Asymmetrically dibridged diiron(III) complexes with aminebis(phenoxide)- based ligands for a magnetostructural study

Article abstract of DOI:10.1002/ejic.201001340

The ligands [N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminoacetic acid] (H₃L¹), [N,N-bis(2-hydroxy-3,5-dimethylbenzyl)aminoacetic acid] (H₃L²), [N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N-(2- pyridylmethyl)amine]

Full text of DOI:10.1002/ejic.201001340

FULL PAPER
DOI: 10.1002/ejic.201001340
Asymmetrically Dibridged Diiron(III) Complexes with
Aminebis(phenoxide)-Based Ligands for a Magnetostructural Study
Thomas Weyhermüller,[a] Rita Wagner,[a] and Phalguni Chaudhuri*[a]
Keywords: Transition metals / Iron / Magnetic properties / N,O ligands / Through-bond interactions
The ligands [N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)amino-
acetic acid] (H3L1), [N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-
aminoacetic acid] (H3L2), [N,N-bis(2-hydroxy-3,5-dimeth-
ylbenzyl)-N-(2-pyridylmethyl)amine] (H2L3) and [meth-
ylamino-N,N-bis(6-tert-butyl-4-methyl-2-methylenephenol)]
(H2L4) were used to investigate their coordination properties
toward FeIII. The ligand H3L1 yields mononuclear iron(III)
complexes, complexes 1–3, whereas the asymmetrically di-
plexes 1–3 were isolated with H3L1, whereas H3L2 bearing
the less bulky methyl groups results in the diferric(III) com-
plex 4. Complexes 4–8, containing two d5 high-spin ferric
centres, possess a diamagnetic St = 0 ground state that is at-
tained through intramolecular antiferromagnetic coupling
between the two paramagnetic centres. We have focused our
investigation largely on the magnetostructural correlation of
the asymmetrically dibridged (hydroxo, methoxo, acetate) di-
bridged diiron(III) complexes 4–8 were isolated with the li- ferric(III) complexes 4–8 and compared these to similar com-
gands H3L2, H2L3 and H2L4. The complexes 1–8 have been
structurally, magnetochemically (2–290 K) and spectroscopi-
cally investigated. Presumably as an effect of the sterically
demanding tert-butyl groups, only mononuclear FeIII com-
pounds reported in the literature. The spin-exchange ability
of the groups such as –OH, –OMe, –OR and –OPh is also
compared.
[methylamino-N,N-bis(6-tert-butyl-4-methyl-2-methylene-
phenol)] (H2L4) were selected for the investigations
(Scheme 1).
Introduction
The study of complexes with phenol-containing ligands
is of unabated interest to inorganic chemists because of
their relevance to apparently dissimilar fields, such as bio-
inorganic chemistry,[1] molecular magnetism[2] and cataly-
sis.[3] Thus, aminebis(phenolate) ligands (potential O,N,O
donors) have attracted the attention of researchers for use
in the field of phosphorus chemistry[4] and for catalytic ole-
fin polymerisation involving group IV metal complexes;[5]
we have used such ligands in the field of transition metal
chemistry.[6] With an aim to scrutinise the effect of an ad-
ditional pendent arm (containing a donor atom) on the li-
gating property of the resulting tetradentate ligand towards
late transition metal ions, particularly iron, we argued for a
carboxylate and a pyridine donor group. Additionally, we
wished to extend our investigations on the effect of steric
hindrance of the resulting tripodal ligands by substituting
the methyl groups on the phenol rings by tert-butyl groups.
Thus, the ligands [N,N-bis(3,5-di-tert-butyl-2-hydroxy-
benzyl)aminoacetic acid] (H3L1), [N,N-bis(2-hydroxy-3,5-di-
methylbenzyl)aminoacetic acid] (H3L2), [N,N-bis(2-hydroxy-
3,5-dimethylbenzyl)-N-(2-pyridylmethyl)amine] (H2L3) and
[a] Max Planck Institute for Bioinorganic Chemistry,
Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany
Fax: +49-208-306-3951
E-mail: chaudhuri@mpi-muelheim-mpg.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001340.
Scheme 1.
Eur. J. Inorg. Chem. 2011, 2547–2557
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. Weyhermüller, R. Wagner, P. Chaudhuri
FULL PAPER
We herein report the isolation, and structural and magne-
tochemical characterisation of complexes 1–8 with a special
focus on the magnetostructural correlation of the asymmetri-
cally dibridged diiron(III) complexes 4–8.
Results and Discussion
The ligands H3L1, H3L2, H2L3 and H2L4 were prepared
by a one-pot Michael condensation reaction according to a
modified procedure reported in the literature.[4,5a] The purity
of the ligands was confirmed by liquid chromatography to be
1
Ͼ98%. The IR and H NMR spectroscopic and MS data
(see Exp. Section) are in agreement with the available litera-
ture data and hence no further discussion is necessary here.
Methanolic solutions of the ligands were treated with dif-
ferent iron salts and Et3N in suitable ratios and the solutions
were heated to reflux to yield complexes 1–8 in reasonable
yields. Depending on the nature of the metal salts, different
products were isolated.
Figure 1. ORTEP diagram of 3 with ellipsoids drawn at the 40%
probability level.
Selected IR data for complexes 1–8 are given in the Exp.
Section. The peaks that convey most information are ν(OH),
which are very sharp in the pure ligands, occurring in the
3629–3125 cm–1 region, and vanish after complex formation
indicating that the phenol character of the ligands has been
lost. Complexes 1–4 exhibit strong to moderate vibrations be-
tween 1655–1617 (νasOCO), 1476–1470 (νsOCO) and 857–
840 cm–1 (νδOCO) as observed earlier for carboxylato com-
plexes.[11] For complexes 5 and 6, the vibrations of the pyr-
idine ring occur at 1445 and 1450 cm–1, respectively. There
are several peaks in the 3000–2800 cm–1 region due to the
ν(C–H) vibrations found in the normal range for methyl and
tert-butyl groups.
Although mass spectrometric measurements do not leave
any doubt about the mononuclearity for complexes 1–3 and
dinuclearity for complexes 4–8, unambiguous detailed char-
acterisation providing atom-connectivity could not be ob-
tained from the MS data. Selected MS data for the complexes
are given in the Experimental Section.
Table 1. Bond lengths [Å] and angles [°] for 3·CH3OH.
Fe(1)–O(1)
Fe(1)–O(15)
Fe(1)–O(40)
Fe(1)–O(18)
Fe(1)–N(8)
O(1)–C(1)
O(15)–C(15)
1.8782(12)
1.8802(13)
1.9118(12)
1.9937(14)
2.2343(14)
1.3503(19)
1.344(2)
O(1)–Fe(1)–O(15)
O(1)–Fe(1)–O(40)
O(15)–Fe(1)–O(40)
O(1)–Fe(1)–O(18)
O(15)–Fe(1)–O(18)
O(40)–Fe(1)–O(18)
O(1)–Fe(1)–N(8)
O(15)–Fe(1)–N(8)
O(40)–Fe(1)–N(8)
O(18)–Fe(1)–N(8)
112.71(6)
100.95(5)
100.02(6)
127.61(6)
117.19(6)
85.50(6)
88.05(5)
90.73(5)
161.90(6)
76.61(6)
mid with N(8) of [L1]3– and O(40) of the methoxide ion as
the apices; the N(8)–Fe(1)–O(40) angle is 161.88(7)°. The tri-
gonality index[12] τ = [(φ1 – φ2)/60], in which φ1 and φ2 are the
two largest L–M–L angles of the coordination sphere, has
been calculated for Fe(1) to be 0.573 for 3 (τ = 1 for a perfect
X-ray Diffraction Studies
Single crystals of deep brown [(L1)Fe(OH2)] (1), [(L1)- trigonal bipyramid and τ = 0 for a perfect square pyramid).
Fe(C2H5OH)] (2) and [(L1)Fe(OCH3)][(C2H5)3NH] (3) were The corresponding τ values for 1 and 2 are 0.715 and 0.679,
obtained either from a methanol or ethanol solution by slow respectively, confirming the distorted trigonal bipyramidal
evaporation at ambient temperature. As the molecular struc- character of both iron centres. The bond length between the
tures of neutral 1, 2 and the anion in 3 are very similar, we FeIII ion and the tertiary apical nitrogen atom N(8), which is
confine our discussion only to that of 3. Pertinent crystallo- situated trans to the methoxide oxygen O(40), of 2.2343(14) Å
graphic data for all three compounds are summarised in is about 0.06 Å longer for 3 than the distances between Fe(1)
Table 8. An ORTEP diagram of 3 is shown in Figure 1 and and the neutral oxygen atoms, originating from EtOH for 2
selected bond lengths and angles are summarised in Table 1. and H2O for 1, as is expected. Accordingly, the Fe(1)–O(40)
The corresponding information for compounds 1 and 2 can bond length is shorter for 3 [1.912(2) Å] than the correspond-
be found in Figures S1 and S2, and Tables S1 and S2, respec- ing FeIII–O bonds in 1 and 2 and falls in the range of bond
tively, in the Supporting Information.
lengths previously determined for terminally bound meth-
The structure of 3 consists of one [(L1)Fe(OCH3)]– anion, oxides in structurally characterised FeIII complexes.[13] The
one non-coordinated [(C2H5)3NH]+ cation and a methanol phenolates O(15) and O(1), together with the carboxylate
molecule of crystallisation. The overall geometry around the oxygen O(18), define the equatorial plane of the five-coordi-
Fe(1) centre is best described as a distorted trigonal bipyra- nate Fe(1). The FeNO4 coordination sphere is not very com-
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Asymmetrically Dibridged Diiron(III) Complexes
mon for FeIII with O,N-based ligation; octahedral coordina- Table 2. Bond lengths [Å] and angles [°] for 4·2.5CH3CN·H2O.[a]
tion is by far the most common. The Fe–N and Fe–O bond
Fe(1)···Fe(2)
3.118(1)
3.127(1)
1.920(2)
1.935(2)
2.003(2)
2.030(2)
2.093(2)
2.192(2)
1.875(2)
1.968(2)
2.038(2)
2.038(2)
2.053(2)
2.208(3)
1.922(2)
1.935(2)
2.000(2)
2.043(2)
2.065(2)
2.194(2)
1.872(2)
1.967(2)
2.043(2)
2.044(2)
2.052(2)
2.189(2)
99.21(9)
97.84(8)
96.48(8)
168.89(9)
91.28(9)
77.21(8)
87.99(8)
166.29(9)
94.05(8)
82.51(8)
90.50(9)
90.49(9)
168.11(9)
93.07(9)
77.72(8)
95.08(9)
97.51(9)
167.12(9)
168.74(9)
77.82(8)
90.14(8)
100.70(9)
89.68(9)
85.32(9)
88.10(8)
87.34(9)
104.69(9)
78.69(9)
86.07(9)
162.95(9)
102.48(9)
100.95(9)
96.69(9)
99.44(9)
96.24(8)
172.70(9)
89.90(8)
76.63(8)
89.02(8)
166.85(8)
94.48(8)
85.19(8)
90.89(9)
lengths are consistent with a d5 high-spin electron configura-
tion of Fe(1), along with variable-temperature magnetic
susceptibility measurements. The methanol molecule of crys-
tallisation is hydrogen bonded to the carboxylate oxygen
O(19) with an O(19)···HO(60) intermolecular distance of
2.74 Å. There is also a hydrogen bond between the methoxide
oxygen O(40) and the triethylammonium cation with an
O(40)···HN(50) contact of 2.65 Å. The bond lengths and
angles in the cation [(C2H5)3NH]+ of 3 seem reasonable and
do not warrant any special discussion.
Fe(3)···Fe(4)
Fe(1)–O(18)
Fe(1)–O(28)
Fe(1)–O(48)
Fe(1)–O(100)
Fe(1)–O(4)
Fe(1)–N(1)
Fe(2)–O(58)
Fe(2)–O(100)
Fe(2)–O(34)
Fe(2)–O(48)
Fe(2)–O(200)
Fe(2)–N(31)
Fe(3)–O(78)
Fe(3)–O(88)
Fe(3)–O(108)
Fe(3)–O(300)
Fe(3)–O(64)
Molecular Structure of 4·2.5CH3CN·H2O
The asymmetric unit of 4 consists of two discrete crystallo-
graphically independent dinuclear monoanions and two tri-
ethylammonium monocations as well as two solvent mole-
cules of crystallisation. The metrical parameters for the two
crystallographically independent molecules in the anion ob-
served for 4 are very similar and only one ORTEP diagram
is shown (Figure 2); selected bond lengths and angles are
summarised in Table 2. Both iron atoms, Fe(1) and Fe(2), are
in distorted octahedral environments with FeNO5 coordina-
tion spheres, although the nature of oxygen donor atoms is
Fe(3)–N(61)
Fe(4)–O(118)
Fe(4)–O(300)
Fe(4)–O(94)
Fe(4)–O(400)
Fe(4)–O(108)
Fe(4)–N(91)
O(18)–Fe(1)–O(28)
O(18)–Fe(1)–O(48)
O(28)–Fe(1)–O(48)
O(18)–Fe(1)–O(100)
O(28)–Fe(1)–O(100)
different. The two FeNO5 coordination polyhedra share the O(48)–Fe(1)–O(100)
O(18)–Fe(1)–O(4)
edge that contains a phenoxo oxygen O(48) and a hydroxo
O(28)–Fe(1)–O(4)
group O(100), resulting in a four-membered Fe2O2 ring. The
O(48)–Fe(1)–O(4)
dihedral angle between the planes comprising Fe(1)–O(48)–
O(100)–Fe(1)–O(4)
Fe(2) and Fe(1)–O(100)–Fe(2) is 14.7°. The carboxylate oxy-
gen O(4) and the phenolate oxygen O(28), being in mutually
trans positions, form axial linkages for Fe(1), whereas N(1),
phenolate O(18), bridging phenolate O(48) from the second
[L2]3– ligand and a hydroxo oxygen O(100) constitute the
equatorial plane for Fe(1). On the other hand, the equatorial
donor atoms for Fe(2) are four oxygen atoms: O(48), O(100),
O(18)–Fe(1)–N(1)
O(28)–Fe(1)–N(1)
O(48)–Fe(1)–N(1)
O(100)–Fe(1)–N(1)
O(4)–Fe(1)–N(1)
O(58)–Fe(2)–O(100)
O(58)–Fe(2)–O(34)
O(100)–Fe(2)–O(34)
O(58)–Fe(2)–O(48)
O(100)–Fe(2)–O(48)
O(34)–Fe(2)–O(48)
O(58)–Fe(2)–O(200)
O(100)–Fe(2)–O(200)
O(34)–Fe(2)–O(200)
O(48)–Fe(2)–O(200)
O(58)–Fe(2)–N(31)
O(100)–Fe(2)–N(31)
O(34)–Fe(2)–N(31)
O(48)–Fe(2)–N(31)
O(200)–Fe(2)–N(31)
Fe(2)–O(100)–Fe(1)
Fe(1)–O(48)–Fe(2)
O(78)–Fe(3)–O(88)
O(78)–Fe(3)–O(108)
O(88)–Fe(3)–O(108)
O(78)–Fe(3)–O(300)
O(88)–Fe(3)–O(300)
O(108)–Fe(3)–O(300)
O(78)–Fe(3)–O(64)
O(88)–Fe(3)–O(64)
O(108)–Fe(3)–O(64)
O(300)–Fe(3)–O(64)
O(78)–Fe(3)–N(61)
Figure 2. Molecular structure of 4 with ellipsoids drawn at the 40%
probability level.
Eur. J. Inorg. Chem. 2011, 2547–2557
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T. Weyhermüller, R. Wagner, P. Chaudhuri
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Table 2. (continued)
the dimeric unit with a crystallographic twofold inversion
symmetry is shown in Figure 3. The structure consists of an
edge-shared bioctahedron for the diiron(III) core. The asym-
metric unit contains half of the dimer and consequently the
geometries of the two iron centres are identical, with ligation
provided by the O(30) and O(30)* atoms of the bridging hy-
droxy ligands, two phenolate oxygen atoms O(1) and O(17),
one amine nitrogen N(9) and one pyridine nitrogen N(24)
belonging to the terminal ligand [L3]2–, resulting in a dis-
torted octahedral arrangement for each iron centre. The sym-
metrically dibridged four-membered Fe2(μ-OH)2 ring is
planar. The pyridine nitrogen N(24) is in the trans position
O(88)–Fe(3)–N(61)
O(108)–Fe(3)–N(61)
O(300)–Fe(3)–N(61)
O(64)–Fe(3)–N(61)
O(118)–Fe(4)–O(300)
O(118)–Fe(4)–O(94)
O(300)–Fe(4)–O(94)
O(118)–Fe(4)–O(400)
O(300)–Fe(4)–O(400)
O(94)–Fe(4)–O(400)
O(118)–Fe(4)–O(108)
O(300)–Fe(4)–O(108)
O(94)–Fe(4)–O(108)
O(400)–Fe(4)–O(108)
O(118)–Fe(4)–N(91)
O(300)–Fe(4)–N(91)
O(94)–Fe(4)–N(91)
O(400)–Fe(4)–N(91)
O(108)–Fe(4)–N(91)
Fe(4)–O(300)–Fe(3)
Fe(3)–O(108)–Fe(4)
89.86(9)
167.27(9)
92.27(9)
78.18(8)
96.60(9)
97.37(9)
165.75(9)
99.54(9)
90.83(8)
84.11(8)
170.25(8)
77.13(8)
89.37(8)
88.12(9)
87.43(9)
103.98(9)
79.46(9)
162.87(9)
86.88(9)
102.44(9)
101.01(9)
Table 3. Bond lengths [Å] and angles [°] for complex 5.[a]
Fe(1)–O(1)
Fe(1)–O(17)
Fe(1)–O(30)
Fe(1)–O(30)#1
Fe(1)–N(24)
Fe(1)–N(9)
1.9033(8)
1.9350(7)
1.9846(8)
2.0853(8)
2.2053(9)
2.2265(9)
96.47(3)
99.53(3)
99.33(3)
91.22(3)
172.30(3)
79.59(3)
166.04(3)
89.12(3)
92.12(3)
83.32(3)
91.40(3)
88.31(3)
165.81(3)
91.24(3)
75.94(3)
100.41(3)
[a] Symmetry transformations used to generate equivalent atoms.
O(1)–Fe(1)–O(17)
O(1)–Fe(1)–O(30)
O(17)–Fe(1)–O(30)
O(1)–Fe(1)–O(30)#1
O(17)–Fe(1)–O(30)#1
O(30)–Fe(1)–O(30)#1
O(1)–Fe(1)–N(24)
O(17)–Fe(1)–N(24)
O(30)–Fe(1)–N(24)
O(30)#1–Fe(1)–N(24)
O(1)–Fe(1)–N(9)
O(17)–Fe(1)–N(9)
O(30)–Fe(1)–N(9)
O(30)#1–Fe(1)–N(9)
N(24)–Fe(1)–N(9)
Fe(1)–O(30)–Fe(1)#1
carboxylate O(34) and phenolate O(58). Interestingly, O(200)
of the coordinated water molecule and N(31) are in axial po-
sitions for Fe(2), thus indicating different coordination site-
occupancy for two [L2]3– anionic ligands; one [L2]3– is ligated
solely to Fe(1), whereas the second one shares a phenoxo oxy-
gen O(48) between Fe(1) and Fe(2) resulting in the asymmet-
rically bridged diiron complex 4. The largest deviation from
the idealised 90° inter-bond angles is found within the five-
and four-membered rings containing Fe(1) with the O(4)–
Fe(1)–N(1) and O(48)–Fe(1)–O(100) at 77.72(8)° and
77.21(8)° angles, respectively. Similar deviations of inter-bond
angles are also observed for Fe(2). It is noteworthy that the
Fe(2)–O(48) bond length of 2.038(2) Å, involving a bridging
phenolate oxygen, is, as expected, longer. This probably re-
sults in a comparatively short trans-positioned Fe(2)–O(58)
bond length of 1.875(2) Å. The Fe–N and Fe–O bond lengths
are consistent with the ferric (d5 high spin) formulation for
both the Fe(1) and Fe(2) centres. The d5 high-spin electron
configuration has also been confirmed by both magnetic
susceptibility and Mössbauer (80 K, zero-field) measure-
ments (δFe = 0.52 mms–1; ΔEQ = 1.15 mms–1). The Fe(1)
···Fe(2) separation of 3.118 Å is in the normal range.
[a] Symmetry transformations used to generate equivalent atoms:
#1 –x + 1, –y, –z + 1.
It should be pointed out that the H3L2 ligand with methyl
substituents results in the facile isolation of dinuclear com-
plexes, such as 4, whereas the sterically demanding ligand
H3L1 containing tert-butyl derivatives with exactly the same
donor atoms yields only mononuclear complexes, such as 1–
3. Our attempts to isolate dinuclear products with the ligand
H3L1 are yet to be successful.
X-ray Structure of 5
The structure of complex 5 consists of neutral, well-iso-
lated dinuclear units of [(L3)2FeIII2(μ-OH)2]. Selected bond
lengths and angles are listed in Table 3. An ORTEP view of probability level.
Figure 3. Molecular structure of 5 with ellipsoids drawn at the 50%
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Asymmetrically Dibridged Diiron(III) Complexes
to the phenolate O(1) yielding an O(1)–Fe(1)–N(24) angle of Table 4. Bond lengths [Å] and angles [°] for 6.
166.01(4)°. The Fe–O and Fe–N bond lengths are in confor-
Fe(1)–O(17)
Fe(1)–O(1)
Fe(1)–O(60)
Fe(1)–O(70)
Fe(1)–N(24)
Fe(1)–N(9)
Fe(2)–O(31)
Fe(2)–O(47)
Fe(2)–O(60)
Fe(2)–O(72)
Fe(2)–N(54)
Fe(2)–N(39)
1.900(6)
1.945(5)
1.984(5)
2.067(5)
2.218(7)
2.221(6)
1.899(6)
1.960(5)
1.963(5)
2.078(5)
2.177(7)
2.230(6)
mation with the high-spin d5 electron configuration for the
ferric centres in 5, which corroborates with the magnetic mea-
surements. The Fe···Fe separation of 3.128 Å is comparable
with those reported for other dihydroxo-bridged diferric(III)
complexes.
Molecular Structure of 6
O(17)–Fe(1)–O(1)
O(17)–Fe(1)–O(60)
O(1)–Fe(1)–O(60)
O(17)–Fe(1)–O(70)
O(1)–Fe(1)–O(70)
O(60)–Fe(1)–O(70)
O(17)–Fe(1)–N(24)
O(1)–Fe(1)–N(24)
O(60)–Fe(1)–N(24)
O(70)–Fe(1)–N(24)
O(17)–Fe(1)–N(9)
O(1)–Fe(1)–N(9)
O(60)–Fe(1)–N(9)
O(70)–Fe(1)–N(9)
N(24)–Fe(1)–N(9)
O(31)–Fe(2)–O(47)
O(31)–Fe(2)–O(60)
O(47)–Fe(2)–O(60)
O(31)–Fe(2)–O(72)
O(47)–Fe(2)–O(72)
O(60)–Fe(2)–O(72)
O(31)–Fe(2)–N(54)
O(47)–Fe(2)–N(54)
O(60)–Fe(2)–N(54)
O(72)–Fe(2)–N(54)
O(31)–Fe(2)–N(39)
O(47)–Fe(2)–N(39)
O(60)–Fe(2)–N(39)
O(72)–Fe(2)–N(39)
N(54)–Fe(2)–N(39)
Fe(2)–O(60)–Fe(1)
Fe(1)–O(70)–C(71)
Fe(2)–O(72)–C(71)
102.0(2)
94.5(2)
94.0(2)
93.8(2)
162.6(2)
92.0(2)
164.8(2)
83.9(2)
99.1(2)
79.1(2)
88.2(2)
88.5(2)
175.8(2)
84.7(2)
77.8(3)
103.1(2)
93.6(2)
94.4(2)
90.6(2)
164.2(2)
92.5(2)
164.0(2)
85.5(2)
99.2(2)
79.4(2)
89.5(2)
87.5(2)
175.9(2)
84.8(2)
77.3(2)
128.1(3)
126.2(5)
127.9(5)
Figure 4 displays a perspective view of the discrete neutral
dinuclear units [(L3)2FeIII2(μ-OCH3)(μ-OOCCH3)] and their
atom labelling scheme. Selected bond lengths and angles are
listed in Table 4. The structural data concerning the [L3]2–-
containing parts of the complex are in good agreement with
those of previous studies[6e,7c] using the same ligand and do
not warrant any detailed description. The Fe(1) and Fe(2)
iron atoms are asymmetrically bridged by one methoxy O(60)
and one syn,syn η1:η1:μ2 acetate ligand [O(70) and O(72)].
Each iron centre is in a distorted octahedral environment
with an FeN2O4 coordination sphere. The bond lengths
Fe(1)–O(70) 2.067(5) Å and Fe(2)–O(72) 2.078(5) Å, involv-
ing the acetate bridging, are in agreement with similar (μ-
acetato)diiron(III) complexes.[14] The bridging methoxy O(60)
constitutes a nearly linear array with bond angles N(9)–
Fe(1)–O(60) of 175.8(2)° and N(39)–Fe(2)–O(60) of 175.9(2)°.
The pyridine nitrogens, N(24) and N(54), are trans disposed
through the iron centres to phenolate oxygens with bond
angles N(24)–Fe(1)–O(17) of 164.8(2)° and N(54)–Fe(2)–
O(31) of 164.0(2)°. The Fe–O and Fe–N bond lengths are
consistent with a d5 high-spin electron configuration for both
FeIII centres with amine nitrogen and phenolate oxygen-do-
nor ligands. The Fe···Fe separation of 3.549 Å is slightly
longer than that for 5 and similar to other complexes.
Crystal Structures of 7·2CH3CN and 8·CH3OH
As complexes 7 and 8 contain the same tridentate [L4]2–
anion with the same substituents, para-disposed methyl and
ortho-substituted tert-butyl to phenol groups, the X-ray struc-
tures are described under the same heading.
Single crystals of deep red-brown dinuclear neutral mole-
cules [(L4)2FeIII2(μ-OH)2]·2CH3CN (7) were obtained from
an acetonitrile solution by slow evaporation. The asymmetric
unit contains half of the dimer and a solvent molecule of
crystallisation. The molecular geometry and the atom label-
ling scheme for the molecule in 7 are shown in Figure 5. Bond
lengths and angles are selectively listed in Table 5. The two
iron centres are bridged by two hydroxo groups, which are
weakly hydrogen bonded to the N(40) and N(40)* atoms
[O(1)···N(40) 2.99 Å] of the solvent acetonitrile. The five-fold
coordination of the iron centres is completed by the dianionic
ligand [L4]2– through the amine nitrogen N(19) and two
Figure 4. ORTEP plot of 6 with ellipsoids drawn at the 50% prob-
ability level.
Eur. J. Inorg. Chem. 2011, 2547–2557
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2551

T. Weyhermüller, R. Wagner, P. Chaudhuri
FULL PAPER
phenolate oxygens O(27) and O(1); this generates two FeNO4
In an attempt to prepare a bis(μ-methoxo)diiron(III) com-
coordination spheres. The geometry of the iron centres is best plex, 7 was recrystallised from a methanolic solution in the
described as a trigonal bipyramid (trigonality index τ = presence of a relatively strong base [nBu4N]OCH3. The asym-
0.839) with three oxygen atoms, namely, the bridging hydroxo metrically bridged (μ-hydroxo) (μ-methoxo) complex
O(1) and the phenoxo oxygens O(11) and O(27), forming the 8·CH3OH was isolated as deep brown crystals. A view of the
equatorial plane; the axial ligation is fulfilled by N(19) and structure is shown in Figure 6 and selected bond lengths and
O(27) [N(19)–Fe(1)–O(1)* 171.69(4)°]. The structures of 7 angles are listed in Table 6. The structure analysis of 8 shows,
and 5 primarily differ from one another in the coordination in contrast to 7, the presence of a five- and a six-coordinate
number of the iron centre, five for 7 and six for 5, which can iron centre [Fe(2) and Fe(1), respectively]. Such dissimilar co-
be attributed to the denticity of the corresponding ancillary ordination numbers for the two iron centres in a diferric(III)
ligands. The bond lengths for the five-coordinate iron centres complex has been observed for another dimethyl-substituted
in 7 are significantly shorter than those for the six-coordinate aminebis(phenolate) ligand.[6a]
iron centres in 5. The Fe–N and Fe–O bond lengths are con-
sistent with a d5 high-spin electron configuration of the iron
in both cases, which is also in agreement with magnetic
susceptibility measurements. The Fe···Fe separation of
3.066 Å in 7 is very similar to that for 5 and other hydroxo-
bridged diferric(III) complexes.
Figure 6. ORTEP diagram of 8 with ellipsoids drawn at the 40%
probability level.
The Fe(1) centre is in a distorted octahedral environment
with an FeNO5 coordination sphere, whereas Fe(2) is in a
distorted trigonal pyramidal FeNO4 environment. Two oxy-
Figure 5. ORTEP diagram of 7·2CH3CN with ellipsoids drawn at the
50% probability level.
gen atoms, a hydroxo O(1) and a methoxy O(2) act as bridg-
ing ligands between the Fe(1) and Fe(2) centres. Each metal
ion is terminally coordinated to the dianionic ligand [L4]2–,
thus rendering them five-coordinate; a methanol molecule
Table 5. Bond lengths [Å] and angles [°] for 7·2CH3CN.[a]
Fe(1)–O(27)
Fe(1)–O(11)
Fe(1)–O(1)
Fe(1)–O(1)#1
Fe(1)–N(19)
O(1)–Fe(1)#1
O(27)–Fe(1)–O(11)
O(27)–Fe(1)–O(1)
O(11)–Fe(1)–O(1)
O(27)–Fe(1)–O(1)#1
O(11)–Fe(1)–O(1)#1
O(1)–Fe(1)–O(1)#1
O(27)–Fe(1)–N(19)
O(11)–Fe(1)–N(19)
O(1)–Fe(1)–N(19)
O(1)#1–Fe(1)–N(19)
Fe(1)–O(1)–Fe(1)#1
1.8570(11)
1.8605(11)
1.9305(10)
2.0037(11)
2.1942(12)
2.0036(11)
121.03(5)
117.54(5)
121.40(5)
96.15(5)
94.41(5)
77.64(5)
89.25(5)
88.14(5)
O(4) completes the sixth coordination site for Fe(1). The se-
vere distortion of Fe(2) from a regular polyhedron (the trigo-
nality index τ = 0.493) is shown by the equatorial angles
O(1)–Fe(2)–O(57) 138.37(6)°, O(57)–Fe(2)–O(41) 113.87(6)°
and O(41)–Fe(2)–O(1) 107.71(6)°. The bridging angles Fe(2)–
O(1)–Fe(1) 102.79(6)° and Fe(2)–O(2)–Fe(1) 103.04(6)° are
very similar. Hydrogen bonds exist between the free methanol
and bridging O(1) [O(70)H···O(1) 2.94 Å], and between coor-
dinated methanol and phenolate O(41) [O(4)H···O(41)
2.72 Å], as exhibited by the dotted lines in Figure 6. For
Fe(1), bridging oxygens O(1) and O(2), and phenolates O(11)
and O(27) constitute the equatorial plane, whereas the amine
N(19) and the methanol O(4) are in the axial sites. The Fe(1)···
Fe(2) separation of 3.128 Å falls in the range of reported val-
ues. The dihedral angle between the planes comprising Fe(1)–
O(1)–Fe(2) and Fe(1)–O(2)–Fe(2) is 28.4°.
94.31(5)
171.72(4)
102.36(5)
[a] Symmetry transformations used to generate equivalent atoms:
#1 – x, –y, –z.
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Asymmetrically Dibridged Diiron(III) Complexes
Table 6. Bond lengths [Å] and angles [°] for 8·CH3OH.
meric paramagnetic impurity (S = 5/2) PI = 0.03% for 6 and
J = –2.1 cm–1, g1 = g2 = 2.00 (fixed) for 7. Similar weak
antiferromagnetic exchange coupling constants have been
evaluated for other complexes and are listed in Table 7. As
the evaluation of coupling constants for such dinuclear com-
Fe(1)–O(11)
1.8893(13)
1.9098(13)
2.0220(13)
2.0678(13)
2.1416(14)
2.1971(15)
1.8518(14)
1.9089(13)
1.9329(13)
1.9731(13)
2.2076(15)
Fe(1)–O(27)
Fe(1)–O(2)
Fe(1)–O(1)
Fe(1)–O(4)
Fe(1)–N(19)
Fe(2)–O(57)
Fe(2)–O(41)
Fe(2)–O(1)
Fe(2)–O(2)
Fe(2)–N(49)
O(11)–Fe(1)–O(27)
O(11)–Fe(1)–O(2)
O(27)–Fe(1)–O(2)
O(11)–Fe(1)–O(1)
O(27)–Fe(1)–O(1)
O(2)–Fe(1)–O(1)
O(11)–Fe(1)–O(4)
O(27)–Fe(1)–O(4)
O(2)–Fe(1)–O(4)
O(1)–Fe(1)–O(4)
O(11)–Fe(1)–N(19)
O(27)–Fe(1)–N(19)
O(2)–Fe(1)–N(19)
O(1)–Fe(1)–N(19)
O(4)–Fe(1)–N(19)
O(57)–Fe(2)–O(41)
O(57)–Fe(2)–O(1)
O(41)–Fe(2)–O(1)
O(57)–Fe(2)–O(2)
O(41)–Fe(2)–O(2)
O(1)–Fe(2)–O(2)
O(57)–Fe(2)–N(49)
O(41)–Fe(2)–N(49)
O(1)–Fe(2)–N(49)
O(2)–Fe(2)–N(49)
Fe(2)–O(1)–Fe(1)
Fe(2)–O(2)–Fe(1)
101.31(6)
92.89(5)
163.81(6)
165.19(6)
93.45(5)
72.32(5)
90.92(6)
87.50(6)
84.51(6)
88.49(5)
89.24(6)
90.25(5)
97.74(6)
91.93(5)
177.73(6)
113.81(6)
138.37(6)
107.72(6)
95.87(6)
96.44(6)
76.32(5)
88.81(6)
91.76(6)
92.82(6)
167.96(6)
102.79(6)
103.04(6)
Figure 7. Plots of μeff versus T for complexes 6 and 7. The solid lines
represent the simulation of the experimental data to the theoretical
equation (see text).
ˆ
Table 7. Selected structural data and exchange coupling constants (H
ˆ
ˆ
= –2JS1·S2) for reported diferric(III) complexes doubly bridged by
two different ligands and for complexes 4–8.
Complexes[a] with
(μ-OX)(μ-OY)
groups[b]
av. Fe–O(bridge)
[Å]
Fe–O–Fe
[°]
J
Ref.
[cm–1]
4 (OH)(OPh)
5 (OH)(OH)
2.01
101.7
100.5
128.1
102.5
102.9
–7.0
–4.1
–14.1
–2.1
–6.0
this
work
this
work
this
work
this
2.034
2.023
1.966
1.999
6 (OMe)(OAc)
7 (OH)(OH)[c]
8 (OH)(OMe)[d]
work
this
Magnetic Susceptibility Measurements
work
[6f]
9 (OH)(OMe)[d]
10 (OMe)(OMe)
11 (OMe)(OMe)
12 (OMe)(OMe)
13 (OMe)(OMe)
14 (OR)(OOCR)
15 (OR)(OOCR)
16 (OH)(OAc)
1.999
2.016
1.997
1.987
1.974
1.971
1.986
1.990
1.985
2.010
2.045
102.4
104.7
104.3
102.0
103.7
130.0
129.3
126.4
104.4
102.8
–5.84
–26.8
–28.6
–7.7
[16]
[16]
[17]
[17]
[14a]
[18]
[19]
[20]
[21]
[22]
Magnetic susceptibility data for polycrystalline samples of
complexes 1–8 were collected in the temperature range 2–
290 K in an applied magnetic field of 1 T. The magnetic prop-
erties of mononuclear complexes 1–3 exhibit, above 20 K, the
essentially temperature-independent magnetic moment values
μeff = 5.90Ϯ0.01 μB, and thus clearly contain high-spin d5
ions, that is high spin FeIII.
–9.5
–14.4
–14.0
–14.3
–12.0
–10.6
17 (OH)(OPh)
18 (OMe)(OR)
19 (OMe)(OAc)
The magnetic behaviour of complexes 4–8 is characteristic
of antiferromagnetically coupled dinuclear complexes. For ex-
ample, at 290 K the μeff value of 6.33 μB (χM·T =
101.0(OAc), –8.3
108.7(OMe)
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[6a]
20 (OH)(OPh)
21 (OMe)(OPh)
22 (OR)(OR)
2.018
2.013
1.972
2.024
2.006
1.967
2.044
2.043
2.054
103.5
102.5
104.1
101.1
105.7
104.5
97.1
–7.4
–8.0
–17.0
–2.2
–10.4
–15.8
+1.2
–12.7
–7.4
5.002 cm3Kmol–1) for
8.379 cm3 Kmol–1) for
6
or of 8.19 μB (χM·T
decreases monotonically with
=
7
23 (OR)(OR)
decreasing temperature until it reaches a value of 0.30 μB
(χM·T = 0.01169 cm3Kmol–1) for 6 or 1.22 μB (χM·T =
0.1862 cm3Kmol–1) for 7 at 2 K (Figure 7). This is a clear
indication of exchange coupling between two paramagnetic
high-spin FeIII centres (SFe = 5/2) with a resulting St = 0
ground state. We used the Heisenberg–Dirac–van Vleck spin-
24 (OPh)(OPh)[c]
25 (OR)(OR)[c]
26 (OPh)(OPh)
27 (OPh)(OPh)
28 (OPh)(OPh)[d]
108.9
104.9
[a] Only the bridging ligands are denoted. The terminal ligands
and the diiron(III) centres have been omitted for clarity. [b] OPh
= phenoxide; OMe = methoxide, OR = alkoxide, OAc = acetate;
monoatomic O-bridging of OAc in 19. [c] Both iron(III)-centres
are five coordinate. [d] Only one of the two iron centres is five
ˆ ˆ
ˆ
Hamiltonian in the form H = –2JS1·S2 for an isotropic ex-
change coupling between two spins S1 and S2. The solid lines
in Figure 7 represent the best fits[15] with the following pa-
rameters: J = –14.1 cm–1, g1 = g2 = 2.00 (fixed) and mono- coordinate.
Eur. J. Inorg. Chem. 2011, 2547–2557
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2553

T. Weyhermüller, R. Wagner, P. Chaudhuri
FULL PAPER
plexes is now a matter of routine work, we refrain from de- similar spin coupling. The weakest coupling in the series is
scribing them in detail. We have chosen to display the results observed for complex 7, containing a bis(μ-hydroxo) bridge,
for complexes 6 and 7 (Figure 7) as they exhibit the strongest as the Fe–O–Fe angle is small in conjunction with the ex-
and the weakest coupling of the present series. Additionally, pected Fe–O bond length (1.966 Å) for five-fold coordination
complex 7 represents a rare case of exchange-coupled five- of FeIII. On the contrary, complex 25 with five-coordinate
coordinate ferric(III) centres.
bis(μ-alkoxo) diferric(III) centres results in a J value of
It has been reported[34b] that different analysis of the same –15.8 cm–1 in spite of the same Fe–O bond length (1.967 Å),
susceptibility data can lead to varying J values, when the g presumably due to the larger bridging angle (104.5°). The fer-
value is allowed to float for a ferric centre. In our case, for romagnetic interaction observed only for complex 26, a bis(μ-
example, the susceptibility data for complex 5 could be simu- phenoxo)diiron(III) compound, has been attributed[29] to the
lated with the same good quality when the g value is floated. smallest Fe–O–Fe angle (97°). In the above discussion, we
Thus, the J values vary between –3.15 with g = 1.883 and have not considered the effect of the groups –H, –R, –Me
–3.66 cm–1 with g = 1.95. Because of the spherical electron and –Ph on the electron density of the bridging oxygen atom.
distribution of high-spin d5 ions, the g value is expected to be Considering the I-effects, it is expected that the methoxo-
equal to 2.00 in accordance with the 6A1 ground state. Hence, bridged compounds should show strongest antiferromagnetic
we have kept a fixed g = 2.00 value for complexes 4–8.
interactions, which is indeed the case as exemplified by com-
Table 7, which includes the diferric(III) complexes of the plexes 9 and 10. No striking correlation is established, how-
present series (4–8), was assembled with consideration of the ever, between only one structural parameter and the magni-
following points: (i) all structurally characterised asymmetri- tude of the exchange coupling.
cally doubly bridged diiron(III) complexes are included; (ii)
The strength of exchange coupling is weaker in the five-
as the non-linear triatomic bridging groups, such as tetra- coordinate than the six-coordinate FeIII species, as exem-
oxometalates,[31] are very weak exchange couplers the di- plified by the J values for complexes 5 and 7, in spite of the
iron(III) complexes bridged by ion such as chromate, molyb- fact that the average Fe–O bond length is shorter and the
date, and sulfate have been excluded; selected triatomic bridg- Fe–O–Fe angle larger in five-coordinate 7 than those in six-
ing through acetate and other carboxylato groups, which are coordinate 5.
also very weak transmitters of spin coupling are listed for the
It is interesting to note that complexes 6, 14, 15 and 16
necessity of comparison with the only known (μ-methoxy)(μ- (Table 7) exhibit very similar exchange couplings (J ≈
acetato)diiron complex 6; (iii) although the symmetrically –14 cm–1), although the Fe–O bond lengths vary in the range
monoatomic(III) dibridged diferric complexes are abun- 1.971–2.023 Å in conjunction with the comparatively larger
dant,[32] only a few of them have been listed to indicate the Fe–O–Fe angles (126–130°).
range of weak ferromagnetism (26) to moderate weak anti-
ferromagnetism (11).
Due to Fe···Fe distances longer than 3 Å in all complexes
listed in Table 7 any correlation between this structural fea-
Conclusion
ture and the exchange coupling constant J can be safely ex-
To conclude, the following points of this study deserve par-
cluded.[33] The presence of ten magnetic orbitals, originating ticular attention.
from five singly occupied d-orbitals per high-spin ferric ion
As an obvious progression of our earlier reports[6] on ligat-
and resulting in a large number of crossed- and direct-ex- ing properties of amine-bis(phenolate) ligands with [O,N,O]
change pathways makes any clear-cut analysis of the magnetic donor atoms, we have studied dimethyl- versus di-tert-butyl-
behaviour for diferric(III) complexes, particularly with weak substituted phenol-based ligands (H3L1 versus H3L2) and ob-
exchange coupling, very difficult. Accordingly, none of the served a noteworthy effect of tert-butyl groups on the nu-
proposed magnetostructural correlations[32,34] relating to the clearity of the ferric(III) compounds. Only mononuclear com-
magnitude of the exchange coupling to either the iron-bridg- plexes 1–3 have been isolated with H3L1 in contrast to the
ing oxygen bond length or the bridging Fe–O–Fe angle can diferric(III) complex 4 with the dimethyl congener H3L2.
satisfactorily rationalise the weak-coupled systems listed in
Emphasis has been given to structural and magnetic stud-
Table 7. Irrespective of the planarity of the bridging Fe2O2 ies of the asymmetrically dibridged diferric(III) complexes 4–
unit, the Fe–O–Fe bridge angle and the Fe–O(bridge) bond 8, whose exchange coupling properties have been compared
length have their independent contributory roles to the overall to those of similar compounds from the literature to judge
exchange coupling J, in which one structural feature can out- the spin exchange ability of the different groups, such as
weigh the other. For example, in complex 6, syn,syn η1:η1- –OH, –OMe, –OR and –OPh. A comparison between five-
acetato bridging yields a large Fe–OMe–Fe bridging angle of and six-coordinate diferric(III) centres, regarding exchange
128.1° resulting in the strongest coupling for the present coupling, is reported. It appears that for the diferric(III) series
series. Similarly, the strength of exchange coupling increases 4–8, the structural feature, the bridging Fe–O–Fe angle, is
with bridging angle for 12–14, in spite of very similar Fe–O a major factor in determining the strength of the exchange
bond lengths of ≈1.98 Å. Indeed, a linear dependence of J on interaction. Inspection of similar compounds from the litera-
the bridging angle for similar complexes has been reported.[17] ture also shows a dependence of J on the Fe–O(bridging)
Additionally, complexes 8 and 9, with nearly identical above- bond length. Thus, no definitive magneto-structural conclu-
mentioned structural parameters, exhibit, as expected, very sion based only on a single structural parameter of the re-
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Eur. J. Inorg. Chem. 2011, 2547–2557

Asymmetrically Dibridged Diiron(III) Complexes
turned red-brown. The solution was refluxed in air for 1 h and filtered
to remove any solid particles. The deep brown microcrystalline solid
that separated after cooling was recrystallised from methanol; yield
110 mg (20%). C32H48FeNO5 (582.59): calcd. C 65.97, H 8.31, N
ported weakly-coupled diiron(III) complexes has been ob-
tained.
2.40, Fe 9.59; found C 66.4, H 8.3, N 2.5, Fe 9.8. IR (KBr): ν =
˜
Experimental Section
2958, 2868, 2678, 1627, 1470, 1439, 1413, 1361, 1305, 1263, 1240,
1204, 1171, 1130, 877, 841, 752, 613 cm–1. ESI-MS (positive mode,
CH3CN solution): m/z (%) = 582 [M+]. UV/Vis [CH3OH]: λmax, nm
(ε, m–1 cm–1): 282 (21.7ϫ103), 381 (10.1ϫ103), 510 (6.0ϫ103).
Materials and Physical Measurements: Reagent or analytical grade
materials were obtained from commercial suppliers and used without
further purification. Elemental analyses (C, H, N, and metal) were
performed by the Microanalytical Laboratory, Mülheim, Germany.
FT-IR spectra of the samples in KBr disks were recorded with a
Perkin–Elmer 2000 FT-IR instrument. Magnetic susceptibilities of
powdered samples were recorded with a SQUID magnetometer in the
temperature range 2–290 K with an applied field of 1 T. Experimental
susceptibility data were corrected for the underlying diamagnetism
using Pascal’s constants. Mass spectra were recorded with either a
Finnigan MAT 8200 (electron ionisation) or a MAT 95 (electrospray
ionisation, ESI-MS) instrument. A Bruker DRX 400 instrument was
used for NMR spectroscopy.
[(L1)FeIII(C2H5OH)] (2): Complex 2 was obtained from 1 after
recrystallisation from an ethanolic solution. C34H52FeNO5 (610.64):
calcd. C 66.88, H 8.58, N 2.29, Fe 9.15; found C 67.0, H 8.6, N 2.2,
Fe 9.1.
[(L1)FeIII(OCH3)][Et3NH] (3): The same protocol used for 1 was used
complex 3 but an excess of Et3N (4.0 mL) was added. Deep-brown
crystals were obtained from a methanol solution for the X-ray struc-
ture determination; yield 0.35 g (≈50%). C39H65FeN2O5 (697.81):
calcd. C 67.13, H 9.39, N 4.02, Fe 8.00; found C 67.0, H 9.3, N 4.1,
Fe 7.9. IR (KBr): ν = 2958, 2905, 2869, 1655, 1474, 1438, 1413, 1387,
˜
Syntheses of Ligands:[4,5a,7] The ligands [N,N-bis(3,5-di-tert-butyl)-2-
hydroxybenzyl]aminoacetic acid (H3L1), (N,N-bis[2-hydroxy-3,5-di-
methylbenzyl]aminoacetic acid) (H3L2), [N,N-bis(2-hydroxy-3,5-di-
methylbenzyl)]-N-(2-pyridylmethyl)amine (H2L3) and [methylamino-
N,N-bis(6-tert-butyl-4-methyl-2-methylenephenol)] (H2L4) were pre-
pared by the one-pot Mannich condensation reaction of phenol with
the appropriate substituents (0.1 mol), formaldehyde 37% (0.1 mol)
and glycine (0.05 mol) for H3L1 and H3L2, 2-(aminomethyl)pyridine
(0.05 mol) for H2L3 or methylamine (0.05 mol) for H2L4 in dry meth-
anol as solvent. The reaction mixture was stirred overnight and then
refluxed for 3 h. On cooling, a white solid was obtained, which was
filtered off, and washed with water and methanol.
1361, 1305, 1267, 1240, 1204, 1170, 1131, 1109, 877, 863, 840, 810,
751, 613 cm–1. ESI-MS (negative mode, CH3OH solution): m/z = 595
[M – Et3NH].
[Et3NH][(L2)2FeIII2(μ-OH)(OH2)] (4): A solution of the ligand H3L2
(0.34 g, 1 mmol) and Et3N (0.4 mL) in methanol (50 mL) was de-
gassed for 15 min. Fe(ClO4)2·6H2O (0.36 g, 1 mmol) was added to
obtain a violet solution, which was refluxed under argon for 0.5 h.
The solution was kept overnight in the air to obtain a microcrystal-
line solid, which was isolated by filtration and air-dried; yield 0.27 g
(≈29%). C46H63O10N3Fe2 (929.73): C 59.43, H 6.83, N 4.52, Fe 12.01;
found C 60.1, H 6.8, N 4.5, Fe 12.2. IR (KBr): ν = 2964, 1617, 1476,
˜
1384, 1307, 1262, 1162, 1121, 1033, 968, 930, 857, 807 cm–1. ESI-MS
(pos. + neg. mode, CH3CN solution): m/z = 809 [M – C6H15NH –
H2O]. X-ray quality deep purple crystals were grown from an acetoni-
trile solution.
The purity of the ligands was checked by liquid chromatography.
1
H3L1: Yield 51%. H NMR (CDCl3, 400 MHz): δ = 1.25–1.40 (36
H), 1.99 (s, 4 H), 3.45 (s, 2 H), 3.77, 3.79 (d, 4 H), 6.88, 6.89 (d, 4
H) ppm. IR (KBr ): ν = 2962 (m), 1705 (s), 1636 (s), 1482 (s), 1361
˜
[(L3)2FeIII2(μ-OH)2] (5): The same protocol as that for 4 was used
for preparing complex 5 by replacing H3L2 with H2L3 and ferrous
perchlorate with anhydrous FeCl2. Deep-brown crystals were ob-
tained from a methanol solution for the X-ray structure determi-
nation; yield 0.48 g (≈54%). C48H54O6N4Fe2 (894.69): C 64.44, H
6.04, N 6.26, Fe 12.48; found C 65.0, H 6.4, N 6.2, Fe 12.3. IR (KBr):
(m), 1292 (m), 1222 (s) cm–1. EI-MS: m/z (%) = 511 (18) [M+], 293
(50) [M – C15H23O], 219 (28) [C15H23O], 203 (100) [C15H21].
1
H3L2: Yield 68%; m.p. 199–201 °C. Nearly insoluble for H NMR
spectroscopy. IR (KBr ): ν = 3456 (m), 3005 (m), 1641 (s), 1491 (s),
˜
1439 (m), 1401 (m), 1386 (s), 1324 (m), 1199 (s), 1159 (m) cm–1. EI-
MS: m/z (%) = 343 (38) [M+], 298 (6) [M – COOH], 209 (67) [M –
C9H11O], 135 (100) [C9H11O].
ν = 3642, 3441, 2991, 2913, 2853, 1607, 1572, 1474, 1445, 1384, 1324,
˜
1310, 1294, 1265, 1161, 958, 856, 818, 802, 759, 596 cm–1. UV/Vis in
CH2Cl2: λmax, nm (ε, m–1 cm–1): 494/6.9ϫ103, 292 (21.8ϫ103).
1
H2L3: Yield 46%; m.p. 122–124 °C. H NMR (400 MHz, CDCl3): δ
[(L3)2FeIII2(μ-OCH3)(μ-OOCCH3)] (6): Isolation of 6 followed the
same procedure as that for 5 using anhydrous Fe(OOCCH3)2 as the
metal salt. Deep violet crystals were obtained from an acetonitrile
solution for X-ray structure determination; yield 0.58 g (≈61%).
C51H58Fe2N4O7 (950.75): calcd. C 64.43, H 6.15, N 5.89, Fe 11.75;
= 2.17–2.25 (12 H), 3.74, 3.82 (6 H), 6.69 (2 H), 6.85 (2 H), 7.10 (1
H), 7.24–7.27 (1 H), 7.66–7.71 (1 H), 8.68–8.69 (1 H) ppm. IR (KBr):
ν = 3125 (m), 3006 (m), 2964 (m), 2849 (m), 2727 (m), 1597 (s), 1571
˜
(m), 1498 (vs), 1431 (s), 1371 (s), 1290 (s), 1223 (vs), 1158 (s), 1028
(s), 1003 (s), 973 (s), 864 (s), 851 (s), 760 (s), 737 (s) cm–1. EI-MS:
m/z (%) = 376 (19) [M+], 284 (56) [M – C5H4N(CH2)], 241 (100)
[M – C6H2(CH3)2(CH2)(OH)].
found C 65.0, H 6.1, N 5.8, Fe 11.6. IR (KBr): ν = 3450, 2857, 1637,
˜
1606, 1571, 1554, 1474, 1450, 1420, 1310, 1276, 1269, 1161, 1094,
857, 821, 803, 760 cm–1. UV/Vis in CH2Cl2: λmax, nm (ε, m–1 cm–1):
527 (9.6ϫ103), 295 (26.5ϫ103). ESI-MS (positive mode, CH2Cl2):
m/z = 877.3 [(L3)2Fe2O], 430 [(L3)Fe].
H L4: Yield 18%; m.p. 99–102 °C. IR (KBr ): ν = 3629 (sharp s),
˜
2
3443 (br. s), 2955, 2918, 2864 (s), 1609 (m), 1481, 1468, 1445 (s), 1388
(m), 1357 (m), 1301 (m), 1280 (m), 1237 (s), 1180 (m), 1126 (m), 1025
(m), 977 (m), 863 (s), 843 (s), 798 (m), 772 (m), 757 (s) cm–1. EI-MS:
m/z (%) = 383 (85) [M+], 207 (100) [M – C6H2(CH3)(C4H9)(CH2)-
OH], 177 [C6H2(CH3)(C4H9)(CH2)(OH)].
[(L4)2FeIII2(μ-OH)2] (7): FeCl2·4H2O (0.198 g, 1 mmol) was added to
a solution of H2L4 (0.384 g, 1 mmol) in methanol (50 mL) under
argon, resulting in a pale-brown solution, to which Et3N (0.5 mL)
was added. The solution was refluxed under argon for 0.5 h, followed
by stirring in the air for 1 h. The precipitated brown microcrystalline
Preparation of Complexes
[(L1)FeIII(H2O)] (1): FeCl2·4H2O (0.19 g, 1 mmol) was added to a solid was separated by filtration and recrystallised twice from an ace-
degassed solution of CH3OH (50 mL) containing H3L1 (0.51 g,
tonitrile solution to yield X-ray quality red-brown crystals of
1 mmol). Upon addition of Et3N (0.4 mL) the colour of the solution 7·2CH3CN; yield 0.4 g (40%). C54H78Fe2N4O6 (990.94): calcd. C
Eur. J. Inorg. Chem. 2011, 2547–2557
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2555

T. Weyhermüller, R. Wagner, P. Chaudhuri
FULL PAPER
65.45, H 7.93, N 5.65, Fe 11.27; found C 65.6, H 7.9, N 5.8, Fe 11.5.
stream of the diffractometer. Graphite-monochromated Mo-Kα radi-
ation (λ = 0.71073 Å) from a Mo-target rotating-anode X-ray source
was used throughout. Final cell constants were obtained from least-
IR (KBr): ν = 3667, 3422, 2956, 2915, 2859, 1607, 1467, 1438, 1414,
˜
1385, 1354, 1298, 1255, 1209, 1152, 1088, 1057, 1031, 926, 859, 832,
807, 626, 605 cm–1. EI-MS: m/z (%) = 890 (15) [(L4)2Fe2O], 714 (100) squares fits of several thousand strong reflections. Crystal faces were
[(L4)2Fe2O – C12H27]. Complex 7 was also obtained from Fe- determined and intensity data were corrected for absorption using
(ClO4)2·6H2O as the starting iron salt.
the Gaussian method in SADABS.[8] Structures were readily solved
by Patterson methods and subsequent difference Fourier techniques.
The Siemens ShelXTL[9] software package was used for solution and
artwork of the structures; ShelXL97[10] was used for the refinement.
All non-hydrogen atoms were anisotropically refined and hydrogen
atoms bound to carbon were placed at calculated positions and re-
fined as riding atoms with isotropic displacement parameters. Acidic
protons bound to oxygen and nitrogen atoms were located from the
difference map and refined with restrained bond length and thermal
displacement parameters. Split atom models were refined for disor-
dered parts, where possible. Split positions were refined with re-
strained bond length, angles and thermal displacement parameters
[(L4)2FeIII2(MeOH)(μ-OH)(μ-OMe)] (8): Complex 8·CH3OH was ob-
tained from 7 through recrystallisation from a methanolic solution in
presence of [nBu4N]OCH3. C53H82Fe2N2O8 (986.95): calcd. C 64.50,
H 8.38, N 2.84, Fe 11.32; found C 65.0, H 8.4, N 2.9, Fe 11.0. IR
(KBr): ν = 3601, 3416, 3301, 2915, 1618, 1466, 1443, 1385, 1352,
˜
1298, 1250, 1210, 1154, 1134, 1089, 1073, 996, 946, 924, 861, 821,
771, 600, 559 cm–1. UV/Vis in CH2Cl2: λmax, nm (ε, m–1 cm–1): 455
(8.3ϫ103), 331 (12.4ϫ103), 283 (24.8ϫ103).
X-ray Crystallographic Data Collection and Refinement of the Struc-
tures: Single crystals for 1, 2, 3·CH3OH, 4·2.5CH3CN·H2O, 5, 6,
7·2CH3CN and 8·CH3OH were coated with perfluoropolyether, using the SADI, SAME and EADP instructions of ShelXL97. Crys-
picked up with nylon loops and were mounted in the nitrogen cold tallographic data of the compounds are listed in Table 8.
Table 8. Crystallographic data for 1, 2, 3·CH3OH, 4·2.5CH3CN·H2O, 5, 6, 7·2CH3CN, 8·CH3OH.
1
2
3·CH3OH
4·2.5CH3CN·H2O
Formula
Fw
C32H48FeNO5
582.56
C34H52FeNO5
610.62
C40H69FeN2O6
729.82
C51H72.5Fe2N5.5O11
1050.34
Space group
a [Å]
b [Å]
c [Å]
Iba2, No. 45
22.557(2)
34.621(3)
8.7112(8)
90
Pbca, No. 61
8.842(2)
23.451(4)
32.700(5)
90
Pca21, No. 29
24.2976(9)
9.2462(4)
19.3926(8)
90
P21/n, No. 14
20.1466(12)
20.2232(12)
26.046(2)
90
α [°]
β [°]
γ [°]
90
90
90
90
90
90
96.769(2)
90
V [Å]
Z
6803.0(10)
8
6780(2)
8
4356.8(3)
4
10537.9(12)
8
T [K]
100(2)
1.138
100(2)
1.196
100(2)
1.113
100(2)
1.324
ρ calcd. [gcm–3]
Refl. collected/2θmax
Unique refl. with IϾ2σ(I)
Parameters/restraints
λ [Å]/μ(Kα) [cm–1]
R1[a]/goodness of fit[b]
wR2[c] [IϾ2σ(I)]
Residual density [eÅ–3]
20223/44.98
4329/3279
371/35
0.71073/4.79
0.0887/1.055
0.2251
7936/46.40
3874/3162
375/0
0.71073/4.83
0.0678/1.156
0.1358
117967/61.96
13826/12943
474/19
0.71073/3.88
0.0450/1.084
0.1072
134334/50.00
18495/13153
1324/17
0.71073/6.13
0.0462/1.053
0.0870
+0.66/–0.28
+0.75/–0.44
+0.78/–0.40
+0.59/–0.41
5
6
7·2CH3CN
8·CH3OH
Formula
Fw
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C48H54Fe2N4O6
894.65
R3, No. 148
32.803(2)
32.803(2)
12.9020(8)
90
90
120
12023.0(13)
9
C51H58Fe2N4O7
950.71
P21/n, No. 14
12.1961(6)
23.0060(13)
16.0454(8)
90
92.623(5)
90
4690.9(4)
4
C54H78Fe2N4O6
990.90
P1, No. 2
C53H82Fe2N2O8
986.91
P1, No. 2
¯
¯
¯
10.229(2)
11.415(2)
12.746(2)
104.644(6)
95.793(7)
101.611(7)
1392.2(4)
1
11.4826(8)
13.6229(12)
18.7220(12)
96.138(4)
92.686(4)
112.844(4)
2670.9(3)
2
γ [°]
V [Å]
Z
T [K]
100(2)
1.112
100(2)
1.346
100(2)
1.182
100(2)
1.227
ρ calcd. [gcm–3]
Refl. collected/2θmax
Unique refl. with IϾ2σ(I)
Parameters/restraints
λ [Å]/μ(Kα) [cm–1]
R1[a]/goodness of fit[b]
wR2[c] [IϾ2σ(I)]
Residual density [eÅ–3]
98327/72.00
12528/9767
278/0
0.71073/5.87
0.0406/1.113
0.0823
42197/45.00
6089/3550
577/0
0.71073//6.74
0.0852/1.054
0.1529
27179/62.02
8790/7033
311/1
0.71073/5.69
0.0417/1.043
0.0895
55642/62.00
16917/14794
616/3
0.71073/5.95
0.0473/1.162
0.1214
+0.55/–0.55
+0.77/–0.50
+0.46/–0.29
+1.25/–0.62
[a] Observation criterion: IϾ2σ(I). R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] GooF = {Σ[w(Fo – Fc )(n – p)]}1/2. [c] wR2 = {Σ[w(Fo – Fc ) ]/
2
2
2
2 2
Σ[w(Fo2)2]}1/2, where w = 1/σ2(Fo2) + (aP)2 + bP, P = (Fo + 2Fc )/3.
2
2
2556
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2547–2557

Asymmetrically Dibridged Diiron(III) Complexes
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CCDC numbers 805368 (for 1), 805369 (for 2), 805370 (for 3), 805371
(for 4), 805372 (for 5), 805373 (for 6), 805374 (for 7) and 805375 (for
8) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this article):
Figures S1 and S2 contain the molecular structures and Tables S1
and S2 selected bond lengths and angles for 1 and 2. Details of the
magnetic SQUID measurements are also included.
[8] SADABS, version 2006/1, Bruker Area Detector Absorption and
Other Correction, G. M. Sheldrick, University of Göttingen,
Germany, 2006.
[9] ShelXTL 6.14 Bruker AXS Inc., Madison, WI, USA 2003.
[10] ShelXL97, G. M. Sheldrick, University of Göttingen, Germany,
1997.
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Received: December 21, 2010
Published Online: April 28, 2011
Eur. J. Inorg. Chem. 2011, 2547–2557
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2557