Preparation and characterisation of [M(CN)4O(pz)]2- complexes (M = Mo or W) and their reactivity towards molecular oxygen

Article abstract of DOI:10.1016/S0277-5387(00)00413-7

The synthesis and characterisation of (PPh₄)₂[M(CN)₄O(pz)]·3H₂O (M = Mo or W; pz = pyrazine) are presented. The salts are reactive towards molecular oxygen, both in solution and in the solid state, with formation of (PPh₄)₂[M(CN)₄O(O₂)]. The X-ray crystal structure of the molybdenum compound confirmed the presence of a peroxo ligand cis to the M=O bond; the O-O bond distance is 1.41 A?. The IR spectra exhibit two absorption bands in the 950-850 cm^-1 region assigned to the terminal M=O group [917 (Mo) and 933 (W) cm^-1] and the peroxo group [893 (Mo) and 871 (W) cm^-1]. The possible mechanism of molecular oxygen uptake by pyrazine complexes is discussed. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00413-7

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1503–1509
2
−
Preparation and characterisation of [M(CN) O(pz)]
complexes
4
(
M=Mo or W) and their reactivity towards molecular oxygen
a
a
a,
b
Dariusz Matoga , Janusz Szklarzewicz , Alina Samotus *, John Burgess ,
b
b
John Fawcett , David R. Russell
a
Department of Inorganic Chemistry, Faculty of Chemistry, Jagiellonian Uni6ersity, R. Ingardena 3, 30-060 Cracow, Poland
b
Department of Chemistry, Uni6ersity of Leicester, Leicester, LE1 7RH, UK
Received 14 February 2000; accepted 29 March 2000
Abstract
The synthesis and characterisation of (PPh ) [M(CN) O(pz)]·3H O (M=Mo or W; pz=pyrazine) are presented. The salts are
4
2
4
2
reactive towards molecular oxygen, both in solution and in the solid state, with formation of (PPh ) [M(CN) O(O )]. The X-ray
4
2
4
2
crystal structure of the molybdenum compound confirmed the presence of a peroxo ligand cis to the MꢀO bond; the OꢁO bond
−
1
distance is 1.41 A
[
,
. The IR spectra exhibit two absorption bands in the 950–850 cm
region assigned to the terminal MꢀO group
−
1
−1
917 (Mo) and 933 (W) cm ] and the peroxo group [893 (Mo) and 871 (W) cm ]. The possible mechanism of molecular oxygen
uptake by pyrazine complexes is discussed. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Molybdenum complexes; Tungsten complexes; Pyrazine complexes; Peroxo ligands; Crystal structures
2
−
n−
1. Introduction
[M(CN) O(H O)] +LL
4 2
(
n+1)−
−
“
[M(CN) O(LL)]
+H O+CN
(4)
3
2
It is well known that in aqueous solutions tetracyan-
4
−
odioxometallates(IV), [M(CN) O ] (M=Mo, W) un-
where LL=2,2%-bipyridyl (bipy), 1,10-phenanthroline
(phen), pyridine-2-carboxylate (pic) and several Schiff
bases [6]. There is also one known example of a neutral
4
2
dergo stepwise protonation reactions
K
1
4
−
+
3−
[
M(CN) O ] +H X [M(CN) O(OH)]
(1)
(2)
4
2
4
K
2
seven-coordinate complex [M(CN) O(LLLL)] formed
2
in the reaction
3
−
+
2−
[
M(CN) O(OH)] +H X [M(CN) O(H O)]
4
4
2
with the equilibrium constants given by: log K =13.00,
2−
1
[
M(CN) O(H O)] +LLLL
4
2
log K =9.70 and log K =11.60, log K =7.76 for
2
1
2
−
M=Mo and W, respectively [1,2]. It is also known that
“[M(CN)
2
O(LLLL)]+H
2
O+2CN
(5)
2
−
the [M(CN) O(H O)]
ion easily undergoes a consid-
4
2
where the tetradentate ligand LLLL=N,N%-bis[1-
pyridin-2-yl)ethylidene]ethane-1,2-diamine [7].
The considerable number of relatively fast reactions
of [M(CN) O(H O)]
us to use a similar procedure to obtain tetracyanocom-
plexes with pyrazine (pz), which forms a variety of
complexes with different transition metals [8,9]. The
pyrazine ligand seemed interesting for us due to its
small dimensions, which imply that the steric factor
does not interfere with its coordination and, moreover,
it can act as a monodentate ligand or as a bridging
ligand. We have found, however, that the only species
formed under our experimental conditions was
erable number of substitution reactions. It reacts with a
variety of monodentate ligands yielding complexes of
(
(
n+2)−
−
general formula [M(CN) O(L)]
, where L=CN ,
2−
4
with different ligands prompted
−
−
−
4
2
NCS , F , N , HCN and pyridine (py) [2–5].
3
2
−
n−
[
M(CN) O(H O)] +L
4 2
(
n+2)−
“
[M(CN) O(L)]
+H O
(3)
4
2
Bidentate ligands are coordinated according to the
reaction
*
Corresponding author. Fax: +48-12-6340-515.
E-mail address: samotus@chemia.uj.edu.pl (A. Samotus).
0
277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00413-7

1
504
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
2
−
[
M(CN) O(pz)] , in which pyrazine is monodentate.
Tetraphenylphosphonium
tetracyanooxoperoxo-
4
The syntheses of new complexes, with their spectro-
scopic and physicochemical characterisation, and their
reactivity towards molecular oxygen are the subject of
this report. There is only one known example of molec-
ular oxygen uptake by a similar type of complex. The
anhydrous salt (PPh ) [Mo(CN) O] has been reported
molybdate(VI), (PPh ) [Mo(CN) O(O )] (3), was pre-
4
2
4
2
pared by extraction of 1.1 g (1.07 mmol) of 1 three
times with 30 ml of acetone and filtering the extracts.
The resulting yellow filtrates were combined and the
solvent was evaporated in air at room temperature
within 20 min. The yellow crystals were washed three
times with ethanol and dried in air. Yield: 0.46 g (46%).
Anal. Found: C, 67.38; H, 4.25; N, 5.94. Calc. for
C H MoN O P : C, 67.39; H, 4.35; N, 6.05%.
4
3
5
[
10] to react with dioxygen according to the equation
IV
(
PPh ) [Mo (CN) O]+O
4 3 5 2
52
40
4
3 2
VI
“
(PPh ) [Mo (CN) O(O )]+(PPh )CN
(6)
4
2
4
2
4
Tetraphenylphosphonium
tetracyanooxoperoxo-
tungstate(VI), (PPh ) [W(CN) O(O )] (4), was prepared
4
2
4
2
In this paper we present the spectroscopic and
by dissolving 0.45 g (0.4 mmol) of 2 in 30 ml of
acetonitrile. After 60 min, the initially yellow solution
turned colourless; it was evaporated in air at room
temperature. The resulting white product was washed
three times with ethanol and dried in air. Yield: 0.35 g
physicochemical characteristics of peroxo complexes of
formula (PPh ) [M(CN) O(O )] obtained by us in the
4
2
4
2
reaction of hydrated pyrazine salts with molecular oxy-
gen. We also discuss the mechanism of molecular oxy-
gen incorporation, our proposal differing from that
presented earlier [11].
(
85%). Anal. Found: C, 61.34; H, 3.92; N, 5.29. Calc.
for C H N O P W: C, 61.55; H, 3.97; N, 5.52%.
52
40
4
3 2
2
.3. Reaction of (PPh ) [M(CN) O(O )] with molecular
2. Experimental
4 2 4 2
oxygen
2
.1. Materials
The salts 3 or 4 were dissolved in acetonitrile and left
in air for about 2 h. In the case of salt 4 no changes
were observed and after evaporation of solvent 4 was
quantitatively recovered. In the case of 3 its yellow
colour disappeared, and after removal of solvent by
K Na[M(CN) O ]·6H O (M=Mo or W) were pre-
3
4
2
2
pared as described in the literature [12], and their purity
confirmed by their IR spectra. Tetraphenylphospho-
nium chloride [(PPh )Cl], pyrazine (pz), triphenylphos-
phine (PPh ) and standard laboratory reagents were of
analytical grade (Sigma, Aldrich) and used as supplied.
4
evaporation a white product of formula (PPh ) -
3
4 2
1
−
[Mo
O
] was isolated. IR: w(MoꢀO)=880 cm
,
2
7
−
1
w(MoꢁOꢁMo)=790 cm . Anal. Found: C, 58.06; H,
3.92; N, 0.06. Calc. for C H Mo O P : C, 58.66; H,
2.2. Syntheses
48
40
2
7 2
4
.07; N, 0.00%.
Tetraphenylphosphonium
tetracyanooxopyrazine-
molybdate(IV) was prepared as its trihydrate,
2.4. Analytical methods and physical measurements
(
PPh ) [Mo(CN) O(pz)]·3H O (1), by dissolving 2.80 g
4
2
4
2
(
5.8 mmol) of K Na[Mo(CN) O ]·6H O and 1.92 g (24
Carbon, nitrogen and hydrogen were determined by
organic microanalysis. UV–Vis absorption spectra
(Shimadzu 2101 PC) were recorded in the normal way.
Reflectance spectra were measured in BaSO4 pellets
versus BaSO4 as a reference (Shimadzu 2101 PC
equipped with ISR-260 attachment). IR spectra were
3
4
2
2
mmol) of pz in 15 ml of water. The pH of the solution
was adjusted to ca. 8 with 1 M HCl and a solution of
4
The resulting green crystals were filtered off, washed
four times with water and dried in air. Yield: 5.80 g
.5 g (12 mmol) of (PPh )Cl in 15 ml of water added.
4
(
97%). Anal. Found: C, 65.80; H, 4.96; N, 8.09. Calc.
measured in KBr pellets on a Bruker IFS 48 spectrom-
1
for C H MoN O P : C, 65.37; H, 4.90; N, 8.17%.
eter. H NMR spectra in CDCl were recorded on a
5
6
50
6
4
2
3
Tetraphenylphosphonium
tungstate(IV) was prepared as its trihydrate, (PPh ) -
tetracyanooxopyrazine
Tesla BS-567A spectrometer. ESR spectra were mea-
sured on a Bruker ELEXSYS spectrometer at room
and liquid nitrogen temperatures using diphenylpicryl-
hydrazide (dpph) as an internal standard. To estimate
4
2
[
W(CN) O(pz)]·3H O (2), by dissolving 3.3 g (5.8
4
2
mmol) of K Na[W(CN) O ]·6H O and 1.92 g (24
3
4
2
2
mmol) of pz in 15 ml of water. The pH of the solution
was adjusted to ca. 7 with 1 M HCl. It was heated to
the amount of paramagnetic centres, VOSO was used
as standard.
4
50°C for ca. 20 min, followed by addition of 4.5 g (12
mmol) of (PPh )Cl in 15 ml of water. The resulting
2.5. Crystal structure determination
4
dark green crystals were filtered off, washed four times
with water and dried in air. Yield: 2.46 g (38%). Anal.
Found: C, 60.40; H, 4.77; N, 7.53. Calc. for
C H N O P W: C, 60.22; H, 4.51; N, 7.52%.
Crystals of 3 suitable for X-ray crystallography were
selected from the recrystallised material prepared as
described above. Crystal data for 3, and a summary of
5
6
50
6
4 2

D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
1505
data collection and structure refinement parameters, are
given in Table 1. The positions of Mo and P atoms
were determined by Patterson methods; the remaining
non-hydrogen atoms were located in successive differ-
ence Fourier syntheses. Hydrogen atoms were included
in the structure factor calculations at idealised positions
and were not refined. In the final refinement cycle all
non-hydrogen atoms were refined anisotropically. The
largest peaks in the final difference maps were located
near the Mo atom. All calculations were performed
The equilibrium lies strongly towards the reac-
tants — formation of the product complex can be
observed only in concentrated solutions in the presence
of a large excess of pyrazine. Complexes 1 and 2 wer+e
isolated as tetraphenylphosphonium salts. The (PPh4)
cation forms insoluble salts only with the product com-
plex thus shifting equilibrium (7) to the right.
The results of analysis and physicochemical measure-
ments indicate that pyrazine acts as a monodentate
ligand; the possibility of its acting as a bridging ligand
between two complex units can be excluded. On the
using SHELXTL
-PC [13]; scattering factors and anoma-
lous dispersion factors were those given in SHELXTL
.
basis of the crystal structure data of all known
n−
[
M(CN) O(L)]
type complexes [3,5,11], the pyrazine
4
ligand is expected to be in the position trans to the
MꢀO bond.
3. Results
Both light green 1 and dark green 2 are diamagnetic.
In the solid state they are stable when kept under
restricted anaerobic conditions. The salts are almost
insoluble in water, but soluble in polar organic solvents
such as MeCN, CH Cl and DMSO. In dilute aqueous
3
.1. Characterisation of the complexes
2
−
The reaction of [M(CN) O(H O)]
with pyrazine
4
2
under appropriate conditions results in aqua ligand
substitution and pyrazine coordination according to the
equation
2
2
solutions, the only species which can be detected by
2
−
visible spectroscopy is the [M(CN) O(H O)]
ion,
4
2
even in the presence of a large excess of pyrazine. In
organic solvents in the presence of air, a fast change of
colour from green to yellow is observed. After evapo-
rating the solvent, yellow 3 and white 4 were isolated,
indicating that the colour change is connected with
molecular oxygen uptake:
2
−
2−
[
M(CN) O(H O)] +pz = [M(CN) O(pz)] +H O
4
2
4
2
(
7)
Table 1
2
−
2−
Crystal data and structure refinement for 3
[
M(CN) O(pz)] +O “[M(CN) O(O )] +pz
(8)
4
2
4
2
Complex
3
Similar colour changes were observed when solid
salts 1 and 2 were kept in air. The final products of
solid state reactions were found to be identical with
those isolated from the solutions. They were found not
to be contaminated with free pyrazine thanks to its high
volatility. The salts 3 and 4 are insoluble in water and
alcohols and soluble in common organic solvents, such
as MeCN, CHCl , CH Cl , DMSO and DMF. As
Empirical formula
Formula weight
Temperature (K)
C52H40N O P Mo
4 3 2
926.76
200(2)
0.71073
triclinic
Wavelength (A
Crystal system
Space group
,
)
P1
(
Unit cell dimensions
3
2
2
a (A
b (A
,
)
)
10.235(4)
10.912(3)
22.557(8)
84.44(2)
82.21(3)
63.64(2)
2234.6(13)
1.377
0.413
0.863, 0.934
952
0.54×0.32×0.08
1.82–26.00
9174
solids they are stable in air at room temperature. In
solution, complex 4 is thermally stable, whereas salt 3
undergoes slow decomposition to (PPh ) [Mo O ] as
,
c (A
,
)
h (°)
i (°)
k (°)
4
2
2
7
confirmed by the elemental analysis and IR spectra of
the product formed.
Molecular oxygen uptake by cyano-oxo complexes of
Volume (A
,
3)
Density (Mg m−3)
Absorption coefficient (mm−1)
Min./max. transmission
F(000)
Crystal size (mm)
r range data collection (°)
Reflection collected
Independent reflections
Observed data [I\2|(I)]
Free parameters
R1
Mo(IV) was found only for anhydrous (PPh ) [Mo-
4 3
(CN) O], and was postulated to be impossible for hy-
5
drated salts [11]. Here we have succeeded, for the first
time, in demonstrating molecular oxygen uptake by the
hydrated oxotetracyano complexes and in isolating the
tungsten salt analogous to (PPh ) [Mo(CN) O(O )] de-
8688
3828
464
0.1127
4 2
4
2
scribed in the literature [10].
3.2. Description of the structure of 3
wR2
0.2437
Goodness-of-fit
Dzmax, Dzmin (e A
1.058
+1.271, −3.017
−3)
The structure of 3 is presented in Fig. 1 and selected
bond distances and angles are collected in Table 2. The
,

1
506
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
in the literature. The Mo(1)ꢁO(1) bond distance is
shorter (1.671(10) compared with 1.694(3) A) and the
Mo(1)ꢁC(4) and Mo(1)ꢁC(3) bond distances are longer
2.296(16) compared with 2.197(4) A and 2.196(11)
compared with 2.167(5) A, respectively). It is difficult to
,
(
,
,
compare the bond angles with the literature data due to
some mistakes in the assignments of angles (e.g. the
O(1)ꢁMo(1)ꢁC(4) angle (O3ꢁMoꢁC4 in their numbering
scheme) was reported to be 88.20°, which is impossible
for atoms in trans positions). It seems, however, that
there are probably some differences in bond angles in
the structures of both salts, though the angle subtended
by the peroxo ligand is the same in both determinations
(
43.1(4)° in 3, 43.2(2) in Ref. [10]). The OꢁO bond
distance in our crystal, 1.410(14) A
in the earlier reference, 1.416(5) A
not differ significantly from that in the analogous com-
plex [Mo(CN) O(O )(hmpa)] , 1.397(10) A [10].
,
,
, is the same as that
; these distances do
,
−
3
2
2
−
3.3. IR spectra
Fig. 1. Structure of the [Mo(CN) O(O) ]
ion in 3.
4
2
Selected IR data for salts 1 to 4 are presented in
Table 3. For comparison data for pyrazine and
anion has a distorted pentagonal bipyramidal geometry
with the OꢁO group cis to the MoꢀO bond. The OꢁO
(
PPh )Cl are also included.
bond distance of 1.41 A
14–17] and thus the formal oxidation state of the
metal is +6.
,
is typical for the peroxo group
4
The IR spectra of salts 1 and 2 show bands at 974
[
−
1
−1
cm (1) and at 969 cm (2) which can be assigned to
the stretching vibrations of the MꢀO group. The pyra-
zine complexes have higher values of w(MꢀO) than
analogous six-coordinate cyano-oxo complexes of
Mo(IV) and W(IV) with monodentate ligands having
A salt of the same formula was obtained earlier by
Arzoumanian et al. [10] in the reaction of
(
PPh ) [Mo(CN) O] in CH Cl with molecular oxygen
4 3 5 2 2
and its structure was determined by X-ray crystal anal-
ysis. There are, however, some differences in the bond
distances and angles between salt 3 and that described
the nitrogen atom in the trans position to the MꢀO
−
bond [3,5,11,18] (for L=N3 , CH CN, HCN band
3
−1
positions are at 953, 940 and 940 cm
L=N3 , NCS , py at 952, 954 and 964 cm
for Mo and for
−
−
−1
for W,
respectively). This suggests weak bonding of pyrazine
and locates the pyrazine ligand between pyridine
Table 2
Selected bond distances (A
,
)
and angles (°) for (PPh ) [Mo-
−1
−1
4
2
(
w(WꢀO)=964 cm ) and H O (w(WꢀO)=980 cm
)
2
(
CN) O(O )] (3), with estimated standard deviations in parentheses
4 2
in the series of ligands arranged according to the de-
Mo(1)ꢁO(1)
Mo(1)ꢁO(3)
Mo(1)ꢁC(2)
Mo(1)ꢁC(4)
C(1)ꢁN(1)
1.671(10)
1.927(9)
2.188(16)
2.296(16)
1.145(18)
1.150(14)
Mo(1)ꢁO(2)
Mo(1)ꢁC(1)
Mo(1)ꢁC(3)
O(2)ꢁO(3)
C(2)ꢁN(2)
C(4)ꢁN(4)
1.914(9)
creasing value of w(WꢀO), viz.
2.156(16)
2.196(11)
1.410(14)
1.156(17)
1.145(17)
−
−
−
−
−
OH \CN \F \N3 \NCS \pyridine
pyrazine\H O
\
2
given in the literature [5]. The analogous series for Mo
complexes [3,10,11,19] is:
C(3)ꢁN(3)
O(1)ꢁMo(1)ꢁO(2) 106.5(4)
O(2)ꢁMo(1)ꢁO(3) 43.1(4)
O(2)ꢁMo(1)ꢁC(1) 81.1(5)
O(1)ꢁMo(1)ꢁC(2) 92.3(5)
O(3)ꢁMo(1)ꢁC(2) 80.9(5)
O(1)ꢁMo(1)ꢁC(3) 87.9(5)
O(3)ꢁMo(1)ꢁC(3) 153.5(5)
C(2)ꢁMo(1)ꢁC(3)
O(2)ꢁMo(1)ꢁC(4) 86.6(4)
C(1)ꢁMo(1)ꢁC(4)
C(3)ꢁMo(1)ꢁC(4)
O(2)ꢁO(3)ꢁMo(1) 68.0(5)
N(2)ꢁC(2)ꢁMo(1) 178.1(13)
N(4)ꢁC(4)ꢁMo(1) 177.6(12)
O(1)ꢁMo(1)ꢁO(3)
O(1)ꢁMo(1)ꢁC(1)
O(3)ꢁMo(1)ꢁC(1)
O(2)ꢁMo(1)ꢁC(2)
C(1)ꢁMo(1)ꢁC(2)
O(2)ꢁMo(1)ꢁC(3)
C(1)ꢁMo(1)ꢁC(3)
O(1)ꢁMo(1)ꢁC(4)
O(3)ꢁMo(1)ꢁC(4)
C(2)ꢁMo(1)ꢁC(4)
O(3)ꢁO(2)ꢁMo(1)
N(1)ꢁC(1)ꢁMo(1)
N(3)ꢁC(3)ꢁMo(1)
106.7(4)
97.0(5)
−
−
−
OH \CN \HCN\CH CN\N3 \pyrazine
3
123.2(5)
123.7(5)
149.7(5)
153.5(5)
75.1(5)
166.2(4)
85.8(4)
83.8(5)
\
H O.
2
In the CN stretching region four bands are observed
(
Table 3). The larger number of bands with respect to
n−
analogous complexes of the [M(CN) O(L)] type indi-
76.5(5)
4
cates lower symmetry for the anions in 1 and 2.
−
1
80.3(5)
78.3(5)
The 400–1800 cm
region is dominated by overlap-
69.0(5)
175.4(13)
176.1(13)
+
ping bands of (PPh4) and pyrazine. There are four
bands assigned to coordinated pyrazine, at 1017, 1037,
−
1
1
147 and 1415 cm
for salt 1 and at 1017, 1040, 1148
−
1
and 1415 cm
for salt 2. The positions of the bands

D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
1507
Table 3
Selected IR absorption bands (cm 1) a
−
Compound w(OꢁO)
w(MꢀO)
w(CꢂN)
Others
7
3
12 s, 816 s, 870 s, 1010 s, 1036 w, 1145 m, 1183 vs, 1351 vs, 1408 vs, 1613 vs,
077 s
1
2
974 s
2085 m, 2088 s,
444 m, 527 vs, 620 vw, 690 vs, 723 vs, 757 m, 763 m, 814 vw, 853 vw, 998 w, 1017
vw, 1027 vw, 1037 w, 1083 vw, 1108 vs, 1147 vw, 1162 vw, 1187 vw, 1316 w, 1338
vw, 1415 m, 1436 vs, 1442 vs, 1484 m, 1586 w, 1620 vw, 3061 vw, 3407 m, b, 3490
w, sh, 3654 w
444 m, 527 vs, 623 vw, 690 vs, 722 vs, 757 m, 763 m, 814 w, 853 vw, 998 m, 1017
vw, 1027 vw, 1040 w, 1085 vw, 1108 vs, 1148 vw, 1162 vw, 1188 w, 1318 w, 1340
vw, 1415 m, 1436 vs, 1441 vs, 1484 s, 1584 w, 1620 vw, 3061 vw, 3400 m, b, 3470
w, sh, 3651 w
2094 s, 2103 vw
969 vs
2071 s, 2079 sh,
082 s, 2092 w
2
3
4
893 s
917 s
933 s
2127 vvw, 2150
vvw, 2201 vvw
415 w, 527 vs, 620 vw, 690 vs, 722 vs, 754 m, 761 m, 852 vw, 996 m, 1027 vw,
1085 vw, sh, 1108 vs, 1164 vw, 1186 vw, 1315 w, 1340 vw, 1437 s, 1443 s, sh, 1484
m, 1586 w, 1615 vw, 3061 vw
422 w, 526 vs, 623 w, 688 vs, 722 vs, 753 m, 762 m, 853 vw, 995 m, 1029 vw, 1086
vw, sh, 1107 vs, 1163 vw, 1185 vw, 1314 w, 1341 vw, 1435 s, 1441 s, 1483 m, 1585
w, 1612 w, 3056 vw
871 m
2132 vvw, 2154
vvw, 2199 vvw
a
s, strong; m, medium; w, weak; v, very; sh, shoulder; b, broad.
are only slightly shifted in comparison with free pyra-
zine to be chemically equivalent. The upfield shift of the
−
1
zine (1010, 1036, 1145 and 1408 cm
[20]) in agree-
signal for salt 1 shows that in CDCl solution weakly
3
ment with weak bonding of pyrazine to the metal
centre.
bonded pyrazine is still coordinated whereas for salt 2
decomposition accompanied by the release of pyrazine
occurs.
The IR spectra of 3 and 4 exhibit bands at 917 and
−
1
9
33 cm , assigned to the respective MꢀO stretching
−1 −1
vibrations. The bands at 893 cm
(3) and 871 cm
3.5. UV–Vis spectra
(
4) correspond to the stretching vibrations of side on
bonded peroxo group [17,21]. The positions of the
w(MꢀO) and w(OꢁO) bands are close to those of peroxo
complexes of molybdenum(VI) and tungsten(VI) with-
out cyanide ligands [14,15,22].
In the solid state reflection spectra show bands at
32, 270, 303 and 650 nm for salt 1 and at 229, 270, 309
2
and 620 nm for salt 2. In aqueous solutions, the bands
observed in the visible region at 610 and 562 nm (for
salts 1 and 2, respectively) can be attributed to the
In the CN stretching region the IR spectra show
bands of very low intensity at 2127, 2150 and 2201
2
−
−
1
−1
[
M(CN) O(H O)]
ions which are formed according
cm
for 3 and 2132, 2154 and 2199 cm
for 4. The
4
2
to equilibrium (7). In organic solvents such as MeCN,
unusually high position of these CN vibrations in com-
parison with the cyano-oxo complexes of Mo(IV) and
W(IV) is consistent with an increase of the formal
oxidation state of the central atom resulting in the
decrease of p-back donation which shifts the CN
stretching band towards higher energy.
CH Cl2 and acetone fast formation of 3 and 4 is
2
observed.
Fig. 2 presents typical spectral changes observed for
a solution of 1 in dichloromethane in the presence of
air, together with the spectrum of 3 in the same solvent.
The decrease of the band at ca. 621 nm, characteristic
of a d–d transition in 1, and the growth of a new band
at ca. 393 nm, connected with dioxygen uptake and
formation of 3, are observed. The diffuse isosbestic
point (Fig. 2) indicates that two species are dominant in
the system.
1
3
.4. H NMR spectra
1
H NMR spectra for solutions of salts 1 and 2
exhibit signals of protons of pyrazine, aromatic rings of
+
(
PPh4) cations and water molecules. The four pyra-
The reflection spectra of salts 3 and 4 consist of
bands originating from the respective cations at 231
and 276 nm and at 237 and 281 nm, and bands at 420
and 336 nm which can be assigned to charge-transfer to
metal (CTTM) transitions. In organic solvents the latter
bands are shifted towards higher energy, e.g. in acetoni-
trile the band maximum is observed at 393 (3) and 323
nm (4) (Fig. 3) with molar absorptivity equal to 465
zine protons give a singlet at 8.48 ppm for 1 and at 8.60
ppm for 2, in comparison with 8.60 ppm for free
pyrazine [23]. One could expect that pyrazine coordi-
nated as a monodentate ligand should exhibit two
doublets of two groups of protons, positioned farther
and closer to the metal centre, respectively [9]. The
singlet resonances for salts 1 and 2 indicate very weak
bonding of pyrazine which causes the protons of pyra-
−
1
−1
and 635 M
cm , respectively.

1
508
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
hydrated salts are not reactive towards molecular oxy-
gen [11]. We have now found that the hydrated tetra-
cyano complexes of Mo(IV) and W(IV) of formula
2
−
[
M(CN) O(pz)]
can react with molecular oxygen,
4
both in solution and in the solid state. The presence of
water molecules does not interfere with the reaction
with dioxygen. This is probably caused by the fact that
the weakly bonded pyrazine ligand does not participate
in the network of hydrogen-bonds. Thus, the best site
for nucleophilic attack of dioxygen seems to be in the
vicinity of pyrazine, whereas the rest of the complex ion
is surrounded by water molecules, hydrogen-bonded to
cyano and oxo ligands, which hinder the reaction with
Fig. 2. Spectral changes of 1 in dichloromethane solution. The arrows
show the direction of changes with time (first spectrum measured
after 30 s from dissolution, time interval=90 s). For comparison the
O . It seems also that the dissociative mechanism pro-
2
−
3
posed by Arzoumanian et al. [11] is not valid. In this
case the incoming dioxygen should be in the trans
spectrum of 3 is given (dashed curve). Conc.=5.3×10
path=1 cm.
M, optical
position to the MꢀO bond, as found for all substitution
n−
reactions leading to the [M(CN) O(L)] (n=2, 3) type
4
complexes. In all these cases the coordinatively unsatu-
2
−
rated [M(CN) O] does not undergo rearrangement in
4
the coordination sphere and does not react with dioxy-
gen. The associative mechanism presented in Scheme 1
seems more probable. In the first step the coordination
of dioxygen occurs with the increase of coordination
number to seven. Such a structure (distorted pentago-
nal bipyramid) was found by us earlier in
[
Mo(CN) O(LLLL)]·H O [7]. Release of pyrazine fol-
2 2
lowed by a rearrangement in which a cyano group is
transferred to the vacant trans position and the second
oxygen of dioxygen is bonded to the metal centre could
lead to the observed cis-tetracyanooxoperoxomolyb-
date(VI) anion.
Fig. 3. Electronic spectra of 3 (solid curve) and of 4 (dashed curve) in
acetonitrile.
3.6. ESR spectra
The ESR spectra for salts 3 and 4 consist of very
weak signals, which are typical for paramagnetic cen-
1
tres M(V)–d [24] with g factors (at room temperature)
equal to 1.970 and 1.915, respectively. In the case of
salt 3 in liquid nitrogen (77 K) an anisotropic signal is
observed with g=1.915 and 1.943 indicating the axial
symmetry of the complex anion. The amount of M(V)
centres in salts 3 and 4 is equal to 0.32 and 0.82%,
respectively. This indicates that to some extent thermal
2
−
charge transfer from the O2
ligand to the metal
−
occurs, resulting in M(V) and O2 formation.
4. Discussion
It was found earlier that anhydrous (PPh ) [Mo-
4
3
(
CN) O] can react with molecular oxygen giving
5
(
PPh ) [Mo(CN) O(O )] [10]. It was also found that this
4
2
4
2
Scheme 1. Proposed mechanism for molecular oxygen incorporation
2
−
reaction is possible only for the anhydrous salt and that
into the [M(CN) O(pz)]
ion.
4

D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
1509
Trans. (1998) 4009. (g) J. Szklarzewicz, A. Samotus, D. Matoga,
J. Burgess, J. Fawcett, D.R. Russell, Coordination Chemistry at
the Turn of the Century, Slovak Technical University Press,
Bratislava, vol. 7, 1999, p. 139.
5
. Supplementary material
Full crystallographic data may be obtained from the
Cambridge Crystallographic Data Centre, where CIF
files have been deposited (deposition no. 139915)
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-
mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
[
[
[
7] J. Szklarzewicz, A. Samotus, J. Burgess, J. Fawcett, D.R. Rus-
sell, J. Chem. Soc., Dalton Trans. (1995) 3057.
8] (a) C. Creutz, H. Taube, J. Am. Chem. Soc. 91 (1969) 3988. (b)
C. Creutz, H. Taube, J. Am. Chem. Soc. 95 (1973) 1086.
9] C.G. Johnson, R.E. Shepherd, Inorg. Chem. 22 (1983) 1117.
10] H. Arzoumanian, J.F. Petrignani, M. Pierrot, F. Ridouane, J.
Sanchez, Inorg. Chem. 27 (1988) 3377.
[11] H. Arzoumanian, M. Pierrot, F. Ridouane, J. Sanchez, Transi-
tion Met. Chem. 16 (1991) 422.
[
[
12] (a) J.P. Smit, W. Purcell, A. Roodt, J.G. Leipoldt, Polyhedron
2 (1993) 2271. (b) A. Roodt, S.S. Basson, J.G. Leipoldt, Poly-
1
Acknowledgements
hedron 13 (1994) 599.
[
[
[
13] G.M. Sheldrick, SHELXTL-PC Release 4.2, Siemens Analytical
This work was supported in part by the Polish Re-
search Committee, KBN, Grant no. 3T09A 057 17.
X-ray Instruments, Madison, WI, 1991.
14] A.C. Dengel, W.P. Griffith, R.D. Powell, A.C. Skapski, J. Chem.
Soc., Dalton Trans. (1987) 991.
15] (a) W.P. Griffith, B.C. Parkin, A.J.P. White, D.J. Williams, J.
Chem. Soc., Dalton Trans. (1995) 3131. (b) N.M. Gresley, W.P.
Griffith, B.C. Parkin, A.J.P. White, D.J. Williams, J. Chem.
Soc., Dalton Trans. (1996) 2039.
References
[
1] (a) E. Hejmo, A. Kanas, A. Samotus, Bull. Acad. Polon. Sci.
Ser. Sci. Chim. 21 (1973) 311. (b) A. Kanas, M. Dudek, A.
Samotus, Bull. Acad. Polon. Sci. Ser. Sci. Chim. 24 (1976) 43.
2] M. Dudek, A. Samotus, Transition Met. Chem. 10 (1985) 271.
3] (a) S.S. Basson, J.G. Leipoldt, I.M. Potgieter, A. Roodt, Inorg.
Chim. Acta 103 (1985) 121. (b) J.G. Leipoldt, R. van Eldik, S.S.
Basson, A. Roodt, Inorg. Chem. 25 (1986) 4639. (c) J.P. Smit,
W. Purcell, A. Roodt, J.G. Leipoldt, J. Chem. Soc., Chem.
Commun. (1993) 1388.
4] (a) J.G. Leipoldt, S.S. Basson, A. Roodt, I.M. Potgieter, S. Afr.
J. Chem. 39 (1986) 179. (b) I.M. Potgieter, S.S. Basson, A.
Roodt, J.G. Leipoldt, Transition Met. Chem. 13 (1988) 209.
5] A. Roodt, J.G. Leipoldt, S.S. Basson, I.M. Potgieter, Transition
Met. Chem. 13 (1988) 336.
[16] (a) M.H. Dickman, M.T. Pope, Chem. Rev. 94 (1994) 569. (b)
M. Sivak, V. Sucha, L. Kuchta, J. Marek, Polyhedron 18 (1998)
93 and references therein. (c) M. Kosugi, S. Hikichi, M. Akita,
Y. Moro-oka, J. Chem. Soc., Dalton Trans. (1999) 1369.
[17] A. Butler, M.J. Clague, G.E. Meister, Chem. Rev. 94 (1994) 625.
[18] A. Roodt, J.G. Leipoldt, S.S. Basson, I.M. Potgieter, Transition
Met. Chem. 15 (1990) 439.
[19] (a) M. Dudek, A. Kanas, A. Samotus, J. Inorg. Nucl. Chem. 41
(1979) 1135. (b) M. Dudek, A. Kanas, A. Samotus, J. Inorg.
Nucl. Chem. 42 (1980) 1701.
[
[
[
[20] B. Klein, J. Berkowitz, J. Am. Chem. Soc. 81 (1959) 5160.
[21] L.J. Bellamy, The Infra-red Spectra of Complex Molecules,
Chapman and Hall, London, 1975, p. 135.
[
[
[22] R. Bandyopadhyay, S. Biswas, S. Guha, A.K. Mukherjee, R.
Bhattacharyya, Chem. Commun. (1999) 1627.
6] (a) S.S. Basson, J.G. Leipoldt, I.M. Potgieter, Inorg. Chim. Acta
1
3
1
8
7 (1984) 71. (b) S.S. Basson, J.G. Leipoldt, I.M. Potgieter,
[23] C.J. Pouchert, J. Behnke, The Aldrich Library of C and H FT
NMR Spectra, vol. III, Aldrich Chemical Company Inc., 1993.
[24] (a) B.R. McGarvey, Inorg. Chem. 5 (1966) 476. (b) R.A.
Pribush, R.D. Archer, Inorg. Chem. 13 (1974) 2556. (b) P.M.
Kiernan, W.P. Griffith, J. Chem. Soc., Dalton Trans. (1975)
2489. (c) T.J. Kemp, M.A. Shand, D. Rehorek, J. Chem. Soc.,
Dalton Trans. (1988) 285. (d) J. Szklarzewicz, A. Samotus, O.
Traverso, S. Sostero, Polyhedron 13 (1994) 1755.
Inorg. Chim. Acta 90 (1984) 57. (c) J.G. Leipoldt, S.S. Basson,
A. Roodt, I.M. Potgieter, Transition Met. Chem. 11 (1986) 323.
(d) J. Szklarzewicz, A. Samotus, N.W. Alcock, M. Moll, J.
Chem. Soc., Dalton Trans. (1990) 2959. (e) B. Burda, J. Burgess,
S.A. Parsons, A. Samotus, J. Szklarzewicz, Transition Met.
Chem. 20 (1995) 291. (f) B. Nowicka, A. Samotus, J. Szk-
larzewicz, F. Heinemann, H. Kisch, J. Chem. Soc., Dalton
.