Sterically crowded azulene-based dication salts as novel guests: synthesis and complexation studies with crown ethers and calixarenes in solution and in the gas phase

Abstract

The 1,4-bis(3-guaiazulenylmethylium)benzene 1 and 1,4-bis[1-(4,6,8-trimethylazulenylmethylium)]benzene 2 dication salts were synthesized via an acid-catalyzed condensation/dehydration protocol with guaiazulene–terephthalaldehyde (2 : 1 ratio), and 4,6,8-trimethylazulene–terephthalaldehyde (2 : 1 ratio) respectively in one-pot processes. A similar condensation reaction with the parent azulene led to an insoluble oligomer that was shown by MALDI-TOF-MS to contain 1,4-bis[(diazulenyl)methylium]benzene as a repeating unit. Dication salts 1 and 2 were fully characterized by 2D NMR and NOE techniques and by electrospray-MS (ES-MS) and MALDI-TOF-MS. NMR studies confirm that the dications are best represented as bis-tropylium species. A delicate balance of electronic (inductive stabilization) and steric influence of the alkyl groups on the seven-membered ring seems to influence the chemo-/regioselectivity of the co-condensation process. NMR titration and T₁ measurements established that, despite its highly crowded structure, dication 1 forms host–guest HG complexes with dibenzo-30-crown-10 (DB30C10) and dibenzo-24-crown-8 (DB24C8) in solution, but fails to complex with the smaller dibenzo-18-crown-6 (DB18C6). The corresponding HG cation–molecule cluster ions were also detected in the gas phase by ES-MS, showing the formation of both dication–crown 1 : 1 and 1 : 2 complexes. Similar complexation of dication salt 2 with DB30C10 was observed via NMR titration and T₁ measurements in solution and by ES–MS in the gas phase. Although solution complexation studies (NMR titration) did not indicate stable complex formation between 1 and p-tert-butyl-methoxycalix[8]arene, their [HG]²⁺ and [H₂G]²⁺ clusters were detectable by ES-MS. Solution decomplexation experiments (HG²⁺ → H + G²⁺) were performed on 1–crown complex by addition of DMSO, acetone, silver tosylate, and tropylium cation salt. Complexation of 1 with DB30C10 was also studied by microcalorimetric titration.