Synthesis and crystal structures of μ-oxido- and μ-hydroxido-bridged dinuclear iron(III) complexes with an N2O donor ligand - A theoretical study on the influence of weak forces on the Fe-O-Fe bridging angle

Article abstract of DOI:10.1002/ejic.201100032

The synthesis and crystal structures of three nonheme diiron(III) complexes with a tridentate N,N,O Schiff-baseligand, 2-({[2-(dimethylamino)ethyl]imino} methyl)phenol (HL), are reported. Complexes [Fe₂OL ₂(NCO)₂] (1a) and

Full text of DOI:10.1002/ejic.201100032

FULL PAPER
DOI: 10.1002/ejic.201100032
Synthesis and Crystal Structures of μ-Oxido- and μ-Hydroxido-Bridged
Dinuclear Iron(III) Complexes with an N2O Donor Ligand – A Theoretical
Study on the Influence of Weak Forces on the Fe–O–Fe Bridging Angle
Rituparna Biswas,[a] Michael G. B. Drew,[b] Carolina Estarellas,[c] Antonio Frontera,*[c] and
Ashutosh Ghosh*[a]
Keywords: Iron / Schiff bases / N,N,O ligands / Crystal structures / Density functional calculations
The synthesis and crystal structures of three nonheme di-
iron(III) complexes with tridentate N,N,O Schiff-base
ligand, 2-({[2-(dimethylamino)ethyl]imino}methyl)phenol
water molecule in the solid state. Structural analyses show
that in 1a the iron atoms are pentacoordinate with a bent Fe–
O–Fe angle [142.7(2)°], whereas in 2 the metal centers are
hexacoordinate with a normal Fe–OH–Fe bridging angle
[137.9(2)°]. The Fe–O–Fe angles in complexes 1a and 1b dif-
fer significantly to those usually shown by (μ-oxido)FeIII com-
plexes. A theoretical study has been performed in order to
rationalize this deviation. Moreover, the influence of the
water molecule observed in the solid-state structure of 1b on
the Fe–O–Fe angle is also analyzed theoretically.
a
(HL), are reported. Complexes [Fe2OL2(NCO)2] (1a) and
[Fe2OL2(SAL)2]·H2O [SAL = o-(CHO)C6H4O–] (1b) are un-
supported μ-oxido-bridged dimers, and [Fe2(OH)L2(HCOO)2-
(Cl)] (2) is a μ-hydroxido-bridged dimer supported by a for-
mato bridging ligand. All complexes have been charac-
terized by X-ray crystallography and spectroscopic analysis.
Complex 1b has been reported previously; however, it has
been reinvestigated to confirm the presence of a crucial
not present, the angle is close to 180°.[13] The angle is obvi-
ously smaller (119–135°) in complexes where a carboxylato
bridge supports the oxido bridge.[14] We have used triden-
tate, N2O donor Schiff-base ligands extensively to synthe-
size both discrete and infinite polynuclear complexes of
CuII and NiII with interesting structures and magnetic
properties.[15] FeIII complexes with such ligands are
scarce,[16–22] although numerous examples have been re-
ported with salen-type N2O2 donor ligands.
Herein we report the synthesis and crystal structures of
two new dinuclear FeIII complexes, [Fe2OL2(NCO)2] (1a)
and [Fe2(OH)L2(HCOO)2(Cl)] (2), which are obtained
when the tridentate Schiff base 2-({[2-(dimethylamino)eth-
yl]imino}methyl)phenol (HL) reacts with a methanol solu-
tion of FeCl3 in the presence of an anionic coligand, cya-
nato (for 1a) or formato (for 2). The same ligand has al-
ready been used to synthesize [Fe2OL2(SAL)2]·H2O [SAL =
o-(CHO)C6H4O–] (1b).[23] We have repeated the synthesis
and crystal structure determination of 1b in order to con-
firm the presence of a water molecule interacting with the
ligand. This water molecule is crucial in the crystal packing
as it influences the Fe–O–Fe angle. Curiously, the structures
of the three species (1a, 1b, and 2) are quite different despite
the fact that the ligand is the same and all are prepared
under similar conditions. Complexes 1a and 1b are unsup-
ported oxido-bridged dimers with very different Fe–O–Fe
angles, whereas compound 2 is a hydroxido-bridged dimer
supported by a formato bridge (see Scheme 1 and Figure 1).
Considering the fact that Fe–O–Fe angles vary widely in
Introduction
The Fe–O–Fe (μ-oxido)diiron unit is a common struc-
tural feature in FeIII compounds.[1] Such compounds are
studied extensively mainly because of their relevance as
model complexes for metalloenzymes or as intermediates in
the catalytic cycles of metalloenzymes. It is well known that
in oxido-bridged hemes[2] and Schiff-base complexes of the
type [Fe(salen)]2X2 [salen = 1,2-bis(salicylideneaminato)eth-
ane] FeIII exists exclusively as a pentacoordinate species
with antiferromagnetically coupled high-spin FeIII ions.[3]
Most of the structurally characterized μ-oxido-bridged di-
iron compounds are either in unsupported conditions, i.e. in
the absence of other bridges, such as FeIII–porphyrins,[4–8]
Schiff-base FeIII–salen,[3a,9–11] and related compounds[11] or
supported by carboxylato and other bridging ligands. Con-
versely, the hydroxido bridge is relatively unusual.[12] A fun-
damental structural component of the chemistry of these
functional FeIII complexes is the variable Fe–O–Fe bridging
angle, which is usually in the range 139–180°. However, in
the majority of complexes where other bridging ligands are
[a] Department of Chemistry, University College of Science,
University of Calcutta,
92 A. P. C. Road, Kolkata 700009, India
[b] School of Chemistry, The University of Reading,
P. O. Box 224, Whiteknights, Reading RG6 6AD, UK
[c] Departament de Química, Universitat de les Illes Balears,
Crta. de Valldemossa km 7.5, 07122 Palma (Baleares), Spain
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100032.
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μ-Oxido- and μ-Hydroxido-Bridged Dinuclear Iron(III) Complexes
Figure 1. Dimeric structures of 1a (left) and 1b (right), which contain crystallographic C2 symmetry. Ellipsoids are plotted at 30%
probability.
unsupported oxido-bridged complexes and no theoretical diamine in a 1:1 molar ratio. FeCl3 reacted with the Schiff-
study has been made to rationalize this variation, we have base ligand in a 1:1 molar ratio, and sodium cyanate was
computed the energy profiles of 1a and 1b by changing the added to the reaction mixture resulting in the formation of
μ-oxido-bridging angle using DFT calculations. We show the dinuclear oxido-bridged iron(III) complex 1a. Similarly,
here that the difference in energy due to bending is not very complex 1b was synthesized by the reaction of FeCl3 and
high, and weak forces, such as π–π interactions or hydrogen the Schiff-base ligand in a 1:1 ratio. Complex 2 was ob-
bonds, can dictate the existence of a particular structure in tained by the reaction of FeCl3, the Schiff-base ligand, and
the solid state by compensating for the difference in energy. sodium formate in a 1:1:2 molar ratio. It is interesting to
Therefore, the flexibility of the Fe–O–Fe motif allows the note that during the formation of 1b, half of the Schiff-base
complexes to adopt the most favorable conformation in or- ligand undergoes hydrolysis, whereas the ligand remains in-
der to maximize the stabilization provided by weak nonco- tact in the synthesis of 1a and 2 although the same condi-
valent interactions.
tions were used throughout. We examined the nature of the
solid crystalline products for all three compounds, and no
visible difference in the nature of the crystalline state was
observed indicating that the solids are not mixtures of dif-
ferent compounds. The results of elemental analyses also
confirm that the compositions of all three compounds are
the same as those determined by X-ray analysis.
FTIR and UV/Vis Spectra of Complexes
The moderately strong and sharp bands at 1614, 1617,
and 1632 cm–1 for 1a, 1b, and 2, respectively, are assigned to
the ν(C=N) azomethine group. Complex 1a shows a strong
absorption band at 2205 cm–1 corroborating the presence of
an N-bonded cyanate group. Complex 1b shows a strong,
broad band, centered at 3424 cm–1 due to the presence of a
water molecule. In complex 2, strong bands at 1581 and
1448 cm–1 are likely to be due to the antisymmetric and
symmetric stretching modes for the carboxylate (formate)
group, respectively. The formulation of 2 as a hydroxido-
bridged compound is also supported by the presence of an
Scheme 1. Synthetic route to 1a, 1b, and 2.
Result and Discussion
Synthesis of Compounds
The tridentate Schiff-base ligand was synthesized by con- O–H stretching band at 3431 cm–1. Moreover, two weak ab-
densation of salicylaldehyde and N,N-dimethylethane-1,2- sorption bands are observed at 844 and 843 cm–1 for 1a
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FULL PAPER
and 1b, respectively. It is well known that the antisymmetric 3.338 Å, which is similar to the π–π stacking distance ob-
stretching mode (ν3), νas(Fe–O–Fe), of a linear or bent Fe– served experimentally in organic crystals (3.3–3.6 Å).[27]
O–Fe system usually occurs in the 900–800 cm–1 region.[24] This issue is discussed further in our theoretical study.
Thus, these two bands indicate the existence of an Fe–O–
Table 1. Distances [Å] and angles [°] in the metal coordination
sphere for 1a and 2.
Fe linkage in both compounds. The FTIR spectra of 1a,
1b, and 2 are shown in Figures S1, S2, S3 (see Supporting
Information).
1a
The electronic absorption spectra of the three complexes
are similar (see Figures S4a, S4b; S5a, S5b; S6a and S6b
in the Supporting Information). The band positions and
intensities are comparable with those found in similar com-
plexes.[23,25] The spectra show a strong band with a hump
(415, 493 nm for 1a; 427, 509 nm for 1b; 411, 510 nm for 2)
assigned to oxido-to-Fe charge transfer (CT). Moreover a
moderately strong band (317 nm for 1a; 318 nm for 1b;
320 nm for 2) is tentatively assigned to amino-to-Fe CT.
The highest energy bands (255, 231 nm for 1a; 260, 233 nm
for 1b; 256 and 234 nm for 2) are attributed to πǞπ* transi-
tions of the ligand, but contributions from less intense ox-
ido-to-Fe CT are also expected in this region.[12]
Fe–O(10)
Fe–O(11)
Fe–N(1)
Fe–N(19)
1.763(1)
1.926(3)
1.958(3)
2.108(4)
2.203(3)
102.11(11)
112.39(14)
95.35(11)
O(10)–Fe–N(19)
O(11)–Fe–N(19)
N(1)–Fe–N(19)
O(10)–Fe–N(22)
O(11)–Fe–N(22)
N(1)–Fe–N(22)
N(19)–Fe–N(22)
109.32(12)
86.17(12)
136.88(13)
93.89(10)
159.97(12)
89.51(14)
77.14(14)
Fe–N(22)
O(10)–Fe–O(11)
O(10)–Fe–N(1)
O(11)–Fe–N(1)
2
Fe(1)–O(31)
Fe(1)–O(10)
Fe(1)–O(4)
Fe(1)–O(1)
Fe(1)–N(39)
Fe(1)–N(42)
1.913(3)
1.934(3)
2.013(3)
2.018(3)
2.116(4)
2.274(4)
Fe(2)–O(11)
Fe(2)–O(10)
Fe(2)–O(3)
Fe(2)–N(19)
Fe(2)–N(22)
Fe(2)–Cl(1)
1.893(3)
1.926(3)
2.081(3)
2.100(4)
2.269(4)
2.368(2)
O(4)–Fe(1)–O(10) 98.98(13)
O(31)–Fe (1)–O(4) 92.34(13)
O(10)–Fe (1)–O(4) 92.67(12)
O(31)–Fe(1)–O(1) 92.07(13)
O(10)–Fe(1)–O(1) 92.59(12)
Description of Crystal Structures of 1a and 2
O(4)–Fe(1)–O(1)
172.53(13)
The structure of 1a with C2 symmetry, is shown in Fig-
ure 1 together with the atom numbering scheme in the
metal coordination sphere. Selected bond lengths and
angles are summarized in Table 1. Complex 1a crystallizes
in the orthorhombic space group Fdd2. The iron atoms are
pentacoordinate, bound to three atoms of the tridentate li-
gand, a nitrogen atom N(1) of an NCO group, and a bridg-
ing oxygen atom O(10) positioned at the twofold axis. The
geometry of the coordination sphere is best considered as a
O(31)–Fe(1)–N(39) 86.73(14)
O(1)–Fe(1)–N(39) 89.94(13)
O(31)–Fe(1)–N(42) 165.56(13)
O(10)–Fe(1)–N(42) 95.45(13)
O(4)–Fe(1)–N(42) 86.50(13)
O(1)–Fe(1)–N(42) 87.72(13)
N(39)–Fe(1)–N(42) 78.83(14)
O(11)–Fe(2)–O(10) 98.84(13)
The structure of 2 is also a dimer as shown in Figure 2.
highly distorted square pyramid with O(10) as the axial Here the two hexacoordinate iron atoms have different co-
atom. The basal plane consists of one oxygen and two ni- ordination spheres. Each is bound to the tridentate ligand
trogen atoms of the ligand [O(11), N(19), N(22)] and one and are bridged by hydroxide and formate ions. However,
nitrogen atom [N(1)] of the terminally coordinated NCO the coordination sphere of Fe(1) is completed by a mono-
group. The four donor atoms in the basal plane show a dentate formate ion and that of Fe(2) by a chloride ion.
tetrahedral distortion with deviations of –0.205(2), 0.221(2), Complex 2 crystallizes in the monoclinic space group
–0.238(2), and 0.222(2) Å for N(1), O(11), N(19), and P21/n. The bridging hydroxide O(10) atom is 1.934(3) Å
N(22), respectively. The iron atom is 0.515(2) Å from this from Fe(1) and 1.926(3) Å from Fe(2) with an Fe(1)–O(10)–
plane in the direction of the axial O(10) atom. The Fe– Fe(2) angle of 137.9(2)°. These dimensions are very dif-
O(10) bond length is 1.763(1) Å, which is in the range found ferent from those of the Fe–O–Fe bridge in 1a. Indeed, the
for other (μ-oxido)diiron complexes (1.75–1.80 Å). How- O(11), N(19), N(22), O(10) equatorial plane for Fe(2) and
ever, unlike other similar oxido-bridged dinuclear iron(III) the O(31), N(39), N(42), O(10) equatorial plane for Fe(1)
complexes, the Fe–O–Fe bond angle [142.7(2)°] in 1a devi- show root mean square deviations of 0.0257 and 0.0233 Å,
ates significantly from 180°.[13] The intermetallic Fe–Fe sep- respectively, with the metal atoms 0.066(2) and 0.025(2) Å
aration is 3.342(1) Å. Bond lengths in the basal plane com- from the planes. It is interesting to note that in 2, the bridg-
prising the N2O2 donor set are Fe–O(11) 1.926(3) Å, Fe– ing hydroxido ligand completes an equatorial plane with
N(1) 1.958(3) Å, Fe–N(19) 2.108(4) Å, and Fe–N(22) the three donor atoms from L with both Fe(1) and Fe(2) in
2.203(3) Å, indicating that the Fe–N distances are signifi- contrast with 1a, where the bridging oxygen atom is in an
cantly longer than the Fe–O bond lengths found in similar axial position relative to L. Distances to L are equivalent
complexes of mixed N–O donor ligands, which may result for the two metal atoms. Thus, Fe(1)–O(31) 1.913(3) Å,
from the greater affinity of iron for oxygen rather than ni- Fe(1)–N(39) 2.116(4), and Fe(1)–N(42) 2.274(4) are equiva-
trogen.[26] The two phenyl rings on either side of the twofold lent to Fe(2)–O(11) 1.893(3) Å, Fe(2)–N(19) 2.100(4), and
axis overlap partially at the C(16)–C(17) bonds. The dis- Fe(2)–N(22) 2.269(4) Å. The distances to the bridging for-
tance between the two bonds (centroid to centroid) is mato ligand are, however, different due to the effect of the
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μ-Oxido- and μ-Hydroxido-Bridged Dinuclear Iron(III) Complexes
Figure 2. Structure of 2 with ellipsoids at 30% probability. The hydrogen bond is shown as a dotted line.
trans atoms with Fe(1)–O(1) 2.018(3) and Fe(2)–O(3) only one such example has been reported.[23] It is also worth
2.081(3) Å. Further distances are to the monodentate for- mentioning that in all previously reported complexes of
mate Fe(1)–O(4) 2.013(3) Å and to the chloride ion Fe(2)– FeIII with this type of tridentate ligands, the FeIII ion is
Cl(1) 2.368(2) Å. As shown in Figure 2, the bridging hy- hexacoordinate, therefore the pentacoordinate FeIII ob-
droxido O(10) forms a hydrogen bond to the unbound oxy- served in 1a is unprecedented in such complexes.
gen atom O(6) from the monodentate formate bound to
Fe(1) with O(10)···O(6) 2.675(4) Å, O(10)–H(10)···O(6)
161°, and H(10)···O(6) 1.88 Å.
Theoretical Studies
In this context we want to emphasize that most of the
As structures 1a and 1b exhibit very different Fe–O–Fe
previously reported FeIII complexes of tridentate Schiff- angles (142 and 166°, respectively), we have focused our
base ligands are bis(Schiff base) complexes with noncoordi- theoretical study on providing a reasonable explanation for
–
–
– [16–18]
nating counteranions such as ClO4 , NO3 , and PF6 .
these values. We have performed theoretical calculations at
Among the mono(Schiff base) complexes, alkoxido bridges the RI-BP86/def2-TZVP level of theory, which is a good
(OMe, OEt) are found in three[19–21] and double phenoxido compromise between the accuracy of the method and the
bridges in two.[19,22] The presence of a single μ-oxido bridge size of the system. For the calculations, we have taken ad-
in 1a, which contains a tridentate N,N,O donor Schiff-base vantage of the C2 symmetry of the structures observed in
ligand is uncommon, and, to the best of our knowledge, the solid state (see Figure 3).
Figure 3. Representation of dimeric X-ray structures of compounds 1a and 1b including the C2 symmetry axes.
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FULL PAPER
Before analyzing the geometric and energetic features of function of the μ-oxido bridge angle. In Table 2 we summa-
compounds 1a and 1b, we have studied the range of Fe– rize the relative energy of 1a imposing several values for the
O–Fe angles in the literature by analyzing the Cambridge Fe–O–Fe angle in the calculations. The variation in energy
Structural Database (CSD). Statistical analysis of structural is represented in Figure 5. It can be observed that the rela-
data retrieved from the CSD provides a better understand- tive energy becomes more positive as the angle increases,
ing of the nature of a variety of covalent and noncovalent indicating that in 1a the most stable geometry does not
interactions and facilitates the identification of frequently correspond to an angle of 180°, as is common in this type
occurring interaction patterns and supramolecular syn- of compound, see Figure 4. It can be observed that the
thons. Therefore, we have performed a search in the CSD structure corresponding to an Fe–O–Fe angle of 180° is just
of compounds containing the μ-oxido bridge Fe–O–Fe im- a local minimum. We have also computed the energy of
posing only one restriction: the absence of additional biden- the system at 140°, see Table 2, and the energy drastically
tate bridging ligands (acetate, phosphate etc.), which would increases by 0.87 kcal/mol. We have examined the geometry
obviously condition the Fe–O–Fe angle. By using the results of 1a at the minimum and an unconventional π–π interac-
of this search, a histogram of this angle has been generated, tion between both aromatic ligands is observed (Figure 6).
which is presented in Figure 4. It is very clear that the pref- That is, the aromatic rings themselves are not stacked; how-
erence for this angle is 180°, and this preference does not ever, since the π-system is extended to the imidic C=N bond
depend upon the coordination number of the Fe ion.
by conjugation, two C–C bonds with partial double-bond
character are stacked. The distance between the middle of
both C–C bonds is 3.3 Å, characteristic of π–π interac-
tions.[28] We have performed the “atoms-in-molecules”
(AIM) analysis of 1a to confirm this feature. The AIM
analysis, by means of the distribution of critical points,
gives an unambiguous confirmation of the presence of an
interaction.[29] The AIM analysis shows a bond critical
point that connects one aromatic carbon atom of one ring
with another aromatic carbon of the other ring, confirming
the interaction (Figure 7). The value of the density at the
bond critical point is 102 ϫρ = 0.547 a.u., which is of the
same magnitude as standard π–π interactions.[30] The pres-
ence of this weak interaction explains the small Fe–O–Fe
angle observed in the solid-state structure of 1a as the en-
ergy at 180° is only 0.86 kcal/mol more positive than that
Table 2. Relative energies [kcal/mol] of different conformers of 1a
at the RI-PB86/def2-TZVP level of theory.
Fe–O–Fe [°]
Erel [kcal/mol]
140
142
145
150
160
170
175
180
0.87
0.00
0.64
1.00
1.07
1.09
0.99
0.86
Figure 4. Histogram of the Fe–O–Fe angles [°] obtained from the
CSD in dinuclear FeIII compounds.
Curiously, 1a has an Fe–O–Fe angle of 142°, which is
considerably smaller than that found in the majority of hits.
In fact, only seven X-ray structures have angles lower than
142.5°. Our theoretical study is focused on explaining the
very small angle observed for the μ-oxido bridge in 1a and
for that observed in 1b, which is due to the presence of a
water molecule. It is clear that in 1b, the geometry of the
dimer is not due to the covalency of the metal–ligand
bonds, but rather to noncovalent interactions that are con-
siderably weaker.
Theoretical Study of 1a
Calculations on 1a have been divided into two parts: the
Figure 5. Graphical representation of the relative energy (Erel) vs.
first is devoted to the study of how the energy changes as a the Fe–O–Fe angle in °.
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μ-Oxido- and μ-Hydroxido-Bridged Dinuclear Iron(III) Complexes
Figure 6. Representation of the structure of 1a with indication of the stacked C–C bonds.
at 142°. This result agrees with the fact that “state of the
art” calculations on the π–π stacking of the benzene dimer
predicts an interaction energy of –1 kcal/mol,[31] which is
similar to the energetic difference between both geometries
(142° and 180°) of 0.86 kcal/mol.
Figure 8. Hydrogen bonds between a water molecule and the phen-
olic oxygen atoms. Distance in Å as found in the crystal structure.
ergy profile is computed, the minimum is found at 180°, as
expected. Therefore, the formation of the hydrogen bonds
compensates for the energy difference between 180° and
166°, which is –1.49 kcal/mol. This explanation has been
confirmed by the calculation of the interaction energy be-
tween the complex and a water molecule, which is
–11.8 kcal/mol. Therefore, the presence of a water molecule
in this compound is crucial to explain the experimental Fe–
O–Fe angle. It is clear from the energy profile of this com-
plex in the absence of a water molecule that the flexibility
of the Fe–O–Fe motif allows the complex to adopt the ener-
getically most favorable conformation by maximizing non-
covalent attractive interactions.
Figure 7. Distribution of critical points in 1a. The bond critical
point (black small sphere) and bond path that characterizes the π–
π interaction is marked by a black circle. Ring critical points are
indicated by small light grey spheres.
Table 3. Relative energies [kcal/mol] of 1b with (Erel_w) and without
(Erel) the water molecule.
Theoretical Study of 1b
Fe–O–Fe [°]
Erel_w [kcal/mol]
Erel [kcal/mol]
In the structure of 1b the Fe–O–Fe angle is clearly in-
fluenced by the presence of a water molecule in the crystal
structure interacting with the phenolic oxygen atoms (see
Figure 8). We have performed two types of calculation on
this structure. First, we studied the influence of the Fe–O–
Fe angle on the relative energy of 1b in the absence and
presence of a water molecule. Secondly, we computed the
interaction energy between a water molecule and the di-
nuclear structure of 1b at the minimum.
160
165
166
170
180
8.62
2.09
0.00
0.01
1.07
7.16
0.92
0.00
–1.11
–1.49
In Table 3 we summarize the relative energy of 1b at dif-
ferent Fe–O–Fe angles. It is interesting to note that the cal-
culation with a water molecule indicates that the minimum
Conclusions
We have synthesized two new nonheme diiron(III) com-
is located at 166°, as observed in the crystal structure. In plexes with a tridentate N,N,O donor Schiff-base ligand (1a
contrast when the water molecule is eliminated and the en- and 2), and we have characterized them experimentally by
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Synthesis of 1b: A methanolic solution (10 mL) of L (5 mmol) was
added to a methanolic solution of FeCl3 (0.811 g, 5 mmol) with
constant stirring. Triethylamine (0.21 mL, 2.5 mmol) was added
dropwise to the solution with constant stirring. The color of the
solution turned to dark-red immediately. The solution was left to
stand in air until dark-red X-ray quality single crystals of 1b ap-
peared at the bottom of the vessel after slow evaporation of the
solvent. Yield 0.62 g (33%, with respect to Fe). C36H42Fe2N4O8
(752.438): calcd. C 56.12, H 5.49, N 7.27; found C 56.01, H 5.40,
elucidating their crystal structures. In addition, we have car-
ried out calculations on structures 1a and 1b at the RI-
BP86/def2-TZVP level of theory in order to study the influ-
ence of noncovalent interactions on the Fe–O–Fe angle. To
accomplish this, we have computed the energy profiles of
both compounds depending on the μ-oxido bridge angle.
Intra- (1a) or internoncovalent (1b) interactions are respon-
sible for the observed differences in the values of the Fe–
O–Fe angle. A CSD search indicates that 180° is the most
abundant situation; however, the energy cost of bending is
not high, and, consequently, it is easily compensated for by
other forces that may exist in the crystal structure.
N 7.17. IR (KBr pellet): ν = 1617.6 cm–1. UV/Vis (CH OH): λ (ε)
˜
3
=
427 (2400), 509 (2450), 318 (6000), 260 (11900), 233
(21400 dm3 mol–1 cm–1) nm.
Synthesis of 2: A methanolic solution (10 mL) of L (5 mmol) was
added to a methanolic solution of FeCl3 (0.811 g, 5 mmol) with
constant stirring. After ca. 15 min, a methanolic solution of sodium
formate (0.680 g, 10 mmol) was added, and the color of the solu-
tion turned to dark-red immediately. The solution was left to stand
in air until dark-red X-ray quality single crystals of 2 appeared at
the bottom of the vessel after slow evaporation of the solvent. Yield
1.08 g (68%). C24H33ClFe2N4O7 (635.69): calcd. C 45.27, H 5.22,
Experimental Section
Starting Materials: Salicylaldehyde, N,N-dimethylethane-1,2-di-
amine, sodium cyanate, and sodium formate were purchased from
commercial sources and used as received. Solvents were of reagent
grade and were used without further purification.
N 8.80; found C 45.18, H 5.10, N 8.72. IR (KBr pellet): ν = 1632.1,
˜
1581.4, 3431.4 cm–1. UV/Vis (CH3OH): λ (ε) = 411 (6800), 510
Physical Measurements: Elemental analyses (C, H, and N) were
performed with a Perkin–Elmer 240C elemental analyzer. IR spec-
tra as KBr pellets (4500–500 cm–1) were recorded with a Perkin–
Elmer RXI FTIR spectrometer. Electronic spectra (1000–200 nm)
were recorded with a Hitachi U-3501 spectrophotometer. The elec-
tronic absorption spectra of complexes 1b and 2 were measured
in CH3OH and that of 1a in CH3CN due to its low solubility in
methanol.
(1600), 320 (32000), 256 (70500), 234 (90400 dm3 mol–1 cm–1) nm.
Crystal Data Collection and Refinement: The independent number
of reflections for 1a, 1b, and 2 are 2206, 4990, and 7603, respec-
tively. They were collected with Mo-Kα radiation at 150 K by using
an Oxford Diffraction X-Calibur CCD System. The crystals were
positioned at 50 mm from the CCD. The number of frames mea-
sured was 321 with a counting time of 10 s. Data analyses were
carried out with the CrysAlis program.[40] The structures were
solved by direct methods with the SHELXS97 program.[41] The
non-hydrogen atoms were refined with anisotropic thermal param-
eters. The hydrogen atoms bound to carbon atoms were included
in geometric positions and given thermal parameters equivalent to
1.2 (or 1.5 for methyl groups) times those of the atom to which
they were attached. Hydrogen atoms of the water molecule in 1b
were located in a difference Fourier map and refined with distance
constraints. Absorption corrections were carried out by using the
ABSPACK program.[42] The structures were refined on F2 to R1 =
Theoretical Methods: The geometries of all complexes studied in
this work were computed at the RI-BP86/def2-TZVP level of
theory with TURBOMOLE version 5.7 using the crystallographic
coordinates.[32] We have used the BP86 method[33] and the def2-
TZVP,[34] basis set because this level of theory has been successfully
used by our group[35] and others[36] to study organometallic com-
plexes theoretically. The RI-DFT method applied to the study of
weak interactions is considerably faster than DFT, and the interac-
tion energies and equilibrium distances are almost identical for
both methods.[37] In addition the RI approximation is very efficient
provided that the functional is of nonhybrid type.[32] We have con-
sidered high-spin density for the FeIII atoms in the calculations.
The AIM analysis[38,29] of the complexes was performed with the
AIM2000 version 2.0 program[39] by using the BP86/def2-TZVP
wavefunction and the geometry obtained from the crystallographic
coordinates.
Table 4. Crystal data and structure refinement of complexes 1a and
2.
1a
2
Empirical formula
M
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C24H30Fe2N6O5
594.24
orthorhombic
Fdd2
31.0762(8)
17.4559(16)
9.731(2)
90
5278.5(12)
8
1.495
1.145
2464
0.063
C24H32ClFe2N4O7
635.69
monoclinic
P21/n
12.9340(7)
12.5115(6)
17.1490(11)
97.905(5)
2748.8(3)
4
1.536
1.202
1316
0.089
Synthesis of 2-({[2-(Dimethylamino)ethyl]imino}methyl)phenol (HL):
Salicylaldehyde (1.05 mL, 10 mmol) and N,N-dimethylethane-1,2-
diamine (1.098 mL, 10 mmol) in methanol (10 mL) were heated un-
der reflux for 1 h. The Schiff-base ligand was not isolated, and the
yellow methanolic solution was used directly for complex forma-
tion.
β [°]
Synthesis of 1a: A methanolic solution (10 mL) of L (5 mmol) was
added to a methanolic solution of FeCl3 (0.811 g, 5 mmol) with
constant stirring. After ca. 15 min, a methanolic solution of sodium
cyanate (0.650 g, 10 mmol) was added, and the color of the solution
turned to dark-red immediately. The solution was left to stand in
air until dark-red X-ray quality single crystals of 1a appeared at
the bottom of the vessel after slow evaporation of the solvent. Yield
1.11 g (75%). C24H30Fe2N6O5 (594.24): calcd. C 48.51, H 5.09, N
V [Å3]
Z
Dcalcd. [gcm–3]
μ [mm–1]
F(000)
R(int)
Total reflections
Unique reflections
I Ͼ 2σ(I)
R1, wR2
T [K]
4625
2206
1370
0.0354, 0.0594
150
16622
7603
2679
0.0574, 0.1235
150
14.14; found C 48.47, H 5.01, N 14.10. IR (KBr pellet): ν = 1614.7,
˜
2205 cm–1. UV/Vis (CH2Cl2): λ (ε) = 415 (4800), 493 (2700), 317
(17000), 255 (38000), 231 (59000 dm3 mol–1 cm–1) nm.
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Eur. J. Inorg. Chem. 2011, 2558–2566

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Acknowledgments
R. B. is thankful to the Council of Scientific and Industrial Re-
search (CSIR), India, for a research fellowship [Sanction no.09/
028 (0746)/2009-EMR-I]. We thank the British Engineering and
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Received: January 11, 2011
Published Online: April 18, 2011
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Eur. J. Inorg. Chem. 2011, 2558–2566