Synthesis of dipolar molecular rotors as linkers for metal-organic frameworks

Article abstract of DOI:10.3762/bjoc.15.132

We report the synthesis of five dicarboxylic acid-substituted dipolar molecular rotors for the use as linker molecules in metalorganic frameworks (MOFs). The rotor molecules exhibit very low rotational barriers and decent to very high permanent, charge free dipole moments, as shown by density functional theory calculations on the isolated molecules. Four rotors are fluorescent in the visible region. The linker designs are based on push-pull-substituted phenylene cores with ethynyl spacers as rotational axes, functionalized with carboxylic acid groups for implementation in MOFs. The substituents at the phenylene core are chosen to be small to leave rotational freedom in solids with confined free volumes. The dipole moments are generated by electron-donating substituents (benzo-1,3-dioxole, benzo-1,4-dioxane, or benzo-2,1,3-thiadiazole annelation) and withdrawing substituents (difluoro, or dicyano substitution) at the opposite positions of the central phenylene core. A combination of 1,4-dioxane annelation and dicyano substitution generates a theoretically predicted, very high dipole moment of 10.1 Debye. Moreover, the molecules are sufficiently small to fit into cavities of 10 ?³. Hence, the dipolar rotors should be ideally suited as linkers in MOFs with potential applications as ferroelectric materials and for optical signal processing.

Full text of DOI:10.3762/bjoc.15.132

Synthesis of dipolar molecular rotors as linkers for
metal-organic frameworks
Sebastian Hamer1, Fynn Röhricht1, Marius Jakoby2, Ian A. Howard2, Xianghui Zhang3,
Christian Näther4 and Rainer Herges*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 1331–1338.
1Otto-Diels-Institut für Organische Chemie, Kiel University,
Otto-Hahn-Platz 4, D-24118 Kiel, Germany, 2Institute of
Microstructure Technology, Karlsruhe Institute of Technology,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany, 3Fakultät für Physik, Universität Bielefeld,Universitätsstr.
25, D-33615 Bielefeld, Germany and 4Institut für Anorganische
Chemie, Kiel University, Max-Eyth-Str. 2, D-24118 Kiel, Germany,
doi:10.3762/bjoc.15.132
Received: 07 February 2019
Accepted: 31 May 2019
Published: 18 June 2019
Associate Editor: T. J. J. Müller
Email:
© 2019 Hamer et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Rainer Herges* - rherges@oc.uni-kiel.de
* Corresponding author
Keywords:
benzothiadiazole; dipolar rotor; fluorescence; large dipole moment;
metal organic framework linker
Abstract
We report the synthesis of five dicarboxylic acid-substituted dipolar molecular rotors for the use as linker molecules in metal-
organic frameworks (MOFs). The rotor molecules exhibit very low rotational barriers and decent to very high permanent, charge
free dipole moments, as shown by density functional theory calculations on the isolated molecules. Four rotors are fluorescent in
the visible region. The linker designs are based on push–pull-substituted phenylene cores with ethynyl spacers as rotational axes,
functionalized with carboxylic acid groups for implementation in MOFs. The substituents at the phenylene core are chosen to be
small to leave rotational freedom in solids with confined free volumes. The dipole moments are generated by electron-donating
substituents (benzo-1,3-dioxole, benzo-1,4-dioxane, or benzo-2,1,3-thiadiazole annelation) and withdrawing substituents (difluoro,
or dicyano substitution) at the opposite positions of the central phenylene core. A combination of 1,4-dioxane annelation and
dicyano substitution generates a theoretically predicted, very high dipole moment of 10.1 Debye. Moreover, the molecules are
sufficiently small to fit into cavities of 10 Å3. Hence, the dipolar rotors should be ideally suited as linkers in MOFs with potential
applications as ferroelectric materials and for optical signal processing.
Introduction
Rotors are among the fundamental functional units in engi- basic building blocks in molecular machines and other molecu-
neering in our macroscopic world, as well as at the molecular lar architectures [1,2]. Equally interesting is the collective be-
level. Molecular rotors have been thoroughly investigated as haviour of ensembles of molecular rotors in two and three
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Beilstein J. Org. Chem. 2019, 15, 1331–1338.
dimensions, i.e., on surfaces and in materials. Different strate- would be far beyond the scope of this paper. Figure 1 merely
gies have been employed to prepare ordered arrays of rotors in gives an approximate idea how large pairwise dipolar interac-
two dimensions, such as the inclusion of rotors in channels on tions could be. However, we dare to draw the conclusion that
the surface of particular crystals [3] on metal surfaces [4-6] or building a ferroelectric MOF at room temperature might not be
in Langmuir–Blodgett films [7]. Several strategies have also a completely unrealistic endeavour.
been pursued to implement molecular rotors in the solid state.
Crystals of linear molecular rotors [8-13], caged rotor crystals Here we report on the synthesis of five different dipolar rotors
and gyroscope like molecules [14-20] as well as organosilicates (Figure 2) that are designed to meet the criteria 1–4 listed
[21] and metal-organic frameworks [22-25] containing molecu- above, for the use as building blocks in the construction of
lar rotors have been synthesized by several groups.
functional MOFs.
Results and Discussion
Molecular rotors with permanent dipole moments can be
oriented by an external electric field as shown by Michl [26,27]
and Price [28,29], or undergo spontaneous ordering by intermo-
lecular dipole–dipole interactions. The ultimate goal is the fab-
rication of an array of dipolar rotors with a ferroelectric ground
state and a Curie temperature above room temperature [30].
Such materials would have a number of exciting optical and
electronic properties and applications such as signal processing
and imaging. Phase transition from a stochastic to an ordered
state with aligned dipoles depends on the arrangement of the
rotors and the strength of their interactions. MOFs [31,32] und
particular SURMOFs [33,34] are ideally suited to achieve an
ordered 3D arrangement and to maximize intermolecular inter-
actions, because the dipolar rotors are used as functional units
as well as building blocks for construction of the lattice (linker).
Towards this end, the dipolar rotors have to meet several
preconditions: 1. the dipole moments should be strong; 2. the
rotors should be small; 3. the barriers to rotation should be
small (<3 kcal mol−1); 4. the chemistry of the dipolar rotors
must be compatible with MOF growth. Preconditions 1. and 2.
are somewhat contradicting each other. In the most simple treat-
ment, the dipole moment is proportional to the point charges
times their distance. However, in a real chemical situation the
distance between the charges cannot be arbitrarily increased
because with increasing lattice parameters controlled MOF
growth becomes increasingly difficult (e.g., interpenetration).
Moreover, large cavities in solids are usually difficult to keep
free from solvents or other impurities that would hamper free
rotation. Hence, a compromise between the size and dipole
moment of the rotor/linker has to be aimed at, and care has to
be taken that the rotational barriers are as low as possible.
Linker design and quantum chemical
calculations
Aiming at high dipole moments our design was inspired by
recent reports of Müllen et al. who reported on very high dipole
moments of 1,2-dicyano-4,5-diamino-substituted phenyl deriva-
tives [35]. Unfortunately, amino substituents are not compati-
ble with MOF growth. Cyano substituents could eventually also
interfere with the coordination chemistry of MOF formation.
We therefore considered benzoannelation with 1,3-dioxole (4)
and 1,4-dioxane (5) units as electron-donating substituents
instead of amino units and substitution with fluorine (1 and 2)
to replace the cyano units. Almost free rotation is provided by
ethynyl units as rotary joints. Carboxylate groups are termi-
nating the axes on both sides because they are known to form
the typical paddle wheel structures in MOFs [36]. Benzo-
annelated 2,1,3-thiadiazoles were introduced (2 and 3) to shift
absorption and fluorescence into the visible region and to
implement interesting optoelectronic properties into the final
MOF structures [37-41]. Prior to synthesis, we calculated the
dipole moments and barriers to rotation of the isolated rotors
1–5 (Table 1 and Figure 1). The rotational barriers of all rotors
are below 2 kcal mol−1. We interpret these data as the lower
limits of the rotational barriers in a MOF environment. Interac-
tions with other linkers or neighbouring dipoles probably will
increase the barriers. The dipole moments of the cyano-substi-
tuted rotors are considerably higher than those with fluoride
substitution. The annelated thiadiazole ring is rather electron
withdrawing and reduces the dipole moment in combination
with fluoride substitution to almost zero. Compound 5 in rela-
tion to its size has an exceptionally high dipole moment of
10.1 D; thus being in the same range as 1,2-dicyano-4,5-
diaminobenzene of Müllen et al. [32] without compromising
MOF compatibility by 4,5-diamino substitution.
Figure 1 shows the interaction of a pair of dipolar rotors with
parallel axes at a distance of 10 Å, which is a typical distance of
the linkers in a MOF. Five different orientations in C2v, C2h and
D2h symmetry are considered. The energy difference between Synthesis of dipolar molecular rotor linkers
the most favourable and the most unfavourable orientation is The key step for the synthesis of all reported linkers is the cou-
9.7 kcal mol−1. It is important to note that the energy states of pling of a substituted aromatic core unit with two ethynyl sub-
3D ensembles of dipoles assuming periodic boundary condi- stituents in para-orientation as spacer units and axis of rotation
tions are extremely difficult to predict. Calculations of this type (Scheme 1). The aromatic core carries the dipole-generating
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Figure 1: Interactions of a pair of dipolar rotors in different orientations. The axes of the rotors are parallel at a distance of 10 Å (terminal H atoms at
the ethynyl units are in a plane and at an intermolecular distance of 10 Å), which is a typical distance in MOF lattices. The structures are fully opti-
mized at the PBE-D3/defSVP level of theory within the point groups C2v, C2h and D2h. Relative energies (Erel) are given in kcal mol−1 and in meV (in
brackets). The arrows indicate the orientation of the dipoles. The calculated dipole moment µ of the rotor (chemical structure on top left) is 8.6 D.
Figure 2: Structures of molecular dipolar rotors/linker molecules 1–5.
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Beilstein J. Org. Chem. 2019, 15, 1331–1338.
Difluoro compound 1
Table 1: DFT calculated dipole moments and rotational barriers of the
dipolar rotors 1–5. Geometry optimizations (symmetry 1–4: C2v; 5: C2)
were performed with B3LYP/aug-cc-pVTZ level of theory and subse-
quent Mullikan dipole moment analysis (analogue to Müllen et al. [32]).
For an appraisal of rotational barriers rotational scans were performed
at the PBE/def2SVP level of density functional theory. For this evalua-
tion both carboxylic groups where fixed in a plane. In all cases, barriers
for rotations are below 2 kcal mol−1. In the absence of intermolecular
interactions there should be thermally excited rotation at room temper-
ature and down to very low temperatures.
Difluorobenzenes have been employed as dipolar rotors by
Garcia-Garibay in crystals [10], as an elongated MOF linker by
Blight and Forgan [25] and for the investigation of rotor dy-
namics and dipole interactions in crystals by Price [28]. The
predicted dipole moment of 1 with 2.6 D is small, but the mole-
cule should be suitable for MOF preparation because the struc-
turally similar parent compound 1,4-benzenedipropynoic acid
has been successfully used for MOF synthesis [43].
Linker
µcalc. [D]
ΔEmax. calc. [kcal mol−1]
The synthesis of linker 1 was straightforward (Scheme 2). The
core unit was synthesized by simple di-iodination of commer-
cially available 1,2-difluorobenzene (6). The lithiation and
subsequent metal iodine exchange has already been described in
the literature [44-46]. While in known procedures 7 is obtained
over two mono-iodination steps, we report here the di-iodina-
tion in a single step. Subsequently, 7 was reacted in a Sono-
1
2
3
4
5
2.6
0.7
6.2
8.6
10.1
1.8
1.2
0.9
1.7
1.6
substituents as well as two halogen atoms for cross coupling. gashira cross coupling with trimethylsilylacetylene to give 1,4-
For all syntheses, either terminal trimethylsilylacetylene or bis(2-trimethylsilylethynyl)-2,3-difluorobenzene (8). Finally, 8
1-trimethylsilyl-2-tributylstannylacetylene was employed for was converted to the dicarboxylic acid 1 using cesium fluoride
the coupling step followed by subsequent direct conversion of under a carbon dioxide atmosphere.
the TMS-protected acetylenes to the dicarboxylic acids using a
Difluoro- and dicyanobenzothiadiazole
method of Kondo and co-workers [42]. Following this proce-
dure, purification of TMS-protected compounds was more con- compounds 2 and 3
venient as compared to the tedious work-up with carboxylic Substituted benzothiadiazole derivatives are well studied in the
acid derivatives directly obtained from cross coupling. Using literature and are of great interest in organic photovoltaics and
this method, dicarboxylic acids could be obtained from simple electronics. As there are no reports of such building blocks in
aqueous work-up and extraction.
MOF systems yet, both derivatives are promising starting mate-
Scheme 1: General synthetic strategy to prepare the dipolar rotors 1–5.
Scheme 2: Synthesis of 3,3'-(2,3-difluoro-1,4-phenylene)dipropiolic acid (1) starting with diiodination of 1,2-difluorobenzene (6), followed by Sono-
gashira reaction with trimethylsilylacetylene and subsequent direct conversion to the dicarboxylic acid.
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Beilstein J. Org. Chem. 2019, 15, 1331–1338.
rials for the preparation of functional materials based on MOF reaction times and metal catalysis in Sonogashira reactions [53].
structures.
In the reported procedure, a bulky substituted acetylene deriva-
tive was used, probably supressing the formation of the phthalo-
Annelated benzothiadiazole linkers could also be obtained cyanine byproduct to some extent. Indeed, 5,6-dicyano-4,7-bis-
following the general synthetic approach as shown in Scheme 3. (2-trimethylsilylethynyl)-2,1,3-benzothiadiazole (12a) could be
Synthesis of the difluoro derivative 2 starts from the known synthesized by Sonogashira coupling in a low yield of 10%
compound 4,7-dibromo-5,6-difluoro-2,1,3-benzothiadiazole (9) alongside with phthalocyanine byproducts. The triisopropyl-
which can be obtained in a four to five step reaction pathway silyl-protected derivative 12b could be obtained in a slightly
from commercial compounds as described several times in the higher yield of 21%, presumably due to the bulkier TIPS-
literature [47-50]. 5,6-Difluoro-4,7-bis(2-trimethylsilylethynyl)- groups. A crystal structure of this compound was obtained (see
2,1,3-benzothiadiazole (10) could be obtained in a Sonogashira Supporting Information File 2). To increase the yield and avoid
reaction with trimethylsilylacetylene. Conversion to the dicar- these side reactions, we reacted diiodo compound 11b with
boxylic acid 2 was achieved using cesium fluoride under a car- 1-trimethylsilyl-2-tributylstannylacetylene in a Stille cross cou-
bon dioxide atmosphere.
pling, where no formation of phthalocyanine or similar side
products was observed. Changing the catalyst from tetrakis(tri-
Not quite as straightforward was the synthesis of the dicyano phenylphosphine)palladium(0) to bis(tri-tert-butylphos-
derivative 3. While the synthesis of 5,6-dicyano-4,7-diiodo- phine)palladium(0) the yield increased to satisfying 90%. For
2,1,3-benzothiadiazole (11b) was reported by Blakey, Marder conversion into the dicarboxylic acid 3, another change of the
and co-workers [51], no cross-coupling reactions using this de- general procedure was necessary. Probably because of the
rivative have been reported yet. Recently, the synthesis of 4,7- extremely electron deficient aromatic system in 12a, desilyl-
dibromo-5,6-dicyano-2,1,3-benzothiadiazole (11a) was also re- ation at room temperature led to polymerisation of the
ported alongside with its use in a Sonogashira reaction [52]. acetylides. No deprotected terminal acetylene was observed. To
The cross coupling following a Sonogashira protocol was prob- avoid polymerization, the solvent was changed to acetonitrile
lematic though in our case. Phthalonitrile (1,2-dicyanobenzene) and the temperature was lowered to 0 °C. Additionally, carbon
units are known to form phthalocyanines and homologues, dioxide was not just used as the reaction atmosphere, but
especially under harsher reaction conditions, such as the long directly bubbled through the solution. Following this procedure,
Scheme 3: Synthesis of 3,3'-(5,6-Difluoro-2,1,3-benzothiadiazol-4,7-diyl)dipropiolic acid (2) and 3,3'-(5,6-Dicyano-2,1,3-benzothiadiazol-4,7-
diyl)dipropiolic acid (3) as well as their silyl protected intermediates.
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3,3'-(5,6-dicyano-2,1,3-benzothiadiazole-4,7-diyl)dipropiolic reaction using copper(I) cyanide was reported before for the
acid (3) was obtained in a satisfying yield of 90%.
preparation of benzodioxole derivative 13a [57,58], but gave far
inferior yields and was hampered by a tedious work-up. The
dicyano compounds 5,6-dicyano-1,3-benzodioxole (14a) and
6,7-dicyano-1,4-benzodioxane (14b) were dibrominated after a
Dicyanobenzodioxole and -benzodioxane
compounds 4 and 5
With the two dicyanodioxoalkylene derivatives 4 and 5, we protocol using dibromoisocyanuric acid in fuming sulfuric acid
aimed at particularly high dipole moments. Cyano groups are [59]. Subsequent Stille cross coupling (to avoid phthalocyanine
known to exhibit the highest electron-withdrawing effect among formation) using conditions established in the synthesis of
the uncharged small functional groups [54]. As electron-donat- linker 3 gave the bis(trimethylsilylethynyl) compounds 15a and
ing substitutents, amino groups exhibit the strongest effects, 15b in nearly quantitative yields. Analogously to the above-de-
especially if tris-alkyl substituted. However, we chose alkoxy scribed systems, conversion into the respective dicarboxylic
substituents for our design, because phenylenediamine (1,2- acids 4 and 5 took place by reaction with cesium fluoride under
diaminobenzene) units are strongly coordinating ligands which a carbon dioxide atmosphere.
interfere with MOF formation. Moreover, alkoxy substituents
provide several advantages in synthesis. While methoxy substit- Conclusion
uents are known and common donors, calculations showed that In summary, we reported here the synthesis of five dipolar
bridged alkoxy substituents such as the benzodioxole and rotors consisting of a push–pull-substituted phenylene core,
benzodioxane groups show an even stronger electron-donating with two ethynyl units in para-position as the rotary axis and
effect than two methoxy substituents while simultaneously two dicarboxylic acids for the use as MOF linkers. Linkers 2
being less sterically demanding as rotors in coordination and 3 contain substituted 2,1,3-benzothiadiazole rotor units,
networks. According to our calculations dioxane derivative 5 with absorption and emission wavelengths in the visible region
exhibits a dipole moment exceeding 10 Debye, which is, to the (see Supporting Information File 1). The dipolar rotors 4 and 5
best of our knowledge, the highest dipole moment reported for exhibit the largest dipole moments known for MOF linkers so
molecules that are suitable as MOF linkers.
far, with 5 even exceeding 10 Debye. Cross-coupling reactions,
particularly Sonogashira reactions of dicyanobenzenes are
Both dipolar rotor units were synthesized according to a proce- known to be accompanied by phthalocyanine formation and
dure outlined in Scheme 4. While the dioxane compound 5 ex- other side products. These problems are avoided by applying
hibits a higher dipole moment, the dioxole compound 4 was ob- Stille conditions using 1-trimethylsilyl-2-tributylstannylacety-
tained in higher yields. Starting from commercially available lene in combination with the bulky palladium catalyst bis(tri-
5,6-dibromo-1,3-benzodioxole (13a) and literature known 6,7- tert-butylphosphino)palladium. The TMS-protected, dicyano-
dibromo-1,4-benzodioxane (13b) [55], dicyanation of the substituted rotors with thiadiazole, dioxole and dioxane annela-
dibromo compounds was achieved with zinc(II) cyanide under tion 12a, and 16 were prepared in yields exceeding 90%. Thus,
palladium catalysis [56]. The typical Rosemund–von-Braun the dipolar rotors 1–5 are now available in gram amounts for
Scheme 4: Synthesis of 3,3'-(5,6-dicyano-1,3-benzodioxole-4,7-diyl)dipropiolic acid (4) and 3,3'-(6,7-dicyano-1,4-benzodioxole-5,8-diyl)dipropiolic
acid (5). DBI = Dibromoisocyanuric acid.
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Supporting Information File 1
Experimental procedures, 1H and 13C NMR spectra of new
compounds as well as UV–vis and fluorescence spectra of
compounds 1–5 and crystallographic data for compound
12b.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-132-S1.pdf]
Supporting Information File 2
cif file for compound 12b.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-132-S2.cif]
Supporting Information File 3
checkcif report for compound 12b.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-132-S3.pdf]
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Acknowledgements
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We are grateful for support by the Deutsche Forschungs-
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ORCID® iDs
Sebastian Hamer - https://orcid.org/0000-0003-0801-8558
Fynn Röhricht - https://orcid.org/0000-0001-9935-9256
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