Oxidative dehydrogenation of Cu(II) complexes with N-Aryl-N-2- hydroxybenzyl- and N-Aryl-N-2-hydroxynaphthylmethyleneamine derivatives

Article abstract of DOI:10.1023/B:RUCO.0000040725.54156.bb

The reactions of Cu²⁺, Co²⁺, and Ni²⁺ ions with N-phenyl-N-2-hydroxybenzyl- and N-phenyl-N-2-hydroxynaphthylmethyleneamine derivatives (HLⁿ, n = 1-8) produced from the derivatives of aniline and aromatic -hydroxyaldehydes are studied. Among the ions studied, only Cu ²⁺ forms stable complexes Cu(Lⁿ)₂ · 2H₂O. The structures of the synthesized compounds are studied by IR, UV, and EPR spectroscopies and differential thermal analysis. The magnetic moments of the Cu(Lⁿ)₂ · 2H₂O complexes are very small and range within 0.43-1.19 μ_B, depending on the ligand structure, which indicates a strong antiferromagnetic interaction between the Cu²⁺ ions. The temperature dependence of the magnetic susceptibility measured for the Cu(L³)₂ ·2H ₂O complex (where HL³ is N-4-methoxyphenyl-N-2- hydroxybenzylamine) is closest to the theoretical curve calculated for the binuclear Cu(II) complexes connected by the intermolecular exchange interaction. The Cu(II) complexes with HLⁿ are shown to undergo oxidative dehydrogenation to form the corresponding metal salicyl-aldiminates. This reaction can occur on heating in the absence of oxygen and is accompanied by the Cu²⁺ → Cu⁺ transition.

Full text of DOI:10.1023/B:RUCO.0000040725.54156.bb

Russian Journal of Coordination Chemistry, Vol. 30, No. 9, 2004, pp. 648–654. Translated from Koordinatsionnaya Khimiya, Vol. 30, No. 9, 2004, pp. 688–694.
Original Russian Text Copyright © 2004 by Usatyuk, Medzhidov, Aydin, Yalchin, Manafova.
Oxidative Dehydrogenation of Cu(II) Complexes
with N-Aryl-N-2-hydroxybenzyl-
and N-Aryl-N-2-hydroxynaphthylmethyleneamine Derivatives
V. G. Usatyuk*, A. A. Medzhidov*, A. Aydin**, B. Yalchin**, and R. A. Manafova*
* Institute of Chemical Problems, Academy of Sciences of Azerbaijan, Baku, Azerbaijan
** Faculty of Arts and Science, Marmara University, Istanbul, Turkey
Received August 7, 2003
Abstract—The reactions of Cu2+, Co2+, and Ni2+ ions with N-phenyl-N-2-hydroxybenzyl- and N-phenyl-N-2-
hydroxynaphthylmethyleneamine derivatives (HLn, n = 1–8) produced from the derivatives of aniline and aro-
matic Ó-hydroxyaldehydes are studied. Among the ions studied, only Cu2+ forms stable complexes Cu(Ln)2 ·
2H2O. The structures of the synthesized compounds are studied by IR, UV, and EPR spectroscopies and differ-
ential thermal analysis. The magnetic moments of the Cu(Ln)2 · 2H2O complexes are very small and range
within 0.43–1.19 µB, depending on the ligand structure, which indicates a strong antiferromagnetic interaction
between the Cu2+ ions. The temperature dependence of the magnetic susceptibility measured for the Cu(L3)2 ·
2H2O complex (where HL3 is N-4-methoxyphenyl-N-2-hydroxybenzylamine) is closest to the theoretical curve
calculated for the binuclear Cu(II) complexes connected by the intermolecular exchange interaction. The Cu(II)
complexes with HLn are shown to undergo oxidative dehydrogenation to form the corresponding metal salicyl-
aldiminates. This reaction can occur on heating in the absence of oxygen and is accompanied by the Cu2+
Cu+ transition.
INTRODUCTION
occur, as a rule, with high selectivity and are convenient
for studying redox reactions of coordinated ligands in the
presence of various oxidants, including oxygen.
Coordination compounds of transition metals with N-
arylsalicylaldimine derivatives (Schiff bases) have been
known long ago [1–8]. However, information on metal
In this work, we synthesized new Cu(II) complexes
complexes with analogs of these bases hydrogenated at with N-aryl-N-2-hydroxybenzyl- and N-aryl-N-2-
the azomethine group is rather scarce [9–13]. In recent hydroxynaphthylmethyleneamine derivatives (çL1–
time, an interest in similar compounds has increased due çL8) produced from the respective Schiff bases and
to their ability to undergo oxidative dehydrogenation at studied their properties and ability to oxidative dehy-
the –CH2–NH– bond [14–17]. Reactions of this type drogenation.
R1 = R2 = H;
HL1
HL2
HL3
R1 = OCH3(meta-); R2 = H
CH2 NH
OH
R1 = OCH3(para-); R2 = H
R1 = Br(meta-); R2 = H
HL4
HL5
R1
R2
R1 = H; R2 = Br(5)
R1 = OCH3(para-); R2 = Br(5) HL6
R1 = Br(meta-); R2 = Br(5)
HL7
CH2 NH
OH
R2
R1
R1 = R2 = H;
HL8
EXPERIMENTAL
radiospectrometer. Thermogravimetric measurements
were carried out on a MOM derivatograph (Hungary).
The magnetic susceptibility of samples in a wide tem-
perature interval (5–300 K) was measured using an
IR spectra were recorded on a Specord M80 spectro-
photometer with KBr pellets. Electronic absorption
spectra were recorded on a Specord M40 spectropho-
tometer. EPR spectra were measured on an RE1306 MPMS-XL instrument (Quantum Design) with the
1070-3284/04/3009-0648 © 2004 åÄIä “Nauka /Interperiodica”

OXIDATIVE DEHYDROGENATION OF Cu(II) COMPLEXES
649
Table 1. Elemental analysis of the HL1–HL8 ligands and their complexes with the Cu2+ ion
Content (found/calculated), %
Compound
HL1
Mp, °C
Molecular formula
C
H
N
Cu
116
C13H13NO
78.15/78.36
73.14/73.34
73.13/73.34
55.97/56.14
55.97/56.14
54.41/54.56
43.60/43.73
81.70/81.90
62.77/62.95
60.31/60.47
60.30/60.47
47.62/47.76
47.64/47.76
46.97/47.11
38.37/38.48
68.30/68.50
6.53/6.58
6.53/6.59
6.54/6.59
4.31/4.35
4.31/4.35
4.54/4.58
3.08/3.11
6.01/6.06
5.64/5.69
5.75/5.80
5.75/5.80
3.98/4.01
3.98/4.01
4.20/4.24
2.95/2.98
5.37/5.41
6.87/7.03
5.96/6.11
5.98/6.11
4.90/5.04
4.91/5.04
4.44/4.55
3.81/3.92
5.50/5.62
5.53/5.65
4.90/5.04
4.92/5.04
4.17/4.28
4.17/4.28
3.83/3.92
3.35/3.45
4.62/4.70
HL2
70.5 C14H15NO2
HL3
128
C14H15NO2
HL4
123.5 C13H12NOBr
HL5
120
113
114
150
145
101
165
154
206
116
175
145
C13H12NOBr
HL6
C14H14NO2Br
HL7
C13H11NOBr2
HL8
C17H15NO
Cu(L1)2 · 2H2O
Cu(L2)2 · 2H2O
Cu(L3)2 · 2H2O
Cu(L4)2 · 2H2O
Cu(L5)2 · 2H2O
Cu(L6)2 · 2H2O
Cu(L7)2 · 2H2O
Cu(L8)2 · 2H2O
C26H28N2O4Cu
C28H32N2O6Cu
C28H32N2O6Cu
C26H26N2O4Br2Cu
C26H26N2O4Br2Cu
C28H30N2O6Br2Cu
C26H24N2O4Br4Cu
C34H32N2O4Cu
12.76/12.81
11.38/11.43
11.38/11.43
9.67/9.72
9.67/9.72
8.85/8.90
7.78/7.83
10.62/10.66
field intensity H = 1 T. Atomic increments [18] were
taken into account in the calculation. X-ray diffraction sized similarly. The results of elemental analysis and
The Cu(II) complexes with çL2–çL8 were synthe-
patterns were obtained on a Dron-4.0M diffractometer.
melting points of all copper complexes synthesized are
presented in Table 1.
N-Aryl-N-2-hydroxybenzyl- and N-aryl-N-2-
We failed to synthesize Co(II) complexes with çLn.
The cobalt complexes with the derivatives of Schiff
bases were obtained under conditions used in the syn-
thesis of the Cu(II) complexes with N-phenyl-N-2-
hydroxybenzylamines. This suggests that the ligand
underwent oxidative dehydrogenation, which was con-
firmed by comparison of the IR spectra of the reaction
products and the corresponding cobalt(II) bis(salicyl-
aldiminates).
hydroxynaphthylmethyleneamine derivatives were
synthesized by the reduction of the corresponding
Schiff bases with sodium borohydride in an alcohol
medium [19].
Synthesis of Cu(L1)2 · 2H2O. (a) A solution of çL1
(10–3 mol, 0.199 g) in methanol (5 ml) was mixed with
a solution of copper acetate (5 × 10–4 mol, 0.100 g) in
methanol (10 ml), and the resulting mixture was heated
to boiling. After cooling, a bright green crystalline
powder precipitated from the resulting dark green solu-
tion. The powder was filtered off, washed with metha-
nol several times, and dried in a vacuum at 75°ë. The
yield was 92%, mp 145°C.
(b) A solution of CuSO4 · 5H2O (5 × 10–4 mol,
0.125 g) in water (10 ml) was added at 50°ë for 10 min
to a solution of the sodium salt of çL1 (10–3 mol,
0.221 g) in water (10 ml). After the whole amount of
CuSO4 · 5H2O was added, the reaction solution was
stirred for 30 min. Green microcrystals that precipitated
were filtered off, washed with a large amount of dis-
An attempt to synthesize a complex from the
sodium salt of çL1 and CoSO4 · 7H2O according to the
procedure described above for the copper complexes
resulted in the precipitation of çL1 and cobalt(II)
hydroxide.
Synthesis of Ni(L1)2 · (CH3COOH)2. A solution of
çL1 (10–3 mol, 0.199 g) in methanol (5 ml) was mixed
with a solution of Ni(CH3COO)2 · 4H2O (5 × 10–4 mol,
0.124 g) in methanol (10 ml). The mixture was heated
to boiling with weak stirring. After cooling, light green
tilled water (300 ml) and methanol (10 ml), and dried in crystals precipitated from the resulting light green solu-
a vacuum at 75°ë. The yield was 90%, mp 145°C. tion. The crystals were separated, washed with metha-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 9 2004

650
USATYUK et al.
Complexation results in disappearance of absorp-
tion bands due to the OH phenol groups at ~2700 cm–1
in the IR spectra of all copper(II) complexes and the
broad bands from the water molecules contained in the
compounds appear instead at ~3400 cm–1. The ν(N–H)
absorption band becomes broader and its intensity
decreases, which indicates that nitrogen is coordinated
by the transition metal ion. The lacking characteristic
bands of the carboxyl group (ν(C=O)) [21] indicates
that acetic acid molecules are absent from the coordina-
tion sphere of the metal. Comparison of the IR spectra
of the complexes synthesized from CuSO4 · 5H2O
shows that they are completely identical to the com-
plexes synthesized from copper acetate.
1.3
4
1.1
0.9
5
3
0.7
0.5
0.3
2
1
0.1
–0.1
50 46 42 38 34 30 26 22 18 14
ν × 10–3, cm–1
The results of thermographic studies show that
water is eliminated from the copper complexes in an
interval of 100–200°ë. Both water molecules are likely
to be in the outer sphere rather than in the first coordi-
nation sphere of the metal.
Fig. 1. Changes in the electronic absorption spectra of
3
Cu(L ) · 2H O on heating at 180°ë: (1) starting complex,
2
2
after (2) 0.5 (3) 1, and (4) 5 min; (5) spectrum of the azome-
thine complex of bis(N-p-methoxyphenyl-N-salicylaldimi-
nato)copper(II).
Based on the data of elemental analysis, IR spectra,
and DTA, we can ascribe the formula Cu(Ln)2 · 2H2O to
the composition of the synthesized complexes.
nol several times, and dried in a vacuum at 75°ë. The
yield was 90%, mp 112°C.
The electronic absorption spectra of methanol solu-
tions of the complexes exhibit slight shifts of the
absorption bands of the “aniline” and “phenol” rings as
compared to the spectra of free ligands (Table 2). Since
the Cu complexes under study are poorly soluble, we
could not detect the bands corresponding to the d–d-
transitions. However, the ligand–metal charge-transfer
bands are pronounced at ~400 nm.
Weak anisotropic signals indicating the axial sym-
metry of the ligand environment of the Cu2+ ion are
observed in the EPR spectra of the polycrystalline cop-
per(II) complexes at room temperature.
The EPR spectra of the copper(II) complexes in
chloroform contain very weak isotropic signal from
63, 65Cu (I = 3/2) with a hyperfine splitting constant of
~75–80 G and giso = 2.07. The signals observed have
very low intensity, which can be caused by a negligible
admixture of monomeric complexes. Note that the EPR
signal in the “middle of the field” characteristic of some
binuclear copper complexes (for example, for carboxy-
lates) is not observed in the case.
For C30H32N2O6Ni
anal. calcd. (%): C, 62.63; H, 5.61; N, 4.87; Ni, 10.20.
Found (%):
C, 62.51; H, 5.59; N, 5.01; Ni, 10.03.
An attempt to recrystallize the complex from etha-
nol resulted in its decomposition to the ligand (çL1)
and nickel(II) hydroxide.
We failed to synthesize nickel complexes from the
sodium salt of çL1 and nickel(II) sulfate. In this case,
we also observed the precipitation of çL1 and
nickel(II) hydroxide.
RESULTS AND DISCUSSION
The IR spectra of the çL1–çL8 ligands contain nar-
row absorption bands characteristic of the secondary
amino group (Table 2). The absorption bands of the OH
phenol groups are shifted to the low-frequency region
and lie at ~2700 cm–1 due to the formation of a strong
hydrogen bond. In some cases, two absorption bands
are observed in this region.
The magnetic moments (µeff) of the copper(II)
complexes at 298 K are strongly decreased and vary
from 0.43 to 1.19 µB, depending on the ligand nature.
The electronic absorption spectra of methanol solu- The temperature dependence of the magnetic suscep-
tions of çL1–çL4 exhibit a series of bands assigned to tibility (χ) for the Cu(L3)2 · 2H2O complex was mea-
two aromatic rings: “aniline” and “phenol” [20]. In the sured in the temperature interval from 5 to 300 K. The
spectrum of HL1, the bands at 245 and 303 nm are magnetic moment equal to 1.19 µB at 298 K decreases
assigned to the “aniline” ring, and the bands at 224 and rapidly with decreasing temperature and reaches zero
276 nm are attributed to the “phenol” ring. The intro- at 165 K. At very low temperatures (below 20 K),
duction of substituents into the “aniline” ring has a paramagnetism increases weakly, most likely, due to
weak effect on the character of bands in the electronic an insignificant admixture of monomeric complexes.
absorption spectra, while the introduction of substitu- The magnetic behavior of the Cu(L3)2 · 2H2O complex
ents into the “phenol” ring leads to considerable does not obey the Bleaney–Bowers equation for iso-
changes in the positions of maxima of the absorption lated binuclear complexes. At the same time, the
bands (Table 2).
experimental dependence of χ on T (K) is close to the
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 9 2004

OXIDATIVE DEHYDROGENATION OF Cu(II) COMPLEXES
Table 2. Magnetic and spectral characteristics of the compounds synthesized
651
IR spectra, cm–1
µeff, µB
(300 K)
Electronic absorption spectra,
max, nm (logε)*
Compound
λ
ν(N–H)
ν(O–H)
ν(M–O) ν(M–N)
HL1
HL2
HL3
HL4
HL5
HL6
HL7
HL8
224ph(3.97), 245a(4.08)
276ph(3.00), 303a(3.00)
221ph(4.17), 247sa(3.96)
280ph(3.56), 298sa(3.04)
225ph(4.04), 241sa(4.07)
274ph(3.52), 305sa(3.04)
220ph(4.16), 252sa(4.05)
281ph(3.45), 303sa(3.11)
210a(4.23), 233sph(4.12)
286sph(3.53)
211sa(4.34), 233sph(4.33)
289sph(3.61), 308sa(3.23)
212sa(4.42), 250sph(4.03)
289sph(3.46), 305sa(3.15)
210n(4.20), 233a(4.35)
289n(3.41), 308a(3.25)
224ph(4.18), 247a(4.27)
274ph(3.80), 300a(3.30)
372ctb(3.14), 403ctb(3.10)
223ph(4.42), 247sa(4.23)
280ph(3.84), 300sa(3.30)
385ctb(3.43), 417ctb(3.44)
221ph(4.67), 240sa(4.58)
275ph(4.22), 302sa(4.00)
368ctb(3.74), 403ctb(3.81)
415ctb(3.75)
3264
2784
2736
2740
2640
2768
2734
2728
2650
2782
2645
2741
2648
2765
2650
2725
2738
3400
3280
3261
3248
3264
3272
3248
3267
3264
Cu(L1)2 · 2H2O
Cu(L2)2 · 2H2O
Cu(L3)2 · 2H2O
0.43
0.78
1.19
364
367
370
453
458
456
3264
3256
3440
3450
Cu(L4)2 · 2H2O
220ph(4.48), 252sa(4.40)
280ph(3.79), 303sa(3.34)
364ctb(2.69)
0.97
3256
3400
365
460
Cu(L5)2 · 2H2O
Cu(L6)2 · 2H2O
210a(4.24), 233sph(4.13)
285sph(3.54), 364ctb(2.67)
211sa(4.34), 233sph(4.33)
288sph(3.61), 303sa(3.35)
308sa(3.23), 364ctb(2.70)
212sa(4.43), 250sph(4.04)
289sph(3.46), 305sa(3.16)
368ctb(2.64)
0.96
0.96
3258
3256
3440
3400
364
367
452
455
Cu(L7)2 · 2H2O
1.05
3264
3260
3450
3448
365
368
458
456
Cu(L8)2 · 2H2O
210n(4.20), 233a(4.35)
289n(3.41), 308a(3.25)
1.04
366ctb(2.63)
a
sa
ph
sph
*
is the aniline fragment, is the aniline fragment with substituents, is the phenol fragment,
is the phenol fragment with substitu-
n
ctb
ents, is the naphthol fragment, and is the ligand–metal charge-transfer band.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 9 2004

652
USATYUK et al.
theoretical curve for the binuclear antiferromagnetic molecular antiferromagnetic exchange interaction
Cu(II) complexes (2J = –300 cm–1) bound by the inter- (2J' = –70 cm–1) [22].
NH
Cu
NH
NH
NH
O
O
O
Cu
O
NH
Cu
NH
NH
NH
O
O
O
Cu
O
R1
NH
NH
CH2
⇒
O
O
R2
X-ray diffraction analysis of the polycrystalline sam- hybridization of the nitrogen atoms (sp3- and sp2-
ples of all copper compounds synthesized showed that
hybridization, respectively) and the presence of π-con-
jugation between the nitrogen and oxygen atoms in the
coordination sphere of the metals in salicylaldiminates
(structure B). This conjugation is lacking in HL1–HL8,
because their nitrogen atom and aromatic ring are con-
they are isostructural to the Cu(L3)2 · 2H2O complex.
The main distinction of the HL1–HL8 ligands (N-
aryl-N-2-hydroxybenzyl- and N-aryl-N-2-hydrox-
ynaphthylmethyleneamine derivatives) (A) from the
corresponding salicylaldiminates (B) is different nected through the methylene group.
NH
Cu
NH
NH
NH
CH
Cu
O
O
N
O
Cu
O
O
O
N
CH
(A)
(B)
It is well known that salicylaldiminates form stable (with the dxy and dyz orbitals of the metal involved in
complexes with almost all transition metal ions [1–6, conjugation) underlies the high stability of salicylaldi-
23]. Our studies showed that the crystalline complexes minates, β-diketonates, and other similar compounds.
can be isolated in pure state only in the case of Cu2+
The absence of conjugation between the heteroat-
ions, while Co2+ and Ni2+ ions do not form stable com-
pounds.
oms in the complexes with the HL1–HL8 ligands is,
most likely, the main reason for the formation of binu-
clear structures between these ligands and the Cu2+ ion.
These structures are characterized by a strong intramo-
lecular antiferromagnetic exchange interaction
between the metal ions through the oxygen atoms of the
phenol group [26].
Undoubtedly, the main reason for instability of the
complexes with the HL1–HL8 ligands compared to the
respective azomethine complexes lies in the lack of
π-conjugation between the coordinating heteroatoms.
Conjugation between the π-bound heteroatoms and
metal ion has been discussed previously in [24, 25]. It
The characteristic feature of the complexes under
can be assumed that the formation of π-conjugated study is their affinity to thermal dehydrogenation,
rings between the conjugated ligands and metal ions which can occur both in air and in the absence of oxy-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 9 2004

OXIDATIVE DEHYDROGENATION OF Cu(II) COMPLEXES
653
2H2O complexes are shown in Fig. 1. Analysis of the IR
spectra of the oxidation products showed their com-
plete identity to the IR spectra of the corresponding
copper salicylaldiminates (Fig. 2).
90
2
Thermal dehydrogenation is also observed in the
absence of oxygen, when the Cu2+ ion was used as an
oxidant. However, in this case, the process does not
occur to the end and involves the reduction of Cu2+ to
Cu+.
80
70
60
50
40
30
20
Thus, the derivatives of N-aryl-N-2-hydroxybenzyl-
and
N-aryl-N-2-hydroxynaphthylmethyleneamines
were shown to be much weaker complexing agents as
compared to corresponding salicylaldiminates. The
main reason for this is the absence of π-conjugation
between the heteroatoms in the coordination sphere of
the metal ion. The Cu(II) complexes with the N-aryl-N-
2-hydroxybenzyl- and N-aryl-N-2-hydroxynaphthyl-
methyleneamine derivatives were found to undergo
selective dehydrogenation to form the corresponding
salicylaldiminate complexes.
1
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were recorded in UV and visible regions. The changes
in the electronic absorption spectra of the Cu(L3)2 ·
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