Nano- and microcrystals of a Mn-based metal-oligomer framework showing size-dependent magnetic resonance behaviors

Article abstract of DOI:10.1039/c0cc03981k

A robust 3D Mn-based metal-organic framework containing metallosalen hexamers is synthesized and its nano- and microcrystals are fabricated by using surfactant-mediated hydrothermal and solvent precipitation methods; the particles exhibit an inverse size-

Full text of DOI:10.1039/c0cc03981k

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COMMUNICATION
Nano- and microcrystals of a Mn-based metal–oligomer framework
showing size-dependent magnetic resonance behaviorsw
Guozan Yuan, Chengfeng Zhu, Yan Liu and Yong Cui*
Received 20th September 2010, Accepted 13th January 2011
DOI: 10.1039/c0cc03981k
A robust 3D Mn-based metal–organic framework containing
metallosalen hexamers is synthesized and its nano- and micro-
crystals are fabricated by using surfactant-mediated hydrothermal
and solvent precipitation methods; the particles exhibit an inverse
size-dependent effect of magnetic resonance relaxivity behaviors.
very high in vivo longitudinal (R1) MR relaxivities by binding
to intracellular proteins.11 Metallosalen complexes have
diverse potential applications in biological processes.12 To
prepare metal–metallosalen polymer particles for MRI, a
dicarboxylate functionalized metalloligand H2[MnL] [H4L =
(R,R)-(ꢀ)-N,N0-bis (3-tert-butyl-5-(carboxyl)salicylidene)-1,2-
diaminocyclohexane)] was chosen, as the linker to form stable
MOFs and to carry a high payload of paramagnetic Mn ions
for MRI. We report here the synthesis of a robust 3D
Mn-based metal–metallosalen oligomer framework with
controllable particle sizes and morphologies, which exhibits
an inverse size-dependent effect of MR properties.
Metal–organic frameworks (MOFs) provide an interesting
avenue to hybrid materials through fine-tuning their composition
and structures by a judicious choice of metal ions and organic
linkers,1 and have promising applications in diverse areas
including storage, catalysis and electro-optics.2 All these
possible applications could undergo remarkable development
if bulk MOF materials were scaled down to micro- and
nanometre regimes.3 For example, nanoscaled MOFs have
recently emerged as promising magnetic resonance imaging
(MRI) agents and drug delivery agents.4 Although several
approaches have been developed to fabricate nano- and
microscaled MOF particles including co-precipitation,
solvothermal, reversed micelles and microwave-assisted
methods,4–7 there are no efficient synthetic routes to manip-
ulate nano- and microcrystallite sizes and shapes of MOFs,
prohibiting a detailed understanding of the systems at a
molecular level.3,7 Moreover, with few exceptions, research
related to size/shape-dependent properties in MOFs has not
yet been performed.8 In contrast, tuning of the chemical and
physical properties of metal and inorganic particles through
size and shape control is a well-known strategy.9
As outlined in Fig. 1, the enantiopure H4L ligand was
prepared in seven steps in an overall 33% yield from the
readily available and cheap 2-tert-butyl-4-methylphenol.
Heating a mixture of Mn(ClO4)2ꢁ6H2O and H4L (2 : 1 molar
ratio) in DMF and water afforded brown crystals of
[MnII3(MnIIIL)6]ꢁ6DMFꢁ6H2O (1) in 80% yield after three
days. The product is stable in air and insoluble in water and
common organic solvents and was formulated based on micro-
analysis and thermogravimetric analysis. The phase purity of
the bulk sample was established by comparison of its observed
and simulated powder X-ray diffraction (PXRD) patterns.
A single-crystal X-ray diffraction study on 1 exhibits a 3D
metallosalen framework that crystallizes in the triclinic chiral
space group P1 with one formula unit in the asymmetric unit.z
The basic building block is a linear trinuclear Mn unit with
adjacent Mn–Mn distances of 3.4998(4) and 3.5245(4) A and a
MRI is a powerful noninvasive tomographic technique that
can detect differences between normal and diseased tissues
based on their varied NMR water proton signals arising from
different water densities and/or nuclear relaxation rates.10
Most presently available MRI contrast agents are highly
paramagnetic complexes, usually Gd3+ and Mn2+ chelates.11
The metal-based contrast agents, however, exhibit modest
longitudinal (R1) relaxivities and large quantities (several
grams per patient) must be administered to get adequate
MR contrast. Amongst them, complexes of Mn2+ can exhibit
School of Chemistry and Chemical Technology, Shanghai Jiao Tong
University, Shanghai200240, China. E-mail: yongcui@sjtu.edu.cn;
Fax: +86 21-5474-1297
w Electronic supplementary information (ESI) available: Details of the
synthetic procedures, spectra data and crystallographic data. CCDC
786168. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0cc03981k
Fig. 1 Schematic showing the synthesis of the ligand H4L.
c
3180 Chem. Commun., 2011, 47, 3180–3182
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Mn–Mn–Mn angle of 179.232(10)1, which are bridged by six
bidentate carboxylate groups of six MnL units. The central
Mn ion is octahedrally coordinated by six oxygen atoms of six
bidentate carboxylate groups from six MnL units, while each
of the two terminal Mn ions are tetrahedrally coordinated by
four oxygen atoms of three bidentate and one monodentate
carboxylate groups from four MnL ligands. The Mn–O bonds
of the central hexacoordinated Mn ion [2.128(7)–2.244(8) A]
are evidently longer than those of the terminal tetra-
coordinated Mn ions [2.011(9)–2.193(8) A].
Of the six independent MnL ligands, four exhibit the
exo-tridentate coordination mode including one monodentate
and one bridging bidentate carboxylate group, and the other
two exhibit the exo-tetradentate coordination mode with two
bridging bidentate groups. In each MnL unit, the metal center
adopts a square-pyramidal geometry with the equatorial plane
occupied by the N2O2 donors of one L ligand and the apical
position by one carboxylate oxygen atom of another L ligand,
with the Mn–N and Mn–O bond lengths ranging from
1.937(11) to 2.001(9) A and from 1.825(9) to 2.219(9) A,
respectively. As a result, six MnL units self-assemble via
complementary coordination of unsaturated metal centers of
each unit to monodentate carboxylate oxygen atoms of other
units into a Mn6L6 hexamer (Fig. 2a). Each Mn3 core in 1 is
thus linked by six Mn6L6 assembles and each hexameric
assemble is linked to six 6-connected Mn3 cores to generate
a chiral 3D hyperbranched framework (Fig. 2b,c). Oligomers
are expected to display enhanced or novel electronic and
magnetic behaviors and are widely studied substances in self-
assembly of well-defined hybrid architectures, but no MOF
derived from metal oligomers has thus far been reported.13
Calculations using PLATON show that 1 has about
B14.9% of the total volume available for guest inclusion.14
TGA revealed that the guest DMF and water molecules could
be removed in the temperature range from 80 to 300 1C and
the frameworks are stable up to 350 1C. Powder XRD experi-
ments indicate that the framework and crystallinity of 1
remain intact upon complete removal of guest molecules
(Fig. S6 in ESIw). CD spectra of 1 obtained from R and S
enantiomers of the H4L ligand are mirror images of each
other, indicative of their enantiomeric nature.
Fig. 2 (a) The self-assembled Mn6L6 hexamer; (b) the hyperbranched
structure around the Mn3 building block; (c) a view of 3D structure
of 1. The Mn3 units are shown in polyhedra.
bromide/DMF/toluene/H2O solution at 80 1C in 42% yield.
After heating Mn(ClO4)2ꢁ6H2O and H4L in DMF/THF/H2O
at 80 1C for 24 h, the mixture was then cooled to room
temperature and injected into distilled water with vigorous
stirring, affording nanoparticles of 1c with diameters of
20–100 nm in 60% yield. PXRD studies showed that powders
1a–c were all highly crystalline and exhibited the same diffrac-
tion patterns as the bulk phase of 1 (Fig. S6 in ESIw).
Solid-state magnetic susceptibility (wM) data on 1 and 1a–c
were collected in a 0.010 T field in the 2.0–300 K range (ESIw,
Fig. S13). The 300 K wMT values of 30.91 (1), 29.91 (1a), 29.31
(1b) and 29.69 (1c) cm3 K molꢀ1 are close to or slightly lower
than the spin only (g = 2) value of 31.13 cm3 K molꢀ1
expected for three MnII and six MnIII ions and
decreases gradually to 11.13 (1), 10.03 (1a), 9.72 (1b) and
9.21 (1c) cm3 K molꢀ1 at 2 K, indicating the presence of
dominant antiferromagnetic interactions.
Micro- and nanoscale crystalline particles of 1 can be
fabricated using surfactant-mediated hydrothermal and
solvent precipitation methods. Microrods of 1a were prepared
by heating an optically transparent microemulsion of
Mn(ClO4)2ꢁ6H2O and H4L (a 2 : 1 molar ratio) in the cationic
cetyltrimethylammonium bromide (CTAB)/DMF/CHCl3/
water system with a water/surfactant molar ratio (w) of
3.1 ꢂ 103 at 60 1C. After 12 h, the powders were isolated in
60% yield by centrifugation and washing with water and
methanol. Scanning electron micrographic (SEM) images of
1a revealed the morphological purity of microrods of 2–3 mm
in length by 500–600 nm in diameter. Note that the shape and
size of the particles were not greatly affected by the w value.
We have also carried out numerous experiments to obtain
other morphologies and sizes of 1 (Fig. 3). For example,
nanorods of 1b of 600–700 nm in length by 200–300 nm in
diameter could be readily obtained by heating Mn(ClO4)2ꢁ6H2O
and H4L (2 : 1 molar ratio) in tetrabutylammonium
The potential utility of particles 1a–c as contrast agents
for MRI was examined. Relaxivity data were obtained on a
c
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Chem. Commun., 2011, 47, 3180–3182 3181

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0.5 T scanner with suspensions of particles 1a–c in water con-
taining 0.2% xanthan gum. Microrods of 1a have a longitudinal
relaxivity (R1) of 41.44 and a transverse relaxivity (R2) of
94.64 mMꢀ1 sꢀ1 on a per formula basis, and nanorods of 1b
caused by the smallest nanoparticles of 1c at the same Mn
concentration. This size/shape-dependent behavior is thus
consistent with the above relaxivity data. Note that the general
reported trend of size dependent MR signals is that as particle
size increases the T1- or T2-weighted (e.g. T1 for Mn or T2 for
Gd ions) MR signal intensity continuously decreases which in
turn appears as darker MR images.15 Further works are in
progress to investigate the inverse size effect of MRI properties
and to evaluate the efficacy of 1 as MR contrast agents in vitro.
In conclusion, we have constructed a robust 3D Mn metal–
oligomer framework based on self-assembled metallosalen
hexamers and developed an efficient route to control synthesis
of its micro- and nanosized crystalline particles that exhibited
size-dependent MR relaxivity behaviors. With a precise
knowledge of their single crystal structures and facile tunability
of their building blocks, the present research holds great
promise in the development of novel MOF materials.
exhibited an R1 of 24.90 and an R2 of 49.80 mMꢀ1 sꢀ1
whereas the smallest nanoparticles of 1c had an R1 of
4.96 and an R2 of 6.14 mMꢀ1 ꢀ1. Thus, an interesting size-
,
s
dependent MR relaxivity behavior was observed for 1: both R1
and R2 increase with the increase of particle sizes (Fig. 4a).
This change trend was further confirmed by measurement of
R1 values on a Mercury plus 400 spectrometer (Fig. S10 in the
ESIw). The level of R1 relaxivities of 1a and 1b is comparable
with those of Mn-based nanoparticles4a,15,16 and may allow
their potential use as T1 contrast agents depending on the MR
pulse sequence employed.15
Indeed, as shown in Fig. 4b, the larger particles of 1a and 1b
are much efficient in enhancing the water signals in
T1-weighted images, whereas significant signal reduction was
This work was supported by NSFC-21025103/20971085,
‘‘973’’ Programs (2007CB209701 and 2009CB930403) and
Shanghai Science and Technology Committee (10DJ1400100).
Notes and references
z Crystal data for 1ꢁ6DMFꢁ6H2O: triclinic, space group: P1, a =
13.663(2) A, b = 18.329(3) A, c = 22.173(4) A, a = 67.204(3)1, b =
88.260(4)1, g = 86.712(3)1, V = 5110.5(15) A3, Z = 1, T = 123 K,
F(000) = 2181, rcalcd = 1.349 g cmꢀ3, m(Mo Ka) = 0.617 mmꢀ1
(l = 0.71073 A), 72 357 measured reflections, 32 894 independent
reflections (Rint = 0.0685), 2291 refined parameters, R1 = 0.0835, and
wR2 = 0.2038 for I > 2s(I), Flack parameter = 0.04(2), GOF =
1.051. CCDC 786168.
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c
3182 Chem. Commun., 2011, 47, 3180–3182
This journal is The Royal Society of Chemistry 2011