Chirality transfer in magnetic coordination complexes monitored by vibrational and electronic circular dichroism

Article abstract of DOI:10.1002/cplu.201300429

Magnetic coordination complexes based on Schiff bases are promising new molecular materials for electronics. Two μ-oxo Fe^III dimeric complexes of enantiomers of Schiff base ligands (N,N′-(1R,2R)-1,2- diphenylethylenebis(salicylideneimine) (H

Full text of DOI:10.1002/cplu.201300429

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DOI: 10.1002/cplu.201300429
Chirality Transfer in Magnetic Coordination Complexes
Monitored by Vibrational and Electronic Circular
Dichroism
[a]
[a]
[a]
[a]
[b]
ˇ
Tao Wu,* Xiao-Peng Zhang, Xiao-Zeng You,* Yi-Zhi Li, and Petr Bour*
Magnetic coordination complexes based on Schiff bases are
promising new molecular materials for electronics. Two m-oxo
FeIII dimeric complexes of enantiomers of Schiff base ligands
(N,N’-(1R,2R)-1,2-diphenylethylenebis(salicylideneimine)
(H2salphen-R, 1a) and N,N’-(1S,2S)-1,2-diphenylethylenebis(sali-
cylideneimine) (H2salphen-S, 1b)) were synthesized; further re-
action with 4-salicylideneamino-1,2,4-triazole (Hsaltrz) led to
enantiomers of two one-dimensional (1D) FeIII coordination
complexes. The structures of these complexes were deter-
mined by X-ray diffraction. Magnetic susceptibility measure-
ments revealed that m-oxo dimeric complexes displayed strong
antiferromagnetic coupling, whereas the 1D complexes exhib-
ited paramagnetic behavior. The chirality of the Schiff bases
transferred to macromolecular chirality of the complexes,
which could be monitored by both vibrational and electronic
circular dichroism (CD) spectroscopic methods. The macro-
scopic handedness was manifested in CD signals attributed to
exciton coupling between the ligands. Thus, the chiral spec-
troscopies were useful to probe the chirality of the complexes,
their structure, and the polymerization degree.
Introduction
Chirality is one of the most prominent properties of biomole-
cules and thus exhibits a primary role in the functioning of
many living systems. It is also becoming increasingly important
from a more industrial point of view, for example, in chiral co-
ordination complexes. They can be used in many domains of
chemistry, such as asymmetric catalysis, chiral recognition,
chiral molecular materials, and metallosupramolecular chemis-
try.[1] However, chirality in coordination chemistry is still rather
unexplored if compared to its role in organic and biomolecular
chemistry.[2] Yet, manipulating the handedness in coordination
complexes is a key for relating their potential properties and
reactivity to their structure and composition.
the case of exciton coupling of ligand in the complex, the
signal can be significantly increased or distorted if compared
with the spectrum of the original chiral component. In this
work, we are particularly interested in how the complexation is
manifested in the spectra of coordination polymers for molec-
ular magnets, not explored by chiral spectroscopies so far.
Molecular magnets based on coordination complexes attract
considerable interest owing to the possibility to fine-tune their
physical properties through versatile chemical synthesis.[4] Gen-
erally, molecular magnetism is compatible with a broad range
of other physicochemical properties,[5] which is helpful for de-
signing multifunctional materials. Particularly interesting to us
is the integration of chirality and magnetism.[6] During the past
few years, enantiomeric chiral magnets (ECMs) have drawn
much interest from both chemists and physicists because of
the forecast[7] and detection in paramagnetic[8] and diamagnet-
ic[9] materials of a cross-effect, which appears exclusively in
enantiopure magnetized systems. This combination of Faraday
effects and natural CD is designated as a magnetochiral effect
that is proportional to the magnetization, and its intensity was
predicted to be profoundly promoted in ECMs. Therefore, it is
fascinating to interpret the synergetic effects between the ge-
ometry and magnetic properties generated from manipulated
chirality.
The transfer of chirality from ligand to metal-ion center is
the most efficient way to asymmetric synthesis of coordination
complexes.[3] The macroscopic chirality of a complex or poly-
mer is caused by the chiral component, which can be seen in
various ways in circular dichroism (CD) spectra. For example, in
[a] Dr. T. Wu, X.-P. Zhang, Prof. X.-Z. You, Prof. Y.-Z. Li
State Key Laboratory of Coordination Chemistry
School of Chemistry and Chemical Engineering
Nanjing National Laboratory of Microstructures
Nanjing University, Nanjing 210093 (P. R. China)
Fax: (+86)25-83314502
E-mail: twu@nju.edu.cn
Circular dichroism spectroscopy[10] arises from the difference
in absorption of left and right circularly polarized light. It in-
cludes electronic CD (ECD) seeing electronic transitions and vi-
brational CD (VCD) reflecting vibrational transitions. Both have
been widely used as convenient methods in stereochemical
analysis of organic and biomolecular systems and coordination
complexes. The ECD of coordination complexes is extremely
significant for monitoring the absolute configuration,[11] but
youxz@nju.edu.cn
ˇ
[b] Prof. P. Bour
Institute of Organic Chemistry and Biochemistry
Academy of Sciences
Flemingovo nꢀm. 2, 16610 Prague 6 (Czech Republic)
Fax: (+420)224-310-177
E-mail: bour@uochb.cas.cz
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300429.
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the vibrational technique displays more local structural infor-
mation.[12]
sequent redissolution in dichloromethane/ethanol mixed solu-
tion, a m-oxo FeIII dimer was obtained as red blocks through
slow evaporation. Normally if the reaction is performed with
the participation of other monodentate Lewis bases, such as
pyridine, thiolate, or chloride, a five-coordinate compound is
formed.[19] Interestingly, altering the solvent from methanol to
pyridine affords analogous m-oxo FeIII dimer [{Fe(salphen-R)}2-m-
O]·3Py (2c); its single-crystal unit incorporates three pyridine
(Py) molecules.
For coordination complexes, VCD usage is still rather limited,
although latest works suggest a great potential here as well.
For example, Nafie et al. successfully implemented VCD spectra
of transition-metal complexes, which afforded good density
functional theory (DFT) simulation compared to experimental
data.[12b] Nafie also systematically analyzed the role of low-lying
excited electronic states[12c] in the enhancement of VCD in
these transition-metal coordination compounds. Sato et al. dis-
cussed the impact of the d-electron configurations of metal
ions of tris(b-diketonato) metal complexes on the VCD
shape.[13] Crassous et al. demonstrated the high sensitivity of
the VCD signal of rhenium complexes for the observation of
parity violation effects.[14] You et al. successfully employed VCD
spectroscopy for monitoring the coordination process of tran-
sition metal to a chiral ligand.[11e] They also evidenced that the
VCD technique is a powerful detector of subtle variances of
chirality in small biologically active systems including metal,[15a]
as well as in detecting small changes of the enantiomeric
excess during the racemization process.[15b]
The reaction of the m-oxo iron(III) complex [{Fe(salphen-R)}2-
m-O] (2a) with coligand 4-salicylideneamino-1,2,4-triazole
(Hsaltrz) follows Equation (1):
n=2½fFeðsalphen-RÞg2-m-Oꢀ þ nHsaltrz !
ð1Þ
½Feðsalphen-RÞðm-saltrzÞꢀn þ n=2 H2O
The unique crystal structure of the 1D FeIII polymer 3a incor-
porates one methanol molecule. The compositions of these
complexes were supported by elemental analysis together
with X-ray crystallographic data, and electronic and infrared
absorption spectra. Note that compounds 1a and 1b are re-
ported enantiomeric molecules, whereas compounds 2a–c, 3a,
and 3b are new species.
Establishing of suitable routines for incorporating chirality in
physicochemical explorations of molecular materials is one of
the ambitious goals for both chemists and physicists. Herein,
we investigate the application of VCD and ECD spectroscopy
to monitoring the chirality transfer in 1D iron(III) Schiff base
complexes. These systems were selected because the Schiff
base complexes are of considerable interest in the design of
molecule-based magnetic materials, owing to the versatile
geometrical modifications leading to magnetic anisotropy al-
ternation.[16] To the best of our knowledge, this work presents
the first attempt at VCD characterization of coordination poly-
mers in solution. CD techniques offer, for example, monitoring
of the exciton coupling, that is, interaction of azomethine
groups conjugated with aromatic chromophores, which occurs
during the polymerization.
Structures of complexes and absolute configurations
Relevant crystallographic data of compounds 2 and 3 are listed
in Table 1; selected bond lengths [ꢁ] and angles [8] of com-
plexes 2 and 3 are listed in Table S1 in the Supporting Informa-
tion. The results are reasonably close to data published for
similar achiral cases.[16d,18b] In the m-oxo dimer complex, the co-
ordination around the central iron(III) atom leads to a distorted
square-pyramidal (SPY) geometry. In the achiral structure, the
two meso-(salphen)2ꢁ ligand skeletons are on the same side,
whereas in the optically active case they lie in opposite direc-
tions with a slight angle distortion owing to the steric hin-
drance. As a result, the bridge oxygen atom looks like
a pseudo-center of inversion (Figure 1). A lower symmetry is
achieved through the addition of a pyridine unit for com-
plex 2c. According to the topographical stereochemistry of the
five-coordinate unit, the absolute configurations of the two
iron atoms in complex 2a are both assigned as L configura-
tion.[20] Analogously, the absolute configurations of the two
iron atoms in the enantiomeric complex 2b both should be of
D configuration.
The 1D polymer shares the zigzag chain.[16d] The coordina-
tion around the central iron(III) atom is an octahedral (OC-6) ar-
rangement; the {FeN3O3} skeleton is in a facial geometry (Fig-
ure 2).[16d,20a] As a result of the sterically hindered diphenyl
group, the Fe···Fe distance (9.98 ꢁ) is much longer than in the
other cases.[16d] The absolute configuration around the central
iron(III) atom of the polymer 3a is L.[20a] The consistent config-
uration manifests that the chirality transferred from the ligand
to the iron(III) atom is consistent during the 1D chain complex-
ation process.
Results and Discussion
Synthesis and general characterization
Enantiomers of Schiff base ligands N,N’-(1R,2R)-1,2-diphenyle-
thylenebis(salicylideneimine) (H2salphen-R; 1a) and N,N’-
(1S,2S)-1,2-diphenylethylenebis(salicylideneimine) (H2salphen-S;
1b) were prepared by condensation reaction of chiral diamines
(1a from the R,R configuration and 1b from the S,S configura-
tion) and salicylaldehyde[18a] (Scheme 1). Both enantiomers
were collected as yellow crystals.
It is generally believed that FeII bis(salicylidene)ethylenedia-
minato (salen)-type complexes are prone to oxidation in air,
which is reflected in the composition of the more stable m-oxo
FeIII dimeric species.[17] The chiral bulky ligand H2salphen-R was
previously supposed to be unable to form a m-oxo dimer
owing to the steric repulsion of the molecular backbone com-
pared to its meso geometry.[18] In addition, synthesis of the
ligand with iron salts under an inert atmosphere is recom-
mended. However, by performing the synthesis in air with sub-
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cMT is 0.945 cm3 Kmolꢁ1 at 300 K
and shows a clear decrease from
300 to 50 K (0.556 cm3 Kmolꢁ1),
which indicates a strong antifer-
romagnetic coupling between
the two iron(III) centers through
the oxo bridge (Fe···Fe distance,
3.49 ꢁ). For complex 3a, the
value of cMT is 4.238 cm3 Kmolꢁ1
at 300 K and there is no distinc-
tive value variance from 300 to
5 K (4.143 cm3 Kmolꢁ1), thus ex-
hibiting a normal paramagnetic
behavior. This indicates that
there is no clear spin coupling
between two iron(III) centers
through the 4-salicylideneamino-
1,2,4-triazolate bridge,[16d] which
is reasonable for the longer
Fe···Fe distance. As a result, the
magnetic property of the 1D
chain can be represented ap-
proximately by the magnetic be-
havior of each unique iron(III)
center.
UV/Vis absorption and ECD
spectra
As expected, the UV/Vis spectra
of the enantiomers of the chiral
pool diamines, the ligands (1a
and 1b), the m-oxo dimer com-
plexes (2a and 2b), and 1D
polymer (3a and 3b) share
almost the same curve, whereas
their ECD spectra nearly look like
mirror images (Figure 4). Al-
though there are several differ-
ences between the crystal struc-
tures of 2a and 2c, there are
minimal variances of the UV/Vis
and ECD spectra in dichlorome-
thane, which indicates that the
conformation of the m-oxo dimer
complex in solution is very
stable.
There is no outstanding chro-
mophore in the chiral pool mol-
ecule, and only one weak ab-
sorption band around 220 nm of
Scheme 1. Synthesis route: from chiral pool diamine to 1D iron(III) coordination polymer.
Magnetic properties
the substituted benzene transition is observed. The R,R-dia-
mine corresponds to a positive Cotton effect (CE) curve,
whereas the S configuration gives a negative one. In the UV/
Vis spectrum of ligand 1a, the intense bands around 230 and
258 nm are assigned to p–p* transition of the azomethine
group, whereas the broad band around 320 nm is the n–p*
The dependencies of magnetic susceptibilities cM and cMT on
the temperature for the m-oxo dimer complex enantiomers 2a
and 2b are almost the same (Figure 3). The difference between
values of cM and cMT for the enantiomers of 1D chain complex-
es 3a and 3b are tiny as well. For complex 2a, the value of
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Table 1. Crystallographic data and structure refinement details for complexes 2 and 3.
2a
2b
2c
3a
3b
formula
Mr
C56H44Fe2N4O5
964.65
C56H44Fe2N4O5
964.65
C56H44Fe2N4O5·3(C5H5N)
1201.95
C37H29FeN6O3·CH3OH
693.55
C37H29FeN6O3·CH3OH
693.55
T [K]
173(2)
173(2)
296(2)
296(2)
296(2)
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
P1
triclinic
P1
triclinic
P21
P21
P21
monoclinic
12.9259(9)
16.2575(11)
15.2354(10)
90.00
105.5370(10)
90.00
2
monoclinic
9.2038(10)
18.315(2)
10.2439(12)
90.00
95.824(2)
90.00
2
monoclinic
9.2005(4)
18.3343(8)
10.2483(5)
90.00
95.8770(10)
90.00
2
13.4093(12)
13.6355(11)
15.1288(14)
66.600(2)
66.931(3)
85.695(2)
2
13.394(3)
13.629(3)
15.108(3)
66.532(3)
66.863(3)
85.502(3)
2
a [8]
b [8]
g [8]
Z
Dcalcd [gcmꢁ3
]
1.378
0.679
1.383
0.681
1.294
0.527
1.341
0.488
1.339
0.488
m [mmꢁ1
]
reflections measured
23808
18418
18796
11875
11817
unique reflections (Rint
)
17329(0.0152)
1207
1.178
0.0570, 0.1478
0.0600, 0.1500
13501(0.0437)
1207
1.004
0.0517, 0.1402
0.0600, 0.1486
11749(0.0269)
766
1.008
0.0426, 0.0869
0.0717, 0.0869
7116(0.0155)
436
1.071
0.0327, 0.0834
0.0370, 0.0862
7635(0.0152)
443
1.047
0.0293, 0.0741
0.0320, 0.0755
number of parameters
goodness-of-fit on F2
[b]
R1,[a] wR2 [I>2s(I)]
[b]
R1,[a] wR2 (all data)
1
2
2
[a] R1 =S(jFojꢁjFcj)/SjFo j. [b] wR2 =[Sw(Fo ꢁFc2)2/Sw(Fo2)2] =
.
transition of the azomethine group.[21] The CE shape in the
ECD spectrum associated with the p–p* and n–p* transitions
of the azomethine chromophore is in good agreement with
the R,R configuration of the diamine skeleton.[22]
B3LYP) or change of the pseudopotential, and it is probably
caused by the complicated electronic structure, beyond the ca-
pabilities of available TDDFT implementations.
The geometry of the central iron(III) shifted from five-coordi-
nate (SPY-5) to six-coordinate (OC-6) during the extension pro-
cedures of m-oxo dimer to 1D framework. The UV/Vis spectrum
of polymer 3a has been considerably modified through the in-
corporation of the saltrz ligand. The p–p* transitions are
around 235 nm, and two new peaks near 270 and 279 nm and
a narrow slope between 290 and 317 nm appear. The n–p*
transition has shifted to 334 nm, perhaps mixed with the saltrz
LMCT transition. The exciton coupling effect of the n–p* chro-
mophore on the ECD shape is almost identical to that of 2a,
that is, it also exhibits a positive peak at longer wavelength
(De1, 386 nm) and a negative peak at shorter wavelength (De2,
345 nm). The amplitude of the couplet is lower
(18 Lmolꢁ1 cmꢁ1). According to an empirical rule for the octa-
hedral complexes,[24] this couplet is related to a L configuration.
The ECD curves of complexes 2a and 3a are almost identical,
so the absolute configuration at the five-coordinate iron(III)
could be considered as the L configuration. These assign-
ments are in agreement with the X-ray data. The region
around 450–600 nm is assigned as the LMCT transition in the
coordination polymer. In the corresponding ECD shape a weak
couplet is detectable. The two salphen planes adopt nearly
perpendicular positions in the 1D zigzag chain (Figure 2),
which could probably induce this intramolecular couplet. In
addition, the new saltrz LMCT transition might contribute. In
the m-oxo dimer unit, the arrangement of the two salphen
plates is approximately parallel at opposite directions
(Figure 1), not favorable for the exciton coupling.[11a,24]
As the m-oxo dimer complex formed, these p–p* and n–p*
transitions displayed a redshift. For complex 2a, the p–p* tran-
sition moved to 241 nm, with a broad slope between 281 and
330 nm. The n–p* transition shifted to 347 nm, and was also
mixed with the oxo to iron(III) charge-transfer (CT) transition.[23]
The exciton coupling effect[11e,24] of the n–p* chromophore is
very large compared to that of the free ligand. It exhibits a pos-
itive peak at a longer wavelength (De1, 386 nm) and a negative
peak at shorter wavelength (De2, 345 nm). The amplitude of
the
couplet
(De1–De2)
is
approximately
up
to
26 Lmolꢁ1 cmꢁ1.[10] The weak absorption around 460–600 nm
corresponds to a salphen ligand to iron(III) CT (LMCT) transitio-
n,[11a,23] accompanied by a very weak negative CE in the ECD
spectrum.
Time-dependent DFT (TDDFT) computations supported the
experimental UVCD spectra only approximately. The calculated
(B3LYP/6-31G**/CPCM(dichloromethane)) spectra of the R,R-di-
amine, ligand 1a, and complex 2a are presented in Figure S4;
for 1a and complex 2a the calculation is compared to experi-
ment in Figure S5. By comparing Figure S4 and Figure 4 we
can see that the trend observed in the spectra is well repro-
duced, that is, the diamine absorbs in the lowest wavelengths
only, with a relatively weak CD, whereas for 1a and 2a the ab-
sorption threshold shifts to longer wavelengths, and the ratio
of CD over absorption increases. Unfortunately, for the com-
pound with iron (2a) the TDDFT method provides many unre-
alistic low-energy (high-wavelength) transitions and the overall
agreement with the experiment is not good (Figure S5). This
failure could not be improved by another functional (CAM-
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Figure 2. Part crystal structure of the zigzag 1D chain framework of coordi-
nation polymer 3a and the absolute configuration around the central iron-
(III) atom. Solvent and hydrogen atoms are omitted for clarity.
1461, 1454, 1412, and 1382 cmꢁ1 (CꢁH deformation), and
1278 cmꢁ1 (CꢁO stretching). The strongest VCD peak
(1454 cmꢁ1) comes from the CꢁH deformations of the ethylene
group (based on DFT calculations), whereas the calculated
most intensive peak occurs around 1656 cmꢁ1, which corre-
sponds to the C=N stretching. Its experimental relative intensi-
ty is smaller, most probably because of uneven band broaden-
ing and the strong hydrogen-bonding interaction between the
C=N bond and the ꢁOH group, and solvation is not considered
completely in the model.
Figure 1. Crystal structure of dinuclear complex 2a and absolution configu-
rations around the central iron(III) atoms. The hydrogen atoms are omitted
for clarity.
Infrared and VCD spectra
Although the concentrations of both dinuclear and 1D com-
plexes are much lower than those of the free ligand for IR and
VCD measurements (Figure 6), the IR and VCD intensities are
much stronger. The intensities of the dinuclear complex are
even stronger than those of the 1D complex. This is explicable
as the m-oxo dimer structure is less strained than that of the
polymerized chain. Most of the IR curves of 3a are similar to
that of 2a, which suggests that the contribution of the saltrz
molecule stretching vibrations is limited. There are no signifi-
cant VCD shape changes except for intensity variances be-
tween the dinuclear geometry and the 1D chain. The results
are consistent with the ECD analysis and verify that the abso-
lute configuration at the central iron(III) atom is kept un-
changed during the polymerization process. The main mea-
sured vibrational absorptions bands of 2a occur at: 1617 cmꢁ1
The diamine provides rather weak IR and VCD spectra (Fig-
ure S6). The calculated energies and Boltzmann population of
the conformers (Figure S7) in chloroform are listed in Table S2.
Three conformers of ligand 1a were selected through the
AM1 method as well (Figure S8). The calculated energies and
Boltzmann population of the conformers in chloroform are
listed in Table S3. Except for the imine groups, the vibrations
of the aromatic skeletons are quite weak in the spectra; there-
fore, two different concentrations were measured. A compari-
son of the experimental IR and VCD spectra of 1a with calcu-
lated data is made in Figure 5. The main absorption of the sal-
phen ligand occurs at 1630 cmꢁ1 (C=N stretching); other peaks
are at 1582 cmꢁ1 (CꢁC stretching of the benzyl rings), 1494,
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shift and a VCD peak. The measured frequency and anisotropy
ratio g factors of 1a, 2a, and 3a are presented in Table 3.
These increases of the anisotropy values indicate that the chir-
ality has been effectively transferred from the ligand to the
complexes, and the signal has also been amplified.
It is generally believed that VCD is less predictable for open-
shell systems.[26] However, recent examples manifest that the
magnetic field perturbation theory for VCD calculation can be
extended to open-shell systems.[11d,e,13] Simulations for IR and
VCD spectra of complex 2a with three different exchange-cor-
relation functionals were performed (Figure 7). The OPBE and
B3PW91 functionals reproduced the IR spectra quite well (e.g.,
CꢁC stretching of the benzyl rings, observed 1543 cmꢁ1; OPBE,
1556, 1558 cmꢁ1; B3PW91, 1583, 1586, 1588 cmꢁ1), whereas the
B3LYP functional provided more reasonable VCD shape simula-
tion (e.g., CꢁH deformations of the ethylene group, observed
1454 cmꢁ1; B3LYP, 1495, 1497 cmꢁ1).
For the simulations of the 1D polymer 3a, monomer and
dimer models were considered (Figures S9 and S10). The ex-
perimental and theoretical (except for OPBE) spectra of the
dimer and monomer are very similar (Figures 8 and 9). This
suggests that the chirality comes from the chiral arrangement
around the central iron ion, that is, one monomeric unit. A
comparison of selected IR chromophore absorptions around
the central iron(III) atom measured in comparison with calcula-
tions for 1a, 2a, and 3a are presented in Table 4. As expected,
the calculated frequency differences of these feature bands be-
tween 2a and the two models of 3a are quite small, which co-
incides with the experimental spectra.
Conclusion
By employing the 4-salicylideneamino-1,2,4-triazolate anion as
bridge, the m-oxo dinuclear complexes were successfully con-
verted to 1D iron(III) coordination complex enantiomers. Both
enantiomers of m-oxo dinuclear iron(III) complexes with optical-
ly active bulky salen-type ligands were obtained. All these
structures and absolute configurations were verified by X-ray
Figure 3. cM (top) and cMT (bottom) versus temperature T for complexes 2a,
2b, 3a, and 3b.
(C=N stretching); 1541 cmꢁ1 (CꢁC stretching of the benzyl
rings); and 1471, 1452, 1444, 1390, and 1342 cmꢁ1 (CꢁH defor-
mation). The feature IR peaks of 3a are: 1621,
1607 cmꢁ1 (C=N stretching); 1543 cmꢁ1 (CꢁC stretch-
ing of the benzyl rings); and 1470, 1454, 1445, 1390,
and 1336 cmꢁ1 (CꢁH deformation). All these vibra-
tional bands are more intense relative to those of the
free ligand. The stretching vibrations of the CꢁO
group (1278 cmꢁ1, 1a) shifted to a higher frequency
(1313 cmꢁ1, 2a; 1310 cmꢁ1, 3a) as the FeꢁO bond
formed.
Table 2. VCD couplet by imine groups for 1a (0.04m), 2a (0.015m), and 3a (0.015m)
(see Figure 6).
De1/n
g1 =De1/e1 De2/n
(Lmolꢁ1 cmꢁ1/cmꢁ1
)
g2 =De2/e2 A=De1–De2
(Lmolꢁ1 cmꢁ1/cmꢁ1
)
1a 0.034/1628
2a 0.232/1608
3a 0.036/1606
2.75ꢂ10ꢁ5 ꢁ0.033/1634
8.98ꢂ10ꢁ5 ꢁ0.214/1625
1.88ꢂ10ꢁ5 ꢁ0.087/1618
3.57ꢂ10ꢁ5 0.067
1.03ꢂ10ꢁ4 0.446
5.76ꢂ10ꢁ5 0.123
The VCD couplet[25] of the imine chromophores in
the region of 1600–1650 cmꢁ1 can be detected
through the whole complexation process. Compari-
son of measured values of De1 (lower wavenumber), De2
(higher wavenumber), anisotropy ratio g factor (also called dis-
symmetry factor, defined as g=De/e),[10] and the amplitude A
of the couplet (defined as De1–De2) of the imine groups in 1a,
2a, and 3a is made in Table 2. In addition, the CꢁH deforma-
tion from the phenylethylene group of the R,R-diamine is ob-
servable following the polymerization process as a tiny band
Table 3. VCD peak by CꢁH deformation of phenylethylene group for R,R-
diamine (0.24m), 1a (0.24m), 2a (0.015m), and 3a (0.015m; see Figure 6).
CꢁH deformations (phenylethylene group)
Frequency
Anisotropy
R,R-diamine
1451
1453
1454
1455
8.19ꢂ10ꢁ5
2.50ꢂ10ꢁ5
3.46ꢂ10ꢁ4
2.39ꢂ10ꢁ4
1a
2a
3a
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the m-oxo dinuclear complex 2a and 1D complex 3a
confirmed the consistency of the L absolute configu-
ration at the central iron(III) atom. The observed
spectral behavior was partially explained by DFT
modeling. This first VCD report on coordination poly-
mers in solution indicates that, in addition to ECD,
VCD spectroscopy is also a useful chiroptical tech-
nique to monitor the polymerization process. It pro-
vided information about the absolute configuration.
In addition, both the electronic and vibrational chiral-
ity is magnified by the regular arrangement of the
chromophores (azomethine groups, n–p* transition
in ECD and C=N stretching in VCD) enabling the
strong exciton coupling.
Experimental Section
General procedures
Elemental analyses for C, H, and N were performed on
an Elementar Vario Micro analyzer. Electronic absorption
spectra were measured on a UV-3600 spectrophotometer
in the region of 200–800 nm. ECD spectra were recorded
on a Jasco J-810 spectropolarimeter in the region of
200–800 nm. IR and VCD spectra were investigated in
the region of 1800–800 cmꢁ1 with a VERTEX 80v Fourier
transform infrared spectrometer equipped with
a PMA 50 VCD/IRRAS module (Bruker, Germany) by using
previous procedures.[15] The photoelastic modulator was
set to 1500 cmꢁ1, the spectral resolution was 4 cmꢁ1, and
the zero filling factor was 4. A demountable cuvette
A145 with KBr windows with a 0.12 or 0.21 mm Teflon
spacer was used. All solution samples were dissolved in
deuterated chloroform. All VCD measurements were col-
lected for 4 h composed of 12 blocks in 20 min. Baseline
correction was performed with the spectra of CDCl3 by
using the same measurement setup as for VCD. Magnet-
ic susceptibilities for crystalline samples were obtained
Figure 4. UV/Vis absorption (top) and ECD (bottom) spectra of chiral pool diamines,
ligand 1, dimer complex 2, and 1D coordination polymer 3 measured in dichloromethane
solution (the concentration of all samples was kept at 6ꢂ10ꢁ4 m).
on a Quantum Design MPMS-SQUID-VSM magnetometer in the
temperature range of 1.8–300 K. Diamagnetic corrections were cal-
culated using Pascal’s constants.
Table 4. Selected experimental IR peaks of the salphen part in compari-
son with simulations for 1a, 2a, and 3a.
All chemicals were purchased from commercial sources, were of
analytical reagent grade, and were used without further purifica-
tion. Enantiomers of Schiff base ligands (N,N’-(1R,2R)-1,2-diphenyle-
thylenebis(salicylideneimine) (H2salphen-R, 1a) and N,N’-(1S,2S)-1,2-
diphenylethylenebis(salicylideneimine) (H2salphen-S, 1b)) were pre-
pared from chiral pool diamine and salicylaldehyde in dried etha-
nol by following procedures described elsewhere.[18a] Bridge ligand
4-(salicylideneamino)-1,2,4-triazole (Hsaltrz) was prepared from 4-
amino-4H-1,2,4-triazole and salicylaldehyde in dried methanol ac-
cording to conditions described elsewhere.[16d]
Experimental
frequency
B3LYP
functional
B3PW91
functional
OPBE
functional
C=N stretching
1a 1634, 1628
2a 1625, 1608
1676, 1659
1699, 1676,
1667, 1642
1686, 1677[a]
1678, 1675,
1673, 1672[b]
–
–
1692, 1683,
1679, 1671
1700, 1691[a]
1693, 1689,
1686,1685[b]
1630, 1621,
1620,1614
1642, 1629[a]
1634, 1633,
1629, 1624[b]
3a 1618, 1606
CꢁO stretching
1a 1278
2a 1313
1300, 1299
1391, 1378,
1356, 1336
–
–
1380, 1375,
1374, 1371
1360, 1358,
1356, 1353
Syntheses
[a] Monomer model. [b] Dimer model.
Dinuclear iron(III) complexes [{Fe(salphen)}2-m-O] 2a and 2b: The
ligand H2salphen-R (1a, 84.2 mg, 0.2 mmol) was suspended in
freshly dried methanol (20 mL) at room temperature, under a pro-
tective nitrogen atmosphere, and Fe(OAc)2 (35 mg, 0.2 mmol,
1 equiv) dissolved in freshly dried MeOH (10 mL) was added to the
ligand suspension. The mixture was stirred for 6 h under reflux.
After cooling to room temperature, the resulting solution was con-
crystallographic analysis. These complexes provided interesting
magnetic properties and exhibited a strong chirality, both in
the ECD and VCD spectra. The similarity of CD spectra between
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Figure 5. IR (top) and VCD (bottom) spectra of 1a measured in CDCl3 in
comparison with DFT simulations.
centrated in vacuo, the residue was extracted with dichlorome-
thane then washed with water in air, and the organic phase was
dried over anhydrous NaSO4 and diluted with ethanol. The red so-
lution was left undisturbed to slowly evaporate. After several days,
red crystals of [{Fe(salphen-R)}2-m-O] (2a) suitable for X-ray crystal-
lography were collected (43 mg, 44.6%). Elemental analysis calcd
(%) for C56H44Fe2N4O5: C 69.72, H 4.60, N 5.81; found: C 69.60, H
4.67, N 5.66. Enantiomer complex [{Fe(salphen-S)}2-m-O] (2b) was
synthesized under the same conditions with ligand H2salphen-S
(1b), and was obtained as red crystals. Elemental analysis calcd (%)
for C56H44Fe2N4O5: C 69.72, H 4.60, N 5.81; found: C 69.13, H 4.67, N
5.61.
Figure 6. IR (top) and VCD (bottom) spectra monitoring the polymerization
process: chirality-poor diamines, ligand 1, dinuclear coordination complex 2,
and 1D coordination polymer 3.
Dinuclear iron(III) complexes [{Fe(salphen-R)}2-m-O]·3Py, 2c: The
ligand H2salphen-R (1a, 84.2 mg, 0.2 mmol) was dissolved in pyri-
dine (20 mL), then Fe(OAc)2 (35 mg, 0.2 mmol, 1 equiv) was added
to the ligand suspension under a nitrogen atmosphere. The mix-
ture was stirred for 6 h at 708C. The cooled red solution was then
filtered in air and left undisturbed to slowly evaporate. After sever-
al days, red needle-shaped crystals of [{Fe(salphen-R)}2-m-O]·3Py
(2c) suitable for X-ray crystallography were collected. Elemental
analysis calcd (%) for C56H44Fe2N4O5·(C5H5N)3: C 69.72, H 4.60, N
5.81; found: C 69.54, H 4.74, N 5.77 (sample dried in vacuo for 12 h
to remove pyridine).
for X-ray analysis were collected after several days (123 mg, 59%).
Elemental analysis calcd (%) for C37H29FeN6O3·CH3OH: C 65.81, H
4.80, N 12.12; found: 65.55, H 4.89, N 11.90 (sample dried in vacuo
for 12 h to remove methanol). Enantiomer 1D complex [Fe(sal-
phen-R)(m-saltrz)]·CH3OH (3b) was synthesized under the same
conditions with dinuclear complex enantiomer [{Fe(salphen-S)}2-m-
O] (2b) and was obtained as black crystals. Elemental analysis
calcd (%) for C37H29FeN6O3·CH3OH: C 65.81, H 4.80, N 12.12; found:
C 65.85, H 5.05, N 11.69 (sample dried in vacuo for 12 h to remove
methanol).
1D iron(III) complexes [Fe(salphen)(m-saltrz)]·CH3OH 3a and 3b:
Ligand Hsaltrz (62 mg, 0.33 mmol) dissolved in freshly dried metha-
nol (5 mL) was added to a suspension (25 mL) of [{Fe(salphen-R)}2-
m-O] (2a; 145 mg, 0.15 mmol) in freshly dried methanol. The red
solution was heated for 6 h under reflux. The resulting solution
was filtered and was left undisturbed to slowly evaporate. Black
crystals of 1D complex [Fe(salphen-R)(m-saltrz)]·CH3OH (3a) suitable
X-ray crystallography
Single-crystal X-ray diffraction measurements of 2a, 2b, 2c, 3a,
and 3b were performed on a Bruker SMART APEX CCD-based dif-
fractometer operating at room temperature. Intensities were col-
lected with graphite-monochromatized MoKa radiation (l=
0.71073 ꢁ) operating at 50 kV and 30 mA, using the w/2q scan
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Figure 7. Measured IR (top) and VCD (bottom) spectra of complex 2a in
Figure 8. Measured IR (top) and VCD (bottom) spectra of complex 3a in
comparison with theoretical calculations.
comparison with theoretical calculations of monomer model.
mode. Data reduction was made with the Bruker SAINT package.[27]
Absorption corrections were performed using the SADABS pro-
gram.[28] The structures were solved by direct methods and refined
on F2 by full-matrix least-squares using SHELXL-97 with anisotropic
displacement parameters for all non-hydrogen atoms in all struc-
tures. All hydrogen atoms of 2a, 2b, 2c, 3a, and 3b were found in
differential maps of electron density and their parameters were re-
fined by using the riding model with the CꢁH distance of 0.95 (CH)
and 0.99 (CH2) ꢁ, respectively, and with Uiso(H)=1.2Ueq(C). All com-
putations were performed using the SHELXTL-97 program pack-
age.[29] Details of the crystal parameters, data collection, and refine-
ment of complexes 2a, 2b, 2c, 3a, and 3b are summarized in
Table 1. CCDC 970062, 970063, 970064, 970065, and 970066 con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Computational methods
The calculations were performed with Gaussian 09 programs.[30]
The conformational search of R,R-diamine and 1a were done with
the AM1 method. The lowest-energy conformers were optimized
at the B3LYP level with the 6-311+ +G (d,p) basis set. The effect
of the solvent was modeled by the conductor-like polarizable con-
tinuum model (CPCM) for chloroform. The calculated energies are
listed in Tables S2 and S3, and the correlated structures are exhibit-
ed in Figures S6 and S7. IR and VCD spectra calculations were per-
formed under the same conditions.
Figure 9. Measured IR (top) and VCD (bottom) spectra of complex 3a in
comparison with theoretical calculations of dimer model.
atoms. Starting geometries of the 2a and 3a models were gener-
ated from crystal structures (Figures S9 and S10). The effect of the
solvent was modeled by the CPCM dielectric model as for the mo-
nomer. IR and VCD spectra calculations were performed under the
same conditions.
For complexes 2a and 3a, both the optimization and the calcula-
tions of IR and VCD spectra were performed under the OBPE/
B3PW91/B3LYP functional combined with the LANL2DZ basis set
and effective core potential (ECP) for FeIII and 6-31G** for other
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Acknowledgements
We thank the National Natural Science Foundation of China
(91022031, 21021062, 21271099) and the National Basic Research
Program of China (2011CB808704) for financial support. We also
thank Dr. Tian-Wei Wang for his help with magnetic measure-
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Keywords: chirality · circular dichroism · iron · magnetic
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Received: December 19, 2013
Revised: January 29, 2014
Published online on February 24, 2014
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