Preparation, characterization, and aquation kinetics of pyridine N-oxide complexes of chromium(III)

Article abstract of DOI:10.1002/ejic.200600260

A series of (H₂O)₅Cr(X-pyO)³⁺ ions (pyO = pyridine N-oxide, X = H, 3-CH₃, 4-CH₃, 4-OCH₃, 4-NO₂) were prepared by the reduction of the corresponding pyridine N-oxide adducts of diperoxochromium(VI) species with acidic ferrous perchlorate. The (H₂O)₅Cr(X-pyO)³⁺ complexes undergo aquation to yield Cr(H₂O)₆³⁺ and X-pyO according to the rate law k_obs = k_o + k ^-1[H⁺]^-1. The values of the rate constants extrapolated to 298 K at 1.0 M ionic strength are: k₀ = 2.80 × 10^-6 s^-1, k_-1 = 1.86 × 10^-8 M s^-1 (X = 4-NO₂); 7.80 × 10^-8, 6.27 × 10^-10 (H); 4.80 × 10^-8, 3.20 × 10 ^-10 (3-CH₃); 3.05 × 10^-8, 1.60 × 10^-10 (4-CH₃); and 2.37 × 10^-9, 4.76 × 10^-11 (4-OCH₃). The reaction of the 4-OCH ₃ complex exhibits two additional terms in the rate law, k ₁[H⁺] + k^-2[H⁺]^-2. The binding of 4-OCH₃-pyO to chromium is suggested to take place through the methoxy group. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600260

FULL PAPER
DOI: 10.1002/ejic.200600260
Preparation, Characterization, and Aquation Kinetics of Pyridine N-Oxide
Complexes of Chromium(III)
Mirjana Kotowski,[a][‡] Radovan Marcec,[a][†] Vjera Butkovic, Andreja Bakac,* and
[a]
[b]
Matko Orhanovic[
a]
Keywords: Pyridine oxide / Chromium / Kinetics / Mechanism / Aquation
Cr(X-pyO)3+ ions (pyO = pyridine N-oxide,
, 4-OCH , 4-NO ) were prepared by the
1.86×10
–8
M
s–1 (X = 4-NO
); 7.80×10 , 6.27×10
–8
–10
(H);
(4-
A series of (H
2
O)
5
2
–
8
–10
–8
–10
X = H, 3-CH , 4-CH
3
3
3
2
4.80×10 , 3.20×10
3
(3-CH ); 3.05×10 , 1.60×10
–
9
–11
reduction of the corresponding pyridine N-oxide adducts of
CH
3
); and 2.37×10 , 4.76×10
3
(4-OCH ). The reaction of
diperoxochromium(VI) species with acidic ferrous perchlo-
the 4-OCH
3
complex exhibits two additional terms in the rate
3
+
+
+ –2
rate. The (H
2
O)
5
Cr(X-pyO) complexes undergo aquation to
1 3
law, k [H ]+k–2[H ] . The binding of 4-OCH –pyO to chro-
3+
yield Cr(H O)
2
–1
6
and X-pyO according to the rate law kobs
=
mium is suggested to take place through the methoxy group.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
+
k
o
+k–1[H ] . The values of the rate constants extrapolated
0
M ionic strength are: k =
= 2.80×10– s , k–1
6
–1
to 298 K at 1.0
Introduction
was not observed, possibly because of its decomposition
on ion-exchange resin. In the reduction of (NH ) Co(X-
3
5
Pyridine N-oxides (X-pyO) are important reagents in pyO)3 by 2-hydroxy-2-propyl radicals, C(CH ) OH, the
+
·
3
2
[
1,2]
[3–5]
thermal
and photochemical
oxidations. Often, al- initial site of attack was shown to be at the aromatic ring
[
6]
[23]
though not always, such reactions involve oxygen-atom of the coordinated X-pyO.
[
7–9]
transfer
with pyridine N-oxide playing a role of either
These studies have now been extended to a series of chro-
[
2,7–12]
3+
oxygen-atom donor
or a promoter of oxygen transfer mium complexes, (H O) Cr(X-pyO) (X = H, 3-CH , 4-
2
5
3
[13–16]
from active donors to the substrate.
Metal catalysis is CH , 4-OCH , 4-NO ). In this first report, we focus on the
3 3 2
frequently involved[
17]
in such processes, which makes preparation and spectral and kinetic characterization of
mechanistic studies of transition-metal complexes of pyri- these complexes. Prior to this work, only the parent ion,
3
+
[24]
dine N-oxides an important area of investigation.
(H O) Cr(pyO) , was known. It was generated by ther-
2 5
3+
A large number of complexes with an array of metal ions mal equilibration of Cr(H O)6 with an excess of pyO in
2
have been prepared and characterized structurally, spectro- slightly acidic aqueous solutions at elevated temperatures,
scopically, electrochemically, and chemically. There are, followed by ion exchange. In the present work, our pre-
however, few reports available on the kinetics and mecha- viously published procedure for the preparation of pyridine
[
18–20]
[22]
nisms of the reactions of such complexes,
especially chromium complexes
was applied to pyridine N-oxide
in aqueous solutions.[
21,22]
Several pentaamminecobalt(III) analogues. This method represents a great advantage over
have been prepared
3
+
[21,22]
ions, (NH ) Co(X-pyO)
and the thermal equilibration method because it yields exclu-
3
5
found to be reasonably stable at room temperature, pro- sively the mono-substituted products.
vided there are no cyano substituents on the pyridine ring,
in which case facile hydrolysis to a bound amide takes
[
21]
3+
place. The reduction of the parent (H O) Co(pyO) by
2
5
Results
chromium(II) is believed to take place by an inner-sphere
2
+
mechanism whereby Cr
attacks at the coordinated pyO
The method used to prepare the complexes in Table 1 is
oxygen, although the expected product, (H O) Cr(pyO) , based on the chemistry that we have developed earlier for
aq
[21]
3+
2
5
[
22]
the pyridine complexes.
By analogy to that work, the
blue paste-like material generated from CrO , X-pyO and
[
[
[
[
a] Rugjer Boskovic Institute,
3
Bijenicka 54, 10000 Zagreb, Croatia
H O is assigned the formula CrO(O ) (X-pyO). Reduction
2
2
2 2
b] Iowa State University, Chemistry Department,
Ames, IA 50011, Iowa, USA
2+
3+
with Feaq
Equation (1).
produces the desired (H O) Cr(X-pyO) ,
2 5
‡] Current address: Max Planck Institute for Chemistry,
5
5128 Mainz, Germany
The chemical composition of the complexes was con-
firmed by chromium and ligand analysis after the complex
had been allowed to aquate. In every case, the [Cr]/[X-pyO]
†] Deceased.
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
2894
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2894–2899

Chromium(III) Complexes with Pyridine N-Oxide
Table 1. UV/Vis spectral data for (H O)
Cr(X-pyO)3+ and X-pyO.
FULL PAPER
2
5
3
+
XpyOH+
(
λ
H
2
O)
5
Cr(X-pyO)
X-pyO
λ
max, [nm] (ε, –1 cm )
–
1
–1 [a][b]
–1 [c]
X
H
max [nm] (ε,  cm )
3
588 (25.7)
5
590 (25.9)
417 (30.5)
414 (30.8)
418 (31.7)
240 (1.15×104)
242 (1.08×104)
246 (1.35×104)
254 (1.15×104)
257 (2.9×10 )
86 (26.0)[
d]
[d]
217 (4.9×10 )
3
4
3
3
4
-CH
3
3
254 (1.17×10 )
263 (3.5×10 )
4
3
2
09 (1.98×10 )
220 (4.2×10 )
4
3
-CH
588 (29.8)
417 (40.9)
256 (1.40×10 )
254 (2.7×10 )
4
3
2
06 (1.87×10 )
226 (8.3×10 )
4
4 [b]
4 [a][b]
4
4
-OCH
-NO
3
582 (27.9)
585 (33.6)
417 (37.9)
sh
262 (1.50×10 )
261 (1.60×10 )
243 (1.20×10 )
4
4
3
2
266 (1.28×10 )
313 (1.25×10 )
280 (3.8×10 )
3
3
2
26 (8.0×10 )
244 (8.2×10 )
4
[a] In 1.0  aqueous HClO . [b] This work. [c] Ref. [25]. [d] Ref. [24].
2
+
3+
CrO(O
2
)
2
(X-pyO)+7Feaq +10H+
Ǟ
pyO) , which exhibited a more complex behavior, see be-
low.
(H
2
O)
5
Cr(X-pyO)3+ +7Feaq3+ (1)
+
–1
k
obs = k
o
+k–1 [H ] ; X = H, 3-CH
3
, 4-CH
3
, 4-NO
2
(2)
ratio was found to be 1.00± 0.02. As expected, the behavior
of the complexes on the cation-exchange resin was consis-
tent with a 3+ charge.
The adherence to Equation (2) is shown for X = 3-CH3
+
in Figure 2. The plot of kobs against 1/[H ] at several tem-
peratures is linear yielding k as an intercept and k as a
0
–1
The UV/Visible spectroscopic data are summarized in
Table 1. The spectrum of the unsubstituted material (X =
H) agrees well with the literature data for a complex ob-
slope. Data for the remaining complexes (X = H, 4-CH3,
and 4-NO ) are presented in the supporting information
2
3
+
(Figures S1–S3; for supporting information see also the
footnote on the first page of this article). The kinetic data
are summarized in Table 2.
tained by anation of Cr(H O)
with pyO followed by ion-
2
6
3
+
exchange to remove Cr(H O)
and cis- and trans-(H O) -
2
6
2
4
3
+ [24]
Cr(pyO)2
.
3+
The kinetics of aquation of (H O) Cr(X-pyO) com-
2
5
plexes were studied as a function of acidity and temperature
at 1.0  ionic strength. As illustrated in Figure 1 for
3
+
(
H O) Cr(pyO) , the spectra exhibit clean isosbestic points
2 5
as expected if the stoichiometry remains unchanged
throughout the reaction, and no long-lived intermediates
are involved.
+
Figure 2. Plot of kobs vs. 1/[H ] for the aquation of (H
2
O)
5
Cr(3-
3
+
3
CH –pyO) at 99.0 °C (circles), 90.0° (squares), 80.1° (diamonds),
and 70.8° (triangles).
The activation parameters, also shown in Table 2, were
obtained by fitting simultaneously all the individual acid-
dependent rate constants kobs for a given complex to the
Eyring expression, Equation (3), where kB and h are the
Boltzman and Planck constants, respectively, T is the abso-
lute temperature, and i = 0 and –1. The fit of the data to
Figure 1. Spectral changes accompanying the aquation of
.16·10–5  (H
O) Cr(pyO) in 0.020  HClO at 1.0  ionic
2 5 4
3+
4
strength and 90 °C. Spectrum 1 was obtained on a fresh sample at
5 °C before the onset of the reaction. Spectra 2 through 6 were
measured at time 0 (arbitrary), 8, 18, 45.5, and 1250 minutes.
2
3+
Equation (3) for (H O) Cr(3-CH –pyO) is shown in Fig-
2
5
3
The standard treatment of the exponential kinetic traces ure 3, and for the rest of the complexes in Figures S4–S7 in
yielded first-order rate constants k which decreased with the supporting information.
obs
+
+
an increase in [H ] according to the expression in Equation
The plots of kobs vs. 1/[H ] for the 4-methoxy complex,
(
2) for all the complexes except for (H O) Cr(4-OCH – Figure 4, exhibited deviations from Equation (2) at both the
2
5
3
Eur. J. Inorg. Chem. 2006, 2894–2899
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2895

M. Kotowski, R. Marcec V. Butkovic, A. Bakac, M. Orhanovic
FULL PAPER
Table 2. Summary of kinetic data for the aquation of (H
2 5
O) Cr(X-
pyO)3+ complexes.
[a]
–
1 [b]
–1 [c]
X
T [°C]
k
0
[s ]
k–1 [ s ]
[
d]
–4
–5
–5
–6
–6
–7
4-NO
2
70.2
4.47(±0.27)×10–
2.92(±0.05)×10
2.58(±0.14)×10
1.15(±0.01)×10
5.20(±0.09)×10
1.31(±0.01)×10
2.76(±0.10)×10
4
4
5
5
6
6
5
4
2
5
–
0.2
0.2
0.1
5.0
1.79(±0.13)×10
5.64(±0.15)×10–
–
1.68(±0.17)×10
–6 [e]
–8 [e]
2.80×10
1.86×10
‡
‡
∆H
0
= 93.9± 2.2
∆H–1 = 129± 3
‡
‡
∆S
0
= –36± 6
∆S–1 = 40± 8
H
99.0
3.7(±1.8)×10–4
6.6(±4.8)×10–
1.2(±4.1)×10–5
1.05(±0.14)×10
1.95(±0.22)×10–5
5
–6
9
8
7
2
0.0
0.0
0.0
5.0
8.98(±0.21)× 10
–6
2.20(±0.07)×10
–5
–7
5.50(±0.05)× 10
–8 [e]
–10 [e]
7.80×10
6.27×10
‡
‡
∆
H
0
0
= 104± 47
= –32± 127
∆H–1 = 125± 4
∆S–1 = –1.6± 10
‡
‡
∆S
–5
4
3
4
-CH
3
98.6
1.80(±0.29)×10–4
1.26(±0.04)×10
2 5 3
Figure 3. Fit of the data for the aquation of (H O) Cr(3-CH –
9
8
7
2
0.0
0.1
0.0
5.0
9.96(±0.81)×10–
5
3.82(±0.04)×10
–6
pyO) to Equation 3 with use of the program Kaleidagraph 3.6.
Index numbers on the x axis identify each unique point at a given
acidity and temperature. Temperature (in °C) for each data set is
indicated.
3+
–
5
–6
2.70(±0.54)×10
1.17(±0.02)×10
–6
–7
6.6(±2.8)×10
3.00(±0.07)×10
–
8 [e]
–10 [e]
3.05×10
1.60×10
‡
‡
∆H
0
= 109± 24
∆H–1 = 139± 3
‡
‡
∆S
0
= –23± 64
∆S–1 = 34± 8
–
4
5
–5
-CH
3
99.0
2.87(±0.13)×10
8.6(±2.2)×10–
4.8(±1.7)×10–5
1.09(±0.50)×10
1.57(±0.02)×10
5
–6
9
8
7
2
0.0
0.1
0.8
5.0
5.75(±0.11)×10
–6
1.47(±0.05)×10
–
–7
3.63(±0.11)×10
–8 [e]
–10 [e]
4.80×10
3.20×10
‡
‡
∆
H
0
0
= 104± 21
∆H–1 = 134± 2
‡
‡
∆S
= –36± 56
∆S–1 = 23± 6
[
f]
–4
–6
-OCH
3
99.3
1.52(±0.10)×10–
5.03(±0.39)×10
2.10(±0.37)×10–
6.20(±0.08)×10
5
5
6
–6
9
8
7
2
0.4
0.0
0.2
5.0
1.98(±0.04)×10
–7
3.08(±0.29)×10
–
–8
3.95(±0.63)×10
8.75(±0.61)×10
–
9 [f]
–11 [f]
2.37×10
4.76(±0.39)×10
‡
‡
∆H
0
= 135± 11
= 43± 29
∆H–1 = 142± 2
‡
‡
∆S
0
∆S–1 = 34± 7
‡
–1
[
∆
a] Ionic strength: 1.0  (HClO
4
+NaClO
4
). Units: ∆H in kJmol ,
‡
–1 –1
+
S in Jmol K . [b] Obtained as intercept of kobs vs. 1/[H ] plots,
+
as in Figure 2. [c] Obtained as a slope of kobs vs. 1/[H ] plots, as in
–
3 –1
Figure 2. [d] Additional data in 1.0  HClO
(
4
:
1.36×10
s
–4
–5
80 °C), 8.43×10 (75 °C), 9.10×10 (55 °C). [e] Calculated from
the activation parameters. [f] At Ͼ5 m [H ], a term directly pro-
+
+
Figure 4. Plot of kobs vs. 1/[H ] for the aquation of (H
2
O)
5
Cr(4-
(4)
+
‡
–1
‡
portional to [H ] also contributes, ∆H
=
1 1
= 104± 43 kJmol , ∆S
3
+
–1
3
OCH –pyO) at 1.0  ionic strength.
–48± 115 Jmol–1 K .
+
+ –1
+ –2
k
1 o 3
obs = k [H ]+k +k–1 [H ] +k–2 [H ] ; X = 4-OCH
(
3)
large errors, especially for the less prominent terms, were
high and low ends of [H ], and required two additional to be expected for a complex fit and a large number of
terms in the rate law, Equation (4), which is analogous to parameters.
+
2
+ [26]
that reported for the azido complex, (H O) CrN
.
To ascertain that the k term is real and not a result of
2
5
3
1
The attempted fit of all the data to the multi-term Eyring misinterpreted deviations from a simpler rate law, several
expression of Equation (3) with i = 1, 0, –1, and –2 failed experiments were conducted at higher acid concentrations
to converge, presumably because the contribution from the at 4.0  ionic strength where k and k terms should domi-
1
0
k–2 term was small and thus subject to large error under all nate. The results are shown in Figure 5, which clearly con-
of the conditions. The activation parameters in Table 2 were firms the expectations. The rate constants at 99.0 °C are k
0
+
–4 –1
–4 –1 –1
obtained by fitting only the data at Ͼ5 m [H ] to a 3-term = 2.20(±0.14)×10
s
and k = 1.16(±0.07)×10

s .
1
variant of Equation (3), i.e., i = 1, 0, and –1. The rather The corresponding values extracted from the more complex
2896
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2894–2899

Chromium(III) Complexes with Pyridine N-Oxide
FULL PAPER
–4
3+
set of data at 1.0  ionic strength are k = 1.52(±0.11)×10
(H O) Cr(X-py) ions. A reasonable mechanism features
2 5
0
–
1
–5 –1 –1
s
and k = 5.6(±1.9)×10

s .
parallel reactions of the pentaaqua complex and its conju-
1
gate base, Equations (5), (6) and (7) leading to the rate law
+
in Equation (8). In the limit K ϽϽ [H ], Equation (8)
a
reduces to the observed form, Equation (2), where k–1
k ЈK .
=
–1
a
(
(
5)
6)
+
Figure 5. Plot of kobs vs. [H ] for the aquation of (H
OCH
–pyO)3+ at 4.0  ionic strength and 99 °C.
2 5
O) Cr(4-
3
(
7)
Discussion
UV/Vis Spectra: The ligand-field transitions for all the
pyridine N-oxide complexes studied (Table 1) are red-
(
8)
[
22]
shifted relative to the corresponding pyridine complexes,
consistent with the greater ligand-field strength of the pyri- have not been determined, but the excellent fit of the data
dines. to Equation (2) throughout the acid range studied places a
With the exception of the 4-OCH derivative, the posi- limit K ϽϽ 10  for all the complexes in the temperature
The acidity constants for the pyridine oxide complexes
–3
3
a
3
+
tions of the UV bands for (H O) Cr(X-pyO) ions are con- range examined.
2
5
siderably different from those of the free ligands. As an ex-
The activation enthalpies are large for both pathways for
3
+
+
ample, (H O) Cr(4-CH –pyO) in 1  H exhibits a maxi- all the complexes. Activation entropies are moderately
2
5
3
4
–1
–1
mum at 246 nm (ε = 1.35×10  cm ). Under the same negative for the k path, and moderately positive for the k
0
–1
conditions, the free ligand has maxima at 254 nm (ε = path. The only exception seems to be the parent pyO com-
3
3
–1 –1
2
.7×10 ) and 226 nm (ε = 8.3×10 ). In neutral solutions, plex, for which ∆S–1‡ is close to zero (–1.6± 10 Jmol K ).
4
these maxima shift to 256 nm (ε = 1.40×10 ) and 206 nm These results are consistent with a strong associative com-
4
III
(
1.9×10 ). Similar spectral differences between the free and ponent in the k path, as is often the case with aqua Cr
0
Cr-bound pyridine N-oxides are observed for the rest of complexes, including the water substitution in Cr-
3
+ [27]
the materials in Table 1. Only the spectrum of (H O) Cr(4- (H O)6
.
The k–1 path is a composite, which makes the
2
–1
5
2
3
+
4
–1
OCH –pyO) (λmax = 262 nm, ε = 1.50×10  cm ) is mechanistic assignment of the kinetic step more difficult to
almost identical to that of the free ligand in H O. This ob- make. Still, given that the ∆S component (associated with
servation suggests that the binding of 4-OCH –pyO to the K ) will be positive, and the overall ∆S is only on the
chromium perhaps takes place at the methoxy oxygen, order of 20–40 Jmol K , one can all but rule out a strong
whereas the rest of the complexes utilize the pyridine oxide dissociative contribution to k . The data are consistent
3
0
2
‡
3
a
–1
–1
–1
–1
oxygen. We rationalize this assignment by noting that coor- with an interchange mechanism in either dissociative or as-
dination via the methoxy group should affect the intrali- sociative regime.
3+
gand charge-transfer transitions to a lesser extent than co-
The Case of (H O) Cr(4-OCH -pyO) : As already men-
2 5 3
ordination via the N-oxo group, or the protonation of the tioned, the aquation of aqua and amine complexes of chro-
N-oxo group. Additional arguments in favor of the unique mium(III) often follows the general rate law of Equation
3
+
+
binding mode in (H O) Cr(4-OCH –pyO) will be pre- (2). The direct dependence on [H ], on the other hand, has
2
5
3
sented shortly.
Kinetics of Aquation of (H O) Cr(X-pyO) (X = H, 3- with an open basic site, i.e. anions such as F , N , SO ,
4
been observed only for those complexes that have a ligand
3
+
–
–
2–
2
5
3
–
– [28–32]
CH , 4-CH , 4-NO ): The two-term rate law of Equation CN , or CH COO .
This path is not expected to oper-
3
3
2
3
3+
(
2) is common to all the complexes with the exception of the ate for (H O) Cr(X-pyO) complexes; the basicity of X-
2 5
4
-methoxy derivative which exhibits two additional terms, pyO oxygen is greatly reduced upon coordination to Cr, and
+
+ –2
k [H ] and k [H ] , Equation (4). The rate law of Equa- the probability of protonation is minimal. The complexes
1
–2
tion (2) is typical for the aquation of pentaaquachro- having X = H, 3-CH , 4-CH , and 4-NO follow this pre-
3
3
2
+
mium(III) complexes, including those of the closely related diction and do not feature a k [H ] term. The 4-OCH de-
1
3
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M. Kotowski, R. Marcec V. Butkovic, A. Bakac, M. Orhanovic
FULL PAPER
+
rivative, on the other hand, has a pronounced k [H ] term
Comparison with Pyridine Complexes: Figure 6 shows a
1
which would be difficult to explain for a complex having a plot of the logarithms of the first-order rate constants k0
Cr–oxopyridine bond, Cr–Opy(4-OCH ). If, however, the against the pK of the protonated ligands for complexes of
3
a
III
ligand is coordinated through the methoxy group, as in Cr– Cr with pyridines and pyridine N-oxides at 70 °C. The
O(CH )pyO, then the protonation at the exposed pyO oxy- correlation was not sought for k , which is a composite of
3
–1
gen becomes more likely. We suggest that bonding through rate and acid dissociation constants.
the methoxy group indeed takes place and is responsible for
Both sets of rate constants in Figure 6 are linearly related
the unique behavior of the 4-OCH –py complex. The UV to the ligand pK , a measure of σ electron donation. In
3
a
spectrum discussed earlier supports this assignment. Even both cases, the greater thermodynamic affinity for the pro-
though the basicity of the methoxy oxygen should be ton results in a kinetically more inert chromium complex,
greatly diminished in the chromium complex as compared i.e. the greater the pK the slower the aquation. When the
a
to the free base, the protonation can still take place at the two series are compared to each other, however, then the
+
high end of [H ] used in this work. The proposed isomer, pK argument breaks down; the less basic pyridine N-oxides
a
3
+
(
H O) Cr–O(CH )pyO , should not be considered an un- bind to the chromium more strongly, at least in the kinetic
2 5 3
III
usual species in that related alcohol complexes of Cr , such sense. One obvious source of the apparent anomalous inert-
as (H O)6–nCr[O(H)CH3]n , are well known and quite ness of pyridine N-oxide complexes is the greater ability of
stable.
The positive ∆S value for (H O) Cr(4-OCH –pyO) ,
3± 29 Jmol K , seems outstanding, and may suggest a
3
+
2
[
33]
pyridine N-oxides to engage in π-bonding with the metal.
‡
3+
0
2
5
3
–
1
–1
4
mechanistic deviation from the associative interchange as- Experimental Section
signed to the rest of the complexes in Table 2. The large
error bars for all the entropy values in Table 2, however,
make such a conclusion uncertain. We also note that an
2 5
Preparation of Complexes. (H O)
Cr(pyO)3+: To a stirred, ice-cold
aqueous solution (2 mL) of 1  pyridine oxide (PyO) and 1  CrO
was added dropwise cold 30% H (2 mol-equiv.). The resulting
3
2
O
2
‡
unexplained positive ∆S0 term was observed in the aqu-
blue paste was immediately treated with a solution containing
which does not appear 7.5 mol-equiv. of ferrous perchlorate and 11 mol-equiv. of HClO4.
The green product solution was loaded onto a column of Dowex
3
+ [22]
ation of (H O) Cr(3-CH –py) ,
2
5
3
outstanding in any other respect.
50W-X8 cation-exchange resin. Iron(III) ions were eluted with
The k path was observed only for (H O) Cr(4-CH O–
–
2
2
5
3
3
+
0.1  NaSCN in 1  HClO . The column was rinsed with 1 
pyO) , implying a greater acidity of the coordinated waters
for this complex. This is a surprising observation and con-
trary to what one might expect on the basis of the strong
electron donation from the 4-methoxy group and the
4
HClO
4
to remove the remaining NaSCN. Chromium(III) species
and collected in several fractions. The
was eluted first, followed by the green (H O)
were eluted with 3  HClO
blue Cr(H
Cr(pyO) . The middle fraction of this complex was used for all
the kinetics and spectral measurements. The same procedure was
4
3
+
2
O)
6
2
5
-
3
+
[34]
greater basicity of 4-CH O–pyO compared to other pyri-
3
dine oxides in this work. It is not clear whether the different
used to prepare the ring-substituted complexes (H
2
O)
), except that
in the case of (H O) Cr(4-NO –pyO) , the solvent water in the
5
Cr-
III
binding mode to Cr would cause the observed shift in the
3+
(
3 3 3 2
X-pyO) (X = 3-CH , 4-CH , 4-CH O and 4-NO
3
+
pKa.
2
5
2
2
initial step was replaced by a 1:1 H O/dioxane mixture to increase
the solubility of the ligand.
Materials: Pyridine N-oxides (pyO, 3-CH
-NO –pyO) were purchased from Fluka and recrystallized twice
from ethanol. 4-CH O–pyO was prepared by refluxing 28 g 4-NO
pyO (Fluka) and 28 g K CO in 200 mL of methanol for
hours.[ The product was recrystallized twice from heptane.
3 3
–pyO, 4-CH –pyO, and
4
2
3
2
–
2
3
35]
8
Solutions of iron(II) perchlorate were prepared by zinc amalgam
III
reduction of the Fe salt. Sodium perchlorate was prepared from
perchloric acid and sodium carbonate. The solid was recrystallized
three times from doubly distilled water. Other chemicals were pur-
chased at the highest purity available and used as received.
2 5
O)
Cr(X-pyO)3+ was deter-
Analyses: The chromium content of (H
mined spectrophotometrically after oxidizing the chromium to
3
–1
–1 [36]
chromate (ε372 = 4.82·10  cm ) with alkaline hydrogen per-
oxide. The concentration of the appropriate pyridine N-oxides gen-
4
erated by hydrolysis of the complex in 0.10  HClO for Ն 10 half-
lives was determined spectrophotometrically.
Kinetic Measurements: The aquation of all the complexes was mon-
itored spectrophotometrically at a wavelength, typically close to
Figure 6. Plot of logk
of Cr(H O)
2 5
0
vs. pK
complexes of pyridines (py, circles) and pyridine N-
oxides (pyO, squares) at 70 °C. The substituents on pyO, in the
order of increasing pK , are 4-NO , H, 3-CH , 4-CH , and 4-
OCH . In the pyridine series, the order is 3-CN, 3-Cl, H, 3-CH
Data are from Table 2 and references [22], [23] and [34].
a
of the free ligands for the aquation
3+
λmax for the free ligand, where the difference in molar absorptivities
for reactants and products was at a maximum. The initial concen-
tration of all the complexes was in the range 3–30 µ. In most
cases, the change in absorbance was monitored continuously inside
a
2
3
3
3
3
.
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Eur. J. Inorg. Chem. 2006, 2894–2899

Chromium(III) Complexes with Pyridine N-Oxide
FULL PAPER
a thermostatted cell compartment of a Cary 17 spectrophotometer.
In some experiments, especially those at Ն 90 °C, the samples were
sealed in glass ampoules and immersed into thermostatted
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gram Kaleidagraph 3.6.
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Received: March 24, 2006
Published Online: May 29, 2006
1943.
Eur. J. Inorg. Chem. 2006, 2894–2899
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2899