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: buxh@nankai.edu.cn;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

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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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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