Synthesis and characterization of a monomeric and a dihydroxo-bridged dimeric complexes of iron(III) with α-benzoinoxime

Article abstract of DOI:10.1016/j.poly.2006.09.029

The reaction of α-benzoinoxime, H₂BNO with FeCl₃ in the presence of Et₃N as a base gives the mononuclear Fe(III) complex, Fe(HBNO)₃ (1). Treatment of 1 with a methanolic solution of KOH at room temperature leads to a dinuclear Fe(III)-Fe(III) complex, [Fe(HBNO)₂OH]₂ (2). The complexes were initially characterized on the basis of their elemental, mass and thermal analyses. The IR studies were useful in assigning the coordination mode of the benzoinoxime ligand to the iron metal. In addition, the presence of a hydroxo-bridge in the dimeric complex 2 is inferred from the IR spectral studies. Room-temperature Mo?ssbauer studies indicated octahedral, high-spin iron(III). Variable-temperature magnetic susceptibility measurements supported the existence of the μ-dihydroxo-bridging structure core, Fe^III(μ-OH)₂Fe^III in the dinuclear complex 2. Theoretical modelling of the magnetic data indicated a weak antiferromagnetic spin exchange between the iron(III) centers (J = -8.35 cm^-1, g = 2.01, ρ = 0.02 and TIP = 1.7 × 10^-4 cm³ mol^-1 for H = -2JS₁ · S₂). The electronic spectra of the complexes revealed two bands due to d-d transitions and one band assignable to an oxygen (p_π) → Fe(d_π*) LMCT transition observed in each complex. An additional charge-transfer transition, assignable to μ-hydroxo(p_π) → Fe(d_π*), was observed for the dimeric complex 2. The structural and vibrational behaviors of these complexes have been elucidated with quantum mechanical methods.

Full text of DOI:10.1016/j.poly.2006.09.029

Polyhedron 26 (2007) 791–796
www.elsevier.com/locate/poly
Synthesis and characterization of a monomeric and a
dihydroxo-bridged dimeric complexes of iron(III) with a-benzoinoxime
*
Attia S. Attia , M.F. El-Shahat
Chemistry Department, Faculty of Science, Ain Shams University, Abbasia, Cairo, Egypt
Received 14 August 2006; accepted 6 September 2006
Available online 27 September 2006
Abstract
The reaction of a-benzoinoxime, H2BNO with FeCl3 in the presence of Et3N as a base gives the mononuclear Fe(III) complex,
Fe(HBNO)3 (1). Treatment of 1 with a methanolic solution of KOH at room temperature leads to a dinuclear Fe(III)–Fe(III) complex,
[Fe(HBNO)2OH]2 (2). The complexes were initially characterized on the basis of their elemental, mass and thermal analyses. The IR
studies were useful in assigning the coordination mode of the benzoinoxime ligand to the iron metal. In addition, the presence of a
hydroxo-bridge in the dimeric complex 2 is inferred from the IR spectral studies. Room-temperature Mo¨ssbauer studies indicated octa-
hedral, high-spin iron(III). Variable-temperature magnetic susceptibility measurements supported the existence of the l-dihydroxo-
bridging structure core, FeIII(l-OH)2FeIII in the dinuclear complex 2. Theoretical modelling of the magnetic data indicated a weak
antiferromagnetic spin exchange between the iron(III) centers (J = ꢀ8.35 cmꢀ1, g = 2.01, q = 0.02 and TIP = 1.7 · 10ꢀ4 cm3 molꢀ1
for H = ꢀ2JS1 Æ S2). The electronic spectra of the complexes revealed two bands due to d–d transitions and one band assignable to
an oxygen (pp) ! Fe(dp*) LMCT transition observed in each complex. An additional charge-transfer transition, assignable to l-hydro-
xo(pp) ! Fe(dp*), was observed for the dimeric complex 2. The structural and vibrational behaviors of these complexes have been
elucidated with quantum mechanical methods.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Iron complexes; Benzoinoxime; Dimer; Dihydroxo bridge; Magnetic studies; Exchange coupling; Semi-empirical calculations
1. Introduction
involved in oxygen storage of hemerythrin and oxygen acti-
vation of methanemonooxygenase [2–4]. Efforts to model
these proteins have simulated the study of complexes con-
taining Fe(l-O)2Fe and Fe(l-OH)2Fe cores [5–7]. In
addition, these systems were treated as models for under-
standing the magnitude of exchange coupling interactions
between the two iron centers and its relation to geometri-
cal parameters [8]. Studies of some diiron complexes have
shown that the oxo-bridge is responsible for strong anti-
ferromagnetic coupling (ꢀ50 > J > ꢀ200 cmꢀ1) and those
complexes with hydroxo, alkoxo, phenoxo-bridges are
weakly coupled (0 > J > ꢀ30 cmꢀ1) [6,8–10].
The importance of iron as an essential element can be
estimated by the wide range of iron proteins and enzymes
playing different roles in biological systems. Proteins con-
taining non-heme iron centers are widely spread in nature.
They perform a broad range of functions, especially acti-
vating dioxygen for the oxidation of various substrates
[1]. Dinuclear iron complexes in particular have biological
relevance since they give valuable information about the
binding mode of dioxygen to the iron sites of proteins
involved in its transport and activation. They provide
structural models for dinuclear sites in several proteins
We reported previously the synthesis and characteriza-
tion of dinuclear chromium and molybdenum complexes
containing oxo and hydroxo-bridged ligands [11,12].
In the present work, we describe the preparation and
characterization of two new iron(III) complexes of the
*
Corresponding author. Tel.: +20 2 6717 839.
E-mail address: attiasattia@hotmail.com (A.S. Attia).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.09.029

792
A.S. Attia, M.F. El-Shahat / Polyhedron 26 (2007) 791–796
a-benzoinoxime ligand. The coordination properties of
the complexes were studied using FTIR, electronic and
Mo¨ssbauer techniques. A magneto-structural correlation
has been studied by variable-temperature magnetic sus-
ceptibility and theoretical modelling of the magnetic
data. Semi-empirical theoretical calculations (MM+ and
parameterized PM3) have been used to predict the most
favored geometry for each complex.
parameterized PM3 method. Available parameters for Fe
metal were implemented in the PM3 semi-empirical method
[18]. The parameterized PM3 was used for the geometry
optimization of the complexes under study. Theoretical har-
monic vibrational frequencies are related to the experimen-
tal fundamentals by the optimum scaling factor k,
determined through a least-squares procedure, given by
P
P
all
2
k ¼ allxtiheormiexpt
=
ðxtiheorÞ , where xitheor and miexpt are
i
i
the ith theoretical harmonic and ith experimental funda-
mental frequencies (in cmꢀ1) [19]. The molecular root mean
square error (rms) was calculated by the square root of the
sum over all the modes of Dmin given in Ref. [19], where Dmin
is the minimized residual for each mode.
OH
N
2.1. Synthesis of tris(benzoinoximato)iron(III),
Fe(HBNO)3 (1)
OH
H
A solution of triethylamine (0.2 mL) in ethanol (5 mL)
was added dropwise to an ethanolic solution (30 mL) of
a-benzoinoxime, H2BNO (0.38 g, 1.67 mmol) with stirring
at room temperature. To the resulting solution, was added
an ethanolic solution (10 mL) of FeCl3 Æ 6H2O (0.15 g,
0.55 mmol) and the mixture was heated at 70 ꢁC for 18 h.
The purple residue was separated from the reaction med-
ium and then washed with cold absolute ethanol and dried.
The residue was dissolved in dichloromethane. Chromato-
graphic separation on a neutral Al2O3 column eluted with
CH2Cl2–ether (90:10% v/v) gave Fe(HBNO)3 as a purple
band. Purple microcrystals of the complex were obtained
by slow evaporation of the solvent with a flow of argon.
The crystals were dried in a vacuum overnight. The prod-
uct was obtained in 0.35 g (87%) yield.
α-benzoinoxime ligand (H2BNO)
2. Experimental
Ferric chloride, FeCl3 Æ 6H2O, and a-benzoinoxime,
H2BNO, were used as purchased from Aldrich chemical
Co. Inc. Solvents were dried according to standard
procedures.
Variable-temperature (10–300 K) magnetic susceptibility
measurements were obtained with an Oxford Faraday mag-
netometer. Mo¨ssbauer measurements were carried out on a
multichannel analyzer in constant acceleration mode. 57Co
in a Rh matrix (20 mc) was used as a radioactive source and
was held at room temperature. The isomer shift was
expressed relative to an iron metal absorber. Infrared spec-
tra (4000–400 cmꢀ1) were recorded as KBr pellets on a
Thermo Nicolet Nexus 670 FTIR spectrometer. The ther-
mal analyses (TGA–DTA) were carried out under nitrogen
with a heating rate of 10 ꢁC/min using a Perkin–Elmer ther-
mal analyzer. The electronic absorption spectra (190–
1100 nm) were measured on a Cary 50 spectrophotometer.
Samples were run on 10ꢀ4 M solutions in DMSO. Elemen-
tal analyses were performed using a Perkin–Elmer 2400
CHN elemental analyzer. The semi-empirical calculations
in this study have been carried out using the Hyperchem
6.01 program package [13]. The Polak–Ribiere version of
the conjugate gradient method was used in all energy mini-
Anal. Calc. for C42H36FeN3O6 (MW = 734.62): C,
68.67; H, 4.94; N, 5.72. Found: C, 68.50; H, 5.10; N,
5.77%. TGA, % weight loss (temp. ꢁC): 61.22 (75–210),
23.26 (213–436).
2.2. Synthesis of l-dihydroxobis{bis(benzoinoximato)-
iron(III)}, [Fe(HBNO)2(l-OH)]2 (2)
To a solution of Fe(HBNO)3 (0.5 g, 0.68 mmol) in
10 mL CH3CN was added 5 mL CH3OH solution of
KOH (0.04 g, 0.71 mmol), and the mixture was stirred for
36 h. The color of the solution turned red and was dried
in vacuum. The residue was washed with hot ethanol
(95%) to remove KHBNO. Then the residue was redis-
solved in CH2Cl2 and subjected to chromatography on sil-
ica gel column with CH2Cl2–ether (80:20% v/v). The red
band was eluted and collected. A dark red solid was
obtained from the eluent by slow evaporation of the sol-
vent with a flow of argon. The solid product was dried in
a vacuum overnight. The product as a red solid was
obtained in 64% (0.23 g) yield.
mization calculatioꢀn1s with a coverage criterion less than
˚
A . First and separately the ben-
0.001 kcal molꢀ1
zoinoxime ligand was optimized and the results obtained
have been applied for the oxime ligand in each calculation
thereafter. The values of Fe–O and Fe–N bond lengths
and Fe–O–Fe bond angle in the literatures were used as
the starting inputs [6,14–17]. Initial optimization of different
geometries of the complexes was performed with molecular
mechanics (MM+). The geometry was further refined using
Anal. Calc. for C56H50Fe2N4O10 (MW = 1050.74): C,
64.01; H, 4.80; N, 5.33. Found: C, 63.92; H, 4.77; N,
5.29%. TGA, % weight loss (temp. ꢁC): 46.41 (61–152),
29.27 (154–326), 10.50 (328–480).

A.S. Attia, M.F. El-Shahat / Polyhedron 26 (2007) 791–796
793
3. Results and discussion
stretching vibration has shifted by 45 cmꢀ1 to lower fre-
quency. In contrast, the bands corresponding to OH and
N–O stretches have shifted to higher frequencies with
respect to the parent ligand. Such behavior is evidently
indicates that the a-benzoinoxime ligands coordinate to
the iron metal through the nitrogen atom of the oxime
group [11,20]. The presence of two bands for the N–O
stretching vibration indicates different types of N–O groups
[21,22]. Consequently, the peaks at 1089 and 1024 cmꢀ1 are
assigned to free N–O and hydrogen-bonded N–O,
respectively.
The stoichiometric reaction between a-benzoinoxime,
H2BNO and FeCl3 in the presence of triethylamine as a
base has yielded the monomeric complex Fe(HBNO)3 (1).
Addition of OHꢀ to complex 1 at room temperature affor-
ded the dimeric product 2. Chromatographic analyses of
both complexes have indicated one isomeric form for each
complex.
The complexes were initially characterized on the basis
of their elemental analyses, mass spectra and thermal anal-
yses (TGA, DTA). The parent ion peak (m/z) was observed
in the mass spectra at 734 and 1051 for complexes 1 and 2,
respectively. The thermal analyses (TGA, DTA) of com-
plex 1 showed a weight loss of 61.22% in the temperature
range 75–210 ꢁC, corresponding to the loss of two mole-
cules of benzoinoxime ligand. The mass loss of 23.26% in
the temperature range 213–436 ꢁC, corresponds to two
phenyl groups and one OH of the third benzoinoxime
ligand. For the dimeric complex 2, three endothermic peaks
were observed. The mass loss of 46.41% in the temperature
range 61–152 ꢁC corresponds to two benzoinoxime ligands
and two OH groups. The other weight losses of 29.27% and
10.50% in the temperature ranges 154–326 and 328–480ꢁC,
respectively, are due to gradual decomposition of the
organic fragments of the other benzoinoxime ligands.
The primary ligand, a-benzoinoxime, showed character-
istic IR bands at 3326, 1670 and 950 cmꢀ1 due to the OH,
C@N and N–O stretching vibrations of the oxime group.
These bands are observed for complex 1 at 3421 (OH),
1625 (C@N), 1089 and 1024 cmꢀ1 (N–O). The C@N
The infrared spectrum of complex 2 revealed two strong
absorptions at 968 and 913 cmꢀ1 that are not present in the
spectrum of the monomeric complex Fe(HBNO)3. In addi-
tion, no band between 800 and 850 cmꢀ1 attributable to an
oxo bridged structural unit is observed in the IR spectrum
of the dimeric complex [Fe(HBNO)2OH]2 [5,10,23]. Fur-
thermore, the similarity in the absorption peaks due to
coordinated benzoinoxime ligand in both the monomeric
and the dimeric complexes rules against any type of ben-
zoinoximate bridging in the dimer. Therefore, the bands
at 968 and 913 cmꢀ1 in the [Fe(HBNO)2OH]2 dimer are
assigned to a vibrational mode associated with the
4þ
Fe2ðOHÞ2 structural unit. Consistent with this assign-
ment, the infrared spectra of [Fe(pic)2OH]2 and
4þ
[Cr(HBNO)(BNIM)OH]2 showed bands due to Fe2ðOHÞ2
and Cr2ðOHÞ2
at 950 and 970 cmꢀ1
, respectively
4þ
[10,12]. Similarly to the monomeric complex 1, a peak
due to stretching vibration of oxime OH is observed at
3422 cmꢀ1 for the dimeric complex. An additional
peak was observed at 3599 cmꢀ1 which is assigned to the
OH
OH
N
OH
O
Fe
O
O
Fe
N
N
N
N
O
N
OH
OH
OH
α-benzoinoximate
≡
O-
O
N
O
HO
1A
1B
OH HO
OH
N
O
N
Fe
O
N
O
H
O
H
O
O
N
O
N
O
N
OH
Fe
Fe
OH
HO
HO
N
O
H
O
H
O
O
N
HO
2A
2B
OH HO
OH HO
N
Fe
O
N
Fe
N
N
Fe
N
N
Fe
N
H
O
H
O
O
O
O
O
O
N
O
O
HO
O
H
O
H
OHHO
HO
2C
2D
Fig. 1. Chemical structures for complexes 1 and 2 used for semi-empirical calculations.

794
A.S. Attia, M.F. El-Shahat / Polyhedron 26 (2007) 791–796
bridging OH stretch. Finally, bands indicative of two types
of N–O groups were observed at 1117 and 1080 cmꢀ1 (free
N–O) and at 1052 and 1024 cmꢀ1 (hydrogen-bonded
N–O). Usually, oxime complexes show two N–O bands
in the range 1190–1260 and 1071–1110 cmꢀ1 for strongly
hydrogen-bonded species, whilst in free and weakly hydro-
gen-bonded species the N–O absorption occurs at ca. 1150
and 1020 cmꢀ1 [21,22,24].
The monomeric complex 1 is expected to have two geo-
metrical isomers; facial 1A and meridional 1B (Fig. 1), but
only one isomer is obtained as indicated by the chromato-
graphic analysis. Now the question is whether the
Fe(HBNO)3 has a facial or meridional configuration. Also,
the dimeric complex 2, is expected to adopt one of four
geometrical isomers; 2A, 2B, 2C or 2D (Fig. 1). The spec-
troscopic techniques gave no information about the geom-
etry of the complex. Hence, it was considered worthwhile
to model the complexes using molecular mechanics
MM+ and PM3 calculations, especially with the lack of
structural analysis due to the poor quality of the crystals
obtained.
The PM3 calculated results for each structure are sum-
marized in Table 1. It can be seen that for the Fe(HBNO)3
complex, the facial geometry 1A is more stable than the
meridional one 1B by 3.8 kcal/mol. These results are in
agreement with the structure of the Co(BMO)3 complex,
whose structure is characterized with a facial configuration
by X-ray diffraction analysis [25]. For the dimeric complex
2, the energy calculated for 2A is more stable than those for
the other configurations.
The most favored structures for the monomeric and
dimeric iron(III) complexes based on semi-empirical PM3
geometry optimization are shown in Fig. 2. In both struc-
tures, the benzoinoxime ligand is coordinated to Fe(III)
with the oxime nitrogen atom and the deprotonated alco-
holic oxygen atom.
The observed and calculated vibrational frequencies for
configurations 1A and 2A are compared in Table 2. The
optimized scale factors determined for 1A and 2A are
0.998407 and 0.997745, respectively. The calculated har-
monic vibrational frequencies compare well with funda-
mental experimental frequencies. The calculated root
mean square error (rms) per molecule is 70.8 and
64.4 cmꢀ1 for 1A and 2A, respectively. The rms values
compare very well with those reported for vibrational fre-
quency calculations using MP2-fc/631G(d) (rms = 63)
and MP2-fc/631G(d,p) (rms = 61) [15].
Fig. 2. The most stable geometries for the Fe(HBNO)3 complex (1A) and
the [Fe(HBNO)2OH]2 complex (2A).
300 K, which is close to the spin-only value (5.92) of
high-spin Fe(III). The magnetic moment showed no signif-
icant temperature-dependence over the temperature range
300–10 K. Such behavior is indicative of a monomeric,
octahedral high-spin Fe(III) complex [23].
The magnetic susceptibility data measured for
Fe(HBNO)3 revealed a magnetic moment of 5.86 B.M. at
Table 1
Calculated results for each structure of the monomeric complex 1 (1A and 1B) and of the dimeric complex 2 (2A, 2B, 2C and 2D)
Calculation resultsa,b (kcal/mol)
1A
1B
2A
2B
2C
2D
Total energy
Heat of formation
ꢀ191659.1
ꢀ284.63
ꢀ191655.3
ꢀ280.83
ꢀ278205.7
ꢀ802.07
ꢀ278149.9
ꢀ746.25
ꢀ278201.5
ꢀ797.84
ꢀ278201.1
ꢀ797.45
a
Total charge = 0 for all complexes.
The metal in all complexes is high-spin Fe(III).
b

A.S. Attia, M.F. El-Shahat / Polyhedron 26 (2007) 791–796
795
Table 2
0.20
Characteristic calculated and observed vibrational frequencies (cmꢀ1) for
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
conformations 1A and 2A
1A
2A
Assignment
Calculated Observed Calculated Observed
3596
3433
3083
3029
2926
2877
1619
1596
3599
3422
3061
3024
2922
2870
1629
1607
m(O–H) bridge
m(O–H) oxime
m(C–H) aromatic
3437
3064
3029
2952
2815
1645
1598
3421
3063
3019
2932
2859
1625
1607
m(C–H) aliphatic
m(C@N)
m(C@C) + ring
deformation
m(N–O)
1108
1076
1048
1011
941
1117
1080
1050
1024
968
1086
1020
1089
1024
0
50
100
150
200
250
300
T (K)
m(Fe–OH–Fe)
910
480
443
913
496
Fig. 3. Temperature dependence of the molar paramagnetic susceptibility,
vm, curve for [Fe(HMNO)2OH]2. The solid line represent the least-squares
fit of the data to the theoretical equation given in the text.
481
441
496
448
m(M–O)
m(M–N)
Magnetic susceptibility data for [Fe(HBNO)2OH]2 indi-
cate weak intramolecular antiferromagnetic spin
a
+
exchange interaction. Both the magnitude and temperature
dependence of the magnetic susceptibility typify the behav-
ior of an S1 = S2 = 5/2 dimer [26]. The effective magnetic
moment drops from 8.55 B.M. per molecule at ꢁ300 K to
4.01 B.M. at ꢁ11 K. The data were analyzed with the
spin–spin interaction model based on the exchange Hamil-
+
+
+
+
+
+
+
+
+ +
+
+
+
+ +
+
+
+
+
++
+
+
+
+ +
+
+
+
++
+
+
+
+ + ++
+
+
++
++
+
+
+
+
+
+
++
+
+
++
+
+
+
+
+
+
+
+
+
+
+
+
++
+ +
+
+
+
+
+
++
+
+ +
+
+
+
+
+
+
+
++ +
+ +++
+
+ + +
+ + +
+ +
+
+
+
+
+
+
++
+
+
+
+
+
+
+
+
+
+
+
++
+
++
+
+
+
+
+
+
+
+
+ +
+
+
+
+
+
++
+
+
+
+
+
+
+
+
+
+
+
+
+
++
++
+ +
+
+
+
+
++
+
+
+
+
+
+
+
+
+
+ +
+
++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
^
^
^
1.5%
tonianðH ¼ ꢀ2 JS1 ꢂ S2Þ. The molar paramagnetic suscep-
tibility of an S1 = S2 = 5/2 exchange coupled Fe(III)–
Fe(III) dimer is given by the following equation [27,28]:
+
+
+
+
+
+
ꢀ
ꢁ
2Nb2g2 v0
2:19g2
T
+
vM
¼
ð1 ꢀ qÞ þ
q þ TIP
v00
+
kT
+
+
+
+
+
v0 ¼ ½ð55 þ 30expðꢀ10J=kTÞ þ 14expðꢀ18J=kTÞ
þ 5expðꢀ24J=kTÞ þ expðꢀ28J=kTÞꢃ
v00 ¼ ½11 þ 9expðꢀ10J=kTÞ þ 7expðꢀ18J=kTÞ
þ 5expðꢀ24J=kTÞ þ 3expðꢀ28J=kTÞ þ expðꢀ30J=kTÞꢃ
+
+
+
+
+
+
+ +
-8
-6
-4
-2
0
2
4
6
8
ð1Þ
VELOCITY (mm/ s)
where, q gauges the amount of magnetically dilute ferric
impurities and the other terms have their usual meanings.
A non-linear least-squares fit to 51 experimental data
Fig. 4. Room-temperature Mo¨ssbauer spectrum of [Fe(HMNO)2OH]2.
points yielded J = ꢀ8.35 0.2 cmꢀ1
,
g = 2.01 0.02,
d = 0.43 mm/s. The values of the quadrupole splitting,
DEQ, and the isomer shift, d, are consistent with the formu-
lation of a complex containing high-spin Fe(III) and are
similar to the Mo¨ssbauer parameters reported for number
of dihydroxo-bridged diiron(III) dimers [30–32].
The electronic absorption spectra recorded for the two
complexes and the parent ligand as well in DMSO solution
over the range 190–1100 nm are summarized in Table 3.
The parent ligand displayed two absorptions at 284 and
405 nm corresponding, respectively, to the intraligand
p–p* and n–p* transitions. The electronic spectra of the
Fe(III) complexes are characterized by a band that can
q = 0.02 0.01 and TIP = 1.7 · 10ꢀ4 0.02 cm3 molꢀ1
with R = 8.29 · 10ꢀ4. The fit is illustrated in Fig. 3. The va-
lue of the coupling constant J is in good agreement with the
values reported for other dihydroxo-bridged Fe(III) dimers
[6,10,29]. They exhibited coupling constants in the range of
ꢀ7 to ꢀ11 cmꢀ1. In contrast, oxo-bridged Fe(III) dimers
are invariably characterized by a coupling constant near
ꢀ100 cmꢀ1 [30].
The Mo¨ssbauer spectrum of the dimeric complex 2 at
298 K is shown in Fig. 4. The spectrum consists of a single
symmetric quadrupole doublet with DEQ = 0.87 mm/s and

796
A.S. Attia, M.F. El-Shahat / Polyhedron 26 (2007) 791–796
Table 3
[5] B.J. Kennedy, K.S. Murray, P.R. Zwack, H. Homborg, W. Kalz,
Inorg. Chem. 24 (1985) 3302.
The electronic absorption data for a-benzoinoxime and its iron(III)
complexes recorded in DMSO
[6] L. Borer, L. Thalken, C. Cecarelli, M. Glick, J.H. Zhang, W.M. Reiff,
Inorg. Chem. 22 (1983) 1719.
[7] Y. Zang, Y. Dong, L. Que Jr., J. Am. Chem. Soc. 117 (1995)
1169.
[8] S.M. Gorun, S.J. Lippard, Inorg. Chem. 30 (1991) 1625.
[9] H. Weihe, H.U. Gudel, J. Am. Chem. Soc. 119 (1997) 6539.
[10] H.J. Schugar, G.R. Rossman, H.B. Gray, J. Am. Chem. Soc. 91
(1969) 4564.
Compound
k, nm (loge)
H2BNO
Fe(HBNO)3
[Fe(HBNO)2OH]2
284 (2.7), 405a (1.5)
305 (4.1), 480a (3.3), 609 (1.2), 794 (1.0)
304 (3.9), 496a (3.4), 563 (3.2) 825 (1.1), 935 (1.2)
a
Shoulder.
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be assigned to a ligand-based p–p* transition, which is nor-
mally the most intense band in the higher energy region of
the spectra. Both complexes displayed a band that can be
assigned to a charge-transfer transition from pp orbitals
of the oxime alcoholic oxygen atom to the half-filled dp*
orbitals of the Fe(III) metal [32,33]. This band is observed
at 480 nm for monomeric complex 1, and at 496 nm for
dimeric complex 2. The red shift of this charge-transfer
band for the dimeric complex with respect to the mono-
meric complex may explain the colors of the DMSO solu-
tions of the two complexes (solutions of complexes 1 and 2
have purple and red colors, respectively). Two low-energy
transitions (loge 6 1.2) were observed in the spectra of
both complexes and most probably are due to spin-forbid-
den d–d transitions. They have been assigned to the
6A1 ! T1 and A1 ! T2 transitions of high-spin Fe(III)
[34]. The dominant feature of the spectrum of the dimeric
complex 2 is an intense transition observed at 563 nm
(loge = 3.2), which is absent in the spectra of both the
monomeric complex and the parent ligand itself. This tran-
sition can be assigned to a charge-transfer transition from
pp orbitals of the hydroxo-bridged oxygen atom to dp*
orbitals of the Fe(III) metal. Consistence with this assign-
ment, a similar transition was observed for number of dihy-
droxo-bridged Fe(III) complexes [32,35].
4
6
4
References
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