Synthesis, structure, and magnetic properties of bithiophene- and terthiophene-linked manganese metal-organic frameworks

Article abstract of DOI:10.1021/ic401305c

A series of metal-organic frameworks (MOFs) containing manganese centers and oligothiophene dicarboxylate linkers have been synthesized: [Mn(3PhT ₂DC)(DMF)_0.45(H₂O)_2.55·1. 55DMF]_n (1), [Mn₆(3HT₂DC)₆(DMF) ₃(H₂O)₅·xDMF·yH ₂O]_n (2), [Mn(T₃DC)(H₂O) ₂]_n (3), [Mn(T₃DC)(H₂O) _1.5]_n (4), and [Mn(Ph₂T₃DC)(DMF) ₂]_n (5) (H₂3PhT₂DC = 3,3'-diphenyl-2,2-bithiophene-5,5'-dicarboxylic acid; H₂3HT ₂DC= 3,3'-dihexyl-2,2'-bithiophene-5,5'-dicarboxylic acid; H ₂T₃DC = 2,2':5',2'-terthiophene-5,5'-dicarboxylic acid; H₂Ph₂T₃DC = 3',4'-diphenyl-2,2':5',2'- terthiophene-5,5'-dicarboxylic acid, DMF = N,N-dimethylformamide). Compound 1 exists as a 2D sheet, 2-4 are 3D frameworks, and 5 is a 1D chain. Compounds 3 and 4 are isomers, and 3-5 are the first examples of crystallographically characterized terthiophene coordination polymers. In the case of 1, 2, and 5, the extended structure is sensitive to β substitution of the oligothiophene linkers. Compounds 1-3 and 5 show antiferromagnetic behavior with typical values of g and J, and 3 exhibits a spin canting transition at 40 K.

Full text of DOI:10.1021/ic401305c

Article
pubs.acs.org/IC
Synthesis, Structure, and Magnetic Properties of Bithiophene- and
Terthiophene-Linked Manganese Metal−Organic Frameworks
Lyndsey D. Earl, Brian O. Patrick, and Michael O. Wolf*
Department of Chemistry, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 Canada
S
* Supporting Information
ABSTRACT: A series of metal−organic frameworks (MOFs)
containing manganese centers and oligothiophene dicarbox-
ylate linkers have been synthesized: [Mn(3PhT2DC)-
(DMF)0.45(H2O)2.55·1.55DMF]n (1), [Mn6(3HT2DC)6-
(DMF)3(H2O)5·xDMF·yH2O]n (2), [Mn(T3DC)(H2O)2]n
(3), [Mn(T3DC)(H2O)1.5
]
(4), and [Mn(Ph2T3DC)-
n
(DMF)2]n (5) (H23PhT2DC = 3,3′-diphenyl-2,2-bithio-
phene-5,5′-dicarboxylic acid; H23HT2DC= 3,3′-dihexyl-2,2′-
bithiophene-5,5′-dicarboxylic acid; H2T3DC = 2,2′:5′,2″-
terthiophene-5,5″-dicarboxylic acid; H2Ph2T3DC = 3′,4′-diphenyl-2,2′:5′,2″-terthiophene-5,5″-dicarboxylic acid, DMF = N,N-
dimethylformamide). Compound 1 exists as a 2D sheet, 2−4 are 3D frameworks, and 5 is a 1D chain. Compounds 3 and 4 are
isomers, and 3−5 are the first examples of crystallographically characterized terthiophene coordination polymers. In the case of 1,
2, and 5, the extended structure is sensitive to β substitution of the oligothiophene linkers. Compounds 1−3 and 5 show
antiferromagnetic behavior with typical values of g and J, and 3 exhibits a spin canting transition at 40 K.
linkers.16 Terthiophene coordination polymers have been
INTRODUCTION
■
synthesized17 but not characterized crystallographically.
While examples of MOFs containing linkers with alkyl chains
have been reported,18 there have been few extensive studies
looking into the influence of functional groups on the extended
structure of coordination polymers. In this Article, we report
how phenyl and n-hexyl substituents as well as thiophene
oligomer length affect the coordination and extended structure
within five new coordination polymers. The first examples of
crystallographically characterized terthiophene coordination
polymers are reported, including a set of isomers. In addition,
the magnetic behavior of these new metal−organic frameworks
and the correlation between magnetic susceptibility and
structure are discussed.
The structural and electronic properties of longer chain
oligothiophenes and polythiophenes are of significant interest
for potential applications including light-emitting and photo-
voltaic devices, electrochromic materials, and chemical sensors.1
Positional and orientational control of oligothiophenes is
important for these potential applications, particularly for
optimizing the efficiency of light-harvesting systems and
controlling orientation-dependent charge transfer.2 Functional-
ization of the β position of oligo- and polythiophenes with
solubilizing and photoactive groups has proved to be a useful
method for manipulating the physical and electronic structure
of these materials.3
The synthesis and characterization of manganese coordina-
tion polymers containing oligothiophene linkers is of particular
interest due to the local4 and global5 structural diversity,
potential mixed valence character,6 electronic activity,7 and
magnetic properties8 of Mn2+ metal−organic frameworks
(MOFs). Although Mn2+ is usually a high-spin d5 metal center
that quenches photoluminescence, the structural data obtained
from these compounds will provide valuable information for
engineering specific solid state interactions of oligothiophenes.
Methodical crystal engineering can be applied via incorpo-
ration of oligothiophene linkers into metal−organic frameworks
to obtain orientational control of coordination polymers.9 A
myriad of MOFs containing thiophene-2,5-dicarboxylic acid
have been reported,10 though few oligothiophene metal−
organic frameworks,11−13 polyhedra,14 or covalent-organic
frameworks15 have been synthesized. Examples of conjugated
triaryl MOFs are limited to those containing p-terphenyl
EXPERIMENTAL SECTION
■
General Methods. MnCl2·4H2O and n-butyllithium were
purchased from Sigma-Aldrich. N,N-Dimethylformamide (DMF) was
purchased from Fisher Scientific. All chemicals were used as received.
H23PhT2DC,11 H23HT2DC,11 and H2T3DC14 were prepared by
1
literature procedures. THF was distilled from Na/benzophenone. H
NMR spectra were collected on either a Bruker AV-300 or AV-400
spectrometer and were referenced to residual solvent. EI mass spectra
were obtained using a Kratos MS-50 mass spectrometer coupled to a
MASPEC data system. Infrared spectra were obtained on a Thermo
Nicolet 6700 with a Smart Orbit accessory in the range 4000−400
cm−1. Thermogravimetric analyses (TGA) were performed using a
Perkin-Elmer Pyris 6 thermogravimentric analyzer under a nitrogen
atmosphere at a rate of 10 °C min−1. Magnetization measurements
Received: May 24, 2013
© XXXX American Chemical Society
A
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Scheme 1
were performed using a Quantum Design MPMS-XL-7S SQUID
magnetometer with an Evercool-equipped liquid helium dewar.
Diamagnetic contributions to the magnetic susceptibility were
corrected for using Pascal’s constants.19 CHN elemental analyses
were performed using a EA1108 elemental analyzer. Powder X-ray
diffraction (PXRD) patterns were obtained on a Bruker D8 Advance
instrument at a scan rate of 5° min−1. TOPOS20 was used to
determine net topology.
HCl, filtered, washed with 0.1 M HCl, methanol, diethyl ether, and
finally dried in vacuo to afford an orange solid (87% yield). Mp > 300
°C. m/z: 488. 1H NMR (300 MHz, d6-DMSO) δ 7.56 (d, J = 3.9 Hz, 2
H), 7.29 (m, 6 H), 7.18 (m, 6 H). FT-IR (cm−1) 3390 (w, br), 3052
(w) 2809 (w), 2521 (w, br), 1651 (s), 1513 (m), 1487 (w), 1440 (s),
1289 (m, br), 1112 (m), 1050 (w), 917 (w), 797 (m), 745 (m), 699
(s), 623 (w), 511 (m), 483 (w). Anal. Calcd for C26H16O4S3: C, 63.91;
H, 3.30. Found: C, 63.46; H, 3.23.
[Mn(3PhT2DC)(DMF)0.45(H2O)2.55·1.55DMF]n (1). MnCl2·4H2O
(0.0198 g, 0.100 mmol) and H23PhT2DC (0.0406 g, 0.100 mmol)
were dissolved in DMF (5 mL). The solution was transferred to a 23
mL Teflon-lined Parr bomb and sealed. The vessel was heated to and
held at 110 °C for 48 h, and then the bomb was cooled at a rate of 3.7
°C h−1. Colorless crystals of [Mn(3PhT2DC)(DMF)0.45(H2O)2.55·-
1.55DMF]n were isolated in 73% yield. FT-IR (cm−1): 3430 (w, br),
2933 (w, br), 1637 (s), 1570 (s), 1528 (m), 1494 (m), 1410 (s), 1372
(s), 1347 (s), 1209 (m), 1109 (m), 1062 (w), 869 (w), 846 (w), 765
(m), 698 (m), 628 (w), 550 (w), 497 (w). Anal. Calcd for
C28H26N2O8.54S2Mn: C, 52.09; H, 4.06; N, 4.34. Found: C, 52.03;
H, 4.61; N, 4.71.
[Mn6(3HT2DC)6(DMF)3(H2O)5·xDMF·yH2O]n (2). MnCl2·4H2O
(0.0198 g, 0.100 mmol) and H23HT2DC (0.0422 g, 0.100 mmol)
were dissolved in DMF (5 mL). The solution was transferred to a 23
mL Teflon-lined Parr bomb and sealed. The vessel was heated to and
held at 110 °C for 48 h, and then the bomb was cooled at a rate of 3.7
°C h−1. Colorless crystals of [Mn6(3HT2DC)6DMF)3H2O)5·xDMF·
yH2O]n were isolated in 56% yield. FT-IR (cm−1): 2925 (m), 2854
(m), 1647 (m), 1565 (m), 1530 (m), 1421 (s), 1386 (s), 1353 (s),
1258 (w), 1195 (w), 1102 (m), 1016 (w), 871 (w), 774 (s), 714 (m),
677 (m), 484 (w). Anal. Calcd for C141H199N3O32S12Mn6: C, 53.55; H,
6.34; N, 1.33. Found: C, 54.40; H, 6.19; N, 2.44. (The discrepancy
between the calculated and experimental elemental analysis is
associated with the presence of noncoordinated solvent that was
removed during the SQUEEZE protocol.)
[Mn(T3DC)(H2O)2]n (3). MnCl2·4 H2O (0.0198 g, 0.100 mmol) and
H2T3DC (0.0336 g, 0.100 mmol) were dissolved in DMF (5 mL). The
solution was transferred to a 23 mL Teflon-lined Parr bomb and
sealed. The vessel was heated to and held at 110 °C for 48 h, and then
the bomb was cooled at a rate of 3.7 °C h−1. Yellow-orange crystals of
[Mn(T3DC)(H2O)2]n were isolated in 81% yield. FT-IR (cm−1): 3212
(m, br), 1547 (m), 1507 (s), 1439 (s), 1380 (s), 1111 (m), 1064 (w),
1034 (m). 858 (m), 785 (m), 769 (s), 699 (w), 676 (w), 631 (w), 534
(w), 476 (m). Anal. Calcd for C14H9O5.5S3Mn: C, 40.39; H, 2.18.
Found: C, 39.63; H, 2.49.
X-ray Crystallography. All crystals were mounted on glass fibers.
Diffraction from 1 and 3 was measured on a Bruker APEX DUO
diffractometer with graphite monochromated Cu Kα radiation.
Diffraction from 2 and 4 was measured on a Bruker APEX DUO
diffractometer with graphite monochromated Mo Kα radiation.
Diffraction from 5 was measured on a Bruker X8 APEX II
diffractometer with graphite monochromated Mo Kα radiation. Data
were collected and integrated using the SAINT software package.21
Data were corrected for absorption effects using the multiscan
technique (SADABS).22 Structures were solved using direct
methods.23 Non-hydrogen atoms were refined anisotropically except
for atoms C37−C38, C43−C44, C59−C60, C84−C88, C107−C110,
C130−C132, C137, C148−C150, N7, and O34 of 2. All non-O−H
hydrogens were placed in calculated positions; O−H hydrogens in 4
were found on the difference map while O−H hydrogens in 1−3 were
not modeled. Refinements for 1−5 were performed using SHELXL-
9724 via the WinGX25 interface. Compound 1 crystallizes with
disordered coordinating and noncoordinating solvent at the O8 site,
and the group was modeled in two orientations. Compound 2
crystallizes with significant disorder in one n-hexyl group (C17−C22),
and the group was modeled in two orientations. Restraints on bond
lengths for n-hexyl groups were employed to maintain reasonable
geometries. There are regions of unresolved electron density in the
void volume of compounds 1 and 2 that could not be appropriately
modeled. The PLATON/SQUEEZE26 program was used to generate
a data set free of electron density in those regions. Crystallographic
details for compounds 1−5 can be found in Table S3.
Synthesis. 3′,4′-Diphenyl-2,2′:5′,2″-terthiophene-5,5″-dicarbox-
ylic Acid (H2Ph2T3DC). 3′,4′-Diphenyl-2,2′:5′,2″-terthiophene27 (0.41
g, 1.0 mmol) was added to THF (20 mL). The flask was cooled to
−78 °C, and n-butyllithium (1.3 mL, 2.1 mmol, 1.6 M in hexanes) was
added dropwise. The yellow solution turned yellow-orange, and then
green during the addition. The reaction mixture was warmed to and
held at 0 °C for one hour, and then cooled to −78 °C. Addition of
excess solid carbon dioxide pellets resulted in the formation of a
vibrant orange opaque suspension. The reaction was allowed to warm
to room temperature overnight. Water (20 mL) was added to quench
the reaction. The aqueous layer was washed with diethyl ether (3 × 10
mL), and the product was precipitated by the addition of excess 1 M
[Mn(T3DC)(H2O)1.5]n (4). Red-orange crystals of 4 were grown from
the solvothermal reaction mixture of 3 and isolated in less than 1%
yield. FT-IR (cm−1): 3590 (w), 1641 (sh), 1626 (m), 1557 (s), 1530
B
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(m), 1509 (m), 1446 (m), 1435 (m) 1361 (vs), 1128 (m), 1066 (m),
1031 (m), 866 (m), 767 (s), 680 (w), 642 (w), 541 (w), 483 (m).
[Mn(Ph2T3DC)(DMF)2]n (5). MnCl2·4H2O (0.0198 g, 0.100 mmol)
and H2Ph2T3DC (0.0488 g, 0.100 mmol) were dissolved in DMF (5
mL). The solution was transferred to a 23 mL Teflon-lined Parr bomb
and sealed. The vessel was heated to and held at 110 °C for 48 h, and
then the bomb was cooled at a rate of 3.7 °C h−1. Dark yellow crystals
of [Mn(Ph2T3DC)(DMF)2]n were isolated in 64% yield. FT-IR
(cm−1): 3089 (w), 2936 (w), 1639 (s), 1563 (s), 1528 (m), 1441 (m),
1379 (s), 1252 (m), 1213 (w), 1105 (m), 1069 (w), 1024 (w), 904
(w), 814 (m), 770 (s), 701 (m), 671 (m), 628 (w), 613 (w), 510 (w),
465 (m). Anal. Calcd for C32H28N2O6S3Mn: C, 55.89; H, 4.10; N,
4.07. Found: C, 54.91; H, 4.09; N, 3.93.
The structure of 1 can be reduced to a 4-connected uninodal
net with sql-Shubnikov tetragonal plane net topology.
Previously synthesized MOFs containing this linker11 are also
2D frameworks; changing the metal center thus far has not
caused changes in dimensionality of 3PhT2DC2− MOFs.
Literature precedent27,28 for the solid state structure of 3,3′-
diphenyl-2,2′-bithiophene and its derivatives11 suggests that the
conformations that are accessible in the solid state will lead to
3PhT2DC2− having a linking angle of 140−150°. Ligands with
such linking angles alone are not conducive to forming 3D
frameworks.29 The presence of additional driving forces (e.g.,
van der Waals interactions, π−π stacking) may help overcome
the disfavored linking angle to form structures with higher
dimensionality.
A 3D framework belonging to the acentric orthorhombic
space group C2221 is formed upon crystallization of
[Mn6(3HT2DC)6(DMF)3(H2O)5·xDMF·yH2O]n (2). The
metal nodes of 2 consist of linear trinuclear manganese centers
coordinated to 3HT2DC2− linkers, DMF, and H2O. Terminal−
center manganese distances range from 3.702(3) to 3.748(3) Å.
The four terminal manganese atoms in the asymmetric unit are
coordinated to two solvent molecules each (Mn1, Mn4, and
Mn6 are coordinated to a DMF and H2O, while Mn3 is
coordinated to two H2O molecules), a μ2-η2:η1 carboxylate, and
two μ2-η1:η1 carboxylates. The central manganese atom is
coordinated to six oxygens from six unique 3HT2DC2− linkers.
Four of the oxygens belong to carboxylates that coordinate in a
μ2-η1:η1 mode. The other two oxygens belong to carboxylates
that bind in a μ2-η2:η1 fashion and coordinate to the terminal
manganese in addition to the central manganese atom. Mn−O
bond lengths are typical: Mn−O carboxylate bond lengths
range from 2.14 to 2.22 Å, while Mn−O solvate bond lengths
range from 2.06 to 2.27 Å. The six 3HT2DC2− within 2 have
annular torsion angles in the regimes of 55° and 90°. Previous
theoretical studies calculate the torsion angle of 3,3′-dihexyl-
2,2′-bithiophene to be 72°,30 suggesting the n-hexyl chains will
drive the thiophene rings of 3HT2DC2− to near perpendicular
angles. 3HT2DC2− adopts two coordination modes within the
solid state structure of 2. Two linkers in the asymmetric unit
have carboxylates that coordinate in a bis-bidentate fashion
(Figure S1b) while the other four 3HT2DC2− linkers in the
asymmetric unit coordinate with a bidentate μ2-η1:η1 mode and
a chelating-bridging μ2-η2:η1 mode (Figure S1c). The six
coordinating 3HT2DC2− linkers bind to four other trinuclear
metal nodes. As illustrated in Figure 2, two 3HT2DC2− connect
to two separate nodes, whereas two sets of two 3HT2DC2− act
as a twisted double pillar and link to one node each.
RESULTS AND DISCUSSION
■
Synthesis. Oligothiophene dicarboxylic acid linkers
(Scheme 1) were synthesized from Kumada cross coupling
reactions followed by carboxylation of the α positions of the
terminal thienyl groups. Compounds 1−3 and 5 were
synthesized directly from solvothermal reactions of MnCl2·
4H2O and an oligothiophene dicarboxylic acid in DMF at 110
°C for 48 h. Solvothermal reaction times of 24 h or less gave
minimal product, and fast cooling rates gave polycrystalline
powder as the product. The low yield of 4 can be partially
credited to most of the reactants being consumed in the
formation of 3. The slow emergence of 4 from the reaction
mixture of 3 is ascribed to the formation of a kinetic product,
whereas 3 may be the thermodynamic product of the reaction.
Table 1 summarizes the preparation and dimensionality of 1−5.
Table 1. Summary of Preparation and Structure Types
compd
preparation
dimensionality
inorganic building unit
1
2
3
4
5
solvothermal
solvothermal
solvothermal
post-solvothermal
solvothermal
2D
3D
3D
3D
1D
mononuclear
linear trinuclear
mononuclear
mononuclear
binuclear
Crystal Structures. Bithiophene MOFs. [Mn(3PhT2DC)-
(DMF)0.45(H2O)2.55·1.55DMF]n (1) forms 2D sheets along the
crystallographic ac-plane. Each asymmetric unit (Figure 1a)
1
1
contains
/ Mn1, /
Mn2, one 3PhT2DC2− linker, two
2 2
coordinating H2O, one noncoordinating DMF, and a
disordered site that is either coordinating water and non-
coordinating DMF (55% occupancy) or coordinating DMF
(45% occupancy). Both manganese sites adopt an octahedral
geometry: Mn1 is coordinated to two trans monodentate
carboxylates (O1) and four solvent molecules (H2O and
DMF), while Mn2 is coordinated to four μ2-η1:η1 carboxylates
(O3 and O4) and two solvent molecules. Figure S1a shows the
linker coordination of 3PhT2DC2− within 1. Syn−anti
carboxylates bridge Mn2 centers to form Mn−carboxylate
chains along the c-axis, and each Mn2 is 4.52(1) Å from the
nearest Mn2. Mn1 sites are not linked directly via carboxylate
bridges, and the distance between two Mn1 atoms is 9.04(2) Å.
Mn1 effectively acts as a bridge for the Mn2 nodes via
3PhT2DC2− linkers. The extended structure of 1 and the
linkage of the manganese environments are shown in Figure 1b.
The 3PhT2DC2− linker is twisted with a torsion angle of
122.49(2)°: the phenyl groups are flanked away from each
other and do not engage in intermolecular interactions.
Noncoordinating solvent and phenyl rings occupy the void
space in the solid state structure.
Compound 2 has a 4-connected uninodal lvt net topology
and a Schlafli symbol of 42.84 when the trinuclear cluster is
̈
treated as a single node. n-Hexyl chains, some of which are
disordered, and noncoordinating solvent fill the voids of the
coordination polymer. There is at least one noncoordinating
DMF in the asymmetric unit, but the exact number and
composition of solvent molecules could not be determined.
The n-hexyl chains of the linkers are not evenly distributed
within the free space of 2 and appear to cluster within this
structure. Exact determination of the extent of aggregation
cannot be assessed due to the disorder of the n-hexyl chains.
However, intraligand repulsion and interligand aliphatic
attraction contribute to the twisting of the 3HT2DC2− linker
and observed grouping of n-hexyl chains.
The long-range configuration of 2 (Figure 2) is dissimilar to
other bithienyl manganese coordination polymers. Compound
C
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Figure 1. Solid state structure of 1. (a) Asymmetric unit of 1. (b) Extended view of 1 on the ac-plane.
1 has phenyl substituents at the β position and is a 2D sheet,
whereas a Mn2+ MOF with unsubstituted bithiophenes,
13
[Mn3(T2DC)3(DMF)4]n (2,2′-bithiophene-5,5′-dicarboxylic
acid), has sxb topology and trinuclear centers that are similar
to those in 2. Although the two compounds share inorganic
b u i l d i n g u n i t s , e a c h t r i n u c l e a r c e n t e r o f
[Mn3(T2DC)3(DMF)4]n is linked to six other centers, whereas
2 is linked to four other centers. Figure 3a illustrates the
simplified coordination of the trinuclear nodes for 2 and
[Mn3(T2DC)3(DMF)4]n. This change in the linkage of the
metal centers has a drastic effect on the overall topology. As
illustrated in Figure 3b, the 4-connected lvt net topology of 2 is
vastly different from the 6-connected sxb topology of
[Mn3(T2DC)3(DMF)4]n. The change in topology is attributed
to van der Waals attraction of the n-hexyl chains11,18 to other n-
hexyl chains that directs the extended structure of 2. This
interaction between chains causes the linkers to twist in a way
that changes the linking angle of the carboxylates which
influences the position and orientation of metal centers.
Terthiophene MOFs. The T3DC2− linker has been
previously used to form a copper−T3DC metal−organic
polyhedron with the T3DC2− linker in a cis,cis conformation
and at a 90° linking angle.14 However, there have been no
reports of crystallographically characterized structures with
terthiophene linkers in a metal−organic framework or
coordination polymer.
Reaction of MnCl2·4H2O and H2T3DC under solvothermal
conditions affords two distinct products: compounds 3
([Mn(T3DC)(H2O)2]n) and 4 ([Mn(T3DC)(H2O)1.5]n).
Both are 3D frameworks of Mn2+, T3DC2−, and coordinating
H2O. The bulk material, 3, crystallizes in the acentric
orthorhombic space group P212121. The asymmetric unit
shown in Figure 3 contains one Mn2+, one T3DC2−, and two
coordinating terminal water molecules. The carboxylate and
water oxygens form an octahedral coordination environment
Figure 2. Solid state structure of 2. (a) Asymmetric unit with n-hexyl
chains removed for clarity. (b) Extended structure of 2 viewed from
the bc-plane.
D
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Figure 3. Simplified coordination environments for the manganese trinuclear nodes of (a) 6-connected [Mn3(T2DC)3(DMF)4]n and (b) 4-
connected compound 2. Colored rods illustrate bithienyl linkers with rods of the same color going to the same trinuclear node. Net topology of (c)
[Mn3(T2DC)3(DMF)4]n and (d) 2.
around the manganese. The T3DC2− linker adopts a μ2-η1:η1
(Figure S1b) binding mode. The syn−anti carboxylate-bridged
Mn−Mn distances are 4.73(2) and 5.04(2) Å. Mn−O
carboxylate distances are 2.142(9)−2.192(8) Å, and Mn−O
water distances are 2.177(9) and 2.207(9) Å. The trans,cis
thiophene rings of 3 have only moderate coplanarity: the S1−
C5−C6−S2 torsion angle is 157.9(9)°, and the S2−C9−C10−
S3 torsion angle is 19.8(9)°. The extended structure in Figure
4b,c shows that the manganese centers form a 2D lattice that
are connected by T3DC2− linkers along the c-axis. When the
manganese centers are treated as nodes, the simplified
framework has a 4-connected lvt topology similar to that of 2.
Crystals of 4 were grown from the reaction mother liquor of
3. Compound 4 crystallizes in the monoclinic space group C2/c
and is a 3D framework (Figure 5). The asymmetric unit
consists of one T3DC2− linker, one Mn, one coordinating
terminal water molecule, and half of a bridging water molecule.
Figure 5a shows the asymmetric unit of 4. Two bis-
monodentate carboxylate oxygens (O1 and O2), a μ2:η2
carboxylate (O3 and O4), one terminal water, and one
bridging water coordinate to the manganese center in an
octahedral geometry. Figure S1a shows the T3DC2− coordina-
tion mode, and Figure 5c shows the simplified coordination
environment for the manganese center. The μ2-η2 Mn−O bond
lengths are slightly elongated (2.199(5)−2.225(5) Å) com-
pared to the bis-monodentate Mn−O bond (2.120(5)−
2.144(5) Å). Similarly, the terminal H2O Mn−O bond is
Figure 4. Structure of 3. (a) The asymmetric unit of 3. (b) View of the
2.157(5) Å compared to 2.262(4) Å for the bridging H2O Mn−
extended structure along the a-axis. (c) View of the carboxylate-
bridged 2D lattice of Mn2+ centers with terthiophene units removed
for clarity.
O bond. The manganese centers are rather close to each other
with separation distances of 3.379(7) and 3.731(7) Å.
The rings of the terthiophene linker of 4 are nearly coplanar.
The S1−C5−C6−S2 torsion angle is 179.2(4)°, and the S2−
C9−C10−S3 torsion angle is −178.8(4)° giving a trans,trans
conformation to the linker. The extended structure of 4 shows
that the T3DC2− linkers are stacked in parallel pairs and are
separated by 3.526(8) Å (S2−S2 contact) to 3.572(9) Å
(terminal ring centroid−centroid) (Figure 5b). These coplanar
pairs arrange in a herringbone orientation throughout the
coordination polymer. When the manganese center and
E
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Figure 5. (a) Asymmetric unit of 4. (b) The extended framework of 4. (c) The simplified coordination environment of the manganese center in 4.
(d) Simplified topology of 4. Gray nodes, Mn; pink nodes, terthiophene.
T3DC2− are treated as nodes, the simplified framework has a 4-
connected umc topology (Figure 5d) and is assigned the
monodentate carboxylate oxygens, 2.150(1)−2.163(1) Å for
the DMF oxygens, and 2.2339(9)−2.3322(9) Å for the
bidentate carboxylate oxygens. The manganese atoms in the
binuclear cluster are separated by 4.505(2) Å.
Schlafli symbol of 43.62.8.
̈
The stacking and orientations of 3, 4, and 2,2′:5′,2′′-
terthiophene (T3)31 differ from each other in the solid state.
The thiophene rings of T3 deviate 6−9° from planarity, and the
closest S−S contact is 3.70 Å. T3 molecules crystallize in a
herringbone orientation and do not engage in any close π−π
interaction. These new terthiophene dicarboxylate MOFs,
along with the copper−T3DC metal−organic polyhedron,
demonstrate the structural versatility of the T3DC2− linker. In
particular, linkers in coordination polymer 4 are more planar
and have closer S−S contacts than in the parent molecule T3.
β substitution of phenyl groups on the central thiophene ring
of the terthiophene linker was explored, resulting in the
synthesis of H2Ph2T3DC and subsequently [Mn(Ph2T3DC)-
(DMF)2]n (5). This coordination network crystallizes in the
The cis,trans thiophene rings of 5 are less coplanar than 3 and
4: the S1−C5−C6−S2 torsion angle is 38.49(6)°, and the S2−
C9−C10−S3 torsion angle is 150.35(7)°. On the basis of the
C1-central thiophene centroid-C14 angle, the linking angle of
Ph2T3DC2− deviates 40° from linearity. The twisted con-
formation of the linker in the solid state and consequential
dimensionality can be connected to the presence of the bulky
phenyl groups at the β position of the central thiophene ring.
Overall, compounds 1−5 demonstrate a sampling of the
crystallographic environments available to oligothiophenes
within a coordination polymer. Although the oligothiophene
ligands have similar linking angles, the presence of phenyl and
n-hexyl functional groups controls the dimensionality of the
extended structures. In addition, compounds containing
terthiophene derivatives are not mere extensions of the
bithiophene analogues. One striking difference between the
bithiophene compound [Mn3(T2DC)3(DMF)4]n and terthio-
phene compounds 3 and 4 is that the terthiophene compounds
form rather dense frameworks lacking noncoordinating solvent.
The propensity for linear trinuclear clusters to form for MOFs
containing bithiophene linkers ([Mn3(T2DC)3(DMF)4]n and
triclinic space group P1 and is composed of 1D chains. Figure 6
̅
shows the metal center coordination as well as the extended
structure of 5. The binding modes of 5 are depicted in Figure
S1e. The chains consist of alternating binuclear manganese
clusters and two Ph2T3DC2− linkers. Two bis-monodentate
carboxylate oxygens, two bidentate carboxylate oxygens, and
two DMF molecules adopt a distorted octahedral geometry
around each crystallographically equivalent manganese center.
The Mn−O distances are 2.1296(9)−2.1367(9) Å for the
F
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to the predicted diffraction patterns from the structure
determined by single crystal X-ray crystallography (Figure
S2). For compounds 1 and 3−5, the predicted and
experimental powder patterns of the bulk material match
well. Due to the small quantity of 4 that was available for
analysis, some peaks in the diffraction pattern of 4 are weaker
than expected but still present. The peaks of the experimental
diffraction pattern of 2 are broad compared to the predicted
pattern, and a loss of crystallinity is credited to structural
collapse upon desolvation of the bulk material.
Thermogravimetric analysis (TGA) was performed on 1−3
and 5 to determine the thermal stability and evaluate the
composition of these materials (Figure S3). A summary of the
analyses is given in Table S1. Noncoordinated solvent is lost
between 50 and 120 °C followed by the loss of coordinating
solvent near 200 °C. Combustion of organic material occurs
before 400 °C for one- and two-dimensional coordination
polymers 1 and 5 and by 450 °C for three-dimensional
frameworks 2 and 3. PXRD was used to determine the identity
of the decomposition products; a mixture of Mn3O4, MnOS,
and trace amounts of other compounds that could not be
identified were found.
Infrared spectroscopy was used to confirm the carboxylate
binding modes observed in 1−5.32 The results are summarized
in Table S2. In 1, the weak CO stretches anticipated for a
noncoordinated oxygen belonging to a η1 carboxylate are
overwhelmed by the CO stretch of DMF. The non-
coordinated oxygen belonging to the μ2:η2 carboxylate of 4
has distinct IR stretches at 1626 and 1641 cm−1. These
stretches are absent in 3.
Figure 6. Solid state structure of 5. (a) Asymmetric unit. (b) Binuclear
metal center.
2) versus mononuclear extended frameworks for terthiophene-
containing compounds (3 and 4) is still under investigation.
Powder X-ray Diffraction, Thermogravimetric Proper-
ties, and Infrared Spectroscopy. The phase purity of
compounds 1−5 was determined using powder X-ray
diffraction (PXRD). The diffraction patterns were compared
Magnetic Properties. Bulk magnetic susceptibilities of
polycrystalline samples of compounds 1−3 and 5 were
measured at 10 kOe in the temperature range 2−300 K.
□
○
Figure 7. χM vs T ( ); χMT vs T ( ) for 1−3 and 5. The solid lines show the best theoretical fit.
G
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Figure 8. Low temperature magnetic susceptibility data for 3: (a) χMT vs T and (b) χM vs T at various fields.
Compound 4 could not be synthesized in sufficient quantities
to perform magnetic susceptibility measurements. The results
of these experiments are shown in Figure 7.
down to 15 K, at which point χ−M1 deviates from linearity. The
antiferromagnetic behavior of 2 is demonstrated by a Curie
constant of C = 11.60 cm3 mol−1 and a Weiss constant of θ =
−17.00 K.
The large distances between trinuclear clusters (12.86(3) Å)
and the terminal manganese within a trinuclear cluster (7.43(2)
Å) are presumed to negate any significant magnetic coupling:
their interactions are assumed to be zero. Although the terminal
manganese atoms are in crystallographically unique locations,
the similar chemical environments and distances to the central
manganese atoms are sufficient grounds for treating the
terminal manganese as equivalent species. Given these
assumptions, the Hamiltonian for these trinuclear clusters is
given in eq 5:
Compound 1 consists of both isolated Mn2+ centers (Mn1)
and carboxylate-bridged 1D Mn2+ chains (Mn2). Antiferro-
magnetic behavior, shown in Figure 7a, was observed down to 2
K. At 300 K, the magnetic susceptibility is 3.91 cm3 K mol−1
which is less than the expected value of 4.37 cm3 K mol−1 for
one isolated Mn2+ center. A Curie constant of C = 4.18 cm3
mol−1 and a Weiss constant of θ = −8.57 K were found,
suggesting the material is antiferromagnetic.
The distance between the carboxylate-bridged Mn2−Mn2
centers of 1 is 4.52(1) Å, while the Mn1−Mn1 centers are well
separated by 9.04(2) Å. It can be assumed that there is
insignificant interchain interaction or exchange between the
Mn1 centers. The molar magnetic susceptibility of 1 can be
modeled as:
̂
̂
̂
̂
̂
H = −2J(S1·S2 + S2·S3)
(5)
The appropriate energy terms34 are inserted into the van Vleck
equation35 (eq 6):
1
2
χM
=
(χ
+ χparamagnet )
chain
(1)
N g2μ 2
3kBT
ΣS(S + 1)(2S + 1)exp(−E/kBT)
A
B
χM
=
·
where χchain describes the Mn2−Mn2 interaction via the Fisher
infinite chain model33 (eq 2):
Σ(2S + 1)exp(−E/kBT)
(6)
N g2μ 2S(S + 1)
where:
⎡
⎤
1 + U
1 − U
A
B
χ
=
⎢
⎣
⎥
⎦
chain
JS(S + 1)
3kBT
(2)
(3)
E =
2
where U is the Langevin function:
per mole of metal ions. Modeling of the magnetic susceptibility
data between 300 and 15 K gives values of g = 2.094(3) and J =
−2.88(3) cm−1 (Figure 7b). These values are similar to those
for other trinuclear manganese systems.36 Although 2 belongs
to an acentric space group, the magnetic behavior of the
material does not reflect this crystallographic assignment. The
lack of anisotropy or spin canting at low temperature can be
associated with the loss of acentricity upon removal from a
solvent-rich environment. Structural collapse and a loss of
crystallinity are observed in the bulk polycrystalline sample
(Figure S2b).
Compound 3 also crystallizes in an acentric space group and
shows a significant deviation from normal antiferromagnetic
behavior below 60 K. At 300 K, the experimental χMT value is
4.45 cm3 mol−1 K, which is slightly higher than expected for an
isolated Mn2+ center. A Curie constant of C = 5.18 cm3 mol−1
and Weiss constant of θ = −43.03 K were found between 60
and 300 K indicating strong antiferromagnetic interactions
within this temperature range (Figure 7c). Above 60 K, the
kBT
JS(S + 1)
U = coth
−
kBT
S(S + 1)
and χparamagnet (eq 4) accounts for the contribution of the
mononuclear Mn1 center:
NAg2μ2S(S + 1)
B
χparamagnet
=
3kBT
(4)
Applying this model to the experimental magnetic susceptibility
data between 300 and 10 K gives g = 1.983(2) and J =
−2.00(3) cm−1 which confirms the antiferromagnetic behavior
of 1.
At 300 K, the experimental χMT value for 2 is 10.64 cm3
mol−1 K which is below the value expected for three isolated
Mn2+ centers. (The molecular weight used to calculate
magnetic susceptibilities does not account for the non-
coordinated solvent that could not be resolved crystallo-
graphically.) Compound 2 adheres to the Curie−Weiss law
H
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Figure 9. FC and ZFC magnetization plots for 3: (a) χMT vs T and (b) χM vs T at 100 Oe.
2N g2μ 2
kBT
magnetic coupling within the 2D lattice can be modeled using
eq 7
ΣS(S + 1)(2S + 1)exp(−E/kBT)
Σ(2S + 1)exp(−E/kBT)
A
B
χM
=
·
(9)
NAg2μ 2S(S + 1)(1 + U)2
B
χM
=
Following the Hamiltonian for a homospin binuclear system,
least-squares analysis of the magnetic susceptibility data gave g
= 2.004(2) and J = −0.594(5) cm−1, showing that the Mn2+ are
weakly coupled through the carboxylate bridges of the binuclear
cluster.
3kBT(1 − U)2
(7)
where U is the previously defined Langevin function (eq 3).
Least-squares analysis of the magnetic susceptibility data gives g
= 2.135(6) and J = −1.94(3) cm−1 which confirms that the 2D
sheets of metal centers are coupled antiferromagnetically.
To help describe the change in magnetic behavior below 60
K, variable field magnetic susceptibilities were measured
(Figure 8). At lower fields, χMT increases slightly to a
maximum found at 32 K before decreasing quickly again.
This behavior is mostly saturated at 10 kOe. Below 25 K,
saturation effects or antiferromagnetic interactions take over,
and a minimum value of 0.50 cm3 K mol−1 is reached at 2 K.
This behavior is attributed to spin canting37 which is a common
phenomenon for materials without an inversion center.38
Figure S4 shows that no remnant magnetization is observed
at 2 K, and the M vs H curve does not increase rapidly at low
field, suggesting that no ferromagnetic ordering is present at 2
K. To further elucidate the identity of the magnetic transition,
zero-field cooled (ZFC) and field cooled (FC) magnetic
susceptibility data were collected at 100 Oe (Figure 9). While
the χMT versus T FC data show a prominent feature in χMT
20−40 K followed by a rapid decrease upon further cooling, the
ZFC cooled data show monotonically decreasing values of χMT
between 20 and 40 K before decreasing rapidly. Divergence of
the ZFC and FC data below Tc = 40 K demonstrates the
irreversibility of the magnetic transition originating from a
magnetically ordered canted state. Convergence of behavior
below 20 K suggests that antiferromagnetic interactions
dominate in this regime.
CONCLUSIONS
■
A series of new bithiophene and terthiophene manganese
metal−organic frameworks have been synthesized and charac-
terized. Compounds 3−5 are the first examples of crystallo-
graphically characterized terthiophene coordination polymers.
Compound 3 is found to be the major product of the reaction
of MnCl2·4H2O and H2T3DC under solvothermal conditions,
while 4 crystallizes from the reaction mother liquor. This work
has shown that alkyl chains cluster in the otherwise void space
of the MOFs. These interactions cause the thiophene rings to
twist to near perpendicular angles and help to shape the
topology of the extended structure. Similarly, phenyl groups do
not provide any driving force for forming a particular extended
structure and cause the linker to have the wrong geometry for
forming 3D frameworks. The magnetic behavior of the
manganese MOFs are sensitive to both the short-range and
extended structure. Compounds 1−3 and 5 exhibit anti-
ferromagnetic behavior and are well modeled by the given
equations that describe the coordination environments. Current
investigations are underway to elucidate the solid state
structures and photophysical properties of d10 Zn2+ terthio-
phene MOFs.
ASSOCIATED CONTENT
■
The plots of χM and χMT versus T for compound 5 are
shown in Figure 7d. Upon cooling from 300 K, χMT is 8.37 cm3
mol−1 K at 300 K, which is less than the expected value for two
isolated Mn2+ centers. The temperature dependence of χ−M1
obeys the Curie−Weiss law above 10 K with a Curie constant
of C = 8.61 cm3 mol−1 and a Weiss constant of θ = −5.14 K.
The Hamiltonian for a homospin binuclear system35 is given in
eq 8:
S
* Supporting Information
Crystallographic data, linker coordination modes, TGA traces
for 1−3 and 5, field dependence on the magnetization of 3,
selected tabulated IR data, and crystallographic data in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
̂
̂
̂
H = −JS1·S2
(8)
Corresponding Author
For a binuclear system, the van Vleck equation (eq 9) takes the
form of:
*E-mail: mwolf@chem.ubc.ca. Phone: (604) 822-1702. Fax:
(604) 822-2847.
I
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council (NSERC) of Canada for funding this research. L.D.E.
thanks D. Savard (Simon Fraser University) for assistance with
collecting the magnetic susceptibility data.
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