New mixed-ligand 8-hydroxyquinolinato iron(III) complexes of dimethylethylenediamine-based aminophenol ligands

Article abstract of DOI:10.1016/j.poly.2016.01.049

Four new mixed ligand iron(III) complexes of N,N′-dimethylethylenediamine based bis(phenol) di-amine (H₂L^NER) and 8-hydroxyquinoline (HQ) ligands have been synthesized and characterized by spectroscopic methods, X-ray diffraction, ma

Full text of DOI:10.1016/j.poly.2016.01.049

Accepted Manuscript
New Mixed-Ligand 8-Hydroxyquinolinato Iron(III) Complexes of Dimethyle-
thylenediamine-based Aminophenol Ligands
Elham Safaei, Narges Naghdi, Andrzej Wojtczak, Zvonko Jagličić
PII:
S0277-5387(16)00082-6
DOI:
http://dx.doi.org/10.1016/j.poly.2016.01.049
POLY 11812
Reference:
Polyhedron
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Accepted Date:
4 November 2015
29 January 2016
Please cite this article as: E. Safaei, N. Naghdi, A. Wojtczak, Z. Jagličić, New Mixed-Ligand 8-Hydroxyquinolinato
Iron(III) Complexes of Dimethylethylenediamine-based Aminophenol Ligands, Polyhedron (2016), doi: http://
dx.doi.org/10.1016/j.poly.2016.01.049
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New Mixed-Ligand 8-Hydroxyquinolinato Iron(III) Complexes
of Dimethylethylenediamine-based Aminophenol Ligands
Elham Safaei a,b Narges Naghdi b, Andrzej Wojtczakc, Zvonko Jagličićd,
,
aDepartment of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran
bInstitute for Advanced Studies in Basic Sciences (IASBS), 45137-66731, Zanjan, Iran
cNicolaus Copernicus University, Department of Chemistry, 87-100, Torun, Poland
dInstitute of Mathematics, Physics and Mechanics & Faculty of Civil and Geodetic
Engineering, University of Ljubljana, Jadranska 19. SI-1000, Ljubljana, Slovenia
Correspondence to: Elham Safaei
E-mail:e.safaei@shirazu.ac.ir
Four new mixed ligand iron(III) complexes of N,N'-dimethylethylenediamine based
bis(phenol) di-amine (H2LNER) and 8-hydroxyquinoline (HQ) ligands have been synthesized
and characterized by spectroscopic methods, X-ray diffraction, magnetic susceptibility
measurements and cyclic voltammetry techniques. X-ray structure analysis revealed iron
complexes with monoclinic or triclinic structures in which the H2LNEX and HQ ligands are
coordinated to the Fe(III) center. The variable temperature magnetic susceptibility indicated a
paramagnetic iron(III) center in the complexes. Electrochemical studies of the complexes
with cyclic voltammetry showed cathodic peaks corresponding to an FeIII to FeII reduction.
An anodic peak was found that corresponded to phenolate ligand oxidation to phenoxyl
radicals in the positive potential regions. To the best of our knowledge, synthesis reports on
mixed-ligand 8-hydroxyquinolinato iron(III) complexes are very rare.
Keywords: Bis(phenol) di-amine; 8-hydroxyquinoline; Tripodal ligands, Enzyme models,
Enzyme-substrate
1

Introduction
Tripodal ligands are tri- and tetradentate ligands which are popular in coordination chemistry
and homogeneous catalysis research areas. As these ligands are polydentate, they have been
used in the synthesis of metal complexes as model complexes for the active site of various
enzymes. Many efforts have been devoted to the design and synthesis of new chelating ligands
with the aim of enhancing their ability to form complexes with metal ions such as copper, iron
and manganese that model the active site of the enzyme galactose oxidase, [1-5] various
dioxygenases, lipoxygenases and phosphatases [6-11]. Lipoxygenases (Scheme 1) are a family
of iron-containing enzymes that catalyze the dioxygenation of polyunsaturated fatty acids into
hydroperoxides using molecular oxygen as an oxidant. These natural compounds play an
important role in plants, animals and fungi physiology, including growth and development,
resistance or response to pest or wounding [12]. The active site of the enzyme contains an iron
atom which is bound by five ligands, three or four of which are histidine residues and one or
two oxygen atoms of carboxylate groups [13-16]. The coordination number of the iron center is
either five or six, with additional hydroxyl or water ligands for a hexacoordinate iron center.
Scheme 1. The active site of lipoxygenase enzyme [17]
2

In soil bacteria, catechol dioxygenase enzymes catalyze the aerobic cleavage of catechol to
aliphatic acids and other dihydroxy aromatics. The active site of these enzymes contains
Fe(II) or Fe(III) ions (Scheme 2). Prior to catechol binding, the ferric or ferro ion is
coordinated by four amino acid residues, i.e. histidine (His) and tyrosine (Tyr) or glutamic
acid and a hydroxyl or H2O molecule. Catecholate binding results in the displacement of non-
amino acid ligands and the active-site geometry of the enzyme-catecholate complexes could
be described as octahedral with mixed ligands [18,19].
Scheme 2. The active site of dioxygenase enzymes [18]
Significant efforts have been made towards the synthesis of iron complexes using tripodal
ligands with exclusively oxygen and nitrogen donor atoms [20-30] that mimic the
architecture of enzyme active sites or mixed ligand complexes of enzyme-substrate adducts.
The main focus of this project was the synthesis, characterization and X-ray structures of a
series of mixed ligand Fe(III) complexes of N,N'-dimethylethylenediamine based bis(phenol)
di-amine (H2LNER) and 8-hydroxy quinolone (HQ) ligands (Scheme 3).
3

Scheme 3. The structure of bis(phenol) di-amine, H2LNER
2. Experimental
2.1. Materials and physical measurements
Reagents or analytical grade materials were obtained from commercial suppliers and used
without further purification, except those for electrochemical measurements. Fourier
transform infrared spectroscopy on KBr pellets was performed on a FT-IR Bruker Vector 22
instrument. UV-Vis absorbance digitized spectra were collected using a CARY 100
spectrophotometer. Magnetic susceptibility was measured for powder samples of solid
materials over the temperature range 2-300 K using a SQUID susceptometer (Quantum
Design MPMS-XL-5) at a constant field of 1000 Oe.
Voltammetric measurements were made with a computer controlled electrochemical system
(ECO Chemie, Utrecht, The Netherlands) equipped with a PGSTA 30 model and driven by
GPES (ECO Chemie). A glassy carbon electrode with a surface area of 0.035 cm2 was used
as a working electrode and a platinum wire served as the counter electrode. The reference
electrode was an Ag wire as a quasi-reference electrode. Ferrocene was added as an internal
standard after completion of the experiment set-up, and potentials are referenced vs. the
ferrocenium/ferrocene couple (Fc+/Fc).
Crystals for the X-ray diffraction experiment were obtained from an EtOH-CH2Cl2 solution.
X-ray diffraction data were collected with an Oxford Sapphire CCD diffractometer using
4

MoKα radiation, λ =0.71073 Å, at 293(2) K, by the ω-2θ method. For FeQLNEOB, the space
group was triclinic P-1. For the other complexes, space groups were assigned as monoclinic
P21/c, based on systematic absences. The structures were solved by direct methods and
refined with the full-matrix least-squares method on F2 with the use of the SHELX97 [31]
program package. Analytical absorption corrections were applied (Table 1) (RED171
package of programs [32] Oxford Diffraction, 2000). No extinction correction was applied.
For all reported structures, hydrogen atoms were located from the electron density maps and
their positions were constrained in the refinement.
2.2. Preparation
All the ligands were synthesized based on literature reports [33].
2.2.1. Synthesis of the FeQLNER complexes
To a stirred mixture of H2LNER (1.00 mmol), 8-hydroxyquinoline (0.14 g, 1.00 mmol) and
triethylamine (0.30 g, 3.00 mmol) in ethanol (50 ml), FeCl3 (0.16 g, 1.00 mmol) was added
under continuous stirring. The reaction mixture was then refluxed for 1 h. The solvent was
evaporated and a red brown powder was obtained. Red brown colored crystals suitable for X-
ray diffraction were obtained by slow evaporation of a dichloromethane and ethanol solution
of the powder.
2.2.1.1. Synthesis of FeQLNEM
Yield: 63%. Anal. calcd. for C31H36FeN3O3 (554.48 g/mol): Fe, 10.02; C, 65.00; H, 6.38; N,
7.10%. Found: Fe, 10.70; C, 65.25; H, 6.54; N, 7.58%. IR (KBr, cm-1): 3779, 3450, 2959,
2907, 2283, 1573, 1466, 1376, 1317, 1273, 1218, 1111, 1026, 973, 875, 831, 782, 742, 611,
516.
2.2.1.2. Synthesis of FeQLNEB
Yield: 55%. Anal. calcd. for C27H24Br4FeN3O3 (813.98 g/mol): Fe, 6.28; C, 39.10; H, 2.49;
N, 4.86%. Found: Fe, 6.31; C, 39.24; H, 2.97; N, 5.06%. IR (KBr, cm-1): 3804, 3435, 2918,
5

1633, 1574, 1451, 1376, 1317, 1271, 1221, 1153, 1106, 1057, 861, 817, 745, 709, 562, 511,
465.
2.2.1.3. Synthesis of FeQLNEOB
Yield: 58%. Anal. calcd. for C37H48FeN3O5 (670.63 g/mol): Fe ,8.02; C, 64.20; H, 7.20; N,
5.80%. Found: Fe, 8.1; C, 65.26; H, 7.21; N, 6.01%. IR (KBr, cm-1): 3439, 2944, 2751, 2678,
2492, 1606, 1460, 1381, 1317, 1268, 1204, 1140, 1104, 1052, 858, 820, 791, 740, 519, 473.
2.2.1.4. Synthesis of FeQLNEC
Yield: 60%. Anal. calcd. for C27H26Cl4FeN3O4 (654.16 g/mol): Fe ,8.02; C, 49.25; H, 3.20;
N, 6.40%. Found: Fe, 8.11; C, 50.08; H, 3.30; N, 6.48%. IR (KBr, cm-1): 3444, 2914, 1580,
1455, 1374, 1317, 1219, 1170, 1055, 865, 828, 751, 571, 515, 459.
3. Results and discussion
3.1. Ligand and complex synthesis and characterization
The strategy applied to prepare the tripodal ligands based on a tertiary amine was the one step
Mannich reaction; N,N'-dimethylethylenediamine was treated with phenol (with different
substituents in the 2 and 4 positions) and paraformaldehyde under solvent free conditions.
Both the ligand and the used reaction method have been reported in literature [33,34]. An
overview of the synthesized FeQLNER complexes is given in Scheme 4. The obtained
complexes have the general formula FeQLNER, in which LNER is one of the dianionic ligands
H2LNEX and “Q” is 8-oxyquinoline (quin). The general method used to prepare these
complexes was to react an iron(III) chloride complex with the tripodal ligand and 8-
oxyquinoline in ethanol, with the addition of 2 equivalents of triethylamine. The complexes
were obtained with high purity and quite a high overall yield.
6

N
OH
R2
N
O
C
H3C
CH3
R1
OH
HO
R2
R2
2
2
N
N
+
+
H
H
N
H
H
R1
R1
N
N
N
Fe
NEt3 , EtOH
O
N
R 2
R2
O
O
FeCl3
HO
R2
R2
OH
N
R1
R1
R1
R1
OH
Scheme 4. The reaction pathway for the synthesis of the complexes
3.1.1. X-ray crystal structures of the FeQLNER complexes
The diffraction experiments and the structure refinements for the complexes are
summarized in Table 1. Selected bond lengths and angles are given in Tables 2 and 3.
Table 1.
Table 2.
Table 3.
The FeQLNEB, FeQLNEC and FeQLNEM complexes crystallize in the monoclinic space
group P21/c, while FeQLNEOB crystallizes in the triclinic space group P-1. For FeQLNEB
,
FeQLNEM and FeQLNEOB, the asymmetric part of the structure consists of a single complex
molecule, while in FeQLNEC it consists of two complex molecules and two water molecules
(Figs. 1-4). For all the complexes, the FeN3O3 coordination sphere has an octahedral
geometry. The Q ligand is bidentately coordinated to the Fe(III) ion via the phenolate O3
(O5 for FeQLNEOB) and pyridine N3 atoms. LNEX acts as a tetradentate ligand with the
phenolate O1 and O2 atoms occupying cis positions. In all the reported complexes, one
phenolate O atom is positioned trans to the hydroxyquinoline O3 atom, and one N atom of
the central ethylenediamine bridge is positioned trans to the hydroxyquinoline N3 atom. The
S,S-N1,N2 configuration is found for the FeQLNEB, FeQLNEM and FeQLNEOB molecules in the
asymmetric unit, although the opposite R,R configuration occurs in the other molecule due to
7

the centrosymmetric space group P21/c. In contrast, for FeQLNEC there is a difference in the
chirality of the N centers between two complex molecules constituting the asymmetric unit of
the structure. For molecule 1, the configuration is S,S-N1,N2, while in molecule 2, the R,R-
N31,N32 configuration is found. Analysis reveals, however, that there is no center of
symmetry or symmetry plane relating these molecules. Interestingly, only a non-
crystallographic two-fold axis can be found with the rotation of [ 1.000 -0.216 0.482] -
175.22o [35], that might relate molecule 1 to molecule 2, but with the chirality of the latter
inverted relative to that observed in the real structure.
Fig. 1- Fig. 4
Similar to other complexes containing LNEB or analogous ligands [33,34,36,37], in the
structures reported here the Fe-O1 and Fe-O2 bonds involving the LNEX phenolate atoms are
shortest within the coordination sphere, ranging from 1.8777(15) to 1.926(3) Å. These values
are significantly shorter than those of Fe1-O1 1.937(9) and Fe1-O2 1.955(8) Å reported for
the FeLNEB(TBC) complex [33], where TBC is tetrabromocatecholate. The Fe-O3 bonds
formed by the phenolate group of Q are significantly longer, with distances ranging from
1.978(2) to 2.009(2) Å. Also in all the complexes reported here, the Fe-N2 and Fe-N1 bonds
formed by the ethylenediamine moiety [2.173(3) to 2.323(3) Å] are significantly longer than
the Fe-N3 bonds formed by the N atom of the hydroxyquinoline ligand [2.132(3) to
2.1651(19) Å] (Table 2). Significant bulk is introduced in the vicinity of the central ion due to
the bidentate coordination of the rigid Q ligand. In particular, the direct coordination of the
ring system via the N3 atom results in deformation of the Fe coordination sphere. As a result,
the longest bond within the coordination sphere of all the complexes is formed by the N atom
of the ethylenediamine bridge, which is positioned cis to both the N and O atoms of the Q
ligand.
8

The valence geometry of the 8-hydroxyquinoline ligand is typical. The valence
geometry of the LNER ligand is typical for such systems, as found in complexes reported
before [33,34,36,37]. In FeQLNEB, the C-Br distances range from C2-Br1 of 1.882(9) to C15-
Br4 of 1.925(10) Å, almost identical to those reported for FeLNEB(TBC) [36]. For FeQLNEC
,
the C-Cl distances range from C2-Cl1 of 1.715(4) to C34-Cl32 of 1.752(4) Å, which and are
similar to those reported for FeLNEC(TBC) [33].
Similar to the FeLNEX or µ-oxo-FeLNEX complexes reported before, in all the FeQLNER
complexes reported here, the conformation of the ethylenediamine bridge is synclinal. In
FeQLNEB and FeQLNEOB, the N1-C8-C9-N2 torsion angle is 54.8(9) and 58.6(3)o,
respectively, and these values are similar to those found in the FeLNEB(TBC) [33] and
FeLNEOB complexes [33]. In FeQLNEM, this torsion angle (61.2(4)o) is larger than those
reported before for FeLNEM [34] or µ-oxo-FeLNEM [36]. In FeQLNEC, the conformation of the
ethylenediamine moiety is also synclinal, in molecule 1 the N1-C8-C9-N2 torsion angle
being 55.4(4)o and in molecule 2 the N31-C38-C39-N32 torsion angle being -57.6(5)o; the
absolute values are almost identical to that reported for FeLNEC-TBC [36].
With such a conformation of the ethylenediamine bridge, the dihedral angle between
the two phenolic rings in FeQLNEB is 80.5(4)o, smaller than that of 85.9o found for
FeLNEB(TBC), and the angles between the N3--C26 best plane of the Q ligand and C1--C6
and C11--C16 phenolic planes are 71.6(3) and 68.0(4)o, respectively. For FeQLNEOB, the
dihedral angle between the two phenolic rings of LNEOB is 75.16(13)o, a value significantly
larger than that of 69.95o found in FeLNEOB [33], and the angles between the N3--C26 Q best
plane and C1--C6 and C11--C16 phenolic planes are 89.79(10) and 88.06(13)o, almost 20º
larger than those found in FeQLNEB. In the FeQLNEM structure, the dihedral angle between
the two phenolic rings of LNEOB is 89.2(2)o, the largest value found for the series of
investigated complexes. It is, however, similar to that of 88.98º reported for µ-ox-FeLNEM
9

[36]. On the other hand, the analogous dihedral angle in FeLNEM is only 74.0o [34]. The
angles between the N3--C26 best plane of the Q ligand and the C1--C6 and C11--C16
phenolic planes are 83.62(17) and 66.57(19)o, the former being slightly smaller than those
found in FeQLNEOB, while the latter is similar to those found in FeQLNEB
.
In FeQLNEC, the dihedral angle between the two phenolic rings of LNECl is 87.40(18)o
in molecule 1 and 82.24(18)o in molecule 2, the values being between that of 80.5(4)o found
for the analogous complex of LNEB and 87.5o found for FeLNEC(TBC) [33], but being
significantly larger than that found for FeLNEC [34]. The Q ligand is almost perpendicular to
the phenolic rings of LNEC, with dihedral angles between the best plane of the Q ligand and
the phenolic C1--C6 and C11--C16 rings in molecule 1 being 83.49(16) and 86.72(15)o, and
the corresponding angles being 83.66(15) and 83.26(15)o in molecule 2. These values are
much larger than those found for FeQLNEB
.
Four chelate rings are formed in the FeQLNER complex molecules and the ring
puckering was analyzed according to Cremer and Pople [38]. In FeQLNEB, the Fe1-O3-C26-
C27-N3 ring is flat, Fe1-N1-C8-C9-N2 is an envelope on C9, ring Fe1-O1-C1-C6-C7-N1 has
a boat conformation, while Fe1-O2-C16-C11-C10-N2 is a screw-boat. For FeQLNEOB, the
conformation of the chelate ring Fe1-O5-C36-C37-N3 is twisted on Fe1-O5 and that of Fe1-
N1-C8-C9-N2 is twisted on C8-C9, Fe1-O1-C1-C6-C7-N1 and Fe1-O2-C16-C11-C10-N2
are a half-chair and an envelope, respectively. In FeQLNEM, the conformation of the chelate
ring Fe1-O5-C36-C37-N3 is an envelope on Fe1, Fe1-N1-C8-C9-N2 is an envelope on C9,
the conformations of both Fe1-O1-C1-C6-C7-N1 and Fe1-O2-C16-C11-C10-N2 are a screw-
boat. In FeQLNEC, the conformations of the chelate rings in molecule 1 are: Fe1-O3-C26-
C27-N3 twisted on N3-Fe1, Fe1-N1-C8-C9-N2 an envelope on C8, Fe1-O1-C1-C6-C7-N1
half-chair, Fe1-O2-C16-C11-C10-N2 a screw-boat. For molecule 2: Fe2-O33-C56-C57-N33
10

is flat, Fe2-N31-C38-C39-N32 an envelope on C39, Fe2-O31-C31-C36-C37-N31 a screw-
boat and Fe2-O32-C46-C41-C40-N32 an envelope.
3.1.2. Electronic absorption spectra of the complexes
The electronic absorption spectra of the complexes were obtained in the range 200-800 nm
(Figure 5, Table 4) and reflect the electronic properties of the iron(III) complexes. The visible
spectra of the complexes are dominated by two important charge transfer (CT) bands. All the
complexes exhibit an intense absorption band (Figure 5, Table 4) at 288-310 nm (104-105 M−1
cm−1) with a shoulder caused by π→π* CT transitions involving the phenolate and 8-
hydroxyquinoline units. The remaining lower energy bands (480-550 nm (104 M−1cm−1)) are
relatively less intense, which indicates that they are associated with the 8-hydroxyquinoline-
to-metal and phenolate-to-metal LMCT transitions [38]. A red shift of the position of the
LMCT bands of complexes to higher wavelengths can be observed when the substituents on
the phenolate groups are electron-donating, which would raise the energy of the frontier
orbitals of the ligand and thus minimize the ligand-to-iron gap.
Figure 5
Table 4
3.1.3. Magnetic susceptibility measurements
In the magnetic susceptibility diagrams, the susceptibilities of the investigated samples
increase with decreasing temperature (Figure 6). The analysis of the susceptibility χ(T) was
performed by applying the Curie-Weiss law,
, for T > 50 K, which gives us
through the Curie-Weiss
information on the magnitude of magnetic moments
constant C. Above T ≈ 15 K, the effective magnetic moments (µeff) of the complexes (Figure
7). are essentially temperature-independent and have µeff values between 5.2-6.3 µB, which is
11

in good agreement with the expected value (5.92 µB) for an isolated S = 5/2 Fe(III) ion [39-
42]. The obtained fitting parameters C, θ and µeff are listed in Table 5.
Small non-zero values of the Curie-Weiss temperatures θ can be regarded only as additional
fitting parameters that slightly improve the fit. No evidence of any antiferromagnetic or
ferromagnetic interactions between the iron magnetic centers can be deduced from the
measured data. A minute temperature dependence of the effective magnetic moment at the
lowest temperature may be attributed to zero-field splitting.
Figure 6
Figure 7
Table 5
3.1.4. Electrochemistry
The cyclic voltammograms (CV) of the complexes have been recorded in CH2Cl2 solutions
containing 0.1 M [(nBu)4N]ClO4 as the supporting electrolyte. The FeQLNER irreversible
metal-centered voltammograms have been observed in the negative potential range, which
corresponds to the FeIII/FeII reduction of the iron complexes. The voltammograms for all the
complexes are typical of mononuclear complexes, since they show only one redox metal-
centered process in the range -0.79 to -1.2 V. This observation is in agreement with the X-ray
analysis results. A typical cyclic voltammogram (CV) for the reduction process of FeQLNEOB
is presented in Fig. 8. Selected results for Epeak for the complexes are collected in Table 6.
The ER value for the FeQLNER complexes decreases in the order FeQLNEOB > FeQLNEM
>
FeQLNEB > FeQLNEC, which means that the two last complexes present less negative redox
potentials than the FeQLNEOB and FeQLNEM complexes, attributed to the most acidic iron(III)
centers in these complexes. This observation may be ascribed to the presence of the chloro
and bromo ligands placed on the phenolate groups, which decreases the electron density and
consequently increases the Lewis acidity of the iron center. FeQLNEOB presents the least acid
12

iron(III) center, related to electron donation of the alkoxide group in the ligand structure. In
addition, nearly similar redox potentials of the iron centers in all the complexes confirms the
similarity of the coordination environment of the iron centers in all the complexes (N2O2).
Our results reveal oxidation peak(s) for the FeQLNER complexes with a similar small
difference in their redox potentials in the positive region (Figure 9). This small difference
also supports that the coordination environment around the metal center in all the complexes
should be similar, as mentioned above. These redox processes might be attributed to the
presence of Fe(III)-phenoxyl radical species [41-43] during the cyclic voltammetry
experiments, a kind of ligand centered redox process in which the phenolate group yields a
phenoxyl radical, though there is no direct evidence available for this yet. Similar
observations have been described previously in other compounds containing tert-butylphenol
ligands, being ascribed as the oxidation of this unit and the generation of the phenoxyl radical
[44-46]. FeQLNEOB showed two quasi-reversible oxidative process, centered on the ligand,
indicating that the first oxidation peak must be due to mono radical cation formation on one
phenolate ring [Fe-OPh•]+ and the second to either dication formation (this seems unlikely) or
to the formation of a second radical cation on the other phenolate ring unit [Fe-OPh••]+. The
second oxidation peak is observed at higher potentials, which means that the second
oxidation is harder relative to the oxidation of the radical cation complex. On the other hand,
the other complexes show just one irreversible oxidative peak. Wieghardt’s group have
proposed that the lack of reversibility of these processes is attributed to a competitive
chemical reaction (polymerization of phenoxyl radicals) which can be induced by the
electrochemical process, resulting in the extinction of the radical species [47]. If we assume
that in FeQLNEOB, the first and second oxidation peaks are related to the formation of [Fe-
OPh•]+ [Fe-OPh••]+ species while in other complexes there is just the production of [Fe-
OPh•]+ species, we can conclude that the FeQLNEOB phenoxyl radicals are much more stable
13

than the corresponding ones of the three other complexes. It considers that the tert-butyl
groups placed on the phenolate rings prevent unwanted chemical reactions of the phenoxyl
radicals, such as polymerization and C-C coupling, due to the high steric effect and inhibition
of the sterically shielded phenoxyl radicals lying too close together. On comparison of the
oxidation potentials of the complexes (Table 6), a red shift in the anodic oxidation peaks has
been observed suggesting that the anodic processes are sensitive to the involved change in the
electron accepting nature of the substituents on the phenolate moieties of the ligands. On the
other hand, the oxidation potentials of the FeQLNEC and FeQLNEB complexes, with electron
accepting ligands such as Cl, are more positive than FeQLNEM and FeQLNEOB (with electron
donating methyl and methoxy groups) due to the harder oxidation of the phenolate ligands to
the corresponding phenoxyl radical cations. These observations may be explained by the fact
that the two last complexes represent the most acidic iron centers and that, due to this, the
electron density is preferentially localized on the aromatic rings, which makes it easier to
carry out their oxidation. Interestingly, FeQLNEOB shows the lowest 1st oxidation potential
compared to the other complexess, so the 2nd oxidation process of FeQLNEOB related to the
formation of a radical from a radical cation appears to align very well with the oxidation peak
potential of the other three compounds with the production of a radial from a neutral
compound. Such steric and electronic effects on the easy production and stability of radicals
in the metalloradical world has been consider by nature for metalloenzymes [18].
Figure 8
Figure 9
Table 6
4. Conclusions
A series of new mixed ligand iron(III) complexes of N,N'-dimethylethylenediamine based
bis(phenol) di-amine (H2LNER) and 8-hydroxyquinolone(HQ) ligands have been synthesized
14

and characterized. The complexes FeQLNEB, FeQLNEC, FeQLNEM and FeQLNEOB have been
structurally characterized by single crystal X-ray diffraction. X-ray structure analysis has
revealed that all the complexes are six coordinate and Fe(III) centers are surrounded by three
phenolic oxygen atoms, comprising two oxygen atoms of H2LNER and another one from the
HQ ligand, and three nitrogen atoms from both H2LNER and HQ ligands. Employing various
ligands with electron-withdrawing or electron donating substituents on the phenolate arms,
we have obtained different complexes, combinations with interesting variations in properties,
which are useful in the study of a structure-activity relationship. The large influence of the
ligand sets and the electron density of the metal centre on both the position of the CT
transitions in the UV-vis spectra and the peak potentials in the CV measurements suggest that
both redox and charge transfer processes are sensitive to the electron accepting properties of
the substituents on the phenolate moieties of the ligands. The magnetic moment
measurements confirm monomer complexes with paramagnetic Fe(III) centres.
Appendix A.
The crystallographic data have been deposited with the Cambridge Crystallographic Data
Centre, with CCDC numbers: 1415983, 1415984, 1415985 and 1415986 for FeQLNEB
,
FeQLNEC, FeQLNEM and FeQLNEOB , respectively. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgments
The authors are grateful to the Institute for Advanced Studies in Basic Sciences (IASBS),
Nicolaus Copernicus, Ljubljana Universities for their valuable help. E. Safaei gratefully
15

acknowledges the support by the Institute for Advanced Studies in Basic Sciences (IASBS)
Research Council under Grant No. G2015IASBS127.
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18

Table 1. Crystal data and structure refinement for FeQLNEB, FeQLNEC, FeQLNEM and FeQLNEOB
FeQLNEB
FeQLNEOB
FeQLNEM
FeQLNEC
Empirical formula
Formula weight
Temperature; K
Wavelength; Å
C27H24Br4FeN3O3
813.98
C37H48FeN3O5
670.63
C31H36FeN3O3
554.48
C54H52Cl8Fe2N6O8
1308.32
293(2)
293(2)
293(2)
293(2)
0.71073
0.71073
0.71073
0.71073
Crystal system,
space group
Unit cell
dimensions;
a, Å
b, Å
c, Å
Monoclinic, P21/c
Triclinic, P-1
Monoclinic, P21/c
Monoclinic, P21/c
11.0934(6)
9.2948(6)
28.0609(17)
11.2421(3)
12.2205(4)
15.1712(5)
75.212(3)
16.9388(11)
8.4293(4)
21.4119(13)
12.2402(2)
21.5512(3)
22.7938(4)
α, º
β, º
96.683(5
74.490(3)
65.101(3)
1797.26(10)
113.255(8)
102.5225(16)
γ, º
Volume; Å3
2873.7(3)
4, 1.881
2808.9(3)
4, 1.311
5869.77(17)
4, 1.480
Z, Calculated
density; Mg/m3
Absorption
2, 1.239
6.121
0.464
0.573
0.916
coefficient; mm-1
F(000)
1588
714
1172
2680
Crystal size; mm
0.50 x 0.45 x 0.31
2.31 to 26.00
0.80 x 0.37 x 0.16
2.15 to 26.00
0.47 x 0.20 x 0.04
2.07 to 28.17
0.41 x 0.27 x 0.21
2.21 to 28.16
Theta range for
data collection; º
Limiting indices
-13<=h<=12,
-11<=k<=6,
-32<=l<=34
-13<=h<=9,
-14<=k<=15,
-18<=l<=18
-20<=h<=21,
-10<=k<=11,
-28<=l<=26
-15<=h<=16,
-28<=k<=26,
-29<=l<=26
Reflections collected 16206 / 5644 [R(int)
11272 / 7042
[R(int) = 0.0344]
26.00 99.7 %
20677 / 6204
[R(int) = 0.0866]
26.00 100.0 %
35538 / 12823 [R(int)
= 0.0291]
/ unique
= 0.0589]
26.00 99.9 %
Completeness to
theta
26.00 99.8 %
Absorption
correction
Max. and min.
transmission
Refinement method
Analytical
Analytical
Analytical
Analytical
0.2514 and 0.1493
0.9291 and 0.7075
0.9802 and 0.7740
0.8306 and 0.7070
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Data / restraints /
parameters
5644 / 0 / 343
7042 / 0 / 415
6204 / 0 / 343
12823 / 0 / 704
Goodness-of-fit on
1.029
1.093
0.855
0.984
F2
Final R indices
[I>2sigma(I)]
R indices (all data)
R1 = 0.0678, wR2 =
0.1616
R1 = 0.1372, wR2 =
0.1884
R1 = 0.0458, wR2
= 0.1260
R1 = 0.0576, wR2
R1 = 0.0586, wR2
= 0.1237
R1 = 0.1441, wR2
= 0.1429
R1 = 0.0532, wR2 =
0.1651
R1 = 0.0848, wR2 =
0.1772
= 0.1340
Largest diff. peak
and hole; e.A-3
2.270 and -1.099
0.535 and -0.324
0.560 and -0.501
1.307 and -0.449
19

Table 2. Selected bond lengths [Å] and angles [deg] for FeQLNEB, FeQLNEOB, FeQLNEM
FeQLNEB
FeQLNEOB
FeLNEM
Bond lengths/ Å
Fe1-O1
Fe1-O2
Fe1-O3
Fe1-N3
Fe1-N2
1.918(5)
1.891(6)
1.984(5)
2.159(7)
2.194(6)
2.323(7)
Fe1-O1
Fe1-O2
Fe1-O5
Fe1-N3
Fe1-N2
Fe1-N1
1.9036(15)
1.8777(15)
2.0077(15)
2.1651(19)
2.2084(17)
2.3066(19)
Fe1-O1
Fe1-O2
Fe1-O3
Fe1-N3
Fe1-N2
Fe1-N1
1.888(3)
1.889(2)
2.009(2)
2.142(3)
2.187(3)
2.323(3)
Fe1-N1
Bond angles/°
O2-Fe1-O1
O2-Fe1-O3
O1-Fe1-O3
O2-Fe1-N3
O1-Fe1-N3
O3-Fe1-N3
O2-Fe1-N2
95.5(3)
96.1(3)
162.6(2)
93.0(3)
88.2(2)
78.4(2)
87.4(2)
O2-Fe1-O1
O2-Fe1-O5
O1-Fe1-O5
O2-Fe1-N3
O1-Fe1-N3
O5-Fe1-N3
O2-Fe1-N2
96.26(7)
93.66(7)
164.93(7)
101.34(7)
89.10(7)
77.83(7)
86.27(6)
O1-Fe1-O2
O1-Fe1-O3
O2-Fe1-O3
O1-Fe1-N3
O2-Fe1-N3
C1-O1-Fe1
C8-N1-Fe1
94.20(11)
169.05(10)
95.28(11)
94.99(12)
97.91(10)
138.1(2)
105.2(2)
C26-O3-Fe1
C27-N3-Fe1
O1-Fe1-N2
117.5(5)
110.3(5)
104.3(2)
O1-Fe1-N2
C8-N1-Fe1
C22-N1-Fe1
99.83(7)
106.98(13)
113.22(14)
C19-N1-Fe1
C7-N1-Fe1
O3-Fe1-N3
112.8(2)
109.8(2)
78.30(11)
O3-Fe1-N2
N3-Fe1-N2
O2-Fe1-N1
O1-Fe1-N1
O3-Fe1-N1
N3-Fe1-N1
C18-N2-Fe1
N2-Fe1-N1
C1-O1-Fe1
C8-N1-Fe1
C17-N1-Fe1
89.0(2)
167.4(2)
165.1(2)
85.0(2)
C7-N1-Fe1
O5-Fe1-N2
N3-Fe1-N2
O2-Fe1-N1
O1-Fe1-N1
O5-Fe1-N1
N3-Fe1-N1
N2-Fe1-N1
C1-O1-Fe1
C23-N2-Fe1
C10-N2-Fe1
110.16(14)
92.09(67)
167.65(7)
164.80(7)
85.79(6)
O1-Fe1-N2
O2-Fe1-N2
O3-Fe1-N2
N3-Fe1-N2
C10-N2-Fe1
C9-N2-Fe1
C20-N2-Fe1
C31-N3-Fe1
O1-Fe1-N1
O2-Fe1-N1
O3-Fe1-N1
96.06(13)
89.62(11)
89.46(12)
166.12(13)
107.3(3)
87.1(2)
101.9(3)
113.2(5)
78.0(2)
87.61(7)
107.3(2)
93.75(7)
113.5(2)
78.54(7)
109.8(2)
131.1(5)
107.9(5)
109.8(6)
139.81(14)
111.59(14)
108.78(12)
84.78(11)
168.70(10)
86.97(11)
C7-N1-Fe1
C9-N2-Fe1
111.3(5)
107.0(5)
C9-N2-Fe1
108.93(14)
129.34(17)
N3-Fe1-N1
N2-Fe1-N1
93.38(10)
79.31(11)
C29-N3-Fe1
20

Table 3. Selected bond lengths [Å] and angles [deg] for FeQLNEC
Molecule 1
Molecule 2
Bond lengths/ Å
Fe1-O2
Fe1-O1
1.912(2)
1.921(2)
1.978(2)
2.147(3)
2.173(3)
2.303(3)
Fe2-O32
Fe2-O31
Fe2-O33
Fe2-N33
Fe2-N32
Fe2-N31
1.916(3)
1.925(3)
1.983(3)
2.132(3)
2.199(3)
2.300(3)
Fe1-O3
Fe1-N3
Fe1-N1
Fe1-N2
Bond angles/°
O2-Fe1-O1
O2-Fe1-O3
O1-Fe1-O3
O2-Fe1-N3
O1-Fe1-N3
O3-Fe1-N3
O2-Fe1-N1
O1-Fe1-N1
91.75(10)
169.77(11)
94.88(11)
92.27(11)
101.52(11)
78.78(10)
101.41(11)
87.38(11)
O32-Fe2-O31
O31-Fe2-O33
O32-Fe2-O33
O31-Fe2-N33
O32-Fe2-N33
O33-Fe2-N33
O31-Fe2-N32
O31-Fe2-N31
92.66(11)
167.39(11)
95.53(12)
90.54(11)
101.88(11)
78.40(11)
103.87(11)
85.69(11)
O3-Fe1-N1
N3-Fe1-N1
86.70(10)
O33-Fe2-N31
N33-Fe2-N32
88.97(12)
163.49(11)
163.31(12)
O2-Fe1-N2
O1-Fe1-N2
O3-Fe1-N2
N3-Fe1-N2
N1-Fe1-N2
C1-O1-Fe1
C17-N1-Fe1
C8-N1-Fe1
C7-N1-Fe1
C18-N2-Fe1
85.11(10)
166.00(11)
90.25(11)
92.24(11)
79.90(11)
133.1(2)
113.5(2)
107.8(2)
108.8(2)
110.0(2)
O32-Fe2-N32
O32-Fe2-N31
O33-Fe2-N32
N33-Fe2-N31
N32-Fe2-N31
C31-O31-Fe2
C47-N31-Fe2
C38-N31-Fe2
C37-N31-Fe2
C48-N32-Fe2
86.03(11)
164.26(12)
86.28(11)
93.79(12)
79.20(12)
133.6(2)
111.7(3)
107.1(2)
111.5(2)
111.8(2)
C9-N2-Fe1
C10-N2-Fe1
107.2(2)
112.0(2)
C39-N32-Fe2
C40-N32-Fe2
107.2(2)
108.6(2)
C16-O2-Fe1
133.7(2)
C46-O32-Fe2
134.1(2)
21

Table 4. λmax/nm (ε/M-1 cm-1) LMCT band of complexes FeQLNER
ε ×10-5(M-1 cm-1)
max(nm)
FeQLNER
λ
max(nm)
ε×10-5(M-1 cm-1)
λ
288
279
(0.8)
480
477
(0.3)
(0.3)
FeQLNEB
FeQLNEC
(0.7)
290
310
(0.6)
(1.3)
556
522
(0.2)
(0.8)
FeQLNEOB
FeQLNEM
Table 5. Calculated parameters C, µeff and θ from the fit of the measured data.
FeQLNER
µ
(BM)
C (emu K/mol)
θ (K)
eff
NEB
NEC
NEM
4.2
-0.93
5.8
FeQL
FeQL
4.1
3.7
5.0
-0.1
0.0
1.0
5.7
5.4
6.3
FeQL
NEOB
FeQL
22

Table 6. Electrode peak potentials for oxidation and reduction of the complexes FeQLNER
measured at ambient temperature in CH2Cl2 solutions and referenced vs. the Fc+/Fc couple.
E1ox/V
E2ox/V
E1red/V
E2red/V
FeQLNEB
FeQLNEC
-
-
-
-
-0.82
-0.79
0.59
0.68
FeQLNEM
FeQLNEOB
-
-
-0.98
0.59
0.25
0.59
-1.2
-
23

Figures
Figure 1. ORTEP diagram and atom labelling scheme for the complex FeQLNEB. Ellipsoids
plotted at the 30% probability level.
Figure 2. ORTEP diagram and atom labelling scheme for the complex FeQLNEOB. Ellipsoids
plotted at the 30% probability level. Hydrogen atoms are omitted for clarity.
24

Figure 3. ORTEP diagram and atom labelling scheme for the complex FeQLNEM. Ellipsoids
plotted at the 30% probability level.
Figure 4. ORTEP diagram and atom labelling scheme for the complex FeQLNEC. Ellipsoids
plotted at the 30% probability level. Hydrogen atoms are omitted for clarity.
25

Figure 5. Electronic absorption spectra of FeQLNER in (10 µM) CH2Cl2 solution
Figure 6. Temperature dependence of the susceptibilityχ(T) of the FeQLNEOB
FeQLNEB, FeQLNEC, and FeQLNEM complexes measured in a magnetic field of H =
1000 Oe
,
26

Figure 7. Temperature dependence of the effective magnetic moments for all four samples
Figure 8. Cyclic voltammogram (reduction process) of the FeQLNEC
complex in CH2Cl2 at -80°C (sc 100 mV s-1)
27

Figure 9. Cyclic voltammogram (oxidation process) of the FeQLNEOB complex in
CH2Cl2 at -80 °C (sc 100 mV s-1)
28

New Mixed-Ligand 8-Hydroxyquinolinato Iron(III) Complexes
of Dimethylethylenediamine-based Aminophenol Ligands
Four new mixed ligand iron(III) complexes of N,N'-dimethylethylenediamine based
bis(phenol) di-amine (H2LNER) and 8-hydroxyquinoline(HQ) ligands have been synthesized
and characterized by spectroscopic methods, X-ray diffraction, magnetic susceptibility
measurements and cyclic voltammetry techniques.
29