Biomimetics of mononuclear and dinuclear Cu(II) and Fe(III) complexes of a newly synthesized piperazyl Mannich base with or without thiocyanate towards catechol

Article abstract of DOI:10.1007/s00706-018-2291-y

Abstract: This paper presents the synthesis and characterization of Cu(II) and Fe(III) complexes of a newly synthesized Mannich base N-[4-hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-yl]methyl]phenyl]acetamide. The spectro-analytical techniques employed includ

Full text of DOI:10.1007/s00706-018-2291-y

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-018-2291-y(0123456789(). -, vo Vl
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ORIGINAL PAPER
Biomimetics of mononuclear and dinuclear Cu(II) and Fe(III) complexes
of a newly synthesized piperazyl Mannich base
with or without thiocyanate towards catechol
Ayowole Olaolu Ayeni1,2
Gareth Mostyn Watkins1
•
Received: 22 February 2018 / Accepted: 14 September 2018
Ó Springer-Verlag GmbH Austria, part of Springer Nature 2018
Abstract
This paper presents the synthesis and characterization of Cu(II) and Fe(III) complexes of a newly synthesized Mannich
base N-[4-hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-yl]methyl]phenyl]acetamide. The spectro-analytical techniques
employed include 1H, 13C NMR, IR, UV spectroscopy, conductivity measurements, and elemental analysis. Both
mononuclear and dinuclear metal complexes were encountered in the study. Utilizing 3,5-di-tert-butyl catechol (3,5-
DTBC) as the substrate, catecholase activity of all four complexes was evaluated with the detailed kinetic studies reported.
The nature of the bonding of the thiocyanate is discussed based on the observation of mCN (2150–2050 cm-1). All the
metal complexes are catalytically active with turn over numbers ranging from 9.47 to 31.86 h-1 in DMF.
Graphical abstract
2.5
2
1.5
1
0.5
0
300
350
400
450
500
550
λ/nm
Keywords p-Acetamidophenol Á Aminomethylation Á Catechol oxidase Á Turnover Á 3,5-DTBC
Introduction
One of such area of researchis that of model coordination
compounds as metalloenzymes with oxidase (oxygenase)
activity and is of particular interest for the development of
bioinspired catalysts for oxidation reactions [3, 4]. In that
context, the type-3 dicopper enzyme catechol oxidase and
catechol dioxygenases use atmospheric dioxygen to achieve
the selective oxidation of catechols to ortho-quinones and
muconic acid, respectively. Metal complexes—Cu(II),
Fe(III), and Mn(II) particularly dinuclear ones are more
popular in this regard [5, 6].
Research into Mannich bases and their metal complexes
are expanding into new frontiers. This class of ligands are
usually NO or NNO donor, bidentate and tridentate
ligands, respectively, and possesses a variety of applica-
tions including biological and catalytic ones [1, 2].
& Ayowole Olaolu Ayeni
aayeni@oauife.edu.ng
Various sources of substrate have been employed in the
synthetic process including amongst others substituted
phenols and acetophenones as revealed in the literature [7].
Syntheses involving substituted acetamidophenols are very
1
Department of Chemistry, Rhodes University,
Grahamstown 6139, South Africa
2
Department of Chemistry, Obafemi Awolowo University,
Ile Ife 22005, Nigeria
123

A. O. Ayeni, G. M. Watkins
rare to come by in the literature and those reported are
listed herein [8–10]. We have reported on the synthesis and
characterization of new Mannich bases of cresols and
acetamidophenol as well as the catecholase activity of
Cu(II) and Fe(III) complexes, while taking into consider-
ation the role played by thiocyanate. The promising results
obtained and previously reported are a motivation for this
study [11–13] (Scheme 1).
Characterization of ligand and its complexes
NMR spectra
1
Data from the characterization of the novel ligand by H
and 13C NMR spectroscopy revealed the formation of a
mono-aminomethylated Mannich base. Notable diagnostic
peaks in the spectra of the ligand include the presence of
the aminomethylated hydrogens and carbons (denoted by
Hf and Cf) at 3.73 (s, 2H) and 61.7 ppm, respectively. The
presence of this signal has been used in distinguishing
mono- from bis-Mannich products [16, 17]. In addition,
two different environments are identified within the
piperazine moiety of the ligand; 2.67 ppm (t, 4H) and
3.57 ppm (t, 4H) for protons and 45.0, 52.2 ppm for the
carbons. Aromatic signals are within (6.63–8.20) ppm and
(107.2–168.2) ppm for hydrogen and carbon atoms,
respectively.
To the best of our knowledge, the several desirable
properties of Mannich bases and their metal complexes
make acetamidophenol-Mannich bases an untapped mine
field. We now, in this study report the synthesis and
characterization of a new Mannich base N-[4-hydroxy-3-
[[4-(pyridin-2-yl)piperazin-1-yl]methyl]phenyl]acetamide
alongside its Cu(II) and Fe(III) complexes. In addition, the
detailed kinetic study of the catecholase activity of all the
metal complexes and the bonding modes of thiocyanate
with its impact on catalytic activity are also reported.
Infrared spectra
Results and discussion
The Mannich base displayed a medium intense band at
3354 cm-1 attributed to the stretching mode of the
hydroxyl group in the ligand. Shifts to lower frequencies
were observed in all the metal complexes indicating the
coordination through the hydroxyl group with the ligand
not deprotonated [18]; however, the shifts were more
pronounced in the thiocyanate complexes. The extent of
the shifts ranged from 165–197 cm-1 in 1 and 3 to
235–274 cm-1 in 2 and 4. Support for the coordination of
the hydroxyl group can be obtained by considering the mC-
O (sharp intense band) as well which shifted a little sig-
nificantly from 1249 cm-1 by 4 cm-1 downward in 1 and
upward in 3 by 6 cm-1 with all the bands appearing
broader. Cases of upward shifts have been previously
reported upon complexation without deprotonation for
some lanthanide complexes of Schiff bases of aniline [19]
and have to do with the impact of complexation on
hydrogen bonding within the ligand. The mCN of the pyr-
idine ring of the ligand shifts downwards from 1657 cm-1
to within 1641–1652 cm-1 in the metal complexes, which
is an evidence to show the involvement of the pyridine
ring of the ligand in complexation with the central metal
ion as previously reported in the literature [20, 21].
In this work, a new Mannich base N-[4-hydroxy-3-[[4-
(pyridin-2-yl)piperazin-1-yl]methyl]phenyl]acetamide has
been synthesized alongside its Cu(II) and Fe(III) com-
plexes. The synthesis of the thiocyanate analogs of the
metal complexes is equally reported. All the synthesized
compounds were characterized by a range of analytical and
spectroscopic techniques including elemental analyses, FT-
IR, UV–Vis, 1H and 13C NMR spectroscopy. All the metal
complexes are colored, air stable, and insoluble in most
solvents except DMF and DMSO.
The molar conductivities (Km) of 1 9 10-3 M solutions
of the complexes in DMSO were measured at 25 °C. The
molar conductance values of the complexes are in the range
17.88–83.30 X-1 cm2 mol-1. From these observed values
of Km, complexes 2 and 4 are considered as 1:1 electrolyte
solutions while 1 and 3 are non-electrolytes [14, 15].
Complexes 1 and 3 are mononuclear with metal–ligand
ratio of 1:1, while the thiocyanate containing metal com-
plexes 2 and 4 are dinuclear with metal–ligand ratio of 2:1.
The data on the characterization of the synthesized
ligand and its transition metal complexes are reported in
Table 1.
Two bands assigned to the stretching modes of the two
mCNC of the piperazine within the ligand are designated as
m1 = 1213 cm-1 and m2 = 1142 cm-1. Considerable low-
ering to within 1180–1162 cm-1 and 1129–1062 cm-1 is
observed for m1 and m2, respectively, with the effect being
most pronounced in complex 2. This observation is evi-
dence of both N-atoms of the piperazine moiety of the
ligand being involved in the complexation [22].
Scheme 1
123

Biomimetics of mononuclear and dinuclear Cu(II) and Fe(III) complexes…
Table 1 Data on the characterization of the synthesized ligand and its transition metal complexes
Complex/ligand
d-
d transition
Assignment
K/
Geometry
M/
g mol-1
Yield/ Color
%
X-1 cm2 mol-1
2
1 [Cu(HL)Cl2]•4H2O
2 [Cu2(HL)(NCS)2(H2O)2]Cl2•2H2O
3 [Fe(HL)Cl3]•6H2O
741a, 733b
687a, 775b
496a
2Eg ? T2g
17.88
78.56
44.16
Octahedral
532.94
53
56
63
Brown
2
2Eg ? T2g
Square Planar 711.54
Light brown
Dark brown
CT from UV
to Vis region
Octahedral
576.80
875.28
326.38
4 Fe2(HL)(NCS)2(H2O)2Cl2]Cl2•6H2O 472b
CT from UV
to Vis region
83.30
Octahedral
45
52
Brown
White
HL
–
CT charge transfer
aDMF
bDMSO
Bonding of the thiocyanate to the central metal ion was
confirmed by the observation of strongly intense bands at
2140 cm-1 (split) in the copper(II) complex and
2051 cm-1 (broad) in the iron(III) complex. These bands
support the following bonding modes: S-bonding and
N-bonding in the copper and iron complexes, respectively
[23, 24]. The bonding modes are supported by the mCS at
868 cm-1 and 737 cm-1 for 2 and 4, respectively, as
reported in the literature [25, 26].
overshadow the d–d transitions in Fe(III) complexes, thus
making conclusions on geometry quite difficult. The pro-
posed structures of the metal complexes are given in Fig. 1.
Catecholase activity
All the metal complexes showed significant catalytic oxi-
dation activity towards 3,5-di-tert-butylcatechol (3,5-
DTBC). This substrate with bulky substituents on the ring
has low quinone–catechol reduction potential, makes it
widely used and is easily oxidized to the corresponding o-
quinone 3,5-DTBQ [32–34], which is highly stable and
showed maximum absorption at 399 nm in DMF.
Electronic spectra
The ligand has a maximum absorption (kmax) at 299 and
289 nm in DMF and DMSO, respectively, attributed to the
n–p* transition [27]. Complex 1 only displays a single d–d
transition at 741 nm in DMF typical of an octahedral
geometry; a similar transition is observed and equally
assigned in DMSO at 733 nm alongside a ligand–metal
charge transfer transition at 454 nm. Complex 2 showed a
single transition at 687 nm attributed to a 4-coordinate
geometry in the coordination sphere with a ligand–metal
charge transfer transition typical of thiocyanates
being observed at 427 nm. In DMSO, the d–d transition
The product 3,5-DTBQ was isolated in moderate yield
(averagely 55.3%) by the slow evaporation of the eluent
1
1
and was identified by H NMR spectroscopy: H NMR
(CDCl3, 300 MHz): d = 1.22 (s, 9H), 1.27 (s, 9H), 6.22 (d,
J = 3.0 Hz, 1H), 6.92 (d, J = 3.0 Hz, 1H) ppm. These
values of chemical shifts are similar to those earlier
reported by Mitra et al. [35]. Figure 2 shows the spectral
change for 2 upon addition of 100-fold 3,5-DTBC
(1 9 10-2 M) observed at an interval of 5 min in DMF.
All the complexes showed saturation kinetics and a treat-
ment based on the Michaelis–Menten model seemed
appropriate. The binding constant (KM), maximum velocity
(Vmax), and rate constant for dissociation of substrates (i.e.,
turnover number, kcat) were calculated for all the com-
plexes using the Lineweaver–Burk graph.
2
observed for 34 at 775 nm is attributed to 2Eg ? T2g
which may suggest an octahedral configuration upon
coordination with the solvent [1].
Complexes 3 and 4 display medium–intense bands at
496, 472 nm in DMF, respectively, assigned to the d–
d transitions and the transitions at 370 and 350 nm for 3
and 4, respectively, in DMSO is assigned to the charge
transfer transition peculiar to a Fe3? ion in an octahedral
configuration [28]. Viswanathan et al. reported that the
electronic spectra of all the iron(III) phenolate complexes
they studied exhibit an intense band in the visible region
(490–550 nm) [5]. This band is assigned to the phenolate
(p1)–Fe(III)(dp) charge transfer transition. Similar obser-
vations are well documented in the literature [29–31]. In
general, the charge transfer bands are known to
The plot of log A!/(A! - At) vs time for all the metal
complex in the presence of 100 equiv. of 3,5-DTBC is
displayed in Fig. 3 above, while Fig. 4 is a plot of the
initial rates versus substrate concentration for the oxidation
of 3,5-DTBC catalyzed by the metal complexes.
The kinetic parameters obtained from the Lineweaver–
Burk plots (Fig. 5) are listed in Table 2. All the metal
complexes show moderately high turnover rates ranging
from 9.47 0.52 to 31.86 1.20 h-1 with the order of
activity as 3 [ 1 [ 4 [ 2. These values are comparable to
123

A. O. Ayeni, G. M. Watkins
1.2
1
1
2
3
4
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
35
Time/min
Fig. 3 A plot of the difference in absorbance vs. time to evaluate the
initial rate of the catalytic oxidation of 3,5-DTBC by 1–4 in DMF
those previously reported in the literature [36, 37] and
considering the fact that rates are lower in coordinating
solvents like DMF owing to the retarded formation of
complex–substrate adduct [38], higher turnover rates are
likely with less coordinating solvent like methanol. How-
ever, complexes 1 and 3 showed higher turnover rates than
those of similar complexes of phenolic Mannich bases
(\ 23 h-1) [39, 40] though lower than the dinuclear ones
(about 78 h-1) possessing better leaving groups like chlo-
rides [41, 42].
Complex 3 exhibited the highest turnover rate amongst
all the Cu(II) and Fe(III) complexes. As the iron(III)
complexes appear to be better catalysts compared to their
copper(II) counterparts, we suggest that the Fe(III)/Fe(II)
reduction process appear more favorable than the Cu(II)/
Cu(I) process particularly in the presence of a more posi-
tively inductive acetamido group.
As previously reported, the non-thiocyanate metal com-
plexes outshine their counterparts in catecholase activity.
The lowering of catecholase activity of metal complexes by
the inclusion of thiocyanate within their coordination sphere
as previously reported by Ramadan et al. [43] and Pajan [44]
is attributed to the poor dissociation (weak leaving group)
of the thiocyanate to create a dissociation results in the
negative impact on catecholase activity. Reports that the
nature of the exogenous bridging ligand in the dimeric
complexes also shows a remarkable influence on the
activity, owing to the fact that a weakly coordinating ligand
may easily be displaced by the incoming catechol, thus
enhancing the activity may be applicable here [45].
Insight into the mechanism of the catalytic reaction
involving the Cu(II) and Fe(III) complexes was gained by
the identification of the presence of H2O2. Iodimetric
titration as performed led to the observation of the iodide
ion as a product of oxidation by hydrogen peroxide. The
Fig. 1 Proposed structures of metal complexes 1–4
2.5
2
1.5
1
0.5
0
300
350
400
450
500
550
λ/nm
Fig. 2 Changes observed in UV–Vis spectra of complex
2
(1 9 10-4 M) in DMF medium upon the addition of 100-fold 3,5-
DTBC (1 9 10-2 M)
123

Biomimetics of mononuclear and dinuclear Cu(II) and Fe(III) complexes…
1
0.030
0.025
0.020
0.015
0.010
0.005
0.000
2
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0.000
0.002
0.004
0.006
0.008
0.010
3
0.000
0.002
0.004
0.006
0.008
0.010
4
[Substrate]/M
[Substrate]/M
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0.05
0.04
0.03
0.02
0.01
0.00
0.000
0.002
0.004
0.006
0.008
0.010
0.000
0.002
0.004
0.006
0.008
0.010
[Substrate]/M
[Substrate]/M
Fig. 4 Plots of the initial rates versus substrate concentration for the oxidation of 3,5-DTBC catalyzed by complexes 1–4
6000000
Conclusion
5000000
The newly synthesized and characterized Mannich mono-
base N-[4-hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-
1
2
3
4
4000000
3000000
2000000
1000000
0
yl]methyl]phenyl]acetamide acts as a tetradentate ligand
with Cu(II) and Fe(III) ions to afford mono- and binuclear
complexes. The coordination and geometry around the
metal center were elucidated by spectroscopic studies and
molar conductivity measurements of all the complexes.
Mono- and binuclear complexes exhibit tetrahedral or
octahedral geometries. Both SCN- and NCS-bonding
modes were identified through IR spectroscopy in the
study. The synthesized metal complexes were evaluated for
their catecholase activity and the results showed that they
were all catalytically active towards the oxidation of cat-
echol to the corresponding o-quinone. Finally, it was
observed that the presence of thiocyanate hinders the ease
0
100
200
300
400
500
600
1/[Substrate]/M-1
Fig. 5 Lineweaver–Burk plots for complexes 1–4
reaction pathway that involves the formation of
semiquinolate intermediate can, therefore, be proposed.
Table 2 Kinetics parameters for
Complex
Vmax/M s-1
Km/M
kcat/h-1
the oxidation of 3,5-DTBC
catalyzed by metal complexes
1–4
1
2
3
4
(7.84 0.44) 9 10-7
(2.63 0.11) 9 10-7
(8.85 0.62) 9 10-7
(5.48 0.12) 9 10-7
(4.03 0.22) 9 10-4
(8.96 0.28) 9 10-4
(8.25 0.49) 9 10-3
(6.82 0.31) 9 10-4
28.22 1.11
9.47 0.52
31.86 1.20
19.73 0.89
123

A. O. Ayeni, G. M. Watkins
299 nm; UV–Vis (DMSO, c = 10-3 mol dm-3):
k = 289 nm.
of complex–substrate adduct formation and, thus, leads to a
reduced turnover rate in the thiocyanate containing binu-
clear complexes.
Synthesis of transition metal complexes
Experimental
The Cu(II) and Fe(III) complexes were prepared by the
following general procedure: 5 mmol of the ligand dis-
solved in 5 cm3 of chloroform was added to an equivalent
amount of the metal salt dissolved in methanol and stirred
at room temperature for 6 h. The synthesis of the thio-
cyanate complexes involved dissolving an equivalent
amount of KSCN in methanol and added to the chloroform
solution of the ligand followed by the addition of the
solution of the metal salt. The desired products were usu-
ally obtained as precipitates after allowing the reaction
mixture to stand, once the reaction is completed. This was
collected and washed with methanol–chloroform mixture
and dried in a desiccator.
Formaldehyde
solution,
p-acetamidophenol,
1-(2-
pyridyl)piperazine, potassium thiocyanate, and 3,5-di-tert-
butylcatechol were purchased from Sigma Aldrich and
used as such. Methanol, isopropanol, and chloroform were
purchased from Sigma Aldrich, are of analytical grade and
used as received without purification. The metal salts were
in the hydrated form, i.e., CuCl2•2H2O (copper(II) chloride
dihydrate) and FeCl3•6H2O (iron(III) chloride hexahy-
drate). Elemental analysis (C, H and N) was obtained using
Elementar Analysensysteme VarioMICRO V1.62 GmbH
analysis System. NMR spectra (1H and 13C NMR) were
acquired in CDCl3 using Bruker AMX 300 MHz spec-
trometer with tetramethylsilane (TMS) as an internal
(N,N,N,O)-[N-[4-Hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-
yl]methyl]phenyl]acetamido]copper(II) chloride tetrahy-
drate ([Cu(HL)Cl2]•4H2O, 1, C18H30Cl2CuN4O6) A solution
of CuCl2•2H2O (5.00 mmol) in 5 cm3 methanol was added
gradually to a solution of the ligand (5.00 mmol) in 5 cm3
chloroform. The reaction mixture was stirred at room
temperature for 6 h. The desired product was obtained as
precipitate after allowing the reaction mixture to stand,
once the reaction is completed. This was collected and
washed with methanol–chloroform mixture and dried in a
desiccator. Yield 1.42 g (53%) of a brown powder. M.p.:
169 °C; IR (ATR-FTIR): m ¼ 3337 (OH), 1641
1
standard for H. Attenuated total reflection Fourier trans-
form infrared (ATR-FTIR) spectra for all the samples were
recorded on a PerkinElmer Spectrum400 spectrophotome-
ter in the range 4000–650 cm-1. Electronic spectra were
recorded for the solutions of the synthesized compounds in
DMF and DMSO on a Perkin Elmer UV–Vis spectropho-
tometer model Lamba 25. Molar conductivities were
measured in DMSO using a 10-3 M solution on AZ 86555
conductivity meter. The melting points were determined on
a Gallenkamp melting point apparatus.
N-[4-Hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-
(C=Npyridine), 1248 (C–O), 1162 and 1120 (C–N–C) cm-1
;
yl]methyl]phenyl]acetamide (HL, C18H22N4O2) 10 mmol
each of 1-(2-pyridyl)piperazine and paraformaldehyde
were stirred in 10 cm3 of isopropyl alcohol for about
30 min over a steam bath followed by the addition of
equimolar quantity of p-acetamidophenol dissolved in the
same solvent. Reflux was continued for another 4 h with
the reaction being monitored by TLC. Standing the reac-
tion mixture for few days at completion led to the forma-
tion of white precipitates which were collected and washed
with ethanol. Recrystallization was carried out in chloro-
form/ethanol mixture to give 1.70 g (52%) white crys-
talline solid of the title substance (Scheme 2). M.p.:
179 °C; 1H NMR (300 MHz, CDCl3): d = 8.19 (d, 1H, m),
7.49 (dd, 1H, c), 7.35 (t, 1H, k), 7.18 (s, 1H, b), 7.09 (dd,
1H, e), 6.79 (d, 1H, j), 6.67 (t, 1H, l), 6.63 (d, 1H, d); 3.73
(s, 2H, f), 3.57 (t, 4H, i, i’), 2.67 (t, 4H, h, h’), 2.17 (s, 3H,
a) ppm; 13C NMR (75 MHz, CDCl3): d = 168.2 (s), 159.3
(s), 154.5 (s), 147.9 (s), 137.60 (s), 129.7 (s), 121.5 (s),
121.2 (s), 116.3 (s), 113.8 (s), 107.8 (s), 61.4 (s), 52.3 (s),
45.0 (s), 24.6 (s) ppm; IR (ATR-FTIR): m ¼ 3534 (OH),
1657 (C=Npyridine), 1249 (C–O), 1213 and 1142 (C–N–C)
cm-1; UV–Vis (DMF, c = 10-3 mol dm-3): k = 270,
UV–Vis (DMF, c = 10-3 mol dm-3): k = 278, 307,
741 nm; UV–Vis (DMSO, c = 10-3 mol dm-3): k = 313,
454, 733 nm; KM = 17.88 X-1 cm2 mol-1
.
(N,N,N,O)-[N-[4-Hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-
yl]methyl]phenyl]acetamido](diaqua)(dithiocyanato)dicop-
per(II) dichloride dihydrate ([Cu2(HL)(NCS)2(H2O)2]Cl2•2H2O,
2, C20H22Cl4Cu2N6O2S2) Compound 2 was prepared in the
same way as 1 with the addition of KSCN (5.00 mmol) in
1 cm3 methanol. Yield 2.01 g (56%) of a light brown
powder. M.p.: [ 250 °C; IR (ATR-FTIR): m ¼ 3299
(OH), 2173, 2156 (CNthiocyanate), 1642 (C=Npyridine), 1244
(C–O), 1180 and 1062 (C–N–C), 868 (CS) cm-1; UV–Vis
(DMF, c = 10-3 mol dm-3): k = 260, 319, 427, 687 nm;
UV–Vis (DMSO, c = 10-3 mol dm-3): k = 269, 324,
775 nm; KM = 78.56 X-1 cm2 mol-1
.
(N,N,N,O)-[N-[4-Hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-
yl]methyl]phenyl]acetamido]iron(III) chloride hexahydrate
[Fe(HL)Cl3]•6H2O, 3, C18H34Cl3FeN4O8) Compound 3 was
prepared in the same way as 1 using FeCl3•6H2O
(5.00 mmol) as starting material. Yield 1.81 g (63%) of a
123

Biomimetics of mononuclear and dinuclear Cu(II) and Fe(III) complexes…
Scheme 2
dark brown powder. M.p.: 123 °C; IR (ATR-FTIR):
(OH), 1638 (C=Npyridine), 1265 (C–O), 1162 and 1112 (C–
monitoring the increase in the absorbance of the product
3,5-DTBQ using 20–100 equivalents of 3,5-DTBC with
10-4 M solutions of the metal complexes.
N–C) cm-1
;
UV–Vis (DMF, c = 10-3 mol dm-3):
k = 269, 304, 496 nm; UV–Vis (DMSO, c = 10-3
mol dm-3):
k = 287, 370 nm;
KM = 44.16 X-1 cm2 mol-1
Kinetic experiments were conducted in duplicates with
20–100 equiv. of 3,5-DTBC. To determine if hydrogen
peroxide was formed as the reduced oxygen species during
the 3,5-DTBC oxidation, reaction mixtures were prepared
as described above in the kinetic experiments. After 1 h of
reaction, an equal volume of water was added and the
quinone formed was extracted three times with dichlor-
omethane. The aqueous layer was acidified with H2SO4 to
pH = 2 to stop further oxidation, followed by the addition
of 1 cm3 of a 10% solution of KI and three drops of 3%
solution of ammonium molybdate. In the presence of
hydrogen peroxide, the following reaction occurs:
.
(N,N,N,O)-[N-[4-Hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-
yl]methyl]phenyl]acetamido](aqua)dichlorodithiocyana-
todiiron(III) chloride hexahydrate) [Fe2(HL)(NCS)2(H2O)2-
Cl2]Cl2•6H2O, 4, C20H34Cl6Fe2N6O8S2) Compound 4 was
prepared in the same way as 3 with the addition of KSCN
(5.00 mmol) in 1 cm3 methanol. Yield 1.98 g (45%) of a
brown powder. M.p.: 182 °C; IR (ATR-FTIR): m ¼ 3260
(OH), 2051 (CNthiocyanate), 1652 (C=Npyridine), 1251 (C–O),
1161 and 1129 (C–N–C), 737 (CS) cm-1; UV–Vis (DMF,
c = 10-3 mol dm-3): k = 272, 302, 472 nm; UV–Vis
(DMSO, c = 10-3 mol dm-3): k = 291, 350 nm;
H2O2 þ 2IÀ þ 2Hþ ! 2H2O þ I2
and with an excess of iodide ions, the triiodide ion is
formed according to the reaction
KM = 83.30 X-1 cm2 mol-1
.
I2ðaqÞ þ IÀ ! I3À
:
Kinetic assays
The increase in the I3- absorption band at 353 nm
(e = 26,000 M-1 cm-1) was monitored by UV–Vis spec-
troscopy [46, 47]. The isolation of the product 3,5-di-tert-
butylquinone (3,5-DTBQ) was achieved and identified by
1H NMR spectroscopy.
Before proceeding into the detailed kinetic study, the
ability of the Cu(II) and Fe(III) complexes to oxidize 3,5-
DTBC was evaluated. For this purpose, 1 9 10-4
mol dm-3 solutions of complexes 1–4 were treated with
1 9 10-2 mol dm-3 (100 equivalents) of 3,5-DTBC under
aerobic condition. The course of the reaction was followed
by UV–Vis spectroscopy. Time-dependent UV–Vis spec-
tral scan was performed in pure DMF. All the complexes
behaved similarly, showing a smooth conversion of 3,5-
DTBC into 3,5-DTBQ which was purified by column
chromatography. The kinetics of the oxidation of 3,5-
DTBC oxidation was determined at room temperature by
Acknowledgements One of the authors (Ayeni Olaolu Ayowole)
thanks the Tertiary Education Trust Fund (TETFund, Nigeria) for
providing a Research Sponsorship and Rhodes University for aca-
demic bursary.
123

A. O. Ayeni, G. M. Watkins
27. Al-Jeboori MJ, Abdul-Ghani AJ, Al-Karawi AJ (2008) Transit
Met Chem 33:925
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