Unexpected participation of nucleophiles in the reaction of palladium(II) acetate with divalent 3d metals

Article abstract of DOI:10.1134/S0023158409030094

The kinetics of reactions of palladium(II) acetate with cobalt(II), nickel(II), and copper(II) acetates were studied by spectrophotometry. These reactions produce heterobimetallic complexes Pd^II(μ-OOCMe) ₄M^II(OH₂

Full text of DOI:10.1134/S0023158409030094

ISSN 0023-1584, Kinetics and Catalysis, 2009, Vol. 50, No. 3, pp. 396–400. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © N.S. Akhmadullina, N.V. Cherkashina, N.Yu. Kozitsyna, A.E. Gekhman, M.N. Vargaftik, 2009, published in Kinetika i Kataliz, 2009, Vol. 50, No. 3,
pp. 417–421.
Unexpected Participation of Nucleophiles in the Reaction
1
of Palladium(II) Acetate with Divalent 3d Metals
N. S. Akhmadullina, N. V. Cherkashina, N. Yu. Kozitsyna, A. E. Gekhman, and M. N. Vargaftik
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
e-mail: wahr36@gmail.com
Received April 19, 2008
Abstract—The kinetics of reactions of palladium(II) acetate with cobalt(II), nickel(II), and copper(II) acetates
II
were studied by spectrophotometry. These reactions produce heterobimetallic complexes Pd (µ-
II
OOCMe) M (OH )(HOOCMe) , where M = Co, Ni, or Cu. These reactions are very slow in carefully dehy-
4
2
2
drated (<0.01% H O) acetic acid, but are considerably enhanced by water or acetonitrile. Our data indicate that
2
the activation of the kinetically inert ring structure of the initial palladium complex Pd (µ-OOCMe) by means
3
6
of the nucleophilic attack of an H O or acetonitrile molecule is the key step of the reaction mechanism.
2
DOI: 10.1134/S0023158409030094
Heterometallic coordination compounds of palla- we synthesized and structurally characterized the first
dium(II) are of both fundamental and practical impor- heterobimetallic acetate complexes of palladium(II)
tance as suitable precursors for both heterometallic with divalent 3d transition metals, tervalent 4f rare-
nanomaterials [1, 2] and catalysts [2–4]. The literature earth metals, and 4s–6s divalent alkaline-earth metals
concerning such complexes is scarce. Recently [5–8], with lantern structures:
Me
C
Me
C
Me
C O
Me
O
C
O
O
Pd
O
Me
O
O
O
C
O
O
O
L
Pd
Pd + M(OOCMe)2(3)
O
Pd
M
O
Ln
O
C
O
O
O
Me
,
(I)
O
C
C
O
C
O
C
Me
Me
Me
Me
I
II
II
II
II
II
M = Co , Ni , Mn , Cu , Zn ; L = OH , NCMe, HOOCMe;
n
2
III
III
III
–
M = Nd , Sm , Eu ; L = OOCMe , OH , THF;
n
2
II
II
II
M = Ca , Sr , Ba ; L = (HOOCMe) .
n
4
Reductive thermolysis easily converts these complexes
This work studies the kinetics of the reaction of
to nanosized intermetallic compounds or alloys [9, 10]. Pd (µ-OOCMe) with 3d-metal acetates, namely,
3
6
Co(OOCMe)2
(II), Ni(OOCMe)
2
(III), and
Reaction (I) occurs with high yields (up to 95%)
when palladium(II) acetate Pd (µ-OOCMe)
complex I) is heated with proper metal acetates in gla-
Cu(OOCMe) (IV), in acetic acid containing controlled
2
3
6
additives of a nucleophilic agent (water, acetonitrile, or
tetrahydrofuran).
(
cial acetic acid. Optimizing the synthesis, we unexpect-
edly discovered that the reaction is very slow in care-
fully dehydrated acetic acid (<0.01% water) but is con-
siderably enhanced by even minor water or acetonitrile
amounts.
EXPERIMENTAL
Palladium acetate Pd (OOCMe) was prepared by
3
6
oxidizing palladium black with nitric acid in glacial
acetic acid as described in [11]; palladium black was
1
To the 80 birthday of Academician I.I. Moiseev.
3
96

UNEXPECTED PARTICIPATION OF NUCLEOPHILES IN THE REACTION
397
prepared by the reduction of PdCl (reagent grade, from Coefficients in Eq. (1) for reaction (I) in the presence of water
2
Reakhim) with sodium borohydride. Then, palladium
3
d metal (M)
keff, l mol min–1
2
–2
β × 10 , l/mol
–3
acetate was repeatedly boiled in glacial acetic acid in
the presence of fresh palladium black portions until
Ni
0.625 ± 0.050
1.00 ± 0.05
2.50 ± 0.10
2.8 ± 0.2
33.7 ± 2.0
3.74 ± 0.20
NO evolution ceased to free it from nitrito complexes
2
Co
Cu
and recrystallized from acetic acid. 3d-Metal acetates
(
Co(OOCMe) · 4H O, Ni(OOCMe) · 4H O, and
2 2 2 2
Cu(OOCMe) · 2H O, all of reagent grade, purchased
2
2
from Acros) were used as received. Commercially
available acetic acid (reagent grade glacial acetic acid
from Reakhim), which contained 1.3% (0.72 mol/l)
water, was purified before use by freezing and boiling
with triacetyl borate followed by fractional distillation.
Water concentration in purified acetic acid was ≤0.07%
OOCMe) M(OH )(HOOCMe) , where M = Co (com-
4
2
2
plex V), Ni (complex VI), and Cu (complex VII), that
were prepared by independent synthesis under the same
3
34
–1
conditions [13]: ε = 1337 l (mol cm) for complex V,
325
280
–1
ε
= 1295 and ε = 1345 l (mol cm) for complexVI,
356
–1
and ε = 1163 l (mol cm) for complex VII.
(
0.04 mol/l) as determined by GLC. The dopants used
were bidistilled water, acetonitrile, and tetrahydrofu-
Accumulation curves for complexes V–VII plotted
ran. Tetrahydrofuran (reagent grade, from Reakhim) on the basis of the above data with account for the extinc-
was purified by boiling with sodium benzophenone tion coefficients of the reagent and product complexes at
ketyl in flowing argon using a routine procedure [12] the working wavelength are exponential up to ~60–80%
and stored in a refrigerator under argon. Acetonitrile conversions for the cobalt complex and ~95% conver-
(
a high purity grade sample from Merck) was used as sions for the nickel and copper complexes. At higher
received.
Water, acetonitrile, or tetrahydrofuran was added to
purified acetic acid gravimetrically and monitored by
GLC on an Agilent 5973 chromatograph/mass spec-
conversions, the product accumulation curves have a
more intricate trends (for unclear reasons), and we used
initial rates in formal data processing.
Our data show that, in the presence of 0.2–0.5 mol/l
trometer (quartz capillary column HP-50, 70-eV ion- water, the rate of reaction (I) increases in proportion to
ization energy). UV–Vis absorption spectra of solutions the palladium(II) acetate concentration (Fig. 1a); the
were recorded on a Cary 50 spectrophotometer in a reaction rate as a function of concentration of the added
quartz cell 10 × 10 mm, where temperature was main- metal is a curve with a plateau (Fig. 1b) and is well lin-
tained at (50–80) ± 0.1°ë with a standard Peltier ther- earized in the coordinates of the Michaelis–Menten
mostat, while stirring with a built-in magnetic stirrer.
A representative kinetic experiment was carried out
equation (Fig. 1c).
The reaction rate increases linearly with increasing
as follows. Weighed samples of complex I (2.02 mg, water concentration (within 0.04–2.10 mol/l) or aceto-
0
.3 mmol as palladium atoms) and complex II, III, or nitrile concentration (within 0.01–0.40 mol/l) but is
IV (0.9 mmol) were each dissolved in acetic acid (5 ml) practically independent of tetrahydrofuran concentra-
and kept for 5 min in separate thermostats. Thermo- tion up to 0.62 mol/l (Fig. 2).
stated solutions of complex I and complexes II–IV
Thus, in the presence of water at 80°C, the rate
1 ml each) were transferred to the spectrophotometer equation is
(
cell, and then the change in absorption intensity was
[
I] [M] [H O]
0 0 2
0
measured over time at the working wavelength (λ =
3
w = k --------------------------------------- .
(1)
0
eff
25, 334, and 356 nm for the reaction with complex II,
1 + β[M]
0
III, and IV, respectively) using the Kinetics program of
the Cary WinUV package (v. 3.0). Primary rate curves
were processed using the OriginPro 7.0 program.
The relevant coefficients are indicated in the table.
For discussing the mechanisms of reactions of coor-
dination compounds in solution, one needs to know not
only the kinetics and structure of solid complexes but
also the state of these complexes in solution. As
opposed to a rich literature on aqueous species of palla-
dium(II) complexes, data on anhydrous media, such as
acetic acid, are scarce. However, for two participants of
reaction (I), namely, cobalt(II) and palladium(II) ace-
tates, such data exist.
RESULTS AND DISCUSSION
The solution changes color in reaction (I), and elec-
tronic spectra offer a convenient means for monitoring
this reaction. The concentrations of complexes I–IV in
–
3
the kinetic experiments were at a level of 10 mol/l,
and the absorption of the working solutions within
2
80–700 nm was initially very low. As the reaction pro-
After Hendriks et al. [14], for example, cobalt(II)
gressed, a strong band associated with the heterometal- acetate in anhydrous acetic acid mostly occurs as
lic complex appeared and increased in intensity over Co(OOCMe) (HOOCMe) octahedral complex mole-
2
4
time. New bands that appeared after the reaction was cules. When water is added, coordinated acetic acid
over practically coincided in position and intensity with molecules are consecutively replaced by OH ligands.
2
bands observed in the UV spectra of acetic acid solu- Unlike this, trinuclear palladium(II) acetate, in which
tions
of
heterometallic
complexes
Pd(µ- palladium(II) atoms are strongly bound by three pairs
KINETICS AND CATALYSIS Vol. 50 No. 3 2009

3
98
AKHMADULLINA et al.
w × 10 , mol l min–1
4
–1
w × 10 , mol l min–1
4
–1
0
0
6
(a)
(
a)
4
3
2
1
1
2
5
4
3
2
1
0
2
4
6
8
10
12
0
0.5
1.0
1.5
(b)
2.0
2.5
4
[
I] × 10 , mol/l
1
1
4
2
w /[H O]/[I], mol l min–1
–
1
0
2
1
1
1
1
1
0
0
0
0
.8
(b)
.6
.4
.2
.0
.8
.6
.4
.2
10
8
1
6
4
2
2
3
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Concentration, mol/l
0
1
2
3
4
5
6
7
8
3
Fig. 2. Reaction rate of palladium(II) acetate I with cobalt
acetate II (a) vs. (1) acetonitrile and (2) water concentration
at 80°C and (b) vs. tetrahydrofuran concentration at 60°C.
[
M] × 10 mol/l
–
4
–4
–
1
[I] = 3 × 10 mol/l, [II] = 9 × 10 mol/l.
[
H O][I]/w , mol min l
0 0
2
0
3
.0
(
c)
w /[MeCN]/[I], mol l min–1
–
1
0
2
2
1
1
0
.5
.0
.5
.0
.5
1
1
2
0
8
6
4
2
0
500
1000 1500 2000 2500 3000
/[II], l/mol
1
Fig. 1. Panel (a): the reaction rate of palladium(II) acetate I
with cobalt acetate II vs. concentration of complex I. Panel
0
1
2
3
4
5
6
7
8
3
[II] × 10 , mol/l
(
b): the same vs. concentration of 3d-metal acetate (1) II,
2) IV, and (3) III. Panel (c): the rate of reaction I + II vs.
(
Fig. 3. Reaction rate of palladium(II) acetate I with cobalt
acetate II vs. concentration of complex II in acetic acid con-
taining 0.21 mol/l MeCN and 0.03 mol/l H O at 80°ë.
[I] = 3 × 10 mol/l.
concentration of complex II in the coordinates of the
Michaelis–Menten equation. Acetic acid contains
2
–
4
0
.29 mol/l H O. T = 80°C.
2
0
KINETICS AND CATALYSIS Vol. 50 No. 3 2009

UNEXPECTED PARTICIPATION OF NUCLEOPHILES IN THE REACTION
399
of acetate bridges, retains its structure in most organic monodentate ligand, and an H O molecule occupies the
2
newly appeared coordination vacancy.
solvents; thus, it has a low reactivity and needs to be
activated to enter reaction (I). Bakhmutov et al. [15]
have recently discovered such a process by NMR; they
demonstrated that, in organic solvents (such as chloro-
form and benzene) containing even minor water
If such a hydrolytic dissociation occurs in the reac-
tion system under study, it will enhance the opening of
the three-membered ring and subsequent chemical
transformations of palladium(II) acetate. If so, our
determined rate laws (the first orders of the reaction in
amounts, at least one acetate bridge in a Pd (µ-
3
complex I and H O and the Michaelis dependence of
2
OOCMe) molecule is subject to hydrolytic dissocia-
6
the reaction rate on the 3d-metal concentration) are rea-
tion. One of the six bridging acetate groups becomes a sonably explained in terms of the kinetic scheme below.
Me
Me
C
Me
C
Me
O
Me
C
Me
C
C
O
Me
C O
Me
O
Pd + H O
Me
C
Me
O
O
O
C
O
C
C O O O O O OH2
k3
O
O
O O
O
O
k2 Me
K1
+ M(COCMe)2
O
M OH2
O
O OH2
Pd
O
Pd
Pd+ MeCOOH
Pd
Pd
Pd
Pd
2
O O
C
Me
Me
O
Pd
Pd
H O
O
O
O
C O
O O O O C
O
O
O
O
O
O
C
C
O
C
C
C
C
O
O
C
Me
Me
C O O
Me
Me
Me
Me
Me
Me
Me
Me
–
2MeCOOH k4
k6 +M(COCMe)2
Me
Me
C
Me
C
Me
C O
O
C
O
O
O
OH
C
O O O O O OH
k5
Me
H O
Me
O
+
MeCOOH
Pd
Pd
Pd
Pd
Pd
O
Pd
O
O
O O O O C
O
C
O
C
C
C O O O
C
Me
C
Me
Me
Me
Me
Me
Scheme 1.
For this scheme, the coefficients of Eq. (1) have the fol- the carboxylate bridge in molecule I. Our experiments
demonstrated that, where even minor amounts of aceto-
nitrile are present, the reaction rate is no longer the
Michaelis function of 3d-metal acetate concentration;
rather, it becomes linear (Fig. 3).
k k
k
2
3
3
lowing meaning: k = K --------- and β = -- -- .
eff
1
k
k
4
4
Presumably, other nucleophilic reagents can also
enhance the reaction of complex I with 3d-metal ace-
tates. Acetonitrile considerably increases the rate of
In this case, both the rate equation (first orders in
reaction (I) (Fig. 2a). Acetonitrile is a sufficiently both reagents and acetonitrile) and the reaction mecha-
strong nucleophile and, likely, also capable of breaking nism are simpler (Scheme 2).
Me
Me
C
Me
C
Me
C
Me
C
Me
O
Me
C O
Me
Me
C O
Me
O
C
O
O
C
C
O
O
O O
C O O O O O NCMe
O
O
O
O
NCMe
Me
H O
Me
O
+MeCN
O
O
+ M(COCMe)2
Pd
Pd
Pd
Pd
Pd
Pd
M NCMe
Pd
Pd
–
O
MeCOOH
Pd
Pd
O
O O
C
O
O
O
O
C O O O O O O C
O
O
O
O
C
O
C
C O
C
C
C
C O O
C
O
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Scheme 2.
The MeCN axial ligand at the 3d metal being resulting complex is dictated by the solvent compo-
readily exchangeable for H O in the presence of sition during the release and crystallization of the
2
even minor water amounts, the composition of the reaction product.
KINETICS AND CATALYSIS Vol. 50 No. 3 2009

4
00
AKHMADULLINA et al.
Unexpectedly, tetrahydrofuran, unlike acetonitrile,
virtually does not influence the rate of reaction (I)
6. Kozitsyna, N.Yu., Nefedov, S.E., Vargaftik, M.N., and
Moiseev, I.I., Mendeleev Commun., 2005, vol. 15,
p. 223.
(
Fig. 2b). This fact can be explained as follows: the
nucleophilic attack of a tetrahydrofuran molecule on
7
8
. Kozitsyna, N.Yu., Nefedov, S.E., Dolgushin, F.M.,
Cherkashina, N.V., Vargaftik, M.N., and Moiseev, I.I.,
Inorg. Chim. Acta, 2006, vol. 359, p. 2072.
complex I is inefficient because of the far larger molec-
ular size of this reagent compared to H O or MeCN and
2
the associated steric hindrances.
. Nefedov, S.E., Kozitsyna, N.Yu., Vargaftik, M.N., and
Moiseev, I.I., Polyhedron, 2009, vol. 28, p. 172.
ACKNOWLEDGMENTS
9. Tkachenko, O.P., Stakheev, A.Yu., Kustov, L.M., Mash-
kovsky, I.V., Berg, M., Grunert, W., Kozitsyna, N.Yu.,
Dobrokhotova, Zh.V., Zhilov, V.I., Nefedov, S.E.,
Vargaftik, M.N., and Moiseev, I.I., Catal. Lett., 2006,
vol. 112, p. 155.
This work was supported by the Russian Foundation
for Basic Research (project nos. 06-03-32578, 09-03-
0514), the Presidential Foundation through the Pro-
gram for Support of the Leading Scientific Schools of
Russia (grant no. NSh-1733.2008.3), and the Presidium
of the Russian Academy of Sciences through the Pro-
gram “Targeted Synthesis of Inorganic Compounds and
Functional Materials Design.”
0
1
0. Kozitsyna, N.Yu., Nefedov, S.E., Dobrokhotova, Zh.V.,
Ikorskii, V.N., Stolyarov, I.P., Vargaftik, M.N., and Moi-
seev, I.I., Ros. Nanotekhnol., 2008, vol. 3, nos. 3–4,
p. 100.
1
1
1
1. Stephenson, T.A., Morehouse, S.M., Powell, A.R., Hef-
fer, J.P., and Wilkinson, G., J. Chem. Soc., 1965, p. 3632.
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