Cyclophane-type fullerene-dibenzo[18]crown-6 conjugates with trans-1, trans-2, and trans-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-sph ere were prepared by Bingel macrocyclization. The trans-1 derivative (+/-)- 1 was obtained in 30% yield, together with a small amount of (+/-)-2 by cyc lization of the dibenzo[18]crown-6(DB18C6)-tethered bis-malonate 4 with C-6 0 (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, st arting from a mixture of tethered bis-malonates with anti (4) and syn (10) bisfunctionalized DB18C6 moieties, afforded the trans-1 ((+/-)-1, 15%), tra ns-2 ((+/-)-2, 1.5%), and trans-3 ((+/-)-3, 20%) isomers (Scheme 2). Variab le-temperature H-1-NMR (VT-NMR) studies showed that the DB18C6 moiety in C- 2-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 H-1 -NMR experiments using the potassium salts of (S)-1,1'-binaphthalene-2,2'-d iyl phosphate ((+/-)-(S)-19) or (+)-(1S)-camphor-10-sulfonic acid ((+)-20) as chiral shift reagents (Fig. 1). The DB18C6 tether in (+/-)-1 is a true c ovalent template: it is readily removed by hydrolysis or transesterificatio n, which opens up new perspectives for molecular scaffolding using trans-1 fullerene derivatives. Characterization of the products 11 (Scheme 3) and 1 8 (Scheme 4) obtained by tether removal unambiguously confirmed the trans-1 addition pattern and the out-out geometry of (+/-)-1. VT-NMR Studies estab lished that (+/-)-2 is a C-2-symmetrical out-out trans-2 and (+/-)-3 a C-1- symmetrical in-out trans-3 isomer. Upon changing from (+/-)-1 to (+/-)-3, t he distance between the DB18C6 moiety and the fullerene surface increases a nd. correspondingly, rotation of the ionophore becomes increasingly facile. The ionophoric properties of (+/-)-1 were investigated with an ion-selecti ve electrode membrane (Fig. 2 and Table 2), and K+ was found to form the mo st 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 lar-ge anodic shift (90 mV) of the first fullerene-centered reduction process, which is attributed to the electrostatic effect of the K+ ion boun d in close proximity to the C-sphere (Fig. 5). Smaller anodic shifts were m easured for the KPF6 complexes of (+/-)-2 (50 mV) and (+/-)-3 (40 mV), in w hich the distance between ionophore and fullerene surface is increased (Tab le 3). The effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of (+/-)-1 were investigated (Table 4). These are t he first-ever observed effects of cation complexation on the redox properti es of the C-sphere in fullerene-crown ether conjugates.