Influence of the nature of the metal ion on the rate of reduction of coordinated azomethine group

Article abstract of DOI:10.1134/S1070363207100155

The kinetics of reduction of the azomethine bond in various Schiff bases and their transition metal complexes with sodium borohydride in dimethylformamide and ethanol solutions was studied. The reduction rate depends on both the structure of the starting

Full text of DOI:10.1134/S1070363207100155

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 10, pp. 1761 1765.
Pleiades Publishing, Ltd., 2007.
Original Russian Text A.A. Medzhidov, S.G. Mamedova, A. Aydin, B. Yalcin, S. Tascioglu, E. Kaki, 2007, published in Zhurnal Obshchei Khimii, 2007,
Vol. 77, No. 10, pp. 1695 1699.
Influence of the Nature of the Metal Ion on the Rate
of Reduction of Coordinated Azomethine Group
A. A. Medzhidova, S. G. Mamedovaa, A. Aydinb,
B. Yalcinb, S. Tascioglub, and E. Kakib
a Institute of Chemical Problems, National Academy of Sciences of Azerbaijan,
Huseyn Javid. av. 29, Baku, AZ1143 Azerbaijan
e-mail: xim205@rambler.ru
b Marmara University, Istanbul, Turkey
Received February 7, 2007
Abstract The kinetics of reduction of the azomethine bond in various Schiff bases and their transition
metal complexes with sodium borohydride in dimethylformamide and ethanol solutions was studied. The
reduction rate depends on both the structure of the starting Schiff bases and the nature of the metal ion. In
transition metal N-phenylsalicylaldiminates, the rate of reduction of the azomethine group increases in the
order Zn(II) < Ni(II) < Cu(II) < Co(II) < VO(II) < Mn(II). Similar trend is observed in other series of metal
complexes with Schiff bases. The revealed trends are opposite to the Irving Williams series of stability of
complexes. This fact suggests that the major factor affecting the rate of reduction of the coordinated azo-
methine bond is the strength of its bonding with the metal ion. Depending on particular metal ion, the com-
plexation can either decelerate or accelerate the reduction.
DOI: 10.1134/S1070363207100155
It is well known that metal complexes catalyze
CH2 CH2
reduction of various organic compounds with sodium
N
N
borohydride. In particular, cobalt(II) -ketoiminate
complexes are used as catalysts for enantioselective
borohydride reduction of ketones [1 4], N-phosphin-
ylimines [4, 5], and , -disubstituted , -unsaturated
carboxamides [4, 6] to the corresponding alcohols,
amines, and saturated carboxamides.
M
CH=N R1
O M/2
CH
CH
O
O
R
R3
R3
R2
R1 = Ph (M[L1]2; R1 = CH2Ph (M[L2]2); R2 = H, R3 = H
The asymmetric reduction of a series of ketones
with borohydride using asymmetric chiral Co(II)
bis(salicylidene)ethylenediamine (salen) complexes
was studied in [7, 8]. It should be noted that all these
complexes used as catalysts contain aldimine groups,
which can be reduced with borohydride to secondary
amines. It is known that the azomethine bond can be
reduced even at 0 C [9]. However, none of these
papers consider the possibility of reduction of the
azomethine group in the metal complex catalysts in
the course of hydrogenation.
(M[L3]); R3
=
OCH3, R2 = H (M[L4]); R2 = R3
=
C(CH3)3 (M[L5]); M = VO(II), Mn(II), Co(II), Ni(II),
Cu(II), Zn(II).
The solvents most commonly used for reduction of
various compounds with borohydride are alcohols and
water. However, in this case concurrent decomposi-
tion of borohydride in the reaction with the solvent
should be taken into account in kinetic studies.
We found that, in anhydrous dimethylformamide,
borohydride does not noticeably decompose during at
least 1-day storage, which consideably simplifies the
kinetic studies, because the borohydride consumption
in the reaction with the solvent can be neglected.
In this study we compared the rates of reduction of
the azomethine bond in complexes of various metals
with Schiff bases derived from salicylaldehyde and
mono- and diamines. We examined the kinetic fea-
tures of the reduction of the starting ligands and metal
complexes and suggested the most probable reduction
mechanism.
The reduction of Schiff bases and their metal com-
plexes in dimethylformamide occurs in the presence
of water in accordance with the general scheme
1761

1762
MEDZHIDOV et al.
NaBH4
+ NaBH3OH
CH NH R
O M/2
CH=N R
O M/2
OH2
The reduction kinetics was monitored by variation
of the optical density in the maximum of the
absorption band of the azomethine group. Figure 1
illustrates variation of the absorption spectrum of the
complex VO[L3] with time.
The reaction rates were measured at large excesses
of sodium borohydride and water relative to the sub-
strate (ligand or complex). Under these conditions, the
experimental plots of lnC of substrates vs. time were
linear (Fig. 2), suggesting the first reaction order with
respect to the substrate concentration.
*
The pseudo-first-order rate constants k evaluated
from the lnC t dependences were used for determin-
ing the reaction order with respect to water and sodi-
um borohydride concentrations. It was found that the
reaction order with respect to these components is also
unity (Figs. 3, 4). Thus, according to the experimental
data, the reduction rate can be described by the fol-
lowing equation:
D
0.8
0.7
0.6
W = kapp[S][H2O][NaBH4],
(1)
where kapp is the reaction rate constant; [S], [H2O],
and [NaBH4] are the concentrations of the substrate,
water, and sodium borohydride, respectively. The
pseudo-first-order rate constant is defined as follows:
0.5
0.4
k = kapp[H2O][NaBH4].
0.3
0.2
The expression for the reduction rate corresponds
to third-order reactions, but in our case such order is
improbable. Among possible reaction mechanisms,
the following mechanism leads to the rate equation
similar to (1):
0.1
0.0
350
400
500 , nm
K
[S NaBH4],
S + NaBH4
k
Fig. 1. Variation with time of the absorption intensity in
SH2 + NaBH3OH,
[S NaBH4] + H2O
3
the electronic spectra of VO[L ]. T 35 C, [NaBH ]
4
W = Kk[S][H2O][NaBH4],
(2)
0.002 M, [H O] 0.073 M. The spectra were recorded at
2
2-min intervals. The arrow denotes the direction of
spectrum evolution. (D) Optical density.
where K is the equilibrium constant of substrate
borohydride complexation, and k is the rate constant
of the limiting step. Equation (1) corresponds to
Eq. (2) at kapp = Kk. The rates of reduction of the
azomethine group in noncoordinated Schiff bases
depend on their structure. Whereas N-phenylsalicyl-
aldimine is reduced at room temperature at a fairly
high rate, the other examined Schiff bases are not
reduced under these conditions (see table).
0
2
4
6
8
t, min
6.8
7.0
7.2
ln C
The reduction rates of the coordinated azomethine
group strongly depend on the nature of the metal ion.
In all the examined series of complexes with Schiff
bases, except N-phenylsalicylaldiminate complexes,
the VO2+ and Mn2+ complexes are reduced most
Fig. 2. Logarithm of the substrate concentration as a
3
function of time for the reduction of VO[L ]. Initial
3
4
concentrations, M: VO[L ] 1 10 , [NaBH ] 0.002,
4
and [H O] 0.073.
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 10 2007

INFLUENCE OF THE NATURE OF THE METAL ION
1763
Kinetic parameters of hydrogenationa of complexesb with sodium borohydride in DMF solution
3
1
1
1
Compound
[NaBH4] 10 , M
[H2O], M
k , s
kapp, l2 mol 2 s
Eapp, kJmol
L1H
0.968
0.968
0.23
0.968
0.968
0.23
0.968
2.02
2.02
2.02
1.7
0.47
0.98
0.968
0.2
4.0
0.968
0.089
0.089
0.21
0.0015
0.0014
0.0005
0.001
0.001
0.001
0.0017
0.0005
0.0002
0.0003
0.00057
0.00053
0.0003
0.005
17.41
16.25
103.5
11.61
11.61
207.03
19.73
2.8
1.12
1.68
4.5
66.33
4.2
33.24
24.92
66.48
24.94
29.0
VO[L1]2
Mn[L1]2
Co[L1]2
Ni[L1]2
Cu[L1]2
Zn[L1]2
VO[L2]2
Co[L2]2
Cu[L2]2
VO[L3]
Mn[L3]
Co[L3]
VO[L4]
Mn[L4]
VO[L5]
Mn[L5]
0.089
0.089
0.021
0.089
0.089
0.089
0.0089
0.073
0.017
0.073
0.089
0.021
0.089
0.073
69.25
38.78
0.831
50.28
27.7
59
309
0.0013
0.00026
0.000145
0.738
2.05
a
2
2
2
2
3
3
3
3
4
4
4
4
4
5
5
5
For L H, Mn[L ] , Ni[L ] , Zn[L ] , L H , Ni[L ], Cu[L ], Zn[L ], L H , Co[L ], Ni[L ], Cu[L ], Zn[L ], L H , Co[L ], Ni[L ],
2
2
2
2
2
2
5
5
b
Cu[L ], and Zn[L ], the absorption maximum of the azomethine bond does not change with time. Geometries of the coordination
1
1
1
1
cores: Co[L ] , distorted tetrahedron [10]; Ni[L ] , planar square [11]; Cu[L ] , planar square tetrahedron [10]; Zn[L ] , tetra-
hedron [10]; Co[L ], planar square [12]; Ni[L ], planar square, dimer [13]; Cu[L ], planar square, dimer [13]; Zn[L ], planar square
2
2
2
2
3
3
3
3
[13].
readily, whereas the Cu(II) and Co(II) complexes are
reduced appreciably more slowly. No noticeable trans-
formations in the examined temperature range are ob-
served with the Ni(II), Pd(II), and Zn(II) complexes.
The Cu(II) complex is reduced by a mechanism that
differs from the mechanism of reduction of the other
complexes. First the Cu(II) ion is reduced to Cu(I),
which is accompanied by decomposition of the com-
plex. A band of the noncoordinated azomethine bond
at 340 nm appears in the electronic absorption spec-
trum, and this is followed by the reduction of this
bond with sodium borohydride.
From the temperature dependence of k (Fig. 5), we
determined the apparent activation energies (see table).
Reduction of N-phenylsalicylaldiminate com-
plexes M[L1]2. Noncoordinated N-phenylsalicylaldi-
1
mine is rapidly (kapp 17.41 l2 mol 1 s ) reduced at
Reduction of N-benzylsalicylaldiminate com-
plexes M[L2]2. N-Benzylsalicylaldimine is not re-
duced with sodium borohydride in DMF solution in
the presence of water at 20 60 C. Complexation with
room temperature.
With all the examined metal complexes of this
ligand, the reduction rate is fairly high (see table).
k
103
k
103
1.0
1.0
0.5
0.0
0.5
0.0
0.019 0.027 0.055 0.073 0.110
[H2O] 102, M
0
4
8
12 16 20
[NaBH4] 102, M
Fig. 3. Pseudo-first-order rate constant k of the reduction
Fig. 4. Pseudo-first-order rate constant k of the reduction
3
3
of VO[L ] at 22 C as a function of [NaBH ]. [H O]
of VO[L ] at 22 C as a function of [H O]. [NaBH ]
4
2
2
4
0.073 M.
0.002 M.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 10 2007

1764
MEDZHIDOV et al.
of the Schiff base. In the case of complexes with
1
N-phenylsalicylaldimine, the hydrogenation rate de-
creases in the following order:
2.9 3.0 3.1 3.2 3.3 103/T, K
6
7
Mn(II) > VO(II) > Co(II) > Cu(II) > Ni(II) > Zn(II).
As for complexes of transition metals with the
other Schiff bases, the Mn(II) and VO(II) complexes
were also reduced most rapidly, the Co(II) and Cu(II)
complexes were reduced more slowly, and the Ni(II)
and Zn(II) complexes were not reactive at all in the
examined temperature range.
8
ln k
Fig. 5. Plot of lnk vs. reciprocal temperature for the
reduction of VO[L ].
3
The noncoordinated azomethine bond in different
free Schiff bases behaves differently. Whereas non-
coordinated N-phenylsalicylaldimine is rapidly re-
duced at room temperature, among salen derivatives
only the methoxy compound (L4H2) is reduced in
DMF at room temperature, with the reaction being
very slow.
VO(II), Mn(II), Co(II), and Cu(II) ions enhances the
reactivity of the ligand, so that the reduction rate
becomes fairly high, especially for the Mn(II) and
VO(II) complexes (see table). At the same time, the
azomethine group in the complexes with Ni(II), Zn(II),
and Pd(II) is not reduced under these conditions, even
at 60 C.
It should also be noted that N-phenylbenzalimine
containing no hydroxy grop is not reduced under the
conditions we used. The enhancing effect of the hy-
droxy group on the reactivity of the azomethine group
may be due to withdrawal of electron density from the
azomethine group, making this group more electro-
philic and hence more reactive toward borohydride, or
to direct participation of the hydroxy group in the
reduction.
Reduction of salen complexes M[L3], M[L4], and
M[L5]. Noncoordinated salen is not reduced under the
conditions of our experiments (DMF + H2O). Salen
coordinated with the VO(II), Mn(II), and Co(II) ions
is active in reduction; the Mn(II) complex is reduced
at the highest rate, whereas the Co(II)[salen] complex
is reduced at a noticeable rate only above 50 C. The
complexes Ni[salen] and Zn[salen], like N-benzylsali-
cylaldiminate complexes of these metals, are inactive
in the reduction.
However, the azomethine bond in N-benzylsalicyl-
aldimine, which is not hydrogenated in the free ligand
under common conditions, is reduced upon coordina-
tion with the Mn(II) and VO(II) ions, which suggests
that these ions exert a catalytic effect.
For 3,5-di-tert-butyl-substituted salen, only the
VO(II) and Mn(II) complexes undergo reduction,
whereas the complexes of the other metals are not
reduced. The methoxy-salen complexes behave simi-
larly. In this case also the Mn(II) complex is the most
active. Somewhat lower reduction rate is observed
with the VO(II) complexes, and the Cu(II) complexes
are reduced very slowly at 60 C. No reduction is
observed under these conditions with the Co(II) and
Ni(II) complexes.
Presumably, the major factor affecting the reactiv-
ity of the azomethine group is the metal nitrogen
bond energy. Indeed, the series of activity of the metal
complexes in the reduction of the azomethine bond is
opposite to the Irving Williams series found for the
stability constants of various metal complexes. Of
course, the systems being compared differ essentially,
because the Irving Williams series described the
strength of metal binding with mono- and bidentate
ligands (1,2-diaminoethane, oxalic acid, glycine, etc.),
whereas our study concerns the reduction of the coor-
dinated azomethine bond. It should also be noted that
the geometry of the chelate core has no noticeable
effect on the capability of the azomethine group for
reduction (see table).
Reduction in ethanol. The rate of reduction of the
azomethine bond in ethanol solutions is considerably
higher than that in dimethylformamide solution. For
example, the rate constant kapp of reduction of Mn[L3]
1
in dimethylformamide is 66.33 l2 mol 2 s , whereas
1
in alcoholic solution it is as high as 106.3 l2 mol 2 s .
The rate of reduction of the vanadyl salen complex in
alcohol is also higher than in DMF. The salen com-
plexes of the other metal ions [Co(II), Ni(II), Cu(II),
Zn(II)] are not reduced at room temperature.
EXPERIMENTAL
Thus, in accordance with the studies performed, the
rate of reduction of the coordinated azomethine group
depends on the nature of the metal ion and structure
Complexes of metals with Schiff bases were pre-
pared by published procedures [10 13].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 10 2007

INFLUENCE OF THE NATURE OF THE METAL ION
1765
Dimethylformamide was purified by fractional
distillation with a benzene water mixture, followed
by distillation at reduced pressure [14]. Kinetic
measurements were performed with Specord M40
(Carl Zeiss, Jena) and UV VIS 240 (Shimadzu) spec-
trophotometers. The reduction progress was monitored
by variation of the optical density in the absorption
maxima of the complexes ( 360 420 nm depending
on particular complex), measured in temperature-
controlled quartz cells in dimethylformamide solution.
The temperature was maintained with an accuracy of
0.5 C.
2. Sugi, K.D., Nagata, T., Yamada, T., and Mukaiva-
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4
nos. 1 2, p. 83.
follows (M): substrates (ligands or complexes) 10 ,
2
3
1
2
8. Sun, W., Chun-Gu Xia, and Pei-Qing Zhao, J. Mol.
Catal., 2002, vol. 184, nos. 1 2, p. 51.
sodium borohydride 10 10 , and water 10 10 .
To determine the initial concentrations of sodium
borohydride, we developed a procedure involving ti-
tration of a solution of NaBH4 in dimethylformamide
with an alcoholic solution of N-phenylsalicylaldimine.
The end of titration corresponded to the moment when
the yellow color of the Schiff base no longer disap-
peared on stirring.
9. Koh, L.L., Ranford, T.O., Robinson, W.T., Svens-
son, J.O., Choo Tan. A.L., and Daqing Wu, Inorg.
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ACKNOWLEDGMENTS
12. Bigotto, A., Costa, G., Mestrony, G., Pellizer, G.,
Puxeddu, A., Relsen-Hofer, E., Stefani, L., and
Tauzher, G., Inorg. Chim. Acta Rev., 1970, vol. 4,
p. 41.
The authors are grateful to the World Federation of
Scientists for the financial support.
13. Shkol’nikova, L.M., Zh. Strukt. Khim., 1970, vol. 11,
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 10 2007