Modulation of N···I and ⁺N−H···Cl⁻···I Halogen Bonding: Folding, Inclusion, and Self-Assembly of Tri- and Tetraamino Piperazine Cyclophanes

The acidity of the crystallization conditions was successfully employed in modulating the balance between the robust intramolecular hydrogen bonding (HB) and intermolecular halogen bonding (XB) observed in large tri- (1) and tetraamino (2) piperazine cyclophanes. A careful crystallization of the title XB acceptor cyclophanes with a strong bidentate XB donor 1,4-diiodotetrafluorobenzene (F4DIB) from CHCl3:MeOH, dimethylformamide (DMF), or HCl:H2O:EtOH resulted in X-ray quality crystals of 1·F4DIB, 2@DMF, 2·2@F4DIB, [1H3]Cl3·(F4DIB)6, and [2H6]Cl6·(F4DIB)2. The intramolecular hydrogen bonding pattern in 1 and 2 was retained in neutral protic and aprotic solvents, and regular N−H···N hydrogen and C−I···N halogen bonding synthons were observed for 1·F4DIB, 2@DMF, and 2·2@F4DIB. Because of its unaffected Pac-Man (alias Pacman) motif, cyclophane 2 manifested a size-selective complexation of DMF over F4DIB under neutral conditions. Acidic conditions led to the protonation of the cyclophanes, and +N−H···Cl−, CPFC−I···Cl−, and CPFC−I···Cl−···I−CPFC synthons appeared as structural elements. The CPFC−I···Cl−···I−CPFC anion resulted in the first XB rotaxane motif which was observed in [1H3]Cl3·(F4DIB)6 where the chloride anion was bound in the center of 1 via EtOH-mediated hydrogen bonding.