Synthesis and structural features of new schiff base complexes of mononuclear MnIV, dinuclear CoIICoIII, and tetranuclear CuII

Article abstract of DOI:10.1002/zaac.201400554

2-(((2-Hydroxy-3-methoxyphenyl)methylene)amino)-2(hydroxymethyl)-1,3-propanediol (LH₄, as abbreviation) reacts with MnCl₂·4H₂O, CoCl₂·6H₂O, and Cu(ClO₄)₂·6H₂O to give

Full text of DOI:10.1002/zaac.201400554

Journal of Inorganic and General Chemistry
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ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201400554
Synthesis and Structural Features of New Schiff Base Complexes of
Mononuclear MnIV, Dinuclear CoIICoIII, and Tetranuclear CuII
Davi F. Back,*[a] Gelson Manzoni de Oliveira,*[a] Cibele Mensch Canabarro,[a] and
Bernardo Almeida Iglesias[a]
Keywords: Schiff bases; Schiff base complexes; Mixed valence complexes; Manganese; Cobalt; Copper
Abstract.
2-(((2-Hydroxy-3-methoxyphenyl)methylene)amino)- molecules of the ligand with one manganese(IV) ion. In the mixed-
2(hydroxymethyl)-1,3-propanediol (LH4, as abbreviation) reacts with valence cobalt complex 2 there is an asymmetry between the coordina-
MnCl2·4H2O, CoCl2·6H2O, and Cu(ClO4)2·6H2O to give the new tion spheres of cobalt(II) and cobalt(III). In the tetramer 3 four cop-
complexes [Mn(LH2)2] (1), [Co2Cl(H2O)(LH2)2]·4H2O (2), and per(II) ions attain a distorted tetrahedral configuration surrounded by
[Cu4(LH2)4(H2O)4] (3). Complex 1 is formed by the assembly of two
four molecules of the ligand.
erally exist in the phenol-imine form,[16,17] and sometimes en-
amine or keto tautomer, and a zwitterionic structure with a
longer N+–H bond.[18]
1 Introduction
Polynuclear complexes of transition metals of the first row
have attracted much attention because of their interesting struc-
tural variety and remarkable properties as catalysts and bio-
inorganic agents. One of the various successful strategies used
to prepare polynuclear transition metal complexes involves the
exploration of different polydentate Schiff bases as starting
materials.
Schiff bases have been used extensively as ligands in the
coordination chemistry.[1,2] In addition, these compounds can
show suitable applications regarding its antitumor activity,[3]
photochromism,[4] and thermochromism[5] by proton displace-
ment between the hydroxyl oxygen atom and the imine nitro-
gen atom.
Many groups are involved in the utilization of the multisite
Schiff-base ligand 2-(((2-hydroxy-3-methoxyphenyl)methyl-
ene)amino)-2-(hydroxymethyl)-1,3-propanediol (as abbrevi-
ation LH4),[19] to prepare 4f clusters, 3d-4f mixed metal
compounds, and a small number of first row polynuclear tran-
sition metal complexes.[20–24] With the aim to provide new
complexes obtained from LH4, we report the synthesis
and the structural characterization of [Mn(LH2)2] (1),
[Co2Cl(LH2)2(H2O)]·4H2O (2), and [Cu4(LH2)4(H2O)4] (3).
2 Experimental Section
Literature search reveals that salicylaldehyde reacts with an
amino group, leading to the formation of Schiff bases.[6,7] Be-
cause of that, salicylaldehyde is a key precursor for a variety
of chelate ligands. o-Vanillin, as substituted derivative of sali-
cylaldehyde, besides their own biological activity,[8,9] and tris
(hydroxymethyl) aminomethane – as an amine, which enables
more than one coordination site,[10] – are good precursors for
the synthesis of polydentate Schiff bases. These Schiff base
ligands allow the formation of both monomers[11] and polynu-
clear transition metal complexes.[7,12]
2.1 General
All manipulations were conducted by use of standard Schlenk flasks.
Elemental analyses for C, H, and N were performed at a Shimadzu
EA 112 microanalysis instrument. IR spectra were recorded with a
Tensor 27 Bruker spectrometer with KBr pellets in the 4000–
400 cm–1 region.
2.2 Preparations
Specifically 2-hydroxy Schiff base ligands are of interest,
mainly due to the existence of O–H···N and N–H···O hydrogen
bonds as models for the study of keto-enol tautomerism.[13–15]
Structurally, Schiff bases derived from salicylaldehyde gen-
2.2.1 Ligand 2-(((2-Hydroxy-3-methoxyphenyl)methylene)-
amino)-2(hydroxymethyl)-1,3-propanediol (LH4)
In accordance with the procedures found in the literature[23,25,26]
LH4was synthesized by condensation of o-vanillin (0.152 g, 1 mmol)
and 2-amino-2-hydroxymethylpropane-1,3-diol (0.121 g, 1 mmol) in
anhydrous methanol (25 mL). The mixture was heated in an oil bath
at 65 °C and stirred for 4 h in a nitrogen atmosphere. After a week
yellow crystals were obtained in the solution. Approx. yield: 43.2%.
Melting point (decomp.): 181 °C. Calcd. C 56.47; H 6.66; N 5.49%;
found: C, 56.47; H, 6.50; N, 5.67%. IR (KBr): 3343 [s, ν(O–H)phenol];
3194 [m, ν(O–H)alcohols]; 2948[w, ν(C–H)sp3)]; 1643 [s, ν(C=N)imine];
* D. F. Back
Fax: +55-55-3220-8031
E-Mail: daviback@gmail.com
* Dr. G. Manzoni de Oliveira
E-Mail: manzonideo@smail.ufsm.br
[a] Departamento de Química - LMI
Universidade Federal de Santa Maria
97105–900, Santa Maria – RS, Brazil
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1501 [vs, ν(C=C)ring]; 1227[s, ν(C–O)methoxy]; 1023 [m, ν(C–O)alcohols
cm–1 (vs. = very strong; s = strong; m = middle; w = weak).
]
2.2.4 [Cu4(LH2)4(H2O)4] (3)
To a solution of LH4 (0.024g, 0.1 mmol) in methanol (20 mL),
Cu(ClO4)2·6H2O (0.037g, 0.1 mmol) was added. After heating at 70 °C
for 0.5 h, triethylamine (0.013 mL, 0.129 mmol) was added to the
green solution. The mixture was stirred and heated for 3.5 h and finally
filtered. The crystallization occurred after 10 d. Approx. yield: 28.7%.
Deep green crystals. Melting point (decomp.): over 260 °C.
C12H17CuNO6: calcd. C 43.0; H 5.07; N 4.18%; found: C 43.15; H
5.12; N 4.29%. IR (KBr): 3452 [m, ν(O–H)alcohols]; 3237 [m,
ν(O–H)alcohols]; 2908 [w, ν(C–H)sp3)]; 1627 [s, ν(C=N)imine]; 1470 [s,
ν(C=C)ring]; 1219 [s, ν(C–O)methoxy]; 1057 [m, ν(C–O)alcohols] cm–1.
2.2.2 [Mn(LH2)2] (1)
MnCl2·4H2O (0.019 g, 0.1 mmol) was added to a solution of LH4
(0.024 g, 0.1 mmol) in methanol (10 mL). After heating at 65 °C for
0.5 h triethylamine (0.013 mL, 0.129 mmol) was added to the brown
solution. The mixture was stirred and heated for 3.5 h and was finally
filtered. The crystallization occurred after 5 d. Approx. yield: 22.7%.
Brown crystals. Melting point (decomp.): 213.9 °C. C24H30MnN2O10:
calcd. C 51.2; H 5.34; N 4.98%; found: C 51.15; H 5.45; N 4.81%.
IR (KBr): 3419 [m, ν(O–H)alcohols]; 1617 [s, ν(C=N)imine]; 1441
[s, ν(C=C)ring]; 1171 [m,ν(C–O)methoxy]; 1037 [s, ν(C–O)alcohols] cm–1.
2.2.5 X-ray Diffraction
2.2.3 [Co2Cl(LH2)2(H2O)]·4H2O (2)
A Bruker CCD X8 APEX II diffractometer was used for the X-ray
structure analyses. The equipment was operated using graphite mono-
chromator and Mo-Kα radiation (λ = 0.71073 Å). The structures were
refined with full-matrix least-squares on F2 using all data (SHELX
crystal structure solution software). The structures were solved with
SHELXS[27] by direct methods. All non-hydrogen atoms were refined
with anisotropic displacement parameters with SHELXL.[27] The
hydrogen atoms were calculated in the idealized positions. More de-
tailed information about the structures determinations of 1–3 are given
in Table 1.
To a solution of LH4 (0.024 g, 0.1 mmol) in methanol (20 mL),
CoCl2·6H2O (0,023 g, 0.1 mmol) was added. After heating for 65 °C
and stirring for 4 h, the green solution was filtered. The green solution
turned brown after 1 d on ambient conditions and after 2 weeks orange
crystals formed in the solution. Approx. yield: 31.3% Melting point
(decomp.): 225.6 °C. C24H40ClCo2N2O15: calcd. C 38.4; H 5.33; N
3.73; found: C, 38.15; H, 5.44; N, 3.55%. IR (KBr): 3332 [m,
ν(O–H)alcohols]; 3142 [m, ν(O–H)alcohols]; 2934[w, ν(C–H)sp3)]; 1632
[s, ν(C=N)imine]; 1440 [s, ν(C=C)ring]; 1219 [s, ν(C–O)methoxy]; 1076
[m, ν(C–O)alcohols] cm–1.
Table 1. Crystal data and structure refinement for 1–3.
1
2
3
Empirical formula
Formula weight
C24H30MnN2O10
561.44
C24H40ClCo2N2O15
749.89
C12H17CuNO6
334.81
T /K
293(2)
100(2)
293(2)
Radiation, λ /Å
Crystal system, space group
Mo-Kα, 0.71073
tetragonal, P41212
Mo-Kα, 0.71073
monoclinic, Cc
M-Kα, 0.71073
tetragonal, I41/a
Unit cell dimensions
a /Å
b /Å
c /Å
α /°
8.1620(2)
8.1620(2)
37.4557(11)
90
11.6492(3)
13.2349(3)
19.6786(6)
90
18.7960(9)
18.7960(9)
15.507(2)
90
β /°
90
90
105.9760(10)
90
2916.79(13)
4, 1.708
1.306
1556
90
90
5478.6(8)
16, 1.624
1.619
2768
γ /°
V /Å3
2495.23(11)
4,1.495
0.590
Z, calculated density /g·cm–3
Absorption coefficient /mm–1
F(000)
1172
Crystal size /mm
θrange /°
Index ranges
0.48ϫ0.34ϫ0.22
2.17–27.09
–10 Յ h Յ 10,
–9 Յ k Յ 10,
–47 Յ l Յ 47
20345
2760 [Rint = 0.0423]
99.9%
Gaussian
0.8588 and 0.7089
full-matrix
least-squares on F2
2760 / 0 / 168
1.222
0.30ϫ0.29ϫ0.21
2.15–27.13
–14 Յ h Յ 14,
–16 Յ k Յ 16,
–24 Յ l Յ 25
21403
6240 [Rint = 0.0355]
99.6%
Gaussian
0.761 and 0.683
full-matrix
least-squares on F2
6240 / 9 / 399
1.008
0.62ϫ0.44ϫ0.40
1.70–27.09
–24 Յ h Յ 19,
–6 Յ k Յ 24,
–19 Յ l Յ 13
4501
2982[Rint = 0.0471]
105.5%
Gaussian
0.522 and 0.432
full-matrix
least-squares on F2
2982 / 0 / 177
0.886
Reflections collected
Reflections unique
Completeness to theta max.
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
R1 = 0.0467,
wR2 = 0.1323
R1 = 0.0536
wR2 = 0.1408
1.173 and –0.723
R1 = 0.0320,
wR2 = 0.0715
R1 = 0.0380,
wR2 = 0.0742
0.359 and –0.506
R1 = 0.0624,
wR2 = 0.1509
R1 = 0.0929,
wR2 = 0.1615
0.533 and –0.356
R indices (all data)
Largest diff. peak and hole /e·Å–3
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Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-1027788, CCDC-1027789, and CCDC-1027819
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk)
Instrumentation
UV/Vis Analysis: UV/Vis spectra were recorded with a Perkin-Elmer
Lambda 191 spectrometer, using acetonitrile solutions.
CV Analysis: Cyclic voltamograms were recorded with a Princeton
Applied Research (PAR) 273 system at room temperature in an argon
atmosphere, in dry acetonitrile (CH3CN). Electrochemical grade tet-
rabutyl ammonium hexafluorophosphate (TBAPF6) was used as sup-
porting electrolyte. A standard three-electrode system was employed
to carry out these experiments: a glassy carbon working electrode; a
platinum wire auxiliary electrode and a platinum wire pseudo-refer-
ence electrode. To monitor the reference electrode, the ferrocenium/
ferrocene redox couple was used as internal reference (E1/2 = 0.40 V
vs. NHE).[28]
Figure 2. Molecular structure of [Co2Cl(LH2)2(H2O)]·4H2O (2). For
clarity, the hydrogen atoms and the solvate molecules are omitted.
3 Results and Discussion
3.1 Crystal Structures
The
structures
of
complexes
[Mn(LH2)2]
(1),
[Co2Cl(LH2)2(H2O)]·4H2O (2), and [Cu4(LH2)4(H2O4)] (3) are
shown in Figure 1, Figure 2, and Figure 3, respectively. Fig-
ure 4 displays the core of tetramer 3. Selected bond lengths
and angles for the three compounds are listed in Table 2,
Table 3, and Table 4. In complex 1 the manganese(IV) ion
presents a distorted octahedral configuration, achieved through
a pair of NO2 donor atoms from two LH4 ligands: the two
phenoxo oxygen atoms (O1 and O1#) and the two oxygen
Figure 3. Molecular structure of [Cu4(LH2)4(H2O)4] (3), with atom
labelling for the asymmetric unit. For clarity, the hydrogen atoms are
are omitted.
atoms of deprotonated alcohol groups (O3 and O3#) occupy
the equatorial positions; two N atoms (N and N#) adopt a trans
arrangement. The oxygen atoms O4, O4# (“2-hydroxymethyl”)
and O5/O5# (“propan-1-ol”) retain their protons. A similar
structure to compound 1 was reported by Zhang et al.[29] in
which does not occur deprotonation of the “1,3-propanediol”
segment of LH4, remaining the manganese ion as MnII.
The molecular structure of the mixed valence cobalt com-
plex 2 (Figure 2) consists of a non-centrosymmetric dimer, in
which two crystallographically independent cobalt ions are
bridged by two deprotonated oxygen atoms of the alcohol
group. Both cobalt ions are hexa-coordinate with distorted oc-
tahedral arrangements. Moreover, the coordination spheres of
Figure 1. Molecular structure of [Mn(LH2)2] (1). For clarity, the
hydrogen atoms are omitted. Symmetry transformations used to gener-
ate equivalent atoms: (#1) y, x,-z+1.
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Table 3. Bond lengths /Å and angles /° for 2.
Bond lengths
Co2–O8
Co2–O4
Co2–O12
Co2–O5
Co2–O10
Co2–Cl
Co1–N2
Co1–N1
Co1–O6
Co1–O1
Co1–O8
Co1–O4
2.057(3)
2.065(3)
2.085(3)
2.152(3)
2.168(3)
2.3902(11)
1.881(3)
1.885(3)
1.897(2)
1.898(3)
1.901(3)
1.904(3)
Bond angles
O5–Co2–O10
N1–Co1–O8
O8–Co2–O4
O6–Co1–O8
O8–Co2–O12
O1–Co1–O8
O4–Co2–O12
N2–Co1–O4
O8–Co2–O5
N1–Co1–O4
O4–Co2–O5
O6–Co1–O4
O12–Co2–O5
O1–Co1–O4
O8–Co2–O10
O8–Co1–O4
O4–Co2–O10
Co1–O4–Co2
O12–Co2–O10
O8–Co2–Cl
O4–Co2–Cl
O12–Co2–Cl
O5–Co2–Cl
O10–Co2–Cl
N2–Co1–N1
N2–Co1–O6
N1–Co1–O6
N2–Co1–O1
N1–Co1–O1
O6–Co1–O1
N2–Co1–O8
Co1–O8–Co2
175.07(12)
90.82(12)
76.72(10)
176.55(12)
173.49(13)
92.41(12)
97.19(13)
91.40(12)
92.54(11)
85.99(13)
86.35(10)
92.89(12)
89.34(12)
176.21(12)
87.27(10)
84.50(11)
88.81(11)
99.20(13)
90.32(12)
96.05(8)
Figure 4. Details of the core of tetramer 3.Symmetry transformations
used to generate equivalent atoms: (Ј) –y+7/4, x–1/4, –z+7/4;
(ЈЈ) y+1/4, –x+7/4, –z+7/4; (ЈЈЈ) –x+4/2, –y+3/2, z.
Table 2. Bond lengths /Å and angles /° for 1.
Bond lengths
Mn–O3
Mn–O1
Mn–N
1.849(2)
1.931(2)
1.991(3)
Bond angles
O3–Mn–O3#1
O3–Mn–O1
O3#1–Mn–O1
O1–Mn–O1#1
O3–Mn–N
O3#1–Mn–N
O1–Mn–N
O1#1–Mn–N
N–Mn–N#1
94.87(16)
171.73(11)
90.21(10)
85.53(13)
84.05(11)
88.83(11)
89.53(10)
98.20(11)
169.49(17)
172.68(8)
90.07(10)
92.89(8)
92.04(9)
175.93(14)
96.54(13)
86.73(12)
86.18(12)
96.25(13)
90.28(12)
85.81(12)
99.58(13)
Symmetry transformations used to generate equivalent atoms: #1 y,
x, –z+1
both cobalt ions are different. The coordination environment
of the trivalent Co1 is formed by a pair of NO2 donor atoms
from two LH4 ligands: the two phenoxo oxygen atoms (O1
and O6), the two oxygen atoms of deprotonated alcohol group and (ЈЈЈ) –x+4/2, –y+3/2, z (see Figure 3 and Figure 4). The
(O4 and O8), and the two imine nitrogen atoms (N1 and N2). copper(II) ions achieve a distorted tetrahedral configuration,
The coordination sphere of divalent Co2 is occupied by two without metal–metal interaction. In the crystal lattice of 3, four
bridge-forming oxygen atoms (O1 and O6) from two LH4 li- oxygen from alcohol groups bridge (each) three copper atoms,
gands, two (protonated) oxygen atoms of alcohol groups from performing four threefold bridges. One (per copper atom) de-
two ligands (O5 and O10), a water molecule (O12), and one protonated phenoxo oxygen, four imine nitrogen atoms, and
chloride ion. The oxidation states of Co1 (III) and Co2 (II) four water molecules complete the distorted octahedral ar-
were assigned on the basis of charge balances and consider- rangement of the metal atoms. The Cu···OH2 distances are
ation of bond lengths (see Table 3). An analogue mixed val- 2.7861(1) Å, being on the frontier of a coordination bond,
ence complex of cobalt was already reported by Ke, Xie and since the sum of the Cu/O van der Waals radii is 2.92 Å.[30]
co-workers.[12]
Although the recommended bond valence parameter for the
The tetramer [Cu(LH2)(H2O)]4 (3) crystallizes in the bond O–Cu2+ is 1.679 Å,[31] the water molecules in 3 are ori-
tetragonal space group I41/a and the equivalent atoms can be ented toward the axial ligand orbitals of the Cu2+ ions, so that
generated from the asymmetric unit by the symmetry transfor- the axial angles with 159.096(2)° are not excessively deviated
mations (Ј) –y+7/4, x–1/4, –z+7/4, (ЈЈ) y+1/4, –x+7/4, –z+7/4 from the axial symmetry.
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Table 4. Bond lengths /Å and angles /° for 3.
Table 5. Redox potential data of studied compounds.
Bond lengths
Compound
E1oxid
E2oxid
E1red
Cu–O1
Cu–O2
Cu–N
Cu–O2’’
Cu–O2’’’
O2–Cu’
1.909(4)
1.9495(6)
1.959(6)
1.9710(8)
2.5635(1)
1.9710(8)
LH4 ligand
+1.35 Va)
+1.03 Va)
+1.00 Va)
+1.01Va)
–
–0.53 Vb)
CoIICoII complex
MnIV complex
CuII complex
+1.30 Va)
+1.30Va)
+1.24 Va)
–
–0,46 Vb)
–
a) Anodic peak. b) Cathodic peak.
Bond angles
to 2.20 V vs. NHE (Table 5). For all compounds, the Fc/Fc+
redox couple of ferrocene was used as the internal standard
(see in instrumentation section).
The aromatic tri-hydroxyl type ligand LH4 shows two
irreversible redox processes as illustrated in Figure 5A. First,
the anodic irreversible peak observed at Epa = +1.35 V can be
assigned to the one-electron oxidation of the Schiff base li-
gand. On the other hand, the only irreversible cathodic peak at
Epc = –0.53 V can be assigned to the monoelectronic reduction
of the Schiff base.[32]
O1–Cu–O2
O1–Cu–N
O2–Cu–N
O1–Cu–O2’’
O2–Cu–O2’’
N–Cu–O2’’
Cu–O2–Cu’
171.93(14)
94.0(2)
84.45(15)
94.88(14)
88.21(3)
165.92(16)
108.87(2)
Symmetry transformations used to generate equivalent atoms:
(‘) –y+7/4, x–1/4, –z+7/4; (‘‘) y+1/4, –x+7/4, –z+7/4;(‘‘‘)
–x+4/2, –y+3/2, z.
The cyclic voltammograms of the manganese complex 1
shows three processes as shown in Figure 5C. A nearly
irreversible reduction occurs at Epc = –0.46 V vs. NHE, which
can be assigned to the MnIV/MnIII redox couple. This rather
3.2 Electrochemical Behavior of Compounds 1–3
The electrochemical behaviors of compounds 1–3 were in- negative and irreversible reduction wave at –0.46 V for the
vestigated by cyclic voltammetry with a glassy carbon elec- MnIV/MnIII indicates a relatively stable MnIV complex, with
trode in a dry CH3CN solution in the range of 1.0ϫ10–3 to the electronic nature of the ligand having hydroxyl and imine
2.5ϫ10–3 m and 0.1 m TBAPF6 as the supporting electrode in groups; this is in agreement with the X-ray structure in this
an argon saturated atmosphere in the potential range –1.00 V work; the hydroxyl and imine groups are reported to be ef-
Figure 5. Cyclic voltammograms of (A) LH4 (B) Co complex, (C) Mn complex, and (D) Cu complex solution of dry acetonitrile, 0.1 m TBAPF6,
using glassy carbon electrode, at scan rate 25 to 200 mV·s–1.
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ficient to stabilizing the higher oxidation MnIV state, thus caus- LMCT, in agreement with the resolved X-ray structure of co-
ing a negative reduction potential for MnIV/MnIII species.[33,34] balt complex 2 (Co1–O1 and Co1–O6). No intervalence transi-
On the other hand, the first oxidation process at Epa = +1.00 V tion at lower energy was observed, thus suggesting that the
can be attributed to the MnIV/MnV redox couple, following to valences are also localized [Co(1)III; Co(2)II] when 2 is dis-
the second oxidation wave at Epa = +1.30 V, which can be solved in CH3CN solution.[12]
assigned to the ligand Schiff base type oxidation.[35]
In the case of tetranuclear CuII complex 3 was observed a
Cobalt complex 2 reveals by cyclic voltammetry two typical UV/Vis spectrum of a copper(II) complex. The lower
irreversible processes at 0.5 V to 2.00 V oxidation range (Fig- energy absorptions can be assigned to d-d transitions at 621 nm
ure 5B). The first anodic peak observed at Epa = +1.03 V can (log ε = 3.01) and are typical of CuII complexes in a highly
be assigned to the one-electron oxidation of the mixed-valence distorted octahedral environment, in agreement with the X-ray
CoIICoIII complex to the corresponding CoIIICoIII species, in structure of complex 3, which reveals that the central copper
which the Co2N2O4 moiety is coordinated by a dichelated li- atoms are in the oxidation state 2+.[36] The band at 375 nm
gand containing N-imine and O-phenolate donor atoms (log ε = 3.76) can be attributed to a charge-transfer
(Co2L).[12] The second oxidation process observed at Epa
+1.30 V can be assigned to the ligand oxidation Schiff base highly-intense remaining band at 279 nm (log ε = 4.23) is as-
type. signed to intraligand transition. At lower energy range (700–
=
Ophenolate Ǟ CuII LMCT transition while the one strong and
In the case of copper complex 3, two irreversible oxidation 900 nm) no intervalence transitions are observed in this cop-
redox waves are observed at 0.75 V to 1.50 V potential range per(II) complex type.
(Figure 5D). The anodic peak observed at Epa = +1.01 V can
be assigned to the one-electron oxidation CuII/CuIII redox cou-
ple. The second anodic peak at Epa = +1.24 V can be attributed
4 Conclusions
to the Schiff base ligand oxidation. These facts can be sup-
posed to the closed-packing central Cu atoms in the structure
and the electronic nature of Schiff base ligand type in the coor-
dination sphere (O-hydroxyl and N-imine donor atoms).[36]
The synthesis and the X-ray structural characterization of
three Schiff base complexes of MnIV, CoIICoIII, and CuII were
discussed, as well as IR and C, H, N analyses of the new
compounds. Experiments on cyclic voltammetry and UV/Vis
spectroscopy were also described to evaluate the redox poten-
tial of the title complexes, as part of preliminary studies, which
includes their use as mimics of the peroxidase in the reaction
of phenol with H2O2 in the presence of an indicator.
3.3 UV/Vis Spectroscopy
The electronic spectra of the ligand LH4 and their com-
plexes 1–3 measured in acetonitrile solution reveal the follow-
ing transitions listed in Table 6, respectively. The LH4 ligand
presents three absorption bands at 261, 329, and 420 nm. The
first high-energy transition can be assigned to the intraligand
πǞπ* transition, following to the nǞπ* C–N imine transition
at 329 nm and in 420 nm a band like charge-transfer transition,
refers to the phenolate group.[32]
Acknowledgements
This work was supported with funds from PRONEX-CNPq/FAPERGS
(Brazil).
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Received: November 25, 2014
Published Online: March 19, 2015
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© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim