The structure and characterization of [1,11-Bis (2-Hydroxybenzy 1) 1,4,8,11-Tetraazaundecane] iron(III) perchlorate

Article abstract of DOI:10.1007/s10870-008-9470-3

The structure of an Fe(III) complex of reduced Schiff base is reported. The title compound, C₂₁H₃₀ClFeN₄O₆ (I), crystallizes in the monoclinic space group P2₁/n with cell constants: a = 9.988(2) A, b = 20.430(5) A, c = 11.415(3) A, β = 105.480(4)°. It contains a six-coordinate FeN₄O ₄ cation where the ligand is a reduced Schiff base resulting from the NaBH₄ reduction of the condensation product between salicylaldehyde and 1,4,8,11-tetraazaundecane. Due to the increased flexibility of the saturated backbone of the ligand compared to the Schiff base from which it was synthesized, the complex adopts a trans-FeN₄O₂ conformation. There is extensive hydrogen bonding between the amine H atoms and the anion O atoms. Springer Science+Business Media, LLC 2008.

Full text of DOI:10.1007/s10870-008-9470-3

J Chem Crystallogr (2009) 39:315–319
DOI 10.1007/s10870-008-9470-3
ORIGINAL PAPER
The Structure and Characterization of [1,11-Bis
(2-Hydroxybenzyl)-1,4,8,11-Tetraazaundecane]
Iron(III) Perchlorate
Teshome B. Yisgedu Æ Yohannes T. Tesema Æ
Yilma Gultneh Æ Ray J. Butcher
Received: 27 April 2006 / Accepted: 12 September 2008 / Published online: 16 October 2008
ꢀ Springer Science+Business Media, LLC 2008
Abstract The structure of an Fe(III) complex of reduced
Schiff base is reported. The title compound,
C21H30ClFeN4O6 (I), crystallizes in the monoclinic space
several years until the discovery that such complexes
exhibited spin-crossover magnetic behavior [3]. Hexaden-
tate linear FeN4O2 ligands derived from the Schiff base
condensation of salicylaldehyde and linear tetramines can
be characterized by the number of linking carbon atoms in
the tetramine backbone (see Fig. 1). The structures of Fe
complexes of Sal222 [3–12], Sal232 [4], Sal323 [4, 13],
and Sal333 [13] have been reported.
˚
group P21/n with cell constants: a = 9.988(2) A,
˚
˚
b = 20.430(5) A, c = 11.415(3) A, b = 105.480(4)ꢁ. It
contains a six-coordinate FeN4O2 cation where the ligand
is a reduced Schiff base resulting from the NaBH4 reduc-
tion of the condensation product between salicylaldehyde
and 1,4,8,11-tetraazaundecane. Due to the increased flexi-
bility of the saturated backbone of the ligand compared to
the Schiff base from which it was synthesized, the complex
adopts a trans-FeN4O2 conformation. There is extensive
hydrogen bonding between the amine H atoms and the
anion O atoms.
When chelating to Fe, as the number of carbon atoms in
the tetramine backbone increases from 6 to 9, the confor-
mation adopted by the ligand changes from a cis-FeN4O2 to
a trans-FeN4O2 arrangement for the phenolic O donors. All
structurally characterized Fe complexes with Sal222 have
adopted the cis-FeN4O2 conformation while all those with
either Sal323 or Sal333 have adopted the trans-FeN4O2
conformation. For Sal232 both conformations have been
observed [4]. Further, it has been observed that the angles
subtended at the Fe center reflect the magnetic properties of
the compound [4, 8] with low-spin compounds having such
angles closer to 90ꢁ and 180ꢁ.
Keywords Linear hexadentate ligands ꢀ
Reduced Schiff base ligands ꢀ Six-coordinate iron (III)
Introduction
Despite the interest outlined above shown in sali-
cyclaldimine complexes with Fe(III) due their interesting
structural and magnetic properties, there have been very
few structures reported on related complexes where the
C = N imine groups have been reduced to C–N–H
amine groups. A search of the Cambridge Structural
Database [14] gives only one example [15] involving an
Fe(III) complex of a reduced Sal323 ligand. As expected,
due to increased flexibility of the saturated amine,
compared to the more rigid Schiff base, and also the
length of the carbon backbone, this compound has
adopted a trans-FeN4O2 conformation. To further char-
acterize such compounds and determine the conformation
adopted the structure of an Fe(III) complex of reduced
Sal232 is reported.
Metal complexes of linear hexadentate ligands have fas-
cinated inorganic chemists since their first report in 1947
[1]. The first such report of a Fe complex of a linear
FeN4O2 ligand derived from the Schiff base condensation
of salicylaldehyde and triethylenetetraamine (Sal222, see
Fig. 1) was in 1955 [2]. However, this interest lapsed for
T. B. Yisgedu ꢀ Y. T. Tesema ꢀ Y. Gultneh ꢀ R. J. Butcher (&)
Department of Chemistry, Howard University, 525 College
Street NW, Washington, DC 20059, USA
e-mail: rbutcher99@yahoo.com; rbutcher@howard.edu
123

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J Chem Crystallogr (2009) 39:315–319
H
N
Complex Synthesis and Characterization
N
N
H
Synthesis of [Fe(III)(L)](ClO4) To 0.85 g of H2L dis-
solved in 10.0 ml of methanol was added 0.58 g of
Fe(ClO4)2 9 H2O. The solution became violet and a red-
purple solid precipitated. This was stirred overnight, the
solid filtered, washed with methanol and dried to give
1.65 g of red powder. Crystallization was effected by slow
evaporation of a DMF solution of the complex. Elemental
analysis: found: C, 47.91; H, 5.89; N, 10.64. Calc. (for
C21H30ClFeN4O6) C, 47.93; H, 5.75; N, 10.65. IR (mineral
oil) 3250, 3289, 3200, 1600, 1565, 1495, 1421, 1371, 1312,
N
Sal222
OH
HO
N
N
H
H
N
N
Sal232
1083, 1048, 1028, 974, 894, 874, 785, 750, 616 cm-1
.
OH
HO
Cyclic Voltammetry and Differential Pulse
Voltammetry
H
N
N
H
Measurements were done on a solution 2.4 9 10-3
M
N
OH
N
Sal323
solution of the complex in acetonitrile containing 0.1 M
[N(Bu4)][PF6] with Ag/AgCl as reference electrode, glassy
carbon as working electrode and platinum wire as counter
electrode were used.
OH
N
N
Room Temperature Magnetic Study
N
N
Fine crystals of the complex (0.0729 g) were packed in to a
magnetic susceptibility tube (1.9 cm) and susceptibility
readings measured using a Johnson–Matthey susceptibility
balance with and without the sample. These readings were
converted to BM units after diamagnetic correction for the
ligand.
Sal333
OH
HO
Fig. 1 Schiff bases derived from linear tetramine
UV/vis Spectroscopy
UV/vis spectrum was taken for 1.14 9 10-4 M of the
complex in acetonitrile on a Hewlett-Packard 8453 A diode
array spectrometer in a 1-cm quartz cuvet.
Experimental
Synthesis
Synthesis of 1,11-Bis(2-Hydroxybenzyl)-1,4,8,11-
Tetraazaundecane (H2L)
X-ray Experimental
A suitable crystal was chosen and attached to a glass rod on a
goniometer head with epoxy cement. This was transferred to
a Siemens P4S diffractometer. All measurements were made
by standard methods [16]. The crystal data is summarized in
Table 1. All H atoms were initially located in a difference
Fourier map. The position of the amine H atoms were
refined freely with Uiso(H) = 1.2Ueq(N). All other H atoms
were placed in geometrically idealized positions and con-
strained to ride on their parent atoms with C–H distances in
To 4.0 g (25 mmol) of 1,4,8,11-tetraazaundecane in
15 ml of ethanol was added 6.1 g (50 mmol) of salicyl-
aldehyde in 10 ml of ethanol drop-wise. The deep yellow
solution was left to stir for half an hour. To this yellow
solution was added a NaBH4 solution (3.0 g NaBH4,
0.4 g NaOH, 40 ml H2O) resulting in a colorless solu-
tion. The volume of the solution was reduced to 20 ml
and extracted with chloroform (3 9 40 ml). The extracts
were combined and dried with Na2SO4. The Na2SO4 was
filtered and the filtrate concentrated to colorless thick oil
(8.1 g, 87%).
˚
the range 0.95–1.00 A and Uiso(H) = 1.2Ueq(C) [17].
Table 2 contains the fractional coordinates and equivalent
isotropic thermal parameters for all refined atoms.
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J Chem Crystallogr (2009) 39:315–319
317
Table 1 Crystal data and
structure refinement for I
Empirical formula
C21H30ClFeN4O6
525.79
Formula weight
Temperature
295(2) K
˚
0.71073 A
Wavelength
Crystal system
Space group
Monoclinic
P21/n
˚
a = 9.988(2) A
Unit cell dimensions
a = 90ꢁ
˚
b = 20.430(5) A
b = 105.480(4)ꢁ
c = 90ꢁ
˚
c = 11.415(3) A
3
˚
2244.8(9) A
Volume
Z
4
Density (calculated)
Absorption coefficient
F(000)
1.556 Mg/m3
0.838 mm-1
1100
0.1 9 0.2 9 0.34 mm3
Crystal size
Theta range for data collection
Index ranges
1.99ꢁ–28.37ꢁ
-13 B h B 13, -27 B k B 26, -15 B l B 15
25,158
Reflections collected
Independent reflections
Completeness to theta = 28.37ꢁ
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I [ 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
5517 [R(int) = 0.1053]
98.1%
Semi-empirical from equivalents
0.6763 and 0.2092
Full-matrix least-squares on F2
5517/0/299
1.003
R1 = 0.0559, wR2 = 0.1352
R1 = 0.0895, wR2 = 0.1534
-3
˚
0.945 and -0.991 e A
˚
Table 2 Comparison of selected metrical parameters for Fe(III) complexes of reduced and unreduced Sal232 ligand (A,ꢁ)
Parameter
R-Sal232a
U-Sal232b
Parameter
R-Sal232
U-Sal232
Fe–O1
1.8746(19)
1.8760(18)
2.004(2)
2.004(2)
2.022(2)
2.017(2)
177.43(8)
89.63(9)
89.95(9)
83.51(9)
89.24(9)
1.886(5)
1.895(5)
1.922(6)
2.017(6)
2.008(6)
1.916(6)
176.8(3)
87.8(3)
O2–Fe–N1
O2–Fe–N2
O2–Fe–N3
O2–Fe–N4
N1–Fe–N2
N1–Fe–N3
N1–Fe–N4
N2–Fe–N3
N2–Fe–N4
N3–Fe–N4
88.53(9)
88.10(8)
90.9(3)
90.9(3)
95.7(3)
87.3(3)
84.0(3)
172.1(3)
100.2(3)
91.5(3)
175.5(3)
84.6(3)
Fe–O2
Fe–N1
98.23(9)
Fe–N2
92.78(9)
Fe–N3
84.59(10)
172.49(9)
98.06(10)
92.33(10)
177.22(10)
84.94(10)
Fe–N4
O1–Fe–O2
O1–Fe–N1
O1–Fe–N2
O1–Fe–N3
O1–Fe–N4
a
91.8(3)
85.9(3)
90.1(3)
Title compound (R-Sal232 is the Fe(III) complex of the reduced Schiff base formed between salicylaldehyde and 1,4,8,11-tetrazaundecane)
b
U-Sal232 is the Fe(III) complex of the Schiff base formed between 2-hydroxybenzophenone and 1,4,8,11-tetrazaundecane (Hayami et al. [4])
Results and Discussion
The title compound, [1,11-bis(2-hydroxybenzyl)-
1,4,8,11-tetraazaundecane]iron(III) perchlorate, C21H30Cl
FeN4O6 (I), contains a six-coordinate FeN4O2 cation where
the ligand (H2L) is the NaBH4 reduction product of the
The ligand was synthesized according to the reaction:
Fig. 2.
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J Chem Crystallogr (2009) 39:315–319
Fig. 2 Synthesis of H2L
HN
NH
H
NH
EtOH
O
NH
HN
HN
+
2
NaBH4
OH
OH
HO
H2N
NH2
H2L
˚
Table 3 Hydrogen bonds for I [A and ꢁ]
D–H…A d(D–H) d(H…A) d(D…A) \(DHA)
N(2A)–H(2AA)…O(4)#1 0.91
N(1B)–H(1BA)…O(2) 0.91
N(2B)–H(2BA)…O(3)#2 0.91
2.16
2.37
2.22
3.055(4) 168.4
3.150(4) 143.4
3.049(4) 151.1
Symmetry transformations used to generate equivalent atoms
#1 x ? 1, y, z
#2 x ? 1/2, -y ? 1/2, z - 1/2
5-membered N1A–Fe–N2A and N1B–Fe–N2B chelate
rings (84.59(10)ꢁ and 94.94(1)ꢁ, respectively.
As discussed above these values indicate a low spin
Fe(III) complex. This is confirmed by the room temper-
ature magnetic moment which was found to be 2.07 BM.
It is of interest to compare this value with that obtained
for other Fe complexes containing the unreduced Sal232
ligand [4]. Four examples have been reported of which
three are high spin (room temperature magnetic moments
ranging from 5.86 to 5.96 BM) and have the O donors in
a cis conformation, while one (II) is low spin (room
temperature magnetic moment of 1.91 BM) with the O
donors in a trans conformation and with similar bond
lengths and angles to I.
Fig. 3 View of the molecule of I showing the atom-labeling scheme.
Displacement ellipsoids are drawn at the 20% probability level.
H atoms are represented by circles of arbitrary size
Schiff base resulting from the condensation of salicylal-
dehyde and 1,4,8,11-tetraazaundecane.
Figure 3 shows the ORTEP representation of the
structure of the cation. As indicated above, Fe(III)
complexes of Schiff bases derived from salicyclaldehyde
derivatives and 1,4,8,11-tetraazaundecane (Sal232) can be
either cis or trans but there is only one example of the
latter conformation, (N,N0-bis(2-(1-(2-phenoxy)benzylide-
neamino)-ethyl)propane-1,2-diamine)iron(III) perchlorate,
II [4]. In the present instance, I has adopted a trans-
FeN4O2 conformation. It is of interest to compare the
metrical parameters of the Fe(III) complexes of the two
types of trans-FeN4O2 complex with both reduced Schiff
base, I, and Schiff base, II (see Table 3); [4]. The major
differences are in the Fe–N bonds to the related imine
and amine N’s (N1A and N1B in I and N1 and N4 in II)
As might be expected there is extensive hydrogen
bonding between the amine H atoms and the perchlorate
anions (see Fig. 4).
UV–Visible Spectroscopy
UV–visible spectrum was taken for 1.14 9 10-4 M of the
complex in acetonitrile shows absorption peaks in the UV–
vis region at kmax(nm), (e M-1 cm-1): 236 (2.0 9 104),
286 (8.7 9 103), 322 (6.4 9 103) and 496 (4.7 9 103).
Cyclic Voltammetry
˚
which are 2.004(2) and 2.017(2) A in I and 1.922(6) and
˚
1.916(6) A in II. The majority of the bond angles sub-
Cyclic voltammetry (and differential pulse voltammetry)
show two reversible redox waves due at E1/2 (versus Ag/
AgCl) of -0.54 and 0.89 V due to Fe(III)/Fe(II) and
Fe(III)/Fe(IV) reversible redox processes.
tended at the Fe are close to either 90ꢁ or 180ꢁ with the
major deviations being due to the smaller bite size of the
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J Chem Crystallogr (2009) 39:315–319
319
Fig. 4 The molecular packing
of I viewed along the b-axis
Acknowledgements RJB wishes to acknowledge the Department of
Defense (DoD-ONR) for partial funding to upgrade the diffractome-
ter. YG would like to acknowledge the financial support of graduate
students by the NIH MBRS program (through the Graduate School of
Howard University) and the EPA GEM program and the Provost
Office of Howard University for support with laboratory supplies.
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