Chiral magnetic metal–organic frameworks of MnII with ...This ournal is c The Royal ociety of...
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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 8543–8545 8543
Chiral magnetic metal–organic frameworks of MnIIwith achiral
tetrazolate-based ligands by spontaneous resolutionw
Xiao-Lan Tong,a Tong-Liang Hu,a Jiong-Peng Zhao,a Yue-Kui Wang,b Hui Zhangc and
Xian-He Bu*a
Received 7th August 2010, Accepted 20th September 2010
DOI: 10.1039/c0cc03111a
The enantiomers of complex 1 (1a and 1b) have been obtained by
spontaneous resolution upon crystallization in the absence of a
chiral source. The enantiomeric nature of 1a and 1b was
confirmed by circular dichroism (CD) spectra and theoretical
investigation.
Recently, the development of new multifunctional materials
that display two or more physical properties in one molecule
has been a topic of growing interest for synthetic chemists.1 In
this field, the design of chiral magnetic systems is of particular
interest for fundamental investigations into the magneto-chiral
effect because of possible applications in magneto-optical
devices. However, only a few optically active magnets have
been reported to date.2–4 For this purpose, carboxylic acids
and tetrazoles, exhibiting a variety of coordination abilities
and the tendency to form architectures with multidimensional
frameworks, are appealing ligands for building simple new
chiral coordination compounds.5 Usually, there are two
methods to build chiral coordination frameworks with building
blocks that have been developed for the construction of
coordination frameworks. One is the possibility of introducing
chiral centers in either metal complexes or ligands, and the
other is the use of achiral ligands with spontaneous resolution
without any chiral auxiliaries to obtain either a chiral network
or enantiomers by spontaneous resolution.6 We have prepared
some chiral complexes by the second method which is
considered the more difficult of the twomethods.5,7 Furthermore,
examples of frameworks with amide ligands of organic acid
containing tetrazolate group have been quite rare up to
now.5d,8 Herein, we use the amide ligand of benzoic acid
containing tetrazolate group and present one pair of
MnII-tetrazole three-dimensional (3D) enantiomorphs, [Mn(L)2]n(1a and 1b), [HL = N-(1H-tetrazol-5-yl)benzamide)], which
exhibits antiferromagnetic interactions between MnII ions.
The nature of enantiomeric 1a and 1b is confirmed by
circular dichroism (CD) spectra measurements and theoretical
investigation.
The reaction of HL and MnCl2�4H2O gives pale pink
crystals of 1 (see ESIw). The phase purity of 1 is confirmed
by XRPD (see Fig. S1, ESIw). During the crystallization of
complex 1, spontaneous resolution occurred and yielded
crystals with chiral space P41212 for 1a and P43212 for 1b
with the absolute structure parameters (flack parameters)
being both +0.02(2).zComplexes 1a and 1b both comprise of one kind of MnII
centre and one deprotonated L. The MnII centre shows a
slightly distorted octahedral geometry and is coordinated by
four N atoms from four different L, and two O atoms from
two of the mentioned four L (Fig. 1a). The related bond
lengths and angles of 1a and 1b are only slightly different.
L acts as tridentate ligand linking two MnII centers, in which
one N atom from the tetrazole and one O atom coordinate to a
MnII centre while the other N atom of the tetrazole ring links
another MnII centre. The distances of the adjacent MnII
centers linked by L is 6.741 A for 1a and 6.714 A for 1b,
respectively. From a topological viewpoint, the nets of 1a and
1b can be rationalized to be 3D dia topological nets with the
metal MnII centers acting as four-connected nodes and L
acting as a linker, and the Schlafli symbol is 66 (Fig. 1b).
The enantiomeric nature of 1a and 1b can be simply
represented by their mirror structures (Fig. 1c).
Fig. 1 (a) The coordination environment of MnII ions in 1a. (b) The
4-connected dia topology for 1a. (c) The enantiomeric nature of 1a
and 1b.
aDepartment of Chemistry, and Tianjin Key Lab on Metal andMolecule-based Material Chemistry, Nankai University,Tianjin 300071, China. E-mail: [email protected];Fax: +86-22-23502458
b Institute of Molecular Science, Shanxi University,Taiyuan 030006, China
cDepartment of Chemistry, Xiamen University, Xiamen 363105, Chinaw Electronic supplementary information (ESI) available: Experimentaldetail, XRPD, crystallographic data and additional figures. CCDC782699 and 782700. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c0cc03111a
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8544 Chem. Commun., 2010, 46, 8543–8545 This journal is c The Royal Society of Chemistry 2010
It is interesting that there exist two kinds of helical chains,
one with a left-handed 41 screw axis and another with a
right-handed 21 screw axis in 1a, and it is the opposite
phenomenon in 1b. Evidently, the initial chirality of the
complexes is due to the screw coordination arrangement of
the achiral ligand around the MnII centers (see Fig. S2, ESIw).The angles of the two six membered rings formed by the MnII
centers and the ligands are 82.001 for 1a and 81.821 for 1b,
respectively. The complexes of 1a and 1b were obtained by
spontaneous resolution upon crystallization in the absence of
any other chiral source. This may provide a rational strategy
for synthesis of chiral coordination polymers by using achiral
ligand and the corresponding metal ions.7a,9
The magnetic measurements were performed on poly-
crystalline samples of complex 1 using a Quantum Design
MPMS XL-7 SQUID magnetometer. The plots of the wmT vs.
T under a 2 kOe external field for 1 are shown in Fig. 2. The
wmT value (4.35 cm3 K mol�1) at 300 K corresponds to the
spin-only value of one octahedral MnII ion with g = 2.0.
During cooling, the value of wmT continues to slowly decrease
until near 100 K; below this temperature, the wmT value
decreases sharply to 0.60 cm3 K mol�1 at 2 K, suggesting
antiferromagnetic behaviour. The appearance of a round peak
in the wm vs. T curve around 4 K indicates antiferromagnetic
coupling (Fig. 2a). The magnetic susceptibility data in the
temperature range of 20–300 K can be well fitted to the
Curie-Weiss law expression where C = 4.45 cm3 mol�1 K
and y = �6.37 K. This further confirms an overall antiferro-
magnetic interaction between the MnII ions (see Fig. S3,
ESIw). The Neel temperature, TN = 3.5 K, was determined
from the sharp peak in dwmT/dT (see Fig. S4, ESIw). At 2 K
the field dependence of the magnetization increases almost
linearly to the highest field measured at 50 kOe, and no
obvious sigmoidal curve is observed. The magnetization value
is 0.59 Nb at 50 kOe, far from the saturation value of 5 Nb for
two MnII ions s = 5/2, which is in agreement with antiferro-
magnetism (Fig. 3b). We attempted to quantitatively analyze
the magnetic behaviour using the HTS model deduced
from the results developed by Rushbrook and Wood for a
Heisenberg antiferromagnet S = 5/2 for a diamond-type
network, but no satisfactory results were obtained. According
to the molecular field theory of antiferromagnetism, there is an
equation describing Y.10
Y = 2S(S + 1)zJ/3k (1)
where Y, S, J, and k have their usual meanings, and z is the
magnetic coordination number of a lattice site. For this sample
y = �6.37 K, S = 5/2 for MnII, and z = 4. Using eqn (1), we
get J = �0.19 cm�1, that is consistent with weak interactions
between the MnII bridged by the N–C–N of the tetrazolate.4
The solid-state circular dichroism (CD) spectra obtained
from KCl pellets further confirmed the optical activity and
enantiomeric nature of complexes 1a and 1b. As shown in
Fig. 3, the CD spectra of 1a and 1b, are nearly mirror images
of each other and indicates the expected formation of the
pair of enantiomeric complexes. In the wavelength range
l = 200–300 nm, 1a shows positive Cotton effects at l = 298
and 215 nm and a negative Cotton effect at l = 250 nm.
Complex 1b shows Cotton effects of the opposite signs to 1a at
the same wavelengths.
Interestingly, the splitting pattern of the solid-state CD
spectra cannot be interpreted using the exciton chirality
method for a single six-coordinated chelate.11 To elucidate
the predominant mechanism of the solid-state CD spectra,
additional calculations have been performed using the exciton
theory for molecular crystals.12 In this theory, the interactions
between transitions on different ligands in the crystal lead to
crystal excitons, and only certain crystal states for k=0 (the Gpoint) are allowed depending on the symmetry of the unit
cell (see ref. 13 for details). For complex 1a, the electronic
transitions of the ligand in the crystal environment have been
calculated at the TDDFT/B3LYP/cc-pVDZ level, and the
results showed that there are two strong p - p* transitions
at 260 nm and 200 nm which are responsible for the crystal
excitons of the complex. The orientation of the corresponding
electric transition dipole moments is shown in Fig. 4a, and the
magnitudes are m1 = 4.268 D and m2 = 5.579 D, respectively.
The eight ligands (four molecules) in the unit cell are labelled
1, 2, . . ., 8, as schematically depicted in Fig. 4b.
Fig. 2 (a) Temperature dependence of wm (red) and wmT (black) for
1 at 2 kOe. (b) The magnetization vs. field plot at 2.0 K for 1.
Fig. 3 The solid-state CD spectra for complexes 1a (red) and 1b
(black).
Fig. 4 (a) The orientation of electric transition dipole moments l1
and l2 located at the centre of mass; (b) the distribution of the eight
ligands represented by l1’s in the unit cell of complex 1a.
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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 8543–8545 8545
The excited-states of the crystal were constructed following
the work of Craig and Walmsley.13 Since the symmetry group
of the Bravais lattice of the crystal is D4, the excited-state
crystal wave functions cPG corresponding to the ligand excited
states Fpi (p = 1,2) for the unit cell are listed (see ESIw for
details). Of these crystal states, only those with symmetries A2
and E give rise to allowed transitions from the ground state
A1. The excited-state interaction energies for the crystal
states were evaluated using transition dipole interactions for
simplicity. The calculated transition wavelengths l (nm),
oscillator f and rotational strengths R (in Debye-Bohr-
Magnetons) for complex 1a are tabulated in Table S1 (see ESIw).The corresponding solid-state CD spectrum is displayed in
Fig. 5, which was generated as a sum of Gaussians, centred at
the calculated wavelengths with integral intensities proportional
to the rotational strengths of the corresponding transitions.
Clearly, the calculated CD spectrum is in good agreement
with the observed one as far as the band shape and the relative
magnitudes are concerned. Based on this agreement the three
observed CD bands can be interpreted as the crystal exciton
bands, mainly arising from the 11A1 - 11E, 11A1 - 11A2 and
11A1 - 31E transitions from long to short wavelengths,
respectively.
In summary, a pair of 3D MnII-tetrazole enantiomorphs
(1, 1a and 1b) have been synthesized by the reaction of an
achiral multidentate ligand and MnCl2�4H2O. They were
characterized and found to exhibit antiferromagnetic inter-
actions between the Mn(II) ions. Also the enantiomeric nature
of 1a and 1b are confirmed by the results of circular dichroism
(CD) spectra measurement and theoretical investigation.
We thank the financial support from the 973 Program of
China (2007CB815305), the NSFC (20773068, 20801029,
21031002), and the NSF of Tianjin, China (10JCZDJC22100).
Notes and references
z Crystallographic data of 1a: C16H12MnN10O2, Mr = 431.30,tetragonal, P41212, a = 10.5842(15) A, b = 10.5842(15) A,c = 15.826(3) A, V = 1772.9(5) A3, Z = 4, rcalcd = 1.616 g cm�3,2ymax = 54.8 (�13 r h r13, �13 r k r 13, �20 r l r 20),T = 293(2) K, Rint = 0.0649, R1 = 0.0432 (I > 2s(I)), wR2 = 0.0749(all data), GOF = 1.220, Flack parameter = 0.02(2), CCDC No:782699; 1b: C16H12MnN10O2, Mr = 431.30, tetragonal, P43212,a = 10.5430(15) A, b = 10.5430(15) A, c = 15.766(3) A,V = 1752.5(5) A3, Z = 4, rcalcd = 1.635 g cm�3, 2ymax = 55.0(�13 r h r13, �13 r k r 13, �20 r l r 20), T = 293(2) K,
Rint = 0.0523, R1 = 0.0324 (I > 2s(I)), wR2 = 0.0696 (all data),GOF = 1.200, Flack parameter = 0.02(2) , CCDC No: 782700.
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Fig. 5 The calculated CD spectrum (left) and exciton energies (right)
of complex 1a.
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