Synthesis, characterization, redox behavior and hydrogenation catalytic activity of bis(N-aryl-3,5-Bu2t-salicylaldiminato) palladium(II) complexes

Article abstract of DOI:10.1016/j.poly.2004.11.020

The synthesis, spectroscopic (¹H NMR, IR, UV-Vis), electron-transfer properties and catalytic reactivity of new palladium(II) complexes with N-aryl-3,5-Bu2t-salicylaldimines prepared from 3,5-Bu2t-salicylaldehyde and o-,p-substituted anilines (X-C₆H ₄NH₂, X = H, F, Cl, Br, CH₃, OCH₃, t-Bu, 5,6-benzo) are reported. Cyclic voltammetry studies of the complexes exhibit an irreversible anodic peak that corresponds to the phenoxide/phenoxyl oxidation. The chemical oxidation of the complexes with (NH₄) ₂Ce(NO₃)₆ in CHCl₃, besides relatively stable Pd^II-phenoxyl radical complexes (g = 2.0083-2.0114), also generate nitroxide radicals exhibiting strongly anisotropic spectra (g_∥ = 2.0061, g_⊥ = 2.0072, A _∥ = 37.5, A_⊥ = 5.38 G) typical for immobilized nitroxide radicals. It has been found that the introduction of t-Bu groups on the salicylic ring increases catalytic activity of towards hydrogenation of nitrobenzene in DMF at room temperature.

Full text of DOI:10.1016/j.poly.2004.11.020

Polyhedron 24 (2005) 319–325
www.elsevier.com/locate/poly
Synthesis, characterization, redox behavior and
hydrogenation catalytic activity of
bis(N-aryl-3; 5-But2-salicylaldiminato)palladium(II) complexes
a,
a
b
c
Veli T. Kasumov *, Esref Tas , F. Koksal , Seniz Ozalp-Yaman
¨
ß
ß
¨
ß
a
Chemistry Department, Faculty of Arts and Sciences, Harran University, Sßanliurfa 63300, Turkey
Physics Department, Ondokuz Mays University, Samsun, Turkey
b
c
Chemistry group of Faculty of Engineering, Atilim University, Ankara, Turkey
Received 1 October 2004; accepted 23 November 2004
Abstract
The synthesis, spectroscopic (1H NMR, IR, UV–Vis), electron-transfer properties and catalytic reactivity of new palladium(II)
complexes with N-aryl-3; 5-But2-salicylaldimines prepared from 3; 5-Bu2t -salicylaldehyde and o-,p-substituted anilines (X–C6H4NH2,
X = H, F, Cl, Br, CH3, OCH3, t-Bu, 5,6-benzo) are reported. Cyclic voltammetry studies of the complexes exhibit an irreversible
anodic peak that corresponds to the phenoxide/phenoxyl oxidation. The chemical oxidation of the complexes with (NH4)2Ce(NO3)6
in CHCl3, besides relatively stable PdII–phenoxyl radical complexes (g = 2.0083–2.0114), also generate nitroxide radicals exhibiting
strongly anisotropic spectra (gi = 2.0061, g^ = 2.0072, Ai = 37.5, A^ = 5.38 G) typical for immobilized nitroxide radicals. It has been
found that the introduction of t-Bu groups on the salicylic ring increases catalytic activity of towards hydrogenation of nitrobenzene
in DMF at room temperature.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Keywords: N-Aryl-3; 5-But2-salicylaldiminepalladium(II) complexes; Spectroscopy; Electrochemistry; Pd(II)–phenoxyl radicals; Nitroxide radicals;
Catalytic hydrogenation
1. Introduction
easily generate M(II)-phenoxyl radical complexes [5,6],
the proceeding unexpected oxidative C–C coupling in
CuII complexes [5b] and reduction via radical intermedi-
ates of CuII and PdII bis-salicylaldiminates upon treat-
ment with PPh3 were also observed [6].
In our previous work, we were interested in how the
bulky But groups would affect the electron-transfer reac-
tivity of this family of complexes if they were introduced
on the salicylaldehyde ring [6d,7a,8a]. It should be noted
that while the chemistry of metal complexes with salen-
type and other polydentate ligands bearing 2; 4-
But2-phenyl moieties has been extensively studied
because of their efficient catalytic activity and their use
as model complexes for metalloproteins having metal
centers and proximal organic radicals in their active site
The design, synthesis and structural characterization
of salicylaldimine complexes is a subject of current inter-
est due to their interesting structural, magnetic, spectral,
catalytic and redox properties, use as models for en-
zymes and various theoretical interest [1–4]. Our interest
in these compounds was associated with the structure,
electron-transfer and catalytic reactivity of the N-aryl-
salicylaldimine transition metal(II) chelates bearing re-
dox-active But2 functionalized aminophenolic and
aniline frames [5,6,7a,8a,8d]. While upon oxidation they
*
Corresponding author. Tel.: +4143128456; fax: +4143121998.
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
0277-5387/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2004.11.020

320
V.T. Kasumov et al. / Polyhedron 24 (2005) 319–325
[4,7], there has been comparatively little work reported
on the metal complexes with bidentate N-aryl-3; 5-But2-
salicylaldimines derived from 3; 5-But2-salicylaldehyde
(3,5-DTBS) [4c,6d,8].
The voltage scan rate during the CV measurements
was 100 mV/s (see Table 4).
2.2. Preparation of the ligands
In the present work, the synthesis, spectroscopy,
chemical and electrochemical oxidation as well as reac-
tivity in the hydrogenation of PhNO2, of palladium(II)
complexes with N-aryl-3; 5-But2-salicylaldimines (LxH)
derived from 3,5-DTBS and substituted anilines (X–
C6H4NH2, where X = H, o,p-F, Cl, Br, CH3, OCH3,
p-But and 5,6-benzo) are reported.
The ligands were N-phenyl-3; 5-But2-salicylaldimine
(L1H), N-2-F-phenyl-3; 5-But2-salicylaldimine (L2H), N-
4-F-phenyl-3; 5-But2-salicylaldimine (L3H), N-2-Cl-
phenyl-3; 5-But2-salicylaldimine (L4H), N-4-Cl-phenyl-3;
5-But2-salicylaldimine (L5H), N-2-Br-phenyl-3; 5-But2-
salicylaldimine (L6H), N-4-Br-phenyl-3; 5-But2-salicyla-]
ldimine (L7H) N-2-CH3-phenyl-3; 5-But -salicylaldimine
2
(L8H), N-4-CH3-phenyl-3; 5-But -salicylaldimine (L9H),
2
2. Experimental
N-2-OCH3-phenyl-3; 5-But2-salicylaldimine (L10H), N-4-
OCH3-phenyl-3; 5-Bu2t -salicylaldimine (L11H), N-
4-But-phenyl-3; 5-But2-salicylaldimine (L12H) and N-5;
6-benzo-3; 5-But2-salicylaldimine (L13H). Their com-
plexes were abbreviated as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
12 and 13, respectively. The ligands were synthesized
by refluxing in the methanol the aniline derivatives with
3,5-DTBS in a 1:1 molar ratio as recently described [8d].
2.1. Materials and equipment
All solvents, aniline derivatives, 2,4-di-t-butylphenol,
hexamethylenetetramine, nitrobenzene, acetic acid,
(NH4)2Ce(NO3)6 and Pd(ac)2 were of reagent grade (Al-
drich) and were used without further purification.
3; 5-But2-salicylaldehyde was prepared from commer-
cially available 2,4-di-t-butyl-phenol according to the lit-
erature [9]. Elemental analyses (C, H, N) and
2.3. Preparation of complexes
1
spectroscopic (IR, UV–Vis, H NMR, ESR) character-
izations were made according to literature [8d,10]. Cyc-
lic voltammetry (CV) measurements were made using
Volta Lab PGZ 301 Dynamic Voltammetry under argon
atmosphere. In this system a SCE (saturated calomel
electrode) was used as a reference electrode (the ferro-
cene/ferrocenium couple oxidation was found to be
0.84 V versus SCE in DMF in our system). Platinum
bead and platinum coil electrodes were employed as
the working and the auxiliary electrodes, respectively.
CV measurements were recorded in DMF and acetoni-
trile (MeCN) at 300 K, and [n-(C4H9)4]BF4 was used
as a supporting electrolyte. The concentration of the
complexes was about 0.001 M for each measurement.
Palladium(II) acetate (0.125 g, 0.5 mmol) was added
as a solid to a stirring warm solution of LxH (1 mmol)
in absolute methanol (40 ml). The resulting mixture
was heated with stirring at ca. 40–45 ꢁC on a water bath
for about 30–40 min. Then the volume of the solution
was reduced to 10–15 ml by slow evaporation at room
temperature. The precipitated solid was collected by fil-
tration and washed with water and then with 2–3 ml
cooled MeOH, dried in air and recrystallized from ace-
tonitrile–CHCl3 mixture (5:1). Some physico-chemical
characteristics are given in Table 1. For comparative
purposes in the catalytic activity studies, bis(N-4-
CH3Ph-salicylaldiminato)Pd(II) was also prepared.
Table 1
Physico-chemical and analytical data for complexes 1–13
Compound
M.p. (ꢁC)
Yield (%)
Formula
Found (calcd.) (%)
C
H
N
1
2
>270
>260
>260
184
47
36
37
38
45
39
42
51
38
46
35
43
49
C
C
42H52N2O2Pd
42H50N2O2F2Pd
70.56 (69.84)
67.23 (66.53)
66.23 (66.53)
64.13 (63.75)
63.18 (63.75)
56.78 (57.18)
57.68 (57.18)
70.75 (70.34)
69.78 (70.34)
67.68 (67.46)
68.15 (67.46)
72.23 (71.88)
73.31 (72.93)
7.69 (7.25)
6.46 (6.64)
6.34 (6.64)
6.87 (6.37)
6.78 (6.37)
5.45 (5.71)
5.64 (5.71)
5.18 (5.51)
6.18 (5.51)
6.89 (7.21)
7.36 (7.21)
7.86 (8.20)
7.12 (6.85)
3.43 (3.87)
3.78 (3.69)
3.94 (3.69)
3.09 (3.54)
3.23 (3.54)
2.98 (3.17)
2.79 (3.17)
3.42 (3.73)
4.12 (3.73)
3.11 (3.57)
3.32 (3.57)
3.68 (3.35)
3.56 (3.40)
3
C42H50N2O2F2Pd
42H50N2O2Cl2Pd
C42H50N2O2Cl2Pd
42H50N2O2Br2Pd
C42H50N2O2Br2Pd
44H56N2O2Pd
4
C
5
180
207
6
C
7
218
194
8
C
9
159
C44H56N2O2Pd
C44H56N2O4Pd
C44H56N2O4Pd
C50H68N2O4Pd
C52H56N2O4Pd
10
11
12
13
a
150a
205
265
>250a
Decomposition.

V.T. Kasumov et al. / Polyhedron 24 (2005) 319–325
321
2.4. Hydrogenation procedure
ing conditions in MeOH, EtOH or CH3COOH, a pro-
gressive decomposition of the formed complexes to
Pd(0) takes place. The analytical data show that the
complexes have a ligand-to-metal ratio of 2:1. All com-
pounds are soluble in polar solvents such as CHCl3,
CH2Cl2, DMF and DMSO.
The hydrogenation of nitrobenzene was carried out in
a thermostatic reaction flask (100 ml) at 25 ꢁC under 760
Torr H2 with vigorous stirring in dry and deoxygenated
25 ml DMF solution. Catalyst (1.0–4.0 · 10ꢀ5 mol) was
added into 25 ml DMF and saturated with H2 for 15–20
min. After addition of NaBH4 (5 · 10ꢀ5 mol) the mix-
ture was stirred for ca. 5 min and PhNO2 (4–8 · 10ꢀ4
mol) was transferred into the vessel. Then the H2 gas
was bubbled again into the flask and the volume of
the absorbed H2 was measured periodically.
3.1. Spectroscopic characterization
The analytical, IR, electronic, 1H and 13C NMR spec-
troscopic characteristics of the ligands were described in a
previous report [8d]. The disappearance of free ligand
absorptions around 2500–2800 cmꢀ1 and lower fre-
quency shifts of m(CH@N) in the IR spectra of complexes
1–12, suggest chelating of LxH via a deprotonated –OH
group and azomethine nitrogen atom (Table 2). A strong
band at about 1528 cmꢀ1, detected in the spectra of com-
plexes 1–12, is assigned to m(C–O) of the coordinated sal-
icylic C–O bond [11]. Surprisingly, the IR and electronic
spectra of (13) are remarkably different from the other
complexes (Table 2).
3. Results and discussion
Our preliminary examination revealed that the pre-
sented LxH ligands easily coordinate Cu(II) [8d], unlike
Ni(II), VO(II), Mn(II), Zn(II) and Cd(II). All of com-
plexes 1–13 can be prepared under mild conditions, only
with lower yields. The reason of this might be the stron-
ger interligand steric hindrance caused by the 3-But
group of the ligands, since the same complication did
not occur during the complexion of salicylaldimines de-
rived from non di-butylated salicylaldehydes and di-But
anilines. When palladium(II) was prepared under reflux-
The electronic spectra of compounds 1–13 (Table 2)
in CHCl3 and DMF (Table 2) consist of very intense
bands due to intraligand n ! p* and p ! p* transitions,
and absorptions at 400–420 and 475–500 nm are as-
signed to metal-to-ligand charge-transfer (MLCT) and
Table 2
IR and electronic absorption spectral data for complexes 1–13
Compound
IR spectra (cmꢀ1
)
Solvent
Electronic spectra k (nm) (log e)
m (C@N)
m (C–O)
1
2
1612
1527
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
EtOH
CHCl3
DMF
CHCl3
260, 305, 408
269, 302, 354a, 399
1615
1613
1616
1615
1602
1605
1611
1615
1612
1526
1527
1528
1527
1527
1526
1529
1528
1527
266, 305, 357, 420a, 450a
269, 302, 355, 409a
3
257, 305, 407, 480a
272, 303, 402, 500a
268, 290a, 350,a 420,a 500a
242, 298, 349,a 410a
271, 309, 356, 420a
271, 302, 354,a 416a
263, 303, 490, 500a
272, 299, 423
4
5
6
7
261, 307, 415
270, 303, 411
263, 299, 415, 480a
8
270, 298, 412
9
262, 306, 350,a 400,a 480a
274, 308, 354,a 399, 424
260,a 299,a 335,a 431a
269, 293,a 358,a 400a
210, 240,a 260, 300, 360, 420
263, 302, 405, 480a
10
11
12
13
a
1614
1613
1623
1528
1528
1518
269, 301, 401, 432, 475a
270, 306, 350,a 400
272, 303, 404
270,a 300,a 320,a 492, 590,a 1008
252(4.6), 300,a380(4.2), 477(4.1), 600(3.05), 1008(2.91)
CHCl3
EtOH
Shoulder.

322
V.T. Kasumov et al. / Polyhedron 24 (2005) 319–325
1A1g ! B1g transitions [12], respectively. Surprisingly,
the electronic spectra of complex 13 in EtOH and CHCl3
are quite different from those of the other complexes 1–
12 (Table 2). The absorbance at 600 nm in the spectra of
13 probably originates from a Pd(dp) ! p* transition.
The band at 1008 nm, which is not characteristic for
bis(salicylaldimine) PdII helates, is similar to the near-
IR region bands (800–1100 nm) recently reported for
(naphthylazo)imidazole-(catecholate) PdII [13a] and
some CuII complexes [13b] and assigned to the inter-li-
gand charge transfer process (LLCT) [13c].
1
(a)
The 1H NMR spectral results obtained for some LxH
ligands and their complexes in CDCl3, with their assign-
ments, are given in Table 3. The proton resonance,
(b)
appearing as
a
broad low intensity singlet at
d = 11.45–14.86 ppm in the spectra LxH due to the
OH/NH protons involved in intramolecular H-bonding,
1
is absent in the H NMR spectra of their complexes. In
contrast to expectation, all protons of azomethine and
salicylic moieties of these complexes were shifted upfield
from the free ligand peaks, while the aniline ring protons
exhibit either downfield shift or remain unchanged (Ta-
ble 3). Similar magnetic shielding effects have been pre-
viously observed in Schiff base complexes of Pd(II),
Zn(II), Co(II) and Cu(II) [14].
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
potential (V)
Fig. 1. (a) Cyclic voltammogram of 1.05 · 10ꢀ3 M (3) in DMF at
room temperature vs. SCE. (b) Cyclic voltammogram of 1.24 · 10ꢀ3 M
(9) in DMF at room temperature vs. SCE.
3.2. Electrochemistry of the complexes
The CV of complexes 1–12 are similar to each other
and are composed of an irreversible oxidation peak
(Fig. 1) which is assigned to the Pd(II)-phenoxide/
Pd(II)-phenoxyl couple within potentials ranging from
1.07 to 1.16 V, except for the complexes 6 and 12. Upon
reversal of scan direction, no cathodic peak was ob-
served for all Pd complexes in our working range from
+1.3 to ꢀ1.0 V. The CV of 1, 9 and 12 were studied in
MeCN versus SCE, as well. The electrochemical behav-
ior of 1 in MeCN was different from the complexes 9
and 12. As shown in Fig. 2, a quasi-reversible oxidation
peak was observed at around +1.29 V versus SCE for (1)
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
Potential (V)
Fig. 2. Cyclic voltammogram of 9.85 · 10ꢀ4 M (1) in acetonitrile at
room temperature vs. SCE.
Table 3
1H NMR spectral data L1H, L7H, L12H and their PdII-complexes (d ppm)
Compound
OH
CH@N
Sal.H
PhH
C(CH3)3
L1H
(1)
13.65
8.63 (s, 1H)
7.89 (d, J = 9.6 Hz, 2H)
7.21 (d, J = 2.2 Hz, 1H)
7.45 (d, J = 2.7 Hz, 1H)
6.93 (d, J = 2.7 Hz, 2H)
7.24 (d, J = 2.6 Hz, 2H)
7.22 (d, J = 2.8 Hz, 1H)
7.47 (d, J = 2.3 Hz, 1H)
6.92 (d, J = 2.3 Hz, 2H)
7.24 (d, J = 2.5 Hz, 2H)
7.22 (d, J = 2.7 Hz, 1H)
7.44 (d, J = 2.8 Hz, 1H)
6.94 (d, J = 2.4 Hz, 2H)
7.22 (d, J = 2.4 Hz, 2H)
7.22–7.46 m
7.47–7.56 m
1.33, 1.48
0.82, 1.22
L7H
(7)
13.49
13.86
8.60 (s, 1H)
7.86 (d, J = 8.7 Hz, 2H)
7.52 m, 7.15 m
7.52–7.55 m
1.33, 1.48
0.81, 1.21
L12
(12)
8.64 (s, 1H)
8.0 (d, J = 7.7 Hz, 2H)
7.21–7.39 m
7.22–7.47 m
1.33, 1.49
1.22, 1.34
1.49

V.T. Kasumov et al. / Polyhedron 24 (2005) 319–325
323
in MeCN. The CV of the other Pd complexes was not
studied in MeCN due to their lower solubility in MeCN
compared to that in DMF. The Epa values of (1)–(12),
unlike Epa of LxH [8d], did not correlate with the elec-
tron-donating/withdrawing power of the aniline
substituents.
The positive slope obtained in the plot of the peak
current (ip) versus the square root of the voltage scan
rate (V1/2) within the range of 100–1000 mV/s indicates
diffusion controlled electron exchange reactions at the
first oxidation peak potentials of all the Pd complexes
[15]. A negative slope was obtained when the current
function [ip/(CV1/2)] for the first oxidation peaks was
plotted against LogV, indicating a reversible electron ex-
change followed by a chemical reaction [15]. Appear-
ance of irreversible peaks in the cyclic voltammogram
can be explained by a higher rate of the chemical reac-
tion following the electrochemical one.
3.3. Chemical oxidation of the complexes
Fig. 3. ESR spectra of the oxidized compounds with (NH4)2Ce(NO3)6
in CHCl3: (a) complex 7 at 300 K; (b) L9H ligand at 130 K; (c) complex
9 at 300 K.
Recently we have found that in the chemical oxidation
of some bis(N-aryl-3,5-di-But-salicylaldiminato) cop-
per(II) complexes with the one-electron oxidant
(NH4)2Ce(NO3)6 in acetonitrile, along with decreasing
of the Cu(II) signal intensity, the generation of directly
coordinated phenoxyl radical complexes are also pro-
duced [8d]. The present investigation of the oxidative
behavior of complexes containing 4-CH3, 4-F and 4-Br,
reveals the formation of strong isotropic radical signals
(Fig. 3(a)) with g = 2.0114 (DH = 22 G), 2.0086
(DH = 10.5 G) and 2.0076 (DH = 12.5 G), respectively,
in the presence of excess oxidant in CHCl3 at 300 K.
The relatively stable behaviors and the increased g-fac-
tors of these radicals compared to that for free 2,4-di-
tert-butylphenoxyl radical (g = 2.0045) [16], suggest sig-
nificant metal-orbital contribution to the SOMO of
phenoxyl radical and the generated radical species can
be assigned to directly coordinated PdII–phenoxyl radical
complexes. It is interesting that under the same condi-
tions upon oxidation of 2-CH3 and 2-Br substituted com-
plexes anisotropically broadened ESR spectra typical for
strongly immobilized nitroxide radicals [17] with param-
eters gi = 2.0061, g^ = 2.0072, Ai = 37.5 G, A^ = 5.38 G,
Aiso = 16.0.9 G and gi = 2.0048, g^ = 2.0064, Ai = 37.5
G, A^ = 4.96 G, Aiso = 15.81 G were observed (Fig.
3(c)). The appearance of such anisotropic spectra sug-
gests that the generated nitroxide radicals are rigidly fixed
in the polycrystalline matrix. Since the reaction mixture
was heterogeneous due to insolubility of the oxidant in
CHCl3 this result is not surprising. When LxH and the
oxidant are mixed in CHCl3 at 300 K and instantly cooled
to 130 K, the isotropic singlet (g = 2.0049–2.0055) assign-
able to the free phenoxyl radicals generated from free li-
gands was observed (Fig. 3(b)).
3.4. Catalytic reduction of nitrobenzene by complexes 1–
13
The present investigation has shown that except for
(6) (X = 2-Br) and (13) (X = 5,6-benzo), all of the com-
plexes exhibit catalytic activity in the hydrogenation of
nitrobenzene under normal pressure of H2, 760 Torr,
in DMF solution at 25 ꢁC,. Nitrobenzene, as identified
by means of IR scanning, was completely reduced to
aniline. Although addition of NaBH4 (1.0–8.0 · 10ꢀ5
mol) to the catalysts solution prior to the introduction
of H2 accelerated their catalytic activity, the hydrogena-
tion generally did not need any preliminary activation
(Table 5, Fig. 4). The conditions, initial rate absorption
of H2 and specific activity for 1–12, as well as for bis(N-
4-CH3Ph-salicylaldiminato)Pd(II), are presented in
Table 5. The course of reduction for some PdLx2 cata-
Table 4
Cyclic voltammetry data (VSR = 100 mV/s) for complexes 1–7, 9, 12 in DMF
Compound
1
2
3
4
5
6
7
9
12
aEa (V)
1.10
1.16
1.14
1.17
1.14
1.16, 1.27
1.15
1.09
1.07, 1.25
a
Ea represents oxidation peak potentials.

324
V.T. Kasumov et al. / Polyhedron 24 (2005) 319–325
Table 5
Conditions, initial rate of H2 absorption, catalytic activity of PdII-
It is thought that the orthogonality of the anilinic and
salicylideneimine planes might cause the lower sensitiv-
ity of the absorption rate of H2 to the electronic factors
on the aniline ring substituents.
complexes at 1 atm. of H2 and at 25 ꢁC
Compound
[Ccat
]
CPhNO2 Initial rate Specific
(mol/l) of H2 activity
Absorption Mol H2/
(10ꢀ3 mol lꢀ1
)
W
In conclusion, the prepared bisðN-aryl-3; 5-But2-
salicylaldimineÞpalladiumðIIÞ complexes exhibit interest-
ing spectral, electron-transfer and catalytic behaviors.
The introduction of the two t-Bu groups on the salicylic
ring increases the catalytic activity and oxidative reactiv-
ity of the PdII–salicylaldinine complexes in the hydroge-
nation of PhNO2. The chemical oxidation of the title
complexes with cerium(IV) nitrate, besides directly coor-
dinated Pd(II)-phenoxyl radical complexes, also gener-
ates nitroxide radicals.
(ml/min)
mol-cat
(min)
(1) (X = H)
(2) (X = 2-F)
a(2)
2.08
1.06
1.32
1.05
1.06
1.27
1.07
0.392
0.156
0.196
0.196
0.245
0.157
0.176
0.176
0.157
5.50
3.67
2.56
6.6
5.41
5.68
3.97
13.20
3.19
12.8
7.67
1.09
3.36
(3) (X = 4-F)
a(3)
1.17
9.9
(7) (X = 4-Br)
(9) (X = 4-CH3)
5.0
(4-CH3Ph-sal)2 Pd 2.02
(12) (X = 4-tBu)
1.13
2.75
1.06
a
Reduction carried out in the absence of NaBH4.
Acknowledgement
400
350
300
250
200
150
100
50
Financial support by the Harran University Research
Fund (HUBAK, Grant No.: 233) is gratefully
acknowledged.
References
Pdts-4F
Pdts-4F*
Pdts-4tBu
Pdts-2F
Pdts-4Br
Pdts-4Me
Pds-4Me
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