Synthesis and characterization of Ni(II) and Cu(II) complexes derived from novel phenolic Mannich bases

Article abstract of DOI:10.3906/kim-1203-67

The novel phenolic Mannich bases 2,2'-(2-hydroxy-4,5- dimethylbenzylazanediyl)diethanol (1) and 2-((2-(4- (2-hydroxy-4,5- dimethylbenzyl)piperazin-1-yl)ethylamino)methyl)-4,5-dimethylphenol (2) have been synthesized in good yield by using a microwave-induced technique from 3,4-dimethylphenol and corresponding amines in the presence of formaldehyde. Metal complexes of 1 and 2 for Ni(II) (3 and 5) and for Cu(II) (4 and 6) have also been prepared and all compounds have been characterized by elemental and spectral analyses. Thermal, magnetic, and electronic studies have also been reported for metal complexes. Electronic spectra and magnetic susceptibility measurements suggested tetrahedral geometry for Ni(II) (3), square planar geometry for Cu(II) (4) complexes of 1, and octahedral geometries for both Ni(II) (5) and Cu(II) (6) complexes of 2. TUeBITAK.

Full text of DOI:10.3906/kim-1203-67

Turk J Chem
Turkish Journal of Chemistry
(2013) 37: 101 – 110
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http://journals.tubitak.gov.tr/chem/
˙
c
ꢀ TUBITAK
doi:10.3906/kim-1203-67
Research Article
Synthesis and characterization of Ni(II) and Cu(II) complexes derived from novel
phenolic Mannich bases
∗
¨
¨
¨
Nurgu¨n BUYUKKIDAN , Salih OZER
Department of Chemistry, Faculty of Arts and Sciences, Dumlupınar University,
43100 Ku¨tahya, Turkey
Received: 29.03.2012
•
Accepted: 04.12.2012
•
Published Online: 24.01.2013
•
Printed: 25.02.2013
Abstract: The novel phenolic Mannich bases 2,2’-(2-hydroxy-4,5-dimethylbenzylazanediyl)diethanol (1) and 2-((2-(4-
(2-hydroxy-4,5-dimethylbenzyl)piperazin-1-yl)ethylamino)methyl)-4,5-dimethylphenol (2) have been synthesized in good
yield by using a microwave-induced technique from 3,4-dimethylphenol and corresponding amines in the presence of
formaldehyde. Metal complexes of 1 and 2 for Ni(II) (3 and 5) and for Cu(II) (4 and 6) have also been prepared and
all compounds have been characterized by elemental and spectral analyses. Thermal, magnetic, and electronic studies
have also been reported for metal complexes. Electronic spectra and magnetic susceptibility measurements suggested
tetrahedral geometry for Ni(II) (3), square planar geometry for Cu(II) (4) complexes of 1, and octahedral geometries
for both Ni(II) (5) and Cu(II) (6) complexes of 2.
Key words: Mannich base, 3,4-dimethylphenol, piperazine, microwave irradiation, metal complexes
1. Introduction
Metal complexes of Mannich bases have been studied extensively in recent years due to selectivity and sensi-
tivity of the ligands toward various metal ions.1−3 The new ligand systems for transition metals have drawn
considerable interest to chelating amino and hydroxy compounds. The compounds containing phenolic Man-
nich base components have a strong tendency to form metal complexes4−7 and exhibit significant anticancer
and antimalarial properties.8,9 The classical Mannich reaction has limited applications and has a number of
serious disadvantages. Therefore, numerous novel Mannich reactions have been developed10,11 to overcome the
drawbacks of the classical method and to avoid environmental problems using catalyst in combination with
surfactant in aqueous medium.12−21
In view of these facts and following the research programs in the field of coordination chemistry, we
report here a microwave-assisted convenient and economic method for the synthesis and characterization of new
Mannich bases 1 and 2, formed by 3-component condensation in good yield when compared with the classical
method. The proposed structures of the synthesized Mannich bases (1 and 2) are given in Scheme 1.
It is well known from the literature that these types of ligands have the ability to form metal complexes
via the N and O atoms. We also report metal complexes of 1 and 2 with Ni(II) (3 and 5) and Cu(II) (4 and
6). The proposed structures of the synthesized complexes 3–6 are shown in Scheme 2.
∗Correspondence: nsakarya@dumlupinar.edu.tr
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H2
CH2CH2OH
CH2CH2OH
C
HN(CH2CH2OH)2
a, b
H3C
H3C
N
OH
H3C
H3C
1
2
+ CH2O
NH2
OH
NH
HO
CH3
CH3
HN
N
H3C
H3C
N
N
OH
a, b
Scheme 1. Conditions of Mannich reaction for 1: a) ethanol, 30–40 ◦ C, 12 h, 59%; b) solvent-free, MW (200 W), 50
min, 85%; and for 2: a) ethanol, 30–40 ◦ C, 60 min, 68%; b) solvent-free, MW (200 W), 35 min, 89%.
H2
C
N
H3C
H3C
Ni
O
H
3
H2
C
O
O
ethanol
r.t.
1 + M(CH3COO)2.nH2O
N
H3C
H3C
O
Cu
O
H
O
4
NH
CH3
CH3
H3C
H3C
ethanol
r.t.
N
N
M(CH3COO)2.nH2O
2 +
.nH2O
O
M
O
OH2
H2O
M = Ni(II) (5), n = 2
M = Cu(II) (6), n = 0
Scheme 2. The proposed structure of complexes 3–6.
2. Experimental section
2.1. General methods and materials
All reagents (Ni(CH3 COO)2 .4H2 O, Cu(CH3 COO)2 .H2 O, 3,4-dimethylphenol, formaldehyde) were of the
highest grade, commercially available, and used without further purification. Elemental analyses for C, H,
and N were performed on a Leco CHNS-932 instrument. IR spectra were recorded on a Bruker Optics vertex
70 FT-IR spectrometer using attenuated total reflectance techniques. Thermal analyses were performed on the
SII Exstar 6000 TG/DTA 6300 model using a platinum crucible with 10 mg of sample. Measurements were
taken in the static air, within a 30–700 ◦ C temperature range. The UV-Vis spectra were carried out with a
Shimadzu UV-2550 spectrometer in the range of 200–900 nm. Magnetic susceptibility measurements at room
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temperature were taken using a Sherwood Scientific Magway MSB MK1 model magnetic balance by the Gouy
method using Hg[Co(SCN)4 ] as calibrant. A domestic microwave oven manufactured by BEKO was used for
the microwave-assisted reactions at the highest power (1200 W) and a 2450-MHz operating frequency.
2.2. General procedure for syntheses of ligands (1 and 2)
Upon mixing of 3,4-dimethylphenol (0.2 mol), amine (0.1 mol), and formaldehyde (0.2 mol), the following
reaction conditions were employed:
a. Reaction mixture was stirred in ethanol at 30–40 ◦ C for 12 h and 1 h for compounds 1 and 2,
respectively.
b. Reaction mixture was irradiated by microwave at 200 MW without solvent for 50 and 35 min for
compounds 1 and 2, respectively.
2.2.1. Synthesis of 2,2’-(2-hydroxy-4,5-dimethylbenzylazanediyl)diethanol (1)
The reaction mixture obtained from both methods was cooled to room temperature and the oily residue was
washed with toluene and dried in air to give pale yellow oil of 1 with 59% (a) and 85% yield (b).
1 H NMR (400 MHz, DMSO-d6): δ 7.48 (br s, 1H, H-7), 6.65 (s, 1H, H-2), 6.39 (s, 1H, H-5), 4.40 (br,
3
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ꢀ
2H, H-13, H-13’), 3.56 (s, 2H, H-10), 3.35 (t, 4H, H-12, H-12’, JH11,11 −H12,12 = 5.18 Hz), 2.46 (t, 4H, H-11,
H-11’, JH11,11 −H12,12 = 5.18 Hz), 2.00 (s, 3H, H-8), 1.98 (s, 3H, H-9); 13 C NMR (400 MHz, DMSO-d6):
δ 149.1 (C-6), 135.7 (C-4), 129.8 (C-2), 125.5 (C-3), 121.3 (C-1), 116.6 (C-5), 59.8 (C-12, C-12’), 57.1 (C-11,
C-11’), 51.2 (C-10), 18.2 (C-8, C-9). IR νmax (cm−1): 3500–3200 (O-H), 3020 (Ar-H), 2987, 2891 (C-H), 1578,
1456, 1443 (Ar C=C), 1119 (C-O); UV-Vis [λ (nm), εmax (L mol−1 cm−1)]: 273 (22180) (π -π *), 324 (810)
(n-π *). Anal. Calc. for C13 H21 NO3 : C, 65.25; H, 8.84; N, 5.85%, found: C, 65.18; H, 8.86; N, 5.79%.
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2.2.2. Synthesis of 2-((2-(4-(2-hydroxy-4,5-dimethylbenzyl)piperazin-1-yl)ethylamino)methyl)-4,5-
dimethylphenol (2)
The residue obtained from both methods was washed and recrystallized from ethanol to give white crystals of
2 with 68% (a) and 89% yield (b).
Mp 131–133 ◦ C. 1 H NMR (400 MHz, DMSO-d6): δ 11.17 (s, 2H, H-15), 9.45 (s, 2H, H-7, H-23), 6.63 (s,
3
ꢀ
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2H, H-2, H-18), 6.37 (s, 2H, H-5, H-21), 4.15 (s, 2H, H-16), 3.77 (s, 2H, H-10), 3.33 (t, 2H, JH11,11 −H12,12
=
3
3
ꢀ
ꢀ
5.01 Hz, H-11, H-11’), 3.12 (t, 2H, JH11,11 −H12,12 = 5.01 Hz, H-12, H-12’), 2.80 (t, 2H, JH13−H14 = 4.98
Hz, H-14), 2.62 (t, 2H, JH13−H14 = 4.98 Hz, H-13), 2.29 (s, 6H, H-9, H-25), 2.18 (s, 6H, H-8, H-24); 13 C
3
NMR (400 MHz, DMSO-d6): δ 154.6 (C-6), 151.2 (C-22), 130.0 (C-4, C-20), 127.5 (C-2), 125.3 (C-18), 117.2
(C-3), 116.5 (C-19), 112.9 (C-1, C-17), 110.3 (C-5), 109.9 (C-21), 56.1 (C-10), 52.9 (C-11, C-11’), 52.0 (C-12,
C-12’), 49.3 (C-13), 48.0 (C-14, C-16), 19.0 (C-9, C-25), 18.2 (C-8, C-24); IR νmax (cm−1): 3500–3200 (O-H),
3020 (Ar-H) 2998, 2896 (C-H), 1615, 1543, 1500 (Ar C=C), 1268 (C-N), 1259 (C-O); UV-Vis [λ (nm), εmax
(L mol−1 cm−1)]: 288 (24,190) (π -π *), 332 (800) (n-π *). Anal. Calc. for C24 H35 N3 O2 : C, 72.51; H, 8.87;
N, 10.57%, found: C, 73.08; H, 8.41; N, 10.26%.
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2.3. The general experimental procedure for complexes (3–6)
A solution of M(CH3 COO)2 .nH2 O (M = Ni and Cu) (2.04 × 10−3 mol) in ethanol (5 mL) was added dropwise
to the solution of ligand (2.04× 10−3 mol) in ethanol (30 mL) with stirring at room temperature. The pH of
the reaction mixture was adjusted by addition of 0.1 M aqueous NaOH solution in the range of 6.0 to 7.0. After
addition of base, the solid complex formed, and then it was filtered, washed with water, and dried in air.
2.3.1. 2,2’-(2-Hydroxy-4,5-dimethylbenzylazanediyl)diethanolatonickel(II) (3)
Green solid with 76% yield. IR νmax (cm−1): 3400–3200 (O-H), 3012 (Ar-H), 2973, 2898 (C-H), 1613, 1543,
1498, 1458 (Ar C=C), 1100 (C-O), 569 (Ni-O), 435 (Ni-N); UV-Vis [λ (nm), εmax (L mol−1 cm−1)]: 248
(15500), 300 (13400), 382 (5410) (π -π *), 523 (235), 613 (175) (d-d). Anal. Calc. for C13 H19 NiNO3 : C, 52.75;
H, 6.47; N, 4.73%, found: C, 52.46; H, 6.38; N, 4.86%.
2.3.2. 2,2’-(2-Hydroxy-4,5-dimethylbenzylazanediyl)diethanolatocopper(II) (4)
Dark green solid with 65% yield. IR νmax (cm−1): 3400–3200 (O-H), 3010 (Ar-H), 2987, 2849 (C-H), 1612,
1551, 1494, 1455 (Ar C=C), 1095 (C-O), 599 (Cu-O), 496 (Cu-N); UV-Vis [λ (nm), εmax (L mol−1 cm−1)]:
271 (14060), 288 (32490), 295 (33690), 389 (2970) (π -π *), 696 (550) (d-d). Anal. Calc. for C13 H19 CuNO3 :
C, 51.90; H, 6.37; N, 4.66%, found: C, 51.26; H, 6.34; N, 5.03%.
2.3.3. Diaqua-2-((2-(4-(4,5-dimethyl-2-oxidobenzyl)piperazin-1-yl)ethylamino)methyl)-4,5-dimet-
hylphenolatonickel(II)dihydrate (5)
Light green solid with 78% yield. IR νmax (cm−1): 3500–3300 (O-H), 3144 (N-H), 3006 (Ar-H), 2987, 2876
(C-H), 1613, 1468 (Ar C=C), 1274 (C-N), 1261 (C-O), 575 (Ni-O), 410 (Ni-N); UV-Vis [λ (nm), εmax
(L mol−1 cm−1)]: 233 (2140), 241 (2450), 262 (2330), 289 (32990) (π -π *), 495 (100) (d-d). Anal. Calc.
for C24 H41 NiN3 O6 : C, 54.77; H, 7.85; N, 7.98%, found: C, 54.15; H, 7.12; N, 8.45; Ni%.
2.3.4. Diaqua-2-((2-(4-(4,5-dimethyl-2-oxidobenzyl)piperazin-1-yl)ethylamino)methyl)-4,5-dimet-
hylphenolatocopper(II) (6)
Brown solid with 72% yield. IR νmax (cm−1): 3550–3300 (O-H), 3142 (N-H), 3008 (Ar-H), 2987, 2876 (C-H),
1612, 1511, 1467, (Ar C=C), 1274 (C-N), 1262 (C-O), 558 (Ni-O), 479 (Ni-N); UV-Vis [λ (nm), εmax (L mol−1
cm−1)]: 227 (10280), 233 (10880), 249 (10850), 257 (11610), 288 (40390), 364 (10710) (π -π *), 815 (200) (d-d).
Anal. Calc. for C24 H37 CuN3 O4 : C, 58.22; H, 7.53; N, 8.49%, found: C, 57.88; H, 7.04; N, 7.77%.
3. Results and discussion
3.1. 1 H NMR and 13 C NMR spectra of 1 and 2
The 1 H and 13 C NMR spectra of compounds 1 and 2 were obtained in DMSO-d6 at room temperature using
TMS as the internal standard; 1 H and 13 C NMR assignments are listed in Tables 1 and 2, respectively. The
1 H NMR spectrum of 1 exhibits a broad singlet at 7.48 ppm due to proton H-7 with 1 H intensity. Aromatic
protons (H-2 and H-5) with 1 H intensity for each are observed at 6.65 and 6.39 ppm. A singlet for protons
H-13 and H-13’ with 2 H intensity is obtained at 4.40 ppm. The singlet signal with 2 H intensity at 3.56 ppm
is assigned to H-10 protons indicating accomplishment of an aminomethylation reaction. The 2 sets of triplets
are observed with 4 H intensity for each at 3.35 ppm (3JH11,11 −H12,12 = 5.18 Hz) for protons H-12 and H-12’
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and 2.46 ppm (3JH11,11 −H12,12 = 5.18 Hz) for protons H-11 and H-11’, respectively. The 2 singlets with 3 H
ꢀ
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intensity for each corresponding to methyl protons (H-8 and H-9) are found at 2.00 and 1.98 ppm, respectively.
The 1 H NMR spectrum of 2 exhibits a singlet at 11.17 ppm due to proton H-15 with 1 H intensity and a broad
singlet at 9.45 due to H-7 and H-23 with 2 H intensity. Aromatic protons (H-2, H-18 and H-5, H-21) with 2 H
intensity for each are observed at 6.63 and 6.37 ppm. Two singlets for protons H-16 and H-10 with 2 H intensity
for each are obtained at 4.15 and 3.77 ppm, respectively, confirming the Mannich aminomethylation reaction.
The 2 sets of triplets are observed with 4 H intensity for each at 3.33 ppm (3JH11,11 −H12,12 =5.01 Hz) for
ꢀ
ꢀ
protons H-11 and H-11’ and 3.12 ppm (3JH11,11 −H12,12 = 5.01 Hz) for protons H-12 and H-12’, respectively.
The protons H-14 (2.80 ppm) and H-13 (2.62 ppm) couple to each other to give triplets with coupling constant
3JH13,14 = 4.98 Hz. The 2 singlets with 6 H intensity for each correspond to methyl protons (H-9, H-25 and
H-8, H-24) found at 2.29 and 2.18 ppm, respectively.
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Table 1. 1 H and 13 C NMR chemical shifts (ppm) with coupling constants (Hz) and assignments for compound 1.
1H NMR data
13C NMR data
H-7
H-2
H-5
H-13, H-13’
H-10
7.48 C-6
149.1
135.7
129.8
125.5
121.3
116.6
6.65 C-4
6.39 C-2
4.40 C-3
3.56 C-1
H-12, H-12’(3JH11,11 −H12,12 = 5.18 Hz) 3.35 C-5
ꢀ
ꢀ
H-11, H-11’(3JH11,11 −H12,12 = 5.18 Hz) 2.46 C-12, C-12’ 59.8
ꢀ
ꢀ
H-8
H-9
2.00 C-11, C-11’ 57.1
1.98 C-10
C-8, C-9
51.2
18.2
The 13 C NMR spectra of 1 and 2 are in good agreement with the expected signals. The 13 C NMR
spectrum of 1 exhibits 10 resonances. Six peaks in the range between 149.1 and 116.6 ppm are assigned to
aromatic carbon atoms. Two peaks are observed in the aliphatic region for C-12 and C-12’ (59.8 ppm) and
C-11 and C-11’ (57.1 ppm), respectively. The other 2 peaks are observed at 51.2 ppm for carbon C-10 and at
18.2 ppm for carbons C-8 and C-9. The 13 C NMR spectrum of 2 displays 10 resonances in the aromatic region
in the range of 154.6–109.9 ppm. The peaks are observed in the aliphatic region for C-10 (56.1 ppm) and for
piperazine ring carbon atoms (C-11, C-11’ and C-12, C-12’) observed at 52.9 and 52.0 ppm, respectively. Two
signals are observed at 49.3 and 48.0 ppm for C-13 and C-14, C-16, respectively. The methyl carbons with 2 C
intensity for each are obtained at 19.0 ppm for C-9, C-25 and 18.2 ppm for C-8, C-24.
3.2. FT-IR measurements
The coordination modes and sites of ligands 1 and 2 to the metal(II) ions were investigated by comparing the
IR spectra of 1 and 2 with their complexes (3 and 5 for Ni(II), 4 and 6 for Cu(II)) (Table 3). The IR spectra
of 1 and 2 show a broad band in the region of 3500–3200 cm−1 , which is assigned to ν (O-H) vibration,22 and
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Table 2. 1 H and 13 C NMR chemical shifts (ppm) with coupling constants (Hz) and assignments for compound 2.
11 12
15
24
8
2
13
16
17
10
18
NH
CH3
1
6
H3C
N
N
3
14
19
OH
7
H3C
9
11' 12'
4
HO
CH3
25
13C NMR data
20
22
5
23
21
1H NMR data
H-15 (s, 1H)
11.17 C-6
9.45 C-22
6.63 C-4, C-20
6.37 C-2
4.15 C-18
3.77 C-3
154.6
151.2
130.0
127.5
125.3
117.2
116.5
H-7, H-23 (s, 2H)
H-2, H-18 (s, 2H)
H-5, H-21 (s, 2H)
H-16 (s, 2H)
H-10 (s, 2H)
H-11, H-11’(t, 2H,
3.33 C-19
3
ꢀ
ꢀ
JH11,11 −H12,12 = 5.01 Hz)
H-12, H-12’(t, 2H,
3.12 C-1, C-17
112.9
3
ꢀ
ꢀ
JH11,11 −H12,12 = 5.01 Hz)
3
H-14 (t, 2H, JH13−H14 = 4.98 Hz) 2.80 C-5
110.3
109.9
56.1
3
H-13(t, 2H, JH13−H14 = 4.98 Hz)
2.62 C-21
2.29 C-10
H-9, H-25 (s, 6H)
H-8, H-24 (s, 6H)
2.18 C-11, C-11’ 52.9
C-12, C-12’ 52.0
C-13
49.3
48.0
19.0
18.2
C-14, C-16
C-9, C-25
C-8, C-24
Table 3. IR spectral data (cm−1) of free ligands (1 and 2) and complexes 3–6.
Assign.
1
2
3
4
5
6
ν(O-H)
3500–3200 3500–3200 3400–3200 3400–3200 3500–3300 3550–3300
(br)
(br)
(br)
(br)
(br)
(br)
ν(N-H)
ν(C-H)ar.
3144(w)
3006(w)
2987(w),
2876(w)
1468(s)
1613(s),
3142(w)
3008(w)
2987(w),
2876(w)
1612(s),
1511(s),
1467(s)
3020(w)
2987(w)
2891(w)
1578(s)
1456(s)
1443(s)
3020(w)
2998(w),
2896 (w)
1615(s),
1543(s),
1500(s)
3012(w)
2973(w)
2898(w)
1613(s)
1543(s)
1498(s)
1458(s)
3010(w)
2987(w)
2849(w)
1612(s)
1551(s)
1494(s)
1455(s)
ν(C-H)aliph.
ν(C=C)
ν(C-N)
ν(C-O)
ν(M-O)
ν(M-N)
1268(s)
1259(s)
1274(s)
1261(s)
575(s)
410(s)
1274(s)
1262(s)
558(s)
479(s)
1119(s)
1100(s)
569(s)
435(s)
1095(s)
599(s)
496(s)
w: weak, br: broad, m: medium, s: strong
the bands due to ν (C–H) and ν (C=C) in regions 2998–2891 and 1615–1443 cm−1 , respectively.
The IR spectra of the metal complexes (3 and 4) of 1 show the broad bands in the region of 3400–
3200 cm−1 consistent with the coordinated ν (O-H) mode. The ν (C=C) stretching vibrations appeared in the
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Table 4. Optical properties of free ligands (1 and 2) and complexes 3–6 in DMSO.
λmax (nm) (ε (L mol−1 cm−1))
1
2
3
4
5
6
273(22,180) 288(24,190) 248(15,500) 271(14,060)
324(810) 332(800) 300(13,400) 288(32,490)
233(2140)
241(2450)
262(2330)
289(32,990) 257(11,610)
495(100)
227(10,280)
233(10,880)
249(10,850)
382(5410)
523(235)
613(175)
295(33,690)
389(2970)
696(550)
DMSO
288(40,390)
364(10,710)
815(200)
region of 1613–1458 cm−1 for 3 and 1612–1455 cm−1 for 4. The shifts in ν (C-O) of the phenolic O-H group,
from 1119 cm−1 in the free ligand to 1100 and 1095 cm−1 in complexes 3 and 4, respectively, indicate the
participation of the O-H group in complex formation.23 Both complexes exhibit bands at 569 and 599 cm−1
,
which are assignable to ν (M–O), and the bands at 435 and 496 cm−1 due to ν (M–N) modes. The ν (M–O)
stretching vibration usually occurs in the higher frequency region and is usually sharper and stronger than
ν (M–N). These observations are in accordance with the structure of the Ni(II) (3) and Cu(II) (4) complexes
with 1, in which the central metal ions acquire a coordination number of 4.22
As seen from Table 3, the IR spectra of complexes 5 and 6 are similar to each other, indicating the similar
structures for the complexes. There are some significant differences between the complexes (5 and 6) and the free
ligand (2) upon chelation, as expected. The broad bands in the region of 3500–3300 cm−1 for 5 and 3550-3300
cm−1 for 6 are due to the ν (O-H) vibrations.22 The only proton of the -NH2 group of the 2-(piperazin-1-
yl)ethanamine takes part in the Mannich aminomethylation reaction. IR spectra of complexes 5 and 6 exhibit
a band at 3144 and 3142 cm−1 , respectively, indicating the uncoordinated N-H group. Complexation takes place
for 2 N atoms of the piperazine ring and the O atom of the deprotonated phenolic O-H group. In comparison
with ligand 2 (1268 cm−1), the ν (C–N) absorption is shifted to higher frequency in complexes 5 and 6 (1274
cm−1), showing the coordination of piperazine nitrogens to the metal(II) ions.23 The band at 1259 cm−1 is
assigned to ν (C–O) of the phenolic O-H group.24 The participation of the deprotonated O-H group is confirmed
by the shift of ν (C–O) towards higher frequency in the spectra of coordination complexes 5 (1261 cm−1) and 6
(1262 cm−1) due to the formation of metal–oxygen bonds.25 These have been further confirmed by the bands at
575 and 410 cm−1 for Ni(II) (5) and at 558 and 479 cm−1 for Cu(II) (6), which may be assigned to the ν (M-O)
and ν (M-N) stretching vibrations of the coordinated O and N atoms of the free ligand (2), respectively.26
All of these characteristic features of the IR studies suggest the proposed structures of the coordination
compounds (3–6) as shown in Scheme 2.
3.3. UV/Vis spectra and magnetic susceptibility
The electronic spectra of compounds 1 and 2 and their complexes (3 and 5 for Ni(II), 4 and 6 for Cu(II)) were
recorded in DMSO solution with 1 × 10−3 M concentration at room temperature (Table 4). The electronic
spectra of the ligands show absorptions at 273 and 324 nm for 1 and at 288 and 332 nm for 2 due to π -π * and
n-π * transitions, respectively.
The Ni(II) complex (3) of 1 exhibits absorptions at 248, 300, and 382 nm that were assigned to π -π *, and
the absorptions at 523 and 613 nm were assigned to d-d transitions of a 4-coordinate tetrahedral geometry.27
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Table 5. Thermogravimetric analysis of complexes 3–6.
Decomposition
temperature
(◦C)
Solid
residue*
(%)
DTGmax
(◦C)
Weight loss
Eliminated
special
Complex
found calculated
3
30–345
345–450
-
35–342
342–550
-
30–284
284–456
456–1000
-
30–177
177–477
477–600
-
256 and 316
35.30
40.20
19.20
32.30
41.70
20.76
13.30
40.00
35.50
11.20
8.00
35.19
39.55
19.83
32.01
41.68
21.12
13.68
39.58
35.57
11.15
7.27
C8H8
C5H10NO2
376
-
223
477
Ni
C6H8O
C7H11NO
-
4 H2O
C16H16
C8H17N3O2
-
2 H2O
C13H18
C11H15N3O2
-
4
5
-
79, 144, and 221
290 and 305
504
Cu
-
Ni
68 and 166
202, 328, and 467 35.40
35.23
44.67
12.83
6
482 and 500
43.82
12.78
-
Cu
*Calculated from the MO residue
Characteristic π − π * transitions are observed in the spectrum of the Cu(II) complex (4) of 1 at 271,
288, 295, and 389 nm.28 The electronic spectrum of 4 also exhibits a broad band at 696 nm attributable to d-d
transitions, which strongly favor square-planar geometry around the Cu(II) ion.26,29
The Ni(II) complex (5) exhibits absorptions at 233, 241, 262, and 289 nm assigned to π -π * transitions
and a broad absorption band at 495 nm due to d-d transitions.
Characteristic π − π * transitions are observed in the spectrum of 6 at 227, 233, 249, 257, 288, and 364
nm.30,31 The electronic spectrum of 6 also exhibits a broad band at 815 nm attributable to d-d transitions,
which strongly distorted the octahedral geometry around the Cu(II) ion.32
The magnetic susceptibility results of transition metal complexes give an indication of the geometry of
the ligands around the central metal ion. The magnetic moment of the Ni(II) complex (3) is 3.37 B.M., which is
in good agreement with a d8 system in a tetrahedral environment.33 The measured magnetic moment value for
4 is 1.86 B.M., which is consistent with the expected spin-only magnetic moment of d9 Cu(II) systems. Thus,
in view of the formulation of the complexes and their magnetic moments, it seems quite reasonable to propose
a tetrahedral environment around the Ni(II) (3) and square-planar geometry around the Cu(II) (4) centers.
The Ni(II) (5) and Cu(II) (6) complexes of 2 are paramagnetic and their magnetic susceptibilities are
3.62 and 1.91 B.M., which indicates 2 and 1 unpaired electrons for Ni(II) (5) and Cu(II) (6), respectively. The
magnetic moments of the complexes at room temperature lie in the range of 3.53–4.26 B.M. for mononuclear
octahedral Ni(II)34 and 1.83–1.96 B.M. for mononuclear octahedral Cu(II) centers.35,36
3.4. Thermal analyses of complexes 3−6
The data for the TG-DTG and DTA curves of compounds 3, 4, 5, and 6 are listed in Table 5. The thermal
decomposition studies of complexes 3 and 4 show no appreciable change at about 200 ◦ C when heated due to
the absence of coordinated or uncoordinated H2 O molecules in 3 and 4.
For compound 3, the first stage, an endothermic peak (DTGmax = 256, 316 ◦ C) between 30 and 345 ◦ C,
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corresponds to the loss of the C8 H8 group (found 35.30, calc. 35.19%). The endothermic second stage (DTGmax
= 376 ◦ C), between 345 and 450 ◦ C, corresponds to the loss of the C5 H10 NO2 group of the ligand (found
40.20, calc. 39.55%). The amount of Ni was calculated from the final decomposition product Ni (found 19.20%,
calc. 19.83%).
For compound 4, the first stage, an endothermic peak (DTGmax = 223 ◦ C) between 35 and 342 ◦ C,
corresponds to the loss of the C6 H8 O group from the ligand (found 32.30, calc. 32.01%). The endothermic
second stage (DTGmax = 477 ◦ C), between 342 and 550 ◦ C, is consistent with the loss of C7 H11 NO from
ligand residue in compound 4 (found 41.70, calc. 41.68%). The amount of Cu was calculated from the final
decomposition product Cu (found 20.76%, calc. 21.12%).
The Ni(II) complex (5) was thermally decomposed in 4 steps. The first step (DTGmax = 79, 144,
221 ◦ C) within the temperature range of 30–280 ◦ C may be attributed to the liberation of coordinated and
uncoordinated water molecules (found 13.30, calc. 13.68%). The second step (DTGmax = 290, 305 ◦ C) occurred
within the temperature range of 284–456 ◦ C with an estimated mass loss of 40.00% (calc. 39.58%), which is
accounted for by the removal of the C16 H16 group. The third step (DTGmax = 504 ◦ C) occurred within the
temperature range of 456–1000 ◦ C, corresponding to the loss of the C8 H17 N3 O2 group of the ligand (found
35.50, calc. 35.57%). The amount of Ni was calculated from the final decomposition product Ni (found 11.20%,
calc. 11.15%).
For compound 6, the first decomposition step (DTGmax = 68, 166 ◦ C) represents the loss of coordinated
water molecules within the temperature range of 30–177 ◦ C (found 8.00, calc. 7.27%). The endothermic second
stage (DTGmax = 202, 328, 467 ◦ C), between 177 and 477 ◦ C, is consistent with the loss of the C13 H18 group
from the ligand residue in compound 6 (found 35.40%, calc. 35.23%). The third step (DTGmax = 482, 500
◦ C) within the temperature range of 477–600 ◦ C corresponds to the loss of the C11 H15 N3 O2 group of the
ligand (found 43.82, calc. 44.67%). The amount of Cu was calculated from the final decomposition product Cu
(found 12.78%, calc. 12.83%).
4. Conclusions
We have synthesized 2 new Mannich bases (1 and 2) with a very simple, efficient, and practical method under
microwave irradiation in the absence of solvent, which gives high yield. We have also synthesized new Ni(II)
complexes 3 and 5 of 1, and Cu(II) complexes 4 and 6 of 2. The elemental and other spectral studies confirm
the bonding of Mannich base 1 to Ni(II) (3) in tetrahedral and Cu(II) (4) in square-planar geometry and the
bonding of 2 to Ni(II) (5) and Cu(II) (6) in octahedral geometry.
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