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

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.

analysis won't show peaks for intact compounds due to the lability of the carboxylic acid groups.Thus, only the decarboxylation fragments of compounds could be found.This being stated, characterization is still confident as compounds used for dicarboxylic acid synthesis are fully characterized, 13 C NMR spectra of 1-5 show the respective carboxylic acid carbon signals, 1 H NMR spectra don't show terminal acetylene signals, IR spectra show carboxylic acid functions and mass spectra show the decarboxylated fragments.

Synthesis of 3,3'-(2,3-difluoro-1,4-phenylene)dipropiolic acid (1)
2,3-Difluoro-1,4-diiodobenzene (7)   Under nitrogen atmosphere, a 2.5 M solution of n-butyllithium in n-hexane (24.0 mL, 60.0 mmol) was added to a solution of diisopropylamine (8.62 mL, 61.0 mmol) in anhydrous THF (100 mL) at −78 °C and stirred for 30 min.1,2-Difluorobenzene (6, 2.40 mL, 24.4 mmol) was then added at −78 °C and the reaction mixture was stirred for another hour.Subsequently, an ice-cooled solution of iodine (12.4 g, 48.7 mmol) in anhydrous THF (50 mL) was added, the cooling removed and the reaction mixture stirred for further 4 hours while warming up to room temperature.A saturated solution of sodium thiosulfate (100 mL) was added, the mixture was extracted with diethylether (2 × 100 mL) and the combined organic layers were washed with saturated sodium thiosulfate solution (2 × 100 mL).The organic layer was dried over magnesium sulfate, the solvent evaporated in vacuo and the crude product purified by S6 column chromatography using silica and n-pentane as eluent (Rf = 0.45) to obtain a colourless solid (6.80 g, 18.6 mmol, 76 %).Trimethylsilylacetylene (0.41 mL, 2.96 mmol) was added and the solution stirred at 70 °C for 16 h.Subsequently, the reaction mixture was filtered through a bed of Celite, the solvent was evaporated in vacuo and the crude product purified by column chromatography using silica and n-hexane as eluent (Rf = 0.44) to obtain a colourless solid (332 mg, 1.08 mmol, 92%).

IV. Crystallographic data for compound 12b
Table S1.Crystal data and structure refinement for herges129.

Identification code herges129
Empirical formula C30H42N4SSi2 Comments Data collection was performed using an IPDS-2 from STOE & CIE.A numerical absorption correction was performed (Tmin(max: 0.9228/0.9828).The structure was solved with SHELXT and structure refinement was done with SHELXL-2014.All non-hydrogen atoms were refined anisotropic.The C-H H atoms were positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and refined isotropic with Uiso(H) = 1.2 Ueq(C) (1.5 for methyl H atoms) using a riding model.Some of the isopropyl C atoms exhibits enlarged anisotropic displacement factors indicative for some static or dynamic disorder, which cannot be resolved.CCDC-1896031contain the supplementary crystallographic data for this paper.These data can be obtained free charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

13 C
Figure S8: 1 H NMR spectrum of compound 12a measured in deuterated chloroform.

13 C
Figure S12: 13 C NMR spectrum of compound 3 measured in deuterated acetone.

Figure S30 :
Figure S30: Crystal structure of 12b with labeling and displacement ellipsoids drawn at the 50% probability level.