Biphenyl and bimesityl tetrasulfonic acid – new linker molecules for coordination polymers

Biphenyl (H 4 BPTS) and bimesityl (H 4 BMTS) 3,3',5,5'-tetrasulfonic acids were prepared by Ullmann type coupling reactions of respective cuprates. The four sulfo groups were installed by nucleophilic displacement at respective tetrabromo biaryl derivatives with sodium isopropyl thiolate. The resulting thioether moieties were cleaved and subsequently oxidized to furnish the title compounds. The new acids H 4 BPTS and H 4 BMTS were used for the preparation of their respective copper salts which allowed for the detailed structural inspection of the tetrasulfonate anions by X-ray methods.


Introduction
2][3] Depending on the nature of the linker the connection may occur in one, two or three dimensions.The compounds with three-dimensional linkage often show high porosity and are then called metal organic frameworks (MOFs).][6][7][8][9][10][11][12] In most cases CPs and MOFs have been prepared using oligocarboxylic acids as linker molecules.As an example of a tetracarboxylic acid, pyromellitic acid (1, Figure 1) had been utilized as a linker with D 2h symmetry, e.g. for the preparation of a material called MIL-121. 13An constitutionally extended example of a compound with this symmetry is 3,3',5,5'-biphenyltetracarboxylic acid (3), where both phenyl rings are coplanar.Its copper salt was reported as so-called MOF-505. 14Introduction of four ortho-methyl groups  4) and H 4 BMTS (6).
In order to extend the structural diversity of coordination polymers, we have initiated a research program on the preparation of new di-and oligosulfonic acids, [15][16][17][18][19] which possess promising and advantageous properties when compared to carboxylic acids, for example, stronger acidity and enhanced thermal stability.In this course we had already reported on the use of benzene tetrasulfonic acid 2; in the solid state, its copper(II) salt formed ladder shaped chains. 20We now wish to extend this structural motif by preparation of biphenyl tetrasulfonic acid 4 (H 4 BPTS) with D 2h symmetry and the respective tilted bimesityl congener H 4 BMTS (6) (C 2v ).

Results and Discussion
The synthesis of D 2h -symmetric tetrasulfonic acid 4 used tetrabromobiphenyl 8 as starting material, which was actually reported before in the literature; 21,22 we however failed to reproduce the procedures used therein.Nevertheless, we could access compound 8 from tribromobenzene 7 via its Lipshutz cuprate 23 (as reported for the respective bimesityl, 24 see below), but we used nitrobenzene as oxidizing reagent 25 instead of benzoquinone.We then proceeded with the formation of tetrathiol 9 as developed before in our laboratories: nucleophilic displacement of bromine by isopropyl thioether functions was followed by reductive cleavage of isopropyl groups.Compound 9 was then oxidized by hydrogen peroxide to yield the tetrasulfonic acid 4 as its tetrahydrate (H  For the synthesis of tetrasulfonic acid 6 we first tried the homocoupling of tribromomesitylene, but we failed.Therefore, we first prepared literature known bimesitylene 26,27 11 from iodomesitylene 28 10 by the above approved protocol.The subsequent tetrabromination proceeded straightforward.Nucleophilic displacement of compound 13 with iPrSNa followed by reductive cleavage with sodium could not be performed in a sequential one flask protocol.We therefore isolated the tetrathioether 12 and degraded the isopropyl groups oxidatively with in situ formed chlorine furnishing the tetrasulfonylchloride 14.Hydrolysis of the latter material with hot water gave the final product 6 again as the tetrahydrate (H 4 BMTS • 4 H 2 O).
The molecular structure of both tetrasulfonic acids H 4 BPTS (4) and H 4 BMTS (6) was established by X-ray crystallography of their respective copper(II) salts, which were obtained from neutralization of aqueous solutions of the respective acids with malachite followed by slow evaporation. In

Conclusions
Two new aromatic tetrasulfonic acids with a biphenyl and bimesityl backbone, H 4 BPTS ( 4) and H 4 BMTS (6), were prepared from tetrabromobiphenyl 8 and -bimesityl 13 in two or three steps, respectively.The key transformations were Ullman-type coupling reaction to achieve the biaryl motif, nucleophilic displacement reactions with sulfur nucleophiles and oxidation to achieve the four sulfo-residues.X-ray crystallographic structure analyses of the copper salts {[Cu(H 2 O) 5 ] 2 (BPTS)]} 15 and {[Cu(H 2 O) 6 ] 2 (BMTS)} 16 revealed the planar (D 2h ) and tilted (C 2v ) symmetries of the two biaryl backbones.Both tetrasulfonic acids will now be applied in our group as linker molecules for the preparation of coordination polymers and metal organic frameworks.

Experimental Section
General.Preparative column chromatography was carried out using Merck SiO 2 (35-70 µm, type 60 A) with n-hexane, tert-butyl methyl ether (MTBE), and CH 2 Cl 2 as TLC was performed on Merck aluminium plates coated with SiO 2 F 254 . 1 H and 13 C NMR spectra were recorded on Bruker Avance ARX 300 and DRX 500 instruments at 23 °C in CDCl 3 or D 2 O. Multiplicities of carbon signals were determined with DEPT experiments.MS and HRMS spectra were obtained with a Finnigan MAT95 (EI) and a Waters Q-TOF Premier (ESI) spectrometer.IR spectra were recorded on a Bruker Tensor 27 spectrometer equipped with a "GoldenGate" diamond ATR unit.Elemental analyses were measured with a Euro EA-CHNS instrument from HEKAtech.The X-ray single crystal investigations were performed with CCD diffractometer (κ-APEX II, Bruker, Karlsruhe, Germany).Suitable crystals were mounted onto a glass needle (Ø 0.1 mm) and placed into a stream of cold N 2 (-153 °C) inside the diffractometer.An absorption correction was applied to the data using the program SADABS-2012/1. 29,303',5,5'-Tetrabromobiphenyl (8).n-Butyllithium (2.5 mL of a 2.5 mol/l solution in n-hexane, 6.3 mmol) was added to a solution of 1,3,5-tribromobenzene (7) (2.00 g, 6.35 mmol) in Et 2 O (40 mL) at -78 °C under inert atmosphere.After 10 min, CuCN (282 mg, 3.18 mmol) was added and the reaction mixture was allowed to warm up to -60 °C within 90 min.Nitrobenzene (0.98 mL, 9.53 mmol) was then added in one portion and the dark solution stirred while warming to ambient temperature (16 h).An aqueous solution of NH 3 (12.5% w/w, 80 mL) was slowly added and the resulting mixture was extracted with MTBE (3 x 50 mL).The combined organic layers were washed with water (50 mL) and brine (50 mL), dried over MgSO 4 , filtered and the solvent was removed in vacuum.Further purification was accomplished by column chromatography (SiO 2 , n-hexane, R f 0.69).The resulting material was washed with acetone (20 mL) to give the product 8 as a colorless solid (722 mg, 1.54 mmol, 48%), mp 190 °C (ref.22 190-191 °C). 1 H NMR (500 MHz, CDCl 3 ): δ 7.70 (t, J 1.7 Hz, 2 H), 7.59 (d, J 1.7 Hz, 4 H) ppm.13  Biphenyl-3,3',5,5'-tetrathiol (9).Sodium 2-propanethiolate (2.76 g, 28.1 mmol) was added to a solution of compound 8 (1.32 g, 2.81 mmol) in DMA (15 mL) under inert atmosphere and the resulting suspension was stirred at 100 °C for 20 h.Sodium (969 mg, 42.1 mmol) was added at the same temperature and the reaction mixture was allowed to stir for 6 h.The conversion was monitored by GC.In case of incomplete conversion, further sodium was added.After full conversion had been achieved, water (10 mL) was slowly and carefully added and the solution was acidified by conc.hydrochloric acid until pH 1 (ca. 3 mL).More water (50 mL) was added and the resulting suspension was extracted with MTBE (3 x 50 mL).The combined organic layers were washed with hydrochloric acid (50 mL, 1 mol/l), water (50 mL) and brine (50 mL), dried over MgSO 4 and filtered.After removal of the solvent in vacuum the residue was washed with hot methanol (100 mL) and the precipitate filtered off to yield the title compound 9 as a slightly yellow solid (466 mg, 1.65 mmol, 59%), mp 130 °C. 1 H NMR (500 MHz, CDCl 3 ): δ 7.18 (s, 6 H), 3.50 (s, 4 H) ppm.13   (30 mL) and MeOH (6 mL).The reaction mixture was allowed to stir at room temperature for 20 h.The volatiles were evaporated and the residue redissolved in water (30 mL) and treated again with hydrogen peroxide (30% solution in H 2 O, 2.17 mL, 21.2 mmol).
After stirring for another 20 h, all volatiles were removed in vacuum to yield the product 4 as a hygroscopic colorless solid (554 mg, 1.01 mmol, 95%), decomp.150-160 °C. 1  (2.9 mL) was added to a suspension of bimesitylene (11) (140 mg, 587 µmol) and iron (9.8 mg, 176 µmol) in CHCl 3 (3.0mL) at ambient temperature.The reaction mixture was allowed to stir for 16 h.Water (20 mL) was then added and the aqueous layer was extracted with CH 2 Cl 2 (3 x 20 mL).The combined organic layers were washed with a saturated solution of Na 2 S 2 O 3 (20 mL) and water (20 mL), dried over MgSO 4 , filtered and the solvent was removed in vacuum.The crude product could be purified by washing with hot acetone (10 mL) to yield the title compound 13 as a colorless solid (290 mg, 524 µmol, 89%), mp 236 °C. 1