Exchange interaction in a dinuclear iron(III) complex of a heptadentate schiff base: Synthesis, crystal structure and magnetic properties

Article abstract of DOI:10.1515/znb-2005-0705

Synthesis, crystallographic characterization and magnetic properties of the new dinuclear iron(III) complex Fe₂L(OCH₃)Cl ₂(L=1,3-bis[N-(3,5-dichloro-2-hydroxybenzylidene)-2-aminoethyl]-2-(3, 5-dichlo ro-2-hydroxyphenyl)imidazolidine) are reported. The structure consists of dinuclear units. The two iron(III) atoms are asymmetrically bridged by a phenoxo and a methoxo group. The iron(III) centers are separated by 3.133(2) ?. The Magnetic susceptibility of the complex was measured over the range 5 - 298 K and the observed data were successfully simulated by the equation based on the spin-Hamiltonian operator H = -JS^→ ₁.S^→₂. Magnetic susceptibility measurements indicate very weak antiferromagnetic coupling between the iron ions with J = -10.8 cm^-1 and g = 2.0.

Full text of DOI:10.1515/znb-2005-0705

Exchange Interaction in a Dinuclear Iron(III) Complex of a Heptadentate
Schiff Base: Synthesis, Crystal Structure and Magnetic Properties
C. T. Zeyreka, A. Elmalib, and Y. Elermanb
a Ankara Nuclear Research and Training Center, Turkish Atomic Energy Authority,
06100 Besevler-Ankara, Turkey
b Department of Engineering Physics, Faculty of Engineering, Ankara University,
06100 Besevler-Ankara, Turkey
Reprint requests to Prof. Dr. A. Elmali. E-mail: elmali@eng.ankara.edu.tr
Z. Naturforsch. 60b, 727 – 731 (2006); received February 4, 2005
Synthesis, crystallographic characterization and magnetic properties of the new dinuclear iron(III)
complex Fe2L(OCH3)Cl2 (L=1,3-bis[N-(3,5-dichloro-2-hydroxybenzylidene)-2-aminoethyl]-2-(3,5-
dichlo ro-2-hydroxyphenyl)imidazolidine) are reported. The structure consists of dinuclear units. The
two iron(III) atoms are asymmetrically bridged by a phenoxo and a methoxo group. The iron(III)
˚
centers are separated by 3.133(2) A. The Magnetic susceptibility of the complex was measured over
the range 5 – 298 K and the observed data were successfully simulated by the equation based on the
ꢀ ꢀ
spin-Hamiltonian operator H = −JS1.S2. Magnetic susceptibility measurements indicate very weak
antiferromagnetic coupling between the iron ions with J = −10.8 cm−1 and g = 2.0.
Key words: Dinuclear Iron(III) Complex, Super-Exchange Interactions,
Antiferromagnetic Coupling, Schiff Base, Imidazolidine
Introduction
In the last decades, the electronic and magnetic
properties of dinuclear iron complexes have been ex-
tensively studied [1 – 4]. They provide structural mod-
els for dinuclear sites in several proteins involved in
oxygen storage of hemerytherin and oxygen activation
of methanemonooxygenase [5 – 8]. In addition, these
systems were treated as model systems for the under-
standing of the size and magnitude of exchange cou-
pling interactions in theoretical considerations. Studies
of some iron complexes have shown that the oxo bridge
is responsible for the strong antiferromagnetic cou-
pling (usually −50 > J > −200 cm−1) [9 – 12] and that
complexes with alkoxo, phenoxo, or hydroxo bridges
are weakly coupled (usually 0 > J > −30 cm−1
)
[11, 13 – 16]. In an attempt to accumulate more data for
further discussions of magneto-structural correlations
for this type of complexes, we present here the synthe-
sis, X-ray structural and magnetic characterization of a
derivative of a closely related ligand, Fe2L(OCH3)Cl2.
Experimental Section
Fig. 1. Formulae of the ligand and its complex.
Preparation of Fe2L(OCH3)Cl2
solution of triethylenetetramine (2.2 g, 15 mmol) in methanol
(20 ml) was added dropwise to a methanolic solution (40 ml)
The dinuclear Fe(III) complex was prepared in two steps.
In the first step, the Schiff base ligand L was synthesized. A
c
0932–0776 / 06 / 0700–0727 $ 06.00 ꢀ 2006 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
Unauthenticated
Download Date | 9/11/18 11:20 AM

728
C. T. Zeyrek et al. · Exchange Interaction in a Dinuclear Iron(III) Complex
Table 1. Crystallographic data for the investigated complex. Table 2. Atomic coordinates and equivalent isotropic dis-
2
˚
placement parameters (A ) of the non-hydrogen atoms.
Formula
C28H24Cl8Fe2N4O4
875.81
monoclinic
P21
Formula weight (g.mol−1
Crystal system
Space group
)
Atom
C1
x
y
z
∗U(eq)
0.7303(3)
0.8495(4)
0.9365(4)
0.9085(4)
0.7962(3)
0.7033(3)
0.5877(3)
0.3671(3)
0.2280(3)
0.1288(3)
0.1327(3)
0.2306(3)
0.3697(3)
0.6093(3)
0.7448(3)
0.8611(3)
0.9927(3)
1.0113(5)
0.8961(4)
0.7595(3)
0.2591(2)
0.3953(3)
0.3765(3)
0.5003(3)
0.6403(3)
0.6554(3)
0.5328(3)
0.4511(3)
0.4868(2)
0.2549(2)
0.2568(2)
0.4937(2)
0.6523(2)
0.5449(2)
0.6558(2)
0.4268(2)
0.33426(8)
0.34235(9)
0.88924(8)
1.01530(7)
0.91845(8)
1.14556(8)
0.82780(8)
0.47898(9)
0.46535(4)
0.46203(5)
0.9766(2)
1.0429(3)
1.0354(4)
0.9539(4)
0.8846(3)
0.8957(3)
0.8205(3)
0.7446(3)
0.7886(2)
0.9085(3)
0.9132(3)
0.7886(2)
0.7403(2)
0.7978(2)
0.8568(2)
0.8221(3)
0.8739(4)
0.9630(4)
0.9997(4)
0.9467(2)
0.7689(2)
0.6976(2)
0.5895(2)
0.5283(3)
0.5714(3)
0.6766(2)
0.7441(2)
1.1366(3)
0.8222(2)
0.8419(2)
0.8433(2)
0.8142(2)
0.9906(2)
0.8478(2)
0.9859(2)
1.0261(2)
1.04532(6)
1.03690(7)
1.14067(6)
0.94008(8)
1.10816(6)
0.82728(6)
0.73157(6)
0.39221(7)
0.93577(4)
0.93949(4)
0.0743(2)
0.0659(2)
0.0239(4)
0.0367(7)
0.0434(8)
0.0412(8)
0.0352(7)
0.0269(5)
0.0265(5)
0.0294(5)
0.0254(5)
0.0299(6)
0.0301(6)
0.0267(5)
0.0293(5)
0.0286(5)
0.0293(5)
0.0356(6)
0.0397(8)
0.0472(9)
0.0405(7)
0.0247(4)
0.0189(4)
0.0200(4)
0.0218(4)
0.0270(5)
0.0291(5)
0.0253(5)
0.0186(4)
0.0452(9)
0.0214(4)
0.0195(4)
0.0175(3)
0.0230(4)
0.0273(4)
0.0175(3)
0.0195(3)
0.0309(4)
0.0367(2)
0.0383(2)
0.0367(2)
0.0356(2)
0.0362(2)
0.0361(2)
0.0364(2)
0.0369(2)
0.0307(2)
0.0316(1)
C2
˚
a [A]
9.187(1)
C3
C4
C5
C6
C7
C8
C9
−0.0018(2)
−0.0690(2)
−0.0619(2)
0.0081(2)
0.0070(2)
0.0466(2)
0.0768(2)
0.1858(2)
0.2903(2)
0.4135(2)
0.4664(2)
0.5249(2)
0.5310(2)
0.5940(2)
0.6033(2)
0.5528(2)
0.4908(2)
0.4774(2)
0.2467(1)
0.2579(2)
0.2564(2)
0.2602(2)
0.2655(2)
0.2698(2)
0.2659(2)
0.2624(3)
0.0611(1)
0.1686(1)
0.3274(1)
0.4655(2)
0.1410(2)
0.2686(1)
0.4174(1)
0.2622(2)
0.04421(6)
0.46808(6)
0.14876(6)
−0.15553(4)
0.42572(6)
0.67739(6)
0.27890(6)
0.25843(6)
0.37205(3)
0.15674(3)
˚
b [A]
12.849(1)
14.569(2)
97.68(1)
1704.4(3)
2
˚
c [A]
β [◦]
3
˚
Vol [A ]
Z
Dcalc [g.cm−3
]
1.707
1.520
µ [cm−1
F(000)
]
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
C26
C27
C28
N1
N2
N3
N4
O1
O2
O3
O4
Cl1
Cl2
Cl3
Cl4
Cl5
Cl6
Cl7
Cl8
Fe1
Fe2
880 ◦
θ Range for data collection
Index ranges
2.60 < θ < 30.15◦
−12 ≤ h ≤ 12
0 ≤ k ≤ 17
0 ≤ l ≤ 20
5123
Reflections collected
Independent reflections
Data / parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
Final R indices (all data)
4880
4880 / 185
1.154
R = 0.0483, wR = 0.9875
R = 0.0591, wR = 0.1138
Flack parameter [x] (Flack, 1983) [31] 0.06(1)
Largest diff. peak and hole
−3
˚
0.367 and −0.584 eA
of 3,5-dichlorosalicylaldehyde (8.6 g, 45 mmol) with stirring
at room temperature. The Schiff base ligand was obtained in
the form of yellow crystals. In the second step, for the prepa-
ration of the Fe(III) complex, a 15 ml quantity of a 0.1 M
solution of CH3ONa in methanol was added to a solution of
FeCl3·6H2O in the same solvent (0.54 g, 2 mmol, in 20 ml).
To the resulting solution, was added with stirring the ligand
L dissolved in methanol (0.61 g, 1 mmol). The dark blue
solution was allowed to evaporate at room temperature for
one day to give prismatic dark blue crystals, which were col-
lected, washed with cold absolute ethanol and dried in air.
The chemical formula of Fe2L(OCH3)Cl2 and L are shown in
Fig. 1. Yield 0.83 g (72.2%) C28H24Cl8Fe2N4O4 (875.81):
calcd. C 38.40, H 2.76, N 6.40, Fe 12.75; found C 38.91,
H 2.86, N 6.49, Fe 12.81. Infrared spectrum (cm−1, KBr
˜
˜
˜
disk): ν (phenolic OH) 3445(w); ν (C=N) 1642(s); ν (phe-
nolic C–O) 1376(s).
X-ray structure determination
∗ U(eq) = (1/3)ΣiΣjUija∗i a∗j aiaj.
A crystal of dimensions 0.40 × 0.18 × 0.02 mm3 was
mounted on a Rigaku AFC7S diffractometer with graphite
˚
˚
0.96 A), and refined using a riding model. H atom displace-
ment parameters were restricted to be 1.2 Ueq of the parent
atom. Selected bond distances and bond angles are listed in
Table 3. An ORTEP view of the molecular structure is given
in Fig. 2 [20, 21]. Crystallographic data (excluding structure
factors) have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no. CCDC
262604 for Fe2L(OCH3)Cl2 [22].
monochromatized Mo-Kα radiation (λ = 0.71073 A). Exper-
imental conditions are summarized in Table 1. The final po-
sitional parameters are presented in Table 2. Data collection,
reduction and corrections for absorption and decomposition
were achieved using TEXAN Single Crystal Structure Anal-
ysis Software [17]. The structure was solved by SHELXS-97
[18] and refined with SHELXL-97 [19]. The positions of the
H atoms bonded to C atoms were calculated (C-H distance
Unauthenticated
Download Date | 9/11/18 11:20 AM

C. T. Zeyrek et al. · Exchange Interaction in a Dinuclear Iron(III) Complex
729
◦
˚
Table 3. Selected bond lenghts (A) and bond angles ( ) char-
acterizing the inner coordination sphere of the iron(III) cen-
ters (see Fig. 2 for labeling scheme adopted).
Table 4. Least-squares pseudomirror plane within the
Fe2L(OCH3)Cl2 molecule, −0.686(4)x −0.139(5)y
+
14.543(3)z = 3.379(4).
Fe1···Fe2
Fe1–N3
Fe1–N4
Fe2–N1
Fe2–N2
Fe1–O2
Fe1–O3
Fe2–O1
Fe2–O2
Fe1–Cl2
Fe2–Cl1
3.133(2)
2.274(2)
2.065(2)
2.085(2)
2.305(2)
2.092(2)
1.899(2)
1.910(2)
2.071(2)
2.312(1)
2.324(1)
O3–Fe1–O4
O3–Fe1–N4
O3–Fe1–O2
O4–Fe1–O2
N4–Fe1–O2
O3–Fe1–N3
O4–Fe1–N3
N4–Fe1–N3
O2–Fe1–N3
O3–Fe1–Cl2
O4–Fe1–Cl2
N4–Fe1–Cl2
O2–Fe1–Cl2
N3–Fe1–Cl2
O1–Fe2–O2
O1–Fe2–N1
O2–Fe2–N1
O2–Fe2–N2
O1–Fe2–Cl1
O2–Fe2–Cl1
N2–Fe2–Cl1
98.04(9)
89.88(9)
92.27(7)
77.01(9)
92.35(8)
168.17(8)
91.29(9)
79.67(8)
82.74(7)
95.81(6)
96.38(8)
93.15(7)
170.24(5)
90.32(6)
92.50(8)
89.15(9)
93.07(8)
81.47(7)
95.66(8)
170.25(5)
91.56(5)
˚
˚
Atom
O2
C22
C24
C26
Cl7
Dev (A)
Atom
C21
C23
C25
C27
Cl8
Dev (A)
−0.050(2)
0.023(2)
0.040(2)
0.019(2)
−0.005(2)
0.0023(2)
0.010(1)
−0.040(2)
0.025(2)
−0.005(2)
−0.019(3)
O4
0.012(2)
C28
2.828(χ T)1/2, where χ is the magnetic susceptibility per
Fe(III) ion.
Fe1–O2–Fe2
Fe1–O4–Fe2
O1–Fe2–O4
O4–Fe2–O2
O4–Fe2–N1
O4–Fe2–N2
N1–Fe2–N2
O4–Fe2–Cl1
N1–Fe2–Cl1
97.63(8)
107.78(9)
98.53(9)
77.73(8)
168.19(9)
91.48(8)
79.72(8)
95.76(7)
92.37(6)
Results and Discussion
X-ray crystal structure
The structure of Fe2L(OCH3)Cl2 consists of dinu-
clear units that are well separated from one another.
The two iron(III) ions are asymmetrically bridged
by phenoxy and methoxy oxygen atoms to form a
four-membered ring. The geometry around the metal
atoms is distorted octahedral, as shown in Fig. 2. The
molecule has a nearly perfect Cs symmetry with the
mirror plane perpendicular to the iron-oxygen ring.
The bridging oxygen atoms O2 and O4 and the car-
bon atoms C21-C27 and C28 lie approximately on this
mirror plane. Deviations of these atoms from the least-
squares pseudomirror plane are given in Table 4. The
dihedral angle between the Fe1O2O4 and Fe2O2O4
planes is 13.1(1)◦. The iron(III) ions are displaced
˚
from the coordination plane by −0.089(1) A for Fe1
···
···
˚
and −0.090(1) A for Fe2. The Fe Fe and O2 O4 sep-
˚
arations are 3.133(2) and 2.531(2) A, respectively. The
bridging angle at the phenoxy oxygen atoms, 97.6(2) ◦,
is smaller than that at the methoxy oxygen atoms,
107.9(1)◦. The Fe–N(imine) bond lengths of 2.065(2)
˚
and 2.085(2) A are significantly shorter than the Fe–
N(amine) bond lengths of 2.274(2) and 2.305(2) A.
˚
The overall geometry distortion is as expected for an
octahedral low-spin d5 iron(III) ion with hexadentate
chelation. The geometry is similar to that of related
binuclear iron complexes [24, 25]. The phenoxy and
methoxy carbon atoms attached to the bridging oxy-
gens are displaced on opposite sides of the bridging
Fig. 2. View of the molecule (numbering of atoms corre-
sponds to Table 2). Displacement ellipsoids are plotted at the
50% probability level. The hydrogen atoms are omitted for
clarity.
Susceptibility measurements
Variable-temperature magnetic susceptibility measure-
ments of a powdered sample were performed with a
QUANTUM Design SQUID magnetometer in the tempera-
ture range 5-298 K. The applied field was about 1 T. Dia-
magnetic corrections of the molar magnetic susceptibility
were applied using Pascal’s constant [23]. The effective
˚
˚
plane by −0.037(2) A for C27 and 0.252(2) A for C28,
respectively.
Magnetic properties
The magnetic susceptibilities of the complex are
shown as a function of temperature in Fig. 3, and the
magnetic moments were calculated by the equation µeff
=
Unauthenticated
Download Date | 9/11/18 11:20 AM

730
C. T. Zeyrek et al. · Exchange Interaction in a Dinuclear Iron(III) Complex
Fig. 3. The molar magnetic susceptibilities χ per iron(III) vs.
temperature T curve.
Fig. 4. Magnetic moment per iron(III) vs. temperature curve.
magnetic moments are shown as a function of temper-
ature in Fig. 4. The magnetic data was fitted using the
expression for molar susceptibility vs. T derived from
magnetic moment per iron(III) at 298.4 K is
7.44 B. M., but 0.69 B. M. at 5 K. The magnetic sus-
ceptibility is at a maximum near 61 K and decreases
rapidly as the temperature is lowered to liquid helium
temperature.
ꢀ ꢀ
the spin exchange Hamiltonian, H = −JS1.S2, where J
is the interaction parameter between two spin carriers
with S1 = S2 = 5/2 as given in eq. (1).
The extent of antiferromagnetism exhibited by
Fe2L(OCH3)Cl2 is in the range so far observed
(7 ≤ |J| ≤ 17 cm−1) for iron(III) complexes involv-
ing Fe2O2 bridging units [13– 16, 25 – 27]. Recently,
Gorun and Lippard suggested a quantitative magne-
tostructural relationship for dinuclear iron(III) cen-
ters, doubly bridged by oxygen atoms [28]. This ex-
ponential relationship, −J = A.exp(BP), correlates
the antiferromagnetic super-exchange coupling con-
stant J with a parameter P, describing the shortest
super-exchange pathway between the two metal cen-
ters. Using the reported values for A(8.763×1011) and
χ = (1−xp)χꢀꢀ +2xpχꢀ +TIP
C
χꢀꢀ = [2exp(2x)+10exp(6x)+28exp(12x)
T
+60exp(20x)+110exp(30x)]
(1)
· [1+3exp(2x)+5exp(6x)+7exp(12x)
+9exp(20x)+11exp(30x)]−1
C
J
NLg2µB2
χꢀ =
,
x =
,
C =
3T
kT
k
˚
B(−12.663) and the value P = 1.965 A as found in the
present study, a value of Jcalc = −13.7 cm−1, is ob-
For a satisfactory fit it was necessary to include
a Curie-Weiss term in order to correct for a para-
magnetic impurity; xp is the molar amount of this
mononuclear impurity. The temperature-independent
paramagnetism (TIP = 400.10−6 cm3/mol for each
iron atom) was also taken into account. The best fit
parameters obtained with eq. (1) by using a standard
least-squares program were J = −10.8 cm−1, xp = 0.1.
An isotropic g value of 2.0 was assumed. The effective
tained in fair agreement with Jobs = −10.7 cm−1
.
The magnetic properties of exchange coupled dinu-
clear complexes of transition metal ions are known to
depend on the particular metal ions, the chemical na-
ture of the bridging ligands, and the bridging geome-
tries. Both bridging angles and bridging ligand-metal
distances as well as electronic factors are important
[13 – 19, 29, 30].
Unauthenticated
Download Date | 9/11/18 11:20 AM

C. T. Zeyrek et al. · Exchange Interaction in a Dinuclear Iron(III) Complex
731
[1] S. J. Lippard, Angew. Chem. 100, 353 (1988).
[2] D. M. Kurtz, Chem. Rew. 90, 585 (1990).
[3] L. Que (Jr.), A. E. True, Progr. Inorg. Chem. 38, 97
(1990).
[4] C. J. O’Conor, Prog. Inorg. Chem. 29, 203 (1982).
[5] S. Sheriff, W. A. Hendrickson, J. L. Smith, J. Mol. Biol.
273 (1987).
[6] R. E. Stenkamp, L. C. Sieker, L. H. Jesen, J. Am.
Chem. Soc. 106, 618 (1994).
[7] A. C. Rosenzweig, C. A. Frederick, S. J. Lippard,
P. Nordlund, Nature 336, 537 (1993).
[8] A. C. Rosenzweig, P. Nordlund, P. M. Takahara, C. A.
Frederick, S. J. Lippard, J. Chem. Biol. 2, 409 (1995).
[9] R. E. Norman, S. Yan, L. Que (Jr.), G. Backes, J. Ling,
J. Sanders-Loehr, J. H. Zhang, C. J. O’Connor, J. Am.
Chem. Soc. 112, 1554 (1990).
[10] R. E. Norman, R. C. Holz, S. Me´nage, C. J. O’Connor,
J. H. Zhang, L. Que (Jr.), Inorg. Chem. 29, 4629
(1990).
[18] G. M. Sheldrick, SHELXS-97, Program for the solu-
tion of crystal structures, Univ. of Go¨ttingen, Germany
(1997).
[19] G. M. Sheldrick, SHELXL-97, Program for the refine-
ment crystal structures, Univ. of Go¨ttingen, Germany
(1997).
[20] L. J. Farrugia, ORTEPIII. J. Appl. Crystallogr. 30, 565
(1997).
[21] L. J. Farrugia, WinGX. Program for Crystallography
Package. J. Appl. Crystallogr. 32, 837 (1999).
[22] Further information may be obtained from: Cam-
bridge Crystallographic Data Center (CCDC), 12
Union Road, Cambridge CB21EZ, UK, by quot-
ing the depository number CCDC-262604 E-mail:
deposit@ccdc.cam.
[23] A. Weiss, H. Witte, Magnetochemie, Verlag Chemie,
Weinheim (1973).
[24] M. Gerloch, F. E. Mabbs, J. Chem. Soc. A, 1900
(1967).
[11] S. M. Gorum, S. Lippard, J. Inorg. Chem. 30, 1625
(1991).
[25] B. Chiari, O. Piovesena, T. Tarantelli, P. E. Zanazzi, In-
org. Chem. 21, 2444 (1982).
[12] H. Weihe, H. U. Gu¨del, J. Am. Chem. Soc. 119, 6539
(1997).
[13] A. Elmali, Y. Elerman, C. T. Zeyrek, I. Svoboda, Z.
Naturforsch. 58b, 433 (2003).
[14] A. Elmali, Y. Elerman, I. Svoboda, H. Fuess, J. Mol.
Struct. 516, 43 (2000).
[26] J. A. Thich, C. Chih Ou, D. Powers, B. Vasiliou,
D. Mastraopolo, J. A. Potenza, H. J. Schugar, J. Am.
Chem. Soc. 98, 1425 (1976).
[27] J. A. Bertrand, P. G. Eller, Inorg. Chem. 13, 125 (1974).
[28] S. M. Gorun, S. J. Lippard, Inorg. Chem. 30, 1625
(1991).
[15] A. Elmali, Y. Elerman, I. Svoboda, Z. Naturforsch.
56b, 897 (2001).
[29] C. T. Zeyrek, A. Elmali, Y. Elerman, I. Svoboda, and
H. Fuess, Z. Naturforsch. 55b, 1067 (2000).
[30] P. J. Hay, J. C. Thibeault, R. Hoffmann, J. Am. Chem.
Soc. 97, 4884 (1975).
[16] A. Elmali, Y. Elerman, I. Svoboda, H. Fuess,
K. Griesar, W. Haase, Z. Naturforsch. 48b, 313 (1993).
[17] Molecular Structure Corporation TEXAN for Win-
dows. Version 1.03. Single Crystal Structure Analy-
sis Software. MSC, 3200 Research Forest Drive, The
Woodlands, 77381, USA (1997).
[31] H. D. Flack, Acta Crystallogr. A39, 876 (1983).
Unauthenticated
Download Date | 9/11/18 11:20 AM