Discrete multiporphyrin pseudorotaxane assemblies from di- and tetravalent porphyrin building blocks

Summary Two pairs of divalent and tetravalent porphyrin building blocks carrying the complementary supramolecular crown ether/secondary ammonium ion binding motif have been synthesized and their derived pseudorotaxanes have been studied by a combination of NMR spectroscopy in solution and ESI mass spectrometry in the gas phase. By simple mixing of the components the formation of discrete dimeric and trimeric (metallo)porphyrin complexes predominates, in accordance to binding stoichiometry, while the amount of alternative structures can be neglected. Our results illustrate the power of multivalency to program the multicomponent self-assembly of specific entities into discrete functional nanostructures.

S4 organic phases were dried over MgSO 4 and evaporated to give a crude solid.

Benzyl-(4-bromobenzyl)amine (10)
Benzylamine (1.158 g, 10.81 mmol, 1 equiv) was added to a solution of 4-bromobenzaldehyde (9, 2.000 g, 10.81 mmol, 1 equiv) in 15 mL of trimethyl orthoformate and stirred overnight. The solvent was removed under reduced pressure to obtain the S7 desired product in decent purity which was used without further purification (2.91 g, 10.80 mmol, 98%). NaBH 4 (0.817 g, 21.59 mmol, 2 equiv) were added to an icecooled solution of the imine (2.960 g, 10.80 mmol, 1 equiv) in a 1:1 mixture of THF/MeOH (30 mL each). The mixture was allowed to warm to room temperature and stirred for 2 h. The volatiles were removed under reduced pressure and the crude product was dissolved in ethyl acetate. The slurry mixture was filtered through a celite plug using ethyl acetate as the solvent. Pure product 10 was obtained after removal of the solvent under reduced pressure (2.910 g, 10.79 mmol, 98%).
After all of the starting material was consumed (TLC, ca 16 h) the solvent was removed under reduced pressure and excess (Boc) 2 O was removed by bulb-to-bulb distillation. The crude product was purified by column chromatography (SiO 2 , petroleum ether/EtOAc 99:1  4:1) affording pure 11 as a colorless oil (2.200 g, 5.87 mmol, 60%).
Afterwards, Pd(PPh 3 ) 4 (31 mg, 0.03 mmol, 0.1 equiv) was added and the solution was degassed again (3 times). After the addition of ethynyltrimethylsilane (0.08 mL, 0.53 mmol, 2 equiv) the solution was stirred at 80 °C for 16 h. When all starting material was consumed (TLC) the mixture was filtered through a plug of celite with ethyl acetate as the eluting solvent. The resulting solution was concentrated under reduced pressure and purified by column chromatography (SiO 2 , petroleum ether/EtOAc 15:1  9:1) to yield 12 as a colourless solid (0.100 g, 0.26 mmol, 96%).
Afterwards the solvent was removed under reduced pressure and the residue was dissolved in CH 2 Cl 2 , washed sequentially with 1 M HCl and brine, dried over MgSO 4 and concentrated under reduced pressure. The crude product was purified by column chromatography (SiO 2, petroleum ether:EtOAc 9:1) to give an yellowish oil that solidifies upon standing (65 mg, 0.23 mmol, 90%).
Afterwards, Pd(PPh 3 ) 4 (11 mg, 0.09 mmol, 0.2 equiv) was added and the solution was degassed again (3 times). After the addition of 8 (75 mg, 0.23 mmol, 5 equiv), the solution was stirred at 80 °C for 3 d under a nitrogen atmosphere. When all starting material was consumed (TLC), the mixture was filtered through a plug of celite with CH 2 Cl 2 as the solvent. The resulting solution was concentrated under reduced pressure and purified by column chromatography (SiO 2 , CH 2 Cl 2 ). The product was precipitated from CH 2 Cl 2 /MeOH by slowly removing CH 2

Target bisammonium porphyrin salt (A2)
Trifluoroacetic acid (3 mL) was added to a stirring solution of 13 (150 mg, 0.10 mmol, 1 equiv) in CH 2 Cl 2 and the resulting solution was stirred for 3 d. When no starting material was left (TLC), the solvent was removed under reduced pressure until dryness, the residue was dissolved in CH 2 Cl 2 and washed with 1 M NaOH solution.
The combined organic phases were dried over MgSO 4 and concentrated under reduced pressure to obtain 14 as a purple solid (120 mg, 0.10 mmol, 97%) which was used without further purification. To a solution of 14 (120 mg, 0.10 mmol, 1 equiv) in 20 mL chloroform/methanol 1:1 was added 3 mL of 1 M HCl solution. After stirring for 5 min the solution was concentrated to dryness under reduced pressure to obtain a green solid. This process was repeated three times to fully protonate the compound. The hydrochloride of 14 (100 mg, 0.07 mmol, 1 equiv) was then dissolved in methanol and NaBArF (196 mg, 0.22 mmol, 3.1 equiv) was added. The desired product was precipitated by adding water to the solution and subsequent centrifugation. The obtained sticky solid was dissolved in MeOH, again precipitated with water and centrifuged. This process was repeated three times. A2 was obtained as a purple solid after lyophilisation from benzene (0.160 mg, 0.02 mmol, 58%).  =163.7, 163.3, 162.4, 135.8, 133.5, 131.3, 131.0,   130.9, 130.6, 130.4, 130.4-130.3 , 129.1 -128.9, 128.5, 126.9, 124.7, 118

3-Formyl-dibenzo[24]crown-8 (20)
3,4-Dihydroxybenzaldehyde (1.381 g, 10.00 mmol, 1 equiv) was dissolved in 100 mL of dry DMF in two necked round-bottomed flask and Cs 2 CO 3 powder (11.40 g, 35.00 mmol, 3.5 equiv) was added. The mixture was allowed to stir for 15 min, after which 19 (5.50 g, 12.66 mmol, 1.1 equiv) dissolved in 100 mL of dry DMF was added dropwise over 2 h at 60 °C. Then the temperature was raised to 85 °C and the reaction mixture was stirred for 5 d at this temperature. Afterwards the solvent was removed under reduced pressure, 100 mL of water were added to the residue and the mixture was extracted with CH 2 Cl 2 three times. The combined organic extracts were dried over anhydrous sodium sulfate and the solvent was removed under reduced pressure to give the crude product. Subsequent purification by column chromatography (SiO 2 , CH 2 Cl 2 /MeOH 98:2) yielded 20 (2.1 g, 4.00 mmol, 40%) as a white powder. Then TEA (1 mL) was added and the major impurities were removed by filtration through a silica plug using CH 2 Cl 2 as eluent. The pure product was obtained by column chromatography (neutral alumina, CH 2 Cl 2 /petrolether 1:3) and preparative GPC as a purple solid (0.80 g, 1.12 mmol, 59%).