Cyclophane-Type Fullerene-dibenzo[18]crown-6 Conjugates withtrans-1,trans-2, andtrans-3 Addition Patterns: Regioselective Templated Synthesis, X-Ray Crystal Structure, Ionophoric Properties, and Cation-Complexation-Dependent Redox Behavior

The fullerene-crown ether conjugates (±)-1 to (±)-3 with trans-1 ((±)-1), trans-2 ((±)-2), and trans-3 ((±)-3) addition patterns on the C-sphere were prepared by Bingel macrocyclization. The trans-1 derivative (±)-1 was obtained in 30% yield, together with a small amount of (±)-2 by cyclization of the dibenzo[18]crown-6(DB18C6)-tethered bis-malonate 4 with C60 (Scheme 1). When the crown-ether tether was further rigidified by K+-ion complexation, the yield and selectivity were greatly enhanced, and (±)-1 was obtained as the only regioisomer in 50% yield. The macrocyclization, starting from a mixture of tethered bis-malonates with anti (4) and syn (10) bisfunctionalized DB18C6 moieties, afforded the trans-1 ((±)-1, 15%), trans-2 ((±)-2, 1.5%), and trans-3 ((±)-3, 20%) isomers (Scheme 2). Variable-temperature 1H-NMR (VT-NMR) studies showed that the DB18C6 moiety in C2-symmetrical (±)-1 cannot rotate around the two arms fixing it to the C-sphere, even at 393 K. The planar chirality of (±)-1 was confirmed in 1H-NMR experiments using the potassium salts of (S)-1,1′-binaphthalene-2,2′-diyl phosphate ((+)-(S)-19) or (+)-(1S)-camphor-10-sulfonic acid ((+)-20) as chiral shift reagents (Fig. 1). The DB18C6 tether in (±)-1 is a true covalent template: it is readily removed by hydrolysis or transesterification, which opens up new perspectives for molecular scaffolding using trans-1 fullerene derivatives. Characterization of the products 11 (Scheme 3) and 18 (Scheme 4) obtained by tether removal unambiguously confirmed the trans-1 addition pattern and the out-out geometry of (±)-1. VT-NMR Studies established that (±)-2 is a C2-symmetrical out-out trans-2 and (±)-3 a C1-symmetrical in-out trans-3 isomer. Upon changing from (±)-1 to (±)-3, the distance between the DB18C6 moiety and the fullerene surface increases and, correspondingly, rotation of the ionophore becomes increasingly facile. The ionophoric properties of (±)-1 were investigated with an ion-selective electrode membrane (Fig. 2 and Table 2), and K+ was found to form the most stable complex among the alkali-metal ions. The complex between (±)-1 and KPF6 was characterized by X-ray crystal-structure analysis (Figs. 3 and 4), which confirmed the close tangential orientation of the ionophore atop the fullerene surface. Addition of KPF6 to a solution of (±)-1 resulted in a large anodic shift (90 mV) of the first fullerene-centered reduction process, which is attributed to the electrostatic effect of the K+ ion bound in close proximity to the C-sphere (Fig. 5). Smaller anodic shifts were measured for the KPF6 complexes of (±)-2 (50 mV) and (±)-3 (40 mV), in which the distance between ionophore and fullerene surface is increased (Table 3). The effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of (±)-1 were investigated (Table 4). These are the first-ever observed effects of cation complexation on the redox properties of the C-sphere in fullerene-crown ether conjugates.