination of 2-pyridinecarbonitrile to the
molybdenum center. The ligand is coordinated in a monodentate
way via the nitrogen atom. It can be either pyridine or nitrile nitro-
gen. This nitrogen atom must occupy the initial position of the
aqua ligand since the aqua ligand is the labile one. This coordina-
tion does not seem to initiate the ligand hydrolysis since no corre-
sponding spectral changes are observed. In the next reaction step, a
cyano ligand, released from the complex, attacks the nitrile group
dotted line represents the voltammogram with ferrocene used as an internal
potential standard. In the upper left corner voltammograms recorded at different
scan speeds are shown.
2
the UV–Vis range were performed in H O–EtOH (2:1 v/v) mixture
at different pH values (8.5, 9.5, 10.5 and 11.5). These pH values
were purposely selected to obtain various ratios of two protonation
forms of the Mo(IV) complex; at pH = 8.5 and 9.5 the

A. Ryniewicz et al. / Polyhedron 45 (2012) 229–237
235
O with 2,3-dicyanopyrazine at different pH. T = 313 K, [Mo] = 1.05 ꢀ 10ꢁ M, [2,3-dicyanopyr-
4
Fig. 11. UV electronic spectra during the reaction of K
3
Na[Mo(CN)
4
O
2
]ꢀ6H
2
azine] = 2.6 ꢀ 10ꢁ M, d = 1 cm, H
3
2
O–EtOH (2: 1 v/v). Spectra measured every 350 s. The arrows indicate the direction of the changes.
with 2,3-dicyanopyrazine at different pH. T = 313 K, [Mo] = 4.0 ꢀ 10ꢁ M, [2,3-
3
Fig. 12. Visible electronic spectra during the reaction of
dicyanopyrazine] = 0.1 M, d = 1 cm, H
K
3
Na[Mo(CN)
4
O
2
]ꢀ6H
2
O
2
O–EtOH (2:1 v/v). Spectra measured every 350 s. The arrows indicate the direction of the changes.
ꢁ
with the formation of pynccn . Since the amount of released cyano
ligand is equal to the amount of resulting 2, it explains the lack of
the catalytical effect of the metal on the ligand transformation and
the small conversion of the nitrile. It seems that the reactive form
is that with a pyridine nitrogen coordinated to the molybdenum
center. This is not a favorable coordination since the nitrile group
has to be located between cyano ligands, otherwise there would
be too big steric hindrance. However, in this case the cyano ligand
is very close to the nitrile group and the nitrilation of the nitrile is
easier. Such a mechanism is also supported by the X-ray crystal
structure of the cyanolysis product, where the pyridine nitrogen
is in the trans position to the oxo ligand. Thus the mechanism
seems to be very similar to that suggested for the nitrilation of
2-pyrazinenitrile [13].

2
36
A. Ryniewicz et al. / Polyhedron 45 (2012) 229–237
Appendix A. Supplementary data
CCDC 873340 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.07.006.
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Scheme 1. Hydrolysis of 2,3-pyrazinecarbodinitrile in the presence of
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[
1

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