Synthesis, physical and electrochemical characterization of mono- and heterobinuclear m-functionalized iron(III) schiff base complexes

Article abstract of DOI:10.1007/s11243-012-9606-3

Mo(NO)T_p * Cl₂ (T_p * = 3,5-dimethyl pyrazole) when reacted with m-functionalized Fe(III) Schiff base complexes; the Schiff base ligands being derived from condensation of 2,4-dihydroxybenzaldehyde or salicylaldehyde with a variety of x,Ω-diamines [1,2-C₆H₄(NH₂)₂, NH₂(CH₂)nNH₂; n = 2-4] affords bimetallic complexes containing two potential reduction centers. The compounds were characterized by physicochemical and spectroscopic methods. It is shown that as the polymethylene carbon chain of the Schiff base backbone increases, the physicochemical and spectroscopic properties also change gradually. Electrochemical data show that the m-functionalized complexes reduce at potentials less cathodic than their psubstituted analogues. It is also shown that the redox potentials are solvent dependent. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11243-012-9606-3

Transition Met Chem (2012) 37:431–437
DOI 10.1007/s11243-012-9606-3
Synthesis, physical and electrochemical characterization
of mono- and heterobinuclear m-functionalized iron(III)
Schiff base complexes
•
Ruth A. Odhiambo Gerald K. Muthakia
•
Stanley M. Kagwanja
Received: 13 June 2011 / Accepted: 2 April 2012 / Published online: 21 April 2012
ꢀ Springer Science+Business Media B.V. 2012
Abstract Mo(NO)Tp*Cl2 (T*p = 3,5-dimethyl pyrazole)
when reacted with m-functionalized Fe(III) Schiff base
complexes; the Schiff base ligands being derived from
condensation of 2,4-dihydroxybenzaldehyde or salicylal-
dehyde with a variety of a,x-diamines [1,2-C6H4(NH2)2,
NH2(CH2)nNH2; n = 2–4] affords bimetallic complexes
containing two potential reduction centers. The compounds
were characterized by physicochemical and spectroscopic
methods. It is shown that as the polymethylene carbon
chain of the Schiff base backbone increases, the physico-
chemical and spectroscopic properties also change gradu-
ally. Electrochemical data show that the m-functionalized
complexes reduce at potentials less cathodic than their p-
substituted analogues. It is also shown that the redox
potentials are solvent dependent.
application in areas such as catalysis [2], various biological
systems [3, 4], polymers [5] and dyes industry [6] has been
reviewed by Kumar and coworkers [7]. Schiff base metal
complexes have been extensively studied due to their
synthetic flexibility, selectivity and sensitivity toward the
central metal atom. The ability to bridge metal centers
allows for the synthesis of discrete, heterocycle-bridged
polynuclear complexes which have been shown to possess
interesting and unusual electrochemical or magnetic
properties [8, 9] because the bridge can mediate exchange
between the metal centers. Previous studies on the syn-
thesis, characterization and electrochemical behavior of
poly-functionalized Schiff base metal complexes of types
L1 and L2 where various 3d metal atoms are linked to
Mo(NO)T*pCl metal fragment centers through Schiff base
ligands have shown that there are weak electronic inter-
actions between the molybdenum centers and the metal
atoms through the bridging ligands [10–13] (Figs. 1, 2).
As a part of our continuing work on the coordination
properties of heterobinuclear complexes linked by asym-
metric tetradentate Schiff base ligands, we herein report the
synthesis, characterization and electrochemistry of com-
plexes of type L3 in which the peripheral redox active
molybdenum nitrosyl functionality is attached to the Schiff
base ligands at the m-position (Fig. 3).
Introduction
In recent years, there has been considerable interest in the
coordination chemistry of Schiff bases with various metal
ions. This is partly due to their capability to act as mul-
tidentate N–N and N–O donors which results in the for-
mation of mono- or polynuclear complexes [1]. Their wide
R. A. Odhiambo (&)
Department of Chemistry, University of Nairobi,
P.O. Box 30197, Nairobi 00100, Kenya
e-mail: raodhiambo@gmail.com
Experimental
Reagents
G. K. Muthakia
Department of Chemistry, Kenyatta University,
P.O. Box 43844, Nairobi 00200, Kenya
All the reagents with an exception of Mo(NO)T*pCl2 were
obtained from Sigma Aldrich Chemical Company Limited
(UK) and used without further purification. The compound
Mo(NO)T*pCl2 was prepared according to the literature
S. M. Kagwanja
Department of Chemistry, Chuka University College,
P.O. Box 109, Chuka 60400, Kenya
123

432
Transition Met Chem (2012) 37:431–437
method, although with slight modifications [14]. All the
solvents used were of analar grade. Whenever required, the
solvents were dried according to the standard literature
procedures [15]. Iron(III) chloride was dried in a desiccator
over phosphorous(V) oxide for 24 h before use. All the
synthetic reactions except those involving the preparation
of Schiff base ligands and their iron(III) mononuclear
derivatives were carried out under nitrogen. All the bime-
tallic complexes were purified through column chroma-
tography using silica gel 60 (70–230 mesh). The purity of
the synthesized compounds was checked by melting point
and TLC techniques.
were recorded in ethanol or DMSO solution (10-3 M) on a
Hitachi U2000 UV/Vis spectrophotometer with
190–1,100 nm wavelength range. IR spectra were recorded
as KBr disks in a Shimadzu FTIR-8400 spectrometer with
1
a range of 4,000–250 cm-1. H NMR spectra were recor-
ded on Varian Mercury 200 MHz NMR spectrometer in
DMSO-d6 solvent, using TMS as an internal standard.
Electrochemical data were obtained with a Metrohm/Eco
Chemie Autolab PGSTAT12 Potentiostat/Galvanostat with
glassy carbon as the working electrode, platinum wire as
the counter electrode and Ag/AgCl as the reference elec-
trode. Solutions in MeCN, DMSO and CH2Cl2 were ca.
1 9 10-3 mol dm-3 in the complex with 0.1 mol dm-3
[n-Bu4N][PF6] as the base electrolyte. Cyclic and differ-
ential pulse voltammetric measurements were taken at scan
rates of 200 and 20 mV/s, respectively. All formal reduc-
tion potentials were taken as an average of the anodic and
cathodic potentials.
Instrumentation
Carbon, hydrogen and nitrogen contents were determined
microanalytically on an elemental analyzer model vario
EL3 (Elementar Analysensysteme GmbH). Mass spectra
were obtained using a Micromass/Waters LCT Mass
Spectrometer while electronic spectra of the compounds
Syntheses
Schiff base ligand [L3, B = C6H4, M = H2, M0 = H]
B'
A solution of o-phenylenediamine (1.2 g, 10.9 mmol) in
ethanol (10 ml) was added dropwise to a mixed solution of
2,4-dihydroxybenzaldehyde (1.5 g, 10.9 mmol) and sali-
cylaldehyde (1.2 ml, 10.9 mmol) in ethanol (30 ml). The
mixture was refluxed for 2 h, filtered while hot, and the
solvent evaporated in vacuo. A red oily product obtained
was first triturated with diethyl ether, thoroughly washed
with diethyl ether and then dried yielding a red solid (yield,
3.0 g, 84 %). The remaining Schiff base ligands [L3,
B = (CH2)n; n = 2–4, M = H2, M0 = H] were prepared in
a similar manner, replacing the o-phenylenediamine with
the appropriate diamine. The solids obtained were then
used in the subsequent preparation of the iron(III) Schiff
base precursors.
N
O
N
O
M
M'O
OM'
L1
Fig. 1 B0 = C6H4, (CH2)n; n = 2, 3, 4, 5; M = Cu, Ni or Pd;
M0 = Mo(NO)T*pCl
B
N
O
N
O
M
M'O
L2
Iron(III) Schiff base precursor [L3, B = C6H4, M = Fe,
M0 = H]
Fig. 2 B = C6H4 or (CH2)n; n = 2–4, M = H2 or Fe; M0 = H or
Mo(NO)T*pCl
A solution of iron(III) chloride (0.4 g, 2.6 mmol) in etha-
nol (30 ml) was added dropwise to a solution of Schiff base
[L3, B = C6H4, M = H2, M0 = H] (0.9 g, 2.6 mmol) in
ethanol (30 ml). The red solution formed on shaking was
refluxed for 48 h to drive the reaction to completion. The
solvent was evaporated in vacuo, the solid formed on
concentration washed with ethanol (3 9 50 ml), and die-
thyl ether (3 9 50 ml) then dry-evaporated to provide a
brown solid (yield; 0.3 g, 32 %). The remaining iron(III)
Schiff base precursors were prepared in a similar manner
by reacting the appropriate Schiff base ligand and iron(III)
chloride, and they gave comparable yields.
B
N
O
N
O
M
M'O
L3
Fig. 3 B = C6H4 or (CH2)n; n = 2–4, M = H2 or Fe; M0 = H or
Mo(NO)T*pCl
123

Transition Met Chem (2012) 37:431–437
433
Table 1 Elemental, physical and mass spectral data [L3, B = C6H4 or (CH2)n; n = 2–4, M = Fe, M0 = H or Mo(NO)Tp*Cl]
Compound
Yield (%) Solventa
Found (Calculated) %
MS data Conductance X-1 cm2 mol-1
M’
B
M
C
H
N
[M]?–Cl DMF
DMSO
MeCN
H
H
H
H
C6H4
Fe 31.90
0
0
0
0
57.1 (56.9) 3.9 (3.3)
51.6 (51.4) 3.8 (3.7)
52.7 (52.6) 4.3 (4.1)
53.8 (53.7) 4.6 (4.5)
6.7 (6.6)
386
337
352
366
11.78
19.07
17.42
14.86
20.22
42.07
30.22
32.61
17.97
25.53
38.45
13.79
12.21
13.34
19.22
12.81
3.91
15.29
12.42
4.28
(CH2)2 Fe 39.80
(CH2)3 Fe 44.80
(CH2)4 Fe 49.70
7.4 (7.5)
7.3 (7.2)
7.1 (7.0)
Mo(NO)T*pCl C6H4
Fe 23.50
0.5 C6H14 49.6 (49.4) 5.9 (5.1) 13.8 (13.7)
0.5 C6H14 46.8 (46.5) 5.4 (5.2) 14.3 (14.4)
0.5 C6H14 47.0 (47.1) 5.5 (5.4) 14.2 (14.1)
0.5 C6H14 47.7 (47.7) 5.6 (5.5) 14.0 (13.9)
10.21
30.59
33.41
15.33
Mo(NO)T*pCl (CH2)2 Fe 43.80
Mo(NO)T*pCl (CH2)3 Fe 40.80
Mo(NO)T*pCl (CH2)4 Fe 44.80
a
Molecules of solvent of crystallization
Molybdated iron(III) Schiff base complex [L3, B = C6H4,
M = Fe, M0 = Mo(NO)T*pCl]
triethylamine. The triethylamine was used to deprotonate
the phenolic hydroxyl group of the Schiff base iron(III)
precursors and trap HCl liberated in the reaction as
Et3NH?Cl-. The binuclear complexes were obtained as red
microcrystalline solids in low yields, 23–45 %, and this
may be attributed to the competing side reactions which
produced the green oxo-bridged {(Mo(NO)T*pCl)2O and
other products [16]. The elemental analytical results for all
the adducts were in good agreement with the proposed
formulae as shown in Table 1. They, however, showed that
the binuclear complexes retain 0.5 mol of C6H14 in their
crystal lattice. ESI mass spectral data (Table 1) for the
iron(III) Schiff base complexes were also consistent
with their formulations. Molar conductivities of 1 9
10-3 mol dm-3 solutions of the complexes at 25 ꢁC in
DMSO, DMF and MeCN lay in the range associated with
non-electrolytes [17, 18].
A mixture of Mo(NO)T*pCl2 (0.3 g, 0.6 mmol) and iron(III)
Schiff base precursor [L3, B = C6H4, M = Fe, M0 = H] in
dry toluene (100 ml) in the presence of a small amount of
Et3N was refluxed for 6 days under nitrogen. The red-
brown solution obtained was filtered while hot and the
solvent evaporated in vacuo to afford a brown solid which
was dissolved in a minimum amount of CH2Cl2 and then
chromatographed on a silica gel column. The predominant
orange fraction was eluted with a mixture of 10 % n-C6H14
in CH2Cl2 (v/v) and the solvent evaporated in vacuo. The
solid obtained was washed with n-C6H14 and then dried to
provide red crystals (yield; 0.12 g, 24 %). Minor quantities
of brown, purple and green species were detected by
chromatography but could not be isolated in sufficient
amount for characterization. The remaining molybdated
binuclear complexes prepared in a similar manner using the
appropriate iron(III) Schiff base precursors and
Mo(NO)T*pCl2 gave comparable yields.
Spectroscopic studies
The principal IR data are summarized in Table 2. The
Schiff base ligands showed characteristic bands associated
with the azomethine mC=N and mphenolic
at ca
C–O
1,628–1,635 cm-1 and 1,278–1,280 cm-1, respectively.
These stretching frequencies showed a bathochromic shift
on coordination to iron, an observation which may be
attributable to the decrease in their bond order when they
coordinate to the metal ion. Other notable bands not
observed in the Schiff base ligands but found in the
mononuclear adducts were in the 405–417 cm-1 and
497–499 cm-1 regions. These may be attributed to mphenolic
Results and discussion
Synthetic studies
The Schiff base ligands were obtained as yellow precipi-
tates except [L3, B = C6H4, M = H2, M0 = H] which was
red. The mononuclear iron(III) Schiff base precursors were
obtained as black solids in moderate yields except [L3,
B = C6H4, M = Fe, M0 = H] which was brown. They
were generally obtained by reacting the preformed Schiff
base ligands with iron(III) chloride in a 1:1 molar ratio in
ethanol. Air- and moisture-stable binuclear complexes
were obtained by reacting the mononuclear iron(III) Schiff
base precursors with Mo(NO)T*pCl2 in a 1:1 molar ratio in
dry toluene in the presence of small amounts of
and mFe/N, respectively [19]. Compared to their
C–O–Fe
corresponding p-analogues [L2, B = C6H4 or (CH2)n;
n = 2–4, M = Fe, M0 = H], mC=N, mphenolic C–O, mphenolic
and mFe/N, stretches for the m-complexes appeared
C–O–Fe
at lower frequencies. Similar observations have been made
in m-functionalized manganese(II) complexes of similar
Schiff base ligands [20]. The IR of the bimetallic complexes
123

434
Transition Met Chem (2012) 37:431–437
Table 2 IR and electronic spectral data [L3, B = C6H4 or (CH2)n; n = 2–4, M = H2 or Fe, M0 = H or Mo(NO)Tp*Cl]
Compound
IR spectral data (cm-1
m(C=N) m(ph.CO) m(MO) m(MN) m(NO) m(BH) kmax (nm), (e) dm3mol-1cm-1
)
UV–Vis spectral data
M’
B
M
H
H
H
H
H
H
H
H
C6H4
H2 1,628
1,279
1,280
1,279
1,278
1,282
1,283
1,280
1,282
1,264
1,263
1,263
1,264
255 (9,964), 281 (5,608), 329 (4,528)
254 (9,210), 282 (7,753), 308 (4,080), 371 (4,174)
252 (7,850), 284 (8,903), 310 (6,344), 370 (4,341)
254 (8,077), 278 (6,304), 306 (4,638), 370 (4,511)
284 (10,511), 332 (8,007), 487 (2,184)
(CH2)2 H2 1,634
(CH2)3 H2 1,633
(CH2)4 H2 1,635
C6H4
Fe 1,617
405
413
416
417
411
416
413
426
497
499
499
499
…b
…b
528
…b
(CH2)2 Fe 1,628
(CH2)3 Fe 1,629
(CH2)4 Fe 1,625
284 (9,223), 321 (6,465), 486 (1,646)
283 (9,933), 320 (6,837), 489 (1,890)
283 (10,823), 325 (7,566), 487 (1,484)
Mo(NO)T*pCl C6H4
Fe 1,608
1,659
1,659
1,659
1,659
2,522 312 (7,547), 434 (1,462), 487 (1,484)
2,522 278 (8,562), 316 (7,840), 430 (1,231), 510 (938)
2,522 272 (9,622), 318 (7,266), 432 (1,506), 496 (640)
2,522 320 (8,319), 434 (1,181), 493 (842)
Mo(NO)T*pCl (CH2)2 Fe 1,608
Mo(NO)T*pCl (CH2)3 Fe 1,607
Mo(NO)T*pCl (CH2)4 Fe 1,608
IR and UV–Vis spectral data for Schiff bases were obtained in EtOH, while those of the monometallic and bimetallic complexes were obtained in
DMSO and CH2Cl2 solvents, respectively
b Broad band
showed strong absorption bands (in CH2Cl2) attributable to
mBH and mNO at 2,522 and 1,659 cm-1, respectively. mC=N
was observed at 1,607–1,608 cm-1. mNO was lower than in
bands at ca 484–489 nm in addition to the intraligand
absorption bands. These bands may be attributed to charge
transfer bands, which usually almost completely obscure
the weak spin forbidden d–d transitions [22]. In the
bimetallic complexes, additional absorption bands consis-
tent with the presence of the Mo(NO)T*pCl-OAr chromo-
phore [23] were observed at ca 430–434 nm. Generally as
the polymethylene carbon chain (B) of the Schiff base
backbone increased, there occurred a small bathochromic
shift of about 4 nm to longer wavelengths. Compared to
their p-analogues, the absorption bands for the m-com-
plexes were observed at lower wavelengths.
the precursor molecule Mo(NO)T*pCl2; mNO = 1,702 cm-1
,
and this may be attributed to the substantial dG–pG back
donation into NO antibonding orbitals which in effect
reduces its bond order. Four characteristic absorption bands
associated with the pyrazolyl groups were also observed in
the range of 1,457–1,558 cm-1
.
The electronic spectral data of all the adducts synthe-
sized are summarized in Table 2. The Schiff base ligands
were dissolved in ethanol, and they showed characteristic
UV–Vis spectra in the 254–371-nm region. The absorption
band observed at ca 254–284 nm is attributable to p–p*
transitions of the benzene ring, while the absorption band
at ca 306–371 nm is attributable to the p–p* transitions of
the C=N group. Absorption bands at ca 370–371 nm may
also be attributed to n–p* transitions of the ligand [21]. The
electronic spectra of all the mononuclear adducts (in
DMSO) showed additional broad and weak absorption
1
The H NMR spectra for the free Schiff base ligands
were run in DMSO-d6 using TMS as an internal standard.
The spectra obtained are generally consistent with their
formulations. All the ligands exhibited signals in the
8.74–8.18 ppm and 7.74–6.10 ppm regions attributable
to azomethine and aromatic protons, respectively. The
signals associated with a-NCH2 protons appear in the
3.58–3.16 ppm region as triplets as a result of coupling to
1
Table 3 H NMR chemical shifts for Schiff bases [L3, B = C6H4, or (CH2)n; n = 2–4, M = H2, M0 = H]
Compound
1H NMR chemical shifts in DMSO-d6
M’
B
M
H
H
C6H4
H2 13.45{1H, s, OH}, 9.13{2H, m, OH}, 8.18{2H, s, 2CHN} 8.16{3H, dd, ArH}, 7.64{4H, s, ArH}, 7.56{4H, m, ArH}
(CH2)2 H2 8.54{1H, s, OH}, 8.33{2H, s, 2CHN}, 7.30{1H, m, ArH}, 7.12{2H, d, ArH}, 6.84{1H, t, ArH}, 6.20{2H, dd, ArH},
6.12{1H, d, ArH}, 3.81{4‘H, t, (CH2-N)2}
H
H
(CH2)3 H2 8.40{1H,d, OH}, 8.18{2H,d, CHN}, 7.95{1H, d, ArH}, 7.03{6H, m, ArH}, 3.41{4H, m, (CH2-N)2}, 1.80{2H, quintet,
C-(CH2)-C}
(CH2)4 H2 8.51{1H, s, OH}, 8.28{2H, s, 2CHN}, 7.30{1H, m, ArH}, 7.11{1H, d, ArH}, 6.84{1H, t, ArH}, 6.19{2H, dd, ArH} 6.10(2H,
d, ArH},1.60{8H, s, (CH2CH2-N)2}
123

Transition Met Chem (2012) 37:431–437
435
Table 4 Cyclic voltammetric data [L3, B = C6H4 or (CH2)n; n = 2–4, M = Fe, M0 = H or Mo(NO)Tp*Cl] in various solvents
Compound
In DMSO
Efa
In MeCN
Efa
In CH2Cl2
Ece
M0
B
M
Eab
Efc
Eae
Efc
Eab
Eaf
H
C6H4
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
-0.658
-0.629
-0.656
-0.676
0.631
0.607
0.666
0.743
H
(CH2)2
(CH2)3
(CH2)4
C6H4
H
H
Mo(NO)T*pCl
Mo(NO)T*pCl
Mo(NO)T*pCl
Mo(NO)T*pCl
-0.772
-0.758
-0.782
-0.798
-0.572
-0.537
-0.564
-0.591
1.122
1.182
1.182
1.188
-0.719
-0.718
-0.745
-0.769
-0.506
-0.522
-0.532
-0.548
0.970
1.008
1.052
1.104
1.215
1.283
1.321
1.386
(CH2)2
(CH2)3
(CH2)4
Sample solution 1 9 10-3 mol dm-3 in 0.1 mol dm-3 [n-Bu4N][PF6] base electrolyte, scan rate 200 mV/s, reading error 10 mV
a
Formal reduction potential (V) for the process Fe3? ? e- ? Fe2?
Anodic peak potential (V) for the oxidation of -OH to phenoxyl radical
b
c
Formal reduction potential (V) for the process Mo(NO)3? ? e- ? Mo(NO)2?
d
Anodic peak potential (V) for oxidation of metal ion and/or ligand
Cathodic peak potential (V) for the process Fe3? ? e- ? Fe2?
e
f
Anodic peak potential (V) for oxidation of ligand
the b-CH2 protons in the central Schiff base backbone. It
was noticeable that all the three phenolic OH protons were
not observable, probably due to exchange broadening. This
phenomenon has been observed by other authors [13, 24].
The OH signals are, however, observed in compound [L3,
B = C6H4, M = H2, M0 = H] where the phenolic OH
protons show a distinct broad peak at 13.45 ppm which
integrates for one proton and as a multiplet centered at
9.13 ppm which integrates for two protons as expected
from the structure. Results showing chemical shifts data for
the free Schiff base ligands are summarized in Table 3.
radical were also observed with the anodic peak potential
falling in the range of 0.631–0.743 V.
The bimetallic complexes in MeCN also exhibited
similar voltammograms with two broad reversible reduc-
tion waves associated with the reduction of molybdenum
and iron(III) centers falling in the potential ranges of
-0.537 to -591 V and -0.758 to -0.798 V, respectively.
The reduction potential associated with the reduction of
molybdenum, when compared to other related complexes
of similar molybdenum centers [25], was found to be more
cathodic by about 80 mV. The reduction potential of iron
was also more cathodic than the mononuclear adduct by
about 130 mV. Multiple scans at varying scan rates yielded
nearly superimposable voltammograms, indicating marked
stability of the reduction processes of both iron and
molybdenum centers. One irreversible oxidization wave
that may be associated with the oxidation of Fe3? ? Fe4?
was observed in the potential range of 1.122 to 1.188 V
(Table 4). These values were more anodic by about 0.4 to
0.5 V than the corresponding mononuclear adducts. This
observation may be attributed to the electron-deficient
Electrochemical studies
The electrochemical properties of the new mononuclear
complexes and their binuclear derivatives were investi-
gated by both cyclic and differential pulse voltammetry
with 200 and 20 mV/s scan rates, respectively, in DMSO,
MeCN and CH2Cl2. Solutions contained ca 1 9
10-3 mol dm-3 of the complex in 0.1 mol dm-3 of
[n-Bu4N][PF6] base electrolyte. The cyclic voltammetric
data are summarized in Table 4. Like their p-analogues
[11], all the mononuclear complexes exhibited similar
cyclic voltammograms in DMSO, with a reversible reduc-
tion wave associated with the Fe3? ? Fe2? reduction
process falling in the potential range of -0.629 to
-0.676 V. These formal reduction potentials were, how-
ever, less cathodic than the p-analogues by 5–10 mV. As
the polymethylene carbon chain (B) of the Schiff base
backbone lengthened from n = 2 to n = 4, a cathodic shift
of about 20 mV was observed. Irreversible oxidation waves
that may be attributed to the oxidation of -OH to a phenoxyl
0.15x10-5
0.13x10-5
0.10x10-5
0.08x10-5
0.05x10-5
-1.100 -1.000 -0.900 -0.800 -0.700 -0.600 -0.500 -0.400 -0.300
E / V
Fig. 4 Differential pulse voltammogram of [L3, B = C6H4, M = Fe,
M0 = Mo(NO)Tp*Cl] in CH2Cl2
123

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Transition Met Chem (2012) 37:431–437
Table 5 Differential pulse voltammetric data [L3, B = C6H4 or (CH2)n; n = 2–4, M = Fe, M0 = H or Mo(NO)Tp*Cl] in various solvents
Compound
In DMSO
Ea (V)
In MeCN
Ea (V)
In CH2Cl2
Ea (V)
M0
B
M
Eb (V)
Eb (V)3
H
C6H4
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
-0.669
-0.656
-0.674
-0.689
H
(CH2)2
(CH2)3
(CH2)4
C6H4
H
H
Mo(NO)T*pCl
Mo(NO)T*pCl
Mo(NO)T*pCl
Mo(NO)T*pCl
-0.729
-0.713
-0.728
-0.739
-0.529
-0.507
-0.527
-0.544
-0.724
-0.741
-0.745
-0.755
-0.503
-0.512
-0.524
-0.535
(CH2)2
(CH2)3
(CH2)4
Sample solution 1 9 10-3 mol dm-3 in 0.1 mol dm-3 [n-Bu4N][PF6] base electrolyte, scan rate 20 mV/s, reading error 10 mV
a
Peak potential for the process Fe3? ? e- ? Fe2?
Peak potential for the process Mo(NO)3? ? e- ? Mo(NO)2?
b
molybdenum center which withdraws electrons from the
iron center, thus making it more difficult to oxidize. The
oxidation peak may also be associated with a ligand oxi-
dation process since these values fall within the typical
range of values that have been obtained by others [21].
When the bimetallic complexes were investigated in
CH2Cl2, a reversible wave associated with the reduction of
the molybdenum center was observed in the potential range
of -0.506 to -0.548 V, while a quasi-reversible wave
associated with the reduction of iron(III) was observed
between -0.718 and -0.769 V for all the complexes
(Table 4). Two irreversible oxidation waves observed in
the potential ranges of 0.944–1.104 V and 1.215–1.386 V
may be attributed to the oxidation of iron and the ligand,
respectively. These observations show that the redox
potentials of this type of electrochemical system are sol-
vent dependent. When the data in Table 4 were compared
with those from the corresponding p-analogues [11], it was
observed that the reduction potentials of the iron and
molybdenum centers are influenced by changing H and
Mo(NO)T*pCl fragments from p- to m-position. This dif-
ference in the reduction potentials may possibly imply that
the m-complexes are easier to reduce, probably due to the
lowest unoccupied molecular orbitals (LUMO’s) of the
m-analogues having less electron charge density than the
LUMO’s of their p-analogues [21].
respectively. In CH2Cl2, the corresponding peak potentials
were observed at -0.503 to -0.535 V and -0.724 to
-0.755 V, respectively. A typical voltammogram is shown
in Fig. 4. These values, similar to what was obtained in cyclic
voltammetry, werefoundtobelesscathodicthanthoseoftheir
p-analogues. The results are depicted in Table 5.
Conclusion
The redox potentials of the metal centers linked by poly-
dentate Schiff base ligands can be influenced by changing
the position of substitution on the benzene ring of the
Schiff base framework from para to meta, and the
m-complexes reduce at potentials less cathodic than their
para analogues.
Acknowledgments We acknowledge the support of the Depart-
ments of Physics and Chemistry, University of Nairobi, for assisting
in the electrochemical and 1H NMR analyses, Department of
Chemistry Jomo Kenyatta University of Agriculture and Technology
for IR analysis, Pyrethrum Board of Kenya Laboratories for UV–Vis
analysis, Sumika Chemical Analyzers, Japan, for C, H, N analysis and
the School of Chemistry University of Sheffield for MS.
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